Optical Glossary:
Essential Terms for ABO & NCLE Exams

Definition: Numerical measure of a lens material's chromatic dispersion, named after Ernst Abbe. The Abbe value (also called V-value or constringence) indicates how much a material separates white light into its component colors. Higher Abbe values mean less chromatic aberration and better optical quality.

Exam Relevance: ABO exam questions test understanding of how material properties affect optical performance. You need to know that CR-39 has an excellent Abbe value of 58, while polycarbonate has a poor Abbe value of 30. This explains why polycarbonate lenses can show more color fringing, especially in peripheral vision. Questions may ask you to compare materials or recommend lenses based on optical quality needs.

Example: CR-39 (Abbe 58) provides superior color fidelity compared to polycarbonate (Abbe 30), making it preferred for patients sensitive to chromatic aberration.

See also: Chromatic Aberration, Refractive Index (n), CR-39

Definition: The eye's ability to change optical power to maintain clear focus on objects at varying distances. This occurs through contraction and relaxation of the ciliary muscle, which changes the shape of the crystalline lens. The lens becomes more convex (steeper) for near vision and more relaxed (flatter) for distance.

Exam Relevance: Critical concept for both ABO and NCLE exams, particularly regarding presbyopia. NCLE questions focus on the physiological mechanism and age-related changes. The amplitude of accommodation decreases predictably with age, typically becoming noticeable around age 40-45. You must understand that presbyopia is not a refractive error but a loss of accommodation.

Example: A 25-year-old can accommodate approximately 10-12 diopters, easily reading at 10cm. A 50-year-old with only 2D of accommodation remaining needs reading glasses for the same task.

See also: Presbyopia, Amplitude of Accommodation, Crystalline Lens

Definition: The additional plus power incorporated into the near portion of multifocal lenses (bifocals, trifocals, or progressives) to compensate for loss of accommodation. Add power is always expressed in plus diopters and represents the difference between distance and near prescriptions.

Exam Relevance: ABO exam frequently tests add power measurement with a lensometer and calculation of near prescription from distance Rx. You must know typical add power ranges from +0.75D to +3.00D, with most presbyopes needing between +1.50D and +2.50D. Questions often involve calculating effective power in the reading segment.

Example: Distance Rx: -2.00 DS, Add: +2.00 → Near Power: Plano. Or: Distance Rx: +1.00 DS, Add: +2.50 → Near Power: +3.50 DS

See also: Bifocal, Progressive Addition Lenses (PALs), Presbyopia, Lensometer (Lensmeter/Focimeter)

Definition: A type of astigmatism where the steepest corneal or refractive meridian is oriented horizontally (at or near 180 degrees ± 30 degrees). The flattest meridian is vertical (at or near 90 degrees). This is opposite to the more common with-the-rule astigmatism pattern.

Exam Relevance: Heavily tested on NCLE exam as it affects contact lens fitting strategy. Against-the-rule (ATR) astigmatism is more common in older patients and presents challenges for soft toric lens fitting due to lens rotation. You need to know that ATR often requires sphere-equivalent fitting with soft lenses or RGP lenses for best results. Expect questions about keratometry readings and fitting approaches.

Example: K readings of 45.00 @ 180 / 42.00 @ 090 indicate 3.00D of against-the-rule corneal astigmatism. The steeper curve is horizontal.

See also: With-the-Rule Astigmatism, Oblique Astigmatism, Keratometry (K Readings), Toric Contact Lens

Definition: An RGP (rigid gas permeable) contact lens fitting relationship where the base curve of the lens closely matches the flattest corneal meridian, typically 0.00 to 0.50 diopters flatter than the flat K reading. This creates optimal lens-to-cornea relationship with proper tear exchange.

Exam Relevance: One of the most frequently tested concepts on NCLE exam. You must be able to identify an alignment fit by its fluorescein pattern: a thin, even band of fluorescein in the mid-periphery with minimal central pooling or bearing. Questions often show fluorescein patterns and ask you to identify the fitting relationship and recommend adjustments if needed.

Example: If flat K = 43.00D, an alignment fit base curve would be 43.00D to 43.50D (7.85mm to 7.75mm radius).

See also: Steep Fit, Flat Fit, Fluorescein Pattern, Base Curve, RGP Lens (Rigid Gas Permeable)

Definition: The maximum amount of accommodation (measured in diopters) that an eye can exert, representing the difference between the far point and near point of clear vision. It decreases predictably with age due to loss of crystalline lens elasticity.

Exam Relevance: NCLE exam questions test calculation and clinical implications for presbyopic patients. Multiple formulas exist, with Hofstetter's formula being most common: Minimum = 15 - 0.25(age), Average = 18.5 - 0.30(age), Maximum = 25 - 0.40(age). ABO may test basic concept related to add power determination.

Example: A 40-year-old has approximately 5-6D amplitude of accommodation remaining (18.5 - 12 = 6.5D by Hofstetter), requiring +1.50 to +2.00 add for comfortable near work.

See also: Accommodation, Presbyopia, Near Point, Add Power

Definition: A condition where the two eyes have significantly different refractive errors, typically defined as a difference of 1.00 diopter or more in sphere, cylinder, or both. This can be in sphere power only (spherical anisometropia), cylinder only (cylindrical anisometropia), or both.

Exam Relevance: ABO exam tests understanding of dispensing challenges and patient adaptation issues. Anisometropia causes unequal image sizes between the two eyes (aniseikonia), which can lead to eyestrain, poor depth perception, and difficulty adapting to spectacles. Questions may involve recommending contact lenses for high anisometropia or discussing lens design options to minimize thickness differences.

Example: OD: -2.00 DS, OS: -5.00 DS represents 3.00D of anisometropia. The patient may experience a 6-9% image size difference between eyes, potentially causing adaptation problems.

See also: Antimetropia, Aniseikonia

Definition: The fluid-filled space between the posterior surface of the cornea and the anterior surface of the iris and crystalline lens. It is filled with aqueous humor and has an average depth of 3.0 to 3.5 millimeters in normal eyes.

Exam Relevance: NCLE anatomy questions test your knowledge of ocular structures and their clinical significance. You need to know that shallow anterior chambers (less than 2.5mm) are a risk factor for angle-closure glaucoma and may be a contraindication for certain contact lens types. Deep anterior chambers are associated with myopia.

See also: Aqueous Humor, Cornea, Iris, Posterior Chamber

Definition: A multi-layer metallic oxide coating applied to lens surfaces that reduces reflections through the principle of destructive interference. Each layer is precisely calculated to be 1/4 wavelength thick, causing reflected light waves to cancel each other out.

Exam Relevance: ABO exam tests benefits, limitations, and care instructions for AR coatings. You must know that AR coating improves light transmission by approximately 8-10%, reduces glare and halos (especially for night driving), and improves cosmetic appearance by eliminating reflections. Questions often cover proper cleaning methods and warranty considerations.

Example: An uncoated lens reflects about 8% of incident light per surface (16% total for both surfaces). AR coating reduces this to less than 1% per surface, dramatically improving clarity and reducing eye fatigue.

See also: lens-coatings

Definition: The absence of the crystalline lens in the eye, most commonly occurring after cataract extraction surgery without intraocular lens (IOL) implantation. Aphakic eyes lack approximately 20 diopters of refractive power and cannot accommodate.

Exam Relevance: NCLE exam questions focus on the special contact lens fitting considerations for aphakic patients. You need to know that aphakic corrections typically require +10 to +14 diopters of plus power in spectacles, but only +8 to +12 diopters in contact lenses due to vertex distance. Aphakic contact lenses must be monitored carefully for corneal health due to reduced corneal sensitivity and higher risk of complications.

Example: A patient who had cataract surgery in 1975 before IOLs were common might wear a +11.00D contact lens in the aphakic eye, providing much better field of view than thick spectacle lenses.

See also: Pseudophakia, Vertex Distance, Cataract, Intraocular Lens (IOL)

Definition: The thinnest edge of a prism, located opposite the base. In ophthalmic prisms, light rays are deviated toward the base and away from the apex. The apex represents the point or edge of the prism with maximum angle.

Exam Relevance: ABO exam tests basic prism concepts and notation. You must understand that light bends toward the prism base, while images appear displaced toward the apex. This is fundamental for understanding how prism corrects eye misalignment and for working with Prentice's Rule.

See also: Prism, Base Direction, Prentice's Rule

Definition: The clear, watery fluid that fills the anterior and posterior chambers of the eye. It is continuously produced by the ciliary body at a rate of approximately 2-3 microliters per minute and drains through the trabecular meshwork at the angle.

Exam Relevance: NCLE anatomy questions test your knowledge of aqueous humor function: providing nutrients to the avascular cornea and lens, removing metabolic waste, maintaining intraocular pressure (IOP), and providing refractive power. You should understand its role in glaucoma pathophysiology.

See also: Anterior Chamber, Ciliary Body, trabecular-meshwork

Definition: A lens design that incorporates continuously changing surface curvature rather than a constant spherical curve. The surface curve gradually flattens from center to edge (in minus lenses) or steepens (in plus lenses), reducing spherical aberration and allowing for thinner, flatter lens designs.

Exam Relevance: ABO exam frequently tests aspheric lens advantages versus traditional spherical designs. You must know the benefits: reduced spherical aberration providing better peripheral vision, flatter front surface for better cosmetics, reduced magnification or minification, and thinner/lighter lenses especially in higher powers. Questions may ask about appropriate patient selection or comparing different lens designs.

Example: A -6.00D aspheric lens can be 20-30% thinner at the edges and provide superior peripheral clarity compared to a traditional spherical -6.00D lens.

See also: Spherical Aberration, atoric, lens-design, Base Curve

Definition: A refractive error where the eye has two different optical powers in perpendicular meridians, causing light to focus at two separate focal points rather than a single point on the retina. This occurs when the cornea or crystalline lens has a toric (football-shaped) rather than spherical surface.

Exam Relevance: Heavily tested on both ABO and NCLE exams. ABO focuses on prescriptions, cylinder transposition, and spectacle lens considerations. NCLE emphasizes keratometry readings, types (with-the-rule, against-the-rule, oblique), and contact lens correction strategies. You must understand the difference between corneal astigmatism (measured with keratometer) and lenticular astigmatism (in the crystalline lens), as well as residual astigmatism in contact lens wearers.

Example: Prescription: -2.00 -1.50 x 090 means -2.00D sphere power at 90° axis (no cylinder there) and -3.50D total power at 180° (perpendicular to axis).

See also: Cylinder Power, Axis, With-the-Rule Astigmatism, Against-the-Rule Astigmatism, Keratometry (K Readings), toric-lens

Definition: An advanced lens design that combines aspheric curves on both front and back surfaces to correct for both spherical aberration and astigmatic aberration. The term "atoric" comes from "aspheric" + "toric," indicating optimization for astigmatic prescriptions.

Exam Relevance: ABO exam tests atoric lens as a premium design option for patients with moderate to high astigmatism. You should know that atoric lenses provide superior optics compared to traditional toric lenses, especially in oblique gaze and peripheral vision. They're particularly beneficial for prescriptions with cylinder power of 2.00D or more.

See also: Aspheric Lens, toric-lens, Spherical Aberration, Astigmatism

Definition: The meridian of a cylindrical lens that contains no cylinder power, measured in degrees from 1 to 180 using the TABO notation system. The axis indicates the orientation of the cylinder correction, with the actual cylinder power acting 90 degrees away from the axis.

Exam Relevance: Critical concept for both ABO and NCLE exams. ABO tests axis measurement with lensometer, axis transposition, and proper lens marking. NCLE emphasizes axis orientation for toric contact lenses and the importance of rotational stability. You must understand that a 5-10 degree axis misalignment can significantly reduce visual acuity in moderate to high astigmats. Common mistake: thinking the axis indicates where the power is (it indicates where there is NO cylinder power).

Example: In prescription -2.00 -1.50 x 090, the axis is at 90° (vertical). The cylinder power of -1.50D acts at 180° (horizontal).

See also: Cylinder Power, Astigmatism, TABO Notation, toric-lens

Definition: In the boxing system, the horizontal width of the lens opening measured at its widest point in millimeters. One of the three key frame measurements along with B (vertical height) and DBL (distance between lenses). Used to calculate frame PD.

Exam Relevance: Essential ABO frame measurement. You must know A measurement is the lens width in boxing system, used in formula: Frame PD = A + DBL. Questions involve frame measurement techniques, calculating frame PD, and determining appropriate frame sizes for prescriptions.

See also: Boxing System, B Measurement, DBL (Distance Between Lenses), Frame PD, Effective Diameter (ED)

Definition: See Amplitude of Accommodation

Exam Relevance: See Amplitude of Accommodation entry for complete information.

See also: Amplitude of Accommodation, Accommodation, Presbyopia

Definition: See Visual Acuity

Exam Relevance: See Visual Acuity entry for complete information.

See also: Visual Acuity, Snellen Chart

Definition: Reduced visual acuity in one or both eyes that cannot be fully corrected with refractive correction, occurring due to abnormal visual development in childhood. Causes include strabismus, significant refractive error, or visual deprivation. Treatment most effective if started before age 7-9.

Exam Relevance: ABO questions test understanding of amblyopia as contraindication for certain lens options. You should know: amblyopic eye won't see 20/20 even with best correction, may affect contact lens vs glasses recommendation, patients need continued use of correction to maintain visual function.

See also: Strabismus, Anisometropia, Visual Acuity

Definition: Any refractive error where light does not focus properly on the retina without correction. Includes myopia, hyperopia, and astigmatism. The opposite of emmetropia (no refractive error).

Exam Relevance: General term for refractive errors tested on both exams. Ametropia requires optical correction (glasses or contacts) to achieve clear vision. Understanding this umbrella term helps organize knowledge of specific refractive errors.

See also: Emmetropia, Myopia (Nearsightedness), Hyperopia (Farsightedness), Astigmatism, Refraction

Definition: A condition where the two eyes perceive images of different sizes, typically caused by anisometropia (different refractive errors). Can cause eyestrain, headaches, and difficulty with binocular vision even when both eyes are corrected.

Exam Relevance: ABO exam tests aniseikonia as complication of anisometropia. You must know: spectacles magnify/minify differently between eyes in anisometropia (especially >2-3D difference), contact lenses reduce aniseikonia because they're at corneal plane. Questions involve when to recommend contacts vs glasses for anisometropes.

See also: Anisometropia, Magnification, contact-lens-advantage

Definition: A form of anisometropia where one eye is myopic and the other hyperopic. Less common than anisometropia with both eyes similar but different powers. Creates significant optical challenges.

Exam Relevance: ABO questions test this as extreme form of anisometropia. Antimetropia creates major image size differences and binocular vision challenges with spectacles. Contact lenses strongly recommended due to reduced aniseikonia.

See also: Anisometropia, Aniseikonia, contact-lens-advantage

Definition: American National Standards Institute standards for ophthalmic products, particularly Z80.1 (prescription lenses) and Z87.1 (safety glasses). Set requirements for optical quality, impact resistance, marking, and performance.

Exam Relevance: ABO exam tests knowledge of applicable standards. Z80.1 covers prescription eyewear tolerances (power, prism, thickness). Z87.1 covers safety eyewear requirements (impact, marking, materials). Questions involve understanding compliance requirements and appropriate applications.

See also: Safety Glasses, Impact Resistance, prescription-tolerance

Definition: The space between the apex of the cornea and the back surface of a contact lens, visible with fluorescein as bright green pooling in RGP fitting. Excessive apical clearance indicates a flat fitting relationship.

Exam Relevance: NCLE concept for RGP fitting evaluation. Apical clearance seen as central bright pooling in fluorescein pattern. Some clearance normal in alignment fit, excessive indicates flat fit. Questions involve fluorescein pattern interpretation.

See also: Flat Fit, Fluorescein Pattern, RGP Lens (Rigid Gas Permeable), Alignment Fit

Definition: A crescent-shaped pattern of corneal epithelial staining at the edge of a contact lens, typically indicating lens edge that is too thick, poorly designed, or damaged. Common with RGP lenses if edge is not properly blended.

Exam Relevance: NCLE complication requiring lens edge modification or replacement. You must recognize arcuate staining pattern and know causes: poor edge design, edge damage (chip), excessive edge thickness, inadequate edge lift. Solution: polish or replace lens, modify edge design.

See also: Corneal Staining, RGP Lens (Rigid Gas Permeable), lens-edge, Fluorescein

Definition: Breaking down oblique cylinder into with-the-rule and against-the-rule components. Advanced optical concept used in some calculations and lens design optimization.

Exam Relevance: Advanced ABO concept that may appear in higher-level questions. Understanding how oblique astigmatism can be resolved into WTR and ATR components helps with complex optical calculations.

See also: Astigmatism, Oblique Astigmatism, Optical Cross

Definition: See TABO Notation

Exam Relevance: See TABO Notation entry for complete information on axis measurement and recording standards.

See also: TABO Notation, Axis, Cylinder Power

Definition: The reciprocal of the distance from the back surface of a lens to its secondary focal point, measured in diopters. This is the effective power of a lens as measured by a lensometer and represents the actual power affecting the eye when the lens is worn at the specified vertex distance.

Exam Relevance: Fundamental ABO concept for lensometry. You must always measure and verify back vertex power (BVP) for spectacle lenses because that's the surface closest to the eye. Front vertex power is only used for high-plus lenses in low vision or contact lens verification. Questions often involve understanding why BVP differs from nominal power in thick lenses.

See also: Front Vertex Power (FVP), Lensometer (Lensmeter/Focimeter), Vertex Distance, effective-power

Definition: In spectacles: the front surface curve of the lens, typically specified as the curve used for a particular power range and affecting lens thickness and aberrations. In contact lenses: the back surface curve that rests on the cornea, measured in millimeters of radius or diopters, critical for proper fit.

Exam Relevance: Important for both exams but with different contexts. ABO: understanding base curve series, compensated curves, and how base curve selection affects lens thickness and performance. NCLE: base curve selection for proper contact lens fit relative to corneal curvature. Questions may test conversion between radius (mm) and diopters, or selecting appropriate base curve based on keratometry.

Example: Spectacle: A -4.00D lens might use a +2.00D base curve. Contact lens: A patient with K readings of 44.00D/45.00D might fit an 8.4mm (40.00D) base curve soft lens.

See also: Keratometry (K Readings), Alignment Fit

Definition: The orientation of prism in a lens, indicated by where the thickest part (base) of the prism is positioned. Described as Base Up (BU), Base Down (BD), Base In (BI), or Base Out (BO). Can also be specified in degrees from 1-360 using the Prentice position notation.

Exam Relevance: ABO exam heavily tests prism notation and base direction. You must know standard conventions: Base In helps exophoria, Base Out helps esophoria, Base Up and Base Down help vertical phorias. Questions involve reading prism prescriptions, calculating induced prism with Prentice's Rule, and understanding how base direction affects light deviation.

Example: 2∆ BU OD means 2 prism diopters base up in the right eye, used to correct right hyperphoria.

See also: Prism, Prentice's Rule, Phoria, Base In Prism (BI), Base Out Prism (BO)

Definition: Prism with its base oriented toward the nose (nasally). Base in prism deviates light rays nasally, causing images to appear displaced temporally, which forces the eye to converge (turn inward) to see the image clearly.

Exam Relevance: ABO questions test understanding of which base direction corrects which eye alignment problem. Base In prism is prescribed for exophoria or exotropia (eye turning outward), as it forces the eye to turn inward to see through the prism. You need to remember: BI helps exo conditions.

Example: 3∆ BI OU prescribes 3 prism diopters base in for both eyes, providing 6∆ total convergence demand to correct exophoria at near.

See also: Base Direction, Base Out Prism (BO), Exophoria, Convergence

Definition: Prism with its base oriented away from the nose (temporally). Base out prism deviates light rays temporally, causing images to appear displaced nasally, which forces the eye to diverge (turn outward) to see the image clearly.

Exam Relevance: ABO exam tests which prism corrects which condition. Base Out prism is prescribed for esophoria or esotropia (eye turning inward), as it forces the eye to turn outward. Less commonly prescribed than Base In since most people have exophoria at near.

Example: 4∆ BO OD prescribes 4 prism diopters base out for the right eye only, helping correct right esophoria.

See also: Base Direction, Base In Prism (BI), Esophoria, Divergence (Eye Movement)

Definition: A lens containing two distinct optical zones with different powers: a distance portion (usually the majority of the lens) and a near reading segment (typically in the lower portion). The two zones are separated by a visible line, with the add power providing additional plus for near tasks.

Exam Relevance: ABO exam extensively tests bifocal types, measurements, and dispensing. You must know common styles: FT-28 (flat-top 28mm), FT-35, round segment, Executive (full-width), and ribbon. Key measurements: segment height, segment width, optical centers. Important concepts: image jump (sudden prismatic effect at segment line), segment placement based on reading distance and frame fit.

Example: FT-28 bifocal is most popular: distance Rx: -2.00 DS, Add +2.00, resulting in Plano power in the 28mm diameter reading segment.

See also: Add Power, Segment Height (Seg Height), Image Jump, Progressive Addition Lenses (PALs), Trifocal

Definition: The total distance between the centers of the two pupils measured in millimeters when the patient is looking at a distant target. This single measurement represents the full interpupillary distance rather than measuring each eye separately.

Exam Relevance: ABO exam tests PD measurement techniques and normal ranges. Adult binocular PD typically ranges from 54mm to 74mm, with average around 62-64mm. You must know proper measurement technique: patient and measurer at same level, patient fixating on distant target, measuring from center of one pupil to center of other. Questions may involve when to use binocular vs monocular PD.

Example: A patient with binocular PD of 64mm needs their lenses centered so optical centers are 32mm from the frame center on each side.

See also: Monocular PD, Pupillary Distance (PD), Optical Center (OC), Decentration

Definition: An RGP contact lens with different curves on both the back surface (to match corneal toricity) and front surface (to correct residual astigmatism). Both surfaces are toric rather than spherical, providing precise correction for eyes with significant corneal astigmatism and residual astigmatism.

Exam Relevance: NCLE questions test when bitoric lenses are indicated and how they differ from simpler designs. Bitoric lenses are needed when a spherical base curve RGP leaves residual astigmatism greater than 0.75D, or when corneal toricity exceeds 2.50-3.00D. You must understand the complex fitting considerations and the difference from front surface toric or back surface toric lenses.

Example: A patient with 3.50D of corneal astigmatism and 2.50D of refractive astigmatism may need a bitoric RGP to achieve optimal vision and fit.

See also: Toric Contact Lens, RGP Lens (Rigid Gas Permeable), Residual Astigmatism, Back Surface Toric

Definition: The laboratory process of temporarily attaching a lens blank to a metal or plastic block holder (chuck) using low-melting-point alloy, tape, or adhesive pads. This allows the lens to be held securely during surfacing, edging, and polishing operations.

Exam Relevance: ABO exam tests basic understanding of lens fabrication steps. You should know that proper blocking is critical for accurate lens centration and preventing damage during processing. The block is positioned according to the optical center location marked during layout.

See also: Edging, Surfacing (Lens), Optical Center (OC), layout

Definition: A tough, acellular layer of the cornea located between the epithelium and the stroma, composed of collagen fibrils. It is approximately 8-14 micrometers thick and provides structural support to the cornea.

Exam Relevance: NCLE anatomy questions test corneal layer structure and clinical significance. Critical fact: Bowman's layer does not regenerate if damaged. Scars in Bowman's layer are permanent and can affect vision. This layer is the target in some refractive surgeries (PRK, LASIK) and can be affected by corneal dystrophies.

See also: corneal-anatomy, Corneal Epithelium, Corneal Stroma, Descemet's Membrane

Definition: The standard frame measurement system that uses the smallest rectangle that completely encloses each lens shape. Measurements include A dimension (horizontal width), B dimension (vertical height), and DBL (distance between lenses), all in millimeters.

Exam Relevance: Critical ABO concept tested extensively. You must know how to measure frames using boxing system and calculate key values: Frame PD = A + DBL, Effective Diameter (ED) = √(A² + B²) for round shapes. Questions involve determining minimum blank size, calculating decentration, and understanding geometric center versus optical center.

Example: Frame marked 52-18-140 means: A=52mm, DBL=18mm, temple=140mm. Frame PD = 52+18 = 70mm.

See also: A Measurement, B Measurement, DBL (Distance Between Lenses), Effective Diameter (ED), Geometric Center (GC)

Definition: A handheld ultraviolet light source with cobalt blue filter used to illuminate fluorescein dye on the eye for assessing contact lens fit, corneal integrity, and tear film. It provides a portable alternative to slit lamp examination.

Exam Relevance: NCLE exam tests knowledge of examination instruments and techniques. Burton lamp is essential for RGP fitting evaluation in office settings without slit lamps. You should know it uses UV light to excite fluorescein, which emits yellow-green light, allowing visualization of the tear layer between lens and cornea.

See also: Fluorescein, Fluorescein Pattern, Slit Lamp (Biomicroscope), rgp-fitting

Definition: In the boxing system, the vertical height of the lens opening measured at its deepest point in millimeters. Used along with A measurement to calculate effective diameter and determine minimum blank size requirements.

Exam Relevance: Important ABO measurement for blank size calculations. Combined with A measurement in formula: ED = √(A² + B²) for round shapes. Larger B measurements require larger blanks and affect seg height positioning in bifocals.

See also: Boxing System, A Measurement, Effective Diameter (ED), Segment Height (Seg Height)

Definition: An RGP contact lens with toric back surface (two different base curves in perpendicular meridians) to match corneal toricity. Provides better fit for high corneal astigmatism but doesn't add cylinder power correction by itself.

Exam Relevance: NCLE concept for RGP toric fitting. Back surface toric conforms to toric cornea but relies on tear lens for astigmatism correction. If residual astigmatism remains, need front surface toric (bitoric). Questions test when each toric type is indicated.

See also: Bitoric Contact Lens, Toric Contact Lens, RGP Lens (Rigid Gas Permeable), corneal-astigmatism

Definition: A soft contact lens used therapeutically to protect damaged corneal epithelium, promote healing, reduce pain, or deliver medication. Not primarily for vision correction. Typically high-Dk silicone hydrogel.

Exam Relevance: NCLE questions test therapeutic lens applications. Bandage lenses indicated for: corneal abrasions, post-surgical healing, recurrent erosions, corneal edema (bullous keratopathy), chemical burns. Requires medical supervision, often with topical antibiotics.

See also: therapeutic-lens, Corneal Abrasion, Silicone Hydrogel

Definition: Prism with base oriented downward. Deviates light downward, causing images to appear displaced upward, forcing the eye to look down. Used to correct hyperphoria (eye deviating upward).

Exam Relevance: ABO vertical prism correction. Base Down corrects hyperphoria. Right hyperphoria corrected with BD OD (or BU OS). Questions test which base direction corrects which vertical deviation.

See also: Base Direction, Base Up Prism (BU), Hyperphoria, vertical-prism

Definition: Prism with base oriented upward. Deviates light upward, causing images to appear displaced downward, forcing the eye to look up. Used to correct hypophoria (eye deviating downward).

Exam Relevance: ABO vertical prism correction. Base Up corrects hypophoria. Left hyperphoria (same as right hypophoria) corrected with BU OD (or BD OS). Understanding vertical prism essential for prism prescriptions.

See also: Base Direction, Base Down Prism (BD), Hypophoria, vertical-prism

Definition: The best visual acuity achievable with optimal refractive correction (glasses or contacts). Used to assess true visual potential and identify pathology. If BCVA is reduced, indicates problem beyond refractive error.

Exam Relevance: Both exams test understanding that BCVA distinguishes refractive error from disease. If patient doesn't achieve expected BCVA (typically 20/20 or 20/25), suggests: amblyopia, cataract, macular degeneration, or other pathology requiring medical evaluation.

See also: Visual Acuity, Refraction, Amblyopia (Lazy Eye)

Definition: The angled edge finish on a lens that allows it to fit securely in a frame groove. Apex of bevel sits in groove. Also refers to safety bevel on rimless/semi-rimless lenses to prevent chipping.

Exam Relevance: ABO exam tests edging concepts. Proper bevel placement critical for lens retention in frame. Standard bevel for full-rim frames (apex in groove), safety bevel for rimless (flat polish to prevent chips), lenticular bevel for high-power carriers.

See also: Edging, lens-fabrication, rimless-mounting

Definition: The ability to use both eyes together as a coordinated team, creating single three-dimensional perception. Requires proper eye alignment, similar acuity in both eyes, and neural integration. Provides depth perception and expanded visual field.

Exam Relevance: ABO questions test binocular vision concepts and factors affecting it. Conditions impairing binocular vision: strabismus, significant anisometropia, amblyopia. Prism may restore binocular function in some cases. Important for understanding patient symptoms and treatment goals.

See also: stereopsis, Depth Perception (Stereopsis), Phoria, Strabismus

Definition: Chronic inflammation of the eyelid margins, often involving meibomian glands. Causes: bacterial (staph), seborrheic, or meibomian gland dysfunction. Symptoms: red lid margins, crusting, irritation. Significantly affects contact lens tolerance.

Exam Relevance: NCLE questions test blepharitis as common cause of contact lens intolerance. You must know: blepharitis causes poor tear quality, increased deposits, discomfort. Management: lid hygiene (warm compresses, lid scrubs), treat underlying cause. May need to discontinue lens wear during flares.

See also: Meibomian Glands, Dry Eye Syndrome, Tear Film, lid-hygiene

Definition: See Circle of Diffusion. The blurred image on the retina from uncorrected refractive error or imperfect focus. Size of blur circle determines perceived image clarity.

Exam Relevance: Optical concept explaining why uncorrected refractive errors cause blurred vision. Larger blur circle = more blur. Corrective lenses reduce blur circle to minimum (diffraction limit).

See also: circle-of-diffusion, Refraction, image-quality

Definition: A lens design where the optical zone is smaller than the full lens diameter, with the remaining outer area serving as a "carrier" for structural support and cosmetic appearance. Used in high-power lenses to reduce weight and thickness.

Exam Relevance: ABO exam tests lenticular designs for high prescriptions. Minus carrier (lenticular minus) has small optical zone for high minus powers, reducing edge thickness. Plus carrier (myodisc) has small optical zone for high plus powers, reducing center thickness and weight. Questions involve advantages, disadvantages, and patient selection.

Example: A -15.00D lenticular lens might have a 40mm optical zone instead of full 70mm, reducing edge thickness by 40-50%.

See also: Lenticular Lens, Myodisc, high-power-lens

Definition: Clouding or opacity of the crystalline lens causing gradual vision loss. Cataracts develop due to protein denaturation and can be nuclear (central), cortical (peripheral), or posterior subcapsular (back surface). Age-related cataracts are most common, but can also be congenital, traumatic, or medication-induced.

Exam Relevance: NCLE questions cover cataracts as part of patient history and contraindications. You should know signs and symptoms: blurred vision, glare, halos, reduced contrast sensitivity, frequent prescription changes. Post-cataract patients (aphakic or pseudophakic) have special contact lens fitting considerations.

See also: Aphakia, Pseudophakia, Crystalline Lens, Intraocular Lens (IOL)

Definition: The thickness of a lens at its optical center, measured in millimeters. For spectacle lenses, typical center thickness is 2.0-2.5mm. For contact lenses, ranges from 0.07mm (ultra-thin soft lenses) to 0.20mm (RGP lenses).

Exam Relevance: ABO: affects lens power accuracy, weight, impact resistance, and cosmetics. Plus lenses are thick in center, minus lenses thin in center. Must meet minimum thickness for safety. NCLE: center thickness affects oxygen transmission (Dk/t), comfort, and lens handling. Questions involve thickness measurement with gauge and effects on lens performance.

See also: Edge Thickness, thickness-gauge, Dk/t (Oxygen Transmissibility), lens-design

Definition: The proper alignment of a lens's optical center with the patient's pupil or visual axis when the frame is worn in the normal position. Proper centration ensures the patient looks through the intended point of the lens, minimizing unwanted prismatic effects.

Exam Relevance: Critical ABO concept. Poor centration induces prism according to Prentice's Rule. You must know how to measure and achieve proper centration considering frame fit, seg height for multifocals, and pantoscopic tilt. Questions often involve calculating induced prism from decentration or determining required lens decentration.

Example: If optical center is 3mm above the pupil in a -4.00D lens, induced prism = 1.2∆ base down per Prentice's Rule.

See also: Optical Center (OC), Decentration, Prentice's Rule, Induced Prism

Definition: The highly vascular middle layer of the eye located between the sclera and retina. It provides blood supply and oxygen to the outer retinal layers and helps absorb scattered light, improving image quality.

Exam Relevance: NCLE anatomy questions test basic ocular structure. You should know that the choroid is part of the uveal tract (along with iris and ciliary body) and that its rich blood supply gives the red color visible in photographs or with transillumination.

See also: Retina, Sclera, Uvea (Uveal Tract), Ciliary Body

Definition: An optical defect where different wavelengths (colors) of light are focused at different points due to dispersion in the lens material. Short wavelengths (blue) focus closer than long wavelengths (red), causing color fringing and reduced image quality.

Exam Relevance: ABO exam tests relationship between chromatic aberration and Abbe value. Lower Abbe value = more chromatic aberration. You need to know which materials have better or worse chromatic aberration: CR-39 (excellent, V=58), Trivex (good, V=43), Polycarbonate (poor, V=30). Questions may ask about patient complaints of color fringing or material recommendations.

See also: Abbe Value, Dispersion, Lens Material, CR-39

Definition: The structure behind the iris consisting of ciliary muscle (controls accommodation) and ciliary processes (produce aqueous humor). The ciliary muscle is ring-shaped and connects to the crystalline lens via zonules.

Exam Relevance: NCLE anatomy questions test structure and function. You must understand dual function: (1) Accommodation - ciliary muscle contracts for near focus, relaxes for distance. (2) Aqueous production - ciliary processes secrete aqueous humor. Questions may involve accommodation mechanism or aqueous humor dynamics.

See also: Accommodation, Aqueous Humor, Crystalline Lens, Zonules (Zonules of Zinn)

Definition: The transparent mucous membrane covering the anterior sclera (bulbar conjunctiva) and inner surface of the eyelids (palpebral conjunctiva). It produces mucin for the tear film and contains goblet cells.

Exam Relevance: NCLE questions test conjunctival anatomy and contact lens-related complications. You should know common conditions: conjunctivitis (inflammation/infection), Giant Papillary Conjunctivitis (GPC from contact lens deposits), pterygium, and pinguecula. The conjunctiva is highly vascular and responds to irritation with redness.

See also: Giant Papillary Conjunctivitis (GPC), Sclera, Tear Film

Definition: A refraction performed while the patient wears their contact lenses to determine if the lens power is correct or needs adjustment. The over-refraction result indicates what power change is needed in the contact lens prescription.

Exam Relevance: Essential NCLE troubleshooting skill. You must know how to interpret over-refraction results: sphere power found indicates needed power change, cylinder found indicates residual astigmatism (common with spherical lenses on astigmatic eyes). Questions often involve calculating new contact lens power based on over-refraction findings.

Example: Patient wears -3.00D contact lenses. Over-refraction shows -1.00D needed. New contact lens power should be -4.00D.

See also: Residual Astigmatism, Spherical Equivalent

Definition: The simultaneous inward rotation of both eyes to maintain single binocular vision when viewing near objects. Convergence is measured in prism diopters and increases as viewing distance decreases.

Exam Relevance: ABO questions test understanding of convergence demands and relationship to prism and PD. You should know that convergence demand increases at near (requiring Base In prism reserve), and that near PD is typically 2-3mm smaller than distance PD. Convergence insufficiency causes near vision problems despite clear acuity.

See also: Binocular Vision, Near PD, Base In Prism (BI), Esophoria

Definition: The clear, dome-shaped anterior surface of the eye providing approximately 40-44 diopters of the eye's total refractive power (about 2/3). The cornea is avascular and receives oxygen from tears and atmosphere, and nutrients from aqueous humor.

Exam Relevance: Critical for both exams with different emphasis. NCLE: Five layers (epithelium, Bowman's, stroma, Descemet's, endothelium), corneal health, oxygen requirements, and contact lens effects. ABO: Corneal power contribution, how spectacle correction relates to corneal curvature. Both: Average corneal diameter 11-12mm, central thickness 0.50-0.55mm.

See also: corneal-anatomy, Keratometry (K Readings), Corneal Epithelium, Corneal Endothelium, Dk (Oxygen Permeability)

Definition: Swelling of the cornea due to fluid accumulation in the stroma, most commonly caused by hypoxia (oxygen deprivation) from contact lens overwear or poor oxygen transmission. Can also result from endothelial dysfunction or trauma.

Exam Relevance: Major NCLE topic for contact lens complications. You must know signs and symptoms: blurred vision, halos around lights, photophobia, corneal haze, and microcysts. Caused by inadequate Dk/t, overnight wear of low-oxygen lenses, or overwear. Requires immediate lens discontinuation and possible change to higher-Dk materials. Questions often involve identifying causes and treatment.

Example: Patient wearing low-Dk soft lenses overnight develops 5% corneal edema, experiencing hazy vision and halos. Treatment: discontinue wear, switch to higher-Dk silicone hydrogel for daily wear only.

See also: Hypoxia, Dk (Oxygen Permeability), Dk/t (Oxygen Transmissibility), corneal-stroma, overwear

Definition: Damage to the corneal epithelium that becomes visible when fluorescein dye is applied, appearing as bright green areas under blue light. Staining indicates epithelial cell loss or damage and can result from various contact lens complications.

Exam Relevance: Critical NCLE concept for identifying and troubleshooting problems. You must know common staining patterns and their causes: 3&9 o'clock staining (inadequate edge lift or dryness in RGP lenses), central staining (tight lens, inadequate movement, or solution toxicity), superior staining (incomplete blinking, GPC), arcuate staining (poor edge design). Questions often show patterns and ask for diagnosis and treatment.

See also: Fluorescein, Corneal Epithelium, solution-toxicity

Definition: Columbia Resin #39, the most common plastic lens material (allyl diglycol carbonate), with refractive index 1.498, specific gravity 1.32, and Abbe value 58. Introduced in the 1940s and remains the gold standard for optical quality.

Exam Relevance: ABO exam frequently tests CR-39 properties and comparisons to other materials. You must know advantages: excellent optical quality (highest Abbe value), good scratch resistance with coating, lower cost, easy to tint. Disadvantages: thicker than high-index materials, heavier than polycarbonate, does not meet high impact standards without treatment. Standard choice for low to moderate prescriptions.

See also: Lens Material, Abbe Value, Refractive Index (n), Polycarbonate

Definition: The natural biconvex transparent lens inside the eye, located behind the iris and suspended by zonules from the ciliary body. Provides approximately 20 diopters of the eye's refractive power and can change shape (accommodate) for near focus.

Exam Relevance: Both exams test structure and function. NCLE emphasizes accommodation mechanism and age-related changes (loss of elasticity causing presbyopia, opacity causing cataracts). ABO focuses on how crystalline lens contributes to refraction and why its removal (aphakia) requires high plus correction. You should know the lens continues growing throughout life, becoming less flexible.

See also: Accommodation, Presbyopia, Cataract, Aphakia, Zonules (Zonules of Zinn)

Definition: The amount of astigmatic correction in a prescription, expressed in diopters with either plus or minus sign. Cylinder power indicates the difference in refractive power between the two principal meridians of an astigmatic eye.

Exam Relevance: Fundamental for both ABO and NCLE. ABO tests cylinder transposition (plus to minus or vice versa), lensometer reading, and cross-cylinder technique. NCLE focuses on corneal cylinder measured with keratometry and how it relates to contact lens correction (masking with RGP, residual astigmatism). Must understand that cylinder power always acts 90° from the axis.

Example: In Rx: -2.00 -1.50 x 090, the cylinder power is -1.50D, making total power -3.50D at 180° and -2.00D at 090°.

See also: Axis, Astigmatism, Transposition (Cylinder), Keratometry (K Readings), Residual Astigmatism

Definition: The middle and thickest layer of the cornea, comprising about 90% of corneal thickness (~500 micrometers). Composed of highly organized collagen lamellae and keratocytes. Transparency depends on precise collagen arrangement and controlled hydration.

Exam Relevance: NCLE anatomy questions test corneal layers and function. You should know that stromal swelling (edema) from hypoxia disrupts collagen spacing causing haze and reduced vision. Stroma can absorb water when oxygen-deprived. Contains no blood vessels or nerves. Refractive surgeries (LASIK, PRK) reshape stroma to correct vision.

See also: corneal-anatomy, Corneal Edema, Bowman's Layer, Descemet's Membrane

Definition: In spectacle lens design, selection of flatter base curves for minus lenses and steeper curves for plus lenses to maintain better optical performance across the power range. Reduces oblique astigmatism and improves peripheral vision.

Exam Relevance: ABO concept for lens design optimization. Modern lenses use compensated base curves rather than single base curve for all powers. Improves optics but affects lens thickness. Questions may involve understanding why different powers use different base curves.

See also: Base Curve, Oblique Astigmatism, lens-design

Definition: Photoreceptor cells in the retina responsible for color vision and high-acuity central vision. Concentrated in the macula, especially fovea. Three types (S, M, L) sensitive to short (blue), medium (green), and long (red) wavelengths.

Exam Relevance: NCLE anatomy questions test photoreceptor types and functions. You must know: cones for color and detail (fovea all cones), rods for peripheral and night vision (absent in fovea). Cone dysfunction causes color blindness or reduced central acuity.

See also: rod-cells, Fovea, Macula, Retina, color-vision

Definition: The overall width of a contact lens measured in millimeters. Soft lenses typically 13.5-14.5mm (larger than cornea to ensure coverage), RGP lenses typically 9.0-10.5mm (smaller than cornea). Affects fit, comfort, and centration.

Exam Relevance: NCLE fitting parameter. You must know: soft lens diameter must fully cover cornea plus overlap onto sclera (~1-2mm), RGP diameter typically 1-2mm smaller than corneal diameter for proper movement. Questions involve selecting appropriate diameter based on corneal measurements.

See also: Soft Contact Lens, RGP Lens (Rigid Gas Permeable), Limbus, Corneal Diameter (Horizontal Visible Iris Diameter - HVID)

Definition: Scratch or injury to the corneal epithelium causing pain, tearing, photophobia, foreign body sensation. Contact lens-related causes: lens insertion/removal trauma, foreign body under lens, overwear, poor lens condition. Stains brightly with fluorescein.

Exam Relevance: NCLE complication requiring immediate attention. You must know: symptoms (severe pain, red eye, tearing, photophobia), treatment (discontinue lens wear, possible bandage lens, antibiotics), prevention (proper handling, clean lenses). Questions test recognition and management.

See also: Corneal Epithelium, Fluorescein, Bandage Contact Lens, lens-handling

Definition: The shape of the anterior corneal surface, measured with keratometry in diopters or millimeters of radius. Normal range approximately 41-46D (7.2-8.2mm radius). Determines corneal refractive power and contact lens base curve selection.

Exam Relevance: Fundamental NCLE concept. Corneal curvature measured by keratometry provides base curve selection guidance, identifies astigmatism type/amount, and explains refractive error origins. Questions involve K reading interpretation and application to fitting.

See also: Keratometry (K Readings), k-readings, base-curve-selection, corneal-astigmatism

Definition: The horizontal width of the visible cornea, measured from limbus to limbus, typically 11.0-12.5mm (average ~11.7mm). Used to select appropriate soft contact lens diameter, which should be 1.5-2.0mm larger than corneal diameter.

Exam Relevance: NCLE measurement for soft lens fitting. You should measure or estimate HVID to select lens diameter. Soft lens should overlap cornea by ~1-2mm all around for stability and comfort. Questions involve diameter selection based on eye size.

See also: Contact Lens Diameter, Limbus, Soft Contact Lens

Definition: The outermost layer of the cornea, 5-6 cells thick (~50 micrometers), providing protective barrier and smooth optical surface. Regenerates continuously from limbal stem cells, complete turnover every 7-10 days. Damage causes staining with fluorescein.

Exam Relevance: Critical NCLE anatomy. Epithelium is first defense against infection and injury. Contact lens complications often involve epithelial damage: staining, erosions, edema. You must understand epithelial regeneration ability (heals quickly) but vulnerability to hypoxia and mechanical trauma.

See also: corneal-anatomy, Corneal Staining, Fluorescein, Corneal Abrasion

Definition: The innermost layer of the cornea, single cell layer that pumps fluid out of stroma maintaining corneal clarity and optimal thickness. Does NOT regenerate - cells lost permanently. Normal count ~2500-3000 cells/mm². Critical for long-term corneal health.

Exam Relevance: Important NCLE anatomy. Endothelium is most vulnerable to long-term contact lens hypoxia. You must know: cells don't regenerate (unlike epithelium), chronic hypoxia causes cell loss (polymegethism, pleomorphism), adequate Dk/t critical for endothelial health. Endothelial damage cumulative and permanent.

See also: corneal-anatomy, Hypoxia, Dk (Oxygen Permeability), Corneal Edema

Definition: Advanced diagnostic technique creating detailed color-coded map of corneal curvature across entire surface. Far more detailed than keratometry (which measures only 2 points). Essential for specialty lens fitting, refractive surgery evaluation, and keratoconus diagnosis.

Exam Relevance: NCLE concept for advanced fitting and diagnosis. While not always available, you should understand topography provides complete corneal shape analysis, identifies irregular astigmatism, assists complex RGP/scleral fitting. Questions may involve interpreting topography color patterns.

See also: Keratometry (K Readings), Keratoconus, Irregular Astigmatism, Scleral Lens

Definition: An open sore on the cornea, typically caused by infection (bacterial, viral, fungal, or parasitic). Most serious contact lens complication. Symptoms: severe pain, redness, discharge, photophobia, white spot on cornea, vision loss. MEDICAL EMERGENCY.

Exam Relevance: Most critical NCLE emergency. You must recognize ulcer symptoms immediately and refer urgently. Usually appears as white/gray infiltrate with overlying epithelial defect. Can rapidly progress causing permanent scarring and vision loss. Prevention: proper hygiene, no sleeping in daily wear lenses, no water exposure.

See also: Microbial Keratitis, Keratitis, corneal-infection, pseudomonas

Definition: A specialized lens used in refraction with equal plus and minus cylinder powers at right angles (e.g., +0.25/-0.25). Used to refine cylinder axis and power during subjective refraction. Has handle at 45° to principal meridians.

Exam Relevance: Refraction instrument both exams should understand conceptually. While opticians don't perform refraction, understanding cross-cylinder helps explain how cylinder in prescriptions is determined and refined.

See also: Refraction, Cylinder Power, Axis, Astigmatism

Definition: Refraction performed after instilling cycloplegic drops (atropine, cyclopentolate, tropicamide) that temporarily paralyze accommodation. Reveals true refractive error without accommodative influence. Essential for children and hyperopes.

Exam Relevance: Both exams should understand difference between manifest (without drops) and cycloplegic refraction. Cycloplegic reveals latent hyperopia and prevents accommodation during testing. Particularly important in children who can accommodate to mask hyperopia.

See also: Refraction, Accommodation, Hyperopia (Farsightedness), manifest-refraction

Definition: Proper handling and care practices to prevent contamination and infection. Critical rules: wash hands before handling, use fresh solution daily, never use water, replace case regularly, follow wearing schedule, don't sleep in daily wear lenses.

Exam Relevance: Essential NCLE patient education topic. You must emphasize: hand washing, no water exposure (tap, shower, swimming), proper case care, solution compliance. Poor hygiene is leading cause of serious infections. Questions test education priorities and troubleshooting.

See also: Microbial Keratitis, lens-case-hygiene, water-exposure

Definition: In bifocal and multifocal lenses, the vertical depth of the reading segment measured from the top of the segment line to the lowest point of the segment. Common D measurements are 28mm (FT-28), 35mm (FT-35), and 25mm (round segments).

Exam Relevance: ABO exam tests bifocal measurements and dispensing. You should know that D measurement affects how much near viewing area is available. Larger D measurements provide more reading area but may be more cosmetically visible. Must be differentiated from segment height, which measures placement on the lens.

See also: Bifocal, Segment Height (Seg Height), multifocal

Definition: Contact lenses designed to be worn once and then discarded, typically for 12-16 hours of wear. No cleaning, disinfection, or storage is required. Represent the healthiest and most convenient contact lens wearing modality.

Exam Relevance: NCLE questions test wearing schedules and patient selection. You must know advantages: lowest risk of infection, no solution needed, best for occasional wear, excellent for allergic patients or GPC, reduced deposit buildup. Disadvantages: higher annual cost for daily wearers, limited parameter availability. Questions involve appropriate patient selection and comparing modalities.

See also: Wearing Schedule, replacement-schedule, Giant Papillary Conjunctivitis (GPC)

Definition: The shortest distance between the two lens shapes in the boxing system, measured in millimeters. Also called bridge size. DBL is measured horizontally at the closest point between the two lens openings.

Exam Relevance: Critical ABO measurement concept. You must know the formula: Frame PD = A + DBL. This is essential for calculating required lens decentration and minimum blank size. Questions often involve calculating frame PD or determining if a lens blank is large enough for a given frame and patient PD.

Example: Frame 52-18 means A=52mm, DBL=18mm, so Frame PD = 52+18 = 70mm. Patient has 64mm PD, requiring 3mm decentration per lens.

See also: Boxing System, A Measurement, Frame PD, Decentration

Definition: The displacement of the optical center of a lens from its geometric center or from the patient's pupil position. Can be intentional (to create prism or to fit a prescription in a specific frame) or unintentional (poor fabrication or adjustment).

Exam Relevance: Major ABO concept linked to Prentice's Rule. You must calculate decentration needed when patient PD differs from frame PD, determine induced prism from decentration, and understand when decentration is acceptable versus problematic. Formula: Decentration per lens = (Frame PD - Patient PD) / 2. Questions often involve calculating induced prism or required blank size.

Example: Frame PD 70mm, Patient PD 64mm: Decentration = (70-64)/2 = 3mm per lens. In -4.00D lens: Induced prism = 1.2∆ per eye.

See also: Prentice's Rule, Induced Prism, Optical Center (OC), Frame PD, Minimum Blank Size (MBS)

Definition: The basement membrane of the corneal endothelium, located between the stroma and endothelium. It is very elastic and strong, approximately 10 micrometers thick in adults, and continues to thicken throughout life.

Exam Relevance: NCLE anatomy questions test corneal layers. You should know that Descemet's membrane is highly elastic and can detach from the stroma in trauma (Descemet's detachment). It is produced by endothelial cells and provides support and barrier function.

See also: corneal-anatomy, Corneal Endothelium, Corneal Stroma

Definition: The unit of measurement for refractive power of lenses. One diopter is the reciprocal of the focal length in meters. A +1.00D lens brings parallel light to focus at 1 meter, +2.00D at 0.5 meters, etc.

Exam Relevance: Fundamental unit for both exams. You must understand the relationship: Power (D) = 1/focal length (meters). Plus lenses converge light (positive power), minus lenses diverge light (negative power). Questions involve calculating focal length from power, understanding vergence changes, and converting between different optical measurements.

See also: Focal Length, Vergence, Lens Power, refractive-power

Definition: Double vision where the patient sees two images of a single object. Can be binocular (occurs only with both eyes open, due to eye misalignment) or monocular (persists with one eye, due to optical issues in that eye).

Exam Relevance: ABO questions test understanding of causes and prism correction. Binocular diplopia results from strabismus, phoria, or cranial nerve palsy, often corrected with prism. Monocular diplopia suggests optical issues like uncorrected astigmatism, cataract, or lens problems. You should know how prism corrects diplopia by aligning images.

See also: Prism, Strabismus, Phoria, Binocular Vision

Definition: The separation of white light into its component wavelengths (colors) when passing through a lens material. Different wavelengths refract by different amounts, with shorter wavelengths (blue) bending more than longer wavelengths (red).

Exam Relevance: ABO exam tests relationship to Abbe value and chromatic aberration. Higher dispersion (lower Abbe value) means more color separation and more chromatic aberration. You must understand that dispersion is an inherent property of optical materials and causes degraded image quality in low-Abbe materials like polycarbonate.

See also: Abbe Value, Chromatic Aberration, Lens Material

Definition: A lens aberration where the magnification varies across the lens field, causing straight lines to appear curved. Minus lenses produce barrel distortion (lines curve outward), plus lenses produce pincushion distortion (lines curve inward).

Exam Relevance: ABO questions test lens aberrations and their effects. You should know that distortion increases with lens power and distance from optical center. Aspheric designs reduce distortion. Patients may notice distortion when moving eyes to look through peripheral lens areas, especially with high powers or when first adapting to glasses.

See also: Spherical Aberration, lens-aberrations, Aspheric Lens, peripheral-vision

Definition: A measure of how much oxygen can pass through a contact lens material, expressed in barrer units (10⁻¹¹ cm² × mlO₂)/(sec × ml × mmHg). The "D" represents diffusion coefficient and "k" represents solubility coefficient of oxygen in the material.

Exam Relevance: Critical NCLE concept for material selection and corneal health. You must know: RGP materials typically 100+ Dk, conventional hydrogel 20-30 Dk, silicone hydrogel 80-140 Dk. Higher Dk allows more oxygen to reach the cornea, reducing risk of hypoxia and edema. However, Dk alone doesn't determine oxygen reaching cornea—must consider lens thickness (Dk/t).

Example: Silicone hydrogel material with Dk of 140 allows 5x more oxygen transmission than conventional hydrogel with Dk of 28.

See also: Dk/t (Oxygen Transmissibility), oxygen-transmissibility, Corneal Edema, Hypoxia, Silicone Hydrogel

Definition: The oxygen transmission of a contact lens accounting for both material permeability (Dk) and center thickness (t). This represents the actual amount of oxygen reaching the cornea through the lens. Higher Dk/t values indicate better oxygen supply.

Exam Relevance: Essential NCLE concept tested extensively. You must know minimum Dk/t requirements: ~24 for daily wear (prevents edema), ~87 for extended wear (maintains corneal health). Calculated by dividing Dk by center thickness in centimeters. A thin lens of moderate Dk material may outperform a thick lens of high Dk material.

See also: Dk (Oxygen Permeability), Oxygen Permeability, Center Thickness (CT), Corneal Edema, Extended Wear Contact Lens

Definition: In datum system frame measurement, the horizontal reference line through the deepest points of the lens shape. Alternative to boxing system. Less commonly used than boxing system in modern practice.

Exam Relevance: ABO may test datum system as alternative measurement method. You should understand the concept and how it differs from boxing system measurements.

See also: datum-system, Boxing System, frame-measurements

Definition: The ability to judge relative distances of objects in three dimensions, primarily achieved through binocular vision. Requires similar visual acuity in both eyes and proper alignment. Measured in seconds of arc (normal ≤40 seconds).

Exam Relevance: ABO questions test factors affecting stereopsis. Conditions reducing depth perception: monovision correction, significant anisometropia, strabismus, amblyopia. Important consideration when recommending monovision for presbyopia - warn about potential depth perception loss.

See also: Binocular Vision, stereopsis, Monovision, Anisometropia

Definition: The principle behind anti-reflective coatings where reflected light waves from different coating layers are out of phase, canceling each other out. Each AR coating layer is 1/4 wavelength thick for specific wavelengths.

Exam Relevance: ABO concept explaining how AR coatings work. Understanding destructive interference helps explain why AR coatings reduce reflections and improve light transmission. Questions may test basic principle.

See also: Anti-Reflective Coating (AR), lens-coatings, Light Transmission

Definition: Advanced lens manufacturing using computer-controlled equipment to generate complex, customized lens surfaces point-by-point. Enables progressive corridor optimization, correction of higher-order aberrations, and personalized designs. Also called free-form surfacing.

Exam Relevance: Modern ABO lab technology. Digital surfacing allows: shorter progressive corridors, wider viewing zones, personalized progressives, complex aspheric designs. You should understand this represents advancement over traditional surfacing enabling better lens performance.

See also: Surfacing (Lens), Progressive Addition Lenses (PALs), Aspheric Lens

Definition: The simultaneous outward rotation of both eyes, opposite of convergence. Required when shifting gaze from near to distant objects. Divergence insufficiency less common than convergence insufficiency but can cause distance diplopia.

Exam Relevance: ABO concept related to binocular vision and prism. Problems with divergence may require Base Out prism. Understanding eye movement terms helps with prism prescription comprehension.

See also: Convergence, Base Out Prism (BO), Binocular Vision

Definition: The eye preferentially used for monocular tasks like looking through a telescope or aiming. Not necessarily the eye with better acuity. Determined by simple sighting tests. Important for monovision contact lens fitting.

Exam Relevance: NCLE concept for monovision fitting. You must know: typically correct dominant eye for distance and non-dominant for near in monovision. Most people (65-70%) are right-eye dominant. Can test dominance with simple pointing test.

See also: Monovision, Presbyopia, Binocular Vision

Definition: Insufficient tear production or excessive tear evaporation causing ocular surface damage and discomfort. Major cause of contact lens intolerance. Types: aqueous deficiency (Sjögren's, aging) or evaporative (MGD most common). Symptoms: dryness, burning, foreign body sensation, fluctuating vision.

Exam Relevance: Critical NCLE topic. You must know: dry eye major reason for contact lens dropout, affects ~50% of lens wearers to some degree. Management: artificial tears, punctal plugs, treat MGD (warm compresses), daily disposables, high-water lenses for some, scleral lenses for severe. Questions test identification and management.

See also: Meibomian Glands, Tear Film, contact-lens-intolerance, artificial-tears

Definition: The laboratory process of cutting a lens blank to the exact shape and size needed to fit into a specific eyeglass frame. Modern edging is done with automated equipment using a frame tracer pattern, but manual edging is still used for some applications.

Exam Relevance: ABO exam tests basic understanding of lens fabrication process. You should know that edging comes after surfacing and blocking, and requires accurate frame measurements. The lens must be edged with proper bevel placement, safety bevel (for rimless/semi-rimless), and sized correctly to fit the frame groove.

See also: Blocking, Surfacing (Lens), Bevel (Lens Edge), frame-tracer

Definition: The minimum diameter of a lens blank required to cut a lens for a given frame, accounting for both the frame size and any decentration needed. ED is twice the distance from the optical center (or geometric center) to the farthest point on the lens shape.

Exam Relevance: Critical ABO calculation for determining minimum blank size. Formula: ED = 2 × √[(distance from OC to farthest edge)²]. For centered lenses in boxing system: ED = √(A² + B²). Questions involve calculating required blank size and whether a given blank is large enough for a prescription and frame combination.

Example: Frame with A=50mm, B=40mm requires ED = √(50² + 40²) = √4100 = 64mm minimum blank diameter.

See also: Boxing System, Minimum Blank Size (MBS), Decentration, A Measurement, B Measurement

Definition: The refractive state where parallel light from distant objects focuses precisely on the retina without accommodation, requiring no corrective lenses. An emmetropic eye has perfect distance vision naturally.

Exam Relevance: Both exams test understanding of refractive states. You must know that emmetropia means zero refractive error at distance (Plano prescription). Most emmetropic people still need reading glasses after age 40-45 due to presbyopia. Emmetropia is the goal of refractive correction with glasses or contact lenses.

See also: Myopia (Nearsightedness), Hyperopia (Farsightedness), Ametropia, Refraction

Definition: A latent tendency for the eyes to turn inward (toward the nose) when binocular fusion is interrupted. The patient maintains alignment through fusional divergence, but may experience eyestrain or diplopia if the phoria exceeds their compensatory ability.

Exam Relevance: ABO questions test understanding of phorias and prism correction. Esophoria is corrected with Base Out (BO) prism, which forces the eyes to diverge. Common at near in presbyopes. You should know symptoms: eyestrain, headaches, intermittent diplopia, especially during prolonged near work.

Example: Patient has 6∆ esophoria at near causing discomfort. Prescribed 3∆ BO OU (base out, both eyes) to reduce fusional demand.

See also: Exophoria, Phoria, Base Out Prism (BO), Strabismus

Definition: A latent tendency for the eyes to turn outward (away from the nose) when binocular fusion is interrupted. The patient maintains alignment through fusional convergence, but may experience eyestrain, especially during near work.

Exam Relevance: ABO exam tests phorias and prism correction. Exophoria is the most common phoria, especially at near. Corrected with Base In (BI) prism. You must understand that uncorrected exophoria at near causes convergence insufficiency symptoms: diplopia, eyestrain, words running together when reading.

Example: Patient with 8∆ exophoria at near prescribed 4∆ BI OU to reduce convergence demand and relieve symptoms.

See also: Esophoria, Phoria, Base In Prism (BI), convergence-insufficiency

Definition: Contact lenses approved by the FDA for continuous wear including sleeping, typically for up to 7 days or 30 days depending on the lens. Requires very high oxygen transmission (Dk/t ≥87) to maintain corneal health during sleep.

Exam Relevance: NCLE questions test wearing schedules, patient selection, and risks. You must know that extended wear significantly increases risk of microbial keratitis (5x higher than daily wear). Only high-Dk silicone hydrogel lenses are approved for extended wear. Requires careful patient selection and frequent follow-up.

See also: Dk/t (Oxygen Transmissibility), daily-wear, Silicone Hydrogel, Microbial Keratitis

Definition: The thickness of a lens at its edge, varies with lens type. Minus lenses are thick at edges and thin in center, plus lenses are thin at edges and thick in center. Measured in millimeters, affects cosmetics, safety, and frame compatibility.

Exam Relevance: ABO questions test understanding of how power and design affect edge thickness. You must know factors affecting edge thickness: lens power, base curve, diameter, material index, lens design (aspheric reduces edge bulk in minus). Questions involve selecting materials and designs to minimize edge thickness in high minus prescriptions.

See also: Center Thickness (CT), Minus Lens (Concave), High-Index Lens Material, Aspheric Lens

Definition: A bifocal design with the reading segment extending across the entire width of the lens, with a straight dividing line. Provides maximum near viewing area but also maximum image jump.

Exam Relevance: ABO exam tests bifocal types. You should know Executive bifocal provides widest near field but creates most image jump. Less commonly prescribed now due to cosmetics and progressive lens availability. Appropriate for patients needing maximum near area for detailed close work.

See also: Bifocal, Image Jump, Flat-Top Bifocal (D-Seg/Straight-Top)

Definition: Contact lens cleaning product containing enzymes (papain, pancreatin, or subtilisin) that break down protein deposits on lens surfaces. Used weekly or as needed to supplement daily cleaning. Particularly important for preventing GPC.

Exam Relevance: NCLE lens care topic. Enzyme cleaners essential for: extended replacement schedules, patients prone to deposits, GPC prevention/management. Modern formulations gentler than older products. Questions involve appropriate use and when to recommend.

See also: Protein Deposits, Giant Papillary Conjunctivitis (GPC), lens-care

Definition: A manifest strabismus where one or both eyes turn inward constantly. Unlike esophoria (latent), esotropia is always present. Can be congenital or acquired. May require surgery, prism, or vision therapy.

Exam Relevance: ABO questions differentiate tropias (constant) from phorias (latent). Esotropia more serious than esophoria, often requires medical intervention beyond simple prism. Large angles typically need surgery.

See also: Strabismus, Esophoria, Exotropia, Prism

Definition: A manifest strabismus where one or both eyes turn outward constantly. Can be constant or intermittent. More common type of horizontal strabismus in adults. May require surgery or prism correction.

Exam Relevance: ABO understanding of constant vs latent eye turning. Exotropia is constant misalignment (vs exophoria which is latent). Questions test recognizing difference and understanding treatment approaches.

See also: Strabismus, Exophoria, Esotropia, Prism

Definition: An RGP contact lens fitting relationship where the base curve is significantly flatter than the flattest corneal meridian (typically more than 0.75D flatter than flat K). Results in excessive lens movement, edge standoff, and peripheral corneal bearing.

Exam Relevance: Important NCLE concept for RGP fitting evaluation. You must recognize flat fit by fluorescein pattern: central pooling of fluorescein, peripheral bearing/touch with dark areas at lens edge. Symptoms: excessive movement, lens decentration, discomfort, possible 3&9 o'clock staining. Solution: steepen base curve.

Example: Flat K = 43.00D, lens BC = 42.00D (1.00D flatter) = flat fit. Fluorescein shows central pooling and edge bearing.

See also: Steep Fit, Alignment Fit, Fluorescein Pattern, RGP Lens (Rigid Gas Permeable)

Definition: An orange dye that fluoresces yellow-green under blue light, used to assess contact lens fit (with RGPs), evaluate corneal integrity, and check tear film quality. Available as sodium fluorescein in solution or impregnated paper strips.

Exam Relevance: Essential NCLE tool for RGP fitting and corneal assessment. You must know proper technique: use preservative-free solution with RGPs (preserved solutions stain soft lenses permanently), view with Burton lamp or slit lamp with cobalt blue filter. Fluorescein reveals lens-to-cornea relationship, corneal staining, and tear breakup time.

See also: Fluorescein Pattern, Burton Lamp, rgp-fitting, Corneal Staining

Definition: The appearance of fluorescein dye between an RGP contact lens and the cornea when viewed under blue light. The pattern reveals the lens-to-cornea fitting relationship and helps determine if adjustments are needed.

Exam Relevance: One of the most heavily tested NCLE concepts. You must interpret patterns: alignment fit (thin even band), flat fit (central pooling, edge bearing), steep fit (central bearing/touch, peripheral pooling), toric fit (uneven pattern). Questions often show images and ask for fitting assessment and recommended changes.

See also: Alignment Fit, Flat Fit, Steep Fit, Fluorescein, RGP Lens (Rigid Gas Permeable)

Definition: The distance from the optical center of a lens to its focal point, where parallel light rays converge (plus lens) or appear to diverge from (minus lens). Measured in meters, focal length is the reciprocal of lens power in diopters.

Exam Relevance: Fundamental optical concept for both exams. You must know the relationship: f = 1/D. Plus lenses have positive focal length (real focus), minus lenses have negative focal length (virtual focus). Questions involve converting between focal length and dioptric power.

See also: Diopter (D), Lens Power, Vergence

Definition: The central depression in the macula of the retina, containing only cone photoreceptors and responsible for sharp, detailed central vision and color perception. The fovea provides maximum visual acuity of 20/20 or better.

Exam Relevance: NCLE anatomy questions test retinal structure and function. You should know that the fovea is avascular, approximately 1.5mm in diameter, and contains the highest density of cones. Any damage to the fovea significantly impacts visual acuity. The fovea is the fixation point for central vision.

See also: Macula, Retina, Cone Cells (Cones), Visual Acuity

Definition: The distance between the geometric centers of the two lens openings in a frame, calculated using the boxing system. Formula: Frame PD = A + DBL, where A is the horizontal lens width and DBL is the distance between lenses.

Exam Relevance: Essential ABO measurement for calculating decentration. You must know Frame PD formula and how to use it to determine if lenses need to be decentered to match patient PD. If patient PD equals frame PD, no decentration needed. If different, calculate decentration per lens: (Frame PD - Patient PD) / 2.

Example: Frame 52-18: Frame PD = 52+18 = 70mm. Patient PD = 64mm. Decentration needed = (70-64)/2 = 3mm per lens.

See also: Boxing System, DBL (Distance Between Lenses), A Measurement, Decentration, Binocular PD

Definition: The reciprocal of the distance from the front surface of a lens to its primary focal point. Rarely used for spectacle lenses, but important for verifying contact lens power and measuring high-plus lenses in low vision applications.

Exam Relevance: ABO exam tests when to use FVP versus BVP. Front vertex power is measured for contact lenses (which sit on the eye with front surface away from cornea) and very high-plus spectacle lenses. For standard spectacles, always measure back vertex power. Questions may involve understanding why different measurement methods are used.

See also: Back Vertex Power (BVP), Lensometer (Lensmeter/Focimeter), contact-lens-verification

Definition: The farthest distance at which an object can be seen clearly without accommodation. For emmetropes, far point is at infinity. For myopes, far point is closer than infinity (negative refractive error). For hyperopes, far point is theoretically behind the eye (positive error).

Exam Relevance: Both exams test basic refractive concepts. Understanding far point helps explain why myopes see near clearly but distance blurred, while hyperopes may accommodate to see distance. The reciprocal of far point distance (in meters) equals the refractive error in diopters.

See also: Near Point, Myopia (Nearsightedness), Hyperopia (Farsightedness), Emmetropia, Refraction

Definition: The most common bifocal design with a D-shaped reading segment having a flat top and curved bottom. Available in various widths: 22mm, 25mm, 28mm (most popular), 35mm, 45mm.

Exam Relevance: Most frequently prescribed bifocal type. ABO exam tests FT advantages: less image jump than Executive, wider near field than round segments, cosmetically acceptable, easy to fit. FT-28 is most popular balancing near area with cosmetics. Questions involve appropriate seg size selection.

Example: FT-28 provides 28mm width reading area, sufficient for most reading tasks while maintaining reasonable cosmetic appearance.

See also: Bifocal, D Measurement, Segment Height (Seg Height), Image Jump

Definition: A bifocal lens design where a higher-index glass segment is fused into a depression in the front surface of the crown glass lens. Creates smooth surface with permanent segment. Contrast to cemented bifocals.

Exam Relevance: ABO questions may test bifocal construction methods, especially for glass lenses. Fused bifocals are more durable than cemented (older design). Most modern plastic bifocals are one-piece (solid) construction.

See also: Bifocal, Glass Lens (Crown Glass)

Definition: U.S. Food and Drug Administration clearance required for contact lenses (classified as medical devices). Specifies approved wearing schedule, replacement frequency, and parameters. Extended wear requires specific FDA approval with high Dk/t.

Exam Relevance: NCLE regulatory concept. You must know: all contact lenses are medical devices requiring FDA clearance, must follow FDA-approved wearing schedules, extended wear requires specific approval (not all lenses approved), can't use lenses off-label without justification.

See also: Extended Wear Contact Lens, daily-wear, Wearing Schedule

Definition: An RGP contact lens with one or more small holes (fenestrations) to improve tear exchange and oxygen transmission. Less commonly used now due to better RGP materials with high Dk. May trap debris.

Exam Relevance: Historical NCLE concept. Fenestrations were used to improve oxygen before high-Dk materials. Modern high-Dk RGPs rarely need fenestrations. You should know the concept but understand it's largely obsolete.

See also: RGP Lens (Rigid Gas Permeable), Dk (Oxygen Permeability), tear-exchange

Definition: A collection of diagnostic contact lenses in various base curves, powers, and diameters used to determine optimal lens parameters before ordering. Essential for RGP fitting, less critical for soft lenses with empirical fitting.

Exam Relevance: NCLE fitting tool. Diagnostic fitting especially important for: RGPs (assess fluorescein pattern, movement, centration), specialty lenses (sclerals, torics), complex fits. Questions involve proper fitting techniques and assessment.

See also: diagnostic-lens, RGP Lens (Rigid Gas Permeable), empirical-fitting

Definition: See Executive Bifocal. Named after Benjamin Franklin who invented bifocals.

Exam Relevance: Historical term for Executive bifocal with straight-line segment across full lens width.

See also: Executive Bifocal (Franklin/E-Style), Bifocal

Definition: A thin, flexible plastic prism that adheres to spectacle lenses to add significant prism power without lens thickness. Made of tiny prism grooves. Useful for temporary prism or very high powers (>10∆). Reduces optical quality.

Exam Relevance: ABO special prism application. Fresnel prisms allow high prism powers that would be impossible to grind into lenses. Used for: temporary prism trials, post-stroke diplopia, very high prescriptions. Disadvantages: visible grooves, reduced clarity, may peel off.

See also: Prism, high-prism, temporary-prism

Definition: An RGP lens with toric front surface (cylinder power ground into front) to correct residual astigmatism. Back surface may be spherical or toric. Requires stabilization to maintain proper axis orientation.

Exam Relevance: NCLE toric concept. Front surface toric adds cylinder correction when spherical RGP leaves residual astigmatism. Requires prism ballast or truncation for stability. Questions test when front surface toric vs bitoric needed.

See also: Bitoric Contact Lens, Residual Astigmatism, RGP Lens (Rigid Gas Permeable), Back Surface Toric

Definition: The neurological process of combining the two slightly different images from each eye into a single, unified perception. Essential for binocular vision and stereopsis. Can be disrupted by significant eye misalignment or image disparity.

Exam Relevance: ABO concept for understanding binocular vision. Fusion requires: similar images from both eyes, proper alignment, adequate acuity in both eyes. Conditions affecting fusion: strabismus, significant anisometropia, high uncorrected error.

See also: Binocular Vision, stereopsis, Strabismus

Definition: The exact middle point of a lens shape as determined by the boxing system - the intersection of the horizontal and vertical midlines of the boxing rectangle. May not coincide with the optical center, especially if decentration is required.

Exam Relevance: ABO exam tests difference between geometric center and optical center. Geometric center is determined by frame shape. Optical center is where light passes undeviated and must align with patient PD. When patient PD differs from frame PD, optical center is decentered from geometric center.

See also: Optical Center (OC), Boxing System, Decentration

Definition: An inflammatory reaction of the upper tarsal conjunctiva characterized by giant papillae (bumps >0.3mm), typically caused by contact lens deposits, especially protein buildup. Results in itching, mucus discharge, lens intolerance, and poor lens surface wettability.

Exam Relevance: Major NCLE complication that appears frequently on exam. You must know signs/symptoms: itching (especially after lens removal), excessive mucus, reduced wearing time, lens coating, giant papillae on upper lid eversion. Treatment: discontinue lens wear temporarily, switch to daily disposables, improve cleaning regimen, consider different lens material.

See also: Protein Deposits, Daily Disposable Contact Lens, lens-deposits, Conjunctiva

Definition: A group of eye diseases characterized by damage to the optic nerve, often (but not always) associated with elevated intraocular pressure. Can lead to progressive, irreversible vision loss starting with peripheral vision. Most common types: open-angle and angle-closure.

Exam Relevance: NCLE questions test basic understanding as part of patient history and contraindications. You should recognize that glaucoma patients may use multiple eye drops daily, which can affect contact lens wear due to preservatives. Shallow anterior chamber is risk factor for angle-closure glaucoma. Contact lenses don't cause glaucoma but may affect IOP measurements.

See also: intraocular-pressure, Anterior Chamber, Optic Nerve

Definition: The distance between the geometric centers of the two lenses in a frame, same as Frame PD (A + DBL). Used in calculating decentration requirements.

Exam Relevance: ABO calculation term that's interchangeable with Frame PD. Questions use this terminology when discussing decentration calculations and minimum blank size determinations.

See also: Frame PD, Geometric Center (GC), Decentration, Boxing System

Definition: Optical lenses made from crown glass (refractive index 1.523), the original lens material. Excellent optical quality but heavy, breakable, and largely replaced by plastic materials.

Exam Relevance: ABO exam tests glass vs plastic comparison. You should know glass advantages: excellent scratch resistance, best optical quality, thin in high plus. Disadvantages: heavy, breaks dangerously, no impact resistance, costly. Rarely prescribed now except for special applications or patient preference.

See also: CR-39, Lens Material, Refractive Index (n)

Definition: See Giant Papillary Conjunctivitis

Exam Relevance: See Giant Papillary Conjunctivitis entry for complete information.

See also: Giant Papillary Conjunctivitis (GPC)

Definition: The gold standard method for measuring intraocular pressure by flattening (applanating) a standard area of the cornea. Used at slit lamp with fluorescein and blue light. Normal IOP 10-21 mmHg.

Exam Relevance: NCLE concept for IOP measurement. While opticians don't perform tonometry, understanding the standard technique helps with: removing lenses before testing, understanding why corneal thickness matters, recognizing when IOP check needed.

See also: Tonometry, intraocular-pressure, Glaucoma, Slit Lamp (Biomicroscope)

Definition: A lens tint that gradually transitions from darker at the top to lighter (or clear) at the bottom. Common in fashion sunglasses and driving glasses. Top blocks bright sky while bottom remains clear for dashboard viewing.

Exam Relevance: ABO tint option for specific uses. Gradient tints good for driving (dark top for glare, clear bottom for dashboard). Not suitable for full sun protection or water sports (light bottom lets glare in). Questions involve appropriate recommendations.

See also: Lens Tint, sunglasses, solid-tint

Definition: Lens materials with refractive index greater than standard plastic (1.498). Common indices: 1.56, 1.60, 1.67, 1.70, 1.74. Higher index allows thinner, lighter lenses for the same prescription power, especially beneficial for higher prescriptions.

Exam Relevance: ABO exam tests understanding of material properties and appropriate recommendations. You must know: higher index = thinner/lighter but also higher cost, lower Abbe value (more chromatic aberration), higher reflectance (requires AR coating), more brittle. Benefits increase with higher prescriptions (over ±3.00D).

Example: A -6.00D lens in 1.74 index is approximately 40% thinner than same prescription in 1.498 CR-39.

See also: Refractive Index (n), Abbe Value, Lens Material, CR-39

Definition: A refractive error where parallel light rays from distant objects focus behind the retina, causing blurred vision. Hyperopes need plus (convex) lenses to converge light onto the retina. May see distance clearly through accommodation when young, but near vision more difficult.

Exam Relevance: Fundamental refractive concept for both exams. ABO: understanding plus lens optics, edge thickness, magnification, and cosmetic concerns. NCLE: fitting considerations (steeper corneas typically), relationship between hyperopia and angle-closure risk, accommodation demands. Questions test ability to identify hyperopic prescriptions and understand correction methods.

Example: Patient with +3.00D hyperopia needs plus lenses to bring focus forward onto retina. Without correction, must accommodate constantly even for distance.

See also: Plus Lens (Convex), Accommodation, Myopia (Nearsightedness), Emmetropia, Refraction

Definition: Insufficient oxygen supply to the cornea, most commonly caused by contact lens wear that blocks atmospheric oxygen from reaching the corneal surface. Leads to corneal edema, neovascularization, and potential permanent damage if chronic.

Exam Relevance: Critical NCLE concept for understanding contact lens complications. You must know signs of hypoxia: corneal edema, hazy vision, halos, microcysts, stromal striae, limbal hyperemia, and neovascularization. Prevention requires adequate Dk/t (minimum 24 for daily wear, 87 for extended wear). Questions involve identifying hypoxic complications and recommending solutions.

See also: Corneal Edema, Dk (Oxygen Permeability), Dk/t (Oxygen Transmissibility), Neovascularization, Cornea

Definition: Traditional soft contact lens material made from hydroxyethyl methacrylate (HEMA) or similar polymers, containing 38-70% water. Oxygen transmission depends primarily on water content. Lower Dk than silicone hydrogels.

Exam Relevance: NCLE questions test material types. Conventional hydrogel advantages: very comfortable, low modulus, less deposit-prone than early silicone hydrogels. Disadvantages: limited Dk (max ~30-35), not suitable for extended wear, more dehydration. Most replaced by silicone hydrogels for full-time wear.

See also: Silicone Hydrogel, Water Content, Dk (Oxygen Permeability), HEMA (Hydroxyethyl Methacrylate)

Definition: A vertical phoria where one eye has a latent tendency to deviate upward when fusion is interrupted. The opposite eye shows relative hypophoria (downward deviation). Corrected with vertical prism.

Exam Relevance: ABO questions test vertical phorias and prism correction. Right hyperphoria corrected with Base Down prism OD (or Base Up OS). Left hyperphoria corrected with Base Down OS (or Base Up OD). Vertical phorias less common than horizontal but more symptomatic.

See also: Phoria, Hypophoria, vertical-prism, Base Up Prism (BU), Base Down Prism (BD)

Definition: A vertical phoria where one eye has a latent tendency to deviate downward when fusion is interrupted. Often described relative to the hyperphoric eye (e.g., right hyperphoria = left hypophoria).

Exam Relevance: ABO exam tests vertical phoria correction. Right hypophoria is same as left hyperphoria, corrected with Base Up prism OD (or Base Down OS). Understanding the reciprocal relationship between hyperphoria and hypophoria is essential.

See also: Phoria, Hyperphoria, vertical-prism

Definition: The polymer material in first successful soft contact lenses (1971) and still used in many conventional hydrogel lenses. Water content typically 38-55%. Moderate oxygen transmission, good comfort, biocompatible.

Exam Relevance: NCLE material science. HEMA is the foundation of conventional hydrogel lenses. You should understand it as the "traditional" soft lens material, now often replaced or enhanced with silicone for higher oxygen transmission.

See also: Hydrogel (Conventional), conventional-hydrogel, Soft Contact Lens

Definition: Prism with base oriented horizontally - either Base In (BI) or Base Out (BO). Used to correct horizontal eye alignment problems (esophoria/exophoria). More commonly prescribed than vertical prism.

Exam Relevance: ABO prism type. Most prescribed prism is horizontal for convergence/divergence issues. Base In for exophoria (most common at near), Base Out for esophoria (less common). Questions test which direction corrects which condition.

See also: Base In Prism (BI), Base Out Prism (BO), vertical-prism, Exophoria, Esophoria

Definition: A manifest vertical strabismus where one eye deviates upward constantly. The fellow eye shows relative hypotropia (downward deviation). Requires vertical prism or surgery. Always specify which eye is hyper.

Exam Relevance: ABO vertical strabismus. Hypertropia is constant upward deviation (vs hyperphoria which is latent). Right hypertropia means right eye higher, corrected with Base Down OD or Base Up OS. Questions test proper notation and correction.

See also: Strabismus, Hyperphoria, Hypotropia, vertical-prism

Definition: A manifest vertical strabismus where one eye deviates downward constantly. Often described relative to hypertropic eye (right hypertropia = left hypotropia).

Exam Relevance: ABO vertical strabismus terminology. Understanding relationship between hypertropia and hypotropia important for proper prism prescription.

See also: Strabismus, Hypertropia, Hypophoria, vertical-prism

Definition: The sudden displacement of an image that occurs when the eyes cross the dividing line of a bifocal or trifocal lens. Caused by the prismatic effect difference between the distance and near portions of the lens at the segment line.

Exam Relevance: Important ABO bifocal concept. You must know that image jump is greatest in high-powered segments and Executive bifocals (full segment width). Reduced by: round-top segments, smaller segments, lower segment placement, progressive lenses (no jump). Questions involve understanding causes and recommending solutions for patients bothered by image jump.

Example: Patient with Executive bifocals and +3.00 add experiences significant image jump when reading. Solution: switch to FT-28 which has much less jump.

See also: Bifocal, Segment Height (Seg Height), Progressive Addition Lenses (PALs), prismatic-effect

Definition: Prismatic effect created when a patient looks through a point on a lens other than the optical center. Calculated using Prentice's Rule: Prism (∆) = Decentration (cm) × Power (D). Always present with decentration in plus or minus lenses.

Exam Relevance: Heavily tested ABO calculation. You must master Prentice's Rule and understand that looking away from optical center in any powered lens creates prism. The amount increases with higher powers and greater decentration. Questions involve calculating induced prism, determining acceptable decentration, and understanding effects on vision.

Example: Patient looks 5mm (0.5cm) above optical center in -4.00D lens: Induced prism = 0.5 × 4 = 2∆ base down.

See also: Prentice's Rule, Decentration, Optical Center (OC), Prism

Definition: An artificial lens implanted inside the eye, typically during cataract surgery to replace the clouded natural crystalline lens. Modern IOLs are usually placed in the capsular bag (posterior chamber) and provide fixed optical power.

Exam Relevance: NCLE questions test IOLs as part of patient history. Patients with IOLs (pseudophakic) have different contact lens fitting considerations than aphakic patients. Modern IOL patients typically need only minor refractive correction with glasses or contacts. You should understand that IOLs don't accommodate, so presbyopic adds are needed.

See also: Pseudophakia, Aphakia, Cataract, Crystalline Lens

Definition: The colored, muscular diaphragm of the eye located behind the cornea and in front of the crystalline lens. Contains the pupil (central opening) and controls its size through dilator and sphincter muscles, regulating light entering the eye.

Exam Relevance: NCLE anatomy questions test iris structure and function. You should know the iris is part of the uveal tract, divides anterior chamber from posterior chamber, and contains pigment determining eye color. Clinical significance: iris damage or defects affect pupil function and light control. Some contact lenses have printed iris patterns for cosmetic purposes.

See also: Pupil, Anterior Chamber, Uvea (Uveal Tract), Ciliary Body

Definition: A lens's ability to withstand impact without breaking or shattering. FDA requires all spectacle lenses sold in the US to pass basic impact test (drop ball test). Higher standards exist for safety glasses and children's eyewear.

Exam Relevance: Critical ABO safety concept. You must know: polycarbonate and Trivex have highest impact resistance (10x CR-39), required for safety glasses and children under 16. CR-39 requires chemical or thermal treatment to pass basic FDA requirements. Glass lenses need special treatment. Questions involve material selection for safety applications.

See also: Polycarbonate, Trivex, Safety Glasses, CR-39

Definition: Visual range between distance and near, typically 16-30 inches (40-75cm), used for computer work, dashboard viewing, and arm's length tasks. Progressives provide this through the corridor, trifocals have a dedicated intermediate segment.

Exam Relevance: ABO questions test multifocal lens designs for intermediate needs. Computer users often need specific intermediate optimization. Progressive lenses provide continuous intermediate but some occupational progressive designs emphasize intermediate zone. Questions involve understanding patient needs and appropriate lens selection.

See also: Progressive Addition Lenses (PALs), Trifocal, occupational-progressive

Definition: The fluid pressure inside the eye, normally 10-21 mmHg, maintained by balance between aqueous humor production and drainage. Elevated IOP is major risk factor for glaucoma. Contact lenses may slightly affect IOP measurements.

Exam Relevance: NCLE questions test basic understanding as part of ocular health. You should know that IOP varies throughout day, increases with prone position, and contact lenses may affect measurement accuracy (should remove lenses for accurate tonometry). Understanding normal range and glaucoma risk important for patient screening.

See also: Aqueous Humor, Glaucoma, Tonometry

Definition: See Refractive Index

Exam Relevance: See Refractive Index entry for complete information on lens material property affecting thickness.

See also: Refractive Index (n), Lens Material, High-Index Lens Material

Definition: Astigmatism created by factors other than the eye itself: lens tilt (pantoscopic/face form angle), oblique ray aberrations through spherical lenses, improper lens mounting, or frame warping. Can be minimized through proper fitting and lens design.

Exam Relevance: ABO concept for understanding fitting-related astigmatism. You must know: excessive pantoscopic tilt induces cylinder axis errors, face-form tilt in wrap frames creates oblique astigmatism, compensated or aspheric designs reduce induced astigmatism from off-axis viewing.

See also: Pantoscopic Tilt, face-form-tilt, Oblique Astigmatism

Definition: Patient's voluntary agreement to contact lens wear after being fully informed of benefits, risks, alternatives, and responsibilities. Essential legal and ethical requirement. Must document education provided and patient understanding.

Exam Relevance: NCLE professional responsibility. You must provide: risks (infection, ulcers, vision loss potential), care requirements, wearing schedule importance, when to seek care, alternatives to contact lenses. Questions test appropriate patient education and documentation.

See also: patient-education, compliance, legal-requirements

Definition: Inflammation of the iris (part of uveal tract). Symptoms: severe pain, photophobia, redness, decreased vision, small pupil. Causes: autoimmune, infection, trauma. Absolute contraindication for contact lens wear during active inflammation.

Exam Relevance: NCLE emergency recognition. Iritis symptoms similar to but distinct from keratitis - requires immediate medical referral. No contact lens wear during active iritis or uveitis. Questions test emergency recognition and appropriate action.

See also: Uveitis, contraindications, Iris, emergency-referral

Definition: Astigmatism where the principal meridians are not perpendicular, or the corneal surface is irregular rather than smoothly toric. Causes: keratoconus, corneal scarring, trauma, surgery. Cannot be fully corrected with spectacles - requires RGP or specialty lenses.

Exam Relevance: Important NCLE concept differentiating from regular astigmatism. You must know: irregular astigmatism best corrected with RGP lenses (mask irregular surface with tear lens), soft lenses don't help, spectacles provide poor correction. Questions involve recognizing need for RGP fitting.

See also: Keratoconus, RGP Lens (Rigid Gas Permeable), Scleral Lens, corneal-scarring

Definition: Inflammation or infection of the cornea, which can be caused by bacteria, viruses, fungi, or parasites (Acanthamoeba). Contact lens wearers have increased risk, especially with poor hygiene, overwear, or sleeping in lenses.

Exam Relevance: Critical NCLE complication. You must know signs/symptoms: eye pain, redness, photophobia, decreased vision, discharge. Microbial keratitis is an emergency requiring immediate referral. Prevention: proper lens care, no water exposure, no sleeping in daily wear lenses, replace lenses on schedule. Questions test risk factors and emergency recognition.

See also: Microbial Keratitis, Corneal Ulcer, overwear, lens-hygiene

Definition: An instrument that measures the curvature of the anterior corneal surface by measuring the size of a reflected image (mires). Provides readings in millimeters of radius or diopters for the two principal meridians, essential for contact lens fitting.

Exam Relevance: Fundamental NCLE instrument. You must understand keratometry technique, how to read and record K readings (e.g., 42.00 @ 180 / 44.00 @ 090), and how to use values for contact lens base curve selection. Questions test reading interpretation, conversion between mm and diopters, and using K values for fitting decisions.

See also: Keratometry (K Readings), Corneal Curvature, base-curve-selection, k-readings

Definition: The measurement of corneal curvature, reported as two values representing the flattest and steepest meridians. Recorded with power (in diopters) and axis (in degrees). Essential for contact lens fitting, especially for RGPs and torics.

Exam Relevance: One of the most important NCLE skills. You must interpret K readings to determine: corneal shape (spherical, toric), amount and type of astigmatism (WTR, ATR, oblique), appropriate base curve selection. Questions involve reading notation (e.g., 43.50 @ 180 / 44.50 @ 090 means 1.00D of WTR astigmatism), calculating corneal toricity, and using for lens selection.

Example: K readings: 43.00 @ 180 / 45.00 @ 090 = 2.00D WTR corneal astigmatism. Flat K is 43.00D, suggesting RGP base curve of 43.00-43.50D for alignment fit.

See also: Keratometer, Base Curve, Corneal Curvature, With-the-Rule Astigmatism, Against-the-Rule Astigmatism

Definition: A progressive corneal disorder where the cornea thins and develops a cone-shaped protrusion, causing irregular astigmatism and reduced vision. Typically begins in teenage years/early 20s. Often requires RGP lenses or specialty lens designs for adequate vision.

Exam Relevance: Important NCLE condition requiring special contact lens fitting. You must know signs: irregular astigmatism, scissoring reflex on retinoscopy, Munson's sign, progressive myopia/astigmatism. RGP lenses often provide best vision by masking irregular cornea. Questions test recognition and appropriate lens recommendations.

See also: Irregular Astigmatism, RGP Lens (Rigid Gas Permeable), Scleral Lens, Corneal Topography

Definition: A mechanical instrument with three pointed legs used to measure the surface power (base curve) of a lens. The center pin moves inward or outward based on the surface curvature, and a scale indicates the power.

Exam Relevance: ABO exam tests proper use of lens clock. You must know: lens clock measures surface power only, not total lens power. Must measure front surface (base curve) and back surface separately. Reading is approximate and affected by lens material refractive index. Used for identifying lens surface curves and verifying base curve markings.

See also: Base Curve, surface-power

Definition: An instrument used to measure the back vertex power, cylinder axis, add power, and prism of spectacle lenses. Essential for verifying prescriptions and marking optical centers for edging.

Exam Relevance: Most critical ABO instrument skill. You must know proper technique: clean lens, position lens correctly (back surface against lens stop), neutralize sphere, neutralize cylinder, read axis, measure add, check for prism. Questions test reading lensometer, marking lenses, measuring add power, and troubleshooting common errors.

See also: Back Vertex Power (BVP), Optical Center (OC), Add Power, prism-measurement

Definition: A lens design with a small optical zone surrounded by a carrier area, used for high-power prescriptions to reduce weight and thickness. The carrier portion has little or no power and provides mechanical support.

Exam Relevance: ABO exam tests lenticular designs for high prescriptions. Minus lenticular reduces edge thickness in high minus powers. Plus lenticular (myodisc) reduces center thickness and weight in high plus powers. Questions involve understanding when to recommend, advantages/disadvantages, and cosmetic considerations.

See also: Carrier Lens, Myodisc, high-power-lens

Definition: Inability to completely close the eyelids, leaving part of the cornea exposed. Can result from Bell's palsy, trauma, or anatomical variations. Increases risk of corneal desiccation and complications with contact lens wear.

Exam Relevance: NCLE questions test contraindications for contact lens wear. Lagophthalmos is relative contraindication due to increased exposure and dryness risk. If fitting, need aggressive lubrication and shorter wearing times. Questions involve patient screening and recognizing risk factors.

See also: Dry Eye Syndrome, contraindications, corneal-exposure

Definition: The substance from which an ophthalmic lens is manufactured. Common materials: CR-39 (n=1.498), polycarbonate (n=1.586), Trivex (n=1.53), high-index plastic (n=1.60-1.74), glass (n=1.523). Each has specific optical and physical properties.

Exam Relevance: Major ABO topic for material selection. You must know properties of each material: index, Abbe value, specific gravity (weight), impact resistance, cost, appropriate applications. Questions involve recommending materials based on prescription, frame, lifestyle, and priorities (thin vs optics vs safety vs cost).

See also: CR-39, Polycarbonate, High-Index Lens Material, Trivex, Refractive Index (n), Abbe Value

Definition: The border zone between the cornea and sclera, approximately 1-2mm wide, containing the transition from clear to white tissue. Contains stem cells that regenerate corneal epithelium and the angle structures where aqueous humor drains.

Exam Relevance: NCLE anatomy questions test limbal zone structure and function. You should know the limbus is where corneal neovascularization typically originates in hypoxia. Soft contact lens diameter is often based on limbus measurements (typically 11.5-12.0mm corneal diameter at limbus).

See also: Cornea, Sclera, Neovascularization, Contact Lens Diameter

Definition: Vision that cannot be corrected to normal with conventional glasses, contact lenses, or medical treatment. Defined as best-corrected visual acuity worse than 20/70 or significant visual field loss. Requires special optical devices and aids.

Exam Relevance: ABO questions may cover basic low vision concepts. You should know that low vision patients may benefit from: high-add readers, magnifiers, telescopic systems, high-intensity lighting, electronic aids. These patients need specialized evaluation and devices beyond standard optical dispensing.

See also: Visual Acuity, Magnification, Legal Blindness

Definition: Color added to lenses for sun protection, fashion, or therapeutic purposes. Types: solid tint (uniform density), gradient tint (darker at top), mirror coating. Density measured in percentage (0-100%). Photochromic lenses automatically tint in UV light.

Exam Relevance: ABO questions test tint options and appropriate uses. You should know: gray/brown/green for sun (minimal color distortion), yellow for contrast enhancement, pink/rose for certain conditions, polarized for glare reduction. Prescription sunglasses typically 75-85% density. Fashion tints lighter (10-40%).

See also: sunglasses, photochromic, polarized, uv-protection

Definition: Definition used for disability benefits: best-corrected visual acuity of 20/200 or worse in the better eye, OR visual field restricted to 20 degrees or less. Not the same as total blindness - many legally blind people have useful vision.

Exam Relevance: Both exams should understand legal blindness definition vs total blindness. Important for: understanding patient limitations, when low vision aids appropriate, legal/social service eligibility.

See also: Visual Acuity, Low Vision, Best Corrected Visual Acuity (BCVA)

Definition: The ability of a lens to converge or diverge light, measured in diopters. Plus power converges (for hyperopia/presbyopia), minus power diverges (for myopia). Also called refractive power or dioptric power.

Exam Relevance: Fundamental concept for both exams. Understanding lens power essential for: reading prescriptions, using lensometer, selecting appropriate corrections, calculating focal length, understanding vergence changes.

See also: Diopter (D), Plus Lens (Convex), Minus Lens (Concave), refractive-power

Definition: The percentage of incident light that passes through a lens. Clear uncoated CR-39 transmits ~92% (8% reflected). AR coating increases transmission to ~99%. Tints reduce transmission proportionally.

Exam Relevance: ABO concept for understanding coating benefits and tint effects. You must know: reflections reduce useful light reaching eye, AR coating improves transmission by reducing reflections, tint percentage indicates amount of light blocked.

See also: Anti-Reflective Coating (AR), Lens Tint, reflections

Definition: The small, specialized area in the center of the retina responsible for sharp central vision. Contains the fovea (highest acuity area) and is rich in cone photoreceptors. Approximately 5-6mm in diameter.

Exam Relevance: NCLE anatomy questions test retinal structure. You should know that the macula provides central vision while peripheral retina handles peripheral vision and motion detection. Macular degeneration (especially age-related) is a leading cause of central vision loss. Any macular pathology significantly impacts reading and fine detail vision.

See also: Fovea, Retina, Cone Cells (Cones), central-vision

Definition: A serious corneal infection caused by bacteria, fungi, or parasites, most commonly associated with contact lens wear. Pseudomonas aeruginosa is the most common bacterial cause. Can rapidly cause corneal scarring and vision loss if untreated.

Exam Relevance: Most serious NCLE complication - requires immediate recognition and referral. You must know risk factors: overnight wear, poor hygiene, water exposure, extended wear schedules. Signs: severe pain, redness, discharge, photophobia, infiltrate/ulcer, decreased vision. EMERGENCY - refer immediately. Questions test risk factor identification and prevention strategies.

See also: Keratitis, Corneal Ulcer, pseudomonas, lens-hygiene, overwear

Definition: The smallest lens blank diameter required to cut a prescription lens for a given frame, accounting for frame size, decentration, and any prism. Must include effective diameter plus any additional decentration or special requirements.

Exam Relevance: Important ABO calculation. You must determine if a blank is large enough for a job. Formula: MBS = ED + total decentration (for both eyes if needed). Questions involve calculating required blank size and determining if a given blank diameter is sufficient.

Example: ED = 55mm, decentration = 3mm per lens: MBS = 55 + 3 = 58mm blank needed (use 60mm stock).

See also: Effective Diameter (ED), Decentration, Boxing System

Definition: The distance from the center of the bridge of the nose to the center of each pupil, measured separately for right and left eyes. Sum of monocular PDs equals binocular PD. More accurate than binocular PD for ensuring proper optical center placement.

Exam Relevance: ABO exam tests when to use monocular vs binocular PD. Monocular PD is essential for: high prescriptions, progressive lenses, when PD is asymmetric, prism prescriptions. Measured from center of bridge, not from center of other pupil. Right and left may differ by 1-3mm.

Example: Right monocular PD = 32mm, Left = 30mm. Binocular PD = 62mm. Patient has asymmetric PD requiring monocular measurements for accuracy.

See also: Binocular PD, Pupillary Distance (PD), Optical Center (OC)

Definition: A refractive error where parallel light rays from distant objects focus in front of the retina, causing blurred distance vision. Myopes need minus (concave) lenses to diverge light and push focus back onto the retina. Near vision often remains clear.

Exam Relevance: Fundamental refractive concept for both exams. ABO: understanding minus lens optics, center thickness, edge thickness, minification, lens design considerations. NCLE: fitting considerations (flatter corneas typically), myopia progression management, relationship between myopia and contact lens motivation. Questions test ability to identify myopic prescriptions and understand correction.

Example: Patient with -4.00D myopia sees clearly at 25cm (1/0.25m = 4D) but needs minus lenses to see distant objects clearly.

See also: Minus Lens (Concave), Hyperopia (Farsightedness), Emmetropia, Refraction, Astigmatism

Definition: A plus lenticular lens design with a small central optical zone (typically 30-40mm) for high-plus prescriptions. The carrier area has zero or minimal power, dramatically reducing center thickness, weight, and magnification compared to full-field high-plus lenses.

Exam Relevance: ABO exam tests myodisc for high hyperopia and aphakia. Advantages: significantly reduced weight and thickness, less magnification, more cosmetically acceptable. Disadvantages: restricted field of view through optical zone, ring scotoma at boundary, cosmetically visible junction. Questions involve appropriate patient selection and counseling.

Example: A +15.00D aphakic correction in myodisc design weighs 60% less than full-field lens and provides better cosmesis.

See also: Lenticular Lens, Carrier Lens, Aphakia, high-power-lens

Definition: The apparent increase in object size viewed through an optical system. Plus lenses magnify, minus lenses minify. Amount depends on lens power, form, and distance from eye. Spectacle magnification approximation: 1% per 0.25D for vertex distance of 12mm.

Exam Relevance: ABO exam tests understanding of magnification effects. You must know: plus lenses make eyes look larger, minus make them smaller, affects cosmetic appearance especially in higher powers. High-index and aspheric designs reduce magnification/minification. Questions involve counseling patients about appearance and expectations.

See also: Plus Lens (Convex), Minus Lens (Concave), cosmetic-considerations

Definition: Specialized sebaceous glands in the eyelids (approximately 30-40 in each lid) that secrete lipids forming the outer layer of the tear film. Lipid layer prevents tear evaporation. Dysfunction causes evaporative dry eye.

Exam Relevance: NCLE questions test tear film components and dry eye causes. Meibomian gland dysfunction (MGD) is very common cause of contact lens intolerance. You should understand that lipid deficiency leads to rapid tear evaporation, dryness, and reduced contact lens comfort. Warm compresses help.

See also: Tear Film, Dry Eye Syndrome, Blepharitis

Definition: A lens that diverges light rays, thinner in the center than at the edges. Minus lenses have negative power (measured in minus diopters) and are used to correct myopia. They minify images.

Exam Relevance: Fundamental for both exams. ABO: understanding minus lens characteristics (thin center, thick edges, minification, cosmetic concerns with edge thickness). NCLE: fitting considerations for myopes. You must know optical properties, appropriate uses, edge thickness reduction strategies (high-index, aspheric, smaller frames).

See also: Myopia (Nearsightedness), minification, Edge Thickness, Plus Lens (Convex)

Definition: A contact lens with multiple focal points or zones to correct presbyopia, providing distance and near vision in the same lens. Designs include simultaneous vision (both distance and near focused simultaneously) or alternating vision (eye looks through different zones).

Exam Relevance: Important NCLE presbyopia solution. You must know design types: simultaneous vision (most common, using concentric zones or aspheric progression), alternating vision (typically RGP with prism ballast or truncation). Success depends on pupil size, motivation, realistic expectations. Not all presbyopes can adapt.

See also: Presbyopia, simultaneous-vision, Monovision, Add Power

Definition: Dilation (enlargement) of the pupil, occurring naturally in dim light or induced by medications (mydriatic drops). Dilated pupils allow more light into the eye but reduce depth of focus.

Exam Relevance: NCLE questions test pupil responses and clinical significance. You should know that certain medications can cause mydriasis (angle-closure risk in susceptible patients), dilated pupils affect vision (especially in bright light), and very large pupils may affect contact lens centration.

See also: Pupil, Miosis, Accommodation

Definition: Constriction (narrowing) of the pupil, occurring naturally in bright light or induced by certain medications. Small pupils increase depth of focus but limit light entering the eye.

Exam Relevance: NCLE questions test pupil function. Miosis naturally occurs with near viewing (near triad: accommodation, convergence, miosis). Some medications cause miosis (e.g., pilocarpine for glaucoma). Small pupils may reduce symptoms of uncorrected refractive error.

See also: Pupil, Mydriasis, Accommodation, pinhole-effect

Definition: A contact lens presbyopia correction strategy where one eye is corrected for distance vision and the other for near vision. Brain adapts to use the appropriate eye for each task. Typically dominant eye for distance.

Exam Relevance: Important NCLE presbyopia solution. You must know: typically dominant eye corrected for distance, non-dominant for near (usually +1.50 to +2.50 add). Advantages: simple, cost-effective, works with spherical lenses. Disadvantages: reduced binocularity, depth perception issues, not suitable for all patients (especially those needing precise binocular vision).

See also: Presbyopia, Multifocal Contact Lens, Dominant Eye (Sighting Eye)

Definition: The closest distance at which an object can be held and still be seen clearly with maximum accommodation. In young people typically 7-10cm, increases with age due to loss of accommodation (presbyopia).

Exam Relevance: Both exams test relationship to accommodation. Near point testing helps determine amplitude of accommodation and need for reading addition. Calculated: Near point (cm) = 100 / Amplitude of Accommodation (D). Questions involve understanding presbyopic changes and add power determination.

See also: Amplitude of Accommodation, Presbyopia, Accommodation, Add Power

Definition: The abnormal growth of new blood vessels into the normally avascular cornea, typically growing from the limbus toward the center. Most commonly caused by chronic corneal hypoxia from contact lens overwear or inadequate oxygen transmission.

Exam Relevance: Important NCLE complication indicating chronic oxygen deprivation. You must know that neovascularization represents permanent corneal damage. Causes: long-term wear of low-Dk lenses, overnight wear, tight lenses, overwear. Treatment: discontinue wear or switch to high-Dk materials, allow regression (vessels may empty but ghost vessels remain).

See also: Hypoxia, Corneal Edema, Dk (Oxygen Permeability), Dk/t (Oxygen Transmissibility), overwear

Definition: The pupillary distance measured when the patient converges on a near target (typically 40cm for reading). Near PD is approximately 2-3mm smaller than distance PD due to convergence. Used for precise bifocal and progressive lens fitting.

Exam Relevance: ABO exam tests PD measurement for multifocals. Near PD important for accurate bifocal seg placement and progressive lens fitting. Not always measured in practice but more accurate than using distance PD alone. Difference between distance and near PD indicates convergence demand.

See also: Pupillary Distance (PD), Binocular PD, Convergence, Bifocal

Definition: Astigmatism where the principal meridians are oriented at approximately 45° or 135° (oblique axes), rather than horizontal or vertical. Most difficult type to correct with soft toric contact lenses due to rotation issues.

Exam Relevance: NCLE questions test astigmatism types and fitting challenges. Oblique astigmatism is less common than WTR or ATR. Presents special challenges for soft toric fitting because lens rotation causes significant visual degradation. Often better corrected with RGP lenses which mask corneal astigmatism.

Example: K readings: 43.00 @ 045 / 45.00 @ 135 = 2.00D oblique astigmatism. Soft toric lens rotation by 5° causes more visual loss than with WTR or ATR.

See also: With-the-Rule Astigmatism, Against-the-Rule Astigmatism, Toric Contact Lens, Axis

Definition: The point in a lens where light passes through without deviation (no prismatic effect). The optical center must be positioned to align with the patient's pupil for proper vision. In single vision lenses, coincides with the point of zero prism.

Exam Relevance: Fundamental ABO concept. You must know how to locate optical center with lensometer, mark it for edging, and calculate required decentration to align with patient PD. Optical center may differ from geometric center when decentration is needed. Questions involve OC marking, decentration calculations, and Prentice's Rule.

See also: Geometric Center (GC), Decentration, Lensometer (Lensmeter/Focimeter), Prentice's Rule

Definition: The nerve bundle (cranial nerve II) that transmits visual information from the retina to the brain. Contains approximately 1.2 million nerve fibers from retinal ganglion cells. Exits the eye at the optic disc (blind spot).

Exam Relevance: NCLE anatomy questions test optic nerve structure and clinical significance. You should know that damage to the optic nerve is irreversible and causes permanent vision loss (as in glaucoma). The optic disc is visible on ophthalmoscopy and assessment of optic nerve health is part of comprehensive eye exams.

See also: Retina, Optic Disc (Optic Nerve Head), Glaucoma, visual-pathway

Definition: Acute corneal hypoxia resulting from wearing contact lenses for excessive hours beyond recommended schedule. Symptoms typically appear several hours after lens removal: severe pain, photophobia, tearing, foreign body sensation, and corneal edema.

Exam Relevance: NCLE questions test recognition and management. Classic presentation: patient wears lenses too long, removes them, feels fine initially, then develops severe pain 2-6 hours later. Treatment: no lens wear until healed, artificial tears, possible antibiotic. Prevention: follow wearing schedule, don't sleep in daily wear lenses.

See also: Hypoxia, Corneal Edema, Wearing Schedule, Dk (Oxygen Permeability)

Definition: See Dk

Exam Relevance: See Dk (Oxygen Permeability) entry for complete information.

See also: Dk (Oxygen Permeability), Dk/t (Oxygen Transmissibility), oxygen-transmissibility

Definition: Prism oriented at an angle other than purely horizontal or vertical (base directions other than straight up, down, in, or out). Can be resolved into horizontal and vertical components for prescription writing or combining prisms.

Exam Relevance: ABO calculation questions test resolving oblique prism. You must be able to break oblique prism into horizontal and vertical components using trigonometry, or combine horizontal and vertical prisms into single oblique prism. Less commonly prescribed but important for understanding prism principles.

See also: Prism, Base Direction, Horizontal Prism, vertical-prism

Definition: A specialized progressive or multifocal lens design optimized for intermediate and near vision, typically with reduced or no distance correction. Designed for computer work and office tasks, providing wider intermediate and near zones than standard progressives.

Exam Relevance: ABO questions test understanding of specialty lens designs. Occupational lenses are NOT for full-time wear (limited distance vision) but excellent for computer users with presbyopia. Wider fields at intermediate and near than standard progressives. Important to counsel patients these are task-specific lenses.

See also: Progressive Addition Lenses (PALs), Intermediate Vision, computer-glasses

Definition: The circular area on the retina where the optic nerve exits the eye and retinal blood vessels enter. Contains no photoreceptors, creating the physiological blind spot. Visible on ophthalmoscopy as a pale circular structure.

Exam Relevance: NCLE anatomy questions test retinal structures. You should know the optic disc is the anatomical blind spot (normally not noticed due to binocular vision and brain filling in). Changes in disc appearance (cupping) indicate glaucoma damage. All nerve fibers funnel through this small area.

See also: Optic Nerve, Retina, blind-spot, Glaucoma

Definition: A diagram tool showing the two principal meridians of a cylinder lens perpendicular to each other, with powers labeled at each meridian. Used to visualize spherocylinder prescriptions and perform transposition. Axis shown as zero-power meridian.

Exam Relevance: Important ABO visualization and calculation tool. You must be able to draw optical crosses from prescriptions, use them for transposition, and understand the relationship between written prescription and the cross representation. Questions often require optical cross to solve transposition or effective power problems.

See also: Transposition (Cylinder), Cylinder Power, Axis, Astigmatism

Definition: A non-surgical procedure using specially designed RGP contact lenses worn overnight to temporarily reshape the cornea, correcting myopia (and sometimes astigmatism). Lenses removed during day, providing clear vision without correction. Effects reverse when discontinued.

Exam Relevance: NCLE questions test understanding of ortho-K as myopia management option, especially for children. You should know: lenses flatten central cornea to reduce myopia, typically for low to moderate myopia (-1.00 to -6.00D), may slow myopia progression in children, requires nightly wear to maintain effect, specialized fitting and monitoring.

See also: RGP Lens (Rigid Gas Permeable), Myopia (Nearsightedness), corneal-reshaping

Definition: The angle formed when the bottom of the lens front is closer to the face than the top, measured in degrees from vertical. Normal pantoscopic tilt is 8-12 degrees and helps align the optical axis with natural downward gaze for reading.

Exam Relevance: ABO exam tests proper frame adjustment and its optical effects. You should know that pantoscopic tilt affects: effective lens power (especially cylinder axis), vertex distance, visual comfort. Excessive tilt can cause cylinder axis errors and poor optical performance. Progressive lenses require precise pantoscopic tilt (10-12°) for proper function.

See also: Vertex Distance, Progressive Addition Lenses (PALs), frame-adjustment

Definition: A latent eye misalignment that is controlled by fusional vergence when both eyes are open. The deviation is only present when binocular fusion is interrupted. Types include esophoria (inward), exophoria (outward), hyperphoria (upward), and hypophoria (downward).

Exam Relevance: ABO questions test understanding of phorias and prism correction. Unlike tropias (constant misalignment), phorias are compensated but may cause eyestrain. Corrected with prism to reduce fusional demand. You must know which base direction corrects which phoria: BO for eso, BI for exo, BU/BD for vertical phorias.

See also: Esophoria, Exophoria, Prism, Strabismus

Definition: A lens that converges light rays, thicker in the center than at the edges. Plus lenses have positive power (measured in plus diopters) and are used to correct hyperopia, presbyopia, and aphakia. They magnify images.

Exam Relevance: Fundamental for both exams. ABO: understanding plus lens characteristics (thick center, thin edges, magnification, weight concerns, cosmetic issues). NCLE: fitting considerations for hyperopes. You must know optical properties, appropriate uses, and how to identify plus lenses by appearance and lensometry.

See also: Hyperopia (Farsightedness), Presbyopia, Add Power, Magnification, Minus Lens (Concave)

Definition: A lightweight, impact-resistant lens material with refractive index 1.586, specific gravity 1.20, and Abbe value 30. Required for children's eyewear and safety glasses due to superior impact resistance (10x more than CR-39).

Exam Relevance: ABO exam frequently tests polycarbonate properties and appropriate uses. You must know advantages: excellent impact resistance, lightweight, UV protection. Disadvantages: poorest optical quality (lowest Abbe value = most chromatic aberration), soft (scratches easily without coating), higher cost than CR-39. Required by FDA for children under 16 and safety applications.

Example: Children's sports glasses should use polycarbonate for safety, despite lower optical quality compared to CR-39.

See also: Lens Material, Abbe Value, Impact Resistance, CR-39, Trivex

Definition: The small space between the posterior surface of the iris and the anterior surface of the crystalline lens, filled with aqueous humor. Aqueous flows from posterior chamber through the pupil into the anterior chamber.

Exam Relevance: NCLE anatomy questions test chamber structure and aqueous flow. You should understand the aqueous humor pathway: produced by ciliary body → posterior chamber → through pupil → anterior chamber → drains through trabecular meshwork. Blockage of flow can cause angle-closure glaucoma.

See also: Anterior Chamber, Aqueous Humor, Ciliary Body, Iris

Definition: Age-related loss of accommodation due to decreased flexibility of the crystalline lens. Typically becomes noticeable around age 40-45 when near vision becomes difficult. Not a refractive error but a normal aging process affecting everyone.

Exam Relevance: Critical concept for both exams. ABO: understanding add power needs, bifocal/progressive options, patient counseling. NCLE: multifocal contact lens fitting, monovision, patient expectations. You must know progression: symptoms start age 40-45, add power increases ~0.25-0.50D every 2-3 years until age 60-65.

Example: A 45-year-old emmetrope (Plano distance) develops presbyopia, needing +1.50 add for reading. By age 55, likely needs +2.25 add.

See also: Accommodation, Amplitude of Accommodation, Add Power, Bifocal, Progressive Addition Lenses (PALs)

Definition: A formula for calculating induced prismatic effect when looking through a point on a lens away from the optical center. Formula: Prism (∆) = Decentration (cm) × Power (D). Essential for understanding decentration effects.

Exam Relevance: One of the most heavily tested ABO calculations. You must master this formula and its applications: calculating induced prism from decentration, determining maximum acceptable decentration, calculating required decentration to achieve prescribed prism, analyzing vertical imbalance in anisometropia. Questions appear in multiple formats requiring full understanding.

See also: Induced Prism, Decentration, Optical Center (OC), Prism

Definition: An optical element that deviates light without focusing it, used to correct eye alignment problems. Prism power is measured in prism diopters (∆), where 1∆ deviates light 1cm at 1 meter distance. Direction indicated by base position (BU, BD, BI, BO).

Exam Relevance: Major ABO topic tested extensively. You must understand: prism notation, base directions, which corrects which eye condition, measuring prism with lensometer, calculating induced prism with Prentice's Rule, combining prisms, resolving oblique prism into horizontal and vertical components. Questions involve prescriptions, calculations, and clinical applications.

Example: 3∆ BI OD corrects exophoria by forcing right eye to converge. Light bends toward base (nasal), image appears displaced toward apex (temporal).

See also: Base Direction, Prentice's Rule, Phoria, Strabismus, Induced Prism

Definition: Multifocal lenses with gradual power progression from distance (top) through intermediate (middle) to near (bottom), without visible lines. Provide continuous focus at all distances but have peripheral distortion in the progression zone.

Exam Relevance: Important ABO topic for modern presbyopic correction. You must know: fitting measurements (seg height, pantoscopic tilt, vertex distance), corridor length, channel width, adaptation issues, peripheral blur zones. Advantages over bifocals: no visible line, continuous vision, cosmetics. Disadvantages: peripheral distortion, adaptation period, higher cost, precise fitting required.

See also: Presbyopia, Add Power, Bifocal, Segment Height (Seg Height)

Definition: The condition of having an artificial intraocular lens (IOL) implanted in the eye, typically after cataract surgery. The IOL replaces the natural crystalline lens. Most modern cataract patients are pseudophakic.

Exam Relevance: NCLE questions test pseudophakia as part of patient history and fitting considerations. Pseudophakic patients typically need minimal distance correction (IOL provides most needed power) but always need reading add since IOLs don't accommodate. Contact lens fitting is straightforward, similar to other patients, unless there are surgical complications.

See also: Aphakia, Intraocular Lens (IOL), Cataract, Presbyopia

Definition: The circular opening in the center of the iris that controls the amount of light entering the eye. Dilates in dim light (mydriasis) via dilator muscle and constricts in bright light (miosis) via sphincter muscle. Normal size 2-8mm depending on lighting.

Exam Relevance: NCLE anatomy questions test pupil function and clinical significance. You should know: normal pupil responses (light reflex, near reflex), pupil size affects depth of focus and image quality, dilated pupils can affect contact lens centra tion, some medications affect pupil size. Anisocoria (unequal pupils) may indicate neurological problems.

See also: Iris, Mydriasis, Miosis

Definition: The distance between the centers of the pupils, measured in millimeters. Can be measured as binocular PD (total distance between both pupils) or monocular PD (from bridge center to each pupil). Essential for proper lens centration.

Exam Relevance: Fundamental ABO measurement skill. You must know: proper measurement technique, normal adult range (54-74mm, average 62-64mm), when to use binocular vs monocular PD, how PD relates to decentration calculations and frame selection. Distance PD typically 2-3mm greater than near PD due to convergence.

See also: Binocular PD, Monocular PD, Optical Center (OC), Decentration

Definition: Abnormal sensitivity to light causing discomfort or pain. Can indicate corneal problems, inflammation (iritis, keratitis), or other eye conditions. Contact lens-related causes include overwear syndrome, corneal abrasion, or UV damage.

Exam Relevance: NCLE questions test symptoms of complications. Photophobia is red flag symptom requiring evaluation. Common with: corneal abrasions, keratitis, overwear syndrome, corneal edema, iritis. You must recognize photophobia as potential emergency sign and know when to refer immediately.

See also: Keratitis, Overwear Syndrome, Corneal Abrasion, Iritis (Anterior Uveitis)

Definition: Zero refractive power, no lens power. Written as "Plano" or "0.00" in prescriptions. Can refer to entire lens (Plano for distance) or one meridian in cylinder prescription.

Exam Relevance: Basic prescription notation for both exams. You must recognize Plano means no correction needed. Common examples: Plano reading glasses (emmetrope needing only reading add), Plano sunglasses (no prescription), or calculated result (like -2.00 + 2.00 add = Plano for near).

See also: Emmetropia, Refraction, prescription-notation

Definition: A method of stabilizing toric soft contact lenses by incorporating prism (typically 1-2∆ base down) to make the bottom of the lens thicker and heavier. Gravity and lid interaction keep lens oriented properly. Most common stabilization method for soft torics.

Exam Relevance: NCLE questions test toric lens stabilization. You must know prism ballast: base down prism adds weight to bottom of lens, combined with dynamic stabilization from lid forces keeps lens oriented. Other methods: thin zones (double slab-off), peri-ballast, accelerated stabilization. Questions involve rotation troubleshooting.

See also: Toric Contact Lens, stabilization, lens-rotation

Definition: The unit of measurement for prism power. One prism diopter deviates light 1 centimeter at a distance of 1 meter (1cm/1m or 1% deviation). Symbol: ∆ (delta).

Exam Relevance: Fundamental ABO prism unit. You must understand the definition and be able to work with prism diopters in Prentice's Rule calculations, prescribed prism, and induced prism from decentration. Questions involve calculations and understanding what prism values mean clinically.

See also: Prism, Prentice's Rule, Base Direction

Definition: Accumulation of tear proteins (primarily lysozyme, lactoferrin, and mucin) on contact lens surfaces. Appears as cloudy or filmy coating, reduces vision, comfort, and wettability. Can trigger giant papillary conjunctivitis (GPC).

Exam Relevance: Major NCLE topic for lens complications and care. You must know: proteins denature on lens surface forming deposits, more common on ionic lenses and conventional hydrogels, requires enzyme cleaners or frequent replacement. GPC commonly caused by protein deposits. Questions involve prevention and management strategies.

See also: Giant Papillary Conjunctivitis (GPC), lens-deposits, Enzyme Cleaner, Daily Disposable Contact Lens

Definition: The process of determining a patient's refractive error and prescribing corrective lenses. Involves objective measurement (retinoscopy, autorefractor) and subjective refinement (trial lenses, phoropter) to achieve best corrected visual acuity.

Exam Relevance: Basic concept for both exams though opticians don't perform refractions. ABO: understanding prescriptions resulting from refraction, what the numbers mean, prescription verification. NCLE: how to use spectacle Rx as starting point for contact lens fitting, understanding manifest vs cycloplegic refraction, over-refraction technique.

See also: Myopia (Nearsightedness), Hyperopia (Farsightedness), Astigmatism, prescription

Definition: A measure of how much a material bends (refracts) light, defined as the ratio of speed of light in vacuum to speed in the material. Higher refractive index allows thinner lenses. Common values: CR-39 = 1.498, polycarbonate = 1.586, high-index ranges from 1.60-1.74.

Exam Relevance: Important ABO concept for lens material selection. You must understand: higher index = thinner/lighter lenses but typically lower Abbe value, higher cost, and higher reflectance. Index selection based on prescription power, frame size, cosmetic concerns. Questions involve material comparison and appropriate recommendations.

Example: A -6.00D lens in 1.67 index is 30% thinner than same Rx in 1.498 CR-39.

See also: High-Index Lens Material, Abbe Value, Lens Material, CR-39

Definition: Astigmatism remaining after a spherical contact lens (especially spherical RGP) corrects corneal astigmatism. Results from lenticular (internal) astigmatism or when corneal astigmatism isn't fully masked by the RGP lens tear layer. Requires toric contact lens if significant (>0.75D).

Exam Relevance: Critical NCLE concept frequently tested. You must understand: RGP lenses mask regular corneal astigmatism via tear lens, but internal astigmatism remains. Check with over-refraction. If residual astigmatism >0.75D found, need front-surface toric or bitoric RGP. Soft spherical lenses don't mask any astigmatism - need soft toric if total astigmatism >0.75D.

Example: Patient with 2.50D corneal astigmatism and 3.00D refractive astigmatism. Spherical RGP masks 2.50D, leaving 0.50D residual = acceptable. If 1.50D residual, need toric.

See also: corneal-astigmatism, RGP Lens (Rigid Gas Permeable), Toric Contact Lens, Bitoric Contact Lens, over-refraction

Definition: The light-sensitive neural tissue lining the back of the eye that converts light into electrical signals sent to the brain via the optic nerve. Contains two types of photoreceptors: rods (peripheral/night vision) and cones (central/color vision).

Exam Relevance: NCLE anatomy questions test retinal structure and function. You should know: retina contains 10 layers, central retina (macula/fovea) provides sharp central vision, peripheral retina handles motion and night vision. Retinal problems (detachment, degeneration) significantly impact vision and may contraindicate contact lens wear.

See also: Macula, Fovea, rods-and-cones, Optic Nerve

Definition: A contact lens made from firm, oxygen-permeable material (typically with Dk 100+) that maintains its shape on the eye. Provides excellent optics, masks corneal astigmatism, allows good oxygen transmission, and is more durable than soft lenses.

Exam Relevance: Major NCLE topic tested extensively. You must know: fitting principles (alignment fit preferred), fluorescein pattern assessment, advantages (best optics, masks astigmatism, durability, less deposits) and disadvantages (initial comfort, adaptation time, easier to lose). Questions cover fitting, troubleshooting, and when to recommend RGPs vs soft lenses.

See also: Alignment Fit, Fluorescein Pattern, Dk (Oxygen Permeability), corneal-astigmatism, Residual Astigmatism

Definition: An objective technique for determining refractive error by observing the reflection (reflex) of light from the retina. Performed with a retinoscope, observing movement and characteristics of the reflex to neutralize with trial lenses.

Exam Relevance: Basic refraction technique both exams should understand conceptually. While opticians don't perform retinoscopy, understanding it explains where prescriptions come from. Retinoscopy provides objective starting point before subjective refraction refinement.

See also: Refraction, objective-refraction, trial-lenses

Definition: The white, opaque, fibrous outer coat of the eye that provides protection and structural support. Continuous with the cornea anteriorly and optic nerve sheath posteriorly. Covered by conjunctiva on visible anterior portion.

Exam Relevance: NCLE anatomy questions test ocular structure. You should know that the sclera maintains eye shape, provides attachment for extraocular muscles, and is relatively avascular (white appearance). Redness visible on the white of the eye is actually conjunctival or episcleral vessel dilation, not scleral inflammation.

See also: Cornea, Conjunctiva, Choroid

Definition: The vertical measurement from the lowest point of the lens (or frame datum line) to the top of the bifocal or trifocal segment. Critical measurement for proper bifocal dispensing, typically set 4-6mm below pupil center for reading.

Exam Relevance: Important ABO bifocal fitting measurement. You must know proper segment height ensures patient looks through distance portion for normal viewing and easily drops eyes into segment for reading. Too high: patient looks through seg while walking (safety issue). Too low: patient must dip head excessively to read. Standard: 4-6mm below pupil for FT-28.

Example: Patient with pupil height of 20mm should have segment top at 14-16mm (4-6mm below pupil) for comfortable FT-28 bifocal use.

See also: Bifocal, D Measurement, Progressive Addition Lenses (PALs)

Definition: A modern soft contact lens material combining hydrogel and silicone components to achieve high oxygen transmission (Dk 80-140+) while maintaining softness and hydration. Allows healthier extended wear than conventional hydrogels.

Exam Relevance: Important NCLE material category. You must know advantages over conventional hydrogel: much higher Dk allowing extended wear, reduced hypoxia risk, better for long wearing hours. Disadvantages: stiffer than conventional hydrogel, may have deposits or dryness issues, higher modulus. Most modern soft lenses are silicone hydrogel.

See also: Dk (Oxygen Permeability), Dk/t (Oxygen Transmissibility), Extended Wear Contact Lens, Soft Contact Lens, conventional-hydrogel

Definition: A binocular microscope combined with a high-intensity adjustable light source that allows detailed magnified examination of the eye's anterior and posterior structures. Essential instrument for contact lens fitting and ocular health assessment.

Exam Relevance: Fundamental NCLE instrument. You should know basic use: magnification ranges, illumination techniques (diffuse, direct, indirect, sclerotic scatter, retro-illumination), what structures can be examined, use with fluorescein for RGP fitting. Questions test understanding of examination techniques and what pathology can be detected.

See also: Fluorescein, Burton Lamp, biomicroscope

Definition: A flexible contact lens made from hydrogel or silicone hydrogel material that conforms to the eye's shape. Contains 38-78% water. Most popular contact lens type due to comfort, easy adaptation, and part-time wear capability.

Exam Relevance: Major NCLE topic. You must know: fitting principles (based on diameter and base curve), material types (conventional hydrogel vs silicone hydrogel), water content effects, replacement schedules (daily, biweekly, monthly), advantages (comfort, adaptation, occasional wear) and disadvantages (deposits, lower oxygen than RGPs, don't mask astigmatism).

See also: Silicone Hydrogel, conventional-hydrogel, Water Content, base-curve-selection

Definition: An optical defect where peripheral light rays focus at different points than central rays due to increasing lens power toward the edges. Causes reduced contrast and halo effects, especially noticeable in low light with dilated pupils.

Exam Relevance: ABO exam tests lens aberrations and their correction. You should know that spherical aberration increases with: higher lens powers, larger pupil size, distance from optical center. Aspheric lens designs significantly reduce spherical aberration by using flatter peripheral curves. Questions involve understanding causes and recommending appropriate lens designs.

See also: Aspheric Lens, lens-aberrations, atoric, Chromatic Aberration

Definition: A simplified lens power that approximates the overall refractive power of a spherocylinder prescription by combining the sphere and half the cylinder power. Formula: SE = Sphere + (Cylinder/2). Used when prescribing spherical contact lenses for low astigmats.

Exam Relevance: Important calculation for both exams. ABO: understanding when spherical equivalent is appropriate approximation. NCLE: frequently used for initial soft contact lens power when corneal astigmatism is ≤0.75D. Questions involve calculating SE and understanding when it provides acceptable vision versus needing toric correction.

See also: Astigmatism, Cylinder Power, Soft Contact Lens

Definition: An RGP contact lens fitting relationship where the base curve is significantly steeper than the flattest corneal meridian (typically steeper by more than 1.00D). Results in restricted lens movement, central bearing, and peripheral pooling of fluorescein.

Exam Relevance: Important NCLE concept for RGP fitting problems. You must recognize steep fit by fluorescein pattern: central dark area (bearing/no fluorescein), bright peripheral pooling, minimal or no movement. Symptoms: awareness of lens, discomfort, possible central corneal staining, reduced vision from central bearing. Solution: flatten base curve.

Example: Flat K = 43.00D, lens BC = 45.00D (2.00D steeper) = steep fit. Fluorescein shows central bearing with dark spot, peripheral pooling.

See also: Flat Fit, Alignment Fit, Fluorescein Pattern, RGP Lens (Rigid Gas Permeable)

Definition: A manifest, constant eye misalignment where the eyes don't align properly. Unlike phoria (latent), strabismus is present even with both eyes open. Types: esotropia (eye turns in), exotropia (out), hypertropia (up). Often requires prism, surgery, or vision therapy.

Exam Relevance: ABO questions test understanding difference between strabismus and phoria, and role of prism correction. Strabismus may be constant or intermittent. Small-angle strabismus may be corrected with prism, but large deviations often need surgery. You should know that strabismus in children can lead to amblyopia if untreated.

See also: Esophoria, Exophoria, Phoria, Prism, Amblyopia (Lazy Eye)

Definition: A large-diameter RGP contact lens (typically 14-24mm) that vaults completely over the cornea and rests on the sclera. Creates a fluid reservoir between lens and cornea. Used for irregular corneas, severe dry eye, and corneal disease.

Exam Relevance: Specialty NCLE lens type for challenging cases. You should know indications: keratoconus, post-surgical irregular corneas, severe dry eye, corneal scarring, graft-vs-host disease. Provides vision by masking irregularity and protects/hydrates cornea with fluid reservoir. Requires specialized fitting.

See also: RGP Lens (Rigid Gas Permeable), Keratoconus, Irregular Astigmatism, corneal-vault

Definition: Protective eyewear meeting ANSI Z87.1 standards for impact resistance and side protection. Required for specific occupational hazards. Must use polycarbonate or Trivex lenses, proper frame design with side shields.

Exam Relevance: ABO exam tests safety eyewear requirements. You must know: Z87 standards, lens material requirements (polycarbonate or Trivex), frame requirements (side protection, appropriate marking), prescription safety glasses must meet same standards. Questions involve identifying appropriate eyewear for occupational use.

See also: Polycarbonate, Trivex, Impact Resistance, ANSI Standards

Definition: The standard eye chart for measuring visual acuity, developed by Herman Snellen. Features rows of letters decreasing in size, with 20/20 line representing normal vision at 20 feet. Each line corresponds to specific acuity level.

Exam Relevance: Basic vision testing tool both exams should understand. You must know Snellen notation interpretation and that 20/20 represents "normal" vision (ability to resolve letters subtending 5 minutes of arc at 20 feet). Alternative charts exist for children (Lea symbols) and illiterates (Landolt C).

See also: Visual Acuity, 20-20-vision

Definition: Adverse reaction to contact lens care solutions, typically caused by preservatives (especially thimerosal, historically, or modern preservatives like polyquad, PHMB). Symptoms: redness, irritation, solution-related staining. Solution: change to different preservative system or hydrogen peroxide.

Exam Relevance: NCLE questions test problem-solving for discomfort. Solution sensitivity common cause of "red eye" in contact lens wearers. You must know: try different preservative system, hydrogen peroxide system (no preservatives in solution worn in eye), or daily disposables (no solutions). Questions involve differential diagnosis and management.

See also: contact-lens-solution, preservatives, Corneal Staining

Definition: The laboratory process of generating and polishing optical curves on lens surfaces. Traditionally involves grinding lens blanks on curve-generating equipment, then fine grinding and polishing to optical quality. Modern labs often use digital surfacing.

Exam Relevance: ABO questions test basic lab processes. You should understand: surfacing creates the prescription curves on lens blanks, typically back surface is surfaced for Rx (front surface is base curve), modern free-form surfacing allows complex designs like progressives.

See also: Blocking, Edging, Base Curve, lens-fabrication

Definition: The international standard system for measuring and recording cylinder axis, where 0/180 degrees is horizontal (temporal right, nasal left) and 90 degrees is vertical (superior). Axis is always written as three digits (090 not 90).

Exam Relevance: Fundamental notation system for both exams. You must understand TABO orientation: axis values 1-180, with 180 horizontal right, 90 vertical, 045 and 135 oblique. Used for recording prescriptions, lensometer readings, keratometry, and toric contact lens axis. Questions test proper axis notation and understanding meridian orientation.

See also: Axis, Cylinder Power, Astigmatism

Definition: The three-layered fluid coating the ocular surface: outer lipid layer (prevents evaporation), middle aqueous layer (hydration and nutrients), inner mucin layer (allows aqueous to spread on hydrophobic epithelium). Critical for corneal health and contact lens success.

Exam Relevance: NCLE questions test tear film structure, function, and contact lens interactions. You must know: normal tear volume ~7 microliters, tear breakup time normally >10 seconds, dry eye significantly impacts contact lens tolerance. Contact lenses float on and interact with tear film. Questions involve dry eye management and understanding tear-lens interactions.

See also: Dry Eye Syndrome, Meibomian Glands, contact-lens-comfort

Definition: A contact lens with two different powers in perpendicular meridians to correct astigmatism. Requires stabilization mechanism (prism ballast, thin zones, truncation) to maintain proper rotational orientation. Available in soft and RGP designs.

Exam Relevance: Major NCLE topic frequently tested. You must know: when toric needed (typically >0.75-1.00D astigmatism), stabilization methods, rotation assessment (LARS rule: Left Add, Right Subtract for soft torics), axis verification, soft vs RGP toric indications. Questions involve fitting, troubleshooting rotation, and calculating power/axis adjustments.

Example: Soft toric rotates 10° left. Using LARS: Left rotation means Add to axis. If intended axis 180, mark at 170, lens rotates left to 180.

See also: Astigmatism, Axis, Bitoric Contact Lens, Prism Ballast

Definition: The mathematical process of converting a prescription from plus cylinder form to minus cylinder form (or vice versa) without changing the actual lens power. Same optical correction, different notation. Formula: new sphere = old sphere + old cylinder, new cylinder = old cylinder changed to opposite sign, new axis = old axis ± 90.

Exam Relevance: Essential ABO calculation skill tested frequently. You must be able to transpose quickly and accurately. Questions involve converting prescriptions, understanding that transposed Rx provides identical correction, and knowing when each form is preferred (minus cylinder more common in US, plus cylinder common internationally).

Example: -2.00 -1.50 x 090 transposes to -3.50 +1.50 x 180 (same optical correction, different notation)

See also: Cylinder Power, Axis, Astigmatism, Optical Cross

Definition: A lens with three distinct optical zones: distance (top), intermediate (middle segment for computer/dashboard distance), and near (bottom segment for reading). Less popular now due to progressive lenses, but useful for specific occupational needs.

Exam Relevance: ABO exam tests trifocal as alternative to bifocals and progressives. You should know advantages: clear vision at three specific distances, less expensive than progressives. Disadvantages: two visible lines, two image jump points, cosmetically visible, limited zone sizes. Appropriate for patients who need specific intermediate distance and reject progressives.

See also: Bifocal, Progressive Addition Lenses (PALs), Intermediate Vision

Definition: A lens material with refractive index 1.53, Abbe value 43-45, and excellent impact resistance comparable to polycarbonate but with better optical quality. Combines safety, optics, and lightweight (specific gravity 1.11 - lightest lens material).

Exam Relevance: ABO exam tests Trivex as alternative to polycarbonate for safety eyewear. You should know advantages: impact resistance like polycarbonate, much better optical quality (Abbe 43 vs 30), lightest material available, excellent for rimless/drill mount. Disadvantages: thicker than polycarbonate or high-index, higher cost than CR-39.

See also: Polycarbonate, Impact Resistance, Abbe Value, Lens Material

Definition: Measurement of intraocular pressure (IOP), typically using applanation tonometry (Goldmann standard), pneumatonometry (air puff), or other methods. Normal IOP is 10-21 mmHg. Elevated IOP is major glaucoma risk factor.

Exam Relevance: NCLE questions test basic understanding of IOP measurement. While opticians don't perform tonometry, you should know: contact lenses may affect measurement accuracy (remove before testing), corneal thickness affects readings, IOP screening important in comprehensive exams.

See also: intraocular-pressure, Glaucoma, Goldmann Applanation Tonometry

Definition: The middle vascular layer of the eye consisting of the iris, ciliary body, and choroid. Highly vascularized and pigmented, provides blood supply to eye structures and controls various eye functions.

Exam Relevance: NCLE anatomy questions test ocular layers. You should know the three components of uvea and their functions: iris (light control), ciliary body (accommodation, aqueous production), choroid (blood supply to retina). Inflammation of uvea (uveitis) is a contraindication for contact lens wear.

See also: Iris, Ciliary Body, Choroid, Uveitis

Definition: Inflammation of the uveal tract (iris, ciliary body, choroid). Can be anterior (iritis), intermediate, posterior, or pan-uveitis. Causes: autoimmune, infection, trauma, or idiopathic. Symptoms: redness, pain, photophobia, decreased vision.

Exam Relevance: NCLE questions test contraindications for contact lens wear. Active uveitis is absolute contraindication - no contact lens wear during active inflammation. You must recognize symptoms and understand patients with history of uveitis need medical clearance before fitting. May permanently affect vision if severe.

See also: Uvea (Uveal Tract), Iritis (Anterior Uveitis), contraindications, inflammation

Definition: The distance from the back surface of a spectacle lens to the front of the cornea, typically 12-14mm. Changes in vertex distance affect effective lens power, especially significant in powers above ±4.00D.

Exam Relevance: Important concept for both exams. ABO: understanding that moving lens closer to eye increases effective power in minus, decreases in plus. Affects frame selection and adjustment. NCLE: critical for converting spectacle Rx to contact lens power - must compensate for different vertex distance. Questions involve calculations and understanding clinical significance.

Example: -10.00D spectacles at 12mm vertex requires approximately -9.00D contact lenses due to vertex distance compensation.

See also: effective-power, spectacle-to-contact-conversion

Definition: The clarity or sharpness of vision, typically measured using Snellen chart notation (e.g., 20/20). The numerator represents test distance (20 feet in US), denominator represents distance at which a person with normal vision could read the same line.

Exam Relevance: Basic concept for both exams. You must understand Snellen notation: 20/20 is normal, 20/40 means patient sees at 20 feet what normal person sees at 40 feet (worse vision), 20/15 is better than normal. Legal blindness is 20/200 or worse in better eye. Questions involve understanding what measurements mean clinically.

Example: 20/60 vision means patient must be at 20 feet to see what normal person sees at 60 feet. Each line on Snellen chart typically represents 10-20% change in acuity.

See also: Snellen Chart, Refraction, Best Corrected Visual Acuity (BCVA)

Definition: The degree of convergence or divergence of light rays, measured in diopters. Parallel rays have zero vergence, converging rays have positive vergence, diverging rays have negative vergence. Lenses change vergence.

Exam Relevance: Fundamental optical concept for both exams. Understanding vergence explains how lenses work: plus lenses add positive vergence (converge light), minus lenses add negative vergence (diverge light). Vergence at any point equals reciprocal of distance to convergence/divergence point.

See also: Diopter (D), Focal Length, Lens Power

Definition: The percentage of water in a hydrogel contact lens by weight, typically ranging from 38% to 78%. Affects oxygen transmission, deposit resistance, and lens handling. Low water (<50%), mid water (50-60%), high water (>60%).

Exam Relevance: NCLE questions test how water content affects lens performance. You must know: in conventional hydrogels, higher water = higher oxygen transmission but also more deposits, greater dehydration, and more fragile. In silicone hydrogels, high Dk achieved through silicone rather than water, so relationship differs. Questions involve material selection and troubleshooting.

See also: conventional-hydrogel, Silicone Hydrogel, Dk (Oxygen Permeability), lens-deposits

Definition: The prescribed pattern of contact lens wear including daily hours and overnight wear approval. Categories: daily wear (removed nightly), flexible wear (occasional overnight with high-Dk lens), extended wear (FDA-approved continuous wear 7-30 days).

Exam Relevance: NCLE questions test appropriate schedules and patient education. You must know: daily wear safest, extended wear increases infection risk 5x, flexible wear compromise for occasional overnight need. Schedule depends on: lens Dk/t, patient needs, corneal health, compliance. Questions involve risk-benefit discussions and schedule recommendations.

See also: daily-wear, Extended Wear Contact Lens, Dk/t (Oxygen Transmissibility), replacement-schedule

Definition: A type of astigmatism where the steepest corneal or refractive meridian is oriented vertically (at or near 90 degrees ± 30 degrees). The flattest meridian is horizontal (at or near 180 degrees). Most common astigmatism pattern in younger patients.

Exam Relevance: Important NCLE concept for keratometry interpretation and contact lens fitting. You must know: WTR is most common (especially under age 40), easier to fit with soft toric lenses due to better stability, changes to ATR with age. Questions test ability to identify WTR from K readings and understand fitting implications.

Example: K readings: 42.00 @ 180 / 44.00 @ 090 = 2.00D with-the-rule astigmatism. Steep meridian at 90° (vertical).

See also: Against-the-Rule Astigmatism, Oblique Astigmatism, Keratometry (K Readings), Corneal Curvature

Definition: Fine suspensory ligaments connecting the ciliary body to the crystalline lens equator, holding the lens in place. When ciliary muscle contracts, zonules relax allowing lens to become more convex for accommodation.

Exam Relevance: NCLE anatomy questions test accommodation mechanism. You must understand: zonules are under tension at rest (lens flat for distance), ciliary contraction relaxes zonules (lens rounds for near). Zonule weakness or rupture can cause lens subluxation or dislocation. Important for understanding how accommodation works.

See also: Accommodation, Ciliary Body, Crystalline Lens, Presbyopia