Lens Materials Comparison: Complete Guide for ABO Exam

Why Lens Materials Matter for Your ABO Exam

Choosing the right lens material is one of the most important decisions you make as a dispensing optician. Material affects thickness, weight, optical quality, impact resistance, cost, and patient satisfaction. The ABO dedicates 10-15 questions to lens materials, testing your knowledge of refractive index, Abbe value, specific gravity, impact resistance standards, and when to recommend each material based on prescription, lifestyle, and budget.

The major lens materials you need to know are CR-39 (standard plastic), polycarbonate, Trivex, and high-index plastics (1.60, 1.67, 1.70, 1.74). Each has strengths and weaknesses. CR-39 offers excellent optical quality and affordability but is thick in high prescriptions. Polycarbonate is thin, lightweight, and impact-resistant but has poor optical quality (low Abbe value). Trivex balances optical quality and impact resistance but costs more. High-index materials are thin but expensive and have varying optical performance.

The exam loves comparison questions: "Which material has the highest Abbe value?" (CR-39), "Which is thinnest for high minus?" (1.74 high-index), "Which is best for children's safety glasses?" (polycarbonate). It also tests trade-offs: if a patient has -8.00 D and wants thin lenses with minimal color fringes, what do you recommend? You must balance refractive index (for thinness) against Abbe value (for optical quality). No material is perfect—every choice is a compromise.

In this guide, you'll learn the key properties of each major lens material, how refractive index and Abbe value affect performance, impact resistance standards (FDA and ANSI Z87.1), specific gravity and weight considerations, when to recommend each material, and how to explain trade-offs to patients. By the end, you'll confidently answer material selection questions and recommend the right lens for any prescription and lifestyle.

Key Lens Material Properties

Before diving into specific materials, understand the properties the ABO tests. These determine material performance and guide your recommendations.

Refractive Index (n)

Refractive index measures how much a material bends light. Higher index = more bending = thinner lenses for the same prescription. CR-39 has n=1.498, polycarbonate n=1.586, high-index materials range from n=1.60 to n=1.74. For a -6.00 D lens, 1.74 high-index is significantly thinner than CR-39. But higher index doesn't automatically mean better—you must consider optical quality too.

Abbe Value (V-value)

Abbe value measures chromatic dispersion—how much a material separates white light into colors. Higher Abbe = less chromatic aberration = better optical quality. CR-39 has Abbe 58, polycarbonate Abbe 30, Trivex Abbe 43-45, high-index materials range from Abbe 32-42. Low Abbe materials (poly, some high-index) create color fringes around lights and at lens edges, especially in high prescriptions.

The ABO tests this relationship constantly: "Which material has the least chromatic aberration?" Answer: CR-39 (highest Abbe). This is the single most important optical quality measure for materials.

Specific Gravity (Density)

Specific gravity measures material density and thus weight. Lower specific gravity = lighter lenses. CR-39 is 1.32, polycarbonate 1.20, Trivex 1.11, high-index materials 1.30-1.47. Trivex is the lightest plastic material—great for rimless or large frames. Polycarbonate is also light. High-index materials are often denser than CR-39 despite being thinner, so the weight advantage isn't as dramatic as thickness reduction suggests.

Impact Resistance

All ophthalmic lenses must pass FDA drop-ball impact testing (basic safety). Polycarbonate and Trivex exceed this standard dramatically—they're used in safety glasses, sports eyewear, and children's glasses because they're nearly unbreakable. CR-39 and high-index plastics meet FDA requirements but are more prone to shattering on severe impact. For safety-critical applications, poly or Trivex are mandatory.

UV Protection

Polycarbonate and Trivex block 100% UVA and UVB inherently (built into the material). CR-39 and high-index plastics require UV coating to achieve full protection. This is a selling point for poly and Trivex—no additional UV treatment needed.

CR-39: The Standard Plastic Lens

CR-39 (Columbia Resin #39) is the baseline plastic lens material. Introduced in the 1940s, it remains the most common material for low to moderate prescriptions. When the exam says "standard plastic," they mean CR-39.

Key Properties

Refractive Index: 1.498 (lowest among plastics). Abbe Value: 58 (highest among plastics—excellent optical quality). Specific Gravity: 1.32 (moderate weight). Impact Resistance: Meets FDA standard but not as strong as poly or Trivex. UV Protection: Requires coating.

Advantages

Best optical quality of all plastic materials (high Abbe = minimal chromatic aberration). Excellent scratch resistance with hard coat. Easy to tint and accepts all coatings well. Most affordable plastic material. Surface quality holds up well over time. No stress-induced birefringence (some materials develop internal stresses that affect optics).

Disadvantages

Thickest plastic material due to low refractive index (1.498). Heavy in high prescriptions. Lower impact resistance than poly or Trivex—not recommended for safety glasses or children. Requires UV coating (not inherent). For high prescriptions (±4.00 D and above), CR-39 creates thick, heavy lenses that look and feel bulky.

When to Recommend CR-39

Low to moderate prescriptions (plano to ±4.00 D) where thickness isn't a concern. Patients prioritizing optical quality over thinness. Budget-conscious patients who don't need high-index. Patients sensitive to chromatic aberration (CR-39 has the least). Full-frame glasses (not safety, sports, or rimless where impact resistance matters).

Polycarbonate: The Impact-Resistant Choice

Polycarbonate is the go-to material for safety, sports, and children's eyewear. It's thin, light, and nearly indestructible. The trade-off? Poor optical quality due to low Abbe value. Polycarbonate is the most common high-index material despite its optical compromises.

Key Properties

Refractive Index: 1.586 (thin). Abbe Value: 30 (lowest—significant chromatic aberration). Specific Gravity: 1.20 (very light). Impact Resistance: Excellent—exceeds ANSI Z87.1 safety standard, 10x more impact-resistant than CR-39. UV Protection: 100% inherent (no coating needed).

Advantages

Extremely impact-resistant—ideal for safety glasses, sports eyewear, children's glasses, and active lifestyles. Thin lenses (high refractive index). Very lightweight (low specific gravity). Inherent 100% UV protection. Good for rimless and semi-rimless frames (strong enough to hold drill mounts). Affordable compared to premium high-index materials.

Disadvantages

Low Abbe value (30) creates noticeable chromatic aberration—color fringes around lights, especially in high prescriptions and peripheral vision. Softer surface than CR-39—scratches more easily, requires hard coat. Stress-induced birefringence can occur in thick areas (internal stresses that affect polarization—visible when viewed through polarized sunglasses). Not ideal for patients prioritizing optical perfection.

When to Recommend Polycarbonate

Children under 18 (safety requirement in many places). Safety glasses and protective eyewear (ANSI Z87.1 compliance). Sports eyewear (impact protection). Rimless and semi-rimless frames (strength for drill mounts). Active lifestyles where impact risk is higher. Patients who prioritize thinness and safety over optical perfection.

Trivex: The Balanced Material

Trivex is a newer material (introduced 2001) that combines polycarbonate's impact resistance with better optical quality. It's lighter than poly but slightly thicker. Trivex is the premium choice when you need safety and optics in one package.

Key Properties

Refractive Index: 1.53 (moderate—thicker than poly, thinner than CR-39). Abbe Value: 43-45 (much better than poly, close to CR-39). Specific Gravity: 1.11 (lightest plastic material). Impact Resistance: Excellent—exceeds ANSI Z87.1, comparable to polycarbonate. UV Protection: 100% inherent.

Advantages

Excellent optical quality (Abbe 43-45)—significantly better than polycarbonate, minimal chromatic aberration. Extremely lightweight (lowest specific gravity of all plastics). Impact-resistant (ANSI Z87.1 safety rated). Inherent 100% UV protection. Superior internal stress characteristics—no birefringence issues like poly. Great for rimless frames (strong and light). Easy to cut and edge.

Disadvantages

Lower refractive index (1.53) than polycarbonate (1.586) or high-index materials—lenses are slightly thicker. More expensive than poly or CR-39. Not as widely available as poly or CR-39. For high prescriptions (±6.00 D+), Trivex may not be thin enough—high-index is better.

When to Recommend Trivex

Patients who need impact resistance (safety, sports, children) but want better optical quality than polycarbonate. Rimless and semi-rimless frames where light weight and strength matter. Patients sensitive to chromatic aberration who also need safety rating. Low to moderate prescriptions (plano to ±4.00 D) where Trivex thickness is acceptable. Premium alternative to polycarbonate when budget allows.

High-Index Materials: Thinner Lenses

High-index materials have refractive indices above 1.60, creating thinner lenses than CR-39 or polycarbonate. They're the solution for high prescriptions where thickness is a cosmetic concern. High-index materials come in several indices: 1.60, 1.67, 1.70, 1.74. Higher index = thinner lenses but often worse optical quality and higher cost.

1.60 High-Index

Refractive Index: 1.60. Abbe Value: 40-42 (good optical quality). Specific Gravity: 1.30. This is the entry-level high-index material—thinner than CR-39 and poly, better optical quality than poly. Good balance of thinness, optics, and cost. Recommended for ±4.00 D to ±6.00 D prescriptions.

1.67 High-Index

Refractive Index: 1.67. Abbe Value: 32-36 (moderate chromatic aberration). Specific Gravity: 1.35-1.37. This is the most popular high-index for strong prescriptions—significantly thinner than 1.60. Recommended for ±6.00 D to ±10.00 D. Optical quality is acceptable but not as good as 1.60. Patients may notice some color fringes in high prescriptions.

1.70 and 1.74 High-Index

Refractive Index: 1.70 or 1.74 (thinnest plastic materials available). Abbe Value: 32-36 (low—noticeable chromatic aberration). Specific Gravity: 1.40-1.47 (dense—heavier than you'd expect). These are premium materials for very high prescriptions (±10.00 D and above). They create the thinnest possible lenses but at significant cost and with optical compromises. 1.74 is about 50% thinner than CR-39 for the same prescription.

High-Index Advantages

Thin lenses for high prescriptions—dramatically improved cosmetics. Lighter than thick CR-39 lenses (despite higher density, the reduced volume wins). Better appearance—edges don't protrude from frame as much. Flat lens profile—reduces "bug-eye" look (plus lenses) or "coke-bottle" look (minus lenses).

High-Index Disadvantages

Lower Abbe values (32-42) create more chromatic aberration than CR-39 or Trivex—color fringes increase with index. Higher cost—1.67 costs 2-3x more than CR-39, 1.74 costs 4-5x more. Higher surface reflections due to index—anti-reflective coating is mandatory for acceptable cosmetics. Heavier than expected due to high density (specific gravity 1.35-1.47). Not as impact-resistant as poly or Trivex.

When to Recommend High-Index

High prescriptions (±4.00 D and above) where thinness is a priority. Patients concerned about lens thickness and cosmetics. Full-frame glasses where impact resistance isn't critical. Patients willing to pay premium for thin lenses. Combine with anti-reflective coating to minimize surface reflections. For very high Rx (±10.00 D+), 1.67-1.74 are essential for acceptable thickness.

Material Comparison Summary

How to Select the Right Material

Material selection balances prescription, lifestyle, priorities, and budget. Here's a decision framework the ABO expects you to understand.

Step 1: Check Safety Requirements

Children under 18? Polycarbonate or Trivex (impact resistance). Safety glasses or sports eyewear? Polycarbonate or Trivex (ANSI Z87.1). If safety is required, choice is made—poly or Trivex. Trivex if budget allows and optical quality matters. Poly if budget is limited or thinness is priority.

Step 2: Assess Prescription Strength

Low Rx (plano to ±2.00 D): CR-39 is fine—thickness not an issue. Moderate Rx (±2.00 to ±4.00 D): CR-39 acceptable, or 1.60 high-index for improved cosmetics. High Rx (±4.00 to ±6.00 D): 1.60 high-index recommended for thinness. Very High Rx (±6.00 to ±10.00 D): 1.67 high-index strongly recommended. Extreme Rx (±10.00 D and above): 1.70-1.74 high-index necessary for acceptable thickness.

Step 3: Consider Patient Priorities

Priority: Optical Quality → CR-39 or Trivex (highest Abbe values). Priority: Thinness → High-index 1.67-1.74 for high Rx. Priority: Light Weight → Trivex (lowest specific gravity). Priority: Budget → CR-39 (most affordable). Priority: Safety → Polycarbonate or Trivex.

Step 4: Frame Considerations

Rimless or semi-rimless frames? Polycarbonate or Trivex (strength for drill mounts). Large frames? Consider weight—Trivex or poly. Small frames? Thickness less critical—CR-39 may work for moderate Rx. Wraparound sports frames? Polycarbonate (impact + thinness).

How the ABO Exam Tests Lens Materials

The ABO includes 10-15 questions on lens materials, covering properties, comparisons, and material selection scenarios. Here's what to expect and how to prepare.

Question Types

Property Questions: "Which material has the highest Abbe value?" Answer: CR-39 (58). "Which plastic material is lightest?" Answer: Trivex (specific gravity 1.11). These test whether you've memorized key properties.

Material Selection: "Best material for children's glasses?" Answer: Polycarbonate or Trivex (impact resistance). "Best material for -10.00 D prescription?" Answer: 1.67 or 1.74 high-index (thinness). These test clinical judgment.

Trade-off Questions: "Patient has -8.00 D, wants thin lenses with minimal color fringes. What material?" Answer: 1.60 high-index (balances thinness with acceptable Abbe 40-42). These test whether you understand compromises.

Study Tips

Memorize key values: CR-39 Abbe 58 (highest), Poly Abbe 30 (lowest), Trivex SG 1.11 (lightest). Create a comparison chart and review it until automatic. Practice material selection scenarios: given a prescription and patient priorities, what material would you recommend? Do 10-15 scenarios covering different Rx ranges, lifestyles, and budgets.

ABO Practice Questions

Test your lens material knowledge with these ABO-style questions. Try to answer before revealing the solutions.

Related ABO Topics

Lens materials connect to several other ABO concepts. Review these topics to strengthen your understanding: