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Thickness impacts safety, cosmetics, and optics. Learn to measure it accurately for both exams.
Lens thickness is more than a cosmetic detail. It affects safety, durability, and optical performance. The ABO and NCLE exams include about 20 questions on thickness measurement, typical ranges, and how thickness changes with lens power.
This guide explains how to use a thickness gauge, interpret readings for different lens types, and apply the information in both dispensing and contact lens contexts.
A lens thickness gauge measures the center or edge thickness of a lens. It can be dial-based or digital. You place the lens between two measurement points and read the thickness in millimeters.
Thickness gauges are used for quality control, safety standards, and verifying lens design—especially with high prescriptions where thickness affects weight and cosmetics.
The gauge includes two contact points (anvil and probe), a dial or digital readout, and a spring mechanism. The lens is placed between the probe and anvil for measurement.
[Image: Labeled diagram of lens thickness gauge]
[Image: Lens thickness gauge measuring center thickness]
Typical center thickness for standard spectacle lenses is 2.0–2.5 mm. High minus lenses have thin centers and thick edges. High plus lenses have thick centers and thin edges.
Contact lens thickness is much thinner: RGP lenses are usually 0.12–0.20 mm, soft lenses range 0.05–0.20 mm.
If your readings fall far outside these ranges, recheck calibration or confirm lens design.
Thickness checks ensure impact resistance and safety. They also help you evaluate whether a lens meets cosmetic expectations and whether a redesign or material change is needed.
In contact lens labs, thickness measurement verifies RGP or soft lens manufacturing quality.
Expect questions on typical thickness ranges, plus/minus lens thickness patterns, and contact lens thickness values.
Memory aid: “Minus = thin center, plus = thick center.”
Answer these to check your understanding.
Typical center thickness for a standard spectacle lens is:
Answer: B. 2.0–2.5 mm
Standard spectacle lenses typically measure around 2.0–2.5 mm in center thickness.
Opterio helps you master thickness measurements and every other instrument topic with exam-style practice.