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The lens clock is small but powerful. Learn how to read base curves and avoid the traps the ABO exam loves.
The Geneva lens measure, often called a lens clock, is a quick way to check lens surface curves. It appears in about 15 questions on the ABO exam, mostly testing your understanding of base curve readings and limitations.
The trick is that the lens clock doesn’t measure true power. It measures surface curve using a standard refractive index of 1.53. If you forget that, you’ll misinterpret the reading.
This guide explains the three-pin contact principle, how to read the dial, and when the measurement is useful (and when it’s not).
The Geneva lens measure is a mechanical device that estimates the curvature of a lens surface. It uses three contact points—two fixed feet and a central probe—to compare the surface to a known standard.
The reading is calibrated assuming a refractive index of 1.53. That means the dial reading represents base curve, not actual lens power. It’s a quick reference tool, not a prescription verifier.
The lens clock has three main components: two fixed pins, one spring-loaded center pin, and a dial gauge. The pins rest on the lens surface while the center pin measures sag.
[Image: Labeled diagram of Geneva lens measure]
[Image: Lens clock placement on convex surface]
A positive reading indicates a convex surface, a negative reading indicates a concave surface. The value represents the surface curve assuming n = 1.53. It does not equal lens power.
Use it to compare lens curves or verify base curves during fabrication. Don’t use it to verify prescriptions.
The lens clock is useful for verifying base curves in the lab, checking a blank, or identifying unknown lens curves quickly. It’s not precise enough for prescription verification but great for rough checks.
Professional tip: use it in combination with lensometer readings when troubleshooting lens complaints.
Expect questions on the index assumption (1.53), the three-pin contact principle, and sign conventions.
Memory aid: “Lens clock reads base curve, not true power.”
Use these to check your understanding.
What refractive index does the Geneva lens measure assume?
Answer: C. 1.53
The Geneva lens measure is calibrated for a refractive index of 1.53.
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