How is the overall diameter of a GP lens typically measured during verification?

• A. Indirectly using a lens loupe/magnifier
• B. Directly with a ruler
• C. By estimation with lighting
• D. Using a PD stick only

Answer: (A)

The key idea is that GP lens diameter is verified indirectly by magnifying the lens to compare its edge against a calibrated reference, rather than trying to read a number off a ruler. A lens loupe or magnifier lets you see the edge of the curved, thin lens clearly and judge whether the overall diameter matches the expected size within tolerance. This avoids distorting the lens shape by flattening it on a ruler and provides a fast, repeatable check in clinic or lab settings.

Direct use of a ruler is not reliable because the lens is curved and flexible, so laying it flat or pressing it to measure with a straight line can change its apparent diameter. Estimating with lighting is too subjective to be dependable, and a PD stick is designed for pupillary distance, not determining lens diameter.