Which instrument is used to measure the base curvature of a lens?

• A. Lens Clock
• B. Lensometer
• C. Refractometer
• D. Vernier Caliper

Answer: (A)

Base curve is the curvature of the lens’s back surface, which matters for how the lens sits in the frame. A lens clock is designed to read that curvature directly. By placing the clock on the lens surface, the readings translate the curvature into a diopter value, giving a quick, practical measure of the base curve. This makes it the best choice for assessing base curvature.

The other instruments aren’t used for this purpose: a lensometer checks the completed lens’s optical power and axis, a refractometer measures refractive index or how light refracts through material, and a Vernier caliper measures straight-line dimensions like thickness or width rather than curvature.