Which instrument is used to verify the base curve radius of a GP lens?

• A. Slit lamp
• B. Radiuscope
• C. Lensometer
• D. Spectacle frame

Answer: (B)

Verifying the base curve radius hinges on measuring the curvature of the GP lens’ back surface with a tool built for that purpose. The base curve is the back-surface radius, and GP lenses must have a precise curve to fit and move correctly on the eye. A radiuscope is designed to measure that curvature directly, giving a readout of the back-curve radius and allowing you to confirm it matches the prescribed value. A slit lamp helps you observe how the lens sits on the eye or edge alignment, but doesn’t provide an exact curvature measurement. A lensometer primarily determines optical power (and sometimes diameter) rather than the back-curve radius. A spectacle frame is unrelated to measuring a contact lens’ curvature. Therefore, the radiuscope is the instrument used to verify the base curve radius.