What are the two ways we can measure the base curve of a GP lens?

• A. Keratometry and Radiuscope
• B. Radiuscope and Drysdale Principle
• C. Pachymeter and Goniometer
• D. Interferometry and Tonometer

Answer: (B)

Base curve is the back-surface curvature of a gas-permeable lens, and knowing it is key to how the lens fits on the eye. You can measure this directly with a Radiuscope, which reads the radius of curvature of the lens’s back surface, giving you the exact base curve value. If a Radiuscope isn’t available, the Drysdale principle offers a practical way to estimate the base curve from corneal measurements and observed fit, using an established fitting relationship to predict which base curve will give the desired lens clearance and centration. Keratometry measures the front surface of the cornea, not the back surface of the GP lens, so it doesn’t directly provide the base curve. The other instruments listed measure aspects like thickness, angles, or intraocular pressure, which aren’t about determining the lens’s base curve.