Which instrument is used to reveal toric base curves on a GP lens, suggesting deformation?

• A. Lensometer
• B. Radiuscope
• C. Keratometer
• D. Lens clock

Answer: (B)

The key idea is detecting toricity in the base curve of a gas-permeable lens. A radiuscope is specifically used to measure the radius of curvature of a contact lens surface and, crucially, to compare curvature in different meridians. When a GP lens deforms, its base curve often becomes toric, meaning the curvature differs along orthogonal axes. The radiuscope lets you observe two distinct radii in those meridians, signaling toricity and thus deformation.

The other instruments don’t target this specific measurement. A lensometer checks the lens’s optical power and axis but not the detailed curvature of the base curve. A keratometer measures corneal curvature, not the lens itself. A lens clock provides a rough estimate of curvature but isn’t designed to reveal toricity of a GP lens’s base curve.