A radiusscope measures base curve by analyzing the distance between real and aerial images.

• A. True
• B. False
• C. Not applicable
• D. It varies with lens diameter

Answer: (A)

Base curve is determined by the curvature of the posterior surface of a contact lens. A radiusscope uses a straightforward optical principle: light that interacts with the lens edge creates two image appearances along the optical axis—one real image where rays actually converge, and one aerial (virtual) image where the extensions of the rays seem to originate. The instrument brings these two images into view and measures the distance between them. That separation is calibrated so it corresponds to the radius of curvature of the lens’s back surface. In other words, the reading you obtain from the radiusscope reflects the base curve because it links image distance to curvature.

So the statement is true because the tool relies on the real-versus-virtual image separation to infer the base curve. Not applicable would not fit since the radiusscope is specifically designed for this measurement, and the idea that it varies with lens diameter isn’t correct because base curve is a property of curvature, not lens size.