How is base curve expressed?

• A. Diameter
• B. Diopter (D) or Radius (mm)
• C. Power
• D. Surface area

Answer: (B)

Base curve describes how curved the back surface of a contact lens is—the curvature is described by its radius of curvature. That radius is typically given in millimeters, which is why expressing base curve as a radius (mm) is standard. In some contexts, people also refer to an equivalent diopter value, since curvature can be related to optical power, but base curve itself is fundamentally about the lens’s shape rather than its corrective strength. This is why the option that lists Diopter (D) or Radius (mm) as the way to express base curve is the best fit. Diameter, power, and surface area pertain to other aspects: diameter is overall width, power is the lens’s corrective strength, and surface area isn’t used to describe the back-surface curvature.