In GP lens verification, which parameter defines the radius of the lens's base curve?

• A. Power
• B. Surface Quality
• C. Base Curve Radius
• D. Lens Edge

Answer: (C)

The key idea here is that the back surface of a gas-permeable lens is defined by its curvature, described by the base curve radius. This radius sets how steep or flat the lens sits on the cornea and how the tear film is distributed beneath it. A smaller base curve radius makes the back surface steeper, pulling the lens closer to the cornea, while a larger radius creates a flatter back surface and a different fit. This parameter is about the geometry of the lens surface that contacts the eye, not about refractive power or surface smoothness. Power relates to how the lens corrects vision, not how curved the back surface is, surface quality refers to the smoothness of the optical surfaces, and the lens edge concerns the outer boundary and comfort—neither defines the base curve’s radius. So, the radius of the base curve is the measure that defines the lens’s back-surface curvature.