RGP Lens Calculator
Power design for rigid gas permeable lenses — tear lens and base curve
Starting parameters
Enter K readings, over-refraction, and base curve. The tear lens between lens and cornea affects net power. Use manufacturer fitting guides for design selection.
Tear lens and base curve
RGP lenses keep their shape; the tear film between the back surface and cornea forms a tear lens. If the lens is flatter than the cornea (BC > K), the tear lens acts minus; if steeper (BC < K), it acts plus. Changing base curve changes tear lens power, so ordered power must change to keep the same net result.
SAM (Steeper Add Minus): Steepening the base curve adds plus tear lens → add minus to ordered power.
FAP (Flatter Add Plus): Flattening the base curve adds minus tear lens → add plus to ordered power.
Around typical K values, ~0.10 mm base curve change ≈ 0.50 D tear lens change. Use the Radius converter to switch between mm and D.