RGP Lens Calculator
Power design for rigid gas permeable lenses — tear lens and base curve
Rigid gas permeable (GP) lens reference: firm, oxygen-permeable lenses ideal for irregular corneas and demanding vision.
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.
Rigid gas permeable lenses at a glance
Rigid gas permeable (RGP, or GP) lenses are firm, oxygen-permeable lenses that hold their shape on the eye. Because they retain a rigid optical surface, they mask corneal irregularity with a tear lens and often deliver crisper vision than soft lenses, making them valuable for irregular corneas (such as keratoconus), high or unusual astigmatism, and demanding visual requirements.
RGP materials are specified by parameters such as base curve, diameter, power and material Dk, and are fitted with diagnostic sets and fluorescein pattern assessment. This page collects RGP reference information to support that process.
When are RGPs preferable to soft lenses?
For irregular corneas, keratoconus, high or residual astigmatism, and cases where soft lenses can't deliver stable vision. The rigid surface plus tear lens neutralises much corneal irregularity.
How is fitting different from soft lenses?
RGP fitting evaluates the tear-film relationship with fluorescein and a diagnostic lens set, adjusting base curve and diameter to achieve an appropriate bearing pattern and movement.
Does oxygen (Dk) still matter for rigid lenses?
Yes. Modern GP materials are oxygen-permeable for corneal health; material Dk remains a key selection factor. See Understanding Dk, Dk/t & oxygen transmissibility.
RGP fitting is a clinical skill guided by diagnostic assessment and manufacturer fitting sets. This page is a reference aid only. See our disclaimer.