Skip to content
ODReference

RGP Lens Calculator

Tear lens power, SAM-FAP adjustments, and ordered power from K readings and over-refraction

Empirical RGP Lens Calculation

Starting Parameters

Refraction

Type numbers to set the value. Press Enter to commit. Press Escape to clear typed input.
Type numbers to set the value. Press Enter to commit. Press Escape to clear typed input.
Type an axis from 1 to 180. Press Enter to commit. Press Escape to clear typed input.

Keratometry

Type numbers to set the value. Press Enter to commit. Press Escape to clear typed input.
Type numbers to set the value. Press Enter to commit. Press Escape to clear typed input.
Type an axis from 1 to 180. Press Enter to commit. Press Escape to clear typed input.
Lens Design
Lens Diameter (toric only)

RGP Tear Lens, SAM-FAP, and Ordered Power Calculator

What this RGP calculator is designed to do

This tool supports power design for rigid gas permeable (RGP) lenses by quantifying the tear lens created between the base curve and the cornea. It connects clinical inputs (K readings, selected base curve, and over-refraction) to an ordered lens power so the ordered result matches what you observed on eye.

It is a power and documentation aid. Manufacturer fitting guides and your slit lamp findings remain the primary references for design selection, edge profile, diameter, and overall fitting strategy.

The tear lens and why base curve changes alter net power

RGP lenses retain their shape instead of draping over the cornea. The space between the back surface of the lens and the anterior cornea fills with tears and forms a tear lens with real refractive power.

The sign of the tear lens depends on whether the lens is steeper or flatter than the cornea:

  • Lens steeper than the cornea (BC < K): the tear lens acts as plus.
  • Lens flatter than the cornea (BC > K): the tear lens acts as minus.

If you change the base curve after assessing fit, the tear lens changes too. This is why the ordered power must change to preserve the same net optical result.

A spherical RGP can typically neutralize up to about 2.00 D of with-the-rule corneal astigmatism through the tear lens alone, because the tear film fills the gap between the spherical back surface and the toric cornea. Beyond that, the tear lens cannot fully correct the cylinder and residual astigmatism appears in the over-refraction, which is when toric designs become relevant.

SAM and FAP as a reliable power adjustment rule

The base curve and tear lens relationship is commonly summarized with SAM and FAP:

  • SAM (Steeper Add Minus): steepening the base curve creates more plus tear lens power, so you add minus to the ordered lens power to keep the net result the same.
  • FAP (Flatter Add Plus): flattening the base curve creates more minus tear lens power, so you add plus to the ordered lens power to keep the net result the same.

Around typical corneal curvatures, a 0.10 mm base curve change is often close to 0.50 D of tear lens change. The calculator applies these relationships directly using your entered values so you do not need to track the adjustments manually. When adjusting diameter along with base curve, remember to flatten by about 0.05 mm (0.25 D) for every 0.4 mm increase in diameter to maintain a similar sagittal relationship, and apply SAM-FAP to the resulting base curve change.

Toric RGP designs: SPE, CPE, and when each applies

When corneal toricity exceeds what a spherical tear lens can neutralize, toric RGP designs distribute cylinder between the back and front surfaces to improve alignment and vision. Labs commonly describe bitoric lenses using two categories:

  • Spherical power effect (SPE) bitoric: when the back surface toricity is approximately 1.5 times the residual astigmatism, the induced cylinder from the back surface effectively cancels the residual and the front surface can remain spherical. The lens rotates on the eye without affecting vision because the power effect is spherical.
  • Cylinder power effect (CPE) bitoric: when the relationship between back surface toricity and residual astigmatism does not produce a spherical power effect, a front surface toric component is needed to correct remaining cylinder. CPE designs must be rotationally stable to maintain axis alignment.
  • Front surface toric (on a spherical base): useful when the cornea is relatively spherical but refractive cylinder is present. Cylinder is placed on the front surface and must be stabilized with prism ballast or truncation to maintain axis.

When you are reconciling K readings, base curve options, and tear lens changes across units, the Radius to Diopter Converter is a helpful companion tool.

Suggested workflow for using this calculator

  1. Start from K readings and select an initial base curve.
  2. Evaluate fit and fluorescein pattern on eye.
  3. Obtain an over-refraction after the lens settles.
  4. Enter K readings, base curve, and over-refraction to estimate the ordered power that preserves the net optical result.
  5. Re-check vision and fit at follow-up and adjust design or parameters as needed.

RGP Design and Power FAQs

What does SAM-FAP mean in RGP fitting?

SAM stands for Steeper Add Minus and FAP stands for Flatter Add Plus. When you steepen the base curve, the tear lens gains plus power, so you add minus to the ordered lens power to keep the net correction the same. When you flatten the base curve, the tear lens gains minus power, so you add plus. Around typical corneal curvatures, each 0.10 mm of base curve change corresponds to roughly 0.50 D of tear lens change.

When should I consider a bitoric RGP instead of a spherical design?

A spherical RGP can typically neutralize up to about 2.00 D of corneal astigmatism through the tear lens. Beyond that, the lens may rock on the eye with a characteristic dumbbell fluorescein pattern, and residual astigmatism appears in the over-refraction. Bitoric designs are often considered around 2.00 to 2.50 D or more of corneal toricity, where they improve alignment and stability while correcting remaining refractive cylinder. If the back surface toricity is about 1.5 times the residual astigmatism, an SPE bitoric may work with a spherical front surface. Otherwise, a CPE bitoric with front surface toric is needed.

How can I estimate tear lens power when changing base curves?

A practical approximation is:

Tear lens (D) ≈ Base curve (D) − flat K (D)

Example: if flat K is 44.00 D and the selected base curve is 45.00 D, the tear lens is about +1.00 D.

To keep the net power the same, apply SAM and FAP. A steeper base curve creates a plus tear lens, so you add minus to the ordered lens power. The calculator performs this adjustment once you enter the values.

Why convert K readings between millimeters and diopters for RGP work?

K readings and base curves are often shown in millimeters, while over-refraction and tear lens changes are easier to interpret in diopters. Converting helps you relate base curve changes to tear lens power. Around typical corneal ranges, a 0.10 mm change is often close to about 0.50 D. The Radius to Diopter Converter links these units so instrument data and ordered parameters stay consistent.

Where does this calculator fit in an RGP fitting workflow?

Use it after you have selected a base curve, evaluated fit, and obtained an over-refraction. The calculator standardizes the tear lens and base curve power adjustments so the ordered lens power matches the optical result you observed. Manufacturer guides still direct the overall lens design, diameter selection, and fitting philosophy.