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RGP Lens Calculator

Empirical RGP base curve, diameter, and power selection from K readings and 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 Calculator and Power Design

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.

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.

Toric RGP designs in plain terms

When corneal toricity is higher, a spherical base curve can become unstable and the tear lens may not provide adequate optical correction. Toric RGP designs distribute cylinder between the back and front surfaces to improve alignment and vision.

  • Front surface toric: useful when the cornea is relatively spherical but refractive cylinder is present. Cylinder is placed on the front surface and must be stabilized to maintain axis.
  • Back surface toric: improves alignment on a toric cornea, but may not fully match the refractive cylinder without an additional front surface component.
  • Bitoric: commonly used when corneal astigmatism is higher. The back surface improves alignment and stability, and the front surface corrects remaining refractive cylinder.

When you are reconciling K readings, base curve options, and tear lens changes, 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

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

Bitoric designs are often considered when corneal astigmatism is higher, commonly around 2.50 D or more. Below that, a spherical RGP can mask some corneal toricity with the tear lens while remaining stable. When toricity is higher, a spherical lens may rock on the eye and show a characteristic dumbbell fluorescein pattern, and a bitoric design can improve alignment and stability while correcting remaining refractive cylinder.

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 and fitting philosophy.