The Contact Lens Effectivity Calculator determines the correct refractive power for contact lenses based on the prescription for spectacle lenses. This calculation is crucial because the position of the contact lens directly on the cornea differs from the position of spectacle lenses, which are placed farther from the eyes. By accounting for this difference, the calculator ensures accurate vision correction and better comfort for contact lens users.
Formula of Contact Lens Effectivity Calculator
The power of the contact lens is calculated using the formula:
Contact Lens Power (CLP) = Spectacle Lens Power (SLP) / (1 – d × SLP)
Detailed Formula Components
- Spectacle Lens Power (SLP):
- This is the refractive power of the lens in diopters (D) used in spectacles.
- Vertex Distance (d):
- The distance from the back of the spectacle lens to the cornea, measured in meters.
- If the distance is provided in millimeters, convert it to meters by dividing by 1000 (e.g., 12 mm = 0.012 m).
- Contact Lens Power (CLP):
- The refractive power required for contact lenses, adjusted based on the vertex distance and spectacle lens power.
Assumptions
- The formula is typically used for powers exceeding ±4.00 D, where the vertex distance significantly affects lens power.
General Terms Table
The following table provides quick references for converting spectacle prescriptions to contact lens power:
| Spectacle Power (SLP) (D) | Vertex Distance (d) (mm) | Adjusted Contact Lens Power (CLP) (D) |
|---|---|---|
| +6.00 | 12 | +5.71 |
| -5.50 | 10 | -5.76 |
| +4.25 | 10 | +4.11 |
| -7.00 | 12 | -7.33 |
| +8.50 | 10 | +8.06 |
These values are approximations based on the formula and typical vertex distances.
Example of Contact Lens Effectivity Calculator
Let’s calculate the contact lens power for a spectacle prescription of +6.00 D with a vertex distance of 12 mm.
- Given Data:
- Spectacle Lens Power (SLP): +6.00 D
- Vertex Distance (d): 12 mm = 0.012 m
- Formula:
Contact Lens Power (CLP) = SLP / (1 – d × SLP) - Substitute Values:
CLP = +6.00 / (1 – 0.012 × +6.00)
CLP = +6.00 / 0.928 ≈ +5.71 D.
Result:
The contact lens power should be approximately +5.71 D.
Most Common FAQs
Contact lenses sit directly on the cornea, whereas spectacle lenses are position farther from the eye. The vertex distance affects the effective lens power, especially for higher prescriptions, necessitating an adjustment.
The formula is most useful for prescriptions above ±4.00 D. For lower prescriptions, the effect of the vertex distance is negligible, and the spectacle power can often be use directly.
Vertex distance is measure as the distance between the back surface of the spectacle lens and the front surface of the cornea. This measurement is typically provided by optometrists during an eye examination.