One 6.00 D curve on seven materials
| Material index | True surface power |
|---|---|
| 1.498 | 5.64 D |
| 1.530 | 6.00 D |
| 1.560 | 6.34 D |
| 1.586 | 6.63 D |
| 1.600 | 6.79 D |
| 1.670 | 7.58 D |
| 1.740 | 8.38 D |
On polycarbonate a lens clock reading of 6.00 D is really 6.63 D; on CR-39 it is slightly less than it reads.
The formulas
True power = tool curve × (n − 1) ÷ 0.530
Tool curve = true power × 0.530 ÷ (n − 1)
Radius (mm) = 530 ÷ tool curve
- n
- Refractive index of the lens material.
- 0.530
- 1.530 − 1: the index that lens clocks and surfacing tools are calibrated for.
The calculator converts from tool curve to true power. The other direction is worked by hand: a −6.00 D true back curve on a 1.67 blank needs a tool of 6.00 × 0.530 ÷ 0.67 = 4.75 D, a radius of 530 ÷ 4.75 = 111.6 mm.
Checking a base curve with a lens clock
- Check that the clock reads zero on a flat surface such as a sheet of glass.
- Hold it upright at the centre of the lens surface, all three pins touching, and read the dial.
- Convert the reading for the lens material: on a 1.67 lens a reading of 6.00 is a true curve of 7.58 D.
When it matters
- Reading a lens clock on any material other than one near 1.53.
- Choosing laps for conventional surfacing of high-index blanks, where a missed conversion is off by more than a dioptre.
- Comparing base curves quoted in true power with ones quoted at 1.530.
Limits. The calculator takes tool curves from 0 to 20 D, without a sign, and converts in one direction. Its Trivex option uses 1.530. Freeform surfacing works from the material's own index, so no conversion is needed there. See the medical disclaimer.
Questions
Why 1.530?
Lens clocks and surfacing tools are calibrated to 1.530, close to the crown glass once used for spectacle lenses. At that index a 530 mm radius is exactly 1.00 D, which keeps tool arithmetic simple.