As requested Haya, I worked up an example for you. One note though... in looking over the last lines (INDEX=), I discovered that the returned value was expressed as just the value to the right of the decimal. So I modified that line to convert to a index that comes out the way you expect it to look like. Works either way, just understand the reading. So here 'tis.
Here is what you know going in;
1) Rx
2) Center Thick
3) Lens Analyzer Readout
1) –2.00 –1.00 x 075
2) 1.7 mm
3) –2.037 –1.027 x 075 (unrounded best)
For this formula cylinder is disregarded, so;
Using a good digital sag gauge (mine is a 40mm ball tip), we measure for TBC and REAR SPHERE (base) values
TBC = 4.26 D (1.631 sag)
REAR = -6.12 D (2.386 sag)
Moving into the formula then….
Frd = ABS(530/4.26) Frd = 124.41315
Rrd = ABS(530/-6.12) Rrd = 86.530612
A1 = 124.41315 – 86.530612 – 1.7 A1 = 36.182533
B1 = 1000 * (124.41315 – 86.530612) + (86.530612 * -2.037) * (124.41315 – 1.7) B1 = 16252.764
C1 = 1000 * 86.530612 * 124.41315 * -2.037 C1 = -21929416.5
Since Power < 0 (minus), then
INDEX = 1+ (-16252.764 + SQRT(16252.764^2 – 4 * (36.182533 * -21929416.5))) / (36.182533 * 2) INDEX = 1.585665109 (or 585.665109 using the original INDEX= from above posting)
Rounded to 3 places past decimal = 1.586. Hence we have a poly lens
Hope this helps. I have now created a small Excel sheet, so my brain can go back to sleep (normal state).
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