Calculating index of a lens

**Sput** · 06-07-2010, 11:31 AM

I am curious to know if it is possible to calculate the index of a lens if you only know the power, the front curve and back curve or do I need other information. Any help would be appreciated.

**HarryChiling** · 06-07-2010, 02:32 PM

Originally Posted by Sput

I am curious to know if it is possible to calculate the index of a lens if you only know the power, the front curve and back curve or do I need other information. Any help would be appreciated.

Using the thin lens formula

$ D_1 + D_2 = Power $

Since

$ D_1 = \frac{n-1}{r_1} \\ D_2 = \frac{n-1}{r_2) $

The our thin lens formula can be written

$ \frac{n-1}{r_1} + \frac{n-1}{r_2} = Power $

and rearranged to

$ (n-1) * (\frac{1}{r_1} + \frac{1}{r_2}) = Power \\ n-1 = \frac{Power}{\frac{1}{r_1} + \frac{1}{r_2}} \\ n = \frac{Power}{\frac{1}{r_1} + \frac{1}{r_2}} + 1 $

If you had the thickness you could factor that in as well using a more accurate power formula then the lensmakers equation, but for most cases this formula will get you close enough to choosing the correct index material.

**manoj_verma** · 06-07-2010, 10:58 PM

ofcourse you can !
as Harry also mentioned ,using the Thin Lens Theory you can calculate the Index Of the Material given !
OR ,Take it in easy way .............

just findout the Back Vertex Power of the Lens with the help of Focimeter & ADD the surface powers together to find the THIN LENS POWER.
NOW ,The Ratio of the thin lens power to the true back vertex power is the same value as the CVF ( CURVE VARIATION FACTOR ) of the Material of the LENS.

IN ABSENCE OF CVF TABLE ,you can go this way.........

use this formula : nT=1+Ft(nLM-1)/Flm

where Ft = true back vertex power of the lens
Flm = sum of the both surface powers ( Thin Lens Power )
nLM = refractive index for which the lens measure has been scaled
nT = REFRACTIVE INDEX OF THE MATERIAL OF THE LENS .

hope you find it easy n usefull !

cheers !

**Darryl Meister** · 06-07-2010, 11:45 PM

Using the curve variation factor will save you the trouble of calculating the individual radii as well. You're essentially eliminating some intermediary steps. This all simplifies to:

$n=0.53\times\frac{F_V}{F_C}+1$

where n is the refractive index, F_V is the back vertex power of the lens as measured with a focimeter and F_C is the power of the lens as measured with a lens clock by adding the front and back surface measurements together. This assumes that the lens clock is calibrated for the standard tooling index of 1.53.

For instance, assume you measure the front and back surfaces of the lens with a lens clock or lens measure, and the total power is -4.75 D. You then measure the lens in a focimeter, and the actual back vertex power is -6.00 D. The refractive index n is approximately:

$n=0.53\times\frac{-6.00}{-4.75}+1$

$n=1.67$

Of course, if the lens has significant center thickness, your results will lose some accuracy. Your lens clock must be properly calibrated as well.