How we gonna to make sure the lens index?

[cnet_baby]

hi can anyone give me a suggestion,how we know about the lens index when the consumer bring them along to us,it just a single vision, can we measure it,how?

[optixbil]

Neutralize the power with your lensometer. Measure the front curve with a lens clock. Measure the rear curves with a lens clock. If the relationship is one to one, it's 1.52 index. Plastic takes a curve of 1.06 times the power etc. Since lens clocks are not that accurate, you should familiarize yourself with glass, poly, and regular plastic. If you're replacing one lens and it's critical to get the exact index, either you call or have the patient call the place the eyeglasses were purchased and ask what they bought. Other than that, sell them what you think is best in a high index lens.

Hope this helps

Bill Belanger

[optigrrl]

Another trick I learned was to look at the surface of the lens with light reflecting on it - poly and hi-indecises will have a rainbow pattern on them. Even with A/R.


They also have different sounds when you drop them lightly on the counters. Do this out of sight...Poly sounds "tinny" while higher index lenses sound "thuddy".

I had my lab send me some breakage lenses in all different materials. I keep them on hand in case I have to do the sound test - I can compare the patient's lens with the sample ones.

I also neutralize them to give me and idea of what the thickness would be in plastic and if it doesn't look thick.

[Snitgirl]

Hi there, you can probably find the formula for determining what the index of refraction is for a lens is on this site.

http://onlineopticianry.com/wordpress/?cat=2


Here is the one that I've been taught: If this is not correct, I am sure someone here will let me know... :D

F = R(n - 1)/(C-1)

Calibration constants:
-----------------------
Plastic Lens Clock C = 1.50
Glass lens Clock: C = 1.53

F = the highest meridian power from lensometer
R = the highest meridian power from the lens clock
C = calibration constant
n = index of refraction

[tmorse]

Snitgirl: You probably meant to place the 'c-1' directly under the 'n-1' in your equation.
Also drop the plastic 1.50 and use c=1.53 as calibration constant for all lens clocks, regardless of lens material. :cheers:

[Snitgirl]

Hi Ted,

Yes, that is what I wanted it to look like and typed it that way but when I hit submit reply, it did not appear that way... :(

Thank you for letting me know to not use the plastic lens clock 1.50, I will make a note in the text book that you have given us.