explaining the varied index of lenses

**beth** · 07-11-2002, 05:21 AM

I have new staff and am trying to easily explain the different indexes of lenses.

We give them the definition of index of refraction/center thickness etc.. However sometimes this needs a more simplified answer(at the beginning)

?'s asked-why is transition a mid-index, but we sell it as regular plastic etc..,

How would you do it?

**chip anderson** · 07-11-2002, 06:40 AM

Ask your lab for a demo. The come with a plate of -2.00, -4.00,
&-6.00's mounted side by side compareing 1.66 & CR 39. Saves most of the explaination and makes a lot of sales to high myopes and presbyopes.

Chip

**Texas Ranger** · 07-11-2002, 09:46 AM

beth, index of refraction is the ratio of speed of light in air compared to that of the transparent material involved, i.e. glass, poly, cr-39 resin, etc. mathematical formulas compensate the differences in surfacing back curves to arrive at the desired rx's. generally, the higher the index, the flatter the curves, thereby arriving at thinner lenses; of course there are trade-offs. and you can't forget about cut in thickness, which may be more important to understand than index. and there is over-kill. we see folks in that have a -.75 sph that someone "sold" them on high index lenses. or moderate plus powers being "sold" high index, them letting them get a frame with an e.d. 5mm higher, they think they're getting a thinner lens, duh!

so · 07-11-2002, 04:30 PM

For a simple explanation to a new employee: The rx is determined by the difference between the front curve of the lens and the back curve. Some materials bend light better than others. Higher index means that the difference between front and back doesn't have to be so much, and the lens will appear thinner in profile.

**eyesguy** · 07-12-2002, 10:43 AM

Hi Beth:

This might seem crazy, but you could try this. Make or buy two plus lenses -- one with the lowest index you can find and one with the highest. Except for index, they should be identical in every way, i.e., same radius on front and back, same thickness, etc. Then take one of those cheap laser pointers and shine it through the same spot on each lens and watch where the red dot hits. You will see that the spot hits closer to the backside of the higher index lens by several inches. This indicate that the higher index lens is stronger even though its geometry is identical. A picture (or demo) is worth a thousand words!

Regards,

eyesguy