P3D Re: Flourite lenses

[Tom Hubin <thubin@xxxxxxxxx>]

Rod Sage wrote:
> 
>       "The color aberration curve of a doublet can be made considerably flatter
>          by using special glasses or fluorite.
> 
> I believe most hard contact lenses are made from fluorite. I always
> assumed the fluorite made them semi-permiable but maybe it was because
> the fluorite gave the lenses a higher refraction index so that thinner
> lenses could be used, or  is it to correct color aberration.
> RDS

Hello Rod,

Hard contact lenses are made of PolyMethylMelacrolate or something like
that. At least that is the material that was used in the 1960s and
1970s. These were not gas permeable. I got out of the ophthalmic
dispensing business in the early 1980s when soft contacts and gas
permeables were just becoming popular. So I am much less familiar with
gas permeable contact lenses. Perhaps this is where flourite is used.

Color aberration can be corrected for 2 wavelengths if two different
materials are used to make a cemented or air spaced doublet. Color
aberation cannot be corrected with any one material unless that material
has an index of refraction which does not vary with wavelength. In other
words, no dispersion. Such materials might exist but they would be very
special.

The index of refraction for most plastics used for common ophthalmic
optics is about 1.50. Cr39, used for plastic eyeglasses, is 1.498. Crown
glass used for eyeglasses is 1.523. 

Unless flourite has an index well over 1.60 it is unlikely that flourite
lenses are much thinner than other contact lenses. The thickness is
likely to be governed by a minimum thickness that is easy to manufacture
and not too fragile. 

Tom Hubin
thubin@xxxxxxxxx


------------------------------