"Dave Liquorice" wrote in message
ll.com...
On 24 Sep 2006 09:58:06 GMT, Bob Eager wrote:

I believe that some prescriptions actually require glass lenses, due to
differences in refractive index and the sheer thickness required. This
may have changed.

It has.


I choose glass lenses and high refractive index glass as well. Plastic
lenses would be more than twice as thick as these glass ones at the
edges, that is they would be over a 1/4" thick.


Plastic lenses reach up to 1.74 refractive index at the moment. Glass can be
obtained in 1.9 - certainly not anywhere near twice as thin.

There are pros and cons for both. Glass lenses are trickier to scratch, but
will "pit" with impact, and will shatter if you fall on your face, and can
shatter on high velocity impact, plastic lenses will not shatter, but will
scratch readily. Loss of eyesight for the sake of new lenses every now and
then due to scratches seems to be a poor pay-off to me.

I never offer glass lenses to my patients unless they have had them last
time, or specifically ask for them. Very occasionally I'll choose a glass
photochromic lens for a patient, as they react better than the plastic ones
(although the gap is narrowing).