At 8:35 PM +0000 7/19/03, olympus-digest wrote:
>Date: Wed, 16 Jul 2003 14:25:55 -0400
>From: "Tom Scales" <tscales@xxxxxxxxxxxxxx>
>Subject: Re: [OM] Plastic Lens question
>
>I'm lost. My plastic lenses are easily 1/3 as thick as comparable glass
>lenses for the same prescription --- at about 10% the weight. What do you
>mean glass is less than 1/2 thick?
It has nothing to do with refractive index. The issue is safety: plastic can
be thin because it doesn't shatter into lots of razor-sharp pieces. Glass has
to be thick enough to be made strong enough it won't break before the owner's
head breaks. The standard test is to drop a one inch diameter steel ball
directly onto the lens from a height of two or three feet (don't recall the
exact height).
As a kid, I tested this with some glass lenses that had just been replaced. I
always got industrial-hard lenses , not just the thinner dress-hard lenses. I
put the old lens convex side up on a wooden board and dropped a 16-ounce steel
hammer on the lens from higher and higher, until the lens broke. As I recall,
it took a drop of a foot or a foot and a half.
I stopped using glass lenses because the glasses got so heavy that the nosepads
dug holes, so now all my lenses are CR-39 plastic. This is the standard
"plastic" eyeglass lens material, the one without any high-priced adjectives.
I like it because it is the most scratch-resistant and chemical-resistant of
the eyeglass lens plastics. In particular, CR39 is essentially immune to
acetone, one of my favorite solvents.
Here are some useful URLs on the characteristics of various opthalmic lens
materials:
<http://www.intercast.it/interte/desbe.html>
<http://www.precision-optical.co.uk/Resources/Jalie/Lens_Data.html>
<http://www.kapleshwar.com/lenses/plastic.asp>
Joe Gwinn
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