From: "Fisher, Nick" 
To: "'atm{at}shore.net'" 
Reply-To: "Fisher, Nick" 


I looked into how plastic lenses are made a while ago and this is what I
found: plastic spectacle lenses can be made from perspex like resins or
from clear polycarbonate. The resin lenses can be cast in glass molds which
are kept at a constant temperature in a water bath while the resin does
it's chemical reaction thing. I didn't find out how polycarbonate lenses
are made but it might be some sort of injection molding. The problem is
stopping the lens from deforming slightly as it sets/cools. Manufacturers
use special surfacing machines that polish the surface of the plastic with
pads and polishing compounds but I don't have any details, they can also be
coated to reduce reflection and resist scratches. Perhaps someone on the
list works in this line ?

As for optical properties perspex and similar plastics have optical
properties very close to window glass, index of refraction about 1.5 etc.
polycarbonate has a higher index of refraction and a higher dispersion as
does styrene so either of these could be used to make a compound lens with
perspex if you calculate the curves properly and in fact I think some cheap
camera lenses are made entirely out of plastic and some more expensive ones
use special cast plastic elements to get aspheric surfaces. Some of the
problems that may occur are different co-efficients of expansion warping
the lens, scratches, slow changes in the plastic as it repeatedly expands
and contracts (perspex will also absorb water) and reflections off surfaces
unless the lenses are coated. I think the optical design program Zemax has
data for plastics in it's database of materials.

Nick


______________________________________________________________________

IMPORTANT: This e-mail, including any attachments, may contain personal,
confidential or privileged information. If you are not the intended
recipient, or if you have received this e-mail in error, please contact the
sender immediately and delete all copies of this e-mail and attachments. If
you are not the intended recipient, you must not use, reproduce,
disseminate or disclose any part of this e-mail or attachments.
______________________________________________________________________

--- BBBS/NT v4.01 Flag-4