Re: polycarb lens problem?


Boxman wrote:

This application is less stringent but is there a practical way of
annealing out the internal stresses caused during the moulding process?

You can run the lens through an annealing oven to remove a lot of the
stresses, but you should expect some shape changes (ie warping) of the
lens when doing this. Polycarbonate faceted lenses are used in plastic
sealed beam headlamps, which is a highly demanding non-imaging

<snip>

Another process that may be available to you is injection compression
molding. In that case, the plastic is shot into the mold with the mold
slightly open from it's final shape and then compressed the rest of the
way to form the final shape. This operation produces very low stress
parts, however it is a more specialized process and thus not as
available and will probably be more expensive. Tooling is more
complicated as well.

If you can share, I am curious to know what exactly what you mean by a
dispersion problem. Are you referring to color fringing or were you
getting incorrect angular/spatial distributions from the plastic part
vs. the glass part. In my experience doing non-imaging optics in
plastic, the molded part stress hasn't been a major optical issue even
if it was very high. For imaging optics however, it is a big concern.

Thanks very much for the response.

The chromatic dispersion is not the problem in this case but rather,
when measured (using a purpose made point source) there is a
relatively large amount of random scattering

i.e. although the majority of the light is going where expected
considering the physical shape, surfaces, and mu of the lens material
etc but a larger than expected (10~15% ) proportion is going all over
the place.
This appears to be related to the internal stress levels. (This is a
qualitative rather than quantitative assement at this time but the
lenses with the most stress levels seem to show a greater amount of
light going 'elsewhere')
The polycarb is up to 9mm thich (max) down to 4mm (don't ask)

This is not a problem that must be solved with some desperation but
more of -- well why not make a better job of it if all we have to do is
XXXXX

Thanks again

Regards

.