Re: [ATM] Alternate Materials

[Dominic-Luc Webb <dlwebb@canit.se>]

On Thu, 26 Jul 2007, Richard F.L.R. Snashall wrote:

> Are there any ATMs actually working in plastic?  Is the processing
> the same?  How about, as in this case, aluminizing it?


For some simpler things, yes, I have tried plastic. You can get
eyeglass stock quite cheap. Someone else should explain proper
way to grind and polish plastic. It is clearly possible to
aluminize plastic, and I recently encountered literature where
aluminzed plastic was used. There are several methods for
aluminizing a surface other than a vacuum chamber. I am pretty
sure the plastic I recently encountered was aluminized by one of
the other methods, perhaps sputtering. I believe the plastic
used for eye glasses is usually some form of polycarbonate (CR-39)
and the refractive index (n) is quite similar to Corning Pyrex,
although, the values for n I have seen (~1.50) were not precise and did not
specify the wavelength. Since this is meant for eyeglasses, I
would assume n is given for a visually useful wavelength
around 500 nm.

Along similar lines, high end laboratory photometric equipment
is more and more frequently built with plastic filters, mirrors,
and polarizers. I discovered this during some recent equipment
purchases when we got demo machines and I needed to operate in
the UV. In some cases, the measurements I wanted to make were
not possible due to the use of these plastics (i.e., strong UV
absorbance). Custom filters usually solved these problems (light
sources and detectors easily handled UV), but for this discussion,
it is noteworthy how extensively plastics have been able to
replace precision optical glass in high end lab equipment. The
use of plastic optics does not stop with the extensive
replacement of glass eyeglass optics these days.

How about more complex optics? Here too plastics are being used.
There are medical imaging devices that use plastic Schmidt
correctors. I investigated the specific case I know about.
This was molded in a metal form. The mirror image of the Schmidt
curve was generated on the metal surfaces.

Getting into plastics will mainly require learning how to work
plastic. In some cases there will be grinding and polishing.
In other cases there will be forms. Aluminizing is possible,
but one should explore how this is best done on plastic.

If you need a lens that intrinsically filters well outside the
visible, this is a good choice. In fact, you can apply dyes to
enhance filtering, kind of like making a Wratten filters (surface
dyeing is also possible). With that, I leave you with a warning.
Plastics like CR-39 that are most used in optics are rarely a good
choice for photometry due to strong UV and IR absorbance.

Dominic-Luc Webb

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