Chris Eddy wrote:
>
> Just one more comment on the issue of light sensitivity.  I had heard of a
> method of using exposed 35mm film, and then ran across an entry in Electronic
> Design December 2 issue on the method.  Dave Johnson from Colorado writes
> that you can expose a roll of 35mm film (preferably Kodacolor 100 ASA) to
> fluorescent light for five seconds, and ask the developer to run the roll
> without making prints.  The graph of response shown in the article shows high
> transmisson up to about 700nM, then a sharp rise in blocking that peaks at
> 880 and is still high well into 1100.  He adds a caveat (sp?).  The film
> filter cannot be used near moisture or where it can be scratched.  I might
> add the danger of heat as well, though it would have to be mighy hot.  But
> for the lab, you could fabricate quite a nice cocoon for your chip.  If the
> film is not conductive, you might even be able to roll the whole chip.  (with
> leads protruding through..)

The above recommendation really surprises me.  To my knowledge,
absorption filters for near IR are really hard to produce by any
method.  There may be some confusion here.  An easy way to obtain a
visible blocking absorption filter is to use either of the following:

1. UNEXPOSED but developed transparency film such as Kodachrome.
2. EXPOSED but developed negative film such as Kodacolor.

These materials will strongly absorb visible light but transmit near IR
very well.  The rise near 700nm is in TRANSMISSIVITY, not 'blocking'.

If you need something more rugged than camera film, visible black IR
transmissive plastic is readily available in both resin and sheet
forms.  Most absorptive color dyes are IR transparent, so the general
recipe is to combine red, green, and blue dyes (or cyan, yellow, and
magenta if you prefer).  Each dye absorbs a different portion of the
visible spectrum and leaves the IR to pass through.  Of course, that's
what you get with camera film, too.  If anyone needs more details about
where to get materials, they can contact me.

.......<snip>

--

Paul Mathews, consulting engineer
AEngineering Co.
optoeng@whidbey.com
non-contact sensing and optoelectronics specialists