>>> exposed film as IR filter

>> B&W or color?

> I took for granted what the other poster said about just using
> exposed film.. I'd guess color as color film has separate layers
> w/ dyes in it that are reactive to different spectrums of light
> (cyan,magenta,yellow)

I believe you want exposed & developed color negative film
or unexposed & developer color reversal film (i.e. slide film).
Color negative film will have an orange tint.  Slide film will
have a black look.

Checking a Kodak data book on color films, the cyan layer's
spectral response curve falls off at 700nm.  IR at 800nm or
so should pass quite well.

Black & white isn't as easy a question as expected.  There's
the old style films, like Kodak Tri-X, based on silver granules.
Those, when exposed and developed, should have a fairly flat
spectral response curve.  I would expect they would attenuate
IR wavelengths as well as visible light.

There are also black & white emulsions based on color film
technology, such as Kodak's T-Max.  I have no idea how they
would respond as an IR filter.

                                                Lee Jones