The filter is basically a 1" diameter piece of clear looking glass that the light enters the diffusion box through. Originally I thought it was just meant to shield the interior of the box from the heat of the halogen bulb, but then I noticed the glass had a faint opalescence to it ...and then a note on the diffusion box itself that said it was equipped with a "infra-red filter." Presumably it's meant to filter out infra-red rather than meant to mimic the effects of infra-red film.

I will indeed do the search you suggest.