It needs to be almost glowing. A soldering iron will emit infrared
radiation in the region where it can be detected by IR film. You may
wish to reference "Rethinking Infrared Photography - Dispelling Myths"
on my Web page.
> I have a question too about infrared: Is it possible to find very fine
> grain 35mm infrared films ? I guess not. I assume the crystals have to be
> very large in order to trap the maximum number of IR photons ...?
Certainly. You can use Konica Infrared film. This is a very slow film
(I use it EI 8 when metered through a #25 filter). The size of the
crystals has more to do with the sensitivity of the film, as opposed to
the portion of the spectrum it is sensitized to. You may wish to
reference "The Components of Photographic Materials" on my Web page.
-- Handmade Photographic Images http://www2.ari.net/glsmyth/