From the YouTube site (or should that be sight?) of this video.
July 28, 2009 — CORRECTION: The description originally stated that exposed film was to be used, but you actually have to use blank, developed film. A chemical used in the development process is what gives the film it's visible light filtering capabilities.In other words - if you can practically see through the garment already this method has possibilities. And even if it doesn't, it might be fun to find an accomplice who would be willing to help you with the experiments. To insure the correct experimental procedure is followed in every experiment it is a requirement that the experimenter dress and undress the model for each iteration of the experiment. And to properly document the experiment videos and photos should be taken at each step. Then you have to write it up. Finally no experiment is really complete until the results are published and replication has confirmed the results. Oh. Yeah. Take measurements.
First off my apologies for the very shoddy camera work and even shoddier narration.
Secondly, this is merely my own personal findings after conducting several very unscientific tests using esotericsean's method of turning an ordinary camcorder into one that only lets in infrared light, allowing the operator to 'see through' various objects including clothing. The tests speak for themselves and yes, by using this method you can actually see what people are wearing under their clothing, provided that the clothing is relatively clingy, and is a thin fabric. Dark clothing works the best, but I've also gotten great results with all colors, provided that the fabric is thin and clingy.
You can buy a Hoya 58mm RM-72 Infrared Filter from Amazon but making your own or just getting a bit of developed blank film from a film processor seems cheaper.
Some books on Infrared Photography to help you on your way.
Cross Posted at Classical Values