A colleague showed up with a thermal camera that attaches to an iOS or Android device. Cost: about 250 USD@2016. His show revived me the interest in trying to remove the IR filter from a (web)camera and see what happens.
I got hold of an old (2004) Creative webcam that works only on WinXP. It is easy to open them up. Not so easy to remove the IR filter: in my attempt I broke the IR filter glass and removed its pieces. The Net is full of tutorials and videos on how to do it, so do your homework and take your time (and wear protective glasses the least!).
Now the camera captures both visible and infrared light. Neat, but not so useful, because you keep getting what looks like over-exposed images with wrong colors.
How to filter the visible spectrum of light? One suggestion online is to use a piece of floppy disk (the magnetic disk inside the plastic, of course). Tried it. Not sure it filters out visible light or it acts as an ND filter (see ND filters in photography). Fail.
A second test was to use the modified camera at night, without any filter. It might work on passing cars, but its frame rate is so slow that cars are just a glob of light caused by long exposure. Fail.
I haven't tried to cover the lens with an old photographic film, the fully exposed frame which is dark. Maybe something will come up.
In any case I think the webcam sensitivity is too low to detect temperature variations at "human" levels, like heat/cold areas of a laptop or an electronic circuit, which would be my main usage at the present time.