Has anyone seen this idea before (or tried to build one)?

http://publiclaboratory.org/tool/thermal-photography

Basic idea is: non-contact IR thermometer, linked to a colour-changing LED which changes colour according to what the IR thermometer is reading at that moment. The LED shines in the direction of whatever you're pointing the thermometer at, so if you point it at something hot, it shines a red light onto that surface, etc.

Then you set a camera up on a long exposure, and you effectively "colour in" the walls or whatever you're looking at by waving the contraption around.

It obviously has its limitations but could be fun to build one! The idea is that it shouldn't cost more than $50 in components.

Some images here:

http://www.flickr.com/groups/thermal-flashlight/pool/with/6502706171/#photo_6502706171

On another thread I posted a link to a site where they were using a low-resolution thermal IR web-cam to overlay the image on a smart phone or tablet camera with the thermal image.

What struck me was that they were not, as far as I could see from the their video, “persisting” the image in any way. IIRC, it was a 64 pixel image which looked like it was arranged as 4x16. It would be better use of the pixels to have a single row of 64 and sweep it up and down across the area using the device's accelerometers to paint an IR image of the area.

Alternatively, you could use a one-pixel (i.e., spot) IR thermometer strapped to the side of a tablet and wave it around to paint an IR overlay over the camera images. Not quite as immediately DIY-easy as the long exposure camera version but quite doable I'd have thought.