Attached Link: http://en.wikipedia.org/wiki/Thermography

I think this link shows how IR relates to heat, it just takes a different type of image sensor to see the lower levels of emission. Its take a pretty high temperature to generate the IR light our digital cameras see, such as the sun or my solding iron. This picture was taken with an r87 IR filter, iso 400, f2.8. 1.5 secs. The working temperature of the tip is 700 degrees F. with no visible light to my eyes in a totally dark room.  My camera gets a little noisy with these settings.