IR Thermography, by James Little
IR Thermography is a lab that makes strong men cry and delicate ladies blush. The physical principle of this lab is that all objects at a finite temperature emit electromagnetic radiation in a wavelength range that is determined by their temperature. For most objects within the temperature ranges found on Earth, this means infrared radiation.
We use a camera that is sensitive to infrared radiation to see this emission. The camera uses solid state detectors, cooled using liquid nitrogen, to detect emitted power in the 8-12 micrometer wavelength range.
Now, it turns out that the emitted power also depends on how transparent or opaque the objects are. All completely opaque objects emit in the same way, and this radiation is called blackbody radiation. Based on the number of photons we detect from something, we can determine its opaqueness (emissivity), so long as we know the temperature. So what we really do in this lab is measure emitted power and determine temperatures and/or emissivities.
Below is an image contributed by Cathy Nicole that shows a normal field of view in the infrared camera. Towards the left you see a black can filled with cold water; to the right is a black can filled with hot water. The "hot" object is emitting much more power in the infrared. The picture below that is a measure of the amount of infrared radiation at every point corresponding to the line that you can see placed across the screen.