Simply put, the FLIR detects heat and converts it into an electronic signal, which is then shown as a coloured thermal image on the built in viewer. The colours range from black, representing the coldest temperature to white representing the hottest. The mid-colours are blue and purple for slightly hotter areas, then red, orange and yellow. This of course depends on which pallet you are using, as some models come with several pallets. The example I gave is from the Iron Pallet, the most commonly used among Paranormal Investigators.

So basically, you’re looking for cold and hot spots/images where they shouldn’t be any. But be careful when using the FLIR as its easy to mistake something natural as paranormal. I’ll give you an example, in fact I purposely did this one myself. Press your hand on a wall for a second or two, then remove it. Now point the FLIR in the direction of where you just had your hand, and you’ll see a ghostly hand print there for several minutes. You can also do this just by leaning against a wall or door. Also be aware of reflective surfaces as you’ll catch your own, or another investigator’s image, which will look pretty convincing on the viewer. Rule of thumb, as with every other type of evidence you get, try and debunk it before reaching a conclusion.