Light energy travels in waves. On one end, there are long waves like radio waves, which are low energy. On the other, there are short waves like gamma rays, which have high energy. Right in the middle is visible light and then next to it, with slightly longer waves, is infrared light. 

The infrared spectrum, which is invisible to the naked eye, splits into three categories: near-infrared, mid-infrared, and thermal-infrared. The first two refer to reflected light while thermal infrared refers to the light emitted by an object, i.e. heat. 

ATN thermal scop https://www.atncorp.com/thermal-scope-thor-hd is equipped with detectors that focus on thermal infrared light and then the device creates a thermogram, so you see the object's form but as it relates to heat.