Thermal Camera

Three categories in infrared light:

• near-infrared (near-IR)
• mid-infrared (mid-IR)
• thermal-infrared (thermal-IR)

The key difference between thermal-IR and the other two is that thermal IR is emitted by an object instead of reflected it. 

The ability to emit energy ranges 0-1, depending on the properties of materials. The blacker material has the value close to 1. The reflective material has a lower value. For instance, polished silver is 0.02, iron has 0.14 if polished, and 0.61 if rested. Regular glass, which effectively blocks thermal radiation, has 0.92.

The thermal radiation also depends on its temperature, the hotter thing emits more thermal radiation.