Thermal cameras can detect and record the heat that radiates (known as infrared radiation, IR) from objects, which is an object’s heat signature. The heat signature that emanates from an object is produced by the speed of atoms moving around in it, so the faster the atom vibration within a given object the hotter the object is. IR does not fall within the visible light spectrum for human eyesight so we cannot see with the naked eye. These cameras capture wavelengths in the light spectrum ranging from ~1µm – 14µm.

Reference: Thermographic Camera – Wikipedia