Infrared cameras can see what human eyes cannot. Such cams create an image based on infrared radiation versus light. Infrared technology spots radiation created by heat. And heat is created when atoms, molecules which are in motion. Everything around us is made of atoms. The more they move around, the more heat is created. That’s why living creatures radiate more heat that can be easily spotted with infrared camera.
In order to make IR camera, you need a specific lens that can turn heat into an electronic signal. This signal produces thermal image on a monitor. These images show heat levels. The warmer area is, the brighter it will be shown on screen. Cold areas are dark. It makes it very easy to spot a human in darkness with such devices like infrared cameras.