Thermal imaging cameras are technically not cameras, but rather sensors that detect heat (also called thermal energy or infrared). Technically, these devices are detecting radiation. The amount of this radiation goes up with the temperature.
With enough precision a thermal imager can see minute differences in heat and represent this as an image (or thermogram) on a screen. The temperature differences detectable on some of the world’s most sophisticated devices can be as small as 0.01°C. Various colors are used to represent temperatures so when you see a black and white thermal image the lighter the color, the hotter the object (newer thermal imagers can invert this or use a wide variety of colors). Human beings, animals and cars generate heat and are usually warmer than their surroundings, allowing the user of a thermal imager to get a good look at them.
Military and Police
One of the most common users of a thermal imager is a law enforcement or military professional. They need to be able to see potential threats without being detected and thermal units give them this chance. The drawback of using thermal imagers in life-and-death situations is that while they’re incredibly effective at detecting people or animals, identification is far more challenging. You may see a man in front of you, but this doesn’t mean you’ll be able to tell if he’s a friend or a foe.
Thermal imaging cameras are one of the most effective tools for surveillance because they work equally well in the day and night. A regular CCTV camera is limited by its need for light, and night vision doesn’t function during the day. The chance to see through smoke and fog also gives thermal a leg up on other surveillance techniques.