Thermal imaging cameras are cameras that see heat instead of light, so they work in complete darkness. They measure the infrared radiation that every warm object gives off. A hot object looks bright in the picture and a cool one looks dark.
Inside the camera, a grid of tiny detectors each measures the heat from one small point of the scene. The camera turns each reading into a brightness or a colour, and the grid of readings becomes the picture. A thermal camera cannot see through glass, because glass blocks this kind of radiation. Its pictures also have fewer pixels than an ordinary camera's.
A thermal camera is used wherever the heat itself is the information. On aircraft and drones, it lets the crew see at night. At borders, it shows people and vehicles in the dark. In defence, it is a sensor that goes along with defence electronics. In industry, it finds faults before they cause trouble.
Say an electrician in Surat checks a factory's electrical panel with a hand-held thermal camera. Most of the joints in the picture show about 40 °C. One joint shows 95 °C. A loose connection is making it hot. He tightens it before it damages the panel. He did not need to switch off the power to find the fault. Panels are often built with electrical enclosures that have an infrared window, so that he can see the inside without opening the door.
The camera's pictures are also read by software. Computer vision for drones and video analytics find people and vehicles in the heat image, so that nobody has to watch the screen all day. Robotic inspection carries the camera along pipes, dams and plants. A counter-drone system can also use it to find a hostile drone at night.
A thermal camera is different from a night-vision camera. Night vision amplifies the small amount of light that is left. A thermal camera needs no light at all.