How Do Infrared Thermometers Work?

Infrared thermometers measure temperature from a distance. This distance can be many miles or a fraction of an inch. Infrared thermometers are often used in circumstances when other sorts of thermometers are not practical. If an object is very fragile or dangerous to be near, for example, an infrared thermometer is a good way to get a temperature from a safe distance.

Infrared thermometers work based on a phenomenon called black body radiation. Anything at a temperature above absolute zero has molecules inside of it moving around. The higher the temperature, the faster the molecules move. As they move, the molecules emit infrared radiation–a type of electromagnetic radiation below the visible spectrum of light. As they get hotter, they emit more infrared, and even start to emit visible light. That is why heated metal can glow red or even white. Infrared thermometers detect and measure this radiation.

Infrared light works like visible light–it can be focused, reflected or absorbed. Infrared thermometers usually use a lens to focus infrared light from one object onto a detector called a thermopile. The thermopile absorbs the infrared radiation and turns it into heat. The more infrared energy, the hotter the thermopile gets. This heat is turned into electricity. The electricity is sent to a detector, which uses it to determine the temperature of whatever the thermometer is pointed at. The more electricity, the hotter the object is.

Ear thermometers are infrared thermometers. The ear drum has about the same temperature as the inside of the body, but it is very sensitive. Touching the ear drum could damage it, so an infrared thermometer measures its temperature from close by–less than an inch away. Infrared thermometers are also used by fire fighters to detect "hot spots" where the fire is burning fiercely. They are even used in manufacturing. Infrared thermometers can help control the machines that put together delicate, temperature sensitive products like electronics to make sure the components are not accidentally damaged.