Pyrometer, an instrument for measuring temperature. Although the term pyrometer is generally considered to apply to instruments that measure high temperatures only, some pyrometers are designed to measure low temperatures. Two common types of pyrometers are the optical pyrometer and the radiation pyrometer.

A heated object gives off electromagnetic radiation. If the object is sufficiently hot, it will give off visible light, ranging from dull red to blue-white. Even if the object is not hot enough to glow, however, it gives off infrared radiation.

An optical pyrometer determines the temperature of a very hot object by the color of the visible light it gives off. The color of the light can be determined by comparing it with the color of an electrically heated metal wire. In one type of pyrometer, the temperature of the wire is varied by varying the strength of the current until the operator of the instrument determines that the color of the wire matches the color of the object. A dial, operated by the current that heats the wire, indicates the temperature.

A radiation pyrometer determines the temperature of an object from the radiation (infrared and, if present, visible light) given off by the object. The radiation is directed at a heat-sensitive element such as a thermocouple, a device that produces an electric current when part of it is heated. The hotter the object, the more current is generated by the thermocouple. The current operates a dial that indicates temperature.