A pyrometer is an instrument used for measuring high temperature – especially those which can’t be measured through ordinary thermometers. For example, pyrometers are used to measure temperature in a furnace. 

          There are two main kinds of pyrometers: the radiation pyrometer and the optical pyrometer. In a radiation pyrometer, the radiation from the hot object is focussed onto a thermopile which is a collection of thermocouples. When the thermopile gets heated due to the intercepted radiation, it produces a voltage. The amount of voltage developed depends upon the temperature. Proper calibration permits this electrical voltage to be converted into the temperature of the hot object.

          Sometimes a bolometer is used instead of a thermopile. A bolometer has two strips of the platinum metal. When the platinum strips heat up, the electrical resistance of the strips changes. The change of resistance can be used to measure the temperature. 

          Such pyrometers are sometimes called resistance pyrometers.

          The optical pyrometers use the light coming from a hot object to measure its temperature. Optical pyrometers, for example, measure the temperature of incandescent bodies by comparing them visually with a calibrated incandescent filament that can be adjusted in temperature. We know that brightness and colour of the light change with the temperature of the object. The light is compared to the light from a tungsten filament lamp. The filament is seen against a background of light from the object. The voltage through the filament is varied. The changing voltage heats or cools the filament, and makes it brighter or dimmer. At a certain voltage, the filament disappears in the background because the two colours are the same. The size of the voltage through the filament can be used to measure the temperature. Apart from the main kinds of pyrometers, two other kinds are recognized as important by physicists. They are: (a) platinum resistance thermometer (b) thermo-electric thermometer. The pyrometers are also used to measure temperature from a long distance by making use of the laws of radiation.