A pyrometer is a device used for measuring high temperatures. Platinum resistance thermometers and pyrometers operating on the principle of the thermocouple have the disadvantage that they must be in contact with the hot body, but optical and radiation pyrometers can be used at a distance. Optical pyrometers estimate temperature from the light intensity in a narrow band of the visible spectrum by optical comparison of a glowing filament with an image of the hot body. Radiation pyrometers focus the body's heat radiation on a responsive thermal element such as a thermocouple.
An optical pyrometer (see illustration) allows very high temperatures to be measured at a distance from the temperature source [A]. It exploits the fact that two solids at the same high temperature radiate light with the same spectrum and show the same color. To measure the heat of a furnace [B], an electrically heated filament  fixed in the tube of a telescope with a special lens  is heated until it glows with the same color as that emitted by the furnace . Comparison with the background image shows if the furnace is hotter [C], as hot [D] or colder [E] than the wire. A meter  indicates the current passing through the wire and this can be calibrated directly in temperature degrees.