The measurement of high temperatures from the amount of radiation emitted, using a pyrometer. Modern narrow-band or spectralpyrometers use infrared-sensitive photoelectric cells behind filters that exclude visible light. In the optical pyrometer (or disappearing filament pyrometer) the image of the incandescent source is focused in the plane of a tungsten filament that is heated electrically. A variable resistor is used to adjust the current through the filament until it blends into the image of the source, when viewed through a red filter and an eyepiece. The temperature is then read from a calibrated ammeter or a calibrated dial on the variable resistor. In the total-radiation pyrometer radiation emitted by the source is focused by a concave mirror onto a blackened foil to which a thermopile is attached. From the e.m.f. produced by the thermopile the temperature of the source can be calculated.
Subjects: Chemistry — Physics.