Optical pyrometer is a temperature measuring instrument based on the principle of spectral radiation. The object will emit light at high temperature. In the wavelength range of visible light (0.35 ~ 0.75), the thermal radiation of high temperature objects is expressed in the form of light, and the intensity of its radiation is similar to that of light. There is a certain relationship between the brightness of light. The spectral radiance of an actual object at a certain wavelength λ is proportional to the spectral radiance emittance.

An optical pyrometer consists of an optical system and an electrical system. The optical system includes objective lens, eyepiece, red filter, bulb, absorption glass, etc. Both the objective lens and the eyepiece can be moved and adjusted. Moving the objective lens can place the image of the measured object on the plane where the filament is located. The purpose of moving the eyepiece is to allow the human eye to see the image of the measured object and the filament clearly at the same time to compare the brightness of the two. The function of the red filter is to form a "monochromator" with the human eye to ensure that the spectral radiance of the two is compared at a certain wavelength (about 0.66). The measuring circuit is used to measure electrical parameters such as current, voltage drop or resistance of the filament corresponding to the brightness of the filament, and finally display the temperature indication. In the figure, the current across the filament is measured. Different types of optical pyrometers have similar structures. The function of the gray absorbing glass is to absorb part of the radiant energy and increase the range while preventing the sublimation of the filament.