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 radiant emittance.

The optical pyrometer consists of two parts, 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.