When measuring temperature with an optical pyrometer, the image seen by the human eye through the eyepiece is shown in the figure. There is a filament on the background of the object to be measured. If you see a bright filament on a dark background, it means that the brightness of the filament is higher than the brightness of the object to be measured, and the filament current should be adjusted to reduce the brightness; if the background is bright, the filament is bright; if it is black, the brightness of the filament is lower than that of the object to be measured The brightness of the filament should be adjusted to increase the brightness. When the filament is dimmed but invisible (i.e. the top of the filament is indistinguishable from the object), the two are equal in brightness and the measurement can be read. In view of this principle, optical pyrometers are also often referred to as filament extinction optical pyrometers.

Optical pyrometers are not suitable for measuring objects with reflected light intensity, and the measurement accuracy is lower than that of thermocouples and thermal resistances. The judgment and adjustment of brightness comparison must be performed manually, and continuous automatic measurement cannot be performed.
Non-bold effect. Since the measured object is a non-absolute black body, the blackness coefficient ελ of the object is not constant, it is related to the wavelength λ, the surface condition of the object and the temperature. Shading coefficients sometimes vary widely, which can adversely affect measurements. Sometimes in order to eliminate the influence of ελ, the conditions of black body radiation can be artificially created.