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 that of the object to be measured, and the filament current should be adjusted to reduce the brightness; if the background is bright and the filament is bright If it is black, the brightness of the filament is lower than the brightness of the object to be measured, and the brightness of the filament should be adjusted to increase. When the filament is dimmed but cannot be seen (that is, the top of the filament is indistinguishable from the object), the brightness of the two is equal, and the measurement result 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 strong reflected light, the measurement accuracy is lower than that of thermocouples and thermal resistances, the judgment and adjustment of brightness comparisons must be performed manually, and they cannot be continuously and automatically measured.