Since the high temperature pyrometer does not need to be in contact with the object to be measured, it can be used in extremely high temperature occasions, sports heat sources, and the temperature measurement of heat sources that cannot be touched or are harmful to the human body and the instrument. The most commonly used optical High temperature pyrometer is the Hidden Filament Optical High temperature pyrometer.

There are two measurement methods: one is to adjust the resistance R to change the brightness of the filament, when it is equal to the brightness of the image of the light source to be measured, the filament disappears on the image of the light source, and the brightness temperature of the object is read from the meter G; or The precise value of the current is measured by the potentiometer with the compensation method, and then the brightness temperature is obtained by calculation. The latter method is suitable for precise temperature measurement.
The second is to keep the brightness of the filament at a constant value, and rotate a piece of absorbing glass whose thickness changes with the angle. When the brightness of the object image is the same as the brightness of the filament, the brightness and temperature value of the object can be read from the corner of the absorbing glass.