Any object at high temperature will project a certain wavelength of electromagnetic waves (radiant energy). The visible light with a wavelength of 0.65μm electromagnetic wave is most sensitive to human eyes, and the radiant energy of this wavelength changes significantly with temperature. The magnitude of visible light energy is reflected in its perception of the brightness of the human eye. The higher the temperature of the object, the stronger the visible light energy it emits, that is, the brighter the visible light seen.

The method of measuring temperature with an optical pyrometer is to compare the brightness of the measured object at a wavelength of 0.65 μm with the brightness of the filament of the pyrometer powered bulb installed inside the meter. When the brightness of the filament of the meter is the same as the brightness emitted by the object to be measured, it means that The filament temperature is the same as the temperature of the object being measured. Because the brightness of the filament is determined by the current passing through the filament, and each current intensity corresponds to a certain filament temperature, the temperature of the filament can be known by measuring the current, that is, the temperature of the object to be measured. Therefore, this pyrometer is also called a hidden wire optical pyrometer.

However, the optical pyrometer uses human eyes to detect the brightness deviation after all, and also manually adjusts the brightness of the standard lamp to eliminate the deviation to achieve a balanced state of the two (the filament image disappears). Obviously, only when the measured object is high temperature, that is, when the red light band (λ=0.65μm or so) in the radiation light has sufficient intensity, can the optical pyrometer work. When the measured object is medium or low temperature, because the red light band in its radiation spectrum is very small, this instrument is powerless. Therefore, the lower limit of the optical pyrometer is generally above 700 °C. Furthermore, due to manual operation, the response cannot be fast and continuous, and it cannot form an automatic adjustment system together with the measured object, so the optical pyrometer cannot meet the requirements of modern automatic control systems.