Optical Pyrometer Detect The Spectrum Composition

This pyrometer is used to measure an incandescent body. The principle for this measurement is that the hotter the body, the spectrum of the radiation will contain more shorter wavelength (visible light). This circuit use infrared filter to separate the visible and infrared spectrum, since the sensor (photodiode) is sensitive for both visible and infrared spectrum. Here is the schematic diagram of the circuit:

The current from the photodiode is converted by the transistor into a logarithmic before subtraction, this make the output proportional to the log of infrared and visible spectrum ratio. This logarithmic operation make it sensitive only to the spectrum composition of the radiation, not to the intensity of the radiation. Without this logarithmic conversion, the distant of the measurement would be a trouble since the distance would affect the intensity. [Circuit’s schematic diagram source: National Semiconductor Application Note]