About the two-color pyrometer (Ratio pyrometer)
As we know, the emitted radiation–temperature relationship for a body depends on the emissivity. This is very difficult to calculate, and therefore in practice all pyrometers have to be calibrated to the particular body they are measuring.
The two-color pyrometer (alternatively known as a ratio pyrometer) is a system which largely overcomes this problem by using a specific arrangement. Radiation from the body is split equally into two parts which are applied to separate narrow-band filters. The outputs from the filters consist of radiation within two narrow bands of wavelengths. Detectors sensitive to these frequencies produce output voltages V1 and V2 respectively. The ratio of these outputs, V1/V2, can be shown to be a function of temperature and to be independent of the emissivity provided that the two wavelengths are close together.