# Two-Color Pyrometer (Ratio Pyrometer)

## 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.

The theoretical basis of the two-color pyrometer is that the output is independent of emissivity because the emissivities at the two wavelengths are equal. This is based on the assumption that the two wavelengths are very close together. In practice, this assumption does not hold and therefore the accuracy of the two-color pyrometer tends to be relatively poor. However, the instrument is still of great use in conditions where the target is obscured by fumes or dust, which is a common problem in the cement and mineral processing industries. Two-color pyrometers typically cost 50–100% more than other types of pyrometer.