Calibration disappearing filament pyrometer

One advantage of a spectral radiation pyrometer is that the emissivity or emittance at only a specific wavelength (e.g. 0.653 pm) is of importance. A non-blackbody source will be less luminescent than a blackbody source at the same temperature. Thus, a falsely low temperature will be determined by sighting a calibrated disappearing filament pyrometer on the non-blackbody. This temperature has been referred to as the brightness temperature . 

Optical pyrometers allow direct measurement of the temperature of an object. The disappearing filament optical pyrometer works by matching the intensity of radiant energy coming from an incandescent source to the intensity of a calibrated filament when both the source and the filament are viewed through a red filter. When the filament and the source intensities are the same, the image of the filament disappears as it is superimposed on the image of the source. An obvious requirement of this technique is that it can be used only to measure temperatures that produce visible incandescence, above about 750°C. 

Optical pyrometers are suitable for measuring temperatures up to 3000 °C and are usually of the disappearing filament type. The heated object is viewed through a telescope which is fitted with an electric lamp. The brightness of the lamp filament is adjusted until its temperature equals that of the object, when the lamp filament disappears . At this point the temperature of the filament, and hence the object, can be obtained from calibrated scales. 

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