Multiple Color Video Pyrometry

Measuring the intensity at more than one wavelength allows the determination of both temperature and emissivity. If the emissivity is not known or varies along a surface, the temperature cannot be determined accurately via a single intensity measurement. Many factors make it difficult to know the value of the emissivity:
If the amount that these factors disturb the emissivity value cannot be quantified, it is necessary to determine both the emissivity and temperature. To illustrate how multiple color pyrometry determines the temperature and emissivity, the measurement of thermal radiation emitted by a tungsten surface will be demostrated with the help of FIG 1.  The real emissivity of tungsten reduces the intensity from that of a black body (the black line) to that of the tungsten surface (red line). Four measurements of the intensity (indicated by the vertical colored lines in FIG 2) supply the data required to determine the temperature and emissivity of the surface.

Tungsten Intensity
FIG 1. Difference between the intensity of a tungsten
surface and a black body.

The temperature and emissivity are determined by fitting the intensity data to models of intensity and emissivity. The two intensity models commonly used are Planck's law and and Wien's approximation.  

Measuring Intensity
FIG 2. Example wavelengths and intensities of measurements
of a four-color pyrometer.

For most metallic surfaces radiating in the visible and infrared spectrums the emissivity or the natural logarithm can be modelled as linear function of wavelength. The assumption of linear emissivity is supported by published data on metallic surfaces, see FIG. 3.

Tungsten Emissivity
FIG. 3.

Multiple Color Video Pyrometer
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