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:
angle that the surface is viewed from
coating by contaminants
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.
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.
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.