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R4 Thermometer
7
Ratio Thermometry
A ratio (or two colour) thermometer measures the radiation from a target
surface at two separate wavelengths. The two signals are then electronically
divided to produce the ratio signal; this signal is taken as a measure of the
target surface temperature:
Ratio Signal = Voltage signal at short wavelength
The individual outputs are obviously dependant on surface emissivity but
if this is equal at both wavelengths, then the ratio is unaffected and the
temperature estimation will be exactly the same as for the black body
calibration. Such a surface is aid to be "grey" at these wavelengths.
For most surfaces, however, the emissivity changes with wavelength and thus
the measured signal ratio is changed by the non-greyness factor:
For a body whose emissivity is larger at the shorter wavelength the non-
greyness factor is >1 and the ratio is also high by this factor. This produces
an artificially high temperature indication which may be corrected by dividing
the measured ratio by the non-greyness factor during the processor stage.
The setting of non-greyness control and the range of values which may be
used are dealt with in the Landmark Signal Processor User Guide.
Page 10
Voltage signal at long wavelength
Emissivity at short wavelength
Emissivity at long wavelength
User Guide
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