User Manual
93
A surface always emits less radiation than would a black body at the same temperature by a
factor called the "emissivity" of the surface.
Emissivity =
Radiation emitted by surface
Radiation emitted by black body
How Do We Deal With Emissivity?
If we ignore the emissivity altogether and infer a temperature from the thermometer output, we
shall get a temperature lower than the true temperature by an amount depending on the value
of emissivity and the characteristics of the thermometer. This temperature is known as the
"apparent" or "brightness" temperature of the surface. If the emissivity is constant, this
temperature rises and falls in exactly the same way as the true temperature and this may be
sufficient for some purposes.
More often the true surface temperature is required. We have:
Actual output = E x output when viewing blackbody
To obtain the true surface temperature we must divide the actual output by the emissivity value
E before we convert to temperature.
This is done by setting the emissivity control to the appropriate value.
