User Manual
Ashley-Edison International Ltd
Page 43
Emssivity
What is Emissivity?
The standard of radiation is "blackbody" radiation which exists in any enclosure
whose walls are at a uniform temperature. The shape of the enclosure and the wall
material have no effect on this radiation, it being a function of temperature alone.
A small hole in such an enclosure only affects this radiation level very slightly. The
area of the hole should not exceed 5% of the area of the walls of the enclosure,
when the radiation passing through the hole is within about one part in a thousand
of the blackbody radiation level defined by the wall temperature. Such a device is
fairly easy to build and forms the source on which all thermometers are
calibrated.
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.
