User manual

USER MANUAL
Page 38
Emissivity
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 for 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.
This is done by setting the emissivity control to the appropriate value.
It is therefore necessary to know the value of the emissivity. There is a vast amount of
data available, unfortunately much of it is confusing because substantially different
values are quoted. This is because emissivity depends upon:
a) The basic material
b) Surface condition - roughness and oxidation