User Manual
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 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.
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
c) Temperature
d) Angle of view
e) Wavelength
