User Manual
94
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
For materials with smooth, clean (unoxidised) surfaces, emissivities are usually in the range
0.05 to 0.50 and are usually very wavelength dependent, being higher at shorter wavelengths.
The appropriate settings for the ThermoPro
TM
TP8 IR Thermal Cameras are given in the
To Next Page
Loading...
