The emissivity of a material (often written e
), is the ratio of
the energy which it radiates by a black body wich radiate
at the same temperature. It is thus a measurement of the capacity of a
body to absorb and re-emit radiated energy.
Emissivity depends on
several factors: the temperature, direction radiation, and its
Emissivities depend on the wavelength to which one takes measurement. You defer to the instructions to know the wavelengths to which your
Caution: the following values depend on the
current nature of material and the conditions of measurements.
Tables of emissivity - metals: Tables of emissivity - not metals:
You can determine an unknown emissivity according to one of the
1. Using a thermocouple or another adapted
method, determine the current temperature of material. Then measure the temperature of the object and regulate emissivity until the
correct temperature is reached. You then determined the exact
emissivity of measured material.
2. To measure temperatures relatively very high (until 260°C), place
on the target an enough large plastic label to cover the spot of
measurement. Then measure its temperature while having regulated
emissivity on 0,95. Measure the temperature of a near zone located on
the object and modify emissivity until obtaining the same temperature.
You then determined the exact emissivity of measured material.
3. If that is possible, apply to a part of the object to measure a black
painting matte posting an emissivity higher than 0,98. Then measure
the temperature of the zone thus painted while having regulated
emissivity on 0,98. Measure the temperature of a near surface located
on the object and modify emissivity until obtaining the same
temperature. You then determined the exact emissivity of measured