Addendum
Determination of Emissivity
93
12. Addendum
12.1. Determination of Emissivity
Emissivity
(applicable for 1-color operations)
is a measure of an object’s ability to absorb and emit
infrared energy. It can have a value between 0 and 1.0. For example a mirror has an emissivity
of 0.1, while the so-
called “Blackbody“ reaches an emissivity value of 1.0. If a higher than actual
emissivity value is set, the output will read low, provided the target temperature is above its
ambient temperature. For example, if you have set 0.95 and the actual emissivity is 0.9, the
temperature reading will be lower than the true temperature.
An object’s emissivity can be det
e
rmined by one of the following methods:
1. Determine the actual temperature of the material using an RTD (PT100), a
thermocouple, or any other suitable method. Next, measure the object’s temperature
and adjust emissivity setting until the correct temperature value is reached. This is the
correct emissivity for the measured material.
2. If possible, apply flat black paint to a portion of the surface of the object. The emissivity
of the paint must be above 0.98. Next, measure the temperature of the painted area
using an emissivity setting of 0.98. Finally, measure the temperature of an adjacent
area on the object and adjust the emissivity until the same temperature is reached. This
is the correct emissivity for the measured material.
12.2. Typical Emissivity Values
The following table provides a brief reference guide for determining emissivity and can be used
when one of the above methods is not practical. Emissivity values shown in the table are only
approximate, since several parameters may affect the emissivity of a material. These include
the following:
1. Temperature
2. Angle of measurement
3. Geometry (plane, concave, convex)
4. Thickness
5. Surface quality (polished, rough, oxidized, sandblasted)
6. Spectral range of measurement
7. Transmissivity (e.g. thin films plastics)
Table 13: Typical Emissivity Values (Metals)
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