Thermalert ET Series Operator's Manual
A-1
APPENDIX A: OBJECT EMISSIVITY
HOW TO DETERMINE OBJECT EMISSIVITY
Emissivity is a measure of an object’s ability to absorb, transmit, and emit infrared
energy. It can have a value from 0 (shiny mirror) to 1.0 (blackbody). If a higher than
actual value of emissivity is set, the output will read low, provided the target temper-
ature is above ambient. For example, if 0.95 is set and the actual emissivity is 0.9, the
reading will be lower than the true temperature when the target temperature is above
ambient.
The emissivity can be determined by one of the following methods, in order of prefer-
ence:
1. Determine the actual temperature of the material using a sensor such as an RTD,
thermocouple or another suitable method. Next, measure the object temperature
and adjust the emissivity setting until the correct value is reached. This is the cor-
rect emissivity for the measured material.
2. For relatively low temperature (up to 260°C or 500°F) objects, place a piece of tape,
such as electrical or masking tape, large enough to cover the field of view, on the
object. Next, measure the tape temperature using an emissivity setting of 0.95.
Finally, measure an adjacent area on the object and adjust the emissivity setting
until the same temperature is reached. This is the correct emissivity for the mea-
sured material.
3. If a portion of the surface of the object can be coated, use a dull black paint, which
will have an emissivity of about 0.98. Next, measure the painted area using an
emissivity setting of 0.98. Finally, measure an adjacent area on the object and
adjust the emissivity setting until the same temperature is reached. This is the cor-
rect emissivity for the measured material.
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 an object. These include the following:
1. Temperature
2. Angle of measurement
3. Geometry (plane, concave, convex, etc.)
4. Thickness
5. Surface quality (polished, rough, oxidized, sandblasted)
6. Spectral region of measurement
7. Transmissivity (i.e., thin film plastics)