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Emissivity is a measure of how effectively a surface can emit energy
as thermal radiation. Surfaces can have an emissivity value ranging
from 0 to 1.0 with 0 being a perfect reflector and 1.0 being a perfect
black body radiator (a black body radiator is an object that absorbs
all incoming radiation and does not reflect any, and then radiates out
energy at the same rate that it receives it). As a general rule, as an
object becomes more reflective, the emissivity value goes down. For
example, shiny aluminium has an emissivity value of 0.05 whereas paint
has a typical emissivity value of 0.95. In summary, emissivity can be
thought of as the ratio of the effectiveness of the surface to emit
thermal radiation compared to a perfect black body radiator. |
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Below is a chart of some emissivity coefficients of commonly measured
items.
As you can see, a majority of commonly measured items have emissivity
values close to 0.95 which is why this is often set as the default.
However to get the highest accuracy results, it is important to adjust
the emissivity setting on your infrared thermometer to match what you
are measuring.
What if you need to measure something like aluminium but you don’t
have an adjustable infrared thermometer?
If you need to measure something with a low emissivity coefficient, then you can stick some black electrical tape on what you wish to measure and then point your infrared thermometer at the tape. The tape should have a fairly similar temperature to the surface it’s attached to but the advantage is that the tape has a high emissivity (0.95). |
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