Understanding Infrared Thermography Reading 3

■ Transmissivity Differences ∆τ
Transmissivity differences can be caused by heat sources behind the target if
the target is partly transparent in the infrared range. These will only be seen if
the target transmissivity is high enough and the heat source is different
enough in temperature from the target to contribute significantly to the total
target radiosity.
■ Target Geometry Differences ∆D
Target geometry differences are caused by multiple reflections within
recesses or concavities on the target surface. They are actually variations in
effective emissivity caused by changes in surface configurations. An example
of this is the apparent temperature gradient in the far corner of an enclosure
that is at a uniform temperature. Geometric differences diminish as target
surface emissivity approaches unity. (blackbody does not affected by G)
Charlie Chong/ Fion Zhang