MikroScan 7600PRO, Руководство оператора

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Section 8
279
MikroScan 7600PRO
Operator’s Manual
Catalog 11180-94
V15.4F 050406
Principle of Thermal Imaging
8.4 Blackbody Type Source and Emissivity
Although a blackbody is actually only a theoretical ideal, an object can be manufactured
whch approxmates t. A law closely related to the blackbody s Krchhoff’s law that
defines reflection, transmission, absorption and radiation.
a = e = 1
Absorptivity equals emissivity, thus emissivity can be described by reflectivity and
transmssvty.
a + t + r = 1
In order to obtain the true temperature of an object, it is necessary to obtain the
emissivity correctly. Therefore, the emissivity of the object has to be measured by using
a blackbody-type source whch s closest to an deal blackbody as possble. The black-
body-type source can be desgned to meet the condtons ponted out by Krchoff where
“the radiation within an isothermal enclosure is blackbody radiation.”
As a blackbody-type source for a measurement must radate outsde of the enclosed sur-
face, a small hole is cut through the wall of the enclosure small enough not to disturb the
blackbody condton. The radaton leavng ths hole should closely approxmate that of
a blackbody. When the diameter of the hole is as 2r and the depth is as L, if L/r is equal
or more than 6, it is used as a blackbody-type source for practical use. The following
figure shows an example of a blackbody-type source based on blackbody conditions.
Key:
a = absorptivity
t = transmissivity
r = reflectivity
e = emissivity