MCL640 Manual
Principles of Thermal Imaging
∂
24
Spectral radiant emittance of a blackbody
The graphs show that wavelength and spectral radiant emittance vary with the temperature.
They also show that as the temperature rises, the peak of spectral radiant emittance is shifting
to shorter wavelengths. This phenomenon is observable in the visible light region as an object at
a low temperature appears red, and as the temperature increases, it changes to yellowish and
then whitish color—thus shifting to shorter and shorter wavelengths as the temperature
increases.
5.4 Blackbody Type Source and Emissivity
Although a blackbody is actually only a theoretical ideal, an object can be manufactured which
approximates it. A law closely related to the blackbody is Kirchhoff’s law that defines reflection,
transmission, absorption and radiation.
a = e = 1
Key:
a
=
absorptivity
t
=
transmissivity
r
=
reflectivity
e
=
emissivity
Absorptivity equals emissivity, thus emissivity can be described by reflectivity and transmissivity.
e + 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 which is closest to an ideal blackbody as possible. The blackbody-type source can be
designed to meet the conditions pointed out by Kirchoff where “the radiation within an
isothermal enclosure is blackbody radiation.”
As a blackbody-type source for a measurement must radiate outside of the enclosed surface, a
small hole is cut through the wall of the enclosure small enough not to disturb the blackbody
condition. The radiation leaving this hole should closely approximate 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.
Содержание MIKRON MCL640
Страница 1: ...MANUAL MIKRON Thermal Imagers MCL640...
Страница 29: ...MCL640 Manual Principles of Thermal Imaging 29...
Страница 30: ...MCL640 Manual Principles of Thermal Imaging 30...
Страница 31: ...MCL640 Manual Principles of Thermal Imaging 31...
Страница 35: ...MCL640 Manual Diagrams 35 7 Diagrams...
Страница 36: ...MCL640 Manual Diagrams 36...
Страница 37: ...MCL640 Manual Diagrams 37...
Страница 38: ...MCL640 Manual Diagrams 38...
