InfRec Thermo Gear G100, Operation Manual

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Chapter 7 Appendix
＜ Blackbody type source and emissivity ＞
Although a blackbody is actually only a theoretical ideal, an object can be formed which
approximates it. A law closely related to the blackbody is Kirchhoff's law that defines reflection,
transmission, absorption and radiation.
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where,
α
: absorptance
( = ε : emissivity)
ρ
: reflectance
α ρ τ + + =
1
τ
: transmittance
Also, absorptance equals emissivity, and so emissivity can be described by reflectance and
transmittance.
In order to obtain a 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 of blackbody-type
source which is nearest to a blackbody as much as possible. So it is necessary to design a
blackbody-type source.
The blackbody-type source can be designed to meet the conditions pointed out by Kirchoff that
“the radiation within an isothermal enclosure is a 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 not to disturb the condition of blackbody. The radiation
leaving this hole should closely simulate that from 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.
Following Fig. 7.3 is an example of a blackbody-type source according to conditions of
blackbody.
Emissivity is the ratio of energy radiated from an object to the exterior and energy radiated from
blackbody. The emissivity varies with the surface condition of the object and also with
temperature variation and wavelength. If this value is not accurate, then the true temperature
Ideal blackbody
L
r
Uniform temperature
Blackbody-type source
Fig.7.3 Example of blackbody-type source
(The larger L/r ratio closer to a blackbody)
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