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Arlon
Online Engineering
Library - TechRef!
Radiation
The radiation emitted by a body is given by
 =   AT4, where
T = the absolute temperature (K or oR)
= 5.67 x 10-8 W/(m2·K4)
[0.173 x 10-8 Btu/(hr-ft2 - oR4)]
= the emissivity of the body
A = the body surface area
For a body (1) which is small compared to its surroundings (2)
12 = A(T14 - T24), where
12 = the net heat transfer rate from the body.
A black body is defined as one which absorbs all energy incident upon it. It also emits radiation at the maximum rate for a body of a particular size at a particular temperature. For such a body:
 = = 1, where
= the absorptivity (energy absorbed/incident energy).
A gray body is one for which = , where
0 < < 1; 0 < < 1;
Real bodies are frequently approximated as gray bodies.
The net energy exchange by radiation between two black bodies, which see each other, is given by
12 = F12 A1 (T14 - T24), where
F12 = the shape factor; 0 <= F12 <= 1.
For any body, +  +  = 1, where
= absorptivity
= reflectivity (ratio of energy reflected to incident energy)
= transmissivity (ratio of energy transmitted to incident energy)
For an opaque body, + = 1
For a gray body, + = 1
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