HANSER Hanser Publishers, Munich • Hanser Gardner Publications ...

Figure 3.13
Black body radiation [1]
Equation 3.58 can also be written as
where cs = 5.77 W/(m 2 • K 4 )
(3.59)
The dependence of the black body radiation on the direction (Figure 3.14) [1] is given
by the cosine law of Lambert
The radiation properties of technical surfaces are defined as (Figure 3.15) [I]:
Reflectivity
Absorptivity
Transmissivity
The sum of these fractions must be unity, or
p + a+8 =1
The transmissivity 8 of opaque solids is zero so that
p + a= 1
The reflectivity of gases p is zero and for those gases which emit and absorb radiation
a+8=l
Figure 3.14
Lambert's law[1]
Figure 3.15
Properties of radiation
(3.60)
(3.61)
(3.62)
(3.63)
Real bodies emit only a fraction of the radiant energy that is emitted by a black body at
the same temperature. This ratio is defined as the emissivity e of the body,
(3.64)