Page 193 - Mechanical Engineers' Handbook (Volume 4)

182 Heat-Transfer Fundamentals

Figure 20 Configuration factor for radiation exchange between surfaces of area dA i and dA j .

Spectral hemispherical absorptivity, , is defined as the radiant energy absorbed per

unit area of the surface, per unit wavelength about the wavelength /G .

Spectral hemispherical transmissivity is defined as the radiant energy transmitted per
unit area of the surface, per unit wavelength about the wavelength /G .

For any surface, the sum of the reflectivity, absorptivity, and transmissivity must equal
unity, i.e.,
1

When these values are averaged over the entire wavelength from 0to they are
referred to as total values. Hence, the total hemispherical reflectivity, total hemispherical
absorptivity, and total hemispherical transmissivity can be written as

Gd /G

0

Gd /G

0
and

Gd /G

0
respectively, where