Lecture 27
Surface Properties
±
Real surfaces do not behave like ideal black bodies
±
non-ideal surfaces are characterized by factors (< 1) - the ratio of the non-ideal
performance to the ideal black body performance
±
these factors can be a function of wavelength (spectral dependence) and
direction (angular dependence)
±
Non-Ideal Radiation Factor
±
emissivity,
ε
±
Non-Ideal Irradiation
±
absorptivity,
α
±
reflectivity,
ρ
±
transmissivity,
τ
5
Lecture 27
Emissivity
±
Emissivity
±
Ratio of the emission of a real body to the ideal emission of a black body
±
can be
²
a function of wavelength (spectral dependence) and direction (angular
dependence)
²
a function of wavelength (spectral dependence) averaged over all directions
²
average over all wavelengths and a function of direction (angular
dependence)
±
Spectral, Directional Emissivity
±
Spectral, Hemispherical Emissivity (directional average)
±
Total, Directional Emissivity (spectral average)
ε
λ
,
θ
,
,
φ
,
T
()
=
I
,
e
,
,
,
T
(
)
I
,
b
,
T
2
2
,
2
00
,
,,
0
,, c
o
ss
i
n
,
,2
,
,
c
o
s
s
i
n
e
bb
Id
d
ET
TT
d
IT
π
λθ
λλ
λθφ
θ θ φ
λε
λπ
==
=
∫∫
∫
,
,
T
=
I
e
,
,
T
(
)
I
b
T
6