MECE 4364 Heat Transfer
Prof. Dong Liu
Department of Mechanical Engineering
University of Houston
1
Lecture 26 – Nov 18, 2010
Lecture 26
Radiation
2

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Lecture 26
Solid Angle measures how large an object appears to an observer looking from a
point which is subtended by the object.
±
A small object nearby may subtend the same solid angle as a larger object
farther away
±
Example: The eclipse!!
Solid Angle
3
2
sin
n
dA
r
d d
θ
θ φ
=
2
sin
n
dA
d
d d
r
ω
θ
θ φ
=
=
Lecture 26
Spectral Intensity
Spectral radiation intensity, I
λ
,e
(
λ
,
θ
,
φ
)
The rate at which radiant energy is emitted at the wavelength
λ
in the (
θ
,
φ
) direction
per unit area of the emitting surface normal to this direction
per unit solid angle about this direction
per unit wavelength interval d
λ
about
λ
the rate at which emission from dA
1
passes through dA
n
(
)(
)
,
1
, ,
cos
e
dq
I
dA
d
d
λ
λ θ φ
θ
ω
λ
=
⋅
4
Consider the rate at which emission from dA
1
passes through dA
n
where
1
cos
n
dA
dA
θ
=
This is how
dA
1
appears
along
θ
,
φ
Unit: W/m
2
-sr-
μ
m