distribution (I,Jm
2
) of energy emitted by a body is based
on Planck’s distribution law (Eq.(1)), for a given wavelength (l, m) and temperature (T,K).
Iðl;TÞ¼
8phc
l
5
e
hc=klT
1
1
ð1Þ
Where,his Planck’s constant (6$6310
34
Js); cis the
speed of light (310
8
ms1
), kis Boltzmann’s constant
(1$3810
23
JK
1
).
The total energy intensity (average power per unit area),
I of an object is the spectral emittance (Il
) integrated over
all wavelengths, i.e.
IT¼
ZN
0
ITdl ð2Þ
IT¼
ZN
0
8phc
l
5
e
hc=klT
1
1
dl ð3Þ
Integration of Eq. (3) gives the StefaneBoltzmann law
(Eq. (4)):
W¼
2p
5
k
4
15c
2
h
3
T
4
¼sT
4
ð4Þ
WhereWis the energy flux emitted per unit area (W/m
2
);c
is the speed of light in a vacuum,his Planck’s constant and
kis Boltzmann’s constant,sis the StefaneBoltzmann constant (5.6710
8
Wm2
K
4
) and Tis temperature (K).
This law is only valid for an idealized black body (i.e. perfect energy absorber); for real objects an additional term,
the object’s emissivity (e), is required:
W¼3sT
4
ð5Þ
The measurement ofWis an indicator of the temperature
of the emitting object. This is the fundamental concept behind thermal imaging.