Page 17 - Applied Photovoltaics

1.2 BLACKBODY RADIATION
A ‘blackbody’ is an ideal absorber, and emitter, of radiation. As it is heated, it starts
to glow; that is, to emit electromagnetic radiation. A common example is when a
metal is heated. The hotter it gets, the shorter the wavelength of light emitted and an
initial red glow gradually turns white.
Classical physics was unable to describe the wavelength distribution of light emitted
from such a heated object. However, in 1900, Max Planck derived a mathematical
expression describing this distribution, although the underlying physics was not
understood until Einstein’s work on ‘quanta’ five years later. The spectral emissive
power of a blackbody is the power emitted per unit area in the wavelength range Ȝ to
Ȝ + dȜ and is given by the Planck distribution (Incropera & DeWitt, 2002),

ʌ
2 hc 2
E Ȝ ,T 5 (1.2)
Ȝ > Ȝ kT hc 1 @exp
where k is Boltzmann’s constant and E has dimensions of power per unit area per unit
wavelength. The total emissive power, expressed in power per unit area, may be
found by integration of Eqn. (1.2) over all possible wavelengths from zero to infinity,
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yielding E = ıT , where ı is the Stefan-Boltzmann constant (Incropera & DeWitt,
2002).

Figure 1.1. Radiation distributions from perfect blackbodies at three different
temperatures, as would be observed at the surface of the blackbodies.

Fig. 1.1 illustrates the radiation distribution for different blackbody temperatures, as
would be observed at the surface of the blackbody. The lowermost curve is that for a
body heated to 3000 K, about the temperature of the tungsten filament in an
incandescent lamp. The wavelength of peak energy emission is about 1 ȝm, in the
infrared. Only a small amount of energy is emitted at visible wavelengths (0.4–
0.8 Pm) in this case, which explains why these lamps are so inefficient. Much higher

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