1. spectral power distribution (SPD) curves of the heat/light source,
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2.  When a coal fire is lit
 (or when the bar of an electric fire is switched
on), it first of all
▪ glows a dull red,
▪ then orange-red,
▪ then yellow;
 eventually it approaches the ‘white-hot’ stage
as the temperature rises.
 At the same time the total amount of energy
emitted rises (the fire gets steadily hotter).
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3.  The radiative power emitted by a
heated body
 is best described by a plot showing the
variation across the electromagnetic
spectrum of the Emittance
 (for example, in watts per square
metre) per unit wavelength.
 Such curves are known as
 the spectral power distribution (SPD)
curves of the heat/light source, and
 Figure 1.5 illustrates
 how these curves change in the visible
region as the temperature of the
heated body rises.
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4.  A Planckian or black body radiator is an
idealised radiation source consisting of a
▪ heated enclosure from which radiation escapes through
an opening whose area is small
▪ compared to the total internal surface area of the
enclosure
 (in practice approximated to by a small hole
in the side of a large furnace).
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5.  The term ‘black body’ was originally used in
recognition that
▪ such a model source would radiate energy perfectly
▪ and conversely would absorb light perfectly,
▪ without reflecting any of it away,
▪ in the manner of an ideal black object.
 Nowadays such a model source is referred to
as an ideal, full or Planckian radiator.
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6.  The Austrian physicist Josef Stefan showed
in 1879 that
 the total radiation emitted
 by such a heated body
 depended only on its temperature
 and was independent of
▪ the nature of the material from which it was
constructed.
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7.  Considerable debate about the spectral
distribution from these
▪ so-called black bodies ensued,
 in which many of the world’s leading
theoretical and practical physicists joined:
these included
▪ Wien,
▪ Jeans
▪ and Lord Rayleigh.
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8.  In 1900, however, the German physicist Max Planck
developed a theoretical treatment that
▪ correctly predicted the form of the spectral power distribution curves
▪ for different temperatures
 (it took the support of Einstein in 1905 to convince the
sceptics).
 Planck’s breakthrough came through the assumption
 that
▪ radiation was not emitted continuously
▪ but only in small packets or quanta,
▪ With the energy of the quantum being directly proportional to the
frequency of the radiation involved
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9.  Planck used his now famous Eqn 1.5 to
derive an expression for
▪ the spectral emittance
▪ from which the SPD curve of the source can be calculated.
 The Planckian radiation expression has the
form of Eqn 1.7:
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10.  Some examples of the SPD curves for
Planckian radiators at different
temperatures based on Eqn 1.7 are shown
in Figure 1.6.
 To accommodate the large ranges of
values involved, Figure 1.6 shows the
power on a logarithmic scale
 (note the units used) plotted against the
wavelength in nm, also on a logarithmic
scale,
 and illustrates how at temperatures below
6000 K most of the energy is concentrated
in the long-wavelength IR or heat region
of the electromagnetic spectrum.
 In fact the emission over the visible
region is only a small part of the total
emission for any of the curves shown.
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11.  The shape of the SPD curve
across the visible region
changes significantly,
 however, from about 1000 K
 at which the colour
appearance of the emitted
radiation is predominantly red
 to 10, 000 K, at which it is
bluish-white (Figure 1.7).
 Between these two limits the
colour changes
 from red,
 through orange-red
 to yellowish-white
 and eventually to bluish-
white, as discussed above.
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12.  The closest approach to the
 ideal equi-energy (ideal white light) source
with constant emittance
▪ across the visible spectrum
▪ occurs somewhere between 5000 and 6000 K.
 Thus we can associate the colour appearance
of the source with the temperature
▪ at which a Planckian radiator will give approximately the same
colour appearance.
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13.  The precise connection between
▪ colour temperature
▪ and Planckian radiator temperature
▪ (and that of correlated colour temperature)
 is best discussed through a plot of
▪ the colour coordinates of the Planckian radiators on a suitable CIE
chromaticity diagram
 The typical 100 W domestic tungsten light bulb has a
▪ colour temperature of about 2800 K.
 That of a tungsten–halogen projector bulb
▪ is about 3100 K,
 whilst that of average daylight from an overcast sky
▪ is about 6500 K.
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