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The
relative amount of energy at different wavelengths
depends upon the temperature at which the
filament is operated. The curve marked 2800 shows
the spectral distribution of energy emitted by incandescentungsten
when operated at a color temperature
of 2800'K. This is approximately the
temperature at which the smaller sizes of commercial
Mazda lamps are operated. In the case of
the larger units such as the 3,000, 5,000 and 10,000-
watt lamps designed specifically for use in motion
picture studios, the filament is operated at a much
higher temperature, in some cases reaching a value
of 3200'K. The distribution of energy in the radiation
from such a lamp is shown by the curve in Fig.
2, designated as 3200. The curve indicates that
there is a great preponderance of radiation in the
long wave-length region o{ the visible spectrum,
that is between 550 and 700 mp. The curve slopes
rapidly downward {or shorter wave-lengths, reaching
a very low value at 300 mp,. The portion of
the radiation lying in the ultra-violet between 300
and 400 mp is relatively small as compared with
that in the visible resion.
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FIGURE
Spectral distribution of energy in radiation from ordinary
carbon arc. The small circles show the distribution of energy
in the radiation from a black body at 40000 K.
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identical, approaches fairly closely that for sunlight
and judeed visually the light from the high intenJity
arc matches approximately sunlight in co-ior. Therl
is a large proportion of the shorter. visible wavelengths
present and although it is not shown in the
figure, it is well known that such sources emit a
relatively high intensity o{ radiation in the ultraviolet
between 300 and 400 mu.
In the case of the flame ur.i, it is difficult to define
the spectral composition in terms of curves similar
to those shown in the illustrations already given.
As stated previously, the spectral composition of
these flame arcs tends to be of the banded or line
type with numerous very high maximum ano very
low minimum intensities at certain wave-lengths.
Spectral composition can in these cases be more
easily shown by dispersing the radiation by means
of a prism or grating and making photographs of
the spectrum thus formed. While these cannot so
easily be interpreted quantitatively as the spectro-
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In Fig. 3 the curve shows the distribution of
energy from the crater of a low intensity carbon
arc, such as has been used extensively in the motion
picture studios and commonly referred to as Kleigs,
spots, elr. A comparison of this curve with that
for incandescent tungsten in Fig. 2 shows the presence
of a relaively greater proportion of the shorter
wave-length radiation. It follows, therefore, that
the light from such an arc is bluer than that emitted
by tungsten.
Fig, 4 shows the spectrophotometric curve for the
high intensity carbon arc, commonly referred to as
the sun arc. In this case the electrical conditions
and the composition of the carbons used are such
that the spectral composition of radiation is markedly
difierent from that emitted by the ordinary low
intensity carbon arc illustrated in Fig. 3. The maximum
has in this case moved over to about 530 mp,
which is approaching the wave-length of the maximum
for sunlight. In fact, the curve, while not
[31 ]
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FIGURE
Wedeie spectrograms showing the relative ristribution of
energy in the radiation from various flame arcs: (A) sunlight,
(B) acetylene ame, (C ) white ame arc, (D) blue flame
arc, (E) yellow flame arc, (F) red flame arc.
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