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is best defined by stating the relatixe intensity at
each wave-length present. This can be determined
by dispersing the radiation into its component parts
and then measuring the intensity at each wavelength
by means of a suitable instrument, such as a
thermopile, radiometer, or photo-electric cell. Wavelength,
it will be remembered, is usually specified
ln terms of a unit called the millimicron (mp.),
which is equivalent to a millionth of a millimeter
(0.00000tmm).
The spectral composition of the radiation emitted
by difierent light sources used in motion picture
work varies enormously. These sources may, for
convenience, be divided into two classes: (a) those
having a continuous spectrum, and (b) those having
a discontinuous or line spectrum. In sources of the
first classification it is found that all possible wavelengths
are present and of this class incandescent
tungsten is typical. In sources of the latter class
radiation is emitted at certain definite wave-lengths
only, all other wave-lengths being entirely absent.
Of this class the Cooper-Hewitt mercury vapor
lamp is a well-known example. Sunlight may also
be classed as of the continuous spectrum type, although
as a matter of fact the spectrum of sunlight
shows numerous very narrow dark lines, due to
absorption in the sun's atmosphere. These lines are
so narrow, however, that for all practical purposes
sunlight may be considered as of the continuous
spectrum type. The radiation from the crater of a
carbon arc is also of the continuous type, but due to
certain selectivity absorption characteristics of the
arc stream (the flame), the spectrum may contain
certain bright bands or wave-length regions showing
an enhancement of intensity. In the case of the
flame arcs, the major portion of the light is emitted
by the arc stream rather than by the crater and the
spectrum is found to be filled with a multitude of
lines and bands which are due to the materials other
than the carbon incorporated in the core. In most
cases these lines and bands lie so close together that
for practical purposes the spectrum of these arcs
may be considered to approach continuity, although
spectroscopic examination shows a certain preponderance
of radiation of some wave-lengths and absence
of others.
The spectral composition of the radiation emitted
by any particular source is clearly shown by means
of the spectrophotometric curve which is obtained
by plotting for each wave-length the intensity of
radiation at that wave-length. Since the final analysis
of why various colors are rendered as they
are under any specified illumination depends essentially
upon this quality factor, it will be profitable
to consider in some detail this characteristic of various
illuminants.
In Fig. .1 the spectrophotometricurve A shows
the relation between intensity and wave-length for
noon sunlight. It will be noted that there is a maximum
at wave-length of approximately 500 mp., this
corresponding to the green region in the spectrum.
From the maximum there is a slight decrease for
longer wave-lengths and a rather rapid decrease for
the shorter wave-lengths. At wave-length 400,
which represents the limit of the visible spectrum
at the short wave-length end, the intensity is only
about one-half of that at the maximum. In the
ultra-violet region between 400 m/, and 300 mp
the decrease is quite rapid so that at wave-length
300 there is but very little energy. Although the
total energy in this ultra-violet region is relatively
small as compared with that in the visible, it is of
considerable importance in photographic work because
photographic materials have relatively high
sensitivity to these shorter wave-lengths. Curve B
in Fig. -Z shows the distribution of energy in the
p
z
300 400 500 600 rco,
W_LEiGTH tN mf.
FIGURE
Spectral distribution of energy in radiation from (A)
noonday sun, (B) north sky.
spectrum of north skylight. It will be noted that
the maximum is at approximately 400 m1, and that
the decrease in the region of long wave-lengths is
rather rapid. It is evident from a comparison of
this curve with that for sunlight (curve A) that
skylight will appear blue, due to the preponderance
of shorter visible wave-lengths as compared with
sunlight. The falling ofi from the point of maximum
intensity in the region of shorter wave-lengths
is also quite rapid, so that at 300 mp the value is
quite low.
In Fig, 2 are shown spectrophotometric curves
for the radiation emitted by incandescentungsten.
0
z
:
J
WAVE_ LEren. rN dl.
FIGURE
Spectral distribution of energy in radiation from tungsten
filament at various tenDeratures.
I
II
t 30l