Theatre Design & Technology - Oct 1972 - 17

Roscolene - Rosco Laboratories plastic color media
series similar in number and to some extent in
colors to Cinemoid but no exact duplication.
The spectrophotometric curve is a diagram which
shows the exact amount of each wavelength of the visible
spectrum that a color sample will transmit or reflect. It is
obtained by a recording spectrophotometer. In the instrument a pure white light produced by a tungsten filament
lamp is beamed through a prism which disperses it into a
spectrum. The broad band of the spectrum strikes the
backside of a movable slit which in turn permits a very
narrow band of light to pass on to strike or pass through
the sample. The slit selects a few millimicrons of wavelengths to pass thus making the light monochromatic. Beginning with the longest wavelengths or the red end, the
movable slit scans the entire length of the visible spectrum. Each segment of the scanning causes each wavelength of Iight to stri ke a phototube. The impu Ises, in the
form of energy, are transmitted to a recorder continuously to produce a curve.
Valuable as actual curves may be for determining the
spectral ingredients of samples, the difficulty of providing
the data makes it impractical to include it in this study.
Such inclusion would involve endless pages of numbers
which would then have to be translated to a graph by the
reader. One is encouraged to make his own recordings.
The data obtained from the spectrophotometric curve
requires dividing the visible spectrum of the recorded
curve into at least 16 wavelength intervals, determi ning
the contribution to stimulation (the percentage of energy
shown on the curve) made by the light at each interval
and summing the results. To describe the computation is
unnecessary since the final data is itemized in the appendix. The color-order system establ ished by the Internationa I Commission On Illumination known as the C.I.E.

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system is based on the principle that the effects of any
color or color stimuli (excluding tints and shades) can be
duplicated by mixing color stimulus from the three primaries in proper proportions. In other words, any color
can be produced by additively mixing set proportions of
the three primary colors. It follows, therefore, that any
color such as that in a filter has within it set proportions
of the three primaries. It follows, also, that when these
primaries are measured, the measurements would represent an accurate description of the color. The tristimulus
values, known as value X, value Y, and value Z measure
the three primaries. The unknown color can, therefore, be
expressed in terms of three number values: X, Y, and Z.
Each number represents one of the primary stimuli. X
represents the amount of a primary which is considered
red. Y represents the amount of a green primary and Z
represents the amount of a blue primary.
By computation the tristimulus values of color samples
and pure spectral colors are converted to trichromatic coefficients. The trichromatic coefficients are represented
by X, '1, and z. For mathematical reasons, only two of
these quantities are independent. Therefore, to specify the
chromaticity of a color,
andy have been establ ished by
the International Commission On Illumination for this
purpose.

The trichromatic coefficients for the spectral colors
have been set by the Commission thus making it possible
to construct the chromaticity diagram. Figure 4 illustrates
its structure. It is laid out by plotting the values on the
vertiCal axis and the '1 values on the horizontal. When
completed, the diagram consists of a locus of the spectral
colors beginning at 400 mu on the left to 700 mu on the
right. The line connecting the locus at 400 mu and 700
represents the unreal colors, that is, the purples and
magentas. Printed working diagrams may be obtained
from the M.I.T. Press, Massachusetts Institute of Technology, Cambridge, Massachusetts. Ask for Chart =23 .
For a more detailed explanation of the colorimetric principles involved refer to Arthur G. Hardy's Handbook of
Colorimetry.

A light source will be located at some point along the
black body locus shown in Figure 4. Note the position of
the standard illuminants. Graphically this locus shows the
position of illuminants as their color temperatures increase, which in turn yields a higher spectral energy distribution.
For a color plate which shows dramatically the relationship of the black body locus to all other colors on a
C.I.E. chromaticity diagram write to the Large Lamp Department of General Electric, Nela Park, Cleveland, Ohio
for their bulletin TP-119 "Light and Color."
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Figure 4

THEATRE DESIGN AND TECHNOLOGY

To study the relationship of color media plot them on
a chromaticity diagram. Begin by establishing the illumi-

OCTOBER, 1972

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