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divided into three equal portions representing approximately
the three primary colors: blue, green,
and red. -ih. dirtitional points are indicated by the
wave-length scale at the bottom of the figure. It
is assumed that this spectrum represents that of
rvhite light, for instance light from the noonday
sun. In the spectrum designated as ..4 absorption of
blue is indicated by the shaded area' The remaining
light consists of red and green which added tog"ther
give a yellow color. Thus the absorption of
6lue reiults in yellow. Complementary colors are
those which when added together result in white.
complementary to each other. Since the absorption
of blue results in yellow, the converse must be true
that the absorption of yellow, that is the red and
'
FIGURE
Diagram illustrating selective absorption of narrow
spectral bands.
green components of white light, will give blue. In
the spectrum designated as B green has been absorbed.
The remaining radiation consists of red
and blue which, when mixed together, produce
magenta ( minus-green ) . In the spectrum designated
as C the shaded area indicates that red has
been absorbed leaving blue-green. Conversely, if
the blue and green be absorbed, red will be obtained.
The absorption bands illustrated in Fig.9 by the
shaded areas represent hypothetical conditions which
are never found to exist in natural objects. Instead,
the selective absorption found in actual practice
occurs in bands such as are illustrated in Fig. 10.
In the upper part of the figure at ,4 the portion
shaded by the diagonal line sloping to the right
represents an ideal absorption band such as can be
realized only in an optical instrument. The removal
of the radiation represented by this shaded area,
IX
which includes the yellow and orange, leaves violet.
The area under the curve shaded by lines sloping to
the left illustrates the absorption band o{ dye giving
a violet color. At B in Fig.70 the two areas shaded
by lines sloping to the right illustrate the production
oi a blue-gi.en color by the sharp-cut absorption of
the extreme violet and the red and orange spectral
reEions, The area under the curve shaded by lines
sloping to the left shows the spectral absorption of
u Jy. giuit g a corresponding blue-green color. The
faci that piactically all colored objects which are
photographed have absorption bands which are not
ubr,rpi but of the gradually cutting type, makes it
possitl" to obtain, in general, much better tonal
rendition than if natural colors were produced by
absorption bands of the abrupt type.
Color in non-luminous objects, there{ore, may be
considered as being due to selective absorption of
the luminous radiation incident thereon. This selective
absorption is best expressed quantitatively in
te rms of spectrophotometric curves which show
reflecting power as a {unction o{ wave-length. The
curves in Fig. 10 are typical of such spectrophotometric
reflection curttes' Practically all pigments
and dyes have absorption characteristics of the
general type illustrated in Fig.10, although in some
Jases the ibsorption either at one or both sides of
the band may be somewhat sharper than shown.
Quantitative data relative to the spectral character-
trulE
lEN67u
FIGURE
Spectral reflection curves of various pig:ments'
XI
FIGURE
Diagram illustrating the difierence between
Practical selective absorption'
X
theoretical
and
t34l
istics of a few colored materials are shown in Figs'
11 and 12. These are taken from a publication by
M. Luckeisht who also gives a large amount o{ data
relative to the colored materials such as dyes, inks,
etc. An examination of the curves in Figs. 11 and
72 reveals some rather interesting features. The
materials represented by these curves may be taken
as representative in a general way of the coloring
mateiials available for producing color in paints,
fabrics, wall papers, etc. It will be noted that the
curves for r.J, orange) and yellow pigments have
high refle-tion factors in the spectral
lelartivelv
1. Luckeish, M. The Physical Basis, of Color
i..hnology. J. Frank. Inst', Julv, 1917' p' 86'