The rules for forming colors by
subtracting are important to understanding how color printing, painting, and
dyeing work. In all of these cases, the
color we see is formed by subtraction.
Most inks and dyes are designed to reflect very little light, so that
most of the incident light passes through the ink or the dye to the material
below and is reflected back again to our eyes. The light passes through the
absorbing material twice, and the color that we see is determined by its
absorption characteristics. (The reflective properties of the underlying
substrate material are also important, of course, since the substrate material
may also absorb some wavelengths are reflect others. To simplify matters, it is
easier to think of the substrate material as reflecting whatever it receives
without alteration and to treat the wavelength-dependence as coming from an
additional layer on top of the simple white reflector.)
Color printing is generally
implemented using multiple subtractive layers, where each layer is composed of
one of the subtractive primaries defined in the previous topic. At each point, the color of the light
that we see is the sum of the wavelengths that make it through all of the
layers at that point. Although it is possible to produce black using all 3
subtractive primaries in equal proportion, that takes a lot of ink, and most
printing processes use a separate black ink for this purpose rather than using
relatively thick layers of the 3 subtractive primaries. Color printing is
therefore often called a 4-color
process for this reason, where the 4 colors are the subtractive primaries
(cyan, magenta and yellow) plus black.
In addition to the fact that the inks
are not ideal primaries, this method cannot be used to produce the
monochromatic, fully saturated colors on the horseshoe color chart itself. This
is the same limitation that we saw in the process of color addition. Also,
recall that the subtraction process can work only with the light that is
incident on it, so that it cannot produce a color that is not present in the
incident light beam. In addition, as the desired color becomes more and more
saturated, the resulting intensity inevitably becomes dimmer, since only a
smaller and smaller fraction of the incident light matches the desired
wavelength. Thus highly saturated colors do not print very well unless special
inks are used. Since each one of these inks must be printed separately, the process is complicated and expensive,
and is usually used only for “coffee-table” art books.
It is difficult to change the overall
intensity of the reflected image, since printing is inherently a binary process
– either the ink is deposited on a point or it is not. The usual solution is
called a “half-tone” which is made by breaking up the image into a large number
of dots. Each dot either receives the ink or does not. Since the dots are very
small, the eye averages the intensity from many adjacent dots, so that the result
is a perceived variation in intensity. This method can be used either with
black ink (to produce a gray scale) or with colored inks to vary the hue and
saturation of the result.