The colored appearance of most
materials is due to selective absorption the material absorbs some
wavelengths very strongly and either transmits or reflects the remainder. The color
is therefore produced by subtraction some wavelength or group of wavelengths
is removed from the incident light. This is in contrast to producing colors by
addition as described in the previous section.
If a light beam is processed by several such
wavelength-selective absorptions, then what remains is the component of the
light beam that is not absorbed by all of the processes. For example, if
unsaturated white light is passed through a substance that transmits only the
wavelength corresponding to the red primary (650 nm) then that is the only
wavelength that emerges. If this beam is now passed through a substance that passes
the green primary (530 nm) and absorbs everything else, then nothing
comes out the output is completely black. This is because the output of the
first process is a wavelength that is absorbed by the second stage, leaving
nothing in the output. This result will be true of any two substances that pass
only a single monochromatic (fully saturated) wavelength and absorb everything
else. The output of the first stage will consist of the single wavelength that
is not absorbed. Unless the second stage passes exactly the same wavelength as
the first stage, the output of the second stage will be black.
If the incident light is unsaturated
white light, then the process of subtraction can be thought of using the ideas
of complementary colors introduced in the previous section. For example, when
unsaturated white light (such as the light produced by an incandescent light
bulb) strikes a yellow object, the object appears yellow in this context because
it absorbs the blue part of the incident light and reflects the yellow light.
(Recall that in the previous section we identified this pair as complementary
colors, since they combine to form white.) Continuing in this way, an object
that absorbs the cyan from a white light will appear red and an object that
absorbs the magenta from a white light will appear green. These colors: yellow, cyan and magenta, are
called the subtractive primaries. They are defined as the answers to the
question, What color must be absorbed from unsaturated white light to produce
one of the 3 additive primaries red, green or blue. Using the color horseshoe
in the previous topic (or on page 245 of the textbook), the subtractive version
of any color can be found by drawing a straight line from the initial color
through the white point to the opposite site of the horseshoe, since the two
colors at the opposite ends of this line will combine to form white.
Substances that generate the
subtractive primaries function by absorption they remove some part of the
wavelength spectrum of the incident light so that what remains has some
specified hue. Based on the definitions above, each one of the subtractive
primary colors could be produced by passing unsaturated white light through an
idea filter which completely absorbed one of the primary colors and passed
everything else. Thus, yellow could be
produced by passing white light through a filter which absorbed blue completely
and passed everything else. Thus:
Yellow= Red + Green = White Blue
Cyan= Green + Blue= White Red
Magenta= Red + Blue= White - Green
If two such filters acted in series then the output
is the part of the input that can pass through both filters. Thus passing
unsaturated white light through a yellow filter absorbs the blue and lets
everything else through. We could think of the result as a combination of only
red and green, since the blue is gone. If the result is then passed through a
cyan filter (which passes only blue and green)
then the red will be absorbed leaving mostly green. Likewise, a magenta
filter absorbs the green from white light, leaving red and blue. If the result
is then passed through a cyan filter, the red will be absorbed and only the
blue will be left.
The subtractive rules can be derived
from these definitions and from the additive rules of the previous section.
Thus,
Cyan + Yellow= (White Red) + (White
Blue)= White (Red + Blue)= (Red + Blue + Green) (Red + Blue)= Green
Cyan + Magenta= (White Red) + (White
Green)= White (Red + Green) = Blue
Yellow + Magenta= (White Blue) +
(White Green)= White (Blue + Green) = Red
Cyan + Yellow + Magenta= (White Red)
+ (White Blue) + (White Green)= White (Red + Blue + Green)= Black
The
left hand side of each relationship describes the combination of two filters
acting in series on white light. In each case, the filter subtracts its color
from the incident light and passes everything else to the next filter in the
sequence. The output of the combination is the only color that is passed by
both filters.
The subtraction process can be used to
produce the same range of colors that can be produced by the addition process
because the two are really different aspects of the same combination rules for
complementary colors that we discussed in the previous topic. However, the
subtraction process cannot create light when none is present to start with. For
example, if, instead of using white light,
monochromatic blue light is passed through a yellow filter, then the
filter absorbs all of the input and nothing comes out the output is black.
The same effect is true for any filter and its complementary color: if the
filter is illuminated with only the complementary color then the incident light
is totally absorbed and the object looks black. In other words, the perceived
color of some object depends very much on the incident light. In particular, if
the object reflects only colors that are not present in the incident beam then
the object will appear black. In general, the perceived color of the object
will be given by the combination of the colors that it reflects and
which are present in the incident beam.
In general, the color that will pass through several subtractive filters is very hard to guess without a lot of detailed knowledge about the transmission characteristics. For example, a yellow filter might be constructed from a material that passes only 570 nm yellow and absorbs everything else. When white light is incident on this filter, everything but the 570 nm energy is absorbed so that the output light is a pure monochromatic yellow color. If this light is passed through a sharp filter of any other color, then nothing will come out and the filter will absorb all of the incident light and look black as a result. However, a yellow filter might also be constructed from a material that absorbs blue and passes red and green. When white light is incident on this filter, the filter absorbs the blue and reflects the red and green, which look like yellow. However, if this yellow is incident on a red filter (or on a material that reflects red), then the red component will be passed and the object will look red; if it is incident on a green filter (or on a material that reflects green) then the green component will be passed and the material will look green.
As with the
additive mixtures discussed in the previous topic, the perceived color from any
mixture depends on the overall intensity as well as on the fractions of the constituents, so that these
rules, which describe hue based only on fractions cannot tell the whole story. Varying
the intensity of a light source is simple in principle, but complicated in
practice. Although you can change the intensity of a standard light bulb by
changing the input power (using a dimmer switch, for example), reducing the
intensity in this way also decreases the temperature of the filament, which
changes its hue towards the red portion of the spectrum. Changing the intensity
in practice therefore means turning off some of the lights completely or
partially blocking them using shutters or gray, neutral density filters, which
absorb all wavelengths equally.