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.