incident on the surface of the green image. The green image will absorb the
red light and result in a lack of visible radiation transition to the eye.
Remember, any two additive primary filters over a single light source will
result in no light being transmitted.
(2) Capabilities of filters. The previous illustration assumes that
the intensity of the light source does not exceed the absorption
capabilities of the filters used.
In theory, a primary filter will only
transmit wavelengths of its own color.
In reality, filters are only
partially selective.
For example, yellow, magenta, and cyan filters are
used extensively in color photography.
While the yellow filter functions
efficiently enough, there are deficiencies in the dyes used to construct
magenta and cyan filters.
(a) Current technology is unable to produce completely efficient
magenta and cyan dyes.
These dyes absorb some radiant energy they should
transmit.
Due to these deficiencies, filters that absorb all the
wavelengths for a given color are presently unavailable.
(b) Another factor to be considered is the expense involved in
producing highly exclusive filters.
The more selective the filter, the
higher the production cost. It is not necessary to use the most expensive
filters in most color photographic tasks. The filters currently in use are
adequately selective in transmission and absorption characteristics.
However, you should be aware of the discrepancies in theoretical results and
those achieved in practical application.
c. Subtractive Process.
Most color processes use some application of
the subtractive principle to reproduce colors.
The increased transmission
capabilities of yellow, magenta, and cyan make them more suitable for making
color transparencies and color prints.
Subtractive filters may be used individually or in pairs to produce the
entire spectrum of colors.
Two subtractive primary filters superimposed
over a single light source will transmit the color they have in common.
Yellow (red-green) and magenta (blue-red) superimposed will pass red light.
It is also evident that any one subtractive color will, since it is composed
of two additive colors, transmit two-thirds of the visible spectrum and
absorb one-third. Each color subtracts one additive primary, hence the term
"subtractive primary." Yellow subtracts blue, magenta subtracts green, and
cyan subtracts red. By using various amounts of yellow, magenta, and cyan
dye in the color emulsion, the subtractive process controls the amount of
blue, green, and red light reaching the eye, thereby controlling the color
of the image reproduced. See figure 1-7.
1-11
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