(1) What a filter does.
To effectively apply filtration techniques
to the photographic process, remember this rule: a filter transmits its own
color(s) and absorbs all others. Filters selectively transmit portions of
A red filter, for instance, appears red
because it transmits the red portion of the visible spectrum while absorbing
the blue and green portions.
(2) Additive primary colors.
All colors can be created by mixing
different amounts of red, green, and blue light.
The eye sees color when
mixtures of the visible radiations stimulate the receptors.
Because red,
green, or blue cannot be reproduced by mixing the other two colors, we call
these primary colors.
Since matching a wide range of colors involves the
addition of colored light, we further refer to the primary colors as
additive primary colors.
When we use mixtures of these colors to produce
color images, we are using the additive color process.
b. Additive Process.
The first color images were produced using the
additive process.
Around 1861, James Clerk Maxwell presented the first
known demonstration of color photography.
Maxwell made three exposures through a filter that transmitted the primary
colors of light.
The resulting black and white negative represented the
parts of the subject that were red, green, and blue. Next, a positive was
made from each black and white negative. Then the positives were projected
through the same color filter used to make the negative.
Using separate
light sources, the positive made from the red negative was projected through
a red filter, the green positive through a green filter, and the blue
positive through a blue filter.
When the three images were superimposed,
they formed a color reproduction of the original subject.
(1) Example of the additive process.
The additive system might be
better explained if we use the following example. Suppose that we use three
separate projectors and project three beams of light, each a different
additive color, onto a screen.
If we arrange the beams of light so that
they partially overlap, we produce white light. By adding red, green, and
blue light, we have combined all the necessary wavelengths to produce the
visual sensation of white light. Figure 1-5 illustrates this example.
(a) When we arrange the lights used in the above example so that
there are areas where only two of the additive colors are combined, we
produce three new colors of light. Where the red and green light overlap,
they combine to make yellow light. The red and blue light would combine
to make magenta light, and the blue and green light would combine to
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