Evening star., 1950-08-27
Camera Angles
The Bosic Theory of Color Photography

filters and you will see that the
green is absorbed by the magenta
filter and the red is taken out
by the cyan filter leaving the
blue-violet only. An overlapping
of the magenta and yellow filters
will result in the absorption Of
the blue-violet by the yellow filter
and the magenta will absorb the
green, leaving only the red rays.
Thus we see that each compile
mentary color absorbs or SUBJ
tracts" one primary color. Hencs
the derivation of the term Sub
tractive process" If two compile.
mentary color filters are super
imposed two primary colors will
be removed and superimposition
of three complementary color
filters will subtract or remove all
the light.
This theory is translated into
practical usage when we consider
the makeup of one of the present
day color films-Kodachrome. It
employs the subtractive method
and the film is so constructed
that only one film is needed by
the photographer instead of the
three employed in the additive
process. This is accomplished by
superimposing three images on
one film. This is the result of hav-
ing three separate emulsions
coated, one over the other, on the
one film base. The layers of emul-
sion are separated by thin layers
of gelatin.
Color Recorded In Layers.
First, as usual, is the film base.
This is coated with an emulsion
which is principally sensitive to
blue-violet and red light. A coat
OF clear gelatin follows. Then an
other layer of emulsion, largely
sensitive to green and blue-violet.
is added. This. in turn, is covered
with another layer of yellow-dyed
gelatin. The last coat Of emulsion
Is sensitive to blue-violet. Thus q
selective sensitivity is achieved
to red. green and blue light rays
by means of the yellow gelatin
filter.
Here is how it works out. When
the light rejecting from the SUBJ
Ject is directed upon the film, the
blue and violet colors are recorded
on the top layer of emulsion. This
emulsion is not sensitive to either
red or green so these colors pass
on. The yellow gelatin filter abb
serbs and blocks the blue-violet
but allows the other two colors to
pass to the next emulsion layer.
This layer is sensitive to green
and so the green light reflected
from the subject is recorded here,
But the middle layer is not SEN
sltive to red so that light con-
tlnues on its way to the bottom
emulsion layer where it is re.
corded. This layer remains un-
affected by the green light to.
which it is inSensitive and, you
will remember. the blue light was
intercepted by the yellow gelatin
filter. Thus the three primary
colors are registered in successive
layers of emulsion of the same
film. *,
! In the hands of the professor.
this triple layer Aim is first de.
veloped to a negative image and
then using dye-coupler develop-
ers (Which will be explained in
another article), each layer is de-
veloped, after second exposure
to colored light, to it's compli-
mentary color. All of the silver
is removed by bleaching process.
Thus the three layers of film, each
dyed in a complimentary color,
can selectively absorb portions of
the light which is used for pro-
lection or transmission and you
see the recorded image in its
original colors.