The basic problems in color separation result in making the switch from the additive system (light color) to the subractive system (pigment color), and in maintaining an accurate color balance. This is most easily done with a copy camera, or in the field with an immobile tripod, by making three separate exposures through each of the following filters onto panchromatic film: #25 red (yields cyan printer), #61 green (yields magenta printer), #47B blue (yields yellow printer). The results a can be enlarged if need be to the necessary state for adaption to whatever process they will be used for.
To make separations from slides it is highly advisable to shoot a frame including the grey scale within the lighting conditions that the images to be separated will be photographed.
Materials for this process should include:
Place the slide with the grey scale in it into the enlarger and test strip for the whole grey scale only, onto the Super XX. Develop it in the DK-50, diluted 1:1, at 68 degrees for 7 minutes. Develop face up carefully as the film is easily scratched; make sure it stays completely beneath the surface of the chemical and agitate constantly. Stop, fix as for any film, wash and dry.
Inspect the test-stripped grey scale versus the original grey scale to find the best match in exposure which holds differentiation between all zones, and choose the proper exposure time. NOTE: When test stripping, it is best to make short exposures with the aperture set at f/11prf/16 as the film is relatively fast and also you will have to open up several stops later to account for the filter factors.
When you have determined the correct exposure, do not change the enlargement size of the image or you will have to re-determine it by test stripping again. Replace the grey scale slide with the image you wish to separate (that image having been shot in the same lighting conditions as your grey scale slide).
Place one of the filters somewhere between the light source and the projection, and add the appropriate amount of exposure (below) to the test strip time to account for the filter density factor. Somehow mark (clipping none, one, or two corners) the unexposed film to represent the filtration of each transparency as it will be hard to determine later. Expose the film and store it in darkness while you repeat the same procedure for each to the other two filters, adjusting the exposure according to the following chart, and marking the film appropriately each time.
|Filter||Yields Transparency For||Factor||Addtnl. Exposure|
|#29 red||cyan printing pigment||8||3 stops|
|#61 green||magenta printing pigment||12||3 2/3 stops|
|#47B blue||blue yellow printing pigment||16||4 stops|
As you can see, the red filter allows only red light to pass, and thus expose and darken the film: the blue and green light is absorbed by the red filter and is recorded as a clear area on the film. So, when the transparency is contact printed using cyan pigment (blue-green) it should be a fairly accurate representation of where the red in the image did not exist as well as where some of the blue and green did exist. Like wise with the rest of the filtration and printing, until a realistic full-color image is produced.
But we're getting ahead of ourselves. First, having properly marked and exposed all three of the transparencies, we must develop them.
The transparencies resulting from the #29 red and #61 green filtration should be developed in DK-50, 1:1, 68 degrees, for 7 minutes, and the #47B blue transparency should be developed under the same conditions, but for 10 minutes. The development of all three can be done at the same time if you're good enough to keep up with it, however I recommend that you process them one at a time so as to avoid accidents, scratches, and waste of time and materials. Stop, fix, wash, and dry as normal for film.
Registration is a problem which can be solved in the exposure stage by simply using a registration board as your easel and pre-punching the film. Later, however, it can be solved by carefully registering by eye on a light table all three transparencies and making pin-holes which can then also be made into the printing surface so that each transparency can be printed in good registration. For gum printing or etching I highly recommend that you pre-shrink and size your paper, otherwise there will be no hope of maintaining registration.
When printing the transparencies print the yellow printer first, then the magenta, and last, the cyan. In gum printing, as it seems difficult or impossible to get process color water color pigments, I recommend substituting a half-and-half combination of chrome and lemon yellow for the yellow pigment, Windsor Newton Red or Permanent Pigment Alizarin Crimson for the magenta pigment, and Windsor Newton Blue or Permanent Pigment Pthalo Blue for the cyan pigment. These colors seem to be reasonably close to the process colors, and more important, produce a fairly good color balance. Care should be taken not to over or under expose and/or wash the successive applications or the balance will be thrown off. Also extreme gentleness in the washing seems advisable as well as the use of a fine grade of paper.