Color Photography - Multi-Layer Color Processes
( Originally Published 1938 )
KODACHROME.—The physical construction of this material is included under "Motion Pictures in Color." This description will be found on page 203, so that we will not repeat it here.
Kodachrome film comes in two types: Regular, for use primarily with daylight. If this type is used with Photoflood a Kodachrome Filter for Photoflood is used. Type A for Photoflood Lamp without filter requires only about one-fourth of the light required when the regular type is used with the blue filter. Type A Kodachrome Film may also be used by day-light with a Type A Kodachrome Filter for daylight. With this filter, it is about 1% times as fast as Regular Kodachrome Film without a filter.
Of great interest now to the commercial and portrait photographer is the possibility that in the near future these films will be put on the market in larger sizes. Up until now (1938) they have been available only in the 35 mm width rolls. Even in the small size rolls, a great deal of work is being done with miniature cameras.
This film when exposed is returned to the Eastman Kodak Company, who process it into a colored transparency. Such a transparency is not only suitable for viewing or projecting, but on account of its grainless property lends itself easily to the making of color separation negatives, from which in turn prints or transparencies in color in various sizes can be made.
It is this use which we think will appeal to all classes of photographers. We will, therefore, confine our text principally to the making of the separation negatives and printing from them.
EXPOSURE AND LIGHTING.—Kodachrome, like all other color film or plates, has less latitude than black and white. Lighting should be according to the instructions for separation negatives, flat without excessive contrast. Outdoor work is best on clear, cloudless days.
For exposure speed and use of haze filter, one had better follow the instructions that come with the film. On account of the possibility of change in this reSpect, specific data are not given here.
MAKING SEPARATION NEGATIVES FROM KODACHROMES.
There are several methods of making separations from Kodachromes, all of which consist, of course, in making three separate negatives, one for each color record through appropriate filters. By one method the Kodachrome transparency is copied (or photographed) by mounting it in front of a light box. Each filter is slid into position over the lens or back of the Kodachrome, before exposure. Thomas Curtis of Hunting-ton Beach, California, makes an outfit for this purpose which can be secured through your dealer. This camera, known as the Curtis Reproduction Camera, by means of its light source and proper filters, is claimed by its maker to give at once a set of separation negatives suitable for printing by the several methods now available.
Another method recommended by the Eastman Kodak Company, which we will describe, is by the use of an enlarger. A lamp such as the 250-watt G-30 Mazda will give a more constant light than the usual Photoflood type. The Kodak Miniature enlarger is suitable. Its bluish heat-absorbing glass causes the filter factors to be more nearly equal.
A light-box open at the bottom, with one side hinged to lift up while placing the film or plate in position, is a good means for blocking out stray light. In the top of the box a hole is made just large enough to pass the cone of light from the enlarger without cutting off the edges.
The Graflex Enlarg-or Printer is also a good device for making the separation negatives. This needs no box to block out stray light.
The Kodachrome film is placed in the enlarging camera together with a grey scale transparency of about eight steps. The filters needed are Wratten No. 29-Red, No. 61-Green, No. 49-Blue. These are placed in turn over the opening in the top of the box or over the lens to give the Blue, Red and Yellow Printer negatives.
Exposure is determined by trial and error, much in the same way as described for making color separations in our first chapter, until an exposure and developing time is found which will give a balanced wedge. Eastman Safety Supersensitive Panchromatic Antihalation Cut Film is recommended. The red and green filter negatives require equal development ; the blue filter negative, more. Usually five minutes for the red and green and seven and a half for the blue will give a suitable gamma of about 0.70, in a tank of DK-50 developer at 65° F. (18° C.). Exposure should be barely enough to hold the necessary shadow detail. A diffused white highlight should read about 1.5 on a densitometer when a gamma of 0.70 is employed. At higher gammas, greater highlight densities are required.
The thoroughly developed and washed film should be wiped carefully front and back with a viscose sponge to eliminate water spots, which will cause uneven color spots in finished pictures.
The finished negatives are now ready for printing by any of the various printing processes previously described.
MASKING FOR COLOR CORRECTION.—The Eastman Kodak Company, Rochester, New York, have recently printed a leaflet, which will be sent on request, de-scribing a method of making corrections on separation negatives for Kodachromes, which is particularly suitable when the intention is to make prints by their Wash-Off Relief method.
THE NEW AGFACOLOR PROCESS.—The new Agfacolor film which is now available in Europe in the 35 mm size for miniature cameras, and which probably will shortly be introduced in this country by the Agfa Ansco Company, is a multilayer film which contains three silver emulSion layers coated on one base. These layers are arranged in the following order: The bottom layer is the red sensitive, the middle layer the green sensitive, and the top layer the blue sensitive. Each layer is separated by a yellow filter layer which disappears during the developing. The emulsion layers contain no dyestuff, but do contain certain colorless dye-coupling components which in subsequent treatment of the film produce color layers which form the complete transparency.
Briefly, the developing and converting operations consist of:
1. Developing, which forms a black and white negative.
The film is exposed to white light and developed a second time in a developer of the para-phenylenediamine type, the oxidation product of which, coupled with the dye component in each layer, forms the sub-tractive color in proportion to the metallic silver reduced.
3. The film, which now contains both the silver and color image, is bleached in a reducer which removes the silver image and leaves the dry image, which is a color reproduction of the subject.
Naturally, separation negatives can be made from this film and prints or transparencies in turn made by the process described in this book. A paper on this product was read by J. L. Forrest and F. M. Wing of the Agfa Ansco Laboratories at the meeting of the Society of Motion-Picture Engineers, 1937, and re-printed in their Journal, November 3, 1937, pages 248-257.