Submitted by Al …
(Adapted from The Encyclopedia of Photography ©1963)
The production and use of coloured couplers marks a great stride forward in the production of colour negatives from which high-quality colour prints can be made without special colour correction processes. Normally, a coupler is a colourless chemical incorporated in the emulsion of films such as
Ektachrome and Anscochrome, or in the developer of films such as Kodachrome, which forms, on combination with the reaction products of development, a coloured dye. By the use of three different couplers, it is possible to form the three subtractive dyes; magenta, cyan, and yellow, in the proper layers of a colour transparency or colour negative.
Prior to 1947, couplers were generally colourless compounds which formed dyes during processing. It was in this year that Eastman Kodak Research Laboratories announced the successful use of coloured couplers in the production of an integral tripack type of colour negative film. This method of colour correction employs a coupler which is originally coloured, but forms an image of a different colour during processing. Since the colour coupler is spread evenly throughout an emulsion layer during manufacture, and only part of the coupler is used during colour development to a negative, the remaining coloured coupler appears as a coloured positive image, while the dye formed during colour development is a coloured negative image. This residual colour positive image acts as a mask during printing.
The colour accuracy obtainable in printing is rather low unless masking techniques are used. This is due to the fact that all magenta colourants absorb considerable blue light, and cyan colourants absorb blue and green light. None of these absorptions is desirable. The magenta should only absorb the green while the cyan colourant should absorb only the red. When a photograph by blue light is made of a colour transparency or colour negative, not only is the yellow dye image recorded (which absorbs blue, as it should) but also the cyan and magenta images. If the dyes in colour transparencies or colour negatives were ideal dyes, only the yellow dye would be recorded on the blue exposure. Ideal dyes would thus only have density (appear dark) to light of the complementary colour.
REAL DYES VERSUS IDEAL DYES
The difference between a set of real dyes and a set of ideal dyes in a colour transparency or colour negative is readily seen by an examination of the schematic tripack colour film shown in Figure 1 below.
The extra undesirable dye absorptions of real dyes must be eliminated by masks if only one layer of the colour film is to be recorded on each one of a set of colour separations. Various techniques of colour correction attempt to obtain this result by the proper use of masks during the printing of the tripack colour film. The masks are generally composed of silver images obtained by exposing the mask emulsion to light of the appropriate colour and developing the image. This procedure is somewhat lengthy and involved. Coloured couplers offer a way of obtaining the same effect very simply without any extra operations on the part of the photographer or printer. The use of coloured masks is restricted to colour negatives, since they cannot be used in a reversal positive transparency to be examined visually without leading to incorrect colour reproduction.
Coloured couplers make it possible to obtain essentially the same result when reproducing a colour negative incorporating coloured couplers as would be possible in reproducing a colour negative composed of ideal dyes. For example, consider single emulsion layer of a colour negative that has been sensitized to red. After colour development, a negative image of cyan dye would exist in this layer. When printing from such an image, we wish to record it only on a red light exposure, or in the red-sensitive layer of a printing material. All cyans will be so recorded by red light. Unfortunately, the cyan will also be weakly recorded with either a blue or green exposure. This defect of cyan dyes cannot at present be eliminated, although it can be overcome or neutralized by masking. The coloured coupler supplies the proper masking automatically.
If we have a cyan-forming coupler that is red before colour development (a coloured coupler), we have a means of overcoming the deficiencies of the cyan image to be developed. As this red coloured coupler combines with the reaction products of development, a cyan negative image is formed. If the coloured coupler in any one area is used up, that area will become all cyan. Wherever colour development does not occur (emulsion not exposed), the red coloured coupler will remain. The result of this operation is that a negative cyan dye image is formed while the unused coloured coupler remaining will appear as a positive red image. See Figure 2 below.
The visual appearance of such an emulsion is an even red before development, while after development to a negative the dark areas will be cyan, the light areas red, with appropriate mixtures of red and cyan between these densities. Thus, the coloured coupler not only produces the usual cyan negative image, but also a residual red positive image. This red positive image acts as a mask when the material is printed.
A red colourant absorbs blue and green light and transmits red light. Blue and green are the very colours that cyan dyes also absorb to some extent, and this absorption is not desirable. If just the right amount of blue and green absorption can be supplied by a mask to balance the undesired absorption of the blue and green light by the cyan dye, then the combination of mask plus cyan will have the same absorption of blue and green at every point of density. Thus, the cyan dye will not be recorded when photographed by blue or green light. The red positive remaining after colour development of a red coloured coupler to a cyan negative will do exactly what is required. In Figure 3 below, it may be observed that the cyan and blue absorptions of the cyan negative dye image and the red positive dye image added together at each density or absorption at every point.
Only the red density varies, and this changes, as it should, proportionately to the quantity of cyan dye present in the negative. In effect, the combination of cyan negative and red positive act in almost exactly the same way as a perfect cyan dye would act, if such a dye were available; the only difference is that the added red acts to increase the printing exposure somewhat.
DEFECT OF MAGENTA DYES
The chief defect of magenta dyes is that they absorb blue light. If a coloured coupler can be found that will produce a magenta dye upon colour development and yet be yellow before development, the same type of masking correction can be supplied for that magenta image that has already been described for the cyan dye. At each point of the picture, a constant quantity of blue light absorption will be present. Where there is a great deal of magenta dye, the blue absorption will be due to this dye. Where little magenta dye is present, the small blue absorption of this dye will have added to it the blue absorption of an appreciable quantity of unused yellow coloured coupler. This constant blue absorption of blue light throughout the pictures allows the deficiencies of the magenta dye to be counteracted and the result obtained is essentially the same as would occur were the magenta dye perfect; that is, if it absorbed only green light.
YELLOW DYES
Most yellow dyes used in dye coupling processes have relatively satisfactory characteristics. They absorb blue light effectively and yet absorb very little red or green light. Yellow dyes thus approach the ideal so closely that coloured couplers are not generally helpful enough to warrant their use.
TO SUM UP
A colour negative material can thus be constructed which will contain two coloured couplers and one uncoloured coupler, and give most satisfactory colour reproduction.
The bottom layer of such a tripack consists of a red-recording emulsion containing a red coloured coupler that, during development, will form a cyan negative image.
The central emulsion layer of the tripack is green-sensitive, and contains a yellow colored coupler that forms a magenta negative when colour-developed.
The upper blue-sensitive emulsion contains a colourless coupler that forms a yellow negative image when colour-developed.
Such a processed film can be printed, and the three dye images are recorded separately on each layer of a colour printing material without any tendency for any one image to register in a layer where it is undesirable. This means that a colour print from such a negative will be cleaner and brighter in colour, and more accurate in rendition, than one made from an uncorrected negative.
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