Color-forming coupler compounds which react during photographic development with the development product of aromatic amino developing agents to form color images are well known. Generally, these color-forming compounds are colorless or substantially colorless. This lack of color is usually desirable when the coupler is to be incorporated in the emulsion layer and the unused coupler remains after formation of the colored image.
UNFORTUNATELY, THE ABSORPTION SPECTRA OF CONVENTIONAL DYES FORMED FROM COLOR-FORMING COUPLER MATERIALS ARE NEVER "CLEAN". Thus, invariably, cyan dyes exhibit various degrees of unwanted absorption in the green and blue regions of the spectrum, while magenta dyes exhibit unwanted absorption in the blue region and to some degree also in the red region of the spectrum. The term "unwanted absorption" is intended herein to mean, with respect to a cyan dye, measurable absorption in the green and blue regions of the visible spectrum. Unwanted absorption is illustrated in FIG. 1, as that portion of the curve above the base line at wavelengths lower than about 600 millimicrons.
Whereas the human eye and brain have the capability of automatically compensating for a certain amount of such unwanted absorption in positive color photographs and transparencies which contain such conventional dyes, machines and equipment for reproducing photographs and transparencies do not have such compensating capability. Hence, photographic materials which must be mechanically reproduced, for example, from color negatives or positive transparencies, must be color-corrected so as to largely overcome the effects of unwanted absorption. Color correction becomes increasingly necessary when it is desired to reproduce the color element by means of a series of negative-positive-negative-positive steps such as those usually used in the preparation of (from valuable "master" films) motion picture films for release.
One very useful method of compensating for the unwanted absorption caused by a dye derived from a given dye-forming coupler involves the inclusion of a special colored coupler into the photographic element (in addition to the image-forming coupler that is to be "color-corrected"). The colored coupler absorbs both green and blue light, in the case of correcting for unwanted absorption in a cyan dye. This colored coupler theoretically is capable of reacting with oxidized color developer (during the color development processing step) to yield the usable cyan image dye while simultaneously losing its ability, in proportion to development, to absorb in the green and blue regions of the spectrum, thereby "correcting" for the unwanted green and blue absorption of the dye derived from the major cyan dye-forming coupler in the photographic element. (See Chapter 13, "Masking and Coloured Couplers" of R. W. G. Hunt's book, The Reproduction of Colour, pages 233, 263, published by John Wiley & Sons, 1967).
Couplers which are in themselves more or less strongly colored by virtue of containing a chromophore group which is split off or destroyed during and by means of the coupling reaction with the result that the original color of the coupler is destroyed and a new dye is formed upon coupling are disclosed in U.S. Pat. Nos. 2,453,661, Nov. 9, 1948; 2,445,169, Nov. 30, 1948; 2,455,170, Nov. 30, 1948; 2,521,908, Sept. 12, 1950; and 2,706,684, Apr. 19, 1955.
Upon development of an emulsion layer containing one of these colored couplers, the original color of the colored coupler is destroyed and a new color is formed by the coupling reaction at those points where development occurs. There is formed in this way a composite dye image consisting of the new dye and the residual colored coupler. The theory of color correction requires that the sum of the absorption of the residual colored coupler and the undesired absorption of the image dye should be as constant as possible. Conventional color correction to date has resulted either in non-ideal (non-uniform) correction, or uniform correction with somewhat higher Dmin than is desirable. (The higher Dmin apparently results from the necessity to use relatively large amounts of certain color-correcting couplers which have relatively low activity.) Another shortcoming that exists with respect to certain of the conventional color-correcting couplers that otherwise exhibit good correction relates to their color prior to their reaction with oxidized color developer. The peak absorption of such colored couplers was often too hypsochromic, by several wavelengths, and thus yielded corrections which were somewhat less than desirable.
The use of certain prior art color-correcting couplers, such as those described in the Examples, below, yield excellent dyes, but are not capable of yielding "ideal" or "uniform" color-correction,. This fact is illustrated by FIGS. 3 and 4. Note that substantially uniform absorption in the green and blue regions (particularly in the green) is not achieved by the materials used in the preparation of FIGS. 3 and 4. Ideal color correction should result in perfectly level "G" and "B" curves in the Figures, such as those marked "ideal" in FIGS. 3 and 4.
Uniform color-correction throughout the development of the color emulsion has been extremely difficult to achieve because of the apparent requirement of uniformity or similarity in each of the many different chemical and physical parameters involved in the color-correcting process. For example, the reaction rates and relative reactivities of each of the couplers in any "color correction" combination must be matched at every stage of the development step, even though the concentration of developer, pH, concentration of reaction by-products and many other aspects of the chemical color development process not only differ widely from one photo-finisher to another, but also differ widely during any single color development operation.