In order to reproduce natural colors by confering spectral sensitivity on a silver halide photographic emulsion, various methine dyes, such as a cyanine dye, a merocyanine dye, etc., are employed individually, as a mixture, or in combination with a supersensitizer.
Useful cyanine dyes are classified as monomethine dyes, trimethine dyes (carbocyanine dyes), pentamethine dyes (dicarbocyanine dyes) or so on according to the length of thier methine chains, and they are employed as spectral sensitizing dyes for a blue-sensitive layer, a green-sensitive layer or a red-sensitive layer according to wavelengths at which they have their own spectral absorption.
Moreover, some of these dyes, e.g., a monomethine dye and a carbocyanine dye, are known to be used as a mixture (e.g., Japanese Patent Publication No. 1762/73, Japanese Patent Application (OPI) Nos. 28826/75, 14313/76 and 153926/83 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application")
In silver halide photographic materials for color photographic printing paper, substantially iodide-free silver halides, such as silver chloride, silver bromide, silver chlorobromide and the like, are preferably employed as silver halide to constitute a lightsensitive emulsion layer. One of the reasons for the preference is that silver halides of the above-described kind can provide contrasty silver halide emulsions capable of producing images of high contrast by using the possible small amount of silver. A deeper reason is that such silver halides can confer a high developing speed and excellent processability on silver halide emulsions. In addition, substantially iodide-free silver halide emulsions are known to have a high desilvering speed in the bleach-fix processing subsequent to the color development processing, and, therefore, to be suitable for color photographic printing paper.
However, silver chlorobromide emulsions, though possessed of excellent properties as described above, tend to cause changes in properties when preserved as finished goods, particularly an increase in fog upon long term preservation. This tendency is observed prominently in silver chlorobromide emulsions having a chloride content raised in order to increase a developing speed, which is a serious problem from the standpoint of supplying products having stable properties. At present, it is an important subject to solve this problem.
As a means for making improvement in keeping quality, several methods have been proposed up to the present. For instance, it is well known that the addition of compounds which have so far been employed as so-called stabilizers, such as azoles, mercapto compounds, azaindenes, etc., brings about the improvement in keeping quality, and examples thereof are described in detail in E. J. Birr, Stabilization of Photographic Silver Halide Emulsions, Focal Press (1974).
As the results of our studies, however, it has been found that in general the above-described azoles, mercapto compounds and azaindenes, which have been employd as a stabilizer, are compounds of the kind which are strongly adsorbed by silver halide grains and, therefore, they can produce marked effects on stabilization of photographic properties and prevention of fog, while when they are used in an increased amount, it happens that they have undesirable side effects also, e.g., desensitization and decrease in contrast, which are caused by a hindrance to the adsorption of the foregoing additives like spectral sensitizing dyes to silver halides, inhibition of desilvering upon bleach-fix processing, and so on. In particular, when used with the intention of preventing silver chlorobromide emulsion from increasing fog upon preservation, the foregoing compounds cannot function as effective means in many cases because of the restriction as described above.