The present invention relates to a silver halide emulsion having such characteristic as high sensitivity and high contrast, which are suitable for use for shooting.  The present invention also relates to a silver halide color photographic light-sensitive material using the emulsion.  Further, the present invention relates to a simple and rapid method for forming a color image by using the light-sensitive material. 
Further, the present invention relates to a silver halide emulsion having high sensitivity and excellent characteristics exhibiting little change of gradation upon exposure to high-intensity illumination.  The present invention also relates to a silver halide color photographic light-sensitive material using the emulsion.  Further, the present invention relates to a simple and rapid color-image-forming process using the light-sensitive material. 
Owing to remarkable development of photographic light-sensitive materials utilizing silver halides, high-quality color images are now easily available.  For example, ordinarily, according to so-called color photography, color prints are obtained by taking a photograph utilizing a color negative film, processing the film, and optically printing the image information that is recorded in the processed color negative film onto color printing paper.  In recent years, this process has made remarkable progress, and large-scale, centralized color laboratories, in which a large quantity of color prints are produced high-efficiently, and the so-called mini labs, which are installed in shops and are designed to use compact and simple printer-processors, have spread widely.  Therefore, anyone can enjoy color photography easily. 
In addition, recently, a new-concept APS system, which uses a color negative film capable of recording various information as magnetic records by utilizing a support coated with a magnetic material, has been introduced into the market.  This system proposes simplicity in handling films and photographic pleasure, such as capability to change the print size by recording information at the time photographs are taken.  In addition, this system proposes a tool for compiling or processing images by reading out image information from a processed negative film by means of a simple scanner.  Such methods enable high-quality image information of silver salt photographs to be digitized easily, and they are making the use of the image information commonplace beyond the traditional scope of enjoyment as photographs. 
The color photography, now in common use, reproduces color by the subtractive color process.  Generally, a color negative film comprises a transparent support and light-sensitive layers formed thereon utilizing silver halide emulsions as light-sensitive elements rendered sensitive to blue, green or red regions, respectively, and containing so-called color couplers capable of producing a yellow, magenta or cyan dye having a complementary hue in each light-sensitive layer.  A color negative film image-wise exposed during photographing is processed in a color developing solution containing an aromatic primary amine developing agent.  At this time, the developing agent develops, i.e., reduces, the exposed silver halide grains, and the oxidized product of the developing agent, which is formed concurrently with the foregoing reduction, undergoes a coupling reaction with the color coupler to form dyes.  The metal silver (developed silver) generated by the development, and the unreacted silver halides, are removed through a bleaching process and a fixing process, respectively.  As a result, a dye image is obtained.  Subsequently, color photographic printing paper, which is a color light-sensitive material comprising a reflective support and light-sensitive layers coated thereon having a combination of light-sensitive wavelength regions and hues to be produced in each layer similar to that of the color negative film, is optically exposed to light through the processed color negative film.  Then, the resultant paper is subjected to the color developing, bleaching and fixing processes, as in the case of the negative film, to obtain a color print having a color image composed of dye images, so that an original scene is reproduced. 
In contrast with these classic image forming processes, recently it has been made possible to convert the image information recorded in a color negative to digital information, using a scanner, and to subject the digital information to various image treatments, so that the image quality of prints to be obtained is upgraded.  Actually, a mini lab system having this technology has been made public. 
Under this situation, as to the image forming process of color negatives, there is a growing demand for a simpler system. 
On the other hand, so-called digital still cameras utilizing a CCD as an imaging element are making rapid progress.  Cameras for amateurs which are mounted with a CCD element having millions or more of pixels have been put on the market for the past several years to obtain image qualities close to those of photographs.  These digital still cameras save a step of developing the film taken, in contrast to usual color photographic systems, and they can produce directly digitized image information.  Therefore, it can be made easy to confirm the image directly on a liquid crystal monitor when taking a photograph and to make use of the resulting digital information variously.  The image information can be transferred to a printer to make a print readily, it can be variously processed using a personal computer, and it makes image transfer through an internet easy.  Along with recent progresses in high density CCDs and in the abilities of equipment treating massive digital data, high quality images worth being appreciated as a photograph have come to be available.  Discussion has been made on the possibilities of these digital still cameras being substituted for general photographing means. 
In this situation, it is desired to further investigate the high sensitivity and high latitude possessed by silver halide light-sensitive materials with the view of further developing a silver salt photographic system in opposition to a digital still camera system.  Although the performance of CCDs used as imaging elements of a digital still camera have been improved remarkably, there is a limitation on the provision of high sensitivity while increasing pixels in elements having a limited size.  Also, it is basically difficult to impart high latitude under the restrictions imposed on an inexpensive and simple camera system.  Hence, if silver halide light-sensitive materials with high sensitivity and latitude are attained and mounted on inexpensive and readily handlable products, e.g., films with a lens, a system attractive to customers will be provided. 
In the meanwhile, it is an urgent problem to make it possible to carry out the developing step, which is a weak point of the silver halide light-sensitive material, more easily and rapidly.  The strength of the digital still camera lies, after all, in the point that liquid development processing is not required.  On the contrary, the development processing of the silver halide light-sensitive material needs private treating equipment and careful control and is hence utilized only in limited bases at present.  This reason is as follows.  The first reason for this is that expertise and skilled operation are necessary due to the requirement of strict control of the composition and the temperature of the solutions in processing baths for the above-mentioned procedure of color development, bleaching and fixation.  The second reason is that equipment to be used exclusively for the developing process is often required, due to substances, contained in the processing solutions, such as color-developing agents and bleaching agents comprising an iron chelate compound and others the discharge of which is regulated from the standpoint of environmental protection.  The third reason is that the currently available systems do not satisfactorily fulfill the requirement for rapid reproduction of recorded images, because the above-mentioned development processes still take time, although this time has been shortened with recent advances in technology. 
Based on the background stated above, a requirement for a technology, which will lessen the load on the environment and contribute to the simplification of the system by establishing a color image formation system without the use of the color developing agents or bleaching agents now in use in current systems, is ever increasing. 
In view of these aspects, many improved technologies have been proposed.  For example, IS and T""s 48th Annual Conference Proceedings, pp.180, disclose a system in which the dye formed in the developing reaction is transferred to a mordant layer and thereafter a light-sensitive material is stripped off, to remove the developed silver and unreacted silver halide without the use of a bleach-fixing bath which has been indispensable to conventional color photographic processing.  However, this technology cannot perfectly solve environmental problems because a developing process using a processing bath containing a color developing agent is still necessary. 
Fuji Photo Film Co., Ltd. has provided Pictrography system which dispenses with a processing solution containing a color developing agent.  In this system, a small amount of water is supplied to a light-sensitive material containing a base precursor, and then the light-sensitive material and an image receiving material are put together face to face and heated, to cause the developing reaction.  This system does not use the aforementioned processing bath and, in this regard, is advantageous with respect to environmental protection.  However, since this system is used in the application where the formed dye is fixed in the dye fixing layer which is then appreciated as a dye image, there has been a demand for a system usable as a recording material for photographing. 
In particular, due to a digital lab system which has rapidly developed recently, there has been an increasing need for a system or recording medium which digitizes photographed image information in a simple and rapid way.  It is believed that, for example, in Digital Lab System Frontier, manufactured by Fuji Photo Film Co., Ltd. (input machine xe2x80x9cHigh-Speed Scanner/Image Processing Work Stationxe2x80x9d Scanner and Image Processor SP-1000 and output machine xe2x80x9cLaser Printer/Paper Processorxe2x80x9d Laser Processor LP-100P), the performance of the system will be enhanced if the photographic negative as input information is processed more simply and rapidly. 
In order to meet such demands, a heat development light-sensitive material system in which the light-sensitive material incorporates a developing agent has been proposed as a photographic negative which can be processed simply and rapidly without placing a heavy load on the environment.  For example, techniques in which photographic light-sensitive materials can be developed by the same simple and rapid processing as in the aforementioned Pictrography system are disclosed in the specifications of JP-A-9-204031 (xe2x80x9cJP-Axe2x80x9d means unexamined published Japanese patent application) and JP-A-9-274295. 
Since this system is used for photographing, the emulsion to be used needs to have a further upgraded sensitivity.  In addition, high-level requirements have been made for the betterment of sensitivity/granularity ratios, sharpness, gradation, and the like. 
A technology for upgrading the sensitivity of a silver halide emulsion is the use of tabular grains.  Advantages of this technology are known to be upgraded sensitivity including the enhancement of spectral sensitizing efficiency by spectral sensitizing dyes, betterment of sensitivity/granularity ratios, enhancement of sharpness owing to the optical properties specific to the tabular grains, enhancement of covering power, and the like. 
The technologies using tabular grain emulsions in a heat development light-sensitive material system in which the light-sensitive material incorporates a developing agent are disclosed in, for example, JP-A-9-274295 and JP-A-10-62932.  However, in these patent applications, no mention is made of the technology of the present invention using a silver halide tabular grain emulsion containing a metal complex having, as a ligand, an organic compound such as a heterocyclic compound in a number more than half of the coordination number of the metal atom. 
Meanwhile, in view of the above-described points, the use of an emulsion containing tabular grains in a heat development system silver halide color photographic light-sensitive material incorporating a color-developing agent, which material is a material for shooting and enables simple and rapid image recording without placing a heavy load on the environment, has been found to present a practically intolerable problem that, when exposed to high-intensity illumination, a change of gradation (softening of tone) tends to occur at the time of heat development in comparison with ordinary development using a conventional developing solution, and the problem is remarkably exasperated particularly when tabular grains each having a large average equivalent-circle diameter (the diameter of a circle equivalent to a projected area of individual grain) are used. 
High sensitization of the silver halide light-sensitive material can be generally attained by increasing the grain size of silver halide grains used as photocells (photosensors).  However, this poses the problem of impaired granularity (graininess) as the grain size increases.  As measures to increase the sensitivity without impairing the granularity, the use of an emulsion comprising tabular grains with a grain thickness smaller for the projected diameter of a grain (the diameter of a circle equivalent to the projected area of a grain) is disclosed in, for instance, the specifications of U.S. Pat.  Nos.  4,434,226 and 4,439,520.  In the descriptions of photographic emulsion grains, the value calculated by dividing the projected diameter of a grain by the thickness of the grain, which value is called as an aspect ratio, is used.  These specifications describe the fact that grains with a high aspect ratio exhibit better sensitivity/graininess ratio than those having low aspect ratios.  In the case of comparing grains having the same grain projected diameter, it is considered that by increasing the aspect ratio, the number of grains can be increased, whereby the granularity can be improved even if the amount of silver to be applied is the same. 
However, it has been clarified that if the aspect ratio of grains is increased and the thickness of the grain is designed to be thin, it is hard to obtain high sensitivity and a deterioration of the contrast is further caused by a reduction in the maximum color density. 
Such a phenomenon, although the way of its appearance differs depending upon the composition and size of emulsion grains, generally starts to appear as a problem when the thickness of a grain is 0.2 xcexcm or less and becomes significant when the thickness of a grain is 0.15 xcexcm or less.  Various techniques have been reported as attempts to solve this problem.  Examples of these techniques may include a technique in which an epitaxial microcrystalline portion having a different halogen composition is formed on the external surface of a grain, especially at the top thereof or such a portion is doped with a 6-cyano iron group complex, as disclosed in the specifications of U.S. Pat.  Nos.  5,536,632 and 5,576,168.  However, it has been confirmed that the use of these techniques is insufficient although an improvement in the sensitivity is observed and a reduction in the contrast is not improved occasionally. 
It has been also confirmed that in a thermal developing treatment as disclosed in the above mentioned JP-A-9-204031 and JP-A-9-274295, in which a photographic light-sensitive material is made to contain a developing agent, overlapped on a processing material containing a basic precursor in the presence of a small amount of water and heated at 60xc2x0 C. or higher, the aforementioned problem offered when the tabular grains having high aspect ratio is used, particularly a reduction in the contrast, becomes more significant. 
As is apparent from the fact mentioned above, an object of the present invention is to provide a silver halide photographic emulsion which produces high contrast and better granularity while it has high sensitivity.  Another object of the present invention is to provide a photographic light-sensitive material of high image quality, using the silver halide photographic emulsion.  Still another object of the present invention is to provide a simple method for forming a color image by using the light-sensitive material. 
Further, another object of the present invention is to provide a silver halide photographic emulsion having high sensitivity and excellent photographic characteristics exhibiting little change of gradation upon exposure to high-intensity illumination.  Still another object of the present invention is to provide a silver halide color photographic light-sensitive material using the emulsion.  A further object of the present invention is to provide a color image forming process which uses the light-sensitive material and which is simple and rapid and places little load on the environment. 
Other and further objects, features, and advantages of the invention will appear more fully from the following description, taken in connection with the accompanying drawing.