A photographic process using silver halide has heretofore been most widely used because it is superior to other photographic processes such as electrophotography and diazo photography in photographic characteristics such as sensitivity and control of gradation. In recent years, a technique for forming images rapidly and in a simplified manner has been developed, in which a light-sensitive material containing silver halide is not wet-developed with a developer, for example, but is dry-developed, for example, by heating.
A heat-developable light-sensitive material is known in the art and is described in, for example, Shashin Kogaku no Kiso (Fundamentals of Photographic Engineering), pp. 553-555 (Corona Co., Ltd., Tokyo, 1979), Eizo Joho (Image Information), April 1978, p. 40, Nebletts Handbook of Photography and Reprography, pp. 32-33 (7th Ed., Van Nostrand Reinhold Company, 1977), U.S. Pat. Nos. 3,152,904, 3,301,678, 3,392,020, 3,457,075, British Pat. Nos. 1,131,108, 1,167,777, and Research Disclosure, June, 1978, pp. 9-15 (RD-17029).
Many methods have been developed to form color images, such as by combining an oxidation product of a developer and a coupler. In connection with this method, U.S. Pat. No. 3,531,286 discloses a combination of a p-phenylenediamine reducing agent and a phenolic or active methylene coupler, U.S. Pat. No. 3,761,270 discloses a p-aminophenol-based reducing agent, Belgian Pat. No. 802,519 and Research Disclosure, Sept. 31, 1975, p. 32, disclose a sulfonamidophenol-based reducing agent, and U.S. Pat. No. 4,021,240 discloses a combination of a sulfonamidophenol-based reducing agent and a 4-equivalent coupler.
Another method forms positive color images by the light-sensitive silver/dye bleaching process. In connection with this method, useful dyes and a bleaching method are described in, for example, Research Disclosure, April, 1976, pp. 30-32 (RD-14433), ibid., December, 1976, pp. 14-15 (RD-15227), and U.S. Pat. No. 4,235,957.
A further method of forming color images by heat development utilizing a compound having a dye portion and capable of releasing a mobile dye in a positive or negative relation to the reduction reaction of silver halide into silver at a high temperature is disclosed in U.S. Pat. Nos. 4,500,626, 4,503,137 and 4,483,914, Japanese Patent Application (OPI) No. 154445/84 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
In many cases, a heat-developable light-sensitive material is developed by heating with a base as a development accelerator. In this development method, however, various problems arise. For example, once a heat-developable light-sensitive material is heated to a high temperature, a long time is required for the temperature to return to an ordinary temperature. As a result, the material is developed excessively and image quality is decreased. Furthermore, even if the same heating pattern is applied for development, slight changes in conditions such as outer temperature, heating temperature, water content of the light-sensitive material, and time produce undesirable variations in development.
Moreover, when a dye formed in or released from the light-sensitive material is transferred to an image receiving element by heating, the problem arises that excessive development is caused by heating to accomplish the transfer of images and then the transferred image is fogged.
In order to overcome the above problems, a diffusion transfer method has been developed. In connection with this technique, a method using acid polymers for neutralization as described in Research Disclosure, Vol. 123, p. 22 and ibid., Vol. 180, p. 18030, British Patent 208272A has been proposed. When this method is applied to a heat-developable light-sensitive material, however, the base is too rapidly neutralized, leading to a reduction in image density. Compounds releasing an acid on heating include an acid component capable of being dissolved at temperatures more than 60.degree. C. or releasing a volatile acid described in Japanese Patent Application (OPI) Nos. 58642/74 and 57452/75. However, when such components are applied to a heat-developable light-sensitive material, they neutralize the base before the start of development by heating, thereby preventing development, leading to a reduction in image density.