Direct positive silver halide photographic materials

#This invention relates to silver halide photographic materials and, more particularly, to direct positive silver halide photographic materials.

#When a silver halide light-sensitive material is exposed to light including the light of a light-sensitive wavelength region for said light-sensitive material and developed, the photographic density or blackening density increases as the amount of exposure increases until it reaches a maximum value. When the amount of exposure is further increased, the photographic density begins to decrease to form, finally, a positive image. Such a phenomenon is generally called solarization. Also, in a silver halide emulsion which is optically or chemically fogged in the production step of the silver halide emulsion, a similar reversal phenomenon by light occurs. The direct positive silver halide photographic emulsion in this invention is the latter type photographic emulsion, that is, a photographic silver halide emulsion which is previously optically or chemically fogged so that the emulsion forms a positive image after ordinary light exposure and ordinary development.

#A previously fogged direct positive silver halide photographic emulsion frequently cannot obtain a sufficiently low minimum density (e.g., a pure white portion is not obtained) with a conventional technique to reduce the contrast of images. Accordingly, sufficient image quality cannot be obtained.

#A gelatino silver bromide emulsion having a mean grain size of about 0.2 micron was prepared by simultaneously adding an aqueous solution of potassium bromide and an aqueous solution of silver nitrate to an aqueous gelatin solution under stirring at 65.degree. C. over a period of about 60 minutes. After removing salts from the silver halide emulsion by a flocculation method using a polyvalent anionic polymer and adjusting the pH of the emulsion to 10, hydrazine was added to the mulsion in an amount of 25 mg per mole of silver in the emulsion followed by heating to 65.degree. C. for 70 minutes to fog the silver halide emulsion. Then, the silver halide emulsion was divided into equal parts. After adding to each emulsion 6-ethoxy-1-methyl-2-(3-nitrostyryl)quinolinium methylsulfate as an electron acceptor in an amount of 500 mg per mole of silver, the phenazine series compound of general formula (I) or (II) and the nitroindazole series compound of general formula (IV) dissolved in methanol were added to the silver halide emulsion according to this invention as shown in Table 1(a). Each of the silver halide emulsions thus prepared was coated on a cellulose triacetate film at a silver coverage of 3 g/m.sup.2, whereby coated Samples 1 to 9 were obtained.

#Each sample was exposed to a mercury lamp through an optical wedge, developed in a developer having the composition shown below for 3 minutes at 20.degree. C., and then fixed to provide a film strip. The density of each film strip thus obtained was measured using a P-type densitometer (made by Fuji Photo Film Co., Ltd.) to provide each characteristic curve. The results of the sensitometry are also shown in Table 1(a).

#______________________________________ Composition of the Developer: ______________________________________ Metol 3.1 g Sodium sulfite 45 g Hydroquinone 12 g Sodium carbonate 79 g Potassium bromide 1.9 g Water to make 1 liter ______________________________________