This invention relates generally to techniques in forming visible pictures or images through the use of so-called nonsilver photosensitive materials or photosensitive materials comprising compounds other than silver salts. More particularly, the invention relates to a process for forming visible pictures or images (hereinafter referred to as "images") of excellent contrast with high sensitivity by treating an image forming agent comprising a polyacid of molybdenum or tungsten or its salts or complex and relates also to an image-forming material for this process.
It is known that heteropolyacids and the like of molybdenum and tungsten as mentioned above, such as phosphomolybdic acid, for example, are reduced by light or heat to develop colors ranging from blue to black. Furthermore, attempts have been and are being made to utilize this phenomenon for formation of images through the use of flash light. One known technique for this purpose comprises placing on a reflecting original a heat-sensitive material formed by applying phosphomolybdic acid, for example, as a coating on a transparent substrate and projecting a flash light through this heat-sensitive material to the reflecting original and imparting heat rays from the black parts of the reflecting original as radiant heat to the heat-sensitive material thereby to impart an image to the heat-sensitive material. (U.S. Pat. No. 2,980,551).
In the practice of this method, however, a number of difficulties are encountered. For example, the resolution or resolving power decreases because of heat diffusion toward the periphery of the image, and only materials which are transparent and, moreover, have low heat conductivity can be used for the support.
In addition, the specifications of British Pat. No. 1,008,537 and U.S. Pat. No. 3,285,746 both disclose the projecting of intense light rays having an energy exceeding a certain threshold value onto a film consisting of a solid solution comprising a polyacid such as phosphomolybdic acid or silicotungstic acid and a polyvinyl alcohol/acetate copolymer or polyvinyl butyral thereby to cause the film to become opaque. From the teaching of these patented inventions, it is understand that the above mentioned material can be used as a photographic material capable of producing an image when irradiated by intense, image-wise light rays. However, since it is necessary to project continuously image light rays of a high intensity required for this process (i.e., because of low sensitivity), the utility of the material as a photographic material must be judged to be low.
As a result of our studies on the coloring or color-developing phenomenon of compounds such as phosphomolybdic acid, we have discovered that this coloring includes two process stages.
More specifically, in the first stage, phosphomolybdic acid or the like is reduced and temporarily undergoes a weak coloring. In the case where this coloring is extremely weak, the image may be a latent image. This primary coloring is attained by the reduction of phosphomolybdic acid or the like as was known heretofore, and for this purpose, a method of irradiating with ultraviolet rays the phosphomolybdic acid coexisting with a reducing agetn or a method of bringing a reducing agent in contact with the phosphomolybdic acid, can be resorted to.
In the second stage, the phosphomolybdic acid which has undergone the above described primary coloring further absorbs light and becomes of dark or black color. A noteworthy point is that this second stage cannot be caused to progress by irradiation with only ultraviolet rays but can be caused to progress only by using light rays having wavelengths longer than the ultraviolet region (visible and infrared rays). While light having a long wavelength of this character has a color darkening effect with respect to phosphomolybdic acid which has undergone the primary coloring, it has no effect whatsoever on phosphomolybdic acid which has not undergone the primary coloring. This property of phosphomolybdic acid is not limited to only this acid but is common to polyacids of molybdenum and tungsten and to salts and complexes thereof as described hereinafter.
On the basis of the above findings, we have carried out further studies. As a result, we have developed an image-forming process of high sensitivity through the use of nonsilver materials and an improved photosensitive material suitable for use in this process.