As diazo processes are concerned, there are various known image formation processes such as dye image formation processes which utilize the photodecomposition of an aromatic diazo compound such as a diazonium salt, a diazosulfonate, an iminoquinone diazide or an iminoquinone diazide, a optionally utilizing a coupling agent.  The process comprises preparation of a printing plate, a proof or a masking film utilizing a curing reaction or a solubitization phenomenon, or preparation of vesicular image formation. 
As photopolymerization processes are concerned, in the field of printing plates, proofing materials, masking films, and resist materials, various known image formation processes utilizing changes insolubility, viscosity, adhesion or the like due to photo-setting exist. 
However, in both diazo processes and photopolymerization processes, no decisive high sensitivity systems sensitive in the visible wavelength range are known. 
In diazo processes, the energy required for photodecomposition of a photodecomposable aromatic diazo compound is generally extremely high.  Furthermore, the wavelength in which the photodecomposable aromatic diazo compound is sensitive is limited to the blue region in the visible light range.  Therefore, the image forming material which can be obtained by using a diazo process is limited to low and ultraviolet sensitive materials. 
Thus, intensive studies have been made concerning the spectral sensitization of photodecomposable aromatic diazo compounds in the visible light range.  For example, it has been known that certain dyes such as riboflavin (M. M. Oreshin, et al.,  Zh Nauchi Prikl Fotgr Kinematogr 26, 323 (1981)), porphyrin (Ando, et al.,  J. Org.  Chem.,  Vol.  37, 2900 (1982)) and chlorophyl can serve as spectral sensitizers for diazonium salts or diazosulfonate.  In other sensitization processes, a diazo process light-sensitive material comprising at least one compound selected from the group consisting of sulfinic acid and sulfinates and a sensitizing dye in an aromatic diazo compound has been known as disclosed in JP-B-50-21247. ( The term "JP-B" as used herein means an "examined Japanese patent publication".) 
However, these processes are disadvantageous in that they are poor in stability and they cannot freely select a spectral wavelength range.  Thus, these processes have not yet been put into practical use.  Furthermore, these systems have little or no effects on the spectral sensitization for photodecomposition of a diazo compound such as naphthoquinonediazide. 
For photopolymerization processes, various spectral sensitization methods have been intensively developed.  However, these spectral sensitization methods are limited to 500 nm.  Thus, no high sensitivity stable systems capable of being spectrally sensitized in a region between 500 nm and the infrared range have been put into practical use. 
One of the approaches taken to overcome these defects is to use a photopolymerization initiator made of a cationic dye/borate anion complex as described in U.S. Pat.  Nos.  4,772,541 and 4,800,149, JP-A-62-143044, JP-A-64-13139, JP-A-64-13140, JP-A-64-13141, JP-A-64-13142, JP-A-64-13143, JP-A-64-13144, JP-A-64-17048, JP-A-64-35548, JP-A-64-40943, JP-A-64-72150, JP-A-64-84245, JP-A-64-88444, JP-A-64-90202, JP-A-1-100536, JP-A-1-138204, JP-A-1-152108 and JP-A-1 152450. ( The term "JP-A" as used herein means an "unexamined published Japanese patent application".)  This system can freely be spectrally sensitized in a region between the blue light range and the infrared range, and exhibits an excellent stability and a high sensitivity, but its sensitivity is still in the order of a few mJ/cm.sup.2.  Thus, it has been desired to further increase the sensitivity of such a system to provide a practical system. 
Further, the use of a photodecoloring compound made of a cationic dye/borate anion complex as described in U.S. Pat.  No.  4,307,182 and JP-A-59-107350, the spectral sensitization of a diazo compound as described in Yamase, Inoue, et al.,  Photo.  Sci.  Eng.,  vol.  19(1), 57(1975), Yamase, et al.,  Bull.  Chem.  Soc.  Japan, vol.  49(1), 351(1976) and Enmanji, J. Inag.  Sci.,  vol.  31(4), 169(1987), and the use of a photopolymerization initiator made of a diazonium salt as described in U.S. Pat.  Nos.  2,976,145 and 1,204,069, EP-0161660, DE 1,269,480B, and JP-A-63-37344 have been reported, but the methods as described in these references have not been satisfactory for the objects of the present invention.