In the field of graphic arts, printing of a printing plate is performed with a set of color separation films formed from a color original by using a lith film. In general, color proofs are formed from color separation films before actual printing work for checking an error in the color separation step and the necessity for color correction. Color proofs are desired to realize high resolution which makes it possible to surely reproduce a half tone image and high performances such as high stability of processing. Further, for obtaining color proofs closely approximating to an actual printed matter, it is preferred to use materials which are used in actual printing as the materials for making color proofs, e.g., the actual printing paper as the base material and pigments as the coloring materials. As the method for forming a color proof, a dry method not using a developing solution is strongly desired.
As the dry method for forming color proofs, a recording system of directly forming color proofs from digital signals has been developed with the spread of electronized system in preprocessing of printing (pro-press field) in recent years. Such electronized system aims at forming in particular high quality color proofs and generally reproduces a dot image of 150 lines/inch or higher. For recording a proof of high image quality from digital signals, laser beams capable of modulation by digital signals and capable of finely diaphragming recording lights are used as recording heads. Therefore, the development of an image-forming material having high recording sensitivity to laser beams and exhibiting high resolution property capable of reproducing highly minute dots is required.
As the recording material to be used in a transfer image-forming method using laser beams, a heat fusion transfer sheet comprising a support having thereon in the order of a light-to-heat (photothermal) converting layer which absorbs laser beams and generates heat, and an image-forming layer which contains a pigment dispersed in components such as a heat fusion type wax and a binder is known (JP-A-5-58045 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)). In an image-forming method using such a recording material, the heat generated at the area of a light-to-heat converting layer irradiated with laser beams fuses an image-forming layer corresponding to the irradiated area, and the fused layer is transferred to an image-receiving sheet arranged on a transfer sheet by lamination, thus a transferred image is formed on the image-receiving sheet.
Further, a thermal transfer sheet comprising a support having provided thereon in the order of a light-to-heat converting layer containing a light-to-heat converting material, an extremely thin heat-peeling layer (from 0.03 to 0.3 μm), and an image-forming layer containing a coloring material is disclosed in JP-A-6-219052. In the thermal transfer sheet, the bonding strength between the image-forming layer and the light-to-heat converting layer bonded via the intervening heat-peeling layer is reduced by laser beam irradiation, as a result, a highly minute image is formed on the image-receiving sheet arranged by lamination on the thermal transfer sheet. The image-forming method by the thermal transfer sheet utilizes so-called ablation, specifically the heat-peeling layer partially decomposes at the area irradiated with laser beams and vaporizes, thereby the bonding strength of the image-forming layer and the light-to-heat converting layer at the irradiated area is reduced and the image-forming layer at that area is transferred to the image-receiving sheet laminated thereon
These image-forming methods have advantages such that an actual printing paper provided with an image-receiving layer (an adhesion layer) can be used as the material of an image-receiving sheet, a multicolor image can be easily obtained by transferring images different in colors in sequence on the image-receiving sheet, and image-forming methods utilizing ablation have an advantage of capable of easily obtaining highly minute image. Therefore, these methods are useful for forming a color proof (DDCP: direct digital color proof) or a highly minute mask image.
For shortening the time required in recording when an image is recorded with laser beams, laser beams comprising multi-beams using a plurality of laser beams are used in recent years. When recording is performed by using a conventional thermal transfer sheet by laser beams comprising multi-beams, there are cases where the image density of the transferred image formed on an image-receiving sheet is insufficient. The reduction of image density is particularly conspicuous in the case of recording with high energy and laser recording. As a result of the investigation by the present inventors, it was found that the reduction of image density was attributable to transfer unevenness caused by irradiation with high energy laser irradiation.
Further improvement of sensitivity and the reduction of temperature and moisture dependency are desired for conventional image-forming materials.