Recent years have seen a remarkable development of facsimile equipment and printers. In particular, these apparatus generally employ a heat-sensitive recording system using a combination of a thermal head and a heat-sensitive recording paper comprising a colorless dye such as Crystal Violet Lactone and a phenol compound coated thereon, as described, e.g., in Japanese Patent Publication No. 14039/70.
This heat-sensitive recording system is advantageous in that the recording paper undergoes a primary color formation which requires no development, simplifies the recording apparatus, and reduces the cost of the recording paper and the recording apparatus, and that it use a quiet non-impact process. Thus, this recording system is widely used in various printers and facsimile equipment. Furthermore, the demand for the recording of half tone objects, such as photographs, has grown in recent years. Such half tone recording requires that the heat-sensitive recording paper exhibit an excellent dot reproducibility, particularly in low density regions.
Heretofore, some methods for improving dot reproducibility have been proposed. For example, Japanese Patent Publication No. 20142/77 describes a method which comprises improving the surface of a heat-sensitive color forming layer to a Bekk smoothness of 200 to 1,000 seconds by supercalendering or the like. Japanese Patent Application (OPI) No. 115255/79 (the term "OPI" as used herein means an "unexamined published Japanese patent application") describes a method which comprises smoothing the surface of a heat-sensitive color forming layer to a Bekk smoothness of 1,100 seconds or more. Japanese Patent Application (OPI) No. 156086/78 discloses a method which comprises improving the surface of a heat-sensitive color forming layer to a surface roughness Ra of 1.2 .mu.m or less and a glossiness of 25% or less. Furthermore, Japanese Patent Application (OPI) No. 83841/79 discloses a method which comprises the use of an undercoated paper as a support. These methods provide some improvement in color density in high density regions. However, these methods leave much to be desired with respect to improving dot reproducibility in low density regions.