"The sensitizer to be used in the present invention is a low molecular weight substance having a melting point of 50??? to 150??? C. If the melting point is lower than 50??? C., the sensitizer will be readily migrated to the dye surface to generate such problem as blocking, etc., while if the melting point exceeds 150??? C., the sensitizing action will be abruptly lowered undesirably." . . . .