1. Field of the Invention
The present invention relates to an inkjet printing apparatus and an inkjet printing method which use color inks containing colorants and an image quality improvement liquid, and more particularly to a technology for reducing gloss unevenness in printed images.
2. Description of the Related Art
There has been growing calls in recent years for the inkjet printing to have a capability to print high quality images on a variety of print mediums. Among the print mediums suited for high quality images, there is coated paper. The coated paper has an ink receiving layer formed on a substrate such as quality paper and film. There are various kinds of coated paper with varying degrees of texture, from glossy paper with a mirror surface to matte paper with a glare-free finish.
For these coated paper, there is a wide range of demands in terms of glossiness of printed images. One such demand is that the printed image be uniform in glossiness over the entire print medium. To meet this demand Japanese Patent No. 4003760 discloses a method that, in an inkjet printing apparatus using color inks and an image quality improvement liquid, alleviates gloss unevenness by adjusting the amount of image quality improvement liquid applied according to the volume of color inks used for printing.
Generally, in areas on glossy paper applied with a small volume of color inks, the level of gloss, which will be described later, is low compared with areas applied with a greater amount of inks. So, Japanese Patent No. 4003760 minimizes the gloss unevenness within the same image by applying a greater amount of image quality improvement liquid to the areas printed with a small volume of inks than to those areas printed with a larger volume of inks to enhance the level of gloss in the areas printed with a small ink volume.
However, as disclosed in Japanese Patent No. 4003760, with the method of making only the gloss level uniform by adjusting the amount of image quality improvement liquid, the uniformity of glossiness in the same image may not be able to be enhanced enough. This is considered due to the fact that the glossiness in an image is affected by not only the uniformity of gloss level but the uniformity of image clarity and that the image clarity and the gloss level change according to the gradation value of the printed areas.
FIG. 1 illustrates how the image clarity and the gloss level vary according to the gradation value. In FIG. 1, “medium” represents a target range of each of the image clarity and the gloss level; “high” represents a range higher than the target range; and “low” represents a range lower than the target range. As shown in FIG. 1, there is a tendency that, when compared with the target range, highlight areas are high in image clarity and low in gloss level, halftone areas are high in both image clarity and gloss level, and shadow areas (high density areas) are medium in image clarity and high in gloss level. This shows that the image clarity as well as the gloss level tends to vary according to the gradation value, which means that the user can recognize gloss unevenness when the image clarity uniformity is low even if the gloss level is uniform. The gloss unevenness in an image becomes particularly distinctive when the gloss level and the image clarity greatly vary between highlight areas and shadow areas.