1.  Field of the Invention
The present invention relates to a process for making a halftone recording for recording a high quality image with a suppressed grainy feeling, as well as an ink tank and a head cartridge fit for halftone recording and an ink-jet apparatus using the same. 
2.  Related Background Art
Recently, OA instruments such as personal computers and word processors have widely spread, thereby rapidly increasing a chance of using such equipment to output not only so-called binary images such as characters and line drawings but also halftone images such as photos or colored images in output of information items inputted. 
For the output of these images, various recording methods and recorders have been developed.  Among them, the ink-jet (IJ) scheme enabling an inexpensive and high quality color image recording has begun to be accepted widely by the market.  With many popular types of recorders including this IJ recorder, halftone records as mentioned above are expressed in terms of areal gradation. 
The areal gradation means a method for executing a halftone reproduction by controlling the recording area onto the recording medium and if an example of IJ recorder is taken for the explanation, this method can be roughly divided into the dot-diameter modulation scheme and the pseudo gradation scheme.  The dot-diameter modulation scheme is a method for controlling the size of a dot on a sheet of paper according to a value of gradation to control the recording area for unit area by controlling the ink-jet amount of a dot to be recorded, whereas devised as the pseudo gradation scheme are well-known gradation reproducing methods such as dither matrix method or ED (Error Diffusion) method. 
However, the existing gradation-reproducing methods mentioned above have the following disadvantages. 
1.  Actually, in the dot-diameter modulation scheme, an ejection amount per dot varies about twice at the utmost in a modulation range and this level of ejection amount modulation is insufficient for a halftone reproducibility.  In comparison of the recorded dot diameter in an ejection amount of 80 ng/dot with that in an ejection amount of 8 ng/dot on paper, dot diameter on the paper surface differs only by about twice though the ejection amount differs by ten times.  Even if somewhat of variations appears without doubt according to the composition of ink, a modulation width of about twice by the ejection amount cannot be sufficient as an areal gradation means by the control of the recorded dot diameter on paper at all. 
2.  The pseudo gradation method, e.g., dither matrix method, is a method for controlling the number of dots recorded in a decided matrix size.  By increasing the matrix size, the gradation reproducibility can also be improved, but the actual resolution lowers and the sharpness of characters or line drawings worsens.  Furthermore, especially in the image portion of a low gradation (highlight part), the grainy feeling of dots takes place as foreign feeling.  Using the ED method mentioned above a lowering of resolution can be moderated in some extent, but the grainy feeling in the highlight part is not improved because it is originated from a dot density. 
There is a recording method using a dilute pale ink in such a manner that the grainy feeling is not noticed, but the use of an ink diluted unduly cannot satisfy the highest density in the dark part though the grainy feeling in the highlight part can be moderated, so that a higher quality is unexpectable as a whole image.  Alternatively, there is a so-called high and low concentration ink recording method in which inks of the same color at a plurality of concentrations are used as a recording ink.  This is a method in which both the grainy feeling in the highlight part and the highest density in the dark part are satisfied by using a sufficiently diluted ink and a sufficiently concentrated ink, but in this case, grainy feeling never fails to appear at a switching portion from a pale ink recording to a deep ink recording in the reproduction process of gradation.  To be specific, when a dot of sufficiently concentrated ink is printed into a highlight image portion recorded by a sufficiently diluted pale ink, grainy feeling never fails to be noticeable due to the difference in contrast between pale ink images and deep ink images, thereby losing an effect of executing a concentration varying recording.  Needless to say, increasing the diluted grades of ink, by use of middle ink diluted to an intermediate concentration between the pale and the deep ink, further use of middle diluted and middle deep ink at a further intermediate concentrations, and so on, can solve such a problem, but has difficulty in practical use because the accompanying increase in recording heads and recording ink species results in a great cost rise.