Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
1999-05-24
2002-03-05
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Controller
C347S042000
Reexamination Certificate
active
06352329
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an ink jet recording apparatus using an ink jet head on which a large number of ink outlet ports are aligned to execute recording by driving the ink jet head in accordance with an image signal and by selectively discharging the ink from each of the ink outlet ports onto a recording medium.
As the ink jet recording apparatus, for example, an on-demand type ink jet printer is well known. In such an ink jet printer, improvement for acceleration is considered important. As the number of ink outlet ports of the ink jet head is increased, printing can be executed at a higher speed. As for the technique of increasing the number of ink outlet ports, an elongated ink jet head is formed by integrally arranging a plurality of head units each having a large number of ink outlet ports aligned thereon so that the alignment of the ink jet heads can be approximately parallel. Thus, the length of the head becomes a line head by corresponding to the width of a recording sheet, and thereby high-speed printing can be realized.
Incidentally, the head unit comprises a nozzle portion
2
in which a large number of ink chambers
1
are formed, a main body portion
4
in which a common ink chamber
3
feeding the ink into each of ink chambers
1
is provided, and an ink feed passage
5
through which the ink is fed to the common ink chamber
3
, as shown in FIG.
10
. Ink drops are discharged from ink outlet ports
6
of the ink chambers
1
by bringing the change of volume into the ink chambers
1
, and dot printing is thereby executed. The ink consumed in the ink chambers
1
is supplemented from the common ink chamber
3
. The control system giving the change of volume into the ink chambers
1
is exemplified by piezoelectric control using distortion of an piezoelectric member, thermal control using heat of an heating element, and the like. The on-demand type printing can be executed by arbitrarily bringing the change of volume into each of ink chambers by the control systems.
Incidentally, the change of volume in the ink chambers
1
during the ink discharging operation is transmitted into the ink chambers as the change of pressure and also propagated into the common ink chamber
3
. When the ink is discharged from a plurality of ink chambers, the ink chambers
1
near the end portions of the ink jet head are largely influenced by the common ink chamber
3
and the volume of the ink discharged from a constant number of ink chambers
1
at the end portions is increased or decreased, which depends on the structure of the ink jet head.
In the ink jet head constituted by a single head unit, even if such a phenomenon occurs, i.e. if the volume of the ink discharged from a constant number of ink chambers at the end portions is changed and thereby the change of optical density slightly occurs, unevenness of the optical density is hardly noticed since the printed portion is located at the end portion.
However, in the case of the ink jet head on which a plurality of head units are integrally aligned, the end portions of the head units are positioned in the middle of the line direction of printing. If the unevenness of optical density occurs at the portions, stripe-shaped unevenness of optical density is noticed.
For example, when printing is executed on the basis of an image signal of a constant value by using an ink jet head on which three head units
71
,
72
and
73
are integrally aligned, as shown in
FIG. 11
, the volume of the discharged ink is increased and the optical density of printing is made higher at the boundaries of the head units
71
,
72
and
73
, and these portions are seen as the stripe-shaped unevenness of optical density. This problem also occurs when the printing is executed on the basis of a general image signal that is not the image signal of a constant value. This stripe-shaped unevenness of optical density does not occur only at the end portions of the head units, but also occurs by irregularity of the ink outlet ports of the ink chambers that is generated during the production of the head units. Thus, in the conventional ink jet head, partial change of the optical density occurs, which appears as the stripe-shaped unevenness of optical density.
Incidentally, in the ink jet head, an image can be only expressed by the ink discharged from one ink chamber, with the number of gradation N as compared with the number of gradation M (M>N≧2) of the input image signal. For this reason, when the number of gradation M is converted into the number of gradation N, pseudo-halftone processing such as the error diffusion method, the dither method and the like needs to be executed. In general, according to the error diffusion method, as compared with the dither method, an image of higher quality can be obtained while the processing speed is low and the circuit dimensions are increased. Therefore, the dither method is often employed when an image of much higher quality is not required. Further, when the optical density is corrected by employing the error diffusion, an error signal for the ink outlet ports to be corrected influences the ink outlet ports that do not have to be corrected, the effect of correction becomes inadequate and deterioration of images is generated. Thus, the error diffusion is not suitable for this kind of correction of the optical density.
BRIEF SUMMARY OF THE INVENTION
Therefore, the object of the present invention is to provide an ink jet recording apparatus capable of reducing the unevenness of optical density caused by change of the ink discharged from the ink outlet ports in a specific area of the ink jet head and thereby improving the quality of images.
According to claim
1
, the present invention provides an ink jet recording apparatus comprising: an ink jet head formed by integrally arranging a plurality of head units each having a large number of ink outlet ports aligned, so that the ink outlet ports in each of the head units can be arranged approximately parallel; a pseudo-halftone processing unit for converting an image signal of M gradation into an image signal of N gradation (M>N) by using dither matrix, so that average optical density of an image area formed in accordance with an image signal corresponding to P number of ink outlet ports at each end portion of the head units constituting the ink jet head can be substantially equivalent to average optical density of an image area formed in accordance with an image signal corresponding to ink outlet ports other than the P number of ink outlet ports; and driving means for driving the ink jet head in accordance with the image signal of N gradation that is output from the pseudo-halftone processing unit, while moving a recording medium in a direction orthogonal with a line direction in which the ink outlet ports of the ink jet heads are aligned, relatively to the ink jet head.
According to the invention of claim
1
, the unevenness of optical density caused by change of the ink discharged from the ink outlet ports in a specific area of the ink jet head can be reduced and thereby the quality of images can be improved.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.
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patent: 4742400 (1988-05-01), Tsuji
patent: 5729277 (1998-03-01), Morrison
patent: 6244681 (2001-06-01), Yano et al.
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patent: 6-155771 (1994-06-01), None
patent: 10-795 (1998-01-01), None
Kubota Atsushi
Watanabe Hidehiro
Barlow John
Frishauf, Holtz Goodman, Langer & Chick, P.C.
Stephens Juanita
Toshiba Tec Kabushiki Kaisha
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