Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
1995-06-05
2002-06-18
Beatty, Robert (Department: 2852)
Incremental printing of symbolic information
Ink jet
Controller
C347S041000
Reexamination Certificate
active
06406114
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to an ink jet recording method and apparatus using an ink jet recording head having a plurality of ejection outlets (nozzles), capable of tone recording.
In an ink jet recording system using the recording head, the ink is ejected to a recording material in accordance with recording signals. The system is widely used because of the low running cost and the quietness. A great number of nozzles are arranged in a line extending perpendicular to the relative movement direction between the recording material and the recording head, and therefore, one scan of the recording head over the recording material can cover a recording width corresponding to the number of nozzles, so that the high speed recording is accomplished relatively easily.
When a tone gradation is to be provided in the ink jet recording system, it will be considered to change the size of liquid droplet ejected. However, there is no practical method for accomplishing this. Usually, therefore, the number of ink droplets per unit area is controlled on the basis of pseudo-half-tone image processing. In another method called “multi-droplet system”, a smaller size ink droplet is used, and a plurality of such ink droplets are deposited substantially at the same point on the recording material to provide one recorded dot, in which the number of ink droplets is changed to reproduce the tone. This system permits the tone recording without reduction of the image resolution, and is particularly effective for the ink jet recording system in which difficulties arise in significantly changing the size of one liquid droplet.
However, in a conventional multi-droplet system, one picture element (pixel) is recorded by a plurality of ink droplets ejected from one and the same nozzle, and therefore, if there is a variation in the sizes of the ink droplets of the individual nozzles, i.e., each individual nozzle ejecting an ink droplet having the same nominal volume/size but different actual volume/size than an ink droplet ejected from other nozzles, a non-uniform image results which includes stripes and/or image density unevenness.
This problem becomes more significant where the number of nozzles of a recording head is increased to expand the recording width covered by one scan in an attempt to accomplish the high speed recording. The increase of the nozzle number and therefore the recording width results in a greater component of the spatial frequency of the unevenness, and therefore, in more conspicuous unevenness. Thus, the image quality is degraded. In the case of the tone recording, the unevenness is so conspicuous that only a several % variation among the ejection quantities of the nozzles is enough for one to recognize stripes caused by the density unevenness.
In order to avoid the problem, the conventional multi-droplet system requires very accurate head manufacturing in order to reduce the variation in the ejection quantities through the individual nozzles. This brings about high cost and low yield. As a method for removing the density unevenness through software, it is effective to change the number of ink ejections to compensate for the variation of volume/size of ejected ink droplets among the nozzles with the aid of image processing of error diffusing method or the like. However, such an image processing system results in increase of the system cost. In addition, even if such a processing is used, the number of ink droplets has to be readjusted if the variation among nozzles in the ink volumes changes with time. This makes the maintenance operation difficult. Furthermore, this method does not work where there is a non-ejection nozzle.
This system also involves the problem that the density unevenness is not sufficiently suppressed when the variation in the ink droplet volume is larger.
In order to accomplish a high quality tone recording of not less than 16 tone gradations in the above-described system, stabilized ink ejections with very small droplets are required. Therefore, the manufacturing accuracy of the recording head has to be very high, so that the manufacturing method is totally different from that for the bi-level recording heads. This results in high cost and low yield.
In the case of multi-droplet system of 3-5 tone gradations, the droplet size, volume or quantity is permitted to be relatively large as compared with that in the case of the 16 or more tone gradations. Therefore, the manufacturing tolerance in the recording head is so large that the same manufacturing method as in the bi-level recording head can be used. The cost can be reduced.
The image provided by the recording head having such a large number of tone gradations is better in the image quality than the image recorded by the bi-level recording head because the grains are not conspicuous. However, as compared with the image provided by the recording head having 16 or the like tone levels, particularly in the grains in the high light portions.
U.S. Pat. No. 4,746,935 proposes multi-tone ink jet printer capable of accomplishing the tone recording on the basis of combinations of 1 pl, 2 pl and 4 pl, for one pixel. According to this proposal, 8 kinds of ink droplet volumes, i.e., 0, 1, 2, 1+2(=3), 4, 1+4(=5), 2+4(=6), 1+2+4(=7), can be provided by three kinds of ink droplets (volume ratio). Therefore, the printing speed is increased as compared with the case where one ink droplet is overlaid 7 times. However, as shown in FIG. 2 of the U.S. '935 patent the curve representing the relationship between the reflection density and the total volume of the ink droplets for one pixel is steep and convex-up. For this reason, even if the differences between adjacent total volumes of the ink for one picture element are the same, the differences, in the reflection density, corresponding thereto, are not the same. Therefore, in the zone where the volume of the ink droplet for one pixel is small, the differences of the adjacent possible reflection densities is large. On the contrary, in the zone where the volume of the ink droplet for one pixel is large, the difference between the possible reflection densities is small. In other words, the volume of the ink droplet does not significantly influence the tone gradation in the zone where the volume of the ink droplet for one pixel is large. In addition, since the number of combinations of different ink droplets for one picture element is large (8 combinations in the case of 1 pl, 2 pl and 4 pl), the image processing circuit becomes complicated with the result of high design and manufacturing cost.
Another problem of the ink jet printer of the U.S. patent is that one recording head has to be provided with the nozzles having different ejection volumes, the difference being as large as 4 times (4 pl/l pl), or 8 times in the case of 1 pl to 8 pl. In this case, the difficulties arise in the manufacturing of the recording head. Generally, the recording head parameters influential to the volume of the ink droplet ejected, a distance between the heater and the ejection outlet, a size of the heater, a configuration of the ink outlet or barrier and an ejection outlet area. In order to change the volume of the ink droplet from 1 to 4, the changes of the heater-outlet distance, the heater area and the ejection outlet area has to be changed. The manufacturing will be difficult only using the conventional practical method. Therefore, in order to accomplish such a recording head, a new process has to be added, with the result of increase of the manufacturing cost.
In the ink jet printer disclosed in the U.S. '935 patent, the ink ejection outlets providing the different ejection volumes (1 pl, 2 pl, and 4 pl) are arranged along a scanning direction of the recording head and closely with each other at the front side of the recording head, so that the plural ink droplet ejections for a given one pixel can be effected through one scan. Therefore, the ink droplets are sequentially overlaid before the previous ink
Beatty Robert
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
LandOfFree
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