Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
2001-01-19
2003-04-15
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Controller
C347S014000
Reexamination Certificate
active
06547362
ABSTRACT:
BACKGROUND OF THE INVENTION
Incremental printers may produce many different kinds of undesired artifacts in printed images. These mainly include:
repeating two-dimensional patterns due to dither-mask periodicity, or periodic relationships between dither and print masks;
progressively expanding or unfolding shapes arising in error diffusion; and
simple banding due to imperfectly abutted swaths, or to printing elements (nozzles, in inkjet printing) that are defective, or progressively inoperative (e.g. clogged, in inkjet printing), or incorrectly aimed.
The present invention addresses the third category, but is not primarily directed to swath abutment as such. Thus a principal target of the invention is malfunction of printing elements, although in some cases this in turn can produce a particular form of swath-abutment failure—and when it does, the present invention can be effective.
(a) Sources of banding—In scanning incremental printers it is well known that striations along the scan axis are a pervasive problem. Early innovations attacked the production of white or light lines due to inadequately precise printing-medium-advance mechanisms, and anomalously colored lines when subtractive primary colorants were superposed in inconsistent sequence.
More recently, the development of very inexpensive techniques for fabrication of inkjet nozzle arrays exceeding two and three centimeters in length has also introduced difficulties in control of aiming at the ends of the arrays. While that particular kind of printing-element malfunction has now been considerably mitigated, it still causes small but stubborn departures of printed swath height from printing-element-array height—and consequent banding.
With the improved control of end-element aiming, focus now shifts to malfunction of elements along the entire length of the array (though this may include the end elements). Artifacts due to these elements, as compared with those addressed earlier at the array ends, are quantitatively much finer—but so are the demands of the marketplace.
The consumer calls for progressively finer image quality, coupled with economy. Consequently a very significant problem remains in relatively subtle banding due to intermediate printing elements that are clogged, weak (e.g. due to firing-component tolerances or fatigue) or, again, mispointed.
(b) Identification of weak printing elements—Some workers in this field (see e.g. the Murcia document and also U.S. Pat. No. 6,010,205 mentioned above) have concentrated upon tactics for correcting known bad nozzles, simply leaving identification of those elements to other artisans. Some workers have proposed to monitor the dot-generating mechanism to predict failure—as for instance in measurement of inkjet nozzle temperature (as in the Allen document) to anticipate malfunction, or in sensing inkdrops in flight (as represented by the patent of Dr. Ix).
At least one earlier effort, represented by the Borrell document mentioned above, treats printing-element failure as a systematic result of environmental factors. Borrell measures parameters of the printer environment with an eye to entirely minimizing the occurrence of element failure.
The most direct approach, however, tries to isolate and quantitatively measure the failure itself—and to do so for each printing element individually. An ideal example of that approach for the inkjet environment appears in the previously mentioned Armijo document.
Armijo forms a test pattern with inkdrops from each nozzle (if functional) arrayed in a respective test group. He can then scan a sensor across each test group to detect functionality of each nozzle alone.
In his test pattern, a failed nozzle appears conspicuously as a missing dot in the overall test pattern. A weak nozzle appears as a dot of less than full, nominal saturation. A slightly misdirected nozzle, however, may be very difficult to detect from his test pattern.
Armijo's technique can be implemented with the naked eye, but is far more powerful when performed automatically and the results applied to initiating corrective action. His strategy provides excellent detailed information about every nozzle—except for incorrect aiming, as noted just above—but is time consuming.
(c) Related uses of sensors—Many different kinds of sensor measurements are made in laboratory and bench tests, or in preparation of color-rendering systems as in the Bockman and Guo documents—as distinguished from automatic measurements made in the field by operational printers. Some such lab measurements may quantify image quality.
The Guo document uses a carriage-mounted sensor to measure color averaged over an area, in color tiles, and applies spectral modeling to determine how to refine halftoning. The Bockman document is likewise addressed to preparing a product line as such, rather than to automatic operational field calibration of finished individual printers.
All such bench and lab uses of sensors are regarded as intrinsically different from routine operational calibration use of sensors in end-user facilities by a finished printer product. The field of the present invention is limited to such latter operational uses.
In general, as mentioned above, earlier approaches to operational determination of printing-element failure have set out to isolate and measure causes as such. Thus for instance the Doval document and the Subirada document both operate sensors over printed patterns to measure swath height and spacing, and then determine how to accommodate any error found.
Subirada in particular uses a bar-type pattern, and such patterns are also known (as in the Sievert and Nelson documents) for determining interpen alignments as well as imperfections—or some adverse results of broad tolerances—within individual printheads. Subirada's invention relates to banding reduction through adjustment of printing-medium advance.
To accomplish this, he performs a fixed matching of the print-medium advance to his measured swath height, or to a fraction of it. The Baker document teaches measurement of color balance with a sensor mounted on an auxiliary sensor carriage.
These earlier sensors ride on carriages which operate in the scan direction. Some of them, however, may be activated for measurements while the print-medium advance mechanism is operating.
(d) Printmode techniques—It is now very well known that image quality can be improved in many ways through scanning multielement printing arrays plural times (rather than only once) over each portion of a printing medium. Although such operation sacrifices throughput, it remains appealing where highest quality is an objective.
Such plural- or multipass printmodes entail laying down in each pass of the printing array (e.g. inkjet pen) only a fraction of the total ink required in each section of the image. Any areas left unaddressed after each pass are completed by one or more later passes.
An intrinsic benefit of this type of printing is a tendency to conceal the edges of each printed swath, and also to hide light lines formed where individual printing elements or groups of elements are not marking fully. Such a tendency is inherent simply because a missing pixel row is somewhat less conspicuous when superimposed on a printed (or partially printed) row of another pass, than when seen against an unprinted (usually white) background of a printing medium.
In liquid-colorant printing systems, plural-pass operation has additional benefits. It tends to control bleed, blocking and cockle by reducing the amount of liquid that is all on the page at any given time, and in addition may facilitate shortening of drying time—thus at least partially offsetting the poorer throughput during the printing process itself.
The specific partial-inking pattern employed in each pass is called a “printmask”, and the way in which these different patterns add up to a single fully inked image is known as a “printmode”. Heretofore, however, it has been recognized that printmodes and printmasks can themselves introduce undesired and conspicuous artifacts.
For example some printmode
Martinez Oscar
Mott James A
Subirada Francesc
Ashen & Lippman
Barlow John
Hewlett--Packard Company
Stewart Jr. Charles W.
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