High-resolution inkjet printing using color drop placement...

Details High-resolution inkjet printing using color drop placement... High-resolution inkjet printing using color drop placement...

1997-03-04

2002-04-09

Barlow, John (Department: 2853)

Incremental printing of symbolic information

Ink jet

Ejector mechanism

C347S015000, C347S043000

Reexamination Certificate

active

06367908

ABSTRACT:

RELATED PATENT DOCUMENTS
Closely related documents include coowned U.S. Pat. No. 4,963,882 entitled “PRINTING OF PIXEL LOCATIONS BY AN INK JET PRINTER USING MULTIPLE NOZZLES FOR EACH PIXEL OR PIXEL ROW”, U.S. Pat. No. 4,965,593 entitled “PRINT QUALITY OF DOT PRINTERS”, U.S. Pat. No. 5,555,006 entitled “INKJET PRINTING: MASK-ROTATION-ONLY AT PAGE EXTREMES; MULTIPASS MODES FOR QUALITY AND THROUGHPUT ON PLASTIC MEDIA”, and U.S. Pat. No. 5,561,449, entitled “POSITION LEADING, DELAY, & TIMING UNCERTAINTY TO IMPROVE POSITION & QUALITY IN BIDIRECTIONAL INKJET PRINTING”; as well as U.S. patent application Ser. No. 08/667,532, entitled “JITTER-FORM BACKGROUND CONTROL FOR MINIMIZING SPURIOUS GRAY CAST IN SCANNED IMAGES” and now issued as U.S. Pat. No. 5,859,928, and two additional U.S. patent applications filed generally concurrently herewith having these Hewlett Packard Company later identified as Ser. No. 08/814,949 and issued as U.S. Pat. No. 6,082,849, entitled “RANDOM PRINTMASKS IN A MULTILEVEL INKJET PRINTER”, and later identified as Ser. No. 08/810,747 and issued as U.S. Pat. No. 6,250,739, entitled “BIDIRECTIONAL COLOR PRINTMODES WITH SEMISTAGGERED SWATHS TO MINIMIZE HUE SHIFT AND OTHER ARTIFACTS”. All these documents in their entireties are hereby incorporated by reference into this document.
FIELD OF THE INVENTION
This invention relates generally to machines and procedures for printing ultrahigh-resolution color text or graphics on printing media such as paper, transparency stock, or other glossy media; and more particularly to a scanning inkjet machine and method that construct text or images from individual ink spots created on a printing medium, in a two-dimensional pixel array. The invention employs print-mode techniques to optimize ultrahigh-resolution color image quality vs. operating time.
BACKGROUND OF THE INVENTION
A previous generation of printing machines and procedures has focused on mixed resolution. These systems most typically have employed about 24 pixels/mm (600 pixel dots per inch, or “dpi”) in a carriage scan direction transverse to the printing medium and 12 pixels/mm (300 dpi) in the print-medium advance direction longitudinal to the printing medium—or 24 pixels/mm for black and 12 pixels/mm for chromatic colors, or relatively tall 12 mm (half-inch) pens for black ink and relatively short 8 mm (third-inch) pens for chromatic colors; or combinations of these and other operating-parameter mixtures.
These mixed-resolution systems have been of interest for obtaining effectively very high quality printing with a minimum of developmental delay. In the continuing highly competitive development of inkjet printer products, the mixed systems have served a very important role because of many difficult problems associated with attaining a full ultrahigh resolution—for example 24 pixel/mm pens, 12 mm tall, for all colorants in a color-printing system.
In the current generation of machines, interest has shifted to solving those many difficult problems. As will be seen, most of such difficulties have been recognized for many years, but tend to be aggravated in the ultra-high-resolution environment.
(a) Throughput and Cost
In a sense many problems flow from these two considerations, since essentially all the problems would evaporate if it did not matter how slow or expensive a printer was. In practice, marketplace pressures have made it crucially important that a printer be both competitively fast (even when printing in a “quality” mode) and competitively economical.
(b) Firing Frequency
Thus for example high throughput in combination with high resolution pushes the capability of economical inkjet nozzles to fire at a high enough repetition rate. An inkjet pen tends to be most stable in operation, and to work best for error hiding, at a low firing frequency.
Horizontal resolution of 24 pixels/mm if printed all in a single pass, however, would require a rather high firing frequency—in fact, for current-day technology, roughly twice the highest frequency of reliable operation in an economical pen. This figure may be expected to change with refinements in pens.
(c) Banding and Pattern Artifacts
These spurious image elements are well known in lower-performance printers, but like other problems can be even more troublesome in the newer generation of devices. It is known, for example, that some banding effects can be reduced by printing highly staggered (i. e., overlapping) swaths—but also that doing so reduces overall throughput proportionately. (A different kind of visible banding, associated with hue shifts, will be discussed below.) Hence, again, high throughput tends to run counter to elimination of banding, and this conflict is aggravated by a requirement for printing at resolution that is twice as fine.
As to pattern defects, the design of dither arrays is a logical culprit and has previously received a great deal of attention in this regard, and may be considered highly refined. Yet heretofore some patterning persists in high-resolution images printed under conditions which should yield the best possible image quality.
Theory suggests that no further advantage can be obtained through dither redesign, and that solutions must be sought elsewhere. Discussion of printmasks in a following subsection of this document will take up this theme again.
Generally speaking, tools for investigating this area heretofore have been inadequate.
(d) Color shift
One important approach to maximizing throughput is to print bidirectionally. In a bidirectional-printing system the pens print while the carriage is traveling in each of its two directions—i. e., across the printing medium, and back.
This technique is well known and successful for printing in monochrome. Workers skilled in this field have recognized, however, that for printing in color a hue shift, or more precisely a color shift, arises as between printing in the two directions.
The reason is that pens are traditionally arranged, physically, on their carriage in a specific sequence. Therefore if two or more of the pens fire while the carriage is moving in one particular direction the different ink colors are laid down one on top of another in a corresponding order—and while the carriage is moving in the opposite direction, in the opposite order.
Usually the first inkdrop of two superposed drops tends to dominate the resulting perceived color, so that for example laying down magenta on top of cyan produces a blue which is biased toward the cyan; whereas printing cyan on top of magenta typically yields a blue which emphasizes magenta. If successive separate swaths—or separately visible color bands, subswaths—are printed while the pen is thus traveling in each of two directions, respectively, the successive swaths or subswaths. Banding that results is often very conspicuous.
For this reason, previous artisans have striven to avoid printing of any superposition-formed secondary colors in more than one order, ever. Printers commercially available under the brand names Encad® and Laser-master®, in particular, employ a tactic that employs brute force to avoid sequence changes: the pens are offset, with respect to the vertical direction, or in other words longitudinally along the printing medium.
They are offset by the full height of each nozzle array—posing, at the outset, significant problems of banding (see discussion following) as between colors. Furthermore, in consequence of the full-height-offset arrangement each of the trailing three pens must print over a color subswath formed in at least one previous scan—from one to three previous scans, depending upon which pen is under consideration.
This system advantageously maintains a fixed color sequence even in bidirectional printing. Use of full-height offset of the pens, however, makes a great sacrifice in other operating parameters. More specifically, the full-height staggered pens have a print zone that is four color bands (subswaths) tall.
Necessarily the overall product size in the direction of printing-medium advance is correspondingly greater, as are weight and cost. In additio

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