Banding reduction in incremental printing, by spacing-apart...

Details Banding reduction in incremental printing, by spacing-apart... Banding reduction in incremental printing, by spacing-apart...

2000-03-01

2002-01-08

Nguyen, Thinh (Department: 2861)

Incremental printing of symbolic information

Ink jet

Ejector mechanism

C347S016000

Reexamination Certificate

active

06336702

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to machines and procedures for printing text or graphics on printing media such as paper, transparency stock, or other glossy media; and more particularly to a scanning thermal-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 image quality.
BACKGROUND OF THE INVENTION
(a) Spatial-frequency effects in banding—A persistent problem in incremental printing is conspicuously visible banding or patterning, which arises from a great variety of causes. Generally these causes are associated with repetitive phenomena that are inherent in the swath-based natured of such printing.
Joan Manel Garcia, in U.S. utility-patent applications Ser. No. 09/150,321 through '323, particularly addresses problems of patterning in the lateral or transverse dimension, i.e. parallel to the scan axis. He points out that such patterning is especially objectionable when it occurs at spatial periodicities to which the human eye is particularly sensitive.
Garcia shows that such banding can be rendered very inconspicuous at normal reading distances by moving its periodicity to roughly 3 cm (1 inch), or preferably a bit longer. This can be accomplished by tiling printmasks of those widths.
Unfortunately that technique is not now readily applicable to the longitudinal dimension—i.e. to the direction parallel to the print-medium advance axis. The reason is that, generally, largest current-day printheads are only about 2½ cm (1 inch) long in that direction.
Within the corresponding available range of spatial frequencies, banding in the lower three-quarters of that range (used in single-pass through four-pass printmodes) is quite conspicuous. Unfortunately the current trend toward reducing the number of passes used for printing each image segment—to enhance overall printing through-put—militates toward use of precisely that part of the range.
(b) Swath-interface effects—Some banding along the print-medium advance axis arises at the interfaces between swaths—due to the advance errors and “PAD” errors mentioned above, and due to ink-media interactions such as coalescence or print-medium expansion. Earlier documents such as Doval's have pointed out that repetitive, small failures of abutment themselves introduce banding (though extremely tiny imprecisions or variations in abutment can be helpful).
Swath-abutment irregularities may represent the single most conspicuous form or type of banding effect. When one swath edge is closely abutted to another, the abutment is almost always imperfect—leading to either a shallow gap between swaths or a shallow overprint where they overlap.
Also the two swaths are generally not exactly the same in darkness or color saturation, adding another element of contrast along the interface. Such problems are aggravated by a high or abrupt gradient of wetness along the edge of a just-deposited swath, when an abutting swath is formed soon after.
(c) Internal effects—Not all banding problems, however, occur at swath boundaries. Some result simply from nozzle PAD problems and these can be entirely internal to the swath.
Internal patterns can be formed by repetitive coincidences of nozzle irregularities. Prior systematic procedures placed particular irregularly-performing pairs (or other groups) of printhead elements into conjunction with respect to the printing medium—over and over.
As an example, the Hewlett Packard Company printer product known as the Model 2000C uses two-pass bidirectional printmodes—each pixel row being printed by two separate nozzles. At 24 rows per millimeter (600 dots per inch, dpi), a 12.7 mm (half inch) pen, has 300 nozzles.
Ordinarily nozzles number
1
and
151
contribute drops to the same image row—using a 6⅓ mm (quarter inch) advance and, again, a two-pass, 300-nozzle printmode. Every 6⅓ mm these same two nozzles are paired (see FIG.
7
and the Table).
If nozzles
1
and
151
when used in combination form a noticeable band effect, this effect is highly visible to the user—because it is present in a repeating pattern, roughly every 6 mm or quarter inch. For example, if both nozzles happen to be directed well away from their nominal target pixel row, then that pixel row will appear unprinted (at least in the particular color in which the head in question prints), rather than the nominal double-printed.
Another kind of band effect can be caused by an interaction of nozzles that are adjacent or nearby. For example assume that nozzle number
5
is aimed “low” (toward the nominal target row for nozzle
6
). If nozzle
6
is aimed accurately, its target row will be double-printed.
If in addition nozzle
156
is also aimed accurately but nozzle
157
is aimed “high” (i.e. both toward the target row for nozzle
156
), then in the printed image the common pixel row for nozzles
6
and
156
will be quadruple-printed—while the adjacent rows above and below will each be single-printed rather than the nominal (double printed).
In short, banding within swaths results from repetitive coincidences between irregularly printing elements within each combination. Patterning arises from repetitive, systematic operation.
Objectionable patterning is subject to quantitative effects. Thus some printmasking approaches to patterning in effect simply dilute repetition within an environment of a greater number of alternative states.
(d) Multipass printmode solutions—Heretofore a common strategy for dealing with all these problems has been to increase the number of passes used to print each image segment. This strategy, however, degrades printing throughput.
It is therefore disadvantageous in the present market, which is increasingly more demanding. This marketplace is characterized by continuously escalating consumer perceptions of what constitutes an acceptable overall image-printing time.
(e) PAD factor—Another kind of band effect arises, particularly with certain pens using tape automated bonding (“TAB”) nozzle arrays, in image areas where adjacent swaths nominally abut. These effects occur because some modern pens are subject to a concentration of aiming errors at the ends of the pen—most classically outboard-aimed nozzles
91
(
FIG. 8
) as distinguished from the great majority of more centrally disposed nozzles
90
.
This higher density of errors, with systematic outboard aim, results from the greater difficulty of maintaining TAB-tape nozzle arrays planar, in comparison with the metal nozzle plates used earlier. In some heads, particularly at the ends of the array, the tape is typically wrapped around the adjacent ends of the printhead—causing the tape to curl very slightly.
The outboard aim in pens of this type increases
93
the overall dimension of the pixel swath in the print-medium-advance axis, beyond the nominal width
92
. Typically this overall increase has been on the order of two or three rows.
As a result, when adjacent swaths
94
,
96
that should neatly abut are printed with a nominal advance of the print-medium-advance mechanism (
FIG. 9
, left-hand “A” view), those swaths will instead overlap slightly. This occurs because an error region
93
(
FIG. 9
, “A” view) in one of the swaths
94
projects into the region
92
′ which should be occupied by the other swath
96
.
Meanwhile a like error region
93
′ extending from that other swath
96
projects into the region which should be occupied by the first swath
94
. As the illustration suggests, these extensions are not limited to the exemplary composite printout
98
of only three swaths
94
-
96
; rather, the phenomenon propagates as at
93
″ to still further swaths above and below.
When these swaths are thus printed with nominal advance of the print-medium-advance mechanism, these effects produce, within the composite printout
98
, a dark band in each overlap area. The darker inking there is usually at the expense of slight lightening within a few pixel rows inboa

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