Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
1998-08-13
2001-08-14
Yockey, David F. (Department: 2861)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
C347S012000, C347S041000
Reexamination Certificate
active
06273549
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to inkjet printers and, in particular, to a technique for maintaining color consistency on a medium.
BACKGROUND
Color inkjet printers are well-known and generally incorporate multiple printheads in a scanning carriage which scans left to right and right to left across a medium while the printheads eject droplets of ink. The printheads are typically housed in one or more print cartridges either containing ink or having ink supplied to them from an external source. The ink is channeled to ink ejection chambers formed on a substrate associated with each printhead. Within each of the ink ejection chambers is an ink ejection element, such as a resistive heater or a piezoelectric element. A nozzle plate resides over each printhead such that each nozzle is aligned over an ink ejection chamber. Each printhead may have hundreds of nozzles for printing at 300 dots per inch or more. As energization signals are provided to the ink ejection elements as the printheads are scanned across the medium, ink droplets are ejected from the nozzles to create a pattern of ink dots to print text or an image. High quality printers typically print in both directions to increase throughput.
Color printers typically include a black printhead, a cyan printhead, a magenta printhead, and a yellow printhead aligned in the scanning carriage so that they scan over the same portion of a medium. Even though an image to be printed may ultimately require a full saturation primary color or a full saturation composite color, it is desirable not to print high density print swaths in a single pass. Such single pass printing is not desirable for many reasons. For example, a defective nozzle or ink ejection element would result in a white horizontal line across the medium. A single pass to deposit all the ink needed for the image may provide too much ink in too short of a time to be absorbed by the medium. This would result in excessive ink bleed, excessive drying times, and cockling (warping) of the medium. Also, a single pass may not be sufficient to provide the desired color saturation.
For at least these reasons, high quality printers use multiple passes, when appropriate, such that only a fraction of the total ink required for the image is deposited in a single pass, and any areas not covered by the first pass are filled by one or more later passes.
Since the arrangement of the color printheads in the scanning carriage is fixed (such as the order: black, cyan, magenta, and yellow as viewed from the front of the printer) and all three primary colors (and possibly black) will generally be printed during each pass for a full color image, yellow ink will always be deposited first going from left to right, and cyan ink will always be deposited first going from right to left. Even though a particular composite color requires a particular ratio of, for example, yellow and cyan, this composite color will appear differently depending upon whether cyan ink overlaps yellow ink or whether yellow ink overlaps cyan ink.
When printing using a multi-pass print mode in both directions, color shift results from two or more overlapping colors of ink being deposited on the medium in different orders. A color shift is any detectable difference in color, including becoming lighter or darker. Because the medium is typically stepped after each pass, color shift is generally apparent by horizontal bands which alternate from lighter to darker (or from one color to another color) due to the colors overlapping in different orders during sequential passes. Prior applications have used large numbers of passes to offset this color shift effect. For example, four or eight total passes, each pass depositing an equal amount of ink, causes the error to be dispersed and largely unnoticed. Multiple pass techniques in an inkjet printer have been described in the following U.S. patents assigned to the present assignee and incorporated herein by reference: U.S. Pat. Nos. 5,555,006; 5,376,958; 5,276,467; and 4,965,593. These patents are also cited for their description of masking particular nozzles during particular passes, referred to as print masking, and to illustrate that such print masking techniques are well-known and need not be described herein in detail.
Another defect from multiple passes in both directions results from bi-directional alignment errors of the carriage whereby dots in one pass intending to overlap dots from a previous pass do not precisely overlap those dots.
What is needed, is a multi-pass technique that mitigates the above-described drawbacks.
SUMMARY
In one embodiment of the present invention, for a two-pass print mode, instead of two or more color (including black) printheads depositing the same amount of ink for each pass, one of the passes (the primary pass) in a predetermined direction deposits a majority of the ink, and other pass (the secondary pass) deposits the remainder of the ink. The primary pass direction may be selected so that the particular order of color inks deposited in the primary pass direction is more desirable than the order of color inks deposited in the secondary pass direction. Hence, the primary pass direction will be fixed for a particular composite color, at least when printing contiguous swaths of the same color. As an example, the left-to-right pass for a particular composite color deposits 60% or more of the ink dots, and the right-to-left pass deposits the remainder of the ink dots.
This technique of depositing more ink dots during a pass in a predetermined direction applies to any number of passes, but is particularly applicable to two-pass print modes.
The majority of the ink should be deposited always in the same direction for minimizing color shift. For minimizing color shift, the choice of a direction can be arbitrary, related to performance, or related to the order of the printheads in the carriage. This majority ink pass will dominate the ink effects and cause color shifts to be less perceptible. The amount of ink per pass may be determined on an empirical basis. Since for a gray scale image portion, or other image portion only using a single primary color, there will not be a color shift, such a non-composite color may still use the more optimal 50% ink per pass. Each color of ink may have a different percentage deposited in each pass based on what is optimal for minimizing color shift.
This technique also works for multi-drop systems where each pixel may consist of multiple drops of the same color ink overlapping one another.
In a preferred embodiment, the medium is incrementally stepped in a direction perpendicular to the direction of scanning by a distance equal to a fraction of the height of a printhead. Thus, the medium will be shifted between each pass, and a portion of the printheads will overlap a printed swath from a prior scan.
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Brenner James M.
Quintana Jason
Wetchler David M.
Hewlett--Packard Company
Yockey David F.
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