Facsimile and static presentation processing – Natural color facsimile – Measuring – testing – and calibrating
Reexamination Certificate
1997-07-31
2001-03-06
Boudreau, Leo H. (Department: 2721)
Facsimile and static presentation processing
Natural color facsimile
Measuring, testing, and calibrating
C358S001900, C358S501000, C347S019000
Reexamination Certificate
active
06198549
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of determining printing misregistration for printers, and more specifically, determining misregistration for each printing station in a printer having multiple printing stations.
2. Description of the Related Art
In order to ensure the best possible print quality from a printer, an operator typically will recalibrate the printer each day. Two of the most important factors that should be calibrated are density and registration. To calibrate the density, the operator will print out a control strip of the different colors (toners) used by the printer. The operator will then use a densitometer to measure the density of each of the patches in the control strip. Then, as a result of the density measurement reading of each of these patches, the operator will make some manual adjustments to the printer to correct for density in the different density ranges. Since maintaining optical density is a very important parameter that the operator needs to control for a printer, most operators have a densitometer for measuring the density.
Also, in a printing process, it is very important that all of the colors are correctly registered since colors are overlaid on top of each other to achieve a desired color as required in the subtractive printing process. If the colors are out of registration, a combination of colors will not lie directly on top of each other, and the quality of the color printing is distorted. In summary, maintaining accurate registration is required to get good color fidelity.
In order to calibrate the printer for registration, the printer will also print out a pattern of very small intersecting lines as shown in FIG.
1
. The patterns are used for the registration of each color of the printer, e.g., cyan, magenta, and yellow, relative to black. The patterns are printed in both the X-direction and the Y-direction to calibrate the registration in both of these directions. For a printer having three colors, e.g., cyan, magenta, and yellow, six targets will be printed: three horizontal and three vertical. A greatly magnified illustration of these targets is shown in FIG.
1
.
Typically, one can only determine the point of intersection with the aid of an eye loop or by using expensive and sophisticated imaging equipment. Ideally, the line should intersect at 0. If the line intersects at +1, the operator has to adjust the registration of the printer to make a correction for +1. Using an eye loop involves a “manual” process of eyeballing whether registration is off by examining the targets of colors that are laid on top of one another. With the aid of an eye loop, it may be difficult to readily determine whether the registration is +1, +2, 0, −1, or −2. Typically, experienced operators can only achieve a registration accuracy within +1 or −1 pel. Basically, these patterns are very difficult to read. For example, typically all of the patterns together, shown in
FIG. 1
, would take up an area of only about 0.5×0.25 inches.
As a typical scenario, an operator will, on a daily basis, use a densitometer to measure printed patches for adjusting the optical density of the printer, and will use an eye loop to measure another group of patterns for adjusting the registration. The patterns, i.e., targets, are typical of what are used in the industry to measure registration and the eyeball is the measuring device. Consequently, this registration process is totally nonautomated.
Although there are a variety of ways for determining misregistration, most of the currently known techniques are similar in that they all compare a line or dot to a reference line, as just described. Then, corrections are made based upon a difference in location with respect to an ideal value or reference distance. The measurement of this variation from the reference distance can be done in an automated manner via sophisticated and expensive image analyzer equipment, or done in a nonautomated manner via the use of a microscope or the equivalent. It may not be cost justified to use expensive image analyzer equipment for determining misregistrations in a relatively low-cost printer. On the other hand, the inaccuracies and lack of ease of use of the manual determinations may not be sufficient, either.
SUMMARY OF THE INVENTION
It is, therefore, an object of this invention to maintain registration for optimizing color fidelity and crispness of a printed image including when an image is made by printing overlapping colors from multiple printing stations.
It is a further object of this invention to determine misregistration without relying on an “eyeball” measurement, and without using expensive and sophisticated equipment.
The system, method, program, and print pattern of this invention allow print misregistration to be detected and controlled by measuring density values using an optical densitometer. In one embodiment, the output of the densitometer is tied into the printer control panel and logic. In other embodiments, the densitometer can be used manually by an operator, in which case the operator will input the density results into the printer for automatic registration adjustment, or the operator will determine the resulting misregistration and make the adjustments manually on the printer. Special print patterns are used that have an output density that can be direly related to the amount of misregistration.
The special overall composite print pattern consists of a first repeating pattern of a printed horizontal bar having a specified width (e.g., 8 pels in the preferred embodiment) followed by a white space having, preferably, but not necessarily, the same specified width. The bars are printed by a first printing station, e.g., for the color black. Superimposed on this repeating pattern printed by the first printing station, but offset in a downward direction by half the width of the bars, is another repeating pattern, printed by another print station, e.g., for the color magenta, of a printed bar followed by a white space, all of which, preferably, but not necessarily, have the same specified width. The second repeating pattern is similar to the first repeating pattern described above except that the superimposed pattern is offset in the opposite direction than the offset direction in the first repeating pattern. This describes the overall composite print pattern for a nominal setup.
The above-described overall composite print pattern is used to determine misregistration in the vertical direction. To determine misregistration in the horizontal direction, a similar overall composite print pattern is printed except that the bars are vertical and the superimposed offsets are in the horizontal direction. A set of these overall composite print patterns, for determining both horizontal and vertical misregistration, are printed using each printing station to determine the misregistration of each printing station.
Initially, for a given printer, and for each printing station therein, the above-described overall composite print pattern, having a first and second repeating pattern, is printed at varying incremental amounts of misregistration, including zero pel misregistration as determined by previously known methods. The optical densities of the first and second repeating pattern are measured using a densitometer. The optical density of the second repeating pattern is then subtracted from the optical density of the first repeating pattern to obtain a density difference for each incremental amount of misregistration. A correlation between density difference and amount of misregistration is then determined for the given printer and, if necessary, for each printing station therein, for each horizontal and vertical direction.
Once the correlation is determined, the misregistration at any given time can be determined for each direction and for each color by 1) printing an overall composite print pattern; 2) measuring the optical densities of the first repeating pattern and the second repeating
Decker William Chesley
Lee Ho Chong
Zable Jack Louis
Boudreau Leo H.
Dawkins Mariyln Smith
International Business Machines - Corporation
Konrad Raynes & Victor
Sherali Ishrat
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