Incremental printing of symbolic information – Thermal marking apparatus or processes – Density control
Reexamination Certificate
1998-12-07
2001-11-06
Tran, Huan (Department: 2861)
Incremental printing of symbolic information
Thermal marking apparatus or processes
Density control
C347S019000
Reexamination Certificate
active
06313857
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a technique for correcting recording density irregularity in image recording by using a line head.
To prepare a printing plate from a color sheet of manuscript to thereby perform printing on a large number of sheets of paper, proof printing is performed and then final printing is performed after the finished state of the proofs is checked. In the proof printing process, a color proof is produced and the image quality of the color proof is checked. The color proof is obtained by the steps of: thermally transferring an image to a receiver sheet by a thermal printer; and transferring the image formed on the receiver sheet to a sheet of paper.
As is known well, in an image recording apparatus such as a thermal printer, or the like, while a glaze formed by arrangement of heating resistors corresponding to one-line's pixels in one direction (a first main scanning direction) is relatively moved, together with a printing medium, in the subsidiary scanning direction substantially perpendicular to the direction of the arrangement of the heating resistors in the condition that the glaze is slightly pressed against the printing medium, the respective heating resistors in the glaze are heated in accordance with image data of a recording image. As a result, an image is thermally transferred to the printing medium to thereby form a recording image.
In the image recording apparatus using the aforementioned recording method, when, for example, a recording image is formed by use of image data having one and the same predetermined-gradation value, recording density irregularity such as shading, or the like, in which the density of the formed recording image varies in accordance with the heating resistors, may occur. This is inevitable because the shape of the glaze of the line head is not always uniform. In some image processing apparatus, density irregularity correction is performed for image data in advance to prevent lowering of image quality caused by density irregularity.
A specific example of the density irregularity correction method is the following method.
First, an image is recorded on the basis of image data having a uniform gradation in the first main scanning direction of the line head. The thus recorded image is scanned by a densitometer such as a scanner, or the like, moving relatively in the subsidiary scanning direction of the line head to thereby detect printing density. The printing density obtained for each pixel is compared with the gradation value of the image data to thereby correct the gradation value of the image data.
In the conventional density irregularity correction method described above, however, printing densities in different positions of the recording image are detected individually by detection elements of the scanner. Accordingly, the printing-density value may vary in accordance with the individual sensitivity characteristic of the detection elements of the scanner. Generally, the sensitivity characteristic of the detection elements of the scanner is not uniform in the strict sense, so that printing densities to be detected as the same value originally are detected as different density values. As a result, the correction values which are set on the basis of the detected printing-density values become incorrect. There arises a problem that appropriate density irregularity correction cannot be made.
SUMMARY OF THE INVENTION
The present invention is designed to provide density irregularity correction methods for correcting density irregularity accurately without influence of individual sensitivity characteristic peculiar to a scanner of an image recording apparatus.
In order to achieve the above object, according to an aspect of the present invention, provided is a method for correcting density irregularity in image recording using a line head, by comprising the steps of: printing a belt-like pattern at a predetermined-gradation value in the first main scanning direction of the line head; relatively moving a line sensor along the first main scanning direction of the line head while aligning a second main scanning direction of the line sensor, which is corresponding to a scanning direction of said line sensor, with the subsidiary scanning direction of the line head to thereby detect printing density of the belt-like pattern; obtaining a correction condition for each pixel position on the basis of the detected printing-density value and the predetermined-gradation value; and correcting image data for image recording on the basis of the correction condition.
Preferably, in the above method, the belt-like pattern includes a plurality of belt-like pattern portions printed at least two gradation values selected in the proximity of the predetermined-gradation value; and averages of printing-density values are obtained for the plurality of belt-like pattern portions respectively and the correction condition is set on the basis of the change ratios of the obtained averages of printing-density values to the gradation values of the belt-like pattern portions.
For example, in the case of belt-like pattern portions printed at two gradation values, a correction condition is set on the basis of the ratio of the difference between averages of printing-density values to the difference between the gradation values. In the case of belt-like pattern portions printed at three or more gradation values, the distribution of printing-density values for the gradation values is subjected to linear approximation, or the like, to obtain change ratios, so that correction conditions are set on the basis of the change ratio.
Preferably, in the above method, the belt-like pattern includes first and second belt-like pattern portions which are printed at a first gradation value as the predetermined-gradation value and at a second gradation value near to the first gradation value, respectively; and averages of printing-density values are obtained for the first and second belt-like pattern portions respectively and the correction condition is set on the basis of the ratio of the difference between the obtained averages to the difference between the first and second gradation values.
Preferably, in the above method, the predetermined-gradation value is any one of gradation values obtained by division of a gradation range up to the maximum gradation value into a plurality of equal stages; and correction conditions for all gradations are set by interpolation on the basis of individual correction conditions set for the respective stages. For example, in the case of image data of 256 gradations, a range of from 0 to 255 is substantially divided into four equal parts so that five gradation values of 0, 64, 128, 192 and 255 are set. The values of 0 and 255 are not corrected and correction conditions for the gradation values of 64, 128 and 192 are obtained. Correction conditions for gradation values of 0 to 255 are set approximately by interpolation on the basis of the obtained individual correction conditions for the gradation values of 64, 128 and 192.
According to another aspect of the present invention, provided is an image recording apparatus comprising a correction value table storage portion for storing a correction value table for gradation value correction determined on the basis of printing-density values of image patterns printed, an image memory for storing image data corrected on the basis of the correction value table, and an image correction control portion for controlling correction of image data, characterized in that density correction data obtained by a method such as will be shown in this invention are stored in the correction value table storage portion.
Preferably, in the above method, an image is recorded, by a thermal head, on a thermal transfer recording material having a substantially transparent heat-sensitive ink layer, the ink layer containing 30 to 70 parts by weight of pigment and 25 to 60 parts by weight of amorphous organic high-molecular polymer having a softening point of from 40° C. to 150° C., the ink la
Fuji Photo Film Co. , Ltd.
Sughrue Mion Zinn Macpeak & Seas, PLLC
Tran Huan
LandOfFree
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