REDUCED BANDING IN BIDIRECTIONAL INK JET PRINTING

Abstract: In the particular embodiments of the invention described in the specification, the banding effect resulting from bidirectional printing in an ink jet printer is eliminated by interlacing the lines (26, 27) produced by the ink jet head (23) during motions in opposite directions so that the combination of the spacing between adjacent lines and the edge raggedness (32) or color contrast of the lines is less than the lower limit required for visual detection of banding.

Description: Description Reduced Banding in Bidirectional Ink Jet Printing Technical Field This invention relates to bidirectional ink jet printing systems and, more particularly, to a new and improved bidirectional ink jet printing system which exhibits a reduced banding effect. Background Art In bidirectional ink jet printing systems, a reciprocating ink jet head having an array of ink jet orifices projects corresponding arrays of ink drops onto a record medium during successive passes in its reciprocating motion adjacent to the record medium. In order to produce a composite image, the record medium is normally advanced a distance corresponding to the effective length of the array of ink jet ori¬ fices between successive passes of the ink jet head. Because the placement of the ink drops on the record medium varies depending on the direction of motion of the ink jet head, however, the image portions produced by successive passes of the ink jet head in opposite directions are often slightly different and therefore tend to produce detectable bands in the composite image. In multicolor ink jet printers, in which the ink jet head has several arrays of orifices arranged to project drops of different colors toward the same locations on the record medium so that an additive color such as green is produced by overprinting drops of one color such as yellow on drops of another color such as cyan, the drops of the second color to be printed generally spread less than the drops of the first color, causing portions of the drops of the first color to be visible, which shifts the hue of the combined colors toward that of the first color. Thus, the hue of a green produced by printing cyan over yellow tends to be shifted toward yellow, while the hue of a green produced by printing yellow over cyan will be shifted towards cyan. Consequently, in bi¬ directional color printing, the bands produced by combinations of colored drops projected from orifice arrays during adjacent passes of the ink jet head in opposite directions will have a difference in hue which produces a detectable color banding in the com¬ posite image. Disclosure of Invention Accordingly, it is an object of the present in¬ vention to provide a bidirectional ink jet printing system which overcomes the above-mentioned disadvan¬ tages of the prior art. Another object of the invention is to provide a bidirectional ink jet printing system in which ink drop placement errors and color variations resulting from printing during motion of the ink jet head in opposite directions are not detectable in the compos¬ ite image. These and other objects of the invention are attained by providing a bidirectional ink jet printing system in which an ink jet head has a plurality of orifices which print a corresponding plurality of lines on an adjacent record medium during each pass of the ink jet head with respect to the medium and wherein at least some lines printed during one pass of the ink jet head are interlaced with lines printed during the succeeding pass of the ink jet head so that the spacing between adjacent lines printed by motion of the ink jet head in opposite directions is less than the minimum required for visual detectability of differences in printing. In one embodiment, an ink jet head having an array of orifices which are spaced at twice the spac- ing of adjacent lines in the ink jet image is advanced a distance equal to an odd number of lines after one pass and a distance equal to a different odd number of lines after the next pass, the total number of lines corresponding to the distance advanced after the suc¬ cessive passes being equal to twice the number of orifices used in printing during each pass. Further objects and advantages of the invention will be apparent from a reading of the following de- scription in conjunction with the accompanying draw¬ ings in which: Brief Description of Drawings Fig. 1 is a schematic diagram showing the lines produced by successive passes of an ink jet head in a conventional system, exaggerated to illustrate the banding effect; Fig. 2 is a graphical representation showing the limit of visual detectability of edge raggedness in an ink jet image as a function of error size and spatial period; Fig. 3 is a graphical representation illustrating the threshold for visual detectability of image-band¬ ing as a function of contrast and spatial period; Fig. 4 is a schematic diagrammatic view illus- trating the operation of one form of ink jet system for eliminating the effects of banding in accordance with the invention; and Fig. 5 is a schematic diagrammatic view illus¬ trating the operation of another form of ink jet sys- tem arranged to eliminate the effects of banding in accordance with the invention. Best Mode for Carrying Out the Invention In the schematic illustration of Fig. 1 showing the operation of a typical prior art ink jet system, an ink jet head 10 has an array of orifices 11 from which ink is ejected drop by drop onto a substrate 12 during successive transverse motions of the head 10 with respect to the substrate 12 to produce a pattern 13 o the substrate. The head 10 is driven in a re¬ ciprocating path adjacent to the substrate as indi- cated by the arrow 14 and, for bidirectional printing, i..e. printing in both directions, the substrate 12 is advanced in the direction of the arrow 15 after each pass of the head 10 a distance 16 which is equal to the effective length of the array of orifices 11 plus the spacing between orifices. In Fig. 1 the location of the head 10 with respect to the substrate 12 prior to each pass subsequent to the first pass is illus¬ trated in phantom. In order to provide high resolution printing, the spacing between the orifices 11 in the ink jet head 10 is as small as possible and, to reduce the spacing between adjacent lines 17 produced by the drops from adjacent orifices 11, the array of orifices in the head 10 may be tilted at an angle to the direction of reciprocating motion 14 of the head rather than being perpendicular thereto as shown in Fig. 1. Alter¬ natively, the head may include two or more adjacent arrays of orifices with the orifices of one array located at positions between those of the orifices of another array. In the schematic illustration of the head 10 in Fig. 1, only eight orifices 11 are illus¬ trated, but it will be understood that the array of orifices in the head may be much larger, totalling, for example, 48 or 96 orifices. In addition, the schematic illustration in Fig. 1 shows lines printed from all orifices during each pass of the head 10 and shows that the substrate is ad¬ vanced between successive head passes by a distance equal to the effective length of the orifice array plus the spacing between orifices in the direction of motion of the substrate. For multicolored printing, however, an ink jet orifice array may include adjacent groups of orifices projecting inks having different subtractive colors. In order to provide color combi¬ nations, such as blue from a combination of magenta and cyan, the arrays of cyan and magenta ink orifices must be arranged and controlled so that one of those inks is printed over the other one on a subsequent pass of the ink jet head. Because the head 10 is moving in the opposite direction during successive passes, the placement of drops laid down from each orifice is slightly differ- ent and, even though the initiation of drop ejection can be varied electronically, it is, as a practical matter, difficult to eliminate the difference in drop placement resulting from bidirectional printing. Con¬ sequently, as shown in Fig. 1, distinct bands of lines 18 are often produced during successive passes of the ink jet head in opposite directions in which the ends of the lines 17 do not line up perfectly but instead are displaced by a distance 19, producing an image constructed from visually perceptible adjacent bands. Fig. 2 illustrates on a logarithmic scale the effect of the spatial period of the bands, i.e.. , twice the band width 16, and the edge raggedness, i.e.., the difference 19 between the location of the ends of adjacent bands on the visual detectability of the bands. In the region below the line 20 in Fig. 2, banding resulting from edge raggedness is not visually detectable, and in the region above the line 20 edge raggedness may be visually detectable. Thus, when the edge raggedness is more than about 0.002 inch (0.05mm), it is visually detectable regardless of the spatial period of the bands and, when the edge ragged¬ ness is about 0.001 inch (0.025mm), it is detectable if the spatial period is between about 0.007 and 1.0 inches (0.175mm and 25mm) whereas, if the edge ragged- ness is about 0.0002 inch (0.0005mm), it is visually detectable only if the spatial period is between about 0.015 and 0.10 inch (0.375 and 2.5mm). Even if the edges of adjacent bands are in per¬ fect alignment, bidirectional ink jet printing may cause variations in density or hue which produce visu¬ ally detectable color banding. For example, as de- scribed above, colors produced by overprinting a com¬ bination of primary colors may differ depending upon the order in which the primary colors are applied to the subs-trate. Fig. 3 illustrates on a logarithmic scale the effect of variations in contrast between adjacent bands, with percent contrast represented on the ordinate scale, and variations in the spatial period represented in inches on the abscissa scale, on the visual detectability of such bands in an ink jet image. The line 21 in Fig. 3 represents the limit of visual detectability when the banding is a square wave function of intensity and the line 22 in Fig. 3 repre¬ sents the limit when the banding is a sinusoidal func¬ tion of intensity. As shown by both the lines 21 and 22, where the contrast between adjacent bands is 10%, banding is detectable for all spatial periods greater than about 0.01 inch (0.25mm), whereas for 1% contrast between adjacent bands, banding is detectable for sinusoidal variations in density only if the spatial period is between about 0.02 and about 0.5 inch (0.5mm and 1.25mm) and for square wave variations of density at all spatial periods above 0.02 inch (0.5mm). In order to eliminate detectable banding result¬ ing not only from edge raggedness but also from den¬ sity variations in accordance with the invention, the spatial period of any bands produced as a result of bidirectional printing should be kept below the threshold for detectability not only with respect to edge raggedness but also with respect to contrast. Preferably, bidirectional printing in accordance with the invention is arranged to produce bands having a spatial period of no more than about 0.01 inch (0.25mm) and, desirably, the spatial period of any bands produced by bidirectional printing are less than 0.005 inch (0.13mm). For this purpose, the ink jet system of the pres¬ ent invention is arranged to interlace the lines pro- duced by successive passes of the ink jet head so that the width of adjacent image portions produced by mo¬ tion of the head in opposite directions is no more than the spacing between adjacent lines of the image. With adjacent line spacing of about 0.003 inch (0.075mm), any bands resulting from bidirectional printing, whether resulting from edge raggedness or color variations, have a spatial cycle which is below the visual threshold of an observer. To provide such interlacing, the ink jet head may, for example, have an array of orifices spaced at twice the intended line spacing in the image and the substrate may be moved by a distance equal to the adjacent line spacing in the image at one end of the bidirectional motion of the head and by a distance equal to the total length of the array minus the spac¬ ing between adjacent image lines at the opposite end of the array. Alternatively, the substrate may be advanced by a distance corresponding to three or five lines, for example, at one end of the head motion and by a distance equal to the total array length minus three or five lines at the opposite end of the head motion. In general, where there are N active orifices in the ink jet head and the total number of lines produced by two successive motions of the head is 2N, the substrate should be advanced by a distance equal to an odd number of image lines at one end of the head motion and by a distance equal to 2N minus that odd number of lines at the opposite end of the head mo¬ tion. Fig. 4 illustrates schematically the arrangement of a typical ink jet system arranged in accordance with the invention. In this instance, an ink jet head 23 has eight orifices 24 spaced by a distance 25 of 0.006 inch (0.15mm) which is twice the intended spac¬ ing between adjacent lines 26 and 27 in the image 28 to be generated by operation of the ink jet head. During the first pass of the head 23 to the right as viewed in Fig. 4, eight image lines 26 spaced by the distance 25 are produced on a substrate 29 and, at the end of that pass, the substrate 29 is advanced by a distance equal to half the orifice spacing 25. There¬ after, as the head 23 moves to the left, it produces a set of eight image lines 27 spaced midway between the image lines 26 produced during the first pass of the head. At the end of that pass, the substrate 29 is advanced a distance equal to 7.5 times the orifice spacing (i.e . ., 15 image lines) and the head then moves to the right as viewed in Fig. 4, producing another set of eight image lines 31. Because of the bidirectional printing, the adja¬ cent ends of the lines 26 and 27 are displaced by a distance 32, producing an edge raggedness in the image. Because the spatial period of the edge ragged¬ ness is equal to the orifice spacing 25, which in this case is 0.006 inch (0.15mm), the raggedness lies in the region below the line 20 in Fig. 2, so that it cannot be visually detected. In a color printing system, the number of ori¬ fices supplied with black ink may be larger than the number supplied with ink of each of the primary colors. In this case, the orifices supplying black ink may be alternated with color ink orifices in the ink jet head so that, when black images are being printed, the printing speed can be twice that obtained when color images are being printed. Fig. 5 illustrates a typical ink jet head ar¬ ranged to produce both black and multicolor printing in accordance with the invention. In this case, a head 33 has an array 34 consisting of 16 orifices arranged to project black ink and three adjacent ar¬ rays 35, 36 and 37 of eight orifices each arranged to project yellow, magenta and cyan inks, respectively, these orifices being aligned with the alternate ori¬ fices in the array 34. When the head 33 is used for bidirectional color printing, all of the colored ink orifices in the arrays 35, 36 and 37 and the eight black ink jet orifices in the array 34 which are aligned with the colored ink orifices are used, and the substrate is advanced after successive passes of the head in the same manner described above with re- spect to Fig. 4 to provide an interlaced image. Thus, any differences in intensity or hue resulting from the reversal of the order of laying down of colored ink drops has a spatial period which is less than the lower limit for visual detection of the banding effect as shown in Fig. 3. When the head 33 is used to print black images, the entire array of black orifices 34 may be utilized if edge raggedness is not a problem. In this case, the substrate is advanced by a distance equal to the length of the array 34 plus the distance between adja¬ cent orifices in that array between successive passes of the head, thereby producing images at twice the speed of color printing. If banding is produced as a result of edge raggedness, only alternate orifices in the array 34 are used, and the lines produced by suc¬ cessive passes of the ink jet head are interlaced in the manner described with respect to Fig. 4. Although the invention has been described herein with reference to specific embodiments, many modifica- tions and variations therein will readily occur to those skilled in the art. Accordingly, all such vari¬ ations and modifications are included within the in¬ tended scope of the invention.

Claims: Claims 1. A bidirectional ink jet printing system compris¬ ing ink jet head means mounted for reciprocal otisn with respect to a substrate, means for advancing a substrate in a direction transverse to the reciprocal motion of the ink jet head means, orifice means in the ink jet head means providing an array of orifices which are spaced in the direction of motion of the substrate so as to produce a plurality of image lines on the substrate during successive reciprocating motions of the ink jet head means, and means for advanc¬ ing the substrate by a distance equal to an odd multiple of half the distance between the adja- cent lines following motion of the ink jet head means in one direction to cause interlacing of lines produced by motion of the ink jet head means in one direction with lines produced from the same set of orifices during motion of the ink jet head means in the opposite direction to pro¬ vide a spacing between the lines printed during successive motions of the ink jet head means in opposite directions which is less than about 0.005 inch (0.125mm). 2. A system according to Claim 1 including means for advancing the substrate at the end of the second motion of the ink jet head by a distance equal to the length of the array of orifices in the direc¬ tion of motion of the substrate plus half the distance in that direction between adjacent ori¬ fices in the array minus the distance the sub¬ strate was advanced at the end of the preceding motion of the ink jet head. 3. A system according to Claim 1 wherein the ink jet head means includes a plurality of arrays of orifice means for overprinting image elements with inks having different colors and wherein the plurality of arrays of orifice means prints adja¬ cent lines during successive motions of the ink jet head means having a spacing and color con¬ trast resulting from overprinting of colors in reverse order which is less than the lower limit for visual detection of banding. 4. A system according to Claim 1 wherein the ink jet head means produces adjacent lines during oppo¬ site successive motions of the ink jet head means so that the nonalignment of the ends of adjacent lines is less than the lower limit for visual detection of edge raggedness. 5. A method for preventing perceptible image banding in bidirectional ink jet printing comprising reciprocating an ink jet head having an array of orifices adjacent to a substrate, printing a first array of lines on the substrate during motion of the ink jet head in one direction with respect to the substrate, moving the substrate in a direction transverse to the motion of the ink jet head by a distance corresponding to an odd multiple of half the distance between the lines of the first array, and printing a second array of lines at least partially interlaced with the lines of the first array during motion of the ink jet head in the opposite direction having a spac¬ ing from the image lines of the first array which is less than about 0.005 inch (0.125mm). 6. A method according to Claim 5 including moving the substrate after the second array is printed in the direction transverse to the motion of the ink jet head by a distance corresponding to the length of the array of orifices in the direction o£ motion of the substrate plus half the distance in that direction between adjacent orifices minus the distance the substrate was advanced after the first array of lines was printed. 7. A method according to Claim 5 including over¬ printing image elements with inks having differ¬ ent colors to produce color contrast between adjacent lines printed by overprinting of colors in reverse order which is less than the lower limit for visual detection of banding. 8. A method according to Claim 5 including printing .-. adjacent interlaced lines of the first and second arrays with a spacing such that the nonalignment of adjacent ends of the adjacent lines is less than the lower limit for visual detection of edge raggedness. 9. A bidirectional ink jet printing system compris¬ ing ink jet head means mounted for reciprocal motion with respect to a substrate, means for advancing a substrate in a direction transverse to the reciprocal motion of the ink jet head means, orifice means in the ink jet head means providing an array of orifices which are spaced in the direction of motion of the substrate so as to produce a plurality of image lines on the substrate during successive reciprocating motions of the ink jet head means, and means for produc¬ ing image lines by motion of the ink jet head means in one direction which are spaced from adjacent lines produced by motion of the ink jet head means in the opposite direction by less than about 0.005 inch (0.125mm). 10. A system according to Claim 9 including means for advancing the substrate at the end of the second motion of the ink jet head means by a distance sufficient to place the first line to be produced by the ink jet head means during the next recip¬ rocating motion adjacent to the last line pro- duced by the ink jet head means during the pre¬ ceding motion of the ink jet head means. 11. A system according to Claim 9 wherein the ink jet head means includes a plurality of arrays of orifice means for overprinting image elements with inks having different colors and wherein the plurality of arrays of orifice means prints adja¬ cent lines during successive motions of the ink jet head means having a spacing and color con¬ trast resulting from overprinting of colors in reverse order which is less than the lower limit for visual detection of banding. 12. A system according to Claim 9 wherein the ink jet head means produces adjacent lines during oppo¬ site successive motions of the ink jet head means so that the nonalignment of the ends of adjacent lines is less than the lower limit for visual detection of edge raggedness. 13. A method for preventing perceptible image banding in bidirectional ink jet printing comprising reciprocating an ink jet head having an array of orifices adjacent to a substrate, printing a first series of lines on the substrate during motion of the ink jet head in one direction with respect to the substrate, and printing a second series of lines at least partially interlaced with the lines of the first series during motion of the ink jet head in the opposite direction having a spacing from the first series of image lines which is less than about 0.005 inch (0.125mm). 14. A method according to Claim 13 including moving the substrate after the second series of lines is printed in the direction transverse to the motion of the ink jet head by a distance sufficient to place the first line to be produced by the ink jet head during the next motion of the head adja¬ cent to the last line produced during the preced¬ ing motion of the head. 15. A method according to Claim 13 including over- printing image elements with inks having differ¬ ent colors and including printing the second series of lines so that the combination of the spacing between adjacent interlaced lines of the first and second series and the color contrast between adjacent lines printed by overprinting of colors in reverse order is less than the lower limit for visual detection of banding. 16. A method according to Claim 13 including printing the second series of lines with a spacing between adjacent interlaced lines of the first and second series such that the nonalignment of adjacent ends of the adjacent lines is less than the lower limit for visual detection of edge raggedness.

Inventors: HOISINGTON PAUL A, HOISINGTON, PAUL, A.
Applicants: SPECTRA INC, SPECTRA, INC.
Application year: 1990
Main classification: B41J2
IPC codes: B41J2, B41J19, G06K15, H04N1