JPH0624877B2 - Inkjet recording method - Google Patents

Description

The present invention relates to a halftone recording method using an inkjet head.

(Prior Art) Conventionally, as a method of performing halftone recording in ink jet recording, when the printing means can only perform constant density, that is, binary recording of whether or not to print, it is called a dither method or a density pattern method. I was using the method.
In these methods, the density of dots is changed according to the density to express pseudo halftones. In such a pseudo halftone recording method, in order to increase the number of gradations, it is necessary to increase the number of dots in the unit matrix that expresses the halftone. Is required. For high-density recording, the practical limit is about 20 dots per mm, and it is technically difficult to achieve higher densities. Further, there has been a problem that the recording time is greatly increased in the high density recording. On the other hand, if one print dot can express multi-valued density levels, the above problem is solved. An on-demand type ink jet recording apparatus is known as one of printing means in which such print dots express a multi-valued level. For example, an on-demand type ink jet head can change the volume of an ink droplet substantially in proportion to the pulse width of a driving pulse. That is, by modulating the drive pulse width in accordance with the input density signal, the area of the printed dots changes and halftone is reproduced. However, in an on-demand type inkjet head, generally, when the drive pulse width is modulated, the flight speed also changes, and the dot hitting positions do not have a constant pitch, and it is difficult to obtain good recording due to gaps and overlapping dots. It was On the other hand, the head disclosed in Japanese Patent Laid-Open No. 60-052352 is an improved head so that the change in the flight speed with respect to the pulse width is reduced, but the head is still within the speed fluctuation that does not affect printing significantly. If limited, the variable range of the drop volume is about 4 times, and the dot area when printed is also about 4 times.
When printing is performed using such dots When recording is performed with dot densities that overlap each other in the maximum dots, a dot area modulation of about 4 times produces a halftone reproduction range of 0.4 to 0.4 in reflection density. Up to about 1.5.
Within this range, continuous recording density can be obtained, but it is not possible to reproduce bright areas with recording density of 0.4 or less.

As a method for solving this, as shown in FIG. 12, when the input density signal is high, dot area modulation is used, and when the input density D _{I which} can be expressed by the minimum droplet is expressed, the dot is formed by the dither method or the like. A method using density modulation is known. That is, halftone recording from low density to high density is performed by selectively using dot area modulation and dot density modulation according to a predetermined density threshold. In this method, the resolution is lowered at low density, but in monochrome recording, when the resolution of medium and high density is high, the overall image quality is not so deteriorated. However, in full-color recording, since the hue is expressed by the mixing ratio of the three primary colors, the deterioration of the resolution is clearly perceived in the portion where the mixing ratio is small, that is, the portion where the density is low, and the image quality deteriorates.

Therefore, as shown in Japanese Patent Laid-Open No. 60-220757, a plurality of inkjet heads having different nozzle diameters are used, and recording density areas corresponding to the respective nozzle diameters are shared so that a wide density range can be obtained as a whole. A method of expressing by modulation is known. For example, as shown in FIG. 13, an ink jet head with a large nozzle diameter records a density range of 0.4 to 1.5, and an ink jet head with a small nozzle diameter that is 1/2 the large nozzle prints a density range of 0.15 to 0.4. It is a thing. This makes it possible to express the density range of 0.15 to 1.5 by using the large nozzles with high resolution dot area modulation.

(Problems to be Solved by the Invention) When recording is performed using nozzles having different diameters by such a conventional method, the dots recorded by each nozzle are on regular grid points determined by a predetermined dot density. When printed accurately, the density pattern of the original image is reproduced, and good recording can be obtained by dot area modulation from low density to high density. However, if the printing deviates from the grid points, the density may change, or the image may be deteriorated due to the pseudo contour. In recording with a single nozzle, the cause of print misalignment is a variation in the ejection speed of the inkjet head due to a droplet ejection cycle or a variation during droplet volume modulation. However, these fluctuations are gradual and do not change discontinuously with respect to the change of the pulse width for the ejection cycle and the drop volume modulation. Therefore, when recording, if the density of a person's face changes smoothly, even if a deviation from the ideal grid point occurs, the distance between dots does not change discontinuously and is smooth. There was almost no deterioration in image quality because it changed to. However, when using heads having different nozzle diameters, it is necessary to further consider the mounting position error of each head, the unevenness of the scanning speed, etc. as factors of the printing position error. The deviation of the printing position due to these appears as a relative deviation between the dot patterns printed by different heads. For example, the two areas 1 and 2 in FIG. 14 (a) are divided by the density value at which the print head switches.
Has a high density and the region 2 has a low density distribution. When such an original image is recorded, a recording having a positional deviation as shown in FIG. In FIG. 3B, the area 3 represents the large nozzle head, and the area 4 represents the area printed by the small nozzle head. In this example, a case is shown in which the printing by the small nozzle head is delayed with respect to the printing by the large nozzle head. In this way, discontinuous misalignment occurs in the area where the nozzles that print are switched,
When the dot spacing is widened, white lines 5 are created due to white spots,
Lines with high density due to overlapping of dots 6
Will occur. These lines are particularly noticeable when the density changes smoothly, which causes a large deterioration in image quality.

An object of the present invention is to solve this problem and provide a high-quality halftone recording method.

(Means for Solving the Problems) The gist of the present invention is to use ink jet heads having different nozzle diameters, and to form ink droplets whose volumes are controlled according to electric signals on grid points arranged at a constant pitch on a recording medium. In the inkjet recording method that performs halftone recording by combining ejection of dot area modulation and dot density modulation, the density of the recorded pixel is monotonic as the input image density signal increases in each of the inkjet heads with different nozzle diameters. The ink jet recording method is characterized in that the ink droplet volume is controlled so as to increase, and one pixel is overwritten with ink droplets from ink jet heads having different nozzle diameters.

(Operation) In the present invention, recording is performed using an inkjet head having a different nozzle diameter and capable of modulating the dot area. FIG. 1 shows an example of typical characteristics of an inkjet head capable of modulating the dot area. Such characteristics are obtained by the head having the structure disclosed in JP-A-60-052352.
This head is capable of printing a dot area approximately proportional to the drive pulse length from the minimum drive pulse length t _w1 to the maximum drive pulse length t _w2 . If the nozzle diameter is different as shown in the figure, the dot area can be changed even with the same drive pulse length. Here, for simplification of description, two types of nozzle diameters D ₁ and D ₂ are used for description. First, when recording is performed in a grid pattern with a constant dot density by a head having a large nozzle diameter D ₁ , as shown by the curve (a) in FIG. 2, the maximum driving pulse length t is larger than the recording density D _L1 at the minimum driving pulse length t _w1 . gradation recording to the recording density _{D H1} in _w2 is obtained. Similarly, when recording with the same density ink using a small nozzle type D ₂ head, it is possible to record from the recording density D _L2 to the recording density D _H2 as shown by the curve (b) in FIG. In order to record a density lower than the recording density of each minimum droplet in each ink jet head, dot density modulation by the minimum droplet is used. As a method of performing dot density modulation, a dither method, a density pattern method, an error diffusion method, or the like can be used. In the present invention, an inkjet head having such recording characteristics is used, and the same pixel is overwritten by a plurality of inkjet heads having different nozzle diameters. That is, when a certain pixel is recorded, the driving pulse length of each inkjet head is determined by the density signal at that point, and the ink from each inkjet head is ejected to the corresponding point on the recording paper to obtain a predetermined recording density. is there. Further, an important point of the present invention is to control the ink ejection amount so that the relationship between the recording density and the input density signal monotonically increases without having a maximum or minimum value in each of the inkjet heads having different nozzle diameters. . Further, in an area where the input density signal is lower than the recording density by the smallest droplet of each inkjet head, recording is performed using dot density modulation with an inkjet head with a small nozzle diameter, and an inkjet head with a large nozzle diameter uses the nozzle diameter as the input density signal. Only when the inkjet head is smaller than the level at which dot area modulation is performed, printing is performed. With such a recording method, from the solid recording density D _L2 to the maximum recording density by the smallest dot of the small nozzle head, all grid points are printed by at least the small nozzle or the droplets from the small nozzle and the large nozzle. Therefore, it is possible to obtain recording without a reduction in resolution in a wide density range. Even with respect to the displacement of the print dot with respect to a predetermined grid point, the recording density is monotonically increased with respect to the input density signal in each inkjet head having a different diameter, and almost all the density range is recorded by any head. Therefore, even if the position shift occurs, the discontinuity point or the density reversal at a specific density does not occur. Therefore, it is possible to obtain high-resolution halftone recording in which the image quality is not deteriorated due to the false contour due to whitening or dot overlap.

(Example) Hereinafter, the present invention will be described in detail with reference to the drawings.

Example 1 FIG. 3 (a) shows the recording density characteristics with respect to the input density signal of the small nozzle head, and FIG. 3 (b) shows the print dot area and dot density with respect to the input density signal. Further, FIG. 4 (a) shows the recording density characteristics with respect to the input density signal of the large nozzle head, and FIG. 4 (b) shows the print dot area and dot density with respect to the input density signal. Further, FIG. 5 shows the recording density characteristics when overwriting is performed with the large nozzle head and the small nozzle head.

In the present embodiment, a density area expressing a halftone is divided into two by a large nozzle head and a small nozzle head, and a portion having a density lower than the input density D _I2 is printed by the small nozzle head.
In a portion where the density is higher than the input density D _I2 , the small nozzle head always prints a dot of a certain size, and the large nozzle head performs gradation recording on the dot. Due to such control, even if the printing position is deviated between the nozzles having different diameters, a white line or a line having a high recording density is not generated.

Embodiment 2 FIG. 6 (a) shows the recording density characteristics of only the small nozzle head with respect to the input density signal of the small nozzle head, and FIG. 6 (b) shows the print dot area and dot density with respect to the input density signal. FIG. 7 (a) shows the recording density characteristics of only the large nozzle head with respect to the input density signal of the large nozzle head, and FIG. 7 (b) shows the print dot area and dot density with respect to the input density signal. FIG. 8 shows the recording density characteristics when overwriting is performed by the large and small nozzle heads by performing the control shown in FIGS. 6 and 7. In the present embodiment, the input density signal representing the halftone is not divided into areas by the large nozzles and the small nozzles, and the halftones are expressed by both of them in almost the entire input density signal range. This embodiment is the first
In comparison with the embodiment of FIG. 5, the input density signal area (D _{I1 to} D _I4 ) in which the dot density modulation is performed in the large nozzle is shifted to the low density direction, and the input density D _I1 in which the dot density modulation in the small nozzle head ends Subsequently, dot density modulation recording with a large nozzle head is started. In this way, when recording, a pattern peculiar to dot density modulation appears only in the bright part, and patterns in large and small nozzles are also continuously connected, and recording with excellent image quality uniformity was obtained.

Embodiment 3 FIG. 9 (a) shows the recording density characteristics of only the small nozzle head with respect to the input density signal of the small nozzle head, and FIG. 9 (b) shows the print dot area and dot density with respect to the input density signal. FIG. 10 (a) shows the print density characteristics of only the large nozzle head for the input density signal of the large nozzle head, and FIG. 10 (b) shows the print dot area and dot density for the input density signal. FIG. 11 shows FIG. 9 and FIG.
The print density characteristics when the large nozzle head and the small nozzle head were used to perform overwriting with the print control shown in FIG. 10 were shown.

In this embodiment, from the density D _{I1 at} which the dot density modulation recording by the small nozzle head ends, the dot density modulation recording by the large nozzle head continues,
Perform to a concentration D _I5 . At this time, with the small nozzle head, recording is performed with a constant density by the smallest droplet from the input density D _I1 to the input density D _I5 . From the input density D _I5 to the input density D _Imax , the large nozzle head and the small nozzle head both increase the dot area at a constant rate. By such printing control, the dot density modulation areas (D _{I1 to} D _I5 ) by the large nozzles are changed to the dot density modulation areas (D _{I1 to} D _I4 ) in the second embodiment.
Narrower than. As a result, a record with less roughness was obtained.

(Effect of the Invention) As described above, according to the present invention, it is possible to control the ejection of ink droplets by using the inkjet heads having different nozzle diameters so that the recording density characteristic with respect to the input density signal of each inkjet head monotonically increases. By performing overwriting with ink jet heads having different nozzle diameters, it becomes possible to perform recording by dot area modulation down to a low density that cannot be recorded by a conventional single nozzle head. In addition, white lines due to dot misalignment and lines due to dot overlap caused by misalignment of dots printed by different nozzles, which are often seen in the conventional method in which printing is performed by selecting nozzles to be printed according to recording density using multiple nozzles, It disappeared by the method of overwriting according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are ink droplet volume modulation characteristics and recording density characteristics with respect to drive pulse length as an example of the characteristics of an inkjet head used in the present invention, FIGS. In each case, the relationship diagram of (a) recording density of the small nozzle head with respect to the input density signal, (b) relationship diagram of dot area and dot density, and (a) recording of the large nozzle head for explaining the first embodiment FIG. 5 is a density characteristic diagram, (b) a relationship diagram of dot area and dot density, FIG. 5 is a recording density characteristic diagram with two large and small nozzles in the first embodiment, and FIGS. 6 to 7 are each the second embodiment. (A) relationship diagram of print density of small nozzle head to input density signal for explanation, (b) relationship diagram of dot area and dot density, and (a) relationship diagram of print density of large nozzle head to input density signal , (B) dot area and FIG. 8 is a relationship diagram of dot density, FIG. 8 is a recording density characteristic diagram of two nozzles of large and small in the second embodiment, and FIGS. 9 to 10 are small nozzles for input density signals for explaining the third embodiment. (A) Relationship between the recording density of the head, (b) Relationship between the dot area and recording density, and (a) Relationship between the recording density of the large nozzle head and the input density signal, (b) Relationship between the dot area and recording density FIG. 11 is a relationship diagram, FIG. 11 is a recording density characteristic diagram with two large and small nozzles in the third embodiment, FIG. 12 is a diagram showing ink droplet control used for halftone recording in single head recording according to the prior art, and FIG. FIG. 14 is a diagram for explaining a halftone recording method using heads having different nozzle diameters according to the prior art, FIG. 14 (a) is an original image pattern having a density distribution, and FIG. 14 (b) is a recording pattern of a two-nozzle head. It is the figure which showed one example. In the figure, 1 is a high density area, 2 is a low density area, 3 is a recording pattern by a large nozzle head, 4 is a recording pattern by a small nozzle head, 5 is a white line, and 6 is a high density line.

Claims (1)

[Claims] 1. An ink jet head having different nozzle diameters is used to eject ink droplets whose volumes are controlled according to electric signals, to a grid point on a recording medium that is arranged at a constant pitch, thereby performing dot area modulation and dot density modulation. In an inkjet recording method in which halftone recording is performed in combination, the ink droplet volume is controlled so that the density of a recorded pixel monotonically increases with respect to an increase in an input image density signal in each inkjet head having different nozzle diameters, and An inkjet recording method, wherein one pixel is overwritten with ink droplets from inkjet heads having different nozzle diameters.