JPS5915363A - Image forming method - Google Patents

Abstract

PURPOSE:To display a natural and elegant picture which is free from ''roughness'', by specifying the rate of region of a gradation level which is expressed by an image forming element of the highest optical density when the picture information having gradations such as characters, patterns, graphs, pictures, etc. are displayed. CONSTITUTION:The rate of gradation property which is expressed by an image forming element of the highest optical density in formation of an image by an ink jet recording method, etc. is set at <=70% entire area of a region having gradations which is displayed by each image forming elements using different types of ink different in density. In this example, plural types of ink having different color densities are used. In other words, an ink jet head unit 10 is used. In this case, inktanks 8 and 9 having heads 6 and 7 containing different types of ink different in density are connected to the head unit 10. This ensures a display of a wide range of gradations and reproduces pictures with high fidelity.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image forming method for expressing so-called image information in a visually perceivable state, in which a two-dimensional floor such as characters, figures, graphs, images, etc. Regarding the image forming method that expresses image information with gradation.
For example, in the field of image information recording, a method of expressing image information including information in a visually perceivable state is as follows: A conventional example of this is to control the number of iron-forming elements having a constant optical density and a constant area provided within the micro-surface abutment and the arrangement state within the micro-surface abutment.

According to this method, if you want to improve the gradation, it is necessary to either increase the area of one pixel itself (pixel area) or to reduce the area of the image forming element that occupies one pixel as much as possible. .

In the former case, it is an undeniable fact that the larger the pixel area, the further the resolution decreases, so this is not desirable from a practical standpoint.

In the latter case, in order to widen the range of gradation expression from gradation levels with higher optical density to gradation levels with lower optical density, it is necessary to increase the color density (optical) of the image forming element and Although it is necessary to reduce the area, if a relatively low-level gradation area is expressed using an image-forming element with a small area and high color density, the image quality will have a ``grainy'' or ``scattered'' appearance. This may result in an image that is difficult to view, resulting in a so-called unnatural image.

In particular, in the case of photographic images, unnaturalness is noticeable in the reproduction of illumination areas and human skin areas, and it is difficult to smoothly and continuously express a wide range of intermediate tones.

On the other hand, for example, ink dots (one of the image forming elements) can be formed by forming flying ink droplets and depositing the ink droplets on a recording medium (for example, paper, plastics, ceramics, etc.). In the inkjet recording method, which records images using
Different color densities (in the case of color recording, for each color)
It has already been proposed to perform recording by using liquid or solid ink.

When using such a recording method to express an image portion with equal optical reflection density (hereinafter referred to as "average reflection density") in a predetermined area, the dot pitch (dot arrangement pitch) must be lower than the desired dot pitch. Expression by forming small-area dots with high-density (high color density) ink and low-density (
Two expressions are possible by forming large-area dots with (low color density) ink. However, the use of inks with different densities is not limited to two types, and the same can be said in the case where two or more types or multiple types of colors are used.) Heigo et al., Kariyo, Even if the average reflection densities of the two displayed images are approximately between the two, when viewing the resulting image, the human eye perceives a large difference in image quality. It appears as if there is a difference. Of course, this will vary depending on the ink density and dot pitch, but in general, images expressed using high-density ink and forming small-area dots will have an accentuated "roughness." This has often been a major cause of loss of naturalness in images in general image expression.

Also, if you try to express the lowest density (lowest gradation level) in image expression by not forming dots, there will be white areas on the screen, and the white areas will be filled with the formation of other dots. The tone of the image clearly changed from the part where the image was removed, which was a factor in lowering the image quality.

The present invention has been made in view of the above-mentioned problems, and is an image forming method capable of expressing images with good image quality, that is, calm and natural images that do not have "roughness" or "jarness". The main purpose is to propose.

In addition, the "image forming element" used in this specification is
For example, it refers to what corresponds to the recorded dots on the recording medium in so-called dot recording, which performs recording by forming dots such as inkjet recording, thermal recording, thermal transfer recording, wire tod recording, and electrostatic recording. . Incidentally, although the same applies to the present invention, when one pixel is constituted by a plurality of image forming elements, an image forming element and one pixel are understood to be different things. In addition, "optical reflection density" and "density (color density)" of ink, which is frequently used in the following explanations,
” refers to the optical reflection density and optical transmission density measured using a commercially available densitometer, respectively. In measuring the reflection density, widely used standard white paper is used as the reference value. The value when using

The image forming method of the present invention is an image forming method that expresses an image with gradation by controlling the area occupied by each of a plurality of image forming elements having different optical densities. The proportion of the gradation level area expressed by each element body is calculated as 7 of the total gradation level area expressed by each image forming element body.
It is characterized by expressing gradation so that it is below 0.

Furthermore, the area of the image forming element expressing the lowest level of gradation among the gradations expressed by the image forming element having the highest optical density is expressed by the image forming element having the lowest optical density. It is characterized by the area K being greater than or equal to the area of the image forming element that expresses the lowest level of gradation.

Furthermore, it is also characterized by arranging the image forming elements having the highest optical density so that the duty ratio is 0.5 or more in any arrangement direction.

According to the image forming method of the present invention, it is possible to easily produce a high-quality, natural-looking, grainy image.

In addition, a wide range of intermediate tones can be expressed smoothly and continuously without losing naturalness in reproducing the Baylert part or the skin part of a person.

Furthermore, even in full-color image expression, high-quality images with sufficient naturalness can be obtained.The present invention will be described in detail below with reference to the drawings.

First, an example of theoretical analysis of image design, which is the basis of the image forming method of the present invention, will be described using a simplified model.

In other words, the target is an image expressed by recording dots of a certain color density, a constant dot pitch, and a constant dot diameter over a sufficiently wide area on the recording medium. We will explain an example in which we consider a one-dimensional model as shown in Figure 2, and analyze it in terms of spatial frequency.

In reality, the dots are arranged two-dimensionally, but since it can be understood in terms of spatial frequency, on a straight line passing through the center of the dots,
You can think dimensionally.

The concentration distribution is as shown in f←) in Fig. 1. X is the position coordinate on the straight line, and y is the optical reflection density at the position X. α is the reflection density of the recording medium (paper, etc.). (Optical reflection density knee 1oyα.), the reflection density of colored (including both achromatic and chromatic) dots is α, (optical reflection density knee loyα).
ogα1), the dot radius is bl, the dot pitch is T, and α−α. −α.

shall be.

Assuming that the dots are lined up over a sufficiently wide area on the recording medium, and the number of dots is, for example, (2N+1), then f
Assuming that N is sufficiently large, the first term of equation +1) can be regarded as a delta function.

Furthermore, since we set ω=〒, if we let FN(ω) when N is sufficiently large be F(ω), we end up with the following. An example of the result of equation (3) is shown in FIG. In this way, in addition to the DC component at 1ω=00, there is also an impulse-like spatial angular frequency component at 2π with a period below. Since the relationship between ω and the spatial frequency f is ω=2πf(4), it is a trap to have an impulse-like spatial frequency component with a period of 1/T on the spatial frequency axis. Rewriting equation (3), it becomes (5) [where f,=〒, l is an integer other than 0]. An example of the result of equation (5) is shown in FIG.

Here, if the duty ratio is defined as h D = -7+61 [however, fo = L is an integer other than 1rao] (force t In equation (7), the first term is a DC component and the average reflection density is (
α, -1. )). Next, the second term is a high frequency component, ! =1 indicates that the value corresponding to the frequency 17T component is taken.

Now, for example, the dot array is set to a frequency f. When trying to express an image by forming a dot array, the frequency band of the image that can actually be expressed by such a dot array is f according to the sampling theorem. It is approximately 72, and frequency components higher than that are considered to be noise. As a practical matter, since the resolution of the human eye is about 1 minute, the normal observation distance, that is, the so-called clear vision distance (25 to 30 α)
The *I# power of the human eye on the recording medium is at most 1
It is about 0 lp/rnm (which means that there are 20 bright and dark lines in order), and therefore, frequency components exceeding 101101p7 can be ignored, so referring to Fig. 3,
If 2fo = 101p/mm, in reality, s
A frequency component of approximately 5 lp/'' will have a large effect on the image quality. Therefore, in practice, the magnitude of the power spectrum p2(fo) at the frequency f will have a large effect on the image quality. It's a good thing to think about.

From formula (8), f. The power spectrum F2 (f
o) When changing the duty ratio from 0 to it, the power spectrum F2 (fa
) becomes a z ine function as shown in Figure 4, and D =
0.5 is the maximum, and it becomes 0 when D=O and D=1.

In other words, when the due-eye ratio is 0.5, the irritation to the eyes is at its maximum and is felt as roughness.

By the way, since the power spectrum can be expressed as in equation (9), it depends on the difference α between the reflection density of the dot itself and the reflection density of the recording medium itself (reflection density difference), and the reflection density difference α is small. The smaller p2(fo) becomes.Therefore, in order to express the same average reflection density, it is considered better to use ink with the lightest possible color density.Actually, formula (7) showing the average reflection density is used. The first term of (α, −αD
) is constant when a and D are changed, 12(
It would be a good idea to check fo).

For example, if the reflection density of a dot on the recording medium is (α, −α
When trying to express an image with the same average reflection density by using ink such that α) and arranging dots such that the duty ratio is D4, [α-(α, -aα)Dα
] = constant (need to be 11. α from 200. Dα = constant (13),
Therefore, the power spectrum F2(fo) ``(fo
) ” (” )2zin2 rc p.

π bekokamiyakua)2 πDaua becomes.

Equation (one side is in the range O≦D≦'1, as shown in FIG. 5).

Since it decreases monotonically, the closer the duty ratio is to 1, the more F
a2(fo) becomes smaller.

In order to express the same average reflection density, it is better to reduce the difference α between the reflectance between the dots and the recording medium (that is, use ink with the lowest possible density) and keep the duty ratio close to 1. is obtained. Looking at it from another perspective, when using high-density ink and when using low-density ink, the power spectrum F4'' (/,) at the duty ratio D = 0.5 as shown in Figure 6. takes the maximum value, but even in that case, the magnitude of the power spectrum F2 (10) is smaller for low-density ink.Therefore, qualitatively speaking, the image quality can be improved by using low-density ink. Also, it has been experimentally confirmed that when using low-density ink, "roughness" is not felt even at a duty ratio of D = 0.5, but with high-density ink, "roughness" is felt. being admired. If A is the value of the minimum power spectrum F" (/11 > where "graininess" is a concern), the area where the duty ratio exceeds A will not have a negative effect on image quality. Therefore, the smaller the area exceeding A, the better. , an image of good quality overall can be obtained.

Furthermore, when considering high frequency components, the power spectrum F
2(/Il+) becomes as shown in Figure 7, but in this case also L
The same thing can be said.

Incidentally, regarding FIG. 6, as an example, the frequency f0 of the dots actually formed is set to 5 dots per 1 m+n (PE
(5PEL in L display), that is, the dot pitch T is 200
In terms of μm, the reflection density is This figure shows the results when an image was formed using ink dots on white paper with a reflectance of about 0.1 (reflectance of about 80%). According to experiments, under exactly the same conditions (same number of bells, same recorded object), the duty ratio is 0.
．． 5, the power spectrum F2 (/6) exceeds the value of A in Figure 6 for ink with a density of about 0.6 (reflectance of formed dots about 10%). .

In addition, although the above theoretical analysis was performed with N sufficiently large,
As for the conclusion, it is good to think that N is 1 or more. Furthermore, although the analysis was conducted based on the relative density of the ink dots to the recording medium, rather than the absolute density of the ink dots, it is generally the case that the recording medium has an extremely low optical density (for example, around O1). Considering that white paper is often used, there is no big difference in the above conclusion even if only the absolute density of the ink dots is considered.

The characteristic curve shown in FIG. 6 is the dot frequency f. , that is, it changes depending on the number of PF3L,
The only thing that can be seen is that the resolving power of the human eye is generally
Since it is about gL, for cases where the number of bells is larger than this, that is, in terms of dot pitch, it is 100 μm or less,
The above-mentioned roughness can be considered as a problem. Specifically, from the above, 4 to 6 PEL is the most problematic (in other words, below this, it is not suitable for expressing the image, and above this, the feeling of graininess gradually increases. ).

Note that although the characteristic curve in FIG. 6 is for black ink, the same tendency exists for other colors as well, although there are differences in degree.

The above has been described using the example of image formation using the inkjet recording method, but it is not limited to the inkjet recording method, but can also be applied to electrophotography, thermal recording (thermal transfer), electrostatic recording, etc. The same applies.

According to the image forming method of the present invention, as described above, the gradation level region H expressed by the image forming element having the highest optical density
H1a) (The ratio of (H, , H, , H,
, 70 of the entire area H (ot
% or less, but in order to more effectively achieve the purpose of the present invention, K preferably has a ratio of ) 1max,
It is desirable that the proportion of Htot is 60% or less, more preferably the proportion of Hma X is 55% or less of Htot.

Next, a preferred example of expressing gradation according to the present invention will be described with reference to FIGS. 8 to 12.

The relationship with the average optical reflection density T5r when the PEL numbers are continuously arranged is shown.

The average optical reflection density Dr herein refers to the value measured using a commercially available densitometer as a reference value for the reflection density of a 10 mm square area of commonly used standard white paper (baryta paper). is set, and the image forming element is continuously arranged with a predetermined number of PELs on the surface of a recording medium such as paper using the densitometer.
This is the value of reflection density measured in the four-sided area as a sampling target.

In FIGS. 8 to 12, the vertical axes are normalized values with the highest value Dr (max) of the measured average optical reflection density Dr set as 1.

In the example shown in FIG. 8, two types of image forming elements E+ -Ex' having different degrees of optical reflection a are used, and the image forming element E, which has a higher reflection density, has the lowest level of gradation. The diameter of the image forming element E, (rnin) expressing the image forming element E, (rnin) and the diameter of the image forming element Ez (min) expressing the lowest level of gradation between the image forming element E, which has a lower reflection density. The case where are equal is shown.

In FIG. 8, the image forming element E1 (the one with higher reflection density) with a reflection density d has an average optical reflection density Dr of 0.4 by modulating its diameter from l to e. It is possible to express gradation in the range of 1.0 to 1.0 continuously and smoothly.

By modulating the diameter of the image forming element E, which has a reflection density d2 (lower reflection density), from l to ls, the average optical reflection density Dr can be adjusted to a level in the range of 0.15 to 0.4. It is possible to express tonality continuously.

In the present invention, each gradation level is actually expressed by the portion shown by the solid line in FIG. It goes without saying that it is possible to express a lower level of gradation, and by further increasing the diameter of the image forming element E2, a higher level of gradation can be expressed (
(The part indicated by the broken line in the figure). This also applies to subsequent similar drawings.

In the example shown in FIG. 8, the image forming element El + -)': 2
is used to express gradation on the high level side and to express gradation on the low level side, respectively, with the point where the average optical reflection density Dr is 0.4 as a boundary.

The gradation when the average optical reflection density Dr is 04 can be expressed by both the image forming element material and the image forming element E2, but in terms of image quality, the image forming element E
It is preferable to express it as .

In particular, when expressing the same gradation level, if the diameters of the two are significantly different, the image quality will be better when expressed with an image forming element with a low reflection density, and a more natural image will be produced. can get. In particular, when expressing gradation using image forming elements with high reflection density, the arrangement of the fusion image forming elements is as follows:
In terms of image quality, it is preferable to express gradation in a region where the duty ratio can be 05 or more. In other words, if the image forming element has a high reflection density and the Tutee ratio is less than 0.5 when expressing pel gradation, the image forming element has a lower reflection density. It is preferable to express the same level of gradation.

The example in FIG. 9 shows three types of image forming elements E! having different reflection densities of 'Fl d4+ dS! l + E49
This is basically the same as the case shown in FIG. 8, except that gradation is expressed using g.

That is, the image forming element E with a low reflection density (reflection density ds) has a diameter of 14 to 1! 7, the average optical reflection density 1) r is 0.1.2 to 0.2.
The image forming element E4, which continuously expresses gradation in the range of 8 and has a medium reflection density (reflection density d4), has a diameter of 14 l.
, by modulating the average optical reflection density Dr
The image forming element E3 has a gradation in the range of 0.28 to 0.45 and a high reflection density (reflection density ds).
, ~l, it is possible to express gradation in the range of average optical reflection density Dr of 0.45 to 1.0, respectively.

A more preferable example is shown in FIGS. 10 and 11.

FIG. 10 shows two types of image forming elements h with different reflection densities.
Figure 11 shows the case where VC1η↓ is used, and the case where three types of image forming elements with different reflection densities are used.

, pto diagram, gl The major difference between the example shown in Figure 1 and the examples shown in Figures 8 and 9 is that the image expressing the lowest level of gradation among the image forming elements of each reflection density. The diameter of the image-forming element is set so that the reflection density is the same as the diameter of the image-forming element.

In the image forming element E0 having a higher optical reflection density,
Image forming element 1 expressing the lowest level of gradation], (m
i n ) is made smaller than the diameter l of the image forming element Et (min) that expresses the lowest level of gradation among the image forming elements E7 with lower optical reflection density. There is. In this way, by configuring the image forming element group so that the minimum diameter of each image forming element is the same as that of the image forming element with high optical reflection density, a more preferable image quality is provided. Images can be displayed.

In FIG. 11, the optical reflection density is tun + dO + dI.

(ds > do, > dI.) Three types of image forming elements E.

EO, h. This is an example of expressing gradation using .

In the case of FIG. 11 as well, the diameter of the image-forming element expressing the lowest level of gradation among each one-quadrant oxygen body is gradually decreased as the optical reflection density becomes lower.

In the case of Fig. 12, it has a high reflection density (oxygen-forming element E
The imaging element ILIt (
! : The diameter of the image forming element that expresses the lowest level of gradation is the same in each of the image forming elements E+s, which expresses the lowest level of gradation among the image forming elements E+s having a low reflection density. An example is shown in which the diameter of the image forming element is smaller than the above.

As explained above, in the present invention, when expressing the same level of gradation, it is expressed by an image forming element with a lower optical reflection density within the range of avoiding a decrease in resolution. By doing so, it is possible to obtain a high-quality image in which the +9'r J brightness is not noticeable and does not give a feeling of "roughness" or "gaginess" to the eyes.

Next, preferred embodiments of the present invention will be specifically described with reference to the accompanying drawings.

In the following explanation, when the present invention is applied to an inkjet recording apparatus that forms flying droplets of colored ink and makes recording by depositing the droplets on a recording medium such as paper, As mentioned above, it goes without saying that the present invention is not limited to such inkjet recording methods, but can be widely applied to electrophotographic devices, thermal recording devices, electrostatic recording devices, etc., as described above. Please note that this is possible.

FIGS. 13 to 15 KFi show an example of an inkjet recording apparatus for embodying the image forming method of the present invention.

FIG. 13 shows an inkjet recording head as a means for obtaining flying dots of ink labeled L. In the figure, l is a glass tube with a tapered tip, and 2
is a cylindrical piezo element circumscribing the glass tube l. 3
is a tubular piezo element, 4 and 5 are electrodes, and by applying a pulse voltage between the electrodes 4 and 5, the piezo element 2
contraction and recovery in the inner diameter direction. At this time, by supplying liquid ink from the direction of arrow B, ink droplets can be ejected from the orifice l-1 at the narrowed end of the glass tube l. Also, depending on the magnitude and width of the voltage signal applied to this piezo element 2, it is possible to change the size of the ejected ink droplet.For example, in this embodiment, the size of the ink droplet on the recording medium can be changed. The dot diameter could be varied within a width approximately three times the diameter.

The range of dot diameter modulation varies depending on the shape and characteristics of the recording head used, the physical properties of the ink used, the driving conditions of the recording head, etc. It is quite possible to take a range of 7 to 8 times as much.

Furthermore, the running temperature of inkjet recording differs depending on the recording method.

In addition to the above-mentioned modulation of the dot diameter, in the present invention, as explained below, multiple types of ink with different color densities are used as the Ill If4 ink (C, therefore, a wide range of gradation is possible). It allows for expression.

For example, in this embodiment, as shown in FIG. 14, an inkjet head is provided with 6,702 heads, each of which is connected to ink tanks 8 and 9 containing ink of different concentrations. By using the unit 10, it is possible to sufficiently express a wide range of tones according to the method described above. In particular, it was possible to faithfully reproduce the 'FEi-true image. In the reproduction of a photographic image, v'i
Silver salt-q was obtained.

In the use of V in the present invention, inks having different densities are not limited to 2fl, and of course two or more types may be used. In addition, the concentration of the plurality of inks having different densities used can be adjusted as desired so that the object of the present invention can be fully achieved.

However, the density of the ink forming the image-forming element with the highest optical density is determined by the ratio of the gradation area expressed by the image-forming element with the highest optical density to the image formed by each of the inks of different densities. It needs to be determined at the ink design stage so that the gradation range expressed by each element body is 70% or less of the total range.

In the case of inkjet recording, the concentration of colorants (dyes, pigments, dyes and pigments) in inks used practically is only a few wt% at most. By adjusting the content of , it is possible to prepare a wide variety of inks with different optical densities.

FIG. 15 shows the configuration of the main parts of a printer equipped with the head unit IO shown in FIG. 13, in which 11 is a platen, 12 is a pulse motor for feeding paper, 13. ,
, &yo head unit l (1') is a motor that scans the attached head carriage 14 with a guide 15 and a screw 16.

16 to 21 show dot diameters and average (optical) dots obtained by forming dots with 5PEL using the inkjet recording apparatus shown in FIGS. 8 to 10.
The relationship with reflection density Dr is shown.

In FIGS. 16 to 21, the vertical axis represents the maximum value Dr (max) of the measured average optical reflection density Dr.
The values are shown as normalized values.

In FIGS. 16 to 21, the vertical axis represents the average reflection density, and the horizontal axis represents the dot diameter.

The solid lines indicate the range in which each ink is used, and the broken lines indicate the range in which the level of gradation in that area can be expressed, but was not used in actual recording.As is clear from Fig. 16, Both when using high concentration ink A16 and when using low concentration ink B16, the dot diameters that can be formed by each head for these inks (Fig. 14 6.7) are both relative to the orifice diameter of 50 μmφ. was 80 μmφ to 245 μmφ. Further, the range of average reflection density that can be expressed by this is 0.3 to 1.0 in the case of high density ink A16, and 0.15 to 0.65 in the case of low density ink B16. As described above, the average reflection densities expressed by the high density ink A16 and the low density ink B16 overlap in the range of 0.3 to 0.65. That is, the average reflection density in this range can be expressed using either ink, but in this example, in order to bring the duty ratio of dots closer to 1 when expressed with high-density ink, the dots of high-density ink are used. The point at which the diameter is 110 μmφ, that is, the average reflection density is 0.45, is set as the switching level.

Therefore, for low density ink B16, the average reflection density is 0.2~
0.4% range (dot diameter 110-180μm), and high density ink A16 has an average reflection density of 0.4%.
The range was from $1.0 to 1.0 (dot diameter 110 to 245 μm), and the minimum dot diameter used was 110 μm for both high and low density inks A16° and B16.

By doing this, the dot pitch is 200 μm (because it is 5 PEL), so the one-dimensional duty ratio of high-density ink A16 is 110 μm even for the smallest diameter dot.
7200 μm=0.55, and when a large number of viewers actually viewed and observed the recorded image, it was verified that the irritation to the eyes was extremely low and the image had a good impression.

Furthermore, there was almost no perceived difference in texture between the area expressed with high-density ink A16 and the area expressed with low-density ink B16, and there was no noticeable roughness.

Furthermore, in this example, as in the second and third examples described below, small dots were formed using the lowest density ink at 5 PEL even in the lowest density area. By designing the image in this way, it was possible to eliminate white areas, prevent the tone of the image from changing, and further improve the image quality.

Next, FIG. 17 shows the dot diameter-average reflection density characteristic in the second embodiment. In the second embodiment,
Although details are not shown in the drawings, the number of inkjet heads shown in FIG. 14 has been changed from two to three, and the number of ink tanks has also been changed from two to three, and each inkjet head has high, medium, and
A device configured to eject ink of three different concentrations (ink A17, ink B17, and ink C17) was used. The ink is low density (ink C17) (
0.2wt, medium density (ink B17) is 0.7
wt%, high concentration (ink A17) is 4.5wt%
The dye content concentration and soda. All three inkjet heads have an orifice diameter of 50μmφ.
Similarly, pat and a are set to 5. As shown in Figure 17, 0.18 to 0.36 for low density ink C17, 0.36 to 0.5 for medium density ink B17, and 0.5 to 1 for high density ink.
．． The average reflection density range of 0 is set to be -.

In this case, the minimum dot diameter for each of high density ink A17, medium density ink B17, and low density ink C17 is 130μ.
It was set as mφ.

The ink A has a higher concentration than that of the first embodiment.
17, and because its density is high, the lower limit of the image design is raised compared to the digital ink.
The duty ratio of the 17 smallest dots is 0.65.
And so. In the case of the image obtained in this example, as with the first real male side, there is no noticeable difference in texture between the different parts of the image formed using three types of ink with different densities. I didn't feel any dryness either. And we were able to obtain the lowest reflection density of 0.18. FIG. 18 shows the diameter-average reflection density characteristics.

Note that the PEL number was similarly set to 5. As shown in Figure 18, we were able to obtain a maximum dot diameter of 290 μmφ with the high-density ink head, making it possible to express an even higher average reflection density than in the previous example. . For high-density ink A] 8, it is not necessary to express it with a dot with a diameter of 135 ttmφ (duty ratio = 0.675) or less, so the radiation density range that can be expressed by using a thick orifice diameter as a head for high-density ink can be further expanded. Furthermore, in combination with the second embodiment, by using ink with three different densities and changing the orifice of each ink head, a wider reflection density range is expressed than in the trap, and an image with a wider range of gradation expression can be obtained. There is no need to explain that it can be obtained.

The present invention also includes such a concept.

19 and 20, respectively, show the dot diameter-average density characteristics in Example 4.5.6.

Figure 21 (rC shown.

In Examples 4 to 6, the difference from the previous examples is that the diameter of the minimum diameter dot formed with the darkest black ink is larger than the diameter of the small diameter dot formed with the lowest density ink. The dot diameter used for image expression was set in a similar manner.

Experimental conditions in Examples 4 to 6 are shown in Table 1 below.

The rest is the same as Examples 1 to 3.

When a photographic image of a woman was reproduced on a cabinet plate under the conditions of Examples 4 to 6 in Table 1, an even better image quality was obtained.In particular, the reproduction of the skin area was rich in naturalness and was easy to observe. When the optic nerve does not feel any fatigue at all, an example of the control circuit for the optical nose section 44 will be described below.

Fig. 22 shows an example of a control circuit when the device shown in Fig. 15 is applied to a printer that prints out color video signals. Sample hold circuit SHR, SHG, S
The synchronization signal 5YNC is input to the system controller 5YSCON. This 5
The video signal is sampled and held according to the timing signal from YSCON. Sample output of each color video signal is provided by signal changeover switch SW width converter AD
C is passed through 1 to 1, and it is stored in the line memories MR, MG, and MB. Next, line memories MR and MG.

When the information in B is subjected to shading processing and undercolor removal processing by the matrix circuit MX, a cyan signal C, a magenta signal M, a yellow signal Y, and a black signal BL are outputted. Now, the output signals of C2M, Y, and BL are from the latch memory M.
C.

MM, MY, MBLK are stored, and their output is
Head control matrix circuit MXC, MXM
, MXY, and MXBL. These matrix circuits convert the output signals of the latch memory into code signals indicating the head to be selected and the voltage value to be applied.

This code signal is sent to the converter DAC, DAM, DAY
.

It is input to DABL and converted into an analog voltage value. This voltage is input to the head drivers AMP1 to AMP8, and the head selected by the head selection signal H8 is driven by a desired timing signal TP to control the amount of ink droplets ejected.

FIG. 23 shows the internal details of the head control matrix circuit MXC for cyan, and also shows the details of the head control matrix circuit MXC for cyan.
FIG. 4 shows the relationship between the voltage applied to the heads H1 and H2 that eject cyan ink and the dot diameter. The relationship between the dot diameter and the average reflection density is shown in the above-mentioned FIG.
A digital value of the voltage applied to each head determined from the characteristics shown in FIG. 24 is output.

FIG. 25 shows the relationship between the input digital value and the output code obtained from the matrix circuit of FIG. 23, the relationship between the code, the selection head, and the applied voltage, and the obtained reflection density. In Figure 25, Hl is low density ink B16
H2 indicates a high-density ink A16 head.

By setting it as 0.2 to 0.0 with low density ink B16.
4″!Average reflection density of A, 0.4 with high density ink A16
It was possible to obtain an average reflection density of %-1.0. or,
Even if the input digital value was "ooooo", small dots were formed with low density ink B16, and white areas were prevented from occurring. Further, the signal from the system controller 5YSCON is sent to the head motor HM via the drivers DRI and DR2.
.

It is added to the paper feed motor LF to control head feed and paper feed, respectively.

FIG. 26 shows details of the circuit of the head drive section of FIG. 22. Control of the inkjet head will be specifically explained using FIG. 26, taking cyan signal processing as an example. A 7-bit digital signal from the matrix circuit MMC shown in FIG. 22 is input to a DA converter DAC that modulates the head applied voltage, and a voltage V corresponding to the digital signal is input.
Generates 8. Further, the head selection signal HS outputted from the matrix circuit MXC is inputted to one input terminal of the antenna G3 and also to one input terminal of the antenna G2 via the inverter Gl. Now, when the signal H8 is at a low level, the head H1 is selected, and when the signal H8 is at a low level, the head H2 is selected. A head drive pulse is input from the system controller 5YSCO,N to the other input terminals of the AND gates G2 and G3. Now, driving of the head H1 when the signal H8 is at a low level will be explained. Since one input terminal of the AND gate G2 is at a high level, when the head drive pulse goes high, the output of the AND gate G2 becomes a high level and the output of the buffer G4 becomes the cover-by level. Therefore, the transistor Tr 3 is turned on, and the transistor Tr 1 is also turned on. Here, a voltage is applied to the head H1 via a resistor R3. As a result, the piezoelectric vibrator contracts in the direction of the inner diameter of the glass tube, and colored droplets are ejected. The amount of colored droplets discharged is controlled by voltage (2).

At this time, the transistor Tr2 is connected to the inverter G6, and the run raster Tr1 is turned off and Tr2 is turned on, so that the charge charged in the head 1 (1) is discharged through the resistor R4, and the piezo element is restored to its original state. Ink droplet ejection is controlled in the following manner.

The above description has been made only for cyan ink, but control circuits are similarly configured for magenta, yellow, and black ink.

Furthermore, although the above description of the control circuit is for the first embodiment explained in FIG. 16, the structure is similar for the second to sixth embodiments explained in FIGS. 17 to 21. be done.

In the above explanation, an example of an inkjet recording method using a cylindrical piezo element has been explained, but in addition to this, for example, Japanese Patent Laid-Open No. 47-2006. 54-59936. Same 55-
The present invention can be applied to any image forming apparatus capable of expressing shades of light and shade, even if it is for recording, thermal recording, etc. according to 27282. Further, although the shape of the image forming element is a circular dot, the present invention is not limited to this, and the present invention can be applied to image forming elements having various shapes. For example, it is also applicable to line pattern expression using scanning lines in television display.

As detailed above, according to the image forming method of the present invention, the irritation to the eyes is alleviated when an image is expressed using a high optical density image forming element with a simple configuration, and the expressed image is j-Roughness 1 no longer bothers me.

The present invention is extremely useful because it is possible to obtain high-gradation, high-quality images with such a simple configuration.

[Brief explanation of the drawing]

Figures 1 to 7 are diagrams for explaining the interrelationships among the density of the image forming element (dot), the duty ratio of the image forming element (dot), the pitch of the image forming element, and the image quality, respectively. 8 to 12 are diagrams showing the relationship between the area of the image forming element and the average optical reflection density, which determine the image design in the image forming method of the present invention, and FIG. FIG. 13 is a schematic cross-sectional view of an inkjet head similar to that used in the embodiment, FIG. 13 is a schematic cross-sectional view of a piezo element, FIG. is an explanatory diagram for explaining the dot diameter-average reflection density characteristics in the first to sixth embodiments shown in FIG. 14, and FIG. 22 is a color diagram to which the first embodiment of the present invention is applied. The control block diagram of the video printer, FIG.
A circuit diagram for explaining the internal details of the head control matrix circuit MXC shown in the figure, #! Figure 24 is an explanatory diagram illustrating the dot diameter-applied voltage characteristics of the cyan ink head.
FIG. 25 is an explanatory diagram for explaining the relationship between the input digital value of the matrix circuit MXC, the output code, the selected head, and the reflection density, and FIG. 26 is a circuit diagram for explaining the details of the head driving section in FIG. 22. It is a diagram. In the figure, 6, 7 and Hi to H8V are ink jet heads, 8 and 9 are ink tanks, 10 is a head unit, 11 is a platen, 12 and 13 are motors, and MX
CU head control matrix circuit and DAC indicate a D/A converter, respectively. 5th M 6th 7th solid image formation y # Diameter of body 1 Diameter of image forming body p Diameter of image form H p

Claims (1)

[Scope of Claims] (1) In an image forming method that expresses an image with gradation by controlling the area occupied by each of a plurality of image forming elements having different optical densities, an image with the highest optical density is provided. The gradation is expressed by making the ratio of the gradation level area expressed by each image forming element body to be 7096 or less of the total gradation level area expressed by each image forming element body. Characteristic image formation method. (21,1 The area of the image forming element that expresses the lowest level of gradation among the gradations expressed by the image forming element with a high optical density is expressed by the image forming element with the lowest optical density. The image forming method according to claim (1), wherein the area of the image forming element is greater than or equal to the area of the image forming element expressing the lowest level of gradation among the gradations. (3) Image forming element with the highest optical density. The image forming method according to claims (1) and (2), wherein the bodies are arranged so that the duty ratio is 0.5 or more in any arrangement direction.