JP2003228203A - Image forming apparatus, its gradation control method and printing system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize excellent gradation control regardless of the number of the levels of input signals in an image forming apparatus, its gradation control method and a printing system. <P>SOLUTION: The image forming apparatus is equipped with a photoreceptor drum 1 on which a latent image corresponding to an image signal is formed, an image transfer body to which a visible image based on the latent image is transferred, a density detection sensor 13 for detecting the density of the visible image, a conversion means for forming a γ conversion table by reading a plurality of patch patterns formed in different density by the sensor 13, and a control means for controlling the gradation of an output image by using the γ conversion table. The apparatus is equipped with a reference density forming means for forming the patch pattern in the specified density being reference on the image transfer body and a comparison means for comparing the density of the patch pattern read by the sensor 13 with the reference density. Then, the γ conversion table is formed so that image density characteristic is different from the signal level of the γ characteristic of the reference according to the result of comparison, and the gradation control is performed by using the γ conversion table. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus, a gradation control method therefor, and a printing system, and more particularly to an image forming apparatus such as a laser beam printer and an electrostatic recording apparatus, a gradation control method therefor, and a printing system. Regarding

[0002]

2. Description of the Related Art FIG. 1 is a schematic block diagram of a conventional color image forming apparatus.

The conventional color image forming apparatus has a digital color image reader section in the upper part and a digital color image printer section in the lower part.

In the reader unit, the original 30 is placed on the original table glass 31, the reflected light image from the original 30 exposed and scanned by the exposure lamp 32 is condensed on the full color sensor 34 by the lens 33 and photoelectrically converted to color. Obtain a color separated image signal. The color-separated image signal is processed by a video processing unit (not shown) via an amplifier circuit (not shown) and sent to the printer section.

In the printer section, a photosensitive drum 1 as an image carrier is rotatably supported in a clockwise direction, and a pre-exposure lamp 11, a corona charger 2, a laser beam exposure optical system 3 and a potential are provided around the photosensitive drum 1. A sensor 12, four developing devices 4y, 4c, 4m, 4bk, a transfer device 5, and a cleaning device 6 are arranged.

The laser beam exposure optical system 3 inputs an image signal from a reader unit, converts it into an optical signal at a laser output unit (not shown), reflects the laser beam at a polygon mirror 3a, and a lens 3b. And an optical image E that linearly scans (raster scans) the surface of the photosensitive drum 1 through the mirror 3c.
Convert to.

When forming an image in the printer unit, first, the photosensitive drum 1 is rotated in the clockwise direction so that the pre-exposure lamp 1
After removing the charge by 1, uniformly charge by the corona charger 2,
The light image E is irradiated for each separated color to form an electrostatic latent image.

Next, a predetermined developing device is operated for each of the separated colors to develop the electrostatic latent image on the photosensitive drum 1, and an image is formed on the photosensitive drum 1 with toner using resin as a base. The developing devices 4y, 4c, 4m, and 4bk are the eccentric cams 2.
By the operations of 4y, 24c, 24m, and 24bk, the photosensitive drum 1 is selectively approached according to each separated color.

Further, the toner image on the photosensitive drum 1 is
The recording material is transferred from the recording material cassette 7 to the recording material supplied to the position facing the photosensitive drum 1 via the transport system and the transfer device 5. In the transfer device 5, in the conventional example, a transfer drum 5f, a transfer charger 5b, an attraction charger 5c for electrostatically attracting a recording material, an attraction roller 5g facing the attraction roller 5g, and an inner charger 5 are provided.
d, a recording material carrying sheet (not shown) made of a dielectric material is integrally formed in a cylindrical shape in an opening area of a peripheral surface of a transfer drum 5f which is rotatably supported and has an outer charger 5e. Is stretched over. A dielectric sheet such as a polycarbonate film is used as the recording material carrying sheet.

As the transfer drum 5f is rotated, the toner image on the photosensitive drum 1 is transferred onto the recording material carried on the recording material carrying sheet by the transfer charger 5b.

In this way, a desired number of color images are transferred to the recording material that is adsorbed and conveyed to the recording material carrying sheet, and a full color image is formed.

In the four-color mode, when the transfer of the four-color toner image is completed in this way, the recording material is separated from the transfer drum 5f by the action of the separation claw 8a, the separation push-up roller 8b and the separation charger 5h, and the heat is removed. The sheet is discharged onto the tray 10 via the roller fixing device 9.

On the other hand, after the transfer, the photosensitive drum 1 is subjected to the image forming process again after the residual toner on the surface is cleaned by the cleaning device 6.

When images are formed on both sides of the recording material,
Immediately after the heat roller fixing device 9 is discharged, the transport path switching guide 19 is driven, and the paper passes through the paper discharge vertical path 20 and the reverse path 21a.
, The recording material is temporarily stopped, and the reversing roller 21b
When the sheet is fed, the trailing edge of the fed sheet is the leading end and the sheet is ejected in the direction opposite to the fed direction, and the recording material is turned over and stocked in the intermediate tray 22. After that, an image is formed on the other surface by the above-described electrophotographic image forming process.

Further, toner is scattered on the recording material carrying sheet on the transfer drum 5f from the photosensitive drum 1, the developing device 4, the cleaning device 6 and the like, and toner is jammed when the recording material is jammed (paper jam). The backup brush 1 faces the fur brush 14 through the fur brush 14 and the recording material carrying sheet, although it is contaminated due to the adhesion, the oil on the recording material sometimes adhering during double-sided image formation, and the like.
5, or after being cleaned by the action of the backup brush 17 facing the oil removing roller 16 via the oil removing roller 16 and the recording material carrying sheet, it is subjected to the image forming process again. Such cleaning is performed at the time of front rotation and at the time of rear rotation, and at any time when a jam occurs.

Further, in the conventional example, the transfer drum eccentric cam 25 is operated and the cam follower 5i integrated with the transfer drum 5f is operated to set the gap between the recording material carrying sheet and the photosensitive drum 1 at a predetermined timing. The configuration is such that a predetermined interval can be set. For example, during standby or when the power is off, the transfer drum 5f and the photosensitive drum 1 are separated from each other, and the rotation of the photosensitive drum 1 can be independently rotated.

Further, each developing device 4y, 4c, 4m, 4b
k is the first and second stirring / conveying means (not shown)
And both are configured to convey the developer in mutually opposite directions. A developing sleeve (not shown) is arranged above the first stirring / conveying means.

In the above series of image forming operations, the developing device 4 operates as follows. When the electrostatic latent image reaches the developing position, a developing bias power source (not shown) applies a developing bias in which an AC voltage and a DC voltage are superimposed to the developing sleeve, and a developing sleeve driving device (not shown).
As a result, the developing sleeve is rotated in a predetermined direction, and the developing device 4 is pressed toward the photosensitive drum 1 by the developing pressure cam 24 to visualize the electrostatic latent image.

The density detection sensor 13 whose schematic configuration is shown in FIG. 2 uses a near-infrared light LED as a light emitting element and a photodiode (PD) as a light receiving element to mount a visualized toner image. The density is detected based on the regular reflection light and the irregular reflection light from the transfer drum 5f. The method will be described below.

Reference numerals 13e, 13f and 13g are photodiodes, and 13h and 13i are prisms. The irradiation light from the LED 13c is separated by the prism 13h into a component vibrating in a direction perpendicular to the incident surface (s-wave light) and a component vibrating in a direction parallel to the incident surface (p-wave light). The s-wave light is applied to the photodiode 13e, and the p-wave light is applied to the toner surface.

The p-wave light incident on the surface of the photoconductor or the intermediate transfer member, which is the base for detecting the density, is almost specularly reflected and passes through the prism 13i as a p-wave to pass through the photodiode 13.
It is incident on f. The p-wave light applied to the toner surface is irregularly reflected into s-wave and p-wave, passes through the prism 13i, the p-wave enters the photodiode 13f, and the specularly reflected light enters, and the s-wave enters the photodiode 13g. , Diffusely reflected light is detected.

Here, FIG. 3 shows the outputs of the photodiodes 13f and 13g with respect to the toner concentration. According to this, it is considered that the diffuse reflection component is actually incident on the photodiode 13f. Therefore, from the output of the photodiode 13f to the photodiode 13g
By subtracting a value obtained by multiplying the output of (1) by a correction coefficient k, that is, the corrected output = (p wave output) −k × (s wave output), a true specular reflection output as shown in FIG. 4 is obtained.

Based on the toner image density detected in this way, the following control is performed in order to maintain gradation.

8 bits, that is, quantized values of levels 0 to 25
Of the 5 input signals, 0, 32, 64, 96, 128,
Toner patches of 160, 192, 224, and 255 levels are formed, read by the density detection sensor 13, subjected to interpolation, inverse conversion, and the like to create a γLUT (γ lookup table).

[0025]

In the above-mentioned conventional γLUT creating method, if the number of input signal levels is too large, the control accuracy is improved, but the control time is increased and the amount of toner consumed by the control is increased. On the other hand, if the number of levels of the input signal is too small, there is a problem that the control accuracy is lowered.

An object of the present invention is to provide an image forming apparatus, a gradation control method therefor, and a printing system which can solve the above problems.

[0027]

In order to achieve the above object, the invention according to claim 1 provides an image carrier on which a latent image corresponding to an image signal is formed and a visible image based on the latent image. An image transfer body to be transferred, a detecting means for detecting the density of the visible image, a converting means for reading a plurality of patch patterns formed with different densities by the detecting means to create a γ conversion table, In an image forming apparatus provided with a control means for controlling the gradation of an output image using a conversion table, a reference density forming means for forming a patch pattern having a predetermined predetermined density on the image transfer body, and the detecting means. Comparing means for comparing the density of the patch pattern read by the reference density with the reference density, and in the converting step, the γ conversion is performed so that the signal level of the reference γ characteristic differs from the image density characteristic in accordance with the comparison result. An image forming apparatus for creating Buru was performed.

According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the converting means, when the read density is higher than the reference density, low density signals of the plurality of patch patterns. When the read density is lower than the reference density by increasing the number of portions higher than the high density signal portion, the γ conversion is performed so that the low density signal portion of the plurality of patch patterns is less than the high density signal portion. An image forming apparatus that creates a table was implemented.

According to a third aspect of the invention, in the image forming apparatus according to the first or second aspect, the reference density is one and the image forming apparatus has an almost intermediate density.

According to a fourth aspect of the present invention, in the image forming apparatus according to the first aspect, the reference densities are a plurality of densities different from each other, and the higher the density of the reference densities, the lower density signals of the plurality of patch patterns. According to the comparison result, the γ conversion is performed such that the number of portions is larger than that of the high density signal portion, and that the lower the density of the reference density is, the smaller the low density signal portion of the plurality of patch patterns is than the high density signal portion. An image forming apparatus that creates a table was implemented.

According to a fifth aspect of the invention, an image carrier on which a latent image corresponding to an image signal is formed, an image transfer member on which a visible image based on the latent image is transferred, and a density of the visible image. Detection means, a conversion means for reading a plurality of patch patterns formed with different densities by the detection means to create a γ conversion table, and a control for controlling the gradation of an output image using the γ conversion table A gradation control method for an image forming apparatus comprising: a reference density forming step of forming a patch pattern having a predetermined reference density on the image transfer body;
A comparing step of comparing the density of the patch pattern read by the detecting means with a reference density; and a converting step using the converting means so that the signal level of the reference γ characteristic differs from the image density characteristic. And a conversion step of creating the γ conversion table.

According to a sixth aspect of the present invention, in the gradation control method according to the fifth aspect, when the read density is higher than the reference density in the converting step, the low density of the plurality of patch patterns is reduced. Create the γ conversion table so that the signal portion is larger than the high-density signal portion,
In the case where the read density is lower than the reference density, the gradation control method for creating the γ conversion table is performed so that the low density signal portion of the plurality of patch patterns is made smaller than the high density signal portion.

The invention according to claim 7 is the gradation control method according to claim 5 or 6, wherein the gradation control method uses one of the reference densities of approximately the intermediate density in the comparison step.

According to an eighth aspect of the present invention, in the gradation control method according to the fifth aspect, the reference density of a plurality of different densities is used in the comparing step, and the density of the reference density is changed in the converting step. The higher the density of the low-density signal portions of the plurality of patch patterns, the larger the density of the high-density signal portion, and the lower the density of the reference density, the smaller the density of the low-density signal portions of the plurality of patch patterns than the high-density signal portion. Then, the gradation control method of creating the γ conversion table according to the comparison result was carried out.

According to a ninth aspect of the present invention, a printing system including a plurality of image forming apparatuses according to any one of the first to fourth aspects is implemented.

[0036]

BEST MODE FOR CARRYING OUT THE INVENTION An image forming apparatus and a gradation control method thereof according to the present invention will be described below in more detail with reference to the drawings. In the following description of the embodiments, the present invention is embodied in the above-described image forming apparatus and the gradation control method thereof, and therefore detailed description of the overall configuration and functions of the image forming apparatus is omitted. Then, the characteristic portion of the present invention will be described.

(First Embodiment) A first embodiment of the present invention will be described.

In the gradation control process of this embodiment, first, a toner patch of level 128 out of levels 0 to 255 is formed as a toner patch of a reference density level. The patch is read by the density detection sensor 13. When the read density is higher than a predetermined density, the signals of the toner patch pattern group of density signals of a plurality of levels are 0, 16, 32, 48, 64, 128, 1
Toner patches are formed as 92 and 255, and a γLUT is created by interpolation, inverse conversion, or the like. On the contrary, when the toner patch of the reference density level of the level 128 is lower than the predetermined density, the signals of the toner patch pattern group of the density signals of a plurality of levels are set to 0, 64, 128, 192, 208, 224, 2
Toner patches are formed as 40 and 255, and a γLUT is created by interpolation, inverse conversion, or the like.

This is based on the following concept.

In a system exhibiting a γ characteristic as shown in FIG. 5 in a certain reference state, when the gradation characteristic changes to the high density side from the reference state, it becomes as shown in FIG. 6, for example. At this time, if the signal level of the toner patch group used for gradation control is a fixed value as in the conventional example, the information of the portion where the gradation change is small in the high density portion is also detected, which is not very effective. It is more effective to use that amount for the detection of the low-density portion where the change in gradation is large. Further, when the gradation characteristic is changed to the low density side from the reference state, for example, it becomes as shown in FIG. At this time, if the signal level of the toner patch group used for gradation control is a fixed value as in the conventional example, the information of the portion where the gradation change is small in the low density portion is also detected, which is not very effective. It is more effective to use that amount for the detection of a high-density portion having a large change in gradation.

Based on the above concept, in the present embodiment, when the reference density level is higher than the predetermined density, the signals of the toner patch pattern group of the density signals of a plurality of levels are set to 0, 16, 32, 48 ,. 64, 128, 192,
255, the toner patch is formed with emphasis on low density. Conversely, when the toner patch of the reference density level is lower than the predetermined density, the signals of the toner patch pattern group of the density signals of a plurality of levels are set to 0, 64, 128, 192. , 208, 224,
240 and 255, a toner patch is formed with emphasis on high density, and a γLUT is created by interpolation, inverse conversion, or the like.

As described above, the image forming apparatus of this embodiment includes an image carrier on which a latent image corresponding to an image signal is formed,
A toner patch pattern having a plurality of levels of density signals, including an image transfer body to which a visible image based on the latent image is transferred after exposure and development, and an optical sensor for detecting the density of the visualized toner image A color image forming apparatus for forming a group, reading with the optical sensor, and creating a γ table, wherein a color image forming apparatus according to a comparison result of a read signal of a toner patch of a reference level and one reference density (predetermined density) By configuring the signal levels of the toner patch pattern groups of the density signals of a plurality of levels to be different in two ways, highly accurate gradation control can be performed without increasing the control time.

(Second Embodiment) This embodiment is an example in which the first embodiment is controlled more finely. The description of the same parts as those in the first embodiment will be omitted.

In the gradation control step of this embodiment, first, a toner patch of level 128 out of levels 0 to 255 is formed as a toner patch of a reference density level. The patch is read by the density detection sensor 13. When the read density is higher than the predetermined density 1, which is set in advance, the signals of the toner patch pattern group of the density signals of a plurality of levels are 0, 8, 16, 24, 82, 140, 1
A toner patch is formed as 98 and 255, and the density is lower than a predetermined density 1 which is predetermined, and a predetermined density 2 (<predetermined density 1).
If it is higher, the signals of the toner patch pattern group of the density signals of a plurality of levels are set to 0, 16, 32, 48, 64, 1
When toner patches 28, 192, and 255 are formed and the density is lower than a predetermined density 2 and higher than a predetermined density 3 (<predetermined density 2), the signals of the toner patch pattern group of the density signals of a plurality of levels are set. 0, 58, 116, 17
When toner patches are formed as Nos. 4, 232, 240, 248, and 255, and the density is lower than the predetermined density 3, the signals of the toner patch pattern group of density signals of a plurality of levels are 0, 64, 128, 192, 208, 224, 2
Toner patches are formed as 40 and 255, and a γLUT is created by interpolation, inverse conversion, or the like.

As described above, the image forming apparatus according to the present embodiment includes an image carrier on which a latent image corresponding to an image signal is formed,
A toner patch pattern having a plurality of levels of density signals, including an image transfer body to which a visible image based on the latent image is transferred after exposure and development, and an optical sensor for detecting the density of the visualized toner image A color image forming apparatus for forming a group, reading by the optical sensor, and creating a γ table, wherein a plurality of reference densities (predetermined densities 1, 2, 3) having different densities from a read signal of a toner patch of a reference level are provided. According to the result of comparison with the above, the signal levels of the toner patch pattern groups of the density signals of the plurality of levels are configured to be different from each other in four ways, so that the control time is not lengthened and compared with the first embodiment. It was possible to perform more accurate gradation control.

(Third Embodiment) This embodiment is an example in which the present invention is applied to cluster printing in which a plurality of printers are centrally managed and output is controlled. The configuration and function of the printer are the same as those of the first and second embodiments, and the description of the same parts will be omitted and the present embodiment will be described.

The present embodiment is a cluster printing system consisting of one server, two RIPs and two printers as shown in the schematic diagram of FIG.

In the cluster printing system, there is a use example in which an output file consisting of 100 pages is output by the printer 1 by 50 pages and the printer 2 by 50 pages in order to improve the total productivity. At that time, if the two printers have different colors, it cannot be said that the system is of high quality.

Therefore, in the present embodiment, the present invention is applied, each device is equipped with a density detection sensor, and the high-accuracy gradation control process described in the first embodiment is provided, so that the density between two different units can be improved. It was possible to provide high-quality cluster printing systems by accurately matching the above.

[0050]

As described above, according to the image forming apparatus, the gradation control method therefor, and the printing system according to the present invention, an image carrier on which a latent image corresponding to an image signal is formed, and exposure and development are performed. In a color image forming apparatus including an image transfer body to which a visible image based on a latent image is transferred via an optical image sensor and an optical sensor for detecting the density of the visualized toner image, a toner patch having a plurality of levels of density signals An image forming apparatus that forms a pattern group, reads it with an optical sensor, and creates a γ table. In an image forming apparatus, the signal level of a toner patch pattern group of a plurality of levels of density signals is changed according to a read signal of a toner patch of a reference level. Thus, there is an effect that highly accurate gradation control can be performed without increasing the control time.

When the read density of the toner patch at the reference level is high, the low density signal portion of the plurality of patch patterns is made larger than the high density signal portion, and when the read density is lower than the reference density. In the above, by creating the γ conversion table so that the low density signal part of the plurality of patch patterns is made smaller than the high density signal part, there is an effect that highly accurate gradation control can be performed without lengthening the control time.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram of an image forming apparatus shown in each embodiment of the present invention and a conventional example.

FIG. 2 is a configuration diagram of a concentration detection sensor in FIG.

FIG. 3 is a characteristic diagram showing the sensor output with respect to the density in the density detection sensor shown in the conventional example.

FIG. 4 is a characteristic diagram showing a correction sensor output with respect to the density in the density detection sensor shown in the conventional example.

FIG. 5 is a characteristic diagram showing a γ characteristic in a reference state of the first embodiment according to the present invention.

FIG. 6 shows γ in a high concentration state according to the first embodiment of the present invention.
It is a characteristic view which shows a characteristic.

FIG. 7 shows γ in a low concentration state according to the first embodiment of the present invention.
It is a characteristic view which shows a characteristic.

FIG. 8 is a schematic diagram showing a cluster printing system according to a third embodiment of the present invention.

[Explanation of symbols]

1 Photosensitive drum (photoreceptor) 2 Corona charger 3 Laser beam exposure optical system 3a polygon mirror 3b lens 3c mirror 4,4y, 4c, 4m, 4bk Developing device 5 Transfer device 5b Transfer charger 5c Adsorption charger 5d Inner charger 5e Outside charger 5f transfer drum 5g suction roller 5h Separation charger 5i cam follower 6 cleaning device 7 Recording material cassette 8a Separation claw 8b roller 9 Heat roller fixing device 10 trays 11 Pre-exposure lamp 12 Potential sensor 13 Concentration detection sensor 13c LED 13e, 13f, 13g Photodiodes 13h, 13i prism 14 fur brush 15,17 Backup brush 16 Oil removal roller 19 Transport path switching guide 20 Paper ejection vertical path 21a Reverse path 21b Reverse roller 22 Intermediate tray 24 Development pressure cam 24y, 24c, 24m, 24bk Eccentric cam 25 Transfer drum eccentric cam 30 manuscripts 31 Platen glass 32 exposure lamp 33 lenses 34 Full color sensor

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) H04N 1/407 H04N 1/40 101E 5C077 F term (reference) 2C061 AQ06 KK18 KK25 KK28 KK32 2C262 AA05 AA24 AB07 BB01 BB36 BC01 BC05 BC07 BC10 EA04 FA13 2H027 DA09 DA10 DE02 DE10 EB04 EC03 EC06 EC07 ZA07 2H030 AA03 AD12 AD16 BB23 BB36 BB46 5C074 AA05 BB17 DD03 DD16 EE11 GG12 5C077 LL04 LL19 MP08 PP15 P22 PQQQQQQQQQQQQQ

Claims (9)

[Claims] 1. An image carrier on which a latent image corresponding to an image signal is formed, an image transfer member on which a visible image based on the latent image is transferred, and a detection means for detecting the density of the visible image. And a conversion unit that reads a plurality of patch patterns formed with different densities by the detection unit to create a γ conversion table, and a control unit that controls the gradation of the output image using the γ conversion table. The image forming apparatus is provided with a reference density creating means for forming a patch pattern having a predetermined reference density on the image transfer body, and a comparing means for comparing the density of the patch pattern read by the detecting means with the reference density. The image forming apparatus is characterized in that, in the converting step, the γ conversion table is created so as to be different from the signal level vs. image density characteristic of the reference γ characteristic according to the comparison result. 2. The image forming apparatus according to claim 1, wherein, when the read density is higher than the reference density, the converting unit converts the low density signal portions of the plurality of patch patterns into high density signal. When the read density is lower than the reference density, the γ conversion table is created so that the low density signal portion of the plurality of patch patterns is less than the high density signal portion. An image forming apparatus characterized by. 3. The image forming apparatus according to claim 1 or 2, wherein the reference density is one, and the density is almost an intermediate density. 4. The image forming apparatus according to claim 1, wherein the reference densities are a plurality of densities different from each other, and the higher the density of the reference densities, the higher the density of the low density signal portions of the plurality of patch patterns. So that the lower the density of the reference density, the smaller the low density signal part of the plurality of patch patterns is made to be less than the high density signal part.
An image forming apparatus, wherein the γ conversion table is created according to the comparison result. 5. An image carrier on which a latent image corresponding to an image signal is formed, an image transfer member on which a visible image based on the latent image is transferred, and a detection means for detecting the density of the visible image. And a conversion unit that reads a plurality of patch patterns formed with different densities by the detection unit to create a γ conversion table, and a control unit that controls the gradation of the output image using the γ conversion table. In a gradation control method of an image forming apparatus, a reference density forming step of forming a patch pattern of a predetermined reference density on the image transfer body, and a density of the patch pattern read by the detecting unit as a reference density. A comparison step of comparing and a conversion of the reference γ according to the comparison result using the conversion means.
The signal level of the characteristic vs.
A gradation control method, comprising: a conversion step of creating a conversion table. 6. The gradation control method according to claim 5, wherein in the converting step, when the read density is higher than the reference density, the low density signal portions of the plurality of patch patterns are set to high density. If the read density is lower than the reference density, the γ conversion table is created so as to be larger than the signal portion, and the low density signal portion of the plurality of patch patterns is made smaller than the high density signal portion. A gradation control method characterized in that the γ conversion table is created. 7. The gradation control method according to claim 5, wherein in the comparing step, one of the reference densities, which is an almost intermediate density, is used. 8. The gradation control method according to claim 5, wherein in the comparing step, the reference densities of a plurality of densities different from each other are used, and in the converting step, the plurality of the reference densities are higher, the plurality of the reference densities are higher. The low density signal part of the patch pattern is made larger than the high density signal part, and the low density signal part of the plurality of patch patterns is made smaller than the high density signal part as the density of the reference density is lower. The gradation control method is characterized in that the γ conversion table is created in accordance with the above. 9. A printing system comprising a plurality of image forming apparatuses according to any one of claims 1 to 4.