JPH10333377A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device having an excellent continuous gradation by always correcting γ property linearly corresponding to the environmental change or the secular change. SOLUTION: This image forming device is provided with a test pattern generating means 40 for generating the image data of a test pattern having gradation, a latent image forming means for forming the latent image of the test pattern, a developing means for visualizing the formed latent image, the image carrier holding the visualized toner image, and a density sensor 25 for detecting the density of the visualized test pattern. Thus the gradation correction is performed based on an output value of the density sensor 25, and the test pattern is generated at least two or more times.

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 for obtaining a high-quality recorded image.

[0002]

2. Description of the Related Art Conventionally, printers based on various principles have been proposed as output terminals of personal computers, workstations, and the like. In particular, an image forming apparatus using an electrophotographic process and a laser scanning technique has a high recording speed and a high print quality. In this regard, printers have a high advantage and are the mainstream of printers. In this method, a laser beam is modulated into a pulse width corresponding to image data to reproduce the gradation of an image.

FIG. 10 is a graph showing general input / output characteristics of an electrophotographic process in a conventional image forming apparatus.
1 is a schematic configuration diagram of a conventional pulse width modulation unit, and FIG. 12 is a graph showing a gradation correction table in conventional image data conversion.

In FIG. 10, the horizontal axis represents image data, and the vertical axis represents image density (reflection density). In FIG. 11, reference numeral 1 denotes a controller, 2 denotes a gradation correction table for linearizing the relationship between input image data and output image density, and 3 denotes a pulse width of data from the gradation correction table 2. The pulse width modulating means 4 for modulating is a laser driver for driving the semiconductor laser. Then, as can be seen from FIG. 12, the gradation correction table 2 is an inverse function with respect to the linear straight line of the γ characteristic before correction.

Hereinafter, a conventional image forming apparatus will be described. In the electrophotographic process in the image forming apparatus, the relationship between the input image data and the output image density, the so-called γ characteristic, becomes non-linear as shown in FIG. In such a device, in order to express continuous gradation, it is indispensable to perform gradation correction for linearizing the γ characteristic.
Conventionally, as shown in FIG. 12, the γ characteristic has been linearly corrected using a gradation correction table 2 in which the γ characteristic is linear. That is, the image data sent from the controller 1 is corrected by the gradation correction table 2 to make the γ characteristic linear. By the way, the gradation correction table 3 may be a fixed table, or a test pattern may be formed on the image carrier by using prescribed image data, and after detecting the γ characteristic with the density sensor, the result may be obtained. In addition, a device that changes the characteristics of a table has been proposed.

[0006]

However, in the conventional configuration using a fixed gradation correction table, when the γ characteristic changes due to environmental change or aging, the table is fixed, so that the γ characteristic is corrected linearly. Can not. In addition, although the gradation correction table is variable, it can cope with environmental changes and changes over time.However, the number of test patterns to be formed is limited, and data that is normally equally divided with respect to the total number of image data is printed. Therefore, as shown in FIG. 10, in the case of an image forming apparatus having a γ characteristic in which the number of image data substantially related to image density is small, effective information obtained from a test pattern is small, and the correction result is rather deteriorated. was there.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an image forming apparatus which always corrects the .gamma. Characteristic linearly in response to environmental changes and temporal changes, and has good continuous gradation.

[0008]

SUMMARY OF THE INVENTION The present invention provides a test pattern generating means for generating image data of a test pattern having shading, a latent image forming means for forming a latent image of the test pattern, and a method for forming the latent image. Developing means for visualizing, an image carrier for holding the visualized toner image, a density sensor for detecting the density of the visualized test pattern, and gradation correction based on the output value of the density sensor And a test pattern is generated at least twice. With this configuration, it is possible to realize an image forming apparatus that constantly corrects the γ characteristic linearly in response to environmental changes and temporal changes, and has good continuous gradation.

[0009]

According to the first aspect of the present invention, there is provided an image forming apparatus, comprising: a test pattern generating means for generating image data of a test pattern having shading; a latent image forming means for forming a latent image of the test pattern; Developing means for developing the latent image, an image carrier for holding the visualized toner image, a density sensor for detecting the density of the visualized test pattern, and an output value of the density sensor. To perform a tone correction to generate a test pattern at least twice.

In the image forming apparatus according to the present invention, the image data of the test pattern generated at the second time or later is within the range of the image data at which the target maximum density is obtained from the image data at which the toner density starts to appear.

In the image forming apparatus according to the third aspect, the image data of the test pattern generated after the second time is image data based on the result of the previous gradation correction.

With these configurations, it is possible to maintain the linearity of the γ characteristic and always obtain a good continuous tone.

An embodiment of the present invention will be described below with reference to the drawings. Here, a latent image formed on a photoconductor by a laser beam or the like is developed by a developing device of each color, and a visualized single-color image is once transferred onto an image forming medium called an intermediate transfer body and synthesized. A so-called intermediate transfer body type image forming apparatus for transferring a composite image on an intermediate transfer body onto a sheet collectively will be mainly described.

FIG. 1 is a configuration diagram of an image forming apparatus according to an embodiment of the present invention. First, a configuration around a photosensitive member will be described. In FIG. 1, reference numeral 5 denotes a closed-loop photosensitive member as a latent image forming unit. The photoreceptor 5 is made of a PET substrate, an aluminum deposition layer, a charge generation layer (CGL), and a charge transport layer (CTL).
And is supported by three photoconductor transport rollers 6, 7, and 8, and is rotated in the direction of arrow A by a drive motor (not shown). Reference numeral 9 denotes a photoconductor position detection mark, which is disposed at one end of the photoconductor 5. Reference numeral 10 denotes a photoconductor position detection sensor that detects the photoconductor position detection mark 9. The photoreceptor 5 has a seam 11, and when forming an image, the seam 11 must be avoided. At this time,
Reference is made to the output of the photoconductor position detection sensor 10.

On the peripheral surface of the photoreceptor 5, a charger 12, an exposure optical system 13, black (K), yellow (Y), magenta (M), and cyan (C) are arranged along a rotation direction indicated by an arrow A. Developing devices (developing means) 14K, 14Y, 1
4M, 14C, an intermediate transfer member 15, a photoconductor cleaning device 16, and a neutralizer 17 are provided.

The charger 12 is composed of a charged wire made of a tungsten wire or the like, a shield plate made of a metal plate, a grid plate or the like (not shown). When a high negative voltage is applied to the charged wire, the charged wire is corona-discharged. When a voltage of, for example, -700 V is applied to the grid plate, the surface of the photoconductor 5 is uniformly charged to a negative potential of about -600 V.

The exposure optical system 13 is composed of a laser driving device, a polygon mirror, a lens system, a motor (scanner motor) for rotating the polygon mirror (not shown), and the like. Form an image. Reference numeral 18 denotes an exposure light beam emitted from the exposure optical system 13. Exposure ray 18
Is obtained by subjecting an image signal from an image data conversion unit (not shown) to pulse width modulation by a laser driving circuit (not shown), and forms an electrostatic latent image corresponding to image data of a specific color on the photoreceptor 5. To form

Each developing unit 14K, 14Y, 14M, 14C
Store black, yellow, magenta, and cyan toners, respectively. Each color developing unit 14K, 14Y, 1
4M and 14C have sleeve rollers 19K, 19Y, 19M and 19C using conductive rubber or the like, and the sleeve rollers 19K, 19Y, 19M and 19C are moved in the forward direction with respect to the arrow A direction of the photoconductor 5. When rotated, the developing device 14
The thinned toner is supplied to the surface of the sleeve roller from the inside of K, 14Y, 14M, and 14C. When the toner is thinned, the toner is negatively charged due to friction. The development of each color is performed by sleeve rollers 19K, 19Y, 19M, 19M.
A negative voltage (development bias) is applied to C, and while rotating the sleeve roller, the color separation / contact cams 20K, 20Y,
By driving a dedicated motor (not shown) corresponding to 0M, 20C, a selected developing device, for example, a black developing device 14K is moved in the direction of the arrow C, and the sleeve roller 19K is moved to the photosensitive member 5.
The contact is made.

The surface potential (bright potential) of the photosensitive member 5 at the portion where the latent image is formed rises to about -50 to -100 V, and the sleeve rollers 19K, 19Y, 19M and 19C apply a negative potential of about -300V. , An electric field is generated from the photoconductor 5 in the direction of the sleeve rollers 19K, 19Y, 19M, and 19C. As a result, the sleeve roller 19
Coulomb force acts on the negatively charged toner on K, 19Y, 19M, and 19C in the opposite direction of the electric field, that is, in the direction of the photoconductor 5, and the toner adheres to the latent image portion formed on the photoconductor 5. On the other hand, since the surface potential (dark potential) of the photosensitive member 5 in the portion where the latent image is not formed is -600 V, the electric field is maintained even when the developing bias is applied.
Y, 19M, and 19C are generated in the direction of the photoconductor 5, so that
The toner does not adhere to the photoconductor 1. The above-described development process is generally referred to as a negative-positive process or a reversal development because the toner adheres (that is, black) to a portion irradiated with light (that is, white).

The photosensitive member cleaning device 16 includes the photosensitive member 5
Are arranged opposite to the photoconductor support roller 8 with the photoconductor 5 interposed therebetween.
Is removed. Seam 1 of photoconductor 5
Numeral 1 is provided at an angle of about 3 ° to 5 ° with respect to the scanning direction of the exposure light beam 18 so as to prevent an image from being disturbed by an impact when the seam 11 passes through the photoconductor cleaning device 16. I have. Therefore, the photoconductor cleaning device 1
Reference numeral 6 does not have a mechanism for separating from and coming into contact with the photoconductor 5.
The static eliminator 17 includes a plurality of red LEDs arranged on a line, and removes a residual potential on the photoconductor 5.

Next, the configuration around the intermediate transfer member 15 will be described. The intermediate transfer body 15 as an image carrier is formed by winding a belt-shaped sheet made of a conductive resin or the like around a base tube (for example, having a diameter of 200 mm) of metal such as aluminum, and is driven by a drive motor (not shown). Rotate in the direction of arrow B,
A medium for forming a full-color image by synthesizing a single-color image. Reference numeral 21 denotes an intermediate transfer member cleaning unit formed of rubber or the like in the form of a blade. The intermediate transfer member cleaning unit 2 forms a synthetic image on the intermediate transfer member 15.
Numeral 1 is separated from the intermediate transfer member 11 and abuts only during cleaning.
The toner remaining without being transferred to the toner is removed. Reference numeral 23 denotes an intermediate transfer member position detection plate, which is arranged on a side surface of the intermediate transfer member 15. Reference numeral 24 denotes an intermediate transfer member position detection sensor that detects a slit provided on the intermediate transfer member position detection plate 23. When forming an image, one of a plurality of slits is selected and used as a reference for an image forming position.

Reference numeral 25 denotes a density sensor in which a light emitting element and a light receiving element are combined, and detects the toner density on the intermediate transfer member 15. The light emission side of the density sensor 25 is connected to a D / A converter (not shown), and the amount of light emission can be changed by setting data in the D / A converter and controlling the current. It has become. The output on the light receiving side is A
/ D conversion port (not shown).

Next, the configuration of the paper feeding system and the fixing system will be described. The recording medium 22 is sent out one sheet at a time from a recording medium cassette 26 by a paper feed roller 27 to a sheet transport path 28. A transfer unit 29 transfers the composite image on the intermediate transfer body 15 to the recording medium 22. The transfer unit 29 is a transfer belt 30 formed of conductive rubber or the like in a belt shape.
A transfer unit 31 for applying a transfer bias for transferring the composite image on the intermediate transfer body 15 to the recording medium 22; and, after the composite image is transferred from the recording medium 22 to the intermediate transfer body 15, And a separator 32 for applying a bias to prevent sticking. A fixing device 33 includes a heat roller 34 having a heat source therein and a pressure roller 35. The fixing device 33 transfers the combined image transferred onto the recording medium 22 with the rotation of the heat roller 34 and the pressure roller 35. The toner is fixed on the recording medium 22 by pressure and heat,
Form a color image.

Electrophotography, including those having the above-described configuration, is generally sensitive to environmental fluctuations and the like, and the gradation characteristics change with time, for example, as the temperature inside the machine rises.
For an image forming apparatus that performs full-color output, it is one of the important technical issues to secure the gradation property and to secure the gray balance when combining the three primary colors of printing, cyan, magenta, and yellow. Various approaches have been taken.

Here, the image forming apparatus of the present embodiment
For example, tone correction is performed at the stage of initialization when the power is turned on. Also, the objective is to reproduce 16 gradations per pixel (600 dpi × 600 dpi). Thereby, 256 gradations / 150 lines can be realized.

Hereinafter, the gradation correction operation will be described in detail. FIG. 2 is a configuration diagram of the gradation correction unit according to one embodiment of the present invention. In FIG. 2, reference numeral 36 denotes an A / D converter 37.
, A D / A converter 37, and an RA 38
M and 39 are ROMs storing programs to be executed by the CPU 36, 40 is test pattern generation means, 41 is S
4 is a gradation correction table including a RAM.

The density sensor 25 is an optical sensor that is disposed to face the intermediate transfer member 15 and includes a light emitting element and a light receiving element. The CPU 36 is configured to control the light amount by changing the forward current of the light emitting element of the density sensor 25 by setting a numerical value to the D / A converter 37. The value that can be set in the D / A converter 37 is 8 bits, and a value from 0 to 255 can be set. The output of the density sensor 25 is C
It is input to the A / D conversion port of PU37.

When the image forming apparatus enters a warm-up period, a main motor (not shown) is started to drive the photosensitive member 5 and the intermediate transfer member 15. However, at this time, the charger 12
And other high-voltage power supplies are not applied. By starting the main motor,
After the photosensitive member 5 and the intermediate transfer member 15 reach a constant speed, the intermediate transfer member 15 is rotated at least once, and the intermediate transfer member 15 is cleaned. Here, as the first stage of the tone correction, the tuning of the density sensor 25 is performed to suppress the variation in the characteristics of the density sensor for each device. Specifically, the light emission amount of the density sensor 25 is determined. Hereinafter, tuning of the density sensor 25 will be described.

When the intermediate transfer body 15 is completely cleaned, the adjustment target value of the background density of the intermediate transfer body is set to, for example, 1.25 V at an analog level, that is, the data after A / D conversion is "64" ( = 1.25V / 5.00V × 2
55). The CPU 36 sets the 8-bit value in the D / A converter 37 in ascending order, starting from the smallest value.
Is compared with the adjustment target value. When the output from the density sensor 25 matches the adjustment target value, the light emission amount setting value is determined, and the value is stored in the RAM 38. When the tuning of the density sensor 25 is completed, the process proceeds to the second stage of gradation correction. In the second stage, the intermediate transfer body 1
The average value of the background density for one round of 5, ie, the highlight reference, is measured, and the result is stored in the RAM 38.

When the acquisition of the highlight reference is completed, the process enters the third stage of gradation correction. In the third stage, the toner density of the test pattern having the gradation formed on the intermediate transfer body 15 is detected for each color, and a table for correcting the γ characteristic of the image forming apparatus is created using the highlight reference.

FIG. 3 shows a test pattern used in the third stage (an example of a test pattern according to an embodiment of the present invention). The test pattern is formed by the test pattern generation means 40 based on an instruction from the CPU 36 when the power is turned on or when appropriate conditions are satisfied. Even if the pattern area is physically deteriorated by forming the same pattern a number of times, the pattern area is formed at the end of the image area so that the image quality is hardly visually noticeable. The number of test patterns is arbitrary, and image data is set in advance so as to form different density patterns.

In this case, a total of 15 test patterns that fit in one circumference of the intermediate transfer member 15 are used. Since the γ characteristic of the image forming apparatus changes between environments and devices, it is necessary to first recognize the general γ characteristic in order to perform correction. That is, the data of the test pattern cannot be a biased value. Therefore, in an apparatus in which the image data is 8 bits, the data of the first test pattern is 11 hexadecimal.
H, the next pattern data is 22H, the next is 33H, ...
At the end, like the FFH, the dynamic range FFH of the image data is divided into fifteen equal parts, and the density is set to increase in order from the top of the image.

In the intermediate transfer member 15, the test pattern formation position is common to each color and the image data is also common, but each color image is formed using a different screen angle for each color and is transferred from, for example, a host computer or the like. The screen angle at the time of printing the processed image data and the screen angle at the time of executing the gradation correction are common for each color.

Next, FIG. 4 shows a general characteristic when the toner of the chromatic component and the achromatic component formed on the intermediate transfer member 15 is detected by the density sensor 25 (FIG. 4). Will be described with reference to FIG. In FIG. 4A, a chromatic component (cyan, magenta, yellow) is shown, and in FIG. 4B, an achromatic component (black) is shown.
3 shows an output example of the density sensor 20 for the test pattern for gradation correction of FIG.

In the case of a color component, the output of the density sensor 25 increases as the density of the test pattern for gradation correction increases. Strictly speaking, the characteristics are different for each color, but there is no difference in that the output of the density sensor 25 monotonically increases as the pattern density increases. On the other hand, when an achromatic component pattern is detected under the same conditions, the output of the density sensor 25 monotonously decreases in accordance with an increase in the pattern density. It is a major feature that the values change in different directions between the chromatic component and the achromatic component in the middle of the graph, that is, the background level of the intermediate transfer body 15 as the pattern density increases.

After the second stage of the tone correction, the test pattern generating means 40 forms an electrostatic latent image of the test pattern shown in FIG. The components required for image formation, such as high voltage, have already been activated, and the preparation for image formation has been completed at this point. After a lapse of a predetermined time, the black developing device 14K comes into contact with the photoreceptor 1 to visualize the test pattern for gradation correction. The visualized black test pattern is transferred to the intermediate transfer body 15 and transported to the density sensor 25.

At this point, the CPU 36 sets the D / A converter 3
7, the light emission amount of the density sensor 25 is set, and reading of the output of the density sensor 25 is started. Density sensor 25
Is output by the CPU 3 at a predetermined sampling cycle.
6 is read. Reading is performed for all image areas, and the CPU 36 immediately stores the reading result in the RAM 38. Thereafter, cyan, magenta, and yellow test patterns are formed on the intermediate transfer member 15 using the same image data as black, and stored in the RAM 38 in the same manner as in the case of black.

As described above, at this point, the intermediate transfer body cleaning device 21 is in the contact state, and the intermediate transfer body 1
5, the density sensor 25 is always cleaned.
Can read a gradation correction pattern for each color. Thus, the density detection result of each test pattern is R
Although the data is stored in the AM 38, this data is merely obtained in order of time from the output of the density sensor 25. Therefore, when the test pattern forming / reading operation is completed,
The image forming apparatus stops all operations of the motors, the charger 12, and the like, and performs data processing.

Since the data in the RAM 38 are all obtained based on the detection of the same slit on the intermediate transfer member position detecting plate 23, the test pattern reading start point is at the same point on the intermediate transfer member 15. Since the time from when the slit of the intermediate transfer member position detecting plate 23 is detected to when the CPU 36 starts taking in the output of the density sensor 25 is fixed, the reading result corresponding to each test pattern position is Obtained easily. First, the values of ten points are summed for one test pattern, and the average value is set as the toner density value of one pattern. Thus, the toner density at each pattern position of the achromatic component and the toner density at each pattern position of the chromatic component can be obtained.

When the density measurement of the test pattern for tone correction of each color is completed, a tone correction table is created using the highlight reference obtained in the second step. From now on, for simplicity,
The data processing of cyan (color component) will be described. The data processing for magenta and yellow is similar to that for cyan, and the data processing for black (achromatic component) is the opposite of the relationship between the toner density of the pattern and the output value of the density sensor compared to the case of the chromatic component. Considering
The same is true. Each pattern position is set to n (n = 1 to 15).
The average value of the background density of the intermediate transfer body 15 (highlight reference) is HL, and the toner density of the gradation pattern is D_C.
[N] (D means Density).

In the cyan data processing, the density data D_C [n] for each pattern, the highlight reference HL_C, and DIF [n] = D_C [n] -H are first applied to all n.
Calculate L and define DIF [n] as the true concentration level. Further, the obtained DIF [n] is converted into the density on the recording paper. The density conversion uses a density conversion table acquired experimentally in advance.

The density conversion table will be described with reference to FIG. FIG. 5 is a graph of a cyan density conversion table according to an embodiment of the present invention. In FIG. 5, the horizontal axis represents the value obtained by A / D conversion of the output of the density sensor 25 for each pattern, and the vertical axis represents the reflection density (Macbeth density) when the same pattern is formed on recording paper. This density conversion table can be easily obtained by detecting the tone correction pattern formed on the intermediate transfer body 15 by the density sensor 25 and transferring and fixing the same pattern on recording paper. Further, if the highlight reference is uniquely determined in the measurement system, the shapes of these graphs hardly change, so that the density on the recording paper can be correctly predicted from the values obtained as described above.

The density conversion table corresponds to the density sensor 25.
Is a table for converting the output of the image into the image density on the recording paper, and thus includes the paper transfer characteristics and the fixing characteristics when the toner image is transferred onto the paper. Therefore, in the case where the paper transfer characteristics fluctuate due to the environment or the like and deteriorate the gradation, if the conversion characteristics of the density conversion table are changed according to the environmental parameters and the like,
Can absorb the effects.

In the above description, the method of detecting the image density from the output of the density sensor 25 has been described. The data of the test pattern for gradation correction, that is, the input is a predetermined value and is known. The relationship between the input data and the density on the recording paper is nothing but the γ characteristic of the image forming apparatus. FIG. 6 (a graph showing the γ characteristic of cyan in one embodiment of the present invention)
The γ characteristics of cyan obtained this time are shown in FIG. In this example, the data is linearly interpolated, but another interpolation method may be used.

Next, a tone correction table is created based on FIG. The target value of the maximum density this time is 1.5 (Macbeth density).
What is necessary is just to find each image data. For example, concentration 0.5
Is 45H. FIG. 7 shows an example of the gradation conversion table created as described above (an example of a cyan gradation correction table according to an embodiment of the present invention).

Next, the relationship between the image data and the gradation correction table will be described with reference to FIG. The CPU 36 transfers the data of the table created earlier to the gradation correction table 41. When the image data (4 bits) 42 output from the controller 1 accesses the address of the gradation correction table 41, the gradation-corrected image data (8 bits)
43 is output from the gradation correction table 41 to the pulse width modulation means 3.

The pulse width modulation means 3 outputs a pulse width corresponding to the image data to the laser driver 4. By accessing the gradation correction table, for example, when the controller 1 outputs the image data (4 bits) 42 in the uniform step, the γ characteristic of the image forming apparatus is canceled by the gradation correction table, and The image density is also a uniform step. The above operation ensures the gradation of the image.

FIG. 8 (a graph showing the result of the first tone correction in one embodiment of the present invention) shows the result of the tone correction obtained as described above. Compared with the γ characteristic in FIG. 6 where the correction is not performed (the same characteristic is obtained even for the equal step of 4-bit data from the controller),
Although there is an effect of correction, only an accuracy of ± 30% is obtained for the target. This deterioration in accuracy is caused by the γ shown in FIG.
As can be seen from the characteristics, the information obtained from the test pattern before the density starts to appear and the test pattern after the target maximum density is useless, and is generated due to a shortage of information required for correction. As described above, since the γ characteristics of the image forming apparatus change between environments and devices, test pattern data cannot be biased in order to perform correction, and overall γ characteristics are recognized. It is necessary and these results are inevitable.

Therefore, in this embodiment, tone correction is further performed so that the result obtained in the first tone correction is reflected on the test pattern in the second tone correction. Specifically, the data of the gradation correction table obtained in the first gradation correction is used as test pattern data, and the above-described correction operation is performed. FIG. 9 (a graph showing the result of the second tone correction in one embodiment of the present invention) shows the result of the second tone correction. As for the correction result, an accuracy of ± 5% with respect to the target was obtained, and a good linearity gradation was reproduced.

As described above, by optimizing the test pattern data, the accuracy of gradation correction can be greatly improved. In addition, the test pattern data in the second tone correction may be data obtained by equally dividing the data corresponding to the target maximum density from the data at which the density starts to appear, or may have a configuration in which the sections desired to be known in detail are dense. Further, the gradation correction may be further repeated. Note that the above tone correction is not limited to when the power of the apparatus is turned on.

[0051]

According to the present invention, it is possible to always linearly correct the γ characteristic in response to environmental changes and changes over time by performing the gradation correction by optimizing the test pattern, and to obtain a stable and excellent gradation characteristic at all times. Can be obtained.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a configuration diagram of a gradation correction unit according to an embodiment of the present invention.

FIG. 3 is a view showing an example of a test pattern according to the embodiment of the present invention;

4A is a graph showing an output of a density sensor according to an embodiment of the present invention; FIG. 4B is a graph showing an output of the density sensor according to an embodiment of the present invention;

FIG. 5 is a graph of a cyan density conversion table according to the embodiment of the present invention;

FIG. 6 is a graph showing γ characteristics of cyan according to an embodiment of the present invention.

FIG. 7 is an exemplary view of a cyan gradation correction table according to the embodiment of the present invention;

FIG. 8 is a graph showing a result of a first tone correction according to the embodiment of the present invention;

FIG. 9 is a graph showing a result of a second tone correction according to the embodiment of the present invention;

FIG. 10 is a graph showing general input / output characteristics of an electrophotographic process in a conventional image forming apparatus.

FIG. 11 is a schematic configuration diagram of a conventional pulse width modulation unit.

FIG. 12 is a graph showing a gradation correction table in conventional image data conversion.

[Description of Signs] 5 Photoconductor 14K Developing Unit (Black) 14Y Developing Unit (Yellow) 14M Developing Unit (Magenta) 14C Developing Unit (Cyan) 15 Intermediate Transfer Body 25 Density Sensor 40 Test Pattern Generating Unit 41 Tone Correction Table ( SRAM)

Claims (3)

[Claims] 1. A test pattern generating means for generating image data of a test pattern having shading, a latent image forming means for forming a latent image of the test pattern, and a developing means for visualizing the formed latent image. An image carrier for holding a visualized toner image, a density sensor for detecting the density of the visualized test pattern, and performing tone correction based on an output value of the density sensor, Is generated at least twice or more. 2. The image forming apparatus according to claim 1, wherein the image data of the test pattern generated after the second time is within the range of the image data having the target maximum density from the image data at which the toner density starts to appear. apparatus. 3. The image forming apparatus according to claim 1, wherein the image data of the test pattern generated after the second time is image data based on the result of the previous tone correction.