Image formation apparatus

Abstract

To perform high-accuracy maximum density correction control under stabilized image formation conditions.SOLUTION: An image formation apparatus includes: an image processing unit that applies pseudo halftone processing corresponding to the image attribute to image data; an image formation unit that is controlled on the basis of image formation conditions and forms an image on the basis of the image data applied with the pseudo halftone processing; an image carrier that carries a detection image; and a detection unit that receives reflected light from the detection image on the image carrier. The image formation apparatus forms detection images for gradation correction, controls the gradation characteristics of the image with the first attribute on the basis of the reflected light from the first detection image corresponding to the first attribute, and controls the gradation characteristics of the image with the second attribute on the basis of the reflected light from the second detection image corresponding to the second attribute. The image formation apparatus forms the detection image for target maximum density correction and generates image formation conditions to correct the target maximum density of the image to be formed on the basis of the reflected light from the detection image for target maximum density correction. Here, the detection image for target maximum density correction is the image applied with error diffusion processing as the pseudo halftone processing.SELECTED DRAWING: Figure 8

Description

Technical Field

[0001] The present invention relates to an image forming apparatus, and more particularly to its calibration technology.

Background Art

[0002] Generally, it is known that the image density varies in an image forming apparatus using an electrophotographic image forming process due to the use environment and deterioration of components within the image forming apparatus. Therefore, recent image forming apparatuses perform image density control (control of image forming conditions) for adjusting image forming conditions such as exposure amount, development bias, and γ correction table, so as to obtain an image with a stable density. In image density control, for example, a detection toner image (toner patch) is formed on an image carrier such as an intermediate transfer body, the toner amount of this toner patch is detected by an optical sensor, and based on the detection result, image forming conditions such as exposure amount, development bias, and γ correction table are adjusted.

[0003] By the way, in an image forming apparatus, the resolution is improved to 600 dpi, 1200 dpi, etc., and a plurality of pixels are used to modulate the image density area-wise to display a pseudo halftone image. The image processing performed in the process of converting this input image data into data of a pseudo halftone image is pseudo halftone processing.

[0004] The input image data can be roughly classified into the following three types of image types. (1) Character / line image (2) Photograph image (3) Graphics image Although there are three types of this input image data, in the character / line image of (1), the shape reproducibility of characters / lines is emphasized. On the contrary, color reproducibility and gradation reproducibility are less important. In contrast, in the photographic image of (2) and the graphics image of (3), color reproducibility and gradation reproducibility are more important than shape reproducibility, respectively, with opposite characteristics. Therefore, a method of switching the line count of pseudo halftone processing has been proposed, such as performing high-line-count screening processing on characters / lines and outlines, and performing low-line-count screening processing on other photographic images, etc. (Patent Document 1).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] As image density control for forming an image with the target maximum density, a method of calibrating using a detection image is known. In this method, for example, a plurality of detection images are formed on an intermediate transfer body based on different plurality of image forming conditions, the plurality of detection images are detected by a detection means, and based on the detection results of each detection image detected by the detection means, the image forming conditions for forming an image with the target density are determined.

[0007] Here, in image density control, color reproducibility and gradation reproducibility are more important than the shape reproducibility of characters / lines. Therefore, a case can be considered where a low-line-count screen suitable for color reproducibility and gradation reproducibility is applied to the detection image formed for controlling the image density.

[0008] However, when using a detection image subjected to low-line screen processing to control image formation conditions, the variation in the detection values of the sensor that detects the detection image is large, and the image density cannot be controlled with high precision.

[0009] The present invention has been made in view of the above circumstances, and an object thereof is to generate, with high precision, image formation conditions for forming an image having a target maximum density.

Means for Solving the Problems

[0010] According to one aspect of the present invention, in order to achieve the above object, there is provided an image forming apparatus for forming an image on a sheet, image processing means for performing pseudo half-tone processing corresponding to the attributes of the image to be formed on the image data; image forming means that is controlled based on image formation conditions and forms an image based on the image data subjected to the pseudo half-tone processing; an image carrier on which a detection image formed by the image forming means is carried; detection means for irradiating light on the detection image on the image carrier and receiving the reflected light from the detection image; causing the image forming means to form a first detection image subjected to pseudo half-tone processing corresponding to a first attribute, and controlling the gradation characteristics of the image of the first attribute to be formed by the image forming means based on the light reception result obtained by receiving the reflected light from the first detection image by the detection means; causing the image forming means to form a second detection image subjected to pseudo half-tone processing corresponding to a second attribute different from the first attribute, and controlling the gradation characteristics of the image of the second attribute to be formed by the image forming means based on the light reception result obtained by receiving the reflected light from the second detection image by the detection means; control means for causing the image forming means to form a detection image for target maximum density correction, and generating, based on the light reception result obtained by receiving the reflected light from the detection image for target maximum density correction by the detection means, the image formation conditions for correcting the target maximum density of the image to be formed by the image forming means. The detection image for the target maximum density correction is an image subjected to error diffusion processing as the pseudo halftone processing regardless of the attributes of the image to be formed. An image forming apparatus is provided, which is characterized by this.

[0011] According to another aspect of the present invention, there is provided an image forming apparatus for forming an image on a sheet, image processing means for subjecting image data to pseudo halftone processing corresponding to the attributes of the image to be formed; image forming means controlled based on image forming conditions and forming an image based on the image data subjected to the pseudo halftone processing; an image carrier on which a detection image formed by the image forming means is carried; detection means for irradiating light on the detection image on the image carrier and receiving reflected light from the detection image; causing the image forming means to form a first detection image subjected to pseudo halftone processing corresponding to a first attribute, and controlling the gradation characteristics of the image of the first attribute to be formed by the image forming means based on the light reception result of receiving the reflected light from the first detection image by the detection means; causing the image forming means to form a second detection image subjected to pseudo halftone processing corresponding to a second attribute different from the first attribute, and controlling the gradation characteristics of the image of the second attribute to be formed by the image forming means based on the light reception result of receiving the reflected light from the second detection image by the detection means; causing the image forming means to form a detection image for target maximum density correction, and control means for generating image forming conditions for correcting the target maximum density of the image to be formed by the image forming means based on the light reception result of receiving the reflected light from the detection image for target maximum density correction by the detection means. The detection image for the target maximum density correction is an image subjected to pseudo halftone processing using a second screen having a higher screen ruling than a first screen used for the pseudo halftone processing corresponding to the first attribute. An image forming apparatus is provided, which is characterized by this.

Advantages of the Invention

[0012] According to the present invention, image formation conditions for forming an image with a target maximum density can be generated with high precision.

Brief Description of the Drawings

[0013]

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Mode for Carrying Out the Invention

[0014] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the invention according to the claims. Although a plurality of features are described in the embodiments, not all of these plurality of features are essential to the invention, and the plurality of features may be arbitrarily combined. Further, in the accompanying drawings, the same or similar configurations are denoted by the same reference numerals, and duplicate explanations are omitted.

[0015] [Embodiment 1] [Image Forming Apparatus] Figs. 1 and 2 are cross-sectional views showing an example of an intermediate transfer type color image forming apparatus 100 according to the present invention. The image forming apparatus 100 is an electrophotographic laser beam multifunction machine that employs a contact charging method and a two-component contact development method.

[0016] As shown in Fig. 1, in the image forming apparatus 100, four image forming stations Pa, Pb, Pc, and Pd as image forming units are arranged along the rotation direction (arrow R) of the intermediate transfer belt 11. Fig. 2 shows a schematic cross-sectional view of the image forming station Pa. Here, the four image forming stations Pa, Pb, Pc, and Pd have the same configuration. As shown in Fig. 2, the image forming station Pa includes a photosensitive drum 1a as a photoreceptor, a charging roller 2a, a developing device 4a, a cleaning device 5a, and a primary transfer device 7a. In the figure, arrow 3a indicates the laser light irradiated from the laser scanner 31 (Fig. 1) to the photosensitive drum 1a. Further, the intermediate transfer belt 11 as an intermediate transfer member is disposed movably in the direction of arrow R so as to pass between the photosensitive drums 1a, 1b, 1c, 1d and the primary transfer devices 7a, 7b, 7c, 7d of the respective image forming stations Pa, Pb, Pc, Pd.

[0017] As shown in Fig. 1, a laser scanner 31 including a light source 32, a polygon mirror 33, and a transmission window 34 is installed in the lower part of the image forming apparatus 100. The laser light emitted from the light source 32 becomes scanning light by the rotation of the polygon mirror 33, and the light beam of the scanning light is deflected by a plurality of reflection mirrors and condensed and exposed onto the generatrix of the charged photosensitive drums 1a, 1b, 1c, 1d by an fθ lens. Thereby, an electrostatic latent image corresponding to the image signal is formed on the photosensitive drum.

[0018] The developing devices 4a, 4b, 4c, and 4d of the four image forming stations Pa, Pb, Pc, and Pd are filled with a two-component developer in which yellow, magenta, cyan, and black non-magnetic toner and magnetic carrier are mixed at a predetermined mixing ratio. The developing devices 4a, 4b, 4c, and 4d form a toner image by developing the electrostatic latent image on the photosensitive drum using toner of each color. The toner images of each color are primarily transferred from the photosensitive drums 1a, 1b, 1c, and 1d onto the intermediate transfer belt 11. Further, the sheet accommodated in the cassette 14 is conveyed to the secondary transfer roller 12, and the toner image carried on the intermediate transfer belt 11 is secondarily transferred onto the sheet. After the toner image is fixed by heating and pressurization in the fixing device 9, it is discharged outside the apparatus as a recorded image.

[0019] Also, a cleaning blade 13 for cleaning the toner (residual toner) adhering to the surface of the intermediate transfer belt 11 is provided downstream of the secondary transfer position in the rotational direction (arrow R) of the intermediate transfer belt 11. Further, the toner (residual toner) remaining on the photosensitive drum 1a is removed from the photosensitive drum 1a by the cleaning device 5a. The same applies to the other image forming stations. The image forming apparatus 100 also includes a patch sensor 50 which is an optical sensor for detecting reflected light from the toner pattern on the intermediate transfer belt 11.

[0020] Also, above the image forming apparatus 100, a reader 200 for reading a document and an operation panel U for receiving operations by the user are provided. The operation panel U has, for example, a liquid crystal touch panel.

[0021] Here, the image forming apparatus 100 forms an image at a process speed of 200 mm / sec. The process speed is the rotational speed of the photosensitive drums 1a, 1b, 1c, and 1d. The process speed can also be said to be the rotational speed of the intermediate transfer belt 11. Or, the process speed can also be said to be the conveyance speed of the sheet passing through the secondary transfer position.

[0022] The image forming apparatus 100 uniformly charges the surface of the photosensitive drum 1a by applying a high voltage to the charging roller 2a. The charging roller 2a has both ends of its core metal rotatably held by bearing members (not shown), and is biased toward the photosensitive drum 1a by a pressing spring 21a, and is pressed against the surface of the photosensitive drum 1a with a predetermined pressing force. Thereby, the charging roller 2a rotates following the rotation of the photosensitive drum 1a. A charging bias voltage under predetermined conditions is applied to the core metal of the charging roller 2a by a high voltage power supply 101a. Thereby, the surface of the rotating photosensitive drum 1a is subjected to contact charging treatment to a predetermined polarity and potential. In the present embodiment, the charging bias voltage for the charging roller 2a is an oscillating voltage obtained by superimposing a DC voltage and an AC voltage. More specifically, it is an oscillating voltage obtained by superimposing a DC voltage and a sinusoidal AC voltage with a frequency of 1.3 kHz and a peak-to-peak voltage Vpp = 1.5 kV. When a DC voltage of -600 V is applied as this charging bias voltage, the surface of the photosensitive drum 1a is uniformly charged to the same -600 V (dark potential Vd) as the DC voltage applied to the charging roller 2a. Note that the charging member for charging the photosensitive drum 1a is not limited to the configuration of the charging roller 2a. The charging member may be, for example, a corona charger that charges the photosensitive drum 1a with a wire to which a charging bias voltage is applied.

[0023] Next, in order to form an electrostatic latent image on the photosensitive drum 1a, the image forming apparatus 100 exposes the surface of the charged photosensitive drum 1a with laser light 3a from a laser scanner 31. The laser scanner 31 is a laser beam scanner using a semiconductor laser as a light source 32.

[0024] Next, the image forming apparatus 100 develops the electrostatic latent image on the photosensitive drum 1a using toner by a developing device 4a. As a result, a toner image (developer image) is formed on the photosensitive drum 1a. The developing device 4a is a developing device that employs a two-component contact developing method in which a magnetic brush using a two-component developer composed of non-magnetic toner and a magnetic carrier housed in a developing container 40a is brought into contact with the photosensitive drum for development. Also, in this embodiment, a negatively charged non-magnetic toner is used. The developing device 4a includes a non-magnetic developing sleeve 41a as a developer carrier. Inside the developing sleeve 41a, a magnet 42a for generating a magnetic field is disposed. The developing sleeve 41a has a part of its outer peripheral surface exposed to the outside of the developing device 4a, and is disposed in close proximity to the photosensitive drum 1a while maintaining the closest distance (S-D gap) at 260 μm. The facing portion between the photosensitive drum 1a and the developing sleeve 41a is a developing region. Also, the developing sleeve 41a is rotationally driven in a rotational direction X that is the same direction as the rotational direction Y of the photosensitive drum 1a during development. The rotating developing sleeve 41a carries and conveys the two-component developer thinned on the developing sleeve 41a by a regulating member 43a. A predetermined developing bias is applied to the developing sleeve 41a from a high voltage power supply 102a. In this embodiment, the developing bias voltage is an oscillating voltage obtained by superimposing a DC voltage and an AC voltage. More specifically, it is an oscillating voltage obtained by superimposing a DC voltage of -450 V and a rectangular wave AC voltage with a frequency of 8.0 kHz and a peak-to-peak voltage Vpp = 1.8 kV. The electrostatic latent image is reversely developed by this developing bias and the electric field of the electrostatic latent image formed on the surface of the photosensitive drum 1a.

[0025] Next, the image forming apparatus 100 primarily transfers the developer image formed on the photosensitive drum 1a to the intermediate transfer belt 11 by a primary transfer device 7a. In this embodiment, the primary transfer device 7a is a transfer roller. The transfer roller 7a is pressed against the photosensitive drum 1a with a predetermined pressing force. A transfer bias having a positive polarity opposite to the charging polarity of the toner, +1 kV in this embodiment, is applied to the transfer roller 7a from a high voltage power supply 103a, and the toner is primarily transferred to the intermediate transfer belt 11.

[0026] As described above, during the rotation process of the photosensitive drum 1a, it is uniformly charged to a predetermined potential (dark part potential Vd) with a predetermined polarity by the charging roller 2a, and is exposed according to the image signal by the laser scanner 31 (laser light 3a). As a result, an electrostatic latent image (bright part potential VL) corresponding to the color component image of the target color image is formed. The electrostatic latent image is developed by the developing roller 42a at the developing position and visualized as a toner image. At this time, a voltage (charging bias voltage) is applied to the charging roller 2a so as to become the dark part potential Vd, and the exposure amount by the laser scanner 31 (laser light 3a) is determined so as to become the bright part potential VL. Also, a developing bias voltage Vdc is applied to the developing roller 42a. Here, the absolute value of the difference between the bright part potential VL and the developing bias voltage Vdc is called the developing contrast.

[0027] [Reader] FIG. 3 is a schematic diagram showing the configuration of the reader 200. The reader 200 includes a first mirror unit 104a, a second mirror unit 104b, an image sensor 105, a lens 115, a motor 116, a document size detection sensor 113, and a home position sensor 106 inside the housing. The first mirror unit 104a includes a document illumination lamp 103 and a first mirror 107a. The second mirror unit 104b includes a second mirror 107b and a third mirror 107c. The first mirror unit 104a and the second mirror unit 104b are driven by the motor 116 and are movable in the Z direction.

[0028] When reading the original document, the motor 116 rotates, causing the first mirror unit 104a and the second mirror unit 104b to move to the home position where the home position sensor 106 is located once. On the platen glass 102, a single original document is fixed by a platen (not shown) or an ADF unit with its reading surface facing the platen glass 102 side. The reader 200 turns on the original document illumination lamp 103 and irradiates light onto the reading surface of the original document 101. The first mirror unit 104a and the second mirror unit 104b deflect the image light from the original document 101 by the first mirror 107a, the second mirror 107b, and the third mirror 107c while moving in the Z direction and guide it to the lens 115. The lens 115 forms an image of the image light on the light-receiving surface of the image sensor 105. The image sensor 105 converts the image light into an electrical signal.

[0029] [Block Diagram] Figure 4 is a schematic functional block diagram of the image forming apparatus 100. The CPU 301 has a function of generating various command signals and performing arithmetic processing in order to operate various sensors and motors of the image forming apparatus 100 in accordance with the electrophotographic process. Also, a memory for storing data is built into the CPU 301. The image data generation unit 302 has a function of converting various image data into signals for laser control and sending control signals to the laser drive units 303a to 303d. The image data generation unit 302 also includes a function of generating various toner patterns for adjustment.

[0030] The laser drive units 303a to 303d have a function of driving the laser elements of the laser scanners 3a to 3d based on the signals sent from the image data generation unit 302 and controlling the lighting and light quantity of the laser. The scanner control unit 306 has a function of controlling the ON / OFF of the illumination lamp 103 inside the reader 200 and the driving of the drive motor 116 in accordance with the command signal of the CPU 301. The scanner image processing unit 305 acquires an electrical signal from the image sensor 105 inside the reader 200, generates an image signal, and sends it to the CPU 301.

[0031] The motor control unit 91 is electrically connected to each drive motor (not shown) and has a function of controlling the drive timing and drive speed. The high-voltage control unit 92 has a function of controlling the output of biases required for the image forming process, such as charging bias, developing bias, and transfer bias.

[0032] Also, the CPU 301 is electrically connected to the cassette 14, the I / F unit 85, and the timer 90, and is further connected to the operation panel U through the I / F unit 85. The CPU 301 can perform image formation using the recording material P stored in the cassette 14. The operation panel U receives operations by the user and is constituted by, for example, a liquid crystal touch panel that functions as the input unit 93 and the display unit 94. Note that the operation panel U may be an external terminal such as a personal computer connected to the image forming apparatus.

[0033] Also, the CPU 301 is electrically connected to the controller 87 and the image processing unit 84. The image information 88 is sent to the CPU 301 through the controller 87. The CPU 301 can form an image by processing the received image information 88 in the image processing unit 84.

[0034] The image processing unit 84 performs pseudo halftone processing on the image information 88 (input image data). The image information 88 (input image data) has information (attribute information) regarding the image type for each region of the image. The types of images (attribute information) are the following three types.

[0035] (1) Character / line image (2) Photograph image (3) Graphics image Here, for (1) the character / line image, importance is attached to the shape reproducibility of characters / lines, and for (2) the photograph image and (3) the graphics image, importance is attached to color reproduction / tonal reproduction.

[0036] The image processing unit 84 performs a screening process using a high-line screen on the input image data of the image information 88 to which attribute information classified into characters and line images is assigned. The high-line screen is, for example, a 230-line / inch screen. The image processing unit 84 generates an image (image data) expressed using a 230-line / inch screen from the input image data of the image information 88 to which attribute information classified into characters and line images is assigned.

[0037] Also, the image processing unit 84 performs a screening process using a low-line screen on the input image data of the image information 88 to which attribute information classified into photographic images and graphics images is assigned. The low-line screen is, for example, a 170-line / inch screen. The image processing unit 84 generates an image (image data) expressed using a 170-line / inch screen from the input image data of the image information 88 to which attribute information classified into photographic images and graphics images is assigned. The screen line number used to reproduce photographic images and graphic images where color reproduction and gradation reproducibility are emphasized is less than the screen line number used to reproduce characters and line images where shape reproducibility is emphasized. Note that the screen line number used to reproduce photographic images and graphic images is preferably 190 lines / inch or less. On the other hand, the screen line number used to reproduce characters and line images is preferably more than 190 lines / inch.

[0038] [Patch Sensor] FIG. 5 is a schematic diagram showing the configuration of the patch sensor 50. The patch sensor 50 is a sensor that measures reflected light from a measurement target. The patch sensor 50 described in this embodiment is a specular reflection type and has a light emitting unit 51 as an irradiation means and a light receiving unit 52 as an output means. The light emitting unit 51 is, for example, an LED, and the light receiving unit 52 is, for example, a photodiode. The patch sensor 50 further has an IC 53 that controls the light emission amount (irradiation light amount) of the light emitting unit 51 as one of the irradiation conditions. The light emitting unit 51 is installed so as to irradiate light at an angle of 45 degrees with respect to the normal of the intermediate transfer belt 11, and irradiates the intermediate transfer belt 11 with light. The light receiving unit 52 is installed at a position symmetric to the light emitting unit 51 around the normal of the intermediate transfer belt 11, receives specularly reflected light from the base or toner image of the intermediate transfer belt 11 which is the irradiation region, and outputs a value corresponding to the light reception result (reflection light level). FIG. 5 shows a case where one patch of the toner pattern image P1 passes through the measurement region of the sensor 50. The CPU 301 converts the measurement result of the reflected light from the toner pattern image P1 measured by the patch sensor 50 (output value of the light receiving unit 52) into a density value. Here, a table (patch detection luminance density conversion table) for converting the output value of the patch sensor 50 corresponding to the toner pattern image P1 into a density value is stored in the image processing unit 84 in advance and is created according to the output characteristics of the patch sensor 50.

[0039] Note that, in this embodiment, the patch sensor 50 has been described as a specular reflection type, but it may also be a diffuse reflection type patch sensor. In the specular reflection type, it is detectable for yellow, magenta, cyan, and black toners, but it cannot be detected accurately until the image printing rate of the toner pattern image P1 reaches 100%. On the other hand, in the diffuse reflection type, it is detectable for yellow, magenta, and cyan toners even when the image printing rate of the toner pattern image P1 is 100%, but it has the characteristic that it cannot be detected for black toner.

[0040] Here, the image printing rate for a certain area may specifically be a value obtained by dividing the cumulative value of the density data for each pixel included in that area by the cumulative value when all pixel values in that area are at the maximum density (or its percentage representation). For example, the image printing rate of an area completely filled with the maximum density (density 100%) of one pixel is 100%, and the image printing rate of an area where no image is formed is 0%.

[0041] [Maximum Density Control by Patch Detection] Generally, density correction control is roughly classified into two types: maximum density correction control that adjusts the development contrast by changing the exposure amount, charging bias voltage, development bias voltage, etc., and gradation correction control that generates a LUT (γ correction table) corresponding to the type of pseudo - halftone process. The exposure amount, charging bias voltage, and development bias voltage are an example of the image formation conditions for correcting the target maximum density of the image to be formed by the image formation stations Pa, Pb, Pc, Pd. The LUT (γ correction table) is an example of the gradation correction conditions for correcting the gradation characteristics of the image to be formed by the image formation stations Pa, Pb, Pc, Pd.

[0042] FIG. 6 is a diagram showing an example of a toner pattern image P1 for development contrast correction used for maximum density adjustment control by patch detection. The toner pattern image P1 functions as a detection image for target maximum density correction. In the present embodiment, the toner pattern image P1 is formed on the intermediate transfer belt 11. Note that the toner pattern image P1 may be formed on the image carrier, and may also be formed on the photosensitive drum 1a. And the patch detection sensor 50 may be arranged to face the image carrier on which the toner pattern image P1 is formed.

[0043] The arrow in Fig. 6 indicates the rotation direction of the intermediate transfer belt 11. The toner pattern image P1 is 25 mm × 25 mm in this example. The toner pattern image P1 is formed under the condition that the development contrast is changed in five steps of Vc1, Vc2, Vc3, Vc4, and Vc5 by changing the exposure amount, charging bias voltage, development bias voltage, etc. for each toner color component of Y, M, C, and K (five patches for each color). A total of 20 toner pattern images P1 are formed in the rotation direction (circumferential direction) of the intermediate transfer belt 11. Note that if the image printing rate is too high, the detection accuracy of the specular reflection type patch sensor 50 will be low. Therefore, in this embodiment, the image printing rate of the toner pattern image P1 is set to 80%. The density relationship between an image printing rate of 80% and 100% is acquired in advance.

[0044] Fig. 7 shows the relationship between the density at an image printing rate of 80% and the density at an image printing rate of 100% acquired in advance. For example, when the target maximum density at an image printing rate of 100% is Dmax_Target = 1.50, the development contrast that gives a density signal value D_80% = 1.20 obtained from the correction toner pattern image formed at an image printing rate of 80% may be determined.

[0045] Fig. 8 is a flowchart of the process for maximum density control by patch detection. The process of this flowchart is realized by the CPU 301 expanding a program stored in the ROM of the image processing unit 84 into a memory such as RAM that the CPU 301 has and then the CPU 301 executing it.

[0046] In step S11, the CPU 301 determines whether to start the maximum density control by patch detection. The activation conditions are when the user or service technician instructs the execution of the maximum density control by patch detection from the operation panel U, and when the CPU 301 determines that the execution of the maximum density control is necessary and starts it when the accumulated number of image forming operations reaches a predetermined value or more.

[0047] In step S12, the CPU 301 forms a toner pattern image P1 used for maximum density control by patch detection. Here, as shown in FIG. 6, the toner pattern image P1 in this example includes five patches with an image printing rate of 80% for each toner color component. The five patches may all have the same image data. For example, when the maximum density is set to 1 and the minimum density is set to 0, intermediate tone image data for five patches uniformly filled with pixels having a density of 0.8 is prepared. Then, screen processing (pseudo-intermediate tone processing) is performed on the intermediate tone image data using, for example, a high-line screen with a screen ruling of 230 to obtain pseudo-intermediate tone image data. Using the obtained pseudo-intermediate tone image data (toner pattern image data), the toner pattern image P1 of each toner color component is formed on the intermediate transfer belt 11. At this time, the imaging conditions (image forming conditions) can be changed for each patch.

[0048] The toner pattern image data may be held in the image data generation unit 302 after being subjected to the above-described screen processing. In this case, in step S12, the toner pattern image P1 may be formed using the held toner pattern image data. Alternatively, the image data generation unit 302 may generate the toner pattern image data each time the toner pattern image P1 is formed. The position of the intermediate transfer belt 11 where the toner pattern image P1 is formed is a position that can be detected by the patch detection sensor 50 when the intermediate transfer belt 11 is driven. Also, the toner patterns of each toner color component are formed so as not to overlap each other. Here, the reason for multiplying the maximum density by 0.8 is as already explained. In this example, a specular reflection type patch detection sensor 50 is used, and the detection accuracy is not high when the image printing rate is 100%. Also, the reason for performing high-line screen processing will be described later with reference to FIGS. 11, 12, 13, 15, 17, 18, etc. Note that the high-line refers to a relatively high line number among multiple screen processing line numbers. In this example, two types of pseudo-intermediate tone processing with screen rulings of 230 and 170 are possible, and the screen processing with a line number of 230, which is a relatively high line number, is called high-line screen processing.

[0049] In the formation of the toner pattern image P1, the development contrast Vc of each patch included in the pattern of each color, the charging bias voltage Vd, the development bias voltage Vdc, and the exposure amount LPW for achieving the same are set as follows. Here, the formation conditions of each patch are indicated by "Dmax", the subsequent characters (Y, M, C, K) indicate the color components, and the subsequent numbers (1 - 5) indicate the indices of the imaging conditions (also referred to as image formation conditions). DmaxY1, DmaxM1, DmaxC1, DmaxK1 (first imaging condition): Vc1, Vd1, Vdc1, LPW1 DmaxY2, DmaxM2, DmaxC2, DmaxK2 (second imaging condition): Vc2, Vd1, Vdc1, LPW2 DmaxY3, DmaxM3, DmaxC3, DmaxK3 (third imaging condition): Vc3, Vd2, Vdc2, LPW2 DmaxY4, DmaxM4, DmaxC4, DmaxK4 (fourth imaging condition): Vc4, Vd3, Vdc3, LPW2 DmaxY5, DmaxM5, DmaxC5, DmaxK5 (fifth imaging condition): Vc5, Vd3, Vdc3, LPW3.

[0050] Here, the charging bias voltage Vd is such that Vd1 = - 500V, Vd2 = - 600V, and Vd3 = - 700V. Considering the fog - removing potential of 150V for the development bias voltage Vdc, the conditions are Vdc1 = - 350V, Vdc2 = - 450V, and Vdc3 = - 550V. And when the exposure amount LPW of the laser light amount is indicated by the surface light amount on the photosensitive drum, the conditions are LPW1 = 0.16 μJ / cm2, LPW2 = 0.24 μJ / cm2, and LPW3 = 0.32 μJ / cm2.

[0051] Fig. 9 shows an example of a table from which the development contrast Vc is obtained from the charging bias voltage Vd and the exposure amount LPW. The development contrast table is created in advance according to the characteristics of the photosensitive drum and stored in the image processing unit 84. In this embodiment, Vc1 = 90V, Vc2 = 160V, Vc3 = 231V, Vc4 = 301V, and Vc5 = 370V. The same applies to other color components.

[0052] In step S13, the CPU 301 calculates the density value of each toner pattern by table calculation for the output value detected by the patch sensor 50 from the toner pattern image P1.

[0053] In step S14, the CPU 301 determines the developing contrast VcA that satisfies TargetA (image printing rate 80%), which is the target density in patch control.

[0054] FIG. 10 shows the relationship between the patch density (vertical axis) of the toner pattern image P1 obtained in step S13 and the developing contrast Vc (horizontal axis). In this example, patches are formed under five image conditions for one color component, and five points corresponding to each toner pattern image are shown in FIG. 10. The CPU 301 calculates the developing contrast VcA that becomes TargetA in patch control by linear interpolation at two points sandwiching the target using the relationship shown in FIG. 10. In this embodiment, for example, the developing contrast VcA corresponding to the target density is VcA = 131V.

[0055] In step S15, imaging condition A at which the development contrast becomes VcA is calculated. Imaging condition A is calculated by linearly interpolating the imaging conditions at two points sandwiching the target. In this example, VcA = 131V is between Vc1 = 90V and Vc2 = 160V. That is, there is an imaging condition to be obtained between the first imaging condition and the second imaging condition. Here, imaging condition 1 is determined by Vc1, Vd1, Vdc1, and LPW1, and imaging condition 2 is determined by Vc2, Vd1, Vdc1, and LPW2. Since the development contrast Vc is a subordinate condition determined by other conditions as shown in FIG. 9, the only difference between imaging condition 1 and imaging condition 2 is that the exposure amount LPW is LPW1 for imaging condition 1 and LPW2 for imaging condition 2. Therefore, for example, referring to the development contrast table in FIG. 9, when the charging bias Vd is Vd1 (for example, -500V), the exposure amount LPW is obtained by interpolation so that the development contrast becomes VcA (for example, 131V). The (target) imaging conditions obtained in this way are a charging bias voltage A = -500V, a development bias A = -350V, and an exposure amount A = 0.21 μJ / cm2. Note that the method for determining the imaging conditions is not limited to this, and includes, for example, a method of calculating by fixing either the charging bias or the exposure amount, or a method of calculating from a table.

[0056] In the above example, between a certain development contrast and the development contrast immediately above or below it, the imaging conditions that determine them are given such that one of them, for example, either the charging bias voltage Vd or the exposure amount LPW, is different and the others are common values. For example, only the exposure amount LPW is different between imaging condition 1 and imaging condition 2, and only the charging bias Vd is different between imaging condition 2 and imaging condition 3. The same applies to other imaging conditions. For this reason, only one item of the imaging conditions determined by interpolation is sufficient, and the imaging conditions can be determined simply and accurately.

[0057] [Tone Correction Control by Patch Inspection] Next, tone correction control will be described. FIG. 21 is a schematic diagram of toner pattern images P2 and P3 as detection images for tone correction used for tone correction control. The toner pattern image P2 is used to generate a LUT (γ correction table) for converting image data when forming a character / line image. The toner pattern image P2 is subjected to screening processing using a high screen ruling. The toner pattern image P3 is used to generate a LUT (γ correction table) for converting image data when forming a photographic image. The toner pattern image P3 is subjected to screening processing using a low screen ruling.

[0058] The toner pattern image P2 includes, for example, six images with tone levels of 10%, 30%, 45%, 60%, 80%, and 100%. The image data (detection image data) of the toner pattern image P2 is stored in a ROM in advance. The CPU 301 reads the detection image data of the toner pattern image P2 from the ROM and inputs it to the image processing unit 84. The image processing unit 84 performs screening processing using a high screen ruling on the detection image data of the toner pattern image P2. As a result, the image forming apparatus 100 forms the toner pattern image P2 on the intermediate transfer belt 11. The CPU 301 detects the toner pattern image P2 by the patch sensor 50, obtains the six densities of the toner pattern image P2, and obtains the tone characteristics from these density detection results. The CPU 301 generates a LUT such that the tone characteristics become ideal tone characteristics as a LUT for a high screen ruling.

[0059] The toner pattern image P3 also includes, for example, six images with a plurality of gradation levels of 10%, 30%, 45%, 60%, 80%, and 100%, similar to the toner pattern image P2. The image data (detection image data) of the toner pattern image P3 is stored in the ROM in advance. The CPU 301 reads out the detection image data of the toner pattern image P3 from the ROM and inputs it to the image processing unit 84. The image processing unit 84 performs screening processing using a low-line screen on the detection image data of the toner pattern image P3. As a result, the image forming apparatus 100 forms the toner pattern image P3 on the intermediate transfer belt 11. The CPU 301 detects the toner pattern image P3 by the patch sensor 50, obtains the six densities of the toner pattern image P3, and obtains the gradation characteristics from these density detection results. The CPU 301 generates a LUT such that the gradation characteristics become ideal gradation characteristics as a LUT for a low-line screen.

[0060] When the image forming apparatus 100 forms a character image, the image processing unit 84 performs screening processing using a high-line screen on the image information 88 (input image data), and converts the screened image data based on the LUT corresponding to the high-line screen. The image data output from the image processing unit 84 is converted into a control signal for laser control in the image data generation unit 302 and transferred to the laser drive units 303a to 303d. As a result, an image is formed by the image forming stations Pa, Pb, Pc, and Pd.

[0061] When the image forming apparatus 100 forms a photo image, the image processing unit 84 performs screening processing using a low-line screen on the image information 88 (input image data), and converts the screened image data based on the LUT corresponding to the low-line screen. The image data output from the image processing unit 84 is converted into a control signal for laser control in the image data generation unit 302 and transferred to the laser drive units 303a to 303d. As a result, an image is formed by the image forming stations Pa, Pb, Pc, and Pd.

[0062] Note that, for example, when the number of sheets printed after the previous toner pattern image P2 is formed reaches the first page number, the CPU 301 forms the toner pattern image P2 on the intermediate transfer belt 11. Further, for example, when the number of sheets printed after the previous toner pattern image P3 is formed reaches the second page number, the CPU 301 forms the toner pattern image P3 on the intermediate transfer belt 11. By forming the toner pattern images P2 and P3 in this way, the LUT is sequentially updated so that the gradation characteristics of the images with different attributes formed by the image forming apparatus 100 become ideal gradation characteristics.

[0063] [Patch Image Processing for Maximum Density Correction] Next, the image processing of the adjustment patch during the maximum density correction control will be described. Note that the image processing described in this embodiment is an example and is not limited thereto.

[0064] In this embodiment, the case where the halftone image processing of the screen processing method is performed on the halftone image data of the output image will be described. By the screen processing, the halftone image data is converted into pseudo halftone image data composed of binarized pixels.

[0065] As described above, the image information 88 (input image data) can be roughly classified into the following three types of image types.

[0066] (1) Character / Line Image (2) Photo Image (3) Graphics Image Here, for (1) the character / line image, the shape reproducibility of the character / line is emphasized, and for (2) the photo image and (3) the graphics image, the color reproduction / gradation reproducibility is emphasized. In this embodiment, the case where high-line screen processing, for example, 230 lines / inch screen processing is performed on the character / line image, and low-line screen processing, for example, 170 lines / inch screen processing is performed on the photo image / graphics image will be described.

[0067] Generally, in the case of an image forming apparatus capable of performing a plurality of types of screen processing, gradation correction control is performed for each screen, and the gradation property for each screen is improved by using a correction table (LUT) for the created input image. By doing so, appropriate processing can be performed on output images having different types of characteristics.

[0068] On the other hand, in maximum density correction control, correction control is not performed for each screen. The maximum density on the output is the density at an image printing rate of 100%, and in the 100% image printing rate portion, halftone processing is not performed. Therefore, due to the difference in halftone processing, the optimum maximum density value and the imaging conditions for achieving it do not change.

[0069] Here, usually, in order to determine the image forming conditions for forming an image with the maximum density, it is preferable that a detection image with an image printing rate of 100% is formed. However, for example, when a detection image is formed using black toner, as the image printing rate increases, the detection accuracy of the optical sensor decreases.

[0070] Therefore, it is conceivable to use a detection image with a slightly reduced printing rate (for example, an image printing rate of 80%). And when the printing rate of the detection image is less than 100%, a detection image reproduced using a low-line screen is formed. This is because in general printed matter, the reproducibility of photographic images is more important than that of characters and line images.

[0071] However, according to the experiments of the inventors, it was found that when a detection image reproduced using a low-line screen is detected using an optical sensor, the variation in the detection signal value of the sensor becomes large. Therefore, when the image forming conditions are controlled based on the detection result of a detection image with a low screen line number, there is a problem that the density of the output image is not stable.

[0072] In the image forming apparatus 100 of the present embodiment, for character and line images, a high-line screen with a screen ruling of 230 lines / inch is used, and for photo images and graphics images, a low-line screen with a screen ruling of 170 lines / inch is used. And in the image forming apparatus 100 of the present embodiment, the screen ruling of the detection image (referred to as a patch image) used for the maximum density correction control is 230 lines / inch. Note that the case where a low-line screen of 170 lines / inch is used will be described as a comparative example.

[0073] FIG. 11 is a graph showing the relationship between the detected density value (horizontal axis) of the correction patch for 20 gradations by the patch sensor 50 and the density value (vertical axis) when the correction patch is output on the sheet P and the density of the output image on the sheet P is actually measured by a colorimeter or the like. The detection image subjected to the high-line screen process of 230 lines / inch, which is a feature of the present embodiment, is detected by the patch sensor 50, and the detected patch image signal value is converted into a density value using the patch detection luminance density conversion table, and the converted value is set on the horizontal axis as the detected density value. Also, the same patch image is transferred onto the sheet P, and the actual density measurement value of the output image on the sheet P output after passing through the fixing process is set on the vertical axis.

[0074] Also, FIG. 12 is a graph showing the relationship between the patch sensor detected density value (horizontal axis) and the patch actual density value (vertical axis) for 20 gradations when a low-line screen of 170 lines / inch is used as a comparative example. Similar to FIG. 11, the horizontal axis of the graph in FIG. 12 is the density value detected by the patch sensor 50 for the patch image formed on the intermediate transfer member, and the vertical axis is the density measurement value of the patch image formed on the sheet P.

[0075] In FIGS. 11 and 12, the relationship when a patch image is formed at Vc = 99V is illustrated by Δ marks as a case where the development contrast is low, and the relationship when a patch image is formed at Vc = 399V is illustrated by □ marks as a case where the development contrast is high. Also, the relationship when a patch image is created at Vc = 141V is illustrated by ○ marks as a case where the development contrast is between them.

[0076] In the maximum density correction control, for an image with an image printing rate of about 80%, the density value D that reaches the target density is approximately D = 1.25. Figures 11 and 12 show how much the measured density value changes depending on the image formation conditions when D = 1.25 is detected by the patch sensor 50.

[0077] Comparing Figure 11 and Figure 12, it can be seen that for the correction patch subjected to high-line screen processing as in this embodiment, the amount of change in the measured density value due to the difference in the image formation conditions indicated by the development contrast is smaller. In the case of a low-line screen, when the detection value by the patch sensor 50 is 1.25, the measured density changes to about D = 1.20 to 1.70 depending on the difference in the image formation conditions. On the other hand, in the case of a high-line screen, when the detection value is 1.25, it can be seen that the variation is suppressed to about D = 1.20 to 1.50. When the difference between the sensor-detected density value and the measured density value becomes large due to the image formation conditions, the control accuracy in density control is greatly reduced, which causes the density, color tone, and gradation to become unstable.

[0078] The cause of such density variation is due to the difference in the latent image state. When creating a patch image using an image with an image printing rate of 100%, there is no difference in the latent image state due to the difference in the halftone processing. However, when the image printing rate becomes 80%, a difference occurs in the latent image state when the halftone processing is different.

[0079] The latent image state in which the density variation is likely to be large is the case where the toner is likely to be laminated in the vertical direction (thickness direction). In a normal reflection type patch sensor, the toner amount is detected as a density value based on the ratio of the toner on the intermediate transfer belt covering the belt base portion. If the amount of toner increases so that the toner is laminated in the vertical direction without the covering ratio of the base changing, the detected density value cannot follow the change in the toner amount, resulting in a large error. In other words, it can be said that the detected density by the patch sensor 50 has a large variation due to the difference in the image formation conditions with respect to the change in density due to the change in the toner amount in the thickness direction.

[0080] Here, FIGS. 14 and 15 are enlarged views of the central portion of the pseudo halftone image data obtained by performing screen processing on a digital image in which pixels with a density of 80% of the maximum density are uniformly distributed so that the image printing rate is a value less than 100%, for example, 80%. The density of 80% of the maximum density is an example, and it is a density lower than the maximum density, and any density may be used as long as the detection accuracy for the black toner image is maintained. FIG. 14 is an example of a digital image subjected to screen processing of 230 lines / inch, which is a feature of the present embodiment. As a comparative example, FIG. 15 is a digital image subjected to screen processing of 170 lines / inch for the same image. Here, for the digital images of FIGS. 14 and 15, a simulation of the latent image state was performed and the potential distribution was calculated under the image forming conditions of charging bias voltage Vd_B = -700V, developing bias voltage Vdc_B = -550V, and exposure amount LPW_B = 0.22 μJ / cm2. FIG. 17 shows the simulation result of the potential distribution on the surface of the image carrier when a simulation of forming a latent image is performed for the digital image of FIG. 14, and FIG. 18 shows the simulation result of the potential distribution on the surface of the image carrier when a simulation of forming a latent image is performed for the digital image of FIG. 15.

[0081] Furthermore, the variation (standard deviation) of the potential distribution in FIGS. 17 and 18 was 46.8V in the case of the high line number screen processing of FIG. 17 and 58.7V in the case of the low line number screen processing of FIG. 18. A large standard deviation means that the potential fluctuation in the latent image state becomes large, and the toner is likely to be laminated in the vertical direction. Therefore, the variation in the detected density by the patch sensor 50 becomes large.

[0082] As described above, in an image forming apparatus that performs a plurality of types of pseudo halftone processes on an input image to form an output, as a correction patch image in the maximum density correction control, a patch image subjected to a higher line number process is used among the screen processes. By doing so, the accuracy of the sensor detected density can be improved, and it becomes possible to perform high-precision maximum density correction control.

[0083] [Modification Example] In the embodiment, the same toner pattern image, i.e., the patch image, is used for all color components. However, since the detection accuracy of an image with an image printing rate of 100% decreases for the Bk (black) image, only the toner pattern image of the black component may be printed at a printing rate of 80%. In this case, the toner pattern images of the other color components may be printed at a printing rate of 100%. Therefore, in this case, for colors other than black, the screen ruling may be either high or low. The same applies to Embodiment 2.

[0084] In this embodiment, as shown in FIG. 6, a toner pattern image P1 in which patches with different image forming conditions are continuously arranged is formed on the intermediate transfer body. However, since it is only necessary to be able to detect the density of patches with different image forming conditions by the patch sensor 50, the patches with different image forming conditions do not have to be continuously arranged. For example, patches of each color component are formed under image forming condition 1, and the density of each patch is detected by the patch sensor 50, and each density value is stored. Then, the image forming condition is changed to image forming condition 2, patches of each color component are formed, and the density of each patch is detected by the patch sensor 50, and each density value is stored. Similarly, if the density of the patches formed by repeating the same process for image forming conditions 3, 4, and 5 is stored, data showing the correlation between the development contrast and the density value of the toner pattern image as shown in FIG. 10 can be obtained. Based on this, the image forming condition that results in the target maximum density may be determined. By doing so, since the region where the image forming condition changes at the boundary between patches is not included, the detection accuracy of the density can be further improved. The same applies to Embodiment 2.

[0085] [Embodiment 2] In Embodiment 1, in the pseudo half-tone processing of the correction patch image in the maximum density correction control, it was explained that a correction patch processed with a higher screen ruling is used rather than a correction patch processed with a lower screen ruling. By doing so, the relationship between the detected density value and the actually measured density value of the correction patch is stabilized, so that the accuracy of the detected density is improved and high-precision density correction control can be implemented. In this embodiment, a method for implementing even higher-precision density correction control will be described. Note that since the image forming apparatus, the image control unit, and the control flow of the maximum density correction in this embodiment are the same as those in Embodiment 1 (FIG. 8 and the like), the description thereof will be omitted.

[0086] In this embodiment, pseudo half-tone processing is performed on character images by high-screen ruling processing at 230 lines / inch, on line images used in CAD, maps, etc. by error diffusion processing, and on photographic images and graphic images by low-screen ruling processing at 170 lines / inch. Further, in this embodiment, pseudo half-tone processing of error diffusion is performed on the correction patch image during maximum density correction control. This will be described below.

[0087] FIG. 13 is a graph showing the relationship between the detected density value (horizontal axis) of a 20-tone correction patch subjected to error diffusion processing by the patch sensor 50 and the density value (vertical axis) when the correction patch is output on the sheet P and the density on the sheet P is actually measured.

[0088] In the maximum density correction control, the density value that becomes the target density is approximately D = 1.25 in the case of an image with an image printing rate of about 80%. As described in Embodiment 1, when low-screen ruling processing is performed, depending on the difference in image formation conditions, the actually measured density changes from 1.20 to 1.70 with respect to the patch sensor detection value of 1.25. When high-screen ruling processing is performed, the variation is suppressed to about D = 1.20 to 1.50. On the other hand, when error diffusion processing, which is a feature of this embodiment, is performed, it can be seen that the variation is suppressed to about D = 1.15 to 1.35 as shown in FIG. 13.

[0089] Similar to Embodiment 1, a simulation of the latent image state was performed on a digital image subjected to error diffusion processing as shown in FIG. 16, and the result of calculating the potential distribution on the image carrier is shown in FIG. 19. The original digital image may be, for example, an image in which pixels having a low density with respect to the maximum density, for example, a density of 80%, are uniformly distributed. When the variation (standard deviation) of the potential in the potential distribution simulation result as shown in FIG. 19 is calculated, it is 39.9 V. From this value, it can be seen that the case where error diffusion processing is performed has the most suppressed potential variation compared to the cases where low-line screen processing and high-line screen processing are respectively performed as pseudo halftone processing (FIG. 20). This means that when error diffusion processing is performed as pseudo halftone processing, the undulations of the latent image state, which are the cause of density variation, can be suppressed to a low level.

[0090] As described above, in the image forming apparatus that performs a plurality of types of pseudo halftone processing on the input image and forms an output, in this embodiment, error diffusion processing is performed as pseudo halftone processing as the correction patch image in the maximum density correction control. By doing so, the accuracy of the sensor detected density can be improved, and high-precision maximum density correction control can be implemented.

[0091] Also, the screen line number of the detection image used for the maximum density correction control in the above embodiment may be more than 190 lines / inch. For example, the screen line number of the detection image used for the maximum density correction control may be 230 lines / inch. In this case, the screen line number of the detection image used for the maximum density correction control is larger than the screen line number used for character images and line images.

[0092] [Other Embodiments] The laser scanner 31 described in the first embodiment and the second embodiment has a configuration in which the photosensitive drum 1a is scanned by a polygon mirror 33 that rotates the laser light from the light source 32. However, the image forming apparatus 100 is not limited to this configuration. The present invention may be used in the image forming apparatus 300 shown in FIG. 22 instead of the image forming apparatus 100.

[0093] FIG. 22 is a schematic cross-sectional view of the image forming apparatus 300. In the following description, the same units as those of the image forming apparatus 100 shown in FIG. 1 are given the same reference numerals, and the description thereof is omitted. In the image forming apparatus 300, four image forming stations Pe, Pf, Pg, and Ph are provided side by side along the sheet conveyance direction as an image forming unit. Each of the image forming stations Pe, Pf, Pg, and Ph forms toner images of a plurality of different colors. For example, the image forming station Pe forms a yellow toner image, the image forming station Pf forms a magenta toner image, the image forming station Pg forms a cyan toner image, and the image forming station Ph forms a black toner image.

[0094] Sheets are supported by the cassettes 109a, 109b, and the manual feed tray 109c. Among the cassettes 109a, 109b, and the manual feed tray 109c, a sheet is fed from a pre-designated supply source and conveyed toward the registration roller 110. The registration roller 110 conveys the sheet to the transfer belt 111 so that the toner images formed at each of the image forming stations Pe, Pf, Pg, and Ph are transferred onto the sheet in an overlapping manner.

[0095] The sheet conveyed to the transfer belt 111 is carried on the transfer belt 111, and the toner images formed at each of the image forming stations Pe, Pf, Pg, and Ph are transferred so as to overlap. As a result, a full-color toner image is formed on the sheet. The sheet onto which the toner image has been transferred is melted and fixed by heat and pressure in the fixing device 9, and is discharged from the image forming apparatus 300 by the discharge roller 112.

[0096] Next, the exposure heads 36 included in each of the image forming units Pe, Pf, Pg, and Ph will be described with reference to FIGS. 23(a) and 23(b). The exposure heads 36 included in each of the image forming units Pe, Pf, Pg, and Ph have the same configuration. Hereinafter, the exposure head 36 included in the image forming unit Pe (hereinafter referred to as the exposure head 36a) will be described.

[0097] The exposure head 36a includes a light-emitting element group 201, a printed circuit board 202 on which the light-emitting element group 201 is mounted, a rod lens array 203, and a housing 204 for attaching the rod lens array 203 and the printed circuit board 202. Note that in the factory, focusing adjustment for adjusting the spot diameter at the light-collecting position of the light-emitting element group 201 to a predetermined size and light quantity adjustment of the light-emitting element group 201 are performed on the exposure head 36a. Also, the light-emitting element group 201 is arranged along a second direction intersecting the rotating direction (first direction) of the photoreceptor 1a. Alternatively, the light-emitting element group 201 is arranged along the surface of the photoreceptor 1a in the direction of the rotation axis of the photoreceptor 1a.

[0098] The exposure head 36a causes the light-emitting element group 201 to emit light based on a signal sent from the image data generation unit 302. The light emitted from the light-emitting element group 201 is condensed onto the photosensitive drum 1a by the rod lens array 203, and the photosensitive drum 1a is exposed. Thereby, an electrostatic latent image corresponding to the image data is formed on the photosensitive drum 1a.

[0099] FIG. 24(a) is a schematic diagram showing the surface opposite to the surface on which the light-emitting element group 201 is mounted (hereinafter referred to as the non-light-emitting element mounting surface). FIG. 24(b) is a schematic diagram showing the surface on which the light-emitting element group 201 is mounted (hereinafter referred to as the light-emitting element mounting surface). The light-emitting element group 201 is composed of a configuration in which 20 light-emitting element array chips 400-1 to 400-20 are arranged in a staggered pattern. In each of the light-emitting element array chips 400-1 to 400-20, the light-emitting elements 602 are arranged at a predetermined pitch in the longitudinal direction and the short-side direction of the chip.

[0100] In one chip, 748 light-emitting elements 602 are arranged in the longitudinal direction of the chip at a pitch of 1200 dpi (approximately 21.16 μm), and multiple rows of these light-emitting elements 602 are arranged in the short transverse direction of the chip. That is, the distance from end to end of the 748 light-emitting points in the longitudinal direction within one chip is approximately 15.8 mm. The light-emitting element group 201 is configured such that 20 of these chips are arranged in the longitudinal direction, enabling 14,960 light-emitting elements to be exposed and allowing image formation corresponding to an image width of approximately 316 mm. The light-emitting element array chips 400-1 to 400-20 are arranged in a staggered pattern as shown in Fig. 24(b).

[0101] Fig. 24(c) shows the state of the boundary between the light-emitting element array chip 400-n and the light-emitting element array chip 400-n+1. Here, n is a natural number from 1 to 19. Even at the boundary between chips, the longitudinal pitch of the light-emitting elements is a pitch of 1200 dpi (approximately 21.16 μm). The interval (S in the figure) between the light-emitting points of adjacent chips is arranged to be approximately 127 μm (6 pixels at 1200 dpi, 4 pixels at 800 dpi). Also, the interval (L in the figure) between the light-emitting points in the longitudinal direction of the exposure head 36a is approximately 21.16 μm (1 pixel at 1200 dpi). In the present invention, it should be noted that the intervals S and L between the light-emitting element array chips do not need to be limited to the above-mentioned values.

[0102] On the non-mounted surface of the light-emitting element, a connector 205 for connecting the control signal lines for the control signals for controlling the light-emitting element array chips 400-1 to 400-20 from the image data generation unit 302 and the power supply lines for power supply is arranged, and each light-emitting element array chip 400-1 to 400-20 is driven via the connector 205.

[0103] When the present invention is applied to the image forming apparatus 300 of this embodiment, image forming conditions for forming an image with the target maximum density can also be generated with high precision.

[0104] [Other Examples] The present invention can also be realized by supplying a program that realizes one or more functions of the above-described embodiments to a system or apparatus via a network or a storage medium, and causing one or more processors in a computer of the system or apparatus to read and execute the program. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

[0105] ● Summary of Embodiment Summarizing the above embodiments, the following inventions are included. (Item 1) An image forming apparatus that forms an image on a sheet, image processing means for performing pseudo halftone processing corresponding to the attributes of the image to be formed on the image data, image forming means controlled based on image forming conditions and forming an image based on the image data subjected to the pseudo halftone processing, an image carrier on which a detection image formed by the image forming means is carried, detection means for irradiating light on the detection image on the image carrier and receiving reflected light from the detection image, causing the image forming means to form a first detection image subjected to pseudo halftone processing corresponding to a first attribute, and controlling the gradation characteristics of the image of the first attribute to be formed by the image forming means based on the light reception result of receiving the reflected light from the first detection image by the detection means, causing the image forming means to form a second detection image subjected to pseudo halftone processing corresponding to a second attribute different from the first attribute, and controlling the gradation characteristics of the image of the second attribute to be formed by the image forming means based on the light reception result of receiving the reflected light from the second detection image by the detection means, control means for causing the image forming means to form a detection image for target maximum density correction, and generating the image forming conditions for correcting the target maximum density of the image to be formed by the image forming means based on the light reception result of receiving the reflected light from the detection image for target maximum density correction by the detection means. The detection image for the target maximum density correction is an image obtained by performing error diffusion processing as the pseudo halftone processing regardless of the attributes of the image to be formed. An image forming apparatus characterized by the above. (Item 2) The first attribute is a photographic image, The second attribute is a character image The image forming apparatus according to item 1, characterized by the above. (Item 3) The screen ruling of the screen used for the pseudo halftone processing corresponding to the first attribute is 190 lines / inch or less, An image forming apparatus characterized in that the screen ruling of the screen used for the pseudo halftone processing corresponding to the second attribute is more than 190 lines / inch. The image forming apparatus according to item 1 or 2, characterized by the above. (Item 4) The detection image for the target maximum density correction is an image formed to have a density lower than the target maximum density. The image forming apparatus according to any one of items 1 to 3, characterized by the above. (Item 5) The image forming means includes a rotating photoreceptor, a charging member for charging the photoreceptor, a light source for exposing the photoreceptor charged by the charging member to form an electrostatic latent image on the photoreceptor, and a developing roller for developing the electrostatic latent image on the photoreceptor using toner. The light source has a plurality of light emitting elements arranged along a second direction intersecting the first direction in which the photoreceptor rotates. The image forming apparatus according to any one of items 1 to 4, characterized by the above. (Item 6) The image forming means includes a photoreceptor, a charging member for charging the photoreceptor based on a charging bias voltage, a light source for exposing the photoreceptor charged by the charging member to form an electrostatic latent image on the photoreceptor, and a developing roller for developing the electrostatic latent image on the photoreceptor using toner. The image forming condition for correcting the target maximum density is the charging bias voltage. The image forming apparatus according to any one of Items 1 to 5, characterized in that... (Item 7) The image forming means includes a photoreceptor, a charging member that charges the photoreceptor, a light source that exposes the photoreceptor charged by the charging member to form an electrostatic latent image on the photoreceptor, and a developing roller that develops the electrostatic latent image on the photoreceptor using toner. The image forming condition for correcting the target maximum density is the developing bias voltage. The image forming apparatus according to any one of Items 1 to 6, characterized in that... (Item 8) The image forming means includes a photoreceptor, a charging member that charges the photoreceptor, a light source that exposes the photoreceptor charged by the charging member to form an electrostatic latent image on the photoreceptor, and a developing roller that develops the electrostatic latent image on the photoreceptor using toner. The image forming condition for correcting the target maximum density is the exposure amount of the light source. The image forming apparatus according to any one of Items 1 to 7, characterized in that... (Item 9) The image processing means converts image data based on gradation correction conditions corresponding to the attribute. The image forming condition for correcting the gradation characteristics is the gradation correction condition. The image forming apparatus according to any one of Items 1 to 8, characterized in that... (Item 10) An image forming apparatus that forms an image on a sheet, comprising: image processing means that performs pseudo halftone processing on image data corresponding to the attribute of the image to be formed; image forming means that is controlled based on image forming conditions and forms an image based on the image data subjected to the pseudo halftone processing; an image carrier on which a detection image formed by the image forming means is carried; detection means that irradiates light on the detection image on the image carrier and receives reflected light from the detection image; Causing the image forming means to form a first detection image subjected to pseudo halftone processing corresponding to the first attribute, and controlling the gradation characteristics of the image of the first attribute to be formed by the image forming means based on the light reception result obtained by the light detection means receiving the reflected light from the first detection image; Causing the image forming means to form a second detection image subjected to pseudo halftone processing corresponding to a second attribute different from the first attribute, and controlling the gradation characteristics of the image of the second attribute to be formed by the image forming means based on the light reception result obtained by the light detection means receiving the reflected light from the second detection image; A control means for causing the image forming means to form a detection image for target maximum density correction, and generating image forming conditions for correcting the target maximum density of the image to be formed by the image forming means based on the light reception result obtained by the light detection means receiving the reflected light from the detection image for target maximum density correction; The detection image for target maximum density correction is an image subjected to pseudo halftone processing using a second screen having a higher screen ruling than a first screen used for pseudo halftone processing corresponding to the first attribute. An image forming apparatus characterized by the above. (Item 11) The pseudo halftone processing using the second screen is the pseudo halftone processing corresponding to the second attribute. The image forming apparatus according to item 10, characterized by the above. (Item 12) The first attribute is a photographic image, The second attribute is a character image The image forming apparatus according to item 10 or 11, characterized by the above. (Item 13) The screen ruling of the first screen is 190 lines / inch or less, The screen ruling of the second screen is higher than 190 lines / inch The image forming apparatus according to any one of items 10 to 12, characterized by the above. (Item 14) The detection image for correcting the target maximum density is an image formed to have a density lower than the target maximum density. The image forming apparatus according to any one of items 10 to 13, characterized in that. (Item 15) The image forming means includes a rotating photoreceptor, a charging member for charging the photoreceptor, a light source for exposing the photoreceptor charged by the charging member to form an electrostatic latent image on the photoreceptor, and a developing roller for developing the electrostatic latent image on the photoreceptor using toner. The light source has a plurality of light emitting elements arranged along a second direction intersecting a first direction in which the photoreceptor rotates. The image forming apparatus according to any one of items 10 to 14, characterized in that. (Item 16) The image forming means includes a photoreceptor, a charging member for charging the photoreceptor based on a charging bias voltage, a light source for exposing the photoreceptor charged by the charging member to form an electrostatic latent image on the photoreceptor, and a developing roller for developing the electrostatic latent image on the photoreceptor using toner. The image forming condition for correcting the target maximum density is the charging bias voltage. The image forming apparatus according to any one of items 10 to 15, characterized in that. (Item 17) The image forming means includes a photoreceptor, a charging member for charging the photoreceptor, a light source for exposing the photoreceptor charged by the charging member to form an electrostatic latent image on the photoreceptor, and a developing roller for developing the electrostatic latent image on the photoreceptor using toner based on a developing bias voltage. The image forming condition for correcting the target maximum density is the developing bias voltage. The image forming apparatus according to any one of items 10 to 16, characterized in that. (Item 18) The image forming means includes a photoreceptor, a charging member for charging the photoreceptor, a light source for exposing the photoreceptor charged by the charging member to form an electrostatic latent image on the photoreceptor, and a developing roller for developing the electrostatic latent image on the photoreceptor using toner. The image forming condition for correcting the target maximum density is the exposure amount of the light source. The image forming apparatus according to any one of items 10 to 17, characterized in that. (Item 19) The image processing means converts image data based on gradation correction conditions corresponding to the attribute. The image forming condition for correcting the gradation characteristics is the gradation correction condition. The image forming apparatus according to any one of items 10 to 18, characterized in that.

[0106] The present invention is not limited to the above embodiments, and various changes and modifications can be made without departing from the spirit and scope of the invention. Therefore, claims are attached to disclose the scope of the invention.

Explanation of Signs

[0107] 1a: Photoreceptor drum, 2a: Charging roller, 31: Laser scanner, 4a: Developing device, 50: Patch sensor, 200: Reader, 301: CPU

Claims

1. An image forming apparatus for forming an image on a sheet, image processing means for performing a pseudo-halftoning process on image data corresponding to the attributes of an image to be formed; an image forming means for forming an image based on the image data that has been subjected to the pseudo halftone processing, the image forming means being controlled based on image forming conditions; an image carrier on which an image for detection formed by the image forming unit is carried; a detection unit that irradiates the image for detection on the image carrier with light and receives reflected light from the image for detection; causing the image forming means to form a first detection image having been subjected to a pseudo-halftone process corresponding to a first attribute, and controlling the gradation characteristics of the image of the first attribute to be formed by the image forming means based on a light reception result of the light reflected from the first detection image being received by the detection means; causing the image forming means to form a second detection image having been subjected to a pseudo-halftone process corresponding to a second attribute different from the first attribute, and controlling the gradation characteristics of the image of the second attribute to be formed by the image forming means based on a light reception result of the light reflected from the second detection image being received by the detection means; a control means for causing the image forming means to form a detection image for correcting the target maximum density, and generating the image forming conditions for correcting the target maximum density of the image to be formed by the image forming means based on a light reception result of the detection means receiving reflected light from the detection image for correcting the target maximum density, The detection image for the target maximum density correction is an image that has been subjected to error diffusion processing as the pseudo halftone processing, regardless of the attributes of the image to be formed.

1. An image forming apparatus comprising:

2. the first attribute is a photographic image; The second attribute is a character image.

2. The image forming apparatus according to claim 1,

3. the screen ruling of the screen used in the pseudo-halftoning process corresponding to the first attribute is 190 lines / inch or less; an image forming apparatus, characterized in that the screen ruling of the screen used in the pseudo-halftone process corresponding to the second attribute is greater than 190 lines / inch; 2. The image forming apparatus according to claim 1,

4. The detection image for the target maximum density correction is an image formed to have a density lighter than the target maximum density.

2. The image forming apparatus according to claim 1,

5. the image forming means includes a rotating photoconductor, a charging member for charging the photoconductor, a light source for exposing the photoconductor charged by the charging member to light in order to form an electrostatic latent image on the photoconductor, and a developing roller for developing the electrostatic latent image on the photoconductor with toner; The light source has a plurality of light emitting elements arranged along a second direction intersecting with a first direction in which the photoconductor rotates.

2. The image forming apparatus according to claim 1,

6. the image forming means includes a photoconductor, a charging member that charges the photoconductor based on a charging bias voltage, a light source that exposes the photoconductor charged by the charging member to light in order to form an electrostatic latent image on the photoconductor, and a developing roller that develops the electrostatic latent image on the photoconductor with toner; The image forming condition for correcting the target maximum density is the charging bias voltage.

2. The image forming apparatus according to claim 1,

7. the image forming means includes a photoconductor, a charging member for charging the photoconductor, a light source for exposing the photoconductor charged by the charging member to light in order to form an electrostatic latent image on the photoconductor, and a developing roller for developing the electrostatic latent image on the photoconductor with toner based on a developing bias voltage; The image forming condition for correcting the target maximum density is the developing bias voltage.

2. The image forming apparatus according to claim 1,

8. the image forming means includes a photoconductor, a charging member for charging the photoconductor, a light source for exposing the photoconductor charged by the charging member to light in order to form an electrostatic latent image on the photoconductor, and a developing roller for developing the electrostatic latent image on the photoconductor with toner; The image forming condition for correcting the target maximum density is the exposure amount of the light source.

2. The image forming apparatus according to claim 1,

9. The image processing means converts the image data based on a gradation correction condition corresponding to the attribute, The image forming conditions for correcting the gradation characteristics are the gradation correction conditions.

2. The image forming apparatus according to claim 1,

10. An image forming apparatus for forming an image on a sheet, image processing means for performing a pseudo-halftoning process on image data corresponding to the attributes of an image to be formed; an image forming means for forming an image based on the image data that has been subjected to the pseudo halftone processing, the image forming means being controlled based on image forming conditions; an image carrier on which an image for detection formed by the image forming unit is carried; a detection unit that irradiates the image for detection on the image carrier with light and receives reflected light from the image for detection; causing the image forming means to form a first detection image having been subjected to a pseudo-halftone process corresponding to a first attribute, and controlling the gradation characteristics of the image of the first attribute to be formed by the image forming means based on a light reception result of the light reflected from the first detection image being received by the detection means; causing the image forming means to form a second detection image having been subjected to a pseudo-halftone process corresponding to a second attribute different from the first attribute, and controlling the gradation characteristics of the image of the second attribute to be formed by the image forming means based on a light reception result of the light reflected from the second detection image being received by the detection means; a control means for causing the image forming means to form a detection image for correcting the target maximum density, and generating image forming conditions for correcting the target maximum density of an image to be formed by the image forming means based on a light reception result of the detection means receiving reflected light from the detection image for correcting the target maximum density, The detection image for the target maximum density correction is an image that has been subjected to pseudo-halftoning using a second screen having a screen ruling greater than that of a first screen used in the pseudo-halftoning process corresponding to the first attribute.

1. An image forming apparatus comprising:

11. The halftone process using the second screen is a halftone process corresponding to the second attribute.

11. The image forming apparatus according to claim 10.

12. the first attribute is a photographic image; The second attribute is a character image.

11. The image forming apparatus according to claim 10.

13. The screen ruling of the first screen is 190 lines / inch or less, The screen ruling of the second screen is greater than 190 lines / inch.

11. The image forming apparatus according to claim 10.

14. The detection image for the target maximum density correction is an image formed to have a density lighter than the target maximum density.

11. The image forming apparatus according to claim 10.

15. the image forming means includes a rotating photoconductor, a charging member for charging the photoconductor, a light source for exposing the photoconductor charged by the charging member to light in order to form an electrostatic latent image on the photoconductor, and a developing roller for developing the electrostatic latent image on the photoconductor with toner; The light source has a plurality of light emitting elements arranged along a second direction intersecting with a first direction in which the photoconductor rotates.

11. The image forming apparatus according to claim 10.

16. the image forming means includes a photoconductor, a charging member that charges the photoconductor based on a charging bias voltage, a light source that exposes the photoconductor charged by the charging member to light in order to form an electrostatic latent image on the photoconductor, and a developing roller that develops the electrostatic latent image on the photoconductor with toner; The image forming condition for correcting the target maximum density is the charging bias voltage.

11. The image forming apparatus according to claim 10.

17. the image forming means includes a photoconductor, a charging member for charging the photoconductor, a light source for exposing the photoconductor charged by the charging member to light in order to form an electrostatic latent image on the photoconductor, and a developing roller for developing the electrostatic latent image on the photoconductor with toner based on a developing bias voltage; The image forming condition for correcting the target maximum density is the developing bias voltage.

11. The image forming apparatus according to claim 10.

18. the image forming means includes a photoconductor, a charging member for charging the photoconductor, a light source for exposing the photoconductor charged by the charging member to light in order to form an electrostatic latent image on the photoconductor, and a developing roller for developing the electrostatic latent image on the photoconductor with toner; The image forming condition for correcting the target maximum density is the exposure amount of the light source.

11. The image forming apparatus according to claim 10.

19. The image processing means converts the image data based on a gradation correction condition corresponding to the attribute, The image forming conditions for correcting the gradation characteristics are the gradation correction conditions.

11. The image forming apparatus according to claim 10.

Images