Image forming apparatus, control method thereof, and program

Abstract

To suppress occurrence of density unevenness due to variations in light volume on a photoreceptor.SOLUTION: Image data, in which image data of a printing object has been halftone-processed, is acquired, and in order to correct variations in light volume on a photoreceptor due to light radiated from a plurality of light emitters, and a plurality of pieces of correction information is generated corresponding to types of factors contributing to variations in light volume. Although the halftone-processed image data is corrected by use of the correction information, the image data of the printing object is not corrected.SELECTED DRAWING: Figure 11

Description

Technical Field

[0001] The present invention relates to a technique for suppressing the occurrence of density unevenness in an electrophotographic image forming apparatus that uses a line head as a light source.

Background Art

[0002] In an electrophotographic image forming apparatus that uses a line head in which a plurality of light emitting elements are arranged as an exposure means, the amount of light may not be uniform for each main scanning position on the photoreceptor. If the amount of light is not uniform in this way, density unevenness in the main scanning direction may occur in the image formed on the recording medium. To suppress the occurrence of this density unevenness, for example, a circuit for correcting the amount of light for each main scanning position on the photoreceptor is provided, and the image data used by the exposure means is corrected by this correction circuit. Specifically, when forming an image on a recording medium, based on a correction profile held in advance in a memory and indicating the difference in the amount of light corresponding to each light emitting element, the image data is corrected so as to turn off the light emitting elements corresponding to the regions where the reproduction density becomes high.

[0003] In this regard, Patent Document 1 discloses a technique for performing a correction process for correcting variations in the amount of light caused by factors other than the light emitting elements on the image data before halftone processing.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the above-mentioned Patent Document 1, since the size of the corrected portion is affected by the halftone processing, in the image data after the halftone processing, the above-mentioned correction was not appropriately reflected.

Means for Solving the Problems

[0006] An image forming apparatus according to one aspect of the present disclosure is an electrophotographic image forming apparatus that uses a line head on which a plurality of light-emitting elements are arranged as a light source, comprising: an acquisition means for acquiring print data of a target to be printed by the electrophotographic image forming apparatus; an execution means for performing halftone processing on the print data acquired by the acquisition means; and a plurality of correction information for correcting variations in light intensity on a photoreceptor due to light irradiated from the plurality of light-emitting elements. Based on a pre-held matrix and a pre-held profile The system comprises a generation means for generating data and a correction means for correcting the halftone-processed print data using the correction information, wherein the plurality of correction information includes a first profile for correcting variations in light intensity caused by the light-emitting element, a second profile for correcting variations in light intensity caused by individual differences in the current source of the light-emitting element, and a third profile for correcting variations in light intensity caused by the positional relationship between the line head and the photoreceptor. Furthermore, the aforementioned matrix holds multiple matrices with different phases, and the referenced matrix changes when either the page or the job changes. It is characterized by the following: [Effects of the Invention]

[0007] According to this disclosure, A plurality of pre-held matrices with different phases, wherein the matrix referenced changes when either a page or a job changes, and a plurality of correction pieces generated based on a pre-held profile, A first profile for correcting variations in light intensity caused by light-emitting elements, a second profile for correcting variations in light intensity caused by individual differences in the current source of the light-emitting elements, and a third profile for correcting variations in light intensity caused by the positional relationship between the line head and the photoreceptor. and including The aforementioned Using multiple correction information , halftone processed print data By correcting , the occurrence of density unevenness due to variations in light intensity on the photoreceptor It can be suppressed. [Brief explanation of the drawing]

[0008] [Figure 1] This is a diagram illustrating the system configuration. [Figure 2] This is a block diagram illustrating the configuration of an image forming apparatus. [Figure 3] This is a cross-sectional view of the printing section of an image forming apparatus. [Figure 4] This is a diagram showing a configuration example of an LED line head. [Figure 5] This is a diagram showing an arrangement example of LED chips. [Figure 6] This is a diagram showing the correspondence relationship between an LED light-emitting element, a rod lens, and a current source. [Figure 7] This is a functional block diagram explaining the configuration of an image processing unit. [Figure 8] This is a diagram showing an example of an HT image and an example of its printed matter without performing light amount correction. [Figure 9] This is a diagram showing an example of an HT image and an example of its printed matter after performing light amount correction. [Figure 10] This is a flowchart showing the flow of processing by an image processing unit. [Figure 11] This is a functional block diagram explaining the configuration of an HT density correction processing unit. [Figure 12] This is a diagram showing an example of a correction profile. [Figure 13] This is a diagram for explaining an example of a matrix and an example of the use of a matrix. [Figure 14] This is a diagram for explaining the generation of a correction signal using correction data and a matrix. [Figure 15] This is a flowchart showing the flow of processing by an HT density correction processing unit. [Figure 16] This is a flowchart showing the detailed flow of processing by a composite profile acquisition unit. [Figure 17] This is a flowchart showing the detailed flow of processing for updating a third correction profile. [Figure 18] This is a schematic diagram of a calibration chart. [Figure 19] This is a functional block diagram explaining the configuration of an HT density correction processing unit. [Figure 20] This is a flowchart showing the flow of processing by an HT density correction processing unit. [Figure 21] This is a flowchart showing the detailed flow of processing by a composite profile acquisition unit. [Figure 22]It is a diagram for explaining the correspondence relationship between matrix data, HT image data, and a threshold value.

Embodiments for Implementing the Invention

[0009] Hereinafter, embodiments for implementing the present disclosure will be described with reference to the drawings. Note that the following embodiments do not limit the present disclosure according to the claims, and not all combinations of the features described in this embodiment are essential for the solution means of the present disclosure. For the same configuration, the same reference numerals will be used for description.

[0010] [Embodiment 1] FIG. 1 is a diagram for explaining the configuration of a system including an image forming apparatus 101 according to this embodiment. This system includes an image forming apparatus 101, a host computer 102, a mobile terminal 103, and a server 104. These apparatuses are connected to a network 105 so as to be able to transmit and receive data to and from each other. The image forming apparatus 101 forms (prints) an image by an electrophotographic method. The image forming apparatus 101 receives image data from a host computer 102, a mobile terminal 103, a server 104, or another image processing apparatus (not shown) via the network 105 and executes printing (image formation). Further, the image forming apparatus 101 can transmit the image data obtained by reading a document by an image reading unit 202 attached thereto to a host computer 102, a mobile terminal 103, or a server 104 via the network 105. Furthermore, the image forming apparatus 101 can realize a copying operation by printing the image data obtained by its reading using a printing unit 206 attached thereto.

[0011] In this embodiment, an example is described in which the image forming apparatus 101 is an image processing device that performs halftone processing on image data to be printed, but it is not limited to this. Image processing such as halftone processing may be performed on a host computer 102 or the like, which is the source of the image data to be printed. In addition, the image forming apparatus 101 and the host computer 102, mobile terminal 103, or server 104, etc., which are the sources of the image data, may cooperate to distribute the processing described below.

[0012] <Configuration of an image forming apparatus> Figure 2 is a functional block diagram illustrating the configuration of the image forming apparatus 101. The image forming apparatus 101 includes a data input unit (receiving unit) 201, an image reading unit 202, a control unit 203, a storage unit 204, a UI (user interface) unit 205, a printing unit 206, and an image processing unit 207.

[0013] The data input unit 201 receives and inputs print data, for example, sent from the server 104, via the network 105. The image reading unit 202 has a scanner and reads the image of the original document to acquire image data of the original document. The control unit 203 is configured to control the operation of the image forming apparatus 101 and has a CPU 208, ROM 209, and RAM 210. The CPU 208 executes a program stored in the ROM 209 to perform the processing described later. The storage unit 204 is a storage device that stores large amounts of data, such as a hard disk drive (HDD). The CPU 208 may be configured to load the program stored in the storage unit 204 into the RAM 210 and execute the processing described later. The UI unit 205 includes an operation panel and a display unit and displays messages to the user and accepts operation instructions from the user. The UI unit 205 may also have a touch panel function.

[0014] The printing unit 206 is a printer engine, and in this embodiment, it forms an image on a recording medium by superimposing toner images of multiple colors (e.g., Cyan / Magenta / Yellow / Black; hereinafter referred to as C / M / Y / K) using an electrophotographic and tandem method. The composition of the colorants is not limited to this. It may be a black-only image forming apparatus, or it may have colors other than the four colors mentioned above. In this embodiment, the printing unit 206 will be described as having a printing resolution of 2400 dpi in the main scanning direction and sub-scanning direction. The above printing resolution is just an example and is not limited to this.

[0015] Furthermore, the printing unit 206 has a ROM 211 for each color line head used for exposure control to the photoreceptor. The ROM 211 stores manufacturing variation information for each line head, such as the assembly position and tilt information of the LED chip measured by a jig in the production process, such as the line head manufacturing process. In addition, the ROM 211 stores a correction profile that summarizes correction values ​​for correcting variations in light intensity. Details of the correction profile will be described later.

[0016] The image processing unit 207 performs various image processing operations on the image data contained in the input print data. The image processing unit 207 may be a processing unit such as hardware specialized for each type of image processing, or it may be configured so that the CPU 208 executes the above-mentioned program to realize these functions.

[0017] <Layout of the printing section> Figure 3 is a cross-sectional view of the printing unit 206 in an image forming apparatus 101 using a tandem electrophotographic method employing an intermediate transfer body 28. The operation of the printing unit 206 will be explained using Figure 3. In the drawings, components provided for each color are indicated by adding an alphabet letter (C / M / Y / K) indicating the respective color to the end of the reference numeral. However, when explaining without distinguishing between colors, this alphabet letter at the end of the reference numeral will be omitted.

[0018] First, let me explain the general procedure. The printing unit 206 exposes the photoreceptor 22 according to the image data processed by the image processing unit 207, forms an electrostatic latent image, and develops the electrostatic latent image to form a monochrome toner image. Then, the printing unit 206 superimposes the monochrome toner images of each color on the intermediate transfer unit 28 to form a multi-color toner image. The printing unit 206 transfers the multi-color toner image to the recording medium 11, and the fixing device 31 fixes the multi-color toner image on the recording medium.

[0019] Next, the details of the configuration of the printing unit 206 will be explained using Figure 3. The injection charger 23 is used to uniformly charge the surface of the photoreceptor 22 to a predetermined potential, and has a sleeve 23S.

[0020] The photoreceptor 22 rotates when the driving force of a drive motor (not shown) is transmitted to it. The drive motor rotates the photoreceptor 22 counterclockwise in Figure 3 according to the image forming operation. The exposure means is configured to form an electrostatic latent image by irradiating the photoreceptor 22 with exposure light from an LED line head 24, which is arranged parallel to the photoreceptor 22, and selectively exposing the surface of the photoreceptor 22. In other words, the LED line head 24 is used as the light source of the exposure means in an electrophotographic image forming apparatus.

[0021] The developer unit 26 is used to visualize the electrostatic latent image on the photoreceptor 22 using monochromatic toner, and has a sleeve 26S. The developer unit 26 can be attached to and detached from the photoreceptor 22.

[0022] The intermediate transfer body 28 rotates clockwise in Figure 3 to receive a monochrome toner image from the photoreceptor 22. As the photoreceptor 22 and the primary transfer roller 27 located opposite it rotate, the monochrome toner image is transferred to the intermediate transfer body 28. By applying an appropriate bias voltage to the primary transfer roller 27 and creating a difference between the rotation speed of the photoreceptor 22 and the rotation speed of the intermediate transfer body 28, the monochrome toner image is efficiently transferred onto the intermediate transfer body 28. This is called primary transfer.

[0023] Furthermore, the monochromatic toner images for each of the CMYK stations are superimposed on the intermediate transfer member 28. The superimposed multicolor toner image is conveyed to the secondary transfer roller 29 as the intermediate transfer member 28 rotates. Also, the recording medium 11 is pinched and conveyed from the paper feed tray 21 to the secondary transfer roller 29, and the multicolor toner image on the intermediate transfer member 28 is transferred to the recording medium 11. At this time, by applying an appropriate bias voltage to the secondary transfer roller 29, the toner image is electrostatically transferred. This is called secondary transfer. The secondary transfer roller 29 contacts the recording medium 11 at position 29a while transferring the multicolor toner image onto the recording medium 11, and separates from the recording medium 11 at position 29b after the printing process.

[0024] The fixing device 31 has a fixing roller 32 for heating the recording medium 11 and a pressure roller 33 for pressing the recording medium 11 against the fixing roller 32 in order to melt and fix the multicolor toner image transferred to the recording medium 11. The fixing roller 32 and the pressure roller 33 are formed in a hollow shape, and heaters 34 and 35 are respectively built inside. The fixing device 31 conveys the recording medium 11 holding the multicolor toner image by the fixing roller 32 and the pressure roller 33, applies heat and pressure, and fixes the toner to the recording medium 11.

[0025] After the toner is fixed, the recording medium 11 is then discharged to a paper discharge tray (not shown) by a discharge roller (not shown), and the image forming operation ends. The cleaning means 30 cleans the toner remaining on the intermediate transfer member 28, and the waste toner remaining after transferring the four-color multicolor toner image formed on the intermediate transfer member 28 to the recording medium 11 is stored in the cleaner container.

[0026] <Configuration of LED line head> FIG. 4 is a diagram showing a configuration example of the LED line head 24 arranged in parallel with the photoreceptor 22. In the present embodiment, the LED line head 24 includes a printed circuit board 40 on which a circuit for supplying various signals for controlling the driving of the LED line head 24 is formed, and a lens array 41. Further, a plurality of LED chips 42 are arranged in a staggered manner on the printed circuit board 40. The size of the LED line head 24 may be, for example, substantially the same size as the photoreceptor 22 in the main scanning direction, or may be smaller than the photoreceptor 22.

[0027] A configuration example of the LED chip 42 will be described with reference to the drawings. FIG. 5 is a diagram schematically showing a configuration example of the LED chip 42. As shown in FIG. 5, 512 LED light-emitting elements 43 are arranged at equal intervals in the LED chip 42. As the LED light-emitting element, for example, an LED (Light Emitting Diode) or an organic LED (Organic Light Emitting Diode) can be used. At the end portions in the LED chip 42, the LED light-emitting elements 43 included in different LED chips 42 are arranged such that the main scanning positions overlap. In FIG. 5, the light-emitting elements are arranged in a state where 4 elements overlap between different chips. Note that the arrangement of the LED chips 42 and the number of light-emitting elements are not limited to this. For example, the LED light-emitting elements 43 may be two-dimensionally arranged in the main scanning direction and the sub-scanning direction. In addition to the description of the configuration of the LED line head 24 below, the factors causing variations in the amount of light on the photoreceptor corrected in the present embodiment will be described.

[0028] First, the correspondence relationship between the LED light-emitting element 43 in the LED chip 42 and a rod lens - current source to be described later will be described with reference to the drawings. FIG. 6 is a diagram showing the correspondence relationship between the LED light-emitting element 43 in the LED chip 42 and the rod lens - current source to be described later. FIG. 6(a) shows the case where there is no temperature change in the LED line head, and FIG. 6(b) shows the case where there is a temperature change in the LED line head. The variations in the amount of light on the photoreceptor caused by the LED line head 24 will be described.

[0029] <Factors Caused by the LED Line Head 24> The lens array 41 is a device provided between the LED chip 42 and the photoreceptor 22 and functions as an imaging lens. The lens array 41 is composed of rod lenses with a refractive index distribution, arranged at a pitch corresponding to, for example, four LED light-emitting elements 43, as shown in Figure 6(a), and images the light emitted from each LED light-emitting element 43 onto the photoreceptor 22. Because the transmittance of the rod lenses differs between the center and the ends, the amount of light on the photoreceptor 22 varies. In this embodiment, a correction value to correct this variation in light amount due to the lens position is provided as a first correction profile 901. In other words, the first correction profile 901 can be said to be correction information for correcting the variation in light amount on the photoreceptor due to the lens position. Also, the printed circuit board 40 expands and contracts due to the temperature inside the LED line head 24. As the printed circuit board 40 expands and contracts, the position of the attached LED light-emitting elements 43 also changes. However, the position of the lens array 41 does not change. As a result, the correspondence between the LED light-emitting element 43 and the rod lens moves parallel to each other while maintaining the distance between the rod lens and the LED light-emitting element 43, as shown in Figures 6(a) and 6(b). Taking this shift in correspondence into consideration, the lens array 41 is provided with a width wider than the LED chip 42. In Figure 6(a), the LED line head 24 further has a rod lens 41001 positioned outside the LED chip 42 at its left end. Furthermore, it has a temperature sensor 212 that detects the temperature inside the LED line head 24, and when the detected temperature changes above a certain level, it triggers the updating of the reference position in the first correction profile 901. The first correction profile 901 stores correction profiles corresponding to all rod lenses in the ROM 211 of the LED line head 24. The correspondence with the LED light-emitting element 43 is adjusted according to the temperature change inside the LED line head 24, and the result of updating the reference position is stored in the RAM 210 of the control unit 203 and used to generate a correction signal. Details of the form of the correction profile and the method of generating the correction signal will be described later.

[0030] On the printed circuit board 40, a ROM 211 storing manufacturing variation information of the LED line head 24 measured in the manufacturing process and a current source for supplying current to the LED light-emitting element 43 are arranged on the back side of the board or the like. Similar to the rod lens, there are a plurality of current sources, and a plurality of LED light-emitting elements 43 correspond to one current source. The correspondence between the current source and the LED light-emitting element 43 is shown in FIG. 6(a). As shown in FIG. 6(a), for example, they are arranged in a pitch corresponding to 8 LED light-emitting elements and 1 current source. In the LED chip 42 of Chip1, the current source 4001 at the left end corresponds to the 8 LED light-emitting elements 43 at the left end, and the current source 4020 at the right end corresponds to the 8 LED light-emitting elements 43 at the right end. In the LED chip 42 of Chip1, the 4 LED light-emitting elements 43 at the right end correspond to the rod lens 41129, and the 4 LED light-emitting elements 43 adjacent to the left of the 4 LED light-emitting elements 43 at the right end correspond to the rod lens 41128 adjacent to the left of the rod lens 41129. Also, the 4 LED light-emitting elements 43 at the left end in the LED chip 42 of Chip2 are arranged so as to overlap with the 4 LED light-emitting elements 43 at the right end in the LED chip 42 of Chip1 in the main scanning direction. In the LED chip 42 of Chip2, the current source 4021 at the left end corresponds to the 8 LED light-emitting elements 43 at the left end. However, due to the individual differences of the above-mentioned current sources, the light emission amounts of the LED light-emitting elements 43 vary, and as a result, the light amounts on the photoreceptor 22 vary. A correction value for suppressing the variation in light amount is provided as the second correction profile 902. The second correction profile 902 can be said to be correction information for correcting the variation in light amount on the photoreceptor due to the individual differences of the current sources. Different from the correspondence between the above-mentioned rod lens and the LED light-emitting element 43, the correspondence between the LED light-emitting element 43 and the current source does not change over time. Therefore, when assembling the LED line head 24, the difference in light emission amount due to the difference in current sources is measured, and the second correction profile 902 created based on the measurement results is held in advance in the ROM 211 for each line head and used for generating a correction signal. The form of the correction profile and the details of the method for generating the correction signal will be described later.

[0031] <Factors other than the LED line head 24> The first and second correction profiles 901 and 902 represent variations in the amount of light emitted by the LED light-emitting elements 43 due to the LED line head 24. Next, we will explain the variations in light intensity on the photoreceptor 22 due to the positional relationship between the LED line head 24 and the photoreceptor 22. For example, eccentricity of the photoreceptor 22 can be cited. If the axis of rotation is tilted with respect to the axis of the photoreceptor 22, the distance from the LED line head 24 to the photoreceptor 22 changes at the main scanning position, causing variations in light intensity on the photoreceptor. Variations in light intensity on the photoreceptor occur at a lower period compared to variations in lens position and current source. In this embodiment, a correction value for correcting variations in light intensity due to the positional relationship between the LED line head 24 and the photoreceptor 22 is provided as a third correction profile 903. The third correction profile 903 can also be said to be correction information for correcting variations in light intensity on the photoreceptor due to the positional relationship between the LED line head 24 and the photoreceptor 22. For example, there is a correction value for each of the 16 light-emitting elements. Furthermore, the positional relationship between the LED line head 24 and the photoreceptor 22 changes over time due to the replacement of components including the photoreceptor 22, and the number of prints made since the replacement. Therefore, the third correction profile 903 is updated triggered by a calibration instruction during component replacement, or by an instruction from a user who has confirmed that density unevenness in the main scanning direction has become apparent. The third correction profile 903 is stored in the memory unit 204. Details of the correction profile form and the method of generating the correction signal will be described later.

[0032] As described above, in the LED line head 24, numerous LED light-emitting elements 43 arranged in the main scanning direction correspond to other components within the LED line head 24 in different units. Therefore, variations in light intensity occur, resulting from the coexistence of multiple different periods. In addition to the LED line head 24, variations in light intensity also occur on the photoreceptor 22 due to the positional relationship between the LED line head 24 and the photoreceptor 22.

[0033] On the other hand, the need to update the correction profile and the trigger for updating differ depending on the factor that causes variations in light intensity on the photoreceptor. Therefore, if a single correction profile is maintained that integrates variations in light intensity on the photoreceptor caused by multiple factors, all data will need to be updated at any one update timing, resulting in a large processing cost for updates. To address this, in this embodiment, correction profiles are maintained for each factor so that only the necessary correction profiles can be updated at the update timing. In other words, multiple types of correction profiles are maintained so that each type of correction profile (each type of correction information) can be updated at a different timing.

[0034] <Configuration of the image processing unit> Next, the configuration of the image processing unit 207, which performs image processing on image data included in the input print data when the image forming apparatus 101 according to this embodiment uses the printing unit 206 to form (print) an image, will be described. Figure 7 is a functional block diagram illustrating the configuration of the image processing unit 207. As mentioned above, the functions of the image processing unit 207 may be implemented by hardware, or by the CPU 208 executing a program.

[0035] The image processing unit 207 includes an input unit 301, a color conversion processing unit 302, a rendering processing unit 303, a gradation correction processing unit 304, a halftone processing unit 305, an output unit 307, and an HT density correction processing unit 306. The "HT" at the beginning of the HT density correction processing unit 306 is an abbreviation for halftone, indicating that it accepts and processes image data that has already undergone halftone processing.

[0036] The input unit 301 receives image data described in PDL (Page Description Language) contained in the print data received by the data input unit 201, for example. The color conversion processing unit 302 converts the image data in the RGB color space to image data in the CMYK color space, for example. The rendering processing unit 303 renders the received PDL data and converts it into image data. The rendering processing unit can switch rendering processing according to the instructions for Fine and Super Fine. When Fine is instructed, image data is generated with a resolution of 600 dpi in the main scanning direction and sub-scanning direction. When Super Fine is instructed, image data is generated with a resolution of 1200 dpi in the main scanning direction and sub-scanning direction. These resolution settings can be instructed by the user via the UI unit 205, and are also selected by the resolution instruction contained in the print data received by the data input unit 201.

[0037] The gradation correction processing unit 304 performs gradation correction so that the printer density characteristics corresponding to the dither matrix of the halftone processing applied to the midtones of the image data reach the target output density for each CMYK color plate. The printer density characteristics described here are obtained by measuring the printed material as follows. Specifically, with the density unevenness and streaks caused by variations in the light intensity of the LED line head 24 corrected by the HT density correction processing unit 306, halftone patches are printed with halftone processing applied to the signal values ​​of each color plate, and the printed material is measured to obtain the characteristics.

[0038] The halftone processing unit 305 performs halftone processing on the image data of each CMYK color plate after gradation correction, and converts it into an N-value quantized dot image (halftone image data) in which the intermediate tones of the image data are expressed as area gradations. Hereinafter, the halftone image data after halftone processing is abbreviated as HT and is referred to as HT image data. In this embodiment, resolution conversion to a printing resolution of 2400 dpi is simultaneously performed. That is, for input image data with a resolution of 600 dpi, in order to perform halftone processing according to a printing resolution of 2400 dpi, the input image data with a resolution of 600 dpi is quadrupled in the main scanning direction and quadrupled in the sub-scanning direction, and halftone processing is performed while repeatedly referring to it. As a feature of this embodiment, at the stage of halftone processing, it is not necessarily required to be at the printing resolution, and in the HT density correction processing unit 306 described later, it is sufficient if it is at the resolution of the printing resolution. For example, a processing unit having a resolution conversion function for converting to the printing resolution may be provided after the halftone processing unit 305.

[0039] The HT density correction processing unit 306 acquires the first and second correction profiles from the ROM 211 of the line head of each color plate in the printing unit 206, and acquires the third correction profile from the storage unit 204, and generates a correction signal for each main scanning position based on these correction profiles. Density correction based on the correction signal for each main scanning position is performed on the image data after halftone processing.

[0040] <Method for Light Quantity Correction by Adjustment of HT Image Data> A method for adjusting variations in light intensity by correcting HT image data will be explained using diagrams. Figure 8 is a diagram illustrating an example of HT image data without light intensity correction and its printed product. Figure 8(a) shows the illuminated and unilluminated pixels in the HT image data, and Figure 8(b) shows the printed product using the HT image data from Figure 8(a). For example, as shown in Figure 8(a), even if an attempt is made to produce a printed product with uniform density based on HT image data in which all LED light-emitting elements 43 are illuminated, if the light intensity on the photoreceptor 22 varies, the following will occur. That is, due to variations in light intensity at each main scanning position of the photoreceptor, the printed product will have varying density at each main scanning position, as shown in Figure 8(b). Therefore, LED light-emitting elements 43 corresponding to areas where the amount of light emitted is relatively higher compared to other areas are probabilistically turned off.

[0041] Figure 9 is a diagram illustrating an example of processed HT image data after light intensity correction and its printed form. Figure 9(a) shows illuminated and unilluminated pixels, and Figure 9(b) shows an example of the HT image according to Figure 9(a). As shown in Figure 9(a), pixels in the HT image corresponding to the central region in the main scanning direction, which is a region with relatively high light emission, are probabilistically turned off, and the difference in light emission per unit area consisting of multiple pixels is controlled to be small. As a result, although the HT image data is non-uniform, the variation in light intensity on the photoreceptor is added, and as shown in Figure 9(b), the difference in density reproduced in the printed material becomes small. In this embodiment, the probability of turning off the pixels is determined according to the correction profile, and the variation in light intensity for each main scanning position is reduced. Details of the HT density correction processing unit 306, which is a feature of this embodiment, will be described later.

[0042] The output unit 307 passes the density correction signal data generated by the HT density correction processing unit 306 to the printing unit 206.

[0043] <Processing flow of image processing unit 207> The processing flow of each processing unit constituting the image processing unit 207 will be explained using diagrams. Figure 10 is a flowchart showing the image processing flow by the image processing unit 207. The processing shown in Figure 10 is achieved by the CPU 208 reading the program stored in the ROM 209, loading it into the RAM 210, and executing it.

[0044] In S1001, the CPU 208 receives the document data from the data input unit 201 and passes it to the rendering unit 303 via the input unit 301 of the image processing unit 207. The rendering unit 303 then converts the input document data into RGB raster image data at a resolution of 600 dpi in the main scanning direction and sub-scanning direction, and supplies this image data to the color conversion unit 302.

[0045] In S1002, the CPU 208 controls the color conversion processing unit 302 to perform the process of converting the generated RGB data to CMYK data. The image data after the color conversion process is passed to the gradation correction processing unit 304.

[0046] In S1003, the CPU 208 controls the gradation correction processing unit 304 to perform gradation correction processing on the image data of each color plate, taking into account the gradation characteristics of the printing unit 206 of the image forming apparatus 101 with respect to the dither matrix of the halftone processing applied to the midtones of the image data. The image data after gradation correction processing is passed to the halftone processing unit 305. There are multiple types of dither matrices that can be applied to the midtones, and the user can specify the type of dither matrix in the UI unit 205. Since the gradation characteristics of the printing unit 206 vary depending on the dither matrix of the halftone processing, it is necessary to switch the gradation correction processing according to the dither matrix. Therefore, gradation correction processing is performed according to the type of dither matrix specified in the UI unit 205.

[0047] In S1004, the CPU 208 controls the halftone processing unit 305 to perform resolution conversion on the CMYK data after gradation correction, and generates HT image data which is 1-bit data indicating on or off of the light emission of the LED light emitting element 43. In the resolution conversion, conversion from 600 dpi to 2400 dpi of the printing resolution is executed. The generated HT image data is sent to the HT density correction processing unit 306.

[0048] In S1005, the CPU 208 controls the HT density correction processing unit 306 to acquire a correction profile from the ROM 211 and the storage unit 204 held in the LED line head 24 for each CMYK color, and generates a correction signal for each main scanning position. Based on the correction signal for the HT image data, the correction processing described using FIGS. 8 and 9 is sequentially executed. The data after the correction processing is passed to the output unit 307. Details of the correction profile and correction values will be described later.

[0049] <Detailed Configuration of HT Density Correction Processing Unit 306> A configuration example of the HT density correction processing unit 306 in the present embodiment will be described using a figure. FIG. 11 is a functional block diagram for explaining the configuration of the HT density correction processing unit 306 in the present embodiment. The HT density correction processing unit 306 has a first correction profile (also referred to as the first correction PF) 901, a second correction profile (also referred to as the second correction PF) 902, and a third correction profile (also referred to as the third correction PF) 903. The HT density correction processing unit 306 has a composite profile acquisition unit (also referred to as the composite PF acquisition unit) 904, a first generation unit 905, a matrix 906 of the first correction signal, a third acquisition unit 907, and a first correction processing unit 910. Based on the profile acquired by the composite profile acquisition unit 904, density correction is performed by these functional units. Further, the HT density correction processing unit 306 has a second generation unit 908, a matrix 909 of the second correction signal, and a second correction processing unit 911. Based on the profile acquired by the third acquisition unit 907, density correction is performed by these functional units. Furthermore, the HT density correction processing unit 306 has an update unit 912. By this functional unit, the third correction profile 903 is updated according to changes over time.

[0050] <First, second, and third correction profiles> The first, second, and third correction profiles will be explained using diagrams. Figure 12 shows example data for the first, second, and third correction profiles.

[0051] The first correction profile 901 is data that holds correction values ​​to correct for variations in the light intensity of the light-emitting elements due to differences in transmittance at each position of the rod lens. In this embodiment, as shown in Figure 12, the first correction profile 1201, which corresponds to the first correction profile 901, holds data corresponding to each of the 512 LED light-emitting elements 43 per chip of the LED chip 42. In other words, the first correction profile 901 can be said to hold data on a per-light-emitting-element basis. As described above, the range in which the LED light-emitting elements 43 and the rod lens shift from their initial positions according to the temperature inside the LED line head is known in advance, and the amount of shift corresponding to the number of rod lenses and the temperature is held. In this embodiment, an example is shown in which a shift of up to one rod lens (four light-emitting elements) occurs due to the expansion and contraction of the printed circuit board 40. Therefore, the first correction profile 1201 holds 520 pieces of data, which is 512 plus 8. The data is held in 4 bits, and 15 is set for the main scanning position where the transmittance is high and the light emission frequency is low in order to reduce the light intensity. Conversely, if the transmittance is low and the light emission frequency of the LED light-emitting element 43 is maintained, then 0 is set. These numbers indicate the probability of the light going out.

[0052] The second correction profile 902 is data that holds correction values ​​to compensate for variations in the light intensity of the LED light-emitting elements 43 due to differences in the current source. In this embodiment, as shown in Figure 12, the second correction profile 1202, which corresponds to the second correction profile 902, holds one data for every eight light-emitting elements. In this embodiment, since there are 512 LED light-emitting elements 43 per LED chip 42, the second correction profile 902 holds 64 data for each LED chip 42. The data is held in 4 bits, similar to the first correction profile 901, and is set to 15 when the current supplied from the current source is strong and the light emission frequency of the LED light-emitting elements 43 is reduced. Conversely, it is set to 0 when the current is weak and the light emission frequency of the LED light-emitting elements 43 is maintained.

[0053] The third correction profile 903 is data that holds correction values ​​to correct for variations in light intensity on the photoreceptor 22 due to the tilt between the LED line head 24 and the photoreceptor 22. In this embodiment, as shown in Figure 12, the third correction profile 1203, which corresponds to the third correction profile 903, holds one data for every 16 light-emitting elements. In this embodiment, since there are 512 LED light-emitting elements 43 per chip of the LED chip 42, 32 data are held per chip. The data is held in 8 bits, and a print chart with all LED light-emitting elements 43 turned on (lit) is output, and the correction value is determined based on the density difference for each main scanning position. The correction value corresponding to the area where the distance between the LED line head 24 and the photoreceptor 22 is far and the light intensity is weak is set to 0, and the correction value corresponding to the area where the distance between the LED line head 24 and the photoreceptor 22 is close and the light intensity is strong is set to greater than 0. The process of generating correction values ​​from the print chart is the same as the process of the update unit 912, and will be described in detail in the process of the update unit 912. Note that the pitch and data volume for each correction profile are not limited to the examples above. For example, the number of corresponding LED light-emitting elements 43 may be uneven depending on the data position. Also, the data may be multi-bit. By multi-biting the data, correction can be performed with finer precision.

[0054] The composite profile acquisition unit 904 includes a first acquisition unit 913, a second acquisition unit 914, and a first synthesis processing unit 915. The first acquisition unit 913 selects a data area corresponding to the LED light-emitting element 43 from the acquired first correction profile based on the first correction profile acquired from the ROM 211 in the LED line head 24 and the temperature information obtained from the temperature sensor 212. The second acquisition unit 914 acquires a second correction profile from the ROM 211 in the LED line head 24. The first synthesis processing unit 915 generates data by combining the data selected by the first acquisition unit 913 and the data acquired by the second acquisition unit 914 on a light-emitting element basis. Details of the composite profile acquisition unit 904 will be described later.

[0055] The first generation unit 905 obtains a threshold corresponding to a light-emitting element by referring to the data of each light-emitting element acquired by the composite profile acquisition unit 904 and the matrix of the first correction signal (also referred to as the first matrix) 906. Then, the first generation unit 905 generates a first correction signal for correcting the HT image data based on the first and second correction profiles 901 and 902. The correction signal in this embodiment is a signal value representing the off-rate for determining which pixels to turn off probabilistically, as described in the above-mentioned method of light intensity correction by adjusting the HT image data. In this embodiment, the first correction signal is generated using the data of each light-emitting element and the matrix of the first correction signal 906, which will be described later.

[0056] <Matrix of the first correction signal> The details of the matrix 906 of the first correction signal will be explained with reference to a figure. Figure 13 is a diagram illustrating the matrix of the first correction signal and an example of its use, with Figure 13(a) showing an example of the matrix of the first correction signal and Figure 13(b) showing an example of its use. The matrix 906 of the first correction signal is matrix data for 128 × 128 pixels. A schematic diagram of the matrix 906 of the first correction signal is shown in Figure 13(a). In this embodiment, matrix data for 128 × 128 pixels is held. When using this matrix data, as shown in Figure 13(b), the matrix data and the shift amount held together are repeatedly arranged and referenced in the vertical and horizontal directions.

[0057] <Generation of the first correction signal> The generation of the first correction signal by the first generation unit 905 will be explained with reference to a figure. Figure 14 is a diagram illustrating the generation of the first correction signal. The correction data 1401 of the main scan position is compared with a threshold 1402 that refers to the matrix of the first correction signal. Pixels where the correction data exceeds the matrix data are set as unlit pixels (0), and the remaining pixels are set as lit pixels (1), generating the first correction signal 1403. A threshold matrix with known blue noise characteristics is used as the matrix. By using blue noise characteristics, the dot period becomes finer than that of an AM screen type dither matrix, and unlit pixels are dispersed. Because the position of unlit pixels is determined by a fine dot period, it is possible to suppress the occurrence of breaks in fine lines and jaggedness of characters due to light intensity correction. Furthermore, it is possible to suppress banding caused by interference between HT image data and the correction signal. Note that the frequency characteristics of the matrix data are not limited to blue noise. Any dot-dispersing type matrix is ​​used in which the corrected dots are less likely to be adjacent and which does not affect jaggedness of fine lines and characters. A threshold matrix with known green noise characteristics or a threshold matrix with pink noise characteristics may be used.

[0058] The third acquisition unit 907 acquires the third correction profile from the storage unit 204 in the control unit 203 and performs a process to convert it into data for each light-emitting element. The conversion process is performed by acquiring the profile value of the main scanning position corresponding to the LED light-emitting element 43.

[0059] The second generation unit 908 refers to the data of each light-emitting element acquired and converted by the third acquisition unit 907 and the matrix of the second correction signal (also called the second matrix) 909. The second generation unit 908 then generates a second correction signal for correcting the HT image data based on the third correction profile 903.

[0060] The format of the matrix 909 of the second correction signal is the same as that of the matrix 906 of the first correction signal, the only difference being the threshold of the matrix data held. The matrix 909 of the second correction signal has known blue noise characteristics, similar to the matrix 906 of the first correction signal, and uses matrix data with a different phase than the matrix 906 of the first correction signal. The matrix 909 of the second correction signal holds matrix data with different phases so that the correction positions from the two correction processes do not overlap. The method of generating the correction data by the second generation unit 908 is the same as the method of generating the first correction signal by the first generation unit 905, and therefore the explanation is omitted.

[0061] The first correction processing unit 910 acquires HT image data from the HT processing unit 305 and the first correction signal from the first generation unit 905. Then, the first correction processing unit 910 performs an AND operation on the acquired HT image data and the acquired first correction signal for each pixel. That is, even if the HT image data is in the illuminated state (1), if the first correction signal is for an unlit pixel (0), the first correction processing unit 910 corrects it to the unlit state (0).

[0062] The second correction processing unit 911 acquires the first light quantity corrected HT image data from the first correction processing unit 910 and acquires the second correction signal from the second generation unit 908. Then, the second correction processing unit 911 performs an AND operation on each pixel between the acquired first light quantity corrected HT image data and the acquired second correction signal. That is, similar to the first correction processing, even if the first light quantity corrected HT image data is in the lit state (1), if the second correction signal is for a pixel that is off (0), the second correction processing unit 911 corrects it to the off state (0).

[0063] The HT image data on which the first and second light quantity corrections have been sequentially performed is sent to the printing unit 206. Then, the printing unit 206 performs image formation on the recording medium based on the processed HT image data.

[0064] Furthermore, the update unit 912 updates the third correction profile 903 based on a user instruction from the UI unit 205. That is, first, based on a user instruction from the UI unit 205, the update unit 912 reads the result of printing the calibration chart previously held in the ROM 209 by the image reading unit 202 and acquires the density distribution for each main scanning position. Then, the update unit 912 updates the third correction profile 903 based on the acquired density distribution for each main scanning position. Based on the position with the lowest density among the main scanning positions, a correction value is calculated such that the emission frequency of the region corresponding to the region darker than the reference becomes lower, and the third correction profile 903 is updated. Details of the update process of the third correction profile will be described later.

[0065] <Processing flow of the HT density correction processing unit 306> The processing flow of the HT density correction processing unit 306 in this embodiment will be explained with reference to the diagram. Figure 15 is a flowchart showing the image processing flow by the HT density correction processing unit 306 in this embodiment. The processing shown in Figure 15 is achieved by the CPU 208 reading the program stored in the ROM 209, loading it into the RAM 210, and executing it. Steps S1501 to S1509 below sequentially select the target toner color and pixel position from the CMYK four-color HT image data and perform processing. The selected pixels are changed and the processing is repeated until processing is completed for all pixels.

[0066] In S1501, the CPU208 obtains the signal value In of the pixel to be processed from the HT image data after halftone processing, which is stored in RAM210.

[0067] In S1502, the CPU 208 retrieves the first correction profile 901 and the second correction profile 902 stored in the ROM 211 of the LED line head 24, and obtains temperature information from the temperature sensor 212 within the LED line head 24. The acquired temperature information is compared with past temperature information stored in the memory unit 204 to determine whether a temperature change greater than a pre-held threshold has occurred. Past temperature information includes, for example, the temperature information stored in the memory unit 204 during the previous processing. If the determination result is that a temperature change greater than a pre-held threshold has occurred, the CPU 208 determines the data for each light-emitting element based on the temperature information from the first correction profile 901 and updates the data stored in the RAM 210. Furthermore, a composite correction value is calculated by combining the data corresponding to the first correction profile 901 stored in the RAM 210 with the data corresponding to the second correction profile 902, and this composite correction value is stored in the RAM 210. Details of the method for calculating the composite profile will be described later.

[0068] In S1503, the CPU 208 retrieves the matrix 906 of the first correction signal held in the RAM 210.

[0069] In S1504, the CPU 208 selects a value Coeff_1st from the composite profile stored in RAM 210, corresponding to the main scan position of the pixel selected for processing. Furthermore, it selects a threshold value Mat_1st from the acquired first correction signal matrix 906, corresponding to the pixel position selected for processing. Then, it calculates the correction signal Corr_1st from this data. The pseudocode for calculating the correction signal Corr_1st is shown below. The calculated result is stored in RAM 210. Both Coeff_1st and Mat_1st are stored as 8 bits and compared. Details of how to calculate the 8-bit Coeff_1st from the 5-bit first correction profile 901 and the second correction profile 902 will be described later.

[0070] Pseudocode: if(Coeff_1st > Mat_1st){ Corr_1st = 0; else{ Corr_1st = 1; } In S1505, the CPU 208 performs an AND operation on In and Corr_1st, which were acquired in S1501, to calculate the HT image data In_trim, which has been corrected by the first correction signal. In other words, the first light intensity correction process is performed. The calculated result is stored in the RAM 210.

[0071] In S1506, the CPU 208 retrieves the third correction profile 903 stored in the memory unit 204. Furthermore, it references the data corresponding to each light-emitting element and converts it into data specific to each light-emitting element. It then references the correction data ProcCoeff corresponding to each light-emitting element and converts it into correction data ProcCoeff_full for each light-emitting element. The converted correction data ProcCoeff_full for each light-emitting element is stored in the RAM 210. For example, the conversion is performed using the following equation (1), where x represents the main scan position. ProcCoeff_full[x] = ProcCoeff[x / 16] ···(1)

[0072] In S1507, the CPU 208 retrieves the matrix 906 of the second correction signal held in the RAM 210.

[0073] In S1508, the CPU 208 selects a value Coeff_2nd corresponding to the pixel position selected for processing from the data corresponding to the third correction profile 903 held in RAM 210. Furthermore, it selects a threshold value Mat_2nd corresponding to the pixel position selected for processing from the acquired matrix 909 of the second correction signal. Then, it calculates the correction signal Corr_2nd from this data. The calculation method is the same as the pseudocode in S1504, so its explanation is omitted.

[0074] In S1509, the CPU 208 performs an AND operation on In_trim and Corr_2nd calculated in S1505 to calculate the HT image data In_trim_2nd, which has been corrected by the second correction signal. In other words, a second light intensity correction process is performed. The calculated HT image data In_trim_2nd, which has been corrected by the second correction signal, is sent to the printing unit 206.

[0075] In the above example, S1501 and S1502 through S1504 are treated as parallel processing, but this is not limited to this. For example, S1501 through S1504 may be processed sequentially. In the above example, S1501 through S1505 and S1506 through S1508 are treated as parallel processing, but this is not limited to this. For example, S1501 through S1508 may be processed sequentially.

[0076] <Detailed processing flow of the synthesis profile acquisition process S1502> The details of the processing (S1502) by the composite profile acquisition unit 904 will be explained using a diagram. Figure 16 is a flowchart showing the detailed flow of processing by the composite profile acquisition unit 904.

[0077] In S1601, the CPU 208 obtains temperature information from the temperature sensor 212, which measures the temperature inside the LED line head 24.

[0078] In S1602, the CPU 208 compares the temperature information obtained in S1601 with past temperature information previously stored in the memory unit 204 to determine whether a temperature difference has occurred that causes the printed circuit board 40 to expand or contract. Past temperature information includes, for example, the temperature information stored in the memory unit 204 in the previous processing. If the determination shows that a temperature difference has occurred that causes the printed circuit board 40 to expand or contract, and there has been a temperature change (YES in S1602), the process proceeds to S1603. On the other hand, if a temperature difference that causes the printed circuit board 40 to expand or contract has not occurred, and there has been no temperature change (NO in S1602), S1603 is skipped and the process proceeds to S1604.

[0079] In S1603, the CPU 208 acquires the temperature information obtained in S1601 and the amount of misalignment between the rod lens and the LED light-emitting element 43, which is stored in the ROM 211 of the LED line head 24 and corresponds to the temperature information. The CPU 208 then selects a correction value corresponding to the current state of the LED light-emitting element 43 from the first correction profile 901 and updates the correction data stored in the RAM 210. The amount of misalignment corresponding to the temperature information is stored in the same units as the main scan resolution. The initial starting position of the correspondence relationship is set to the main scan position shifted by the amount of misalignment, and the correction value is selected. The temperature information stored in the memory unit 204 is also updated to the latest information.

[0080] In S1604, the CPU 208 acquires the first correction profile 901 of the light-emitting element unit, which has already been selected and is stored in the RAM 210.

[0081] In S1605, the CPU 208 retrieves the second correction profile 902 stored in the ROM 211 of the LED line head 24. Furthermore, it refers to the corresponding correction data PowCoeff for each light-emitting element, converts it into correction data PowCoeff_full for each light-emitting element, and stores it in the RAM 210. For example, the conversion is performed using equation (2) below, where x represents the main scan position. PowCoeff_full[x] = PowCoeff[x / 8] ···(2)

[0082] In S1606, CPU208 calculates the composite correction value TotalCoeff[x], which is composite information, using the following pseudocode based on the data shown below. The calculation process for the composite correction value TotalCoeff[x] is performed based on the following data. Specifically, it is performed based on LensCoeff[x], the data corresponding to the first correction profile 901 acquired in S1604, and PowCoeff_full[x], the data corresponding to the second correction profile 902 acquired in S1605. Here, x represents the main scan position. The calculation is performed for each main scan position.

[0083] First, the lens transmittance correction value LensCoeff[x], which corresponds to the first correction profile 901 acquired in S1604, is converted into a light intensity equivalent value Lumi that takes lens transmittance into account. The lower the amount of light emitted from the LED light-emitting element 43 and transmitted through the rod lens, the smaller the correction value becomes. In other words, the correction value and the light intensity are inversely proportional. Therefore, the light intensity equivalent value can be obtained by inverting the correction value.

[0084] Next, the correction value PowCoeff_full[x] due to variations in the current source is converted into a light intensity correction ratio Rate that takes into account the difference in current. The less current supplied to the LED light-emitting element 43, the smaller the correction value becomes. In other words, similar to LensCoeff, the correction value and the light intensity correction ratio are inversely proportional. Therefore, the equivalent value of the light intensity correction ratio can be obtained by inverting the correction value.

[0085] Next, the combined correction value TotalCoeff[x] is determined based on the corrected light intensity (LumiTrim), which is obtained by multiplying the light intensity by the light intensity adjustment ratio. Furthermore, the adjustment value Gain is multiplied when determining TotalCoeff[x]. The adjustment value Gain is used to extend the correction resolution when the amount of data held in the correction profile is small. A value of 1 is set if the correction resolution is sufficient. A value greater than 1 is set if the correction resolution is insufficient.

[0086] Finally, TotalCoeff[x] is clipped so that it fits within the 8-bit threshold range held by the matrix 906 of the first correction signal.

[0087] Pseudocode: Lumi = 16 - LensCoeff[x] / / Correction value → Light intensity Rate = (16 - PowCoeff_full[x]) / 16 / / Correction value → Ratio LumiTrim = Lumi × Rate / / Corrected light amount = Light amount × Ratio TotalCoeff[x] = (16 - LumiTrim) × Gain / / Corrected light amount → Combined corrected value if(TotalCoeff[x]>255){TotalCoeff[x]=255} The first correction profile 901 and the second correction profile are both factors causing variations in light intensity due to the same LED line head 24. In other words, if either condition changes, the variation caused by the remaining factor will have the same period but only the amplitude will change. Therefore, the two conditions are combined by multiplying them together.

[0088] In the above example, steps S1601 to S1604 and S1605 were processed in parallel, but this is not limited to this. For example, steps S1601 to S1605 could be processed sequentially.

[0089] <Processing flow of update unit 912> The details of the processing performed by the update unit 912 will be explained using diagrams. Figure 17 is a flowchart showing the detailed flow of the process of updating the third correction profile by the update unit 912. The process shown in Figure 17 is achieved by the CPU 208 reading the program stored in the ROM 209, loading it into the RAM 210, and executing it. The following processes are performed based on user calibration instructions input from the UI unit 205. For example, calibration is necessary when replacing components including the photoreceptor 22, or when a certain number of prints have been performed and there has been a change in the surface of the photoreceptor over time. Therefore, it is desirable to display a notification prompting the UI unit 205 to perform calibration at the above timings.

[0090] In S1701, the CPU 208 prints the calibration chart 1801, which is stored in the ROM 209, using the printing unit 206. The calibration chart will be explained using a diagram. Figure 18 is a schematic diagram of the calibration chart 1801. As shown in Figure 18, the calibration chart 1801 represents the differences in the colorants recorded by patches 1802, 1803, 1804, and 1805. In this embodiment, the calibration chart 1801 holds the print data for recording using C (cyan) for patch 1802, M (magenta) for patch 1803, Y (yellow) for patch 1804, and K (black) for patch 1805. In each patch, as shown in Figure 8(a), all pixels within the patch are illuminated pixels for each colorant. In this embodiment, only an example where all signal values ​​of each patch are illuminated pixels is shown, but this is not the only example. A calibration chart including intermediate tone patches with illuminated pixels of less than 100% may also be used.

[0091] In S1702, the CPU 208 reads the printed calibration chart (printed chart) 1801 output by S1701 using the image reading unit 202, and stores the resulting signal values ​​in the RAM 210.

[0092] In S1703, the CPU 208 converts the read signal values held in the RAM 210 into density values for each main scanning position using the conversion formula held in the ROM 209. In this embodiment, the resolution of the image reading unit 202 is 600 dpi, and density values for each 600 dpi unit of main scanning are calculated and held in the RAM 210. That is, in this embodiment, one pixel of 600 dpi in main scanning is set as a predetermined area, and correction data is generated so as to reduce density variations. Note that the resolution of the predetermined area is an example and is not limited to the above example. In order to correct finer streaks, a high-resolution read signal value may be used.

[0093] In S1704, the CPU 208 calculates a correction profile. That is, the CPU 208 calculates an updated value ProcCoeff[subx] of a third correction profile for each 150 dpi unit of main scanning based on the density values Dens[scanx][comp] for each 600 dpi unit of main scanning held in the RAM 210. The following is pseudo code for calculating the correction profile. Here, scanx represents the main scanning position in units of 600 dpi, subx represents the main scanning position in units of 150 dpi, and comp represents the ID of the colorant. The calculated updated value ProcCoeff[subx] is held in the RAM 210.

[0094] Pseudo code: for(int c=0; c<4; c++){ / / Repeat for 4 colors of CMYK min = Dens[0][c]; for(int scanx=1; scanx<width_600; scanx++){ / / width_600: Maximum value of main scanning position in units of 600 dpi if(min > Dens[x][c]) min = Dens[x][c]; } } for(int subx=0; subx <width_150; subx+=3){ / / width_150: Maximum value of main scanning position in units of 150 dpi sum = 0; for(int i=0; i <4;i++){ sum = Dens[subx*4+i][c] - min;} ProcCoeff[subx]=sum / 4; } } Finally, in S1705, the CPU 208 updates the value of the third correction profile stored in the memory unit 204 with the updated value ProcCoeff[subx] stored in the RAM 210.

[0095] As described above, in this embodiment, the HT image data after halftone processing is subjected to the following correction process to suppress the occurrence of variations in light intensity on the photoreceptor. Specifically, a correction process to suppress variations in light intensity on the photoreceptor is performed based on a correction signal created using a dot-dispersion type matrix that allows for high-frequency extinguishing than the AM screen type dither matrix used in halftone processing. Therefore, it is possible to perform the correction process without being affected by the low-period dither matrix used in halftone processing, and it is possible to suppress the occurrence of breaks in thin lines and jaggedness of characters. In addition, since a correction profile is maintained for each cause of variations in light intensity, the processing cost due to updating the correction profile can also be suppressed. In other words, it is possible to suppress the occurrence of density unevenness due to variations in light intensity on the photoreceptor.

[0096] In this embodiment, an example was described in which two light intensity corrections are performed after halftone processing, but the number of light intensity corrections is not limited to this. Other correction processes may be added and performed after the second correction process.

[0097] [Embodiment 2] The image forming apparatus according to this embodiment will be described with reference to the figures. However, this embodiment will focus on the differences from Embodiment 1. Figure 19 is a functional block diagram illustrating the configuration of the HT density correction processing unit 306 in the image forming apparatus according to this embodiment.

[0098] In this embodiment, a mode will be described in which correction profiles corresponding to different factors are pre-synthesized and aggregated into one correction process using the synthesized profile. In this embodiment, the synthetic correction value calculation unit 904 for each light-emitting element is replaced by the synthetic profile acquisition unit 916 including the third acquisition unit 907, which is different from the first embodiment. Also, in this embodiment, the correction signal value generation unit is one of the first generation units 905, and the light amount correction unit is one of the first correction processing units 910, which is also different from the first embodiment.

[0099] The HT concentration correction processing unit 306 of this embodiment includes the first correction profile 901, the second correction profile 902, the third correction profile 903, the first generation unit 905, the matrix 906 of the first correction signal, and the first correction processing unit 910. The HT concentration correction processing unit 306 of this embodiment further includes a synthetic profile acquisition unit 916. The synthetic profile acquisition unit 916 includes a third acquisition unit 907, a first acquisition unit 913, a second acquisition unit 914, a first synthetic processing unit 915, and a second synthetic processing unit 917.

[0100] After calculating the first synthetic profile by synthesizing the first and second correction profiles 901 and 902 in the same manner as in the first embodiment, the synthetic profile acquisition unit 916 executes the following processing. That is, the synthetic profile acquisition unit 916 (the second synthetic processing unit 917) generates a second synthetic profile by synthesizing the first synthetic profile and the third correction profile 903. The processing after generating the second synthetic profile is the same as that in the first embodiment, so the description is omitted.

[0101] <Processing Flow of HT Concentration Correction Processing Unit> The processing flow of the HT density correction processing unit 306 in this embodiment will be explained with reference to the figures. Figure 20 is a flowchart showing the image processing flow by the HT density correction processing unit 306 in this embodiment. The processing shown in Figure 20 is achieved by the CPU 208 reading the program stored in the ROM 209, expanding it into the RAM 210, and executing it. In this embodiment, after the processing of S1502 is executed, the processing of S1510, which is different from that in Embodiment 1, is executed. Then, after the processing of S1510, the processing of S1503, S1504, and S1505 is executed, similar to Embodiment 1.

[0102] In S1510, the CPU 208 combines the composite profile calculated in S1502 with the third correction profile 903 obtained in S1607 (details to be described later) to calculate a second composite profile. The calculated second composite profile is stored in the RAM 210. The details of the process in S1510 will be explained using a diagram.

[0103] <Detailed processing flow for synthesis profile acquisition process S1502, S1510> The details of the processing (S1502, S1510) of the composite profile acquisition unit 916 will be explained with reference to the figures. Figure 21 is a flowchart showing the detailed flow of processing by the composite profile acquisition unit 916. S1607 and S1608, which differ from Embodiment 1, will be explained.

[0104] In S1607, the CPU 208 obtains the third correction profile 903 stored in the memory unit 204. In S1608, the CPU 208 performs a second synthesis process using the combined correction value TotalCoeff[x] calculated in S1502 and ProcCoeff_full[x] obtained by converting the third correction profile 903 obtained in S1607 into light-emitting element units. That is, the CPU 208 adds the combined correction value TotalCoeff[x] and the converted correction data ProcCoeff_full[x] for each light-emitting element as shown in equation (3) below to calculate the second combined value TotalCoeff_2nd[x]. In other words, the second combined value TotalCoeff_2nd[x] can also be said to be the synthesis information. Here, x represents the main scan position. The calculation is performed for each main scan position. TotalCoeff[x] and ProcCoeff_full[x] have been converted in advance into 8-bit data for each light-emitting element. The TotalCoeff_2nd[x] is clipped so that it fits within the 8-bit threshold range held in the matrix 906 of the first correction signal. TotalCoeff_2nd[x]=TotalCoeff[x]+ProcCoeff_full[x] ····(3)

[0105] The composite profile calculated in S1502 and the third correction profile 903 represent factors causing variations in light intensity due to different phenomena. In other words, even if one of the conditions changes, the period and amplitude of the variation of the remaining factor remain unaffected. Therefore, the two conditions are added together to form the composite profile.

[0106] As explained above, according to this embodiment, in addition to the influence of the dither matrix used in halftone processing, interference caused by correction processing between different correction profiles can also be suppressed, making it possible to further suppress image quality degradation caused by correction processing for variations in light intensity.

[0107] [Embodiment 3] The image processing apparatus according to this embodiment will be described with reference to the figures. However, this embodiment will focus on the differences from Embodiments 1 and 2. Embodiments 1 and 2 described an example in which a matrix for generating correction signals is commonly maintained for each CMYK color. However, when correction signals are generated using a common matrix for CMYK, the blackout due to correction occurs at the same position, exposing the white of the paper. When the white of the paper is exposed, the blackout due to correction becomes more visible.

[0108] In this embodiment, a method for shifting the position of the unlit pixels for each color will be described. As a method for shifting the position of the unlit pixels for each color, for example, a method may be used in which there are multiple types of matrices with different phases corresponding to each color plate in the printing unit 206, and the matrix being referenced is switched for each color. Alternatively, as a method for shifting the position of the unlit pixels for each color, a processing unit may be provided that shifts the reference start position (reference reference position) of the matrix so that each color has a different phase, and this processing unit shifts the reference start position of the matrix so that each color has a different phase.

[0109] An example of changing the phase by shifting the reference start position will be explained using a diagram. Figure 22 is a diagram for explaining the correspondence between matrix data, HT image data, and thresholds. Figure 22(a) shows the case where the reference start position 2201 is at the position of threshold 16, and Figure 22(b) shows the case where the correspondence between HT image data and thresholds is determined based on the shift amount of the matrix data in Figure 22(a). Figure 22(c) shows the case where the reference start position 2201 is at the position of threshold 32, and Figure 22(d) shows the case where the correspondence between HT image data and thresholds is determined based on the shift amount of the matrix data in Figure 22(c). For the sake of simplicity, the matrix size will be assumed to be 8x8. The reference start position 2201 is set as the origin of the matrix (main scan position = 0, sub scan position = 0), and the data is referenced by arranging it vertically and horizontally and repeating the process based on the shift amount held together with the matrix data.

[0110] By changing the reference starting position 2201, it becomes possible to reference matrices with different phases from the same matrix data, as shown in Figures 22(b) and 22(d).

[0111] As explained above, according to this embodiment, in addition to the influence of the dither matrix used in halftone processing, it is possible to prevent overlapping of pixels that are turned off for each CMYK color, thereby further suppressing image quality degradation due to correction processing for variations in light intensity.

[0112] Furthermore, while embodiments 1 and 2 describe examples where the reference start position 2201 of the matrix for generating the correction signal is always the same, the invention is not limited to this. It is known that LED light-emitting elements 43 degrade and their light output decreases as the illumination time increases. If there is a difference in illumination time among the multiple light-emitting elements on the LED line head 24, degradation progresses locally, causing unevenness. If the reference start position 2201 of the correction signal matrix is ​​always fixed regardless of the number of printed pages, a particular LED light-emitting element 43 is more likely to remain off, making it easy for differences in illumination time to occur. Therefore, a processing unit may be provided that changes the reference start position 2201 of the matrix to a position shifted in the main scanning direction at the timing when either a page or a job changes, such as between pages, at startup, or between jobs.

[0113] Furthermore, the system may have multiple matrices with different phases, and may include a processing unit that changes the referenced matrix when either the page or the job changes.

[0114] [Other embodiments] This disclosure can also be implemented by supplying a program that implements one or more of the functions of the embodiments described above to a system or device via a network or storage medium, and by having one or more processors in the computer of that system or device read and execute the program. It can also be implemented by a circuit (e.g., an ASIC) that implements one or more functions. [Explanation of symbols]

[0115] 905 First generation section 908 Second generation section 910 First correction processing unit 911 Second correction processing unit

Claims

1. An electrophotographic image forming apparatus that uses a line head with multiple light-emitting elements arranged as a light source, An acquisition means for acquiring print data of a print target by the electrophotographic image forming apparatus, An execution means for performing halftone processing on the print data acquired by the acquisition means, Multiple correction pieces of information for correcting variations in light intensity on a photoreceptor caused by light irradiated from the multiple light-emitting elements are generated based on a pre-held matrix and a pre-held profile. Correction means for correcting the halftone-processed print data using the correction information, It has, The plurality of correction information includes a first profile for correcting the variation in light intensity caused by the light-emitting element, a second profile for correcting the variation in light intensity caused by individual differences in the current source of the light-emitting element, and a third profile for correcting the variation in light intensity caused by the positional relationship between the line head and the photoreceptor. The aforementioned matrix holds multiple matrices with different phases, and the referenced matrix changes when either the page or the job changes. An image forming apparatus characterized by the following features.

2. The correction information has a dot period that is finer than the dot period generated in the halftone processed print data. The image forming apparatus according to feature 1.

3. The aforementioned plurality of correction information holds correction values ​​for determining the probability of turning off the plurality of light-emitting elements in different units. The image forming apparatus according to feature 1 or 2.

4. The system further includes an update means for performing an update on at least one of the aforementioned multiple correction pieces of information. The image forming apparatus according to any one of claims 1 to 3.

5. The update means executes the update at different timings for each of the multiple correction information. The image forming apparatus according to feature 4.

6. The timing of the update is either a calibration instruction from the user or a change in the state of the line head. The image forming apparatus according to feature 5.

7. The image forming apparatus according to claim 1, characterized in that the reference reference position that refers to the matrix is ​​changed at the timing when any one of the toner color, page, or job changes.

8. The aforementioned matrix holds multiple matrices with different phases, and the referenced matrix changes when any one of the toner color, page, or job changes. The image forming apparatus according to feature 1.

9. A control method for an electrophotographic image forming apparatus that uses a line head with multiple light-emitting elements arranged as a light source, The acquisition step involves acquiring print data for a print target using the electrophotographic image forming apparatus, An execution step in which halftone processing is performed on the print data acquired in the acquisition step, A generation process for generating multiple correction information for correcting variations in light intensity on a photoreceptor caused by light irradiated from the multiple light-emitting elements, based on a pre-held matrix and a pre-held profile. A correction step of correcting the halftone-processed print data using the correction information, Includes, The plurality of correction information includes a first profile for correcting the variation in light intensity caused by the light-emitting element, a second profile for correcting the variation in light intensity caused by individual differences in the current source of the light-emitting element, and a third profile for correcting the variation in light intensity caused by the positional relationship between the line head and the photoreceptor. The aforementioned matrix holds multiple matrices with different phases, and the referenced matrix changes when either the page or the job changes. A control method for an image forming apparatus, characterized by the following:

10. A program for causing a computer to function as each means of an image forming apparatus according to any one of claims 1 to 8.

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