Image forming apparatus

Abstract

To provide an image forming apparatus that can prevent fluctuation of image density due to a control delay.SOLUTION: An image forming apparatus 2 performs a plurality of types of image processing on image data representing an image according to an attribute of the image, and forms the image on a recording material S on the basis of the image data on which the image processing are performed. The image forming apparatus 2 comprises: an image forming unit 50y that forms an image on a photoconductor drum 51y; and a reflected light quantity sensor 12y that detects image density of an image for measurement according to an attribute for measuring density of the image formed on the photoconductor drum 51y. The image forming apparatus corrects the image data on the basis of a result of detection of the image density of the image for measurement performed by the reflected light quantity sensor 12y, and causes the image forming unit 5y to form an image based on the corrected image data. The image for measurement is formed according to a frequency set by an operation unit 20 in correspondence with the plurality of types of image processing.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to image forming apparatuses such as copiers, printers, and facsimile apparatuses using an electrophotographic method or an electrostatic recording method.

Background Art

[0002] An image forming apparatus has a function of correcting the image density of an image to be formed. When the image forming apparatus outputs a color image, it also has a function of performing gradation correction (γ correction) for stabilizing gradations from the highlight density range to the shadow density range, not only the maximum density. Detection of the gradation property of the image forming apparatus is performed by forming a toner image of a specific halftone pattern on an image carrier and measuring the amount of reflected light of the light irradiated on this halftone pattern by a reflected light amount sensor. Based on the amount of reflected light measured by the reflected light amount sensor, a gradation (γ correction) is obtained that is estimated to obtain an image of a predetermined image density (amount of reflected light).

[0003] An optical sensor including a light emitting element and a light receiving element is used as the reflected light amount sensor. The reflected light amount sensor uses infrared light. By using infrared light, it becomes possible to detect the amount of toner on the image carrier regardless of the color of the toner. The amount of infrared light detected by the reflected light amount sensor has a correlation that is substantially proportional or inversely proportional to the amount of attached toner. Since there is a correlation between the amount of attached toner and the image density of the output image, the image density of the output image can be estimated from the measurement value by the reflected light amount sensor.

[0004] It is preferable that toner quantity detection be performed without interrupting the image forming operation. The image forming apparatus detects the toner quantity by forming a halftone pattern outside the image forming area, for example, between images on each page (so-called paper spaces). However, since the area outside the image forming area is limited, multiple halftone patterns are formed by dividing them between multiple paper spaces. Patent Document 1 discloses a method for controlling image density. In this control method, the image forming apparatus forms a halftone pattern outside the image forming area by dividing it into multiple steps. The image forming apparatus measures the halftone pattern with a reflected light intensity sensor and detects the amount of toner deposited (image density) based on the measurement result. Based on the detection result of the deposited toner quantity, the image forming apparatus corrects the data used for image density control and gradation control. As a result, even when continuously forming images, the image forming apparatus can form multiple halftone patterns and correct the image density and gradation without interrupting the process. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2001-109219 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] Figure 13 is an explanatory diagram illustrating the formation timing of a conventional halftone pattern and image formed in multiple stages. The numbers (No.) in the figure represent the type of image processing used to reproduce the midtone image (for example, the number of patterns used in dithering). Depending on the type of image processing, an image density detection pattern 101, which is a halftone pattern, is prepared. The image density detection pattern 101 corresponding to the image processing corresponding to each number is formed at the timing between sheets of paper.

[0007] Figure 13(a) shows the case where there is one type of image processing and the halftone pattern is formed by dividing it. Figure 13(b) shows the case where there are multiple types of image processing (two types in this case) and the halftone pattern is formed by dividing it. When there are multiple types of image processing, the formation frequency of the image density detection pattern 101 for each type of image processing decreases, and the execution interval becomes longer. As a result, there is a possibility that image density fluctuations may occur due to control delay.

[0008] In view of the above-mentioned problems, the primary objective of the present invention is to provide an image forming apparatus that can suppress image density fluctuations due to control delay. [Means for solving the problem]

[0009] The present invention relates to an image forming apparatus that performs a plurality of types of image processing on image data representing an image to be formed on a recording material according to the attributes of the image to be formed on the recording material, and forms an image on the recording material based on the image processed image data, comprising: an image forming means for forming an image on an image carrier; a transfer means for transferring the image formed on the image carrier to the recording material; a detection means for detecting the image density of a measurement image according to the attributes for measuring the image density formed on the image carrier; a control means for correcting the image data based on the detection result of the image density of the measurement image by the detection means and causing the image forming means to form an image based on the corrected image data; and an input means, wherein the control means causes the image forming means to form the measurement image according to a frequency set by the input means in correspondence with the plurality of types of image processing. Another image forming apparatus of the present invention is an image forming apparatus that performs a plurality of types of image processing on image data representing an image to be formed on a recording material according to the attributes of the image to be formed on the recording material, and forms an image on the recording material based on the image processed image data, comprising: an image forming means for forming an image on an image carrier; a transfer means for transferring the image formed on the image carrier to the recording material; a detection means for detecting the image density of a measurement image according to the attributes for measuring the image density formed on the image carrier; and a control means for correcting the image data based on the detection result of the image density of the measurement image by the detection means, and causing the image forming means to form an image based on the corrected image data, wherein the control means causes the image forming means to form the measurement image according to a frequency set in correspondence with the plurality of types of image processing, and the control means changes the frequency according to the image amount for each attribute of the image based on the image data. [Effects of the Invention]

[0010] According to the present invention, fluctuations in image density due to control delay can be suppressed. [Brief explanation of the drawing]

[0011] [Figure 1] Diagram of the image forming system. [Figure 2] Diagram illustrating the formation position of the pattern for image density detection. [Figure 3] Diagram illustrating the signal processing unit. [Figure 4] A graph showing the relationship between image density and output signal. [Figure 5] An illustrative diagram showing the relationship between the input image density and the output image density of the resulting image. [Figure 6] A flowchart illustrating the image formation process. [Figure 7] (a) and (b) are illustrative diagrams of the setting screen for the automatic tone correction frequency. [Figure 8] A flowchart illustrating the process for determining the formation frequency of patterns used for image density detection. [Figure 9] A flowchart illustrating image formation processes that include correction using multiple types of LUTs. [Figure 10] Flowchart representing image formation processing. [Figure 11] Graph exemplifying image density transition. [Figure 12] Flowchart representing the process of determining a screen for performing image density correction. [Figure 13] (a) and (b) are explanatory diagrams of a halftone pattern formed by being divided into multiple times and the image formation timing.

Mode for Carrying Out the Invention

[0012] Hereinafter, preferred embodiments of this invention will be exemplarily and detailedly described with reference to the drawings.

[0013] (First Embodiment) FIG. 1 is a configuration diagram of the image forming system of the present embodiment. The image forming system 1 of the present embodiment includes an image forming apparatus 2, an operation unit 20, and a reader 8. The image forming apparatus 2 is a tandem type full-color image forming apparatus adopting an intermediate transfer method that can form a full-color image by an electrophotographic method. However, the image forming apparatus 2 is not limited to the tandem type and may be other methods. Further, the image forming apparatus 2 is not limited to an image forming apparatus capable of forming a full-color image and may be capable of forming only monochrome (black and white or monocolor) images. The image forming apparatus 2 can be specified for various uses such as a printer, various printing machines, a copying machine, a facsimile apparatus, a multifunction machine, etc.

[0014] The operation unit 20 is provided on the upper part of the main body 10 of the image forming apparatus 2. The operation unit 20 is a user interface including an input interface and an output interface. The operation unit 20 of the present embodiment is a liquid crystal operation panel with a touch sensor (touch panel display) and is used for inputting a print job and various settings of the image forming system 1. When making various settings, a setting change screen for "automatic gradation correction frequency" described later is used. A user or an operator can input various conditions such as the type of image to be formed, the number of printed sheets, the automatic gradation correction frequency, etc. using the operation unit 20.

[0015] The reader 8 is an image reading device including an image reading unit 80, an ADF (Auto Document Feeder) 81, and a platen glass 82. The reader 8 reads an image from a document continuously conveyed by the ADF 81 or a document placed on the platen glass 82 by the image reading unit 80. The reader 8 transmits the reading result of the document image to the image forming apparatus 2. For example, when the image forming system 1 performs a copying process, the reader 8 reads an image from a document, and the image forming apparatus 2 prints the reading result of the document image by the reader 8 on a recording material S.

[0016] The image forming apparatus 2 includes a paper feeding unit 4, an image forming unit 5, and a control unit 30. In the present embodiment, a case where there is one paper feeding unit 4 will be described, but a plurality of paper feeding units 4 may be provided. The paper feeding unit 4, the image forming unit 5, and the control unit 30 are provided in the main body 10 of the image forming apparatus 2.

[0017] The image forming unit 5 forms an image based on image data on a recording material S fed from the paper feeding unit 4 and conveyed through a conveyance path L. The image forming unit 5 has image forming units 50y, 50m, 50c, 50k, toner bottles 41y, 41m, 41c, 41k, exposure units 42y, 42m, 42c, 42k, an intermediate transfer unit 44, a secondary transfer unit 45, and a fixing unit 46.

[0018] The image forming unit 50y forms a yellow (y) image. The image forming unit 50m forms a magenta (m) image. The image forming unit 50c forms a cyan (c) image. The image forming unit 50k forms a black (k) image. For elements having the same or corresponding functions or configurations provided corresponding to these four image forming units 50y, 50m, 50c, 50k, the y, m, c, k at the end of the symbol indicating that it is an element for any color may be omitted and described collectively. Note that the image forming apparatus 2 can form a monochromatic or multi-color image such as a black monochromatic image using the image forming units 50 for some of the monochromatic or four colors.

[0019] The image forming unit 50 includes a photosensitive drum 51, which is a drum-type (cylindrical) photoreceptor (electrophotographic photoreceptor), a charging roller 52, which is a roller-type charging member, a developer 40, a pre-exposure unit 54, and a cleaning blade 55, which is a cleaning member. The image forming unit 50 forms a toner image on the intermediate transfer belt 44b, which will be described later. The image forming unit 50 is integrated as a process cartridge and is detachable from the main body 10 of the image forming apparatus 2. A reflected light intensity sensor 12, used to detect the image density of the toner image formed on the photosensitive drum 51, is positioned near the photosensitive drum 51.

[0020] The photosensitive drum 51 is a first image carrier on which an electrostatic image (electrostatic latent image) or toner image is formed. The photosensitive drum 51 is rotatable around the drum axis. In this embodiment, the photosensitive drum 51 is a negatively charged organic photoreceptor (OPC) with an outer diameter of 30 mm. When the image forming operation is started, the photosensitive drum 51 is driven to rotate at a predetermined process speed (peripheral speed) in the direction of the arrow in the figure (counterclockwise) by a drive source (not shown).

[0021] The surface of the rotating photosensitive drum 51 is uniformly charged by the charging roller 52. In this embodiment, the charging roller 52 is a rubber roller that contacts the surface of the photosensitive drum 51 and rotates in conjunction with the rotation of the photosensitive drum 51. A charging bias power supply (not shown) is connected to the charging roller 52. The charging bias power supply applies a charging bias to the charging roller 52 during the charging process.

[0022] The surface of the charged photosensitive drum 51 is scanned and exposed by the exposure unit 42 based on image data. Scanning exposure forms an electrostatic image on the photosensitive drum 51. In this embodiment, the exposure unit 42 is a laser scanner. The exposure unit 42 modulates and emits laser light based on the separated color image data output from the control unit 30, and scans and exposes the surface of the photosensitive drum 51.

[0023] The electrostatic image formed on the photosensitive drum 51 is developed by the toner supplied by the developer 40. The development of the electrostatic image forms a toner image on the photosensitive drum 51. In this embodiment, the developer 40 contains a two-component developer (also simply called "developer") comprising non-magnetic toner particles (toner) and magnetic carrier particles (carrier). Toner is supplied to the developer 40 from the toner bottle 41.

[0024] The developing unit 40 has a developing sleeve 24. The developing sleeve 24 is made of a non-magnetic material such as aluminum or non-magnetic stainless steel (aluminum in this embodiment). Inside the developing sleeve 24, a magnetic roller, which is a roller-shaped magnet, is fixed to the body of the developing unit 40 so as not to rotate. The developing sleeve 24 carries the developer and transports it to the developing area facing the photosensitive drum 51. A developing bias power supply (not shown) is connected to the developing sleeve 24. The developing bias power supply applies a developing bias to the developing sleeve 24 during development. In this embodiment, the normal charging polarity of the toner, which is the charging polarity of the toner during development, is negative.

[0025] The intermediate transfer unit 44 is positioned opposite four photosensitive drums 51y, 51m, 51c, and 51k. The intermediate transfer unit 44 has an intermediate transfer belt 44b, which is an endless belt serving as a second image carrier. The intermediate transfer belt 44b is wrapped around several rollers, including a drive roller 44a, a driven roller 44d, primary transfer rollers 47y, 47m, 47c, and 47k, and a secondary transfer inner roller 45a. The intermediate transfer belt 44b is rotatable and carries the toner image.

[0026] The drive roller 44a is rotationally driven by a drive source such as a motor (not shown) to rotate the intermediate transfer belt 44b. The driven roller 44d is a tension roller that controls the tension of the intermediate transfer belt 44b to a constant level. The driven roller 44d is biased by a spring (not shown) or the like to push the intermediate transfer belt 44b toward its outer surface. The driven roller 44d applies a tension of approximately 2 to 5 kgf to the intermediate transfer belt 44b in the process direction. The secondary transfer inner roller 45a constitutes the secondary transfer section 45, as will be described later. The intermediate transfer belt 44b is driven by the drive roller 44a to rotate in the direction of the arrow in the figure (clockwise) at a predetermined peripheral speed corresponding to the peripheral speed of the photosensitive drum 51. The intermediate transfer unit 44 has a belt cleaner 60 near the drive roller 44a, positioned opposite the intermediate transfer belt 44b.

[0027] The primary transfer rollers 47y, 47m, 47c, and 47k are roller-type primary transfer members. The primary transfer rollers 47y, 47m, 47c, and 47k are positioned opposite the corresponding photosensitive drums 51y, 51m, 51c, and 51k, respectively. The primary transfer roller 47 clamps the intermediate transfer belt 44b between itself and the corresponding photosensitive drum 51. As a result, the intermediate transfer belt 44b comes into contact with the photosensitive drum 51, forming a primary transfer section (primary transfer nip section) 48 between itself and the photosensitive drum 51.

[0028] The toner image formed on the photosensitive drum 51 is transferred to the intermediate transfer belt 44b in the primary transfer unit 48 by the action of the primary transfer roller 47. In this embodiment, a positive primary transfer voltage is applied to the primary transfer roller 47, thereby transferring the negatively polarized toner image on the photosensitive drum 51 to the intermediate transfer belt 44b. For example, when forming a full-color image, the toner images formed on each of the photosensitive drums 51y, 51m, 51c, and 51k are transferred to the intermediate transfer belt 44b in such a way that they are sequentially superimposed. A primary transfer power supply (not shown) is connected to the primary transfer roller 47. During transfer, the primary transfer power supply applies a DC voltage to the primary transfer roller 47 as a primary transfer bias, which is the opposite polarity to the charging polarity of the toner (positive polarity in this embodiment). A voltage detection sensor for detecting the output voltage and a current detection sensor for detecting the output current are connected to the primary transfer power supply. In this embodiment, four primary transfer power supplies are provided, corresponding to each of the primary transfer rollers 47y, 47m, 47c, and 47k. Therefore, the primary transfer voltages applied to the primary transfer rollers 47y, 47m, 47c, and 47k can be controlled individually.

[0029] The surface of the photosensitive drum 51 is electrostatically discharged by the pre-exposure unit 54 after the toner image has been transferred. Toner that remains on the photosensitive drum 51 without being transferred to the intermediate transfer belt 44b during the transfer process is removed from the surface of the photosensitive drum 51 by the cleaning blade 55 and collected in a collection container (not shown). The cleaning blade 55 is a plate-shaped member that contacts the photosensitive drum 51 with a predetermined pressing force. The tip of the cleaning blade 55 on its free end is in a counter-direction, facing upstream in the rotational direction of the photosensitive drum 51, and is in contact with the surface of the photosensitive drum 51.

[0030] The intermediate transfer belt 44b of this embodiment has a two-layer structure consisting of a base layer and a surface layer from the inner circumferential surface side. Suitable materials for the base layer include resins such as polyimide and polycarbonate, or various types of rubber, with an appropriate amount of carbon black added as an antistatic agent. For the surface layer, a base material can be one type of resin material such as polyurethane, polyester, or epoxy resin, or two or more elastic materials such as elastic rubber, elastomer, or butyl rubber. Note that the intermediate transfer belt 44b is not limited to a two-layer structure; it may also have a single-layer structure with the base layer described above.

[0031] On the outer circumferential surface of the intermediate transfer belt 44b, a secondary transfer outer roller 45b is positioned opposite the secondary transfer inner roller 45a and, together with the secondary transfer inner roller 45a, constitutes the secondary transfer section 45. The secondary transfer outer roller 45b contacts the intermediate transfer belt 44b and forms a secondary transfer section (secondary transfer nip section) 45n between itself and the intermediate transfer belt 44b. The secondary transfer outer roller 45b sandwiches the intermediate transfer belt 44b between itself and the secondary transfer inner roller 45a. The toner image formed on the intermediate transfer belt 44b is secondary transferred onto the recording material S in the secondary transfer section 45n by the action of the secondary transfer section 45. In this embodiment, by applying a positive polarity secondary transfer voltage to the secondary transfer outer roller 45b, the negative polarity toner image on the intermediate transfer belt 44b is secondary transferred onto the recording material S, which is being transported sandwiched between the intermediate transfer belt 44b and the secondary transfer outer roller 45b. The recording material S is fed from the paper feed unit 4 in parallel with the toner image formation operation described above, and is transported to the secondary transfer unit 45n by the register roller 11 provided on the transport path L, in timing with the toner image on the intermediate transfer belt 44b. Any toner, paper dust, or other adhering materials that remain on the intermediate transfer belt 44b without being transferred to the recording material S during secondary transfer are removed from the surface of the intermediate transfer belt 44b by the belt cleaner 60 and collected.

[0032] As described above, the secondary transfer unit 45 has a secondary transfer inner roller 45a as an opposing member and a secondary transfer outer roller 45b which is a roller-type secondary transfer member. In this embodiment, the secondary transfer outer roller 45b is constructed by covering the periphery of a metal core with an elastic layer of ion-conductive foamed rubber. A secondary transfer power supply (not shown) is connected to the secondary transfer outer roller 45b. During secondary transfer, the secondary transfer power supply applies a DC voltage to the secondary transfer outer roller 45b as a secondary transfer bias, which is the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner. The metal core of the secondary transfer inner roller 45a is connected to ground potential. When recording material S is supplied to the secondary transfer unit 45n, a constant voltage controlled secondary transfer voltage with the opposite polarity to the normal charging polarity of the toner is applied to the secondary transfer outer roller 45b.

[0033] In this embodiment, the secondary transfer inner roller 45a is grounded and a voltage is applied to the secondary transfer outer roller 45b from the secondary transfer power supply. However, conversely, a voltage may be applied to the secondary transfer inner roller 45a from the secondary transfer power supply and the secondary transfer outer roller 45b may be grounded. In this case, a DC voltage with the same polarity as the normal charging polarity of the toner is applied to the secondary transfer inner roller 45a.

[0034] The recording material S onto which the toner image has been transferred is transported to the fuser 46 by the secondary transfer outer roller 45b. The fuser 46 has a fuser roller 46a and a pressure roller 46b. The fuser roller 46a has a built-in heater as a heat source. The recording material S, which carries the unfixed toner image, is heated and pressurized by being sandwiched and transported between the fuser roller 46a and the pressure roller 46b. As a result, the toner image is fixed (melted and solidified) onto the recording material S. In this way, an image is formed on the recording material S. The temperature of the fuser roller 46a (fixing temperature) is detected by a sensor (not shown). The control unit 30 controls the temperature of the fuser roller 46a to a predetermined temperature based on the detection result of the fixing temperature.

[0035] In the case of single-sided printing, the recording material S on which the image has been formed is output directly from the transport unit 6 to the paper output unit 9. In the case of double-sided printing, the recording material S on which the image has been formed on one side is transported to the inversion transport path 7. In the inversion transport path 7, the recording material S on which the image has been formed on the first side is turned over and transported again to the secondary transfer unit 45n. The recording material S transported from the inversion transport path 7 to the secondary transfer unit 45n has the toner image transferred to the second side, which is the opposite side from the first side, and after the toner image is fixed by the fuser unit 46, it is output from the transport unit 6 to the paper output unit 9. In this way, the image forming apparatus 2 of this embodiment is capable of automatic double-sided printing, in which an image is formed on both sides of a single recording material S.

[0036] Figure 2 is an explanatory diagram of the formation position of the image density detection pattern 101. In Figure 2, the vicinity of the photosensitive drum 51y and the intermediate transfer belt 44b is shown in magnified view. In this embodiment, the image density detection pattern 101 is formed outside the image formation area, which is sandwiched between the leading edge of the rear image 102b and the trailing edge of the forward image 102a, among the continuous images formed on the surface of the photosensitive drum 51y during continuous printing. The image density detection pattern 101 includes a halftone pattern and is a measurement image for measuring image density. Note that the area outside the image formation area is not limited to the region sandwiched between the leading edge of the rear image 102a and the trailing edge of the forward image 102b.

[0037] For example, depending on the size of the image to be formed, the image density detection pattern 101 may be formed in an area such as the side of the image. However, if the image for the job is wide, such as an image for a 13 x 19 inch paper size, it is necessary to provide an image-forming area wider than the paper width in order to form the image density detection pattern 101 outside the paper. This increases the size of the main body 10, increasing the installation area of ​​the image forming system 1 and increasing the cost of each part. Therefore, in many cases, as shown in this embodiment, it is preferable to form the image density detection pattern 101 near the center in the depth direction and outside the image forming area between the leading edge of image 102b and the trailing edge of image 102a.

[0038] The amount of toner (image density) adhering to the image density detection pattern 101 is detected by measuring the amount of reflected light from the image density detection pattern 101 with a reflected light intensity sensor 12y. The reflected light intensity sensor 12y is an optical sensor consisting of a light-emitting element and a light-receiving element. The light-emitting element irradiates light onto the photosensitive drum 51y. The light-receiving element detects only the specularly reflected light from the photosensitive drum 51y that is irradiated from the light-emitting element. The reflected light intensity sensor 12y measures the amount of reflected light from the image density detection pattern 101 in this way. The reflected light intensity sensor 12y is positioned between the development position by the developer unit 40y and the primary transfer unit 48y in a location where the amount of toner can be measured. The amount of reflected light is measured at the timing when the image density detection pattern 101, formed outside the image formation area on the photosensitive drum 51y, passes through the measurement range of the reflected light intensity sensor 12y. Based on the amount of reflected light, a yellow gradation correction (γ correction) is obtained, which is estimated to yield a predetermined constant image density (amount of reflected light).

[0039] Similarly, for the other photosensitive drums 51m, 51c, and 51k, the reflected light intensity sensors 12m, 12c, and 12k are positioned to measure the amount of toner between the development position by the developer units 40m, 40c, and 40k and the primary transfer unit 48m, 48c, and 48k. The image density of the image (toner image) formed on the photosensitive drums 51m, 51c, and 51k is detected by the reflected light intensity sensors 12m, 12c, and 12k. Based on the detected image density (reflected light intensity), magenta, cyan, and black gradation correction (gamma correction) is obtained, which is estimated to yield a predetermined constant image density (reflected light intensity).

[0040] Figure 3 is an explanatory diagram of the signal processing unit that processes the detection result (output signal) from the reflected light intensity sensor 12. The signal processing unit 70 is included in the control unit 30. The signal processing unit 70 includes an A / D converter 15, a density conversion unit 16, and a CPU (Central Processing Unit) 17. The reflected light intensity sensor 12 receives reflected light (infrared light) from the photosensitive drum 51 and converts the received reflected light (infrared light) into an electrical signal for output. The electrical signal can take a voltage value of, for example, 0 to 5 [V] depending on the amount of reflected light received by the reflected light intensity sensor 12. The electrical signal is input to the A / D converter 15.

[0041] The A / D converter 15 converts the electrical signal acquired from the reflected light intensity sensor 12 into, for example, an 8-bit digital signal. The 8-bit digital signal is transmitted to the density conversion unit 16. The density conversion unit 16 converts the digital signal acquired from the A / D converter 15 into an image density signal. The density conversion unit 16 has a table 16a that shows the relationship between the digital signal corresponding to the reflected light intensity (output signal) and the image density signal (image density). The digital signal is converted into an image density signal according to the table 16a.

[0042] The CPU 17 is connected to the operation unit 20, ROM (Read Only Memory) 18, RAM (Random Access Memory) 19, I / O interface 21, and LUT (Look Up Table) 25. The CPU 17 performs image formation processing by executing the control program stored in the ROM 18. The RAM 19 provides a workspace for the CPU 17 during processing.

[0043] This section describes image density detection and gradation control during continuous printing using the image forming system 1 configured as described above. Figure 4 is a graph showing the relationship between the image density of the image (toner image) formed on the photosensitive drum 51 and the output signal from the reflected light intensity sensor 12. Here, the relationship between image density and the output signal is shown when the image density of the image density detection pattern 101 formed on the photosensitive drum 51 is changed stepwise by the area gradation of each color. When no toner is attached to the photosensitive drum 51, the voltage value of the electrical signal output from the reflected light intensity sensor 12 is set to 5[V] (255 levels in an 8-bit digital signal). As the area coverage rate by each color of toner increases and the image density becomes darker, the voltage value of the electrical signal output from the reflected light intensity sensor 12 decreases.

[0044] The density conversion unit 16 stores a dedicated table 16a for each color, each having these characteristics. Using the dedicated table 16a, the density conversion unit 16 converts the electrical signal (output signal) output from the reflected light intensity sensor 12 into an image density signal for each color. This allows for accurate reading of the image density of the image density detection pattern 101 for each color.

[0045] In this embodiment, the CPU 17 forms an image density detection pattern 101 outside the image formation area during normal image formation according to the print job, detects the image density, and corrects the table data of the LUT 25 as needed. The LUT 25 is a table of correction conditions for setting the writing image halftone density of yellow, cyan, magenta, and black during image formation in order to obtain an output with an appropriate image density for the image data representing the image to be formed. Even when forming the image density detection pattern 101, the CPU 17 sets the appropriate image signal amount using the LUT 25 for the image data amounts of the four colors cyan, magenta, yellow, and black in the image data, just as with normal images.

[0046] The table data of LUT25 used when forming the image density detection pattern 101 is equivalent to the table data used during normal image formation at that time. In other words, the correction results from previous image density correction control are used as the table data of LUT25.

[0047] For example, when the input image density (image density indicated by the image data) of the image density detection pattern 101 is 128 levels and the target value of the output image density is 128, the image data is corrected by the LUT 25 so that there is no discrepancy between the output image density and the target value. However, the image characteristics of the image forming apparatus 2 are unstable and can change at any time. The table data of the LUT 25 is corrected based on the discrepancy ΔD between the input image density and the measurement result (output image density) of the formed image. Figure 5 is an example diagram of the input image density and the output image density of the formed image. The discrepancy ΔD is the difference between the target value obtained from the image density detection pattern 101 formed using the previous LUT 25 and the image density obtained from the image density detection pattern 101 formed using the new LUT 25.

[0048] In this embodiment, a LUT correction table for correcting the shift of LUT 25 is stored in RAM 19 in advance. During control, the CPU 17 calculates a correction amount equal to the shift amount ΔD for all image density signals in the previous LUT correction tables stored in RAM 19. Based on the calculated correction amount, the CPU 17 creates the current LUT correction table by correcting the previous LUT correction table by the shift amount ΔD. This rewriting (correction) of LUT 25 is performed for each color at the timing when the creation of a LUT correction table corresponding to the shift amount ΔD is completed.

[0049] Figure 6 is a flowchart illustrating the image formation process, including the creation of a LUT correction table. This process is performed in conjunction with the normal image formation process for each print job. Figure 6 illustrates the process when an image is formed using a single type of image processing.

[0050] When processing according to the print job is started, the CPU 17 corrects the table data of LUT 25 based on the following equation (1) using the LUT correction table obtained from the previous processing (S21). The CPU 17 sets the corrected table data to LUT 25 (S22). The CPU 17 performs image formation using this LUT 25 (S23). LUT = LUT table + Previous LUT correction table …(1)

[0051] After image formation, the CPU 17 forms an image density detection pattern 101 outside the image formation area (between images) on the photosensitive drum 51, which is the area between the trailing edge of the image and the leading edge of the next image. The CPU 17 controls the reflected light intensity sensor 12 to read the image density detection pattern 101 (S24). Based on the reading result of the image density detection pattern 101, the CPU 17 calculates the deviation amount ΔD between the image density of the image density detection pattern 101 and the target image density (S25). Based on the calculated deviation amount ΔD and the previous LUT correction tables, the CPU 17 creates a new LUT correction table (S26).

[0052] The CPU 17 determines whether to continue the print job (S27). For example, the CPU 17 determines whether to continue the print job based on whether image formation has been completed for all pages specified in the print job. If there are still pages remaining to be image-formed as specified in the print job, the CPU 17 decides to continue the print job. If the print job is to be continued (S27:N), the CPU 17 repeats the processes from S21 onward until the print job is terminated. If the print job is not to be continued (S27:Y), the CPU 17 terminates the print job.

[0053] As described above, Figure 6 illustrates the case where LUT25 is corrected and image formation is performed using one type of image processing. Next, we will explain the case where LUT25 is corrected and image formation is performed using multiple types of image processing.

[0054] The image forming apparatus 2 of this embodiment is capable of forming images from image data that has undergone multiple types of image processing. The attributes of the input image data include image attributes, graphic attributes, text attributes, etc. The image forming apparatus 2 determines the appropriate image processing (dithering) for the image data according to the image attributes. For example, for image attributes, the image forming apparatus 2 uses a relatively low-line dither called a gradation screen to perform image processing that emphasizes the uniformity and stability of halftones. For text attributes, the image forming apparatus 2 uses a relatively high-line dither called a resolution screen to perform image processing that accurately reproduces edges. In this way, the image forming apparatus 2 changes the image processing content (the screen used) based on the image attributes.

[0055] In this embodiment, we will explain using an example where a 189-line dither is used as the grayscale screen for image and graphic attributes, and a 233-line dither is used as the resolution screen for text attributes. In this embodiment, the same number of lines of dither are used for all colors, but this is not particularly limited, and different numbers of lines of dither may be used for each color. In any case, multiple numbers of lines of dither are used for each color.

[0056] Because the area between images (outside the image formation area) on the photosensitive drum 51 is narrow, it is not possible to form all the image density detection patterns 101 for each color image processing in a single area between images. For this reason, the image density detection patterns 101 are divided and formed by distributing them across multiple areas between images (outside the image formation area).

[0057] For example, between each image in the first six images printed in continuous printing, an image density detection pattern 101 is formed by dividing it five times using a 189-line dither, which is a gradation screen used for four-color image attributes and graphic attributes. Next, between each sheet of paper in the sixth and eleventh image prints, the image density detection pattern 101 is formed by dividing it using a 233-line dither, which is a resolution screen used for text attributes. Then, between each sheet of paper in the eleventh through sixteenth image prints, the image density detection pattern 101 is again formed by dividing it using a 189-line dither, which is a gradation screen used for image attributes and graphic attributes. The same operation is then repeated sequentially.

[0058] Furthermore, while it is standard practice to prepare a LUT 25 for each dither (189 lines, 233 lines) and control each independently, independent control for each color is also performed. In other words, in this embodiment, the image forming apparatus 2 has a total of eight LUTs 25 consisting of four colors × two dithers, and forms eight types of image density detection patterns 101. The image forming apparatus 2 calculates each shift amount (density change) Δ?n (where n is a number representing the type of dither which differs for each color) and corrects the LUT 25.

[0059] When multiple dither image density detection patterns 101 are formed sequentially, the control frequency of each color dither decreases by the number of dither types (the frequency is halved in the case of two types). Some users may desire a higher formation frequency of the dither image density detection patterns 101 for images where image density and color stability are important. Therefore, in this embodiment, the formation frequency of the image density detection patterns 101 for each dither is made variable.

[0060] Figure 7 is an example of the setting screen for automatic gradation correction frequency. This setting screen sets the formation frequency of the image density detection pattern 101. This setting screen is displayed on the operation unit 20. The user can input the priority for each dither using this setting screen. The control frequency (formation frequency of the image density detection pattern 101) is determined according to the input priority. For example, in the initial setting state shown in Figure 7(a), the standard button is selected. In the screen shown in Figure 7(b), the gradation screen is set to have priority. In the case of Figure 7(b), for example, out of 5 control cycles, the dither control frequency for the gradation screen is 4 times and the dither control frequency for the resolution screen is 1 time.

[0061] Figure 8 is a flowchart illustrating the process for determining the formation frequency of the image density detection pattern 101. This process is performed prior to the start of the image formation process.

[0062] The CPU 17 determines whether the "Automatic Gradation Correction Frequency" mode setting button has been changed from the default "Standard" setting (S31). If the mode setting button has not been changed (S31:N), the CPU 17 terminates processing. If the mode setting button has been changed (S31:Y), the CPU 17 obtains the control priority for each dither input (S32). The CPU 17 determines the control frequency for each dither based on the obtained priority (S33). The CPU 17 stores the determined control frequency for each dither in the RAM 19.

[0063] Figure 9 is a flowchart representing the image formation process, including correction of multiple types of LUTs 25. The CPU 17 reads the control frequency for each dither (the formation frequency of the image density detection pattern 101) stored in the RAM 19 (S41). Based on the read control frequency for each dither, the CPU 17 sets the order in which to form the image density detection pattern 101 for each dither and each color (S42). The formation order of the image density detection pattern 101 is determined in an order that matches the control frequency for each dither, for example, in the order of yellow, magenta, cyan, and black. The CPU 17 performs the image formation process with LUT correction for each dither (S43) and then terminates this process. Figure 10 is a flowchart representing the image formation process in S43.

[0064] The CPU 17 obtains the number n, which represents the type of image processing to be corrected by the LUT, and the total number of controls for each dither (ALL) (S51). Here, the initial value of the number n is "1". The CPU 17 uses the LUT correction table obtained from the previous control to correct the table data of LUT 25 based on formula (1) (S52). The CPU 17 sets the corrected table data in LUT 25 (S53). The CPU 17 uses this LUT 25 to perform dithering on the image data and form an image according to the print job (S54).

[0065] The CPU 17 forms an image density detection pattern 101 outside the image formation area (between images) on the photosensitive drum 51, which is the region between the trailing edge of the previously formed image and the leading edge of the next image. The CPU 17 reads the image density of the image density detection pattern 101 using the reflected light intensity sensor 12 (S55). The CPU 17 calculates the difference ΔDn between the read image density and the target image density (S56). The CPU 17 creates a LUT correction table using the calculated difference ΔDn (step S57). The creation of the LUT correction table is performed as follows: The CPU 17 calculates the current LUT correction table corresponding to the difference ΔDn by multiplying all image density signals from the previous LUT correction table stored in RAM 19 by ΔDn.

[0066] The CPU 17 determines whether to continue the print job (S58). For example, the CPU 17 determines whether to continue the print job based on whether image formation has been completed for all pages specified in the print job. If there are still pages remaining to be image-formed as specified in the print job, the CPU 17 determines to continue the print job. If the print job is to be continued (S58:N), the CPU 17 determines whether the number n is less than the total number ALL (S59). If the number n is less than the total number ALL (S59:Y), the CPU 17 increments the number n by 1 (S60) and repeats the process from S52 onwards until the print job is terminated. If the number n has reached the total number ALL (S59:N), the CPU 17 sets the number n to its initial value of "1" (S61) and repeats the process from S52 onwards until the print job is terminated. If the print job is not to be continued (S58:Y), the CPU 17 terminates the print job.

[0067] In this way, the CPU 17 determines the control frequency of each dither based on the information input from the operation unit 20. The CPU 17 then performs image formation operations based on the control results of each dither.

[0068] Figure 11 is a graph illustrating the image density changes of an image using dithering of a grayscale screen applied to an image with an image attribute. The solid line a illustrates the image density when the dithering control frequency is changed. The dashed line b illustrates the image density when the dithering control frequency is not changed. It can be seen that the change in image density is less when the dithering control frequency is changed (solid line a) compared to when it is not changed (dashed line b). In this way, the stability of image density of images with image attributes that users consider important is greatly improved. Images with text attributes to which resolution screen dithering is applied may experience large fluctuations in image density. However, since images with text attributes are images of characters, the impact of fluctuations in image density is relatively small.

[0069] Image forming machines, especially those used as commercial printing presses, are often used for different purposes depending on the job. Such image forming machines perform printing jobs that are specialized for images where gradation screens are applied, such as photobooks, where image attributes make up a large portion of the content, and printing jobs that are specialized for images where resolution screens are applied, such as novels, where text attributes make up a large portion of the content. For this reason, the image forming machine of this embodiment is effective when used as a commercial printing press by specifying dithering to suppress variations in image density.

[0070] However, fluctuations in image density of non-priority screens cannot be ignored. To address this, the image density detection result of the resolution screen is estimated from the image density detection result of the gradation screen by comparing the characteristic difference in the output characteristics of the image density of the gradation screen and the resolution screen. This allows for simultaneous correction of the LUT correction tables for both the gradation screen and the resolution screen. The measured result differs from the estimated result when the resolution screen is actually formed as a halftone pattern and detected by the reflected light intensity sensor 12. Therefore, although the accuracy of image density correction depends on the characteristic difference of the screens, it is possible to suppress fluctuations in image density compared to conditions without correction.

[0071] Thus, the image forming system 1 (image forming apparatus 2) of the first embodiment can always obtain highly stable images that meet the user's needs by varying the control frequency according to the type of image processing.

[0072] (Second Embodiment) The configuration of the image forming system 1 in the second embodiment is the same as in the first embodiment, so a description will be omitted. The image forming apparatus 2 included in the image forming system 1 of the second embodiment controls at the optimal frequency at all times by appropriately changing the control frequency according to the amount of each image attribute of the input image data, thereby obtaining a good transition of image density.

[0073] In this embodiment, the image quantity is determined by a video count value obtained by accumulating the image density value of each pixel based on the image data. The CPU 17 calculates the video count value at the timing of forming the image density detection pattern 101 and forms the dither image density detection pattern 101 that has the maximum value of the video count value. The image data has attribute information stored by the application that created the image data. In addition, the image data generated by the reader 8 is assigned attributes by the conventional image area separation process.

[0074] For example, the CPU 17 detects the video count value of the image data being printed at the time it forms the yellow image density detection pattern 101. The video count value of the part of the gradation screen where dithering is performed is set to "152", and the video count value of the part of the resolution screen where dithering is performed is set to "58". In this case, the gradation screen part occupies a large area, and the visual sensitivity of image density changes is high. For this reason, the CPU 17 determines that it is necessary to increase the frequency of image density correction control and executes image density correction control using the yellow gradation screen image density detection pattern 101. When the image density correction control using the image density detection pattern 101 is complete, the CPU 17 resets the accumulated value (video count value) to "0".

[0075] The CPU 17 detects the video count value at the timing of forming the next yellow image density detection pattern 101. If the detected video count value is "26" on the grayscale screen and "84" on the resolution screen, the CPU 17 performs image density correction control using the yellow image density detection pattern 101 on the resolution screen. Similar processing is performed for other colors besides yellow.

[0076] If there is no significant difference between the video count value of the grayscale screen and the video count value of the resolution screen, the CPU 17 determines that the two screens have equivalent visual sensitivity to image density changes. In this case, the CPU 17 performs image density correction control by alternately using the two types of screen processing.

[0077] Figure 12 shows a flowchart representing the image processing required to create the image density detection pattern 101, specifically the process of determining the screen on which image density correction will be performed. If the video count value of the grayscale screen is "A" and the video count value of the resolution screen is "B", the difference ratio C (in units: %) of the video count values ​​is calculated using the following formula (2). C=|A‐B| / ((A+B) / 2)×100 …(2)

[0078] The CPU 17 performs image density correction control by alternately using the gradation screen and the resolution screen at equal frequency as long as the difference ratio C (%) of the video count value is less than or equal to the threshold (in this case, 10%) (C ≤ 10%) (S71:Y). The CPU 17 determines whether the previous image density correction control was performed using the gradation screen (S72). If the previous image density correction control was performed using the gradation screen (S72:Y), the CPU 17 decides to perform the current image density correction control using the resolution screen (S73). If the previous image density correction control was not performed using the gradation screen (S72:N), the CPU 17 decides to perform the current image density correction control using the gradation screen (S75). Having determined which screen to use, the CPU 17 resets the video count value to "0" (S77).

[0079] For example, if the video count value A of the grayscale screen is "205" and the video count value B of the resolution screen is "194", and the difference ratio C is 6% (≤ threshold 10%), the CPU 17 determines which screen was used for the previous image density correction control. If the previous image density correction control was performed using the grayscale screen, the CPU 17 performs the current image density correction control using the resolution screen and resets the video count value to "0". If the difference ratio C of the video count value is within the threshold at the timing of the formation of the next image density detection pattern, the CPU 17 performs image density correction control using the grayscale screen.

[0080] If the difference ratio C(%) of the video count values ​​is greater than the threshold (10%) (C>10%) (S71:N), the CPU 17 compares the video count value "A" of the gradation screen with the video count value "B" of the resolution screen (S74). If the video count value "A" of the gradation screen is greater than the video count value "B" of the resolution screen (A>B) (S74:Y), the CPU 17 decides to perform the image density correction control on the gradation screen (S75). If the video count value "A" of the gradation screen is less than or equal to the video count value "B" of the resolution screen (A≦B) (S74:N), the CPU 17 decides to perform the image density correction control on the resolution screen (S76). Having determined the screen, the CPU 17 resets the video count value to "0" (S77).

[0081] The image forming apparatus 2 of the second embodiment can increase the control frequency of dither images, which are used more frequently in the overall image. As a result, a highly stable image can be obtained from an overall perspective. In the second embodiment, the screen type used for image density correction was determined for each color. However, by performing image density correction control for all four colors—yellow, magenta, cyan, and black—by comprehensively determining the screen type for all four colors, it is possible to prevent phenomena such as color shifts in any one of the four colors.

[0082] In the first and second embodiments, each time the image density of the image density detection pattern 101 formed by a specific image processing (dithering) is detected, only the LUT corresponding to that specific dithering is rewritten. In the first embodiment, the image density of other screens is estimated using the characteristic difference for each screen. Alternatively, the LUT corresponding to other ditherings may also be rewritten by multiplying the LUT corresponding to a specific dithering by a predetermined coefficient (feedback rate). For example, if the shift amount ΔD as a result of the dithering patch on the grayscale screen is "15", the LUT correction table for that screen is rewritten and corrected by an amount equivalent to the shift amount ΔD value "15". Furthermore, the LUT correction table for the resolution screen, which is another dithering, is rewritten by multiplying the shift amount ΔD value "15" by 0.8 (feedback coefficient). This improves the responsiveness of image density control.

Claims

1. An image forming apparatus that corrects image density using a detection image formed by performing image processing, image processing means for performing image processing on image data; a conversion means for converting the image data based on a conversion condition corresponding to the type of image processing to be performed on the image data by the image processing means; an image forming unit that forms an image on an image carrier based on the image data converted by the conversion unit; a transfer means for transferring the image formed on the image carrier onto a recording material; a detection means for detecting the detection image formed on the image carrier; a control means for controlling the image forming means to continuously form images of a plurality of pages on the image carrier in accordance with a print job, and for controlling the image forming means to form the detection image in an area between a rear end of an image corresponding to a preceding page formed on the image carrier and a front end of an image corresponding to a page subsequent to the preceding page; a generating means for generating a first conversion condition corresponding to the first type of image processing based on a detection result by the detecting means of a first detection image of N gradations that has been subjected to a first type of image processing, and for generating a second conversion condition corresponding to the second type of image processing based on a detection result by the detecting means of a second detection image of N gradations that has been subjected to a second type of image processing different from the first type, the control means has a first mode in which a group of first detection images of N gradations by the first type of image processing and a group of second detection images of N gradations by the second type of image processing are formed at least once each in the M number of the intervening regions, and a second mode in which the number of times the group of first detection images is formed in the M number of the intervening regions is different from the number of times the group of first detection images is formed in the M number of the intervening regions in the first mode, the control means causes the image forming means to form the detection image based on a mode selected from a plurality of modes including the first mode and the second mode. Image forming device.

2. The number of times that the group of the first detection images is formed in the M number of the intervening regions in the second mode is greater than the number of times that the group of the first detection images is formed in the M number of the intervening regions in the first mode.

2. The image forming apparatus according to claim 1.

3. comprising an input means for setting which of the first type of image processing and the second type of image processing is to be prioritized; the control means, in the second mode, sets a number of times that a group of detection images is formed by the image processing set to be prioritized more than a number of times that a group of other detection images is formed.

2. The image forming apparatus according to claim 1.

4. The image processing device further includes a display on which a setting screen for setting whether the first type of image processing or the second type of image processing is to be prioritized is displayed; The input means selects a priority image processing from the setting screen, The control means sets the image processing selected from the setting screen by the input means so as to have priority.

4. The image forming apparatus according to claim 3.

5. the control means determines the number of times the first detection image group and the number of times the second detection image group are formed in accordance with an image amount for each attribute of the image data.

2. The image forming apparatus according to claim 1.

6. The control means calculates a video count value by accumulating image density values ​​for each pixel based on the image data, and determines the number of times to form the first detection image group and the number of times to form the second detection image group based on the calculated video count value.

2. The image forming apparatus according to claim 1.

Images