JP2023135437A - Image forming apparatus - Google Patents

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

The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile machine using an electrophotographic method or an electrostatic recording method.

The image forming apparatus has a function of correcting the image density of the image to be formed. When outputting a color image, the image forming apparatus has a function to perform not only maximum density but also gradation correction (γ correction) to stabilize the gradation from the highlight density area to the shadow density area. . The gradation of an image forming device is detected by forming a toner image of a specific halftone pattern on an image carrier and measuring the amount of light reflected from the halftone pattern using a reflected light amount sensor. be exposed. Based on the amount of reflected light measured by the amount of reflected light sensor, a gradation (γ correction) that is estimated to provide an image with a predetermined image density (amount of reflected light) is obtained.

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, the amount of toner on the image carrier can be detected regardless of the color of the toner. The amount of infrared light detected by the reflected light amount sensor has a correlation that is approximately proportional or inversely proportional to the amount of attached toner. Since there is a correlation between the amount of adhered toner and the image density of the output image, the image density of the output image can be estimated from the measured value by the reflected light amount sensor.

It is preferable that the amount of toner be detected without interrupting the image forming operation. The image forming apparatus detects the amount of toner by forming a halftone pattern outside the image forming area, for example, between images on each page (so-called between sheets). However, since the area outside the image forming area is limited, a plurality of halftone patterns are divided and formed between a plurality of sheets. Patent Document 1 discloses a method for controlling image density. In this control method, the image forming apparatus forms a halftone pattern in multiple steps outside the image forming area. The image forming apparatus measures the halftone pattern with a reflected light amount sensor, and detects the amount of attached toner (image density) based on the measurement result. The image forming apparatus corrects data used for image density control and gradation control based on the detection result of the amount of attached toner. As a result, even when forming images continuously, the image forming apparatus can form multiple halftone patterns and correct image density and gradation without interrupting processing. .

Japanese Patent Application Publication No. 2001-109219

FIG. 13 is an explanatory diagram of the conventional halftone pattern and image formation timing that are formed in multiple steps. The numbers in the figure represent the type of image processing (for example, the number of patterns used in dither processing) used as a method for reproducing a halftone image. An image density detection pattern 101, which is a halftone pattern, is prepared depending on the type of image processing. Image density detection patterns 101 corresponding to the image processing corresponding to each number are formed at the timing between sheets.

FIG. 13A shows a case where there is only one type of image processing and a halftone pattern is divided and formed. FIG. 13B shows a case where there are multiple types of image processing (here, two types) and a halftone pattern is divided and formed. When there are multiple types of image processing, the frequency of formation of the image density detection pattern 101 for each type of image processing decreases, and the execution interval becomes longer. Therefore, image density fluctuations may occur due to control delay.

SUMMARY OF THE INVENTION In view of the above-mentioned problems, a main object of the present invention is to provide an image forming apparatus that can suppress fluctuations in image density due to control delays.

The image forming apparatus of the present invention performs a plurality of types of image processing on image data representing an image according to attributes of the image to be formed on a recording material, and forms an image on the recording material based on the image processed image data. An image forming apparatus 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 onto the recording material; a detection means for detecting the image density of the measurement image according to the attribute for measuring image density, and correcting the image data based on the detection result of the image density of the measurement image by the detection means; control means for causing the image forming means to form an image based on the image data that has been processed; and input means; Accordingly, the image forming means is configured to form the measurement image.
Another image forming apparatus of the present invention 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, and forms an image on the recording material based on the image-processed image data. an image forming apparatus that forms an image on an image carrier, an image forming device that forms an image on an image carrier, a transfer device that transfers the image formed on the image carrier onto the recording material, and an image forming device that forms an image on the image carrier. a detection means for detecting the image density of the measurement image according to the attribute for measuring the image density, and correcting the image data based on the detection result of the image density of the measurement image by the detection means. , a control means for causing the image forming means to form an image based on the corrected image data, the control means controlling the image forming means according to frequencies set corresponding to the plurality of types of image processing. The measuring image is formed in the measurement image, and the control means changes the frequency according to the image amount for each attribute of the image based on the image data.

According to the present invention, image density fluctuations due to control delays can be suppressed.

FIG. 1 is a configuration diagram of an image forming system. FIG. 3 is an explanatory diagram of the formation position of an image density detection pattern. FIG. 3 is an explanatory diagram of a signal processing section. A graph showing the relationship between image density and output signal. FIG. 3 is a diagram illustrating an input image density and an output image density of a formed image. Flowchart showing image formation processing. (a) and (b) are illustrative diagrams of automatic tone correction frequency setting screens. 5 is a flowchart showing a process for determining the frequency of formation of an image density detection pattern. 5 is a flowchart showing image forming processing including correction of multiple types of LUTs. Flowchart showing image formation processing. A graph illustrating image density transition. 5 is a flowchart showing a process for determining a screen on which image density correction is to be performed. (a) and (b) are explanatory diagrams of the formation timing of a halftone pattern and an image that are formed by dividing into a plurality of times.

Hereinafter, preferred embodiments of the present invention will be described in detail by way of example with reference to the drawings.

(First embodiment)
FIG. 1 is a configuration diagram of an image forming system according to this embodiment. The image forming system 1 of this embodiment includes an image forming apparatus 2, an operation section 20, and a reader 8. The image forming apparatus 2 is a tandem-type full-color image forming apparatus that employs an intermediate transfer method and is capable of forming a full-color image using an electrophotographic method. However, the image forming apparatus 2 is not limited to the tandem type, and may be of other types. Further, the image forming apparatus 2 is not limited to an image forming apparatus capable of forming a full-color image, but may be capable of forming only a monochrome (black and white or monochrome) image. The image forming apparatus 2 can be used for various purposes such as a printer, various printing machines, copying machines, facsimile machines, and multifunction peripherals.

The operation unit 20 is provided at the top 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 this embodiment is a liquid crystal operation panel (touch panel display) with a touch sensor, and is used for inputting print jobs and various settings of the image forming system 1. When making various settings, a setting change screen for "automatic gradation correction frequency", which will be described later, is used. A user or operator can use the operation unit 20 to input various conditions such as the type of image to be formed, the number of copies to be printed, and the frequency of automatic gradation correction.

The reader 8 is an image reading device that includes an image reading section 80 , an ADF (Auto Document Feeder) 81 , and a document table glass 82 . The reader 8 uses an image reading unit 80 to read images from a document that is continuously transported by the ADF 81 or a document placed on a document table glass 82 . The reader 8 transmits the reading result of the image of the document 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 result of reading the image of the document by the reader 8 onto a recording material S.

The image forming apparatus 2 includes a paper feeding section 4, an image forming section 5, and a control section 30. In this embodiment, a case will be described in which there is one paper feed section 4, but a plurality of paper feed sections 4 may be provided. The paper feed section 4, the image forming section 5, and the control section 30 are provided within the main body 10 of the image forming apparatus 2.

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

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 with the same or corresponding functions or configurations provided corresponding to these four image forming units 50y, 50m, 50c, and 50k, the y at the end of the code indicates that the element is for one of the colors. , m, c, and k may be omitted for comprehensive explanation. Note that the image forming apparatus 2 is capable of forming a monochrome or multicolor image, such as a monochromatic black image, by using the image forming unit 50 for a single color or some of the four colors. .

The image forming unit 50 includes a photosensitive drum 51 that is a drum-shaped (cylindrical) photosensitive member (electrophotographic photosensitive member), a charging roller 52 that is a roller-shaped charging member, a developing device 40, a pre-exposure device 54, and a cleaning member. It has a cleaning blade 55 that is . The image forming unit 50 forms a toner image on an intermediate transfer belt 44b, which will be described later. The image forming unit 50 is integrally formed as a process cartridge and is removable from the main body 10 of the image forming apparatus 2 . A reflected light amount sensor 12 used for detecting the image density of a toner image formed on the photosensitive drum 51 is arranged near the photosensitive drum 51 .

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

The surface of the rotating photosensitive drum 51 is uniformly charged by a 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 as the photosensitive drum 51 rotates. A charging bias power source (not shown) is connected to the charging roller 52. The charging bias power supply applies a charging bias to the charging roller 52 during charging processing.

The charged surface of the photosensitive drum 51 is scanned and exposed by the exposure device 42 based on image data. An electrostatic image is formed on the photosensitive drum 51 by scanning exposure. The exposure device 42 is a laser scanner in this embodiment. The exposure device 42 modulates and emits laser light based on the separated color image data output from the control section 30, and scans and exposes the surface of the photosensitive drum 51.

The electrostatic image formed on the photosensitive drum 51 is developed by being supplied with toner by the developing device 40 . By developing the electrostatic image, a toner image is formed on the photosensitive drum 51. In this embodiment, the developing device 40 contains a two-component developer (also simply referred to as "developer") that includes non-magnetic toner particles (toner) and magnetic carrier particles (carrier). The developing device 40 is supplied with toner from a toner bottle 41.

The developing device 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). A magnet roller, which is a roller-shaped magnet, is arranged inside the developing sleeve 24 so as not to rotate with respect to the main body of the developing device 40 . The developing sleeve 24 carries developer and conveys it to a developing area facing the photosensitive drum 51 . A developing bias power source (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 polarity.

The intermediate transfer unit 44 is arranged to face the four photosensitive drums 51y, 51m, 51c, and 51k. The intermediate transfer unit 44 includes an intermediate transfer belt 44b that is an endless belt and serves as a second image carrier. The intermediate transfer belt 44b is wound around a plurality of rollers such as a drive roller 44a, a driven roller 44d, primary transfer rollers 47y, 47m, 47c, and 47k, and a secondary inner transfer roller 45a. The intermediate transfer belt 44b is rotatable while carrying a toner image.

The drive roller 44a is rotationally driven by a drive source such as a motor (not shown), and rotates the intermediate transfer belt 44b. The driven roller 44d is a tension roller that controls the tension of the intermediate transfer belt 44b to be constant. The driven roller 44d is biased by a spring (not shown) or the like so as to push the intermediate transfer belt 44b toward the outer peripheral surface. A tension of about 2 to 5 kgf is applied to the intermediate transfer belt 44b in the process progress direction by the driven roller 44d. The secondary transfer inner roller 45a constitutes a secondary transfer section 45 as described later. A driving force is transmitted to the intermediate transfer belt 44b by the driving roller 44a, and the intermediate transfer belt 44b is rotated in the direction of the arrow (clockwise) in the figure at a predetermined circumferential speed corresponding to the circumferential speed of the photosensitive drum 51. The intermediate transfer unit 44 includes a belt cleaner 60 near the drive roller 44a and at a position facing the intermediate transfer belt 44b.

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 arranged facing the corresponding photosensitive drums 51y, 51m, 51c, and 51k, respectively. The primary transfer roller 47 sandwiches the intermediate transfer belt 44b with the corresponding photosensitive drum 51. As a result, the intermediate transfer belt 44b comes into contact with the photosensitive drum 51 and forms a primary transfer portion (primary transfer nip portion) 48 with the photosensitive drum 51.

The toner image formed on the photosensitive drum 51 is transferred onto the intermediate transfer belt 44b by the action of the primary transfer roller 47 in the primary transfer section 48. In this embodiment, by applying a positive primary transfer voltage to the primary transfer roller 47, a negative toner image on the photosensitive drum 51 is transferred onto the intermediate transfer belt 44b. For example, when forming a full-color image, toner images formed on each of the photosensitive drums 51y, 51m, 51c, and 51k are transferred onto the intermediate transfer belt 44b in a superimposed manner. A primary transfer power source (not shown) is connected to the primary transfer roller 47 . During transfer, the primary transfer power supply applies a DC voltage having a polarity opposite to the charging polarity of the toner (positive polarity in this embodiment) to the primary transfer roller 47 as a primary transfer bias. A voltage detection sensor that detects an output voltage and a current detection sensor that detects an output current are connected to the primary transfer power source. 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 individually controlled.

After the toner image is transferred, the surface of the photosensitive drum 51 is neutralized by a pre-exposure device 54 . Toner remaining on the photosensitive drum 51 without being transferred to the intermediate transfer belt 44b during transfer 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 comes into contact with the photosensitive drum 51 with a predetermined pressing force. The cleaning blade 55 has its free end side in contact with the surface of the photosensitive drum 51 in a counter direction facing upstream in the rotational direction of the photosensitive drum 51 .

The intermediate transfer belt 44b of this embodiment has a two-layer structure including a base layer and a surface layer from the inner peripheral surface side. As the material constituting the base layer, a material made of resin such as polyimide or polycarbonate or various rubbers containing an appropriate amount of carbon black as an antistatic agent can be suitably used. For the surface layer, for example, one type of resin material such as polyurethane, polyester, or epoxy resin, or two or more types of elastic materials such as elastic material rubber, elastomer, butyl rubber, etc., is used as a base material. Note that the intermediate transfer belt 44b is not limited to a two-layer structure, and may have a single-layer structure including the above-mentioned base layer.

On the outer peripheral surface side of the intermediate transfer belt 44b, an outer secondary transfer roller 45b, which together with the inner secondary transfer roller 45a constitutes the secondary transfer section 45, is arranged at a position facing the inner secondary transfer roller 45a. The secondary transfer outer roller 45b contacts the intermediate transfer belt 44b and forms a secondary transfer portion (secondary transfer nip portion) 45n with the intermediate transfer belt 44b. The outer secondary transfer roller 45b and the inner secondary transfer roller 45a sandwich the intermediate transfer belt 44b. The toner image formed on the intermediate transfer belt 44b is secondarily transferred onto the recording material S by the action of the secondary transfer section 45 in the secondary transfer section 45n. In this embodiment, by applying a positive secondary transfer voltage to the secondary transfer outer roller 45b, a negative toner image on the intermediate transfer belt 44b is transferred between the intermediate transfer belt 44b and the secondary transfer outer roller 45b. The image is secondarily transferred onto the recording material S which is conveyed while being sandwiched between the two. The recording material S is fed from the paper feeding unit 4 in parallel with the above-described toner image forming operation, and is timed with the toner image on the intermediate transfer belt 44b by the registration rollers 11 provided on the conveyance path L, and is transferred to the secondary image. It is transported to the transfer section 45n. Attachments such as toner and paper dust remaining on the intermediate transfer belt 44b without being transferred to the recording material S during the secondary transfer are removed from the surface of the intermediate transfer belt 44b by the belt cleaner 60 and collected.

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

Note that in this embodiment, the inner secondary transfer roller 45a is grounded and a voltage is applied to the outer secondary transfer roller 45b from the secondary transfer power source; however, on the contrary, the inner secondary transfer roller 45a is The secondary transfer outer roller 45b may be grounded by applying a voltage from the transfer power source. In this case, a DC voltage having the same polarity as the normal charging polarity of the toner is applied to the secondary transfer inner roller 45a.

The recording material S onto which the toner image has been transferred is conveyed to the fixing device 46 by the secondary transfer outer roller 45b. The fixing device 46 includes a fixing roller 46a and a pressure roller 46b. The fixing roller 46a has a built-in heater as a heat source. The recording material S carrying the unfixed toner image is heated and pressurized by being conveyed while being held between the fixing roller 46a and the pressure roller 46b. As a result, the toner image is fixed (melted, fixed) on the recording material S. As described above, an image is formed on the recording material S. Note that the temperature of the fixing roller 46a (fixing temperature) is detected by a sensor (not shown). The control unit 30 controls the temperature of the fixing roller 46a to a predetermined temperature based on the detection result of the fixing temperature.

In the case of single-sided printing, the recording material S on which an image has been formed is outputted as is from the conveyance section 6 to the paper discharge section 9. In the case of double-sided printing, the recording material S with an image formed on one side is conveyed to the reversal conveyance path 7. In the reversing conveyance path 7, the recording material S with an image formed on its first surface is turned over and conveyed again to the secondary transfer section 45n. The recording material S transported from the reverse transport path 7 to the secondary transfer section 45n has a toner image transferred to the second surface opposite to the first surface, and is transported after the toner image is fixed by the fixing device 46. It is outputted from the section 6 to the paper discharge section 9. In this manner, the image forming apparatus 2 of the present embodiment is capable of performing automatic double-sided printing in which images are formed on both sides of one sheet of recording material S.

FIG. 2 is an explanatory diagram of the formation position of the image density detection pattern 101. In FIG. 2, the vicinity of the photosensitive drum 51y and the intermediate transfer belt 44b is displayed in an enlarged manner. In this embodiment, among the continuous images formed on the surface of the photosensitive drum 51y during continuous printing, image density detection is performed outside the image forming area sandwiched between the leading edge of the rear image 102b and the rear end of the leading image 102a. A pattern 101 is formed. The image density detection pattern 101 includes a halftone pattern and is a measurement image for measuring image density. Note that the outside of the image forming area is not limited to the area sandwiched between the leading end of the rear image 102a and the rear end of the forward image 102b.

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

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 the reflected light amount sensor 12y. The reflected light amount 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 regular reflection of the light emitted from the light-emitting element by the photosensitive drum 51y. The reflected light amount sensor 12y thus measures the amount of reflected light from the image density detection pattern 101. The reflected light amount sensor 12y is arranged at a position where the amount of toner can be measured between the development position by the developing device 40y and the primary transfer section 48y. The amount of reflected light is measured at the timing when the image density detection pattern 101 formed outside the image forming area on the photosensitive drum 51y passes through the measurement range of the reflected light amount sensor 12y. Based on the amount of reflected light, a yellow gradation correction (γ correction) that is estimated to provide a predetermined constant image density (amount of reflected light) is obtained.

Similarly, for the other photosensitive drums 51m, 51c, and 51k, reflected light amount sensors 12m, 12c, and 12k measure the amount of toner between the development position of the developing devices 40m, 40c, and 40k, and the primary transfer portions 48m, 48c, and 48k. Placed in a measurable location. The image density of the images (toner images) formed on the photosensitive drums 51m, 51c, and 51k is detected by reflected light amount sensors 12m, 12c, and 12k. Based on the detected image density (reflected light amount), magenta, cyan, and black gradation corrections (γ correction) that are estimated to provide a predetermined constant image density (reflected light amount) are obtained.

FIG. 3 is an explanatory diagram of a signal processing section that processes the detection result (output signal) by the reflected light amount sensor 12. The signal processing section 70 is included in the control section 30. The signal processing section 70 includes an A/D converter 15, a density conversion section 16, and a CPU (Central Processing Unit) 17. The reflected light amount sensor 12 receives reflected light (infrared light) from the photosensitive drum 51, converts the received reflected light (infrared light) into an electrical signal, and outputs the electrical signal. The electric 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 amount sensor 12. The electrical signal is input to A/D converter 15.

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

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

Image density detection and gradation control during continuous printing by the image forming system 1 having such a configuration will be described. FIG. 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 amount sensor 12. As shown in FIG. Here, the relationship between the 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 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 amount sensor 12 is set to 5 [V] (255 levels with an 8-bit digital signal). As the area coverage by the toner of each color increases and the image density increases, the voltage value of the electrical signal output from the reflected light amount sensor 12 decreases.

In the density conversion unit 16, a table 16a dedicated to each color having such characteristics is registered. The density conversion unit 16 converts the electrical signal (output signal) output from the reflected light amount sensor 12 into an image density signal for each color using a table 16a dedicated to each color. Thereby, the image density of the image density detection pattern 101 of each color can be read with high accuracy.

In this embodiment, the CPU 17 forms an image density detection pattern 101 outside the image forming area during normal image formation according to a print job, detects the image density, and corrects the table data of the LUT 25 as needed. The LUT 25 contains correction conditions for setting the yellow, cyan, magenta, and black writing image halftone densities during image formation in order to obtain a product with appropriate image density for the image data representing the image to be formed. This is the table. When forming the image density detection pattern 101, the CPU 17 sets an appropriate image signal amount using the LUT 25 for the image data amount of the four colors of image data, cyan, magenta, yellow, and black, in the same way as for normal images. do.

The table data of the LUT 25 used when forming the image density detection pattern 101 is equivalent to the table data during normal image formation at that time. In other words, the correction results from the previous image density correction control are used as the table data of the LUT 25.

For example, when the input image density (image density specified by image data) of the image density detection pattern 101 is 128 level and the target value of the output image density is 128, the deviation from the target value of the output image density is The image data is corrected by the LUT 25 so that it disappears. However, the image characteristics of the image forming apparatus 2 are unstable and may constantly change. The table data of the LUT 25 is corrected based on the deviation amount ΔD between the input image density and the measurement result of the formed image (output image density). FIG. 5 is an exemplary diagram of the input image density and the output image density of the formed image. The deviation amount 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. It is.

In this embodiment, an LUT correction table for correcting this deviation of the LUT 25 is stored in the RAM 19 in advance. During control, the CPU 17 calculates a correction amount corresponding to the deviation amount ΔD for all image density signals of the LUT correction table up to the previous time stored in the RAM 19. The CPU 17 creates a current LUT correction table by correcting the previous LUT correction table by the deviation amount ΔD based on the calculated correction amount. Such rewriting (correction) of the LUT 25 is performed at the timing when the creation of the LUT correction table according to the deviation amount ΔD is completed for each color.

FIG. 6 is a flowchart showing image forming processing including LUT correction table creation processing. This processing is executed along with normal image forming processing according to the print job. FIG. 6 illustrates an example of processing in which an image is formed by one type of image processing.

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

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

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 for all pages specified in the print job has been completed. If there remain pages for image formation instructed by the print job, the CPU 17 determines to continue the print job. If the print job is to be continued (S27:N), the CPU 17 repeats the processes from S21 onwards until the print job is finished. If the print job is not to be continued (S27: Y), the CPU 17 ends the print job.

In this way, FIG. 6 describes the case where image formation is performed by correcting the LUT 25 using one type of image processing. Next, a case where image formation is performed by correcting the LUT 25 using a plurality of types of image processing will be described.

The image forming apparatus 2 of this embodiment is capable of forming an image using image data that has been subjected to a plurality of types of image processing. Image attributes indicated by input image data include image attributes, graphic attributes, text attributes, and the like. The image forming apparatus 2 determines appropriate image processing (dither processing) for the image data according to the attributes of the image. For example, the image forming apparatus 2 uses dithering with a relatively low number of lines called a gradation screen for image attributes, and performs image processing that emphasizes uniform stability of halftones. For text attributes, the image forming apparatus 2 uses a dither with a relatively high number of lines called a resolution screen to perform image processing to accurately reproduce edge portions. In this way, the image forming apparatus 2 changes the content of image processing (the screen to be used) based on the image attributes.

In this embodiment, an example will be described in which a 189-line dither is used as a gradation screen for image attributes and graphic attributes, and a 233-line dither is used as a resolution screen for text attributes. Note that in this embodiment, dither with the same number of lines is used for all colors, but this is not particularly limited, and dither with a different number of lines may be used for each color. In any case, multiple lines of dithering are used for each color.

Since the area between images (outside the image forming area) on the photosensitive drum 51 is narrow, it is not possible to form the image density detection pattern 101 for all image processing of each color in the area between one image. For this purpose, the image density detection pattern 101 is divided and formed in areas between a plurality of images (outside the image forming area).

For example, between the first to sixth images that are continuously printed, a 189-line dither, which is a gradation screen used for four-color image attributes and graphic attributes, is used to adjust the image density. The detection pattern 101 is formed in five divisions. Next, an image density detection pattern 101 is divided and formed between the 6th and 11th sheets of paper during image printing using a 233-line dither, which is a resolution screen used for text attributes. . Next, the image density detection pattern 101 is divided again between the 11th to 16th sheets of paper during image printing using a 189-line dither, which is a gradation screen used for image attributes and graphic attributes. It is formed by Thereafter, similar operations are repeated one after another.

In addition, it goes without saying that LUTs 25 are prepared for each dither (189 lines, 233 lines) and controlled independently, but also independently controlled for each color. That is, in this embodiment, the image forming apparatus 2 has a total of eight LUTs 25 consisting of four colors x 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 (n is a number representing a different dither type for each color) and corrects the LUT 25.

When a plurality of dithered image density detection patterns 101 are sequentially formed, the control frequency of dithering for each color is reduced by the dither type (in the case of two types, the frequency is halved). Some users desire that the dither image density detection pattern 101 be formed more frequently for images in which image density and color stability are important. Therefore, in this embodiment, the frequency of forming the image density detection pattern 101 in each dither is made variable.

FIG. 7 is an exemplary diagram of a setting screen for automatic gradation correction frequency. Using this setting screen, the frequency of forming the image density detection pattern 101 is set. 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 (the frequency of forming the image density detection pattern 101) is determined according to the input priority. For example, in the initial setting state shown in FIG. 7(a), the standard button is selected. The screen shown in FIG. 7(b) is set to give priority to the gradation screen. In the case of FIG. 7B, the control frequency is, for example, out of 5 times, the dither control frequency for the gradation screen is 4 times, and the control frequency for the resolution screen dither is 1 time.

FIG. 8 is a flowchart illustrating the process of determining the frequency of formation of the image density detection pattern 101. This process is performed prior to starting the image forming process.

The CPU 17 determines whether the mode setting button for "automatic gradation correction frequency" has been changed from the default "standard" setting (S31). If the mode setting button has not been changed (S31:N), the CPU 17 ends the process. If the mode setting button has been changed (S31: Y), the CPU 17 acquires the input control priority for each dither (S32). The CPU 17 determines the control frequency for each dither based on the acquired priority (S33). The CPU 17 stores the determined control frequency for each dither in the RAM 19.

FIG. 9 is a flowchart showing image forming processing including correction of a plurality of types of LUTs 25. As shown in FIG. The CPU 17 reads the control frequency for each dither (the frequency of forming the image density detection pattern 101) stored in the RAM 19 (S41). The CPU 17 sets the order of forming the image density detection pattern 101 for each dither and for each color based on the read control frequency for each dither (S42). The order in which the image density detection pattern 101 is formed is determined, for example, in the order of yellow, magenta, cyan, and black in accordance with the control frequency for each dither. The CPU 17 performs image formation processing with LUT correction for each dither (S43), and ends this processing. FIG. 10 is a flowchart showing the image forming process in S43.

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

The CPU 17 forms an image density detection pattern 101 outside the image forming area (between images) on the photosensitive drum 51, which is an area 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 amount sensor 12 (S55). The CPU 17 calculates the deviation amount ΔDn between the read image density and the target value image density (S56). The CPU 17 creates an LUT correction table using the calculated deviation amount ΔDn (step S57). The LUT correction table is created as follows. That is, the CPU 17 multiplies the total image density signal of the previous LUT correction table stored in the RAM 19 by ΔDn to create the current LUT correction table according to the deviation amount ΔDn.

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 for all pages specified in the print job has been completed. If there remain pages for image formation instructed by the print job, the CPU 17 determines to continue the print job. When continuing the print job (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 processes from S52 onwards until the print job is finished. If the number n has reached the total number ALL (S59:N), the CPU 17 sets the number n to the initial value "1" (S61), and repeats the processes from S52 onwards until the print job is finished. . If the print job is not to be continued (S58: Y), the CPU 17 ends the print job.

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 performs an image forming operation based on the control results of each dither.

FIG. 11 is a graph illustrating an image density transition of an image using tone screen dithering applied to an image attribute image. A solid line a illustrates the image density when the dither control frequency is changed. A broken line b illustrates the image density when the dither control frequency is not changed. It can be seen that the change in image density is smaller when the dither control frequency is changed (solid line a) than when it is not changed (broken line b). In this way, the stability of the image density of images with image attributes that the user considers to be important is greatly improved. Images with text attributes to which resolution screen dithering is applied may have large variations in image density. However, since images with text attributes are images of characters, etc., the influence of fluctuations in image density is relatively small.

In particular, image forming apparatuses used as commercial printing machines are often used for printing with different purposes for each job. Such image forming apparatuses are used for image-specific print jobs such as photo books, which use a gradation screen that mostly has image attributes, and resolution screens that have mostly text attributes, such as novels. Run a print job that is specific to the image to which it is applied. Therefore, for an image forming apparatus used as a commercial printing machine, the image forming apparatus of this embodiment is effective by specifying dithering to suppress image density fluctuations.

However, variations in image density on non-priority screens may not be ignored. As a countermeasure, the detection result of the image density of the resolution screen is estimated from the detection result of the image density of the gradation screen based on the characteristic difference between the image density output characteristics of the gradation screen and the resolution screen. As a result, the LUT correction tables for the gradation screen and the resolution screen are corrected at the same time. Compared to the result detected by the reflected light amount sensor 12 after actually forming a resolution screen as a halftone pattern, there are differences between the actual measurement and the estimated result. Therefore, although there is a problem in that the accuracy of image density correction depends on the characteristic difference between the screens, it is possible to suppress fluctuations in image density more than under the condition without correction.

In this way, the image forming system 1 (image forming apparatus 2) of the first embodiment can always produce highly stable images that meet the user's needs by making the control frequency variable depending on the type of image processing. Obtainable.

(Second embodiment)
The configuration of the image forming system 1 of the second embodiment is the same as that of the first embodiment, so a description thereof will be omitted. The image forming apparatus 2 included in the image forming system 1 of the second embodiment constantly performs control at an optimal frequency by appropriately changing the control frequency according to the image amount for each image attribute of the input image data. to obtain a good image density transition.

In this embodiment, the image amount is a video count value obtained by integrating image density values for each pixel based on image data. The CPU 17 calculates the video count value at the timing of forming the image density detection pattern 101, and forms the dithered image density detection pattern 101 having the maximum value. Image data has attribute information stored by an application that creates the image data. Further, the image data generated by the reader 8 is given attributes by image area separation processing, which is a conventional technique.

For example, the CPU 17 detects the video count value of the image data being printed at the timing when the yellow image density detection pattern 101 is formed. It is assumed that the video count value of the portion where the gradation screen is dithered is "152" and the video count value of the portion where the resolution screen is dithered is "58". In this case, the gradation screen portion occupies a large area, and the visibility of image density fluctuations is high. For this purpose, 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 image density detection pattern 101 of the yellow gradation screen. When the image density correction control using the image density detection pattern 101 is completed, the CPU 17 resets the integrated value (video count value) to "0".

The CPU 17 detects the video count value at the timing when the next yellow image density detection pattern 101 is formed. When the detected video count value is "26" for the gradation screen and "84" for the resolution screen, the CPU 17 performs image density correction control using the image density detection pattern 101 for the yellow resolution screen. Similar processing is performed for colors other than yellow.

If there is no large difference between the video count value of the gradation screen and the video count value of the resolution screen, the CPU 17 determines that the two screens have the same visual sensitivity to image density fluctuations. In this case, the CPU 17 executes image density correction control by alternately using two types of screen processing.

FIG. 12 is a flowchart showing image processing for creating the image density detection pattern 101, that is, processing for determining a screen on which image density correction is to be performed. When the video count value of the gradation screen is "A" and the video count value of the resolution screen is "B", the difference ratio C (unit: %) of the video count values is calculated by the following formula (2). .
C=|A-B|/((A+B)/2)×100...(2)

The CPU 17 alternately displays the gradation screen and the resolution screen at the same frequency while the difference ratio C (%) of the video count value is less than or equal to the threshold value (here, 10%) (C≦10%) (S71: Y). Image density correction control is executed using the image density correction control. The CPU 17 determines whether or not the previous image density correction control was performed using gradation screen processing (S72). If the previous image density correction control was performed using the gradation screen process (S72: Y), the CPU 17 determines that the current image density correction control will be performed using the resolution screen (S73). If the previous image density correction control was not performed using gradation screen processing (S72:N), the CPU 17 determines that the current image density correction control will be performed using gradation screen processing (S75). Having determined the screen, the CPU 17 resets the video count value to "0" (S77).

For example, if the video count value A of the gradation screen is "205", the video count value B of the resolution screen is "194", and the difference ratio C is 6% (≦threshold value 10%), the CPU 17 Determine the screen used for density correction control. If the previous image density correction control was executed using the gradation screen process, the CPU 17 executes 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 values is within the threshold value at the next forming timing of the pattern for image density detection, the CPU 17 executes image density correction control using gradation screen processing.

If the difference ratio C (%) of the video count values is larger than the threshold (10%) (C>10%) (S71:N), the CPU 17 sets the video count value "A" of the gradation screen and the video count of the resolution screen. The value "B" is compared (S74). If the video count value "A" of the gradation screen is larger than the video count value "B" of the resolution screen (A>B) (S74: Y), the CPU 17 performs the current image density correction control on the gradation screen. is determined (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 determines that the current image density correction control is to be performed on the resolution screen. Determine (S76). Having determined the screen, the CPU 17 resets the video count value to "0" (S77).

The image forming apparatus 2 of the second embodiment can increase the control frequency of the dithered image, which is used more frequently in the entire image. Therefore, an image with high stability can be obtained from a comprehensive viewpoint. In the second embodiment, the type of screen used for image density correction is determined for each color. However, by performing image density correction control for the four colors (yellow, magenta, cyan, and black) based on a screen type that is comprehensively determined, phenomena such as color shift in any one of the four colors can be prevented. be able to.

In the first and second embodiments, each time the image density of the image density detection pattern 101 formed by a specific image process (dither) is detected, only the LUT corresponding to that specific dither is rewritten. In the first embodiment, the image density of other screens is estimated using the characteristic difference for each screen. Alternatively, LUTs corresponding to other dithers may be rewritten by multiplying the LUT corresponding to a specific dither by a predetermined coefficient (feedback rate). For example, if the shift amount ΔD is "15" as a result of patching by dithering the gradation screen, the LUT correction table is rewritten by an amount corresponding to the value "15" of the shift amount ΔD to correct it. Furthermore, the LUT correction table for the resolution screen, which is another dither, is rewritten by multiplying the value of the deviation amount ΔD by 0.8 (feedback coefficient), "15". Thereby, the responsiveness of image density control can be improved.

Claims (9)

An image forming apparatus that performs multiple types of image processing on image data representing an image according to attributes of the image to be formed on a recording material, and forms an image on the recording material based on the image-processed image data, the image forming apparatus 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 attribute for measuring the image density formed on the image carrier;
a control unit that corrects the image data based on a detection result of the image density of the measurement image by the detection unit, and causes the image forming unit to form an image based on the corrected image data;
an input means;
The control means causes the image forming means to form the measurement image according to a frequency set by the input means corresponding to the plurality of types of image processing.
Image forming device. further comprising a display on which the frequency setting screen is displayed;
The input means selects priority image processing from the plurality of types of image processing using the setting screen;
The control means determines the frequency so as to give priority to the type of image processing selected by the input means,
The image forming apparatus according to claim 1. The control means sets an order for forming measurement images corresponding to the processing content of the image processing based on the determined frequency, and causes the image forming means to form the measurement images in the set order. do,
The image forming apparatus according to claim 2. The control means causes the image forming means to continuously form images on the image carrier according to a print job, and to form images in an area between a trailing edge of the previously formed image and a leading edge of the next image. forming the measurement image;
The image forming apparatus according to any one of claims 1 to 3. The control means is characterized in that the measurement image is divided and formed.
The image forming apparatus according to claim 4. An image forming apparatus that performs multiple types of image processing on image data representing an image according to attributes of the image to be formed on a recording material, and forms an image on the recording material based on the image-processed image data, the image forming apparatus 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 attribute for measuring the image density formed on the image carrier;
a control unit that corrects the image data based on the detection result of the image density of the measurement image by the detection unit and causes the image forming unit to form an image based on the corrected image data;
The control means causes the image forming means to form the measurement image according to a frequency set corresponding to the plurality of types of image processing,
The control means is characterized in that the frequency is changed according to the image amount for each attribute of the image based on the image data.
Image forming device. The control means calculates a video count value by integrating image density values for each pixel based on the image data, and changes the frequency based on the calculated video count value.
The image forming apparatus according to claim 6. The control means causes the image forming means to continuously form images on the image carrier according to a print job, and to form images in an area between a trailing edge of the previously formed image and a leading edge of the next image. forming the measurement image;
The image forming apparatus according to claim 6 or 7. The control means is characterized in that the measurement image is divided and formed.
The image forming apparatus according to claim 8.

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