Image forming apparatus

Abstract

To provide an image forming apparatus that reduces the entire time required for a calibration operation while maintaining the reading accuracy of an optical sensor.SOLUTION: An image forming apparatus 100 comprises: a reader 160 that reads an image formed on recording paper; a white reference plate that is used for calibration of the reader 160; and a controller 110 that performs calibration of the reader 160 with black shading adjustment performed by causing the reader 160 to read the white reference plate in a first state and white shading adjustment performed by causing the reader 160 to read the white reference plate in a second state. The controller 110 performs the black shading adjustment at a longer execution interval than the white shading adjustment.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an image forming apparatus that reads an image printed on a printed matter and performs processing to stabilize the quality of an image formed based on the reading result.

Background Art

[0002] In an image forming apparatus that forms an image using an electrophotographic process, due to changes over time and environmental changes, the characteristics of each process of charging, developing, transferring, and fixing change, and the image density and color of the image formed on the printed matter may change. Therefore, image stabilization control is performed in the image forming apparatus. The image stabilization control is control in which a detection image for detecting the image density formed on the image carrier is detected by an optical sensor, and the image forming conditions are adjusted based on the detection result so that the image printed on the printed matter has an appropriate image density. The image forming conditions are various settings at the time of image formation, such as the charge amount of the image carrier and the emission energy amount of the laser that scans the image carrier.

[0003] The image stabilization control is control performed based on the image density of the image before being transferred to the recording paper. Therefore, the change in the image density of the image after being transferred to the recording paper is not controlled. For example, the variation in the transfer efficiency when transferring an image from a photoreceptor or an intermediate transfer body to the recording paper due to the influence of environmental fluctuations affects the image density of the image after being transferred to the recording paper. For these reasons, in the above-described image stabilization control, the image density of the image formed on the final printed matter is not stable.

[0004] On the other hand, Patent Document 1 discloses an image forming apparatus that forms a detection image for detecting the image density on a recording paper, detects the detection image by an optical sensor after the image is fixed, and adjusts the image density based on the detection result. The detection image is formed in a region where an image (user image) corresponding to the print job on the recording paper is formed. Since the detection image is formed on the same recording paper as the user image, real-time image density adjustment becomes possible every time an image formation corresponding to the print job is performed.

[0005] For optical sensors, line sensors such as CCD (Charge Coupled Device) and CIS (Contact Image Sensor) are adopted. After the output value is adjusted by a calibration operation using a white reference plate, the line sensor performs a reading operation of a detection image, enabling highly accurate measurement of image density.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] In a configuration where an image formed on a recording paper is read by a line sensor, the line sensor is affected by the radiant heat from the recording paper. When temperature fluctuations occur in the line sensor due to radiant heat, the light emission amount of the line sensor and thermal deformation of internal optical components occur. This leads to a decrease in the reading accuracy of the line sensor. It is possible to prevent such a decrease in reading accuracy by increasing the frequency of the calibration operation. However, a detection image cannot be read during the calibration operation. Therefore, an increase in the frequency of the calibration operation leads to a decrease in the real-time performance of image density adjustment.

[0008] The present invention has been made in view of the above problems, and a main object thereof is to provide an image forming apparatus that shortens the overall time required for a calibration operation while maintaining the reading accuracy of an optical sensor.

Means for Solving the Problems

[0009] The image forming apparatus of the present invention includes a reading unit that reads an image formed on a recording sheet, a reference plate used for calibration of the reading unit, a first calibration operation in which the reading unit reads the reference plate in a first state, and a second calibration operation in which the reading unit reads the reference plate in a second state, and a control unit that calibrates the reading unit. The reading means includes a light emitting means and a light receiving means. In the first state, the reading means reads the reference plate in a state where the light emitting means emits light to irradiate the reference plate with light. In the second state, the reading means reads the reference plate in a state where the light emitting means is turned off. The control unit is characterized in that the first calibration operation is performed at an execution interval longer than that of the second calibration operation.

Effects of the Invention

[0010] According to the present invention, it is possible to shorten the overall time required for the calibration operation while maintaining the reading accuracy of the reading unit.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

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Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[0012] Hereinafter, embodiments will be described in detail with reference to the drawings. Note that the following embodiments do not limit the invention according to the claims, and not all combinations of the features described in the embodiments are essential for the solution means of the invention.

[0013] (Printing System) FIG. 1 is a configuration explanatory diagram of a printing system including the image forming apparatus according to the present embodiment. The printing system 1 includes an image forming apparatus 100 and a host computer 101. The image forming apparatus 100 and the host computer 101 are communicably connected via a network 105. The network 105 is composed of a communication line such as, for example, a LAN (Local Area Network), a WAN (Wide Area Network), or a public communication line. Note that a plurality of the image forming apparatus 100 and the host computer 101 may be respectively connected to the network 105.

[0014] The host computer 101 is, for example, a server apparatus, and transmits a print job to the image forming apparatus 100 via the network 105. The print job includes various types of information necessary for printing, such as image data, the type of recording paper used for printing, the number of printed sheets, and an instruction for double-sided or single-sided printing.

[0015] The image forming apparatus 100 includes a controller 110, an operation panel 120, a paper feeding unit 140, a printer 150, and a reader 160. The controller 110, the operation panel 120, the paper feeding unit 140, the printer 150, and the reader 160 are communicably connected to each other via a system bus 116. The image forming apparatus 100 controls the operation of the printer 150 based on a print job acquired from the host computer 101 by the controller 110, and forms an image corresponding to the image data on the recording paper.

[0016] The controller 110 controls the operation of each unit of the image forming apparatus 100. The controller 110 is an information processing apparatus including a ROM (Read Only Memory) 112, a RAM (Random Access Memory) 113, and a CPU (Central Processing Unit) 114. The controller 110 includes a communication control unit 111 and a storage 115. Each module is communicably connected to each other via the system bus 116.

[0017] The communication control unit 111 is a communication interface that communicates with the host computer 101 and other devices via the network 105. The storage 115 is a large-capacity storage device such as an HDD (Hard Disk Drive) or an SSD (Solid State Drive). The storage 115 stores various data used for computer programs and image formation processing (printing processing). The CPU 114 executes the computer programs stored in the ROM 112 and the storage 115 to control the operation of the image forming apparatus 100. The RAM 113 provides a work area when the CPU 114 executes a computer program.

[0018] The operation panel 120 is a user interface and includes an input interface and an output interface. The input interface is, for example, operation buttons, numeric keys, a touch panel, etc. The output interface is, for example, a display such as an LCD (Liquid Crystal Display), a speaker, etc. The user can input a print job, commands, print settings, etc. to the image forming apparatus 100 via the operation panel 120. The operation panel 120 displays a setting screen and the state of the image forming apparatus 100 on the display.

[0019] The paper feeding unit 140 includes a plurality of paper feeding stages described later for accommodating recording paper. The paper feeding unit 140 feeds the type of recording paper instructed in the print job from the paper feeding stage that accommodates the recording paper. A plurality of sheets of recording paper (recording paper bundles) are accommodated in the paper feeding stage, and the paper is fed in order from the topmost recording paper. The paper feeding unit 140 conveys the recording paper fed from the paper feeding stage to the printer 150. The same type of recording paper may be accommodated in each paper feeding stage, or different types of recording paper may be accommodated.

[0020] Printer 150 generates a printed matter by printing an image on a recording sheet supplied from a paper feeding unit 140 based on image data included in a print job. Reader 160 is an image reading device that reads an image from the printed matter generated by printer 150 and transmits the reading result to controller 110. The image that reader 160 reads is an image (detection image) for adjusting the image forming conditions when printer 150 performs image formation. Controller 110 detects an image state such as image quality from the reading result of the detection image by reader 160, and adjusts the image forming conditions based on the detected image state. In the present embodiment, controller 110 adjusts the image forming conditions based on the image density detected from the detection image.

[0021] (Image forming apparatus) FIG. 2 is a configuration diagram of image forming apparatus 100. Image forming apparatus 100 includes paper feeding stages 140a to 140e, printer 150, reader 160, and finisher 190 in order from the upstream side in the conveyance direction of the recording sheet. Paper feeding stages 140a to 140e constitute paper feeding unit 140. Finisher 190 is a post-processing device that performs post-processing on the printed matter generated by printer 150. Finisher 190 performs, for example, staple processing, sorting processing, or cutting processing (described later) on a plurality of printed matters.

[0022] Printer 150 includes a plurality of image forming units that form images of different colors. Printer 150 of the present embodiment includes four image forming units in order to form images of four colors: yellow (Y), magenta (M), cyan (C), and black (K). Each image forming unit only has a different color of the image to be formed, and performs the same operations with the same configuration.

[0023] One image forming unit includes a photosensitive drum 153, a charger 220, an exposure device 223, and a developing device 152. The photosensitive drum 153 is a drum-shaped photoreceptor having a photosensitive layer on its surface, and is rotationally driven in the direction of arrow R1 by a motor (not shown) around the drum axis. The charger 220 charges the surface (photosensitive layer) of the rotating photosensitive drum 153. The exposure device 223 exposes the charged surface of the photosensitive drum 153 with laser light. The laser light scans the surface of the photosensitive drum 153 in the drum axis direction of the photosensitive drum 153. The direction in which the laser light scans the surface of the photosensitive drum 153 is the main scanning direction (depth direction in FIG. 2) of the printer 150. Thereby, an electrostatic latent image is formed on the surface of the photosensitive drum 153. The developing device 152 develops the electrostatic latent image using a developer (toner). Thereby, an image (toner image) in which the electrostatic latent image is visualized is formed on the surface of the photosensitive drum 153.

[0024] The printer 150 includes an intermediate transfer belt 154 onto which the toner images generated by each image forming unit are transferred. The intermediate transfer belt 154 is rotationally driven in the direction of arrow R2. The toner images of each color are transferred from the photosensitive drum 153 at a timing corresponding to the rotation of the intermediate transfer belt 154. Thereby, a full-color toner image in which the toner images of each color are superimposed is formed on the intermediate transfer belt 154. The full-color toner image is conveyed by the rotation of the intermediate transfer belt 154 to a nip portion formed by the intermediate transfer belt 154 and the transfer roller 221. The full-color toner image is transferred to the recording paper by the nip portion.

[0025] The recording paper is stored in the paper feed stages 140a, 140b, 140c, 140d, 140e of the paper feed unit 140, and is fed according to the timing of image formation by each image forming unit. The paper feed stage that is the source of the recording paper is indicated by the print job. The recording paper is conveyed to the nip portion at the timing when the full-color toner image is conveyed to the nip portion formed by the intermediate transfer belt 154 and the transfer roller 221. Thereby, the toner image is transferred to a predetermined position on the recording paper. The conveyance direction of the recording paper is the sub-scanning direction orthogonal to the main scanning direction.

[0026] Printer 150 includes a first fuser 155 and a second fuser 156 that fix the toner image onto the recording paper by heating and pressurizing. The first fuser 155 includes a fusing roller with a built-in heater and a pressure belt for pressing the recording paper against the fusing roller. The fusing roller and the pressure belt are driven by a motor (not shown) to sandwich and convey the recording paper. The second fuser 156 is disposed downstream of the first fuser in the conveyance direction of the recording paper. The second fuser 156 is used to increase the gloss of the image on the recording paper that has passed through the first fuser 155 and ensure fixability. The second fuser 156 includes a fusing roller with a built-in heater and a pressure roller with a built-in heater. Depending on the type of recording paper, the second fuser 156 may not be used. In this case, the recording paper is not conveyed to the second fuser 156 but is conveyed to the conveyance path 130. Therefore, a flapper 131 for guiding the recording paper to either the conveyance path 130 or the second fuser 156 is provided on the downstream side of the first fuser 155.

[0027] Downstream of the position where the conveyance path 130 merges on the downstream side of the second fuser 156, a conveyance path 135 and a discharge path 139 are provided. Therefore, at the position where the conveyance path 130 merges on the downstream side of the second fuser 156, a flapper 132 for guiding the recording paper to either the conveyance path 135 or the discharge path 139 is provided. The flapper 132 guides, for example, the recording paper with an image formed on the first side to the conveyance path 135 in the duplex printing mode. The flapper 132 guides, for example, the recording paper with an image formed on the first side to the discharge path 139 in the face-up paper discharge mode. The flapper 132 guides, for example, the recording paper with an image formed on the first side to the conveyance path 135 in the face-down paper discharge mode.

[0028] The recording paper conveyed to the conveyance path 135 is conveyed to the inversion unit 136. The recording paper conveyed to the inversion unit 136 makes a switchback to reverse the conveyance direction after the conveyance operation is once stopped. The recording paper is guided from the inversion unit 136 by the flapper 133 to either the conveyance path 135 or the conveyance path 138. The flapper 133 guides the recording paper that has made a switchback to the conveyance path 138 to print an image on the second side, for example, in the double-sided printing mode. The recording paper conveyed to the conveyance path 138 is conveyed toward the nip portion between the intermediate transfer belt 154 and the transfer roller 221. As a result, the front and back of the recording paper are reversed when passing through the nip portion, and image formation on the second side is performed. The flapper 133 guides the recording paper that has made a switchback to the conveyance path 135, for example, in the face-down paper discharge mode. The recording paper conveyed to the conveyance path 135 by the flapper 133 is guided to the discharge path 139 by the flapper 134.

[0029] The recording paper on which an image is formed by the printer 150 is conveyed from the discharge path 139 to the reader 160. The reader 160 is an image reading device that reads a detection image of the image density formed on the recording paper. Note that a user image corresponding to the print job is printed on the recording paper together with the detection image. The recording paper conveyed from the printer 150 to the reader 160 is conveyed through the conveyance path 313 in the reader 160. The reader 160 includes a document detection sensor 311 and line sensor units 312a and 312b in the conveyance path 313. A flow-through glass 314a and a white reference plate 315a are disposed between the line sensor unit 312a and the conveyance path 313. A flow-through glass 314b and a white reference plate 315b are disposed between the line sensor unit 312b and the conveyance path 313. The reader 160 reads the detection image printed on the recording paper by the line sensor units 312a and 312b while conveying the recording paper through the conveyance path 313.

[0030] The original document detection sensor 311 is, for example, an optical sensor having a light-emitting element and a light-receiving element. The original document detection sensor 311 detects the leading edge in the conveyance direction of the recording paper being conveyed along the conveyance path 313. The detection result of the leading edge of the recording paper by the original document detection sensor 311 is transmitted to the controller 110. Based on the detection timing of the leading edge of the recording paper by the original document detection sensor 311, the controller 110 starts the reading operation by the reader 160 (line sensor units 312a, 312b).

[0031] The line sensor units 312a, 312b read the detection images formed on the recording paper being conveyed. The detection images are printed on both the front and back surfaces of the recording paper. The line sensor units 312a, 312b are provided at positions sandwiching the conveyance path 313 in order to read the detection images formed on both sides in a single pass of the paper. When performing image density adjustment, the image forming apparatus 100 reads the detection images by the line sensor units 312a, 312b, and detects the image densities of both the front and back surfaces from the reading results. The controller 110 controls the image forming process based on the detection result of the image density so that the image printed on the output printed matter has an appropriate image density. The line sensor units 312a, 312b are moved by a drive unit (not shown) to positions where they can read the white reference plates 315a, 315b during a calibration operation described later, and are moved to positions where they can read the recording paper during detection of the image density.

[0032] (Reader) FIG. 3 is a configuration explanatory diagram of the reader 160. In addition to the line sensor units 312a, 312b and the original document detection sensor 311, the reader 160 includes an image memory 303 and an image density detection processing unit 305. The operations of the line sensor units 312a, 312b, the image memory 303, the image density detection processing unit 305, and the original document detection sensor 311 are controlled by the CPU 114 of the controller 110.

[0033] The line sensor units 312a and 312b include line sensors 301a and 301b, memories 300a and 300b, and AD converters 302a and 302b. The line sensors 301a and 301b are, for example, CIS. The memories 300a and 300b store correction information such as the variation in light amount between pixels, the step between pixels, and the distance between pixels of the corresponding line sensors 301a and 301b. The AD converters 302a and 302b acquire analog signals that are the reading results by the line sensors 301a and 301b. The AD converters 302a and 302b convert the acquired analog signals into digital signals and transmit them to the image density detection processing unit 305. The digital signals are RGB image data.

[0034] The image density detection processing unit 305 calculates the average value (average luminance value) of the RGB luminance values of the detection target image portion from the RGB image data acquired from the line sensor units 312a and 312b. The image density detection processing unit 305 transmits the calculated average luminance value to the CPU 114. The image density detection processing unit 305 is composed of semiconductor devices such as an FPGA (Field-Programmable Gate Array) and an ASIC (Application Specific Integrated Circuit). The image memory 303 stores the image data necessary for image processing in the CPU 114.

[0035] (Line sensor) FIG. 4 is a configuration explanatory diagram of the line sensor 301a. The line sensor 301b has the same configuration. The line sensor 301a is an optical sensor including light emitting parts 400a and 400b, light guides 402a and 402b, a lens array 403a, and a sensor chip group 401. The line sensor 301a is substantially a rectangular parallelepiped and reads an image with the longitudinal direction as the main scanning direction.

[0036] The light emitting units 400a and 400b are light sources composed of, for example, LEDs (Light Emitting Diodes) that emit white light. The light guide 402a has the light emitting unit 400a disposed at an end, and irradiates the light emitted from the light emitting unit 400a toward the recording paper. The light guide 402b has the light emitting unit 400b disposed at an end, and irradiates the light emitted from the light emitting unit 400b toward the recording paper. The light guides 402a and 402b are formed linearly in the main scanning direction. Therefore, the line sensor 301 irradiates light linearly in the main scanning direction. The main scanning direction of the line sensor unit 312 is the same as the main scanning direction of the printer 150.

[0037] The lens array 403a is an optical system that guides the reflected light of the light irradiated from the light emitting units 400a and 400b by the recording paper to the sensor chip group 401a. The sensor chip group 401a is a light receiving unit in which a plurality of photoelectric conversion elements (sensor chips) are arranged linearly in the main scanning direction. One sensor chip reads an image of one pixel. The plurality of sensor chips in the present embodiment have a three-line configuration. An R (red) color filter is applied to one line, a G (green) color filter is applied to another line, and a B (blue) color filter is applied to another line. The light guided by the lens array 403a forms an image on the light receiving surface of each sensor chip of the sensor chip group 401a.

[0038] The light emitted from the light emitting units 400a and 400b diffuses inside the light guides 402a and 402b, and is emitted from the portions having curvature, illuminating the entire area in the main scanning direction of the recording paper. The light guide 402a and the light guide 402b are arranged with the lens array 403a interposed therebetween in the sub-scanning direction orthogonal to the main scanning direction. Therefore, the line sensor 301a has a two-side illumination configuration that irradiates light from two directions in the sub-scanning direction with respect to the lens array 403a (image reading line). The sub-scanning direction of the line sensor unit 312a is the same as the sub-scanning direction of the printer 150.

[0039] (Calibration operation) The calibration operations of the line sensors 301a and 301b will be described. Here, the calibration operation of the line sensor 301a will be described, but the calibration operation of the line sensor 301b is performed in the same manner.

[0040] There are two calibration operations according to the states (here, the light-emitting states) of the light-emitting units 400a and 400b. The first calibration operation is an operation of reading the white reference plate 315a in a state where the light-emitting units 400a and 400b emit light and irradiate the white reference plate 315a with light, and correcting so that the output value of each pixel (sensor chip) becomes a target value. In the present embodiment, such calibration performed by causing the light-emitting units 400a and 400b to emit light is called "white shading adjustment". The correction methods include adjustment of the light-emitting time of the light-emitting units 400a and 400b, adjustment of the reading time of the line sensor 301a, and correction using a digital correction value (difference from the target value). In the present embodiment, the light-emitting time of the light-emitting units 400a and 400b and the reading time of the line sensor 301a are set in advance. Therefore, in the white shading adjustment, correction is performed using a digital correction value.

[0041] The second calibration operation is an operation called dark voltage correction of reading the white reference plate 315a with the light-emitting units 400a and 400b turned off and correcting the output value of each pixel (sensor chip) to a predetermined value (for example, 0 [V]). In the present embodiment, such calibration performed by turning off the light-emitting units 400a and 400b is called "black shading adjustment". By performing white shading adjustment and black shading adjustment, accurate detection of the image density by the line sensor 301a is enabled.

[0042] White shading adjustment and black shading adjustment are performed before the operation of reading the detection image. The controller 110 executes white shading adjustment and black shading adjustment at a predetermined time interval. During such calibration (white shading adjustment and black shading adjustment), the operation of reading the detection image is suspended. Therefore, the real-time performance of the adjustment of the image density deteriorates.

[0043] FIG. 5 is an explanatory diagram of the output change of the line sensor 301a when white shading adjustment is performed after black shading adjustment. When shading adjustment is required, the line sensor unit 312a moves to the reading position of the white reference plate 315a and performs black shading adjustment with the light emitting units 400a and 400b turned off. Thereafter, the line sensor unit 312a turns on the light emitting units 400a and 400b to perform white shading adjustment. Immediately after the light emitting units 400a and 400b are turned on, since the light emission amount is not stable, the output of the line sensor 301a fluctuates without being stable. Therefore, white shading adjustment needs to be performed after a predetermined time has elapsed since the light emitting units 400a and 400b are turned on. Due to this waiting time, the time required for the calibration operation increases.

[0044] On the other hand, black shading adjustment is less affected by environmental changes than white shading adjustment. This is due to the structure of the line sensor 301a, and in the case of white shading adjustment, it can be mentioned that there are more factors related to the components of the line sensor 301a than in black shading. Therefore, black shading adjustment does not need to be performed as frequently as white shading adjustment. From this, by making the time interval for performing black shading adjustment longer than that for white shading adjustment, it is possible to reduce the time required for the calibration operation. For example, white shading adjustment is performed every 5 minutes, and black shading adjustment is performed every 10 minutes. Thereby, it is possible to reduce the overall time required for the calibration operation compared to the case where black shading adjustment is performed at the same time interval as white shading adjustment.

[0045] (Image for detecting image density) FIG. 6 is an exemplary diagram of a detection image for detecting the image density formed on the recording paper. In the present embodiment, a detection image is added to the user image according to the print job. When image formation according to the print job is performed, the line sensor units 312a and 312b read the detection image from all the recording papers. The controller 110 adjusts the image density based on the reading result of the detection image by the line sensor units 312a and 312b. Therefore, real-time adjustment of the image density is possible.

[0046] The hatched portion is the printing area of the user image. The detection image is formed along the edge of the recording paper. In FIG. 6, the detection images are formed at both ends in the main scanning direction of the recording paper. The detection image has two-color test images 601 and 602 formed along one side, and the other two-color test images 603 and 604 formed along the other side. The test images 601 to 604 of the present embodiment are the four colors of yellow, magenta, cyan, and black, but are not limited thereto. The test images 601 to 604 of each color are printed prior to the user image.

[0047] The test images 601 to 604 are each composed of a combination of images with gradually changing image densities. In FIG. 6, the test images 601 to 604 are each composed of seven images arranged in the sub-scanning direction. The images at both ends in the sub-scanning direction have the same image density and the highest image density. The area where the test images 601 to 604 are formed is the area that is finally cut by the finisher 190. Therefore, the detection image is not printed on the final printed matter.

[0048] A predetermined range around the test images 601 to 604 is white. The white area has a longer distance from the user image to the test images 601 to 604 in the sub-scanning direction than in the main scanning direction. That is, the white area in the conveyance direction of the recording paper is larger than the white area in the reading direction of the recording paper. This makes the influence of the reflection from the user image when reading the detection image constant. Note that the positions of the test images 601 to 604 in FIG. 6 are merely examples and are not limited thereto.

[0049] (Image density detection process) FIG. 7 is a flowchart showing an image density detection process. This process is performed when an image forming process is carried out. Here, it is started when an instruction such as a document size or a printing mode is input from the operation panel 120 by the user.

[0050] The CPU 114 obtains an instruction from the operation panel 120, and performs mode setting (S500) by setting each device with information necessary for the print job based on the instruction and storing various parameters included in the instruction in the RAM 113. The CPU 114 waits for a copy start instruction from the operation panel 120 after mode setting (S501: N).

[0051] When the CPU 114 obtains a copy start instruction (S501: Y), it starts a printing process. The CPU 114 that has started the printing process initializes the page count value P to 0 (S502). The page count value P represents the number of sheets of recording paper on which an image has been printed. Next, the CPU 114 performs a calibration operation (black shading adjustment and white shading adjustment) on the line sensor units 312a and 312b (S503). When the calibration operation is completed, the CPU 114 initializes both the first timer T1 for white shading adjustment and the second timer T2 for black shading adjustment to 0 (S504). After initialization, the CPU 114 starts counting with the first timer T1 and the second timer T2.

[0052] In the copying process, the image forming apparatus 100 reads the image of the document by a scanner (not shown). The CPU 114 causes the printer 150 to print the document image read by the scanner on the recording paper together with the detection image (S505). The recording paper on which the image is formed is conveyed from the printer 150 to the reader 160. The document detection sensor 311 of the reader 160 detects the leading edge of the recording paper conveyed from the printer 150. The output signal of the document detection sensor 311 changes its signal value when detecting the leading edge of the recording paper. The CPU 114 increments the page count value P by 1 in response to the change in the signal value of the output signal of the document detection sensor 311 (S506).

[0053] Triggered by the change in the signal value of the output signal of the document detection sensor 311, the CPU 114 detects the image density of the detection image printed on the recording paper by the line sensor units 312a and 312b and the image density detection processing unit 305 (S507). Here, the luminance value may be detected instead of the image density. The CPU 114 derives a correction value for correcting the density deviation of the image density based on the detected image density (S508). Based on this correction value, the image forming conditions are adjusted to achieve stabilization of the image density at the next printing. The correction value is the difference between the detected image density and the reference image density value.

[0054] The CPU 114 that has derived the correction value determines whether the time measured by the first timer T1 is equal to or greater than the first predetermined time Ttar1 (S509). If the time measured by the first timer T1 is less than the first predetermined time Ttar1 (S509: N), the CPU 114 determines whether the page count value P is equal to or greater than the predetermined number of sheets Pmax for detecting the end of the image density (S511). The number of sheets Pmax for detecting the end is a preset number. If the page count value P is less than the number of sheets Pmax for detecting the end of the image density (S511: N), the CPU 114 repeats the processes after S505. If the page count value P is equal to or greater than the number of sheets Pmax for detecting the end of the image density (S511: Y), the CPU 114 ends the process.

[0055] When the time measured by the first timer T1 is greater than or equal to the first predetermined time Ttar1 (S509: Y), the CPU 114 determines whether the time measured by the second timer T2 is greater than or equal to the second predetermined time Ttar2 (S510). The second predetermined time Ttar2 is a time longer than the first predetermined time Ttar2.

[0056] When the time measured by the second timer T2 is greater than or equal to the second predetermined time Ttar2 (S510: Y), the CPU 114 performs black shading adjustment and white shading adjustment (S512). After performing the black shading adjustment and the white shading adjustment, the CPU 114 initializes the first timer T1 and the second timer T2 to 0 (S513), and performs the process of S511.

[0057] When the time measured by the second timer T2 is less than the second predetermined time Ttar2 (S510: N), the CPU 114 performs only white shading adjustment (S514). After performing the white shading adjustment, the CPU 114 initializes the first timer T1 to 0 (S515), and performs the process of S511.

[0058] In this way, when the second predetermined time Ttar2 longer than the first predetermined time Ttar1 has elapsed, black shading adjustment and white shading adjustment are performed. When a time longer than the first predetermined time Ttar1 and shorter than the second predetermined time Ttar2 has elapsed, only white shading adjustment is performed. That is, white shading adjustment is performed at intervals of the first predetermined time Ttar1, and white shading adjustment and black shading adjustment are performed at intervals of the second predetermined time Ttar2.

[0059] In this way, the execution interval of white shading adjustment is set shorter than that of black shading adjustment. Therefore, the number of times of black shading adjustment is reduced compared to the case where black shading adjustment and white shading adjustment are performed every time during calibration, and the overall time required for calibration is shortened. On the other hand, in this embodiment, the influence of environmental changes is more likely to be received by white shading adjustment than by black shading adjustment. Since white shading adjustment is performed more frequently than black shading adjustment, the influence of environmental changes on the line sensor units 312a and 312b is suppressed. Therefore, the adjustment of image density is performed based on the detection results of the accurate detection images by the line sensor units 312a and 312b, and accurate image density adjustment is realized.

[0060] In this embodiment, an example in which the execution interval of white shading adjustment is shorter than that of black shading adjustment has been described, but this relationship may be reversed. That is, the execution interval of black shading adjustment may be set shorter than that of white shading adjustment. Also in this case, the overall time required for calibration is shortened.

[0061] Which of the execution intervals of black shading adjustment and white shading adjustment is shortened is determined by the magnitude of the influence of environmental changes on the detection accuracy of the line sensor units 312a and 312b. By performing the shading adjustment with a greater influence of environmental changes more frequently with a shorter execution interval, the influence of environmental changes on the detection accuracy of the line sensor units 312a and 312b is reduced.

[0062] Although the image density adjustment has been described in this embodiment, this embodiment can also be applied when adjusting other image formation conditions. That is, (1) the printer 150 forms a detection image for image formation condition adjustment on the recording paper. (2) The line sensor units 312a and 312b detect the detection image formed on the recording paper. (3) Based on the detection results of the detection image by the line sensor units 312a and 312b, the image formation conditions are adjusted. As long as the image formation conditions are adjusted by the above processes (1) to (3), high-precision adjustment can be achieved in a short time by applying this embodiment.

Claims

1. A reading means for reading an image formed on a recording paper; A reference plate used for calibration of the reading means; A control means for calibrating the reading means by a first calibration operation of causing the reading means to read the reference plate in a first state and a second calibration operation of causing the reading means to read the reference plate in a second state; The reading means includes a light emitting means and a light receiving means; In the first state, the reading means reads the reference plate in a state where the light emitting means emits light to irradiate the reference plate with light, and in the second state, the reading means reads the reference plate in a state where the light emitting means is turned off; The control means is characterized in that the first calibration operation is performed at an execution interval longer than that of the second calibration operation; An image forming apparatus.

2. The reading means includes a light emitting means and a light receiving means; In the first state, the reading means reads the reference plate in a state where the light emitting means is turned off, and in the second state, the reading means reads the reference plate in a state where the light emitting means emits light to irradiate the reference plate with light, which is characterized in that; The image forming apparatus according to Claim 1.

3. Further comprising an image forming means for forming an image on a recording paper based on predetermined image forming conditions; The control means is characterized in that a correction value used for adjusting the image forming conditions is derived based on a reading result by the reading means of a detection image for adjusting the image forming conditions formed on the recording paper; The image forming apparatus according to Claim 1 or 2.

4. The control means is characterized in that the image forming means forms on the recording paper a detection image corresponding to the detection image and a print job, and the reading means reads the detection image formed on the recording paper; The image forming apparatus according to claim 3.

5. The image forming means forms the detection image on the recording paper with priority over the image according to the print job. The image forming apparatus according to claim 4.

6. The apparatus further comprises cutting means for cutting the area where the detection image is formed from the recording paper after the detection image is read by the reading means. The image forming apparatus according to any one of claims 3 to 5.

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