JP2024070047A - Image reading device, and image forming apparatus - Google Patents

Abstract

To appropriately generate and update black reference data and white reference data while preventing a reduction in the efficiency of document reading, thereby maintaining image quality.SOLUTION: In an image forming apparatus 10, a control unit 49 generates white reference data and black reference data based on output from a CIS 34 and stores them in memory 47A, causes the CIS 34 to start reading a plurality of documents, generates the black reference data based on the output from the CIS 34 during reading of each document, when determining that there is a large difference between the black reference data generated during the reading of each document and the black reference data stored in the memory 47A, suspends reading of the next document, generates the white reference data and the black reference data based on the output from the CIS 34, stores the data and updates in the memory 47A, and resumes reading of the next document.SELECTED DRAWING: Figure 4

Description

The present invention relates to an image reading device and an image forming device equipped with an image sensor that reads an image of a document, and in particular to a technique for appropriately updating white reference data and black reference data used in shading correction applied to image data representing an image of a document read by the image sensor.

In an image reading device, for example, while a document is pulled out from a document tray and transported, the document is irradiated with light from a light source and an image of the document is read by an image sensor. In addition, white reference data is generated based on the output of the image sensor when a white reference member illuminated by light from the light source is being read, and black reference data is generated based on the output of the image sensor when the light source is turned off, and shading correction is performed on image data showing the image of the document read by the image sensor based on the white reference data and black reference data. Examples of image sensors include contact image sensors (CIS) and CCDs.

For example, in the case of a CIS, shading correction is applied to image data representing the image of the original document read by the CIS because the conversion efficiency of each operational amplifier connected to the output side of each photoelectric conversion element is different.

In addition, in an image reading device equipped with an automatic document feeder (ADF), white reference data and black reference data are generated, and then continuous reading operations of multiple documents are started, and shading correction based on the white reference data and black reference data is sequentially performed on the image data representing the image of each document.

Here, the image sensor has temperature characteristics. The characteristics of the CIS also change depending on self-heating and the ambient temperature. For this reason, if white reference data and black reference data are generated when the image reading device is turned on or before the start of continuous reading operations of multiple documents, and this white reference data and black reference data are used continuously, the white reference data and black reference data will not be updated even though the characteristics of the CIS have changed, and appropriate shading correction will not be applied to the image data, resulting in a decrease in image quality.

For example, in the image reading device described in Patent Document 1, white reference data and black reference data are generated and updated one by one during the inter-paper time from when the trailing edge of a document in the transport direction passes the reading position to when the leading edge of the following document in the transport direction passes the reading position.

In addition, in the image reading device described in Patent Document 2, white reference data and black reference data are generated when the image reading device is turned on, and then black reference data is generated during the paper interval, and the white reference data is corrected based on the amount of change in the black reference data generated during this paper interval.

JP 2017-163453 A JP 2019-134351 A

However, in Patent Document 1, white reference data and black reference data are generated one by one for each paper interval, which reduces the efficiency of document reading. For black reference data, the light source is turned off and the output of the CIS is captured, so the processing time for generating the black reference data is short. In contrast, for white reference data, motor control is performed to move the white reference member in order to smooth out the effects of dirt on the white reference member read by the CIS, so the processing time for generating the white reference data is long. Therefore, the paper interval needs to be lengthened according to the processing time for generating the white reference data, resulting in a situation where the efficiency of document reading is reduced.

In addition, in Patent Document 2, black reference data is generated during the paper interval, and the white reference data is corrected based on the black reference data generated during this paper interval, so there is no need to read the white reference member using the CIS during the paper interval, there is no need to extend the paper interval, and there is no decrease in document reading efficiency. However, the method of correcting the white reference data based on the black reference data generated during the paper interval makes it impossible to obtain accurate white reference data, which reduces the accuracy of shading correction and leads to a decrease in image quality.

The present invention has been developed in consideration of the above circumstances, and aims to maintain image quality by accurately generating and updating black base data and white reference data while minimizing the decrease in document reading efficiency during continuous reading operations of multiple documents.

An image reading device according to one aspect of the present invention includes a document transport unit that transports a document, an image sensor that reads an image of the document being transported by the document transport unit, a memory that stores white reference data and black reference data used for shading correction, a control unit that generates the white reference data and the black reference data based on the output of the image sensor and stores them in the memory, starts reading of a plurality of documents by the image sensor, and acquires the latest black reference data based on the output of the image sensor while each document is being read, and acquires the latest black reference data generated while each document is being read for image data representing the image of the document read by the image sensor. and an image processing unit that performs shading correction based on the white reference data and the white reference data stored in the memory. The control unit determines whether to update the white reference data and the black reference data stored in the memory based on the difference between the black reference data generated during the reading of each document and the black reference data stored in the memory. If it is determined that the white reference data and the black reference data should be updated, the control unit interrupts the reading of the document by the document transport unit and the image sensor, obtains new white reference data based on the output of the image sensor, stores the new white reference data and the latest black reference data at this point in the memory to update them, and then resumes reading of the document.

An image forming apparatus according to one aspect of the present invention includes the image reading device of the present invention described above, and an image forming section that forms an image of the document read by the image reading device on recording paper.

According to the present invention, when performing continuous reading operations on multiple documents, it is possible to accurately generate and update black reference data and white reference data while suppressing a decrease in document reading efficiency, thereby maintaining image quality.

1 is a cross-sectional view showing an image forming apparatus to which an image reading apparatus according to an embodiment of the present invention is applied. FIG. 2 is a cross-sectional view showing an image reading device in the image forming apparatus. FIG. 1 is a perspective view showing an external appearance of an image reading device. 1 is a block diagram showing a main internal configuration of an image forming apparatus; 11 is a graph showing the relationship between the frequency of generating white reference data and document reading efficiency and image quality. FIG. 1 is a diagram illustrating a schematic diagram of a CIS composed of multiple sensor chips. 1A to 1C are a table and a graph showing black reference data, white reference data, and pixel data of an arbitrary image sensor, and a diagram showing a shading correction formula. 1A and 1B are diagrams showing a table and a graph showing changed black reference data, white reference data, and changed pixel data of an arbitrary image sensor, and a shading correction formula. 11A and 11B are diagrams showing a table and a graph showing changed black reference data, changed white reference data, and changed pixel data of an arbitrary image sensor, and a shading correction formula. 11 is a table showing the relationship between the update frequency of the white reference data and the number of documents read per minute. 11 is a flowchart showing a control procedure for generating black reference data for each paper interval time, and updating the white reference data and the black reference data when the amount of change in the black reference data increases. 12 is a table showing timings for updating the white reference data and the black reference data according to the control procedure of FIG. 11 .

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view showing an image forming apparatus to which an image reading device according to one embodiment of the present invention is applied. As shown in FIG. 1, the image forming apparatus 10 includes an image reading device 11 and an image forming unit 12.

The image reading device 11 has an imaging element (e.g., a contact image sensor) that optically reads the image of the document, and the analog output of this imaging element is converted into a digital signal to generate image data representing the image of the document.

The image forming unit 12 prints an image represented by the image data on a recording sheet, and includes an image forming unit 3M for magenta, an image forming unit 3C for cyan, an image forming unit 3Y for yellow, and an image forming unit 3Bk for black. In each of the image forming units 3M, 3C, 3Y, and 3Bk, the surface of the photosensitive drum 4 is uniformly charged, the surface of the photosensitive drum 4 is exposed to light, an electrostatic latent image is formed on the surface of the photosensitive drum 4, the electrostatic latent image on the photosensitive drum 4 is developed into a toner image, and the toner image on the photosensitive drum 4 is transferred to the intermediate transfer belt 5. As a result, a color toner image is formed on the intermediate transfer belt 5. This color toner image is secondarily transferred to the recording sheet P transported from the paper feed unit 14 through the transport path 8 in the nip area Np between the intermediate transfer belt 5 and the secondary transfer roller 6.

After this, the recording paper P is heated and pressurized by the fixing device 15 to fix the toner image on the recording paper P by thermocompression, and the recording paper P is then discharged to the discharge tray 17 via the discharge rollers 16.

Next, the image reading device 11 of this embodiment will be described. FIG. 2 is a cross-sectional view that shows the image reading device 11. FIG. 3 is a perspective view that shows the exterior of the image reading device 11, showing the document transport unit 20 in an open state.

As shown in FIG. 2, the image reading device 11 includes a document transport unit 20 and a reading unit 30. The document transport unit 20 includes a document tray 22, a pickup roller 25, a transport roller 26, a paper discharge roller 27, a paper discharge tray 28, and a white reference plate 33. The reading unit 30 includes a first platen glass 31, a second platen glass 32, and a contact image sensor (CIS) 34.

Two hinges 38 (shown in FIG. 3) are provided at a distance from one end of the top surface of the reading unit 30, and these hinges 38 support the document transport unit 20 so that it can be opened and closed, allowing the user to open and close the document transport unit 20.

The image reading device 11 is switched between a first mode in which an image of the original M placed on the second platen glass 32 is read, and a second mode in which an image of the original M is read while the original M is being transported by the original transport unit 20. For example, a first sensor (not shown) that detects the original M placed on the second platen glass 32, and a second sensor (not shown) that detects the original M placed on the original tray 22 of the original transport unit 20 are provided. When the original M is detected by the first sensor, the first mode is set, and when the original M is detected by the second sensor, the second mode is set.

In the first mode, the document transport unit 20 is opened, and the second platen glass 32 of the reading unit 30 is open, and the document M is placed on the second platen glass 32. The document transport unit 20 is then closed, and the document transport unit 20 presses the document M placed on the second platen glass 32. In the reading unit 30, while moving the CIS 34 in the sub-scanning direction X, light from a light source built into the CIS 34 is irradiated onto the document M through the second platen glass 32, and the light reflected by the document M is made incident on a line sensor built into the CIS 34, and the image of the document M is repeatedly read by this line sensor in the main scanning direction Y (the direction perpendicular to the sub-scanning direction X).

In the second mode, when the document transport unit 20 is closed and multiple documents M are placed on the document tray 22, each document M is sequentially pulled out of the document tray 22 by the pickup roller 25, and each document M is transported sequentially to the first platen glass 31 by the transport roller 26. Each document M passes over the first platen glass 31 and is discharged one by one to the discharge tray 28 by the discharge roller 27. In the reading unit 30, the CIS 34 is positioned below the first platen glass 31, and each time a document M passes over the first platen glass 31, light from a light source built into the CIS 34 is irradiated onto the document M through the first platen glass 31, and the light reflected by the document M is made incident on a line sensor built into the CIS 34, and the image of the document M is repeatedly read in the main scanning direction Y by this line sensor.

Figure 4 is a block diagram showing the internal configuration of the image forming device 10. As shown in Figure 4, the image forming device 10 includes an image reading device 11, an image forming unit 12, an operation unit 41, a display unit 42, a touch panel 44, an image memory 45, a storage unit 46, and a control unit 48. These components are capable of transmitting and receiving data or signals to and from each other via a bus.

The operation unit 41 includes physical keys such as a numeric keypad, a decision key, and a start key. The display unit 42 includes a liquid crystal display (LCD) or an organic light-emitting diode (OLED) display.

A touch panel 44 is disposed on the screen of the display unit 42. The touch panel 44 is a so-called resistive film type or capacitive type touch panel, which detects contact (touch) of a user's finger or the like with the touch panel 44 together with the contact position, and outputs a detection signal indicating the coordinates of the contact position to a control unit 49 of the control unit 48, which will be described later.

The image memory 45 stores image data representing the image of the original document read by the image reading device 11.

The storage unit 46 is a large-capacity storage device such as an SSD (Solid State Drive) or HDD (Hard Disk Drive), and stores various application programs and various data.

The control unit 48 is composed of a processor, a RAM (Random Access Memory), a ROM (Read Only Memory), etc. The processor is, for example, a CPU (Central Processing Unit), an ASIC, or an MPU (Micro Processing Unit). The control unit 48 functions as a control unit 49 and an image processing unit 47 by the processor executing a control program stored in the ROM or memory unit 46.

The control unit 49 provides overall control of the image forming device 10. The control unit 49 is connected to the image reading device 11, the image forming unit 12, the operation unit 41, the display unit 42, the touch panel 44, the image memory 45, and the storage unit 46, and controls the operation of these components and transmits and receives signals or data between these components.

The control unit 49 serves as a processing unit that executes various processes required for image formation by the image forming device 10. The control unit 49 also accepts operation instructions input by the user based on a detection signal output from the touch panel 44 or the operation of a physical key on the operation unit 41. For example, the control unit 49 accepts touch operations on a GUI (Graphical User Interface) displayed on the screen of the display unit 42 through the touch panel 44. Furthermore, the control unit 49 has a function of controlling the display operation of the display unit 42.

The image processing unit 47 writes and reads image data to the image memory 45, performs various processing of the image data, and performs shading correction using white reference data and black reference data.

Memory 47A is, for example, a non-volatile memory built into control unit 48, and stores white reference data and black reference data.

In the image forming device 10 configured as described above, for example, when a user places a document on the second platen glass 32 of the reading unit 30 of the image reading device 11, the first sensor detects the document M placed on the second platen glass 32, and the control unit 49 sets the first mode. Then, when the user operates the start key of the operation unit 41, the control unit 49 causes the CIS 34 to read the image of the document M on the second platen glass 32. The control unit 49 inputs the output of the CIS 34 and generates image data showing the image of the document M. The image processing unit 47 performs shading correction on this image data based on the white reference data and the black reference data, and stores this image data in the image memory 45. This image data is input from the image memory 45 to the image forming unit 12, and the image of the document M shown by this image data is formed on the recording paper P by the image forming unit 12 under the control of the control unit 49.

When the user places multiple documents M on the document tray 22 of the document transport unit 20 of the image reading device 11, the second sensor detects the documents M placed on the document tray 22, and the control unit 49 sets the second mode. Then, when the user operates the start key of the operation unit 41 to input a document reading instruction, the control unit 49 causes the document transport unit 20 to sequentially pull out and transport each document M from the document tray 22, and causes the CIS 34 to sequentially read the image of each document M. The control unit 49 generates image data showing the image of the document M each time it inputs the output of the CIS sensor 34. The image processing unit 47 performs shading correction on the image data of each document M based on the white reference data and the black reference data, and stores this image data in the image memory 45. The control unit 49 causes the image forming unit 12 to form an image on the recording paper P based on this image data stored in the image memory 45.

On the other hand, the control unit 49 generates white reference data and black reference data used for shading correction based on the output of the CIS 34 and stores them in the memory 47A. For the white reference data, the control unit 49 positions the CIS 34 below the first platen glass 31, turns on the light source built into the CIS 34, irradiates the light from this light source through the first platen glass 31 onto the white reference board 33, causes the line sensor built into the CIS 34 to read the white reference board 33, and causes the control unit 49 to generate white reference data indicating the output level of each image sensor of the line sensor and stores this white reference data in the memory 47A.

In addition, with regard to the black reference data, the control unit 49 causes the CIS 34 to perform reading using the built-in line sensor while the built-in light source of the CIS 34 is turned off, and the control unit 49 generates black reference data indicating the output level of each image sensor of the line sensor, and stores this black reference data in memory 47A.

In the second mode described above, immediately after the user operates the start key on the operation unit 41 to input an instruction to read a document, the control unit 49 drives the CIS 34 to generate white reference data and black reference data and stores them in the memory 47A. Alternatively, the control unit 49 may drive the CIS 34 to generate white reference data and black reference data and store them in advance in the memory 47A at a timing when an instruction to read a document is not input and document reading is not performed, that is, before the document is read.

After that, under the control of the control unit 49, the document transport unit 20 sequentially pulls out and transports each document M from the document tray 22, and the CIS 34 sequentially reads the image of each document M. The control unit 49 inputs the output of the CIS 34 indicating the image of the first document M, and generates image data indicating the image of the first document M. The image processing unit 47 then performs shading correction on this image data based on the white reference data and black reference data stored in the memory 47A, and stores this image data in the image memory 45. That is, for the image data indicating the image of the first document M, the image processing unit 47 uses the black reference data stored in the memory 47A at this time as the latest black reference data for shading correction. Under the control of the control unit 49, the image forming unit 12 forms the image of the first document M indicated by this image data on the recording paper P.

The characteristics of CIS 34 change depending on self-heating and the ambient temperature. Therefore, if shading correction is applied to image data representing the second or subsequent pages of original M by continuously using the white and black reference data stored in memory 47A before the start of the continuous reading operation of each original in the second mode, the white and black reference data will not be updated despite the change in the characteristics of CIS 34, and appropriate shading correction will not be applied to the image data, which may result in a decrease in image quality.

For this reason, in this embodiment, when the control unit 49 sets the second mode and sequentially reads the images of multiple documents M by the CIS 34, during the period from when the CIS 34 finishes reading the image of the document M to when it starts reading the image of the next document M (hereinafter referred to as the inter-page time), it turns off the light source built into the CIS 34, reads the image by the line sensor built into the CIS 34, and generates black reference data indicating the output level of each image sensor of the line sensor. The image processing unit 47 performs shading correction on the image data showing the image of the second or subsequent document M using the latest black reference data generated at this time during the inter-page time immediately before the CIS 34 starts reading the image of the second or subsequent document M, and the white reference data stored in the memory 47A at this time. Therefore, black reference data is generated every time the inter-page time occurs, and this generated black reference data is used for shading correction.

However, since the acquisition of the white reference data requires the CIS 34 to read the white reference plate 33, the processing time required for generating the data is long. Therefore, if the reading of the document M is interrupted to secure this processing time, the efficiency of reading the document M is reduced. For example, if the CIS 34 is moved so that the CIS 34 can read the unsoiled parts of the white reference plate 33, the time required to move the CIS 34 is required, and the processing time for generating the white reference data is further increased. Alternatively, in a configuration using a white shading roller instead of the white reference plate 33, the time required to rotate the shading roller is required so that the CIS 34 can read the unsoiled parts of the shading roller, and the processing time for generating the white reference data is increased. As is clear from the graph G1 in FIG. 5, the more frequently the white reference data is generated, the better the image quality is, but the lower the reading efficiency is.

Therefore, the control unit 49 generates black reference data for each inter-paper time, calculates the difference between this generated black reference data and the black reference data stored in memory 47A at this point, and if this difference is equal to or greater than a preset threshold, determines that the white reference data and black reference data stored in memory 47A should be updated (the time to update the white reference data and black reference data has arrived). When this determination is made, the control unit 49 suspends the reading of the next document M by the CIS 34.

While the reading of the next document M is suspended, the control unit 49 controls the driving of the CIS 34 and the light source to obtain white reference data indicating the output level of each image sensor of the line sensor when the light source is on, which is obtained by the CIS 34 reading the white reference board 33, and black reference data indicating the output level of each image sensor of the line sensor when the light source is off, and stores the white reference data and black reference data generated by this acquisition in the memory 47A to update them. At this time, the control unit 49 may not obtain the black reference data, but may store the black reference data obtained most recently between sheets in the memory 47A to update them. When the white reference data and black reference data stored in the memory 47A are updated, the control unit 49 causes the CIS 34 to resume reading the next document M.

Therefore, the white reference data stored in memory 47A is updated only when the difference between the black reference data generated for each paper interval and the black reference data stored in memory 47A (black reference data acquired before document reading begins) becomes equal to or exceeds a threshold value, that is, when the amount of change in the black reference data increases. Since this is not performed for every paper interval when reading each document, the number of times the white reference data is updated is reduced, and a decrease in the efficiency of reading the document M is prevented.

The amount of change in the black reference data also changes depending on the heat generated by CIS 34 and the ambient temperature, and the amount of change in the white reference data also changes. Therefore, when the amount of change in the black reference data increases, the amount of change in the white reference data also increases. If the white reference data and black reference data stored in memory 47A are updated at this time, shading correction can be performed with high accuracy (accuracy can be maintained).

In other words, when the second mode is set and multiple documents M are read continuously, the white reference data and black reference data stored in memory 47A can be accurately generated and updated while suppressing a decrease in the efficiency of reading the documents M, thereby maintaining image quality.

Next, we will explain in more detail how to update the white and black reference data stored in memory 47A.

Here, the CIS 34 is configured by arranging 12 sensor chips 34-1 to 34-12 in the main scanning direction as shown in FIG. 6. Each of the sensor chips 34-1 to 34-12 has 432 image sensors and light sources arranged at a density of 600 dpi, and reads a pixel row consisting of 432 pixels. Each of these sensor chips 34-1 to 34-12 has different photoelectric conversion efficiency and temperature characteristics.

For this reason, white reference data indicating the output level of each image sensor when the light source is on and black reference data indicating the output level of each image sensor when the light source is off are generated separately for each sensor chip 34-1 to 34-12, and shading correction based on the white reference data and black reference data is applied to the value of each pixel read by the sensor chip.

For example, as shown in table HA and graph GA in FIG. 7, when black reference data values "0", "0" are generated indicating the output level of any image sensor in each of sensor chips 34-1, 34-2, and white reference data values "80", "70" are generated indicating the output level of that image sensor in each of sensor chips 34-1, 34-2, even if images of the same density are read by the image sensors in each of sensor chips 34-1, 34-2 and the data values (pixel values) of these images are "40" and "35", the data values of each pixel are corrected to "50", "50" based on the shading correction formula (1) shown in FIG. 7, and the data values of each pixel become the same.

However, if the characteristics of each sensor chip 34-1, 34-2 change and, during the above-mentioned paper interval, the black reference data values "10", "10" indicating the output level of any image sensor of each sensor chip 34-1, 34-2 are generated and acquired as shown in table HB and graph GB in FIG. 8, but the white reference data values "80", "70" indicating the output level of that image sensor of each sensor chip 34-1, 34-2 are maintained as they are, pixels of the same density are read by the image sensor of each sensor chip 34-1, 34-2, and when the data values of these pixels change to "50" and "45", the data values of each pixel are corrected to "57", "58" based on the shading correction formula (1), and the data values of each pixel become inaccurate.

Therefore, the control unit 49 calculates the difference "10" between the values "0", "0" of the black reference data of each sensor chip 34-1, 34-2 in table HA of Fig. 7 and the values "10", "10" of the black reference data of each sensor chip 34-1, 34-2 in table HB of Fig. 8, and compares this difference "10" with a preset threshold value. Then, if the difference "10" is equal to or greater than the threshold value, the control unit 49 determines that the white reference data and black reference data should be updated, suspends the reading of the next document M, and during the suspension, obtains and generates the white reference data and recorded black reference data at this time, and updates the white reference data and recorded black reference data stored in memory 47A. In this case, as shown in Table HC and Graph GC of FIG. 9, the black reference data values "10", "10" indicating the output level of any image sensor of each sensor chip 34-1, 34-2 are generated and updated, and the white reference data values "90", "80" indicating the output level of that image sensor of each sensor chip 34-1, 34-2 are generated and updated. Then, when images of the same density are read by the image sensors of each sensor chip 34-1, 34-2, even if the data values (pixel values) of these images change to "50", "45", the data values of each pixel are corrected to "50", "50" by the shading correction formula (1) using the updated white reference data and black reference data, and the data values of each pixel become accurate.

When the amount of change in the black reference data generated during the paper interval increases in this way, the amount of change in the white reference data also increases, so the reading of the next document M is interrupted and the white reference data and black reference data are updated to maintain the accuracy of the shading correction.

Table HD in FIG. 10 shows the correspondence between the update frequency of the white reference data and the reading efficiency of the original M relative to the number of pages read of the original M. As is clear from Table HD in FIG. 10, the number of pages read of the original M is the greatest when the white reference data is never updated, and the number of pages read of the original M is the least when the white reference data is updated each time one page of the original M is read. In this embodiment, the update frequency of the white reference data is appropriately set to maintain the reading efficiency of the original M while suppressing degradation of image quality.

Next, the control procedure for generating black reference data for each paper interval and updating the white reference data and black reference data when the amount of change in the black reference data increases will be explained with reference to the flowchart shown in FIG. 11.

When the user places multiple documents M on the document tray 22 of the document transport unit 20 of the image reading device 11, the control unit 49 sets the second mode (S101). When the second mode is set, the control unit 49 generates white reference data indicating the output level of each image sensor when the light source is on and black reference data indicating the output level of each image sensor when the light source is off for each sensor chip 34-1 to 34-12 of the CIS 34, and stores them in the memory 47A (S102). The control unit 49 initializes the number n of documents M read by the CIS 34 to "0" (S103), and also initializes the white reference data generation flag FG to off (S104).

In response to a user's operation of the start key on the operation unit 41, the control unit 49 causes the document transport unit 20 of the image reading device 11 to pull out and transport the document M from the document tray 22, and causes the CIS 34 to read the image of the document M (S105). When the control unit 49 inputs the output of the CIS sensor 34, it generates image data representing the image of the document M. The image processing unit 47 applies shading correction to this image data based on the white reference data and black reference data stored in the memory 47A (S106), and stores this image data in the image memory 45. The control unit 49 causes the image forming unit 12 to form, on recording paper P, an image of the document M represented by the image data stored in the image memory 45.

Then, the control unit 49 determines whether the white reference data generation flag FG is off (S107). If the control unit 49 determines that the white reference data generation flag FG is off (S107 "off"), it counts up the number n of originals M read by the CIS 34 to "1" (S108). The control unit 49 generates black reference data indicating the output level of each image sensor when the light source is off for each sensor chip 34-1 to 34-12 of the CIS 34 during the inter-paper time until the next original M is pulled out from the original tray 22 by the original transport unit 20 and transported (S109).

The control unit 49 compares the number of read sheets n, "1," with a preset number of sheets N (N is a natural number, for example, "2") (S110). The value of N can be changed as appropriate based on an instruction input by operating the operation unit 41, for example.

When the control unit 49 determines that the number of read sheets n has not reached the fixed number N (S110 "No"), it determines whether there is another document M on the document tray 22 (S111). When the control unit 49 determines that there is another document M on the document tray 22 (S111 "Yes"), it causes the document transport unit 20 to pull out the next document M from the document tray 22 and transport it, and causes the CIS 34 to read the image of the next document M (S112). The image processing unit 47 obtains image data representing the image of the document M from the CIS sensor 34, and performs shading correction on this image data using the black reference data generated in S109 and the white reference data stored in the memory 47A at this time (S113), and stores this image data in the image memory 45. The control unit 49 causes the image forming unit 12 to form an image of the document M represented by the image data on the recording paper P. After this, the process returns to S107. After this, the processes of S107 to S110 are repeated.

The processes of S107 to S110 are repeated, and in S110, the control unit 49 compares the number of read sheets n with a fixed number of sheets N (N=2) (S110), and when it is determined that the number of read sheets n has reached the fixed number of sheets N (S110 "Yes"), the control unit 49 compares the black reference data generated in S109 with the black reference data stored in the memory 47A at this point, and calculates the difference X between the two black reference data (S114). For example, the control unit 49 calculates the average value of the output of each image sensor indicated by the generated black reference data for each sensor chip 34-1 to 34-12 of the line sensor built into the CIS 34, and also calculates the average value of the output of each image sensor indicated by the black reference data stored in the memory 47A, and calculates the difference X between the two average values. Then, the control unit 49 determines whether the calculated difference X is equal to or greater than a preset threshold value SV (S115).

If the respective differences calculated in S114 are less than the threshold value SV (S115 "No"), the control unit 49 determines that the amount of change in the black reference data is small and the amount of change in the white reference data is also small, and initializes the number n of read pages of the document M to "0" (S116).
The control unit 49 then determines whether or not there is an original M on the original tray 22 (S117), and when it determines that there is an original M on the original tray 22 (S117 "Yes"), it causes the original transport unit 20 to pull out the next original M from the original tray 22 and transport it, and causes the CIS 34 to read the image of the next original M (S118). The image processing unit 47 acquires image data showing the image of the original M from the CIS sensor 34, performs shading correction on this image data using the black reference data generated in S109 and the white reference data stored in the memory 47A at this time (S119), and stores this image data in the image memory 45. The control unit 49 causes the image forming unit 12 to form an image of the original M shown by the image data on the recording paper P. After this, the process returns to S107, and the process from S107 is repeated.

Therefore, when the amount of change in the black reference data is small and the amount of change in the white reference data is also small, the number of pages n of the original M to be read is initially set to "0", CIS 34 continues to read the original M, and each time the number of pages n of the original M to be read reaches a certain number N (N=2), the difference X between the black reference data generated during the paper interval and the black reference data stored in memory 47A is calculated. Then, if this difference X is less than the threshold value SV, the number of pages n of the original M to be read is initially set to "0", and CIS 34 repeats reading the original M.

For example, as shown in Table HE of FIG. 12, when the second original M is read and shading correction is performed, the amount of change in the black reference data generated during the inter-paper time before the second original M is read is small, so reading of the third original M continues, and when the fourth original M is read and shading correction is performed, the amount of change in the black reference data generated during the inter-paper time before the fourth original M is read is also small, so reading of the fifth original M continues.

If the difference calculated in S114 is equal to or greater than the threshold value SV (S115 "Yes"), the control unit 49 determines that the amount of change in the black reference data is large and that the amount of change in the white reference data is also large, that is, the amount of change in the black reference data and the white reference data has become large to the extent that the quality of the image obtained by shading correction is lower than the expected quality, and switches the white reference data generation flag FG from OFF to ON (S120). At this time, the next document M is not read, and the process returns to S107.

When the process returns to S107 in this way, the control unit 49 determines in S107 that the white reference data generation flag FG is on (S107 "on"), stops the document transport unit 20 from drawing out and transporting the next document M from the document tray 22, and interrupts the reading of the next document M by the CIS 34 (S121). The control unit 49 acquires and generates white reference data indicating the output level of each image sensor when the light source is on and black reference data indicating the output level of each image sensor when the light source is off for each sensor chip 34-1 to 34-12 of the CIS 34, and stores the newly generated white reference data and black reference data in memory 47A to update them (S122).

After this, the control unit 49 initializes the number n of originals M to be read to "0" (S123), initializes the white reference data generation flag FG to off (S124), and then determines whether or not there is an original M on the original tray 22 (S125). When the control unit 49 determines that there is an original M on the original tray 22 (S125 "Yes"), it causes the original transport unit 20 to pull out the next original M from the original tray 22 and transport it, and causes the CIS 34 to read the image of the next original M (S126). The image processing unit 47 acquires image data representing the image of the original M from the CIS sensor 34, performs shading correction on this image data using the updated white reference data and black reference data stored in the memory 47A (S127), and stores this image data in the image memory 45. The control unit 49 causes the image forming unit 12 to form an image of the original M represented by the image data on the recording paper P. Then, the process returns to S107.

Therefore, when the change in the black reference data is large and the change in the white reference data is also large, that is, when the change in the black reference data and the white reference data becomes large enough that the quality of the image obtained by shading correction is lower than the expected quality, the white reference data generation flag FG is switched from off to on, the document transport unit 20 stops drawing out and transporting the next document M from the document tray 22, the CIS 34 stops reading the next document M, and the white reference data and the black reference data are generated and stored in the memory 47A and updated. Note that the black reference data for updating may be (i) the black reference data newly acquired at the time of interruption, or (ii) the black reference data acquired between the most recent sheets (stored in a storage unit such as the memory 47A) may be used as the black reference data for updating, without acquiring new black reference data at the time of interruption. In other words, the "latest black reference data at this point" in the claims includes both of the black reference data (i) and (ii) above. After this, the number n of originals M to be read is initialized to "0", the white reference data generation flag FG is initialized to off, and the drawing and transport of the next original M is resumed, and the next original M is read by the CIS 34.

For example, as shown in the table of FIG. 12, when the sixth original M is read and shading correction is performed, the amount of change in the black reference data generated during the inter-paper time before reading the sixth original M is large, so the white reference data generation flag FG is turned on, the reading of the seventh original M is interrupted, white reference data and black reference data are generated and stored in memory 47A for update, the white reference data generation flag FG is turned off, and the seventh original M is read and shading correction is performed.

If the control unit 49 determines that there is no document M on the document tray 22 (S111 "No", S117 "No", or S125 "No"), the process shown in FIG. 11 ends.

In this manner, in the image reading device 11 of this embodiment, before the second mode is set and multiple documents M are successively read, the control unit 49 generates white reference data and black reference data and stores them in memory 47A. When reading of each document M begins, black reference data is generated for each inter-paper time, and this generated black reference data is compared with the black reference data stored in memory 47A at this point. If the difference between the two black reference data is equal to or greater than the threshold value SV, the reading of the document M is interrupted and the white reference data and black reference data stored in memory 47A are updated. This makes it possible to maintain image quality while minimizing the decrease in the efficiency of reading the document M.

In the above embodiment, the control unit 49 calculates the average value of the output of each image sensor indicated by the black reference data for each sensor chip 34-1 to 34-12, calculates the average value of the output of each image sensor indicated by the black reference data stored in memory 47A, calculates the difference X between the two average values, and compares the calculated difference X between the average values with the threshold value SV. Alternatively, the control unit 49 may select the maximum value from among the outputs of all image sensors indicated by the black reference data for each sensor chip 34-1 to 34-12, and may select the maximum value from among the outputs of all image sensors indicated by the black reference data stored in memory 47A, calculate the difference between the two maximum values, and compare the calculated difference between the maximum values with the threshold value SV.

Note that the configuration and processing of the above embodiment described using Figures 1 to 12 are merely examples of the present invention, and are not intended to limit the present invention to these configurations and processing.

10 Image forming apparatus 11 Image reading apparatus 12 Image forming section 20 Document conveying section 30 Reading section 34 Contact image sensor (CIS)
41 Operation section 42 Display section 44 Touch panel 45 Image memory 46 Storage section 47 Image processing section 47A Memory 48 Control unit 49 Control section

Claims (5)

a document transport unit that transports a document;
an image sensor that reads an image of the document being transported by the document transport unit;
a memory for storing white reference data and black reference data used for shading correction;
a control unit that generates the white reference data and the black reference data based on an output of the image sensor, stores the white reference data and the black reference data in the memory, starts reading a plurality of documents by the image sensor, and acquires the latest black reference data based on the output of the image sensor while reading each of the documents;
an image processing unit that performs shading correction on image data representing an image of the document read by the image sensor based on the latest black reference data generated during the reading of each document and the white reference data stored in the memory;
The control unit determines whether or not to update the white reference data and black reference data stored in the memory based on a difference between the black reference data generated during the reading of each document and the black reference data stored in the memory, and if it determines to update, interrupts the reading of the document by the document transport unit and the image sensor, obtains new white reference data based on the output of the image sensor, stores the new white reference data and the latest black reference data at this point in the memory to update them, and then resumes reading of the document. The image reading device according to claim 1, wherein the control unit calculates the difference between the black reference data generated during the reading of each document and the black reference data stored in the memory, and when the difference is equal to or greater than a preset threshold, determines to update the white reference data and the black reference data stored in the memory. The image reading device according to claim 1, wherein the control unit starts reading a plurality of documents by the image sensor, counts the number of documents read, and each time the counted number increases by a preset number, determines whether or not to update the white reference data and black reference data stored in the memory based on the difference between the latest black reference data at that time and the black reference data stored in the memory. The image sensor is a contact image sensor including a plurality of sensor chips arranged to read a pixel row composed of a plurality of pixels;
the control unit generates white reference data and black reference data for each of the sensor chips and stores them in the memory, starts reading a plurality of documents by each of the sensor chips, and acquires the latest black reference data for each of the documents based on the output of each of the sensor chips during the reading of each document;
the image processing unit performs shading correction for each of the sensor chips on image data representing an image of the document read by each of the sensor chips, based on the latest black reference data generated during the reading of each of the documents and the latest white reference data stored in the memory;
2. The image reading device according to claim 1, wherein the control unit calculates, for each of the sensor chips, a difference between each of the black reference data acquired during the reading of each of the documents and each of the black reference data stored in the memory, and when any of the differences is equal to or greater than a preset threshold, determines to update the black reference data and the white reference data of each of the sensor chips stored in the memory, interrupts the reading of the documents by the document transport unit and the image sensor, acquires new white reference data for each of the sensor chips, stores the new white reference data and the latest black reference data at this point in the memory to update them, and then resumes the reading of the documents. An image reading device according to any one of claims 1 to 4,
an image forming section that forms an image of the document read by the image reading device on a recording sheet;