Patent Publication Number: US-8995856-B2

Title: Image processing device, image processing method and print system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image processing device, an image processing method and a print system for correcting density unevenness in the sub-scanning direction, which is caused in a print device. 
     2. Description of Related Art 
     In a print device for printing an image on recording paper, density unevenness is caused due to various types of factors. In order to reduce the density unevenness, the following correction process is generally carried out. In the correction process, a test image is printed on recording paper, and the density unevenness is measured by optically reading the printed image. Then, the correction data for cancelling the measured density unevenness is prepared, and the image data to be printed is corrected in accordance with the prepared correction data. 
     The density unevenness is caused in the main scanning direction which is perpendicular to the conveying direction of the recording paper and in the sub-scanning direction which is the conveying direction of the recording paper. Therefore, in order to precisely correct the density unevenness, it is desired to correct the density unevenness not only in the main scanning direction but also in the sub-scanning direction. 
     For example, in Japanese Patent Application Publication No. 2009-192896, as a method for reducing the density unevenness both in the main scanning direction and in the sub-scanning direction, the following technology is disclosed. In the technology, test charts having each constant density in the main scanning direction and in the sub-scanning direction are printed on a plurality of pages so as to gradually change the density of the test chart printed in every page, and the density unevenness of each density is measured in the main scanning direction and in the sub-scanning direction to correct the density unevenness. 
     On the other hand, because there are various types of factors of the density unevenness, it is effective to correct the density unevenness which is caused by a main factor. In Japanese Patent Application Publication No. 2010-220182, the following technology is disclosed. In the technology, from an output image or an image processed in an image forming process, such as an image formed on a photoconductive drum or an intermediate transfer belt, the density unevenness is resolved into the factors thereof and the factors of the density unevenness are extracted. Then, by sequentially correcting each of the extracted factors in series, the contribution ratio of each factor to the entire density unevenness is estimated. 
     The density unevenness in the sub-scanning direction is caused by, for example, the non-uniformity of rotating parts of the print device, such as a photoconductive drum, a intermediate transfer belt, various types of rollers and the like, and the switching on/off of a fixing heater. Further, the density unevenness in the sub-scanning direction is often periodically caused. 
     Conventionally, in case that the density unevenness in the sub-scanning direction is corrected, for example, as shown in  FIG. 15 , a test image  202  which is printed on one sheet of recording paper  201  and which has the even density is optically read to obtain the measuring data. In accordance with the obtained measuring data, the profile  203  of the density unevenness in the sub-scanning direction is prepared. Then, by carrying out the frequency analysis or the like for the profile  203 , the periodic density unevenness in the sub-scanning direction is extracted and corrected. 
     However, in the density unevenness in the sub-scanning direction, there is density unevenness having a long period which cannot be contained in one sheet of recording paper  201 . Therefore, in the conventional method using the measuring data obtained from the test image  202  printed on one sheet of recording paper  201 , the above long period density unevenness in the sub-scanning direction cannot be removed. 
     As the factor of the long period density unevenness in the sub-scanning direction, for example, parts of the print device, which have the lengths longer than that of one sheet of recording paper, such as an intermediate transfer belt, the switching-on timing of a fixing heater, the toner supply timing and the like are considered. 
     In the technology disclosed in Japanese Patent Application Publication No. 2009-192896, a plurality of sheets of recording paper are used. However, the density unevenness for one density is measured by using only one sheet of recording paper. Further, in Japanese Patent Application Publication No. 2010-220182, the measuring data is obtained, by using one sheet of recording paper. These technologies are not suitable for the long period density unevenness in the sub-scanning direction. 
     SUMMARY 
     To achieve at least one of the abovementioned objects, an image processing device reflecting one aspect of the present invention comprises: 
     a sample output unit to instruct a predetermined print device for printing an image on conveyed recording paper, to successively print test images for detecting a density unevenness in a sub-scanning direction which is a conveying direction of the recording paper, on a plurality of sheets of the recording paper; 
     a profile preparing unit to prepare a plurality of profiles of the density unevenness in the sub-scanning direction in the plurality of sheets of the recording paper, from measuring data obtained by optically reading the test images which are successively printed on the plurality of sheets of the recording paper; 
     an analyzing unit to analyze the plurality of profiles in the plurality of sheets of the recording paper, which are prepared by the profile preparing unit, by arranging the profiles in a printing order apart from each other so as to correspond to intervals of the sheets of the recording paper, and to detect a long period density unevenness in the sub-scanning direction, which has a long period extending through the plurality of sheets of the recording paper; and 
     a correction data preparing unit to prepare correction data for removing the long period density unevenness in the sub-scanning direction, which is detected by the analyzing unit. 
     Preferably, the analyzing unit prepares data in which the long period density unevenness in the sub-scanning density is removed from the profiles prepared by the profile preparing unit, and detects a short period density unevenness in the sub-scanning direction, which has a short period contained in one sheet of the recording paper, by analyzing the prepared data, and 
     the correction data preparing unit prepares correction data for removing the detected short period density unevenness in the sub-scanning direction. 
     Preferably, the analyzing unit calculates an in-page density distribution which is always caused on a same position in the sub-scanning direction in one sheet of the recording paper, from the plurality of profiles in the plurality of sheets of the recording paper, which are prepared by the profile preparing unit, and detects the long period density unevenness in the sub-scanning density after the in-page density distribution is removed. 
     Preferably, the analyzing unit calculates the intervals of the sheets from the number of sheets on which the test image is printed by the print device in unit time, a convey speed of the recording paper and a length of the recording paper in the sub-scanning direction. 
     Preferably, the sample output unit instructs the print device to print predetermined marks on the plurality of sheets of the recording paper on which the test images are printed, so as to arrange the marks in the sub-scanning direction at regular intervals including the intervals of the sheets, and 
     the analyzing unit calculates the intervals of the sheets in accordance with positions of the marks printed on the plurality of sheets of the recording paper. 
     Preferably, a sampling pattern of the measuring data for preparing the profiles of the density unevenness in the sub-scanning direction is changed according to a type of the density unevenness in the sub-scanning direction to be detected. 
     Preferably, the sample output unit instructs the print device to print the test images on the recording paper out of an effective image area. 
     Preferably, the image processing device further comprises a factor specifying unit to specify a factor of the density unevenness in the sub-scanning direction in accordance with a period of the detected density unevenness in the sub-scanning direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinafter and the accompanying drawings given by way of illustration only, and thus are not intended as a definition of the limits of the present invention, and wherein: 
         FIG. 1  is a view showing the mechanical schematic structure of the print system including the function of the image processing device according to the embodiment; 
         FIG. 2  is a block diagram showing the electric schematic structure of the print system according to the embodiment; 
         FIGS. 3A and 3B  are views showing an example of the test charts for measuring the density unevenness in the sub-scanning direction; 
         FIG. 4  is a flowchart showing the process for measuring and correcting the long period density unevenness in the sub-scanning direction in the print system according to the embodiment; 
         FIG. 5  is a view showing the profiles for ten test charts which are successively printed, so as to overlap the profiles; 
         FIGS. 6A to 6C  are views showing the situation in which the profiles for ten test charts which are successively printed are arranged in the printing order in view of the intervals of the sheets of recording paper, and the like; 
         FIG. 7  is a view showing the profiles which are arranged in view of the intervals of the sheets of recording paper, so as to be related to the timings at which the specific portion reaches the secondary transfer position; 
         FIG. 8  is an explanatory view showing the situation in which the marks are put on the intermediate transfer belt at the constant interval which does not depend on the timing of conveying the recording paper; 
         FIGS. 9A and 9B  are explanatory views showing an example of the case in which the interval of the sheets is calculated in accordance with the marks put on the recording paper; 
         FIG. 10  is a view showing an example of the case in which the sub-scanning belt and the marks are printed out of the effective image area on the recording paper; 
         FIG. 11  is a flowchart showing the process for measuring and correcting the long period density unevenness and the short period density unevenness in the sub-scanning direction in the print system according to the embodiment; 
         FIGS. 12A and 12B  are the profiles relating to ten test charts which are successively printed and the profile (in-page incline) in which the profiles relating to ten test charts are averaged in one sheet of recording paper; 
         FIG. 13  is a flowchart showing the process for correcting the long period density unevenness and the short period density unevenness in the sub-scanning direction after the density unevenness in the sub-scanning direction, which is always caused on the same position in the recording paper, is removed; 
         FIG. 14  is a view showing the test image and the sampling timings thereof in case that the profiles corresponding to the long period density unevenness and the short period density unevenness in the sub-scanning direction, are prepared; and 
         FIG. 15  is a view showing the density unevenness in the sub-scanning direction, which is caused in one sheet of recording paper, and the sampling timings thereof. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     Hereinafter, a preferred embodiment of the present invention will be explained with reference to the accompanying drawings. 
       FIG. 1  is a view showing the mechanical schematic structure of the print system  10  including the function of the image processing device according to the embodiment. The print system  10  has a print function for printing out an image on recording paper in accordance with print data input from an external device via a network or the like, a copy function for printing out an image on recording paper in accordance with image data obtained by optically reading an original, and the like. The print system  10  is not required to carry out color printing, and may be a system for carrying out monochrome printing. 
     The print system  10  comprises an intermediate transfer belt  11  having a predetermined width, which is circularly bridged endlessly, four image forming units  12 Y,  12 M,  12 C and  12 K for forming single color toner images having the colors of yellow (Y), magenta (M), cyan (C) and black (K) respectively, on the intermediate transfer belt  11 , a paper feeding unit  13  for feeding the recording paper, a conveying unit for conveying the fed recording paper, a fixing device  15 , a belt cleaning device  19 , a reading unit  18  for optically reading an image printed on the recording paper, and the like. 
     In the image forming units  12 Y,  12 M,  12 C and  12 K, the colors of the toners to be used are different from each other. These image forming units have the same structure. Each of the image forming units  12 Y,  12 M,  12 C and  12 K comprises a cylindrical photoconductive drum  16  as an electrostatic latent image carrier on which an electronic latent image is formed. Around the cylindrical photoconductive drum  16 , a charging device, a developing device, a transfer device, a cleaning device and the like are provided. In each of the image forming units  12 Y,  12 M,  12 C and  12 K, a laser unit  17  having a laser diode, a polygon mirror, various types of lenses, mirrors and the like, is provided. 
     In each of the image forming units  12 Y,  12 M,  12 C and  12 K, the photoconductive drum  16  is rotated in a constant direction by a driving unit which is not shown in the drawing. The charging device uniformly charges the photoconductive drum  16 . The laser unit  17  forms an electronic latent image on the surface of the photoconductive drum  16  by scanning the photoconductive dram  16  with the laser light which is switched on/off in accordance with the image data of the corresponding color. 
     The laser light repeatedly scans the photoconductive drum  16  in the axial direction thereof. By rotating the photoconductive drum  16 , the two-dimensional electronic latent image is formed on the photoconductive drum  16 . The direction (axial direction of the photoconductive drum) in which the laser light scans the surface of the photoconductive drum  16  is the main scanning direction. The direction in which the photoconductive drum  16  is rotated is the sub-scanning direction. 
     The developing device visualizes the electrostatic latent image on the photoconductive drum  16  by using the toner. The toner image formed on the surface of the photoconductive drum  16  is transferred to the intermediate transfer belt  11  at the portion where the photoconductive drum  16  contacts with the intermediate transfer belt  11 . The cleaning device removes and collects the toner which remains on the surface of the photoconductive drum  16  by rubbing the remaining toner with a blade or the like after the transfer of the toner image. 
     The intermediate transfer belt  11  is wound so as to be bridged by a plurality of rollers, and is rotated in the direction of the arrow A in the drawing. In a process of the rotation of the intermediate transfer belt  11 , the images (toner images) of the respective colors are formed on the intermediate transfer belt  11  so as to overlap the images in the order of the color Y, the color M, the color C and the color K by the image forming units  12 Y,  12 M,  12 C and  12 K. Thereby, the color image is composed. This color image is transferred from the intermediate transfer belt  11  to the recording paper at a second transferring position Q. The toner which remains on the intermediate transfer belt  11  after the transfer is removed by the belt cleaning device  19  provided on the downstream side of the second transferring position Q. 
     On the intermediate transfer belt  11 , the width direction of the intermediate transfer belt  11  is the main scanning direction. The rotating direction of the intermediate transfer belt  11  is the sub-scanning direction. On the recording paper, the conveying direction of the recording paper is the sub-scanning direction. The direction (the width direction of the recording paper) perpendicular to the conveying direction is the main scanning direction. 
     The paper feeding unit  13  comprises a plurality of paper feed trays  13   a  that contain the recording paper used in printing. The paper feeding unit  13  feeds the recording paper sheet by sheet from the selected paper feed tray  13   a  toward the conveying unit  14 . The conveying unit  14  has a function of conveying the recording paper fed from the paper feed tray  13   a  so as to pass the recording paper through the second transferring position Q and the fixing device  15 , and discharging the recording paper to a discharge tray through a reading position of the reading unit  18 . The conveying unit  14  is configured by conveying rollers and a guide unit that form a conveying passage, and by a motor that drives the conveying rollers. 
     The print system  10  further comprises a control board  20  for controlling the operations of the print system  10  and a scanner unit  21  for reading the original set by a user, an operation panel unit  22  for receiving the operation from the user and for displaying various types of windows, and the like. 
       FIG. 2  is a block diagram showing the electric schematic structure of the print system  10 . The print system  10  comprises a CPU (Central Processing Unit)  31  for controlling the whole operation of the print system  10 . The CPU  31  is connected with a ROM (Read On Memory)  33 , a RAM (Random Access Memory)  34 , a nonvolatile memory  35 , an HDD (hard disk drive)  36 , the operation panel unit  22 , the scanner unit  21 , a network I/F (interface) unit  37 , an image processing unit  38 , the reading unit  18 , a printer engine unit  40 , and the like via a bus  32 . The CPU  31 , the ROM  33 , the RAM  34 , the nonvolatile memory  35 , the image processing unit  38  and the like are mounted on the control board  20 . 
     By the CPU  31 , a middleware, application programs and the like are executed on an OS (Operating System) program as a base. In the ROM  33 , various types of programs are stored. By executing the processes by the CPU  31  in accordance with these programs, various types of functions of the print system  10  are realized in addition to the operation relating to the correction of the density unevenness in the sub-scanning direction. 
     The RAM  34  is used as a work memory that temporarily stores various types of data when the CPU  31  executes the programs, an image memory that stores image data, and the like. 
     The nonvolatile memory  35  is a rewritable memory (flash memory) in which the contents are stored even if the print system  10  is turned off. In the nonvolatile memory  35 , the unique information of the print system  10 , various types of setting information, the correction data for correcting the density unevenness, and the like are stored. 
     The HDD  36  is a large-capacity nonvolatile storing device. In the HDD  36 , print data, image data, data for printing test charts described later, and the like are stored. 
     The printer engine unit  40  has a function of printing an image (forming an image) on the recording paper. The printer engine unit  40  comprises the intermediate transfer belt  11 , the image forming units  12 Y,  12 M,  12 C and  12 K, the paper feeding unit  13 , the conveying unit  14 , the fixing device  15 , and the like that are shown in  FIG. 1 . The printer engine unit  40  operates on the basis of the control from the CPU  31 . 
     The reading unit  18  comprises a light source for irradiating the recording paper passing through the reading position with light, a line image sensor for reading an image on the paper sheet line by line in the width direction of the recording paper (main scanning direction) by receiving the reflected light from the recording paper, and the like. The reading unit  18  repeats the operation of reading an image by one line in the width direction of the recording paper conveyed by the conveying unit  14  (main scanning direction). Thereby, the reading unit  18  reads the image formed on the recording paper conveyed in the sub-scanning direction, as a two-dimensional image. The reading width of the reading unit  18  in the main scanning direction may be optionally set as long as the test image (sub-scanning belt) which is described later can be read. 
     The operation panel unit  22  comprises a liquid crystal display (LCD), a touch panel that is provided on a screen of the liquid crystal display and that detects the coordinate position pressed by a pen, a finger or the like, an operation switch unit, such as a numerical keypad and a start key, and the like. The operation panel unit  22  displays various types of operation windows, setting windows and system state windows, and receives various types of operations such as job input and setting from a user. 
     The scanner unit  21  has the function of optically reading an image on the original to obtain image data. The scanner unit  21  comprises a light source for irradiating the original with the light, a line image sensor for reading an image on the original line by line in the width direction of the original by receiving the reflected light, a moving unit for sequentially moving the reading position line by line in the longitudinal direction of the original, an optical system having lenses, mirrors and the like for guiding the reflected light from the original to the line image sensor and focusing the reflected light on the line image sensor, a converting unit for converting an analog image signal outputted from the line image sensor into digital image data, and the like. 
     Further, the scanner unit  21  comprises an automatic document feeder that feeds the original sheet by sheet from a stack of the original, set on an original tray to convey the original via the reading position to a paper discharge position. The scanner unit  21  can successively read the original, having a plurality of sheets, which is set to the original tray. 
     The network I/F unit  37  communicates with an external device connected via a network such as a LAN (Local Area Network). For example, the network I/F unit  37  receives print data from the external device. 
     The image processing unit  38  performs the rasterization processing for converting print data into image data, compression/decompression processing of image data and the like, in addition to the processings, such as enlargement/reduction and rotation of the image. Further, in accordance with the correction data, the image processing unit  38  carries out the process of correcting the density unevenness in the main scanning direction and in the sub-scanning direction, for the image data to be printed out by the printer engine unit  40 . 
     The print system  10  successively prints the test images for measuring the density unevenness in the sub-scanning direction, on a plurality of sheets of recording paper. Then, in the print system  10 , the measuring data obtained by optically reading the test images printed on a plurality of sheets of recording paper (data indicating the density of the read test image), are arranged in the printing order apart from each other so as to correspond to the intervals of the sheets of recording paper. By analyzing the measuring data, the density unevenness in the sub-scanning direction, which has a period longer than the length of one sheet of recording paper, is detected. 
       FIG. 3A  shows an example of the recording paper (test chart)  50  on which a test image  52  for measuring the density unevenness in the sub-scanning direction is printed. The test image  52  (also referred to as “sub-scanning belt”) having the evenness density is printed in a belt form having a predetermined width extending in the main scanning direction, throughout the entire length extending in the sub-scanning direction of the recording paper  50  (from the front end to the rear end). The width of the sub-scanning belt  52  may be set so as to be thinner than the sub-scanning belt  52  shown in the drawings as long as the density of the sub-scanning belt  52  can be measured. The sub-scanning belt  52  is not required to be arranged on the middle of the recording paper in the width direction, and may be arranged on the position corresponding to the reading position of the reading unit  18 . 
       FIG. 3B  shows the situation in which a plurality of test charts  50  which are successively printed are arranged at the intervals D at which the sheets of the recording paper are conveyed. 
       FIG. 4  shows the process for measuring and correcting the long period density unevenness in the sub-scanning direction in the print system  10 . Firstly, a plurality of test charts  50  on which the test images  52  are printed, are successively printed out (Step S 101 ). At this time, the test image  52  of each test chart  50  is optically read by the recording unit  18  to obtain the measuring data indicating the density of the test image  52 . Then, the profile for indicating the density unevenness in the sub-scanning direction is prepared in each recording paper. 
     The number of the sheets of the test charts  50  to be output is set so as to obtain the data having the distance which is longer than the period of the part having the longest period among the parts of the print system  10 , which could influence the density unevenness in the sub-scanning direction. For example, in case that the intermediate transfer belt  11  is a part having the longest period and has the length of 2 m, when the length of one test chart  50  is 0.5 m and the interval of the sheets of the test charts  50  is 0.1 m, at least four test charts  50  (0.5 m×4+0.1 m×3=2.3 m) are successively printed out. 
     In this case, in order to correct both of the long period density unevenness in the sub-scanning direction, which has the period longer than the length of one sheet of the recording paper, and the short period density unevenness in the sub-scanning direction, which has the period shorter than the length of one sheet of the recording paper, the measuring data are sampled at the sampling interval corresponding to the short period density unevenness in the sub-scanning direction. The measuring data may be separately sampled at each of the sampling intervals which correspond to the long period density unevenness and the short period density unevenness. The sampling of the measuring data will be explained below. 
       FIG. 5  shows the graph of the profiles of the measuring data relating to ten test charts so as to overlap the profiles. 
     In  FIG. 4 , the profiles for the test charts obtained in Step S 101  are arranged in the printing order apart from each other so as to correspond to the intervals D of the sheets of recording paper (Step S 102 ). The arranged profiles are shown in  FIG. 6A . 
     The distance of the interval D can be calculated by the number of sheets of the test charts  50  which are printed out in unit time, the convey speed of the recording paper and the length of the test chart  50 . For example, in case that the number of the sheets on which the charts are printed in 1 minute is 30, the length of the test chart  50  is 0.5 m and the convey speed is 0.3 m/sec, the interval D is calculated by the equation (0.3 m/sec×60 sec−0.5 m×30)/(30−1)=0.1034 m. 
     The data which are positioned in the intervals D are interpolated. By analyzing one graph obtained by interpolating the data, the long period density unevenness in the sub-scanning direction is extracted. Then, the correction data for cancelling the extracted long period density unevenness is obtained (Step S 103  in  FIG. 4 ). 
     As a method for interpolating the data positioned in the intervals D, for example, (1) the data is interpolated by a straight line, (2) the front end of the next recording paper is connected with the rear end of the previous recording paper by extending the fluctuation of the density near the rear end of the previous recording paper, and the like. The data may be interpolated by an optional method. Even though the data is simply interpolated by a straight line, the interval D is sufficiently short as compared with the period of the long period density unevenness in the sub-scanning direction. Therefore, the analysis of the long period density unevenness in the sub-scanning direction is hardly influenced. 
     As described above, by carrying out the frequency analysis for one graph obtained by interpolating the data, the long period density unevenness in the sub-scanning direction is calculated.  FIG. 6B  shows the graph  61  of the extracted long period density unevenness in the sub-scanning direction so as to overlap the data shown in  FIG. 6A . 
     The correction data for cancelling the above long period density unevenness in the sub-scanning direction (the correction data for flatting the graph by being superposed on the long period density unevenness in the sub-scanning direction) is prepared.  FIG. 6C  shows the situation in which the data shown in  FIG. 6A  is corrected by using the correction data. 
     The characteristic of the long period density unevenness in the sub-scanning direction may be obtained as an approximate curve from the data of a plurality of program files arranged apart from each other so as to correspond to the intervals D, without interpolating the data in the intervals D. 
     Next, in accordance with the period of the long period density unevenness in the sub-scanning direction, the parts or the mechanisms which are the factors of the long period density unevenness, are specified (Step S 104  in  FIG. 4 ). For example, in case that the factor of the long period density unevenness is the intermediate transfer belt  11 , the length of the period of the long period density unevenness is approximately coincident with the length of the intermediate transfer belt  11 . 
     In this case, the parts and the mechanisms which can be the factors of the long period density unevenness, are previously picked up. The length of the period of each of the parts and the mechanisms which can be the factors of the long period density unevenness, is estimated. A table in which each factor is related to each estimated length is previously prepared and stored. Then, by referring the table, the factor of the long period density unevenness in the sub-scanning direction is specified. 
     The factors of the long period density unevenness include various types of rollers for supporting the intermediate transfer belt  11  or forming the conveying passage, a development sleeve for stirring the toner in the developing device, the photoconductive drum  16 , the switching on/off of a heater of the fixing device and the like, in addition to the above-described intermediate transfer belt  11 . 
     When the test charts  50  are successively printed in Step S 101 , the phase of the periodic motion of each of the parts and the mechanisms which can be the factors of the density unevenness in the sub-scanning direction is detected by a sensor and is related to the measuring data. With respect to the on/off control carried out for the heater of the fixing device by the CPU  31 , the timing information of the on/off control is related to the measuring data. 
     For example, a sensor for detecting that the specific portion of the intermediate transfer belt  11  reaches the secondary transfer position Q, is provided. The CPU  31  relates the detecting timing at which the sensor detects that the specific portion of the intermediate transfer belt  11  reaches the secondary transfer position Q, to the measuring data obtained by reading the portion of the test image  52  of the recording paper with the reading unit  18 , which is positioned on the secondary transfer position Q at the detecting timing. 
       FIG. 7  shows the profiles which are arranged in view of the intervals of the sheets of recording paper, so as to be related to the timings  71  at which the specific portion reaches the secondary transfer position Q. Thereby, the phase relation between the position of the intermediate transfer belt  11  and the period of the density unevenness in the sub-scanning direction, which is caused due to the intermediated transfer belt  11 , is grasped. In the correction data prepared in Step S 103 , the information indicating the identification name of the part or the mechanism which is the factor of the long period density unevenness in the sub-scanning direction, to be corrected by using the prepared correction data and the phase relation, is added. 
     When the normal printing is carried out, the CPU  31  corrects the long period density unevenness in the sub-scanning direction by using the above correction data (Steps S 105  and S 106  in  FIG. 4 ). From the information added to the correction data, the CPU  31  recognizes the part or the mechanism which is the factor of the long period density unevenness in the sub-scanning direction, and detects the driving timing (phase) of the recognized part or mechanism by using the sensor or the like (Step S 105 ). Then, by applying the correction data so as to be synchronized with the phase of the recognized part or mechanism, the CPU  31  corrects the long period density unevenness in the sub-scanning direction, which is caused due to the recognized part or mechanism (Step S 106 ). In this correction, for example, the density value of the image data to be printed is corrected by using the correction data which is synchronized with the phase. 
     As described above, a plurality of test charts  50  are successively output, and the profiles for the test charts  50 , which indicate the density unevenness in the sub-scanning direction, are prepared by reading the test charts  50 , and are analyzed by arranging them apart from each other so as to correspond to the intervals D of the sheets of the test charts  50 . Therefore, it is possible to detect and correct the long period density unevenness in the sub-scanning direction, which is not contained in one sheet of the recording paper. 
     Further, because the test image is printed on actual recording paper, it is possible to obtain the test charts  50  in which the density unevenness caused due to all of the factors relating to the printing, is indicated. Therefore, it is possible to correct the density unevenness caused due to all of the factors. In case that the test image is formed on the intermediate transfer belt  11  as the toner image and the profile is prepared by optically reading, the toner image, it is not possible to measure the influence of the density unevenness caused after the toner image is transferred from the intermediate transfer belt  11  to the recording paper. For example, the measuring data on which the density unevenness in the sub-scanning direction, which is caused at the fixing, or the difference in the characteristic of the density unevenness according to the type of paper is reflected, cannot be obtained. The correction for the above density unevenness cannot be carried out. 
     Further, in the embodiment, because the final output image which is printed on the recording paper is measured to correct the density unevenness, it is possible to accurately separate and recognize the factors of the density unevenness. For example, even though both of the long period density unevenness in the sub-scanning direction, which is caused due to the intermediate transfer belt  11  and the long period density unevenness in the sub-scanning direction, which is caused due to the fixing device are simultaneously caused, it is possible to separate and extract the long period density unevenness according to each factor. In case that the profile is prepared from the toner image formed on the intermediate transfer belt  11 , the recording paper can be saved. However, as described above, it is not possible to accurately separate the factors of the density unevenness. Therefore, the density unevenness in the sub-scanning direction cannot be precisely corrected. 
     Next, the method for measuring the interval of the sheets will be exemplified. 
     In this embodiment, the marks are printed on the recording paper as scales. In detail, as shown in  FIG. 8 , the marks  73  are put on the image carrier, such as the intermediate transfer belt  11 , the photoconductive drum  16  or the like, at the constant interval which does not depend on the timing of conveying the recording paper. The marks  73  are printed so as to add the serial numbers. The marks  73  function as the graduated scale disposed on a plurality of test charts arranged at the interval of the sheets, so as to traverse the above intervals. 
       FIG. 9A  shows the sub-scanning belt  52  printed on the first sheet of the recording paper and the marks  73  (scale) printed along the sub-scanning belt  52 . 
       FIG. 9B  shows the situation, in which the interval between the first sheet and the second sheet is calculated by using the mark  73  which is printed on the first sheet of the recording paper and the mark  73  which is printed on the second sheet of the recording paper. From the distance L 1  from the mark  73   a  indicating the number “2” which is the closest to the rear end of the first sheet in the marks  73  printed on the first sheet, to the rear end of the first sheet, the distance L 2  from the mark  73   b  indicating the number “4” which is the closest to the front end of the second sheet in the marks  73  printed on the second sheet, to the front end of the second sheet and the distance L 3  from the mark  73   a  indicating the number“2” to the mark  73   b  indicating the number “4”, the interval D of the sheets can be calculated by the equation D=L 3 −L 1 −L 2 . In case that the interval of the sheets is roughly known, the precise interval can be calculated even though the numbers are not attached to the marks  73 . 
     The sub-scanning belt  52  and the marks (scale)  73  and the like for preparing the profile of the density in the sub-scanning direction may be printed on a portion except the effective image area  75  of the recording paper as shown in  FIG. 10 . The effective image area  75  is an area in which the normal print image is printed. For example, the outside area of the effective image area  75  is an area which is removed by being trimmed and an area in which the crop marks for positioning the trimming position are printed. 
     In case that the sub-scanning belt  52  and the marks  73  are printed in the portion except the effective image area  75 , it is possible to obtain the profiles for measuring the density unevenness in the sub-scanning direction when the user instructs the print system  10  to carry out the normal printing. Thereby, the recording paper can be saved as compared with the case in which the test chart is printed on the separate recording paper. Further, because the profiles can be obtained during the normal printing to measure the density unevenness, when the situation of the caused density unevenness is changed, the correction data can be immediately updated to correct the caused density unevenness. 
     Next, the case in which both of the long period density unevenness in the sub-scanning direction and the short period density unevenness in the sub-scanning direction are corrected, will be explained. 
       FIG. 11  shows the process in the above case. The Steps S 201  to S 204  are the same as the Steps S 101  to S 104  shown in  FIG. 4 . The explanations thereof are appropriately omitted. 
     By arranging and analyzing the profiles obtained by reading a plurality of test charts  50  which are successively printed, the long period density unevenness in the sub-scanning direction is extracted and the correction data for the extracted density unevenness is prepared (Steps S 201  to S 203 ). Then, from the length of the period of the long period density unevenness in the sub-scanning direction, the part or the mechanism which is the factor of the long period density unevenness is specified (Step S 204 ). 
     Next, by correcting the profiles by using the prepared correction data, the data obtained by removing the long period density unevenness in the sub-scanning direction from the profiles is prepared. Then, the short period density unevenness in the sub-scanning direction is extracted in accordance with the data obtained by removing the long period density unevenness, and the correction data for the short period density unevenness is prepared (Step S 205 ). As a method for extracting the short period density unevenness in the sub-scanning direction and preparing the correction data for the short period density unevenness, a known method may be used. 
     Next, from the length of the period of the short period density unevenness in the sub-scanning direction, the part or the mechanism which is the factor of the short period density unevenness is specified (Step S 206 ). When the test charts are successively printed in Step S 201 , the phase of the periodic motion of each of the parts and the mechanisms which can be the factor of the long period density unevenness in the sub-scanning direction or the factor of the short period density unevenness in the sub-scanning direction, is detected by a sensor, and is related to the measuring data. In each of the correction data prepared in Steps S 203  and S 205 , the information indicating the identification name of the part or the mechanism which is the factor of the long period density unevenness or the short period density unevenness in the sub-scanning direction, to be corrected by the prepared correction data and the phase relation, is added. 
     When the normal printing is carried out, the CPU  31  corrects the long period density unevenness and the short period density unevenness in the sub-scanning direction by using the above correction data for the long period density unevenness and the above correction data for the short period density unevenness (Steps S 206  to S 208 ). At this time, from the information added to each of the correction data the CPU  31  recognizes the part or the mechanism which is the factor of the long period density unevenness and the part or the mechanism which is the factor of the short period density unevenness, respectively, and detects the driving timing (phase) of the recognized part or mechanism by using the sensor (Step S 207 ). 
     Then, the CPU  31  prepares the synthesized correction data obtained by overlapping the correction data for the long period density unevenness, which is synchronized with the phase of the part or the mechanism which is the factor of the long period density unevenness in the sub-scanning direction, and the correction data for the short period density unevenness, which is synchronized with the phase of the part or the mechanism which is the factor of the short period density unevenness in the sub-scanning direction. By using the synthesized correction data, the CPU  31  corrects the long period density unevenness and the short period density unevenness in the sub-scanning direction, which are caused due to the recognized part or mechanism (Step S 208 ). In this correction, the density value of the image data to be printed is corrected by using the synthesized correction data which is synchronized with the phases. 
     As described above, the long period density unevenness in the sub-scanning direction is detected from the profiles arranged in view of the intervals of the sheets. The data is prepared by removing the long period density unevenness from the profiles. Then, the short period density unevenness in the sub-scanning direction is removed by analyzing the data prepared by removing the long period density unevenness. Therefore, it is possible to accurately remove the long period density unevenness and the short period density unevenness in the sub-scanning direction. That is, it is hard to influence the analysis and the detection of the long period density unevenness in the sub-scanning direction by the short period density unevenness in the sub-scanning direction. On the other hand, it is easy to influence the analysis and the detection of the short period density unevenness in the sub-scanning direction by the long period density unevenness in the sub-scanning direction. Therefore, by previously detecting and removing the long period density unevenness in the sub-scanning direction, it is possible to more precisely detect and correct the short period density unevenness in the sub-scanning direction. 
     Next, the case in which the long period density unevenness and the short period density unevenness in the sub-scanning direction are corrected after removing the density unevenness in the sub-scanning direction, which is always caused on the same position in the recording paper, will be explained. 
     The density unevenness in the sub-scanning direction includes the density unevenness caused on the constant position in the recording paper. For example, by averaging the profiles relating to ten test charts which are successively printed and having the same length from the front end of the recording paper as shown in  FIG. 12A , the averaged profile ( FIG. 12B ) is prepared in one sheet of the recording paper. In the averaged profile, the long period density unevenness and the short period density unevenness in the sub-scanning direction are removed, that is, the influence due to the density unevenness in the sub-scanning direction which does not depend on the position in the page, is eliminated. In the averaged profile, only the density unevenness (in-page density distribution) which depends on the position in the page is shown. In the example shown in  FIG. 12B , a gentle density incline (in-page incline) is shown from the front end to the rear end of one sheet of recording paper. In  FIG. 12B , the graph  81  of the averaged profile and the graph  82  which is obtained by approximating the graph  81  by a line, are shown. 
     By previously detecting and removing the density unevenness in the sub-scanning direction (in-page density distribution) which depends on the position in the page, the accuracy of the analysis and the correction of the long period density unevenness in the sub-scanning direction, which are carried out after removing the in-page density distribution can be improved. 
       FIG. 13  shows the process for correcting the long period density unevenness and the short period density unevenness in the sub-scanning direction after the density unevenness in the sub-scanning direction, which is always caused on the same position in the recording paper, is removed. In  FIG. 13 , the steps which are the same as those of the flowchart shown in  FIG. 11  are denoted by the same step numbers as those of  FIG. 11 . The difference between the flowcharts of  FIG. 11  and  FIG. 13  is that Step S 201 B is inserted between Step S 201  and Step S 202  in  FIG. 13 . 
     In Step S 201 B, by averaging the profiles relating to a plurality of test charts  50  which are successively printed and having the same length from the front end of the recording paper, the averaged profile is prepared in one sheet of the recording paper. Then, by analyzing the averaged profile, the density unevenness in the sub-scanning direction, which is always caused on the same position in the recording paper, is calculated. In this case, as shown in  FIG. 12B , the in-page incline is caused, and the graph is obtained by approximating the in-page incline by a line. 
     Next, the data which is corrected so as to cancel the in-page incline in each profile, is obtained. 
     In Step S 202  and the following steps, the process is carried out by using the profiles in which the in-page incline is corrected. 
     Next, in case that the long period density unevenness and the short period density unevenness in the sub-scanning direction are measured, the test chart and the sampling thereof will be explained by using  FIG. 14 . 
     In case that the shape of the short period density unevenness in the sub-scanning direction is measured, it is required to measure the density unevenness by a fine pitch to a certain degree. For example, in case that the period of the short period density unevenness in the sub-scanning direction is 20 cm, in order to accurately measure the density unevenness, it is preferable to set at least five sampling points (measurement points) within the range of 20 cm. On the upper side of the test chart  90  shown in  FIG. 14 , the test image (sub-scanning belt)  52  for measuring the short period density unevenness in the sub-scanning direction and the sampling pattern thereof are shown. The circles shown in the drawing indicate the sampling points. 
     In case of the long period density unevenness in the sub-scanning direction, it is possible to use the test image  52  for measuring the short period density unevenness in the sub-scanning direction and the sampling pattern thereof as they are. However, the sampling may be carried out by a rougher pitch. For example, when about four sampling points are set in one sheet of the recording paper, it is possible to measure the shape of the long period density unevenness in the sub-scanning direction. In the test chart  90  of  FIG. 14 , the second test image  52 B which is shown below the test image  52  is a test image for measuring the long period density unevenness in the sub-scanning direction. In the second test image  52 B, each density patch is printed only in the portion of the sampling point for the long period density unevenness, which is set by a rough pitch. 
     In accordance with the period of the density unevenness in the sub-scanning direction to be measured, the type of the test image and the sampling pattern may be automatically changed. For example, in case that only the long period density unevenness in the sub-scanning direction is corrected, the type of the test image and the sampling pattern may be automatically changed so as to measure the density unevenness by using the second test pattern  52 B shown in  FIG. 14  at the rough sampling pattern. In case that both of the long period density unevenness and the short period density unevenness in the sub-scanning direction are corrected, the type of the test image and the sampling pattern may be automatically changed so as to measure the long period density unevenness and the short period density unevenness by using the test pattern  52  shown in  FIG. 14  at the fine sampling pattern. Further, in case that both of the long period density unevenness and the short period density unevenness in the sub-scanning direction are corrected, the measurement for the long period density unevenness in the sub-scanning direction is carried out by using the second test image  52 B at the rough sampling pattern, and the measurement for the short period density unevenness in the sub-scanning direction is carried out by using the test image  52  at the fine sampling pattern, respectively. 
     As described above, the embodiment is explained by using the drawings. However, in the present invention, the concrete configuration is not limited to the above embodiment. In the present invention, various modifications of the above embodiment or the addition of various functions or the like to the embodiment can be carried out without departing from the gist of the invention. 
     In the print system  10  according to the embodiment, the test chart  50  is read by the reading unit  18  disposed on the conveying passage of the subsequent stage of the fixing device  15 . However, for example, a user may set the stack of a plurality of printed test charts  50  on the original tray of the scanner unit  21 , and the profiles may be prepared by reading the test charts  50  with the scanner unit  21 . Further, the measuring data obtained by reading the test charts  50  with an external reading device may be received via the network I/F unit  37  or the like to prepare the profiles. 
     Further, in the embodiment, the print system  10  comprising the reading unit  18  and the printer engine unit  40  as a united device, is described. However, the device for preparing the correction data may be constructed as an image processing device. That is, the device for preparing the correction data may be constructed as an image processing device having a function of instructing a predetermined print device to successively print a plurality of test charts  50  (sample output unit); a function of preparing profiles by inputting measuring data obtained by reading a plurality of printed test charts  50  with an external reading device (profile preparing unit); a function of analyzing the profiles by arranging them in view of the intervals of the sheets and detecting the long period density unevenness and the short period density unevenness in the sub-scanning direction (analyzing unit); and a function of preparing the correction data for removing the detected long period density unevenness and the detected short period unevenness in the sub-scanning direction (correction data preparing unit). Alternatively, the above image processing device may further have a function of specifying a factor of each period of density unevenness (factor specifying unit). The prepared correction data is input to the print device and is used for correcting the density unevenness in the sub-scanning direction. 
     The test image is not limited to the image exemplified in the embodiment. For example, a plurality of sub-scanning belts having different densities from each other may be printed on one sheet of recording paper. In this case, it is possible to measure the density unevenness in the sub-scanning direction with respect to a plurality of densities at once. 
     The update of the correction data is preferably carried out at the timing when the density unevenness in the sub-scanning direction is changed. For example, when a curtain amount of component degradation is caused, when parts are exchanged, or when the environment change is caused above a certain level, the correction data is preferably updated. 
     The image forming method is not limited to the electrophotographic method which is exemplified in the embodiment. An ink jet method or the like may be used. 
     One of the objects of the above embodiment is to provide an image processing device, an image processing method and a print system, which can correct the long period density unevenness in the sub-scanning direction, which extends through a plurality of sheets of recording paper. 
     In the above embodiment, the image processing device instructs a predetermined print device to successively print a plurality of test charts for measuring the density unevenness in the sub-scanning direction, and arranges the profiles of the measurement data obtained by optically reading the test charts in the printing order in view of the intervals of the sheets of recording paper. Then, the image processing device analyzes the arranged measurement data and detects the long period density unevenness in the sub-scanning direction, which extends through a plurality of sheets of recording paper, to prepare the correction data for removing the detected long period density unevenness. 
     In the above embodiment, the image processing device firstly detects the long period density unevenness in the sub-scanning direction as described above. Next, the image processing device prepares the data in which the detected long period density unevenness in the sub-scanning direction is removed from the profiles and analyzes the data to detect the short period density unevenness in the sub-scanning direction. It is hard to influence the detection of the long period density unevenness in the sub-scanning direction by the short period density unevenness in the sub-scanning direction. On the other hand, it is easy to influence the detection of the short period density unevenness in the sub-scanning direction by the long period density unevenness in the sub-scanning direction. Therefore, by analyzing and removing the density unevenness in order of the long period density unevenness and the short period density unevenness, it is possible to more precisely detect the short period density unevenness in the sub-scanning direction. 
     In the above embodiment, firstly, the in-page density distribution which is always caused on the same position in the sub-scanning direction in one sheet of recording paper, is detected and removed from the profiles. Then, the long period density unevenness and the short period density unevenness in the sub-scanning direction are detected and removed. 
     In the above embodiment, from the number of sheets of the test charts which are printed out in unit time (for example, 30 sheets/min), the convey speed of the recording paper and the length of the recording paper in the sub-scanning direction, the interval of the sheets is calculated. 
     In the above embodiment, the marks are printed on a plurality of sheets of recording paper, on which the test charts are successively printed, at regular intervals including the intervals of the sheets of recording paper, and the distance of the interval of the sheets is calculated from the positions of the marks. For example, from the distance L 1  from the mark A which is put on the first sheet of the recording paper near the rear end of the first sheet, to the rear end of the first sheet, the distance L 2  from the mark B which is put on the second sheet of the recording paper near the front end of the second sheet, to the front end of the second sheet and the distance L 3  from the mark A to the mark B, the interval (L 3 −L 1 −L 2 ) of the sheets is calculated. 
     In the above embodiment, for example, the sampling pattern of the test image is changed according to whether the case in which the long period density unevenness in the sub-scanning direction is detected or the case in which the short period density unevenness in the sub-scanning direction is detected. 
     In the above embodiment, the test image is printed on an outside area of the effective image area, which is trimmed off and discarded. 
     In the above embodiment, for example, the part or the mechanism in which the period of the rotation or the motion is approximately coincident with the period of the density unevenness in the sub-scanning direction, is specified as the factor of the above density unevenness in the sub-scanning direction. 
     In the above embodiment, the print system comprises the image processing device for preparing the correction data for the long period density unevenness in the sub-scanning direction, and the print device to be corrected. In the print system, the print device corrects the density unevenness in the sub-scanning direction therein by using the correction data prepared by the image processing device. 
     In the above embodiment, the correcting unit of the print device detects the phase of the part which is the factor of the density unevenness having a certain period in the sub-scanning direction, and applies the correction data for removing the above density unevenness, so as to synchronize the correction data with the detected phase. Thereby, the above density unevenness is corrected. 
     According to the image processing device, the image processing method and the print system, it is possible to correct the long period density unevenness in the sub-scanning direction, which extends through a plurality of sheets of recording paper. 
     The present U.S. patent application claims the priority of Japanese Patent Application No. 2012-141007, filed on Jun. 22, 2012, according to the Paris Convention, and the entirety of which is incorporated herein by reference for correction of incorrect translation.