JP2012195715A - Image forming apparatus and method of correcting grayscale reproduction characteristics - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively correct grayscale reproduction characteristics for each of multiple grayscale reproduction modes.SOLUTION: An image forming apparatus has a first grayscale reproduction mode and a second grayscale reproduction mode. The apparatus forms a first test pattern image showing grayscale reproduction characteristics as to a first grayscale range in the first grayscale reproduction mode, and a second test pattern image showing grayscale reproduction characteristics as to a second grayscale range which is a part of the first grayscale range in the second grayscale reproduction mode, and detects a density of the first test pattern image and a density of the second test pattern image. The apparatus corrects the grayscale reproduction characteristics as to the whole area of the first grayscale range in the first grayscale reproduction mode based on the density of the first test pattern image, and corrects the grayscale reproduction characteristics as to the whole area of the first grayscale range in the second grayscale reproduction mode based on the density of the second test pattern image and a density of part corresponding to a range other than the second grayscale range within the first grayscale range in the first test pattern image.

Description

  The present invention relates to an image forming apparatus having a plurality of gradation reproduction modes and a method for correcting gradation reproduction characteristics for such an image forming apparatus.

  An electrophotographic image forming apparatus has an image stabilization function that automatically adjusts an electrophotographic process when the power is turned on or when the cumulative number of image formation reaches a set value. In general, in this automatic adjustment, a predetermined test pattern image is formed, and the gradation reproduction characteristics are corrected based on the measurement result of the density of the test pattern image. For example, in a recent image forming apparatus that reproduces a halftone by dithering by replacing each pixel of an input image with a screen such as a line or halftone dot, if the measured density is lighter than an appropriate value, it becomes darker. The correspondence relationship between the input gradation value and the screen area ratio applied thereto is corrected so as to become lighter.

  An image forming apparatus having a plurality of gradation reproduction modes is known. In this type of image forming apparatus, it is necessary to correct gradation reproduction characteristics for each of a plurality of gradation reproduction modes. Patent Document 1 stores a gradation correction table corresponding to one light emission output method in an image forming apparatus having a plurality of gradation reproduction modes with different light emission output methods for exposing a photoreceptor, and other light emission. Regarding the output method, a technique is described in which a difference between a gradation correction table corresponding to the output method and the gradation correction table is stored, thereby reducing a memory storage capacity necessary for storing correction data. Japanese Patent Laid-Open No. 2004-228688 discloses a frequency of correction during 1200 dpi operation in an image forming apparatus capable of switching the resolution between 600 dpi and 1200 dpi by changing the process speed, when the operation is performed at 600 dpi. It has been proposed to increase the frequency of correction.

JP-A-5-328130 JP 2009-122709 A

  The aspect of the change in gradation reproduction characteristics caused by factors such as the usage environment (humidity and temperature) and operating time of the image forming apparatus is not necessarily the same among the plurality of gradation reproduction modes. In the technique in which the gradation correction data of one mode is diverted to other modes as in the prior art of Patent Document 1, correct correction is performed according to the aspect of change in gradation reproduction characteristics that differs for each mode. I can't. However, when a test pattern image showing the gradation reproducibility of the entire input gradation range is formed for each of the plurality of gradation reproduction modes and the gradation reproduction characteristics are examined, the color used for forming the test pattern image Material (mainly toner) consumption increases. In particular, a color image forming apparatus forms a test pattern image for each of the colors yellow, magenta, cyan, and black. Therefore, compared to an apparatus that forms only a monochrome image, more color materials are formed. Will be consumed by. Since the test pattern image is sufficiently smaller than a normal image formed by the image forming apparatus, the increase in consumption of the color material is never large. However, although it is slight, the running cost of the image forming apparatus is increased. Further, by forming a large number of test pattern images corresponding to the entire input gradation range, there is a possibility that a dead time during which the user cannot use the image forming apparatus may be extended.

  In view of such circumstances, an object of the present invention is to provide an apparatus and a method for efficiently correcting gradation reproduction characteristics for each of a plurality of gradation reproduction modes.

  An image forming apparatus that achieves the above object is an image forming apparatus having a first gradation reproduction mode and a second gradation reproduction mode, wherein the first gradation range is the whole or a part of the input gradation range. A first test pattern image showing characteristics of gradation reproduction in the first gradation reproduction mode for the second gradation reproduction for the second gradation range that is a part of the first gradation range An image forming unit that forms a second test pattern image showing characteristics of gradation reproduction by mode, and input tone values that are included in the first test pattern image formed by the image forming unit and that correspond to each other, A density detector that detects densities of a plurality of different portions and a density of a plurality of portions that are included in the second test pattern image formed by the image forming unit and that have different input tone values corresponding to each other; A first correction unit that corrects the gradation reproduction characteristics of the first gradation reproduction mode for the entire range of the first gradation range based on the density of the first test pattern image detected by the recording density detection unit. And the second gradation range of the first gradation range in the first test pattern image detected by the density detection unit and the density of the second test pattern image detected by the density detection unit And a second correction unit that corrects the tone reproduction characteristics of the second tone reproduction mode for the entire range of the first tone range based on the density of the portion corresponding to the range other than.

  According to the present invention, a test pattern image is formed for only one gradation reproduction mode without forming a test pattern image showing gradation reproducibility over the entire input gradation range for each of a plurality of gradation reproduction modes. Compared with the case where it does, the gradation reproducibility of several gradation reproduction modes can be correct | amended appropriately.

1 is a diagram illustrating a schematic configuration of an image forming apparatus according to an embodiment of the present invention. It is a graph which shows the outline | summary of correction | amendment of a gradation reproduction characteristic. It is a figure which shows typically the state in which the test pattern image was formed. It is a figure which shows the screen in connection with selection of the gradation range which should measure a gradation reproduction characteristic. It is a figure which shows typically the correction method of a gradation reproduction characteristic. 3 is a block diagram showing components related to correction of gradation reproduction characteristics in the image forming apparatus 1. FIG. It is a figure which shows the flow of the image processing at the time of image formation. It is a figure which shows the flow of an image stabilization process. It is a figure which shows the flow of the range setting process which sets the gradation range of test pattern formation.

  An image forming apparatus 1 illustrated in FIG. 1 is an information device called an MFP (Multifunction Peripheral) or a multifunction device having various uses including copying and network printing. The image forming apparatus 1 includes a tandem printer engine 10 that forms a color or monochrome image by electrophotography. The printer engine 10 uses four imaging units 11, 12, 13, and 14 to form toner images of four colors of yellow (Y), magenta (M), cyan (C), and black (K) in parallel. can do. The imaging units 11 to 14 each have one photosensitive drum 15, 16, 17, and 18 as an image carrier. In each of the imaging units 11 to 14, components necessary for forming a toner image such as a developing device, a print head, a charging charger, and a cleaner are arranged around a cylindrical photosensitive member. The image forming apparatus 1 operates as follows in response to a color print request from an external device (typically a personal computer), for example.

  The electric circuit 100 of the image forming apparatus 1 generates exposure data for determining an exposure pattern of each color of Y, M, C, and K based on image data given from an external device. The exposure data is sent to the print heads of the imaging units 11-14. With the photoconductor drums 15 to 18 rotating, the print head irradiates the charged photoconductor with laser light. At this time, the print head controls the light source according to the exposure data, thereby performing main scanning on the photosensitive member. A latent image is formed on the photosensitive member by main scanning and sub-scanning (rotation of the photosensitive drums 15 to 18). The developing units of the imaging units 11 to 14 adsorb the toner to the photoconductor to form a toner image corresponding to the latent image. The toner image is primarily transferred to an intermediate transfer belt 26 that moves at a constant speed with respect to the photosensitive drums 15 to 18. The timings of latent image formation and primary transfer for the four colors are set so that the four color toner images overlap on the intermediate transfer belt 26 in an appropriate positional relationship without being shifted from each other. In parallel with the operation of forming four toner images per sheet surface, a sheet (not shown) taken out from the sheet stocker 41 by the sheet feeding roller 31 is conveyed. The paper is temporarily held by the pair of registration rollers 34 and fed to the secondary transfer position at an appropriate timing. The secondary transfer position is a position where the intermediate transfer belt 26 and the pressure roller 35 face each other so as to form a nip portion having a predetermined gap. Toner sheets of four colors overlapped from the intermediate transfer belt 26 are secondarily transferred onto the sheet passing through the secondary transfer position. Thereafter, the sheet passes through the inside of the fixing unit 29 and is discharged onto the discharge tray 46. When the paper passes through the fixing unit 29, the toner is melted by heating and pressurization, and the toner image is fixed on the paper as a print image. In parallel with the paper discharge, the toner remaining on the intermediate transfer belt 26 during the secondary transfer is removed by a cleaner 27 disposed on the upstream side of the photosensitive drum 15 and stored in a waste toner container 28.

  The image forming apparatus 1 executes image stabilization processing at a predetermined time, and corrects tone reproduction characteristics of color and monochrome image formation. The gradation reproduction characteristics are affected by environmental conditions such as humidity and temperature, and changes with time of electrophotographic process mechanism elements typified by a photoreceptor. For this reason, when turning on the power, returning from sleep, when the cumulative number of image formation reaches the set value, or detecting an aperiodic and drastic change in environmental conditions that seems to have changed the installation location, etc. The tone reproduction characteristics are corrected. At that time, a test pattern image, which is a toner image showing reproducibility of a plurality of gradations within the input gradation range, is formed on the intermediate transfer belt 26 and is disposed between the photosensitive drum 18 and the secondary transfer position. The density of the test pattern image is detected by the optical density sensor 62. Thereafter, the test pattern image is removed from the intermediate transfer belt 26 by the cleaner 27 without being secondarily transferred.

  The graph of FIG. 2 shows an outline of correction of gradation reproduction characteristics. It is desirable that the relationship between the input tone value and the density of the image to be formed is a proportional relationship over the entire input tone range, as indicated by the one-dot chain line in the figure. For this reason, when the tone reproduction characteristics actually measured by detecting the density of the test pattern image are, for example, the characteristics indicated by the dashed curve in the figure, the correction represented by the solid tone correction curve in the figure is performed. It is. In other words, the image is so reproduced that the measured density is darker for the input tone value that is lighter than the appropriate value, and the input tone value that is darker than the appropriate value is reproduced for the light eye. The setting of the forming operation is corrected. If the measured gradation characteristic is expressed by a function f (x), the gradation correction curve is, for example, 1 / f (x). Here, x is a gradation value before gradation correction. In the present embodiment, the gradation reproduction characteristics are corrected by increasing or decreasing the input gradation value as will be described later.

  Hereinafter, the correction of the gradation reproduction characteristics in the image forming apparatus 1 will be described in more detail.

  FIG. 3 schematically shows a state in which a test pattern image is formed. A total of eight test pattern images 51K, 52K, 51M, 52M, 51C, 52C, 51Y, and 52Y are formed on the both sides of the intermediate transfer belt 26 in the width direction (main scanning direction). Four test pattern images 51K, 52K, 51M, 52M are arranged in a line along the moving direction of the intermediate transfer belt 26 on one side (upper side in the figure), and the density of these patterns is a pair of density sensors 62, 62. Detected by one of the two. Similarly, four test pattern images 51C, 52C, 51Y, and 52Y are arranged in a line on the other side (lower side in the drawing) of the intermediate transfer belt 26. The density of these patterns is detected by the other density sensor 62.

  The test pattern image 51K and the test pattern image 52K are single color toner images formed using black (K) toner, and the test pattern image 51M and the test pattern image 52M are formed using magenta (M) toner. This is a single color toner image. The test pattern image 51C and the test pattern image 52C are single color toner images formed using cyan (C) toner, and the test pattern image 51Y and the test pattern image 52Y are formed using yellow (Y) toner. This is a single color toner image. In this way, two test pattern images are formed for each of the four colors. The image forming apparatus 1 is provided with first and second gradation reproduction modes, ie, a low resolution mode and a high resolution mode. Because it is.

  Since the screen used for reproducing the halftone differs between the low resolution mode and the high resolution mode, there may be a difference in gradation reproduction characteristics due to environmental conditions and other factors. For this reason, a test pattern image is formed for each mode, and the gradation reproduction characteristics are measured. In FIG. 3, relatively long strip-shaped test pattern images 51K, 51M, 51C, and 51Y are images formed using a screen for a low resolution mode. The remaining strip-shaped test pattern images 52K, 52M, 52C, and 52Y are images formed using a screen for a high resolution mode.

  The test pattern images 51K, 51M, 51C, and 51Y for the low resolution mode are gradation pattern images that become gradually lighter from the front side to the rear side in the moving direction, and the input gradation values corresponding to each of them are mutually different. Consists of multiple parts of different and same size. In FIG. 3, as a representative, in the K test pattern image 51K, portions 511, 512, 513, 514, 516, 517, 518, and 519 are drawn. Actually, the test pattern images 51K, 51M, 51C, and 51Y respectively have a total of 256 portions corresponding to the gradation values of “0” to “255” represented by 8 bits, for example. A front end portion 511 in the movement direction is a so-called solid portion corresponding to the maximum gradation value “255”, and a rear end portion 519 in the movement direction is a so-called white portion corresponding to the minimum gradation value “0”. It is. That is, the test pattern images 51K, 51M, 51C, and 51Y show gradation reproducibility for the entire input gradation range. However, it is not always necessary to correspond to the entire input gradation range. For example, when the remaining range excluding the minimum gradation value “0”, the maximum gradation value “255” or the vicinity thereof is set as the correction target range. A test pattern image corresponding to the correction target range may be formed. An example of the size of each part corresponding to the input gradation value is one-to-one. The length in the main scanning direction is 256 dots at 600 dpi, and the length in the sub-scanning direction is 16 dots at 600 dpi. It is.

  The test pattern images 52K, 52M, 52C, and 52Y for the high resolution mode are also gradation pattern images that become gradually lighter from the front side to the rear side in the moving direction, and the input gradation values corresponding to each of them are mutually different. Consists of multiple parts of different and same size. In FIG. 3, as a representative, portions 521, 522, 523, and 524 are drawn in the K test pattern image 52K. The test pattern images 52K, 52M, 52C, and 52Y do not correspond to the entire correction target range that is the whole or part of the input gradation range, but only correspond to a part of the correction target range. In the present embodiment, the gradation range corresponding to the test pattern images 52K, 52M, 52C, and 52Y is a range on the low gradation side of the input gradation range, specifically, the gradation value from the minimum gradation value “0”. The range is up to “63”. The test pattern images 52K, 52M, 52C, and 52Y each have a total of 64 portions corresponding to the gradation values within this range. Compared with the test pattern image for the high resolution mode, the total number of corresponding gradation values is as small as 1/4, and the area of the entire pattern is small. Therefore, the toner consumed in forming the test pattern images 52K, 52M, 52C, and 52Y is Few.

  FIG. 4 shows a screen related to selection of a gradation range in which gradation reproduction characteristics are to be measured. The left two screens A13 and A3 in FIG. 4 are 16 × 16 matrix dither patterns used in the low resolution mode (for example, 600 dpi). The area ratio of the screen A13 is 13%, and the area ratio of the screen A3 is 3%. Further, the two screens B13 and B3 on the right side in FIG. 4 are 32 × 32 matrix dither patterns used in the high resolution mode (eg, 1200 dpi). The area ratio of the screen B13 is 13%, and the area ratio of the screen B3 is 3%. A square filled with black in the matrix in the figure represents a region (print dot) to which toner should be attached.

  When the screen A13 and the screen B13, both of which have an area ratio of 13%, are compared, the arrangement of print dots is the same. Although illustration is omitted, even with a screen having an area ratio larger than 13%, the arrangement of print dots is the same or similar between the low resolution mode and the high resolution mode. The reason is that a method of changing the area ratio by changing the size of the printed dots without changing the spatial frequency as much as possible is applied to the generation of the screen.

  However, when the screen A3 and the screen B3, both having an area ratio of 3%, are compared, there is a large difference in the arrangement of print dots. The number of print dots on the screen A3 in the low resolution mode is 8, whereas the number of print dots on the screen B3 in the high resolution mode is 32, and the size of each print dot is 1/4 that of the screen A3. is there. This difference occurs because the print dots are arranged so as to satisfy the restriction that the spatial frequency is not changed as much as possible in the screen B3 in the high resolution mode, whereas in the screen A3 in the low resolution mode, the difference is generated. This is because an area ratio of 3% is realized by lowering the spatial frequency without satisfying such restrictions. If there is a difference in the arrangement of print dots, a difference in tone reproducibility is considered to occur. Therefore, in order to correct the gradation reproduction characteristics, it is necessary to measure the gradation reproduction characteristics by separately forming test pattern images for the low resolution mode and the high resolution mode.

  Further, when the two screens B13 and B3 in the high resolution mode are compared, the spatial frequency and the screen angle are the same, but the size of so-called isolated dots apart from the other print dots is smaller on the screen B3. Generally, in an electrophotographic process, gradation reproducibility tends to become unstable as the number of isolated dots becomes smaller. Therefore, it is necessary to form a test pattern image and measure the gradation reproduction characteristics for the low density side range that corresponds to the screen with a small area ratio in the input gradation range. As described above, the image forming apparatus 1 forms the test pattern images 52K, 52M, 52C, and 52Y corresponding to the low density range from the minimum gradation value “0” to the gradation value “63”.

  FIG. 5 schematically shows a method for correcting gradation reproduction characteristics. Here, correction for one color will be described with reference to FIGS. The remaining three colors are corrected in the same manner as described below.

  Based on the detected density shown in the graph of FIG. 5A, the image forming apparatus 1 is shown in the tone correction table T1 for the low resolution mode shown in FIG. 5B and in FIG. 5C. The gradation correction table T2 for the high resolution mode is updated. The black circle in FIG. 5A represents the density detected by the density sensor 62, and the broken-line circle represents the virtual density in the gradation range that is not actually measured in the high resolution mode.

  For example, during the period in which the K test pattern images 51K and 52K (see FIG. 3) pass through the detection position by the density sensor 62 at a constant speed, the densities of the above-described many portions in the test pattern images 51K and 52K are sequentially detected. . At time t1 (255), the density of the leading portion 511 of the test pattern image 51K is detected, at time t1 (64), the density of the portion corresponding to the input gradation value “64” is detected, and at time t1 (0). The density of the rear end portion 519 of the test pattern image 51K is detected. At time t2 (63), the density of the leading portion 521 of the test pattern image 52K is detected, and at time t2 (0), the density of the trailing end portion 524 of the test pattern image 52K is detected.

  The gradation correction table T1 for the low resolution mode is a corrected output gradation value (correction value) obtained by calculation or referring to a lookup table based on the densities of all portions detected from the test pattern image 51K. Updated to show. At this time, a correction value is obtained for each of all gradation values within the input gradation range.

On the other hand, the gradation correction table T1 for the high resolution mode includes the density of all portions detected from the test pattern image 52K and the portions corresponding to the input gradation values “0” to “64” in the test pattern image 51K. It is updated to indicate a correction value obtained based on the density. At this time, a correction value is obtained for each of all gradation values within the input gradation range. In other words,
In the high resolution mode, the gradation reproduction characteristics are corrected by using part of the density detection data in the low resolution mode. As described with reference to FIG. 4, the diverted density detection data is data corresponding to the high density side gradation range in which the gradation reproducibility in both modes is considered to be substantially the same.

  FIG. 6 is a block diagram showing components related to the correction of the gradation reproduction characteristics in the image forming apparatus 1.

  The main controller 60 responsible for overall control of the image forming apparatus 1 performs predetermined control in response to a user instruction from the operation panel 62 and an operation request from an external device communicating via the communication interface 66. The main controller 60 includes a central processing unit (CPU) as a computer that executes control programs and various applications, a read only memory (ROM) that stores the control program, and a random access memory (RAM) that is a work area for executing the program. Have. In a print job using the printer engine 10 among various jobs given to the image forming apparatus 1 which is an MFP, the main controller 60 causes the printer controller 70 to control the printer engine 10 and print data suitable for the printer engine 10. Instruct the generation of. As a print job, the contents to be printed are stored in a network printing job described in a page description language (PDL), a copy job using the image scanner 20, and a hard disk drive which is the internal storage 68. There is a job to print a document.

  The printer controller 70 includes hardware including a CPU and software that allows the CPU to realize a predetermined function. As functional elements implemented by software, the printer controller 70 includes a language analysis unit 71, a rasterization unit 72, a gradation correction unit 73, and a screen processing unit 74. Further, the printer controller 70 includes a pattern formation control unit 75, a density detection unit 76, a first correction unit 77, and a second correction unit 78 as functional elements that are deeply involved in correction of gradation reproduction characteristics. For example, the printer controller 70 captures the PDL data D10 in a network printing job.

  First, basic functions of the printer controller 70 related to image formation will be described by taking a network printing job as an example.

  The language analysis unit 71 interprets the PDL data D10, generates intermediate data D11 representing the content to be rendered, and passes it to the rasterization unit 72. The contents to be drawn described in PDL are roughly classified into three attributes (types) of “text” representing characters, “graphics” representing graphics and lines, and “image” representing raster images. In the process of generating the intermediate data D11, the language analysis unit 71 partitions an image for each page to be drawn into single or plural blocks having the same attribute and a continuous portion as one block. Then, the result is transmitted to the rasterizing unit 72.

  The rasterizing unit 72 generates raster format image data (frame) D12 corresponding to each of the four colors of YMCK based on the intermediate data D11 from the language analyzing unit 71. In this case, optimization for each block is performed for color conversion from RGB to YMCK. Specifically, a conversion parameter that prioritizes saturation enhancement over tone reproduction is applied to characters and graphics, and a conversion parameter that prioritizes gradation reproduction over saturation enhancement is applied to images.

In accordance with the resolution mode specified in the PDL data D10, the gradation correcting unit 73
According to the gradation correction table T1 or the gradation correction table T2, the gradation value of each pixel of each image data D12 of four colors from the rasterizing unit 72 is corrected. As a result, the difference between the actual gradation reproduction characteristics of the image forming apparatus 1 and the appropriate gradation reproduction characteristics (see FIG. 2) is compensated, and the gradation of the image data D12 is reproduced appropriately.

  The screen processing unit 74 generates binary image data D14 to be given to the printer engine 10 by applying an appropriate screen to the pixels of the corrected image data D13 delivered from the gradation correction unit 73. The screen data SA indicating the screen for the low resolution mode including the screens A3 and A13 and the screen data SB indicating the screen for the high resolution mode including the screens B3 and B13 are stored in the nonvolatile memory in advance. The screen processing unit 74 refers to the screen data SA or the screen data SB according to the resolution mode. Based on the image data D14 generated by the screen processing unit 74, the printer engine 10 controls exposure by the printer heads of the imaging units 11-14.

  Next, functions of the printer controller 70 relating to correction of gradation reproduction characteristics will be described.

  The pattern formation control unit 75 reads the test pattern data D40 from the nonvolatile memory and sends it to the printer engine 10 at a time when a predetermined image stabilization process is to be performed, and the test pattern images 51K, 52K, 51M, and 52M described above. , 51C, 52C, 51Y, 52Y are formed in the printer engine 10. The cooperative operation of the pattern formation control unit 75 and the printer engine 10 corresponds to the function of the image forming unit that forms a test pattern image. Further, the pattern formation control unit 75 changes the setting of the gradation range for actually measuring the reproducibility of the high resolution mode according to the environmental conditions. When the setting is changed, a test pattern image having a portion corresponding to each gradation value in the changed gradation range is formed.

  The density detector 76 samples the output of the density sensor 62 at a timing synchronized with the formation of the test pattern image, and the input tone values in the test pattern images 51K, 52K, 51M, 52M, 51C, 52C, 51Y, and 52Y are mutually different. Concentration detection data is generated by quantizing the concentrations of different portions.

  The first correction unit 77 obtains correction values for the gradation reproduction characteristics of the four colors based on the density detection data of the test pattern images 51K, 51M, 51C, 51Y for the low resolution mode, and the gradation correction table T1. Update. Further, the second correction unit 78 is based on the density detection data of the test pattern images 52K, 52M, 52C, and 52Y for the high resolution mode and a part of the density detection data of the test pattern image for the low resolution mode (FIG. 5). (Ref.) The correction values of the gradation reproduction characteristics of the four colors are obtained, and the gradation correction table T2 is updated.

  FIG. 7 shows the flow of image processing during image formation. When the print controller 70 receives RGB image data (S31), the print controller 70 executes color conversion processing to convert the image data into 8-bit CMYK image data (S32). The print controller 70 refers to the gradation correction tables T1 and T2 that are corrected in an appropriate image stabilization process, and corrects the gradation value (input gradation value) of the image data of each color of CMYK (S33). Screen processing is executed as halftone processing for binarizing each pixel of the corrected image data (S34).

  FIG. 8 shows the flow of image stabilization processing. If it is time to perform the image stabilization process (YES in S40), the print controller 70 and the printer engine 10 cooperate to test the test pattern images 51K, 51M, 51C, 51Y for the low resolution mode and the high resolution mode. Test pattern images 52K, 52M, 52C and 52Y are formed side by side as shown in FIG. 3 (S41). The print controller 70 detects the density of the test pattern image (S42), measures the current gradation reproduction characteristics, and obtains a gradation reproduction correction value (S43, S44). Then, the print controller 70 updates the gradation correction tables T1 and T2 (S45).

  FIG. 9 shows a flow of range setting processing executed by the pattern formation control unit 75. As described above, when the number of isolated dots on the screen used for the screen processing is small, the gradation reproducibility varies greatly with environmental changes. Further, the fluctuation is more noticeable in a high-temperature, high-humidity or low-temperature, low-humidity environment than in a normal temperature and normal humidity environment. Therefore, in order to stabilize the image, the gradation range to be actually measured is switched in the gradation reproduction mode (in this embodiment, the high resolution mode) that measures the reproducibility of only a part of the input gradation range according to the temperature and humidity. It is valid. The pattern formation control unit 75 checks whether detected values by a temperature sensor and a humidity sensor (not shown) in the image forming apparatus 1 are within a normal temperature and humidity range (S101), and forms using the screen B for the high resolution mode. The gradation ranges of the test pattern images 52K, 52M, 52C, and 52Y to be set are gradation values “0” to “63” if the detected value is within the normal range (S102), and the detected value is within the normal range. If not, the gradation values are set to “0” to “127” (S103). That is, when it is assumed that the variation in gradation reproducibility is large, the gradation range in which the gradation reproducibility is actually measured by forming a test pattern image is expanded as compared with the normal case. As a result, it is possible to more accurately correct the appropriate gradation reproduction characteristics.

  According to the above embodiment, the measurement result of the density in the gradation range that can obtain stable gradation reproducibility regardless of the environment is applied to gradation correction in both the low resolution mode and the high resolution mode. Even in the high resolution mode in which the gradation reproducibility of the entire gradation range is not measured, the reproduction characteristics of the entire input gradation range can be corrected appropriately.

  In the above-described embodiment, the measurement data of the gradation reproduction characteristics in the low resolution mode is used for correcting the gradation correction table T2 in the high resolution mode. However, the high resolution mode corresponding to the entire input gradation range is used. The test pattern image may be formed, and a part of the density measurement result may be used to correct the tone correction table T1 in the low resolution mode. That is, when having a plurality of gradation reproduction modes, a mode for actually measuring gradation reproducibility over the correction target range (first gradation range) that is the entire input gradation range or a range close thereto, and the correction target range The selection of the mode for actually measuring the gradation reproducibility of a part of (second gradation range) is optional. When the tone reproduction mode is 3 or more, at least one mode is set as a mode for actually measuring the tone reproducibility of a part of the correction target range, and a plurality of modes are measured for the tone reproducibility of the entire correction target range. The configuration of the mode to be applied corresponds to the application of the present invention.

  In addition to temperature and humidity, the gradation range in which the density is actually measured by the test pattern image may be changed according to the cumulative operation conditions such as the developing device rotation speed and the photoreceptor rotation speed. When image stabilization is performed, in addition to the above-mentioned periods including when the power is turned on and when returning from sleep, when the amount of change in temperature / humidity, developer speed, photoreceptor speed, etc. exceeds the threshold after the previous implementation May be determined.

  The screen (dither pattern) A and the screen (dither pattern) B do not need to have both the number of screen lines and the screen angle or one of them. The present invention is applied when using a plurality of types of dither patterns in which a common tendency is seen in a change in gradation reproducibility in a part of the gradation range and a different tendency is found in a change in gradation reproducibility in the remaining part. Can be applied.

1 Image forming apparatus 51K, 51M, 51C, 51Y Test pattern image (first test pattern image)
52K, 52M, 52C, 52Y Test pattern image (second test pattern image)
10 Printer engine (image forming unit)
62 Density sensor 76 Density detection unit 511, 512, 514, 514 part 516, 517, 518, 519 part 521, 522, 523, 524 part 77 first correction part 78 second correction part T1, T2 gradation correction table A3 A13 Screen (Dither pattern)
B3, B13 Screen (Dither pattern)
75 Pattern formation control unit (range setting unit)

Claims (9)

An image forming apparatus having a first gradation reproduction mode and a second gradation reproduction mode,
A first test pattern image showing the characteristics of gradation reproduction in the first gradation reproduction mode for the first gradation range that is the whole or part of the input gradation range, and a part of the first gradation range An image forming unit that forms a second test pattern image showing characteristics of gradation reproduction in the second gradation reproduction mode for the second gradation range;
Densities of a plurality of portions included in the first test pattern image formed by the image forming unit and having different input gradation values corresponding to the first test pattern image, and the second test pattern image formed by the image forming unit A density detection unit that detects the density of a plurality of portions that are included in each of the input gradation values corresponding to each other, and
A first correction unit that corrects gradation reproduction characteristics of the first gradation reproduction mode for the entire first gradation range based on the density of the first test pattern image detected by the density detection unit. When,
The density of the second test pattern image detected by the density detector and the first gray scale range of the first test pattern image detected by the density detector other than the second gray scale range A second correction unit that corrects the gradation reproduction characteristics of the second gradation reproduction mode for the entire range of the first gradation range based on the density of the portion corresponding to the range. Image forming apparatus. The image forming apparatus according to claim 1, wherein an area of the second test pattern image is smaller than an area of the first test pattern image. The image forming apparatus according to claim 2, wherein the image forming unit forms the first test pattern image and the second test pattern image side by side on a forming surface. The first and second gradation reproduction modes are modes for reproducing a halftone by a dither method, and the first gradation reproduction mode and the second gradation reproduction mode are within the first gradation range. The image forming apparatus according to claim 2, wherein at least a part of a plurality of dither patterns applied to each input gradation value is different. The first gradation reproduction mode is a low resolution mode in which a dither pattern for forming a first resolution image is applied, and the second gradation reproduction mode is an image having a second resolution higher than the first resolution. The image forming apparatus according to claim 4, wherein the image forming apparatus is a high resolution mode in which a dither pattern for forming is applied. The image forming apparatus according to claim 5, wherein the second gradation range is a low density side range in the first gradation range. The minimum size of the region to which the color material represented by the dither pattern applied in the second gradation reproduction mode should be adhered is the color material represented by the dither pattern applied in the first gradation reproduction mode. The image forming apparatus according to claim 6, wherein the image forming apparatus is smaller than a minimum size of a region to be formed. The image forming apparatus according to claim 1, further comprising a range setting unit that sets the second gradation range according to at least one of an operating environment of the image forming apparatus and a cumulative number of image formations. A method for correcting gradation reproduction characteristics in an image forming apparatus having a first gradation reproduction mode and a second gradation reproduction mode,
A first test pattern image showing the characteristics of gradation reproduction in the first gradation reproduction mode for the first gradation range that is the whole or part of the input gradation range, and a part of the first gradation range Forming a second test pattern image showing the characteristics of gradation reproduction in the second gradation reproduction mode for the second gradation range,
Densities of a plurality of portions included in the formed first test pattern image and corresponding input gradation values are different from each other, and included in the second test pattern image formed by the image forming unit and Detecting the density of each of a plurality of portions having different input gradation values corresponding to
Based on the density of the first test pattern image detected by the density detector, the gradation reproduction characteristics of the first gradation reproduction mode for the entire first gradation range are corrected,
The density of the second test pattern image detected by the density detector and the first gray scale range of the first test pattern image detected by the density detector other than the second gray scale range A gradation reproduction characteristic correction method, comprising: correcting gradation reproduction characteristics of the second gradation reproduction mode for the entire first gradation range based on a density of a portion corresponding to the range.

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