JP2008261909A - Image density control device and image forming apparatus having same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To ensure sharp, high quality image quality of images of various forms and to avoid an inversion phenomenon of image density regardless of the device characteristics. <P>SOLUTION: The image density control device includes: a binary processing section 22 for subjecting a multivalue image signal to a binary process; a peripheral density level acquiring section 24 that acquires the peripheral density level of a pixel within an observation window, having a target pixel as its reference, based on the gradation value of a binary signal acquired by the binary processing section; and an exposure control value acquiring section 25 that obtains an exposure control value for the target pixel according to the peripheral density level of the target pixel acquired by the peripheral density level acquiring section. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image density control device for controlling the density of a toner image obtained by developing a latent image formed on a photosensitive member by an exposure means with toner, and image formation provided with this type of image density control device. It relates to the device.

  In an electrophotographic image forming apparatus (printing apparatus, facsimile apparatus, copying machine, etc.), the density of the toner image changes due to the deterioration of the photoreceptor and the environmental conditions, so that an appropriate density can be stably obtained. In this way, image density control (image stabilization control) for controlling image forming conditions such as the amount of laser light is performed. As such image density control, there is a technique for controlling image forming conditions using a plurality of types of test patterns. Widely used (see Patent Documents 1 and 2).

  Image density control using such a test pattern is very effective in obtaining an appropriate and stable image density against deterioration of the photoreceptor and changes in environmental conditions, but for various types of images. It is difficult to say that the demand for ensuring clear and high-quality image quality is sufficiently satisfied, and a configuration capable of ensuring clear and high-quality image quality for various types of images is desired.

  Therefore, the applicant of the present application has previously proposed an image density control device aimed at ensuring clear and high-quality image quality for various types of images (see Patent Document 3).

This image density control device uses a test pattern that can optimize the thickness of the toner layer by paying attention to the fact that the thickness of the toner layer affects the image density in a solid portion that is entirely black. In the halftone where white and black areas are mixed, the toner layer thickness can be optimized by focusing on the fact that the toner layer thickness affects the image density. The exposure condition is determined using the test pattern.
JP-A-3-279971 (FIGS. 3 and 4) Japanese Patent Laid-Open No. 2001-80113 (FIGS. 3, 4, and 5) JP 2004-306590 A (FIGS. 22 and 23)

  However, in a relatively low density halftone area, the image density is determined by the thickness of the toner layer and is hardly affected by the thickness of the toner layer. As the density approaches the solid part, the image density is greatly affected by not only the thickness of the toner layer but also the thickness of the toner layer. For this reason, assuming uniform exposure conditions in halftone, the toner layer thickness is optimized in a high density halftone area, but the toner layer thickness is not optimized, and the toner layer thickness is excessive. In this case, a phenomenon occurs in which the image density is reversed between the solid portion and the halftone portion.

  Therefore, in order to avoid such an image density reversal phenomenon, in the image density control apparatus proposed in Patent Document 3, the development conditions are optimized by controlling the development contrast potential. Depending on the characteristics of the device such as the agent and the development system, it may be difficult to say that the demand for ensuring high quality image quality is sufficiently satisfied, and improvement is desired.

  The present invention has been devised to solve such problems of the prior art, and its main purpose is to ensure clear and high-quality image quality for various forms of images. Another object of the present invention is to provide an image density control apparatus and an image forming apparatus configured to be able to avoid an image density reversal phenomenon regardless of the characteristics of the apparatus.

  According to the present invention, a binarization processing unit that performs binarization processing on a multivalued image signal, and a pixel of interest based on a gradation value of the binary signal acquired by the binarization processing unit. The peripheral density level acquisition means for acquiring the peripheral density level for the pixels in the observed window, and the exposure control value of the target pixel according to the peripheral density level of the target pixel acquired by the peripheral density level acquisition means And a required exposure control value acquisition means.

  According to the present invention, since the exposure control value is obtained according to the peripheral density level indicating the peripheral density state of the target pixel, it is possible to ensure clear and high quality image quality for various forms of images. Since the exposure control value is obtained according to the correspondence relationship between the peripheral density level obtained by a predetermined interpolation method and the exposure control value with reference to the optimum exposure control value for each test pattern, the phenomenon that the image density is reversed occurs. Can be avoided.

  A first invention made to solve the above problems is a binarization processing means for performing binarization processing on a multi-valued image signal, and a level of the binary signal acquired by the binarization processing means. Peripheral density level acquisition means for acquiring a peripheral density level for a pixel in the observation window with reference to the target pixel based on the tone value, and the peripheral density level of the target pixel acquired by the peripheral density level acquisition means And an exposure control value acquisition means for obtaining the exposure control value of the target pixel according to the above.

  According to this, since the exposure control value is obtained according to the peripheral density level indicating the density state around the pixel of interest, it is possible to ensure clear and high quality image quality for various forms of images.

  According to a second aspect of the present invention for solving the above problem, the peripheral density level is the number of black pixels in the observation window.

  According to this, the calculation processing for obtaining the peripheral density level is simplified, and the control time can be shortened. In the present invention, in addition to simply counting the number of black pixels in this way, it is possible to obtain a peripheral density level by weighting according to the positional relationship between the pixel in the observation window and the target pixel. .

  According to a third aspect of the present invention for solving the above-mentioned problems, the optimum exposure control value for each test pattern is obtained based on the density of the toner image for each of the plurality of test patterns formed on the photoconductor. Correspondence that obtains the correspondence between the peripheral density level and the exposure control value by a predetermined interpolation method on the basis of the optimum exposure control value for each test pattern acquired by the optimum value acquisition means and the optimum value acquisition means for each pattern And setting means.

  According to this, since the exposure control value is obtained by verifying the relationship between the actual toner image density and the exposure control value with the test pattern, an appropriate density can be obtained without being affected by the deterioration of the photosensitive member or the change of the environmental conditions. It can be obtained stably. In addition, since the exposure control value is obtained according to the correspondence relationship between the peripheral density level and the exposure control value obtained by a predetermined interpolation method with reference to the optimum exposure control value for each test pattern, a phenomenon in which the image density is reversed occurs. You can avoid that.

  According to a fourth aspect of the present invention for solving the above-described problems, in the optimum value acquisition unit for each pattern, the optimum exposure control value of the test pattern for halftone control and the test pattern for solid portion control is obtained, and the correspondence In the relationship setting means, the exposure control value of a predetermined reference peripheral density level belonging to the halftone area is set as the optimum exposure control value of the test pattern for halftone control, and the exposure control value of the peripheral density level corresponding to the solid density is set. The exposure control value of the peripheral density level belonging to the high density halftone area between the two is determined by a predetermined interpolation method as the optimum exposure control value of the solid pattern control test pattern.

  According to this, the exposure control value is set so as to gradually increase or decrease as the peripheral density level increases while avoiding the phenomenon that the image density is reversed between the solid portion and the halftone portion. And appropriate control can be performed.

  According to a fifth aspect of the present invention for solving the above-described problems, the correspondence setting means uniformly sets exposure control values of peripheral density levels belonging to a low density halftone area lower than the reference peripheral density level to the halftone level. It is set as the optimal exposure control value acquired by the test pattern for control.

  According to this, an exposure control value suitable for the low density halftone region can be obtained.

  According to a sixth aspect of the present invention for solving the above-mentioned problems, the test pattern for solid portion control is a solid black test pattern filled with black, and the test pattern for halftone control is white. A black-and-white test pattern is set in which the area ratio between the white area and the black area is set so that the area and the black area are mixed and the determination accuracy of the thickness of the toner image is improved.

  According to this, since the thickness of the toner layer can be accurately evaluated with the solid black test pattern, an exposure control value suitable for the solid portion can be obtained. In addition, since it is possible to evaluate the thickness of the toner image with a black-and-white test pattern, it is possible to obtain an exposure control value suitable for a halftone, and in particular, to appropriately set the area ratio between the white area and the black area. Thus, the thickness of the toner image can be determined with high accuracy, and a more appropriate exposure control value can be obtained.

  In this case, the reference peripheral density level may be set to a substantially central peripheral density level.

  According to a seventh aspect of the present invention for solving the above-described problems, the optimum value for each pattern obtaining unit obtains an optimum exposure control value of a test pattern for isolated point control, and the corresponding relationship setting unit obtains a peripheral density level. The exposure control value of the isolated point with the lowest is the optimum exposure control value of the test pattern for the isolated point control.

  According to this, in the case of an isolated point having the lowest peripheral density level, that is, the number of peripheral black pixels is 0, an exposure control value suitable for the isolated point can be obtained by using a test pattern dedicated to the isolated point. .

  In an eighth aspect of the present invention made to solve the above-described problem, the isolated point control test pattern includes a plurality of isolated points arranged such that one black pixel is surrounded by white pixels. And

  According to this, an exposure control value suitable for an isolated point can be obtained.

  According to a ninth aspect of the invention made to solve the above problems, the interpolation method is based on linear interpolation.

  According to this, calculation processing for interpolation is simplified, and the control time can be shortened.

  In a tenth aspect of the invention made to solve the above-mentioned problem, the observation window is set to a size including at least one screen pitch in the binary screen processing performed by the binarization processing means. The configuration.

  According to this, it is possible to avoid the problem that the peripheral density level changes greatly according to the positional relationship between the target pixel and the screen line and appropriate control cannot be performed.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic sectional view showing an image forming apparatus according to the present invention. This image forming apparatus scans around the photosensitive drum 1 with a charging roller 2 that uniformly charges the image forming surface of the photosensitive drum 1 and an exposure laser beam on the image forming surface of the photosensitive drum 1. A developing device including an LSU (laser scanning unit, exposure means) 3 for forming an electrostatic latent image and a developing roller 4 for developing the electrostatic latent image on the image forming surface of the photosensitive drum 1 with toner. 5, a transfer roller 6 for transferring a toner image formed on the image forming surface of the photosensitive drum 1 onto a recording sheet, and a cleaning blade 7 for cleaning the image forming surface of the photosensitive drum 1. After the recording paper in the paper supply unit 8 is fed between the photosensitive drum 1 and the transfer roller 6, it is discharged to the paper discharge unit 10 through the fixing device 9. The image forming apparatus further includes an original reading unit 11 for copying and facsimile transmission.

  FIG. 2 is a block diagram showing a schematic configuration of an image density control unit in the image forming apparatus shown in FIG. The image density control unit (image density control device) 21 includes a binarization processing unit (binarization processing unit) 22, an image memory 23, a peripheral density level acquisition unit (peripheral density level acquisition unit) 24, and an exposure. A control value acquisition unit (control value acquisition unit) 25, a pattern-specific optimum value acquisition unit (pattern-specific optimal value acquisition unit) 26, a correspondence setting unit 27, and a correspondence table storage unit 28 are provided.

  In the optimum value acquisition unit 26 for each pattern, the optimum value for each test pattern is determined based on the detection result of the photosensor (density detection means) 29 that detects the density of the toner image for each of the plurality of test patterns formed on the photosensitive drum 1. Processing for acquiring an exposure control value is performed. The correspondence setting unit 27 performs a process of obtaining a correspondence between the peripheral density level and the exposure control value by a predetermined interpolation method based on the optimum exposure control value acquired by the pattern-specific optimum value acquisition unit 26. Information regarding the correspondence between the peripheral density level and the exposure control value is stored in the correspondence table storage unit 28 as a correspondence table.

  In the binarization processing unit 22, binarization processing is performed on the input multi-value image signal, and pixel values obtained thereby are stored in an image memory 23 that stores pixel values of a required number of lines. The The peripheral density level acquisition unit 24 reads the pixel value from the image memory 23 and acquires the peripheral density level of the target pixel based on the gradation value for each pixel in the predetermined observation window with the target pixel as a reference. Is done. In the exposure control value acquisition unit 25, according to the peripheral density level and exposure control value correspondence table stored in the correspondence table storage unit 28, the attention control value is acquired from the peripheral density level of the target pixel acquired by the peripheral density level acquisition unit 24. Processing for obtaining the exposure control value of the pixel is performed.

  The pixel-by-pixel exposure control signal output from the exposure control value acquisition unit 25 is input to the laser modulation circuit 31, and the LSU 3 is output from the laser drive circuit 32 in response to the control signal output from the laser modulation circuit 31. The laser exposure operation is controlled. The laser modulation circuit 31 controls the light source lighting time for each pixel according to the pulse width modulation (PWM) control, that is, the exposure control value acquired by the exposure control value acquisition unit 25.

  FIG. 3 shows the state of the toner image formed on the image forming surface of the photosensitive drum 1 shown in FIG. FIG. 4 shows a test pattern used in the image density control unit 21 shown in FIG. Here, three types of test patterns for isolated point control, halftone control, and solid portion control are used.

  At the isolated point shown in FIG. 3A, the thickness of the toner image formed on the photosensitive material (image forming surface) 41 becomes a problem. In this case, the isolated point control shown in FIG. Since the thickness of the toner image at the isolated point can be accurately evaluated by the test pattern, an exposure control value suitable for the isolated point can be obtained.

  This isolated point control test pattern is a plurality of isolated points in which a single black pixel is surrounded by white pixels. Here, only one pixel in a square region composed of 2 × 2 pixels is arranged. Becomes a black pixel, and one black pixel is surrounded by white pixels.

  In the low density halftone region shown in FIG. 3B, the image density changes in accordance with the ratio between the exposed surface of the photoconductor 41 and the toner image, and the thickness of the toner layer affects the image density. In this case, since the thickness of the toner image can be accurately evaluated by the test pattern for halftone control shown in FIG. 4B, an exposure control value suitable for the halftone can be obtained.

  This test pattern for halftone control is a black and white test pattern in which a white area and a black area are mixed. Here, a black area and a white area in which three pixels of a square area composed of 2 × 2 pixels are black pixels. Are regularly arranged alternately, and the black areas are adjacent to each other only in an oblique direction, and the area ratio between the white area and the black area is set so as to improve the accuracy of determination of the thickness of the toner image. .

  In the high density halftone area shown in FIG. 3C, as in the low density halftone area, the thickness of the toner layer affects the image density, but the test pattern for halftone control shown in FIG. If the thickness of the toner image is simply controlled by this, the toner layer becomes excessively thick, and a phenomenon occurs in which the image density is reversed between the solid portion and the halftone portion. For this reason, as will be described in detail below, the optimum exposure control value based on the test pattern for halftone control smoothly changes toward the optimum exposure control value based on the test pattern for solid portion control shown in FIG. The exposure control value is set to

  In the solid portion shown in FIG. 3D, the thickness of the toner layer affects the image density, and the image density changes according to the thickness of the toner layer. In this case, the solid portion control shown in FIG. With this test pattern, the thickness of the toner layer can be accurately evaluated, so that an exposure control value suitable for the solid portion can be obtained. This solid part control test pattern is a solid black test pattern filled with black as a whole.

  For these test patterns, the average density of the whole is detected by the photo sensor 29. At this time, image formation of the test pattern is performed a plurality of times while changing the exposure control value stepwise, and exposure control for each test pattern is performed. The density of a plurality of toner images having different values is detected by the photo sensor 29. The sensor output value acquired in this way is compared with a predetermined output target value by the optimum value acquisition unit 26 for each pattern, and an optimum exposure control value that can realize this output target value is obtained for each test pattern.

  FIG. 5 is a schematic diagram for explaining the outline of processing performed by the image density control unit 21 shown in FIG. In the peripheral density level acquisition unit 24, when the target pixel is a black pixel, a process of counting the number of black pixels in the observation window centered on the target pixel is performed as the peripheral density level, and the exposure control value acquisition unit 25 Processing for obtaining an exposure control value from the peripheral density level (number of peripheral black pixels) is performed with reference to the correspondence table. On the other hand, when the target pixel is a white pixel, the exposure control value acquisition unit 25 sets the exposure control value to 0.

  In the optimum value acquisition unit 26 for each pattern, a process for acquiring the optimum exposure control value of the test pattern for isolated point control, the test pattern for halftone control, and the test pattern for solid portion control shown in FIG. Based on the optimum exposure control value for each test pattern, the correspondence setting unit 27 creates a correspondence table.

  In the correspondence table, the peripheral density level is divided into four areas of an isolated point area, a low density halftone area, a high density halftone area, and a solid density area, and an exposure control value for each peripheral density level is set.

  First, in the isolated point area, that is, in the area where the pixels in the observation window around the target pixel are all white pixels and the peripheral density level (number of peripheral black pixels) is the lowest, the corresponding exposure control value is set for the isolated point control. The optimum exposure control value (duty A) acquired with the test pattern. Thereby, an exposure control value suitable for an isolated point can be obtained. Here, only the case where the pixels in the observation window are all white pixels (peripheral density level = 0) is defined as the isolated point region, but the isolated point region also includes the case where there are only a few black pixels in the observation window. It is also possible to set so that

  On the other hand, in the solid density area, that is, the area where the pixels in the observation window around the target pixel are all black pixels and the peripheral density level (number of peripheral black pixels) is the highest, the corresponding exposure control value is The optimum exposure control value (duty C) of the test pattern. Thereby, the exposure control value suitable for the solid portion can be obtained. Here, only the case where all the pixels in the observation window are black pixels (peripheral density level = 24) is the solid density area, but the solid density area also includes a case where there are only a few white pixels in the observation window. It is also possible to set so that

  Further, the exposure control value of the peripheral density level belonging to the low density halftone region lower than the predetermined reference peripheral density level is uniformly set to the optimum exposure control value (duty B) acquired by the test pattern for halftone control. Thereby, an exposure control value suitable for the low density halftone region can be obtained.

  On the other hand, the exposure control value of the peripheral density level belonging to the high density halftone area higher than the reference peripheral density level is the optimum exposure control value (duty B) acquired by the test pattern for halftone control and the test pattern for solid portion control. The optimum exposure control value (duty C) is determined by a predetermined interpolation method so as to gradually increase or decrease as the peripheral density level (the number of peripheral black pixels) increases. Thereby, it is possible to avoid a phenomenon in which the image density is reversed between the solid portion and the halftone portion.

  6 and 7 show an example of the correspondence table created by the correspondence setting unit 27 shown in FIG. 2. FIG. 6 shows a case where the resolution is 600 dpi, and FIG. 7 shows a case where the resolution is 1200 dpi. Show.

  When the resolution is 600 dpi, the observation window is an area of 5 × 5 pixels centered on the target pixel, and the peripheral density level (the number of peripheral black pixels) is divided into 25 levels of 0 to 24. Here, the reference peripheral density level for distinguishing the high density halftone area and the low density halftone area is set to 12 which is substantially the center (about 50% of the maximum level), and the low density halftone area is 1 of the peripheral density level. The range of ˜12, the high density halftone region is the range of 13-23 of the peripheral density level.

  When the resolution is 1200 dpi, the observation window is an area of 9 × 9 pixels centered on the target pixel, and the peripheral density level (number of peripheral black pixels) is divided into 81 steps of 0 to 80. Here, the reference peripheral density level for distinguishing the high density halftone area and the low density halftone area is set to 40, which is substantially the center (about 50% of the maximum level), and the low density halftone area is 1 of the peripheral density level. The high density halftone region is in the range of 41 to 79 of the peripheral density level.

  FIGS. 8 and 9 are graphs showing the correspondence table created by the correspondence setting unit 27 shown in FIG. 2. FIG. 8 shows the case of linear interpolation, and FIG. 9 shows the case of curve interpolation. ing. In the case of linear interpolation, an exposure control value for each peripheral density level belonging to the high density halftone region is calculated by an interpolation formula using a linear function, and in the case of curve interpolation, an interpolation formula using a quadratic function or the like. As a result, the image density is not reversed between the high density halftone area and the solid density area, and the density correction that smoothly changes is performed.

For example, in the case of linear interpolation, the optimum exposure control value (duty B, duty C) for each test pattern for halftone control and solid portion control, the total number of pixels N in the observation window, the number of black pixels Nb, the black of the reference peripheral density level From the number of pixels Nm = (N−1) / 2, the interpolation equation for obtaining the exposure control value dutyX is:
dutyX = (dutyC−dutyB) × (Nb−Nm) / Nm + dutyB
It becomes.

  The magnitude relationship between the optimum exposure control values (duty B, duty C) for each test pattern for halftone control and solid portion control may be reversed depending on the characteristics and state of the apparatus. ), The optimal exposure control value for the solid pattern test pattern (duty C = 128, for 8 bits and 256 gradations, the same applies below) is used as the optimal exposure control value for the halftone control test pattern (duty B). = 180). In the example shown in FIG. 8B, the optimum exposure control value (duty C = 240) of the solid pattern control test pattern is the optimum exposure control value (duty B = 200) of the test pattern for halftone control. It is getting bigger.

  FIG. 10 shows a procedure for setting the size of the observation window shown in FIG. The observation window is set to a size that includes at least one screen pitch in the screen processing performed by the binarization processing unit 22, and the size of the observation window varies with the number of screen lines corresponding to the resolution. Here, 5 × 5 pixels centered on the target pixel at 600 dpi / 150 lpi, and 9 × 9 pixels centered on the target pixel at 1200 dpi / 200 lpi.

  In screen processing, black pixels are gathered linearly according to the number of screen lines to create a binary image, and black pixels are unevenly distributed according to the number of screen lines. The peripheral density level (number of surrounding black pixels) changes greatly depending on the positional relationship between the pixel and the pixel of interest within or outside the linear black pixel region, and appropriate control cannot be performed. By setting the size to a range wider than one screen pitch, it is possible to eliminate the influence of uneven black pixels depending on the number of screen lines.

  FIG. 11 shows a change state of the gamma characteristic before and after the control performed by the image density control unit 21 shown in FIG. Here, in each of the error diffusion and screen processing methods in the binarization processing unit 22, a high density state and an image in which the image density becomes high even if an image is formed with the same exposure control value according to the state of the developing device, etc. The density (Macbeth density) of the actually formed image is measured while changing the gradation on the electronic data in two states of low density where the density is low.

  Comparing before control and after control, in either case of the error diffusion processing shown in (A) and the screen processing shown in (B), the overall image density changes in a decreasing direction, but in the high density range. Therefore, a constant density characteristic can be ensured irrespective of whether the apparatus is dark or light, and stable image formation is possible.

  FIG. 12 is a block diagram showing an example of a color image forming apparatus according to the present invention. In this color image forming apparatus, a plurality of process units 121 a to 121 d for each color of yellow (Y), magenta (M), cyan (C), and black (K) are arranged along the intermediate transfer belt 122. Then, writing is performed on the photosensitive drums 123a to 123d of these process units 121a to 121d by the LSU 124 and then developed by the developing units 125a to 125d, whereby each color on the photosensitive drums 123a to 123d. A toner image for each color is formed, and a color image is obtained by sequentially transferring and synthesizing the toner images for each color to the intermediate transfer belt 122, and the recording paper of the paper feeding unit is transferred to the intermediate transfer belt. A color toner image is transferred between the belt 122 and the secondary transfer roller 126 and transferred onto the recording paper.

  In this color image forming apparatus, a test pattern for each color is created in each of the process units 121a to 121d. This test pattern image formation is performed while the intermediate transfer belt 122 is running, and after the test patterns for the respective colors formed on the respective photosensitive drums 123a to 123d are sequentially transferred onto the intermediate transfer belt 122, The density of the test pattern on the intermediate transfer belt 122 is sequentially detected by the image density sensor 127.

  The image density control unit 131 calculates the optimum exposure control value for each test pattern based on the output value of the image density sensor 127, and further relates to the correspondence between the peripheral density level and the exposure control value. A correspondence table is created for each color. Then, when the image signal is input, the peripheral density level of the target pixel is obtained for each color, and the exposure control value for each pixel is obtained for each color from the peripheral density level according to the correspondence table. The laser driving operation of the LSU 124 is controlled by the laser driving circuit 133 in response to a control signal input to the modulation circuit 132 and output from the laser modulation circuit 132 accordingly.

  The image density control apparatus and the image forming apparatus including the image density control apparatus according to the present invention can ensure clear and high-quality image quality for various types of images, and can control the image density regardless of the characteristics of the apparatus. An image density control device for controlling the density of a toner image obtained by developing a latent image formed on a photosensitive member by exposure means with toner, which has an effect of avoiding a reverse phenomenon, and an image of this type It is useful as an image forming apparatus provided with a density control device, for example, a printer, a facsimile machine, a copying machine, a multifunction machine, and the like.

Schematic sectional view showing an image forming apparatus according to the present invention. 1 is a block diagram showing a schematic configuration of an image density control unit in the image forming apparatus shown in FIG. The figure which shows the state of the toner image formed on the image creation surface of the photosensitive drum shown in FIG. The figure which shows the test pattern used with the image density control part shown in FIG. FIG. 2 is a schematic diagram for explaining the outline of processing performed by the image density control unit shown in FIG. The figure which shows the corresponding | compatible table produced in the corresponding | compatible relationship setting part shown in FIG. The figure which shows the corresponding | compatible table produced in the corresponding | compatible relationship setting part shown in FIG. The figure which shows the correspondence table created by the correspondence setting part shown in FIG. 2 in a graph The figure which shows the correspondence table created by the correspondence setting part shown in FIG. 2 in a graph The figure explaining the point which sets the size of the observation window shown in FIG. The figure which shows the change condition of the gamma characteristic before and behind control performed by the image density control part shown in FIG. 1 is a block diagram showing an example of a color image forming apparatus according to the present invention.

Explanation of symbols

1 Photosensitive drum 3 LSU (exposure means)
21 Image density controller (image density controller)
22 Binarization processing unit (binarization processing means)
24 Ambient concentration level acquisition unit (Ambient concentration level acquisition means)
25 Exposure control value acquisition unit (control value acquisition means)
26 Optimum value acquisition unit for each pattern (optimum value acquisition means for each pattern)
27 Correspondence setting unit 28 Corresponding table storage unit 29 Photo sensor (density detection means)
31 Laser modulation circuit (exposure control means)
32 Laser drive circuit

Claims (11)

Binarization processing means for performing binarization processing on multi-value image signals;
Peripheral density level acquisition means for acquiring a peripheral density level for a pixel in the observation window based on the target pixel based on the gradation value of the binary signal acquired by the binarization processing means;
An image density control apparatus comprising: an exposure control value acquisition unit that obtains an exposure control value of the target pixel according to the peripheral density level of the target pixel acquired by the peripheral density level acquisition unit.   The image density control apparatus according to claim 1, wherein the peripheral density level is the number of black pixels in the observation window. An optimum value acquisition unit for each pattern for acquiring an optimum exposure control value for each test pattern based on the density of the toner image for each of the plurality of test patterns formed on the photoreceptor;
Correspondence setting means for obtaining a correspondence relationship between the peripheral density level and the exposure control value by a predetermined interpolation method on the basis of the optimum exposure control value for each test pattern acquired by the optimum value acquisition means for each pattern. The image density control apparatus according to claim 1. In the optimum value acquisition means for each pattern, an optimal exposure control value of a test pattern for halftone control and a test pattern for solid portion control is acquired,
In the correspondence setting means, the exposure control value of a predetermined reference peripheral density level belonging to the halftone area is set as the optimum exposure control value of the test pattern for halftone control, and the exposure control of the peripheral density level corresponding to the solid density is performed. The value is the optimum exposure control value of the test pattern for the solid portion control, and the exposure control value of the peripheral density level belonging to the high density halftone area between the two is obtained by a predetermined interpolation method. The image density control apparatus according to claim 3.   In the correspondence setting means, the exposure control value of the peripheral density level belonging to the low density halftone area lower than the reference peripheral density level is uniformly set to the optimum exposure control value acquired by the test pattern for halftone control. The image density control apparatus according to claim 4.   The solid part control test pattern is a solid black test pattern filled with black, and the halftone control test pattern includes a mixture of white and black areas and a toner image that is thick. 6. The image density control apparatus according to claim 4, wherein the image density control apparatus is a black-and-white test pattern in which an area ratio between a white area and a black area is set so as to improve discrimination accuracy. In the optimum value acquisition means for each pattern, an optimum exposure control value of a test pattern for isolated point control is obtained,
4. The image density according to claim 3, wherein, in the correspondence setting means, the exposure control value of the isolated point having the lowest peripheral density level is set as the optimum exposure control value of the test pattern for the isolated point control. Control device.   8. The image density control apparatus according to claim 7, wherein the isolated point control test pattern includes a plurality of isolated points arranged such that one black pixel is surrounded by white pixels.   The image density control apparatus according to claim 3, wherein the interpolation method is linear interpolation.   2. The image density control apparatus according to claim 1, wherein the observation window is set to a size including at least one screen pitch of the binary screen processing performed by the binarization processing means. .   An image forming apparatus comprising the image density control device according to claim 1.

Images