JP5272535B2 - Exposure control apparatus and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain proper reproducibility of an image by further improving the gradation and the resolution of the image further. <P>SOLUTION: When bit map data is acquired from an image processing part 60, a selection part 71 supplies the bit map data, to which sort information, such as, "photographic image" and "graphic", such as, graphic art is allotted to a halftone processing part 72, and the halftone processing part 72 performs screen processing. A pulse width modulation part 73 corrects the exposure light, subjected to a pulse width modulation system obtained from the bit map data according to film thickness or the like of a charge receptor, and generates a PWM signal. Meanwhile, an intensity modulation part 74 acquires bit map data to which sort information, such as, "line image" or "character image" is allotted from the selecting part 71, and corrects the exposure light subjected to an intensity modulation system by a correction amount, in accordance with the sort of image, the line width of the line image or the character image and the film thickness of the charge receptor, and generates an intensity-modulated signal. A laser driver 75 drives an exposure device 513, based on the modulated signals. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to an exposure control apparatus and an image forming apparatus.

In an electrophotographic image forming apparatus, an exposure device that forms an electrostatic latent image on the surface of a photoconductor expresses a gradation value of each pixel as a method of expressing a gradation value of each pixel. A width modulation method and an intensity modulation method that expresses gradation according to the intensity of exposure light are known. By the way, there are various types of images represented by image data such as character images, line images, photographic images, graphic arts, etc., and in order to improve the reproducibility of the image, the gradation is changed depending on the type of image. There is a case where emphasis is particularly placed on sex, and on the contrary, importance is placed on resolution. Therefore, for example, Patent Document 1 discloses that PWM (Pulse Width Modulation: Pulse) generated in accordance with a pulse width modulation method depending on whether to emphasize gradation or resolution based on the analysis of image data. The selection of a (width modulation) signal is disclosed. Patent Document 2 discloses that an intensity modulation (power modulation) method is adopted for a region representing a photographic image, and a pulse width modulation method is adopted for a region representing a character image.
JP 2000-341517 A JP-A-6-217120

The technique disclosed in Patent Document 1 employs a pulse width modulation method both when emphasizing gradation and when emphasizing resolution. The technique disclosed in Patent Document 2 The intensity modulation method is employed when importance is attached to the gradation, and the pulse width modulation method is adopted when importance is attached to the resolution. By the way, when an electrostatic latent image is formed according to the pulse width modulation method when a toner image corresponding to high resolution image data is to be formed, for example, a thin line electrostatic latent image having a line width of one pixel. Cannot be formed, and the toner image may be partially interrupted. Further, the intensity modulation method has a problem that it is difficult to precisely control the intensity of exposure light in order to form a toner image that faithfully reproduces the gradation value of a pixel. Therefore, the techniques disclosed in Patent Documents 1 and 2 cannot reproduce the delicate balance between gradation and resolution, and as a result, the reproducibility of the image may deteriorate.
The present invention has been made in view of such circumstances, and an object of the present invention is to further improve the gradation and resolution of an image and to obtain a good reproducibility of the image.
To obtain a good reproducibility.

In order to solve the above-described problem, the present invention provides an acquisition unit that acquires image data including a gradation value of each pixel, type information indicating a type of an image represented by the image data, and a pixel based on a light pulse width. In accordance with a pulse width modulation system that expresses the tone value of the first, a charge receptor is irradiated with light according to the image data acquired by the acquisition means, and a first electrostatic latent image is formed on the charge receptor. In accordance with the exposure means and the intensity modulation method that expresses the gradation value of the pixel by the light intensity, the charge receptor is irradiated with light according to the image data acquired by the acquisition means, and the charge receptor is electrostatically applied to the charge receptor. Either the first or second exposure means is selected as a second exposure means for forming a latent image and an exposure means for forming the electrostatic latent image based on the type information acquired by the acquisition means. Selecting means and said selecting means Then, when the second exposure unit is selected, a correction unit that corrects the light intensity according to the image data to be irradiated to the second exposure unit with a correction amount according to the type information. A correction amount that increases the light intensity when the line width of the image represented by the image data is smaller than a first threshold, in addition to the correction according to the type information. And when the line width is equal to or larger than a second threshold value that is larger than the first threshold value , the exposure control is performed with a correction amount that reduces the light intensity. Providing equipment .

In the exposure control apparatus of the present invention, the type information includes a type of a line image or a character image, and the selection unit, if the type information acquired by the acquisition unit represents the line image or character image, It is preferable to select the second exposure means. In this case, if the type information acquired by the acquisition unit represents a type other than the line image and the character image, the selection unit selects the first exposure unit and the second exposure. The maximum intensity of light emitted by the means is greater than the maximum intensity of light emitted by the first exposure means .

  The exposure control apparatus according to the present invention further comprises a film thickness detecting means for detecting the film thickness of the charge receptor, wherein the correction means is detected by the film thickness detecting means in addition to the correction according to the type information. It is preferable that correction is performed with a correction amount that reduces the intensity of light emitted by the selected exposure unit as the film thickness is reduced. Further, whether the recording medium to which the developed electrostatic latent image is transferred is a recording medium having a coating layer on the recording surface or a recording medium having no coating layer. Detecting means for detecting, in addition to the correction according to the type information, the correction means selects the recording medium provided with the coating layer according to the result of detection by the detecting means. A correction amount that corrects the intensity of light irradiated by the exposed exposure means with a correction amount that makes it smaller than the intensity of light irradiated to a recording medium not provided with the coating layer. It is preferable to correct by.

  Further, in the exposure control apparatus of the present invention, when the image represented by the image data is a white image that is an image represented by surrounding a region where no colored pixel is arranged by a region where the colored pixel is arranged, The acquisition unit includes an image width changing unit that increases an image width of an area in which the colored pixels are not arranged according to at least one of the line width and size of the image and the gradation value of the colored pixel. Preferably, the image data processed by the image width changing means is acquired.

  According to another aspect of the present invention, there is provided the exposure control apparatus according to any one of the above-mentioned configurations, the charge receptor, a charging unit that charges the surface of the charge receptor, and the surface of the charge receptor charged by the charging unit. The image forming apparatus further comprises: a developing unit that develops the electrostatic latent image formed by the exposure control device with a developer; and a transfer unit that transfers the image developed by the developing unit to a recording medium. Providing equipment.

  According to the present invention, it is possible to further improve the gradation and resolution of an image and obtain a good reproducibility of the image.

Embodiments of the present invention will be described below with reference to the drawings.
(1) Configuration FIG. 1 is a block diagram illustrating a configuration of the image forming apparatus 1. As shown in FIG. 1, the image forming apparatus 1 includes a control unit 10, a storage unit 20, a communication unit 30, a UI (User Interface) unit 40, an image forming unit 50, an image processing unit 60, and an exposure. And a control unit 70.

  The control unit 10 includes a CPU (Central Processing Unit) 11 that controls the entire apparatus, a RAM (Random Access Memory) 12 that provides a work area for work, and a ROM (Read Only Memory) 13 that stores various control programs. Arithmetic processing is performed according to the procedure described in the control program. The ROM 13 stores a correction amount used for correcting exposure light irradiated by an exposure device 513 of the image forming unit 50 described later. The storage unit 20 is a storage device such as an HDD (Hard Disk Drive) and stores various data such as image data. The communication unit 30 is an interface device for performing communication via a network such as a LAN (Local Area Network), and receives image data from an external terminal such as a computer (not shown) and outputs various types of information. . The UI unit 40 is a user interface such as a touch panel that allows a user to input an operation and perform notification including display for the user. The image processing unit 60 performs predetermined image processing on image data supplied from an external apparatus 100 described later, which is an apparatus other than the image forming apparatus 1.

FIG. 2 is a diagram schematically illustrating the structure of the image forming apparatus 1. The configuration of the image forming unit 50 will be described with reference to FIG. The image forming unit 50 includes image forming units 51Y, 51M, 51C, 51K, an intermediate transfer belt 52, a secondary transfer device 53, and a fixing device 54. However, the image forming units 51Y, 51M, 51C, and 51K all have the same configuration except that the colors of the toner images to be formed are different, such as yellow, magenta, cyan, and black. Therefore, hereinafter, when there is no need to distinguish between them, these are collectively referred to as “image forming unit 51”.
In addition to the photosensitive drum 511, the image forming unit 51 includes a charger 512, an exposure device 513, a developing device 514, a primary transfer device 515, and a cleaning device 516 arranged around the photosensitive drum 511. The photoconductive drum 511 has, for example, two to three photoconductive layers including a charge generation layer and a charge transport layer made of OPC (Organic Photo Conductor) as a charge acceptor on a conductive substrate. This is an image carrier formed by the above method. The photosensitive drum 511 is rotated in the direction of arrow A (clockwise direction) shown in the drawing by the driving mechanism (not shown) around the central axis. The charger 512 is a primary charging scorotron that uniformly charges the surface of the photosensitive drum 511, and charges the surface of the photosensitive drum 511 to a predetermined potential.

  The exposure device 513 irradiates the polygon mirror with exposure light and irradiates the reflected light onto the surface of the photosensitive drum 511, thereby reducing the surface of the predetermined photosensitive drum 511 to a predetermined potential by photoconductivity. To form an electrostatic latent image. The scanning direction of the exposure light is equal to the axial direction of the photosensitive drum, and this direction is referred to as “main scanning direction” of the image forming unit 50. The direction orthogonal to the main scanning direction is referred to as “sub-scanning direction” of the image forming unit 50.

  The developing device 514 develops the electrostatic latent image using toner contained in the developer. When the toner image formed on the surface of the photoconductive drum 511 reaches a position between itself and the photoconductive drum 511 as the photoconductive drum 511 rotates, the primary transfer unit 515 uses electrostatic force or the like. The toner image is transferred to the intermediate transfer belt 52. The cleaning device 516 removes residual toner and paper dust on the surface of the photosensitive drum 511. The secondary transfer unit 53 transfers each color toner image (development image) transferred to the intermediate transfer belt 52 to the recording paper (recording medium) conveyed at that timing. The fixing device 54 fixes the toner image transferred to the recording paper.

  In addition, the image forming unit 50 includes a film thickness detection unit 80. The film thickness detection unit 80 detects the film thickness of the charge receptor provided on the surface of the photosensitive drum 511 and is provided in the vicinity of the photosensitive drum 511. The film thickness detector 80 sends a current to the photosensitive drum 511 and detects the current value of the current flowing through the photosensitive drum 511. Then, a film thickness signal indicating the detected current value is output to the control unit 10. The controller 10 obtains the film thickness of the charge acceptor based on the film thickness signal. That is, when the current value of the current flowing through the photosensitive drum 511 is constant, the film thickness decreases, and when the thickness of the charge receptor of the photosensitive drum 511 changes, the current flowing through the photosensitive drum 511 changes. Therefore, the film thickness of the charge receptor can be obtained by measuring the current value of the current flowing through the photosensitive drum 511.

Next, the image processing unit 60 and the exposure control unit 70 will be described.
FIG. 3 is a block diagram showing the configuration of the image processing unit 60 and the exposure control unit 70. As shown in FIG. 3, the image processing unit 60 is roughly divided into a raster data generation unit 61, a color conversion processing unit 62, a refinement processing unit 63, and a gradation correction unit 64. The arrows in the figure indicate the data flow.

  The raster data generation unit 61 performs rasterization based on image data described in a page description language (PDL: Page Description Language) supplied from the external device 100, and performs red (R), green (G), blue ( Bmap-format image data (hereinafter referred to as “bitmap data”) in which the gradation values of each pixel are represented by the three color components of B) is generated. Here, it is assumed that bitmap data having a resolution of 600 dpi is generated. “Dpi” is an abbreviation of “dots / inch” which means the number of pixels per inch. In addition, the image data acquired by the raster data generation unit 61 includes information for designating the type of the image. Based on this, the raster data generation unit 61 applies the image data to each pixel of the bitmap data. The type information indicating the type is assigned, and the coordinate value of each pixel and the type information assigned to the pixel are associated and written to the RAM 12 of the control unit 10. The type information includes “photo image”, “graphic”, “line image”, and “character image”, which are designated in advance by the user of the external apparatus 100.

  The color conversion processing unit 62 performs color space conversion processing on the bitmap data. Specifically, the color conversion processing unit 62 applies a lookup table stored in the storage unit 20 or the ROM 13 to bitmap data in which the gradation value of each pixel is expressed in RGB format, and Y, After calculating the three color components of M and C, a known under color removal process is performed to calculate the K color component. By this color conversion process, the image data is converted into bitmap data composed of four color components Y, M, C, and K corresponding to the color of the toner image formed by each image forming unit 51. The color component included in the bitmap data represents a gradation value corresponding to each color toner.

  The refinement processing unit 63 performs a process for improving the definition of the image represented by the bitmap data. Specifically, the refinement processing unit 63 applies a smoothing filter of a predetermined size to execute a smoothing process that reduces a difference in light and shade of an image or an expansion process for enhancing a specific image. Or execute. The dilation processing refers to other pixels in the vicinity of the target pixel. If at least one of the neighboring pixels has a tone value higher than the tone value of the target pixel (that is, a dark pixel), This refers to the process of replacing the gradation value with the gradation value of the neighboring pixels.

The gradation correction unit 64 corrects the gradation conversion characteristic representing the density of the toner image (the coverage of toner on the recording paper per unit area) with respect to the gradation value of the bitmap data. Here, the gradation correction unit 64 applies a lookup table stored in the storage unit 20 or the ROM 13 to the bitmap data to correct the gamma characteristic.
The above is the description of the image processing unit 60.

  Next, the exposure control unit 70 will be described. The exposure control unit 70 controls driving of the exposure apparatus 513. As shown in FIG. 2, the exposure control unit 70 is roughly divided into a selection unit 71, a halftone processing unit 72, a pulse width modulation unit 73, an intensity modulation unit 74, and a laser driver 75. The arrows in the figure indicate the data flow.

The selection unit 71 converts the bitmap data acquired from the gradation correction unit 64 of the image processing unit 60 into halftone processing unit 72 or intensity modulation according to the type information indicating the type of image designated for each pixel. Either one of the parts 74 is selected and output. The selection unit 71 has a function of selecting an exposure light modulation method for forming an electrostatic latent image in accordance with the type information. Specifically, the selection unit 71 includes type information “line image” representing a straight line such as a ruled line or a line image representing a line such as a curve, various characters such as hiragana, katakana, kanji, alphabet, and “+ Bitmap data to which type information “character image” representing a character image representing a character such as a symbol such as “” or “」 ”is assigned is output to the intensity modulator 74. Such line images and character images are images in which resolution is particularly important.
On the other hand, the selection unit 71 includes type information “graphic image” representing a photographic image meaning a photograph, or type information “graphic” representing an image (hereinafter referred to as “graphic”) meaning a picture, graphic, graphic art, or the like. Is assigned to the halftone processing unit 72. This graphic is, for example, an image like graphic art drawn by a user using a drawing application using a mouse, a pen device, or the like. That is, the selection unit 71 outputs bitmap data of type information other than “character image” and “line image” to the halftone processing unit 72. Such photographic images and graphics are images in which gradation is particularly important.
The selection unit 71 may be configured such that the control unit 10 realizes a function equivalent to this. In this case, the control unit 10 selects the output destination of the bitmap data according to the type information. .

  The halftone processing unit 72 performs screen processing on the bitmap data acquired from the selection unit 71. The screen processing is processing in which multi-value data is converted into binary data, and gradation is expressed by the number of pixels per predetermined unit area of a predetermined colored pixel represented by the binary data. There are screen shapes such as halftone dots and rows. Here, a row screen is used. In this case, gradation is determined by a plurality of rows formed by arranging predetermined pixels in a certain direction. Is represented. By determining the screen angle and the number of screen lines of this row-shaped screen according to the gradation value of each pixel, the halftone of the bitmap data is expressed in a pseudo manner. The halftone processing unit 72 stores a lookup table for performing screen processing for each of the type information “photo image” and “graphic” of the supplied bitmap data. The halftone processing unit 72 performs screen processing by applying a lookup table according to the type information to the bitmap data. Specifically, in general, a photographic image uses more colors than a graphic, and gradation is particularly important, so that the photographic image can express halftone more faithfully. However, it is preferable to use a parameter lookup table with high conversion accuracy. Of course, in this case, the same lookup table may be applied to the bitmap data of the type information “photo image” and “graphic”. During screen processing, the halftone processing unit 72 reads out this lookup table from the storage unit 20 or the ROM 13 to its own RAM, and performs screen processing.

  The pulse width modulation unit 73 generates a PWM signal so as to form an electrostatic latent image according to the pulse width modulation method, based on the bitmap data of the type information “photo image” and “graphic”. Specifically, the pulse width modulation unit 73 is a PWM signal for causing the laser driver 75 to form an electrostatic latent image in accordance with a pulse width modulation method that represents the gradation value of each pixel according to the pulse width of the exposure light. Is generated. The pulse width modulation unit 73 increases the pulse width to increase the density of the toner image as the pixel gradation value increases, and decreases the toner image density by decreasing the pulse width as the gradation value decreases. The pulse width modulation unit 73 may control the maximum intensity of the exposure light irradiated when forming the electrostatic latent image, which will be described in detail later.

  The intensity modulation unit 74 generates an intensity modulation signal so as to form an electrostatic latent image according to the intensity modulation method for the bitmap data of the type information “line image” and “character image”. Specifically, the intensity modulation unit 74 generates an intensity modulation signal for causing the laser driver 75 to form an electrostatic latent image in accordance with an intensity modulation method that represents a gradation value of each pixel according to the intensity of exposure light. To do. The intensity modulating unit 74 increases the intensity of the exposure light to increase the density of the toner image as the gradation value of the pixel increases, and decreases the intensity of the exposure image by decreasing the intensity of the exposure light as the gradation value decreases. To do.

Each of the pulse width modulation unit 73 and the intensity modulation unit 74 includes a RAM, and stores a correction amount corresponding to the type information supplied from the control unit 10 in the RAM. In accordance with the correction amount, the pulse width modulation unit 73 converts at least one of the pulse width and intensity of the exposure light determined according to the gradation value of the bitmap data data with a correction amount according to the type information and the bitmap data. After correction, a modulation signal is generated. The intensity modulator 74 also generates the modulation signal after correcting the exposure light intensity determined according to the bitmap data with the correction amount according to the type information and the bitmap data.
The laser driver 75 drives the exposure apparatus 513 based on a PWM signal or intensity modulation signal (hereinafter, may be collectively referred to as “modulation signal”) acquired from the pulse width modulation unit 73 or the intensity modulation unit 74. Then, an electrostatic latent image corresponding to the modulation signal is formed on the photosensitive drum 511. The laser driver 75 functions as a first exposure unit when forming an electrostatic latent image based on the PWM signal generated by the pulse width modulation unit 73, and converts the intensity modulation signal generated by the intensity modulation unit 74 into the intensity modulation signal. When forming an electrostatic latent image based on this, it functions as a second exposure means.
The above is the description of the configuration of the image processing unit 60 and the exposure control unit 70.

Here, FIG. 4 shows an ideal toner image density (hereinafter referred to as “input density”) corresponding to the gradation value of the pixel when an electrostatic latent image is formed according to each of the pulse width modulation method and the intensity modulation method. This is a graph schematically showing the relationship between Cin and the density (hereinafter referred to as “output density”) Cout of a toner image actually formed.
As shown in FIG. 4, when the pulse width modulation method is adopted, the input density Cin and the output density Cout have a substantially linear relationship as shown by the solid line. In the pulse modulation method, halftones are expressed based on the pulse width of the exposure light in the main scanning direction. By appropriately controlling this pulse width, the density corresponding to the gradation value is faithfully maintained in the entire density range. The expressed toner image can be formed.

  On the other hand, when the intensity modulation method is adopted, as illustrated by the one-dot chain line, in the low concentration region where the input concentration Cin is 5% to 20% and in the high concentration region where the input concentration Cin is 80% to 100%, It can be seen that the change in the output density Cout with respect to the change in the input density Cin is very small. In such a density range, the difference in density of the toner image due to the difference in gradation value does not appear clearly, so that a toner image with a color collapse is formed. Further, as shown in the figure, the change amount of the output density Cout with respect to the change of the input density Cin is large in the medium density region where the input density Cin is 20% to 80%. However, it is difficult to precisely control the intensity of the exposure light emitted by the exposure apparatus 513 according to the gradation value of the pixel, for example, 256 gradations. It is difficult to faithfully express the corresponding toner image. Therefore, when the intensity modulation method is adopted, a hard toner image with strong contrast of light and darkness and color contrast is formed, and defects such as pseudo contours are generated. As a result, the gradation of the toner image may be deteriorated.

  For these reasons, the intensity modulation method is not adopted when forming an image that emphasizes gradation, such as a photographic image or graphic. On the other hand, the number of colors included in a line image or a character image is generally small, and since it is required that the outline, the line width, and the like be expressed faithfully, the resolution is particularly emphasized.

  For line images and character images, a technique is used in which the density of the edge portion is lowered in order to increase the sharpness. Here, FIG. 5 shows an image representing the edge portion of the line image or character image before screen processing (FIG. 5A), and the edge when it is assumed that the pulse width modulation method is adopted after the screen processing. The toner image corresponding to the portion ((b) in the figure) is shown. As shown in FIG. 6A, in the case of a black line image or character image, the edge portion is expressed in gray. When an electrostatic latent image is formed in accordance with the pulse width modulation method, screen processing is performed as shown in FIG. 5B (here, a row screen is used). This cannot contribute to increasing the sharpness of the edge portion of the toner image, and the image quality deteriorates. For these reasons, it is preferable to employ the intensity modulation method when importance is attached to the resolution, such as a line image or a character image.

(2) Generation of Modulation Signal As a precondition for generating the modulation signal, the exposure control unit 70 sets the maximum intensity E1 of the exposure light when forming the electrostatic latent image according to the intensity modulation method to the electrostatic level according to the pulse width modulation method. It is made larger than the maximum intensity E2 of the exposure light when forming the latent image. The maximum intensity E1 of the exposure light when this intensity modulation method is adopted is that the line width used for the line image or the character image is one pixel, and the angle of the line with respect to the main scanning direction (or the sub-scanning direction) is A sufficient strength is set so that the toner image is not interrupted when reproduced at 45 °. This is because the inventors have found that the line of the above condition in the line image and the character image shown on the left side of FIG. 6 tends to be broken in a broken line shape in the toner image as shown on the right side of the figure. It is. On the other hand, if the maximum intensity E2 of exposure light when forming an electrostatic latent image in accordance with the pulse width modulation method is increased as E1, the development amount of the toner image becomes too large and the development becomes excessive. . In this case, the toner is diffused in the process of forming an image. For example, the toner images stick together in an unintended manner on a line-shaped screen, and the density becomes high, thereby reproducing a good gradation. Can not do it. Even if the shape of the screen is a halftone dot, the halftone dot may be larger than the intended size and the density may increase. Therefore, the relationship of E2> E1 is established at the maximum intensity.

Next, the correction performed by the pulse width modulation unit 73 and the intensity modulation unit 74 on the exposure light specified from the bitmap data will be specifically described.
(2-1) Pulse width modulation unit 73
The pulse width modulation unit 73 corrects the exposure light determined from the bitmap data with a correction amount according to the detection result of the film thickness detection unit 80, and generates a PWM signal according to the corrected exposure light.
Here, the relationship between the film thickness of the charge acceptor provided on the surface of the photosensitive drum 511 detected by the film thickness detector 80 and the exposure light correction amount will be described.
Increasing the film thickness of the charge receptor provided on the surface of the photoconductive drum 511 can extend the lifetime. However, in the photosensitive drum 511 having a thick film, in particular, when the exposure light is irradiated when the film thickness of the charge receptor is large as in the case of a new article, the image formation is repeated or the electrostatic latent image is formed. In order to improve reproducibility, the charge density in the charge receptor is greatly different from the case where the surface is processed or worn so that the surface becomes smooth and the film thickness is small. Naturally, the smaller the film thickness, the larger the charge density. As a result, when the electrostatic latent image formed on the surface of the photosensitive drum 511 is developed, the development becomes excessive. This diffuses the screen and increases the density of the toner image. Therefore, in order to prevent the toner image from becoming too dark, a correction amount is stored in the ROM 13 of the control unit 10 so that the exposure light intensity decreases as the charge receptor film thickness decreases.

  As described above, when the pulse width modulation unit 73 generates the PWM signal, in addition to the correction according to the bitmap data and the type information, the intensity of the exposure light according to the bitmap data depends on the film thickness. The PWM signal is generated after the correction amount is corrected to be small. Accordingly, since the intensity of exposure light decreases as the film thickness decreases, it is possible to form a toner image with a substantially constant density by avoiding excessive development regardless of the film thickness.

  Further, as described above, while gradation is considerably emphasized in a photographic image, the number of colors is generally smaller in a graphic in which an electrostatic latent image is formed according to the same pulse width modulation method than in a photographic image. Although the gradation may be slightly inferior, the saturation and the sharpness of the outline are more important than the photographic image. Therefore, in the case of graphics, the correction amount for the screen image may not be set as precisely as in the case of a photographic image, but the sharpness of the edge portion is increased so as to ensure the resolution. Thus, it is preferable to set a correction amount that increases the intensity of exposure light to some extent.

(2-2) Intensity modulation unit 74
When acquiring the bitmap data, the intensity modulating unit 74 corrects the intensity of the exposure light with the correction amount corresponding to the line width (thickness) of these line images and character images. Hereinafter, the correction according to the line width will be specifically described.

First, when the line width of the line image is very small, such as 1 pixel or 2 pixels, the line width of the corresponding toner image becomes narrower than intended. May occur.
FIG. 7 is a graph showing an experimentally obtained relationship between the line width of a line image and the line width of a toner image in bitmap data having a resolution of 600 dpi. In FIG. 7, the horizontal axis represents the line width (pixels) of the line image, and the vertical axis represents the line width (μm; micrometer) of the toner image actually formed. As illustrated by the broken line, the line width of the toner image should increase proportionally as the line width of the line image increases. However, as shown by the solid line in the figure, when the line width of the line image is extremely thin such as 1 pixel or 2 pixels, the line width of the toner image actually formed is larger than the line width of the line image. Will be much smaller.

The reason for this will be described with reference to FIG.
FIG. 8 shows an electrostatic latent image corresponding to a line image having a line width (approximately 20 millimeters) of 472 pixels (dot) (FIG. 8A) and a static image representing a line image having a line width of 1 pixel (dot). It is a figure explaining the relationship between the electric potential of each electrostatic latent image, and the electric potential of a developing bias at the time of forming an electrostatic latent image (the figure (b)).
As shown in FIG. 5A, in the case of a line image having a somewhat large line width, the potential difference between the corresponding electrostatic latent image potential and the developing bias potential of the developing device 514 is VR. A sufficiently large potential difference of approximately VR is ensured over almost the entire area, and a toner image can be formed on the entire electrostatic latent image. On the other hand, as shown in FIG. 5B, in the case of a line image having a line width of one pixel, the potential at the end of the electrostatic latent image is not sufficiently lowered from the charged potential, and the development bias and The potential difference is quite small. This is because the beam diameter of the exposure light irradiated by the exposure apparatus 513 has a certain size, but the intensity is not uniform over the entire area, and the transition is made so that the intensity decreases radially from the center. Based. Even if the potential difference VR is to be secured, the line width of the line image is small, so that only VR1 (<VR) can be secured at most. For this reason, since the potential difference from the developing bias is considerably small in the end region of the electrostatic latent image, the line width of the toner image is reduced because the end region is not developed.

From the above situation, when the line width is small, the intensity modulation unit 74 corrects the exposure light with a correction amount set so that the intensity of the exposure light is increased as the line width of the line image is smaller.
FIG. 9 is a diagram schematically showing the relationship between the line width of a line image and the correction amount with respect to the intensity of exposure light. In FIG. 9, the horizontal axis represents the line width (pixel) of the line image, and the vertical axis represents the exposure light intensity correction amount. As shown in the figure, the correction amount is set so that the correction amount increases as the line width of the line image decreases. For example, in the case of forming a toner image representing a line image having a line width of one pixel, the potential of the electrostatic latent image is changed as indicated by a one-dot chain line shown in FIG. Further reduction can be achieved. As a result, a potential difference from the developing bias can be ensured in a wider area, so that the line width of the toner image can be prevented from being reduced. Further, as the line width of the line image becomes larger as 2, 3 and 4 pixels, the influence of thinning of the line width becomes smaller, so the correction amount becomes gradually smaller. When the number of pixels is 5 or 6, the correction amount is set to “0” because there is almost no influence from the reduction of the line width of the line image as in the case where the line width is large (FIG. 8A). Yes. That is, when the line width of the line image or the character image is smaller than the threshold value (first threshold value), the intensity modulation unit 74 performs correction with a correction amount that increases the intensity of the exposure light.

  On the other hand, a toner image having a line width of one pixel or two pixels is also used in a column-shaped screen used for a photographic image or graphic, but the pulse width modulation unit 73 does not correct the line width. This is because if the image constituting the screen is corrected, the density of the toner image becomes high and the gradation is deteriorated.

Second, when the line width of the line image is increased to some extent, image deterioration may occur in which the line width in the toner image becomes thinner than intended.
FIG. 10 is a graph showing an experimentally obtained relationship between the line width of the toner image corresponding to the line image and the line width of the toner image formed according to the line image. In the figure, the horizontal axis represents the line width (μm) of the line image, and the vertical axis represents the line width (μm) of the actually formed toner image. As can be seen from the figure, in the line image, the line width is approximately 30 μm to 450 μm, and the line width of the toner image is 30 μm to 50 μm larger than intended. This occurs when the toner image is diffused when the toner image is pressed in the transfer of the toner image by the primary transfer device 515 or in the fixing process by the fixing device 54.

In order to prevent such a defect from occurring, the intensity modulation unit 74 reduces the exposure intensity to reduce the line width based on the correction amount set based on the tendency shown in FIG. Correct as follows. For example, as shown in FIG. 10, even if the intensity modulator 74 specifies exposure light with a line image having a line width of 150 μm, the actually formed toner image has a line width of approximately 180 μm. Therefore, in order to form a toner image with a line width of 150 μm, for example, as indicated by an arrow r, the intensity modulation unit 74 adjusts the exposure light so as to have an intensity corresponding to a line image with a line width of 120 μm. Correct to reduce the intensity. As a result, the line width of the actually formed toner image can be set to an intended size (150 μm).
That is, the intensity modulation unit 74 corrects the exposure light so that the intensity of the exposure light is reduced when the line width of the line image or character image is greater than or equal to the second threshold (however, greater than the first threshold). Correct with. However, when the line width is large to some extent, even if the toner image diffuses and the line width becomes large, it is hardly noticeable. Therefore, if the line width becomes larger than a certain value, the exposure intensity correction amount gradually increases as the line width increases. A correction amount for reducing the value may be set.

  In addition, when the intensity modulation unit 74 generates an intensity modulation signal, the correction considering the charge density due to the film thickness of the charge receptor of the photosensitive drum 511 is performed in the same manner as the correction performed by the pulse width modulation unit 73. Do. Since this correction is the same as the correction performed by the pulse width modulation unit 73 described above, description thereof is omitted here.

  On the other hand, the character image is also corrected according to the line width in the same manner as described above. However, in a character image, the line width of the character often depends on its size (font size), so the exposure control unit 70 may correct the exposure light with a correction amount corresponding to the font size. . For example, in a font size in which the line width of a character image is about 1 to 6 pixels, the intensity modulating unit 74 increases the intensity of exposure light as the font size decreases so that the line width of the toner image does not decrease. It is preferable to correct with a small correction amount, and to reduce the correction amount as the font size increases. In addition, for a font size that may cause image degradation that increases the line width of the character image and increases the line width of the toner image, the intensity modulation unit 74 may correct the exposure light with a correction amount that decreases the intensity of the exposure light.

(3) Operation Next, the operation of the image forming apparatus 1 will be described with reference to the flowchart shown in FIG.
In the external apparatus 100, when the execution of the image forming process is instructed by the user, the execution instruction is transmitted together with the PDL data from the external apparatus 100. Therefore, the control unit 10 of the image forming apparatus 1 is notified by the communication unit 30. Receive.

First, the control unit 10 supplies image data to the image processing unit 60 based on this execution instruction, and the image processing unit 60 interprets this to generate bitmap data (step S1). The image processing unit 60 assigns type information to each pixel of the generated bitmap data (step S2), and associates the coordinate value of each pixel with the type information assigned to the pixel in the RAM 12 of the control unit 10. Write. Then, the image processing unit 60 performs various image processing such as color conversion processing, refinement processing, and gradation correction processing on the bitmap data (step S3).
Subsequently, when the exposure control unit 70 acquires bitmap data from the image processing unit 60 and acquires type information from the RAM 12 of the control unit 10, the exposure control unit 70 determines the type of image based on the type information assigned to each pixel. (Step S4). When the selection control unit 71 determines that the type information of the bitmap data is “photographic image” or “graphic” (step S4; photographic image / graphic), the exposure control unit 70 performs electrostatic latent image according to the pulse width modulation method. In this case, the halftone processing unit 72 executes screen processing on the bitmap data. Subsequently, the exposure control unit 70 obtains the exposure light obtained from the bitmap data after the screen processing by the pulse width modulation unit 73 according to the type information acquired from the ROM 13 of the control unit 10 and stored in its own RAM. Correction is performed with the correction amount to generate a PWM signal (step S5).

On the other hand, when the selection unit 71 determines that the type information of the bitmap data is “line image” or “character image” (step S4; character image / line image), the exposure control unit 70 performs electrostatic according to the intensity modulation method. In order to form a latent image, the exposure light obtained from the bitmap data by the intensity modulation unit 74 is corrected by a correction amount according to the type information acquired from the ROM 13 of the control unit 10 and stored in its own RAM, and the intensity A modulation signal is generated (step S6).
Subsequently, the exposure control unit 70 supplies the PWM signal generated by the pulse width modulation unit 73 or the intensity modulation signal generated by the intensity modulation unit 74 to the laser driver 75, and based on this, the exposure apparatus 513. Drive. As a result, an electrostatic latent image is formed on the photosensitive drum 511, and the control unit 10 causes the developing unit 514 to develop the electrostatic latent image, and thereafter forms a toner image on the recording paper through a transfer and fixing process (Step S1). S7).

  According to the embodiment described above, the image forming apparatus 1 employs a pulse width modulation method for forming a toner image with an emphasis on gradation for photographic images and graphics, and has a resolution for line images and character images. An intensity modulation method is adopted to form an important toner image. Further, the image forming apparatus 1 corrects the exposure light with a correction amount according to the bitmap data and the type information, or with a correction amount according to the film thickness of the charge receptor provided on the photosensitive drum 511. As a result, the gradation and resolution of the image can be further improved, and good image reproducibility can be obtained.

(4) Modifications The above embodiment may be modified as follows. Specifically, the following modifications are mentioned, for example. These modifications can be appropriately combined with each other.
(4-1) Modification 1
In the embodiment described above, the pulse width modulation unit 73 and the intensity modulation unit 74 correct the pulse width and intensity of the exposure light according to the type of image and the film thickness of the charge receptor. However, based on other conditions. The exposure light may be corrected with the correction amount set as described above. This specific example will be described below.
(4-1-1) Types of Paper Various types of recording paper to which toner images are transferred are used. However, recording paper having a coating layer on the recording surface of a recording medium (hereinafter referred to as “coating”). In some cases, recording paper (hereinafter referred to as “non-coated paper”) without a coating layer is used. In a non-coated paper made of pulp fiber, when a toner image is formed, the toner enters the fiber layer and the line width tends to be reduced. On the other hand, in coated paper, toner is placed on the surface of the light-coated layer used as a coating material, such as a paint containing a dye such as calcium carbonate, a thermoplastic resin, a rubber latex, or a polymer material. The line width is difficult to decrease. Therefore, when using exposure light that is optimally set to form a toner image having an intended density or line width for non-coated paper that is inexpensive and frequently used, if coated paper is used, In some cases, the density may be too high, the screen structure may be crushed, or the line width of the line image may be increased. Therefore, when using coated paper, the pulse width modulation unit 73 and the intensity modulation unit 74 correct the exposure light with a correction amount that reduces the intensity of the exposure light compared to the case of non-coated paper. It is preferable.

  In order to realize this, the image forming apparatus according to this modification detects the type of paper, and corrects the intensity of exposure light and the pulse width in the pulse width modulation method with a correction amount corresponding thereto. As a method of detecting the paper type, each type of paper is stored in a paper feed tray classified for each paper type, and the control unit 10 feeds paper as instructed by the external device 100. There is a method of detecting the type of recording paper using a tray. In the ROM 13 of the control unit 10, the intensity of the exposure light is smaller in the coated paper having the coating layer provided on the recording surface than in the non-coated paper in which the coating layer is not provided. The set correction amount is stored. When the modulation signal is generated, the exposure control unit 70 is a coated paper having a coating layer provided on the recording surface or a non-coated paper having no coating layer. Is detected, and the exposure light intensity obtained from the bitmap data is corrected by a correction amount corresponding to the recording paper.

(4-1-2) Environment The exposure light may be corrected with a correction amount according to the environment in which the image forming apparatus 1 is installed. For example, the charging potential tends to increase as the temperature increases. Therefore, a temperature sensor is provided around the photosensitive drum 511, and the controller 10 increases the potential difference between the electrostatic latent image and the developing bias as the temperature measured by the temperature sensor increases from the reference temperature. In order to ensure, it is preferable to correct the exposure light with a correction amount set so as to increase the intensity of the exposure light. Further, since the potential of the electrostatic latent image tends to decrease as the humidity increases, the control unit 10 corrects the exposure light with a correction amount set so that the intensity of the exposure light decreases as the humidity increases. You may do it.

(4-2) Modification 2
In the embodiment described above, in the type information corresponding to each of the character image and the line image, an area where the colored pixels are not arranged (hereinafter, “white”) by an area where the colored pixels are arranged (hereinafter, “ground color portion”). A “white-out image” that is an image expressed by surrounding the “out-cut portion”) may be provided. FIG. 12 is a diagram showing a white image IMG representing the letter “A” as an example of a white image. The ground color portion IMG1 illustrated by hatching is an area where colored pixels are arranged, and the white area IMG2 which is the other area is an area where no colored pixels are arranged, and has the shape of the letter “A”. Yes.
In a blank image, the ground color portion IMG1 is often composed of pixels having a relatively high gradation value so that a human can easily recognize it. Therefore, when the toner image corresponding to the white image is formed, the toner image corresponding to the ground color portion IMG1 has a high density, and when the white image is formed on the recording paper, the region corresponding to the white portion IMG2. In some cases, the toner protrudes and adheres. The reason for this is the same as the reason why the line width is increased as described above. For example, if the film thickness is large, the charge spreads before moving to the surface of the photosensitive drum 511. Based. This causes a defect that the line width (image width) of the white portion is reduced or crushed.

  In order to solve this problem, the refinement processing unit 63 of the image processing unit 60 performs image processing for increasing the line width of the white portion for white images that satisfy a predetermined condition. Specifically, when the refinement processing unit 63 obtains the type information corresponding to the white image from the control unit 10, the refinement processing unit 63 performs expansion processing on the bitmap data representing the white portion and expands it. In this way, the occurrence of the above-described defects can be avoided by increasing the line width by inserting white pixels in the white portions. As a method for increasing the line width, a predetermined number of white pixels may be inserted into the white portions to increase the line width.

The number of pixels to be inserted in order to increase the line width of the white portion may be determined by the line width of the white portion, the size of the character image, the gradation value of the colored pixel in the ground color portion, or the like. For example, when the line width of the white portion is as large as several hundred pixels, even if the toner image protrudes into the region corresponding to the white portion IMG2, it can be said that there is almost no influence of the defect. Therefore, for example, the processing for increasing the line width is performed only when the line width is 1 to 4 pixels, and the number of pixels to be inserted is increased as the line width is decreased. In addition, in the character image, the larger the size of the notification, the larger the line width, so the influence of the defect becomes smaller. Therefore, when the character size is smaller than the predetermined character size, the smaller the size, the larger the number of pixels to be inserted. If the character size is larger than the predetermined value, the line width may not be increased. In addition, since the density of the formed toner image increases as the gradation value of the colored pixel increases, more toner tends to protrude into the white area. Therefore, the higher the gradation value of the colored pixel, the more the toner image is inserted. It is better to increase the number of pixels.
Based on such various conditions, the refinement processing unit 63 determines the number of pixels to be inserted, increases the image width of the outline image, and then outputs bitmap data. If the same effect can be obtained, the refinement processing unit 63 does not correct the bitmap data, but the exposure control unit 70 corrects the exposure light intensity so as to correspond to this correction. May be.

  Further, the refinement processing unit 63 of the image processing unit 60 may perform correction so as to thin out the colored pixels of the ground color portion from the boundary portion between the ground color portion and the whiteout portion. This is because if there is a high-density toner image adjacent to the white portion, this toner tends to protrude into the region corresponding to the white portion. As a specific method, a smoothing filter may be used. For example, the refinement processing unit 63 assigns a filter coefficient to each of “3 × 3 pixels” centered on the target pixel, for example, sets the filter coefficient of the target pixel that is the central pixel of this pixel group to “5”, and the like. Filter processing is performed using a filter having a pixel filter coefficient of “−0.5”. By this filter processing, the tone value of the colored pixel in the ground color portion at the boundary with the white portion is reduced by the negative filter coefficient, so that the density of the toner image corresponding to this boundary portion can be lowered. . Thereby, it can suppress that the line | wire width of a white part becomes small.

(4-3) Modification 3
In the embodiment described above, the image forming apparatus performs the image forming process based on the start instruction of the image forming process received by the communication unit 30. However, the image forming apparatus includes an image reading apparatus like a copier, Bitmap data generated by reading an original by the image reading apparatus may be used as bitmap data for image formation. In this case, the image forming unit may cause the user to specify the type of image via the UI unit. Further, the image forming apparatus may acquire bitmap data from a storage medium such as smart media or a USB (Universal Serial Bus) memory.

(4-4) Modification 4
Further, the image forming apparatus may not specify the type information for each pixel of the bitmap data, may assign one type information to the entire image represented by the bitmap data, or each of the partial images included in the image. Type information may be assigned to. When assigning to a partial image, the image forming apparatus may determine the type of the partial image specified by adopting a labeling process or the like, and assign type information.

(4-5) Modification 5
In the above-described embodiment, the type information is classified into one of “photo image”, “graphic”, “line image”, and “character image”, but other type information may be provided. In short, the image forming apparatus 1 employs screen processing in the first type of image where gradation is important, and follows a pulse width modulation method suitable for expressing the gradation value of bitmap data. An electrostatic latent image is formed. On the other hand, the image forming apparatus 1 forms an electrostatic latent image in accordance with an intensity modulation method suitable for expressing a gradation value by the intensity of exposure light in the second type of image in which resolution is important. In particular, as long as the image is a type of image defect that cannot be ignored, such as a line image or a character image, where the line width is deviated from the ideal or the line is interrupted, other than the line image and the character image. Even for a type of image, this problem can be solved by correcting the line width of the toner image by correcting the intensity of exposure light in the same manner as in the embodiment.
With the configuration of the modified example 5, it is possible to further improve the gradation and resolution and obtain a good reproducibility of an image even for an image other than the forward inclined type.

1 is a block diagram illustrating a configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a diagram illustrating in detail the structure of the image forming apparatus according to the embodiment. 2 is a block diagram showing a configuration of an image processing unit and an exposure control unit according to the embodiment. FIG. It is the graph which represented typically the relationship between the input density | concentration at the time of employ | adopting a pulse width modulation system and an intensity | strength modulation system. FIG. 4 is a diagram illustrating an image representing an edge portion of a line image or a character image and a state of an edge portion of a toner image formed when a pulse width modulation method is employed. It is a figure explaining the mode of the toner image formed by employ | adopting the image which bit map data represents, and a pulse width modulation system. 6 is a graph showing the relationship between the line width of a line image and the line width of a toner image. It is a figure explaining the relationship between the electric potential of an electrostatic latent image according to the line width of a line image, and the electric potential of a developing bias. FIG. 6 is a diagram schematically showing a relationship between a line width of a line image and a correction amount with respect to exposure light intensity in the image forming apparatus according to the embodiment. 6 is a graph showing the relationship between the line width of a line image and the line width of a toner image. 5 is a flowchart illustrating processing executed in the image forming apparatus according to the embodiment. It is a figure showing an example of the white image.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 10 ... Control part, 11 ... CPU, 12 ... RAM, 13 ... ROM, 100 ... External device, 20 ... Memory | storage part, 30 ... Communication part, 40 ... UI part, 50 ... Image forming part, 51 Image forming unit, 511, photosensitive drum, 512, charger, 513, exposure device, 514, developing device, 515, primary transfer device, 516, cleaning device, 517, intermediate transfer belt, 52, intermediate transfer belt, 53 ... secondary transfer device, 54 ... fixing device, 60 ... image processing unit, 61 ... raster data generation unit, 62 ... color conversion processing unit, 63 ... definition processing unit, 64 ... tone correction unit, 70 ... exposure control unit , 71 ... selection unit, 72 ... halftone processing unit, 73 ... pulse width modulation unit, 74 ... intensity modulation unit, 75 ... laser driver, 80 ... film thickness detection unit.

Claims (7)

Acquisition means for acquiring image data including a gradation value of each pixel and type information indicating a type of an image represented by the image data;
In accordance with a pulse width modulation method that expresses the gradation value of a pixel by the pulse width of light, the charged charge receptor is irradiated with light according to the image data acquired by the acquisition means, and the electrostatic charge is applied to the charge receptor. First exposure means for forming an image;
In accordance with an intensity modulation method that expresses the gradation value of a pixel by the intensity of light, the charge receptor is irradiated with light according to the image data acquired by the acquisition unit, and an electrostatic latent image is formed on the charge receptor. A second exposure means;
A selection unit that selects either the first or second exposure unit as an exposure unit that forms the electrostatic latent image based on the type information acquired by the acquisition unit;
When the second exposure unit is selected by the selection unit, a correction unit that corrects the light intensity corresponding to the image data to be irradiated to the second exposure unit by a correction amount corresponding to the type information It equipped with a door,
In addition to the correction according to the type information, the correction means corrects with a correction amount that increases the light intensity when the line width of the image represented by the image data is smaller than a first threshold. Then, when the line width is equal to or larger than the second threshold value that is larger than the first threshold value , the exposure control apparatus corrects with a correction amount that reduces the light intensity . Before SL type information includes a type of the line image or a character image,
The exposure control apparatus according to claim 1, wherein the selection unit selects the second exposure unit if the type information acquired by the acquisition unit represents the line image or the character image. . Before Symbol selection means, as long as the type information acquired by the acquiring unit represents a type other than the line image and the character image, and selects said first exposure means,
The exposure control apparatus according to claim 2 , wherein a maximum intensity of light irradiated by the second exposure unit is larger than a maximum intensity of light irradiated by the first exposure unit. Comprising a thickness detecting means for detecting a film thickness before Symbol charge receptor,
In addition to the correction according to the type information, the correction unit corrects the intensity of light emitted by the selected exposure unit as the film thickness detected by the film thickness detection unit decreases. The exposure control apparatus according to claim 1, wherein the exposure control apparatus corrects by an amount. Detecting whether a recording medium on which the developed image prior Symbol electrostatic latent image is transferred, whether the recording surface as a recording medium coating layer is provided, or a recording medium the coating layer was not provided Detecting means for
In addition to the correction according to the type information, the correction means is irradiated by the selected exposure means on the recording medium provided with the coating layer according to the result of detection by the detection means. The light intensity is corrected with a correction amount so as to be smaller than the light intensity when the recording medium not provided with the coating layer is irradiated. Exposure control device. Image represented by pre Symbol image data, the region of arranging the colored pixels, in the case of white unplug image is an image expressed by enclosing a region that does not place a colored pixel, the line width of the image, the size and the In accordance with at least any one of the gradation values of the colored pixels, the image width changing means for increasing the image width of the area where the colored pixels are not arranged,
The exposure control apparatus according to claim 1, wherein the acquisition unit acquires the image data processed by the image width changing unit. An exposure control device according to any one of 請 Motomeko 1 to 6,
The charge acceptor;
Charging means for charging the surface of the charge receptor;
Developing means for developing the electrostatic latent image formed by the exposure control device on the surface of the charge receptor charged by the charging means with a developer;
An image forming apparatus comprising: a transfer unit that transfers an image developed by the developing unit to a recording medium.

Images