JP2007062098A - Image formation method and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem of generation of an image trouble such that a density difference is brought about between both ends in a main scanning direction when gradation reproduction is carried out using a threshold matrix of a dot concentration type in an image forming apparatus which forms a latent image by performing reciprocative scanning in the main scanning direction by a deflection mirror face while irradiating a latent image carrier surface driven in the sub-scanning direction by light beams in the shape of a spot. <P>SOLUTION: An image is formed by using N×1 cells. The cell has N (N≥2) pixels in the main scanning direction, and also has one pixel in the sub-scanning direction. For example, when 5×1 cells CL51 are used as shown in Fig. 11, no difference is generated in a range (spot latent image) where the 5×1 cells CL51 respectively arranged at a wide pitch side and a narrow pitch side are exposed by the spot. The generation of the image trouble that the density difference is brought about between both ends in the main scanning direction X although the image of an equal density is to be formed can be suppressed, thus enabling good gradation reproduction. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus for forming an image by scanning a surface of a latent image carrier driven in the sub-scanning direction in a main scanning direction substantially perpendicular to the sub-scanning direction, and an image forming method in the apparatus. It is about.

  This type of image forming apparatus includes a latent image carrier, an exposure unit, and a developing unit, and forms a toner image on the latent image carrier as follows. In other words, the light source of the exposure unit is controlled based on the image data indicating the toner image, and the light beam from the light source is deflected in the main scanning direction by the deflector of the exposure unit while forming a light beam on the latent image carrier in a spot shape. By scanning, a latent image corresponding to the image data is formed on the surface of the latent image carrier. The latent image is developed with toner to form a toner image.

  Further, in order to reduce the size and speed of the deflector, it has been conventionally proposed to use the deflecting mirror surface as a deflector by vibrating the surface (see Patent Document 1). That is, in this apparatus, the deflection mirror supported by the torsion bar is vibrated, and the light beam emitted from the light source is reflected by the deflection mirror and reciprocally scanned on the surface of the line image carrier.

JP 2002-182147 A (the third page and FIGS. 9 and 10)

  In such an image forming apparatus, the light beam from the light source can be scanned on the latent image carrier in both the forward and backward directions in the main scanning direction. However, in such an apparatus that uses a deflection mirror to scan spots in both the forward path and the backward path, there are the following problems in reproducing gradation using a dot concentration type threshold matrix. That is, in gradation reproduction using a dot concentration type threshold matrix, a single cell is formed using a plurality of pixels, and a pixel corresponding to a gradation level among a plurality of pixels of the cell is formed according to the threshold matrix. Only by performing exposure and development to form dots, gradation is realized by changing the size of a halftone dot constituted by such dots in accordance with the gradation level. That is, when the gradation level is low, a small number of pixels are exposed and developed to form small halftone dots, and when the gradation level is high, a large number of pixels are exposed and developed to form large halftone dots. Therefore, the gradation level to be realized is determined by the size of the halftone dots.

  However, in the image forming apparatus as described above, since the light beam is scanned in both directions in the main scanning direction while forming a light beam on the surface of the latent image carrier, the scanning pitch in the sub scanning direction is constant. In other words, the degree of spot overlap in the sub-scanning direction varies. In other words, the spot overlap in the sub-scanning direction increases at a narrow scanning pitch in the sub-scanning direction, whereas the spot overlap in the sub-scanning direction decreases at a wide scanning pitch in the sub-scanning direction. Therefore, even when forming halftone dots corresponding to the same gradation level, the gradation level is lowered because the halftone dots formed are small where the scanning pitch is narrow, whereas the halftone dots are formed where the scanning pitch is wide. Since the halftone dot is large, the gradation level becomes high. As a result, as will be described in detail later, the halftone dots formed in the cells arranged at both ends in the main scanning direction are in between the two ends even though halftones of the same gradation level are formed. As a result, there is a case in which an image detrimental effect that a density difference occurs between both ends may occur.

  SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems. A latent image is obtained by reciprocating scanning in the main scanning direction by a deflecting mirror surface while irradiating the surface of a latent image carrier driven in the sub-scanning direction in a spot shape. In the image forming apparatus for forming the image, even if the tone reproduction is performed using a dot concentration type threshold matrix, the above-described image adverse effect is suppressed and a technique for realizing a good tone reproduction is provided. It is said.

  The image forming method according to the present invention includes a latent image carrier whose surface is driven in the sub-scanning direction, and a first beam in the main scanning direction that is substantially perpendicular to the sub-scanning direction with the light beam from the light source by the oscillating deflection mirror surface. A plurality of spot latent images are formed by irradiating the surface of the latent image carrier in a spot shape with a light beam scanned in the first direction. In addition, a latent image forming unit that forms a plurality of spot latent images by irradiating the surface of the latent image carrier with a light beam that scans in the second direction, and develops each of the plurality of spot latent images to form dots. In an image forming apparatus including a developing unit to be formed, an image is formed by expressing a gradation by forming a halftone dot by dots for a cell having a plurality of pixels using a dot concentration type threshold matrix. With image forming method In order to achieve the above object, an image is formed using N × 1 cells having N pixels (N ≧ 2) in the main scanning direction and one pixel in the sub scanning direction as cells. It is characterized by doing.

  The image forming apparatus according to the present invention has a latent image carrier whose surface is driven in the sub-scanning direction and a light beam from the light source in the main scanning direction substantially orthogonal to the sub-scanning direction by the oscillating deflection mirror surface. It is configured to be capable of scanning in both the first direction and the second direction opposite to the first direction, and a plurality of spot latent images are formed by irradiating the surface of the latent image carrier in a spot shape with a light beam scanned in the first direction. A latent image forming section that forms a plurality of spot latent images by irradiating the surface of the latent image carrier with a light beam that is formed and scanned in the second direction, and developing each of the plurality of spot latent images; An image forming apparatus including a developing unit that forms dots and using a dot-concentrated threshold matrix to express gradation by forming halftone dots with dots for cells having a plurality of pixels And the above purpose In order to achieve this, an image is formed using N × 1 cells having N pixels (N ≧ 2) in the main scanning direction and one pixel in the sub-scanning direction as cells. .

  In the invention thus configured (image forming method and image forming apparatus), the first direction in the main scanning direction and the second direction opposite to the first direction while irradiating the light beam onto the latent image carrier in a spot shape. It is configured to be able to scan in both directions. For example, when forming a line latent image on the latent image carrier as shown in FIG. 6, the light beam is scanned in the first direction of the main scanning direction while being irradiated in a spot shape on the latent image carrier. A plurality of spot latent images are arranged along the main scanning direction to form a line latent image LI (+ X), while a light beam is scanned in a second direction opposite to the first direction to form a plurality of spot latent images. Line latent images LI (-X) are formed side by side along the main scanning direction. As a result, the line latent images LI (+ X) and LI (−X) are alternately formed in the sub-scanning direction.

  In the invention configured as described above, the light beam is reciprocated in the main scanning direction on the surface of the latent image carrier, and the surface of the latent image carrier is driven in the sub-scanning direction substantially orthogonal to the main scanning direction. is doing. Therefore, the scanning trajectory of the light beam on the latent image carrier (spot latent image formation trajectory) is, for example, shown by a one-dot chain line in FIG. 7, and the scanning pitch in the sub-scanning direction is not constant. Such non-uniformity of the scanning pitch in the sub-scanning direction is particularly noticeable near the end of the scanning locus in the main scanning direction. Therefore, when gradation reproduction is performed using a dot concentration type threshold matrix as in the image forming method or the image forming apparatus according to the present invention, the halftone density is high at one end in the main scanning direction and the halftone density is at the other end. There is a case where an image detrimental effect such as a decrease in density occurs. The reason for this will be described in detail below.

  FIG. 8 shows a 5 × 2 cell (5 × 2 cell) of 5 pixels in the main scanning direction and 2 pixels in the sub scanning direction and a threshold matrix (5 × 2) having 1 to 10 threshold values corresponding to the 5 × 2 cells. FIG. 9 is a diagram for explaining the reason why the above-described image detriment occurs. In this specification, a cell having N pixels in the main scanning direction and M pixels in the sub-scanning direction is referred to as an “N × M cell”, and a threshold matrix corresponding to the N × M cell is referred to as an “N × M threshold matrix. ". Here, the alternate long and short dash line in FIG. 9 indicates the scanning trajectory of the light beam on the latent image carrier (the formation trajectory of the spot latent image), the solid ellipse indicates the area exposed by the spot, and the solid rectangle indicates 5 × 2. Represents a cell. First, the operation of the 5 × 2 threshold matrix will be described with reference to FIG. 8 before explaining the reason why the image adverse effect occurs using FIG. 9.

  FIG. 8A is a diagram showing a 5 × 2 threshold matrix, and FIG. 8B is a diagram showing growth of halftone dots in a 5 × 2 cell when the 5 × 2 threshold matrix is used. The 5 × 2 threshold value matrix shown in FIG. 8A is a dot concentration type threshold value matrix, and the size of the halftone dot is grown as the gradation level increases. Specifically, as the gradation level increases from 0% to 100%, as shown in FIG. 8B, among the 10 pixels of the 5 × 2 cell CL52, A halftone dot grows by forming dots by exposure and development according to each gradation level. Here, the gradation level 0% corresponds to a state in which no dots are formed in the pixels, and the gradation level 100% corresponds to a state in which dots are formed in all the pixels. As the gradation level increases from 0% to 100%, the number of pixels forming dots is increased proportionally. For example, when the gradation level is 40%, dots are formed in four pixels corresponding to a threshold value of 4 or less in the 5 × 2 threshold matrix, and when the gradation level is 80%, the threshold value of the 5 × 2 threshold matrix. A dot is formed in 8 pixels corresponding to 8 or less. Then, the above-described image adverse effect when gradation reproduction is performed using such a 5 × 2 threshold matrix will be described with reference to FIGS.

  As described above, the scanning trajectory of the light beam (spot latent image formation trajectory) is indicated by, for example, a one-dot chain line in FIG. 7, and the scanning pitch in the sub-scanning direction is not constant. For example, as it goes from the left end to the right end in FIG. 7, the scanning pitch in the sub-scanning direction changes from “wide pitch” which is wide to “narrow pitch” where the scanning pitch is narrow, and vice versa. It changes to “wide pitch”. When a gradation such as a 5 × 2 cell having a plurality of pixels in the sub-scanning direction is arranged with respect to such a scanning line and a gradation is expressed by a dot concentration type threshold matrix, FIG. As shown in the figure, an adverse effect of the image will occur.

  FIG. 9 shows a case where a halftone having a gradation level of 20% is formed. In this case, a spot latent image is formed only on two pixels corresponding to the threshold values 1 and 2 among the ten pixels constituting the 5 × 2 cell CL52, and a light beam corresponding to the spot latent image is formed. Are relatively separated on the wide pitch side (left hand side in FIG. 9), but relatively close on the narrow pitch side (right hand side in FIG. 9). Therefore, as shown in the “latent image formation” column of FIG. 9, the pitch of the spot latent image formed in these pixels in the sub-scanning direction Y is wide on the wide pitch side, whereas on the narrow pitch side. It is narrower. The two dots obtained by developing the latent image thus formed with toner are relatively separated on the wide pitch side as shown in the “halftone dot” column of FIG. As a result, the halftone dots formed as a collection of these dots are relatively large. On the other hand, two dots are relatively close on the narrow pitch side. As a result, the halftone dots formed as a collection of these dots are relatively small. Due to the difference in the pitch of the scanning locus in this way, “halftone dot” and “small dot” are formed even though halftone dots corresponding to the same gradation level are formed. That is, although the halftone dots having the same gradation are being formed for the cells arranged between both ends in the main scanning direction X, the size of the halftone dots actually formed in each cell is different. As a result, there arises an image detrimental effect that a density difference occurs between both ends in the main scanning direction X in spite of trying to form an image having the same density.

  In order to deal with such image defects, in the present invention, an N × 1 cell having N pixels (N ≧ 2) in the main scanning direction and one pixel in the sub scanning direction is used as a cell. Form. Therefore, the size of halftone dots formed in each cell arranged between both ends in the main scanning direction X does not differ. Therefore, it is possible to suppress the occurrence of an image defect that a density difference occurs between both ends in the main scanning direction X even though an image having the same density is being formed, and to express gradation well. Become. The reason for this will be described using the examples shown in FIGS.

  FIG. 10A is a diagram showing a 5 × 1 threshold matrix of one pixel in the sub-scanning direction, and FIG. 10B is a halftone dot in the 5 × 1 cell CL51 when the 5 × 1 threshold matrix is used. It is a figure which shows the growth of. The 5 × 1 threshold value matrix shown in FIG. 10A is a dot concentration type threshold value matrix, and the size of the halftone dot is increased as the gradation level increases. Specifically, as the gradation level increases from 0% to 100%, as shown in FIG. 10B, among the five pixels of the 5 × 1 cell CL51, A halftone dot grows by forming dots by exposure and development according to each gradation level. Here, the gradation level 0% corresponds to a state in which no dots are formed in the pixels, and the gradation level 100% corresponds to a state in which dots are formed in all the pixels. As the gradation level increases from 0% to 100%, the number of pixels forming dots is increased proportionally. For example, when the gradation level is 40%, dots are formed in two pixels corresponding to a threshold value of 2 or less in the 5 × 1 threshold matrix MTX51, and when the gradation level is 80%, the 5 × 1 threshold matrix MTX51. A dot is formed on four pixels corresponding to a threshold value of 4 or less.

  FIG. 11 is a diagram showing the effect of the present invention. Here, in FIG. 11, the alternate long and short dash line represents a scanning line, the solid line ellipse with a white interior represents a spot latent image, and the solid line ellipse with a slanted interior represents a dot. A solid rectangle indicates a 5 × 1 cell CL51. In the example of FIG. 11, among two 5 × 1 cells CL51 arranged adjacent to each other in the sub-scanning direction Y, the 5 × 1 cell CL51 on the downstream side in the sub-scanning direction Y in FIG. A halftone is formed. In this case, a spot latent image is formed only on one pixel corresponding to the threshold value 1 among the five pixels constituting the 5 × 1 cell CL51. As shown in FIG. 11, since the 5 × 1 cell CL51 has only one pixel in the sub-scanning direction Y, the sub-scan is performed in the cell as in the case where a halftone of 20% is formed in the 5 × 2 cell CL52 described above. Two spot latent images are not formed side by side in the direction Y. Therefore, the distance in the sub-scanning direction Y between the spot latent images formed in each of the pixels arranged adjacent to each other in the sub-scanning direction Y as shown in the “latent image formation” column of FIG. The phenomenon that it is different between cells arranged at both ends of X cannot occur. In other words, as shown in the “latent image formation” column in FIG. 11, the spot latent image for the pixel is corresponding to the position where the scanning line passes for each pixel on the wide pitch side and the narrow pitch side. Even if the position at which the image is formed is shifted in the sub-scanning direction Y, the 5 × 1 cell arranged on each of the wide pitch side and the narrow pitch side is exposed to the spot (spot latent image). There is no difference. Therefore, there is no difference in the size of halftone dots obtained by forming dots in such an exposure area. Therefore, it is possible to suppress the occurrence of an image defect that a density difference occurs between both ends in the main scanning direction X even though an image having the same density is being formed, and good gradation reproduction is possible.

  As described above, it has been found that by using one pixel N × 1 cell in the sub-scanning direction Y, the density difference between both ends of the main scanning direction X can be eliminated and good gradation reproduction is possible. The N × 1 cell has the following effects in addition to the above effects. That is, the N × 1 cell is particularly suitable when the form of the image to be formed is a character or the like and a high spatial resolution is required. The reason for this will be described with reference to FIG. 12 by comparing an N × 1 cell with 1 in the sub-scanning direction and an N × M cell having two or more M pixels in the sub-scanning direction. FIG. 12 shows a case where N × 1 cells (left of FIG. 12) and N × M cells (right of FIG. 12) are arranged in a region DM having a predetermined area. As described above, the N × 1 cell has only one pixel in the sub-scanning direction Y, whereas the N × M cell has M pixels in the sub-scanning direction Y. Therefore, the number of cells that can be arranged in the region DM is N The × 1 cell is M times the N × M cell. As described above, when tone reproduction is performed using a dot concentration type threshold matrix, one halftone dot is formed by one cell having a plurality of pixels. Therefore, in an N × 1 cell in which more cells can be arranged in a region DM having a predetermined area, an image can be formed in the region DM using M times as many halftone dots as compared to an N × M cell. . That is, the N × 1 cell can realize a spatial resolution M times that of the N × M cell and can form a clear image with a clear outline. In this way, when image formation is performed using N × 1 cells, it is possible to eliminate the density difference at both ends in the main scanning direction X and realize good gradation reproduction, and at the same time, the outline is clear with high spatial resolution. A beautiful image can be formed.

  On the other hand, there are cases where it is required to reproduce gradations at many gradation levels, as in the case of photographs and the like. That is, there are cases where the realization of finer gradation reproduction (high gradation resolution) is the most important by expressing multi-level gradation levels using more pixels. In such a case, it is required to realize finer gradation reproduction (high gradation resolution) using an N × M cell having a larger number of pixels. As described above, the required contents differ depending on the form of the image to be formed. Therefore, in order to perform optimal image formation according to such a difference in image mode, the image forming method and the image forming apparatus according to the present invention may be configured as follows.

  The image forming method according to the present invention includes a latent image carrier whose surface is driven in the sub-scanning direction, and a first beam in the main scanning direction that is substantially perpendicular to the sub-scanning direction with the light beam from the light source by the oscillating deflection mirror surface. A plurality of spot latent images are formed by irradiating the surface of the latent image carrier in a spot shape with a light beam scanned in the first direction. In addition, a latent image forming unit that forms a plurality of spot latent images by irradiating the surface of the latent image carrier with a light beam that scans in the second direction, and develops each of the plurality of spot latent images to form dots. In an image forming apparatus including a developing unit to form, an image is formed by expressing a gradation by forming a halftone dot with the dot for a cell having a plurality of pixels using a dot concentration type threshold matrix. Image formation In order to achieve the above object, as a cell, an N × 1 cell having N pixels (N ≧ 2) in the main scanning direction and one pixel in the sub-scanning direction, and the main scanning direction N × M cells having N (N ≧ 2) pixels and M (M ≧ 2) pixels in the sub-scanning direction are prepared in advance according to the form of the image to be formed. One of the N × 1 cells and the N × M cells is selectively used to form the image.

  The image forming apparatus according to the present invention has a latent image carrier whose surface is driven in the sub-scanning direction and a light beam from the light source in the main scanning direction substantially orthogonal to the sub-scanning direction by the oscillating deflection mirror surface. It is configured to be capable of scanning in both the first direction and the second direction opposite to the first direction, and a plurality of spot latent images are formed by irradiating the surface of the latent image carrier in a spot shape with a light beam scanned in the first direction. A latent image forming section that forms a plurality of spot latent images by irradiating the surface of the latent image carrier with a light beam that is formed and scanned in the second direction, and developing each of the plurality of spot latent images; An image forming apparatus including a developing unit that forms dots and using a dot-concentrated threshold matrix to express gradation by forming halftone dots with dots for cells having a plurality of pixels And the above purpose To achieve this, N × 1 cells having N (N ≧ 2) pixels in the main scanning direction and one pixel in the sub-scanning direction, and N (N ≧ 2) in the main scanning direction, are used as cells. ) Pixels and N × M cells having M (M ≧ 2) pixels in the sub-scanning direction, and N × 1 cells and N × M cells depending on the form of the image to be formed One of them is selectively used to form the image.

  In the invention (image forming method and image forming apparatus) configured as described above, an N × 1 cell having N (N ≧ 2) pixels in the main scanning direction and one pixel in the sub-scanning direction; N × M cells having N (N ≧ 2) pixels in the main scanning direction and M (M ≧ 2) pixels in the sub-scanning direction are prepared in advance, and the form of the image to be formed Accordingly, one of the N × 1 cell and the N × M cell is selectively used to form the image. That is, N × 1 cells and N × M cells having different sizes and different numbers of pixels are prepared, and more appropriate cells are selectively used according to the form of the image. Accordingly, it is possible to appropriately respond to different requests depending on the image mode as described above, and to perform optimal image formation according to each image mode.

<First Embodiment>
A. Apparatus Configuration FIG. 1 is a diagram showing a first embodiment of an image forming apparatus according to the present invention. FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus of FIG. This image forming apparatus is a so-called tandem type color printer, and yellow (Y), magenta (M), cyan (C), and black (K) four-color photoconductors 2Y, 2M, and 2C as latent image carriers. 2K is provided in the apparatus main body 5. The apparatus forms a full-color image by superimposing the toner images on the photoreceptors 2Y, 2M, 2C, and 2K, or forms a monochrome image using only the black (K) toner image. That is, in this image forming apparatus, when a print command is given to the main controller 11 from an external device such as a host computer in response to an image formation request from the user, an engine controller is responded to the print command from the CPU 111 of the main controller 11. 10 controls each part of the engine unit EG to print an image corresponding to the print command on a sheet S such as copy paper, transfer paper, paper, and an OHP transparent sheet.

  The engine unit EG includes a charging unit, a developing unit, an exposure unit (latent image forming unit), and a cleaning unit corresponding to each of the four photoconductors 2Y, 2M, 2C, and 2K. As described above, for each toner color, there is provided a photoconductor (latent image carrier), a charging unit, a developing unit (developing unit), an exposure unit, and a cleaning unit, and an image forming unit that forms a toner image of the toner color. Is provided. The configuration of these image forming means (photosensitive member, charging unit, developing unit, exposure unit, and cleaning unit) is the same for all color components. Therefore, the configuration relating to yellow will be described here, and the other color components will be described. Are denoted by corresponding reference numerals, and description thereof is omitted.

  The photoreceptor 2Y is rotatably provided in the arrow direction (sub-scanning direction) in FIG. More specifically, the drive motor MT is mechanically connected to one end of the photoreceptor 2Y. The motor control unit 105 electrically connected to the drive motor MT controls the drive motor MT. As a result, the photoreceptor 2Y rotates.

  A charging unit 3Y, a developing unit 4Y, and a cleaning unit (not shown) are arranged around the photosensitive member 2Y driven in this way along the rotation direction. The charging unit 3Y is composed of, for example, a scorotron charger, and uniformly charges the outer peripheral surface of the photoreceptor 2Y to a predetermined surface potential by applying a charging bias from the charging control unit 103. The photosensitive member 2Y charged by the charging unit 3Y is irradiated with the scanning light beam Ly from the exposure unit 6Y (latent image forming unit) toward the outer peripheral surface. As a result, an electrostatic latent image corresponding to the yellow image data included in the print command is formed on the photoreceptor 2Y. Thus, the exposure unit 6Y operates in response to a control command from the exposure control unit 102Y. The configuration and operation of the exposure unit 6 (6Y, 6M, 6C, 6K) and the exposure control unit 102 (102Y, 102M, 102C, 102K) will be described in detail later.

  The electrostatic latent image formed in this way is developed with toner by the developing unit 4Y (developing unit). The developing unit 4Y contains yellow toner. When a developing bias is applied from the developing device controller 104 to the developing roller 41Y, the toner carried on the developing roller 41Y partially adheres to each surface portion of the photoreceptor 2Y according to the surface potential. As a result, the electrostatic latent image on the photoconductor 2Y becomes visible as a yellow toner image. As the developing bias applied to the developing roller 41Y, a DC voltage or a voltage obtained by superimposing an AC voltage on the DC voltage can be used. In particular, the photosensitive member 2Y and the developing roller 41Y are spaced apart from each other. In a non-contact development type image forming apparatus that develops toner by flying toner with a voltage waveform in which an alternating voltage such as a sine wave, a triangular wave, or a rectangular wave is superimposed on a direct current voltage in order to efficiently fly the toner Is desirable.

  The yellow toner image developed by the developing unit 4Y is primarily transferred onto the intermediate transfer belt 71 of the transfer unit 7 in the primary transfer region TRy1. The color components other than yellow are configured in the same manner as yellow, and magenta toner images, cyan toner images, and black toner images are formed on the photoreceptors 2M, 2C, and 2K, respectively, and primary transfer is performed. Primary transfer is performed on the intermediate transfer belt 71 in the regions TRm1, TRc1, and TRk1, respectively.

  The transfer unit 7 includes an intermediate transfer belt 71 stretched between two rollers 72 and 73, and a belt driving unit (not shown) that rotates the intermediate transfer belt 71 in a predetermined rotation direction R2 by rotating the roller 72. ). In addition, a secondary transfer roller 74 is provided at a position facing the roller 73 with the intermediate transfer belt 71 interposed therebetween. The secondary transfer roller 74 is configured to be brought into contact with and separated from the surface of the belt 71 by an electromagnetic clutch (not shown). Yes. When a color image is transferred to the sheet S, the primary transfer timing is controlled to superimpose the toner images to form a color image on the intermediate transfer belt 71, and the color image is taken out from the cassette 8 to be intermediate. The color image is secondarily transferred onto the sheet S conveyed to the secondary transfer region TR2 between the transfer belt 71 and the secondary transfer roller 74. On the other hand, when a monochrome image is transferred to the sheet S, only the black toner image is formed on the photoreceptor 2K, and the monochrome image is secondarily transferred onto the sheet S conveyed to the secondary transfer region TR2. In addition, the sheet S that has received the secondary transfer of the image in this way is conveyed toward the discharge tray portion provided on the upper surface portion of the apparatus main body via the fixing unit 9.

  The surface potential of each of the photoreceptors 2Y, 2M, 2C, and 2K after primary transfer of the toner image to the intermediate transfer belt 71 is reset by a neutralizing unit (not shown), and the toner remaining on the surface is further removed. Is removed by the cleaning unit, and then charged by the charging units 3Y, 3M, 3C, and 3K.

  In the vicinity of the roller 72, a transfer belt cleaner 75, a density sensor 76 (FIG. 2), and a vertical synchronization sensor 77 (FIG. 2) are arranged. Among these, the cleaner 75 can be moved toward and away from the roller 72 by an electromagnetic clutch (not shown). Then, the blade of the cleaner 75 is in contact with the surface of the intermediate transfer belt 71 that is stretched over the roller 72 while moving to the roller 72 side, and the toner that remains on the outer peripheral surface of the intermediate transfer belt 71 after the secondary transfer. Remove. The density sensor 76 is provided to face the surface of the intermediate transfer belt 71 and measures the optical density of the patch image formed on the outer peripheral surface of the intermediate transfer belt 71. Further, the vertical synchronization sensor 77 is a sensor for detecting the reference position of the intermediate transfer belt 71, and is a synchronization signal output in association with the rotational drive of the intermediate transfer belt 71 in the sub-scanning direction, that is, a vertical synchronization signal. It functions as a vertical sync sensor for obtaining Vsync. In this apparatus, the operation of each part of the apparatus is controlled based on the vertical synchronization signal Vsync in order to align the operation timing of each part and to superimpose toner images of each color accurately. Further, a color misregistration sensor 78 is disposed between the rollers 72 and 73, and detects the color misregistration amount of each color toner image.

  In FIG. 2, reference numeral 113 denotes an image memory provided in the main controller 11 for storing image data given from an external device such as a host computer via the interface 112, and reference numeral 106 is executed by the CPU 101. A ROM for storing calculation data, control data for controlling the engine unit EG, and the like, and a reference numeral 107 are RAMs for temporarily storing calculation results in the CPU 101 and other data. Reference numeral 108 denotes an FRAM (ferroelectric memory) for storing information on the usage status of each part of the engine.

  FIG. 3 is a main scanning sectional view showing a configuration of an exposure unit (latent image forming unit) provided in the image forming apparatus of FIG. The exposure unit 6Y (6M, 6C, 6K) has an exposure housing 61. A single laser light source 62Y is fixed to the exposure housing 61, and a light beam can be emitted from the laser light source 62Y.

  FIG. 4 is a diagram showing signal processing blocks in the image forming apparatus of FIG. The configurations and operations of the exposure unit 6 and the exposure control unit 102 will be described in detail below with reference to FIGS. The configuration of the exposure unit 6 and the exposure control unit 102 is the same for all color components, so the configuration relating to yellow will be described here, and the other color components will be denoted by corresponding reference numerals and description thereof will be omitted.

  The exposure unit 6Y (6M, 6C, 6K) has an exposure housing 61. A single laser light source 62Y is fixed to the exposure housing 61, and a light beam can be emitted from the laser light source 62Y. Then, the laser light source 62Y is ON / OFF controlled in accordance with the image signal as described below, and a light beam modulated in accordance with the image data is emitted from the laser light source 62Y. Hereinafter, a description will be given with reference to FIG.

  In this image forming apparatus, when an image signal is input from an external device such as the host computer 100, the main controller 11 performs predetermined signal processing on the image signal. The main controller 11 includes functional blocks such as a color conversion unit 114, an image processing unit 115, two types of line buffers 116A and 116B, a scanning mode switching unit 116C, a pulse modulation unit 117, a gradation correction table 118, and a correction table calculation unit 119. It has.

  In addition to the CPU 101, the ROM 106, the RAM 107, and the exposure control unit 102 shown in FIG. 2, the engine controller 10 detects a gradation characteristic that detects a gradation characteristic indicating the gamma characteristic of the engine unit EG based on the detection result of the density sensor 76. Part 123 is provided. In the main controller 11 and the engine controller 10, these functional blocks may be configured by hardware, or may be realized by software executed by the CPUs 111 and 101.

  In the main controller 11 to which the image signal is given from the host computer 100, the color conversion unit 114 converts the RGB gradation data indicating the gradation level of the RGB component of each pixel in the image corresponding to the image signal into the corresponding CMYK. Conversion into CMYK gradation data indicating the gradation level of the component. The CMYK gradation data output from the color conversion unit 114 is input to the image processing unit 115.

  The image processing unit 115 executes the following processing for each color component. FIG. 5 is a diagram showing the configuration of the image processing unit 115. The image processing unit 115 includes a gradation correction unit 1151 and a halftone processing unit 1152. The gradation correction unit 1151 performs gradation correction on the gradation data of each pixel input from the color conversion unit 114 to generate correction gradation data, and then the halftone processing unit 1152 performs the correction gradation data. Is subjected to halftoning processing to generate halftone gradation data. That is, the gradation correction of the gradation correction unit 1151 is performed by referring to the gradation correction table 118 registered in advance in the nonvolatile memory and inputting each pixel from the color conversion unit 114 according to the gradation correction table 118. The gradation data is converted into corrected gradation data indicating the corrected gradation level. The purpose of the gradation correction is to compensate for the change in the gamma characteristic of the engine unit EG configured as described above, and to keep the overall gamma characteristic of the image forming apparatus always ideal. In other words, in this type of image forming apparatus, the gamma characteristic of the apparatus varies from apparatus to apparatus, and even in the same apparatus, depending on the usage status. Therefore, in order to eliminate the influence of such gamma characteristic variations on the image quality, a gradation control process is executed to update the contents of the gradation correction table 118 based on the actual measurement result of the image density at a predetermined timing.

  In this gradation control process, for each toner color, a gradation patch gradation image prepared in advance for measuring the gamma characteristic is formed on the intermediate transfer belt 71 by the engine unit EG, and each gradation patch is obtained. The image density of the image is read by the density sensor 76, and based on a signal from the density sensor 76, the gradation characteristic detection unit 123 associates the gradation level of each gradation patch image with the detected image density ( The gamma characteristics of the engine unit EG are created and output to the correction table calculation unit 119 of the main controller 11. Then, the correction table calculation unit 119 compensates the actually measured gradation characteristic of the engine unit EG based on the gradation characteristic given from the gradation characteristic detection unit 123 to obtain an ideal gradation characteristic. The tone correction table data is calculated, and the content of the tone correction table 118 is updated to the calculation result. Thus, the gradation correction table 118 is changed and set. By doing so, this image forming apparatus can form an image with stable quality regardless of variations in gamma characteristics of the apparatus and changes over time.

  With respect to the corrected gradation data corrected as described above, the halftone processing unit 1152 generates halftone gradation data using a so-called dot concentration type threshold matrix MTX. That is, by forming one cell using a plurality of pixels and forming dots by performing exposure and development only on pixels corresponding to the gradation level among the plurality of pixels of the cell according to the threshold matrix MTX, The gradation is realized by changing the size of the halftone dots formed by such dots in accordance with the gradation level. Specific halftone processing will be described later in the section “B. Halftone processing”. The halftone gradation data generated by the halftone processing unit 1152 is input to two types of line buffers 116A and 116B.

  These line buffers 116A and 116B are common in that they store halftone gradation data constituting one line image data output from the image processing unit 15, but the readout order of gradation data is different. That is, the forward line buffer 116A outputs halftone gradation data constituting one line image data in the forward direction from the head, whereas the reverse line buffer 116B outputs in the reverse direction from the end. is there.

  The halftone gradation data thus output is input to the scanning mode switching unit 116C, and only the halftone gradation data output from one line buffer based on the scanning mode switching signal is scanned at an appropriate timing. 116C is output to pulse modulation section 117. In addition, the gradation data is input to the pulse modulation unit 117 at the timing and order corresponding to each color component by the scanning mode switching unit 116C.

  The gradation data after halftoning input to the pulse modulation unit 117 is a multi-value signal indicating the size and arrangement of toner dots (dots) of each color to be attached to each pixel. The modulation unit 117 uses the halftone gradation data to create a video signal for pulse width modulating the exposure laser pulse of each color image of the engine unit EG, and the engine controller 10 via a video interface (not shown). Output to. Based on this video signal, the laser light source 62Y of the exposure unit 6 is ON / OFF controlled. The same applies to the other color components.

  Next, returning to FIG. In the exposure housing 61, a collimator lens 631, a cylindrical lens 632, a deflector 65, and a scanning lens 66 are provided to scan and expose the light beam from the laser light source 62Y onto the surface (not shown) of the photoreceptor 2Y. It has been. That is, the light beam from the laser light source 62Y is shaped into collimated light of an appropriate size by the collimator lens 631, and then incident on the cylindrical lens 632 having power only in the sub-scanning direction Y. Then, by adjusting the cylindrical lens 632, the collimated light is imaged in the vicinity of the deflection mirror surface 651 of the deflector 65 in the sub-scanning direction Y. Thus, in this embodiment, the collimator lens 631 and the cylindrical lens 632 function as the beam shaping system 63 that shapes the light beam from the laser light source 62Y.

  The deflector 65 is formed by using a micromachining technique in which a micromachine is integrally formed on a semiconductor substrate by applying a semiconductor manufacturing technique, and includes a vibrating mirror that resonates and oscillates. That is, in the deflector 65, the light beam can be deflected in the main scanning direction X by the deflecting mirror surface 651 that resonates and vibrates. More specifically, the deflecting mirror surface 651 is pivotally supported around a swing shaft (torsion spring) that is substantially orthogonal to the main scanning direction X, and responds to an external force applied from an operating portion (not shown). Swings sine around the swing axis. This actuating unit applies an electrostatic, electromagnetic or mechanical external force to the deflection mirror surface 651 based on a mirror drive signal from a mirror drive unit (not shown) of the exposure control unit 102 to mirror the deflection mirror surface 651. Swing at the frequency of the drive signal. Note that any driving method such as electrostatic adsorption, electromagnetic force, or mechanical force may be adopted as the driving method by the operating unit, and since these driving methods are well known, description thereof is omitted here.

  Further, in the apparatus configured as described above, the light beam can be reciprocated in the main scanning direction, that is, the light beam can be scanned in both the (+ X) direction and the (−X) direction. . As described above, the gradation data constituting one line image data is temporarily stored in the storage unit (line buffers 116A and 116B), and the scanning mode switching unit 116C performs gradation data at an appropriate timing and order. Is supplied to the pulse modulation unit 117. For example, when the direction is changed to the (+ X) direction, as shown in FIG. 6A, the beam spot is read out from the line buffer 116A in the order of gradation data DT1, DT2,. Is irradiated onto the photoconductor 2 in the first direction (+ X) to form a line latent image LI (+ X). On the other hand, when the direction is switched to the (−X) direction, as shown in FIG. 6B, the grayscale data DTn, DT (n−1),. A beam spot is irradiated onto the photoconductor 2 in the second direction (−X) based on each gradation data, and a line latent image LI (−X) is formed.

B. Halftone Processing As described in the above section “A. Device Configuration”, in the first embodiment, the halftone processing unit 1152 performs dot concentration type thresholding on the corrected gradation data output from the gradation correction unit 1151. Halftone gradation data is generated using the matrix MTX (FIG. 5). Hereinafter, the halftone process according to the present embodiment will be described with reference to FIGS. FIG. 13 is a diagram showing halftone processing performed by the halftone processing unit 1152. In the first embodiment, a 5 × 1 cell CL51 of 5 pixels in the main scanning direction X and 1 pixel in the sub-scanning direction Y is used, and the gradation level among a plurality of pixels included in the 5 × 1 cell CL51 is handled. Only pixels are exposed and developed to form dots. At this time, to which pixel a dot is to be formed is determined by a comparison result between the corrected gradation data and a threshold value matrix MTX described below. That is, for each pixel, the correction gradation data is compared with the threshold value of the 5 × 1 threshold matrix MTX51, and if the correction gradation data is equal to or larger than the threshold value of the corresponding pixel, a spot latent image is formed in the pixel portion. If it is less, a spot latent image is not formed. For example, when the gradation level of the correction gradation data is 40%, a spot is formed in the shaded portion of the 5 × 1 cell CL51 in FIG.

  FIG. 10 is a diagram illustrating the operation of the 5 × 1 threshold matrix MTX51. FIG. 10A is a diagram showing a 5 × 1 threshold matrix MTX51, and FIG. 10B shows the growth of halftone dots in the 5 × 1 cell CL51 when the 5 × 1 threshold matrix MTX51 is used. FIG. A 5 × 1 threshold value matrix MTX15 shown in FIG. 10A is a dot concentration type threshold value matrix, and the size of a halftone dot is increased as the gradation level increases. Specifically, as the gradation level increases from 0 to 100%, as shown in FIG. 10B, among the five pixels of the 5 × 1 cell CL51, the hatched pixels correspond to each gradation level. As a result of the exposure, halftone dots grow from the center of the cell.

C. Cell Arrangement Method FIG. 14 is a diagram illustrating an arrangement method of the 5 × 1 cell CL51 in the first embodiment. In the first embodiment, 5 × 1 cells CL51 are arranged adjacent to each other in the main scanning direction X and the sub-scanning direction Y.

D. Effects of the Invention in the First Embodiment In the first embodiment, as described in “B. Halftone processing” above, gradation reproduction is realized using a 5 × 1 cell CL51 of one pixel in the sub-scanning direction Y. ing. Therefore, for example, as shown in FIG. 11, out of two 5 × 1 cells CL51 arranged adjacent to each other in the sub-scanning direction Y, the grayscale level of the 5 × 1 cell CL51 on the downstream side in the sub-scanning direction Y in FIG. Even when a halftone of level 20% is formed, since the 5 × 1 cell CL51 has only one pixel in the sub-scanning direction Y, the 5 × 2 cell CL52 shown in the “halftone dot” column of FIG. The two spot latent images are not formed side by side in the sub-scanning direction Y in the cell as in the case where a halftone of 20% is formed. Therefore, the distance in the sub-scanning direction Y between the spot latent images formed in each of the pixels arranged adjacent to each other in the sub-scanning direction Y as shown in the “latent image formation” column of FIG. The phenomenon that it is different between cells arranged at both ends of X cannot occur. In other words, as shown in the “latent image formation” column in FIG. 11, the spot latent image for the pixel is corresponding to the position where the scanning line passes for each pixel on the wide pitch side and the narrow pitch side. Even if the position where the image is formed is shifted in the sub-scanning direction Y, the range in which the 5 × 1 cell CL51 arranged on each of the wide pitch side and the narrow pitch side is exposed by the spot (spot latent image) There is no difference. Therefore, there is no difference in the size of halftone dots obtained by forming dots in such an exposure area. Therefore, it is possible to suppress the occurrence of an image defect that a density difference occurs between both ends in the main scanning direction X even though an image having the same density is being formed, and good gradation reproduction is possible.

  Further, as in the first embodiment, when image formation is performed using the 5 × 1 cell CL51, the following effects are obtained. That is, as described with reference to FIG. 12 in the above “Means for Solving the Problems”, a 5 × 1 cell having one pixel in the sub-scanning direction Y has 5 pixels having M pixels (M ≧ 2) in the sub-scanning direction Y. Higher spatial resolution can be realized compared to × M cells. Therefore, when image formation is performed using the 5 × 1 cell CL51, there is further an effect that a clear image with a clear outline can be formed.

Second Embodiment
As described above, according to the first embodiment, since the 5 × 1 cell CL51 of one pixel is used in the sub-scanning direction Y, the density difference between both ends in the main scanning direction X is eliminated and a good level is obtained. It was found that not only tone reproduction can be realized, but also high spatial resolution can be realized. Therefore, 5 × 1 cell CL51 of one pixel in the sub-scanning direction Y is particularly suitable for an image mode that requires a high spatial resolution, such as a line drawing of characters or the like. However, there are cases where it is required to reproduce gradations with more gradation levels, as in the case of photographs and the like. That is, there are cases where the realization of finer gradation reproduction (high gradation resolution) is the most important by expressing multi-level gradation levels using more pixels. In such a case, it is required to realize finer gradation reproduction (high gradation resolution) using, for example, a 5 × 5 cell CL55 having a larger number of pixels. However, in this case, since 5 × 5 cell CL55 which is a larger cell is used, the spatial resolution is lower than that of 5 × 1 cell CL51. As described above, the required contents differ depending on the form of the image to be formed. Therefore, the second embodiment of the present invention is configured as follows in order to perform optimal image formation in accordance with such a difference in image mode. However, only the characteristic part of the second embodiment will be described below, and the description of the same part as the other first embodiment will be omitted.

  15 and 16 are diagrams showing a second embodiment of the image forming method and the image forming apparatus according to the present invention. In the second embodiment, two types of cells are prepared in advance, and the type of cell used for gradation reproduction is changed according to the image mode to be formed. That is, the CPU 111 determines the form of the image to be formed from the image data transmitted to the CPU 111 via the I / F (FIG. 2). More specifically, in the second embodiment, the image mode includes a high spatial resolution mode that prioritizes the spatial resolution and a high gradation resolution mode that prioritizes the gradation resolution among the spatial resolution and the gradation resolution. The CPU 111 determines whether the image mode to be formed is the high spatial resolution mode or the high gradation resolution mode. The determination result is sent to the halftone processing unit 1152 as an image mode signal. Then, the halftone processing unit 1152 performs halftone processing on the corrected gradation data sent from the gradation correction unit 1151 according to the image mode signal.

  FIG. 16 is a diagram illustrating halftone processing in the second embodiment. In the second embodiment, 5 × 1 cell CL51 (FIG. 16A) is used when the image mode signal sent from the CPU 111 is in the high spatial resolution mode, while 5 × 5 cell when in the high gradation resolution mode. CL55 (FIG. 16B) is used. Next, halftone processing for each mode will be described.

  When the image mode is the high spatial resolution mode as described above, gradation reproduction is performed using the 5 × 1 cell CL51, and the dot concentration type shown in FIG. A 5 × 1 threshold matrix MTX51 is used. That is, only the pixel corresponding to the gradation level among the plurality of pixels included in the 5 × 1 cell CL51 is exposed and developed to form dots. At this time, to which pixel a dot is to be formed is determined by a comparison result between the corrected gradation data and the threshold matrix MTX51. That is, for each pixel, the correction gradation data is compared with the threshold value of the 5 × 1 threshold matrix MTX51, and if the correction gradation data is equal to or larger than the threshold value of the corresponding pixel, a spot latent image is formed in the pixel portion. If it is less, a spot latent image is not formed. For example, when the gradation level of the correction gradation data is 40%, a spot is formed in the hatched portion of the 5 × 1 cell CL51 in FIG. In the second embodiment, a plurality of 5 × 1 cells CL51 are arranged adjacent to each of the main scanning direction X and the sub-scanning direction Y, as shown in FIG.

  When the image mode is the high gradation resolution mode as described above, gradation reproduction is performed using the 5 × 5 cell CL55, and the dot concentration shown in FIG. The type 5 × 5 threshold matrix MTX55 is used. That is, only the pixel corresponding to the gradation level among the plurality of pixels of the 5 × 5 cell CL55 is exposed and developed to form dots. At this time, to which pixel a dot is to be formed is determined by a comparison result between the corrected gradation data and the threshold matrix MTX55. That is, for each pixel, the correction gradation data is compared with the threshold value of the 5 × 5 threshold matrix MTX55, and if the correction gradation data is equal to or larger than the threshold value of the corresponding pixel, a spot latent image is formed in the pixel portion. If it is less, a spot latent image is not formed. For example, when the gradation level of the correction gradation data is 40%, a spot is formed in the hatched portion of the 5 × 5 cell CL55 in FIG. In the second embodiment, a plurality of 5 × 5 cells CL55 are arranged adjacent to each of the main scanning direction X and the sub-scanning direction Y, as shown in FIG.

  As described above, in the second embodiment, one of the 5 × 1 cell CL51 and the 5 × 5 cell CL55 is selectively used as the cell used for gradation reproduction according to the mode of image data to be formed. Yes. In the high spatial resolution mode in which spatial resolution is prioritized, image formation is performed using 5 × 1 cells CL51 of one pixel in the sub-scanning direction Y. Therefore, as in the case of the first embodiment described above, as shown in the “latent image formation” column of FIG. 11, the positions where the scanning lines pass for each pixel are different on the wide pitch side and the narrow pitch side. Correspondingly, even if the position where the spot latent image is formed with respect to the pixel is shifted in the sub-scanning direction Y, the 5 × 1 cell CL51 arranged on each of the wide pitch side and the narrow pitch side is spotted. No difference occurs in the exposed range (spot latent image). Therefore, there is no difference in the size of halftone dots obtained by forming dots in such an exposure area. Therefore, it is possible to suppress the occurrence of an image defect that a density difference occurs between both ends in the main scanning direction X even though an image having the same density is being formed, and good gradation reproduction is possible. Further, since one pixel 5 × 1 cell CL51 is used in the sub-scanning direction Y, a high spatial resolution can be realized and a clear and clear image can be formed.

  Furthermore, in the second embodiment, in the high gradation resolution mode, gradation reproduction is performed using a 5 × 5 cell CL55 having two or more pixels in the sub-scanning direction Y. As described above, by performing gradation expression by the 5 × 5 cell CL55 having more pixels than the 5 × 1 cell CL51, it is possible to realize more gradation levels. Therefore, finer gradation reproduction (high gradation resolution) is possible. As described above, in the second embodiment, one of the high spatial resolution mode and the high gradation resolution mode is selectively executed according to the mode of the image data, so that an optimum image can be formed according to the difference in the image mode. It becomes.

<Others>
The present invention is not limited to the above-described embodiment, and various modifications other than those described above can be made without departing from the spirit of the present invention. For example, in the second embodiment, the type of cell to be used is selected depending on whether the mode of the image to be formed is the high spatial resolution mode or the high gradation resolution mode. However, the criteria for selecting cells to be used for image formation are not limited to this. For example, as shown in FIG. 18, as a form of an image, a line drawing mode in which a central element constituting an image is a character, and an image is configured. When the mode of the image to be formed is the line drawing mode, the 5 × 1 cell CL51 is used, and when the image is to be formed, the 5 × 5 cell CL55 is used. It may be configured. With this configuration, when forming a line drawing, an image can be formed with a high spatial resolution, so a line drawing with a clear outline can be formed. When forming a photograph, an image with a high gradation resolution can be formed. Since it can be formed, a beautiful photograph can be formed.

  In the first and second embodiments, as shown in FIG. 10, exposure development is performed on the entire area of each pixel constituting the 5 × 1 cell CL51 in accordance with the gradation level when performing gradation reproduction. Is going. Therefore, in the 5 × 1 cell CL51 composed of five pixels, only six gradation levels of 0%, 20%, 40%, 60%, 80%, and 100% can be realized. However, the above-described 5 × 1 cell CL51 is subjected to so-called PWM modulation to adjust the lighting time of the light beam, so that the light beam is exposed only to a part of each pixel of the 5 × 1 cell CL51. You may do it. Thus, by adjusting the lighting time of the light beam, 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% as shown in FIG. 11 and 100% gradation levels can be realized, which is preferable. However, in FIG. 19, the square represents a pixel and the light beam is exposed to the gray portion.

  In the high spatial resolution mode in the first embodiment and the second embodiment, a 5 × 1 cell CL51 is used as a cell having one pixel in the sub-scanning direction Y, but the cell type is not limited to this. However, the number of pixels in the main scanning direction X can be arbitrarily set, for example, 3 × 1 cell or 6 × 1 cell. In short, the number of pixels in the main scanning direction X may be two or more.

  In the high gradation resolution mode in the second embodiment, the 5 × 5 cell CL55 is used. However, the types of cells that can be used in the high gradation resolution mode are not limited thereto. In short, any cell that has two or more pixels in the main scanning direction X and two or more pixels in the sub-scanning direction Y may be used. However, as the number of pixels increases, more gradation levels can be realized.

  In the first and second embodiments, a plurality of 1 × 5 cells CL15 are arranged adjacent to the main scanning direction X and the sub scanning direction Y, and cells adjacent to each other in the sub scanning direction Y are arranged in the main scanning direction. The cells are arranged without being shifted from each other in X, but the method of arranging the cells is not limited to this, and for example, adjacent to each other in the sub-scanning direction Y as shown in FIGS. The cells to be operated may be arranged with a few pixels shifted from each other in the main scanning direction X.

  In the second embodiment, a plurality of 5 × 5 cells CL55 are arranged adjacent to each other in the main scanning direction X and the sub-scanning direction Y, and cells adjacent to each other in the sub-scanning direction Y are arranged in the main scanning direction X. However, the arrangement of the cells is not limited to this, and the cells adjacent to each other in the sub-scanning direction Y may be shifted by several pixels in the main scanning direction X. .

  In the second embodiment, a plurality of 5 × 5 cells CL55 are arranged adjacent to each other in the main scanning direction X and the sub scanning direction Y, and cells adjacent to each other in the main scanning direction X are arranged in the sub scanning direction Y. However, the arrangement of the cells is not limited to this, and the cells adjacent to each other in the main scanning direction X may be arranged with a few pixels shifted from each other in the sub-scanning direction Y. .

  In the above embodiment, the present invention is applied to a so-called tandem color printer. However, the scope of the present invention is not limited to this, and for example, only a so-called four-cycle printer or single color printing is used. The present invention can also be applied to a monochrome printer.

  In the above embodiment, the present invention is applied to an image forming apparatus that temporarily forms a color image on an intermediate transfer medium such as an intermediate transfer belt and then transfers the color image to the sheet S. The present invention is also applicable to an apparatus that forms a color image by directly superimposing toner images on a sheet.

  In the above embodiment, the vibrating deflection mirror surface 651 is formed by using a micromachining technique. However, the method of manufacturing the deflection mirror surface is not limited to this, and the vibrating deflection mirror surface is used. The present invention can be applied to all so-called image forming apparatuses in which a light beam is deflected to scan the latent image carrier.

  Next, examples of the present invention will be shown. However, the present invention is not limited by the following examples as a matter of course, and it is of course possible to implement the present invention with appropriate modifications within a range that can meet the gist of the preceding and following descriptions. They are all included in the technical scope of the present invention.

  In the following, in order to show the effect of the present invention, a halftone image is formed by a case where a halftone image is formed by Examples 1 to 6, which are embodiments of the present invention, and a comparative example using a conventional gradation reproduction method. The result when compared with the case where it forms is demonstrated.

  FIG. 20 is a diagram illustrating threshold matrixes corresponding to the cells used in Examples 1 to 6 and the comparative example. In the first to sixth embodiments, a 5 × 1 cell CL51 having 5 pixels in the main scanning direction X and 1 pixel in the sub-scanning direction Y is used, and the dot concentration type 5 × shown in FIG. A one-threshold matrix MTX51 was used. In the comparative example, 5 × 4 cells of 5 pixels in the main scanning direction X and 4 pixels in the sub-scanning direction Y are used, and a dot concentration type threshold matrix MTX54 shown in FIG. It was.

  In each of Examples 1 to 6 and Comparative Example, a cyan halftone image (gradation) having a predetermined density is created on both ends of the paper (both ends in the main scanning direction X) using Microsoft's Word. The concentration difference between them was measured. In Examples 1 to 3 and Comparative Example, halftone images (gradations) were created when the density of the image to be formed was set to four types of set densities of 20%, 40%, 60%, and 80%. In Examples 4 to 6, the so-called PWM modulation described above with reference to FIG. 19 is performed to adjust the lighting time of the light beam, thereby adjusting the density of the image to be formed for the 5 × 1 cell CL51. Four types of set densities of 10%, 30%, 50%, and 70% are set. In addition, for the concentration measurement, a spectrolino manufactured by Gretag Macbeth was used, and an average of 5 points was adopted as a measured value in consideration of measurement error. The spatial resolution was 600 dpi in both the main scanning direction X and the sub-scanning direction Y. Then, as will be described below, the cell arrangement method used in each of the examples or comparative examples is changed.

  FIG. 21 is a diagram showing a cell arrangement method in the first and fourth embodiments. In the first and fourth embodiments, a plurality of 5 × 1 cells CL51 are arranged adjacent to each other in the main scanning direction X and the sub-scanning direction Y.

  FIG. 22 is a diagram showing a cell arrangement method in the second and fifth embodiments. In the second and fifth embodiments, a plurality of 5 × 1 cells CL51 are arranged adjacent to each other in the main scanning direction X and the sub-scanning walk Y. Further, the 5 × 1 cells CL51 adjacent to each other in the sub-scanning direction Y are shifted from each other by one pixel in the main scanning direction X.

  FIG. 23 is a diagram showing a cell arrangement method in the third and sixth embodiments. In Examples 3 and 6, a plurality of 5 × 1 cells CL51 are arranged adjacent to each other in the main scanning direction X and the sub-scanning walk Y. Further, 5 × 1 cells CL51 adjacent to each other in the sub-scanning direction Y are arranged so as to be shifted from each other by 2 pixels in the main scanning direction X.

  FIG. 24 is a diagram showing a cell arrangement method in a comparative example. In the comparative example, a plurality of 5 × 4 cells CL54 are arranged adjacent to each other in the main scanning direction X and the sub-scanning direction Y.

  Table 1 shows the results of actual measurement of the density of each set density (%) halftone image (gradation) formed on the left and right ends (both ends in the main scanning direction) of the paper in Example 1. In FIG. 25, the horizontal axis represents the set density (%) of the halftone image, and the vertical axis represents the actually measured value of the halftone image density formed on the left and right ends of the paper at the horizontal axis set density (%). The result shown in 1 is plotted.

  Table 2 shows the results of actual measurement of the density of each set density (%) halftone image (gradation) formed on the left and right ends (both ends in the main scanning direction) of the paper in Example 2. In FIG. 26, the horizontal axis represents the set density (%) of the halftone image, and the vertical axis represents the measured value of the halftone image density formed on the left and right ends of the paper with the set density (%) on the horizontal axis. The result shown in 2 is plotted.

  Table 3 shows the results of actual measurement of the density of the halftone image (gradation) of each set density (%) formed on the left and right ends (both ends in the main scanning direction) of the paper in Example 3. In FIG. 27, the horizontal axis is the set density (%) of the halftone image, and the vertical axis is the measured value of the halftone image density formed on the left and right ends of the paper with the set density (%) on the horizontal axis. The result shown in 3 is plotted.

  Table 4 shows the results of actual measurement of the density of the halftone image (gradation) of each set density (%) formed on the left and right ends (both ends in the main scanning direction) of the paper in the comparative example. In FIG. 28, the horizontal axis represents the set density (%) of the halftone image, and the vertical axis represents the measured value of the halftone image density formed on the left and right ends of the paper at the horizontal axis set density (%). 4 is a plot of the results shown in FIG.

  Table 5 shows the results of actual measurement of the density of the halftone image (gradation) of each set density (%) formed on the left and right ends (both ends in the main scanning direction) of the paper in Example 4. In FIG. 29, the horizontal axis represents the set density (%) of the halftone image, and the vertical axis represents the measured value of the halftone image density formed on the left and right ends of the paper with the set density (%) on the horizontal axis. 5 is a plot of the results shown in FIG.

  Table 6 shows the result of actually measuring the density of the halftone image (gradation) of each set density (%) formed on the left and right ends (both ends in the main scanning direction) of the sheet in Example 5. In FIG. 30, the horizontal axis represents the halftone image set density (%), and the vertical axis represents the measured value of the halftone image density formed on the left and right ends of the paper at the horizontal axis set density (%). 6 is a plot of the results shown in FIG.

  Table 7 shows the results of actual measurement of the density of the halftone image (gradation) of each set density (%) formed on the left and right ends (both ends in the main scanning direction) of the paper in Example 6. In FIG. 31, the horizontal axis represents the set density (%) of the halftone image, and the vertical axis represents the measured value of the halftone image density formed on the left and right ends of the paper with the set density (%) on the horizontal axis. 7 is a plot of the results shown in FIG.

  As can be seen from FIG. 28, in the comparative example in which the halftone image is formed using the conventional gradation reproduction method, the halftone image formed at the left end is formed at the right end at any of the set densities of 40% to 80%. It can be seen that the measured value of the image density is higher than the halftone image. Compared with this, in FIGS. 25 to 27 corresponding to the first to third embodiments which are embodiments of the present invention, the actually measured value of the image density of the halftone image formed at the left end and the image of the halftone image formed at the right end. It can be seen that the measured value of the concentration is almost the same value. That is, it can be seen that Examples 1 to 3 of the present invention have a lower left-right density difference than the comparative example. As described above, according to the first to third embodiments of the present invention, it has been found that good gradation reproduction can be realized by suppressing the image adverse effect that the density difference occurs between both ends in the main scanning direction X.

  29 to 31 corresponding to Examples 4 to 6 which are embodiments of the present invention, the actual measured value of the image density of the halftone image formed at the left end and the actual measured value of the image density of the halftone image formed at the right end. As can be seen from FIG. In this way, by adjusting the lighting time of the light beam by performing so-called PWM modulation, a dot concentration type threshold matrix is used for the 5 × 1 cell CL51, and the density of the image to be formed is 10%, 30 Even when the ratio is set to%, 50%, or 70%, the effect of the present invention is achieved in that good gradation reproduction can be realized by suppressing the image detrimental effect of density difference between both ends in the main scanning direction X. I found out.

1 is a diagram illustrating a first embodiment of an image forming apparatus according to the present invention. FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus in FIG. 1. FIG. 2 is a main scanning sectional view showing a configuration of an exposure unit in the image forming apparatus of FIG. 1. FIG. 2 is a diagram showing a signal processing block in the image forming apparatus of FIG. 1. The figure which shows the structure of an image processing unit. FIG. 2 is a diagram showing a line latent image formed by the image forming apparatus of FIG. 1. The figure which shows the scanning pitch in this invention. Explanatory drawing of the subject of this invention. Explanatory drawing of the subject of this invention. The figure which shows operation | movement of the threshold value matrix in 1st Embodiment. Explanatory drawing of the effect of this invention. Explanatory drawing of the effect of this invention. The figure which shows the halftone process in 1st Embodiment. The figure which shows arrangement | positioning of the cell in 1st Embodiment. The figure which shows the structure of the image processing unit in 2nd Embodiment. The figure which shows the halftone process in 2nd Embodiment. The figure which shows arrangement | positioning of the cell in 2nd Embodiment. The figure which shows the halftone process in another embodiment. Explanatory drawing of gradation reproduction by PWM modulation for 5 × 1 cells. The figure which shows the threshold value matrix used in an Example. The figure which shows the cell arrangement | positioning of Example 1,4. The figure which shows the cell arrangement | positioning of Example 2,5. The figure which shows the cell arrangement | positioning of Example 3,6. The figure which shows the cell arrangement | positioning of a comparative example. FIG. 6 is a diagram showing actual measurement results in Example 1. FIG. 6 is a diagram showing actual measurement results in Example 2. FIG. 10 is a diagram showing actual measurement results in Example 3. The figure which shows the actual measurement result in a comparative example. FIG. 6 is a diagram showing actual measurement results in Example 4. FIG. 6 is a diagram showing actual measurement results in Example 5. FIG. 10 is a diagram showing actual measurement results in Example 6.

Explanation of symbols

  2, 2Y, 2M, 2C, 2K ... photosensitive member (latent image carrier), 6, 6Y, 6M, 6C, 6K ... exposure unit, 60 ... horizontal synchronization sensor, 62, 62Y, 62M, 62C, 62K ... laser light source 71: Intermediate transfer belt 651: Deflection mirror surface DT1, DT2, DT (n-1), DTn: Gradation data, Ly, Lm, Lc, Lk: Scanning light beam, LI (+ X), LI ( -X) ... line latent image, MT ... drive motor, X ... main scanning direction, Y ... sub-scanning direction, MTX, MTX51, MTX54, MTX55 ... threshold matrix, CL, CL51, CL54, CL55 ... cell

Claims (6)

A latent image carrier whose surface is driven in the sub-scanning direction and a vibrating deflection mirror surface cause the light beam from the light source to pass in the first direction in the main scanning direction substantially orthogonal to the sub-scanning direction and opposite to the first direction. A plurality of spot latent images are formed by irradiating the surface of the latent image carrier in a spot shape with a light beam that scans in the first direction, and also in the second direction. A latent image forming unit that forms a plurality of spot latent images by irradiating the surface of the latent image carrier with a light beam to be scanned in a spot shape, and a development that forms dots by developing each of the plurality of spot latent images Forming an image by using a dot-concentrated threshold matrix and forming a halftone dot by the dot for a cell having a plurality of pixels to form an image. Way ,
An image is formed using N × 1 cells having N pixels (N ≧ 2) in the main scanning direction and one pixel in the sub-scanning direction as the cells. Method. A latent image carrier whose surface is driven in the sub-scanning direction and a vibrating deflection mirror surface cause the light beam from the light source to pass in the first direction in the main scanning direction substantially orthogonal to the sub-scanning direction and opposite to the first direction. A plurality of spot latent images are formed by irradiating the surface of the latent image carrier in a spot shape with a light beam that scans in the first direction, and also in the second direction. A latent image forming unit that forms a plurality of spot latent images by irradiating the surface of the latent image carrier with a light beam to be scanned in a spot shape, and a development that forms dots by developing each of the plurality of spot latent images Forming an image by using a dot-concentrated threshold matrix and forming a halftone dot by the dot for a cell having a plurality of pixels to form an image. Way ,
As the cell, N × 1 cell having N pixels (N ≧ 2) in the main scanning direction and one pixel in the sub-scanning direction, and N (N ≧ 2) in the main scanning direction. And an N × M cell having M pixels (M ≧ 2) in the sub-scanning direction in advance,
An image forming method, wherein the image is formed by selectively using one of the N × 1 cell and the N × M cell according to an aspect of an image to be formed. As an aspect of the image, it has a high spatial resolution mode that prioritizes spatial resolution and a high gradation resolution mode that prioritizes gradation resolution among spatial resolution and gradation resolution,
The image forming method according to claim 2, wherein the N × 1 cell is used when the mode of an image to be formed is the high spatial resolution mode, and the N × M cell is used when the image mode is the high gradation resolution mode. As an aspect of the image, there is a line drawing mode in which the central element constituting the image is a character, and a photo mode in which the central element constituting the image is a photograph,
3. The image forming method according to claim 2, wherein the N × 1 cell is used when the mode of the image to be formed is the line drawing mode, and the N × M cell is used when the mode is the photographic mode. A latent image carrier whose surface is driven in the sub-scanning direction and a vibrating deflection mirror surface cause the light beam from the light source to pass in the first direction in the main scanning direction substantially orthogonal to the sub-scanning direction and opposite to the first direction. A plurality of spot latent images are formed by irradiating the surface of the latent image carrier in a spot shape with a light beam that scans in the first direction, and also in the second direction. A latent image forming unit that forms a plurality of spot latent images by irradiating the surface of the latent image carrier with a light beam to be scanned in a spot shape, and a development that forms dots by developing each of the plurality of spot latent images A dot concentration type threshold matrix, and forming an image by expressing a gradation by forming a halftone dot with the dot for a cell having a plurality of pixels,
An image is formed using N × 1 cells having N pixels (N ≧ 2) in the main scanning direction and one pixel in the sub-scanning direction as the cells. apparatus. A latent image carrier whose surface is driven in the sub-scanning direction and a vibrating deflection mirror surface cause the light beam from the light source to pass in the first direction in the main scanning direction substantially orthogonal to the sub-scanning direction and opposite to the first direction. A plurality of spot latent images are formed by irradiating the surface of the latent image carrier in a spot shape with a light beam that scans in the first direction, and also in the second direction. A latent image forming unit that forms a plurality of spot latent images by irradiating the surface of the latent image carrier with a light beam to be scanned in a spot shape, and a development that forms dots by developing each of the plurality of spot latent images A dot concentration type threshold matrix, and forming an image by expressing a gradation by forming a halftone dot with the dot for a cell having a plurality of pixels,
As the cell, N × 1 cell having N pixels (N ≧ 2) in the main scanning direction and one pixel in the sub-scanning direction, and N (N ≧ 2) in the main scanning direction. And N × M cells having M (M ≧ 2) pixels in the sub-scanning direction,
An image forming apparatus, wherein the image is formed by selectively using one of the N × 1 cell and the N × M cell according to an aspect of an image to be formed.

Images