JP4581859B2 - Image forming apparatus and control method thereof - Google Patents

Description

  The present invention relates to an image forming apparatus that scans a surface of a latent image carrier driven in a sub-scanning direction in a main scanning direction substantially orthogonal to the sub-scanning direction and forms an image, and a control method for the apparatus. Is.

  This type of image forming apparatus includes a photoconductor, an exposure unit, and a development unit, and forms a toner image on the photoconductor as follows. That is, 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 by the deflector of the exposure unit so that the spot is scanned on the photosensitive member surface in the main scanning direction. A latent image corresponding to the image data is formed on the surface of the photoreceptor. 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 reciprocated on the surface of the photoreceptor.

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

  In such an image forming apparatus, it is possible to selectively switch between a double-sided scanning mode in which a light beam from a light source is scanned on the photoreceptor in both the forward path and the backward path, and a single-sided scanning mode in which only one of them is scanned. Thus, it is possible to form an image according to the printing mode. For example, when printing characters etc. that do not require high resolution, a spot is scanned on the photoreceptor only in either the forward path or the backward path to form an image with a low resolution, as in a photograph or the like. When it is necessary to reproduce the gradation, the spot can be scanned on the surface of the photoconductor in both the forward pass and the return pass to increase the resolution.

  As means for realizing the above-described gradation reproduction, a line screen that realizes gradation reproduction by changing the line width of a line extending in a predetermined direction according to the gradation, or discretely arranged in a predetermined direction. A halftone dot screen or the like that realizes gradation reproduction by growing the size of the halftone dots according to the gradation can be used.

  However, when the latent image is formed on the photosensitive member in the double-sided scanning mode in the configuration in which the single-sided scanning mode and the double-sided scanning mode can be switched as described above, an image resulting from the fact that the scanning pitch in the sub-scanning direction is not constant. Detrimental effects may occur. The reason will be described in detail. In the image forming apparatus as described above, a latent image is formed by scanning a spot on the surface of the photosensitive member. However, in the double-sided scanning mode, the scanning pitch in the sub-scanning direction is not constant, and the spots overlap in the sub-scanning direction. The degree of variation will vary. 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, for example, when forming a line extending in a predetermined direction in order to realize gradation reproduction by a multi-line screen, the line becomes thin in a portion where the overlap of spots in the sub-scanning direction is large, while the spot in the sub-scanning direction The line becomes thicker in the part where the overlap of is small. Therefore, an unnecessary pattern in which the line becomes thin or thick corresponding to the non-uniformity of the scanning pitch in the sub-scanning direction is generated, and an image detrimental effect that good gradation reproduction cannot be obtained occurs. There was a case. In addition, the same image problem may occur even when gradation reproduction is performed by a halftone screen in which halftone dots are discretely arranged in a predetermined direction.

  The present invention has been made in view of the above problems, and is an image forming apparatus that forms a latent image by scanning a spot in the main scanning direction by a polarizing mirror that vibrates on a latent image carrier driven in the sub-scanning direction. An object of the present invention is to provide a technology that can selectively switch between the one-side scanning mode and the two-side scanning mode, and that can prevent the above-described image adverse effects and realize good gradation reproduction even in the two-side scanning mode. Yes.

According to the control method of the image forming apparatus of the present invention, the latent image carrier that is driven in the first direction and the light beam is reflected by the oscillating deflection mirror so that the latent image carrier is substantially orthogonal to the first direction. A latent image forming means for forming a latent image by scanning in the second direction, a one-side scanning mode in which the latent image carrier is scanned only in one of the second directions, and both the one and the other opposite to the one An image forming apparatus comprising a scanning mode control means for selectively switching between both-side scanning modes for scanning the image, and a method for controlling an image forming apparatus that performs gradation reproduction using a line screen, in order to achieve the above object In addition, the angle between each line of the line screen and the first direction is smaller in the double-sided scanning mode than in the single-sided scanning mode.
At this time, the first direction may be the sub-scanning direction, and the second direction may be the main scanning direction.
According to another aspect of the present invention, there is provided a method for controlling an image forming apparatus, comprising: a latent image carrier whose surface is driven in a sub-scanning direction; and a deflection mirror that reflects a light beam emitted from a light source. Latent image forming means for forming a latent image by scanning the surface of the latent image carrier in a main scanning direction substantially perpendicular to the sub-scanning direction, and a spot on the surface of the latent image carrier in the first direction of the main scanning direction. In an image forming apparatus comprising a single-side scanning mode for scanning only and a scanning mode control means for selectively switching between the first direction and a double-sided scanning mode for scanning in both the first direction and the second direction opposite to the first direction. A method of controlling an image forming apparatus that performs gradation reproduction using a line screen, and in order to achieve the above object, the angle formed between each line of the line screen and the sub-scanning direction is set in the double-sided scanning mode and in the single-sided scanning mode. ratio It is characterized by small and.

Also, the image forming apparatus according to the present invention reflects the light beam by the latent image carrier driven in the first direction and the oscillating deflection mirror, and is substantially orthogonal to the first direction. A latent image forming unit that scans in the second direction to form a latent image; a one-sided scanning mode that scans the latent image carrier in only one of the second directions; and both the one and the other opposite to the one There is provided an image forming apparatus that includes a scanning mode control means for selectively switching between scanning modes on both sides of scanning, and performs gradation reproduction by a line screen, and in order to achieve the above object, each line of the line screen and the first line The angle formed between the two-sided scanning mode is smaller than that in the one-sided scanning mode.
At this time, the first direction may be the sub-scanning direction, and the second direction may be the main scanning direction.
The image forming apparatus according to another aspect of the present invention also includes a latent image carrier whose surface is driven in the sub-scanning direction and a deflection mirror that oscillates a light beam emitted from a light source, thereby reflecting the spot as a latent image. Latent image forming means for forming a latent image by scanning the surface of the carrier in the main scanning direction substantially perpendicular to the sub-scanning direction, and scanning the spot on the surface of the latent image carrier only in the first direction of the main scanning direction. And a scanning mode control means for selectively switching between the one-side scanning mode and the both-side scanning mode in which scanning is performed in both the first direction and the second direction opposite to the first direction, and gradation reproduction is performed by a line screen. In order to achieve the above object, the image forming apparatus is characterized in that the angle formed between each line of the line screen and the sub-scanning direction is smaller in the double-sided scanning mode than in the single-sided scanning mode.

  In the invention configured as described above (the image forming apparatus control method and the image forming apparatus), the one-side scanning mode in which the spot is scanned only in the first direction (one side) in the main scanning direction (second direction), and the first It is configured to be able to selectively switch between a double-sided scanning mode for scanning in one direction (one) and a second direction (the other) opposite to the first direction. For example, when a line latent image is formed on the latent image carrier as shown in FIG. 7, a line latent image LI (() is generated by a spot scanned in the first direction (one side) in the main scanning direction (second direction). + X) is formed, while the line latent image LI (−X) is formed by the spot scanned in the second direction (the other) opposite to the first direction (the one). Therefore, in the double-sided scanning mode in which the spot used for forming the latent image is scanned in the first direction (one) and the second direction (the other), the line latent images LI (+ X) and LI (−X) are in the sub-scanning direction (first). In one direction). In contrast, in the one-side scanning mode in which the spot is scanned only in either the first direction (one) or the second direction (the other), only one of the line latent images LI (+ X) and LI (-X) is detected. Are formed in the sub-scanning direction (first direction).

  In the invention configured as described above, the spot is reciprocated in the main scanning direction (second direction) on the surface of the latent image carrier, and the main scanning direction (second direction) on the surface of the latent image carrier. ) In the sub-scanning direction (first direction) that is substantially orthogonal. Therefore, the scanning trajectory of the spot on the latent image carrier in the double-sided scanning mode is as shown by the one-dot chain line in FIG. 10, and therefore the scanning pitch in the sub-scanning direction (first direction) is not constant. . Such non-uniformity of the scanning pitch in the sub-scanning direction (first direction) is particularly noticeable in the vicinity of the end of the scanning locus in the main scanning direction (second direction). On the other hand, in the present invention, gradation reproduction is realized by a line screen that changes the line width of a line extending in a predetermined direction according to the gradation. Therefore, as shown in FIG. 11, when a line latent image extending in a predetermined direction is formed in order to realize gradation reproduction in the double-sided scanning mode, it corresponds to the non-uniformity of the scanning pitch in the sub-scanning direction (first direction). Thus, a thin part and a thick part of the line latent image are periodically generated, and an unnecessary pattern is generated. In FIG. 11, a solid line represents a scanning line, and a circle represents a spot formed on the surface of the photoreceptor. Such a pattern hinders the realization of a good gradation reproduction and causes image adverse effects.

  However, the present invention is configured such that the angle formed between each line of the line screen and the sub-scanning direction (first direction) is smaller in the double-sided scanning mode than in the single-sided scanning mode. Note that the angle formed by each line and the sub-scanning direction (first direction) is the smallest angle among the angles formed by intersecting each line and the sub-scanning direction (first direction). With this configuration, in the double-sided scanning mode, the thin portion and the thick portion of the line latent image periodically correspond to the non-uniformity of the scanning pitch in the sub-scanning direction (first direction). It becomes possible to suppress appearing. The reason for this will be described with reference to FIG. In FIG. 12, the alternate long and short dash line indicates the scanning line, the square indicates the latent image formed at a narrow or wide scanning pitch, and the solid line indicates the direction in which each line of the line screen extends. In FIG. 12, the angle formed between the line screen and the sub-scanning direction (first direction) is made smaller as going from (a) to (d). In FIG. 12A where the angle is the largest, the line width DT of the portion with the narrow scanning pitch is significantly different from the line width DF of the portion with the wide scanning pitch. However, as the angle formed between each line of the line screen and the sub-scanning direction (first direction) is reduced as shown in FIGS. 12C and 12D, the line width DT and the scanning pitch of the narrower scanning pitch. It can be seen that the difference from the line width DF of the wide portion of the line is reduced. In particular, in FIG. 12D, it can be seen that the line width DF and the line width DT are substantially the same. Thus, the difference between the line width DT of the narrow scanning pitch portion and the line width DF of the wide scanning pitch portion reduces the angle formed between each line of the line screen and the sub-scanning direction (first direction). Can be suppressed. Therefore, in the present invention, the angle formed between each line of the line screen and the sub-scanning direction (first direction) is smaller in the double-sided scanning mode than in the single-sided scanning mode. Occasionally, the thin and thick portions of the line latent image can be prevented from appearing periodically corresponding to the non-uniformity of the scanning pitch in the sub-scanning direction. Therefore, satisfactory gradation reproduction can be realized even in the double-sided scanning mode.

  Further, the angle formed between each line of the line screen and the first direction (sub-scanning direction) may be set to approximately 0 degrees in the double-sided scanning mode. In this case, as shown in FIG. 12D, the line latent image has a width in the main scanning direction (second direction) corresponding to the non-uniformity of the scanning pitch in the sub-scanning direction (first direction). Fluctuations can be completely suppressed.

The image forming apparatus control method according to the present invention includes a latent image carrier that is driven in a first direction and a deflecting mirror that oscillates and reflects the light beam to the latent image carrier in the first direction. Latent image forming means for forming a latent image by scanning in a substantially orthogonal second direction, one-sided scanning mode for scanning the latent image carrier in only one of the second directions, the one and the other opposite to the one In the image forming apparatus comprising a scanning mode control means for selectively switching between both-side scanning modes for scanning in both directions, halftone dots lined up discretely in the first array direction and the second array direction as the gradation increases A method of controlling an image forming apparatus that reproduces gradation using a halftone dot screen that grows an image, and in order to achieve the above object, an angle formed by a first direction among a first arrangement direction and a second arrangement direction is A large array direction and the first Angle between direction Metropolitan, both sides scanning mode is characterized by small compared to the one-side scanning mode.
At this time, the first direction may be the sub-scanning direction, and the second direction may be the main scanning direction.
The image forming apparatus control method according to another aspect of the present invention includes a latent image carrier whose surface is driven in the sub-scanning direction and a spot reflected by a deflection mirror that vibrates a light beam emitted from a light source. Latent image forming means for forming a latent image by scanning the surface of the latent image carrier in a main scanning direction substantially perpendicular to the sub-scanning direction, and a spot on the surface of the latent image carrier in the first direction of the main scanning direction. An image forming apparatus comprising: a one-side scanning mode that scans only one; and a scanning mode control unit that selectively switches between the first direction and a two-side scanning mode that scans in both the first direction and the second direction opposite to the first direction. A method of controlling an image forming apparatus that performs gradation reproduction by a halftone screen that grows halftone dots that are discretely arranged in a first arrangement direction and a second arrangement direction as the tone increases. First, Direction and the angle between the alignment direction and the subscanning direction angle is large the sub-scanning direction in the second arrangement direction, both side scanning mode is characterized by small compared to the one-side scanning mode.

Also, the image forming apparatus according to the present invention reflects the light beam by the latent image carrier driven in the first direction and the oscillating deflection mirror, and is substantially orthogonal to the first direction. A latent image forming unit that scans in the second direction to form a latent image; a one-sided scanning mode that scans the latent image carrier in only one of the second directions; and both the one and the other opposite to the one Scanning mode control means for selectively switching between both-side scanning modes for scanning, and a halftone screen for growing halftone dots that are discretely arranged in the first arrangement direction and the second arrangement direction as the gradation increases. An image forming apparatus that performs tone reproduction, and in order to achieve the above-described object, the first direction is formed by an arrangement direction having a large angle with the first direction out of the first arrangement direction and the second arrangement direction. One side scan when angle is double side scan mode It is characterized by small compared to the time over de.
At this time, the first direction may be the sub-scanning direction, and the second direction may be the main scanning direction.
An image forming apparatus according to another aspect of the present invention includes a latent image carrier whose surface is driven in the sub-scanning direction and a deflection mirror that oscillates a light beam emitted from a light source to generate a latent image. Latent image forming means for forming a latent image by scanning the surface of the carrier in the main scanning direction substantially perpendicular to the sub-scanning direction, and scanning the spot on the surface of the latent image carrier only in the first direction of the main scanning direction. Scanning mode control means for selectively switching between the one-side scanning mode and the two-sided scanning mode for scanning in both the first direction and the second direction opposite to the first direction. An image forming apparatus that performs gradation reproduction by a halftone screen that grows halftone dots that are discretely arranged in the arrangement direction and the second arrangement direction. In order to achieve the above object, the first arrangement direction and the second arrangement direction Out of the sub-scanning direction The angle between the angle is greater sequence direction and the sub Nasu, both sides scanning mode is characterized by small compared to the one-side scanning mode.

In the invention thus configured (control method for image forming apparatus and image forming apparatus), a halftone screen for growing halftone dots that are discretely arranged in the first arrangement direction and the second arrangement direction as the gradation increases. To reproduce the gradation. In other words, in the present invention, gradation is not realized by changing the width of a line latent image extending in a predetermined direction like the above-described line screen, but the size of halftone dots discretely arranged in two directions. The gradation is realized by changing the height. In this way, the line screen and the halftone screen have a difference between a continuous line and a set of discretely arranged points. However, in the halftone screen, as in the case of the multiline screen, in the double-sided scanning mode, an image in which a periodic pattern is generated in the halftone dot arrangement direction corresponding to the non-uniformity of the scanning pitch in the sub-scanning direction. Bad effects may occur. Such image defects are remarkable in the arrangement direction where the angle formed with the sub-scanning direction (first direction) is large. However, in the present invention, the angle formed by the growth direction and the sub-scanning direction (first direction), which are large with the sub-scanning direction (first direction) of the first and second arrangement directions, In the double-sided scanning mode, it is configured to be smaller than in the single-sided scanning mode. Thereby, in the double-sided scanning mode, it is possible to prevent an image defect that a periodic pattern is generated in the arrangement direction of halftone dots corresponding to the non-uniformity of the scanning pitch in the sub-scanning direction (first direction). . Therefore, good gradation reproduction can be realized even in the double-sided scanning mode.

<First Embodiment>
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 (recording medium) S such as a copy sheet, a transfer sheet, a sheet, and an OHP transparent sheet.

  In the engine unit EG, a charging unit, a developing unit, an exposure unit, and a cleaning unit are provided for each of the four photoconductors 2Y, 2M, 2C, and 2K. As described above, for each toner color, an image forming unit is provided that includes a photoreceptor, a charging unit, a developing unit, an exposure unit, and a cleaning unit, and forms a toner image of the toner color. 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. Thus, in this embodiment, the photoconductor 2Y is driven by transmitting the driving force from the drive motor MT only to one end side of the photoconductor 2Y. In this embodiment, the arrangement position of the drive motor MT, the horizontal synchronization sensor 60 described later, and the scanning direction of the light beam are set to satisfy a predetermined relationship. This point will be described in detail later.

  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. Then, the scanning light beam Ly is irradiated from the exposure unit 6Y toward the outer peripheral surface of the photoreceptor 2Y charged by the charging unit 3Y. 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 corresponds to the “latent image forming unit” of the present invention, and operates in accordance with 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. 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.

  3 is a main scanning sectional view showing the structure of the exposure unit equipped in the image forming apparatus of FIG. 1, FIG. 4 is a view showing the scanning region of the light beam in the exposure unit of FIG. 3, and FIG. FIG. 3 is a diagram illustrating a signal processing block in one image forming apparatus. Hereinafter, the configurations and operations of the exposure unit 6 and the exposure control unit 102 will be described in detail with reference to these drawings. 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. As shown in FIG. 4, the laser light source 62Y is electrically connected to the light source driving unit 1021 of the exposure control unit 102Y. Then, the light source driving unit controls ON / OFF of the laser light source 62Y according to the image signal as follows, 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. That is, gradation correction and halftoning processing are performed on the gradation data of each pixel input from the color conversion unit 114. That is, the image processing unit 115 refers to the gradation correction table 118 registered in advance in the non-volatile memory, and in accordance with the gradation correction table 118, the input gradation data of each pixel from the color conversion unit 114 is Conversion to corrected gradation data indicating the corrected gradation level is performed. 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.

  The image processing unit 115 performs halftoning processing on the corrected gradation data thus corrected, and inputs the halftone gradation data to the two types of line buffers 116A and 116B. In the present embodiment, halftoning processing using a line screen is performed. In a multi-line screen, gradation reproduction is performed by changing the line width of a plurality of lines extending in a predetermined direction according to the gradation. FIG. 6 shows this state. For example, gradation reproduction is realized by increasing the line width as shown in FIGS. 6A to 6C as the gradation increases.

  These line buffers 116A and 116B are common in that they store halftone gradation data (image information) constituting one line image data output from the image processing unit 15, but the readout order of gradation data is the same. 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. The main reason for providing the two types of line buffers 116A and 116B in this way is to cope with the difference in the spot scanning mode depending on the printing mode, as will be described later. 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. Thus, in this embodiment, the line buffers 116A and 116B and the scanning mode switching unit 116C correspond to the “scanning mode control means” of the present invention.

  The gradation data after halftoning input to the pulse modulation unit 117 is a multilevel signal indicating the size and arrangement of toner dots of each color to be attached to each pixel, and the pulse modulation unit 117 that has received such data. 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 outputs it to the engine controller 10 via a video interface (not shown). . Upon receiving this video signal, the light source driving unit 1021 of the exposure control unit 102Y performs ON / OFF control of the laser light source 62Y of the exposure unit 6. The same applies to the other color components.

  Next, returning to FIGS. 3 and 4, the description will be continued. 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 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 vibrates. 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.

  The light beam deflected by the deflecting mirror surface 651 of the deflector 65 is deflected toward the scanning lens 66 at the maximum amplitude angle θmax as shown in FIG. In this embodiment, the scanning lens 66 is configured so that the F values are substantially the same in the entire effective image region IR of the photoreceptor 2. Accordingly, the light beam deflected toward the scanning lens 66 is focused on the effective image area IR on the surface of the photosensitive member 2 through the scanning lens 66 with substantially the same spot diameter. As a result, a line-shaped latent image extending in the main scanning direction X is formed on the effective image area IR of the photosensitive member 2 by scanning the light beam in parallel with the main scanning direction X. In this embodiment, the scanning area SR that can be scanned by the deflector 65 is set wider than the effective image area, as shown in FIG. The effective image area IR is located substantially at the center of the scanning area SR and is substantially symmetric with respect to the optical axis. Further, the symbol θir in the figure indicates the amplitude angle of the deflection mirror surface 651 corresponding to the end of the effective image region IR, and the symbol θs indicates the amplitude angle of the deflection mirror surface 651 corresponding to the horizontal synchronization sensor 60 described below. Is shown.

  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 switched to the (+ X) direction, as shown in FIG. 7A, the spot data is read from the line buffer 116A in the order of the gradation data DT1, DT2,. The photosensitive drum 2 is irradiated in the first direction (+ X) to form a line latent image LI (+ X). On the other hand, when switching to the (−X) direction, as shown in FIG. 7B, the grayscale data DTn, DT (n−1),. A spot is irradiated onto the photosensitive member 2 in the second direction (−X) based on each gradation data, and a line latent image LI (−X) is formed. For this reason, the spot scanning mode can be made different for each printing mode or for each line as follows. More specifically, in this embodiment, information about the resolution (resolution information) included in the print command is temporarily stored in the RAM 107. When high resolution printing is instructed, the spot is scanned in the effective image area IR in the (+ X) direction to form a latent image in the effective image area IR, and the spot is effective in the (−X) direction. A latent image is formed by executing a so-called double-sided scanning mode in which the operation of scanning the image region IR and alternately forming the latent image in the effective image region IR is repeated. On the other hand, when low resolution printing is instructed, a so-called one-side scanning mode is executed in which only the operation of scanning the spot in the (+ X) direction to the effective image area IR and forming a latent image in the effective image area IR is repeated. To form a latent image. Thus, in this embodiment, the spot scanning mode is switched between high resolution printing and low resolution printing based on the resolution information.

  In this embodiment, the scanning direction and the arrangement position of the drive motor MT are set in advance so as to satisfy the following relationship. That is, the drive motor MT is disposed on the downstream side in the scanning direction (+ X). Also, as shown in FIG. 3, the end of the scanning path of the scanning light beam is guided to the horizontal synchronization sensor 60 by the folding mirror 69 on the upstream side in the scanning direction (+ X). The folding mirror 69 is disposed at the end of the scanning region SR on the upstream side in the scanning direction (+ X), and moves on the upstream side in the scanning direction (+ X) within the scanning region SR and outside the effective image region IR. The scanning light beam to be guided is guided to the horizontal synchronization sensor 60. A signal is output from the horizontal synchronization sensor 60 at a timing when the scanning light beam is received by the horizontal synchronization sensor 60 and passes through the sensor position (amplitude angle θs). Thus, in this embodiment, the horizontal synchronization sensor 60 is used as a horizontal synchronization reading sensor for obtaining a synchronization signal when the light beam scans the effective image area IR in the main scanning direction X, that is, the horizontal synchronization signal Hsync. The latent image forming operation is controlled based on the horizontal synchronization signal Hsync. Hereinafter, a latent image forming operation in the apparatus according to the present embodiment will be described.

  When a print command is input from an external device such as the host computer 100, a latent image is formed on each photoconductor according to the flowchart shown in FIG. 8, and a color image is formed based on each latent image. That is, in step S10, resolution information included in the print command is acquired (information acquisition step). Then, based on the resolution information, it is determined whether the print command requests high resolution printing or low resolution printing (step S11).

  If “YES” is determined in the step S11, that is, if it is determined that the low-resolution printing is performed, steps S16 to S19 are executed to form an image with a low resolution, and the image is transferred to the sheet S and the printing process is ended. First, in step S16, the one-side scanning mode is set (scanning mode setting step). Next, the sub-scanning line screen angle in the one-side scanning mode (one-side scanning line screen angle) is set (step S17). Here, the sub-scanning line screen angle is an angle formed by each line of the line screen of each color component and the sub-scanning direction Y. In this embodiment, as shown in FIG. 9A, the sub-scanning line screen angle MYK at the time of one-side scanning of each color component yellow (Y), magenta (M), cyan (C), and black (K). , MMK, MCK, MKK are set. The reason why the sub-scanning line screen angles MYK, MMK, MCK, and MKK during one-side scanning of each color component are different is to suppress the occurrence of so-called moire fringes. Such moire fringes are known to occur when a sub-scanning line screen angle shifts between colors. In order to make the moire fringes inconspicuous, it has been confirmed from experience that it is optimal that the sub-scanning line screen angle is shifted by about 30 ° between the two colors. Since yellow (Y) is less noticeable to human eyes than other colors, sub-scanning line screen angles of cyan (C), magenta (M), and black (K) other than yellow (Y) Are shifted by 30 °. Therefore, in this embodiment, when setting the one-side scanning sub-scanning line screen angle, the yellow (Y) one-side scanning sub-scanning line screen angle MYK is 90 °, and the cyan (C) one-side scanning sub-scanning is performed. The line screen angle MCK was set to 75 °, the black (K) one-side scanning sub-scanning line screen angle MKK was set to 45 °, and the magenta (M) one-side scanning sub-scanning line screen angle MMK was set to 15 °.

  Further, a scanning mode switching signal corresponding to the scanning mode determined as described above is given to the scanning mode switching unit 116C of the main controller 11 (step S18). On the other hand, the scanning mode switching unit 116C that has received these instructions forms a latent image line by line by fixing the read timing and order of gradation data from the line buffer. That is, only the spot scanned in the first direction while being read out from the forward direction line buffer 116A at an appropriate timing and forward direction (that is, in the order of gradation data DT1, DT2,... DTn) and optically modulated based on each gradation data. Is scanned on the photosensitive member 2 to form a latent image (step S19). In this way, a so-called one-side scanning mode is executed, and a latent image is formed at a low resolution. Further, the latent image formed in this way is developed with toner to form a four-color toner image, and is superimposed on the intermediate transfer belt 71 to form a color image, and then the color image is formed on the sheet S. The image is transferred and the low resolution printing is completed.

  If “NO” is determined in the step S11, that is, it is determined that high resolution printing is performed, steps S12 to S15 are executed to form an image with high resolution, transferred to the sheet S, and the printing process is ended. First, in step S12, the double-sided scanning mode is set (scanning mode setting step). Next, a sub-scanning line screen angle in the double-sided scanning mode (sub-scanning line-screen angle during double-sided scanning) is set (step S13). In this embodiment, as shown in FIG. 9B, the sub-scanning line screen angle MYR at the time of both-side scanning of each color component yellow (Y), magenta (M), cyan (C), and black (K). , MMR, MCR, MKR. That is, the angle between the color components is kept constant, and for each color component, the sub-scanning line screen angle at the time of double-side scanning is set to be smaller than the sub-scanning line screen angle at the time of one-side scanning. More specifically, the yellow (Y) side scanning sub-scanning line screen angle MYR is 75 °, the cyan (C) side scanning sub-scanning line screen angle MCR is 30 °, and the black (K) both sides. The sub-scanning line screen angle MKR at the time of scanning is set to 60 °, and the sub-scanning line screen angle MMR at the time of both-side scanning of magenta (M) is set to 0 °.

  Further, a scanning mode switching signal corresponding to the scanning mode determined as described above is given to the scanning mode switching unit 116C of the main controller 11 (step S14). On the other hand, upon receiving these instructions, the scanning mode switching unit 116C alternately switches the timing and order of reading out the gradation data from the line buffer for each line. Thereby, a high-resolution latent image is formed as follows. That is, the spot is scanned in the effective image area IR in the (+ X) direction to form a latent image in the effective image area IR, and the spot is scanned in the effective image area IR in the (−X) direction. The operation of forming an image is repeated alternately (step S15). Thus, a so-called double-sided scanning mode is executed to form a latent image with high resolution. Further, the latent image formed in this way is developed with toner to form a four-color toner image, and is superimposed on the intermediate transfer belt 71 to form a color image, and then the color image is formed on the sheet S. The image is transferred and high-resolution printing ends.

  As described above, in this embodiment, the angle between each line of the line screen and the sub-scanning direction (sub-scanning line screen angle) is smaller in the double-sided scanning mode than in the single-sided scanning mode. The sub-scanning line screen angle in the scanning mode is set. Therefore, it is possible to prevent the thin and thick portions of the line latent image from appearing periodically corresponding to the non-uniformity of the scanning pitch in the sub-scanning direction in the double-sided scanning mode. Therefore, satisfactory gradation reproduction can be realized even in the double-sided scanning mode. The reason will be described in detail. In the present embodiment, as described above, the spot is reciprocated in the main scanning direction on the surface of the photoconductor 2 and driven on the surface of the photoconductor 2 in the sub-scanning direction substantially orthogonal to the main scanning direction. Accordingly, the scanning trajectory of the spot on the surface of the photosensitive member 2 in the double-sided scanning mode is as shown by the alternate long and short dash line in FIG. 10, and thus 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. On the other hand, in the present embodiment, gradation reproduction is realized by a line screen that changes the line width of a line extending in a predetermined direction according to the gradation. Therefore, as shown in FIG. 11, when a line latent image extending in a predetermined direction is formed in order to realize gradation reproduction in the double-sided scanning mode, the line latent image corresponds to the non-uniformity of the scanning pitch in the sub-scanning direction. A thin line portion and a thick portion are periodically generated, and an unnecessary pattern is generated. In FIG. 11, a solid line represents a scanning line, and a circle represents a spot formed on the surface of the photoreceptor. Such a pattern hinders the realization of a good gradation reproduction and causes image adverse effects.

  However, in this embodiment, each scanning mode is such that the angle formed between each line of the line screen and the sub-scanning direction (sub-scanning line screen angle) is smaller in the double-sided scanning mode than in the single-sided scanning mode. The sub-scanning line screen angle at is set. With this configuration, it is possible to prevent the thin and thick portions of the line latent image from appearing periodically corresponding to the non-uniformity of the scanning pitch in the sub-scanning direction in the double-sided scanning mode. It becomes possible. The reason for this will be described with reference to FIG. In FIG. 12, the alternate long and short dash line indicates the scanning line, the square indicates the latent image formed at a narrow or wide scanning pitch, and the solid line indicates the direction in which each line of the line screen extends. In FIG. 12, the sub-scanning line screen angle MI is decreased as going from (a) to (d). In FIG. 12A where the angle is the largest, the line width DT of the portion with the narrow scanning pitch is significantly different from the line width DF of the portion with the wide scanning pitch. However, as the sub-scanning line screen angle MI is reduced as shown in FIGS. 12C and 12D, the difference between the line width DT of the narrow scanning pitch portion and the line width DF of the wide scanning pitch portion decreases. You can see that As described above, the difference between the line width DT of the narrow scanning pitch portion and the line width DF of the wide scanning pitch portion can be suppressed by reducing the sub-scanning line screen angle MI. Therefore, in the present embodiment, the angle formed between each line of the line screen and the sub-scanning direction (sub-scanning line screen angle MI) is smaller in the double-sided scanning mode than in the single-sided scanning mode. In the double-sided scanning mode, it is possible to prevent the thin and thick portions of the line latent image from appearing periodically corresponding to the non-uniformity of the scanning pitch in the sub-scanning direction. Therefore, satisfactory gradation reproduction can be realized even in the double-sided scanning mode.

  In this embodiment, the magenta (M) sub-scanning line screen angle MMR in the double-sided scanning mode is 0 °. In this case, as shown in FIG. 12D, the line width DF and the line width DT are substantially the same. That is, in this embodiment, for magenta (M), the angle formed by each line of the line screen in the double-sided scanning mode and the sub-scanning direction (sub-scanning line screen angle during double-sided scanning) is set to 0 °. The effect on the gradation reproducibility caused by the non-uniformity of the scanning pitch in the sub-scanning direction in the double-sided scanning mode is completely suppressed, and good gradation reproduction is realized.

<Second Embodiment>
In the first embodiment, halftoning processing is performed using a line screen, whereas in the second embodiment, halftoning processing is performed using a halftone screen. As shown in FIG. 13, the halftone screen realizes gradation reproduction by growing halftone dots that are discretely arranged in predetermined two directions D1 and D2 in accordance with the gradation. Here, as it goes to (a) to (c) of FIG. 13, the gradation corresponds to a darker one. In addition, since the basic composition of the apparatus concerning 2nd Embodiment is the same as 1st Embodiment, it attaches | subjects the same and equivalent code | symbol, and abbreviate | omits description.

  FIG. 14 is a flowchart illustrating the operation of the image forming apparatus according to the second embodiment. In the second embodiment, when a print command is input from an external device such as the host computer 100, a latent image is formed on each photoconductor according to the flowchart shown in FIG. 14, and a color image is generated based on each latent image. Is formed. That is, in step 20, resolution information included in the print command is acquired (information acquisition process). Based on the resolution information, it is determined whether the print command requests high resolution printing or low resolution printing (step S21).

  If “YES” is determined in the step S21, that is, if it is determined that low resolution printing is requested, the images are formed by executing the steps S26 to S29, transferred to the sheet S, and the printing process is ended. First, in step S26, the one-side scanning mode is set (scanning mode setting step). Next, the sub-scanning halftone screen angle in the one-side scanning mode (sub-scanning halftone screen angle during one-side scanning) is set (step S27). Here, the sub-scanning halftone screen angle is an angle formed by the direction in which the halftone dots of the halftone screen of each color component are arranged and the sub-scanning direction Y. As described above, since halftone dots are arranged in two predetermined directions in the halftone screen, there are two sub-scanning halftone screen angles. Therefore, in the present embodiment, for example, black (K) as a representative example, as shown in FIG. 15A, two sub-scanning halftone screen angles AKK1 and AKK2 are set during one-side scanning. Further, a scanning mode switching signal corresponding to the scanning mode determined as described above is given to the scanning mode switching unit 116C of the main controller 11 (step S28). On the other hand, the scanning mode switching unit 116C that has received these instructions forms a latent image line by line by fixing the read timing and order of gradation data from the line buffer. That is, only the spot scanned in the first direction while being read out from the forward direction line buffer 116A at an appropriate timing and forward direction (that is, in the order of gradation data DT1, DT2,... DTn) and optically modulated based on each gradation data. Is scanned on the photosensitive member 2 to form a latent image (step S29). Thus, a so-called one-side scanning mode is executed to form a latent image. Further, the latent image formed in this way is developed with toner to form a four-color toner image, and is superimposed on the intermediate transfer belt 71 to form a color image, and then the color image is formed on the sheet S. The character is printed after being transferred.

  If “NO” is determined in the step S21, that is, it is determined that high resolution printing is required, the steps S22 to S25 are executed to form an image with a high resolution, transferred to the sheet S, and the printing process is ended. . First, in step S22, the double-sided scanning mode is set (scanning mode setting step). Next, an angle (sub-scanning halftone screen angle) formed by the direction in which the halftone dots of the halftone screen of each color component are arranged and the sub-scanning direction Y in the double-sided scanning mode is set (step S23). In this embodiment, as shown in FIG. 15B, black (K) double-side scanning sub-scanning halftone screen angles AKR1 and AKR2 are set. That is, in the present embodiment, as shown in FIG. 15B, the black (K) is formed by the arrangement direction of the larger arrangement angle between the two arrangement directions and the sub-scanning direction Y and the sub-scanning direction Y. The sub-scan halftone screen angle in the double-sided scanning mode is set so that the angle is smaller in the double-sided scanning mode than in the single-sided scanning mode. In other words, in the one-side scanning mode, the sub-scanning halftone screen angle AKK1 is set as shown in FIG. 15A, and in the both-side scanning mode, as shown in FIG. Sub scanning halftone screen angle AKR1, AKR2 which is larger of both sides scanning subscanning halftone screen angle AKR1 is the larger one of the two side scanning subscreen halftone screen angles AKK1, AKK2. It is set to be smaller than the time sub-scanning halftone screen angle AKK1. Here, the broken line in FIG. 15B indicates the arrangement direction of the halftone dots in the one-side scanning mode for comparison. In the present embodiment, the arrangement direction with the smaller angle formed with the sub-scanning direction Y is common to the one-side scanning mode and the two-side scanning mode. That is, the one-side scanning sub-scanning halftone screen angle AKK2 and the two-side scanning sub-scanning halftone screen angle AKR2 are the same. In this embodiment, the sub-scanning halftone screen angle is changed only for black K between the double-sided scanning mode and the single-sided scanning mode.

  Further, a scanning mode switching signal corresponding to the scanning mode determined as described above is given to the scanning mode switching unit 116C of the main controller 11 (step S24). On the other hand, upon receiving these instructions, the scanning mode switching unit 116C alternately switches the timing and order of reading out the gradation data from the line buffer for each line. Thereby, a latent image is formed as follows. That is, the spot is scanned in the effective image area IR in the (+ X) direction to form a latent image in the effective image area IR, and the spot is scanned in the effective image area IR in the (−X) direction. The operation of forming a latent image is alternately repeated (step S25). In this way, a so-called double-sided scanning mode is executed to form a latent image. Further, the latent image formed in this way is developed with toner to form a four-color toner image, and the color image is formed on the sheet S after being superimposed on the intermediate transfer belt 71 to form a color image. Transfer is completed and photo printing is completed.

  As described above, in this embodiment, the angle formed between the arrangement direction in which the angle between the two halftone dot arrangement directions is large with the sub-scanning direction and the sub-scanning direction is compared with that in the one-side scanning mode in the double-sided scanning mode. The sub-scan halftone screen angle is set so as to be smaller. That is, in the double-sided scanning mode, the larger one of the two side-scanning sub-scanning halftone screen angles is the larger of the two-sided scanning sub-scanning halftone screen angles, and the larger of the two one-sided scanning sub-scanning halftone screen angles Is set to be smaller than the sub-scanning halftone screen angle during one-side scanning. Therefore, it is possible to suppress the appearance of an unnecessary pattern corresponding to the non-uniformity of the scanning pitch in the sub-scanning direction in the double-sided scanning mode. Therefore, satisfactory gradation reproduction can be realized even in the double-sided scanning mode. The reason will be described in detail. In the present embodiment, as described above, the spot is reciprocated in the main scanning direction on the surface of the photoconductor 2 and driven on the surface of the photoconductor 2 in the sub-scanning direction substantially orthogonal to the main scanning direction. Accordingly, the scanning trajectory of the spot on the surface of the photosensitive member 2 in the double-sided scanning mode is as shown by the alternate long and short dash line in FIG. 10, and thus 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 remarkable in the vicinity of the end of the scanning locus in the main scanning direction X. On the other hand, in the present embodiment, gradation reproduction is realized by a halftone screen that grows halftone dots that are discretely arranged in two arrangement directions. Accordingly, as shown in FIG. 16A, when halftone dots arranged in a predetermined direction are formed in order to realize gradation reproduction in the double-sided scanning mode, it corresponds to the non-uniformity of the scanning pitch in the sub-scanning direction. The halftone dot widths DT and DF in the vertical direction with respect to the arrangement direction of the halftone dots are periodically changed to generate an unnecessary pattern. In FIG. 16, a one-dot chain line represents a scanning line, a square represents a halftone dot formed at a narrow or wide scanning pitch, and a solid line represents the arrangement direction of the halftone dots.

  Further, the periodic fluctuation of the halftone dot width as described above depends on the angle formed by the halftone dot arrangement direction and the sub-scanning direction. This will be described with reference to FIG. In FIG. 16, the angle formed by the halftone dot arrangement direction and the sub-scanning direction Y (sub-scanning halftone screen angle AI) decreases as going from (a) to (d). In FIG. 16A where the angle is the largest, the halftone dot width DT of the narrow scan pitch portion and the halftone dot width DF of the wide scan pitch portion are significantly different. However, as shown in FIGS. 16C and 16D, as the angle between the arrangement direction of the halftone screen and the sub-scanning direction (sub-scanning halftone screen angle AI) becomes smaller, the halftone dot in the narrower scanning pitch. It can be seen that the difference between the width DT and the halftone dot width DF at the wide scanning pitch is reduced. Thus, the difference between the halftone dot width DT at the narrow scanning pitch portion and the halftone dot width DF at the wide scanning pitch portion becomes smaller as the sub-scanning halftone screen angle AI is smaller. Therefore, in the present embodiment, of the two arrangement directions existing on the halftone screen, the arrangement direction and the sub-scanning direction are arranged with respect to the arrangement direction in which the angle formed with the sub-scanning direction is large and the variation in the halftone dot width is large. The sub-scan halftone screen angle is set so that the angle formed is smaller in the double-sided scanning mode than in the single-sided scanning mode. That is, the larger one of the two sub-scanning halftone screen angles is set smaller in the double-sided scanning mode than in the one-sided scanning mode. Therefore, it is possible to suppress the appearance of an unnecessary pattern corresponding to the non-uniformity of the scanning pitch in the sub-scanning direction in the double-sided scanning mode. Therefore, satisfactory gradation reproduction can be realized even in the double-sided scanning mode.

<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 first embodiment, for all the color components yellow (Y), magenta (M), cyan (C), and black (K), the angle formed between each line of the line screen and the sub-scanning direction (sub-scanning). The sub-scanning line screen angle is set so that the line screen angle) is smaller in the double-sided scanning mode than in the single-sided scanning mode, but specific color components can be selected and executed as appropriate. For example, the setting of the sub-scanning line screen angle may be executed only for a color component having a large sub-scanning line screen angle. Further, the setting of the sub-scanning line screen angle may be executed only for the color components in which generation of unnecessary patterns is conspicuous corresponding to the non-uniformity of the scanning pitch as described above.

  In the first embodiment, only for black (K), the angle formed by the arrangement direction having a large angle with the sub-scanning direction and the sub-scanning direction of the two halftone dot arrangement directions of the halftone screen is set on both sides. The sub-scanning halftone screen angle is set so that it is smaller in the scanning mode than in the one-side scanning mode, but the scope of application of the present invention is not limited to black (K), The present invention can also be applied to the color components yellow (Y), magenta (M), and cyan (C). For example, the present invention may be applied to all color components yellow (Y), magenta (M), cyan (C), and black (K).

  In the first or second embodiment, the two-sided scanning mode and the one-sided scanning mode are switched based on the required resolution. However, the criterion for switching the scanning mode is not limited to this. The present invention is applicable to all image forming apparatuses configured to be able to switch between the one-side scanning mode.

  In the first and second embodiments, the spot is scanned only in the (+ X) direction in the one-side scanning mode. However, the spot may be scanned in the (−X) direction. In short, the spot may be configured to be unidirectionally scanned in the first direction (+ X) or the second direction (−X) in the main scanning direction X.

  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 needless to say that the present invention can be implemented with appropriate modifications within a range that can meet the gist of the preceding and following descriptions. These are all included in the technical scope of the present invention.

  In all of the embodiments described below, gradation reproduction is performed using a so-called dither method. Here, before explaining each embodiment, gradation reproduction by the dither method will be described with reference to FIG. In the dither method, gradation reproduction is performed by arranging a threshold matrix as shown in FIG. 17A, for example, as shown in FIG. Each element of the threshold value matrix corresponds to one pixel, and each element has a predetermined threshold value. Then, the density value of the correction gradation data is compared with the threshold value of each element of the threshold value matrix, and it is determined whether or not a latent image is formed on the pixel corresponding to each element. Specifically, for example, when the density value of the correction gradation data is 4, a latent image is formed on a pixel corresponding to an element having a threshold value of 4 or less. When the density value of the correction gradation data is 8, a latent image is formed on a pixel corresponding to an element having a threshold value of 8 or less. In this way, gradation reproduction is performed by changing the number of pixels forming a latent image in accordance with each density value.

<First embodiment>
In the first embodiment, gradation reproduction is performed using a multi-line screen, and for black K, the sub-scan line screen angle in the single-side scan mode is 45 °, and the sub-scan line screen angle in the double-side scan mode is 26.6. It is switched with °. Hereinafter, means for realizing a line screen having such a sub-scanning line screen angle will be described with reference to FIGS. 17 and 18. FIG.

  FIG. 17 is an explanatory diagram of a means for realizing the sub-scanning line screen angle in the one-side scanning mode in the first embodiment. In this embodiment, the gradation table shown in FIG. 17A is arranged as shown in FIG. 17B, and the size of one pixel is 42.3 μm in both the main scanning direction and the sub-scanning direction. With this configuration, as shown in FIG. 17C, a line screen is realized in which a line extending at 45 ° with respect to the sub-scanning direction becomes thicker as the density value increases.

  FIG. 18 is an explanatory diagram for the means for realizing the sub-scanning line screen angle in the double-sided scanning mode in the first embodiment. In the present embodiment, the gradation table shown in FIG. 18A is arranged as shown in FIG. 18B, the size of one pixel is 42.3 μm in the main scanning direction, and the both-side scanning mode is in the sub-scanning direction. Corresponding to switching to 21.2 μm, which is approximately half of the one-side scanning mode. With this configuration, as shown in FIG. 18C, a line screen is realized in which the line extending 26.6 ° with respect to the sub-scanning direction becomes thicker as the density value increases.

  As described above, in the first embodiment, for black (K), the sub-scanning line screen in the single-sided scanning mode is set to 45 °, whereas the sub-scanning line screen in the double-sided scanning mode is set to 26.6 °. It is said. As described above, in the first embodiment, the angle formed between each line of the black (K) line screen and the sub-scanning direction (sub-scanning line screen angle) is compared in the double-sided scanning mode and in the single-sided scanning mode. It is configured to be smaller. Therefore, it is possible to prevent the thin and thick portions of the line latent image from appearing periodically corresponding to the non-uniformity of the scanning pitch in the sub-scanning direction in the double-sided scanning mode. Therefore, satisfactory gradation reproduction can be realized even in the double-sided scanning mode.

<Second embodiment>
In the second embodiment, gradation reproduction is performed by a halftone screen, and the sub-scanning halftone screen angle in the one-side scanning mode is set to 14.04 ° and 75.7 ° for yellow (Y), and the two-side scanning mode is used. The sub-scan halftone screen angle is switched between 18.43 ° and 71.57 °. The means for realizing a halftone screen having such a sub-scanning halftone screen angle will be described below with reference to FIGS.

  FIG. 19 is an explanatory diagram for means for realizing the sub-scanning halftone screen angle in the one-side scanning mode in the second embodiment. In this embodiment, the gradation table shown in FIG. 19A is arranged as shown in FIG. 19B, and the size of one pixel is 42.3 μm in both the main scanning direction and the sub-scanning direction. With this configuration, as shown in FIG. 19C, halftone dots arranged in the directions of 14.04 ° and 75.7 ° with respect to the sub-scanning direction grow as the density value increases. A halftone screen is realized.

  FIG. 20 is an explanatory diagram of a means for realizing the sub-scanning halftone screen angle in the double-sided scanning mode in the second embodiment. In this embodiment, the gradation table shown in FIG. 20A is arranged as shown in FIG. 20B, the size of one pixel is 42.3 μm in the main scanning direction, and the double-sided scanning mode is in the sub-scanning direction. Corresponding to switching to 21.2 μm, which is approximately half of the one-side scanning mode. With this configuration, as shown in FIG. 20C, halftone dots arranged in the direction of 18.43 ° and 71.57 ° with respect to the sub-scanning direction grow as the density value increases. A halftone screen is realized.

  As described above, in the second embodiment, for yellow (Y), the angle formed by the sub-scanning direction, which is the larger angle between the two dot arrangement directions and the sub-scanning direction, is set in the one-side scanning mode. Is 75.7 ° in the double-sided scanning mode. As described above, in the second embodiment, for yellow Y, the angle formed between the arrangement direction having a large angle with the sub-scanning direction and the sub-scanning direction among the two dot arrangement directions is one side in the double-sided scanning mode. It is configured to be smaller than in the scanning mode. That is, the larger one of the two sub-scanning halftone screen angles of the halftone screen is set to be smaller in the double-sided scanning mode than in the single-sided scanning mode. Therefore, it is possible to prevent the halftone dot width from periodically changing and generating unnecessary patterns. Therefore, satisfactory gradation reproduction can be realized even in the double-sided scanning mode.

1 is a diagram showing an 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. The figure which shows the scanning area | region of the light beam in the exposure unit of FIG. FIG. 2 is a diagram showing a signal processing block in the image forming apparatus of FIG. 1. Explanatory drawing of a line screen. FIG. 2 is a diagram showing a line latent image formed by the image forming apparatus of FIG. 1. 6 is a flowchart illustrating the operation of the image forming apparatus according to the first embodiment. The figure which shows the subscanning line screen angle of 1st Embodiment. The figure which shows the scanning pitch at the time of double-sided scanning mode. Explanatory drawing about the pattern resulting from the nonuniformity of a scanning pitch. The figure which shows the relationship between the angle which a line screen and the subscanning direction make, and line | wire width. Explanatory drawing of a halftone screen. 9 is a flowchart showing the operation of the image forming apparatus according to the second embodiment. The figure which shows the sub-scanning halftone screen angle of 2nd Embodiment. The figure which shows the relationship between the angle which the arrangement | sequence direction of a halftone dot and the subscanning direction make, and halftone dot width. The figure which shows the subscanning line screen angle of the single side scanning mode of 1st Example. The figure which shows the subscanning line screen angle of the double-sided scanning mode of 1st Example. The figure which shows the subscanning halftone screen angle of the one side scanning mode of 2nd Example. The figure which shows the subscanning halftone screen angle of the both-sides scanning mode of 2nd Example.

Explanation of symbols

  2, 2Y, 2M, 2C, 2K ... photosensitive member (latent image carrier), 6, 6Y, 6M, 6C, 6K ... exposure unit (latent image forming unit), 60, 60A to 60C ... horizontal synchronization sensor (detector) 62, 62Y, 62M, 62C, 62K ... laser light source, 71 ... intermediate transfer belt (transfer medium), 651 ... deflection mirror surface, DT1, DT2, DT (n-1), DTn ... gradation data (image information) ), IR ... effective image area, Ly, Lm, Lc, Lk ... scanning light beam, LI (+ X), LI (-X) ... line latent image, MT ... drive motor (drive means), PT, PT1, PT2 , PT3 ... scanning pitch, SR ... scanning region, X ... main scanning direction, Y ... sub-scanning direction

Claims (6)

A latent image carrier driven in a first direction;
Latent image forming means for forming a latent image by scanning in a second direction substantially perpendicular to the first direction on the latent image bearing member reflects the light beam by the deflection mirror to vibrate,
And a scan mode control means for selectively switching the sides scan mode to scan the one side scanning mode and the one and the other of both the one and opposite said scanning only one of the second direction on the latent image bearing member A control method of an image forming apparatus that performs gradation reproduction by a line screen in an image forming apparatus provided,
An image forming apparatus control method, wherein an angle formed between each line of the line screen and the first direction is smaller in the double-sided scanning mode than in the single-sided scanning mode. The method of controlling an image forming apparatus according to claim 1, wherein an angle formed between each line of the line screen and the first direction is approximately 0 degrees in the double-side scanning mode. The method according to claim 1, wherein the first direction is a sub-scanning direction, and the second direction is a main scanning direction. A latent image carrier driven in a first direction;
Latent image forming means for forming a latent image by scanning in a second direction substantially perpendicular to the first direction on the latent image bearing member reflects the light beam by the deflection mirror to vibrate,
And a scan mode control means for selectively switching the sides scan mode to scan the one side scanning mode and the one and the other of both the one and opposite said scanning only one of the second direction on the latent image bearing member In the image forming apparatus provided, the method of controlling the image forming apparatus performs gradation reproduction using a halftone screen that grows halftone dots that are discretely arranged in the first arrangement direction and the second arrangement direction as the gradation increases. And
Of the first arrangement direction and the second arrangement direction, the angle formed between the first direction and the first direction is larger than the angle formed between the first direction and the first direction. And a small control method for the image forming apparatus. A latent image carrier driven in a first direction;
Latent image forming means for forming a latent image by scanning in a second direction substantially perpendicular to the first direction on the latent image bearing member reflects the light beam by the deflection mirror to vibrate,
And a scan mode control means for selectively switching the sides scan mode to scan the one side scanning mode and the one and the other of both the one and opposite said scanning only one of the second direction on the latent image bearing member An image forming apparatus that performs gradation reproduction with a line screen,
An image forming apparatus, wherein an angle formed between each line of the line screen and the first direction is smaller in the double-sided scanning mode than in the single-sided scanning mode. A latent image carrier driven in a first direction;
Latent image forming means for forming a latent image by scanning in a second direction substantially perpendicular to the first direction on the latent image bearing member reflects the light beam by the deflection mirror to vibrate,
And a scan mode control means for selectively switching the sides scan mode to scan the one side scanning mode and the one and the other of both the one and opposite said scanning only one of the second direction on the latent image bearing member An image forming apparatus that performs gradation reproduction by a halftone screen that grows halftone dots that are discretely arranged in the first arrangement direction and the second arrangement direction as the gradation increases.
Of the first arrangement direction and the second arrangement direction, the angle formed between the first direction and the first direction is larger than the angle formed between the first direction and the first direction. An image forming apparatus characterized by being small.

Images