JP2006256148A - Scanning exposure device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a scanning exposure device and an image forming apparatus each capable of forming a high quality image in a resolution realized by dots having sizes smaller than the diameter of a laser beam, wherein variation in density due to deviation of an imaging position on a scanned face or a reciprocity failure. <P>SOLUTION: A plurality of beam-writing positions on a photosensitive body are set and a beam-writing start position is shifted by each scanning line (or each scanning line group). As a result, it is possible to adjust an aspect ratio of a produced image. As a thickness (a width of a line in a sub-scanning direction) of one dot line can be changed, it is possible, for example, to shift the writing start position so as to make the aspect ratio to be one. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a scanning exposure apparatus and an image forming apparatus, and more particularly to a scanning exposure apparatus that turns on a semiconductor laser based on image information and scans and exposes a photoconductor with a laser beam output from the semiconductor laser. The present invention relates to an image forming apparatus including the apparatus.

  An electrostatic system using a laser is generally used for an image forming apparatus such as a copying machine or a printer. In an image forming apparatus using this laser (mainly a semiconductor laser, hereinafter referred to as LD), a scanning exposure apparatus is used which exposes a photosensitive member with a laser beam to form an electrostatic latent image.

  Specifically, an electrostatic latent image is formed by scanning a laser beam modulated on the basis of image data onto a photoconductor whose surface is uniformly charged by a scanning exposure apparatus. After the toner is supplied to the electrostatic latent image and developed, the transfer paper is superimposed on the developed toner image, and the toner is electrostatically adsorbed to the surface of the transfer paper and transferred. Thereafter, heat or pressure is applied to the transfer paper to fix the transferred toner image, thereby forming an image.

  Here, an LD driving method in the scanning exposure apparatus will be described. When forming an electrostatic latent image with a laser beam, the formation of the electrostatic latent image differs depending on the light intensity (light quantity) of the laser beam, so that the laser beam has a predetermined light intensity (a predetermined output light quantity). As such, it is necessary to drive the LD. Further, as shown in FIG. 11, the LD has a characteristic of outputting coherent light when the drive current becomes a predetermined current value (hereinafter referred to as “threshold current”) Ith or more.

  In addition, in order to form an electrostatic latent image on the photosensitive member, a laser beam modulated based on an image signal (Video signal) based on image data, that is, an ON / OFF signal indicating ON / OFF of the LD is output. There is a need.

  Therefore, as shown in FIG. 12, the LD driving circuit 400 includes a current source 404 for supplying a current corresponding to a desired output intensity to the LD 402, and a laser beam output from the LD 402 based on image data. A switching circuit 406 is provided for modulation. The current source 404 supplies a current having a current value corresponding to the set voltage to the LD 402 via the switching circuit 406. The switching circuit 406 supplies / stops the current to the LD 402 based on the Video signal. . Thereby, it is possible to output a laser beam modulated based on image data having a desired light intensity. Such a modulation method is generally called a pulse width modulation (PWM) method.

  On the other hand, there is an intensity modulation (PAM) method as a modulation method that positively utilizes the fact that the formation of a latent image varies depending on the amount of laser output. In the image formation by the PAM method, an image is formed by changing the light emission intensity (that is, output light amount) of the LD based on the image data.

  With the recent progress in digitization technology and the development of image processing technology, the resolution of image forming devices has been increasing, and the resolution (mainly, the resolution of forming images with dots smaller than the laser beam diameter that can be realized with a scanning exposure device). There is an increasing demand for a pure writing density, in other words, a writing density in the sub-scanning direction).

FIG. 13 shows the relationship between the laser beam diameter and resolution of a general scanning exposure apparatus using an LD having an output beam wavelength of 780 nm. In general, the laser beam diameter is defined as the diameter of a point that is 1 / e ² (e: base of natural logarithm) of the light intensity at the center of the beam.

  As shown in FIG. 13, the laser beam diameter of a general scanning exposure apparatus is 60 to 80 nm, whereas the dot size (1 dot size) is a resolution of 600 dpi (dot per inch). Is required to be about 42 μm, and in order to obtain a resolution of 1200 dpi, it is required to be about 21 μm. That is, even at the current main resolution of 600 dpi, the dot size is already smaller than the laser beam diameter.

  When the dot size is smaller than the laser beam diameter in this way, the ratio of the scanning line thickness in the main scanning direction and the sub-scanning direction (hereinafter referred to as “aspect ratio”) and the reproducibility of the intermediate color are remarkably deteriorated. The image quality will deteriorate.

  Hereinafter, the aspect ratio will be described in detail. The ideal aspect ratio (normal value) is 1, that is, as shown in FIG. 14A, even if a line having the same number of dots is drawn in the vertical direction (sub-scanning direction), it is drawn in the horizontal direction (main scanning direction). However, the ideal state (normal state) is the same thickness.

  In general, in image formation by the PWM method, the thickness of a line drawn in the horizontal direction (horizontal line) mainly depends on the laser beam diameter Ds (see FIG. 14C) in the sub-scanning direction. The thickness of the line (vertical line) drawn in the vertical direction mainly depends on the lighting time of the LD, and the lighting time of one dot becomes shorter as the resolution is increased.

  Therefore, when the dot size is smaller than the laser beam diameter, the horizontal line has the thickness determined by the laser beam diameter Ds in the sub-scanning direction as shown in FIG. It is thicker than the dot size, and the vertical line becomes thinner.

  More specifically, when the electrostatic latent image is developed, a larger amount of toner adheres to a portion where the exposure amount of the photoreceptor is larger, and a smaller amount of toner adheres if the exposure amount is smaller. When the laser beam is scanned and exposed to the photoconductor, as shown in FIG. 15, when the LD is driven with an ideal light output waveform, the exposure amount of the photoconductor near the LD lighting start position is larger than the predetermined exposure amount. Also lower. When the exposure amount is small, the latent image formed on the photoconductor becomes shallow, and the amount of toner adhering is insufficient for the amount of toner necessary for printing (shaded area), so-called image skipping occurs. For this reason, the vertical line formed by turning on the LD for a short time becomes thinner as the resolution increases.

  If the LD lighting time is lengthened, it is possible to make the vertical line thicker, but the lighting time of 1 dot is naturally determined by the optical design depending on the resolution, so the LD lighting time can be increased even if the vertical line is thickened. You can't make it too long. Also, if the light intensity (output light amount) of the LD is reduced, the horizontal line can be made thinner, but the vertical line also becomes thinner accordingly.

  Eventually, the higher the resolution, the shorter the lighting time for one dot determined by the optical design and the narrower the vertical line, so the aspect ratio is getting worse. In addition, the shape of one dot formed by a toner image tends to be vertically long as the resolution is increased, and the reproducibility of one dot is deteriorated.

  Further, the deterioration of the aspect ratio and the reproducibility of 1 dot also affects the reproducibility of the halftone. For example, when dither area gradation is used, the number of pixels to be filled with dots in one screen cell consisting of a plurality of vertical and horizontal pixels (subpixels) as shown in FIG. The shade is expressed by the painted area.

  However, if the shape of one dot becomes vertically long as described above, as shown in FIG. 16B, the pixel cannot be filled well, and the filling area in one screen cell becomes small, and the high The light reproducibility will deteriorate. In addition, as shown in FIG. 16C, adjacent dots in the vertical direction overlap each other, the smoothness of density is disturbed (so-called tone jump), and when dots are arranged in the horizontal direction, the density becomes high. In addition, even when a line screen is used, highlight reproducibility is impaired for the same reason.

  Although it is conceivable to solve these problems by the PAM method, the higher the resolution, the faster the operation time is required, and more current sources are required to cope with the minute light intensity setting. . Therefore, the configuration of the drive circuit becomes complicated and the amount of power consumption increases, which is disadvantageous in terms of cost.

  Although it is conceivable to make the laser beam diameter narrow, it is not possible to easily change the laser beam diameter from the viewpoint of its characteristics and cost in the LD that has a wavelength of 780 nm that is currently generally used. Even if an LD having a wavelength shorter than 780 nm can be used, if the resolution is further increased, the same problem will be encountered.

  At present, these problems are made inconspicuous by changing the process conditions of electrophotography. However, as the resolution increases further in the future, it is easily expected that it will be difficult to solve with the process conditions alone. Solution is needed.

  As a method for solving this, there is a technique described in Patent Document 1, for example. In Patent Document 1, by utilizing the fact that the state of the latent image to be formed varies depending on the exposure amount of the photoconductor, exposure is performed with a light intensity stronger than usual, so that conventionally only a shallow and narrow latent image is formed. This makes it possible to obtain a deep and wide latent image for those that could not. Specifically, an overshoot is generated at the start of LD lighting, and the rising light intensity is made stronger than the steady light intensity (see FIG. 17A) (see FIG. 17B), so that the aspect ratio and intermediate color are increased. It has been shown to improve the reproducibility of.

However, in the means for generating this overshoot, the light amount at the time of overshoot must be set below the high amount range where the LD is destroyed, and the light amount upper limit value inherent to the LD is inherent. Since the amount of light during normal operation must be set low, an LD with a large output must be selected as a result. Moreover, measures such as VCCI are also required due to overshoot.
JP 2001-96794 A

  The present invention has been made to solve the above-described problems, and an object thereof is to provide a scanning exposure apparatus and an image forming apparatus capable of forming a high-quality image at a resolution in which the dot size is smaller than the laser beam diameter. And

  According to the first aspect of the present invention, there is provided a scanning exposure apparatus that turns on a semiconductor laser based on image information, and scans and exposes a photosensitive member with a laser beam output from the semiconductor laser. There are a plurality of write start positions in the main scanning direction.

  That is, since there are a plurality of laser beam writing start positions on the photosensitive member, that is, scanning line starting positions, a wide latent image is formed on the photosensitive member in the scanning direction (main scanning direction). be able to. For this reason, the line width in the sub-scanning direction is increased, the aspect ratio is improved, and a high-quality image can be obtained.

  According to a second aspect of the present invention, in the first aspect of the present invention, the scanning line group composed of a plurality of the laser beams is deflected and scanned by one deflection scanning, and the sub-scanning direction orthogonal to the main scanning direction is performed. The write start position is different in the scanning line group adjacent to the.

  In this way, by changing the writing start position for each adjacent scanning line group, the cycle in which the line width in the sub-scanning direction fluctuates is shortened, so that a higher quality image can be obtained.

  The invention according to claim 3 is characterized in that, in the invention according to claim 1 or 2, the writing start position is different in a scanning line adjacent to the sub-scanning direction orthogonal to the main scanning direction.

  In this way, by changing the writing start position for each adjacent scanning line, for example, the cycle in which the line width in the sub-scanning direction fluctuates is shorter than in the configuration in which the writing start position is changed for each scanning line group. Therefore, a higher quality image can be obtained.

  According to a fourth aspect of the present invention, in the invention according to any one of the first to third aspects, the writing start position periodically varies in a sub-scanning direction orthogonal to the main scanning direction. Features.

  As a result, a higher quality image can be obtained as compared with a configuration in which the writing start position varies without periodicity.

  According to the invention described in claim 5, in the invention described in any one of claims 1 to 4, the scanning line interval on the photosensitive member determined by the writing density in the sub-scanning direction orthogonal to the main scanning direction is A. When the beam diameter on the photosensitive member is B, 2A ≦ B.

  In this way, by satisfying 2A ≦ B, it is possible to obtain a high-quality image without interrupting the scanning line in the sub-scanning direction.

  According to a sixth aspect of the present invention, in the first aspect of the present invention, the scanning line interval on the photosensitive member determined by the writing density in the sub-scanning direction orthogonal to the main scanning direction is A. When the deviation amount of the writing position in the sub-scanning direction is C, 0 <C ≦ 2A.

  In this way, by satisfying 0 <C ≦ 2A, it is possible to prevent the undulation of each scanning line in the sub-scanning direction and obtain a higher quality image.

  The invention according to claim 7 is characterized in that, in the invention according to any one of claims 1 to 6, the writing start position is changed according to the type of image.

  Since the writing start position can be changed and optimized according to the type of image, a higher quality image can be obtained.

  The invention described in claim 8 is characterized in that, in the invention described in claim 7, the writing start position is made constant for each scanning line in the image image, and is made different for each scanning line in the text image.

  As a result, in a text image in which the deviation from the normal value of the aspect ratio is likely to occur, a high-quality image can be obtained by optimizing the aspect ratio. In addition, in the case of an image image in which a difference in aspect ratio does not become apparent as compared with a text image, it is possible to easily obtain an image with a constant writing start position.

  According to the ninth aspect of the present invention, the scanning exposure apparatus according to any one of the first to eighth aspects and the electrostatic latent image formed on the photosensitive member by the scanning exposure apparatus are electrostatically Development means for developing and forming a toner image on the photosensitive member; and transfer means for transferring the toner image to a transfer member and forming an image on the transfer member.

  Therefore, the electrostatic latent image formed on the photosensitive member by the scanning exposure device is developed by the developing means, and a toner image is formed. This toner image is transferred to a transfer member by a transfer unit, and an image is formed on the transfer member.

  Since the scanning exposure apparatus according to any one of claims 1 to 8 is provided, the line width in the sub-scanning direction is increased, the aspect ratio is improved, and a high-quality image can be obtained.

  Since the present invention has the above-described configuration, a high-quality image can be formed at a resolution in which the dot size is smaller than the laser beam diameter.

  Next, an example of an embodiment according to the present invention will be described in detail with reference to the drawings.

(overall structure)
FIG. 1 shows an electrophotographic image forming apparatus 10 to which the present invention is applied. As shown in FIG. 1, the image forming apparatus 10 is covered with a casing 12.

  A paper tray 14 is disposed below the image forming apparatus 10. In the paper tray 14, for example, paper 16 of a desired size such as B5 size, B4 size, A4 size, or A3 size is accommodated. A half-moon roller 18A is disposed in the vicinity of the paper discharge portion in the paper tray 14. The half moon roller 18A feeds the sheets 16 supplied to the sheet tray 14 one by one in order from the upper layer. The sheet 16 sent out from the sheet tray 14 is conveyed by a plurality of conveying roller pairs 20 between a photosensitive member 22 and a transfer charging member 24 described later.

  On one side surface of the image forming apparatus 10 (the right side surface in FIG. 1), a manual feed tray 26 for manually inserting the paper 16 as needed is disposed. Similar to the paper tray 14 described above, a half-moon roller 18B is disposed in the vicinity of the paper discharge portion in the manual feed tray 26 so that the paper 16 can be fed out from the upper layer one by one.

  On the side surface (left side surface in FIG. 1) of the image forming apparatus 10 facing the side surface on which the manual feed tray 26 is disposed, a discharge tray 28 for discharging the paper 16 on which a desired image is formed is provided. .

  An image forming unit 30 is provided in the casing 12. An image forming unit 30 is a cylindrical photosensitive drum (hereinafter simply referred to as “photosensitive member”) 22 that rotates at a constant speed in the direction of arrow A shown in FIG. The laser beam (see arrow B in FIG. 1) is sensitized based on data (the image forming apparatus 10 in the present embodiment targets a black and white image, and is converted into grayscale image data by performing image processing). A scanning exposure device 32 that irradiates the body 22 and a fixing device 34 that fixes a desired image on the paper 16 are included. In the present embodiment, the rotational peripheral speed of the photosensitive member 22 is 120 mm / s.

  A charger 36 is disposed in the vicinity of the peripheral surface of the photoreceptor 22. The charger 36 is configured to uniformly charge the photosensitive member 22. Specifically, an AC voltage having a peak-to-peak value of 2 kV and a grid voltage of −500 V are applied to the charger 36, and the surface of the photosensitive member 22 that has passed through the charging portion by the charger 36 is In this way, it is charged to -500V.

  The photosensitive member 22 uniformly charged by the charger 36 is irradiated with a laser beam by being rotated in the direction of arrow A shown in FIG. As a result, a latent image is formed on the photoreceptor 22. Specifically, positive charges are generated from the photosensitive layer of the photosensitive member 22 in the exposed portion, and a −150 V latent image is formed. The latent image potential of −150 V referred to here is a convergence potential when the entire surface is exposed by the scanning exposure device 32, that is, when the entire image area is scanned.

  Further, a developing unit 38 that supplies toner to the photosensitive member 22 is disposed opposite to the circumferential surface of the photosensitive member 22 on the downstream side in the rotation direction of the photosensitive member 22 from the irradiation position of the laser beam by the scanning exposure device 32. Has been. The toner supplied from the developing unit 38 is attached to the portion irradiated with light from the direction of arrow B shown in FIG. As a result, a toner image is formed on the photoreceptor 22.

  Specifically, the developing device 38 includes a developing roll 39 disposed with a spacing of 0.35 mm with respect to the surface of the photoreceptor 22, and a thin layer of toner is carried on the surface of the developing roll 39 to provide the photoreceptor 22. It is rotating according to the rotation. A developing bias voltage in which a DC voltage and an AC voltage are superimposed is applied to the developing roll 39. Specifically, a rectangular wave having a DC component of −400 V, an AC voltage of 2400 Hz, a peak-to-peak value of 1.6 kV, and a duty ratio of 50:50 is used.

  The latent image formed on the photosensitive member 22 has a background portion of −500 V and a latent image portion of −150 V. Toner charged to a negative polarity by a developing bias voltage (−400 V) is used in the background portion. An electric field force is applied from 22 to the developing roll 39 side and from the developing roll 39 to the photosensitive member 22 side in the latent image portion. By this electric field force, the toner is electrostatically attracted to the latent image portion of the photosensitive member 22 to form a toner image.

  In this embodiment, the image forming apparatus is configured such that the photoconductor 22 and the developing roll 39 are not in contact with each other. However, a contact type image forming apparatus in which the photoconductor 22 and the developing roll 39 are in contact may be used. .

  Further, the toner is a magnetic one-component toner that is charged to a negative polarity by passing through a pressure contact portion with a layer thickness regulating member pressed against the developing roll 39 when the developing roll 39 rotates. In the case of a magnetic developer (toner), the toner is constrained and held on the surface of the developing roll 39 by a magnet member included in the developing roll 39, but non-magnetic toner and secondary component toner are also charged by the charge of each particle. It is held by the generated mirror image force on the surface of the developing roll 39. Needless to say, development is performed using the difference between the potential of the surface of the photosensitive member 22 and the developing bias potential applied to the developing roll 39. As long as it uses an AC voltage, it is not limited to a magnetic one-component toner, and may be a non-magnetic toner or a secondary component toner. Also, the charging polarity is set to the minus side in the present embodiment, but may be set to the plus side.

  A charging member 24 for transfer is disposed on the downstream side in the rotation direction of the photosensitive member 22 (position where the axial center of the photosensitive member 22 hangs down) from the position where the developing unit 38 is disposed, facing the peripheral surface of the photosensitive member 22. ing. The transfer charging member 24 transfers the toner image formed on the photosensitive member 22 to the paper 16.

  A cleaner 40 is disposed opposite to the photosensitive member 22 on the downstream side in the rotational direction of the photosensitive member 22 from the position where the transfer charging member 24 is disposed. The toner remaining on the surface of the photoreceptor 22 after the transfer is removed by the cleaner 40.

  The paper 16 to which the toner image is transferred is conveyed in the direction of arrow C shown in FIG. A fixing unit 34 including a pressure roller 42 and a heating roller 44 is disposed on the downstream side of the photosensitive member 22 in the conveyance direction of the paper 16. The fixing device 34 heats and pressurizes the paper 16 on which the conveyed toner image is transferred, and melts and fixes the toner. In other words, the fixing device 34 performs a so-called fixing process, and a predetermined image is formed on the paper 16.

(Detailed configuration of scanning exposure apparatus)
Next, the scanning exposure apparatus 32 will be described with reference to FIG. FIG. 2 shows a schematic configuration of the scanning exposure apparatus 32. The scanning exposure device 32 outputs a single laser beam.

  The scanning exposure device 32 includes an LD 50 as a light source, and a rotating polygon mirror 52 that reflects the laser beam emitted from the LD 50 and irradiates the photosensitive member 22 with the laser beam.

  The LD 50 is connected to an LD driving circuit 54 and controlled so that a laser beam is emitted based on image data. A collimator lens 56 is disposed on the downstream side in the traveling direction of the laser beam emitted from the LD 50. The collimator lens 56 converts the laser beam emitted from the LD 50 from a diffused beam to a parallel beam. The laser beam converted into parallel rays by the collimator lens 56 is incident on the rotary polygon mirror 52 via the cylinder lens 58.

  The rotating polygon mirror 52 is formed in a regular polygonal shape (regular hexagon in this embodiment) having a plurality of reflecting surfaces 52A on the side surface, and the incident laser beam converges on the reflecting surface 52A. ing.

  The rotary polygon mirror 52 is pivotally attached to a motor (not shown), and rotates in the direction of arrow D about the rotary shaft 60. That is, the incident angle of the laser beam on each reflecting surface 52A is continuously changed and deflected. Accordingly, the photosensitive member 22 is irradiated with a laser beam by scanning in the axial direction of the photosensitive member 22 (in the direction indicated by the arrow E in FIG. 2, hereinafter referred to as “main scanning direction”).

  In the traveling direction of the laser beam reflected by the rotary polygon mirror 52, an fθ lens 62 composed of a first lens 62A and a second lens 62B is disposed. With this fθ lens 62, the scanning speed when irradiating the photosensitive member 22 with the laser beam becomes equal, and an imaging point is formed on the peripheral surface of the photosensitive member 22.

  The laser beam that has passed through the fθ lens 62 is bent by the reflection mirror 64 and applied to the photosensitive member 22. A mirror 66 is arranged in the traveling direction of the laser beam and on the upstream side in the main scanning direction (the leftmost end direction of the photosensitive member 22 in FIG. 2). A photodetector 68 is disposed in the direction of reflection of the laser beam by the mirror 66.

  Each time the photosensitive member 22 is scanned in the axial direction, the laser beam traveling in the leftmost direction of the photosensitive member 22 is reflected by the mirror 66 and enters the photodetector 68. That is, the photodetector 68 can detect the irradiation start timing (so-called SOS: Start of Scan) for each line of the photosensitive member 22 by the scanning exposure device 32.

  In the image forming apparatus 10 of the present embodiment, when scanning exposure is performed with a laser beam by the scanning exposure device 32 to form an image, one pixel is formed by turning on the LD 50 for 21 μm movement time. That is, as shown in FIG. 3, the LD 50 is extinguished by moving the beam center by 21 μm in the scanning direction with respect to one theoretical pixel.

  Therefore, on the photosensitive member 22, there is a most overlapped portion where the beam is always irradiated from the start to the end of the irradiation of the laser beam, and the photoelectric effect is the largest and the most positive charges are generated in the most overlapped portion. Therefore, the latent image potential rises most. In the developing unit 38, as described above, the development bias is used to develop an arbitrary level of the latent image potential as a threshold value. Conventionally, the surface of the photosensitive drum 22 is scanned when a line of 1 dot (pixel) width (hereinafter referred to as “1 dot line”) is scanned in the main scanning direction and when a 1 dot line is scanned in the sub scanning direction. As shown in the shaded area in FIG. 4, the width of the latent image potential at the most overlapped part of the laser beams formed in FIG. 4 clearly differs (the actual latent image potential threshold value is It is set to an optimum value according to the development characteristics of the forming apparatus).

  Here, in the scanning exposure apparatus 32 of this embodiment, a plurality of beam writing positions on the photosensitive member 22 are set, and the beam writing start position is made different for each scanning line (or for each scanning line group described later). It can be (shifted).

  FIG. 5 shows an actual electrostatic latent image (upper stage) and an image of one dot line in the sub-scanning direction after development (lower stage) when the beam writing start position on the photosensitive member 22 is shifted for each scanning line in this way. ) Is conceptually shown. In FIG. 5 (and FIG. 6 described later), gray shades represent the depth of the latent image as an image. In FIG. 5, in order to make it easy to understand how the latent image potential of one dot line in the sub-scanning direction changes and the thickness of one dot line where the latent image is developed and fixed on the paper, the threshold value is set. It is shown as 80 μm and 1 dot 21 μm. FIG. 5C shows a case where the conventional writing position is not shifted. FIG. 5B shows a case where the writing start position is shifted every other line, and FIG. 5A shows a case where 21 μm is written every other line. This is a case where the start position is shifted.

  As can be seen by comparing FIGS. 5A to 5C, the aspect ratio of the obtained image can be adjusted by shifting the writing position in the main scanning direction, and the thickness of one dot line (sub-scanning). The line width in the direction can be changed. Since the normal value of the aspect ratio is generally set to 1, in the example shown in FIG. 5, when the 21 μm writing start position is shifted as shown in (A), an image with an aspect ratio of 1 is obtained.

  6A and 6B show one-dot lines when the writing position in the main scanning direction is shifted when the threshold value is 80 μm and the diameter of one dot is 21 μm, as in the example shown in FIG. The electrostatic latent image (gray portion) and the thickness of the developed one dot line are conceptually shown. However, unlike the case of FIG. 9, the scanning line group is composed of two lines. The writing start position is made different for each group, that is, every two lines.

  As described above, even when the writing start position is changed every two lines, the aspect ratio of the obtained image can be adjusted, and the line width in the sub-scanning direction can be changed. The number of lines constituting the scanning line group is not limited, and may be three lines or more in addition to the above two lines (in this respect, in the example shown in FIG. 5, one scanning line group is constituted by one line). Can also be considered). However, as the number of lines constituting the scanning line group increases, the cycle in which the line width in the sub-scanning direction fluctuates also increases. From the viewpoint of obtaining a higher quality image, it is preferable to reduce the number of lines. As in the example shown in FIG. 5, it is particularly preferable to shift the write start position for each line.

  In order to change the writing start position in the main scanning direction, that is, to change the beam writing position on the photosensitive member 22 for each scanning line, the timing for sending image information from the controller based on the SOS is set to the scanning line. You should change every. In addition, when a so-called Dual LD that emits two laser beams from one LD is used, the two beam positions in the main scanning direction when irradiating the photosensitive member 22 are shifted by a predetermined value. You can do it. Further, a combination of changing the timing of sending image information from the controller and shifting the positions of the two beams may be used.

  It goes without saying that the exposure start position of each beam on the photosensitive member may be varied using a scanning exposure apparatus having a combination of a plurality of LDs that emit a plurality of single beams.

  FIGS. 7, 8 and 9 show examples in which the writing start position is changed when scanning one, two, and four beams during one deflection scanning. 11A to 13B, (A) is an example in which the writing start position is shifted between the odd-numbered line and the even-numbered line in the sub-scanning direction, and (B) is the first and second in the sub-scanning direction. The 5th, 6th, ..., 3rd and 4th lines. An example in which the writing start position is shifted with respect to the seventh, eighth,... Lines, (C) shows the first, fourth, fifth, eighth,. Second, third. An example in which the writing start position is shifted with respect to the sixth, seventh,... Line, (D) is the first, fifth line, second, third, sixth, seventh in the sub-scanning direction. ,... And the fourth, 87th,. In any example, since the writing start position is shifted with periodicity in the sub-scanning direction, a higher quality image can be obtained as compared with the configuration in which the writing start position is shifted without having such periodicity. Can do.

  FIG. 10 conceptually shows an electrostatic latent image when the beam diameter is substantially equal to the scanning line interval in the sub-scanning direction. As described above, when the beam diameter B is substantially the same as the scanning line interval A in the sub-scanning direction on the photosensitive member 22, one dot line in the sub-scanning direction is interrupted. In order to avoid this, at least 2A ≦ B, that is, the beam diameter B needs to be twice the scanning line interval A. It is preferable to set the scanning line interval A and the beam diameter B so as to satisfy this condition, because one dot line in the sub-scanning direction is continuous without interruption and a high-quality image can be obtained.

  Further, if the deviation amount C of the writing position in the main scanning direction exceeds twice the scanning line interval A, one dot line in the sub scanning direction undulates and the image quality deteriorates, which is not preferable. In order to avoid this, at least C ≦ 2A, that is, the shift amount C must be in a range not exceeding twice the scanning line interval A. It is preferable to set the scanning line interval A and the shift amount C so as to satisfy this condition, because the line undulation in the sub-scanning direction is eliminated and a high-quality image can be obtained. As a matter of course, the deviation amount C needs to be larger than 0 (zero). Therefore, the condition lent to the deviation amount C is 0 <C ≦ 2A.

  In the present invention, the writing position may be randomly shifted as viewed in the sub-scanning direction, but it is preferable to periodically change the writing position because a higher quality image can be obtained.

  In the present invention, the image for shifting the writing start position is not particularly limited. However, in practice, depending on the type of image, there may be a significant deviation from the normal value of the aspect ratio of the image. Therefore, it is preferable that the writing start position is shifted and optimized in accordance with the type of image because a higher quality image can be obtained. For example, in a so-called text image, the deviation from the normal value of the aspect ratio tends to be noticeable. Therefore, it is possible to obtain a high-quality image by shifting the writing start position to optimize the aspect ratio. On the other hand, in the so-called image image, since the deviation of the aspect ratio is less obvious compared to the text image, the image may be easily obtained with the writing start position fixed.

1 is a schematic diagram illustrating an overall configuration of an image forming apparatus according to an embodiment of the present invention. It is the schematic which shows the scanning exposure apparatus of one Embodiment of this invention. It is a conceptual diagram which shows the formation principle of 1 dot by an image forming apparatus. It is a conceptual diagram which shows the 1 dot line of the main / sub-scanning direction formed in the normal case. In this invention, it is a conceptual diagram which shows the electrostatic latent image and image of 1 dot line at the time of shifting the writing start position for every line, (A) is the case where it shifts by 21 micrometers (equivalent to 1 dot at 1200 dpi) , (B) shows a case of shifting by 10 μm, and (C) shows a case of no shifting. In this invention, it is a conceptual diagram which shows the electrostatic latent image and the image of 1 dot line when the writing start position is shifted every two lines, and (A) is when shifted by 10 μm (corresponding to 1 dot at 1200 dpi) , (B) shows a case of shifting by 21 μm. (A) to (D) are explanatory diagrams respectively showing examples of write start positions in the main scanning direction when one beam is deflected and scanned in one scan. (A) to (D) are explanatory diagrams respectively showing examples of writing start positions in the main scanning direction when two beams are deflected and scanned in one scan. (A) to (D) are explanatory diagrams respectively showing examples of write start positions in the main scanning direction when four beams are deflected and scanned in one scan. It is a conceptual diagram which shows an electrostatic latent image in case a beam diameter is equal to a scanning line space | interval. It is a figure which shows the output characteristic of general LD. 1 is a circuit diagram showing a general LD drive circuit used in an image forming apparatus. It is explanatory drawing which shows the relationship between a laser beam diameter and resolution. It is explanatory drawing for demonstrating an aspect ratio, (A) is the ideal state of the image which formed the vertical line and the horizontal line with the same number of dots, (B) is formed when dot size is smaller than a laser beam diameter. (C) shows the spot shape of the laser beam. It is explanatory drawing which shows that the aspect ratio and the reproducibility of 1 dot deteriorate in a normal exposure state with a scanning exposure apparatus. It is explanatory drawing which shows that the reproducibility of a halftone falls in a normal exposure state with a scanning exposure apparatus, (A) is a figure for demonstrating an area gradation, (B), (C) is a laser beam. It is a figure which shows the half-filled state in 1 screen in case dot size is smaller than a diameter. (A) is an output waveform of the LD when the LD is driven by a conventional LD drive circuit (without an overshoot circuit), and (B) is an output of the LD when the LD is driven by an LD drive circuit (with an overshoot circuit). It is a graph which shows a waveform.

Explanation of symbols

10 Image forming device 16 Paper (transfer body)
24 Charger for transfer (transfer means)
32 Scanning exposure equipment (exposure means)
34 Fixing device 36 Charging device 38 Developing device (developing means)
50 LD (semiconductor laser)
52 Rotating polygon mirror 54 LD drive circuit (semiconductor laser drive circuit)
62 fθ lens (imaging means)
Dt Laser beam diameter in the main scanning direction Ds Laser beam diameter in the sub-scanning direction

Claims (9)

A scanning exposure apparatus that turns on a semiconductor laser based on image information and scans and exposes a photoconductor with a laser beam output from the semiconductor laser,
There are a plurality of write start positions in the main scanning direction of the laser beam on the photoconductor.
A scanning exposure apparatus. A scanning line group composed of a plurality of the laser beams is deflected and scanned by one deflection scanning, and the writing start position is different in the scanning line group adjacent to the sub-scanning direction orthogonal to the main scanning direction.
The scanning exposure apparatus according to claim 1, wherein: The writing start position is different in the scanning lines adjacent in the sub-scanning direction orthogonal to the main scanning direction.
The scanning exposure apparatus according to claim 1 or 2, wherein The writing start position periodically varies in the sub-scanning direction orthogonal to the main scanning direction.
The scanning exposure apparatus according to any one of claims 1 to 3, wherein When the scanning line interval on the photosensitive member determined by the writing density in the sub-scanning direction orthogonal to the main scanning direction is A and the beam diameter on the photosensitive member is B, 2A ≦ B.
The scanning exposure apparatus according to any one of claims 1 to 4, wherein the scanning exposure apparatus is characterized. When the scanning line interval on the photosensitive member determined by the writing density in the sub-scanning direction orthogonal to the main scanning direction is A and the deviation amount of the writing position in the sub-scanning direction is C, 0 <C ≦ 2A.
The scanning exposure apparatus according to any one of claims 1 to 5, wherein Changing the writing start position according to the type of image,
The scanning exposure apparatus according to any one of claims 1 to 6, wherein: The writing start position is constant for each scanning line in the image image, and is different for each scanning line in the text image.
The scanning exposure apparatus according to claim 7. A scanning exposure apparatus according to any one of claims 1 to 8,
Developing means for electrostatically developing an electrostatic latent image formed on the photosensitive member by the scanning exposure device, and forming a toner image on the photosensitive member;
Transfer means for transferring the toner image to a transfer member and forming an image on the transfer member;
An image forming apparatus comprising: