JP2007171457A - Optical scanner and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical scanner capable of switching a mode while a low price and a high picture quality are maintained, and to provide an image forming apparatus. <P>SOLUTION: When beam intervals of one scan portion is congested compared with an ideal beam pitch, influence on a matrix can be equalized regardless of a writing start line (what beam the scan started from) as shown in fig. 3. Whether the scan started from a first beam as shown in fig. 3(a) or from a third beam as in shown fig. 3(b), the intervals D are equally narrowed in pixels PX, therefore the toner concentration is evenly varied and no difference in the concentration level is caused in either instance when color register (skew compensation) is performed in image processing. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  In the present invention, after a plurality of light beams emitted from a single light source arranged two-dimensionally are deflected by a single deflector, the optical path is separated and a plurality of photoconductors are respectively scanned and exposed with a plurality of light beams. The present invention relates to an optical scanning device and an image forming apparatus using the optical scanning device.

  2. Description of the Related Art Conventionally, in a tandem color laser printer, a method of scanning a plurality of photoconductors by separating a plurality of laser beams emitted from a single light source and deflected by a single deflector has been devised (for example, , See Patent Document 1).

  In the above-mentioned Patent Document 1, as the light source 114, as shown in FIG. 10 (a), for example, eight light sources for four colors of Y (yellow), M (magenta), C (cyan), and K (black), respectively. An example using four light emitting point groups (32 in total) for emitting light beams, three light emitting point groups (YMC) for emitting four light beams as shown in FIG. A full-color mode in which four photoconductors (YMCK) are scanned with a total of 20 light emitting point groups (K) that emit one light beam, and each YMCK is scanned with four light beams. An example of a configuration in which an image is formed at different process speeds without changing the number of rotations of the deflector by switching the monochrome mode for scanning with the light beams (eight) is described.

  However, in the above optical scanning device, since the number of light beams emitted from the light emitting point is an even number, when performing color registration correction (color misregistration correction) by image data processing, a correction defect (density) due to the image structure. There is a problem that a step) occurs. If there is an error in the writing pitch of multiple beams that are scanned simultaneously, when the color registration correction of the SKEW component is performed by processing the image data, a difference occurs in the pixel filling method due to the selection of the writing line, and the division in the main scanning direction A density step is generated between them.

  As an even example, a case where an image forming apparatus that performs scanning exposure with a writing resolution of 2400 dpi forms an image of a 50% halftone of 600 dpi with eight light beams will be described. When there is no error in the interval between the first to eighth beams, that is, when there is no clogging / extension in the scan beam joint as shown in FIG. 11 (nominal state), color registration correction by image processing (skew correction) Even if the image data is shifted for each section in the main scanning direction to execute the above, there is no gap or overlap between the nth scan and the (n + 1) th scan, so that the density due to the section is as shown in FIG. There is no difference.

  However, as shown in FIG. 12A, when the beam interval for one scan is smaller than the ideal beam pitch, when processing 600 dpi data at a writing resolution of 2400 dpi, color registration by image processing is performed. When correction (skew correction) is performed, for example, when the writing line is scanned from the beam 1; when scanning from the beam 3; and when scanning from the beam 3; FIG. There is a problem that a density step occurs as shown in FIG.

  This is because when the pitch of a plurality of beams to be scanned simultaneously is clogged, depending on the selection of the writing beam, a pitch error is a gap generated between pixels (FIG. 12A), and a gap generated in a pixel is This is because the connection of dots in a pixel differs from that in FIG. 12B, resulting in a difference in developability and transferability, and a difference in printout density depending on the section.

  In the above example, the maximum number of light beams for scanning the photosensitive member is 8, so there is a limit to the process speed at which an image can be formed at 2400 dpi. However, if the number of light beams is further increased in order to improve the process speed, the number of light source and laser driving devices increases, and the device itself becomes complicated and expensive.

An object of the present invention is to provide an optical scanning apparatus and an image forming apparatus that can cope with a high process speed by mode switching while maintaining low cost and high image quality.
Japanese Patent Laid-Open No. 2005-70067

  In consideration of the above-described facts, an object of the present invention is to provide an optical scanning device and an image forming apparatus that can switch modes while maintaining low cost and high image quality.

  The optical scanning device according to claim 1, wherein a light source in which an odd number of light emitting point groups arranged on a straight line at a predetermined angle with respect to the main scanning direction are arranged at a distance in the sub-scanning direction; and the light source A deflector for deflecting a plurality of light beams emitted from the optical system; and an optical system for separating the optical paths of the plurality of light beams and scanning the plurality of photosensitive members with the plurality of light beams, respectively. Optical scanning device.

    In the invention with the above configuration, even if color registration correction is performed by image data processing, it is possible to prevent an image density step in the main scanning direction that occurs when an even number of light emission point groups are used.

  The optical scanning device according to claim 2, wherein a light source in which an odd number of light emitting point groups arranged on a straight line at a predetermined angle with the main scanning direction are arranged at a distance in the sub-scanning direction, and the light source A deflector that deflects a plurality of light beams emitted from the optical system, and an optical system that separates optical paths of the plurality of light beams and scans a plurality of photoconductors with the plurality of light beams, respectively. Of these, the specific light emission point group has a larger number of light emission points than the other light emission point groups.

  In the invention with the above configuration, even when color registration correction is performed by image data processing, it is possible to prevent the image density step in the main scanning direction that occurs when an even number of light emission point groups are used, and to maintain high image quality. Mode switching is possible.

  According to a third aspect of the present invention, in the optical scanning device according to the second aspect, the specific light emitting point group is arranged at a substantially central position in the sub scanning direction on the light source, and the light emitting point group is arranged in the sub scanning direction. It is characterized by being arranged at intervals.

  In the invention of the above configuration, the degree of freedom in design / quality can be ensured by equalizing the separation pitches of the plurality of light beams and optically setting the conditions for each color.

  According to a fourth aspect of the present invention, there is provided the optical scanning device according to the second aspect, wherein the specific light emitting point group and the other light emitting point group at the same position in the main scanning corresponding direction are the other light emitting points. The light emitting points of the light emitting point group are characterized in that the light emitting point group pitch in the sub-scanning corresponding direction is constant.

  In the invention with the above configuration, it is possible to achieve both optical performance in the color mode and beam separation in the monochrome mode.

  According to a fifth aspect of the present invention, in the optical scanning device according to the second aspect, the specific light emission point group is disposed on the light source at an end portion in the sub-scanning direction, and is emitted from the specific light emission point group. The optical beam is split at the most downstream side of the optical path.

  In the invention with the above configuration, since the optical path is divided at the most downstream side on the optical path, the optical path can be separated with a wide light beam interval, and both stable light emitting point operation and beam separation can be achieved.

  An image forming apparatus according to a sixth aspect includes the optical scanning device according to any one of the second to fifth aspects, and forms a processed image by switching between a full color mode and a monochrome mode having a higher processing speed. In the monochrome mode, only the specific light emission point group is used.

  In the invention with the above-described configuration, it is possible to realize both the full color mode and the processing capability in the monochrome mode in which a speed increase is required.

  According to a seventh aspect of the present invention, in the image forming apparatus according to the sixth aspect, in the full color mode, seven light beams are respectively provided on four photosensitive members corresponding to four colors of yellow, magenta, cyan, and black. Therefore, in the monochrome mode, scanning exposure is performed on the black photosensitive member with the writing resolution of 2400 dpi by 11 light beams.

  In the invention having the above-described configuration, the cost can be reduced by minimizing the number of light emission points of the light scanning device that performs scanning exposure at a writing resolution of 2400 dpi by switching between the full color mode and the monochrome mode for forming an image at a higher speed than the full color mode.

  The optical scanning device according to claim 8, wherein light emitting point posts arranged on a straight line that forms an angle with the main scanning direction are arranged in a plurality of rows at a predetermined distance in the sub scanning direction, and the light emitting point posts A light source is provided in which only selected ones are connected so that an odd number can be turned on per row.

  In the invention with the above-described configuration, by combining the light emitting point posts according to the purpose, any number of light emitting point combinations can be realized, and the same parts can be diverted to various types, so that the cost can be reduced.

  Since the present invention is configured as described above, an optical scanning apparatus and an image forming apparatus that can switch modes while maintaining low cost and high image quality can be obtained.

<Basic configuration>
1 and 2 show an optical scanning device according to a first embodiment of the present invention.

  As shown in FIG. 1, the optical scanning device 10 provided in the color laser printer includes four photoconductors 12Y, 12M, 12K, and 12C serving as scanning objects, and laser beam groups LY, LM, and A latent image is formed by irradiating LK and LC. The latent images formed on the photoreceptors 12Y, 12M, 12K, and 12C are developed with yellow (Y), magenta (M), black (K), and cyan (C) toners, respectively. Then, the toner on each photoconductor is transferred to a transfer belt (not shown). At this time, the toners of the respective colors are superimposed to form a full-color image and transferred to a recording medium such as plain paper.

  The optical scanning device 10 includes a light source 14, a pre-deflection optical system 16, a polygon mirror 18 as a deflecting unit, and a scanning optical system 20, and four laser beam groups LY, LM, LK, LC from a single light source 14. Is then deflected and scanned by a single surface of the polygon mirror 18, and then each laser beam group is separated in the scanning optical system and image-scanned on the four photoconductors 12Y, 12M, 12K, and 12C. The deflection scanning direction by the rotation of the polygon mirror of the optical scanning device is referred to as a main scanning corresponding direction, and the direction orthogonal to the deflection scanning direction is referred to as a sub scanning corresponding direction. That is, in the photoconductors 12Y, 12M, 12K, and 12C, a direction corresponding to the axial direction is referred to as a main scanning corresponding direction, and a direction corresponding to the rotation direction is referred to as a sub scanning corresponding direction.

  As shown in FIG. 2, the light source 14 is a surface emitting laser beam in which a total of 32 light emitting points P of 7 columns × 3 rows + 11 columns × 1 row are arranged two-dimensionally in the main scanning corresponding direction and the sub scanning corresponding direction. This is an array, and laser beams 12C, 12K, 12M, and 12Y are emitted in order from the top row, and seven photoconductors 12C, 12K, 12M, and 12Y are formed in the color mode, that is, when an image is formed with four color materials of YMCK. Scanning is performed with a laser beam (range surrounded by “CL” in the figure).

  Further, in the monochrome mode using only K, 11 light emitting points P (range surrounded by “MC” in the figure) are used so that high-speed processing can be performed without increasing the rotational speed of the polygon mirror 18. .

At this time, the conditions are, for example, the process speed in the color mode; 220 mm / s
Resolution: 2400 dpi
Polygon mirror; 6 surfaces 回 転 Rotation speed: 29,696 rpm
Scanning lens f; 330mm
Video frequency: 193.9 MHz
In monochrome mode, process speed; 320mm / s
Resolution: 2400 dpi
Polygon mirror; 6 surfaces 回 転 Rotation speed: 27,487 rpm
Scanning lens f; 330mm
Video frequency: 179.5 MHz
It is. In the case of 8x4 case (conventional example), resolution: 2400 dpi
Polygon rotation speed: 37,795 rpm
Video frequency: 246.8 MHz
It is.
The resolution of 2400 dpi is a resolution necessary for correcting the color registration without generating a correction defect by correcting the image data. The number of faces of the polygon mirror 18 is 6 in a standard manner in consideration of the balance between the number of faces and the size. The number of surfaces to be used (polygon mirrors with 8 or more surfaces are increased in diameter to increase the motor load, which is undesirable from the viewpoint of power consumption and reliability).

  The focal length of 330 mm is determined from the optical path length required to route the optical path inside the tandem optical scanning device that scans the light beams for four colors on the same surface of the deflector. +1) The scanning was adjacent exposure in which the exposure is continuously performed in the sub-scanning direction (not overlapping). This is because when double exposure is performed, the image forming speed falls to ½.

  Furthermore, the upper limit of the rotational speed of the motor that drives the polygon mirror is 30,000 rpm, and if this upper limit is exceeded, countermeasures such as vibration / temperature rise / noise are required and the cost becomes high. The upper limit of the video transfer speed is 200 MHz. If this upper limit is exceeded, the transmission waveform quality is deteriorated and radio wave countermeasures are required.

  In the conventional case of 8 columns × 4 rows, the polygon mirror 18 (deflector), which is thick, has a rotation speed of 38,000 rpm and a video frequency exceeding 250 MHz, so monochrome operation at 2400 dpi (320 mm / s) Is practically difficult and has a limited application range. Although it is conceivable to set the monochrome mode without color registration correction to 1200 dpi, there is a problem that a pitch adjustment mechanism by rotating the light source is necessary.

In contrast, in the present invention, mode (process speed) switching at 2400 dpi can be realized up to 320 mm / s without increasing the number of beams. Further, since the color mode is written with an odd number of beams, there is no density step due to the color registration correction.
<Principle of step countermeasures>
As shown in FIG. 2, by setting the number of light emission points on the light source 14 used for one scan, that is, the number of beams in one scan, to an odd number, the influence on the pixel matrix regardless of the writing line (from which beam scanning is started). Even when color registration correction (skew correction) is performed by image processing, no density step is generated.

  First, when the beam interval is nominal, that is, in an ideal state where the scan beam interval is not clogged / extended as shown in FIG. Since there is no gap or overlap between the scan and the (n + 1) th scan, there is no difference from the conventional eight beams in that a density difference due to the section does not occur even when a 50% halftone is formed at 600 dpi.

  Next, conventionally, for example, when 8 beams are used, when the beam interval for one scan is smaller than the ideal beam pitch as shown in FIG. 12A, 600 dpi data is processed at a writing resolution of 2400 dpi. At this time, when color registration (skew correction) is performed by image processing, for example, when the writing line is scanned from the beam 1 = FIG. 12A and when scanning from the beam 3 is compared, FIG. When scanning from the beam 3, there is a problem that the density of the pixel PX is lowered because the gap D is sandwiched in the pixel PX, and a density step is generated as shown in FIG.

  On the other hand, in the present invention, when the beam interval for one scan is smaller than the ideal beam pitch, the influence on the matrix does not depend on the writing line (from which beam is started) as shown in FIG. Can be made uniform.

That is, even when scanning is performed from the first beam as shown in FIG. 3A or when scanning is performed from the third beam as shown in FIG. 3B, the gap D is equally sandwiched in the pixels PX. Therefore, the density changes equally between the two, and no density step occurs even when color registration (skew correction) is performed in image processing.
<Numerical basis>
As described above, the resolution, the number of polygon mirror surfaces, the focal length of the imaging lens, the scanning method, and the like are determined based on mechanical requirements. In addition, the upper limit of the motor speed and the upper limit of the video transfer speed are also restricted for reasons such as quality and cost.

  At this time, a process speed of 220 mm / s in the color mode and 320 mm / s in the monochrome mode is required to realize the processing capability of 50 sheets / minute of color and 75 sheets / minute of monochrome, which are typically required as a medium-speed color machine. It becomes.

  The number of beams that satisfy the above conditions and do not generate a density step is shown in the table of FIG. That is, in the color mode, the processing capacity of 50 sheets / min is satisfied, the upper limit (MPA) of the motor rotation speed is 30,000 rpm or less, the video transfer speed is suppressed to 200 MHz or less, and the smallest number of beams is 7 from the table. It turns out that it is a book.

  In monochrome mode, the processing capacity is 75 sheets / minute, the upper limit of the motor rotation speed (MPA) is 30,000 rpm or less, the video transfer speed is suppressed to 200 MHz or less, and the smallest number of beams is 11 from the table. It can be seen that it is.

For the above reasons, the present invention employs a light emitting point array that emits seven beams for Y, M, and C and eleven beams for K. As a result, it is possible to obtain an optical scanning device that does not cause a density difference even if color registration (skew correction) is performed by image processing while ensuring reliability while suppressing an increase in cost.
<Separation pitch priority>
FIG. 5 shows a light emission point arrangement of a light source according to the second embodiment of the present invention.

  As shown in FIG. 5, the light source 14 is a surface emitting laser beam in which a total of 32 light emitting points P of 7 columns × 3 rows + 11 columns × 1 row are arranged two-dimensionally in the main scanning corresponding direction and the sub scanning corresponding direction. This is an array, and laser beams 12C, 12K, 12M, and 12Y are emitted in order from the top row, and seven photoconductors 12C, 12K, 12M, and 12Y are formed in the color mode, that is, when an image is formed with four color materials of YMCK. Scanning is performed with a laser beam (range surrounded by “CL” in the figure). Further, in the monochrome mode using only K, 11 light emitting points P (range surrounded by “MC” in the figure) are used so that high-speed processing can be performed without increasing the rotational speed of the polygon mirror 18. The point is the same as in the first embodiment.

  In this embodiment, in order to make the separation pitches S1 to S3 for each beam group 12 constant, the arrangement pitches L1 to L3 of the light emitting points P on the light source 14 are changed for each color.

In other words, as shown in FIG. 5, by setting L1 (C to K) = L2 (K to M)> L3 (M to Y), the separation pitches S1, S2, and S3 are made equal, and optically equivalent conditions for each color are set. The design freedom / quality can be secured.
<Equal priority of light emission point position>
FIGS. 6 and 7 show the light emission point arrangement and the optical system of the light source according to the third embodiment of the present invention.

  As shown in FIG. 6, the light source 14 is a surface emitting laser beam in which a total of 32 light emitting points P of 7 columns × 3 rows + 11 columns × 1 row are arranged two-dimensionally in the main scanning corresponding direction and the sub scanning corresponding direction. This is an array, and laser beams 12K, 12C, 12M, and 12Y are emitted in order from the top row, and seven photoconductors 12K, 12C, 12M, and 12Y are formed in the color mode, that is, when an image is formed with four color materials of YMCK. Scanning is performed with a laser beam (range surrounded by “CL” in the figure). Further, in the monochrome mode using only K, 11 light emitting points P (range surrounded by “MC” in the figure) are used so that high-speed processing can be performed without increasing the rotational speed of the polygon mirror 18. .

  In this embodiment, in order to make the arrangement pitches L1 to L3 of the light emitting points P on the light source 14 constant, the separation pitches S1 to S3 for each beam group 12 are changed for each color.

  That is, by setting L1 (K to C) = L2 (C to M) = L3 (M to Y) as shown in FIG. 5, the light emitting point operating in the color mode (the range surrounded by “CL” in the figure). The group (K) having a large number of light emitting points is arranged at the end.

For this reason, the separation pitch is S1 (K to C) <S2 (C to M) = S3 (M to C). In this case, the separation pitch of K and C is narrowed, so it is necessary to earn a separation margin by some method. is there. Therefore, as shown in FIG. 7, the photoconductor 12K is arranged at the position farthest from the polygon mirror 18 so as to prevent the interference between K and C. As a result, the separation position of the K group having a large number of light emitting points is farthest from the polygon mirror and can be separated in a state in which the light beam diverges, and both stable light emitting point operation and beam separation can be achieved.
<Separation pitch priority>
FIG. 8 shows a light emission point arrangement of a light source according to the fourth embodiment of the present invention.

  As shown in FIG. 8, the light source 14 is a surface emitting laser beam in which a total of 32 light emitting points P of 7 columns × 3 rows + 11 columns × 1 row are arranged two-dimensionally in the main scanning corresponding direction and the sub scanning corresponding direction. This is an array, and laser beams 12K, 12C, 12M, and 12Y are emitted in order from the top row, and seven photoconductors 12K, 12C, 12M, and 12Y are formed in the color mode, that is, when an image is formed with four color materials of YMCK. Scanning is performed with a laser beam (range surrounded by “CL” in the figure). Further, in the monochrome mode using only K, 11 light emitting points P (range surrounded by “MC” in the figure) are used so that high-speed processing can be performed without increasing the rotational speed of the polygon mirror 18. The point is the same as in the third embodiment.

In the present embodiment, the separation pitches S1 to S3 for each beam group 12 and the arrangement pitches L1 to L3 of the light emitting points P on the light source 14 are uniform. Further, in the K having a larger number of light emitting points, the arrangement is made such that the additional light emitting point P + is extended to the side not affected by the separation pitch S, that is, the side away from the optical axis. In the figure, beam separation in the monochrome mode is ensured by disposing it on the left side (upper side). By arranging the light emitting points used in the color mode (range surrounded by “CL” in the figure) at equal intervals, both the optical performance in the color mode and the beam separation in the monochrome mode are achieved.
<Select use>
FIG. 9 shows a light emission point arrangement of a light source according to the fifth embodiment of the present invention.

  In the present embodiment, the light emitting point posts P0 provided in the light source 14 in advance are selectively connected and used as the necessary light emitting points P.

  That is, when priority is given to the separation pitches S1 to S3 for each beam group 12, the pitch can be increased by connecting them as in CL1 and using the light source 14 obliquely.

Alternatively, if it is desired to have the same light emission point arrangement as in the fourth embodiment of FIG. 8, the light source 14 that is the same component is lighted by changing the connection by connecting the light emission points as shown in CL2 in the figure. The point arrangement can be changed and used according to various purposes such as stability and beam separation priority. As a result, the number of parts can be reduced and the same parts can be diverted to various types, so that the cost can be reduced.
<Others>
In addition, this invention is not limited to said embodiment.

  For example, although the above embodiment is applied to a four-color full-color printer, the present invention is not limited to this. For example, a printer having three colors or less may be used, or five or more colors may be used.

  In addition, when configuring a full-color image forming apparatus equipped with two optical scanning devices that use two deflected beams from the same deflecting surface, correction defects are generated by configuring all light emitting point groups with odd numbers. Color registration correction can be performed without causing the color registration to occur.

  The image recording in the present invention is not limited to recording characters and images on recording paper. That is, the present invention can be used for general image recording used for industrial purposes.

It is a figure which shows the optical scanning device based on this invention. It is a figure which shows the light emission point arrangement | positioning which concerns on 1st Embodiment of this invention. It is a figure which shows the relationship between the beam pitch and image density which concern on this invention. It is a figure which shows the number of beams and processing capacity which concern on 1st Embodiment of this invention. It is a figure which shows the light emission point arrangement | positioning which concerns on 2nd Embodiment of this invention. It is a figure which shows the light emission point arrangement | positioning which concerns on 3rd Embodiment of this invention. It is a figure which shows the optical system arrangement | positioning which concerns on 3rd Embodiment of this invention. It is a figure which shows the light emission point arrangement | positioning which concerns on 4th Embodiment of this invention. It is a figure which shows the light emission point arrangement | positioning which concerns on 5th Embodiment of this invention. It is a figure which shows the conventional light emission point arrangement | positioning. It is a figure which shows the relationship between the conventional beam pitch and image density. It is a figure which shows the relationship between the conventional beam pitch and image density.

Explanation of symbols

10 Optical Scanning Device 12 Photoconductor (Scanned Object)
14 Light source 16 Pre-deflection optical system 18 Polygon mirror (deflection means)
20 Scanning optical system 22 Coupling lens 28 Aspherical lens (first scanning optical system)
30 plane mirror group (separation means)
32 Toroidal lens (second scanning optical system)
34Y First plane mirror Y
34M First plane mirror M
34C First plane mirror C
34K first plane mirror D
36Y Second plane mirror Y
36M Second plane mirror M
36C Second plane mirror C
36K second plane mirror D

Claims (8)

A light source in which a plurality of light emitting point groups arranged on a straight line at a predetermined angle with the main scanning direction are arranged at a distance in the sub-scanning direction;
A deflector for deflecting a plurality of light beams emitted from the light source;
An optical scanning apparatus comprising: an optical system that separates optical paths of the plurality of light beams and scans the plurality of photosensitive members with the plurality of light beams, respectively. A light source in which a plurality of light emitting point groups arranged on a straight line at a predetermined angle with the main scanning direction are arranged at a distance in the sub-scanning direction;
A deflector for deflecting a plurality of light beams emitted from the light source;
An optical system that separates the optical paths of the plurality of light beams and scans the plurality of photoconductors with the plurality of light beams, respectively, and the specific light emission point group of the light emission point groups is more than the other light emission point groups. An optical scanning device having a large number of light emitting points. 3. The optical scanning device according to claim 2, wherein the specific light emission point group is disposed substantially at the center in the sub scanning direction on the light source, and the light emission point groups are disposed at equal intervals in the sub scanning direction. Among the specific light emission point groups, the light emission points in the same position as the other light emission point groups and the light emission points of the other light emission point groups have a constant light emission point group pitch in the sub-scanning corresponding direction. The optical scanning device according to claim 2. The specific light emission point group is disposed on the light source at the end in the sub-scanning direction,
3. The optical scanning device according to claim 2, wherein the light beam emitted from the specific light emitting point group is divided on the most downstream side on the optical path. 6. The optical scanning device according to claim 2, wherein the processed image is formed by switching between a full color mode and a monochrome mode having a higher processing speed, and in the monochrome mode, the specific light emission point group is formed. An image forming apparatus using only the image forming apparatus. In the full color mode, writing light is 2400 dpi on the black photosensitive member by seven light beams respectively on four photosensitive members corresponding to four colors of yellow, magenta, cyan, and black, and in the monochrome mode by eleven light beams. The image forming apparatus according to claim 6, wherein scanning exposure is performed. A plurality of light emitting point posts arranged on a straight line that forms an angle with the main scanning direction are arranged at a predetermined distance in the sub scanning direction, and only selected ones of the light emitting point posts are odd per row. An optical scanning device comprising a light source connected in a state that can be individually turned on.