JP2017207539A - Optical scanner and image forming apparatus using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a compact optical scanner that suppresses a reduction in image quality and is manufactured at a low cost, and an image forming apparatus using the same.SOLUTION: In an optical scanner that has a principal ray orthogonal to a scanning area in a scanning target surface as a reference line and has the scanning area on the scanning target surface brought close to a light source side with respect to the reference line, an imaging optical system includes a first imaging optical element and a second imaging optical element closer to the scanning target surface than the first imaging optical element. When the distances from the reference line to an end of the first imaging optical element on the same side of the light source and to an end on the opposite of the light source are defined as L1u and L1l, respectively, and the distances from the reference line to an end of the second imaging optical element on the same side of the light source and to an end on the opposite of the light source are defined as L2u and L2l, respectively, the conditions of 0.9≤L1u/L1l≤1.1, L2u>L2l, and L1l≤L2l are satisfied.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical scanning device, and is particularly suitable for an image forming apparatus such as a laser beam printer having an electrophotographic process, a digital copying machine, or a multifunction printer (multifunction printer).

  In order to reduce the size and cost of the optical scanning device used in the image forming apparatus, the light beams from the first and second light sources are deflected by a common deflector, and the first light is deflected by the deflected light beams. Further, there is known one that scans a second surface to be scanned (Patent Document 1).

  Further, it is known in the optical scanning device to reduce the distance from the light source to the deflector by providing beam combining means in the middle of the optical path from the plurality of light sources to the deflector and reducing the distance between the optical paths. (Patent Document 2).

JP 2004-294886 A JP 2004-13021 A

  Here, in the optical scanning device sharing the deflector as in Patent Document 1, the control substrate of the first and second light sources is integrated (commonized), and the incident light that causes the light beam from the light source to enter the deflector. In some cases, the first and second incident optical systems that are incident on different deflection surfaces are arranged close to each other. In this case, when the optical axes of the first and second incident optical systems are substantially parallel when projected onto the main scanning section, the optical axes of the first and second incident optical systems have a large angle (non-parallel). Therefore, it is difficult to make the entire optical scanning device compact.

  In addition, when two incident light beams are incident obliquely on the deflection surface from the sub-scanning direction as incident optical systems corresponding to different scanning surfaces (overlapping as one incident optical system when projected onto the main scanning section), the following reasons Therefore, it becomes difficult to make the entire optical scanning device compact.

  In other words, in order to suppress scanning surface bending and image formation performance degradation, it is necessary to reduce the oblique incident angle with respect to the deflection surface. On the other hand, the imaging spot on the scanning surface is required in order to cope with higher image quality. It is necessary to reduce the diameter. Then, since the light flux width in the main scanning direction on the exit surface of the collimator lens in the incident optical system increases, it is necessary to increase the outer diameter of the collimator lens. Further, in order to enlarge the developing device (toner container) in the image forming apparatus, it is necessary to increase the optical path length of the imaging optical system. In this case as well, the imaging spot diameter on the scanning surface is reduced. In order to maintain, it is necessary to increase the outer diameter of the collimator lens.

  For this reason, in order to prevent interference between adjacent members in the sub-scanning direction in the incident optical system, it is necessary to ensure a long optical path length from the light source to the deflector, and thus it is difficult to make the entire optical scanning device compact. Become.

  Here, if the beam combining means of Patent Document 2 is used to reduce the distance between the light beams of the first and second projecting optical systems in the sub-scanning direction to reduce the distance from the light source to the deflector, the number of parts is reduced. Increased costs and high assembly accuracy demands lead to increased costs.

  An object of the present invention is to provide a compact and low-cost optical scanning device that suppresses deterioration in image quality and an image forming apparatus using the same.

In order to achieve the above object, an optical scanning device according to the present invention deflects a light beam from a light source and optically scans a scanning region on a scanned surface in a main scanning direction, and is deflected by the deflector. An imaging optical system for condensing a light beam on the scanning region, wherein a main light beam orthogonal to the scanning region among the light beams deflected by the deflector in a main scanning section is used as a reference An angle formed between the principal ray incident on the first end portion on the same side as the light source in the main scanning direction of the scanning region and the reference line is θu (deg), and the first end portion and the reference line Ldu (mm), the angle between the principal ray incident on the second end opposite to the light source in the main scanning direction of the scanning region and the reference line is θl (deg), and the second end When the distance between the part and the reference line is Ldl (mm) ,
θu> θl
Ldu> Ldl
Meets the conditions
The imaging optical system includes a first imaging optical element and a second imaging optical element closer to the scanned surface than the first imaging optical element, and in the main scanning direction, The distances from the reference line to the end of the first imaging optical element on the same side as the light source and the end opposite to the light source are L1u and L1l, respectively, and the second connection from the reference line. When the distances to the end of the image optical element on the same side as the light source and the end on the opposite side of the light source are L2u and L2l,
0.9 ≦ L1u / L1l ≦ 1.1
L2u> L2l
L1l ≦ L2l
It satisfies the following condition.

  An image forming apparatus according to the present invention includes the optical scanning device.

  According to the present invention, it is possible to provide a low-cost and compact optical scanning device that suppresses deterioration in image quality and an image forming apparatus using the same.

(A) is a main scanning sectional view of an optical scanning device according to an embodiment of the present invention, and (b) is a sub-scanning sectional view. It is explanatory drawing of the image forming apparatus carrying the optical scanning device which concerns on embodiment of this invention. It is a sub-scan sectional view schematically showing the arrangement of an incident optical system that is obliquely incident in the sub-scan direction. It is the figure which showed typically arrangement | positioning of the incident optical system when the diameter of a condensing element becomes large. It is the figure which showed typically arrangement | positioning of the incident optical system at the time of setting an oblique incident angle small. It is the figure which showed typically arrangement | positioning of the incident optical system which added the beam synthesis means and shortened the length of the incident optical system. It is a figure explaining that the optical path length increases as the cartridge capacity increases. FIG. 6 is a main scanning sectional view of the optical scanning device as the cartridge capacity increases. It is a schematic diagram explaining the shape of the imaging lens which concerns on embodiment of this invention, and generation | occurrence | production of yaw. 6 is a graph showing changes in focus position and dot position in the main scanning direction and sub-scanning direction when yaw occurs. It is a main scanning sectional view of an optical scanning device concerning a 2nd embodiment. It is the figure which showed typically the pressure which acts in a cavity at the time of shaping | molding. It is a graph which shows the influence on the optical performance by the eccentricity of each surface of an imaging lens. It is the figure which showed typically the mode of positioning of the 1st imaging lens made from resin, and the adhesion | attachment. It is the figure which showed the gate arrangement | positioning and the flow of resin typically. It is the figure which showed typically the example which observed the lens manufactured by injection molding by crossed Nicols. It is a main scanning sectional view of an optical scanning device concerning a 3rd embodiment. It is a main scanning sectional view of an optical scanning device concerning a 4th embodiment.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

<< First Embodiment >>
(Image forming device)
FIG. 2 is a schematic diagram of a main part of a color image forming apparatus 60 equipped with an optical scanning device according to an embodiment of the present invention. This is a color image forming apparatus that records image information on the scanned surface, which is the drum surface of the photosensitive drums 71, 72, 73, and 74 that are image carriers in the optical scanning device 61. In FIG. 2, reference numerals 31, 32, 33, and 34 denote developing devices, and 51 denotes a conveyor belt. Note that a transfer device (not shown) for transferring the toner image developed by each developing device and a fixing device (not shown) for fixing the transferred toner image onto the transfer material are provided.

  The color image forming apparatus 60 receives R (red), G (green), and B (blue) color signals from an external device 52 such as a personal computer. Each color signal output from the external device by the printer controller 53 in the apparatus is converted into each image signal (dot data) as C (cyan), M (magenta), Y (yellow), and B (black) image data. Is done. These image signals are input to the optical scanning device 61. From these optical scanning devices, light beams 41, 42, 43, 44 modulated in accordance with the respective image signals are emitted, and these light beams are irradiated on the photosensitive surfaces of the photosensitive drums 71, 72, 73, 74. Scans in the main scanning direction.

  As described above, the color image forming apparatus 60 uses the light scanning device 61 to convert the electrostatic latent images of the respective colors to the corresponding drum surfaces of the photosensitive drums 71, 72, 73, and 74 using the light beams based on the respective image signals. Form on top. Thereafter, a single full color image is formed by multiple transfer onto a recording material.

  As the external device 52, for example, a color image reading apparatus including a CCD sensor may be used. In this case, the color image reading apparatus and the color image forming apparatus 60 constitute a color digital copying machine.

(Optical scanning device)
The optical scanning device according to the present embodiment includes an incident optical system that converts the condensing state of one or a plurality of light beams emitted from one or a plurality of light sources and guides them to a deflector, and a deflector that deflects and scans the light beams. Have. Further, the light beam deflected and scanned by the deflecting surface of the deflector is condensed on the surface to be scanned by the imaging optical system.

  Hereinafter, an optical scanning device according to an embodiment of the present invention will be described with reference to the drawings. It should be noted that the drawings shown below may be drawn at a scale different from the actual scale so that the present invention can be easily understood.

  In the following description, the main scanning direction corresponds to a direction perpendicular to the rotation axis of the deflector and the optical axis of the optical system, and the sub-scanning direction corresponds to a direction parallel to the rotation axis of the deflector. . The main scanning section corresponds to a section perpendicular to the sub-scanning direction, and the sub-scanning section corresponds to a section perpendicular to the main scanning direction.

  In FIG. 1A schematically showing a main scanning section, in the light source unit 200, 1 is a light source (laser light source). 2 is a condensing element, 3 is a cylindrical lens, 4 is a stop, and these constitute an incident optical system.

  The first and second incident optical systems (FIG. 1A) in which the optical axes for allowing the first and second light beams to enter the deflector in the main scanning section are parallel to each other will be described later with reference to FIG. Each of them is composed of two incident optical systems that allow two light beams to enter in the sub-scanning direction. In other words, the first incident optical system is configured as each incident optical system that causes the third and fourth light beams to enter the deflector, and each optical axis in the main scanning section is light of the first incident optical system. Overlapping as an axis (first optical axis).

  Similarly, the second incident optical system is configured as each incident optical system that causes the fifth and sixth light beams to enter the deflector, and each optical axis in the main scanning section is that of the second incident optical system. Overlapping as the optical axis (second optical axis).

  In FIG. 1A, reference numeral 5 denotes a polygon mirror as a deflector. An image forming optical system 6 includes a first image forming lens 6a as a first image forming optical element, and a second image forming optical element closer to the scanning surface (photosensitive member) 8 than the first image forming optical element. As a second imaging lens 6b, and mirrors M1, M2, M3, and M4 (FIG. 1B) that bend the optical path. 7 is a cover glass, and 800 is an optical box.

  The light source 1 is a laser light source having one or a plurality of light emitting points, and is a single beam laser having one light emitting part on a single chip or a multi-beam laser having a plurality of light emitting parts. Although it is an infrared laser with a wavelength λ = 790 nm, it is not necessarily limited to this, and a red laser with a wavelength λ = 650 nm or a blue laser with λ = 430 nm may be used. The divergent light beam emitted from the light source 1 is converted into substantially parallel light within the main scanning section by the light collecting element 2.

  The substantially parallel light converted by the condensing element 2 is converted into a light beam that converges in the sub-scanning direction by the cylindrical lens 3, and is condensed near the deflection surface (reflection surface) of the deflector 5. Therefore, the light beam forms a line image in the vicinity of the deflection surface.

  The diaphragm 4 has an opening of an elliptical shape, a rectangular shape, an oval shape, a rhombus shape, etc., and may be determined according to the wavelength of the light source used and the required size and shape of the beam spot. In addition, there are two diaphragms, for example, a slit member that extends in the sub-scanning direction to limit the light beam in the main scanning direction and a slit member that extends in the main scanning direction to restrict the light beam in the sub-scanning direction. You may divide and install in a slit member.

  When the number of laser emission points used for the light source 1 is large, jitter can be reduced by providing a slit member that restricts the light beam in the main scanning direction in the vicinity of the deflector 5. In addition, when there are many light emitting points of the laser used for the light source 1, the slit member that restricts the light beam in the sub-scanning direction is positioned conjugate with the position of the first imaging lens 6a or the second imaging lens 6b (generally the light source 1). And between the light condensing elements 2), the uniformity of the printing intervals of a plurality of beams is improved. In consideration of such a technical viewpoint, a diaphragm divided into a plurality of members may be employed.

  The deflector 5 composed of a plurality of reflecting surfaces is rotationally driven by a drive system (not shown). The incident light beam guided to the deflector 5 by the incident optical system is deflected and scanned by an arbitrary reflecting surface that is rotationally driven, and is guided to the imaging optical system 6 (fθ lens).

  Next, the operation of the imaging optical system 6 will be described. The imaging optical system 6 (fθ lens) has a two-lens configuration including a first imaging lens 6a made of glass or plastic (resin) and a second imaging lens 6b made of resin. The light beam reflected and deflected by the deflector 5 is condensed on the surface to be scanned 8 to form a beam spot and scan the surface to be scanned 8 at a constant speed. The resin-made imaging lens is manufactured by a known molding technique in which a mold is filled with a resin and is taken out of the mold after cooling. Thereby, it can be manufactured at a lower cost than when a glass imaging lens is used.

  The imaging optical system 6 is designed with a known power arrangement. For example, the first imaging lens 6a is configured as a lens having power mainly in the main scanning direction. The lens surface shape is a shape expressed by a known function. The shape in the main scanning section is symmetric with respect to the optical axis, and in the sub-scanning direction, the entrance surface and the exit surface may be substantially non-power (refractive power is substantially zero) with the same curvature. Power may be given accordingly.

  One second imaging lens 6b is an aspherical lens having power mainly in the sub-scanning direction, and the lens surface shape is an aspherical shape expressed by a known function. In the second imaging lens 6b, the power in the sub-scanning section is larger than the power in the main-scanning section, and the shape in the main-scanning section is asymmetric (optical surface) with respect to the reference axis (optical axis) as will be described later. The shape of the lens (aspheric expression) is symmetric, but is asymmetric as the outer diameter of the lens. The main scanning direction near the axis is substantially non-power (refractive power is substantially zero).

  The shape of the sub-scanning cross section is a substantially flat surface where the curvature of the entrance surface is extremely loose, and the exit surface is a convex shape in which the curvature gradually changes from on-axis to off-axis, and is symmetrical with respect to the optical axis. . The incident light beam is mainly responsible for image formation in the sub-scanning direction and correction of some distortion in the main scanning direction. The imaging relationship in the sub-scanning direction by the imaging optical system 6 is a so-called tilt correction system in which the deflecting / reflecting surface 5a and the surface to be scanned 8 are substantially conjugate.

  The cover glass 7 is inclined so as to have an angle with respect to the incident light beam in the sub-scan section. This is to prevent the surface reflected light from the cover glass 7 from returning to the light source 1. This is because if the surface reflected light returns to the light source 1, the laser oscillation in the light source 1 becomes unstable and the amount of light may fluctuate.

  In the optical scanning device, it is necessary to control the light emission amount of the laser light source 1 to a predetermined amount. Therefore, the light beam emitted from the laser light source 1 is monitored by a light amount detection sensor, and the APC (Auto) for controlling the laser drive current is controlled. (Power Control) is performed.

  Next, synchronization detection will be described. A synchronization detection light is guided to a synchronization detection light receiving sensor (not shown) by a synchronization detection optical system (not shown) for a light beam having an image height outside the effective image area on the scanned surface 8 to control the timing of print writing. The synchronous detection optical system can be achieved by a known configuration. Further, the scanning optical system is not necessarily configured as described above. For example, the scanning optical system may be asymmetric with respect to the optical axis in order to further improve the imaging performance.

  The incident optical system, the scanning optical system, and the synchronization detection optical system are accurately held in the optical box 800 to form the optical scanning device 61 (FIG. 2).

(Oblique incidence optical system)
FIG. 3 is a diagram schematically showing a sub-scanning section of the oblique incidence optical system as the incidence optical system in the present embodiment. Here, the oblique incident optical system refers to an incident optical system that allows a light beam to be incident on one deflection surface obliquely from the sub-scanning direction.

  In order to reduce the number of deflectors 5, a single deflecting surface 5a of the deflector 5 deflects a plurality of light beams in the same direction. Further, in order to reduce the size of the deflector 5 in the sub-scanning direction, the deflector 5 is made incident on the same deflecting surface 5a at an angle in the sub-scanning direction.

  In FIG. 1B, a plurality of light beams deflected and scanned by the same deflection surface 5a pass through the shared first imaging lens 6a, and after one passes through the first imaging lens 6a, 2 passes through the imaging lens 6b and passes through the mirror M4 and the cover glass 7. Then, the light is condensed on the photosensitive drum 8A which is the first surface to be scanned.

  The other passes through the second imaging lens 6b via the mirrors M1 and M2, and further converges on the photosensitive drum 8B, which is the second surface to be scanned, via the mirror M3 and the cover glass 7. Similarly, a plurality of light beams deflected and scanned by another same deflection surface are condensed on the photosensitive drum 8C as the third scanned surface and on the photosensitive drum 8D as the fourth scanned surface.

  In this way, a plurality of constraints deflected by the same deflection surface are separated by a folding mirror or the like in the scanning optical system, and the corresponding scanned surface is scanned with each light beam.

Incidentally, in the incident optical system, it is necessary to be able to arrange the upper and lower elements in the sub-scanning direction without interfering with each other. In FIG. 3, for example, consider a case where the vertical separation distance is determined by the light source unit 200 that is most likely to be the largest among the components that include the barrel and constitute the incident optical system. Assuming that the height of the light source 1 in the sub-scanning direction is H and the distance in the main scanning direction from the same deflection surface 5a to the light source 1 is L, the principal ray angle θ _PG of the light beam incident on the same deflection surface is generally given by Set as shown.

  The arrangement of the incident optical system will be described in detail later in connection with an increase in cartridge capacity.

(Range of optical scanning in main scanning section and shape of imaging optical system)
FIG. 1A is a schematic diagram of a main scanning section of the optical scanning device of the present embodiment, and a principal ray when a light beam deflected and scanned by the deflector 5 is orthogonal to the surface to be scanned 8 is used as a reference line. Then, the angle formed by the principal ray and the reference line at the first end on the same side as the light source 1 in the scanning area on the scanned surface is θu (deg), and the principal ray position on the scanned surface is Ldu (mm). .

Further, the angle formed between the principal ray and the reference line at the second end opposite to the light source 1 is θl (deg), and the principal ray position on the scanned surface is Ldl (mm). At this time, the scanning field angle of the scanning light (scanning beam) by the light emission timing control and the scanning light (scanning beam) deviate from the imaging optical element (imaging lens) in the imaging optical system so as to satisfy the following relationship. A scanning area is set when it passes through and reaches the surface to be scanned.
θu> θl (1)
Ldu> Ldl (2)
That is, the conditions of Expression (1) and Expression (2) are satisfied.

  Further, in the imaging optical system 6, the length from the reference line of the second imaging lens 6b closest to the scanned surface 8 to the end (outer end) on the same side as the light source 1 is L2u (mm), The length to the end (outer end) opposite to the light source 1 is L2l (mm). At this time, the shape of the second imaging lens 6b is set so as to satisfy the following relationship.

L2u> L2l (3)
About this, it is more preferable to satisfy the following relationship.

1.1 <L2u / L2l (3a)
The fact that it is not preferable to fall outside the range of the expression (3) will be described in detail later from the viewpoint of miniaturization.

  On the other hand, in the imaging optical system 6, the length from the reference line of the first imaging lens 6a closest to the deflector 5 to the end (outer end) on the same side as the light source 1 is L1u (mm), and the light source The length to the end (outer end) opposite to 1 is L1l (mm).

  At this time, the shape of the first imaging lens 6a satisfies L1u≈L1l (hereinafter, approximately equal is represented by “≈” symbol). This corresponds to satisfying the following relationship.

0.9 ≦ L1u / L1l ≦ 1.1 (4)
The fact that it is not preferable to fall outside the range of the formula (4) will be described later in detail from the viewpoint of performance and mounting accuracy.

  The length on the side opposite to the light source 1 with respect to the reference lines of the first imaging lens 6a and the second imaging lens 6b is as follows in order to minimize Lr shown in FIG. I try to satisfy the relationship.

L1l ≦ L2l (5)
With the configuration of the imaging optical system 6 described above, in this embodiment, the length Lr (FIG. 1A) in the direction parallel to the scanned surface 8 of the optical box 800 from the reference line on the side opposite to the light source 1 is reduced. can do. That is, the minimum length Lin + Lr in the direction parallel to the scanned surface 8 of the theoretical optical box 800 can be reduced. Here, Lin is the length in the direction parallel to the scanned surface 8 of the optical box 800 from the reference line on the same side as the light source 1. This can contribute to the downsizing of the optical scanning device 61 (FIG. 2) and the entire image forming apparatus.

(Specific description of the first imaging lens 6a)
In the present embodiment, the first imaging lens 6a closest to the deflector 5 in the imaging optical system 6 is entirely composed of a refractive surface, and a glass lens is used to improve stability against temperature changes. ing. The glass lens is not necessarily a shape to be rotated from the viewpoint of processing and evaluation, but it is desirable that the glass lens has a symmetrical shape with respect to a certain axis, for example, each axis in the main scanning direction and the sub-scanning direction.

  From the viewpoint of performance, the first imaging lens 6a is symmetric, but when the first imaging lens 6a processed with L1u≈L1l is set to L1u> L1l so as to correspond to the scanning region, It is necessary to perform additional cutting, which leads to an increase in cost. That is, in the present embodiment, it is desirable that L1u≈L1l.

  Further, as shown below, it is desirable that L1u≈L1l from the viewpoint of the mounting accuracy of the first imaging lens 6a. Similarly to the second imaging lens 6b, when the shape of the first imaging lens 6a is L1u> L1l, a step is generated in the positioning reference in the optical axis direction provided at both ends of the lens as shown in FIG. 9B. Two reference planes of different X1 and X2 are created.

  On the other hand, when the shape of the first imaging lens 6a is L1u≈L1l, as shown in FIG. 9 (a), the positioning reference in the optical axis direction provided at both ends of the lens is on the same plane, and a single reference Surface X is created. When there are a plurality of reference planes when the first imaging lens 6a is attached to the optical box 800, the attachment accuracy is reduced as compared with the case where the first imaging lens 6a is on the same plane, and the main scanning cross section of the first imaging lens 6a. The amount of yaw generation, which is internal rotation (rotation in the paper of FIG. 9), increases.

  Here, by using specific numerical values, the amount of yaw that is assumed is compared when the shape of the first imaging lens 6a is L1u> L1l and when L1u≈L1l. When the shape of the first imaging lens 6a is L1u> L1l, the positioning reference interval Ly in the left and right optical axis directions in FIG. 9B is 22 mm, and the management accuracy of the relative difference between the left side and the right side is different from the positioning reference. Is ± 0.05 mm, and the amount of yaw generated is ± 7.81 ′.

  On the other hand, when the shape of the first imaging lens 6a is L1u≈L1l, the positioning reference interval Ly in the left and right optical axis directions in FIG. 9A is 23 mm, and the management accuracy of the relative difference between the left side and the right side is the positioning reference. Are on the same plane and are ± 0.01 mm. Therefore, the amount of yaw generated is ± 1.49 ′.

  Further, for the second imaging lens 6b in which L2u> L2l is set to reduce the size of the entire apparatus, as shown in FIG. 9 (d), the management accuracy of the relative difference between the left side and the right side has a level difference of ± 0.05 mm. However, the distance Ly between the left and right is as large as 130 mm. Therefore, the amount of yaw generated is ± 1.32 ′.

  FIG. 10 shows vertical amounts of changes dm, ds, dy, and dz in the main scanning direction focus position, sub-scanning direction focus position, main scanning direction dot position, and sub-scanning direction dot position when a certain amount of yaw occurs. It is the graph which showed the position on a to-be-scanned surface on the axis | shaft on the horizontal axis. The solid line graph represents the sensitivity of the first imaging lens 6a, and the broken line graph represents the sensitivity of the second imaging lens 6b.

  The influence on the optical performance when the yaw is generated is larger in the first imaging lens 6a. Also, the first imaging lens 6a has a smaller left-right distance Ly, and the influence of the management accuracy of the relative difference between the left side and the right side on the amount of yaw generation is greater. Therefore, the positioning reference in the left and right optical axis directions of the first imaging lens 6a should be arranged on the same plane, and the shape is preferably L1u≈L1l.

(Specific description of the second imaging lens 6b)
In the imaging optical system 6, the second imaging lens 6b closest to the scanned surface 8 is composed of all refractive surfaces made of resin (plastic) that can be manufactured at low cost, unlike the first imaging lens 6a made of glass. Is a lens to be used. Resin lenses are manufactured by injection molding, and the degree of freedom with respect to the lens shape is higher than that of glass lenses.

  In order to manufacture a resin lens at a lower cost, it is possible to reduce the number of materials and increase the number of moldings per shot. Since the second imaging lens 6b closest to the surface to be scanned 8 has a relatively large lens size, the number of shots per molding shot depends on the capacity of the molding machine, mainly the mold size and mold clamping that can be mounted. Power is rate limiting. From the viewpoint of reducing the material and increasing the number of moldings per shot, it is desirable that the second imaging lens 6b be downsized. Therefore, when θu> θl, the shape of the second imaging lens 6b is L2u> L2l.

  When the lens shape is asymmetric with respect to the optical axis, the lens thickness at each end is different, and the thickness increases as the distance from the optical axis is shorter. In injection molding, the resin material is preferably poured from the thicker side, so the gate during molding is preferably arranged on the side opposite to the light source 1 with respect to the reference line.

  Here, the gate position of the resin lens in the present embodiment is arranged at one position on the end in the longitudinal direction. FIG. 15 is a diagram schematically illustrating the gate arrangement and the resin flow during molding. FIGS. 15B and 15D have a plurality of gates, and welds are generated where the resin filled from the gates collides with each other, leading to a reduction in image quality. It is good to have a place. FIG. 15A is filled with resin from the longitudinal direction, and FIG. 15C is filled with resin from the short direction. From the viewpoint of the resin filling property of the entire lens as a molded product, it is preferable to fill the resin from the longitudinal direction. Therefore, the gate is arranged at one place in the longitudinal end portion.

(Effect of this embodiment)
As described above, in the present embodiment, L1u≈L1l from the viewpoint of performance and the like, and the first imaging lens 6a is set so as to satisfy the relationship of L1l ≦ L2l in order to minimize Lr shown in FIG. The size is set. If this embodiment is adopted, the minimum length Lin + Lr (FIG. 1A) in the direction parallel to the surface to be scanned 8 of the optical box 800 can be reduced, which can contribute to the miniaturization of the entire apparatus.

  Furthermore, since the effect of the present embodiment is more effective from the viewpoint of increasing the cartridge capacity in the optical scanning apparatus employing the above-described oblique incidence optical system, the case where the oblique incidence optical system is employed will be described.

  FIG. 7A is a sub-scan sectional view of a conventional optical scanning device, and reference numerals 31 and 32 denote developing devices. FIG. 7B is a sub-scan sectional view of the optical scanning device in which the capacity of the cartridge (particularly the toner container integrated with the developing device) is increased in order to meet demands such as mass printing. FIGS. 8A and 8B are schematic views of main scanning sections corresponding to the respective parts. The distance from the deflection point of the conventional optical scanning device shown in FIG. 8A to the surface to be scanned (hereinafter, the optical path length of the scanning system) is 215.21 mm, and the fθ coefficient is 187 mm / rad. The main numerical values are shown in Table 1. At this time, the light flux width in the main scanning direction on the exit surface of the light collecting element 2 is 3.62 mm.

  On the other hand, the optical path length of the scanning optical system of the optical scanning device shown in FIG. 8B in which the capacity of the developing device is increased is 240 mm, and the fθ coefficient is 210 mm / rad. The main numerical values are shown in Table 2.

  Here, the main scanning vertical magnification becomes 110.3 times, and the optical system becomes very sensitive to the assembly error. Therefore, the main scanning vertical magnification should be suppressed to 100 times or less in consideration of the assembly process. Table 3 shows main numerical values of the optical scanning device in which the main scanning vertical magnification is suppressed. In the optical scanning device shown in Table 3, in order to reduce the main scanning longitudinal magnification, the collimator lens (the light condensing element 2) is changed to one having a long focal length, which increases the total length of the incident optical system. Become.

  Here, in an optical scanning device that employs an oblique incidence optical system, it is necessary to be able to arrange the upper and lower elements in the sub-scanning direction without interfering with each other. In an ordinary optical scanning device, an incident optical system is arranged as shown in FIG. 3 (full length L of the incident optical system). On the other hand, in the optical scanning device in which the capacity of the cartridge (particularly the toner container) is increased, it is necessary to increase the beam diameter in order to keep the imaging spot diameter small, and each element of the incident optical system is large. (FIG. 4). In FIG. 4, the total length of the incident optical system is increased to L2 as the condensing elements 2u and 2l are increased in diameter.

  5 is a diagram schematically showing the arrangement of the incident optical system when the oblique incident angle is set smaller than that in FIG. 3, and FIG. 6 shows the length of the incident optical system with the addition of beam combining means. It is the figure which showed typically arrangement | positioning of the incident optical system which shortened. In FIG. 5, the condensing elements 2u and 2l interfere with each other as the condensing elements 2u and 2l increase in diameter. In FIG. 6, the cost increases due to an increase in the number of parts, ie, beam combining means, and a requirement for high assembly accuracy.

  In FIG. 4, the total length of the incident optical system is increased to L2 as the condensing elements 2 u and 2 l are increased, so that in FIG. 1A, the incident optical system has a direction parallel to the scanned surface 8. The length Lin is increased. The theoretical minimum length Lin + Lr in the direction parallel to the surface to be scanned 8 of the optical box 800 becomes larger as the incident optical system becomes longer. However, according to the present embodiment, even if the optical path length of the incident optical system is increased, the incident optical system can be accommodated in a smaller optical scanning device without increasing the number of components.

  That is, regarding the theoretical minimum length Lin + Lr in the direction parallel to the scanned surface 8 of the optical box 800, Lr can be reduced even if Lin increases. As a result, the minimum length Lin + Lr in the direction parallel to the scanned surface 8 of the optical box 800 can be reduced while coping with the increase in the capacity of the cartridge (particularly the toner container), which can contribute to downsizing of the entire apparatus. .

  According to the present embodiment, the second imaging lens 6b closest to the scanning surface is reduced in size by an asymmetric length along the effective area, while the first imaging lens 6a closest to the deflector is ineffective. Priority is given to performance with a symmetrical length even if left behind. According to the present embodiment, it is possible to provide a low-cost and compact optical scanning device that suppresses deterioration in image quality and an image forming apparatus using the same.

<< Second Embodiment >>
FIG. 11 is a schematic diagram of a main scanning section of the optical scanning device according to the second embodiment of the present invention. In the present embodiment, the difference from the first embodiment is that a resin (plastic) lens that can be manufactured at low cost is employed for the first imaging lens 6a.

  As described in the first embodiment, also in this embodiment, θu> θl, Ldu> Ldl, L2u> L2l, and L1l ≦ L2l are set to reduce the size of the entire apparatus. Even if a plastic lens is used which has a higher degree of freedom with respect to its shape than a glass lens, the shape of the first imaging lens 6a is L1u≈L1l.

  As described in the first embodiment, it is desirable that L1u≈L1l from the viewpoint of the performance and the mounting accuracy of the first imaging lens 6a.

  Further, from the viewpoint of manufacturing the first imaging lens 6a made of resin, it is desirable that L1u≈L1l as shown below. Since the first imaging lens 6a has imaging performance in the main scanning direction and fθ characteristics as shown in FIG. 11, the first imaging lens 6a has a main scanning sectional shape close to an arc. FIG. 12 schematically shows the force acting on the mold in the pressure-holding process when the first imaging lens 6a is injection molded. Solid arrows indicate the distribution of forces acting on the specular frames 401s and 401d from the first imaging lens 6a, which is a molded product, and the white arrows indicate the direction of the forces received by the specular frames 401s and 401d. .

  In FIG. 12A showing the case where the shape of the first imaging lens 6a is L1u> L1l, the force acting on the mirror piece from the molded product is distributed asymmetrically on the left and right, and the mirror piece is in the optical axis direction of the lens. On the other hand, it receives force in an oblique direction. At this time, the clamping force applied from the molding machine to the mold is a direction along the optical axis of the lens.

  Thus, if the direction of the clamping force and the direction of the force received by the mirror surface piece are different, the mirror surface piece may be laterally displaced, which may cause problems such as surface eccentricity of the lens as a molded product and a decrease in durability of the mold.

  FIG. 13 shows the amounts of change dm, ds, dy of the main scanning direction focus position, sub-scanning direction focus position, main scanning direction dot position, and sub-scanning direction dot position when a certain amount of lateral displacement of the lens surface occurs. It is the graph which showed the position on the to-be-scanned surface on the horizontal axis | shaft on dz. The solid line graph indicates the incident surface of the first imaging lens 6a, the alternate long and short dash line graph indicates the exit surface of the first imaging lens 6a, the broken line graph indicates the incident surface of the second imaging lens 6b, and the dotted line graph indicates the second connection. The sensitivity of each exit surface of the image lens 6b is shown. If each surface of the first imaging lens 6a is laterally shifted, the optical performance is deteriorated.

  On the other hand, in FIG. 12B showing the case where the shape of the first imaging lens 6a is L1u≈L1l, the force acting on the mirror piece from the molded product is distributed symmetrically on the left and right, and the mirror piece is the optical axis of the lens. It receives force in the direction along the direction. In this case, the direction of the clamping force and the direction of the force received by the mirror piece coincide with each other, which is advantageous in terms of maintaining the performance of the lens as a molded product and durability of the mold. Therefore, the shape of the first imaging lens 6a made of resin is preferably L1u≈L1l from the viewpoint of manufacturing.

  In the first embodiment, it has been described that the second imaging lens 6b having a relatively large size is preferably downsized from the viewpoint of reducing the material and increasing the number of moldings per shot. However, since the first imaging lens 6a is relatively small in size, even if the shape is set to L1u> L1l, the material reduction effect is poor. In addition to this, the number taken per shot is not the clamping force as the capacity of the molding machine, but the cavity arrangement in the mold is rate limiting.

  Taking the above into consideration, considering the overall performance and cost, the shape of the first imaging lens 6a made of resin is preferably L1u≈L1l.

(Stability against temperature change)
Next, stability against temperature change will be described. Among the components constituting the optical scanning device, main portions that generate heat during operation of the device are a drive system (not shown) that rotates the deflector 5, a light source 1, and a substrate (not shown) that controls the light source 1. Can be mentioned. The first imaging lens 6a closest to the deflector 5 (that is, the heat source) undergoes a large temperature change compared to other imaging lenses. In the present embodiment, the first imaging lens 6a is made of resin, and is likely to expand / contract due to temperature changes.

  FIG. 14 is a diagram schematically showing how the first imaging lens 6 a is held in the optical box 800. In FIG. 14A, the position of the first imaging lens 6a in the main scanning direction is determined by the shapes 301L and 301R formed in the optical box 800, and the shapes 805L and 805R similarly formed in the optical box 800 are determined. The position in the optical axis direction is determined. The adhesives 302L and 302R are bonded and fixed in the vicinity of the shapes 805L and 805R.

  At this time, the distances from the optical axis of the first imaging lens 6a to the positioning shapes 301L and 301R are both equal to A. Similarly, the distances to the fixing positions 302L and 302R by the adhesive are both B and equal to each other.

  In FIG. 14B, the bottom surface of the first imaging lens 6a is fixed to the optical box 800 with adhesives 302L and 302R, but the distance to the positioning shapes 301L and 301R is A, adhesive. The distance to the fixed positions 302L and 302R is B.

  FIG. 14C shows the position of the first imaging lens 6a in the main scanning direction when the first imaging lens 6a is assembled to the optical box 800 by determining the position in the main scanning direction by an abutting reference 301E using a tool or the like. It is fixed to the optical box 800 with adhesive adhesives 302L and 302R. Thereafter, the abutting reference 301E is retracted and fixed only with an adhesive. The distances to the fixing positions 302L and 302R by the adhesive are both B and equal to each other.

  Thus, the place where the first imaging lens 6a is fixed to the optical box 800 (that is, the restraint when expansion / contraction occurs when the temperature changes) is set as the optical axis of the first imaging lens 6a overlapping the reference line. By arranging them symmetrically, the influence on the optical performance due to the temperature change can be suppressed.

  Further, it is more desirable to make the weight shared by the above-mentioned adhesives equal. Also from this viewpoint, the shape of the first imaging lens 6a is preferably L1u≈L1l.

  Based on the above description, L1u≈L1l in the present embodiment indicates a case corresponding to one or more of the following. That is, the positioning reference in the optical axis direction exists on the same plane, the difference in length between L1u and L1l is approximately within 10%, the same side as the light source 1 with respect to the optical axis of the first imaging lens 6a, The difference in volume on the opposite side is approximately within 5%.

  Further, L2u> L2l indicates a case corresponding to any one or more of the following. That is, the positioning reference in the optical axis direction is not located on the same plane, or the difference in length between L2u and L2l is 10% or more, or the same side as the light source 1 with respect to the optical axis of the second imaging lens 6b, The difference in volume on the opposite side is either 5% or more.

(Position of gate part)
In FIG. 11 schematically showing the present embodiment, the gate part when the first imaging lens 6a made of resin is molded is the same as the gate part when the second imaging lens 6b made of resin is molded. It is arranged on the opposite side of the light source 1 with respect to the line.

  As shown in FIG. 16, a resin lens manufactured by injection molding generally has a strain remaining on the gate portion side, that is, birefringence is likely to occur. FIG. 16 is a diagram schematically illustrating an example in which a lens manufactured by injection molding is observed with crossed Nicols.

  In the present embodiment, the light beam passing region in the main scanning direction of the second imaging lens 6b is wide on the same side as the light source 1 and narrower on the opposite side to the light source 1, while the first imaging lens 6a has a narrow side. As described above, the shape is L1u≈L1l. That is, an unused area of the first imaging lens 6a is generated on the side opposite to the light source 1. By aligning the unused area and the direction of the gate portion where birefringence is likely to occur, deterioration of optical performance can be suppressed.

  As described above, according to this embodiment, it is possible to provide a low-cost and compact optical scanning device that suppresses deterioration in image quality. That is, it can contribute to high image quality and downsizing of the entire apparatus.

<< Third Embodiment >>
FIG. 17 is a schematic diagram of a main scanning section of an optical scanning device according to a third embodiment of the present invention. In the present embodiment, the difference from the second embodiment is that the gate portion at the time of molding the first imaging lens 6a made of resin (plastic) is arranged on the same side as the light source 1 with respect to the reference line. That is. This arrangement is adopted from two viewpoints.

  The first viewpoint is print image quality. As described above, a resin lens manufactured by injection molding generally has distortion remaining on the gate portion side, that is, birefringence is likely to occur. The shape of the second imaging lens 6b is L2u> L2l from the viewpoint of downsizing the entire apparatus. Since it is asymmetric with respect to the optical axis, the lens thickness at each end is different, and the thickness becomes larger as the distance from the optical axis is shorter. In injection molding, the resin material is preferably poured from the thicker side, so the gate during molding is preferably arranged on the side opposite to the light source 1 with respect to the reference line.

  At this time, the influence of birefringence generated by the second imaging lens 6b occurs only on the side opposite to the light source 1 with respect to the reference line, resulting in uneven printing image quality. Therefore, the gate portion when molding the first imaging lens 6a made of resin is arranged in a direction different from that of the second imaging lens 6b, that is, on the same side as the light source 1 with respect to the reference line. The influence of birefringence generated by the first imaging lens 6a occurs on the same side as the light source 1 with respect to the reference line. In this way, by generating the influence of birefringence of the first imaging lens 6a and the second imaging lens 6b in different directions, it is possible to obtain a uniform and good print image quality.

  The second viewpoint is assemblability. A resin lens manufactured by injection molding generally faces a gate from the direction of force acting on a molded product in an injection process for filling a resin material into a mold cavity and a subsequent pressure holding process. The surface on the side (hereinafter referred to as the anti-gate side) is the most accurate. Therefore, the positioning reference plane in the main scanning direction when the first imaging lens 6a is assembled to the optical box 800 is provided on the opposite gate side of the first imaging lens 6a. At the time of assembly, the first imaging lens 6a is pressurized from the gate side so that the first gate lens 6a abuts against the optical box 800.

  In the present embodiment, the distance from the end opposite to the light source 1 with respect to the reference line of the first imaging lens 6a to the wall surface of the optical box 800 is reduced, while the distance to the scanned surface 8 of the incident optical system is reduced. Due to the length in the parallel direction, a large distance is secured on the same side as the light source 1. In order to assemble easily and accurately, it is desirable to pressurize the first imaging lens 6a from the same side as the light source 1 that can ensure a sufficient stroke of a tool or the like when the first imaging lens 6a is abutted against the optical box 800. In other words, it is desirable that the gate portion at the time of molding the first imaging lens 6a is disposed on the same side as the light source 1 with respect to the reference line.

  As described above, this embodiment can also provide a low-cost and compact optical scanning device that suppresses deterioration in image quality. That is, it can contribute to high image quality and downsizing of the entire apparatus.

<< Fourth Embodiment >>
FIG. 18 shows an optical scanning device designed with a wider angle of view (for example, A3 width). When only a part of the angle of view (for example, A4 width is used), for example, an area to be used for A4 width is arbitrarily selected. It is the figure which showed typically the case where selection is performed. That is, assuming various cases such as when an optical scanning device designed in the past with A3 width is diverted to an A4 width optical scanning device, or when an A3 width optical scanning device and an A4 width optical scanning device are designed in common. Yes.

  When a part of a wide scanning region is selected, any of the following three patterns S1, S2, and S3 can be selected as a scanning method in FIG.

S1 (scans widely on the same side as the light source 1): θu> θl and Ldu> Ldl
S2 (scan evenly around the reference line): θu = θl and Ldu = Ldl
S3 (widely scans the opposite side of the light source 1): θu <θl and Ldu <Ldl
The most expectable miniaturization of the entire apparatus is S1.

  Further, from the viewpoint of optical performance, the optical performance is better when the angle formed by the incident light and the scanning light is smaller. Therefore, S1 is preferable. On the other hand, if the difference between Ldl and Ldu is increased, adverse effects such as asymmetry of the optical performance are caused. Since it appears, there are some aspects where S2 is desirable. The amount by which the scanning area is moved toward the light source 1 may be selected while evaluating various performances.

  Further, from the viewpoint of manufacturing, the first imaging lens 6a is smaller than the second imaging lens 6b, so that the number of moldings per one shot is large, the molding cycle time is shortened, and both unit time per unit time. There is a difference in the production number. Therefore, when the production quantity increases, the number of molds for molding the second imaging lens 6b may be 2 to 4 times the number of molds for molding the first imaging lens 6a. .

  When the shape of the first imaging lens 6a is L1u≈L1l, the first imaging lens 6a is used in common while the second imaging lens 6b is optimally shaped for each product as follows. Production can be realized.

A) Use a product corresponding to S2 as the second imaging lens 6b in the A3 width product. B) Use a product corresponding to S1 as the second imaging lens 6b in the A4 width product prioritizing miniaturization. C) In the A4 width image quality priority product, the second imaging lens 6b corresponding to S2 is used. As described above, this embodiment can also provide a low-cost and compact optical scanning device that suppresses image quality degradation. That is, it can contribute to high image quality and downsizing of the entire apparatus.

(Modification)
As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to these embodiment, A various deformation | transformation and change are possible within the range of the summary.

(Modification 1)
In the above-described embodiment, the case where the optical axes of the first and second incident optical systems are substantially parallel to each other when projected onto the main scanning section is shown. However, the optical axes of the first and second incident optical systems are A large angle may be provided (non-parallel). In this case, Lin at the minimum length Lin + Lr in the direction parallel to the surface to be scanned 8 of the theoretical optical box 800 is reduced, so that the size can be further reduced.

(Modification 2)
Among the plurality of imaging lenses in the imaging optical system described above, the number of the molding molding lenses made of resin is arranged on the same side as the light source with respect to the reference line, and the light source with respect to the reference line Alternatively, the difference between the numbers arranged on the opposite side may be 1 or less.

(Modification 3)
In the above-described embodiment, the configuration is configured as θu>θl> 0 and Ldu>Ldl> 0. However, the configuration may be configured as θu> θl = 0 and Ldu> Ldl = 0.

DESCRIPTION OF SYMBOLS 1 ... Light source 5 ... Deflector 6 ... Imaging lens system (f (theta) lens system), 6a ... First imaging lens, 6b ... Second imaging lens

Claims (21)

A deflector that deflects a light beam from a light source and optically scans a scanning region on a surface to be scanned in a main scanning direction; and an imaging optical system that focuses the light beam deflected by the deflector on the scanning region. An optical scanning device,
Within the main scanning section, the principal ray orthogonal to the scanning region of the light beam deflected by the deflector is used as a reference line and is incident on the first end portion on the same side as the light source in the main scanning direction of the scanning region. The angle between the principal ray and the reference line is θu (deg), the distance between the first end portion and the reference line is Ldu (mm), and the first of the scanning region opposite to the light source in the main scanning direction. When the angle between the principal ray incident on two ends and the reference line is θl (deg), and the distance between the second end and the reference line is Ldl (mm),
θu> θl
Ldu> Ldl
Meets the conditions
The imaging optical system includes a first imaging optical element, and a second imaging optical element closer to the scanned surface than the first imaging optical element,
In the main scanning direction, the distances from the reference line to the end of the first imaging optical element on the same side as the light source and the end opposite to the light source are L1u and L1l, respectively. When the distances to the end of the second imaging optical element on the same side as the light source and the end on the opposite side of the light source are respectively L2u and L2l,
0.9 ≦ L1u / L1l ≦ 1.1
L2u> L2l
L1l ≦ L2l
An optical scanning device characterized by satisfying the following condition. 1.1 <L2u / L2l
The optical scanning device according to claim 1, wherein the following relationship is satisfied.   3. The first and second incident optical systems for entering the different deflecting surfaces of the deflector as the incident optical system for causing the light beam from the light source to enter the deflector. 4. Optical scanning device.   The first incident optical system includes third and fourth incident optical systems that are obliquely incident on the deflector in the sub-scanning direction, and the second incident optical system is inclined with respect to the deflector in the sub-scanning direction. The optical scanning device according to claim 3, further comprising incident fifth and sixth incident optical systems. The deflector deflects the third and fourth light fluxes and the fifth and sixth light fluxes so that the first and second scanned surfaces and the third and fourth scanned surfaces are in the main scanning direction, respectively. Light scan to
The first and second imaging optical elements are provided corresponding to the first and second scanned surfaces and the third and fourth scanned surfaces, respectively. Optical scanning device.   The first imaging optical element provided corresponding to the first and second scanned surfaces is shared, and the first imaging provided corresponding to the third and fourth scanned surfaces. 6. The optical scanning device according to claim 5, wherein the optical element is shared.   Within the main scanning section, the optical axes of the respective incident optical systems that allow the third and fourth light beams to enter the deflector overlap as the first optical axis, and the fifth and sixth light beams overlap the deflector. 7. The optical scanning according to claim 5, wherein the optical axes of the respective incident optical systems incident on the optical system overlap as a second optical axis, and the first and second optical axes are provided in parallel to each other. apparatus.   Within the main scanning section, the optical axes of the respective incident optical systems that allow the third and fourth light beams to enter the deflector overlap as the first optical axis, and the fifth and sixth light beams overlap the deflector. 7. The light according to claim 5, wherein the optical axes of the respective incident optical systems incident on the light beam overlap with each other as a second optical axis, and the first and second optical axes are provided non-parallel to each other. Scanning device.   9. The optical scanning device according to claim 1, wherein an incident optical system that causes a light beam from the light source to enter the deflector includes a condensing element that converts a state of the light beam. 10.   The light according to any one of claims 1 to 9, wherein the first and second imaging optical elements are first and second imaging lenses each having a refractive surface. Scanning device.   The optical scanning device according to claim 10, wherein at least one of the first and second imaging lenses is made of resin.   The optical scanning device according to claim 11, wherein the first imaging lens is made of glass, and the second imaging lens is made of resin.   The optical scanning device according to claim 11, wherein the first and second imaging lenses are made of resin.   14. The optical scanning device according to claim 12, wherein in the second imaging lens, a molding gate is disposed on a side opposite to the light source with respect to the reference line.   14. The optical scanning device according to claim 13, wherein in the first imaging lens, a molding gate is arranged on the same side as the light source with respect to the reference line.   14. The optical scanning device according to claim 13, wherein in the first imaging lens, a molding gate is disposed on the side opposite to the light source with respect to the reference line.   The position where the first imaging lens is constrained in a direction parallel to the surface to be scanned is a position symmetric with respect to the reference line. Optical scanning device.   Of the first and second imaging lenses, the number of gates at the time of molding in the resin lens is the number arranged on the same side as the light source with respect to the reference line, and the reference line 18. The optical scanning device according to claim 11, wherein the difference in the number of the light sources disposed on the side opposite to the light source is 1 or less.   The optical scanning device according to claim 1, wherein the first imaging optical element has a larger power in the main scanning direction than the second imaging optical element.   20. The optical scanning device according to claim 1, a developing device that develops an electrostatic latent image formed on the scanned surface by the optical scanning device as a toner image, and the developed toner An image forming apparatus comprising: a transfer device that transfers an image to a transfer material; and a fixing device that fixes the transferred toner image to the transfer material.   20. The optical scanning device according to claim 1, and a printer controller that converts a color signal output from an external device into image data and inputs the image data to the optical scanning device. Image forming apparatus.