JP2011048085A - Scanning optical apparatus and electrophotographic image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a scanning optical apparatus and an electrophotographic image forming apparatus in which one-side magnification difference at an elevated temperature is reduced and a color shift in a main scanning direction is suppressed with a simple configuration. <P>SOLUTION: For positioning a plurality of imaging members 21, 22, which image a luminous flux from a light source 2 deflected and scanned with a rotary polygon mirror 10 onto an electrophotographic photoreceptor 82, in the main scanning direction S1, a positioning part 21a of the imaging member 21 closest to the rotary polygon mirror among the plurality of imaging members is disposed on the side, with respect to the optical axis OA of the closest imaging member 21, at which the temperature of the imaging member 21 rises during the operation of the closest scanning optical apparatus 50, and the distance L21 in the main scanning direction between the optical axis OA of the closest imaging member 21 and the positioning part 21a is larger than the distance L22 in the main scanning direction between the optical axis OA of the other imaging member 22 and the positioning part 22a of the other imaging member 22. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a scanning optical apparatus and an electrophotographic image forming apparatus having a plurality of imaging members that form an image of a light beam deflected and scanned by a rotating polygon mirror on an electrophotographic photosensitive member.

2. Description of the Related Art Conventionally, an electrophotographic image forming apparatus (hereinafter referred to as an image forming apparatus) is a scanning optical apparatus for forming an electrostatic latent image by forming a light beam on an electrophotographic photoreceptor (hereinafter referred to as a photoreceptor). Is provided. The scanning optical device exposes the photosensitive member to laser light to form an electrostatic latent image on the photosensitive member. The electrostatic latent image is developed with a developer (hereinafter referred to as toner) to form a developer image (hereinafter referred to as toner image). After the toner image is transferred onto the recording medium, it is heated and pressurized by a fixing device and fixed on the recording medium. In this way, an image is formed on the recording medium.
The scanning optical device has an optical box (hereinafter referred to as a housing). The housing is provided with a light source, a rotating polygon mirror, an fθ lens, and a folding mirror. The light source emits laser light based on the image information. The rotary polygon mirror deflects and scans the laser light emitted from the light source and directs it toward the fθ lens. The fθ lens scans the laser beam polarized and scanned by the rotating polygon mirror at a constant speed. The folding mirror reflects the laser beam from the fθ lens and irradiates the photosensitive member through the slit-shaped opening of the housing. The fθ lens spot-forms laser light on the photoconductor to form an electrostatic latent image on the photoconductor. Here, the fθ lens is positioned with high accuracy in the housing so that the laser beam can scan the photosensitive member at a constant speed.

  In recent years, there has been a demand for speeding up image formation. In order to speed up image formation, it is necessary to rotate the rotary polygon mirror at high speed. As the rotary polygon mirror rotates at high speed, heat generated by the motor for rotating the rotary polygon mirror increases, and the temperature in the scanning optical device rises. Since the housing and the fθ lens are thermally expanded due to the temperature rise, the optical axis position of the fθ lens may change and optical performance such as aberration may be deteriorated. In addition, a one-magnification difference in which the magnification of the left and right images differs from the center of the image due to thermal expansion of the fθ lens may occur. In order to solve such a problem, Patent Document 1 discloses that a plurality of fθ lenses have a plurality of positioning portions in the main scanning direction fitted between a pair of positioning ribs provided in the housing. The positioning of an fθ lens is disclosed. This prevents the optical axis shift in the main scanning direction of the plurality of fθ lenses due to temperature changes. Japanese Patent Laid-Open No. 2004-260688 makes constant the product of the distance between the positioning unit in the main scanning direction of each of the plurality of fθ lenses and the optical axis and the linear expansion coefficient of each lens member of the plurality of fθ lenses. Disclosure. Accordingly, even when the linear expansion coefficients of the lens members are different, the degree of expansion or contraction in the main scanning direction of each of the plurality of fθ lenses due to temperature change can be made substantially the same. In Patent Document 2, when the linear expansion coefficients of the plurality of fθ lenses are the same, the distances between the positioning portions in the main scanning direction of the plurality of fθ lenses and the optical axis are equal.

JP 2003-207734 A Japanese Patent Laid-Open No. 9-105878

During the operation of the scanning optical device, the temperature of the housing of the scanning optical device rises. Since the temperature distribution in the housing is not uniform, there is a difference in the amount of deformation due to thermal expansion among the plurality of imaging members. Further, the temperature distribution in one imaging member is not uniform. Due to a non-uniform temperature rise in one imaging member, a difference in deformation due to thermal expansion occurs between one part of the imaging member and the other part. There is a possibility that a half magnification difference may occur due to a difference in deformation amount due to thermal expansion of the imaging member.
The present invention provides a scanning optical apparatus and an electrophotographic image forming apparatus that can reduce the difference in half magnification at the time of temperature increase with a simple configuration.

In order to achieve the above object, a scanning optical apparatus according to the present invention includes a light source, a rotating polygon mirror that deflects and scans a light beam emitted from the light source in a main scanning direction, and a light beam that is deflected and scanned by the rotating polygon mirror. A plurality of imaging members that form an image on the electrophotographic photosensitive member, a housing in which the light source, the rotary polygon mirror, and the plurality of imaging members are disposed, and each of the plurality of imaging members A positioning portion provided in each of the plurality of imaging members to position the lens in the main scanning direction with respect to the housing, and is closest to the rotary polygon mirror among the plurality of imaging members The imaging member positioning portion is provided on the side where the temperature of the nearest imaging member becomes higher during the operation of the scanning optical device with respect to the optical axis of the nearest imaging member, and the nearest imaging image The optical axis of the member and the positioning portion; The distance in the main scanning direction is configured to be larger than the distance in the main scanning direction between the optical axis of the other imaging member and the positioning portion of the other imaging member. .
An electrophotographic image forming apparatus according to the present invention includes the scanning optical device.

  According to the scanning optical apparatus and the electrophotographic image forming apparatus of the present invention, it is possible to reduce the half magnification difference at the time of temperature increase with a simple configuration.

1 is a diagram illustrating an image forming apparatus according to a first exemplary embodiment. 1 is a diagram illustrating a scanning optical device according to a first embodiment. The figure which shows the 1st attaching part of a 1st imaging lens. The figure which shows the 2nd attachment part of a 2nd imaging lens. FIG. 6 is a diagram showing temperature changes of the first and second imaging lenses of Example 1. FIG. 4 is a diagram illustrating an image forming apparatus according to a second exemplary embodiment. FIG. 6 is a diagram illustrating a scanning optical device according to a second embodiment. FIG. 6 is a diagram showing a temperature change of the first imaging lens of Example 2. FIG. 6 is a front view of a first imaging lens of Example 2.

[Example 1]
(Image forming device)
FIG. 1 is a diagram illustrating an electrophotographic image forming apparatus according to a first embodiment. The electrophotographic image forming apparatus according to the first embodiment includes a scanning optical device that forms an image on the electrophotographic photosensitive member with a light beam deflected and scanned by a rotary polygon mirror. Here, an electrophotographic image forming apparatus (hereinafter referred to as an image forming apparatus) forms an image on a recording medium using an electrophotographic image forming process. Examples of the image forming apparatus include an electrophotographic copying machine, an electrophotographic printer (for example, a color laser beam printer, a color LED printer), a facsimile apparatus, and a word processor. Hereinafter, Example 1 will be described by taking a tandem type color printer as an example of the image forming apparatus. FIG. 1A is a cross-sectional view of the image forming apparatus. FIG. 1B is a cross-sectional view showing the scanning optical device and the image forming unit.

  The image forming apparatus 100 is provided with four image forming units (image forming units) 81 (81Bk, 81C, 81M, 81Y) arranged in a line at regular intervals. The image forming unit 81Bk forms a black image. The image forming unit 81C forms a cyan image. The image forming unit 81M forms a magenta image. The image forming unit 81Y forms a yellow image. Each image forming unit 81 is provided with a drum-type electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) 82 (82a, 82b, 82c, 82d). In this embodiment, the electrophotographic photosensitive member has a drum shape, but may have a belt shape. A photoconductor layer is provided as a photosensitive layer on the drum or belt. Examples of the material for the photosensitive layer include amorphous selenium, zinc oxide, cadmium sulfide, amorphous silicon, and organic photoconductive materials. The photoconductive drum 82 has a photoconductor layer of a negatively charged OPC photoconductor (organic photoconductor) on an aluminum drum base. A primary charger 83, a developing device 84, a transfer roller 85, and a drum cleaner device 86 are disposed around each photosensitive drum 82. A scanning optical device 50 (50a, 50b, 50c, 50d) is installed below the primary charger 83 (83a, 83b, 83c, 83d) and the developing device 84 (84a, 84b, 84c, 84d). The developing devices 84a, 84b, 84c, and 84d contain black toner, cyan toner, magenta toner, and yellow toner, respectively. A transfer roller 85 (85a, 85b, 85c, 85d) as a primary transfer unit brings the intermediate transfer belt 87 into contact with the photosensitive drum 82, and a primary transfer portion between the intermediate transfer belt 87 and the photosensitive drum 82. T1 is formed.

  The photosensitive drum 82 is rotationally driven in a direction indicated by an arrow (a clockwise direction in FIG. 1A) at a predetermined process speed by a driving device (not shown). A primary charger 83 as primary charging means uniformly charges the surface of the photosensitive drum 82 to a predetermined negative potential by a charging bias applied from a charging bias power source (not shown). The scanning optical device 50 as an exposure device irradiates the uniformly charged surface of the photosensitive drum 82 with scanning light E (E1, E2, E3, E4) in accordance with an image signal, so that the surface of the photosensitive drum 82 is statically exposed. An electrostatic latent image is formed. The developing device 84 develops (makes a visible image) the electrostatic latent image formed on the photosensitive drum 82 with toner of each color to form a toner image. The transfer roller 85 transfers the toner image on the photosensitive drum 82 onto the intermediate transfer belt 87 at the primary transfer portion T1. The drum cleaner 86 (86a, 86b, 86c, 86d) has a cleaning blade for removing residual toner remaining on the surface of the photosensitive drum 82 after the primary transfer from the surface of the photosensitive drum 82.

  The intermediate transfer belt 87 is stretched between a pair of belt conveyance rollers 88 and 89, and is rotated (moved) in the direction of arrow A (counterclockwise direction in FIG. 1A). The intermediate transfer belt 87 is made of a dielectric resin such as a polycarbonate, a polyethylene terephthalate resin film, a polyvinylidene fluoride resin film, or the like. The belt conveyance roller 88 causes the intermediate transfer belt 87 to contact the secondary transfer roller 90 to form a secondary transfer portion T <b> 2 between the intermediate transfer belt 87 and the secondary transfer roller 90. A belt cleaning device 91 is provided outside the intermediate transfer belt 87 and in the vicinity of the belt conveying roller 89. The belt cleaning device 91 removes and collects residual toner remaining on the surface of the intermediate transfer belt 87 from the surface of the intermediate transfer belt 87 after the secondary transfer.

  A registration detection sensor (hereinafter referred to as a registration sensor) 71 that is a color misregistration detector detects a registration correction pattern (hereinafter referred to as a registration correction pattern) of each color formed on the intermediate transfer belt 87. Detects the amount of color misregistration.

  The paper feed cassette 92 stores a recording medium. The recording medium is an image on which an electrophotographic image forming apparatus forms an image, and includes, for example, paper, an OHP sheet, and the like. Hereinafter, the recording medium is referred to as a sheet. The sheets in the sheet feeding cassette 92 are fed one by one by a sheet feeding roller 93 to a registration roller pair (hereinafter referred to as a registration roller pair) 94. The sheet is temporarily stopped by the registration roller pair 94. Thereafter, the conveyance of the sheet is started in synchronization with the toner image on the intermediate transfer belt 87, and the toner image is transferred to a predetermined position on the sheet by the secondary transfer portion T2. The sheet on which the toner image is transferred at the secondary transfer portion T2 is heated and pressed by the fixing device 95, and the toner image is fixed on the sheet. The sheet on which the toner image is fixed is conveyed by the conveyance roller pair 96 and is discharged onto the paper discharge tray 98 by the paper discharge roller pair 97.

(Scanning optical device)
FIG. 2 is a diagram illustrating the scanning optical device 50 according to the first embodiment. FIG. 2A is a plan view showing the overall configuration of the scanning optical device 50. FIG. 2B is a cross-sectional view of the incident optical system. The scanning optical device 50 performs optical writing scanning on the photosensitive drum 82. The main scanning direction of the scanning optical device 50 is the direction indicated by the arrow S1 in FIG. The sub-scanning direction is a direction perpendicular to the paper surface of FIG. 2A and is a direction indicated by an arrow S2 in FIGS. 1B and 2B. The scanning optical device 50 includes a laser holder 1 that holds a semiconductor laser diode (single beam laser) 2 that is a light source.

(Laser holder)
As shown in FIG. 2B, the semiconductor laser diode (hereinafter referred to as “LD”) 2 is press-fitted into the lens barrel holding portion 1 a of the laser holder 1. A distal end portion 1aL of the lens barrel holding portion 1a is provided with a diaphragm portion 1c corresponding to the LD2. The diaphragm 1c shapes the light beam emitted from the LD 2 into a desired optimum beam shape. A collimator lens 6 is provided on the exit side of the stop 1c to convert the light beam that has passed through the stop 1c into a substantially parallel light beam. Two adhesive portions 1e extend forward from the distal end portion 1aL of the lens barrel holding portion 1a. The two adhesive portions 1 e are provided on both sides of the lens 6 in the main scanning direction in order to fix the collimator lens 6. The irradiation position and focus of the lens 6 are adjusted while detecting the optical characteristics of the laser beam. When the position of the lens 6 is determined, the lens 6 is bonded and fixed to the bonding portion 1e by irradiating the ultraviolet curable adhesive with ultraviolet rays. The electric circuit board 4 is provided with a laser drive circuit. The substrate 4 is electrically connected to the LD2. The BD lens 9 forms an image of a light beam reflected by a polygon mirror (hereinafter referred to as a rotating polygon mirror) 10 on a light receiving surface of the BD sensor 5. The BD sensor 5 provided on the substrate 4 detects the light beam reflected by the rotary polygon mirror 10 and passed through the BD lens 9, and outputs a synchronization signal in the main scanning direction. Based on this synchronization signal, the timing of the scanning start position at the image edge is adjusted.

(Laser holder positioning)
As shown in FIG. 2A, the optical element of the scanning optical device 50 is arranged inside an optical box (hereinafter referred to as a housing) 40. The side wall of the housing 40 is provided with a long hole portion that is continuous with the fitting hole portion 40a and the hole portion 40a for positioning the laser holder 1 and that is long in the sub-scanning direction S2. The laser holder 1 is attached to the housing 40 by fitting a fitting portion 1m provided on the outer shape of the lens barrel holding portion 1a into the hole portion 40a.
The cylindrical lens 8 has a predetermined refractive power only in the sub-scanning direction S2. The cylindrical lens 8 condenses the parallel light flux from the collimator lens 6 in a linear shape. The main scanning direction described above represents the direction of the arrow S1 in which the laser beam emitted from the LD 2 is scanned by the rotary polygon mirror 10.

  The rotary polygon mirror 10 is rotated by the scanner motor M in the arrow direction (clockwise direction) in FIG. The light beam emitted from the LD 2 is deflected and scanned by the deflecting / reflecting surface 10 r of the rotary polygon mirror 10. The fθ lens is composed of a plurality of imaging members. Specifically, the fθ lens includes a first imaging lens (closest imaging member) 21 and a second imaging lens (another imaging member) 22 as resin-molded optical elements. The first imaging lens 21 is an imaging member closest to the rotary polygon mirror 10. The fθ lens scans the light flux of the LD 2 reflected by the rotary polygon mirror 10 at a constant speed, and spot-forms the light flux on the photosensitive drum 82. The first imaging lens 21 is composed of a cylinder lens and has refractive power (power) in the main scanning direction S1. The second imaging lens 22 corrects the light beam in the sub-scanning direction S2.

(Fixing part of the imaging lens)
FIG. 3 is a view showing a first attachment portion 41 for fixing the first imaging lens 21 to the housing. FIG. 3A is a plan view of the first attachment portion 41 provided in the housing 40. The bottom plate 42 of the housing 40 is provided with a first elongated hole 43a for positioning the first imaging lens 21 in the main scanning direction S1. The first elongated hole 43a as the main scanning direction positioning locking portion is formed long along the optical axis OA. The first optical axis direction abutting portion 44 a and the second optical axis direction abutting portion 44 b for positioning the first imaging lens 21 in the optical axis direction (focus direction) OA are provided on the bottom plate 42. A first sub-scanning direction abutting portion 45a and a second sub-scanning direction abutting portion 45b for positioning the first imaging lens 21 in the sub-scanning direction (height direction) are provided on the bottom plate. The first elongated hole 43a, the first optical axis direction abutting portion 44a, the second optical axis direction abutting portion 44b, the first sub-scanning direction abutting portion 45a, and the second sub-scanning direction abutting portion 45b are first mounted. Part 41 is configured. FIG. 3B is a front view of the first attachment portion 41. FIG. 3B shows the first imaging lens 21 before being attached. On the bottom surface 21f of the first imaging lens 21, a positioning projection 21a as a main scanning direction positioning portion is provided. The protruding portion 21 a is inserted into the first long hole 43 a of the housing 40. The protrusion 21a can move in the first long hole 43a in the optical axis direction. However, the movement of the protrusion 21a in the main scanning direction S1 is restricted by the first elongated hole 43a. Therefore, the first imaging lens 21 is positioned in the main scanning direction S <b> 1 with respect to the housing 40 by the engagement between the protrusion 21 a and the first elongated hole 43 a. The protrusion 21a serves as a positioning reference for the first imaging lens 21 in the main scanning direction S1.

  The protrusion 21a is provided by being shifted from the optical axis OA of the first imaging lens 21 by a distance L21 on the downstream side in the main scanning direction S1. During the operation of the scanning optical device 50, the temperature of the downstream portion of the first imaging lens 21 in the main scanning direction S1 is higher than that of the upstream portion. The portion on the downstream side of the first imaging lens 21 that is on the higher temperature side has a large amount of elongation due to thermal expansion. If the positioning projection 21a is provided at the position of the optical axis OA of the first imaging lens 21, the length of the upstream portion and the length of the downstream portion of the first imaging lens 21 with respect to the optical axis OA are greatly different. End up. As a result, a single magnification difference that causes a difference in magnification between the left and right images with respect to the image center occurs. Therefore, in this embodiment, the protrusion 21a is provided by being shifted from the optical axis OA of the first imaging lens 21 by a distance L21 on the downstream side in the main scanning direction S1. That is, the length in the main scanning direction S1 of the portion on the downstream side with respect to the protrusion 21a is shorter than the length of the portion on the upstream side with respect to the protrusion 21a. Therefore, the elongation amount of the downstream portion having a high temperature but a short length can be made substantially the same as the elongation amount of the upstream portion having a low temperature but a long length. Thereby, during the operation of the scanning optical device 50, the left and right extension amounts of the first imaging lens 21 with respect to the optical axis OA can be made substantially the same. Therefore, generation | occurrence | production of a half magnification difference can be reduced. The distance L21 is a distance obtained by shifting the protrusion 21a from the optical axis OA so that the left and right extension amounts of the first imaging lens 21 with respect to the optical axis OA become substantially the same during the operation of the scanning optical device 50.

  A first sub-scanning direction positioning portion 21b is provided at one end of the bottom surface 21f of the first imaging lens 21, and a second sub-scanning direction positioning portion 21c is provided at the other end. FIG. 3C is a plan view of the first imaging lens 21 attached to the first attachment portion 41. A first optical axis direction positioning portion 21d is provided at one end portion on the incident surface side 21g of the first imaging lens 21, and a second optical axis direction positioning portion 21e is provided at the other end portion. One end portion on the exit surface side 21h of the first imaging lens 21 is urged toward the incident surface side in the optical axis direction by a spring (first optical axis direction urging member) 46a. The other end of the exit surface side 21h of the first imaging lens 21 is urged toward the incident surface side in the optical axis direction by a spring (second optical axis direction urging member) 46b. The first optical axis direction positioning portion 21d abuts on the first optical axis direction abutting portion 44a, and is pressed against the abutting portion 44a by the urging force of the spring 46a. The second optical axis direction positioning portion 21e is in contact with the second optical axis direction abutting portion 44b and is in pressure contact with the abutting portion 44b by the urging force of the spring 46b. Therefore, the first imaging lens 21 is positioned in the optical axis direction with respect to the first mounting portion 41. FIG. 3D is a front view of the first imaging lens 21 attached to the first attachment portion 41. One end of the top surface 21k of the first imaging lens 21 is urged downward by a spring (first sub-scanning direction urging member) 47a. The other end of the top surface 21k is biased downward by a spring (second sub-scanning direction biasing member) 47b. The first sub-scanning direction positioning portion 21b is in contact with the first sub-scanning direction abutting portion 45a and is pressed against the abutting portion 45a by the urging force of the spring 47a. The second sub-scanning direction positioning portion 21c is in contact with the second sub-scanning direction abutting portion 45b and is in pressure contact with the abutting portion 45b by the urging force of the spring 47b. Accordingly, the first imaging lens 21 is positioned with respect to the first mounting portion 41 in the sub-scanning direction (height direction).

  FIG. 4 is a diagram illustrating a second attachment portion for fixing the second imaging lens to the housing. FIG. 4A is a plan view of the second attachment portion 51 provided in the housing 40. The bottom plate 42 of the housing 40 is provided with a second elongated hole 53a for positioning the second imaging lens 22 in the main scanning direction S1. The second elongated hole 53a as the main scanning direction positioning locking portion is formed long along the optical axis OA. The third optical axis direction abutting portion 54a and the fourth optical axis direction abutting portion 54b for positioning the second imaging lens 22 in the optical axis direction (focus direction) OA are provided on the bottom plate 42. A third sub-scanning direction abutting portion 55a and a fourth sub-scanning direction abutting portion 55b for positioning the second imaging lens 22 in the sub-scanning direction (height direction) are provided on the bottom plate. The second elongated hole 53a, the third optical axis direction abutting portion 54a, the fourth optical axis direction abutting portion 54b, the third sub-scanning direction abutting portion 55a, and the fourth sub-scanning direction abutting portion 55b are provided in the second attachment. Part 51 is configured. FIG. 4B is a front view of the second attachment portion 51. FIG. 4B shows the second imaging lens 22 before being attached. The bottom surface 22f of the second imaging lens 22 is provided with a positioning projection 22a as a main scanning direction positioning portion. The protrusion 22 a is inserted into the second long hole 53 a of the housing 40. The protrusion 22a can move in the second long hole 53a in the optical axis direction. However, the movement of the protrusion 22a in the main scanning direction S1 is restricted by the second elongated hole 53a. Therefore, the second imaging lens 22 is positioned in the main scanning direction S1 with respect to the housing 40 by the engagement between the protrusion 22a and the second elongated hole 53a. The protrusion 22a serves as a positioning reference for the second imaging lens 22 in the main scanning direction S1.

  The protrusion 22a is provided by being shifted from the optical axis OA of the second imaging lens 22 by a distance L22 on the downstream side in the main scanning direction S1. During the operation of the scanning optical device 50, the temperature of the downstream portion of the second imaging lens 22 in the main scanning direction S1 is higher than that of the upstream portion. The portion on the downstream side of the first imaging lens 21 that is on the higher temperature side has a large amount of elongation due to thermal expansion. In the present embodiment, the length in the main scanning direction S1 of the portion on the downstream side with respect to the protrusion 22a is shorter than the length of the portion on the upstream side with respect to the protrusion 22a. Therefore, the elongation amount of the downstream portion having a high temperature but a short length can be made substantially the same as the elongation amount of the upstream portion having a low temperature but a long length. Thereby, during the operation of the scanning optical device 50, the left and right extension amounts of the second imaging lens 22 with respect to the optical axis OA can be made substantially the same. Therefore, generation | occurrence | production of a half magnification difference can be reduced. The distance L22 is a distance obtained by shifting the protrusion 22a from the optical axis OA so that the left and right extension amounts of the second imaging lens 22 with respect to the optical axis OA become substantially the same during the operation of the scanning optical device 50.

  A third sub-scanning direction positioning portion 22b is provided at one end of the bottom surface 22f of the second imaging lens 22, and a fourth sub-scanning direction positioning portion 22c is provided at the other end. FIG. 4C is a plan view of the second imaging lens 22 attached to the second attachment portion 51. A third optical axis direction positioning portion 22d is provided at one end of the incident surface side 22g of the second imaging lens 22, and a fourth optical axis direction positioning portion 22e is provided at the other end. One end portion on the exit surface side 22h of the second imaging lens 22 is urged toward the incident surface side in the optical axis direction by a spring (third optical axis direction urging member) 56a. The other end of the exit surface side 22h of the second imaging lens 22 is urged toward the entrance surface in the optical axis direction by a spring (fourth optical axis direction urging member) 56b. The third optical axis direction positioning portion 22d abuts on the third optical axis direction abutting portion 54a, and is pressed against the abutting portion 54a by the urging force of the spring 56a. The fourth optical axis direction positioning portion 22e is in contact with the fourth optical axis direction abutting portion 54b and is pressed against the abutting portion 54b by the urging force of the spring 56b. Accordingly, the second imaging lens 22 is positioned in the optical axis direction with respect to the second mounting portion 51. FIG. 4D is a front view of the second imaging lens 22 attached to the second attachment portion 51. One end of the top surface 22k of the second imaging lens 22 is urged downward by a spring (third sub-scanning direction urging member) 57a. The other end of the top surface 22k is urged downward by a spring (fourth sub-scanning direction urging member) 57b. The third sub-scanning direction positioning portion 22b is in contact with the third sub-scanning direction abutting portion 55a and is pressed against the abutting portion 55a by the urging force of the spring 57a. The fourth sub-scanning direction positioning portion 22c is in contact with the fourth sub-scanning direction abutting portion 55b and is in pressure contact with the abutting portion 55b by the urging force of the spring 57b. Accordingly, the second imaging lens 22 is positioned in the sub-scanning direction (height direction) with respect to the second mounting portion 51.

  Since the first and second imaging lenses 21 and 22 are biased by the first and second mounting portions 41 and 51 by springs (biasing members), the imaging lenses expand as the temperature rises. However, no great stress is applied to the imaging lens. Therefore, the optical performance of the optical scanning device can be maintained. For example, when the imaging lens is fixed to the housing with an adhesive, the lens and the housing have different linear expansion coefficients, so that the amount of deformation between the lens and the housing varies with increasing temperature. Due to the difference in the deformation amount, stress is generated in the lens, and the optical performance of the lens may be changed. In this embodiment, since the lens is urged to the positioning position by the urging member, the lens is not easily affected by the stress caused by the difference in deformation amount.

  The temperature rise of the first and second imaging lenses 21 and 22 during the operation of the optical scanning device 50 is caused by the rotating polygon mirror 10, the driving circuit 10 a for driving the rotating polygon mirror 10, and the heat source such as the scanner motor M. to cause. In the main scanning direction S1, which region of the first imaging lens 21 and the second imaging lens 22 heats up relative to the other regions depends on the heat source and the first and second imaging lenses 21, It differs depending on the positional relationship with 22. However, it is possible to specify a region where the temperature rises relatively during design. Therefore, in this embodiment, the positions of the protrusions 21a and 22a and the positions of the first and second long holes 43a and 53a are determined based on the result of simulation at the time of design. Alternatively, the temperature of the first and second imaging lenses 21 and 22 is measured by rotating the rotary polygon mirror 10, and the positions of the projections 21a and 22a and the first and second oblong holes are determined based on the measurement results. The positions of 43a and 53a are determined. That is, the distance L21 and the distance L22 are determined based on the result of simulation at the time of design and / or the measurement result of the temperature distribution.

In this embodiment, the relationship between the distance L21 and the distance L22 is
L21> L22
Meet. The distance L21 between the positioning projection 21a in the main scanning direction S1 with respect to the housing 40 of the first imaging lens 21 closest to the rotary polygon mirror 10 and the optical axis OA is the positioning projection 22a of the other second imaging lens 22. And the distance L22 between the optical axis OA and the optical axis OA.

  FIG. 5 shows temperature changes with respect to time of the upstream side portion and the downstream side portion in the main scanning direction S1 of the first imaging lens 21 and the second imaging lens 22 of the first embodiment. As shown in FIG. 5, both the first imaging lens 21 and the second imaging lens 22 have a higher temperature in the downstream portion than in the upstream portion in the main scanning direction S1. Therefore, the projections 21a and 22a are provided on the imaging lenses 21 and 22, respectively, shifted from the optical axis OA to the downstream side in the main scanning direction S1. Thereby, it is possible to reduce the bias of thermal expansion due to the bias temperature rise of the imaging lens with respect to the positioning portion. In particular, the first imaging lens 21 close to the rotary polygon mirror 10 has a large temperature difference between the upstream portion and the downstream portion in the main scanning direction S1. Therefore, the protrusion 21a is shifted from the optical axis OA to the first imaging lens 21 by shifting the distance L21 from the optical axis OA to the downstream side in the main scanning direction S1, which is the higher temperature distribution, so that the relationship of L21> L22 is satisfied. Is provided. With such a simple configuration, it is possible to reduce the bias of thermal expansion due to the bias temperature rise of the imaging lens with respect to the positioning portion. Then, it is possible to reduce the occurrence of a single magnification difference that causes a difference in magnification between the left and right images with respect to the image center.

  Moreover, since the first imaging lens 21 has a refractive power in the main scanning direction S1, the amount of spot position deviation in the main scanning direction S1 due to thermal expansion is larger than that of the second imaging lens 22. . Therefore, by satisfying the relationship of L21> L22 described above, it is possible to more effectively reduce the occurrence of the half magnification difference.

(Exposure of photosensitive drum)
Next, a process of irradiating the photosensitive drum 82 with the light beam emitted from the LD 2 as the scanning light E and exposing the photosensitive drum 82 will be described.
In the scanning optical device 50a, the size of the cross section of the light beam emitted from the LD 2 is limited by the diaphragm 1c of the laser holder 1 (FIG. 2B). The light beam is converted into a substantially parallel light beam by the collimator lens 6 and enters the cylindrical lens 8 (FIG. 2A). The light beam incident on the cylindrical lens 8 is transmitted as it is in the main scanning section, converges in the sub-scanning section, and forms an almost linear image on the reflecting surface 10r of the rotary polygon mirror 10. The light beam is emitted from the rotating polygon mirror 10 while being deflected and scanned by rotating the rotating polygon mirror 10. The light beam emitted from the rotary polygon mirror 10 passes through the BD slit portion 1 s provided in the BD lens 9 and the laser holder 1 and is received by the BD sensor 5. The BD sensor 5 detects the light beam emitted from the LD 2 and outputs a synchronization signal. Based on the synchronization signal, the timing of the scanning start position of the image edge by the LD 2 is adjusted. The light beam emitted from the LD 2 with the timing adjusted passes through the first imaging lens 21. Thereafter, the light beam passes through the second imaging lens 22 and is irradiated onto the photosensitive drum 82a as the scanning light E1, thereby exposing the photosensitive drum 82a.
The scanning optical devices 50b, 50c, and 50d have the same configuration as the scanning optical device 50a. Similarly, the light beams from the scanning optical devices 50b, 50c, and 50d are irradiated onto the photosensitive drums 82b, 82c, and 82d as scanning lights E2, E3, and E4, and the photosensitive drums 82b, 82c, and 82d are exposed.

(Image forming operation)
Next, an operation when image formation is performed in the image forming apparatus 100 will be described.
When a print start signal is input to a control unit (not shown) of the image forming apparatus 100, the scanning optical device 50 emits a laser beam based on the image information. The light beam is irradiated as scanning light E onto the photosensitive drum 82 to expose the photosensitive drum 82. Since the process of exposing the photosensitive drum 82 by the light beam from the scanning optical device 50 has already been described, the description thereof is omitted here. Positioning protrusions 21a and 22a as positioning portions for the first and second imaging lenses are separated from the optical axis OA by distances L21 and L22, respectively. Therefore, the half magnification difference of the scanning lights E1 to E4 due to the temperature rise of the imaging lens is reduced. Further, the image forming apparatus 100 is provided with a registration sensor 71 as a color misregistration amount detector facing the intermediate transfer belt 87. The registration sensor 71 detects the color misregistration amount by detecting registration correction patterns for each color formed on the intermediate transfer belt 87. The color shift between the top margin and the side margin is corrected by electrically correcting the image data writing timing. The color shift due to the magnification factor is corrected by matching the magnification by minutely changing the image clock frequency.

  Image formation will be described. The photosensitive drum 82 uniformly charged by the primary charger 83 is exposed by the scanning optical device 50 to form an electrostatic latent image on the photosensitive drum 82. The developing roller of the developing device 84 attaches the toner of each color frictionally charged in the developing device 84 to the electrostatic latent image, and forms a toner image of each color on the photosensitive drum 82. The toner images of the respective colors are transferred from the photosensitive drum 82 to the intermediate transfer belt 87 at the primary transfer nip T1, and are superimposed. On the other hand, sheets are fed from the sheet cassette 92 by the sheet feed roller 93 one by one. When the sheet is conveyed to the registration roller pair 94, it stops once. The registration roller pair 94 starts conveying the sheet in time with the toner image on the intermediate transfer belt 87 so that the toner image is transferred to a predetermined position on the sheet at the secondary transfer portion T2. At the secondary transfer portion T2, the toner image is transferred from the intermediate transfer belt 87 onto the sheet. The sheet on which the toner image is transferred is conveyed to the fixing device 95. In the fixing device 95, the toner image on the sheet is fixed on the sheet by heat and pressure. The sheet on which the image is formed is transported by a transport roller pair 96 and a paper discharge roller pair 97 and discharged onto a paper discharge tray 98.

  In this embodiment, the scanning light beams E1 to E4 have a reduced one-magnification difference even when the temperature rises. The overall magnification deviation is corrected electrically. Therefore, a high quality image with a small amount of color shift in the main scanning direction can be obtained.

  As described above, in the first embodiment, the positioning protrusions 21a and 22a as the positioning portions of the first imaging lens 21 and the second imaging lens 22 are mainly located on the side having a higher temperature distribution from the optical axis OA. It was shifted and provided downstream in the scanning direction S1. The distance between the positioning projection 21a and the optical axis OA for positioning the first imaging lens 21 with respect to the housing 40 in the main scanning direction S1 is L21. The distance between the positioning projection 22a and the optical axis OA for positioning the second imaging lens 22 with respect to the housing 40 in the main scanning direction S1 is L22. As a result, it is possible to efficiently reduce the bias of thermal expansion due to the uneven temperature increase of the imaging lens with respect to the positioning portion, which is caused by the temperature difference between the upstream side and the downstream side of the imaging lens in the main scanning direction. Further, the distance L 21 of the first imaging lens 21 closest to the rotating polygon mirror 10 is larger than the distance L 22 of the second imaging lens 22 far from the rotating polygon mirror 10. Thereby, it is possible to efficiently reduce the deviation of thermal expansion caused by the partial temperature increase of the imaging lens with respect to the positioning portion, which is caused by the temperature difference between the upstream portion and the downstream portion in the main scanning direction. With such a simple configuration, it is possible to reduce the bias of thermal expansion due to the bias temperature rise of the imaging lens with respect to the positioning portion. In addition, it is possible to reduce the occurrence of a single magnification difference that causes a difference in magnification between the left and right images with respect to the image center. Furthermore, in a color image forming apparatus, a high quality image with a small amount of color shift in the main scanning direction can be obtained. Furthermore, it is possible to provide an image forming apparatus capable of forming an image in which the amount of color misregistration in the main scanning direction is suppressed even when the speed of the image forming apparatus is increased.

[Example 2]
A second embodiment in which the present invention is applied to a tandem color image forming apparatus (hereinafter referred to as an image forming apparatus) will be described below. FIG. 6 is a diagram illustrating the image forming apparatus according to the second embodiment. FIG. 6A is a cross-sectional view of the image forming apparatus. FIG. 6B is a cross-sectional view showing the scanning optical device and the image forming unit. FIG. 7 is a diagram illustrating the scanning optical apparatus according to the second embodiment. FIG. 7A is a plan view showing the overall configuration of the scanning optical apparatus. FIGS. 7B and 7C are cross-sectional views of the incident optical system. FIG. 8 is a diagram illustrating a temperature change of the first imaging lens of Example 2. FIG. 9 is a front view of the first imaging lens of Example 2. FIG.

(Image forming device)
In the image forming apparatus 200 according to the second embodiment, the same components as those of the image forming apparatus 100 according to the first embodiment are denoted by the same reference numerals, and the description thereof is omitted.

(Scanning optical device)
The scanning optical device 150 uses a single polygon mirror (hereinafter referred to as a rotating polygon mirror) 10 to cause four light beams from four semiconductor laser diodes (single beam lasers) 2, 3, 12, and 13 to be four photosensitive drums. 82 (82a, 82b, 82c, 82d). As shown in FIG. 7A, the main scanning direction of the scanning optical device 150 is the direction indicated by the arrow S1. The main scanning direction S1 of the first imaging lens 21 and the main scanning direction S1 of the first imaging lens 31 are opposite to each other in FIG. The sub-scanning direction is a direction perpendicular to the paper surface of FIG. 7A and is a direction indicated by an arrow S2 in FIGS. 6B, 7B, and 7C. Laser holders 1 and 11 are attached to the scanning optical device 150.

(Laser holder)
As shown in FIG. 7B, the laser holder 1 holds semiconductor laser diodes (hereinafter referred to as LDs) 2 and 3 which are light sources. The LDs 2 and 3 are press-fitted into the lens barrel holding parts 1a and 1b, respectively. The lens barrel holders 1a and 1b are arranged so that the optical paths OP2 and OP3 of the LDs 2 and 3 intersect with the basic plane FP opposite to each other and intersect the sub-scanning direction S2 at the same predetermined angle ± θ. The axis is inclined. Here, the basic plane FP is a plane perpendicular to the reflecting surface 10r with the rotation axis 10b of the rotary polygon mirror 10 shown in FIG. The lens barrel holding portions 1a and 1b are partially integrated with each other to reduce the interval between the LDs 2 and 3 and hold the LDs 2 and 3 close to each other. The distal end portions 1aL and 1bL of the lens barrel holding portions 1a and 1b are provided with aperture portions 1c and 1d corresponding to the LD2 and LD3, respectively. The aperture portions 1c and 1d shape the light beams emitted from the LDs 2 and 3 into desired optimum beam shapes, respectively. Collimator lenses 6 and 7 for converting the light beams that have passed through the diaphragm portions 1c and 1d into substantially parallel light beams are provided on the exit side of the diaphragm portions 1c and 1d. Two adhesive portions 1e and 1f extend forward from the front end portions 1aL and 1bL of the lens barrel holding portions 1a and 1b, respectively. The two adhesive portions 1 e are provided on both sides of the lens 6 in the main scanning direction in order to fix the collimator lens 6. Similarly, the two adhesive portions 1 f are provided on both sides of the lens 7 in the main scanning direction in order to fix the lens 7. Here, the irradiation position and focus of the lenses 6 and 7 are adjusted while detecting the optical characteristics of the laser light. When the positions of the lenses 6 and 7 are determined, the lenses 6 and 7 are bonded and fixed to the bonding portions 1e and 1f, respectively, by irradiating the ultraviolet curable adhesive with ultraviolet rays. The electric circuit board 4 is provided with a laser drive circuit. The substrate 4 is electrically connected to the LDs 2 and 3. As shown in FIG. 7A, the BD lens 9 images the light beam reflected by the rotary polygon mirror 10 on the light receiving surface of the BD sensor 5. The BD sensor 5 provided on the substrate 4 detects the light beam reflected by the rotary polygon mirror 10 and passed through the BD lens 9, and outputs a synchronization signal in the main scanning direction. Based on this synchronization signal, the timing of the scanning start position at the image edge is adjusted.

  As shown in FIG. 7C, the laser holder 11 has the same structure as the laser holder 1. The laser holder 11 holds the LDs 12 and 13 that are light sources. The LDs 12 and 13 are press-fitted into the lens barrel holding portions 11a and 11b, respectively. Here, the lens barrel holders 11a and 11b are arranged so that the optical paths OP12 and OP13 of the LDs 12 and 13 intersect with the basic plane FP at the same predetermined angle ± θ in the sub-scanning direction S2, respectively. 13 optical axes are inclined. The lens barrel holding portions 11a and 11b are partially integrated with each other to reduce the interval between the LDs 12 and 13 and hold the LDs 12 and 13 close to each other. At the front end portions 11aL and 11bL of the lens barrel holding portions 11a and 11b, diaphragm portions 11c and 11d corresponding to the LDs 12 and 13, respectively, are provided. The aperture portions 11c and 11d shape the light beams emitted from the LDs 12 and 13 into desired optimum beam shapes, respectively. Collimator lenses 16 and 17 for converting the light beams that have passed through the diaphragm portions 11c and 11d into substantially parallel light beams are provided on the exit side of the diaphragm portions 11c and 11d, respectively. Two adhesive portions 11e and 11f extend forward from the front end portions 11aL and 11bL of the lens barrel holding portions 11a and 11b, respectively. The lenses 16 and 17 are bonded and fixed in the same manner as the lenses 6 and 7 of the laser holder 1, respectively. The electric circuit board 14 is provided with a laser drive circuit. The substrate 14 is electrically connected to the LDs 12 and 13.

(Laser holder positioning)
As shown in FIG. 7A, the optical element of the scanning optical device 150 is arranged inside an optical box (hereinafter referred to as a housing) 140. The side wall of the housing 140 is connected to the fitting hole portions 140a and 140b and the fitting hole portions 140a and 140b for positioning the laser holders 1 and 11, and is provided with a long long hole portion in the sub-scanning direction S2. . The laser holder 1 is attached to the housing 140 by fitting a fitting portion 1m provided on the outer shape of the lens barrel holding portions 1a and 1b into the fitting hole 140a. Further, the laser holder 11 is attached to the housing 140 by fitting a fitting portion 11m provided on the outer shape of the lens barrel holding portions 11a and 11b into the fitting hole 140b. In this way, the fitting portions 1m and 11m are fitted in the fitting holes 140a and 140b, respectively, and the laser holders 1 and 11 are attached to the housing 140. Therefore, the LDs 2, 3, 12, and 13 and the housing The positional relationship with the optical component stored in 140 can be accurately guaranteed.

(BD sensor)
The cylindrical lenses 8 and 18 have a predetermined refractive power only in the sub-scanning direction S2. The lens 8 is integrally formed with a lens portion corresponding to the light beam emitted from the LDs 2 and 3. The lens 18 is integrally formed with a lens portion corresponding to the light beam emitted from the LDs 12 and 13. The BD lenses 9 and 19 image the light beams reflected by the rotary polygon mirror 10 on the light receiving surfaces of the BD sensors 5 and 15, respectively. In this embodiment, BD sensors 5 and 15 are provided at positions corresponding to the LDs 2 and 12, respectively. BD sensors corresponding to LD3 and 13 are not provided. This is because LD2 and 3 are provided in one laser holder 1 in the sub-scanning direction S2, and therefore the timing of the scanning start position of the image edge by LD3 can be the same as that of LD2. Similarly, since the LDs 12 and 13 are provided in one laser holder 11 in the sub-scanning direction S2, the timing of the scanning start position of the image edge by the LD 13 can be the same as that of the LD 12.

(Optical path of laser light)
The rotary polygon mirror 10 is rotated in the arrow direction (clockwise direction) in FIG. 7A at a constant speed by the scanner motor M, and deflects and scans the light beam emitted from the LD. The drive circuit 10 a is provided on the control board of the rotary polygon mirror 10. The fθ lens is composed of a plurality of imaging members. Specifically, the fθ lens is composed of first imaging lenses (imaging members) 21 and 31 and second imaging lenses (imaging members) 22, 23, 32 and 33 as resin-molded optical elements. Become. As shown in FIG. 6B, the first imaging lens 21 and the second imaging lenses 22 and 23, together with the second imaging lenses 22 and 23, carry out spot imaging on the photosensitive drums 82a and 82b with constant speed scanning. The light beams emitted from the LDs 2 and 3 are incident on the first imaging lens 21 at different angles ± Θ. The first imaging lens 21 is composed of a cylinder lens having a refractive power (power) in the main scanning direction. With respect to the sub-scanning direction S2, the light beam of LD2 is imaged on the photosensitive drum 82a by the second imaging lens 22. The light beam of the LD 3 is imaged on the photosensitive drum 82 b by the second imaging lens 23. The folding mirrors 24, 25, and 26 reflect the light beam in a predetermined direction. The final folding mirror 24 directs the light flux of LD2 from the second imaging lens 22 to the photosensitive drum 82a. The separation folding mirror 25 directs the light beam of the LD 3 from the first imaging lens 21 to the second imaging lens 23. The final folding mirror 26 directs the light beam of LD3 from the second imaging lens 23 toward the photosensitive drum 82b. As described above, the separation folding mirror 25 and the final folding mirror 26 reflect the light flux of the LD3 a plurality of times, so that the small optical path can be effectively used to make the optical path length the same as the light flux of the LD2. On the other hand, on the opposite side of the rotary polygon mirror 10, the first imaging lens 31, the second imaging lenses 32 and 33 corresponding to the LDs 12 and 13, the final folding mirror 34 of the LD 13, the separation folding mirror 35 of the LD 12, and the final A folding mirror 36 is arranged. As described above, the separation folding mirror 35 and the final folding mirror 36 reflect the light flux of the LD 12 a plurality of times, so that a small space can be effectively used to have the same optical path length as the light flux of the LD 13. Therefore, the scanning optical device 150 can be made compact.

(Fixing part of the imaging lens)
Since the structure of the attachment parts of the first imaging lenses 21, 31 and the second imaging lenses 22, 23, 32, 33 is substantially the same as the first attachment part 41 and the second attachment part 51 of the first embodiment, Description is omitted.
A positioning projection 21 a as a positioning portion provided on the bottom surface of the first imaging lens 21 is inserted into a first elongated hole (not shown) as a main scanning direction positioning locking portion provided in the housing 140. . The first imaging lens 21 is positioned with respect to the housing 140 in the main scanning direction S <b> 1 by the engagement between the protrusion 21 a and the first elongated hole. The protrusion 21a serves as a positioning reference for the first imaging lens 21 in the main scanning direction S1. The positioning projection 21a is provided by being shifted from the optical axis OA of the first imaging lens 21 by the first distance L21 on the downstream side in the main scanning direction S1.

The first imaging lens (one imaging member) 21 and the first imaging lens (another imaging member) 31 are provided on the opposite sides with respect to the rotation axis 10b of the rotary polygon mirror 10, respectively. A positioning protrusion 31 a as a positioning portion provided on the bottom surface of the first imaging lens 31 is inserted into a first elongated hole (not shown) provided in the housing 140. The first imaging lens 31 is positioned with respect to the housing 140 in the main scanning direction S <b> 1 by the engagement between the protrusion 31 a and the first elongated hole. The protrusion 31a serves as a positioning reference for the first imaging lens 31 in the main scanning direction S1. The positioning projection 31a is provided by being shifted from the optical axis OA of the first imaging lens 31 by the second distance L31 on the downstream side in the main scanning direction S1.
In the present embodiment, the relationship between the first distance L21 and the second distance L31 is
L21 <L31
Meet.

  FIG. 8 shows temperature changes with respect to time of the upstream side portion and the downstream side portion in the main scanning direction S1 of the first imaging lenses 21 and 31 of the second embodiment. As shown in FIG. 8, both the first imaging lens 21 and the first imaging lens 31 have a higher temperature in the downstream portion than in the upstream portion in the main scanning direction S1. Therefore, the projections 21a and 31a are provided on the imaging lenses 21 and 31 while being shifted from the optical axis OA to the downstream side in the main scanning direction S1, respectively. Thereby, it is possible to reduce the bias of thermal expansion due to the bias temperature rise of the imaging lens with respect to the positioning portion. In particular, the first imaging lens 31 is on the upstream side of the drive circuit 10a that is a heat source in the main scanning direction S1. On the other hand, the first imaging lens 21 is on the downstream side of the drive circuit 10a in the main scanning direction S1. For this reason, the first imaging lens 31 is more greatly affected by the drive circuit 10 a as a heat source than the first imaging lens 21. As a result, the first imaging lens 31 has a large temperature difference between the upstream side and the downstream side in the scanning direction. Accordingly, the projection 31a is shifted from the optical axis OA by the second distance of L31 to the downstream side in the main scanning direction S1, which is the higher temperature distribution, so that the relationship of L21 <L31 is satisfied. 31 is provided. By doing so, even when the imaging lenses are arranged on both sides of the rotary polygon mirror 10, it is possible to reduce the bias of thermal expansion due to the uneven temperature rise of the imaging lens with respect to the positioning portion with a simple configuration. In addition, it is possible to reduce the occurrence of a single magnification difference that causes a difference in magnification between the left and right images with respect to the image center. Furthermore, the difference in the amount of misregistration between the colors can be reduced, and the color misregistration due to the difference in half magnification can be reduced.

  The first imaging lens 21 and the first imaging lens 31 may be common parts. As shown in FIG. 9, the positioning protrusion 21a protruding in the sub-scanning direction S2 from the one surface 21f of the common first imaging lens 21 (31) is shifted from the optical axis OA by the first distance L21. Further, the positioning protrusion 31a protruding in the direction opposite to the sub-scanning direction S2 from the other surface 21k of the common first imaging lens 21 (31) is shifted from the optical axis OA by the second distance L31. The second distance L31 is different from the first distance L21. The protrusion 31a is provided on the side opposite to the protrusion 21a in the main scanning direction S1 with respect to the optical axis OA. The common first imaging lens 21 (31) can be selectively used with the positioning projection 21a on the lower side when positioning the first imaging lens 21 on the housing 140 as the first imaging lens 21. In addition, the common first imaging lens 21 (31) can be selectively used with the protrusion 31a on the lower side when positioning the first imaging lens 31 on the housing 140. That is, the designer can select which of a plurality of positioning portions is used according to the position (positioning position) where the common first imaging lens 21 is installed. By using common parts, cost can be reduced. Further, in many types of scanning optical devices, the common first imaging lens 21 (31) can be used according to the amount of temperature increase. As shown in FIG. 9, one protrusion 21a, 31a is provided on each of the bottom surface 21f and the top surface 21k of the common first imaging lens 21 (31). However, a plurality of positioning projections may be provided on each of the bottom surface 21f and the top surface 21k of the first imaging lenses 21 and 31.

  The positioning of the second imaging lenses 22 and 23 in the main scanning direction S1 with respect to the housing 140 is not shown, but is attached to the downstream side of the main scanning direction S1 from the optical axis OA by a distance shorter than L21. ing. Although the positioning of the second imaging lenses 32 and 33 in the main scanning direction S1 with respect to the housing 140 is not shown, the second imaging lenses 32 and 33 are attached to the downstream side of the main scanning direction S1 from the optical axis OA while being shifted by a shorter distance than L31. .

(Top lid)
The upper lid 141 (FIG. 6B) is attached so as to cover the upper opening of the housing 140. The upper lid 141 seals the scanning optical device 150 and prevents dust and toner from entering the scanning optical device 150. The upper lid 141 is provided with slit-like openings 142 (142a, 142b, 142c, 142d) at positions corresponding to the respective photosensitive drums 82. The opening 142 is closed by a dustproof glass 143 (143a, 143b, 143c, 143d), which is a transparent member, and prevents dust and toner from entering the scanning optical device 150. The scanning light E exposes each photosensitive drum 82 through the dust-proof glass 143.

(Exposure of photosensitive drum)
Next, a process in which light beams emitted from the LDs 2, 3, 12, and 13 are irradiated onto the respective photosensitive drums 82 as scanning lights E1, E2, E3, and E4 and the respective photosensitive drums 82 are exposed will be described.

  The luminous fluxes emitted from the LDs 2 and 3 are limited in size by the diaphragm portions 1c and 1d of the laser holder 1 (FIG. 7B). The luminous flux is converted into a substantially parallel luminous flux by the collimator lenses 6 and 7, and enters the lens portions 8a and 8b of the cylindrical lens 8 (FIG. 7A). The light beam incident on the cylindrical lens 8 is transmitted as it is in the main scanning section, converges in the sub-scanning section, and forms a substantially linear image on the same reflecting surface 10r of the rotary polygon mirror 10. At this time, it is obliquely incident with an angle ± θ in the sub-scanning direction. Then, the rotary polygon mirror 10 is rotated and deflected and scanned, and is emitted with an angle ± θ in the sub-scanning direction. Of the two light beams emitted from the rotary polygon mirror 10, the light beam emitted from the LD 2 passes through the BD lens 9 and is received by the BD sensor 5. The BD sensor 5 detects the light beam emitted from the LD 2 and outputs a synchronization signal, and adjusts the timing of the scanning start position of the image edge by the LD 2 and 3. Here, since LD2 and LD3 are provided in one laser holder 1 in the sub-scanning direction, the timing of the scanning start position of the image edge by LD3 can be the same as that of LD2. The light beams emitted from the LDs 2 and 3 after the timing adjustment pass through the first imaging lens 21. Thereafter, the light beam emitted from the LD 2 passes through the second imaging lens 22 and is reflected by the final folding mirror 24 (FIG. 6B). The light beam reflected by the final folding mirror 24 passes through the dust-proof glass 143a and is irradiated on the photosensitive drum 82a as scanning light E1, thereby exposing the photosensitive drum 82a. On the other hand, the light beam emitted from the LD 3 is reflected downward by the separation folding mirror 25, then passes through the second imaging lens 23 and is reflected by the final folding mirror 26. The light beam reflected by the final folding mirror 26 passes through the dust-proof glass 143b and is irradiated onto the photosensitive drum 82b as the scanning light E2 to expose the photosensitive drum 82b.

  Similarly, the size of the cross section of the light beam emitted from the LDs 12 and 13 is limited by the diaphragm portions 11c and 11d of the laser holder 11 (FIG. 7C). The luminous flux is converted into a substantially parallel luminous flux by the collimator lenses 16 and 17, and enters the lens portions 18a and 18b of the cylindrical lens 18 (FIG. 7A). The light beam incident on the cylindrical lens 18 is transmitted as it is in the main scanning section, and converges in the sub-scanning section to form an almost linear image on the same reflecting surface 10r of the rotary polygon mirror 10. At this time, it is obliquely incident with an angle ± θ in the sub-scanning direction. Then, the rotary polygon mirror 10 rotates and is emitted with an angle ± θ in the sub-scanning direction while performing deflection scanning. Of the two light beams emitted from the rotary polygon mirror 10, the light beam emitted from the LD 12 and reflected by the rotary polygon mirror 10 passes through the BD lens 19 and is received by the BD sensor 15. The BD sensor 15 detects the light beam emitted from the LD 12 and outputs a synchronization signal, and adjusts the timing of the scanning start position of the image edge by the LDs 12 and 13. Here, since the LDs 12 and 13 are provided in one laser holder 11 in the sub-scanning direction, the timing of the scanning start position of the image end portion by the LD 13 can be the same as that of the LD 12. The light beams emitted from the LDs 12 and 13 after the timing adjustment pass through the first imaging lens 31. Thereafter, the light beam emitted from the LD 12 is reflected downward by the separation folding mirror 35, then passes through the second imaging lens 33 and is reflected by the final folding mirror 36 (FIG. 6B). The light beam reflected by the final folding mirror 36 passes through the dust-proof glass 143c and is irradiated on the photosensitive drum 82c as the scanning light E3 to expose the photosensitive drum 82c. On the other hand, the light beam emitted from the LD 13 passes through the second imaging lens 32 and is reflected by the final folding mirror 34. The light beam reflected by the final folding mirror 34 passes through the dust-proof glass 143d and is irradiated on the photosensitive drum 82d as the scanning light E4 to expose the photosensitive drum 82d.

(Image forming operation)
Since the image forming operation in the image forming apparatus 100 is the same as that in the first embodiment, the description thereof is omitted.

As described above, the scanning optical device 150 according to the second embodiment exposes different photosensitive drums by deflecting and scanning the laser beam in both directions with the rotating shaft 10b of the rotary polygon mirror 10 interposed therebetween. The positioning portion (positioning protrusion) in the main scanning direction S1 with respect to the housing 140 of the first imaging lenses 21 and 31 of the scanning optical device 150 is shifted from the optical axis OA by a different distance downstream in the main scanning direction with a high temperature distribution. Provide. Thereby, it is possible to efficiently reduce the bias of thermal expansion due to the uneven temperature rise of the imaging lens, which is caused by the temperature difference between the upstream portion and the downstream portion in the main scanning direction. Further, due to the influence of the drive circuit 10a that is a heat source, the first imaging lens 31 that is upstream of the drive circuit 10a in the main scanning direction from the first imaging lens 21 that is downstream of the drive circuit 10a in the main scanning direction is The temperature difference between the upstream portion and the downstream portion in the main scanning direction becomes large. For this reason, the second distance L31 between the optical axis and the positioning unit in the main scanning direction of the first imaging lens 31 with respect to the housing 140 is set as the second distance L31. It is larger than the first distance L21 with the shaft. Thereby, it is possible to efficiently reduce the bias of thermal expansion due to the uneven temperature rise of the imaging lens, which is caused by the temperature difference between the upstream portion and the downstream portion in the main scanning direction. By doing so, even when the imaging lens is disposed in both directions of the rotary polygon mirror 10, it is possible to reduce the bias of thermal expansion due to the uneven temperature rise of the imaging lens with respect to the positioning unit with a simple configuration. Further, it is possible to reduce the occurrence of a half magnification difference that causes a difference in magnification between the left and right images with respect to the image center. Furthermore, it is possible to provide an image forming apparatus capable of forming an image in which the amount of color misregistration in the main scanning direction is suppressed even when the speed of the image forming apparatus is increased.
In Examples 1 and 2, the imaging lens constituting the fθ lens is resin-molded. In general, a resin-molded lens has a larger amount of thermal expansion due to temperature rise than a glass lens, and thus tends to cause a half magnification difference due to partial temperature rise. However, according to the present invention, even in the case of a resin-molded lens, it is possible to more effectively reduce the bias of thermal expansion due to the bias temperature rise of the imaging lens with respect to the positioning portion. Therefore, generation of the half magnification difference and color shift due to the half magnification difference can be effectively reduced.

  In the scanning optical apparatus according to the first and second embodiments, the positioning unit in the main scanning direction with respect to the casing of the second imaging lens is attached to be shifted from the optical axis by a distance L22 on the downstream side in the main scanning direction. Yes. However, the present invention is not limited to this. L22 = 0, that is, the positioning portion in the main scanning direction with respect to the housing of the second imaging lens may be on the optical axis.

2, 3, 12, 13 ... LD (light source), 10 ... rotating polygon mirror, 82 ... photosensitive drum (electrophotographic photosensitive member), 21, 31 ... first imaging lens (imaging member), 22, 32 ... Second imaging lens (imaging member), 21a, 22a, 31a ... positioning projection (positioning part), 40, 140 ... casing, 50, 150 ... scanning optical device, OA ... optical axis, L21, L22, L31 …distance

Claims (8)

A light source;
A rotating polygon mirror that deflects and scans the light beam emitted from the light source in the main scanning direction;
A plurality of imaging members for imaging the light beam deflected and scanned by the rotary polygon mirror on the electrophotographic photosensitive member;
A housing in which the light source, the rotary polygon mirror, and the plurality of imaging members are disposed;
In the scanning optical device having a positioning portion provided in each of the plurality of imaging members to position each of the plurality of imaging members in the main scanning direction with respect to the housing,
The positioning portion of the imaging member closest to the rotary polygonal mirror among the plurality of imaging members has a temperature of the closest imaging member during operation of the scanning optical device with respect to the optical axis of the closest imaging member. Is provided on the higher side,
The distance in the main scanning direction between the optical axis of the nearest imaging member and the positioning portion is the distance between the optical axis of the other imaging member and the positioning portion of the other imaging member. A scanning optical device characterized in that it is larger than the distance in the main scanning direction. Multiple light sources;
A rotary polygon mirror that deflects and scans each light beam emitted from the plurality of light sources in a main scanning direction;
A plurality of imaging members for imaging the light beams deflected and scanned by the rotary polygon mirror on the respective electrophotographic photosensitive members;
A housing in which the plurality of light sources, the rotary polygon mirror, and the plurality of imaging members are disposed;
In the scanning optical device having a positioning portion provided in each of the plurality of imaging members to position each of the plurality of imaging members in the main scanning direction with respect to the housing,
One imaging member of the plurality of imaging members and another imaging member of the plurality of imaging members are provided on opposite sides with respect to the rotation axis of the rotary polygon mirror, respectively.
The positioning portion of the one imaging member is provided on the side where the temperature of the one imaging member becomes higher during the operation of the scanning optical device with respect to the optical axis of the one imaging member,
The positioning portion of the other imaging member is provided on the side where the temperature of the other imaging member becomes higher during the operation of the scanning optical device with respect to the optical axis of the other imaging member. And
The first distance in the main scanning direction between the optical axis of the one imaging member and the positioning portion of the one imaging member is the optical axis of the other imaging member and the other And a second distance in the main scanning direction between the image forming member and the positioning portion. And a drive circuit for driving the rotary polygon mirror.
The one imaging member is disposed on the downstream side of the drive circuit in the main scanning direction,
The other one imaging member is disposed on the upstream side of the drive circuit in the main scanning direction,
The first distance between the optical axis of the one imaging member arranged on the downstream side of the driving circuit and the positioning portion is the other one of the connections arranged on the upstream side of the driving circuit. The scanning optical apparatus according to claim 2, wherein the scanning optical apparatus is smaller than the second distance between the optical axis of the image member and the positioning portion. Each of the plurality of imaging members has a plurality of positioning portions,
By using at least one of the plurality of positioning portions according to the positioning positions of the plurality of imaging members, each of the plurality of imaging members is positioned in the main scanning direction with respect to the housing. The scanning optical device according to claim 1, wherein the scanning optical device is a scanning optical device. The housing includes a plurality of positioning portions corresponding to the plurality of imaging members,
By using at least one of the plurality of positioning portions according to the positioning positions of the plurality of imaging members, each of the plurality of imaging members is positioned in the main scanning direction with respect to the housing. The scanning optical apparatus according to claim 1, wherein the scanning optical apparatus is characterized in that:   The scanning optical apparatus according to claim 1, wherein the plurality of imaging members are resin-molded optical elements. Each of the plurality of imaging members includes an optical axis direction positioning unit that positions the corresponding imaging member in the optical axis direction, and a sub scanning direction positioning unit that positions the corresponding imaging member in the sub scanning direction. And
The housing includes an optical axis abutting portion for positioning the corresponding imaging member in the optical axis direction, and a sub-scanning direction abutting portion for positioning the corresponding imaging member in the sub-scanning direction. And
The scanning optical device further includes:
An optical axis direction biasing member that presses the optical axis direction positioning portion against the optical axis direction abutting portion;
The scanning optical apparatus according to claim 1, further comprising: a sub-scanning direction biasing member that presses the sub-scanning direction positioning portion against the sub-scanning direction abutting portion. In an electrophotographic image forming apparatus having an image forming unit for forming an image on a recording medium,
The image forming unit includes:
A plurality of electrophotographic photoreceptors;
An electrophotographic image forming apparatus comprising: the scanning optical apparatus according to claim 1 for exposing the plurality of electrophotographic photosensitive members.

Images