Optical scanning device and image forming apparatus

Abstract

The invention discloses an optical scanning device and an image forming apparatus. An optical scanning apparatus includes a deflector including a first deflection surface configured to deflect a first light beam and a second light beam to scan a first scanned surface and a second scanned surface in a main scanning direction, respectively, and a first optical system and a second optical system configured to direct the first light beam and the second light beam deflected by the first deflection surface to the first scanned surface and the second scanned surface, where the first optical system and the second optical system include a common first optical element disposed on a first optical path and a second optical path, wherein the first optical system comprises a second optical element located between the first optical element and the first scanned surface, and wherein the second optical system comprises a third optical element located between the first optical element and the second scanned surface, and wherein the first optical element includes first and second optical portions into which the first and second light beams are incident.

Description

Technical Field

[0001] The present disclosure relates to an optical scanning device, and more particularly to an optical scanning device applicable to an image forming apparatus such as a laser beam printer (LBP), a digital copying machine, and a multifunction printer (MFP). Background Art

[0002] In recent years, miniaturization of an optical scanning device for an image forming apparatus has been required. However, if an attempt is made to reduce the size of the optical scanning device, the internal space of the optical scanning device becomes narrow, and it becomes difficult to deploy optical elements without interfering with each other.

[0003] Japanese Patent Application Laid-Open No. 2018-128516 discusses an optical scanning device in which a multi-stage lens that integrates a plurality of lens surfaces arranged in the sub-scanning direction is employed, and the lens surfaces of the multi-stage lens are configured in various different shapes. This increases the degree of freedom in layout and reduces the number of optical components. Summary of the Invention

[0004] According to some embodiments, an optical scanning device includes a deflector including a first deflection surface configured to deflect a first light beam and a second light beam to scan a first scanned surface and a second scanned surface in a main scanning direction, respectively; and a first optical system and a second optical system configured to guide the first light beam and the second light beam deflected by the first deflection surface to the first scanned surface and the second scanned surface, respectively, wherein the first optical system and the second optical system include a common first optical element deployed on a first optical path and a second optical path extending from the first deflection surface to the first scanned surface and the second scanned surface, respectively, wherein the first optical system includes a second optical element located between the first optical element and the first scanned surface on the first optical path, wherein the optical system includes a third optical element located between the first optical element and the second scanned surface on the second optical path, and wherein the first optical element includes a first optical portion and a second optical portion on which the first light beam and the second light beam are incident.

[0005] More features of the present disclosure will become apparent from the following description of exemplary embodiments with reference to the accompanying drawings. Brief Description of the Drawings

[0006] Figure 1A is a sub-scanning cross-sectional view of an optical scanning device according to a first exemplary embodiment. Figure 1B is an optical path diagram of the optical scanning device according to the first exemplary embodiment in a main scanning cross-section. Figure 1C is an optical path diagram of the optical scanning device according to the first exemplary embodiment in a sub-scanning cross-section.

[0007] Figure 2A and Figure 2B is a diagram showing a sub-scanning cross section of a multi-stage lens according to a first exemplary embodiment and a comparative example.

[0008] Figure 3 is a graph showing the step height at the interface of the lens surface of a multi-stage lens according to a first practical example.

[0009] Figure 4A and Figure 4B is a graph showing the field curvature of an imaging optical system according to a first practical example.

[0010] Figure 5A and Figure 5B is a graph showing the fθ characteristic of an imaging optical system according to a first practical example.

[0011] Figure 6A and Figure 6B is a graph showing the scan line curvature of an imaging optical system according to a first practical example.

[0012] Figure 7A is a sub-scanning cross-sectional view of an optical scanning device according to a second exemplary embodiment. Figure 7B is an optical path diagram of an optical scanning device in a main scanning cross section according to a second exemplary embodiment. Figure 7C is an optical path diagram of an optical scanning device in a sub-scanning cross section according to a second exemplary embodiment.

[0013] Figure 8 is a graph showing the step height at the interface of the lens surface of a multi-stage lens according to a second practical example.

[0014] Figure 9A and Figure 9B is a graph showing the field curvature of an imaging optical system according to a second practical example.

[0015] Figure 10A and Figure 10B is a graph showing the fθ characteristic of an imaging optical system according to a second practical example.

[0016] Figure 11A and Figure 11B is a graph showing the scan line curvature of an imaging optical system according to a second practical example.

[0017] Figure 12A is an optical path diagram of an optical scanning device in a main scanning cross section according to a third exemplary embodiment. Figure 12B is an optical path diagram of an optical scanning device in a sub-scanning cross section according to a third exemplary embodiment.

[0018] Figure 13 is a sub-scanning cross-sectional view of an imaging optical system included in an optical scanning device according to a third exemplary embodiment.

[0019] Figure 14 is a schematic diagram for describing the change in light rays caused by the eccentricity of the deflector.

[0020] Figure 15A is an optical path diagram of an optical scanning device in a main scanning cross-section according to a fourth exemplary embodiment. Figure 15B is an optical path diagram of an optical scanning device in a sub-scanning cross-section according to a fourth exemplary embodiment.

[0021] Figure 16 is a sub-scanning cross-sectional view of an imaging optical system included in an optical scanning device according to a fourth exemplary embodiment.

[0022] Figure 17A and Figure 17B is an optical path diagram of an optical scanning device in a main scanning cross-section according to a fifth exemplary embodiment. Figure 17C is a sub-scanning cross-sectional view of an incident optical system of an optical scanning device according to a fifth exemplary embodiment. Figure 17D is a sub-scanning cross-sectional view of an imaging optical system of an optical scanning device according to a fifth exemplary embodiment.

[0023] Figure 18 is a diagram for describing a first optical element according to a fifth exemplary embodiment.

[0024] Figure 19 is a diagram for describing the optical performance according to a fifth exemplary embodiment.

[0025] Figure 20A and Figure 20B is an optical path diagram of an optical scanning device in a main scanning cross-section according to a sixth exemplary embodiment. Figure 20C is a sub-scanning cross-sectional view of an incident optical system of an optical scanning device according to a sixth exemplary embodiment. Figure 20D is a sub-scanning cross-sectional view of an imaging optical system of an optical scanning device according to a sixth exemplary embodiment.

[0026] Figure 21 is a diagram for describing a first optical element according to a sixth exemplary embodiment.

[0027] Figure 22 is a sub-scanning cross-sectional view of a color image forming apparatus. Detailed Description

[0028] Hereinafter, an optical scanning device according to the present exemplary embodiment will be described in detail with reference to the accompanying drawings. Note that, for facilitating understanding of the present exemplary embodiment, the drawings may be drawn in a scale different from the actual scale.

[0029] Figure 1A is a sub-scanning cross-sectional view of the main components of the optical scanning device 100 according to the first exemplary embodiment. Figure 1B is an optical path diagram of the main components of the optical scanning device 100 in the main scanning cross-section according to the first exemplary embodiment. Figure 1C is an optical path diagram of the main components of the optical scanning device 100 in the sub-scanning cross-section according to the first exemplary embodiment.

[0030] In the following description, the main scanning direction (Y direction) refers to the direction perpendicular to the rotation axis (or swing axis) of the deflector and the optical axis of the imaging optical system (the direction in which the light beam is reflected and deflected [deflected and scanned] by the rotating polygon mirror). The sub-scanning direction (Z direction) refers to the direction parallel to the rotation axis (or swing axis) of the deflector. The main scanning cross-section refers to the cross-section including the optical axis and perpendicular to the sub-scanning direction. The sub-scanning cross-section refers to the cross-section perpendicular to the main scanning direction.

[0031] The optical scanning device 100 according to the present exemplary embodiment includes light sources 1A (first light source), 1B (second light source), 1C (third light source), and 1D (fourth light source), incident optical systems LA, LB, LC, and LD, a deflector 5, imaging optical systems SA (first optical system), SB (second optical system), SC (third optical system), and SD (fourth optical system), and mirrors M1, M2, M3, M'1, M'2, and M'3. Optical elements such as prisms and lenses having reflective surfaces may be used instead of mirrors as reflective elements. A prism may be used instead of a lens as a refractive element.

[0032] In the optical scanning device 100 according to the present exemplary embodiment, the imaging optical systems SA and SB and the imaging optical systems SC and SD are positioned such that a single deflector 5 is located therebetween. The single deflector 5 deflects and scans four light beams RA (first light beam), RB (second light beam), RC (third light beam), and RD (fourth light beam) to scan corresponding scanned surfaces 8A (first scanned surface), 8B (second scanned surface), 8C (third scanned surface), and 8D (fourth scanned surface). The single deflector 5 is shared by the four light beams RA, RB, RC, and RD, and a sub-scanning inclined incident optical system is used, in which the light beams are incident on the deflector at an inclined angle in the sub-scanning direction. The advantage of the sub-scanning inclined incident optical system is that the deflected and reflected light beams can be separated without increasing the size of the deflection surface of the optical deflector in the sub-scanning direction.

[0033] In the imaging optical system SA, the light beam RA (first light beam) deflected by the common deflection surface (first deflection surface) of the deflector (quadrilateral polygon mirror) 5 serving as a deflection unit passes through the first optical section 6A and the lens 7A in sequence, and is then folded back by the mirror M1 (first reflecting element) and guided to the surface 8A to be scanned. The first optical section 6A is part of a multi-stage lens serving as a first optical element (first refracting element). The lens 7A serves as a second optical element (second refracting element). In the imaging optical system SB, the light beam RB (second light beam) deflected and reflected by the deflection surface of the deflector 5 passes through the second optical section 6B, and is then folded back by the mirror M2 (second reflecting element) and passes through the lens 7B. The light beam RB is then folded back by the mirror M3 and reaches the surface 8B to be scanned. The second optical section 6B is part of a multi-stage lens. The lens 7B serves as a third optical element (third refracting element). In the figure, when the principal ray of the light beam (axial light beam) reaching the axial image height on the surface to be scanned is deflected, the incident point (deflection point) of the principal ray on the deflection surface is denoted by C0, and will be referred to as the axial deflection point or simply the deflection point hereinafter. The plane (reference plane) intersecting the deflection point C0 and perpendicular to the rotation axis of the deflector 5 is denoted by P0. The light beams RA and RB incident on the deflection surface intersect at the deflection point C0 and are deflected in the sub-scanning section. The length of the optical path from the deflection point C0 to each surface to be scanned will be referred to as the optical path length of each imaging optical system hereinafter. The optical paths from the deflection surface to the surfaces 8A and 8B to be scanned will be referred to as the first optical path and the second optical path, respectively.

[0034] In the imaging optical system SD (SC), the optical path is routed similarly to the imaging optical system SA (SB). Specifically, in the imaging optical system SC, the light beam RC (third light beam) deflected and reflected by the deflection surface (second deflection surface) of the deflector 5 passes through the third optical section 6C, and is then folded back by the mirror M'2 (fourth reflecting element) and passes through the lens 7C. The light beam RC is then folded back by the mirror M'3 (fifth reflecting element) and reaches the surface 8C to be scanned. The third optical section 6C is part of a multi-stage lens serving as a fourth optical element (fourth refracting element). The lens 7C serves as a fifth optical element (fifth refracting element). In the imaging optical system SD, the light beam RD (fourth light beam) deflected by the deflection surface of the deflector 5 passes through the fourth optical section 6D and the lens 7D, and is then folded back by the mirror M'1 (sixth reflecting element) and guided to the surface 8D to be scanned. The fourth optical section 6D is part of a multi-stage lens. The lens 7D serves as a sixth optical element (sixth refracting element). The optical paths from the deflection surface to the surfaces 8C and 8D to be scanned will be referred to as the third optical path and the fourth optical path, respectively.

[0035] The imaging optical systems SA and SB according to the present exemplary embodiment will be described. The imaging optical systems SA and SB are each composed of a plurality of lenses. In the imaging optical system SA (SB), the lens (optical portion) optically closest to the deflector 5 is referred to as lens 6A (6B), and the lens optically closest to the scanned surface 8A (8B) is referred to as lens 7A (7B). As used herein, "optically" means "in a state where the optical path is unfolded".

[0036] The lenses (optical portions) 6A and 6B according to the present exemplary embodiment are arranged in the sub-scanning direction and constitute a multi-stage lens (shared first optical element) in which the incident surface and the exit surface are integrally formed. This configuration enables the first optical path and the second optical path corresponding to the light beams RA and RB to share the lens. Thereby, the number of optical components is reduced to reduce the size and cost of the optical scanning device 100.

[0037] In the multi-stage lens according to the present exemplary embodiment, at least the incident surface or the exit surface of the lenses 6A and 6B has a lens surface shape that is asymmetric in the sub-scanning direction with respect to the reference plane P0. The upper and lower portions of the multi-stage lens with respect to the reference plane P0 have different shapes in both the main scanning section (generatrix shape) and the sub-scanning section (sagittal shape). As used herein, the generatrix shape refers to the lens surface shape within the main scanning section including the optical axis. By configuring at least the incident surface or the exit surface of the lenses 6A and 6B to have different lens surface shapes, the lenses 7A and 7B are positioned such that the optical positions of the lenses 7A and 7B are different from that of the deflection point C0 while maintaining good optical performance of the imaging optical systems SA and SB. This increases the degree of freedom in layout.

[0038] Compared with the case where the lenses 7A and 7B are located at the same optical position as the deflection point C0, the lens 7B is thus optically closer to the scanned surface than the lens 7A. This results in avoiding interference between the lens 7B and the light beam RA in a small space and reducing the size of the optical scanning device 100.

[0039] Figure 2B A multi-stage lens according to a comparative example (conventional example) is illustrated, in which a plurality of optical portions are positioned such that their optical axes (surface vertices) are offset from the center and different from each other in the sub-scanning direction. If the lenses in the multi-stage lens have different lens surface shapes, then at the interface between the lens surfaces of the multi-stage lens, the step height in the optical axis direction increases.

[0040] In the optical scanning device 100 according to the present exemplary embodiment, as Figure 2AAs shown, lenses 6A and 6B are integrated into a multi-stage lens such that the optical axes of lenses 6A and 6B are located at the same position, i.e., they are not offset from each other in the sub-scanning direction. Although the lens surfaces of the multi-stage lens have different shapes, the step height at the interface of the lens surfaces is thus caused only by the difference in the generatrix shape and is not affected by the difference in the sagittal shape. This reduces the step height at the interface of the multi-stage lens, thereby advantageously improving the molding stability of lenses 6A and 6B.

[0041] The multi-stage lens of the optical scanning device 100 according to the present exemplary embodiment desirably satisfies the following inequality (1):

[0042] 0.01 ≤ |Xmax| ≤ 1.0, (1)

[0043] where Xmax (mm) is the maximum value (maximum step height) of the step (deviation in the optical axis direction) across the entire interface between lens surfaces with different lens surface shapes.

[0044] By satisfying inequality (1), it is possible to reduce the difference in imaging performance between optical paths caused by the difference in the lens surface shapes of the multi-stage lens. Setting the maximum value higher than the upper limit of inequality (1) results in too large a step height at the interface, causing the lens surface to be significantly deformed and strained due to the thermal deformation stress generated near the step caused by the step during molding. This affects the effective area of the lens surface through which the light beam passes and deteriorates the wavefront aberration. Setting the maximum value lower than the lower limit of inequality (1) results in too small a shape change amount introduced between the lens shapes, reducing the range within which lenses 7A and 7B can be freely arranged while maintaining the imaging performance of imaging optical systems SA and SB. This makes it difficult to achieve miniaturization and imaging performance in a compatible manner.

[0045] The multi-stage lens more desirably satisfies inequality (1a):

[0046] 0.01 ≤ |Xmax| ≤ 0.5. (1a)

[0047] The multi-stage lens even more desirably satisfies inequality (1b):

[0048] 0.02 ≤ |Xmax| ≤ 0.2. (1b)

[0049] The imaging optical systems SC and SD according to the present exemplary embodiment have configurations and optical operations similar to those of the imaging optical systems SA and SB. Lenses (optical parts) 6C and 6D are arranged in the sub-scanning direction and constitute a multi-stage lens (a shared fourth optical element) in which the incident surface and the exit surface are integrally formed. The incident surfaces of the optical parts 6C and 6D are independent of each other and have respective different surface vertices. Similarly, the exit surfaces of the optical parts 6C and 6D are also independent of each other and have respective different surface vertices. Thereby, the number of components of the lens is reduced. Since the lenses 6C and 6D have respective different lens surface shapes, and the lenses 7C and 7D are located at respective different optical positions with respect to the deflection point C0, interference between the lens 7C and the light beam RD is avoided in a small space while maintaining good optical characteristics of the imaging optical systems SC and SD. This results in a reduction in the size of the optical scanning device 100.

[0050] The optical scanning device 100 according to the present exemplary embodiment is configured such that in the sub-scanning cross section, the sub-scanning tilt incident angles of the imaging optical systems SA (SB) and the sub-scanning tilt incident angles of the imaging optical systems SD (SC) are rotationally symmetric about the main scanning axis by 180°. The main scanning axis refers to an axis that passes through the intersection of the rotation axis of the deflector 5 and the optical axes of the imaging optical systems SA and SD (SB and SC) and is parallel to the main scanning direction. The optical parts 6A and 6D (the first optical part and the fourth optical part) are shaped such that if one of the optical parts 6A and 6D is rotated by 180° (about the main scanning axis) in the sub-scanning cross section, it matches the other optical part. Therefore, even if, like the sagittal tilt configuration, the sagittal shape is asymmetric with respect to the optical axis, the lenses 6A and 7A and the lenses 6D and 7D have the same lens surface shape, and the sagittal tilt configuration is used to correct both the scanning line curvature and the distorted wavefront aberration in a compatible manner in a conventional sub-scanning tilt incident optical system. Similarly, the lenses 6B and 7B and the lenses 6C and 7C have the same lens surface shape. Therefore, the multi-stage lens integrating the lenses 6A and 6B and the multi-stage lens integrating the lenses 6C and 6D are configured as shared optical components. Moreover, the lenses 7A and 7D and the lenses 7B and 7C are configured as optical components of corresponding identical shapes. This results in a reduction in the types of optical components.

[0051] In the optical scanning device 100 according to the present exemplary embodiment, the imaging optical systems SA and SD are optically equivalent, and the imaging optical systems SB and SC are optically equivalent. This optically equivalent configuration minimizes color misregistration when the optical scanning device 100 is used in an image forming apparatus. Moreover, since the fθ characteristics are made the same, a shared image clock is used to reduce the cost of the circuit board.

[0052] Therefore, the optical scanning device 100 according to the present exemplary embodiment achieves good imaging performance in a manner compatible with miniaturization and reduction in the types of components.

[0053] (First Practical Example)

[0054] Now, the optical scanning device 100 according to the first practical example will be described. A description of the configuration of the optical scanning device 100 according to the present practical example that is similar to the configuration of the optical scanning device 100 according to the foregoing exemplary embodiment will be omitted.

[0055] In the optical scanning device 100 according to the present practical example, the light beams RA and RB emitted from the respective light sources 1A and 1B are respectively obliquely incident on the deflection surface of the deflector 5 at angles of αsA = +2.7° and αsB = -2.7° with respect to the reference plane P0 in the sub-scanning section. Similarly, the light beams RC and RD emitted from the respective light sources 1C and 1D are respectively obliquely incident on the deflection surface of the deflector 5 at angles of αsC = +2.7° and αsD = -2.7° with respect to the reference plane P0 in the sub-scanning section.

[0056] An excessively large tilt incident angle makes it difficult to correct the spot distortion caused by the distorted wavefront aberration. An excessively small tilt incident angle makes it difficult to separate the optical paths.

[0057] The optical scanning device 100 according to this practical example uses semiconductor lasers as the light sources 1A, 1B, 1C, and 1D.

[0058] In the optical scanning device 100 according to this practical example, the incident optical systems LA, LB, LC, and LD include anamorphic lenses 2A, 2B, 2C, and 2D, sub-scanning aperture diaphragms 3A, 3B, 3C, and 3D, and main-scanning aperture diaphragms 4A, 4B, 4C, and 4D.

[0059] The anamorphic lenses 2A, 2B, 2C, and 2D have anamorphic exit surfaces, where the radii of curvature in the main-scanning direction and the sub-scanning direction are designed to be different so that desired light beams are formed in both the main-scanning direction and the sub-scanning direction. In the main-scanning section, the anamorphic lenses 2A, 2B, 2C, and 2D convert the light beams RA, RB, RC, and RD emitted from the respective light sources 1A, 1B, 1C, and 1D into parallel light beams. As used herein, the parallel light beams are not limited to strictly parallel light beams, but include approximately parallel light beams, such as weakly divergent light beams and weakly convergent light beams. In the sub-scanning section, the anamorphic lenses 2A and 2B respectively converge the light beams RA and RB emitted from the light sources 1A and 1B near the deflection surface of the deflector 5. Similarly, in the sub-scanning section, the anamorphic lenses 2C and 2D respectively converge the light beams RC and RD emitted from the light sources 1C and 1D near the deflection surface of the deflector 5. The anamorphic lenses 2A and 2B have refractive incident surfaces for temperature compensation.

[0060] The sub-scanning aperture diaphragms 3A, 3B, 3C, and 3D respectively limit the beam diameters of the light beams RA, RB, RC, and RD passing through the anamorphic lenses 2A, 2B, 2C, and 2D in the sub-scanning direction. Similarly, the main-scanning aperture diaphragms 4A, 4B, 4C, and 4D respectively limit the beam diameters of the light beams RA, RB, RC, and RD passing through the anamorphic lenses 2A, 2B, 2C, and 2D in the main-scanning direction. The aperture diameters are designed to form light spots with a desired light spot diameter on the scanned surfaces 8A (yellow [Y]), 8B (magenta [M]), 8C (cyan [C]), and 8D (black [K]).

[0061] The optical scanning device 100 according to the actual example is designed such that in the main-scanning cross-section, the chief rays of the light beams RA and RB passing through the incident optical systems LA and LB and incident on the deflection surface form an angle α of 78° with the optical axes of the corresponding imaging optical systems SA and SB. Similarly, the optical scanning device 100 is designed such that in the main-scanning cross-section, the chief rays of the light beams RC and RD passing through the incident optical systems LC and LD and incident on the deflection surface form an angle α of 78° with the optical axes of the corresponding imaging optical systems SC and SD.

[0062] In the optical scanning device 100 according to the actual example, the incident optical systems LA, LB, LC, and LD have the same configuration and the same distance in the optical axis direction. In the optical scanning device 100 according to the actual example, the anamorphic lenses 2A and 2B and the anamorphic lenses 2C and 2D are constituted by resin lenses integrally formed respectively, so as to reduce the number of optical components, thereby reducing costs. However, the effects of the actual example are not limited to such a configuration. In the optical scanning device 100 according to the actual example, the optical components are deployed in a shared layout, thereby reducing the types of component holding units and the types of assembly tools, so as to improve productivity.

[0063] The deflector 5 is a four-sided polygon mirror with an outside diameter of 10 mm. The deflector 5 is rotated at a constant speed by a motor, so that the scanned surfaces 8A, 8B, 8C, and 8D are scanned. This realizes an optical scanning device that can achieve simultaneous scanning corresponding to the four colors Y, M, C, and K when installed on an image forming apparatus. The imaging optical systems SA, SB, SC, and SD are configured such that in the sub-scanning cross-section, the deflection surface 5A of the deflector 5 and the scanned surfaces 8A, 8B, 8C, and 8D are in an optically conjugate relationship for surface distortion correction. When using a deflector having a plurality of deflection surfaces (such as a polygon mirror), a surface distortion correction optical system is generally adopted because the tilt angle of the deflection surface in the sub-scanning direction varies from one deflection surface to another.

[0064] Tables 1, 2, 3, and 4 below illustrate the specifications, optical layout, and lens surface shapes of the optical scanning device 100 according to this actual example. Table 1 describes the specifications and lens layout of the incident optical system LA and the imaging optical system SA. Table 2 describes the lens surface shapes of the incident optical system LA and the imaging optical system SA. Table 3 describes the specifications and lens layout of the incident optical system LB and the imaging optical system SB. Table 4 describes the lens surface shapes of the incident optical system LB and the imaging optical system SB.

[0065] Tables 1 and 3 also illustrate the lens layouts of the incident optical system LC and the imaging optical system SC and the lens layouts of the incident optical system LD and the imaging optical system SD. The specifications and lens surface shapes of the incident optical system LC and the imaging optical system SC and the specifications and lens surface shapes of the incident optical system LD and the imaging optical system SD are identical to the specifications and lens surface shapes of the input optical system LB and the imaging optical system SB and the input optical system LA and the imaging optical system SA, respectively. Therefore, their descriptions will be omitted. The optical layout sections of Tables 1 and 3 describe the coordinates of the reflection points of the light beams RA and RB traveling toward the image center (axial image height) in the main scanning direction on the scanned surface on the respective mirrors.

[0066] [Table 1]

[0067]

[0068]

[0069]

[0070] [Table 2]

[0071]

[0072]

[0073]

[0074]

[0075] [Table 3]

[0076]

[0077]

[0078] [Table 4]

[0079]

[0080]

[0081]

[0082] The incident surfaces of the anamorphic lenses 2A, 2B, 2C, and 2D according to this actual example are rotationally asymmetric diffractive surfaces, where the phase function Φ of the diffraction grating is expressed by the following equation:

[0083]

[0084] k is the order of diffraction, and here k = 1. λ is the wavelength, and here λ = 790 nm.

[0085] The generatrix shape (the shape of the lens surface in the main scanning section) of the lens surfaces of the lenses 6A, 6B, 6C, and 6D and the lenses 7A, 7B, 7C, and 7D according to this actual example is an aspherical shape expressed by a function of up to the 10th order described below. Taking the intersection of each lens surface (optical surface) with the optical axis as the origin, the axis in the optical axis direction as the X-axis, and the axis orthogonal to the X-axis in the main scanning section as the Y-axis, the generatrix shape X is represented by the following equation:

[0086]

[0087] In this exemplary embodiment, the X-axis is defined with the direction of light travel as the +X side, and the Y-axis is defined with the light source side of the optical axis as the +Y side.

[0088] Here, taking the intersection of each lens surface with the optical axis of the optical part as the origin, the X-axis, Y-axis, and Z-axis respectively refer to the optical axis, the axis orthogonal to the optical axis in the main scanning section, and the axis orthogonal to the optical axis in the sub-scanning section. R is the radius of curvature of the generatrix, K is the eccentricity, and B i (i = 1, 2,..., 10) are aspherical coefficients.

[0089] The sagittal shape (the shape of the lens surface in the sub-scanning section at a given image height) of the lens surfaces of the lenses 6A, 6B, 6C, and 6D and the lenses 7A, 7B, 7C, and 7D according to this actual example is an aspherical shape expressed by the following equation:

[0090]

[0091] Here, S is the sagittal shape defined in a plane perpendicular to the main scanning section and including the normal of the generatrix at each position along the generatrix direction. m i,j are aspherical coefficients. The term composed of the first-order function of Z provides the tilt amount in the sagittal direction. In other words, the sagittal tilt amount according to this actual example corresponds to m 0,1 . Therefore, the sagittal tilt surface refers to m 0,1A non-zero surface. A sagittal inclined surface refers to an optical surface in a sub-scanning cross-section including the optical axis where the normal line of the generatrix is inclined with respect to the optical axis (not parallel to the optical axis). Here, the generatrix refers to the intersection line of the optical surface and the main scanning cross-section. Since y = 0 on the optical axis, the sagittal inclination amount (the inclination of the normal line of the generatrix with respect to the optical axis) in the sub-scanning cross-section including the optical axis is expressed as m 0,1 The sagittal inclined surface (sagittal inclination changing surface) has an aspherical coefficient m 2,1 and the sagittal inclination amount changes with the position Y in the main scanning direction.

[0092] As expressed by the following equation, the sagittal radius of curvature r' continuously changes with the Y coordinate of the lens surface:

[0093]

[0094] Here, r is the sagittal radius of curvature on the optical axis, and E i (i = 1, 2,..., 16) are sagittal change coefficients.

[0095] As can be seen from Table 2 and Table 4, in the optical scanning device 100 according to this actual example, the exit surfaces of the lenses 6A (6D) and the optical parts 6B (6C) have different generatrix shapes, sagittal shapes, and sagittal inclined shapes. Therefore, the two optical parts 6A (6D) and 6B (6C) located above and below the sub-scanning direction of the multi-stage lens have different aspherical coefficients. Even if the optical parts 7A (7D) and 7B (7C) are located at different optical positions from the deflection point C0, corresponding different optimal surface shapes are configured to correct the optical characteristics of the imaging optical systems SA (SD) and SB (SC).

[0096] As described above, at least a pair of incident surfaces or a pair of exit surfaces of the first optical part and the second optical part are offset (have a step) in the optical axis direction at the interface. The incident surfaces of the lenses 6A (6D) and the optical parts 6B (6C) are optically closer to the deflection point C0 than the exit surfaces. In the sub-scanning cross-section, the distance between the light beams RA (RD) and RB (RC) on the incident surfaces of the lenses 6A (6D) and the optical parts 6B (6C) is narrow. If there is a step (offset in the optical axis direction) at the interface of the incident surface, the deformation or strain of the lens surface near the step caused by the thermal deformation stress may have a large impact. Moreover, when the light beam moves up and down, halos are more likely to appear at the step at the interface of the multi-stage lens. Therefore, in the optical scanning device 100 according to this actual example, as can be seen from Table 2 and Table 4, the incident surfaces of the lenses 6A (6D) and the optical parts 6B (6C) are designed to have the same shape so that no step is generated at the interface of the lens surfaces of the multi-stage lens.

[0097] In the optical scanning device 100 according to the present practical example, the function expressing the surface shape of the optical portion is defined by the aforementioned defining formula. However, this is not restrictive, and other defining formulas can be used.

[0098] Figure 3 FIG. shows a step at the interface of the exit surface of the multi-stage lens in the optical scanning device 100 according to the present practical example. In Figure 3 it, a positive value (negative value) indicates that the generatrix of the optical portion 6A is farther from (closer to) the deflector 5 in the optical axis direction than the generatrix of the lens 6B in the optical axis direction.

[0099] As can be seen from Figure 3 in the optical scanning device 100 according to the present practical example, the step height at the interface of the exit surface of the multi-stage lens is at most ±0.04 mm, which satisfies the inequalities (1), (1a), and (1b). In the present practical example, by configuring the lenses 6A and 6B to have the same thickness, the step height is reduced. The shape difference between the exit surfaces of the lenses 6A and 6B is reduced to a sufficiently low level such that the multi-stage lens can be integrally formed without problems and satisfies various imaging characteristics.

[0100] Figure 4A and Figure 4B are graphs showing the field curvature (defocus characteristics) of the optical scanning device 100 according to the present practical example in the main scanning direction and the sub-scanning direction. Figure 4A corresponds to the light beam RA. Figure 4B corresponds to the light beam RB. In the present practical example, the effective image width (the width of the effective scanning region on the scanned surface) is W = ±163 mm. As Figure 4A and Figure 4B shown, the field curvature of both the imaging optical systems SA and SB in the main scanning direction and the sub-scanning direction is well corrected on the image plane.

[0101] Figure 5A and Figure 5B are graphs showing the fθ characteristic dy of the optical scanning device 100 according to the present practical example. Figure 5A corresponds to the light beam RA. Figure 5B corresponds to the light beam RB. The fθ characteristic dy refers to the difference obtained by subtracting the ideal image height from the position where the light beam actually arrives. The fθ characteristics dy of the imaging optical systems SA and SB are well corrected.

[0102] Figure 6A and Figure 6BIllustrated is the dependence of the position in the main scanning direction of the scanning line curvature dz of the optical scanning device 100 according to the present practical example on the scanned surfaces 8A and 8B. As used herein, the scanning line curvature dz refers to the difference between the imaging position in the sub-scanning direction at each image height on the scanned surface and the imaging position in the main scanning direction at the axial image height. As Figure 6A and Figure 6B shown in, the scanning line curvature dz of both the imaging optical systems SA and SB is well corrected on the image plane.

[0103] As described above, in the optical scanning device 100 according to the present exemplary embodiment, the imaging optical systems SA and SD are optically equivalent, and the imaging optical systems SB and SC are optically equivalent. Although the description of the imaging optical systems SC and SD is omitted, the imaging performance of the imaging optical systems SC and SD is thus similarly well corrected.

[0104] As described above, in the present practical example, the lenses 6A and 6B that are optically closest to the deflector 5 are configured as multi-stage lenses. The lens surfaces of the multi-stage lenses have different surface shapes, and the lenses 7A and 7B are located at optically different positions. This configuration increases the degree of freedom of the lens layout and enables miniaturization. Moreover, the imaging optical systems SA and SD are composed of lenses of the same shape, and the imaging optical systems SB and SC are composed of lenses of the same shape. This results in a reduction in the number of component types.

[0105] Such an optical scanning device 100 enables a reduction in the number of component types and miniaturization of the optical elements, while reducing the step height at the interface of the multi-stage lenses for stable moldability and good imaging performance.

[0106] Figure 7A is a sub-scanning cross-sectional view of the main components of the optical scanning device 200 according to the second exemplary embodiment. Figure 7B is an optical path diagram of the main components of the optical scanning device 200 according to the second exemplary embodiment in the main scanning cross-section. Figure 7C is an optical path diagram of the main components of the optical scanning device 200 according to the second exemplary embodiment in the sub-scanning cross-section.

[0107] Except for using the lenses 27A to 27D instead of the lenses 7A to 7D, the optical scanning device 200 according to the present exemplary embodiment has the same configuration as that of the optical scanning device 100 according to the first exemplary embodiment. Similar components will be described with the same reference numerals.

[0108] The imaging optical systems SA and SB according to the present exemplary embodiment each consist of a plurality of lenses. In the imaging optical system SA (SB), the lens optically closest to the deflector will be referred to as lens 6A (6B), and the lens optically closest to the scanned surface will be referred to as lens 27A (27B).

[0109] The lenses 6A and 6B according to the present exemplary embodiment are arranged in the sub-scanning direction to form a multi-stage lens in which the incident surface and the exit surface are integrally formed. In the multi-stage lens according to the present exemplary embodiment, at least the incident surface or the exit surface of the lenses 6A and 6B has a lens surface shape that is asymmetric in the sub-scanning direction with respect to the reference plane P0. The upper and lower portions of the multi-stage lens with respect to the reference plane P0 have different shapes in the main scanning section (generatrix shape) and the sub-scanning section (sagittal shape). By configuring at least the incident surface or the exit surface of the lenses 6A and 6B to have different lens surface shapes, the optical path lengths of the imaging optical systems SA and SB are made different while maintaining good optical performance of the imaging optical systems SA and SB. This results in an increase in the degree of freedom in layout.

[0110] In the optical scanning device 200 according to the present exemplary embodiment, the imaging optical system SA has an optical path length smaller than that of the imaging optical path length SB. This makes it possible to reduce the size of the optical scanning device 200 in the drum arrangement direction as compared with the case where the imaging optical systems SA and SB have the same optical path length.

[0111] In the optical scanning device 200 according to the present exemplary embodiment, as in the first exemplary embodiment, the lenses 6A and 6B are integrally formed into a multi-stage lens such that the optical axes of the lenses 6A and 6B are located at the same position, that is, the optical axes of the lenses 6A and 6B do not deviate from each other in the sub-scanning direction. Therefore, even if the lens surfaces of the multi-stage lens have correspondingly different shapes, the step height at the interface of the lens surfaces is caused only by the shape difference between the generatrix shapes, and the step height is not affected by the difference between the sagittal shapes. This results in a reduction in the step height at the interface of the multi-stage lens, thereby advantageously improving the molding stability of the lenses 6A and 6B.

[0112] The multi-stage lens of the optical scanning device 200 according to the present exemplary embodiment also satisfies the inequality (1). The multi-stage lens desirably satisfies the inequality (1a) and more desirably satisfies the inequality (1b).

[0113] The imaging optical systems SC and SD according to this exemplary embodiment have configurations and optical operations similar to those of the imaging optical systems SA and SB. The lenses 6C and 6D are arranged in the sub-scanning direction to form a multi-stage lens in which the incident surface and the exit surface are integrally formed. This reduces the number of lens components. The lenses 6C and 6D have different lens surface shapes, and the imaging optical system SD has an optical path length shorter than that of the imaging optical system SC. This makes it possible to reduce the size of the optical scanning device 200 in the drum arrangement direction.

[0114] Now, the incident angle of the chief ray of the light beam on the deflection surface in the sub-scanning cross-section (the angle formed between the main scanning cross-section and the chief ray) will be referred to as the sub-scanning tilt incident angle. The optical scanning device 200 according to this exemplary embodiment is configured such that in the sub-scanning cross-section, the sub-scanning tilt incident angles corresponding to the imaging optical systems SA (SB) and the sub-scanning tilt incident angles corresponding to the imaging optical systems SD (SC) are rotationally symmetric by 180° about an axis passing through the intersection of the rotation axis of the deflector 5 and the optical axis of the imaging optical system and parallel to the main scanning direction. Therefore, even if, like the sagittal tilt shape, the sagittal shape is asymmetric about the optical axis, the lenses 6A and 27A and the lenses 6D and 27D have the same lens surface shape, and the sagittal tilt shape is used in a conventional sub-scanning tilt incident optical system to correct the scanning line curvature and the distorted wavefront aberration in a compatible manner. Similarly, the lenses 6B and 27B and the lenses 6C and 27C have the same lens surface shape. Therefore, the multi-stage lens integrating the lenses 6A and 6B and the multi-stage lens integrating the lenses 6C and 6D are configured as common optical components. Moreover, the lenses 27A and 27D and the lenses 27B and 27C are configured as corresponding optical components of the same shape. This results in a reduction in the types of optical components.

[0115] In the optical scanning device 200 according to this exemplary embodiment, the imaging optical systems SA and SD are optically equivalent, and the imaging optical systems SB and SC are optically equivalent. This optically equivalent configuration minimizes color misregistration when the optical scanning device 200 is used in an image forming apparatus. Moreover, since the fθ characteristics are made the same, a common image clock is used to reduce the cost of the circuit board.

[0116] Therefore, the optical scanning device 200 according to this exemplary embodiment achieves good imaging characteristics in a manner compatible with miniaturization and reduction in the types of components.

[0117] (Second Practical Example)

[0118] Now, the optical scanning device 200 according to the second actual example will be described. The description of the configuration of the optical scanning device 200 according to this actual example, which is similar to the configurations of the optical scanning device 100 according to the foregoing exemplary embodiment and the optical scanning device 200 according to this exemplary embodiment, will be omitted.

[0119] Tables 5, 6, 7, and 8 below illustrate the specifications, optical layouts, and lens surface shapes of the optical scanning device 200 related to this actual example. Table 5 describes the specifications and lens layouts of the incident optical system LA and the imaging optical system SA. Table 6 describes the lens surface shapes of the incident optical system LA and the imaging optical system SA. Table 7 describes the specifications and lens layouts of the incident optical system LB and the imaging optical system SB. Table 8 describes the lens surface shapes of the incident optical system LB and the imaging optical system SB.

[0120] Tables 5 and 7 also illustrate the lens layouts of the incident optical system LC and the imaging optical system SC and the lens layouts of the incident optical system LD and the imaging optical system SD. The specifications and lens surface shapes of the incident optical system LC and the imaging optical system SC and the specifications and lens surface shapes of the incident optical system LD and the imaging optical system SD are respectively equivalent to the specifications and lens surface shapes of the input optical system LB and the imaging optical system SB and the specifications and lens surface shapes of the input optical system LA and the imaging optical system SA. Therefore, their descriptions will be omitted. The optical layout parts of Tables 5 and 7 describe the coordinates of the reflection points of the light beams RA and RB traveling toward the image center (axial image height) in the main scanning direction on the scanned surface on the corresponding mirrors.

[0121] [Table 5]

[0122]

[0123]

[0124] [Table 6]

[0125]

[0126]

[0127]

[0128] [Table 7]

[0129]

[0130]

[0131]

[0132] [Table 8]

[0133]

[0134]

[0135]

[0136]

[0137] As can be seen from Table 6 and Table 8, in the optical scanning device 200 according to this actual example, the exit surfaces of the lens 6A (6D) and the optical section 6B (6C) have different generatrix shapes, sagittal shapes, and sagittal tilt shapes. Therefore, the two optical sections 6A (6D) and 6B (6C) located above and below the sub-scanning direction of the multi-stage lens have different aspherical coefficients. Even if the imaging optical systems SA and SB have different optical path lengths, corresponding different optimal surface shapes are configured to correct the optical characteristics of the imaging optical systems SA (SD) and SB (SC).

[0138] The entrance surfaces of the lens 6A (6D) and the optical section 6B (6C) are optically closer to the deflection point C0 than the exit surfaces. In the sub-scanning cross-section, the distance between the light beams RA (RD) and RB (RC) on the entrance surfaces of the lens 6A (6D) and the optical section 6B (6C) is narrow. If there are steps at the interfaces between the entrance surfaces of the multi-stage lens, or if the upper lens surface and the lower lens surface in the sub-scanning direction have different shapes, then the deformation or strain of the lens surface near the steps caused by thermal deformation stress may have a large impact. Moreover, when the light beam moves up and down, halos are more likely to appear at the steps at the interfaces of the multi-stage lens. Therefore, in the optical scanning device 200 according to this actual example, as can be seen from Table 6 and Table 8, the entrance surfaces of the lens 6A (6D) and the optical section 6B (6C) are designed to have the same shape so that there are no steps at the interfaces of the lens surfaces of the multi-stage lens.

[0139] In the optical scanning device 200 according to this actual example, the function expressing the surface shape of the optical section is defined by the aforementioned defining formula. However, this is not restrictive, and other defining formulas can be used.

[0140] Figure 8 Illustrated is a step at the interface of the exit surface of the multi-stage lens in the optical scanning device 200 according to this actual example. In Figure 8 , a positive value (negative value) indicates that the generatrix of the optical section 6A is farther from (closer to) the deflector 5 in the optical axis direction than the generatrix of the lens 6B in the optical axis direction.

[0141] As can be seen fromFigure 8 As can be seen, in the optical scanning device 200 according to the present actual example, the step height at the interface of the exit surface of the multi-stage lens is at most -0.194 mm, which satisfies the inequalities (1), (1a), and (1b). The shape difference between the exit surfaces of the lenses 6A and 6B is reduced to a sufficiently low level such that the multi-stage lens can be integrally formed without problems and various imaging characteristics are satisfied.

[0142] Figure 9A and Figure 9B are graphs showing the field curvature (defocus characteristics) of the optical scanning device 200 according to the present actual example in the main scanning direction and the sub-scanning direction. Figure 9A Corresponds to the light beam RA. Figure 9B Corresponds to the light beam RB. In the present actual example, the effective image width (the width of the effective scanning region on the scanned surface) is W = ±163 mm. As Figure 9A and Figure 9B shown, the field curvature of both the imaging optical systems SA and SB in the main scanning direction and the sub-scanning direction is well corrected on the image plane.

[0143] Figure 10A and Figure 10B are graphs showing the fθ characteristic dy of the optical scanning device 200 according to this actual example. Figure 10A Corresponds to the light beam RA. Figure 10B Corresponds to the light beam RB. The fθ characteristic dy refers to the difference obtained by subtracting the ideal image height from the position where the light beam actually arrives. The fθ characteristics dy of the imaging optical systems SA and SB are well corrected.

[0144] Figure 11A and Figure 11B illustrate the dependence of the scanning line curvature dz on the position in the main scanning direction of the optical scanning device 200 according to the present exemplary embodiment on the scanned surfaces 8A and 8B. Here, the scanning line curvature dz refers to the difference between the imaging position in the sub-scanning direction at each image height on the scanned surface and the imaging position in the main scanning direction at the axial image height. As Figure 11A and Figure 11B shown, the scanning line curvatures dz of both the imaging optical systems SA and SB are well corrected on the image plane.

[0145] As described above, in the optical scanning device 200 according to the present exemplary embodiment, the imaging optical systems SA and SD are optically equivalent, and the imaging optical systems SB and SC are optically equivalent. Although the description of the imaging optical systems SC and SD is omitted, the imaging performance of the imaging optical systems SC and SD is similarly well corrected.

[0146] As described above, in the present practical example, the lenses 6A and 6B that are optically closest to the deflector 5 are configured as multi-stage lenses. The lens surfaces of the multi-stage lenses have different surface shapes, and the imaging optical systems SA and SB have different optical path lengths. This configuration increases the degree of freedom in lens layout and enables miniaturization. Moreover, the imaging optical systems SA and SD are composed of lenses of the same shape, and the imaging optical systems SB and SC are composed of lenses of the same shape. This results in a reduction in the number of component types.

[0147] This optical scanning device 200 enables a reduction in the number of component types and miniaturization of optical elements, while reducing the step height at the interface of the multi-stage lenses for stable moldability and good imaging performance.

[0148] Figure 12A FIG. is an optical path diagram of the main components of the optical scanning devices 10 and 20 according to the third exemplary embodiment in the main scanning section. Figure 12B FIG. is an optical path diagram of the main components of the optical scanning devices 10 and 20 according to the third exemplary embodiment in the sub-scanning section.

[0149] The optical scanning devices 10 and 20 according to the present exemplary embodiment include a first light source 101 and a second light source 201, a first anamorphic collimator lens 102 and a second anamorphic collimator lens 202, a first sub-scanning aperture stop 103 and a second sub-scanning aperture stop 203, and a first main-scanning aperture stop 104 and a second main-scanning aperture stop 204.

[0150] The optical scanning devices 10 and 20 according to the present exemplary embodiment further include a deflector 1, a first fθ lens 106 and 206 (first imaging element), and a second fθ lens 107 and 207 (second imaging element and third imaging element).

[0151] The first fθ lens 106 is located on the optical path between the deflector 1 and the second fθ lens 107. The first fθ lens 206 is located on the optical path between the deflector 1 and the second fθ lens 207.

[0152] A semiconductor laser is used as the first light source 101 and the second light source 201.

[0153] The first anamorphic collimator lens 102 and the second anamorphic collimator lens 202 convert the light beams RA and RB (first light beam and second light beam) emitted from the first light source 101 and the second light source 201 into parallel light beams in the main scanning section and converge the light beams RA and RB in the sub-scanning direction. Here, the parallel light beams are not limited to strictly parallel light beams, but include approximately parallel light beams such as weakly divergent light beams and weakly convergent light beams.

[0154] The first sub-scanning aperture diaphragm 103 and the second sub-scanning aperture diaphragm 203 limit the beam diameters of the light beams RA and RB passing through the first anamorphic collimator lens 102 and the second anamorphic collimator lens 202 in the sub-scanning direction.

[0155] The first main-scanning aperture diaphragm 104 and the second main-scanning aperture diaphragm 204 limit the beam diameters of the light beams RA and RB passing through the first anamorphic collimator lens 102 and the second anamorphic collimator lens 202 in the main-scanning direction.

[0156] Therefore, the light beams RA and RB emitted from the first light source 101 and the second light source 201 converge only near the deflection surface of the deflector 1 in the sub-scanning direction, respectively, and are formed into line images that are long in the main-scanning direction.

[0157] The deflector 1 is rotated in the direction of arrow A in the figure by a driving unit (such as a motor) (not shown), so that the deflector 1 deflects the incident light beams RA and RB. For example, the deflector 1 is composed of a polygon mirror.

[0158] The first fθ lens 106 and the second fθ lens 107 are anamorphic imaging lenses having different focal powers in the main-scanning section and the sub-scanning section. The first fθ lens 106 and the second fθ lens 107 converge (guide) the light beam RA deflected by the deflection surface of the deflector 1 onto the first scanned surface 108.

[0159] The first fθ lens 206 and the second fθ lens 207 are anamorphic imaging lenses having different focal powers in the main-scanning section and the sub-scanning section. The first fθ lens 206 and the second fθ lens 207 converge (guide) the light beam RB deflected by the deflection surface of the deflector 1 onto the second scanned surface 208.

[0160] In the optical scanning device 10 according to the present exemplary embodiment, the first incident optical system 45a is composed of the first anamorphic collimator lens 102, the first sub-scanning aperture diaphragm 103, and the first main-scanning aperture diaphragm 104. In the optical scanning device 20, the second incident optical system 55a is composed of the second anamorphic collimator lens 202, the second sub-scanning aperture diaphragm 203, and the second main-scanning aperture diaphragm 204.

[0161] In the optical scanning device 10 according to the present exemplary embodiment, the first imaging optical system 45b is composed of the first fθ lens 106 and the second fθ lens 107. In the optical scanning device 20, the second imaging optical system 55b is composed of the first fθ lens 206 and the second fθ lens 207.

[0162] The refractive powers of the second fθ lenses 107 and 207 in the sub-scanning cross-section are higher than those of the first fθ lenses 106 and 206 in the sub-scanning cross-section, that is, they are the highest in the first imaging optical system 45b and the second imaging optical system 55b, respectively.

[0163] The light beam RA emitted from the emission point of the first light source 101 is converted into a parallel light beam by the first anamorphic collimator lens 102.

[0164] Then, the converted light beam RA passes through the first sub-scanning aperture stop 103, passes through the first main-scanning aperture stop 104, and is incident on the deflector 1.

[0165] The light beam RA emitted from the first light source 101 and incident on the deflector 1 is deflected and scanned by the deflector 1, and then is converged on the first scanned surface 108 by the first imaging optical system 45b, and scans the first scanned surface 108 at a constant speed.

[0166] The light beam RB emitted from the emission point of the second light source 201 is converted into a parallel light beam by the second anamorphic collimator lens 202.

[0167] The converted light beam RB then passes through the second main-scanning aperture stop 203, passes through the second main-scanning aperture stop 204, and is incident on the deflection surface of the deflector 1.

[0168] The light beam RB emitted from the second light source 201 and incident on the deflection surface of the deflector 1 is deflected and scanned by the deflector 1, and then is converged on the second scanned surface 208 by the second imaging optical system 55b, and scans the second scanned surface 208 at a constant speed.

[0169] Since the deflector 1 rotates in the direction of arrow A in the figure, the deflected and scanned light beams RA and RB scan the first scanned surface 108 and the second scanned surface 208 in the direction of arrow B in the figure, respectively.

[0170] The deflection point (axial deflection point) of the principal ray of the axial light beam on the deflection surface of the deflector 1 is represented by C0. The deflection point C0 is used as a reference point for the first imaging optical system 45b and the second imaging optical system 55b.

[0171] In this exemplary embodiment, the first photosensitive drum 108 and the second photosensitive drum 208 are used as the first scanned surface 108 and the second scanned surface 208, respectively.

[0172] By rotating the first photosensitive drum 108 and the second photosensitive drum 208 in the sub-scanning direction at each main-scanning exposure, the exposure distributions of the first photosensitive drum 108 and the second photosensitive drum 208 in the sub-scanning direction are formed.

[0173] The following Tables 9 to 11 illustrate the characteristics of the first incident optical system 45a and the second incident optical system 55a, and the first imaging optical system 45b and the second imaging optical system 55b of the optical scanning devices 10 and 20 according to the present exemplary embodiment.

[0174] [Table 9]

[0175]

[0176]

[0177] [Table 10]

[0178]

[0179]

[0180]

[0181]

[0182] [Table 11]

[0183]

[0184]

[0185]

[0186]

[0187] Next, the effects of the optical scanning devices 10 and 20 according to the present exemplary embodiment will be described.

[0188] Figure 13 Illustrated is the folded layout of the optical scanning devices 10 and 20 according to the present exemplary embodiment using reflection elements.

[0189] As Figure 13 shown, the first imaging optical system 45b includes mirrors 109 and 110. The second imaging optical system 55b includes mirror 209.

[0190] Reflection elements having vapor deposition films are used as mirrors 109, 110, and 209.

[0191] In the present exemplary embodiment, the light emitted from the first fθ lens 106 of the first imaging optical system 45b is deflected and reflected by the mirror 109, passes through the second fθ lens 107, is deflected and reflected by the mirror 110, and is guided to the photosensitive drum 108. The light emitted from the second fθ lens 207 of the second imaging optical system 55b is deflected and reflected by the mirror 209 and is guided to the photosensitive drum 208.

[0192] If the distance between the photosensitive drums 108 and 208 is reduced to miniaturize the image forming apparatus, and the second fθ lenses 107 of the first imaging optical system 45b and the second fθ lens 207 of the second imaging optical system 55b are at optically equivalent distances from the deflector 1, then the second fθ lenses 107 and 207 interfere with the light beams RA and RB.

[0193] In the present exemplary embodiment, to solve this problem, the second fθ lens 107 of the first imaging optical system 45b is positioned closer to the deflector 1 than the second fθ lens 207 of the second imaging optical system 55b.

[0194] This layout prevents interference between the fθ lenses and the light beams while reducing the size of the image forming apparatus.

[0195] However, since the second fθ lenses 107 and 207 of the first imaging optical system 45b and the second imaging optical system 55b are located at different positions, the first fθ lenses 106 and 206 of the corresponding imaging optical systems 45b and 55b are expected to have different powers in the sub-scanning direction so that the imaging optical systems 45b and 55b have substantially the same sub-scanning magnification.

[0196] Table 12 below illustrates the characteristics of the first fθ lenses 106 and 206 and the second fθ lenses 107 and 207 according to the present exemplary embodiment.

[0197] [Table 12]

[0198]

[0199]

[0200] In the present exemplary embodiment, the sagittal curvature (curvature in the sub-scanning cross-section) of the exit surface of the first fθ lens 106 of the optical scanning device 10 on the optical axis (near the axis) is 55.261. The sagittal curvature of the exit surface of the first fθ lens 206 of the optical scanning device 20 near the axis is 25.004.

[0201] Therefore, by making the sagittal curvatures of the exit surfaces of the first fθ lenses 106 and 206 different, the above-described compact configuration is achieved.

[0202] Figure 14 Schematically illustrates the optical path when the deflector 1 is misaligned according to this exemplary embodiment.

[0203] Figure 14 The upper part of... is a schematic diagram showing the optical path when the deflector 1 is in the ideal position. Figure 14 The lower part of... is a schematic diagram showing the optical path when the deflector 1 is positioned to deviate from the ideal position.

[0204] In the figure, the light ray L1 is incident on the deflector 1. When the deflector 1 is in the ideal position, the light ray L2 is generated. When the deflector 1 is positioned to deviate from the ideal position, the light ray L3 is generated.

[0205] The amount of change in the deviation of the light irradiation position on the photosensitive drum caused by the positional deviation due to the assembly error of the deflector 1 varies according to the tilt incident angle on the deflector 1.

[0206] When the deflector 1 rotates in the direction of arrow A in... Figure 12A the deflection surface appears and disappears in the sub-scanning direction. To reduce the distribution of the deviation in the main scanning direction, the sagittal curvature is changed, which further reduces the deviation of the irradiation position on the photosensitive drum.

[0207] In this exemplary embodiment, as shown in Tables 10 and 11, the exit surfaces of the first fθ lenses 106 and 206 are surfaces whose sagittal curvature changes in the main scanning direction.

[0208] In the sub-scanning cross-section, it is desirable to simultaneously satisfy the following inequalities (2) and (3):

[0209] |θ2| ≥ |θ1|, and (2)

[0210] -2.5 < θ2 / θ1 < 2.5, (3)

[0211] where θ1 is the incident angle of the principal ray of the first light beam RA corresponding to the optical scanning device 10 on the deflection surface of the deflector 1, and θ2 is the incident angle of the principal ray of the second light beam RB corresponding to the optical scanning device 20 on the deflection surface of the deflector 1. Thus, the difference in the irradiation position deviation caused by the optical scanning devices 10 and 20 on the photosensitive drums 108 and 208 is reduced.

[0212] The incident angles θ1 and θ2 more desirably satisfy the following inequality (3a), and even more desirably satisfy the inequality (3b):

[0213] -2.0 < θ2 / θ1 < 2.0, and (3a)

[0214] -1.5 < θ2 / θ1 < 1.5. (3b)

[0215] It is even more desirable that θ1 = θ2.

[0216] In this exemplary embodiment, the incident angle θ1 of the principal ray of the optical scanning device 10 in the sub-scanning direction is 2.7°. The incident angle θ2 of the principal ray of the optical scanning device 20 in the sub-scanning direction is -2.7°.

[0217] If the deflector 1 of the optical scanning device 10 moves 15 μm in the optical axis direction, then the light ray L3 moves 1.5 μm from the light ray L2 on the photosensitive drum 108. If the deflector 1 of the optical scanning device 20 moves 15 μm in the optical axis direction, then the light ray L3 moves -1.5 μm from the light ray L2 on the photosensitive drum 208.

[0218] The relative difference is 3 μm. At a resolution of 600 dpi, the influence of this difference on a 42.3 μm pitch is approximately 7%, and it has little impact on the image quality.

[0219] In this exemplary embodiment, |θ2| = |θ1|, and θ2 / θ1 = -1. This satisfies |θ2| ≥ |θ1| and -2.5 < θ2 / θ1 < 2.5.

[0220] Therefore, the amount of positional deviation caused by the optical scanning devices 10 and 20 on the photosensitive drum is reduced.

[0221] In this exemplary embodiment, the optical scanning devices 10 and 20 respectively include an optical path including two reflecting elements 109 and 110 and an optical path including one reflecting element 209, where the signs of the inclined incident angles are opposite. In this configuration, the deviations on the photosensitive drums 108 and 208 caused by the positional deviation of the deflector 1 occur in the same direction.

[0222] A configuration where θ2 / θ1 < 0 and the difference in the number of reflecting elements included in the optical scanning devices 10 and 20 is odd further reduces the amount of positional deviation.

[0223] Therefore, the amount of positional deviation caused by the optical scanning devices 10 and 20 on the photosensitive drums 108 and 208 is further reduced.

[0224] Here, the relative difference is 0 μm, which results in a further reduction in the impact on the image quality.

[0225] As a modification of this exemplary embodiment, in the case where θ1 is 2.7° and θ2 is -6.7°, θ2 / θ1 = -2.48 is obtained.

[0226] In this case, if the deflector 1 of the optical scanning device 10 moves 15 μm in the optical axis direction, the light ray L3 moves 1.5 μm from the light ray L2 on the photosensitive drum 108. If the deflector 1 of the optical scanning device 20 moves 15 μm in the optical axis direction, the light ray L3 moves -3.72 μm from the light ray L2 on the photosensitive drum 208.

[0227] The relative difference is 5.22 μm. At a resolution of 600 dpi, the influence of this difference on a 42.3 μm pitch is approximately 12.3%.

[0228] In this modification example, since θ2 / θ1 < 0, if the difference in the number of reflection elements included in the optical scanning devices 10 and 20 is odd, the amount of positional deviation is further reduced.

[0229] As another modification example of the present exemplary embodiment, when θ1 is 1.1° and θ2 is 2.7°, θ2 / θ1 = 2.45 is obtained.

[0230] In this case, if the deflector 1 of the optical scanning device 10 moves 15 μm in the optical axis direction, the light ray L3 moves 0.6 μm from the light ray L2 on the photosensitive drum 108. If the deflector 1 of the optical scanning device 20 moves 15 μm in the optical axis direction, the light ray L3 moves 1.5 μm from the light ray L2 on the photosensitive drum 208.

[0231] The relative difference is 0.9 μm. At a resolution of 600 dpi, the influence of this difference on a 42.3 μm pitch is approximately 2.1%.

[0232] In this modification example, since θ2 / θ1 > 0, if the difference in the number of reflection elements included in the optical scanning devices 10 and 20 is even, the amount of positional deviation is further reduced.

[0233] In this case, if the deflector 1 of the optical scanning device 10 moves 15 μm in the optical axis direction, the light ray L3 moves 1.5 μm from the light ray L2 on the photosensitive drum 108. If the deflector 1 of the optical scanning device 20 moves 15 μm in the optical axis direction, the light ray L3 moves -3.72 μm from the light ray L2 on the photosensitive drum 208.

[0234] In view of miniaturization and reduction of image quality differences, the first fθ lenses 106 and 206 used in the present exemplary embodiment are desirably composed of integrally formed lenses.

[0235] Even if the incident surface of the first fθ lenses 106 and 206 has a different sagittal curvature from the exit surface, an effect similar to that of the present exemplary embodiment can be obtained.

[0236] Using the foregoing configuration, the optical scanning devices 10 and 20 according to the present exemplary embodiment provide a compact optical scanning device while reducing the difference in image quality.

[0237] Figure 15A It is an optical path diagram of the main components of the optical scanning device 30 according to the fourth exemplary embodiment in the main scanning section. Figure 15B It is an optical path diagram of the main components of the imaging optical system included in the optical scanning device 30 according to the fourth exemplary embodiment in the sub-scanning section.

[0238] The optical scanning device 30 according to the present exemplary embodiment includes a first light source 301, a second light source 401, a third light source 501, and a fourth light source 601, a first anamorphic collimator lens 302, a second anamorphic collimator lens 402, a third anamorphic collimator lens 502, and a fourth anamorphic collimator lens 602, a first sub-scanning aperture stop 303, a second sub-scanning aperture stop 403, a third sub-scanning aperture stop 503, and a fourth sub-scanning aperture stop 603, and a first main-scanning aperture stop 304, a second main-scanning aperture stop 404, a third main-scanning aperture stop 504, and a fourth main-scanning aperture stop 604.

[0239] The optical scanning device 30 according to the present exemplary embodiment further includes a deflector 1, first fθ lenses 306, 406, 506, and 606 (first imaging elements), second fθ lenses 307 and 407 (second and third imaging elements), and second fθ lenses 507 and 607 (second and third imaging elements).

[0240] The first fθ lens 306 is located on the optical path between the deflector 1 and the second fθ lens 307. The first fθ lens 406 is located on the optical path between the deflector 1 and the second fθ lens 407. The first fθ lens 506 is located on the optical path between the deflector 1 and the second fθ lens 507. The first fθ lens 606 is located on the optical path between the deflector 1 and the second fθ lens 607.

[0241] Semiconductor lasers are used as the first light source 301, the second light source 401, the third light source 501, and the fourth light source 601.

[0242] The first anamorphic collimator lens 302, the second anamorphic collimator lens 402, the third anamorphic collimator lens 502, and the fourth anamorphic collimator lens 602 convert the light beams RC, RD, RE, and RF (the first light beam, the second light beam, the third light beam, and the fourth light beam) emitted from the first light source 301, the second light source 401, the third light source 501, and the fourth light source 601 into parallel light beams in the main scanning section, and converge the light beams RC, RD, RE, and RF in the sub-scanning direction. Here, the parallel light beams are not limited to strictly parallel light beams, but include substantially parallel light beams, such as weakly divergent light beams and weakly convergent light beams.

[0243] The first sub-scanning aperture stop 303, the second sub-scanning aperture stop 403, the third sub-scanning aperture stop 503, and the fourth sub-scanning aperture stop 603 limit the beam diameters of the light beams RC, RD, RE, and RF passing through the first anamorphic collimator lens 302, the second anamorphic collimator lens 402, the third anamorphic collimator lens 502, and the fourth anamorphic collimator lens 602 in the sub-scanning direction.

[0244] The first main-scanning aperture stop 304, the second main-scanning aperture stop 404, the third main-scanning aperture stop 504, and the fourth main-scanning aperture stop 604 limit the beam diameters of the light beams RC, RD, RE, and RF passing through the first anamorphic collimator lens 302, the second anamorphic collimator lens 402, the third anamorphic collimator lens 502, and the fourth anamorphic collimator lens 602 in the main-scanning direction.

[0245] In this way, the light beams RC, RD, RE, and RF emitted from the first light source 301, the second light source 401, the third light source 501, and the fourth light source 601 are each converged only in the sub-scanning direction near the deflection surface of the deflector 1, and are formed into line images that are long in the main-scanning direction.

[0246] The deflector 1 is rotated in the direction of arrow A in the figure by a driving unit (such as a motor) not shown, so that the deflector 1 deflects the incident light beams RC, RD, RE, and RF. For example, the deflector 1 is composed of a polygon mirror.

[0247] The first fθ lens 306 and the second fθ lens 307 are anamorphic imaging lenses having different focal powers in the main scanning section and the sub-scanning section. The first fθ lens 306 and the second fθ lens 307 converge (guide) the light beam RC deflected by the deflection surface of the deflector 1 onto the first scanned surface 308.

[0248] The first fθ lens 406 and the second fθ lens 407 are anamorphic imaging lenses having different refractive powers in the main scanning section and the sub-scanning section. The first fθ lens 406 and the second fθ lens 407 converge (guide) the light beam RD deflected by the deflection surface of the deflector 1 onto the second scanned surface 408.

[0249] The first fθ lens 506 and the second fθ lens 507 are anamorphic imaging lenses having different refractive powers in the main scanning section and the sub-scanning section. The first fθ lens 506 and the second fθ lens 507 converge (guide) the light beam RE deflected by the deflection surface of the deflector 1 onto the third scanned surface 508.

[0250] The first fθ lens 606 and the second fθ lens 607 are anamorphic imaging lenses having different refractive powers in the main scanning section and the sub-scanning section. The first fθ lens 606 and the second fθ lens 607 converge (guide) the light beam RF deflected by the deflection surface of the deflector 1 onto the fourth scanned surface 608.

[0251] In the optical scanning device 30 according to the present exemplary embodiment, the first incident optical system 65a is composed of a first anamorphic collimator lens 302, a first sub-scanning aperture stop 303, and a first main-scanning aperture stop 304. The second incident optical system 75a is composed of a second anamorphic collimator lens 402, a second sub-scanning aperture stop 403, and a second main-scanning aperture stop 404. The third incident optical system 85a is composed of a third anamorphic collimator lens 502, a third sub-scanning aperture stop 503, and a third main-scanning aperture stop 504. The fourth incident optical system 95a is composed of a fourth anamorphic collimator lens 602, a fourth sub-scanning aperture stop 603, and a fourth main-scanning aperture stop 604.

[0252] In the optical scanning device 30 according to the present exemplary embodiment, the first imaging optical system 65b is composed of a first fθ lens 306 and a second fθ lens 307. The second imaging optical system 75b is composed of a first fθ lens 406 and a second fθ lens 407. The third imaging optical system 85b is composed of a first fθ lens 506 and a second fθ lens 507. The fourth imaging optical system 95b is composed of a first fθ lens 606 and a second fθ lens 607.

[0253] The refractive powers of the second fθ lenses 307, 407, 507, and 607 in the sub-scanning section are respectively higher than the refractive powers of the first fθ lenses 306, 406, 506, and 606 in the main scanning section, that is, they are the highest in the first imaging optical system 65b, the second imaging optical system 75b, the third imaging optical system 85b, and the fourth imaging optical system 95b respectively.

[0254] The light beam RC emitted from the emission point of the first light source 301 is converted into a parallel light beam by the first anamorphic collimator lens 302 in the main scanning section and converges in the sub-scanning direction.

[0255] The resulting light beam RC passes through the first sub-scanning aperture diaphragm 303, passes through the first main-scanning aperture diaphragm 304, and is incident on the deflection surface of the deflector 1.

[0256] The light beam RC emitted from the first light source 301 and incident on the deflection surface of the deflector 1 is deflected and scanned by the deflector 1, and then converged on the first scanned surface 308 by the first imaging optical system 65b, and scans the first scanned surface 308 at a constant speed.

[0257] The light beam RD emitted from the emission point of the second light source 401 is converted into a parallel light beam by the second anamorphic collimator lens 402 in the main scanning section and converges in the sub-scanning direction.

[0258] The resulting light beam RD passes through the second sub-scanning aperture diaphragm 403, passes through the second main-scanning aperture diaphragm 404, and is incident on the deflection surface of the deflector 1.

[0259] The light beam RD emitted from the second light source 401 and incident on the deflection surface of the deflector 1 is deflected and scanned by the deflector 1, and then converged on the second scanned surface 408 by the second imaging optical system 75b, and scans the second scanned surface 408 at a constant speed.

[0260] The light beam RE emitted from the emission point of the third light source 501 is converted into a parallel light beam by the third anamorphic collimator lens 502 in the main scanning section and converges in the sub-scanning direction.

[0261] The resulting light beam RE passes through the third sub-scanning aperture diaphragm 503, passes through the third main-scanning aperture diaphragm 504, and is incident on the deflection surface of the deflector 1.

[0262] The light beam RE emitted from the third light source 501 and incident on the deflection surface of the deflector 1 is deflected and scanned by the deflector 1, and then converged on the third scanned surface 508 by the third imaging optical system 85b, and scans the third scanned surface 508 at a constant speed.

[0263] The light beam RF emitted from the emission point of the fourth light source 601 is converted into a parallel light beam by the fourth anamorphic collimator lens 602 in the main scanning section and converges in the sub-scanning direction.

[0264] The resulting light beam RF passes through the fourth sub-scanning aperture diaphragm 603, passes through the fourth main-scanning aperture diaphragm 604, and is incident on the deflection surface of the deflector 1.

[0265] A light beam RF emitted from a fourth light source 601 and incident on a deflection surface of a deflector 1 is deflected and scanned by the deflector 1, then converged by a fourth imaging optical system 95b on a fourth scanned surface 608, and scans the fourth scanned surface 608 at a constant speed.

[0266] Since the deflector 1 rotates in the direction of arrow A in the figure, the deflected and scanned light beams RC, RD, RE, and RF scan the first scanned surface 308, the second scanned surface 408, the third scanned surface 508, and the fourth scanned surface 608 in the direction of arrow B in the figure, respectively.

[0267] Deflection points (axial deflection points) of chief rays of axial light beams on the deflection surface of the deflector 1 are represented by D0 and E0. The deflection points D0 and E0 are used as reference points for a first imaging optical system 65b, a second imaging optical system 75b, a third imaging optical system 85b, and a fourth imaging optical system 95b.

[0268] In the present exemplary embodiment, a first photosensitive drum 308, a second photosensitive drum 408, a third photosensitive drum 508, and a fourth photosensitive drum 608 are used as the first scanned surface 308, the second scanned surface 408, the third scanned surface 508, and the fourth scanned surface 608.

[0269] By rotating the first photosensitive drum 308, the second photosensitive drum 408, the third photosensitive drum 508, and the fourth photosensitive drum 608 in a sub-scanning direction at each main scanning exposure, an exposure distribution on the first photosensitive drum 308, the second photosensitive drum 408, the third photosensitive drum 508, and the fourth photosensitive drum 608 in the sub-scanning direction is formed.

[0270] The following Tables 13 to 15 illustrate characteristics of a first incident optical system 65a, a second incident optical system 75a, a third incident optical system 85a, and a fourth incident optical system 95a and a first imaging optical system 65b, a second imaging optical system 75b, a third imaging optical system 85b, and a fourth imaging optical system 95b of an optical scanning device 30 according to the present exemplary embodiment.

[0271] [Table 13]

[0272]

[0273]

[0274]

[0275] [Table 14]

[0276]

[0277]

[0278]

[0279]

[0280] [Table 15]

[0281]

[0282]

[0283]

[0284]

[0285] The radius of curvature r' in the sub-scanning cross-section changes continuously with the y-coordinate of the lens surface.

[0286] Next, the effects of the optical scanning device 30 according to the present exemplary embodiment will be described.

[0287] Figure 16 The folded layout of the optical scanning device 30 according to the present exemplary embodiment using a reflective element is illustrated.

[0288] As Figure 16 shown, the first imaging optical system 65b includes mirrors 309 and 310. The second imaging optical system 75b includes mirror 409. The third imaging optical system 85b includes mirrors 509 and 510. The fourth imaging optical system 95b includes mirror 609.

[0289] Reflective elements having vapor deposition films are used as mirrors 309, 310, 409, 509, 510, and 609.

[0290] In the present exemplary embodiment, the light emitted from the first fθ lens 306 of the first imaging optical system 65b is deflected and reflected by the mirror 309, passes through the second fθ lens 307, is reflected and deflected by the mirror 310, and is guided to the photosensitive drum 308. The light emitted from the second fθ lens 407 of the second imaging optical system 75b is deflected and reflected by the mirror 409 and is guided to the photosensitive drum 408. The light emitted from the first fθ lens 506 of the third imaging optical system 85b is deflected and reflected by the mirror 509, passes through the second fθ lens 507, is deflected and reflected by the mirror 510, and is guided to the photosensitive drum 508. The light emitted from the second fθ lens 607 of the fourth imaging optical system 95b is deflected and reflected by the mirror 609 and is guided to the photosensitive drum 608.

[0291] If the distance between the photosensitive drums 308 and 408 is reduced in order to miniaturize the image forming apparatus, and the second fθ lenses 307 and 407 of the first imaging optical system 65b and the second imaging optical system 75b are at optically equivalent distances from the deflector 1, then the second fθ lenses 307 and 407 interfere with the light beams RC and RD.

[0292] If the distance between the photosensitive drums 508 and 608 is reduced, and the second fθ lenses 507 and 607 of the third imaging optical system 85b and the fourth imaging optical system 95b are at optically equivalent distances from the deflector 1, then the second fθ lenses 507 and 607 interfere with the light beams RE and RF.

[0293] To solve this problem, in the present exemplary embodiment, the second fθ lens 307 of the first imaging optical system 65b is positioned closer to the deflector 1 than the second fθ lens 407 of the second imaging optical system 75b. The second fθ lens 507 of the third imaging optical system 85b is positioned closer to the deflector 1 than the second fθ lens 607 of the fourth imaging optical system 95b.

[0294] This layout prevents interference between the fθ lenses and the light beams, while reducing the size of the image forming apparatus.

[0295] However, since the second fθ lenses 307 and 407 of the first imaging optical system 65b and the second imaging optical system 75b are located at different positions, the first fθ lenses 306 and 406 of the imaging optical systems 65b and 75b desirably have different focal powers in the substrate scanning direction so that the sub-scanning magnification (imaging magnification in the sub-scanning cross section) of the corresponding imaging optical systems is substantially the same.

[0296] Table 16 illustrates the characteristics of the first fθ lenses 306, 406, 506, and 606 and the second fθ lenses 307, 407, 507, and 607 according to the present exemplary embodiment.

[0297] [Table 16]

[0298]

[0299] In the present exemplary embodiment, the exit surfaces of the first fθ lenses 306 and 506 of the optical scanning device 30 have a sagittal curvature of 54.586 on the optical axis (near the axis). The exit surfaces of the first fθ lenses 406 and 606 have a sagittal curvature of 20.586 near the optical axis.

[0300] Therefore, by making the sagittal curvatures of the exit surfaces of the first fθ lenses 306 and 506 different from the sagittal curvatures of the exit surfaces of the first fθ lenses 406 and 606, the above-described compact configuration is achieved.

[0301] In the present exemplary embodiment, as shown in Tables 14 and 15, the exit surfaces of the first fθ lenses 306, 406, 506, and 606 are surfaces whose sagittal curvature changes in the main scanning direction.

[0302] By satisfying |θ2|≥|θ1| and -2.5 < θ2 / θ1 < 2.5, the difference in the irradiation position deviation caused by the optical scanning device 30 on the photosensitive drums 308 and 408 is reduced, where θ1 is the incident angle of the principal ray of the first light beam RC on the deflector 1 in the sub-scanning direction, and θ2 is the incident angle of the principal ray of the second light beam RD in the sub-scanning direction.

[0303] In the present exemplary embodiment, the incident angle θ1 of the principal ray of the first light beam RC in the sub-scanning direction is 2.7°. The incident angle θ2 of the principal ray of the second light beam RD in the sub-scanning direction is -2.7°.

[0304] If the deflector 1 of the optical scanning device 30 moves 15 μm in the optical axis direction, then the principal ray moves 1.5 μm on the photosensitive drum 108, -1.5 μm on the photosensitive drum 408, 1.5 μm on the photosensitive drum 508, and -1.5 μm on the photosensitive drum 608.

[0305] The relative difference is 3 μm. At a resolution of 600 dpi, the influence of this difference on a 42.3-μm pitch is approximately 7%, and the image quality is not greatly affected.

[0306] In the present exemplary embodiment, |θ2| = |θ1| and θ2 / θ1 = -1. This satisfies |θ2|≥|θ1| and -2.5 < θ2 / θ1 < 2.5.

[0307] Therefore, the amount of position deviation caused by the optical scanning device 30 on the photosensitive drums 308, 408, 508, and 608 is reduced.

[0308] In the present exemplary embodiment, the optical device 30 includes an optical path including two reflecting elements 309 and 310 and an optical path including one reflecting element 409, where the signs of the oblique incident angles are opposite. With this configuration, the deviations on the photosensitive drums 308 and 408 caused by the position deviation of the deflector 1 occur in the same direction.

[0309] In the present exemplary embodiment, the optical device 30 includes an optical path including two reflecting elements 509 and 510 and an optical path including one reflecting element 609, where the signs of the oblique incident angles are opposite. With this configuration, the deviations on the photosensitive drums 508 and 608 caused by the position deviation of the deflector 1 occur in the same direction.

[0310] A configuration where θ2 / θ1 < 0 and the difference between the numbers of reflecting elements included in the optical path of the optical scanning device 30 is odd further reduces the amount of positional deviation.

[0311] Therefore, the amount of positional deviation caused by the optical scanning device 30 on the photosensitive drum is further reduced.

[0312] In the present exemplary embodiment, the relative difference is 0 μm, and the influence on image quality is further reduced.

[0313] As a modification of the present exemplary embodiment, in the case where θ1 is 2.7° and θ2 is -6.7°, θ2 / θ1 = -2.48 is obtained.

[0314] In this case, if the deflector 1 of the optical scanning device 30 is moved 15 μm in the optical axis direction, the principal ray moves 1.5 μm on the photosensitive drum 308, -3.72 μm on the photosensitive drum 408, 1.5 μm on the photosensitive drum 508, and -3.72 μm on the photosensitive drum 608.

[0315] The relative difference is 5.22 μm. At a resolution of 600 dpi, the influence of this difference on a 42.3 - μm pitch is approximately 12.3%.

[0316] In this modification, since θ2 / θ1 < 0, if the difference between the numbers of reflecting elements included in the optical scanning device 30 is odd, the amount of positional deviation is further reduced.

[0317] As another modification of the present exemplary embodiment, in the case where θ1 is 1.1° and θ2 is 2.7°, θ2 / θ1 = 2.45 is obtained.

[0318] In this case, if the deflector 1 of the optical scanning device 30 is moved 15 μm in the optical axis direction, the principal ray moves 0.6 μm on the photosensitive drum 308, 1.5 μm on the photosensitive drum 408, 0.6 μm on the photosensitive drum 508, and 1.5 μm on the photosensitive drum 608.

[0319] The relative difference is 0.9 μm. At a resolution of 600 dpi, the influence of this difference on a 42.3 - μm pitch is approximately 2.1%.

[0320] In this modification, since θ2 / θ1 > 0, if the difference between the numbers of reflecting elements included in the optical scanning device 30 is even, the amount of positional deviation is further reduced.

[0321] In this case, if the deflector 1 of the optical scanning device 30 moves 15 μm in the optical axis direction, the light beam L3 moves 1.5 μm from the light beam L2 on the photosensitive drum 308. The light beam L3 moves -3.72 μm from the light beam L2 on the photosensitive drum 408.

[0322] In view of miniaturization and reduction of image quality differences, the first fθ lenses 306 and 406 and the first fθ lenses 506 and 606 used in the present exemplary embodiment are desirably composed of integrally formed lenses.

[0323] Due to the increased degree of freedom in layout, in view of miniaturization, the fθ lenses 306 and 307 and the fθ lenses 506 and 507 and the fθ lenses 406 and 407 and the fθ lenses 606 and 607 are desirably composed of corresponding different lenses.

[0324] Even if the incident surfaces of the first fθ lenses 306, 406, 506, and 606 have different sagittal curvatures like the exit surfaces, effects similar to those of the present exemplary embodiment can be obtained.

[0325] With the foregoing configuration, the optical scanning device 30 according to the present exemplary embodiment provides a compact optical scanning device while reducing image quality differences.

[0326] Figure 17A and Figure 17B are optical path diagrams in the main scanning cross section of the main components of the optical scanning device 100 according to the fifth exemplary embodiment. Figure 17C and Figure 17D are sub-scanning cross-sectional views of the incident optical system and the imaging optical system included in the optical scanning device 100 according to the fifth exemplary embodiment.

[0327] The optical scanning device 100 according to the present exemplary embodiment includes a first light source 1a and a second light source 1b, a first anamorphic lens 2a and a second anamorphic lens 2b, and a first aperture stop 3a and a second aperture stop 3b.

[0328] The optical scanning device 100 according to the present exemplary embodiment further includes a deflector 4, a first fθ lens (first optical element) 5, second fθ lenses 6a and 6b, and reflection members 71a, 71b, and 72a.

[0329] A semiconductor laser is used as the first light source 1a and the second light source 1b.

[0330] The first anamorphic lens 2a and the second anamorphic lens 2b have different positive refractive powers (refractive forces) in the main scanning direction and the sub-scanning direction, respectively, so that the light beams emitted from the light sources 1a and 1b are converted into approximately parallel light beams in the main scanning direction and converge in the sub-scanning direction.

[0331] The first aperture stop 3a and the second aperture stop 3b limit the beam diameters of the light beams RA and RB emitted from the first light source 1a and the second light source 1b.

[0332] In this way, the light beams RA and RB emitted from the first light source 1a and the second light source 1b converge only in the sub-scanning direction near the deflection surface 41 of the deflector 4 and are formed into line images that are long in the main scanning direction.

[0333] The deflector 4 is rotated in the direction of arrow A in the figure by a driving unit (such as a motor) (not shown), thereby deflecting the light beams RA and RB incident on the deflector 4. The deflector 4 is composed of, for example, a polygon mirror.

[0334] The first fθ lens 5 and the second fθ lenses 6a and 6b are anamorphic imaging lenses having different focal powers in the main scanning section and the sub-scanning section. The first fθ lens 5 and the second fθ lenses 6a and 6b converge (guide) the light beams RA and RB deflected by the deflection surface 41 of the deflector 4 onto the first scanning surface 8a and the second scanning surface 8b.

[0335] The first fθ lens 5 is a multi-stage lens in which the first optical portion 5a and the second optical portion 5b are arranged in the sub-scanning direction. More specifically, the incident surface of the fθ lens 5 is composed of the incident surface of the first optical portion 5a and the incident surface of the second optical portion 5b. The exit surface of the fθ lens 5 is composed of the exit surface of the first optical portion 5a and the exit surface of the second optical portion 5b. The exit surfaces of the first optical portion 5a and the second optical portion 5b have correspondingly different lens surface shapes.

[0336] The reflection members 71a, 71b, and 72a are units for reflecting light beams. Evaporation mirrors are used as the reflection members 71a, 71b, and 72a.

[0337] In the optical scanning device 100 according to the present exemplary embodiment, the first incident optical system 75a is composed of the first anamorphic lens 2a and the first aperture stop 3a. The second incident optical system 75b is composed of the second anamorphic lens 2b and the second aperture stop 3b.

[0338] In the optical scanning device 100 according to the present exemplary embodiment, the first imaging optical system 85a is composed of the first optical portion 5a of the first fθ lens 5 and the second fθ lens 6a. The second imaging optical system 85b is composed of the second optical portion 5b of the first fθ lens 5 and the second fθ lens 6b.

[0339] In the optical scanning device 100 according to the present exemplary embodiment, the optical axes of the first incident optical system 75a and the second incident optical system 75b form angles of -3.0° and +3.0° with the main scanning section in the sub-scanning section, respectively.

[0340] The light beam RA emitted from the emission point of the first light source 1a passes through the first aperture stop 3a, and then is converted into a parallel beam in the main scanning direction and converged in the sub-scanning direction by the first anamorphic lens 2a.

[0341] Then, the obtained light beam RA is incident on the deflection surface 41 of the deflector 4 from above in the sub-scanning direction.

[0342] The light beam RA emitted from the first light source 1a and incident on the deflection surface 41 of the deflector 4 is deflected by the deflector 4, and then converged on the first scanned surface 8a by the first imaging optical system 85a, and scans the first scanned surface 8a at a constant speed.

[0343] The light beam RB emitted from the emission point of the second light source 1b passes through the second aperture stop 3b, and then is converted into a parallel beam in the main scanning direction and converged in the sub-scanning direction by the second anamorphic lens 2a.

[0344] Then, the obtained light beam RB is incident on the deflection surface 41 of the deflector 4 from below in the sub-scanning direction.

[0345] The light beam RB emitted from the second light source 1b and incident on the deflection surface 41 of the deflector 4 is deflected by the deflector 4, and then converged on the second scanned surface 8b by the second imaging optical system 85b, and scans the second scanned surface 8b at a constant speed.

[0346] Since the deflector 4 rotates in the direction of arrow A in the figure, the deflected light beams RA and RB scan the first scanned surface 8a and the second scanned surface 8b in the direction of arrow B in the figure, respectively.

[0347] The deflection point (axial deflection point) of the chief ray of the axial light beam on the deflection surface 41 of the deflector 4 is denoted by C0. In the sub-scanning direction, the light beams RA and RB emitted from the first light source 1a and the second light source 1b intersect at the deflection point C0. The deflection point C0 also serves as a reference point for the first imaging optical system 85a and the second imaging optical system 85b. The plane (reference plane) that intersects the deflection point C0 and is perpendicular to the rotation axis of the deflector 4 is denoted by P0. In the following description, the length of the optical path from the deflection point C0 to the scanned surface will be referred to as the optical path length.

[0348] In the present exemplary embodiment, the first photosensitive drum 8a and the second photosensitive drum 8b are used as the first scanned surface 8a and the second scanned surface 8b.

[0349] An exposure distribution of the first photosensitive drum 8a and the second photosensitive drum 8b in the sub-scanning direction is formed by rotating the first photosensitive drum 8a and the second photosensitive drum 8b in the sub-scanning direction during each main scan exposure.

[0350] Tables 17 and 18 below illustrate the characteristics of the first incident optical system 75a and the second incident optical system 75b, and the first imaging optical system 85a and the second imaging optical system 85b of the optical scanning device 100 according to the present exemplary embodiment.

[0351] [Table 17]

[0352]

[0353]

[0354]

[0355]

[0356]

[0357] Table 18

[0358]

[0359]

[0360]

[0361]

[0362]

[0363] Next, the effects of the optical scanning device 100 according to the present exemplary embodiment will be described. In the present exemplary embodiment, the imaging optical systems 85a and 85b have different optical path lengths. Compared with the case where the optical path lengths are the same, this improves the degree of freedom in the layout of optical components, and enables the light beams RA and RB to be converged (guided) on the photosensitive drums 8a and 8b while preventing interference between the optical components and the light beams. This results in a reduction in the size of the optical scanning device 100. To achieve this configuration, as shown in Tables 17 and 18, the first fθ lens 5 has different generatrix shapes and sagittal shapes for the imaging optical systems 85a and 85b, and thus has an asymmetrical shape in the sub-scanning direction with respect to the reference plane P0 (interface) as shown in Figure 18 and is asymmetrical in the sub-scanning direction with respect to the reference plane P0 (interface).

[0364] The shapes (generatrix shapes) of the incident surface and the exit surface of the first fθ lens 5 in the main scanning direction are desirably symmetric about the optical axis. This reduces the difference in optical performance between the imaging optical systems 85a and 85b having different optical path lengths. Here, at least a pair of generatrix shapes that are asymmetric in the sagittal shape and symmetric about the optical axis is sufficient. Another pair may appropriately have a generatrix shape that is asymmetric about the optical axis. By satisfying the following inequality (4), the difference in optical performance between the imaging optical systems 85a and 85b having different optical path lengths is reduced:

[0365] 0.8 < β1 / β2 < 1.2, (4)

[0366] where β1 and β2 are the sub-scanning magnifications of the imaging optical systems 85a and 85b, respectively. Figure 19 The figure shows the field curvature sensitivity of the imaging optical systems 85a and 85b when the first fθ lens 5 is offset by 10 μm in the Y direction. Deterioration of the image quality is prevented by reducing the difference in optical performance between the imaging optical systems 85a and 85b.

[0367] In the present exemplary embodiment, as shown in Tables 17 and 18, the incident surface and the exit surface of the first fθ lens 5 are symmetric in the main scanning direction. β1 = -2.05 and β2 = -2.46, and β1 / β2 is 0.82. This satisfies the inequality (4) and prevents deterioration of the image quality. The sub-scanning magnifications β1 and β2 more desirably satisfy the following inequality (4a):

[0368] 0.8 < β1 / β2 < 1.0. (4a)

[0369] With the foregoing configuration using the first fθ lens 5, the optical scanning device 100 according to the present exemplary embodiment thus achieves miniaturization in a manner compatible with preventing deterioration of the image quality by reducing the difference in optical performance between the imaging optical systems 85a and 85b.

[0370] Figure 20A and Figure 20B are the optical path diagrams in the main scanning cross-section of the optical scanning device 200 according to the sixth exemplary embodiment. Figure 20C is the sub-scanning cross-sectional view of the incident optical system included in the optical scanning device 200 according to the sixth exemplary embodiment. Figure 20D is the sub-scanning cross-sectional view of the imaging optical system included in the optical scanning device 200 according to the sixth exemplary embodiment.

[0371] The optical scanning device 200 according to this exemplary embodiment includes a first light source 1a, a second light source 1b, a third light source 1c, and a fourth light source 1d, a first anamorphic lens 2a, a second anamorphic lens 2b, a third anamorphic lens 2c, and a fourth anamorphic lens 2d, and a first aperture stop 3a, a second aperture stop 3b, a third aperture stop 3c, and a fourth aperture stop 3d

[0372] The optical scanning device 200 according to this exemplary embodiment further includes a deflector 4, first fθ lenses 5 and 5', second fθ lenses 6a, 6b, 6c, and 6d, and reflecting members 71a, 71b, 72b, 71c, 72c, and 71d.

[0373] Semiconductor lasers are used as the first light source 1a, the second light source 1b, the third light source 1c, and the fourth light source 1d.

[0374] The first anamorphic lens 2a, the second anamorphic lens 2b, the third anamorphic lens 2c, and the fourth anamorphic lens 2d have different positive refractive powers (refractive forces) in the main scanning direction and the sub-scanning direction, respectively, such that the light beams RA, RB, RC, and RD (the first light beam, the second light beam, the third light beam, and the fourth light beam) emitted from the first light source 1a to the fourth light source 1d are converted into approximately parallel light beams in the main scanning direction and converge in the sub-scanning direction. Here, the parallel light beams are not limited to strictly parallel light beams, but include approximately parallel light beams such as weakly divergent light beams and weakly convergent light beams.

[0375] The first aperture stop 3a, the second aperture stop 3b, the third aperture stop 3c, and the fourth aperture stop 3d limit the beam diameters of the light beams RA to RD passing through the first anamorphic lens 2a to the fourth anamorphic lens 2d.

[0376] Therefore, the light beams RA and RB emitted from the first light source 1a and the second light source 1b converge only in the sub-scanning direction near the first deflection surface 41 of the deflector 4 and are formed into a line image that is long in the main scanning direction.

[0377] The light beams RC and RD emitted from the third light source 1c and the fourth light source 1d converge only in the sub-scanning direction near the second deflection surface 42 of the deflector 4 and are formed into a line image that is long in the main scanning direction.

[0378] The deflector 4 is rotated in the direction of arrow A in the figure by a driving unit (such as a motor) (not shown), so as to deflect the light beams LA to RD incident on the deflector 4. The deflector 4 is composed of, for example, a polygon mirror.

[0379] The first fθ lens 5 and the second fθ lenses 6a and 6b are deformed imaging lenses having different focal powers in the main scanning section and the sub-scanning section. The first fθ lens 5 and the second fθ lenses 6a and 6b converge (guide) the light beams RA and RB deflected by the first deflection surface 41 of the deflector 4 onto the first scanned surface 8a and the second scanned surface 8b.

[0380] The first fθ lens 5' and the second fθ lenses 6c and 6d are deformed imaging lenses having different focal powers in the main scanning section and the sub-scanning section. The first fθ lens 5' and the second fθ lenses 6c and 6d converge (guide) the light beams RC and RD deflected by the second deflection surface 42 of the deflector 4 onto the third scanned surface 8c and the fourth scanned surface 8d.

[0381] The first fθ lens 5 is a multi-stage lens in which the first optical portion 5a and the second optical portion 5b are arranged in the sub-scanning direction. More specifically, the incident surface of the first fθ lens 5 is composed of the incident surface of the first optical portion 5a and the incident surface of the second optical portion 5b. The exit surface of the first fθ lens 5 is composed of the exit surface of the first optical portion 5a and the exit surface of the second optical portion 5b. The exit surfaces of the first optical portion 5a and the second optical portion 5b are shaped to have different sagittal tilt amounts, and each is a sagittal tilt change surface in which the sagittal tilt amount changes in the main scanning direction.

[0382] The first fθ lens 5' is a multi-stage lens in which the first optical portion 5c (third optical portion) and the second optical portion 5d (fourth optical portion) are arranged in the sub-scanning direction. More specifically, the incident surface of the first fθ lens 5' is composed of the incident surface of the third optical portion 5c and the incident surface of the fourth optical portion 5d. The exit surface of the first fθ lens 5' is composed of the exit surface of the third optical portion 5c and the exit surface of the fourth optical portion 5d. The exit surfaces of the third optical portion 5c and the fourth optical portion 5d are shaped to have different sagittal tilt amounts, and each is a sagittal tilt change surface in which the sagittal tilt amount changes in the main scanning direction.

[0383] The reflection members 71a, 71b, 72b, 71c, 72c, and 71d are units for reflecting light beams. Evaporation mirrors are used as the reflection members 71a, 71b, 72b, 71c, 72c, and 71d.

[0384] In the optical scanning device 200 according to the present exemplary embodiment, the first imaging optical system 85a is composed of the first optical portion 5a of the first fθ lens 5 and the second fθ lens 6a. The second imaging optical system 85b is composed of the second optical portion 5b of the first fθ lens 5 and the second fθ lens 6b.

[0385] The third imaging optical system 85c is composed of a third optical part 5c of the first fθ lens 5' and a second fθ lens 6c. The fourth imaging optical system 85d is composed of a fourth optical part 5d of the first fθ lens 5' and a second fθ lens 6d.

[0386] In the optical scanning device 200 according to the present exemplary embodiment, the optical axes of the first incident optical system 75a and the second incident optical system 75b form angles of +2.7° and -2.7° with the main scanning section in the sub-scanning section, respectively.

[0387] The optical axes of the third incident optical system 75c and the fourth incident optical system 75d form angles of -2.7° and +2.7° with the main scanning section in the sub-scanning section, respectively.

[0388] The first light beam RA and the second light beam RB emitted from the emission points of the first light source 1a and the second light source 1b are converted into parallel light beams in the main scanning direction and converged in the sub-scanning direction by the first anamorphic lens 2a and the second anamorphic lens 2b.

[0389] The obtained first light beam RA and second light beam RB pass through the first aperture stop 3a and the second aperture stop 3b, respectively, and are incident on the first deflection surface 41 of the deflector 4 from above and below in the sub-scanning direction.

[0390] The first light beam RA and the second light beam RB emitted from the first light source 1a and the second light source 1b and incident on the first deflection surface 41 of the deflector 4 are deflected by the deflector 4, and then converged on the first scanned surface 8a and the second scanned surface 8b by the first imaging optical system 85a and the second imaging optical system 85b, and scan the first scanned surface 8a and the second scanned surface 8b at a constant speed.

[0391] The third light beam RC and the fourth light beam RD emitted from the emission points of the third light source 1c and the fourth light source 1d are converted into parallel light beams in the main scanning direction and converged in the sub-scanning direction by the third anamorphic lens 2c and the fourth anamorphic lens 2d.

[0392] The obtained third light beam RC and fourth light beam RD pass through the third aperture stop 3c and the fourth aperture stop 3d, respectively, and are incident on the second deflection surface 42 of the deflector 4 from below and above in the sub-scanning direction.

[0393] The third light beam RC and the fourth light beam RD emitted from the third light source 1c and the fourth light source 1d and incident on the second deflection surface 42 of the deflector 4 are deflected by the deflector 4, and then converged on the third scanned surface 8c and the fourth scanned surface 8d by the third imaging optical system 85c and the fourth imaging optical system 85d, and scan the third scanned surface 8c and the fourth scanned surface 8d at a constant speed.

[0394] Since the deflector 4 rotates in the direction of arrow A in the figure, the deflected light beams RA and RB scan the first scanned surface 8a and the second scanned surface 8b in the direction of arrow B in the figure, respectively. The deflected light beams RC and RD scan the third scanned surface 8c and the fourth scanned surface 8d in the direction of arrow D in the figure, respectively.

[0395] The deflection point (axial deflection point) of the principal ray of the axial light beam on the first deflection surface 41 of the deflector 4 is denoted by C0. In the sub-scanning direction, the light beams RA and RB emitted from the first light source 1a and the second light source 1b intersect at the deflection point C0. The deflection point C0 serves as a reference point for the first imaging optical system 85a and the second imaging optical system 85b.

[0396] The deflection point (axial deflection point) of the principal ray of the axial light beam on the second deflection surface 42 of the deflector 4 is denoted by E0. In the sub-scanning direction, the light beams RC and RD emitted from the third light source 1c and the fourth light source 1d intersect at the deflection point E0. The deflection point E0 serves as a reference point for the third imaging optical system 85c and the fourth imaging optical system 85d.

[0397] The plane (reference plane) that intersects the deflection points C0 and E0 and is perpendicular to the rotation axis of the deflector 4 is denoted by P0. In the following description, the lengths of the optical paths from the deflection point C0 to the scanned surfaces 8a and 8b and the lengths of the optical paths from the deflection point E0 to the scanned surfaces 8c and 8d will be referred to as the optical path lengths of the imaging optical systems 85a, 85b, 85c, and 85d.

[0398] In the present exemplary embodiment, the first photosensitive drum 8a, the second photosensitive drum 8b, the third photosensitive drum 8c, and the fourth photosensitive drum 8d are used as the first scanned surface 8a, the second scanned surface 8b, the third scanned surface 8c, and the fourth scanned surface 8d.

[0399] By rotating the first photosensitive drum 8a to the fourth photosensitive drum 8d in the sub-scanning direction during each main scan exposure, an exposure distribution of the first photosensitive drum 8a to the fourth photosensitive drum 8d in the sub-scanning direction is formed.

[0400] Tables 19 and 20 below illustrate the characteristics of the first incident optical systems 75a to 75d and the first imaging optical systems 85a to 85d of the optical scanning device 200 according to the present exemplary embodiment.

[0401] [Table 19]

[0402]

[0403]

[0404]

[0405]

[0406]

[0407]

[0408]

[0409] [Table 20]

[0410]

[0411]

[0412]

[0413]

[0414]

[0415]

[0416]

[0417] The radius of curvature r' in the sub-scanning cross-section changes continuously with the y-coordinate of the lens surface.

[0418] Next, the effects of the optical scanning device 200 according to the present exemplary embodiment will be described. The description of the effects similar to those of the optical scanning device 100 according to the fifth exemplary embodiment will be omitted.

[0419] The optical scanning device 200 according to the present exemplary embodiment uses a single deflector 4 to scan four scanned surfaces 8a, 8b, 8c, and 8d.

[0420] The distance on the optical path from the deflection point C0 to the incident surface of the second fθ lens 6a is different from the distance on the optical path from the deflection point C0 to the incident surface of the second fθ lens 6b.

[0421] The distance on the optical path from the deflection point E0 to the incident surface of the second fθ lens 6c is different from the distance on the optical path from the deflection point E0 to the incident surface of the second fθ lens 6d.

[0422] Compared with the case where the optical path lengths are the same, this improves the degree of freedom in the layout of the optical components and enables the light beam to be converged (guided) onto the photosensitive drum while preventing interference between the optical components and the light beam. This results in a reduction in the size of the optical scanning device 200. To achieve such a configuration, as shown in Tables 19 and 20, the first fθ lenses 5 and 5' of the imaging optical systems 85a and 85b and the imaging optical systems 85c and 85d have different generatrix shapes and different sagittal shapes, and thus have an asymmetric shape in the sub-scanning direction with respect to the reference plane P0.

[0423] The same lenses are used as the first fθ lenses 5 and 5'. Figure 21 The left side part of Figure 21 is a main scanning cross-sectional view of the first fθ lenses 5 and 5' included in the optical scanning device 200 according to the present exemplary embodiment. Each of the first fθ lenses 5 and 5' has a gate portion at any outer end in the main scanning direction. As used herein, the gate portion refers to the portion (protrusion) corresponding to the resin inlet of the mold when the lens is injection-molded. The gate portion is formed at either end of each lens in the main scanning direction. As Figure 20A and Figure 20B shown, the first fθ lens 5 is deployed such that its gate portion is on the side of the first light source 1a and the second light source 1b. The first fθ lens 5' is deployed such that its gate portion is on the side away from the third light source 1c and the fourth light source 1d. In other words, the gate portions of the first fθ lens 5 (the first optical element) and the first fθ lens 5' (the fourth optical element) are on opposite sides of the optical axis in the main scanning cross-section. This helps to reduce the difference in optical performance between the optical systems.

[0424] Figure 21 The right side part of

[0424] is a sub-scanning cross-sectional view of the first fθ lenses 5 and 5' included in the optical scanning device 200 according to the present exemplary embodiment. The first fθ lenses 5 and 5' are positioned using the same lens holder 51. Even if there is an error in the dimension between the reference plane P0 and the lens holders 51 and 52 due to the manufacturing of the lenses, positioning the first fθ lenses 5 and 5' using the lens holder 51 makes the error amount in the Z direction the same between the first fθ lenses 5 and 5' in the imaging optical systems 85a and 85b and the imaging optical systems 85c and 85d. This results in a reduction in the difference in optical performance between the imaging optical systems 85a and 85b and the imaging optical systems 85c and 85d.

[0425] As described above, in the optical scanning device 200 according to the present exemplary embodiment, the use of the aforementioned first fθ lenses 5 and 5' reduces the difference in optical performance between the imaging optical systems 85a, 85b, 85c, and 85d, and provides an optical scanning device of an even smaller size.

[0426] [Image forming apparatus]

[0427] Figure 22 This is a sub-scanning cross-sectional view of the main components of a color image forming apparatus 90 equipped with an optical scanning device 100 according to any one of the exemplary embodiments.

[0428] The image forming apparatus 90 is a tandem type color image forming apparatus that uses the optical scanning device 100 to record image information on the surface of a photosensitive drum as an image bearing member.

[0429] The image forming apparatus 90 includes an optical scanning device 100, photosensitive drums (photosensitive members) 23, 24, 25, and 26 serving as image bearing members, and developing devices 15, 16, 17, and 18. The image forming apparatus 90 further includes a conveyor belt 91, a printer controller 93, and a fixing device 94.

[0430] Red (R), green (G), and blue (B) color signals (code data) output from an external device 92 such as a personal computer are input to the image forming apparatus 90.

[0431] The printer controller 93 in the image forming apparatus 90 converts the input color signals into corresponding image data (dot data) of C, M, Y, and K.

[0432] The converted image data is input to the optical scanning device 100. The optical scanning device 100 emits light beams 19, 20, 21, and 22 modulated based on the respective image data. The photosensitive surfaces of the photosensitive drums 23, 24, 25, and 26 are exposed by the light beams 19, 20, 21, and 22.

[0433] A charging roller (not shown) that uniformly charges the surfaces of the photosensitive drums 23, 24, 25, and 26 is deployed in contact with the surfaces. The optical scanning device 100 irradiates the surfaces of the photosensitive drums 23, 24, 25, and 26 charged by the charging roller with the light beams 19, 20, 21, and 22.

[0434] As described above, the light beams 19, 20, 21, and 22 are modulated based on the image data of each color. An electrostatic latent image is formed on the surfaces of the photosensitive drums 23, 24, 25, and 26 by the irradiation of the light beams 19, 20, 21, and 22. The formed electrostatic latent image is developed into a toner image by the developing devices 15, 16, 17, and 18 deployed in contact with the photosensitive drums 23, 24, 25, and 26.

[0435] The toner images developed by the developing devices 15 to 18 are transferred in a superimposed manner to an unshown sheet (material to be transferred) conveyed on the conveyor belt 91 by an unshown transfer roller (transfer device) opposed to the photosensitive drums 23 to 26, thereby forming a full-color image.

[0436] Therefore, the sheet with the unfixed toner image transferred thereon is further conveyed to the fixing device 94 behind the photosensitive drums 23, 24, 25, and 26 (on the left in Figure 22 ). The fixing device 95 consists of a fixing roller and a pressure roller. The fixing roller includes a fixing heater (not shown) inside, and the pressure roller is deployed to press against the fixing roller. The sheet conveyed from the transfer section is pressurized and heated at the nip portion between the fixing roller and the pressure roller, so that the unfixed toner image on the sheet is fixed. A discharge roller (not shown) is further deployed behind the fixing roller. The discharge roller discharges the fixed sheet to the outside of the image forming apparatus 90.

[0437] The color image forming apparatus 90 uses the optical scanning device 100 to record an image signal (image information) on the photosensitive surfaces of the photosensitive drums 23, 34, 25, and 26 corresponding to the C, M, Y, and K colors, and prints a color image at high speed.

[0438] For example, a color image reading device having a charge-coupled device (CCD) sensor can be used as the external device 92. In this case, the color image reading device and the color image forming apparatus 90 constitute a color digital copying machine.

[0439] Although the desired exemplary embodiments have been described above, the present invention is not limited to these exemplary embodiments, and various modifications and changes can be made within the scope of the gist of the present disclosure. The configurations of the above exemplary embodiments can be combined with each other. That is, the configuration adopted in one exemplary embodiment can be adopted in another exemplary embodiment, or can be adopted as needed even if not adopted.

Claims

1. An optical scanning device, comprising: a deflector including a first deflecting surface configured to deflect the first light beam and the second light beam to scan the first scanned surface and the second scanned surface, respectively, in a main scanning direction; as well as a first optical system and a second optical system configured to guide the first light beam and the second light beam deflected by the first deflecting surface to the first scanned surface and the second scanned surface, The first optical system and the second optical system include a common first optical element, and the common first optical element is disposed on a first optical path and a second optical path extending from the first deflecting surface to the first scanned surface and the second scanned surface, respectively. The first optical system includes a second optical element located between the first optical element and the first scanned surface on the first optical path, wherein the second optical system comprises a third optical element located between the first optical element and the second scanned surface on the second optical path, and The first optical element includes a first optical portion and a second optical portion into which the first light beam and the second light beam are incident.

2. The optical scanning device according to claim 1, wherein at least a pair of incident surfaces or a pair of exit surfaces of the first optical portion and the second optical portion are offset in the optical axis direction at their interfaces.

3. The optical scanning device according to claim 2, wherein the following inequality is satisfied: 0.01≤|Xmax|≤1.0, Wherein Xmax (mm) is the maximum value of the offset of the at least one pair of incident surfaces or the pair of exit surfaces of the first optical portion and the second optical portion at the interface in the optical axis direction.

4. The optical scanning device according to claim 1, wherein incident surfaces of the first optical portion and the second optical portion have the same shape.

5. The optical scanning device according to claim 1, wherein optical path lengths from the first deflecting surface to the second optical element and the third optical element are different. 6 . The optical scanning device according to claim 1 , wherein optical path lengths from the first deflecting surface to the first scanned surface and the second scanned surface are different.

7. The optical scanning device according to claim 1, wherein the following inequality is satisfied: |θ2|≥|θ1|, and -2.5<θ2 / θ1<2.5, wherein θ1 is the incident angle of the principal ray of the first light beam on the first deflecting surface in the sub-scan section and θ2 is the incident angle of the principal ray of the second light beam on the first deflecting surface in the sub-scan section.

8. The optical scanning device according to claim 1, wherein the at least one pair of incident surfaces or one pair of exit surfaces of the first optical portion and the second optical portion have different curvatures on the optical axis in a sub-scanning section.

9. The optical scanning device according to claim 1, wherein a curvature of the at least one pair of incident surfaces or one pair of exit surfaces of the first optical portion and the second optical portion in a sub-scanning section changes in a main-scanning direction. 10 . The optical scanning device according to claim 9 , wherein curvatures of the pair of exit surfaces of the first optical portion and the second optical portion in the sub-scanning section vary in the main scanning direction.

11. The optical scanning device according to claim 1, wherein a distance from the second optical element to the first scanned surface is different from a distance from the third optical element to the second scanned surface.

12. The optical scanning device according to claim 1, The following inequalities are satisfied: θ2 / θ1<0, and The difference between the number of reflective elements disposed between the first deflecting surface and the first scanned surface and the number of reflective elements disposed between the first deflecting surface and the second scanned surface is an odd number.

13. The optical scanning device according to claim 1, The following inequalities are satisfied: θ2 / θ1>0, and The difference between the number of reflective elements disposed between the first deflecting surface and the first scanned surface and the number of reflective elements disposed between the first deflecting surface and the second scanned surface is an even number.

14. The optical scanning device according to claim 1, wherein the at least one pair of incident surfaces or one pair of exit surfaces of the first optical portion and the second optical portion have mutually asymmetric shapes in a sub-scanning section. 15 . The optical scanning device according to claim 14 , wherein the pair of shapes having mutually asymmetric shapes in the sub-scan section have shapes symmetric about the optical axis in the main-scan section.

16. The optical scanning device according to claim 1, wherein the following inequality is satisfied: 0.8<β1 / β2<1.2， Wherein β1 and β2 are the imaging magnifications of the first optical system and the second optical system in the sub-scanning section, respectively.

17. The optical scanning device according to claim 1, wherein the first optical element includes an optical surface whose normal line of a generating line is not parallel to the optical axis in a sub-scanning section including the optical axis.

18. The optical scanning device according to claim 1, wherein the at least one pair of incident surfaces or one pair of exit surfaces of the first optical portion and the second optical portion have different shapes in a main scanning section.

19. The optical scanning device according to claim 1, wherein the deflector includes a second deflecting surface configured to deflect the third light beam and the fourth light beam to scan the third scanned surface and the fourth scanned surface, respectively, in the main scanning direction, wherein the optical scanning device further comprises a third optical system and a fourth optical system, wherein the third optical system and the fourth optical system are configured to guide the third light beam and the fourth light beam deflected by the second deflecting surface to the third scanned surface and the fourth scanned surface, wherein the third optical system and the fourth optical system include a common fourth optical element, and the common fourth optical element is disposed on a third optical path and a fourth optical path extending from the second deflecting surface to the third scanned surface and the fourth scanned surface, respectively; wherein the third optical system comprises a fifth optical element located between the fourth optical element and the third scanned surface on the third optical path, wherein the fourth optical system comprises a sixth optical element located between the fourth optical element and the fourth scanned surface on the fourth optical path, and The fourth optical element includes a third optical portion and a fourth optical portion into which the third light beam and the fourth light beam are incident.

20. The optical scanning device according to claim 19, wherein the first optical portion and the fourth optical portion have the same shape if one of the first optical portion and the fourth optical portion is rotated 180° relative to the other in a sub-scanning section.

21. The optical scanning device according to claim 19, wherein the first optical element and the fourth optical element each include a gate portion disposed at either end thereof in the main scanning direction, and The gate portions of the first optical element and the fourth optical element are located on opposite sides of the optical axis in the main scanning section.

22. An image forming device comprising: An optical scanning device according to any one of claims 1 to 21; as well as A developing device configured to develop the electrostatic latent image formed by the optical scanning device into a toner image.

23. An image forming apparatus, comprising: An optical scanning device according to any one of claims 1 to 21; as well as A printer controller is configured to convert code data output from an external device into an image signal and input the image signal to the optical scanning apparatus.

Images