Optical scanner and image formation device

Abstract

To provide a small optical scanner having good imaging performance.SOLUTION: An optical scanner comprises: a deflector including a first deflection surface for deflecting first and second light fluxes and scanning each of first and second scanned surfaces in a main scanning direction; and first and second optical systems guiding the first and second light fluxes deflected by the first deflection surface to the first and second scanned surfaces. The first and second optical systems include a common first optical element arranged on first and second optical paths spanned from the first deflection surface to each of the first and second scanned surfaces. The first optical system includes a second optical element arranged between the first optical element and the first scanned surface on the first optical path. The second optical system includes a third optical element arranged between the first optical element and the second scanned surface on the second optical path. The first optical element includes first and second optical parts to which the first and second light fluxes are made incident.SELECTED DRAWING: Figure 1A

Description

Technical Field

[0001] The present invention relates to an optical scanning device, and is particularly suitable for image forming devices such as laser beam printers (LBPs), digital copiers, and multifunction printers (MFPs).

Background Art

[0002] In recent years, miniaturization of optical scanning devices used in image forming devices has been demanded. However, when attempting to miniaturize an optical scanning device, the space inside the optical scanning device becomes narrow, making it difficult to arrange optical elements so as not to interfere with each other.

[0003] Patent Document 1 discloses an optical scanning device that employs a multi-stage lens in which a plurality of lens surfaces are stacked in the sub-scanning direction and integrated, and by making the shapes of the lens surfaces of the multi-stage lens different from each other, an increase in the degree of freedom of arrangement and a reduction in the number of optical components are achieved.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] An object of the present invention is to provide an optical scanning device that is small in size and has good imaging performance.

Means for Solving the Problems

[0006] As one aspect of the present invention for achieving the above object, an optical scanning device includes a deflector including a first deflection surface that deflects first and second light beams to scan respective first and second scanned surfaces in a main scanning direction, and first and second optical systems that guide the first and second light beams deflected by the first deflection surface to the first and second scanned surfaces. The first and second optical systems have a common first optical element disposed on first and second optical paths from the first deflection surface to the respective first and second scanned surfaces. The first optical system has a second optical element disposed between the first optical element on the first optical path and the first scanned surface. The second optical system has a third optical element disposed between the first optical element on the second optical path and the second scanned surface. The first optical element includes first and second optical portions into which the first and second light beams enter.

Advantages of the Invention

[0007] According to the present invention, it is possible to provide an optical scanning device that is small in size and has good imaging performance.

Brief Description of the Drawings

[0008]

Figure 1A

Figure 1B

Figure 1C

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Embodiments for Carrying Out the Invention

[0009] Hereinafter, the optical scanning device according to the present embodiment will be described in detail with reference to the accompanying drawings.

[0010] Note that the drawings shown below may be drawn at scales different from the actual ones in order to facilitate understanding of the present embodiment.

[0011] [First Embodiment] FIG. 1A is a sub-scanning cross-sectional view of a main part of the optical scanning device 100 according to the first embodiment. FIG. 1B is an optical path development diagram in the main scanning cross-section of the main part of the optical scanning device 100 according to the first embodiment. FIG. 1C is an optical path development diagram in the sub-scanning cross-section of the main part of the optical scanning device 100 according to the first embodiment.

[0012] In the following description, the main scanning direction (Y direction) is 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 (deflection scanning) by the rotating polygon mirror). The sub-scanning direction (Z direction) is the direction parallel to the rotation axis (or swing axis) of the deflector. Also, the main scanning cross-section is a cross-section that includes the optical axis and is perpendicular to the sub-scanning direction. The sub-scanning cross-section is a cross-section perpendicular to the main scanning direction.

[0013] The optical scanning device 100 of the present 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. Note that, instead of the mirrors, optical elements such as lenses or prisms having a reflecting surface may be used as reflecting elements. Also, instead of each lens, a prism or the like may be used as a refracting element.

[0014] In the optical scanning device 100 of the present embodiment, the imaging optical systems SA and SB and the imaging optical systems SC and SD are arranged with a single deflector 5 interposed therebetween, and 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 the corresponding scanned surfaces 8A (first scanned surface), 8B (second scanned surface), 8C (third scanned surface), and 8D (fourth scanned surface). A so-called sub-scanning oblique incidence optical system is used in which a single deflector 5 is shared by a plurality of light beams and the light beams are incident on the deflector obliquely from the sub-scanning direction. The advantage of the sub-scanning oblique incidence optical system is that it enables separation of the deflected and reflected light beams without increasing the size of the deflector surface in the sub-scanning direction.

[0015] In the imaging optical system SA, the light beam RA (first light beam) deflected by the common deflection surface 5A (first deflection surface) of the deflector (4-sided polygon mirror) 5, which is a deflection means, passes through the first optical part 6A, which is part of a multi-stage lens as the first optical element (first refractive element), and the lens 7A as the second optical element (second refractive element) in sequence, is then reflected back by the mirror M1 (first reflection element), and is guided to the surface 8A to be scanned. Also, in the imaging optical system SB, the light beam RB (second light beam) deflected and reflected by the deflection surface 5A of the deflector 5 passes through the second optical part 6B, which is part of a multi-stage lens, is then reflected back by the mirror M2 (second reflection element), and passes through the lens 7B as the third optical element (third refractive element). Then, it is reflected back by the mirror M3 (third reflection element) and reaches the surface 8B to be scanned. Here, C0 in the figure is the incident point (deflection point) of the chief ray of the light beam (axial light beam) reaching the axial image height on the surface to be scanned on the deflection surface when the chief ray is deflected, and will be referred to as the axial deflection point (or deflection point C0) hereinafter. P0 is a plane (reference plane) passing through the deflection point C0 and perpendicular to the rotation axis of the deflector 5. The light beams RA and RB incident on the deflection surface 5A intersect and are deflected at the deflection point C0 within the sub-scanning cross-section. Hereinafter, the length of the optical path from the deflection point C0 to each surface to be scanned is defined as the optical path length of each imaging optical system. Also, the optical paths from the deflection surface 5A to the surface 8A to be scanned and the surface 8B to be scanned are defined as the first optical path and the second optical path, respectively.

[0016] Also in the imaging optical system SD (SC), the same way of routing the optical path as that of the imaging optical system SA (SB) is performed. Specifically, in the imaging optical system SC, the light beam RC (the third light beam) deflected and reflected by the deflection surface 5'A (the second deflection surface) of the deflector 5 passes through the third optical unit 6C which is part of a multi-stage lens as the fourth optical element (the fourth refractive element), is then folded back by the mirror M'2 (the fourth reflecting element), and passes through the lens 7C as the fifth optical element (the fifth refractive element). Then, it is folded back by the mirror M'3 (the fifth reflecting element) and reaches the surface to be scanned 8C. Also, in the imaging optical system SD, the light beam RD (the fourth light beam) deflected by the deflection surface 5'A of the deflector 5 passes through the fourth optical unit 6D which is part of a multi-stage lens and the lens 7D as the sixth optical element (the sixth refractive element), is then folded back by the mirror M'1 (the sixth reflecting element), and is guided to the surface to be scanned 8D. The optical paths from the deflection surface 5'A to the surface to be scanned 8C and the surface to be scanned 8D respectively are defined as the third optical path and the fourth optical path.

[0017] The imaging optical systems SA and SB in this 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 unit) optically closest to the deflector is defined as the lens 6A (6B), and the lens optically closest to the surface to be scanned is defined as the lens 7A (7B). Note that in this specification, "optically" means "in the state when the optical path is unfolded".

[0018] The lenses (optical units) 6A and 6B of this embodiment are arranged in the sub-scanning direction and constitute a multi-stage lens (a common first optical element) in which the incident surface and the exit surface of each are integrally formed. By doing so, the lenses can be shared in the first and second optical paths corresponding to the light beams RA and RB, reducing the number of optical components and achieving miniaturization and cost reduction of the optical scanning device 100. And in this embodiment, optical elements different from the multi-stage lens are individually arranged in each optical path. In this way, by arranging separate optical elements for each optical path, it becomes easy to reduce the difference in imaging performance for each optical path caused by the shape difference of each optical surface of the multi-stage lens.

[0019] In the multi-stage lens of the present embodiment, at least one of the incident surfaces and the exit surfaces 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, and the upper shape and the lower shape with respect to the reference plane P0 are different in both the main scanning cross-section (generatrix shape) and the sub-scanning cross-section (fillet shape). Here, the generatrix shape refers to the lens surface shape in the main scanning cross-section including the optical axis. By making at least one of the lens surface shapes of the incident surfaces and the exit surfaces of the lenses 6A and 6B different from each other, while maintaining the optical performance of each imaging optical system well, the optical positions from the deflection point C0 to the respective lenses 7A and 7B can be made different from each other, and the lenses 7A and 7B can be arranged, increasing the degree of freedom in arrangement.

[0020] As a result, compared with the case where the optical positions of the lenses 7A and 7B from the optical deflection point C0 are arranged at the same position, by arranging the lens 7B at a position optically closer to the scanned surface than the lens 7A, interference between the lens 7B and the light beam RA can be avoided in a small space, achieving miniaturization of the optical scanning device 100.

[0021] In the multi-stage lens of the comparative example (conventional example) shown on the right side of FIG. 2, the optical axes (apexes of the surfaces) of the plurality of optical parts are arranged eccentrically in the sub-scanning direction so as to be different from each other. When the lens surface shapes are different for each lens of the multi-stage lens, the amount of step in the optical axis direction becomes large at the boundary part of the lens surfaces of the multi-stage lens.

[0022] Therefore, in the optical scanning device 100 of the present embodiment, as shown on the left side of FIG. 2, the optical axes of the lenses 6A and 6B are both at the same position, that is, the multi-stage lens is integrally formed by the lenses 6A and 6B in an arrangement where the optical axes of the lenses 6A and 6B are not eccentric with respect to each other in the sub-scanning direction. Thereby, even if the lens surfaces of the multi-stage lens have different shapes from each other, the amount of step at the lens surface boundary part is only the shape difference of the generatrix shape, and the amount of step is not affected by the difference in the fillet shape. Thereby, the amount of step at the boundary part of the multi-stage lens can be reduced, and the molding stability of the lenses 6A and 6B is improved well.

[0023] In the multi-stage lens of the optical scanning device 100 according to the present embodiment, when the maximum value (maximum step amount) of the step (shift in the optical axis direction) over the entire boundary portion of the lens surfaces with different lens surface shapes is Xmax (mm), it is desirable to satisfy the following conditional expression (1). 0.01 ≦ |Xmax| ≦ 1.0 (1)

[0024] In the optical scanning device disclosed in Patent Document 1, it is difficult to reduce the difference in imaging performance for each optical path caused by the difference in the lens surface shapes of the multi-stage lens. On the other hand, in the present embodiment, by satisfying the conditional expression (1), it is made easy to reduce the difference in the imaging performance. If the upper limit value of the conditional expression (1) is exceeded, the step amount at the boundary portion becomes large, and the lens surface deformation and the occurrence of distortion due to the thermal deformation stress in the vicinity of the step, which occur due to the step during molding, become large, affecting the effective area of the lens surface through which the light beam passes, and the wavefront aberration deteriorates. Also, if it is below the lower limit of the conditional expression (1), the amount by which the lens surface shapes can be made different from each other becomes small, and the range in which the lenses 7A and 7B can be freely arranged while maintaining the imaging performance in both the imaging optical systems SA and SB becomes small, making it difficult to achieve both miniaturization and imaging performance.

[0025] Furthermore, it is more preferable to satisfy the conditional expression (1a). 0.01 ≦ |Xmax| ≦ 0.5 (1a)

[0026] Furthermore, it is more preferable to satisfy the conditional expression (1b). 0.02 ≦ |Xmax| ≦ 0.2 (1b)

[0027] Also, in the imaging optical systems SC and SD in this embodiment, they have the same configuration and optical action as the imaging optical systems SA and SB. Lenses (optical units) 6C and 6D are arranged in the sub-scanning direction, and constitute a multi-stage lens (a common fourth optical element) in which the respective incident surfaces and exit surfaces are integrally formed. The incident surfaces of the optical units 6C and 6D are independent of each other, and each has a different surface vertex. Similarly, the exit surfaces of the optical units 6C and 6D are independent of each other, and each has a different surface vertex. This reduces the number of lens components. By making the lens surface shapes of lenses 6C and 6D different from each other and arranging the optical positions of lenses 7C and 7D from the deflection point C0 at different positions, while maintaining the optical performance of each imaging optical system well, interference between lens 7C and light beam RD is avoided in a small space, achieving miniaturization of the optical scanning device 100.

[0028] In the sub-scanning cross-section of the optical scanning device 100 of this embodiment, the sub-scanning oblique incident angles of the imaging optical system SA (SB) and the sub-scanning oblique incident angles of the imaging optical system SD (SC) have a 180-degree rotational symmetry relationship about an axis (main scanning axis) parallel to the main scanning direction passing through the intersection of the rotation axis of the deflector 5 and the optical axis of the imaging optical system. Also, the optical units 6A and 6D (the first and fourth optical units) have the same shape as each other when one is rotated 180 degrees (centered on the main scanning axis) in the sub-scanning cross-section. As a result, even if the sub-ray shape is asymmetric with respect to the optical axis, such as the sub-ray tilt shape used for achieving both correction of scanning line curvature and wavefront aberration twist in a conventionally proposed sub-scanning oblique incident optical system, the lens surface shapes of lenses 6A, 7A and lenses 6D, 7D can be made the same. Similarly, the lens surface shapes of lenses 6B, 7B and lenses 6C, 7C can be made the same. For this reason, a multi-stage lens formed by integrating lens 6A and lens 6B and a multi-stage lens formed by integrating lens 6D and lens 6C can be made common as optical components. Furthermore, lenses 7A and 7D, and lenses 7B and 7C can be made the same shape as each other as optical components. By doing so, the types of optical components can be reduced.

[0029] In the optical scanning device 100 according to this embodiment, the imaging optical systems SA and SD are optically equivalent to each other, and the imaging optical systems SB and SC are also optically equivalent to each other. By making them optically equivalent to each other in this way, color misregistration when the optical scanning device 100 is used in an image forming apparatus can be minimized. Also, since the fθ characteristics can be made the same, the image clock can be made common, and the cost of the circuit board can be reduced.

[0030] As described above, in the optical scanning device 100 according to this embodiment, good imaging performance, miniaturization, and reduction in the number of component types can be achieved simultaneously.

[0031] (Example 1) Hereinafter, the optical scanning device 100 according to an embodiment of the present invention will be described. Regarding the optical scanning device 100 according to this embodiment, description of configurations equivalent to those of the optical scanning device 100 according to the above-described embodiment will be omitted.

[0032] In the optical scanning device 100 according to this embodiment, the light beams RA and RB emitted from the light sources 1A and 1B respectively are obliquely incident on the deflection surface 5A 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 direction. Similarly, the light beams RC and RD emitted from the light sources 1C and 1D respectively are obliquely incident on the deflection surface 5A 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 direction.

[0033] Here, if the oblique incident angle is too large, it becomes difficult to correct the collapse of the spot due to the twist of the wavefront aberration. On the other hand, if the oblique incident angle is too small, it becomes difficult to separate the respective optical paths.

[0034] In the optical scanning device 100 according to the embodiment, semiconductor lasers are used as the light sources 1A, 1B, 1C, and 1D.

[0035] In the optical scanning device 100 according to the embodiment, the incident optical systems LA, LB, LC, and LD include anamorphic lenses 2A, 2B, 2C, and 2D, sub-scanning aperture stops 3A, 3B, 3C, and 3D, and main-scanning aperture stops 4A, 4B, 4C, and 4D.

[0036] The anamorphic lenses 2A, 2B, 2C, and 2D have an anamorphic surface as the exit surface, and are set so as to have a desired light beam in the main scanning direction and the sub-scanning direction by making the radius of curvature different in the main scanning direction and the sub-scanning direction. The anamorphic lenses 2A, 2B, 2C, and 2D each convert the light beams RA, RB, RC, and RD emitted from the light sources 1A, 1B, 1C, and 1D, respectively, in the main scanning cross-section into parallel light beams. Here, the parallel light beam includes not only a strictly parallel light beam but also a substantially parallel light beam such as a weakly divergent light beam or a weakly convergent light beam. Further, the anamorphic lenses 2A, 2B, 2C, and 2D each condense the light beams RA and RB emitted from the light sources 1A and 1B, respectively, in the sub-scanning cross-section in the vicinity of the deflection surface 5A of the deflector 5. Similarly, the anamorphic lenses 2C and 2D each condense the light beams RC and RD emitted from the light sources 1C and 1D, respectively, in the sub-scanning cross-section in the vicinity of the deflection surface 5'A of the deflector 5. Further, temperature compensation is performed by providing a diffraction surface on the incident surface side of the anamorphic lenses 2A and 2B.

[0037] The sub-scanning aperture stops 3A, 3B, 3C, and 3D each limit the light beam diameter in the sub-scanning direction of the light beams RA, RB, RC, and RD that have passed through the anamorphic lenses 2A, 2B, 2C, and 2D. Similarly, the main-scanning aperture stops 4A, 4B, 4C, and 4D each limit the light beam diameter in the main-scanning direction of the light beams RA, RB, RC, and RD that have passed through the anamorphic lenses 2A, 2B, 2C, and 2D. Each aperture diameter is set so as to have a desired spot diameter on the scanned surfaces 8A(Y), 8B(M), 8C(C), and 8D(K).

[0038] In addition, in the optical scanning device 100 according to the embodiment, in the main scanning cross-section, the angles α formed between the principal rays of the light beams RA and RA that pass through the incident optical systems LA and LB respectively and enter the deflection surface 5A, and the optical axes of the imaging optical systems SA and SB respectively, are designed to be 78°. Similarly, in the main scanning cross-section, the angles α formed between the principal rays of the light beams RC and RD that pass through the incident optical systems LC and LD respectively and enter the deflection surface 5'A, and the optical axes of the imaging optical systems SC and SD respectively, are designed to be 78°.

[0039] In the optical scanning device 100 according to the embodiment, the incident optical systems LA, LB, LC, and LD have the same configuration as each other, and the distances in the optical axis direction are also the same. In the optical scanning device 100 according to the embodiment, the anamorphic lenses 2A and 2B, and the anamorphic lenses 2C and 2D are formed of resin lenses integrally molded with each other, aiming to reduce costs by reducing the number of optical components. However, the effects of the embodiment are not limited to this configuration. Thus, in the optical scanning device 100 according to the embodiment, by sharing the arrangement of the optical components, the types of component holding parts and the types of assembly tools are reduced, improving productivity.

[0040] The deflector 5 is a polygon mirror having a four-sided configuration with an outer circumscribed circle radius of 10 mm. The deflector 5 scans the scanned surfaces 8A, 8B, 8C, and 8D by rotating at a constant speed by a motor. As a result, an optical scanning device capable of simultaneously scanning corresponding to four colors of yellow (Y), magenta (M), cyan (C), and black (K) when mounted on an image forming apparatus is achieved. The imaging optical systems SA, SB, SC, and SD perform keystone correction by optically conjugating between the deflection surfaces 5A and 5'A of the deflector 5 and the scanned surfaces 8A, 8B, 8C, and 8D in the sub-scanning cross-section. When using a deflector having a plurality of deflection surfaces such as a polygon mirror, since the tilt angles of the deflection surfaces in the sub-scanning direction are different for each deflection surface, it is common to employ a keystone correction optical system.

[0041] Next, the specifications, optical arrangements, and lens surface shapes of the optical scanning device 100 according to this embodiment are shown in Tables 1, 2, 3, and 4 below. Here, Table 1 shows the specifications and lens arrangements of the incident optical system LA and the imaging optical system SA, and Table 2 shows the lens surface shapes of the incident optical system LA and the imaging optical system SA. Further, Table 3 shows the specifications and lens arrangements of the incident optical system LB and the imaging optical system SB, and Table 4 shows the lens surface shapes of the incident optical system LB and the imaging optical system SB.

[0042] The lens arrangements of the incident optical system LC and the imaging optical system SC, and the incident optical system LD and the imaging optical system SD are shown in accordance with Tables 1 and 3. Note that the specifications and lens surface shapes of the incident optical system LC and the imaging optical system SC, and the incident optical system LD and the imaging optical system SD are omitted because they correspond to the incident optical system LB and the imaging optical system SB, and the incident optical system LA and the imaging optical system SA, respectively. In the columns of the optical arrangements in Tables 1 and 3, the coordinates of the reflection points of the light beams RA and RB at each mirror toward the image center (on-axis image height) in the main scanning direction on the surface to be scanned are shown.

[0043]

Table 1

[0044]

Table 2

[0045]

Table 3

[0046]

Table 4

[0047] The incident surfaces of the anamorphic lenses 2A, 2B, 2C, and 2D according to this example are rotationally asymmetric diffractive surfaces, and the phase function Φ of the diffraction grating is represented by the following equation.

[0048] [Number]

[0049] k is the diffraction order, and here k = 1 is assumed. Also, λ is the wavelength, and here λ = 790 nm is assumed.

[0050] The generatrix shapes (the shapes of the lens surfaces in the main scanning cross-section) of the lenses 6A, 6B, 6C, and 6D, and the lenses 7A, 7B, 7C, and 7D according to this embodiment are aspherical shapes that can be expressed as functions up to the 10th order as shown in the following formula. Here, taking the intersection of each lens surface (optical surface) and each 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 cross-section as the Y-axis, the generatrix shape X is expressed by the following formula. However, in this embodiment, the +X side of the X-axis is the light propagation direction, and the +Y side of the Y-axis is the light source side with respect to the optical axis.

[0051] [Number]

[0052] Here, taking the intersection of each lens surface and the optical axis of each optical part as the origin, the optical axis, the axis orthogonal to the optical axis in the main scanning cross-section, and the axis orthogonal to the optical axis in the sub-scanning cross-section are respectively the X-axis, the Y-axis, and the Z-axis. Also, R is the generatrix curvature radius, K is the eccentricity, and Bi (i = 1, 2,..., 10) are aspherical coefficients.

[0053] Also, the fillet shapes (the shapes of the lens surfaces in the sub-scanning cross-section at an arbitrary image height) of the lenses 6A, 6B, 6C, and 6D, and the lenses 7A, 7B, 7C, and 7D according to this embodiment are aspherical shapes as shown in the following formula.

[0054] [Number]

[0055] Here, S is a sub-line shape defined in a plane perpendicular to the main scanning cross-section and including the normal line of the bus bar at each position in the bus bar direction, and mi,j is an aspherical coefficient. The term consisting of a first-order function of Z is a term that gives the tilt amount in the sub-line direction. That is, the sub-line tilt amount in the present embodiment is m 0,1 corresponding to. Therefore, the sub-line tilt plane refers to a plane where m 0,1 is not zero. The sub-line tilt plane indicates an optical surface in the sub-scanning cross-section including the optical axis where the normal line on the bus bar is inclined with respect to the optical axis (non-parallel to the optical axis). Here, the bus bar refers to the intersection line of the optical surface and the main scanning cross-section. Since y = 0 on the optical axis, the sub-line tilt amount (the inclination of the normal line on the bus bar with respect to the optical axis) in the sub-scanning cross-section including the optical axis is m 0,1 represented by. Also, m 2,1 has an aspherical coefficient, and the sub-line tilt plane (sub-line tilt change plane) is such that the sub-line tilt amount of each optical surface changes according to the position y in the main scanning direction.

[0056] In addition, the sub-line curvature radius r' continuously changes as follows according to the Y coordinate of the lens surface.

[0057]

Equation

[0058] Here, r is the sub-line curvature radius on the optical axis, and Ei (i = 1, 2,..., 16) are sub-line change coefficients.

[0059] In the optical scanning device 100 according to the present embodiment, as can be seen from Table 2 and Table 4, the lens 6A (6D) and the optical unit 6B (6C) have different bus bar shapes, sub-line shapes, and sub-line tilt shapes for the exit surface. In this way, by making the aspherical coefficients different between the two optical units 6A (6D) and the optical unit 6B (6C) in the vertical direction of the sub-scanning direction of the multi-stage lens, even if the optical positions from the deflection point C0 are arranged at different positions between the optical unit 7A (7D) and the optical unit 7B (7C), they are configured in different optimal surface shapes so as to correct the optical characteristics of the imaging optical system SA (SD) and the imaging optical system SB (SC).

[0060] As described above, at least one pair of the incident surface and the exit surface of each of the first and second optical units is displaced (has a step) in the optical axis direction at the boundary portion. Here, the incident surface of the lens 6A (6D) and the optical unit 6B (6C) is optically closer to the deflection point C0 than the exit surface, and the interval between the light beam RA (RD) and the light beam RB (RC) is narrow in the sub-scanning cross section of the incident surface of the lens 6A (6D) and the optical unit 6B (6C). Therefore, if there is a step (displacement in the optical axis direction) at the boundary portion of the incident surface, the influence of lens surface deformation and the generation of distortion due to thermal deformation stress near the step, and the influence of separation due to the step at the boundary of the multi-stage lens when the light beam swings up and down are likely to occur. Therefore, in the optical scanning device 100 according to the present embodiment, as can be seen from Tables 2 and 4, the incident surfaces of the lens 6A (6D) and the optical unit 6B (6C) have the same shape, and there is no step at the boundary of the lens surface of the multi-stage lens.

[0061] Note that in the optical scanning device 100 according to the present embodiment, the functional formula of the surface shape of each optical unit is defined by the above-defined formula, but it is not limited thereto, and another defined formula may be used.

[0062] FIG. 3 shows the step at the boundary of the exit surface of the multi-stage lens of the optical scanning device 100 according to the present embodiment. In FIG. 3, when the position of the generatrix of the optical unit 6A in the optical axis direction is farther (closer) from the deflector 5 than the position of the generatrix of the lens 6B in the optical axis direction, it is defined as positive (negative).

[0063] As can be seen from FIG. 3, in the optical scanning device 100 according to the present embodiment, the maximum step amount at the boundary of the exit surface of the multi-stage lens is ±0.04 mm, and the conditional expressions (1), (1a), and (1b) are satisfied. In the present embodiment, by making the thicknesses of the lenses 6A and 6B the same, the step amount is reduced. The shape difference between the exit surface of the lens 6A and the exit surface of the lens 6B is suppressed to be sufficiently small to a level that causes no problem in the integral molding of the multi-stage lens, and various imaging performances are satisfied.

[0064] FIG. 4 is a graph showing the field curvature (defocus characteristics) in the main scanning direction and the sub-scanning direction by the optical scanning device 100 according to the present embodiment. FIG. 4(a) corresponds to the light beam RA, and FIG. 4(b) corresponds to the light beam RB. In the present embodiment, the effective width of the image (the width of the effective scanning region on the scanned surface) is W = ±163 mm. As shown in FIGS. 4(a) and 4(b), it can be seen that for both of the imaging optical systems SA and SB, the field curvature in the main scanning direction and the sub-scanning direction is well corrected for the image plane.

[0065] FIG. 5 is a graph showing the fθ characteristic dy of the optical scanning device 100 according to the embodiment. FIG. 5(a) corresponds to the light beam RA, and FIG. 5(b) corresponds to the light beam RB. Regarding the fθ characteristic dy, it shows the difference obtained by subtracting the ideal image height from the position where the light beam actually reaches. It can be seen that the fθ characteristics dy of the imaging optical systems SA and SB are well corrected.

[0066] FIGS. 6(a) and (b) show the main scanning direction position dependence of the scanning line curvature dz on the scanned surfaces 8A and 8B by the optical scanning device 100 according to the present embodiment. Here, the scanning line curvature dz means the difference between the imaging position in the sub-scanning direction at each image height and the imaging position in the sub-scanning direction at the on-axis image height on the scanned surface. As shown in FIGS. 6(a) and (b), it can be seen that for both of the imaging optical systems SA and SB, the scanning line curvature is well corrected for the image plane.

[0067] Note that as described above, in the optical scanning device 100 according to the present embodiment, the imaging optical systems SA and SD are optically equivalent to each other, and the imaging optical systems SB and SC are also optically equivalent to each other. Therefore, the description of the imaging optical systems SC and SD is omitted, but also in the imaging optical systems SC and SD, similarly, the imaging performance is well corrected.

[0068] As described above, in the present embodiment, the lenses 6A and 6B that are optically closest to the deflector are multi-stage lenses. On the lens surfaces of the multi-stage lenses, the surface shapes are made different from each other, and the optical positions of the lenses 7A and 7B are shifted from each other. By configuring in this way, the degree of freedom in arranging the lenses is increased, making it possible to miniaturize. Also, in the imaging optical systems SA and SD, and the imaging optical systems SB and SC, by making each lens have the same shape, it is possible to reduce the number of component types.

[0069] According to the light scanning device 100, while miniaturizing and reducing the number of component types of the optical elements, it is possible to reduce the step amount at the boundary of the multi-stage lens, stabilize the moldability, and achieve good imaging performance.

[0070] [Second Embodiment] FIG. 7A is a sub-scanning cross-sectional view of a main part of the light scanning device 200 according to the second embodiment. Also, FIG. 7B is an optical path development view in the main scanning cross-section of the main part of the light scanning device 200 according to the second embodiment. FIG. 7C is an optical path development view in the sub-scanning cross-section of the main part of the light scanning device 200 according to the second embodiment.

[0071] Note that the light scanning device 200 according to the present embodiment has the same configuration as the light scanning device 100 according to the first embodiment, except that lenses 27A to 27D are used instead of lenses 7A to 7D. Therefore, the same reference numerals are given to the same members for explanation.

[0072] The imaging optical systems SA and SB in the present embodiment are each composed of a plurality of lenses. In the imaging optical system SA (SB), the lens that is optically closest to the deflector is the lens 6A (6B), and the lens that is optically closest to the surface to be scanned is the lens 27A (27B).

[0073] The lenses 6A and 6B of the present embodiment are multi-stage lenses arranged in the sub-scanning direction and integrally form their respective incident surfaces and exit surfaces. In the multi-stage lenses of the present embodiment, at least one of the incident surfaces and the exit surfaces 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, and the upper shape and the lower shape with respect to the reference plane P0 are different in both the main scanning cross-section (generatrix shape) and the sub-scanning cross-section (filament shape). By making at least one of the lens surface shapes of the incident surfaces and the exit surfaces of the lenses 6A and 6B different from each other, while maintaining the optical performance of each imaging optical system well, the optical path lengths of the imaging optical systems SA and SB can be made different from each other, and the degree of freedom in arrangement can be increased.

[0074] In the optical scanning device 200 according to the present embodiment, the optical path length of the imaging optical system SA is shorter than the optical path length of the imaging optical system SB. Thereby, compared with the case where the optical path lengths of the imaging optical systems SA and SB are the same, miniaturization of the size of the optical scanning device 200 in the drum arrangement direction is achieved.

[0075] Similar to the first embodiment, in the optical scanning device 200 according to the present embodiment, the optical axes of the lenses 6A and 6B are both at the same position, that is, the multi-stage lens is integrally formed by the lenses 6A and 6B in an arrangement such that the optical axes of the lenses 6A and 6B are not eccentric with respect to each other in the sub-scanning direction. Thereby, even if the lens surfaces of the multi-stage lens have different shapes from each other, the amount of step at the lens surface boundary portion is only the shape difference of the generatrix shape, and the amount of step is not affected by the difference in the filament shape. Thereby, the amount of step at the boundary portion of the multi-stage lens can be reduced, and the molding stability of the lenses 6A and 6B is improved well.

[0076] Also, in the multi-stage lens of the optical scanning device 200 of the present embodiment, the conditional expression (1) is satisfied. It is more preferable to further satisfy the conditional expression (1a), and it is more preferable to further satisfy the conditional expression (1b).

[0077] Also, in the imaging optical systems SC and SD in the present embodiment, they have the same configuration and optical action as the imaging optical systems SA and SB. The lenses 6C and 6D are arranged in the sub-scanning direction and are multi-stage lenses in which their respective incident surfaces and exit surfaces are integrally formed, reducing the number of lens components. The lens surface shapes of the lenses 6C and 6D are made different from each other, and the optical path length of the imaging optical system SD is made shorter than the optical path length of the imaging optical system SC, achieving miniaturization of the size of the optical scanning device 200 in the drum arrangement direction.

[0078] Here, the incident angle of the principal ray of the light beam with respect to the deflection plane in the sub-scanning cross-section (the angle formed by the main scanning cross-section and the principal ray) is defined as the sub-scanning oblique incident angle. At this time, in the sub-scanning cross-section of the optical scanning device 200 of the present embodiment, the sub-scanning oblique incident angles corresponding to the imaging optical systems SA (SB) and the sub-scanning oblique incident angles corresponding to the imaging optical systems SD (SC) are rotationally symmetric by 180 degrees about an axis parallel to the main scanning direction passing through the intersection of the rotation axis of the deflector 5 and the optical axis of the imaging optical system. Thereby, even if the shape of the sub-ray is asymmetric with respect to the optical axis, such as the sub-ray tilt shape used for achieving both correction of scanning line curvature and torsional aberration in a conventionally proposed sub-scanning oblique incident optical system, the lens surface shapes of the lenses 6A, 27A and the lenses 6D, 27D can be made the same. Similarly, the lens surface shapes of the lenses 6B, 27B and the lenses 6C, 27C can be made the same. For this reason, the multi-stage lens in which the lenses 6A and 6B are integrated and the multi-stage lens in which the lenses 6D and 6C are integrated can be made common as optical components. Furthermore, the lenses 27A and 27D, and the lenses 27B and 27C can be made the same shape as optical components respectively. By doing so, the types of optical components can be reduced.

[0079] In the optical scanning device 200 according to the present embodiment, the imaging optical systems SA and SD are optically equivalent to each other, and the imaging optical systems SB and SC are also optically equivalent to each other. In this way, by making them optically equivalent to each other, color misregistration when the optical scanning device 200 is used in an image forming apparatus can be minimized. Also, since the fθ characteristics can be made the same, the image clock can be made common, and the cost of the circuit board can be suppressed.

[0080] As described above, in the optical scanning device 200 according to the present embodiment, it is possible to achieve both good imaging performance and miniaturization and reduction in the number of component types.

[0081] (Example 2) Hereinafter, the optical scanning device 200 according to an embodiment of the present invention will be described. Regarding the optical scanning device 200 according to the present embodiment, descriptions of configurations equivalent to those of the scanning device 100 according to the above-described first embodiment and the optical scanning device 200 according to the present embodiment will be omitted.

[0082] The specifications, optical arrangements, and lens surface shapes of the optical scanning device 200 according to the present embodiment are shown in Tables 5, 6, 7, and 8 below. Here, Table 5 shows the specifications and lens arrangements of the incident optical system LA and the imaging optical system SA, and Table 6 shows the lens surface shapes of the incident optical system LA and the imaging optical system SA. Further, Table 7 shows the specifications and lens arrangements of the incident optical system LB and the imaging optical system SB, and Table 8 shows the lens surface shapes of the incident optical system LB and the imaging optical system SB.

[0083] The lens arrangements of the incident optical system LC and the imaging optical system SC, and the incident optical system LD and the imaging optical system SD are shown in accordance with Tables 5 and 7. Note that the specifications and lens surface shapes of the incident optical system LC and the imaging optical system SC, and the incident optical system LD and the imaging optical system SD are omitted because they correspond to the incident optical system LB and the imaging optical system SB, and the incident optical system LA and the imaging optical system SA, respectively. In the columns of the optical arrangements in Tables 5 and 7, the coordinates of the reflection points of the light beams RA and RB at each mirror toward the image center (axial image height) in the main scanning direction on the surface to be scanned are shown.

[0084]

Table 5

[0085]

Table 6

[0086]

Table 7

[0087]

Table 8

[0088] In the optical scanning device 200 according to this embodiment, as can be seen from Table 6 and Table 8, in the optical units 6A (6D) and the optical units 6B (6C), the exit surfaces have different bus-bar shapes, sub-ray shapes, and sub-ray tilt shapes from each other. In this way, by making the aspherical coefficients different from each other between the two optical units 6A (6D) and the optical units 6B (6C) above and below in the sub-scanning direction of the multi-stage lens, even if the optical path length of the imaging optical system SA and the optical path length of the imaging optical system SB are different from each other, they are configured in different optimal surface shapes so as to correct the optical characteristics of the imaging optical system SA (SD) and the imaging optical system SB (SC).

[0089] Further, the entrance surfaces of the lenses 6A (6D) and the optical units 6B (6C) are optically closer to the deflection point C0 than the exit surfaces, and the interval between the light beam RA (RD) and the light beam RB (RC) is narrow in the sub-scanning cross section of the entrance surfaces of the lenses 6A (6D) and the optical units 6B (6C). Therefore, if the upper and lower lens surfaces in the sub-scanning direction have different shapes such that there is a step at the boundary of the entrance surface of the multi-stage lens, the influence of lens surface deformation and the generation of distortion due to thermal deformation stress near the step, and the influence of separation due to the step at the boundary of the multi-stage lens when the light beam sways up and down are likely to occur. Therefore, in the optical scanning device 200 according to this embodiment, as can be seen from Table 6 and Table 8, the entrance surfaces of the lenses 6A (6D) and the optical units 6B (6C) have the same shape, and there is no step at the boundary of the lens surfaces of the multi-stage lens.

[0090] Note that in the optical scanning device 200 according to this embodiment, the functional expressions of the surface shapes of the respective optical units are defined by the above-defined expressions, but it is not limited thereto, and other defined expressions may be used.

[0091] FIG. 8 shows the step at the boundary of the exit surface of the multi-stage lens of the optical scanning device 200 according to the present embodiment. In FIG. 8, the position of the optical axis direction of the generatrix of the optical unit 6A is defined as positive (negative) when it is farther (closer) from the deflector 5 than the position of the optical axis direction of the generatrix of the lens 6B.

[0092] As can be seen from FIG. 8, in the optical scanning device 200 according to the present embodiment, the maximum step amount at the boundary of the exit surface of the multi-stage lens is -0.194 mm at most, satisfying the conditional expressions (1), (1a) and (1b). The shape difference between the exit surface of the lens 6A and the exit surface of the lens 6B is suppressed to a sufficiently small level without problems in the integrally molding of the multi-stage lens, and various imaging performances are satisfied.

[0093] FIG. 9 is a graph showing the field curvature (defocus characteristics) in the main scanning direction and the sub-scanning direction by the optical scanning device 200 according to the present embodiment. FIG. 9(a) corresponds to the light beam RA, and FIG. 9(b) corresponds to the light beam RB. In the present embodiment, the effective width of the image (the width of the effective scanning region on the surface to be scanned) is W = ±163 mm. As shown in FIGS. 9(a) and 9(b), it can be seen that for both of the imaging optical systems SA and SB, the field curvature in the main scanning direction and the sub-scanning direction are both well corrected for the image surface.

[0094] FIG. 10 is a graph showing the fθ characteristic dy of the optical scanning device 200 according to the embodiment. FIG. 10(a) corresponds to the light beam RA, and FIG. 10(b) corresponds to the light beam RB. Regarding the fθ characteristic dy, it shows the difference obtained by subtracting the ideal image height from the position where the light beam actually reaches. It can be seen that the fθ characteristics dy of the imaging optical systems SA and SB are well corrected.

[0095] Figs. 11(a) and (b) show the main scanning direction position dependency of the scanning line curvature dz on the scanned surfaces 8A and 8B by the optical scanning device 200 according to this embodiment. Here, the scanning line curvature dz means the difference between the imaging position in the sub-scanning direction at each image height and the imaging position in the sub-scanning direction at the on-axis image height on the scanned surface. As shown in Figs. 11(a) and (b), it can be seen that for both of the imaging optical systems SA and SB, the scanning line curvature is well corrected for the image plane.

[0096] In addition, as described above, in the optical scanning device 200 according to this embodiment, the imaging optical systems SA and SD are optically equivalent to each other, and the imaging optical systems SB and SC are also optically equivalent to each other. Therefore, although the description of the imaging optical systems SC and SD is omitted, similarly, the imaging performance is well corrected in the imaging optical systems SC and SD.

[0097] As described above, in this embodiment, the lenses 6A and 6B that are optically closest to the deflector are multi-stage lenses, and on the lens surfaces of the multi-stage lenses, the surface shapes are made different from each other, and the optical path lengths of the imaging optical systems SA and SB are made different from each other. By configuring in this way, the degree of freedom in the lens arrangement is increased, and miniaturization is made possible. Also, by making each lens have the same shape in the imaging optical systems SA and SD, and the imaging optical systems SB and SC, the reduction of the number of component types is made possible.

[0098] According to such an optical scanning device 200, while miniaturizing and reducing the number of component types of the optical elements, the step amount at the boundary of the multi-stage lens can be made small, the moldability can be stabilized, and good imaging performance can be achieved.

[0099] [Third Embodiment] Fig. 12A shows an optical path development view in the main scanning cross section of the main part of the optical scanning devices 10 and 20 according to the third embodiment. Fig. 12B shows an optical path development view in the sub-scanning cross section of the main part of the optical scanning devices 10 and 20 according to the third embodiment.

[0100] The optical scanning devices 10 and 20 according to this embodiment include first and second light sources 101 and 201, first and second anamorphic collimator lenses 102 and 202, first and second sub-scanning diaphragms 103 and 203, and first and second main-scanning diaphragms 104 and 204.

[0101] Further, the optical scanning devices 10 and 20 according to this embodiment include a deflector 1, first fθ lenses 106 and 206 (first imaging elements), and second fθ lenses 107 and 207 (second and third imaging elements).

[0102] On the optical path, the first fθ lens 106 is disposed between the deflector 1 and the second fθ lens 107, and the first fθ lens 206 is disposed between the deflector 1 and the second fθ lens 207.

[0103] As the first and second light sources 101 and 201, semiconductor lasers or the like are used.

[0104] The first and second anamorphic collimator lenses 102 and 202 convert the light beams LC and LD (first and second light beams) emitted from the first and second light sources 101 and 201 into parallel light beams within the main scanning plane and converge them in the sub-scanning direction. Here, the parallel light beams include not only strict parallel light beams but also substantially parallel light beams such as weakly divergent light beams and weakly convergent light beams.

[0105] The first and second sub-scanning diaphragms 103 and 203 limit the beam diameters of the light beams LA and LB in the main scanning direction that have passed through the first and second anamorphic collimator lenses 102 and 202.

[0106] The first and second main-scanning diaphragms 104 and 204 limit the beam diameters of the light beams LA and LB in the main scanning direction that have passed through the first and second anamorphic collimator lenses 102 and 202.

[0107] In this way, the light beams LA and LB emitted from the first and second light sources 101 and 201 are respectively condensed only in the sub-scanning direction in the vicinity of the deflection plane of the deflector 1 and imaged as a line image long in the main scanning direction.

[0108] The deflector 1 deflects the incident light beams LA and LB by rotating in the direction of arrow A in the figure by driving means such as a motor (not shown). The deflector 1 is composed of, for example, a polygon mirror or the like.

[0109] The first fθ lens 106 and the second fθ lens 107 are anamorphic imaging lenses having different powers in the main scanning cross-section and the sub-scanning cross-section, and condense (guide) the light beam LA deflected by the deflection plane of the deflector 1 onto the first scanned surface 108.

[0110] The first fθ lens 206 and the second fθ lens 207 are anamorphic imaging lenses having different powers in the main scanning cross-section and the sub-scanning cross-section, and condense (guide) the light beam LB deflected by the deflection plane of the deflector 1 onto the second scanned surface 208.

[0111] In the optical scanning device 10 according to the present embodiment, the first incident optical system 45a is constituted by the first anamorphic collimator lens 102, the first sub-scanning aperture 103, and the first main-scanning aperture 104. In the optical scanning device 20, the second incident optical system 55a is constituted by the second anamorphic collimator lens 202, the second sub-scanning aperture 203, and the second main-scanning aperture 204.

[0112] Also, in the optical scanning device 10 according to the present embodiment, the first imaging optical system 45b is constituted by the first fθ lens 106 and the second fθ lens 107. In the optical scanning device 20, the second imaging optical system 55b is constituted by the first fθ lens 206 and the second fθ lens 207.

[0113] Note that the refractive power in the sub-scanning cross-section of the second fθ lenses 107 and 207 is stronger than that in the sub-scanning cross-section of the first fθ lenses 106 and 206, that is, the strongest among the first and second imaging optical systems 45b and 55b.

[0114] The light beam LA emitted from the light-emitting point of the first light source 101 is converted into a parallel light beam by the first anamorphic collimator lens 102.

[0115] Then, the converted light beam LA is condensed in the sub-scanning direction by the first anamorphic collimator lens 102, passes through the first sub-scanning aperture stop 103 and the first main-scanning aperture stop 104, and enters the deflector 1.

[0116] The light beam LA emitted from the first light source 101 and incident on the deflector 1 is deflected and scanned by the deflector 1, and then condensed on the first scanned surface 108 by the first imaging optical system 45b, and scans the first scanned surface 108 at a constant speed.

[0117] The light beam LB emitted from the light-emitting point of the second light source 201 is converted into a parallel light beam by the second anamorphic collimator lens 202.

[0118] Then, the converted light beam LB is condensed in the sub-scanning direction by the second anamorphic collimator lens 202, passes through the second sub-scanning aperture stop 203 and the second main-scanning aperture stop 204, and enters the deflection plane of the deflector 1.

[0119] The light beam LB emitted from the second light source 201 and incident on the deflection plane of the deflector 1 is deflected and scanned by the deflector 1, and then condensed on the second scanned surface 208 by the second imaging optical system 55b, and scans the second scanned surface 208 at a constant speed.

[0120] Note that since the deflector 1 is rotating in the direction of arrow A in the figure, the deflected and scanned light beams LA and LB scan the first and second scanned surfaces 108 and 208 in the direction of arrow B in the figure, respectively.

[0121] Further, C0 is the deflection point (axial deflection point) on the deflection surface of the deflector 1 with respect to the chief ray of the on-axis light beam. Further, the deflection point C0 serves as the reference point of the first and second imaging optical systems 45b and 55b.

[0122] In this embodiment, the first and second scanned surfaces 108 and 208 are the first and second photosensitive drums 108 and 208.

[0123] Also, the creation of the exposure distribution in the sub-scanning direction on the first and second photosensitive drums 108 and 208 is achieved by rotating the first and second photosensitive drums 108 and 208 in the sub-scanning direction for each main-scanning exposure.

[0124] Next, various characteristics of the first and second incident optical systems 45a and 55a and the first and second imaging optical systems 45b and 55b of the optical scanning devices 10 and 20 according to this embodiment are shown in Tables 9 to 11 below.

[0125]

Table 9

[0126]

Table 10

[0127]

Table 11

[0128] Next, the effects of the optical scanning devices 10 and 20 according to this embodiment will be described.

[0129] FIG. 13 shows the folded arrangement using the reflection elements of the optical scanning devices 10 and 20 according to this embodiment.

[0130] As shown in FIG. 13, the first imaging optical system 45b includes reflection mirrors 109 and 110, and the second imaging optical system 55b includes a reflection mirror 209.

[0131] As the reflecting mirrors 109, 110, and 209, reflecting elements having vapor-deposited films or the like are used.

[0132] In this embodiment, the light emitted from the first fθ lens 106 of the first imaging optical system is deflected and reflected by the reflecting mirror 109, deflected and reflected by the reflecting mirror 110 via the second fθ lens 107, and guided to the photosensitive drum 108. The light emitted from the second fθ lens 207 of the second imaging optical system is deflected and reflected by the reflecting mirror 209 and guided to the photosensitive drum 208.

[0133] When the distance between the photosensitive drums 108 and 208 is reduced to miniaturize the image forming apparatus, if the second fθ lens of the first imaging optical system and the second fθ lens of the second imaging optical system are arranged at optically equivalent distances with respect to the deflector 1, they interfere with the respective light beams LA and LB.

[0134] In this embodiment, in order to solve this problem, the second fθ lens 107 of the first imaging optical system is arranged closer to the deflector 1 than the second fθ lens 207 of the second imaging optical system.

[0135] By arranging them in this way, it is possible to miniaturize the image forming apparatus while avoiding interference between the fθ lens and the light beam.

[0136] However, since the positions of the second fθ lenses are different between the first imaging optical system and the second imaging optical system, in order to make the sub-scanning magnifications substantially coincide in each imaging optical system, it is necessary to arrange the first fθ lenses of the respective imaging optical systems with power distributions in different sub-scanning directions.

[0137] The characteristics of the first fθ lenses 106 and 206 and the second fθ lenses 107 and 207 according to this embodiment are shown in Table 12 below.

[0138]

Table 12

[0139] In this embodiment, the sub-ray curvature (curvature in the sub-scanning cross-section) on the optical axis (near the axis) of the exit surface of the first fθ lens 106 of the optical scanning device 10 is 55.261, and the sub-ray curvature near the axis of the exit surface of the first fθ lens 206 of the optical scanning device 20 is 25.004.

[0140] In this way, by making the sub-ray curvatures of the exit surfaces of the first fθ lenses 106 and 206 different, a compact configuration as described above is achieved.

[0141] FIG. 14 schematically shows the optical path when displacement of the deflector 1 according to this embodiment occurs.

[0142] The upper side of FIG. 14 is a schematic view showing when the deflector 1 is in an ideal position, and the lower side of FIG. 14 is a schematic view showing when the deflector 1 is in a position deviated from the ideal position.

[0143] In the figure, L1 indicates the light ray incident on the deflector 1, L2 indicates the light ray when the deflector 1 is in an ideal position, and L3 indicates the light ray when the deflector 1 is in a position deviated from the ideal position.

[0144] The deviation of the light irradiation position on the photosensitive drum with respect to the displacement due to the assembly error or the like of the deflector changes depending on the oblique incident angle with respect to the deflector.

[0145] Further, when the deflector 1 rotates in the direction of arrow A in FIG. 12A, the entrance and exit of the deflection plane occur in the sub-scanning direction. Therefore, in order to reduce the distribution of the deviation in the main scanning direction, by giving a change in sub-ray curvature, it is possible to further reduce the deviation of the irradiation position on the photosensitive drum.

[0146] In this embodiment, as shown in Tables 10 and 11, the exit surfaces of the first fθ lenses 106 and 206 are surfaces where the sub-ray curvature changes in the main scanning direction.

[0147] In the sub-scanning cross section, when the incident angle of the principal ray of the first light beam corresponding to the optical scanning device 10 with respect to the deflection plane of the deflector 1 is θ1, and the incident angle of the principal ray of the second light beam corresponding to the optical scanning device 20 with respect to the deflection plane of the deflector 1 is θ2, it is desirable to simultaneously satisfy the following conditional expressions (2) and (3). Thereby, the difference in the irradiation position shift generated on the photosensitive drums 108 and 208 in the optical scanning devices 10 and 20 can be reduced. |θ2|≧|θ1| (2) -2.5 < θ2 / θ1 < 2.5 (3)

[0148] Furthermore, it is preferable to satisfy the following conditional expressions (3a) and (3b) in order, and it is more preferable to set θ1 = θ2. -2.0 < θ2 / θ1 < 2.0 (3a) -1.5 < θ2 / θ1 < 1.5 (3b)

[0149] In this embodiment, the incident angle θ1 of the principal ray of the optical scanning device 10 in the sub-scanning direction is 2.7°, and the incident angle θ2 of the principal ray of the optical scanning device 20 in the sub-scanning direction is -2.7°.

[0150] When the deflector 1 of the optical scanning device 10 moves 15 μm in the optical axis direction, on the photosensitive drum 108, the light ray L3 moves 1.5 μm with respect to the light ray L2. When the deflector 1 of the optical scanning device 20 moves 15 μm in the optical axis direction, on the photosensitive drum 208, the light ray L3 moves -1.5 μm with respect to the light ray L2.

[0151] The relative difference is 3 μm. When the resolution is 600 dpi, for a 42.3 μm pitch, the influence is about 7%, and the influence on the image quality is minor.

[0152] In the case of this embodiment, |θ2| = |θ1|, θ2 / θ1 = -1, and |θ2|≧|θ1| and -2.5 < θ2 / θ1 < 2.5 are satisfied.

[0153] As a result, the amount of position shift on the photosensitive drum in the optical scanning devices 10 and 20 can be reduced.

[0154] Furthermore, in this embodiment, the signs of the oblique incident angles are different between the optical scanning device 10 and the optical scanning device 20, and there are an optical path including two reflecting elements 109 and 110 and an optical path including one reflecting element 209. With this configuration, due to the displacement of the deflector 1, the photosensitive drums 108 and 208 are displaced in the same direction.

[0155] θ2 / θ1 < 0, and by making the difference in the number of reflecting elements included in the optical paths of the optical scanning device 10 and the optical scanning device 20 an odd number, a configuration is achieved in which the amount of displacement can be further reduced.

[0156] As a result, the amount of displacement on the photosensitive drums in the optical scanning devices 10 and 20 can be further reduced.

[0157] In this case, the relative difference is 0 μm, and the influence on the image quality can be further reduced.

[0158] Also, as a modification of this embodiment, for example, if θ1 is 2.7° and θ2 is -6.7°, then θ2 / θ1 = -2.48.

[0159] In this case, when the deflector 1 of the optical scanning device 10 moves 15 μm in the optical axis direction, on the photosensitive drum 108, the light beam L3 moves 1.5 μm with respect to the light beam L2, and when the deflector 1 of the optical scanning device 20 moves 15 μm in the optical axis direction, on the photosensitive drum 208, the light beam L3 moves -3.72 μm with respect to the light beam L2.

[0160] The relative difference is 5.22 μm, and when the resolution is 600 dpi, it has an influence of about 12.3% on a 42.3 μm pitch.

[0161] In the case of this modification, since θ2 / θ1 < 0, the amount of displacement can be further reduced by making the difference in the number of reflecting elements included in the optical paths of the optical scanning device 10 and the optical scanning device 20 an odd number.

[0162] Also, as a modification of this embodiment, for example, if θ1 is 1.1° and θ2 is 2.7°, then θ2 / θ1 = 2.45.

[0163] In this case, when the deflector 1 of the optical scanning device 10 moves 15 μm in the optical axis direction, on the photosensitive drum 108, the light beam L3 moves 0.6 μm with respect to the light beam L2. When the deflector 1 of the optical scanning device 20 moves 15 μm in the optical axis direction, on the photosensitive drum 208, the light beam L3 moves 1.5 μm with respect to the light beam L2.

[0164] The relative difference is 0.9 μm. When the resolution is 600 dpi, for a 42.3 μm pitch, it has an influence of about 2.1%.

[0165] In the case of this modification, since θ2 / θ1 > 0, the number difference of the reflection elements included in the optical paths of the optical scanning device 10 and the optical scanning device 20 being even can reduce the amount of misalignment more.

[0166] In this case, when the deflector 1 of the optical scanning device 10 moves 15 μm in the optical axis direction, on the photosensitive drum 108, the light beam L3 moves 1.5 μm with respect to the light beam L2. When the deflector 1 of the optical scanning device 20 moves 15 μm in the optical axis direction, on the photosensitive drum 208, the light beam L3 moves -3.72 μm with respect to the light beam L2.

[0167] The first fθ lenses 106 and 206 used in this embodiment are preferably configured to be lenses integrally formed in terms of miniaturization and reduction of image quality differences.

[0168] Also, even when the incident surfaces of the first fθ lenses 106 and 206 have different sub-ray curvatures similar to the exit surfaces, the same effects as in this embodiment can be obtained.

[0169] Thus, in the optical scanning devices 10 and 20 according to this embodiment, by adopting the above-described configuration, it is possible to provide a compact optical scanning device while reducing the difference in image quality.

[0170] [Fourth Embodiment] FIG. 15A shows an optical path development view of the main scanning cross section of the main part of the optical scanning device 30 according to the fourth embodiment. FIG. 15B shows an optical path development view of the sub-scanning cross section of the main part of the imaging optical system included in the optical scanning device 30 according to the fourth embodiment.

[0171] The optical scanning device 30 according to this embodiment includes first, second, third, and fourth light sources 301, 401, 501, and 601, first, second, third, and fourth anamorphic collimator lenses 302, 402, 502, and 602, first, second, third, and fourth sub-scanning apertures 303, 403, 503, and 603, and first, second, third, and fourth main-scanning apertures 304, 404, 504, and 604.

[0172] In addition, the optical scanning device 30 according to this embodiment includes a deflector 1, first fθ lenses 306, 406, 506, and 606 (first imaging elements), and second fθ lenses 307 and 407 (second and third imaging elements), 507 and 607 (second and third imaging elements).

[0173] On the optical path, the first fθ lens 306 is disposed between the deflector 1 and the second fθ lens 307, the first fθ lens 406 is disposed between the deflector 1 and the second fθ lens 407, the first fθ lens 506 is disposed between the deflector 1 and the second fθ lens 507, and the first fθ lens 606 is disposed between the deflector 1 and the second fθ lens 607.

[0174] As the first, second, third, and fourth light sources 301, 401, 501, and 601, semiconductor lasers or the like are used.

[0175] The first, second, third, and fourth anamorphic collimator lenses 302, 402, 502, and 602 convert the light beams LC, LD, LE, and LF (first, second, third, and fourth light beams) emitted from the first, second, third, and fourth light sources 301, 401, 501, and 601 into parallel light beams within the main-scanning cross section and condense them in the sub-scanning direction. Here, the parallel light beams include not only strict parallel light beams but also substantially parallel light beams such as weakly divergent light beams and weakly convergent light beams.

[0176] The first, second, third, and fourth sub-scanning apertures 303, 403, 503, and 603 limit the beam diameters of the light beams LC, LD, LE, and LF in the sub-scanning direction that have passed through the first, second, third, and fourth anamorphic collimator lenses 302, 402, 502, and 602.

[0177] The first, second, third, and fourth main-scanning apertures 304, 404, 504, and 604 limit the beam diameters of the light beams LC, LD, LE, and LF in the main-scanning direction that have passed through the first, second, third, and fourth anamorphic collimator lenses 302, 402, 502, and 602.

[0178] In this way, the light beams LC, LD, LE, and LF emitted from the first, second, third, and fourth light sources 301, 401, 501, and 601 are each condensed only in the sub-scanning direction near the deflection plane of the deflector 1 and are imaged as a long line image in the main-scanning direction.

[0179] The deflector 1 rotates in the direction of arrow A in the figure by driving means such as a motor (not shown), and the deflector 1 deflects the incident light beams LC, LD, LE, and LF. The deflector 1 is composed of, for example, a polygon mirror or the like.

[0180] The first fθ lens 306 and the second fθ lens 307 are anamorphic imaging lenses having different powers in the main-scanning cross-section and the sub-scanning cross-section, and condense (guides) the light beam LC deflected by the deflection plane of the deflector 1 onto the first scanned surface 308.

[0181] The first fθ lens 406 and the second fθ lens 407 are anamorphic imaging lenses having different powers in the main-scanning cross-section and the sub-scanning cross-section, and condense (guides) the light beam LD deflected by the deflection plane of the deflector 1 onto the second scanned surface 408.

[0182] The first fθ lens 506 and the second fθ lens 507 are anamorphic imaging lenses having different powers in the main-scanning cross-section and the sub-scanning cross-section, and condense (guides) the light beam LE deflected by the deflection plane of the deflector 1 onto the first scanned surface 508.

[0183] The first fθ lens 606 and the second fθ lens 607 are anamorphic imaging lenses having different powers in the main scanning section and the sub-scanning section, and condense (guide) the light beam LF deflected by the deflection surface of the deflector 1 onto the second surface to be scanned 608.

[0184] In the optical scanning device 30 according to the present embodiment, a first incident optical system 65a is configured by the first anamorphic collimator lens 302, the first sub-scanning aperture 303, and the first main scanning aperture 304. Then, a second incident optical system 75a is configured by the second anamorphic collimator lens 402, the second sub-scanning aperture 403, and the second main scanning aperture 404. Then, a third incident optical system 85a is configured by the third anamorphic collimator lens 502, the third sub-scanning aperture 503, and the third main scanning aperture 504. Then, a fourth incident optical system 95a is configured by the fourth anamorphic collimator lens 602, the fourth sub-scanning aperture 603, and the fourth main scanning aperture 604.

[0185] Also, in the optical scanning device 30 according to the present embodiment, a first imaging optical system is configured by the first fθ lens 306 and the second fθ lens 307. Then, a second imaging optical system is configured by the first fθ lens 406 and the second fθ lens 407. Then, a third imaging optical system is configured by the first fθ lens 506 and the second fθ lens 507. Then, a fourth imaging optical system is configured by the first fθ lens 606 and the second fθ lens 607.

[0186] Note that the refractive power in the sub-scanning section of the second fθ lenses 307, 407, 507, and 607 is stronger than the refractive power in the sub-scanning section of the first fθ lenses 306, 406, 506, and 606, that is, the strongest among the first, second, third, and fourth imaging optical systems.

[0187] The light beam LC (95c) emitted from the light emitting point of the first light source 301 is converted into a parallel light beam in the main scanning section by the first anamorphic collimator lens 302 and condensed in the sub-scanning direction.

[0188] Then, the converted light beam LC passes through the first sub-scanning aperture stop 303, passes through the first main-scanning aperture stop 304, and is incident on the deflection plane 305 of the deflector 1.

[0189] The light beam LC emitted from the first light source 301 and incident on the deflection plane 305 of the deflector 1 is deflected and scanned by the deflector 1, and then condensed on the first scanned surface 308 by the first imaging optical system, and scans the first scanned surface 308 at a constant speed.

[0190] The light beam LD (95d) emitted from the light-emitting point of the second light source 401 is converted into a parallel light beam in the main-scanning cross section by the second anamorphic collimator lens 402 and condensed in the sub-scanning direction.

[0191] Then, the converted light beam LD passes through the second sub-scanning aperture stop 403, passes through the second main-scanning aperture stop 404, and is incident on the deflection plane of the deflector 1.

[0192] The light beam LD emitted from the second light source 401 and incident on the deflection plane of the deflector 1 is deflected and scanned by the deflector 1, and then condensed on the second scanned surface 408 by the second imaging optical system, and scans the second scanned surface 408 at a constant speed.

[0193] The light beam LE (95e) emitted from the light-emitting point of the third light source 501 is converted into a parallel light beam in the main-scanning cross section by the third anamorphic collimator lens 502 and condensed in the sub-scanning direction.

[0194] Then, the converted light beam LE passes through the third sub-scanning aperture stop 503, passes through the third main-scanning aperture stop 504, and is incident on the deflection plane of the deflector 1.

[0195] The light beam LE emitted from the third light source 501 and incident on the deflection plane of the deflector 1 is deflected and scanned by the deflector 1, and then condensed on the third scanned surface 508 by the third imaging optical system, and scans the third scanned surface 508 at a constant speed.

[0196] The light beam LF(95f) emitted from the light-emitting point of the fourth light source 601 is converted into a parallel light beam within the main scanning plane by the fourth anamorphic collimator lens 602 and is condensed in the sub-scanning direction.

[0197] Then, the converted light beam LF passes through the fourth sub-scanning aperture stop 603, passes through the fourth main-scanning aperture stop 604, and enters the deflection plane of the deflector 1.

[0198] The light beam LF emitted from the fourth light source 601 and incident on the deflection plane of the deflector 1 is deflected and scanned by the deflector 1, and then is condensed onto the fourth scanned surface 608 by the fourth imaging optical system, and scans the fourth scanned surface 608 at a constant speed.

[0199] Since the deflector 1 is rotating in the direction of arrow A in the figure, the deflected and scanned light beams LC, LD, LE, and LF scan the first, second, third, and fourth scanned surfaces 308, 408, 508, and 608 in the direction of arrow B in the figure, respectively.

[0200] Also, D0 and E0 are the deflection points (on-axis deflection points) on the deflection plane of the deflector 1 with respect to the chief ray of the on-axis light beam. Also, the deflection points D0 and E0 serve as the reference points of the first, second, third, and fourth imaging optical systems.

[0201] In this embodiment, the first, second, third, and fourth scanned surfaces 308, 408, 508, and 608 use the first, second, third, and fourth photosensitive drums 308, 408, 508, and 608.

[0202] Also, the creation of the exposure distribution in the sub-scanning direction on the first, second, third, and fourth photosensitive drums 308, 408, 508, and 608 is achieved by rotating the first, second, third, and fourth photosensitive drums 308, 408, 508, and 608 in the sub-scanning direction for each main-scanning exposure.

[0203] Next, the characteristics of the first, second, third, and fourth incident optical systems 65a, 75a, 85a, and 95a and the first, second, third, and fourth imaging optical systems of the optical scanning device 30 according to this embodiment are shown in Tables 13 to 15 below.

[0204] [Table 13]

[0205] [Table 14]

[0206] [Table 15]

[0207] Also, the radius of curvature r' in the sub-scanning cross-section changes continuously according to the y-coordinate of the lens surface.

[0208] Next, the effects of the optical scanning device 30 according to this embodiment will be described.

[0209] FIG. 16 shows the folding arrangement using the reflection elements of the optical scanning device 30 according to this embodiment.

[0210] As shown in FIG. 16, the first imaging optical system includes reflection mirrors 309 and 310, the second imaging optical system includes reflection mirror 409, the third imaging optical system includes reflection mirrors 509 and 510, and the second imaging optical system includes reflection mirror 609.

[0211] For the reflection mirrors 309, 310, 409, 509, 510, and 609, reflection elements having a vapor deposition film or the like are used.

[0212] In this embodiment, the light emitted from the first fθ lens 306 of the first imaging optical system is deflected and reflected by the reflection mirror 309, deflected and reflected by the reflection mirror 310 through the second fθ lens 307, and guided to the photosensitive drum 308. The light emitted from the second fθ lens 407 of the second imaging optical system is deflected and reflected by the reflection mirror 409 and guided to the photosensitive drum 408. The light emitted from the first fθ lens 506 of the third imaging optical system is deflected and reflected by the reflection mirror 509, passed through the second fθ lens 507, deflected and reflected by the reflection mirror 510, and guided to the photosensitive drum 508. The light emitted from the second fθ lens 607 of the fourth imaging optical system is deflected and reflected by the reflection mirror 609 and guided to the photosensitive drum 608.

[0213] When the distance between the photosensitive drums 308 and 408 is reduced to miniaturize the image forming apparatus, arranging the second fθ lens of the first imaging optical system and the second fθ lens of the second imaging optical system at optically equivalent distances with respect to the deflector 1 causes interference with the respective light beams LC and LD.

[0214] Also, when the distance between the photosensitive drums 508 and 608 is reduced, arranging the second fθ lens of the third imaging optical system and the second fθ lens of the fourth imaging optical system at optically equivalent distances with respect to the deflector 1 causes interference with the respective light beams LE and LF.

[0215] In this embodiment, to solve this problem, the second fθ lens 307 of the first imaging optical system is arranged closer to the deflector 1 than the second fθ lens 407 of the second imaging optical system, and the second fθ lens 507 of the third imaging optical system is arranged closer to the deflector 1 than the second fθ lens 607 of the fourth imaging optical system.

[0216] By arranging in this way, it becomes possible to avoid interference between the fθ lens and the light beam while miniaturizing the image forming apparatus.

[0217] However, since the positions of the second fθ lenses are different between the first imaging optical system and the second imaging optical system, in order to make the sub-scanning magnification (imaging magnification in the sub-scanning cross-section) substantially coincide in each imaging optical system, it is necessary to arrange the first fθ lenses of each imaging optical system with different power distributions in different sub-scanning directions.

[0218] The characteristics of the first fθ lenses 306, 406, 506, and 606 and the second fθ lenses 307, 407, 507, and 607 according to this embodiment are shown in Table 16 below.

[0219]

Table 16

[0220] In this embodiment, the child ray curvature in the vicinity of the optical axis on the exit surface of the first fθ lenses 306 and 506 of the optical scanning device 30 is 54.586, and the child ray curvature in the vicinity of the optical axis on the exit surface of the first fθ lenses 406 and 606 is 20.586.

[0221] In this way, by making the child ray curvatures of the exit surfaces of the first fθ lenses 306 and 506 and 406 and 606 different, a small-sized configuration as described above is achieved.

[0222] In this embodiment, as shown in Tables 14 and 15, the exit surfaces of the first fθ lenses 306, 406, 506, and 606 are surfaces where the child ray curvature changes in the main scanning direction.

[0223] When the incident angle of the light beam of the optical scanning device 30 with respect to the deflector 1 in the sub-scanning direction is θ1 and the incident angle of the light beam in the sub-scanning direction is θ2, by setting |θ2| ≥ |θ1| and -2.5 < θ2 / θ1 < 2.5, the difference in the irradiation position shift generated on the photosensitive drums 308 and 408 in the optical scanning device 30 can be reduced.

[0224] In this embodiment, the incident angle θ1 of the principal ray of the first light beam RC in the sub-scanning direction is 2.7°, and the incident angle θ2 of the principal ray of the second light beam RD in the sub-scanning direction is -2.7°.

[0225] When the deflector 1 of the optical scanning device 30 moves 15 μm in the optical axis direction, the principal ray moves 1.5 μm on the photosensitive drum 308, -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.

[0226] The relative difference is 3 μm. When the resolution is 600 dpi, it has an impact of about 7% on a 42.3 μm pitch, and the impact on the image quality is minor.

[0227] In the case of this embodiment, |θ2| = |θ1|, θ2 / θ1 = -1, and |θ2| ≥ |θ1| and -2.5 < θ2 / θ1 < 2.5 are satisfied.

[0228] As a result, the amount of displacement on the photosensitive drum in the optical scanning device 30 can be reduced.

[0229] Furthermore, in this embodiment, in the optical scanning device 30, although the signs of the oblique incident angles are different, due to the two reflecting elements 309 and 310 included in the optical path of the optical scanning device 30 and the one reflecting element 409 included in the optical path, the photosensitive drums 308 and 408 are displaced in the same direction due to the displacement of the deflector 1.

[0230] Also, in the optical scanning device 30, although the signs of the oblique incident angles are different, due to the two reflecting elements 509 and 510 included in the optical path of the optical scanning device 30 and the one reflecting element 609 included in the optical path, the photosensitive drums 508 and 608 are displaced in the same direction due to the displacement of the deflector 1.

[0231] θ2 / θ1 < 0, and by making the difference in the number of reflecting elements included in the optical path of the optical scanning device 30 an odd number, a configuration is achieved in which the amount of displacement can be further reduced.

[0232] As a result, the amount of displacement on the photosensitive drum in the optical scanning device 30 can be further reduced.

[0233] In this case, the relative difference is 0 μm, and the impact on the image quality can be further reduced.

[0234] Also, as a modification example of this embodiment, for example, when θ1 is 2.7° and θ2 is -6.7°, θ2 / θ1 = -2.48.

[0235] In this case, when the deflector 1 of the optical scanning device 30 moves 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.

[0236] The relative difference is 5.22 μm. When the resolution is 600 dpi, it has an influence of about 12.3% on a 42.3 μm pitch.

[0237] In the case of this modification example, since θ2 / θ1 < 0, the amount of positional deviation can be reduced more when the difference in the number of reflecting elements included in the optical path of the optical scanning device 30 is odd.

[0238] Also, as a modification example of this embodiment, for example, when θ1 is 1.1° and θ2 is 2.7°, θ2 / θ1 = 2.45.

[0239] In this case, when the deflector 1 of the optical scanning device 30 moves 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.

[0240] The relative difference is 0.9 μm. When the resolution is 600 dpi, it has an influence of about 2.1% on a 42.3 μm pitch.

[0241] In the case of this modification example, since θ2 / θ1 > 0, the amount of positional deviation can be reduced more when the difference in the number of reflecting elements included in the optical path of the optical scanning device 30 is even.

[0242] In this case, when the deflector 1 of the optical scanning device 30 moves 15 μm in the optical axis direction, on the photosensitive drum 308, the light beam L3 moves 1.5 μm with respect to the light beam L2, and on the photosensitive drum 408, the light beam L3 moves -3.72 μm with respect to the light beam L2.

[0243] The first fθ lenses 306, 406, 506, and 606 used in this embodiment are more preferably configured to be formed of integrally molded lenses in terms of miniaturization and reduction of image quality differences.

[0244] Also, the fθ lenses 306 and 307, 506 and 507, 406 and 407, and 606 and 607 are more preferably configured with different lenses respectively because the degree of freedom in arrangement increases, which is more favorable for miniaturization.

[0245] Moreover, even when the incident surfaces of the first fθ lenses 306, 406, 506, and 606 have different sub-ray curvatures similar to the exit surfaces, the same effects as in this embodiment can be obtained.

[0246] Thus, in the optical scanning device 30 according to this embodiment, by adopting the configuration shown above, it is possible to provide a compact optical scanning device while reducing the difference in image quality.

[0247] [Fifth Embodiment] FIGS. 17A and 17B show an optical path development diagram in the main scanning cross-section of the main part of the optical scanning device 100 according to the fifth embodiment. FIGS. 17C and 17D respectively show 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 embodiment.

[0248] The optical scanning device 100 according to this embodiment includes first and second light sources 1a and 1b, first and second anamorphic lenses 2a and 2b, and first and second aperture stops 3a and 3b.

[0249] Also, the optical scanning device 1 according to this embodiment includes a deflector 4, a first fθ lens (first optical element) 5, second fθ lenses 6a and 6b, and reflecting members 71a, 71b, and 72a.

[0250] As the first and second light sources 1a and 1b, semiconductor lasers or the like are used.

[0251] The first and second anamorphic lenses 2a and 2b convert the light beams emitted from the light sources 1a and 1b into substantially parallel light beams in the main scanning direction and condense them in the sub-scanning direction, and have different positive powers (refractive powers) independently in the main scanning direction and the sub-scanning direction, respectively.

[0252] The first and second aperture stops 3a and 3b limit the beam diameters of the light beams LA and LB emitted from the first and second light sources 1a and 1b.

[0253] In this way, the light beams LA and LB emitted from the first and second light sources 1a and 1b are condensed only in the sub-scanning direction in the vicinity of the deflection plane 41 of the deflector 4 and are imaged as a long line image in the main scanning direction.

[0254] The deflector 4 deflects the light beams LA and LB incident on the deflector 4 by rotating in the direction of arrow A in the figure by a driving means such as a motor (not shown). Note that the deflector 4 is composed of, for example, a polygon mirror or the like.

[0255] The first fθ lens 5, the second fθ lenses 6a and 6b are anamorphic imaging lenses having different powers in the main scanning cross-section and the sub-scanning cross-section. Then, the first fθ lens 5, the second fθ lenses 6a and 6b condense (guide) the light beams LA and LB deflected by the deflection plane 41 of the deflector 4 onto the first and second surfaces to be scanned 8a and 8b.

[0256] Here, the first fθ lens 5 is a multi-stage lens in which a first optical unit 5a and a second optical unit 5b are arranged side by side in the sub-scanning direction. That is, the incident surface of the first fθ lens 5 is composed of the incident surface of the first optical unit 5a and the incident surface of the second optical unit 5b, and the exit surface of the first fθ lens 5 is composed of the exit surface of the first optical unit 5a and the exit surface of the second optical unit 5b. And the exit surfaces of the first optical unit 5a and the second optical unit 5b have different lens surface shapes from each other.

[0257] The reflecting members 71a, 71b, and 72a are means for reflecting a light beam, and a vapor deposition mirror or the like is used.

[0258] In the optical scanning device 100 according to the present embodiment, a first incident optical system 75a is constituted by the first anamorphic lens 2a and the first aperture stop 3a. And a second incident optical system is constituted by the second anamorphic lens 2b and the second aperture stop 3b.

[0259] Also, in the optical scanning device 100 according to the present embodiment, a first imaging optical system 85a is constituted by the first optical unit 5a of the first fθ lens 5 and the second fθ lens 6a. And a second imaging optical system is constituted by the second optical unit 5b of the first fθ lens 5 and the second fθ lens 6b.

[0260] Note that in the optical scanning device 100 according to the present embodiment, the optical axes of the first and second incident optical systems 75a are respectively at angles of -3.0 degrees and +3.0 degrees with respect to the main scanning cross section in the sub-scanning cross section.

[0261] Note that the "different angles from each other" in the present embodiment includes two angles having the same absolute value but different signs from each other.

[0262] The light beam LA emitted from the light emitting point of the first light source 1a is converted by the first anamorphic lens 2a into a parallel light beam in the main scanning direction and converged in the sub-scanning direction after passing through the first aperture stop 3a.

[0263] Then, the converted light beam LA is incident on the deflection surface 41 of the deflector 4 from the upper side in the sub-scanning direction.

[0264] The light beam LA 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 condensed on the first scanned surface 8a by the first imaging optical system 85a, and scans the first scanned surface 8a at a constant speed.

[0265] The light beam LB emitted from the emission point of the second light source 1b is converted by the second anamorphic lens 2a to a parallel light beam in the main scanning direction and a converging light beam in the sub-scanning direction after passing through the second aperture stop 3b.

[0266] Then, the converted light beam LB is incident on the deflection surface 41 of the deflector 4 from the lower side in the sub-scanning direction.

[0267] The light beam LB 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 condensed on the second scanned surface 8b by the second imaging optical system, and scans the second scanned surface 8b at a constant speed.

[0268] Since the deflector 4 is rotating in the direction of arrow A in the figure, the deflected light beams LA and LB scan the first and second scanned surfaces 8a and 8b in the direction of arrow B in the figure, respectively.

[0269] Also, C0 is the deflection point (axial deflection point) on the deflection surface 41 of the deflector 4 with respect to the chief ray of the on-axis light beam. In the sub-scanning direction, the light beams LA and LB emitted from the first and second light sources 1a and 1b intersect each other at the deflection point C0. Also, the deflection point C0 serves as the reference point of the first and second imaging optical systems 85a and. P0 is a plane (reference plane) passing through the deflection point C0 and perpendicular to the rotation axis of the deflector 4. Hereinafter, the length of the optical path from the deflection point C0 to each scanned surface is referred to as the optical path length.

[0270] In this embodiment, the first and second scanned surfaces 8a and 8b are the first and second photosensitive drums 8a and 8b.

[0271] Also, the creation of the exposure distribution in the sub-scanning direction on the first and second photosensitive drums 8a and 8b is achieved by rotating the first and second photosensitive drums 8a and 8b in the sub-scanning direction for each main scanning exposure.

[0272] Next, various characteristics of the first and second incident optical systems 75a and the first and second imaging optical systems 85a of the optical scanning device 100 according to the present embodiment are shown in Tables 17 to 18 below.

[0273]

Table 17

[0274]

Table 18

[0275] Next, the effects of the optical scanning device 100 according to the present embodiment will be described. In this embodiment, the optical path length of the imaging optical system 85a and the optical path length of the imaging optical system are different from each other. As a result, compared with the case where each optical path length is the same, the degree of freedom in arranging optical components is improved, and it is possible to condense (guide light) onto the photosensitive drum while avoiding interference between the optical components and the light beam, and miniaturization of the optical scanning device can be achieved. To achieve the above configuration, the first fθ lens 5 has an asymmetrical shape in the sub-scanning direction with respect to the reference plane P0 (boundary surface) shown in FIG. 18 because the generatrix shape and the child line shape are different in the imaging optical systems 85a and as shown in Tables 17 and 18.

[0276] In addition, it is desirable that the shapes (generatrix shapes) in the main scanning cross sections of the incident surface and the exit surface of the first fθ lens 5 be symmetric with respect to the optical axis. This reduces the difference in optical performance between the imaging optical systems 85a and 85b with different optical path lengths. At this time, it is only necessary that the generatrix shapes be symmetric with respect to the optical axis for at least pairs in which the child ray shapes are asymmetric with respect to each other, and for other pairs, the generatrix shape may be asymmetric with respect to the optical axis as necessary. Also, when the sub-scanning magnifications of the imaging optical systems 85a and 85b are β1 and β2, respectively, by satisfying the following formula (4), the difference in optical performance between the imaging optical systems 85a and 85b with different optical path lengths is reduced. FIG. 19 shows the sensitivity of image surface curvature of the imaging optical systems 85a and 85b when the first fθ lens 5 is eccentric by 10 μm in the Y direction. By reducing the difference in optical performance between the imaging optical systems 85a and 85b, it is possible to suppress image quality degradation. 0.8 < β1 / β2 < 1.2 (4)

[0277] In the present embodiment, as described in FIGS. 17 and 18, the first fθ lens 5 has an incident surface and an exit surface that are symmetric in the main scanning direction, β1 = -2.05, β2 = -2.46, and β1 / β2 is 0.82, and since the conditional expression (4) is satisfied, image quality degradation is suppressed. Furthermore, it is preferable to satisfy the following conditional expression (4a). 0.8 < β1 / β2 < 1.0 (4a)

[0278] Therefore, in the optical scanning device 100 according to the present embodiment, by adopting the configuration using the first fθ lens 5 as described above, it is possible to achieve both miniaturization and reduction of the difference in optical performance between the imaging optical systems 85a and 85b and suppression of image quality degradation.

[0279] [Sixth Embodiment] FIGS. 20A and 20B show an optical path development diagram in the main scanning cross section of the optical scanning device 200 according to the sixth embodiment. FIG. 20C shows a sub-scanning cross-sectional view of the incident optical system included in the optical scanning device 200 according to the sixth embodiment. FIG. 20D shows a sub-scanning cross-sectional view of the imaging optical system included in the optical scanning device 200 according to the sixth embodiment.

[0280] The optical scanning device 200 according to this embodiment includes first, second, third, and fourth light sources 1a, 1b, 1c, and 1d, first, second, third, and fourth anamorphic lenses 2a, 2b, 2c, and 2d, and first, second, third, and fourth aperture stops 3a, 3b, 3c, and 3d.

[0281] The optical scanning device 200 according to this embodiment further includes a deflector 4, first fθ lenses 5 and 5', and second fθ lenses 6a, 6b, 6c, and 6d, and reflecting members 71a, 71b, 72b, 71c, 72c, and 71d.

[0282] As the first, second, third, and fourth light sources 1a, 1b, 1c, and 1d, semiconductor lasers or the like are used.

[0283] The first, second, third, and fourth anamorphic lenses 2a, 2b, 2c, and 2d convert light beams LA, LB, LC, and LD (first, second, third, and fourth light beams) emitted from the first to fourth light sources 1a to 1d into substantially parallel light beams in the main scanning direction and converge them in the sub-scanning direction, and have different positive powers (refractive powers) independently in the main scanning direction and the sub-scanning direction. Here, the parallel light beam includes not only a strictly parallel light beam but also a substantially parallel light beam such as a weakly divergent light beam or a weakly convergent light beam.

[0284] The first, second, third, and fourth aperture stops 3a, 3b, 3c, and 3d limit the beam diameters of the light beams LA to LD that have passed through the first to fourth anamorphic lenses 2a to 2d.

[0285] In this way, the light beams LA and LB emitted from the first and second light sources 1a and 1b are converged only in the sub-scanning direction in the vicinity of the first deflection surface 41 of the deflector 4 and are imaged as a long line image in the main scanning direction.

[0286] Also, the light beams LC and LD emitted from the third and fourth light sources 1c and 1d are converged only in the sub-scanning direction in the vicinity of the second deflection surface 42 of the deflector 4 and are imaged as a long line image in the main scanning direction.

[0287] The deflector 4 deflects the light beams LA to LD incident on the deflector 4 by rotating in the direction of arrow A in the figure by driving means such as a motor (not shown). The deflector 4 is composed of, for example, a polygon mirror or the like.

[0288] The first fθ lens 5, the second fθ lenses 6a and 6b are anamorphic imaging lenses having different powers in the main scanning section and the sub-scanning section. Then, the first fθ lens 5, the second fθ lenses 6a and 6b condense (guide) the light beams LA and LB deflected by the first deflection surface 41 of the deflector 4 onto the first and second scanned surfaces 8a and 8b.

[0289] Also, the first fθ lens 5', the second fθ lenses 6c and 6d are anamorphic imaging lenses having different powers in the main scanning section and the sub-scanning section. Then, the first fθ lens 5', the second fθ lenses 6c and 6d condense (guide) the light beams LC and LD deflected by the second deflection surface 42 of the deflector 4 onto the third and fourth scanned surfaces 8c and 8d.

[0290] Here, the first fθ lens 5 is a multi-stage lens in which the first optical part 5a and the second optical part 5b are arranged side by side in the sub-scanning direction. That is, the entrance surface of the first fθ lens 5 is composed of the entrance surface of the first optical part 5a and the entrance surface of the second optical part 5b, and the exit surface of the first fθ lens 5 is composed of the exit surface of the first optical part 5a and the exit surface of the second optical part 5b. And the exit surfaces of the first optical part 5a and the second optical part 5b have shapes with different sub-ray tilt amounts from each other, and are sub-ray tilt change surfaces in which the sub-ray tilt amounts change according to the main scanning direction.

[0291] Further, the first fθ lens 5' is a multi-stage lens in which the first optical unit 5c (third optical unit) and the second optical unit 5d (fourth optical unit) are arranged side by side in the sub-scanning direction. That is, the incident surface of the first fθ lens 5' is composed of the incident surface of the third optical unit 5c and the incident surface of the fourth optical unit 5d, and the exit surface of the first fθ lens 5' is composed of the exit surface of the third optical unit 5c and the exit surface of the fourth optical unit 5d. And the exit surfaces of the third optical unit 5c and the fourth optical unit 5d have shapes with different sub-ray tilt amounts from each other, and are each a sub-ray tilt change surface in which the sub-ray tilt amount changes according to the main scanning direction.

[0292] The reflecting members 71a, 71b, 72b, 71c, 72c, and 71d are means for reflecting a light beam, and a vapor deposition mirror or the like is used.

[0293] Also, in the optical scanning device 200 according to the present embodiment, a first imaging optical system 85a is configured by the first optical unit 5a of the first fθ lens 5 and the second fθ lens 6a. And a second imaging optical system is configured by the second optical unit 5b of the first fθ lens 5 and the second fθ lens 6b.

[0294] Also, a third imaging optical system 85c is configured by the third optical unit 5c of the first fθ lens 5' and the second fθ lens 6c, and a fourth imaging optical system 85d is configured by the fourth optical unit 5d of the first fθ lens 5' and the second fθ lens 6d.

[0295] Note that in the optical scanning device 200 according to the present embodiment, the optical axes of the first and second incident optical systems 75a and are each at an angle of +2.7 degrees and -2.7 degrees with respect to the main scanning plane in the sub-scanning plane.

[0296] Also, the optical axes of the third and fourth incident optical systems 75c and 75d are each at an angle of -2.7 degrees and +2.7 degrees with respect to the main scanning plane in the sub-scanning plane.

[0297] The first and second light beams LA and LB emitted from the light emitting points of the first and second light sources 1a and 1b are converted by the first and second anamorphic lenses 2a and 2b so as to be parallel light beams in the main scanning direction and converged in the sub-scanning direction.

[0298] Then, the converted first and second light beams LA and LB pass through the first and second aperture stops 3a and 3b and enter the first deflection surface 41 of the deflector 4 from the lower side in the sub-scanning direction.

[0299] Then, the first and second light beams LA and LB emitted from the first and second light sources 1a and 1b and incident on the first deflection surface 41 of the deflector 4 are deflected by the deflector 4 and then converged onto the first and second scanned surfaces 8a and 8b by the first and second imaging optical systems 85a and 85b, and scan the first and second scanned surfaces 8a and 8b at a constant speed.

[0300] The third and fourth light beams LC and LD emitted from the light emitting points of the third and fourth light sources 1c and 1d are converted by the third and fourth anamorphic lenses 2c and 2d so as to be parallel light beams in the main scanning direction and converged in the sub-scanning direction.

[0301] Then, the converted third and fourth light beams LC and LD pass through the third and fourth aperture stops 3c and 3d and enter the second deflection surface 42 of the deflector 4 from the lower side and the upper side in the sub-scanning direction, respectively.

[0302] Then, the third and fourth light beams LC and LD emitted from the third and fourth light sources 1c and 1d and incident on the second deflection surface 42 of the deflector 4 are deflected by the deflector 4 and then converged onto the third and fourth scanned surfaces 8c and 8d by the third and fourth imaging optical systems 85c and 85d, and scan the third and fourth scanned surfaces 8c and 8d at a constant speed.

[0303] Since the deflector 4 is rotating in the direction of arrow A in the figure, the deflected light beams LA and LB scan the first and second scanned surfaces 8a and 8b in the direction of arrow B in the figure, respectively. And the deflected light beams LC and LD scan the third and fourth scanned surfaces 8c and 8d in the direction of arrow D in the figure, respectively.

[0304] Also, C0 is the deflection point (axial deflection point) on the first deflection surface 41 of the deflector 4 with respect to the principal ray of the on-axis light beam. In the sub-scanning direction, the light beams LA and LB emitted from the first and second light sources 1a and 1b intersect each other at the deflection point C0. And the deflection point C0 is the reference point of the first and second imaging optical systems 85a and.

[0305] Also, E0 is the deflection point (axial deflection point) on the second deflection surface 42 of the deflector 4 with respect to the principal ray of the on-axis light beam. In the sub-scanning direction, the light beams LC and LD emitted from the third and fourth light sources 1c and 1d intersect each other at the deflection point E0. And the deflection point E0 is the reference point of the third and fourth imaging optical systems 85c and 85d.

[0306] Also, P0 is a plane (reference plane) passing through the deflection points C0 and E0 and perpendicular to the rotation axis of the deflector 4. Hereinafter, 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 are defined as the optical path lengths of the imaging optical systems 85a, 85b, 85c, and 85d.

[0307] In this embodiment, the first, second, third, and fourth scanned surfaces 8a, 8b, 8c, and 8d are the first, second, third, and fourth photosensitive drums 8a, 8b, 8c, and 8d.

[0308] Also, the creation of the exposure distribution in the sub-scanning direction on the first to fourth photosensitive drums 8a to 8d is achieved by rotating the first to fourth photosensitive drums 8a to 8d in the sub-scanning direction for each main scanning exposure.

[0309] Next, the characteristics of the first to fourth incident optical systems 75a to 75d and the first to fourth imaging optical systems 85a to 85d of the optical scanning device 200 according to the present embodiment are shown in Tables 19 to 20 below.

[0310] [Table 19]

[0311] [Table 20]

[0312] Also, the radius of curvature r' in the sub-scanning cross-section changes continuously according to the y-coordinate of the lens surface.

[0313] Next, the effects of the optical scanning device 200 according to the present embodiment will be described. Note that the description of the same effects as those of the optical scanning device 100 according to the fifth embodiment will be omitted.

[0314] In the optical scanning device 200 according to the present embodiment, one deflector 4 can scan four scanned surfaces 8a, 8b, 8c, and 8d.

[0315] Also, the distance on the optical path from the deflection point C0 to the incident surface of the second fθ lens 6a and the distance on the optical path from the deflection point C0 to the incident surface of the second fθ lens 6b are different from each other.

[0316] Also, the distance on the optical path from the deflection point E0 to the incident surface of the second fθ lens 6c and the distance on the optical path from the deflection point E0 to the incident surface of the second fθ lens 6d are different from each other.

[0317] As a result, compared with the case where each optical path length is the same, the degree of freedom in arranging optical components is improved, and it is possible to condense (guide light) onto the photosensitive drum while avoiding interference between the optical components and the light beam, achieving miniaturization. To achieve the above configuration, as shown in Tables 19 and 20, the first fθ lenses 5 and 5' of the imaging optical systems 85a, 85c, and 85d have different bus shapes and child line shapes, and are asymmetrical in the sub-scanning direction with respect to the reference plane P0.

[0318] Also, the first fθ lenses 5 and 5' use the same lens. The upper side of FIG. 21 shows a main scanning cross-sectional view of the first fθ lenses 5 and 5' provided in the optical scanning device 200 according to the present embodiment. The first fθ lenses 5 and 5' have gate portions at either outer end in the main scanning direction. Here, the gate portion refers to the portion (protrusion) corresponding to the resin injection port in the mold when each lens is injection-molded, and is provided at one of both ends of each lens in the main scanning direction. As shown in FIGS. 20A and 20B, the first fθ lens 5 is arranged such that the gate portion comes to the side of the first and second light sources 1a and 1b, and the first fθ lens 5' is arranged such that the gate portion comes in the direction opposite to the third and fourth light sources 1c and 1d. That is, the gate portion of the first fθ lens 5 (the first optical element) and the gate portion of the first fθ lens 5' (the fourth optical element) are located on opposite sides with respect to the optical axis in the main scanning cross-section. This makes it easier to reduce the difference in optical performance in each optical system.

[0319] The lower side of FIG. 21 shows a sub-scanning cross-sectional view of the first fθ lenses 5 and 5' provided in the optical scanning device 200 according to the present embodiment. The first fθ lenses 5 and 5' use the same seating surface 51, and the first fθ lenses 5 and 5' are arranged. Therefore, even if there are errors in P0 to the seating surfaces 51 and 52 during the manufacture of the lenses, by using and arranging the seating surface 51, the error amounts in the Z direction of the first fθ lenses 5 and 5' become the same error amounts for the imaging optical systems 85a, 85c, and 85d, and the difference in optical performance of the imaging optical systems 85a, 85c, and 85d can be suppressed.

[0320] As described above, in the optical scanning device 200 according to the present embodiment, by using the first fθ lenses 5 and 5' as described above, the difference in optical performance in the imaging optical systems 85a, 85c, and 85d can be reduced, and an optical scanning device that achieves further miniaturization can be provided.

[0321] [Image forming apparatus] FIG. 22 shows a main part sub-scanning cross-sectional view of a color image forming apparatus 90 equipped with an optical scanning device 100 according to any of the embodiments.

[0322] The image forming apparatus 90 is a tandem type color image forming apparatus that records image information on each photosensitive drum surface, which is an image carrier, using the optical scanning device 100.

[0323] The image forming apparatus 90 includes an optical scanning device 100, photosensitive drums (photoconductors) 23, 24, 25, 26 as image carriers, and developing devices 15, 16, 17, 18. The image forming apparatus 90 also includes a conveyance belt 91, a printer controller 93, and a fixing device 94.

[0324] R (red), G (green), and B (blue) color signals (code data) output from an external device 92 such as a personal computer are input to the image forming apparatus 90.

[0325] The input color signals are converted into C (cyan), M (magenta), Y (yellow), and K (black) image data (dot data) by a printer controller 93 in the image forming apparatus 90.

[0326] Each of the converted image data is input to the optical scanning device 100. Then, light beams 19, 20, 21, 22 modulated according to each image data are emitted from the optical scanning device 100, and the photosensitive surfaces of the photosensitive drums 23, 24, 25, 26 are exposed by these light beams.

[0327] A charging roller (not shown) is provided so as to contact the surfaces of the photosensitive drums 23, 24, 25, and 26 to uniformly charge them. Then, light beams 19, 20, 21, and 22 are irradiated onto the surfaces of the photosensitive drums 23, 24, 25, and 26 charged by the charging roller by the optical scanning device 100.

[0328] As described above, the light beams 19, 20, 21, and 22 are modulated based on the image data of each color, and an electrostatic latent image is formed on the surfaces of the photosensitive drums 23, 24, 25, and 26 by irradiating 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 disposed so as to contact the photosensitive drums 23, 24, 25, and 26.

[0329] The toner image developed by the developing devices 15 to 18 is multi-transferred onto a sheet of paper (transfer material) not shown that is conveyed on the conveyance belt 91 by a transfer roller (transfer device) not shown disposed so as to face the photosensitive drums 23 to 26, and a single full-color image is formed.

[0330] As described above, the sheet of paper onto which the unfixed toner image has been transferred is further conveyed to a fixing device 94 behind the photosensitive drums 23, 24, 25, and 26 (the left side in FIG. 22). The fixing device 94 is composed of a fixing roller having a fixing heater (not shown) inside and a pressure roller disposed so as to be in pressure contact with the fixing roller. The sheet of paper conveyed from the transfer unit is heated while being pressed at the pressure contact portion of the fixing roller and the pressure roller, whereby the unfixed toner image on the sheet of paper is fixed. Further, a paper discharge roller not shown is disposed behind the fixing roller, and the paper discharge roller discharges the fixed sheet of paper outside the image forming apparatus 90.

[0331] 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, 24, 25, and 26 corresponding to the colors C, M, Y, and K, and prints color images at high speed.

[0332] As the external device 92, for example, a color image reading device equipped with a CCD sensor may be used. In this case, a color digital copier is configured by this color image reading device and the color image forming device 90.

[0333] As described above, the preferred embodiments have been explained, but the present invention is not limited to these embodiments, and various modifications and changes are possible within the scope of the gist. Also, the configurations of the above-described embodiments can be combined with each other. That is, regarding the configuration adopted in a certain embodiment, it can be adopted as needed whether it is adopted in other embodiments or not.

[0334] Note that the embodiments of the present invention include the following configurations.

[0335] (Configuration 1) A deflector including a first deflection surface that deflects the first and second light beams to scan each of the first and second scanned surfaces in the main scanning direction; First and second optical systems that guide the first and second light beams deflected by the first deflection surface to the first and second scanned surfaces; The first and second optical systems have a common first optical element disposed on the first and second optical paths from the first deflection surface to each of the first and second scanned surfaces; The first optical system has a second optical element disposed between the first optical element and the first scanned surface on the first optical path; The second optical system has a third optical element disposed between the first optical element and the second scanned surface on the second optical path; The first optical element includes first and second optical portions into which the first and second light beams are incident. A light scanning device characterized by this.

[0336] (Configuration 2) The light scanning device according to Configuration 1, wherein at least one pair of the incident surface and the exit surface of each of the first and second optical portions is displaced in the optical axis direction at the boundary portion.

[0337] (Configuration 3) When the maximum value of the amount of deviation in the optical axis direction at the boundary portion of at least one pair of the incident surface and the exit surface of each of the first and second optical units is Xmax (mm), 0.01 ≦ |Xmax| ≦ 1.0 The optical scanning device according to Configuration 2, characterized by satisfying the conditional expression.

[0338] (Configuration 4) The optical scanning device according to any one of Configurations 1 to 3, characterized in that the shapes of the incident surfaces of the first and second optical units are identical to each other.

[0339] (Configuration 5) The optical scanning device according to any one of Configurations 1 to 4, characterized in that the optical path lengths from the first deflection surface to the second and third optical elements are different from each other.

[0340] (Configuration 6) The optical scanning device according to any one of Configurations 1 to 5, characterized in that the optical path lengths from the first deflection surface to the first and second surfaces to be scanned are different from each other.

[0341] (Configuration 7) In the sub-scanning cross section, when the incident angle of the principal ray of the first light beam with respect to the first deflection surface is θ1 and the incident angle of the principal ray of the second light beam with respect to the first deflection surface is θ2, |θ2| ≧ |θ1| -2.5 < θ2 / θ1 < 2.5 The optical scanning device according to any one of Configurations 1 to 6, characterized by satisfying the conditional expression.

[0342] (Configuration 8) The optical scanning device according to any one of Configurations 1 to 7, characterized in that, for at least one pair of the incident surface and the exit surface of each of the first and second optical units, the curvatures on the optical axis in the sub-scanning cross section are different from each other.

[0343] (Configuration 9) The optical scanning device according to any one of Configurations 1 to 8, wherein for at least one pair of the incident surface and the exit surface of each of the first and second optical units, the curvature in the sub-scanning cross section changes in the main scanning direction.

[0344] (Configuration 10) The optical scanning device according to Configuration 9, wherein for the pair of the exit surfaces of each of the first and second optical units, the curvature in the sub-scanning cross section changes in the main scanning direction.

[0345] (Configuration 11) The optical scanning device according to any one of Configurations 1 to 10, wherein the distance from the second optical element to the first scanned surface and the distance from the third optical element to the second scanned surface are different from each other.

[0346] (Configuration 12) θ2 / θ1 < 0 The optical scanning device according to any one of Configurations 1 to 11, wherein the difference between the number of reflecting elements disposed between the deflection surface and the first scanned surface and the number of reflecting elements disposed between the first deflection surface and the second scanned surface satisfies the conditional expression and is an odd number.

[0347] (Configuration 13) θ2 / θ1 > 0 The optical scanning device according to any one of Configurations 1 to 12, wherein the difference between the number of reflecting elements disposed between the first deflection surface and the first scanned surface and the number of reflecting elements disposed between the first deflection surface and the second scanned surface satisfies the conditional expression and is an even number.

[0348] (Configuration 14) The optical scanning device according to any one of Configurations 1 to 13, wherein for at least one pair of the incident surface and the exit surface of each of the first and second optical units, the shapes in the sub-scanning cross section are asymmetrical with respect to each other.

[0349] (Configuration 15) The optical scanning device according to configuration 14, wherein for one pair in which the shapes in the sub-scanning cross section are asymmetric with respect to each other, the shape in the main scanning cross section is symmetric with respect to the optical axis.

[0350] (Configuration 16) When the imaging magnifications in the sub-scanning cross sections of the first and second optical systems are β1 and β2, respectively, 0.8 < β1 / β2 < 1.2 The optical scanning device according to any one of configurations 1 to 15, characterized by satisfying the conditional expression.

[0351] (Configuration 17) The first optical element includes an optical surface in which the normal line on the generatrix in the sub-scanning cross section including the optical axis is non-parallel to the optical axis, the optical scanning device according to any one of configurations 1 to 16.

[0352] (Configuration 18) For at least one pair of the incident surface and the exit surface of the first and second optical parts, respectively, the shapes in the main scanning cross section are different from each other, the optical scanning device according to any one of configurations 1 to 17.

[0353] (Configuration 19) The deflector includes a second deflection surface that deflects the third and fourth light beams to scan the third and fourth scanned surfaces in the main scanning direction, The third and fourth optical systems that guide the third and fourth light beams deflected by the second deflection surface to the third and fourth scanned surfaces, The third and fourth optical systems have a common fourth optical element arranged on the third and fourth optical paths from the second deflection surface to the third and fourth scanned surfaces, respectively, The third optical system has a fifth optical element arranged between the fourth optical element on the third optical path and the third scanned surface, The fourth optical system has a sixth optical element arranged between the fourth optical element on the fourth optical path and the fourth scanned surface, The fourth optical element includes third and fourth optical parts on which the third and fourth light beams are incident, the optical scanning device according to any one of Configurations 1 to 18.

[0354] (Configuration 20) The first and fourth optical parts have the same shape as each other when one of them is rotated 180 degrees in the sub-scanning cross section, the optical scanning device according to Configuration 19.

[0355] (Configuration 21) Each of the first and fourth optical elements includes a gate part provided at either one of both ends in the main scanning direction, and the gate parts of each of the first and fourth optical elements are located on opposite sides with respect to the optical axis in the main scanning cross section, the optical scanning device according to Configuration 19 or 20.

[0356] (Configuration 22) An image forming apparatus comprising the optical scanning device according to any one of Configurations 1 to 21, and a developing device that develops an electrostatic latent image formed on the first to fourth surfaces to be scanned by the optical scanning device into a toner image.

[0357] (Configuration 23) An image forming apparatus comprising the optical scanning device according to any one of Configurations 1 to 21, and a printer controller that converts code data output from an external device into an image signal and inputs the image signal to the optical scanning device.

Explanation of Reference Numerals

[0358] 5 Deflector 6A First optical part 6B Second optical part 7A Second optical element 7B Third optical element 100 Optical scanning device

Claims

1. A deflector including a first deflection surface that deflects first and second light beams to scan respective first and second scanned surfaces in a main scanning direction; first and second optical systems that guide the first and second light beams deflected by the first deflection surface to the first and second scanned surfaces; the first and second optical systems having a common first optical element disposed on first and second optical paths extending from the first deflection surface to the first and second scanned surfaces, respectively; the first optical system having a second optical element disposed between the first optical element and the first scanned surface on the first optical path; the second optical system having a third optical element disposed between the first optical element and the second scanned surface on the second optical path; the first optical element including first and second optical portions into which the first and second light beams enter, a light scanning device.

2. The light scanning device according to claim 1, wherein at least one pair of the incident surface and the exit surface of each of the first and second optical portions is displaced in the optical axis direction at a boundary portion.

3. When a maximum value of a displacement amount in the optical axis direction at a boundary portion of at least one pair of the incident surface and the exit surface of each of the first and second optical portions is Xmax (mm), 0.01 ≦ |Xmax| ≦ 1.0 The light scanning device according to claim 2, satisfying the conditional expression.

4. The light scanning device according to claim 1, wherein the shapes of the incident surfaces of the first and second optical portions are identical to each other.

5. The light scanning device according to claim 1, wherein optical path lengths from the first deflection surface to the second and third optical elements are different from each other.

6. The light scanning device according to claim 1, wherein optical path lengths from the first deflection surface to the first and second scanned surfaces are different from each other.

7. In a sub-scanning cross section, when an incident angle of a principal ray of the first light beam with respect to the first deflection surface is θ1 and an incident angle of a principal ray of the second light beam with respect to the first deflection surface is θ2, |θ2| ≧ |θ1| -2.5 < θ2 / θ1 < 2.5 The light scanning device according to claim 1, satisfying the conditional expression.

8. The light scanning device according to claim 1, wherein, for at least one pair of the incident surface and the exit surface of each of the first and second optical portions, curvatures on the optical axis in a sub-scanning cross section are different from each other.

9. The optical scanning device according to claim 1, wherein for at least one pair of the incident surface and the exit surface of each of the first and second optical units, the curvature in the sub-scanning cross section varies in the main scanning direction.

10. The optical scanning device according to claim 9, wherein for the pair of the exit surfaces of each of the first and second optical units, the curvature in the sub-scanning cross section varies in the main scanning direction.

11. The optical scanning device according to claim 1, wherein the distance from the second optical element to the first scanned surface and the distance from the third optical element to the second scanned surface are different from each other.

12. θ2 / θ1 < 0 The optical scanning device according to claim 1, satisfying the conditional expression, and the difference between the number of reflection elements arranged between the first deflection surface and the first scanned surface and the number of reflection elements arranged between the first deflection surface and the second scanned surface is an odd number.

13. θ2 / θ1 > 0 The optical scanning device according to claim 1, satisfying the conditional expression, and the difference between the number of reflection elements arranged between the first deflection surface and the first scanned surface and the number of reflection elements arranged between the first deflection surface and the second scanned surface is an even number.

14. The optical scanning device according to claim 1, wherein for at least one pair of the incident surface and the exit surface of each of the first and second optical units, the shapes in the sub-scanning cross section are asymmetric with respect to each other.

15. The optical scanning device according to claim 14, wherein for one pair in which the shapes in the sub-scanning cross section are asymmetric with respect to each other, the shape in the main scanning cross section is symmetric with respect to the optical axis.

16. When the imaging magnifications in the sub-scanning cross sections of the first and second optical systems are β1 and β2, respectively, ０．８＜β１／β２＜１．２ The optical scanning device according to claim 1, characterized by satisfying the conditional expression.

17. The first optical element includes an optical surface in which the normal line on the generatrix in the sub-scanning cross section including the optical axis is non-parallel to the optical axis, according to the optical scanning device of claim 1.

18. The optical scanning device according to claim 1, wherein for at least one pair of the incident surface and the exit surface of each of the first and second optical units, the shapes in the main scanning cross section are different from each other.

19. The deflector includes a second deflection surface that deflects the third and fourth light beams to scan the respective third and fourth scanned surfaces in the main scanning direction. A third and a fourth optical system for guiding the third and fourth light beams deflected by the second deflection surface to the third and fourth scanned surfaces are provided. The third and fourth optical systems have a common fourth optical element disposed on third and fourth optical paths extending from the second deflection surface to the third and fourth scanned surfaces, respectively. The third optical system has a fifth optical element disposed between the fourth optical element and the third scanned surface on the third optical path. The fourth optical system has a sixth optical element disposed between the fourth optical element and the fourth scanned surface on the fourth optical path. The light scanning device according to claim 1, wherein the fourth optical element includes third and fourth optical portions into which the third and fourth light beams are incident. **Claim 20** The light scanning device according to claim 19, wherein the first and fourth optical portions have the same shape when one of them is rotated 180 degrees in a sub-scanning cross section. **Claim 21** Each of the first and fourth optical elements includes a gate portion provided at either one of both ends in a main scanning direction, and the gate portions of each of the first and fourth optical elements are located on opposite sides with respect to the optical axis in a main scanning cross section. The light scanning device according to claim 19. **Claim 22** An image forming apparatus comprising the light scanning device according to any one of claims 1 to 21 and a developing device that develops an electrostatic latent image formed by the light scanning device into a toner image. **Claim 23** An image forming apparatus comprising the light scanning device according to any one of claims 1 to 21 and a printer controller that converts code data output from an external device into an image signal and inputs the image signal to the light scanning device.

Images