Optical scanner and image formation apparatus

Abstract

To provide an optical scanner capable of improving imaging performance and achieving preferable image quality.SOLUTION: An optical scanner 5 includes a light source 53, a deflector 54, a first lens 551, and a second lens 552. The light source 53 is arranged at a position where the light beam enters a deflecting surface of the deflector 54 at an oblique angle in a sub-scanning direction. In the first lens 551 and the second lens 552, the surface shape of each of an incident surface and an emission surface of the light beam is defined by a main scanning direction shape formula and a sub-scanning direction shape formula that include different coefficients on one side and the other side relative to the central part in a main scanning direction. The sub-scanning direction shape formula includes the curvature radius in the sub-scanning direction defined by a sub-scanning direction curvature radius definition formula as a variable. The sub-scanning direction curvature radius definition formula is expressed by a polynomial equation with the coordinates in the main scanning direction as the variable and includes a primary term of the variable.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to an optical scanning device and an image forming apparatus.

Background Art

[0002] An electrophotographic image forming apparatus such as a copying machine or a printer may include an optical scanning device that exposes the surface of a photosensitive drum, which is an image carrier, while scanning it with a light beam to form an electrostatic latent image on the surface of the photosensitive drum. In the optical scanning device, various techniques for improving imaging performance have been proposed.

[0003] For example, a conventional optical scanning device disclosed in Patent Document 1 has a plurality of rotationally asymmetric lenses. The shape of the generatrix connecting the sub-ray vertices of these lenses is constituted by a curve that is curved in the sub-scanning direction. By having a curved generatrix of the lens, the rotation of the light beam incident with an angular field in the main scanning direction is eliminated, and good imaging performance can be obtained.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In an optical scanning device, the light beam irradiated near the central portion in the main scanning direction hits both one side and the other side of the central portion of the lens in the main scanning direction, and is refracted and condensed. As a result, in the prior art, there has been a problem that a shift occurs in the irradiation position in the sub-scanning direction near the central portion of the lens in the main scanning direction. As a result, there has been a concern that the imaging performance deteriorates and the image quality is affected.

[0006] The present invention has been made in view of the above points, and an object thereof is to provide an optical scanning device and an image forming device capable of improving imaging performance and obtaining suitable image quality.

Means for Solving the Problems

[0007] In order to solve the above problems, the optical scanning device of the present invention includes a light source, a deflector, a first lens, and a second lens. The light source emits a light beam. The deflector has a deflection plane for deflecting the light beam. The first lens and the second lens are arranged on the optical path of the light beam deflected by the deflector, extend in the main scanning direction and the sub-scanning direction of the light beam, and the light beam passes through them in sequence. The light source is arranged at a position where the light beam is incident obliquely with respect to the deflection plane in the sub-scanning direction. The first lens and the second lens are defined by a main scanning direction shape formula and a sub-scanning direction shape formula including coefficients different on one side and the other side with respect to the central portion in the main scanning direction of the incident surface and the exit surface of the light beam, respectively. The sub-scanning direction shape formula includes, as a variable, the radius of curvature in the sub-scanning direction defined by a sub-scanning direction radius of curvature definition formula. The sub-scanning direction radius of curvature definition formula is represented by a polynomial having the coordinates in the main scanning direction as a variable and includes a first-order term of the variable.

Effects of the Invention

[0008] According to the configuration of the present invention, it is possible to suppress the deviation of the irradiation position in the sub-scanning direction near the central portion in the main scanning direction of the first lens and the second lens. Thereby, it is possible to improve the imaging performance and obtain suitable image quality.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

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Figure 10

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Figure 12

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Figure 15

Embodiments for Carrying Out the Invention

[0010] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the following content.

[0011] FIG. 1 is a schematic cross-sectional side view of the image forming apparatus 1 according to the embodiment. As an example of the image forming apparatus 1 of the present embodiment, it is a tandem color printer that receives image data and a print command related to a print job from an external computer and transfers a toner image onto a sheet S using an intermediate transfer belt 71. The image forming apparatus 1 may be a so-called multifunction device having functions such as printing, scanning (image reading), and facsimile transmission.

[0012] As shown in FIG. 1, the image forming apparatus 1 includes a paper supply unit 3, a paper conveyance unit 4, an optical scanning device 5, an image forming unit 6, a transfer unit 7, a fixing unit 8, a paper discharge unit 9, and a control unit 10 provided in its main body 2.

[0013] The paper supply unit 3 is disposed at the bottom of the main body 2. The paper supply unit 3 stores a plurality of sheets (recording media) S before printing and separates and feeds out the sheets S one by one during printing. The paper conveyance unit 4 extends vertically along the side wall of the main body 2. The paper conveyance unit 4 conveys the sheet S fed out from the paper supply unit 3 to the secondary transfer unit 73 and the fixing unit 8, and further discharges the sheet S after fixing from the paper discharge port 4a to the paper discharge unit 9.

[0014] The optical scanning device 5 is disposed at the upper part of the main body 2. The optical scanning device 5 irradiates a laser beam controlled based on image data toward the image forming unit 6. The detailed configuration of the optical scanning device 5 will be described later.

[0015] The image forming unit 6 is disposed below the optical scanning device 5 and above the intermediate transfer belt 71. The image forming unit 6 includes a yellow image forming unit 6a, a cyan image forming unit 6b, a magenta image forming unit 6c, and a black image forming unit 6d. These four image forming units 6 have the same basic configuration. Thus, in the following description, the identification symbols "a", "b", "c", "d" representing each color may be omitted unless particularly limited.

[0016] The image forming unit 6 includes a photosensitive drum (image carrier) 61 that is rotatably supported in a predetermined direction (counterclockwise in FIG. 1). The image forming unit 6 further includes a charging unit, a developing unit, and a drum cleaning unit that are arranged along the rotation direction around the photosensitive drum 61. Note that a primary transfer unit 72 is arranged between the developing unit and the drum cleaning unit.

[0017] The photosensitive drum 61 has a photosensitive layer formed on its outer peripheral surface. The charging unit charges the outer peripheral surface of the photosensitive drum 61 to a predetermined surface potential. The optical scanning device 5 exposes the outer peripheral surface of the photosensitive drum 61 charged by the charging unit, and forms an electrostatic latent image of the original image with attenuated charge on the outer peripheral surface of the photosensitive drum 61. The developing unit supplies toner to the electrostatic latent image on the outer peripheral surface of the photosensitive drum 61 for development to form a toner image. Each of the four image forming units 6 forms a toner image of a different color. The drum cleaning unit removes and recovers deposits such as toner remaining on the outer peripheral surface of the photosensitive drum 61 after the toner image is primarily transferred to the outer peripheral surface of the intermediate transfer belt 71. In this way, the image forming unit 6 forms an image (toner image) to be later transferred to the paper S.

[0018] The transfer unit 7 includes an intermediate transfer belt 71, primary transfer units 72a, 72b, 72c, 72d, a secondary transfer unit 73, and a belt cleaning unit 74. The intermediate transfer belt 71 is arranged below the four image forming units 6 and above the paper supply unit 3. The intermediate transfer belt 71 is an endless intermediate transfer body that is rotatably supported in a predetermined direction (clockwise in FIG. 1), and the toner images formed on the outer peripheral surface of the photosensitive drum 61 in each of the four image forming units 6 are sequentially stacked and primarily transferred. The four image forming units 6 are arranged in a so-called tandem method in a row from the upstream side to the downstream side in the rotation direction of the intermediate transfer belt 71.

[0019] The primary transfer units 72a, 72b, 72c, and 72d are disposed below the respective color image forming units 6a, 6b, 6c, and 6d with the intermediate transfer belt 71 therebetween. The secondary transfer unit 73 is disposed upstream of the fixing unit 8 with respect to the paper conveyance direction of the paper conveyance unit 4 and downstream of the four image forming units 6a, 6b, 6c, and 6d with respect to the rotation direction of the intermediate transfer belt 71. The belt cleaning unit 74 is disposed downstream of the secondary transfer unit 73 with respect to the rotation direction of the intermediate transfer belt 71.

[0020] The primary transfer unit 72 transfers the toner image formed on the outer peripheral surface of the photosensitive drum 61 to the outer peripheral surface of the intermediate transfer belt 71. In other words, the toner image is primarily transferred to the outer peripheral surface of the intermediate transfer belt 71 by the respective primary transfer units 72a, 72b, 72c, and 72d for each color. Then, as the intermediate transfer belt 71 rotates, the toner images of the four image forming units 6 are successively and continuously transferred and overlapped onto the intermediate transfer belt 71 at a predetermined timing, whereby a color toner image in which yellow, cyan, magenta, and black toner images are overlapped is formed on the outer peripheral surface of the intermediate transfer belt 71.

[0021] The color toner image on the outer peripheral surface of the intermediate transfer belt 71 is transferred to the paper S that has been fed in synchronization by the paper conveyance unit 4 at the secondary transfer nip portion formed in the secondary transfer unit 73. The belt cleaning unit 74 removes and cleans the attachments such as toner remaining on the outer peripheral surface of the intermediate transfer belt 71 after the secondary transfer. In this way, the transfer unit 7 transfers (records) the toner image formed on the outer peripheral surface of the photosensitive drum 61 to the paper S.

[0022] The fixing unit 8 is disposed above the secondary transfer unit 73. The fixing unit 8 heats and presses the paper S onto which the toner image has been transferred to fix the toner image to the paper S.

[0023] The paper discharge unit 9 is disposed above the transfer unit 7. The paper S onto which the toner image has been fixed and the printing has been completed is conveyed to the paper discharge unit 9. The paper discharge unit 9 allows the printed paper (printed matter) to be taken out from above.

[0024] The control unit 10 includes a CPU, an image processing unit, a storage unit, and other electronic circuits and components (none of which are shown). Based on the control programs and data stored in the storage unit, the CPU controls the operations of the respective components provided in the image forming apparatus 1 to perform processes related to the functions of the image forming apparatus 1. Each of the paper supply unit 3, the paper conveyance unit 4, the optical scanning device 5, the image forming unit 6, the transfer unit 7, and the fixing unit 8 receives individual commands from the control unit 10 and cooperates to perform printing on the paper S. The storage unit is composed of a combination of a non-volatile storage device such as a program ROM (Read Only Memory) and a data ROM, and a volatile storage device such as a RAM (Random Access Memory).

[0025] Subsequently, the configuration of the optical scanning device 5 will be described with reference to FIGS. 2 to 6. FIG. 2 is a schematic cross-sectional side view of the optical scanning device 5 of the image forming apparatus 1 in FIG. 1. FIG. 3 is a perspective view of the optical scanning device 5 in FIG. 2. FIG. 4 is a cross-sectional view showing the locations of the light source 53 and the deflector 54 of the optical scanning device 5 in FIG. 2.

[0026] In this book, for the sake of convenience in explanation, the main scanning direction is the Y direction (the depth direction of the paper surface in FIG. 2), the sub-scanning direction is the X direction (the vertical direction in FIG. 2), and the traveling direction of light with respect to the deflector 54 is the Z direction (the horizontal direction from left to right in FIG. 2). Each of the X direction, the Y direction, and the Z direction is orthogonal to each other. Regarding the Y direction, which is the main scanning direction, with the reflection point of light on the deflector 54 as the origin, the direction from the deflector 54 to the opposite direction of the light source 53 is the Y1 direction, and the direction from the deflector 54 to the light source 53 is the Y2 direction. The X direction, which is the sub-scanning direction, coincides with the direction in which the rotation axis of the deflector 54 extends. Regarding each of the deflector 54, the first lens 551, and the second lens 552, the X direction has the center of the length in the sub-scanning direction as the origin, and one side direction with respect to the origin (for example, the upward direction in FIG. 2) is the X1 direction, and the other side direction (for example, the downward direction in FIG. 2) is the X2 direction. Speaking in detail about the Z direction, one side direction with respect to the deflector 54 is the Z1 direction, and the other side direction, which is the opposite direction, is the Z2 direction.

[0027] The optical scanning device 5 is used to expose the outer peripheral surfaces of the photosensitive drums 661a, 61b, 61c, and 61d of the respective color image forming units 6a, 6b, 6c, and 6d while scanning them with a light beam, and to form an electrostatic latent image on the outer peripheral surface.

[0028] As shown in FIGS. 2 and 3, the optical scanning device 5 includes a housing 51, a light source 53, a deflector 54, and an optical member 55.

[0029] The housing 51 is formed in a bottomed box shape having an opening on one side in the X direction (sub-scanning direction, the vertical direction in FIG. 2), which is the upper side in FIG. 2. The opening of the housing 51 is closed by a cover member (not shown). Note that FIG. 3 omits the drawing of the cover member. The cover member is formed in a substantially plate shape and is attached to the opening of the housing 51 to cover the internal space of the optical scanning device 5.

[0030] The housing 51 houses the light source 53, the deflector 54, and the optical member 55. A window portion 51w provided with a light transmissive member (not shown) is arranged at the bottom 51b of the housing 51. The window portion 51w allows the light beams La, Lb, Lc, and Ld, which will be described later and are emitted from the light source 53, to pass through when they head toward the scanned surfaces of the photosensitive drums 61a, 61b, 61c, and 61d.

[0031] The light source 53 is disposed inside the housing 51, for example, near the side wall of the housing 51. The light source 53 has, for example, a laser diode designed to emit a light beam of laser light in the visible region. The light source 53 is provided with four light emitting modules 53M each including four independent laser diodes that emit the light beams La, Lb, Lc, and Ld to be irradiated onto the respective four photosensitive drums 61a, 61b, 61c, and 61d. Further, the light source 53 includes a collimator lens and a cylindrical lens (both not shown) through which the light beams La, Lb, Lc, and Ld emitted from the laser diode pass.

[0032] The four light-emitting modules 53M are arranged such that, in the Z direction (the left-right horizontal direction in FIG. 2), two are arranged in the Z1 direction with the position of the rotation axis of the deflector 54 as the center, and two are arranged in the Z2 direction. Further, the four light-emitting modules 53M are arranged such that, in the X direction (the sub-scanning direction, the up-down direction in FIG. 2), two are arranged above the center in the sub-scanning direction of the deflection plane 54m of the deflector 54 to be described later, and two are arranged below it. That is, the four light-emitting modules 53M are located at substantially the center of the housing 51 in the Z direction (the left-right horizontal direction in FIG. 2) and face the deflector 54 when viewed from the Y direction (the depth direction of the paper surface in FIG. 2), and are arranged at four locations corresponding to the respective vertices of a rectangular shape.

[0033] That is, as shown in FIG. 4, the light source 53 is arranged at a position where the light beam is incident obliquely with respect to the deflection plane 54m in the sub-scanning direction (X direction, the up-down direction in FIG. 4).

[0034] The light beams La, Lb, Lc, and Ld emitted from the light source 53 pass through a collimator lens and a cylindrical lens and are incident on the deflection plane 54m around the deflector 54. The collimator lens converts the light beams La, Lb, Lc, and Ld emitted from the light source 53 into substantially parallel light within the main scanning cross-section. The cylindrical lens converges the light beams La, Lb, Lc, and Ld in the sub-scanning direction (X direction) and condenses them near the deflection plane 54m of the deflector 54. As a result, the light beams La, Lb, Lc, and Ld are formed as a line image near the deflection plane 54m of the deflector 54.

[0035] The deflector 54 is inside the housing 51 and is arranged, for example, at substantially the center of the housing 51. The deflector 54 is a polygon mirror configured in a regular polygon shape in plan view, and is provided with a plurality of deflection planes (reflective surfaces) 54m for deflecting light beams around it. The deflector 54 rotates around an axis perpendicular to the bottom 51b of the housing 51 by a motor (not shown).

[0036] Each of the light beams La, Lb, Lc, and Ld emitted from the light source 53 is incident on the deflection surface 54m around the deflector 54 in a state where it is shifted by a small angle in the normal direction with respect to the bottom 51b (sub-scanning direction (X direction), vertical direction in FIG. 2). The deflector 54 deflects the light beams La, Lb, Lc, and Ld emitted from the light source 53 on the deflection surface 54m. Specifically, while rotating, the deflector 54 reflects the light beams La, Lb, Lc, and Ld on its deflection surface 54m and guides them to the optical member 55 while deflecting them in the main scanning direction (Y direction).

[0037] The optical member 55 is disposed on the optical paths of the light beams La, Lb, Lc, and Ld deflected (reflected) by the deflector 54 inside the housing 51. The optical member 55 includes, for example, a first lens 551, a second lens 552, and folding mirrors 553 and 554. The first lens 551, the second lens 552, and the folding mirrors 553 and 554 extend in the main scanning direction (Y direction) of the light beams La, Lb, Lc, and Ld.

[0038] Both the first lens 551 and the second lens 552 are so-called fθ lenses, and each of the light beams La, Lb, Lc, and Ld reflected by the deflector 54 is deflected at a constant speed in the main scanning direction (Y direction). The light beams La, Lb, Lc, and Ld pass through these lenses in the order of the first lens 551 and the second lens 552.

[0039] The folding mirrors 553 and 554 change the optical paths of the light beams La, Lb, Lc, and Ld. Specifically, the folding mirror 553 reflects each of the light beams La, Lb, Lc, and Ld in a predetermined direction, passes them through the window portion 51w, and causes them to reach the surfaces of the photosensitive drums 61a, 61b, 61c, and 61d, which are the surfaces to be scanned, for imaging. The folding mirror 554 is disposed on the optical paths of the light beams Lb and Lc. The folding mirror 554 guides the light beams Lb and Lc that have passed through the first lens 551 and the second lens 552 to the folding mirror 553.

[0040] Next, the detailed configuration of the optical scanning device 5 will be described. FIG. 5 is a top view showing the schematic configuration of the first lens 551 and the second lens 552 of the optical scanning device 5 in FIG. 2. FIG. 6 is a schematic perspective view for explaining the bus shape of the first lens 551 and the second lens 552 of the optical scanning device 5 in FIG. 2. FIG. 7 is an explanatory diagram showing the incident light hitting the deflector 54 of the optical scanning device 5 in FIG. 2. FIG. 8 is an explanatory diagram showing the incident light hitting the first lens 551 and the second lens 552 of the optical scanning device 5 in FIG. 2. In the following description, the light beams La, Lb, Lc, and Ld may sometimes be simply referred to as "light beam L".

[0041] As shown in FIG. 5, the light beam L emitted from the light source 53 is deflected (reflected) on the deflection surface 54m of the deflector 54 and passes through the first lens 551 and the second lens 552 in this order. In the following description, regarding the main scanning direction (Y direction), with the light reflection point in the deflector 54 as the origin (0), the Y1 direction side going from the deflector 54 in the opposite direction to the light source 53 may be referred to as the +Y side (plus Y side), and the Y2 direction side going from the deflector 54 toward the light source 53 may be referred to as the -Y side (minus Y side).

[0042] The surface shapes of the incident surface and the exit surface of the light beam L of the first lens 551 and the second lens 552, which are fθ lenses, are defined by the following formulas (1) to (12). Formulas (1) to (6) are the surface shape definition formulas in the -Y side region in the main scanning direction (Y direction), and formulas (7) to (12) are the surface shape definition formulas in the +Y side region in the main scanning direction (Y direction).

[0043]

Equation

[0044]

Equation

[0045] Equation (1) is the main scanning direction shape equation in the -Y side region of the first lens 551 and the second lens 552, and Zm_m represents the sag amount in the main scanning direction in the -Y side region. In Equation (1), rm_m represents the main scanning direction radius of curvature, km_m represents the main scanning direction conic constant, and Am 4 to Am 10 represent the aspherical coefficients. Equation (2) is the sub-scanning direction shape equation in the -Y side region of the first lens 551 and the second lens 552, and Zs_m represents the sag amount in the sub-scanning direction in the -Y side region. Equation (3) is the sub-scanning direction radius of curvature definition equation in the -Y side region of the first lens 551 and the second lens 552, and rs_m represents the sub-scanning direction radius of curvature in the -Y side region. In Equation (3), Bm 1 to Bm 10 represent the coefficients. Equation (4) is the sub-scanning direction conic constant definition equation in the -Y side region of the first lens 551 and the second lens 552, and ks_m represents the sub-scanning direction conic constant in the -Y side region. Equation (5) is the sub-scanning direction generatrix position definition equation in the -Y side region of the first lens 551 and the second lens 552, and x_m represents the sub-scanning direction generatrix position in the -Y side region. In Equation (5), Cm 2 to Cm 10 represent the coefficients. As shown in Figure 6, the sub-scanning direction generatrix position gp1 curves in the sub-scanning direction (X direction). Equation (6) is the total sag amount definition equation in the -Y side region of the first lens 551 and the second lens 552, and Z_m represents the total sag amount in the -Y side region.

[0046] Equation (7) is the main scanning direction shape equation in the +Y side region of the first lens 551 and the second lens 552, and Zm_p represents the sag amount in the main scanning direction in the +Y side region. In Equation (7), rm_p represents the main scanning direction radius of curvature, km_p represents the main scanning direction conic constant, and Ap 4 to Ap 10represents the aspherical coefficient up to. Equation (8) is the sub-scanning direction shape equation in the +Y side region of the main scanning direction of the first lens 551 and the second lens 552, and Zs_p represents the sub-scanning direction sag amount in the +Y side region. Equation (9) is the sub-scanning direction radius of curvature definition equation in the +Y side region of the main scanning direction of the first lens 551 and the second lens 552, and rs_p represents the sub-scanning direction radius of curvature in the +Y side region. In Equation (9), B_p 1 from B_p 10 represents the coefficient up to. Equation (10) is the sub-scanning direction conic constant definition equation in the +Y side region of the main scanning direction of the first lens 551 and the second lens 552, and ks_p represents the sub-scanning direction conic constant in the +Y side region. Equation (11) is the sub-scanning direction generatrix position definition equation in the +Y side region of the main scanning direction of the first lens 551 and the second lens 552, and x_p represents the sub-scanning direction generatrix position in the +Y side region. In Equation (11), C_p 2 from C_p 10 represents the coefficient up to. Similar to the -Y side region, the sub-scanning direction generatrix position is curved in the sub-scanning direction (X direction) (see Fig. 6). Equation (12) is the total sag amount definition equation in the +Y side region of the main scanning direction of the first lens 551 and the second lens 552, and Z_p represents the total sag amount in the +Y side region.

[0047] In the main scanning direction shape equations (Equations (1) and (7)) of the -Y side region and the +Y side region respectively, the main scanning direction radii of curvature are equal to each other (rm_m = rm_p), and the main scanning direction conic constants are equal to each other (km_m = km_p). In the sub-scanning direction radius of curvature definition equations (Equations (3) and (9)) of the -Y side region and the +Y side region respectively, the 0th order terms of the position y are equal to each other (rs_m 0 = rs_p 0 ), and the coefficients of the 1st order terms of the position y are equal to each other (B_m 1 = B_p 1 ). In the sub-scanning direction conic constant definition equations (Equations (4) and (10)) of the -Y side region and the +Y side region respectively, the 0th order terms of the position y are equal to each other (ks_m 0 = ks_p 0)。In the definitions of the sub-scanning direction generatrix positions (Equations (5) and (11)) for the -Y side region and the +Y side region respectively, the zero-order terms of the position y are equal to each other (C_m 0 =C_p 0 )。

[0048] On the left side of FIG. 7, the upper incident light beam LU and the lower incident light beam LL of the present embodiment that are incident on the deflection surface 54m of the deflector 54 and the light beam L0 of the comparative example that is incident parallel to the main scanning direction (Y direction) are schematically drawn. On the other hand, on the right side of FIG. 7, the ray passing positions of the light beams LU, LL, and L0 on the first lens 551 are shown. In the present embodiment, since the upper incident light beam LU is incident obliquely on the deflection surface 54m from above, it curves so as to be convex downward in the vicinity of the central portion in the main scanning direction (Y direction). Further, since the lower incident light beam LL is incident obliquely on the deflection surface 54m from below, it curves so as to be convex upward in the vicinity of the central portion in the main scanning direction (Y direction). Furthermore, the light beams LU and LL have different degrees of curvature on the +Y side and -Y side with respect to the center (y = 0) in the main scanning direction (Y direction).

[0049] Therefore, in the main scanning direction shape formulas (Equations (1) and (7)), the sub-scanning direction shape formulas (Equations (2) and (8)), the sub-scanning direction radius of curvature definition formulas (Equations (3) and (9)), the sub-scanning direction conic constant definition formulas (Equations (4) and (10)), and the sub-scanning direction generatrix position definition formulas (Equations (5) and (11)) for the -Y side region and the +Y side region respectively, except for the radius of curvature, conic constant, coefficient of the first-order term, and zero-order term defined to be equal as described above, the corresponding coefficients between the -Y side region and the +Y side region are different numerical values. For example, specifically, in the main scanning direction shape formulas (Equations (1) and (7)) for the -Y side region and the +Y side region respectively, the coefficients (A_m 4 , A_p 4 ) of the fourth-order term of the position y are different from each other. Also, for example, in the sub-scanning direction radius of curvature definition formulas (Equations (3) and (9)) for the -Y side region and the +Y side region respectively, the coefficients (B_m 2 , B_p 2 ) of the second-order term of the position y are different from each other.

[0050] As described above, for the first lens 551 and the second lens 552, the surface shapes of the incident surface and the exit surface of the light beam L respectively include coefficients different in the -Y side region and the +Y side region with respect to the central portion in the main scanning direction (Y direction). They are defined by the main scanning direction shape formulas (Equations (1) and (7)) and the sub-scanning direction shape formulas (Equations (2) and (8)).

[0051] And the sub-scanning direction shape formulas (Equations (2) and (8)) include, as variables, the sub-scanning direction curvature radii (rs_m, rs_p) defined by the sub-scanning direction curvature radius definition formulas (Equations (3) and (9)). The sub-scanning direction curvature radius definition formulas (Equations (3) and (9)) are represented by polynomials with the coordinates (position y) in the main scanning direction (Y direction) as variables, and include the first-order terms of the variables. Specifically, the sub-scanning direction curvature radius definition formulas (Equations (3) and (9)) for the -Y side region and the +Y side region respectively are B_m 1 y and B_p 1 y and include them.

[0052] FIG. 8 shows the state of the light beam L on the first lens 551 and the second lens 552 near the central portion in the main scanning direction (Y direction). Near the central portion in the main scanning direction (Y direction) of the first lens 551 and the second lens 552, light hits each of the -Y side region and the +Y side region, is refracted, and is condensed.

[0053] FIG. 9 is a graph showing the irradiation positions in the sub-scanning direction (X direction) in the optical scanning apparatuses of the embodiment and the comparative example. The horizontal axis of FIG. 9 indicates the image height [mm] in the main scanning direction of the lens, and the vertical axis indicates the irradiation position [μm] in the sub-scanning direction.

[0054] The fθ lens of the optical scanning device of the comparative example shown in FIG. 9 has a sub-scanning direction radius of curvature definition formula represented by a polynomial with the coordinates in the main scanning direction as variables and does not include the first-order term of the variable. As a result, as shown in FIG. 9, in the vicinity of the image height of 0 [mm] (near the center in the main scanning direction) of the fθ lens of the comparative example, a large deviation occurs in the irradiation position in the sub-scanning direction, and it can be seen that the irradiation position swings greatly to the minus side and the plus side.

[0055] On the other hand, the first lens 551 and the second lens 552, which are the fθ lenses of the optical scanning device 5 of the embodiment shown in FIG. 9, have sub-scanning direction radius of curvature definition formulas (Equations (3) and (9)) represented by a polynomial with the coordinates (position y) in the main scanning direction (Y direction) as variables, and include the first-order terms (B_m 1 y, B_p 1 y) of the variable. As a result, as shown in FIG. 9, in the vicinity of the image height of 0 [mm] (near the center in the main scanning direction (Y direction)) of the first lens 551 and the second lens 552 of the embodiment, no large deviation occurs in the irradiation position in the sub-scanning direction (X direction). The first lens 551 and the second lens 552 of the embodiment have the irradiation position in the sub-scanning direction (X direction) within a relatively small swing over the entire range of the image height.

[0056] As in the above configuration, the sub-scanning direction radius of curvature definition formulas (Equations (3) and (9)) are represented by a polynomial with the coordinates (position y) in the main scanning direction (Y direction) as variables, and include the first-order terms (B_m 1 y, B_p 1 y) of the variable, so that the deviation of the irradiation position in the sub-scanning direction (X direction) can be suppressed near the center in the main scanning direction (Y direction) of the first lens 551 and the second lens 552. As a result, the optical scanning device 5 can improve the imaging performance and obtain a suitable image quality.

[0057] Also, the first-order terms of the sub-scanning direction radius of curvature definition formulas (Equations (3) and (9)) have equal coefficients (B_m 1 =B_p 1) Regarding this, it will be described with reference to FIGS. 10 to 15.

[0058] FIGS. 10 and 11 are graphs showing the relationship between the position in the main scanning direction and the light beam diameter in the main scanning direction for the comparative example and the embodiment. FIGS. 12 and 13 are graphs showing the relationship between the position in the main scanning direction and the light beam diameter in the sub-scanning direction for the comparative example and the embodiment. FIGS. 14 and 15 are graphs obtained by magnifying the vicinity of the central portion of the graphs of FIGS. 12 and 13. The horizontal axis of these figures indicates the position [mm] in the main scanning direction of the lens, and the vertical axis indicates the light beam diameter [μm] in the main scanning direction or the sub-scanning direction.

[0059] In the fθ lens of the optical scanning device of the comparative example shown in FIGS. 10, 12, and 14, the coefficient of the first-order term of the sub-scanning direction curvature radius definition formula is different between the -Y side region and the +Y side region with respect to the central portion in the main scanning direction. As a result, as shown in FIGS. 10 and 14, it can be seen that the size of the light beam diameter changes rapidly in the vicinity of the central portion in the main scanning direction of the fθ lens of the comparative example.

[0060] On the other hand, in the first lens 551 and the second lens 552, which are the fθ lenses of the optical scanning device 5 of the embodiment shown in FIGS. 11, 13, and 15, the coefficient of the first-order term of the sub-scanning direction curvature radius definition formula (Equations (3) and (9)) is equal between the -Y side region and the +Y side region with respect to the central portion in the main scanning direction (X direction) (B_m 1 =B_p 1 ). As a result, as shown in FIGS. 11, 13, and 15, in the first lens 551 and the second lens 552 of the embodiment, the size of the light beam diameter changes smoothly over the entire region in the main scanning direction.

[0061] As in the above configuration, the coefficient of the first-order term of the sub-scanning direction curvature radius definition formula (Equations (3) and (9)) is equal between the -Y side region and the +Y side region with respect to the central portion in the main scanning direction (X direction) (B_m 1 =B_p 1)As a result, it is possible to suppress a local and rapid change in the size of the light beam diameter over the entire main scanning direction (Y direction) of the first lens 551 and the second lens 552. Thereby, the optical scanning device 5 can improve the imaging performance and obtain a suitable image quality.

[0062] Also, in the sub-scanning direction curvature radius definition formulas (Formulas (3) and (9)), the degrees of all terms other than the first-order term are even numbers. For example, when an odd-degree term is included in the sub-scanning direction curvature radius definition formula, it has been found that the curvature radius in the sub-scanning direction changes more smoothly over the entire main scanning direction. However, it has also been found that, in exchange for this, a local and rapid change is likely to occur in the curvature radius in the sub-scanning direction. In the configuration of the above-described embodiment, by making the degrees of all terms other than the first-order term even in the sub-scanning direction curvature radius definition formula, it becomes possible to preferentially take measures to suppress a local and rapid change in the sub-scanning direction curvature radius.

[0063] As described above, the embodiments of the present invention have been described. However, the scope of the present invention is not limited to this, and various modifications can be made without departing from the gist of the invention.

[0064] For example, in the present embodiment, the first lens 551 and the second lens 552 are arranged on both sides (Z1 side and Z2 side) in the Z direction (the left-right horizontal direction in FIG. 2) of the optical scanning device 5, but the configuration is not limited to such. The first lens 551 and the second lens 552 may be arranged only on one side in the Z direction.

Industrial Applicability

[0065] The present invention can be used in an optical scanning device and an image forming device.

Explanation of Signs

[0066] 1 Image forming device 5 Optical scanning device 6, 6a, 6b, 6c, 6d Image forming unit 51 Housing 53 Light source 54 Deflector 54m Deflection plane 55 Optical member 61, 61a, 61b, 61c, 61d Photoconductor drum (image carrier) 71 Intermediate transfer belt 551 First lens 552 Second lens

Claims

1. A light source that emits a light beam, A deflector having a deflection plane for deflecting the light beam, A first lens and a second lens that are arranged on the optical path of the light beam deflected by the deflector and extend in the main scanning direction and the sub-scanning direction of the light beam, and through which the light beam passes in sequence, Comprising: The light source is arranged at a position where the light beam is incident obliquely with respect to the deflection plane in the sub-scanning direction, The first lens and the second lens are defined by a main scanning direction shape formula and a sub-scanning direction shape formula that include coefficients different on one side and the other side with respect to the central portion in the main scanning direction for the incident surface and the exit surface of the light beam respectively, The sub-scanning direction shape formula includes, as a variable, the radius of curvature in the sub-scanning direction defined by a sub-scanning direction radius of curvature definition formula, The sub-scanning direction radius of curvature definition formula is represented by a polynomial having the coordinates in the main scanning direction as variables, and includes a first-order term of the variable. A light scanning device characterized by this.

2. The light scanning device according to Claim 1, characterized in that the coefficients of the first-order term of the sub-scanning direction radius of curvature definition formula are equal on one side and the other side with respect to the central portion in the main scanning direction.

3. The light scanning device according to Claim 1, characterized in that the degrees of all terms other than the first-order term of the sub-scanning direction radius of curvature definition formula are even numbers.

4. A light scanning device according to any one of Claims 1 to 3, An image forming unit having an image carrier on which an electrostatic latent image is formed on the outer peripheral surface when irradiated with the light beam, An image forming apparatus characterized by comprising this.

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