Optical scanning apparatus and image forming apparatus having the same

Abstract

To provide a compact optical scanning device while employing the UFS method. [Solution] The optical scanning device according to the present invention comprises a deflector that deflects a light beam from a light source to scan the surface to be scanned in the main scanning direction, and a first optical system that guides the light beam deflected by the deflector to the surface to be scanned, wherein in the main scanning cross-section, the width of the light beam immediately before it enters the deflector is smaller than the width of the deflection surface of the deflector, and in the main scanning cross-section, only a portion of the light beam that enters the deflector reaches multiple image heights on the surface to be scanned via the deflection surface, and when the distance between the image height closest to the axial image height on one side of the multiple image heights and the axial image height is Y1 (mm), and the distance between the image height closest to the axial image height on the other side of the multiple image heights and the axial image height is Y2 (mm), -0.005≦(Y2-Y1) / (Y2+Y1)≦0.005 It is characterized by satisfying the following conditions.

Description

Technical Field

[0001] The present invention relates to an optical scanning device, and more particularly to an optical scanning device suitably used in an image forming apparatus such as a laser beam printer (LBP), a digital copier, or a multifunction printer (MFP).

Background Art

[0002] Conventionally, an optical scanning device has been used as an exposure device mounted in an image forming apparatus such as a laser beam printer that uses an electrophotographic process. The optical scanning device can be classified into an underfield scan (UFS) method and an overfield scan (OFS) method according to the relationship between the size of the incident light beam incident on the deflector and the size of the deflection plane.

[0003] Specifically, in the UFS method, the width of the incident light beam incident on the deflector in the main scanning cross-section is smaller than the width of the deflection plane of the deflector, while in the OFS method, the width of the incident light beam incident on the deflector in the main scanning cross-section is larger than the width of the deflection plane of the deflector. Patent Document 1 discloses an optical scanning device that employs the UFS method.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Therefore, an object of the present invention is to provide a compact optical scanning device while adopting the UFS method.

Means for Solving the Problems

[0006] To achieve the above objective, an optical scanning device as one aspect of the present invention comprises a deflector that deflects a light beam from a light source to scan the surface to be scanned in the main scanning direction, and a first optical system that guides the light beam deflected by the deflector to the surface to be scanned, wherein in the main scanning cross-section, the width of the light beam immediately before it enters the deflector is smaller than the width of the deflection surface of the deflector, and in the main scanning cross-section, only a portion of the light beam that enters the deflector reaches multiple image heights on the surface to be scanned via the deflection surface, and when the distance between the image height closest to the axial image height on one side of the multiple image heights and the axial image height is Y1 (mm), and the distance between the image height closest to the axial image height on the other side of the multiple image heights and the axial image height is Y2 (mm), -0.005≦(Y2-Y1) / (Y2+Y1)≦0.005 It is characterized by satisfying the following conditions. Furthermore, in order to achieve the above objective, an optical scanning device, as another aspect of the present invention, comprises a deflector that deflects a light beam from a light source to scan the surface to be scanned in the main scanning direction, and a first optical system that guides the light beam deflected by the deflector to the surface to be scanned, characterized in that, in the main scanning cross-section, the width of the light beam immediately before it enters the deflection surface of the deflector is smaller than the width of the deflection surface, and in the main scanning cross-section, when the normal to the deflection surface forms a predetermined angle with respect to the optical axis of the first optical system, only a portion of the light beam entering the deflector is deflected by the deflection surface and reaches the surface to be scanned. [Effects of the Invention]

[0007] According to the present invention, a compact optical scanning device can be provided while employing the UFS method. [Brief explanation of the drawing]

[0008] [Figure 1] A cross-sectional view of the main scanning and a partially sub-scanning of the optical scanning apparatus according to the first embodiment. [Figure 2] This figure shows the dependence of the position of the deflection point on the deflection plane of a polygon mirror on the rotation angle. [Figure 3] This figure shows the dependence of the incident position of each ray on the rotation angle in a polygon mirror. [Figure 4]This figure shows the scan lines formed on the scanned surface when the polygon mirror is malfunctioning in the optical scanning apparatus according to the first embodiment. [Figure 5] A cross-sectional view of the main scanning and a partially sub-scanning of the optical scanning apparatus according to the second embodiment. [Figure 6] This figure shows the scan lines formed on the scanned surface when the polygon mirror is malfunctioning in the optical scanning apparatus according to the second embodiment. [Figure 7] A sub-scanning cross-sectional view of the main parts of a monochrome image forming apparatus and a color image forming apparatus according to an embodiment. [Modes for carrying out the invention]

[0009] The optical scanning apparatus according to this embodiment will be described in detail below with reference to the attached drawings. Note that the drawings shown below may be drawn to a different scale than the actual dimensions in order to facilitate understanding of this embodiment. In the following explanation, the main scanning direction is the direction perpendicular to the rotation axis of the polygon mirror 5 and the optical axis of the imaging optical system 7 (the direction in which the light beam is deflected by the polygon mirror 5), and the sub-scanning direction is the direction parallel to the rotation axis of the polygon mirror 5. Furthermore, the main scanning cross-section is the cross-section parallel to the main scanning direction and the optical axis of the imaging optical system 7 (perpendicular to the sub-scanning direction), and the sub-scanning cross-section is the cross-section parallel to the sub-scanning direction and the optical axis of the imaging optical system 7 (perpendicular to the main scanning direction).

[0010] This embodiment relates to an optical scanning device, and more particularly to an optical scanning device that records image information by deflecting a light beam emitted from a light source using a polygon mirror as a deflector and scanning it on a surface to be scanned via an imaging optical system. Furthermore, the optical scanning apparatus according to this embodiment is suitably used in image forming apparatuses such as printers, digital copiers, and multifunction printers that have an electrophotographic process.

[0011] [First Embodiment] Conventionally, when newly designing an optical scanning device, it is easier to suppress the initial investment and the total cost of the product by reusing conventional parts as much as possible while satisfying the specifications. For example, when higher speed and higher image quality are required compared to the conventional ones, it is possible to suppress the initial investment by increasing the number of deflection surfaces of the polygon mirror while reusing the conventional optical elements.

[0012] On the other hand, if the number of deflection surfaces of the polygon mirror is simply increased, it may cause the polygon mirror to become larger, or the position of the deflection point with respect to the light beam incident on the deflection surface may shift, resulting in optical performance degradation such as image plane curvature. Here, consider the case of using a polygon mirror having five deflection surfaces (hereinafter referred to as a five-sided polygon) instead of a polygon mirror having four deflection surfaces (hereinafter referred to as a four-sided polygon).

[0013] In such a case, by setting the width of each deflection surface of the five-sided polygon in the main scanning cross-section to be equal to that of the four-sided polygon, it is possible to make it difficult for the light beam to be kicked at the ends of each deflection surface of the five-sided polygon. On the other hand, in that case, the distance between the rotation center and the center of each deflection surface in the five-sided polygon increases compared to that of the four-sided polygon.

[0014] For example, in the case of a four-sided polygon having an outer diameter of 20 mm (the length of the diagonal of a square), the width in the main scanning cross-section of each deflection surface is 14.142 mm, and the distance between the rotation center and the center of each deflection surface is 7.071 mm. Also, in a five-sided polygon having a width of 14.142 mm in the main scanning cross-section of each deflection surface so as to be the same as the four-sided polygon, the outer diameter (the length of the diagonal of a regular pentagon) is 24.060 mm, and the distance between the rotation center and the center of each deflection surface is 9.732 mm. <​Furthermore, by using pentagonal polygons instead of tetrahedral polygons so that the width of each deflection surface does not change within the main scanning cross-section, the area projected within the main scanning cross-section becomes 200 mm². 2 From 344mm 2 As the number increases, the weight also increases. Therefore, if the drive unit is not changed, the time it takes for the five-faced polygon to reach a predetermined rotational speed when rotated by the drive unit will be longer than the time it takes for the four-faced polygon, which is undesirable.

[0016] Furthermore, by using pentahedrons instead of tetrahedrons to ensure that the width of each deflection surface within the main scanning cross-section does not change, the distance between the rotation center and the center of each deflection surface also increases by 2.661 mm. Therefore, even if the positions of the deflection points that deflect the light beam to reach a predetermined image height on a predetermined deflection plane are made to coincide between the four-faced polygon and the five-faced polygon, the positions of the deflection points on other deflection planes will differ because the positions of the rotation centers are different.

[0017] Therefore, when using a pentagonal polygon instead of a tetrahedron to ensure that the width of each deflection plane within the main scanning cross-section does not change, the position at which the deflected light beam passes through the imaging optical system will also be different from its position in the tetrahedron. Consequently, optical performance such as the focus position will also change. Furthermore, if a pentagonal polygon is used instead of the tetrahedron polygon to prevent the rotation center from changing, the position of each deflection point on each deflection plane will change, resulting in a more significant change in optical performance.

[0018] Therefore, we consider using a pentahedron polygon instead of a tetrahedron so that the distance between the rotation center and the center of each deflection surface does not change. For example, if a pentahedron polygon is used instead of a tetrahedron polygon with an outer diameter of 20 mm so that the distance between the rotation center and the center of each deflection surface does not change, the outer diameter will change from 20 mm to 17.481 mm.

[0019] Furthermore, the width of each deflection plane within the main scanning plane changes from 14.142 mm to 10.275 mm, and the area projected within the main scanning plane is 200 mm². 2 From 182mm 2 As it becomes smaller, its weight also decreases. Therefore, if the drive unit is not changed, the time it takes for the five-faced polygon to reach a predetermined rotational speed when rotated by the drive unit will be shorter than the time it takes for the four-faced polygon to reach that predetermined speed.

[0020] Furthermore, if a pentahedron polygon is used instead of a tetrahedron polygon so that the distance between the center of rotation and the center of each deflection surface does not change, and the position of the center of rotation does not change, the position of the deflection points on each deflection surface does not change, and therefore the optical performance, such as the focal position, does not change either. On the other hand, when using a pentahedron polygon instead of a tetrahedron polygon with an outer diameter of 20 mm so that the distance between the rotation center and the center of each deflection surface does not change, the width of each deflection surface within the main scanning cross-section is reduced from 14.142 mm to 10.275 mm, as described above.

[0021] Therefore, depending on the beam width of the incident light beam incident on the pentagonal polygon in the main scanning direction and the fθ coefficient of the imaging optical system, there is a risk that a portion of the incident light beam may be deflected when it is deflected at the edges of each deflection surface of the pentagonal polygon. When a portion of the light beam is deflected and reaches the scanning surface, the amount of light in that beam decreases in proportion to the amount that has been deflected. This results in uneven density in the image formed on the scanning surface, leading to a decrease in image quality.

[0022] Therefore, the objective of this embodiment is to provide an optical scanning device that can suppress such degradation of image quality. Figures 1(a) and 1(b) show a schematic main scanning cross-sectional view and a partially schematic sub-scanning cross-sectional view of the optical scanning device 50 according to the first embodiment, respectively. The optical scanning device 50 according to this embodiment includes a light source 1, a sub-scanning aperture 2, an anamorphic collimator lens 3, a main scanning aperture 4, a polygon mirror 5, a first imaging lens 7a, a second imaging lens 7b, and dustproof glass 8.

[0023] As the light source 1, a single light-emitting point, such as a semiconductor laser, is used. As will be described later, the light source 1 is positioned shifted in the main scanning direction so that the single light-emitting point of the light source 1 is not located on the optical axis of the incident optical system 6. The sub-scan diaphragm 2 has a rectangular opening and restricts the beam width of the light beam emitted from the light source 1 in the sub-scanning direction. The beam width of the light beam emitted from the light source 1 in the main scanning direction is also restricted once it passes through the sub-scan diaphragm 2. The rectangular opening formed in the sub-scan aperture 2 has a size of 2.40 mm in the main scanning direction and 1.29 mm in the sub-scanning direction.

[0024] The anamorphic collimator lens 3 converts the light beam that has passed through the sub-scan aperture 2 into a parallel light beam within the main scan cross-section, and into a converged light beam within the sub-scan cross-section. In this context, the term "parallel beam" includes not only strictly parallel beams but also approximate parallel beams such as weakly converging beams and weakly diverging beams. Furthermore, in the optical scanning device 50 according to this embodiment, temperature compensation is performed by forming the incident surface of the anamorphic collimator lens 3 as the diffraction surface.

[0025] The main scanning aperture 4 has a rectangular opening and further restricts the beam width in the main scanning direction of the light beam that has passed through the anamorphic collimator lens 3. The rectangular opening formed in the main scanning aperture 4 has a size of 2.96 mm in the main scanning direction, and no structure is formed in the sub-scanning direction to restrict the light beam width.

[0026] The polygon mirror 5 is a rotating polyhedron mirror that functions as a deflector, directing the light beam that has passed through the main scanning aperture 4 toward the scanning surface 9. The polygon mirror 5 also has five deflection surfaces 5a and an outer diameter of 17.481 mm.

[0027] The polygon mirror 5 then rotates at a constant speed in the direction of arrow PA in Figure 1(a) by a drive unit such as a motor (not shown). In the optical scanning device 50 according to this embodiment, in the main scanning cross-section, the width of the light beam immediately before it enters the deflection surface 5a of the polygon mirror 5 is smaller than the width of the deflection surface 5a.

[0028] The first imaging lens 7a and the second imaging lens 7b guide (focus) the light beam deflected by the deflection surface 5a of the polygon mirror 5 onto the scanning surface 9. The dustproof glass 8 prevents foreign matter such as dust from entering the housing (not shown) of the optical scanning device 50 according to this embodiment from the side of the scanned surface 9, and also prevents noise generated in the drive unit that drives the polygon mirror 5 from leaking to the outside.

[0029] In the optical scanning device 50 according to this embodiment, the incident optical system 6 (second optical system) is formed by the sub-scanning aperture 2, the anamorphic collimator lens 3, and the main scanning aperture 4. Furthermore, in the optical scanning device 50 according to this embodiment, an imaging optical system 7 (first optical system) having fθ characteristics is formed by the first imaging lens 7a and the second imaging lens 7b. Furthermore, the imaging optical system 7 forms a so-called distortion correction optical system that optically conjugates the deflection surface 5a of the polygon mirror 5 and the scanned surface 9 within the sub-scanning cross-section.

[0030] In the optical scanning device 50 according to this embodiment, the light beam (divergent light beam) emitted from the light source 1, which is optically modulated according to the image information, passes through a rectangular opening provided in the sub-scanning aperture 2, thereby partially blocking light in the sub-scanning direction and restricting the width of the light beam in the sub-scanning direction. Next, the light beam that has passed through the sub-scan aperture 2 is converted into a substantially parallel light beam within the main scanning cross-section by the anamorphic collimator lens 3, and is focused within the sub-scan cross-section so that a line image elongated in the main scanning direction is formed near the deflection surface 5a of the polygon mirror 5.

[0031] The light beam that has passed through the anamorphic collimator lens 3 is partially blocked in the main scanning direction by passing through a rectangular opening provided in the main scanning aperture 4, thereby restricting the width of the light beam in the main scanning direction, before it is incident on the deflection surface 5a of the polygon mirror 5. The light beam deflected by the deflection surface 5a of the polygon mirror 5 is focused into a spot on the scanning surface 9 by the first imaging lens 7a and the second imaging lens 7b.

[0032] The light beam, thus focused into a spot, is scanned at a constant speed along the direction of arrow PB in Figure 1(a), i.e., the main scanning direction, as the polygon mirror 5 rotates in the direction of arrow PA in Figure 1(a). As a result, an image is recorded on the photosensitive surface of the photosensitive drum, which is a recording medium positioned at the location of the scanned surface 9.

[0033] Next, Table 1 below shows the various values ​​such as the radius of curvature, interplanar spacing, and refractive index of each optical element provided in the optical scanning device 50 according to this embodiment, within the main scanning cross-section and the sub-scanning cross-section.

[0034] [Table 1]

[0035] Note that in Table 1, R Y R is the radius of curvature within the main scanning plane. Z is the radius of curvature within the sub-scanning cross-section, D is the distance between adjacent optical planes, and n is the refractive index for a light beam with a wavelength of 792 nm.

[0036] Furthermore, the aspherical coefficients of the incident and exit surfaces of the first imaging lens 7a and the second imaging lens 7b, respectively, provided in the optical scanning device 50 according to this embodiment are shown in Table 2 below. Note that in Table 2, E+X is "×10 +X This indicates that, and the same applies to the following tables.

[0037] [Table 2]

[0038] Specifically, the shapes of the incident and exit surfaces of the first imaging lens 7a and the second imaging lens 7b, respectively, provided in the optical scanning device 50 according to this embodiment, within the main scanning cross-section are represented by the following equation (1).

number

[0039] In equation (1), the direction parallel to the optical axis of the imaging optical system 7 is defined as the X direction, the main scanning direction as the Y direction, and the sub-scanning direction as the Z direction. This definition is also applied to the following equations. Furthermore, the shapes of the incident and exit surfaces of the first imaging lens 7a and the second imaging lens 7b, respectively, within the sub-scanning cross-section of the optical scanning device 50 according to this embodiment are expressed by the following equation (2).

number

[0040] In equation (2), S(Y,Z) represents the amount of sag at coordinate (Y,Z) from the generatrix shape at coordinate Y, with the origin being the vertex of each optical surface. The actual shape of the optical surface is X+S(Y,Z). Also, in equation (2), r Z ' represents the radius of curvature in the sub-scan cross-section at coordinate Y on the generatrix of each optical surface, and is specifically expressed by the following equation (3).

number

[0041] Furthermore, the phase function φ of the diffraction plane formed on the incident surface of the anamorphic collimator lens 3 provided in the optical scanning device 50 according to this embodiment is expressed by the following equation (4).

number

[0042] In equation (4), λ is the design wavelength (790 nm), and C and E are the phase coefficients shown in Table 3 below.

[0043] [Table 3]

[0044] In the diffraction grating on the diffraction surface formed on the incident surface of the anamorphic collimator lens 3, a step is provided at a coordinate where the optical path length differs by a height equivalent to the wavelength, at coordinates where the phase function φ is an integer multiple of 2π. Furthermore, the coordinates of the vertices of each optical surface and the angles of the surface normals in the optical scanning device 50 according to this embodiment are shown in Table 4 below.

[0045] [Table 4]

[0046] Note that the angles of the surface normals of the first imaging lens 7a and the second imaging lens 7b shown in Table 4 do not take into account the aspherical shape defined by the aspherical coefficient shown in Table 2. Furthermore, regarding the position of the light source 1 shown in Table 4, it is taken into consideration that it is positioned with a shift of 0.278 mm in a direction perpendicular to the optical axis of the incident optical system 6 and away from the scanned surface 9, as will be described later.

[0047] Next, the characteristic configuration and effects of the optical scanning device 50 according to this embodiment will be described. Specifically, we consider the case where a polygon mirror 5 with an inscribed circle radius of 7.071 mm and five deflection surfaces 5a is used instead of a polygon mirror 5' with an inscribed circle radius of 7.071 mm and four deflection surfaces 5a.

[0048] However, the following configuration is not limited to this example, and can also be applied to cases where, for example, a polygon mirror having a predetermined inscribed radius and six deflection surfaces 5a is used instead of a polygon mirror having a predetermined inscribed radius and five deflection surfaces 5a. In other words, the configuration shown below can be applied when using a polygon mirror having a predetermined inscribed radius and (N+1) deflection surfaces 5a instead of a polygon mirror having a predetermined inscribed radius and N deflection surfaces 5a.

[0049] Furthermore, the configuration shown below can also be applied when using a polygon mirror having a second inscribed circle radius and (N+1) deflection surfaces 5a instead of a polygon mirror having a first inscribed circle radius and N deflection surfaces 5a. Table 5 shows the specifications for Polygon Mirror 5', Polygon Mirror 5, and Polygon Mirror 5''.

[0050] [Table 5]

[0051] Specifically, the polygon mirror 5' has four deflection surfaces 5a and an inscribed circle radius of 7.071 mm. Furthermore, while the polygon mirror 5 has the same inscribed circle radius of 7.071 mm as the polygon mirror 5', it has five deflection surfaces 5a. Furthermore, the polygon mirror 5'' has five deflection surfaces 5a, while having a width of 14.142 mm within the main scanning cross-section of the same deflection surface 5a as the polygon mirror 5'.

[0052] For example, in the polygon mirror 5'', the diameter of the inscribed circle is larger while the width of the deflection surface 5a within the main scanning cross-section is maintained compared to the polygon mirror 5'', so under certain conditions, the incident light beam may be deflected. In other words, in order to prevent the incident light beam from being deflected under any conditions in the polygon mirror 5'', the width of the deflection surface 5a within the main scanning cross-section must be increased in proportion to the increase in the diameter of the inscribed circle.

[0053] On the other hand, with the polygon mirror 5'', the width of the deflection surface 5a within the main scanning cross-section is maintained, while the area projected within the main scanning cross-section has already increased by more than 1.7 times compared to the polygon mirror 5''. In other words, because the weight of Polygon Mirror 5'' has increased compared to Polygon Mirror 5'', the time required to reach a predetermined rotation speed when Polygon Mirror 5'' is rotated will also increase. Furthermore, increasing the power of the drive mechanism used to rotate the polygon mirror 5'' in order to suppress the increase in that time would be undesirable because it would increase costs.

[0054] Furthermore, even if polygon mirrors 5' and 5'' are arranged so that the positions of the deflection points on the deflection surface 5a with respect to the principal ray of the light beam scanning a predetermined image height coincide, the positions of the rotation centers will be different due to the difference in the size of the inscribed circle diameters. Therefore, when polygon mirrors 5' and 5'' are rotated by the same angle relative to each other, the positions of the deflection points on the deflection surface 5a will be different.

[0055] For simplicity, let's consider the case where the angle between the optical axis of the incident optical system 6 and the optical axis of the imaging optical system 7 within the main scanning plane is 90°. Furthermore, consider the case where polygon mirrors 5' and 5'' are arranged such that the positions of the deflection points on the deflection surface 5a with respect to the principal ray of the light beam scanning the on-axis image height (hereinafter referred to as the on-axis light beam) coincide with each other. In this case, the rotation center of the polygon mirror 5'' will be shifted by 2.661 mm, which is the difference in the radius of the inscribed circle, along a 45° direction relative to the rotation center of the polygon mirror 5'' (the direction away from the light source 1 and the scanned surface 9).

[0056] Next, consider the case where the predetermined polygon mirror is rotated such that the angle between the normal to the predetermined deflection surface 5a and the optical axis of the incident optical system 6 becomes smaller by φ[°] with respect to the reference angle at which the predetermined deflection surface 5a deflects the on-axial light beam. Furthermore, the inscribed circle diameter of the predetermined polygon mirror is r, and the position of the principal ray of the light beam incident on the predetermined polygon mirror is shifted by a from the rotation center of the predetermined polygon mirror 5 toward the scanned surface 9.

[0057] First, when the angle φ is 0°, the position of the deflection point on the deflection surface 5a (i.e., the on-axial deflection point) of the light beam incident on a predetermined polygon mirror with respect to the principal ray is separated from the rotation center of the predetermined polygon mirror by the following amount in a direction parallel to the optical axis of the incident optical system 6.

number

[0058] Now, let's consider the case where a given polygon mirror rotates by φ[°] as described above. At this time, the position of the deflection point on the deflection surface 5a with respect to the principal ray of the light beam incident on the predetermined polygon mirror will be separated from the rotation center of the predetermined polygon mirror by the following amount in a direction parallel to the optical axis of the incident optical system 6.

number

[0059] Therefore, when a predetermined polygon mirror rotates by φ[°], the position of the deflection point on the deflection surface 5a with respect to the principal ray of the light beam incident on the predetermined polygon mirror will shift towards the light source 1 by the following amount in a direction parallel to the optical axis of the incident optical system 6.

number

[0060] Here, if the shift amount a in polygon mirror 5' is 5 mm, then the shift amount a in polygon mirror 5'' can be calculated as shown in equation (5) below.

number

[0061] The dependence of the position of the deflection point on the deflection surface 5a with respect to the principal ray of the light beam incident on each of the polygon mirrors 5' and 5'' on the rotation angle φ in a direction parallel to the optical axis of the incident optical system 6 is shown in Figure 2. Figure 2 shows the relative position of the deflection point with respect to the axial deflection point on the deflection plane 5a with respect to the principal ray of the axial beam at each rotation angle φ, that is, the amount of shift of the axial deflection point relative to that position.

[0062] As shown in Figure 2, when the rotation angle φ is 0°, the positions of the on-axial deflection points on the deflection surface 5a for the principal rays of the on-axial light beam incident on each of the polygon mirrors 5' and 5'' in a direction parallel to the optical axis of the incident optical system 6 coincide with each other. On the other hand, when the rotation angle φ is not 0°, the positions of the deflection points on the deflection surface 5a for the principal rays of the light beam incident on each of the polygon mirrors 5' and 5'' in a direction parallel to the optical axis of the incident optical system 6 do not coincide with each other.

[0063] Therefore, the incident positions in the imaging optical system 7 for light beams other than the on-axis light beam deflected by the deflection surface 5a in each of the polygon mirrors 5' and 5'' will be different. As a result, image field curvature occurs, leading to a decrease in optical performance.

[0064] Here, the amount of shift of the deflection point on the deflection surface 5a shown in Figure 2 is calculated only from the inscribed circle diameter r of the polygon mirror, the shift amount a indicating the incident position of the light beam, and the rotation angle φ of the polygon mirror, as described above, and does not depend on the number of deflection surfaces 5a of the polygon mirror. That is, consider polygon mirrors 5' and 5, which have different numbers of deflection surfaces 5a as shown in Table 1, but have the same inscribed circle diameter r.

[0065] In this case, by making the shift amount a the same for both polygon mirror 5' and polygon mirror 5, the positions of the deflection points on the deflection surface 5a in a direction parallel to the optical axis of the incident optical system 6 will coincide for any rotation angle φ. In other words, if polygon mirror 5' and polygon mirror 5 are positioned so that their rotation centers are at the same position, image field curvature will not occur, thus avoiding a decrease in optical performance.

[0066] On the other hand, as shown in Table 1, in polygon mirror 5, where the number of deflection surfaces 5a is increased while the inscribed circle diameter remains the same compared to polygon mirror 5', the width of the deflection surface 5a within the main scanning cross-section is smaller. Therefore, with the polygon mirror 5, depending on the scanning angle, the width of the incident light beam, and the angle of incidence within the main scanning cross-section, there is a risk that a portion of the light beam scanning near the outermost image height may be blocked.

[0067] Figure 3(a) shows the dependence of the incident position of the principal and marginal rays of the light beam incident on polygon mirror 5' and polygon mirror 5, respectively, on the rotation angle φ. Here, the angle between the optical axis of the incident optical system 6 and the optical axis of the imaging optical system 7 within the main scanning cross-section is 90°, the shift amount a is 5 mm, and the beam width of the incident light beam in the main scanning direction is 3 mm.

[0068] Furthermore, the vertical axis in Figure 3(a) shows the position of the deflection point on the deflection surface 5a when the position of the center of the polygon mirror 5' in the direction parallel to the optical axis of the incident optical system 6 at each rotation angle φ is set to 0 mm. The dashed line ±7.071 mm shown in Figure 3(a) indicates the position of the edge of the deflection surface 5a of the polygon mirror 5' in a direction parallel to the optical axis of the incident optical system 6. Furthermore, the dotted line ±5.137 mm shown in Figure 3(a) indicates the position of the edge of the deflection surface 5a of the polygon mirror 5 in a direction parallel to the optical axis of the incident optical system 6.

[0069] As shown in Figure 3(a), when the rotation angle φ in the polygon mirror 5' becomes smaller than -20.5°, a portion of the incident light beam is blocked. On the other hand, with Polygon Mirror 5, if the rotation angle φ becomes smaller than -14.6°, a portion of the incident light beam is blocked.

[0070] Furthermore, as shown in Figure 3(a), it can be seen that when the rotation angle φ in the polygon mirror 5 becomes greater than +33.0°, a portion of the incident light beam is blocked. On the other hand, with the polygon mirror 5', the incident light beam is not blocked even when the rotation angle φ increases to +35°.

[0071] Therefore, in the optical scanning device 50 according to this embodiment, when the number of deflection surfaces 5a is changed while maintaining the size of the inscribed circle diameter r, the shift amount a is changed so that the positive rotation angle φ and the negative rotation angle φ at which the light beam begins to be deflected are the same. For example, if the shift amount a is changed from 5 mm to 6 mm in polygon mirror 5' and polygon mirror 5, the dependence of the incident position of the principal and marginal rays of the incident ray on the rotation angle φ, as shown in Figure 3(a), changes as shown in Figure 3(b).

[0072] In other words, as shown in Figure 3(b), when the shift amount a in the polygon mirror 5 is changed from 5 mm to 6 mm, it can be seen that a portion of the incident light beam is blocked when the rotation angle φ becomes less than -22° or greater than +22°. Therefore, when the shift amount a is changed from 5mm to 6mm in polygon mirror 5, the area where the incident light beam is blocked increases on the positive side of the rotation angle φ, while the area where the incident light beam is blocked decreases on the negative side.

[0073] Furthermore, since roughly the same range can be used on both the positive and negative sides of the rotation angle φ, convenience is improved. The shift amount a can be changed using several methods as shown below.

[0074] For example, the shift amount a can be changed by shifting the positions of the light source 1 and the incident optical system 6 in the main scanning direction, or by shifting the position of the rotation center of the polygon mirror 5 in a direction parallel to the optical axis of the imaging optical system 7. The above method is suitable when changing the number of deflection surfaces 5a of the polygon mirror 5, and also when changing the housing that holds each optical element and the polygon mirror 5. On the other hand, when using multiple polygon mirrors 5, each with a different number of deflection surfaces 5a, in a single housing, it is necessary to provide multiple positions for holding the light source 1, each optical element, and each polygon mirror 5 when using the above method. Therefore, the structure of the housing becomes complicated, which is undesirable.

[0075] Alternatively, for example, the aperture can be provided as a separate component from the housing, and the shift amount a can be changed by changing only the position of the aperture. In this method, even when multiple polygon mirrors 5 with different numbers of deflection surfaces 5a are used in a single housing, only multiple positions for holding the aperture are required, thus suppressing the complexity of the housing structure.

[0076] Alternatively, the shift amount a can be changed by shifting only the position of light source 1 in the main scanning direction. In this case, unlike the method described above, instead of changing the incident position of the light beam on the deflection surface 5a of the polygon mirror 5, the incident angle of the light beam on the deflection surface 5a of the polygon mirror 5 is changed.

[0077] In this case, if an aperture is placed between the light source and the collimator lens, the shift amount a can be changed without significantly changing the angle of incidence of the light beam to the deflection surface 5a of the polygon mirror 5, provided that the distance between the collimator lens and the polygon mirror 5 is sufficiently large. Furthermore, even when an aperture is positioned between the collimator lens and the polygon mirror 5, if the distance between the aperture and the polygon mirror 5 is sufficiently large, the shift amount a can be changed without significantly changing the angle of incidence of the light beam to the deflection surface 5a of the polygon mirror 5.

[0078] In particular, in a housing where the illumination position and focus of the light beam can be adjusted by three-dimensionally adjusting the position of light source 1, the position of light source 1 can be shifted without making any changes to the housing itself, making it the optimal method among the above methods. Therefore, in the optical scanning device 50 according to this embodiment, the shift amount a is changed by shifting the position of the light source 1 in the main scanning direction.

[0079] In other words, in the optical scanning device 50 according to this embodiment, when projected into the main scanning cross-section, the center of the light-emitting surface of the light source 1 is not on the optical axis of the incident optical system 6. In this context, the light-emitting surface of light source 1 is a surface that is perpendicular to the optical axis of the incident optical system 6 and contains all the light-emitting points. The center of this light-emitting surface can be defined as the position of a single light-emitting point, or the midpoint of the line segment connecting the two most distant light-emitting points.

[0080] On the other hand, the amount of shift in the position of light source 1 required to change the shift amount a may be several times larger than the amount of shift in the position of light source 1 during the conventional three-dimensional adjustment described above. Therefore, it is preferable that the housing of the optical scanning device 50 according to this embodiment is provided in such a way that the position of the light source 1 can be shifted by a sufficiently large amount. The position of the light source 1 in the housing of the optical scanning device 50 according to this embodiment can be shifted, for example, by providing a movable holding member for holding the light source 1, or by changing the position in which the light source 1 is fixed to the holding member by adhesive or the like.

[0081] Furthermore, in the optical scanning device 50 according to this embodiment, by making the positive scanning angle and the negative scanning angle, which cause the light beam of the polygon mirror 5 to start to be deflected, the following effects can be obtained. Generally, when a light beam is deflected by the polygon mirror 5, the amount of light in that beam decreases, causing unevenness in light intensity on the scanned surface 9.

[0082] Such unevenness in light intensity on the scanned surface 9 can be reduced by increasing the light output of the light source 1 as needed. In this case, if the incident position of the light beam on the polygon mirror 5 shifts according to the tolerance, the timing for increasing the light output of such light source 1 may be shifted.

[0083] Therefore, it is preferable to correct only the light intensity of the light beam scanning near the edge of the printing area, i.e., near the outermost image height, where changes in light intensity, i.e., correction unevenness, are not noticeable even if the timing of such light intensity correction is off. In other words, if the positive scanning angle and the negative scanning angle at which the light beam of the polygon mirror 5 begins to be kicked out differ significantly from each other, as in the conventional method, the light beam will begin to be kicked out on one side at the edge of the printing area, i.e., in the area far from the outermost image height, and it will be necessary to correct the amount of light.

[0084] Furthermore, if the timing of light intensity correction is off in areas away from the edges of the printed area, i.e., in the intermediate image height, correction inconsistencies will occur, resulting in a decrease in image quality. Specifically, in the optical scanning device 50 according to this embodiment, it is preferable that the following conditions (6) and (7) are satisfied. Y3 - Y1 ≤ 5.00 ···(6) Y4 - Y2 ≤ 5.00 ···(7)

[0085] In condition (6), Y1 is the distance [mm] in the main scanning direction between the image height closest to the on-axis image height among the image heights reached by the light beam deflected by the deflection surface 5a of the polygon mirror 5 on the negative side of the main scanning direction and the on-axis image height. In other words, Y1 is the distance between the image height that is closest to the axial image height among the at least one image height reached when only a portion of the light beam incident on the polygon mirror 5 in the main scanning cross-section on one side of the main scanning direction with respect to the on-axial image height is deflected by the deflection surface 5a, and the axial image height. To put it another way, Y1 is the distance between the image height closest to the on-axis image height on one side of the multiple image heights that only a portion of the light beam incident on the polygon mirror 5 reaches via the deflection surface 5a, and the on-axis image height. In condition (6), Y3 is the distance [mm] in the main scanning direction between the off-axis image height (second off-axis image height) and the on-axis image height on the negative side of the main scanning direction.

[0086] In condition (7), Y2 is the distance [mm] in the main scanning direction between the image height closest to the on-axis image height and the on-axis image height, which are reached by the light beam deflected by the deflection surface 5a of the polygon mirror 5 on the positive side of the main scanning direction. In other words, Y2 is the distance between the image height closest to the axial image height among at least one image height reached when only a portion of the light beam incident on the polygon mirror 5 in the main scanning cross-section is deflected by the deflection surface 5a on the other side of the main scanning direction relative to the on-axial image height, and the axial image height. To put it another way, Y2 is the distance between the on-axis image height and the on-axis image height, which is the closest image height on the other side of the on-axis image height among the multiple image heights that only a portion of the light beam incident on the polygon mirror 5 reaches via the deflection surface 5a.

[0087] In condition (7), Y4 is the distance [mm] in the main scanning direction between the off-axis image height (first off-axis image height) and the on-axis image height on the positive side of the main scanning direction. In conditions (6) and (7), the on-axis image height is defined as the origin of the coordinates on the scanned surface 9, the side where light source 1 is located is defined as the positive side of the main scanning direction, and the side where light source 1 is not located is defined as the negative side of the main scanning direction.

[0088] In the optical scanning device 50 according to this embodiment, if conditions (6) and (7) are satisfied and the resulting light intensity unevenness, i.e., density unevenness, is minor, the deterioration of image quality can be sufficiently suppressed. Furthermore, in the optical scanning device 50 according to this embodiment, if the following conditions (6a) and (7a) are satisfied, and the resulting light intensity unevenness, i.e., density unevenness, is minor, the deterioration of image quality can be suppressed more effectively. Y3 - Y1 ≤ 3.00 ···(6a) Y4 - Y2 ≤ 3.00 ···(7a)

[0089] Therefore, when increasing the number of deflection surfaces 5a while maintaining the inscribed circle diameter compared to polygon mirror 5', such as polygon mirror 5, it is sufficient to consider whether at least conditions (6) and (7) are satisfied.

[0090] Specifically, in the optical scanning device 50 according to this embodiment, the fθ coefficient of the imaging optical system 7 is 134 mm / radian, so the scanning angle by the polygon mirror 5 when scanning an image height of ±108.00 mm, which is the furthest off-axis image height, is ±23.089°. On the other hand, as mentioned above, the width of the deflection surface 5a of the polygon mirror 5 within the main scanning cross-section is small, at 10.275 mm.

[0091] Therefore, if the position of the light source 1 is not shifted, when the light beam reaching the region between the negative image height of -94.36 mm and the furthest off-axis image height of -108.00 mm on the negative side of the main scanning direction on the scanned surface 9 is deflected by the deflection surface 5a of the polygon mirror 5, a portion of it is blocked. As a result, when the light beam reaching the furthest off-axis image height of -108.00 mm is deflected by the deflection surface 5a of the polygon mirror 5, 13.3% of the incident light beam is kicked out, causing a decrease in light intensity.

[0092] Therefore, in the optical scanning device 50 according to this embodiment, the light source 1 is positioned with a shift of 0.278 mm in a direction perpendicular to the optical axis of the incident optical system 6 and away from the scanned surface 9. As a result, when the light beam reaching the region between the image height of 105.02 mm and the outermost image height of 108.00 mm on the positive side of the main scanning direction on the scanned surface 9 is deflected by the deflection surface 5a of the polygon mirror 5, a portion of it is kicked out.

[0093] Furthermore, when the light beam reaching the region between the negative image height of -105.04 mm and the furthest off-axis image height of -108.00 mm on the negative side of the main scanning direction on the scanned surface 9 is deflected by the deflection surface 5a of the polygon mirror 5, a portion of it will be kicked out. As a result, when the light beam reaching the furthest off-axis image height of 108.00 mm is deflected by the deflection surface 5a of the polygon mirror 5, 1.8% of the incident light beam is kicked out, causing a decrease in light intensity.

[0094] Furthermore, when the light beam reaching the furthest off-axis image height of -108.00 mm is deflected by the deflection surface 5a of the polygon mirror 5, 2.8% of the incident light beam is kicked out, resulting in a decrease in light intensity. In other words, in the optical scanning device 50 according to this embodiment, Y1=105.04, Y2=105.02, Y3=108.00, and Y4=108.00, so conditions (6), (6a), (7), and (7a) are satisfied.

[0095] This makes it possible to make the regions reached by the light beam that has been partially kicked out on both the positive and negative sides of the main scanning direction on the scanned surface 9 substantially the same. Furthermore, the percentage of light beam that is deflected by the deflection surface 5a of the polygon mirror 5 when it reaches the furthest off-axis image height of -108.00 mm can be reduced from 13.3% to 2.8%.

[0096] Furthermore, in the optical scanning device 50 according to this embodiment, since Y3-Y1=2.96mm and Y4-Y2=2.98mm, the area reached by the light beam that is partially kicked out on both the positive and negative sides of the main scanning direction on the scanned surface 9 can be sufficiently reduced to 5mm or less. As a result, even if density unevenness caused by vignetting or image streaks caused by timing discrepancies when correcting light intensity occur, they will not be noticeable, thus suppressing a decrease in image quality.

[0097] Furthermore, if the housing is formed so that the light source 1, each optical element, and the polygon mirror 5 are movable, the positions of the screw holes for fixing them may be too close together, or there may be limitations on the movable range of the light source 1. In this case, it becomes difficult to make the positive scanning angle and the negative scanning angle exactly the same as the scanning angle at which the light beam of the polygon mirror 5 begins to be deflected.

[0098] In that case, even if the positive scanning angle and the negative scanning angle at which the light beam of the polygon mirror 5 begins to be deflected are not exactly the same, it is sufficient for the effect to be achieved if they are similar to a certain extent. Specifically, the effects of this embodiment can be fully realized if the following condition (8) is satisfied.

number

[0099] If the upper limit of condition (8) is exceeded or the lower limit is exceeded, it becomes difficult to sufficiently reduce the percentage of light beam that is deflected by the deflection surface 5a of the polygon mirror 5 when the light beam reaching the off-axis image height of 108.00 mm or -108.00 mm is deflected. In addition, on one side of the scanning surface 9 in the main scanning direction, the area reached by the light beam that is partially kicked out widens towards the on-axial image height. As a result, if light intensity correction is not performed, density unevenness becomes more noticeable, leading to a decrease in image quality.

[0100] Furthermore, when correcting light intensity, if the correction timing is off, image streaks that are generated become more visible in the center of the scanning area, resulting in a decrease in image quality. In the optical scanning device 50 according to this embodiment, it is preferable that the following condition (8a) is satisfied instead of condition (8).

number

[0101] Furthermore, in the optical scanning device 50 according to this embodiment, it is more preferable that the following condition (8b) is satisfied instead of condition (8a).

number

[0102] As described above, when shifting the luminous beam in the main scanning direction by shifting the light source 1 in the main scanning direction, it is preferable to shift the light source 1 so that it moves away from the surface to be scanned 9. In other words, in the optical scanning device 50 according to this embodiment, it is preferable to shift the position of the light source 1 such that the center of the light-emitting surface of the light source 1 is positioned on the opposite side of the scanned surface 9 with respect to a cross section that includes the optical axis of the incident optical system 6 and is parallel to the sub-scanning direction. This is because, as shown in Figures 3(a) and (b) above, the light beam scanning the image height on the anti-light source side in the region where the rotation angle φ is negative is more susceptible to being deflected than the light beam scanning the image height on the light source side in the region where the rotation angle φ is positive.

[0103] In this case, if the light beam is incident so as to move away from the rotation center of the polygon mirror 5, the light beam scanning the image height on the anti-light source side will be less likely to be deflected. This can be seen from the fact that, as shown in Figures 3(a) and (b) above, changing the shift amount a in the polygon mirror 5 from 5 mm to 6 mm increases the region in which the light beam is not blocked in the region where the rotation angle φ is negative.

[0104] Then, by shifting only the position of the light source 1 so that it is further away from the scanning surface 9, the light beam incident on the anamorphic collimator lens 3 from the light source 1 will be emitted from the anamorphic collimator lens 3 in a direction that is relatively closer to the scanning surface 9. As a result, the light beam emitted from the anamorphic collimator lens 3 can be incident on the polygon mirror 5 at a position relatively far from the center of rotation.

[0105] On the other hand, if the position of the light source 1 is shifted as described above, the position at which the light beam deflected by the polygon mirror 5 enters the imaging optical system 7 will also shift. Furthermore, if the deflection surface 5a in the polygon mirror 5 is tilted in the sub-scanning direction, scan line curvature occurs on the scanned surface 9.

[0106] In other words, the distortion correction function in the optical scanning device 50 according to this embodiment is reduced. If the distortion correction function is sufficient, the coordinates in the sub-scanning direction of the light beam irradiation position at the farthest off-axis image height on the positive and negative sides of the main scanning direction on the scanned surface 9 are substantially the same as those in the sub-scanning direction of the light beam irradiation position at the on-axis image height.

[0107] Generally, such scan line curvature is reduced by making the illumination position of the light beam in the sub-scan direction the same at the off-axis image height corresponding to the largest scan angle on the positive side and the off-axis image height corresponding to the largest scan angle on the negative side. On the other hand, if the amount of shift in the incident position of the light beam on the polygon mirror 5 increases as described above, the illumination position of the light beam in the sub-scanning direction at the far-off image height on both sides will differ significantly from one another, resulting in a significant increase in scan line curvature.

[0108] Therefore, in the optical scanning device 50 according to this embodiment, it is preferable that the following condition (9) is satisfied when the normal of the deflection surface 5a of the polygon mirror 5 is deviated by an angle of 2' with respect to the main scanning cross-section within a cross-section parallel to the sub-scanning direction that includes the normal of the deflection surface 5a of the polygon mirror 5.

number

[0109] In condition (9), Z3 is the distance in the sub-scanning direction between the point where the light beam reaches at the negative off-axis image height in the main scanning direction and the optical axis of the imaging optical system 7. In condition (9), Z4 is the distance in the sub-scanning direction between the position where the light beam reaches at the farthest off-axis image height on the positive side in the main scanning direction and the optical axis of the imaging optical system 7. Furthermore, Z0 is the distance in the sub-scanning direction between the optical axis of the imaging optical system 7 and the destination position in the sub-scanning direction that is furthest away in the sub-scanning direction from the destination position at one of the furthest off-axis image heights corresponding to the larger of the two values ​​of Z3 and Z4 among the destination positions of the deflected light beam at each image height. Furthermore, in condition (9), Max(Z3,Z4) represents the larger of the two values ​​of Z3 and Z4.

[0110] If the upper limit of condition (9) is exceeded, the amount of scan line curvature becomes too large, causing variations in the density of scan lines depending on the image height, resulting in noticeable density unevenness. On the other hand, if the value falls below the lower limit of condition (9), the positive scanning angle and the negative scanning angle at which the light beam of the polygon mirror 5 begins to be kicked out are no longer the same, due to insufficient shift in the incident position of the light beam on the polygon mirror 5. As a result, the light beam begins to be kicked out in areas far from the edges of the printing area, i.e., at the intermediate image height, and the image quality deteriorates because the correction unevenness becomes noticeable when the timing of light intensity correction is off.

[0111] Figure 4 shows the scan lines formed on the scanned surface 9 in the optical scanning device 50 according to this embodiment, when the normal of the deflection surface 5a of the polygon mirror 5 is obscured by an angle of 2' with respect to the main scanning cross-section within a cross-section parallel to the sub-scanning direction that includes the normal of the deflection surface 5a of the polygon mirror 5. Specifically, the vertical axis in Figure 4 shows the coordinates of the light beam's arrival position on the scanned surface 9 in the sub-scanning direction, and the horizontal axis shows the image height.

[0112] As shown in Figure 4, in the optical scanning device 50 according to this embodiment, Z3 = -0.63 μm, Z4 = 1.77 μm, and Z0 = -1.21 μm (at an image height of -40 mm), and condition (9) is satisfied.

[0113] The above describes a method for increasing the number of deflection surfaces 5a of the polygon mirror 5 in an optical scanning device 50 that uses a light source having a single light-emitting point as the light source 1. Next, we consider a method for increasing the number of deflection surfaces of a polygon mirror in an optical scanning device that uses a light source with multiple light-emitting points.

[0114] First, consider the case where the multiple light-emitting points are optically located at the same position within the main scanning plane, but are separated from each other in the sub-scanning direction, for example, by being equally spaced apart. In this case, since the multiple light beams emitted from each light-emitting point travel along the same optical path within the main scanning cross-section, the same configuration as when a light source with a single light-emitting point is used can be applied.

[0115] On the other hand, if multiple light-emitting points are optically positioned at different locations within the main scanning cross-section, the positions of each light beam incident on the polygon mirror 5 within the main scanning cross-section will be different from each other. Therefore, in multiple light beams from multiple light-emitting points that reach the same predetermined image height, there will be light beams that are blocked by the deflection surface 5a of the polygon mirror 5 and light beams that are not blocked.

[0116] In this case, for each of the multiple light beams, the image height closest to the on-axis image height among the image heights reached by the light beams deflected by the deflection surface 5a of the polygon mirror 5 should be made to coincide between the positive and negative sides of the main scanning direction. In other words, we just need to make the minimum value of distance Y1 and the minimum value of distance Y2 for each of the multiple luminous beams coincide with each other.

[0117] This allows the image height at which light intensity correction is performed to be brought as close as possible to the edges of the scanning area, thereby suppressing the degradation of image quality due to uneven correction. Furthermore, the light beam that is blocked for each image height in the wide area on the positive side of the main scanning direction is blocked only for each image height in the narrow area on the negative side of the main scanning direction. On the other hand, the light beam that is blocked for each image height in a wide area on the negative side of the main scanning direction is blocked only for each image height in a narrow area on the positive side of the main scanning direction.

[0118] Therefore, it is sufficient that a predetermined amount of light beam that reaches the off-axis image height on the positive side of the main scanning direction and is not blocked by the deflection surface 5a of the polygon mirror 5 is blocked by the deflection surface 5a of the polygon mirror 5 when it reaches the off-axis image height on the negative side of the main scanning direction. This makes it easier to balance the image heights at which the image begins to be kicked off by the deflection surface 5a of the polygon mirror 5 on both the positive and negative sides of the main scanning direction, thereby suppressing the degradation of image quality due to correction unevenness.

[0119] Furthermore, the width of the light beam projected onto the deflection surface 5a of the light beam incident on the polygon mirror 5 is larger when scanning the negative image height in the main scanning direction compared to when scanning the positive image height in the main scanning direction. Therefore, the light beam reaching the positive image height in the main scanning direction is more easily blocked by the deflection surface 5a of the polygon mirror 5 compared to the light beam reaching the negative image height in the main scanning direction.

[0120] On the other hand, as described above, the light beam that reaches the negative image height in the main scanning direction has a large beam width when projected within the deflection surface 5a, so the effect of vignetting is small. Therefore, in the optical scanning device 50 according to this embodiment, it is preferable that the following condition (10) is satisfied. 1 ≤ n1 ≤ n2 ···(10)

[0121] In condition (10), n1 is the number of luminous beams that reach the off-axis image height on the positive side of the main scanning direction as they are deflected by the deflection surface 5a of the polygon mirror 5. In condition (10), n2 is the number of luminous beams that reach the off-axis image height on the negative side of the main scanning direction as they are deflected by the deflection surface 5a of the polygon mirror 5.

[0122] If, different from the conditional expression (10), n2 < n1 is satisfied, the decrease in the amount of light at the outermost image height on the plus side in the main scanning direction becomes larger than the decrease in the amount of light at the outermost image height on the minus side in the main scanning direction. Therefore, it is necessary to increase the correction amount of the light amount. Also, if, different from the conditional expression (10), n1 < 1 is satisfied, any light beam reaching the outermost image height on the plus side in the main scanning direction is not kicked when being deflected by the deflection surface 5a of the polygon mirror 5, so the decrease in the amount of light on the scanned surface becomes unbalanced. In the optical scanning device 50 according to the present embodiment, since n1 = 1 and n2 = 1, the conditional expression (10) is satisfied.

[0123] Also, in the optical scanning device 50 according to the present embodiment, it is preferable that the following conditional expression (11) is satisfied.

Equation

[0124] In the conditional expression (11), N is the number of deflection surfaces 5a of the polygon mirror 5, and θ is the angle [°] in the main scanning cross section between the optical axis of the incident optical system 6 and the optical axis of the imaging optical system 7. If it exceeds the upper limit value of the conditional expression (11), the light beam incident on the polygon mirror 5 is easily kicked, so in a region away from the end of the printing region, that is, at the intermediate image height, the light beam starts to be kicked. On the other hand, if it is below the lower limit value of the conditional expression (11), the incident optical system 6 and a BD (Beam Detection) optical system (not shown) get too close to each other, making the arrangement of both difficult.

[0125] In the optical scanning device 50 according to the present embodiment, since θ = 87.5° and N = 5, the conditional expression (11) is satisfied. Also, in the optical scanning device 50 according to the present embodiment, it is preferable that the following conditional expression (12) is satisfied.

Equation

[0126] In condition (12), L is the distance [mm] along the optical axis of the incident optical system 6 between the light-emitting surface of the light source 1 and the on-axial deflection point on the deflection surface 5a of the polygon mirror 5, and W is the distance between the two off-axis image heights on the scanned surface 9, i.e., Y3 + Y4. If the upper limit of condition (12) is exceeded, the size of the incident optical system 6 increases, which in turn makes the optical scanning device 50 according to this embodiment larger, making it difficult to secure space in the image forming apparatus to mount the optical scanning device 50 according to this embodiment.

[0127] On the other hand, if the value falls below the lower limit of condition (12), it becomes difficult to sufficiently shift the light beam in the main scanning direction even if the light source 1 is shifted in the main scanning direction as described above. In the optical scanning device 50 according to this embodiment, since L=84.4, Y3=108.00, and Y4=108.00, condition (12) is satisfied.

[0128] As described above, in the optical scanning device 50 according to this embodiment, the light source 1 is positioned with a shift of 0.278 mm in a direction perpendicular to the optical axis of the incident optical system 6 and away from the scanned surface 9. This is larger than the shift of the light source 1 of several tens of micrometers that occurs during normal adjustments for the arrival position of each light beam on the scanned surface 9.

[0129] In the optical scanning device 50 according to this embodiment, in order to shift the light source 1 significantly in this manner, for example, a holding member that holds the light source 1 may be provided in the housing so as to be movable. Furthermore, in the optical scanning device 50 according to this embodiment, the light source 1 may be fixed in place by adhesive, taking into consideration the above-mentioned shift position.

[0130] When using a polygon mirror 5 with a small width within the main scanning cross-section of the deflection surface 5a, as in the optical scanning device 50 according to this embodiment, it is required to shift the light source 1 by a shift amount that is about an order of magnitude larger than normal adjustment. On the other hand, the optical scanning device 50 according to this embodiment is configured to allow the light source 1 to be shifted by a large amount, so a polygon mirror 5' having four deflection surfaces 5a can also be used. In addition, when using only the polygon mirror 5 having five deflection surfaces 5a in the optical scanning device 50 according to this embodiment, the area in which the light source 1 can be placed may be shifted to take into consideration the above-mentioned shift amount.

[0131] As described above, in the optical scanning device 50 according to this embodiment, by adjusting the small polygon mirror 5 so that the light beam is appropriately deflected so as to satisfy the above condition (8), it is possible to suppress the deterioration of image quality due to density unevenness that occurs on the scanned surface 9.

[0132] [Second Embodiment] Figures 5(a) and 5(b) show a schematic main scanning cross-sectional view and a partially schematic sub-scanning cross-sectional view of the optical scanning apparatus 60 according to the second embodiment, respectively. In this embodiment, the optical scanning device 60 has the same configuration as the optical scanning device 50 according to the first embodiment, except that a light source 11 is provided instead of light source 1. Therefore, the same reference numerals are used for the same components, and their descriptions are omitted.

[0133] Specifically, the light source 11 provided in the optical scanning device 60 according to this embodiment has two light-emitting points. The two light-emitting points are positioned 90 μm apart from each other on a straight line in a direction parallel to the optical axis of the incident optical system 6, rotated 5.4° counterclockwise with respect to the main scanning cross-section when viewed from the polygon mirror 5 side.

[0134] This makes it possible to make the spacing between each scan line formed on the scanned surface 9 by the two light-emitting points uniform. Furthermore, in accordance with the arrangement of the two light-emitting points described above, the amount of shift of the light source 11 in the direction perpendicular to the optical axis of the incident optical system 6 and away from the scanned surface 9 is changed to 0.26 mm compared to the optical scanning device 50 according to the first embodiment.

[0135] Furthermore, in the optical scanning device 60 according to this embodiment, the width of the rectangular opening formed in the main scanning aperture 4 in the main scanning direction is somewhat smaller, at 2.8 mm, compared to the optical scanning device 50 according to the first embodiment. In the optical scanning device 60 according to this embodiment, the two light beams emitted from each of the two light-emitting points of the light source 11 pass through a common sub-scanning aperture 2 and a main scanning aperture 4. Therefore, the incident positions of the two light beams upon the polygon mirror 5 are different in the main scanning direction.

[0136] Next, Table 6 below shows the various values ​​such as the radius of curvature, interplanar spacing, and refractive index of each optical element provided in the optical scanning device 60 according to this embodiment, within the main scanning cross-section and the sub-scanning cross-section.

[0137] [Table 6]

[0138] Furthermore, the aspherical coefficients of the incident and exit surfaces of the first imaging lens 7a and the second imaging lens 7b, respectively, provided in the optical scanning device 60 according to this embodiment, are shown in Table 7 below.

[0139] [Table 7]

[0140] The aspherical shapes of the incident and exit surfaces of the first imaging lens 7a and the second imaging lens 7b, respectively, provided in the optical scanning device 60 according to this embodiment, are represented by the above equations (1) to (3). Furthermore, the phase function φ of the diffraction plane formed on the incident surface of the anamorphic collimator lens 3 provided in the optical scanning device 60 according to this embodiment is expressed by the above equation (4), and the phase coefficients C and E are shown in Table 8 below.

[0141] [Table 8]

[0142] Furthermore, the coordinates of the vertices of each optical surface and the angles of the surface normals in the optical scanning device 60 according to this embodiment are shown in Table 9 below.

[0143] [Table 9]

[0144] Note that the angles of the surface normals of the first imaging lens 7a and the second imaging lens 7b shown in Table 9 do not take into account the aspherical shape defined by the aspherical coefficient shown in Table 7. Furthermore, regarding the position of the light source 11 shown in Table 9, it is taken into consideration that it is positioned with a shift of 0.26 mm in a direction perpendicular to the optical axis of the incident optical system 6 and away from the scanned surface 9. Furthermore, the position of the light source 11 shown in Table 9 is indicated as the midpoint of the positions of the two light-emitting points.

[0145] Here, of the two light-emitting points of the light source 11, the one that is further away from the scanned surface 9 in a direction perpendicular to the optical axis of the incident optical system 6 will be referred to as the first light-emitting point 11a, and the other light-emitting point will be referred to as the second light-emitting point 11b. The light beam emitted from the first light-emitting point 11a is referred to as the first light beam, and the light beam emitted from the second light-emitting point 11b is referred to as the second light beam.

[0146] In the optical scanning device 60 according to this embodiment, the fθ coefficient of the imaging optical system 7 is 134 mm / radian, so the scanning angle by the polygon mirror 5 when scanning an image height of ±108.00 mm, which is the furthest off-axis image height, is ±23.089°. On the other hand, the width of the deflection surface 5a of the polygon mirror 5 within the main scanning cross-section is small, at 10.275 mm.

[0147] Therefore, if the position of the light source 11 is not shifted, when the first light beam that reaches the region between the image height of -99.03 mm and the outermost image height of -108.00 mm on the negative side of the main scanning direction on the scanned surface 9 is deflected by the deflection surface 5a, a portion of it is kicked out. Furthermore, if the position of the light source 11 is not shifted, a portion of the second light beam reaching the region between the negative image height of -95.14 mm and the furthest off-axis image height of -108.00 mm on the negative side of the main scanning direction on the scanned surface 9 will be deflected by the deflection surface 5a.

[0148] In other words, when scanning the outermost image height of -108.00 mm on the negative side of the main scanning direction on the scanned surface 9, the second light beam is deflected more than the first light beam when deflected by the deflection surface 5a. Specifically, when the second beam of light, which reaches an off-axis image height of -108.00 mm, is deflected by the deflection surface 5a of the polygon mirror 5, 13.2% of the incident light beam is kicked out, resulting in a decrease in light intensity.

[0149] Therefore, in the optical scanning device 60 according to this embodiment, the light source 11 is positioned with a shift of 0.26 mm in a direction perpendicular to the optical axis of the incident optical system 6 and away from the scanned surface 9. As a result, the first light beam that reaches any image height on the negative side of the main scanning direction on the scanned surface 9 will not be blocked when deflected by the deflection surface 5a. On the other hand, when the first light beam that reaches the region between the image height of 106.07 mm and the outermost image height of 108.00 mm on the positive side of the main scanning direction on the scanned surface 9 is deflected by the deflection surface 5a, a portion of it is kicked out.

[0150] Furthermore, the second light beam that reaches any image height on the positive side of the main scanning direction on the scanned surface 9 will not be blocked when deflected by the deflection surface 5a. On the other hand, when the second light beam that reaches the region between the negative image height of -106.55 mm and the furthest off-axis image height of -108.00 mm on the negative side of the main scanning direction on the scanned surface 9 is deflected by the deflection surface 5a, a portion of it is kicked out.

[0151] In other words, in the optical scanning device 60 according to this embodiment, when the normal of the deflection surface 5a forms a predetermined angle (first angle) with respect to the optical axis of the imaging optical system 7 within the main scanning cross-section, only a portion of the first light beam incident on the polygon mirror 5 is deflected by the deflection surface 5a. Then, a portion of the deflected first light beam reaches the first off-axis image height on one side. On the other hand, when the deflection surface 5a is at the predetermined angle within the main scanning plane, all of the second light beam incident on the polygon mirror 5 is deflected by the deflection surface 5a and reaches the first off-axis image height.

[0152] Furthermore, within the main scanning plane, when the normal to the deflection plane 5a forms another predetermined angle (second angle) with respect to the optical axis of the imaging optical system 7, all of the first light beam incident on the polygon mirror 5 is deflected by the deflection plane 5a and reaches the second off-axis image height on the other side. On the other hand, when the deflection surface 5a is at another predetermined angle within the main scanning plane, only a portion of the second light beam incident on the polygon mirror 5 is deflected by the deflection surface 5a and reaches the second off-axis image height.

[0153] As a result, when the first beam of light reaching the furthest off-axis image height of 108.00 mm is deflected by the deflection surface 5a of the polygon mirror 5, 1.2% of the incident beam of light is kicked out, causing a decrease in light intensity. Furthermore, when the second beam of light, which reaches an off-axis image height of -108.00 mm, is deflected by the deflection surface 5a of the polygon mirror 5, 1.5% of the incident light beam is kicked out, resulting in a decrease in light intensity.

[0154] This makes it possible to make the region reached by the first light beam, which is partially blocked on the positive side of the main scanning direction on the scanned surface 9, and the region reached by the second light beam, which is partially blocked on the negative side of the main scanning direction on the scanned surface 9, substantially the same. Furthermore, the percentage of stray light when the first luminous beam reaching an off-axis image height of 108.00 mm and the second luminous beam reaching an off-axis image height of -108.00 mm are deflected by the deflection surface 5a of the polygon mirror 5 can be reduced to 1.2% and 1.5%, respectively.

[0155] In the optical scanning device 60 according to this embodiment, it is preferable that the above-mentioned conditions (6) and (7) are satisfied, and it is more preferable that conditions (6a) and (7a) are satisfied. Y1 is defined as the distance [mm] in the main scanning direction between the image height closest to the on-axis image height and the on-axis image height, respectively, of the image heights reached by the first and second light beams, which are deflected by the deflection surface 5a of the polygon mirror 5 on the negative side of the main scanning direction. Furthermore, Y2 is defined as the distance [mm] in the main scanning direction between the image height closest to the on-axis image height and the on-axis image height, respectively, of the image heights reached by the first and second light beams, which are deflected by the deflection surface 5a of the polygon mirror 5 on the positive side of the main scanning direction.

[0156] In other words, in the optical scanning device 60 according to this embodiment, Y1=106.55, Y2=106.07, Y3=108.00, and Y4=108.00, so conditions (6), (6a), (7), and (7a) are satisfied. Therefore, the area reached by the light beam that is partially kicked out on both the positive and negative sides of the main scanning direction on the scanned surface 9 can be sufficiently reduced to 5 mm or less. As a result, even if density unevenness caused by vignetting or image streaks caused by timing discrepancies when correcting light intensity occur, they will not be noticeable, thus suppressing a decrease in image quality.

[0157] Furthermore, in the optical scanning device 60 according to this embodiment, the above condition (8) is satisfied, it is preferable that the above condition (8a) is satisfied, and it is more preferable that the above condition (8b) is satisfied. In the optical scanning device 60 according to this embodiment, Y1 = 106.55 and Y2 = 106.07, so conditions (8), (8a), and (8b) are satisfied.

[0158] Furthermore, in the optical scanning device 60 according to this embodiment, it is preferable that the above condition (9) is satisfied for both the first luminous beam and the second luminous beam. Figure 6 shows the scan lines formed on the scanned surface 9 in the optical scanning device 60 according to this embodiment, when the normal of the deflection surface 5a of the polygon mirror 5 is obscured by an angle of 2' with respect to the main scanning cross-section within a cross-section parallel to the sub-scanning direction that includes the normal of the deflection surface 5a of the polygon mirror 5.

[0159] Specifically, the vertical axis in Figure 6 shows the coordinates of the light beam's arrival position on the scanned surface 9 in the sub-scanning direction, and the horizontal axis shows the image height. In reality, the scan lines produced by the first and second light beams are offset by approximately one pixel, but this offset is removed in Figure 6.

[0160] As shown in Figure 6, in the optical scanning device 60 according to this embodiment, the first luminous beam has Z3 = -0.69 μm, Z4 = 1.26 μm, and Z0 = -1.67 μm (at an image height of -50 mm), and condition (9) is satisfied. Furthermore, for the second luminous beam, Z3 = -0.39 μm, Z4 = 2.30 μm, and Z0 = -2.32 μm (at an image height of -40 mm), so condition (9) is satisfied.

[0161] Furthermore, in the optical scanning device 60 according to this embodiment, it is preferable that the above condition (10) is satisfied. As described above, in the optical scanning device 60 according to this embodiment, the light source 11 has a first light-emitting point 11a and a second light-emitting point 11b.

[0162] Then, the first light beam emitted from the first light-emitting point 11a is deflected by the deflection surface 5a to scan the area near the outermost image height on the positive side of the main scanning direction on the scanned surface 9, and a portion of it is kicked out. On the other hand, the first luminous beam is not blocked even when deflected by the deflection surface 5a to scan any image height on the negative side of the main scanning direction on the scanned surface 9.

[0163] Furthermore, when the second light beam emitted from the second light-emitting point 11b is deflected by the deflection surface 5a to scan the area near the furthest off-axis image height on the negative side of the main scanning direction on the scanned surface 9, a portion of it is kicked out. On the other hand, the second luminous beam is not blocked even when deflected by the deflection surface 5a to scan any image height on the positive side of the main scanning direction on the scanned surface 9.

[0164] This makes it possible to make the region reached by the first light beam, which is partially blocked on the positive side of the main scanning direction on the scanned surface 9, and the region reached by the second light beam, which is partially blocked on the negative side of the main scanning direction on the scanned surface 9, substantially the same. As a result, the amount of light beam deflection on both the positive and negative sides of the main scanning direction on the scanned surface 9 can be reduced. In other words, in the optical scanning device 60 according to this embodiment, since n1=1 and n2=1, condition (10) is satisfied.

[0165] In this embodiment, the optical scanning device 60 uses a light source 11 having two light-emitting points, but it is not limited to this, and a light source having four light-emitting points may be used to increase the resolution of the image. For example, if a light source is used that has four light-emitting points spaced 30 μm apart on a straight line rotated 5.4° counterclockwise with respect to the main scanning cross-section when viewed from the polygon mirror 5 side, parallel to the optical axis of the incident optical system 6, the resolution in the sub-scanning direction will be doubled.

[0166] Here, in a light source having four light-emitting points, the light-emitting point furthest from the scanned surface 9 in a direction perpendicular to the optical axis of the incident optical system 6 is referred to as the first light-emitting point, and the light-emitting point closest to the scanned surface 9 is referred to as the second light-emitting point. Furthermore, on the straight line in which the four light-emitting points are located, the light-emitting point adjacent to the first light-emitting point is referred to as the third light-emitting point, and the light-emitting point adjacent to the second light-emitting point is referred to as the fourth light-emitting point.

[0167] Furthermore, the light beams emitted from the first, second, third, and fourth light-emitting points are referred to as the first, second, third, and fourth light beams, respectively. Then, the light source having the four light-emitting points described above is positioned by shifting it by 0.26 mm in a direction perpendicular to the optical axis of the incident optical system 6 and away from the scanned surface 9, similar to the light source 11.

[0168] At this time, when only the first of the first to fourth luminous beams is deflected by the deflection surface 5a to scan the outermost image height on the positive side of the main scanning direction on the scanned surface 9, a portion of it is kicked out. On the other hand, when the second and fourth light beams of the first to fourth light beams are deflected by the deflection surface 5a to scan the outermost image height on the negative side of the main scanning direction on the scanned surface 9, a portion of them is kicked out.

[0169] Specifically, when the fourth beam of light is deflected by the deflection surface 5a to reach the region between the negative image height of -107.88 mm and the furthest off-axis image height of -108.00 mm on the negative side of the main scanning direction on the scanned surface 9, a portion of it is kicked out. Then, when the fourth beam is deflected by the deflection surface 5a to reach the off-axis image height of -108.00 mm on the negative side of the main scanning direction on the scanned surface 9, 0.1% of the incident beam is kicked out.

[0170] In other words, even when a light source having four light-emitting points as described above is used instead of the light source 11 in the optical scanning device 60 according to this embodiment, condition (10) is satisfied because n1=1 and n2=2. This makes it possible to make the region reached by the first light beam, which is partially blocked on the positive side of the main scanning direction on the scanned surface 9, and the region reached by the second light beam, which is partially blocked on the negative side of the main scanning direction on the scanned surface 9, substantially the same. As a result, the amount of light beam deflection on both the positive and negative sides of the main scanning direction on the scanned surface 9 can be reduced.

[0171] Furthermore, in the optical scanning device 60 according to this embodiment, it is preferable that the above condition (11) is satisfied, and since θ = 87.5° and N = 5, condition (11) is satisfied. Furthermore, in the optical scanning device 60 according to this embodiment, it is preferable that the above condition (12) is satisfied, and since L=84.4, Y3=108.00 and Y4=108.00, condition (12) is satisfied.

[0172] As described above, in the optical scanning device 60 according to this embodiment, by adjusting the small polygon mirror 5 so that the light beam is appropriately deflected so as to satisfy the above condition (8), it is possible to suppress the deterioration of image quality due to density unevenness that occurs on the scanned surface 9.

[0173] The values ​​of each conditional expression in the optical scanning apparatus according to the first and second embodiments are shown in Table 10 below.

[0174] [Table 10]

[0175] [Monochrome Image Forming Apparatus] Figure 7(a) shows a schematic sub-scanning cross-sectional view of a monochrome image forming apparatus 104 equipped with an optical scanning device according to the first or second embodiment.

[0176] The monochrome image forming apparatus 104 receives code data Dc output from an external device 117, such as a personal computer. The input code data Dc is then converted into image data (dot data) Di by the printer controller 111 in the monochrome image forming apparatus 104.

[0177] Next, the image data Di is input to the optical scanning unit 100, which is an optical scanning device according to the first or second embodiment. Then, a light beam 103 modulated according to the image data Di is emitted from the optical scanning unit 100, and the photosensitive surface of the photosensitive drum 101 is scanned in the main scanning direction by the emitted light beam 103.

[0178] Furthermore, the photosensitive drum 101, which is an electrostatic latent image carrier (photoreceptor), is rotated clockwise by the motor 115. As the photosensitive drum 101 rotates in this manner, the photosensitive surface of the photosensitive drum 101 moves in the sub-scanning direction relative to the light beam 103.

[0179] Above the photosensitive drum 101, a charging roller 102 is provided so as to contact the photosensitive surface of the photosensitive drum 101, which uniformly charges the photosensitive surface of the drum 101. Then, a light beam 103 scanned by the light scanning unit 100 is irradiated onto the photosensitive surface of the photosensitive drum 101, which has been charged by the charging roller 102.

[0180] As described above, the light beam 103 is modulated based on the image data Di, and an electrostatic latent image is formed on the photosensitive surface of the photosensitive drum 101 by irradiating it with the modulated light beam 103. The formed electrostatic latent image is then developed as a toner image by a developer unit 107, which is positioned further downstream in the rotational direction of the photosensitive drum 101 than the irradiation position of the light beam 103, and is in contact with the photosensitive drum 101.

[0181] The toner image developed by the developing unit 107 is transferred onto the paper 112, which is the transfer material, by a transfer roller 108 that is positioned below the photosensitive drum 101 and facing the photosensitive drum 101. The paper 112 is stored in the paper cassette 109 located in front of the photosensitive drum 101 (on the right side in Figure 7(a)), but it can also be fed manually.

[0182] A paper feed roller 110 is provided at the end of the paper cassette 109, and the paper 112 inside the paper cassette 109 is fed into the transport path by the paper feed roller 110. The paper 112, onto which the unfixed toner image has been transferred as described above, is then transported to the fuser 150 located behind the photosensitive drum 101 (on the left side in Figure 7(a)).

[0183] The fuser 150 is formed by a fuser roller 113 having a fuser heater (not shown) inside, and a pressure roller 114 disposed so as to be in contact with the fuser roller 113. Then, the paper 112 that has been transported from the transfer roller 108 is heated while being pressurized at the contact point between the fixing roller 113 and the pressure roller 114, thereby fixing the unfixed toner image on the paper 112.

[0184] Furthermore, a paper discharge roller 116 is located behind the fuser 150, and the fixed paper 112 is discharged to the outside of the monochrome image forming apparatus 104 by the paper discharge roller 116. Although not shown in Figure 7(a), the printer controller 111 not only performs the data conversion described above, but also controls various components within the monochrome image forming apparatus 104, such as the motor 115, and the polygon motor within the optical scanning unit 100.

[0185] The recording density of the monochrome image forming apparatus 104 is not particularly limited, but higher image quality is required as the recording density increases. Therefore, the above configuration of the optical scanning unit 100, which is an optical scanning device according to the first or second embodiment, is effective when the recording density of the monochrome image forming apparatus 104 is 1200 dpi or higher.

[0186] [Color Image Forming Apparatus] FIG. 7(b) shows a schematic partial sub-scanning cross-sectional view of a color image forming apparatus 260 including the optical scanning device according to the first or second embodiment.

[0187] The color image forming apparatus 260 is a tandem type color image forming apparatus in which four optical scanning devices according to the first or second embodiment record image information on the photosensitive surfaces of four photosensitive drums, which are image carriers, in parallel. The color image forming apparatus 260 includes optical scanning devices 211, 212, 213, and 214 according to the first or second embodiment, and photosensitive drums 221, 222, 223, and 224. The color image forming apparatus 260 also includes developing devices 231, 232, 233, and 234, a conveyance belt 251, a printer controller 253, and a fixing device 254.

[0188] As shown in FIG. 7(b), color signals of R (red), G (green), and B (blue) output from an external device 252 such as a personal computer are input to the color image forming apparatus 260. The input color signals are converted by a printer controller 253 provided in the color image forming apparatus 260 into respective image data (dot data) of C (cyan), M (magenta), Y (yellow), and K (black).

[0189] Next, the image data is input to the optical scanning devices 211, 212, 213, and 214, and modulated light beams 241, 242, 243, and 244 are emitted from the optical scanning devices 211, 212, 213, and 214 according to the respective image data. The photosensitive surfaces of the photosensitive drums 221, 222, 223, and 224 are scanned in the main scanning direction by the emitted light beams 241, 242, 243, and 244.

[0190] A charging roller (not shown) for uniformly charging the photosensitive surfaces of the photosensitive drums 221, 222, 223, and 224 is provided so as to contact the photosensitive surfaces. Then, the photosensitive surfaces of the photosensitive drums 221, 222, 223, and 224, which have been charged by the charging roller, are irradiated with light beams 241, 242, 243, and 244 by the light scanning devices 211, 212, 213, and 214.

[0191] As described above, the light beams 241, 242, 243, and 244 are modulated based on image data of each color, and by irradiating with the light beams 241, 242, 243, and 244, electrostatic latent images are formed on the photosensitive surfaces of the photosensitive drums 221, 222, 223, and 224. The formed electrostatic latent image is then developed as a toner image by developing units 231, 232, 233, and 234, which are arranged to contact the photosensitive drums 221, 222, 223, and 224.

[0192] Next, the toner image developed by the developing units 231 to 234 is transferred in multiple layers onto a sheet of paper (transfer material) (not shown) that is transported on a transport belt 251 by a transfer roller (transfer unit) (not shown) positioned opposite the photosensitive drums 221 to 224. This forms a single full-color image. The paper onto which the unfixed toner image has been transferred is then transported to the fuser 254 located behind the photosensitive drums 221, 222, 223, and 224 (on the left side in Figure 7(b)).

[0193] The fuser 254 is formed by a fuser roller having a fuser heater (not shown) inside and a pressure roller disposed to press against the fuser roller. Then, the paper transported from the transfer roller is heated while being pressurized by the contact area between the fixing roller and the pressure roller, thereby fixing the unfixed toner image on the paper.

[0194] Furthermore, a paper discharge roller (not shown) is located behind the fuser 254, and this paper discharge roller ejects the fixed paper to the outside of the color image forming apparatus 260. Optical scanning devices 211, 212, 213, and 214 correspond to the colors C (cyan), M (magenta), Y (yellow), and K (black).

[0195] The optical scanning devices 211, 212, 213, and 214 each record image signals (image information) on the photosensitive surfaces of the photosensitive drums 221, 222, 223, and 224 in parallel, thereby printing color images at high speed. As the external device 252, for example, a color image reading device equipped with a CCD sensor may be used.

[0196] In this case, a color digital copier is formed by the color image reading device and the color image forming device 260. Although preferred embodiments have been described above, the invention is not limited to these embodiments, and various modifications and changes are possible within the scope of its essence.

[0197] This embodiment includes the following configuration. (Configuration 1) An optical scanning device comprising a deflector that deflects a light beam from a light source to scan the surface to be scanned in the main scanning direction, and a first optical system that guides the light beam deflected by the deflector to the surface to be scanned, wherein in the main scanning cross-section, the width of the light beam immediately before it enters the deflector is smaller than the width of the deflection surface of the deflector, and in the main scanning cross-section, only a portion of the light beam that enters the deflector reaches multiple image heights on the surface to be scanned via the deflection surface, and when Y1 (mm) is the distance between the image height closest to the axial image height on one side of the multiple image heights and the axial image height, and Y2 (mm) is the distance between the image height closest to the axial image height on the other side of the multiple image heights and the axial image height, the condition -0.005 ≤ (Y2 - Y1) / (Y2 + Y1) ≤ 0.005 is satisfied. (Configuration 2) An optical scanning device comprising a deflector that deflects a light beam from a light source to scan a surface to be scanned in the main scanning direction, and a first optical system that guides the light beam deflected by the deflector to the surface to be scanned, wherein in the main scanning cross-section, the width of the light beam immediately before it enters the deflection surface of the deflector is smaller than the width of the deflection surface, and in the main scanning cross-section, when the normal to the deflection surface forms a predetermined angle with respect to the optical axis of the first optical system, only a portion of the light beam entering the deflector is deflected by the deflection surface and reaches the surface to be scanned. (Configuration 3) An optical scanning device according to Configuration 2, characterized in that when Y1 (mm) is the distance between the on-axis image height and the image height closest to the on-axis image height on one side of the multiple image heights to which only a portion of the light beam incident on the deflector reaches via the deflection surface, and Y2 (mm) is the distance between the on-axis image height and the image height closest to the on-axis image height on the other side of the multiple image heights to the on-axis image height, the condition -0.005 ≤ (Y2 - Y1) / (Y2 + Y1) ≤ 0.005 is satisfied. (Configuration 4) An optical scanning apparatus according to Configuration 1 or 3, characterized in that when the distance between the first off-axis image height and the on-axis image height on one side is Y3 (mm) and the distance between the second off-axis image height and the on-axis image height on the other side is Y4 (mm), the conditions Y3 - Y1 ≤ 5.00 and Y4 - Y2 ≤ 5.00 are satisfied. (Configuration 5) An optical scanning device according to any one of Configurations 1 to 4, comprising a second optical system that causes a light beam from a light source to be incident on a deflector, wherein when projected onto the main scanning cross-section, the center of the light-emitting surface of the light source is not on the optical axis of the second optical system. (Configuration 6) The optical scanning apparatus according to Configuration 5, characterized in that the center of the light-emitting surface is located on the opposite side of the surface to be scanned with respect to a cross section parallel to the sub-scanning direction that includes the optical axis of the second optical system. (Configuration 7) Let Z3 be the distance in the sub-scanning direction between the reaching position at the first off-axis image height on one side of the light beam deflected by the deflection plane and the optical axis of the first optical system, let Z4 be the distance in the sub-scanning direction between the reaching position at the second off-axis image height on the other side of the light beam deflected by the deflection plane and the optical axis of the first optical system, and let Z0 be the distance in the sub-scanning direction between the reaching position that is furthest away in the sub-scanning direction from the reaching position at one of the first and second off-axis image heights corresponding to the larger of Z3 and Z4, and the optical axis of the first optical system, and when the angle that the normal makes with respect to the main scanning cross-section in a cross-section parallel to the sub-scanning direction that includes the normal of the deflection plane, 0.30≦(Z3+Z4) / {Z0+Max(Z3,Z4)}≦0.82 An optical scanning apparatus according to any one of configurations 1 to 6, characterized in that it satisfies the following conditions. (Configuration 8) An optical scanning device according to any one of Configurations 1 to 7, characterized in that the number of light-emitting points included in the light source is one. (Configuration 9) An optical scanning apparatus according to any one of Configurations 1 to 7, characterized in that, in the main scanning cross-section, when the normal of the deflection plane forms a first angle with respect to the optical axis of the first optical system, only a portion of the first luminous beam from the first luminous beam incident on the deflector is deflected by the deflection plane and reaches a first off-axis image height on one side, while the entire second luminous beam from the second luminous beam incident on the deflector is deflected by the deflection plane and reaches a first off-axis image height, and in the main scanning cross-section, when the normal of the deflection plane forms a second angle with respect to the optical axis of the first optical system, the entire first luminous beam incident on the deflector is deflected by the deflection plane and reaches a second off-axis image height on the other side, while only a portion of the second luminous beam incident on the deflector is deflected by the deflection plane and reaches a second off-axis image height. (Configuration 10) An optical scanning apparatus according to Configuration 9, wherein the light source is located on one side, and in the main scanning cross-section, when the normal to the deflection plane is at a first angle, the number of luminous beams that reach a first off-axis image height is n1, where only a portion of the multiple luminous beams from multiple light-emitting points incident on the deflector are deflected by the deflection plane, and the number of luminous beams that reach a first off-axis image height is n1, and in the main scanning cross-section, when the normal to the deflection plane is at a second angle, the number of luminous beams that reach a second off-axis image height is n2, where only a portion of the multiple luminous beams incident on the deflector are deflected by the deflection plane, and the condition 1 ≤ n1 ≤ n2 is satisfied. (Configuration 11) An optical scanning device according to any one of Configurations 1 to 10, comprising a second optical system that causes a light beam from a light source to be incident on a deflector, wherein when the number of deflection surfaces is N and the angle between the optical axis of the first optical system and the optical axis of the second optical system in the main scanning cross-section is θ(°), the device satisfies the condition (720 / N) / (N-1)+45 < θ < (720 / N) / (N-1)+60. (Configuration 12) An optical scanning device according to any one of Configurations 1 to 11, comprising a second optical system that causes a light beam from a light source to be incident on a deflector, wherein the distance on the optical axis of the second optical system between the light-emitting surface of the light source and the on-axis deflection point on the deflection surface is L, and the distance between the first off-axis image height on one side and the second off-axis image height on the other side is W, and the condition 0.30 ≤ L / W ≤ 0.70 is satisfied. (Configuration 13) An optical scanning device according to any one of Configurations 1 to 12, characterized in that the deflector is a polyhedron that rotates around a rotation axis. (Configuration 14) An image forming apparatus comprising: an optical scanning device as described in any one of Configurations 1 to 13; a developer that develops an electrostatic latent image formed on a surface to be scanned by the optical scanning device as a toner image; a transfer device that transfers the developed toner image to a transfer material; and a fuser that fixes the transferred toner image to the transfer material. (Configuration 15) An image forming apparatus comprising an optical scanning device described in any one of Configurations 1 to 13, and a printer controller that converts a signal output from an external device into image data and inputs it to the optical scanning device. [Explanation of symbols]

[0198] 1 light source 5 Polygon mirror (deflector) 5a Deflection plane 7 Imaging optical system (first optical system) 9 Scanned surface 50 Optical scanning device

Claims

1. A deflector that deflects the light beam from the light source to scan the surface to be scanned in the main scanning direction, The system comprises a first optical system that guides the light beam deflected by the deflector to the surface to be scanned, In the main scanning section, the width of the light beam immediately before it enters the deflector is smaller than the width of the deflection surface of the deflector. In the main scanning section, at multiple image heights on the scanned surface, only a portion of the light beam incident on the deflector reaches through the deflection surface. When Y1 (mm) is the distance between the image height closest to the on-axis image height on one side of the plurality of image heights and the on-axis image height, and Y2 (mm) is the distance between the image height closest to the on-axis image height on the other side of the plurality of image heights and the on-axis image height, -0.005≦(Y2-Y1) / (Y2+Y1)≦0.005 An optical scanning device characterized by satisfying the following conditions.

2. When the distance between the first off-axis image height and the on-axis image height on one side is Y3 (mm), and the distance between the second off-axis image height and the on-axis image height on the other side is Y4 (mm), Y3-Y1≦5.00 Y4-Y2≦5.00 The optical scanning apparatus according to claim 1, characterized in that it satisfies the following conditions.

3. The system includes a second optical system that directs the light beam from the light source onto the deflector, The optical scanning apparatus according to claim 1, characterized in that when projected onto the main scanning cross-section, the center of the light-emitting surface of the light source is not on the optical axis of the second optical system.

4. The optical scanning apparatus according to claim 3, characterized in that the center of the light-emitting surface is located on the opposite side of the surface to be scanned with respect to a cross section parallel to the sub-scanning direction that includes the optical axis of the second optical system.

5. Let Z3 be the distance in the sub-scanning direction between the reaching position at the first off-axis image height on one side of the light beam deflected by the deflection surface and the optical axis of the first optical system, let Z4 be the distance in the sub-scanning direction between the reaching position at the second off-axis image height on the other side of the light beam deflected by the deflection surface and the optical axis of the first optical system, and let Z0 be the distance in the sub-scanning direction between the reaching position that is furthest from the reaching position at one of the first and second off-axis image heights corresponding to the larger of Z3 and Z4, and the optical axis of the first optical system, and when the angle that the normal makes with respect to the main scanning cross-section in a cross-section parallel to the sub-scanning direction that includes the normal of the deflection surface, 0.30≦(Z3+Z4) / {Z0+Max(Z3, Z4)}≦0.82 The optical scanning apparatus according to claim 1, characterized in that it satisfies the following conditions.

6. The optical scanning apparatus according to claim 1, characterized in that the number of light-emitting points included in the light source is one.

7. The light source has a plurality of light-emitting points, including a first and a second light-emitting point. In the main scanning cross-section, when the normal to the deflection surface forms a first angle with respect to the optical axis of the first optical system, only a portion of the first luminous beam from the first light-emitting point incident on the deflector is deflected by the deflection surface and reaches the first off-axis image height on one side, while the entirety of the second luminous beam from the second light-emitting point incident on the deflector is deflected by the deflection surface and reaches the first off-axis image height. The optical scanning apparatus according to claim 1, characterized in that, in the main scanning cross-section, when the normal of the deflection surface forms a second angle with respect to the optical axis of the first optical system, the entire first light beam incident on the deflector is deflected by the deflection surface and reaches the second off-axis image height on the other side, while only a portion of the second light beam incident on the deflector is deflected by the deflection surface and reaches the second off-axis image height.

8. The light source is located on one of the sides, In the main scanning plane, when the normal to the deflection plane is at the first angle, the number of luminous beams that reach the first off-axis image height is n1, where only a portion of the multiple luminous beams from the multiple light-emitting points incident on the deflector are deflected by the deflection plane. In the main scanning plane, when the normal to the deflection plane is at the second angle, the number of luminous beams that reach the second off-axis image height is n2, where only a portion of the multiple luminous beams incident on the deflector are deflected by the deflection plane. 1 ≤ n1 ≤ n2 The optical scanning apparatus according to claim 7, characterized in that it satisfies the following conditions.

9. The system includes a second optical system that directs the light beam from the light source onto the deflector, When the number of deflection surfaces is N, and the angle between the optical axis of the first optical system and the optical axis of the second optical system in the main scanning cross-section is θ (°), (720 / N) / (N-1)+45<θ<(720 / N) / (N-1)+60 The optical scanning apparatus according to claim 1, characterized in that it satisfies the following conditions.

10. The system includes a second optical system that directs the light beam from the light source onto the deflector, When L is the distance on the optical axis of the second optical system between the light-emitting surface of the light source and the on-axial deflection point on the deflection surface, and W is the distance between the first off-axis image height on one side and the second off-axis image height on the other side, 0.30 ≤ L / W ≤ 0.70 The optical scanning apparatus according to claim 1, characterized in that it satisfies the following conditions.

11. The optical scanning apparatus according to claim 1, characterized in that the deflector is a polyhedron that rotates around a rotation axis.

12. A deflector that deflects the light beam from the light source to scan the surface to be scanned in the main scanning direction, The system comprises a first optical system that guides the light beam deflected by the deflector to the surface to be scanned, In the main scanning cross-section, the width of the light beam immediately before it enters the deflection surface of the deflector is smaller than the width of the deflection surface. An optical scanning apparatus characterized in that, in the main scanning cross-section, when the normal of the deflection surface forms a predetermined angle with respect to the optical axis of the first optical system, only a portion of the light beam incident on the deflector is deflected by the deflection surface and reaches the surface to be scanned.

13. An image forming apparatus comprising: an optical scanning apparatus according to any one of claims 1 to 12; a developer for developing an electrostatic latent image formed on the surface to be scanned by the optical scanning apparatus as a toner image; a transfer unit for transferring the developed toner image to a transfer material; and a fuser for fixing the transferred toner image to the transfer material.

14. An image forming apparatus comprising an optical scanning device according to any one of claims 1 to 12, and a printer controller that converts a signal output from an external device into image data and inputs it to the optical scanning device.

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