Optical scanner and image forming apparatus

Abstract

To provide an optical scanner that reduces deterioration of optical performance due to birefringence, while favorably correcting the optical performance by using a child line tilt change surface for an imaging optical system.SOLUTION: An optical scanner has a deflector that deflects a light beam from a light source to scan a scanning target surface in a main scanning direction, and a first imaging optical element that guides the light beam from the deflector to the scanning target surface, is closest to the scanning target surface, and is made of resin. The thickness in an optical axis direction of the first imaging optical element on a main scanning cross section including an optical axis changes in the main scanning direction. The first imaging optical element includes a child line tilt change surface. In an area on one side of the child line tilt change surface relative to the optical axis in the main scanning direction, a position in the main scanning direction where the interval in the optical axis direction between one end and the other end becomes maximum in a sub-scanning direction of an effective area of the child line tilt change surface on a sub-scanning cross section, a position in the main scanning direction where the thickness in the optical axis direction of the first imaging optical element becomes maximum on the main scanning cross section, and the maximum image height in the main scanning direction on the scanning target surface, are appropriately set.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Conventionally, in order to achieve miniaturization in optical scanning devices for color image forming devices, an optical system (sub-scanning oblique incidence system) has been adopted in which multiple light beams emitted from multiple light sources are obliquely incident on a deflector in the sub-scanning cross section. Patent Document 1 discloses a technique for correcting scanning line curvature and wavefront aberration in a sub-scanning oblique incidence system by making the optical surface of an imaging optical element a sagittal tilt changing surface. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-140011 Summary of the Invention [Problem to be solved by the invention]

[0004] However, when a sagittal tilt varying surface is used as in Patent Document 1, the thickness of the imaging optical element in the optical axis direction in the sub-scanning cross section becomes non-uniform in the sub-scanning direction. As a result, the amount of birefringence in the sub-scanning cross section of the imaging optical element changes in the sub-scanning direction, and optical performance deteriorates when the position of the light beam passing through the imaging optical element fluctuates in the sub-scanning direction due to a placement error or the like. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an optical scanning device having good optical performance. [Means for solving the problem]

[0005] In order to achieve the above object, an optical scanning device of the present invention includes a deflector that deflects a light beam from a light source to scan a surface to be scanned in a main scanning direction, and an imaging optical system that includes at least one imaging optical element that guides the light beam from the deflector to the surface to be scanned, wherein a first imaging optical element of the at least one imaging optical element that is closest to the surface to be scanned is made of a resin material, a thickness of the first imaging optical element in the optical axis direction in a main scanning cross section including the optical axis varies in the main scanning direction, the first imaging optical element includes a sagittal tilt changing surface, and in an area on one side of the optical axis in the main scanning direction of the sagittal tilt changing surface, a position with respect to the optical axis in the main scanning direction at which a distance in the optical axis direction between one end and the other end of an effective area of ​​the sagittal tilt changing surface in the sub-scanning cross section is maximum is defined as y |Δs|max1 y is the position with respect to the optical axis in the main scanning direction at which the thickness of the first imaging optical element in the main scanning cross section in the optical axis direction is maximum. dmax1、 When the maximum image height in the main scanning direction on the scanned surface is W1, 0.0≦|y |Δs|max1 -y dmax1 | / W1≦0.1 The present invention is characterized by satisfying the following conditions. [Effects of the Invention]

[0006] According to the present invention, it is possible to provide an optical scanning device in which the optical performance is well corrected by using a sagittal tilt varying surface in the imaging optical system, while reducing the deterioration of the optical performance due to birefringence. [Brief explanation of the drawings]

[0007] [Figure 1] 1A is a sub-scanning sectional view of an optical scanning device according to a first embodiment, FIG. 1B is a development view of a main-scanning sectional view, and FIG. 1C is a development view of a sub-scanning sectional view. [Figure 2] FIG. 2A is a sub-scanning cross-sectional view of the imaging lens according to the first embodiment, and FIG. 2B is a diagram for explaining |Δs|(y) and d(y). [Figure 3] 1A and 1B are diagrams showing (a) |Δs|(y) and (b) d(y) of the imaging lens according to the first embodiment. [Figure 4]3A and 3B are diagrams showing curvature of field of the optical scanning device according to the first embodiment. [Figure 5] FIG. 2 is a diagram showing the fθ characteristic of the optical scanning device according to the first embodiment. [Figure 6] 3A and 3B are diagrams illustrating scanning line curvature of the optical scanning device according to the first embodiment. [Figure 7] 3A and 3B are diagrams showing a 45° astigmatism of the optical scanning device according to the first embodiment. [Figure 8] 3A and 3B are diagrams showing an image plane illuminance distribution of the optical scanning device according to the first embodiment. [Figure 9] 10A is a sub-scanning sectional view of an optical scanning device according to a second embodiment, FIG. 10B is a development view of a main-scanning sectional view, and FIG. 10D is a development view of a sub-scanning sectional view. [Figure 10] 10A and 10B are diagrams showing an image plane illuminance distribution of an optical scanning device according to a second embodiment. [Figure 11] FIG. 2 is a sub-scanning sectional view of the color image forming apparatus. DETAILED DESCRIPTION OF THE INVENTION

[0008] The optical scanning device according to the present embodiment will be described in detail below with reference to the accompanying drawings. Note that the drawings may be drawn at a scale different from the actual scale in order to facilitate understanding of the present embodiment. In the following description, the main scanning direction (Y direction) is the direction perpendicular to the rotation axis (or oscillation axis) of the deflector and the optical axis (X direction) of the optical system (the direction in which the light beam is reflected and deflected (deflectively scanned) by the rotating polygon mirror). The sub-scanning direction (Z direction) is the direction parallel to the rotation axis (or oscillation axis) of the deflector. Furthermore, the main scanning cross section is a cross section perpendicular to the sub-scanning direction. Furthermore, the sub-scanning cross section is a cross section perpendicular to the main scanning direction.

[0009] [First embodiment] 1(a), (b), and (c) are a sub-scanning partial cross-sectional view, a main-scanning partial cross-sectional development, and a sub-scanning partial cross-sectional development, respectively, of an optical scanning device 100 according to the first embodiment. The optical scanning device 100 of this embodiment includes a light source 1A, an incident optical system LA, a deflector 5, an imaging optical system SA (first imaging optical system), and a reflecting mirror (reflecting optical element) M1.

[0010] The optical scanning device 100 of this embodiment uses a so-called sub-scanning oblique incidence optical system in which a light beam RA is deflected by a single deflector 5 to scan the scanned surface 8A, and the light beam RA is incident on the deflector 5 at an angle in the sub-scanning direction. A semiconductor laser or the like is used as the light source 1A. The number of light emitting points of the light source 1A may be one or more. The incident optical system LA includes an anamorphic lens 2A, a sub-scanning aperture stop 3A, and a main-scanning aperture stop 4A.

[0011] The anamorphic lens 2A converts the light beam RA emitted from the light source 1A into a parallel light beam in the main scanning cross section and focuses the light in the sub-scanning direction. Note that the parallel light beam here includes not only a strictly parallel light beam but also a substantially parallel light beam such as a weakly divergent light beam or a weakly convergent light beam. A collimator lens or a cylindrical lens may be used instead of the anamorphic lens 2A.

[0012] Each of the sub-scanning aperture stops 3A limits the diameter of the light beam RA in the sub-scanning direction after passing through the anamorphic lens 2A. Similarly, each of the main-scanning aperture stops 4A limits the diameter of the light beam RA in the main-scanning direction after passing through the sub-scanning aperture stop 3A.

[0013] The deflector 5 is rotated in the direction of arrow A in the drawing by a driving means such as a motor (not shown), whereby the deflector 5 deflects the incident light beam RA and scans the scanned surface 8A in the direction of arrow B in the drawing. The deflector 5 is formed of, for example, a polygon mirror. In the imaging optical system SA, the deflected light beam RA deflected and reflected by the deflecting surface 5A of the deflector 5 passes through imaging lenses 6A and 7A, is turned back by a reflecting mirror M1, and is guided to the scanned surface 8A. The imaging optical system SA includes an imaging lens 7A (first imaging optical element) as an optical element located closest to the scanned surface 8A on the optical path of the deflected light beam RA deflected and reflected by the deflecting surface 5A.

[0014] The reflecting mirror M1 is a means for reflecting the light beam, and may be a vapor-deposited mirror, etc. The effect of this embodiment is not limited by the number of reflecting mirrors, and the number of reflecting mirrors may be changed as appropriate. Here, C0 in the figure is the deflection point (axial deflection point) when the chief ray of the axial light beam is deflected, and P0 is a plane (reference plane) that passes through the deflection point C0 and is perpendicular to the rotation axis of the deflector 5. The light beam RA that is incident on the deflecting surface 5a intersects with the main scanning cross section at the deflection point C0 in the sub-scanning cross section and is deflected. Hereinafter, the length of the optical path from the deflection point C0 to each scanned surface will be referred to as the optical path length of each imaging optical system.

[0015] Next, the specifications, optical layout, and optical surface shape of the optical scanning device 100 according to this embodiment are shown in Tables 1 to 3 below. Table 1 shows the specifications and lens layout of the incident optical system LA and the imaging optical system SA, and Tables 2 and 3 show the optical surface shapes of the incident optical system LA and the imaging optical system SA. The optical layout column in Table 1 shows the coordinates of the reflection points of the light beam RA, which is directed toward the image center (axial image height) in the main scanning direction on the scanned surface 8A, at each reflecting mirror.

[0016] In Tables 1 and 2, the intersection of each optical surface and the optical axis is taken as the origin, and the optical axis direction, the axis perpendicular to the optical axis in the main scanning cross section, and the axis perpendicular to the optical axis in the sub-scanning cross section are taken as the x-axis, y-axis, and z-axis, respectively. However, the x-axis indicates the direction of light travel on the +x side, and the y-axis indicates the light source side with respect to the optical axis on the +y side. Also, in Table 3, "Ex" means "×10 -x " means.

[0017] [Table 1]

[0018] [Table 2]

[0019] [Table 3]

[0020] Temperature compensation is performed by forming a diffractive surface on the entrance surface of the anamorphic lens 2A, but the effects of this embodiment are not limited to this configuration. The entrance surface of the anamorphic lens 2A is a rotationally asymmetric diffractive surface, and the phase function Φ of the diffraction grating is expressed by the following equation (1):

number

[0021] The imaging lenses 6A and 7A according to this embodiment are optical elements made of resin, and the meridional shape of each optical surface (the shape of the optical surface in the main scanning cross section) is an aspherical shape in which the position x in the optical axis direction can be expressed as a function of up to tenth order relative to the position y in the main scanning direction, as shown in Equation (2).

number

[0022] where R is the generatrix radius of curvature, K is the eccentricity, and B i (i=1, 2, ..., 10) are aspherical coefficients. The imaging lenses 6A and 7A according to this embodiment are optical elements made of a resin material, but they are not limited to being made of resin alone and may contain components other than resin, such as inorganic particles (as long as the main component is resin). In this specification, the term "generatrix" refers to the shape of the optical element in the main scanning cross section that includes the optical axis.

[0023] Furthermore, the sagittal shape of each optical surface of the imaging lenses 6A and 7A according to this embodiment (the shape of the optical surface in the sub-scanning cross section at any image height (y), z) is an aspherical shape as shown in the following formula (3).

number

[0024] The sagittal curvature radius r′ is the curvature radius in the sub-scan section, and changes continuously according to the y coordinate of the optical surface as shown in the following equation.

number

[0025] Next, the effects of the optical scanning device 100 according to this embodiment will be described. The optical scanning device 100 according to this embodiment employs a sub-scanning oblique incidence optical system, and it is necessary to correct the scanning line curvature and the wavefront aberration difference in the azimuth ±45° direction (45° astigmatism) that occur due to the sub-scanning oblique incidence optical system. Therefore, as shown in Tables 2 and 3, the entrance surface and exit surface of the imaging lens (first imaging optical element) 7A according to this embodiment are aspherical with aspherical coefficients m i,1 ≠0 is included, and correction is performed on the sagittal tilt change plane where the sagittal tilt amount changes in the main scanning direction (y-axis direction). In this specification, a sagittal tilt varying surface is an optical surface in which the amount of sagittal tilt varies from on-axis to off-axis.

[0026] The entrance surface and exit surface of the imaging lens 7A of this embodiment are formed by sagittal tilt changing surfaces that are inclined in different directions relative to a plane perpendicular to the optical axis in the sub-scanning cross section.

[0027] By using a sagittal tilt varying surface, it is possible to correct the scanning line curvature by appropriately setting the amount of sagittal tilt at each light ray passing position and controlling the irradiation position on the scanned surface 8A. Similarly, it is also possible to correct 45° astigmatism by setting the tilt of the optical surface in accordance with the tilt of the incident wavefront. In other words, by using the entrance surface and exit surface as sagittal tilt varying surfaces, both the scanning line curvature and 45° astigmatism are effectively corrected.

[0028] However, after correcting for the scanning line curvature and 45° astigmatism, if the sagittal tilt of the entrance surface and the sagittal tilt of the exit surface have different signs, as shown in Figure 2(a), the thickness of the imaging lens 7A in the optical axis direction in the sub-scanning cross section will be uneven (uneven thickness). When a lens with such an uneven thickness is injection molded, the amount of birefringence generally varies in the sub-scanning direction. If this occurs, if the position of the light beam passing through the imaging lens 7A varies in the sub-scanning direction due to an assembly error or other reason, the birefringence also varies, potentially resulting in fluctuations and degradation of optical performance. When a laser is used as the light source 1A, the polarization state of the light beam RA changes due to birefringence, and the reflectivity of the light beam changes due to the polarization reflection characteristics of the reflecting mirror M1, resulting in uneven light intensity on the scanned surface 8A.

[0029] Therefore, when the optical surface of the imaging lens 7A according to this embodiment is divided into a positive region (y≧0, one side region) and a negative region (y<0, opposite side region) in terms of the position y in the main scanning direction on the optical surface, the optical surface satisfies conditional expression (5) in at least one of the regions. 0.0≦|y |Δs|max -y dmax | / W≦0.1 (5)

[0030] where y |Δs|max is the y (position relative to the optical axis in the main scanning direction) at which |Δs|(y) is maximum when |Δs|(y) is the distance (absolute value of the difference in position) in the optical axis direction between one end and the other end of the effective area of ​​the optical surface in the sub-scanning section at the main scanning direction position y on the optical surface. dmaxis the position y (position relative to the optical axis in the main scanning direction) at which the thickness (d(y)) of the imaging lens on the generatrix at position y in the main scanning direction is maximum. W is the maximum image height in the main scanning direction on the scanned surface.

[0031] More specifically, in the region on one side of the optical axis in the main scanning direction of the sagittal tilt changing surface, the position with respect to the optical axis in the main scanning direction at which the distance in the optical axis direction between one end and the other end in the sub-scanning direction of the effective region of the sagittal tilt changing surface in the sub-scanning cross section is maximum is defined as y |Δs|max1 y is the position with respect to the optical axis in the main scanning direction at which the thickness of the imaging lens 7A in the optical axis direction in the main scanning cross section is maximum. dmax1 When the maximum image height on one side of the main scanning direction on the scanned surface is W1, 0.0≦|y |Δs|max1 -y dmax1 | / W1≦0.1 Meet the following conditions.

[0032] In addition, in the region on one side of the sagittal tilt changing surface with respect to the optical axis in the main scanning direction, the position with respect to the optical axis in the main scanning direction at which the distance in the optical axis direction between one end and the other end in the sub-scanning direction of the effective region of the sagittal tilt changing surface in the sub-scanning cross section is maximum is defined as y |Δs|max2 y is the position with respect to the optical axis in the main scanning direction at which the thickness of the imaging lens 7A in the optical axis direction in the main scanning cross section is maximum. dmax2 , when the maximum image height on the opposite side of the main scanning direction on the scanned surface is W2, 0.0≦|y |Δs|max2 -y dmax2 | / W2≦0.1 Meet the following conditions.

[0033] Here, W = 163 mm (scan width on the scanned surface: 326 mm). |Δs|max and y dmax is shown in Figure 2(b).

[0034] The effective optical surface area is an area that is set appropriately taking into consideration the area where the light beam passes on the optical surface of the optical element according to design (light beam use area) and fluctuations in the light beam passing position due to manufacturing errors, assembly errors, etc. In this example, the width of the light beam use area in the sub-scanning direction is 3 mm (±1.5 mm from the optical axis), while the width of the effective optical surface area in the sub-scanning direction is 6 mm (±3 mm from the optical axis).

[0035] In the optical scanning device 100 of this embodiment, the sagittal tilt varying surface is designed to satisfy conditional expression (5) so that the y position where |Δs|(y) of the optical surface becomes large and the y position where the thickness along the optical axis on the generatrix becomes large are close in the y direction. By satisfying conditional expression (5), the thickness along the optical axis on the generatrix is ​​increased, resulting in a thickness deviation shape due to sagittal tilt that reduces the thickness deviation ratio, thereby reducing the difference in the amount of birefringence generated in the sub-scanning direction. Here, the thickness deviation ratio is the ratio of the maximum thickness to the minimum thickness of the optical element along the optical axis. As a result, the sagittal tilt varying surface effectively corrects the scanning line curvature and 45° astigmatism while suppressing fluctuations in optical performance due to birefringence, thereby suppressing degradation of optical performance.

[0036] If conditional expression (5) is not satisfied, the thickness deviation ratio increases, and the difference in the amount of birefringence in the sub-scanning direction increases, so that the fluctuation in optical performance due to assembly error increases, resulting in degradation of the optical performance. Furthermore, it is more preferable that conditional expression (5a) be satisfied. 0.0≦|y |Δs|max -y dmax | / W≦0.05 (5a) Furthermore, when the position y in the main scanning direction is divided into a positive region and a negative region, it is more preferable that conditional expression (5) or conditional expression (5a) be satisfied in either region (both regions).

[0037] Furthermore, it is more preferable that the conditional formula (5) or the conditional formula (5a) be satisfied on both the entrance surface and the exit surface of the imaging lens 7A. Furthermore, when the optical surface of the imaging lens 7A according to this embodiment is divided into a positive region (y≧0) and a negative region (y<0) in terms of the position y in the main scanning direction on the optical surface, the optical surface satisfies conditional expression (6) in at least one of the regions. 0.0≦|y |Δs|max -y dmax | / |y max |≦0.12 (6) where y max is the y position (position relative to the optical axis in the main scanning direction) of the end of the optical surface effective area of ​​the imaging lens 7A in the main scanning direction.

[0038] More specifically, in the region on one side of the optical axis in the main scanning direction of the sagittal tilt changing surface, the position with respect to the optical axis in the main scanning direction at which the distance in the optical axis direction between one end and the other end in the sub-scanning direction of the effective region of the sagittal tilt changing surface in the sub-scanning cross section is maximum is defined as y |Δs|max1 y is the position with respect to the optical axis in the main scanning direction at which the thickness of the imaging lens 7A in the optical axis direction in the main scanning cross section is maximum. dmax1 , the position of one end of the effective area of ​​the sagittal tilt changing surface in the main scanning direction relative to the optical axis in the main scanning direction is defined as y max1 When 0.0≦|y |Δs|max1 -y dmax1 | / |y max1 |≦0.12 Meet the following conditions.

[0039] In addition, in the region opposite to one side of the sagittal tilt changing surface with respect to the optical axis in the main scanning direction, the position with respect to the optical axis in the main scanning direction at which the distance in the optical axis direction between one end and the other end in the sub-scanning direction of the effective region of the sagittal tilt changing surface in the sub-scanning cross section is maximum is defined as y |Δs|max2 y is the position with respect to the optical axis in the main scanning direction at which the thickness of the imaging lens 7A in the optical axis direction in the main scanning cross section is maximum. dmax2 , the position of the end of the effective area of ​​the sagittal tilt changing surface on the opposite side of the main scanning direction with respect to the optical axis in the main scanning direction is defined as y max2 When 0.0≦|y |Δs|max2 -y dmax2 | / |y max2 |≦0.12 Meet the following conditions.

[0040] Here, y max= 90 mm (width of the optical surface effective area in the main scanning direction is 180 mm). The width of the light beam use area in the main scanning direction is 170 mm (within a range of ±85 mm from the optical axis position). If the upper and lower limits of condition (6) are not met, the thickness deviation ratio increases, increasing the difference in the amount of birefringence in the sub-scanning direction. This increases the fluctuation in optical performance due to assembly errors, resulting in degradation of optical performance. Furthermore, it is more preferable that conditional expression (6a) be satisfied. 0.0≦|y |Δs|max -y dmax | / |y max |≦0.09 (6a) Furthermore, it is more preferable that conditional expression (6b) be satisfied. 0.0≦|y |Δs|max -y dmax | / |y max |≦0.06 (6b) Furthermore, when the position y in the main scanning direction is divided into a positive region and a negative region, it is more preferable that conditional expression (6), (6a), or (6b) be satisfied in both regions.

[0041] So far, we have explained the sagittal tilt varying surface for correcting the scanning line curvature and 45° astigmatism that occur when a sub-scanning oblique incidence optical system is used, but the present invention can be applied to any imaging optical system that includes a sagittal tilt varying surface, and is not limited to sub-scanning oblique incidence optical systems. For example, in a so-called in-deflection-plane incidence optical system in which light is incident perpendicular to the rotation axis of the deflector in the sub-scan cross section, ghosts are generated by reflection at the optical surface of the imaging lens. The present invention can also be applied to a configuration in which at least one optical surface of the imaging lens is made a sagittal tilt varying surface, thereby controlling the ghost optical path so that the ghost does not reach the surface to be scanned.

[0042] FIG. 3 shows |Δs|(y) and d(y) on the optical surface of the imaging lens 7A according to this embodiment. |Δs|max、 y dmax、 y max、 W 、 |y |Δs|max -y dmax |, |y |Δs|max -y dmax| / W and |y |Δs|max -y dmax | / |y max The numerical values ​​of | are shown in Table 4. As shown in Table 4, the imaging lens 7A according to this embodiment satisfies conditional expression (5) on both the entrance surface and the exit surface in both the region where the position y in the main scanning direction is positive (y≧0) and negative (y<0). Furthermore, as shown in Table 4, the imaging lens 7A according to this embodiment satisfies conditional expression (6) on both the entrance surface and the exit surface in both the positive region (y≧0) and the negative region (y<0) of the position y in the main scanning direction. Also, as shown in Table 4, |y |Δs|max -y dmax | is at most about 11mm.

[0043] [Table 4] As shown in Table 4, the maximum distance |Δs|(y |Δs|max ) (mm) satisfies conditional expression (7). 0.2≦|Δs|(y |Δs|max )≦1.2 (7)

[0044] If the lower limit of conditional expression (7) is exceeded, the absolute value of the sagittal tilt amount becomes small, making it impossible to sufficiently correct the scanning line curvature and 45° astigmatism, resulting in a deterioration of optical performance. Conversely, if the upper limit of conditional expression (7) is exceeded, the absolute value of the sagittal tilt amount becomes large, increasing the thickness deviation ratio, thereby increasing the amount of change in birefringence in the sub-scanning direction and resulting in a deterioration of optical performance. Furthermore, it is more preferable that conditional expression (7a) be satisfied. 0.23≦|Δs|(y |Δs|max )≦1.15 (7a)

[0045] The imaging lens (second imaging optical element) 6A according to this embodiment has an exit surface that is a sagittal tilt varying surface, as shown in Table 3. The sagittal tilt varying surface makes it possible to vary the angle of the exiting light beam RA in the main scanning direction, thereby correcting the scanning line curvature on the entrance surface of the imaging lens 7A that occurs in the sub-scanning oblique incidence optical system. By correcting the scanning line on the entrance surface of the imaging lens 7A so that it passes near the generatrix, it is possible to reduce the influence of birefringence fluctuation in the sub-scanning direction due to the sagittal tilt varying surface of the imaging lens 7A, thereby reducing degradation of optical performance.

[0046] Next, the optical performance of the optical scanning device 100 according to this embodiment will be described. 4 is a graph showing the field curvature in the main scanning direction and the sub-scanning direction of the optical scanning device 100 according to this embodiment. As shown in FIG. 4, it can be seen that the field curvature in the main scanning direction and the sub-scanning direction of the optical scanning device 100 according to this embodiment is well corrected.

[0047] Fig. 5 is a graph showing the fθ characteristic dy of the optical scanning device 100 according to this embodiment. The fθ characteristic dy shows the difference obtained by subtracting the ideal height from the position where the light beam actually reaches. As shown in Fig. 5, it can be seen that the fθ characteristic dy of the optical scanning device 100 according to this embodiment is well corrected.

[0048] 6 shows the image height dependency of the scanning line curvature dz on the scanned surface 8A of the optical scanning device 100 according to this embodiment. Here, the scanning line curvature dz means the difference between the imaging position in the sub-scanning direction at each image height and the imaging position in the sub-scanning direction at the on-axis image height on the scanned surface 8A. As shown in FIG. 6, it can be seen that the scanning line curvature is well corrected in the optical scanning device 100 according to this embodiment.

[0049] 7 shows the wavefront aberration difference (45° astigmatism) in the azimuth ±45° directions of the optical scanning device 100 according to this embodiment. As shown in FIG. 7, it can be seen that the 45° astigmatism is well corrected in the optical scanning device 100 according to this embodiment.

[0050] 8 shows the image height dependency (image plane illuminance distribution) of the light amount on the scanned surface 8A of the optical scanning device 100 according to this embodiment. Here, the light amount at an image height of 0 mm is normalized to 1. The image plane illuminance distribution when the imaging lens 7A according to this embodiment is moved ±0.0 mm, +0.5 mm, and −0.5 mm in the sub-scanning direction is also shown.

[0051] As shown in Figure 8, the fluctuation in image plane illuminance is within ±10% relative to the axis, and fluctuations in the amount of light on the scanned surface 8A are sufficiently suppressed. To further suppress fluctuations in the amount of light on the scanned surface 8A, an electrical correction means may be used to adjust the amount of light from the light source 1A based on a pre-measured image plane illuminance distribution. Furthermore, in the optical scanning device 100 according to this embodiment, even if the light beam RA passing through the imaging lens 7A fluctuates by ±0.5 mm due to assembly errors or the like, the maximum fluctuation in image plane illuminance at each image height is 0.02 or less, and changes in the image plane illuminance distribution are sufficiently reduced.

[0052] When using the above-described electrical correction means, if the image plane illuminance distribution varies significantly for each optical scanning device, a correction coefficient for correcting the light intensity must be applied individually to each optical scanning device, which complicates the correction means and increases the manufacturing cost of the optical scanning device. Alternatively, even if a uniform correction coefficient is applied, the correction residual of the image plane illuminance distribution becomes large. The optical scanning device 100 according to this embodiment sufficiently reduces variations in the image plane illuminance distribution due to assembly errors, etc., so that even if a uniform correction coefficient is used by an electrical correction means for correcting the image plane illuminance distribution, the correction residual can be reduced, allowing the use of an electrical correction means with a simple configuration.

[0053] As described above, in the optical scanning device 100 according to this embodiment, the scanning line curvature and 45° astigmatism are corrected well, while fluctuations in optical performance due to birefringence are suppressed, and deterioration of optical performance is reduced.

[0054] [Second embodiment] An optical scanning device 200 according to a second embodiment of the present invention will now be described. Fig. 9(a) is a partial cross-sectional view in the sub-scanning direction of the optical scanning device 200 according to the second embodiment. Fig. 9(b) is a partial cross-sectional development in the main scanning direction of the optical scanning device 200 according to the second embodiment. Fig. 9(c) is a partial cross-sectional development in the sub-scanning direction of the optical scanning device 200 according to the second embodiment.

[0055] The optical scanning device 200 according to this embodiment differs from the optical scanning device 100 according to the first embodiment in that four surfaces to be scanned 8A, 8B, 8C, and 8D can be scanned simultaneously by a common deflector 5. The optical scanning device 200 of this embodiment includes light sources 1A, 1B, 1C, 1D, incident optical systems LA, LB, LC, LD, a deflector 5, imaging optical systems SA, SB, SC, SD, and reflecting mirrors M1, M2, M3, M'1, M'2, M'3.

[0056] Each of the light sources 1A, 1B, 1C, and 1D uses a semiconductor laser or the like, similar to the light source 1A according to the first embodiment. The number of light emitting points of the light sources 1A, 1B, 1C, and 1D may be one or more.

[0057] Each of the incident optical systems LA, LB, LC, and LD in this embodiment has the same configuration and optical function as the incident optical system LA in the first embodiment, except that the combinations of oblique incidence angles in the main scanning direction and sub-scanning direction are different. Note that components such as optical elements and diaphragms may be integrated between adjacent optical systems. For example, the main scanning aperture diaphragms 4A and 4B may be integrated to form a single diaphragm with a single aperture.

[0058] The light beams RA and RB emitted from the light sources 1A (first light source) and 1B (second light source) are incident on the deflection surface 5a of the deflector 5 via the incident optical system (first incident optical system) LA and the incident optical system (second incident optical system) LB. The light beams RC and RD emitted from the light sources 1C and 1D are incident on the deflection surface 5b of the deflector 5 via the incident optical systems LC and LD. At this time, the deflection surface 5a onto which the light beams RA and RB from the light sources 1A and 1B are incident is different from the deflection surface 5b onto which the light beams RC and RD from the light sources 1C and 1D are incident at the same time.

[0059] The incident optical systems LA and LB are arranged so that their optical axes are inclined with respect to the main-scanning cross section in order to cause the light beams RA and RB to be obliquely incident on the deflecting surface 5a in the sub-scanning cross section. This allows the light beams RA and RB to be separated and guided to the corresponding scanned surfaces 8A and 8B. In this embodiment, to equalize the optical performance of each optical path, the optical axes of the incident optical systems LA and LB are configured so that the absolute values ​​of the oblique incident angles with respect to the main-scanning cross section are equal but the signs are different. However, it is sufficient that at least one of the absolute values ​​and signs of the inclination angles of the optical axes are different from each other, and the absolute values ​​may be different or the signs may be equal, as necessary. The same applies to the incident optical systems LC and LD.

[0060] Each of the imaging lenses 6A and 6B is an imaging lens similar to the imaging lens 6A according to the first embodiment. Each of the imaging lenses 7A and 7B is an imaging lens similar to the imaging lens 7A according to the first embodiment. The exit surface of each of the imaging lenses 6A and 6B is a multi-stage toric surface made up of two toric surfaces arranged in the sub-scanning direction. The imaging lenses 6A and 7A form an imaging optical system SA, and the imaging lenses 6B and 7B form an imaging optical system SB. The imaging optical systems SC and SD have the same configuration as the imaging optical systems SA and SB.

[0061] Reflecting mirror M1 is arranged between imaging lens 7A and scanned surface 8A, reflecting mirror M2 between imaging lens 6B and imaging lens 7B, and reflecting mirror M3 between imaging lens 7B and scanned surface 8B. Light beam RA emitted from light source 1A and deflected by deflecting surface 5a is guided to scanned surface (first scanned surface) 8A via imaging lens 6A, imaging lens 7A, and reflecting mirror M1 in that order.

[0062] A light beam RB emitted from light source 1B and deflected by deflecting surface 5a is guided to scanned surface 8B (second scanned surface) via imaging lens 6B, reflecting mirror M2, imaging lens 7B, and reflecting mirror M3 in that order. Reflecting mirrors M'1, M'2, and M'3 are similarly arranged, and a light beam RD emitted from light source 1D and deflected by deflecting surface 5b is guided to scanned surface 8D via imaging lens 6D, imaging lens 7D, and reflecting mirror M'1 in that order. A light beam RC emitted from light source 1C and deflected by deflecting surface 5b is guided to scanned surface 8C via imaging lens 6C, reflecting mirror M'2, imaging lens 7C, and reflecting mirror M'3 in that order.

[0063] Here, on each side of the deflector 5, the optical paths leading to the scanned surfaces 8A and 8D located spatially (physically) farther from the deflector 5 are referred to as the "outer optical paths," while the optical paths leading to the scanned surfaces 8B and 8C located spatially closer to the deflector 5 are referred to as the "inner optical paths." In this case, only one reflective mirror is arranged in the outer optical path, and two reflective mirrors are arranged in the inner optical path. By varying the number of reflective elements between the outer and inner optical paths in this way, it is possible to match the optical path lengths in all optical paths, avoid interference between each optical element and the optical path, and simplify manufacturing. However, the number of reflective mirrors is not limited to this and can be determined appropriately depending on the spacing between the scanned surfaces, the arrangement of each imaging element, etc.

[0064] The specifications, optical layout, and optical surface shapes of the optical scanning device 200 according to this embodiment are shown in the following Tables 5 to 11. Here, Tables 5 to 9 show the specifications and lens layouts of the incident optical systems LA to LD and the imaging optical systems SA to SD, and Tables 10 and 11 show the optical surface shapes of the incident optical systems LA to LD and the imaging optical systems SA to SD.

[0065] [Table 5]

[0066] [Table 6]

[0067] [Table 7]

[0068] [Table 8]

[0069] [Table 9]

[0070] [Table 10]

[0071] [Table 11]

[0072] The entrance and exit surfaces of the imaging lenses 7A, 7B, 7C, and 7D of this embodiment are formed as sagittal tilt changing surfaces that are inclined in different directions relative to a plane perpendicular to the optical axis in the sub-scanning cross section.

[0073] Next, the effects of the optical scanning device 200 according to this embodiment will be described. |Δs|max、 y dmax , y max、 W 、 |y |Δs|max -y dmax |, |y |Δs|max -y dmax | / W and |y |Δs|max -y dmax | / y dmax The field curvature in the main scanning direction and sub-scanning direction, the fθ characteristic dy, the scanning line curvature dz, and the 45° astigmatism are the same as those in the optical scanning device 100 according to the first embodiment, and therefore will be omitted.

[0074] Figure 10 shows the image height dependency (image plane illuminance distribution) of the light quantity on the scanned surface of the optical scanning device 200 according to this embodiment. Figures 10(a) to 10(d) show the image plane illuminance distribution on the scanned surfaces 8A to 8D. Here, the light quantity at the central image height in the main scanning direction on each scanned surface is normalized to 1. Also shown are the image plane illuminance distributions when the imaging lenses 7A, 7B, 7C, and 7D according to this embodiment are moved ±0.0 mm, +0.5 mm, and -0.5 mm in the sub-scanning direction, respectively.

[0075] As shown in Figures 10(a) to 10(d), the fluctuation in image plane illuminance is within ±10% relative to the axis, and fluctuations in the amount of light on the scanned surface are sufficiently suppressed. To further suppress fluctuations in the amount of light on the scanned surface, an electrical correction means may be used to adjust the light amount of the light source based on a pre-measured image plane illuminance distribution. In the optical scanning device 200 according to this embodiment, even if the light beam passing through the imaging lenses 7A, 7B, 7C, and 7D fluctuates by ±0.5 mm due to assembly errors or the like, the maximum fluctuation in image plane illuminance at each image height is 0.01 or less, and changes in the image plane illuminance distribution are sufficiently reduced.

[0076] Although the preferred embodiments have been described above, the present invention is not limited to these embodiments and various modifications and changes are possible within the scope of the gist thereof.

[0077] [Image forming device] FIG. 11 shows a sub-scanning cross-sectional view of a main part of a color image forming apparatus 90 equipped with the optical scanning device 100 according to the first embodiment or the optical scanning device 200 according to the second embodiment. The color image forming device 90 can be configured to have four optical scanning devices 100 according to the first embodiment, or one optical scanning device 200 according to the second embodiment, and is a color image forming device that records image information on the surface of each photosensitive drum, which is an image carrier.

[0078] The color image forming apparatus 90 includes an optical scanning device 100 according to the first embodiment or an optical scanning device 200 according to the second embodiment, photosensitive drums (photosensitive bodies) 23, 24, 25, and 26 as image carriers, and developing units 15, 16, 17, and 18. The color image forming apparatus 90 also includes a conveyor belt 91, a printer controller 93, and a fixing unit 94.

[0079] Color image forming apparatus 90 receives as input R (red), G (green), and B (blue) color signals (code data) output from an external device 92 such as a personal computer. The input color signals are converted by a printer controller 93 in the color image forming apparatus 90 into image data (dot data) for each of C (cyan), M (magenta), Y (yellow), and K (black).

[0080] Each converted image data is input to the optical scanning device 100 or 200. Then, the optical scanning device 100 or 200 emits light beams 19, 20, 21, and 22 modulated according to each image data, and the photosensitive surfaces of the photosensitive drums 23, 24, 25, and 26 are exposed to these light beams.

[0081] Charging rollers (not shown) that uniformly charge the surfaces of the photosensitive drums 23, 24, 25, and 26 are provided in contact with the surfaces. Then, the surfaces of the photosensitive drums 23, 24, 25, and 26 that have been charged by the charging rollers are irradiated with light beams 19, 20, 21, and 22 by the optical scanning device 100 or 200.

[0082] As described above, the light beams 19, 20, 21, and 22 are modulated based on image data for each color, and electrostatic latent images are formed on the surfaces of the photosensitive drums 23, 24, 25, and 26 by irradiating them with the light beams 19, 20, 21, and 22. The formed electrostatic latent images are developed into toner images by the developing units 15, 16, 17, and 18, which are arranged so as to contact the photosensitive drums 23, 24, 25, and 26.

[0083] The toner images developed by the developing devices 15 to 18 are transferred in multiple layers onto a sheet of paper (transfer material) (not shown) transported on a conveyor belt 91 by transfer rollers (transfer devices) (not shown) arranged opposite the photosensitive drums 23 to 26, forming a single full-color image.

[0084] The paper onto which the unfixed toner image has been transferred in this manner is further transported to a fuser 94 located behind the photosensitive drums 23, 24, 25, and 26 (on the left side in FIG. 11). The fuser 94 is composed of a fuser roller having an internal fuser heater (not shown) and a pressure roller arranged to be in pressure contact with the fuser roller. The paper transported from the transfer section is heated and pressurized at the contact point between the fuser roller and the pressure roller, thereby fixing the unfixed toner image on the paper. Further behind the fuser roller is a paper ejection roller (not shown), which ejects the fixed paper out of the color image forming apparatus 90.

[0085] The color image forming device 90 uses an optical scanning device 100 or 200 to record image signals (image information) on the photosensitive surfaces of the photosensitive drums 23, 24, 25, and 26 corresponding to each of the colors C, M, Y, and K, and prints color images at high speed. For example, a color image reading device equipped with a CCD sensor may be used as the external device 92. In this case, the color image reading device and the color image forming device 90 constitute a color digital copying machine.

[0086] The disclosure of this embodiment includes the following configurations and methods. (Configuration 1) a deflector that deflects a light beam from a light source to scan the surface to be scanned in a main scanning direction; an imaging optical system including at least one imaging optical element that guides the light beam from the deflector to the surface to be scanned, a first imaging optical element, which is closest to the scanned surface among the at least one imaging optical element, is made of a resin material; a thickness of the first imaging optical element in the optical axis direction in a main scanning cross section including the optical axis varies in the main scanning direction; the first imaging optical element includes a sagittal tilt varying surface; In the region on one side of the optical axis in the main scanning direction of the sagittal tilt changing surface, the position with respect to the optical axis in the main scanning direction at which the distance in the optical axis direction between one end and the other end of the effective region of the sagittal tilt changing surface in the sub-scanning section is maximum is defined as y |Δs|max1 y is the position with respect to the optical axis in the main scanning direction at which the thickness of the first imaging optical element in the main scanning cross section in the optical axis direction is maximum. dmax1 When the maximum image height in the main scanning direction on the scanned surface is W1, 0.0≦|y |Δs|max1 -y dmax1 | / W1≦0.1 An optical scanning device characterized by satisfying the following conditions. (Configuration 2) In the region of the sagittal tilt changing surface on the opposite side to the one side with respect to the optical axis in the main scanning direction, the position with respect to the optical axis in the main scanning direction at which the distance in the optical axis direction between one end and the other end of the effective region of the sagittal tilt changing surface in the sub-scanning cross section is maximum is defined as y |Δs|max2 y is the position with respect to the optical axis in the main scanning direction at which the thickness of the first imaging optical element in the main scanning cross section in the optical axis direction is maximum. dmax2 When the maximum image height in the main scanning direction on the scanned surface is W2, 0.0≦|y |Δs|max2 -y dmax2 | / W2≦0.1 2. The optical scanning device according to configuration 1, wherein the following conditions are satisfied: (Configuration 3) a deflector that deflects a light beam from a light source to scan the surface to be scanned in a main scanning direction; an imaging optical system including at least one imaging optical element that guides the light beam from the deflector to the surface to be scanned, a first imaging optical element, which is closest to the scanned surface among the at least one imaging optical element, is made of a resin material; a thickness of the first imaging optical element in the optical axis direction in a main scanning cross section including the optical axis varies in the main scanning direction; the first imaging optical element includes a sagittal tilt varying surface; In the region on one side of the optical axis in the main scanning direction of the sagittal tilt changing surface, the position with respect to the optical axis in the main scanning direction at which the distance in the optical axis direction between one end and the other end of the effective region of the sagittal tilt changing surface in the sub-scanning section is maximum is defined as y |Δs|max1 y is the position with respect to the optical axis in the main scanning direction at which the thickness of the first imaging optical element in the main scanning cross section in the optical axis direction is maximum. dmax1 , the position of the end of the effective area of ​​the sagittal tilt changing surface in the main scanning direction relative to the optical axis in the main scanning direction is defined as y max1 When 0.0≦|y |Δs|max1 -y dmax1 | / |y max1 |≦0.12 An optical scanning device characterized by satisfying the following conditions. (Configuration 4) In the region of the sagittal tilt changing surface on the opposite side to the one side with respect to the optical axis in the main scanning direction, the position with respect to the optical axis in the main scanning direction at which the distance in the optical axis direction between one end and the other end of the effective region of the sagittal tilt changing surface in the sub-scanning cross section is maximum is defined as y |Δs|max2 y is the position with respect to the optical axis in the main scanning direction at which the thickness of the first imaging optical element in the main scanning cross section in the optical axis direction is maximum. dmax2 , the position of the end of the effective area of ​​the sagittal tilt changing surface in the main scanning direction relative to the optical axis in the main scanning direction is defined as y max1 When 0.0≦|y |Δs|max2 -y dmax2 | / |y max2 |≦0.12 4. The optical scanning device according to configuration 3, wherein the following conditions are satisfied: (Configuration 5) 5. The optical scanning device according to any one of configurations 1 to 4, wherein the entrance surface and exit surface of the first imaging optical element are the sagittal tilt changing surfaces. (Configuration 6) 6. The optical scanning device according to configuration 5, wherein the entrance surface and the exit surface are inclined in different directions relative to a plane perpendicular to the optical axis in the sub-scan cross section. (Configuration 7) The sagittal tilt changing surface is configured such that the maximum value of the distance in the optical axis direction between one end and the other end in the sub-scanning direction of the effective area of ​​the sagittal tilt changing surface in the sub-scanning cross section is |Δs|(y |Δs|max ) (mm), 0.2≦|Δs|(y |Δs|max )≦1.2 7. The optical scanning device according to any one of configurations 1 to 6, characterized in that the following conditions are satisfied: (Configuration 8) 8. The optical scanning device according to any one of configurations 1 to 7, further comprising an incident optical system that makes a light beam from a light source obliquely incident on the deflector in a sub-scan cross section. (Configuration 9) the at least one imaging optical element includes a second imaging optical element disposed at a position closer to the deflector than the first imaging optical element on the optical path of the light beam; 9. The optical scanning device according to any one of configurations 1 to 8, wherein at least one of the entrance surface and the exit surface of the second imaging optical element is the sagittal tilt changing surface. (Configuration 10) 9. The optical scanning device according to any one of configurations 1 to 8, wherein the deflector deflects light beams from first and second light sources to scan first and second scanned surfaces in the main scanning direction. (Configuration 11) 11. The optical scanning device according to configuration 10, comprising first and second incident optical systems that make the light beams from the first and second light sources obliquely incident on the deflector at different angles in the sub-scanning cross section. (Configuration 12) The optical scanning device described in configuration 11, characterized in that the first and second incident optical systems cause light beams from the first and second light sources to be obliquely incident on the deflector from different sides of the main scanning cross section including the deflector. (Configuration 13) 13. An image forming apparatus comprising: an optical scanning device according to any one of configurations 1 to 12; a developing unit that develops an electrostatic latent image formed on the scanned surface by the optical scanning device into a toner image; a transfer unit that transfers the developed toner image to a transfer material; and a fixing unit that fixes the transferred toner image to the transfer material. (Configuration 14) 13. An image forming apparatus comprising: an optical scanning device according to any one of configurations 1 to 12; and a printer controller that converts code data output from an external device into an image signal and inputs the image signal to the optical scanning device. [Explanation of symbols]

[0087] 100 Optical scanning device LA entrance optical system SA imaging optical system (imaging optical system) 1A light source 2A Anamorphic Lens (Incoming Optical System) 3A Sub-scanning aperture stop (incident optical system) 4A Main scanning aperture stop (incident optical system) 5 Deflector 6A Imaging lens (imaging optical system) 7A Imaging lens (imaging optical system, first imaging optical element) 8A Scanned surface

Claims

1. a deflector that deflects a light beam from a light source to scan the surface to be scanned in a main scanning direction; an imaging optical system including at least one imaging optical element that guides the light beam from the deflector to the surface to be scanned, a first imaging optical element, which is closest to the scanned surface among the at least one imaging optical element, is made of a resin material; a thickness of the first imaging optical element in the optical axis direction in a main scanning cross section including the optical axis varies in the main scanning direction; the first imaging optical element includes a sagittal tilt changing surface; In a region on one side of the optical axis in the main scanning direction of the sagittal tilt changing surface, the position with respect to the optical axis in the main scanning direction at which the distance in the optical axis direction between one end and the other end in the sub-scanning direction of the effective region of the sagittal tilt changing surface in the sub-scanning cross section is maximum is defined as y ｜Δｓ｜ｍａｘ１ y is the position with respect to the optical axis in the main scanning direction at which the thickness of the first imaging optical element in the main scanning cross section in the optical axis direction is maximum. ｄｍａｘ１ When the maximum image height in the main scanning direction on the scanned surface is W1, 0.0≦|y ｜Δｓ｜ｍａｘ１ -y ｄｍａｘ１ | / W1≦0.1 An optical scanning device characterized by satisfying the following conditions.

2. In the region of the sagittal tilt changing surface on the opposite side to the one side with respect to the optical axis in the main scanning direction, the position with respect to the optical axis in the main scanning direction at which the distance in the optical axis direction between one end and the other end in the sub-scanning direction of the effective region of the sagittal tilt changing surface in the sub-scanning cross section is maximum is defined as y ｜Δｓ｜ｍａｘ２ y is the position with respect to the optical axis in the main scanning direction at which the thickness of the first imaging optical element in the main scanning cross section in the optical axis direction is maximum. ｄｍａｘ２ , where W2 is the maximum image height in the main scanning direction on the scanned surface, 0.0≦|y ｜Δｓ｜ｍａｘ２ -y ｄｍａｘ２ | / W2≦0.1 2. The optical scanning device according to claim 1, wherein the following conditions are satisfied:

3. a deflector that deflects a light beam from a light source to scan the surface to be scanned in a main scanning direction; an imaging optical system including at least one imaging optical element that guides the light beam from the deflector to the surface to be scanned, a first imaging optical element, which is closest to the scanned surface among the at least one imaging optical element, is made of a resin material; a thickness of the first imaging optical element in the optical axis direction in a main scanning cross section including the optical axis varies in the main scanning direction; the first imaging optical element includes a sagittal tilt changing surface; In a region on one side of the optical axis in the main scanning direction of the sagittal tilt changing surface, the position with respect to the optical axis in the main scanning direction at which the distance in the optical axis direction between one end and the other end in the sub-scanning direction of the effective region of the sagittal tilt changing surface in the sub-scanning cross section is maximum is defined as y ｜Δｓ｜ｍａｘ１ y is the position with respect to the optical axis in the main scanning direction at which the thickness of the first imaging optical element in the main scanning cross section in the optical axis direction is maximum. ｄｍａｘ１ , the position of the end of the effective area of ​​the sagittal tilt changing surface in the main scanning direction with respect to the optical axis in the main scanning direction is defined as y ｍａｘ１ When 0.0≦|y ｜Δｓ｜ｍａｘ１ -y ｄｍａｘ１ | / |y ｍａｘ１ |≦0.12 An optical scanning device characterized by satisfying the following conditions.

4. In the region of the sagittal tilt changing surface on the opposite side to the one side with respect to the optical axis in the main scanning direction, the position with respect to the optical axis in the main scanning direction at which the distance in the optical axis direction between one end and the other end in the sub-scanning direction of the effective region of the sagittal tilt changing surface in the sub-scanning cross section is maximum is defined as y ｜Δｓ｜ｍａｘ２ y is the position with respect to the optical axis in the main scanning direction at which the thickness of the first imaging optical element in the main scanning cross section in the optical axis direction is maximum. ｄｍａｘ２ , the position of the end of the effective area of ​​the sagittal tilt changing surface in the main scanning direction with respect to the optical axis in the main scanning direction is defined as y ｍａｘ１ When 0.0≦|y ｜Δｓ｜ｍａｘ２ -y ｄｍａｘ２ | / |y ｍａｘ２ |≦0.12 4. The optical scanning device according to claim 3, wherein the following conditions are satisfied:

5. 2. The optical scanning device according to claim 1, wherein the entrance surface and exit surface of said first imaging optical element are said sagittal tilt changing surfaces.

6. 6. The optical scanning device according to claim 5, wherein the entrance surface and the exit surface are inclined in different directions relative to a plane perpendicular to the optical axis in the sub-scan cross section.

7. The sagittal tilt changing surface is such that the maximum value of the distance in the optical axis direction between one end and the other end in the sub-scanning direction of the effective area of ​​the sagittal tilt changing surface in the sub-scanning cross section is |Δs|(y ｜Δｓ｜ｍａｘ ) (mm), 0.2≦|Δs|(y ｜Δｓ｜ｍａｘ )≦1.2 2. The optical scanning device according to claim 1, wherein the following conditions are satisfied:

8. 2. The optical scanning device according to claim 1, further comprising an incident optical system that causes a light beam from a light source to be obliquely incident on said deflector in a sub-scan cross section.

9. the at least one imaging optical element includes a second imaging optical element disposed at a position closer to the deflector than the first imaging optical element on the optical path of the light beam, 2. The optical scanning device according to claim 1, wherein at least one of the entrance surface and the exit surface of said second imaging optical element is said sagittal tilt changing surface.

10. 2. The optical scanning device according to claim 1, wherein the deflector deflects light beams from the first and second light sources to scan the first and second surfaces to be scanned in the main scanning direction.

11. 11. The optical scanning device according to claim 10, further comprising first and second incident optical systems that make the light beams from the first and second light sources obliquely incident on the deflector at different angles in the sub-scanning cross section.

12. 12. The optical scanning device according to claim 11, wherein the first and second incident optical systems make the light beams from the first and second light sources obliquely incident on the deflector from different sides of a main scanning cross section including the deflector.

13. 13. An image forming apparatus comprising: an optical scanning device according to claim 1; a developing unit that develops an electrostatic latent image formed on the scanned surface by the optical scanning device into a toner image; a transfer unit that transfers the developed toner image to a transfer material; and a fixing unit that fixes the transferred toner image to the transfer material.

14. 13. An image forming apparatus comprising: the optical scanning device according to claim 1; and a printer controller that converts code data output from an external device into an image signal and inputs the image signal to the optical scanning device.

Images