Optical scanning apparatus and image forming apparatus including the same

Abstract

To provide an entire optical scanning apparatus and an image forming apparatus, in which downsizing of an entire optical scanning apparatus is facilitated.SOLUTION: An optical scanning apparatus 100 comprises a deflector 105 that deflects a light flux from a light source 102 and scans a surface to be scanned 101 in a main scanning direction, and an imaging optical system 106 that guides the light flux deflected by the deflector 105 to the surface to be scanned 101. The speed of scanning with the light flux on the surface to be scanned 101 has a non-uniform velocity. On the surface to be scanned 101, when the absolute value of an outermost off-axis image height is defined as |Ymax|, and an intermediate image height at which the speed of scanning with the light flux becomes maximum as |Y|, a conditional expression of 0.70≤|Y| / |Ymax|<1.00 is satisfied.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Conventionally, there has been a demand for miniaturization of optical scanning devices used in image forming apparatuses. Patent Document 1 describes an optical scanning device that realizes miniaturization of the entire device by configuring an imaging optical system for guiding a light beam from a deflector to a surface to be scanned so that the surface to be scanned is optically scanned at a non-uniform speed. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-016144 Summary of the Invention [Problem to be solved by the invention]

[0004] Here, in order to further reduce the size of the optical scanning device, it is necessary to further reduce the thickness of the imaging optical elements that make up the imaging optical system.

[0005] SUMMARY OF THE INVENTION An object of the present invention is to reduce the overall size of an optical scanning device and an image forming apparatus equipped with the same. [Means for solving the problem]

[0006] In order to achieve the above object, one aspect of the present invention provides an optical scanning device comprising: 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 guides the light beam deflected by the deflector to the surface to be scanned, wherein the scanning speed of the light beam on the surface to be scanned is non-uniform, and where the absolute value of the most off-axis image height on the surface to be scanned is |Ymax| and the intermediate image height at which the scanning speed of the light beam is maximum is |Y|, the optical scanning device satisfies the condition 0.70≦|Y| / |Ymax|<1.00. [Effects of the Invention]

[0007] According to the present invention, it is possible to reduce the size of the entire optical scanning device and the image forming apparatus including the same. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a schematic diagram of a main part of an optical scanning device according to a first embodiment; [Figure 2] FIG. 1 is a diagram showing the curvature of an imaging optical element according to Example 1. [Figure 3] Power ratio to on-axis power in Example 1 [Figure 4] Scanning speed ratio to on-axis according to Example 1 [Figure 5] Schematic diagram explaining edge thickness [Figure 6] Curvature of the imaging optical element according to Example 2 [Figure 7] Scanning speed ratio to on-axis according to the second embodiment [Figure 8] Power ratio to on-axis power in Example 2 [Figure 9] Curvature of the imaging optical element according to the third embodiment [Figure 10] Scanning speed ratio to on-axis according to the third embodiment [Figure 11] Power ratio to on-axis power according to the third embodiment [Figure 12] Image forming device DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. Note that the drawings may be drawn at a scale different from the actual scale for convenience. In addition, the same reference numerals are used to designate the same components in the drawings, and redundant explanations will be omitted.

[0010] 1 is a schematic diagram of a main scanning cross section (XY cross section) of an optical scanning device 100 according to an embodiment of the present invention. The optical scanning device 100 includes a deflector 105 that deflects a light beam from a light source 102 to scan a surface 101 to be scanned in the main scanning direction (Y direction), and an imaging optical system 106 that guides the light beam deflected by the deflector 105 to the surface 101 to be scanned.

[0011] In the following description, the optical axis of the imaging optical system 106 is defined as the X-axis, and the traveling direction of the light beam from the imaging optical system 106 on the optical axis is defined as the +X-direction. The main scanning direction (Y-direction) is a direction (direction in which the scanned surface 101 is scanned) perpendicular to the rotation axis of the deflector 105 and the optical axis direction (X-direction) of the imaging optical system 106, and the scanning start side and scanning end side of the scanned surface 101 with respect to the optical axis (X-axis) are defined as the +Y-side and -Y-side, respectively. The sub-scanning direction (Z-direction) is a direction parallel to the rotation axis of the deflector 105. The main scanning cross section (XY cross section) is a cross section that includes the optical axis (X-axis) and is parallel to the main scanning direction (cross section perpendicular to the sub-scanning direction). The sub-scanning cross section (ZX cross section) is a cross section that is parallel to the optical axis (X-axis) and the sub-scanning direction (cross section perpendicular to the main scanning direction).

[0012] Furthermore, the image height on the optical axis of the imaging optical system 106 on the scanned surface 101 (the intersection of the scanned surface 101 and the optical axis) is defined as the on-axis image height Y0, and other image heights are defined as off-axis image heights. Of the off-axis image heights, the image heights corresponding to both ends of the effective area (image formation area) on the scanned surface 101 are defined as the most off-axis image heights ±Ymax, and the image height between the on-axis image height Y0 and the most off-axis image height ±Ymax is defined as the intermediate image height. The on-axis image height Y0 corresponds to the center (Y=0) of the effective area on the scanned surface 101, and the most off-axis image height on the scanning start side relative to the on-axis image height Y0 is defined as +Ymax, and the most off-axis image height on the scanning end side is defined as -Ymax.

[0013] 1 shows only the most off-axis light beams L1 and L3 that reach the most off-axis image height +Ymax (scanning start position) and the most off-axis image height −Ymax (scanning end position), respectively, and the on-axis light beam L2 that reaches the on-axis image height Y0 (center position), among the light beams from the light source 102, and omits other light beams. Also, for each of the light beams L1, L2, and L3, only the chief ray and marginal ray are shown, and other light rays are omitted.

[0014] A typical constant speed scanning optical scanning device is configured so that when light beams emitted at constant intervals from a light source are deflected by a deflector rotating at a constant speed, they reach the scanned surface at equal intervals. That is, the imaging optical system in a constant speed scanning optical scanning device has a distortion aberration (fθ characteristic) in which the rotation angle (scanning angle) of the deflector is proportional to the image height in the main scanning direction on the scanned surface so that the passing light beam scans the scanned surface at a constant speed. Here, in order to form a good image (spot) in the effective area on the scanned surface, it is necessary to properly correct the field curvature of the imaging optical system over the entire effective area.

[0015] However, in order to ensure uniform speed of optical scanning while properly correcting field curvature, it is necessary to make the shape of each optical surface in the imaging optical system different on the optical axis in the main-scan cross section from that on the other side. Furthermore, if the imaging optical elements constituting the imaging optical system are located close to the deflector in order to reduce the size of the optical scanning device, the shape of each optical surface becomes steep. In this case, coma aberration increases and the thickness of each imaging optical element increases, making it difficult to sufficiently reduce the size of the entire device.

[0016] Therefore, the imaging optical system 106 according to this embodiment is configured so that the light beam passing through it does not satisfy the uniform velocity requirement on the scanned surface 101 (scans at a non-uniform velocity). In other words, the imaging optical system 106 has a partial magnification (local magnification in the main scanning direction) such that the arrival position of the light beam on the scanned surface 101 is shifted from the arrival position when scanning at a uniform velocity. That is, the optical scanning device 100 according to this embodiment employs a non-uniform velocity scanning method in which the scanning velocity of the light beam differs between the on-axis image height and the off-axis image height. This makes it possible to shorten the distance between the imaging optical system 106 and the deflector 105 and the scanned surface 101 while maintaining imaging performance. Furthermore, since it is not necessary to design the power of the imaging optical system 106 to perform uniform velocity scanning, it is possible to suppress an increase in the thickness of the imaging optical elements constituting the imaging optical system 106. This makes it possible to achieve a smaller diameter for the optical scanning device 100.

[0017] When a non-uniform scanning method is adopted, the scanning position (scanning distance per unit time) at an off-axial image height is elongated depending on the deviation (partial magnification deviation) of the partial magnification at the off-axial image height from the partial magnification at the on-axial image height. Therefore, scanning the scanned surface 101 without considering this partial magnification deviation may result in degradation of the image formed on the scanned surface 101 (degradation of printing performance). Therefore, it is desirable to suppress such degradation of printing performance by controlling the light emission of the light source 102 using a control unit (not shown). Specifically, it is desirable to electrically correct at least one of the scanning position and scanning time on the scanned surface 101 by controlling at least one of the modulation timing (light emission timing) and modulation time (light emission time) of the light source 102 according to the partial magnification deviation. This corrects the partial magnification deviation and image degradation, enabling good printing performance to be achieved, similar to when the fθ characteristic is satisfied. In this case, to ensure better printing performance, it is desirable to keep the partial magnification deviation of the imaging optical system 106 within 2% at all image heights.

[0018] When using a constant-speed scanning system, it is necessary to intentionally generate distortion by increasing the power (refractive power) at the edges of the imaging optical system compared to the center so that the light beams reach equidistantly near the most off-axis image height on the scanned surface. On the other hand, when using a non-constant-speed scanning system, it is not necessary to increase the power at the edges of the imaging optical system as in the constant-speed scanning system. This allows the thickness of the edges of the imaging optical elements constituting the imaging optical system to be thinner than when using a constant-speed scanning system. However, simply reducing the power at the edges of the imaging optical system results in a monotonically increasing scanning speed from the on-axis image height to the most off-axis image height, which increases the beam diameter (spot diameter) near the most off-axis image height, potentially resulting in poor printing performance. Furthermore, attempting to thin the entire imaging optical element while maintaining imaging performance can result in the thickness of the edges (edges) of the effective area of ​​the imaging optical element becoming too thin, potentially resulting in molding defects in the imaging optical element.

[0019] Therefore, in the optical scanning device 100 according to this embodiment, the imaging optical system 106 is configured so that the scanning speed is maximized at an intermediate image height between the on-axis image height and the most off-axis image height on the scanned surface 101. Specifically, when the absolute value of the most off-axis image height on the scanned surface 101 is |Ymax| and the absolute value of the intermediate image height at which the scanning speed is maximized is |Y|, the optical scanning device 100 according to this embodiment satisfies the following conditional expression (1): 0.70≦|Y| / |Ymax|<1.00 (1)

[0020] When conditional expression (1) is satisfied, a graph of the scanning speed versus image height on the scanned surface 101 has a maximum value at intermediate image heights ±Y. This allows the entire imaging optical element constituting the imaging optical system 106 to be thinned while ensuring sufficient thickness at the edges of the imaging optical element. If the lower limit of conditional expression (1) is not met, it becomes necessary to increase the power from the center to the middle of the imaging optical element. In this case, to achieve good printing performance across the entire effective area, it becomes necessary to significantly thin the edges of the imaging optical element, making it difficult to manufacture the imaging optical element. Furthermore, if the upper limit of conditional expression (1) is exceeded, the scanning speed increases monotonically from the on-axial image height Y0 to the most off-axial image height Ymax, which increases the spot diameter near the most off-axial image height, making it difficult to achieve good printing performance.

[0021] Furthermore, it is preferable to satisfy the following conditional formula (1a), and it is more preferable to satisfy the following conditional formula (1b): 0.72≦|Y| / |Ymax|≦0.98 (1a) 0.75≦|Y| / |Ymax|≦0.95 (1b)

[0022] It is desirable that the imaging optical system 106 be configured so that the scanning speed of the light beam at the outermost off-axis image heights ±Ymax on the scanned surface 101 is faster than the scanning speed of the light beam at the on-axial image height Y0. If the scanning speed at the outermost off-axis image heights ±Ymax is made slower than the on-axial image height Y0 while satisfying conditional expression (1), it becomes necessary to either significantly reduce the scanning speed at the outermost off-axis image heights ±Ymax or increase the scanning speed at the on-axial image height Y0 to close to the maximum scanning speed. In this case, the change in scanning speed near the outermost off-axis image heights ±Ymax becomes too large, or the change in scanning speed across the entire scanned surface 101 becomes too small, which could result in poor printing performance.

[0023] Furthermore, it is desirable that the imaging optical system 106 be configured so that the scanning speed of the light beam on the scanned surface 101 changes monotonically between the intermediate image height ±Y, the on-axis image height Y0, and the most off-axis image height ±Ymax. Here, "between the intermediate image height ±Y, the on-axis image height Y0, and the most off-axis image height ±Ymax" refers to the distance between the intermediate image height ±Y and the on-axis image height Y0, the distance between the intermediate image height +Y on the scanning start side and the most off-axis image height +Ymax, and the distance between the intermediate image height -Y and the most off-axis image height -Ymax on the scanning end side. In other words, when the scanning speed versus image height on the scanned surface 101 is represented as a graph, it is desirable that the graph have a minimum value only at the on-axis image height Y0 and a maximum value only at the intermediate image height +Y on the scanning start side and the intermediate image height -Y on the scanning end side.

[0024] This configuration makes it possible to prevent good printing performance from being lost due to extreme changes in scanning speed between image heights. In this case, it is preferable that the imaging optical system 106 is configured so that the scanning speed of the light beam on the scanned surface 101 monotonically increases from the on-axial image height Y0 toward the intermediate image heights ±Y, and monotonically decreases from the intermediate image heights ±Y toward the most off-axial image heights ±Ymax. This makes it possible to prevent good printing performance from being lost, for the same reasons as those explained above.

[0025] Furthermore, when the maximum value (maximum value) of the scanning speed of the light beam on the scanned surface is Vmax and the scanning speed of the light beam at the outermost off-axis image height ±Ymax is Ve, it is desirable that the optical scanning device 100 according to this embodiment satisfies the following conditional expression (2): 0.85≦Ve / Vmax<1.00 (2)

[0026] If the lower limit of conditional expression (2) is not reached, it becomes necessary to extremely increase the power at the end of the imaging optical element, which makes the thickness (edge ​​thickness) of the end of the effective area of ​​the imaging optical element too thin, which may result in molding defects during manufacturing. Also, if the upper limit of conditional expression (2) is exceeded, the scanning speed of the light beam on the scanned surface 101 increases monotonically between the on-axial image height Y0 and the most off-axial image heights ±Ymax, which increases the spot diameter near the most off-axial image heights, which may result in poor printing performance.

[0027] Furthermore, it is preferable to satisfy the following conditional expression (2a), and it is more preferable to satisfy the following conditional expression (2b): 0.88≦Ve / Vmax≦0.98 (2a) 0.90≦Ve / Vmax≦0.96 (2b)

[0028] Furthermore, when the minimum value (local minimum value) of the scanning speed of the light beam on the surface to be scanned is Vmin, it is desirable that the optical scanning device 100 according to this embodiment satisfy the following conditional expression (3). 1.10≦Vmax / Vmin<1.40 (3)

[0029] If the lower limit of conditional expression (3) is exceeded, the difference between the minimum and maximum scanning speeds becomes too small, approaching a constant speed scanning method, which may make it difficult to thin the imaging optical element. Also, if the upper limit of conditional expression (3) is exceeded, the maximum scanning speed becomes too large, which may make it difficult to suppress an increase in spot diameter near the most off-axis image height.

[0030] Furthermore, it is preferable to satisfy the following conditional expression (3a), and it is more preferable to satisfy conditional expression (3b): 1.15≦Vmax / Vmin<1.35 (3a) 1.20≦Vmax / Vmin<1.30 (3b)

[0031] Furthermore, when the thickness of the imaging optical element on the optical axis is Do and the thickness (edge ​​thickness) of the imaging optical element at the position where the most off-axis light ray passes through is Dc, it is desirable that the optical scanning device 100 according to this embodiment satisfies the following conditional expression (4): 0.46≦Dc / Do≦0.60 (4)

[0032] If the lower limit of conditional expression (4) is exceeded, the edge thickness of the imaging optical element becomes too thin, which may result in molding defects in the imaging optical element, whereas if the upper limit of conditional expression (4) is exceeded, it may become difficult to thin the entire imaging optical element.

[0033] Furthermore, it is preferable to satisfy the following conditional expression (4a), and it is more preferable to satisfy conditional expression (4b): 0.47≦Dc / Do≦0.58 (4a) 0.48≦Dc / Do≦0.55 (4b)

[0034] A number of examples based on the above-described embodiment will now be described in detail.

[0035] [Example 1] Hereinafter, an optical scanning device 100 according to a first embodiment of the present invention will be described. The optical scanning device 100 according to this embodiment has the same configuration as the optical scanning device 100 according to the above-described embodiment, and therefore, redundant description will be omitted.

[0036] 1, an optical scanning device 100 according to this embodiment includes a light source 102, a diaphragm 103 that regulates the light beam from the light source 102, an incident optical system 104 that guides the light beam from the light source 102 to a deflector 105, and the above-mentioned deflector 105 and imaging optical system 106. In the optical scanning device 100, the light beam emitted from the light source 102 passes through an opening provided in the diaphragm 103 and is guided to a deflection surface of the deflector 105 by the incident optical system 104. Note that the light source 102, the diaphragm 103, and the incident optical system 104 do not necessarily have to be included in the optical scanning device 100; for example, these may be configured as external lighting devices.

[0037] The light source 102 has a substrate and a light-emitting element (light-emitting point) provided on the substrate. For example, a semiconductor laser can be used as the light source 102, and the number of light-emitting points may be one or more. The light source 102 according to this embodiment is an edge-emitting laser, and is configured with a single light-emitting point provided on the substrate. When the light source 102 has multiple light-emitting points, it is desirable to use a VCSEL (Vertical Cavity Surface Emitting Laser). Alternatively, an LED (Light Emitting Diode) may be used as the light source 102. In this embodiment, it is assumed that the wavelength of the light beam emitted from the light source 102 is 780 nm, but a light source that emits a light beam with another wavelength may also be used.

[0038] The diaphragm 103 is a component for shaping the shape of the light beam reaching the deflector 105 and limiting the amount of light by blocking a portion of the light beam from the light source 102. Of the light beam passing through an opening provided in the diaphragm 103, a ray passing through the center of the opening is a chief ray, and a ray passing through the extreme edge of the opening is a marginal ray. In this embodiment, a rectangular diaphragm having a rectangular opening is used as the diaphragm 103, but the shape of the opening is not limited to this. For example, an elliptical diaphragm having an elliptical opening or a circular diaphragm having a circular opening may also be used as the diaphragm 103.

[0039] The incident optical system 104 according to this embodiment has an anamorphic lens as an optical element (incident optical element) that converts the convergence of the light beam from the light source 102. An anamorphic lens is a lens that has different powers (refractive powers) in the main scanning cross section and the sub-scanning cross section. The anamorphic lens according to this embodiment converts the divergent light beam from the light source 102 into a substantially parallel light beam or a substantially convergent light beam in the main scanning cross section, and collects the light on the deflection surface of the deflector 105 in the sub-scanning cross section.

[0040] Here, it is desirable that the anamorphic lens in the incident optical system 104 be disposed closer to the deflector 105 (the -Y side) than the front principal plane of the anamorphic lens. Such an arrangement allows the light beam emitted from the anamorphic lens to become a substantially convergent light beam in the main scanning cross section, making it easier to thin the imaging optical elements that make up the imaging optical system 106. Although the incident optical system 104 in this embodiment is configured with only a single optical element (anamorphic lens), the incident optical system 104 may also have multiple optical elements. For example, the incident optical system 104 may be configured with two optical elements, a collimator lens and a cylindrical lens, so that each optical element is responsible for converting the convergence in the main scanning cross section and collecting light in the sub-scanning cross section.

[0041] The deflector 105 is rotated at a constant speed in the direction of arrow A in the figure by a drive unit (e.g., motor) not shown, and deflects the light beam from the incident optical system 104 using multiple deflecting surfaces (reflecting surfaces). The light beam from the deflector 105 passes through the entrance surface 106a (the optical surface on the deflector 105 side) and the exit surface 106b (the optical surface on the scanned surface 101 side) of the imaging optical element in order, scanning the effective area of ​​the scanned surface 101 sequentially from the scanning start side (+Y side) to the scanning end side (-Y side). When the optical scanning device 100 is applied to an image forming apparatus described below, the effective area of ​​the scanned surface 101 becomes the image formation area (printing area). In this embodiment, a rotating polygon mirror having four deflecting surfaces is used as the deflector 105, but the number of deflecting surfaces is not limited to this. Furthermore, instead of the rotating polygon mirror, an oscillating mirror with one or two oscillating deflecting surfaces may be used. As the oscillating mirror, for example, a MEMS (Micro Electro Mechanical Systems) can be used.

[0042] The imaging optical system 106 according to this embodiment forms an image of the light emitting point of the light source 102 on the scanned surface 101 or its vicinity in both the main scanning section and the sub-scanning section by focusing the light beam deflected by the deflector 105 on the scanned surface 101. At this time, by arranging the imaging optical system 106 so that the deflecting surface or its vicinity and the scanned surface 101 or its vicinity have a conjugate relationship in the sub-scanning section, it is possible to reduce the scanning position deviation on the scanned surface 101 when the deflecting surface is tilted due to a placement error or the like (compensation for surface tilt).

[0043] Although the imaging optical system 106 according to this embodiment is configured with only a single imaging optical element, the imaging optical system 106 may also have multiple imaging optical elements. However, in order to reduce the size of the entire apparatus, it is desirable to configure the imaging optical system 106 with only a single imaging optical element, as in this embodiment. The imaging optical element according to this embodiment is an anamorphic lens (toric lens) having different powers in the main scanning cross section and the sub-scanning cross section.

[0044] The anamorphic lenses constituting the incident optical system 104 and the imaging optical system 106 according to this embodiment are plastic molded lenses formed by injection molding a resin material, which allows for significant cost reduction compared to using glass lenses. Furthermore, using plastic molded lenses makes it easier to mold diffractive and aspherical surfaces, improving productivity and optical performance. However, if necessary, the lenses constituting the incident optical system 104 and the imaging optical system 106 may also be glass lenses. The anamorphic lenses according to this embodiment are made of a resin material called K22R (trademark: Zeonex) manufactured by Zeon Corporation, but the resin material is not limited to this.

[0045] For each of the entrance surface 106a and the exit surface 106b of the imaging optical element according to this embodiment, the shape (meridian shape) in the main-scan cross section including the surface vertex is expressed by the following equation (aspheric equation). Here, a local coordinate system is defined in which the intersection of the surface vertex of each optical surface (lens surface) and each optical axis is the origin, the axis in the optical axis direction is the X-axis, the axis perpendicular to the X-axis in the main-scan cross section is the Y-axis, and the axis perpendicular to the X-axis and Y-axis is the Z-axis (to be distinguished from the global coordinate system in FIG. 1). Furthermore, the meridian shapes on the scanning start side (+Y side) and the scanning end side (-Y side) with respect to the optical axis (X-axis) are expressed by different aspheric equations.

[0046]

number

[0047]

number

[0048] where R is the radius of curvature (generatrix radius of curvature) on the optical axis in the main scanning section, and K, B3, B4, B6, B8, B 10 ,B 12 are aspherical coefficients in the main scanning cross section. The suffix s added to the aspherical coefficient B indicates that it is the scanning start side (+Y side), and the suffix e indicates that it is the scanning end side (-Y side). The aspherical coefficients B4 to B5 are 12 By making the values ​​of .gamma. and .gamma. different from each other, the meridional shape can be made asymmetric in the main scanning direction with respect to the optical axis.

[0049] Furthermore, the radius of curvature r' (sagittal radius of curvature) in the sub-scanning cross section at each position (each image height) in the main scanning direction of each optical surface of the imaging optical system 106 according to this embodiment is expressed by the following equation (continuous function): The above-mentioned local coordinate system is also assumed in the following equation, and the sagittal radius of curvature on the scanning start side (+Y side) and the sagittal radius of curvature on the scanning end side (-Y side) with respect to the optical axis (X axis) are expressed by different aspherical equations.

[0050]

number

[0051]

number

[0052] where r is the radius of curvature on the optical axis in the sub-scan section, and E i is the sagittal variation coefficient. Sagittal variation coefficient E i The suffix s added to indicates the scanning start side (+Y side), and the suffix e indicates the scanning end side (-Y side). Note that the sagittal curvature radius (sagittal shape) of each optical surface can be rephrased as the surface shape on a cross section perpendicular to the main scanning cross section that includes a surface normal on the generatrix at each position in the main scanning direction.

[0053] Table 1 shows the specifications of the optical scanning device 100 according to this embodiment. Note that each distance listed in Table 1 indicates a value on the optical axis of the incident optical system 104 or the imaging optical system 106. The distance between the deflector 105 and each component varies depending on the rotation angle of the deflector 105. However, Table 1 shows the distance to the intersection (axial deflection point) between the chief ray of the axial light beam L2 and the deflection surface when the axial light beam L2 is deflected to reach the axial image height Y0. The "angle of incident chief ray" indicates the angle between the chief ray of the light beam that is emitted from the incident optical system 104 and incident on the deflection surface and the optical axis of the imaging optical system 106. The "coordinates of the rotation center of the deflector" indicate the coordinates (position) of the rotation center (rotation axis) of the deflector 105 when the axial deflection point is set as the origin. In this embodiment, the maximum scanning angle and the most off-axis image height (effective scanning width) are symmetrical with respect to the optical axis of the imaging optical system 106, and in Table 1, the values ​​on one side of the optical axis are positive and the values ​​on the other side are negative.

[0054] [Table 1]

[0055] The shape of each optical surface of the imaging optical element according to this embodiment is shown in Table 2. In Table 2, the suffix s added to each of the "entrance surface" and "exit surface" indicates the scanning start side (+Y side), and the suffix e indicates the scanning end side (-Y side). Also, ".E±N" means "×10 ±N As shown in Table 2, the shapes of the entrance surface and exit surface of the imaging optical element according to this embodiment are different between the scanning start side and the scanning end side in the main scanning cross section, in other words, asymmetric with respect to the optical axis.

[0056] [Table 2]

[0057] FIG. 2 is a graph showing the curvature of each optical surface of the imaging optical element according to this embodiment in the main scanning cross section for each position in the main scanning direction. In FIG. 2, the vertical axis indicates the local curvature in the main scanning cross section of the imaging optical element, the horizontal axis indicates the position in the main scanning direction (Y direction) relative to the optical axis (X axis), the solid line indicates the graph for the incident surface, and the dashed line indicates the graph for the exit surface. In FIG. 2, when the sign of the curvature is positive, it indicates that the center of curvature of the optical surface is located on the +X side (the side of the scanned surface 101) when the vertex of the optical surface is used as the reference. Furthermore, when the sign of the curvature is negative, it indicates that the center of curvature of the optical surface is located on the -X side (the side of the deflector 105) when the vertex of the optical surface is used as the reference. Note that the curvature of each optical surface at each position in the main scanning direction can be calculated from the radius of curvature at a local position on the optical surface, and therefore can be derived using the aspherical formula described above.

[0058] 2, on the optical axis of the imaging optical element according to this embodiment, the signs of the curvatures of the entrance surface and the exit surface are both negative, and therefore the centers of curvature of each are located on the side of the deflector 105. Therefore, on the optical axis, the entrance surface is concave and the exit surface is convex, and the imaging optical element has a meniscus shape. This makes it easy to thin the central portion of the imaging optical element.

[0059] Furthermore, the curvature of the exit surface of the imaging optical element changes so that it approaches 0 (so that its absolute value decreases) toward an intermediate position approximately ±10 mm away from the position on the optical axis. In this way, by ensuring a sufficient curvature near the optical axis of the exit surface while varying the absolute value of that curvature so that it decreases toward intermediate image heights, it is possible to thin the center portion of the imaging optical element while ensuring a sufficient edge thickness. This facilitates both miniaturization of the entire device and simplification of manufacturing the imaging optical element.

[0060] 3 is an enlarged view of a portion of the imaging optical system (imaging optical element) 106 and the most off-axis light beam L1 in FIG. 1. In FIG. 3, the intersections of the marginal ray (most off-axis ray) L1a on the +Y side of the most off-axis light beam L1 with the entrance surface and exit surface of the imaging optical element are indicated as x1 and x2, respectively. The distance Dc in the X direction between the intersection points x1 and x2 corresponds to the thickness (edge ​​thickness) of the imaging optical element at the most off-axis position. From FIG. 3, it can be seen that in this embodiment, the edge thickness Dc of the imaging optical element is approximately half the thickness on the optical axis, ensuring a sufficient edge thickness.

[0061] Fig. 4 is a graph showing the power (refractive power) in the main scanning cross section of the imaging optical element according to this embodiment for each position in the main scanning direction. In Fig. 4, in order to make it easier to understand the change in the power of the imaging optical element in the main scanning direction, the value of the off-axis power (normalized power) when the on-axis power is set to 1 is shown in correspondence with the image height on the scanned surface 101. In other words, the normalized power here indicates the ratio (power ratio) of the off-axis power to the power (refractive power) on the optical axis. In Fig. 4, the vertical axis indicates the local normalized power in the main scanning cross section of the imaging optical element, and the horizontal axis indicates the image height relative to the optical axis (X axis).

[0062] 4, the solid line is a graph corresponding to this embodiment, the dashed line is a graph corresponding to Comparative Example 1, and the two-dot chain line is a graph corresponding to Comparative Example 2. Comparative Examples 1 and 2 are assumed to be configurations in which only the imaging optical element in the optical scanning device 100 according to this embodiment is changed. Specifically, Comparative Example 1 is assumed to be an imaging optical element in the case where the optical scanning device 100 is of a constant speed scanning type, and Comparative Example 2 is assumed to be an imaging optical element in the case where the scanning speed of the optical scanning device 100 monotonically increases from the on-axis image height to the most off-axis image height.

[0063] As shown in Fig. 4, the normalized power of the imaging optical element according to Comparative Example 1 increases significantly from near the intermediate image height, which is half the most off-axis image height, toward the most off-axis image height in order to achieve a constant speed scanning system. On the other hand, in Comparative Example 2, which uses a non-constant speed scanning system, the increase in normalized power from near the intermediate image height toward the most off-axis image height can be suppressed compared to Comparative Example 1. Furthermore, in this example, the image height at which the normalized power begins to increase is significantly shifted toward the most off-axis image height compared to Comparative Example 2, and therefore the normalized power near the most off-axis image height can be significantly reduced. This makes it possible to thin the center portion of the imaging optical element while ensuring a sufficient edge thickness compared to Comparative Examples 1 and 2.

[0064] Fig. 5 is a graph showing the scanning speed of the light beam for each image height on the scanned surface 101. In Fig. 5, the vertical axis indicates the scanning speed (normalized scanning speed) at an off-axial image height when the scanning speed at an on-axial image height is set to 1, and the horizontal axis indicates the image height with respect to the optical axis (X-axis). In other words, the normalized scanning speed indicates the ratio (scanning speed ratio) of the scanning speed at an off-axial image height to the scanning speed at an on-axial image height. This scanning speed ratio corresponds to the ratio (partial magnification ratio) of the off-axial partial magnification to the on-axis partial magnification of the imaging optical system 106.

[0065] As shown in FIG. 5, the normalized scanning speed in this embodiment is minimum (and extremely small) at the on-axis image height Y0 (image height 0 mm) and maximum (and extremely large) at the intermediate image height Y (image height 101 mm). In other words, in this embodiment, the normalized scanning speed at the most off-axis image height Ymax is neither minimum nor maximum. In this way, by configuring the scanning speed to be maximum at an image height lower than the most off-axis image height, it is possible to suppress an increase in spot diameter near the most off-axis image height. This makes it possible to achieve better printing performance than a configuration in which the scanning speed increases monotonically from the on-axis image height to the most off-axis image height.

[0066] [Example 2] An optical scanning device 100 according to a second embodiment of the present invention will be described below. The optical scanning device 100 according to this embodiment differs from the optical scanning device 100 according to the first embodiment described above in the shape and arrangement of the imaging optical element. The rest of the configuration is the same as that of the first embodiment, so a description thereof will be omitted.

[0067] Table 3 shows the shape of each optical surface of the imaging optical element according to this example.

[0068] [Table 3]

[0069] As in FIG. 2, FIG. 6 shows the change in curvature in the main-scan cross section of each optical surface of the imaging optical element according to this embodiment. Similarly to FIG. 4, FIG. 7 shows the change in normalized power of the imaging optical element according to this embodiment. Similarly to FIG. 5, FIG. 8 shows the change in normalized scanning speed on the scanned surface 101. As in Example 1, on the optical axis of the imaging optical element according to this embodiment, the entrance surface is concave and the exit surface is convex, and the imaging optical element has a meniscus shape. Furthermore, the curvature of the exit surface of the imaging optical element changes to approach 0 from the position on the optical axis toward an intermediate position approximately ±10 mm away. Furthermore, the image height at which the normalized power of the imaging optical element begins to increase is sufficiently close to the most off-axis image height.

[0070] As shown in Fig. 8, in this embodiment, the position of the intermediate image height at which the scanning speed reaches the maximum value Vmax is closer to the on-axial image height Y0 than in Example 1. Therefore, the scanning speed Ve at the most off-axial image height is lower than in Example 1. In contrast, as shown in Fig. 7, the imaging optical element according to this embodiment is configured so that the normalized power at the most off-axial image height is greater than in Example 1. This makes it possible to thin the central portion of the imaging optical element while ensuring a sufficient edge thickness.

[0071] [Example 3] An optical scanning device 100 according to a third embodiment of the present invention will be described below. The optical scanning device 100 according to this embodiment differs from the optical scanning devices 100 according to the first and second embodiments in the shape and arrangement of the imaging optical element. Other configurations are the same as those of the first embodiment, and therefore description thereof will be omitted.

[0072] The shape of each optical surface of the imaging optical element according to this embodiment is shown in Table 4. Similarly to FIG. 2, changes in curvature in the main scanning cross section of each optical surface of the imaging optical element according to this embodiment are shown in FIG.

[0073] [Table 4]

[0074] As in FIG. 2, FIG. 9 shows the change in curvature of each optical surface of the imaging optical element according to this embodiment in the main-scan cross section. As in FIG. 4, FIG. 10 shows the change in normalized power of the imaging optical element according to this embodiment. As in FIG. 5, FIG. 11 shows the change in normalized scanning speed on the scanned surface 101. As in Examples 1 and 2, on the optical axis of the imaging optical element according to this embodiment, the entrance surface is concave and the exit surface is convex, and the imaging optical element has a meniscus shape. Furthermore, the curvature of the exit surface of the imaging optical element changes to approach zero from a position on the optical axis toward an intermediate position approximately ±10 mm away. Furthermore, the image height at which the normalized power of the imaging optical element begins to increase is sufficiently close to the most off-axis image height.

[0075] As shown in Fig. 11, in this embodiment, the position of the intermediate image height at which the scanning speed reaches the maximum value Vmax is closer to the on-axial image height Y0 than in Example 2. Therefore, the scanning speed Ve at the most off-axial image height is lower than in Example 2. In contrast, as shown in Fig. 10, the imaging optical element according to this embodiment is configured so that the normalized power at the most off-axial image height is greater than in Example 2. This makes it possible to thin the central portion of the imaging optical element while ensuring a sufficient edge thickness.

[0076] Table 5 shows the values ​​of the middle sides of the conditional expressions (1) to (3) in each of the above-mentioned embodiments.

[0077] [Table 5]

[0078] [Image forming device] 12 is a schematic diagram (ZX cross-sectional view) of a main part of an image forming apparatus 600 according to an embodiment of the present invention. The image forming apparatus 600 is a tandem-type color image forming apparatus that records image information on the photosensitive surfaces (scanned surfaces) of four photosensitive drums (photoconductors) in parallel using an optical scanning unit 500.

[0079] The image forming apparatus 600 includes a printer controller (controller) 530, an optical scanning unit 500, photosensitive drums 210, 220, 230, and 240 as image carriers, developing units 310, 320, 330, and 340, a conveyor belt 510, and a fixing unit 540. The optical scanning unit 500 may be configured to include four optical scanning devices according to any one of the first to third embodiments. In this case, the optical scanning unit 500 is disposed so that the sub-scanning direction coincides with the Z direction, which is the rotation direction of each of the photosensitive drums 210 to 240.

[0080] 12, R (red), G (green), and B (blue) color signals are output from an external device 520 such as a personal computer. Each color signal is converted by a printer controller 530 into Y (yellow), M (magenta), C (cyan), and K (black) image signals (image data, dot data), which are then input to the optical scanning unit 500. Note that the printer controller 530 not only converts the signals described above, but also controls each part of the image forming apparatus 600, such as a motor, which will be described later.

[0081] The optical scanning unit 500 scans the photosensitive surfaces of the photosensitive drums 210-240 in the main scanning direction (Y direction) with light beams 410, 420, 430, and 440 modulated according to the image data. Each of the photosensitive drums 210-240 is rotated clockwise by a motor (not shown), and with this rotation, each photosensitive surface moves in the sub-scanning direction (Z direction) relative to the light beams 410-440. Each of the photosensitive surfaces, which have been charged by a charging roller (not shown), is exposed to the light beams 410-440, thereby forming an electrostatic latent image on each photosensitive surface.

[0082] Thereafter, the electrostatic latent images corresponding to each color formed on the photosensitive surfaces of the photosensitive drums 210 to 240 are developed into toner images of the respective colors by the respective developing units 310 to 340. The toner images of each color are then multiply transferred by a transfer unit (not shown) onto a transfer material conveyed by a conveyor belt 510, and then fixed by a fixing unit 540. Through the above process, one full-color image is formed.

[0083] The optical scanning unit 500 may be configured, for example, by arranging two optical scanning devices that simultaneously scan two surfaces to be scanned with one deflector, or by arranging one optical scanning device that simultaneously scans four surfaces to be scanned with one deflector. Also, a color digital copier may be configured by connecting a color image reading device equipped with a line sensor such as a CCD sensor or a CMOS sensor to the image forming apparatus 600 as the external device 520. Alternatively, a monochrome image forming apparatus may be configured by employing the optical scanning unit 500 consisting of a single optical scanning device.

[0084] Although the preferred embodiments and examples of the present invention have been described above, the present invention is not limited to these embodiments and examples, and various combinations, modifications, and changes are possible within the scope of the gist of the present invention.

[0085] For example, the optical scanning device 100 of this embodiment may include a synchronization detection means used to determine the scan start position (image writing start position) on the scanned surface 101. A light receiving unit (sensor) that receives a light beam deflected by the deflector 105 can be used as the synchronization detection means. Based on the output (signal) from the light receiving unit, the control unit controls the light emission timing of the light source 102, thereby determining the scan start position on the scanned surface 101. When the synchronization detection means is used, the light receiving element in the light receiving unit and the light emitting element in the light source 102 may be provided on the same substrate.

[0086] The embodiments of the present invention include the following configurations.

[0087] (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 that guides the light beam deflected by the deflector to the surface to be scanned, the scanning speed of the light beam on the surface to be scanned is non-uniform; On the scanned surface, when the absolute value of the most off-axis image height is |Ymax| and the intermediate image height at which the scanning speed of the light beam is maximum is |Y|, 0.70≦|Y| / |Ymax|<1.00 An optical scanning device characterized by satisfying the following conditional expression:

[0088] (Configuration 2) 2. The optical scanning device according to configuration 1, wherein the scanning speed of the light beam at the most off-axis image height on the surface to be scanned is faster than the scanning speed of the light beam at an axial image height.

[0089] (Configuration 3) 3. The optical scanning device according to configuration 1 or 2, wherein the scanning speed of the light beam on the surface to be scanned varies monotonically between the intermediate image height, the on-axis image height, and the most off-axis image height.

[0090] (Configuration 4) The optical scanning device described in configuration 3, characterized in that the scanning speed of the light beam on the scanned surface monotonically increases as the image height moves from an on-axis image height to the intermediate image height, and monotonically decreases as the image height moves from the intermediate image height to the most off-axis image height.

[0091] (Configuration 5) 5. The optical scanning device according to any one of configurations 1 to 4, wherein the imaging optical system includes only one imaging optical element.

[0092] (Configuration 6) 6. The optical scanning device according to any one of configurations 1 to 5, wherein the entrance surface of the imaging optical element is concave on the optical axis, and the exit surface of the imaging optical element is convex.

[0093] (Configuration 7) The optical scanning device according to any one of configurations 1 to 6, further comprising an optical element that converts the convergence of the light beam from the light source, and the refractive power of the optical element is different between the main scanning cross section and the sub-scanning cross section.

[0094] (Configuration 8) 8. The optical scanning device according to configuration 7, wherein the optical element is disposed closer to the deflector than a front principal plane of the optical element.

[0095] (Configuration 9) 9. The optical scanning device according to any one of configurations 1 to 8, wherein the imaging optical system has an imaging optical element including an optical surface whose curvature in the main scanning cross section changes in the main scanning direction.

[0096] (Configuration 10) 10. The optical scanning device according to configuration 9, wherein the absolute value of the curvature of the optical surface in the main scanning cross section approaches 0 as the distance from the optical axis increases in the main scanning direction.

[0097] (Configuration 11) When the maximum value of the scanning speed of the light beam on the scanned surface is Vmax and the scanning speed of the light beam at the most off-axis image height on the scanned surface is Ve, 0.85≦Ve / Vmax<1.00 11. The optical scanning device according to any one of configurations 1 to 10, wherein the following condition is satisfied:

[0098] (Configuration 12) When the minimum and maximum scanning speeds of the light beam on the surface to be scanned are Vmin and Vmax, respectively, 1.10≦Vmax / Vmin<1.40 12. The optical scanning device according to any one of configurations 1 to 11, wherein the following condition is satisfied:

[0099] (Configuration 13) When the thickness of the imaging optical element of the imaging optical system on the optical axis is Do and the thickness of the imaging optical element at the position where the most off-axis ray passes is Dc, 0.46≦Dc / Do≦0.60 13. The optical scanning device according to any one of configurations 1 to 12, wherein the following condition is satisfied:

[0100] (Configuration 14) 14. An image forming apparatus comprising: an optical scanning device according to any one of configurations 1 to 13; 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.

[0101] (Configuration 15) 14. An image forming apparatus comprising: an optical scanning device according to any one of configurations 1 to 13; and a printer controller that converts data output from an external device into an image signal and inputs the image signal to the optical scanning device. [Explanation of symbols]

[0102] 100 Optical scanning device 101 scanned surface 102 Light source 105 Deflector 106 Imaging Optical System

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 that guides the light beam deflected by the deflector to the surface to be scanned, the scanning speed of the light beam on the surface to be scanned is non-uniform; On the scanned surface, when the absolute value of the most off-axis image height is |Ymax| and the intermediate image height at which the scanning speed of the light beam is maximum is |Y|, 0.70≦|Y| / |Ymax|<1.00 An optical scanning device characterized by satisfying the following conditional expression:

2. 2. The optical scanning device according to claim 1, wherein the scanning speed of the light beam at the most off-axis image height on the surface to be scanned is faster than the scanning speed of the light beam at an axial image height.

3. 2. The optical scanning device according to claim 1, wherein the scanning speed of the light beam on the surface to be scanned varies monotonically between the intermediate image height, the on-axis image height, and the most off-axis image height.

4. 4. The optical scanning device according to claim 3, wherein the scanning speed of the light beam on the scanned surface monotonically increases as the image height moves from the axial image height to the intermediate image height, and monotonically decreases as the image height moves from the intermediate image height to the most off-axis image height.

5. 2. The optical scanning device according to claim 1, wherein the imaging optical system includes only one imaging optical element.

6. 6. The optical scanning device according to claim 5, wherein the entrance surface of said imaging optical element is concave on the optical axis, and the exit surface of said imaging optical element is convex.

7. 2. The optical scanning device according to claim 1, further comprising an optical element for converting the degree of convergence of the light beam from the light source, the optical element having different refractive powers in the main scanning cross section and the sub-scanning cross section.

8. 8. The optical scanning device according to claim 7, wherein the optical element is disposed closer to the deflector than a front main plane of the optical element.

9. 9. The optical scanning device according to claim 1, wherein the imaging optical system has an imaging optical element including an optical surface whose curvature in a main scanning cross section changes in the main scanning direction.

10. 10. The optical scanning device according to claim 9, wherein an absolute value of the curvature of the optical surface in the main scanning cross section approaches 0 as the distance from the optical axis increases in the main scanning direction.

11. When the maximum value of the scanning speed of the light beam on the scanned surface is Vmax and the scanning speed of the light beam at the most off-axis image height on the scanned surface is Ve, 0.85≦Ve / Vmax<1.00 9. The optical scanning device according to claim 1, wherein the following condition is satisfied:

12. When the minimum and maximum scanning speeds of the light beam on the surface to be scanned are Vmin and Vmax, respectively, 1.10≦Vmax / Vmin<1.40 9. The optical scanning device according to claim 1, wherein the following condition is satisfied:

13. When the thickness of the imaging optical element of the imaging optical system on the optical axis is Do and the thickness of the imaging optical element at the position where the most off-axis ray passes is Dc, 0.46≦Dc / Do≦0.60 9. The optical scanning device according to claim 1, wherein the following condition is satisfied:

14. 9. 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.

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

Images