Optical scanning device

Abstract

To provide a compact optical scanner with which it is possible to suppress the changes of spot diameter and illuminance attributable to an OFS system.SOLUTION: An optical scanner 100 pertaining to the present invention comprises: a light source 1; a deflector 5 for deflecting a beam from the light source 1 and scanning the surface 7 to be scanned, in the main scan direction; and an image forming optical system 85 for guiding the beam deflected by the deflector 5 to the surface 7 to be scanned. The width of the beam in the main scan cross section when entering a deflection surface 51 of the deflector 5 is larger than the width of the deflection surface 51, and refractive power in the main scan cross section of the image forming optical system 85 is different between a first position where an on-axis beam passes through and a second position where the outermost off-axis beam passes through.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an optical scanning device, and is particularly suitable for an image forming apparatus such as a laser beam printer or a multi-function printer (multi-functional printer) using an electrophotographic process.

Background Art

[0002] Conventionally, in order to achieve high-speed printing and high-definition printing, a so-called OFS (overfield scan) type optical scanning device including a deflector composed of a deflection plane having a width smaller than the width in the main scanning direction of an incident light beam has been used. And in such an optical scanning device, since the width and energy of the light beam that is reflected and deflected change according to the angle of the deflection plane as the deflection plane moves while changing the angle in the incident light beam, the spot diameter changes according to the image height on the scanned surface and the illuminance also changes. Patent Document 1 discloses an optical scanning device in which an optical element having both a phase shift function and a diffraction function and having characteristics in which both the phase shift amount and the diffraction efficiency are asymmetric with respect to the optical axis is provided in an incident optical system, thereby suppressing fluctuations in the spot diameter and illuminance according to such an image height.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the optical scanning device disclosed in Patent Document 1, since it is necessary to add a complex-shaped optical element having a phase shift surface and a diffraction grating surface to the incident optical system, the device becomes large-sized. Therefore, the present invention aims to provide a compact optical scanning device that can suppress fluctuations in spot diameter and illuminance associated with the OFS method. [Means for solving the problem]

[0005] The optical scanning device according to the present invention comprises a polygon mirror that rotates at a constant angular velocity to deflect a light beam from a light source and scan the surface to be scanned in the main scanning direction, and an imaging optical system that guides the light beam deflected by the polygon mirror to the surface to be scanned. Within the main scanning cross-section, the width of the light beam incident on the deflection surface of the polygon mirror is greater than the width of the deflection surface. When the light beam directed toward the on-axis image height, which is the image height on the optical axis of the imaging optical system on the surface to be scanned, is called the on-axis light beam, and the light beam directed toward the outermost image height on the surface to be scanned is called the outermost light beam, the refractive force of the imaging optical system within the main scanning cross-section is: The monotonically changing beam width cancels out the monotonically changing beam width caused by the monotonically changing reflection angle of the beam due to the deflection surface. The first position through which the on-axis light beam passes and the second position through which the off-axis light beam passes are different from each other, the scanning speed of the light beam on the scanned surface is different for the on-axis image height and the off-axis image height, and on the scanned surface, the first region from the off-axis image height to the on-axis image height on one side with respect to the optical axis is longer than the second region from the off-axis image height to the on-axis image height on the other side, and the imaging optical system is characterized by guiding the light beam to the first and second regions. [Effects of the Invention]

[0006] According to the present invention, it is possible to suppress fluctuations in spot diameter and illuminance associated with the OFS method and to provide a compact optical scanning device. [Brief explanation of the drawing]

[0007] [Figure 1] A cross-sectional view of the main scanning area of ​​the optical scanning apparatus according to the first embodiment. [Figure 2] A diagram illustrating the definitions of each angle in the optical scanning device according to the first embodiment. [Figure 3]This figure shows the spot diameter ratio, spot movement speed, and spot diameter changes, as well as the DIST characteristics, in the optical scanning device according to the first embodiment. [Figure 4] A cross-sectional view of the main scanning area of ​​the optical scanning apparatus according to the second embodiment. [Figure 5] This figure shows the spot diameter ratio, spot movement speed, and spot diameter changes, as well as the DIST characteristics, in the optical scanning device according to the second embodiment. [Figure 6] A cross-sectional view of the main scanning area of ​​the optical scanning apparatus according to the third embodiment. [Figure 7] This figure shows the spot diameter ratio, spot movement speed, and spot diameter changes, as well as the DIST characteristics, in the optical scanning device according to the third embodiment. [Figure 8] A sub-scanning cross-sectional view of the main part of the image forming apparatus according to the embodiment. [Figure 9] A cross-sectional view of the main scanning device disclosed in Patent Document 1. [Modes for carrying out the invention]

[0008] The optical scanning apparatus according to this embodiment will be described in detail below with reference to the attached drawings. Note that the drawings shown below may be drawn to a different scale than the actual dimensions in order to facilitate understanding of this embodiment. In the following, the direction parallel to the optical axis of the imaging optical system 85 is defined as the X-axis, the main scanning direction as the Y-axis, and the sub-scanning direction as the Z-axis.

[0009] [First Embodiment] Conventionally, optical scanning devices have been used as exposure devices for image forming equipment such as laser beam printers that use electrophotographic processes. In an optical scanning device, the light beam emitted from the light source, modulated according to the image signal from the PC, is guided by the incident optical system to a deflector such as a polygon mirror (rotating multifaceted mirror), and then deflected by the deflection reflecting surface of the deflector. The deflected light beam is then focused into a spot on the photosensitive drum, which serves as the scanning surface, by an imaging optical system having Fθ characteristics. As the deflector rotates, the spot formed on the scanning surface is scanned, and the exposure and recording of image information is performed.

[0010] Furthermore, various color image forming apparatuses have been proposed that form color images by scanning multiple photosensitive drums using multiple optical scanning devices. In recent years, in response to the demand for faster and higher-resolution printing, the OFS (Overfill Scan) method, which involves multi-faceted polygon mirrors, has been proposed.

[0011] Figure 9 shows a cross-sectional view of the main scanning device 500 disclosed in Patent Document 1.

[0012] As shown in Figure 9, the optical scanning device 500 disclosed in Patent Document 1 consists of a light source 10, an incident optical system composed of a coupling lens and the like, a polygon mirror (rotating polyface mirror) 16 as a deflector, and an imaging optical system (Fθ lens) 18. Furthermore, the polygon mirror 16 provided in the optical scanning device 500 is a polyhedron having 12 deflection reflection surfaces.

[0013] The incident light beam I emitted from the light source 10 is guided toward the polygon mirror 16 by an incident optical system composed of optical elements 30 and the like. Here, the incident light beam I has a beam width wider than the width of the deflection reflection surface of the polygon mirror 16 in the main scanning direction.

[0014] As the polygon mirror 16 rotates, a predetermined deflection reflecting surface moves through the incident light beam I, cutting off the incident light beam I, thereby reflecting and deflecting the incident light beam I towards the imaging optical system 18. Subsequently, the reflected and deflected light beam scans the printing area on the photosensitive drum 20, which is the scanning surface, forming a beam spot through the imaging performance of the imaging optical system 18, thereby performing printing.

[0015] A scanning method that uses an incident light beam with a beam width wider than the width of the deflection reflecting surface in the main scanning direction, as in the optical scanning device 500, is called the OFS method. On the other hand, a scanning method that uses an incident light beam with a beam width sufficiently smaller than the width of the deflection reflecting surface in the main scanning direction is called the UFS (Under-Fielded Scan) method.

[0016] In optical scanning devices using the OFS method, miniaturization is easier even when the polygon mirror is multifaceted compared to the UFS method. Therefore, it is possible to achieve higher printing speed and resolution without increasing the size of the optical scanning device. On the other hand, in optical scanning devices using the OFS method, the deflection reflecting surface moves while changing angle within the incident light beam I, so the width and energy of the reflected and deflected light beam change according to the angle of the deflection reflecting surface.

[0017] As a result, the spot diameter and illuminance on the scanned surface change according to the image height. Furthermore, if the spot diameter and illuminance fluctuate at each image height on the scanned surface, a problem arises in that a uniform printed image cannot be obtained when used in an image forming apparatus.

[0018] In contrast, the optical scanning device 500 disclosed in Patent Document 1 attempts to solve the above problem by providing an optical element 30 in the incident optical system that has both a phase shift function and a diffraction function, and in which both the amount of phase shift and the diffraction efficiency are asymmetric with respect to the optical axis.

[0019] However, the optical scanning device 500 using the OFS method disclosed in Patent Document 1 has the following problems. First, because it is necessary to add an optical element 30 with a complex shape that has both a phase shift surface and a diffraction grating surface, it becomes difficult to miniaturize and reduce the cost of the device.

[0020] Furthermore, the difficulty of manufacturing the optical element 30 that achieves a predetermined phase shift amount increases, making it difficult to manufacture such an optical element 30 at a low cost. Specifically, a surface shape is required in which the phase changes asymmetrically by 2π across the optical axis within the effective portion of the optical element 30. It is difficult to manufacture optical elements 30 with such complex and precise optical surface shapes accurately by injection molding using a mold, and it is also difficult to reduce costs when manufacturing them using cutting and polishing.

[0021] Furthermore, significant problems arise from providing an optical element 30 that has multiple regions with different diffraction efficiencies. Specifically, it is difficult to manufacture an optical element 30 having two regions where the diffraction efficiency of the zeroth-order light is 100% and 75%, respectively.

[0022] Furthermore, as described in paragraph

[0053] of Patent Document 1, the intensity distribution of the incident light beam I incident on the optical element 30 is a Gaussian distribution according to the FFP of the laser used in the light source 10. Here, it is known that laser FFP has tolerances, and typically has a wide variation range of 9° to 16° and 23° to 36°.

[0023] Therefore, the energy density within the light beam spanning the region where the diffraction efficiency is 100% and the region where it is 75% in the optical element 30 changes depending on the laser, making it difficult to precisely control the energy within the incident light beam I. Furthermore, if the energy density changes within the incident light beam I, the shape of the spot on the scanning surface, i.e., the photosensitive drum 20, will also be distorted.

[0024] Furthermore, if higher-order diffracted light is ignored in the region where the diffraction efficiency of the 0th-order light in the optical element 30 is 75%, the ±1st-order diffracted light has 25% of the energy and reaches the photosensitive drum 20 as flare or ghosting, resulting in a decrease in printing performance. Furthermore, in order to suppress such a decrease in printing performance, it is necessary to provide a light-shielding wall to properly block such flares and ghosting.

[0025] In view of the above-mentioned problems, this embodiment aims to provide an optical scanning device that can reduce fluctuations in spot diameter and non-uniformity of image plane illumination that occur when using the OFS method, which achieves high speed and high resolution, without increasing the number of components.

[0026] Figure 1 shows a cross-sectional view of the main scanning of the optical scanning device 100 according to the first embodiment. Figure 2 also shows a diagram illustrating the definition of each angle in the optical scanning device 100 according to this embodiment.

[0027] The optical scanning device 100 according to this embodiment includes a light source 1, an aperture 2, a coupling lens 3, a deflector 5, a first imaging lens 61, and a second imaging lens 62.

[0028] Aperture 2 regulates the beam width of the light beam emitted from light source 1 in both the main scanning direction and the sub-scanning direction. Furthermore, the width of aperture 2 in the main scanning direction is set to be sufficiently wide so that the width of the light beam incident on the deflector 5 in the main scanning direction is wider than the width within the main scanning cross-section of the deflection surface (deflection reflection surface) 51 of the deflector 5.

[0029] The coupling lens 3 is an anamorphic lens having different powers in the sub-scanning plane and the main scanning plane, respectively. The light beam that passes through aperture 2 is converted into a parallel light beam within the main scanning area by coupling lens 3, and is focused within the sub-scanning area. Here, the term "parallel light beam" includes not only strictly parallel light beams, but also approximate parallel light beams such as weakly diverging light beams and weakly converging light beams.

[0030] The deflector 5 is a 10-face polygon mirror, and rotates at a predetermined constant angular velocity using a drive motor (not shown) to deflect and scan the incident light beam. In this embodiment of the optical scanning device 100, as described above, a light beam with a width in the main scanning direction that is wider than the width within the main scanning cross-section of the deflection surface 51 is incident on the deflector 5. In other words, the width of the light beam incident on the deflection surface 51 of the deflector 5 within the main scanning plane is greater than the width of the deflection surface 51 of the deflector 5. Therefore, the width of the light beam deflected by the deflector 5 in the main scanning direction is determined by the width of the deflection surface 51 within the main scanning cross-section. This method is called the OFS (overfilled scan) method.

[0031] The first imaging lens 61 and the second imaging lens 62 guide (focus) the light beam deflected by the deflector 5 onto the scanning surface 7.

[0032] In the optical scanning device 100 according to this embodiment, with the above configuration, the light beam emitted from the light source 1 passes through the aperture 2 and coupling lens 3, converting it into a parallel light beam (or weakly focused light beam) within the main scanning cross-section, and focusing it within the sub-scanning cross-section so as to form a line image (focus line) on the deflection surface 51 of the deflector 5. The light beam deflected by the deflector 5 then passes through the first imaging lens 61 and the second imaging lens 62 and is focused onto the scanning surface 7, and the scanning surface 7 is scanned as the deflector 5 rotates at a constant speed.

[0033] In the optical scanning device 100 according to this embodiment, the incident optical system 75 (first optical system) is composed of an aperture 2 and a coupling lens 3, and the imaging optical system 85 (second optical system) is composed of a first imaging lens 61 and a second imaging lens 62. In this embodiment, the optical scanning device 100 uses an anamorphic lens as the coupling lens 3, but a rotationally symmetric coupling lens and a cylinder lens that has power in the sub-scanning cross-section may be used instead.

[0034] As described above, the optical scanning device 100 according to this embodiment employs an OFS method in which the incident light beam is converted by the incident optical system 75 so that the width of the light beam incident on the deflector 5 in the main scanning direction is wider than the width within the main scanning cross-section of the deflection surface 51. And as shown in FIG. 2, the optical axis (X i axis) of the incident optical system 75 and the optical axis (X axis) of the imaging optical system 85 are both arranged within the main scanning section, and the X i axis forms an incident angle θ i with respect to the X axis. In other words, in the optical scanning device 100 according to the present embodiment, within the main scanning section, the traveling direction LP1 of the light beam when incident on the deflector 5 and the optical axis (X axis) of the imaging optical system 85 are non-parallel to each other. However, in the optical scanning device 100 according to the present embodiment, both the X i axis and the X axis do not necessarily have to be arranged within the main scanning section.

[0035] Also, in the optical scanning device 100 according to the present embodiment, the scanned surface 7 has a first scanning region 71 (First domain) and a second scanning region 72 (Second domain) on each of the side where the incident optical system 75 is arranged (light source side) and the side where it is not arranged (anti-light source side) across the on-axis image height 711 (Y = 0). That is, when the coordinates in the Y direction of the outermost off-axis image height 710 on the +Y side, the on-axis image height 711, and the outermost off-axis image height 712 on the -Y side are Y max+ , 0, and biY max- respectively, the first scanning region 71 is the length from the outermost off-axis image height 710 on the +Y side to the on-axis image height 711 |Y max+ |(Y max+ (The absolute value of) and is the region having this length. Also, the second scanning region 72 is the length from the outermost off-axis image height 712 on the -Y side to the on-axis image height 711 |Y max- |(Y max- (The absolute value of) and is the region having this length. And in the optical scanning device 100 according to the present embodiment, a beam spot is formed by condensing the light beam deflected by the deflector 5 onto the scanned surface 7 by the imaging optical system 85, and is scanned from the outermost off-axis image height 710 on the +Y side to the outermost off-axis image height 712 on the -Y side as the deflector 5 rotates.

[0036] Furthermore, in the optical scanning device 100 according to this embodiment, the following condition (1) is satisfied. |Y max+ |>|Y max- | ...(1) Thus, the distance from the on-axis image height of 711 to the outermost image height of 710 on the +Y side. |Y max+ | The distance from the on-axis image height 711 to the off-axis image height 712 on the -Y side. |Y max- | By setting it to a larger value, the power of the imaging optical system 85 can be monotonically varied from on the optical axis to off the optical axis. As will be described in detail later, by monotonically changing the power of the imaging optical system 85, the monotonically changing light beam width associated with the monotonically changing reflection angle of the light beam by the deflection surface 51 can be canceled out, thereby suppressing fluctuations in the spot diameter.

[0037] In this embodiment, the optical scanning device 100 does not provide a second scanning area 72, i.e., Y max- Alternatively, you may set it to =0 and set only the first scanning region 71 as the scanning range of the surface to be scanned 7. By setting the scanning range of the scanned surface 7 in this way, it becomes unnecessary to design the first imaging lens 61 and the second imaging lens 62 constituting the imaging optical system 85 asymmetrically with respect to the optical axis, making it possible to design the printing position with high precision.

[0038] Next, the specifications of the optical scanning device 100 according to this embodiment are shown in Tables 1-1 to 1-3 below. In Table 1-3, "Ex" represents "×10 -x It means "...".

[0039] [Table 1-1] [Table 1-2] [Table 1-3]

[0040] In the optical scanning apparatus 100 according to this embodiment, the generatrix shape (shape within the main scanning cross-section) of the output surface of the coupling lens 3, and the incident and output surfaces of the first imaging lens 61 and the second imaging lens 62, respectively, is expressed by the following equation (2).

number

[0041] Furthermore, in equation (2), R is the radius of curvature (generatrix radius) within the main scanning section, and K, B4, B6, B8 and B 10 This is the aspherical coefficient. Note that the aspherical coefficients B4, B6, B8 and B 10 For this, the values ​​on the +Y side and the -Y side may be different. This makes it possible to create an asymmetrical shape for the generatrix of the optical surface in the main scanning direction, with respect to the optical axis. Note that in Table 1-3 above, the aspherical coefficient on the +Y side (light source side) is B. 4u B 6u B 8u and B 10u This is expressed as follows, and the aspherical coefficient on the -Y side (anti-light source side) is B 4l B 6l B 8l and B 10l This is how it is expressed.

[0042] Furthermore, the sub-line shapes (shapes within the sub-scanning cross-section) of the exit surface of the coupling lens 3, and the incident and exit surfaces of the first imaging lens 61 and the second imaging lens 62, respectively, are expressed by the following equation (3).

number

[0043] The sub-line shape referred to here includes the surface normal on the generatrix at each position in the main scanning direction, and also represents the surface shape within a cross-section perpendicular to the main scanning cross-section. Also, M in equation (3) jk This is the aspherical coefficient. In the optical scanning device 100 according to this embodiment, all aspherical coefficients M are present on all optical surfaces. jk Although it is set to 0, this is not the only way to reduce aberrations; at least one aspherical coefficient M jk You may set it to a value other than 0. Specifically, the first-order term of Z in equation (3) is a term that contributes to the amount of tilt (sub-line tilt) within the sub-scanning section of the optical surface. Therefore, the aspherical coefficient M on the +Y side (light source side) 0_1u ~M 16_1u And the aspherical coefficient M on the -Y side (anti-light source side) 0_1l ~M 16_1l By making these two parameters different from each other, the sub-line tilt amount can be changed asymmetrically in the main scanning direction.

[0044] Furthermore, r' shown in equation (3) represents the radius of curvature (sub-line radius of curvature) in the sub-scan cross-section at a position Y away from the optical axis in the main scanning direction, and is expressed by the following equation (4).

number

[0045] Here, r is the radius of curvature of the sub-line on the optical axis, and E2, E4, E6, E8 and E 10 This is the aspherical coefficient (sub-line change coefficient). Note that the aspherical coefficients E2 to E 10 For this, the values ​​on the +Y side and the -Y side may be different. This allows the aspherical quantity of the sublinear shape to be set asymmetrically across the optical axis in the main scanning direction. Note that in Table 1-3 above, the aspherical coefficient on the +Y side (light source side) is E 2u , E 4u , E 6u , E 8u and E 10u This is expressed as E, where E is the aspherical coefficient on the -Y side (anti-light source side). 2l , E 4l , E 6l , E 8l and E 10l This is how it is expressed. Furthermore, although equation (4) only includes even-order terms of Y, odd-order terms of Y may also be included.

[0046] Next, a characteristic configuration of the optical scanning device 100 according to this embodiment will be described. As shown in Figure 2, within the main scanning plane, the optical axis (Xi axis) of the incident optical system 75 is at an angle θ with respect to the optical axis (X axis) of the imaging optical system 85. i They form a line and intersect at the origin O. Then, the angle that the normal LP2 of the deflection surface 51 of the deflector 5 makes with respect to the optical axis of the imaging optical system 85 is θ. p When this is the case, the angle θ p This changes with the rotation of the deflector 5.

[0047] Furthermore, when the direction of propagation of the principal rays of the light beam deflected by the deflection surface 51 is denoted by θ, the angle (angle of view) that the principal rays make with respect to the optical axis of the imaging optical system 85 is θ. max+ From θ max- It changes up to that point. Here, θ max+ θ is the angle that the direction of propagation of the principal ray of the light beam L10, when deflected by the deflection surface 51 toward the off-axis image height 710, makes with respect to the optical axis of the imaging optical system 85. max- This is the angle that the direction of propagation of the principal ray of the light beam L12 makes with respect to the optical axis of the imaging optical system 85 when it is deflected by the deflection surface 51 so as to be directed toward the off-axis image height 712.

[0048] At this time, the angle θ i , θp The following relationship, expressed in equation (5), is satisfied between and θ.

number

[0049] As shown in Table 1-1, in the optical scanning device 100 according to this embodiment, θ max- and θ i These are set to -8.712° (-0.152 radians) and 63° (1.100 radians), respectively, and the number N of the deflection surfaces 51 of the deflector 5 is 10. Therefore, in the optical scanning device 100 according to this embodiment, the following condition (6) is satisfied.

number

[0050] If condition (6) is not met, when the surface to be scanned 7 is scanned by the light beam reflected by a predetermined deflection surface 51 of the deflector 5, the light beam reflected by an adjacent deflection surface 51 reaches the scanning area on the surface to be scanned 7 as a ghost, which is undesirable. Thus, in the optical scanning device 100 according to this embodiment, it is preferable to set the off-axis image heights 712 and 710 asymmetrically with respect to the optical axis so that condition (6) is satisfied.

[0051] As described above, the optical scanning device 100 according to this embodiment uses an OFS method in which the deflection surface 51 moves and reflects and deflects within an incident light beam with a wide beam width in the main scanning direction. Therefore, the beam width in the main scanning direction of the light beam directed toward each image height on the scanned surface 7 changes according to the reflection angle θ by the deflection surface 51. Specifically, the beam width in the main scanning direction of the light beam incident on each main scanning direction position of the imaging optical system 85 increases monotonically from the light beam incident on the on-axis position of the imaging optical system 85 toward the on-axis image height 711 to the light beam incident on a predetermined off-axis position (outside-axis position) of the imaging optical system 85 toward the off-axis image height 710.

[0052] On the other hand, the first imaging lens 61 and the second imaging lens 62 that constitute the imaging optical system 85 each have an aspherical shape and are designed to have different focal lengths depending on the position in the main scanning direction. Specifically, the focal length of the imaging optical system 85 at each main scanning direction position increases monotonically from the position through which the principal ray (on-axis ray) of the light beam directed toward the on-axis image height 711 passes (on-axis position, first position) to the position through which the principal ray (out-of-axis ray) of the light beam directed toward the outermost image height 710 passes (out-of-axis position, second position) (i.e., the power (refractive force) decreases monotonically).

[0053] Therefore, the distance the spot moves on the scanned surface 7 as the deflector 5 rotates and scans in a unit of time increases monotonically from the on-axis image height 711 to the off-axis image height 710. In other words, in the optical scanning device 100 according to this embodiment, the imaging optical system 85 has non-uniform speed scanning characteristics.

[0054] In the optical scanning device 100 according to this embodiment, by adopting the above configuration, it is possible to suppress changes in the spot diameter in the main scanning direction at each image height on the scanned surface 7, as shown in detail below.

[0055] As described above, since the optical scanning device 100 according to this embodiment employs the OFS method, the width of the light beam deflected by the deflection surface 51 in the main scanning direction (within the main scanning cross-section) changes according to the field of view θ. Here, when the circumscribed diameter of the deflector 5 is φ and the number of deflection surfaces 51 of the deflector 5 is N, the width W0 of the deflection surface 51 within the main scanning cross-section is expressed by the following equation (7).

number

[0056] Furthermore, the angle that the optical axis of the incident optical system 75 makes with respect to the normal LP2 of the deflection plane 51 is θ. i -θ pTherefore, the beam width W in the main scanning direction of the light beam deflected by the deflection surface 51 is expressed by the following equation (8).

number

[0057] Substituting equation (5) into equation (8), we obtain the following equation (9).

number

[0058] That is, from equation (8), the beam width in the main scanning direction of the light beam incident on each main scanning direction position of the imaging optical system 85 is from the beam deflected toward the on-axis image height 711 to the beam deflected toward the outermost image height 710, i.e., the angle θ p The coefficient increases monotonically as the angle θ (in equation (9)) increases.

[0059] Furthermore, the spot diameter SPOT in the main scanning direction at each image height on the scanned surface 7 is expressed by the following equation (10).

number

[0060] In this embodiment, the optical scanning device 100 employs the OFS method, and the optical beam width W is expressed as in equation (9). By substituting equation (9) into equation (10), the following equation (11) is obtained.

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[0061] Next, in a conventional optical scanning device using the OFS method, the image height Y has a DISTORTION characteristic (hereinafter referred to as the DIST characteristic) as shown in the following equation (12). Y=Fθ ···(12) Here, F is referred to as the Fθ coefficient and is equal to the focal length f(θ=0) at the on-axis position of the imaging optical system 85.

[0062] In other words, in a conventional optical scanning device using the OFS method, there is a constant-velocity scanning relationship between the amount of change in the field of view θ dθ due to the deflector 5 rotating at a constant velocity per unit time and the distance dY that the spot moves as it scans the scanned surface 7, as shown in equation (13) below.

number

[0063] Here, let dW(θ) be the ratio of the beam width in the main scanning direction of the light beam incident on the imaging optical system 85 at each main scanning direction position to the beam width in the main scanning direction of the light beam incident on the axial position of the imaging optical system 85. Furthermore, dSPOT(θ) is defined as the ratio of the spot diameter in the main scanning direction at each image height on the scanned surface 7 to the spot diameter in the main scanning direction at the on-axial image height on the scanned surface 7.

[0064] At this point, equation (14) is obtained based on equation (11).

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[0065] Thus, in conventional optical scanning devices using the OFS method, the ratio of spot diameters dSPOT(θ) at each field of view depends on the ratio of light beam widths dW(θ) at each field of view according to equation (14).

[0066] Based on equation (14), the dependence of the spot diameter ratio dSPOT on the field of view θ is shown by the dashed line in Figure 3(a). As shown in Figure 3(a), the dSPOT changes by approximately 23% across the entire field of view θ. In conventional optical scanning devices using the OFS method, fluctuations in the light beam width W were the cause of fluctuations in the spot diameter SPOT in the main scanning direction.

[0067] On the other hand, the optical scanning device 100 according to this embodiment has DIST characteristics as shown in the following equation (15).

number

[0068] In other words, there is a non-uniform scanning relationship between the amount of change in the field of view θ dθ due to the deflector 5 rotating at a constant velocity per unit time and the distance dY that the spot moves as it scans the scanned surface 7, as shown in equation (16) below, and the scanning speed of the light beam on the scanned surface 7 differs depending on the image height.

number

[0069] Figure 3(b) shows the DIST characteristics of a conventional optical scanning device and the DIST characteristics of the optical scanning device 100 according to this embodiment. As shown in Figure 3(b), in conventional optical scanning devices, the image height Y changes with respect to the field of view θ using a linear function shown in equation (12), whereas in the optical scanning device 100 according to this embodiment, it changes using a polynomial function shown in equation (15). Then, as the field of view θ increases, the difference between each DIST characteristic increases, and each DIST coefficient α i It is set.

[0070] Figure 3(c) also shows the dependence of the moving speed dY / dθ on the field of view θ in a conventional optical scanning device shown by equation (13), and the dependence of the moving speed dY / dθ on the field of view θ in the optical scanning device 100 according to this embodiment shown by equation (16). As shown in Figure 3(c), in conventional optical scanning devices, the moving speed dY / dθ is a constant value F regardless of the field of view θ, whereas in the optical scanning device 100 according to this embodiment, the moving speed dY / dθ increases monotonically as the field of view θ increases.

[0071] Thus, in the optical scanning apparatus 100 according to this embodiment, the imaging optical system 85 has a non-uniform speed scanning DIST characteristic such that the moving speed dY / dθ increases monotonically as the field of view θ increases from the on-axis image height 711 to the off-axis image height 710. Then, when df(θ) is the ratio of the focal length f(θ) at each main scanning direction position of the imaging optical system 85 to the focal length f(0) at the on-axis position of the imaging optical system 85, the following equation (17) is satisfied.

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[0072] Then, from equation (17), the focal length f(θ) at each main scanning direction position of the imaging optical system 85 can be expressed as shown in equation (18) below.

number

[0073] Thus, in the optical scanning apparatus 100 according to this embodiment, as the field of view θ increases from the on-axis image height 711 to the off-axis image height 710, the power of the imaging optical system 85 within the main scanning cross-section decreases, that is, the focal length f(θ) within the main scanning cross-section becomes longer.

[0074] In other words, in the optical scanning device 100 according to this embodiment, equations (11) and (14) can be rewritten as equations (19) and (20) below.

number

number

[0075] Then, based on equation (20), the dependence of the spot diameter ratio dSPOT on the field of view θ when dW(θ) is set to 1 is shown by the dotted line in Figure 3(a).

[0076] Then, based on equation (20), the dependence of dSPOT on the field of view θ when both dW(θ) and df(θ) change is shown by the solid line in Figure 3(a). As shown in Figure 3(a), it can be seen that the change in the spot diameter ratio dSPOT(θ) when the light beam width ratio dW(θ) changes, and the change in the spot diameter ratio dSPOT(θ) when the focal length ratio df(θ) changes, cancel each other out.

[0077] Specifically, it can be seen that the variation in the spot diameter ratio dSPOT(θ), which was approximately 23% across the entire field of view θ due to a change in the light beam width ratio dW(θ), can be reduced to approximately 11% by changing the focal length ratio df(θ). In other words, in conventional optical scanning devices using the OFS method, the spot diameter ratio dSPOT(θ) changed by approximately 23% across the entire field of view θ, but in the optical scanning device 100 according to this embodiment, the change in the spot diameter ratio dSPOT(θ) can be reduced to approximately 11% across the entire field of view θ.

[0078] Furthermore, it is preferable that the optical scanning device 100 according to this embodiment is designed to satisfy the following condition (21) at any field of view θ from the on-axis image height to the off-axis image height.

number

number

[0079] In this embodiment, the optical scanning device 100 uses the constant A and the DIST coefficient α. i It has been determined that the conditions (21) and (21a) are satisfied.

[0080] Figure 3(d) shows the size of the spot diameter (SPOT) in the main scanning direction at each image height on the scanned surface 7, calculated using the specifications shown in Tables 1-1 to 1-3 in the optical scanning apparatus 100 according to this embodiment.

[0081] As shown in Figure 3(d), in the optical scanning apparatus 100 according to this embodiment, the spot diameter SPOT has a value of 67.5 micrometers to 74.5 micrometers across the entire image height from the outermost image height 712 to the outermost image height 710. In other words, the spot diameter (SPOT) has a variation range of approximately 10.3% across the entire image height. Here, the variation range of approximately 10.3% shown in Figure 3(d) is smaller than the reduced variation range of approximately 11% for the spot diameter ratio dSPOT(θ) shown in Figure 3(a) because the calculation shown in Figure 3(d) includes the effects of residual aberrations such as wavefront aberration and partial magnification.

[0082] Furthermore, when examining the change in light intensity at each image height on the scanned surface 7 in the optical scanning device 100 according to this embodiment, the expression df(θ) / dW(θ) shown in equation (20) is equivalent to the F number at the field of view θ. In other words, in the optical scanning device 100 according to this embodiment, by canceling out the change in the ratio of the light beam width dW(θ) and the change in the ratio of the focal length df(θ) as described above, it is possible to suppress the change in the F number of the spot scanned on the scanned surface 7. This suppresses changes in the amount of light at each image height on the scanned surface 7, thereby reducing non-uniformity in the image plane illumination distribution.

[0083] In the optical scanning device 100 according to this embodiment, the field of view θ to the off-axis image height 710 is as shown in equation (1). max+ and the angle of view θ to the outermost image height of 712. max- These values ​​are asymmetrical with respect to the optical axis of the imaging optical system 85. On the other hand, in the optical scanning device 100 according to this embodiment, as shown in Table 1-3, the shapes of the optical surfaces of the first imaging lens 61 and the second imaging lens 62 are symmetrical with respect to the optical axis of the imaging optical system 85.

[0084] As described above, in the optical scanning device 100 according to this embodiment, when the light beam is deflected by the deflection surface 51 of the deflector 5 and scanned on the surface to be scanned 7, the field of view θ is θ max+ From θ max- It changes up to that point. Then, the beam width W of the light beam incident on the imaging optical system 85 in the main scanning direction is given by θ as shown in equation (9). max+ From θ max- It changes monotonically over that period.

[0085] Therefore, in order to cancel out the change in the light beam width W and the change in the focal length f of the imaging optical system 85 as described above, the focal length f of the imaging optical system 85 must also be changed monotonically. Here, the field of view θ max+ and θ max-If the two are set to have values ​​that are symmetrical with respect to the optical axis of the imaging optical system 85, then in order to monotonically change the focal length f, the shapes of the optical surfaces of the first imaging lens 61 and the second imaging lens 62 must be designed to be significantly asymmetrical with respect to the optical axis of the imaging optical system 85.

[0086] Therefore, in the optical scanning device 100 according to this embodiment, in order to reduce the burden when manufacturing such an asymmetrical optical surface, the field of view θ max+ and θ max- The two values ​​are set to be asymmetrical with respect to the optical axis of the imaging optical system 85.

[0087] In the optical scanning device 100 according to this embodiment, the distance from the on-axis image height 711 to the off-axis image height 710 is as shown in equation (1). |Y max+ | The distance from the on-axis image height 711 to the outermost image height 712. |Y max- | It's set to a larger value. In other words, the angle of view θ to the outermost image height 710 on the light source side where the incident optical system 75 is located. max+ While increasing the angle of view θ to the outermost image height of 712 on the anti-light source side max- By reducing the size of the component, the device has been made more compact.

[0088] Furthermore, in the optical scanning device 100 according to this embodiment, in order to make the shapes of the optical surfaces of the first imaging lens 61 and the second imaging lens 62 symmetrical with respect to the optical axis of the imaging optical system 85, the DIST coefficients α2, α4, ... of the even-order terms in the polynomial showing the DIST characteristics represented by equation (15) may be set to 0, while the DIST coefficients α3, α5, ... of the odd-order terms may each be set to predetermined constants.

[0089] As described above, the optical scanning device 100 according to this embodiment can suppress fluctuations in spot diameter that occur in the OFS method used to achieve high-speed and high-definition printing without increasing the number of parts, and can also reduce non-uniformity in the image plane illumination distribution.

[0090] [Second Embodiment] Figure 4 shows a cross-sectional view of the main scanning of the optical scanning device 200 according to the second embodiment. Since the optical scanning device 200 according to this embodiment has the same configuration as the optical scanning device 100 according to the first embodiment, the same reference numerals are used for the same components and their descriptions are omitted.

[0091] Furthermore, the specifications of the optical scanning device 200 according to this embodiment are shown in Tables 2-1 to 2-3 below.

[0092] [Table 2-1] [Table 2-2] [Table 2-3]

[0093] The definitions of the local coordinate system for each optical surface and the surface shape of each optical surface are the same as those for the optical scanning device 100 according to the first embodiment.

[0094] Figure 5(a) shows the dependence of the ratio of spot diameters dSPOT in the main scanning direction to the field of view θ in the optical scanning device 200 according to this embodiment. Specifically, the dependence of the spot diameter ratio dSPOT on the field of view θ when the luminous flux width ratio dW(θ) changes based on equation (14) is shown by the dashed line in Figure 5(a).

[0095] As shown in Figure 5(a), the variation in dSPOT is approximately 19% across the entire field of view θ, indicating that the variation in the light beam width W is a factor in the variation of the spot diameter SPOT in the main scanning direction.

[0096] In the optical scanning device 200 according to this embodiment, similar to the optical scanning device 100 according to the first embodiment, the power of the imaging optical system 85 within the main scanning cross-section decreases as the field of view θ increases from the on-axis image height 711 to the off-axis image height 710, that is, the focal length f(θ) within the main scanning cross-section becomes longer. Based on equation (20), the dotted line in Figure 5(a) shows the dependence of the ratio of spot diameters dSPOT with respect to the field of view θ, i.e., the fact that the beam width does not change at each field of view, i.e., when dW(θ) is set to 1.

[0097] Then, based on equation (20), the dependence of dSPOT on the field of view θ when both dW(θ) and df(θ) change is shown by the solid line in Figure 5(a). As shown in Figure 5(a), it can be seen that the change in the spot diameter ratio dSPOT(θ) when the ratio of the luminous flux width dW(θ) changes, and the change in the spot diameter ratio dSPOT(θ) when the ratio of the focal length df(θ) changes, cancel each other out.

[0098] Specifically, it can be seen that the variation in the spot diameter ratio dSPOT(θ), which was approximately 19% across the entire field of view θ due to the change in the light beam width ratio dW(θ), can be reduced to almost 0% by changing the focal length ratio df(θ). In other words, in conventional optical scanning devices using the OFS method, the spot diameter ratio dSPOT(θ) changed by approximately 19% across the entire field of view θ. However, in the optical scanning device 200 according to this embodiment, the change in the spot diameter ratio dSPOT(θ) can be reduced to almost 0% across the entire field of view θ.

[0099] Furthermore, the optical scanning device 200 according to this embodiment is designed to satisfy conditions (21) and (21a) at any field of view θ from the on-axis image height to the off-axis image height.

[0100] Figure 5(b) shows the DIST characteristics of a conventional optical scanning device and the DIST characteristics of the optical scanning device 200 according to this embodiment. As shown in Figure 5(b), in conventional optical scanning devices, the image height Y changes linearly with respect to the field of view θ as shown in equation (12), whereas in the optical scanning device 200 according to this embodiment, the image height Y changes polynomially with respect to the field of view θ as shown in equation (15). Then, as the field of view θ increases, the difference between each DIST characteristic increases, and each DIST coefficient α i It is set.

[0101] Figure 5(c) also shows the dependence of the moving speed dY / dθ on the field of view θ in a conventional optical scanning device shown by equation (13), and the dependence of the moving speed dY / dθ on the field of view θ in the optical scanning device 200 according to this embodiment shown by equation (16). As shown in Figure 5(c), in conventional optical scanning devices, the moving speed dY / dθ is a constant value F regardless of the field of view θ, whereas in the optical scanning device 200 according to this embodiment, the moving speed dY / dθ increases monotonically as the field of view θ increases.

[0102] Figure 5(d) also shows the size of the spot diameter (SPOT) in the main scanning direction at each image height on the scanned surface 7, calculated using the specifications shown in Tables 2-1 to 2-3 for the optical scanning device 200 according to this embodiment.

[0103] As shown in Figure 5(d), in the optical scanning apparatus 200 according to this embodiment, the spot diameter SPOT has a value of 69.7 micrometers to 72.5 micrometers across the entire image height from the outermost image height 712 to the outermost image height 710. In other words, the spot diameter (SPOT) has a variation range of approximately 4.0% across the entire image height. Here, the reason why the fluctuation range of approximately 4.0% shown in Figure 5(d) is greater than the reduced fluctuation range of approximately 0% for the spot diameter ratio dSPOT(θ) shown in Figure 5(a) is that the calculation shown in Figure 5(d) includes residual aberrations such as wavefront aberration and partial magnification.

[0104] As described above, the optical scanning device 200 according to this embodiment can further suppress the fluctuation in spot diameter that occurs in the OFS method used to achieve high-speed and high-definition printing without increasing the number of parts, and can further reduce non-uniformity in the image plane illumination distribution.

[0105] [Third Embodiment] Figure 6 shows a cross-sectional view of the main scanning area of ​​the optical scanning device 300 according to the third embodiment. The optical scanning device 300 according to this embodiment has the same configuration as the optical scanning device 100 according to the first embodiment, except that it uses a single imaging lens 63 instead of the first imaging lens 61 and the second imaging lens 62. Therefore, the same reference numerals are used for the same components and their descriptions are omitted.

[0106] Furthermore, the specifications of the optical scanning device 300 according to this embodiment are shown in Tables 3-1 to 3-3 below.

[0107] [Table 3-1] [Table 3-2] [Table 3-3]

[0108] The definitions of the local coordinate system for each optical surface and the surface shape of each optical surface are the same as those for the optical scanning device 100 according to the first embodiment.

[0109] Figure 7(a) shows the dependence of the ratio of spot diameters dSPOT in the main scanning direction to the field of view θ in the optical scanning device 300 according to this embodiment. Specifically, the dependence of the spot diameter ratio dSPOT on the field of view θ when the luminous flux width ratio dW(θ) changes based on equation (14) is shown by the dashed line in Figure 7(a).

[0110] As shown in Figure 7(a), the variation in dSPOT is approximately 23% across the entire field of view θ, indicating that the variation in the light beam width W is a factor in the variation of the spot diameter SPOT in the main scanning direction.

[0111] In the optical scanning device 300 according to this embodiment, similar to the optical scanning device 100 according to the first embodiment, the power of the imaging optical system 85 within the main scanning cross-section decreases as the field of view θ increases from the on-axis image height 711 to the off-axis image height 710, that is, the focal length f(θ) within the main scanning cross-section becomes longer. Based on equation (20), the dotted line in Figure 7(a) shows the dependence of the ratio of spot diameters dSPOT with respect to the field of view θ, i.e., the fact that the beam width does not change at each field of view, i.e., when dW(θ) is set to 1.

[0112] Then, based on equation (20), the dependence of dSPOT on the field of view θ when both dW(θ) and df(θ) change is shown by the solid line in Figure 7(a). As shown in Figure 7(a), it can be seen that the ratio of spot diameters dSPOT(θ) when the ratio of light beam widths dW(θ) changes and the ratio of spot diameters dSPOT(θ) when the ratio of focal length df(θ) changes cancel each other out.

[0113] Specifically, it can be seen that the variation in the spot diameter ratio dSPOT(θ), which was approximately 23% across the entire field of view θ due to a change in the light beam width ratio dW(θ), can be reduced to approximately 11% by changing the focal length ratio df(θ). In other words, in conventional optical scanning devices using the OFS method, the spot diameter ratio dSPOT(θ) changed by approximately 23% across the entire field of view θ, but in the optical scanning device 300 according to this embodiment, the change in the spot diameter ratio dSPOT(θ) can be reduced to approximately 11% across the entire field of view θ.

[0114] Furthermore, the optical scanning device 300 according to this embodiment is designed to satisfy conditions (21) and (21a) at any field of view θ from the on-axis image height to the off-axis image height.

[0115] Figure 7(b) shows the DIST characteristics of a conventional optical scanning device and the DIST characteristics of the optical scanning device 300 according to this embodiment. As shown in Figure 7(b), in conventional optical scanning devices, the image height Y changes linearly with respect to the field of view θ as shown in equation (12), whereas in the optical scanning device 300 according to this embodiment, the image height Y changes polynomially with respect to the field of view θ as shown in equation (15). Then, as the field of view θ increases, the difference between each DIST characteristic increases, and each DIST coefficient α i It is set.

[0116] Figure 7(c) also shows the dependence of the moving speed dY / dθ on the field of view θ in a conventional optical scanning device shown by equation (13), and the dependence of the moving speed dY / dθ on the field of view θ in the optical scanning device 300 according to this embodiment shown by equation (16). As shown in Figure 7(c), in conventional optical scanning devices, the moving speed dY / dθ is a constant value F regardless of the field of view θ, whereas in the optical scanning device 300 according to this embodiment, the moving speed dY / dθ increases monotonically as the field of view θ increases.

[0117] Figure 7(d) also shows the size of the spot diameter (SPOT) in the main scanning direction at each image height on the scanned surface 7, calculated using the specifications shown in Tables 3-1 to 3-3 for the optical scanning device 300 according to this embodiment.

[0118] As shown in Figure 7(d), in the optical scanning apparatus 300 according to this embodiment, the spot diameter SPOT has a value of 68.2 micrometers to 76.4 micrometers across the entire image height from the outermost image height 712 to the outermost image height 710. In other words, the spot diameter (SPOT) has a variation range of approximately 12.0% across the entire image height. Here, the variation range of approximately 12.0% shown in Figure 7(d) is greater than the reduced change of approximately 11% in the spot diameter ratio dSPOT(θ) shown in Figure 7(a) because the calculation shown in Figure 7(d) includes residual aberrations such as wavefront aberration and partial magnification.

[0119] As described above, the optical scanning device 300 according to this embodiment can suppress fluctuations in spot diameter that occur in the OFS method used to achieve high-speed and high-definition printing without increasing the number of parts, and can also reduce non-uniformity in the image plane illumination distribution.

[0120] [Image forming apparatus] Figure 8 shows a sub-scanning cross-sectional view of the main part of an image forming apparatus (electrophotographic printer) 104 equipped with an optical scanning device according to any of the first to third embodiments.

[0121] As shown in Figure 8, the image forming apparatus 104 receives code data Dc from an external device 117 such as a personal computer. The input code data Dc is then converted into image data (dot data) Di by the printer controller 111 within the device.

[0122] The converted image data Di is input to an optical scanning unit 400, which is an optical scanning device according to any of the first to third embodiments. Then, a light beam (light flux) 103 modulated according to the image data Di is emitted from the light scanning unit 400, and the photosensitive surface of the photosensitive drum 101 is scanned in the main scanning direction by the light beam 103.

[0123] The photosensitive drum 101, which is an electrostatic latent image carrier (photoreceptor), is rotated clockwise by the motor 115. As a result of this rotation, the photosensitive surface of the photosensitive drum 101 moves in a sub-scanning direction that is perpendicular to the main scanning direction with respect to the light beam 103.

[0124] A charging roller 102 is provided above the photosensitive drum 101 so as to contact the surface of the photosensitive drum 101, uniformly charging the surface of the drum. Then, a light beam 103 scanned by the light scanning unit 400 is irradiated onto the surface of the photosensitive drum 101, which has been charged by the charging roller 102.

[0125] As described above, the light beam 103 is modulated based on the image data Di, and an electrostatic latent image is formed on the surface of the photosensitive drum 101 by irradiating it with the light beam 103. The formed electrostatic latent image is then developed as a toner image by a developer unit 107, which is positioned to contact the photosensitive drum 101 downstream of the irradiation position of the light beam 103 within the rotational cross-section of the photosensitive drum 101.

[0126] The toner image developed by the developing unit 107 is transferred onto the paper 112, which is the material to be transferred, by a transfer roller (transfer unit) 108, which is positioned below the photosensitive drum 101 and facing the photosensitive drum 101. Although the paper 112 is stored in the paper cassette 109 located in front of the photosensitive drum 101 (on the right side in Figure 8), it can also be fed manually. Then, the paper 112 inside the paper cassette 109 is fed into the transport path by the paper feed roller 110 located at the end of the paper cassette 109.

[0127] As described above, the paper 112 onto which the unfixed toner image has been transferred is then transported to the fuser located behind the photosensitive drum 101 (on the left in Figure 8). The fuser consists of a fuser roller 113 having a fuser heater (not shown) inside and a pressure roller 114 arranged to press against the fuser roller 113.

[0128] Then, the paper 112 that has been transported from the transfer roller 108 is heated while being pressurized by the contact area between the fixing roller 113 and the pressure roller 114, thereby fixing the unfixed toner image on the paper 112. Furthermore, a paper discharge roller 116 is located behind the fuser, and the fixed paper 112 is discharged to the outside of the image forming apparatus 104.

[0129] Although not shown in Figure 8, the printer controller 111, in addition to the data conversion described above, also controls various components within the image forming apparatus 104, such as the motor 115, and components within the optical scanning unit 400, such as the polygon motor. Furthermore, although this example shows the application of the optical scanning device according to any of the first to third embodiments to an image forming apparatus 104 that prints in monochrome, it goes without saying that it can also be applied to a color image forming apparatus that scans and draws multiple photoreceptors with multiple light beams. In this case, for example, in a color image forming apparatus that superimposes four-color images, it is sufficient to provide optical scanning devices according to any of the four first to third embodiments in parallel.

[0130] Although preferred embodiments have been described above, the invention is not limited to these embodiments, and various modifications and changes are possible within the scope of its essence. [Explanation of Symbols]

[0131] 1 light source 5 Deflector 51 Deflection surface 7 Scanned surface 85 Imaging Optical System 100 Optical scanning device

Claims

1. A polygon mirror rotates at a constant angular velocity to deflect the light beam from the light source and scan the surface to be scanned in the main scanning direction, The system comprises an imaging optical system that guides the light beam deflected by the polygon mirror to the surface to be scanned, Within the main scanning cross-section, the width of the light beam when incident on the deflection surface of the polygon mirror is greater than the width of the deflection surface. When the light beam directed toward the on-axis image height, which is the image height on the optical axis of the imaging optical system on the scanned surface, is defined as the on-axis light beam, and the light beam directed toward the outermost image height on the scanned surface is defined as the outermost light beam, The refractive power within the main scanning cross-section of the imaging optical system changes monotonically to cancel out the monotonic change in the beam width due to the monotonic change in the reflection angle of the light beam by the deflection surface, and differs from that of the first position through which the on-axis light beam passes and the second position through which the off-axis light beam passes. The scanning speed of the light beam on the scanned surface differs from that of the on-axis image height and the off-axis image height. In the scanned surface, the first region on one side with respect to the optical axis, from the off-axis image height to the on-axis image height, is longer than the second region on the other side, from the off-axis image height to the on-axis image height. The imaging optical system is characterized by guiding the light beam to the first and second regions.

2. The optical scanning apparatus according to claim 1, characterized in that the refractive force changes monotonically between the first position and the second position.

3. The optical scanning apparatus according to claim 2, characterized in that the refractive force at the second position is smaller than the refractive force at the first position.

4. The optical scanning apparatus according to any one of claims 1 to 3, characterized in that the scanning speed changes monotonically between the on-axis image height and the off-axis image height.

5. The optical scanning apparatus according to claim 4, characterized in that the scanning speed at the furthest off-axis image height is greater than the scanning speed at the on-axis image height.

6. The optical scanning apparatus according to any one of claims 1 to 5, characterized in that, within the main scanning cross-section, the direction of propagation of the light beam when incident on the deflection surface and the optical axis of the imaging optical system are non-parallel to each other.

7. The optical scanning apparatus according to any one of claims 1 to 6, comprising an incident optical system for directing the light beam from the light source onto the deflection surface of the polygon mirror, wherein the incident optical system is located on one side.

8. Within the main scanning cross-section, the angle that the direction of propagation of the off-axis luminous beam toward the off-axis image height on the other side, when deflected by the deflection surface, makes with respect to the optical axis of the imaging optical system is θ. ｍａｘ－ , the angle that the optical axis of the incident optical system makes with respect to the optical axis of the imaging optical system is θ ｉ When the number of deflection surfaces of the polygon mirror is N, i ｉ -θ ｍａｘ－ <4π / N The optical scanning apparatus according to claim 7, characterized in that it satisfies the following conditions.

9. The optical scanning apparatus according to claim 7 or 8, characterized in that the optical axes of the incident optical system and the imaging optical system are parallel to the main scanning cross-section.

10. Within the main scanning cross-section, when the direction of propagation of the light beam deflected by the deflection surface is defined as θ, the angle that the deflected light beam makes with respect to the optical axis of the imaging optical system is θ, the ratio of the width of the off-axis light beam deflected by angle θ to the width of the on-axis light beam incident on the imaging optical system is dW(θ), and the ratio of the focal length at the position through which the off-axis light beam passes is df(θ) to the focal length at the first position of the imaging optical system, 0.90<df(θ) / dW(θ)<1.10 An optical scanning apparatus according to any one of claims 1 to 9, characterized in that it satisfies the following conditions.

11. The optical scanning apparatus according to any one of claims 1 to 10, characterized in that the first region is on the same side as the light source with respect to the optical axis of the imaging optical system.

12. The optical scanning apparatus according to any one of claims 1 to 11, characterized in that, in the optical element included in the imaging optical system, the region through which the light beam toward the first region passes is longer than the region through which the light beam toward the second region passes.

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

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

Images