WO2023003598A1

WO2023003598A1 - Optical scanner to reduce deterioration of beam spot

Info

Publication number: WO2023003598A1
Application number: PCT/US2022/016797
Authority: WO
Inventors: Jongwuk KU
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2021-07-20
Filing date: 2022-02-17
Publication date: 2023-01-26
Also published as: KR20230013957A

Abstract

An optical scanner includes a light source unit, a deflector comprising at least three first coupling portions; an imaging optical system to form an image by scanning deflected light from the deflector on an exposure object; and a frame comprising at least three second coupling portions respectively corresponding to the at least three first coupling portions, wherein an assembly reference line is a shorter line among a first line between two second coupling portions relatively close to a rotational axis of the deflector from among the at least three second coupling portions and a second line perpendicular to a line from another second coupling portion to the first line, and positions of the at least three second coupling portions are such that the assembly reference line faces a direction in which an error of a diameter of beam spots formed on the exposure object is within a reference error.

Description

OPTICAL SCANNER TO REDUCE DETERIORATION OF BEAM SPOT DUE TO TILT OF OPTICAL DEFLECTOR BACKGROUND [0001] Electrophotographic image forming apparatuses develop an electrostatic latent image formed on a photoconductor into a toner image. Electrophotographic image forming apparatuses then transfer the toner image to a printing medium and fix the same to print an image. In electrophotographic image forming apparatuses, an optical scanner which irradiates, to a photoconductor, light modulated according to image information is used. Optical scanners deflect light irradiated from a light source unit in a main scanning direction by using a deflector. The deflector includes a motor and a deflection mirror coupled to a rotational axis of the motor. The deflection mirror includes a reflection surface that reflects light irradiated from the light source unit. As the deflection mirror is rotated, an angle between the light and the reflection surface is varied, and accordingly, the light may be scanned in the main scanning direction. The light reflected by the reflection surface is formed as an image on the photoconductor by an fθ lens (imaging optical system) in the form of spots. BRIEF DESCRIPTION OF THE DRAWINGS [0002] FIG.1 is a schematic perspective view of an example of an optical scanner. [0003] FIG.2 illustrates a relationship between a tilt amount of a deflector and a distance between two coupling portions, in an example. [0004] FIG. 3 is a plan view illustrating an example of determining a position of second coupling portions. [0005] FIG. 4 is a plan view illustrating an example of determining a position of second coupling portions. [0006] FIG. 5 is a plan view illustrating an example of determining a position of the second coupling portions. [0007] FIG. 6 is a plan view illustrating an example of determining a position of the second coupling portions. [0008] FIG.7 is a graph showing a simulation result of a diameter of beam spots formed on an exposure object, in a main scanning direction Y, according to variation in θ1, in an example. [0009] FIG.8 is a graph showing a simulation result of a diameter of beam spots formed on an exposure object, in a sub-scanning direction X, according to variation in θ1, in an example. [0010] FIG.9 is a graph showing a simulation result of a diameter of beam spots formed on an exposure object, in the main scanning direction Y, when θ1 is -25° and 45°, respectively, in an example. [0011] FIG.10 is a graph showing a simulation result of a diameter of beam spots formed on an exposure object, in the main scanning direction Y, when θ1 is --80° and 45°, respectively, in an example. [0012] FIG.11 is a schematic perspective view of an example of a deflector. [0013] FIG.12 is a schematic structural diagram of an example of a tilt direction adjusting portion. [0014] FIG.13 is a schematic structural diagram of an example of a tilt direction adjusting portion. DETAILED DESCRIPTION [0015] Electrophotographic image forming apparatuses form an electrostatic latent image by scanning, onto a photoconductor, light modulated according to an image signal, by using an optical scanner, and develop the electrostatic latent image into a visible toner image. Electrophotographic image forming apparatuses then transfer the toner image to a printing medium to fix the same and print an image. The optical scanner includes a deflector deflecting light in a main scanning direction and an imaging optical system scanning the deflected light onto a photoconductor (exposure object) at a constant speed to form an image. A tilt of the deflector may affect a shape of beam spots formed on an exposure object. To reduce deterioration of beam spots, a method of increasing a thickness of the imaging optical system in a traveling direction of light may be considered. A method of reducing sensitivity of the shape of beam spots due to a tilt of the deflector by providing a relatively long optical path length from a light source to an exposure object may be considered. According to the above methods, however, the costs may be increased due to the increase in the price of the imaging optical system or the increase in the overall size of the optical scanner. [0016] According to the optical scanner according to the present example, deterioration of beam spots due to a tilt of a deflector may be reduced by optimizing an assembly direction of a deflector. The deflector may include at least three coupling portions coupled to a frame of the optical scanner. When there is a difference in heights of the coupling portions, the deflector is tilted. A tilt direction of the deflector affects an amount of deterioration of beam spots formed on an exposure object. Deterioration of beam spots is caused by a tilt of the deflector because an incident tilt angle between a reflection surface of the deflector and a light beam incident on the reflection surface (an angle in a thickness direction of the reflection surface) is varied. When coupling the deflector to a frame of the optical scanner, by adjusting an assembly direction such that a tilt direction of the deflector is within a certain angle range with respect to a line perpendicular to a main scanning direction, a variation in an incident tilt angle may be minimized to thereby minimize deterioration of beam spots. [0017] For example, a plurality of first coupling portions may be included on the deflector. A plurality of second coupling portions corresponding to the plurality of first coupling portions may be included in the frame. At least three first coupling portions and at least three second coupling portions may be provided. The first coupling portions may be coupling holes formed in a supporting member supporting deflector, for example, a driving motor and a polygonal rotating mirror, for example. The second coupling portions may be, for example, coupling bosses to which a coupling member such as a screw is coupled. A line which connects two coupling bosses from among a plurality of coupling bosses that are relatively close to a rotational axis of the deflector and a line that is perpendicular to the first line from the other boss from among the plurality of coupling bosses are referred to as a first line and a second line, respectively. A shorter line among the first line and the second line is defined as an assembly reference line. Positions of the plurality of coupling bosses may be determined such that the assembly reference line faces a direction in which an error of a diameter of beam spots formed on an exposure object is within a reference error. According to this configuration, deterioration of the shape of beam spots due to a tilt of a deflector may be minimized without adjusting a thickness of an imaging optical system in a light proceeding direction or an optical path length. [0018] For example, the assembly reference line may face a quadrant opposite a light source unit with respect to a third line that is orthogonal to the main scanning direction. An angle θ1 between the assembly reference line and the third line may satisfy -90° ≤ θ1 ≤ -15°. Here, a negative sign denotes an opposite side of the light source unit with respect to the third line. The assembly reference line may also face the light source unit with respect to the third line. In this case, the angle θ1 between the assembly reference line and the third line may satisfy 75° ≤ θ1 ≤ 90°. [0019] For example, the optical scanner may include a height adjusting portion adjusting a height of at least one of the plurality of first coupling portions. The height adjusting portion may adjust a height of two first coupling portions that are relatively close to a rotational axis of the deflector from among the plurality of first coupling portions. For example, the height adjusting portion may include an elastic member applying an elastic force to the deflector in a direction to be apart from the coupling bosses and a coupling member that is coupled to the coupling bosses through the coupling holes. By adjusting a degree of coupling of the coupling member, a height of the first coupling portion (coupling hole) may be adjusted. Accordingly, a tilt amount and a tilt direction of the deflector may be adjusted. [0020] For example, when an angle between a direction in which a rotational axis of the deflector is inclined and the third line orthogonal to the main scanning direction is θ2, the optical scanner may satisfy -90° ≤ θ2 ≤ -15° or 75° ≤ θ2 ≤ 90°. By adjusting a height of at least one of the plurality of first coupling portions (coupling holes) by using the tilt direction adjusting portion, the deflector may be assembled in the frame such that conditions of -90° ≤ θ2 ≤ -15° or 75° ≤ θ2 ≤ 90° are satisfied. Here, a negative sign denotes the opposite side of the light source unit with respect to the third line. [0021] Hereinafter, examples of the optical scanner will be described. Elements having the same functions will be labeled with the same reference numerals, and repeated description thereof will be omitted. [0022] FIG.1 is a schematic perspective view of an example of an optical scanner. Referring to FIG. 1, an optical scanner 1 may include a light source unit 20 emitting a light beam LB, a deflector 10 deflecting and scanning the light beam LB in a main scanning direction Y, and an imaging optical system 24 scanning the deflected light beam LB onto an exposure object 40 at a constant speed to form an image. The light beam LB may be reflected by a reflection mirror 25 and pass through an opening 31 provided in a frame 30 to be incident on the exposure object 40. The deflector 10 includes at least three coupling portions 12, 13, and 14 coupled to the frame 30. [0023] For example, the light source unit 20 may include a light source 21, a first optical member 22, and a second optical member 23. The light source 21 may be, for example, a laser light source. The first optical member 22 may include a collimating lens converting light emitted from the light source 21, into parallel light. The second optical member 23 condenses the parallel light onto a reflection surface 16a of the deflector 10, which will be described later. The second optical member 23 may include, for example, a cylindrical lens having refractive power in a sub-scanning direction (X). Although not illustrated in the drawings, the first optical member 22 and the second optical member 23 may be implemented using a single plastic anamorphic lens. [0024] The deflector 10 may include a motor 15 and a polygonal rotating mirror 16 coupled to a rotational axis of the motor 15. The polygonal rotating mirror 16 may include at least one reflection surface 16a. The deflector 10 may include a support plate 11 on which the motor 15 is supported. The support plate 11 is coupled to the frame 30. A plurality of first coupling portions may be provided in the support plate 11. A plurality of second coupling portions corresponding to the plurality of first coupling portions may be provided in the frame 30. For example, the optical scanner 1 may include at least three first coupling portions and at least three second coupling portions corresponding thereto. The first coupling portions may be, for example, coupling holes. The coupling holes 12, 13, and 14 are provided in the support plate 11. The second coupling portions may be, for example, coupling bosses. For example, three coupling bosses 32, 33, and 34 respectively corresponding to the coupling holes 12, 13, and 14 may be provided on the frame 30. A coupling member 50, for example, a screw, may be coupled to the coupling bosses 32, 33, and 34 through the coupling holes 12, 13, and 13 to assemble the deflector 10 to the frame 30. [0025] The light beam LB irradiated from the light source unit 20 is deflected by the deflector 10 in the main scanning direction Y. The imaging optical system 24 forms the light beam LB deflected by the deflector 10 into an image on an outer circumferential surface of the exposure object 40, that is, a scanning surface. An optical axis of the imaging optical system 24 is a Z-direction orthogonal to the main scanning direction Y. The imaging optical system 24 may be an f-θ lens that scans the light beam LB onto the exposure object 40 at a constant speed to form an image. The imaging optical system 24 may have an optical shape in consideration of a distance between the imaging optical system 24 and the deflector 10 and a distance between the deflector 10 and the exposure object 40. [0026] While heights of the coupling bosses 32, 33, and 34 respectively corresponding to the coupling holes 12, 13, and 14 may be equal to each other, the heights of the coupling bosses 32, 33, and 34 may not be equal according to manufacturing errors, injection molding conditions of the frame 30, or the like. In this case, the deflector 10 assembled to the frame 30 may be tilted. A tilt of the deflector 10 affects an incident position of the light beam LB on the imaging optical system 24 in the sub-scanning direction X and an optical path from the reflection surface 16a to the exposure object 40. Thus, beam spots formed on the exposure object 40 may be deteriorated. For example, the optical scanner 1 is optically designed such that beam spots on the exposure object 40 have a certain reference diameter, and diameters of the beam spots may be not uniform according to positions thereof on the exposure object 40 in the main scanning direction Y. The non-uniform diameters of beam spots as above may cause a non-uniform density of an image printed by an imaging forming apparatus and degrade the quality of printed images. [0027] A tilt amount of the deflector 10 may rely on a difference between heights of two second coupling portions (coupling bosses) that are relatively close to the rotational axis 15a of the deflector 10. Also, the smaller a distance between two second coupling portions, the greater may be the tilt amount of the deflector 10. [0028] FIG. 2 illustrates a relationship between a tilt amount of the deflector 10 and a distance between two coupling portions, for example. In FIG. 2, the deflector 10 is illustrated with lines A1 and A2. Referring to FIG. 2, three first coupling portions, for example, coupling bosses 32a, 32b, and 32c are illustrated. A height difference between the coupling bosses 32a and 32b and a height difference between the coupling bosses 32a and 32c is h1 and is equal. A distance d1 between the coupling bosses 32a and 32b is smaller than a distance d2 between the coupling bosses 32a and 32c. Regarding a tilt angle t1 of the deflector 10 when the deflector 10 is coupled to the coupling bosses 32a and 32b and a tilt angle t2 of the deflector 10 when the deflector 10 is coupled to the coupling bosses 32a and 32c, t1 > t2. That is, with a smaller a distance between two coupling portions, a height difference between the two coupling portions greatly affects a tilt amount of the deflector 10. Also, with closer positions of two coupling portions to a rotational axis of the deflector 10, a height difference between the two coupling portions greatly affects a tilt amount of the deflector 10. [0029] A tilt direction of the deflector 10 affects an incident tilt angle of the light beam LB that is irradiated from the light source unit 20 and incident on the reflection surface 16a of the deflector 10. Considering these aspects, when assembling the deflector 10 to the frame 30, two second coupling portions that affect a tilt of the deflector 10 relatively greatly may be selected and a line connecting the two second coupling portions may be oriented in a certain appropriate direction to thereby minimize a variation in the incident tilt angle of the light beam LB incident on the reflection surface 16a and minimize deterioration of beam spots on the exposure object 40. [0030] According to the optical scanner 1 of the present example, among a first line connecting two second coupling portions that are relatively close to the rotational axis 15a of the deflector 10 from among at least three second coupling portions and a second line which is a perpendicular line to the first line from another one of the at least three second coupling portions, a shorter line is an assembly reference line, and a direction which the assembly reference line faces may be determined such that an error of a diameter of a beam formed on the exposure object 40 is within a reference error. The reference error of the beam diameter may be determined in consideration of a degree of non-uniformity of a density of a printed image. For example, the reference error of the beam diameter may be about 15 %. An assembly direction of the deflector 10 may be determined based on positions of a plurality of second coupling portions. Hereinafter, examples will be described. [0031] FIG. 3 is a plan view illustrating an example of determining a position of second coupling portions. In FIG. 3, an Y-Z plane is a main scanning plane. Referring to FIG.3, among the three coupling bosses 32, 33, and 34, the coupling bosses 32 and 33 are relatively close to the rotational axis 15a of the deflector 10. A line connecting the coupling bosses 32 and 33 is referred to as a first line L1. A perpendicular line drawn from another coupling boss 34 to the first line L1 or an extension line of the first line L1 is a second line L2. In FIG.3, a length of the first line L1 is shorter than a length of the second line L2. Thus, the first line L1 is an assembly reference line. Hereinafter, the first line L1 will be referred to as the assembly reference line L1. Positions of the coupling bosses 32, 33, and 34 are determined such that the assembly reference line L1 faces a direction in which an error of a diameter of beam spots formed on the exposure object 40 is within a reference error. The reference error may be, for example, about 10 μm. [0032] The positions of the coupling bosses 32, 33, and 34 may be determined such that the assembly reference line L1 faces a quadrant QA opposite the light source unit 20 with respect to a third line L3 orthogonal to the main scanning direction Y, an origin point OP of the third line L3 being on an incident point of the light beam LB on the reflection surface 16a. By placing the deflector 10 on the frame 30 such that the coupling holes 12, 13, and 13 respectively correspond to the coupling bosses 32, 33, and 34 and coupling the coupling member 50 to the coupling bosses 32, 33, and 34 through the coupling holes 12, 13, and 13, the deflector 10 is inclined toward the assembly reference line L1. When an inclination direction of the deflector 10 faces the quadrant QA, a variation in an incident tilt angle between the reflection surface 16a and the light beam LB irradiated from the light source unit 20, by a tilt of the deflector 10, that is, a variation in an angle between the reflection surface 16a and the light beam LB in a direction X orthogonal to the main scanning plane YZ is relatively small. However, a position at which the light beam LB is incident on the reflection surface 16a is changed to the direction X orthogonal to the main scanning plane YZ. Accordingly, beam spots formed on the exposure object 40 may be distorted. Thus, a range of an angle between the assembly reference line L1 and the third line L3 in the quadrant QA needs to be determined in consideration of reference error. [0033] When an angle between the assembly reference line L1 and the third line L3 is θ1, and θ1 is about -90°, the incident tilt angle of the light beam LB that is irradiated from the light source unit 20 and is incident on the reflection surface 16a of the deflector 10 is hardly varied. Thus, there is hardly an error in a diameter of beam spots on the exposure object 40. Here, a negative sign in front of an angle indicates an opposite side of the light source unit 20 with respect to the third line L3. When an absolute value of θ1 is smaller than 90, deterioration of a diameter of beam spots formed on the exposure object 40 starts to occur. The range of θ1 may be determined such that diameter deterioration of beam spots, that is, an error of a diameter of beam spots, is within a reference error. The reference error may be, for example, 10 μm. For example, θ1 may satisfy Expression 1 below. [0034] -90° ≤ θ1 ≤ -15° -----(Expression 1) [0035] FIG. 4 is a plan view illustrating an example of determining a position of second coupling portions. In FIG. 4, a Y-Z plane is a main scanning plane. Referring to FIG.4, among coupling bosses 32', 33', and 34', coupling bosses 32' and 33' are relatively close to the rotational axis 15a of the deflector 10. A line connecting the coupling bosses 32' and 33' is referred to as a first line L1. A perpendicular line drawn from another coupling boss 34' to the first line L1 or an extension line of the first line L1 is a second line L2. In FIG. 4, a length of the second line L2 is shorter than the first line L1. Thus, the second line L2 is an assembly reference line. Positions of the coupling bosses 32’, 33’, and 34’ may be determined such that the assembly reference line L2 faces a quadrant QA opposite the light source unit 20 with respect to a third line L3 orthogonal to the main scanning direction Y, an origin point OP of the third line L3 being on an incident point of the light beam LB on the reflection surface 16a. An angle θ1 between the assembly reference line L2 and the third line L3 may satisfy Expression 1 above. By placing the deflector 10 on the frame 30 such that coupling holes 12', 13', and 14' respectively correspond to the coupling bosses 32', 33', and 34' and coupling the coupling member 50 to the coupling bosses 32', 33', and 34' through the coupling holes 12', 13', and 14', the deflector 10 is inclined toward the assembly reference line L2. [0036] When the assembly reference line L2 faces the quadrant QA which is opposite the light source unit 20 with respect to the third line L3, as described above, a variation in an incident tilt angle between the light beam LB and the reflection surface 16a according to a variation in θ1 is not great (that is, the variation in θ1 is not large). Thus, a range of θ1 within which a beam diameter error is within a reference error is relatively broad (or large). Accordingly, when the assembly reference line L2 faces the quadrant QA, the freedom of layout of components of the deflector 10 and the optical scanner 1 may be extended. That is, when assembly reference line L2 faces quadrant QA, there may be extended freedom of layout of components of the deflector 10 and the optical scanner 1. [0037] According to circumstances, an assembly reference line may be arranged to face a quadrant QB at the side of the light source unit 20, with respect to a third line L3 orthogonal to the main scanning direction Y, an origin point OP of the third line L3 being on an incident point of the light beam LB on the reflection surface 16a. [0038] FIG. 5 is a plan view illustrating an example of determining a position of the second coupling portions. In FIG. 5, a Y-Z plane is a main scanning plane. Referring to FIG.5, among the three coupling bosses 32, 33, and 34, the coupling bosses 32 and 33 are relatively close to the rotational axis 15a of the deflector 10. A line connecting the coupling bosses 32 and 33 is referred to as a first line L1. A perpendicular line drawn from another coupling boss 34 to the first line L1 or an extension line of the first line L1 is a second line L2. In FIG.5, a length of the first line L1 is shorter than a length of the second line L2. Thus, the first line L1 is an assembly reference line. When determining positions of the coupling bosses 32, 33, and 34, the assembly reference line L1 may be determined to face the quadrant QB at the light source unit 20 with respect to a third line L3 orthogonal to the main scanning direction Y, an origin point OP of the third line L3 being on an incident point of the light beam LB on the reflection surface 16a. A direction of the assembly reference line L1 here is determined such that an error of a diameter of beam spots formed on the exposure object 40 is within a reference error. A range of an angle θ1 between the assembly reference line L1 and the third line L3 may be determined such that an error of a diameter of beam spots is within a reference error. The reference error may be, for example, 10 μm. For example, θ1 may satisfy Expression 2 below. [0039] 75° ≤ θ1 ≤ 90°---(Expression 2) [0040] FIG. 6 is a plan view illustrating an example of determining a position of the second coupling portions. In FIG. 6, a Y-Z plane is a main scanning plane. Referring to FIG. 6, among the three coupling bosses 32', 33', and 34', the coupling bosses 32' and 33' are relatively close to the rotational axis 15a of the deflector 10. A line connecting the coupling bosses 32' and 33' is referred to as a first line L1. A perpendicular line drawn from another coupling boss 34' to the first line L1 or an extension line of the first line L1 is a second line L2. In FIG. 6, a length of the second line L2 is shorter than a length of the first line L1. Thus, the second line L2 is an assembly reference line. When determining positions of the coupling bosses 32', 33', and 34', the assembly reference line L2 may be determined to face the quadrant QB at the light source unit 20 with respect to a third line L3 orthogonal to the main scanning direction Y, an origin point OP of the third line L3 being on an incident point of the light beam LB on the reflection surface 16a. An angle θ1 between the assembly reference line L2 and the third line L3 may satisfy Expression 2 above. [0041] The deflector 10 is inclined toward the assembly reference line. When an inclination direction of the deflector 10 faces the quadrant QB at the light source unit 20, a variation in an incident tilt angle between the reflection surface 16a and the light beam LB irradiated from the light source unit 20, by a tilt of the deflector 10, that is, a variation in an angle between the reflection surface 16a and the light beam LB in a direction orthogonal to the main scanning plane YZ is relatively great. Thus, a range of θ1 within which an error of a diameter of beam spots is within a reference error is relatively narrow (or small). [0042] FIG.7 is a graph showing a simulation result of a diameter of beam spots formed on the exposure object 40, in the main scanning direction Y, according to variation in θ1, in an example. FIG.8 is a graph showing a simulation result of a diameter of beam spots formed on the exposure object 40, in the sub-scanning direction X, according to variation in θ1, in an example. [0043] In FIGS. 7 and 8, a reference resolution is 600 dpi, and a reference diameter of beam spots is about 70 μm. A horizontal axis in FIGS.7 and 8 denotes a position on the exposure object 40 in the main scanning direction Y. Here, a negative sign in the horizontal axis indicates the opposite side of the light source unit 20 with respect to the third line L3. A vertical axis denotes a diameter of beam spots. Referring to FIGS.7 and 8, when θ1 satisfies the range of Expression 1 or Expression 2, an error in a beam diameter is about 10 μm. [0044] FIG.9 is a graph showing a simulation result of a diameter of beam spots formed on the exposure object 40, in the main scanning direction Y, when θ1 is - 25° and 45°, respectively, in an example. FIG.10 is a graph showing a simulation result of a diameter of beam spots formed on the exposure object 40, in the main scanning direction Y, when θ1 is -80° and 45°, respectively, in an example. [0045] Referring to FIGS. 9 and 10, when θ1 is 45°, a maximum error of a diameter of beam spots in the main scanning direction Y is about 20 μm, and a relatively large diameter deviation of beam spots is caused according to a position in the main scanning direction Y. In this regard, when θ1 is -25°, which satisfies Expression 1 or is 80°, which satisfies Expression 2, an error of a diameter of beam spots in the entire main scanning direction Y is within 10 μm. [0046] The error of the diameter of beam spots may occur by a tilt of the deflector 10 during a process of assembling the deflector 10 to the frame 30 or by a tilt of the rotational axis 15a in a manufacturing process of the deflector 10 itself. By assembling the deflector 10 to the frame 30 such that a tilt direction of the rotational axis 15a is a certain direction, beam diameter error due to the tilt of the rotational axis 15a may be reduced. [0047] FIG.11 is a schematic perspective view of an example of the deflector 10. Referring to FIG. 11, the rotational axis 15a may be tilted in a manufacturing process of the deflector 10. In this case, the deflector 10 may be assembled to the frame 30 such that the angle θ2 of the tilt direction of the rotational axis 15a with respect to the third line L3 orthogonal to the main scanning direction Y satisfies Equation 3 or Equation 4. Here, a negative sign indicates the opposite side of the light source unit 20 with respect to the third line L3. [0048] -90° ≤ θ2 ≤ -15° -----(Expression 3) [0049] 75° ≤ θ2 ≤ 90°---(Expression 4) [0050] For example, as illustrated in FIG. 11, when an angle between a line L4 obtained by projecting the rotational axis 15a onto a main scanning plane (or a scanning plane) (YZ plane) and the third line L3 orthogonal to the main scanning direction Y is θ2, θ2 satisfies Expression 3 or Expression 4 above. According to the above configuration, the same effects as those described above with reference to FIGS.1 through 10 may be obtained. [0051] The optical scanner 1 illustrated in FIG. 11 may include a tilt direction adjusting portion which adjusts the tilt direction of the deflector 10. For example, after assembling the deflector 10 to the frame 30, a height of at least one of the plurality of first coupling portions of the deflector 10 may be adjusted to satisfy Expression 3 or Expression 4. The tilt direction adjusting portion may adjust a height of at least one of the plurality of first coupling portions with respect to at least one of the second coupling portions. For example, the tilt direction adjusting portion may adjust a height of two coupling holes that are relatively close to the rotational axis 15a of the deflector 10 from among a plurality of first coupling portions, for example, three coupling holes 12'', 13'', and 14''. For example, heights of the coupling holes 12'' and 13'' may be adjusted in the example illustrated in FIG. 11. The tilt direction adjusting portion may also adjust heights of all of, for example, the three coupling holes 12'', 13'', and 14''. [0052] FIGS.12 and 13 are schematic structural diagrams of an example of a tilt direction adjusting portion (height adjusting portion). Referring to FIGS. 12 and 13, an example of the tilt direction adjusting portion may include an elastic member applying an elastic force to the deflector 10 in a direction to be apart from coupling bosses 32'', 33'', and 34'' and the coupling member 50 coupled to the coupling bosses 32'', 33'', and 34'' through the coupling holes 12'', 13'', and 14''. The elastic member may include a U-shaped plate spring 60 arranged between a lower surface of the support plate 11 and the frame 30, as illustrated in FIG.12. The elastic member may include a compression coil spring 65 arranged between the lower surface of the support plate 11 and the frame 30, as illustrated in FIG. 13. A form of the elastic member is not limited to a plate spring or a coiled spring. The coupling member 50 may be a screw to be coupled to the coupling boss 32’’. The coupling member 50 is coupled to the coupling bosses 32'', 33'', and 34'' through the coupling holes 12'', 13'', and 14''. Here, by adjusting a coupling amount of the coupling member 50, heights of the coupling holes 12'', 13'', and 14'' with respect to the frame 30 may be adjusted. By setting a relatively small coupling amount of the coupling member 50, a height from the frame 30 to the coupling holes 12'', 13'', and 14'' is increased, and by setting a relatively large coupling amount of the coupling member 50, a height from the frame 30 to the coupling holes 12'', 13'', and 14'' is decreased. According to this configuration, by adjusting a tilt direction of the rotational axis 15a of the deflector 10 to satisfy Expression 3 or Expression 4, the error of the diameter of beam spots on the exposure object 40 may be reduced. [0053] The tilt direction adjusting portion illustrated in FIGS. 12 and 13 may be applied to reduce the tilt amount of the deflector 10 in FIGS.3 through 6. In this case, the tilt direction adjusting portion is referred to as a height adjusting portion. The height adjusting portion may adjust a height of at least one of a plurality of first coupling portions, for example, at least three first coupling portions. For example, the height adjusting portion may adjust heights of two coupling holes that are relatively close to the rotational axis 15a of the deflector 10 from among the plurality of first coupling portions, for example, three coupling holes. For example, heights of the coupling holes 12 and 13 may be adjusted in the example illustrated in FIGS.3 and 5. For example, heights of the coupling holes 12'' and 13'' may be adjusted in the example illustrated in FIGS.4 and 6. According to this configuration, by reducing a tilt amount of the deflector 10, the error of the diameter of beam spots on the exposure object 40 may be reduced. [0054] It should be understood that examples described herein should be considered in a descriptive sense and not for purposes of limitation. Descriptions of features or aspects within each example should typically be considered as available for other similar features or aspects in other examples. While one or more examples have been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope as defined by the following claims.

Claims

WHAT IS CLAIMED IS: 1. An optical scanner comprising: a light source unit to emit a light beam; a deflector to deflect and scan the light beam in a main scanning direction and comprising at least three first coupling portions; an imaging optical system to form an image by scanning the deflected light on an exposure object at a constant speed; and a frame comprising at least three second coupling portions respectively corresponding to the at least three first coupling portions, wherein an assembly reference line is a shorter line among a first line between two second coupling portions relatively close to a rotational axis of the deflector from among the at least three second coupling portions and a second line perpendicular to a line from another second coupling portion among the at least three second coupling portions to the first line, and positions of the at least three second coupling portions are such that the assembly reference line faces a direction in which an error of a diameter of beam spots formed on the exposure object is within a reference error. 2. The optical scanner of claim 1, wherein the assembly reference line faces a quadrant opposite the light source unit with respect to a third line that is orthogonal to the main scanning direction. 3. The optical scanner of claim 2, wherein an angle θ1 between the assembly reference line and the third line satisfies Expression 1 below: -90° ≤ θ1 ≤ -15° ---(Expression 1), where a negative sign denotes an opposite side of the light source unit with respect to the third line. 4. The optical scanner of claim 1, wherein the assembly reference line faces a quadrant at the light source unit with respect to a third line that is orthogonal to the main scanning direction, and an angle θ1 between the assembly reference line and the third line satisfies Expression 2 below: 75°≤θ1≤90°---(Expression 2) 5. The optical scanner of claim 1, further comprising a height adjusting portion to adjust a height of at least one of the at least three first coupling portions. 6. The optical scanner of claim 5, wherein the height adjusting portion adjusts a height of two first coupling portions that are relatively close to a rotational axis of the deflector from among the at least three first coupling portions. 7. The optical scanner of claim 5, wherein the first coupling portion comprises a coupling hole, the second coupling portion comprises a coupling boss corresponding to the coupling hole, and the height adjusting portion comprises: an elastic member to apply an elastic force to the deflector in a direction to be apart from the coupling boss; and a coupling member coupled to the coupling boss through the coupling hole. 8. An optical scanner comprising: a light source unit to emit a light beam; a deflector to deflect and scan the light beam in a main scanning direction, the deflector comprising a plurality of coupling holes; and the frame having a plurality of coupling bosses respectively corresponding to the plurality of coupling holes, wherein, a line connecting two coupling bosses that are relatively close to a rotational axis of the deflector from among the plurality of coupling bosses is a first line, a perpendicular line drawn from another coupling boss among the plurality of coupling bosses to the first line is a second line, and an angle θ1 between an assembly reference line which is a shorter line among the first line and the second line and a third line orthogonal to the main scanning direction satisfies Expression 1 or Expression 2 below: -90° ≤ θ1 ≤ -15° ---(Expression 1), 75° ≤ θ1 ≤ 90°---(Expression 2), where a negative sign denotes an opposite side of the light source unit with respect to the third line. 9. The optical scanner of claim 8, further comprising a height adjusting portion to adjust a height of at least one of the plurality of coupling holes. 10. The optical scanner of claim 9, wherein the height adjusting portion adjusts a height of two first coupling portions that are relatively close to a rotational axis of the deflector from among the plurality of coupling holes. 11. The optical scanner of claim 9, wherein the height adjusting portion comprises: an elastic member to apply an elastic force to the deflector in a direction to be apart from the coupling bosses; and a coupling member coupled to the coupling bosses through the coupling holes. 12. An optical scanner comprising: a light source unit to emit a light beam; a deflector to deflect and scan the light beam in a main scanning direction, the deflector comprising a plurality of first coupling portions; a frame comprising second coupling portions respectively corresponding to the plurality of first coupling portions; an imaging optical system to form an image by scanning the deflected light onto an exposure object at a constant speed; and a tilt direction adjusting portion to adjust an inclination direction of a rotational axis of the deflector, wherein an angle θ2 between the inclination direction of the rotational axis of the deflector and a third line orthogonal to the main scanning direction satisfies Expression 3 or Expression 4 below: -90° ≤ θ2 ≤ -15° ---(Expression 3) 75° ≤ θ2 ≤ 90°---(Expression 4) where a negative sign denotes an opposite side of the light source unit with respect to the third line. 13. The optical scanner of claim 12, wherein the tilt direction adjusting portion adjusts a height of at least one of the plurality of first coupling portions with respect to at least one of the second coupling portions. 14. The optical scanner of claim 13, wherein the tilt direction adjusting portion adjusts a height of two first coupling portions that are relatively close to the rotational axis of the deflector from among the plurality of first coupling portions. 15. The optical scanner of claim 13, wherein the first coipling portion comprises a coupling hole, and the second coupling portion comprises a coupling boss corresponding to the coupling hole, and wherein the tilt direction adjusting portion comprises: an elastic member to apply an elastic force to the deflector in a direction to be apart from the coupling boss; and a coupling member coupled to the coupling boss through the coupling hole.