Optical deflector, optical scanning device including optical deflector, and image forming apparatus including optical scanning device

An optical deflector is configured such that a distance between a circumscribed circle of a rotary polyhedron centered on an axis of the rotary polyhedron and an inner peripheral surface of a peripheral wall of a cover member in a radial direction of the rotary polyhedron is largest at both of circumferential ends of an opening of the cover member.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2018-222217 filed on Nov. 28, 2018, the entire contents of which are incorporated herein by reference.

BACKGROUND

The technology disclosed herein relates to an optical deflector, an optical scanning device including an optical deflector, and an image forming apparatus including an optical scanning device.

An optical deflector is installed in, e.g., an image forming apparatus. Such an optical deflector includes a rotary polyhedron and a cover covering the rotary polyhedron, and the cover has an opening facing a peripheral surface of the rotary polyhedron. A light emitted from a light source is applied to the peripheral surface of the rotary polyhedron through the opening of the cover, and the rotary polyhedron deflects the light while rotating about an axis thereof to scan the light on an object to be irradiated, i.e., an image carrier, through the opening. Thereby, an electrostatic latent image is formed on a surface of the image carrier.

Since the cover of the optical deflector of this type is a non-closed type having an opening as described above, noises generated by rotation of the rotary polyhedron leak out of the cover through the opening. Accordingly, in the conventional art, the opening is formed as small as possible so as to reduce the noises.

SUMMARY

An aspect of the present disclosure provides an optical deflector including a rotary polyhedron and a cover member. The cover member covers the rotary polyhedron. The cover member has an opening facing a peripheral surface of the rotary polyhedron. A light beam emitted from a light source is applied to the peripheral surface of the rotary polyhedron through the opening of the cover member. The rotary polyhedron deflects the light beam while rotating about an axis thereof to scan the light beam on an object to be irradiated through the opening.

The optical deflector is configured such that a distance between a circumscribed circle of the rotary polyhedron centered on the axis of the rotary polyhedron and an inner peripheral surface of a peripheral wall of the cover member in a radial direction of the rotary polyhedron is largest at both of circumferential ends of the opening.

Another aspect of the present disclosure provides an optical scanning device including the optical deflector and the light source.

Another aspect of the present disclosure provides an image forming apparatus including the optical scanning device and the object to be irradiated. The object to be irradiated is an image carrier having a surface on which an electrostatic latent image is to be formed.

DETAILED DESCRIPTION

Hereinafter, example embodiments of the technology disclosed herein will be described in detail on the basis of the drawings. It should be noted that the technology disclosed herein is not limited to the embodiments described below.

FIG. 1shows a schematic configuration of an image forming apparatus1according to an example embodiment. The image forming apparatus1is a tandem-type color printer and has an image formation part3in a box-shaped casing2. The image formation part3forms by transfer an image on a sheet P on the basis of image data transmitted from an external device such as a computer connected to a network or the like. An optical scanning device4that radiates a laser beam is disposed under the image formation part3. A transfer belt5is disposed above the image forming part3. A sheet storage unit6that stores sheets P is disposed under the optical scanning device4, and a manual sheet feed unit7is disposed at a lateral side of the sheet storage unit6. A fixing unit8that performs a fixing process on the image formed on the sheet P is disposed at a lateral side of and above the transfer belt5. Reference numeral9indicates a sheet discharge unit which is disposed at the top of the casing2and to which the sheet P subjected to the fixing process in the fixing unit8is discharged.

The image formation part3includes four image forming units10Y,10M,10C, and10Bk aligned along the transfer belt5. The image forming units10Y,10M,10C, and10Bk respectively form yellow (Y), magenta (M), cyan (C), and black (Bk) toner images on the basis of image information transmitted from the external device. In the description below, reference numerals are suffixed with the letters “Y”, “M”, “C,” and “Bk” when the color correspondence need to be distinguished from each other, while the letters are omitted when the color correspondence does not need to be distinguished from each other.

Each image forming unit10Y,10M,10C,10Bk has a photosensitive drum11. The photosensitive drum11serves as an image carrier and an object to be irradiated. A charger12is disposed directly under each photosensitive drum11. A developing device13is disposed at one lateral side of each photosensitive drum11. A primary transfer roller14is disposed directly above each photosensitive drum11. A cleaning unit15is disposed on the other lateral side of each photosensitive drum11. The cleaning unit15cleans a peripheral surface of the photosensitive drum11.

The peripheral surface of each photosensitive drum11is uniformly, electrically charged by the charger12. The electrically charged peripheral surface of each photosensitive drum11is irradiated with a laser beam corresponding to each color, which is based on the image data input from the aforementioned computer or the like and is radiated from the optical scanning device4, so that an electrostatic latent image is formed on the peripheral surface of each photosensitive drum11. A developer is supplied to the electrostatic latent image from the developing device13, so that a yellow, magenta, cyan, or black toner image is formed on the peripheral surface of each photosensitive drum11. The toner images are superimposed together and transferred to the transfer belt5by a transfer bias applied to the primary transfer roller14.

Reference numeral16indicates a secondary transfer roller disposed below the fixing unit8. The secondary transfer roller16is in contact with the transfer belt5. A sheet P conveyed along a sheet conveyance path17from the sheet storage unit6or the manual sheet feed unit7passes between the secondary transfer roller16and the transfer belt5. In this process, the toner images on the transfer belt5are transferred to the sheet P by a transfer bias applied to the secondary transfer roller16.

The fixing unit8has a heat roller18and a pressure roller19, and heats and presses the sheet P with the sheet P interposed between the heat roller18and the pressure roller19, thereby fixing the toner images transferred on the sheet P to the sheet P. The sheet P subjected to the fixing process is discharged to the sheet discharge unit9. Reference numeral20indicates a reverse conveyance path which is used in duplex printing to reverse the sheet P discharged from the fixing unit8.

[Configuration of Optical Scanning Device4]

Next, the configuration of the optical scanning device4is generally described with reference toFIGS. 2 to 4.

As shown inFIG. 2, the optical scanning device4scans a light beam LY for yellow, a light beam LM for magenta, a light beam LC for cyan, and a light beam LBk for black on the peripheral surfaces of the photosensitive drums11Y,11M,11C, and11Bk, respectively, so that an electrostatic latent image is formed thereon.

The optical scanning device4has an incidence optical system including light sources40Y,40M,40C, and40Bk respectively emitting the light beams LY, LM, LC, and LBk (seeFIGS. 3 and 4), one polygon mirror41shared for the four light beams LY, LM, LC, and LBk, and an image-formation optical system for forming an image of each light beam LY, LM, LC, LBk deflected and scanned by the polygon mirror41on the peripheral surface of the corresponding photosensitive drum11Y,11M,11C,11Bk and scanning the light beam LY, LM, LC, LBk on the peripheral surface of the photosensitive drum11Y,11M,11C,11Bk (seeFIGS. 2 and 4).

As shown inFIGS. 3 and 4, the incidence optical system includes four collimator lenses42respectively provided with respect to the light sources40Y,40M,40C, and40Bk, an aperture (not illustrated) adjusting the light beams LY, LM, LC, and LBk having passed through the collimator lenses42to a predetermined optical path width, and a cylindrical lens43focusing the light beams LY, LM, LC, and LBk having passed through the aperture onto a deflecting surface40aof the polygon mirror41.

As shown inFIG. 2, the image-formation optical system includes a first image forming lens45adisposed at a optical-path downstream side of the polygon mirror41, a plurality of second image forming lenses45b, and turning mirrors46ato46d. Each image forming lens45a,45bis composed of, for example, fθ lens.

The operation of the thus-configured optical scanning device4is described with reference toFIGS. 2 to 4. As shown inFIG. 3, the light beams LY, LM, LC, and LBk emitted from the light sources40Y,40M,40C, and40Bk are collimated by the collimator lenses42, and then made incident on the cylindrical lens43. The light beams LY, LM, LC, and LBk made incident on the cylindrical lens43are emitted in the form of collimated light beams in a main-scanning cross section (a cross section perpendicular to a sub-scanning direction) and in the form of convergent light beams in a sub-scanning cross section (a cross section extending along the sub-scanning direction), and made obliquely incident on the deflecting surface41aof the polygon mirror41so that images thereof are formed thereon. In this process, in order to facilitate separation of the optical paths of the four light beams LY, LM, LC, and LBk deflected by the polygon mirror41, the light beams LY, LM, LC, and LBk are made incident at different angles on the deflecting surface41aas seen in the sub-scanning cross section (seeFIG. 3).

As shown inFIG. 4, the light beams LY, LM, LC, and LBk made incident on the deflecting surface41aof the polygon mirror41are each scanned at a constant angular velocity by the polygon mirror41, and then converted into a constant-velocity scanning light by the first image forming lens45a. The light beams LY, LM, LC, and LBk having passed through the first image forming lens45aare each reflected by one or more turning mirrors46ato46bdisposed in their respective optical paths (seeFIG. 2), and they are respectively guided to the peripheral surfaces of the photosensitive drums11Y,11M,11C, and11Bk through the second image forming lenses45b.

[Configuration of Optical Deflector47]

As shown inFIGS. 5 and 6, the polygon mirror41is united with a drive motor44(illustrated only inFIG. 6) and a board48to constitute one optical deflector47. Further, the optical deflector47has a cover member50covering the polygon mirror41.

The optical deflector47is enclosed in an optical housing49that forms an outer wall of the optical scanning device4. As shown inFIG. 5, the optical housing49has a flat mount surface49atherein. The optical deflector47is fixed to the mount surface49a.

As illustrated inFIG. 6, the drive motor44includes a cylindrical motor body44aand a drive shaft44b. The drive shaft44bprotrudes upward from the top of the motor body44a.

The motor body44ais fixed to the board48. The board48has a connector48a, a capacitor48b, an integrated circuit48cfor driving, etc. (seeFIG. 8) mounted thereon. The board is fixed to the mount surface49aof the optical housing49.

As shown inFIG. 4, the polygon mirror41is a polygonal mirror having a regular hexagonal cross section. The polygon mirror41has six deflecting surfaces41athat respectively correspond to six sides of the regular hexagon. The light beams LY, LM, LC, and LBk emitted from the light sources40Y,40M,40C, and40Bk are applied to the deflecting surfaces41a. The polygon mirror41is mounted to the upper end of the drive shaft44bso as to rotate together with the drive shaft44b.

[Configuration of Cover Member50]

As illustrated inFIG. 5, the cover member50is fixed to the mount surface49aof the optical housing49to cover the polygon mirror41and the drive motor44. The cover member50functions to prevent noises caused by rotation of the polygon mirror41and to prevent dust and the like from adhering to the polygon mirror41.

The cover member50has a first cover part51, a second cover part52, and an extending part53. As shown inFIG. 6, the first cover part51covers the polygon mirror41from above to define a first space S1in which the polygon mirror41is disposed. The first cover part51has a peripheral wall51ahaving a circular cross section and surrounding the polygon mirror41, and a top plate51bclosing the top of the peripheral wall51a.

As shown inFIG. 7, the peripheral wall51aof the first cover part51has a rectangular opening51cformed therein which extends in the circumferential direction. The opening51cguides the light beams LY, LM, LC, and LBk emitted from the light sources40to the inside of the first cover part51and guides the light beams LY, LM, LC, and LBk deflected by the deflecting surfaces41aof the polygon mirror41to the outside of the first cover part51. The opening51calso serves as an opening through which a flow of air generated by rotation of the polygon mirror41passes. The opening51cis described in detail later.

The second cover part52has a flat-case shape which is trapezoidal in plan view. The second cover part52defines a second space S2in which the motor body44aof the drive motor is enclosed (seeFIG. 6). The second cover part52is joined to the first cover part51. The bottom of the first cover part51is entirely opened, so that the first space S1in the first cover part51continuously communicates with the second space S2in the second cover part52.

The second cover part52that has a trapezoidal shape in plan view is arranged such that the shorter one of the parallel sides of the trapezoid faces the first image forming lens45a. A wall extending downward from the shorter side of the second cover part52has a rectangular cutout52aformed therein which is opened at the lower side thereof. This cutout52aand the mount surface49aform a rectangular opening T1. The opening T1communicates between the inside and the outside of the second space S2.

As shown inFIG. 8, the extending part53is formed on the opposite side to the first image forming lens45aside of the second cover part52. The extending part53is joined to the center of the side of the second cover part52. An opening T2and an opening T3are respectively formed at both sides of the extending part53. The openings T2and T3function to cause a flow of air generated by rotation of the polygon mirror41to pass therethrough. This function enables the drive motor44driving the polygon mirror41as well as electronic components around the drive motor44to be cooled.

Side walls of the second cover part52corresponding to the pair of oblique sides of the trapezoid are each bent so as to form a plate-shaped bracket54. The bracket54has an attachment hole54aformed therein. The extending part53also has a plate-shaped bracket55. The bracket55has an attachment hole55aformed therein. The cover member50is fixed to the mount surface49aby bolts inserted in the attachment holes54aand55a.

[Variation of Pressure of Air Inside First Cover Part51]

Referring toFIG. 4, when the polygon mirror41rotates, the air inside the first cover part51is pushed in the rotating direction by the vertices of the polygon mirror41. The air is blown out of the first cover part51through a first clearance K1between a first opening end A of the opening51cand the deflecting surfaces41aof the polygon mirror41. Further, air is sucked into the first cover part51through a second clearance K2between a second opening end B of the opening51cand the deflecting surfaces41aof the polygon mirror41.

The first clearance K1and the second clearance K2are varied by rotation of the polygon mirror41since the peripheral wall51aof the first cover part51has a circular cross section and the polygon mirror41has a regular hexagonal cross section.

Therefore, the pressure of air between the first opening end A of the opening51cand the deflecting surfaces41aof the polygon mirror41(i.e., air in the first clearance K1), which pressure is hereinafter referred to as “first pressure”, varies cyclically with rotation of the polygon mirror41(seeFIG. 9). Further, the pressure of air between the second opening end B of the opening51cand the deflecting surfaces41aof the polygon mirror41(i.e., air in the second clearance K2), which pressure is hereinafter referred to as “second pressure”, varies cyclically with rotation of the polygon mirror41(seeFIG. 10). The first pressure and the second pressure are each subjected to one-cycle variation during each one-surface rotation of the polygon mirror41(each 60° rotation of the polygon mirror41).

Embodiment 1 of Noise Prevention Structure of Optical Deflector47

The peripheral wall51aof the first cover part51covering the polygon mirror41has the opening51cformed therein which lets noises generated by rotation of the polygon mirror41leak out of first cover part51through the opening51c. Therefore, in this embodiment, such noises are reduced by applying an inventive idea to the arrangement and structure of the first cover part51and the structure of the opening51c.

Specifically, as shown inFIG. 11, the optical deflector is configured such that a distance d between a circumscribed circle G of the polygon mirror41and an inner peripheral surface of the peripheral wall51aof the first cover part51in the radial direction of the polygon mirror41is largest at both circumferential ends of the opening51c(i.e., at the first opening end A and at the second opening end B). To realize this configuration, in this embodiment, an axis U of the peripheral wall51aof the first cover part51is positioned so as to be shifted to the opening51cside from an axis O of the polygon mirror41by a predetermined distance δ.

This configuration enables the first clearance K1and the second clearance K2to be wider than in the case where the axis U of the peripheral wall51aof the first cover part51is coincident with the axis O of the polygon mirror41. Thereby, the amplitude of the variation of the pressure of air passing through each clearance K1, K2is reduced. Consequently, noises caused by the pressure variation are reduced.

Further, as shown inFIG. 11, the opening51cis formed such that an opening angle θ1of the opening51ccentered on the axis O of the polygon mirror41satisfies Equation 1 below:
θ1(360°/the number of surfaces of the polygon mirror 41)×n(1),
where n is a natural number less than the number of surfaces of the polygon mirror41.

FIG. 11shows a cross section of the peripheral wall51aof the first cover part51, which is a cross section perpendicular to the axial direction of the polygon mirror41, i.e., a main-scanning cross section, with n being 1. Since the number of surfaces of the polygon mirror41is 6, θ160° is obtained on the basis of Equation (1) above. Therefore, the opening angle θ1in this embodiment is set to 60°. When the opening angle θ1of the opening51cis 60°, a reduced noise level is achieved. The reasons therefor are as follows.

[A] The opening51cis formed such that the opening angle θ1satisfies Equation (1); therefore, as shown inFIG. 11, when one of the six vertices of the polygon mirror41is positioned in the vicinity of the first opening end A, another one of the vertices is positioned in the vicinity of the second opening end B. When the middle of a pair of adjacent vertices of the polygon mirror41is positioned in the vicinity of the opening end A, the middle of another pair of adjacent vertices is positioned in the vicinity of the opening end B.

[B] That is to say, the first clearance K1and the second clearance K2always have substantially the same length during rotation of the polygon mirror41, and the first pressure and the second pressure therefore show substantially the same value (absolute value). However, since the first clearance K1allows air to be blown out of the first cover part51and, in contrast, the second clearance K2allows air to be sucked into the first cover part51, the variation of the first pressure and the variation of the second pressure are shifted in phase from each other by 180° (0.5 cycle) (seeFIGS. 9 and 10).

[C] Therefore, a sound generated at the air outlet in the vicinity of the first clearance K1and a sound generated at the air inlet in the vicinity of the second clearance K2are also shifted in phase from each other by 180°. As a result thereof, these sounds cancel out each other, so that a reduced noise level is achieved. Thus, noises generated by rotation of polygon mirror41are sufficiently reduced even though the first cover part51is a non-sealed type.

When n is 2, θ1is 120°. When n is 3, θ1is 180°. When n is a natural number less than the number of surfaces of the polygon mirror41, i.e., 2, 3, . . . , the same actions and effects as those described as [A]-[C] above are provided. That is to say, when the opening51cis formed such that the opening angle θ1satisfies Equation (1) above, a reduced noise level is obtained. As shown inFIG. 12, the inventors conducted an experiment to confirm that a reduced noise level was achieved when the opening angle θ1was 60°, 120°, or 180°.

FIG. 13shows the relation between a combined amplitude and a phase difference (a phase difference between a first waveform representing the variation of the first pressure and a second waveform representing the variation of the second pressure). The combined amplitude means the amplitude of a waveform obtained by combining the first waveform and the second waveform. The amplitudes of the first and second waveforms (the aptitudes of the fundamental waveforms) are each 1.

InFIG. 13, the combined amplitude is smaller than the amplitudes of the fundamental waveforms, i.e., smaller than 1, in the range where the phase difference is 0.33 cycle to 0.67 cycle. When the phase difference is 0.5 cycle as in this embodiment, the combined amplitude is smallest; therefore, the noise level is smaller than those of the noise caused by the first pressure and the noise caused by the second pressure.

That is to say, in this embodiment, the phase difference between the first waveform representing the variation of the first pressure (seeFIG. 9) and the second waveform representing the variation of the second pressure (seeFIG. 10) is determined such that the amplitude of the combined waveform of the first waveform and the second waveform is smaller than both the amplitude of the first waveform and the amplitude of the second waveform. Further, the opening angle θ1of the opening51cis set to an angle corresponding to the thus-determined phase difference (in this embodiment, 60°).

Modification of Embodiment 1

The opening51cmay be formed such that the opening angle θ1of the opening51ccentered on the axis O of the polygon mirror41satisfies Equations (2) and (3) below:
θ1>((360°/the number of surfaces of the rotary polyhedron)×n)×0.83  (2)
θ1<((360°/the number of surfaces of the rotary polyhedron)×n)×1.17  (3),
where n is a natural number less than the number of surfaces of the polygon mirror41.

When n is 1, 49.8°<θ1<70.2° is obtained on the basis of Equations (2) and (3) (the number of surfaces of the polygon mirror41is 6). In the range of 49.8°<θ1<70.2°, a reduced noise level is achieved. The reasons therefor are as follows.

Equations (2) and (3) mean that the opening angle θ1of the opening51cis in a range of plus or minus 17% with respect to the opening angle θ1of the opening51cin Embodiment 1 above. This means that the phase difference between the first waveform representing the variation of the first pressure and the second waveform representing the variation of the second pressure in this modification is increased by 0.17 cycle and decreased by 0.17 cycle with respect to the phase difference in Embodiment 1 above. In the range of 49.8°<θ1<70.2°, the two waveforms show a phase difference of 0.33-0.67 cycle since the phase difference in Embodiment 1 above is 0.5 cycle.

As seen fromFIG. 13, in the range where the phase difference is in the range of 0.33-0.67 cycle, the combined amplitude is smaller than the amplitudes of the fundamental waveforms, i.e., smaller than 1; therefore, the noise level is smaller than those of the noise caused by the first pressure and the noise caused by the second pressure. Thus, in this modification, the noises generated by rotation of the polygon mirror41are sufficiently reduced even though the first cover part51is a non-sealed type.

FIG. 14shows Embodiment 2. Embodiment 2 is different from Embodiment 1 in the structure for causing the distance d to be largest at the circumferential ends of the opening51c.

That is to say, in Embodiment 2, the peripheral wall51aof the first cover part51is composed of an arcuate wall51dhaving an arc-shaped cross section, a first joined wall51e, and a second joined wall51f. The first and second joined walls51eand51fare respectively joined to circumferential ends of the arcuate wall51d.

The arcuate wall51dis formed to surround the polygon mirror41. The arcuate wall51dhas an axis U coincident with the axis O of the polygon mirror41. That is to say, the arcuate wall51dis formed coaxially with the polygon mirror41.

The first joined wall51eand the second joined wall51fare formed to be bilaterally symmetrical inFIG. 14. The curvature of the first and second articulated walls51eand51fis smaller than that of the arcuate wall51d.

The first joined wall51eextends from one circumferential end toward the other circumferential end of the arcuate wall51dand extends outside an inscribed circle I of the arcuate wall51din the radial direction. Similarly, the second joined wall51fextends from the other circumferential end toward the one circumferential end of the arcuate wall51dand extends outside the inscribed circle I of the arcuate wall51din the radial direction. The distal end of the first joined wall51eforms one circumferential end (the first opening end A) of the opening51c, while the distal end of the second articulated wall51fforms the other circumferential end (the second opening end B) of the opening51c.

This configuration enables the first clearance K1and the second clearance K2to be wider in the radial direction than in the case where the first and second joined walls51eand51fare formed to extend along the inscribed circle I. Thereby, the same actions and effects as those in Embodiment 1 above are achieved.

Further, differently from Embodiment 1, the axis U of the arcuate wall51dof the first cover part51does not need to be eccentric to the axis O of the polygon mirror41. Therefore, differently from Embodiment 1, the width of the air flow path (the distance between the inner peripheral surface of the peripheral wall51and the deflecting surfaces41aof the polygon mirror41) at the opposite side to the opening51cside in the first cover part51is not narrowed. Therefore, the air flow path around the polygon mirror41has a sufficient width, which provides high heat dissipation in the first cover part51.

Note that, for the same reasons as those in Embodiment 1, it is preferred that the opening angle θ1of the opening51csatisfies Equation (1) above. Further, for the same reasons as those in the above-described modification of Embodiment 1, the opening angle θ1may be set to satisfy Equations (2) and (3).

In the thus-configured first cover part51according to Embodiment 2, the air flow separates at the boundary between the arcuate wall51dand the first joined wall51e(one end C of the arcuate wall51d) and at the boundary between the arcuate wall51dand the second joined wall51f(the other end D of the arcuate wall51d) because the curvature of the inner peripheral surface of the first cover part51changes there. Therefore, noises are likely to be generated there.

To solve this problem, in this embodiment, an angle θ2formed by lines connecting each circumferential end of the arcuate wall51dto the axis U of the arcuate wall51dis set to satisfy Equation (4) below. In Equation (4), n is a natural number less than the number of surfaces of the polygon mirror41.
θ2(360°/the number of surfaces of the polygon mirror 41)×n(4)

Equation (4) is obtained by replacing θ1with θ2in Equation (1) that is used in Embodiment 1. When Equation (4) is satisfied, variation of the pressure of air at the one end C of the arcuate wall51dand variation of the pressure of air at the other end D of the arcuate wall51dcancel out each other. Therefore, generation of noises at the points where the curvature of the inner peripheral surface of the first cover part51changes is suppressed.

In the example shown inFIG. 14, θ1is set to 60° on the basis of Equation (1), where n=1 and the number of surfaces of the rotary polyhedron is 6. On the other hand, θ2is set to 120° on the basis of Equation (4), where n=2 and the number of surfaces of the rotary polyhedron is 6.

The angle θ2may be set to satisfy Equations (5) and (6) below. In Equations (5) and (6), n is a natural number less than the number of surfaces of the polygon mirror41.
θ2>((360°/the number of surfaces of the rotary polyhedron)×n)×0.83  (5)
θ2<((360°/the number of surfaces of the rotary polyhedron)×n)×1.17  (6)

Equations (5) and (6) are obtained by replacing θ1with in Equations (2) and (3) that is used in the above-described modification of Embodiment 1. When θ2is set in this range, a noise reduction effect is achieved on the same reasons as those in the above-described modification of Embodiment 1.

Modification 1 of Embodiment 2

FIG. 15corresponds toFIG. 14which shows Modification 1 of Embodiment 2 is shown. In the description below, components identical to those inFIG. 14are denoted by the same reference numerals as those used inFIG. 14so that the description thereof is omitted.

In this Modification 1, the first joined wall51eand the second joined wall51eare each curved in the direction opposite to the direction in which the arcuate wall51dis curved. Further, the first joined wall51eand the second joined wall51fare formed to respectively extend from the ends of the arcuate wall51dtoward the top ofFIG. 15and to spread outward in the lateral direction (the main-scanning direction).

In the example shown inFIG. 15, θ2is set to 120° on the basis of Equation (4), wherein n=2 and the number of surfaces of the rotary polyhedron is 6. θ1is set to an angle which permits movement of a light beam in the main-scanning direction, such as an angle of 100 to 110°.

This configuration enables the first opening end A and second opening end B of the opening51cto be located outside and greatly away from the inscribed circle I of the arcuate wall51din the radial direction. Therefore, the amplitude of the variation of the pressure of the air passing through each clearance K1, K2is minimized, so that generation of noises is suppressed.

Further, this configuration enables the opening51cto have a greater circumferential width, which prevents a light beam as scanning from hitting the circumferential ends of the opening51c.

Note that, even when the opening angle θ1of the opening51cdoes not satisfy Equation (1) or Equations (2) and (3), noises caused by the flow of air passing through each clearance K1, K2are so small as to be ignorable as compared with noises generated at each end C, D of the arcuate wall51dbecause the first clearance K1and the second clearance K2are sufficiently wide. The noises generated at each end C, D of the arcuate wall51dis sufficiently reduced when the angle θ2satisfies Equation (4) or Equations (5) and (6).

Modification 2 of Embodiment 2

In this modification, the first joined wall51eand the second joined wall51fare each linearly formed as viewed in the axial direction. Further, the first joined wall51eand the second joined wall51feach extend in the direction of a tangent line to the arcuate wall51das viewed in the axial direction. In the example shown inFIG. 16, θ1is set to 60° on the basis of Equation (1), wherein n=1 and the number of surfaces of the rotary polyhedron is 6. On the other hand, θ2is set to 120° on the basis of Equation (4), wherein n=2 and the number of surfaces of the rotary polyhedron is 6.

As compared with the configuration according to Embodiment 2 (the configuration shown inFIG. 14), the configuration according to Modification 2 enables the first opening end A and second opening end B of the opening51cto be located outside and greatly away from the inscribed circle I of the arcuate wall51din the radial direction. Therefore, the amplitude of the variation of the pressure of the air passing through each clearance K1, K2is minimized, so that generation of noises is suppressed.

Further, since the first joined wall51eand the second joined wall51feach extend in the direction of a tangent line to the arcuate wall51das viewed in the axial direction, the flow of air at each end C, D of the arcuate wall51dis smoothed, so that generation of noises is suppressed.

Modification 3 of Embodiment 2

In this modification, the first joined wall51eand the second joined wall51fare each linearly formed as viewed in the axial direction, and they are respectively coupled to the ends of the arcuate wall51dsuch that they are bent from the ends of the arcuate wall51d.

In the configuration according to Modification 3 of Embodiment 2, as compared with the configurations according to Modifications 1 and 2, the direction of the wall is suddenly changed at each end C, D of the arcuate wall51d. Because of this, noises are likely to be generated at the ends C and D of the arcuate wall51d. Therefore, applying the technology disclosed herein to the peripheral wall51ahaving a shape as in this modification to determine the angle θ2on the basis of Equations (4) to (6) is particularly helpful for suppressing noises. Note that, in the example shown inFIG. 17, θ1is set to 30° and θ2is set to 60°.

Other Embodiments

The technology disclosed herein encompasses the following configuration with respect to Embodiment 1.

The polygon mirror41in the above-described embodiments and the modifications thereof is formed to have a hexagonal cross section; however, the present disclosure is not limited to such a polygon mirror. For example, a polygon mirror having a pentagonal cross section is possible.

Further, the foregoing description describes an example where the optical deflector47is applied to the optical scanning device4installed in a printer; however, the present disclosure is not limited to the example. For example, the optical deflector47may be applied to, for example, a copying machine, a facsimile, or a multifunction peripheral/printer/product (MFP).