JP2012230313A - Optical scanner and image forming apparatus equipped with the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical scanner for performing constant-speed scanning while minimizing the variation of a beam radius in a main scanning direction on a scanning surface.SOLUTION: There is provided an optical scanner 13 which comprises: a laser diode (light source) 25; an MEMS mirror 28 for deflecting light beams emitted from the laser diode 25; and a plurality of scanning lenses 29 and 30 for imaging light beams deflected by the MEMS mirror 28 on a photoreceptor drum (scanning surfaces)2a (2b to 2d). The optical characteristics of the scanning lens 29 closest to the MEMS mirror 28 are set so that the light beams are incident to the scanning lens 29 by deflection angle characteristics represented by the following expression: θ(t)=Asinωt+Asin(nωt) and the light beams are imaged on the photoreceptor drum (scanning surfaces)2a (2b to 2d) so as to satisfy the following expression.

Description

  The present invention relates to an optical scanning device using a MEMS mirror as a deflecting unit and an image forming apparatus such as a copying machine or a printer provided with the optical scanning device.

  In an image forming apparatus such as a copying machine or a printer, an image carrier whose surface is uniformly charged by a charger is optically scanned by an optical scanning device, and an electrostatic latent image corresponding to image information is formed on the surface. The The electrostatic latent image is developed by a developing device using toner as a developer to be visualized as a toner image. The toner image is transferred onto a sheet by a transfer device and then heated and fixed by a fixing device. A series of image forming operations is completed when the sheet is pressed and fixed on the sheet, and the sheet on which the toner image is fixed is discharged out of the apparatus.

  Conventionally, a polygon mirror or a galvanometer mirror is exclusively used as a deflector for scanning a light beam in an optical scanning device. In order to achieve higher resolution images and high-speed printing, these polygon mirrors are used. And galvanometer mirrors need to be rotated at higher speeds.

  However, if the polygon mirror or galvanometer mirror is rotated at a high speed, problems such as heat generation and noise due to bearing durability, windage loss, and the like are limited.

  Accordingly, in recent years, a deflector using silicon micromachining (MEMS) technology has been developed. For example, a micromirror (hereinafter referred to as a “MEMS mirror”) and a torsion beam supporting the micromirror are integrated into a Si substrate. This MEMS mirror is formed by applying an AC voltage between the movable electrode on the MEMS mirror side and the fixed electrode on the fixed side, and resonating the MEMS mirror while twisting the torsion beam by electrostatic attraction generated between the two electrodes. There has been proposed a method of reciprocating vibration using the resonance (see, for example, Patent Document 1).

  According to the above method, the MEMS mirror can reciprocally vibrate (sinusoidal vibration) using resonance, so that it is possible to operate at high speed, and it is possible to obtain an advantage that noise and power consumption can be kept low.

  By the way, in order to scan a light beam at a constant speed on a scanning surface in a relatively simple optical scanning device using a MEMS mirror having a single deflection surface, the light beam deflected and scanned by the MEMS mirror is scanned with an arc sine characteristic. It needs to be condensed by the lens. In this case, constant speed scanning is possible, but a problem arises in that the image quality deteriorates because the beam diameter in the main scanning direction on the scanning surface changes from the scanning center to the scanning end. Here, the scanning end is a scanning position corresponding to the end of the image area.

Therefore, using a MEMS mirror having a plurality of reflecting surfaces that vibrate independently, the following formula:
θ (t) = A ₁ sin ωt + A ₂ sin (nωt)
Where n = integer (2 or 3)
θ: deflection angle (angle formed by scanning lens optical axis and light beam)
A ₁ , A ₂ : amplitude of each component
ω: angular frequency
t: As represented by time, an optical scanning device that vibrates by superimposing a sine wave having an integral multiple of the fundamental sine wave is used, and is closer to a constant angular velocity than a MEMS mirror having one deflection surface. In this state, the scanning surface is deflected and scanned. According to this, the fluctuation in the main scanning direction of the beam diameter on the scanning surface can be suppressed to be small, and a state close to constant speed scanning can be realized. The reason is that if the image height when the intersection of the scanning surface and the scanning lens optical axis is 0 is y, the beam diameter in the main scanning direction is proportional to dy / dθ and the scanning speed is equal to dy / dt. It is.

  Since the characteristic of the scanning lens ties the deflection angle θ and the image height y, the beam diameter in the main scanning direction is determined by the scanning lens. Since the characteristic of a typical fθ lens is y = fθ (f is a focal point), dy / dθ = f, and the beam diameter does not vary with respect to time t and image height y.

  However, since dy / dt = dy / dθ · dθ / dt, if dy / dθ is constant, dy / dt is not constant unless dθ / dt is constant, and constant-speed scanning is not realized. That is, in order to achieve both the suppression of beam diameter fluctuation and constant speed scanning with a simple optical system, it is necessary to use a deflector and an fθ lens such that the deflection angle is a constant angular velocity, such as a polygon mirror. The closer the deflection angle characteristic is to a constant angular velocity, the smaller the variation in scanning speed and the variation in beam diameter.

In Patent Document 2, the optical characteristic of the scanning lens is not shown by a specific expression, but dy / dθ and dθ / dt at the scanning center portion y0, the scanning end portion y _max, and y _0.5 between them. Each of the following magnitude relationships:
Dθ _max / dt _max <dθ ₀ / dt ₀ (the angular velocity of the deflection angle is smaller at the end than at the center)
Dy ₀ / dθ ₀ <dy _max / dθ _max (the beam diameter on the scanning surface is larger at the end than at the center)
Dθ ₀ / dt ₀ <dθ _0.5 / dt _0.5 (the angular velocity of the deflection angle is smaller at the center than at the middle part)
Dy _0.5 / dθ _0.5 <dy ₀ / dθ ₀ (the beam diameter on the scanning surface is larger at the center than at the middle)
Has been proposed to use a MEMS mirror as a deflector without degrading the image quality.

JP-A-4-21218 JP 2008-083688 A

  Factors that deteriorate the image quality described in Patent Document 2 are fluctuations in beam diameter and fluctuations in scanning speed. In Patent Document 2, the limit values of the beam diameter fluctuation and the scanning speed fluctuation are defined as ± 25% for the beam diameter fluctuation (Claim 2) and ± 10% for the scanning speed fluctuation (Claim 3). ing. In addition, it is desirable to set the ratio of the actual scanning time to be higher than the time required for one cycle of the MEMS mirror.

  However, satisfying the above-described magnitude relationship regarding dy / dθ and dθ / dt does not mean that the scanning time can be further maximized without deteriorating the image quality. In addition, fluctuations in scanning speed are more sensitive to image quality than fluctuations in beam diameter, and an image is degraded by fluctuations of several% in scanning speed.

  The present invention has been made in view of the above problems, and an object of the present invention is an optical scanning apparatus capable of performing constant speed scanning while suppressing fluctuations in the beam diameter in the main scanning direction on the scanning surface. The present invention provides an image forming apparatus including the above.

In order to achieve the above object, a first aspect of the present invention is directed to a light source that emits a light beam, a MEMS mirror that deflects the light beam emitted from the light source, and a light beam deflected by the MEMS mirror on a scanning plane. In an optical scanning device having a plurality of scanning lenses for imaging,
The optical characteristic of the scanning lens closest to the MEMS mirror is expressed as follows:
θ (t) = A ₁ sin ωt + A ₂ sin (nωt)
Where n = 2 or 3,
θ: angle formed by the optical axis of the scanning lens and the light beam A ₁ , A ₂ : amplitude of deflection angle
ω: angular frequency
t: incident with a deflection angle characteristic expressed in time, and the following formula:

ここに、γ：像高
ｆ：焦点距離
を満たすように光ビームを走査面上に結像させるよう設定したことを特徴とする。

Where γ: image height
f: The optical beam is set to form an image on the scanning surface so as to satisfy the focal length.

  According to a second aspect of the present invention, in the first aspect of the invention, the beam diameter in the main scanning direction on the scanning plane is the largest at the scanning center and the scanning end, and the minimum diameter is 0.9 times the maximum diameter. It is large.

  According to a third aspect of the present invention, in the first aspect of the present invention, the beam diameter in the main scanning direction on the scanning surface is the smallest at the center of the scanning, the largest at the scanning end, and the maximum diameter is 1. It is smaller than 1 time.

  According to a fourth aspect of the present invention, the optical scanning device according to any one of the first to third aspects is provided.

According to the present invention, the deflection angle θ (t) is given by
θ (t) = A ₁ sin ωt + A ₂ sin (nωt)
In the optical scanning device provided with the MEMS mirror represented by

By using a scanning lens having optical characteristics represented by (Equation 1), the image height y is given by the following formula:
y = f (A ₁ + nA ₂ ) ωt
It can be seen that constant speed scanning is possible.

  Further, the beam diameter D in the main scanning direction on the scanning surface is expressed by the following formula:

で表され、偏向角θの振幅Ａ_１とＡ_２の比によって傾向が異なる。具体的には、正弦波であるＡ_２＝０からＡ_２を負の方向に大きくしていくと（等角速度に近づく）とビーム径Ｄの変動は、
（１）走査中央部が最も小さく、走査端部に向かうに連れて単調に増加する
（２）走査中央部から走査端部に向かって一旦小さくなって極小値を示した後に単調に増加する
傾向を示す。

The tendency varies depending on the ratio of the amplitudes A ₁ and A ₂ of the deflection angle θ. Specifically, when A ₂ is increased in the negative direction from A ₂ = 0, which is a sine wave (approaching an equal angular velocity), the fluctuation of the beam diameter D is:
(1) The scan center is the smallest, and monotonously increases toward the scan end. (2) The value tends to monotonically increase after decreasing from the scan center to the scan end and showing a minimum value. Indicates.

Therefore, by appropriately setting the ratio of the amplitudes A ₁ and A ₂ of the deflection angle θ,
(1) The beam diameter in the main scanning direction on the scanning plane is the largest at the scanning center and the scanning end, and the minimum diameter is larger than 0.9 times the maximum diameter. (2) The beam in the main scanning direction on the scanning plane. The diameter is the smallest at the scanning center, the largest at the scanning end, and the maximum diameter can be smaller than 1.1 times the minimum diameter. Can be kept small within a range of ± 10%.

  As a result, according to the present invention, it is possible to perform constant speed scanning while suppressing fluctuations in the beam diameter in the main scanning direction on the scanning surface.

1 is a side sectional view of an image forming apparatus (color laser printer) according to the present invention. It is a top view of the main scanning direction which shows the structure of the optical scanning device principal part which concerns on this invention. It is a figure which shows the relationship between the linearity with respect to image height, and a beam diameter ratio.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

[Image forming apparatus]
FIG. 1 is a cross-sectional view of a color laser printer as an embodiment of an image forming apparatus according to the present invention. 1M, a cyan image forming unit 1C, a yellow image forming unit 1Y, and a black image forming unit 1K are arranged in tandem at regular intervals.

  Each of the image forming units 1M, 1C, 1Y, and 1K is provided with photosensitive drums 2a, 2b, 2c, and 2d, which are image carriers, and a charger 3a is disposed around each of the photosensitive drums 2a to 2d. 3b, 3c, 3d, developing devices 4a, 4b, 4c, 4d, transfer rollers 5a, 5b, 5c, 5d and drum cleaning devices 6a, 6b, 6c, 6d, respectively.

  Here, the photosensitive drums 2a to 2d are drum-shaped photosensitive members, and are rotationally driven at a predetermined process speed in a direction indicated by an arrow (clockwise) by a driving motor (not shown). The chargers 3a to 3d uniformly charge the surfaces of the photosensitive drums 2a to 2d to a predetermined potential by a charging bias applied from a charging bias power source (not shown).

  Further, the developing devices 4a to 4d contain magenta (M) toner, cyan (C) toner, yellow (Y) toner, and black (K) toner, respectively, and are formed on the photosensitive drums 2a to 2d. In addition, the toner of each color is attached to each electrostatic latent image to visualize each electrostatic latent image as a toner image of each color.

  The transfer rollers 5a to 5d are arranged so as to be able to contact the photosensitive drums 2a to 2d via the intermediate transfer belt 7 in the respective primary transfer portions. Here, the intermediate transfer belt 7 is stretched between a driving roller 8 and a tension roller 9 and is disposed on the upper surface side of each of the photosensitive drums 2a to 2d. The driving roller 8 is a secondary roller. The transfer unit is disposed so as to be in contact with the secondary transfer roller 10 via the intermediate transfer belt 7. A belt cleaning device 11 is provided in the vicinity of the tension roller 9.

  Incidentally, toner containers 12a, 12b, 12c, and 12d for supplying toner to the developing devices 4a to 4d are arranged in a line above the image forming units 1M, 1C, 1Y, and 1K in the printer main body 100. It is installed.

  Further, below the image forming units 1M, 1C, 1Y, and 1K in the printer main body 100, a total of four optical scanning devices (lasers) according to the present invention are provided corresponding to the image forming units 1M, 1C, 1Y, and 1K. A scanner unit (LSU)) 13 is disposed, and a paper feed cassette 14 is detachably installed at the bottom of the printer main body 100 below the scanner unit (LSU). A plurality of sheets (not shown) are stacked and accommodated in the sheet cassette 14, and in the vicinity of the sheet cassette 14, a pickup roller 15 that extracts sheets from the sheet cassette 14, and the extracted sheets A feed roller 16 and a retard roller 17 are provided for separating the components and feeding them one by one to the conveyance path S.

  In addition, the conveyance path S extending in the vertical direction on the side of the printer main body 100 includes a conveyance roller pair 18 that conveys a sheet, and the secondary transfer counter roller 8 and the second roller at a predetermined timing after the sheet is temporarily held. A registration roller pair 19 is provided to be supplied to a secondary transfer portion that is a contact portion with the next transfer roller 10. Next to the transport path S, another transport path S ′ used for forming an image on both sides of the sheet is formed, and a plurality of reversing roller pairs 20 are appropriately used for the transport path S ′. Are provided at regular intervals.

  By the way, the conveyance path S arranged in the vertical direction on one side of the printer main body 100 extends to the paper discharge tray 21 provided on the upper surface of the printer main body 100, and the fixing device 22 and the discharge path are disposed in the middle. Paper roller pairs 23 and 24 are provided.

  Next, an image forming operation by the color laser printer having the above configuration will be described.

  When an image formation start signal is issued, the photosensitive drums 2a to 2d are driven to rotate at a predetermined process speed in the direction of the arrow (clockwise) in the image forming units 1M, 1C, 1Y, and 1K, and these photosensitive drums 2a. ˜2d are uniformly charged by the chargers 3a to 3d. Each optical scanning device 13 emits a laser beam modulated by a color image signal for each color, irradiates the surface of each photosensitive drum 2a-2d with each laser beam, and each color on each photosensitive drum 2a-2d. The electrostatic latent images corresponding to the color image signals are respectively formed.

  First, magenta toner is attached to the electrostatic latent image formed on the photosensitive drum 2a of the magenta image forming unit 1M by the developing device 4a to which a developing bias having the same polarity as the charged polarity of the photosensitive drum 2a is applied. The electrostatic latent image is visualized as a magenta toner image. The magenta toner image is moved in the direction indicated by the arrow in the primary transfer portion (transfer nip portion) between the photosensitive drum 2a and the transfer roller 5a by the action of the transfer roller 5a to which a primary transfer bias having a polarity opposite to that of the toner is applied. Primary transfer is performed on the intermediate transfer belt 7 that is rotationally driven.

  The intermediate transfer belt 7 on which the magenta toner image has been primarily transferred as described above moves to the next cyan image forming unit 1C. In the cyan image forming unit 1C as well, the cyan toner image formed on the photosensitive drum 2b is transferred to the magenta toner image on the intermediate transfer belt 7 in the primary transfer portion in the same manner as described above.

  Similarly, yellow and black toners respectively formed on the photosensitive drums 2c and 2d of the yellow and black image forming units 1Y and 1K on the magenta and cyan toner images superimposed and transferred on the intermediate transfer belt 7, respectively. The images are sequentially superimposed at each primary transfer portion, and a full-color toner image is formed on the intermediate transfer belt 7. The transfer residual toner which is not transferred onto the intermediate transfer belt 7 and remains on the photosensitive drums 2a to 2d is removed by the drum cleaning devices 6a to 6d, and the photosensitive drums 2a to 2d are prepared for the next image formation. It is done.

  Then, in accordance with the timing at which the leading end of the full-color toner image on the intermediate transfer belt 7 reaches the secondary transfer portion (transfer nip portion) between the drive roller 8 and the secondary transfer roller 10, The sheet fed to the conveyance path S by the feed roller 16 and the retard roller 17 is conveyed to the secondary transfer unit by the registration roller pair 19. Then, a full-color toner image is collectively transferred from the intermediate transfer belt 7 onto the sheet conveyed to the secondary transfer unit by the secondary transfer roller 10 to which a secondary transfer bias having a polarity opposite to that of the toner is applied. .

  Thus, the sheet on which the full-color toner image has been transferred is conveyed to the fixing device 22, and the sheet on which the full-color toner image has been fixed by being heated and pressurized and thermally fixed on the surface of the sheet. A pair of paper discharge rollers 23 and 24 are discharged onto the paper discharge tray 21 to complete a series of image forming operations. The transfer residual toner that is not transferred onto the sheet and remains on the intermediate transfer belt 7 is removed by the belt cleaning device 11, and the intermediate transfer belt 7 is prepared for the next image formation.

[Optical scanning device]
Next, details of the configuration of the optical scanning device 13 according to the present invention will be described with reference to FIG. Since all the four optical scanning devices 13 have the same configuration, only one optical scanning device 13 will be described below.

  FIG. 2 is a plan view showing the configuration of the main part of the optical scanning device according to the present invention. The illustrated optical scanning device 13 includes a laser diode 25 as a light source, and the light beam is emitted from the laser diode 25. A collimator lens 26, a cylindrical lens 27, and a MEMS mirror 28, which is a deflecting unit, are arranged in a straight line along the direction.

  The collimator lens 26 is a condensing lens that condenses a light beam that is a divergent light beam emitted from the laser diode 25, and the cylindrical lens 27 is a lens having a specific power only in the sub-scanning direction, The light beam passes through the collimator lens 26 and functions as a line image on the deflection surface of the MEMS mirror 28 in the sub-scan section.

  The MEMS mirror 28 is integrally formed on a Si substrate using micromachining technology (MEMS technology) such as etching or film formation, and a plurality of (two or three) deflection surfaces having different natural frequencies are formed. Have. Each vibration surface resonates at each natural frequency by superimposing driving and reciprocally vibrates (sine vibration), and deflects and scans by reflecting the light beam that has passed through the cylindrical lens 27. Note that an aluminum film or the like is formed on each deflection surface (reflection surface) of the MEMS mirror 28 to increase the reflectance.

  As shown in FIG. 2, two arc-shaped scanning lenses 29 and 30 are disposed along the traveling direction of the light beam deflected by the MEMS mirror 28. The scanning lenses 29 and 30 have fθ characteristics for condensing the light beam deflected by the MEMS mirror 28 on the photosensitive drum 2a (2b, 2c, 2d) that is a scanning surface at a constant speed. .

  Thus, in the optical scanning device 13 shown in FIG. 2, when the laser diode 25 is ON / OFF controlled according to the image data, a light beam modulated in accordance with the image data is emitted from the laser diode 25. The light beam is condensed by passing through the collimator lens 26 and then formed as a line image on each deflection surface of the MEMS mirror 28 in the sub-scan section by passing through the cylindrical lens 27. The light beam incident on the cylindrical lens 27 passes through the cylindrical lens 27 as it is in the main scanning section.

  As described above, the light beam imaged linearly on each deflection surface of the MEMS mirror 28 is deflected in the main scanning direction as each deflection surface of the MEMS mirror 28 reciprocally vibrates at the respective natural frequency due to resonance. The deflected light beam passes through the scanning lenses 29 and 30 to form an image on the photosensitive drum 2a (2b to 2d) of each image forming unit 1M (1C, 1Y, 1K) shown in FIG. The photosensitive drum 2a (2b to 2d) is reciprocally scanned in the main scanning direction with one end of the effective scanning range R shown in FIG. 2 as a writing point (scanning end) P1 and the other end as a writing end point (scanning end) P2. . Then, as described above, electrostatic latent images corresponding to the color image signals of the respective colors are formed on the respective photosensitive drums 2a (2b to 2d).

Thus, in the present embodiment, the optical characteristics of the scanning lens 29 on the side close to the MEMS mirror 28 are expressed by the following formula:
θ (t) = A ₁ sin ωt + A ₂ sin (nωt)
Where n = 2 or 3,
θ: angle formed by the optical axis of the scanning lens and the light beam A ₁ , A ₂ : amplitude of each component
ω: angular frequency
t: incident with a deflection angle characteristic expressed in time, and the following formula:

ここに、γ：像高
ｆ：焦点距離
を満たすように光ビームを感光ドラム２ａ（２ｂ〜２ｄ）走査面上に結像させるよう設定している。

Where γ: image height
f: The light beam is set to form an image on the scanning surface of the photosensitive drum 2a (2b to 2d) so as to satisfy the focal length.

Therefore, in the present embodiment, the deflection angle θ (t) is expressed by the following formula:
θ (t) = A ₁ sin ωt + A ₂ sin (nωt)
In the optical scanning device 13 including the MEMS mirror 28 represented by

By using the scanning lens 29 having the optical characteristics represented by (Equation 3), the image height y is expressed by the following formula:
y = f (A ₁ + nA ₂ ) ωt
It can be seen that constant speed scanning is possible.

  The beam diameter D in the main scanning direction on the photosensitive drum 2a (2b to 2d) is expressed by the following formula:

で表され、偏向角θの振幅Ａ_１とＡ_２の比によって傾向が異なる。具体的には、正弦波であるＡ_２＝０からＡ_２を負の方向に大きくしていくと（等角速度に近づく）とビーム径Ｄの変動は、
（１）走査中央部が最も小さく、走査端部に向かうに連れて単調に増加する
（２）走査中央部から走査端部に向かって一旦小さくなって極小値を示した後に単調に増加する
傾向を示す。

The tendency varies depending on the ratio of the amplitudes A ₁ and A ₂ of the deflection angle θ. Specifically, when A ₂ is increased in the negative direction from A ₂ = 0, which is a sine wave (approaching an equal angular velocity), the fluctuation of the beam diameter D is:
(1) The scan center is the smallest, and monotonously increases toward the scan end. (2) The value tends to monotonically increase after decreasing from the scan center to the scan end and showing a minimum value. Indicates.

Therefore, by appropriately setting the ratio of the amplitudes A ₁ and A ₂ of the deflection angle θ,
(1) The beam diameter in the main scanning direction on the scanning plane is the largest at the scanning center and the scanning end, and the minimum diameter is larger than 0.9 times the maximum diameter. (2) The beam in the main scanning direction on the scanning plane. The diameter is the smallest at the center of scanning, the largest at the scanning end, and the maximum diameter is smaller than 1.1 times the minimum diameter.
Therefore, the fluctuation of the beam diameter D in the main scanning direction on the photosensitive drum 2a (2b to 2d) can be suppressed to a range within ± 10%.

  As a result, according to the optical scanning device 13 of the present invention, it is possible to perform constant speed scanning while suppressing the fluctuation of the beam diameter D in the main scanning direction on the photosensitive drum 2a (2b to 2d).

Here, the following three conditions:
(1) The beam diameters at the image height center y = 0 and the image height end y = 110 mm are equal and maximum.
(2) The minimum value of the beam diameter is 90% of the maximum value.
(3) The maximum value θ _max of the deflection angle θ is θ _max = 45 deg.
Table 1 shows the results of calculating the focal length f of the scanning lens and the amplitudes A ₁ and A ₂ of the deflection angle θ.

而して、走査レンズ２９の焦点距離ｆと偏向角θの振幅Ａ_１，Ａ_２を表１のように設定したときの走査速度の変動（リニアリティ）と感光ドラム２ａ（２ｂ〜２ｄ）上での主走査方向のビーム径比は像高に対して図３に示す傾向を示す。

Thus, when the focal length f of the scanning lens 29 and the amplitudes A ₁ and A ₂ of the deflection angle θ are set as shown in Table 1, the fluctuation in the scanning speed (linearity) and on the photosensitive drum 2a (2b to 2d). The beam diameter ratio in the main scanning direction shows the tendency shown in FIG. 3 with respect to the image height.

  As is apparent from FIG. 3, by appropriately setting the focal length f of the scanning lens 29 and the amplitudes A1 and A2 of the deflection angle θ, the fluctuation (linearity) of the scanning speed can be reduced to almost 0% and the photosensitivity can be obtained. Variations in the beam diameter in the main scanning direction on the drum 2a (2b to 2d) can be suppressed to within ± 10%.

  Although the present invention has been described with respect to a mode in which the present invention is applied to a color laser printer and an optical scanning device provided in the color laser printer, the present invention is not limited to any other image forming apparatus including a monochrome printer and a copying machine. Of course, the present invention can be similarly applied to the optical scanning device provided for this.

1M Magenta image forming unit 1C Cyan image forming unit 1Y Yellow image forming unit 1K Black image forming unit 2a to 2d Photosensitive drum (scanning surface)
3a to 3d charger 4a to 4d developing device 5a to 5d transfer roller 6a to 6d drum cleaning device 7 intermediate transfer belt 8 drive roller 9 tension roller 10 secondary transfer roller 11 belt cleaning device 12a to 12d toner container 13 optical scanning device 14 Paper cassette 15 Pickup roller 16 Feed roller 17 Retard roller 18 Transport roller pair 19 Registration roller pair 20 Transport roller pair 21 Paper discharge tray 22 Fixing device 23, 24 Paper discharge roller pair 25 Laser diode (light source)
26 Collimator lens 27 Cylindrical lens 28 MEMS mirror 29, 30 Scan lens P1 Writing point P2 Writing end point R Effective scanning range S, S 'Transport path

Claims (4)

In an optical scanning device comprising: a light source that emits a light beam; a MEMS mirror that deflects the light beam emitted from the light source; and a plurality of scanning lenses that image the light beam deflected by the MEMS mirror on a scanning surface;
The optical characteristic of the scanning lens closest to the MEMS mirror is expressed as follows:
θ (t) = A ₁ sin ωt + A ₂ sin (nωt)
Where n = 2 or 3,
θ: angle formed by the optical axis of the scanning lens and the light beam A ₁ , A ₂ : amplitude of each component
ω: angular frequency
t: incident with a deflection angle characteristic expressed in time, and the following formula:

Where γ: image height
f: An optical scanning device characterized in that the light beam is set to form an image on the scanning surface so as to satisfy the focal length.   2. The optical scanning device according to claim 1, wherein the beam diameter in the main scanning direction on the scanning surface is the largest at the center of scanning and the scanning end, and the minimum diameter is larger than 0.9 times the maximum diameter.   2. The light according to claim 1, wherein the beam diameter in the main scanning direction on the scanning surface is the smallest at the center of the scanning, the largest at the scanning end, and the maximum diameter is smaller than 1.1 times the minimum diameter. Scanning device. An image forming apparatus comprising the optical scanning device according to claim 1.

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