JP2006235369A - Optical scanner and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To ensure rigidity of a lens located more on the scanned surface side from a deflecting means such as a polygon mirror by enabling the deflecting means to rotate fast with low energy by reducing the size of the deflecting means in its axis of rotation. <P>SOLUTION: An optical scanner configured so that pieces of luminous flux deflected by a common deflecting means 7 on the same side about its axis of rotation are separated closely to one another in a vertical scanning direction and each of sets of scanning optical systems has a plurality of lenses 8 and 10 between the deflecting means 7 and corresponding surfaces to be scanned, while the 1st lens and 2nd lens such that the power of the 1st lens 8 in a horizontal scanning direction is larger than the power of the 2nd lens 10 in the horizontal scanning direction and the power of the 1st lens in the vertical scanning lens is less than the power of the 2nd lens 10 in the vertical scanning direction are present as individual lenses in each scanning optical system satisfies large/small relations Wm>Wn and hm<hn, where Wm is the thickness of the 1st lens 8 in the optical axis, Wn the thickness of the 2nd lens 10 in the optical axis, hm the thickness of the 1st lens 10 in the vertical scanning direction, and hn the thickness of the 2nd lens 10 in the vertical scanning direction. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical scanning device and an image forming apparatus.

  Recently, as an image forming apparatus such as a digital color copying machine, a color laser printer, or a color laser plotter, for example, four photosensitive drums are arranged in the recording paper conveyance direction, and each of these photosensitive drums is exposed simultaneously to be latent. An image is formed, and these latent images are visualized by developing units using developers of different colors such as yellow, magenta, cyan, and black (hereinafter, Y, M, C, K), and then the visible images. A so-called four-drum tandem system that obtains a color image by sequentially superimposing and transferring images on the same recording paper is being put into practical use.

The 4-drum tandem system is advantageous for high-speed image formation because both a color image and a monochrome image can be output at the same speed as compared to a 1-drum system image forming apparatus.
It has also been proposed to use a common deflecting means for a plurality of sets of optical systems for optically scanning a plurality of photosensitive drums (Patent Documents 1 to 3, etc.). In this way, it is advantageous to reduce the cost to share the deflecting means for a plurality of sets of optical systems.

  Polygon mirrors are generally used as deflection means, but polygon mirrors shared by multiple sets of optical systems tend to be large in size in the direction of the rotation axis. When rotating, driving energy increases and driving sound also increases.

  In order to reduce the drive energy of the polygon mirror, it is only necessary to reduce the dimension of the polygon mirror in the direction of the rotation axis. However, in this case, “sub-scanning” of a plurality of light beams deflected on the same side with respect to the rotation axis of the polygon mirror. In the vicinity of the polygon mirror, the “interval in the sub-scanning direction” between the lenses that optically act on each deflected light beam is also reduced in the vicinity of the polygon mirror.

  As a result, the thickness of the lens in the sub-scanning direction is reduced. Since the lens arranged between the polygon mirror and the surface to be scanned is generally a long lens that is long in the main scanning direction, when the thickness in the sub-scanning direction is reduced, its rigidity is reduced and the lens is easily deformed.

JP2002-250880 Japanese Patent Laid-Open No. 10-186252 JP 2003-262812 A

  In view of the circumstances as described above, in the optical scanning device as described above, the present invention makes it possible to reduce the size of the deflecting means such as a polygon mirror in the direction of the rotation axis so that high-speed rotation can be performed with low energy, and moreover from the deflecting means. It is an object to ensure the rigidity of the lens on the scanned surface side.

  The optical scanning device according to the present invention deflects the light beams from N (≧ 2) light sources by a common deflecting means, and N deflected surfaces through N sets of scanning optical systems. , And the light beams deflected on the same side with respect to its rotation axis are close to each other in the sub-scanning direction. Each set of the scanning optical system has a plurality of lenses between the deflecting means and the corresponding scanned surface.

When a plurality of lenses included in each set of scanning optical systems is the i-th lens (i = 1, 2,...) Counted from the deflection means side, scanning is arranged on the same side with respect to the rotation axis of the deflection means. In the set of optical systems, the optical path toward the corresponding scanned surface is separated between the jth lens (j ≧ 1) and the j + 1th lens, and m ≦ j and n> j,
Power in the main scanning direction of the mth lens> Power in the main scanning direction of the nth lens Power in the subscanning direction of the mth lens <Power in the subscanning direction of the nth lens is scanned by the mth lens and the nth lens. It exists as a separate lens in the optical system.

The optical scanning device according to claim 1 has such a configuration.
Thickness in the optical axis direction of the mth lens: Wm, Thickness in the optical axis direction of the nth lens: Wn,
The thickness of the m-th lens in the sub-scanning direction: hm, and the thickness of the n-th lens in the sub-scanning direction: hn
But big and small:
Wm> Wn, hm <hn
It is characterized by satisfying.

  In the optical scanning device according to claim 1, it is preferable that the light beam width in the sub-scanning direction of the light beam passing through the m-th lens is smaller than the light beam width in the sub-scanning direction of the light beam passing through the n-th lens. 2).

  The image forming apparatus of the present invention is an image forming apparatus having the optical scanning device according to claim 1.

  If the explanation is supplemented slightly, the optical scanning device of the present invention deflects the light beams from N (≧ 2) light sources by a common deflecting means, and passes the deflected light beams through N sets of scanning optical systems. Therefore, for example, two light sources are used with N = 2, and the light from the two light sources is emitted from the light source. A toner obtained by simultaneously scanning and separately scanning two photosensitive drums that form “separate surfaces to be scanned” with a light beam, writing latent images, and developing these two types of latent images separately. Images can be superimposed and transferred on the same recording paper to form “an image obtained by combining toner images formed on the respective photosensitive drums”. In this case, by changing the color of the toner for developing each latent image, it is possible to form an image of two colors or three colors (one color is an overlap of toner images of different colors).

  Further, when N = 3 or 4, three or four photosensitive drums are used, and the latent image formed on each photosensitive drum is converted into yellow, magenta, cyan, or yellow, magenta, cyan, and black color toners. It is also possible to make visible images and transfer these visible images onto the same recording paper to form a color image.

A tandem type image forming apparatus can be realized by arranging the two, three, or four photosensitive drums in the recording paper conveyance direction.
Further, the light flux emitted from each of the N light sources may be a single light flux or a plurality of light fluxes. By radiating a plurality of light beams from each light source, the surface to be scanned can be optically scanned by the “multi-beam scanning method”.

  As the deflecting means, a polygon mirror can be preferably used, but the present invention is not limited to this, and a rotating single mirror, a rotating dihedral mirror, a galvanometer mirror, or the like can also be used. When a polygon mirror is used as the deflecting means, all of the light beams from the N light sources may be deflected “on the same side with respect to the rotation axis”, or the light beams from the N light sources may be deflected on both sides of the rotation axis. You may make it distribute and deflect. The optical scanning device described in Patent Document 1 is an example in the case of “dividing and deflecting four light beams from four light sources by two on both sides of the rotation axis of a common polygon mirror”. Further, the optical scanning device described in Patent Document 3 is an example in which “four light beams from four light sources are deflected on the same side of the rotation axis of a common polygon mirror”.

As described above, in the optical scanning device of the present invention, the set of scanning optical systems arranged on the same side with respect to the rotation axis of the deflecting means corresponds to the scanned object between the jth lens (j ≧ 1) and the j + 1th lens. The optical path toward the surface is separated, and m ≦ j and n> j,
Power in the main scanning direction of the mth lens> Power in the main scanning direction of the nth lens Power in the subscanning direction of the mth lens <Power in the subscanning direction of the nth lens is scanned by the mth lens and the nth lens. It exists as a separate lens in the optical system. The mth lens is closer to the deflecting means, and the power in the main scanning direction is larger than the power in the main scanning direction of the nth lens, so that the thickness of the lens can be increased. Since the m-th lens is provided at “a position where optical paths toward different scanning surfaces are not largely separated”, the dimension in the rotation axis direction of the deflecting unit cannot be reduced unless the thickness in the sub-scanning direction is reduced. However, even if the m-th lens has a small thickness in the sub-scanning direction corresponding to the rotation axis direction of the deflecting means, the thickness in the optical axis direction: Wm can be larger than the thickness in the optical axis direction of the n-th lens: Wn. Therefore, “the rigidity necessary for the m-th lens” can be ensured by the thickness Wm in the optical axis direction.

  Further, at the position where the nth lens is disposed, the optical paths of the adjacent light beams are largely separated, so the restrictions on the size of the nth lens are loose. However, since the “nth lens” has a small “power in the main scanning direction”, the thickness Wn in the lens optical axis direction cannot be increased. However, using the fact that the n-th lens has “relaxing restrictions on size”, the thickness in the sub-scanning direction: hn is set larger than the thickness in the sub-scanning direction of the m-th lens: hm. “Thickness necessary for the nth lens” can be ensured by the thickness in the scanning direction: hn.

  In this case, as in the optical scanning device according to claim 2, the light beam width in the sub-scanning direction of the light beam passing through the m-th lens is smaller than the light beam width in the sub-scanning direction of the light beam passing through the n-th lens. Then, it is easy to reduce the size of the deflecting means in the rotation axis direction, and the optical layout can be reduced.

  As described above, in the optical scanning device according to the present invention, the size of the deflecting means such as a polygon mirror can be reduced in the rotation axis direction, so that high-speed rotation with low energy is possible, and the surface to be scanned is more than the deflecting means. The rigidity of the lens on the side can be ensured.

Embodiments will be described below. First, an example of an image forming apparatus to which the present invention can be applied will be described with reference to FIGS.
FIG. 2 shows a state in which the optical system portion of the optical scanning device is viewed from the sub-scanning direction, that is, the rotational axis direction of the polygon mirror 7 which is a deflecting means. For simplicity of illustration, the illustration of the mirror for bending the optical path on the optical path from the polygon mirror 7 to the surface to be scanned, which is the optical scanning position, is omitted, and the optical path is drawn to be a straight line.

  This optical scanning device is for N = 4, and each of the four scanned surfaces is optically scanned with one light beam. The four scanned surfaces are actually photoconductive photosensitive drums 11Y, 11M, 11C, and 11K. The electrostatic latent images formed on these four photosensitive drums are magenta, yellow, Visualized separately with cyan and black toners to form a color image.

  In FIG. 2, reference numerals 1Y, 1M, 1C, and 1K denote semiconductor lasers as “light sources”. The semiconductor lasers 1Y and 1M are arranged so as to overlap in the sub-scanning direction which is a direction orthogonal to the drawing. The intensity of the semiconductor laser 1M is modulated by “an image signal corresponding to a magenta image”, and the intensity of the semiconductor laser 1Y is modulated by an “image signal corresponding to a yellow image”.

  Similarly, the semiconductor lasers 1C and 1K are also arranged so as to overlap in the sub-scanning direction. The semiconductor laser 1C is intensity-modulated by “an image signal corresponding to a cyan image”, and the semiconductor laser 1K is “corresponding to a black image”. The intensity is modulated by the “image signal”.

  The light flux emitted from each of the semiconductor lasers 1Y and 1M is converted into a parallel light flux by the coupling lenses 3Y and 3M (arranged in the sub-scanning direction, and the light flux from each semiconductor laser is incident), and the aperture 12Y. , 12M (arranged so as to overlap in the sub-scanning direction), and after beam shaping, cylindrical lenses 5Y and 5M arranged in the sub-scanning direction (arranged so as to overlap in the sub-scanning direction). Are condensed in the sub-scanning direction and enter the polygon mirror 7. A line image long in the main scanning direction by the cylindrical lenses 5Y and 5M is formed in the vicinity of the deflection reflection surface of the polygon mirror 7.

  The light beams deflected by the polygon mirror optical deflector 7 are transmitted through the first lenses 8Y and 8M and the second lenses 10Y and 10M, respectively, and light spots are formed on the scanned surfaces 11Y and 11M by the action of these lenses. These are optically scanned.

  Similarly, the light beams emitted from the semiconductor lasers 1C and 1K are converted into parallel light beams by the coupling lenses 3C and 3K, and after passing through the apertures 12C and 12K, the beams are shaped, and then the cylindrical lenses 5C arranged in the sub-scanning direction. 5K is condensed in the sub-scanning direction, is incident on the polygon mirror 7 and deflected, and passes through the first lenses 8C and 8K and the second lenses 10C and 10K, respectively. , 11K, and light scanning is performed.

  The portion indicated by reference numeral 20 in FIG. 3 is an optical scanning device, which is the portion described with reference to FIG. As shown in FIG. 3, one of the light beams deflected on the upper side of the polygon mirror 7 is guided to the photosensitive drum 11M through the optical path bent by the optical path bending mirrors mM1, mM2, and mM3, and the other light beam. Is guided to the photosensitive drum 11C by the optical path bent by the optical path bending mirrors mC1, mC2, and mC3.

  One of the light beams deflected on the lower side of the polygon mirror 7 is guided to the photosensitive drum 11Y by the optical path bent by the optical path folding mirror mY, and the other light beam is bent by the optical path folding mirror mK. The light is guided to the photosensitive drum 11K through the optical path.

  Accordingly, the four photosensitive drums 11Y, 11M, 11C, and 11K are optically scanned by the four light beams from the N = 4 semiconductor lasers 1Y, 1M, 1C, and 1K. Each of the photosensitive drums 11Y to 11K is rotated at a constant speed in the clockwise direction, and is uniformly charged by the charging rollers TY, TM, TC, and TK that form a charging unit, and is subjected to optical scanning of the corresponding light beam, respectively, yellow, magenta, cyan Each black color image is written and a corresponding electrostatic latent image (negative latent image) is formed.

  These electrostatic latent images are reversed and developed by developing devices GY, GM, GC, and GK, respectively, and a yellow toner image, a magenta toner image, a cyan toner image, and a black toner image are respectively formed on the photosensitive drums 11Y, 11M, 11C, and 11K. It is formed.

  These color toner images are transferred onto a “transfer sheet” (not shown). That is, the transfer sheet is conveyed by the conveyance belt 17, the yellow toner image is transferred from the photosensitive drum 11Y by the transfer device 15Y, and the magenta is transferred from the photosensitive drums 11M, 11C, and 11k by the transfer devices 15M, 15C, and 15K, respectively. The toner image, cyan toner image, and black toner image are sequentially transferred.

  In this way, a yellow toner image to a black toner image are superimposed on the transfer sheet to compose a color image synthetically. This color image is fixed on the transfer sheet by the fixing device 19 to obtain a color image.

  In the image forming apparatus shown in FIGS. 2 and 3, as shown in FIG. 3, the two light beams deflected on the same side of the rotation axis by the polygon mirror have a sufficient interval in the direction of the rotation axis. Therefore, the polygon mirror 7 has a large size in the rotation axis direction.

  FIG. 1 is a diagram for explaining an embodiment in which the present invention is applied to an image forming apparatus of the type described with reference to FIGS. FIG. 1A shows a characteristic part of this embodiment. In order to clarify the correspondence with FIG. 2 and FIG. 3, the same reference numerals as those in FIG. 2 and FIG.

  In FIG. 1 (a), the polygon mirror 7 has a smaller size in the direction of the rotation axis (sub-scanning direction) than that described with reference to FIGS. 2 and 3, and the first lenses 8M and 8Y are accordingly made. Also, the thickness in the sub-scanning direction is reduced. Although not shown in FIG. 1, the first lenses 8C and 8K are also made thinner in the sub-scanning direction.

  On the other hand, the second lenses 10M and 10Y (10C and 10K not shown) are also increased in thickness in the sub-scanning direction (thickness in the horizontal direction in the figure).

  The lens denoted by reference numeral 8 in FIG. 1B is drawn on behalf of the first lenses 8M to 8K. In addition, the lens denoted by reference numeral 10 in FIG. 1C is drawn on behalf of the second lenses 10M to 10K.

  As shown in FIG. 1B, the thickness of the first lens 8 in the optical axis direction is “8TH” and the thickness in the sub-scanning direction is “8D”. As shown in FIG. 1C, the thickness of the second lens 10 in the optical axis direction is “10TH” and the thickness in the sub-scanning direction is “10D”.

These satisfy the magnitude relationship of 8TH> 10TH and 8D <10D.
That is, by satisfying this relationship, the first lens 8 has a rigidity necessary for the first lens 8 by increasing the thickness 8TH in the optical axis direction even if the thickness 8D in the sub-scanning direction is reduced. Can be secured. In the second lens 10, even if the thickness in the optical axis direction: 10TH is reduced, the rigidity necessary for the second lens 10 can be secured by increasing the thickness in the sub-scanning direction: 10D.

  Hereinafter, specific examples related to the embodiment described with reference to FIG. 1 will be given. As described above, the arrangement of the optical system is the same as that shown in FIGS.

"Light source" Semiconductor laser Light source wavelength: 655nm
"Coupling lens"
Focal length: 27mm
Coupling action: Collimating action "Cylindrical lens"
Focal length in the sub-scanning direction: 70.6mm
"Polygon mirror"
Number of deflecting reflecting surfaces: 6
Inscribed circle radius: 18mm
Incidence from each light source to the deflecting / reflecting surface of the polygon mirror is performed within a plane orthogonal to the rotation axis of the polygon mirror (referred to as a deflecting / reflecting surface), and the average incident angle of the light beam incident from the light source side to the deflecting / reflecting surface (Average of the angle of inclination of the incident light beam that scans both ends of the effective scanning area with the rotation of the deflecting reflecting surface with respect to the main scanning direction) Deflected light beam α: 37.275 degrees.

  The lens data after the polygon mirror is shown below.

  The first surface of the first lens 8 and both surfaces of the second lens 10 are expressed by equations (1) and (2).

Shape in the main scanning plane (virtual flat section including the optical axis and parallel to the main scanning direction)
"Main scanning non-arc shape"
The surface shape in the main scanning plane is a non-arc shape, the paraxial radius of curvature in the main scanning plane on the optical axis is Rm, the distance in the main scanning direction from the optical axis is Y, the cone constant is K, and the higher order coefficient Are A1, A2, A3... And the depth in the optical axis direction is X, and is expressed by the following polynomial.

X = (Y ² / Rm) / [1 + √ {1- (1 + K) (Y / Rm) ² }] +
+ A1 · Y + A2 · Y ² + A3 · Y ³ + A4 · Y ⁴ + A5 · Y ⁵ + ·· (1).

  The curvature in the sub-scanning plane (virtual flat section perpendicular to the main scanning direction) changes according to the distance Y in the main scanning direction from the optical axis. This change is expressed by the following equation (2) (“sub-scanning curvature equation”).

Cs (Y) = 1 / Rs (0) +
+ B1 · Y + B2 · Y ² + B3 · Y ³ + B4 · Y ⁴ + ·· (2).

The second surface of the first lens 8 is a coaxial aspheric surface and is expressed by the following equation.
"Coaxial aspheric surface"
The paraxial radius of curvature on the optical axis is R, the distance in the main scanning direction from the optical axis is Y, the cone constant is K, the higher order coefficients are A1, A2, A3, A4,. X is represented by the following polynomial.

X = (Y ² / R) / [1 + √ {1- (1 + K) (Y / Rm) ² }] +
+ A1 · Y + A2 · Y ² + A3 · Y ³ + A4 · Y ⁴ + A5 · Y ⁵ + ·· (3).

"Shape of the first surface of the first lens 8"
Rm = −279.9
Rs = −61.0
K = -2.900,000E + 01
A4 = 1.755765E-07
A6 = −5.491789E-11
A8 = 1.087700E-14
A10 = -3.183245E-19
A12 = −2.6635276E-24
B1 = −2.066347E-06
B2 = 5.727737E-06
B3 = 3.152201E-08
B4 = 2.280241E-09
B5 = −3.729852E-11
B6 = −3.283274E-12
B7 = 1.765590E-14
B8 = 1.3729295E-15
B9 = −2.889722E-18
B10 = -1.984531E-19
In the above notation, for example, “−1.984431E-19” means “−1.984431 × 10 ⁻¹⁹ ”. The same applies to the following.

"Shape of the second surface of the first lens 8"
R = −83.6
K = −0.549157
A4 = 2.7484446E-07
A6 = −4.502346E-12
A8 = -7.366455E-15
A10 = 1.803003E-18
A12 = 2.727900E-23.

“Shape of the first surface of the second lens 10”
Rm = 6950
Rs = 110.9
K = 0.000000E + 00
A4 = 1.549648E-08
A6 = 1.292741E-14
A8 = −8.811446E-18
A10 = −9.182312E-22
B1 = −9.593510E-07
B2 = −2.135322E-07
B3 = −8.079549E-12
B4 = 2.390609E-12
B5 = 2.881396E-14
B6 = 3.693775E-15
B7 = -3.258754E-18
B8 = 1.814487E-20
B9 = 8.72085E-23
B10 = -1.3340807E-23.

"Shape of the second surface of the second lens 10"
Rm = 766
Rs = −68.22
K = 0.000000E + 00
A4 = -1.150396E-07
A6 = 1.096926E-11
A8 = −6.5542135E-16
A10 = 1.9844381E-20
A12 = −2.411512E−25
B2 = 3.644079E-07
B4 = −4.847051E-13
B6 = −1.666159E-16
B8 = 4.534859E-19
B10 = −2.819319E-23
The refractive indexes of the materials of the first and second lenses at the used wavelength are all 1.52724.

The optical arrangement is shown below.
Distance d1: 64 mm from the deflection surface of the polygon mirror to the first surface of the first lens 8
Center thickness d2 of the first lens 1: 22.6 mm
Distance d3 from the second surface of the first lens 8 to the first surface of the second lens 10: 75.9 mm
Center thickness d4 of second lens 10: 4.9 mm
Distance d2: 158.7 mm from the second surface of the second lens 10 to the surface to be scanned
The F number in the sub-scanning direction at the re-peripheral image height and the central image height is as follows.

Image height 150 mm: 41.5
Image height 0 mm: 40.4
Image height -150 mm: 41.0
The magnitude relationship between the power of the first lens 8 and the power of the second lens 10 is
Main scanning direction power of the first lens 8> Main scanning direction power of the second lens 10 Sub-scanning direction power of the first lens 8 <Sub-scanning direction power of the second lens 10

The thickness of the lens shown in FIGS. 1B and 1C is as described above.
Thickness in the optical axis direction of the first lens 8: 8TH = 22.6 mm
The thickness of the first lens 8 in the sub-scanning direction: 8D = 6.4 mm
Thickness 10TH of second lens 10 in the optical axis direction: 4.9 mm
Thickness of second lens 10 in the sub-scanning direction: 10D = 14.3 mm
It is.

  Although not shown in FIGS. 1 to 3, the polygon mirror 7 is housed in a soundproof housing, and the light beam from the light source is incident on the polygon mirror 7 through the soundproof glass provided in the window of the housing and deflected. The light beam enters the first lens 8 through the soundproof glass.

  The soundproof glass is a parallel plate glass having a refractive index of 1.514 and a thickness of 1.9 mm, and is in the main scanning direction within the deflection rotation surface (the surface perpendicular to the rotation axis of the polygon mirror and deflected by the deflected light beam). In contrast, it is inclined 16 degrees and 1.3 degrees with respect to the sub-scanning direction.

That is, the optical scanning apparatus described in the above embodiment deflects the light beams from N (= 4) light sources 1Y to 1K by the common deflecting means 7, and N sets of scanning optics for the deflected light beams. An optical scanning device that guides light to N (= 4) scanned surfaces 11Y to 11K through systems 8Y, 10Y to 8K, and 10K, and simultaneously scans the N scanned surfaces, and has a common deflection. The deflected light beams deflected by the means 7 on the same side with respect to the rotation axis are separated from each other in the sub-scanning direction, and each set of the scanning optical system includes a plurality of lenses 8 between the deflecting means and the corresponding scanned surface. , And the plurality of lenses is an i-th lens (i = 1, 2,...) Counted from the deflecting means side. System set corresponds between the jth lens (j = 1) and the j + 1th lens That the optical path is separated toward the surface to be scanned,
m (= 1) ≦ j, n (= 2)> j,
Power in the main scanning direction of the mth lens 8> Power in the main scanning direction of the nth lens 10 Power in the subscanning direction of the mth lens 8 <Power of the nth lens 10 in the subscanning direction In the optical scanning device in which the n lens 10 exists as a separate lens in each scanning optical system,
Thickness in the optical axis direction of the mth lens: Wm (= 8TH = 22.6 mm), Thickness in the optical axis direction of the nth lens: Wn (10TH = 4.9 mm), Thickness in the sub-scanning direction of the mth lens Length: hm (8D = 6.4 mm), nth lens thickness in the sub-scanning direction: hn (10D = 14.3 mm)
Wm> Wn, hm <hn
(Claim 1).

  Further, since the power in the sub-scanning direction is small in the first lens 8, the deflected light beam that has passed through the first lens 8 is incident on the second lens 10 while diverging weakly. That is, the width of the light beam passing through the m-th lens 8 in the sub-scanning direction is smaller than the width of the light beam passing through the n-th lens 10 in the sub-scanning direction.

  Accordingly, by mounting the optical scanning device according to the above embodiment on the image forming apparatus having the configuration as shown in FIGS. 2 and 3, an image forming apparatus having the optical scanning device according to claim 1 or 2 (claim 3). ) Can be implemented.

  In the image forming apparatus described above, the optical scanning of each photosensitive drum is performed by the “single beam method”, but it is natural that it can be performed by the “multi-beam method”.

It is a figure for demonstrating the characteristic part of this invention. It is a figure for demonstrating an example of the image forming apparatus which can apply this invention. FIG. 3 is a diagram illustrating an optical arrangement after a deflection unit of the image forming apparatus of FIG. 2.

Explanation of symbols

7 Common deflection means 8 First lens 10 Second lens

Claims (3)

The light beams from N (≧ 2) light sources are deflected by a common deflecting unit, and the deflected light beams are guided to N scanned surfaces via N sets of scanning optical systems. An optical scanning device that simultaneously scans a surface to be scanned,
The deflected light beams deflected by the common deflecting means on the same side with respect to the rotation axis are separated from each other in the sub-scanning direction,
Each set of scanning optical systems has a plurality of lenses between the deflecting means and the corresponding scanned surface,
When these multiple lenses are counted from the deflecting means side to be the i-th lens (i = 1, 2,...), The set of scanning optical systems arranged on the same side with respect to the rotation axis of the deflecting means is jth. The optical path toward the corresponding scanned surface is separated between the lens (j ≧ 1) and the j + 1th lens,
As m ≦ j and n> j,
Power in the main scanning direction of the mth lens> Power in the main scanning direction of the nth lens Power in the subscanning direction of the mth lens <Power in the subscanning direction of the nth lens is scanned by the mth lens and the nth lens. In an optical scanning device that exists as a separate lens in the optical system,
Thickness in the optical axis direction of the mth lens: Wm, Thickness in the optical axis direction of the nth lens: Wn,
The thickness of the m-th lens in the sub-scanning direction: hm, and the thickness of the n-th lens in the sub-scanning direction: hn
But big and small:
Wm> Wn, hm <hn
An optical scanning device characterized by satisfying The optical scanning device according to claim 1,
An optical scanning device, wherein a light beam width in a sub-scanning direction of a light beam passing through an m-th lens is smaller than a light beam width in a sub-scanning direction of a light beam passing through an n-th lens.   An image forming apparatus comprising the optical scanning device according to claim 1.

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