JP2000180749A - Optical scanner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical scanner which can be made inexpensive and small in size and where positional deviation between plural beams can be restrained. SOLUTION: Beams LA and LB from a semiconductor laser are made incident on an fθ lens 26 at different angles respectively, reach a deflection surface 28A, are reflected by the surface 28A, pass through the fθ lens 26 again, are respectively reflected and separated by reflection mirrors 32A and 32B, and reflected by corresponding cylindrical mirrors 34A and 34B, and formed into images on photoreceptor drums 30A and 30B. For the beam LA, whose incident angle in a subscanning direction on the surface 28A is large, conjugate magnification is set to be small, while assuming S1a>S1b and S2a<S2b. Thus, the difference of the curve of a scanning line is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical scanning device, and more particularly, to a laser copying machine, a laser copying machine, which scans different positions on the same photosensitive member using a plurality of light beams and scans a plurality of photosensitive members to record an image. The present invention relates to an optical scanning device such as a printer and a laser facsimile.

[0002]

2. Description of the Related Art Printers, digital copiers and the like which record a multicolor image by exposing and scanning different positions of the same photosensitive member or different photosensitive members with a plurality of light beams (laser beams) to form an electrostatic latent image are widely used. Are known. For example, an optical scanning device that exposes and scans different scan surfaces with a plurality of beams has been proposed (JP-A-56-161566,
See JP-A-2-58014).

As shown in FIG. 9, in an optical scanning apparatus for exposing and scanning different scan surfaces with a plurality of beams, light beams from a plurality of light sources are transmitted through a modulator and a beam shaping optical system provided for each light source. It is incident on the same rotating polygon mirror. Each light beam deflected by the rotary polygon mirror passes through the same fθ lens, is separated by a plane mirror, and is incident on a cylindrical lens provided for each light beam, and then scanned at different positions. The latent image formed by the scanning exposure forms a multicolor image by developing different colors. The different scanning positions can be different positions on the same recording medium or different recording materials.

[0004] In this scanning apparatus, the two light beams are made incident at different angles in the sub-scanning direction on the fθ lens in order to spatially separate the two light beams after exiting the fθ lens. However, fθ
When the light enters the lens at an angle in the sub-scanning direction, the scanning line is curved (so-called Bow). For this reason, it is disclosed that the curvature of the bow-shaped scanning line is corrected by disposing the cylindrical lens in front of the surface to be scanned.

Meanwhile, a so-called front-incidence double-pass scanning optical system has been proposed as an advantage in terms of miniaturization of the rotary polygon mirror and securing of left-right symmetry of optical performance (Japanese Patent Laid-Open No. 9-9).
No. 6773). In this front-incidence double-pass scanning optical system, the fθ lens having a curvature only in the main scanning direction is arranged at an angle to prevent stray light due to fθ lens surface reflection of incident light, which is likely to occur in the double-pass optical system, so that it is inexpensive and good. Optical performance can be secured.

[0006]

However, in the front-incidence scanning optical system, Bow which occurs when a light beam is incident on the deflecting surface at a predetermined angle in the sub-scanning direction to separate the incident light beam and the outgoing light beam is monochromatic. In the case of a recording apparatus, a bow up to about several hundred μm can hardly be recognized and there is no problem in practical use. However, in a recording apparatus in which different positions are exposed and scanned with a plurality of light beams to form images of different colors, a plurality of light beams are interposed. Bo
When an image is formed in different colors with different degrees of w, even a difference of several tens of μm is recognized as a color shift. That is, as shown in FIG. 10, the scanning line A shown in FIG.
And the scanning line B shown in FIG. 10 (B) has a different curvature, and when combined as a multicolor image shown in FIG. 10 (C),
It will be easily recognized as a color shift. Also, Bo
The difference in w is that the amount of color misregistration changes between the center of the image and the end of the image, resulting in color unevenness, making it impossible to obtain a good image.

To prevent this bow, Japanese Patent Application Laid-Open No.
No. 151566 proposes to position the focal point of a cylindrical lens in front of the surface to be scanned. However, since the surface to be scanned is arranged at the focal point of the cylindrical lens, Bow due to oblique incidence on the rotary polygon mirror cannot be completely corrected.

In Japanese Patent Application Laid-Open No. 2-58014, an emission principal plane of an fθ lens is arranged near a deflection surface, and an anamorphic condensing lens having a toric surface arranged in front of a scanning surface is provided for each light beam. Bow due to oblique incidence on the deflector while reducing the field curvature in the sub-scanning direction
Is shown to be corrected. However, the main output plane must be near the deflecting / reflecting surface, which limits the degree of freedom of the fθ lens design, and the anamorphic condensing lens is placed in front of the surface to be scanned, which increases the size and is difficult to manufacture.

In Japanese Patent Application Laid-Open No. Hei 2-289816, the parallel flat plate is rotated in the sub-scanning corresponding direction so that
A technique for correcting ow has been proposed. However, when the plane-parallel plate is rotated, the fθ characteristic changes, and a color shift occurs in the main scanning direction.

The present invention has been made in view of the above-described circumstances, and provides an optical scanning device capable of suppressing the displacement between a plurality of light beams at low cost and miniaturization.

[0011]

SUMMARY OF THE INVENTION The present inventor has found that when a multicolor image is recorded by exposing and scanning different positions on the same photosensitive member or different photosensitive members with a plurality of light beams, a curved scanning line is formed. The present invention has been achieved by paying attention to the fact that if present, a color shift that is easily recognized is generated.

According to the present invention, there is provided a deflecting means for reflecting a light beam incident so that an optical axis is inclined by a predetermined angle in a sub-scanning direction intersecting with the main scanning direction and deflecting the light beam in the main scanning direction;
A scanning optical system that focuses a plurality of light beams reflected at different angles in the sub-scanning direction by the deflecting unit on a surface to be scanned so that light beams are respectively scanned. The same first optical system that forms an optical system in the main scanning direction with respect to the plurality of light beams, and a plurality of second optical systems that correspond to each of the plurality of light beams and converge in the sub-scanning direction The scanning optics in the sub-scanning direction as a relationship between a deflection position by the deflecting means and a surface to be scanned, so that a difference in scanning line curvature caused by incidence of a light beam so that the optical axis is inclined is suppressed. The conjugate magnification of the system is set to be different between a plurality of light beams.

In the present invention, the plurality of light beams reflected at different angles in the sub-scanning direction are focused by the scanning optical system so that the light spot is scanned as a scanning line on the surface to be scanned. For example, it can be focused so that the light spot is scanned at different positions. This scanning optical system includes a first optical system and a second optical system. The first optical system focuses a plurality of light beams in the main scanning direction, and the second optical system focuses each of the plurality of light beams in the sub-scanning direction. Since the light beam is incident on the deflecting means so that the optical axis is inclined by a predetermined angle in the sub-scanning direction, the light beam is incident on the scanning optical system so that the optical axis is also inclined. Accordingly, the scanning line formed on the surface to be scanned is curved (a so-called bow occurs). Therefore, the conjugate magnification of the scanning optical system in the sub-scanning direction is set to be different between a plurality of light beams as a relationship between the deflection position and the surface to be scanned so as to suppress the difference in scanning line curvature. For example, the conjugate magnification corresponding to a light beam having a large bow is reduced, and the conjugate magnification corresponding to a light beam having a small bow is increased. As a result, the amount of Bow on the surface to be scanned can be matched or reduced. As described above, by suppressing the difference in curvature between a plurality of light beams with respect to the curvature of a scanning line generated when scanning a light beam, it is possible to reduce color misregistration and color unevenness that are easily visible without adding a new optical element. Can be.

In the optical scanning device according to the present invention, the conjugate magnification of the deflecting means on the light beam side having a large reflection angle can be set smaller than the conjugate magnification on the light beam side having a small reflection angle.

The scanning line formed on the surface to be scanned has a larger curvature as the angle of reflection by the deflecting means increases. That is, as the angle of incidence on the deflecting means increases, the curvature increases. This is because curvature of the deflection scanning surface caused by oblique incidence on the deflection surface in the sub-scanning direction increases. As a result, by reducing the conjugate magnification corresponding to a light beam having a large reflection angle (incident angle) in the sub-scanning direction of the deflecting surface, it is possible to reduce or match the difference in scanning line curvature between the plurality of light beams. According to this, since the conjugate magnification of each light beam after the deflecting unit is set to be different, it is possible to suppress the Bow difference and to obtain a good color image without color shift.

In the optical scanning device according to the present invention, the conjugate magnification on the light beam side where the angle between the optical axis of the deflected light beam and the optical axis of the first optical system is large in the sub-scanning direction is deflected. The angle between the optical axis of the light beam and the optical axis of the first optical system can be set smaller than the light beam having a smaller angle.

In the optical scanning device, stray light may occur or the size of the light beam due to main scanning may change. It is known that the first optical system represented by the fθ lens is tilted in the sub-scanning direction in order to prevent such stray light and increase in the beam diameter at the main scanning end and to reduce the absolute value of Bow. I have. By tilting the first optical system in this way, for example, the absolute value of Bow becomes smaller, but the difference does not change. For this reason, in the sub-scanning direction, the conjugate magnification on the light beam side where the angle between the optical axis of the deflected light beam and the optical axis of the first optical system is large is set to the optical axis of the deflected light beam and the first optical system. By setting the angle formed with the optical axis to be smaller than that of the small light beam, it is possible to reduce or match the scanning line curvature difference between the plurality of light beams. According to this, the conjugate magnification on the light beam side where the angle between the optical axis of the first optical system such as the fθ lens and the deflected light beam is large can be set to be small, so that in the so-called double-pass front incidence optical system, fθ is used to prevent stray light and the like.
Even in an optical system in which a lens is tilted, a Bow difference is reduced to 0.
Alternatively, there is an effect that can be reduced.

Further, the optical scanning device of the present invention has opening means for limiting the size of each light beam corresponding to each light beam, and the opening means can have different widths in the sub-scanning direction.

If the magnification from the deflecting surface to the surface to be scanned is different for each light beam, if the optical system from the light source to the deflecting surface is shared or constituted by the same components, the magnification of each light beam in the sub-scanning direction will be different. As a result, a difference occurs in the beam diameter in the sub-scanning direction on the surface to be scanned. Therefore, the width of the opening of the opening means provided corresponding to each light beam is made different in the sub-scanning direction, and the beam diameter in the sub-scanning direction is made to be substantially the same between the light beams, so that on the surface to be scanned. There is no difference in beam diameter in the sub-scanning direction. According to this,
By configuring the aperture width in the sub-scanning direction corresponding to each light beam to be different, it is possible to prevent a difference in light beam diameter in the sub-scanning direction due to a difference in conjugate magnification.

Further, the optical scanning device of the present invention further comprises a light source for causing the light beam to enter the deflecting means, and the magnification in the sub-scanning direction from the light source to the surface to be scanned can be made substantially the same. . That is, the conjugate magnification corresponding to each light beam is made different, and the magnification in the sub-scanning direction of the optical element from the light source to the surface to be scanned is substantially the same.

If the beam diameters in the sub-scanning direction are made substantially the same by changing the aperture width, the amount of vignetting of the light beam due to the opening becomes different, and the transmittance differs between the light beams. For this reason, when trying to expose with the same energy, a difference occurs in the amount of emitted light. The variation in xerography (Xerography) and the original variation of the optical scanning device, and the difference in the amount of emitted light between a plurality of light beams, increases the range of the amount of emitted light, and the light source (eg, laser) is expensive. become. In an optical scanning device that performs simultaneous scanning so as to form a plurality of scanning lines using a plurality of light sources such as a laser array, the intervals between the scanning lines are different. Therefore, by making the magnification in the sub-scanning direction from the light source to the surface to be scanned substantially the same, the beam diameter in the sub-scanning direction can be made substantially the same, and the emitted light amount and the interval between the scanning lines can be made substantially the same. As described above, by making the magnification in the sub-scanning direction from the light source to the surface to be scanned substantially equal for each light beam, it is possible to prevent a color shift due to a Bow difference, to improve the light amount use efficiency, and to improve the light beam diameter in the sub-scanning direction. Has the effect of equalizing Further, even in a simultaneous scanning optical system using an LD array or the like, there is an effect that the scanning line interval can be made substantially equal between the colors.

Further, in the optical scanning device according to the present invention, the plurality of second optical systems may be constituted by a reflective optical element having a cylindrical surface, and each of the incident angles of the light beam to the reflective optical element may be different. preferable.

To make the conjugate magnification different, it is necessary to make the focal length and arrangement of the second optical system different. To change the focal length by the refractive optical element, it is necessary to change the radius of curvature or the refractive index, and it is not possible to use common components. Therefore, by configuring each of the plurality of second optical systems with a reflective optical element having a cylindrical surface, the components can be shared and the design can be made at a lower cost. Further, by making the angles at which the respective light beams enter the second optical system constituted by the reflective optical element having a cylindrical surface different, the elements having the same radius of curvature can be used. in this way,
By making the angle of incidence on the cylindrical mirror different, there is an effect that the cost can be reduced by using the cylindrical mirror that is the same part.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In the present embodiment, the present invention is applied to an optical scanning device that employs two light beams as a plurality of light beams, is deflected by a rotating polygon mirror, and scans and forms an image.

[First Embodiment] FIG. 2 is a perspective view schematically showing an optical scanning device according to the present embodiment.
FIG. 3 is a diagram viewed in the direction of the arrow Ey (sub-scanning direction) in FIG. 2. As shown in FIGS. 2 and 3, the optical scanning device according to the present embodiment irradiates a rotating polygon mirror 28 as a deflecting unit that deflects a plurality of light beams, and a light beam reflected and deflected by the rotating polygon mirror 28. The photosensitive drums 30A and 30B coated with a photosensitive member for image recording, and the laser diode assemblies 20A and 20B for emitting light beams, respectively. The rotary polygon mirror 28 is formed in a regular polygonal prism shape, and its side surface is formed of a plane mirror functioning as a deflection surface 28A. The rotary polygon mirror 28 is installed so that it is rotated in the direction of arrow P at a predetermined angular velocity by a driving means such as a motor (not shown) around a substantially vertical rotation axis O, and the light beam from the light source is deflected at a constant angular velocity. ing.

In the following description, the plane formed by the trajectory of the light beam reflected and deflected by the rotary polygon mirror 28 is defined as the main scanning plane, and the direction formed by intersecting the main scanning plane with the surface of the photosensitive drum. A main scanning direction, and a direction intersecting (particularly orthogonal to) the main scanning plane is defined as a sub-scanning direction. Further, the surface of the photoconductor drum is set as the surface to be scanned.

First laser diode assembly 20A
A semiconductor laser 14A as a light source that emits a divergent light beam that is a diffused light beam whose divergence angle in the direction corresponding to the main scanning direction is larger than the divergence angle in the direction corresponding to the sub-scanning direction;
It comprises a collimator lens 16A for shaping a light beam emitted from the semiconductor laser 14A, and an aperture stop 18A for beam forming, that is, for forming the light beam into a desired spot diameter. The semiconductor laser 14A is controlled to be turned on and off by a modulation unit (not shown) in accordance with an image signal. This laser diode assembly 20A emits the light beam LA.

A cylindrical lens 22A is provided on the exit side of the aperture stop 18A. The cylindrical lens 22A is provided so as to converge the transmitted light beam LA only in the sub-scanning direction, and converges the light beam LA in the main scanning direction by converging on the deflection surface 28A of the rotary polygon mirror 28 or in the vicinity thereof. This is a shaping optical system for forming an image as an elongated line image.

On the axis of the light beam LA emitted from the laser diode assembly 20A and on the exit side of the cylindrical lens 22A, there is disposed a plane mirror 24A for causing the light beam LA to enter the rotary polygon mirror 28 from the front. .

Between the plane mirror 24A and the rotary polygon mirror 28, the light beam LA reflected and deflected by the rotary polygon mirror 28 is focused on the photosensitive drum 30A as a light spot to form an image, and the imaged light is formed. An fθ lens system 26 for moving the spot at a constant speed on the surface of the photosensitive drum 30 by deflection scanning is arranged. This fθ
The lens system 26 includes a first lens 26F and a second lens 26
S, and a rotating polygon mirror 28 by a plane mirror 24A.
The light beam LA incident from the front and the light beam LA reflected and deflected by the rotary polygon mirror 28 are both incident.

That is, the light beam LA passes through the fθ lens system 26 twice before and after the reflection and deflection by the rotary polygon mirror 28. This optical system as a whole forms a so-called front-incidence double-pass optical system. ing. In order to prevent the light incident on the rotating polygon mirror 28 and the reflected light from the rotating polygon mirror 28 from overlapping, the direction of the incident light on the rotating polygon mirror 28 is orthogonal to the rotation axis of the rotating polygon mirror 28. The direction is inclined in the sub-scanning direction from the moving direction.

Second laser diode assembly 20B
Is a semiconductor laser 14B and a collimator lens 16B similar to the first laser diode assembly 20A.
And an aperture stop 18B. This laser diode assembly 20B emits a light beam LB.
A cylindrical lens 22 is provided on the exit side of the aperture stop 18B.
B and the plane mirror 24B are arranged in order, and the light beam LB is f
It is configured to reach the θ lens system 26.

As described above, in this embodiment, the two light beams LA and L are transmitted twice before and after the reflection and deflection by the rotary polygon mirror 28.
B passes through the fθ lens system 26, and the present optical system as a whole forms a so-called front-incidence double-pass optical system.

On the exit side of the fθ lens system 26 of the light beam LA reflected and deflected by the rotary polygon mirror 28, a reflecting mirror 32A,
A cylindrical mirror 34A and a reflecting mirror 36A are provided in order. The light beam LA reflected and deflected by the rotary polygon mirror 28 is reflected by the reflecting mirror 32A, reflected by the cylindrical mirror 34A, and reflected by the reflecting mirror 3A.
The light is reflected by 6A and reaches the photosensitive drum 30A.

Similarly, a reflecting mirror 32B, a cylindrical mirror 34B, and a reflecting mirror 36B are sequentially provided on the emission side of the fθ lens system 26 of the light beam LB reflected and deflected by the rotary polygon mirror 28. Light beam LB reflected and deflected by rotating polygon mirror 28
Is reflected by the reflecting mirror 32B, is reflected by the cylindrical mirror 34B, is reflected by the reflecting mirror 36B, and reaches the photosensitive drum 30B.

In this embodiment, the fθ lens system 26
Functions as an imaging optical element that forms an image of the light beam that has been deflected and scanned on the surface of the photosensitive drum (surface to be scanned). That is, the imaging optical element is a first optical system that mainly has an image forming action in the main scanning direction and has a function of converting a light beam deflected at a constant angular velocity to a constant velocity, and passes each light beam in common. .

Each of the cylindrical mirrors 34A and 34B functions as a second optical system for each light beam. In other words, the second optical system mainly has an image forming action in the sub-scanning direction and converts the light flux converged linearly on the deflecting reflection surface into the fθ lens system 26.
The image is formed on the surface of the photosensitive drum (scanned surface) by the combined refracting power of the light beam and the light beam.

Therefore, a plurality of (two in this embodiment) light beams enter the deflecting reflection surface at different angles in the sub-scanning direction after passing through the shaping optical system (the cylindrical lenses 22A and 22B). Each light beam deflected by the rotary polygon mirror 28 has the same fθ
After being incident on the lens 26 at different angles and undergoing an imaging action, it is emitted, separated by different reflecting mirrors 32A and 32B, incident on cylindrical mirrors 34A and 34B corresponding to each light beam, and guided by the reflecting mirrors 36A and 36B. Different photosensitive drum 30
A and 30B are exposed by scanning.

The photosensitive drum 30A is formed in a slender, substantially cylindrical shape having a surface coated with a photosensitive material sensitive to a light beam. The photoreceptor drum 30A is moved in a direction along the main scanning direction of the light beam scanned by the rotary polygon mirror 28 (arrow Q in FIG. 2).
Direction), the photosensitive drums 30 </ b> A are arranged so that their longitudinal directions substantially coincide with each other. The photosensitive drum 30A is configured to rotate in the direction of arrow S at a predetermined constant rotation speed about a rotation axis by a driving unit (not shown).

Since the photosensitive drum 30B has the same configuration as the photosensitive drum 30A, the description is omitted.

Each of the photosensitive drums 30A and 30B produces a latent image when exposed, and a multicolor visible image is developed by developing different colors corresponding to the latent images on each photosensitive drum. can get. By transferring this visible image to the same recording medium, a multicolor print can be obtained.

In this embodiment, the number of photosensitive drums is two.
Although the case of each of them will be described, a recording device of four colors can be formed by adding a scanning device and a photosensitive drum having the same configuration.

Lens 26 corresponds to a first optical system of the present invention, and cylindrical mirrors 34A and 34B correspond to a second optical system of the present invention. The aperture stops 18A and 18B correspond to the aperture means of the present invention.

FIG. 1 is a developed view of a light beam in the sub-scanning direction in the optical scanning device according to the present embodiment. The light beams LA and LB are incident on the fθ lens 26 (first optical system) at different angles, pass through the fθ lens 26, and pass through the deflection surface 28.
A. The light beams LA and LB reflected by the deflecting surface 28A pass through the fθ lens 26 again, and are reflected and separated by the reflecting mirrors (folding mirrors) 32A and 32B, respectively. Thereafter, the light beams LA, L are provided to the cylindrical mirrors 34A, 34B (second optical system) provided corresponding to the light beams LA, LB.
Each of B is incident. Each of the light beams LA and LB reflected by the cylindrical mirrors 34A and 34B forms an image on the photosensitive drums 30A and 30B, which are surfaces to be scanned.

The cylindrical mirror 3 functioning as a second optical system
4A and 34B are not limited to cylindrical reflection elements, but may be cylindrical lenses or deformed cylindrical lenses whose curvature in the sub-scanning direction changes between the scanning center and the scanning end.

In the above description, since the deflecting surface 28A and the surface to be scanned of the photosensitive drum are conjugate in the sub-scanning direction,
The distance S1a extends from the conjugate image position (conjugate plane 28X) of the 8A fθ lens system 26 (first optical system) to the cylindrical mirror 34A (second optical system), and the cylindrical mirror 34B (second plane) extends from the conjugate image position (conjugate plane 28X). 2 optical system) to the distance S1b, the cylindrical mirror 34
A distance from A to the surface to be scanned is a distance S2a, and a distance from the cylindrical mirror 34B to the surface to be scanned is a distance S2b.

In the above, the deflecting surface 2 of the light beams LA and LB
The incident angle in the sub-scanning direction with respect to 8A is such that the light beam LA is large. In this case, as shown in FIG. 4, by setting the incident angles α _A and α _B (α _A > α _B ) of the light beams LA and LB, the light beams LA having a large incident angle are formed by the trajectory of the light beam due to the deflection and reflection. The surface to be deflected will be larger. As a result, the curvature of the scanning line increases on the side of the light beam LA having a large incident angle.

Therefore, in this embodiment, the distance S2a for the light beam LA is made smaller than the distance S2b for the light beam LB, and the cylindrical mirrors 34A and 34A are formed so as to form an image on the photosensitive drum.
The radius of curvature of each of the 34B is set. That is, the conjugate magnification on the side of the light beam having a large incident angle in the sub-scanning direction on the deflection surface 28A is set small. As a result, the degree of the scanning line curvature can be changed, and the difference in the scanning line curvature between the light beams LA and LB on the photosensitive drum surface (scanned surface) can be reduced.

More specifically, since the deflecting surface 28A and the surface of the photosensitive drum (scanned surface) are conjugate in the sub-scanning direction, the deflecting surface is shifted from the conjugate image position by the first optical system (fθ lens system 26) to the second image. Distance S to optical system (cylindrical mirrors 34A and 34B)
1. The relationship with the distance S2 from the second optical system to the surface to be scanned is represented by a conjugate magnification β (β = S2 / S1). That is, the conjugate magnification βa (βa = S2a / S
1a), and the conjugate magnification βb (βb =
S2b / S1b). Since the optical path length from the deflecting surface 28A to the surface to be scanned is the same for the light beam LA and the light beam LB,
The distances are in the relationship of S1a> S1b and S2a <S2b. As a result, the conjugate magnification of the light beams LA and LB has a relationship of | βa | <| βb |.

As described above, in this embodiment, the deflection surface 28
Since the conjugate magnification on the light beam side where the angle of incidence in A in the sub-scanning direction is large is set small, the degree of scanning line curvature can be changed for each light beam, reducing the difference in scanning line curvature between light beams. And a Bow difference can be reduced. As a result, it is possible to obtain a good multicolor image in which color shift and color unevenness are suppressed.

[Second Embodiment] In the present embodiment, the present invention is applied to the case where the Bow difference is suppressed by the angular relationship between the deflected light beam and the optical axis of the fθ lens system. Since this embodiment has substantially the same configuration as the above embodiment, the same portions are denoted by the same reference numerals and detailed description thereof will be omitted. In the present embodiment, a description will be given in comparison with a conventional optical scanning device.

As shown in FIG. 5, in the optical scanning device of the present embodiment, the light beams LA and LB are incident on the fθ lens 26 (first optical system) at different angles, pass through the fθ lens 26, and deflect. 28A. In the present embodiment, the light beam LA is incident on the deflection surface 28A at an incident angle α _A = 2.4 degrees,
The light beam LB is set to be incident at an incident angle α _B = 1.5 degrees. Each light flux L reflected by the deflection surface 28A
After passing through the fθ lens 26 again, A and LB are reflected and separated by the reflecting mirrors (folding mirrors) 32A and 32B, respectively, and thereafter enter the cylindrical mirrors 34A and 34B (second optical system) respectively. , 34B are formed on the photosensitive drums 30A, 30B, which are the surfaces to be scanned.

Each of the first lens 26F and the second lens 26S of the fθ lens system 26 of the present embodiment is arranged to be inclined with respect to its optical axis. Specifically, it is arranged so as to be tilted by 7 degrees with respect to a plane perpendicular to the plane (trajectory plane) formed by the deflected light beam, and to be inclined in a direction in which the angle formed with the incident light beam becomes larger. This is to prevent the beam diameter in the main scanning direction from increasing at the scanning end, and to suppress the surface reflection by the lens surface in the double-pass optical system from entering the surface to be scanned as stray light. It is.

Therefore, Bow caused by oblique incidence is changed to f
The over-correction is performed by the θ lens system 26, and the amount of Bow generated is smaller for a light beam having a larger incident angle on the deflection surface 28A. That is, since the fθ lens system 26 is set so that the optical axis forms an angle with the incident light beam, the deflecting surface 28A relatively deviates from the deflected light beam.
The light beam side having a larger incident angle with respect to the fθ lens system 26 has a smaller angle with the fθ lens system 26, the amount of overcorrection is small, and the generated Bow is small.

Thus, the Bow difference can be reduced by reducing the conjugate magnification of the light beam LB having a relatively small angle of incidence on the deflecting surface 28A but a large angle between the fθ lens system 26 and the deflected light beam. it can.

Table 1 below shows main optical data after the conventional deflecting surface, and Table 2 shows main optical data after the deflecting surface of the present embodiment.

[0057]

[Table 1]

[0058]

[Table 2]

The conventional optical scanning device comprises cylindrical mirrors 34A and 34B.
Use the same parts having a common radius of curvature 210.79 mm. Further, from the fθ lens system 26, the cylindrical mirrors 34A, 34A
The distance to B is 229 mm, and the cylindrical mirrors 34A and 34
The distance from B to each of the photoconductors 30A and 30B is also 145.6.
mm, and the conjugate magnification at this time is about -0.51 for both the light beam LA and the light beam LB. FIG. 6 shows the measurement result of the color shift due to the Bow difference at this time. The color misregistration between the two colors caused by the Bow difference increases from the scanning center to the edge, and occurs at a maximum image height of 22 μm. Scan density 60
When 0 dots / inch, the scanning pitch is 42.3 μm
Then, the thickest half dot color shift occurs, and the color shift is visually recognized.

On the other hand, in this embodiment, the distance from the fθ lens system 26 of the light beam LA to the cylindrical mirror 34A is as short as 215 mm and the distance from the cylindrical mirror 34A to the photosensitive drum 30A is as long as 159.2 mm with respect to the light beam LΒ. Configured. At this time, the conjugate magnification βa is -0.58 for the light beam LA and -0.51 for the light beam LB. FIG. 7 shows a measurement result of the color misregistration due to the Bow difference in such a configuration. As can be understood from the figure, the Bow difference is a maximum of 2 μm, which is about 1/10 as compared with the conventional optical scanning device.

As described above, in the present embodiment, the fθ lens system 26 is arranged to be inclined with respect to the optical axis,
Since the conjugate magnification on the side of the light beam LB where the angle between the θ lens system 26 and the deflected light beam is large is set small, a Bow difference generated between the light beams can be suppressed, and a color shift due to the Bow difference can be suppressed. it can.

[Third Embodiment] In this embodiment, the light beam diameter in the sub-scanning direction on the surface to be scanned is taken into consideration. In this embodiment, since the configuration is substantially the same as that of the above-described embodiment, the same portions are denoted by the same reference numerals and detailed description thereof will be omitted.

In the above embodiment, the color shift is reduced by changing the magnification between the deflecting surface and the surface to be scanned between the light beams, but all the optical systems before the rotary polygon mirror 28 are shared (shared). Or the same component), the magnification of each light beam in the sub-scanning direction is different. That is, the optical magnification from the semiconductor laser to the photosensitive drum (scanned surface) is different, and the light beam diameter in the sub-scanning direction is different on the photosensitive drum (scanned surface).

For example, assuming that the aperture stop 18A, 18B has a width in the sub-scanning direction of 1.6 mm in the optical system of the second embodiment, when the conjugate magnification is common, the width in the sub-scanning direction determined by 1 / e ² intensity is obtained. While the light beam diameter is about 55 μm, if the conjugate magnification is adjusted as described above, the light beam LA becomes 63 μm.

Therefore, in the present embodiment, the aperture stop provided corresponding to each light beam so that the light beam diameter in the sub-scanning direction substantially coincides with each other on the photosensitive drum (scanned surface). The width in the sub-scanning direction is different. Specifically, the width of the aperture stop 18A of the light beam LA in the sub-scanning direction is set to 1.
Set to 8 mm.

By doing so, the light beam diameter of the light beam LA in the sub-scanning direction can be set to 55 μm, and there is no difference in the light beam diameter in the sub-scanning direction on the photosensitive drum (scanned surface). . Therefore, according to the present embodiment, it is possible to obtain an optical scanning device in which there is no difference in the light beam diameter in the sub-scanning direction.

[Fourth Embodiment] The present embodiment takes into account the amount of light flux. In this embodiment,
Since the configuration is substantially the same as that of the above-described embodiment, the same portions are denoted by the same reference numerals, and detailed description is omitted.

In the above embodiment, the magnification from the deflecting surface to the surface to be scanned is made different for each light beam to reduce color misregistration, or the aperture width of the aperture stop is adjusted to change the light beam diameter difference in the sub-scanning direction. However, if the aperture width is made different for each light beam, a difference occurs in the amount of light passing through the aperture stop. For this reason, in order to maintain the same light amount, it is necessary to make the emission light amount of the semiconductor laser different, and the light amount variable range of each light beam becomes different.

Usually, when a semiconductor laser is used,
There is a phenomenon that a light amount is reduced when a pulse corresponding to one scanning line is turned on with a constant drive current (a so-called droop). It is known that this phenomenon occurs remarkably when the amount of emitted light is lower than the rated output, and is considered when determining the lower limit of the laser output when actually using the semiconductor laser. For example, most semiconductor lasers having an oscillation wavelength near the near infrared used in laser printers and the like have an output lower limit of about 1 mW. Quality image cannot be obtained. Also, if the light emission exceeds the maximum rating, the laser diode end face will be destroyed or the drive current will increase, resulting in a loss of reliability.Therefore, the upper limit of the output must be set so as not to exceed the maximum rating. There is. Therefore, it is necessary to set the emission light range of the semiconductor laser within a range where the maximum rating and the droop are allowable.

However, the transmittance of the scanning optical system, the divergence angle of the semiconductor laser, and the visualization method (for example, Xerogram)
Since there is a variation in phy), when there is a difference in the amount of transmitted light in the aperture stop as described above, a semiconductor laser having a large maximum rating is required, which increases the cost.

Therefore, in the present embodiment, the magnification from the light source (semiconductor laser) to the surface to be scanned (photosensitive drum) in the sub-scanning direction is made substantially the same for each light beam.

Specifically, the radius of curvature of the cylindrical lens 22A (shaping optical system) is 54.734 mm, and the cylindrical lens 22B
Is 53.626 mm. by this,
The magnification in the sub-scanning direction is 6.35 for both the light beam LA and the light beam LB.
Becomes The optical system after the rotary polygon mirror 28 is the same as that of the second embodiment, and the aperture widths of the aperture stops 18A and 18B in the sub-scanning direction are both 1.6 mm.

With the above configuration, the optical system transmittance of the light beam LA is approximately equal to 20.9 and the optical system transmittance of the light beam LB is approximately 20.8, and the light beam diameter in the sub-scanning direction is the light beam LA, the light beam. Both LBs are 55 μm.

As a light source, an LD array having a light emitting point interval of 20 μm can be used instead of two semiconductor lasers. In this case, with the above configuration, the scanning line interval on the surface to be scanned is 127 μm, and an optical system that simultaneously scans a plurality of lines by 3 lines interlaced scanning at a setting of a scanning line density of 600 dpi can be configured. It is possible to provide a simultaneous scanning optical system in which scanning line intervals do not differ.

[Fifth Embodiment] This embodiment is intended to more easily change the conjugate magnification. In addition,
In the present embodiment, since the configuration is substantially the same as that of the above-described embodiment, the same portions are denoted by the same reference numerals and detailed description thereof will be omitted.

As shown in FIG. 8, in the present embodiment, the incident angles of the light beams on the cylindrical mirrors 34A and 34B are set to be different.

More specifically, the fθ lens system 26 has a surface whose optical axis is perpendicular to the deflecting surface 28A (the rotation axis O of the rotating polygon mirror 28).
(A plane perpendicular to the plane) is tilted by 7.2 degrees in a direction in which the angle formed by the incident light beam becomes larger. Also,
The angle between the incident light beam and the plane perpendicular to the rotation axis O of the rotary polygon mirror 28 is set so that the light beam LA is 2.8 degrees and the light beam LB is 1.5 degrees so that the light beam can be easily separated after exiting the fθ lens system 26. did.

Here, the cylindrical mirror 3 used as the second optical system
The focal length of 4A and 34B is {−2 / (R × cos α)}.
Can be represented by Here, α is the incident angle of the cylindrical mirror, and R is the radius of curvature of the cylindrical mirror.

For this reason, the focal length can be made different by setting the incident angle α to be different even for the same radius of curvature. Therefore, in the present embodiment, by setting the incident angle to be different for each light beam, Bo is achieved while reducing the cost by using a cylindrical mirror of the same part having the same radius of curvature.
The conjugate magnification is made different so as to eliminate the w difference.

Table 3 below shows the main optical data of the present embodiment after the deflection surface.

[0081]

[Table 3]

In order to eliminate the Bow difference, the conjugate magnification after the rotary polygon mirror is set to 1.21 of the light beam LB.
Double it. Therefore, in the present embodiment, the conjugate magnification of the light beam LA is -0.526, and the conjugate magnification of the light beam B is -0.4
The focal length of the cylindrical mirror 34A (second optical system) is 97.3 mm, and the focal length of the cylindrical mirror 34B is 90.
4 and the distance from the cylindrical mirror to the surface to be scanned is 149.75 mm for the light beam LA and 130.75 mm for the light beam LB.
It is arranged so as to be 68 mm.

As described above, in this embodiment, since the conjugate magnification is made different by changing the focal length and arrangement of the cylindrical mirror as the second optical system, the Bow difference can be reduced with a simple structure at low cost. Is substantially zero, and a good image without color shift can be obtained.

[0084]

As described above, according to the present invention, since the conjugate magnification of each light beam after the deflecting means is set to be different, the Bow difference can be suppressed, and a good color image without color shift can be obtained. The effect is that it can be obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram for explaining a conjugate magnification in an optical scanning device according to a first embodiment of the present invention.

FIG. 2 is a configuration diagram of an optical scanning device according to the first embodiment of the present invention.

FIG. 3 is a side view showing an optical arrangement in the sub-scanning direction of the optical scanning device.

FIG. 4 is an explanatory diagram of occurrence of Bow due to oblique incidence on a deflecting reflection surface.

FIG. 5 is a side view schematically showing an optical scanning device according to a second embodiment.

FIG. 6 is a diagram showing a conventional color shift due to a Bow difference.

FIG. 7 is a diagram illustrating a color shift due to a Bow difference according to the second embodiment.

FIG. 8 is a side view schematically showing an optical scanning device according to a fifth embodiment.

FIG. 9 is a configuration diagram of a conventional optical scanning device.

FIG. 10 is an explanatory diagram for explaining occurrence of color misregistration due to a difference in scanning line curvature.

[Explanation of symbols]

 LA, LB Light beam 14A, 14B Semiconductor laser 16A, 16B Collimator lens 18A, 18B Aperture stop 22A, 22B Cylindrical lens 26 fθ lens system 28 Rotating polygon mirror 28A Deflection surface 28X Conjugation surface 30A, 30B Photoconductor drum 32A, 32B Reflecting mirror 34A , 34B cylindrical mirror

Claims (6)

[Claims] A deflecting means for reflecting a light beam incident so that an optical axis is inclined by a predetermined angle in a sub-scanning direction intersecting with the main scanning direction and deflecting the light beam in the main scanning direction; A scanning optical system that focuses on a surface to be scanned such that light spots are respectively scanned for a plurality of light beams reflected at different angles in a direction, the optical scanning device comprising: A first optical system that forms an image of the light beam in the main scanning direction, and a plurality of second optical systems that correspond to each of the plurality of light beams and converge in the sub-scanning direction, wherein the optical axis is A plurality of conjugate magnifications of the scanning optical system in the sub-scanning direction as a relationship between the deflection position by the deflecting unit and the surface to be scanned, so that a difference in scanning line curvature caused by making the light beam incident so as to be inclined is suppressed. light Optical scanning device is characterized in that set differently between. 2. The optical scanning device according to claim 1, wherein the conjugate magnification of the deflecting means on the side of the light beam having a large reflection angle is set smaller than the conjugate magnification of the side of the light beam having a small reflection angle. 3. The conjugate magnification on the side of the light beam having a large angle between the optical axis of the deflected light beam and the optical axis of the first optical system in the sub-scanning direction is determined by the optical axis of the deflected light beam and the first optical system. 2. The optical scanning device according to claim 1, wherein an angle formed by the optical axis of the system is set smaller than a light beam having a small value. 4. The optical scanning device according to claim 1, further comprising opening means for limiting the size of each light beam corresponding to each light beam, wherein the opening means has a different width in the sub-scanning direction. apparatus. 5. The apparatus according to claim 1, further comprising a light source for inputting a light beam to the deflecting unit, wherein a magnification in a sub-scanning direction from the light source to the surface to be scanned is substantially the same. Optical scanning device. 6. The apparatus according to claim 1, wherein each of the plurality of second optical systems is formed of a reflective optical element having a cylindrical surface, and each of the incident angles of the light beam to the reflective optical element is made different. 2. The optical scanning device according to 1.