JP2017090592A - Optical scanning device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical scanning device that includes a multi-step image formation lens having a light flux vignetted by a boundary line or prevented from being incident upon a region on an opposite side.SOLUTION: An optical scanning device comprises: a deflector that deflects first and second light fluxes to optically scan first and second scanned surfaces in a main scan direction; first and second incidence optical systems that allow the first and second light fluxes to be incident upon a deflection surface of the deflector from a mutually same side with respect to a main scan cross section surface including an on-axis deflection point at a mutually different angle of incidence within a sub-scan cross section surface; and an image formation optical system that guides the first and second light fluxes deflected by the deflector to the first and second scanned surfaces. The image formation optical system has an image formation optical element that includes first and second optical surfaces through which each of the first and second light fluxes passes, in which an interface between the first and second optical surfaces is away from the main scan cross sectional surface including the on-axis deflection point at an edge part rather than a central part in the main scan direction.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical scanning apparatus, and is suitable for an image forming apparatus such as a laser beam printer (LBP), a digital copying machine, or a multifunction printer (MFP).

  2. Description of the Related Art Conventionally, in an optical scanning device, in order to reduce the number of parts and to reduce the size of the entire device with a simple configuration, an optical system (hereinafter referred to as sub-scanning oblique incidence optical) that is obliquely incident on a polygon mirror in the sub-scanning direction. System) is used.

Patent Document 1 discloses an optical scanning device in which a plurality of light beams are incident on the same polygon mirror at different sub-scanning oblique incident angles. In addition, the optical scanning device of Patent Document 1 is provided with a multistage imaging lens in which two imaging optical elements are integrally molded so as to be arranged in the sub-scanning direction.
In the multistage imaging lens provided in the optical scanning device of Patent Document 1, the trajectories of a plurality of scanning light beams are not taken into consideration. Therefore, there is a problem that the scanning light beam is vignetted at the boundary portion of the multistage imaging lens or enters the opposite region, resulting in a deterioration in imaging performance.

JP 2008-191435 A

  The present invention provides an optical scanning device including a multistage imaging lens in which a light beam is prevented from being vignetted by a boundary line or incident on an opposite region.

  The optical scanning device according to the present invention includes a deflector that deflects the first and second light beams to optically scan the first and second scanned surfaces in the main scanning direction, and the first and second in the sub-scan section. First and second incident optical systems that cause two light beams to enter the deflecting surface of the deflector at different incident angles, and first and second scanned light beams deflected by the deflector. An imaging optical system for guiding light to the surface, and the first and second incident optical systems deflect the first and second light beams from the same side with respect to the main scanning section including the on-axis deflection point. The imaging optical system includes imaging optical elements including first and second optical surfaces through which the first and second light beams pass, respectively. The optical surfaces of the first and second optical surfaces are discontinuously connected to each other in the sub-scan section, and the boundary between the first and second optical surfaces is the middle in the main scanning direction. You were end than part, characterized in that apart from the main scanning cross-section including the axial deflection point.

According to the present invention, it is possible to provide an optical scanning device including a multistage imaging lens in which a light beam is prevented from being vignetted by a boundary line or incident on an opposite region.

FIG. 3 is a main scanning sectional view of the optical scanning device according to the first embodiment. FIG. 3 is a sub-scan sectional view of the imaging optical system according to the first embodiment. FIG. 3 is a sub-scan sectional view of the incident optical system according to the first embodiment. The figure which looked at the incident optical system concerning a first embodiment from the light source side. The figure showing the locus | trajectory of the scanning light beam on the boundary line and multistage lens surface which concern on 1st embodiment. The figure which showed the bus-line shape of the entrance plane and exit surface of the 1st area | region and 2nd area | region of the imaging lens which concern on 1st embodiment. The figure which showed the main scanning direction position dependence of the child-line tilt amount of the entrance plane and exit surface of the 1st area | region and 2nd area | region of the imaging lens which concern on 1st embodiment. The figure which showed the subscanning cross-sectional shape of the entrance plane in each position of the main scanning direction of the imaging lens which concerns on 1st embodiment. The figure which showed the subscanning cross-sectional shape of the output surface in each position of the main scanning direction of the imaging lens which concerns on 1st embodiment. The figure which showed the subscanning cross-sectional shape of the entrance plane in each position of the main scanning direction of the imaging lens which concerns on 2nd embodiment. The figure which showed the subscanning cross-sectional shape of the output surface in each position of the main scanning direction of the imaging lens which concerns on 2nd embodiment. The figure showing the boundary line of the multistage lens surface concerning 3rd embodiment, and the locus | trajectory of the scanning light beam on a multistage lens surface. The figure showing the boundary line of the multistage lens surface concerning 4th embodiment, and the locus | trajectory of the scanning light beam on a multistage lens surface. The figure showing the boundary line of the multistage lens surface concerning 5th embodiment, and the locus | trajectory of the scanning light beam on a multistage lens surface. The figure showing the boundary line of the multistage lens surface concerning 6th embodiment, and the locus | trajectory of the scanning light beam on a multistage lens surface. The figure showing the boundary line of the multistage lens surface concerning 7th embodiment, and the locus | trajectory of the scanning light beam on a multistage lens surface. 1 is a schematic view of a main part of a color image forming apparatus equipped with an optical scanning device according to an embodiment of the present invention.

  Hereinafter, an optical scanning device according to an embodiment of the present invention will be described with reference to the drawings. It should be noted that the drawings shown below may be drawn at a scale different from the actual scale so that the present invention can be easily understood.

  In the following description, the main scanning direction corresponds to the direction perpendicular to the rotation axis (or oscillation axis) of the rotating polygon mirror and the optical axis of the imaging optical system, that is, the direction in which the light beam is deflected and scanned by the rotating polygon mirror. To do. The sub-scanning direction corresponds to a direction perpendicular to the optical axis direction of the imaging optical system and the main scanning direction, that is, a direction parallel to the rotation axis (or swing axis) of the rotary polygon mirror. The main scanning section corresponds to a plane parallel to the main scanning direction and the optical axis of the imaging optical system, that is, a plane perpendicular to the sub-scanning direction. The sub-scanning section corresponds to a plane perpendicular to the main scanning direction.

[First embodiment]
The optical scanning device according to the first embodiment of the present invention includes an incident optical system LA1, LA2, LA3, LA4 (hereinafter sometimes referred to collectively as LA), a polygon mirror 4 as a rotating polygon mirror, and imaging optics. A system LB and a synchronization detection optical system (BD optical system) LC are provided.
The incident optical system LA converts the light beam emitted from the light source into a desired shape, and the polygon mirror 4 rotates to deflect and scan the light beam converted into the desired shape by the incident optical system LA. The imaging optical system LB focuses the light beam deflected and reflected by the polygon mirror 4 on the scanned surface 8, and the BD optical system LC determines the irradiation start timing of the light beam on the scanned surface 8.

  FIG. 1 is a main scanning sectional view of the optical scanning device 100 according to the first embodiment. FIG. 2 shows a sub-scan sectional view of the imaging optical system LB of the optical scanning device 100 according to the first embodiment. FIG. 3 shows a sub-scan sectional view of the incident optical systems LA1, LA2, LA3, LA4 of the optical scanning device 100 according to the first embodiment. FIG. 4 shows a view of the incident optical systems LA1, LA2, LA3, LA4 of the optical scanning device 100 according to the first embodiment viewed from the light source 1 side.

  Next, the configuration of each optical system will be described with reference to FIGS.

Each of the incident optical systems LA1, LA2, LA3, and LA4 includes a light source 1, a cover glass 9, an aperture stop 2, and an anamorphic collimator lens 3.
The light source 1 is a semiconductor laser having a light emitting point.
The cover glass 9 is disposed adjacent to the light source 1 to protect the light source 1 from dust.
The aperture stop 2 cuts out the light beam emitted from the light source 1 into a desired shape, and is a rectangular stop in this embodiment. However, the aperture stop 2 does not affect the present invention even if it is an aperture having an arbitrary shape such as an ellipse as required.
The anamorphic collimator lens 3 changes the convergence of the light beam emitted from the light source 1 and cut into a desired shape by the aperture stop 2. Here, the anamorphic collimator lens 3 is substantially parallel light in the main scanning direction and in the sub scanning direction. Is converted into a light beam that is condensed near the deflection surface 4a of the polygon mirror 4. The substantially parallel light includes weakly divergent light, weakly convergent light, and parallel light. Further, instead of the anamorphic collimator lens 3, a collimator lens that converts the light beam emitted from the light source 1 into substantially parallel light, and the substantially parallel light is condensed only in the sub-scanning direction in the vicinity of the deflection surface 4 a of the polygon mirror 4. Cylindrical lenses may be arranged in series.

As can be seen from FIG. 1, the optical axes of the incident optical systems LA <b> 1 and LA <b> 3 are in the same plane parallel to the rotation axis of the polygon mirror 4. Then, the light beams from the incident optical systems LA1 and LA3 are arranged in such a manner that the cross section (main scanning cross section) perpendicular to the rotation axis of the polygon mirror 4 has the same incident angle (projection onto the cross section). 4 is incident on the deflecting surface 4a. Further, the optical axes of the incident optical systems LA2 and LA4 are in the same plane parallel to the rotation axis of the polygon mirror 4. Then, the light beams from the incident optical systems LA2 and LA4 are arranged in such a manner that the cross section (main scanning cross section) perpendicular to the rotation axis of the polygon mirror 4 has the same incident angle (projection onto the cross section). 4 is incident on the deflecting surface 4a.
As can be seen from FIGS. 3 and 4, the incident optical systems LA1, LA2, LA3, and LA4 each have a deflection surface 4a of the polygon mirror 4 at different incident angles in the sub-scan section (projection onto the section). Is incident. Specifically, the signs of the incident angles on the deflection surface 4a in the sub-scan section of the optical axes LA1 and LA2 are the same, and the signs of the angles of incidence on the deflection surface 4a in the sub-scan section of the optical axes of LA3 and LA4. Are the same and differ from the signs of the incident angles of LA1 and LA2. Due to the difference in the incident angle on the deflecting surface 4a in the sub-scanning cross section, the light beam can be separated by the light beam separating means described later and guided to each scanned surface.

  The polygon mirror 4 is a rotating polygon mirror as a deflector, has a plurality of deflection surfaces (reflection surfaces) 4a, and rotates (rotates) at a constant speed in the direction of arrow A. In the present embodiment, the polygon mirror 4 is a rectangular four-sided mirror and has four deflection surfaces 4a. The light beam emitted from the incident optical system LA is deflected and scanned by the rotating polygon mirror 4 and guided to the imaging optical system LB.

The imaging optical system LB includes a first imaging lens 5a and a second imaging lens (imaging optical element) for condensing the light beam deflected and scanned (reflected and deflected) by the polygon mirror 4 in the vicinity of the scanned surface 8. 5b1 and 5b2. The first imaging lens 5a and the second imaging lenses 5b1, 5b2 have a positive refractive power in the main scanning section and the sub-scanning section. However, the refractive powers in the main scanning section and the sub-scanning section are different from each other. In the main scanning direction, it has an fθ characteristic, and the light beam deflected and scanned by the polygon mirror 4 is placed in a position proportional to the deflection angle (rotation angle of the deflector) in the main scanning direction. In FIG. 5, light can be condensed near the scanned surface 8 at regular intervals. The imaging optical system LB compensates for the surface tilt of the deflection surface 4a by optically conjugating the vicinity of the deflection surface 4a of the polygon mirror 4 and the surface 8 to be scanned.
In the present embodiment, the surface to be scanned 8 is composed of photosensitive drum surfaces corresponding to four colors of yellow (Y), magenta (M), cyan (C), and black (K), which are 8y, 8m, and 8m, respectively. They shall be referred to as 8c and 8k. The light beams emitted from the incident optical systems LA1, LA2, LA3, and LA4 enter the scanned surfaces 8k, 8c, 8y, and 8m, respectively.
The first imaging lens 5a of the imaging optical system LB is an imaging lens common to all the scanned surfaces 8y, 8m, 8c, and 8k. The second imaging lens 5b1 is an imaging lens for a light beam directed toward the photosensitive drums 8c and 8k, and the second imaging lens 5b2 is an imaging lens for a light beam directed toward the photosensitive drums 8y and 8m.
The folding mirror 6 as the light beam separating means guides light to each scanned surface by appropriately separating and reflecting the light beam directed to each scanned surface. The folding mirror 6a separates the light beam incident on the imaging lens 5b1 and the light beam incident on the imaging lens 5b2. The folding mirrors 6 and 6a are not shown in FIG.

The BD optical system LC as a synchronization detection optical system includes a BD lens 10 and a BD sensor 11.
The BD lens 10 is a toric lens having different refractive powers in the main scanning section and the sub-scanning section.
The BD sensor 11 is composed of a photodiode and has a light receiving surface of about φ1.5 mm.
The light beam deflected and reflected at a predetermined angle by the deflecting surface 4a of the polygon mirror 4 is incident on the BD lens 10 to be condensed in the main scanning direction near the BD sensor 11 and spread in the sub-scanning direction. Is converted to By condensing in the main scanning direction in this manner, the synchronization timing (BD signal) of the image information can be detected with high accuracy, and by spreading in the sub scanning direction, it is caused by dust or the like on the BD sensor 11. It is possible to prevent a decrease in detection accuracy.
By acquiring the BD signal when the light beam reaches a desired position on the BD sensor 11, it is possible to control the rotational speed of the polygon mirror 4 and the image writing timing on the scanned surface 8.

The optical scanning device 100 according to the present embodiment is configured as described above. That is, based on the image information, the light flux is emitted from the light source 1 of each of the incident optical systems LA1 to LA4 after intensity modulation, passes through the incident optical systems LA1 to LA4, and then enters the same deflection surface 4a of the polygon mirror 4. The light beam incident on the deflecting surface 4a of the polygon mirror 4 from the incident optical systems LA1 to LA4 is deflected and scanned by the polygon mirror 4 and condensed at a desired position on the scanned surface 8 by the imaging optical system LB. The image information is recorded.
In the optical scanning device 100 according to the present embodiment, the incident optical systems LA1 to LA4 are arranged in the sub-scanning direction so that the respective optical axes have different angles with respect to the main scanning section. The light beams emitted from the incident optical systems LA1 to LA4 are incident on the same deflecting surface 4a and deflected and scanned, so that the optical elements are shared, and space saving and cost reduction are achieved.

  Table 1 shows a main configuration of each optical member included in the optical scanning device 100 according to the present embodiment.

  Table 2 shows the incident optical systems LD1 to LD4 included in the optical scanning device 100 according to the present embodiment, and R, D, and N of the imaging optical systems 5a, 5b1, and 5b2 corresponding thereto. Here, R is the radius of curvature of the surface, D is the surface vertex spacing, and N is the refractive index.

  Table 3 shows the curvature radii R and r of the anamorphic collimator lens 3 of each of the incident optical systems LA1 to LA4 included in the optical scanning device 100 according to the present embodiment, and the phase function of the diffraction grating. Yes.

  However, the phase function Φ is expressed as the following formula (1).

  Here, k is the diffraction order, and here k = 1. Further, λ is a wavelength, and here, λ = 790 nm.

  Table 4 shows aspheric coefficients of the imaging lens 5a and the imaging lenses 5b1 and 5b2 of the imaging optical system LB provided in the optical scanning device 100 according to the present embodiment.

  However, the aspherical shapes of the imaging lens 5a and the imaging lenses 5b1, 5b2 are defined by the following equations (2) to (5).

  In the imaging optical system LB, the intersection of the curved surface of the imaging lens and the optical axis is the origin, the optical axis direction is the X axis, the axis perpendicular to the optical axis in the main scanning section is the Y axis, and the light in the sub-scanning section is light. The axis orthogonal to the axis is taken as the Z axis. In this case, when the intersection line between the XY plane and the curved surface is a bus, and the intersection line between the XZ plane and the curved surface is a child line, the bus shape is expressed by the following equation (2).

  On the other hand, a child wire shape is represented by the following formula (3).

  Therefore, the actual surface shape x is represented by the following formula (4).

    x = X + S (4)

  Here, the radius of curvature r ′ of the child line that changes depending on the value of Y is expressed by the following equation (5).

1 / r ′ = 1 / r + E ₂ Y ² + E ₄ Y ⁴ + E ₆ Y ⁶ + E ₈ Y ⁸ + E ₁₀ Y ¹⁰ (5)

  However, in Equation (3) and Equation (5), Y ≧ 0 is set to upper, and Y ≦ 0 is set to lower, and the aspheric coefficient is set individually for each.

Further, the photosensitive drums K, C, Y, and M are irradiated with the light beams that have passed through the imaging lenses 5b1k, 5b1c, 5b2y, and 5b2m, respectively.
Therefore, the imaging lens 5b1 is configured by integrally coupling the imaging lenses 5b1k and 5b1c, and the imaging lens 5b2 is configured by integrally coupling the imaging lenses 5b2y and 5b2m.
In other words, the imaging lenses 5b1 and 5b2 are composed of multistage lens surfaces having two upper and lower regions represented by different aspherical expressions. For example, the imaging lenses 5b1 and 5b2 include a first region whose sub-wire shape can be expressed by Equation (3) and a second region whose sub-wire shape can be expressed by Equation (3). (Of course, the coefficients of the equations in the first region and the second region are not completely the same). In addition, the imaging lenses 5b1 and 5b2 have a first region in which the shape of the child line can be expressed by the equation (3) and a second region in which the shape of the child wire can be expressed by an aspheric expression other than the equation (3). May be provided.
A boundary portion between the upper and lower regions of the imaging lenses 5b1 and 5b2 will be described later.

The surface eccentricity in Table 4 is a value representing the amount of eccentricity of the optical axis of each lens surface of the imaging lens 5b with respect to the optical axis of the imaging lens 5a.
For example, as shown in Table 4, the fact that the surface decentering amount of the ninth surface of the imaging lens 5b1k is +8.0 mm in the Z direction indicates that the optical axis of the ninth surface of the imaging lens 5b1k is that of the imaging lens 5a. This indicates that the optical axis is eccentric by 8.0 mm on the upper side in the Z-axis direction.
In the present embodiment, the surface shape is defined by the function defined above, but the present invention is not limited to this.

Here, as shown in FIGS. 2 and 3, a plane that includes a deflection point at the on-axis image height and is perpendicular to the rotation axis of the polygon mirror is defined as a reference plane G.
At this time, the optical axis of the imaging lens 5a is included in the reference plane G.

As can be seen from FIG. 3, the incident optical systems LA1 to LA4 are incident on the deflecting surface 4a at different incident angles (hereinafter referred to as sub-scanning oblique incident angles) with respect to the reference plane G in the sub-scanning direction.
Here, of the two light beams incident on the common imaging lens from the same side with respect to the reference plane G, the one having the larger absolute value of the sub-scanning oblique incident angle is defined as the first light beam, and the sub-scanning oblique incidence is performed. The smaller absolute value of the angle is defined as the second light flux. Here, since the imaging lens 5b1 and the imaging lens 5b2 have substantially the same configuration, only the imaging lens 5b1 will be described.
At this time, the light beam from the incident optical system LA1 toward the photosensitive drum K is the first light beam, and α1 = + 4.2 ° is set when the sub-scanning oblique incident angle of the incident optical system LA1 is α1. Further, the light beam from the incident optical system LA2 toward the photosensitive drum C is the second light beam. When the sub-scanning oblique incident angle of the incident optical system LA2 is defined as α2, α2 = + 1.5 ° is set.

In the present embodiment, by making the absolute values of the sub-scanning oblique incident angles of the first light beam and the second light beam incident on the polygon mirror 4 different from each other, an optical path is asymmetrically set in the sub-scanning direction with respect to the reference plane G. It is possible to turn.
Specifically, the imaging lens 5b1 in FIG. 2 can be eccentrically arranged on the upper side in the sub-scanning direction with respect to the reference plane G, and the first luminous flux and the second luminous flux incident on the imaging lens 5b1. Can be biased upward in the sub-scanning direction with respect to the reference plane G.
Further, by setting the signs of the sub-scanning oblique incident angles of the first light beam and the second light beam incident on the polygon mirror 4 to be the same, the first light beam and the second light beam incident on the imaging lens 5b1. It is possible to deviate the optical path of the luminous flux upward in the sub-scanning direction with respect to the reference plane G.
Similarly, the optical paths of the light beam (second light beam) of the incident optical system LA3 and the light beam (first light beam) of the incident optical system LA4 incident on the imaging lens 5b2 are in the sub-scanning direction with respect to the reference plane G. Can be biased downward.
In this embodiment, since a part of the imaging optical system LB can be arranged asymmetrically with respect to the reference plane G in this way, the degree of freedom in handling the optical path can be increased. As a result, it is possible to guide a plurality of light beams after passing through the imaging lenses 5b1 and 5b2 onto each scanned surface with the minimum folding mirror 6 while achieving miniaturization of the optical scanning device.

In this embodiment, since the imaging optical system LB can be configured in this way, two light beams incident on the imaging lens 5b1 in the vicinity of the optical path separation folding mirror 6a and 2 incident on the imaging lens 5b2. It is possible to sufficiently separate the two light beams.
In the present embodiment, the four light beams are separated in advance in the sub-scanning direction upper and lower optical paths by the folding mirror 6a before the imaging lenses 5b1 and 5b2. Thereby, the position of the common imaging lens 5a can be arranged closer to the surface to be scanned 8, and the sub-scanning magnification can be lowered.
As a result, it is possible to reduce the influence of image formation performance deterioration due to mounting errors and molding variations of each lens, and a good image can be formed.

  The scanning field angle in the main scanning direction is θ (rad), the sub-scanning oblique incident angle is α (rad), and the distance from the deflection surface 4a to the virtual plane perpendicular to the optical axis of the imaging lens 5a is L (mm). Then, the scanning field angle dependency Z (θ) of the light ray height is expressed by the following equation (6).

In other words, in the case of an oblique incidence optical system, the ray height in the sub-scanning direction increases as it goes off-axis (that is, the absolute value of θ increases), and the ray height increases as the oblique incidence angle α increases. The absolute value of the height increases.
The relationship that the absolute value of the light height in the sub-scanning direction is larger in the sub-scanning direction than on the axis does not change even when passing through the imaging lens 5a having a weak positive power in the sub-scanning direction. Therefore, the incident position of the light beam on the imaging lenses 5b1 and 5b2 in the sub-scanning direction is curved so as to increase from on-axis to off-axis.

  For example, as described above, light beams from the incident optical systems LA1 and LA2 having different absolute values of the sub-scanning oblique incident angle but the same sign enter the imaging lens 5b1. For this reason, the passing positions of the imaging lenses 5b1k and 5b1c of the light beams from the incident optical systems LA1 and LA2 are curved by different magnitudes in the same direction from the axis toward the axis.

  Next, the lens surface shapes of the imaging lenses 5b1 and 5b2 will be described.

As described above, in the present embodiment, among the two light beams having the same sign of the sub-scanning oblique incident angle incident on the polygon mirror 4 and different absolute values, the light beam having the larger sub-scanning oblique incident angle is used as the first light beam. The light flux and the smaller light flux are defined as the second light flux. Further, a region through which the first light beam passes in the imaging lens 5b1 is defined as a first region, and a region through which the second light beam passes is defined as a second region.
If the absolute values of the sub-scanning oblique incident angles of the two light beams are the same, the first region and the second region are configured by optical surfaces that are symmetrical in the sub-scanning direction with the reference plane G in between. As a result, the imaging performance can be corrected. However, when the absolute values of the oblique incident angles in the sub-scanning direction of the first light beam and the second light beam are different as in the present embodiment, the aberration amounts due to the conical scan are different from each other. Therefore, the conventional optical surface shape symmetrical in the sub-scanning direction cannot completely correct the scanning line curvature and wavefront aberration caused by the conical scan for both the first light beam and the second light beam.
Therefore, in the present embodiment, the sub-scanning cross-sectional shapes of the first region and the second region are vertically asymmetric with respect to both the entrance surface and the exit surface of the optical surfaces of the imaging lenses 5b1 and 5b2. It is set to become. In other words, a boundary is formed between the first region and the second region with respect to both the entrance surface and the exit surface of the optical surfaces of the imaging lenses 5b1 and 5b2. The first region and the second region are two regions whose surface shapes are respectively defined by different mathematical formulas.

FIGS. 5A and 5B respectively show the boundary line of the multistage lens surface and the trajectory of the scanning light beam on the multistage lens surface when the entrance surface and the exit surface of the imaging lens 5b1 are projected onto a cross section perpendicular to the optical axis. Represents. In the discussion here, it is assumed that the optical path is expanded, that is, the folding mirrors 6 and 6a are not present.
In FIG. 5, the position of the reference plane G along the sub-scanning direction is Z = 0, and the point where the light beam intersects the optical axis of the imaging lens 5a is Y = 0. Further, since the following discussion is the same for the imaging lens 5b2, the description of the imaging lens 5b2 is omitted.

For the imaging lens 5b1, the region through which the first light beam from the incident optical system LA1 passes is the first region, and the region through which the second light beam from the incident optical system LA2 passes is the second region. It is.
The shapes of the entrance surface and the exit surface of the first region of the imaging lens 5b1 can be expressed by the aspheric coefficient of 5b1k in Table 4, and the shapes of the entrance surface and the exit surface of the second region are shown in Table 4. It can be represented by an aspherical coefficient of 5b1c.

In this embodiment, as can be seen from Table 4, the bus bar shape, the child wire curvature shape, and the child wire tilt shape are different between the first region (5b1k) and the second region (5b1c).
In this way, the first area and the second area have different aspheric coefficients, and the different incident surfaces and exit surfaces are different from each other so as to be optimal for correcting the optical characteristics of each light beam. It consists of shapes.
For this reason, the sub-scanning cross-sectional shape of the entrance surface and the exit surface is discontinuous at the boundary between the first region and the second region, that is, the entrance surface and the exit surface are the first in the sub-scan section. A boundary line is provided between the region and the second region.

As described above, in accordance with the equation (6), the trajectories of the principal ray and the marginal ray of the first light flux and the second light flux are both set to the on-axis deflection point from the center of the lens to the lens end in the main scanning direction. Curved in a direction away from the main scanning cross section. Here, the on-axis deflection point is defined as a point at which the principal ray of the light beam from the incident optical system is deflected and reflected by the deflection surface when going to the center of the image.
In the conventional multistage lens, the trajectory of the scanning light beam is not taken into consideration, and the boundary line shape when projected onto a cross section perpendicular to the optical axis is set to a straight line. For this reason, when the scanning light beam fluctuates up and down, there is a problem in that the imaging performance deteriorates due to vignetting at the boundary line at the end of the lens in the main scanning direction or incident on the opposite region.
Therefore, in the present embodiment, the boundary line between the first region and the second region when projected onto a cross section perpendicular to the optical axis moves along the axial deflection point from the center to the end in the main scanning direction. Is curved in a direction away from the main scanning section including.
In other words, the boundary between the entrance surfaces and the exit surfaces of the first region and the second region of the imaging lens 5b1 is closer to the end portion than the center portion in the main scanning direction. Is away from the main scanning section including the on-axis deflection point toward the side opposite to the side where the light beam enters the deflection surface.
Thereby, it is possible to prevent the scanning light flux incident on the first region and the second region from entering the opposite region across the boundary line when projected onto the cross section perpendicular to the optical axis.
Here, the central portion of the imaging lens 5b1 in the main scanning direction corresponds to the position where the axial light beam passes, and the end portion of the imaging lens 5b1 in the main scanning direction is the most off-axis light beam. Corresponds to the passing position.

  Next, the boundary line conditional expression in the present embodiment will be described.

First, the sub-scanning oblique incident angles of the first light beam and the second light beam are α1 and α2, respectively. Therefore, the relationship between α1 and α2 is set so as to satisfy both the following expressions (7) and (8).
0 <α1 × α2 (7)
| Α2 | <| α1 | (8)

Also, T is the distance from the on-axis deflection point to the multistage lens in the reference plane G. The number of lenses arranged between the deflection surface 4a and the multistage lens surface is m (m lenses), and each lens is sequentially arranged from the deflection surface 4a to the i-th lens (i = 0, 1,..., M). ), The thickness of the i-th lens is di, and the refractive index is ni.
Further, an arbitrary position in the main scanning direction of the multistage lens surface when the on-axis deflection point is the origin is Y, and the position in the subscanning direction at the position Y in the main scanning direction of the boundary line in the subscanning section of the multistage lens surface Let Hr (Y). At this time, in the present embodiment, the shape of the boundary line when projected onto a cross section perpendicular to the optical axis is set so as to satisfy the following conditional expression (9).

  Next, derivation of conditional expression (9) will be described.

First, for the sake of simplicity, let us consider a case where no lens is disposed between the deflecting surface 4a and the multistage lens surface (that is, there is no region where the refractive index is n ≠ 1).
At that time, if the optical path length from the on-axis deflection point of the light beam reaching an arbitrary position Y in the main scanning direction of the multistage lens surface in the main scanning section is t (Y), t (Y) can be expressed by the following approximate expression (10).

  Here, assuming that the height in the sub-scanning direction at the midpoint of the trajectories of the two light beams conically scanned on the multistage lens surface at the sub-scanning oblique incident angles α1 and α2 is h0 (Y), h0 (Y) is It can be expressed by the approximate expression (11).

  Here, when Expression (10) is substituted into Expression (11), the following Expression (12) is obtained.

Considering the height in the sub-scanning direction at the midpoint of the locus of the two light beams represented by Expression (12) and the sub-scanning direction light beam diameters of the two light beams, the distance from the two light beams is the same. Coordinate points can be calculated. A curve connecting these coordinate points is an ideal boundary line.
However, although the sub-scanning direction light beam diameters of the two scanning light beams on the imaging lens 5b1 are slightly different, they are the same spot diameter on the scanned surface 8, and from the scanned surface 8 to the imaging lens. The optical path lengths up to 5b1 are substantially the same. For this reason, the difference in beam diameter can be ignored because it is sufficiently small.
Therefore, when there is no lens between the deflecting surface 4a and the multi-stage lens surface, the ideal boundary line shape can be expressed by Expression (12).

  Next, consider a case where a lens is disposed between the deflection surface 4a and the multistage lens surface (that is, a region where the refractive index is n ≠ 1).

  As described above, the number of lenses disposed between the deflecting surface 4a and the multistage lens surface is m, and each lens is sequentially arranged from the deflecting surface 4a to the i-th lens (i = 0, 1,..., M). Where the thickness of each lens is di and the refractive index is ni. The wall thickness di depends on the position Y in the main scanning direction, but is ignored here.

  In the main scanning section, the optical path length from the on-axis deflection point of the light beam reaching an arbitrary position Y in the main scanning direction of the multistage lens surface to the arrival position on the multistage lens surface when passing through the i-th lens Let the internal optical path length be di (Y). At that time, di (Y) can be expressed by the following approximate expression (13).

  Therefore, in the case where no lens is disposed between the deflecting surface 4a and the multistage lens surface, the subpoint of the middle point of the trajectories of the two light beams scanning on the multistage lens surface with the sub-scanning oblique incident angles α1 and α2 respectively. The amount of deviation Δhi (Y) in the scanning direction height is represented by the following equation (14).

  Here, when Expression (13) is substituted into Expression (14), the following Expression (15) is obtained.

  Accordingly, the height h (Y) in the sub-scanning direction of the midpoint of the trajectories of the two light beams passing through the first lens to the m-th lens and scanning on the multistage lens surface, that is, the sub-line of the ideal boundary line. The height in the scanning direction can be expressed by the following equation (16) using h0 (Y) and Δhi (Y).

  Here, when Expression (12) and Expression (15) are substituted into Expression (16), the following Expression (17) is obtained.

  In the present embodiment, it is preferable that the height Hr (Y) in the sub-scanning direction of the boundary line in the sub-scan section of the multistage lens surface satisfies the following conditional expression (18).

If Hr (Y) falls below the lower limit value of the conditional expression (18), the scanning light beam on the lower side in the sub-scanning direction of the two light beams is shaken due to a lens mounting error or the like and is vignetted at the boundary line. On the other hand, if Hr (Y) exceeds the upper limit value of the conditional expression (18), the scanning light beam on the upper side in the sub-scanning direction out of the two light beams is shaken due to a lens attachment error or the like and is vignetted at the boundary line. In such cases, good imaging performance cannot be obtained.
Therefore, in the present embodiment, by setting the boundary line of the multistage lens surface so as to satisfy the conditional expression (18), vignetting due to the boundary line between the two scanning light beams is prevented, and good imaging performance is achieved. An imaging lens arrangement that is optimal for miniaturization can be made possible. Thereby, the miniaturization of the optical scanning device is achieved.

  In the present embodiment, it is more preferable that the height Hr (Y) of the boundary line of the multi-stage lens surface matches the height H (Y) of the ideal boundary line in the sub-scanning direction. Specifically, the height Hr (Y) of the boundary line of the multistage lens surface coincides with the height H (Y) of the ideal boundary line over the entire lens mirror surface in the main scanning direction. More preferably.

In the present embodiment, the shape of the boundary line of the incident surface of the imaging lens 5b1 is α1 = + 4.2 degrees, α2 = + 1.5 degrees, T = 102.925 mm, m = 1, n1 = 1.524. , D1 = 8 mm, it is configured with a curved shape represented by the equation (17).
In the present embodiment, the shape of the boundary line of the exit surface of the imaging lens 5b1 is α1 = + 4.2 degrees, α2 = + 1.5 degrees, T = 108.150 mm, m = 2, n1 = 1.524. , D1 = 8 mm, n2 = 1.524, d2 = 5.225 mm, and a curved shape represented by Expression (17). It should be noted that the thickness d2 is an average value of the thicknesses of the imaging lenses 5b1k and 5b1c. Note that when the values of T, ni, and di are different between the first region and the second region, the average value of both is taken.

In this embodiment, the sub-scanning oblique incident angles α1 and α2 are set so as to satisfy the following conditional expressions (19) and (20).
0.5 ° ≦ | α2 | ≦ 2.5 ° (19)
3.5 ° ≦ | α1 | ≦ 5.0 ° (20)

  In this embodiment, since the oblique incident angle α1 of the light beam toward the photosensitive drum K is +4.2 degrees and the oblique incident angle α2 of the light beam toward the photosensitive drum C is +1.5 degrees, the conditional expression (19) And (20) are each satisfied.

If | α1 |> 5.0 degrees, the scanning line curvature amount and wavefront aberration caused by conical scanning become excessively large, and these cannot be corrected sufficiently satisfactorily, and a good image is formed. Can not be.
Further, when | α1 | <3.5 degrees or | α2 |> 2.5 degrees, the following problems occur. That is, the light beam separation between the light beam to the photosensitive drum K and the light beam to the photosensitive drum C is insufficient on the multistage lens surface of the imaging lens 5b1. As a result, when the light beam passage height is shifted due to tolerance, each light beam is vignetted on the opposite surface by entering the boundary surface of the multistage lens surface or entering the opposite surface across the boundary line. . For this reason, spot deterioration on the surface to be scanned due to ghosting of unnecessary ghost light or luminous flux occurs, and a good image cannot be obtained.
Also, when | α2 | <0.5 degrees, the effect of asymmetrically deflecting the optical path with respect to the reference plane G in the sub-scanning direction becomes too small, so that the second light beam directed to the photosensitive drum C is another second light beam. The optical path cannot be separated from the light beam toward the photosensitive drum Y, which is the second light beam. Therefore, the optical path of the imaging optical system cannot be bent compactly as shown in FIG.
Therefore, in this embodiment, by setting the sub-scanning oblique incident angles α1 and α2 so as to satisfy the conditional expressions (19) and (20), the vignetting of the light beam due to the boundary line is prevented and the oblique incident angle is large. Therefore, it is possible to prevent deterioration of the imaging performance. Thereby, it is possible to obtain an optical scanning device that is compact and can achieve good imaging performance.

In this embodiment, for example, as shown in FIGS. 5A and 5B, in order to prevent the scanning light beam from being vignetted by the boundary line, the boundary line is moved from the center to the end in the main scanning direction. It is curved in a direction away from the main scanning section including the on-axis deflection point.
In addition, by making the bending direction of the boundary line on the entrance surface and the exit surface in the same direction in this way, it is possible to suppress the deviation of the height in the sub-scanning direction between the boundary line on the entrance surface and the boundary line on the exit surface as much as possible. Can do. Thereby, the imaging lenses 5b1 and 5b2 can be formed in a simple lens shape, and the lens surface deformation at the time of injection molding can be suppressed small.

  As a method for producing a multistage lens such as the imaging lenses 5b1 and 5b2 of the present embodiment, there are a method of injection-molding the lens with a mold or a method of directly cutting the lens. In any method, when the shape of the boundary line which is a discontinuous portion is complicated, it becomes extremely difficult to process the lens mold or the lens itself, and there are problems such as an increase in the cost of the lens and a decrease in accuracy of the formed surface. Arise. For this reason, it is important to make the shape of the boundary line as simple as possible.

Therefore, in the present embodiment, in order to form the boundary line between the imaging lenses 5b1 and 5b2 with a simple curved shape, the shapes of the entrance surface and the exit surface are set as follows.
First of all, the shape of the generatrix of the entrance surfaces of the imaging lenses 5b1 and 5b2 is designed so as not to have an inflection point. Thereby, the main scanning direction dependency of the height of the scanning light beam on the entrance surface and the exit surface in the sub-scanning direction can be a simple curved shape.

  FIGS. 6A and 6B show the generatrix shapes of the entrance surface and the exit surface of the first region and the second region of the imaging lens 5b1, respectively.

  As can be seen from FIGS. 6A and 6B, the entrance surfaces of the first and second regions of the imaging lens 5b1 are set to a simple concave shape so that the generatrix does not have an inflection point. Has been.

Second, the entrance surfaces of the imaging lenses 5b1 and 5b2 are sub-tilt tilt changing surfaces in which the plane inclination of the sub-line changes depending on the position in the main scanning direction. The change amount of the sub-line tilt is set to be small. Yes.
The angle at which the scanning light beam is refracted in the sub-scanning direction on the incident surface is determined by the sub-line tilt and the sub-line curvature at each position in the main scanning direction.
For this reason, in this embodiment, the amount of change in the sub-line tilt across the main scanning direction of the incident surfaces of the imaging lenses 5b1 and 5b2, that is, the difference between the maximum value and the minimum value is set to be 100 minutes or less. Yes.
Thereby, the main scanning direction dependency of the height of the scanning light beam on the exit surface in the sub-scanning direction can be a simple curved shape.

  FIGS. 7A and 7B show the position dependency of the sub-tilt direction of the entrance surface and the exit surface of the first region and the second region of the imaging lens 5b1 in the main scanning direction, respectively.

  As can be seen from FIGS. 7A and 7B, the change in the amount of tilt of the child line across the entire main scanning direction on the incident surface of the imaging lens 5b1 is set small.

In addition, when a lens having a strong refractive power in the sub-scanning direction is disposed in the optical path until the scanning light beam enters the imaging lenses 5b1 and 5b2, it enters the imaging lenses 5b1 and 5b2 due to an attachment error of the lens. The height of the scanning light beam to be shifted in the sub-scanning direction is shifted.
For this reason, in this embodiment, the imaging lens 5a is substantially non-powered in the sub-scanning direction, and the imaging lenses 5b1 and 5b2 have a refractive power in the positive sub-scanning direction.
Thereby, even when the mounting error of the imaging lens 5a is large, the deviation of the height of the scanning light beam in the sub-scanning direction is sufficiently suppressed, and the light beam is prevented from vignetting at the boundary line between the imaging lenses 5b1 and 5b2. be able to.
However, the present invention is not limited to this configuration, and if the strongest refractive power in the sub-scanning direction is given to the imaging lenses 5b1 and 5b2, the effect of the present invention can be sufficiently obtained.

When a multi-stage lens is manufactured by injection molding, if there is a large step in the optical axis direction at the boundary line between the first region and the second region, the surface direction in the sub-line direction will be near the boundary line and in the scanning beam passage region. Increases the imaging performance.
Therefore, in the present embodiment, the direction of the step in the optical axis direction between the lens surfaces of the first region and the second region is reversed between the central portion and the end portion along the main scanning direction. Design the lens surface.
Thereby, the magnitude | size of a level | step difference can be restrained small in the whole lens area.

FIG. 8 shows the sub-scanning cross-sectional shape of the incident surface at each position in the main scanning direction of the imaging lens 5b1 according to this embodiment.
FIG. 9 shows the sub-scanning cross-sectional shape of the exit surface at each position in the main scanning direction of the imaging lens 5b1 according to this embodiment.

As shown in FIG. 8 and FIG. 9, on the entrance surface and the exit surface of the imaging lens 5b1, the first region and the second region have a step in the optical axis direction. Connected discontinuously. And it turns out that the direction of this level | step difference is reverse by the center part and an edge part along the main scanning direction.
In other words, the positions of the end of the first region of the imaging lens 5b1 on the second region side and the end of the second region on the first region side in the optical axis direction of the imaging lens 5b1. The relationship is reverse between the central portion and the end portion of the imaging lens 5b1 in the main scanning direction.
Further, as can be seen from FIGS. 8 and 9, in the present embodiment, the boundary line is formed by shifting the surface vertex positions of the entrance surface and the exit surface of the first region and the second region of the imaging lens 5b1 in the optical axis direction. By reducing the level difference, the amount of surface habit caused by the level difference can be reduced.

Further, in this embodiment, as can be seen from Table 4, the incident surface and the exit surface of the imaging lens 5b1 are optical surfaces (subordinate tilt changes) in which the amount of surface inclination in the sub-scanning section changes along the main scanning direction. Surface).
In this embodiment, by using such an optical surface, the surface inclination in the sub-scanning direction can be set independently of the bus shape at any position in the main scanning direction of the imaging lens. Thus, it is possible to satisfactorily correct the wavefront aberration caused by the conical scan.
However, the present invention is not limited to this configuration, and in a multistage lens having a plurality of asymmetric regions, if the boundary line connecting the plurality of regions is set to a non-linear shape as described above, the effect of the present invention is sufficiently satisfied. be able to.

[Second Embodiment]
As described above, the entrance surface and the exit surface of the first region and the second region of the imaging lens 5b1 of the present embodiment are asymmetric with respect to the boundary line in the sub-scanning cross section. There is a step in the optical axis direction, and there is a portion where the surface inclination is discontinuous.
Therefore, when the imaging lens 5b1 is molded by injection molding, if there is a step or a discontinuous portion in the tilt of the surface, the lens surface may be deformed and deformed in the vicinity by thermal deformation stress during injection molding. A habit is likely to occur.
Therefore, by connecting the first area and the second area with a spline curve, the step in the optical axis direction of the boundary line and the discontinuity of the tilt of the surface are eliminated, and problems caused by the discontinuity are suppressed. May be.

FIG. 10 shows the sub-scanning cross-sectional shape of the incident surface at each position in the main scanning direction of the imaging lens 5b1 according to the second embodiment.
FIG. 11 shows the sub-scanning cross-sectional shape of the exit surface at each position in the main scanning direction of the imaging lens 5b1 according to the second embodiment.

As shown in FIGS. 10 and 11, as in the first embodiment, the first region and the second region are stepped in the optical axis direction on the entrance surface and the exit surface of the imaging lens 5 b 1. Are connected by a boundary line. And it turns out that the direction of this level | step difference is reverse by the center part and an edge part along the main scanning direction.
Further, the boundary line connecting the entrance surface and the exit surface of the first region and the second region of the imaging lens 5b1 according to the second embodiment is a spline curve.

In the sub-scan section, when the incident surface and the exit surface of the first region and the second region which are asymmetric are connected by a spline curve, the spline curve is locally connected at the connection portion between the first region and the second region. Irregularities with a very small radius of curvature always occur.
Even when the scanning light beam is vignetted by the uneven portion, the imaging performance is deteriorated as in the case where the boundary line is a step as in the first embodiment. Therefore, it is possible to prevent the imaging performance from being deteriorated by curving the uneven portion along the main scanning direction in the same manner as the boundary line of the first embodiment.
That is, by using the spline curve, the uneven portion at the connection between the spline curve and the first region and the second region can be reduced even if the discontinuity between the first region and the second region is eliminated. By curving along the main scanning direction, the effects of the present invention can be sufficiently obtained.

  The present invention can exhibit an effective effect as long as the lens surfaces of a plurality of regions in the multistage imaging lens are asymmetric.

  Up to this point, only the imaging lens 5b1 has been described, but the imaging lens 5b2 can be configured in the same manner.

In the first embodiment, the boundary line is configured with a simple curved shape as shown in, for example, Expression (17), but the present invention is not limited to this. The boundary line connecting the entrance surface and the exit surface of the first region and the second region is even in the direction away from the main scanning section including the axial deflection point at the end rather than the center in the main scanning direction. If it exists, the effect of this invention can fully be acquired.
In the first and second embodiments, in order to prevent interference between the light beams emitted from the respective light sources, the light beams of the incident optical systems LA2 and LA4 and the light beams of the incident optical systems LA1 and LA3 in the main scanning section are deflected. The incident angles on the surface 4a were set to 85 degrees and 75 degrees. However, the incident angle may be unified to a single value. When the incident angle to the deflecting surface 4a in the main scanning section is unified to a single value, the imaging lenses 5b1 and 5b2 can be designed to have the same shape, and the imaging lens can be formed with only one type of lens mold. Since 5b1 and 5b2 can be molded, the lens manufacturing cost can be further reduced.

In the first and second embodiments, the light beam of the incident optical system LA1 and the light beam of the incident optical system LA2 are incident on the deflecting surface 4a at different oblique incident angles of 4.2 ° and 1.5 ° in the sub-scanning direction. However, the effects of the present invention can be obtained even when oblique incidence is performed at the same angle. Similarly, the effect of the present invention can be obtained even when the light beam of the incident optical system LA3 and the light beam of the incident optical system LA4 are obliquely incident on the deflecting surface 4a at the same angle in the sub-scanning direction.
Moreover, although the light source 1 of the first and second embodiments is configured by a semiconductor laser having one light emitting point, it may be configured by a monolithic multi-beam laser having a plurality of light emitting points.
In addition, the deflection unit 4 of the first and second embodiments is configured by one polygon mirror, but may be configured by a plurality of polygon mirrors arranged in the sub-scanning direction.

In the imaging lens 5a of the first and second embodiments, the non-power is set in the sub-scanning direction to reduce the degradation of imaging performance due to the eccentricity of the imaging lens 5a with respect to the sub-scanning direction. You may have power in the scanning direction.
Further, in the first and second embodiments, as shown in FIG. 2, a folding mirror 6a is disposed after the imaging lens 5a, and the four light beams are separated into two sets of light beams by two, Although the light is incident on the imaging lenses 5b1 and 5b2, respectively, the effect of the present invention is not limited to this configuration. For example, the effects of the present invention can be obtained even when the folding mirror 6a is eliminated, the imaging lenses 5b1 and 5b2 are integrally formed, and the multistage imaging lens 5b having four regions respectively corresponding to the four light beams is used. it can.
In the first and second embodiments, in order to prevent interference between the holding units of the plurality of light sources and between the adjustment tools, the main scanning sections of the light beams of the incident optical systems LA2 and LA4 and the light beams of the incident optical systems LA1 and LA3, respectively. The incident angle to the deflection surface 4a is set to a different angle. However, the incident angles may be unified. In that case, four light sources are arranged in a line in the sub-scanning direction. The effects of the present invention can also be obtained by using a combining means such as a combining prism or mirror for combining the optical paths of the respective incident optical systems.

[Third embodiment]
12 (a) and 12 (b) respectively show the boundary line between the multistage lens surface and the multistage lens surface when the entrance surface and the exit surface of the imaging lens 5b1 according to the third embodiment are projected onto a cross section perpendicular to the optical axis. It represents the trajectory of the upper scanning beam. In the discussion here, it is assumed that the optical path is expanded, that is, the folding mirrors 6 and 6a are not present.
In FIG. 12, the position of the reference plane G along the sub-scanning direction is Z = 0, and the point where the light beam intersects the optical axis of the imaging lens 5a is Y = 0. Further, since the following discussion is the same for the imaging lens 5b2, the description of the imaging lens 5b2 is omitted.

  Unlike the first and second embodiments, the boundary line according to this embodiment is not a curved shape, but is composed of two straight lines having one bending point. Since other configurations are the same as those in the first and second embodiments, the same reference numerals are given and description thereof is omitted.

  Table 5 and Table 5 below show the relationship between the main scanning direction coordinate Y (mm) and the sub-scanning direction coordinate Z (mm) of the boundary lines of the entrance surface and the exit surface of the imaging lens 5b1 according to the third embodiment. It is shown in FIG.

  Note that the angle of the boundary line in the table is defined as an angle formed by the boundary line with respect to the Y axis, and a counterclockwise direction is positive.

Specifically, each boundary line between the entrance surface and the exit surface of the imaging lens 5b1 according to the third embodiment has one inflection point at the central portion in the main scanning direction (Y = 0). It is composed of two straight lines with different inclinations.
Here, an angle formed between two boundary lines (two straight lines in the present embodiment) sandwiching a bending point when projected onto a cross section perpendicular to the optical axis is defined as a bending angle. When the bending angle is large, it becomes difficult to process the lens mold or the lens itself. Therefore, it is preferable that the bending angle is small.

Specifically, in the present embodiment, on the incident surface of the imaging lens 5b1, the boundary angle is 0.63 degrees and −100 ≦ Y <0 in the region of 0 ≦ Y ≦ 100 across the bending point. In this area, the angle of the boundary line is set to -0.63 degrees.
Therefore, it can be seen that the bending angle is 0.63 + 0.63 = 1.26 degrees, which is set sufficiently small.
In the present embodiment, the number of bending points is minimized by providing the bending point only in the central portion (Y = 0) in the main scanning direction, and vignetting of the scanning light beam due to the boundary line is prevented.
In this embodiment, the bending angle is set as small as 1.26 degrees. However, if the bending angle can be suppressed to 10 degrees or less, the effect of the present invention can be sufficiently obtained.

  Similarly, it can be seen that the bending angle on the exit surface of the imaging lens 5b1 is 0.52 + 0.52 = 1.04 degrees, which is set sufficiently small.

  Also in this embodiment, since the height Hr (Y) of the boundary line in the sub-scanning direction is set to satisfy the conditional expression (18), vignetting of the scanning light beam due to the boundary line is prevented over the entire lens. And good imaging performance can be obtained.

[Fourth embodiment]
FIGS. 13A and 13B respectively show the boundary line between the multistage lens surface and the multistage lens surface when the entrance surface and the exit surface of the imaging lens 5b1 according to the fourth embodiment are projected onto a cross section perpendicular to the optical axis. It represents the trajectory of the upper scanning beam. In the discussion here, it is assumed that the optical path is expanded, that is, the folding mirrors 6 and 6a are not present.

  Unlike the third embodiment, the boundary line according to the present embodiment is composed of four straight lines having three bending points. Since other configurations are the same as those in the first and second embodiments, the same reference numerals are given and description thereof is omitted.

  Table 7 and Table 7 below show the relationship between the main scanning direction coordinate Y (mm) and the sub-scanning direction coordinate Z (mm) of the boundary lines of the entrance surface and the exit surface of the imaging lens 5b1 according to the fourth embodiment, respectively. It is shown in FIG.

  As shown in Tables 7 and 8, it can be seen that also in the fourth embodiment, the bending angle can be kept small as in the third embodiment.

In the present embodiment, the imaging lenses 5b1 and 5b2 are manufactured by pouring resin into a lens mold and injection molding.
In addition, the entrance and exit surfaces of the multistage imaging lenses 5b1 and 5b2 are arranged in the sub-scanning direction so that their shapes are different corresponding to the first and second regions above and below the sub-scanning direction across the boundary line. It is formed by two lens mirror pieces arranged side by side.
In a mold structure with two lens mirror pieces arranged side by side, the boundary line is curved or bent in a direction that is monotonously separated from the main scanning section including the on-axis deflection point, as the boundary line goes from the center to the end. The width in the sub-scanning direction at the lens end of the mirror piece is reduced.
If the width of the mirror surface piece is narrow, sufficient rigidity cannot be obtained, and there is a problem that the molding stability of the lens surface shape is deteriorated because the end portion of the mirror surface piece is deformed during lens forming.
Therefore, in this embodiment, the boundary line has at least two or more bending points. Then, at the bending point on the most end side in the main scanning direction, when the boundary line on the end side rather than the boundary line on the center side across the bending point is projected onto a cross section perpendicular to the optical axis direction The angle formed with respect to the main scanning direction is set to be small.
Thereby, in this embodiment, the subscanning direction width | variety in the lens edge part of a mirror surface piece can fully be ensured, and the shaping | molding stability of a lens surface shape can fully be maintained.

[Fifth embodiment]
FIGS. 14A and 14B show the boundary line between the multistage lens surface and the multistage lens surface when the entrance surface and the exit surface of the imaging lens 5b1 according to the fifth embodiment are projected on a cross section perpendicular to the optical axis, respectively. It represents the trajectory of the upper scanning beam. In the discussion here, it is assumed that the optical path is expanded, that is, the folding mirrors 6 and 6a are not present.

  Unlike the third and fourth embodiments, the boundary line according to this embodiment is composed of five straight lines having four bending points. Note that none of the four bending points is provided in the central portion (Y = 0) in the main scanning direction. Since other configurations are the same as those in the first and second embodiments, the same reference numerals are given and description thereof is omitted.

  Table 9 and Table 9 below show the relationship between the main scanning direction coordinate Y (mm) and the sub-scanning direction coordinate Z (mm) of the boundary lines of the entrance surface and the exit surface of the imaging lens 5b1 according to the fifth embodiment, respectively. 10 shows.

By using four bending points, each bending angle can be further reduced, and when the boundary line at the center and both ends is projected on a cross section perpendicular to the optical axis, it can be made parallel to the main scanning direction. .
As a result, the rotational attitude of the mirror surface piece about the optical axis center can be controlled more easily, and the surface eccentricity error of the lens surface can be further reduced.

[Sixth embodiment]
15A and 15B show the boundary line between the multistage lens surface and the multistage lens surface when the entrance surface and the exit surface of the imaging lens 5b1 according to the sixth embodiment are projected on a cross section perpendicular to the optical axis, respectively. It represents the trajectory of the upper scanning beam. In the discussion here, it is assumed that the optical path is expanded, that is, the folding mirrors 6 and 6a are not present.

  Unlike the third to fifth embodiments, the boundary line according to this embodiment has a curved shape only in the region through which the scanning light beam passes, and has a linear shape in the outer region. Since other configurations are the same as those in the first and second embodiments, the same reference numerals are given and description thereof is omitted.

  Table 11 and Table 11 below show the relationship between the main scanning direction coordinate Y (mm) and the sub-scanning direction coordinate Z (mm) of the boundary lines of the entrance surface and the exit surface of the imaging lens 5b1 according to the sixth embodiment, respectively. 12 shows.

  In this embodiment, the shape of the boundary line is curved so that only the region through which the scanning light beam passes prevents the scanning light beam from entering the region on the opposite side across the boundary line and does not cause vignetting at the boundary line. Shape. And in the area | region outside the area | region through which a scanning light beam passes, it is made into the simple linear shape so that it may be easy to design a specular piece with high precision.

[Seventh embodiment]
FIGS. 16A and 16B show the boundary line between the multistage lens surface and the multistage lens surface when the entrance surface and the exit surface of the imaging lens 5b1 according to the seventh embodiment are projected on a cross section perpendicular to the optical axis, respectively. It represents the trajectory of the upper scanning beam. In the discussion here, it is assumed that the optical path is expanded, that is, the folding mirrors 6 and 6a are not present.

  Unlike the third to sixth embodiments, the boundary line according to the present embodiment is configured by three straight lines having two bending points, and the bending angle at each bending point is 0 degree. Since other configurations are the same as those in the first and second embodiments, the same reference numerals are given and description thereof is omitted.

  Table 13 and Table 13 below show the relationship of the sub-scanning direction coordinate Z (mm) to the main scanning direction coordinate Y (mm) of the boundary lines of the entrance surface and the exit surface of the imaging lens 5b1 according to the seventh embodiment, respectively. 14 shows.

  By setting the bending angle to 0 degree, it becomes easy to increase the accuracy of the angle of each surface of the mirror surface piece, and the rotation posture of the optical axis center of the mirror surface piece can be controlled more easily.

  As described above, in the multistage imaging lens 5b1 of the optical scanning device according to the third to seventh embodiments, the main scanning direction position dependency of the height of the boundary line in the sub-scanning direction has at least one bending point. It is expressed by a straight line group, a curve group, or a combination of straight lines and curves.

  As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to these embodiment, A various deformation | transformation and change are possible within the range of the summary.

  For example, in the present invention, one light beam is obliquely incident on each of the first region and the second region of one imaging lens with the same sign and different incident angles, but the present invention is not limited to this. Absent. For example, one angle of each light beam may be 0 degree, that is, perpendicular incidence.

  FIG. 17 is a schematic diagram of a main part of a color image forming apparatus 60 equipped with the optical scanning device 100 according to the embodiment of the present invention.

The color image forming apparatus 60 according to the present embodiment scans four light beams by the optical scanning apparatus 100 and records image information on the photoconductors 21 to 24 as image carriers in parallel. A color image forming apparatus.
In FIG. 17, reference numeral 60 denotes a color image forming apparatus, and 100 denotes an optical scanning apparatus according to an embodiment of the present invention. 21, 22, 23 and 24 are photosensitive drums as image carriers for forming images of different colors, 31, 32, 33 and 34 are developing units corresponding to the photosensitive drums, and 51 is a conveyor belt. is there.

  In FIG. 17, the color image forming apparatus 60 receives R (red), G (green), and B (blue) color signals (code data) from an external device 52 such as a personal computer. These color signals are converted into image data (dot data) of Y (yellow), M (magenta), C (cyan), and K (black) by a printer controller 53 in the apparatus. These image data are input to the optical scanning device 100. The light scanning device 100 emits light beams 41, 42, 43, and 44 that are modulated in accordance with each image data, and the photosensitive surfaces of the photosensitive drums 21, 22, 23, and 24 are mainly formed by these light beams. Scanned in the scanning direction.

In the color image forming apparatus 60 in the present embodiment, the optical scanning device 100 scans each of the four light beams corresponding to each color of Y (yellow), M (magenta), C (cyan), and B (black). The In parallel, image signals (image information) are recorded on the photosensitive surfaces of the photosensitive drums 21, 22, 23, and 24, and a color image is printed at high speed.
In the color image forming apparatus 60 in the present embodiment, as described above, the optical scanning apparatus 100 uses the light beams based on the respective image data, and the electrostatic drum images 21, 22, 23 corresponding to the electrostatic latent images of the respective colors, 24 is formed on the photosensitive surface. Thereafter, the electrostatic latent images of the respective colors are developed as toner images by the developing devices 31, 32, 33, and 34, and are transferred onto the transfer material conveyed by the conveying belt 51 by a transfer device (not shown). Then, the toner image transferred by a fixing device (not shown) is fixed on the transfer material to form one full-color image.

  As the external device 52, for example, a color image reading device including a CCD sensor may be used. In this case, the color image reading apparatus and the color image forming apparatus 60 constitute a color digital copying machine.

  According to the present invention, in an optical scanning device using a multistage lens in which a plurality of imaging optical elements are integrally molded, it is possible to achieve both improvement in the degree of freedom in arrangement and achievement of good imaging performance. As a result, it is possible to provide an optical scanning device that can achieve low cost and compactness and can form a good image, and an image forming apparatus including the same.

4 Polygon mirror (deflector)
5a, 5b1, 5b2 Imaging lens (imaging optical element)
8y, 8m, 8c, 8k Photosensitive drum (scanned surface)
100 Optical scanning devices LA1, LA2, LA3, LA4 Incident optical system LB Imaging optical system

Claims (13)

A deflector for deflecting the first and second light beams to optically scan the first and second scanned surfaces in the main scanning direction;
First and second incident optical systems that cause the first and second light beams to enter the deflecting surface of the deflector at different incident angles within a sub-scanning section;
An imaging optical system for guiding the first and second light beams deflected by the deflector to the first and second scanned surfaces;
The first and second incident optical systems cause the first and second light beams to enter the deflection surface from the same side with respect to a main scanning section including an on-axis deflection point,
The imaging optical system includes an imaging optical element including first and second optical surfaces through which the first and second light beams pass,
The first and second optical surfaces are discontinuously connected to each other in the sub-scanning section,
The optical scanning device characterized in that the boundary between the first and second optical surfaces is farther from the main scanning section including the on-axis deflection point at the end than the center in the main scanning direction. A deflector for deflecting the first and second light beams to optically scan the first and second scanned surfaces in the main scanning direction;
First and second incident optical systems that cause the first and second light beams to enter the deflecting surface of the deflector at different incident angles within a sub-scanning section;
An imaging optical system for guiding the first and second light beams deflected by the deflector to the first and second scanned surfaces;
The first and second incident optical systems cause the first and second light beams to enter the deflection surface from the same side with respect to a main scanning section including an on-axis deflection point,
The imaging optical system includes an imaging optical element including first and second optical surfaces connected to each other in a sub-scan section through which each of the first and second light beams passes.
When the first and second optical surfaces are defined by different aspherical expressions, the boundary between the first and second optical surfaces is on-axis deflected toward the end rather than the center in the main scanning direction. An optical scanning device characterized by being separated from a main scanning section including a point.   The boundary between the first and second optical surfaces is closer to the end than the center in the main scanning direction, and the first and second light fluxes are relative to the main scanning section including the axial deflection point. The optical scanning device according to claim 1, wherein the optical scanning device is separated toward a side opposite to a side incident on the deflecting surface. The incident angles of the first and second light beams in the sub-scanning section are α1 and α2, respectively, the distance from the on-axis deflection point to the imaging optical element in the main scanning section is T, and from the deflector Of the m lenses arranged up to the imaging optical element, the thickness and refractive index of the i-th lens are di and ni, respectively, and the on-axis deflection point is the origin The position Hr (Y) of the boundary in the main scanning direction position Y in the sub-scanning direction is expressed by the following conditional expression 0 ≦ α1 × α2 (except α1 = 0 and α2 = 0)

The optical scanning device according to claim 1, wherein:   Positions of the end portion of the first optical surface on the second optical surface side and the end portion of the second optical surface on the first optical surface side in the optical axis direction of the imaging optical element 5. The optical scanning device according to claim 1, wherein the relationship is reverse between a central portion and an end portion in the main scanning direction.   The shape of the boundary is represented by a group of straight lines, a group of curves, or a group including at least one straight line and at least one curve having at least one inflection point. An optical scanning device according to claim 1. The incident angles α1 and α2 are defined by the following conditional expression: 0.5 ° ≦ | α2 | ≦ 2.5 °
3.5 ° ≦ | α1 | ≦ 5.0 °
The optical scanning device according to claim 4, wherein:   8. The optical scanning device according to claim 1, wherein the generatrix shape of the incident surface of the imaging optical element does not have an inflection point. 9.   9. The optical scanning device according to claim 1, wherein a difference between a maximum value and a minimum value of a child-line tilt of an incident surface of the imaging optical element is 100 minutes or less.   10. The light according to claim 1, wherein the first and second incident optical systems cause light beams to enter the deflector from different angles in the main scanning section. Scanning device.   The optical scanning device according to claim 1, wherein the first and second optical surfaces of the imaging optical element are asymmetrical.   12. The optical scanning device according to claim 1, and an electrostatic latent image formed on a photosensitive surface of a photoconductor disposed on the scanned surface by the optical scanning device as a toner image. An image forming apparatus comprising: a developing device that transfers the developed toner image onto a transfer material; and a fixing device that fixes the transferred toner image onto the transfer material.   The image forming apparatus according to claim 12, further comprising a printer controller that converts code data input from an external device into an image signal and inputs the image signal to the optical scanning device.

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