JP4418567B2 - Multi-beam scanning optical system and image forming apparatus using the same - Google Patents

Description

[0001]
[Technical field to which the invention belongs]
The present invention relates to an optical scanning device, and in particular, reflects and deflects a light beam emitted from a light source means by a polygon mirror as an optical deflector, and optically scans a surface to be scanned through an imaging optical system to record image information. In a multi-beam optical scanning optical system suitable for image forming apparatuses such as laser beam printers and digital copying machines having an electrophotographic process as described above, the scanning line spacing changes asymmetrically due to manufacturing errors and assembly errors of optical components. The present invention relates to an optical scanning device that can always obtain a good image with no pitch unevenness.
[0002]
[Prior art]
FIG. 20 is a sectional view in the main scanning direction of a conventional multi-beam scanning device. A plurality of light beams modulated and emitted from the plurality of light source means 1 are converted into substantially parallel light beams by a collimator lens 2, and the light beam is limited by an aperture stop 3 to have a predetermined refractive power only in the sub-scan section. 4 is incident. Of the substantially parallel light beam incident on the cylindrical lens 4, the light beam is emitted as it is in a substantially parallel light beam in the main scanning section. In the sub-scan section, the light beam converges and forms a substantially linear image on the deflection reflection surface 5a of the polygon mirror 5 (optical deflector). The light beam reflected and deflected by the polygon mirror is guided to the scanned surface 7 (photosensitive drum surface) via the imaging optical system 6 having the fθ characteristic, and the scanned surface is rotated by rotating the polygon mirror. 7 (photosensitive drum surface) is optically scanned to record image information.
[0003]
[Problems to be solved by the invention]
However, the optical components constituting the light source means 1, polygon mirror 5, and imaging optical system 6 have inclinations and parallel shifts (manufacturing errors and assembly errors) that occur during manufacturing and assembly, and are introduced onto the surface to be scanned. The plurality of emitted light beams are optically scanned at positions different from the design. Specifically, regarding the pitch interval of the scanning lines (lines) drawn by each light beam, the amount of deviation from the design value differs for each light beam of the multi-beam and for each scanning position on the surface to be scanned 7. The pitch interval widens or narrows depending on the scanning position and appears as pitch unevenness on the image.
[0004]
On the other hand, when optical scanning is performed on the surface to be scanned with small manufacturing and assembly errors in each optical component, the amount of deviation from the design value can be reduced and the change in line pitch interval can be kept small. Improving accuracy and assembly accuracy will lead to significant cost increases.
[0005]
In view of this, the present invention has an object to reduce the sensitivity to manufacturing errors and assembly errors in each optical component to suppress the change amount of the line pitch interval to be small and to always form a good image without pitch unevenness.
[0006]
[Means for Solving the Problems]
In order to solve the above problems, the present invention provides light source means in which a plurality of light emitting points are arranged at intervals in the main scanning direction, and an incident optical system for condensing a multi-beam emitted from the light source means. And deflecting means for reflecting and deflecting a multi-beam emitted from the light source means, and at least one image forming the multi-beam reflected and deflected by the deflecting means as a spot on a scanned surface which is a photosensitive member In a multi-beam scanning optical system having an fθ imaging optical system constituted by lenses, one light beam of the multi-beams is MBA, an adjacent light beam is MBB, and the light beams MBA and MBB are different from each other in the main scanning direction. When reaching the same position on the surface to be scanned through the optical path, the imaging magnification in the sub-scanning direction of the imaging lens in the light beam MBA is βla, and the imaging The imaging magnification in the sub-scanning direction of the optical system behind the lens is βl′a, the imaging magnification in the sub-scanning direction of the imaging lens in the light beam MBB is βlb, and the optical system behind the imaging lens is When the imaging magnification in the sub-scanning direction is βl′b and the pixel density in the sub-scanning direction on the surface to be scanned is A (dpi), Expression (1) is satisfied.
[0007]
[Expression 7]
In the present invention, in the above-described multi-beam scanning optical system, one light beam of the multi-beams is MBA and the adjacent light beam is MBB, and the light beams MBA and MBB pass through different optical paths in the main scanning direction. When reaching the same position on the surface to be scanned, the imaging magnification in the sub-scanning direction of the lens surface of the imaging lens in the light beam MBA is βsa, and the optical system behind the lens surface is connected in the sub-scanning direction. The image magnification is βs′a, the imaging magnification in the sub-scanning direction of the lens surface of the imaging lens in the light beam MBB is βsb, and the imaging magnification in the sub-scanning direction of the optical system behind the lens surface is βs′b. When the pixel density in the sub-scanning direction on the surface to be scanned is A (dpi), Expression (8) is satisfied.
[0008]
[Equation 8]
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
[0010]
[Embodiment 1]
FIG. 1 is a main scanning sectional view of the multi-beam scanning device according to the first embodiment. Reference numeral 1 denotes a semiconductor laser array as a light source means. Two light beams emitted from the semiconductor laser array 1 are converted into parallel light beams by one collimator lens 2 and pass through a diaphragm 3a that defines a light beam width in the sub-scanning direction. A cylindrical lens 4 having a positive refractive power only in the sub-scanning direction forms an image as a line image that is long in the main scanning direction. Then, the two light beams emitted from the semiconductor laser array 1 pass through a diaphragm 3b that defines the light beam width in the main scanning direction disposed behind the cylindrical lens 4 so that the deflecting reflection surface 5a comes near the line image. Is reflected and deflected by a polygon mirror 5 which is a deflecting means disposed on the surface of the photosensitive drum 7 through an imaging optical system 6 composed of two imaging lenses 6a and 6b. Lighted. As the polygon mirror 5 rotates in the direction of the arrow in the figure, the surface to be scanned 7 is optically scanned.
[0011]
FIG. 2 is a table showing a specific example of the multi-beam scanning apparatus according to the first embodiment. Here, a semiconductor laser array 1 having two light emitting points is used. As shown in this table, the semiconductor laser array 1 is arranged so that the interval in the sub-scanning direction when the two light beams emitted from the respective light emitting points reach the surface to be scanned 7 matches the interval corresponding to the pixel density. It is adjusted by rotating 9398 degrees. At this time, the two light emitting points are arranged at an interval of 4.616 μm in the sub-scanning direction and at the same time at an interval of 89.882 μm in the main scanning direction.
[0012]
FIG. 3 is an expression representing the bus direction corresponding to the main scanning direction. In this equation, the intersection with the optical axis is the origin, the optical axis direction is the x axis, the axis orthogonal to the optical axis in the main scanning plane is the y axis, and the axis orthogonal to the optical axis in the sub scanning plane is the z axis. It is said.
[0013]
FIG. 4 is an equation representing a sub-line direction corresponding to the sub-scanning direction (direction including the optical axis and orthogonal to the main scanning direction).
[0014]
FIG. 5 is a sub-scan sectional optical path diagram of the multi-beam scanning device according to the first embodiment. The light beam reflected and deflected by the deflecting / reflecting surface 5a of the polygon mirror is imaged on the scanned surface 7 by the imaging lens 6a and the imaging lens 6b. At this time, if a parallel shift Δ in the sub-scanning direction due to an assembly error occurs in the imaging lens 6a, the light beam that tends to be reflected by the deflecting / reflecting surface 5a of the polygon mirror is condensed to the point p ′ by the imaging lens 6a. The image is formed at a point p on the scanned surface 7 by the imaging lens 6b.
[0015]
The point p on the surface to be scanned 7 is shifted from the design image forming position by Z in the sub scanning direction, the imaging magnification of the imaging lens 6a in the sub scanning direction is β1, and the imaging lens 6b in the sub scanning direction is in the sub scanning direction. When the imaging magnification is βl ′, it is expressed by Expression (10).
[0016]
[Equation 9]
Here, when one of the two light beams emitted from the two light emitting points is A and the other is B, the imaging magnifications in the sub-scanning direction of the imaging lens 6a in the light beams A and B are βla and βlb, respectively. When the imaging magnification of the lens 6b in the sub-scanning direction is set to βl′a and βl′b, respectively, the imaging lens 6a reaches the scanned surface 7 when the parallel shift Δ in the sub-scanning direction due to an assembly error occurs. Deviation amounts Za and Zb from the design value of the position to be expressed are expressed by equations (20) and (30), respectively.
[0017]
[Expression 10]
[0018]
## EQU11 ##
At this time, the absolute value of the change amount Δd of the line pitch interval is given by Expression (40).
[0019]
[Expression 12]
In general, the assembly error is at most 0.1 (mm), and when the sensitivity to the change in the line pitch interval is ρ, this is smaller than 10% of the pixel density A (dpi) in the sub-scanning direction. In order to suppress, it is sufficient that the expression (50) is satisfied.
[0020]
[Formula 13]
Here, an imaging optical system composed of two imaging lenses has been taken as an example. However, when there is no optical system behind the imaging lens as in the case of a single imaging lens, It is only necessary to set the imaging magnification βl ′ = 1 behind the imaging lens, and when there is an optical system composed of a plurality of optical components behind the imaging lens, their combined imaging The magnification may be given to βl ′. The imaging magnification is a substantial imaging magnification along the optical path of each light beam.
[0021]
The same applies to the parallel shift in the sub-scanning direction of each lens surface due to a manufacturing error. The imaging magnifications in the sub-scanning direction of the surface of the imaging lens 6a on the polygon mirror 5 side in the light beams A and B are βsa and βsb, respectively. When the imaging magnifications in the combined sub-scanning direction from the surface on the scanned surface 7 side of the imaging lens 6a to the surface on the scanned surface 7 side of the imaging lens 6b are βs′a and βs′b, respectively, In order to suppress the change amount of the line pitch interval due to the manufacturing error of the surface of the image lens 6a on the polygon mirror 5 side to be smaller than 10% of the pixel density A (dpi) in the sub-scanning direction, Expression (60) is satisfied. That's fine.
[0022]
[Expression 14]
FIG. 6 is a cross-sectional view of the optical system from the semiconductor laser array 1 to the deflecting / reflecting surface 5a of the polygon mirror 5, that is, the incident optical system in the main scanning direction. The two light beams A and B emitted from the two light emitting points 9a and 9b of the semiconductor laser array 1 are converted into parallel light beams by the collimator lens 2 and the light beam width in the main scanning direction is determined by the diaphragm 3b, and the deflecting / reflecting surface 5a of the polygon mirror 5 is determined. To reach. At this time, since the light emitting points 9a and 9b are spaced apart in the main scanning direction, the two light beams A and B exit the collimator lens 2 at a certain angle, and the principal rays intersect each other at the center of the stop 3b and deflect. Since it reaches the reflecting surface 5a, the principal rays of the light beams A and B have a certain interval on the deflecting reflecting surface 5a.
[0023]
The distance between the light emitting points in the main scanning direction is dms (mm), the focal length (in the main scanning direction) of the collimator lens is fco (mm), and the optical path length from the stop to determine the beam width in the main scanning direction to the deflecting reflection surface When Lap (mm) is set, the distance dmp (mm) in the main scanning direction of the principal rays of the light beams A and B on the deflecting reflection surface is expressed by Expression (70).
[0024]
[Expression 15]
FIG. 7 is a cross-sectional view of the optical system from the polygon mirror 5 to the scanned surface 7, that is, the imaging optical system 6, in the main scanning direction. When the light beams A (solid line) and B (broken line) reflected and deflected by the deflecting / reflecting surface 5a are parallel, the imaging optical system 6 forms an image at the same position on the scanned surface 7 in the main scanning direction. However, as described above, since the light beams A and B are incident on the deflecting / reflecting surface 5a at different angles, it is necessary to rotate the polygon mirror 5 by a minute angle in order to form an image at the same position on the scanned surface 7. In the multi-beam scanning device shown in the table as a specific example of the first embodiment, the angle is 6.271 (minutes). Actually, this is achieved by shifting the writing timing of the two light beams A and B by a minute time.
[0025]
Although the light beams A and B after the polygon mirror 5 are incident on the imaging lens surface at a certain interval while being parallel, the incident angle θi in the main scanning direction, the optical path length Lli, and the child are different because the positions incident on the imaging lens surface are different. A difference occurs in the linear curvature radius Rs. These are factors that change the imaging magnification in the sub-scanning direction. In particular, when an aspherical surface is used for the imaging lens surface, there is a portion where the difference in the incident angle in the main scanning direction becomes considerably large. The difference in image magnification also increases.
[0026]
Therefore, in the multi-beam scanning device according to the first embodiment, the stop 3b for determining the light beam width in the main scanning direction is disposed in the vicinity of the deflecting / reflecting surface 5a so that the distance between the two light beams A and B is narrowed and incident on the imaging lens surface. The difference in the imaging magnification is reduced by suppressing the difference in the incident angle at, and the sensitivity to the change amount of the line pitch interval when a manufacturing error and an assembly error occur in each optical component is reduced.
[0027]
Here, since the two light beams A and B are imaged at the same position in the main scanning direction on the scanned surface 7 by the imaging optical system 6, the two light beams A and B are incident on the scanned surface 7. If the angle a formed by the light beam is small, the incident angle difference at the position where it enters the imaging lens surface is also small. Therefore, the incident optical system may be configured so that the angle a satisfies the expression (80).
[0028]
[Expression 16]
Substituting equation (70) into equation (80) leads to equation (90).
[0029]
[Expression 17]
Here, k is the fθ coefficient of the imaging optical system 6, and if the incident optical system is configured so as to satisfy the equation (90), the change amount of the line pitch interval when a manufacturing error and an assembly error occur in each optical component. Can be suppressed within 10% of the predetermined pitch interval.
[0030]
FIG. 8 is a table showing the configuration of the incident optical system in the conventional multi-beam scanning device and the multi-beam scanning device of Embodiment 1 and the angle formed by the principal rays of the two light beams A and B when entering the scanned surface 7. is there.
[0031]
FIG. 9 shows that in the conventional multi-beam scanning device and the multi-beam scanning device of the present embodiment, the surface on the scanning surface 7 side of the fθ lens 6b is shifted by +10 μm in the sub-scanning direction due to manufacturing errors, and the fθ lens is caused by assembly errors. It is a graph which shows the variation | change_quantity of the line pitch space | interval when 6a carries out -10 micrometer parallel shift. The fθ coefficient k when the scanning length SL is expressed as k × f × θ is 136.2 mm.
[0032]
By reducing the optical path length Lap from the stop 3b for determining the light beam width in the main scanning direction to the reflection deflection surface 5a of the polygon mirror 5 from Lap = 76.3 mm to Lap = 25.0 mm, the maximum value of the line pitch interval change amount Was reduced from the conventional 3.23 μm to 1.08 μm, as shown in the figure. When this is compared with a ratio to a predetermined line pitch interval (21.2 μm at 1200 dpi) determined by the pixel density, the change amount of the line pitch interval was equivalent to 15.3% in the conventional multi-beam scanning device. In the multi-beam scanning apparatus according to the first embodiment, the amount of the two light beams A is reduced to 5.1%, and is incident on the scanned surface 7 with the diaphragm 3b that determines the light beam width in the main scanning direction close to the reflection deflection surface 5a. It can be seen that the effect of reducing the angle of the main scanning direction formed by the B principal ray is reduced.
[0033]
FIG. 10 is a table showing the maximum sensitivity ρ with respect to the change in the line pitch interval for each image height of each optical component at this time. This also satisfies the conditional expressions shown in the expressions (50) and (60).
[0034]
The diaphragm 3b that determines the light beam width in the main scanning direction is more effective when it is closer to the deflecting / reflecting surface 5a. Therefore, it is desirable to dispose the diaphragm 3b closest to the polygon mirror 5 among the optical components constituting the incident optical system. It is arranged between the polygon mirror 5.
[0035]
Further, it is desirable that the aperture stop 3a for determining the light beam width in the sub-scanning direction is disposed before the cylindrical lens 4 that moves during focus adjustment in the sub-scanning direction. In the first embodiment, the diaphragm 3a is disposed between the collimator lens 2 and the cylindrical lens 4. is doing.
[0036]
Not only the two light beams but also the light source means that emits n light beams may be used as long as the above formulas are satisfied for the adjacent light beams.
[0037]
[Embodiment 2]
FIG. 11 is a cross-sectional view in the main scanning direction of the multi-beam scanning device according to the second embodiment. The difference from the first embodiment is that the diaphragm 3b for determining the light beam width in the main scanning direction and the diaphragm 3a for determining the light beam width in the sub-scanning direction are the same diaphragm 3, and the diaphragm 3 is located immediately after the cylindrical lens 4. It is the point which is arranged in.
[0038]
As described above, the range in which the pitch unevenness is not noticeable is represented by the formula (100).
[0039]
[Formula 18]
Here, A is the pixel density (dpi), and Δd is the line pitch interval change amount (mm). The incident optical system satisfies the threshold (40), and the width of the light beam in the main scanning direction and the sub-scanning direction is determined by one diaphragm 3, thereby reducing the number of parts and reducing the cost.
[0040]
FIG. 12 is a table showing the configuration of the incident optical system in the conventional multi-beam scanning device and the multi-beam scanning device of Embodiment 2 and the angle formed by the principal rays of the two light beams A and B when entering the scanned surface 7. is there.
[0041]
FIG. 13 shows a conventional multi-beam scanning apparatus and the multi-beam scanning apparatus according to the present embodiment in which the surface on the scanned surface 7 side of the fθ lens 6b is shifted +10 μm in parallel in the sub-scanning direction due to a manufacturing error, It is a graph which shows the variation | change_quantity of the line pitch space | interval when 6a carries out -10 micrometer parallel shift. As shown in this graph, the maximum value of the change amount of the line pitch interval in the second embodiment is 2.1 μm, which corresponds to 9.9% of the line pitch interval determined by the pixel density.
[0042]
Even if the number of parts is reduced in this way, the incident optical system satisfies Expression (30), and therefore Expression (100) is also satisfied.
[0043]
[Embodiment 3]
The third embodiment differs from the first embodiment in that the aperture 3b determines the luminous flux width in the main scanning direction by shortening the interval in the main scanning direction of the light emitting point of the light source means and extending the focal length of the collimator lens in the main scanning direction. This is a point where the angle formed by the incident two light beams A and B is narrowed. In Embodiment 3, the aperture 3b for determining the beam width in the main scanning direction is arranged in the vicinity of the polygon mirror, and the angle formed by the two beams A and B incident on the aperture 3b is reduced, so that manufacturing errors and assembly of optical components are reduced. The sensitivity of the error to the line pitch interval error is reduced.
[0044]
FIG. 14 is a table showing the configuration of the incident optical system in the conventional multi-beam scanning device and the multi-beam scanning device of the third embodiment and the angle formed by the principal rays of the two light beams A and B when entering the scanned surface 7. is there.
[0045]
FIG. 15 shows that in the multi-beam scanning apparatus according to the first and third embodiments, the surface on the surface to be scanned 7 of the fθ lens 6b is shifted by +10 μm in the sub-scanning direction due to a manufacturing error, and the fθ lens 6a is −− due to an assembly error. It is a graph which shows the variation | change_quantity of the line pitch space | interval at the time of 10 micrometers parallel shift. In the third embodiment, the angle α = 6.246 (minutes) formed by the two light beams A and B incident on the stop 3b that determines the light beam width in the main scanning direction, and the first embodiment (α = 12.542 minutes). Since the angle a formed by the light beams A and B when entering the surface to be scanned 7 is reduced by about half, the maximum value of the change amount of the line pitch interval when the manufacturing error and the assembly tolerance occur is the embodiment. 1 is 1.08 μm, but in the third embodiment, it is reduced to 0.60 μm. Further, the ratio of the change amount of the line pitch interval to the predetermined interval corresponding to the pixel density is also changed from 5.1% to 2.6%, and the two light beams A and B incident on the stop 3b that determines the light beam width in the main scanning direction. It can be seen that the sensitivity to the line pitch interval change amount is reduced by narrowing the angle formed.
[0046]
When the angle α (rad) formed by the two light beams A and B that enter the stop 3b that determines the light beam width in the main scanning direction, there is a relationship of Expression (110).
[0047]
[Equation 19]
Here, when Expression (50) is substituted into Expression (20), Expression (120) is derived.
[0048]
[Expression 20]
Further, when Expression (60) is satisfied, Expression (40) can be satisfied.
[0049]
Further, in the multi-beam scanning apparatus in which a plurality of light beams are incident on the polygon mirror via a single incident optical system as in the third embodiment, there is a relationship of Expression (130).
[0050]
[Expression 21]
In the third embodiment, as shown in the equation (130), the focal length fco of the collimator lens 2 in the main scanning direction is extended, and the angle α formed when the two light beams A and B are incident on the stop 3b is reduced to reduce the manufacturing error and the assembly error. The sensitivity to the change amount of the line pitch interval when this occurs is further reduced from the first embodiment.
[0051]
[Embodiment 4]
FIG. 16 is a cross-sectional view in the main scanning direction of the multi-beam scanning device according to the fourth embodiment.
[0052]
Reference numeral 1 denotes a semiconductor laser array as light source means. Two light beams emitted from the semiconductor laser array 1 are converted into parallel light beams by a collimator lens 2 and pass through a diaphragm 3 that determines the light beam width in the main scanning direction and the sub scanning direction. Thus, a cylindrical lens 4 having a positive refractive power only in the sub-scanning direction forms a line image that is long in the main scanning direction. Then, the two light beams emitted from the semiconductor laser array are reflected and deflected by the polygon mirror 5 which is a deflecting means arranged so that the deflecting and reflecting surface 5a is in the vicinity of the line image, and the two fθ lenses 6a and 6b. The light is guided onto the photosensitive drum surface 7 which is a surface to be scanned through the imaging optical system 6 configured. As the polygon mirror 5 rotates in the direction of the arrow in the figure, the surface to be scanned 7 is optically scanned.
[0053]
FIG. 17 is a table showing a specific example of the multi-beam scanning apparatus according to the fourth embodiment. The semiconductor laser array 1 is adjusted so that the interval in the sub-scanning direction when the two light beams A and B emitted from the respective light emitting points of the semiconductor laser array 1 reach the surface to be scanned 7 coincides with the interval corresponding to the pixel density. It is adjusted by rotating 2.97 degrees. At this time, the two light emitting points are arranged at intervals of 2.33 μm in the sub-scanning direction, and at the same time, at intervals of 89.879 μm in the main scanning direction as shown in this table.
[0054]
FIG. 18 is a graph showing a difference in incident angle at a position where the two light beams A and B deflected by the polygon mirror 5 are incident on the surface of the fθ lens 6b on the scanning surface 7 side.
[0055]
FIG. 19 shows a case where the surface on the scanned surface 7 side of the fθ lens 6b is shifted by +10 μm in parallel in the sub-scanning direction due to manufacturing errors and the fθ lens 6a is shifted by −10 μm in parallel due to assembly errors in the conventional example and the fourth embodiment. It is a graph which shows the variation | change_quantity of a line pitch space | interval.
[0056]
In Embodiment 4, the difference between the incident angles of the two light beams A and B is 0 at the maximum because the generatrix shape of the surface on the scanned surface 7 side of the fθ lens 6b having the strongest refractive power in the sub-scanning direction is concentric. The line spacing change amount is reduced to −1.19 μm (5.6%) while keeping it as small as .0023 (rad).
[0057]
When the rate of change τ of the imaging lens bus slope is reduced, the sensitivity of the change amount of the line pitch interval when a manufacturing error and an assembly error occur can be reduced.
[0058]
Equation (140) is satisfied when Δθi is the difference in incident angle at the position where the two light beams A and B are incident on the fθ lens surface when they are formed at the same image height in the main scanning direction on the scanned surface 7. Thus, since the bus shape is concentric, the sensitivity to the change amount of the line pitch interval when the manufacturing error and the assembly error occur is reduced.
[0059]
[Expression 22]
Although the manufacturing error and the assembly error are replaced with the parallel shift, the change amount of the line pitch interval can be reduced by the above-described configuration with respect to the inclination.
[0060]
【The invention's effect】
According to the present invention described above, the multi-beam scanning is performed so that the relative positional relationship in which the multi-beam is scanned on the surface to be scanned is not destroyed even if the parallel shift and tilt of the lens and the lens surface occur due to the manufacturing error and the assembly error. By configuring the apparatus, it is possible to provide a multi-beam scanning apparatus capable of always obtaining a good image without pitch unevenness and an image forming apparatus using the same.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view in the main scanning direction of the multi-beam scanning apparatus according to the first embodiment of the present invention. FIG. 2 is a table showing a specific example of the multi-beam scanning apparatus according to the first embodiment. FIG. 4 is a formula showing a bus corresponding to the main scanning direction. FIG. 5 is a formula showing a bus corresponding to the sub-scanning direction. FIG. 6 is a cross-sectional view of the incident optical system of Embodiment 1 in the main scanning direction. 7 is a cross-sectional view in the main scanning direction of the imaging optical system according to the first embodiment. FIG. 8 is a table showing angles formed by the two light beams according to the first embodiment. FIG. 10 is a table showing the relationship between the sensitivity of an optical component and the amount of change in line pitch in the first embodiment. FIG. 11 is a main scan of the multi-beam scanning apparatus according to the second embodiment of the present invention. Cross-sectional view of direction [FIG. 12] formed by the two light beams of the second embodiment FIG. 13 is a graph showing the relationship between the manufacturing / assembly error and the line pitch change amount in the second embodiment. FIG. 14 is a table showing the angle formed by the two light beams in the third embodiment. FIG. 16 is a cross-sectional view in the main scanning direction of the multi-beam scanning device according to the fourth embodiment. FIG. 17 is a cross-sectional view of the multi-beam scanning device according to the fourth embodiment. Table showing a specific example. FIG. 18 is a graph showing the relationship between the difference in angle between the two light beams of the fourth embodiment and the image height. FIG. 19 shows the relationship between the manufacturing / assembly error and the line pitch variation in the fourth embodiment. FIG. 20 is a graph showing the relationship. FIG. 20 is a cross section in the main scanning direction of a conventional multi-beam scanning apparatus.
1 Semiconductor laser array 2 Condensing lens (collimator lens)
3 Diaphragm 3a Diaphragm 3b for determining the beam width in the sub-scanning direction Diaphragm 4 for determining the beam width in the main scanning direction 4 Cylindrical lens 5 Deflection means (5a: deflection reflection surface)
6 Imaging optical system (fθ lens)
7 Photosensitive drum surface

Claims (4)

A light source means having a plurality of light emitting points arranged at intervals in both the main scanning direction and the sub-scanning direction; a deflecting means for deflecting and scanning a plurality of light beams emitted from the plurality of light emitting points; An incident optical system that guides a plurality of light beams emitted from a light emitting point to the deflecting unit; an imaging optical system that forms an image on the scanned surface by deflecting and scanning a plurality of light beams deflected by the deflection surface of the deflecting unit; A multi-beam scanning optical system comprising:
The incident optical system includes, in order from the light source means, a condensing lens that collects each of the plurality of light beams emitted from the plurality of light emission points, and each of the plurality of light beams emitted from the plurality of light emission points. A diaphragm for determining the beam width in the main scanning direction,
An arbitrary light beam among the plurality of light beams is defined as a first light beam MBA, adjacent to the first light beam MBA in the main scanning direction, and at the same position in the main scanning direction on the surface to be scanned and the first light beam MBA. When the reaching light beam is defined as the second light beam MBB, the first image forming optical element disposed at a position closest to the deflecting means constituting the image forming optical system through which the first light beam MBA passes is provided. The imaging magnification in the sub-scanning direction is β1a, and is located behind the first imaging optical element disposed at a position closest to the deflecting means constituting the imaging optical system through which the first light beam MBA passes. An imaging magnification in the sub-scanning direction of the optical system is β1′a, and the first imaging optical element disposed at a position closest to the deflecting means constituting the imaging optical system through which the second light beam MBB passes. The imaging magnification in the sub-scanning direction is β1b, and the second light The imaging magnification in the sub-scanning direction of the optical system on the rear side of the first imaging optical element arranged closest to the deflecting means constituting the imaging optical system through which the bundle MBB passes is β1′b , The pixel density A (dpi) in the sub-scanning direction on the surface to be scanned, the optical path length from the stop to the deflection surface of the deflection means, Lap (mm), and the distance in the main scanning direction of the light source means to dms (mm) ), When the focal length of the condenser lens in the main scanning direction is fco (mm), and the fθ coefficient of the imaging optical system is k (mm / rad) ,
Multibeam scanning optical system characterized by satisfying The imaging optical system includes, in order from the deflection unit side, the first imaging optical element and a second imaging optical element having a power in the sub-scanning direction larger than that of the first imaging optical element. multi-beam scanning optical system according to claim 1 that. The multi-beam scanning optical system according to claim 2, wherein a bus bar shape of the second imaging optical element is concentric in a main scanning section. A laser beam printer having a multi-beam scanning optical system according to any one of claims 1 to 3.