JP5098491B2 - Optical scanning device - Google Patents

Description

  The present invention relates to an optical scanning device, and more particularly to an optical scanning device that scans a plurality of scanned surfaces using a single deflector with a plurality of light beams modulated based on image data.

  In recent years, in an image forming apparatus such as a full-color copying machine or printer, four photoconductors are juxtaposed corresponding to each color of Y (yellow), M (magenta), C (cyan), and K (black). The tandem method in which images of the respective colors formed on the respective photoconductors are transferred to an intermediate transfer belt and synthesized is the mainstream. In this type of tandem image forming apparatus, for example, there is an optical scanning device that draws an image by simultaneously scanning four light beams using a single deflector (polygon mirror) on each photoconductor. It is installed.

  As this type of optical scanning device, after separating four light beams into the same reflecting surface of the deflector and separating the optical path by a common lens through which the four light beams pass after deflection and a mirror in Patent Documents 1 and 2. There has been proposed a technique for printing a color image at a high speed by providing individual lenses through which each of the four light beams passes to simultaneously irradiate the photoconductors corresponding to four colors with the light beams.

  However, in the optical scanning device, when the four light beams are condensed at the same height in the sub-scanning direction on the deflector, the optical paths of the respective light beams are combined in front of the deflector and again after being deflected by the deflector. Since it is necessary to separate the optical paths from each other at the separation location, the inclination of the light beam incident on the deflector in the sub-scanning direction (hereinafter referred to as the incident angle in the sub-scanning direction) increases, and the deflector is troublesome. There is a problem in that the image quality deteriorates due to jitter caused by this and uneven pitch due to the error in the distance between the axial surfaces of the deflectors.

On the other hand, if the four light beams incident on the deflector are set parallel to each other, jitter due to tilting of the deflector and uneven pitch due to the distance error between the axial surfaces of the deflector will not occur, but the deflector must be enlarged. Is necessary and has a problem incurring cost increase. Also, in order to facilitate separation of each optical path after deflection by the deflector while concentrating the four light beams on the deflector at the same height in the sub-scan direction, the incident angles in the sub-scan direction are set at unequal intervals. If the angle difference between the light flux separating the optical path on the side closer to the deflector and the light flux adjacent to the deflector is increased, the lens is manufactured by optimizing each individual lens. As a result, there is a problem that the number of molds for use must be increased and the initial investment increases, and if the number of molds is decreased, the scanning line curvature and imaging performance are degraded compared to the case of individually optimizing the lens. However, the image quality deteriorates.
JP 2003-75751 A JP 2004-70190 A

  SUMMARY OF THE INVENTION An object of the present invention is to provide an optical scanning device that increases the degree of freedom of arrangement of optical elements such as lenses, is inexpensive, and has good performance.

In order to achieve the above object, an optical scanning device according to the present invention includes:
With four light sources,
A deflector having a reflecting surface for simultaneously reflecting the light beams emitted from the four light sources on the same surface;
A first mirror that reflects two of the four light beams deflected by the deflector;
A second mirror that reflects one of the two light beams reflected by the first mirror;
A third mirror for reflecting one of the two light beams not reflected by the first mirror;
A first lens through which the light beam reflected by the first mirror and reflected by the second mirror is transmitted;
A second lens that transmits a light beam reflected by the first mirror but not reflected by the second mirror;
A third lens that transmits the light beam reflected by the third mirror without being reflected by the first mirror;
A fourth lens through which a light beam not reflected by the first mirror but not reflected by the third mirror passes;
With
The deflector rotates around one axis, and its reflecting surface is flat and parallel to the rotation center axis.
Two of the light beam reflected by the first mirror, the are different from one another rather than an angle of 0 ° which forms a plane perpendicular to the rotational center axis when entering the deflector and the angle is perpendicular the Either upward or downward relative to the plane ,
The absolute value of the angle formed by one of the two light beams not reflected by the first mirror and the plane perpendicular to the rotation center axis when entering the deflector is reflected by the first mirror. Of the two light beams, the absolute value of the angle is equal to the angle of the large light beam, and the angles are opposite to each other with respect to the vertical plane, and the chief rays intersect after being deflected by the deflector,
The other one of the two light beams not reflected by the first mirror reflects the absolute value of the angle formed by a plane perpendicular to the rotation center axis when entering the deflector by the first mirror. Of the two luminous fluxes, the absolute value of the angle is equal to that of the small luminous flux, and the angles are opposite to each other with respect to the vertical plane, and the chief rays intersect before being deflected by the deflector To do,
It is characterized by.

  That is, in the optical scanning device according to the present invention, of the four light beams, two light beams having a large incident angle in the sub-scanning direction are crossed after being deflected by the deflector, and two light beams having a small incident angle in the sub-scanning direction are intersected. Are crossed in front of the deflector, and the first mirror reflects the former one light beam and the latter one light beam, so that a large margin for optical path separation by the first mirror is set. can do. Therefore, a sufficient optical path separation margin can be secured even if the maximum value of the sub-scanning direction incident angles of the four light beams is set smaller than when the sub-scanning direction incident angles are equally spaced. Jitter and pitch unevenness due to the incident angle in the scanning direction can be reduced. In addition, since the absolute values of the incident angles in the sub-scanning direction are two types, only two types of individual lenses are required for each of the four optical paths, and the four individual lenses are arranged at unequal intervals in the sub-scanning direction. Compared to the case where each is optimized, the types of lens working molds can be reduced, and the cost can be reduced.

  Hereinafter, embodiments of an optical scanning device according to the present invention will be described with reference to the accompanying drawings.

(Refer 1st Example and FIGS. 1-7)
As for the first embodiment of the optical scanning device according to the present invention, FIG. 1 shows a three-dimensional arrangement relationship, and FIG. 2 shows an optical path in a sub-scanning direction section from the polygon mirror 5 to each of the photosensitive drums 13a to 13d.

  This optical scanning device is configured as an exposure unit of an image forming apparatus using tandem electrophotography, and forms images of respective colors (yellow, magenta, cyan, and black) on four photosensitive drums 13a to 13d. It is configured as follows. The four-color images (electrostatic latent images) formed on the photoconductive drums 13a to 13d are developed with toner, and then primary-transferred / combined on an intermediate transfer belt (not shown), on the recording material. Secondary transferred. This type of image forming process is well known and will not be described.

  In this optical scanning device, the light source section is composed of four laser diodes 1a to 1d and collimator lenses 2a to 2d. The light beams (diffused light) emitted from the laser diodes 1a to 1d are converted into parallel light by the collimator lenses 2a to 2d and reflected by the synthesis mirror 3 so that the traveling directions of the light beams are the same in the main scanning direction Y. Thereafter, the light is converged only in the sub-scanning direction Z by the cylindrical lens 4 and is incident on the same reflecting surface of the polygon mirror 5. The polygon mirror 5 is rotationally driven by a motor (not shown) around the central axis 5a, and the reflecting surface composed of six surfaces is parallel to the central axis 5a.

  Regarding the traveling direction of the light flux from the polygon mirror 5, the first lens 6, the first mirror 7, the second and third lenses 8a and 8b, the second and third mirrors 9a and 9b, the fourth to seventh. Lenses 10a to 10d, fourth to seventh mirrors 11a to 11d, and dust-proof window glasses 12a to 12d are arranged.

  The four light beams simultaneously deflected by the same reflecting surface of the polygon mirror 5 are refracted by the first lens 6 and then separated by the first mirror 7 into the light beams Ba and Bb and the light beams Bc and Bd. The light beams Ba and Bb are folded back by the first mirror 7 and refracted by the second lens 8a. Further, the light beams Bc and Bd that are not folded back by the first mirror 7 are refracted by the third lens 8b. The second lens 8a and the third lens 8b have the same shape.

  Of the light beams Ba and Bb transmitted through the second lens 8a, the light beam Ba is folded back by the second mirror 9a and separated into the light beams Ba and Bb. Of the light beams Bc and Bd transmitted through the third lens 8b, the light beam Bc is folded back by the third mirror 9b and separated into the light beams Bc and Bd.

  Thereafter, the light beams Ba, Bb, Bc, and Bd are transmitted through the fourth to seventh lenses 10a to 10d individually arranged in the respective optical paths, and are turned back by the fourth to seventh mirrors 11a to 11d to be displayed on the window. Images are formed on the photosensitive drums 13a to 13d through the glasses 12a to 12d, and scanned in the main scanning direction Y. The optical surfaces of the fourth to seventh lenses 10a to 10d all have the same shape.

  In addition, since the suffixes a to d of the respective symbols correspond to those attached to the laser diodes 1 a to 1 d and the collimator lenses 2 a to 2 d and those attached to the individual lenses 10 a to 10 d and thereafter, a to a on the light source side. Although d is arranged in order, the arrangement of a and b is switched on the photosensitive drums 13a to 13d side.

  FIG. 3 shows only the principal rays of the light beams Ba, Bb, Bc, and Bd by enlarging the optical path from the polygon mirror 5 to the first mirror 7. Each light beam Ba, Bb, Bc, Bd is condensed only in the sub-scanning direction Z in the vicinity of the reflection surface of the polygon mirror 5. At the condensing position, the two light beams Ba, Bc and Bb, Bd have the same height in the sub-scanning direction Z, and the incident angles in the sub-scanning direction are opposite. Focusing on the principal ray, the light beams Bb and Bc are 1.0 ° and the light beams Ba and Bd are 2.6 ° with respect to the plane perpendicular to the rotation center axis 5a.

  In the case of an optical scanning device that supports the tandem method, the thickness of the polygon mirror 5 can be reduced by reducing the thickness of the polygon mirror 5 by aligning the height at which each light beam is incident on the polygon mirror 5, and the cost of the mirror 5 itself and the drive motor can be reduced. .

  FIG. 4 shows the optical path in the sub-scanning direction Z in the first embodiment, from the combining mirror 3 to the polygon mirror 5. The composite mirror 3 has a planar reflection surface, and each light beam has a different angle in the sub-scanning direction Z.

  FIG. 5 shows only the principal rays of the light beams Ba, Bb, Bc and Bd by enlarging the vicinity of the reflection surface of the polygon mirror 5. The respective light beams are substantially equally spaced in the sub-scanning direction Z on the composite mirror 3. The condensing position of each light beam differs depending on the cylindrical lens 4 because the combining mirror 3 is slightly inclined in the sub-scanning direction Z.

  In the first embodiment described above, the construction data of each optical element constituting each optical path is shown in Tables 1 to 4 shown below. It can be seen that the composite mirror 3 has an inclination in the sub-scanning direction Z because the Z component of the X vector is not zero. The focal length of the cylindrical lens 4 in the sub-scanning direction Z is 90 mm, and the condensing height is separated by 0.4 mm on the polygon mirror 5. For this reason, the inclination angle of the incident light on the cylindrical lens 4 in the sub-scanning direction Z is ± 0.2 / 90 (unit is radians).

  Further, when the incident light on the combining mirror 3 is horizontal, the inclination angle of the combining mirror 3 is α for the main scanning direction Y and the angle between the incident light and the mirror surface normal line, and the outgoing light for the sub-scanning direction Z. When the inclination angle is β, it can be expressed by β / (2cos α). Although it is difficult to understand in FIG. 4 because the inclination angle is small, the light beams Bd and Bb are inclined upward and the light beams Bc and Ba are inclined downward after being reflected by the composite mirror 3. On the polygon mirror 5, two upwards are concentrated on the upper side and two downwards are concentrated on the lower side.

  It is possible to obtain the same effect by arranging one cylindrical lens after each collimator lens 2a to 2d and then arranging the combining mirror 3 after that. However, as in the first embodiment, the number of components can be reduced by providing the common cylindrical lens 4 at the subsequent stage of the combining mirror 3.

  Further, as shown in FIG. 5, the two light beams Bb and Bc having a small incident angle in the sub-scanning direction intersect in the sub-scanning direction Z before entering the polygon mirror 5. As shown in FIG. 3, the two light beams Ba and Bd having a large incident angle in the sub-scanning direction intersect in the sub-scanning direction Z after being reflected by the polygon mirror 5.

  With this configuration, the light beams that are substantially equally spaced in the sub-scanning direction Z on the composite mirror 3 are converted into two upper light beams Ba and Bb and two lower light beams on the first mirror 7. There is a slight gap between Bc and Bd. On the synthesis mirror 3, it is advantageous that the four light beams have to be synthesized, so that they are equidistant. Since only the upper two light beams have to be reflected on the first mirror 7, it is advantageous that the upper light beams Ba and Bb and the lower light beams Bc and Bd are open.

  Assuming that four light beams are condensed at the same height on the polygon mirror 5, if the incident angles in the sub-scanning direction are made the same, the separation margin at the first mirror 7 becomes small. Conversely, if the same separation margin is set, the incident angle in the sub-scanning direction must be increased. It has been conventionally known that when the incident angle in the sub-scanning direction increases, the pitch unevenness due to the distance error between the axes of the polygon mirror 5 and the jitter due to the surface tilt increase and the image quality deteriorates.

  Also, when all four light beams are configured to intersect the sub-scanning direction Z before the polygon mirror 5, if the same separation margin is set in the first mirror 7, the incident angle in the sub-scanning direction is changed. Otherwise, the margin on the composite mirror 3 is reduced between the upper light beams Ba and Bb and the lower light beams Bc and Bd. On the contrary, if the interval on the composite mirror 3 is made the same, the incident angle in the sub-scanning direction becomes large and the above-mentioned problem occurs, and the difference in the sub-scanning direction Z on the polygon mirror 5 increases. As a result, it is necessary to increase the thickness of the polygon mirror 5, resulting in an increase in cost. In order to reduce the thickness of the polygon mirror 5 as much as possible, as described above, at the position where the light is condensed in the sub-scanning direction Z in the vicinity of the reflection surface of the polygon mirror 5, the height of the two upper and lower light beams in the sub-scanning direction Z is increased. It is desirable that the lengths are substantially matched.

  FIG. 6 shows a light beam passage range on the first mirror 7. The passage range is long in the main scanning direction Y because the polygon mirror 5 is rotated by the image forming range. The upper light beams Ba and Bb and the lower light beams Bc and Bd are open, but the light beams Ba and Bb and the light beams Bc and Bd are connected to each other.

  FIG. 7 shows a passing range of the light beams Bc and Bd that are not reflected by the first mirror 7 on the third mirror 9b. Since the passing ranges of the two light beams Bc and Bd are separated on the third mirror 9b, it is possible to separate them. As apparent from FIG. 3, the light beams Bc and Bd intersect between the polygon mirror 5 and the first lens 6 in the sub-scanning direction Z. In the first embodiment, the light beams Bb and Bc having a small incident angle in the sub-scanning direction are intended to make light beam separation advantageous by opening as much as possible on the first mirror 7. Therefore, it is preferable that the light beams Ba and Bd having a large incident angle in the sub-scanning direction be emitted outside before the first mirror 7 and intersect the light beams Bb and Bc having a small incident angle in the sub-scanning direction.

Table 5 shows free-form surface coefficient data of the entrance surface of the first lens 6, and Table 6 shows free-form surface coefficient data of the exit surface of the first lens 6. Further, Table 7 shows free-form surface coefficient data of the exit surfaces of the second and third lenses 8a and 8b, and Table 8 shows free-form surface coefficient data of the exit surfaces of the fourth to seventh lenses 10a to 10d. In these tables, ^En is ^x10 -n.

  The first lens 6 has a free-form surface for both the entrance surface and the exit surface, and the other lenses have a flat entrance surface and a free-form exit surface. These free-form surfaces are calculated by the free-form surface equation shown in Equation (1). In addition, a coefficient not marked in each table is 0.

  By the way, the first lens 6, the second and third lenses 8a and 8b, and the fourth to seventh lenses 10a to 10d arranged at the rear stage of the polygon mirror 5 are all made of resin, and are refracted at the wavelength used. The rate is 1.572. The refractive index at the used wavelength of the window glasses 12a to 12d is 1.511.

  The first lens 6 uses only even-order terms in the sub-scanning direction Z and has a symmetrical shape with respect to the sub-scanning direction Z. The symmetry plane is perpendicular to the rotation center axis 5a of the polygon mirror 5, and the intersection of the two light beams Bb and Bc having a small incident angle in the sub-scanning direction shown in FIG. The intersection of the two light beams Ba and Bb having a large incident angle in the sub-scanning direction and the height in the sub-scanning direction Z are the same. Accordingly, the upper light beams Ba and Bb and the lower light beams Bc and Bd at the rear stage of the first lens 6 are also symmetric with respect to the symmetry plane. After the upper light beams Ba and Bb are reflected by the first mirror 7, the light beams Ba and Bb and the light beams Bc and Bd are optically equivalent.

  The second and third lenses 8a and 8b have the same shape as described above, and have the same shape including not only the optical surface but also the outer shape. Therefore, a lens manufactured with the same mold can be used, and the performance difference can be suppressed small.

  The fourth to seventh lenses 10a to 10d have the same optical surface, but the outer shapes of the fourth lens 10a and the sixth lens 10c are the same, and the fifth lens 10b and the seventh lens 10c. The lens 10d is the same, and the front two and the rear two are shifted in the sub-scanning direction Z.

(Refer 2nd Example and FIGS. 8-14)
With respect to the second embodiment of the optical scanning device according to the present invention, FIG. 8 shows a three-dimensional arrangement relationship, and FIG. 9 shows an optical path in the sub-scanning direction from the polygon mirror 5 to each of the photosensitive drums 13a to 13d.

  This optical scanning apparatus is the same as the first embodiment in that it is configured as an exposure unit of an image forming apparatus based on tandem electrophotography, and is intended to reduce the number of optical components. Therefore, the difference from the first embodiment is that the free-form mirror array 14 is used in place of the synthesis mirror 3 and the cylindrical lens 4 and that the second and third lenses 8a and 8b are omitted. . Further, as will be described later, the arrangement and the number of mirrors are devised, so that the reflection mode at the third mirror 9b is different from that in the first embodiment, and the photosensitive drums 13c and 13d are opposite to those in the first embodiment. Is arranged.

  FIG. 10 shows only the principal rays of the light beams Ba, Bb, Bc, and Bd by enlarging the optical path from the polygon mirror 5 to the first mirror 7, and corresponds to FIG.

  FIG. 11 shows the optical path in the sub-scanning direction Z in the second embodiment, from the free-form surface mirror array 14 to the polygon mirror 5, and corresponds to FIG.

  FIG. 12 is an enlarged view of the vicinity of the reflecting surface of the polygon mirror 5, showing only the principal rays of the light beams Ba, Bb, Bc, and Bd, and corresponds to FIG.

  FIG. 13 shows a light beam passage range on the first mirror 7. FIG. 14 shows a passing range of the light beams Bc and Bd that are not reflected by the first mirror 7 on the third mirror 9b. These correspond to FIGS. 6 and 7, respectively.

  Next, in the second embodiment, construction data of each optical element constituting each optical path is shown in Tables 9 to 12 shown below. Referring to the respective coordinates of the free-form mirror array 14, the coordinate value in the sub-scanning direction Z is 0.2 or −0.2, which is the height of the focal line in the sub-scanning direction Z by the respective reflecting surfaces. It shows.

  The optical path after passing through each collimator lens 2a-2d is horizontal, and the height of each reflecting surface of the free-form mirror array 14 is also equal to the height of each collimator lens 2a-2d.

  Table 13 shows free-form surface coefficient data of the entrance surface of the first lens 6, and Table 14 shows free-form surface coefficient data of the exit surface of the first lens 6. Further, Table 15 shows free-form surface coefficient data of the exit surfaces of the fourth and seventh lenses 10a and 10d, and Table 16 shows free-form surface coefficient data of the exit surfaces of the fifth and sixth lenses 10b and 10c.

  The first lens 6 has a free-form surface for both the entrance surface and the exit surface, and the other lenses have a flat entrance surface and a free-form exit surface. In the free-form surface of the first lens 6, only a zero-order term is used for Z on both surfaces. The fact that the term of the first order or higher is not used indicates that the cross section of the surface shape in the sub-scanning direction Z is a straight line. Also in the second embodiment, the upper two light beams Ba and Bb and the lower two light beams Bc and Bd are symmetrical in the subsequent stage of the first lens 6, and the upper light beams Ba and Bb are symmetric by the first mirror 7. After being reflected, the light beams Ba and Bb and the light beams Bc and Bd are optically equivalent.

  In the free-form surfaces of the fourth to seventh lenses 10a to 10d, zero-order, first-order, and second-order Z are used. By using the Z1 order term, the scanning line curvature and the wavefront twist caused by the incident angle in the sub-scanning direction are corrected. When the incident angles in the sub-scanning direction are different, the generation amounts of these aberrations are different, so that it is necessary to use different lenses.

  In the second embodiment, the arrangement and number of mirrors are devised. That is, for the two light beams Bb and Bd and the light beams Ba and Bc having the same absolute value of the incident angle in the sub-scanning direction, the former is an odd number and the latter is an even number, thereby the fourth lens 10a and the seventh lens 10d. The light flux before incidence is made equivalent, and the light flux before incidence is made equivalent for the fifth lens 10b and the sixth lens 10c. Therefore, the fourth lens 10a and the seventh lens 10d have the same shape, and the fifth lens 10b and the sixth lens 10c have the same shape. When the incident angles in the sub-scanning direction are different for each of the four light beams, two types are sufficient as compared to the need for four types of lenses, so that high precision can be achieved with a small amount of mold investment.

(Other examples)
The optical scanning device according to the present invention is not limited to the above-described embodiments, and can be variously modified within the scope of the gist thereof.

1 is a three-dimensional layout diagram illustrating a first embodiment of an optical scanning device according to the present invention. It is sectional drawing of the subscanning direction which shows the optical path structure from the polygon mirror in 1st Example to the photoreceptor drum. It is an optical path figure from the polygon mirror in the 1st example to the 1st mirror. It is an optical path figure from the synthetic | combination mirror in 1st Example to a polygon mirror. It is an optical path figure of the reflective surface vicinity of the polygon mirror in 1st Example. It is explanatory drawing which shows the light beam passage range on the 1st mirror in 1st Example. It is explanatory drawing which shows the light beam passage range on the 3rd mirror in 1st Example. It is a three-dimensional arrangement | positioning figure which shows 2nd Example of the optical scanning device based on this invention. It is sectional drawing of the subscanning direction which shows the optical path structure from the polygon mirror in 2nd Example to the photoconductive drum. It is an optical path diagram from the polygon mirror in the 2nd example to the 1st mirror. It is an optical path figure from the free-form surface mirror array in a 2nd Example to a polygon mirror. It is an optical path figure of the reflective surface vicinity of the polygon mirror in 2nd Example. It is explanatory drawing which shows the light beam passage range on the 1st mirror in 2nd Example. It is explanatory drawing which shows the light beam passage range on the 3rd mirror in 2nd Example.

Explanation of symbols

1a to 1d Laser diode 5 Polygon mirror (deflector)
6 ... 1st lens 7 ... 1st mirror 8a ... 2nd lens 8b ... 3rd lens 9a ... 2nd mirror 9b ... 3rd mirror 10a-10d ... 4th-7th lens 11a-11d ... fourth to seventh mirrors 13a to 13d ... photosensitive drum

Claims (8)

With four light sources,
A deflector having a reflecting surface for simultaneously reflecting the light beams emitted from the four light sources on the same surface;
A first mirror that reflects two of the four light beams deflected by the deflector;
A second mirror that reflects one of the two light beams reflected by the first mirror;
A third mirror for reflecting one of the two light beams not reflected by the first mirror;
A first lens through which the light beam reflected by the first mirror and reflected by the second mirror is transmitted;
A second lens that transmits a light beam reflected by the first mirror but not reflected by the second mirror;
A third lens that transmits the light beam reflected by the third mirror without being reflected by the first mirror;
A fourth lens through which a light beam not reflected by the first mirror but not reflected by the third mirror passes;
With
The deflector rotates around one axis, and its reflecting surface is flat and parallel to the rotation center axis.
Two of the light beam reflected by the first mirror, the are different from one another rather than an angle of 0 ° which forms a plane perpendicular to the rotational center axis when entering the deflector and the angle is perpendicular the Either upward or downward relative to the plane ,
The absolute value of the angle formed by one of the two light beams not reflected by the first mirror and the plane perpendicular to the rotation center axis when entering the deflector is reflected by the first mirror. Of the two light beams, the absolute value of the angle is equal to the angle of the large light beam, and the angles are opposite to each other with respect to the vertical plane, and the chief rays intersect after being deflected by the deflector,
The other one of the two light beams not reflected by the first mirror reflects the absolute value of the angle formed by a plane perpendicular to the rotation center axis when entering the deflector by the first mirror. Of the two luminous fluxes, the absolute value of the angle is equal to that of the small luminous flux, and the angles are opposite to each other with respect to the vertical plane, and the chief rays intersect before being deflected by the deflector To do,
An optical scanning device characterized by the above. Any one of the luminous fluxes has an absolute value of an angle different from a plane perpendicular to the rotation center axis when entering the deflector, and the angle is opposite to the vertical plane. The optical scanning device according to claim 1, wherein the light beam and the height in the sub-scanning direction incident on the deflector are equal.   3. The optical scanning device according to claim 1, wherein the principal rays of the two light beams reflected by the first mirror intersect before being reflected by the first mirror.   A fifth lens through which the four light beams pass is disposed in the optical path between the deflector and the first mirror, and the fifth lens has power in a plane including the rotation center axis of the deflector. 4. The optical scanning device according to claim 1, wherein the optical scanning device is not provided.   A fifth lens through which the four light beams pass is disposed in the optical path between the deflector and the first mirror, and the fifth lens is a plane perpendicular to the rotation center axis of the deflector. It has a symmetrical shape with respect to one, and the plane of symmetry includes two intersections of two sets of light beams having the same absolute value of the angle formed with the plane when entering the deflector. The optical scanning device according to claim 1. A sixth lens is disposed in the optical path after the first mirror of the two light fluxes reflected by the first mirror;
A seventh lens is disposed in the optical path after passing through the first mirror of the two light beams not reflected by the first mirror;
The sixth lens and the seventh lens have the same shape;
The optical scanning device according to claim 1, wherein:   7. The optical scanning device according to claim 1, wherein the four light beams incident on the deflector pass through an eighth lens having a cylindrical surface and a flat surface.   4. The four light beams incident on the deflector are respectively collected in a linear shape by individual free curved surface reflecting surfaces, and the four free curved surface reflecting surfaces are integrally formed. The optical scanning device according to claim 6.

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