JP2020179529A - Image forming device - Google Patents

Abstract

To provide an image forming device that has both-side telecentricity and uniform magnification while keeping an imaging state, where luminous point groups do not exist on the same plane.SOLUTION: A imaging magnification of an imaging optics is set to -1 and conjugate lengths of the imaging optics are different at positions in a sub direction and increase or decrease monotonically depending on the positions in the sub direction. First and second lenses have two lens surfaces having curve surfaces and the two lens surfaces have two common symmetrical surfaces, where at least one lens surface is non-axisymmetric with a symmetric line in which the two common symmetric surfaces cross each other. The lens surface closer to a diaphragm and the outer lens surface away from the diaphragm are in identical shapes which turn over from each other with respect to a light advancing direction of main light ray and have equal core diameters. A center of the diaphragm is arranged on a principal point interval line connecting main points close to the diaphragm to each other, the symmetric line has an angle which is not zero with respect to the principal point interval line, and the symmetric lines of the lenses are parallel to each other. A center of a luminous point group is arranged so as not to be arranged on a straight line passing on the outer principal point close to the luminous point group of the first lens and extending in parallel to the principal point interval line.SELECTED DRAWING: Figure 2

Description

The present invention relates to an image forming apparatus having an optical writing unit, and more particularly to an image forming apparatus including an imaging optical system for forming an image of a light emitting point group on a light receiving surface.

Conventionally, an optical book including a light emitting substrate in which a plurality of light emitting point groups composed of a plurality of light emitting points are arranged in a main direction and a plurality of sub directions, and a lens array in which imaging lenses are arranged one-to-one with each light emitting point group An image forming apparatus having a recess is known. In the optical writing unit of such an image forming apparatus, the light emitted from the light emitting point is set as a desired beam spot at a desired position on the photoconductor by interposing an imaging lens.

As an image forming apparatus, for example, as described in Patent Document 1 below, a plurality of imaging lenses whose optical axes are parallel to each other are used to emit light from a plurality of light emitting point groups corresponding to the plurality of imaging lenses. There is a technique for forming an image and drawing. At this time, if a plurality of light emitting point groups are arranged on the same glass substrate, it is easy to obtain relative position accuracy, while if a drive circuit or the like is arranged on the same glass substrate, the drive circuit or the like is placed on the glass substrate. The area occupied may increase and the unit may become large.

In order to reduce the size of the unit, it is conceivable that the light emitting point groups are not arranged on the same glass substrate but are arranged so that the positions of the light emitting point groups in the optical axis direction are different. In this case, each light emitting point group is arranged. Since the distances from the group to the lens array are different, it is difficult to achieve bilateral telecentricity and uniform magnification of the optical system while maintaining the image formation state.

Japanese Unexamined Patent Publication No. 2009-51194

The present invention has been made in view of the above background technology, and includes an optical writing unit capable of achieving bilateral telecentricity and uniform magnification while maintaining an image formation state when light emitting point groups are not on the same plane. It is an object of the present invention to provide an image forming apparatus.

In order to solve the above problems, the image forming apparatus according to the present invention has a photoconductor having a surface transported in a sub-direction substantially orthogonal to the main direction, and a light emitting substrate having a plurality of light emitting point groups arranged in two dimensions. And a plurality of imaging optical systems for forming an image of light from a plurality of light emitting point groups at different positions on the photoconductor, the imaging magnification of the imaging optical system is -1, and the rotation of the photoconductor is provided. When viewed from the direction of the axis, the conjugate lengths of the plurality of imaging optical systems differ depending on the position in the sub-direction, and monotonically increase or decrease depending on the position in the sub-direction. Each imaging optical system has a first lens, an aperture, and a second lens in order from the light emitting point group side, and the first and second lenses each have two lens surfaces having curved surfaces. The two lens planes have two common symmetric planes, and at least one of the two lens planes is axisymmetric with respect to the line of symmetry where the two common symmetric planes intersect, and the first and second lenses , The aperture side lens surface on the side closer to the aperture has the same shape inverted with respect to the light traveling direction of the principal light, and the outer lens surface on the side far from the aperture has the same shape inverted with respect to the light traveling direction, and the core thickness. Are equal, and in the first and second lenses, the center of the aperture is located on the principal point interval line connecting the principal points on the aperture side close to the aperture, and the angle at which the symmetry line is not zero with respect to the principal point interval line is set. The symmetric lines of the first and second lenses are parallel to each other, and the center of the emission point group is on a straight line extending parallel to the principal point interval through the outer principal point near the emission point group of the first lens. It is placed avoiding.

According to the image forming apparatus, by making the lens surfaces of the first and second lenses non-axially symmetric but free curved surfaces, the mismatch of the image planes is reduced and the position of the light emitting point group is set to the light emitting point group of the first lens. By setting appropriately so as to avoid a straight line that passes through the outer principal point on the side closer to the main point and extends parallel to the line between the principal points, the defect in the imaging state due to the free curved surface of the lens surface is eliminated and the image area A good imaging state can be obtained as a whole.

In one specific aspect of the present invention, in the image forming apparatus, the center of the light emitting point group is the symmetry of the first lens and the straight line extending parallel to the main point interval through the outer principal point of the first lens. Located between the lines. In this case, a good imaging state can be obtained by appropriately setting the position of the light emitting point group according to the tilt direction of the first lens.

In another aspect of the present invention, the lens surfaces of the first and second lenses are point-symmetric with respect to the center of the diaphragm when viewed from the direction of the rotation axis of the photoconductor. When there is only a diaphragm between the first and second lenses, the asymmetric aberration component can be removed by arranging the first and second lenses so as to be point-symmetric with respect to the center of the diaphragm.

In yet another aspect of the present invention, for a plurality of imaging optics, the first lens has resin lens portions on both sides of a single first lens substrate, and the second lens is a single first lens. 2 It has resin lens portions on both sides of the lens substrate. If a plurality of lens portions are formed on a single lens substrate and the positions of the lens substrates are adjusted and held, the configuration becomes simpler than in the case of arranging and holding the positions of the individual lens portions. Since the relative positions of the 1st and 2nd lenses are fixed, it is easy to obtain accuracy.

In yet another aspect of the present invention, the first and second lens substrates are arranged at an angle with respect to the line between the principal points of the first and second lenses. In this case, the first and second lenses of each imaging optical system can be easily arranged at predetermined positions having different sub-directions according to the conjugate length of each imaging optical system.

In yet another aspect of the present invention, the tilt direction of the normal of the first lens substrate and the tilt direction of the symmetric lines of the first and second lenses are different from each other with respect to the line between the main points of the first and second lenses. It is the same, and the inclination direction of the normal line of the second lens substrate and the inclination direction of the symmetric lines of the first and second lenses are the same. In this case, the tilting direction of the lens portions constituting the first and second lenses is the same as the tilting direction of the first and second lens substrates.

In yet another aspect of the present invention, the first lens substrate and the second lens substrate have different inclination angles with respect to the line between the principal points. Since the tilt angles of the first and second lens substrates differ depending on the position of the first and second lens substrates from the light emitting point group to the photoconductor, the first lens substrate and the second lens substrate are used. Are not placed in parallel.

In yet another aspect of the present invention, the tilt angle of the symmetric line with respect to the principal point interval is the tilt angle of the normal of the first lens substrate and the tilt angle of the normal of the second lens substrate with respect to the principal point interval. Between. By setting the tilt angle of the lens portions constituting the first and second lenses between the tilt angle of the first lens substrate and the tilt angle of the second lens substrate, the lens portions constituting the first and second lenses can be tilted. The difference in thickness can be reduced.

It is a partial cross-sectional view which shows the schematic structure of the image forming apparatus of an embodiment. It is sectional drawing explaining the structure of the optical print head which constitutes an image formation unit. (A) is a conceptual perspective view for explaining the structure of the light emitting element, (B) is a diagram for explaining a light emitting point group provided in the light emitting element, and (C) is a light emitting point group and a lens. It is a figure explaining the arrangement of. (A) is a conceptual diagram for explaining the optical system constituting the optical printhead of FIG. 2, and (B) is a conceptual diagram for explaining the optical system constituting the optical printhead of the comparative example. It is sectional drawing explaining the structure of the optical printhead of the comparative example. (A) is a diagram showing the curvature of field of the central imaging optical system of the example, and (B) is a diagram showing the curvature of field of the central imaging optical system of the comparative example. (A) to (C) are diagrams showing the PSF for the central imaging optical system of the example, and (D) to (F) show the PSF for the central imaging optical system of the comparative example. It is a figure.

[Embodiment]
Hereinafter, the image forming apparatus according to the present invention will be described with reference to the drawings. As shown in FIG. 1, the image forming apparatus 100 of the embodiment is used as, for example, a digital copier or the like, and an image reading unit 10 for reading a color image formed on a document D and an image corresponding to the document D are printed on paper P. The image forming unit 20 formed in the image forming unit 20, the feeding unit 40 for feeding the paper P to the image forming unit 20, the conveying unit 50 for conveying the paper P, and the control unit 90 for comprehensively controlling the operation of the entire apparatus. including.

The image forming unit 20 includes an image forming unit 70Y, 70M, 70C, 70K provided for each of the yellow, magenta, cyan, and black colors, an intermediate transfer unit 81 on which a toner image obtained by synthesizing each color is formed, and a toner. A fixing portion 82 for fixing the image is provided.

Of the image forming unit 20, the image forming unit 70Y is a portion that forms a Y (yellow) color image, and is a photoconductor drum 71, a charging unit 72, an optical print head (optical writing device) 73, and a developing unit 74. Etc. are provided. The photoconductor drum 71 forms a Y-color toner image, and the charging portion 72 is arranged around the photoconductor drum 71 to charge the surface of the photoconductor drum 71 as a photoconductor by corona discharge, and the optical print head 73. Irradiates the photoconductor drum 71 with light corresponding to the image of the Y color component, and the developing unit 74 attaches the toner of the Y color component to the surface of the photoconductor drum 71 to obtain toner from the electrostatic latent image. Form an image. The photoconductor drum 71 has a cylindrical shape and rotates around the rotation axis RX. The cylindrical surface of the photoconductor drum 71 is a light receiving surface 71a for forming an image by the optical print head 73.

Since the other image forming units 70M, 70C, and 70K have the same structure and function as the image forming unit 70Y for Y color except that the colors of the images to be formed are different, the description of these structures and the like will be omitted. The image forming unit 70 means any unit among the four color image forming units 70Y, 70M, 70C, and 70K, and as elements adapted to each color, the photoconductor drum 71, the charging portion 72, and the light It includes a print head 73 and a developing unit 74.

FIG. 2 is a side view of the optical print head 73. Note that FIG. 2 also shows a part of the cylindrical photoconductor drum 71. The optical print head 73 is exposed to light from a light emitting element 73a having light emitting regions 3a, 3b, 3c forming a two-dimensionally arranged light emitting point cloud group DG and light from the light emitting point cloud group DG of the light emitting point groups 3a, 3b, 3c. An optical system 73b having imaging optical systems 2a, 2b, and 2c for forming an image on the light receiving surface 71a of the body drum (photoreceptor) 71 is provided. A glass flat plate (glass flat plate or protective cover) 5 g is provided between the imaging optical systems 2a, 2b, 2c and the photoconductor drum 71. The flat plate 5g and the exterior (not shown) cover the imaging optical systems 2a, 2b, 2c, and prevent dust from adhering to the lenses constituting the imaging optical systems 2a, 2b, 2c. ..

The imaging optical systems 2a, 2b, and 2c are arranged at equal intervals in the directions of the upper and lower ± Z axes. The imaging optical systems 2a, 2b, and 2c are telecentric on both sides and have the same imaging magnification (for example, -1x), but do not have exactly the same shape, but are on the left and right ± X axes of the drawing. The size is slightly different in terms of direction. Here, the Y-axis parallel to the rotation axis RX of the photoconductor drum 71 corresponds to the main scanning direction or the main direction, is orthogonal to the rotation axis RX of the photoconductor drum 71, and is the optical axis AX (the light referred to here). The axis AX extends perpendicularly to the main ray from the light emitting point ED to the first lens 5d, which will be described later), and corresponds to the sub-scanning direction or the sub-direction. That is, in the optical print head 73, light emitting regions 3a, 3b, and 3c are provided at three locations in the vertical sub-scanning direction, and imaging optical systems 2a, 2b, and 2c are provided corresponding to the respective light emitting regions 3a, 3b, and 3c. The imaging optical systems 2a, 2b, and 2c are positioned and fixed to the light emitting element 73a in a state of being mutually positioned by a holder (not shown).

As shown in FIG. 3A, the light emitting element 73a has a three-layer structure, and three light emitting substrates 76a, 76b, 76c are laminated in the X-axis direction parallel to the optical axis AX. By dividing the light emitting element 73a into three light emitting substrates 76a, 76b, 76c and arranging a drive circuit behind the light emitting point group DG, the unit can be miniaturized. On each light emitting substrate 76a, 76b, 76c, light emitting regions 3a, 3b, 3c are formed at predetermined intervals along reference lines SXa, SXb, SXc extending in the Y direction. That is, the light emitting regions 3a, 3b, and 3c are arranged two-dimensionally. The light emitting region 3b on the upper layer side (that is, the upper side in FIG. 2) is arranged so as to be shifted in the direction away from the photoconductor drum 71 on the −X side with respect to the intermediate light emitting region 3a, and is arranged on the lower layer side (that is, FIG. 2). The light emitting region 3c (lower side) is arranged so as to be shifted in the direction closer to the photoconductor drum 71 on the + X side with respect to the intermediate light emitting region 3a.

FIG. 3B is a partially enlarged view from behind explaining the arrangement of the light emitting point group DG or the light emitting point ED formed in the single light emitting region 3a. The light emitting points ED constituting the light emitting point group DG are arranged at equal intervals in the Y direction corresponding to the main scanning direction, and are also arranged at equal intervals in the direction inclined with respect to the Z direction corresponding to the sub scanning direction. .. The diameter of one light emitting point ED is, for example, 30 μm, and the main direction is arranged at a pitch of 10.6 μm corresponding to one dot of 2400 dpi. Although not shown, there are 32 light emitting point EDs per row, and a total of 128 light emitting point EDs are arranged in a parallelogram matrix in 4 rows as a whole.

As shown in FIG. 3C, in the light emitting element 73a, the light emitting set SCn (n = 1, 2, 3, ...) Consisting of three adjacent light emitting regions 3a, 3b, 3c is repeated in the Y direction. They are evenly spaced. Note that FIG. 2 corresponds to the AA cross section of FIG. 3C. The light emitting regions 3a, 3b, and 3c constituting the light emitting group SCn are arranged at different positions in the main scanning direction or the Y direction, and are arranged at different positions in the sub scanning direction or the Z direction. Similarly, in the optical system 73b, imaging sets LCn (n = 1, 2, 3, ...) Consisting of three adjacent imaging optical systems 2a, 2b, 2c are repeatedly arranged in the Y direction at equal intervals. Has been done. The imaging optical systems 2a, 2b, and 2c constituting the imaging group LCn are arranged at different positions in the main scanning direction or the Y direction, and are arranged at different positions in the sub-scanning direction or the Z direction. In a specific example, about 100 imaging optical systems 2a, 2b, 2c are provided per row extending in the Y direction, and a total of 287 imaging optical systems 2a, 2b, 2c or light emitting regions 3a are provided as a whole. , 3b, 3c are arranged in a parallelogram matrix.

Each of the light emitting substrates 76a, 76b, and 76c constituting the light emitting element 73a is a bottom emission type organic EL element in which the light emitting points ED are two-dimensionally arranged on the glass plate 73q. Each light emitting point group DG constituting the light emitting element 73a is projected as a projected image group PG on the light receiving surface 71a, and the projected image group PG includes a large number of projected image PDs corresponding to a large number of light emitting point EDs.

Returning to FIG. 2, in the optical system 73b, each imaging optical system 2a, 2b, 2c has a first lens 5d, an aperture 5e, and a second lens 5f, respectively. The first lens 5d collimates the light rays LB from the light emitting regions 3a, 3b, and 3c. The second lens 5f collects the light rays LB from the first lens 5d and projects the images of the light emitting regions 3a, 3b, and 3c on the light receiving surface 71a of the photoconductor drum 71. As a result, in the light receiving regions 4a, 4b, 4c on the light receiving surface 71a, a projection image group PG having the same pattern as the light emitting point cloud group DG of the two-dimensional arrangement formed in the light emitting regions 3a, 3b, 3c is formed. Will be done. As will be described in detail later, the plurality of first lenses 5d constituting the optical system 73b are arranged two-dimensionally, and their centers are arranged on the same plane (second plane PLb). The plurality of second lenses 5f constituting the optical system 73b are arranged two-dimensionally, and their centers are arranged on the same plane (third plane PLc). Further, the plurality of diaphragms 5e constituting the optical system 73b are arranged two-dimensionally, and their centers are arranged on the same plane (fourth plane PLd).

Since the imaging optics 2a, 2b, and 2c are telecentric on both sides and the imaging magnification is -1x, the connection on the light emitting point group DG, the first lens 5d, the aperture 5e, the second lens 5f, and the photoconductor drum 71. The image points are approximately evenly spaced. Further, since the drive circuit of another light emitting point group DG is arranged behind a certain light emitting point group DG, the three light emitting point group DGs with respect to the three imaging optical systems 2a, 2b, and 2c are substantially straight lines ( Or they are arranged on the same plane), and the straight line is inclined with respect to the light traveling direction of the main ray or the optical axis AX. The distance from the light emitting point group DG to the imaging point on the photoconductor drum 71 is as long as the upper imaging optical system 2b. Since the photoconductor drum 71 has a cylindrical shape instead of a flat surface, the distance (or) from the light emitting point group DG of each imaging optical system 2a, 2b, 2c to the position in the sub-scanning direction to the imaging point on the photoconductor drum 71. The change (conjugate length of the imaging optical system 2a, 2b, 2c) is not linear, but increases or decreases monotonically.

As shown in FIG. 2, the first lens 5d is a convex lens, and more specifically, the lens shape of the first lens 5d is biconvex in the vicinity of the optical axis. In the illustrated example, the first lens 5d is formed by forming or laminating resin lens portions 5i and 5j on both sides of a common first lens substrate 5h made of glass or the like. If a plurality of lens portions 5i and 5j are formed on a single first lens substrate 5h to adjust and hold the position of the first lens substrate 5h, the positions of the individual lens portions 5i and 5j can be arranged and maintained. The configuration is simpler than the case of holding the lens, and the relative position of the first lens 5d is fixed, so that accuracy can be easily obtained. The first lens 5d arranged at three different positions with respect to the sub-scanning direction constitutes the first lens array A1. The diaphragm 5e has a plurality of openings 5s formed in a diaphragm substrate 5p which is a light-shielding body. The diaphragms 5e arranged at three different positions with respect to the sub-scanning direction constitute the diaphragm array A3. The second lens 5f is a convex lens, and more specifically, the lens shape of the second lens 5f is biconvex in the vicinity of the optical axis. In the illustrated example, the second lens 5f has resin lens portions 5m and 5n formed on both sides of a common second lens substrate 5k made of glass or the like. The second lens 5f arranged at three different positions with respect to the sub-scanning direction constitutes the second lens array A2.

FIG. 4A is a conceptual diagram illustrating the light emitting point group DG, the first lens 5d, and the periphery of the second lens 5f. In FIG. 4A, for convenience of explanation, the rotation angles of the first and second lenses 5d and 5f shown in FIG. 2 are enlarged. In FIG. 4A, the first and second lenses 5d and 5f show a cross section of only the lens surfaces S1 to S4. The two short straight lines in the center of the figure indicate the aperture 5e. Further, two symbols "+" between the lens surfaces S1 to S4 of the lenses 5d and 5f indicate the principal points T1 to T4 of the lenses 5d and 5f.

The first lens 5d has two common planes of symmetry SE1 and SE3, and is non-rotational symmetric, vertically symmetric, and horizontally symmetric. Further, the second lens 5f has two common symmetric planes SE2 and SE3, and is non-rotational symmetric, vertically symmetric and left and right symmetric. Of the common symmetric planes, the common symmetric planes SE1 and SE2 (planes perpendicular to the paper surface) that are vertically symmetrical with respect to the light traveling direction of the main ray are the planes that pass through the principal points T1 and T2 of the first lens 5d and the second lens 5f. It is a surface passing through the principal points T3 and T4. In the first and second lenses 5d and 5f, the vertically symmetrical common symmetric planes SE1 and SE2 are not parallel to the light traveling direction but are inclined to the upper right. The symmetrical common symmetric plane SE3 (plane on the paper surface) with respect to the light traveling direction passes through the center C1 of the aperture 5e and is a plane perpendicular to the vertically symmetrical common symmetric planes SE1 and SE2. Further, with respect to the first and second lenses 5d and 5f, the two common symmetric surfaces SE1 (or the lens surfaces S1 and S3 on the light source (or light emitting point ED) side and the lens surfaces S2 and S4 on the photoconductor drum 71 side are used. Although SE2) and SE3 are in agreement, when the first lens 5d and the second lens 5f are compared, the vertically symmetrical common symmetric planes SE1 and SE2 are not in agreement and are shifted vertically. In the present embodiment, the first and second lenses 5d and 5f constituting the imaging optical systems 2a, 2b and 2c have four lens surfaces S1 to S4 having curved surfaces. At least one of the two lens surfaces constituting each lens 5d and 5f is non-axisymmetric with respect to the symmetry lines E1 and E2 at which the two common symmetric surfaces SE1 (SE2) and SE3 intersect. Specifically, for example, the lens surfaces S1 to S4 are all free curved surfaces.

The first and second lenses 5d and 5f have the same shape in which the aperture side lens surfaces S2 and S3 close to the aperture 5e are inverted with respect to the light traveling direction of the main light beam, and have the same core thickness. Further, the first and second lenses 5d and 5f have the same shape in which the outer lens surfaces S1 and S4 far from the diaphragm 5e are inverted with respect to the light traveling direction of the main light beam, and have the same core thickness. Further, the lens surfaces S1 to S4 of the first and second lenses 5d and 5f are point-symmetrical with respect to the center C1 of the diaphragm 5e when viewed from the direction of the rotation axis RX of the photoconductor drum 71. When there is only a diaphragm 5e between the first and second lenses 5d and 5f, asymmetric aberration is caused by arranging the first and second lenses 5d and 5f so as to be point-symmetric with respect to the center C1 of the diaphragm 5e. Ingredients can be removed.

Of the principal points T1 to T4 on the lenses 5d and 5f, the straight line connecting the principal points T2 and T3 on the aperture 5e side (the principal point interval line LL1) passes through the center C1 of the aperture 5e. That is, in the first and second lenses 5d and 5f, the center C1 of the aperture 5e is arranged on the main point interval line LL1 connecting the aperture side principal points T2 and T3 close to the aperture 5e. Further, the symmetry lines E1 and E2 have a non-zero angle with respect to the principal point interval line LL1, and the symmetry lines E1 and E2 of the first and second lenses 5d and 5f are parallel to each other.

As shown in FIGS. 2 and 4A, the first and second lens substrates 5h and 5k of the first and second lens arrays A1 and A2 are lines between the principal points of the first and second lenses 5d and 5f. It is arranged at an angle with respect to LL1. The inclination direction of the normal NL1 of the first lens substrate 5h and the inclination of the symmetric lines E1 and E2 of the first and second lenses 5d and 5f with respect to the principal point interval LL1 of the first and second lenses 5d and 5f. The direction is the same, specifically, it is in the upper right direction on the paper. Further, with respect to the principal point interval LL1 of the first and second lenses 5d and 5f, the inclination direction of the normal NL2 of the second lens substrate 5k and the symmetric lines E1 and E2 of the first and second lenses 5d and 5f. It is the same as the inclination direction of, and specifically, it is the upper right direction on the paper. In other words, the tilting directions of the lens portions 5i, 5j, 5m, and 5n constituting the first and second lenses 5d and 5f are the same as the tilting directions of the first and second lens substrates 5h and 5k. Further, the first lens substrate 5h and the second lens substrate 5k have different inclination angles with respect to the principal point interval line LL1. The inclination angles of the first and second lens substrates 5h and 5k differ depending on the position of the first and second lens substrates 5h and 5k from the light emitting group point DG to the photoconductor drum 71. The 1-lens substrate 5h and the 2nd lens substrate 5k are not arranged in parallel.

Further, the inclination angles of the symmetric lines E1 and E2 with respect to the principal point interval line LL1 are the inclination angle of the normal line NL1 of the first lens substrate 5h and the inclination angle of the normal line NL2 of the second lens substrate 5k with respect to the principal point interval line LL1. It is between the angle. By setting the tilt angle of the lens portions 5i, 5j, 5m, 5n constituting the first and second lenses 5d, 5f between the tilt angle of the first lens substrate 5h and the tilt angle of the second lens substrate 5k, The difference in thickness of the lens portions 5i, 5j, 5m, and 5n constituting the first and second lenses 5d and 5f can be reduced. Further, the degree of inclination of the lens portions 5i, 5j, 5m, and 5n with respect to the first and second lens substrates 5h and 5k can be reduced without deteriorating the imaging state. In the present embodiment, the lens portions 5i, 5j, 5m, and 5n of the first and second lenses 5d and 5f are tilted at an angle close to the tilt angle of the diaphragm 5e, respectively. The tilt angle of the first lens substrate 5h constituting the first lens 5d is closer to the tilt angle of the aperture 5e than that of the second lens substrate 5k constituting the second lens 5f, and the lens portions 5i, 5j, 5m and 5n are tilted according to the tilt angle of the first lens substrate 5h. That is, even if the first and second lens substrates 5h and 5k of the first and second lens arrays A1 and A2 have different tilt angles, the tilt angles of the lens portions 5i, 5j, 5m and 5n, and thus the first lens unit. The tilt angles of the second lenses 5d and 5f as a whole are the same.

It is assumed that the inclinations of the lens portions 5i, 5j, 5m, and 5n are matched to the corresponding lens substrates with respect to the first and second lens substrates 5h and 5k of the first and second lens arrays A1 and A2, respectively. The tilt angle is different between the first lens 5d and the second lens 5f, and asymmetric aberrations are present, which worsens the imaging state.

As shown in FIG. 4 (A), the center of the light emitting point group DG is shifted downward with respect to the extension line of the line extending horizontally from the outer principal point T1 near the light emitting point group DG of the first lens 5d. It is placed in a position. That is, the center of the light emitting point group DG is arranged avoiding the straight line LL2 which passes through the outer principal point T1 close to the light emitting point group DG of the first lens 5d and extends parallel to the principal point interval line LL1. Specifically, the center of the light emitting point group DG is between the straight line LL2 that passes through the outer principal point T1 of the first lens 5d and extends parallel to the principal point interval line LL1 and the symmetry line E1 of the first lens 5d. positioned. A good imaging state can be obtained by appropriately setting the position of the light emitting point group DG according to the tilt direction of the first lens 5d.

FIG. 4A shows a light beam LB traveling from the center of the light emitting point group DG toward the center C1 of the diaphragm 5e. In FIG. 4A, the light ray LB emitted from the light emitting point group DG travels substantially in parallel until it enters the lens surface S1 of the first lens 5d, and the light ray LB emitted from the lens surface S4 of the second lens 5f is a photoconductor. It proceeds substantially in parallel until it enters the drum 71. Further, the light beam LB is not horizontal between the first lens 5d and the second lens 5f, and is the inclination direction of the common symmetric planes SE1 and SE2 that are vertically symmetrical with respect to the light traveling direction of the main light rays of the lenses 5d and 5f, that is, the paper surface. Incline to the upper right.

In the comparative example shown in FIG. 5, of the imaging optical systems 2a, 2b, and 2c, the first and second lenses 5d and 5f constituting the imaging optical system 2a, 2b, and 2c have four lens surfaces S1 to S4 having curved surfaces. All of these four lens surfaces S1 to S4 are axially symmetric aspherical surfaces, and the rotational symmetry axes RT are all coincident. Further, the rotational symmetry axes RTs of the three imaging optical systems 2a, 2b, and 2c are parallel to each other. The center of the light emitting point group DG of each imaging optical system 2a, 2b, 2c is on an extension of the rotational symmetry axis RT. Further, the center of the imaging point on the photoconductor drum 71 of each imaging optical system 2a, 2b, 2c is on the extension line of the rotational symmetry axis RT.

In the comparative example shown in FIG. 5, the angle difference between the optical axis AX and the first and second lens substrates 5h and 5k of the first and second lens arrays A1 and A2 is that the lens portions 5i, 5j, 5m and 5n are the first. The angles of inclination with respect to the 1st and 2nd lens substrates 5h and 5k are shown. In the case of the imaging optical systems 2a, 2b, 2c of the comparative example, the thicknesses of the lens portions 5i, 5j, 5m, 5n are different between the upper imaging optical system 2b and the lower imaging optical system 2c, and in particular. The difference in thickness is large in the upper imaging optical system 2b.

In another comparative example shown in FIG. 4B, the lens surfaces S1 to S4 of the first and second lenses 5d and 5f constituting the optical print head 73 of the present embodiment are not free curved surfaces but axisymmetric aspherical surfaces. .. Although the lenses 5d and 5f of the comparative example are rotationally symmetric, the rotational symmetry axis K1 of the first lens 5d and the rotational symmetry axis K2 of the second lens 5f do not match, and the rotational symmetry axes K1 and K2 are It is in a state of being inclined with respect to the optical axis AX and shifting in parallel with each other. In the comparative example, the center of the light emitting point group DG is arranged on a straight line LL2 that passes through the outer principal point T1 of the first lens 5d and extends parallel to the principal point interval line LL1.

In FIG. 4B, the light ray LB traveling horizontally from the center of the light emitting point group DG to the lens surface S1 of the first lens 5d also travels horizontally between the first lens 5d and the second lens 5f, and the second lens The light beam LB emitted from the lens surface S4 of 5f and directed to the imaging point on the photoconductor drum 71 also travels horizontally.

In the present embodiment, in each of the imaging optical systems 2a, 2b, and 2c, the first lens array A1, the second lens array A2, and the aperture array A3 have a flat plate shape as a whole, and these members A1 to A3 are light emitting points. The optical path from the group DG to the imaging point on the photoconductor drum 71 is arranged so as to divide it into substantially four equal parts. In this case, the tangent of the angle θa formed by the plane perpendicular to the optical axis AX and the same plane (first plane PLa) connecting the centers of the light emitting point group DG is, for example, 1. Further, the tangent of the angle θb formed by the plane perpendicular to the optical axis AX and the same plane (second plane PLb) connecting the center of the first lens 5d of the first lens array A1 is, for example, about 3/4. .. The tangent of the angle θd formed by the plane perpendicular to the optical axis AX and the same plane (fourth plane PLd) connecting the centers of the diaphragm 5e of the diaphragm array A3 is, for example, about 1/2. The tangent of the angle θc formed by the plane perpendicular to the optical axis AX and the same plane (third plane PLc) connecting the centers of the second lens 5f of the second lens array A2 is, for example, about 1/4.

According to the image forming apparatus described above, the lens surfaces S1 to S4 of the first and second lenses 5d and 5f constituting the imaging optical systems 2a to 2c are not axially symmetric but are free curved surfaces to form an image surface. Appropriately to reduce the discrepancy and avoid the position of the light emitting point group DG on the straight line LL2 which extends parallel to the main point interval line LL1 through the outer principal point T1 on the side close to the light emitting point group DG of the first lens 5d. By setting, it is possible to eliminate the defect of the imaging state due to the free curved surface of the lens surfaces S1 to S4 and obtain a good imaging state in the entire image region.

〔Example〕
Hereinafter, specific examples of the optical system 73b incorporated in the image forming apparatus 100 will be described. The optical system 73b of the embodiment is the same as that shown in FIG. In the following tables, the upper imaging optical system means the imaging optical system 2b in FIG. 2, the central imaging optical system means the imaging optical system 2a in FIG. 2, and the lower imaging optical system. The optical system means the imaging optical system 2c of FIG. The flat plates constituting the first lens array A1, the aperture array A3, and the second lens array A2 are common to the imaging optical systems 2a to 2c. The light emitting point cloud DG shows the coordinates at the center in the table. The photoconductor drum 71 has a cylindrical shape with a radius of 25 mm and is common to the three imaging optical systems 2a, 2b, and 2c as surfaces. However, in the table, the surface of the photoconductor drum 71 is referred to as the optical axis AX. The position and inclination at the intersection are shown. Looking at the aspherical coefficient, the absolute values are the same and the signs are opposite across the aperture 5e, so the lens surfaces of the first and second lenses 5d and 5f have the same shape and are in opposite directions. I understand.

中央の結像光学系２ａの非球面形状について表２にまとめた。記載した非球面は、いずれも自由曲面であり、球面項はなく、形状式は、Ｘ、Ｙ、Ｚに対応するローカル座標をｘ、ｙ、ｚとして

である。なお、表に無い非球面係数ａ_ｉｊはすべて０である。これらの点は以下でも同様である。
［表２］

[1-a: Central imaging optical system]
Hereinafter, the data of the central imaging optical system 2a will be described. Table 1 summarizes the coordinates of the surface vertices of the lens surfaces of the first lens 5d, the diaphragm 5e, and the second lens 5f that constitute the central imaging optical system 2a. The unit of distance is mm and the unit of angle is degrees. The refractive index of the lens substrate is 1.5145 with respect to light having a wavelength of 650 nm, and the refractive index of the resin lens portion provided between the lens surface and the lens substrate is 1.5285. The imaging magnification β is -1 in all optical systems.
[Table 1]

Table 2 summarizes the aspherical shape of the central imaging optical system 2a. All of the aspherical surfaces described are free-form surfaces, have no spherical term, and the shape formula has local coordinates corresponding to X, Y, and Z as x, y, and z.

Is. The aspherical coefficient _aij not shown in the table is all 0. These points are the same in the following.
[Table 2]

上側の結像光学系２ｂの自由曲面形状について表４にまとめた。
［表４］

[1-b: Upper imaging optical system]
Hereinafter, the data of the upper imaging optical system 2b will be described. Table 3 summarizes the coordinates of the surface vertices of the lens surfaces of the first lens 5d, the diaphragm 5e, and the second lens 5f constituting the upper imaging optical system 2b.
[Table 3]

Table 4 summarizes the free curved surface shape of the upper imaging optical system 2b.
[Table 4]

下側の結像光学系２ｃの自由曲面形状について表６にまとめた。
［表６］

[1-c: Lower imaging optical system]
Hereinafter, the data of the lower imaging optical system 2c will be described. Table 5 summarizes the coordinates of the surface vertices of the lens surface of the first lens 5d, the diaphragm 5e, and the second lens 5f that constitute the lower imaging optical system 2c.
[Table 5]

Table 6 summarizes the free-form surface shape of the lower imaging optical system 2c.
[Table 6]

FIG. 6A shows the curvature of field of the optical printhead 73 of the embodiment. On the other hand, FIG. 6B shows the curvature of field of the optical print head of the comparative example. Here, the comparative example shows the data for the optical print head having the configuration shown in FIG. 4 (B). In the embodiment, by using a free curved surface for the lens surface, the image planes match and the curvature of field is corrected. In the comparative example, there is a dissociation of the image plane between the main scanning direction and the sub scanning direction, and the curvature of field is also different.

7 (A) to 7 (C) show the PSF (Point spread function) of the optical print head 73 of the embodiment. The actual light emitting point of the optical printhead 73 is a surface light source, but what is shown here is a beam profile calculated by a point light source. FIG. 7 (A) is calculated by placing a point light source at a position corresponding to the left end of the light emitting point group DG in the main scanning direction, and FIG. 7 (B) corresponds to the center of the light emitting point group DG in the main scanning direction. The calculation was made by placing a point light source at the position where the light source is located, and FIG. 7 (C) shows the calculation by placing the point light source at a position corresponding to the right end in the main scanning direction of the light emitting point group DG.

7 (D) to 7 (F) show the PSF of the optical print head 73 of the comparative example. The surface shapes of the imaging optical systems 2a, 2b, and 2c of the comparative example are free curved surfaces, and only the arrangement of the light emitting point cloud group DG is the same as that of the optical printhead of the comparative example shown in FIG. 4 (B). In FIGS. 7 (D) to 7 (F), a good imaging state is obtained at the center of the light emitting point group DG, but the imaging state is deteriorated at the left end and the right end of the light emitting point group DG.

Although the image forming apparatus as a specific embodiment has been described above, the image forming apparatus according to the present invention is not limited to the above. For example, in the above embodiment, the number of imaging optical systems constituting the optical system 73b is not limited to three, but may be two or four or more.

In the above embodiment, the specific example regarding the number and arrangement of the light emitting point EDs constituting the light emitting point group DG is merely an example, and the number and arrangement of the light emitting point EDs can be changed according to the application and purpose.

In the above embodiment, the diaphragm 5e has openings 5s formed in the diaphragm substrate 5p, but a large number of openings are formed in the light-shielding portion made of a layered light-shielding body formed on one side of the diaphragm substrate made of glass. It may be the one that was done.

In the above embodiment, the light emitting substrates 76a, 76b, and 76c are arranged in close contact with each other, but they may be arranged at intervals. Further, the length and thickness of the light emitting substrates 76a, 76b, 76c can be appropriately changed.

In the above embodiment, the lens surfaces on both sides of the first and second lenses 5d and 5f are free curved surfaces, but only one of the lens surfaces may be a free curved surface.

2a, 2b, 2c ... Imaging optical system, 4a-4c ... Light receiving region, 3a, 3b, 3c ... Light emitting region, 4a, 4b, 4c ... Light receiving region, 5d ... First lens, 5f ... Second lens, 5g ... Flat plate, 5h ... 1st lens substrate, 5k ... 2nd lens substrate, 5i, 5j, 5m, 5n ... lens unit, 5p ... aperture substrate, 5s ... opening, 10 ... image reading unit, 20 ... image forming unit, 40 ... Feeding unit, 50 ... Conveying unit, 70, 70Y, 70M, 70C, 70K ... Image forming unit, 71 ... Photoreceptor drum, 71a ... Light receiving surface, 72 ... Charging part, 73 ... Optical print head, 73a ... Light emitting element, 73b ... Optical system, 74 ... Development unit, 76a, 76b, 76c ... Light emitting substrate, 81 ... Intermediate transfer unit, 82 ... Fixing unit, 90 ... Control unit, 100 ... Image forming device, A1 ... First lens array, A2 ... 2nd lens array, A3 ... aperture array, AX ... optical axis, DG ... emission point group, SE1, SE2, SE3 ... common symmetry plane, E1, E2 ... symmetry line, ED ... emission point, K1, K2 ... rotational symmetry axis , LB ... ray, LL1 ... principal point interval, PD ... projected image, PG ... projected image group, RT ... rotational symmetry axis, S1, S4 ... outer lens surface, S2, S3 ... aperture side lens surface, T1, T4 ... Outer principal point, T2, T3 ... Main point on the aperture side

Claims (8)

A photoconductor having a surface transported in a sub-direction substantially orthogonal to the main direction,
A light emitting board having a plurality of light emitting point groups arranged in two dimensions,
A plurality of imaging optical systems for forming an image of light from the plurality of light emitting point clouds at different positions on the photoconductor, and
With
The imaging magnification of the imaging optical system is -1.
When viewed from the direction of the rotation axis of the photoconductor, the conjugated lengths of the plurality of imaging optical systems differ depending on the position in the sub-direction, and monotonically increase or decrease depending on the position in the sub-direction.
Of the plurality of imaging optical systems, each imaging optical system has a first lens, an aperture, and a second lens in order from the light emitting point group side.
The first and second lenses each have two lens surfaces having a curved surface.
The two lens planes have two common symmetric planes, and at least one of the two lens planes is axisymmetric with respect to a line of symmetry where the two common symmetric planes intersect.
The first and second lenses have the same shape in which the aperture-side lens surfaces close to the aperture are inverted with respect to the light traveling direction of the main light beam, and the outer lens surfaces far from the aperture are inverted with respect to the light traveling direction. Shape and equal core thickness
In the first and second lenses, the center of the diaphragm is arranged on the principal point interval line connecting the diaphragm side principal points close to the diaphragm, and the symmetry line is not zero with respect to the principal point interval line. It has an angle, and the symmetry lines of the first and second lenses are parallel to each other.
An image forming apparatus characterized in that the center of the light emitting point group is arranged avoiding a straight line extending in parallel with the line between the main points through an outer main point close to the light emitting point group of the first lens. .. The center of the light emitting point group is characterized by being located between a straight line extending parallel to the main point interval line passing through the outer principal point of the first lens and the symmetric line of the first lens. The image forming apparatus according to claim 1. One of claims 1 and 2, wherein the lens surfaces of the first and second lenses are point-symmetrical with respect to the center of the diaphragm when viewed from the direction of the rotation axis of the photoconductor. The image forming apparatus according to one item. For the plurality of imaging optical systems, the first lens has resin lens portions on both sides of a single first lens substrate, and the second lens is provided on both sides of a single second lens substrate. The image forming apparatus according to any one of claims 1 to 3, wherein the image forming apparatus has a lens portion made of resin. The image forming apparatus according to claim 4, wherein the first and second lens substrates are arranged so as to be inclined with respect to the main point interval line of the first and second lenses. The inclination direction of the normal line of the first lens substrate and the inclination direction of the symmetric line of the first and second lenses are the same with respect to the main point interval line of the first and second lenses. The image forming apparatus according to claim 5, wherein the inclination direction of the normal line of the second lens substrate is the same as the inclination direction of the symmetric line of the first and second lenses. The image forming apparatus according to any one of claims 4 and 5, wherein the first lens substrate and the second lens substrate have different inclination angles with respect to the line between the principal points. The inclination angle of the symmetric line with respect to the principal point interval is between the inclination angle of the normal line of the first lens substrate and the inclination angle of the normal line of the second lens substrate with respect to the principal point interval line. The image forming apparatus according to claim 7.

Images