JP7161145B2 - image forming device - Google Patents

Description

The present invention relates to an image forming apparatus having an optical writing section, and more particularly to an image forming apparatus having an image forming optical system for forming an image of light emitting points on a light receiving surface.

2. Description of the Related Art Conventionally, an optical writer comprising a light-emitting substrate on which a plurality of light-emitting point groups consisting of a plurality of light-emitting points are arranged in a main direction and a sub-direction, and a lens array in which an imaging lens is arranged to face each light-emitting point group in a one-to-one correspondence. 2. Description of the Related Art An image forming apparatus having an insert portion is known. In the optical writing unit of such an image forming apparatus, the light emitted from the light emitting point is formed into a desired beam spot at a desired position on the photoreceptor by interposing an imaging lens.

As an image forming apparatus, for example, as described in Patent Document 1 below, a plurality of imaging lenses having parallel optical axes 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 imaging 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. The area occupied increases, and there is a risk that the size of the unit will increase.

In order to reduce the size of the unit, it is conceivable to arrange the light emitting point groups so that the positions of the light emitting point groups are different in the optical axis direction instead of arranging them on the same glass substrate. In this case, each light emitting point The distances from the groups to the lens array will be different, making it difficult to achieve bilateral telecentricity and uniform magnification of the optical system while maintaining imaging conditions.

JP 2009-51194 A

The present invention has been made in view of the above-described background art, and includes an optical writing unit that can achieve both-side telecentricity and uniform magnification while maintaining an imaging state when light-emitting point groups are not on the same plane. An object of the present invention is to provide an image forming apparatus.

In order to solve the above problems, an image forming apparatus according to the present invention provides a photoreceptor having a surface that is conveyed in a sub-direction substantially perpendicular to the main direction, and a light-emitting substrate having a plurality of light-emitting point groups arranged two-dimensionally. and a plurality of imaging optical systems for forming images of the light from the plurality of light emitting point groups at different positions on the photoreceptor, wherein the imaging magnification of the imaging optical system is -1, and the rotation of the photoreceptor When viewed from the axial direction, the conjugate lengths of the plurality of imaging optical systems are different 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, a diaphragm, and a second lens in order from the light emitting point group side, and the first and second lenses each have two curved lens surfaces, The two lens surfaces have two common planes of symmetry, and at least one of the two lens surfaces is axisymmetric with respect to a line of symmetry intersecting the two common planes of symmetry, and the first and second lenses are , the aperture-side lens surface on the side closer to the aperture has the same shape in which the light traveling direction of the principal ray is reversed, and the outer lens surface on the far side from the aperture has the same shape in which the light traveling direction is reversed, and the thickness are equal, and in the first and second lenses, the center of the diaphragm is placed on the principal point line connecting the diaphragm-side principal points close to the diaphragm, and the line of symmetry forms a non-zero angle with the principal point line The lines of symmetry of the first and second lenses are parallel to each other, and the center of the light emitting point group is on a straight line extending parallel to the line between the principal points passing through the outer principal point near the light emitting point group of the first lens placed to avoid

According to the above image forming apparatus, the lens surfaces of the first and second lenses are not axially symmetrical but are free-form surfaces, thereby reducing the mismatch of the image planes, and the position of the light emitting point group is shifted to the position of the light emitting point group of the first lens. By properly setting the straight line extending parallel to the line between the principal points passing through the outer principal point on the side closer to the , the image area can be corrected by eliminating the poor image formation caused by the free-form lens surface. A good imaging condition can be obtained as a whole.

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

In another aspect of the present invention, 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 rotational axis of the photoreceptor. When there is only the diaphragm between the first and second lenses, the asymmetrical aberration components can be removed by arranging the first and second lenses so as to be symmetrical about the center of the diaphragm.

According to still another aspect of the present invention, for a 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 has a single 2 Lenses made of resin are provided 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 when the positions of the individual lens portions are arranged and held. Since the relative positions of the first and second lenses are fixed, it is easy to achieve accuracy.

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

In still another aspect of the present invention, the tilt direction of the normal to the first lens substrate and the tilt direction of the line of symmetry of the first and second lenses with respect to the line between the principal points of the first and second lenses are The direction of inclination of the normal to the second lens substrate and the direction of inclination of the symmetry lines of the first and second lenses are the same. In this case, the tilt directions of the lens portions forming the first and second lenses are the same as the tilt directions of the first and second lens substrates.

In still another aspect of the present invention, the first lens substrate and the second lens substrate have different angles of inclination with respect to the line between principal points. Since the inclination angles of the first and second lens substrates differ depending on the positions of the first and second lens substrates from the light emitting point group to the photosensitive member, the first lens substrate and the second lens substrate are not arranged in parallel.

In still another aspect of the present invention, the inclination angle of the line of symmetry with respect to the line between principal points is 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 line between principal points. between By setting the tilt angle of the lens portions forming 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 forming the first and second lenses A difference in thickness can be reduced.

1 is a partial cross-sectional view showing a schematic configuration of an image forming apparatus according to an embodiment; FIG. 3 is a cross-sectional view for explaining the structure of an optical print head that constitutes the image forming unit; FIG. (A) is a conceptual perspective view illustrating the structure of a light emitting element, (B) is a diagram illustrating 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 arrangement|positioning of. (A) is a conceptual diagram for explaining an optical system forming the optical print head of FIG. 2, and (B) is a conceptual diagram for explaining an optical system forming an optical print head of a comparative example. FIG. 5 is a cross-sectional view for explaining the structure of an optical print head of a comparative example; (A) is a diagram showing the curvature of field for the central imaging optical system of the example, and (B) is a diagram showing the curvature of field for the central imaging optical system of the comparative example. (A)-(C) are diagrams showing the PSF for the central imaging optical system of the example, and (D)-(F) are diagrams showing the PSF for the central imaging optical system of the comparative example. It is a diagram.

[Embodiment]
An image forming apparatus according to the present invention will be described below with reference to the drawings. As shown in FIG. 1, an image forming apparatus 100 of the embodiment is used as, for example, a digital copier or the like. an image forming unit 20 for forming images, a paper feeding unit 40 for feeding the paper P to the image forming unit 20, a transport unit 50 for transporting the paper P, and a control unit 90 for comprehensively controlling the operation of the entire apparatus. including.

The image forming section 20 includes image forming units 70Y, 70M, 70C, and 70K provided for each of yellow, magenta, cyan, and black colors, an intermediate transfer section 81 for forming a toner image synthesized from each color, and toner. and a fixing section 82 for fixing the image.

The image forming unit 70Y of the image forming section 20 forms a Y (yellow) image, and includes a photosensitive drum 71, a charging section 72, an optical print head (optical writing device) 73, and a developing section 74. etc. The photoreceptor drum 71 forms a Y-color toner image, and the charging unit 72 is arranged around the photoreceptor drum 71 to charge the surface of the photoreceptor drum 71 as a photoreceptor by corona discharge. irradiates the photoreceptor 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 photoreceptor drum 71 to remove the toner from the electrostatic latent image. form an image. The photoreceptor drum 71 has a cylindrical shape and rotates around the rotation axis RX. The cylindrical surface of the photosensitive drum 71 serves as a light receiving surface 71a on which an image formed by the optical print head 73 is formed.

The other image forming units 70M, 70C, and 70K have the same structure and function as the Y-color image forming unit 70Y except that the colors of the images to be formed are different. Note that the image forming unit 70 means any one of the four color image forming units 70Y, 70M, 70C, and 70K, and the elements adapted to each color are the photosensitive drum 71, the charging section 72, and the light source. A print head 73 and a developing section 74 are provided.

2 is a side view of the optical print head 73. FIG. Note that FIG. 2 also shows a portion of the cylindrical photosensitive drum 71 . The optical print head 73 has a light-emitting element 73a having light-emitting regions 3a, 3b, and 3c forming light-emitting point groups DG arranged two-dimensionally, and the light from the light-emitting point groups DG of the light-emitting regions 3a, 3b, and 3c. and an optical system 73b having imaging optical systems 2a, 2b, and 2c for forming images on the light receiving surface 71a of the body drum (photoreceptor) 71, respectively. Between the imaging optical systems 2a, 2b, 2c and the photosensitive drum 71, a flat glass plate (glass flat plate or protective cover) 5g is provided. The imaging optical systems 2a, 2b, and 2c are covered by the flat plate 5g and an exterior (not shown) to prevent dust from adhering to the lenses constituting the imaging optical systems 2a, 2b, and 2c. .

The imaging optical systems 2a, 2b, and 2c are arranged at regular intervals in the directions of the ±Z-axes. The imaging optical systems 2a, 2b, and 2c are telecentric on both sides and have the same imaging magnification (for example, -1). The size is slightly different in the direction. Here, the Y-axis parallel to the rotation axis RX of the photoreceptor drum 71 corresponds to the main scanning direction or main direction, is perpendicular to the rotation axis RX of the photoreceptor drum 71, and is perpendicular to the optical axis AX (here, light The axis AX is based on the principal ray from the light emitting point ED to the first lens 5d, which will be described later. The Z axis extending perpendicular to the axis corresponds to the sub-scanning direction or the sub-direction. That is, in the optical print head 73, the light emitting areas 3a, 3b, and 3c are provided at three locations in the vertical sub-scanning direction, and the imaging optical systems 2a, 2b, and 2c are provided correspondingly. The imaging optical systems 2a, 2b, and 2c are positioned and fixed to the light emitting element 73a while being mutually positioned by holders (not shown).

As shown in FIG. 3A, the light-emitting element 73a has a three-layer structure, in which three light-emitting substrates 76a, 76b, and 76c are stacked 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, and 76c and arranging the drive circuit behind the light-emitting point group DG, the size of the unit can be reduced. Light-emitting regions 3a, 3b, 3c are formed at predetermined intervals along reference lines SXa, SXb, SXc extending in the Y direction on the respective light-emitting substrates 76a, 76b, 76c. That is, the light emitting regions 3a, 3b, 3c are arranged two-dimensionally. The light-emitting region 3b on the upper layer side (that is, the upper side in FIG. 2) is shifted in the −X direction and away from the photosensitive drum 71 with respect to the middle light-emitting region 3a, and is arranged on the lower layer side (that is, in FIG. 2). ) is shifted in the +X direction toward the photosensitive drum 71 with respect to the intermediate light emitting region 3a.

FIG. 3(B) is a partially enlarged view from the back explaining the arrangement of the light emitting point group DG or the light emitting points 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 a 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 they are arranged at a pitch of 10.6 μm corresponding to one dot of 2400 dpi in the main direction. Although not shown, there are 32 light-emitting points ED per row, and a total of 128 light-emitting points ED in four rows are arranged in a parallelogram matrix.

As shown in FIG. 3C, in the light-emitting element 73a, a light-emitting group SCn (n=1, 2, 3, . They are evenly spaced. Note that FIG. 2 corresponds to the AA section of FIG. 3(C). The light-emitting regions 3a, 3b, and 3c forming the light-emitting set SCn are arranged at different positions in the main scanning direction or the Y direction and at different positions in the sub-scanning direction or the Z direction. Similarly, in the optical system 73b, an imaging group LCn (n=1, 2, 3, . It is The imaging optical systems 2a, 2b, and 2c forming the imaging set LCn are arranged at different positions in the main scanning direction or the Y direction and 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. , 3b and 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 onto the light receiving surface 71a as a projection image group PG, and the projection image group PG includes a large number of projection images PD corresponding to a large number of light emitting points ED.

Returning to FIG. 2, in the optical system 73b, each imaging optical system 2a, 2b, 2c has a first lens 5d, a diaphragm 5e, and a second lens 5f. The first lens 5d collimates the light beams LB from the light emitting regions 3a, 3b, 3c. The second lens 5f converges the light beam LB from the first lens 5d and projects images of the light emitting areas 3a, 3b and 3c onto the light receiving surface 71a of the photosensitive drum 71. FIG. As a result, in the light-receiving areas 4a, 4b, and 4c on the light-receiving surface 71a, a projection image group PG having the same pattern as the two-dimensional array of light-emitting point groups DG formed in the light-emitting areas 3a, 3b, and 3c is formed. be done. As will be described in detail later, the plurality of first lenses 5d forming 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 forming the optical system 73b are two-dimensionally arranged, and their centers are arranged on the same plane (third plane PLc). A plurality of diaphragms 5e forming the optical system 73b are arranged two-dimensionally, and their centers are arranged on the same plane (fourth plane PLd).

Since the imaging optical systems 2a, 2b, and 2c are telecentric on both sides and the imaging magnification is −1, the light-emitting point group DG, the first lens 5d, the diaphragm 5e, the second lens 5f, and the focus on the photosensitive drum 71 The image points are approximately equally spaced. In addition, since drive circuits for other light emitting point groups DG are arranged behind a certain light emitting point group DG, the three light emitting point groups DG for the three imaging optical systems 2a, 2b, and 2c are generally straight lines ( or the same plane), and the straight line thereof is inclined with respect to the light traveling direction of the principal ray or the optical axis AX. The distance from the light emitting point group DG to the imaging point on the photosensitive drum 71 is longer in the upper imaging optical system 2b. Since the photoreceptor drum 71 is cylindrical rather than flat, the distance (or The conjugate length of the imaging optical systems 2a, 2b, 2c) is not linear, but monotonously increases or decreases.

As shown in FIG. 2, the first lens 5d is a convex lens. 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 and the position of the first lens substrate 5h is adjusted and held, the positions of the individual lens portions 5i and 5j can be arranged and held. The configuration is simpler than in the case of holding, and since the relative position of the first lens 5d is fixed, it is easy to achieve accuracy. The first lenses 5d arranged at three different positions in the sub-scanning direction constitute a first lens array A1. The diaphragm 5e is obtained by forming a plurality of apertures 5s on a diaphragm substrate 5p, which is a light shielding body. The apertures 5e arranged at three different positions in the sub-scanning direction constitute an aperture 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 is formed by forming lens portions 5m and 5n made of resin on both sides of a common second lens substrate 5k made of glass or the like. The second lenses 5f arranged at three different positions in the sub-scanning direction constitute a second lens array A2.

FIG. 4A is a conceptual diagram for explaining the periphery of the light emitting point group DG, the first lens 5d, and 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 cross sections of only the lens surfaces S1 to S4. Two short straight lines in the center of the drawing indicate the diaphragm 5e. Two symbols "+" between the lens surfaces S1 to S4 of the lenses 5d and 5f indicate principal points T1 to T4 of the lenses 5d and 5f.

The first lens 5d has two common planes of symmetry SE1 and SE3, which are non-rotationally symmetrical, vertically symmetrical, and horizontally symmetrical. In addition, the second lens 5f has two common planes of symmetry SE2 and SE3, which are non-rotationally symmetrical, vertically symmetrical, and horizontally symmetrical. Of the common planes of symmetry, common planes of symmetry SE1 and SE2 (planes perpendicular to the plane of the paper) that are vertically symmetrical with respect to the direction of travel of the principal ray are planes passing through the principal points T1 and T2 of the first lens 5d and planes of the second lens 5f. It is a plane passing through principal points T3 and T4. In the first and second lenses 5d and 5f, the common planes of symmetry SE1 and SE2, which are vertically symmetrical, are not parallel to the direction of travel of light, but are inclined upward and to the right. A common plane of symmetry SE3 (a plane on the paper surface) that is bilaterally symmetrical with respect to the light traveling direction is a plane that passes through the center C1 of the diaphragm 5e and is perpendicular to the common planes of symmetry SE1 and SE2 that are vertically symmetrical. Further, regarding each of the first and second lenses 5d and 5f, two common planes of symmetry SE1 ( Although SE2) and SE3 match, when the first lens 5d and the second lens 5f are compared, the common planes of symmetry SE1 and SE2, which are vertically symmetrical, do not match and are shifted vertically. In this embodiment, the first and second lenses 5d, 5f constituting the imaging optical systems 2a, 2b, 2c have four curved lens surfaces S1 to S4. At least one of the two lens surfaces forming each of the lenses 5d and 5f is axisymmetric with respect to the lines of symmetry E1 and E2 intersecting the two common planes of symmetry SE1 (SE2) and SE3. 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 diaphragm-side lens surfaces S2 and S3 near the diaphragm 5e are opposite to each other with respect to the traveling direction of the principal ray, and have the same thickness. The outer lens surfaces S1 and S4 of the first and second lenses 5d and 5f, which are far from the stop 5e, have the same shape in which they are opposite to each other with respect to the traveling direction of the principal ray, and have the same thickness. Further, when viewed from the direction of the rotation axis RX of the photosensitive drum 71, 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 there is only the diaphragm 5e between the first and second lenses 5d and 5f, the asymmetrical aberration components can be removed.

Of the principal points T1 to T4 on the lenses 5d and 5f, a straight line (line LL1 between principal points) connecting the diaphragm-side principal points T2 and T3 on the side of the diaphragm 5e passes through the center C1 of the diaphragm 5e. That is, in the first and second lenses 5d and 5f, the center C1 of the diaphragm 5e is arranged on the inter-principal point line LL1 connecting the diaphragm-side principal points T2 and T3 near the diaphragm 5e. Also, the symmetry lines E1 and E2 have a non-zero angle with respect to the principal point 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 aligned with the lines between the principal points of the first and second lenses 5d and 5f. It is arranged obliquely with respect to LL1. Inclination direction of the normal line NL1 of the first lens substrate 5h with respect to the line LL1 between the principal points of the first and second lenses 5d and 5f, and inclination of the symmetry lines E1 and E2 of the first and second lenses 5d and 5f The direction is the same as the direction, and specifically, it is the upper right direction on the paper surface. In addition, the direction of inclination of the normal NL2 of the second lens substrate 5k with respect to the line LL1 between the principal points of the first and second lenses 5d and 5f, and the symmetry lines E1 and E2 of the first and second lenses 5d and 5f. is the same as the inclination direction of , and specifically, it is the upper right direction in the plane of the paper. In other words, the tilt directions of the lens portions 5i, 5j, 5m and 5n forming the first and second lenses 5d and 5f are the same as the tilt directions of the first and second lens substrates 5h and 5k. In addition, the first lens substrate 5h and the second lens substrate 5k have different inclination angles with respect to the line LL1 between principal points. Since the inclination angles of the first and second lens substrates 5h and 5k differ depending on the positions of the first and second lens substrates 5h and 5k from the light emission group point DG to the photosensitive drum 71, The first lens substrate 5h and the second lens substrate 5k are not arranged in parallel.

In addition, the inclination angles of the symmetry lines E1 and E2 with respect to the line LL1 between the principal points 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 line LL1 between the principal points. The angle is between By setting the tilt angles of the lens portions 5i, 5j, 5m, and 5n constituting the first and second lenses 5d and 5f between the tilt angle of the first lens substrate 5h and the tilt angle of the second lens substrate 5k, It is possible to reduce the difference in thickness between the lens portions 5i, 5j, 5m and 5n that constitute the first and second lenses 5d and 5f. Also, the degree of inclination of the lens portions 5i, 5j, 5m, 5n with respect to the first and second lens substrates 5h, 5k can be reduced without deteriorating the imaging state. In this embodiment, the lens portions 5i, 5j, 5m and 5n of the first and second lenses 5d and 5f are each tilted at an angle close to the tilt angle of the diaphragm 5e. The inclination angle of the first lens substrate 5h constituting the first lens 5d is closer to the inclination angle of the diaphragm 5e than the second lens substrate 5k constituting the second lens 5f. 5m and 5n are inclined according to the inclination angle of the first lens substrate 5h. That is, even if the tilt angles of the first and second lens substrates 5h and 5k of the first and second lens arrays A1 and A2 are different, the tilt angles of the lens portions 5i, 5j, 5m and 5n, and thus the first And the inclination angles of the second lenses 5d and 5f as a whole are the same.

If 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 amount of inclination differs between the first lens 5d and the second lens 5f, and an asymmetrical aberration exists, deteriorating the imaging state.

As shown in FIG. 4A, the center of the light emitting point group DG is shifted downward with respect to the extension of the line horizontally extending from the outer principal point T1 near the light emitting point group DG of the first lens 5d. placed in position. In other words, the center of the light emitting point group DG is arranged to avoid the straight line LL2 extending parallel to the inter-principal point line LL1 through the outer principal point T1 near the light emitting point group DG of the first lens 5d. Specifically, the center of the light emitting point group DG is between a straight line LL2 extending parallel to the inter-principal point line LL1 passing through the outer principal point T1 of the first lens 5d and the line of symmetry E1 of the first lens 5d. positioned. By appropriately setting the position of the light emitting point group DG according to the tilt direction of the first lens 5d, a good imaging state can be obtained.

FIG. 4A shows a light ray 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 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 travels toward the photoreceptor. It travels substantially parallel until it hits the drum 71 . Also, the light ray LB is not horizontal between the first lens 5d and the second lens 5f, but is inclined in the direction of inclination of the common planes of symmetry SE1 and SE2 of the lenses 5d and 5f, which are vertically symmetrical with respect to the direction of travel of the principal ray of light, that is, the plane of the paper. , incline to the right and proceed.

In the comparative example shown in FIG. 5, first and second lenses 5d and 5f constituting imaging optical systems 2a, 2b and 2c have four curved lens surfaces S1 to S4. These four lens surfaces S1 to S4 are all axially symmetrical aspherical surfaces, and the rotationally symmetrical axes RT are all coincident. Further, the rotationally symmetrical axes RT of the three imaging optical systems 2a, 2b, 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 the extension line of the rotational symmetry axis RT. Further, the center of the image forming point on the photosensitive drum 71 of each of the image forming optical systems 2a, 2b, 2c is on the extension line of the axis of rotational symmetry 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 It shows the angle of inclination with respect to the first and second lens substrates 5h and 5k. 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 differ between the upper imaging optical system 2b and the lower imaging optical system 2c. The thickness difference 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 this embodiment are axisymmetric aspherical surfaces instead of free curved surfaces. . The lenses 5d and 5f of the comparative example are rotationally symmetrical, but the rotationally symmetrical axis K1 of the first lens 5d and the rotationally symmetrical axis K2 of the second lens 5f do not match, and the respective rotationally symmetrical axes K1 and K2 are This is a state in which they are shifted parallel to each other while being inclined with respect to the optical axis AX. 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 line LL1 between the principal points.

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, A light ray LB emitted from the lens surface S4 of 5f and directed to an image forming point on the photosensitive drum 71 also travels horizontally.

In this embodiment, in each imaging optical system 2a, 2b, 2c, the first lens array A1, the second lens array A2, and the diaphragm array A3 are flat as a whole, and these members A1 to A3 are light emitting points. It is arranged so as to divide the optical path from the group DG to the image forming point on the photosensitive drum 71 into approximately four equal parts. In this case, the tangent of the angle θa between 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 1, for example. Also, the tangent of the angle θb between the plane perpendicular to the optical axis AX and the same plane (second plane PLb) connecting the centers of the first lenses 5d of the first lens array A1 is, for example, about 3/4. . The tangent of the angle θd between the plane perpendicular to the optical axis AX and the same plane (fourth plane PLd) connecting the centers of the diaphragms 5e of the diaphragm array A3 is, for example, about 1/2. The tangent of the angle θc between the plane perpendicular to the optical axis AX and the same plane (third plane PLc) connecting the centers of the second lenses 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 symmetrical but are free curved surfaces. Inconsistency is reduced, and the position of the light emitting point group DG is appropriately avoided on the straight line LL2 extending parallel to the line LL1 between the principal points through the outer principal point T1 on the side closer to the light emitting point group DG of the first lens 5d. By setting the lens surfaces S1 to S4 as free-form surfaces, it is possible to eliminate the poor image forming state and obtain a good image forming state over the entire image area.

〔Example〕
Specific examples of the optical system 73b incorporated in the image forming apparatus 100 will be described below. The optical system 73b of the example is the same as that shown in FIG. In the following tables, the upper imaging optical system means the imaging optical system 2b of FIG. 2, the middle imaging optical system means the imaging optical system 2a of FIG. Optical system means the imaging optical system 2c of FIG. The flat plates forming the first lens array A1, the diaphragm array A3, and the second lens array A2 are common to the imaging optical systems 2a to 2c. The light emission point group DG indicates the central coordinates in the table. The photosensitive drum 71 has a cylindrical shape with a radius of 25 mm, and has a surface common to the three imaging optical systems 2a, 2b, and 2c. The position and tilt at the intersecting position are shown. Looking at the aspheric coefficients, the absolute values are the same and the signs are opposite across the diaphragm 5e. 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]
The data of the central imaging optical system 2a will be described below. 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 degree. For light with a wavelength of 650 nm, the lens substrate has a refractive index of 1.5145, and the resin lens portion provided between the lens surface and the lens substrate has a refractive index of 1.5285. Also, the imaging magnification β is −1 in all optical systems.
[Table 1]

Table 2 summarizes the aspheric shape of the central imaging optical system 2a. All of the aspherical surfaces described are free-form surfaces with no spherical terms.

is. All aspheric coefficients _aij not shown in the table are 0. These points are the same below.
[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]
The data of the upper imaging optical system 2b will be described below. 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 that constitute the upper imaging optical system 2b.
[Table 3]

Table 4 summarizes the free-form 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]
The data of the lower imaging optical system 2c will be described below. Table 5 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 imaging optical system 2c on the lower side.
[Table 5]

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

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

7A to 7C show the PSF (Point Spread Function) of the optical print head 73 of the embodiment. Although the actual light emitting point of the optical print head 73 is a surface light source, the beam profile shown here is calculated with a point light source. FIG. 7A 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. 7B is the center of the light emitting point group DG in the main scanning direction. FIG. 7C was calculated by placing a point light source at a position corresponding to the right end of the light emitting point group DG in the main scanning direction.

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 group DG is the same as that of the optical print head of the comparative example shown in FIG. 4(B). In FIGS. 7(D) to 7(F), a good image forming state is obtained at the center of the light emitting point group DG, but the image forming state deteriorates at the left end and 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-described embodiment, the number of imaging optical systems constituting the optical system 73b is not limited to three, and may be two or four or more.

In the above-described embodiment, specific examples regarding the number and arrangement of the light emitting points ED constituting the light emitting point group DG are merely examples, and the number and arrangement of the light emitting points ED can be changed according to the application and purpose.

In the above embodiment, the diaphragm 5e has the openings 5s formed in the diaphragm substrate 5p. It may be

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. Also, the length and thickness of the light emitting substrates 76a, 76b, and 76c can be changed as appropriate.

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

2a, 2b, 2c... Imaging optical system 4a-4c... Light receiving area 3a, 3b, 3c... Light emitting area 4a, 4b, 4c... Light receiving area 5d... First lens 5f... Second lens 5g... Flat plate 5h First lens substrate 5k Second lens substrate 5i, 5j, 5m, 5n Lens section 5p Diaphragm substrate 5s Aperture 10 Image reading section 20 Image forming section 40 Paper feeding unit 50 Conveying unit 70, 70Y, 70M, 70C, 70K Image forming unit 71 Photoconductor drum 71a Light receiving surface 72 Charging unit 73 Optical print head 73a Light emitting element 73b... Optical system 74... Developing unit 76a, 76b, 76c... Light emitting substrate 81... Intermediate transfer unit 82... Fixing unit 90... Control unit 100... Image forming apparatus A1... First lens array A2... Second lens array A3... Diaphragm array AX... Optical axis DG... Emission point group SE1, SE2, SE3... Common plane of symmetry E1, E2... Line of symmetry ED... Emission point K1, K2... Axis of rotational symmetry , LB... Light ray LL1... Line between principal points PD... Projected image PG... Group of projected images RT... Axis of rotational symmetry S1, S4... Outside lens surface S2, S3... Stop side lens surface T1, T4... Outer principal points, T2, T3... Restriction side principal points

Claims (8)

a photoreceptor having a surface transported in a secondary direction substantially perpendicular to the primary direction;
a light-emitting substrate having a plurality of light-emitting point groups arranged two-dimensionally;
a plurality of imaging optical systems for forming images of the light from the plurality of light emitting point groups at different positions on the photoreceptor;
with
The imaging magnification of the imaging optical system is -1,
When viewed from the direction of the rotation axis of the photoreceptor, the conjugate lengths of the plurality of imaging optical systems differ depending on the positions in the sub-direction and monotonically increase or decrease according to the positions in the sub-direction,
Each imaging optical system among the plurality of imaging optical systems has a first lens, a diaphragm, and a second lens in order from the light emitting point group side,
The first and second lenses each have two curved lens surfaces,
the two lens surfaces have two common planes of symmetry, and at least one of the two lens surfaces is axisymmetric with respect to a line of symmetry intersecting the two common planes of symmetry;
The first and second lenses have the same shape in which the aperture-side lens surfaces close to the aperture are opposite to each other with respect to the light traveling direction of the principal ray, and the outer lens surfaces far from the aperture have the same shape in which they are opposite to each other with respect to the light traveling direction. with the same core thickness,
In the first and second lenses, the center of the diaphragm is arranged on a principal point line connecting principal points near the diaphragm, and the line of symmetry is not zero with respect to the principal point line. angled and the lines of symmetry of the first and second lenses are parallel to each other;
The image forming apparatus, wherein the center of the light emitting point group is arranged avoiding a straight line extending parallel to the line between the principal points passing through an outer principal point near the light emitting point group of the first lens. . The center of the light emitting point group is positioned between a straight line passing through the outer principal point of the first lens and extending parallel to the line between the principal points and the line of symmetry of the first lens. The image forming apparatus according to claim 1. 3. 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 rotational axis of the photoreceptor. 1. The image forming apparatus according to item 1. For the plurality of imaging optical systems, the first lens has resin lens parts on both sides of a single first lens substrate, and the second lenses have lens parts on both sides of a single second lens substrate. 4. The image forming apparatus according to claim 1, further comprising a resin lens. 5. The image forming apparatus according to claim 4, wherein the first and second lens substrates are arranged to be inclined with respect to the line between the principal points of the first and second lenses. the direction of inclination of the normal to the first lens substrate and the direction of inclination of the line of symmetry of the first and second lenses with respect to the line between the principal points of the first and second lenses are the same; 6. The image forming apparatus according to claim 5, wherein the inclination direction of the normal to the second lens substrate and the inclination direction of the symmetry lines of the first and second lenses are the same. 6. The image forming apparatus according to claim 4, wherein the first lens substrate and the second lens substrate have different angles of inclination with respect to the line between principal points. An inclination angle of the line of symmetry with respect to the line between principal points is between an inclination angle of a normal line of the first lens substrate and an inclination angle of a normal line of the second lens substrate with respect to the line between principal points. 8. The image forming apparatus according to claim 7, wherein:

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