JP2010162849A - Line head and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a line head which can achieve highly accurate exposure processing, and to provide an image forming apparatus which can obtain high-quality images. <P>SOLUTION: The line head 13 has a light emitting element group 71 comprised of a plurality of light emitting elements 74 arranged in one direction, and an imaging optical system 60 which images the light from the light emitting elements 74 that constitute the light emitting element group 71. The imaging optical system 60 has an axis of symmetry (light axis 601) when seen on a cross section that includes its light axis 601 and in the one direction. Moreover, the imaging optical system is configured to have focal points FP1 and FP2 present shifting in the light axis direction respectively on the imaging optical system 60 side and the opposite side to an imaging point FP0 in the vicinity of the axis of symmetry of the imaging optical system 60. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a line head and an image forming apparatus.

2. Description of the Related Art Image forming apparatuses such as copying machines and printers using an electrophotographic system are provided with exposure means for forming an electrostatic latent image by exposing the outer surface of a rotating photoreceptor. As such an exposure means, a line head having a structure in which a plurality of light emitting elements are arranged in the direction of the rotation axis of a photoreceptor is known (for example, see Patent Document 1).
As such a line head, for example, Patent Document 1 discloses an optical information writing device in which a plurality of LED array chips each having a plurality of LEDs (light emitting elements) are arranged in one direction.

In such an optical information writing device, a plurality of LEDs of each LED array chip are arranged in the rotation axis direction of the photosensitive member, and a convex lens element (imaging optical system) is provided for each LED array chip. A convex lens element images light from each LED of the LED array chip.
In such a line head disclosed in Patent Document 1, the imaging performance of the convex lens element decreases as it moves away from the optical axis due to the curvature of field of the convex lens element. The spot diameter of light from the LED provided proximal to the axis is different from the spot diameter of light from the LED provided distal to the optical axis of the convex lens element. As a result, the latent image formed on the surface of the photoreceptor is provided distal to the optical axis of the convex lens element and the pixel formed by the light from the LED provided proximal to the optical axis of the convex lens element. A density difference occurs between the pixels formed by the light from the LEDs and density unevenness occurs.
In addition, the positional relationship between the image surface of the convex lens element and the irradiated surface (photosensitive member surface) may be shifted or fluctuated due to an assembly error of the line head with respect to the image forming apparatus main body, the eccentricity of the photosensitive member, etc. Density unevenness occurs.

Japanese Patent Laid-Open No. 2-4546

  An object of the present invention is to provide a line head capable of realizing a highly accurate exposure process and to provide an image forming apparatus capable of obtaining a high-quality image.

Such an object is achieved by the present invention described below.
The line head of the present invention includes a light emitting element disposed in the first direction,
An imaging optical system that forms an image of light emitted from the light emitting element;
The imaging optical system includes a rotationally symmetric lens,
In the first direction cross section passing through the symmetry axis of the lens, the imaging optical system has a longitudinal aberration whose sign is inverted with respect to the image plane.

In the line head according to the aspect of the invention, it is preferable that the imaging optical system is plane-symmetric with respect to a second direction cross section including the symmetry axis and orthogonal to the first direction.
In the line head according to the aspect of the invention, the imaging optical system may be configured such that a difference between a maximum value and a minimum value of longitudinal aberration in the first direction cross section of the imaging optical system is emitted from the light emitting element. It is preferable that it is larger than the minimum spot diameter of the light converged after passing through.

In the line head according to the aspect of the invention, it is preferable that the rotationally symmetric lens is a multifocal lens having different focal points.
In the line head according to the aspect of the invention, the lens includes a first region provided so as to include an intersection with the symmetry axis of the lens, and a second region provided around the first region. It is preferable that the lens surface has a lens surface, and the shape of the lens surface is defined by different definition formulas for the first region and the second region.
In the line head of the present invention, the imaging optical system has a plurality of lenses arranged in the direction of the symmetry axis,
It is preferable that the lens located closest to the light emitting element among the plurality of lenses has the first region and the second region.

The image forming apparatus of the present invention includes a latent image carrier on which a latent image is formed,
A line head that exposes the latent image carrier to form the latent image, and
The line head is
A light emitting device arranged in a first direction;
An imaging optical system that forms an image of light emitted from the light emitting element;
The imaging optical system includes a rotationally symmetric lens,
In the first direction cross section passing through the symmetry axis of the lens, the imaging optical system has a longitudinal aberration whose sign is inverted with respect to the latent image carrier.

According to the line head of the present invention having the above configuration, when the light emitted from the light emitting element is imaged by the imaging optical system, it covers a relatively wide range in the optical axis direction near the image plane. The spot diameter can be made almost constant. For this reason, even if the positional relationship in the optical axis direction between the image surface and the irradiated surface fluctuates or shifts, the fluctuation of the spot diameter on the irradiated surface is suppressed, and as a result, the density unevenness of the formed latent image is reduced. Can be suppressed. In this way, the line head of the present invention can realize high-precision exposure processing.
Further, according to the image forming apparatus of the present invention, it is possible to obtain a high-quality image in which density unevenness is suppressed by realizing the high-precision exposure processing as described above.

1 is a schematic diagram illustrating an overall configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a partial cross-sectional perspective view of a line head provided in the image forming apparatus shown in FIG. 1. It is the sectional view on the AA line in FIG. It is a figure which shows the positional relationship of a lens and a light emitting element when the line head shown in FIG. 2 is planarly viewed. FIG. 3 is a cross-sectional view (a cross section along a first direction) showing an imaging optical system provided in the line head shown in FIG. 2. It is a figure which shows the lens by the side of the light emitting element with which the imaging optical system shown in FIG. 5 was equipped. It is a figure for demonstrating the effect | action of the lens shown in FIG. It is a figure for demonstrating the effect | action of the imaging optical system shown in FIG. It is a figure which shows the imaging optical system with which the line head concerning the Example of this invention was equipped. It is a graph which shows the longitudinal aberration of the imaging optical system with which the line head concerning the Example of this invention was equipped. 6 is a graph showing spot diameters in the vicinity of the photoreceptor surface (image plane) in each of the imaging optical system of the line head of the example of the present invention and the imaging optical system of the line head of the comparative example.

Hereinafter, a line head and an image forming apparatus according to the present invention will be described in detail based on preferred embodiments shown in the accompanying drawings.
FIG. 1 is a schematic diagram showing an overall configuration of an image forming apparatus according to an embodiment of the present invention, FIG. 2 is a partial sectional perspective view of a line head provided in the image forming apparatus shown in FIG. 1, and FIG. 4 is a cross-sectional view taken along line AA in FIG. 2, FIG. 4 is a diagram showing the positional relationship between the lens and the light emitting element when the line head shown in FIG. 2 is viewed in plan, and FIG. 5 is provided in the line head shown in FIG. FIG. 6 is a diagram showing a lens on the light emitting element side provided in the imaging optical system shown in FIG. 5, and FIG. 7 is a sectional view showing the imaging optical system obtained. FIG. 8 is a diagram for explaining the operation of the lens shown in FIG. 6, and FIG. 8 is a diagram for explaining the operation of the imaging optical system shown in FIG. Hereinafter, for convenience of explanation, the upper side in FIGS. 1 to 3 and FIG. 5 is referred to as “upper” and the lower side is referred to as “lower”.

(Image forming device)
An image forming apparatus 1 shown in FIG. 1 is an electrophotographic printer that records an image on a recording medium P through a series of image forming processes including a charging process, an exposure process, a developing process, a transfer process, and a fixing process. In the present embodiment, the image forming apparatus 1 is a color printer that employs a so-called tandem method.
As shown in FIG. 1, the image forming apparatus 1 includes an image forming unit 10 for a charging process, an exposure process, and a developing process, a transfer unit 20 for a transfer process, and a fixing unit for a fixing process. 30, a transport mechanism 40 for transporting a recording medium P such as paper, and a paper feed unit 50 that supplies the recording medium P to the transport mechanism 40.

The image forming unit 10 includes an image forming station 10Y that forms a yellow toner image, an image forming station 10M that forms a magenta toner image, an image forming station 10C that forms a cyan toner image, and a black toner image. Four image forming stations including an image forming station 10K to be formed are provided.
Each of the image forming stations 10Y, 10C, 10M, and 10K has a photosensitive drum (photoconductor) 11 that is a latent image carrier that carries an electrostatic latent image. A charging unit 12, a line head (exposure unit) 13, a developing device 14, and a cleaning unit 15 are arranged around the photosensitive drum 11 (outer peripheral side) along the rotation direction. The image forming stations 10Y, 10C, 10M, and 10K have substantially the same configuration except that the color of the toner used is different.

The photosensitive drum 11 has a cylindrical shape as a whole, and can rotate around the axis in the direction of the arrow in FIG. A photosensitive layer (not shown) is provided near the outer peripheral surface (cylindrical surface) of the photosensitive drum 11. The outer peripheral surface of the photosensitive drum 11 has a light receiving surface 111 that receives light L (emitted light) from the line head 13 (see FIG. 2).
The charging unit 12 uniformly charges the light receiving surface 111 of the photosensitive drum 11 by corona charging or the like.

  The line head 13 receives image information from a host computer such as a personal computer (not shown) and irradiates the light L toward the light receiving surface 111 of the photosensitive drum 11 in response to the image information. When the light L is irradiated on the light receiving surface 111 of the uniformly charged photosensitive drum 11, a latent image (electrostatic latent image) corresponding to the irradiation pattern of the light L is formed on the light receiving surface 111. The configuration of the line head 13 will be described in detail later.

The developing device 14 has a storage unit (not shown) that stores toner, and supplies toner from the storage unit to the light receiving surface 111 of the photosensitive drum 11 that carries an electrostatic latent image. To do. Thereby, the latent image on the photosensitive drum 11 is visualized (developed) as a toner image.
The cleaning unit 15 has a rubber cleaning blade 151 that abuts on the light receiving surface 111 of the photosensitive drum 11, so that the toner remaining on the photosensitive drum 11 after the primary transfer described later is scraped off and removed by the cleaning blade 151. It has become.

The transfer unit 20 collectively transfers the toner images of the respective colors formed on the photosensitive drums 11 of the image forming stations 10Y, 10M, 10C, and 10K as described above to the recording medium P.
In each of the image forming stations 10Y, 10C, 10M, and 10K, the charging unit 12 charges the light receiving surface 111 of the photosensitive drum 11 and the line head 13 exposes the light receiving surface 111 while the photosensitive drum 11 rotates once. Then, supply of toner to the light receiving surface 111 by the developing device 14, primary transfer to the intermediate transfer belt 21 by pressure contact with a primary transfer roller 22 described later, and cleaning of the light receiving surface 111 by the cleaning unit 15 are sequentially performed.

The transfer unit 20 includes an endless belt-like intermediate transfer belt 21, and the intermediate transfer belt 21 includes a plurality of (four in the configuration shown in FIG. 1) primary transfer rollers 22, drive rollers 23, and driven rollers 24. It is stretched, and is driven to rotate in the direction of the arrow shown in FIG.
Each primary transfer roller 22 is disposed opposite to the corresponding photosensitive drum 11 via an intermediate transfer belt 21, and transfers a single color toner image on the photosensitive drum 11 to the intermediate transfer belt 21 (primary transfer). It is like that. At the time of primary transfer, the primary transfer roller 22 is applied with a primary transfer voltage (primary transfer bias) having a polarity opposite to the charging polarity of the toner.

On the intermediate transfer belt 21, a toner image of at least one of yellow, magenta, cyan, and black is carried. For example, when a full-color image is formed, four color toner images of yellow, magenta, cyan, and black are sequentially transferred onto the intermediate transfer belt 21 to form a full-color toner image as an intermediate transfer image.
Further, the transfer unit 20 includes a secondary transfer roller 25 disposed to face the driving roller 23 via the intermediate transfer belt 21, and a cleaning unit 26 disposed to face the driven roller 24 via the intermediate transfer belt 21. have.

  The secondary transfer roller 25 transfers a single-color or full-color toner image (intermediate transfer image) formed on the intermediate transfer belt 21 to a recording medium P such as paper, film, or cloth supplied from the paper supply unit 50. (Secondary transfer). The secondary transfer roller 25 is pressed against the intermediate transfer belt 21 and applied with a secondary transfer voltage (secondary transfer bias) during secondary transfer. During such secondary transfer, the drive roller 23 also functions as a backup roller for the secondary transfer roller 25.

The cleaning unit 26 has a rubber cleaning blade 261 that contacts the surface of the intermediate transfer belt 21, and the toner remaining on the intermediate transfer belt 21 after the secondary transfer is scraped off and removed by the cleaning blade 261. It has become.
The fixing unit 30 includes a fixing roller 301 and a pressure roller 302 that is pressed against the fixing roller 301, and is configured such that the recording medium P passes between the fixing roller 301 and the pressure roller 302. Yes. The fixing roller 301 has a built-in heater for heating the outer peripheral surface of the fixing roller, and can heat and press the recording medium P passing therethrough. The fixing unit 30 having such a configuration heats and presses the recording medium P that has received the secondary transfer of the toner image, and fuses the toner image to the recording medium P to fix it as a permanent image.

The conveyance mechanism 40 includes a registration roller pair 41 that conveys the recording medium P while feeding the recording medium P to the secondary transfer portion between the secondary transfer roller 25 and the intermediate transfer belt 21 described above, and fixing by the fixing unit 30. Conveying roller pairs 42, 43, and 44 for nipping and conveying the processed recording medium P are provided.
When such a transport mechanism 40 forms an image on only one surface of the recording medium P, the transport mechanism 40 sandwiches and transports the recording medium P fixed on one surface by the fixing unit 30 by the transport roller pair 42. Then, it is discharged outside the image forming apparatus 1. When forming an image on both surfaces of the recording medium P, the recording medium P fixed on one surface by the fixing unit 30 is once sandwiched by the conveying roller pair 42 and then the conveying roller pair 42 is driven to reverse. Then, the pair of conveying rollers 43 and 44 are driven, the recording medium P is turned upside down and returned to the registration roller pair 41, and an image is formed on the other surface of the recording medium P by the same operation as described above.
The paper feeding unit 50 includes a paper feeding cassette 51 that stores unused recording media P, and a pickup roller 52 that feeds the recording media P from the paper feeding cassette 51 to the registration roller pair 41 one by one. .

(Line head)
Here, the line head 13 will be described in detail. In the following, for convenience of explanation, the longitudinal direction (first direction) of the long lens array 6 is referred to as “main scanning direction”, and the width direction (second direction) is referred to as “sub-scanning direction”. .
As shown in FIG. 3, the line head 13 is disposed below the photosensitive drum 11 so as to face the light receiving surface 111. The line head 13 includes, from the photosensitive drum 11 side, a lens array (first lens array) 6 ′, a spacer 84, a lens array (second lens array) 6, a light shielding member (first light shielding member) 82, a diaphragm. A member (aperture stop) 83, a light shielding member (second light shielding member) 81, and the light emitting element array 7 are arranged in this order, and these members are accommodated in the casing 9.

In the line head 13, the light L emitted from the light emitting element array 7 is narrowed by the diaphragm member 83, and then passes through the lens array 6 ′ and the lens array 6 in this order to irradiate the light receiving surface 111 of the photosensitive drum 11. Is done.
As shown in FIG. 2, each of the lens array 6 and the lens array 6 ′ is composed of a plate-like body whose outer shape is long.

As shown in FIG. 3, a plurality of lens surfaces (convex curved surfaces) 62 are formed on the lower surface (incident surface) on which the light L of the lens array 6 is incident. On the other hand, the upper surface (emission surface) from which the light L of the lens array 6 is emitted is a flat surface.
That is, in the lens array 6, a plurality of lenses 64, which are planoconvex lenses having a light curved surface on the incident side of the light L and a flat surface on the light emitting side of the light L, are arranged. Here, portions of the lens array 6 other than the lenses 64 constitute a support portion 65 that supports the lenses 64.

Similarly, a plurality of lens surfaces (convex curved surfaces) 62 ′ are formed on the lower surface (incident surface) on which the light L of the lens array 6 ′ is incident, corresponding to the plurality of lens surfaces 62 described above. On the other hand, the upper surface (outgoing surface) from which the light L of the lens array 6 ′ is emitted is a flat surface.
That is, in the lens array 6 ′, a plurality of lenses 64 ′, which are planoconvex lenses having a light curved surface on the incident side of the light L and a flat surface on the light emitting side of the light L, are arranged. Here, the portions other than the lenses 64 ′ of the lens array 6 ′ constitute a support portion 65 ′ that supports the lenses 64 ′.

The corresponding pair of lenses 64 and lens 64 ′ constitutes an imaging optical system 60 that forms an image of light emitted from each light emitting element 74 of the corresponding light emitting element group 71 (see FIGS. 5 and 6). ). The imaging optical system 60 (particularly the lens surface shape of the lenses 64 and 64 ′) will be described in detail later.
Hereinafter, the arrangement of the lenses 64 will be described. Note that the arrangement of the lens 64 ′ (arrangement in plan view) is the same as the arrangement of the lens 64, and thus description thereof is omitted.

As shown in FIG. 4, the lenses 64 are arranged in a plurality of rows in the main scanning direction (first direction), and in the sub-scanning direction (second scanning direction orthogonal to the main scanning direction and the optical axis direction of the lens 64, respectively. Multiple lines are arranged in the direction).
More specifically, the plurality of lenses 64 are arranged in a matrix of 3 rows and n columns (n is an integer of 2 or more). Hereinafter, among the three lenses 64 belonging to one row (lens row), the lens 64 located at the center is referred to as a “lens 64b” and is located on the left side in FIG. 3 (upper side in FIG. 4). The lens 64 is referred to as “lens 64a”, and the lens 64 positioned on the right side in FIG. 3 (lower side in FIG. 4) is referred to as “lens 64c”. In addition, regarding the lens 64 ′ paired with the lens 64, the lens 64 ′ corresponding to the lens 64a is referred to as “lens 64a ′”, the lens 64 ′ corresponding to the lens 64b is referred to as “lens 64b ′”, and the lens 64c. The corresponding lens 64 ′ is referred to as “lens 64c ′”.

In the present embodiment, among the plurality of lenses 64 (64a to 64c) belonging to one row, the lens 64b closest to the center side in the sub-scanning direction is closest to the light receiving surface 111 of the photosensitive drum 11. The line head 13 is installed in the image forming apparatus 1 so that Thereby, the setting of the optical characteristics of the plurality of lenses 64 is facilitated.
As shown in FIGS. 2 and 4, in each lens row, the lenses 64 a to 64 c are sequentially arranged at equal distances in the main scanning direction (right direction in FIG. 4). That is, in each lens row, the line connecting the lens centers of the lenses 64a to 64c is inclined at a predetermined angle with respect to the main scanning direction and the sub-scanning direction.

In the cross section shown in FIG. 3, in the three lenses 64 belonging to one lens row, that is, the lenses 64 a to 64 c, the lens 64 a and the lens 64 c have the optical axes 601 of the lenses 64 a and the optical axes 601 of the lenses 64 b. Are arranged symmetrically. The lenses 64a to 64c are arranged so that their optical axes 601 are parallel to each other.
The constituent material of the lens arrays 6 and 6 ′ is not particularly limited as long as it can exhibit the optical characteristics as described above. For example, a resin material and / or a glass material is preferably used. It is done.

  As this resin material, various resin materials can be used. For example, a liquid crystal polymer such as polyamide, thermoplastic polyimide, polyamideimide aromatic polyester, polyolefin such as polyphenylene oxide, polyphenylene sulfide, polyethylene, modified polyolefin, polycarbonate, acrylic (Methacrylic), Polymethylmethacrylate, Polyethylene terephthalate, Polybutylene terephthalate and other polyesters, Polyether, Polyetheretherketone, Polyetherimide, Polyacetal and other thermoplastic resins, Epoxy resins, Phenol resins, Urea resins, Melamine resins Thermosetting resins such as saturated polyester resins and polyimide resins, photocurable resins, etc. are mentioned, and one or more of these are combined. It is possible to have.

Among such resin materials, resin materials such as thermosetting resins and photo-curing resins have the advantage of a relatively high refractive index, and also have a relatively low thermal expansion coefficient, and expansion (deformation) due to heat. ), A material that is unlikely to be modified or deteriorated.
Examples of the glass material include various glass materials such as soda glass, crystalline glass, quartz glass, lead glass, potassium glass, borosilicate glass, and non-alkali glass. When 72 is made of a glass material, a relative displacement between the light emitting element and each lens due to temperature fluctuations can be prevented by using a glass material having a linear expansion coefficient substantially equal to the glass material.

  Further, when the lens array 6 is configured by combining the resin material and the glass material as described above, for example, as described later, a glass substrate made of a glass material is used as the support portion 65, and on one surface thereof, A lens 64 may be formed by forming a resin layer made of a resin material and molding the lens surface 62 on the surface of the resin layer opposite to the glass substrate (see FIGS. 5 and 6). The lens array 6 can also be formed by, for example, providing a plurality of convex portions projecting in a convex curved shape made of a resin material on one surface of a flat plate member (substrate) made of a glass material. Can be formed.

As shown in FIGS. 2 and 3, a spacer 84 is provided between the lens array 6 and the lens array 6 ′. The lens array 6 and the lens array 6 ′ are bonded via a spacer 84.
The spacer 84 has a function of regulating a gap length that is a distance between the lens array 6 and the lens array 6 ′.

The spacer 84 has a frame shape so as to correspond to the outer peripheral portion of the lens array 6 and the outer peripheral portion of the lens array 6 ′, and is joined to each of the outer peripheral portions. The spacer 84 is not limited to the above-described frame shape as long as it can perform the above-described function. For example, one of the outer peripheral portions of the lens arrays 6 and 6 ′ facing each other is disposed. It may be configured by a pair of members so as to correspond only to the portion corresponding to the side, or it is configured such that a through-hole corresponding to the optical path is formed in a plate-like member like light shielding members 81 and 83 described later. May be.
The constituent material of the spacer 84 is not particularly limited as long as it can exhibit the functions described above, and a resin material, a metal material, a glass material, a ceramic material, or the like can be used.

As shown in FIG. 3, the light emitting element array 7 is installed on the light incident side of the lens array 6 via a light shielding member 82, a diaphragm member 83, and a light shielding member 81. The light emitting element array 7 includes a plurality of light emitting element groups (light emitting element groups) 71 and a support plate (head substrate) 72.
The support plate 72 supports each light emitting element group 71, and is configured by a plate-like body having a long outer shape. The support plate 72 is disposed in parallel with the lens array 6.

The support plate 72 is longer in the main scanning direction than the lens array 6 in the main scanning direction. The length of the support plate 72 in the sub-scanning direction is also set longer than the length of the lens array 6 in the sub-scanning direction.
The constituent material of the support plate 72 is not particularly limited, but when the light emitting element group 71 is provided on the back side of the support plate 72 as in the present embodiment (that is, when a bottom emission type light emitting element is used as the light emitting element 74). ), Transparent materials such as various glass materials and various plastics are preferably used. In addition, when using a top emission type light emitting element as the light emitting element 74, the constituent material of the support plate 72 is not limited to a material having transparency, for example, various metal materials such as aluminum and stainless steel, and various glasses. A material, various plastics, etc. can be used individually or in combination. When the support plate 72 is made of various metal materials or various glass materials, heat generated by light emission of each light emitting element 74 can be efficiently radiated through the support plate 72. Further, when the support plate 72 is made of various plastics, it contributes to weight reduction of the support plate 72.

A box-shaped storage portion 73 that opens to the support plate 72 side is installed on the back side of the support plate 72. In this housing portion 73, a plurality of light emitting element groups 71, conductive wires (not shown) electrically connected to these light emitting element groups 71 (each light emitting element 74), or each light emitting element 74 is driven. A circuit (not shown) is housed.
The plurality of light emitting element groups 71 are arranged in a matrix of 3 rows and n columns (n is an integer equal to or greater than 2) spaced apart from each other, corresponding to the plurality of lenses 64 described above (for example, see FIG. 4). ). Each light emitting element group 71 is composed of a plurality (eight in this embodiment) of light emitting elements 74.

The eight light emitting elements 74 constituting each light emitting element group 71 are arranged along the lower surface 721 of the support plate 72 shown in FIG. The light L emitted from the eight light emitting elements 74 is condensed (imaged) on the light receiving surface 111 of the photosensitive drum 11 via the corresponding lens 64.
Further, as shown in FIG. 4, the eight light emitting elements 74 are spaced apart from each other, arranged in four columns in the main scanning direction, and arranged in two rows in the sub scanning direction. As described above, the eight light emitting elements 74 are arranged in a matrix of 2 rows and 4 columns. Two adjacent light emitting elements 74 belonging to one row (light emitting element row) are arranged shifted in the main scanning direction.

In the eight light emitting elements 74 having a matrix of 2 rows and 4 columns in this way, the light emitting elements 74 adjacent in the main scanning direction are complemented by one light emitting element 74 in the next row.
For example, there is a limit to arranging the eight light emitting elements 74 in one row as densely as possible. However, by arranging the eight light emitting elements 74 to be shifted as described above, the arrangement density of these light emitting elements 74 is reduced. Can be higher. Thereby, when an image is recorded on the recording medium P, the recording density on the recording medium P can be further increased. Therefore, it is possible to obtain the recording medium P having a high resolution, a multi-gradation and a clear image.

The eight light emitting elements 74 belonging to one light emitting element group 71 are arranged in a matrix of 2 rows and 4 columns in the present embodiment, but the present invention is not limited to this, and for example, a matrix of 4 rows and 2 columns. May be arranged.
As described above, the plurality of light emitting element groups 71 are spaced apart from each other and arranged in a matrix of 3 rows and n columns. As shown in FIG. 4, the three light emitting element groups 71 belonging to one row (light emitting element group row) are arranged at equal intervals in the main scanning direction (right direction in FIG. 4).

In the light emitting element group 71 having a matrix of 3 rows and n columns in this way, the interval between the adjacent light emitting element groups 71 is set so that the light emitting element group 71 in the next row and the light emitting element group 71 in the next row have the same distance. Complements sequentially.
For example, there is a limit in arranging the plurality of light emitting element groups 71 in one row as densely as possible. However, by arranging the plurality of light emitting element groups 71 so as to be shifted as described above, these light emitting element groups 71 are arranged. The arrangement density of can be made higher. Thereby, coupled with the fact that the eight light-emitting elements 74 in one light-emitting element group 71 are shifted and arranged, when an image is recorded on the recording medium P, the recording density on the recording medium P can be increased. Therefore, it is possible to obtain the recording medium P having higher resolution, multi-gradation, good color reproducibility and carrying a clearer image.

Each light emitting element 74 is an organic EL element (organic electroluminescence element) having a bottom emission structure. Note that the light emitting element 74 is not limited to an element having a bottom emission structure, and may be an element having a top emission structure. In this case, as described above, the support plate 72 is not required to have optical transparency.
When each light emitting element 74 is an organic EL element, the interval (pitch) between the light emitting elements 74 can be set to be relatively small. Thereby, when an image is recorded on the recording medium P, the recording density on the recording medium P becomes relatively high. In addition, each light emitting element 74 can be formed with highly accurate dimensions and positions by using various film forming methods. Therefore, the recording medium P carrying a clearer image can be obtained.

In the present embodiment, each light emitting element 74 is configured to emit red light. Here, examples of the constituent material of the light emitting layer that emits red light include (4-dicyanomethylene) -2-methyl-6- (paradimethylaminostyryl) -4H-pyran (DCM) and Nile red. It is done. In addition, each light emitting element 74 is not limited to being configured to emit red light, and may be configured to emit monochromatic light of other colors or white light. Thus, in the organic EL element, the light L emitted from the light emitting layer can be appropriately set to monochromatic light of an arbitrary color according to the constituent material of the light emitting layer.
In general, the spectral sensitivity characteristics of a photosensitive drum used in an electrophotographic process is set to have a peak in the region from red to the near infrared, which is the emission wavelength of a semiconductor laser. It is preferable to use materials.

As shown in FIG. 3, a light shielding member 82, a diaphragm member 83, and a light shielding member 81 are installed between the lens array 6 and the light emitting element array 7.
The light shielding members 81 and 82 prevent crosstalk of the light L between the adjacent light emitting element groups 71, respectively.
In such a light shielding member 81, a plurality of through holes (openings) 811 are formed through the light shielding member 81 in the vertical direction (thickness direction) in FIG. 3. Each of these through holes 811 is disposed at a position corresponding to each lens 64.

Similarly, the light shielding member 82 is formed with a plurality of through holes 821 that penetrate the light shielding member 82 in the vertical direction (thickness direction) in FIG. Each of these through holes 821 is disposed at a position corresponding to each lens 64 described above.
The through holes 811 and 821 each form an optical path from the light emitting element group 71 to the corresponding lens 64. The through holes 811 and 821 each have a circular shape in plan view, and include eight light emitting elements 74 of the light emitting element group 71 corresponding to the through holes 811 and 821 inside.
In addition, although each through-hole 811 and 821 has comprised the cylindrical shape in the structure shown in FIG. 3, it is not limited to this, For example, you may comprise the truncated cone shape expanded toward upper direction.

A diaphragm member 83 is installed between the light shielding members 81 and 82.
The diaphragm member 83 is an aperture diaphragm that limits the light L incident on the lens 64 from the light emitting element group 71 to a predetermined amount.
The diaphragm member 83 is plate-shaped or layered, and a plurality of through-holes (openings) 831 are formed through the diaphragm member 83 in the vertical direction (thickness direction) in FIG. These through-holes 831 are respectively arranged at positions corresponding to the respective lenses 64 (that is, the above-described through-holes 811 and 821).

Further, the through hole 831 of the aperture member 83 is circular in plan view, and the diameter thereof is smaller than the diameter of the through hole 811 of the light shielding member 81 described above.
Such a diaphragm member 83 is preferably set to a relatively short distance from the lens 64. Thereby, even if it is between the light emitting elements 74 from which the distance from the optical axis 601 differs (even if an angle of view is different), light can be entered into the substantially same area | region of the lens 64. FIG.

Such light shielding members 81 and 82 and the diaphragm member 83 also have a function of defining the distance, positional relationship, and posture between the lens array 6 and the support plate 72 with high accuracy.
The distance between the lens surface 62 of each lens 64 and the corresponding light emitting element group 71 is an important condition (element) for determining the vertical position in FIG. 3 of the imaging position of the imaging optical system 60 described later. It is. Therefore, as described above, when the light shielding members 81 and 82 and the diaphragm member 83 function also as a spacer that regulates the gap length, which is the distance between the lens array 6 and the light emitting element array 7, with high accuracy, A highly reliable image forming apparatus 1 is obtained.

Moreover, it is preferable that the light shielding members 81 and 82 and the diaphragm member 83 each have a dark color such as black, brown, or amber at least on the inner peripheral surface.
The constituent materials of the light shielding members 81 and 82 and the diaphragm member 83 are not particularly limited as long as they do not transmit light. For example, various colorants, metal materials such as chromium and chromium oxide, Examples thereof include carbon black and a resin kneaded with a colorant.

As shown in FIGS. 2 and 3, the lens array 6, the light emitting element array 7, the spacer 84, the light shielding members 81 and 82, and the diaphragm member 83 are collectively stored in the casing 9. The casing 9 includes a frame member (casing body) 91, a lid member (back cover) 92, and a plurality of clamp members 93 that fix the lid member 92 to the frame member 91 (see FIG. 3).
As shown in FIGS. 2, 5, and 6, the frame member 91 has a long overall shape.

Further, the frame member 91 has a frame shape, and as shown in FIG. 3, the frame member 91 is formed with a lumen portion 911 that opens to the upper side and the lower side thereof. The width of the lumen portion 911 gradually decreases from the lower side to the upper side in FIG.
In the lumen portion 911, the lens array 6 ′, the spacer 84, the lens array 6, the light shielding member 82, the diaphragm member 83, the light shielding member 81, and the light emitting element array 7 are fitted, and these are fixed with, for example, an adhesive. Has been. As a result, the lens array 6 ′, the spacer 84, the lens array 6, the light shielding member 82, the diaphragm member 83, the light shielding member 81, and the light emitting element array 7 are collectively held by the frame member 91. The lens array 6, the light shielding member 82, the diaphragm member 83, the light shielding member 81, and the light emitting element array 7 are positioned in the main scanning direction and the sub scanning direction.

Here, the upper surface 722 of the support plate 72 of the light emitting element array 7 is in contact with (in contact with) the stepped portion 915 formed on the wall surface of the lumen portion 911 and the lower surface of the second light shielding member 81. Yes. A lid member 92 is fitted into the lumen portion 911 from below.
The lid member 92 is formed of a long member having a recess 922 into which the storage portion 73 is inserted. The upper end surface of the lid member 92 sandwiches the edge portion of the support plate 72 of the light emitting element array 7 with the boundary portion 915 of the frame member 91.

Further, the lid member 92 is pressed upward by each clamp member 93. Thereby, the lid member 92 is fixed to the frame member 91. Further, the positional relationship between the light emitting element array 7, the light shielding members 81, 82, the diaphragm member 83, and the lens array 6 in the main scanning direction, the sub scanning direction, and the vertical direction in FIG. 3 is fixed by the pressed lid member 92. Is done.
A plurality of clamp members 93 are preferably arranged at equal intervals along the main scanning direction. Thereby, the frame member 91 and the lid member 92 can be uniformly clamped along the main scanning direction.

The clamp member 93 has a substantially U shape in the cross section shown in FIG. 3 and is formed by bending a metal plate. Both end portions of the clamp member 93 form claw portions 931 that are bent inward. Each claw portion 931 is engaged with the shoulder portion 916 of the frame member 91.
A curved portion 932 that is curved upward in an arch shape is formed in the intermediate portion of the clamp member 93. The top portion of the curved portion 932 is in pressure contact with the lower surface of the lid member 92 in a state where each claw portion 931 is engaged with the shoulder portion 916 as described above. Thereby, the lid member 92 is biased upward in a state where the bending portion 932 is elastically deformed.

In addition, when each clamp member 93 holding the frame member 91 and the lid member 92 is removed, the lid member 92 can be removed from the frame member 91. Thereby, maintenance such as replacement and repair of the light emitting element array 7 can be performed.
Moreover, it does not specifically limit as a constituent material of the frame member 91 and the cover member 92, For example, the constituent material similar to the support plate 72 can be used. It does not specifically limit as a constituent material of the clamp member 93, For example, aluminum and stainless steel are mentioned. The clamp member 93 may be made of a hard resin material.
Furthermore, although not shown, spacers that protrude upward are provided at both ends of the frame member 91 in the longitudinal direction. This spacer regulates the distance between the light receiving surface 111 and the lens array 6.

(Imaging optics)
Here, the imaging optical system 60 of the line head 13 will be described in detail with reference to FIGS.
As described above, in the line head 13, the pair of lenses 64 and 64 ′ corresponding to the light emitting element group 71 including the light emitting elements 74 arranged in the first direction (first direction) are arranged in parallel in the optical axis direction. (Disposed in the direction of the axis of symmetry of the lens 64). As shown in FIG. 5, the pair of lenses 64 and 64 ′ constitute an imaging optical system 60 that forms an image of the light L from the light emitting elements 74 belonging to the corresponding light emitting element group 71.

  5 shows a cross section (hereinafter referred to as “main direction cross section” or “first direction cross section”) parallel to the optical axis direction (third direction) and the main scanning direction (first direction) of each imaging optical system 60. "). Hereinafter, the imaging optical system 60 including the pair of lenses 64a and 64a ′ is referred to as an “imaging optical system 60a” as necessary, and includes the pair of lenses 64b and 64b ′. The imaging optical system 60 is referred to as “imaging optical system 60b”, and the imaging optical system 60 including the pair of lenses 64c and 64c ′ is referred to as “imaging optical system 60c”.

The imaging optical system 60 forms an image of the light L after passing through the through hole 831 (aperture stop) of the diaphragm member 83 in the vicinity of the light receiving surface 111 of the photoconductor 11. In the present embodiment, the imaging optical system 60 is image side telecentric.
Here, the imaging optical system 60 includes a rotationally symmetric lens 64 as will be described later, and is a longitudinal section whose sign is inverted with respect to an image plane I to be described later in a first direction cross section passing through the symmetry axis of the lens 64. Has aberrations.

  Such an imaging optical system 60 has an axis of symmetry when viewed in a cross section (main cross section) including the optical axis 601 and along the main scanning direction (first direction). In the present embodiment, the symmetry axis of the imaging optical system 60 and the optical axis 601 coincide. That is, the imaging optical system 60 is plane-symmetric with respect to a cross section including the symmetry axis (optical axis 601) and orthogonal to the first direction (hereinafter referred to as “sub-direction cross section” or “second direction cross section”). is there. Thereby, the imaging characteristics of the imaging optical system 60 as will be described later can be realized easily and reliably.

The imaging optical system 60 only needs to have an axis of symmetry in the cross section in the main direction as described above, and the axis of symmetry may be different from the optical axis 601. In addition, the imaging optical system 60 may or may not be rotationally symmetric with respect to the optical axis 601, but in the following description, for convenience of explanation, the imaging optical system 60 is relative to the optical axis 601. It demonstrates as what is rotationally symmetric.
The imaging optical system 60 has an imaging point FP1 that is shifted to the imaging optical system 60 side with respect to the imaging point FP0 in the vicinity of the symmetry axis of the imaging optical system 60 and an imaging point that is shifted to the opposite side. It is configured to have a point FP1.

That is, when the light emitted from the light emitting element is incident, the imaging optical system 60 forms an image at different positions (imaging points FP0, FP1, FP2) depending on the portion through which the light passes through the imaging optical system 60. . In other words, the imaging optical system 60 has a plurality of imaging points FP0, FP1, and FP2 formed at different positions in the optical axis direction (having longitudinal aberration).
Here, the imaging point FP0 is obtained when light emitted from a point (object point) where the lower surface 721 of the support plate 72 and the optical axis 601 intersect is incident on the vicinity of the optical axis 601 of the imaging optical system 60. This is the position (paraxial imaging point) where the beam of (emitted light) intersects the optical axis 601. The imaging point FP1 is obtained when light emitted from a point (object point) where the lower surface 721 of the support plate 72 and the optical axis 601 intersect is incident on the imaging optical system 60 via the diaphragm member 83. This is the position closest to the imaging optical system 60 among the positions where the light beam (emitted light) intersects the optical axis 601. The imaging point FP2 is obtained when light emitted from a point (object point) where the lower surface 721 of the support plate 72 and the optical axis 601 intersect is incident on the imaging optical system 60 through the diaphragm member 83. Of the positions where the (emitted light) light beam intersects the optical axis 601, this is the position most distant from the imaging optical system 60.

That is, the imaging optical system 60 has longitudinal aberration on the imaging optical system 60 side and the opposite side when the imaging point FP0 is used as a reference. That is, the imaging optical system 60 has a longitudinal aberration in which the sign is inverted with respect to the image plane I. The difference between the maximum value and the minimum value of the longitudinal aberration corresponds to the distance G1 between the imaging points FP1 and FP2.
In such an imaging optical system 60, an imaging point located closest to the imaging point FP1 located most distal to the imaging optical system 60 among the plurality of imaging points FP0, FP1, and FP2. Between the point FP2, the spot diameter of the light L from the light emitting element 74 can be made small and substantially constant. In particular, when the imaging optical system 60 is positioned at the imaging point FP0 in the vicinity of the optical axis 601 between the imaging point FP1 and the imaging point FP2, other opticals necessary for the imaging optical system 60 are obtained. While satisfying the characteristics, the distance G1 (difference between the maximum value and the minimum value of longitudinal aberration) can be increased between the image formation point FP1 and the image formation point FP2. As a result, when the light emitted from the light emitting element 74 is imaged by the imaging optical system 60, the spot diameter can be made substantially constant over a relatively wide range in the optical axis direction near the image plane.

Therefore, even if the positional relationship in the optical axis direction (third direction) between the image surface and the light receiving surface 111 that is the irradiated surface fluctuates or shifts, the variation in the spot diameter on the light receiving surface 111 is suppressed, As a result, the density unevenness of the formed latent image can be suppressed.
In addition, the imaging optical system 60 has a distance G1 in the optical axis direction between the imaging point FP2 located most distal to the imaging optical system 60 and the imaging point FP1 located most proximal (that is, the coupling point). The difference between the maximum value and the minimum value of longitudinal aberration in the first direction cross section of the image optical system 60 is preferably larger than the minimum spot diameter (minimum spot size) of the light emitted from the light emitting element 74. Thereby, the fluctuation | variation of the spot diameter in the light-receiving surface 111 as mentioned above can be suppressed effectively.

The imaging optical system 60 having such characteristics can be realized by including multifocal lenses having different focal points.
In the present embodiment, the lens 64 and a multifocal lens having a plurality of focal points are used, and the lens 64 ′ is a single focal point lens having a single focal point, so that the imaging optical system 60 performs a plurality of imaging as described above. It is configured to have points FP0, FP1, and FP2.

As shown in FIG. 6A, the lens 64 is formed on a support portion 65 made of, for example, a glass material. As shown in FIG. 6B, the lens 64 has a lens surface 62 on the side opposite to the support portion 65.
As shown in FIG. 7, the lens surface 62 of the lens 64 is formed such that the lens 64 has a plurality of focal points fp0, fp1, and fp2 whose positions in the optical axis direction are different from each other.

  Here, the focal point fp0 is a position where a light beam (emitted light) intersects the optical axis 601 when light parallel to the optical axis 601 (light from infinity) enters the vicinity of the optical axis 601 of the lens 64. (Paraxial focus). Further, the focal point fp1 is the most relative to the lens 64 among the positions where the light beam (emitted light) intersects the optical axis 601 when light parallel to the optical axis 601 enters the lens 64 via the diaphragm member 83. This is the proximal position. Further, the focal point fp <b> 2 is the most relative to the lens 64 among the positions where the light beam of the light (emitted light) intersects the optical axis 601 when light parallel to the optical axis 601 enters the lens 64 through the diaphragm member 83. This is the distal position.

That is, the lens 64 has longitudinal aberration on the lens 64 side and the opposite side when the focal point fp0 is used as a reference. The difference between the maximum value and the minimum value of the longitudinal aberration corresponds to the distance g between the focal points fp1 and fp2.
More specifically, as shown in FIG. 6, the lens surface 62 of the lens 64 has a circular first region 62a provided at the center (including the intersection with the symmetry axis of the lens 64). And an annular second region 62b provided around the first region 62a. In FIG. 6, a light passing area that has passed through the diaphragm member 83 (aperture stop) is indicated by a broken line.

  The surface shape of the first region 62a and the surface shape of the second region 62b are defined by different definition formulas. As this definition formula, for example, the definition formula (rotationally symmetric aspherical surface) represented by the following formula 1 can be used (more specifically, refer to the examples described later). Thereby, the lens 64 having the above-described characteristics can be realized relatively easily and reliably.

ここで、上記の数１で表わされる定義式において、
ｚ：光軸方向（第３の方向）での座標
ｒ：光軸からの距離
ｃ：光軸上曲率
Ｋ：コーニック定数
Ａ〜Ｃ、Δ：非球面係数
である。

Here, in the definition formula represented by the above equation 1,
z: coordinates in the optical axis direction (third direction) r: distance from the optical axis c: curvature on the optical axis K: conic constants A to C, Δ: aspheric coefficient.

Further, the coefficients A to C and Δ in the above definition formula are as described above for the imaging optical system 60 according to the focal length of the imaging optical system 60, the shape of the lens surface 62 ′ of the lens 64 ′, and the like. It is appropriately set so as to have a plurality of image forming points.
Also, different definition formulas can be given to the first region 62a and the second region 62b by making at least one of the coefficients A to C and Δ of the above definition formula different.

In addition, the optical axis in said definitional expression is a symmetry axis of a rotationally symmetric lens.
Further, it is preferable that the first region 62a and the second region 62b have substantially the same area in the light passage region a. Thereby, even if the position of the image plane and the light receiving surface 111 in the optical axis direction fluctuates, the light amount unevenness (density unevenness) of the spot formed on the light receiving surface 111 can be suppressed.

Further, as described above, the imaging optical system 60 has a plurality (two) of lenses 64 and 64 ′ arranged in parallel in the optical axis direction. By having the lens surface 62 including the first region 62a and the second region 62b as described above, even between the light emitting elements 74 having different distances from the optical axis 601 (even if the angle of view is different). The imaging optical system 60 can stably exhibit the characteristics described above.
In addition, since the lens surface 62 including the first region 62a and the second region 62b is provided on the light emitting element 74 side of the lens 64, the characteristic fluctuation due to the angle of view can be suppressed also in this respect.

On the other hand, like the lens 64, the lens 64 ′ is formed on a support portion 65 ′ made of, for example, a glass material. And lens 64 'has lens surface 62' on the opposite side to support part 65 '.
The lens surface 62 ′ of the lens 64 ′ may be a spherical surface or an aspheric surface, and the surface shape can be defined by one defining formula. As the definition formula, for example, a definition formula (xy polynomial plane) represented by the following formula 2 can be used (more specifically, refer to an example described later).

ここで、上記の数２に示される式において、ｒ２＝ｘ２＋ｙ２であり、
ｘ：主走査方向（第１の方向）での座標
ｙ：副走査方向（第２の方向）での座標
ｚ：光軸に平行な面に対するサグ量
ｃ：光軸上曲率
Ｋ：コーニック定数
Ａ〜Ｉ：非球面係数
である。

Here, in the equation shown in the above equation 2, r2 = x2 + y2,
x: coordinate in the main scanning direction (first direction) y: coordinate in the sub-scanning direction (second direction) z: sag amount with respect to the plane parallel to the optical axis c: curvature on the optical axis K: conic constant A ~ I: aspheric coefficient.

Further, the coefficients A to I of the above-described definition formulas represent a plurality of imaging points as described above by the imaging optical system 60 according to the focal length of the imaging optical system 60, the shape of the lens surface of the lens 64, and the like. It is suitably set so as to have
In the imaging optical system 60 configured as described above, each of the four light emitting elements 74 (74a, 74b, 74c, 74d) arranged in one direction in the main scanning direction as shown in FIG. After the emitted light L (L1, L2, L3, L4) passes through the diaphragm member 83, it passes through the lens 64 and the lens 64 ′ sequentially. As a result, the lights L1, L2, L3, and L4 are imaged (condensed) in the vicinity of the light receiving surface 111 of the photoreceptor 11 as shown in FIG.

At this time, due to the action of the imaging optical system 60 having a plurality of imaging points as described above, the light L1 has a plurality of imaging positions IFP10 and IFP11 whose positions in the traveling direction (third direction) are different from each other. The image is formed by the IFP 12.
Here, the imaging position IFP10 is a position where light rays passing near the optical axis 601 form an image (condensate) when the light L1 emitted from the light emitting element 74a is incident on the lens 64 via the diaphragm member 83 ( Paraxial imaging point). Further, at the imaging position IFP11, when the light L1 emitted from the light emitting element 74a is incident on the lens 64 via the diaphragm member 83, the light beam passing through the first region 62a of the lens 64 is imaged (condensed). This is the position that is closest to the imaging optical system 60 among the positions. Further, at the imaging position IFP12, when the light L1 emitted from the light emitting element 74a is incident on the lens 64 through the diaphragm member 83, the light beam passing through the second region 62b of the lens 64 is imaged (condensed). This position is the most distal to the imaging optical system 60.

  Similarly, the light L2 is imaged at a plurality of imaging positions IFP20, IFP21, and IFP22 whose positions in the traveling direction (third direction) are different from each other. The light L3 is imaged at a plurality of imaging positions IFP30, IFP31, and IFP32 whose positions in the traveling direction (third direction) are different from each other. The light L4 is imaged at a plurality of imaging positions IFP40, IFP41, and IFP42 whose positions in the traveling direction (third direction) are different from each other.

Each of the lights L1, L2, L3, and L4 imaged by the imaging optical system 60 has a spot diameter of almost a wide range (distance G1) in the optical axis direction near the image plane. It is constant.
The imaging optical system 60 is installed such that the imaging position IFP10, the imaging position IFP20, the imaging position IFP30, and the imaging position IFP40 are located in the vicinity of the light receiving surface 111, respectively.

  As a result, even if the positional relationship in the optical axis direction (third direction) between the image plane I and the light receiving surface 111 that is the irradiated surface fluctuates or deviates, the imaging position IFP11 and the imaging position IFP12 The light receiving surface 111 is positioned between the imaging position IFP21 and the imaging position IFP22, between the imaging position IFP31 and the imaging position IFP32, and between the imaging position IFP41 and the imaging position IFP42.

In this way, the line head 13 can suppress the variation of the spot diameter on the light receiving surface 111, and as a result, can suppress the density unevenness of the formed latent image.
FIG. 8 illustrates a case where the imaging optical system 60 has a curvature of field. The imaging position IFP10 of the light L1, the imaging position IFP20 of the light L2, the imaging position IFP30 of the light L3, and the light L4 are connected. The image position IFP40 is located on the curved image surface I. Thereby, the imaging positions IFP10 and IFP40 and the imaging positions IFP20 and IFP30 are shifted from each other in the optical axis direction.

  More specifically, as shown in FIG. 5, four light emitting elements 74 (74a, 74b, 74c, 74d) arranged in one direction in the main scanning direction are arranged on the optical axis 601 of the imaging optical system 60. On the other hand, there are two light emitting elements 74b and 74c located proximally and two light emitting elements 74a and 74d located distally. Accordingly, the light emitting elements 74a and 74d and the light emitting elements 74b and 74c have different angles of view, and the image forming positions IFP10 and IFP40 and the image forming positions IFP20 and IFP30 have an optical axis due to curvature of field of the image forming optical system 60. The direction (third direction) may be shifted.

Even in such a case, the distance G1 (difference between the maximum value and the minimum value of longitudinal aberration) between the image formation point FP1 and the image formation point FP2 described above is larger than the maximum value G2 of the shift amount. large. As a result, even if the image plane I and the light receiving surface 111 of the imaging optical system 60 are slightly displaced in the optical axis direction, the light spot from the light emitting element 74 located proximal to the optical axis 601 on the light receiving surface 111. The difference between the diameter and the spot diameter of the light from the light emitting element 74 located distal to the optical axis 601 can be suppressed.
Further, even if the positional relationship between the image surface I and the light receiving surface 111 of the imaging optical system 60 is shifted or fluctuated due to an assembly error of the line head 13 with respect to the main body of the image forming apparatus 1 or the eccentricity of the photosensitive drum 11, In the light receiving surface 111, fluctuations in the spot diameter of light from the light emitting element 74 can be suppressed.

The line head and the image forming apparatus of the present invention have been described above with respect to the illustrated embodiment. However, the present invention is not limited to this, and each part constituting the line head and the image forming apparatus has the same function. It can be replaced with any configuration that can be exhibited. Moreover, arbitrary components may be added.
The lens array is not limited to a plurality of lenses arranged in a matrix of 2 rows and n columns, and may be arranged in a matrix of 3 rows and n columns, 4 rows and n columns, for example.

One imaging optical system may be composed of a plurality of lenses, and may be composed of one or three or more lens surfaces.
In the above-described embodiment, for convenience of explanation, the light emitting elements are arranged in 1 row and n columns. However, the present invention is not limited to this, and the light emitting elements are 2 rows n columns, 3 rows n columns, and the like. May be arranged in a matrix.

Hereinafter, specific examples of the present invention will be described.
(Example)
A line head having an imaging optical system as shown in FIG. 9 was prepared. FIG. 9 is a cross-sectional view (main-direction cross-sectional view) showing the imaging optical system provided in the line head according to the example of the present invention.
The line head of this embodiment has the same configuration as the line head shown in FIGS. 3 and 5 except that three light emitting elements 74 are arranged in the main scanning direction.

Here, in the cross section in the main direction, the three light emitting elements 74 arranged in the main scanning direction are arranged so as to be symmetric with respect to the optical axis.
Further, a glass material was used as a constituent material of the support portions 65 and 65 ′, and a resin material was used as a constituent material of the lenses 64 and 64 ′.
Table 1 shows the surface configuration of the imaging optical system of the line head.

  As shown in FIG. 9, in Table 1, the surface S1 is a boundary surface (light source surface) between the light emitting element 74 and the support plate 72, and the surface S2 is a surface opposite to the light emitting element 74 of the support plate 72 (glass). The surface S3 is a surface (aperture stop) on the light emitting element 74 side of the diaphragm member 83, the surface S4 is a lens surface 62 (resin portion incident surface) of the lens 64, and the surface S5 is a support with the lens 64. The boundary surface (resin-glass interface surface) with the portion 65, the surface S6 is the surface opposite to the lens 64 of the support portion 65 (glass substrate emission surface), and the surface S7 is the lens surface 62 ′ of the lens 64 ′. (Resin portion incident surface), surface S8 is a boundary portion (resin-lens boundary surface) between lens 64 'and support portion 65', and surface S9 is a surface on the opposite side of lens 64 'of support portion 65' ( The glass substrate emission surface) and the surface S10 are the light receiving surface 111 (image surface).

  The surface interval d1 is the interval between the surfaces S1 and S2, the surface interval d2 is the interval between the surfaces S2 and S3, the surface interval d3 is the interval between the surfaces S3 and S4, and the surface interval d4 is the surface. The distance between the surface S5 and the surface S5, the surface distance d5 is the distance between the surface S5 and the surface S6, the surface distance d6 is the distance between the surface S6 and the surface S7, the surface distance d7 is the distance between the surface S7 and the surface S8, The surface interval d8 is the interval between the surfaces S8 and S9, and the surface interval d9 is the interval between the surfaces S9 and S10.

The reference wavelength refractive index is a refractive index at each surface with respect to light having a reference wavelength.
Further, the wavelength (reference wavelength) of light emitted from the light emitting element 74 is set to 690 nm, the object-side numerical aperture is set to 0.153, the entire width of the object-side pixel group in the main scanning direction is set to 1.176 mm, and the object-side pixel is set. The total width of the group in the sub-scanning direction was 0.127 mm.
Further, the lens surface 62 of the lens 64 has a first region in the range of radius 0 to 0.604 mm centered on the optical axis, and a second region in the range outside the radius 0.604 about the optical axis. The surface shape of each region was defined using the coefficients shown below in the definition formula shown in Equation 1 above.

<The coefficient of the definition formula of the 1st field of lens surface 62>
c = 1 / 1.498749
K = -0.99993244
A = −0.0825629
B = 0.083801118
C = −0.1
Δ = 0.0

<The coefficient of the definition formula of the 2nd field of lens surface 62>
c = 1 / 1.517423
K = -1.21004
A = −0.007269
B = 0.0
C = 0.0
Δ = 0.0013885889
Further, the surface shape of the lens surface 62 ′ of the lens 64 ′ is defined by using the following coefficients in the definition formula shown in the above-described formula 2.

<The coefficient of the definition formula of lens surface 62 '>
c = 1 / 1.41337
K = -3.8946025
A = 0.03959898
B = 0.035508266
C = 0.11256865
D = 0.2034097
E = 0.1094741
F = −0.07921190
G = −0.2126654
H = −0.2376198
I = −0 · 0781115926
The imaging optical system thus obtained has longitudinal aberrations as shown in FIG. In FIG. 10, the horizontal axis shows the left side as the light source side and the right side as the image side when the longitudinal aberration near the optical axis is 0 (reference), and the vertical axis shows the diaphragm member 83 (aperture stop). The separation distance from the optical axis of the light beam that has passed through is shown.

(Comparative example)
A line head was produced in the same manner as in the previous embodiment except that the surface shape of the lens surface 62 of the lens 64 was the same as the surface shape of the lens surface 62 ′ of the lens 64 ′.

(Evaluation)
FIG. 11 shows changes in spot diameter depending on the position of the obtained imaging optical system in the optical axis direction for each of the above-described examples and comparative examples. In FIG. 11, (a) relates to an example of the present invention, and (b) relates to a comparative example.
As can be seen from FIG. 11, in the line head (imaging optical system) of the example according to the present invention, the change of the spot diameter in the vicinity of the minimum spot diameter could be suppressed as compared with the line head of the comparative example.
Further, when each of the line heads of the example and the comparative example is incorporated in an image forming apparatus as shown in FIG. 1 to form an image, the image forming apparatus according to the example forms an image according to the comparative example. Compared with the apparatus, a high-quality image without unevenness could be obtained.

  DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus 6 ... 2nd lens array 6 '... 1st lens array 60, 60a, 60b, 60c ... Imaging optical system 601 ... Optical axis 61 ... Lower surface (incident surface) 62, 62' ... Lens surface 62a, 62b ... areas 64, 64 ', 64a, 64a', 64b, 64b ', 64c, 64c' ... lenses 65, 65 '... lens support 7 ... light emitting element array 71 ... light emitting element group (light emitting element group) 72 ... support plate (head substrate) 721 ... lower surface 722 ... upper surface 73 ... storage section 74, 74a, 74b, 74c, 74d ... light emitting element 82 ... first light shielding member 81 ... second light shielding member 83 ... aperture member 811, 821 831 ... Through-hole 84 ... Spacer 9 ... Casing 91 ... Frame member (casing body) 911 ... Lumen 915 ... Boundary part (step part) 916 ... Shoulder part DESCRIPTION OF SYMBOLS 2 ... Cover member (back cover) 922 ... Recessed part 93 ... Clamp member 931 ... Claw part 932 ... Curved part 10 ... Image forming unit 10C, 10K, 10M, 10Y ... Image forming station 11 ... Photosensitive drum (photoconductor) 111 ... Light reception Surface 12 ... Charging unit 13 ... Line head (exposure unit) 14 ... Developing device 15 ... Cleaning unit 151 ... Cleaning blade 20 ... Transfer unit 21 ... Intermediate transfer belt 22 ... Primary transfer roller 23 ... Drive roller 24 ... Driving roller 25 ... Second Next transfer roller 26 ... Cleaning unit 261 ... Cleaning blade 30 ... Fixing unit 301 ... Fixing roller 302 ... Pressure roller 40 ... Conveying mechanism 41 ... Registration roller pair 42, 43, 44 ... Conveying roller pair 50 ... Feeding unit 51 ... Feeding Paper cassette 52 ... Pickup roller a ... Passing area P ... Recording medium S1 to S10 ... Surface g, G1 ... Distance G2 ... Maximum value I ... Image plane L1, L2, L3, L4 ... Light FP0, FP1, FP2 ... Focus point fp0, fp1, fp2: Focus IFP10, IFP11, IFP12, IFP20, IFP21, IFP22, IFP30, IFP31, IFP32, IFP40, IFP41, IFP42 ... Imaging position

Claims (7)

A light emitting device arranged in a first direction;
An imaging optical system that forms an image of light emitted from the light emitting element;
The imaging optical system includes a rotationally symmetric lens,
The line head according to claim 1, wherein the imaging optical system has a longitudinal aberration in which a sign is inverted with respect to an image plane in the first direction cross section passing through the symmetry axis of the lens.   2. The line head according to claim 1, wherein the imaging optical system is plane-symmetric with respect to a second direction cross section including the symmetry axis and perpendicular to the first direction.   In the imaging optical system, the difference between the maximum value and the minimum value of longitudinal aberration in the cross section in the first direction of the imaging optical system is emitted from the light emitting element and converged through the imaging optical system. The line head according to claim 1, wherein the line head is larger than a minimum spot diameter of light.   The line head according to claim 1, wherein the rotationally symmetric lens is a multifocal lens having different focal points.   The lens has a lens surface including a first region provided so as to include an intersection with the symmetry axis of the lens, and a second region provided around the first region, 5. The line head according to claim 1, wherein the shape of the lens surface is defined by a definition formula in which the first region and the second region are different from each other. The imaging optical system has a plurality of lenses arranged in the direction of the symmetry axis,
The line head according to claim 5, wherein a lens located closest to the light emitting element among the plurality of lenses has the first region and the second region. A latent image carrier on which a latent image is formed;
A line head that exposes the latent image carrier to form the latent image, and
The line head is
A light emitting device arranged in a first direction;
An imaging optical system that forms an image of light emitted from the light emitting element;
The imaging optical system includes a rotationally symmetric lens,
The image forming apparatus according to claim 1, wherein the imaging optical system has a longitudinal aberration in which a sign is inverted with respect to the latent image carrier in the first direction cross section passing through the symmetry axis of the lens.

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