JP2010162852A - 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 a main scan 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 lenses 64 which have a common axis of symmetry when seen on each of a first cross section that includes the light axis 601 of the imaging optical system and is in the main scan direction and a second cross section that includes the light axis 601 and is perpendicular to the first cross section. The lens 64 has a refracting power on the axis of symmetry in the second cross section smaller than a refracting power on the axis of symmetry in the first cross section. <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 are arranged in one direction.
In such an optical information writing device, a convex lens element is provided corresponding to each LED array chip, and the light from the LEDs provided in each LED array chip is imaged by the convex lens element.

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 distal to the axis is larger than the spot diameter of light from the LED provided distal to the optical axis of the convex lens element.
Therefore, 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. An apparent density difference occurs between pixels formed by light from the LED, and as a result, density unevenness occurs in the finally obtained image.

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 an imaging optical system having a first symmetry plane perpendicular to the first direction and a second symmetry plane perpendicular to the second direction orthogonal to the first direction;
A light emitting element disposed in parallel or substantially parallel to the first direction,
The imaging optical system has a virtual intersection line between the first symmetry surface and the second symmetry surface, and the refractive power in the first direction is different from the refractive power in the second direction,
The imaging point of the first symmetry plane is located farther from the imaging optical system than the imaging point of the second symmetry plane of the imaging optical system at the virtual intersection line. To do.

In the line head according to the aspect of the invention, the imaging optical system includes two lenses arranged in a third direction orthogonal or substantially orthogonal to the first direction and the second direction,
It is preferable that the refractive power in the second direction of the lens closer to the light emitting element among the two lenses at the virtual intersection is smaller than the refractive power in the first direction.
In the line head according to the aspect of the invention, it is preferable that the imaging optical system is image side telecentric.

In the line head of the present invention, the light emitting element includes a first light emitting element and a second light emitting element arranged in the first direction,
The imaging optical system projects light emitted from the first light emitting element and light emitted from the second light emitting element onto a light receiving surface,
A spot area on the light receiving surface of the light emitted from the first light emitting element and a spot area on the light receiving surface of the light emitted from the second light emitting element are equal or substantially equal. It is preferable that
In the line head according to the aspect of the invention, the first light emitting element is located closest to the virtual intersection among the light emitting elements, and the second light emitting element is the virtual intersection among the light emitting elements. It is preferably located farthest from the line.

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
An imaging optical system having a first symmetry plane perpendicular to the first direction and a second symmetry plane perpendicular to the second direction orthogonal to the first direction;
A light emitting element disposed in parallel or substantially parallel to the first direction,
The imaging optical system has a virtual intersection line between the first symmetry surface and the second symmetry surface, and the refractive power in the first direction is different from the refractive power in the second direction,
The imaging point of the first symmetry plane is located farther from the imaging optical system than the imaging point of the second symmetry plane of the imaging optical system at the intersecting line. .

According to the line head of the present invention having the above-described configuration, it is possible to suppress the uneven spot size on the projection surface (light receiving surface) due to the curvature of field between the light emitting elements having different angles of view. it can. Therefore, it is possible to form a high-quality latent image with reduced unevenness. 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 for demonstrating the image formation point in the image formation optical system 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 field curvature of the imaging optical system with which the line head concerning the Example of this invention was equipped. It is a graph which shows the curvature of field of the imaging optical system with which the line head concerning the comparative example (conventional example) was equipped.

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 cross-sectional view (cross-section including the optical axis and along the first direction) showing the imaging optical system, and FIG. 6 is a diagram for explaining an imaging point in the imaging optical system shown in FIG. 7 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 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 photoconductor 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 photoconductor 11. The outer peripheral surface of the photosensitive member 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 photoreceptor 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 photoconductor 11 in response to the image information. When the light receiving surface 111 of the uniformly charged photoreceptor 11 is irradiated with light L, 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 part (not shown) for storing toner, and supplies the toner from the storage part to the light receiving surface 111 of the photosensitive member 11 carrying the electrostatic latent image. To do. Thereby, the latent image on the photoconductor 11 is visualized (developed) as a toner image.
The cleaning unit 15 includes a rubber cleaning blade 151 that abuts on the light receiving surface 111 of the photoconductor 11, and scrapes and removes toner remaining on the photoconductor 11 after the primary transfer described later by the cleaning blade 151. It has become.

The transfer unit 20 transfers the toner images of the respective colors formed on the photoconductors 11 of the image forming stations 10Y, 10M, 10C, and 10K as described above to the recording medium P at a time.
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 member 11 and the line head 13 exposes the light receiving surface 111 while the photosensitive member 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 photoreceptor 11 via an intermediate transfer belt 21, and transfers (primary transfer) a single color toner image on the photoreceptor 11 to the intermediate transfer belt 21. 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 photoconductor 11 so as to face the light receiving surface 111. The line head 13 includes 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 from the photosensitive member 11 side. 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 photoconductor 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 ′.
Then, the corresponding pair of lenses 64 and 64 ′ constitute an imaging optical system 60 that forms an image of light 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 located 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 (64 a to 64 c) belonging to one row, the lens 64 b closest to the center side in the sub-scanning direction is closest to the light receiving surface 111 of the photoconductor 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 FIG. 5). 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 82 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 member 11 through 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 point 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, a pair of lenses 64 and 64 ′ corresponding to the light emitting element group 71 are arranged in parallel in the optical axis direction. 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 is a cross-sectional view (hereinafter referred to as “main-direction cross section”) parallel to the optical axis direction (third direction) and the main scanning direction (first direction) of each imaging optical system 60. The figure is shown. 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 photosensitive member 11 (projects onto the light receiving surface 111). In the present embodiment, the imaging optical system 60 is image side telecentric.

Here, the imaging optical system 60 is plane-symmetric (mirror symmetry) with respect to a symmetry plane perpendicular to the main scanning direction (first direction), and the imaging optical system 60 is sub-scanning direction (second scanning direction). Plane symmetry (mirror symmetry) with respect to a plane of symmetry perpendicular to the direction of. The line of intersection between the two symmetry planes is called the symmetry axis.
As described above, the imaging optical system 60 has the first symmetry plane perpendicular to the first direction and the second symmetry plane perpendicular to the second direction orthogonal to the first direction.
When the imaging optical system 60 is rotationally symmetric, the above-described symmetry axis coincides with the optical axis.

Strictly speaking, the optical axis of the imaging optical system 60 may not be defined when the imaging optical system 60 is not rotationally symmetric. However, in the following description, for the sake of convenience, the above-described symmetry axis may be described as the optical axis. is there.
When light is emitted from the point (object point) where the lower surface 721 of the support plate 72 and the optical axis 601 intersect, the imaging optical system 60 forms an image of this light at the imaging point FP.

The imaging optical system 60 forms an image of the light L emitted from the light emitting element 74 at an imaging point IFP near the light receiving surface 111. More specifically, as shown in FIG. 6, the imaging optical system 60 forms an image of the light L emitted from the light emitting element 74 at the imaging point IFPS on the SS line cross section, and the TT An image is formed at the image point IFPT in a line section.
Here, the SS cross section is a plane (meridional cut plane) including the light emission position (object point) of the light emitting element 74 and the optical axis 601. The TT line cross section is a surface (spherical cut surface) that includes the principal ray of the light beam emitted from the light emitting element 74 and is orthogonal to the SS line cross section (meridional cut surface).

In particular, in the imaging optical system 60, as will be described in detail later, the lens 64 has a refractive power on the optical axis 601 (symmetric axis) in the sub-direction section (second section) having a main-direction section (first section). It is smaller than the refractive power on the optical axis 601 (symmetry axis) in the cross section. That is, the imaging optical system 60 has a virtual intersection line between the first symmetry plane and the second symmetry plane, the refractive power in the first direction differs from the refractive power in the second direction, and the virtual intersection. The imaging point of the first symmetry plane is located farther from the imaging optical system 60 than the imaging point of the second symmetry plane of the imaging optical system 60.
As a result, the imaging optical system 60 reduces the spot size difference on the projection surface (light receiving surface 111) due to the curvature of field between the light emitting elements 74 having different angles of view (separation distance from the optical axis 601). Can be suppressed. Therefore, the line head 13 can form a high-quality latent image with suppressed density unevenness.

  More specifically, as shown in FIG. 7, the imaging optical system 60 focuses the light L1 emitted from the light emitting element 74a on the imaging points IFPS1 and IFPT1, and the light emitted from the light emitting element 74b. L2 is imaged at imaging points IFPS2 and IFPT2, the light L3 emitted from the light emitting element 74c is imaged at imaging points IFPS3 and IFPT3, and the light L4 emitted from the light emitting element 74d is imaged at IFPS4 and IFPT4. To form an image.

  Here, the image formation points IFPS1, IFPS2, IFPS3, and IFPS4 are image formation points on the above-described SS line cross section (meridional cut surface) at each angle of view. The imaging points IFPT1, IFPT2, IFPT3, and IFPT4 are imaging points on the above-described TT line cross section (spherical cut surface) at each angle of view. In the present specification, the image forming point refers to a position where the spot size of light (width in the cross section) is minimized by the image forming action.

Such imaging points IFPS1, IFPS2, IFPS3, and IFPS4 are located on an image plane IS that is curved so that the light emitting element 74 side is convex as shown in FIG. Here, the imaging point IFPS1 and the imaging point IFPS4 are positioned so as to be symmetrical via the optical axis 601, and the imaging point IFPS2 and the imaging point IFPS3 are symmetrical via the optical axis 601. positioned.
Further, the imaging points IFPT1, IFPT2, IFPT3, and IFPT4 are located on an image plane IT that is substantially parallel to the light receiving surface 111 and substantially flat as shown in FIG. Here, the imaging point IFPT1 and the imaging point IFPT4 are positioned so as to be symmetric via the optical axis 601, and the imaging point IFPT2 and the imaging point IFPT3 are symmetrical via the optical axis 601. positioned.

The image plane IS is located on the opposite side of the image plane IT from the imaging optical system 60 (on the opposite side of the light emitting element 74) in the vicinity of the optical axis 601. Thereby, even if the image plane IS is curved so that the light emitting element 74 side is convex, it is between the imaging point IFPS1 and the imaging point IFPT1 (between the imaging point IFPS4 and the imaging point IFPT4). The difference between the distance G1 and the distance G2 between the imaging point IFPS2 and the imaging point IFPT2 (between the imaging point IFPS3 and the imaging point IFPT3) can be reduced. In addition, the amount of deviation of the image planes IS and IT with respect to the light receiving surface 111 can be suppressed as a whole (over the entire angle of view).
As a result, even if the light emitting elements 74a and 74d and the light emitting elements 74b and 74c have different angles of view, the spot size on the light receiving surface 111 of the light L1 (L4) and the light receiving surface 111 of the light L2 (L3). The difference from the spot size can be reduced.

In this way, in the imaging optical system 60, the refractive power on the optical axis 601 (symmetric axis) in the sub-direction section (second section) of the lens 64 is changed to the main-direction section (first section) of the lens 64. By making the refractive power smaller than the refractive power on the optical axis 601 (symmetry axis), the projection surface (light reception) caused by field curvature between the light emitting elements 74 having different angles of view (separation distance from the optical axis 601). It is possible to suppress a difference in spot size (width in each cross section of the main direction cross section and the sub direction cross section) on the surface 111).
Therefore, the line head 13 can suppress the density difference between the pixels of the latent image, and as a result, can form a high-quality latent image with reduced density unevenness.
Here, the imaging optical system 60 has a spot area on the light receiving surface 111 of the lights L1 and L4 emitted from the light emitting elements 74a and 74d and a light receiving surface 111 of the lights L2 and L3 emitted from the light emitting elements 74b and 74c. It is preferable that the area of the spot is equal. Thereby, the density difference between the pixels of the latent image can be more reliably suppressed.

The light emitting elements 74 b and 74 c are light emitting elements that are positioned closest to the optical axis 601 of the imaging optical system 60 among the plurality of light emitting elements 74, and the light emitting elements 74 a and 74 d are the light emitting elements 74 of the plurality of light emitting elements 74. Among these, the light emitting element is located farthest from the optical axis 601 of the imaging optical system 60. In the light emitting elements 74b and 74c and the light emitting elements 74a and 74d having such a relationship, the light emitting elements 74b and 74c have the least influence of field curvature, and the light emitting elements 74a and 74d have the largest influence of field curvature. Therefore, the difference in density between the pixels of the latent image formed on the light receiving surface 111 is obtained by making the spot area on the light receiving surface 111 of the lights L1 and L4 equal to the area of the spot on the light receiving surface 111 of the lights L2 and L3. (Apparent density difference) can be effectively suppressed.
Further, the lens 64 having the refractive power as described above is located closest to the light emitting element 74 among the plurality (two) of lenses forming the imaging optical system 60. Thereby, the light from each light emitting element 74 can be easily and reliably passed on the symmetry axis of the lens surface 62 of the lens 64, and the above-described effects can be exhibited.

The imaging optical system 60 having such characteristics can be realized by appropriately setting the shape of the lens surface 62 of the lens 64 and the shape of the lens surface 62 ′ of the lens 64 ′.
As shown in FIG. 5, the lens 64 is formed on a support portion 65 made of, for example, a glass material. The lens 64 has a lens surface 62 on the side opposite to the support portion 65.

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 '.
As the defining formulas that define the shape of the lens surface 62 of the lens 64 and the shape of the lens surface 62 ′ of the lens 64 ′, for example, a defining formula (anamorphic aspherical surface) represented by the following equation 1 is used. (See more specific examples below). Thereby, the lenses 64 and 64 ′ having the above-described characteristics can be realized relatively easily and reliably.

Here, in the definition formula represented by the above equation 1,
z: coordinate in the symmetric axis direction (third direction) x: coordinate in the main scanning direction (first direction) y: coordinate in the sub-scanning direction (second direction) CUX: main direction on the symmetric axis Curvature CUY in the cross section: Curvature KX, KY: Conic constants A to C in the sub-direction cross section on the symmetry axis.

Further, the coefficients CUX, CUY, A to C, etc. of the above-described definition formulas represent the curved image planes IS and IT as described above according to the focal length of the imaging optical system 60 and the like. It sets suitably so that it may form.
Also, different definition formulas are given to the lens surface 62 of the lens 64 and the lens surface 62 ′ of the lens 64 ′ by making at least one of the coefficients CUX, CUY, A to C, etc. of the above definition formula different. be able to.
Further, the shape of the lens surface 62 of the lens 64 is set such that the refractive power is different between the cross section in the main direction and the cross section in the sub direction as described above, but the shape of the lens surface 62 ′ of the lens 64 ′ is The curvature in the main direction cross section and the sub direction cross section may be the same or different.

According to the line head 13 including the imaging optical system 60 as described above, the difference in spot size on the projection surface (light receiving surface 111) due to the curvature of field between the light emitting elements 74 having different angles of view. Can be suppressed. Therefore, it is possible to form a high-quality latent image with suppressed density unevenness. In this way, the line head 13 can realize highly accurate exposure processing.
In addition, according to the image forming apparatus 1 including such a line head 13, a high-quality image with reduced density unevenness can be obtained by realizing a highly accurate exposure process.

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.
In the embodiment described above, the case where the organic EL is used as the light emitting element 74 has been described as an example. However, the light emitting element 74 may be an LED (light emitting diode).

Hereinafter, specific examples of the present invention will be described.
(Example)
A line head having an imaging optical system as shown in FIG. 8 was prepared. FIG. 8 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. 8, 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 (glass) on the opposite side of the support plate 72 from the light emitting element 74. The substrate exit surface), the surface S3 is a surface on the light emitting element 74 side (aperture stop) 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 (projected 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 1.155 mm, and the object-side pixel is set. The total width of the group in the sub-scanning direction was 0.127 mm, and the magnification of the imaging optical system 60 was −0.5039.
In addition, the surface shapes of the lens surface 62 (surface S4) and the lens surface 62 ′ (surface S7) were defined using the coefficients shown below in the above-described definition formula (1).

<The coefficient of the definition formula of the lens surface 62>
CUX = 1 / 1.487066
CUY = 1 / 1.498313
KX = -1.0
KY = −1.0
A = −0.000398224
B = 0.001717732
C = −0.001779401

<Coefficient of definition formula of lens surface 62 '>
CUX = 1 / 1.197729
CUY = 1 / 1.19101068
KX = -1.0
KY = −1.0
A = −0.00893896
B = −0.011078889
C = −0.01825088

(Comparative example)
Except for setting the CUY of the definition formula of the lens surface 62 (S4) to be the same as CUX and setting the CUY of the definition formula of the lens surface 62 ′ (S7) to be the same as CUX, it is the same as the above-described embodiment. .
(Evaluation)
The imaging optical system of the example obtained in this way has a field curvature as shown in FIG. Further, the imaging optical system of the comparative example has a field curvature as shown in FIG. In FIGS. 9 and 10, the horizontal axis indicates the deviation of the imaging point with the left side as the light source side and the right side as the image side when the vicinity of the imaging point near the optical axis is 0 (reference). It is the field curvature shown. The image plane (imaging point) at the meridional cut plane (sagittal) is indicated by a solid line, and the image plane (imaging point) at a spherical cut plane (tangential) is indicated by a broken line.

In the embodiment of the present invention, the deviation of the image plane is improved as compared with the comparative example, and the change in spot size for each light emitting element is reduced.
Further, in the above-described embodiment, when the spot size (width in the main direction cross section, width in the sub direction cross section) on the light receiving surface 111 of the photoconductor 11 is evaluated by simulation, it is proximal to the optical axis (optical axis). The spot size of the light from the light emitting element 74 in the vicinity) is 21.4 μm in the width in the main scanning direction and 22.8 μm in the cross section in the sub direction, and the light emission is distal (end side) with respect to the optical axis. The spot size of the light from the element 74 was 21.2 μm in the main scanning direction and 22.4 μm in the cross section in the sub direction, and a uniform spot size was obtained.
Further, if the image forming apparatus according to the embodiment in which the line head of the embodiment is incorporated in the image forming apparatus as shown in FIG. 1, a high-quality image with reduced unevenness can 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 62, 62' ... Lens surface 64, 64a, 64b, 64c 64 ', 64a', 64b ', 64c' ... lens 65, 65 '... support portion 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 part 74, 74a, 74b, 74c, 74d ... Light emitting element 81 ... Second light shielding member 811, 821, 831 ... Through hole 82 ... First light shielding member 83 ... Diaphragm member 84 ... Spacer 9 ... Casing 91 ... Frame member (casing body) 911 ... Lumen portion 915 ... Boundary portion (stepped portion) 916 ... Shoulder portion 92 ... Lid member (back cover) 922 ... Recessed portion 93 ... Clamp Material 931 ... Claw part 932 ... Curved part 10 ... Image forming unit 10C, 10K, 10M, 10Y ... Image forming station 11 ... Photosensitive drum (photoconductor) 111 ... Light receiving surface 12 ... Charging unit 13 ... Line head (exposure unit) 14 DESCRIPTION OF SYMBOLS Development apparatus 15 ... Cleaning unit 151 ... Cleaning blade 20 ... Transfer unit 21 ... Intermediate transfer belt 22 ... Primary transfer roller 23 ... Drive roller 24 ... Driven roller 25 ... Secondary 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 ... Paper feed unit 51 ... Paper feed cassette 52 ... Pickup roller P ... Recording medium S1- 10 ... surface IFPS1~IFPS4, IFPT1~IFPT4, FP ... imaging points G1, G2 ... distance L1, L2, L3, L4 ... light IS, IT ... image surface

Claims (6)

An imaging optical system having a first symmetry plane perpendicular to the first direction and a second symmetry plane perpendicular to the second direction orthogonal to the first direction;
A light emitting element disposed in parallel or substantially parallel to the first direction,
The imaging optical system has a virtual intersection line between the first symmetry surface and the second symmetry surface, and the refractive power in the first direction is different from the refractive power in the second direction,
The imaging point of the first symmetry plane is located farther from the imaging optical system than the imaging point of the second symmetry plane of the imaging optical system at the virtual intersection line. Line head to do. The imaging optical system has two lenses arranged in a third direction orthogonal or substantially orthogonal to the first direction and the second direction;
2. The line head according to claim 1, wherein a refractive power in the second direction of a lens closer to a light emitting element among the two lenses at the virtual intersection is smaller than a refractive power in the first direction.   The line head according to claim 1, wherein the imaging optical system is image side telecentric. The light emitting element includes a first light emitting element and a second light emitting element arranged in the first direction,
The imaging optical system projects light emitted from the first light emitting element and light emitted from the second light emitting element onto a light receiving surface,
A spot area on the light receiving surface of the light emitted from the first light emitting element and a spot area on the light receiving surface of the light emitted from the second light emitting element are equal or substantially equal. The line head according to any one of claims 1 to 3.   The first light emitting element is located closest to the virtual intersection line among the light emitting elements, and the second light emitting element is farthest from the virtual intersection line among the light emitting elements. The line head according to claim 4, which is located at 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
An imaging optical system having a first symmetry plane perpendicular to the first direction and a second symmetry plane perpendicular to the second direction orthogonal to the first direction;
A light emitting element disposed in parallel or substantially parallel to the first direction,
The imaging optical system has a virtual intersection line between the first symmetry surface and the second symmetry surface, and the refractive power in the first direction is different from the refractive power in the second direction,
The imaging point of the first symmetry plane is located farther from the imaging optical system than the imaging point of the second symmetry plane of the imaging optical system at the intersecting line. Image forming apparatus.

Images