Line head and image forming apparatus

An image forming apparatus includes a latent image carrier on which a latent image is formed; and a line head. The line head includes light-emitting elements arranged in a first direction; an aperture diaphragm; and an optical system that images light emitted from the light-emitting elements on a latent image carrier. The aperture diaphragm and the optical system are arranged in a second direction that is orthogonal to or substantially orthogonal to the first direction; and among the lenses included in the optical system, a lens located at the position closest to the aperture diaphragm is a multifocal lens.

BACKGROUND

1. Technical Field

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

2. Related Art

Electrophotographic image forming apparatuses such as copying machines or printers are provided with an exposure unit that performs an exposure process on an outer surface of a rotating photoconductor so as to form an electrostatic latent image thereon. As the exposure unit, a line head having a structure in which a plurality of light-emitting elements is arranged in the direction of the rotation axis) of the photoconductor is known (for example, see JP-A-2-4546)

As the line head, for example, JP-A-2-4546 describes an optical information writer in which a plurality of LED array chips with a plurality of LEDs (light-emitting elements) is arranged in one direction.

In the optical information writer, the plurality of LEDs of each of the LED array chips is arranged in the direction of the rotation axis of the photoconductor. Convex lens elements (optical systems) are provided so as to correspond to the respective LED array chips. The convex lens elements image the light from the respective LEDs of each of the LED array chips.

In the line head described in JP-A-2-4546, due to the image-surface curvature of the convex lens element, the imaging capability of the convex lens element decreases as it becomes distant from the optical axis. On the surface of the photoconductor, a spot size (diameter) of light from an LED which is located close to the optical axis of the convex lens element is different from a spot size of light from an LED which is located distant from the optical axis of the convex lens element. As a result, the concentration of the latent image formed on the surface of the photoconductor becomes different between pixels, which are formed by the light from the LED located close to the optical axis of the convex lens element, and pixels, which are formed by the light from the LED located distant from the optical axis of the convex lens element, whereby concentration unevenness occurs.

Furthermore, the positional relationship between the image surface of the convex lens element and the light irradiation surface (the surface of the photoconductor) is offset or varied due to errors in mounting the line head onto the body of the image forming apparatus, eccentricity of the photoconductor, or the like. In this respect, concentration unevenness will occur.

SUMMARY

An advantage of some aspects of the invention is that it provides a line head capable of performing a high-accuracy exposure process and an image forming apparatus capable of obtaining a high-quality image.

The above-described advantage is achieved by the following aspects and embodiments of the invention.

According to an aspect of the invention, there is provided a line head including: light-emitting elements arranged in a first direction; an aperture diaphragm; and an optical system that images light emitted from the light-emitting elements on an image surface, wherein: the aperture diaphragm and the optical system are arranged in a second direction that is orthogonal to or substantially orthogonal to the first direction; and among the lenses included in the optical system, a lens located at the position closest to the aperture diaphragm is a multifocal lens.

In an embodiment of the line head of the above aspect of the invention, the multifocal lens may have a lens surface including a first region and a second region which are defined by different definition formulas.

In another embodiment of the line head of the above aspect of the invention, the first region may be provided in a central portion of the lens surface, and the second region may be provided so as to surround the periphery of the first region.

In another embodiment of the line head of the above aspect of the invention, the lens surface may have a rotationally symmetrical shape.

In another embodiment of the line head of the above aspect of the invention, the lens surface including the first region and the second region may be a lens surface which is located at the position closest to the light-emitting elements.

In another embodiment of the line head of the above aspect of the invention, the first region may have a larger area than the second region.

In another embodiment of the line head of the above aspect of the invention, the optical system may have imaging points which are located at different positions in the second direction. A distance in the second direction between a imaging point, which is located furthest from the optical system in the second direction among the imaging points, and a imaging point, which is located closest to the optical system in the second direction, may be larger than the minimum spot size of light which is emitted from the light-emitting elements to converge in the optical system.

According to another aspect of the invention, there is provided an image forming apparatus including: a latent image carrier on which a latent image is formed; and a line head, the line head including: light-emitting elements arranged in a first direction; an aperture diaphragm; and an optical system that images light emitted from the light-emitting elements on a latent image carrier, wherein: the aperture diaphragm and the optical system are arranged in a second direction that is orthogonal to or substantially orthogonal to the first direction; and among the lenses included in the optical system, a lens located at the position closest to the aperture diaphragm is a multifocal lens.

According to the line head of the aspects and embodiments of the invention having the above-described configuration, since the optical system has the lens (multifocal lens) having a plurality of focal points, when the light emitted from the light-emitting element is imaged by the optical system, it is possible to make the spot size of the light substantially constant over a relatively wide range in the optical axis direction in the vicinity of the image surface. Therefore, even when the positional relationship in the optical axis direction between the image surface and the light irradiation surface, is changed or offset, it is possible to prevent a variation of the spot size on the light irradiation surface. As a result, it is possible to prevent concentration unevenness in the formed latent image. In particular, since the lens (multifocal lens) having a plurality of focal points is located closest to the side of the aperture diaphragm (the side of the light-emitting elements), the optical system can reliably exhibit the above-described characteristics even when the light-emitting elements are located at different distances from the optical axis (namely, even when the angles of view are different). Therefore, the line head of the invention is able to realize a high-accuracy exposure process.

Moreover, according to the image forming apparatus of the aspect of the invention, by realizing the above-described high-accuracy exposure process, it is possible to obtain a high-quality image in which concentration unevenness is suppressed.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, a line head and an image forming apparatus according to preferred embodiments of the invention will be described in detail with reference to the accompanying drawings.

FIG. 1is a schematic view illustrating the entire configuration of an image forming apparatus according to an embodiment of the invention.FIG. 2is a partially sectional perspective view illustrating a line head included in the image forming apparatus illustrated inFIG. 1.FIG. 3is a cross-sectional view taken along the line III-III ofFIG. 2.FIG. 4is a plan view of the line head illustrated inFIG. 2, illustrating the positional relationship between lenses and light-emitting elements.FIG. 5is a cross-sectional view, taken along the first direction, of an optical system included in the line head illustrated inFIG. 2.FIGS. 6A and 6Bare views illustrating a light-emitting element-side lens included in the optical system illustrated inFIG. 5.FIG. 7is a view for describing the operation of the lens illustrated inFIGS. 6A and 6B.FIG. 8is a view for describing the operation of the optical system illustrated inFIG. 5. In the following description, it is assumed that an upper side inFIGS. 1 to 3andFIG. 5is “upper” or “upward” and a lower side in the drawings is “lower” or “downward” for convenience of explanation.

Image Forming Apparatus

An image forming apparatus1illustrated inFIG. 1is an electrophotographic printer that records an image on a recording medium P by a series of image forming processes including an electrical charging process, an exposure process, a developing process, a transferring process, and a fixing process. In the present embodiment, the image forming apparatus1is a so-called tandem type color printer.

As illustrated inFIG. 1, the image forming apparatus1includes: an image forming unit10for the electrical charging process, the exposure process, the developing process; a transfer unit20for the transferring process; a fixing unit30for the fixing process; a transport mechanism40for transporting the recording mediums P, such as paper; and a paper feed unit50that supplies the recording medium P to the transport mechanism40.

The image forming unit10has four image forming stations: an image forming station10Y that forms a yellow toner image, an image forming station10M that forms a magenta toner image, an image forming station10C that forms a cyan toner image, and an image forming station10K that forms a black toner image.

Each of the image forming stations10Y,10C,10M, and10K has a photosensitive drum (photoconductor)11which is a latent image carrier that carries an electrostatic latent image thereon. A charging unit12, a line head (exposure unit)13, a developing unit14, and a cleaning unit15are provided around the periphery (outer peripheral side) of the photosensitive drum11along a rotating direction thereof. The image forming stations10Y,10C,10M, and10K have substantially the same configurations except that they use toner of different colors.

The photosensitive drum11has a cylindrical shape as an overall shape and is configured to be rotatable around an axial line thereof along the direction indicated by the arrow inFIG. 1. A photosensitive layer (not shown) is formed in the vicinity of the outer peripheral surface (cylindrical surface) of the photosensitive drum11. The outer peripheral surface of the photosensitive drum11forms a light receiving surface111that receives light L (emitted light) from the line head13(refer toFIG. 2).

The charging unit12uniformly charges the light receiving surface111of the photosensitive drum11by corona charging or the like.

The line head13receives image information from a host computer (not shown) such as a personal computer and irradiates the light L towards the light receiving surface111of the photosensitive drum11in response to the image information. When the light L is irradiated to the uniformly charged light receiving surface111of the photosensitive drum11, a latent image (electrostatic latent image) corresponding to an irradiation pattern of the light L is formed on the light receiving surface111. The configuration of the line head13will be described in detail later.

The developing unit14has a reservoir (not shown) storing toner therein and supplies toner from the reservoir to the light receiving surface111of the photosensitive drum11that carries the electrostatic latent image and applies toner thereon. As a result, the latent image on the photosensitive drum11is visualized (developed) as a toner image.

The cleaning unit15has a cleaning blade151, which is made of rubber and makes abutting contact with the light receiving surface111of the photosensitive drum11, and is configured to remove toner, which remains on the photosensitive drum11after a primary transfer to be described later, by scraping the remaining toner with the cleaning blade151.

The transfer unit20is configured to collectively transfer toner images corresponding to respective colors, which are formed on the photosensitive drums11of the image forming stations10Y,10M,10C, and10K described above, onto the recording medium P.

In each of the image forming stations10Y,10C,10M, and10K, electrical charging of the light receiving surface111of the photosensitive drum11performed by the charging unit12, exposure of the light receiving surface111performed by the line head13, supply of toner to the light receiving surface111performed by the developing unit14, primary transfer to an intermediate transfer belt21, caused by pressure between the intermediate transfer belt21and a primary transfer roller22, which will be described later, and cleaning of the light receiving surface111performed by the cleaning unit15are sequentially performed while the photosensitive drum11rotates once.

The transfer unit20has the intermediate transfer belt21having an endless belt shape. The intermediate transfer belt21is stretched over the plurality (four in the configuration illustrated inFIG. 1) of primary transfer rollers22, a driving roller23, and a driven roller24. The intermediate transfer belt21is driven to rotate in the direction indicated by the arrow illustrated inFIG. 1and at approximately the same speed as a circumferential speed of the photosensitive drum11by rotation of the driving roller23.

Each primary transfer roller22is provided opposite the corresponding photosensitive drum11with the intermediate transfer belt21interposed therebetween and is configured to transfer (primary transfer) a monochrome toner image on the photosensitive drum11to the intermediate transfer belt21. At the time of primary transfer, a primary transfer voltage (primary transfer bias), which has an opposite polarity to that of electrically charged toner is applied to the primary transfer roller22.

A toner image corresponding to at least one of the colors yellow, magenta, cyan, and black is carried on the intermediate transfer belt21. For example, when a full color image is formed, toner images corresponding to the four colors yellow, magenta, cyan, and black are sequentially transferred onto the intermediate transfer belt21so as to overlap one another so that a full color toner image is formed as an intermediate transfer image.

In addition, the transfer unit20has a secondary transfer roller25, which is provided opposite the driving roller23with the intermediate transfer belt21interposed therebetween, and a cleaning unit26, which is provided opposite the driven roller24with the intermediate transfer belt21interposed therebetween.

The secondary transfer roller25is configured to transfer (secondary transfer) a monochrome or full-color toner image (intermediate transfer image), which is formed on the intermediate transfer belt21, to the recording medium P such as paper, a film, or cloth, which is supplied from the paper feed unit50. At the time of secondary transfer, the secondary transfer roller25is pressed against the intermediate transfer belt21, and a secondary transfer voltage (secondary transfer bias) is applied to the secondary transfer roller25. The driving roller23also functions as a backup roller of the secondary transfer roller25at the time of this secondary transfer.

The cleaning unit26has a cleaning blade261, which is made of rubber and makes abutting contact with a surface of the intermediate transfer belt21, and is configured to remove toner, which remains on the intermediate transfer belt21after the secondary transfer, by scraping the remaining toner with the cleaning blade261.

The fixing unit30has a fixing roller301and a pressure roller302pressed against the fixing roller301and is configured such that the recording medium P passes between the fixing roller301and the pressure roller302. In addition, the fixing roller301is provided with a heater which is provided at the inside thereof so as to heat an outer peripheral surface of the fixing roller301so that the recording medium P passing between the fixing roller301and the pressure roller302can be heated and pressed. By the fixing unit30having such a configuration, the recording medium P having a secondary-transferred toner image thereon is heated and pressed, such that the toner image is heat-fixed on the recording medium P as a permanent image.

The transport mechanism40has a resist roller pair41, which transports the recording medium P to a secondary transfer position while calculating the timing of paper feeding to the secondary transfer position between the secondary transfer roller25and the intermediate transfer belt21described above, and transport roller pairs42,43, and44which pinch and transport only the recording medium P, on which the fixing process in the fixing unit30has been completed.

When an image is formed on only one surface of the recording medium P, the transport mechanism40pinches and transports the recording medium P, in which one surface thereof has been subjected to the fixing process by the fixing unit30, using the transport roller pair42and discharges the recording medium P to the outside of the image forming apparatus1. When images are formed on both surfaces of the recording medium P, the recording medium P in which one surface thereof has been subjected to the fixing process by the fixing unit30is first pinched by the transport roller pair42. Then, the transport roller pair42is reversely driven and the transport roller pairs43and44are driven so as to reverse the recording medium P upside down and transport the recording medium P back to the resist roller pair41. Then, another image is formed on the other surface of the recording medium P by the same operation as described above.

The paper feed unit50is provided with a paper feed cassette51, which stores therein the recording medium P which has not been used, and a pickup roller52that feeds the recording medium P from the paper feed cassette51toward the resist roller pair41one at a time.

Line Head

Next, the line head13will be described in detail. In the following description, the longitudinal direction (first direction) of a long lens array6will be referred to as a “main-scanning direction” and the width direction (second direction) of the lens array6will be referred to as a “sub-scanning direction” for convenience of explanation.

As illustrated inFIG. 3, the line head13is arranged below the photosensitive drum11so as to oppose the light receiving surface111of the photosensitive drum11. The line head13includes a lens array (first lens array)6′, a spacer84, the lens array (second lens array)6, a light shielding member (first light shielding member)82, a diaphragm member (aperture diaphragm)83, a light shielding member (second light shielding member)81, and a light-emitting element array7, which are sequentially arranged in that order from the side of the photosensitive drum11and are accommodated in a casing9.

In the line head13, the light L emitted from the light-emitting element array7is collimated by the diaphragm member83and sequentially passes through the lens array6′ and the lens array6to be irradiated onto the light receiving surface111of the photosensitive drum11.

As illustrated inFIG. 2, the lens arrays6and6′ are formed of a planar member having a long appearance.

As illustrated inFIG. 3, a plurality of lens surfaces (convex surfaces)62is formed on a lower surface (incidence surface) of the lens array6on which the light L is incident. On the other hand, an upper surface (emission surface) of the lens array6from which the light L is emitted is configured as a flat surface.

That is to say, the lens array6includes a plurality of plano-convex lenses64, each of the lenses having a convex surface on a surface on which the light L is incident and a flat surface on a surface from which the light L is emitted. Here, a portion of the lens array6excluding the respective lenses64constitutes a support portion65that supports each of the lenses64.

Similarly, on a lower surface (incidence surface) of the lens array6′ on which the light L is incident, a plurality of lens surfaces (convex surfaces)62′ is formed so as to correspond to the plurality of lens surfaces62described above. On the other hand, an upper surface (emission surface) of the lens array6′ from which the light L is emitted is configured as a flat surface.

That is to say, the lens array6′ includes a plurality of plano-convex lenses64′, each of the lenses having a convex surface on a surface on which the light L is incident and a flat surface on a surface from which the light L is emitted. Here, a portion of the lens array6′ excluding the respective lenses64′ constitutes a support portion65′ that supports each of the lenses64′.

A plurality of lens pairs64and64′ constitutes an optical system60that images light emitted from corresponding light-emitting elements74of a light-emitting element group71(seeFIGS. 5 and 6). The optical system60(particularly, the shapes of the lens surfaces of the lenses64and64′) will be described in detail later.

The arrangement of the lenses64will be described. Since the lenses64′ have the same arrangement (in plan view) as the lenses64, the description thereof will be omitted.

As illustrated inFIG. 4, the lenses64are arranged in plural columns in the main-scanning direction (first direction), and are arranged in plural rows in the sub-scanning direction (second direction) which is orthogonal to the main-scanning direction and the optical axis direction of the lenses64.

More specifically, the plurality of lenses64are arranged in a matrix of three rows by n columns (n is an integer of two or more). In the following description, among the three lenses64belonging to one column (lens array), the lens64positioned in the middle will be referred to as a “lens64b”, the lens64positioned at a left side inFIG. 3(upper side inFIG. 4) will be referred to as a “lens64a”, and the lens64positioned at a right side inFIG. 3(lower side inFIG. 4) will be referred to as a “lens64c”. In the lenses64′ which are paired with the lenses64, the lens64′ corresponding to the lens64awill be referred to as a “lens64a′”, the lens64′ corresponding to the lens64bwill be referred to as a “lens64b′”, and the lens64′ corresponding to the lens64cwill be referred to as a “lens64c′”.

In the present embodiment, the line head13is mounted on the image forming apparatus1so that, among the plural lenses64(64ato64c) belonging to one column, the lens64bpositioned closest to the center in the sub-scanning direction is arranged at the position closed to the light receiving surface111of the photosensitive drum11. By doing so, the optical characteristics of the plurality of lenses64can be configured easily.

As illustrated inFIGS. 2 and 4, in each lens column, the lenses64ato64care sequentially arranged so as to be offset by an equal distance in the main-scanning direction (right direction inFIG. 4). That is, in each lens column, a line that connects the centers of the lenses64ato64cto one another is inclined at a predetermined angle with respect to the main-scanning direction and the sub-scanning direction.

When seen from the cross section illustrated inFIG. 3, the three lenses64belonging to one lens column, namely the lenses64aand64c, are arranged such that the optical axes601of the lenses64aand64care symmetrical with respect to the optical axis601of the lens64b. Moreover, the optical axes601of the lenses64ato64care arranged in parallel to each other.

Although the constituent materials of the lens arrays6and6′ are not particularly limited as long as they exhibit the optical characteristics described above, the lens arrays6and6′ are preferably formed of a resin material and/or a glass material, for example.

As the resin material, various kinds of resin materials can be used. Examples thereof include liquid crystal polymers such as polyamides, thermoplastic polyimides and polyamideimide aromatic polyesters; polyolefins such as polyphenylene oxide, polyphenylene sulfide and polyethylene; polyesters such as modified polyolefins, polycarbonate, acrylic (methacrylic) resins, polymethyl methacrylate, polyethylene terephthalate and polybutylene terephthalate; thermoplastic resins such as polyethers, polyether ether ketones, polyetherimide and polyacetal; thermosetting resins such as epoxy resins, phenolic resins, urea resins, melamine resins, unsaturated polyester resins and polyimide resins; photocurable resins; and the like. These can be used individually or in combination of two or more species.

Among these resin materials, resin materials such as thermosetting resins and photocurable resins are preferred because such materials have a relative low thermal expansion coefficient and are rarely thermally expanded (deformed), modified or deteriorated, in addition to the advantages of a relative high refractive index.

In addition, as the glass material, various kinds of glass materials, such as soda glass, crystalline glass, quartz glass, lead glass, potassium glass, borosilicate glass, alkali-free glass, and the like may be mentioned. When a later-described supporting plate72of the light-emitting element array7is formed of a glass material, the lens arrays6and6′ are preferably formed of a glass material having approximately the same linear expansion rate as the above glass material. By doing so, the positional misalignment of the respective lenses relative to the light-emitting elements due to temperature variation can be prevented.

When the lens array6is formed by using a combination of the described resin material and glass material, a glass substrate formed of a glass material may be used as the support portion65, for example, as will be described later. In this case, a resin layer formed of a resin material may be formed on one surface of the glass substrate, and the lens surface62may be formed on the other surface of the glass substrate opposite the resin layer, thus forming the lens64(seeFIGS. 5 and 6). In addition, the lens array6may be obtained, for example, by forming a plurality of convex portions, which is formed of a resin material and protrudes in a convex surface shape, on one surface of a flat plate-like member (substrate) which is formed of a glass material.

As illustrated inFIGS. 2 and 3, a spacer84is provided between the lens arrays6and6′. The lens arrays6and6′ are bonded together via the spacer84.

The spacer84has a function of regulating a gap length that is a distance between the lens arrays6and6′.

The spacer84has a frame shape which corresponds to the outer peripheral portions of the lens arrays6and6′ and is bonded to these peripheral portions. The spacer84is not limited to being a frame-shaped member as long as it has the above-described function. The spacer84may be configured as a pair of members which correspond to one of the opposing sides of the outer peripheral portions of the lens arrays6and6′. Alternatively, the spacer84may be configured as a planar member having through-holes formed therein so as to correspond to optical paths, similar to light shielding members81and82which will be described later.

Although the constituent materials of the spacer84are not particularly limited as long as they exhibit the above-described function, a resin material, a metallic material, a glass material, a ceramics material, and the like can be used, for example.

As illustrated inFIG. 3, at a side of the lens array6on which the light L is incident, the light-emitting element array7is provided with the light shielding member82, a diaphragm member83, and the light shielding member81interposed therebetween. The light-emitting element array7has a plurality of groups of light-emitting elements (light-emitting element groups)71and a supporting plate (head substrate)72.

The supporting plate72is configured to support each of the light-emitting element groups71and is formed of a planar member having a long appearance. The supporting plate72is arranged in parallel to the lens array6.

In addition, the length of the supporting plate72in the main-scanning direction is larger than that of the lens array6in the main-scanning direction. The length of the supporting plate72in the sub-scanning direction is also set to be larger than that of the lens array6in the sub-scanning direction.

Although the constituent materials of the supporting plate72are not particularly limited, when the light-emitting element groups71are provided on the bottom surface side of the supporting plate72(that is, bottom emission-type light-emitting elements are used as the light-emitting elements74), the supporting plate72is preferably formed of transparent materials such as various kinds of glass materials or various kinds of plastics. When top emission-type light-emitting elements are used as the light-emitting elements74, the constituent materials of the supporting plate72are not limited to the transparent materials, various kinds of metallic materials, such as aluminum or stainless steel, various kinds of glass materials, various kinds of plastics, and the like may be used individually or in combination thereof. When the supporting plate72is formed of various kinds of metallic materials or various kinds of glass materials, heat generated by the emission of the light-emitting elements74can be efficiently dissipated through the supporting plate72. When the supporting plate72is formed of various kinds of plastics, the weight of the supporting plate72can be reduced.

A box-shaped accommodation portion73which is open to the supporting plate72is provided on the bottom surface side of the supporting plate72. The plurality of light-emitting element groups71, wiring lines (not shown) electrically connected to the light-emitting element groups71(the respective light-emitting elements74), or circuits (not shown) used for driving the respective light-emitting elements74are accommodated in the accommodation portion73.

The plurality of light-emitting element groups71are separated from each other and arranged in a matrix of three rows by n columns (n is an integer of two or more) so as to correspond to the plurality of lenses64described above (for example, seeFIG. 4). Each of the light-emitting element groups71is configured to include a plurality (8 in the present embodiment) of light-emitting elements74.

The eight light-emitting elements74that constitute each of the light-emitting element groups71are arranged along a lower surface721of the supporting plate72illustrated inFIG. 3. The light L emitted from each of the eight light-emitting elements74is focused (imaged) on the light receiving surface111of the photosensitive drum11through the corresponding lens64.

In addition, as illustrated inFIG. 4, the eight light-emitting elements74are separated from each other and are arranged in four columns in the main-scanning direction and in two rows in the sub-scanning direction. Thus, the eight light-emitting elements74are arranged in a matrix of two rows by four columns. The two adjacent light-emitting elements74belonging to one column (column of light-emitting elements) are arranged so as to be offset from each other in the main-scanning direction.

In the eight light-emitting elements74which form a matrix of two rows by four columns, two light-emitting elements74which are adjacent to each other in the main-scanning direction are supplemented by one light-emitting element74in the next row.

There is a limitation in arranging the eight light-emitting elements74as closely as possible in one row, for example. However, it is possible to increase further the arrangement density of the light-emitting elements74by arranging the eight light-emitting elements74so as to be offset from each other as described above. In this way, the recording density of the recording medium P when an image is recorded on the recording medium P can be increased further. As a result, it is possible to obtain the recording medium P carrying thereon an image which has high resolution and multiple gray-scale levels and is clear.

In addition, although the eight light-emitting elements74belonging to one light-emitting element group71are arranged in a matrix of two rows by four columns in the present embodiment, the arrangement shape is not limited thereto. For example, the eight light-emitting elements74may be arranged in a matrix of four rows by two columns.

As described above, the plurality of light-emitting element groups71are arranged in a matrix of three rows by n columns so as to be separated from each other. As illustrated inFIG. 4, the three light-emitting element groups71belonging to one column (column of light-emitting element groups) are arranged so as to be offset from each other by an equal distance in the main-scanning direction (right direction inFIG. 4).

Thus, in the light-emitting element groups71which form a matrix of three rows by n columns, the gaps between adjacent light-emitting element groups71are sequentially supplemented by the light-emitting element group71of the next row and the light-emitting element group71of the subsequent row.

There is a limitation in arranging the plurality of light-emitting element groups71as closely as possible in one row, for example. However, it is possible to increase further the arrangement density of the light-emitting element groups71by arranging the plurality of light-emitting element groups71so as to be offset from each other as described above. In this way, due to the synergetic effect, along with the fact that the eight light-emitting elements74within one light-emitting element group71are arranged so as to be offset from each other, the recording density of the recording medium P when an image is recorded on the recording medium P can be increased further. As a result, it is possible to obtain a recording medium P carrying thereon an image which has higher resolution, multiple gray-scale levels, and high color reproducibility and is clearer.

The light-emitting elements74are bottom emission-type organic electroluminescence (EL) element. The light-emitting elements74are not limited to the bottom emission-type elements and may be top emission-type elements. In this case, the supporting plate72is not required to have optically transparent properties as described above.

When the light-emitting elements74are organic EL elements, the gaps (pitches) between the light-emitting elements74can be set to be relatively small. In this way, the recording density of the recording medium P when an image is recorded on the recording medium P can be made relatively high. In addition, the light-emitting elements74can be formed with highly accurate sizes and at highly accurate positions by using various film-forming methods. As a result, it is possible to obtain the recording medium P carrying thereon a clearer image.

In the present embodiment, all of the light-emitting elements74are configured to emit red light. Here, as examples of the constituent materials of a light-emitting layer which emits red light, (4-dicyanomethylene)-2-methyl-6-paradimethylaminostyryl)-4H-pyrane (DCM), Nile Red and the like can be mentioned. In addition, the light-emitting elements74are not limited to those configured to emit red light, but may be configured to emit monochromatic light of another color 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 in accordance with the constituent materials of the light-emitting layer.

Since the spectral sensitivity characteristic of the photosensitive drum used in the electrophotographic process is generally set to have a peak in a wavelength range of a red wavelength, which is the emission wavelength of a semiconductor laser, to a near-red wavelength, it is preferable to use the materials capable of emitting red light as described above.

As illustrated inFIG. 3, the light shielding member82, the diaphragm member83, and the light shielding member81are provided between the lens array6and the light-emitting element array7.

The light shielding members81and82are configured to prevent crosstalk of the light L between the adjacent light-emitting element groups71.

A plurality of through-holes (openings)811is formed in the light shielding member81so as to pass through the light shielding member81in the up and down direction (thickness direction) ofFIG. 3. These through-holes811are arranged at positions corresponding to the respective lenses64.

Similarly, a plurality of through-holes821is formed in the light shielding member82so as to pass through the light shielding member82in the up and down direction (thickness direction) ofFIG. 3. These through-holes821are arranged at positions corresponding to the respective lenses64.

Each of the through-holes811and821is configured to form an optical path which extends from the light-emitting element group71to the corresponding lens64. In addition, each of the through-holes811and821has a circular shape in plan view thereof and includes therein the eight light-emitting elements74of the light-emitting element group71corresponding to each of the through-holes811and821.

Although the through-holes811and821have a cylindrical shape in the configuration illustrated inFIG. 3, the invention is not limited thereto. For example, the through-holes811and821may have a circular truncated cone shape which expands upward.

The diaphragm member83is provided between the light shielding members81and82.

The diaphragm member83is an aperture diaphragm that restricts the amount of light L incident on the lens64from the light-emitting element group71to a predetermined amount. That is to say, the diaphragm member83regulates the outer diameter of the light L emitted from the light-emitting element74.

The diaphragm member83has a planar or layered shape, and a plurality of through-holes (openings)831is formed in the diaphragm member83so as to pass through the diaphragm member83in the up and down direction (thickness dimension) ofFIG. 3. These through-holes831are arranged at positions corresponding to the lenses64(namely, the above-described through-holes811and821).

In addition, each of the through-holes831of the diaphragm member83has a circular shape in plan view thereof and has a diameter smaller than that of the through-holes811of the light shielding member81described above.

The diaphragm member83is preferably configured to set the distance to the lens64so as to be relatively small. By doing so, light emitted from light-emitting elements74which are located at different distances from the optical axis601(that is, even when the light-emitting elements74are located at different angles of view) can be made incident to approximately the same region of the lens64.

The diaphragm member83is provided between the optical system60, which will be described later, and the light-emitting element group71. Therefore, even when light is emitted from light-emitting elements74having different angles of view, the light can be made incident to a desired region of the lens64of the optical system60, which will be described later.

The light shielding members81and82and the diaphragm member83also have a function of regulating the distance, positional relationship, and attitude between the lens array6and the supporting plate72with high accuracy.

The distance between the lens surface62of each lens64and the corresponding light-emitting element group71is an important condition (element) that determines the position in the up and down direction ofFIG. 3of the imaging position of the optical system60which will be described later. Therefore, as described above, when the light shielding members81and82and the diaphragm member83function as the spacer that regulates the gap length which is the distance between the lens array6and the light-emitting element array7, it is possible to obtain the image forming apparatus1which is highly precise and reliable.

Moreover, the light shielding member81and82and the diaphragm member83preferably have at least an inner peripheral surface thereof which has a dark color such as black, brown, or dark blue.

Although the constituent materials of the light shielding members81and82and the diaphragm member83are not particularly limited as long as they are not optically transparent, various kinds of coloring agents, metallic materials such as chrome or chromic oxides, resins having mixed therein carbon black or coloring agents, and the like can be mentioned as examples thereof.

As illustrated inFIGS. 2 and 3, the lens array6, the light-emitting element array7, the spacer84, the light shielding members81and82, and the diaphragm member83are collectively accommodated in the casing9. The casing9has a frame member (casing body)91, a lid member (bottom lid)92, and a plurality of clamp members93which fixedly secures the frame member91to the lid member92(seeFIG. 3).

The frame member91has a generally long shape, as illustrated inFIGS. 2,5, and6.

In addition, the frame member91has a frame shape, and an inner cavity portion911that is open to the upper and lower sides of the frame member91is formed in the frame member91as illustrated inFIG. 3. The width of the inner cavity portion911gradually decreases upwardly from the lower side ofFIG. 3.

The lens array6′, the spacer84, the lens array6, the light shielding member82, the diaphragm member83, the light shielding member81, and the light-emitting element array7are inserted in the inner cavity portion911, and they are fixed by adhesive, for example. In this way, the lens array6′, the spacer84, the lens array6, the light shielding member82, the diaphragm member83, the light shielding member81, and the light-emitting element array7are collectively held on the frame member91, such that the positions in the main and sub-scanning directions of the lens array6′, the spacer84, the lens array6, the light shielding member82, the diaphragm member83, the light shielding member81, and the light-emitting element array7are determined.

Here, an upper surface722of the supporting plate72of the light-emitting element array7is in contact (abutting contact) with a stepped portion915, which is formed on a wall surface of the inner cavity portion911, and the lower surface of the second light shielding member81. The lid member92is inserted into the inner cavity portion911from the lower side.

The lid member92is formed of a lengthy member having a recess portion922in which the accommodation portion73is inserted at an upper side thereof. The edge portions of the supporting plate72of the light-emitting element array7are pinched between the upper end surface of the lid member92and the boundary portion915of the frame member91.

Moreover, the lid member92is pressed upward by each of the clamp members93. In this way, the lid member92is fixed to the frame member91. In addition, by the pressed lid member92, the positional relationships among the light-emitting element array7, the light shielding members81and82, the diaphragm member83, and the lens array6in the main-scanning direction, the sub-scanning direction, and the up and down direction ofFIG. 3are fixed.

The clamp members93are preferably arranged in plural numbers at equal intervals in the main-scanning direction. Accordingly, the frame member91and the lid member92can be pinched uniformly in the main-scanning direction.

The clamp member93is approximately U shaped in the cross section illustrated inFIG. 3and is formed by folding a metallic plate. Both ends of the clamp member93are bent inward to form claw portions931. The claw portions931are engaged with shoulder portions916of the frame member91.

In addition, a curved portion932which is curved upward in an arch shape is formed in the middle portion of the clamp member93. The apex of the curved portion932is in pressure-contact with the lower surface of the lid member92in a state where the claw portions931are engaged with the shoulder portion916. In this way, the curved portion932urges the lid member92upwardly in a state where the curved portion932is elastically deformed.

In addition, when the clamp members93which pinch the frame member91and the lid member92are detached, the lid member92can be detached from the frame member91. Then, it is possible to perform maintenance, such as replacement and repair, for the light-emitting element array7.

Furthermore, the constituent materials of the frame member91and the lid member92are not particularly limited, and the same constituent materials as the supporting plate72may be used, for example. The constituent materials of the clamp member93are not particularly limited, and aluminum or stainless steel may be used, for example. In addition, the clamp member93may also be formed of a hard resin material.

Moreover, although not illustrated in the drawings, the frame member91has spacers which are provided at both ends in the longitudinal direction thereof so as to protrude upward. The spacers are configured to regulate the distance between the light receiving surface111and the lens array6.

Optical System

Next, the optical system60of the line head13will be described in detail with reference toFIGS. 5 to 8.

As described above, in the line head13, a pair of lenses64and64′ corresponding to the light-emitting element group71are arranged in the optical axis direction. As illustrated inFIG. 5, this pair of lenses64and64′ constitutes the optical system60that images the light emitted from the light-emitting elements74belonging to the corresponding light-emitting element group71on an image surface I.

FIG. 5illustrates a view of the optical system60taken along a cross section (hereinafter referred to as a “main-cross section”) which is parallel to the optical axis direction (second direction) and the main-scanning direction (first direction). In the following description, if necessary, the optical system60formed by a pair of lenses64aand64a′ will be referred to as an “optical system60a”, the optical system60formed by a pair of lenses64band64b′ will be referred to as an “optical system60b”, and the optical system60formed by a pair of lenses64cand64c′ will be referred to as an “optical system60c”.

The optical system60is configured to image the light L having passed through the through-holes (aperture diaphragm)831of the diaphragm member83in the vicinity of the light receiving surface111of the photoconductor11. In the present embodiment, the optical system60is arranged to be telecentric on the image side.

Here, the optical system60has an axis of symmetry when seen on a cross section (main-cross section) which contains the optical axis601and is taken along the main-scanning direction (first direction). In the present embodiment, the axis of symmetry of the optical system60is identical to the optical axis601. Due to this configuration, the imaging characteristics of the optical system60which will be described later can be realized relatively easily and reliably.

The optical system60may not have the axis of symmetry as described above, and the axis of symmetry may not be identical to the optical axis601. Furthermore, although the optical system60may not be rotationally symmetrical to the optical axis601, in the following description, the optical system60will be described as being rotationally symmetrical to the optical axis601, for convenience of explanation.

The optical system60is configured so as to have an imaging point FP0which is located in the vicinity of the axis of symmetry of the optical system60, an imaging point FP1which is located offset toward the side of the optical system60with respect to the imaging point FP0, and an imaging point FP2which is located offset toward the opposite side.

That is to say, in the optical system60, when light is incident from the light-emitting element74, the light is imaged at different positions (imaging points FP0, FP1, and FP2) depending on the portion of the optical system60through which the light passes. In other words, the optical system60has a plurality of imaging points FP0, FP1, and FP2which are formed at different positions in the optical axis direction (that is to say, the optical system60has a longitudinal aberration).

Here, the imaging point FP0is a position (paraxial imaging point) at which, when the ray of light emitted from a virtual light-emitting element located on the optical axis601is incident in the vicinity of the optical axis601of the optical system60, the emitted ray of light intersects the optical axis601. The imaging point FP1is the position closest to the optical system60among the positions at which, when the off-axis ray of light emitted from the virtual light-emitting element located on the optical axis601is incident to the optical system60via the diaphragm member83, the emitted ray of light intersects the optical axis601. The imaging point FP2is the position farthest from the optical system60among the positions at which, when the off-axis ray of light emitted from the virtual light-emitting element located on the optical axis601is incident to the optical system60via the diaphragm member83, the emitted ray of light intersects the optical axis601.

That is to say, the optical system60has a longitudinal aberration on the side of the optical system60and the opposite side with respect to the imaging point FP0. Here, the difference between the maximum value and the minimum value of the longitudinal aberration corresponds to the distance G1between the imaging point FP1and the imaging point FP2.

In the optical system60, the spot size of the light L from the light-emitting element74can be made to be small and substantially constant for the ray of light imaged at a imaging point located between the imaging point FP1and the imaging point FP2which are respectively located furthest from and closest to the optical system60, among the plurality of imaging points FP0, FP1, and FP2. In particular, by making sure that the optical system60has the imaging point FP0in the vicinity of the optical axis601so as to be located between the imaging point FP1and the imaging point FP2, it is possible to increase the distance G1between the imaging point FP1and the imaging point FP2(namely, the difference between the maximum value and the minimum value of the longitudinal aberration) while satisfying other optical characteristics needed by the optical system60. As a result, when the light emitted from the light-emitting elements74is imaged by the optical system60, it is possible to make the spot size substantially constant over a relatively wide range in the optical axis direction in the vicinity of the image surface.

Therefore, even when the positional relationship in the optical axis direction (second direction) between the image surface and the light receiving surface111, which is the light irradiation surface, is changed or offset, it is possible to prevent a variation of the spot size on the light receiving surface111. As a result, it is possible to prevent concentration unevenness in the formed latent images.

Furthermore, the optical system60is preferably configured such that the distance G1in the optical axis direction between the imaging point FP2and the imaging point FP1, which are respectively located furthest from and closest to the optical system60(namely, the difference between the maximum value and the minimum value of the longitudinal aberration), is larger than the minimum spot diameter (minimum spot size) of the light emitted from the light-emitting element74(namely, the light converging in the optical system60). By doing so, it is possible to effectively prevent the above-described variation of the spot size on the light receiving surface111.

The optical system60having such characteristics can be realized by a multifocal lens having different focal points.

In the present embodiment, the lenses64are configured as multifocal lenses having a plurality of focal points, and the lenses64′ are configured as single focus lenses having a single focal point so that the optical system60is configured to have the plurality of above-described imaging points FP0, FP1, and FP2.

As illustrated inFIG. 6A, the lens64is formed on the support portion65which is formed of a glass material, for example. As illustrated inFIG. 6B, the lens64has a lens surface62on an opposite side to the support portion65.

The lens surface62of the lens64has a rotationally symmetrical shape and is formed so that the lens64has a plurality of focal points fp0, fp1, and fp2which are located at different positions in the optical axis direction, as illustrated inFIG. 7.

Here, the focal point fp0is a position (paraxial focal point) at which, when light parallel to the optical axis601is incident in the vicinity of the optical axis601of the lens64, the ray of the light (the emitted light) intersects the optical axis601. The focal point fp1is the position closest to the lens64among the positions at which, when light parallel to the optical axis601is incident to the lens64via the diaphragm member83, the ray of the light (the emitted light) intersects the optical axis601. The focal point fp2is the position farthest from the lens64among the positions at which, when light parallel to the optical axis601is incident to the lens64via the diaphragm member83, the ray of the light (the emitted light) intersects the optical axis601.

That is to say, the lens64has a longitudinal aberration on the side of the lens64and the opposite side with respect to the focal point fp0. The difference between the maximum value and the minimum value of the longitudinal aberration corresponds to the distance g between the focal points fp1and fp2.

By providing the lens64, which is a multifocal lens, in the vicinity of the aperture diaphragm (the diaphragm member83), it is possible to obtain an advantage of increasing a defocus region where a change in the spot size is similarly small for both the light-emitting element74which is located in the vicinity of the optical axis601and the light-emitting element74which is located off the optical axis601.

More specifically, as illustrated inFIGS. 6A and 6B, the lens surface62of the lens64has a first circular region62awhich is defined at a central portion thereof and a second ring-shaped region62bwhich is defined at a position so as to surround the periphery of the first region62a. InFIG. 6, a region, through which the light having passed through the diaphragm member (aperture diaphragm)83passes, is indicated by broken lines.

The surface shape of the first region62aand the surface shape of the second region62bare defined by different definition formulas. As the definition formula, a definition formula (rotationally symmetrical aspheric surface) expressed by Formula 1 below can be used, for example (see Examples below for more details). In this way, the lens64having the above-described characteristics can be realized relatively easily and reliably.

Z=cr21+1-(1-K)⁢c2⁢r2+Ar4+Br6+Cr8+Δ(Formula⁢⁢1)
In the definition formula expressed by Formula 1 above,
z: coordinate in optical axis direction (second direction)
r: distance from optical axis
c: curvature on optical axis
K: conic coefficient
A to C, Δ: aspheric coefficient

The respective coefficients A to C and Δ of the definition formula are appropriately set in accordance with the focal distance of the optical system60, the shape of the lens surface62′ of the lens64′, and the like so that the optical system60has a plurality of above-described imaging points.

When at least one of the coefficients A to C and Δ of the definition formula is changed, the first region62aand the second region62bwill be expressed by different definition formulas.

The optical axis in the definition formula refers to the axis of symmetry of a rotationally symmetrical lens.

The size of the first region62ais larger than the size of the second region62b. By doing so, the size of the first region62awithin the light passing region a can be made to be substantially the same as the size of the second region62bwithin the light passing region a. As a result, even when the positions in the optical axis direction of the image surface and the light receiving surface111are changed, light amount unevenness (concentration unevenness) of the spots formed on the light receiving surface111can be suppressed.

In particular, as described above, the optical system60has a plurality (two) of lenses64and64′ which are arranged in the optical axis direction thereof. Moreover, the lens64which is located closest to the side of the light-emitting elements74has the above-described lens surface62having the first region62aand the second region62b. Therefore, the optical system60can reliably exhibit the above-described characteristics even when the light-emitting elements74are located at different distances from the optical axis601(namely, even when the angles of view are different).

Moreover, since the lens surface62including the first region62aand the second region62bis provided on the side of the lens64close to the light-emitting elements74, it is possible to suppress characteristic variation due to an angle of view.

Similar to the lens64, the lens64′ is formed on a support portion65′ which is formed of a glass material, for example. The lens64′ has a lens surface62′ on an opposite side to the support portion65′.

The lens surface62′ of the lens64′ may be a spherical surface or an aspheric surface, and a surface shape thereof can be defined by one definition formula. As the definition formula, a definition formula (xy polynomial surface) expressed by Formula 2 below can be used, for example (see Examples below for more details).

Z=cr21+1-(1+K)⁢c2⁢r2+Ax2+By2+Cx4+Dx2⁢y2+Ey4+Fx6+Gx4⁢y2+Hx2⁢y4+Iy6(Formula⁢⁢2)
In the definition formula expressed by Formula 2 above,
r2=x2+y2, and
x: coordinate in main-scanning direction (first direction)
y: coordinate in sub-scanning direction
z: sag amount on plane parallel to optical axis
c: curvature on optical axis
K: conic coefficient
A to I: aspheric coefficient

The respective coefficients A to I of the definition formula are appropriately set in accordance with the focal distance of the optical system60, the shape of the lens surface62of the lens64, and the like so that the optical system60has a plurality of above-described imaging points.

In the optical system60having the above-described configuration, the light L (L1, L2, L3, and L4) emitted from the four light-emitting elements74(74a,74b,74c, and74d), which are linearly arranged in the main-scanning direction as illustrated inFIGS. 5 and 6, are sequentially permitted to pass through the lens64and the lens64′ after passing through the diaphragm member83. In this way, the respective light L1, L2, L3, and L4are imaged (focused) in the vicinity of the light receiving surface111of the photoconductor11as illustrated inFIG. 8.

At that time, by the above-described function of the optical system60having a plurality of imaging points, the light L1is imaged at a plurality of imaging positions IFP10, IFP11, and IFP12which are located at different positions in its travelling direction (second direction).

Here, the imaging position IFP10is a position (paraxial imaging point) at which, when the light L1emitted from the light-emitting element74ais incident to the lens64via the diaphragm member83, the ray of light passing through the vicinity of the optical axis601is imaged (focused). The imaging position IFP11is the position closest to the optical system60among the positions at which, when the light L1emitted from the light-emitting element74ais incident, to the lens64via the diaphragm member83, the ray of light passing through the first region62aof the lens64is imaged (focused). The imaging position IFP12is the position furthest from the optical system60among the positions at which, when the light L1emitted from the light-emitting element74ais incident to the lens64via the diaphragm member83, the ray of light passing through the second region62bof the lens64is imaged (focused).

Similarly, the light L2is imaged at a plurality of imaging positions IFP20, IFP21, and IFP22which are located at different positions in its travelling direction (second direction). Moreover, the light L3is imaged at a plurality of imaging positions IFP30, IFP31, and IFP32which are located at different positions in its travelling direction (second direction). Furthermore, the light L4is imaged at a plurality of imaging positions IFP40, IFP41, and IFP42which are located at different positions in its travelling direction (second direction).

The respective light L1, L2, L3, and L4imaged by the optical system60will have their spot sizes which are, substantially constant over a relatively wide range (distance G1) in the optical axis direction in the vicinity of the image surface.

The optical system60is configured so that the respective imaging positions IFP10, IFP20, IFP30, and IFP40are located in the vicinity of the light receiving surface111.

Therefore, even when the positional relationship in the optical axis direction (second direction) between the image surface I and the light receiving surface111, which is the light irradiation surface, is changed or offset, the light receiving surface111is positioned between the imaging positions IFP11and IFP12, between the imaging positions IFP21and IFP22, between the imaging positions IFP31and IFP32, and between the imaging positions IFP41and IFP42.

In this way, with the line head13it is possible to prevent variation of the spot size on the light receiving surface111. As a result, it is possible to prevent concentration unevenness of formed latent images.

FIG. 8illustrates a case where the optical system60has an image-surface curvature. Specifically, the imaging position IFP10of the light L1, the imaging position IFP20of the light L2, the imaging position IFP30of the light L3, and the imaging position IFP40of the light L4are located on a curved image surface I. Therefore, the imaging positions IFP10and IFP40and the imaging positions IFP20and IFP30are offset from each other in the optical axis direction.

More specifically, as illustrated inFIGS. 5 and 6, among the four light-emitting elements74(74a,74b,74c, and74d) arranged linearly in the main-scanning direction, two light-emitting elements74band74care located at positions close to the optical axis601of the optical system60, and the other two light-emitting elements74aand74dare located at positions distant from the optical axis601. The light-emitting elements74aand74dand the light-emitting elements74band74chave different angles of view. As a result, there is a case where the imaging positions IFP10and IFP40and the imaging positions IFP20and IFP30are sometimes offset in the optical axis direction (second direction) due to the image-surface curvature of the optical system60.

In such a case, the above-described distance G1between the imaging point FP1and the imaging point FP2(namely, the difference between the maximum value and the minimum value of the longitudinal aberration) is larger than the maximum value G2of the offset amount. Therefore, even when the image surface I of the optical system60and the light receiving surface111are slightly offset in the optical axis direction, it is possible to decrease the difference on the light receiving surface111between the spot size of the light from the light-emitting element74which is positioned closer to the optical axis601and the spot size of the light from the light-emitting element74which is positioned distant from the optical axis601.

Furthermore, even when the positional relationship between the image surface I of the optical system60and the light receiving surface111is offset or varied due to errors in mounting the line head13onto the body of the image forming apparatus1, eccentricity of the photosensitive drum11, or the like, it is possible to prevent a variation of the spot size on the light receiving surface111, of the light from the light-emitting elements74.

In particular, since the lens64which is located closest to the side of the light-emitting elements74has the above-described lens surface62having the first region62aand the second region62b, it is possible to prevent occurrence of a change in the above-mentioned advantage of suppressing a variation in the spot size between the light L1, L2, L3, and L4(specifically, between the light L1and L4, and between the light L2and L3). Due to such a configuration, the line head13can exhibit excellent exposure characteristics in which the concentration unevenness is suppressed.

Having described the line head and the image forming apparatus according to the embodiments of the invention, the invention is not limited thereto. Each of the components provided in the line head and the image forming apparatus can be replaced with a component having an arbitrary configuration capable of realizing the same function. In addition, an arbitrary structure may be added.

Furthermore, in the lens arrays, a plurality of lenses is not limited to being arranged in a matrix of two rows by n columns. For example, a plurality of lenses in each of the lens arrays may be arranged in a matrix of three rows by n columns, four rows by n columns, and the like.

Moreover, one optical system may be configured by a plurality of lenses, and may be configured to have one or three or more lens surfaces.

Furthermore, in the above-described embodiment, although the light-emitting elements are described as being arranged in a matrix of one row by n columns for convenience of explanation, the arrangement is not limited to this, and the light-emitting elements may be arranged in a matrix of two rows by n columns, three rows by n columns, and the like.

EXAMPLES

Hereinafter, specific examples of the invention will be described.

Example

A line head having the optical system as illustrated inFIG. 9was produced.FIG. 9is a cross-sectional view taken along the main-cross section, illustrating the optical system included in the line head according to Example of the invention.

The line head of the present example had the same configuration as the line head illustrated inFIGS. 3 and 5, except that three light-emitting elements74were arranged in the main-scanning direction.

Here, in the main-cross section, the three light-emitting elements74arranged in the main-scanning direction were arranged symmetrically to the optical axis.

Moreover, a glass material was used as the constituent material of the support portions65and65′, and a resin material was used as the constituent material of the lenses64and64′.

The surface configuration of the optical system of the line head is shown in Table 1.

As illustrated inFIG. 9, in Table 1, a surface S1is a boundary surface (light source plane) of the light-emitting element74and the supporting plate72, a surface S2is a surface (emission surface of a glass substrate) of the supporting plate72opposite to the light-emitting element74, a surface S3is a surface (aperture diaphragm) of the diaphragm member83close to the light-emitting element74, a surface S4is the lens surface62(incidence surface of a resin portion) of the lens64, a surface S5is a boundary surface (resin-glass boundary surface) of the lens64and the support portion65, a surface S6is a surface (emission surface of the glass substrate) of the support portion65opposite to the lens,64, a surface S7is the lens surface62′ (incidence surface of the resin portion) of the lens64′, a surface S8is a boundary portion (resin-glass boundary surface) of the lens64′ and the support portion65′, a surface S9is a surface (emission surface of the glass substrate) of the support portion65′ opposite to the lens64′, and a surface S10is the light receiving surface111(image surface).

Moreover, a surface spacing d1is a spacing between the surface S1and the surface S2, a surface spacing d2is a spacing between the surface S2and the surface S3, a surface spacing d3is a spacing between the surface S3and the surface S4, a surface spacing d4is a spacing between the surface S4and the surface S5, a surface spacing d5is a spacing between the surface S5and the surface S6, a surface spacing d6is a spacing between the surface S6and the surface S7, a surface spacing d7is a spacing between the surface S7and the surface S8, a surface spacing d8is a spacing between the surface S8and the surface S9, and a surface spacing d9is a spacing between the surface S9and the surface S10.

Furthermore, the reference wavelength refractive indexes refer to the refractive indexes on the respective surfaces facing the light having the reference wavelength.

The wavelength (reference wavelength) of the light emitted from the light-emitting element74was 690 nm, the object-side numerical aperture was 0.153, the total width of the object-side pixel group in the main-scanning direction was 1.176 mm, the total width of the object-side pixel group in the sub-scanning direction was 0.127 mm, and the optical magnification of the optical system60was −0.5039.

Furthermore, the lens surface62of the lens64was configured such that a range of regions within a radius of 0 to 0.604 mm around the optical axis was defined as the first region, and a range of regions outside the radius 0.604 mm around the optical axis was defined as the second region. The surface shapes of the respective regions were defined using the coefficients shown below in the definition formula given by Formula 1.

Coefficients of the definition formula of the first region of the lens surface62

Coefficients of the definition formula of the second region of the lens surface62

Furthermore, the surface shape of the lens surface62′ of the lens64′ was defined using the coefficients shown below in the definition formula given by Formula 2.

Coefficients of the definition formula of the lens surface62′

The optical system obtained in the above-described manner had a longitudinal aberration as shown inFIG. 10. InFIG. 10, the horizontal axis is defined such that, when the 0 (reference) point of the horizontal axis corresponds to a longitudinal aberration in the vicinity of the optical axis, the left side is the light source side and the right side is the image side. The vertical axis represents the separation distance of the ray of light having passed through the diaphragm member (aperture diaphragm)83from the optical axis.

Comparative Example

A line head was produced similar to the above-described example, except that the surface shape of the lens surface62of the lens64was made identical to the surface shape of the lens surface62′ of the lens64′.

Evaluation

FIGS. 11A and 11Brespectively illustrate changes in the spot sizes at various positions in the optical axis direction of the optical systems of the example and the comparative example.FIG. 11Ais for the example of the invention, andFIG. 11Bis for the comparative example.

As is obvious fromFIGS. 11A and 11B, the line head (optical system) of the example according to the invention was better able to suppress a change in the spot size in the vicinity of the minimum spot size than the line head of the comparative example.

Moreover, the line heads of the example and the comparative example were mounted on the image forming apparatuses as shown inFIG. 1, and images were formed using the respective image forming apparatuses. With the image forming apparatus of the example, it was possible to obtain higher-quality images in which concentration unevenness was not observed, compared to the image forming apparatus of the comparative example.

The entire disclosure of Japanese Patent Applications No. 2009-009384, filed on Jan. 19, 2009 is expressly incorporated by reference herein.