1. Technical Field
The present invention relates to an exposure head that exposes an exposure surface or an image carrier having a curvature by converging light emitted from light emitting elements onto the exposure surface or the image carrier. The invention also relates to an image forming apparatus including the exposure head.
2. Related Art
Exposure heads that expose an exposure surface by converging light emitted from light emitting elements onto the exposure surface using an optical system have been known. Exposure heads have been generally used to expose an exposure surface having a curvature, such as a peripheral surface of a photosensitive drum (image carrier). JP-A-2008-036937 discloses an exposure head (a “line head” in the Publication) including a plurality of optical systems disposed at different positions with respect to a direction in which the exposure surface has a curvature (a “sub-scanning direction” in the Publication). In this exposure head, each the optical systems converges a light emitted from a light emitting element at a position facing the optical system in the optical axis direction.
However, in the aforementioned exposure head, the optical systems converge the light at different positions with respect to the direction in which the exposure surface has a curvature. Therefore, the position at which one optical system converges a light on the exposure surface and the position at which another optical system converges a light on the exposure surface may be displaced from each other with respect to the optical axis direction. As a result, the sizes of converged light formed on the exposure surface by the optical systems may become different from each other. Such a difference between the sizes of the converged light formed by the optical systems may cause a defective and uneven exposure.
An advantage of some aspects of the invention is that, in an exposure head and an image forming apparatus including the exposure head, the exposure head including a plurality of optical systems that converge light at different positions with respect to a direction in which an exposure surface has a curvature, the aforementioned difference between the sizes of converged light is suppressed and a good exposure is realized.
An image forming apparatus according to an aspect of the invention includes an image carrier having a curvature in a first direction; and an exposure head including a first light emitting element that emits a light having a wavelength λ11 and a light having a wavelength λ12, a first optical system that converges each of the light emitted from the first light emitting element onto the image carrier, a second light emitting element, and a second optical system that converges a light emitted from the second light emitting element onto the image carrier, wherein a position at which the first optical system converges each of the light and a position at which the second optical system converges the light are different from each other with respect to the first direction, wherein the first optical system focuses the light having the wavelength λ11 at an imaging position P11 and focuses the light having the wavelength λ12 at an imaging position P12, the imaging position P11 and the imaging position P12 being different from each other with respect to an optical axis direction of the first optical system, and wherein a distance Δ1 between the imaging position P11 and the imaging position P12 with respect to the optical axis direction of the first optical system is equal to or larger than a distance d between an intersection point I1 and an intersection point I2 with respect to the optical axis direction of the first optical system, the intersection point I1 being a point at which the optical axis of the first optical system intersects the image carrier, the intersection point I2 being a point at which an optical axis of the second optical system intersects the image carrier.
An exposure head according to another aspect of the invention includes a first light emitting element that emits a light having a wavelength λ11 and a light having a wavelength λ12; a first optical system that converges each of the light emitted from the first light emitting element onto an exposure surface having a curvature in a first direction; a second light emitting element; and a second optical system that converges a light emitted from the second light emitting element onto the exposure surface, wherein a position at which the first optical system converges each of the light and a position at which second optical system converges the light are different from each other with respect to the first direction, wherein the first optical system focuses the light having the wavelength λ11 at an imaging position P11 and focuses the light having the wavelength λ12 at an imaging position P12, the imaging position P11 and the imaging position P12 being different from each other with respect to an optical axis direction of the first optical system, and wherein a distance Δ1 between the imaging position P11 and the imaging position P12 with respect to the optical axis direction of the first optical system is equal to or larger than a distance d between an intersection point I1 and an intersection point I2 with respect to the optical axis direction of the first optical system, the intersection point I1 being a point at which the optical axis of the first optical system intersects the exposure surface, the intersection point I2 being a point at which an optical axis of the second optical system intersects the exposure surface.
In the image forming apparatus and the exposure head, the first optical system and the second optical system converge light onto an image carrier (exposure surface) having a curvature in the first direction. The position at which the first optical system converges the light on the surface of the image carrier surface and the position at which the second optical system converges the light on the surface of the image carrier are different from each other with respect to the first direction. Thus, the position of the converged light formed by the first optical system on the surface of the image carrier and the position of the converged light formed by the second optical system on the surface of the image carrier are displaced from each other in the optical axis direction. As a result, the sizes of the converged light formed by these optical systems may become different from each other.
The image forming apparatus and the exposure head according to aspects of the invention includes the first light emitting element that emits a light having a wavelength λ11 and a light having a wavelength λ12. The first optical system focuses the light having the wavelength λ11 at the imaging position P11 and focuses the light having the wavelength λ12 at the imaging position P12, the imaging position P11 and the imaging position P12 being different from each other with respect to the imaging position P12. That is, the first optical system focuses the light from the first light emitting element at the imaging positions P11 and P12, which are separated from each other by the distance Δ1 in the optical axis direction. As a result, an effect is obtained in that the apparent depth of focus of the first optical system is increased. The distance Δ1 is equal to or larger than the distance d, which is a distance between the intersection point IS1, at which the optical axis of the first optical system intersects the image carrier (exposure surface), and the intersection point IS2, at which the optical axis of the second optical system intersects the surface of the image carrier (exposure surface), in the optical axis direction. Therefore, the apparent depth of focus of the first optical system can be made sufficiently larger than the displacement between the position of converged light formed by the first optical system and the position of the converged light formed by the second optical system, so that the difference between the sizes of the converged light is suppressed and an even and good exposure can be realized.
It is preferable that the first light emitting element have an emission spectrum having peaks at the wavelength λ11 and at the wavelength λ12. In this case, the apparent depth of focus is efficiently increased, whereby a better exposure can be realized.
According to the aspects of the invention, the distance Δ1 between the imaging position P11 and the imaging position P12 of the first optical system in the optical axis direction is equal to or larger than the distance d, whereby an advantage is obtained in that the difference in the sizes of the converged light formed by the first optical system and the second optical system is suppressed. However, if the distance Δ1 is too large, aberration of the converged light increases and the imaging performance deteriorates, so that an uneven exposure or a decrease in the resolution may occur. It is preferable that the image forming apparatus include an aperture diaphragm disposed in the first optical system, and an expression
Δ1≦|m|×D/tan(u)
be satisfied, where D is a diameter of the first light emitting element with respect to a second direction that is perpendicular to the first direction, m is a magnification of the first optical system with respect to the second direction, and u is an image-side angular aperture that is half an angle between two lines connecting an image point of the first optical system and ends of a diameter of an entrance pupil. In this case, influence on the imaging performance such as aberration can be suppressed, so that a better exposure can be realized.
As with the first optical system, the apparent depth of focus of the second optical system may be increased. That is, it is preferable that the second light emitting element emit a light having a wavelength λ21 and a light having a wavelength λ22, the second optical system focus the light having the wavelength λ21 at an imaging position P21 and focus the light having the wavelength λ22 at an imaging position P22, the imaging position P21 and the imaging position P22 being different from each other with respect to the optical axis direction of the second optical system, and a distance Δ2 between the imaging position P21 and the imaging position P22 with respect to the optical axis direction of the second optical system be equal to or larger than the distance d. In this case, the apparent depth of focus of the second optical system can be made sufficiently larger than the displacement between the converged light formed by the first optical system and the converged light formed by the second optical system in the optical axis direction. By making the apparent depths of focus of the first optical system and the second optical system sufficiently larger than the displacement between the converged light formed by the first and the second optical systems, the difference in the sizes of the converged light formed by these optical systems can be more reliably suppressed, so that a better exposure can be realized.
It is preferable that three or more optical systems including the first optical system and the second optical system be arranged in the first direction, the three or more optical system converging light at different positions with respect to the first direction. With this structure, there is a large difference between the imaging point of the optical system having an optical axis that is farthest from the center of curvature of the image carrier and the imaging position of the optical system having an optical axis that is nearest to the center of curvature of the image carrier in the optical axis direction. The difference in the sizes of the converged light is significant between these optical systems. Therefore, it is preferable that one of the optical axis of the first optical system and the optical axis of the second optical system be nearest to a center of curvature of the image carrier among optical axes of the three or more optical systems, and the other of the optical axis of the first optical system and the optical axis of the second optical system be farthest from the center of curvature of the image carrier among the optical axes of the three or more optical systems. In this case, the difference in the sizes of the converged light between the optical system having an optical axis that is farthest from the center of curvature of the image carrier and the imaging position of the optical system having an optical axis that is nearest to the center of curvature is suppressed, so that a good exposure can be realized.
As described above, if the distance Δ1 between the imaging position P11 and the imaging position P12 in the optical axis direction of the first optical system is too large, there may be an influence on the imaging performance such as aberration. The influence on the imaging performance such as aberration may be suppressed by decreasing the distance Δ1. For this purpose, it is preferable that the distance d be decreased, because, in this case, the magnitude of the distance Δ1 can be decreased while satisfying the condition that the distance Δ1 is equal to or larger than the distance d. Thus, the following structure may be used.
That is, it is preferable that (2N+2) optical systems (where N is an integer equal to or greater than 1) including the first optical system and the second optical system be arranged in the first direction with a distance therebetween, and the one of the first optical system and the second optical system be located in an (N+1)th or an (N+2)th position from an end of the (2N+2) optical systems. In this case, because the distance d is decreased, the distance Δ1 can be decreased while satisfying the condition that the distance Δ1 is equal to or larger than the distance d, whereby an influence on the imaging performance such as aberration can be easily suppressed.
It is preferable that (2N+1) optical systems (where N is an integer equal to or greater than 1) including the first optical system and the second optical system be arranged in the first direction with a distance therebetween, and the one of the first optical system and the second optical system be located in an (N+1)th position from an end of the (2N+1) optical systems. In this case, because the magnitude of the distance d is limited, the magnitude of the distance Δ1 can be limited while satisfying the condition that the distance Δ1 is equal to or larger than the distance d, whereby an influence on the imaging performance such as aberration can be easily suppressed.
The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements.
As described above, with an exposure head and an image forming apparatus including the exposure head, the exposure head including a plurality of optical systems that converges light at different positions with respect to a direction in which an exposure surface has a curvature, the sizes of the converged light formed by the plurality of optical systems may become different from each other. Hereinafter, the cause of the difference between the sizes of converged light and measures to deal therewith will be first described, and the embodiments will be described in detail.
As measures to deal with such a problem, the following structure can be used. In the structure illustrated in
In the case described above, the apparent depth of focus of the optical system OS1, which is farther from the center of curvature CT of the exposure surface ES, is increased. However, the same advantage can be obtained if the apparent depth of focus of the optical system OS2, which is nearer to the center of curvature CT of the exposure surface ES, is increased (
The distance d can be calculated from the following expression
d=(R2−Bβ2)1/2−(R2−Bα2)1/2) (expression 1),
where R is the radius of curvature of the exposure surface ES, Bα is the distance between the optical axis OAα of the optical system OSα and the center of curvature CT of the exposure surface ES, and Bβ is the distance between the optical axis OAβ of the optical system OSβ and the center of curvature CT of the exposure surface ES. The distance between an optical axis and the center of curvature is the distance between the optical axis and a line that is parallel to the optical axis and passes through the center of curvature.
The optical axis of an optical system will be described before describing the embodiments. The optical axis of an optical system can be obtained as follows. When an optical system is symmetric (mirror symmetric) with respect to a plane perpendicular to the sub-scanning direction SD (first direction) and symmetric (mirror symmetric) with respect to a plane perpendicular to the main scanning direction MD (second direction), the optical system has a first symmetry plane that is perpendicular to the first direction and has a second symmetry plane that is perpendicular to the second direction. The optical axis can be obtained as the intersection of the first symmetry plane and the second symmetry plane. In particular, if the optical system is rotationally symmetric, the intersection of the first symmetry plane and the second symmetry plane coincides with the axis of rotational symmetry, and the optical axis can be obtained as this axis of rotational symmetry.
An electrical component box 5, which is disposed in a housing body 3 of the image forming apparatus, contains a power circuit substrate, the main controller MC, the engine controller EC, and the head controller HC. An image forming unit 7, a transfer belt unit 8, and a sheet feed unit 11 are disposed in the housing body 3. A secondary transfer unit 12, a fixing unit 13, and a sheet guide 15 are disposed on the right side of the housing body 3 in
The image forming unit 7 includes four image forming stations Y (yellow), M (magenta), C (cyan), and K (black), each forming an image of a corresponding color. Each of the image forming stations Y, M, C, and K includes a photosensitive drum 21 having a cylindrical shape and having a surface with a predetermined length in the main scanning direction MD. Each of the image forming stations Y, M, C, and K forms a toner image of a corresponding color on the surface of the photosensitive drum 21. The photosensitive drums 21 is disposed in such a manner that the axis thereof extends in a direction parallel to or substantially parallel to the main scanning direction MD. Each of the photosensitive drums 21 is connected to a dedicated drive motor that rotates the photosensitive drum 21 at a predetermined speed in a direction indicated by an arrow D21 in
The charger 23 includes a charging roller having a surface made of elastic rubber. The charging roller rotates while being in contact with the surface of the photosensitive drum 21 at a charging position. As the photosensitive drum 21 rotates, the charging roller is rotated by the photosensitive drum 21 in a driven direction at a peripheral speed. The charging roller is connected to a charge bias generator (not shown). The charging roller, which is supplied with a charge bias from the bias generator, charges the surface of the photosensitive drum 21 at the charging position at which the charger 23 contacts the photosensitive drum 21.
The line head 29 is disposed at a distance from the photosensitive drum 21. The longitudinal direction of the line head 29 is parallel to or substantially parallel to the main scanning direction MD. The lateral direction of the line head 29 is parallel to or substantially parallel to the sub-scanning direction SD. The line head 29 includes a plurality of light emitting elements, and each of the light emitting elements emits a light in accordance with the video data VD supplied by the head controller HC. The charged surface of the photosensitive drum 21 is irradiated with the light emitted from the light emitting elements, whereby an electrostatic latent image is formed on the surface of the photosensitive drum 21.
The developing section 25 includes a development roller 251 having a surface for bearing toner thereon. The development roller 251 is electrically connected to a development bias generator (not shown) that applies a development bias to the development roller 251. The developing bias moves the charged toner from the development roller 251 to the photosensitive drum 21 at the development position at which the development roller 251 contacts the photosensitive drum 21. Thus, the electrostatic latent image, which has been formed by the line head 29, is developed.
The toner image, which has been developed at the development position, is transported in the rotation direction D21 of the photosensitive drum 21. Subsequently, the toner image is primarily transferred to the transfer belt 81 at a primary transfer position TR1 at which the transfer belt 81 contacts the photosensitive drum 21.
In the embodiment, the photosensitive-body cleaner 27, which contacts the surface of the photosensitive drum 21, is disposed downstream of the primary transfer position TR1 and upstream of the charger 23 with respect to the rotation direction D21 of the photosensitive drum 21. The photosensitive-body cleaner 27 contacts the surface of the photosensitive drum 21 and removes residual toner remaining on the surface of the photosensitive drum 21 after the primary transfer.
The transfer belt unit 8 includes a drive roller 82, a driven roller 83 (blade facing roller), which is disposed on the left side of the drive roller 82 in
On the other hand, in the monochrome mode, the color primary transfer rollers 85Y, 85M, and 85C are separated from the image forming stations Y, M, and C respectively facing them. Only the monochrome primary transfer roller 85K is located adjacent to the image forming station K, so that only the monochrome image forming station K contacts the transfer belt 81. As a result, the primary transfer position TR1 is formed only between the monochrome primary transfer roller 85K and the image forming station K. The primary transfer bias generator applies a primary transfer bias to the primary transfer roller 85K at an appropriate time, so that a toner image formed on the surface of a photosensitive drum 21K is transferred to the transfer belt 81 at the primary transfer position TR1. As a result, a monochrome image is formed.
The transfer belt unit 8 includes a downstream guide roller 86 that is disposed downstream of the monochrome primary transfer roller 85K and upstream of the drive roller 82. The downstream guide roller 86 contacts the transfer belt 81 at a position on an internal common tangent line formed by the monochrome primary transfer roller 85K and the photosensitive drum 21K of the image forming station K at the primary transfer position TR1 at which the monochrome primary transfer roller 85K and the photosensitive drum 21K contact each other.
The drive roller 82 rotates the transfer belt 81 in the direction indicated by the arrow D81 and also serves as a backup roller of the secondary transfer roller 121. The peripheral surface of the drive roller 82 is covered with a rubber layer having a thickness of about 3 mm and a volume resistivity lower than 1000 kΩcm. The rubber layer is grounded through a metal shaft and serves as a conductive path of a secondary transfer bias that is supplied by the secondary transfer bias generator (not shown) through the secondary transfer roller 121. By forming the rubber layer, which has high friction and shock absorption, on the drive roller 82, transmission of an impact that occurs when a sheet enters a contact portion (secondary transfer position TR2) between the drive roller 82 and a secondary transfer roller 121 to the transfer belt 81 is suppressed, whereby degradation of the quality of an image can be prevented.
The sheet feed unit 11 includes a sheet feed cassette 77, which can hold a stack of sheets, and a sheet feed section that includes a pickup roller 79 that feeds the sheets one by one from the sheet feed cassette 77. When a sheet is fed from the sheet feed section by the pickup roller 79, a pair of registration rollers 80 adjust timing to feed the sheet, and the sheet is fed to the secondary transfer position TR2 along the sheet guide 15.
The secondary transfer roller 121 can be made to contact or to be separated from the transfer belt 81, driven by a secondary transfer roller drive mechanism (not shown). The fixing unit 13 includes a heating roller 131 and a pressure section 132. The heating roller 131 is rotatable and includes a heating element such as a halogen heater. The pressure section 132 presses and urges the heating roller 131. The sheet guide 15 guides the sheet, on which an image has been secondarily transferred, to a nip portion formed between the heating roller 131 and a pressure belt 1323 of the pressure section 132. An image is thermally fixed at the nip portion at a predetermined temperature. The pressure section 132 includes two rollers 1321 and 1322 and the pressure belt 1323 looped over the two rollers. A surface of the pressure belt 1323 extending between the rollers 1321 and 1322 is pressed against the peripheral surface of the heating roller 131 so as to enlarge the nip portion between the heating roller 131 and the pressure belt 1323. The sheet, that has been subjected the fixing operation, is transported to an output tray 4 disposed on an upper surface of the housing body 3.
This apparatus includes a cleaner section 71 that faces the blade facing roller 83. The cleaner section 71 includes a cleaner blade 711 and a waste toner box 713. An edge of the cleaner blade 711 contacts the blade facing roller 83 with the transfer belt 81 therebetween so as to remove foreign substances, such as residual toner and paper dust, which remain on the transfer belt 81 after the secondary transfer. The foreign substances that have been removed are recovered in the waste toner box 713.
The head substrate 293 is formed of a glass substrate that transmits light. A plurality of light emitting elements E, which are bottom emission organic EL (Electro-Luminescence) devices, are formed on the head-substrate back surface 293-t and sealed with a sealing member 294 (
In further detail, this arrangement can be described as follows. In each light emitting element group EG, fifteen light emitting elements E are disposed at different positions with respect to the longitudinal direction LGD. The distance between the light emitting elements E that are adjacent to each other in the longitudinal direction LGD is an inter-element pitch Pel (in other words, in each light emitting element group EG, fifteen light emitting elements E are arranged at the pitch Pel in the longitudinal direction LGD). The plurality of light emitting element groups EG are separately arranged in the longitudinal direction LGD at an inter-group pitch Peg, which is larger than the inter-element pitch Pel, thereby forming the light emitting element group line GRa. Three light emitting element group lines GRa, GRb, and GRc are separately disposed with a distance Dt therebetween in the lateral direction LTD. Moreover, the light emitting element group lines GRa, GRb, and GRc are shifted from each other by a distance Dg in the longitudinal direction LGD.
The inter-element pitch Pel can be obtained as the distance between the geometric barycenters of two light emitting elements E that are adjacent to each other in the longitudinal direction LGD. The inter-group pitch Peg can be obtained as the distance, in the longitudinal direction LGD, between the geometric barycenter of a light emitting element E that is at a front end of the light emitting element group EG with respect to the longitudinal direction LGD and the geometric barycenter of a light emitting element E that is at a back end of an adjacent light emitting element group EG with respect to the longitudinal direction LGD. The distance Dg can be obtained as the distance between the geometric barycenters of two light emitting element groups EG that are adjacent to each other in the longitudinal direction LGD. The distance Dt can be obtained as the distance between the geometric barycenters of two light emitting element groups EG that are adjacent to each other in the lateral direction LTD.
Thus, the plurality of light emitting element groups EG are separately arranged on the head-substrate back surface 293-t. On the other hand, a head-substrate front surface 293-h is attached to the second side of the head frame 291 with respect to the thickness direction TKD with an adhesive. The head-substrate front surface 293-h is in contact with the light blocking member 297 disposed in the head frame 291. A second side of the light blocking member 297 with respect to the thickness direction TKD is attached to the head-substrate front surface 293-h with an adhesive. Light guide holes 2971 extend through the light blocking member 297 in the thickness direction TKD. The light guide holes 2971 are circular in plan view when viewed from the thickness direction TKD, and the inner walls thereof are black plated. Each of the light guide holes 2971 corresponds to one of the light emitting element groups EG. That is, one light guide hole 2971 is formed for one light emitting element group EG. Thus, the light blocking member 297 is attached to the head-substrate front surface 293-h in such a manner that the light guide hole 2971 is open toward the light emitting element group EG.
The light blocking member 297 is provided in order to prevent so-called stray light from entering the lenses LS1 and LS2. Each of the light emitting element groups EG includes a dedicated optical system constituted by a pair of the lenses LS1 and LS2. When using such a structure, it is desirable that a light enter only the optical system constituted by LS1 and LS2 of the light emitting element group EG that is an emission source thereof and be focused. However, a part of the light may not enter the optical system constituted by LS1 and LS2 of the light emitting element group EG that is the emission source thereof. This part of the light becomes stray light. If such stray light enters the optical system constituted by LS1 and LS2 of the light emitting element group EG that is not the emission source thereof, a so-called ghost may be generated. In order to prevent this, in the embodiment, the light blocking member 297 is disposed between the light emitting element group EG and the optical system constituted by LS1 and LS2. The light blocking member 297 has the light guide hole 2971 that has a black-plated inner wall and that is open toward the light emitting element group EG. Therefore, most of the stray light is absorbed by the inner wall of the light guide hole 2971. As a result, ghost is suppressed and a good exposure operation can be realized.
On a first side of the light blocking member 297 with respect to the thickness direction TKD, a first lens array LA1, which is substantially flat-plate shaped, is supported between side portions 291A and 291B of the head frame 291 in the lateral direction LTD. On the back surface of the first lens array LA1, the first lenses LS1 (LS1a, LS1b, and LS1c) are formed so as to correspond to the light emitting element groups EG. That is, one first lens LS1 faces one light emitting element group EG. Thus, in the first lens array LA1, a plurality of first lenses LS1 are arranged in three lines in a staggered manner. In other words, three first lenses LS1 (LS1a, LS1b, and LS1c) that are disposed adjacent to each other in the main scanning direction MD (longitudinal direction LGD) are disposed at different positions with respect to the sub-scanning direction SD (lateral direction LTD). In
On a first side of the first lens array LA1 with respect to the thickness direction TKD, a second lens array LA2, which is substantially flat-plate shaped, is supported between the side portions 291A and 291B in the lateral direction LTD of the head frame 291. On the back surface of the second lens array LA2, the second lenses LS2 (LS2a, LS2b, and LS2c) are formed so as to correspond to the light emitting element groups EG. That is, one second lens LS2 faces one light emitting element group EG. Thus, in the second lens array LA2, a plurality of second lenses LS2 are arranged in three lines in a staggered manner. In other words, the second lenses LS2 (LS2a, LS2b, and LS2c) that are disposed adjacent to each other in the main scanning direction MD (longitudinal direction LGD) are disposed at different positions with respect to the sub-scanning direction SD (lateral direction LTD). In
Each of the lens arrays LA1 and LA2 includes a light-transmissive lens array substrate SB made of glass. The lenses LS1 and LS2, which are made of resin, are formed on a back surface SB-t of the lens array substrate SB. That is, the first lenses LS1 (LS1a, LS1b, and LS1c), which are made of resin, are formed on the back surface of the substrate SB of the first lens array LA1 (in the same plane). The second lenses LS2 (LS2a, LS2b, and LS2c), which are made of resin, are formed on the back surface of the substrate SB of the second lens array LA2. The lens arrays LA1 and LA2 can be formed by using an existing method, such as a method of using a metal mold. With this method, a metal mold having concave portions corresponding to the shapes of the lenses LS1 and LS2 is made to contact the back surface SB-t of the lens array substrate SB, and a photo-curable resin is injected into a space between the metal mold and the lens array substrate SB. Subsequently, the photo-curable resin is irradiated with light so that the resin is cured, thereby forming the lenses LS1 and LS2 on the lens array substrate SB.
Thus, three optical systems, that is, the upstream optical system constituted by LS1a and LS2a, the middle optical system constituted by LS1b and LS2b, and the downstream optical system constituted by LS1c and LS2c are disposed at different positions with respect to the sub-scanning direction SD. The optical axes OAa, OAb, and OAc of the three optical systems (such as that constituted by LS1a and LS2a) are parallel to each other, and parallel to the optical axis direction Doa illustrated in
Each of the upstream optical system constituted by LS1a and LS2a, the middle optical system constituted by LS1b and LS2b, and the downstream optical system constituted by LS1c and LS2c converges a light emitted from the light emitting element E on the peripheral surface of the photosensitive drum 21. These optical systems converge light at the vicinities of intersection points 1a, 1b, and 1c of the peripheral surface of the photosensitive drum 21 and the optical axes OAa, OAb, and OAc, respectively (
The peripheral surface of the photosensitive drum 21 has a finite curvature. The optical axis OAb of the middle optical system passes through the center of curvature CT21 of the photosensitive drum 21. The optical axis OAa of the upstream optical system constituted by LS1a and LS2a and the optical axis OAc of the downstream optical system constituted by LSlc and LS2c are located on lateral sides of the optical axis OAb of the middle optical system at a distance L1s in the sub-scanning direction SD. As a result, an intersection point Ib, at which the optical axis OAb of the middle optical system intersects the peripheral surface of the photosensitive drum 21, is displaced from the intersection point Ia, at which the optical axis OAa of the upstream optical system intersects the peripheral surface of the photosensitive drum 21, and from the intersection point Ic, at which the optical axis OAc of the downstream optical system intersects the peripheral surface of the photosensitive drum 21, by a distance d in the optical axis direction Doa.
That is, the upstream optical system constituted by LS1a and LS2a forms the spot SP in the vicinity of the intersection point Ia and the middle optical system constituted by LS1b and LS2b forms the spot SP in the vicinity of the intersection point Ib, the intersection points Ia and Ib being displaced from each other by the distance d in the optical axis direction. The same relationship exists between the downstream optical system constituted by LS1c and LS2c and the middle optical system constituted by LS1b and LS2b. Owing to the displacement by the distance d, the size of the spot SP formed by the upstream optical system constituted by LS1a and LS2a and the size of the spot SP formed by the middle optical system constituted by LS1b and LS2b may become different from each other, and the size of the spot formed by the downstream optical system constituted by LS1a and LS2a and the size of the spot SP formed by the middle optical system constituted by LS1b and LS2b may become different from each other.
In order to prevent this, in the embodiment, the apparent depths of focus of the optical systems are increased. That is, in the embodiment, the light emitting elements E have an emission spectrum having peaks at wavelengths λ1 and λ2. Each of the upstream optical system constituted by LS1a and LS2a, the middle optical system constituted by LS1b and LS2b, and the downstream optical system constituted by LS1c and LS2c focuses a light having the wavelength λ1 and a light having the wavelength λ2 at different positions with respect to the optical axis direction Doa. As the light emitting element E, for example, an organic EL device described in JP-A-10-237439 can be used. To be specific, the organic EL device has an emission spectrum having peaks at wavelengths of 463 nm and 534 nm.
As illustrated in
The upstream optical system constituted by LS1a and LS2a and the middle optical system constituted by LS1b and LS2b have the same optical structure. Therefore, the imaging positions Pa1 and Pb1 are the same in the optical axis direction Doa, and the imaging position Pa2 and Pb2 are the same in the optical axis direction Doa. Therefore, the imaging positions Pa1 and Pb1 are in a first imaging plane IP1 that is perpendicular to the optical axis direction Doa, and the imaging positions Pa2 and Pb2 are in a second imaging plane IP2 that is perpendicular to the optical axis direction Doa. The distance between the first imaging plane IP1 and the second imaging plane IP2 is the distance Δ. The distance Δ is equal to or larger than the distance d, which is the distance between the intersection point Ia and the intersection point Ib in the optical axis direction Doa. Both the intersection points Ia and Ib are located between the first imaging plane IP1 and the second imaging plane IP2.
Thus, in the first embodiment, the light emitting element E emits a light having the wavelength λ1 and a light having the wavelength λ2. The optical system constituted by LS1a and LS2a, for example, focuses the light having the wavelengths λ1 and the light having the wavelength λ2 at imaging positions Pa1 and Pa2 that are separated from each other by the distance Δ in the optical axis direction Doa. Thus, an effect is obtained in that the apparent depth of focus of the optical system constituted by LS1a and LS2a is increased. The distance Δ is equal to or larger than the distance d. Therefore, for the same reason that is described in the section “A. Cause of Difference between the Sizes of Converged Light and Measures to deal therewith”, the difference between the sizes of the spots SP formed by the optical systems are suppressed, whereby a good exposure can be realized.
Moreover, in the first embodiment, the apparent depths of focus of the upstream optical system constituted by LS1a and LS2a and the middle optical system constituted by LS1b and LS2b are sufficiently increased relative to the displacement between the spots SP formed by the optical systems in the optical axis direction Doa. Thus, the difference between the sizes of the spots SP formed by the optical systems can be more reliably suppressed, whereby a better exposure can be realized. The same relationship and advantage apply to the downstream optical system constituted by LS1c and LS2c and the middle optical system constituted by LS1b and LS2b having the structure same as that described above.
In the first embodiment, the light emitting element E has an emission spectrum having peaks at the wavelengths λ1 and λ2. Thus, the apparent depth of focus is effectively increased, whereby a better exposure can be realized.
In the first embodiment, the imaging position of the light having the wavelength λ1 and the imaging position of the light having the wavelength λ2 are separated from each other by the distance Δ in the optical axis direction Doa. In other words, the distance Δ between the first imaging plane IP1 and the second imaging plane IP2 in the optical axis direction Doa is equal to or larger than the distance d, so that the difference between the sizes of the spots SP formed by the optical systems is suppressed. However, if the distance Δ is too large, aberration of the spot SP becomes large and an imaging performance deteriorates, so that exposure may become uneven and the resolution may decrease. Therefore, a second embodiment has the following structure, in addition to the structure the same as that of the first embodiment. Needless to say, the second embodiment has the same advantage as that of the first embodiment, because the second embodiment include the structure the same as that of the first embodiment.
Δ≦|m|×D/tan(u) (expression 2)
is satisfied, where D is a diameter of the light emitting element E with respect to the main scanning direction MD, m is a lateral magnification of the optical system with respect to the main scanning direction MD, and u is an image-side angular aperture that is half the angle between two lines connecting an image point and ends of a diameter of an entrance pupil. Thus, an influence on the imaging performance such as aberration is suppressed, so that a better exposure can be realized.
With this structure, there is a large displacement dmx between the intersection points Ia and Ib in the optical axis direction Doa, where the intersection point Ia is a point at which the peripheral surface of the photosensitive drum 21 intersects the optical axis OAa, which is farthest from the center of curvature CT21, and the intersection point Ib is a point at which the peripheral surface of the photosensitive drum 21 intersects the optical axis OAb, which is nearest to the center of curvature CT21. Owing to the large displacement dmx, between the upstream optical system constituted by LS1a and LS2a and the middle optical system constituted by LS1b and LS2b, the difference between the positions at which the spots SP are formed differ greatly in the optical axis direction Doa. Therefore, the difference between the sizes of the spots SP is significant between the upstream optical system constituted by LS1a and LS2a and the middle optical system constituted by LS1b and LS2b.
Thus, it is preferable that the apparent depth of focus be increased for at least one of the upstream optical system constituted by LS1a and LS2a and the middle optical system constituted by LS1b and LS2b. That is, by making the distance Δ, which is the distance between the imaging position of the light having the wavelength λ1 and the imaging position of the light having the wavelength λ2 in the optical axis direction Doa, equal to or larger than the distance dmx, the difference between the sizes of the spots formed by the upstream optical system constituted by LS1a and LS2a and the middle optical system constituted by LS1b and LS2b can be suppressed, whereby a good exposure can be realized.
Moreover, the third embodiment has the following operational advantage. As described above, if the distance Δ between the imaging position of the light having the wavelength λ1 and the imaging position of the light having the wavelength λ2 is too large, there may be an influence on the imaging performance such as aberration. The influence on the imaging performance such as aberration may be suppressed by decreasing the distance Δ. For this purpose, it is preferable that the distance d be decreased, because, in this case, the distance Δ can be decreased while satisfying the condition that the distance Δ is equal to or larger than the distance dmx. In the line head 29 of the third embodiment, (2N+1) optical systems (where N is an integer equal to or greater than 1, and N=1 in the third embodiment) are arranged in the sub-scanning direction SD at a distance L1s therebetween, and the optical system constituted by LS1b and LS2b that are nearest to the center of curvature CT21 are located at the (N+1)th position from and end of the (2N+1) optical systems. In this case, because the distance d is decreased, the distance Δ can be decreased while satisfying the condition that the distance Δ is equal to or larger than the distance dmx, whereby an influence on the imaging performance such as aberration can be easily suppressed.
As illustrated in
With this structure, there is a large displacement dmx between the intersection points Id and Ib in the optical axis direction Doa, where the intersection point Id is a point at which the peripheral surface of the photosensitive drum 21 intersects the optical axis OAd, which is farthest from the center of curvature CT21, and the intersection point Ib is a point at which the peripheral surface of the photosensitive drum 21 intersects the optical axis OAb, which is nearest to the center of curvature CT21. Owing to the large displacement dmx, between the optical system constituted by LS1d and LS2d and the optical system constituted by LS1b and LS2b, the difference between the positions at which the spots SP are formed differ greatly in the optical axis direction Doa. Therefore, the difference between the sizes of the spots SP is significant between the optical system constituted by LS1d and LS2d and the optical system constituted by LS1b and LS2b.
Thus, it is preferable that the apparent depth of focus be increased for at least one of the optical system constituted by LS1d and LS2d and the optical system constituted by LS1b and LS2b. That is, by making the distance Δ, which is the distance between the imaging position of the light having the wavelength λ1 and the imaging position of the light having the wavelength λ2 in the optical axis direction Doa, equal to or larger than the distance dmx, the difference between the sizes of the spots formed by the optical system constituted by LS1d and LS2d and the middle optical system constituted by LS1b and LS2b can be suppressed, whereby a good exposure can be realized.
Moreover, the fourth embodiment has the following operational advantage. As described above, if the distance Δ between the imaging position of the light having the wavelength λ1 and the imaging position of the light having the wavelength λ2 is too large, there may be an influence on the imaging performance such as aberration. The influence on the imaging performance such as aberration may be suppressed by decreasing the distance Δ. For this purpose, it is preferable that the distance d be decreased, because, in this case, the distance Δ can be decreased while satisfying the condition that the distance Δ is equal to or larger than the distance dmx. In the line head 29 of the fourth embodiment, (2N+2) optical systems (where N is an integer equal to or greater than 1, and N=1 in the fourth embodiment) are arranged in the sub-scanning direction with a distance L1s therebetween, and the optical system constituted by LS1b and LS2b that is nearest to the center of curvature CT21 are located at the (N+1)th or the (N+2)th position from and end of the (2N+2) optical systems. In this case, because the distance d is decreased, the distance Δ can be decreased while satisfying the condition that the distance Δ is equal to or larger than the distance dmx, whereby an influence on the imaging performance such as aberration can be easily suppressed.
In the embodiments, the line head 29 corresponds to the “exposure head” of the invention, the photosensitive drum 21 corresponds to the “image carrier” of the invention, the sub-scanning direction SD corresponds to the “first direction” of the invention, the main scanning direction corresponds to the “second direction” of the invention, and the peripheral surface of the photosensitive drum 21 corresponds to the “exposure surface” of the invention. In the description of
The invention is not limited to the embodiments described above, and the embodiments can be modified in various ways within the spirit and scope of the invention.
In this modification, the photosensitive belt 21B is looped over two rollers 28 that extend in the main scanning direction MD. The photosensitive belt 21B is rotated in a predetermined rotation direction D21 by a drive motor (not shown). The charger 23, the line head 29, the developing section 25, and the photosensitive-body cleaner 27 are disposed around the photosensitive belt 21B in the rotation direction D21. These members perform charging, forming of a latent image, and developing of toner.
In this modification, the line head 29 is disposed so as to face a looped-over portion of the photosensitive belt 21B at which the photosensitive belt 21B is looped over one of the rollers 28. The rollers 28 are cylindrical. Therefore, the looped-over portion of the photosensitive belt 21B has a finite curvature. The line head 29 is disposed so as to face the looped-over portion for the following reason. That is, an extended portion of the photosensitive belt 21B flutters to a greater degree than the looped-over portion. By disposing the line head 29 so as to face the looped-over portion that flatters to a smaller degree than the extended portion, the distance between the line head 29 and the surface of the photosensitive belt 21B can be stabilized.
However, because the surface of the photosensitive at the looped-over portion has a finite curvature in the sub-scanning direction SD, defective exposure may occur as described above. Therefore, by applying the invention to an image forming apparatus having the structure illustrated in
In the embodiments, the peak strengths of the light emitting element at the wavelengths λ1 and λ2 are not specified. However, the peak strengths at the wavelengths λ1 and λ2 may be greater than half the maximum value of the emission spectrum. In this case, the depth of focus can be more effectively increased.
In the embodiments, the optical system forms an inverted reduced image with a negative magnification having an absolute value smaller than 1. However, the magnification of the optical system is not limited thereto. The magnification may be positive and may have an absolute value equal to or larger than 1.
In the embodiments, the lenses are arranged in the lens arrays LA1 and LA2 in three or four lines in a staggered manner. However, the arrangement of the lenses is not limited thereto.
In the third and fourth embodiments, the integer N is 1. However, the integer N is not limited to 1, and may be equal to or larger than 2.
In the embodiments, the optical systems are arranged at a distance L1s in the sub-scanning direction SD. However, the optical systems may not be arranged at a regular distance.
In the embodiments, the lenses LS1 and LS2 are formed on the back surfaces of the lens arrays LA1 and LA2. However, the lenses LS1 and LS2 may be formed, for example, on the front surfaces of the lens arrays LA1 and LA2.
In the embodiments, the lens arrays LA1 and LA2 include the light transmissive substrates SB1 and SB2, which are made of glass, and the lenses LSa1, LSa2, and the like, which are made of resin. However, the lens arrays LA1 and LA2 may be integrally formed.
In the first embodiment, the plurality of light emitting element groups EG are arranged in three lines in a staggered manner. However, the arrangement of the plurality of light emitting element groups EG is not limited thereto.
In the embodiments, fifteen light emitting element E constitutes the light emitting element group EG. However, the number of the light emitting elements E that constitute the light emitting element group EG is not limited thereto.
In the embodiments, the plurality of light emitting elements E included the light emitting element group EG are arranged in two lines in a staggered manner. However, the arrangement of the plurality of light emitting elements E in the light emitting element group EG is not limited thereto.
In the embodiments, bottom emission organic EL devices are used as the light emitting elements E. However, top emission organic EL devices may be used as the light emitting elements E. Alternatively, light emitting diodes (LEDs) other than the organic EL devices may be used as the light emitting elements E.
In the embodiments, the light emitting element E has an emission spectrum with peaks at the wavelengths λ1 and λ2. However, it is not necessary that the light emitting element E have peaks at the wavelengths λ1 and λ2. As long as the light emitting element E can emit light having the wavelength λ1 and light having the wavelength λ2, the depth of focus can be increased.
An example of the invention will be described below. However, the invention is not limited to the example, and can be modified within the spirit an scope of the invention, and such modification are included in the technical scope of the invention.
As illustrated in lens data of
Next, a case in which the values in
As illustrated in
The imaging position of the light having the wavelength of 610 nm and the imaging position of the light having the wavelength of 670 nm were displaced from each other in the optical axis direction by Δ=30 μm. Therefore, as illustrated in
Likewise, the imaging position of the light having the wavelength of 565 nm and the imaging position of the light having the wavelength of 715 were displaced from each other in the optical axis direction by Δ=60 μm. Therefore, as illustrated in
Moreover, in any of
This distance Δ satisfied the expression 2, so that influence on the imaging performance such as aberration was suppressed, whereby a better exposure could be realized. That is, the right hand side of the expression 2 was
|−0.7056|×28.6 μm/tan(17.6°)=63.6 μm.
The distance Δ (=30 μm, 60 μm) between the imaging positions illustrated in
The entire disclosure of Japanese Patent Applications No. 2009-147862, filed on Jun. 22, 2009 is expressly incorporated by reference herein.
Number | Date | Country | Kind |
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2009-147862 | Jun 2009 | JP | national |