JP7087980B2 - Image forming device - Google Patents

Description

The present invention relates to an image forming apparatus including a photoconductor and an optical print head, and more specifically, an image forming apparatus including a light emitting element having a group of light emitting points arranged in two dimensions, and also with respect to a sub-scanning direction. The present invention relates to an image forming apparatus having an improved image forming state.

Patent Document 1 below discloses an image forming apparatus using a line head that projects a row of light emitting elements onto an irradiated surface using a microlens array to form a row of imaged spots. In the line head disclosed in Patent Document 1, illuminant blocks including one or more rows of light emitting elements in which a plurality of light emitting elements are arranged in an array in the first direction are spaced apart from each other in at least the first direction. A lens array in which one positive lens system is arranged corresponding to each illuminant block is arranged on the emission side of a plurality of illuminant arrays, and a writing surface is provided on the image forming side of the lens array. The positive lens system constituting the lens array is composed of a telephoto optical system in which two groups of positive lenses are arranged in confocal, and an aperture aperture is arranged in the confocal surface of the telescopic optical system. It is characterized by being.
This line head technique is a technique of forming and drawing light from a group of light emitting points corresponding to each optical system by using a plurality of optical systems whose optical axes are parallel to each other.

However, according to this technology, if the light emitting point cloud is formed on the same glass substrate, it is easy to obtain the accuracy of the relative position, but if the drive circuit etc. is built on the same glass substrate, the circuit is on the glass substrate. There is a concern that the area occupied by the glass will increase and the unit will become larger. Further, the image quality of the formed image is not always sufficient, and it is considered that there is room for improvement.
Therefore, it is desired to develop a technique that enables further improvement of the formed image as the unit is miniaturized.

Japanese Unexamined Patent Publication No. 2009-51194

The present invention has been made in view of the above problems and situations, and the problem to be solved thereof is an image forming apparatus provided with a light emitting element having a group of light emitting points arranged in two dimensions, and the sub-scanning direction is also construed. It is to provide the image forming apparatus which improved the image state.

In order to solve the above problems, the present invention is provided on the lens substrate in the process of examining the relative positional relationship between the light emitting point, the lens, the photoconductor surface, etc. in the optical system of the optical print head provided in the image forming apparatus. We have found that the tilt of the lens and the like have a great influence on the imaging state in the sub-scanning direction, and have arrived at the present invention.
That is, the above-mentioned problem according to the present invention is solved by the following means.

1. 1. At least, an image forming apparatus including a photoconductor and an optical print head.
The optical print head has a light emitting element having a light emitting point cloud arranged two-dimensionally, and
It has an optical system that forms an image of light from the light emitting point group at different positions on the light receiving surface of the photoconductor for each light emitting point.
The image magnification of the optical system is -1, and the image magnification is -1.
A plurality of sets of the light emitting point group and the optical system exist, and include those having different positions in the sub-scanning direction.
When viewed from the direction of the rotation axis of the photoconductor, the conjugate length of each of the optical systems varies depending on the position in the sub-scanning direction, and monotonically increases or decreases depending on the position in the sub-scanning direction.
For each of the above optical systems, there is one lens on the upstream side and one lens on the downstream side of the aperture.
Each of the two lenses has a common axis of rotational symmetry on two planes with curvature.
For the two lenses, the surface on the side closer to the aperture has the same shape, the surface on the side farther from the aperture has the same shape, and the core thickness is the same.
The center of the aperture is arranged on a straight line connecting the two principal points on the side close to the aperture of the two lenses, and the straight line connecting the two principal points on the side close to the aperture and the said. An image forming apparatus characterized in that the rotational symmetry axes of the lenses have a non-zero angle, and the rotational symmetry axes of the two lenses are parallel to each other.

2. 2. The image forming apparatus according to item 1, wherein the four lens surfaces of the two lenses are point-symmetrical with respect to the center of the diaphragm.

3. 3. For the plurality of optical systems, all the lenses on the upstream side of the diaphragm are formed by stacking resins on one glass substrate, while all the lenses on the downstream side of the diaphragm are made of resin on one glass substrate. The image forming apparatus according to the first item or the second item, wherein the images are formed by stacking the lenses.

4. The image forming apparatus according to item 3, wherein the glass substrate is not perpendicular to a straight line connecting the two principal points on the side close to the diaphragm of the two lenses.

5. The lens of the lens with respect to the direction in which the normal line of the glass substrate is tilted with respect to the straight line connecting the two principal points on the side close to the aperture and the straight line connecting the two principal points on the side close to the aperture. The image forming apparatus according to item 4, wherein the direction in which the axis of rotational symmetry is tilted is the same.

6. The image forming apparatus according to item 4 or 5, wherein the glass substrate on the upstream side and the glass substrate on the downstream side are not parallel to each other.

7. The angle at which the axis of rotational symmetry of the lens is tilted with respect to the straight line connecting the two main points on the side close to the aperture is upstream of the straight line connecting the two main points on the side close to the aperture. Item 6. The image forming apparatus according to Item 6, wherein the normal line of the glass substrate on the side is between an angle at which the normal line is tilted and the normal line of the glass substrate on the downstream side is tilted.

According to the above means of the present invention, it is possible to provide an image forming apparatus provided with a light emitting element having a light emitting point group arranged two-dimensionally and having an improved image forming state also in a sub-scanning direction. ..
(Basic concept about the present invention and its effects)
Hereinafter, in order to clarify the characteristics of the present invention, the basic ideas and the like leading to the present invention will be described.
The technique disclosed in Patent Document 1 has the above-mentioned problems.
Therefore, as one of the solutions to the above problems, a group of light emitting points having the same position in the sub-scanning direction is formed on one glass substrate, and a group of light emitting points having different positions in the sub-scanning direction is formed on another glass substrate. The unit is formed on the top, and the distance from the photoconductor of the glass substrate is changed so that the drive circuit of another glass substrate is arranged behind the light emitting point cloud on one glass substrate. Technology for miniaturization is conceivable.

However, when such a configuration is adopted, the conjugated length of the optical system is different depending on the position in the sub-scanning direction. When the optical system is configured so that the lenses at different positions are formed by laminating resin on a common glass plate, and the glass plate faces the photoconductor, from the final surface of the lens to the photoconductor. However, the distance from the light emitting point group to the front surface of the lens changes according to the difference in the conjugate length, which makes it difficult to make the magnifications uniform.

Further, there is a technique for making the optical system telecentric in order to prevent the size of the image from changing when the distance between the light source and the photoconductor changes with respect to the lens. In order to make the optical system telecentric, it is necessary to set the positional relationship between the lens and the aperture according to the focal length of the partial optical system before and after the aperture, but all optics are on the photoconductor side where the difference in conditions is small. While it is possible to make the system telecentric at the same time, it is difficult to make the system telecentric at the same time on the light source side where the difference in distance is large. In order to make the magnification uniform and the light source side to be telecentric, there is a method to stop forming the lens on the common lens substrate and divide it into multiple lenses, but if the position shifts relatively, the image In a situation where the light source substrate is divided into multiple parts and must be adjusted and held individually, the configuration of the lens holding mechanism becomes extremely complicated and adjustment becomes very difficult. ..

Therefore, while maintaining a common axis of symmetry for the lens surface, the lens substrate is slanted and the light source substrates are laminated, while the magnification of the optical system is the same, and a telecentric optical system is used on both the light source side and the photoconductor side. The technology to be realized is conceivable.
However, when the glass substrate is tilted without tilting the lens surface, the lens surface is tilted when viewed with the glass substrate as a reference, and there is a difference in thickness in the sub-scanning direction. In particular, in the lens on the light source side of the diaphragm, the inclination of the glass substrate with respect to the photoconductor becomes large, so that there arises a problem that the difference in the thickness of the resin on the glass substrate becomes large.

Therefore, the present invention has been reached through the above considerations and experiments. That is, the rotational symmetry axes of the front and back surfaces of the lens on the upstream side and the lens on the downstream side of the aperture are in the same state by the means described in the column of "Means for Solving the Problem". The upstream lens and the downstream lens can be rotated by the same angle as they are, and if the same shape is rotated 180 degrees, the asymmetrical aberration component generated by tilting the lens will be on the upstream side. And on the downstream side, they cancel each other out, and only symmetrical components remain. For example, it is possible to obtain a better image formation state as compared with the case where only one is tilted or the case where it is tilted at a different angle.

A partial cross-sectional view showing a schematic configuration of an image forming apparatus of an embodiment. Conceptual diagram illustrating the structure and optical path of the optical printhead that constitutes the image forming unit Conceptual diagram illustrating the structure and optical path of the optical print head of the comparative example Schematic diagram showing the positional relationship between the light source and the lens in the embodiment of the present invention.

In the image forming apparatus of the present invention, the optical print head has a light emitting element having a light emitting point group arranged in two dimensions, and light from the light emitting point group is different for each light emitting point on the light receiving surface of the photoconductor. It has an optical system that forms an image at a position, the imaging magnification of the optical system is -1, and there are a plurality of pairs of the light emitting point group and the optical system, including those having different positions in the sub-scanning direction. When viewed from the direction of the rotation axis of the photoconductor, the conjugate length of each of the optical systems varies depending on the position in the sub-scanning direction, and monotonically increases or decreases depending on the position in the sub-scanning direction. For each of the optical systems, there is one lens each on the upstream side and the downstream side of the aperture, and each of the two lenses has two planes having a curvature having a common axis of rotational symmetry, and the two lenses have the same axis of rotational symmetry. Regarding the lenses, the surface on the side close to the aperture has the same shape, the surface on the side far from the aperture has the same shape, and the core thickness is the same, and the side of the two lenses near the aperture. The center of the aperture is arranged on a straight line connecting the two main points of the lens, and the straight line connecting the two main points on the side close to the aperture and the axis of rotational symmetry of the lens have a non-zero angle. The two lenses are characterized in that the axes of rotational symmetry are parallel to each other.
This feature is a technical feature common to each of the following embodiments.

In the image forming apparatus of the present invention, the lens on the upstream side and the lens on the downstream side of the aperture can rotate by the same angle while keeping the axes of rotational symmetry on the front and back surfaces in the same state. Therefore, if the upstream lens and the downstream lens have the same shape and are rotated 180 degrees, the asymmetric aberration components generated by tilting the lens cancel each other out on the upstream side and the downstream side. Only symmetric components remain, and it is possible to obtain a better image formation state as compared with the case where only one is tilted or tilted at a different angle, for example.

In an embodiment of the present invention, it is preferable that the four lens surfaces of the two lenses are point-symmetrical with respect to the center of the diaphragm.
This is because when there is only an aperture between the two lenses, it is better to arrange the upstream and downstream lenses so that they are point-symmetrical with respect to the center of the aperture in order to remove the asymmetric component. ..

Further, for the plurality of optical systems, the lenses on the upstream side of the diaphragm are all formed by stacking resins on one lens substrate, while the lenses on the downstream side of the diaphragm are all on one glass substrate. It is preferably formed by stacking resins on the surface.
If multiple lenses are formed on one glass substrate and the positions of the glass substrates are adjusted and held, the configuration is simpler than when they are separated, and the relative positions are fixed. This is because it is easy to obtain accuracy. It is desirable to form all the lenses on the upstream side of the aperture on one glass substrate and all the lenses on the downstream side of the aperture on one glass substrate. Some glass substrates have a light emitting point cloud or drive circuit formed on them, and some have a lens formed by laminating resin. From now on, a light emitting point cloud group is used to distinguish them. The glass substrate on which the lens is formed is referred to as a "light emitting substrate", and the glass substrate on which a lens is formed is referred to as a "lens substrate".

Further, it is preferable that none of the lens substrates is perpendicular to the straight line connecting the two principal points on the side close to the aperture of the two lenses.
In the present invention, the unit can be miniaturized by dividing the light emitting board and arranging the drive circuit behind the light emitting point cloud. At this time, the drive circuit is arranged on the same side with respect to the light emitting point cloud. It is smaller in the sub-scanning direction than when it is mixed with the one in the opposite direction. At that time, the conjugate length monotonically increases or decreases depending on the position in the sub-scanning direction. At that time, if lenses at different positions in the sub-scanning direction are formed on a common lens substrate as described above, the distance from the photoconductor to the lens is required to make the magnifications uniform for optical systems having different conjugate lengths. However, it is desirable to tilt the lens substrate so that it is approximately the same as the ratio of the conjugate lengths. At that time, all the glass substrates are in an inclined state.

In an embodiment of the present invention, a direction in which the normal line of the lens substrate is tilted with respect to a straight line connecting the two principal points on the side close to the aperture and the two principal points on the side close to the aperture are defined. It is preferable that the direction in which the axis of rotational symmetry of the lens is tilted with respect to the straight line to be connected is the same.

Further, it is preferable that the lens substrate on the upstream side and the lens substrate on the downstream side are not parallel to each other.
Since the angle of inclination differs depending on the position of the lens substrate from the light emitting point to the photoconductor, there is a difference in angle between the two lens substrates.

Further, the angle at which the axis of rotational symmetry of the lens is tilted with respect to the straight line connecting the two main points on the side close to the aperture is the same as the straight line connecting the two main points on the side close to the aperture. It is preferable that the normal line of the lens substrate on the upstream side is between the angle at which the normal line of the lens substrate is tilted and the normal line of the lens substrate on the downstream side is tilted.

As mentioned earlier, the angle at which the lens surface is tilted must be the same on the upstream side and the downstream side, so tilting the lens surface according to the angle of the lens substrate is limited to either the upstream side or the downstream side. You will not be able to. Therefore, although the difference in the thickness of the resin layer cannot be completely eliminated, it is desirable to rotate the lens surface at an angle between the two lens substrates in order to suppress the difference in thickness as much as possible.

Hereinafter, the present invention, its constituent elements, and a mode for carrying out the present invention will be described in detail.
(Overview of the image forming apparatus of the present invention)
The image forming apparatus of the present invention is at least an image forming apparatus including a photoconductor and an optical print head.
The optical print head has a light emitting element having a light emitting point cloud arranged two-dimensionally, and
It has an optical system that forms an image of light from the light emitting point group at different positions on the light receiving surface of the photoconductor for each light emitting point.
The image magnification of the optical system is -1, and the image magnification is -1.
A plurality of sets of the light emitting point group and the optical system exist, and include those having different positions in the sub-scanning direction.
When viewed from the direction of the rotation axis of the photoconductor, the conjugate length of each of the optical systems varies depending on the position in the sub-scanning direction, and monotonically increases or decreases depending on the position in the sub-scanning direction.
For each of the above optical systems, there is one lens on the upstream side and one lens on the downstream side of the aperture.
Each of the two lenses has a common axis of rotational symmetry on two planes with curvature.
For the two lenses, the surface on the side closer to the aperture has the same shape, the surface on the side farther from the aperture has the same shape, and the core thickness is the same.
The center of the aperture is arranged on a straight line connecting the two principal points on the side close to the aperture of the two lenses, and the straight line connecting the two principal points on the side close to the aperture and the said. The axis of rotational symmetry of the lens has a non-zero angle, and the axes of rotational symmetry of the two lenses are parallel to each other.

[Embodiment]
Hereinafter, an example of an embodiment of the image forming apparatus according to the present invention will be described with reference to the drawings. The technical scope of the present invention is not limited to the following examples of embodiments.

(Structure of the entire image forming apparatus)
As shown in FIG. 1, the image forming apparatus 100 according to the present embodiment is used as, for example, a digital copier, and has an image reading unit 10 for reading a color image formed on a document D and an image corresponding to the document D. An image forming unit 20 formed on the paper P, a feeding unit 40 for feeding the paper P to the image forming unit 20, a conveying unit 50 for conveying the paper P, and a control unit for comprehensively controlling the operation of the entire apparatus. Includes 90 and.

The image forming unit 20 includes an image forming unit 70Y, 70M, 70C, 70K provided for each of the cyan, magenta, yellow, and black colors, an intermediate transfer unit 81 on which a toner image obtained by synthesizing each color is formed, and a toner. It is provided with a fixing portion 82 for fixing an image.

Of the image forming units 20, the image forming unit 70Y is a portion that forms a Y (yellow) color image, and includes a photoconductor 71, a charging unit 72, an optical print head (optical writing device) 73, a developing unit 74, and the like. It is equipped with. The photoconductor 71 forms a Y-color toner image, the charging portion 72 is arranged around the photoconductor 71 to charge the surface of the photoconductor 71 by corona discharge, and the optical print head 73 is attached to the photoconductor 71. On the other hand, the light corresponding to the image of the Y color component is irradiated, and the developing unit 74 forms the toner image from the electrostatic latent image by adhering the toner of the Y color component to the surface of the photoconductor 71. The photoconductor 71 has a cylindrical shape and rotates around the rotation axis RX. The cylindrical surface of the photoconductor 71 is a light receiving surface 71a for forming an image by the optical print head 73.

Since the other image forming units 70M, 70C, and 70K have the same structure and function as the image forming unit 70Y for Y color except that the colors of the images to be formed are different, the description of these structures and the like will be omitted. The image forming unit 70 means any unit among the four color image forming units 70Y, 70M, 70C, and 70K, and the photoconductor 71, the charging portion 72, and the optical print are the elements adapted to each color. It includes a head 73 and a developing unit 74.

(Composition of optical print head)
FIG. 2 is a conceptual side view (hereinafter, “in the following,” illustrating the structure of the optical print head (also referred to as “optical writing device”) 73 of the image forming unit 70 and the optical path in the optical print head 73. It is also referred to as an "optical path diagram"), and is a diagram viewed from the direction of the rotation axis of the photoconductor 71. The photoconductor 71 has a cylindrical shape, but a part thereof is shown in the figure.

The light source is a bottom emission type organic electroluminescence element (also referred to as “organic EL element”), and a light emitting point group 3 composed of a plurality of light emitting points is two-dimensionally arranged on a glass plate 4. The vertical direction in the figure corresponds to the sub-scanning direction, but there are light emitting point clouds at three points, and there is an optical system corresponding to each.

Each optical system consists of two convex lenses 1 and 2. The lenses 1 and 2 are formed by laminating a resin on the lens substrates 5a and 5b, respectively. The lens on the light source side is formed on one common lens substrate 5a, and the lens on the photoconductor side is formed on another common lens substrate 5b. Further, between the lens 2 on the photoconductor side and the photoconductor 71, there is a window 5c which is a flat glass plate. The window 5c and an exterior (not shown) cover the two lens substrates so that dust does not adhere to the lens.

FIG. 3 is an optical path diagram of a comparative example. Looking at one of the three optical systems shown, there are four planes with curvature, all of which are axisymmetric aspherical surfaces and all axes of rotational symmetry coincide. Further, the axes of rotational symmetry of the three optical systems are parallel to each other. The center of the light emitting point group (DG) constituting the light emitting point 3 is on the extension line of the axis of rotational symmetry, and the center of the imaging point on the photoconductor 71 is also on the extension line of the axis of rotational symmetry. The optical system is telecentric on both sides and has a magnification of -1x. In order to realize this, the light emitting point cloud group DG, the upstream lens 1, the diaphragm 6, the downstream lens 2, and the imaging points 7 on the photoconductor 71 are substantially equidistant to each other. Since the drive circuit of another light emitting point cloud is arranged behind one light emitting point cloud, the three light emitting point clouds corresponding to the three illustrated optical systems are arranged in a straight line. The straight line is tilted with respect to the optical axis.

The distance from the light emitting point group to the imaging point 7 on the photoconductor is longer toward the upper side of the figure. Due to the cylindrical shape of the photoconductor rather than the flat surface, the above change in distance with respect to the position in the sub-scanning direction is not linear, but increases monotonically. Since the two lens substrates 5a and 5b and the diaphragm 6 are flat plates and are arranged so as to roughly divide the light emitting point 3 and the imaging point 7 on the photoconductor into four equal parts, the angle formed with the optical axis deviates from the vertical. When the tangent (tangent) is calculated for each angle, when the light emitting point (group) is 1, the upstream lens substrate is about 3/4, the aperture is about 1/2, and the downstream lens substrate. Is about 1/4. The difference in angle between the optical axis and the lens substrate directly indicates the amount at which the lens surface is tilted with respect to the lens substrate. The thickness of the resin portion on the lens substrate is different between the upper side and the lower side in the figure, and the difference is particularly large in the lens on the upstream side.

On the other hand, in the embodiment of the present invention, the axes of rotational symmetry of the lenses 1 and 2 are tilted so as to match the angle close to the angle of the diaphragm 6. As a result, the difference in thickness is greatly reduced in the upstream lens. In the downstream lens, the direction of tilt is opposite, but the difference in thickness has not improved.
If the tilts are adjusted for the respective lens substrates 5a and 5b, the tilts will be different between the upstream lens and the downstream lens, and asymmetric aberrations will remain, deteriorating the imaging state. It ends up.
Therefore, in the present invention, even if the angles of the lens substrates 5a and 5b are different, the angles of the lens tilts are aligned on the upstream side and the downstream side, and the lens with respect to the lens substrates 5a and 5b is provided without deteriorating the imaging state. The aim is to reduce the tilt. For that purpose, it is desirable that the tilt of the lens is an angle between the two lens substrates 5a and 5b.

FIG. 4 is a schematic diagram showing the positional relationship between the light source and the lens in the embodiment of the present invention. For the sake of explanation, the rotation angle of the lens is made larger than that in FIG. The lens showed a cross section only on the surface with curvature. The two short straight lines in the center of the figure indicate the aperture. Two "ten" marks between the lens surfaces indicate the principal point of the lens. Of the two principal points on each lens, the straight line connecting the two principal points on the aperture side passes through the center of the aperture.

Further, the center of the light emitting point cloud 3 is located on the extension line of the line extending horizontally from the outer principal point of the lens on the left side of the figure. The light beam that flies horizontally from the center of the emission point group 3 shifts upward in the figure due to the tilted lens, but it flies horizontally after the lens, passes through the center of the aperture 6, and then rises again with another lens. It shifts in the direction and is ejected horizontally. The center of the imaging point 7 is located on the extension of a straight line extending horizontally from the outer principal point of the right lens.

Hereinafter, specific examples of the optical system of the optical print head 73 incorporated in the image forming apparatus of the present invention will be described.

Tables I (1) to (III) numerically show the optical system of the optical print head 73 shown in FIG. 2 as an example of the embodiment of the present invention. The center, upper side, and lower side are shown respectively.
The flat plate has the same coordinates as the central optical system. For the light emitting point group 3, Table I shows the coordinates of the center of the light emitting point cloud.
Further, the photoconductor 71 has a cylindrical shape with a radius of 25 mm and is common to all three optical systems as a surface, but in the table, the position and inclination of the position where the photoconductor surface intersects the optical axis are shown. Looking at the aspherical coefficient, it can be seen that the absolute values are the same and the signs are opposite across the aperture, so they have the same shape and are in opposite directions.
The refractive index is 1.5145 for the lens substrate with respect to the wavelength of 650 nm, and a resin is used between the lens surface and the lens substrate, and the refractive index is 1.5285. The image magnification is -1 for all optical systems.

I About the central optical system (relative positional relationship)
Table I (1) shows the relative positional relationship of the central optical system (imaging system) in XYZ coordinates. The unit of distance is mm.

(About aspherical shape)
Table I (2) summarizes the aspherical coefficients for the aspherical shape of each lens of the central optical system. The described aspherical surfaces are all axially symmetric aspherical surfaces, have no spherical surface term, and the shape formula follows the following formula (I) with the local coordinates corresponding to X, Y, and Z as x, y, and z. The aspherical coefficients _ai not shown in the table are all 0. These points are the same in the following.

II About the upper optical system (relative positional relationship)
Table II (1) shows the relative positional relationship of the central optical system (imaging system) in XYZ coordinates. The unit of distance is mm.

(About aspherical shape)
Table II (2) summarizes the aspherical coefficients for the aspherical shape of each lens of the upper optical system. The described aspherical surfaces are all axially symmetric aspherical surfaces, have no spherical surface term, and the shape formula follows the above formula (I) with the local coordinates corresponding to X, Y, and Z as x, y, and z. The aspherical coefficients _ai not shown in the table are all 0. These points are the same in the following.

III About the lower optical system (relative positional relationship)
Table III (1) shows the relative positional relationship of the central optical system (imaging system) in XYZ coordinates. The unit of distance is mm.

(About aspherical shape)
Table III (2) summarizes the aspherical coefficients for the aspherical shape of each lens of the lower optical system. The described aspherical surfaces are all axially symmetric aspherical surfaces, have no spherical surface term, and the shape formula follows the above formula (I) with the local coordinates corresponding to X, Y, and Z as x, y, and z. The aspherical coefficients _ai not shown in the table are all 0. These points are the same in the following.

As can be seen from the numerical values of the coordinates of the optical system shown in Tables I to III above, the optical print head of the image forming apparatus of the present invention has a light emitting element having a group of light emitting points arranged in two dimensions and the light emitting points. It has an optical system that forms an image of light from the group at different positions on the light receiving surface 7 of the photoconductor at each light emitting point 3, the imaging magnification of the optical system is -1, and the light emitting point group. When viewed from the direction of the rotation axis of the photoconductor 71, the conjugate length of each of the optical systems is in the sub-scanning direction. The angle differs depending on the position, and increases or decreases monotonically according to the position in the sub-scanning direction. For each of the optical systems, one lens is provided on the upstream side and the downstream side of the aperture 6 (lens 1 and lens 2). The two lenses each have a common axis of rotational optics on two surfaces having curvature, and the surfaces of the two lenses on the side closer to the aperture have the same shape and the aperture. The surface on the side far from the optics has the same shape, the core thickness is the same, and the center of the optics is arranged on a straight line connecting the two main points on the side close to the optics of the two lenses. The straight line connecting the main points on the side close to the aperture and the axis of rotational optics of the lens have a non-zero angle, and the axes of rotational optics of the two lenses are parallel to each other.

As shown in the examples, it is preferable that the four lens surfaces of the two lenses are point-symmetrical with respect to the center of the diaphragm.
Further, regarding the plurality of optical systems, all the lenses on the upstream side of the diaphragm are formed by stacking resins on one lens substrate, while all the lenses on the downstream side of the diaphragm are made of resin on one lens substrate. It is preferable that the lenses are formed by stacking the lenses.
Further, it is preferable that none of the lens substrates is perpendicular to the straight line connecting the two principal points on the side close to the aperture of the two lenses.

Further, as shown in FIG. 4, the direction in which the normal line of the lens substrate is tilted with respect to the straight line connecting the two principal points on the side close to the aperture and the two principal points on the side close to the aperture are connected. It is preferable that the direction in which the axis of rotational symmetry of the lens is tilted with respect to the straight line is the same.
Further, it is preferable that the lens substrate on the upstream side and the lens substrate on the downstream side are not parallel to each other, and the axis of rotational symmetry of the lens is tilted with respect to a straight line connecting two main points on the side close to the aperture. The angle is such that the normal line of the lens substrate on the upstream side is tilted with respect to the straight line connecting the two main points on the side close to the aperture, and the normal line of the lens substrate on the downstream side is tilted. It is preferably between the angles.
That is, the image forming apparatus of the present invention can improve the image formation state also in the sub-scanning direction by satisfying the above-mentioned constitutional requirements.

1 Optical lens 1
2 Optical lens 2
3 Light emitting points consisting of a light emitting point group 4 Glass plates 5a, 5b Lens substrate (glass substrate)
5c Window 6 Aperture 7 Imaging point on photoconductor 71 100 Image forming device 10 Image reading unit 20 Image forming unit 40 Paper feeding unit 50 Transport unit 70 Image forming unit 71 Photoreceptor 72 Charging unit 73 Optical print head (optical writing) Device)
74 Development section

Claims (7)

At least, an image forming apparatus including a photoconductor and an optical print head.
The optical print head has a light emitting element having a light emitting point cloud arranged two-dimensionally, and
It has an optical system that forms an image of light from the light emitting point group at different positions on the light receiving surface of the photoconductor for each light emitting point.
The image magnification of the optical system is -1, and the image magnification is -1.
A plurality of sets of the light emitting point group and the optical system exist, and include those having different positions in the sub-scanning direction.
When viewed from the direction of the rotation axis of the photoconductor, the conjugate length of each of the optical systems varies depending on the position in the sub-scanning direction, and monotonically increases or decreases depending on the position in the sub-scanning direction.
For each of the above optical systems, there is one lens on the upstream side and one lens on the downstream side of the aperture.
Each of the two lenses has a common axis of rotational symmetry on two planes with curvature.
For the two lenses, the surface on the side closer to the aperture has the same shape, the surface on the side farther from the aperture has the same shape, and the core thickness is the same.
The center of the aperture is arranged on a straight line connecting the two principal points on the side close to the aperture of the two lenses, and the straight line connecting the two principal points on the side close to the aperture and the said. An image forming apparatus characterized in that the rotational symmetry axes of the lenses have a non-zero angle, and the rotational symmetry axes of the two lenses are parallel to each other. The image forming apparatus according to claim 1, wherein the four lens surfaces of the two lenses are point-symmetrical with respect to the center of the diaphragm. For the plurality of optical systems, all the lenses on the upstream side of the diaphragm are formed by stacking resins on one glass substrate, while all the lenses on the downstream side of the diaphragm are made of resin on one glass substrate. The image forming apparatus according to claim 1 or 2, wherein the image forming apparatus is formed by stacking the above. The image forming apparatus according to claim 3, wherein the glass substrate is not perpendicular to a straight line connecting the two principal points on the side close to the diaphragm of the two lenses. The lens of the lens with respect to the direction in which the normal line of the glass substrate is tilted with respect to the straight line connecting the two principal points on the side close to the aperture and the straight line connecting the two principal points on the side close to the aperture. The image forming apparatus according to claim 4, wherein the direction in which the axis of rotational symmetry is tilted is the same. The image forming apparatus according to claim 4, wherein the glass substrate on the upstream side and the glass substrate on the downstream side are not parallel to each other. The angle at which the axis of rotational symmetry of the lens is tilted with respect to the straight line connecting the two main points on the side close to the aperture is upstream of the straight line connecting the two main points on the side close to the aperture. The image forming apparatus according to claim 6, wherein the normal line of the glass substrate on the side is between an angle at which the normal line is tilted and the normal line of the glass substrate on the downstream side is tilted.

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