JP4495942B2 - Imaging optical system, image forming apparatus, printer and image reading apparatus - Google Patents

Description

  The present invention relates to an imaging optical system for a line light source of an electrophotographic image forming apparatus used for a copying machine or a printer.

FIG. 6 is a conceptual diagram showing how minute light sources such as LEDs arranged in a line are imaged onto a photoconductor using an imaging optical system. The figure (a) is a side view, and the figure (b) is a perspective view.
In the figure, reference numeral 1 denotes a linear light source, 2 denotes an imaging optical system, and 3 denotes a light receiving surface such as a photoconductor.
The light beam L0 emitted from the light emitting point 1a of the line light source 1 forms an image on the light receiving surface placed on the image forming surface (sometimes referred to simply as an image surface) via the image forming optical system 2.
As the imaging optical system 2, in order to reduce the size of the apparatus, a so-called lens array in which a plurality of optical elements are arranged in a line with their optical axes parallel to each other as shown in the figure is often used. In this case, if the image is an inverted image, the images of adjacent optical elements are individually inverted, so that the images are not connected to each other, and the light source line cannot be correctly formed. Therefore, the imaging optical system used here must be an erecting real image system. In the case of an erect real image, the image directions of adjacent optical elements are correctly connected, and if the imaging magnification is made equal, the light source line can be correctly imaged.

Conventionally, a refractive index distribution type rod lens array (trade name SELFOC) has been used as such an optical system. The gradient index rod has a self-convergence because the light beam incident on the end face travels in a sine wave in the rod, and an inverted image is obtained when the length is set to a half wavelength or less of the sine wave. If the length is set to about twice that, an erect image can be obtained. Place the light source and the image plane symmetrically with respect to the rod, and select the length of the rod so that an inverted image is formed at the center in the length direction of the rod. can get.
The rod lens is formed so that the refractive index distribution curve is substantially a quadratic curve, but the aberration cannot be sufficiently removed due to the influence of higher-order terms. In addition, since the numerical aperture (referred to as NA) indicating the range in which light can be captured is small, it is difficult to obtain a bright image. Due to these problems, this method cannot be used for high resolution.

There is a proposal of an optical system intended for high resolution (see, for example, Patent Document 1). In this optical system, a large number of optical systems having the same configuration are arranged in two rows in a staggered pattern to form a line shape, and a linear light source is arranged in the middle of the two rows. The reason why the optical systems are arranged in a staggered pattern is to make the light quantity distribution on the image plane as uniform as possible. The individual optical system is formed of, for example, a four-lens configuration of thin lenses having the same shape with a focal length of F. Each lens is arranged at the same F interval as the focal length, and the light source and the image plane are arranged at a distance of 2F from the end lens. In this configuration, the middle of the four lenses is the intermediate imaging plane, and when the light source is not on the optical axis, the distance from the optical axis of the light source (object height) from the optical axis of the image on the intermediate imaging plane. The height (image height) is exactly half. Since the optical system is configured symmetrically with respect to the intermediate image plane, the image formed on the intermediate image plane passes through the rest of the optical system, and then at the same height as the object height on the image plane, the same as the light source Connect light source images of size.
In many optical systems, each first lens is formed in an array as a common plate, and each of the second, third, and fourth lenses is also formed in a plate shape. A plurality of optical systems is configured by overlapping lens arrays with a predetermined interval.
It has been shown that, instead of the configuration of four thin lenses, a configuration of two thick lenses having an inter-plane distance of F is equivalent to an imaging effect. It is also shown that if the symmetry with respect to the intermediate image plane is not broken, an optical system that can obtain the same effect can be configured even if the focal lengths of both surfaces of the thick lens are not necessarily equal.

  In the above optical system, in order to increase MTF (Modulation of Transfer Function) indicating resolution, a partition plate provided with a through hole corresponding to the lens position is interposed as a light shielding means between the lens plates. However, since the lens pitch of the lens array is very small compared to its length, if a separate partition plate is sandwiched between the two, unless both are made with very high accuracy, it may be undesirable depending on the location due to the deviation of the pitch. “Kicking” of the luminous flux occurs. Even if this is not the case, if the two materials are different, the same may occur due to the difference in the linear expansion coefficient due to temperature changes. It is difficult to select a material with the same linear expansion coefficient between a transparent lens plate and a black partition plate for shading.

Japanese Unexamined Patent Publication No. 2000-212445 (page 3, FIG. 4)

An object is to realize a novel imaging optical system that can be used in a high-resolution image forming apparatus.

The invention according to claim 1 is an imaging optical system for forming an erecting equal-magnification image of a line-shaped light source, wherein the line-shaped light source, the first to fourth lens plates, and a thin light shielding means And have.
In each of the first to fourth lens plates, a plurality of single lenses are arranged and integrated into a lens array of at least two lens rows in a staggered pattern in the line direction of the line-shaped light source, and the lens The arrangement of the single lenses forming the row is the same.
In the lens arrays of the first to fourth lens plates, single lenses whose arrangements correspond to each other constitute a coaxial imaging lens system.
The second lens plate and the third lens plate have a lens array formed on one side and the other side is a flat surface, and the light shielding means is sandwiched between the flat surfaces of the second and third lens plates.

  The lens arrays of the first and second lens plates jointly form an inverted real image of the linear light source as an intermediate image on the plane of the second lens plate, and the lenses of the third and fourth lens plates. The array forms an erecting equal-magnification image of a linear light source using the inverted real image as an object.

  The light-shielding means is formed with a “slit-shaped opening having a line direction of a line-shaped light source as a longitudinal direction” at an imaging position of an intermediate image by a single lens of each of the coaxial axes of the first and second lens plates. The light that forms an erecting equal-magnification image is allowed to pass to the third lens plate side, and the light other than the aperture is a light-shielding part that blocks stray light from forming an erecting equal-magnification image. To do.

According to a second aspect of the present invention, in the imaging optical system according to the first aspect, the length corresponding to the line direction of the line-shaped light source of the slit-shaped opening of the light shielding means is the second lens plate. It is equal to "the length of the string corresponding to the image height of an inverted real image formed including the lens surface" of the lens surface of the lens array .
According to a third aspect of the present invention, in the imaging optical system according to the first or second aspect, the shape of the plurality of lens plates is “symmetrical with respect to the image plane of the intermediate image” .
The invention according to claim 4 is the imaging optical system according to any one of claims 1 to 3,
In one or more of the first to fourth lens plates, the planar portion around the single lens is shielded by the light shielding means.
According to a fifth aspect of the invention, the linear light source in the imaging optical system according to any one of the first to fourth aspects is any one of “semiconductor laser, LED, EL element, optical waveguide, and optical fiber”. One is arranged in a line shape ” .

  A sixth aspect of the present invention is an image forming apparatus using the imaging optical system according to any one of the first to fifth aspects.
  A seventh aspect of the present invention is a copying machine using the image forming apparatus according to the sixth aspect.
  The invention described in claim 8 is a printer using the image forming apparatus described in claim 6.
  A ninth aspect of the present invention is an image reading apparatus using the imaging optical system according to any one of the first to fifth aspects.

According to the present invention , a novel imaging optical system that can be used in a high-resolution image forming apparatus can be obtained.

  An equal magnification erecting real image optical system composed of a plurality of lens plates disposed between a line-shaped light source and an image plane, each of the plurality of lens plates having a plurality of single lenses is symmetrical with respect to the intermediate image plane, A part of the lens plate has a flat portion that coincides with the intermediate image plane, and a light-shielding means having a slit-like opening is provided at the position where the image of the line-shaped light source is formed on the flat portion. A light shielding means is also provided in the surrounding flat portion.

FIG. 1 is a view showing an embodiment of the light shielding means of the present invention.
(A) is a front view, (b) is an AA arrow view of (a), and (c) is an outline of a cross section parallel to the optical axis passing through SA in (b). FIG. However, the holding part 4 is omitted in FIGS. In both cases, hatching of the fracture surface was omitted to clarify the light rays.
In the figure, reference numeral 10 denotes an optical system, 11 denotes a first lens plate, 12 denotes a second lens plate, 13 denotes a third lens plate, 14 denotes a fourth lens plate, 15 denotes an intermediate image plane, and S0 denotes a light shielding unit. Show. The thickness of the light shielding means S0 is exaggerated.
In FIG. 2A, the dotted line indicates the relative position of the line light source.
Each lens plate of the first to fourth are formed in an elongated plate shape, single lens of a plurality of identical shape each, in this example forms a lens array are arranged in "2 rows of staggered". The line light source is arranged so as to correspond to exactly the middle of the two lens rows. That is, the line light source is arranged so as to occupy the position of the dotted line in the direction orthogonal to the drawing of FIG.

Here, an array called a staggered pattern refers to an array in which two columns have the same array pitch, but one array and the other array are shifted by a half pitch with respect to the longitudinal direction of the array. In the example of FIG. 6A, an arrangement is made such that any three single lenses located at the shortest distance from each other are positioned at the apex of a regular triangle. In order to increase the use efficiency of the light amount, the lens array can be made into three by the same arrangement method. In this case, the position of the line light source is made to correspond to the center lens array.
Regardless of whether the single lens is a spherical lens or an aspheric lens, the openings have the same diameter .
The first lens plate 11 and the fourth lens plate 14 are configured as biconvex lenses, and the second lens plate and the third lens plate are configured as plano-convex lenses. When the first to fourth lens plates are overlapped with each other in a predetermined relationship, each single lens is formed in a positional relationship corresponding to each other, and the corresponding four single lenses are coaxial. Configured to form a set of lens systems. In a set of lens systems, each single lens is referred to as a first lens to a fourth lens, corresponding to each lens plate.
That is, in each of the first to fourth lens plates 11 to 14, a plurality of single lenses are arranged in a zigzag pattern in a line direction of a line-shaped light source and are integrated into a lens array of two lens rows. In addition, in the lens arrays of the first to fourth lens plates 11 to 14 having the same “array of single lenses forming a lens array”, an “imaging lens system in which the single lenses corresponding to the arrays are coaxial” Is configured.
In the example shown in the figure, the first lens and the fourth lens are configured symmetrically with respect to the intermediate image plane 15, and the second lens and the third lens are configured symmetrically as well.

A part of the light beam emitted from the light emitting unit 1a of the line-shaped light source 1 enters the convex lens surface on the front surface side of the first lens plate 11, exits from the convex lens surface on the back surface side, and is on the surface side of the second lens plate. Is incident on the convex lens surface and forms an image on a flat surface portion on the back surface side. In this optical path, the power of each lens is distributed so that the principal ray is incident on the second lens plate and becomes parallel to the optical axis.
The light beam that has passed through the intermediate imaging plane reaches the image plane 3 through the third lens plate 13 and the fourth lens plate 14 and forms an image. The optical path from the intermediate image plane 15 to the image plane 3 is substantially symmetric with respect to the optical path from the light emitting point 1 a to the intermediate image plane 15 and the intermediate image plane 15.
As shown in FIG. 5B, the light beam from the light emitting unit 1a can be incident on the plurality of lens systems of the first lens plate 11 simultaneously. Light beams incident on different lens systems from the same point are imaged at the same point on the image plane 3.

For an arbitrary lens system, the image on the intermediate image plane 15 of the line-shaped light source 1 can be formed into a line shape with an image height according to the image magnification determined by the first lens plate 11 and the second lens plate 12. . The line direction of this image is parallel to the line direction of the light source 1, and the line thickness is obtained by multiplying the line thickness of the light source 1 by the imaging magnification. For the lens system, light should not pass through except for the line-shaped image. However, the first lens plate 11 has a flat portion other than the single lens, and the light source 1 has this flat portion. The incident light is subjected to an optical action that is completely different from the desired optical action of the lens system, and thus reaches the intermediate imaging plane 15 as so-called stray light.
The stray light eventually reaches the image plane 3 and may be superimposed on the light source image as light other than the imaging light, which may cause a reduction in resolving power. Therefore, in order to block stray light as much as possible, the light shielding means S0 shields light other than the position where the line-shaped image of the intermediate imaging surface 15 is formed. With respect to the line direction, for the reason described later, as shown in FIG. 5C, an opening is made so that light can be transmitted to a range substantially equal to the chord length of the entrance opening of the lens system corresponding to the image height. Shield light from other areas.

FIG. 2 shows the shape of the light shielding means. For reference, the corresponding position of the light source is indicated by a dotted line and the corresponding position of the lens system is indicated by a broken line so that the positional relationship can be understood.
In the figure, symbol SA indicates the opening, and L indicates the length of the opening in the line direction.
As shown in the figure, the opening SA is formed in a plurality of slit shapes. The ratio of the distance from the center of each lens system to the line light source and the distance from the center of the lens system to the center of the aperture is the imaging magnification.
The principal ray of the light beam emitted from each light emitting point of the light source 1 is parallel to the optical axis after entering the second lens. Accordingly, even if an image outside the light source corresponding to this light beam is originally formed, the light beam cannot pass through the third lens. Therefore, the length of the chord corresponding to the image height of the line image at the opening of the second lens. An image having a size equal to that is the limit of the size of a practical image. The limit of the size of an image that can be practically used is a limit in which the amount of light passing therethrough becomes zero. Therefore, if the aperture is approximately equal to this length, even if it is slightly smaller, it does not significantly affect the imaging performance. On the contrary, if it is longer than that, for example, if it is made to be continuous with an adjacent opening, undesired stray light may enter, which is not so preferable.
In this example, since all the single lenses have the same diameter, the aperture of the second lens is equal to that of the first lens.
Assuming that the image height is equal to two-thirds of the radius of the single lens, the length L in the line direction of the opening of the light shielding means S0 is about 0.75 times the diameter of the single lens.
Depending on the magnification of the lens system, for example, assuming that the imaging magnification at the intermediate imaging surface 15 is 1/2 and the image height is as described above, the size of the single lens aperture is about The light flux from the light source can be captured up to 1.5 times longer.

  Although the light shielding means S0 may be formed of a thin plate, a difference in linear expansion coefficient from the lens system becomes a problem. Therefore, in the present invention, the second lens plate or the third lens is used by, for example, a printing method. Form directly on the flat part of the plate. There is also a method using a photoresist. For example, a photoresist is uniformly applied to a flat surface portion, a light source, a first lens plate, and a second lens plate are arranged in a predetermined positional relationship, and light is emitted from the light source to expose the photoresist in a desired opening shape. The light shielding means S0 may be formed by removing the resist in the exposed portion and then dyeing the remaining photoresist black. During exposure, stray light also strikes the photoresist at the same time, but due to the sensitivity of the photoresist, only where the energy is concentrated, such as imaging light, can be removed in a later step.

FIG. 3 is a diagram for explaining a reference example .
In the figure, reference numerals S1 to S6 denote light shielding means, respectively.
All flat portions other than the single lens of each lens plate are shielded by the light shielding means Sn (n is 1 to 6). Since the light beam necessary for the image plane 3 is only the light beam correctly passing through the lens system, all the light beam passing through the plane portion may be blocked. This embodiment can be applied when the second lens plate and the third lens plate are formed as an integral biconvex lens. The method for forming the light shielding means is the same as that shown in the first embodiment.
All of S1 to S6 are not necessarily provided. For example, only S1 to S3 may be provided, and S4 to S6 may be omitted or vice versa. Since the lens system is symmetric with respect to the intermediate image plane 15, if only the plane portion of the lens plate on one side is properly shielded, the light beam passing through the plane portion of the lens plate on the other side will not reach the image plane. .
For the same reason, for example, it is possible to omit other light shielding means while leaving only S1, S3, and S5. In short, since it is symmetrical with respect to the intermediate image plane 15, it is only necessary to select one from each of the corresponding light shielding means, that is, each combination of S1 and S6, S2 and S5, and S3 and S4.
The same applies to an asymmetric lens system described later when the shape of one lens system is proportional to the shape of the other lens system with respect to the intermediate image plane.
When the second lens plate and the third lens plate are formed separately, more reliable light shielding can be expected if the light shielding means S0 of the first embodiment is also employed in the second embodiment. .

FIG. 4 is a diagram for explaining another reference example .
In the figure, reference numerals 16 and 17 denote light shielding auxiliary plates, and S7 and S8 denote light shielding means, respectively.
The light shielding auxiliary plates 16 and 17 are plates formed of a transparent member made of the same material as the first to fourth lens plates, and light shielding means is formed leaving a desired opening. The method for forming the light shielding means is as described above. The shape of the opening varies depending on the arrangement position of the light shielding means, but generally the width and length are larger than the opening of the light shielding means S0 of the first embodiment. In this configuration, the second lens plate and the third lens plate may be integrated or separated. In the case of a separate body, when the contact surface of the second lens plate and the third lens plate is flat, the light shielding means S0 shown in the first embodiment can be used in combination. When both the second lens and the third lens are biconvex, a light shielding means similar to S0 is disposed at the position of the intermediate image plane with a light shielding auxiliary plate similar to the light shielding auxiliary plate 16 interposed therebetween. Can do. Here, the light shielding auxiliary plate is made of the same material as the lens plate. However, the difference in expansion and contraction between the two does not have to be large due to temperature changes. It doesn't matter.

The light shielding auxiliary plate may be made thicker than the thickness shown in the drawing, and may be used as a spacer / holding portion that supports each lens plate outside the lens portion. Since the optical path length changes when a transparent plate is sandwiched in the optical path of the optical system, the thickness of the light shielding auxiliary plate must be taken into consideration in designing.
When the light shielding auxiliary plate is used, the number of parts is naturally increased, so that the number of parts used is reduced as much as possible, and a method of providing a light shielding means on the plane portion around the single lens as shown in S1 to S6 in Example 2 is used together. preferable.
As shown in the above embodiments, the light-shielding means according to the present invention is formed in direct contact with the lens plate or the light-shielding auxiliary plate of the same material by printing or other methods, so that even if there is a temperature change. The light shielding means does not shift and the desired light shielding can always be achieved.

FIG. 5 is a graph obtained by calculating the MTF of the imaging optical system having the lens configuration and the light shielding unit shown in FIG.
In the figure, the thick line is a curve showing the MTF on the image plane of the single lens system of the light source closest to the lens system, and the thin line is a curve showing the same MTF of the light source farthest from the lens system. Reference symbol T represents an MTF curve (solid line) in the tangential direction, and R represents an MTF curve (one-dot chain line) in the radial direction.
The outline of the specifications of the lens system used for the calculation is as follows. The single lens on each surface is a spherical surface, the radius is 2 mm, the distance between the first surface and the second surface is 1.2 mm, and the second surface and the third surface. The distance between the surfaces (air interval) was 0.5 mm, the distance between the third surface and the intermediate image surface was 1.6 mm, and the refractive index of the material was 1.49.
In the figure, the light source wavelength was set to 560 nm, and the spatial frequency was calculated as MTF for 49 lp / mm which is almost close to 1200 dpi.
When the image plane position is set correctly, that is, when the defocus amount is 0, the MTF is at least about 58%. At ± 0.15 mm, which is a design tolerance of the defocus amount, the positive side is lower, and the minimum is about 23%.

All of the optical systems shown in FIG. 1 and below were “symmetrical with respect to the intermediate image plane” , but this was selected for ease of production and cost reduction. Is not limited to the symmetrical shape as described above. In short, the product of the imaging magnification at the intermediate image plane of the light source and the imaging magnification at the final image plane of the image on the intermediate image plane should be 1 even if it is asymmetric. For example, if the image on the intermediate image plane is reduced by a factor of two, if the image formation magnification from there to the image plane 3 is doubled, the final image size will be Equal to size.
Such an optical system is possible, for example, with respect to the intermediate image plane, as long as the lens system configuration on one side is proportionally expanded or reduced with respect to the configuration of the lens system on one side. It is. Furthermore, it is obvious that asymmetry can be allowed for the number of lens systems and the number of lens surfaces as long as the condition of an upright real image is not broken. However, the symmetric shape is better from the viewpoint of luminous flux utilization efficiency. The asymmetric shape is used when, for some reason, it is desired to make the distance from the light source to the light incident surface different from the distance from the light exit surface to the image surface.
For example, it can be used to secure a space for placing a sensor for detecting reflected light on the image plane side when a light source is uniformly emitted and an original to be read is placed on the image plane and used as an image reading apparatus.

Even in the case of an asymmetric optical system, the light shielding means of the present invention can be applied in the same manner as in the symmetric optical system. The symmetric and asymmetric shapes here refer to the state before the light shielding means is applied.
Although the single lenses formed on each lens plate have been described as having the same diameter, for example, if the lens diameter on the light beam exit side is larger than the lens diameter on the light beam incident surface side of the first lens plate, the use of the light beam Efficiency may increase. Thus, depending on the configuration, including the asymmetric type, the single lens diameter between the lens plates may not necessarily be the same. Of course, the same lens plate needs to have the same diameter in the same plane.

It is a figure which shows one Embodiment of the imaging optical system of this invention. It is a figure which shows the shape of a light-shielding means. It is a figure for demonstrating a reference example . It is a figure for demonstrating another reference example . It is the graph which computed MTF of the imaging optical system which has a lens structure shown in FIG. 1, and a light-shielding means by calculation. It is a conceptual diagram which shows a mode that the micro light source arranged in the line form is imaged on a photoreceptor etc. using an imaging optical system.

1 Line-shaped light source
2 Optical system
3 Image plane
10 Optical system
11 First lens plate (first lens plate)
12 Second lens plate (second lens plate)
13 Third lens plate (third lens plate)
14 Fourth lens plate (fourth lens plate)
15 Intermediate imaging plane
S0 shading means

Claims (9)

An imaging optical system that forms an erecting equal-magnification image of a line-shaped light source,
A linear light source, first to fourth lens plates, and a thin light-shielding means;
In each of the first to fourth lens plates, a plurality of single lenses are arranged and integrated in a lens array of at least two lens rows in a staggered pattern in the line direction of the line-shaped light source, and The arrangement of the single lenses forming the lens array is the same,
In the lens arrays of the first to fourth lens plates, single lenses whose arrangements correspond to each other constitute a coaxial imaging lens system,
The second lens plate and the third lens plate have a lens array formed on one side and the other side is a flat surface, and the light shielding means is sandwiched between the flat surfaces of the second and third lens plates,
The lens arrays of the first and second lens plates jointly form an inverted real image of the line-shaped light source as an intermediate image on the plane of the second lens plate,
The lens arrays of the third and fourth lens plates form an erecting equal-magnification image of the line-shaped light source using the inverted real image as an object,
The light shielding means forms a slit-like opening having a line direction of a line-like light source as a longitudinal direction at a position where the intermediate image is formed by a single lens of each of the coaxial axes of the first and second lens plates. The light that forms the erecting equal-magnification image is allowed to pass to the third lens plate side, and the light shielding portion other than the opening is a stray light with respect to the imaging of the erecting equal-magnification image. An imaging optical system characterized by blocking the light that becomes The imaging optical system according to claim 1,
The length corresponding to the line direction of the line-shaped light source of the slit-shaped opening of the light shielding means is imaged including the lens surface of the lens surface of the lens forming the lens array of the second lens plate. An imaging optical system characterized by being equal to the length of a string corresponding to the height of an inverted real image . The imaging optical system according to claim 1 or 2,
An imaging optical system, wherein the plurality of lens plates are symmetrical with respect to an imaging plane of the intermediate image . The imaging optical system according to any one of claims 1 to 3,
  An imaging optical element characterized in that, in one or more of the first to fourth lens plates, a planar portion around the single lens is shielded by another light shielding means. The imaging optical system according to any one of claims 1 to 4,
The line-shaped light source is an imaging optical system in which any one of a semiconductor laser, an LED, an EL element, an optical waveguide, and an optical fiber is arranged in a line . An image forming apparatus using the imaging optical system according to any one of claims 1 to 5 . A copier using the image forming apparatus according to claim 6 . A printer using the image forming apparatus according to claim 6 . An image reading apparatus using the imaging optical system according to any one of claims 1 to 5 .

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