Erecting equal-magnification lens array plate, optical scanning unit, and image reading device

An erecting equal-magnification lens array plate includes: a first lens array plate provided with a plurality of first lenses arranged on a first surface and a plurality of second lenses arranged on a second surface opposite to the first surface; and a second lens array plate provided with a plurality of third lenses arranged on a third surface and a plurality of fourth lenses arranged on a fourth surface opposite to the third surface. The first and second lens array plates form a stack such that the second surface and the third surface face each other. The erecting equal-magnification lens array plate receives light from a linear light source facing the first surface and forms an erect equal-magnification image of the linear light source on an image plane facing the fourth surface. An annular slope is formed around each second lens and each third lens.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an erecting equal-magnification lens array plate used in image reading devices, and to an optical scanning unit and an image reading device in which the erecting equal-magnification lens array plate is used.

2. Description of the Related Art

Some image reading devices such as scanners are known to use erecting equal-magnification optical systems. Erecting equal-magnification optics are capable of reducing the size of devices better than reduction optics. In the case of image reading devices, an erecting equal-magnification optical system comprises a linear light source, an erecting equal-magnification lens array, and a line image sensor.

A rod lens array capable of forming an erect equal-magnification image is used as an erecting equal-magnification lens array in an erecting equal-magnification optical system. Normally, a rod lens array comprises an arrangement of rod lenses in the longitudinal direction (main scanning direction of the image reading device) of the lens array. By increasing the number of rows of rod lenses, the light transmissibility is improved and unevenness in the amount of light transmitted is reduced. Due to price concerns, it is common to use one or two rows of rod lenses in a rod.

Meanwhile, an erecting equal-magnification lens array plate could be formed as a stack of a plurality of transparent lens array plates built such that the light axes of individual convex lenses are aligned, where each transparent lens array plate includes a systematic arrangement of micro-convex lenses on one or both surfaces of the plate. Since an erecting equal-magnification lens array plate such as this can be formed by, for example, injection molding, erecting equal-magnification lens arrays in a plurality of rows can be manufactured at a relatively low cost.[Patent Document No. 1] JP2005-37891

An erecting equal-magnification lens array plate lacks a wall for beam separation between adjacent lenses. Therefore, there is a problem of stray light wherein a light beam diagonally incident on an erecting equal-magnification lens array plate travels diagonally inside the plate and enters an adjacent convex lens, creating ghost noise as it leaves the plate.

SUMMARY OF THE INVENTION

The present invention addresses the aforementioned disadvantage and a purpose thereof is to provide an erecting equal-magnification lens array plate capable of reducing ghost noise and to provide an optical scanning unit and an image reading device in which the erecting equal-magnification lens array plate is used.

To address the issue described above, the erecting equal-magnification lens array plate comprises: a first lens array plate provided with a plurality of first lenses systematically arranged on a first surface and a plurality of second lenses systematically arranged on a second surface opposite to the first surface; and a second lens array plate provided with a plurality of third lenses systematically arranged on a third surface and a plurality of fourth lenses systematically arranged on a fourth surface opposite to the third surface. The first lens array plate and the second lens array plate form a stack such that the second surface and the third surface face each other to ensure that a combination of the lenses aligned with each other form a coaxial lens system, and the erecting equal-magnification lens array plate receives light from a linear light source facing the first surface and forms an erect equal-magnification image of the linear light source on an image plane facing the fourth surface. In this erecting equal-magnification lens array plate, an annular slope is formed around each second lens and/or each third lens.

According to the embodiment, at least a part of the light beam diagonally incident from the linear light source on the first lens array plate is refracted or reflected by the annular slope formed around each second lens and/or each third lens so that the likelihood of the light beam directly passing through the second lens array plate is reduced. This will reduce ghost noise more successfully than in the case where a flat part is provided between the second lenses and between the third lenses.

The slope may be formed to radially extend from the outer periphery of each second lens and/or each third lens in a tapered shape. The angle of the slope may be defined so that a light beam incident on the slope is totally reflected. The height of the slope may be equal to the sag of the second lens and/or the third lens.

The erecting equal-magnification lens array plate may further comprise: a first light-shielding wall having a plurality of first through holes aligned with the first lenses, and provided on the first surface such that each of the first through holes is located directly opposite to the corresponding first lens; and a second light-shielding wall having a plurality of second through holes aligned with the fourth lenses, and provided on the fourth surface such that each of the second through holes is located directly opposite to the corresponding fourth lens. At least one of the first through hole and the second through hole may comprise: a lateral wall portion; an annular inner projection portion provided to project from an end of the lateral wall portion facing the lens; and an annular outer projection portion provided to project from an end of the lateral wall portion opposite to the end facing the lens, wherein the inner projection portion and the outer projection portion may not be formed with a surface parallel to an optical axis.

The inner projection portion may be formed to be tapered. The outer projection portion may also be formed to be tapered.

Another embodiment of the present invention relates to an optical scanning unit. The optical scanning unit comprises: a linear light source configured to illuminate an image to be read; the erecting equal-magnification lens array plate according to claim1configured to condense light reflected by the image to be read; and a line image sensor configured to receive light transmitted through the erecting equal-magnification lens array plate.

According to the embodiment, the optical scanning unit is configured using the aforementioned erecting equal-magnification lens array plate and so can read an erect equal-magnification image in which ghost noise is reduced.

Still another embodiment of the present invention relates to an image reading device. The device comprises: the aforementioned optical scanning unit; and an image processing unit configured to process an image signal detected by the optical scanning unit.

According to the embodiment, the image reading device is configured using the aforementioned optical scanning unit so that high-quality image data in which ghost noise is suitably reduced can be obtained.

Optional combinations of the aforementioned constituting elements, and implementations of the invention in the form of methods, apparatuses, systems, computer programs, data structures, and recording mediums may also be practiced as additional modes of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows an image reading device100according to an embodiment of the present invention. As shown inFIG. 1, the image reading device100comprises an optical scanning unit10, a glass plate14on which a document G is placed, a driving mechanism (not shown) for driving the optical scanning unit10, and an image processing unit (not shown) for processing data read by the optical scanning unit10.

The optical scanning unit10comprises a linear light source16for illuminating a document G placed on a glass plate14, an erecting equal-magnification lens array plate11for condensing light reflected from the document G, a line image sensor (photoelectric transducer)20for receiving light condensed by the erecting equal-magnification lens array plate11, and a housing12for housing the linear light source16, the erecting equal-magnification lens array plate11, and the line image sensor20.

The housing12is substantially rectangular solid in shape. A first recess12aand a second recess12bare formed in the upper part of the housing12and a third recess12cis formed in the lower part. The housing12is formed injection-molding a resin. By forming the housing12by injection molding, the housing12can be formed easily at a low cost. The linear light source16is diagonally fixed inside the first recess12a. The linear light source16is secured such that the optical axis of the illuminating light passes through the intersection point of the optical axis Ax of the erecting equal-magnification lens array plate11and the top surface of the glass plate14.

The erecting equal-magnification lens array plate11is fitted in the second recess12b. A substrate22provided with the line image sensor20is fitted in the third recess12c. The substrate22is secured such that the top surface thereof is in contact with a step12dprovided in the third recess12c.

As described later, the erecting equal-magnification lens array plate11comprises a stack of a first lens array plate24and a second lens array plate26such that pairs of corresponding lenses form coaxial lens systems, where each lens array plate is formed with a plurality of convex lenses on both planes of the plate. The first lens array plate24and the second lens array plate26are held by a holder30in a stacked state. The erecting equal-magnification lens array plate11is installed in the image reading device100such that the longitudinal direction thereof is aligned with the main scanning direction and the lateral direction thereof is aligned with the sub-scanning direction.

The erecting equal-magnification lens array plate11is configured to receive linear light reflected from the document G located above and form an erect equal-magnification image on an image plane located below, i.e., a light-receiving surface of the line image sensor20. The image reading device100can read the document G by scanning the optical scanning unit10in the sub-scanning direction.

FIG. 2shows a partial cross section of the optical scanning unit10in the main scanning direction. Referring toFIG. 2, the vertical direction represents the main scanning direction and the depth direction represents the sub-scanning direction.FIG. 3shows a cross section of the optical scanning unit10in the sub-scanning direction. Referring toFIG. 3, the vertical direction represents the sub-scanning direction and the depth direction represents the main scanning direction.

As described above, the erecting equal-magnification lens array plate11comprises a stack of the first lens array plate24and the second lens array plate26. The first lens array plate24and the second lens array plate26are rectangular plates having a thickness of t. A plurality of convex lenses are arranged on both surfaces of the plate. In other words, a plurality of first lenses24aare systematically arranged on a first surface24cof the first lens array plate24, and a plurality of second lenses24bare systematically arranged on a second surface24dopposite to the first surface24c. A plurality of third lenses26aare systematically arranged on a third surface26cof the second lens array plate26, and a plurality of fourth lenses26bare systematically arranged on a fourth surface26dopposite to the third surface26c. According to the embodiment, it is assumed that the first lens24a, the second lens24b, the third lens26a, and the fourth lens26bare spherical in shape. Alternatively, the lenses may have aspherical shapes.

The first lens array plate24and the second lens array plate26are formed by injection molding. Preferably, each of the first lens array plate24and the second lens array plate26is formed of a material amenable to injection molding, having high light transmittance in a desired wavelength range, and having low water absorbability. Desired materials include cycloolefin resins, olefin resins, norbornene resins, and polycarbonate.

FIG. 4is a top view showing the first surface24cof the first lens array plate24. As shown inFIG. 4, the plurality of first lenses24aare arranged in a single line on the first surface24cat a lens pitch P in the longitudinal direction of the first lens array plate24. The lens diameter D of the first lens24ais configured to be smaller than the lens pitch P. Therefore, a flat part24enot formed with a lens is provided between adjacent first lenses24a.

FIG. 5is a top view showing the second surface24dof the first lens array plate24. As shown inFIG. 5, the plurality of second lenses24bare arranged in a single line on the second surface24din the longitudinal direction of the first lens array plate24at a lens pitch P identical to the lens pitch of the first lenses24a. The lens diameter D of the second lens24bis configured to be identical to that of the first lens24a. The first lens24aand the second lens24bmay have different lens diameters.

FIG. 6is a perspective view of the second surface24dof the first lens array plate24. As shown inFIGS. 2,3,5, and6, an annular slope24fis formed on the second surface24dof the first lens array plate24to surround each second lens24b. Each slope24fis formed to radially extend from the outer periphery of each second lens24bin a tapered shape. The shape of the slope24fcan be expresses as a mortar. The angle of the slope24fis defined to totally reflect light incident on the slope.

The slope24fis formed by providing a mortar-shaped hole in the second surface24d. The second lens24bis provided at the center of the mortar-shaped hole. As shown inFIGS. 5 and 6, the adjacent slopes24fare in contact with each other at the upper peripheries thereof. Thus,FIG. 2, which shows a section of the optical scanning unit10in the main scanning direction, shows that a triangular convex portion is formed between adjacent second lenses24b.

In further accordance with the embodiment, the top of the second lens24band the upper end of the slope24fare at the same height. In other words, it is ensured that the sag of the second lens24bis equal to the height of the slope24f. The slope24fis formed with the same material as that of the second lens24band integrated with the second lens24b.

Basically, the second lens array plate26has the same structure as the first lens array plate24. As in the second surface24dof the first lens array plate24, the plurality of third lenses26ahaving a lens diameter D are arranged on the third surface26cof the second lens array plate26at a lens pitch P. As in the second surface24dof the first lens array plate24, an annular slope26fis formed on the third surface26cto surround each third lens26a. As in the first surface24cof the first lens array plate24, the plurality of fourth lenses26bhaving a lens diameter D are arranged on the fourth surface26dof the second lens array plate26at a lens pitch P. A flat part26enot formed with a lens is provided between adjacent fourth lenses26b. The third lens26aand the fourth lens26bmay have different lens diameters. It is desirable that the first lens24aand the fourth lens26bhave the same lens diameter and the second lens24band the third lens26ahave the same lens diameter.

The first lens array plate24and the second lens array plate26form a stack such that the second surface24dand the third surface26cface each other to ensure that a combination of the first lens24a, the second lens24b, the third lens26a, and the fourth lens26bassociated with each other form a coaxial lens system. According to the embodiment the first lens array plate24and the second lens array plate26form a stack such that the top of the second lens24band that of the third lens26aare in contact with each other. As described above, the top of the second lens24band the third lens26ais at the same height as the upper end of the slopes24fand26f, respectively. Thus, the upper end of the slope24fand that of the slope26fare in contact with each other as shown inFIG. 2. In the following description, the combination of the first lens24a, the second lens24b, and third lens26a, and the fourth lens26bassociated with each other are dealt as one lens system. The light axes of the lenses belonging to a given lens system are aligned. The slopes24fand26fformed around the second lens24band the third lens26a, respectively, are also included in the lens system.

As described above, the first lens array plate24and the second lens array plate26are held by a holder30in a stacked state. As shown inFIG. 1, the holder30is formed as a hollow quadrangular prism. The first lens array plate24and the second lens array plate26are inserted therein. Alternatively, the hollow quadrangular prism may be divided into two parts so that the two parts accommodate the first lens array plate24and the second lens array plate26, respectively. A plurality of first through holes30caligned with the plurality of first lenses24aare formed in the first surface part30aof the holder30. A plurality of second through holes30daligned with the plurality of fourth lenses26bare formed in the second surface part30bof the holder30opposite to the first surface part30a. The first through holes30cand the second through holes30dare cylindrical through holes.

The first through holes30cand the second through holes30dhave the same shape and are arranged in a line at the same pitch in the longitudinal direction of the first surface part30aand the second surface part30b, respectively. The central axes of the corresponding two through holes are aligned. The diameter of each of the first through holes30cand the second through holes30dis configured to be substantially the same as or slightly smaller than the diameter D of the first lenses24aand the fourth lenses26b. The pitch of arrangement of the first through holes30cand the second through holes30dis identical to the lens pitch P of the first lenses24aand the fourth lenses26b.

The first surface part30aand the second surface part30bof the holder30are formed as one piece using a light shielding material. The assembly may be formed by, for example, injection molding. Preferably, the shielding material is amenable to injection molding and is highly capable of shielding light in a required wavelength band. For example, the shielding material may be a black ABS resin.

In a state where the first lens array plate24is inserted into the holder30, the first through holes30cof the first surface part30adirectly face the respective first lenses24a. According to the embodiment, the first lenses24aare laid in the respective first through holes30c. Further, in a state where the second lens array plate26is inserted into the holder30, the second through holes30dof the second surface part30bdirectly face the respective fourth lenses26b. According to the embodiment, the fourth lenses26bare laid in the respective second through holes30dof the second surface part30b.

By producing the assembly as described above, the area on the first surface24cof the first lens array plate24outside the first lenses24ais covered by the first surface part30aof the holder30. Further, the area on the fourth surface26dof the second lens array plate26outside the fourth lenses26bis covered by the second surface part30bof the holder30.

The first surface part30aand the second surface part30bof the holder30are formed by using a light shielding material. Accordingly, the portion of the first surface part30asurrounding the first lens24afunctions as a first light shielding wall30ethat prevents stray light from being incident on the first lens24a. The portion of the second surface part30bsurrounding the fourth lens26bfunctions as a second light shielding wall30fthat prevents stray light from being incident from the fourth lens26b.

By adjusting the thickness of the first surface part30aand the second surface part30bof the holder30, the height of the first light shielding wall30eand the second light shielding wall30fcan be changed. The height of the first light shielding wall30eand the second light shielding wall30fis desirably set to remove light entering at an angle larger than a predetermined maximum angle of view.

The erecting equal-magnification lens array plate11as configured above is built in the image reading device100such that the distance from the first lens24ato the document G and the distance from the fourth lens26bto the line image sensor20are equal to a predetermined working distance WD.

A description will now be given of elimination of ghost noise using the erecting equal-magnification lens array plate11according to the embodiment. Before describing the erecting equal-magnification lens array plate11, a comparative example will be shown.FIG. 7shows an erecting equal-magnification lens array plate211according to the comparative example. In the erecting equal-magnification lens array plate211according to the comparative example, no slopes are formed around the second lenses24band the third lenses26a. Therefore, between adjacent second lenses24band between adjacent third lenses26aare located flat parts24hand26h, respectively. The other aspects of the erecting equal-magnification lens array plate211are identical to those of the erecting equal-magnification lens array plate11according to the embodiment.

It will be assumed that a light beam diagonally enters one of the first lenses24afrom the document G. As shown inFIG. 7, the light beam traveling toward the lens center of the first lens24afrom a given point on the document G will be referred to as primary light beam L1(dotted line), the light beam traveling toward the nearer edge of the first lens24awill be denoted by L2(solid line), and the light beam traveling toward the farther edge of the first lens24awill be denoted by L3(dashed line).

As shown inFIG. 7, the light beam L2is removed by the first light shielding wall30e. Meanwhile, the primary light beam L1and the light beam L3are incident on the first lens24a, travel past the flat part24hof the second surface24dand the flat part26hof the third surface26c, respectively, before exiting from the fourth lens26bof the lens system adjacent to the lens system on which the beam is incident. The light exiting from the fourth lens26bis not removed by the second light shielding wall30fand is incident on the line image sensor20, creating ghost noise.

FIG. 8shows, by way of one example, how ghost noise is eliminated by the erecting equal-magnification lens array plate11according to the embodiment. As in the case of the comparative example ofFIG. 7, it will be assumed that a light beam diagonally enters one of the first lenses24aof the erecting equal-magnification lens array plate11from the document G. The light beam traveling toward the lens center of the first lens24afrom a given point on the document G will be referred to as primary light beam L1(dotted line), and the light beam traveling toward the nearer edge of the first lens24awill be denoted by L2(solid line). The light beam L3traveling toward the farther edge of the first lens24awill be discussed with reference toFIG. 9in order to illustrate the light path of the beam more clearly.

In this example, the lens pitch P=0.65 mm, the lens diameter D=0.53 mm, and the lens sag=0.085 mm. The sag of the lens is equal to the height of the slope. In this case, if the slopes24fand26fare formed such that adjacent slopes24fare in contact with each other and adjacent slopes26fare in contact with each other, the angle θ of the slopes24fand26fwill be 55°. The angle θ of the slope is defined with respect to a plane perpendicular to the optical axis Ax of the erecting equal-magnification lens array plate11.

As shown inFIG. 8, the light beam L2is removed by the first light shielding wall30e. The primary light beam L1is incident on the first lens24a. The primary light beam L1is totally reflected by the slope24fformed around the second lens24bof the lens system on which the beam is incident. The totally reflected primary light beam L1exits the slope24fbelonging to the adjacent lens system before being incident on the second lens array plate26. The primary light beam L1is refracted by the third lens26aon the third surface26cbefore being removed by the second light shielding wall30f. Therefore, the primary light beam L1is not incident on the line image sensor20and does not create ghost noise.

FIG. 9shows a light path of the light beam L3(dashed line) traveling from a given point on the document G toward the farther edge of the first lens24aunder the same condition as that ofFIG. 8.

The light beam L3incident on the first lens24ais totally reflected by the slope24fbelonging to the lens system adjacent to the lens system on which the beam is incident. The totally reflected light beam L3is refracted by the second lens24bof the lens system on which the beam is incident before being removed by the first light shielding wall30e. Therefore, the light beam L3is not incident on the line image sensor20and does not create ghost noise.

FIG. 10shows, by way of another example, how ghost noise is eliminated by the erecting equal-magnification lens array plate11according to the embodiment. In the example ofFIG. 10, the lens pitch P=0.7 mm, the lens diameter D=0.53 mm, and the lens sag=0.085 mm. The sag of the lens is equal to the height of the slope. In this case, if the slopes24fand26fare formed such that adjacent slopes24fare in contact with each other and adjacent slopes26fare in contact with each other, the angle θ of the slopes24fand26fwill be 45°. The paths of the primary light beam L1(dotted line) and the light beam L2(solid line) will be discussed in this example, too. The light beam L3will be discussed with reference toFIG. 11.

As shown inFIG. 10, the light beam L2is removed by the first light shielding wall30e. The primary light beam L1is incident on the first lens24a. The primary light beam L1is totally reflected by the slope24fof the lens system on which the beam is incident. In this example, the totally reflected primary light beam L1exits the slope24fbelonging to the adjacent lens system before being incident on the second lens24bof the adjacent lens system. The primary light beam L1is refracted by the second lens24bbefore being removed by the first light shielding wall30e. Therefore, the primary light beam L1is not incident on the line image sensor20and does not create ghost noise in this example as in the above example.

FIG. 11shows the light path of the light beam L3occurring under the same condition as that ofFIG. 10. The light beam L3incident on the first lens24ais totally reflected by the slope24fbelonging to the lens system adjacent to the lens system on which the beam is incident. The totally reflected light beam L3is refracted by the second lens24bof the lens system on which the beam is incident before being removed by the first light shielding wall30e. Therefore, the light beam L3is not incident on the line image sensor20and does not create ghost noise.

FIG. 12shows, by way of still another example, how ghost noise is eliminated by the erecting equal-magnification lens array plate11according to the embodiment. In the example ofFIG. 12, the lens pitch P=0.8 mm, the lens diameter D=0.53 mm, and the lens sag=0.085 mm. The sag of the lens is equal to the height of the slope. In this case, if the slopes24fand26fare formed such that adjacent slopes24fare in contact with each other and adjacent slopes26fare in contact with each other, the angle θ of the slopes24fand26fwill be 32°. The paths of the primary light beam L1(dotted line) and the light beam L2(solid line) will be discussed in this example, too.

As shown inFIG. 12, the light beam L2is removed by the first light shielding wall30e. The primary light beam L1is incident on the first lens24a. The primary light beam L1is totally reflected by the slope24fbelonging to the lens system on which the beam is incident. In this example, the totally reflected primary light beam L1is totally reflected by the slope24fbelonging to the next lens system again. The primary light beam L1totally reflected twice is removed by the first light shielding wall30e. Therefore, the primary light beam L1is not incident on the line image sensor20and does not create ghost noise in this example as in the above example. Under the condition ofFIG. 12, no beams are incident on the first lens24aand then incident on the slope24fof the immediately adjacent lens system.

FIG. 13shows, by way of yet another example, how ghost noise is eliminated by the erecting equal-magnification lens array plate11according to the embodiment. In the example ofFIG. 13, the lens pitch P=0.85 mm, the lens diameter D=0.53 mm, and the lens sag=0.085 mm. The sag of the lens is equal to the height of the slope. In this case, if the slopes24fand26fare formed such that adjacent slopes24fare in contact with each other and adjacent slopes26fare in contact with each other, the angle θ of the slopes24fand26fwill be 28°. The paths of the primary light beam L1(dotted line) and the light beam L2(solid line) will be discussed in this example, too.

As shown inFIG. 13, the light beam L2is removed by the first light shielding wall30e. The primary light beam L1is incident on the first lens24a. The primary light beam L1is totally reflected by the slope24fof the lens system on which the beam is incident. In this example, the totally reflected primary light beam L1is not incident on the other slope24fand removed by the first light shielding wall30e. Therefore, the primary light beam L1is not incident on the line image sensor20and does not create ghost noise in this example as in the above example. Under the condition ofFIG. 13, no beams are incident on the first lens24aand then incident on the slope24fof the immediately adjacent lens system.

FIG. 14shows, by way of still another example, how ghost noise is eliminated by the erecting equal-magnification lens array plate11according to the embodiment.FIG. 14shows a light path occurring on the assumption that a beam exits the line image sensor20. As shown inFIG. 14, the light beam traveling toward the lens center of the fourth lens26bwill be referred to as primary light beam L1(dotted line), and the light beam traveling toward one of the edges of the fourth lens26bwill be denoted by L2(solid line). The conditions such as lens pitch are the same as those ofFIG. 13.

As shown inFIG. 14, the light beam L2is removed by the second light shielding wall30f. The primary light beam L1is incident on the fourth lens26b. The primary light beam L1is totally reflected by the slope26fof the lens system on which the beam is incident. In this example, the totally reflected primary light beam L1is not incident on the other slope26fand removed by the second light shielding wall30f.

As described above, the light beam exiting from the line image sensor20does not reach the document G. Since the erecting equal-magnification lens array plate11according to the embodiment is a symmetrical optical system, this means that there are no paths for beams from the document G to the line image sensor20. This is explained by the principle of reversibility of light-path.

As described above with reference toFIGS. 8-14, at least a part of the beams diagonally incident on the first lens array plate24from the document G is totally reflected by the slopes24fand26fof the erecting equal-magnification lens array plate11according to the embodiment. The totally reflected primary light beam is removed by the first light shielding wall30eor the second light shielding wall30f. As a result, ghost noise is reduced more successfully than in the comparative example shown inFIG. 7so that an erect equal-magnification image with higher quality can be formed.

An attempt to remove the diagonally incident beam in the comparative example ofFIG. 7will require providing a light shielding member that covers the flat part24hof the second surface24dand the flat part26hof the third surface26c. The erecting equal-magnification lens array plate11of the embodiment does not require the light shielding members with the result that the number of components is reduced and the manufacturing cost is reduced.

In the above-described embodiment, the slopes24fand26fare formed in the second and third surfaces24dand26c, respectively. Alternatively, ghost noise can be reduced by forming a slope in one of the surfaces. However, formation of the slopes24fand26fin the second and third surfaces24dand26d, respectively, is favorable in that ghost noise is reduced most successfully.

In the embodiment described above, the slopes24fand26fare formed such that the height thereof is identical to the sag of the second and third lenses24band26a, respectively. Alternatively, ghost noise can be suitably reduced by ensuring that the height of the slopes24fand26fis 80% of the sag of the second and third lenses24band26a, respectively, or more.

A description will now be given of the angle of slope necessary to create total reflection in the slope.FIG. 15shows a light path of a light beam L4incident on the erecting equal-magnification lens array plate11at the maximum angle of incidence Y. The light beam L4is refracted when it is incident on the first lens24a. The angle with respect to the optical axis of the lens is Y′, which is smaller than the angle Y. Given that the refractive index of the first lens array plate24is n, the angle Y′ is given by the expression (1) below.
Y′=Y/n(1)

There are two patterns of incidence of the light beam L4on the slope.FIG. 16shows that the light beam L4reaches a slope24f1belonging to the lens system on which the beam is incident.FIG. 17shows that the light beam L4reaches a slope24f2belonging to the lens system adjacent to the lens system on which the beam is incident. Given that the angle of incidence on the slope24f1in the case ofFIG. 16is u, the angle of incidence on the slope24f2in the case ofFIG. 17is v, and the angle of slope is θ, the angles of incidence u and v are given by the expressions (2) and (3) below.
u=θ+Y′(2)
v=θ−Y′(3)
The condition in which the angles of incidence u and v are larger than the angle of total reflection is given by the expressions (4) and (5) below.
u=θ+Y′>arcsin(1/n)  (4)
v=θ−Y′>arcsin(1/n)  (5)
Modifying (4) and (5), we obtain the expressions (6) and (7) below showing the conditions for the angle θ of slope in which the light beam L4is totally reflected by the slope.
θ>arcsin(1/n)−Y′(6)
θ>arcsin(1/n)+Y′(7)
Based on the expressions (6) and (7), it is known that the angle θ of slope should meet the expression (7) for total reflection to occur in both of the incidence patterns ofFIGS. 16 and 17. However, as the angle θ of slope becomes smaller and the lens pitch becomes larger than a predetermined value, the light beam incident on a given lens system cannot enter the adjacent lens system. Accordingly, the case ofFIG. 16need only be considered to determine the minimum value of the angle θ of slope necessary for total reflection. Therefore, the necessary angle θ is determined by the expression (6).

An example of the erecting equal-magnification lens array plate11will be shown below.FIG. 18shows a result of simulation of noise ratio performed in the erecting equal-magnification lens array plate11shown inFIG. 2. A ray tracing simulation was conducted. The entirety of the erecting equal-magnification lens array plate11is illuminated in the main scanning direction by a 90° Lambertian emission from a point light source. The amount of imaging light arriving at a specified point on the image plane is designated as the amount of imaging light transmitted. The amount of light arriving elsewhere is designated as the amount of light transmitted as noise. The illumination and measurement are conducted on a line extending in the main scanning direction. A noise ratio is defined as a sum of the amount of light transmitted as noise divided by the amount of imaging light transmitted. The ghost noise ratio and the flare noise ratio are determined separately. The total noise ratio is a sum of the ghost noise ratio and the flare noise ratio. Flare noise is created as the light beam reflected by the light shielding wall reaches the image plane.

The conditions of simulation are such that the lenses are arranged in a single row, the lens's working distance WD=3.3 mm, the plate thickness t of the first and second lens array plates is such that t=1.6 mm, the lens pitch P=0.65 mm, the lens diameter of the first and fourth lenses=0.47 mm, the lens diameter of the second and third lenses=0.53 mm, the refractive index n=1.53, the sag of the first and fourth lenses=0.07 mm, the sag of the second and third lenses=0.085 mm, the height h of the first and second light shielding walls is such that h=0.66 mm, the height of the light shielding wall-the sag=0.59 mm, the aperture of the first and second through holes=0.47 mm, the viewing field radius=0.92 mm, and the maximum angle of incidence=15.3°. The height of slope is equal to the sag of the second and third lenses and is 0.085 mm. Therefore, the angle θ of the slope is 55°. Incorporating the conditions for this simulation into the expression (7) above, we obtain θ>arcsin(1/1.53)+15.3/1.53=50.8°, demonstrating that the angle θ of slope calculated in the simulation meets the expression (7).

FIG. 18also shows a result of simulation conducted in the erecting equal-magnification lens array plate according to comparative examples. The erecting equal-magnification lens array plate211according to comparative example 1 as shown inFIG. 7is provided with a flat part24hbetween adjacent second lenses24band with a flat part26hbetween adjacent third lenses26a. The erecting equal-magnification lens array plate according to comparative example 2 is provided with a light shielding member (not shown) to cover the flat parts24hand26hof the comparative example 1 ofFIG. 7. The light shielding member according to comparative example 2 is sandwiched between the first lens array plate24and the second lens array plate26. The light shielding member is provided with a plurality of third through holes in which the second lenses24band the third lenses26aare laid. The aperture of the third through hole is 0.53 mm. The other conditions of simulation are identical to those of the embodiment.

The simulation reveals that the ghost noise ratio according to the embodiment is 0.00%. The ratio is significantly small compared with the ghost noise ratio=18.22% according to comparative example 1. This demonstrates that the embodiment provides satisfactory ghost noise reduction. It also shows that the embodiment provides substantially identical noise elimination performance in reference to the ghost noise ratio=0.00% according to comparative example 2 where a light shielding member is provided.

The simulation result shows that the slopes24fand26fformed around the second lenses24band the third lenses26a, respectively, are effective to eliminate ghost noise but are not so effective to eliminate flare noise. Therefore, it is necessary to add an additional feature to eliminate flare noise. A description will now be given of an exemplary configuration to eliminate flare noise.

FIG. 19shows an erecting equal-magnification lens array plate311provided with a feature to eliminate flare noise. In the erecting equal-magnification lens array plate311shown inFIG. 19, the shape of the first through hole30cand the second through hole30din the first and second light shielding walls30eand30f, respectively, is different from that of the erecting equal-magnification lens array plate11shown inFIG. 2, etc. Further, a light shielding member312is provided between the first and second lens array plates24and26in the erecting equal-magnification lens array plate311in order to address ghost noise. The light shielding member312is provided with a plurality of third through holes312ain which the second and third lenses24band26aare laid.

As shown inFIG. 19, each first through hole30cof the first light-shielding wall30eis provided with a cylindrical lateral wall portion30jprovided upright so as to surround a space above the first lens24a, an annular inner projection portion30hprovided at the end of the lateral wall portion30jfacing the first lens24a, and an outer projection portion30iprovided at the end of the lateral wall portion30jfacing the document G. The inner projection portion30hand the outer projection30iare provided so as to project from the inner circumferential edge of the lateral wall portion30jtoward the center of the hole.

As shown inFIG. 19, an aperture having an aperture diameter ID (hereinafter, inner aperture diameter ID) is formed inside the inner projection portion30h, and an aperture having an aperture diameter OD (hereinafter, referred to as outer aperture diameter OD) is formed inside the outer projection portion30i. In the erecting equal-magnification lens array plate311, the inner projection portion30hand the outer projection portion30iare formed such that the portions have the identical height. Therefore, given that the inner diameter of the lateral wall portion30jis denoted by MD, the inner aperture diameter ID=the outer aperture diameter OD<the inner diameter MD.

The inner projection portion30hand the outer projection portion30iare formed such that there are no surfaces parallel to the optical axis Ax of the lens system. More specifically, the inner projection portion30his tapered such that the inner diameter is progressively larger from the edge facing the first lens24atoward the center of the first through hole30cin the direction of height. The outer projection portion30iis tapered such that the inner diameter is progressively larger from the end facing the document G toward the center of the first through hole30cin the direction of height.

As in the first light shielding wall30e, each second through hole30dof the second light-shielding wall30fis provided with a cylindrical lateral wall portion30mprovided upright so as to surround a space above the fourth lens26b, an annular inner projection portion30kprovided at the end of the lateral wall portion30mfacing the fourth lens26b, and an outer projection portion30lprovided at the end of the lateral wall portion30mfacing the line image sensor20. The inner projection portion30kand the outer projection30lare provided so as to project from the inner circumferential edge of the lateral wall portion30mtoward the center of the hole. The shapes of the lateral wall portion30m, the inner projection unit30k, and the outer projection portion30lof the second through hole30dare identical to those of the first through hole30cso that a detailed description is omitted.

A description will now be given of flare noise elimination in the erecting equal-magnification lens array plate311according to the embodiment. Before describing the erecting equal-magnification lens array plate311ofFIG. 19, a comparative example will be shown.FIG. 20shows how flare noise is produced in an erecting equal-magnification lens array plate411according to the comparative example. In the erecting equal-magnification lens array plate411according to the comparative example, the first through holes30cof the first light-shielding wall30eand the second through holes30dof the second light-shielding wall30fare simply cylindrically formed. Inner projection portions or outer projection portions are not formed. In other words, the inner diameter of the first through holes30cand the second through holes30din the erecting equal-magnification lens array plate411remains constant in the direction of height of the through holes.

First, a light beam L5(solid line) emitted from a point60on the document G located on the optical axis of the first lens24awill be discussed. Normally, the light beam L5about to be incident on the first lens array plate24at an angle of incidence larger than an angle of the light to be imaged is absorbed by the lateral wall of the first through hole30cof the first light shielding wall30e. However, the light beam L5is not completely absorbed even if a light absorbing material is used. The light beam L5is partly incident on the first lens24adue to Fresnel reflection. This is because, the Fresnel reflectance for an angle of incidence close to 90° of the light beam L5incident on the lateral wall of the first through hole30cis extremely large.

As shown inFIG. 20, the reflected light beam L5is transmitted through the first lens24a, the second lens24b, the third lens26a, and the fourth lens26bbefore being incident on the line image sensor20, causing flare noise.

Secondly, a light beam L6(broken line) emitted from a point62on the document G outside the optical axis of the first lens24awill be discussed. The light beam L6is partly reflected by the lateral wall of the first through hole30cby Fresnel reflection. As shown inFIG. 20, the reflected light beam L6is transmitted through the first lens24a, the second lens24b, the third lens26a, and the fourth lens26bbefore being incident on the line image sensor20, causing flare noise.

Flare noise produced by the reflection by the first light shielding wall30eis described with reference toFIG. 20. Flare noise is also produced by the reflection by the second light shielding wall30f.

FIG. 21shows flare noise elimination in the erecting equal-magnification lens array plate311shown inFIG. 19. First, as in the case of the comparative example ofFIG. 20, the light beam L5(solid line) emitted from the point60on the document G located on the optical axis of the first lens24awill be discussed. In erecting equal-magnification lens array plate311, the light beam L5is incident on the inner projection portion30hof the first through hole30c. Since the interior surface of the inner projection portion30his formed as a tapered surface inclined with respect to the optical axis, the light beam L5reflected by the inner projection portion30his reflected multiple times in the first through hole30cbefore being eliminated. Therefore, the light beam L5does not reach the line image sensor20so that flare noise due to the light beam L5is not produced. The same discussion applies to the light beam L6(broken line) emitted from the point62outside the optical axis.

The action of reducing flare noise by the inner projection portion30hof the first through hole30cis described with reference toFIG. 21. Flare noise is similarly reduced by the inner projection portion30kof the second through hole30d.FIGS. 19 and 21show that the inner projection portion and the outer projection portion are provided in both the first light shielding wall and the second light shielding wall. However, flare noise is suitably reduced by providing the inner projection portion and the outer projection portion in one of the first and second light shielding walls. Further, flare noise is suitably reduced by providing one of the inner and outer projection portions in the through holes.

FIG. 22shows an erecting equal-magnification lens array plate511produced by providing the erecting equal-magnification lens array plate11shown inFIG. 2with a feature to eliminate flare noise. In the erecting equal-magnification lens array plate511according to this embodiment, the lateral wall portion30j, the inner projection portion30h, and the outer projection portion30isuch as those ofFIG. 19are formed in the first through hole30cof the first light shielding wall30eof the holder30. Further, the second through hole30dof the second light shielding wall30fof the holder30is formed with the lateral wall portion30m, the inner projection unit30k, and the outer projection portion30l. Flare noise is reduced according to the structure as described above.

In further accordance with the erecting equal-magnification lens array plate511, an annular slope24fis formed on the second surface24dof the first lens array plate24so as to surround each second lens24b. Further, an annular slope26fis formed on the third surface26cof the second lens array plate26so as to surround each third lens26a. The slopes24fand26freduce ghost noise. Thus, the embodiment provides an erecting equal-magnification lens array plate capable of reducing ghost noise and flare noise.

A description will be given of an exemplary embodiment of the erecting equal-magnification lens array plate511.FIG. 23shows a result of simulation of noise ratio performed in the erecting equal-magnification lens array plate511shown inFIG. 22. As in the case ofFIG. 18, the ghost noise ratio and the flare noise ratio are determined separately. A sum of the ghost noise ratio and the flare noise ratio represents the total noise ratio. The conditions of simulation are such that the inner diameter MD=0.6 mm, the inner aperture diameter ID=0.47 mm, the outer aperture diameter OD=0.47 mm, and the taper angle of the inner projection portion and the outer projection portion=45°. The other conditions are the same as those ofFIG. 18.

FIG. 23also shows a result of simulation conducted in the erecting equal-magnification lens array plate according to comparative examples. The erecting equal-magnification lens array plate311shown inFIG. 19is defined as comparative example 4. The erecting equal-magnification lens array plate311shown inFIG. 19with the light shielding member312removed is defined as comparative example 3.

The simulation reveals that the flare noise ratio according to the embodiment is 0.04%, which is significantly smaller than 45.15% indicated inFIG. 18. The ghost noise ratio is 0.00% as in the case ofFIG. 18so that the total ratio in the erecting equal-magnification lens array plate511is 0.04%, which is quite small. This shows that the erecting equal-magnification lens array plate511provides substantially identical noise elimination performance as comparative example 4 (total noise ratio=0.03%).

FIG. 24shows a result of simulation of noise ratio performed in the erecting equal-magnification lens array plate511shown inFIG. 22such that the angle of the slope is varied; The height of the slope is maintained constant (0.085 mm). The angle of slope is varied from 55° to 20° by varying the lent pitch from 0.65 mm to 1.00 mm. The simulation result shows that the ghost noise ratio remains unchanged even if the angle of slope is varied from 55° to 20°. However, the flare noise ratio tends to increase as the angle of slope is decreased. Therefore, the larger the angle of slope, the better.

FIG. 25shows a variation of embodiment concerning the slope portion.FIG. 6shows that the slope24fis formed by providing the second surface24dwith a mortar-like hole. Meanwhile, in the embodiment shown inFIG. 25, the slope24fis formed by providing an annular projection portion24gon the second surface24d. A mortar-like slope is formed inside the projection portion24g. The slope configured according to this example will also eliminate ghost noise suitably.

FIG. 26shows a variation of embodiment concerning the erecting equal-magnification lens array plate.FIGS. 5 and 6show that the upper edges of adjacent slopes are in contact with each other. As a result, the neighborhood of the area of contact between adjacent slopes will be thin. As a result, the mechanical strength is slightly decreased. To address this in this embodiment, spacing is provided between adjacent slopes24fand between adjacent slopes26fso as to form flat parts24iand26i. This eliminates thin portions so that the mechanical strength of the erecting equal-magnification lens array plate is improved.

According to this variation, however, the light beam passing through the flat parts24iand26imay create ghost noise. It is therefore necessary to design the flat parts24iand26iaccording to the variation so that the light beam at the maximum angle of incidence does not enter the flat parts24iand26i. This can be achieved by extending the lens pitch in accordance with the size of the flat parts24iand26i.

For example, the embodiment described above requires defining the angle of the slope so that the incident light beam is totally reflected. However, the angle of the slope need not necessarily to meet the condition for total reflection. Any slope formed at least around each second lens and/or third lens causes at least a part of the light beam diagonally incident on the slope to be refracted or reflected. This will decrease the likelihood of the light beam exiting from the second lens array plate so that ghost noise is reduced more successfully than in the comparative example ofFIG. 7.

According to the above-described embodiment, the top of the second lens and the top of the third lens are in contact with each other. The present invention is equally useful where the second lens and the third lens are spaced apart, creating a space therebetween.