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

An erecting equal-magnification lens array plate includes: 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. A plurality of V grooves are formed in an area between adjacent second lenses on the second surface in the erecting equal-magnification lens array plate.

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 image writing devices.

2. Description of the Related Art

Some image reading devices such as scanners are known to use erecting equal-magnification optics. 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 linear 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 proportion of light transmitted is improved and unevenness in the amount of light transmitted is reduced. Due to cost concerns, it is common to use one or two rows of rod lenses in an array.

Meanwhile, an erecting equal-magnification lens array plate could be formed as a stack of two transparent lens array plates built such that the optical axes of individual convex lenses are aligned, where each transparent lens array plate includes a systematic arrangement of micro-convex lenses on both surfaces of the plate. Since an erecting equal-magnification lens array plate such as this can be formed by, for example, injection molding, an erecting equal-magnification lens array can be manufactured at a relatively low cost.

An erecting equal-magnification lens array plate lacks a wall for ray separation between adjacent lenses. Therefore, there is a problem in that a ray diagonally incident on an erecting equal-magnification lens array plate travels diagonally inside the plate and enters an adjacent convex lens, creating noise (referred to as ghost noise) as it leaves the plate.

There is known an erecting equal-magnification lens array plate in which a light-shielding wall is provided between the two lens array plates in order to reduce ghost noise (see, for example, patent document No. 1).[patent document No. 1] JP2009-069801

However, when a light-shielding wall is provided between two lens array plates, the number of components may be increased so that the manufacturing cost may be increased.

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 without providing a light-shielding wall between lens array plates and to provide an optical scanning unit, an image reading device, and an image writing device in which the erecting equal-magnification lens array plate is used.

To address the aforementioned purpose, the erecting equal-magnification lens array plate according to an embodiment of the present invention 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, wherein 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 an erect equal-magnification image of an object on the first surface side is formed on an image plane facing the fourth surface. In the erecting equal-magnification lens array plate, a plurality of V grooves are formed in an area between adjacent second lenses on the second surface and/or an area between adjacent third lenses on the third surface.

The V grooves may be formed to extend substantially parallel to the main scanning direction of the erecting equal-magnification lens array plate. The total width of the V grooves in the sub-scanning direction may be equal to or more than an aperture size of the first lenses. the adjacent V grooves are contiguous with each other at their ends in the sub-scanning direction.

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 original to be read; the erecting equal-magnification lens array plate configured to condense light reflected by the original to be read; and a linear image sensor configured to receive light transmitted by the erecting equal-magnification lens array plate.

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

Yet another embodiment of the present invention relates to an image writing device. The device comprises an LED array comprising an array of a plurality of LED's; the aforementioned erecting equal-magnification lens array plate for condensing light emitted from the LED array; and a photosensitive drum for receiving the light transmitted through the erecting equal-magnification lens array plate.

Optional combinations of the aforementioned constituting elements, and implementations of the invention in the form of methods, apparatuses, and systems 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 a housing102, a glass plate14on which a document G is placed, an optical scanning unit10accommodated in the housing102, 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 linear image sensor (photoelectric transducer)20for receiving light condensed by the erecting equal-magnification lens array plate11, and a case (not shown) for fixing the linear light source16, the erecting equal-magnification lens array plate11, and the linear image sensor20.

The linear light source16is a light source emitting a substantially straight light. The linear light source16is secured such that the optical axis thereof passes through the intersection of the optical axis Ax of the erecting equal-magnification lens array plate11and the top surface of the glass plate14. The light exiting from the linear light source16illuminates the document G placed on the glass plate14. The light illuminating the document G is reflected by the document G toward the erecting equal-magnification lens array plate11.

The erecting equal-magnification lens array plate11comprises a stack of a first lens array plate24and a second lens array plate26built such that pairs of corresponding lenses form a coaxial lens system, where each lens array plate is formed with a plurality of convex lenses on both surfaces of the plate, as described later. The first lens array plate24and the second lens array plate26are held by a holder (not shown) in 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 linear image sensor20. The image reading device100can read the document G by scanning document G with the optical scanning unit10in the sub-scanning direction.

FIG. 2shows a cross section of the optical scanning unit10in the main scanning direction. Referring toFIG. 2, the vertical direction in the illustration represents main scanning direction (longitudinal direction) of the erecting equal-magnification lens array plate11and the depth direction in the illustration represents the sub-scanning direction (lateral direction).

As described above, the erecting equal-magnification lens array plate11comprises a stack of the first lens array plate24and the second lens array plate26. Each of the first lens array plate24and the second lens array plate26is a rectangular plate and is provided with an arrangement of a plurality of convex lenses on both sides thereof.

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 absorption. Desired materials include cycloolefin resins, olefin resins, norbornene resins, and polycarbonate.

A plurality of first lenses24aare arranged in a single line on a first surface24c(one of the surfaces of the first lens array plate24) in the longitudinal direction of the first lens array plate24. A plurality of second lenses24bare arranged in a single line on a second surface24dof the first lens array plate24opposite to the first surface24cin the longitudinal direction of the first lens array plate24. As shown inFIG. 2, the lens diameter of the second lenses24bis smaller than the lens diameter of the first lenses24ain this embodiment.

A plurality of third lenses26aare arranged in a single line on a third surface26c(one of the surfaces of the second lens array plate26) in the longitudinal direction of the second lens array plate26. A plurality of fourth lenses26bare arranged in a single line on a fourth surface26dopposite to the third surface26cin the longitudinal direction of the second lens array plate26. As shown inFIG. 2, the lens diameter of the third lenses26ais smaller than the lens diameter of the fourth lenses26bin this embodiment. The lens diameter of the third lenses26ais equal to the lens diameter of the second lenses24b, and the lens diameter of the fourth lenses26bis equal to the lens diameter of the first lenses24a.

In this 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 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 lens26baligned with each other form a coaxial lens system. In other words, the first and second lens array plates24and26form a stack such that the optical axes of the first, second, third, and fourth lenses24a,24b,26a, and26baligned with each other are aligned.

A first surface light-shielding wall30is provided on the first surface24cof the first lens array plate24. The first surface light-shielding wall30is a light-shielding member of a film form made of a light-shielding material and is formed with a plurality of first surface through holes30a. The first surface through holes30aare arranged in a single line in the longitudinal direction of the first surface light-shielding wall30so as to be in alignment with the first lenses24aof the first lens array plate24. The hole diameter of the first surface through hole30ais equal to the effective diameter of the first lens24a. The first surface light-shielding wall30is provided on the first surface24csuch that each first surface through hole30ais located directly opposite to the corresponding first lens24a. In other words, the first surface light-shielding wall30is provided on the first surface24csuch that the central axis of each first surface through hole30ais aligned with the optical axis of the corresponding first lens24a. As shown inFIG. 2, an area24e(hereinafter, also referred to as “first surface flat area24e”) on the first surface24coutside the effective region of the first lenses24ais covered by the first surface light-shielding wall30. The term “effective region of a lens” refers to a portion having the function of a lens. The first surface light-shielding wall30shields light not contributing to imaging. The first surface light-shielding wall30may be formed by printing the first surface24cwith a light-shielding pattern using a light-absorbing material such as black ink.

A fourth surface light-shielding wall32is provided on the fourth surface26dof the second lens array plate26. The fourth surface light-shielding wall32is a light-shielding member of a plate form made of a light-shielding material and is formed with a plurality of fourth surface through holes32a. The fourth surface through holes32aare arranged in a single line in the longitudinal direction of the fourth surface light-shielding wall32so as to be in alignment with the fourth lenses26bof the second lens array plate26. The fourth surface through hole32ais cylindrically formed and the hole diameter thereof is equal to the effective diameter of the fourth lens26b. The fourth surface light-shielding wall32is provided on the fourth surface26dsuch that each fourth surface through hole32ais located directly opposite to the corresponding fourth lens26b. In other words, the fourth surface light-shielding wall32is provided on the fourth surface26dsuch that the central axis of each fourth surface through hole32ais aligned with the optical axis of the corresponding fourth lens26b. As shown inFIG. 2, an area26e(hereinafter, also referred to as “fourth surface flat area26e”) on the fourth surface26doutside the effective region of the fourth lenses26bis covered by the fourth surface light-shielding wall32.

Preferably, the fourth surface light-shielding wall32may be formed by, for example, injection molding, using a light absorbing material such as black ABS resin. Alternatively, the fourth surface light-shielding wall32may be formed by stacking a black resin paint.

In this specification, the first surface light-shielding wall30is configured in a “film form” and the fourth surface light-shielding wall32is configured in a “plate form”. This means that the first surface light-shielding wall30is far thinner than the fourth surface light-shielding wall32. In other words, the term “film form” means that the thickness is negligibly small.

FIG. 3is a cross section of the erecting equal-magnification lens array plate11along A-A inFIG. 2. Referring toFIG. 3, the vertical direction in the illustration represents sub-scanning direction (lateral direction) of the erecting equal-magnification lens array plate11and the depth direction in the illustration represents the main scanning direction (longitudinal direction).FIG. 4is a front view of the second surface24dof the first lens array plate24. The feature of the erecting equal-magnification lens array plate11according to the embodiment is found in an area24f(hereinafter, “second surface inter-lens area24f”) between adjacent second lenses24bon the second surface24dof the first lens array plate24. Therefore, the A-A line ofFIG. 2is drawn such that a part thereof passes through the second surface inter-lens area24f. In the embodiment, an area26f(hereinafter, “third surface inter-lens area26f”) between adjacent third lenses26aon the third surface26cof the second lens array plate26is formed as a flat surface.

As shown inFIGS. 3 and 4, the second surface inter-lens area24fis formed with a plurality of V grooves40in the embodiment. The V grooves40have a function of deflecting light incident on the second surface inter-lens area24fin the sub-scanning direction of the erecting equal-magnification lens array plate11. The direction of deflection can be controlled by changing the angle of inclination of the slope of the V grooves40.

As shown inFIGS. 3 and 4, the plurality of V grooves40are formed to extend substantially parallel to the main scanning direction of the erecting equal-magnification lens array plate11. The V grooves40are arranged such that adjacent V grooves are contiguous with each other at their ends in the sub-scanning direction. The V grooves40may be provided by forming the second surface24dwith concave portions having a triangular cross section or forming the second surface24dwith convex portions having a triangular cross section.

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 linear image sensor20are equal to a predetermined working distance.

A description will now be given of the operation of the erecting equal-magnification lens array plate11according to the embodiment. Before describing the operation of the erecting equal-magnification lens array plate11, a comparative embodiment will be shown.

FIG. 5shows a cross section of an erecting equal-magnification lens array plate511according to the first comparative embodiment in the main scanning direction. The erecting equal-magnification lens array plate511according to the first comparative embodiment differs from the erecting equal-magnification lens array plate11according to the embodiment in that an intermediate light-shielding wall512is provided between the first lens array plate24and the second lens array plate26. In the erecting equal-magnification lens array plate511according to the first comparative embodiment, the second surface inter-lens area24fis formed as a flat surface. The other aspects are identical to those of the erecting equal-magnification lens array plate11according to the embodiment.

The intermediate light-shielding wall512is a light-shielding member of a plate form made of a light-shielding material and is formed with a plurality of intermediate through holes512a. The intermediate light-shielding wall512is arranged such that each intermediate through hole512ais located directly opposite to the corresponding second and third lenses24band26a. As shown inFIG. 5, the second surface inter-lens area24fand the third surface inter-lens area26fare covered by the intermediate light-shielding wall512.

FIG. 5shows optical paths of a ray L1(broken line) and a ray L2(chain line) emitted from the document G. Both of the rays L1and L2are diagonally incident on the erecting equal-magnification lens array plate511. As shown inFIG. 5, the rays L1and L2are absorbed or at least attenuated by the intermediate light-shielding wall512. By using the erecting equal-magnification lens array plate511according to the first comparative embodiment, an erect equal-magnification image in which ghost noise is reduced can be formed.

FIG. 6shows a cross section of the erecting equal-magnification lens array plate611according to the second comparative embodiment in the main scanning direction.FIG. 7shows a cross section of the erecting equal-magnification lens array plate611according to the second comparative embodiment in the sub-scanning direction. As shown inFIGS. 6 and 7, the configuration of the erecting equal-magnification lens array plate611according to the second comparative embodiment is derived from removing the intermediate light-shielding wall512from the erecting equal-magnification lens array plate511according to the first comparative embodiment.

FIG. 6also shows optical paths of the ray L1(broken line) and the ray L2(chain line) diagonally incident on the erecting equal-magnification lens array plate611.FIG. 7shows optical paths of a ray L3(broken line) and a ray L4(chain line). L3and L4inFIG. 7represent the extent of the rays L1and L2inFIG. 6in the sub-scanning direction. Since no intermediate light-shielding walls are provided between the first and second lens array plates24and26in the second comparative embodiment, the rays L1and L2are transmitted through the second surface inter-lens area24fand the third surface inter-lens area26fand are incident on the linear image sensor20. Therefore, ghost is created in the erecting equal-magnification lens array plate611according to the second comparative embodiment. The rays L1and L2originate ghost noise and so are stray light.

FIG. 8shows the operation of the erecting equal-magnification lens array plate11according to the embodiment.FIG. 8is a cross section of the erecting equal-magnification lens array plate11along A-A inFIG. 2. As inFIG. 7,FIG. 8shows optical paths of the ray L3(broken line) and the ray L4(chain line).

As shown inFIG. 8, the erecting equal-magnification lens array plate11according to the embodiment is configured such that the rays L3and L4are deflected in the sub-scanning direction of the erecting equal-magnification lens array plate11by the V grooves40formed in the second surface inter-lens area24f. The rays L3and L4deflected by the V grooves40are absorbed or at least attenuated by the fourth surface light-shielding wall32provided on the fourth surface of the second lens array plate26and so do not reach the linear image sensor20. Therefore, by using the erecting equal-magnification lens array plate11according to the embodiment, an erect equal-magnification image in which ghost noise is reduced can be formed without providing an intermediate light-shielding wall. Consequently, the number of components is reduced so that the erecting equal-magnification lens array plate11according to the embodiment is implemented at a lower cost than the erecting equal-magnification lens array plate511according to the comparative embodiment shown inFIG. 5.

As shown inFIG. 7, the optical path of the stray light originating ghost noise is similar to that of the imaging light except that the stray light passes through the second surface inter-lens area24fand the third surface inter-lens area26f. Therefore, the width of the second surface inter-lens area24fin the sub-scanning direction through which the stray light could pass does not exceed the aperture size of the first lens24a. Accordingly, as shown inFIG. 3, the width W1of the V grooves40in the sub-scanning direction need only be equal to or more than the aperture size D1of the first lens24a. The aperture size D1of the first lens24ais equal to the hole diameter of the first surface through hole30aof the first surface light-shielding wall30.

FIG. 9shows an erecting equal-magnification lens array plate911according to an alternative embodiment of the present invention. The erecting equal-magnification lens array plate911according to an alternative embodiment differs from the erecting equal-magnification lens array plate11shown inFIG. 3in that a plurality of V grooves42are formed in the third surface inter-lens area26fof the third surface26c. The second surface inter-lens area of the second surface24dis formed as a flat surface.

A description will be given of the operation of the erecting equal-magnification lens array plate911according to the alternative embodiment. As inFIG. 7, FIG.9shows optical paths of the ray L3(broken line) and the ray L4(chain line). As shown inFIG. 9, the rays L3and L4are transmitted by the first lens array plate24before being incident on the third surface inter-lens area26fof the second lens array plate26. The rays L3and L4incident on the third surface inter-lens area26fare deflected in the sub-scanning direction of the erecting equal-magnification lens array plate911by the V grooves42formed in the third surface inter-lens area26f. The rays L3and L4deflected by the V grooves42are absorbed or at least attenuated by the fourth surface light-shielding wall32provided on the fourth surface of the second lens array plate26and so do not reach the linear image sensor20. Therefore, by using the erecting equal-magnification lens array plate911according to the alternative embodiment, an erect equal-magnification image in which ghost noise is reduced can be formed without providing an intermediate light-shielding wall. Consequently, the number of components is reduced so that the erecting equal-magnification lens array plate911according to the embodiment is implemented at a lower cost than the erecting equal-magnification lens array plate511according to the comparative embodiment shown inFIG. 5.

FIG. 10shows an erecting equal-magnification lens array plate1011according to a second alternative embodiment of the present invention. The erecting equal-magnification lens array plate1011according to the second alternative embodiment is configured such that the V grooves40are formed to cause the ray incident on the second surface inter-lens area24fto be reflected toward the first surface24c. As inFIG. 7,FIG. 10shows optical paths of the ray L3(broken line) and the ray L4(chain line). By adjusting the angle of inclination of the slope of the V grooves40, the rays L3and L4incident on the second surface inter-lens area24fcan be reflected toward the first surface24c. According to the second alternative embodiment, the stray light can be directed in a direction opposite to the linear image sensor20. Therefore, the stray light is more properly prevented from being incident on the linear image sensor20so that ghost noise is more successfully reduced.

FIG. 11shows a relationship between the angle of inclination of the slope of the V grooves in the second surface and the angle of ray deflection. A discussion will be given of how a ray L5(broken line) incident on the V groove40, which is formed to have an isosceles triangle cross section, at an angle of 0° is deflected. The ray having an angle of incidence of 0° means a ray perpendicularly incident on the erecting equal-magnification lens array plate. As shown inFIG. 11, the angle formed by the slope of the V groove40and the surface parallel to the first lens array plate24will be defined as an angle of inclination ε. The V groove40shown inFIG. 11is formed such that the angle of inclination ε of one of the slopes40ais equal to the angle of inclination ε of the other slope40b. The angle formed by the line perpendicular to the first lens array plate24and the ray L5leaving the slope of the V groove40will be defined as an angle of deflection γ of the ray L5. For example, if the ray L5leaves the first lens array plate24perpendicularly to the plate, the angle of deflection γ=0°. If the ray L5leaves the first lens array plate24parallel to the plate, the angle of deflection γ=90°. If the ray L5is reflected perpendicularly to the first lens array plate24, the angle of deflection γ=180°. It will be assumed here that the refractive index of the first lens array plate24is 1.53. In this case, the critical angle θc of the slope of the V groove40will be 40.8°.

FIG. 12shows an optical path occurring when the angle of inclination ε of the slope of the V groove in the second surface is equal to or less than the critical angle θc. In this case, the ray L5incident on the slope of the V groove40is refracted by the slope of the V groove40and is directed toward the second lens array plate, as shown inFIG. 12.

FIG. 13shows an optical path occurring when the angle of inclination ε of the slope of the V groove in the second surface is more than the critical angle θc.FIG. 13shows an optical path occurring when the angle of inclination ε=45°. In this case, the ray L5totally reflected by the slope of the V groove40is totally reflected again by the slope of the adjacent V groove40before being directed toward the first surface (angle of deflection γ=180°. The optical path ofFIG. 13results because the angle of inclination ε=45°. The optical path of the ray L5totally reflected by the slope of the V groove40varies depending on the value of the angle of inclination ε. For example, if the angle of inclination ε=70°, the ray L5totally reflected by the slope of a given V groove40is refracted by the slope of the adjacent V groove40before being directed to the second lens array plate.

FIG. 14shows a relationship between the angle of inclination ε of the slope of the V groove in the second surface and the angle of ray deflection γ. As described with reference toFIG. 10, ghost noise is more successfully reduced if the ray L5is reflected toward the first surface (i.e., if the angle of deflection≧90°). Ghost noise is most successfully reduced when the angle of deflection γ=180°. Therefore, it is most desirable to ensure that the angle of inclination ε of the V groove40is 45°. As shown inFIG. 14, the angle of deflection γ varies depending on the value of the angle of inclination ε. The range of the angle of deflection γ effective to reduce ghost noise varies depending on the lens design. Therefore, the angle of inclination ε may be appropriately determined in accordance with the lens design.

FIG. 15shows a relationship between the angle of inclination ε of the slope of the V groove in the third surface and the angle of ray deflection γ. In this case, the ray enters the second lens array plate via air so that the ray is not totally reflected. As shown inFIG. 15, as the angle of inclination ε of the slope of the V groove in the third surface is increased, the angle of deflection γ is also increased. In the case of the V groove in the third surface, too, the range of the angle of deflection γ effective to reduce ghost noise varies depending on the lens design. Therefore, the angle of inclination ε may be appropriately determined in accordance with the lens design.

FIGS. 11-15depict cases where the angle of inclination of one of the slopes of the V groove is equal to the angle of inclination of the other slope. However, one of the slopes of the V groove need not be equal to the other slope. For example, one of the slopes of the V groove may differ from the other slope in the angle of inclination, depending on the position of the V groove. Further, the angle of inclination of the V grooves may not be uniform. The angle may differ depending on the position of the V groove. By adjusting the angle of inclination appropriately depending on the position where the V groove is located, ghost noise is more suitably reduced.

In the erecting equal-magnification lens array plate11shown inFIG. 3, the second surface24dis formed with the V grooves40. In the erecting equal-magnification lens array plate911shown inFIG. 9, the third surface26cis formed with the V grooves42. However, both the second surface and the third surface may be formed with V grooves. In this case, the stray light is deflected at both the second and third surfaces so that ghost noise is more successfully reduced. In this case, however, the width W1of the V groove in the sub-scanning direction is desirably equal to the aperture size D1of the first lens24a. This is because the V grooves in the second surface24dand/or the third surface26cmight create additional ghost.

In the embodiments ofFIGS. 3 and 9, the first surface light-shielding wall30is a light-shielding member of a film form, and the fourth surface light-shielding wall32is a light-shielding member of a plate form. Alternatively, both the first surface light-shielding wall30and the fourth surface light-shielding wall32may be a light-shielding member of a plate form. By increasing the height of the light-shielding wall thus, ghost noise is more successfully reduced. Still alternatively, the first surface light-shielding wall30may be a light-shielding member of a plate form, and the fourth surface light-shielding wall32may be a light-shielding member of a film form. Still alternatively, the first surface through hole30aof the first surface light-shielding wall30may be formed as a circular truncated cone. In other words, the diameter of the opening of the first surface through hole30afacing the document and that of the opening at the first surface may be different. Still alternatively, the fourth surface through hole32aof the fourth surface light-shielding wall32may be formed as a circular truncated cone. In other words, the diameter of the opening of the fourth surface through hole32afacing the image plane and that of the opening at the fourth surface may be different.

FIG. 16shows an erecting equal-magnification lens array plate1611according to a third alternative embodiment of the present invention. The erecting equal-magnification lens array plate1611according to the third alternative embodiment differs from the erecting equal-magnification lens array plate11shown inFIG. 3in that the first surface light-shielding wall30is a light-shielding member of a plate form and the fourth surface light-shielding wall is removed.

As inFIG. 7,FIG. 16shows optical paths of the ray L3(broken line) and the ray L4(chain line). In the erecting equal-magnification lens array plate1611according to the third alternative embodiment, the rays L3and L4are deflected in the sub-scanning direction of the erecting equal-magnification lens array plate1611by the V grooves40formed in the second surface inter-lens area24f, as shown inFIG. 16. The rays L3and L4deflected by the V grooves40travel away from the linear image sensor20after passing through the second lens array plate26and so are not incident on the linear image sensor20. Therefore, by using the erecting equal-magnification lens array plate1611according to the third alternative embodiment, an erect equal-magnification image in which ghost noise is reduced can be formed without providing an intermediate light-shielding wall or the fourth surface light-shielding wall. Consequently, the number of components is reduced so that the erecting equal-magnification lens array plate can be implemented at a reduced cost.

In the erecting equal-magnification lens array plate1611shown inFIG. 16, the first surface light-shielding wall30cannot be removed. In the erecting equal-magnification lens array plate11shown inFIG. 3, the first surface light-shielding wall30cannot be removed. Unlike the linear image sensor20, the document G has a large extent in the sub-scanning direction. Therefore, there are a plurality of paths for stray light traveling from the document G to the linear image sensor20so that stray light cannot be successfully prevented from entering the linear image sensor20merely by providing V grooves.

In the erecting equal-magnification lens array plates according to the respective embodiments, a gap is provided between the first lens array plate and the second lens array plate. Alternatively, the gap may be removed. In other words, the erecting equal-magnification lens array plates may be configured such that the second lens on the second surface and the third lens on the third surface are in contact with each other. In this case, too, ghost noise is reduced without providing a light-shielding wall between the lens array plates, by forming a plurality of V grooves in the second surface inter-lens area and/or the third surface inter-lens area.

A description will now be given of exemplary embodiments of the present invention. A simulation of noise ratio was conducted in the first exemplary embodiment, second exemplary embodiment, and the comparative exemplary embodiment of the present invention. More specifically, a ray tracing simulation was conducted. The entirety of the erecting equal-magnification lens array plate is 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 calculation of the amount of light 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 a sum of the amount of imaging light transmitted.

FIG. 17shows an erecting equal-magnification lens array plate1711according to the first exemplary embodiment of the present invention. As shown inFIG. 17, the erecting equal-magnification lens array plate1711is configured such that the second surface inter-lens area24fis formed with a plurality of V grooves40. The erecting equal-magnification lens array plate1711is also configured such that the first surface light-shielding wall30and the fourth surface light-shielding wall32of a plate form are provided. No intermediate light-shielding walls are provided. The first surface through hole30aand the fourth surface through hole32aare formed as a circular truncated cone.

The conditions for simulation in the erecting equal-magnification lens array plate1711according to the first exemplary embodiment are such that the conjugation length TC=9.9 mm, the thickness of the first and second lens array plates24and26(hereinafter, lens thickness)=1.05 mm, the pitch of arrangement of the first through fourth lenses (hereinafter, lens arrangement pitch)=0.7 mm, the lens diameter of the first lenses24a=0.6 mm, the lens diameter of the second lenses24b=0.4 mm, the lens diameter of the third lenses26a=0.35 mm, the lens diameter of the fourth lenses26b=0.6 mm, the gap between the first lens array plate24and the second lens array plate26(hereinafter, gap)=0.8 mm, the refractive index of the first and second lens array plates24and26=1.53, the height of the first surface light-shielding wall30=0.7 mm, the height of the fourth surface light-shielding wall32=0.7 mm, the diameter of the opening of the first surface light-shielding wall30facing the document=0.45 mm, the diameter of the opening of the first surface light-shielding wall30at the first surface=0.51 mm, the diameter of the opening of the fourth surface light-shielding wall32facing the image plane=0.45 mm, and the diameter of the opening of the fourth surface light-shielding wall32at the fourth surface=0.51 mm.

FIG. 18shows an erecting equal-magnification lens array plate1811according to the second exemplary embodiment of the present invention. As shown inFIG. 18, the erecting equal-magnification lens array plate1811is configured such that the second surface inter-lens area24fis formed with a plurality of V grooves40. The erecting equal-magnification lens array plate1811is also configured such that the first surface light-shielding wall30is provided. No intermediate light-shielding walls are provided and the fourth surface light-shielding wall32is not provided. The first surface through hole30ais formed as a circular truncated cone.

The conditions for simulation in the erecting equal-magnification lens array plate1811according to the second exemplary embodiment are such that the conjugation length TC=9.9 mm, the thickness of the first and second lens array plates24and26(hereinafter, lens thickness)=1.05 mm, the pitch of arrangement of the first through fourth lenses (hereinafter, lens arrangement pitch)=0.7 mm, the lens diameter of the first lenses24a=0.6 mm, the lens diameter of the second lenses24b=0.4 mm, the lens diameter of the third lenses26a=0.35 mm, the lens diameter of the fourth lenses26b=0.6 mm, the gap between the first lens array plate24and the second lens array plate26(hereinafter, gap)=0.8 mm, the refractive index of the first and second lens array plates24and26=1.53, the height of the first surface light-shielding wall30=0.7 mm, the diameter of the opening of the first surface light-shielding wall30facing the document=0.45 mm, and the diameter of the opening of the first surface light-shielding wall30at the first surface=0.51 mm.

FIG. 19shows an erecting equal-magnification lens array plate1911according to a comparative exemplary embodiment. The erecting equal-magnification lens array plate1911is configured such that the first surface light-shielding wall30, the fourth surface light-shielding wall32, and the intermediate light-shielding wall512of a plate form are provided. The first surface through hole30a, the fourth surface through hole32a, and the intermediate through hole512aare formed as a circular truncated cone. No V grooves are provided in the erecting equal-magnification lens array plate1911.

The conditions for simulation in the erecting equal-magnification lens array plate1911according to the comparative exemplary embodiment are such that the conjugation length TC=9.9 mm, the thickness of the first and second lens array plates24and26(hereinafter, lens thickness)=1.05 mm, the pitch of arrangement of the first through fourth lenses (hereinafter, lens arrangement pitch)=0.7 mm, the lens diameter of the first lenses24a=0.6 mm, the lens diameter of the second lenses24b=0.4 mm, the lens diameter of the third lenses26a=0.35 mm, the lens diameter of the fourth lenses26b=0.6 mm, the gap between the first lens array plate24and the second lens array plate26(hereinafter, gap)=0.8 mm, the refractive index of the first and second lens array plates24and26=1.53, the height of the first surface light-shielding wall30=0.7 mm, the height of the fourth surface light-shielding wall32=0.7 mm, the height of the intermediate light-shielding wall512=0.80 mm, the diameter of the opening of the first surface light-shielding wall30facing the document=0.45 mm, the diameter of the opening of the first surface light-shielding wall30at the first surface=0.51 mm, the diameter of the opening of the fourth surface light-shielding wall32facing the image plane=0.45 mm, the diameter of the opening of the fourth surface light-shielding wall32at the fourth surface=0.51 mm, the diameter of the opening of the intermediate light-shielding wall512at the second surface=0.65 mm, and the diameter of the opening of the intermediate light-shielding wall512at the third surface=0.35 mm.

FIG. 20shows a result of simulation in the comparative exemplary embodiment, the first exemplary embodiment, and the second exemplary embodiment. The shape of the V grooves of the first and second exemplary embodiments is varied as in (1)-(4) below. The noise ratio is computed in the case where the total width of the V grooves in the sub-scanning direction W1=0.51 mm and in the case where W1=200 mm.

In order to demonstrate improvement in noise ratio, the noise ratio is computed in the case where no V grooves are formed in the first and second exemplary embodiments.

The simulation result in the first exemplary embodiment shows that the noise ratio occurring when no V grooves are formed is as high as 38.57%. In contrast, the noise ratio occurring when V grooves are formed is dramatically reduced in all of the cases (1)-(4), both when the total width of the V grooves in the sub-scanning direction W1=0.51 mm and when W1=200 mm. In particular, the noise ratio in the conditions (1), (3), and (4) is lower than the noise ratio=1.07% in the comparative exemplary embodiment, in which an intermediate light-shielding wall is provided. The simulation result shows that the plurality of V grooves formed in the second surface inter-lens area are useful to reduce ghost noise.

The simulation result in the second exemplary embodiment shows that the noise ratio occurring when no V grooves are formed is as high as 97.14%. In contrast, the noise ratio occurring when V grooves are formed is dramatically reduced in all of the cases (1)-(4), both when the total width of the V grooves in the sub-scanning direction W1=0.51 mm and when W1=200 mm. In particular, in the case where the total width of the V grooves in the sub-scanning direction W1=0.51 mm, the noise ratio in all of the conditions (1)-(4) is lower than the noise ratio=1.07% in the comparative exemplary embodiment in which an intermediate light-shielding wall is provided. The simulation results show that, by forming a plurality of V grooves in the second surface inter-lens area, ghost noise is reduced without providing the fourth surface light-shielding wall. In the case where the total width of the V grooves in the sub-scanning direction W1=200 mm, the noise ratio is increased in the conditions (1) and (3) and exceeds 1.07% (i.e., the ratios are 3.53% and 2.65%, respectively). This shows that it is desirable that the total width of the V grooves in the sub-scanning direction W1be equal to the aperture size D1of the first lenses.

FIG. 21shows an image writing device200according to another embodiment of the present invention. As shown inFIG. 21, the image writing device200comprises an LED array206comprising an array of a plurality of LED's, a substrate204on which the LED array206is mounted, a control unit202configured to control the LED array206, the aforementioned erecting equal-magnification lens array plate11for condensing light emitted from the LED array206, a photosensitive drum208for receiving the light transmitted through the erecting equal-magnification lens array plate11, and a housing210for accommodating the components. InFIG. 21, the developer device, the transferring device, etc. provided around the photosensitive drum208are omitted from the illustration. The explanation given above of the image reading device100also applies to the image writing device by replacing the document G of the image reading device100shown inFIG. 1by the photosensitive drum208in the image writing device200and further replacing the linear image sensor20of the image reading device100by the LED array206in the image writing device200.

The image writing device200is provided with an LED print head which uses LED's as light sources. When an LED print head is used, pixels correspond one to one to light-emitting sources so that no mechanisms for scanning are necessary. Therefore, the size and weight of the device can be reduced as compared with a laser raster output scanner (ROS) system in which a laser light source and a polygon mirror are combined.

In the related art, a rod lens array is used in an erecting equal-magnification lens array plate in a device in which an LED print head is used. By using the erecting equal-magnification lens array plate11according to the present invention, the cost of the image writing device200can be reduced. By using the erecting equal-magnification lens array plate11according to the present invention, a high-quality image in which flare noise is reduced can be formed on the photosensitive drum208.

In the embodiment described, lenses on the respective lens surfaces are arranged in a single row in the main scanning direction. Alternatively, lenses may be arranged in two or more rows in the main scanning direction or arranged in a square array to reduce ghost noise equally.