JP2002365536A - Lens for image reading and image reader having the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lens for reading an image, which maintains a high aperture ratio and has high resolution and a wide angle of view of half-viewing angle of 25 degrees or larger, and to provide an image reader using the same. SOLUTION: The lens for reading the image has four lenses: a first meniscus- shaped lens which has positive refractive power and turns a convex face toward the document surface side, a second lens which has a diaphragm and a lens having two concave surfaces, a third lens having two convex lens surfaces and a fourth meniscus-shaped lens which turns a convex face toward the image surface side in the order starting from the document surface side. When the focal distance of the overall system in a dominant wavelength is fall , the radius of curvature of the lens surface at the image surface side of the first lens is rf , the refractive index in the dominant wavelength of the material of the first lens is nf , the radius of curvature of the lens surface at the document surface side of the second lens is rr , the refractive index in the dominant wavelength of the material of the second lens is nr , the distance between the first lens and the diaphragm on the optical axis is df and the distance between the diaphragm and the second lens on the optical axis is dr , each conditional expression is satisfied.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reading lens and an image reading apparatus having the same, and more particularly to a method for reading image information of a document with high accuracy by using a one-dimensional image sensor such as a CCD. It is suitable for devices such as image scanners, digital copiers and facsimile machines.

[0002]

2. Description of the Related Art In an image reading lens used in an image reading apparatus such as an image scanner, a digital copying machine or a facsimile, image information of a document is read by a CC which is a reading element.
It is necessary to reduce and project with high precision on the D plane to form an image. For this purpose, for example, the aperture efficiency must be 95% or more, and various aberrations including spherical aberration must be sufficiently corrected.

Further, in order to reduce the size of the apparatus main body, it is desired to widen the angle of the image reading lens and reduce the distance between object images. It is also required that the number of lenses be as small as possible for simplification of the entire optical system and cost. Further, it is demanded that chromatic aberration is particularly well corrected in an image reading lens such as a color scanner.

Generally, when the image reading lens is widened, the field curvature, astigmatism, distortion and chromatic aberration of magnification increase in accordance with the angle of view, and it is difficult to obtain sufficient contrast at all angles of view. There is.

[0005] Conventionally, as a lens type of an image reading lens suitable for use in such a wide angle range, for example, a Tessar type, a Topogone type, and a telephoto type are known.

[0006] A Tessar type image reading lens is compact and can correct various aberrations well, but the upper limit of the half angle of view is one.
There is a problem that it is extremely difficult to correct astigmatism at a wide angle of about 8 degrees or more.

As a topogon-type image reading lens, for example, a half field angle of about 20 degrees can be used as proposed in Japanese Patent Application Laid-Open No. 7-294812.
There is a problem that it is difficult to suppress the occurrence of chromatic aberration of magnification.

As a telephoto type image reading lens, Japanese Patent Application Laid-Open No. 2000-2499 previously proposed by the present applicant has been proposed.
No. 13 is known. This publication realizes an image reading lens having a simple configuration with a wide angle of 22 ° or more at a half angle of view.

In addition, in particular, in recent years, the resolution of image reading apparatuses has been increased, and in addition to the above demands, higher resolution has been required.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to provide an image reading lens having a high resolution while maintaining a high aperture ratio while having a simple structure, and an image reading apparatus having the same.

[0011]

According to a first aspect of the present invention, there is provided an image reading lens comprising a meniscus first lens G1 having a positive refractive power with a convex surface facing the original surface, and an aperture stop S in order from the original surface. A second lens G2 with both lens surfaces concave, a third lens G3 with both lens surfaces convex, and a meniscus-shaped fourth lens G4 with the convex surface facing the image surface side. The focal length of the whole system is f _all , and the first lens G1
The radius of curvature of the lens surface on the image plane side r _f, the first lens G1 of
Is the refractive index of the material at the principal wavelength of n _f , and the second lens G2
The curvature radius r _r of the lens surface of the original surface side, the second lens G
The refractive index of the material at the principal wavelength of 2 is n _r , and the first lens G
The distance on the optical axis between 1 and the stop S is d _f , and the stop S and the second stop
When the distance on the optical axis between the lens G2 has a d _r

[0012]

(Equation 2)

It is characterized by satisfying the following conditions.

According to a second aspect of the present invention, in the first aspect, the aperture efficiency of the image reading lens is 95% or more.

According to a third aspect of the present invention, there is provided an image reading apparatus, wherein image information of a document is formed on a reading means surface by using the image reading lens according to the first or second aspect.

[0016]

FIG. 1, FIG. 2, and FIG. 3 are lens sectional views of numerical examples 1, 2, and 3 of an image reading lens according to the present invention, respectively, and FIG. FIG. 4 is a diagram illustrating various aberrations (spherical aberration, astigmatism, distortion, and chromatic aberration of magnification) of Numerical Examples 1, 2, and 3 described below of the image reading lens of the present invention.

In the lens cross-sectional view, the left side is the enlargement side (longer conjugate point) and the document surface P side (side on which the read image is provided) and the right side is the reduction side (short conjugate point).
On the image plane Q side (for example, the side provided with a CCD as a photoelectric conversion element).

LG is an image reading lens, P is a document surface (object surface), on which image information for reading is formed. Q is an image plane on which imaging means such as a CCD and a photosensitive surface are placed. C1 is a contact glass, and C2 is a cover glass.

The image reading lens LG forms a reduced image of the image information on the document surface P on an image pickup means (image plane) Q, and the image information is read by the image pickup means Q.

The image reading lens LG in FIGS. 1, 2 and 3 has a meniscus shape having a positive refractive power (hereinafter abbreviated as "positive") with a convex surface facing the document surface in order from the document surface. The first lens G1, the stop S, the second lens G2 having both lens surfaces concave, the third lens G3 having both lens surfaces convex, and the fourth meniscus lens G4 having the convex surface facing the image surface side.
It is composed of a telephoto type having two lenses.

In the present invention, the focal length of the entire system at the principal wavelength is f _all , the radius of curvature of the lens surface on the image plane side of the first lens G1 is r _f , and the refractive index of the material at the principal wavelength of the first lens G1 is n. _f , the radius of curvature of the lens surface on the original side of the second lens G2 is r _r , the refractive index of the material at the main wavelength of the second lens G2 is n _r , on the optical axis between the first lens G1 and the stop S the distance d _f, and a distance on the optical axis between the aperture stop S and the second lens G2 has a d _r

[0022]

(Equation 3)

The conditions are set so as to satisfy the following conditions.

The dominant wavelength is a wavelength substantially at the center of gravity of the lens in the use environment.
It is desirable to use e-line (wavelength 546 nm).

In this embodiment, the image reading lens LG is composed of a four-group, four-telephoto type lens having a predetermined shape as shown in FIGS. By setting the refractive powers of the lens surfaces R2 and R3 as in the conditional expressions (1) and (2), the aperture efficiency is 100%, the resolution is high, and the wide angle of view is a half angle of view of 25 ° or more. And an image reading lens having excellent optical performance is obtained. It should be noted that even if the aperture efficiency is not 100%, 9
If it is 5% or more, the object of the present invention can be almost achieved.

The image reading lens LG of Numerical Example 1 has a necessary and sufficient F / No (F number) for an image reading apparatus, and various aberrations from on-axis to off-axis as shown in FIG. Sufficient correction is performed, and high imaging performance is obtained.

The image reading lens LG of Numerical Example 2 uses an aspheric surface in addition to the structure of the present invention. This aspheric surface is proposed by the present inventor in Japanese Patent Application Laid-Open No. 2000-171705, has a refractive power that is rotationally asymmetric with respect to the optical axis, and has a lens surface on the original surface side of the fourth lens G4. R
8. As a result, despite the half angle of view reaching 29 °, various aberrations are sufficiently corrected both on and off the axis, achieving a high resolution, and an F-number 5.5 bright image reading lens. are doing.

The image reading lens LG of Numerical Embodiment 3 uses an aspherical surface having a refractive power rotationally symmetric with respect to the optical axis in addition to the aspherical surface used in Numerical Embodiment 2. As a result, a high resolving power is realized and a distortion is reduced, while realizing an extremely wide angle where the half angle of view exceeds 33 °.
In Numerical Example 3, an aspherical surface having a rotationally symmetric refractive power is formed on a lens surface R8 of the fourth lens G4 on the original surface side.
An aspheric surface having rotationally asymmetric refractive power is formed on the lens surface R9 on the image plane side.

Next, an expression representing the shape of the above-mentioned aspherical surface is shown. These shapes have an aspherical surface that has rotationally asymmetric refractive power with respect to the optical axis.The shape of the aspherical surface is the origin at the intersection of the lens surface and the optical axis, the optical axis direction is the x axis, and the axis perpendicular to the optical axis in the main scanning section Is the y-axis, and the axis orthogonal to the optical axis in the sub-scanning section is the z-axis.

[0030]

(Equation 4)

[0031] Here, R is the radius of curvature _{k y, B 4, B 6} , B 8, B 10 is represented by the aspheric coefficients becomes equation.

The sagittal shape S is a curve having a plane perpendicular to the generatrix as a cross section on the generatrix and having a center of curvature of a radius r 'in an xy plane.

[0033]

(Equation 5)

Where r ₀ is the sagittal radius of curvature on the optical axis and R =
r ₀ D ₂ , D ₄ , D ₆ , D ₈ , and D ₁₀ are represented by an aspheric coefficient.

The shape of the aspherical surface having a rotationally symmetric refractive power is

[0036]

(Equation 6)

[0037] Here, R is the radius of curvature _{k y, B 4, B 6} , B 8, B 10 is represented by the aspheric coefficients becomes equation.

Next, the technical meaning of each of the conditional expressions (1) and (2) of the present invention will be described.

Conditional expression (1) represents the lens surface R before and after the stop S.
2, the refractive power of R3, and the conditional expression (2) relates to the position of the stop S.

If any one of the numerical ranges of the conditional expressions (1) and (2) is out of the range, it is difficult to correct higher-order off-axis aberrations and the resolution is reduced. In particular, as for conditional expression (2), if the lower limit is exceeded, the edge thickness of the first lens G1 cannot be sufficiently obtained, and the aperture efficiency may decrease. If the upper limit is exceeded, the aperture of the fourth lens G4 increases, Not only is it difficult to maintain the opening efficiency of 100%, but also the cost may increase, which is not good.

In this embodiment, the refractive power of the lens surfaces R2 and R3 before and after the stop S and the position of the stop S are set so as to satisfy the conditional expressions (1) and (2). The generation is easily canceled out, and a high resolution is obtained even at a high angle of view.

In the present invention, it is more desirable to set the above conditional expressions (1) and (2) as follows.

[0043]

(Equation 7)

In the present invention, the image reading apparatus having the high resolution is downsized by forming an image reading apparatus using the image reading lens.

Next, numerical examples of the present invention will be described. In the numerical examples, i indicates the order from the original side, Ri is the radius of curvature of the i-th surface, Di is the i-th and i + 1-th optical material thickness and air gap, and Ndi and νdi are the i-th surface. Are the refractive index and Abbe number of the optical material. Table 1 shows the relationship between the above-mentioned conditional expressions and various numerical values in the numerical examples.
It is shown in FIG.

[0046]

[Table 1]

[0047]

[Table 2]

[0048]

[Table 3]

[0049]

[Table 4]

FIG. 7 shows Embodiment 1 in which any one of the image reading lenses of Numerical Examples 1, 2, and 3 of the present invention is used in an image reading apparatus (image scanner) having a 1: 2 scanning optical system. It is a principal part schematic diagram.

In the figure, an original loading table 72 made of transparent glass is arranged at an upper open portion of the housing 80.
An original 71 is placed on the original 71 and a pressure plate 81 is placed on the original 71 from above.
, The original 71 is fixed. First, a xenon lamp 74 as a light source emits light to illuminate the surface of the document 71.

The xenon lamp 74 is provided with a light emitting window called an aperture on the side of the tube, and the direction and shape of the xenon lamp 74 are set so that a light beam is emitted from the aperture toward the original 71. A reflecting member (reflecting shade) 73 is arranged around the xenon lamp in order to make illumination more efficient and improve image quality.
The light flux that does not directly go to the reading position 1 is picked up to increase the illumination light rate. An appropriate shape is set as the reflection member 73, mainly by making the metal member a mirror surface. These are integrally formed as a first mirror base 82, and are arranged in the direction of arrow C (sub-scanning direction) in the figure along the surface of the original 71.
Is scanned.

The light beam illuminating the original 71 is reflected by the original 71 and travels to the first mirror 75. The reflected light contains image information of the original 71, and a second mirror 76 for guiding this light flux to the photoelectric conversion element 79 as reading means.
A third mirror 77 is provided. The second mirror 76 and the third mirror 77 are also an integral structure (C-shaped mirror)
And is constituted by a second mirror base 83.

When scanning (reading) a document, the first mirror table 82 scans the document 71 by a motor (not shown), and the second mirror table 83 follows at half the speed. With this structure, the optical distance between the original 71 being scanned and the image reading lens (imaging lens) 78 is maintained. Thus, the image reading lens 78
As a result, the light beam from the document 71 is imaged on the surface of the photoelectric conversion element 79 arranged in a conjugate relationship.

The photoelectric conversion element 79 is constituted by a one-dimensional line sensor (CCD) or the like in which a plurality of elements are arranged in the main scanning direction, and its position on the original surface is defined along the main scanning direction. As a result, the main scanning direction of the original 71 placed on the original loading table 72 is read once, and the first and second originals are sequentially read.
The mirror tables 82 and 83 scan the original 71 in the sub-scanning direction.

In this embodiment, the imaging lens of the present invention is applied to an image reading apparatus having a 1: 2 scanning optical system.
The present invention is not limited to this, and can be applied to the integrated (flatbed) image reading apparatus shown in FIG.

In FIG. 8, the light beam radiated from the illuminating means 4 illuminates the original 1 directly or via the reflector 3, and the reflected light beam from the original 1 is reflected by first, second, third and fourth reflecting mirrors. The optical path is bent inside the carriage 11 through 5, 6, 7, and 8, and the one-dimensional CC is
An image is formed on the surface of a linear image sensor 10 (hereinafter, referred to as “CCD”) such as D. The image information of the document 1 is read by moving the carriage 11 in the direction of arrow C (sub-scanning direction) shown in the figure by a sub-scanning motor (not shown). The CCD 10 in FIG. 1 has a configuration in which a plurality of light receiving elements are arranged in a one-dimensional direction (main scanning direction).

In this embodiment, the image reading lens of the present invention is applied to an image scanner. However, the present invention is not limited to this, and can be applied to devices such as a digital copying machine and a facsimile.

[0059]

According to the present invention, as described above, the lens configuration of each lens group of the four-photograph type telephoto type image reading lens is appropriately set and each conditional expression is satisfied, so that a high aperture ratio is obtained. , And a high-resolution image reading lens having a wide angle of view of 25 ° or more at a half angle of view can be achieved.

Further, by forming an image reading apparatus using this image reading lens, it is possible to achieve a reduction in the size of the image reading apparatus having a high resolution.

[Brief description of the drawings]

FIG. 1 is a sectional view of a lens according to a numerical example 1 of the present invention.

FIG. 2 is a sectional view of a lens according to a numerical example 2 of the present invention.

FIG. 3 is a sectional view of a lens according to a numerical example 3 of the present invention.

FIG. 4 is a diagram illustrating various aberrations of Numerical Example 1 of the present invention.

FIG. 5 is a diagram illustrating various aberrations of Numerical Example 2 of the present invention.

FIG. 6 is a diagram showing various aberrations of Numerical Example 3 of the present invention.

FIG. 7 is a schematic diagram of a main part when the imaging lens of the present invention is applied to an image reading apparatus.

FIG. 8 is a schematic diagram of a main part when the imaging lens of the present invention is applied to an image reading device.

[Explanation of symbols]

 LG Image reading lens G1 First lens G2 Second lens G3 Third lens G4 Fourth lens C1 Contact glass C2 Cover glass S Aperture OB Document surface IP Image surface 71 Document 72 Document loading table 73 Reflecting member 74 Light source 75 First mirror 76 Second mirror 77 Third mirror 78 Imaging lens 79 Photoelectric conversion element 80 Housing 81 Pressure plate 82 First mirror base 83 Second mirror base

Continued on the front page F term (reference) 2H087 KA08 KA18 KA19 LA01 PA04 PA17 PB04 QA02 QA07 QA12 QA22 QA25 QA37 QA41 QA46 RA05 RA06 RA13 RA32 RA42 RA44 5B047 AA01 BB02 BC05 5C051 AA01 BA03 DA03 DB01 DC01

Claims (3)

[Claims] 1. A meniscus-shaped first lens G1 having a positive refractive power with a convex surface facing the original surface side, an aperture S, a second lens G2 having both concave surfaces, and both lens surfaces being sequentially arranged from the original surface side. It has four lenses, a convex third lens G3 and a meniscus fourth lens G4 with the convex surface facing the image surface side. The focal length of the entire system at the main wavelength is f _all , and the first lens G
The radius of curvature of the lens surface on the image plane side of the first lens is _rf , and the first lens G is
The refractive index of the material at the main wavelength of 1 is n _f , and the second lens G
The radius of curvature of the lens surface on the original surface side of No. 2 is r _r , the refractive index of the material at the main wavelength of the second lens G2 is n _r , and the distance on the optical axis between the first lens G1 and the stop S is d _f , Equation 1] when the distance on the optical axis was d _r between the stop S and the second lens G2 An image reading lens which satisfies the following conditions. 2. The aperture efficiency of the image reading lens is 95.
2. The image reading lens according to claim 1, wherein the ratio is not less than%. 3. An image reading apparatus, wherein image information of a document is formed on a reading means surface by using the image reading lens according to claim 1 or 2.