JP2005266771A - Image reading lens and image reader - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make chromatic aberrations satisfactory, while attaining wide field angle by employing four-group, five-element constitution as the lens constitution, and to obtain optical performance which is suitable for a read of a color image, while making the constitution small-sized and inexpensive. <P>SOLUTION: The lens comprises, in the order starting from the object side, a negative meniscus lens L<SB>1</SB>which is a concave surface to the image side, a positive meniscus lens L<SB>2</SB>which is a convex surface to the object side, a stop 1, a biconcave lens L<SB>3</SB>, a biconvex lens L<SB>4</SB>, and a negative meniscus lens L<SB>5</SB>which is concave to the object side, L<SB>3</SB>and L<SB>4</SB>cemented together. Further, five conditional expressions prescribing respective values r<SB>1</SB>/f, f<SB>2</SB>/.f, f<SB>3</SB>/f, r<SB>8</SB>/r<SB>9</SB>, and θ<SB>gd</SB>(f: focal length of whole system, f<SB>2</SB>: focal length of L<SB>2</SB>, f<SB>3</SB>: composite focal length of L<SB>3</SB>and L<SB>4</SB>, r<SB>1</SB>: radius of curvature of the object-side surface of L<SB>1</SB>, r<SB>8</SB>: radius of curvature of the image-side surface, r<SB>9</SB>: radius of curvature of the object-side surface, abnormality dispersion θ<SB>gd</SB>=(N<SB>g</SB>-N<SB>d</SB>)/(N<SB>F</SB>-N<SB>C</SB>) of a constitution material of L<SB>2</SB>), are satisfied. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reading lens and an image reading apparatus for reading an image on a document, such as a facsimile or an image scanner, and more particularly to an image reading of a four-group five-lens configuration suitable for reading a color image. And an image reading apparatus using the same.

  2. Description of the Related Art Conventionally, there has been known an image reading apparatus in which an image on a document is formed on a light receiving element such as a CCD via an optical system and image information is read. In recent years, such image reading apparatuses, particularly image scanners, have been required to be downsized and inexpensive, and accordingly, image reading lenses used in these apparatuses have also been downsized and reduced in price. Etc. are demanded.

  Conventionally, various types of image reading lenses have been known. However, for example, those described in Patent Documents 1 and 2 below are known as being composed of four groups and five lenses. These are configured to be able to be reduced in size and cost.

JP-A-9-127414 Japanese Patent Laid-Open No. 5-264900

  In recent years, in image reading apparatuses such as facsimiles and image scanners, there is an urgent need to cope with high resolution, and the number of light receiving elements such as CCDs tends to increase significantly. As the number of pixels increases, the overall size of the light-receiving element increases, and the imaging magnification approaches the same magnification from the reduction side. However, as described above, there is a proposition of downsizing the apparatus. It is indispensable to use a wide-angle lens.

  Further, since there is an increasing demand for higher definition in reading color images, it is strongly desired to provide an image reading lens that can improve chromatic aberration correction.

  However, the one described in Patent Document 1 has a narrow half angle of view of 18 to 20 degrees and does not meet the demand for the wide angle of view.

  On the other hand, the one described in Patent Document 2 has a relatively wide angle of view of 23.5 degrees, but is sufficient for correcting chromatic aberration of magnification because it is intended for reading monochrome images. It is not.

  The present invention has been made in view of such circumstances, and can achieve a good chromatic aberration while achieving a wide angle of view, and is suitable for reading a color image while achieving miniaturization and cost reduction. It is an object of the present invention to provide an image reading lens and an image reading apparatus capable of obtaining optical performance.

  The image reading lens of the present invention, in order from the object side, a first lens composed of a negative meniscus lens having a concave surface facing the image side, a second lens composed of a positive meniscus lens having a convex surface directed to the object side, It comprises a stop, a third lens made of a biconcave lens, a fourth lens made of a biconvex lens, and a fifth lens made of a negative meniscus lens having a concave surface facing the object side, and the third lens and the fourth lens are It is characterized by being joined together.

Moreover, it is preferable that the following conditional expressions (1) to (4) are satisfied.

_{0.22 <r 1 / f <0.28} ··· (1)
_{0.40 <f 2 / f <0.51} ··· (2)
0.33 <f ₃ /f<0.42 (3)
1.14 <r ₈ / r ₉ <1.31 (4)

However,
f : Focal length of entire lens system f ₂ : focal length of second lens f ₃ : combined focal length of third lens and fourth lens r ₁ : radius of curvature of object side surface of first lens r ₈ : fourth lens Radius of curvature of the image side surface of the lens r ₉ : radius of curvature of the object side surface of the fifth lens

Furthermore, it is preferable that the material forming the second lens satisfies the following conditional expression (5).

θ _gd > −2.083 × 10 ⁻³ × ν _d +1.366 (5)

However,
θ _gd = (N _g −N _d ) / (N _F −N _C )
N _g : Refractive index in g line N _d : Refractive index in d line N _F : Refractive index in F line N _C : Refractive index in C line ν _d : Abbe number in d line

  Furthermore, the image reading apparatus of the present invention is characterized by using the above-described image reading lens.

  According to the image reading lens of the present invention and the image reading apparatus using the same, the chromatic aberration can be improved while achieving a wide angle of view by forming each lens of the four groups and five elements into a predetermined shape. In addition, optical performance suitable for reading a color image can be obtained while further reducing the size and cost.

  Furthermore, the above-mentioned effect can be made better by satisfying a predetermined conditional expression.

  Hereinafter, an image reading lens and an image reading device including the image reading lens according to an embodiment of the present invention will be described with reference to the drawings.

  As shown in FIG. 6, the image reading lens 10 according to the present embodiment is used as an image reading optical system of an image reading apparatus 20 such as an image scanner. The image reading device 20 includes the image reading lens 10 between a glass plate 4 on which an original 3 is placed and a cover glass 6 of a line-shaped CCD 5 in which CCD elements are arranged in one to several lines. Further, the illumination device 7 is arranged on the glass reading plate 10 side of the glass plate 4. Then, the image on the document 3 is read by moving the document 3 in the direction of the arrow A orthogonal to the optical axis of the image reading lens 10 and the arrangement direction of the line CCDs 5.

  FIG. 1 shows the basic lens configuration of Example 1. Here, the lens configuration of this embodiment will be described with the configuration shown in FIG. 1 as a representative example.

The image reading lens according to the present embodiment includes a four-group five-lens lens system including five lenses L _{1 to} L ₅ , and the diaphragm 1 is located between the second lens L ₂ and the third lens L ₃ . The light beam incident along the optical axis X from the object side is imaged at an imaging position on the imaging surface 2.

Here, the five lenses L ₁ ~L _5, in order from the object side, a first lens L ₁ of a negative meniscus lens having a concave surface facing the image side, a positive meniscus having a convex surface directed toward the object side A second lens L _{2 made} of a lens, a stop 1, a third lens L ₃ of a biconcave lens, a fourth lens L ₄ of a biconvex lens, and a fifth lens made of a negative meniscus lens having a concave surface facing the object made L _5, the third lens _{L 3} and the fourth lens _{L 4} constitute a cemented lens.

  The image reading lens of the present embodiment configured as described above can achieve a wide angle of view while achieving good chromatic aberration, and can read a color image while achieving miniaturization and cost reduction. Optical performance suitable for the above can be obtained.

  Note that one reason that the image reading lens of the present embodiment can improve chromatic aberration correction is that the entire lens system has a substantially symmetrical configuration with the aperture 1 interposed therebetween.

Further, the image reading lens according to the present embodiment is configured to satisfy the following conditional expressions (1) to (5).

_{0.22 <r 1 / f <0.28} ··· (1)
_{0.40 <f 2 / f <0.51} ··· (2)
0.33 <f ₃ /f<0.42 (3)
1.14 <r ₈ / r ₉ <1.31 (4)
θ _gd > −2.083 × 10 ⁻³ × ν _d +1.366 (5)

However,
f : Focal length of entire lens system f ₂ : focal length of second lens f ₃ : combined focal length of third lens and fourth lens r ₁ : radius of curvature of object side surface of first lens r ₈ : fourth lens Radius of curvature of the image-side surface of the lens r ₉ : radius of curvature of the object-side surface of the fifth lens θ _gd = (N _g −N _d ) / (N _F −N _C )
N _g : Refractive index in g line N _d : Refractive index in d line N _F : Refractive index in F line N _C : Refractive index in C line ν _d : Abbe number in d line

Hereinafter, the technical significance of each of the conditional expressions (1) to (5) will be described.
Condition (1) relates to the radius of curvature of the first lens L ₁ on the object side, it is possible to improve the correction of distortion and lateral chromatic aberration by satisfying the conditional expression.

  That is, in this conditional expression, if the upper limit is exceeded, distortion cannot be corrected satisfactorily, and if the lower limit is exceeded, lateral chromatic aberration cannot be corrected satisfactorily.

Further, the conditional expression (2) relates to the focal length of the second lens L _2, it is possible to improve the correction of lateral chromatic aberration and the spherical aberration by satisfying the conditional expression.

  That is, in this conditional expression, if the upper limit is exceeded, the lateral chromatic aberration cannot be corrected satisfactorily, and if the lower limit is not reached, spherical aberration cannot be corrected satisfactorily.

Further, the conditional expression (3) relates to the third lens L ₃ combined focal length of the fourth lens L _4, possible to improve the correction of the coma aberration and curvature of field by satisfying the conditional expression Can do.

  That is, in this conditional expression, if the upper limit is exceeded, coma cannot be corrected well, and if the lower limit is not reached, curvature of field cannot be corrected well.

The conditional expression (4) defines the relationship between the curvature radius of the image side surface of the fourth lens L ₄ and the curvature radius of the fifth lens L _{5 on} the object side, and satisfies this conditional expression. Thus, it is possible to improve correction of spherical aberration and field curvature.

  That is, in this conditional expression, if the upper limit is exceeded, spherical aberration cannot be corrected satisfactorily, and if the lower limit is exceeded, field curvature cannot be corrected satisfactorily.

Further, the conditional expression (5) relates to anomalous dispersion material which forms the second lens L _2, by using a material that satisfies this condition as a lens glass material of the second lens L _2, on the axis Chromatic aberration and lateral chromatic aberration can be corrected satisfactorily.

  Hereinafter, the image reading lens of the present embodiment will be described more specifically with reference to the drawings and numerical values using Examples 1 to 4. In each of the following embodiments, the focal length is converted to 100 mm, and actually, the focal length is used under optimum conditions depending on the document size and resolution.

<Example 1>
As shown in FIG. 1, the image reading lens of this example includes a first lens L _{1 formed of} a negative meniscus lens having a concave surface on the image side and a positive surface having a convex surface on the object side. A second lens L _{2 made} of a meniscus lens, a diaphragm 1, a third lens L _{3 made} of a biconcave lens having a strong curvature surface facing the image side, and a biconvex lens having a strong curvature surface facing the image side a fourth lens L _4, which is, and a fifth lens L ₅ formed of a negative meniscus lens having a concave surface on the object side, the third lens L ₃ and the fourth lens L ₄ constitute a cemented lens .

In this embodiment, the radius of curvature R (mm) of each lens surface, the center thickness of each lens and the air gap D (mm) between the lenses, and the refractive index N of each lens at the d-line, g-line, F-line, and C-line. The lower part of Table 1 shows _d , N _g , N _F , N _C and the Abbe number ν _d and the material of each lens. However, in Table 1, the symbols R, D, N _d , N _g , N _F , N _C , ν _d and the numbers corresponding to the materials are sequentially increased from the object side (tables described later). 2-4 is the same).

  Further, in the upper part of Table 1, the focal length f, the F number, the reference wavelength λ, the entire lens system of the entire lens system in the present embodiment (in the corresponding examples for the upper stages of Tables 2 to 4 described later), Values of the magnification β and the angle of view 2ω are shown.

In this example, as shown in Table 5 below, all the conditional expressions (1) to (4) are satisfied. Furthermore, using a material that satisfies conditional expression (5) to the second lens L ₂ as shown in Table 5 below.

  FIG. 2 is an aberration diagram (spherical aberration, astigmatism, distortion, lateral chromatic aberration) showing various aberrations of the image reading lens according to the present embodiment. This aberration diagram is for a focal length of 100 mm, including a glass plate with a thickness of 5.84 mm on the object side and 1.46 mm on the image plane side in the optical path. As is apparent from this figure, according to the present embodiment, an image reading lens having good imaging performance up to the periphery of the field of view can be obtained.

<Example 2>
The image reading lens of this embodiment is configured in substantially the same manner as that of the first embodiment.
In this embodiment, the radius of curvature R (mm) of each lens surface, the center thickness of each lens and the air gap D (mm) between the lenses, and the refractive index N of each lens at the d-line, g-line, F-line, and C-line. _d , N _g , N _F , N _C and the Abbe number ν _d and material of each lens are shown in the lower part of Table 2 below.

In this example, as shown in Table 5 below, all the conditional expressions (1) to (4) are satisfied. Furthermore, using a material that satisfies conditional expression (5) to the second lens L ₂ as shown in Table 5 below.

  FIG. 3 is an aberration diagram (spherical aberration, astigmatism, distortion, lateral chromatic aberration) showing various aberrations of the image reading lens according to this example. This aberration diagram is for a focal length of 100 mm, including a glass plate with a thickness of 5.84 mm on the object side and 1.46 mm on the image plane side in the optical path. As is apparent from this figure, according to the present embodiment, an image reading lens having good imaging performance up to the periphery of the field of view can be obtained.

<Example 3>
The image reading lens of this embodiment is configured in substantially the same manner as that of the first embodiment.
In this embodiment, the radius of curvature R (mm) of each lens surface, the center thickness of each lens and the air gap D (mm) between the lenses, and the refractive index N of each lens at the d-line, g-line, F-line, and C-line. The lower part of Table 3 below shows _{d 1} , N _g , N _F , N _C and the Abbe number ν _d and the material of each lens.

本実施例では、下記表５に示すように、条件式（１）〜（４）を全て満足している。さらに、下記表５に示すように第２レンズＬ_２に条件式（５）を満足する材質を使用している。

In this example, as shown in Table 5 below, all the conditional expressions (1) to (4) are satisfied. Furthermore, using a material that satisfies conditional expression (5) to the second lens L ₂ as shown in Table 5 below.

FIG. 4 is an aberration diagram (spherical aberration, astigmatism, distortion, lateral chromatic aberration) showing various aberrations of the image reading lens according to this example. This aberration diagram shows a state in which a focal length is 100 mm and a glass plate having a thickness of 5.85 mm on the object side and 1.46 mm on the image plane side is included in the optical path. As is apparent from this figure, according to the present embodiment, an image reading lens having good imaging performance up to the periphery of the visual field can be obtained.

<Example 4>
The image reading lens of this embodiment is configured in substantially the same manner as that of the first embodiment.

In this embodiment, the radius of curvature R (mm) of each lens surface, the center thickness of each lens and the air gap D (mm) between the lenses, and the refractive index N of each lens at the d-line, g-line, F-line, and C-line. _d , N _g , N _F , N _C and the Abbe number ν _d and material of each lens are shown in the lower part of Table 4 below.

  In this example, as shown in Table 5 below, all the conditional expressions (1) to (4) are satisfied.

  FIG. 5 is an aberration diagram (spherical aberration, astigmatism, distortion, lateral chromatic aberration) showing various aberrations of the image reading lens according to this example. This aberration diagram shows a state in which a focal length is 100 mm and a glass plate having a thickness of 5.85 mm on the object side and 1.46 mm on the image plane side is included in the optical path. As is apparent from this figure, according to the present embodiment, an image reading lens having good imaging performance up to the periphery of the field of view can be obtained.

Here, Table 5 shows the values of r ₁ / f, | f ₂ | / f, f ₃ / f, r ₈ / r ₉ , and θ _gd corresponding to the conditional expressions in the above embodiments.

The image reading lens and the image reading apparatus according to the present invention are not limited to those in the above-described embodiments, and various modifications can be made. For example, the third lens L ₃ and the fourth lens L can be changed. ₄ can be configured such that a surface with a strong curvature is directed toward the object side, and the curvature radius R of the lens and the axial top surface distance D can be appropriately changed.

1 is a schematic diagram showing a basic configuration of an image reading lens according to Embodiment 1 of the present invention. Each aberration diagram of the lens according to Example 1 (spherical aberration, astigmatism, distortion, lateral chromatic aberration) Each aberration diagram of the lens according to Example 2 (spherical aberration, astigmatism, distortion, lateral chromatic aberration) Each aberration diagram of the lens according to Example 3 (spherical aberration, astigmatism, distortion, lateral chromatic aberration) Each aberration diagram of the lens according to Example 4 (spherical aberration, astigmatism, distortion, lateral chromatic aberration) 1 is a schematic diagram showing an image reading apparatus according to an embodiment of the present invention.

Explanation of symbols

L ₁ ~L ₅ lens R ₁ to R of curvature of ₁₀ the lens surface radius D ₁ to D ₉ lens spacing (lens thickness)
X Optical axis 1 Aperture 2 Imaging surface 3 Original 4 Glass plate 5 Line CCD
6 CCD cover glass 7 Illumination device 10 Image reading lens 20 Image reading device

Claims (4)

In order from the object side, a first lens composed of a negative meniscus lens having a concave surface facing the image side, a second lens composed of a positive meniscus lens having a convex surface facing the object side, a stop, and a third lens composed of a biconcave lens. A lens, a fourth lens composed of a biconvex lens, and a fifth lens composed of a negative meniscus lens having a concave surface facing the object side, wherein the third lens and the fourth lens are cemented with each other. An image reading lens.
2. The image reading lens according to claim 1, wherein the following conditional expressions (1) to (4) are satisfied.
_{0.22 <r 1 / f <0.28} ··· (1)
_{0.40 <f 2 / f <0.51} ··· (2)
0.33 <f ₃ /f<0.42 (3)
1.14 <r ₈ / r ₉ <1.31 (4)
However,
f : Focal length of entire lens system f ₂ : focal length of second lens f ₃ : combined focal length of third lens and fourth lens r ₁ : radius of curvature of object side surface of first lens r ₈ : fourth lens Radius of curvature of the image side surface of the lens r ₉ : radius of curvature of the object side surface of the fifth lens
3. The image reading lens according to claim 1, wherein a material forming the second lens satisfies the following conditional expression (5).
θ _gd > −2.083 × 10 ⁻³ × ν _d +1.366 (5)
However,
θ _gd = (N _g −N _d ) / (N _F −N _C )
N _g : Refractive index in g line N _d : Refractive index in d line N _F : Refractive index in F line N _C : Refractive index in C line ν _d : Abbe number in d line
An image reading apparatus using the image reading lens according to claim 1.

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