JP2006113087A - Reading lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a reading lens whose performance degradation caused by the change of ambient temperature is little though it includes a plastic lens, and to provide an image scanner using the same. <P>SOLUTION: The reading lens OP forming the image of an original is equipped with a front group GrF, a diaphragm ST and a rear group GrR in order from an original side. The front group GrF is constituted of two or more lenses and the rear group GrR is constituted of two or more lenses, and the front group GrF and the rear group GrR are respectively have at least one plastic lens. The reading lens satisfies a conditional expression: 1<¾Fn/Fp¾<1.7 (Fp: the sum of the refractive power of the plastic lens having positive refractive power and Fn: the sum of the refractive power of the plastic lens having negative refractive power). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reading lens and an image reading apparatus. For example, an image reading apparatus (especially an image used for a digital copying machine, a facsimile, a scanner, etc.) having a one-dimensional image sensor such as a line CCD (Charge Coupled Device). Reading device) and a reading lens suitable for the reading device.

Conventionally known reading lenses are often made of glass. For this reason, in order to achieve good aberration correction with a brightness of about F5, about six lenses are required, and there is a limit to cost reduction in this respect. If glass is replaced with plastic and the lens surface is made aspherical, it is possible to reduce the number of sheets and reduce costs. However, a plastic lens has a larger linear expansion coefficient than a glass lens, and a refractive index change due to a temperature change is large. Therefore, when the environmental temperature changes, the curvature of field increases and the performance deteriorates. Therefore, when a plastic lens is used, it is necessary to compensate for performance changes due to temperature changes. Examples of the reading lens considering temperature compensation include those described in Patent Documents 1 and 2.
JP 2002-314767 A JP 9-43508 A

  However, the reading lens described in Patent Document 1 uses only one plastic lens, and the reduction in field curvature fluctuation is insufficient. In the reading lens described in Patent Document 2, three plastic lenses are used. However, only the back focus change accompanying the temperature change is mentioned, and the reduction of the field curvature fluctuation is insufficient.

  The present invention has been made in view of such a situation, and an object of the present invention is to provide a reading lens that includes a plastic lens and has little performance deterioration due to a change in environmental temperature, and an image reading apparatus using the reading lens. is there.

In order to achieve the above object, a reading lens according to a first aspect of the present invention is a reading lens for forming an image of a document, and includes a front group, a diaphragm, and a rear group in order from the document side. The rear group is composed of two or more lenses, each of the front group and the rear group has at least one plastic lens, and satisfies the following conditional expression (1) It is characterized by that.
1 <| Fn / Fp | <1.7 (1)
However,
Fp: Sum of refractive powers of plastic lenses having positive refractive power,
Fn: Sum of refractive powers of plastic lenses having negative refractive power,
It is.

  In the reading lens of the second invention, in the first invention, the front group is composed of two lenses, a positive lens made of a plastic material and a negative lens made of a glass material, and the rear group is made of a glass material. The positive lens comprises a negative lens made of a plastic material, and a negative lens made of a plastic material.

  In the reading lens of the third invention, in the first invention, the front group is composed of two lenses, a positive lens made of a plastic material and a negative lens made of a plastic material, and the rear group is made of a glass material. The positive lens is composed of a positive lens and a negative lens made of a plastic material.

  An image reading apparatus according to a fourth aspect of the present invention is an illumination device that illuminates a document, and a reading lens according to any one of the first to third aspects that forms an image of the document illuminated by the illumination device, And a one-dimensional image sensor that converts an optical image formed by the reading lens into an electrical signal.

  According to the present invention, it is possible to realize a reading lens that includes a plastic lens and has little performance deterioration due to a change in environmental temperature, and an image reading apparatus using the reading lens.

  Hereinafter, a reading lens embodying the present invention and an image reading apparatus using the same will be described with reference to the drawings. 1 and 3 show optical configurations of the reading lens OP according to the first and second embodiments, respectively, in optical cross sections. FIG. 5 shows a schematic configuration of an image reading apparatus equipped with the reading lens OP shown in FIGS. In FIG. 1 and FIG. 3, the surface with Si (i = 1, 2, 3,...) Is the i-th surface counted from the object side, and the surface with Si marked with *. Is an axisymmetric aspheric surface, and the surface marked with $ is a free-form surface.

  The image reading apparatus shown in FIG. 5 is an optical apparatus for image reading used in devices such as a digital copying machine, a facsimile machine, and a scanner, and illuminates IL that illuminates a document (that is, object) GK, and the illuminating apparatus IL. A reading lens OP that forms an image of the original GK illuminated with a one-dimensional image sensor SR (for example, a line CCD) that converts an optical image formed by the reading lens OP into an electrical signal. ing. When the original GK on the original platen glass P1 is illuminated for a long time in the main scanning direction by the illumination device IL, the reflected light is bent in the optical path by a plurality of plane mirrors, and the sub-scanning direction (main scanning direction and By moving in the direction perpendicular to the surface normal direction), the entire surface of the original image is guided to the reading lens OP. The image of the original GK illuminated by the illumination device IL is formed on the one-dimensional image sensor SR by the reading lens OP. Then, the optical image IM formed by the reading lens OP is converted into an electrical signal by the one-dimensional image sensor SR. Note that OB is an object surface on which an image of the original GK is formed, and IM is an image surface formed on the light receiving surface (image reading unit) of the one-dimensional image sensor SR.

  As shown in FIGS. 1 and 3, the reading lens OP includes a front group GrF, a stop ST, and a rear group GrR in order from the original GK side. An original table glass P1 is positioned on the object side of the reading lens OP, and a cover glass P2 of an image sensor SR (for example, a line CCD) is positioned on the image side of the reading lens OP. In the first embodiment (FIG. 1), the front group GrF is composed of a positive lens L1 made of a plastic material and a negative lens L2 made of a glass material, and the rear group GrR is a positive lens made of a glass material. The lens L3 includes a negative lens L4 made of a plastic material and a negative lens L5 made of a plastic material. In the second embodiment (FIG. 3), the front group GrF is composed of a positive lens L1 made of a plastic material and a negative lens L2 made of a plastic material, and the rear group GrR is a positive lens made of a glass material. It consists of two lenses, a lens L3 and a negative lens L4 made of a plastic material.

As in each of the embodiments, a front group, a diaphragm, and a rear group are provided in order from the document side, the front group includes two or more lenses, and the rear group includes two or more lenses. In a reading lens in which the rear group has at least one plastic lens, it is desirable that the following conditional expression (1) is satisfied.
1 <| Fn / Fp | <1.7 (1)
However,
Fp: Sum of refractive powers of plastic lenses having positive refractive power,
Fn: Sum of refractive powers of plastic lenses having negative refractive power,
It is.

  As described above, the plastic lens has a larger linear expansion coefficient than the glass lens, and also has a large refractive index change due to temperature change. For this reason, when the environmental temperature changes, the curvature of field increases, causing performance degradation. Therefore, when a plastic lens is used as the reading lens, it is important to suppress performance deterioration due to a temperature change. Conditional expression (1) defines a preferable condition range for temperature compensation of a reading lens including a plastic lens. By satisfying conditional expression (1), it becomes possible to reduce the field curvature fluctuation at the time of temperature change while using two or more plastic lenses. Therefore, if a reading lens satisfying conditional expression (1) is used in the image reading apparatus, the image reading apparatus can be reduced in weight, size, cost, and performance. Below the lower limit of conditional expression (1), the field curvature fluctuation in the minus direction (the direction in which the image plane approaches the lens) increases. On the contrary, if the upper limit of conditional expression (1) is exceeded, the field curvature fluctuation in the plus direction (the direction in which the image plane moves away from the lens) increases.

  As described above, satisfying conditional expression (1) makes it possible to reduce performance deterioration due to environmental temperature changes even in a reading lens including a plastic lens. In particular, satisfying conditional expression (1) in the lens configuration as in the first and second embodiments is more preferable in order to achieve a good balance between the effect of using a plastic lens and optical performance. . That is, as in the first embodiment, the front group is composed of two lenses, a positive lens made of plastic material and a negative lens made of glass material, and the rear group is made of glass material and positive lens and plastic material. It is preferable that the negative lens made of a plastic lens and a negative lens made of a plastic material are used. Further, as in the second embodiment, the front group is composed of a positive lens made of a plastic material and a negative lens made of a plastic material, and the rear group is made of a glass material and the plastic lens. It is preferable that the lens is composed of two lenses including a negative lens made of

  Hereinafter, the reading lens embodying the present invention will be described more specifically with reference to construction data and the like. Examples 1 and 2 listed here are numerical examples corresponding to the first and second embodiments, respectively, and are lens configuration diagrams showing the first and second embodiments (FIGS. 1 and 2). 3) shows the lens configuration, optical path, and the like of the corresponding first and second embodiments.

  Tables 1 and 2 show the construction data of Examples 1 and 2, respectively. In the basic optical configuration (i: surface number) from the object surface OB to the image surface IM in Tables 1 and 2, Si (i = 0, 1, 2, 3,...) Is counted from the object side. The i-th surface, ri (i = 0, 1, 2, 3,...) Is the radius of curvature of the surface Si (unit: mm), di (i = 0, 1, 2, 3,...) Indicates the axial upper surface distance (unit: mm) between the i-th surface Si and the (i + 1) -th surface Si + 1 counted from the object side, and Ni (i = 1, 2, 3,...) , νi (i = 1, 2, 3,...) respectively indicate a refractive index (Nd) and an Abbe number (νd) with respect to the d-line of the optical material positioned at the axial upper surface distance di. In Tables 1 and 2, f, FNO, and m respectively indicate the focal length (unit: mm), F-number, and magnification of the entire system, and | Fn / Fp | is the corresponding value of conditional expression (1). Is shown.

In Tables 1 and 2, the surface Si marked with * is an axisymmetric aspherical surface (aspherical refractive optical surface, surface having a refractive action equivalent to an aspherical surface, etc.), and the surface vertex is the origin. It is defined by the following equation (AS) using a local orthogonal coordinate system (X, Y, Z). Tables 1 and 2 show the aspheric surface data of each example. However, the coefficient of the term without description is 0, and E−n = × 10 ⁻ⁿ for all data.

…(AS)

… (AS)

However, in the formula (AS)
X: Displacement amount in the optical axis AX direction at the position of height h (h ² = Y ² + Z ² ) (based on the surface vertex),
Y: Y coordinate of the height from the optical axis AX,
Z: Z coordinate of height from optical axis AX,
c: curvature at the surface vertex,
k: conic constant,
A _i : aspheric coefficient,
It is.

In Tables 1 and 2, the surface Si marked with $ is a free-form surface, and the following formula (FS) using a local orthogonal coordinate system (X, Y, Z) with the surface vertex as the origin: Defined. Tables 1 and 2 show the free-form surface data of each example. However, the coefficient of the term without description is 0, and E−n = × 10 ⁻ⁿ for all data.

…(FS)

… (FS)

However, in the formula (FS)
X: Displacement amount in the optical axis AX direction at the position of height h (h ² = Y ² + Z ² ) (based on the surface vertex),
Y: Y coordinate of the height from the optical axis AX,
Z: Z coordinate of height from optical axis AX,
c: curvature at the surface vertex,
k: conic constant,
A _ij : free-form surface coefficient,
It is.

  2 and 4 are aberration diagrams of Examples 1 and 2. (A) is a spherical aberration diagram (LONGITUDINAL SPHERICAL ABER.), (B) is an astigmatism diagram (ASTIGMATIC FIELD CURVES), and (C) is distortion. It is an aberration diagram (DISTORTION). The spherical aberration diagram (A) shows the amount of spherical aberration with respect to the d-line (wavelength 587.56 nm) indicated by a broken line, the amount of spherical aberration with respect to the C-line (wavelength 656.28 nm) indicated by a solid line, and the g-line (wavelength 435.84 nm) indicated by a one-dot chain line. The amount of spherical aberration is expressed as the amount of deviation in the optical axis AX direction from the paraxial image plane (unit: mm, horizontal scale: -0.20 to 0.20 mm), and the vertical axis indicates the incident height to the pupil. The value normalized by the maximum height (that is, the relative pupil height) is represented. In the astigmatism diagram (B), two-dot chain lines Y1, Y2, and Y3 are tangential image planes for C line, d line, and g line, and solid line X1, broken line X2, and one-dot chain line X3 are C line, d line, and g line. The sagittal image plane is expressed by the amount of deviation in the optical axis AX direction from the paraxial image plane (unit: mm, horizontal axis scale: -0.20 to 0.20 mm), and the vertical axis is the object height (OBJ HT, unit: mm, vertical scale: 0 to -150.00 mm). In the distortion diagram (C), the horizontal axis represents the distortion with respect to the d-line (unit:%, horizontal axis scale: -0.100 to 0.100%), and the vertical axis represents the object height (OBJ HT, unit: mm, vertical axis). Scale: 0 to -150.00 mm).

The lens block diagram of 1st Embodiment (Example 1). FIG. 6 is an aberration diagram of Example 1. The lens block diagram of 2nd Embodiment (Example 2). FIG. 6 is an aberration diagram of Example 2. 1 is a schematic diagram schematically showing an overall configuration of an image reading apparatus.

Explanation of symbols

P1 Document glass P2 Cover glass OP Reading lens GrF Front group GrR Rear group L1 1st lens L2 2nd lens L3 3rd lens L4 4th lens L5 5th lens GK Document ST Aperture OB Object surface IM Image surface AX Optical axis

Claims (4)

A reading lens for forming an image of a document, comprising a front group, an aperture, and a rear group in order from the document side, wherein the front group is composed of two or more lenses, and the rear group is composed of two or more lenses A reading lens, wherein each of the front group and the rear group has at least one plastic lens, and satisfies the following conditional expression (1):
1 <| Fn / Fp | <1.7 (1)
However,
Fp: Sum of refractive powers of plastic lenses having positive refractive power,
Fn: Sum of refractive powers of plastic lenses having negative refractive power,
It is.   The front group is composed of a positive lens made of plastic material and a negative lens made of glass material, and the rear group is made up of a positive lens made of glass material, a negative lens made of plastic material, and a negative lens made of plastic material. The reading lens according to claim 1, wherein the reading lens includes three lenses.   The front group consists of two lenses, a positive lens made of plastic material and a negative lens made of plastic material, and the rear group consists of two lenses, a positive lens made of glass material and a negative lens made of plastic material. The reading lens according to claim 1, wherein the reading lens is provided.   An illumination device that illuminates the original, and an image of the original illuminated by the illumination device, and the optical image formed by the read lens according to any one of claims 1 to 3 An image reading apparatus comprising: a one-dimensional image pickup device that converts into a typical signal.

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