JP2015176002A - Image read lens, image reader, and image formation apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image read lens with high performance that is adaptive to smaller pixels with a small number of lenses while using one aspherical lens, and has various aberrations excellently corrected.SOLUTION: An image read lens has a front-group lens system Gf arranged on an object side and a rear-group lens system Gr arranged on an image side, respectively. The front-group lens system Gf comprises four or five glass spherical lenses, including a first meniscus lens E1 convex to the object side and a second biconcave lens E2, arranged in order from the object side. The rear-group lens system Gr is formed of a rear-group lens Gr comprising one plastic aspherical lens in an aspherical shape such that an image-side lens surface is concave to the image side nearby the axis and gradually becomes convex to the image side toward the peripheral part. The lens system satisfies at least a conditional expression of [1] 0°≤θ<30°, where θ is the absolute value of the angle to the optical axis of a main light beam made incident on an imaging element.

Description

  The present invention relates to an image reading lens suitable for capturing an original image in an image reading apparatus such as an image scanner, an image forming apparatus such as a facsimile and a digital copying machine, and an image reading using such an image reading lens. The present invention relates to an apparatus and an image forming apparatus using such an image reading apparatus.

An image reading apparatus used for reading an original image in an image scanner, a facsimile machine, a digital copying machine or the like reduces an image of an original to be read by an image reading lens for reading, and forms the reduced optical image on a CCD. (Charge Coupled Device) Image information is converted into electronic image data by imaging with a solid-state imaging device such as an image sensor.
Conventionally, a so-called Gaussian image reading lens widely used as such an image reading lens can achieve a high resolution with a relatively large aperture. However, in order to achieve high performance by obtaining high image quality with few aberrations with such a Gaussian configuration, a large number of lenses are required.
Corrects axial chromatic aberration in a wide wavelength range with a relatively small number of lenses, corrects field curvature at a wide angle, improves aperture efficiency with a large aperture, and corrects various aberrations. As a small-sized and low-cost reading lens that can obtain high contrast in a high spatial frequency region, for example, there is an image reading lens disclosed in Japanese Patent Application Laid-Open No. 2012-145839.
However, the image reading lens disclosed in Patent Document 1 has a small aspherical amount on the image surface side lens surface, a weak convex surface shape at the peripheral portion, and an incident angle to the image sensor exceeding 50 °. Therefore, in the image reading lens of Patent Document 1, there is a possibility that the diffraction image of the peripheral part will spread. In order to reduce the pixel size of the image sensor, increase the number of pixels, and achieve high image quality, it is important to suppress the spread of the diffraction image in the peripheral portion. In this sense, the image reading lens disclosed in Patent Document 1 does not support the reduction of pixels. Further, when a specific example in Patent Document 1 is viewed, the cost tends to be high, such as using three or more aspheric lenses.

As described above, the image reading lens disclosed in Patent Document 1 has a small amount of aspherical surface on the image surface side lens surface, a weak convex surface shape at the peripheral portion, and an incident angle to the image sensor exceeding 50 °. There is a risk that the diffraction image of the peripheral part will spread. In addition, the image reading lens of Patent Document 1 can sufficiently cope with reducing the pixel size of the image pickup device, increasing the number of pixels, achieving high image quality, and suppressing the spread of the diffraction image in the peripheral portion. Can not. In other words, the image reading lens of Patent Document 1 does not sufficiently cope with the reduction in pixels, and the cost tends to increase.
The present invention has been made in view of the above-described circumstances, and uses a single aspherical lens, and with a small number of lenses equivalent to or less than that of a conventional Gaussian lens, can cope with a reduction in the number of pixels, and has excellent aberrations. An object of the present invention is to provide a high-performance image reading lens in which the above is corrected.

In order to achieve the above-described object, an image reading lens according to the present invention is provided.
An image reading lens for reading a document image formed on a light receiving surface of an image sensor,
Arrange the front lens group on the object side and the rear lens system on the image side.
The front group lens system is any one of four and five lenses including at least a meniscus first lens having a convex surface facing the object side and a second lens composed of a biconcave lens. Consists of one glass spherical lens,
The rear lens group system is composed of a single plastic aspherical lens having an aspherical shape in which the image side lens surface is concave on the image side with a paraxial axis and gradually convex toward the image side toward the periphery. Consists of a rear lens group,
The maximum field angle of the principal ray of e-line incident on the first lens of the front group lens system is 50 ° or more,
The absolute value of the angle of the e-line incident on the image sensor with respect to the optical axis of the principal ray is θ, and the inclination of the normal of the lens surface of the rear lens group to the Y-axis at each height (that is, the normal is the optical axis). _Is 90 °), that is, the inclination of the normal to the Y-axis at the maximum ray effective height of the image side lens surface of the rear group lens is ψ _{R2 — 1.0,} and the image side lens surface of the rear group lens The inclination of the normal to the Y axis at 70% of the maximum effective ray height is ψ _{R2 — 0.7,} and the inclination of the normal to the Y axis of the maximum ray effective height of the object side lens surface of the rear lens group is ψ _{R1 — 1.} and _.0, the inclination and [psi _{R1_0.7} with respect to the normal line of the Y-axis at the maximum effective ray height of 70% of the object-side lens surface of the rear lens group, the maximum light of the image-side lens surface of the rear group lens effective Paired with the normal Y-axis at 30% of the height That the inclination and ψ _{R2_0.3,} the inclination relative to the normal of the Y axis in the optical axis of the image-side lens surface of the rear lens group and ψ _{R2_0.0,} maximum ray effective height of the object side lens surface of the rear lens group The inclination of the normal to the Y axis at 30% is ψ _{R1 — 0.3,} and the inclination of the normal to the Y axis of the optical axis of the object side lens surface of the rear lens group is ψ _{R1 — 0.0} .
Conditional expression:
[1] 0 ° ≤ θ <30 °
[2] _{1 <- {(ψ R2_1.0 -ψ} R2_0.7) - (ψ R1_1.0 -ψ R1_0.7)} / {(ψ R2_0.3 -ψ R2_0.0) - (ψ R1_0.3 −ψ _{R1 — 0.0} )} <7
It is characterized by satisfying both.

According to the present invention, a high-performance image reading that uses a single aspherical lens and corrects various aberrations in a small number of lenses equivalent to or less than that of a conventional Gaussian lens in response to a reduction in the number of pixels. A lens can be provided.
That is, according to the image reading lens of the present invention, an image reading lens for reading an original image formed on the light receiving surface of the image sensor, the front group lens system on the object side and the rear group lens system on the image side And each of the front group lens systems includes at least a meniscus first lens having a convex surface facing the object side in order from the object side and a second lens composed of a biconcave lens. And 5 glass spherical lenses, and the rear lens group system has a concave shape on the image side when the image side lens surface is paraxial, and a convex shape on the image side gradually toward the periphery. It is composed of a rear lens group consisting of a single plastic aspheric lens having an aspheric shape.
The maximum angle of view of the principal ray of e line incident on the first lens of the front group lens system is set to 50 ° or more, and the absolute value of the angle of the principal ray of e line incident on the image sensor with respect to the optical axis is calculated. The inclination of the normal surface of the lens surface of the rear lens group at each height with respect to the Y axis (that is, 90 ° if the normal is parallel to the optical axis) is ψ, that is, the image side lens surface of the rear lens group The slope of the normal to the Y axis at the maximum effective ray height is ψ _{R2 — 1.0,} and the inclination of the normal to the Y axis at 70% of the maximum ray effective height of the image side lens surface of the rear lens group is ψ _{R2 — 0. .7} , the inclination of the normal to the Y-axis of the maximum ray effective height of the object side lens surface of the rear lens group is _{ψR1_1.0,} and the maximum ray effective height of the object side lens surface of the rear lens group is the inclination ψ with respect to the normal line of the Y-axis in 70% And _{1_0.7,} the inclination relative to the normal of the Y-axis in 30% of the maximum effective ray height of the image-side lens surface of the rear lens group and [psi _{R2_0.3,} the optical axis of the image-side lens surface of the rear lens group The inclination of the normal to the Y axis is ψ _{R2 — 0.0} , the inclination of the normal to the Y axis at 30% of the maximum effective ray height of the object side lens surface of the rear lens group is ψ _{R1 — 0.3} , and The inclination of the normal to the Y axis of the optical axis of the object side lens surface of the rear lens group is ψ _{R1 — 0.0} ,
Conditional expression:
[1] 0 ° ≤ θ <30 °
[2] _{1 <- {(ψ R2_1.0 -ψ} R2_0.7) - (ψ R1_1.0 -ψ R1_0.7)} / {(ψ R2_0.3 -ψ R2_0.0) - (ψ R1_0.3 −ψ _{R1 — 0.0} )} <7
By satisfying together,
A single aspherical lens is used, and with a small number of lenses, high performance can be obtained by appropriately correcting various aberrations in response to a reduction in the number of pixels.

It is sectional drawing which shows the structure of the principal part of Example 1 which is an Example of the image reading lens which concerns on the 1st Embodiment of this invention. FIG. 2 is an aberration diagram illustrating spherical aberration, astigmatism, distortion, and coma aberration of the image reading lens in FIG. 1. It is sectional drawing which shows the structure of the principal part of Example 2 which is an Example of the image reading lens which concerns on the 2nd Embodiment of this invention. FIG. 4 is an aberration diagram showing spherical aberration, astigmatism, distortion and coma aberration of the image reading lens in FIG. 3. It is sectional drawing which shows the structure of the principal part of Example 3 which is an Example of the image reading lens which concerns on the 3rd Embodiment of this invention. FIG. 6 is an aberration diagram showing spherical aberration, astigmatism, distortion aberration, and coma aberration of the image reading lens in FIG. 5. It is sectional drawing which shows the structure of the principal part of Example 4 which is an Example of the image reading lens which concerns on the 4th Embodiment of this invention. FIG. 8 is an aberration diagram showing spherical aberration, astigmatism, distortion, and coma aberration of the image reading lens in FIG. 7. It is sectional drawing which shows the structure of the principal part of Example 5 which is an Example of the image reading lens which concerns on the 5th Embodiment of this invention. FIG. 10 is an aberration diagram showing spherical aberration, astigmatism, distortion, and coma aberration of the image reading lens in FIG. 9. It is sectional drawing which shows the structure of the principal part of Example 6 which is an Example of the image reading lens which concerns on the 6th Embodiment of this invention. FIG. 12 is an aberration diagram illustrating spherical aberration, astigmatism, distortion, and coma aberration of the image reading lens in FIG. 11. It is a typical sectional view showing the notional composition of the principal part of the image reading device concerning a 7th embodiment of the present invention. It is a typical sectional view showing the notional composition of the principal part of the image reading device concerning an 8th embodiment of the present invention. It is a typical sectional view showing the notional composition of the principal part of the image forming device concerning a 9th embodiment of the present invention.

Hereinafter, based on an embodiment of the present invention, an image reading lens, an image reading apparatus, and an image forming apparatus according to the present invention will be described in detail with reference to the drawings. Before describing specific embodiments and examples, characteristics of the image reading lens, the image reading apparatus, and the image forming apparatus according to the present invention will be described first.
In the image reading lens according to the present invention, a front group lens system is disposed on the object side, and a rear group lens system is disposed on the image side. The front group lens system includes any one of four and five lenses including at least a meniscus first lens having a convex surface facing the object side and a second lens composed of a biconcave lens in order from the object side. One glass spherical lens is used. The rear group lens system is composed of one plastic aspherical lens having an aspherical shape in which the image side lens surface is concave on the image side in the paraxial direction and gradually convex toward the image side toward the periphery. The maximum field angle of the principal ray of the e-line incident on the first lens of the front group lens system is set to 50 ° or more, and the following conditional expressions [1] and [2] ], A high-performance, low-cost image-reading lens that is small in size, maintains a high MTF (Modulation Transfer Function) in the peripheral portion, and is small enough to correct aberrations satisfactorily. Yes (corresponding to claim 1).

[1] 0 ° ≤ θ <30 °
[2] _{1 <- {(ψ R2_1.0 -ψ} R2_0.7) - (ψ R1_1.0 -ψ R1_0.7)} / {(ψ R2_0.3 -ψ R2_0.0) - (ψ R1_0.3 −ψ _{R1 — 0.0} )} <7
Here, θ represents the absolute value of the angle of the e-line incident on the image sensor with respect to the optical axis of the principal ray, and ψ represents the inclination of the normal of the lens surface of the rear lens group at each height with respect to the Y-axis. (That is, 90 ° if the normal is parallel to the optical axis), that is, ψ _{R2 — 1.0} is the inclination of the normal to the Y axis at the maximum ray effective height of the image side lens surface of the rear group lens, ψ _{R2 — 0 .7} is the inclination of the normal to the Y axis at 70% of the maximum effective ray height of the image side lens surface of the rear lens group, and ψ _{R1 — 1.0} is the maximum effective ray of the object side lens surface of the rear lens group. The inclination of the normal to the Y axis in height, ψ _{R1 — 0.7} is the inclination of the normal to the Y axis in 70% of the maximum ray effective height of the object side lens surface of the rear lens group, and ψ _{R2 — 0.3} is Maximum ray effective height of the image side lens surface of the rear lens group Inclination with respect to the normal line of the Y-axis in 30%, [psi _{R2_0.0} the inclination relative to the normal of the Y axis in the optical axis of the image-side lens surface of the rear lens group, [psi _{R1_0.3} is the rear lens group The inclination of the normal to the Y axis at 30% of the maximum effective ray height of the object side lens surface, and _{ψR1_0.0} is the inclination of the normal to the Y axis of the optical axis of the object side lens surface of the rear lens group. , Respectively.

  Conditional expression [1] defines the incident angle of the light beam from the image reading lens to the image sensor. In order to aim at low cost, it is very effective to reduce the size of the image reading lens. Further, the downsizing of the image reading lens contributes to the downsizing of the image reading device itself. It is effective to use an image sensor having a small pixel size as a means for achieving the downsizing of the image reading lens. However, in order to obtain a high image quality with a small pixel size, it is required to achieve a high MTF in a higher spatial frequency region than in the case of the conventional pixel size. However, when the spatial frequency is increased, the theoretical limit value of the MTF is reduced. In addition, the spread of the off-axis diffraction image is inversely proportional to the cosine cube of the incident angle with respect to the diffraction image on the optical axis on the meridional plane. That is, in order to obtain high image quality with a small-pixel image sensor, it is particularly important to reduce the incident angle with respect to the off-axis image sensor so that the resolution of the peripheral portion is not reduced. Therefore, by arranging the aspherical lens with the shape as described above as the rear group lens, the position where the exit pupil position is moved to the object side while correcting astigmatism, the periphery that enters the off-axis image sensor The incident angle of the principal ray of light can be reduced, and the spread of the diffraction image in the peripheral portion can be suppressed. By setting the specific incident angle within the range defined by the conditional expression [1], it is possible to obtain good imaging performance while keeping the theoretical limit value of the MTF at the peripheral portion high.

Conditional expression [2] defines the shape of the rear lens group. Variable conditional expression (2) _{- {(ψ R2_1.0 -ψ R2_0.7)} - (ψ R1_1.0 -ψ R1_0.7)} / {(ψ R2_0.3 -ψ R2_0.0) - (ψ R1_0 _.3 −ψ _{R1 — 0.0} )} falls below the lower limit value of the conditional expression [2], the aspherical convex shape in the periphery of the rear lens group becomes insufficient, and the periphery incident on the off-axis image sensor It becomes impossible to suppress the size of the incident angle of light, and the diffraction image spreads around the periphery, making it difficult to obtain good imaging performance. Further, if the value of the variable in conditional expression [2] exceeds the upper limit value of conditional expression [2], the convex shape becomes too large with respect to the concave shape on the paraxial axis of the rear group lens, which is better for the rear group lens. It is difficult to achieve a correct aberration correction.
That is, by satisfying the above-described configuration and conditional expressions [1] and [2], the number of aspherical lenses is one, and aberration correction can be performed satisfactorily with the number of lenses equal to or less than the Gaussian type. In addition, even in an image reading lens using an image sensor with small pixels, the MTF in the peripheral portion can be kept high, and high image quality can be obtained.

Furthermore, the image reading lens according to the present invention can achieve better off-axis aberration correction by satisfying the following conditional expression [3] (corresponding to claim 2).
[3] 0.10 <(φb−φa) / ds <0.45
Where φa is the effective diameter of the image side lens surface of the final lens of the front group lens system, φb is the effective diameter of the object side lens surface of the rear group lens, and ds is the front group lens system. And the distance on the optical axis between the rear lens group and the rear group lens.
Conditional expression [3] indicates that the difference between the effective diameter of the image side lens surface of the final lens of the front group lens system and the effective diameter of the object side lens surface of the rear group lens is between the front group lens system and the rear group lens. The ratio to the distance on the optical axis is defined. When the value of the variable (φb−φa) / ds of the conditional expression [3] exceeds the upper limit value of the conditional expression [3], light rays are incident on the rear group lens from the final lens of the front group lens system with an excessive incident angle. In order to form an image on the image sensor, off-axis rays suddenly bend in the direction parallel to the optical axis on the object-side lens surface and the image-side lens surface of the rear lens group. It becomes difficult to obtain. On the other hand, if the value of the variable in the conditional expression [3] is below the lower limit value of the conditional expression [3], light rays are incident at an incident angle that is too small for the rear lens group. For this reason, in order to form an image on the image pickup device, abrupt bending of off-axis rays in the direction perpendicular to the optical axis occurs on the object-side lens surface and the image-side lens surface of the rear lens group, thereby achieving good aberration correction. It becomes difficult.

In other words, by setting the value of the above-mentioned variable within the range defined by the conditional expression [3], it is possible to suppress a sharp bending of the light beam between the front group lens system, the rear group lens, and the image pickup device, and a high image. Quality can be achieved.
Furthermore, the image reading lens according to the present invention can achieve better off-axis aberration correction by satisfying the following conditional expression [4] (corresponding to claim 3).
[4] 0.5 <θj / θi <1.2
Here, θi is the maximum incident angle with respect to the optical axis of the principal ray of e-line incident on the first lens of the front group lens system, and θj is e line emitted from the final lens of the front group lens system. Represents the maximum emission angle of the principal ray with respect to the optical axis.
Conditional expression [4] defines the ratio of the exit angle from the final lens of the front lens group to the incident angle of the front lens system on the first lens. If the value of the ratio θj / θi, which is a variable of the conditional expression [4], is out of the range defined in the conditional expression [4], the front group lens system with respect to the incident angle to the first lens of the front group lens system In addition to bending the off-axis rays too much in the lens, the off-axis rays are bent sharply in the rear group lens, and the weight of aberration correction for one rear group lens is too strong, achieving good aberration correction. Difficult to do. By setting the value of the variable in the conditional expression [4] within the range specified in the conditional expression [4], the aberration correction is performed in the front group lens system, and the ray angle with respect to the image pickup element is used in the rear group lens. Therefore, it is possible to achieve good aberration correction and obtain high image quality.

Furthermore, the image reading lens according to the present invention can achieve better off-axis aberration correction by satisfying the following conditional expression [5] (corresponding to claim 4).
[5] 0.05 <Bf / Da <0.12
Here, Bf is the back focus, that is, the distance on the optical axis between the image side lens surface of the rear group lens and the image sensor, and Da is the first lens of the front group lens system. The distance on the optical axis between the object side lens surface and the image sensor is shown.
Conditional expression [5] defines the ratio of the back focus to the total lens length including the image sensor. If the value of the ratio Bf / Da, which is a variable of the conditional expression [5], exceeds the upper limit value of the conditional expression [5], the rear group lens is too far away from the image sensor, and the height of the off-axis principal ray is increased. Therefore, it becomes difficult to efficiently correct curvature of field. Further, the separation of the light flux for each image height is weakened, and the effect of correcting off-axis aberrations by the aspherical surface is weakened. If the value of the variable in the conditional expression [5] is below the lower limit value of the conditional expression [5], the degree of freedom of the aspherical shape convex to the image side in the peripheral part of the image side lens surface of the rear lens group is reduced and good. It is difficult to achieve a correct aberration correction. In addition, the distance between the rear lens group and the image sensor becomes too short, making it difficult to adjust the distance, and high image quality cannot be obtained.

Furthermore, the image reading lens according to the present invention can achieve further miniaturization by satisfying the following conditional expression [6] (corresponding to claim 5).
[6] 0.2 <ds / D <0.6
Here, ds represents the distance on the optical axis between the front group lens system and the rear group lens, and D represents the total lens length.
Conditional expression [6] defines the size of the distance on the optical axis between the front group lens system and the rear group lens with respect to the total length of the image reading lens. If the value of the variable ds / D of the conditional expression [6] exceeds the upper limit value of the conditional expression [6], the total length of the image reading lens increases, resulting in an increase in size. If the value of the variable in conditional expression [6] is below the lower limit value of conditional expression [6], the diameter of the front lens group will increase, leading to an increase in the size of the image reading lens and an increase in cost. End up.
Further, in the image reading lens according to the present invention, it is desirable to form an aspherical surface on the object side lens surface of the rear lens group (corresponding to claim 6). By making the rear lens group aspherical on both sides, it is possible to achieve good aberration correction while reducing the incident angle with respect to the image sensor.

Furthermore, since the light-receiving element array is arranged on the image plane as an image pickup element, the image reading lens according to the present invention has an outer shape of the rear lens group that is not rotationally symmetric with respect to the optical axis, that is, non-rotationally symmetric. (Corresponding to claim 7), it may be a strip shape long in the main scanning direction of the image reading scan (corresponding to claim 8).
With the configuration described above, the lens diameter of the rear lens group may become enormous in order to obtain good image quality. However, when a light receiving element array is used as the image pickup element, the arrangement of the light receiving elements It is only necessary to secure a width through which light rays pass in only one direction of the main scanning direction. Therefore, in the direction orthogonal to the arrangement of the light receiving elements, the height of the image reading lens can be made smaller than the lens diameter, and the size can be reduced.
Of course, the outer shape of the rear lens group can be rotationally symmetric with respect to the optical axis, and in this case, it is possible to use an area sensor to simultaneously read the entire screen of the image.
Furthermore, an image reading apparatus according to the present invention includes an illumination system that illuminates a document, an imaging lens that reduces and forms reflected light of the document illuminated by the illumination system, and a document that is imaged by the imaging lens. An image reading apparatus that photoelectrically converts an image, and using the image reading lens described above as the imaging lens, the image reading device is small and has high performance corresponding to small pixels that can be corrected well. An image reading apparatus including an image reading lens can be configured (corresponding to claim 9).

Furthermore, the image forming apparatus according to the present invention can form a high-performance image forming apparatus corresponding to small pixels that are small and can be satisfactorily corrected by using the above-described image reading apparatus. Corresponding to item 10).
[Specific Embodiments and Examples]
Next, further detailed embodiments and examples (numerical examples) of the image reading lens, the image reading apparatus, and the image forming apparatus according to the present invention will be described. First, with regard to the first to sixth embodiments as specific embodiments of the image reading lens according to the present invention, Examples 1 to 6 as specific examples, respectively. It will be described together.
1 and 2 are views for explaining an image reading lens according to Example 1 as the first embodiment of the present invention. 3 and 4 are diagrams for explaining an image reading lens according to Example 2 as the second embodiment of the present invention. FIG. 5 and FIG. 6 are for explaining an image reading lens according to Example 3 as the third embodiment of the present invention. 7 and 8 are diagrams for explaining an image reading lens according to Example 4 as the fourth embodiment of the present invention. 9 and 10 are diagrams for explaining an image reading lens according to Example 5 as the fifth embodiment of the present invention. FIGS. 11 and 12 illustrate an image reading lens according to Example 6 as the sixth embodiment of the present invention.

In the first to sixth embodiments, that is, the image reading lenses of Examples 1 to 6, all have a front group lens system on the object side and a rear group lens system on the image side. The front lens group system is configured by arranging four or five meniscus-shaped first lenses having a convex surface facing the object side and a second lens composed of a biconcave lens in order from the object side. The rear lens group is composed of a single glass spherical lens, and has an aspherical shape in which the image side lens surface has a concave shape on the image side in the paraxial direction and gradually becomes convex on the image side toward the periphery. It is composed of a rear group lens composed of a single plastic aspheric lens.
Aberrations in the image reading lenses of Examples 1 to 6 are sufficiently corrected. That is, by configuring the image reading lens as in the first to sixth embodiments according to the present invention, it is possible to ensure very good image performance. It is clear from the examples.

The meanings of symbols common to the first to sixth embodiments are as follows.
F: F value (F number)
r: radius of curvature D: surface spacing Ne: refractive index of e-line νe: Abbe number of e-line Y: object height φ: effective diameter of optical surface K: conic constant of aspheric surface C: reciprocal of paraxial radius of curvature (near Axial curvature) (1 / r)
H: height from the optical axis A _4: 4-order aspherical coefficients A _6: 6-order aspherical coefficients A _8: 8-order aspherical coefficients A _10: 10-order aspherical coefficients A _12: 12 following Aspheric coefficient A ₁₄ : 14th-order aspheric coefficient A ₁₆ : 16th-order aspheric coefficient A ₁₈ : 18th-order aspheric coefficient m: Imaging magnification The aspheric shape used here is the reciprocal of the paraxial radius of curvature. (Paraxial curvature) is C, the height from the optical axis is H, the conic constant is K, the above-mentioned aspheric coefficients are used, and X is the aspheric amount in the optical axis direction, 7], and a shape is specified by giving a paraxial radius of curvature, a conical constant, and an aspherical coefficient.

次に、上述した本発明に基づく、具体的な実施の形態および実施例を詳細に説明する。
以下に述べる本発明の第１の実施の形態の実施例１〜第６の実施の形態の実施例６は、本発明に係る画像読取レンズの数値例による具体的な構成の実施例である。
〔第１の実施の形態〕
まず、上述した本発明の第１の実施の形態としての具体的な実施例１に係る画像読取レンズを詳細に説明する。

Next, specific embodiments and examples based on the present invention described above will be described in detail.
Examples 1 to 6 of the first embodiment of the present invention to be described below are examples of specific configurations based on numerical examples of the image reading lens according to the present invention.
[First Embodiment]
First, the image reading lens according to the specific example 1 as the first embodiment of the present invention described above will be described in detail.

Example 1 is an example of a specific configuration of the image reading lens according to the first embodiment of the present invention. FIG. 1 and FIG. 2 are examples of the first embodiment of the present invention. 1 is a diagram for explaining an image reading lens according to No. 1;
FIG. 1 shows the configuration of the longitudinal section of the optical system of the image reading lens of Example 1 according to the first embodiment of the present invention.
The image reading lens shown in FIG. 1 includes a first lens E1, a second lens E2, a third lens E3, a fourth lens E4, a fifth lens E5, a sixth lens E6, an aperture stop AD, a contact glass CTG, and a cover glass CVG. It has.
In FIG. 1, each optical element constituting the optical system of the image reading lens is sequentially contact glass CTG, first lens E1, and second lens from the image original side that is the subject, that is, from the object side to the image plane side. E2, an aperture stop AD, a third lens E3, a fourth lens E4, a fifth lens E5, a sixth lens E6, and a cover glass CVG are disposed.
The contact glass CTG is a glass composed of generally parallel plates, such as a document placement glass, on which an image document is closely placed.

The first lens E1 is a positive meniscus lens having a convex surface facing the object side and convex toward the object side. The second lens E2 is a negative lens composed of a biconcave lens with a concave surface having a smaller radius of curvature than the object side facing the image side.
The aperture stop AD is disposed between the second lens E2 and the third lens E3.
The third lens E3 is a negative meniscus lens having a concave surface directed toward the object side. The fourth lens E4 is a positive lens composed of a biconvex lens having a convex surface having a smaller radius of curvature than the object side and facing the image side. The fifth lens E5 is a negative meniscus lens having a concave surface directed toward the object side.
The first lens E1, the second lens E2, the aperture stop AD, the third lens E3, the fourth lens E4, and the fifth lens E5 constitute a front group lens system Gf. Each of the first lens E1, the second lens E2, the third lens E3, the fourth lens E4, and the fifth lens E5 is composed of, for example, a glass spherical lens.
The sixth lens E6 is a negative meniscus lens having a concave surface directed toward the object side, and aspheric surfaces are formed on both the object side and the image side. The sixth lens E6 is a single rear group lens system Gr. Is a rear group lens. The sixth lens E6 as the rear group lens system Gr is an aspherical lens having an image side lens surface that is concave on the image side in the paraxial direction and is convex on the image side toward the periphery. The plastic aspherical lens.

The cover glass CVG is a CCD (charge coupled device) image sensor or a CMOS (complementary metal oxide semiconductor) image sensor that captures an optical image of an original image formed by the image reading lens and obtains electronic image data. Generally, it is a sealing glass made of a parallel plate for protecting the imaging surface of a solid-state imaging device. When various optical filters such as an optical low-pass filter and an infrared cut filter, or a dummy glass that replaces them, these are also included in the cover glass CVG.
That is, an image reading lens body is composed of a front group lens system Gf composed of the first lens E1 to the fifth lens E5 and a rear group lens system Gr composed of the sixth lens E6, and is in close contact with the contact glass CTG. An optical image of an image original, which is an object to be imaged, is formed behind the cover glass CVG.
FIG. 1 also shows the surface numbers of the optical surfaces. In addition, each reference code in FIG. 1 is used in common for corresponding parts in the second to sixth embodiments in order to avoid complication of explanation due to an increase in the number of digits of the reference code. Therefore, even if the same reference numerals as those in FIGS. 3, 5, 7, 9, and 11 are given, the configurations are not necessarily the same as those of the second to sixth embodiments.

  In the first embodiment, the F value (F number) F and the object height Y are F = F4.59 and Y = 152.4, the imaging magnification m is 0.047244, and the pixel size is 2 μm. The optical characteristics of each optical element are as shown in Table 1 below. In Table 1, the contact glass is CTG, the aperture stop is AD, the cover glass is CVG, the front group lens system is Gf, the rear group lens system is Gr, the first lens is E1, the second lens is E2, and the third lens. Is shown as E3, the fourth lens as E4, the fifth lens as E5, and the sixth lens as E6, and BF represents the back focus, and these are shown in Examples 2 to 6 (Table 4, Table 4). The same applies to 7, Table 10, Table 13, and Table 16).

In Table 1, the lens surface with the surface number indicated by adding “* (asterisk)” to the surface number is an aspherical surface. That is, in Table 1, each optical surface of the 14th surface and the 15th surface to which “*” is attached is an aspheric surface, and the parameters of each aspheric surface in Equation [7] are as shown in Table 2 below. . In the aspheric coefficient, “En” represents “power of 10”, that is, “× 10 ⁿ ”, for example, “E-05” represents “× 10 ⁻⁵ ”. The same applies to the other embodiments.

  In this case, the value corresponding to the variable in each conditional expression and the corresponding conditional expression and back focus, that is, the value of the distance Bf on the optical axis between the image side lens surface of the rear lens group and the imaging element is It becomes as Table 3 below and satisfies the conditional expressions [1] to [6].

FIG. 2 shows aberration diagrams of spherical aberration, astigmatism, distortion aberration, and coma aberration in Example 1. In these aberration diagrams, the broken line in the spherical aberration diagram represents the sine condition, the solid line in the astigmatism diagram represents sagittal, and the broken line represents meridional. The same applies to the aberration diagrams of the other examples.
[Second Embodiment]
Next, the image reading lens according to Example 2 as the second embodiment of the present invention described above will be described in detail.

Example 2 is an example of a specific configuration of the image reading lens according to the second embodiment of the present invention, and FIGS. 3 and 4 are examples as the second embodiment of the present invention. 2 is a view for explaining an image reading lens according to No. 2;
FIG. 3 shows a conceptual configuration of a longitudinal section of the optical system of the image reading lens of Example 2 according to the second embodiment of the present invention.
The image reading lens shown in FIG. 3 includes a first lens E1, a second lens E2, a third lens E3, a fourth lens E4, a fifth lens E5, a sixth lens E6, an aperture stop AD, a contact glass CTG, and a cover glass CVG. It has.
In FIG. 3, each optical element constituting the optical system of the image reading lens is in order from the object side to the image plane side, contact glass CTG, first lens E1, second lens E2, aperture stop AD, third lens. A lens E3, a fourth lens E4, a fifth lens E5, a sixth lens E6, and a cover glass CVG are disposed.

The contact glass CTG is a glass composed of generally parallel plates, such as a document placement glass, on which an image document is closely placed.
The first lens E1 is a positive meniscus lens having a convex surface directed toward the object side. The second lens E2 is a negative lens composed of a biconcave lens having a concave surface having a smaller radius of curvature than the object side and directed toward the image side.
The aperture stop AD is disposed between the second lens E2 and the third lens E3.
The third lens E3 is a positive meniscus lens having a convex surface facing the image side. The fourth lens E4 is a positive lens composed of a biconvex lens with a surface having a smaller radius of curvature than the image side facing the object side. The fifth lens E5 is a negative meniscus lens having a concave surface directed toward the object side.
The first lens E1, the second lens E2, the aperture stop AD, the third lens E3, the fourth lens E4, and the fifth lens E5 constitute a front group lens system Gf. The first lens E1, the second lens Each of the lenses E2, the third lens E3, the fourth lens E4, and the fifth lens E5 is composed of, for example, a glass spherical lens.
The sixth lens E6 is a negative meniscus lens having a concave surface facing the image side, and aspherical surfaces are formed on both the object side and the image side. The sixth lens E6 is a single rear group lens system Gr. Is a rear group lens. The sixth lens E6 as the rear lens group is a single aspherical plastic that has an image side lens surface that is concave on the image side in the paraxial direction and is convex toward the image side toward the periphery. An aspheric lens is used.

The cover glass CVG protects the imaging surface of a solid-state imaging device such as a CCD image sensor or a CMOS image sensor that captures an optical image of a document image formed by the image reading lens and obtains electronic image data. Generally, it is a sealing glass made of parallel flat plates. When various optical filters such as an optical low-pass filter and an infrared cut filter, or a dummy glass that replaces them, these are also included in the cover glass CVG.
That is, an image reading lens body is constituted by a front group lens system Gf composed of the first lens E1 to the fifth lens E5 and a rear group lens Gr composed of the sixth lens E6, and is closely mounted on the contact glass CTG. An optical image of an image original, which is an object to be imaged, is formed behind the cover glass CVG.
FIG. 3 also shows the surface numbers of the optical surfaces. In FIG. 3, each reference symbol is used in common for the corresponding portions in the first embodiment and the third to sixth embodiments in order to avoid complication of explanation due to an increase in the number of digits of the reference symbol. 1, 5, 7, 9, and 11, the reference numerals common to those in FIGS. 1, 5, 7, 9, and 11 are not necessarily the same as those in the corresponding first and third to sixth embodiments.
In Example 2, the F value F and the object height Y are F = F4.59 and Y = 152.4, the imaging magnification m is 0.047244, and the pixel size is 2 μm. . The optical characteristics of each optical element are as shown in Table 4 below.

  In Table 4, the lens surface with the surface number indicated by adding “*” to the surface number is an aspherical surface. That is, in Table 4, the 14th and 15th optical surfaces marked with “*” are aspherical surfaces, and the parameters of each aspherical surface in Equation [7] are as shown in Table 5 below. is there.

  In this case, the value corresponding to the variable in each conditional expression, the corresponding conditional expression, and the value of the distance Bf on the optical axis between the image side lens surface of the rear lens group and the imaging element are as shown in Table 6 below. Conditional expressions [1] to [6] are satisfied, respectively.

FIG. 4 shows aberration diagrams of spherical aberration, astigmatism, distortion, and coma aberration in Example 2.
[Third Embodiment]
Next, an image reading lens according to a specific example 3 as the third embodiment of the present invention described above will be described in detail.

Example 3 is an example of a specific configuration of the image reading lens according to the third embodiment of the present invention, and FIGS. 5 and 6 are examples of the third embodiment of the present invention. 3 is a diagram for explaining an image reading lens according to No. 3;
FIG. 5 schematically shows a conceptual configuration of a longitudinal section of the optical system of the image reading lens of Example 3 according to the third embodiment of the present invention.
The image reading lens shown in FIG. 5 includes a first lens E1, a second lens E2, a third lens E3, a fourth lens E4, a fifth lens E5, a sixth lens E6, an aperture stop AD, a contact glass CTG, and a cover glass CVG. It has.
In FIG. 5, each optical element constituting the optical system of the image reading lens is sequentially contact glass CTG, first lens E1, second lens E2, aperture stop AD, third, from the object side to the image surface side. A lens E3, a fourth lens E4, a fifth lens E5, a sixth lens E6, and a cover glass CVG are disposed.
The contact glass CTG is a glass composed of generally parallel plates, such as a document placement glass, on which an image document is closely placed.
The first lens E1 is a positive meniscus lens having a convex surface directed toward the object side. The second lens E2 is a negative lens composed of a biconcave lens having a concave surface having a smaller radius of curvature than the object side and directed toward the image side.

The aperture stop AD is disposed between the second lens E2 and the third lens E3.
The third lens E3 is a positive meniscus lens having a concave surface directed toward the object side. The fourth lens E4 is a positive lens composed of a biconvex lens having a convex surface having a smaller radius of curvature than the image side and facing the object side. The fifth lens E5 is a negative meniscus lens having a concave surface directed toward the object side.
The first lens E1, the second lens E2, the aperture stop AD, the third lens E3, the fourth lens E4, and the fifth lens E5 constitute a front group lens system Gf. Each of the first lens E1, the second lens E2, the third lens E3, the fourth lens E4, and the fifth lens E5 is composed of, for example, a glass spherical lens.
The sixth lens E6 is a negative meniscus lens having a concave surface facing the image side, and aspherical surfaces are formed on both the object side and the image side. The sixth lens E6 is a single rear group lens system Gr. Is a rear group lens. The sixth lens E6 as the rear lens group is a single aspherical plastic that has an image side lens surface that is concave on the image side in the paraxial direction and is convex toward the image side toward the periphery. An aspheric lens is used.

The cover glass CVG protects the imaging surface of a solid-state imaging device such as a CCD image sensor or a CMOS image sensor that captures an optical image of a document image formed by the image reading lens and obtains electronic image data. Generally, it is a sealing glass made of parallel flat plates. When various optical filters such as an optical low-pass filter and an infrared cut filter, or a dummy glass that replaces them, these are also included in the cover glass CVG.
That is, an image reading lens body is composed of a front group lens system Gf composed of the first lens E1 to the fifth lens E5 and a rear group lens system Gr composed of the sixth lens E6, and is in close contact with the contact glass CTG. An optical image of an image original, which is an object to be imaged, is formed behind the cover glass CVG.
FIG. 5 also shows the surface numbers of the optical surfaces.
In Example 3, the F value F and the object height Y are F = F4.59 and Y = 152.4, the imaging magnification m is 0.047244, and the pixel size is 2 μm. . The optical characteristics of each optical element are as shown in Table 7 below.

  In Table 7, the lens surface with the surface number indicated by adding “*” to the surface number is an aspherical surface. That is, in Table 7, each optical surface of the 14th surface and the 15th surface to which “*” is attached is an aspheric surface, and the parameters of each aspheric surface in Expression [7] are as shown in Table 8 below. .

  In this case, the value corresponding to the variable in each conditional expression, the corresponding conditional expression, and the value of the distance Bf on the optical axis between the image side lens surface of the rear lens group and the imaging element are as shown in Table 9 below. Conditional expressions [1] to [6] are satisfied, respectively.

FIG. 6 shows aberration diagrams of spherical aberration, astigmatism, distortion, and coma aberration in Example 3.
[Fourth Embodiment]
Next, an image reading lens according to a specific example 4 as the above-described fourth embodiment of the present invention will be described in detail.

Example 4 is an example of a specific configuration of the image reading lens according to the fourth embodiment of the present invention, and FIGS. 7 and 8 are examples of the fourth embodiment of the present invention. 4 is a view for explaining an image reading lens according to No. 4;
FIG. 7 schematically shows a conceptual configuration of a longitudinal section of the optical system of the image reading lens of Example 4 according to the fourth embodiment of the present invention.
The image reading lens shown in FIG. 7 includes a first lens E1, a second lens E2, a third lens E3, a fourth lens E4, a fifth lens E5, a sixth lens E6, an aperture stop AD, a contact glass CTG, and a cover glass CVG. It has.
In FIG. 7, each optical element constituting the optical system of the image reading lens is sequentially contact glass CTG, first lens E1, aperture stop AD, second lens E2, third lens from the object side to the image surface side. A lens E3, a fourth lens E4, a fifth lens E5, a sixth lens E6, and a cover glass CVG are disposed.
The contact glass CTG is a glass composed of generally parallel plates, such as a document placement glass, on which an image document is closely placed.
The first lens E1 is a positive meniscus lens having a concave surface facing the image side.

The aperture stop AD is disposed between the first lens E1 and the second lens E2.
The second lens E2 is a negative lens composed of a biconcave lens having a concave surface having a smaller radius of curvature than the object side and directed toward the image side. The third lens E3 is a positive meniscus lens having a concave surface directed toward the object side. The fourth lens E4 is a positive lens composed of a biconvex lens having a convex surface having a smaller radius of curvature than the object side and facing the image side. The fifth lens E5 is a negative meniscus lens having a concave surface directed toward the object side.
The first lens E1, the aperture stop AD, the second lens E2, the third lens E3, the fourth lens E4, and the fifth lens E5 constitute a front group lens system Gf. The first lens E1 and the second lens Each of the lenses E2, the third lens E3, the fourth lens E4, and the fifth lens E5 is composed of a glass spherical lens, for example.
The sixth lens E6 is a biconcave lens in which a concave surface having a smaller radius of curvature than the image side is directed to the object side, and an aspheric surface is formed on the image side. The sixth lens E6 is a single lens. This is a rear group lens constituting the rear group lens system Gr. The sixth lens E6 as the rear group lens is a single plastic non-spherical shape in which the image side lens surface is formed concave on the image side in the paraxial direction and formed convex toward the image side toward the periphery. It consists of a spherical lens.

The cover glass CVG protects the imaging surface of a solid-state imaging device such as a CCD image sensor or a CMOS image sensor that captures an optical image of a document image formed by the image reading lens and obtains electronic image data. Generally, it is a sealing glass made of parallel flat plates.
That is, an image reading lens body is composed of a front group lens system Gf composed of the first lens E1 to the fifth lens E5 and a rear group lens Gr composed of the sixth lens E6, and is closely mounted on the contact glass CTG. An optical image of an image original, which is an object to be imaged, is formed behind the cover glass CVG.
FIG. 7 also shows the surface numbers of the optical surfaces.
In Example 4, the F value F and the object height Y are F = F4.59 and Y = 152.4, respectively, the imaging magnification m is 0.047244, and the pixel size is 2 μm. . The optical characteristics of each optical element are as shown in Table 10 below.

  In Table 10, the lens surface with the surface number indicated by adding “*” to the surface number is an aspherical surface. That is, in Table 10, the fifteenth surface marked with “*” is an aspheric surface, and the parameters of each aspheric surface in Equation [7] are as shown in Table 11 below.

  In this case, the value corresponding to the variable in each conditional expression, the corresponding conditional expression, and the value of the distance Bf on the optical axis between the image side lens surface of the rear lens group and the imaging element are as shown in Table 12 below. Conditional expressions [1] to [6] are satisfied, respectively.

FIG. 8 shows aberration diagrams of spherical aberration, astigmatism, distortion aberration, and coma aberration in Example 4.
[Fifth Embodiment]
Next, the image reading lens according to Example 5 as the fifth embodiment of the present invention described above will be described in detail.

Example 5 is an example of a specific configuration of the image reading lens according to the fifth embodiment of the present invention, and FIGS. 9 and 10 are examples of the fifth embodiment of the present invention. 5 is a diagram for explaining an image reading lens according to No. 5;
FIG. 9 shows a conceptual configuration of a longitudinal section of the optical system of the image reading lens of Example 5 according to the fifth embodiment of the present invention.
The image reading lens shown in FIG. 9 includes a first lens E1, a second lens E2, a third lens E3, a fourth lens E4, a fifth lens E5, a sixth lens E6, an aperture stop AD, a contact glass CTG, and a cover glass CVG. It has.
In FIG. 9, each optical element constituting the optical system of the image reading lens is sequentially contact glass CTG, first lens E1, aperture stop AD, second lens E2, third lens from the object side to the image surface side. A lens E3, a fourth lens E4, a fifth lens E5, a sixth lens E6, and a cover glass CVG are disposed.
The contact glass CTG is a glass composed of generally parallel plates, such as a document placement glass, on which an image document is closely placed.

The first lens E1 is a positive meniscus lens having a concave surface facing the image side.
The aperture stop AD is disposed between the first lens E1 and the second lens E2.
The second lens E2 is a negative lens composed of a biconcave lens having a concave surface having a smaller radius of curvature than the object side and directed toward the image side. The third lens E3 is a positive meniscus lens having a concave surface directed toward the object side.

The fourth lens E4 is a positive meniscus lens having a concave surface directed toward the image side, and the fifth lens E5 is a negative meniscus lens having a concave surface directed toward the image side. The two lenses, the fourth lens E4 and the fifth lens E5, are closely bonded to each other and joined together to form a cemented lens composed of two lenses.
The first lens E1, the aperture stop AD, the second lens E2, the third lens E3, the fourth lens E4, and the fifth lens E5 constitute a front group lens system Gf. The first lens E1 and the second lens Each of the lenses E2, the third lens E3, the fourth lens E4, and the fifth lens E5 is composed of a glass spherical lens, for example.

The sixth lens E6 is a negative meniscus lens having a concave surface facing the image side, and aspherical surfaces are formed on both the object side and the image side. The sixth lens E6 is a single rear group lens system Gr. Is a rear group lens. The sixth lens E6 as the rear lens group is a single aspherical plastic that has an image side lens surface that is concave on the image side in the paraxial direction and is convex toward the image side toward the periphery. An aspheric lens is used.
The cover glass CVG protects the imaging surface of a solid-state imaging device such as a CCD image sensor or a CMOS image sensor that captures an optical image of a document image formed by the image reading lens and obtains electronic image data. Generally, it is a sealing glass made of parallel flat plates.
That is, an image reading lens body is composed of a front group lens system Gf composed of the first lens E1 to the fifth lens E5 and a rear group lens system Gr composed of the sixth lens E6, and is in close contact with the contact glass CTG. An optical image of an image original, which is an object to be imaged, is formed behind the cover glass CVG.
FIG. 9 also shows the surface numbers of the optical surfaces.
In Example 5, the F value F and the object height Y are F = F4.59 and Y = 152.4, the imaging magnification m is 0.047244, and the pixel size is 2 μm. . The optical characteristics of each optical element are as shown in Table 13 below.

  In Table 13, the lens surface with the surface number indicated by adding “*” to the surface number is an aspherical surface. That is, in Table 13, the thirteenth and fourteenth surfaces marked with “*” are aspheric surfaces, and the parameters of each aspheric surface in Equation [7] are as shown in Table 14 below.

  In this case, the value corresponding to the variable in each conditional expression, the corresponding conditional expression, and the value of the distance Bf on the optical axis between the image side lens surface of the rear lens group and the imaging element are as shown in Table 15 below. Conditional expressions [1] to [6] are satisfied, respectively.

FIG. 10 shows aberration diagrams of spherical aberration, astigmatism, distortion, and coma aberration in Example 5.
[Sixth Embodiment]
Next, the image reading lens according to Example 6 as the sixth embodiment of the present invention described above will be described in detail.

Example 6 is an example of a specific configuration of the image reading lens according to the sixth embodiment of the present invention, and FIGS. 11 and 12 are examples of the sixth embodiment of the present invention. 6 is a diagram for explaining an image reading lens according to No. 6;
FIG. 11 shows a conceptual configuration of a longitudinal section of the optical system of the image reading lens of Example 6 according to the sixth embodiment of the present invention.
The image reading lens shown in FIG. 11 includes a first lens E1, a second lens E2, a third lens E3, a fourth lens E4, a fifth lens E5, an aperture stop AD, a contact glass CTG, and a cover glass CVG. .
In FIG. 11, each optical element constituting the optical system of the image reading lens is sequentially contact glass CTG, first lens E1, aperture stop AD, second lens E2, and third lens from the object side to the image plane side. A lens E3, a fourth lens E4, a fifth lens E5, and a cover glass CVG are disposed. This sixth embodiment is not a six-sheet configuration as in the other embodiments, but a five-sheet configuration.
The contact glass CTG is a glass composed of generally parallel plates, such as a document placement glass, on which an image document is closely placed.
The first lens E1 is a positive meniscus lens having a concave surface facing the image side.

The aperture stop AD is disposed between the first lens E1 and the second lens E2.
The second lens E2 is a negative lens composed of a biconcave lens having a concave surface having a smaller radius of curvature than the object side and directed toward the image side. The third lens E3 is a positive lens composed of a biconvex lens having a convex surface having a smaller radius of curvature than the object side and facing the image side. The fourth lens E4 is a negative meniscus lens having a concave surface directed toward the object side.
The first lens E1, the aperture stop AD, the second lens E2, the third lens E3, and the fourth lens E4 constitute a front lens group Gf. The first lens E1, the second lens E2, and the third lens. Each of the lenses E3 and the fourth lens E4 is constituted by a glass spherical lens, for example.
The fifth lens E5 is a biconcave lens in which a concave surface having a smaller radius of curvature than the image side is directed to the object side, and aspheric surfaces are formed on both the object side and the image side. E5 is a rear group lens constituting the rear group lens system Gr as a single unit. The fifth lens E5 as the rear lens group is a single aspherical plastic having an image side lens surface that is concave on the image side in the paraxial direction and is convex on the image side toward the periphery. An aspheric lens is used.

The cover glass CVG protects the imaging surface of a solid-state imaging device such as a CCD image sensor or a CMOS image sensor that captures an optical image of a document image formed by the image reading lens and obtains electronic image data. Generally, it is a sealing glass made of parallel flat plates.
That is, an image reading lens body is composed of a front group lens system Gf composed of the first lens E1 to the fourth lens E4 and a rear group lens system Gr composed of the fifth lens E5, and is closely adhered to the contact glass CTG. An optical image of an image original, which is an object to be imaged, is formed behind the cover glass CVG.
FIG. 11 also shows the surface numbers of the optical surfaces.

  In Example 6, the F value F and the object height Y are F = F4.59 and Y = 152.4, respectively, the imaging magnification m is 0.047244, and the pixel size is 2 μm. . The optical characteristics of each optical element are as shown in Table 16 below.

  In Table 16, the lens surface with the surface number indicated by adding “*” to the surface number is an aspherical surface. That is, in Table 16, the twelfth and thirteenth surfaces marked with “*” are aspheric surfaces, and the parameters of each aspheric surface in Equation [7] are as shown in Table 17 below.

In this case, the value corresponding to the variable in each conditional expression, the corresponding conditional expression, and the value of the distance Bf on the optical axis between the image side lens surface of the rear lens group and the imaging element are as shown in Table 18 below. Conditional expressions [1] to [6] are satisfied, respectively.

FIG. 12 shows aberration diagrams of spherical aberration, astigmatism, distortion aberration, and coma aberration in Example 6.
[Seventh Embodiment]
Next, an image reading lens such as the first to sixth embodiments according to the first to sixth embodiments of the present invention described above is adopted as an imaging lens for image reading. An image reading apparatus according to the seventh embodiment of the present invention will be described.
FIG. 13 is a diagram for explaining a specific configuration of the image reading apparatus according to the seventh embodiment of the present invention.
FIG. 13 schematically shows a conceptual configuration of a longitudinal section of an image reading apparatus according to the seventh embodiment of the present invention. In the image reading apparatus according to the seventh embodiment, the image reading lenses according to Examples 1 to 6 of the first to sixth embodiments described above are used for image reading. Used as a lens.

An image reading apparatus 100 illustrated in FIG. 13 includes a contact glass 101, a first traveling body 103, a second traveling body 104, an image reading lens 105, and a line sensor 106. The first traveling body 103 includes an illumination light source 103a, a light source mirror 103b, and a first mirror 103c, the second traveling body 104 includes a second mirror 104a and a third mirror 104b, and the line sensor 106 performs color separation. As means, photoelectric conversion elements 106a, 106b, and 106c that have red (R), green (G), and blue (B) filters and are arranged in three rows on one chip to form a three-line CCD sensor are arranged. I have. The image reading lens 105 is configured using the image reading lens according to the first to sixth embodiments described above.
In FIG. 13, a document TD as a reading target on which an image to be read is written is placed on a contact glass 101, which is a flat document placing glass as a document table, with the reading surface turned flat. The first traveling body 103 disposed below the contact glass 101 holds an illumination light source 103a, a light source mirror 103b, and a first mirror 103c that are long in a direction orthogonal to the drawing of FIG. It moves at a constant traveling speed V from a position shown as the traveling body 103 to a position shown as the first traveling body 103 ′.

The illumination light source 103a is an elongated light source whose longitudinal direction is a direction orthogonal to the drawing of FIG. As the illumination light source 103a, a tube lamp such as a halogen lamp, a Xe (so-called xenon) lamp, or a fluorescent lamp such as a cold cathode tube may be used, and point light sources such as LEDs (light emitting diodes) are arranged in a line. A linear light source using a light guide that converts a point light source into a linear light source may be used, or a surface-emitting light source typified by organic EL (electroluminescence) may be used as an elongated shape. . The illumination light source 103a is controlled to emit light when the first traveling body 103 is moved and displaced in the right direction in FIG. The light emitted from the illumination light source 103a is guided by a semi-cylindrical light source mirror 103b that is long in the direction orthogonal to the drawing of FIG. 13, and is orthogonal to the drawing of FIG. 13 on the original TD placed on the contact glass 101. A slit-like portion that is long in the main scanning direction is illuminated.
The first mirror 103 c is held by the first traveling body 103 in a state where the mirror surface is inclined 45 degrees with respect to the document placement surface of the contact glass 101.
The second traveling body 104 holds a second mirror 104a and a third mirror 104b that are long in a direction orthogonal to the drawing of FIG. The second traveling body 104 is synchronized with the displacement of the first traveling body 103 at a constant traveling speed V / 2 (that is, half the speed of the first traveling body 103) to the position indicated as the second traveling body 104 ′. Displace.

The reflected light from the illuminated portion of the document TD (reflected light from the image) is reflected by the first mirror 103 c provided on the first traveling body 103 and then the second mirror provided on the second traveling body 104. 104a and further sequentially reflected by the third mirror 104b and incident on the image reading lens 105. The image reading lens 105 causes the imaging surface (input surface) of the line sensor 106 to pass through the cover glass of the line sensor 106 as an imaging device. ) A reduced optical image of the original image is formed on top.
That is, the first mirror 103c, the second mirror 104a, and the third mirror 104b constitute a reflection optical system. The first traveling body 103 and the second traveling body 104 are caused to travel in the directions of the arrows (rightward in the drawing) by driving means (not shown). If the traveling speed of the first traveling body 103 at this time is V, the traveling speed of the second traveling body 104 is V / 2, and while the first traveling body 103 travels a predetermined amount, The first traveling body 103 moves by half of the moving amount. By this traveling, the first traveling body 103 and the second traveling body 104 are displaced to the positions indicated by broken lines in the drawing. The illumination light source 103a, the light source mirror 103b, and the first mirror 103c move integrally with the first traveling body 103, and illuminate and scan the entire document TD on the contact glass 101. As described above, since the moving speed ratio between the first traveling body 103 and the second traveling body 104 is V: V / 2 = 2: 1, the optical path length from the original scanned portion to the image reading lens 105 is illuminated. Is kept almost unchanged.

The line sensor 106, which is an image sensor, includes three line-like photoelectric conversion elements 106a, 106b, and 106c each having red (R), green (G), and blue (B) filters as color separation means on one chip. This is a three-line CCD sensor arranged in a row, and the original image is converted into an image signal as the original TD is illuminated and scanned. In this way, reading of the document TD is executed, and the color image of the document TD is read by color separation into three primary colors of red, green, and blue.
Such an image reading apparatus 100 is an apparatus that reads an image in full color, and reads the color separation means that is provided in the line sensor 116 and includes red (R), green (G), and blue (B) filters. It is in the imaging optical path of the lens 105.
As described above, the imaging light beam incident on the image reading lens 105 forms a reduced image of the document TD on the light receiving surface of the line sensor 106 that is an image pickup device by the image forming action of the image reading lens 105. In this case, the line sensor 106 is a CCD line sensor, in which minute photoelectric conversion units are closely arranged in a direction orthogonal to the drawing of FIG. 13, and an original image is converted into a pixel unit in accordance with illumination scanning of the original TD. Output as an electrical signal. As described above, the line sensor 106 separates the image formed into three colors (red, green, and blue), reads the color information, and combines the electrical signals converted by the photoelectric conversion units of the respective colors. A color original can be read. This electric signal is converted into an image signal through signal processing such as A / D (analog-digital) conversion, and stored in a memory (not shown) as necessary.

As described above, by using the image reading lens according to the present invention for the image reading lens 105 shown in FIG. 13, the image reading apparatus can be downsized.
The color separation method is not limited to the configuration described above, and a color separation prism or filter is selectively inserted between the image reading lens and the line sensor, and red (R), green (G), and blue. A configuration for color separation into (B), or a configuration for illuminating a document by sequentially turning on red (R), green (G), and blue (B) light sources can be used.
That is, the image reading apparatus 100 according to the seventh embodiment of the present invention uses the image reading lenses of Examples 1 to 6 of the first to sixth embodiments described above as imaging lenses. It is the image reading apparatus 100 used as. The image reading apparatus 100 may read the document information in full color by providing a color separation function in the imaging optical path of the imaging lens.

[Eighth Embodiment]
Next, an image reading lens such as the first to sixth embodiments according to the first to sixth embodiments of the present invention described above is adopted as an imaging lens for image reading. An image reading apparatus according to the eighth embodiment of the present invention will be described.
As an image reading apparatus, an illumination unit that illuminates a document on a contact glass in a slit shape, a line sensor, a plurality of mirrors that form an imaging optical path from an illuminated part of the document to the line sensor, and an imaging optical path An image reading unit configured to integrate an image reading lens disposed in the image reading unit, and to read and scan the document by driving the image reading unit relative to the document by a driving unit; You can also This is the second configuration of the image reading apparatus according to the eighth embodiment of the present invention.
That is, according to the eighth embodiment of the present invention, which is configured by adopting the image reading lens according to the first to sixth embodiments of the present invention described above as an imaging lens for image reading. The image reading apparatus will be described with reference to FIG.

FIG. 14 schematically shows a conceptual configuration of a longitudinal section of an image reading apparatus according to the eighth embodiment of the present invention. Also in the image reading apparatus according to Example 8, the image reading lenses according to Examples 1 to 6 of the first to sixth embodiments described above are used as an imaging lens for image reading. .
An image reading apparatus 110 shown in FIG. 14 includes a contact glass 101, an image reading unit 113, an image reading lens 114, and a line sensor 115.
The image reading unit 113 includes a first illumination light source 113a, a first light source mirror 113b, a second illumination light source 113c, a second light source mirror 113d, a first mirror 113e, a second mirror 113f, and a third mirror 113g. The line sensor 115 has red (R), green (G) and blue (B) filters as color separation means, and is arranged in three lines on one chip to form a three-line CCD sensor. Photoelectric conversion elements 115a, 115b, and 115c are provided. The image reading lens 114 is configured using the reading lens according to Examples 1 to 6 of the first to sixth embodiments described above.

In FIG. 14, a document TD as a reading target on which an image to be read is written is placed on a contact glass 101, which is a flat document placing glass as a document table, with the reading surface turned flat. The image reading unit 113 arranged below the contact glass 101 has a first illumination light source 113a, a first light source mirror 113b, a second illumination light source 113c, a second illumination light source 113a that are long in a direction orthogonal to the drawing of FIG. The light source mirror 113d, the first mirror 113e, the second mirror 113f, and the third mirror 113g are held, and the image reading lens 114 and the line sensor 115 are also provided integrally with the image reading unit 113.
Each of the first illumination light source 113a and the second illumination light source 113c is an elongated light source whose longitudinal direction is a direction orthogonal to the drawing of FIG. The first illumination light source 113a and the second illumination light source 113c are, like the illumination light source 103a in FIG. 13, a tube lamp such as a halogen lamp, a Xe (so-called xenon) lamp, or a fluorescent lamp such as a cold cathode tube. Or a linear light source using a light guide that converts a point light source into a line light source, or an organic EL (light emitting diode). It is also possible to use a surface-emitting light source typified by electroluminescence as an elongated shape. These first and second illumination light sources 113a and 113c are also controlled to emit light when the image reading unit 113 is moved and displaced in the right direction in FIG.

The light emitted from the first and second illumination light sources 113a and 113c is guided by the first light source mirror 113b and the second light source mirror 113d that are long in the direction perpendicular to the drawing of FIG. A slit-like portion that is long in the main scanning direction orthogonal to the drawing of FIG. 14 in the document TD placed on the contact glass 101 is illuminated.
The first mirror 113e, the second mirror 113f, and the third mirror 113g are all long in the direction perpendicular to the drawing of FIG. 14, the mirror surface is inclined 45 degrees with respect to the document placement surface of the contact glass 101, and the mirror surfaces are mutually It is held in the image reading unit 113 as being tilted orthogonally.
While the image reading unit 113 moves at a constant speed from the position of the image reading unit 113 shown in FIG. 14 to the position shown as the image reading unit 113 ′, the reflected light from the illuminated portion of the document TD (the reflected light from the image) ) Are sequentially reflected by a first mirror 113e provided in the image reading unit 113, a second mirror 113f provided in the image reading unit 113, and further a third mirror 113g provided in the image reading unit 113. The incident light enters the lens 114, and the image reading lens 114 forms a reduced optical image of the original image on the imaging surface (input surface) of the line sensor 115 through the cover glass of the line sensor 115 as an imaging device.

That is, the first mirror 113e, the second mirror 113f, and the third mirror 113g constitute a reflection optical system. The image reading unit 113 is caused to travel to the right in the drawing in FIG. 14 by driving means (not shown).
Therefore, the document TD is illuminated and scanned while the image reading unit 113 is displaced to the position indicated as the image reading unit 113 ′. When the document TD is illuminated and scanned, the reflected light of the illumination light from the document TD is sequentially reflected by the first mirror 113e, the second mirror 113f, and the third mirror 113g, and enters the image reading lens 114 as an imaging light beam. To do.
At this time, since all of the first mirror 113e, the second mirror 113f, and the third mirror 113g are integrally held by the image reading unit 113, the illuminated portion of the document TD during the illumination scanning of the document TD is removed. The optical path length to the image reading lens 114 is constant.
The imaging light beam incident on the image reading lens 114 reduces and forms an image of the document TD on the light receiving surface of the line sensor 115 serving as an image sensor by the imaging action of the image reading lens 114. The image formed on the light receiving surface of the line sensor 115 is converted into an electric signal in the same manner as the image reading apparatus described above, and the document information is read.

[Ninth Embodiment]
Next, an image forming apparatus according to a ninth embodiment of the present invention configured by using the image reading apparatus according to the seventh embodiment or the eighth embodiment of the present invention described above will be described.
FIG. 15 schematically shows a conceptual configuration of a longitudinal section of an image forming apparatus according to the ninth embodiment of the present invention. In the image forming apparatus according to the ninth embodiment, the image reading apparatus shown in FIG. 13 according to the seventh embodiment described above is used for image reading.
The image forming apparatus illustrated in FIG. 15 includes an image reading apparatus 100 and an image forming unit 200. The image reading apparatus 100 has the same configuration as that in FIG. 13, and the same reference numerals are given to the same portions as those in FIG. 13, and detailed description thereof will be omitted. That is, the image reading apparatus 100 includes a contact glass 101, a first traveling body 103, a second traveling body 104, an image reading lens 105, and a line sensor 106. The first traveling body 103 includes an illumination light source 103a, a light source mirror 103b, and a first mirror 103c, the second traveling body 104 includes a second mirror 104a and a third mirror 104b, and the line sensor 106 performs color separation. As means, photoelectric conversion elements 106a, 106b, and 106c that have red (R), green (G), and blue (B) filters and are arranged in three rows on one chip to form a three-line CCD sensor are arranged. I have. The image reading lens 105 is configured using the image reading lens according to Examples 1 to 6 of the first to sixth embodiments described above.

As the image reading apparatus 100, an illumination unit that illuminates the document TD on the contact glass 101 in a slit shape, a line sensor 106, and a plurality of imaging optical paths that form an imaging optical path from the illuminated portion of the document TD to the line sensor 106. An image reading unit is configured by integrating the mirror and the image reading lens 105 disposed on the imaging optical path, and the original is read and scanned by running the image reading unit relative to the original by the driving means. It is also possible to use an image reading apparatus in the form described above. That is, instead of the image reading apparatus shown in FIG. 13 according to the seventh embodiment, the image reading apparatus shown in FIG. 14 according to the eighth embodiment of the present invention is used to configure the image forming apparatus. You may do it.
The image forming unit 200 is located below the image reading device 100, and includes a photoreceptor 210, a charging roller 211, a developing device 213, a transfer belt unit 214, a cleaning device 215, a fixing device 216, an optical scanning device 217, and the like. A cassette 218, a registration roller pair 219, a signal processing unit 220, a tray 221, and a paper feed roller 222 are provided. The transfer belt unit 214 includes a transfer voltage application roller 214a and a transfer roller 214b.

In FIG. 15, the image signal output from the three-line line sensor 106 of the image reading apparatus 100 is sent to the signal processing unit 220 of the image forming unit 200 and processed in the signal processing unit 220, that is, a writing signal, that is, It is converted into a signal for writing each color of yellow (Y), magenta (magenta) (M), cyan (C) and black (K).
The image forming unit 200 includes a photoconductive photosensitive member 210 formed in a cylindrical shape as a latent image carrier, around which a charging roller 211 as a charging unit, a turret type developing device 213, and a transfer belt unit. 214 and a cleaning device 215 are provided. As a charging means, a corona charger can be used instead of the charging roller 211.
The optical scanning device 217 receives a signal for writing from the signal processing unit 220 and performs writing on the photosensitive member 210 by optical scanning. Further, the optical scanning device 217 performs optical scanning of the photosensitive member 210 between the charging roller 211 and the developing device 213.
When image formation is performed, the photoconductive photosensitive member 210 is rotated at a constant speed in the clockwise direction in the drawing, and the surface thereof is uniformly charged by the charging roller 211, and exposure by optical writing of the laser beam of the optical scanning device 217 is performed. In response, an electrostatic latent image is formed. The formed electrostatic latent image is a so-called negative latent image, and the image portion is exposed.

The image writing is performed sequentially in the order of yellow (Y) image, magenta (M) image, cyan (C) image, and black image (K) in accordance with the rotation of the photoreceptor 210. The image is developed with each developing unit Y (developing with yellow toner), M (developing with magenta toner), C (developing with cyan toner), and K (developing with black toner) of the turret type developing device 213. The color toner images obtained are sequentially reversed and developed to be visualized as positive images, and the obtained color toner images are sequentially transferred onto the transfer belt 214 by the transfer voltage application roller 214a, and the color toner images described above are transferred onto the transfer belt 214. Overlapped to form a color image.
The cassette 218 containing the transfer sheet S as a recording medium is detachable from the main body of the image forming apparatus. When the cassette 218 is mounted as shown in the figure, the uppermost sheet of the stored transfer sheet S is fed by a paper feed roller. The transfer paper S that has been taken out and fed by 222 and the fed transfer paper S is caught by the registration roller pair 219.
The registration roller pair 219 feeds the transfer sheet S to the transfer unit at the timing when the color image of the toner on the transfer belt unit 214 moves to the transfer position. The transferred transfer paper S is superimposed on the color image at the transfer portion, and the color image is electrostatically transferred by the action of the transfer roller 214b. The transfer roller 214b presses and transfers the color image to the transfer sheet S during transfer.

The transfer sheet S on which the color image has been transferred is sent to the fixing device 216, where the color image is fixed, passes through a conveyance path by a guide means (not shown), and is trayd by a pair of paper discharge rollers (not shown). 221 is discharged. Each time an image of each color toner is transferred, the surface of the photoreceptor 210 is cleaned by the cleaning device 215 to remove residual toner, paper dust, and the like.
That is, the image forming apparatus according to the ninth embodiment of the present invention uses the image reading lenses of Examples 1 to 6 of the first to sixth embodiments as an imaging lens. An image forming apparatus configured using an image reading apparatus.
Needless to say, the image forming apparatus according to the present invention is not limited to the configuration for forming a color image, but can also be configured to form a monochrome image.
In addition, an image reading apparatus using the image reading lens according to the present invention described above in a space-saving image forming apparatus of in-body discharge type in which paper output is formed between the image reading apparatus and the image forming unit. By using it, the image reading apparatus can be thinned, and the distance from the image forming unit is widened, thereby improving the visibility of the output paper and the like to the operator, thereby facilitating the work. There is also.
In the description of the present invention, all circular lenses are used. However, in order to reduce the size of the lens, particularly the height in the height direction of the image reading apparatus, a strip with the upper and lower sides of the lens cut. An image reading lens according to the present invention can also be configured using a shape or oval lens.

As will be clear from the above detailed description, according to the first aspect of the present invention, the image reading lens is small and maintains the MTF in the peripheral portion high by reducing the incident angle of the chief ray to the image sensor. Can be provided.
According to the second, third, fourth, and sixth aspects of the invention, a higher-performance image reading lens can be provided.
According to the fifth, seventh, and eighth aspects of the invention, a smaller image reading lens can be provided.
According to the ninth aspect of the present invention, it is possible to provide an image reading lens that is small in size and has a high MTF in the peripheral portion by reducing the incident angle of the principal ray with respect to the imaging device. It is possible to realize an image reading apparatus that supports high image quality, small size, light weight, and low cost.
According to the tenth aspect of the present invention, it is possible to provide an image reading apparatus that is small in size and has a high MTF in the peripheral portion by reducing the incident angle of the chief ray with respect to the image sensor. An image forming apparatus with high image quality, small size, light weight, and low cost can be realized.

E1 1st lens E2 2nd lens E3 3rd lens E4 4th lens E5 5th lens E6 6th lens AD Aperture stop CTG Contact glass CVG Cover glass Gf Front group lens system Gr Rear group lens system 100,110 Image reader 101 Contact glass 103 First traveling body 104 Second traveling body 105 Image reading lens 106 Line sensor 103a Illumination light source 103b Light source mirror 106, 115 Line sensors 106a, 106b, 106c, 115a, 115b, 115c Photoelectric conversion element 113 Image reading unit 114 Image Reading lens 115 Line sensor 113a First illumination light source 113b First light source mirror 113c Second illumination light source 113d Second light source mirror 113e First mirror 113f Second mirror 113g Third mirror 2 DESCRIPTION OF SYMBOLS 0 Image forming part 210 Photoconductor 211 Charging roller 213 Developing device 214 Transfer belt part 215 Cleaning device 216 Fixing device 217 Optical scanning device 218 Cassette 219 Registration roller pair 220 Signal processing unit 221 Tray 222 Paper feed roller 214a Transfer voltage application roller 214b Transfer roller

JP 2012-145839 A

Claims (10)

An image reading lens for reading a document image formed on a light receiving surface of an image sensor,
Arrange the front lens group on the object side and the rear lens system on the image side.
The front group lens system is any one of four and five lenses including at least a meniscus first lens having a convex surface facing the object side and a second lens composed of a biconcave lens. Consists of one glass spherical lens,
The rear lens group system is composed of a single plastic aspherical lens having an aspherical shape in which the image side lens surface is concave on the image side with a paraxial axis and gradually convex toward the image side toward the periphery. Consists of a rear lens group,
The maximum field angle of the principal ray of e-line incident on the first lens of the front group lens system is 50 ° or more,
The absolute value of the angle of the e-line incident on the image sensor with respect to the optical axis of the principal ray is θ, and the inclination of the normal of the lens surface of the rear lens group to the Y-axis at each height (that is, the normal is the optical axis). _Is 90 °), that is, the inclination of the normal to the Y-axis at the maximum ray effective height of the image side lens surface of the rear group lens is ψ _{R2 — 1.0,} and the image side lens surface of the rear group lens The inclination of the normal to the Y axis at 70% of the maximum effective ray height is ψ _{R2 — 0.7,} and the inclination of the normal to the Y axis of the maximum ray effective height of the object side lens surface of the rear lens group is ψ _{R1 — 1.} and _.0, the inclination and [psi _{R1_0.7} with respect to the normal line of the Y-axis at the maximum effective ray height of 70% of the object-side lens surface of the rear lens group, the maximum light of the image-side lens surface of the rear group lens effective Paired with the normal Y-axis at 30% of the height That the inclination and ψ _{R2_0.3,} the inclination relative to the normal of the Y axis in the optical axis of the image-side lens surface of the rear lens group and ψ _{R2_0.0,} maximum ray effective height of the object side lens surface of the rear lens group The inclination of the normal to the Y axis at 30% is ψ _{R1 — 0.3,} and the inclination of the normal to the Y axis of the optical axis of the object side lens surface of the rear lens group is ψ _{R1 — 0.0} .
Conditional expression:
[1] 0 ° ≤ θ <30 °
[2] _{1 <- {(ψ R2_1.0 -ψ} R2_0.7) - (ψ R1_1.0 -ψ R1_0.7)} / {(ψ R2_0.3 -ψ R2_0.0) - (ψ R1_0.3 −ψ _{R1 — 0.0} )} <7
An image reading lens characterized by satisfying both. The effective diameter of the image side lens surface of the final lens of the front group lens system is φa, the effective diameter of the object side lens surface of the rear group lens is φb, and between the front group lens system and the rear group lens system If the distance on the optical axis is ds,
Conditional expression:
[3] 0.10 <(φb−φa) / ds <0.45
The image reading lens according to claim 1, wherein: The maximum incident angle with respect to the optical axis of the principal ray of e line incident on the first lens of the front group lens system is θi, and the maximum angle with respect to the optical axis of the principal ray of e line emitted from the final lens of the front group lens system Let the injection angle be θj,
Conditional expression:
[4] 0.5 <θj / θi <1.2
The image reading lens according to claim 1, wherein the image reading lens satisfies the following. The distance on the optical axis between the image side lens surface of the rear group lens and the image sensor is Bf, and the light between the object side lens surface of the first lens of the front group lens system and the image sensor. The distance on the axis is Da,
Conditional expression:
[5] 0.05 <Bf / Da <0.12
The image reading lens according to claim 1, wherein: The distance on the optical axis between the front group lens system and the rear group lens is ds, and the total lens length is D.
Conditional expression:
[6] 0.2 <ds / D <0.6
The image reading lens according to claim 1, wherein:   The image reading lens according to claim 1, wherein an aspheric surface is formed on an object side lens surface of the rear group lens.   The image reading lens according to claim 1, wherein an outer shape of the rear group lens is non-rotationally symmetric with respect to an optical axis.   The image reading lens according to claim 7, wherein an outer shape of the rear group lens is a strip shape that is long in a direction corresponding to a main scanning direction of image reading scanning. An illumination system that illuminates the document;
An imaging lens that forms a reduced image of the reflected light of the original illuminated by the illumination system;
An image sensor that photoelectrically converts a document image formed by the imaging lens;
An image reading apparatus comprising:
An image reading apparatus, wherein the imaging lens is configured using the image reading lens according to any one of claims 1 to 8.   An image forming apparatus comprising the image reading apparatus according to claim 9.

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