JP4957409B2 - Reading lens - Google Patents

Description

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

Conventionally, as a document reading lens for an image reading apparatus mounted on a scanner, a digital copying machine or the like, a type based on a spherical system (Gauss type, orthometa type, xenoter type, etc.) has been widely adopted. However, with recent demands for higher density, higher speed, smaller size, and the like for image reading apparatuses, there has been an increasing demand for higher performance and wider angle of view for reading lenses. In particular, in a unit scan type color image reading apparatus which is mainstream in the A4 area, it is necessary to downsize the unit itself while maintaining high performance. For this purpose, a small sensor with a minute pixel pitch is used, and the reduction ratio of the optical system is reduced. It is necessary to set a large (projection magnification) and a short distance between object images. Along with this, high-resolution reading lenses that support color and wide angle of view are required. In order to realize this, for example, in Patent Documents 1 and 2, in a lens system having a reduction ratio of about 1/9 times, an anamorphic aspheric surface is adopted as a part of the lens system, and peripheral aberrations are effective even at a wide angle of view. A technique for correcting the above has been proposed.
JP 2001-100095 A JP 2002-214528 A

  In the prior art described in Patent Documents 1 and 2, the anamorphic aspheric lens is used to correct monochromatic aberrations, but it is difficult to correct axial chromatic aberrations. In a lens system having a large reduction ratio, in order to ensure MTF (modulation transfer function) performance corresponding to a desired spatial frequency on the original side, it is necessary to ensure MTF performance for a higher spatial frequency on the reduction side. There is. However, generally, as the spatial frequency increases, the diffraction limit level that can be realized by the optical system itself decreases. For this reason, in such a lens system, it is difficult to ensure the desired MTF performance on the document side simply by correcting the monochromatic aberration, and therefore it is necessary to correct the chromatic aberration as much as possible.

  The present invention has been made in view of such a situation, and an object of the present invention is to provide a reading lens capable of securing a high MTF with a color correspondence and a wide angle of view with a projection magnification of about 1/9 ×, and the use thereof. Another object of the present invention is to provide an image reading apparatus.

In order to achieve the above object, a reading lens according to a first aspect of the present invention is a reading lens for forming an image of a document on a one-dimensional image pickup element in order to read image information of the document. A first group having paraxial power; a second group having negative paraxial power; a third group having positive paraxial power; and a fourth group having negative paraxial power; The total number of lenses constituting the entire system is four, the lens having at least one anamorphic aspheric surface is included in the fourth group, and the positive lens in the first group has at least one aspheric surface. And satisfying the following conditional expressions (1) and (2), the lens having the anamorphic aspherical surface is made of a plastic material, the other lenses are made of a glass material, and the following conditional expression (3) It is characterized by satisfaction .
νd> 62 (1)
θgf> -0.0016 × νd + 0.6415 (2)
0.05 <| φpla | / | φtotal | <0.3… (3)
However,
νd: Abbe number,
θgf is the partial dispersion ratio, and the partial dispersion ratio θgf is expressed by the formula: θgf = (Ng-NF) / (NF-NC)
Ng: refractive index for g-line,
NF: refractive index for F-line,
NC: Refractive index for C-line,
φpla: Power of plastic lens with anamorphic aspheric surface,
φtotal: Power of the entire reading lens system
It is.

  According to a second aspect of the present invention, there is provided the reading lens according to the first aspect, further comprising an aperture stop between the second group and the third group.

  According to a third aspect of the present invention, there is provided the reading lens according to the first aspect, further comprising an aperture stop between the first group and the second group.

According to a fourth aspect of the present invention, in the reading lens according to any one of the first to third aspects, the positive lens in the third group satisfies the following conditional expressions (4) and (5): And
νd> 62 (4)
θgf> -0.0016 × νd + 0.6415 (5)
However,
νd: Abbe number,
θgf is the partial dispersion ratio, and the partial dispersion ratio θgf is expressed by the formula: θgf = (Ng-NF) / (NF-NC)
Ng: refractive index for g-line,
NF: refractive index for F-line,
NC: Refractive index for C-line,
It is.

According to a fifth aspect of the present invention, there is provided the reading lens according to the fourth aspect , wherein the positive lens in the third group has at least one aspheric surface.

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

  According to the present invention, it is possible to effectively achromatic (that is, correct axial chromatic aberration) over a wide wavelength range with a small number of lenses. In addition, off-axis aberrations can be effectively corrected, and as a result, a high MTF can be secured at a wide angle of view. Therefore, it is possible to ensure a high MTF in a reading lens corresponding to a color and a wide angle of view with a projection magnification of about 1/9 ×.

Hereinafter, a reading lens, an image reading apparatus and the like according to the present invention will be described with reference to the drawings. A reading lens according to the present invention is a reading lens that forms an image of a document on a one-dimensional image pickup element in order to read image information of the document, and includes a first group having positive paraxial power in order from the enlargement side. , A second group having negative paraxial power, a third group having positive paraxial power, and a fourth group having negative paraxial power, and having at least one anamorphic aspheric surface The fourth lens unit has a lens having the same. The positive lens in the first group has at least one aspherical surface and satisfies the following conditional expressions (1) and (2).
νd> 62 (1)
θgf> -0.0016 × νd + 0.6415 (2)
However,
νd: Abbe number,
θgf is the partial dispersion ratio, and the partial dispersion ratio θgf is expressed by the formula: θgf = (Ng-NF) / (NF-NC)
Ng: refractive index for g-line,
NF: refractive index for F-line,
NC: Refractive index for C-line,
It is.

  As described above, it is preferable to arrange anamorphic aspheric surfaces in the final group. If the anamorphic aspherical surface is arranged in the final group, the power for each image height can be effectively controlled at a position where the light fluxes of the respective image heights are relatively separated. Therefore, it becomes easy to correct the curvature of field with a relatively weak power even at a wide angle of view. The fact that it can be configured with weak power also has the advantage that performance degradation is less likely to occur even if an arrangement error occurs.

  As described above, it is preferable that the positive lens in the first group satisfies a desired material condition, that is, satisfies the conditional expressions (1) and (2). In order to effectively correct the axial chromatic aberration over a wide area, it is preferable to use a material that satisfies the conditional expressions (1) and (2) for the positive lens having strong power. In the reading lens according to the present invention, since the power of the positive lens in the first group becomes relatively strong, it is easy to correct axial chromatic aberration over a wide area by satisfying conditional expressions (1) and (2). . If the conditional expressions (1) and (2) are not satisfied, the amount of axial chromatic aberration in a wide area increases, and the MTF performance deteriorates due to the influence of axial chromatic aberration.

  As described above, it is preferable that the positive lens in the first group has at least one aspheric surface. A material that satisfies the conditional expressions (1) and (2) is generally in a region having a low refractive index. If the lens is placed in a relatively powerful group, the curvature of the lens becomes too strong. As a result, it becomes difficult to correct various aberrations including spherical aberration. Therefore, if the degree of freedom is increased by disposing an aspherical surface on the positive lens in the first group, it becomes easy to efficiently correct aberrations without increasing the number of lenses.

  With the characteristic configuration described above, it is possible to effectively achromatic (that is, correct axial chromatic aberration) over a wide wavelength range with a small number of lenses. In addition, off-axis aberrations can be effectively corrected, and as a result, a high MTF can be secured at a wide angle of view. Therefore, a high MTF can be ensured in a reading lens compatible with a color and a wide angle of view with a projection magnification of about 1/9 ×. Conditions for obtaining these effects in a well-balanced manner and achieving higher optical performance and the like will be described below.

When trying to reduce the number of lenses in the reading lens according to the present invention, not only the first group but also the power of the third group positive lens becomes stronger. Therefore, for the positive lens in the third group, it is preferable to use a material that satisfies the same conditions as described above. In other words, it is preferable that the positive lens in the third group satisfies the following conditional expressions (4) and (5). Furthermore, it is preferable that the positive lens in the third group also has at least one aspherical surface, like the positive lens in the first group.
νd> 62 (4)
θgf> -0.0016 × νd + 0.6415 (5)
However,
νd: Abbe number,
θgf is the partial dispersion ratio, and the partial dispersion ratio θgf is expressed by the formula: θgf = (Ng-NF) / (NF-NC)
Ng: refractive index for g-line,
NF: refractive index for F-line,
NC: Refractive index for C-line,
It is.

  It is desirable to have an aperture stop between the second group and the third group, or to have an aperture stop between the first group and the second group. By disposing the aperture stop between the first group and the second group or between the second group and the third group, it is possible to improve the balance between the lens system size reduction and aberration correction. . That is, by disposing the aperture stop at the center of the lens system, it is easy to downsize the lens system itself in a balanced manner, and it becomes easy to effectively correct distortion aberration and lateral chromatic aberration.

It is desirable that the lens having the anamorphic aspherical surface is made of a plastic material, and the other lenses are made of a glass material, and satisfies the following conditional expression (3).
0.05 <| φpla | / | φtotal | <0.3… (3)
However,
φpla: Power of plastic lens with anamorphic aspheric surface,
φtotal: Power of the entire reading lens system
It is.

  As described above, it is preferable in manufacturing that the lens having an anamorphic aspherical surface is made of a plastic material (that is, a resin material). In other words, the anamorphic aspheric lens is desirably a plastic lens (that is, a resin lens). When an anamorphic aspherical lens is actually manufactured, molding is generally adopted as in the case of a rotationally symmetric aspherical lens. At present, resin materials are cheaper and more accurate than glass materials, but if resin materials are used easily, the characteristics of resin materials change greatly when temperature changes. As a result, the variation of the focus position at the time of temperature change becomes large. The conditional expression (3) defines a condition for satisfactorily suppressing this defocus.

  Conditional expression (3) defines a preferable condition range regarding the power of the plastic lens. If the upper limit of conditional expression (3) is exceeded, the amount of focus movement increases in the direction of shortening the lens back with respect to the temperature rise. As a result, it swings in the opposite direction to the expansion of the actual machine on which the reading lens is mounted, and the performance is extremely deteriorated. On the other hand, if the lower limit of conditional expression (3) is exceeded, the power of the anamorphic aspheric lens becomes too small, making it difficult to correct field curvature.

  In order from the enlargement side, a first group having positive paraxial power, a second group having negative paraxial power, a third group having positive paraxial power, and a fourth group having negative paraxial power. As a specific lens configuration of the reading lens composed of groups, it is preferable that each group has a single lens configuration. That is, the number of lenses constituting the entire system is preferably four. By using one aspherical lens that satisfies the above-mentioned material conditions as the first group and one anamorphic aspherical lens as the final group, the fourth group, high performance can be achieved even when the number of lenses is limited to four. It is possible to balance with widening the angle of view.

  Next, specific optical configurations of the reading lens will be described in more detail with reference to first to third embodiments of the reading lens. 1 to 3 show optical configurations of first to third embodiments of the reading lens OP, respectively, in optical cross sections. These reading lenses OP are reading lenses that form an image of a document on a one-dimensional image pickup element (IM: image plane) in order to read image information of the document (IM: image plane). Four lenses: a first lens L1 having a second lens L2 having a negative paraxial power, a third lens L3 having a positive paraxial power, and a fourth lens L4 having a negative paraxial power. It consists of In any of the embodiments, the first lens L1, the second lens L2, and the third lens L3 are glass lenses, and the fourth lens L4 is a plastic lens (in FIGS. Shown with p.) An original table glass P1 is positioned on the enlargement side of the reading lens OP, and a cover glass P2 of an image sensor (for example, a line CCD) is positioned on the reduction side of the reading lens OP.

  In the first embodiment (FIG. 1), the first lens L1 is a positive meniscus lens convex on the magnifying side whose magnifying side surface is an aspheric surface, the second lens L2 is a biconcave negative lens, The lens L3 is a biconvex positive lens, the fourth lens L4 is a negative meniscus lens that is concave on the enlargement side, both surfaces of which are anamorphic aspheric surfaces, and an aperture stop between the second lens L2 and the third lens L3. ST is arranged. In the second embodiment (FIG. 2), the first lens L1 is a biconvex positive lens whose magnifying side surface is an aspheric surface, the second lens L2 is a negative meniscus lens concave on the reduction side, The lens L3 is a biconvex positive lens whose reduction side surface is an aspheric surface, and the fourth lens L4 is a negative meniscus lens concave on the enlargement side whose both surfaces are anamorphic aspheric surfaces, and the second lens L2 and the third lens L3. An aperture stop ST is disposed between the two. In the third embodiment (FIG. 3), the first lens L1 is a biconvex positive lens whose amplifying side surface is an aspheric surface, the second lens L2 is a biconcave negative lens, and the third lens L3 is The fourth lens L4 is a negative meniscus lens that is concave on the magnifying side and whose both sides are anamorphic aspheric surfaces. The fourth lens L4 is between the first lens L1 and the second lens L2. Is provided with an aperture stop ST.

  FIG. 16 schematically shows a schematic configuration example of an image reading apparatus equipped with a reading lens OP. The image reading apparatus shown in FIG. 16 is an optical apparatus for image reading used in devices such as a scanner, a digital copying machine, and a facsimile, and includes an illumination unit 11 that illuminates a document (that is, an object) and the illumination unit 11. A reading lens OP that forms an image of an illuminated original, and a one-dimensional image sensor that converts an optical image (image plane IM in FIGS. 1 to 3) formed by the reading lens OP into an electrical signal. And a three-line CCD 12 (Q: CCD pixel array vertical direction).

  An illumination unit 11, reflection mirrors 13a to 13e, a reading lens OP, and a three-line CCD 12 are arranged in one housing 14, and a document image is scanned by scanning the housing 14 at a constant speed V. It is configured to read. This will be described in more detail. A document is set between the document cover 15 and the document table glass P1, and the document is illuminated by the illumination unit 11 through the document table glass P1 for a long time in the main scanning direction. Reflected light from the document is guided to the reading lens OP over the entire surface of the document image by bending the optical path by the reflection mirrors 13a to 13e and moving the housing 14 in the sub-scanning direction (constant speed V). The original image illuminated by the illumination unit 11 is formed on the three-line CCD 12 by the reading lens OP. The optical image IM formed by the reading lens OP is converted into an electrical signal by the three-line CCD 12.

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

In the construction data of each example, in order from the left column, the surface number, the radius of curvature r (mm), the surface interval d (mm) on the axis, the refractive index Nd for the d-line, the Abbe number νd for the d-line, the optical Indicates the sign of the element. The surface with * in the surface number is an aspherical surface (i.e., rotationally symmetric aspherical surface), and the following equation (AS) using a local Cartesian coordinate system (x, y, z) with the surface vertex as the origin Defined by Also, the surface numbered with # is an anamorphic aspheric surface, and is defined by the following formula (BS) using a local Cartesian coordinate system (x, y, z) with the surface vertex as the origin. . Aspherical data and anamorphic aspherical data for each example are also shown. However, the coefficient of the term without description is 0, and En = × 10 ⁻ⁿ for all data.
z = (c · h ² ) / [1 + √ {1− (1 + k) · c ² · h ² }] + A · h ⁴ + B · h ⁶ + C · h ⁸ + D · h ¹⁰ + E · h ¹² + F · h ¹⁴ + G ・ h ¹⁶ + H ・ h ¹⁸ + J ・ h ²⁰ … (AS)

…(BS)

… (BS)

However,
h: height (h ² = x ² + y ² ) in the direction perpendicular to the z-axis (optical axis AX)
z: The amount of sag in the direction of the optical axis AX at the position of height h (based on the surface vertex),
c: curvature at the surface vertex (the reciprocal of the radius of curvature r),
k: conic constant,
A, B, C, D, E, F, G, H, J: Aspheric coefficients of 4th order, 6th order, 8th order, 10th order, 12th order, 14th order, 16th order, 18th order, 20th order,
Cj: coefficient of x ^m y ⁿ [j = {(m + n) ² + m + 3n} / 2 + 1],
It is.

  As the refractive index of the optical material constituting each optical element, the refractive index NC for the C line (wavelength 656.28 nm), the refractive index Nd for the d line (wavelength 587.56 nm), and the refractive index Ne for the e line (wavelength 546.07 nm) , The refractive index NF for the F-line (wavelength 486.13 nm) and the refractive index Ng for the g-line (wavelength 435.84 nm), respectively. As various data, the focal length (mm), the F number (effective F number), the projection magnification, the reading width (mm), and the distance between object images (mm) regarding the e-line are shown. Table 1 shows values corresponding to the conditional expressions and related data of each example.

  FIGS. 4, 6, and 8 are longitudinal aberration diagrams corresponding to Examples 1, 2, and 3, respectively. In each figure, in order from the left, spherical aberration diagrams (LONGITUDINAL SPHERICAL ABER.), Astigmatism diagrams (ASTIGMATIC) FIELD CURVES) and distortion diagram (DISTORTION). The spherical aberration diagram shows the amount of spherical aberration for the C line (wavelength 656.2800 nm) indicated by the solid line, the amount of spherical aberration for the e line (wavelength 546.0700 nm) indicated by the broken line, and the amount of spherical aberration for the g line (wavelength 435.8400 nm) indicated by the alternate long and short dash line Is expressed by the amount of deviation in the optical axis AX direction from the paraxial image plane (unit: mm, horizontal axis scale: -0.200 to 0.200 mm), and the vertical axis indicates the incident height to the pupil at its maximum height. The value normalized by (ie, relative pupil height). In the astigmatism diagram, two-dot chain lines Y1, Y2, and Y3 are tangential image surfaces for C line, e line, and g line, and solid line X1, broken line X2, and one-dot chain line X3 are sagittal images for C line, e line, and g line. The surface is represented by the amount of deviation in the optical axis AX direction from the paraxial image plane (unit: mm, horizontal axis scale: -0.200 to 0.200 mm), and the vertical axis is the object height (OBJ HT, unit: mm, vertical) Axis scale: 0 to -110.00 mm). In the distortion diagrams, the horizontal axis represents the distortion (unit:%, horizontal axis scale: -0.100% to 0.100%) with respect to the C line (solid line), e line (dashed line), and g line (dashed line). The axis represents the object height (OBJ HT, unit: mm, vertical scale: 0 to -110.00 mm).

  5, 7, and 9 are lateral aberration diagrams corresponding to Examples 1, 2, and 3, respectively. In each lateral aberration diagram, the left column (Y-FAN) is a lateral aberration diagram with a tangential beam, and the right column (X-FAN) is a lateral aberration diagram with a sagittal beam. The solid line represents the lateral aberration with respect to the C line, the broken line represents the lateral aberration with respect to the e line, and the alternate long and short dash line represents the lateral aberration with respect to the g line. Each lateral aberration diagram shows lateral aberration (mm) at an image height ratio (half angle of view ω °) represented by RELATIVE FIELD in (X, Y) in each diagram. The image height ratio is a relative image height obtained by normalizing the image height y ′ with the maximum image height y′max (y′max corresponds to OBJ HT = −110.00 mm).

  10, 12, and 14 are chromatic aberration diagrams of magnification corresponding to Examples 1, 2, and 3, respectively, and FIGS. 11, 13, and 15 are on axes corresponding to Examples 1, 2, and 3, respectively. It is a chromatic aberration diagram. In the lateral chromatic aberration diagram, lateral chromatic aberration (Lateral Color) with respect to the C line (solid line), e line (dashed line), and g line (dashed line) is shown for each image height y ′. In the longitudinal chromatic aberration diagram, the wavelength ( The longitudinal chromatic aberration is indicated by the back focus difference (LB) with respect to nm).

An example of the specifications of an image reading apparatus equipped with the reading lens OP of each embodiment is given below.
Lens configuration: L1 ... positive glass lens, L2 ... negative glass lens,
L3: Positive glass lens, L4: Negative plastic lens CCD Pixel pitch: 4.7 μm 3-line CCD
Reading magnification: -0.1110236 (corresponding to 600 dpi)
Scanning document width: A4 short side width equivalent (220mm)
Effective F-number: 5.1-5.4

Example 1
Unit: mm
Surface data surface number rd Nd νd Optical element
1 ∞ 3.000 1.516330 64.1 P1
2 ∞ Any
3 * 11.772 6.16 1.487490 70.2 L1
4 82.262 0.37
5 -205.709 1.20 1.672700 32.1 L2
6 11.641 0.25
7 (ST) ∞ 0.42
8 16.423 3.77 1.620411 60.3 L3
9 -13.001 10.50
10 # -8.981 1.89 1.530481 55.7 L4
11 # -11.858 4.24
12 ∞ 0.70 1.516 330 64.1 P2
13 ∞ 0.20
Image plane ∞

Aspheric data 3rd surface
k = 1.06872E-01,
A = -1.24788E-04, B = -1.79013E-06, C = -1.66988E-08, D = -1.42092E-10, E ~ J = 0

Anamorphic aspheric data 10th surface
k = 0,
C11 = 1.28086E-03, C13 = 1.70744E-03, C15 = -1.19089E-04,
C24 = 3.67969E-05, C26 = -9.48176E-06, C28 = 6.32189E-06,
C45 = -1.34463E-08
11th page
k = 0,
C11 = 2.25202E-03, C13 = 2.53146E-03, C15 = -1.05488E-04,
C24 = 2.11777E-05, C26 = -1.80646E-05, C28 = 4.34998E-06,
C45 = -1.82761E-08

Refractive index optical element of optical material NC Nd Ne NF Ng
656.28nm 587.56nm 546.07nm 486.13nm 435.84nm
P1, P2 1.513855 1.516330 1.518251 1.521905 1.526213
L1 1.485344 1.487490 1.489147 1.492285 1.495963
L2 1.666606 1.672700 1.677651 1.687564 1.700113
L3 1.617275 1.620411 1.622865 1.627566 1.633149
L4 1.527670 1.530481 1.532747 1.537194 1.542587

Various data focal length: 20.25mm (e line)
F number (effective F number): 4.4 (5.0)
Projection magnification: -0.1110236
Reading width: 220mm
Distance between objects: 230mm

Example 2
Unit: mm
Surface data surface number rd Nd νd Optical element
1 ∞ 3.000 1.516330 64.1 P1
2 ∞ Any
3 * 10.168 1.89 1.496999 81.5 L1
4 -70.728 1.28
5 137.513 1.28 1.613397 44.3 L2
6 7.682 0.31
7 (ST) ∞ 0.79
8 26.140 3.73 1.496999 81.5 L3
9 * -9.135 11.75
10 # -9.381 1.20 1.530481 55.7 L4
11 # -12.563 4.50
12 ∞ 0.70 1.516 330 64.1 P2
13 ∞ 0.20
Image plane ∞

Aspheric data 3rd surface
k = 2.89484E-01,
A = -2.93660E-04, B = -6.77606E-06, C = 5.87705E-08, D = -1.15787E-09, E ~ J = 0
9th page
k = 0,
A = -1.69524E-04, B = -7.63377E-06, C = 2.77286E-08, D = -1.01430E-08, E ~ J = 0

Anamorphic aspheric data 10th surface
k = 0,
C11 = 1.31506E-03, C13 = 7.48593E-04, C15 = -1.83721E-04,
C24 = 2.74108E-04, C26 = 1.22878E-05, C28 = -3.17911E-07,
C41 = -4.52046E-06, C43 = -1.66336E-07, C45 = 3.07368E-08
11th page
k = 0,
C11 = 2.00034E-03, C13 = 1.48719E-03, C15 = -1.88745E-04,
C24 = 2.59406E-04, C26 = 2.14924E-06, C28 = 9.33438E-07,
C41 = -3.54122E-06, C43 = -1.21526E-07, C45 = 6.07494E-09

Refractive index optical element of optical material NC Nd Ne NF Ng
656.28nm 587.56nm 546.07nm 486.13nm 435.84nm
P1, P2 1.513855 1.516330 1.518251 1.521905 1.526213
L1 1.495136 1.496999 1.498455 1.501231 1.504506
L2 1.609248 1.613397 1.616690 1.623105 1.630910
L3 1.495136 1.496999 1.498455 1.501231 1.504506
L4 1.527670 1.530481 1.532747 1.537194 1.542587

Various data focal length: 20.27mm (e line)
F number (effective F number): 4.8 (5.4)
Projection magnification: -0.1110236
Reading width: 220mm
Distance between objects: 230mm

Example 3
Unit: mm
Surface data surface number rd Nd νd Optical element
1 ∞ 3.000 1.516330 64.1 P1
2 ∞ Any
3 * 9.845 3.07 1.496999 81.5 L1
4 -94.337 1.28
5 (ST) ∞ 0.20
6 -25.104 1.88 1.672700 32.1 L2
7 65.229 0.80
8 178.392 4.82 1.496999 81.5 L3
9 * -12.751 8.78
10 # -10.644 1.96 1.530481 55.7 L4
11 # -13.781 4.50
12 ∞ 0.70 1.516 330 64.1 P2
13 ∞ 0.20
Image plane ∞

Aspheric data 3rd surface
k = -6.92516E-02,
A = -4.19061E-05, B = 1.51518E-06, C = -8.16996E-08, D = 1.38360E-09, E ~ J = 0
9th page
k = 2.21936E + 00,
A = 3.64424E-04, B = 3.65646E-06, C = 2.11184E-07, D = -2.16160E-09, E ~ J = 0

Anamorphic aspheric data 10th surface
k = 0,
C11 = -5.40557E-03, C13 = 1.72839E-03, C15 = 4.76815E-05,
C24 = 3.56013E-04, C26 = -9.25594E-06, C28 = -5.95264E-07,
C41 = -3.83571E-06, C43 = 1.39549E-07, C45 = 2.34604E-08
11th page
k = 0,
C11 = -8.81589E-03, C13 = 2.43696E-03, C15 = -2.98954E-05,
C24 = 4.44867E-04, C26 = -1.36693E-05, C28 = -3.14143E-07,
C41 = -3.67858E-06, C43 = 7.84118E-08, C45 = 3.31286E-09

Refractive index optical element of optical material NC Nd Ne NF Ng
656.28nm 587.56nm 546.07nm 486.13nm 435.84nm
P1, P2 1.513855 1.516330 1.518251 1.521905 1.526213
L1 1.495136 1.496999 1.498455 1.501231 1.504506
L2 1.666606 1.672700 1.677651 1.687564 1.700113
L3 1.495136 1.496999 1.498455 1.501231 1.504506
L4 1.527670 1.530481 1.532747 1.537194 1.542587

Various data focal length: 20.35mm (e line)
F number (effective F number): 4.8 (5.4)
Projection magnification: -0.1110236
Reading width: 220mm
Distance between objects: 230mm

The optical path figure of 1st Embodiment (Example 1). The optical path figure of 2nd Embodiment (Example 2). The optical path figure of 3rd Embodiment (Example 3). FIG. 3 is a longitudinal aberration diagram of Example 1. FIG. 4 is a lateral aberration diagram of Example 1. FIG. 6 is a longitudinal aberration diagram of Example 2. FIG. 4 is a lateral aberration diagram of Example 2. FIG. 6 is a longitudinal aberration diagram of Example 3. FIG. 4 is a lateral aberration diagram of Example 3. FIG. 3 is a chromatic aberration diagram of magnification of Example 1. FIG. 3 is an on-axis chromatic aberration diagram of Example 1. FIG. 3 is a chromatic aberration diagram of magnification of Example 2. FIG. 6 is an axial chromatic aberration diagram of Example 2. FIG. 4 is a chromatic aberration diagram of Example 3 for magnification. FIG. 6 is an on-axis chromatic aberration diagram of Example 3. FIG. 2 is a schematic diagram schematically illustrating a schematic configuration example of an image reading apparatus.

Explanation of symbols

P1 Document glass P2 Cover glass OP Reading lens L1 First lens (first group, positive lens)
L2 Second lens (second group)
L3 3rd lens (3rd group, positive lens)
L4 4th lens (4th group)
ST Aperture stop IM Image plane (optical image)
11 Lighting unit (lighting device)
12 3-line CCD (one-dimensional image sensor)
13a, 13b, 13c, 13d, 13e Reflective mirror 14 Housing 15 Document cover AX Optical axis

Claims (6)

A reading lens that forms an image of a document on a one-dimensional image sensor for reading image information of the document, and has a first group having a positive paraxial power and a negative paraxial power in order from the enlargement side. The second group, a third group having a positive paraxial power, and a fourth group having a negative paraxial power, the total number of lenses constituting the entire system is four, and an anamorphic aspheric surface A lens having at least one surface in the fourth group, a positive lens in the first group having at least one aspherical surface, and satisfying the following conditional expressions (1) and (2): And a lens having the anamorphic aspheric surface is made of a plastic material, and the other lens is made of a glass material, and satisfies the following conditional expression (3) :
νd> 62 (1)
θgf> -0.0016 × νd + 0.6415 (2)
0.05 <| φpla | / | φtotal | <0.3… (3)
However,
νd: Abbe number,
θgf is the partial dispersion ratio, and the partial dispersion ratio θgf is expressed by the formula: θgf = (Ng-NF) / (NF-NC)
Ng: refractive index for g-line,
NF: refractive index for F-line,
NC: Refractive index for C-line,
φpla: Power of plastic lens with anamorphic aspheric surface,
φtotal: Power of the entire reading lens system
It is.   The reading lens according to claim 1, further comprising an aperture stop between the second group and the third group.   The reading lens according to claim 1, further comprising an aperture stop between the first group and the second group. The reading lens according to any one of claims 1 to 3 , wherein the positive lens in the third group satisfies the following conditional expressions (4) and (5).
νd> 62 (4)
θgf> -0.0016 × νd + 0.6415 (5)
However,
νd: Abbe number,
θgf is the partial dispersion ratio, and the partial dispersion ratio θgf is expressed by the formula: θgf = (Ng-NF) / (NF-NC)
Ng: refractive index for g-line,
NF: refractive index for F-line,
NC: Refractive index for C-line,
It is. The reading lens according to claim 4, wherein the positive lens in the third group has at least one aspheric surface. An illumination device that illuminates the document, and an image of the document illuminated by the illumination device is formed, and an optical image formed by the reading lens according to any one of claims 1 to 5 is electrically generated. An image reading apparatus comprising: the one-dimensional image sensor that converts the signal into a typical signal.

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