JP4932208B2 - Image reading lens and image reading apparatus - Google Patents

Description

  The present invention relates to an image reading imaging lens used in an optical system of an image reading apparatus such as a facsimile or an image scanner, and more particularly to an image reading lens for image reduction or image enlargement and an image reading apparatus using the same. About.

An imaging lens for image reading used in a facsimile, an image scanner, or the like of a type in which an original image is reduced or enlarged on an imaging device such as a CCD (Charge Coupled Device) is used. Basically, it is required to have a high resolving power at magnification, a large amount of peripheral light, and a small distortion. In addition, along with the recent demand for downsizing and cost reduction of the entire optical apparatus, it is also required to make the size compact and to reduce the manufacturing cost. A reading lens described in Patent Document 1 is known as an imaging lens that can meet such requirements.
Japanese Patent No. 3335295

  The reading lens described in Patent Document 1 has a three-lens configuration of one glass lens and two resin lenses, and a positive diffractive lens surface having a chromatic aberration correction effect is formed on at least one surface of the resin lens. Thus, a compact and inexpensive optical system is achieved while correcting chromatic aberration well. However, although the reading lens described in Patent Document 1 has a wide angle of view in which the half angle of view exceeds 23 degrees as an example, the amount of residual aberration is actually large, and further in recent years. It is difficult to say that the performance can withstand high resolution and the accompanying miniaturization of the image sensor. Further, in this reading lens, the power of the diffractive action of the diffractive lens surface is relatively large, so it is estimated that there are a large number of ring zones with respect to the effective diameter of the diffractive lens surface. Less is desirable.

  The present invention has been made in view of such problems, and the object thereof is to realize a compact and inexpensive lens system with a small number of lenses, and to achieve a wide angle of view and high quality of a read image. In particular, it is an object of the present invention to provide an image reading lens and an image reading apparatus that can satisfactorily correct axial chromatic aberration and lateral chromatic aberration at the same time.

An image reading lens according to the present invention includes, in order from the object side, a first lens composed of a positive meniscus lens having a convex surface facing the object side, a second lens composed of a negative meniscus lens having a concave surface facing the object side, A third lens composed of a positive lens having a convex surface facing the image side, a diffractive lens surface is formed on the object side surface of the second lens, and is configured to satisfy the following conditions: Is.
0.012 <L / f <0.04 (5)
However,
L: the optical path length through which the outermost ray of the outermost field angle passes through the second lens
f: Focal length of the entire system
And

  An image reading apparatus according to the present invention includes the image reading lens according to the present invention.

In the image reading lens and the image reading apparatus according to the present invention, the diffractive lens surface is formed on the object-side surface of the second lens while the number of lenses is small, that is, the three-lens configuration. In particular, the longitudinal chromatic aberration and the lateral chromatic aberration are favorably corrected at the same time so that the quality of the image can be improved.
Furthermore, the following preferable conditions are appropriately adopted according to the required specifications and the like to be satisfied, so that better performance suitable for image reading can be obtained.

The image reading lens according to the present invention is preferably configured to satisfy the following conditions.
3.6 <| f12 / f3 | <6.4 (1)
M / D <3.0 (2)
(Ν1 + ν3) / 2−ν2> 21 (3)
However,
f12: Composite focal length of the first lens and the second lens f3: Focal length of the third lens M: Number of ring zones of the diffractive lens surface D: Effective diameter (mm) of the diffractive lens surface
ν i is the dispersion of the i-th lens.

The image reading lens according to the present invention is also preferably configured to satisfy the following conditions.
0.62 <| (f2 · χ1) / (f1 · χ2) | <0.98 (4 )
However it was,
f1: Focal length of the first lens f2: Focal length of the second lens χi: Function defined by the following expression of the i-th lens χi = αi− (1 / (ni−1)) · (dn / dt) i
.alpha.i: linear expansion coefficient of the i-th lens ni: refractive index of the e-line of the i-th lens (dn / dt) i: refractive index temperature coefficient of the e-line of the i-th lens
And

  The image reading lens according to the present invention may further include a diaphragm disposed between the first lens and the second lens.

  In the image reading lens according to the present invention, the first lens and the second lens are preferably resin lenses.

  According to the image reading lens or the image reading apparatus of the present invention, in order from the object side, the first lens is a positive meniscus lens having a convex surface facing the object side, and the negative meniscus lens having a concave surface facing the object side. And a third lens composed of a positive lens having a convex surface facing the image side, and a diffractive lens surface is formed on the object side surface of the second lens. While realizing a compact and inexpensive lens system with a small number of lenses, it is possible to widen the angle of view and to improve the quality of the read image. In particular, axial chromatic aberration and lateral chromatic aberration can be corrected well at the same time.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 shows a configuration example of an image reading lens according to an embodiment of the present invention. This configuration example corresponds to the lens configuration of a first numerical example (FIGS. 2A and 2B) described later. In FIG. 1, the symbol Ri indicates the radius of curvature of the i-th surface that is numbered so as to increase sequentially toward the image side (imaging side), with the most object-side component surface being first. . A symbol di indicates a surface interval on the optical axis Z1 between the i-th surface and the i + 1-th surface.

  The image reading lens 1 is suitably used for an optical system of an image reading apparatus such as a facsimile or an image scanner. The image reading lens 1 includes, in order from the object side along the optical axis Z1, a first lens L1 including a positive meniscus lens having a convex surface facing the object side, and a negative meniscus lens having a concave surface facing the object side. A second lens L2 and a third lens L3 made of a positive lens having a convex surface facing the image side. The object side surface of the second lens L2 is a diffractive lens surface. A diaphragm St is disposed between the first lens L1 and the second lens L2. It is preferable that the first lens L1 and the second lens L2 are, for example, resin lenses having both aspheric surfaces, and the third lens L3 is a glass lens.

  Flat optical members 11 and 12 are disposed on the object reading side (image reading side) and the image forming surface side (imaging surface side) of the image reading lens 1. An imaging element such as a CCD (not shown) is disposed on the image plane. The optical members 11 and 12 are a glass plate, various optical filters, a cover glass of an image sensor such as a CCD, and the like as a document placement table in an image reading apparatus to be described later.

The image reading lens 1 is preferably configured to satisfy the following conditions.
3.6 <| f12 / f3 | <6.4 (1)
M / D <3.0 (2)
(Ν1 + ν3) / 2−ν2> 21 (3)
However,
f12: Combined focal length of the first lens L1 and the second lens L2 f3: Focal length of the third lens L3 M: Number of ring zones of the diffractive lens surface D: Effective diameter (mm) of the diffractive lens surface
νi: i-th lens dispersion

The image reading lens 1 is preferably configured to satisfy the following conditions.
0.62 <| (f2 · χ1) / (f1 · χ2) | <0.98 (4)
0.012 <L / f <0.04 (5)
However,
f1: Focal length of the first lens L1 f2: Focal length of the second lens L2 L: Optical path length through which the outermost ray at the outermost field angle passes through the second lens L2 f: Focal length of the entire system

Particularly in the conditional expression (4), χi (χ1 or χ2) represents a function defined by the following expression of the i-th lens.
χi = αi− (1 / (ni−1)) · (dn / dt) i
αi: Linear expansion coefficient of i-th lens ni: Refractive index of e-line of i-th lens (dn / dt) i: Refractive index temperature coefficient of e-line of i-th lens

  FIG. 18 shows a configuration example of an image reading apparatus using the image reading lens 1. The image reading apparatus 10 is a reflective original type image scanner, and emits illumination light toward a document placing table 3 on which a document 2 to be read is placed and a document 2 on the document placing table 3, for example, in a line shape. The light source 5 and an image sensor 4 such as a CCD for capturing an image of the document 2 are provided. The image reading lens 1 is disposed between the document 2 and the image sensor 4. In this image reading apparatus 10, the light source 5 sequentially irradiates the entire surface of the document 2 over the entire surface of the document table 3 along the direction orthogonal to the extending direction (the direction indicated by the arrow A). To do. Reflected light from the document 2 is imaged on the image sensor 4 by the reading lens 1 and taken in as image information by the image sensor 4.

  The image reading lens 1 is applicable not only to the reflective original type but also to a transparent original type image reading apparatus. In the case of the transparent original type, a transparent original such as a negative film or a positive film is placed on the transparent original placing table as the original 2. Then, illumination light is irradiated from the back side of the document placing table toward the transmission original, and the transmitted light is imaged on the image sensor 4 by the reading lens 1 and taken in as image information by the image sensor 4.

Next, operations and effects of the image reading lens 1 configured as described above will be described.
In this image reading lens 1, the diffractive lens surface is formed on the object side surface of the second lens L 2, although the number of lenses is small, that is, a three-lens configuration. In particular, longitudinal chromatic aberration and lateral chromatic aberration are favorably corrected at the same time. The diffractive lens surface is preferably disposed immediately before or after the stop surface in order to reduce the secondary spectrum of chromatic aberration while keeping the lateral chromatic aberration small. In this image reading lens 1, since the diffractive lens surface is formed on the surface immediately after the stop St, chromatic aberration can be corrected well. Further, by making the first lens L1 and the second lens L2 resin lenses, it becomes easy to process the aspherical surface and the diffractive lens surface. Further, by satisfying the above-mentioned conditional expressions as appropriate according to the required specifications and the like, better performance suitable for image reading can be obtained.

  Conditional expression (1) defines an appropriate ratio between the combined focal length f12 of the first lens L1 and the second lens L2 and the focal length f3 of the third lens L3 that is a glass lens. If the lower limit of conditional expression (1) is not reached, when the first lens L1 and the second lens L2 are made of resin lenses, the power decreases, so that the amount of movement of the focus position due to temperature change can be reduced, but aberration correction Is not preferable because the required resolution performance cannot be obtained. If the upper limit is exceeded, it is advantageous for aberration correction, but it is not preferable because the amount of focus movement due to temperature change becomes large.

  Conditional expression (2) relates to the number of zonal zones of the diffractive lens surface. If the upper limit is exceeded, it is advantageous for aberration correction, but the number of zonal zones with respect to the effective diameter increases and workability deteriorates.

  Conditional expression (3) defines an appropriate relationship between the dispersion of the first lens L1 and the third lens L3 having positive power and the dispersion of the second lens L2 having negative power. If the lower limit of conditional expression (3) is not reached, the dependence on the diffractive lens surface with respect to chromatic aberration correction becomes high and it becomes difficult to ensure processability, which is not preferable.

Conditional expression (4) indicates that the powers φ1 (= 1 / f1) and φ2 (= 1 / f2) of the first lens L1 and the second lens L2, and
α- (1 / (n-1)) · dn / dt
The ratio of the product for each lens with the functions χ1 and χ2 relating to the temperature characteristics of each lens defined in (1) is specified. Whether the value falls below the lower limit or exceeds the upper limit of the range of the conditional expression (4), the amount of movement of the focus position due to the temperature change becomes large, and good performance cannot be obtained. To get better performance,
0.7 <| (f2 · χ1) / (f1 · χ2) | <0.85 (4A)
It is desirable to be in the range.

Conditional expression (5) defines an appropriate ratio between the optical path length L through which the outermost ray at the outermost field angle passes through the second lens L2 and the focal length f of the entire lens system. If the lower limit of conditional expression (5) is not reached, the moldability of the second lens L2 on which the diffractive lens surface is formed deteriorates, which is not preferable. Exceeding the upper limit is not preferable because high frequency performance deteriorates due to the influence of birefringence. In particular, when using a lens material having a large birefringence, the upper limit is
L / f <0.033 (5A)
It is desirable that

  As described above, according to the image reading lens 1 according to the present embodiment, by effectively using the diffractive lens surface, while realizing a compact and inexpensive lens system with a small number of lenses of three lenses, A wide angle of view (for example, a half angle of view of 23 ° or more) and high quality of a read image can be achieved, and particularly, axial chromatic aberration and lateral chromatic aberration can be corrected well at the same time.

  Next, specific numerical examples of the image reading lens according to the present embodiment will be described. Hereinafter, five numerical examples (Examples 1 to 5) will be described together.

  FIGS. 2A and 2B show specific lens data (Example 1) corresponding to the configuration of the image reading lens 1 shown in FIG. In particular, FIG. 2A shows basic lens data, and FIG. 2B shows data related to diffraction and aspheric surfaces. Similarly, FIGS. 3A and 3B show lens data of the image reading lens according to the second embodiment, and FIGS. 4A and 4B show images according to the third embodiment. Lens data of the reading lens is shown, FIGS. 5A and 5B show lens data of the image reading lens according to Example 4, and FIGS. 6A and 6B show the lens data. 10 shows lens data of an image reading lens according to Example 5. The basic lens configuration of the image reading lens according to Examples 2 to 5 is substantially the same as that of the image reading lens 1 shown in FIG.

  In the column of the surface number Si in the lens data shown in FIGS. 2A, 3A, 4A, 5A, and 6A, image reading according to each embodiment is performed. For the lens for use, the i-th (i = 1 to 1) is assigned such that the surfaces of the constituent elements closest to the object side including the optical members 11 and 12 and the aperture stop St are first, and increase sequentially toward the image side. 11) shows the surface number. In the column of the curvature radius Ri, the value of the curvature radius of the i-th surface from the object side is shown in correspondence with the reference symbol Ri in FIG. Similarly, the column of the surface interval di indicates the interval on the optical axis between the i-th surface Si and the i + 1-th surface Si + 1 from the object side. The unit of the value of the curvature radius Ri and the surface interval di is millimeter (mm). The column of nej indicates the value of the refractive index for the e-line (wavelength 546.07 nm) of the j-th (j = 1 to 5) optical element from the object side. In the column of νdj, the Abbe number value of the d-line (wavelength 587.6 nm) of the j-th optical element from the object side is shown.

  In the image reading lens according to each example, both surfaces S3 and S4 of the first lens L1 and both surfaces S6 and S7 of the second lens L2 are aspherical. The object side surface S6 of the second lens L2 is a diffractive lens surface. The basic lens data in FIGS. 2A, 3A, 4A, 5A, and 6A includes the curvature near the optical axis as the radius of curvature of these aspheric surfaces. The numerical value of the radius is shown.

2B, FIG. 3B, FIG. 4B, FIG. 5B, and FIG. 6B show diffraction and aspheric data in the image reading lens according to each example. In the numerical values shown as aspherical data, the symbol “E” indicates that the subsequent numerical value is a “power exponent” with a base of 10, and the numerical value represented by an exponential function with the base of 10 is Indicates that the value before “E” is multiplied. For example, “1.0E-02” indicates “1.0 × 10 ⁻² ”.

As the aspheric surface data, the values of the coefficients A _i and K in the aspheric surface expression represented by the following expression (A) are described. More specifically, Z is the length (mm) of a perpendicular line drawn from a point on the aspheric surface at a height h from the optical axis to the tangential plane (plane perpendicular to the optical axis) of the apex of the aspheric surface. Show.
Z = C · h ² / {1+ (1−K · C ² · h ² ) ^1/2 } + A ₄ · h ⁴ + A ₆ · h ⁶ + A ₈ · h ⁸ + A ₁₀ · h ¹⁰ + A ₁₂ · h ¹² + A ₁₄・ h ¹⁴ + A ₁₆・ h ¹⁶ …… (A)
However,
Z: Depth of aspheric surface (mm)
h: Distance from the optical axis to the lens surface (height) (mm)
K: Conic constant C: Paraxial curvature = 1 / R
(R: paraxial radius of curvature)
A _i : i-th aspherical coefficient (i = 4, 6, 8, 10, 12, 14, 16)

Further, as the data of the diffractive lens surface, the value of the i-th order coefficient D _i (i = 2, 4, 6, 8, 10) in the phase difference function Φ expressed by the following equation (B) is described.
Φ = D ₂ · h ² + D ₄ · h ⁴ + D ₆ · h ⁶ + D ₈ · h ⁸ + D ₁₀ · h ¹⁰ ...... (B)

  FIG. 7 shows a summary of the values for the above-described conditional expressions for each example. As can be seen from FIG. 7, the value of each example is within the numerical range of each conditional expression.

  FIGS. 8A to 8D respectively show spherical aberration, astigmatism, distortion (distortion aberration), and lateral chromatic aberration in the image reading lens according to Example 1 at normal temperature. FIGS. 9A to 9D show similar aberrations at a high temperature (normal temperature + 20 ° C.). Each aberration diagram shows an aberration with the e-line as a reference wavelength, but in the spherical aberration diagram, astigmatism diagram and lateral chromatic aberration diagram, the g-line (wavelength 435.8 nm), C-line (wavelength 656.3 nm). , Aberrations for the s-line (wavelength 852.11 nm) are also shown. In the astigmatism diagram, the solid line indicates the sagittal direction and the broken line indicates the tangential direction. FNO. Indicates an F value, and ω indicates a half angle of view.

  Similarly, various aberrations at normal temperature for Example 2 are shown in FIGS. 10A to 10D, and various aberrations at high temperature are shown in FIGS. 11A to 11D. Similarly, FIGS. 12A to 12D show various aberrations of Example 3 at normal temperature, and FIGS. 13A to 13D show various aberrations at high temperature. Similarly, FIGS. 14A to 14D show various aberrations at room temperature for Example 4, and FIGS. 15A to 15D show various aberrations at high temperature. Similarly, FIGS. 16A to 16D show various aberrations of Example 5 at normal temperature, and FIGS. 17A to 17D show various aberrations at high temperature.

  As can be seen from the numerical data and aberration diagrams described above, in each example, by effectively using the diffractive lens surface, various aberrations are corrected well, and a wide field angle of 23 ° or more is achieved with a half field angle. It has been realized. In particular, axial chromatic aberration and lateral chromatic aberration can be corrected well at the same time. In addition, aberration fluctuation at the time of temperature change is also suppressed.

  In addition, this invention is not limited to the said embodiment and each Example, A various deformation | transformation implementation is possible. For example, the radius of curvature, the surface interval, and the refractive index of each lens component are not limited to the values shown in the above numerical examples, and may take other values.

1 is a lens cross-sectional view illustrating a configuration example of an image reading lens according to an embodiment of the present invention and corresponding to Example 1. FIG. FIG. 4 is a diagram illustrating lens data of an image reading lens according to Example 1, where (A) shows basic lens data, and (B) shows data related to diffraction and aspherical surfaces. FIG. 6 is a diagram illustrating lens data of an image reading lens according to Example 2, wherein (A) shows basic lens data, and (B) shows data related to diffraction and aspherical surfaces. FIG. 6 is a diagram illustrating lens data of an image reading lens according to Example 3, where (A) shows basic lens data, and (B) shows data related to diffraction and aspherical surfaces. FIG. 10 is a diagram illustrating lens data of an image reading lens according to Example 4, where (A) shows basic lens data, and (B) shows data related to diffraction and aspherical surfaces. FIG. 10 is a diagram illustrating lens data of an image reading lens according to Example 5, wherein (A) shows basic lens data, and (B) shows data related to diffraction and aspherical surfaces. It is the figure which showed the value regarding a conditional expression collectively about each Example. FIG. 4 is an aberration diagram showing various aberrations of the image reading lens according to Example 1 at normal temperature, where (A) is spherical aberration, (B) is astigmatism, (C) is distortion, and (D) is chromatic aberration of magnification. Show. FIG. 4 is an aberration diagram showing various aberrations of the image reading lens according to Example 1 at a high temperature; (A) shows spherical aberration, (B) shows astigmatism, (C) shows distortion, and (D) shows chromatic aberration of magnification. Show. FIG. 6 is an aberration diagram showing various aberrations of the image reading lens according to Example 2 at normal temperature, where (A) is spherical aberration, (B) is astigmatism, (C) is distortion, and (D) is chromatic aberration of magnification. Show. FIG. 6 is an aberration diagram showing various aberrations of the image reading lens according to Example 2 at a high temperature; (A) shows spherical aberration, (B) shows astigmatism, (C) shows distortion, and (D) shows chromatic aberration of magnification. Show. FIG. 6 is an aberration diagram showing various aberrations of the image reading lens according to Example 3 at normal temperature, where (A) is spherical aberration, (B) is astigmatism, (C) is distortion, and (D) is chromatic aberration of magnification. Show. FIG. 6 is an aberration diagram showing various aberrations of the image reading lens according to Example 3 at a high temperature, in which (A) shows spherical aberration, (B) shows astigmatism, (C) shows distortion, and (D) shows chromatic aberration of magnification. Show. FIG. 6 is an aberration diagram showing various aberrations of the image reading lens according to Example 4 at normal temperature, where (A) is spherical aberration, (B) is astigmatism, (C) is distortion, and (D) is chromatic aberration of magnification. Show. FIG. 6 is an aberration diagram showing various aberrations of the image reading lens according to Example 4 at a high temperature, in which (A) is spherical aberration, (B) is astigmatism, (C) is distortion, and (D) is chromatic aberration of magnification. Show. FIG. 10 is an aberration diagram showing various aberrations of the image reading lens according to Example 5 at normal temperature, where (A) is spherical aberration, (B) is astigmatism, (C) is distortion, and (D) is chromatic aberration of magnification. Show. FIG. 10 is an aberration diagram showing various aberrations of the image reading lens according to Example 5 at a high temperature. (A) shows spherical aberration, (B) shows astigmatism, (C) shows distortion, and (D) shows chromatic aberration of magnification. Show. 1 is a schematic configuration diagram illustrating a configuration example of an image reading apparatus according to an embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Image reading lens, 10 ... Image reading apparatus, 11, 12 ... Optical member, L1 ... 1st lens, L2 ... 2nd lens, L3 ... 3rd lens, St ... Diaphragm, Ri ... i-th from the object side The radius of curvature of the lens surface, di: the distance between the i-th and i + 1-th lens surfaces from the object side, Z1, the optical axis.

Claims (7)

In order from the object side, a first lens composed of a positive meniscus lens having a convex surface facing the object side, a second lens composed of a negative meniscus lens having a concave surface facing the object side, and a positive lens having a convex surface directed to the image side A third lens comprising a lens is disposed,
A diffractive lens surface is formed on the object side surface of the second lens ,
Furthermore, it is configured to satisfy the following conditions:
0.012 <L / f <0.04 (5)
An image reading lens.
However,
L: the optical path length through which the outermost ray of the outermost field angle passes through the second lens
f: Focal length of the entire system
And The image reading lens according to claim 1, further configured to satisfy the following condition.
3.6 <| f12 / f3 | <6.4 (1)
M / D <3.0 (2)
(Ν1 + ν3) / 2−ν2> 21 (3)
However,
f12: Composite focal length of the first lens and the second lens f3: Focal length of the third lens M: Number of ring zones of the diffractive lens surface D: Effective diameter (mm) of the diffractive lens surface
ν i is the dispersion of the i-th lens. Further, it is configured to satisfy the following condition: 0.62 <| (f2 · χ1) / (f1 · χ2) | <0.98 (4)
The image reading lens according to claim 1, wherein the lens is an image reading lens.
However,
f1: Focal length of the first lens f2: Focal length of the second lens χi: Function defined by the following expression of the i-th lens χi = αi− (1 / (ni−1)) · (dn / dt) i
αi: Linear expansion coefficient of i-th lens ni: Refractive index of e-line of i-th lens (dn / dt) i: Refractive index temperature coefficient of e-line of i-th lens Furthermore, it is configured to satisfy the following conditions:
L / f <0.033 (5A)
The image reading lens according to claim 1, wherein the lens is an image reading lens.
However,
L: Optical path length through which the outermost light beam at the outermost field angle passes through the second lens f: The focal length of the entire system. The image reading lens according to any one of claims 1 to 4, further comprising a diaphragm disposed between the first lens and the second lens. The image reading lens according to claim 1, wherein the first lens and the second lens are resin lenses.   An image reading apparatus comprising the image reading lens according to claim 1.

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