JP2003195165A - Lens for reading document and document reader using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a lens for reading a document, which has, specially, its chromatic aberration among various aberrations excellently corrected over a wide band, a small Fno (that is, light) (F number: ≤4.0) and a large aperture, and high MTF to high spatial frequencies, and provide a document reader using the lens. <P>SOLUTION: This lens for imaging image information on a document surface lighten by a light source, on a read element surface has 1st, 2nd, 3rd, and 4th lenses in order from the document surface, also has a diffracting optical element in the optical path, and meets respective conditional expressions. In the conditional expressions, N<SB>1</SB>is the refractive index of the material of the 1st lens, (f) the focal length of the whole system, ϕ<SB>d</SB>the refracting power of a diffracting optical element surface, and L<SB>t</SB>the sum of the absolute values of an on-axis chromatic aberration coefficient LR to a wavelength longer than the reference wavelength of the whole system in the absence of the diffracting optical element and an on-axis chromatic aberration coefficient LB to a wavelength shorter than the reference wavelength. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an original reading lens having a diffractive optical element and an original reading apparatus using the same, and is particularly bright Fno (F number) while maintaining a high aperture ratio up to a peripheral angle of view. It is suitable for digital reading devices such as digital copying machines and image scanners, in which each aberration is corrected in a well-balanced manner and which has high resolution.

[0002]

2. Description of the Related Art Generally, in a digital copying machine, an image scanner or the like, image information of a document illuminated by an illuminating device is reduced and imaged on a reading element surface by a document reading lens, and the document surface is sequentially scanned. The image information is read as digital data and converted into a signal that can be electrically processed. A one-dimensional solid-state image pickup device (line sensor) is often used as a reading device for such a digital copying machine or an image scanner.

Further, in a document reading lens used in a digital copying machine, an image scanner or the like, as a color reading element, for example, a three-line sensor in which three line sensors (CCD) are arranged on the same substrate is often adopted. is doing. In this structure, light receiving elements having red (R), green (G), and blue (B) color filters are arranged in three rows on one chip, and a color image is formed on the light receiving element surface. Information is signaled.

In the original reading lens used in this digital copying machine, image scanner, etc., in order to read a color original satisfactorily, RGB of 3 on the light receiving element surface is used.
It is necessary to match the image forming positions of the respective color lights of the primary colors in the optical axis direction. Therefore, chromatic aberration must be corrected well for each color. This chromatic aberration needs to be corrected extremely accurately.

Further, in such a document reading lens,
In general, an image is required to have a high contrast in a high spatial frequency region, and in view of compactness and high-speed reading, it is required to be small in size, have a high angle of view, and have an aperture efficiency close to 100% up to the peripheral part of the angle of view. . Further, it is naturally required that the structure is simple and the cost is low.

As an original reading lens satisfying such a demand, for example, there is a symmetrical lens system in which a plurality of lenses are arranged substantially symmetrically at the center of the diaphragm. As this symmetric type lens system, for example, there is a Gauss type lens having a structure of 4 elements in 6 groups.

In general, a symmetrical lens system has a feature that an image can be easily obtained with a particularly high resolution and high image quality.

[0008]

When the image information on the original surface is reduced and imaged on the sensor surface by the original reading lens and the image information is read as electronic information by the signal from the sensor, the entire original surface is read. It becomes important to form a high resolution image on the sensor surface.

As a document reading lens, for example, there is a tesser type document reading lens having a lens configuration of 3 elements in 4 groups. This original reading lens has various aberrations such as spherical aberration, field curvature aberration, and distortion aberration corrected relatively well,
Moreover, since the number of lenses is small, the cost is low.

However, a large amount of astigmatism at the intermediate image height and off-axis coma aberration tend to remain. For this reason,
The tesser type document reading lens is used in a document reading device having a relatively low resolution and a narrow angle of view.

Although this tesser-type original reading lens is corrected to a certain level with respect to chromatic aberration, particularly axial chromatic aberration, it cannot be said that the lens system for reading a color image is sufficiently satisfactory. It was

In particular, the axial chromatic aberration is overcorrected on the short wavelength side of the visible wavelength region, and on the contrary is undercorrected on the long wavelength side, and it is difficult to satisfactorily correct it in a wide visible wavelength region. Had the following spectrum). Therefore, when such a document reading lens is used in a color image reading device such as an image scanner, the focus positions of R (red), G (green), and B (blue) are slightly different. There is a problem that the read image is deteriorated.

On the other hand, a Gauss type lens having a lens configuration of 4 groups and 6 elements as the original reading lens has more aberrations than the tesser type due to the large number of lenses, but the cost is increased. .

Regarding chromatic aberration, it has the same residual chromatic aberration (secondary spectrum) as the above-mentioned lens for reading a document of the tesser type. Therefore, there is a problem in that the focus position is different for each color of R, G, and B, and the read image is deteriorated.

In order to correct this residual chromatic aberration with respect to a wavelength in a wide band, it is necessary to use a glass (anomalous dispersion glass) having an extraordinary partial dispersion with a small change in refractive index even in a wide band wavelength. In general, the glass has a problem that it is expensive and difficult to process.

Various document reading lenses using a diffractive optical element have been proposed to reduce chromatic aberration. However, this document reading lens has a problem that a read image is deteriorated by flare light of orders other than the designed value. As a method of reducing this flare light,
For example, as shown in FIGS. 14 and 15, diffraction gratings 105 and 10 made of two kinds of resins having different dispersion characteristics are provided on a substrate 104.
There is a method of laminating 6 together.

However, there is a problem in that it is difficult to manufacture the type having the different depths d ₁ and d _{2 of the} diffraction grating shown in FIG. 14, and the type shown in FIG. 15 has a depth d of the diffraction gratings 105 and 106. Is as deep as 80 to 90 μm, and there is a problem that a ray having an angle of view is reflected by the wall portion of the diffraction grating.

As a lens system, for example, Japanese Patent Laid-Open No. 10-339843 can be cited. The Fno (F number) of the lens is as dark as 4.5, and the reduction ratio is 0.15 to 0.19. Reduction ratio is 0.22 because it is small
If the above is the case, there is a problem that the amount of aberration increases.

According to the present invention, by properly disposing the diffractive optical element in the lens system, chromatic aberration among various aberrations is satisfactorily corrected over a wide band, and Fno (F number = 4. (0 or less) is bright and has a large diameter, and a document reading lens having a high MTF with respect to a high spatial frequency and a document reading apparatus using the same are provided.

[0020]

According to another aspect of the invention, there is provided an original reading lens for forming an image on a reading element surface of image information on an original surface illuminated by a light source. In order from the surface side, a meniscus-shaped first lens having a positive refractive power whose convex surface faces the document surface side, a second meniscus-shaped negative lens having a convex surface facing the document surface side, a diaphragm, It has a meniscus-shaped negative third lens having a convex surface facing the reading element side and a meniscus-shaped fourth lens having a positive refractive power facing the reading element side, and diffractive optics in the optical path. An element, the refractive index of the material of the first lens is N ₁ , the focal length of the entire system is f, the refractive power of the diffractive optical element surface is φ _d , and the reference of the entire system in the absence of the diffractive optical element Longitudinal wavelength chromatic aberration coefficient L on the long wavelength side of the wavelength
When L _t is the sum of the absolute values of _R and the axial chromatic aberration coefficient L _B of the wavelength on the short wavelength side with respect to the reference wavelength, 1.65 <N ₁ (1) 0.06 <f × phi _d is characterized in <0.1 ‥‥‥ (2) 0.007 <satisfying the L _t <0.012 ‥‥‥ (3) becomes a condition.

The invention of claim 2 is characterized in that, in the invention of claim 1, the diffractive optical element is arranged immediately before or after the diaphragm.

According to a third aspect of the present invention, in the first or second aspect of the invention, the diffractive optical element is arranged closer to the original surface side than the diaphragm.

According to a fourth aspect of the present invention, in the diffractive optical element according to the first, second or third aspects, the diffraction grating surfaces of the grating portion having positive power and the grating portion having negative power are opposed to each other. It is characterized in that it is a laminated type diffractive optical element that is joined together.

According to a fifth aspect of the present invention, in the fourth aspect, the diffractive optical element is characterized in that the grating portion is formed on a parallel plate.

According to a sixth aspect of the present invention, in the fourth aspect of the invention, the diffractive optical element has the grating portion formed on a curved surface.

According to a seventh aspect of the present invention, there is provided a document reading device which forms image information on a document surface on a reading element surface by using the document reading lens according to any one of the first to sixth aspects. It is characterized by that.

[0027]

1 to 3 are lens cross-sectional views of Embodiments 1 to 3 of a document reading lens according to the present invention, each showing a case suitable for a document reading apparatus. 4 and 5
Are aberration diagrams when the imaging magnification β = −0.22 of the first embodiment of the present invention, and FIGS. 6 and 7 respectively show imaging magnification β = −0.22 of the second embodiment of the present invention. FIG. 8 is a diagram of various aberrations, and FIGS. 8 and 9 are diagrams of aberrations when the imaging magnification β = −0.22 according to the third embodiment of the present invention.

In the lens cross-sectional view, the left side is the enlargement side (the one having a longer conjugate point) on the drawing side, the document surface T side (the side on which the read image is provided), and the right side is the reduction side (the shorter the conjugate point).
And the image plane IP side (for example, the side on which a CCD as a reading element is provided).

1 to 3, PL is a document reading lens, which forms image information on the document surface T illuminated by a light source such as a Xe lamp (xenon lamp) on the reading element IP surface. There is. T is a document surface on which image information is formed. IP is a line sensor (CCD) as a reading element. Gi is an i-th lens forming the document reading lens PL, and SP is a diaphragm.

GP is a laminated type diffractive optical element, and is a first diffraction grating GP having a positive power grating portion on a parallel plate.
1 and a second diffraction grating GP2 having a negative power grating portion on a parallel plate are joined together with their diffraction grating surfaces facing each other.

In each embodiment, a single-layer diffractive optical element may be used.

Among the i-th lenses Gi constituting the document reading lens PL, G1 is a meniscus-shaped first lens having a positive refractive power whose convex surface faces the document surface side, and G2 has a convex surface facing the document surface side. Second meniscus lens with negative refractive power, G
Reference numeral 3 denotes a meniscus-shaped third lens having a negative refractive power whose convex surface faces the line sensor side, and G4 denotes a meniscus-shaped fourth lens whose convex surface faces a line sensor side and having a positive refractive power.

In the first to third embodiments, the second lens G2
And an aperture stop SP between the third lens G3 and the second lens G3.
A laminated diffractive optical element GP is provided between the lens G2 and the stop SP.
Are arranged.

In each embodiment, the refractive index of the material of the first lens G1 is N ₁ , the focal length of the entire system is f, the refractive power of the laminated diffractive optical element GP is φ _d , and the laminated diffractive optical element G is
Reference wavelength of the whole system in the absence of P (e line = 543n
m) and the longitudinal chromatic aberration coefficient L _R of the wavelength (622 nm) on the long wavelength side and the wavelength (4
58 nm) and the sum of absolute values with the axial chromatic aberration coefficient L _B is L
_{t (L t = | L R} | + | L B |) when the (. However, the axial chromatic aberration coefficient is a value obtained when normalizing the focal length of the image reading lens 1) 1.65 <N ₁ ... (1) 0.06 <f x φ _d <0.1 ... (2) 0.007 <L _t <0.012 ... (3) One or more of the following conditions: Is set to satisfy.

Next, the technical meanings of the conditional expressions (1) to (3) will be described.

Conditional expression (1) is an expression for defining the glass material of the first lens G1, and when a glass material having a refractive index less than the numerical value is used, the first lens is used in the case of a lens system having a bright Fno (F number). The size of G1 becomes large and the entire lens system becomes large. Therefore, the cost of the entire lens system increases, which is not good.

Conditional expression (2) is an expression for satisfactorily correcting the axial chromatic aberration. If the upper limit value is exceeded, the refractive power φ _d of the laminated diffractive optical element becomes large, and the axial chromatic aberration is overcorrected. In addition, the difference in the grating pitch of the grating portion between the central portion and the peripheral portion of the laminated diffractive optical element increases, which makes manufacturing difficult, which is not preferable.

When the value goes below the lower limit, the refractive power φ _{d of the} surface of the laminated diffractive optical element becomes small, the axial chromatic aberration is undercorrected, and the influence of diffracted light other than the first-order diffracted light increases, resulting in flare. It is not good because it will be.

Conditional expression (3) is an expression for defining the axial chromatic aberration in the absence of the laminated diffractive optical element GP. If the upper limit value is exceeded, when the laminated diffractive optical element GP is inserted, However, the spherical aberration is deteriorated and the balance with other various aberrations cannot be obtained, which is not good. On the other hand, if the lower limit is exceeded, correction of lateral chromatic aberration will be insufficient when the laminated diffractive optical element GP is inserted, which is not preferable.

Further, in the present invention, it is preferable to set the conditional expressions (1) to (3) as follows.

1.68 <N ₁ (1 ′) 0.061 <f × φ _d <0.08 (2 ′) 0.008 <L _t <0.01 (3) ′) In the present invention, in each of the embodiments, the laminated diffractive optical element GP is arranged near the diaphragm SP and on the incident surface (original surface) side. Specifically, it is arranged immediately before the aperture stop SP. This is because by arranging the laminated diffractive optical element GP in the vicinity of the stop SP, the passing position of the off-axis light beam from the optical axis is low, and the adverse effect on the off-axis aberration is minimized, and the axial chromatic aberration is minimized. This is for efficiently correcting.

Further, since the grating portion is formed on the parallel plate, the distance from the stop SP to the diffraction grating surface must be separated by at least the thickness of the parallel plate. Since the coefficient of chromatic aberration of magnification tends to become worse as the diffraction grating surface moves away from the stop, if the laminated diffractive optical element GP is arranged on the exit surface side of the stop SP, that is, the line sensor IP side, spherical aberration and image surface will be reduced. When the bending aberration, the distortion aberration, and the axial chromatic aberration are properly suppressed, the chromatic aberration of magnification is insufficiently corrected because the number of lenses is small.

Therefore, taking into consideration that both the lateral chromatic aberration and the axial chromatic aberration are corrected well at all angles of view in the line sensor IP, the laminated diffractive optical element GP is set to the diaphragm S.
It is corrected by arranging it on the incident surface side from P, that is, on the document surface side.

Further, here, the lattice portion is formed on the parallel plate, but the same effect can be obtained by forming the lattice portion on a surface having a curvature.

The grating pitch of the grating portions of the first and second diffraction gratings GP1 and GP2 used in each embodiment is λ ₀ as the reference wavelength (d line) of the light source, h is the distance from the optical axis, and the phase coefficient is Is C _{2 i} (i = 1, 2 ...) And the phase is φ (h), φ (h) = 2π / λ ₀ · (C ₂ · h ² + C ₄ · h ⁴ + C ₆ · h ⁶
+ ··· + C _2i · h ²ⁱ ). In each embodiment, the primary light is the design value.

FIG. 10 and FIG. 11 are explanatory views for explaining the laminated diffractive optical element GP according to the present invention.

As shown in FIG. 10, the first diffraction grating GP1
Is formed on the surface of the parallel plate glass 101 with a positive power grid portion 102 made of an ultraviolet curable resin,
Similarly, in the second diffraction grating GP2, a grating portion 104 made of an ultraviolet curable resin and having a negative power is formed on the surface of the parallel plate glass 103. Further, the diffraction grating surface 10
The laminated diffractive optical element GP is achieved by arranging 2a and 104a so as to face each other and fixing the peripheries with an adhesive 105.

As a similar effect, as shown in FIG. 11, the first diffraction grating GP constituting the diffractive optical element GP.
The first and second diffraction gratings GP2 may be formed by integrally molding the substrate 101 (103) and the grating portion 102 (104). For example, a plastic optical material integrally injection-molded may be used.

The UV-curable resin of the lattice portion 102 used in each embodiment uses a product name RC8922 (Dainippon Ink and Chemicals, Inc.), and the UV-curable resin of the lattice portion 104 is a product. Name UV1000 (Mitsubishi Chemical Corporation)
Are using.

In each embodiment, when the grating depths of the diffraction gratings of the first and second grating portions 102 and 104 are d ₁ and d ₂ respectively, the grating depths d ₁ and d ₂ are { _{n 1 (λ a) -1}} d 1 - {n 2 (λ a) -1} d 2 = mλ a ·· (4) {n 1 (λ b) -1} d 1 - {n 2 (λ _b ) -1} d ₂ = mλ _b ··· (5) where n _{1 is} the refractive index of the material of the first grating part 102 n _{2 is} the refractive index of the material of the second grating part 104 is λ _a : the first Set wavelength (μm) λ _b : second set wavelength (μm) m: 1, 2, 3, ...

In the combination of the above materials, (4),
Using the equation (5), the design order is set as m = 1, and the depths of the lattices of the first and second lattice portions 102 and 104 are respectively d ₁ = 1.
It is set to 0.295 (μm) and d ₂ = 7.432 (μm).

FIG. 12 is a schematic view of a main part when the document reading lens of the present invention is applied to a document reading device of a digital color copying machine.

In the figure, reference numeral 2 denotes a document table glass, on which the document 1 is placed. An illumination light source 4 is composed of, for example, a halogen lamp, a fluorescent lamp, a xenon lamp, or the like. A reflecting shade 3 reflects the light flux from the illumination light source 4 to efficiently illuminate the original.
Reference numerals 5, 6, and 7 denote first, second, and third reflecting mirrors in this order, and the optical path of the light flux from the original 1 is bent inside the main body. Reference numeral 8 denotes the document reading lens according to any one of the above-described first to third embodiments, which forms a light flux based on the image information of the document 1 on the surface of the reading element 9. Reference numeral 9 is a line sensor (CCD) as a reading element. 10 is the main body, 11
Is a pressure plate.

In this embodiment, the luminous flux emitted from the illumination light source 4 illuminates the original 1 directly or via the reflection shade 3, and the reflected light from the original 1 is reflected by the first, second and third reflection mirrors 5. , 6, 7, the optical path of the light flux is bent inside the main body, and an image is formed on the CCD 9 surface by the document reading lens 8. At this time, the image information of the original 1 is read by electrically scanning the main scanning direction while the first, second, and third reflecting mirrors 5, 6, and 7 move in the sub-scanning direction. At this time, the second and third reflection mirrors 6 and 7 are moved by half the movement amount of the first reflection mirror 5 to move the original 1 and CC.
The distance from D9 is constant.

In the present embodiment, the original reading lens of the present invention is applied to the image reading apparatus having the 1: 2 scanning optical system, but the present invention is not limited to this, and for example, an integrated type (flat bed type) shown in FIG. The present invention can be applied to the image reading apparatus described above in the same manner as the above-described embodiment.

In FIG. 13, the luminous flux emitted from the illumination means 24 illuminates the original 21 directly or via the reflection shade 23, and the reflected luminous flux from the original 21 is divided into the first, second, third,
The optical path is bent inside the carriage 31 via the fourth reflection mirrors 25, 26, 27 and 28, and the linear image sensor 30 such as a one-dimensional CCD is read by the original reading lens 29.
An image is formed on the surface (hereinafter referred to as "CCD"). Then, the carriage 31 is moved in the direction of arrow C (sub-scanning direction) shown in the drawing by a sub-scanning motor (not shown) to read the image information of the document 21. The CCD 30 in the figure has a configuration in which a plurality of light receiving elements are arranged in a one-dimensional direction (main scanning direction).

Although the original reading lens of the present invention is applied to the original reading device of the digital color copying machine in the present embodiment, the present invention is not limited to this, and is also applied to various color original reading devices such as color image scanners. can do.

Next, Numerical Examples 1 to 3 corresponding to the first to third embodiments of the present invention will be shown. In the numerical example, i indicates the order of the surfaces from the original surface, Ri is the radius of curvature of each surface, and D
i is the i-th and (i + 1) th optical member thickness or the air gap, and Ni and νi are the d-line refractive index and Abbe number of the i-th optical member. f is the focal length, Fno is the F number, ω is the half angle of view, and β is the magnification. Table 1 shows the relationship between some of the above-mentioned conditional expressions and various numerical values in the numerical examples.
Shown in.

[0059]

[Outer 1]

[0060]

[Outside 2]

[0061]

[Outside 3]

[0062]

[Table 1]

[0063]

According to the present invention, by properly disposing the diffractive optical element in the vicinity of the diaphragm of the lens system as described above, chromatic aberration among various aberrations, particularly chromatic aberration, can be well corrected over a wide range, and the number of constituent elements is small. Regardless, it is possible to achieve a document reading lens having a high Fno (F number = 4.0 or less) and a large aperture and a high MTF with respect to a high spatial frequency, and a document reading device using the same.

[Brief description of drawings]

FIG. 1 is a lens cross-sectional view of Embodiment 1 of the present invention.

FIG. 2 is a lens cross-sectional view of Embodiment 2 of the present invention.

FIG. 3 is a lens cross-sectional view of Embodiment 3 of the present invention.

FIG. 4 is an aberration diagram of the first embodiment of the present invention.

FIG. 5 is an aberration diagram of Embodiment 1 of the present invention.

FIG. 6 is an aberration diagram of a second embodiment of the present invention.

FIG. 7 is an aberration diagram of Embodiment 2 of the present invention.

FIG. 8 is an aberration diagram of a third embodiment of the present invention.

FIG. 9 is an aberration diagram of a third embodiment of the present invention.

FIG. 10 is an explanatory view of a laminated diffractive optical element according to the present invention.

FIG. 11 is an explanatory view of a laminated diffractive optical element according to the present invention.

FIG. 12 is a schematic view of a main part when the document reading lens of the present invention is applied to a document reading device.

FIG. 13 is a schematic view of a main part when the document reading lens of the present invention is applied to a document reading device.

FIG. 14 is an explanatory diagram of a conventional laminated diffractive optical element.

FIG. 15 is an explanatory diagram of a conventional laminated diffractive optical element.

[Explanation of symbols]

PL original reading lens G1 first lens G2 second lens G3 Third lens G4 4th lens GP1 First diffraction grating GP2 Second diffraction grating GP Stacked Diffractive Optical Element SP aperture T original IP reading element (image plane) d d line g g line ΔS sagittal image plane ΔM meridional image plane f Focal length ω angle of view Fno F number 1 manuscript 2 Platen glass 3 reflective shade 4 Illumination light source 5 First reflection mirror 6 Second reflection mirror 7 Third reflective mirror 8 Original reading lens 9 reading element 10 body 11 pressure plate 101, 103 substrate 102 first lattice part 104 Second lattice part 102a, 104a Diffraction grating surface 105 adhesive

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 2H087 KA08 KA18 LA01 NA14 PA04                       PA17 PB04 QA02 QA07 QA12                       QA22 QA26 QA32 QA42 QA46                       RA32 RA42 RA46                 2H108 AA01 AA02 CB01 DA06 GA04                 5C051 AA01 BA03 DA03 DB01 DB22                       DC04 DC07 EA01 FA01

Claims (7)

[Claims] 1. A document reading lens for forming image information on a document surface illuminated by a light source on a reading element surface, wherein a meniscus shape having a convex surface facing the document surface side in order from the document surface side. Of the positive refractive power, the second lens of meniscus negative refractive power with the convex surface facing the document surface side, the diaphragm, of the meniscus negative refractive power with the convex surface facing the reading element side of A third lens, a meniscus-shaped fourth lens having a positive refractive power facing the reading element side, and a diffractive optical element in the optical path, and the refractive index of the material of the first lens is N ₁ , the focal length of the entire system is f, the refractive power of the diffractive optical element surface is φ _d , and the axial chromatic aberration coefficient L _R of the wavelength on the long wavelength side with respect to the reference wavelength of the entire system in the absence of the diffractive optical element the sum of the absolute value of the axial chromatic aberration coefficient L _B of the wavelength of the short wavelength side with respect to the reference wavelength and When the _{t, 1.65 <N 1 0.06 <} f × φ d <0.1 0.007 < document reading lens that satisfies the L _t <0.012 following condition. 2. The document reading lens according to claim 1, wherein the diffractive optical element is arranged immediately before or after the diaphragm. 3. The document reading lens according to claim 1, wherein the diffractive optical element is arranged closer to the document surface than the diaphragm. 4. The diffractive optical element is a laminated diffractive optical element in which a grating portion having a positive power and a grating portion having a negative power are joined with their diffraction grating surfaces facing each other. The document reading lens according to claim 1, 2 or 3. 5. The document reading lens according to claim 4, wherein the diffractive optical element has the grating portion formed on a plane parallel plate. 6. The document reading lens according to claim 4, wherein the diffractive optical element has the grating portion formed on a curved surface. 7. A document reading device, wherein image information on a document surface is formed on a reading element surface by using the document reading lens according to claim 1. Description: