JP2001083414A - Lens for reading image - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a lens for reading an image which is constituted of the small number of lenses, whose various aberration are sufficiently corrected and which is provided with high optical performance. SOLUTION: This lens is provided with the 1st positive meniscus lens 1 whose convex surface faces a document surface, a diaphragm SP and the 2nd negative meniscus lens 2 whose convex surface faces an image surface, in this order from the document surface. The lens surface of the 1st lens facing the document surface and that of the 2nd lens facing the image surface are constituted of the aspherical surface and a diffraction optical element is added to at least one lens surface of the 1st and the 2nd lenses.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reading lens, and more particularly, to an image reading apparatus such as an image scanner, a film scanner, or a digital copying machine, in which various aberrations are satisfactorily corrected with a small number of lens elements such as two in two groups. The present invention relates to an image reading lens suitable for an apparatus.

[0002]

2. Description of the Related Art Conventionally, an image forming apparatus forms an image on a document surface at a predetermined reduction magnification on a solid-state image sensor array (line sensor) surface by an image reading lens, and reads the image information. For example, various devices such as an image scanner, a film scanner, and a digital copying machine have been proposed. An image reading lens used in such an apparatus is required to have a small number of lens components, to have a small lens system as a whole, and to have high optical performance.

Here, a high-performance image reading lens has a spherical aberration, a coma aberration, a field curvature aberration, and the like that are sufficiently corrected, and has a high contrast, a deep depth of focus, and a small field curvature on an image forming surface. It is a lens. Furthermore, in the case of an image reading lens used in a color image reading apparatus, it is necessary that axial chromatic aberration, chromatic aberration of magnification, and the like be sufficiently corrected.

Conventionally, an image reading lens of this type includes a convex lens, a concave lens, and a cemented lens of a concave lens and a convex lens in order from the document surface (object) side.
A four-element, six-element, four-group, six-element structure, in which a convex lens and a cemented lens of a convex lens and a concave lens are arranged approximately symmetrically with respect to the aperture, are used. Have been.

On the other hand, an image reading lens in which the entire lens system is small and inexpensive is a lens in which a light-weight and low-cost material such as plastic is used and the above-mentioned various aberrations are satisfactorily corrected with a small number of lenses. For example, various types of image reading lenses having two groups and two elements have been conventionally proposed as such lenses.

[0006]

However, the above-mentioned conventional image reading lens has the following various problems.

First, in an image reading lens having four lenses in three groups or six in four groups, various aberrations such as spherical aberration, coma aberration and field curvature aberration are sufficiently corrected, but chromatic aberration, particularly The axial chromatic aberration was corrected to a certain level, but was not sufficiently satisfactory. That is, the axial chromatic aberration is overcorrected on the short wavelength side of the visible wavelength range, is undercorrected on the long wavelength side, and is not completely corrected over a wide range of the visible wavelength range. ). Therefore, when such an image reading lens is used in, for example, a color image reading apparatus, R (red), G
There is a problem in that the focus positions for the (green) and B (blue) color lights are slightly different, and the read image quality is degraded.
Also, because of the large number of lens components, it has been difficult to reduce the size and cost of the entire apparatus.

On the other hand, in an image reading lens having two lenses and two groups, an aspherical surface is frequently used, and although the above-mentioned various aberrations are sufficiently corrected, no consideration is given to chromatic aberration at all. Can be used only for a lens for monochrome reading using. That is, although the number of lens components is small, the size is small and the cost is low, but the chromatic aberration is not sufficiently corrected. Therefore, there is a problem that it is difficult to use the lens for an image reading lens for a color image reading apparatus.

According to the present invention, various kinds of aberrations can be obtained by appropriately configuring an image reading lens with a small number of lenses such as two in two groups and adding a diffractive optical element to at least one lens surface of the two lenses. In particular, it is an object of the present invention to provide a high-performance and small-sized image reading lens which can sufficiently correct axial chromatic aberration and can be used for a color image reading apparatus.

[0010]

According to a first aspect of the present invention, there is provided an image reading lens comprising: a meniscus-shaped positive first lens having a convex surface facing the document surface in order from the document surface; an aperture; and an image surface. The second lens has a meniscus negative second lens having a convex surface, and the lens surface of the first lens on the original surface side and the lens surface of the second lens on the image surface side are aspherical. In addition, a diffractive optical element is added to at least one lens surface of the first and second lenses.

According to a second aspect of the present invention, in the first aspect of the present invention, in the image reading lens, the focal length of the entire system is f, the focal length of the first lens is f ₁ , and the first and second lenses are the same.
Abbe numbers of the materials of the two lenses are ν ₁ and ν, respectively.
_2, satisfies _{0.35 <f 1 / f <0.55} 25 <ν 1 -ν 2 0.015 <f · φ d <0.05 condition: when the power of the diffractive optical element was phi _d It is characterized by:

According to a third aspect of the present invention, in the first or second aspect, the diffractive optical element is added to at least one of the two lens surfaces of the first lens.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1 to 4 are lens sectional views of Numerical Examples 1 to 4 of the present invention, respectively, and FIGS. 5 to 8 are Numerical Examples 1 to 4 of the present invention, respectively. 9 to 12 are lateral aberration diagrams of Numerical Examples 1 to 4 of the present invention, respectively, which will be described later.

1 to 4, 10, 20, 30, 4
Reference numeral 0 denotes an image reading lens, which is a meniscus-shaped positive first lens (convex lens) 1 having a convex surface facing the original surface P in order from the original surface (object) P side, an aperture SP, and an image surface Q
The lens has two meniscus-shaped negative second lenses (concave lenses) 2 with convex surfaces directed to the sides. The first lens 1 has a lens surface on the original surface side, and the second lens 2 has an image surface side on the image surface side. The lens surface is formed of an aspherical surface, and a diffractive optical element is added to at least one of the first and second lenses.

The diffractive optical element is the first diffractive optical element in FIGS. 1, 2 and 4.
It is added to the lens surface of the lens 1 on the image side, and in FIG. 3 is added to the lens surface of the first lens 1 on the original side. This diffractive optical element (DOE) is what is called a multi-phase level, and may be manufactured by repeating etching using a photomask or may be manufactured by cutting.

Next, the optical function and effect of this embodiment will be described.

In general, in order to correct the field curvature aberration and the chromatic aberration, a combination of two convex lenses having a positive refractive power and concave lenses having a negative refractive power is required.

The image reading lens according to the present embodiment basically has a two-group structure consisting of a convex lens and a concave lens. Spherical aberration and coma can be corrected satisfactorily by introducing an aspherical surface on the outer surface of the convex lens and the concave lens, that is, the surface that is away from the stop (pupil) position and has high on-axis and off-axis light ray passing positions. are doing.

On the other hand, chromatic aberration can be corrected to some extent by a combination of a convex lens and a concave lens, but there is a limit in a general combination of optical glass.
There is residual chromatic aberration (secondary spectrum).

Therefore, in the present embodiment, in order to correct the residual chromatic aberration, a diffractive optical element is added to at least one lens surface of a convex lens (first lens) and a concave lens (second lens), thereby obtaining axial chromatic aberration. Is corrected favorably.

This diffractive optical element has an optical characteristic that the sign of the Abbe number is negative, and a chromatic aberration opposite to the chromatic aberration caused by the refraction of the ordinary refraction type lens occurs.
By combining a normal refractive lens and a diffractive optical element, chromatic aberration can be favorably corrected.

In the present embodiment, spherical aberration, coma aberration, field curvature aberration, chromatic aberration, etc. are satisfactorily corrected by appropriately setting the lens configuration and lens shape of the image reading lens as described above. .

In this embodiment, the image reading lens is used.
Where f is the focal length of the entire system and f is the focal length of the first lens.
₁ , The Abbe number of the material of the first and second lenses is sequentially ν
₁ , Ν _Two , The power of the diffractive optical element is φ_d 0.35 <f₁ /F<0.55‥‥‥(1) 25 <ν₁ −ν_Two ‥‥‥ (2) 0.015 <f ・ φ_d The condition of <0.05 ‥‥‥ (3) is satisfied.

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

Conditional expression (1) defines the ratio of the focal length f ₁ of the first lens to the focal length f of the entire system, and relates to various aberrations other than chromatic aberration. Exceeding the lower limit is not preferable because it becomes difficult to correct spherical aberration, coma aberration, and field curvature aberration in a well-balanced manner.

Conditional expression (2) relates to chromatic aberration. If conditional expression (2) is not satisfied, the residual chromatic aberration in the refracting system will increase, so that the power of the diffractive optical element will inevitably increase, and spherical aberration and image quality will increase. It is difficult to satisfactorily correct chromatic aberration without affecting surface curvature aberration and the like, which is not good.

Condition (3) defines the ratio of the power φ _d (1 / focal length) of the diffractive optical element to the focal length f of the whole system. The power of the optical element is increased, the chromatic aberration is overcorrected, and the difference in pitch between the central portion and the peripheral portion is increased as the surface shape.
If the lower limit of conditional expression (3) is exceeded, the power φ _d of the diffractive optical element becomes small, chromatic aberration is insufficiently corrected, and flare components other than the first-order diffracted light increase.

[Other Embodiments] Numerical Examples 1 to 4 described above.
The diffraction grating shape 101 of the diffractive optical element portion in FIG.
The kinoform shape shown in FIG. FIG. 14 shows the wavelength dependence of the first-order diffraction efficiency of the diffractive optical element shown in FIG. The actual configuration of the diffraction grating is as follows. An ultraviolet curable resin is applied to the surface of the base material 102, and the wavelength of 530 nm is applied to the resin portion.
Grating 10 having a grating thickness d such that the next diffraction efficiency becomes 100%
3 is formed.

As is apparent from FIG. 14, the diffraction efficiency at the design order decreases as the distance from the optimized wavelength of 530 nm increases, while the 0th-order and second-order diffracted lights near the design order increase. This increase in diffracted light other than the design order causes a flare, which leads to a reduction in the resolution of the optical system.

FIG. 15 shows the MTF characteristics with respect to the spatial frequency in the case where the above-mentioned Numerical Example 1 is prepared with the lattice shape shown in FIG. From this figure, it can be seen that the MTF in the low frequency region is lower than a desired value.

Therefore, as another embodiment, a laminated diffraction grating shown in FIG. 16 is used as the grating shape of the diffractive optical element in the embodiment of the present invention. FIG. 17 shows the wavelength dependence of the first-order diffraction efficiency of the diffractive optical element having this configuration. As a specific configuration, an ultraviolet curable resin (nd = 1.49) is provided on a base material.
9, a first diffraction grating 104 composed of νd = 54), and another ultraviolet curable resin (nd = 1.598,
νd = 28) is formed. In this combination of materials, the grating thickness d1 of the first diffraction grating portion is d1 = 13.8 μm, and the grating thickness d2 of the second diffraction grating portion is d = 10.5 μm. By using a diffraction grating having a laminated structure as shown in FIG. 17, the diffraction efficiency of the design order has a high diffraction efficiency of 95% or more over the entire use wavelength range.

FIG. 18 shows M for the spatial frequency in this case.
4 shows TF characteristics. By using a diffraction grating with a laminated structure,
The low-frequency MTF is improved, and a desired MTF characteristic is obtained. As described above, by using a diffraction grating having a laminated structure as the diffractive optical element of the embodiment of the present invention, the optical performance is further improved.

The material of the above-mentioned diffractive optical element having a laminated structure is not limited to an ultraviolet curable resin, but other plastic materials can be used. Depending on the base material, the first diffraction grating portion 104 can be directly formed. It may be a substrate.
Further, it is not necessary that the grating thicknesses be different, and depending on the combination of materials, the two grating thicknesses can be made equal as shown in FIG. In this case, since the grating shape is not formed on the surface of the diffractive optical element, it is excellent in dust resistance, the workability of assembling the diffractive optical element is improved, and a more inexpensive optical system can be provided.

Next, numerical embodiments 1 to 4 of the present invention will be described. In Numerical Examples 1 to 4, f is the focal length, Fno is the F-number, β is the magnification, and ri is the i-th lens element in order from the document surface (object) side.
The radius of curvature of the lens surface, di is the i-th lens thickness and air gap from the document surface side, and ni and νi are the refractive index of the glass of the i-th lens and the Abbe number in order from the document surface side.

In the aspherical shape, the optical axis direction is the X axis, the height in the direction perpendicular to the optical axis is H, the intersection point between the vertex of the lens and the X axis is the origin, and r is the paraxial radius of curvature of the lens surface. A, B, C,
When D and E are aspheric coefficients, respectively,

[0036]

(Equation 1)

Is expressed by the following equation.

The diffraction surface of the diffractive optical element has a phase function of φ
As (H), when the height from the optical axis is H, the wavelength is λ, and the phase coefficients are C ₂ , C ₄ , C ₆ , C ₈ , and C ₁₀ respectively,

[0039]

(Equation 2)

This is expressed by the following equation.

In the aberration diagrams, the solid line represents the reference wavelength e.
The two-dot chain line is a g-line with a wavelength of 433 nm, and the one-dot chain line is a d-line with a wavelength of 630 nm.

Table 1 shows the relationship between the above-mentioned conditional expressions (1) to (3) and various numerical values in the numerical examples.

(Numerical Example 1) f = 31.02 Fno = 6.2 β = -0.857 * r ₁ = 6.239 d ₁ = 3.61 n ₁ = 1.63246 ν ₁ = 63.8 * r ₂ = 14.919 (DOE plane) d ₂ = 0.38 r ₃ = stop _{(SP) d 3 = 0.57 n} 2 = 1.72250 ν 2 = 29.2 r 4 = -5.473 d 4 = 4.12 * r 5 = -10.524 * aspherical r ₁ aspherical surface coefficient r ₂ aspherical surface coefficients A = 3.56580 × 10 ^-3 A = 0 B = 6.85547 × 10 ^-4 B = 1.04647 × 10 ^-3 C = 5.11124 × 10 ^-5 C = 2.07682 × 10 ^-4 D = -2.02789 × 10 ^-6 D = -3.81153 × 10 ^-5 E = 2.94303 × 10 ^-7 E = 6.60737 × 10 ^-6 r _5- plane aspherical coefficient r _2- plane phase coefficient A = 7.79486 × 10 ^-4 C ₂ = -5.17468 × 10 ^-4 B = 2.26185 × 10 ^-4 C ₄ = 4.57263 × 10 ^-5 C = 2.61035 × 10 ^-5 C ₆ = -4.50112 × 10 ^-5 D = -3.96086 × 10 ^-6 C ₈ = 1.57800 × 10 ^-5 E = 2.23315 × 10 ^-7 C ₁₀ = -1.90291 × 10 ^-6 (Numerical Example 2) f = 30.99 Fno = 6.2 β = -0.857 * r ₁ = 9.991 d ₁ = 4.78 n ₁ = 1.63246 ν ₁ = 63.8 r ₂ = 30.975 (DOE surface) d ₂ = 0.32 r ₃ = Aperture (SP) d ₃ = 0.70 n ₂ = 1.72250 ν ₂ = 29.2 r ₄ = -5.228 d ₄ = 4.18 * r ₅ = -11.071 * Aspheric surface r _1- plane aspheric coefficient r _5- plane aspheric coefficient A = 1.87829 × 10 ^-2 A = -5.01648 × 10 ^{-3 B = 5.31558 × 10 -4 B} = 2.38892 × 10 -4 C = 1.13883 × 10 -5 C = 3.08624 × 10 -6 D = 6.41452 × 10 -7 D = 1.21164 × 10 -6 E = 4.31297 × 10 - ⁹ E = -7.91730 × 10 ^-8 r ₂ plane phase coefficient C ₂ = -6.30030 × 10 ^-4 C ₄ = 4.69069 × 10 ^-5 C ₆ = -1.09462 × 10 ^-5 C ₈ = 3.90636 × 10 ^-6 C ₁₀ = -4.45734 × 10 ^-7 (Numerical example 3) f = 31.03 Fno = 6.2 β = -0.857 * r ₁ = 6.444 (DOE plane) d ₁ = 3.18 n ₁ = 1.63246 ν ₁ = 63.8 * r ₂ = 13.254 d ₂ = 0.61 r ₃ = Aperture (SP) d ₃ = 0.60 n ₂ = 1.72250 ν ₂ = 29.2 r ₄ = -5.552 d ₄ = 4.06 * r ₅ = -10.892 * Aspheric surface r ₁ surface aspherical coefficient r ₂ surface non Spherical coefficient A = 1.00336 × 10 ^-2 A = 0 B = 9.42399 × 10 ^-4 B = 1.36745 × 10 ^-3 C = 5.18011 × 10 ^-5 C = 1.60776 × 10 ^-4 D = -6.97473 × 10 ^-7 D = -1.71626 × 10 ^-5 E = 3.08865 × 10 ^-7 E = 4.53421 × 10 ^-6 r _5- plane aspherical coefficient r _1- plane phase coefficient A = 2.52600 × 10 ^-4 C ₂ = -3.28885 × 10 ^-4 B = 2.49284 _{^{× 10 -4 C 4 = 9.79888 ×}} 10 -6 C = 2.20345 × 10 -5 C 6 = -4.35091 × 10 -6 D = -3.33450 × 10 -6 C 8 = 6.31878 × 10 -7 E = 1.98045 × 10 - ⁷ C ₁₀ = -4.50247 × 10 ^-8 (Numerical example 4) f = 31.00 Fno = 6.2 β = -0.85 7 * r ₁ = 6.034 d ₁ = 3.33 n ₁ = 1.69400 ν ₁ = 56.3 * r ₂ = 19.178 (DOE plane) d ₂ = 0.28 r ₃ = Aperture (SP) d ₃ = 0.49 n ₂ = 1.72250 ν ₂ = 29.2 r ₄ = -5.461 d ₄ = 4.71 * r ₅ = -10.033 * Aspheric surface r _1- plane aspherical coefficient r _2- plane aspherical coefficient A = -3.19862 × 10 ^-3 A = 0 B = 6.57482 × 10 ^-4 B = 8.86602 × 10 ^-4 C = 5.99460 × 10 ^-5 C = 1.19611 × 10 ^-4 D = -2.78137 × 10 ^-6 D = -3.29487 × 10 ^-5 E = 4.03351 × 10 ^-7 E = 6.07813 × 10 ^-6 r ₅ aspherical surface coefficient r ₂ surface phase coefficient ^{A = 9.09793 × 10 - 3 C} 2 = -5.36469 × 10 -4 B = 3.28101 × 10 -4 C 4 = 3.77223 × 10 -5 C = 2.40705 × 10 -5 C 6 = -2.74963 × 10 ^-5 D = -3.20768 × 10 ^-6 C ₈ = 9.99486 × 10 ^-6 E = 1.67736 × 10 ^-7 C ₁₀ = -1.26914 × 10 ^-6

[0044]

[Table 1]

[0045]

According to the present invention, as described above, the image reading lens is appropriately constituted by a small number of lens elements such as two groups and two lenses, and the diffractive optical element is provided on at least one lens surface of the two lenses. By adding, it is possible to sufficiently correct various aberrations, especially axial chromatic aberration, and to achieve a high-performance and compact image reading lens that can be used in a color image reading apparatus.

[Brief description of the drawings]

FIG. 1 is a sectional view of a lens according to a numerical example 1 of the present invention.

FIG. 2 is a sectional view of a lens according to a numerical example 2 of the present invention.

FIG. 3 is a sectional view of a lens according to a numerical example 3 of the present invention.

FIG. 4 is a sectional view of a lens according to a numerical example 4 of the present invention.

FIG. 5 is a longitudinal aberration diagram according to Numerical Example 1 of the present invention.

FIG. 6 is a longitudinal aberration diagram according to Numerical Example 2 of the present invention.

FIG. 7 is a longitudinal aberration diagram of Numerical Example 3 of the present invention.

FIG. 8 is a longitudinal aberration diagram of Numerical Example 4 of the present invention.

FIG. 9 is a lateral aberration diagram of Numerical Example 1 of the present invention.

FIG. 10 is a lateral aberration diagram of Numerical Example 2 of the present invention.

FIG. 11 is a lateral aberration diagram of a numerical example 3 of the present invention.

FIG. 12 is a lateral aberration diagram of a numerical example 4 of the present invention.

FIG. 13 is an explanatory view of a diffractive optical element according to the present invention.

FIG. 14 is an explanatory diagram of a wavelength dependence characteristic of the diffractive optical element according to the present invention.

FIG. 15 is an MTF characteristic diagram of the diffractive optical element according to the present invention.

FIG. 16 is an explanatory diagram of a diffractive optical element according to the present invention.

FIG. 17 is an explanatory diagram of a wavelength dependence characteristic of the diffractive optical element according to the present invention.

FIG. 18 is an MTF characteristic diagram of the diffractive optical element according to the present invention.

FIG. 19 is an explanatory view of a diffractive optical element according to the present invention.

[Explanation of symbols]

 10, 20, 30, 40 Image reading lens 1 First lens (convex lens) 2 Second lens (concave lens) SP stop ΔM Meridional image plane ΔS Sagittal image plane

Claims (3)

[Claims] 1. A meniscus-shaped positive first lens having a convex surface facing the original surface side, an aperture, and a meniscus-shaped negative second lens having a convex surface facing the image surface side. A lens surface of the first lens on the document surface side and a lens surface of the second lens on the image surface side are aspherical surfaces, and at least one of the first and second lenses An image reading lens characterized in that a diffractive optical element is added to the surface. 2. In the image reading lens, the focal length of the entire system is f, the focal length of the first lens is f ₁ , and the Abbe numbers of the materials of the first and second lenses are respectively ν. _{1, ν 2, 0.35 <f} 1 / f <0.55 25 <ν 1 -ν 2 0.015 <f · φ d <0.05 condition: when the power of the diffractive optical element and phi _d 2. The image reading lens according to claim 1, wherein the following condition is satisfied. 3. The image reading lens according to claim 1, wherein the diffractive optical element is added to at least one of the two lens surfaces of the first lens.