JPH11183794A - Optical lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical lens saving a space, having a short focal distance and capable of compensating chromatic aberration even by the single lens by forming aspherical surfaces on both surfaces and a diffraction surface on one side. SOLUTION: Both surfaces (refraction surfaces) 2, 3 of an optical lens 1 are formed to be aspherical surfaces and one side of the surfaces is formed to be a diffraction surface 4. When this lens is a single optical lens 1,, since the aspherical surfaces are formed on both surfaces (refraction surfaces) 2, 3 and the diffraction surface 4 is formed on one side, various aberrations such as the distortion aberration are compensated. Consequently, the chromatic aberration of an image position is compensated while it is impossible for a refractive single lens heretofore. Thus, this lens is used for an image pickup device composed of an optical system having a very short focal length such as a small camera as the optical lens 1. Namely, even if it is at least a single lens, the optical lens capable of compensating the chromatic aberration, saving the space and having a short focal distance is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical lens used for imaging an imaging device such as a small camera.

[0002]

2. Description of the Related Art Such an optical lens system used for imaging uses a combination of a plurality of lenses to correct aberrations. In order to efficiently correct aberrations, the lenses are formed as aspherical surfaces, and the number of lens combinations is reduced.

As described above, an optical lens system for imaging is often made compact and housed in a small-sized imaging device. For example, it is described in JP-B-8-20595.

[0004]

The reason that a plurality of lenses need to be combined in this manner is that chromatic aberration cannot be theoretically corrected by a single refractive lens, and therefore, in a small-sized imaging device or the like. This is disadvantageous when the mounting space for the lens is small. Therefore, an object of the present invention is to provide a space-saving and short-focus optical lens that can correct chromatic aberration even with at least a single lens.

[0005]

According to a first aspect of the present invention, there is provided an optical lens having an aspheric surface formed on both surfaces and a diffractive surface formed on one surface. According to the second aspect, there is provided an optical lens including a lens having an aspherical surface formed on both surfaces, and a flat plate having a diffraction surface formed on one surface and having the diffraction surface in contact with one surface of the lens.

According to a third aspect of the present invention, in the optical lens according to the first or second aspect, the shape of the aspheric surface is such that an optical axis is an x-axis, and an axis perpendicular to the optical axis and passing through a vertex of the lens surface is a y-axis. then, when the value x of the x coordinates for y coordinates r is the curvature of the reference spherical surface C _o, k a conic constant, the aspherical coefficient is a,

[0007]

(Equation 2) Is represented by

According to a fourth aspect, in the optical lens according to the first or second aspect, the power of the diffractive surface is φ ₁ , the power of the aspherical surface is φ ₂ and φ ₃ , the power of the entire system is φ, and the lens thickness is φ. d
Then, the following condition is satisfied: 0.075 ≦ φ ₁ /φ≦0.095 0.4 ≦ φ ₂ /φ≦0.5 0.55 ≦ φ ₃ /φ≦0.72 6.4 ≦ φd ≦ 9.6 To be satisfied.

According to a fifth aspect, in the optical lens according to the first or second aspect, the diffractive surface is formed in a blazed shape, the refractive index of the lens material is n, the main wavelength is λ _o , and the natural number is m. Then, the blaze depth is represented by mλ _o / (n−1).

[0010]

(1) Hereinafter, a first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram of a single optical lens. The optical lens 1 has both surfaces 2 and 3 formed as aspherical surfaces, and one surface thereof is formed as a diffraction surface 4. In addition, both surfaces 2 and 3 each act as a refraction surface.

FIG. 1 shows an optical path from the stop 5 to the CCD image sensor 6 via the optical lens 1 when the optical lens 1 is used in a small camera, for example. The shape of the aspherical surfaces 2 and 3 of the aspherical lens is such that when the optical axis is the x-axis and the axis perpendicular to the optical axis and passing through the vertex of the lens surface is the y-axis, the value x of the x coordinate with respect to the y coordinate r is When the curvature of the reference sphere is C _o , the conic constant is k, and the aspheric coefficient is A,

[0012]

(Equation 3) Is represented by

The specifications of the single optical lens 1 are shown in FIG.
And FIG. The F number of the entire system is 4, the focal length f is 5 mm, and the angle of view 2ω is 48 deg. However, in this optical lens 1, the power of the diffraction surface is φ
₁ , the power of the aspherical surface is φ ₂ , φ ₃ , the power of the whole system is φ,
When the lens thickness is d and the power φ ₁ of these diffraction surfaces, the powers φ ₂ , φ _{3 of the} aspheric surface, and the power φ of the entire system are obtained, the reciprocal of the focal length is called power in the first place. ₁ is φ ₁ = 0.01737 (2), and the power of the aspherical surface φ ₂ is φ ₂ = {(1.49115-1) / 1｝ × {(1 /5.43899)−(1/∞)}… (3).

The power of the aspherical surface φ ₃ is represented by a model of a plano-convex lens having only the aspherical surface 3 as follows: φ ₃ = {(1.49115-1) / 1｝ × {(1 / ∞) − (1 / 3.78315)} (4) )
Becomes

The power φ of the whole system is as follows: φ = 1 / f = 1/5 = 0.2 (5)
Becomes

The single optical lens 1 has the following characteristics: 0.075 ≦ φ ₁ /φ≦0.095 0.4 ≦ φ ₂ /φ≦0.5 0.55 ≦ φ ₃ /φ≦0.726 0.4 ≦ φd ≦ 9.6 (6)

On the other hand, the diffraction surface 4 formed on one side of the optical lens 1 is formed in a ring-shaped diffraction groove, ie, a blaze shape, as shown in FIG. The number of ring zones NO is determined by the following equation. Here, the aperture diameter is D, and the main wavelength is λ.
_o, and the focal length due only to the blaze and f _b, is represented by _{^{NO = D 2 / 8λ o f}} b ... (7).

For the single optical lens 1 of the present embodiment, D = 5.4 mm, λ _o = 500 nm, f _b = 5.
Since it is 7.57 mm, the number of ring zones NO is 126.
Further, as shown in FIG. 4, the relationship between the blaze and the distance from the center is represented by the following equation. N is the number of zones.

[0019]

(Equation 4)

In the case of the single optical lens 1 of the present embodiment, r ₁ = 0.240 μm. Then, the blaze depth d _max is represented by d _max = λ _o / (n−1) (10). Here, n is the refractive index of the glass material.

In the case of the present embodiment, n = 1.49115
Therefore, d _max is 0.00102. In the case of such a single optical lens 1, since aspherical surfaces are formed on both surfaces 2 and 3 and a diffractive surface 4 is formed on one of these surfaces, various aberrations including distortion can be corrected. Chromatic aberration at the image position, which was impossible with a single lens, can be corrected. Thereby, it can be used as the optical lens 1 for imaging in an imaging device such as a small camera having an optical system with an extremely short focal length.

FIG. 5 shows the wavelength dependence of the back focus. As can be seen from this figure, the fluctuation of the back focus is within 0.02 mm at a wavelength of 400 to 650 nm.

The diffraction efficiency ε by the diffraction surface 4 at this time is
Is represented by ε = sin c ² [π (λ _o / λ−1)] (11), where λ is the wavelength and λ _o is the design wavelength of the blaze.

FIG. 6 shows the wavelength dependence of the diffraction efficiency ε. As can be seen from this figure, a good diffraction efficiency ε of 0.81 or more is obtained over the entire wavelength range of 400 to 650 nm. Further, before designing the diffractive surface 4, by adjusting the two aspheric surfaces 2, 3, spherical aberration, astigmatism, distortion,
Coma aberration and the like can be reduced. 7 shows spherical aberration, FIG. 8 shows astigmatism, and FIG. 9 shows distortion.

As described above, although the optical lens 1 is a single lens, it can correct chromatic aberration at the image position and reduce other aberrations. This lens is used alone in a small camera or the like, and an image in the visible light region can be obtained. Images can be taken. (2) Next, a second embodiment of the present invention will be described with reference to the drawings.

The second embodiment is an optical lens 1 having an aspherical surface on both surfaces 2 and 3 and a diffraction surface 4 on one of the surfaces as shown in FIG.
The place of difference from the first embodiment is where the formation of the depth d _max of the blaze as represented by the following equation.

D _max = mλ _o / (n−1) (12) where m is a natural number. The diffraction efficiency ε due to such a blaze is represented by ε = sin c ² [π ｛m (λ _o / λ) −k｝] (13). Here, k is the diffraction order.

FIG. 10 shows the diffraction efficiency ε by the diffraction surface 4.
Is shown. However, the natural number m is 15. (3) Next, a third embodiment of the present invention will be described with reference to the drawings.

FIG. 11 is a structural view of the optical lens. The optical lens 10 includes an aspheric single lens 13 having both surfaces 11, 12 formed as aspheric surfaces, and a flat plate 15 having a diffraction surface 14 formed on one surface. In addition, both sides 11, 12
Act as refractive surfaces, respectively.

The flat plate 15 is arranged such that the diffraction surface 14 faces the aspherical single lens 13 and the diffraction surface 14 is in contact with one surface 11 of the aspherical single lens 13. Aspheric single lens 13
The F-number, focal length f, angle of view 2ω, and other specifications of the entire system are the same as those in the first embodiment.

The diffraction surface 14 is formed in a blazed shape of a diffraction groove in an annular zone. Then, the relationship between the blaze and the distance from the center is expressed in the same manner as the above equations (8) and (9),
Further, the blaze depth d _max is also expressed in the same manner as in the above equation (10).

Accordingly, the optical lens 10 has a value of 0.075 ≦ φ ₁ /φ≦0.095 0.4 ≦ φ ₂ /φ≦0.5 0.55 ≦ φ ₃ / φ ≦ 0.72 6.4 ≦ φd ≦ 9.6 (14)

As described above, if the optical lens 10 is used, an aspheric single lens 13 having both surfaces 11, 12 formed as aspheric surfaces;
Since it is composed of the flat plate 15 having the diffractive surface 14 formed on one side, various aberrations including distortion can be corrected as in the first embodiment, and an image which cannot be obtained with a refraction single lens. Chromatic aberration at the position can be corrected. Thus, imaging can be performed by using the imaging lens such as a small camera as the imaging optical lens 10.

Further, since the flat plate 15 having the diffractive surface 14 is used, it is useful when it is difficult to form a diffractive surface on the aspherical single lens 13, for example. Furthermore, diffraction surface 1
The flat plate 15 on which 4 is formed can also serve as a cover glass for the optical lens 10, for example.

[0035]

As described above, according to the present invention, it is possible to provide a space-saving and short-focus optical lens capable of correcting chromatic aberration even with at least a single lens.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a first embodiment of a single optical lens according to the present invention.

FIG. 2 is a diagram showing specifications of the single optical lens.

FIG. 3 is a diagram showing specifications of the single optical lens.

FIG. 4 is a diagram showing a blaze shape of a diffraction surface in the single optical lens.

FIG. 5 is a diagram showing wavelength dependence of a back focus.

FIG. 6 is a diagram showing wavelength dependence of diffraction efficiency.

FIG. 7 is a diagram showing spherical aberration.

FIG. 8 is a diagram showing astigmatism.

FIG. 9 is a diagram showing distortion.

FIG. 10 is a diagram showing diffraction efficiency of a second embodiment of the optical lens according to the present invention.

FIG. 11 is a configuration diagram showing a third embodiment of the optical lens according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Optical lens, 2, 3 ... Both surfaces (refractive surface), 4 ... Diffractive surface, 10 ... Optical lens, 11, 12 ... Both surfaces (refractive surface), 13 ... Aspherical single lens, 14 ... Diffractive surface, 15 ... Flat plate .

Claims (5)

[Claims] 1. An optical lens having an aspheric surface formed on both surfaces and a diffractive surface formed on one surface. 2. An optical system comprising: a lens having an aspherical surface formed on both surfaces; and a flat plate having a diffraction surface formed on one surface and having the diffraction surface in contact with one surface of the lens. lens. 3. The shape of the aspheric surface is such that an optical axis is an x-axis,
Assuming that the axis perpendicular to the optical axis and passing through the vertex of the lens surface is the y-axis, the value x of the x-coordinate with respect to the y-coordinate r is given by C _{o for} the curvature of the reference sphere, k for the conic constant, and A for the aspheric coefficient. , [Equation 1] The optical lens according to claim 1, wherein the optical lens is represented by: 4. If the power of the diffraction surface is φ ₁ , the power of the aspheric surface is φ ₂ and φ ₃ , the power of the entire system is φ, and the lens thickness is d, 0.075 ≦ φ ₁ / φ ≦ 0 0.095 0.4 ≦ φ ₂ /φ≦0.5 0.55 ≦ φ ₃ /φ≦0.72 6.4 ≦ φd ≦ 9.6 The following condition is satisfied. An optical lens as described. 5. The method according to claim 5, wherein the diffractive surface is formed in a blaze shape.
And the refractive index of the lens material n, a dominant wavelength lambda _o, when the natural number m, the depth of blaze, according to claim 1 or 2, characterized by being represented by mλ _o / (n-1) Optical lens.