Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
2000-03-01
2002-08-06
Chang, Audrey (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S565000, C359S566000, C359S571000
Reexamination Certificate
active
06429972
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a diffraction lens and the method of designing the same, in particular, to a diffraction lens having a diffraction relief on a lens surface and the method of designing the diffraction lens.
For a diffraction lens, it has been heretofore known a lens which has a diffraction relief for generating diffraction on a lens surface. For example, in respect of a diffraction lens for use in an optical pick up device, an achromatic objective lens and a two-focus objective lens which utilize the characteristics peculiar to diffraction have been proposed.
The shape of the lens surface of these diffraction lenses is optimized by a method of adding an optical path difference function on the virtual basic aspherical surface or basic spherical surface by the high refractive index method or phase function method, and after that, it is transformed into the actual shape of a diffraction relief having steps from the optical path difference function.
The positions of the steps in the direction perpendicular to the optical axis can be obtained as the height such that the optical path difference function varies by an amount of an integral number times the wavelength, and the amount of variation in the direction of the optical axis is approximated by a function form in order to be easily treated. For example, in the publication of TOKKAIHEI 10-186231, it is disclosed that the amount of displacement in a diffraction relief and the amount of displacement in a basic aspherical surface with a diffraction relief added are approximated by a polynomial with a distance from the optical axis made as a variable.
However, the above-described method of approximation using a polynomial has a problem that the working of diffraction lenses is likely to become complicated because of the large number of the terms in the function [form].
SUMMARY OF THE INVENTION
This invention has been done in view of it that the respective surfaces of the diffraction annular zones can be approximated as surfaces formed by rotation around the respective predetermined points on the virtual basic aspherical surface or basic spherical surface, and an object of this invention is to provide the method of designing a diffraction lens wherein a high-precision approximate shape can be obtained by a simple expression in the process of transformation from the optical path difference function to the actual shape of a diffraction relief having steps. Further, another object of this invention is to provide a diffraction lens which has a diffraction relief with a simple shape and is made by a high-precision expression.
The above-described objects can be accomplished by any one of the following structures and methods:
1. A diffractive lens, comprises
an optical axis;
a lens surface having a peripheral portion, and
a diffractive relief provided on the lens surface,
wherein the diffractive relief is shaped in annular zones, and
wherein following conditional formulas (1) and (2) are satisfied:
h
i−1
≦h<h
i
(1)
x=f
(
h−h
ti
)+
S
(2)
where
i is a suffix showing an ordinal number of each of the diffractive annular zones obtained by counting each diffractive annular zone in such a manner that a diffractive annular zone including the optical axis is counted as a first diffractive annular zone and other diffractive annular zones are counted respectively in consecutive order from the optical axis toward the peripheral portion;
h is a distance between a point and X axis in which, when the optical axis is deemed as X axis, the distance is from X axis to the point in a direction perpendicular to X axis;
h
i
is a distance from X axis to a border between i-th diffractive annular zone and (i+1)-th diffractive annular zone which are counted from the optical axis in the above manner, provided that h
0
=0;
f(h) is a function of h;
h
ti
is a constant in which at least one of h
ti
is not zero; and
S is a term characterizing an aspherical surface.
It may be preferable that S is zero, because a highly precise diffractive lens may be produced more easily.
2. The diffractive lens described in paragraph 1, wherein f(h) is a following conditional formula (3):
f
(
h
-
h
ti
)
=
(
h
-
h
ti
)
2
/
r
ti
1
+
1
-
(
h
-
h
ti
)
2
/
r
ti
2
+
x
ti
-
r
ti
(
3
)
where r
ti
is a constant and x
ti
is a constant.
3. The diffractive lens described in paragraph 1, wherein the diffractive relief is shaped in a sawtooth.
4. The diffractive lens described in paragraph 3, wherein a difference in a direction parallel to the optical axis between a cross sectional shape of the diffractive relief and a shape represented by a following general formula (4) is not larger than 0.2 of a basic wavelength:
x
(
h
)
=
Φ
D
(
h
)
cos
θ
cos
α
{
n
-
n
′
cos
(
θ
-
θ
′
)
}
+
x
B
(
h
)
(
4
)
where
x
B
(h) represents a shape of a basic aspherical surface or a basic spherical surface;
&PHgr;
D
(h) represents an optical path difference produced by providing the diffractive relief on the lens surface;
&thgr; is an angle made between a ray, which comes from an object point on the optical axis and is incident on a diffractive surface, and a normal line on the diffractive surface.;
&thgr;′ is an angle made betwee a ray, which comes from the object point on the optical axis and emerges from the diffraction surface, and a normal line on the diffractive surface;
&agr; is an angle made between the optical axis and a normal line on the diffractive surface;
n is a refractive index of the incident side of the diffractive surface; and
n′ is a refractive index of the emerging side of the diffractive surface.
5. A method of manufacturing a mold to produce a diffractive lens having a diffractive relief on a lens surface, comprises steps of:
inputting data regarding a shape of the diffractive relief into a computer provided to a mold processing machine; and
processing a metal block by the mold processing machine controlled by the computer based on the inputted data
so that the mold to produce the diffractive lens is manufactured,
wherein the data are represented by following conditional formulas (1) and (2):
h
i−1
≦h<h
i
(1)
x=f
(
h−h
ti
) (2)
where
i is a suffix showing an ordinal number of each of the diffractive annular zones obtained by counting each diffractive annular zone in such a manner that a diffractive annular zone including the optical axis is counted as a first diffractive annular zone and other diffractive annular zones are counted respectively in consecutive order from the optical axis toward the peripheral portion;
h is a distance between a point and X axis in which, when the optical axis is deemed as X axis, the distance is from X axis to the point in a direction perpendicular to X axis;
h
i
is a distance from X axis to a border between i-th diffractive annular zone and (i+1)-th diffractive annular zone which are counted from the optical axis in the above manner, provided that h
0
=0;
f(h) is a function of h; and
h
ti
is a constant in which at least one of h
ti
is not zero.
6. A method of producing a diffractive lens having a diffractive relief on a lens surface, comprises steps of:
inputting data regarding a shape of the diffractive relief into a computer provided to a mold processing machine;
processing a metal block by the mold processing machine controlled by the computer based on the inputted data
so that the mold to produce the diffractive lens is manufactured, and
producing the diffractive lens by injecting molding a melted plastic with use of the mold;
wherein the data are represented by following conditional formulas (1) and (2):
h
i−1
≦h<h
i
(1)
x=f
(
h−h
ti
) (2)
where
i is a suffix showing an ordinal number of each of the diffractive annular zones obtained by coun
Kiriki Toshihiko
Ota Kohei
Chang Audrey
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Konica Corporation
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