U.S. patent application number 13/529942 was filed with the patent office on 2012-10-18 for optical element and optical system.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. Invention is credited to Takeharu Okuno, Daisuke Sano, Kazue Uchida.
Application Number | 20120262794 13/529942 |
Document ID | / |
Family ID | 41725073 |
Filed Date | 2012-10-18 |
United States Patent
Application |
20120262794 |
Kind Code |
A1 |
Uchida; Kazue ; et
al. |
October 18, 2012 |
OPTICAL ELEMENT AND OPTICAL SYSTEM
Abstract
The optical element includes a base member configured to have an
optical surface, a concave-convex structure configured to have an
average pitch smaller than a shortest wavelength of a use
wavelength range, and an intermediate layer formed between the
optical surface and the concave-convex structure, made of a
material different from that of the concave-convex structure, and
having a refractive index between those of the base member and the
material of the concave-convex structure. The optical surface is
formed into a shape having a rotational symmetry axis. A thickness
of the intermediate layer or each of thicknesses of the
intermediate layer and the concave-convex structure varies so as to
increase as a distance from the rotational symmetry axis increases.
The optical element has good anti-reflection performance not only
at a central part of the optical surface having a small curvature
radius but also at a peripheral part thereof.
Inventors: |
Uchida; Kazue;
(Utsunomiya-shi, JP) ; Okuno; Takeharu;
(Utsunomiya-shi, JP) ; Sano; Daisuke;
(Utsunomiya-shi, JP) |
Assignee: |
CANON KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
41725073 |
Appl. No.: |
13/529942 |
Filed: |
June 21, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12548754 |
Aug 27, 2009 |
8226250 |
|
|
13529942 |
|
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Current U.S.
Class: |
359/601 |
Current CPC
Class: |
Y10T 428/2457 20150115;
G02B 1/02 20130101; G02B 1/118 20130101; G02B 1/115 20130101 |
Class at
Publication: |
359/601 |
International
Class: |
G02B 5/00 20060101
G02B005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 29, 2008 |
JP |
2008-222899 |
Aug 7, 2009 |
JP |
2009-184144 |
Claims
1. An optical element comprising: a base member configured to have
an optical surface; a concave-convex structure configured to have
an average pitch smaller than a shortest wavelength of a use
wavelength range; and an intermediate layer formed between the
optical surface and the concave-convex structure, made of a
material different from that of the concave-convex structure, and
having a refractive index between those of the base member and the
material of the concave-convex structure, wherein the optical
surface is formed into a shape having a rotational symmetry axis,
and wherein a thickness of the intermediate layer or each of
thicknesses of the intermediate layer and the concave-convex
structure varies so as to increase as a distance from the
rotational symmetry axis increases.
2. The optical element according to claim 1, wherein the
concave-convex structure varies in structure in its thickness
direction.
3. The optical element according to claim 1, wherein the following
condition is satisfied:
.lamda./(8.times.n).ltoreq.Dc.ltoreq.(2.times..lamda.)/n where Dc
represents the thickness of the intermediate layer at a position of
the rotational symmetry axis, n represents the refractive index of
the intermediate layer, and .lamda. represents the shortest
wavelength of the used wavelength range.
4. The optical element according to claim 1, wherein the following
conditions are satisfied: 0.3.ltoreq.n.times.sin(.theta.(h))<1
Dc<D(h)<Dc/cos(.theta.(h)) where Dc represents the thickness
of the intermediate layer at a position of the rotational symmetry
axis, n represents the refractive index of the intermediate layer,
D(h) represents the thickness of the intermediate layer at a
h-position away from the rotational symmetry axis by a distance h,
and .theta.(h) represents an angle that a light ray reaching the
intermediate layer at the h-position in parallel with the
rotational symmetry axis forms in the intermediate layer with a
normal to a concave-convex structure-side most superficial surface
of the intermediate layer at the h-position.
5. An optical system comprising: an optical element; wherein the
optical element comprising: a base member configured to have an
optical surface; a concave-convex structure configured to have an
average pitch smaller than a shortest wavelength of a use
wavelength range; and an intermediate layer formed between the
optical surface and the concave-convex structure, made of a
material different from that of the concave-convex structure, and
having a refractive index between those of the base member and the
material of the concave-convex structure, wherein the optical
surface is formed into a shape having a rotational symmetry axis,
and wherein a thickness of the intermediate layer or each of
thicknesses of the intermediate layer and the concave-convex
structure varies so as to increase as a distance from the
rotational symmetry axis increases.
6. The optical system according to claim 5, wherein the following
conditions are satisfied: 0.3.ltoreq.n.times.sin(.psi.(h))<1
Dc<D(h)<Dc/cos(.psi.(h)) where Dc represents the thickness of
the intermediate layer at a position of the rotational symmetry
axis, n represents the refractive index of the intermediate layer,
D(h) represents the thickness of the intermediate layer at a
h-position away from the rotational symmetry axis by a distance h,
and .psi.(h) represents an angle that a light ray having an average
incident angle among all light rays passing through the h-position
of the intermediate layer in the optical system forms in the
intermediate layer with a normal to a concave-convex structure-side
most superficial surface of the intermediate layer at the
h-position.
7. The optical system according to claim 5, further comprising: an
aperture stop; wherein the following conditions are satisfied:
0.3.ltoreq.n.times.sin(.zeta.(h))<1
Dc<D(h)<Dc/cos(.zeta.(h)) where Dc represents the thickness
of the intermediate layer at a position of the rotational symmetry
axis, n represents the refractive index of the intermediate layer,
D(h) represents the thickness of the intermediate layer at a
h-position away from the rotational symmetry axis by a distance h,
and .zeta.(h) represents an angle that a light ray passing through
the h-position of the intermediate layer and a center of the
aperture stop in the optical system forms in the intermediate layer
with a normal to a concave-convex structure-side most superficial
surface of the intermediate layer at the h-position.
8. An optical apparatus comprising: an optical system configured to
include an optical element; wherein the optical element comprising:
a base member configured to have an optical surface; a
concave-convex structure configured to have an average pitch
smaller than a shortest wavelength of a use wavelength range; and
an intermediate layer formed between the optical surface and the
concave-convex structure, made of a material different from that of
the concave-convex structure, and having a refractive index between
those of the base member and the material of the concave-convex
structure, wherein the optical surface is formed into a shape
having a rotational symmetry axis, and wherein a thickness of the
intermediate layer or each of thicknesses of the intermediate layer
and the concave-convex structure varies so as to increase as a
distance from the rotational symmetry axis increases.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 12/548,754 filed Aug. 27, 2009, which claims
priority to Japanese Patent Application Nos. 2009-184144, filed on
Aug. 7, 2009 and 2008-222899, filed on Aug. 29, 2008, each of which
is hereby incorporated by reference herein in its entirety.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to an optical element having a
reflection suppressing function (anti-reflection function).
[0003] Optical elements such as lenses used in optical systems are
manufactured by using a transparent base material such as optical
glass or optical plastic. Such a transparent base material has a
large refractive index, so that a reflectance thereof is high. A
high reflectance of the transparent base material reduces an amount
of effective light reaching an image surface, and generates
unnecessary reflection which causes ghost or flare. Therefore, it
is necessary to provide an anti-reflection function to the optical
element manufactured by using the transparent base material.
[0004] To provide the anti-reflection function to the optical
element, an anti-reflection film is generally formed on a surface
of the optical element (transparent base material). The
anti-reflection film is formed by laminating thin film layers on
the surface of the transparent base material, based on a general
optical interference theory.
[0005] Methods for forming anti-reflection films include a dry
method (vacuum film forming method) and a wet method (wet film
forming method). The dry method coats a surface of the transparent
base material with metal fluoride or metal oxide having a low
refractive index, as with a vapor deposition method and a
sputtering method. Further, the wet method applies coating liquid
containing a low refractive index material to the surface of the
transparent base material by a dipping method, a spin coat method
or the like, and then dries or fires it.
[0006] A thickness (hereinafter also referred to "film thickness")
of the anti-reflection film is frequently designed such that an
anti-reflection effect becomes largest at a central part of the
optical element (hereinafter referred to as lens), that is, at a
portion where an incident angle of a light ray is 0 degree. The
anti-reflection film designed as above has a uniform film thickness
over the entire lens surface.
[0007] However, when a light ray vertically enters the central part
of the lens on which the anti-reflection film is formed and light
rays enter a peripheral part of the lens in parallel with the above
light ray, incident angles of the light rays become large at the
peripheral part. Therefore, anti-reflection performance at the
peripheral part becomes lower than that of the central part of the
lens.
[0008] Japanese Patent Laid-open No. 2004-333908 discloses an
anti-reflection film having an optical film thickness which makes
its reflectance lowest for an entering/emerging light ray at
arbitrary positions on a lens surface. According to a general
optical interference theory, an optical path length difference
between reflected light at a surface of the anti-reflection film
and reflected light at a boundary surface between the
anti-reflection film and the transparent base material is an
odd-numbered times of one-half of a wavelength of the light.
Accordingly, these reflected lights interfere with each other to be
mutually weakened. Japanese Patent Laid-open No. 2004-333908
utilizes this theory. According to the theory, it is necessary to
increase the thickness of the anti-reflection film from the central
part of the lens toward the peripheral part thereof.
[0009] However, the anti-reflection effect of the anti-reflection
film manufactured based on the general optical interference theory
depends on the thickness thereof, so that a difference between an
actual film thickness and a designed film thickness makes it
impossible to obtain a satisfactory anti-reflection effect.
Therefore, a highly accurate film thickness control is required for
forming the anti-reflection film.
[0010] Further, another method for providing the anti-reflection
function to the optical element forms a structure having a
concave-convex shape finer than a wavelength of entering light
(hereinafter referred to as "use wavelength") on the surface of the
transparent base material.
[0011] In the concave-convex structure finer than the use
wavelength, the entering light behaves as if the structure is a
uniform medium since the entering light cannot recognize the
concave-convex shape. The concave-convex structure has a refractive
index according to a volume ratio of a material forming the
concave-convex shape, thereby showing a low refractive index that
cannot be obtained by normal materials. Consequently, the use of
such a concave-convex structure can achieve a higher
anti-reflection performance than that of anti-reflection films made
of low refractive index materials.
[0012] Methods for forming the above described concave-convex
structure include a method applying a film in which fine particles
having a particle diameter smaller than a use wavelength are
dispersed onto a surface of a transparent base material (refer to
Japanese Patent No. 3135944), and a method forming a periodic
concave-convex structure by pattern formation by using a
microfabrication apparatus (refer to Japanese Patent Laid-open No.
50-70040). Further, the methods include a method forming a
concave-convex structure of petal-shaped alumina by using a sol-gel
method (refer to Japanese Patent Laid-open No. 09-202649).
[0013] However, formation of such a concave-convex structure
requires complicated processes. Further, the concave-convex
structure is formed by using some limited materials, which reduces
a degree of freedom in design of the refractive index. Therefore,
there is a problem that high anti-reflection performance of the
concave-convex structure can be obtained only when using
transparent base materials having limited refractive indexes.
[0014] Japanese Patent Laid-open No. 2005-275372 discloses a method
providing, between a concave-convex structure and a transparent
base material, a thin film layer (intermediate layer) formed of a
material having a refractive index between those of a material
forming the concave-convex structure and the transparent base
material. This disclosed method changes the refractive index
gradually from the concave-convex structure to the transparent base
material, which can reduce reflection at the boundary surface of
the transparent base material. Further, selection of a material
forming the thin film layer can provide a lot of options in
selecting the transparent base material.
[0015] The dry method described above such as the sputtering method
and the vapor deposition method arranges a vapor deposition source
such that it faces the central part of the lens to form a thin film
layer. This arrangement enables formation of an anti-reflection
film whose anti-reflection performance becomes the maximum at the
central part of the lens as designed.
[0016] However, when anti-reflection film formation is performed by
the dry method on a lens surface having a small curvature radius,
an incident angle of a vapor deposition material increases toward
the peripheral part, thus reducing the film thickness toward the
peripheral part. Generally, when a film thickness at an incident
angle of 0 degree is defined as D, a film thickness at an incident
angle of 60 degrees is about D.times.cos(60.degree.), which is
about half of the film thickness at the incident angle of 0 degree.
Consequently, it is difficult to make the film thickness at the
peripheral part larger than that at the central part of the lens
based on the optical interference theory by using the dry method. A
method disposing a mask can make the film thickness at the
peripheral part than that at the central part of the lens, which,
however, requires huge equipment.
[0017] On the other hand, the wet method described above such as
the dipping method and the spin coat method has a low
controllability of the film thickness, which makes it difficult to
achieve a highly accurate film thickness control.
[0018] Further, the anti-reflection structure disclosed in Japanese
Patent Laid-open No. 2005-275372 in which the thin film layer is
disposed between the concave-convex structure and the transparent
base material has better antireflection characteristics in a large
wavelength band in comparison with the anti-reflection structure
constituted only by the anti-reflection film formed based on the
interference theory or the concave-convex structure. However, in
the disclosed anti-reflection structure, small reflection occurring
at the peripheral part of the lens surface having a small curvature
radius generates ghost and flare. Thus, further development in
anti-reflection performance is desired.
SUMMARY OF THE INVENTION
[0019] The present invention provides an optical element having
good anti-reflection performance not only at a central part of an
optical surface having a small curvature radius, but also at the
peripheral part thereof. Further, the present invention provides an
optical system using the optical element and an optical apparatus
including the optical system.
[0020] The present invention provides as an aspect thereof an
optical element including a base member configured to have an
optical surface, a concave-convex structure configured to have an
average pitch smaller than a shortest wavelength of a use
wavelength range, and an intermediate layer formed between the
optical surface and the concave-convex structure, made of a
material different from that of the concave-convex structure, and
having a refractive index between those of the base member and the
material of the concave-convex structure. The optical surface is
formed into a shape having a rotational symmetry axis. A thickness
of the intermediate layer or each of thicknesses of the
intermediate layer and the concave-convex structure varies so as to
increase as a distance from the rotational symmetry axis
increases.
[0021] The present invention provides as another aspect thereof an
optical system using the above described optical element and an
optical apparatus using the optical system.
[0022] Other aspects of the present invention will become apparent
from the following description and the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a cross-sectional view showing a basic
configuration of an optical element that is an embodiment of the
present invention.
[0024] FIG. 2 shows reflectance characteristics of each of first
and second embodiments of the present invention and those of each
of first through third comparative examples.
[0025] FIG. 3 shows reflectance characteristics of each of third
and fourth embodiments of the present invention and those of a
fourth comparative example.
[0026] FIG. 4 is a cross-sectional view showing a configuration of
an image-pickup optical system that is a fifth embodiment of the
present invention.
DESCRIPTION OF THE EMBODIMENTS
[0027] Exemplary embodiments of the present invention will
hereinafter be described with reference to the accompanying
drawings.
[0028] Before describing specific embodiments, characteristics
common to optical elements of the embodiments will be described.
FIG. 1 shows a basic configuration of the optical element. FIG. 1
schematically shows a concave-convex structure and an intermediate
layer.
[0029] The optical element of each embodiment includes a lens 11 as
a transparent base material (base member) having a concave lens
surface as an optical surface, a thin film layer 12 as an
intermediate layer formed on the optical surface, and a
concave-convex structure layer 13 as a concave-convex structure
including a fine concave-convex shape formed on the thin film layer
12.
[0030] The thin film layer 12 is made of a material different from
that of the concave-convex structure layer 13, and disposed between
the lens surface and the concave-convex structure layer 13. The
thin film layer 12 may have a single layer structure or a
multi-layer structure in which two or more thin film layers made of
mutually different materials are laminated. In other words, it is
sufficient that at least one thin film layer is formed between the
lens surface and the concave-convex structure layer 13. In a case
where the thin film layer 12 is made into a single structure, the
thin film layer 12 is made of a material different from that of the
concave-convex structure layer 13 and has a refractive index
between those of the lens 11 and the material forming the
concave-convex structure layer 13. In a case where the thin film
layer 12 is made into a multi-layer structure, it is sufficient
that at least one of the two or more thin film layers is made of a
material different from that of the concave-convex structure layer
13 and has a refractive index between those of the lens 11 and the
material forming the concave-convex structure layer 13.
[0031] Further, the lens surface of the lens 11 has a shape having
a rotational symmetry axis, that is, the lens surface has a
rotationally symmetric shape.
[0032] Moreover, in the optical element of each embodiment, a
thickness (hereinafter also referred to as "film thickness") of the
thin film layer 12 or each of thicknesses of the thin film layer 12
and the concave-convex structure layer 13 varies so as to increase
as a distance from the rotational symmetry axis increases.
[0033] In FIG. 1, Dc denotes a film thickness of the thin film
layer 12 at a center of the lens 11, that is, at a position of the
rotational symmetry axis (hereinafter referred to as "optical
axis") of the lens 11. The position of the rotational symmetry axis
of the lens 11 is a position (hereinafter referred to as "optical
axis position") where the optical axis crosses the lens surface. In
a case where the thin film layer 12 has a multi-layer structure, Dc
represents a film thickness of each thin film layer in the
multi-layer structure.
[0034] Further, h denotes a distance in a direction orthogonal to
the optical axis (or a direction along the lens surface) from the
optical axis position. D(h) denotes a film thickness of the thin
film layer 12 at a position of the distance h from the optical axis
position (hereinafter referred to as "h-position").
[0035] Further, .phi.(h) denotes an angle that a light ray reaching
the thin film layer 12 at the h-position in parallel with the
optical axis forms in a most superficial surface side outside of
the thin film layer 12 (that is, in a concave-convex structure
layer side outside of the thin film layer 12) with a normal to a
most superficial surface of the thin film layer 12 at the
h-position. The "most superficial surface" of the thin film layer
12 is a concave-convex structure-side superficial surface thereof,
in other words, an uppermost surface closest to the concave-convex
structure layer 13.
[0036] Moreover, .theta.(h) denotes an angle that a light ray
reaching the thin film layer 12 at the h-position in parallel with
the optical axis forms in the thin film layer 12 (in the
intermediate layer) with the normal to the most superficial surface
of the thin film layer 12 at the h-position.
[0037] When n denotes a refractive index of the thin film layer 12,
.phi.(h) and .theta.(h) satisfy the following Snell's equation:
sin(.phi.(h))=n.times.sin(.theta.(h)).
[0038] As described above, in a peripheral part of the lens 11
(hereinafter also referred to as "lens peripheral part") having a
curvature, incident angles of light rays increase, and therefore
anti-reflection performance is deteriorated in comparison with that
at a central part of the lens 11 (hereinafter also referred to as
"lens central part"). Changing a film thickness of an
anti-reflection film according to a position on a lens surface such
that a typical optical interference theory is satisfied makes it
possible to provide good anti-reflection performance. However, the
anti-reflection performance of the anti-reflection film based on
the typical optical interference theory largely depends on the film
thickness, so that even a small difference of an actual film
thickness from a designed film thickness deteriorates the
anti-reflection performance.
[0039] Accordingly, in each embodiment, the thin film layer 12 is
provided between the concave-convex structure layer 13 and the lens
11 (lens surface).
[0040] The concave-convex structure layer 13 is formed by a
concave-convex structure whose average pitch is sufficiently
smaller than that of a shortest wavelength in a use wavelength
range that is a wavelength range of light entering the optical
element. The pitch means, when one convex portion and one concave
portion adjacent thereto are defined as one set of concave and
convex portions, a distance between two sets of concave and convex
portions adjacent to each other.
[0041] Further, a configuration (shape) of the concave-convex
structure layer 13 varies in its thickness direction. Specifically,
a width of the convex portion increases from a light entrance side
on which light reaches the optical element toward a thin film layer
12 side (or a lens 11 side), a width of the concave portion
decreases from the thin film layer 12 side toward the light
entrance side. Therefore, the concave-convex structure layer 13 can
be considered as a structure whose refractive index varies
continuously in its thickness direction. The continuous variation
of the refractive index generates in the structure innumerable
reflected lights whose amplitudes are small, and the reflected
lights interfere with each other to be mutually reduced. Reflected
lights generated at a boundary surface between the thin film layer
12 and the lens 11 mutually interfere innumerably to be mutually
reduced in the structure, so that the anti-reflection performance
has a little dependency on film thickness accuracy.
[0042] Consequently, the optical element of each embodiment is more
insensitive to the film thickness in comparison with an optical
element having a conventional anti-reflection film. As a result,
the optical element of each embodiment has a certain margin
(tolerance) in the film thickness accuracy in the lens peripheral
part at which incident angles of light rays are large.
[0043] The above described configuration can realize an optical
element having satisfactory anti-reflection performance not only at
the lens central part having a small curvature radius but also at
the lens peripheral part and excellent mass productivity.
[0044] It is desired that the optical element of each embodiment
satisfy, for .theta.(h) which satisfies:
0.3.ltoreq.n.times.sin(.theta.(h))<1 (1),
the following condition:
Dc<D(h)<Dc/cos(.theta.(h)) (2).
[0045] Further, it is more desired that the optical element of each
embodiment satisfy the following condition instead of the above
condition (2):
(Dc/2).times.(1/cos(.theta.(h))+1)<D(h)<Dc/cos(.theta.(h))
(3).
[0046] Moreover, it is more preferable that the optical element of
each embodiment satisfy the following condition:
.lamda./(8.times.n).ltoreq.Dc.ltoreq.(2.times..lamda.)/n (4)
where .lamda. represents the shortest wavelength of the use
wavelength range.
[0047] Furthermore, in a case where the optical element of each
embodiment is used in an optical system such as an image-forming
optical system or an observation optical system, it is preferable
that the optical element satisfy, for .psi.(h) which satisfies:
0.3.ltoreq.n.times.sin(.psi.(h))<1 (5)
the following condition:
Dc<D(h)<Dc/cos(.psi.(h)) (6)
where .psi.(h) represents an angle that a light ray having an
average incident angle among all light rays passing through the
h-position of the thin film layer 12 in an optical system forms in
the thin film layer 12 (in the intermediate layer) with a normal to
the most superficial surface of the thin film layer 12 at the
h-position.
[0048] Further, it is more preferable that the optical element
satisfy the following condition instead of the condition (6):
(Dc/2).times.(1/cos(.psi.(h))+1)<D(h)<Dc/cos(.psi.(h))
(7).
[0049] Still further, in a case where the optical element of each
embodiment is used in an optical system, such as an image-forming
optical system or an observation optical system, including an
aperture stop, it is preferable that the optical element satisfy,
for .zeta.(h) which satisfies:
0.3.ltoreq.n.times.sin(.zeta.(h))<1 (8),
the following condition:
Dc<D(h)<Dc/cos(.zeta.(h)) (9)
where .zeta.(h) represents an angle that a light ray passing
through the h-position of the thin film layer 12 and a center of
the aperture stop in the optical system forms in the thin film
layer 12 (in the intermediate layer) with a normal to the most
superficial surface of the thin film layer 12 at the
h-position.
[0050] Further, it is preferable that the optical element satisfy
the following condition instead of the condition (9):
(Dc/2).times.(1/cos(.zeta.(h))+1)<D(h)<Dc/cos(.zeta.(h))
(10).
[0051] Satisfying at least one of the above described conditions
(2), (3), (6), (7), (9) and (10) makes it possible to more surely
obtain an optical element having good anti-reflection performance
also in the lens peripheral part and excellent mass
productivity.
[0052] The thin film layer 12 in each embodiment can be formed by
arbitrary methods. For example, the thin film layer 12 may be
formed by a dry method (vacuum film forming method) such as a
sputtering method or a vapor deposition method. Alternatively, the
thin film layer 12 may be formed by a wet method (wet film forming
method) such as a dipping method or a spin coat method using
sol-gel coating liquid.
[0053] Each of the above-described methods forms, after forming the
thin film layer 12 on the lens surface of the lens 11, the
concave-convex structure layer 13 on the most superficial surface
of the thin film layer 12. The concave-convex structure layer 13
can also be formed by arbitrary methods such as a method applying a
film in which fine particles having a particle diameter smaller
than the use wavelength are dispersed onto the most superficial
surface of the thin film layer 12 or a method forming a
concave-convex structure of petal-shaped alumina by using a sol-gel
method.
[0054] Further, the material for forming the thin film layer 12 is
not particularly limited if the material is different from that of
the concave-convex structure layer 13. However, it is preferable
that the material contain at least one of zirconia, silica, titania
or zinc oxide. It is also preferable that the concave-convex
structure layer 13 be mainly made of alumina.
[0055] Next, description will be made of the method forming the
thin film layer 12 and the concave-convex structure layer 13 by
using the spin coat method.
[0056] To form the thin film layer 12 on the lens surface of the
lens 11, an SiO.sub.2 sol liquid and a TiO.sub.2 sol liquid are
first mixed and stirred. The mixed liquid as an
SiO.sub.2--TiO.sub.2 coating liquid is applied onto the lens
surface by using the spin coat method. After the application of the
mixed liquid, the lens 11 is subjected to a heating process at
several hundred degrees and further subjected to a drying process,
thereby obtaining a transparent amorphous SiO.sub.2/TiO.sub.2 film.
The refractive index of the thin film layer 12 is decided depending
on a molar ratio of the SiO.sub.2 sol liquid and TiO.sub.2 sol
liquid mixed with each other. The film thickness of the thin film
layer 12 is controlled by a rotation speed of the lens 11 or a
viscosity of the coating liquid used in the spin coat method.
[0057] Then, a sol-gel coating liquid containing alumina is applied
on the most superficial surface of the thin film layer 12 to form a
gel film thereon. Subsequently, the gel film is dipped into heated
water to precipitate plate crystals mainly containing alumina, and
thereby the concave-convex structure is formed whose convex
portions are formed by the precipitated crystals.
[0058] The dipping of the gel film into heated water provides
peptization or the like to part of a superficial layer of the gel
film formed by applying the sol-gel coating liquid containing
alumina, and thereby components included in the gel film liquate
out from the part. Differences in solubility of respective
hydroxides for the heated water precipitate the plate crystals
mainly containing alumina on the superficial surface layer of the
gel film, and then the precipitated crystals grow to form the
concave-convex structure. A preferred temperature of the heated
water is a temperature from 40 degrees to 100 degrees. A preferred
heated water treatment time is between about 5 minutes and about 24
hours.
[0059] In each embodiment, the lens 11 as the transparent base
material is formed of optical glass or optical plastic. The
transparent base material may be formed into any shape, such as a
plate shape, a film shape or a sheet shape, other than a lens shape
as far as the transparent base material has an optical surface and
is formed into a shape suitable for a final intended purpose.
[0060] The specific embodiments will be hereinafter described
below. The use wavelength range of each embodiment is a wavelength
range of 400 to 700 nm, and a central wavelength of the use
wavelength range is 550 nm. However, those values are only for the
sake of examples. Embodiments of the present invention are not
limited thereto.
Embodiment 1
[0061] In a first embodiment (Embodiment 1) of the present
invention, the film thickness of the thin film layer 12 having a
single layer structure varies so as to gradually increase from the
optical axis position toward an outer edge of the lens peripheral
part, and the thickness of the concave-convex structure layer 13 is
uniform over the entire lens surface.
[0062] In Embodiment 1, a meniscus lens having an outer diameter of
51 mm and a refractive index of 2.0 was used as the transparent
base material. After applying the SiO.sub.2--TiO.sub.2 coating
liquid onto a concave lens surface of the lens having a diameter of
38 mm and a curvature radius of 20.3 mm by the spin coat method,
the heating process and the drying process were performed to form
the thin film layer 12. Then, the sol-gel coating liquid containing
alumina was applied on the thin film layer 12 to form a gel film,
and followed by dipping it into heated water to precipitate plate
crystals mainly containing alumina. Thus, the concave-convex
structure layer 13 was formed.
[0063] The thickness and the refractive index of the respective
layers were measured by using an ellipsometer. The same measurement
is performed also in the other embodiments described later.
[0064] The thickness of the concave-convex structure layer 13 was
160 nm at every position on the lens surface. Further, the
refractive index of the concave-convex structure layer 13
continuously reduced from 1.3 to 1.0 from the thin film layer side
toward the light entrance side (air side). The refractive index n
of the thin film layer 12 was 1.64, and the film thickness thereof
smoothly increased from the optical axis position toward the outer
edge of the lens peripheral part.
[0065] Table 1 shows the distance h from the optical axis position,
.phi.(h) at the h-position, the film thickness D(h) of the thin
film layer 12, Dc/cos(.theta.(h)) in the condition (2), the
thickness Ds(h) of the concave-convex structure layer 13, and an
average reflectance R.sub.a(h) in the wavelength range from 400 to
700 nm in Embodiment 1.
[0066] As shown in Table 1, the film thickness of the thin film
layer 12 in Embodiment 1 satisfies the condition (2).
[0067] FIG. 2 shows reflectance characteristics in the wavelength
range from 400 to 700 nm at a position of a distance h=10.2 mm.
Embodiment 1 achieved very good reflectance characteristics in
which the reflectance was 0.6% or less in the wavelength range from
400 to 700 nm.
[0068] Further, as shown in Table 1, Embodiment 1 achieved very
good average reflectance characteristics in which an average
reflectance Ra(10.2) at the distance h=10.2 mm was 0.12% and an
average reflectance Ra(17.6) at a distance h=17.6 mm was 2.08% in
the wavelength range from 400 to 700 nm.
Embodiment 2
[0069] A second embodiment (Embodiment 2) of the present invention
will describe a case where each of the film thickness of the thin
film layer 12 having a single layer structure and the thickness of
the concave-convex structure layer 13 varies so as to gradually
increase from the optical axis position toward the outer edge of
the lens peripheral part.
[0070] Embodiment 2 formed the thin film layer 12 on a meniscus
lens (lens surface) and formed the concave-convex structure layer
13 on the thin film layer 12, by a similar method to that in
Embodiment 1.
[0071] The thickness of the concave-convex structure layer 13
smoothly increased from the optical axis position toward the outer
edge of the lens peripheral part. The refractive index of the
concave-convex structure layer 13 continuously reduced from 1.3 to
1.0 from the thin film layer side toward the light entrance side.
The refractive index n of the thin film layer 12 was 1.64, and the
film thickness thereof smoothly increased from the optical axis
position toward the outer edge of the lens peripheral part.
[0072] Table 1 shows h, .phi.(h), D(h), Dc/cos(.theta.(h)), Ds(h)
and the average reflectance R.sub.a(h) in Embodiment 2.
[0073] As shown in Table 1, the film thickness of the thin film
layer 12 in Embodiment 2 satisfies the condition (2).
[0074] Further, as shown in FIG. 2, Embodiment 2 achieved very good
reflectance characteristics in which the reflectance at a position
of a distance h=10.2 mm was 0.6% or less in the wavelength range
from 400 to 700 nm.
[0075] Further, as shown in Table 1, Embodiment 2 achieved very
good average reflectance characteristics in which an average
reflectance Ra(10.2) at the distance h=10.2 mm was 0.11% and an
average reflectance Ra(17.6) at a distance h=17.6 mm was 1.18% in
the wavelength range from 400 to 700 nm.
Embodiment 3
[0076] A third embodiment (Embodiment 3) of the present invention
will describe a case where the film thickness of the thin film
layer 12 having a multi-layer (two-layer) structure varies so as to
gradually increase from the optical axis position toward the outer
edge of the lens peripheral part and the thickness of the
concave-convex structure layer 13 is uniform over the entire lens
surface.
[0077] Embodiment 3 formed the thin film layer 12 on a meniscus
lens (lens surface) and formed the concave-convex structure layer
13 on the thin film layer 12, by a similar method to that in
Embodiment 1.
[0078] The thickness of the concave-convex structure layer 13 was
195 nm at every position on the lens surface. Further, the
refractive index of the concave-convex structure layer 13
continuously reduced from 1.36 to 1.0 from the thin film layer side
toward the light entrance side. A refractive index n.sub.1 of a
thin film layer of the two-layered thin film layer 12 closer to the
concave-convex structure layer 13 was 1.64, and a refractive index
n.sub.2 of another thin film layer closer to the lens surface was
1.74. Thus, in the case where the thin film layer 12 is formed into
a multi-layer structure, it is preferable to form the thin film
layer 12 such that the refractive indexes of the respective thin
film layer decrease from a lens side to a concave-convex structure
layer side in order to suppress Fresnel reflection. Further, the
film thickness of each of the two thin film layers smoothly
increased from the optical axis position toward the outer edge of
the lens peripheral part.
[0079] Table 2 shows h, .phi.(h), a film thickness D.sub.1(h) of
the thin film layer of the two-layered thin film layer 12 closer to
the concave-convex structure layer 13 and Dc.sub.1/cos(.theta.(h))
in the condition (2) in Embodiment 3. Further, Table 2 shows a film
thickness D.sub.2(h) of the thin film layer of the two-layered thin
film layer 12 closer to the lens surface, Dc.sub.2/cos(.theta.(h))
in the condition (2), Ds(h) and an average reflectance Ra(h).
[0080] As shown in Table 2, the film thickness of each thin film
layer of the two-layered thin film layer 12 in Embodiment 3
satisfies the condition (2).
[0081] FIG. 3 shows reflectance characteristics in the wavelength
range from 400 to 700 nm at a position of a distance h=10.2 mm.
Embodiment 3 achieved very good reflectance characteristics in
which the reflectance was 0.5% or less in the wavelength range from
400 to 700 nm.
[0082] Further, as shown in Table 2, Embodiment 3 achieved very
good average reflectance characteristics in which an average
reflectance Ra(10.2) at the distance h=10.2 mm was 0.32% and an
average reflectance Ra(17.6) at a distance h=17.6 mm was 2.16% in
the wavelength range from 400 to 700 nm.
Embodiment 4
[0083] A fourth embodiment (Embodiment 4) of the present invention
will describe a case where each of the film thickness of the thin
film layer 12 having a multi-layer (two-layer) structure and the
thickness of the concave-convex structure layer 13 varies so as to
gradually increase from the optical axis position toward the outer
edge of the lens peripheral part.
[0084] Embodiment 4 formed the thin film layer 12 on a meniscus
lens (lens surface) and formed the concave-convex structure layer
13 on the thin film layer 12, by a similar method to that in
Embodiment 1.
[0085] The thickness of the concave-convex structure layer 13
smoothly increased from the optical axis position toward the outer
edge of the lens peripheral part. Further, the refractive index of
the concave-convex structure layer 13 continuously reduced from
1.36 to 1.0 from the thin film layer side toward the light entrance
side. A refractive index n.sub.1 of a thin film layer of the
two-layered thin film layer 12 closer to the concave-convex
structure layer 13 was 1.64, and a refractive index n.sub.2 of
another thin film layer closer to the lens surface was 1.74. The
film thicknesses of those two thin film layers smoothly increased
from the optical axis position toward the outer edge of the lens
peripheral part.
[0086] Table 2 shows h, .phi.(h), a film thickness D.sub.1(h) of
the thin film layer of the two-layered thin film layer 12 closer to
the concave-convex structure layer 13 and Dc.sub.1/cos(.theta.(h))
in the condition (2) in Embodiment 4. Further, Table 2 shows a film
thickness D.sub.2(h) of the thin film layer of the two-layered thin
film layer 12 closer to the lens surface, Dc.sub.2/cos(.theta.(h))
in the condition (2), Ds(h) and an average reflectance Ra(h).
[0087] As shown in Table 2, the film thickness of each thin film
layer of the two-layered thin film layer 12 in Embodiment 4
satisfies the condition (2).
[0088] Further, as shown in FIG. 3, Embodiment 4 achieved very good
reflectance characteristics in which the reflectance at a position
of a distance h=10.2 mm was 0.5% or less in the wavelength range
from 400 to 700 nm.
[0089] Further, as shown in Table 2, Embodiment 4 achieved very
good average reflectance characteristics in which an average
reflectance Ra(10.2) at the distance h=10.2 mm was 0.27% and an
average reflectance Ra(17.6) at a distance h=17.6 mm was 1.16% in
the wavelength range from 400 to 700 nm.
[0090] Comparative examples 1 to 4 for Embodiments 1 to 4 will
hereinafter be described.
Comparative Example 1
[0091] A first comparative example (Comparative Example 1) will
describe a case where both of the film thickness of the thin film
layer having a single layer structure and the thickness of the
concave-convex structure layer are uniform over the entire lens
surface, for the sake of comparison with Embodiments 1 and 2.
[0092] Comparative Example 1 formed the thin film layer on a
meniscus lens (lens surface) and formed the concave-convex
structure layer on the thin film layer, by a similar method to that
in Embodiment 1.
[0093] The thickness of the concave-convex structure layer was 160
nm at every position on the lens surface. Further, the refractive
index of the concave-convex structure layer continuously reduced
from 1.3 to 1.0 from the thin film layer side toward the light
entrance side. The refractive index of the thin film layer was
1.64, and the film thickness thereof was constantly 65 nm over the
entire lens from the optical axis position to the outer edge of the
lens peripheral part.
[0094] Table 1 shows h, .phi.(h), D(h), Dc/cos (.theta.(h)), Ds(h)
and an average reflectance Ra(h) in Comparative Example 1.
[0095] As shown in Table 1, the film thickness of the thin film
layer of Comparative Example 1 does not satisfy the condition
(2).
[0096] FIG. 2 shows reflectance characteristics in the wavelength
range from 400 to 700 nm at a position of a distance h=10.2 mm.
Comparative Example 1 provided a reflectance higher than those in
Embodiments 1 and 2 in the wavelength range from 500 to 700 nm.
[0097] Further, as shown in Table 1, in Comparative Example 1, an
average reflectance Ra(10.2) at the distance h=10.2 mm was 0.16%
and an average reflectance Ra(17.6) at a distance h=17.6 mm was
2.13% in the wavelength range from 400 to 700 nm.
[0098] As seen from the above, the anti-reflection performance of
Comparative Example 1 is lower than those of Embodiments 1 and
2.
Comparative Example 2
[0099] A second comparative example (Comparative Example 2) will
describe a case where the film thickness of the thin film layer
having a single layer structure varies so as to extremely increase
from the optical axis position toward the outer edge of the lens
peripheral part and the thickness of the concave-convex structure
layer is uniform over the entire lens surface, for the sake of
comparison with Embodiments 1 and 2.
[0100] Comparative Example 2 formed the thin film layer on a
meniscus lens (lens surface) and formed the concave-convex
structure layer on the thin film layer, by a similar method to that
in Embodiment 1.
[0101] The thickness of the concave-convex structure layer was 160
nm at every position on the lens surface. Further, the refractive
index of the concave-convex structure layer continuously reduced
from 1.3 to 1.0 from the thin film layer side toward the light
entrance side. The refractive index of the thin film layer was
1.64, and the film thickness thereof was remarkably increased from
the optical axis position toward the outer edge of the lens
peripheral part.
[0102] Table 1 shows h, .phi.(h), D(h), Dc/cos (.theta.(h)), Ds(h)
and an average reflectance Ra(h) in Comparative Example 2.
[0103] As shown in Table 1, the film thickness of the thin film
layer of Comparative Example 2 does not satisfy the condition
(2).
[0104] FIG. 2 shows reflectance characteristics in the wavelength
range from 400 to 700 nm at a position of a distance h=10.2 mm.
Comparative Example 2 provided a reflectance higher than those in
Embodiments 1 and 2 in the wavelength range from 400 to 670 nm.
[0105] Further, as shown in Table 1, in Comparative Example 2, an
average reflectance Ra(10.2) at the distance h=10.2 mm was 0.85%
and an average reflectance Ra(17.6) at a distance h=17.6 mm was
4.96% in the wavelength range from 400 to 700 nm.
[0106] As seen from the above, the anti-reflection performance of
Comparative Example 2 is lower than those of Embodiments 1 and
2.
Comparative Example 3
[0107] A third comparative example (Comparative Example 3) will
describe a case where the thin film layer having a single layer
structure is formed by the vacuum vapor deposition method, and
thereafter the concave-convex structure layer is formed by the spin
coat method, for the sake of comparison with Embodiments 1 and
2.
[0108] The film thickness of the thin film layer varies so as to
decrease from the optical axis position toward the outer edge of
the lens peripheral part, and the thickness of the concave-convex
structure is uniform over the entire lens surface.
[0109] In Comparative Example 3, an alumina (Al2O3) layer having a
refractive index of 1.64 as a thin film layer was formed on a
meniscus lens (lens surface) having the same shape as that of the
meniscus lens in Embodiment 1 by the vacuum vapor deposition
method. Then, the concave-convex structure layer was formed on the
alumina thin film layer by the spin coat method using the sol-gel
coating liquid.
[0110] The thickness of the concave-convex structure layer was 160
nm at every position on the lens surface. Further, the refractive
index of the concave-convex structure layer continuously decreased
from 1.3 to 1.0 from the thin film layer side to the light entrance
side, as with Embodiments 1 and 2. The film thickness of the thin
film layer smoothly decreased from 65 nm at the optical axis
position toward the outer edge of the lens peripheral part.
[0111] Table 1 shows h, .phi.(h), D(h), Dc/cos (.theta.(h)), Ds(h)
and an average reflectance Ra(h) in Comparative Example 3.
[0112] As shown in Table 1, the film thickness of the thin film
layer of Comparative Example 3 does not satisfy the condition
(2).
[0113] FIG. 2 shows reflectance characteristics in the wavelength
range from 400 to 700 nm at a position of a distance h=10.2 mm.
Comparative Example 3 provided a reflectance about twice as high as
those in Embodiments 1 and 2 in the wavelength range from 400 to
700 nm.
[0114] Further, as shown in Table 1, in Comparative Example 3, an
average reflectance Ra(10.2) at the distance h=10.2 mm was 0.41%
and an average reflectance Ra(17.6) at a distance h=17.6 mm was
4.17% in the wavelength range from 400 to 700 nm.
[0115] As seen from the above, the anti-reflection performance of
Comparative Example 3 is lower than that of Embodiments 1 and
2.
Comparative Example 4
[0116] A fourth comparative example (Comparative Example 4) will
describe a case where the thin film layer having a two-layer
structure is formed by the vacuum vapor deposition method, and
thereafter the concave-convex structure layer is formed by the spin
coat method, for the sake of comparison with Embodiments 3 and 4.
The film thickness of the thin film layer varies so as to decrease
from the optical axis position toward the outer edge of the lens
peripheral part, and the thickness of the concave-convex structure
is uniform over the entire lens surface.
[0117] In Comparative Example 4, two layers including a magnesium
oxide thin film layer having a refractive index of 1.74 and an
alumina (Al2O3) thin film layer having a refractive index of 1.64
were formed on a meniscus lens (lens surface) having the same shape
as that of the meniscus lens in Embodiment 1 by the vacuum vapor
deposition method. Then, the concave-convex structure layer was
formed on the alumina thin film layer by the spin coat method using
the sol-gel coating liquid.
[0118] The thickness of the concave-convex structure layer was 195
nm at every position on the lens surface. Further, the refractive
index of the concave-convex structure layer continuously decreased
from 1.36 to 1.0 from the thin film layer side to the light
entrance side, as with Embodiments 3 and 4. The film thickness of
the magnesium oxide thin film layer at the optical axis position
was 43 nm, and that of the alumina thin film layer at the optical
axis position was 79 nm. These film thicknesses smoothly decreased
from the optical axis position toward the outer edge of the lens
peripheral part.
[0119] Table 2 shows h, .phi.(h), a film thickness D1(h) and
Dc1/cos(.theta.(h)) of the magnesium oxide thin film layer of the
two-layered structure thin film layer closer to the concave-convex
structure layer in Comparative Example 4. Further, Table 2 shows a
film thickness D2(h) and Dc2/cos(.theta.(h)) of the alumina thin
film layer of the two-layered structure thin film layer closer to
the lens surface. Moreover, Table 2 shows Ds(h) and an average
reflectance Ra(h).
[0120] As shown in Table 2, each of the film thicknesses of the two
thin film layers of Comparative Example 4 does not satisfy the
condition (2).
[0121] FIG. 3 shows reflectance characteristics in the wavelength
range from 400 to 700 nm at a position of a distance h=10.2 mm.
Comparative Example 4 provided a reflectance twice or more than
those of the Embodiments 3 and 4 in the wavelength range from 400
to 700 nm.
[0122] Further, as shown in Table 2, in Comparative Example 4, an
average reflectance Ra(10.2) at the distance h=10.2 mm was 0.78%
and an average reflectance Ra(17.6) at a distance h=17.6 mm was
2.70% in the wavelength range from 400 to 700 nm.
[0123] As seen from the above, the anti-reflection performance of
Comparative Example 4 is lower than that of Embodiments 3 and
4.
[0124] Although not shown as a specific comparative example, the
inventor found that an anti-reflection structure including a thin
film layer having a two-layer structure and a concave-convex
structure layer also provides low anti-reflection performance in a
case where the film thickness of the thin film layer is uniform
over the entire lens surface and in a case where the film thickness
of the thin film layer remarkably increases from the optical axis
position toward the outer edge of the lens peripheral part.
[0125] As described above, each of Embodiments 1 to can realize an
optical element having high anti-reflection performance not only at
the lens central part whose curvature radius is small, but also at
the lens peripheral part and excellent mass productivity.
TABLE-US-00001 TABLE 1 DISTANCE THICKNESS FROM FILM OF CONCAVE-
AVERAGE OPTICAL AXIS .phi. (h) THICKNESS Dc/cos CONVEX SHAPE
REFLECTANCE h (mm) (.degree.) (nm) (.theta. (h)) D.sub.s (h) (nm)
R.sub.a (h) (%) Embodiment 1 0.0 0 65.0 (=Dc) 65.0 160 0.11 10.2 30
68.0 68.2 160 0.12 17.6 60 76.0 76.4 160 2.08 Embodiment 2 0.0 0
65.0 (=Dc) 65.0 160 0.11 10.2 30 68.0 68.2 164 0.11 17.6 60 76.0
76.4 188 1.18 Comparative 0.0 0 65.0 (=Dc) 65.0 160 0.11 Example 1
10.2 30 65.0 68.2 160 0.16 17.6 60 65.0 76.4 160 2.13 Comparative
0.0 0 65.0 (=Dc) 65.0 160 0.11 Example 2 10.2 30 98.0 68.2 160 0.85
17.6 60 126.0 76.4 160 4.96 Comparative 0.0 0 65.0 (=Dc) 65.0 160
0.11 Example 3 10.2 30 56.3 68.2 160 0.41 17.6 60 32.5 76.4 160
4.17
TABLE-US-00002 TABLE 2 DISTANCE FILM FILM THICKNESS FROM THICKNESS
THICKNESS OF CONCAVE- AVERAGE OPTICAL AXIS .phi. (h) D.sub.1 (h)
D.sub.2 (h) Dc.sub.1/cos Dc.sub.2/cos CONVEX SHAPE REFLECTANCE h
(mm) (.degree.) (nm) (nm) (.theta. (h)) (.theta. (h)) D.sub.s (h)
(nm) R.sub.a (h) (%) Embodiment 3 0.0 0 43 (=DC.sub.1) 79
(=Dc.sub.2) 43.0 79.0 195 0.25 10.2 30 45.0 82.0 45.2 82.5 195 0.32
17.6 60 50.0 90.0 50.8 91.1 195 2.61 Embodiment 4 0.0 0 43
(=DC.sub.1) 79 (=Dc.sub.2) 43.0 79.0 203 0.25 10.2 30 45.0 82.0
45.2 82.5 203 0.27 17.6 60 50.0 90.0 50.8 91.1 224 1.69 Comparative
0.0 0 43 (=DC.sub.1) 79 (=Dc.sub.2) 43.0 79.0 195 0.25 Example 4
10.2 30 37.0 68.0 45.2 82.5 195 0.78 17.6 60 21.5 39.5 50.8 91.1
195 2.70
Embodiment 5
[0126] FIG. 4 shows an image pickup optical system (image-forming
optical system) using the optical element described in each of
Embodiments 1 to 4. The image pickup optical system is used in an
optical apparatus such as a digital camera, a video camera, and an
interchangeable lens.
[0127] In FIG. 4, reference numeral 43 denotes an image pickup
plane at which a solid-state image pickup element (photoelectric
conversion element) such as a CCD sensor or a CMOS sensor is
disposed. Reference numeral 42 denotes an aperture stop.
[0128] Reference numeral 44 denotes a lens as the optical element.
On at least one of an entrance surface and an exit surface of the
lens 44, an anti-reflection structure 41 (illustrated by plural
dots in FIG. 4) is formed which is constituted by the thin film
layer 12 and the concave-convex structure layer 13 described in
Embodiments 1 to 4.
[0129] The use wavelength range of the image-pickup optical system
of this embodiment is a visible range. The shortest wavelength in
the use wavelength range is 400 nm.
[0130] It is preferable that .psi.(h) in this embodiment be an
angle that a light ray whose incident angle is an average value of
incident angles of all light rays passing through the position away
from the optical axis position by the distance h forms with the
normal to the most superficial surface of the thin film layer.
[0131] Further, it is preferable that .zeta.(h) in the optical
system of this embodiment be an angle that a light ray passing
through the position away from the optical axis position by the
distance h and a center of the aperture stop 42 forms in the thin
film layer with the normal to the most superficial surface of the
thin film layer.
[0132] Furthermore, the present invention is not limited to these
embodiments and various variations and modifications may be made
without departing from the scope of the present invention.
* * * * *