U.S. patent application number 13/755307 was filed with the patent office on 2013-05-30 for optical element and optical apparatus.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. The applicant listed for this patent is CANON KABUSHIKI KAISHA. Invention is credited to Daisuke Sano.
Application Number | 20130135743 13/755307 |
Document ID | / |
Family ID | 41725070 |
Filed Date | 2013-05-30 |
United States Patent
Application |
20130135743 |
Kind Code |
A1 |
Sano; Daisuke |
May 30, 2013 |
OPTICAL ELEMENT AND OPTICAL APPARATUS
Abstract
The optical element includes a base member, and a first layer
which is formed on the base member and whose refractive index for a
central use wavelength .lamda. changes in a thickness direction of
the first layer by 0.05 or more. The first layer has an
anti-reflection function and satisfies n.sub.t=n.sub.i+0.1
(n.sub.s-n.sub.i),
0.5.ltoreq.[n{t(n.sub.t)/2}-n.sub.t]/[n.sub.s-n{t(n.sub.t)/2}]0.8,
.lamda./4.ltoreq.t(n.sub.t).ltoreq.2.lamda. and
1.0.ltoreq.n.sub.i.ltoreq.1.1. n.sub.i represents a refractive
index of a most light entrance side part of the first layer for the
central use wavelength, n.sub.s represents a refractive index of a
most base member side part of the first layer for the central use
wavelength, t(n.sub.o) represents an optical film thickness of the
first layer at which the refractive index thereof for the central
use wavelength is n.sub.o, and n{t.sub.o} represents a refractive
index of the first layer for the central use wavelength at a
position where the optical film thickness is t.sub.o.
Inventors: |
Sano; Daisuke;
(Utsunomiya-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CANON KABUSHIKI KAISHA; |
Tokyo |
|
JP |
|
|
Assignee: |
CANON KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
41725070 |
Appl. No.: |
13/755307 |
Filed: |
January 31, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12546949 |
Aug 25, 2009 |
8373930 |
|
|
13755307 |
|
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Current U.S.
Class: |
359/580 |
Current CPC
Class: |
G02B 1/113 20130101;
G02B 1/11 20130101 |
Class at
Publication: |
359/580 |
International
Class: |
G02B 1/11 20060101
G02B001/11 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 29, 2008 |
JP |
2008-222900 |
Claims
1. An optical element comprising: a base member; and a first layer
which is formed on the base member and whose refractive index for a
central use wavelength .lamda. changes in a direction of a
thickness of the first layer by 0.05 or more, wherein the first
layer has an anti-reflection function and satisfies the following
conditions: n t = n i + 0.1 ( n s - n i ) ##EQU00004## 0.5 .ltoreq.
n { t ( n t ) / 2 } - n t n s - n { t ( n t ) / 2 } .ltoreq. 0.8
##EQU00004.2## .lamda. 4 .ltoreq. t ( n t ) .ltoreq. 2 .lamda.
##EQU00004.3## 1.0 .ltoreq. n i .ltoreq. 1.1 ##EQU00004.4## where
n.sub.i represents a refractive index of a most light entrance side
part of the first layer for the central use wavelength, n.sub.s
represents a refractive index of a most base member side part of
the first layer for the central use wavelength, t(n.sub.o)
represents an optical film thickness of the first layer at which
the refractive index thereof for the central use wavelength is
n.sub.o, and n{t.sub.o} represents a refractive index of the first
layer for the central use wavelength at a position where the
optical film thickness is t.sub.o.
2. The optical element according to claim 1, wherein at least one
optical interference layer is formed between the base member and
the first layer.
3. The optical element according to claim 2, wherein a refractive
index of at least one of the optical interference layer for the
central use wavelength is between a refractive index of the base
member and n.sub.s.
4. The optical element according to claim 1, wherein the refractive
index of the first layer changes more gently on a light entrance
side than on a base member side.
5. The optical element according to claim 1, wherein the first
layer is formed by plural structure portions each smaller than the
central use wavelength.
6. An optical apparatus comprising: an optical element which
comprises: a base member; and a first layer which is formed on the
base member and whose refractive index for a central use wavelength
.lamda. changes in a direction of a thickness of the first layer by
0.05 or more, wherein the first layer has an anti-reflection
function and satisfies the following conditions: n t = n i + 0.1 (
n s - n i ) ##EQU00005## 0.5 .ltoreq. n { t ( n t ) / 2 } - n t n s
- n { t ( n t ) / 2 } .ltoreq. 0.8 ##EQU00005.2## .lamda. 4
.ltoreq. t ( n t ) .ltoreq. 2 .lamda. ##EQU00005.3## 1.0 .ltoreq. n
i .ltoreq. 1.1 ##EQU00005.4## where n.sub.i represents a refractive
index of a most light entrance side part of the first layer for the
central use wavelength, n.sub.s represents a refractive index of a
most base member side part of the first layer for the central use
wavelength, t(n.sub.o) represents an optical film thickness of the
first layer at which the refractive index thereof for the central
use wavelength is n.sub.o, and n{t.sub.o} represents a refractive
index of the first layer for the central use wavelength at a
position where the optical film thickness is t.sub.o.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to an optical element provided
with a structure having a reflection suppressing effect
(anti-reflection function), and to an optical apparatus including
the optical element.
[0002] In general, on at least one surface of an optical element
formed of a transparent member, a thin film having an
anti-reflection function is formed by using a film forming method
typified by vapor deposition and sputtering. However, such a film
forming method limits materials that can be used for film
formation, which makes it difficult to obtain a thin film having an
arbitrary refractive index.
[0003] Japanese Examined patent publication No. 61-51283 discloses
a method for virtually obtaining a thin film having an intermediate
refractive index by selectively introducing a thin film having a
high refractive index and a thin film having a low refractive index
and by appropriately setting thicknesses of the introduced thin
films.
[0004] Further, another method forms an anti-reflection structure
constituted by plural structure portions (protrusions) each smaller
than a use wavelength (that is, a wavelength of light entering the
optical element) on at least one surface of an optical element. The
most famous anti-reflection structure is a moth-eye structure. A
surface of the moth-eye structure provides a very low reflectance
due to a microstructure unique to the moth-eye structure.
[0005] In a microstructure whose each structure portion is smaller
than the use wavelength, light of the use wavelength cannot
recognize the microstructure and therefore behaves as if entering a
uniform medium. The microstructure has a refractive index according
to a volume ratio of a material forming the microstructure. The use
of this can realize a microstructure having a low refractive index
that cannot be obtained by using normal materials. The use of such
a low refractive index microstructure can achieve a higher
performance anti-reflection function.
[0006] Japanese Patent Laid-Open No. 2005-62674 discloses an
anti-reflection structure formed by the above-described
microstructure in which protrusions have a tapered shape toward a
surface side (light entrance side). In such an anti-reflection
structure, a refractive index gradually reduces from a base member
side of an optical element toward the surface side thereof.
[0007] Japanese Patent Laid-Open No. 2003-240904 discloses a
microstructure constituted by plural protrusions each having a
shape in which, in comparison of a most convex portion and a most
concave portion of the protrusion, the most convex portion is
sharper than the most concave portion. Such a shape of the
protrusion makes change in refractive index at a superficial
surface part or a boundary part between the microstructure and the
base member more gently, which reduces the reflectance of the
microstructure.
[0008] However, the film virtually having the intermediate
refractive index disclosed in Japanese Examined patent publication
No. 61-51283 is inferior in a broadband characteristic since the
film is formed by using a high refractive index material.
[0009] Japanese Patent Laid-Open No. 2005-62674 does not disclose
an optimal refractive index structure though it discloses a
microstructure formed so as to be tapered in order to gradually
change the refractive index.
[0010] Further, the microstructure disclosed in Japanese Patent
Laid-Open No. 2003-240904 is focused only on the change of the
refractive index at a boundary surface, which generates part where
the refractive index greatly changes, and therefore a good
broadband characteristic cannot be obtained.
[0011] Thus, the film disclosed in Japanese Examined patent
publication No. 61-51283 and the microstructures disclosed in
Japanese Patent Laid-Open No. 2005-62674 and Japanese Patent
Laid-Open No. 2003-240904 can realize an anti-reflection function
under restricted conditions. However, the anti-reflection function
is inferior in the broadband characteristic as well as in an
incident angle characteristic.
SUMMARY OF THE INVENTION
[0012] The present invention provides an optical element having a
reflection suppressing function (anti-reflection function)
excellent in a broadband characteristic and an incident angle
characteristic, and an optical apparatus including the optical
element.
[0013] The present invention provides as one aspect thereof an
optical element including a base member, and a first layer which is
formed on the base member and whose refractive index for a central
use wavelength .lamda. changes in a direction of a thickness of the
first layer by 0.05 or more. The first layer has an anti-reflection
function and satisfies the following conditions:
n t = n i + 0.1 ( n s - n i ) ##EQU00001## 0.5 .ltoreq. n { t ( n t
) / 2 } - n t n s - n { t ( n t ) / 2 } .ltoreq. 0.8 ##EQU00001.2##
.lamda. 4 .ltoreq. t ( n t ) .ltoreq. 2 .lamda. ##EQU00001.3## 1.0
.ltoreq. n i .ltoreq. 1.1 ##EQU00001.4##
[0014] where n.sub.i represents a refractive index of a most light
entrance side part of the first layer for the central use
wavelength, n.sub.s represents a refractive index of a most base
member side part of the first layer for the central use wavelength,
t(n.sub.o) represents an optical film thickness of the first layer
at which the refractive index thereof for the central use
wavelength is n.sub.o, and n{t.sub.o} represents a refractive index
of the first layer for the central use wavelength at a position
where the optical film thickness is t.sub.o.
[0015] The present invention provides as another aspect thereof an
optical apparatus including the above-described optical
element.
[0016] Other aspects of the present invention will become apparent
from the following description and the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 shows a refractive index structure of a graded layer
in each embodiment of the present invention.
[0018] FIG. 2 shows a basic structure of an optical element of each
embodiment.
[0019] FIG. 3 shows a structure of an optical element in which a
layer having a uniform refractive index is formed on a base
member.
[0020] FIG. 4 shows a refractive index structure in a case where
the uniform refractive index layer is formed on the base
member.
[0021] FIG. 5 shows a refractive index structure of a graded layer
whose refractive index changes uniformly with respect to an optical
film thickness.
[0022] FIG. 6 shows a reflectance characteristic in a case where
the graded layer shown in FIG. 5 is added to a base member.
[0023] FIG. 7 shows a reflectance characteristic in a case where
the graded layer shown in FIG. 1 is added to a base member.
[0024] FIG. 8 shows a structure of an optical element in a case
where a graded layer is formed as a microstructure layer in each
embodiment.
[0025] FIG. 9 shows a refractive index structure of a graded layer
of Embodiment 1.
[0026] FIG. 10 shows a reflectance characteristic of Embodiment
1.
[0027] FIG. 11 shows a refractive index structure of a graded layer
of Embodiment 2.
[0028] FIG. 12 shows a refractive index structure of an optical
element of Embodiment 2.
[0029] FIG. 13 shows a reflectance characteristic of Embodiment
2.
[0030] FIG. 14 shows a refractive index structure of a graded layer
of Embodiment 3.
[0031] FIG. 15 shows a refractive index structure of an optical
element of Embodiment 3.
[0032] FIG. 16 shows a reflectance characteristic of Embodiment
3.
[0033] FIG. 17 shows a refractive index structure of a graded layer
of Embodiment 4.
[0034] FIG. 18 shows a reflectance characteristic of Embodiment
4.
[0035] FIG. 19 shows a refractive index structure of a graded layer
of Comparative Example 1.
[0036] FIG. 20 shows a reflectance characteristic of Comparative
Example 1.
[0037] FIG. 21 shows a refractive index structure of a graded layer
of Comparative Example 2.
[0038] FIG. 22 shows a reflectance characteristic of Comparative
example 2.
[0039] FIG. 23 schematically shows a digital camera using the
optical element of each embodiment.
[0040] FIG. 24 shows a refractive index structure of a graded layer
of Embodiment 5.
[0041] FIG. 25 shows a reflectance characteristic of Embodiment
5.
[0042] FIG. 26 shows a refractive index structure of a graded layer
of Embodiment 6.
[0043] FIG. 27 shows a reflectance characteristic of Embodiment
6.
[0044] FIG. 28 shows a refractive index structure of a graded layer
of Comparative Example 3.
[0045] FIG. 29 shows a reflectance characteristic of Comparative
Example 3.
DESCRIPTION OF THE EMBODIMENTS
[0046] Exemplary embodiments of the present invention will be
described hereinafter with reference to the accompanying
drawings.
[0047] First, description will be made of common matters to
embodiments (Embodiments 1 to 6) described below before specific
description thereof.
[0048] Each embodiment describes an example in which a use
wavelength range is 400 to 700 nm or 300 to 1000 nm and a central
wavelength thereof (hereinafter referred to as central use
wavelength) is 550 nm or 650 nm. However, the use wavelength range
and the central use wavelength are not limited to the above
wavelength range and wavelength, and may be other wavelength range
and wavelength.
[0049] FIG. 2 shows a basic configuration common to optical
elements of Embodiment 1 to 6. Reference numeral 021 denotes a
first layer, which is hereinafter referred to as "graded layer".
The graded layer means a layer whose refractive index changes in a
z-direction that is a thickness direction of the layer (also
referred to as "layer thickness direction" or "film thickness
direction"). The graded layer 021 has a reflection suppressing
function (in other words, an anti-reflection function).
[0050] Reference numeral 022 denotes a base member (base material)
which corresponds to a main body (transmissive member) of the
optical element including the graded layer 021. FIG.2 shows a case
where the graded layer 021 is formed on one surface of the base
member 022. However, the graded layer 021 may be formed on both
surfaces of the base member 022 (in other words, it is only
necessary that the graded layer be formed on at least one surface
of the base member).
[0051] Reference numeral 023 schematically shows a refractive index
structure of the graded layer 021. A horizontal axis shows
refractive index n of the graded layer 021, and a vertical axis
shows optical film thickness t of the graded layer 021.
[0052] FIG. 1 shows the above-mentioned refractive index structure
023 in more detail. However, the refractive index structure shown
in the FIG. 1 is rotated by 90 degrees with respect to that shown
in FIG. 2. A horizontal axis in the FIG. 1 shows the optical film
thickness t and a vertical axis therein shows the refractive index
n. A solid line 011 shows change of the refractive index of the
graded layer 021 with respect to the optical film thickness. In the
horizontal axis showing the optical film thickness t, an origin O
denotes a boundary surface between the base member 022 and the
graded layer 021, and t.sub.i represents a total optical film
thickness of the graded layer 021.
[0053] When viewed from a base member side, the refractive index n
of the graded layer 021 changes from n.sub.s to n.sub.i. n.sub.i
represents a refractive index of a most surface side part (that is,
a most superficial surface) of the graded layer 021 for the central
use wavelength. The most surface side part of the graded layer 021
can be also said as a most light entrance side part. Moreover,
n.sub.s represents a refractive index of a most base member side
part of the graded layer 021 for the central use wavelength.
[0054] The refractive index of the graded layer 021 changes more
gently in the light entrance side part thereof than in the base
member side part thereof. This is the same in FIGS. 9, 11, 14, 17,
24 and 26 used later.
[0055] Each embodiment defines n.sub.t by using n.sub.s and
n.sub.i. n.sub.t is shown by the following expression (1).
n.sub.t=n.sub.i+0.1(n.sub.s-n.sub.i) (1)
[0056] Each embodiment satisfies a condition shown by the following
expression (2):
0.5 .ltoreq. n { t ( n t ) / 2 } - n t n s - n { t ( n t ) / 2 }
.ltoreq. 0.8 ( 2 ) ##EQU00002##
where t(n.sub.o) represents an optical film thickness of the graded
layer 021 at which the refractive index thereof for the central use
wavelength is n.sub.o, and n{t.sub.o} represents a refractive index
of the graded layer 021 for the central use wavelength at a
position where the optical film thickness is t.sub.o.
[0057] The condition shown by the expression (2) relates to a
degree of the change of the refractive index of the graded layer
021 from a part (position) where the optical film thickness is
t(n.sub.t) to a part(position) where the optical film thickness is
t(n.sub.t)/2.
[0058] The anti-reflection function can be explained by
interference of light waves. This is the same in the graded layer
whose refractive index changes thereinside in its thickness
direction. In the explanation by the light wave interference, a
change amount of the refractive index shows an amplitude of the
light wave, and the optical film thickness shows a phase difference
amount of the light waves.
[0059] FIG. 3 shows an example of a layer having a uniform
refractive index in the thickness direction. Reference numeral 032
denotes a base member, and reference numeral 031 denotes a thin
film layer having a uniform refractive index in the thickness
direction. Reference numeral 033 schematically shows a refractive
index structure of the thin film layer 031.
[0060] FIG. 4 shows the above-mentioned refractive index structure
033 in more detail. However, the refractive index structure shown
in the FIG. 4 is rotated by 90 degrees with respect to that shown
in FIG. 3. A horizontal axis in FIG. 4 shows optical film thickness
t and a vertical axis therein shows refractive index n. Reference
numeral 041 denotes a refractive index and an optical film
thickness of the thin film layer 031.
[0061] Reference numeral 042 shows a light wave reflected at a
surface of the thin film layer 031, and reference numeral 043 shows
a light wave reflected at a boundary surface between the thin film
layer 031 and the base member 032. FIG. 4 shows interference of the
light waves 042 and 043 when the optical film thickness t.sub.s of
the thin film layer 031 is .lamda./4. In this case, the light waves
042 and 043 have a phase difference amount of about .lamda./2,
thereby negating each other. As a result, the thin film layer 031
functions as an anti-reflection film.
[0062] On the other hand, FIG. 5 shows an example of a refractive
index structure of an optical element having a graded layer. FIG. 5
shows a case where the refractive index uniformly changes with
respect to the optical film thickness. Reference numeral 051
denotes the change of the refractive index with respect to the
optical film thickness. Reference numeral 052 denotes part of light
waves (reflected waves) reflected at respective positions on the
graded layer.
[0063] Although the graded layer shown in FIG. 5 does not include a
clear boundary surface, the light waves are reflected at respective
parts where the refractive index changes. In other words,
innumerable lights are reflected in the graded layer shown in FIG.
5, and the reflected waves 052 shown in FIG. 5 are part of the
innumerable lights. An anti-reflection characteristic of such a
graded layer is obtained by superposition of all reflected waves
052 from the respective optical film thicknesses.
[0064] Each embodiment makes it a condition that a difference
between n.sub.s and n.sub.i is 0.05 or more, in other words, the
refractive index of the graded layer for the central use wavelength
changes by 0.05 or more in the thickness direction. This is a
condition for providing the anti-reflection characteristic by the
interference of the light waves in the graded layer.
[0065] Further, this condition also means that the refractive index
greatly changes in the graded layer. The great change of the
refractive index provides a great influence on a reflectance
characteristic. Therefore, each embodiment is characterized by
satisfying the conditions shown by the expressions (1) and (2).
[0066] In the expression (1), n.sub.t shows a value between the
refractive indexes n.sub.s and n.sub.i. The expression (2) shows a
gradient of the refractive index between n.sub.t and n.sub.s. The
expression (2) means that the change of the refractive index
(n.sub.s-n{t(n.sub.t)/2}) in a base member side part of the graded
layer more inside than a surface side part thereof provides a
greater influence on the light wave interference than the change of
the refractive index (n{t(n.sub.t)/2}-n.sub.t) in the surface side
part.
[0067] Moreover each embodiment is characterized in that the
optical film thickness t(n.sub.t) of the graded layer for the
central use wavelength .lamda. satisfies a condition shown by the
following expression (3):
.lamda. 4 .ltoreq. t ( n t ) .ltoreq. 2 .lamda. ( 3 )
##EQU00003##
[0068] The optical film thickness t(n.sub.t) of the graded layer
smaller than .lamda./4 does not cause sufficient light wave
interference in the graded layer, so that the graded layer cannot
function as an anti-reflection film. On the other hand, the optical
film thickness t(n.sub.t ) larger than 2.lamda. makes it very
difficult to form the graded layer shown in FIG. 5.
[0069] It is more preferable that the range of the expression (3)
be a range from .lamda./3 to 3.lamda./2 (.lamda./3 or more and
3.lamda./2 or less). It is still more preferable that the range of
the expression (3) be a range from 3.lamda./8 to 5.lamda./4
(3.lamda./8 or more and 5.lamda./4 or less).
[0070] Furthermore, each embodiment is characterized by satisfying
a condition shown by the following expression (4):
1.0.ltoreq.n.sub.i.ltoreq.1.1 (4)
[0071] When n.sub.i exceeds the upper limit of the expression (4),
a difference between a refractive index of a tip end of the graded
layer shown in FIGS. 5 and that of air becomes large, and thereby
light reflected at a boundary surface of the graded layer and the
air increases. Therefore, it becomes difficult to reduce the
reflectance in the entire wavelength range.
[0072] FIG. 6 shows an example of a reflectance characteristic in a
case where the graded layer shown in FIG. 5 is added to the base
member. A horizontal axis shows wavelength .lamda. and a vertical
axis shows reflectance R. Reference numeral 061 denotes a
reflectance characteristic of only the base member, and reference
numeral 062 denotes the reflectance characteristic in the case
where the graded layer shown in FIG. 5 is added to the base member.
The refractive index of the base member is n.sub.s.
[0073] In the case shown in FIG. 6, the value of the expression (2)
for the graded layer shown in FIG. 5 is 1.0, which does not satisfy
the condition shown by the expression (2). In this reflectance
characteristic, while the reflectance for a specific wavelength is
almost 0, the reflectance in other wavelengths is increased.
[0074] On the other hand, FIG. 7 shows a reflectance characteristic
in a case where the graded layer 021 shown in FIG. 2 is added to
the base member. A horizontal axis shows wavelength .lamda. and a
vertical axis shows reflectance R. Reference numeral 071 denotes a
reflectance characteristic of only the base member, and reference
numeral 072 denotes the reflectance characteristic in the case
where the graded layer 021 is added to the base member. The
refractive index of the base member is n.sub.s.
[0075] In the case shown in FIG. 7, the value of the expression (2)
for the graded layer 021 is 0.52, which satisfies the condition
shown by the expression (2). The reflectance characteristic in this
case has two minimum values, which shows that an extremely low
reflectance is achieved in a broad wavelength range as compared
with the reflectance characteristic shown in FIG. 6.
[0076] The satisfaction of the conditions shown by the expressions
(1), (2), (3) and (4) enables acquisition of reflection suppressing
performance (anti-reflection performance) with an excellent
broadband characteristic.
[0077] On the other hand, when the value of the expression (2) is
smaller than 0.5, the change of the refractive index in the base
member side part of the graded layer further than the surface side
part whose refractive index is low becomes large, which makes it
difficult to reduce the reflectance. Further, it is difficult to
obtain a W-letter shaped reflectance characteristic, so that the
broadband characteristic is deteriorated. In addition, when the
value of the expression (2) is 0.8 or more, the refractive index
changes nearly linearly with respect to the optical film thickness,
and therefore the broadband characteristic is deteriorated.
[0078] It is more preferable that the range of the value of the
expression (2) be a range from 0.55 to 0.75 (0.55 or more and 0.75
or less). It is still more preferable that the range of the value
of the expression (2) be a range from 0.58 to 0.7 (0.58 or more and
0.7 or less).
[0079] In order to add a high performance anti-reflection function
to various glass materials having mutually different refractive
indexes, it is necessary to adjust the refractive index of the
graded layer so as to adapt it to the respective glass materials.
Thus, in each embodiment, it is preferable to provide at least one
optical interference layer between the base member that is a glass
material and the graded layer to form a laminated structure by
using the graded layer and the optical interference layer.
[0080] Adjusting a refractive index and a film thickness of the
optical interference layer in such a laminated structure can
realize a high performance anti-reflection laminated structure
usable for various glass materials, without adjusting the
refractive index of the graded layer.
[0081] Moreover, it is preferable that a refractive index of at
least one of the above-described optical interference layer for the
central use wavelength .lamda. be a refractive index between the
refractive index of the base member and n.sub.s. This can realize a
refractive index structure in which the refractive index gradually
decreases from the base member to the most superficial surface of
the graded layer. Such a refractive index structure prevents
increase of light reflection caused due to oblique incidence of
light, which makes it possible to obtain an anti-reflection
laminated structure excellent in the broadband characteristic and
an oblique incidence characteristic.
[0082] Furthermore, in each embodiment, it is preferable to form
the graded layer into a microstructure layer constituted by plural
structure portions each smaller than the central use wavelength
.lamda.. FIG. 8 shows an example of a structure in which such a
microstructure layer is formed on the base member. Reference
numeral 081 denotes the microstructure layer (graded layer), and
reference numeral 082 denotes the base member.
[0083] In a microstructure whose each structure portion is
sufficiently smaller than the central use wavelength .lamda., as
described before, light entering the microstructure cannot
recognize the microstructure itself and therefore behaves as if
entering a homogeneous medium. In this case, the entering light
shows a characteristic as if the microstructure is averaged.
[0084] An effective refractive index n.sub.e of the microstructure
layer 081 is shown by the following expression (5):
n.sub.e={ff.times.n.sub.m.sup.2+(1-ff)}.sup.1/2 (5)
[0085] where n.sub.m represents a refractive index of a material
forming the microstructure, and ff represents a volume filling rate
(filling factor) of the microstructure. This method for calculating
the effective refractive index n.sub.e is called as a first-order
effective refractive index method, and it is the easiest method for
converting the microstructure (microstructure layer 081) into the
effective refractive index.
[0086] On the other hand, the effective refractive index of the
microstructure layer 081 changes depending on a pitch and a
three-dimensional shape of the microstructure. For instance, in a
case where n.sub.m is 2.3 and ff is 0.5, the effective refractive
index n.sub.e calculated by the expression (5) is about 1.77. In
contrast thereto, when an analysis is made by using a rigorous
coupled wave analysis (RCWA) method on an assumption that the pitch
of the microstructure changes from 80 nm to 150 nm, the effective
refractive index n.sub.e changes from 1.73 to 1.79. Therefore, it
is difficult to calculate the effective refractive index accurately
from only a cross-sectional shape and ff of the microstructure.
[0087] Thus, each embodiment calculates change of the effective
refractive index of the microstructure layer (graded layer) 081
with respect to its optical film thickness. Spectral ellipsometry
is used as one of methods for calculating the effect refractive
index. The spectral ellipsometry is a method in which irradiations
of linearly polarized lights of mutually different wavelengths onto
a specimen are performed to measure a ratio of polarized
reflectances and a phase delay amount, and then a refractive index
and film thickness model closest to the measurement results is
calculated. This method makes it possible to confirm whether or not
the microstructure satisfies the condition of the expression (2)
even if details of the microstructure are unclear. If the
expression (2) is satisfied, the graded layer may be a thin film
layer and may be a microstructure layer.
[0088] Further, methods for obtaining the graded layer of each
embodiment include a method in which a comparatively sparse film
formed uniformly is soaked in a transparent sol-gel solution for a
long time and then the soaked film is sintered, and a method in
which, during film formation by a binary film forming method, a
mixture ratio is gradually changed. If the graded layer satisfies
the above-described refractive index conditions, the graded layer
can be produced by any manufacturing process.
[0089] An optical element on which the above-described graded layer
is formed can be used for various optical apparatuses. FIG. 23
shows a digital camera that is an optical apparatus using the
optical element of each embodiment.
[0090] Reference numeral 20 denotes a camera body, and reference
numeral 21 denotes an image-pickup optical system including a lens
that is the optical element of each embodiment. The image-pickup
optical system 21 includes plural lenses, and at least one thereof
may be the optical element of each embodiment. Reference numeral 22
denotes a solid-state image pickup element (photoelectric
conversion element) such as a CCD sensor or a CMOS sensor which
receives an object image formed by the image-pickup optical system
21, the solid-state image-pickup element 22 being provided in a
camera body 20. The solid-state image-pickup element 22
photoelectrically converts the object image to generate image
information corresponding to the object image.
[0091] Reference numeral 23 denotes a memory that records therein
the image information. Reference numeral 24 denotes an electronic
viewfinder constituted by a liquid crystal display panel and the
like, which enables observation of the image information (that is,
the object image).
[0092] The image-pickup optical system thus constituted using the
optical element of each embodiment can realize a camera that
suppresses unnecessary reflection in the image-pickup optical
system and thereby has high optical performance.
[0093] The optical element of each embodiment may also be used for
a viewfinder optical system of a camera, an illumination optical
system of a liquid crystal projector, a projection optical system
thereof, and the like. The optical element having the
above-described anti-reflection structure can sufficiently increase
an amount of light being transmitted through the optical element
and can well suppress generation of ghost or flare due to
unnecessary reflection.
[0094] Hereinafter, Embodiments (simulation examples) 1 to 6 and
Comparative Examples 1 to 3 will be described.
Embodiment 1
[0095] In Embodiment 1, S-BSM14 (glass) made by OHARA Co., Ltd was
used as a base member. The refractive index thereof for the
wavelength 650 nm was 1.6033. A graded layer having a refractive
index structure shown in FIG. 9 was formed on the base member.
n.sub.s was 1.530, n.sub.i was 1.0, and t.sub.i was 350 nm.
[0096] From the expression (1), n.sub.t was 1.053, and the value of
the expression (2) was 0.72. FIG. 10 shows the reflectance
characteristic of Embodiment 1. The reflectance characteristic
included two minimum values, and a low reflectance of 0.2% or less
was achieved in the entire visible wavelength range.
Embodiment 2
[0097] In Embodiment 2, S-BSM14 made by OHARA Co., Ltd was used as
a base member. The refractive index thereof for the wavelength 550
nm was 1.6088. An optical interference layer (refractive index:
1.50 and optical film thickness: 113 nm) was formed on the base
member, and further a graded layer having a refractive index
structure shown in FIG. 11 was formed on a surface of the base
member. FIG. 12 shows the refractive index structure of the optical
element of Embodiment 2. Part where the optical film thickness is
negative shows the refractive index of the base member. n.sub.s was
1.38, n.sub.i was 1.0, and t.sub.i was 485 nm.
[0098] From the expression (1), n.sub.t was 1.038, and the value of
the expression (2) was 0.50. FIG. 13 shows the reflectance
characteristic of Embodiment 2. The reflectance characteristic
included two minimum values, and a low reflectance of 0.1% or less
was achieved in the entire visible wavelength range.
Embodiment 3
[0099] In Embodiment 3, S-LAH55 (glass) made by OHARA Co., Ltd was
used as a base member. The refractive index thereof for the
wavelength 550 nm was 1.8390. An optical interference layer
(refractive index: 1.56 and optical film thickness: 105 nm) was
formed on the base member, and further a graded layer having a
refractive index structure shown in FIG. 14 was formed on a surface
of the base member. The graded layer was formed as a microstructure
layer by first forming a thin film consisting primarily of aluminum
oxide by a sol-gel method and then hydrothermally treating the thin
film. This microstructure layer was analyzed with spectral
ellipsometry. FIG. 15 shows the refractive index structure of the
optical element of Embodiment 3. Part where the optical film
thickness is negative shows the refractive index of the base
member. n.sub.s was 1.39, n.sub.i was 1.0, and t.sub.i was 250
nm.
[0100] From the expression (1), n.sub.t was 1.039, and the value of
the expression (2) was 0.53. FIG. 16 shows the reflectance
characteristic of Embodiment 3. A low reflectance of 0.4% or less
was achieved in the entire visible wavelength range.
Embodiment 4
[0101] In Embodiment 4, S-BSM14 made by OHARA Co., Ltd was used as
a base member. The refractive index thereof for the wavelength 650
nm was 1.6033. A graded layer having a refractive index structure
shown in FIG. 17 was formed on the base member. n.sub.s was 1.530,
n.sub.i was 1.0, and t.sub.i was 350 nm.
[0102] From the expression (1), n.sub.t was 1.053, and the value of
the expression (2) was 0.51. FIG. 18 shows the reflectance
characteristic of Embodiment 4. The reflectance characteristic
included two minimum values, and a low reflectance of 0.4% or less
was achieved in the entire visible wavelength range.
Embodiment 5
[0103] In Embodiment 5, S-BSM14 made by OHARA Co., Ltd was used as
a base member. The refractive index thereof for the wavelength 650
nm was 1.6033. A graded layer having a refractive index structure
shown in FIG. 24 was formed on the base member. n.sub.s was 1.530,
n.sub.i was 1.0, and t.sub.i was 350 nm.
[0104] From the expression (1), n.sub.t was 1.053, and the value of
the expression (2) was 0.78. FIG. 25 shows the reflectance
characteristic of Embodiment 5. The reflectance characteristic
included two minimum values, and a low reflectance of 0.5% or less
was achieved in the entire visible wavelength range.
Embodiment 6
[0105] In Embodiment 6, S-BSM14 made by OHARA Co., Ltd was used as
a base member. The refractive index thereof for the wavelength 650
nm was 1.6033. A graded layer having a refractive index structure
shown in FIG. 26 was formed on the base member. n.sub.s was 1.530,
n.sub.i was 1.0, and t.sub.i was 500 nm.
[0106] From the expression (1), n.sub.t was 1.053, and the value of
the expression (2) was 0.51. FIG. 27 shows the reflectance
characteristic of Embodiment 6. The reflectance characteristic
included two minimum values, and a low reflectance of 0.4% or less
was achieved in the entire visible wavelength range.
COMPARATIVE EXAMPLE 1
[0107] In Comparative Example 1, S-BSM14 made by OHARA Co., Ltd was
used as a base member. The refractive index thereof for the
wavelength 550 nm was 1.6088. A graded layer having a refractive
index structure shown in FIG. 19 was formed on the base member.
n.sub.s was 1.51, n.sub.i was 1.0, and t.sub.i was 440 nm. The
graded layer shown in FIG. 19 was formed into a film whose
refractive index linearly changes with respect to its physical film
thickness.
[0108] From the expression (1), n.sub.t was 1.051. The value of the
expression (2) was 1.19, which does not satisfy the condition of
the expression (2). FIG. 20 shows the reflectance characteristic of
Comparative Example 1. Comparative Example 1 is inferior in the
broadband characteristic as compared with Embodiment 1.
COMPARATIVE EXAMPLE 2
[0109] In Comparative Example 2, S-BSM14 made by OHARA Co., Ltd was
used as a base member. The refractive index thereof for the
wavelength 650 nm was 1.6033. A graded layer having a refractive
index structure shown in FIG. 21 was formed on the base member.
n.sub.s was 1.53, n.sub.i was 1.0, and t.sub.i was 350 nm.
[0110] From the expression (1), n.sub.t was 1.053. The value of the
expression (2) was 0.48, which does not satisfy the condition of
the expression (2). FIG. 22 shows the reflectance characteristic of
Comparative Example 2. Comparative Example 2 shows that its
reflectance characteristic greatly changes as compared with
Embodiment 1 even though the optical film thickness of Comparative
Example 2 is equal to that of Embodiment 1.
COMPARATIVE EXAMPLE 3
[0111] In Comparative Example 3, S-BSM14 made by OHARA Co., Ltd was
used as a base member. The refractive index thereof for the
wavelength 650 nm was 1.6033. A graded layer having a refractive
index structure shown in FIG. 28 was formed on the base member.
n.sub.s was 1.53, n.sub.i was 1.0, and t.sub.i was 500 nm.
[0112] From the expression (1), n.sub.t was 1.053. The value of the
expression (2) was 0.48, which does not satisfy the condition of
the expression (2). FIG. 29 shows the reflectance characteristic of
Comparative Example 3. Comparative Example 3 shows that its
reflectance characteristic greatly changes as compared with
Embodiment 1 even though the optical film thickness of Comparative
Example 3 is equal to that of Embodiment 1.
[0113] Table 1 collectively shows numerical values of Embodiments 1
to 6, and Table 2 collectively shows numerical values of
Comparative Examples 1 to 3.
TABLE-US-00001 TABLE 1 EMBODI- EMBODI- EMBODI- EMBODI- EMBODI-
EMBODI- MENT 1 MENT 2 MENT 3 MENT 4 MENT 5 MENT 6 GRADED n.sub.s
1.530 1.380 1.390 1.530 1.530 1.530 LAYER n.sub.i 1.000 1.000 1.000
1.000 1.000 1.000 f.sub.i [nm] 350 485 250 350 350 500 OPTICAL
REFRACTIVE INDEX -- 1.500 1.560 -- -- -- INTERFERENCE OPTICAL FILM
-- 113 105 -- -- -- LAYER THICKNESS [nm] BASE GLASS S-BSM14 S-BSM14
S-LAH55 S-BSM14 S-BSM14 S-BSM14 MEMBER REFRACTIVE INDEX 1.6033
1.6088 1.8390 1.6033 1.6033 1.6033 .lamda. [nm] 650 550 550 650 650
650 n.sub.t 1.05 1.04 1.04 1.05 1.05 1.05 f(n.sub.t) [nm] 304 262
205 304 349 428 .lamda. 0.47.lamda. 0.48.lamda. 0.37.lamda.
0.47.lamda. 0.54.lamda. 0.66.lamda. n {f(n.sub.t)} 1.25 1.15 1.16
1.22 1.26 1.22 CONDITION (2) 0.72 0.50 0.53 0.51 0.78 0.51
TABLE-US-00002 TABLE 2 COMPARATIVE COMPARATIVE COMPARATIVE EXAMPLE
1 EXAMPLE 2 EXAMPLE 3 GRADED n.sub.s 1.511 1.530 1.530 LAYER
n.sub.i 1.000 1.000 1.000 t.sub.i [nm] 440 350 500 OPTICAL
REFRACTIVE INDEX -- -- -- INTERFERENCE OPTICAL FILM THICKNESS -- --
-- LAYER [nm] BASE GLASS S-BSM14 S-BSM14 S-BSM14 MEMBER REFRACTIVE
INDEX 1.6088 1.6033 1.6033 .lamda. [nm] 550 650 650 n.sub.t 1.05
1.05 1.05 t (n.sub.t) [nm] 201 308 370 .lamda. 0.36.lamda.
0.47.lamda. 0.57.lamda. n {t(n.sub.t)} 1.30 1.20 1.20 CONDITION (2)
1.19 0.48 0.48
[0114] While the present invention has been described with
reference to exemplary embodiments, it is to be understood that the
invention is not limited to the disclosed exemplary embodiments.
The scope of the following claims is to be accorded the broadest
interpretation so as to encompass all modifications, equivalent
structures and functions.
[0115] This application claims the benefit of Japanese Patent
Application No. 2008-222900, filed on Aug. 29, 2008, which is
hereby incorporated by reference herein in its entirety.
* * * * *