U.S. patent application number 09/468864 was filed with the patent office on 2002-12-26 for diffractive optical element and photographic optical system having the same.
Invention is credited to NAKAI, TAKEHIKO.
Application Number | 20020196545 09/468864 |
Document ID | / |
Family ID | 26577753 |
Filed Date | 2002-12-26 |
United States Patent
Application |
20020196545 |
Kind Code |
A1 |
NAKAI, TAKEHIKO |
December 26, 2002 |
DIFFRACTIVE OPTICAL ELEMENT AND PHOTOGRAPHIC OPTICAL SYSTEM HAVING
THE SAME
Abstract
To obtain a diffractive optical element of high diffraction
efficiency over a wide range of wavelengths with no conspicuous
color flare and a photographic optical system having the
diffractive optical element, at least two diffraction gratings of
different materials in dispersion are stratified, the first order
is chosen as the design order and two wavelengths which, when
multiplied one times, amounts to the maximum optical path length
difference in the grating structure are used as the design
wavelengths, wherein each of the values of the plurality of design
wavelengths is determined so as to make white or nearly white flare
caused by the diffracted light in the zero and second orders.
Inventors: |
NAKAI, TAKEHIKO;
(KAWASAKI-SHI, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Family ID: |
26577753 |
Appl. No.: |
09/468864 |
Filed: |
December 21, 1999 |
Current U.S.
Class: |
359/569 |
Current CPC
Class: |
G02B 5/1866
20130101 |
Class at
Publication: |
359/569 |
International
Class: |
G02B 005/18 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 24, 1998 |
JP |
10-376552 |
Dec 3, 1999 |
JP |
11-344369 |
Claims
1. A photographic optical system comprising a diffractive optical
element having a grating structure in which a plurality of
diffraction gratings made from at least two kinds of materials
different in dispersion from each other are laminated, and having a
plurality of design wavelengths, a maximum optical path length
difference in the grating structure being integer times each of the
plurality of design wavelengths, said diffractive optical element
satisfying the following conditions for the group of design
wavelengths .lambda.0:0<E1(.lambda.0)+E2(.lambda.0)+E3-
(.lambda.0)<0.040<max{E1(.lambda.0), E2(.lambda.0),
E3(.lambda.0)}-min {E1(.lambda.0), E2(.lambda.0),
E3(.lambda.0)}<0.02w- here max{E1(.lambda.0), E2(.lambda.0),
E3(.lambda.0)} represents a maximum value among E1(.lambda.0),
E2(.lambda.0) and E3(.lambda.0), and min{E1(.lambda.0),
E2(.lambda.0), E3(.lambda.0)} represents a minimum value among
E1(.lambda.0), E2(.lambda.0) and E3(.lambda.0), 5 E1 ( 0 ) = D m -
1 ( 0 , ) L ( ) F1 ( ) T ( ) + D m + 1 ( 0 , ) L ( ) F1 ( ) T ( ) D
m ( 0 , ) L ( ) F1 ( ) T ( ) E2 ( 0 ) = D m - 1 ( 0 , ) L ( ) F2 (
) T ( ) + D m + 1 ( 0 , ) L ( ) F2 ( ) T ( ) D m ( 0 , ) L ( ) F2 (
) T ( ) E3 ( 0 ) = D m - 1 ( 0 , ) L ( ) F3 ( ) T ( ) + D m + 1 ( 0
, ) L ( ) F3 ( ) T ( ) D m ( 0 , ) L ( ) F3 ( ) T ( ) where D m ( 0
, ) L ( ) F1 ( ) T ( ) = D m ( 0 , ) L ( ) F2 ( ) T ( ) = D m ( 0 ,
) L ( ) F3 ( ) T ( ) where D.sub.m-1(.lambda.0,.lambda.),
D.sub.m(.lambda.0,.lambda.) and D.sub.m+1(.lambda.0,.lambda.)
represent diffraction efficiencies for a wavelength .lambda. in the
(m-1)st, m-th and (m+1)st orders, respectively, in said diffractive
optical element where the m-th order is taken as a design order and
the wavelength .lambda.0 is taken as the group of design
wavelengths, L(.lambda.) represents a spectral characteristic for
the wavelength .lambda. of a light source, F1(.lambda.),
F2(.lambda.), F3(.lambda.) represent spectral sensitivity
characteristics of light receiving means for detecting light in
respective wavelength regions in an image pickup means, where
F1(.lambda.), F2(.lambda.) and F3(.lambda.) are arranged in order
from the shorter of wavelengths at which spectral sensitivities
become maximum, and T(.lambda.) represents a transmittance for the
wavelength .lambda. of said photographic optical system.
2. A photographic optical system according to claim 1, wherein the
shortest design wavelength .lambda.01 among the plurality of design
wavelengths of said diffractive optical element satisfies the
following condition:400 nm.ltoreq..lambda.01<455 nm.
3. A photographic optical system according to claim 1, wherein the
longest design wavelength .lambda.0L among the plurality of design
wavelengths of said diffractive optical element satisfies the
following condition:550 nm.ltoreq..lambda.0L.ltoreq.620 nm.
4. A photographic optical system according to claim 1, wherein an
interval .DELTA..lambda.0.a between adjacent two of the plurality
of design wavelengths of said diffractive optical element satisfies
the following condition:.DELTA..lambda.0.a.ltoreq.220 nmwhere
.DELTA..lambda.0.a=.lambd-
a.0.a+1-.lambda.0.a1.ltoreq.a.ltoreq.L-1where L is the number of
the plurality of design wavelengths.
5. A photographic optical system according to claim 1, wherein at
least one of the plurality of diffraction gratings differs in
orientation of grating from the other diffraction gratings.
6. A photographic optical system according to claim 1, wherein a
predetermined wavelength region of said diffractive optical element
is a visible spectrum.
7. A photographic optical system according to claim 1, wherein the
plurality of diffraction gratings are layered on a substrate and,
when the plurality of diffraction gratings are consecutively
numbered, from the diffraction grating nearest to the substrate, as
the first diffraction grating, the second diffraction grating and
up to the i-th diffraction grating, a material from which the first
diffraction grating is made is the same as a material of the
substrate.
8. A photographic optical system according to claim 1, wherein the
plurality of diffraction gratings are arranged in intimate contact
or in closely spaced relation.
9. A diffractive optical element having a grating structure in
which a plurality of diffraction gratings made from at least two
kinds of materials different in dispersion from each other are
laminated, and having a plurality of design wavelengths, a maximum
optical path length difference in the grating structure being
integer times each of the plurality of design wavelengths, flare
caused by diffracted rays in orders other than the plurality of
design wavelengths being made white or a color near to white.
10. A diffractive optical element according to claim 9, wherein the
shortest design wavelength .lambda.01 among the plurality of design
wavelengths satisfies the following condition:400
nm.ltoreq..lambda.01<- ;455 nm.
11. A diffractive optical element according to claim 9, wherein the
longest design wavelength .lambda.0L among the plurality of design
wavelengths satisfies the following condition:550
nm.ltoreq..lambda.0L.lt- oreq.620 nm.
12. A diffractive optical element according to claim 9, wherein the
plurality of diffraction gratings are arranged in intimate contact
or in closely spaced relation.
13. A photographic optical system comprising an optical system
including a diffractive optical element according to any one of
claims 9 to 12, and a plurality of light receiving means whose
wavelength regions at peak sensitivity are different from each
other, and an amount of light of each of diffracted rays in orders
other than the plurality of design wavelengths is controlled by the
plurality of light receiving means so that the flare becomes white
or a color near to white.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a diffractive optical
element to be used either at a plurality of wavelengths, or with
light in a predetermined band, and a photographic optical system
having the diffractive optical element and, more particularly, to a
diffractive optical element suited to be used as a part of a
photographic optical system using three or more light beams of
different original colors in forming a color image.
[0003] 2. Description of Related Art
[0004] Conventionally, as one of methods for correcting chromatic
aberrations of an optical system, there is known a method of
combining two glass materials (lenses) which differ in dispersion
from each other.
[0005] Unlike this method of reducing the chromatic aberrations by
selectively using two glass materials, it has been known to provide
the optical system with a diffractive optical element (hereinafter
also called the "diffraction grating") as made up in either one of
the lens surfaces thereof or somewhere else. Such a method of
reducing the chromatic aberrations is disclosed in, for example,
"International Lens Design Conference" in SPIE Vol. 1354 (1990),
Japanese Laid-Open Patent Applications No. Hei 4-213421 and No. Hei
6-324262 and U.S. Pat. No. 5,044,706. This method is attained by
utilizing such a physical phenomenon that, for a refractive surface
and a diffractive surface in an optical system, if their refractive
powers are of the same sign, chromatic aberrations for the rays of
light of a certain reference wavelength occur in the opposite
directions. Further, with such a diffractive optical element, when
its periodical structure is changed in pitch as it can be done
freely, an effect similar to an aspherical lens is produced.
Therefore, the diffractive optical element has an additional great
advantage of reducing even mono-chromatic aberrations.
[0006] Here, it is in refraction that one ray, even after having
refracted, remains one ray. In diffraction, on the other hand, one
ray brakes up to a plurality of rays in different orders of
diffraction. Therefore, for a case of using the diffractive optical
element in an optical system, there is need to make determination
of the grating structure so that a light beam of the useful
wavelength region concentrates on a particular order (hereinafter
also referred to as the "design order"). In a situation when light
concentrates on the particular order, the intensities of the
diffracted rays in the other orders become low. If the intensity is
"0", the corresponding diffracted ray becomes not existing.
[0007] In order to make useful the above-described advantage of the
diffractive optical element, it becomes necessary throughout the
entire range of predetermined wavelengths including design
wavelengths that the diffraction efficiency for the rays in the
design order is sufficiently high. It should also be pointed out
that the rays having the other orders than the design order focus
themselves at different places than the rays of the design order
do, becoming flare (light). In an optical system employing the
diffractive optical element, therefore, it is of great importance
to consider the spectral distribution, too, of diffraction
efficiencies of the rays in the design order fully and, further,
the behavior of even more rays which are in the other orders than
the design order (or the useless diffracted rays).
[0008] FIG. 19 shows a diffractive optical element 1 in which a
diffraction grating 3 is made up in one layer on a substrate 2.
With such a diffractive optical element 1 formed on a surface of an
optical system, the rays in particular orders diffract with
diffraction efficiencies shown in FIG. 20. The values of the
diffraction efficiency are in percentage of the diffracted amount
of light at every wavelength to the transmitted amount of light.
The reflected light from the grating boundary or the like is not
taken into account in the evaluation, because the explanation
becomes complicated. In FIG. 20, the abscissa represents the
wavelength and the ordinate represents the diffraction efficiency.
This diffractive optical element 1 is so designed that the
diffraction efficiency in the first order (a solid line curve in
FIG. 20) is highest in the predetermined wavelength region. That
is, the design order is the first one. Furthermore, the diffraction
efficiencies in the orders near to the first one (or (1.+-.1)st
orders, namely, zero order and second order) are also depicted for
comparison. As shown in FIG. 20, it is in the design order that the
diffraction efficiency has the highest value at a certain
wavelength (hereinafter referred to as the "design wavelength") and
becomes gradually lower toward the ends of the whole spectrum. This
decrease of the diffraction efficiency in the design order is
translated into an increase of the amount of diffracted rays in the
other orders, becoming flare. In addition, in a case where two or
more diffraction gratings are used, in particular, the lowering of
the diffraction efficiency at the other wavelengths than the design
wavelength leads to reduction of the transmittance.
[0009] To diminish the lowering of the diffraction efficiency, many
previous proposals have been made.
[0010] For example, Japanese Laid-Open patent Application No. Hei
9-127322 discloses a diffractive optical element made up in such a
way that, as shown in FIG. 21, three different materials of
different kinds (for three layers 4, 8 and 5 of diffraction
gratings) and two different grating thicknesses d1 and d2 (for the
bottom and top gratings 4 and 5) are appropriately selected and
that the bottom and top diffraction gratings of an equal pitch
distribution are juxtaposed. By this construction and arrangement,
a high diffraction efficiency in the design order is realized over
the entire visible region, as shown in FIG. 23.
[0011] Also, a diffractive optical element capable of diminishing
the lowering of the diffraction efficiency has been proposed in
Japanese Laid-Open Patent Application No. Hei 10-133149. As shown
in FIG. 22, this diffractive optical element has two layers
superimposed one upon another. For the stratification of the layers
4 and 5 in cross-section, the refractive indices and dispersions of
their materials and the thicknesses of the gratings in them are
made optimum, thus realizing a high diffraction efficiency in the
design order over the entire range of visible spectrum.
[0012] In another Japanese Laid-Open Patent Application No. Hei
10-104411, with the use of a diffractive optical element of the
kinoform type shown in FIG. 19, the grating thickness is adjusted
to shift the design wavelength as desired, thus reducing the amount
of needless diffracted light in the orders near to the design
order.
[0013] Of the prior known techniques described above, the one
proposed in Japanese Laid-Open patent Application No. Hei 9-127322
has greatly improved the diffraction efficiency in the design
order. Therefore, the proportion of the diffracted rays in the
other orders than the design order, or the needless diffracted
rays, too, is improved. So, the diffractive optical element
produces lesser flare. However, color flare is appreciable in the
obtained image. Also, there is no detailed description about the
color appearance of flare and the amount of flare.
[0014] Meanwhile, Japanese Laid-Open Patent Application No. Hei
10-104411 is concerned with the grating having one diffractive
surface like that shown in FIG. 19 (hereinafter called the
"mono-layer DOE" for Diffractive Optical Element). With this
regard, it suggests the influence of the color flare due to the
light in the needless orders. However, as far as the diffractive
optical element in the stratified form of two or more layers
(hereinafter called the "stratified multilayer DOE") is concerned,
nothing is said about the flare.
[0015] Using the stratified multilayer DOE described above, the
optical system has succeeded in greatly reducing the flare from
that when the mono-layer DOE is in use. However, this does not mean
that the useless diffracted light is not present at all. So, it is,
though little, left to exist. In application to a type of optical
system which does not suffer changes of the photo-taking
(light-projecting) condition (for example, the reader lens in the
copying machine and the projection lens in the liquid crystal
projector), the influence of flare is depressed to a negligible
level by the stratified multilayer DOE. However, after having
conducted many investigations, the inventor of the present
invention has found that, for the film camera or video camera, as
various photographic conditions are encountered, it sometimes
happens that the little remaining of flare gives a serious problem.
To show an example, in a case where a light source exists in the
scene to be photographed, a correct exposure is usually made not on
the light source, but on an object of photographic interest other
than the light source. Accordingly, the light source is shot in an
over-exposure. For example, assuming that the exposure to the light
source is 100 times greater than the correct exposure, then even if
the flare is left as little as 2%, because the flare of the light
source, too, is 100 times intensified, the flare gets a light
amount 2 times as large as the correct exposure. Therefore, it is
sure that the flare appears in the picture which will be taken.
[0016] As described above, in application of the stratified
multilayer DOE to the optical system in the film camera or video
camera, the flare becomes problematic with some possibility, no
matter however little it may be. In particular, if the flare
component has a wavelength dependency, color flare is produced even
in the case of the stratified multilayer DOE, being similar to the
color light characteristics based on the mono-layer DOE in Japanese
Laid-Open Patent Application No. Hei 10-104411.
BRIEF SUMMARY OF THE INVENTION
[0017] An object of the present invention is to provide a
diffractive optical element having no prominent color flare due to
the diffracted rays in the needless orders, and a photographic
optical system having the diffractive optical element.
[0018] To attain the above object, in accordance with a first
aspect of the invention, there is provided a photographic optical
system comprising a diffractive optical element having a grating
structure in which a plurality of diffraction gratings made from at
least two kinds of materials different in dispersion from each
other are laminated, and having a plurality of design wavelengths,
a maximum optical path length difference in the grating structure
being integer times each of the plurality of design wavelengths,
the diffractive optical element satisfying the following conditions
for each of the design wavelengths .lambda.0:
0<E1(.lambda.0)+E2(.lambda.0)+E3(.lambda.0)<0.04 (1)
0<max{E1(.lambda.0), E2(.lambda.0),
E3(.lambda.0)}-min{E1(.lambda.0), E2(.lambda.0),
E3(.lambda.0)}<0.02 (2)
[0019] where max{E1(.lambda.0), E2(.lambda.0), E3(.lambda.0)}
represents a maximum value among E1(.lambda.0), E2(.lambda.0) and
E3(.lambda.0), and min{E1(.lambda.0), E2(.lambda.0), E3(.lambda.0)}
represents a minimum value among E1(.lambda.0), E2(.lambda.0) and
E3(.lambda.0), where 1 E1 ( 0 ) = D m - 1 ( 0 , ) L ( ) F1 ( ) T (
) + D m + 1 ( 0 , ) L ( ) F1 ( ) T ( ) D m ( 0 , ) L ( ) F1 ( ) T (
) E2 ( 0 ) = D m - 1 ( 0 , ) L ( ) F2 ( ) T ( ) + D m + 1 ( 0 , ) L
( ) F2 ( ) T ( ) D m ( 0 , ) L ( ) F2 ( ) T ( ) E3 ( 0 ) = D m - 1
( 0 , ) L ( ) F3 ( ) T ( ) + D m + 1 ( 0 , ) L ( ) F3 ( ) T ( ) D m
( 0 , ) L ( ) F3 ( ) T ( ) where D m ( 0 , ) L ( ) F1 ( ) T ( ) = D
m ( 0 , ) L ( ) F2 ( ) T ( ) = D m ( 0 , ) L ( ) F3 ( ) T ( )
[0020] where
[0021] D.sub.m-1(.lambda.0,.lambda.) , D.sub.m(.lambda.0,.lambda.)
and D.sub.m+1(.lambda.0,.lambda.): diffraction efficiencies for a
wavelength .lambda. in the (m-1)st, m-th and (m+1)st orders,
respectively, in the diffractive optical element where the m-th
order is taken as a design order and the wavelength .lambda.0 is
taken as the design wavelength,
[0022] L(.lambda.): a spectral characteristic for the wavelength
.lambda. of a light source,
[0023] F1(.lambda.), F2(.lambda.), F3(.lambda.): spectral
sensitivity characteristics of light receiving means for detecting
light in respective wavelength regions in an image pickup means,
where F1(.lambda.), F2(.lambda.) and F3(.lambda.) are arranged in
order from the shorter of wavelengths at which spectral
sensitivities become maximum, and
[0024] T(.lambda.): a transmittance for the wavelength .lambda. of
the photographic optical system.
[0025] Another feature in the first aspect of the invention is that
the shortest design wavelength .lambda.01 among the plurality of
design wavelengths of the diffractive optical element satisfies the
following condition:
400 nm.ltoreq..lambda.01.ltoreq.455 nm.
[0026] Another feature in the first aspect of the invention is that
the longest design wavelength .lambda.0L among the plurality of
design wavelengths of the diffractive optical element satisfies the
following condition:
550 nm.ltoreq..lambda.0L.ltoreq.620 nm.
[0027] Another feature in the first aspect of the invention is that
an interval .DELTA..lambda.0.a between adjacent two of the
plurality of design wavelengths of the diffractive optical element
satisfies the following condition:
.DELTA..lambda.0.a.ltoreq.220 nm
[0028] where .DELTA..lambda.0.a=.lambda.0.a+1-.lambda.0.a
1.ltoreq.a.ltoreq.L-1
[0029] where L is the number of the plurality of design
wavelengths.
[0030] Another feature in the first aspect of the invention is that
at least one of the plurality of diffraction gratings differs in
orientation of grating from the other diffraction gratings.
[0031] Another feature in the first aspect of the invention is that
a useful wavelength region of the diffractive optical element is a
visible spectrum.
[0032] Another feature in the first aspect of the invention is that
the plurality of diffraction gratings are layered on a substrate
and, when the plurality of diffraction gratings are consecutively
numbered, from the diffraction grating nearest to the substrate, as
the first diffraction grating, the second diffraction grating and
up to the i-th diffraction grating, a material from which the first
diffraction grating is made is the same as a material of the
substrate.
[0033] Another feature in the first aspect of the invention is that
the plurality of diffraction gratings are arranged in intimate
contact or in closely spaced relation.
[0034] In accordance with a second aspect of the invention, there
is provided a diffractive optical element having a grating
structure in which a plurality of diffraction gratings made from at
least two kinds of materials different in dispersion from each
other are laminated, and having a plurality of design wavelengths,
a maximum optical path length difference in the grating structure
being integer times each of the plurality of design wavelengths,
flare caused by diffracted rays in orders other than the plurality
of design wavelengths being made white or a color near to
white.
[0035] Another feature in the second aspect of the invention is
that the shortest design wavelength .lambda.01 among the plurality
of design wavelengths of the diffractive optical element satisfies
the following condition:
400 nm.ltoreq..lambda.01.ltoreq.455 nm.
[0036] Another feature in the second aspect of the invention is
that the longest design wavelength .lambda.0L among the plurality
of design wavelengths of the diffractive optical element satisfies
the following condition:
550 nm.ltoreq..lambda.0L.ltoreq.620 nm.
[0037] Another feature in the second aspect of the invention is
that the plurality of diffraction gratings are arranged in intimate
contact or in closely spaced relation.
[0038] Another feature in the second aspect of the invention is
that a photographic optical system comprises an optical system
including the diffractive optical element, and a plurality of light
receiving means whose wavelength regions at peak sensitivity are
different from each other, and an amount of light of each of
diffracted rays in orders other than the plurality of design
wavelengths is controlled by the plurality of light receiving means
so that the flare becomes white or a color near to white.
[0039] The above and further aspects and features of the invention
will become apparent from the following detailed description of
preferred embodiments thereof taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0040] FIG. 1 is a front view of the main parts of a diffractive
optical element according to a first embodiment of the
invention.
[0041] FIG. 2 is a cross-sectional view taken along A-A' line in
FIG. 1 to explain the forms of diffraction gratings in the
diffractive optical element according to the first embodiment of
the invention.
[0042] FIG. 3 is a graph for explaining the diffraction efficiency
in the design order of the diffractive optical element according to
the first embodiment of the invention.
[0043] FIG. 4 is a graph for explaining the diffraction
efficiencies in the needless orders of the diffractive optical
element according to the first embodiment of the invention.
[0044] FIG. 5 is a graph for explaining the diffraction efficiency
in the design order of a diffractive optical element according to
an example of modification of the first embodiment of the
invention.
[0045] FIG. 6 is a graph for explaining the diffraction
efficiencies in the needless orders of the diffractive optical
element shown in FIG. 5.
[0046] FIG. 7 is a graph for explaining the diffraction efficiency
in the design order of a diffractive optical element of another
example of modification of the first embodiment of the
invention.
[0047] FIG. 8 is a graph for explaining the diffraction
efficiencies in the needless orders of the diffractive optical
element shown in FIG. 7.
[0048] FIG. 9 is a graph for explaining the diffraction efficiency
in the design order of a diffractive optical element of a further
example of modification of the first embodiment of the
invention.
[0049] FIG. 10 is a graph for explaining the diffraction
efficiencies in the needless orders of the diffractive optical
element shown in FIG. 9.
[0050] FIG. 11 is a table showing the relationship between the
design wavelength and the amount of flare.
[0051] FIG. 12 is a graph for explaining the spectral
characteristic of the common color film.
[0052] FIG. 13 is a graph for explaining the spectral
characteristic of a white light source.
[0053] FIG. 14 is a graph for explaining the spectral transmittance
of a lens.
[0054] FIG. 15 is a graph for explaining the spectral
characteristic of the photographic optical system except for the
DOE in the first embodiment of the invention.
[0055] FIG. 16 is a front view of a diffractive optical lens in the
invention.
[0056] FIG. 17 is a schematic diagram of a photographic optical
system according to a second embodiment of the invention.
[0057] FIG. 18 is a schematic diagram of an observation optical
system according to a third embodiment of the invention.
[0058] FIG. 19 is a sectional view of a conventional example of
the, grating form (triangular wave form).
[0059] FIG. 20 is a graph for explaining the diffraction efficiency
of the conventional example.
[0060] FIG. 21 is a sectional view of a conventional example of a
stratified multilayer type diffractive optical element.
[0061] FIG. 22 is a sectional view of another conventional example
of the stratified multilayer type diffractive optical element.
[0062] FIG. 23 is a graph for explaining the diffraction efficiency
of the conventional example of the stratified multilayer type
diffractive optical element.
DETAILED DESCRIPTION OF THE INVENTION
[0063] Hereinafter, preferred embodiments of the invention will be
described in detail with reference to the drawings.
[0064] FIG. 1 is a front elevation view showing a diffractive
optical element according to a first embodiment of the invention.
The diffractive optical element 1 is composed of a substrate 2 and
a diffraction grating 3 formed on the surface of the substrate 2.
FIG. 2 is a fragment of the cross-section taken along A-A' line of
FIG. 1. This illustration of FIG. 2 is considerably deformed in the
depth of the grooves of the diffraction grating. In the first
embodiment, the diffraction grating is composed of a first layer 4
and a second layer 5 formed on the substrate 2. At a boundary
between the first layer 4 and the air 8 there is a first
diffraction grating surface 6. At a boundary between the second
layer 5 and the air 8 there is a second diffraction grating surface
7. The diffractive optical element 1 has a structure of the first
and second diffraction gratings 4 and 5.
[0065] Next, the first embodiment is explained about the
diffraction efficiency of the diffractive optical element 1.
Conventionally, as shown in FIG. 19, the diffraction grating 1 is
of transmission type with one layer 3. The diffraction efficiency
of the diffractive optical element becomes highest (100%) at the
design wavelength .lambda.0 under a condition that, as the light
beam is normally incident on the substrate 2, the difference in the
length of optical path of the optics between the rays passing
therethrough to the peak and valley, respectively, of the
diffraction grating 3 is equal to integer times as much as the
design wavelength. Hence, the following equation is
established:
(n01-1)d=m.lambda.0 (3)
[0066] where n01 is the refractive index for the wavelength
.lambda.0 of the material of the diffraction grating 3, d is the
grating thickness, and m is the design order of diffraction (the
design order). Also, .lambda.0 is the design wavelength.
[0067] Since the equation (3) includes the term of wavelengths, the
sign of equality is established only at the design wavelength. At
the other wavelengths, the diffraction efficiency drops from the
maximum value. For an arbitrary wavelength .lambda., the
diffraction efficiency is expressed by the following equation
(4):
.eta.(.lambda.)=sinc.sup.2[.PI.{M-(n1(.lambda.)-1)d/.lambda.}]
(4)
[0068] where M is the diffraction order for which to evaluate the
diffraction efficiency, and n1(.lambda.) is the refractive index
for the wavelength .lambda. of the material of the diffraction
grating 3.
[0069] Even with the use of two or more layers in the diffractive
optical element, the fundamental optical characteristics as the
diffraction gratings are identical. In order to integrate all the
layers to function as one diffraction grating, therefore, every
diffraction grating is first treated with the rays passing
respectively through the peak and valley of its grooves to obtain
the optical path length difference of the optics. For all the
gratings, determination is then made such that the total sum of
their differences coincides with integer times the wavelength. In
the first embodiment, for the case shown in FIG. 2, the diffraction
efficiency in the design order m becomes highest under a condition
expressed by the following equation:
(n01-1)d1.+-.(n02-1)d2=m.lambda.0 (5)
[0070] where n01 is the refractive index for the design wavelength
.lambda.0 of the material of the first diffraction grating 4, n02
is the refractive index for the design wavelength .lambda.0 of the
material of the second diffraction grating 5, d1 and d2 are
respectively the thicknesses of the first and second diffraction
gratings 4 and 5, and m is the design order. Here, the ray
diffracted to the left as viewed from the diffracted rays in the
zero order in FIG. 2 is assumed to be positive in diffraction
order, and the ray diffracted to the right is assumed to be
negative in diffraction order. Then, for each layer, the symbol of
.+-. in the equation (5) should be read for plus when the grating
thickness decreases from the left to the right, as in the case of
the first diffraction grating 4. Conversely, when the grating
thickness increases from the left to the right as in the case of
the second diffraction grating 5, the symbol of .+-. should be read
for minus.
[0071] In the arrangement shown in FIG. 2, for a wavelength
.lambda. other than the design wavelength .lambda.0, the
diffraction efficiency is given by the following expression:
.eta.(.lambda.)=sinc.sup.2[.PI.{M-{(n1(.lambda.)-1)d1.+-.(n2(.lambda.)-1)d-
2 }/.lambda.}]=sinc.sup.2[.PI.{M-.PHI.(.lambda.)/.lambda.}] (6)
[0072] where
.PHI.(.lambda.)=(n1(.lambda.)-1)d1.+-.(n2(.lambda.)-1)d2, and where
M is the diffraction order for which to evaluate the diffraction
efficiency, n1(.lambda.) is the refractive index for the wavelength
.lambda. of the material of the first diffraction grating 4,
n2(.lambda.) is the refractive index for the wavelength .lambda. of
the material of the second diffraction grating 5, and d1 and d2 are
the thicknesses of the first and second diffraction gratings 4 and
5, respectively.
[0073] Although, in FIG. 2, the diffraction grating surfaces 6 and
7 are formed at the boundaries with air, the invention is not
confined thereto. As illustrated in FIG. 21, the diffraction
grating surface may take its place at a boundary between two
different materials.
[0074] Next, the color flare and its cause or the diffracted light
in the needless orders (or the orders other than the design order)
are described below. For the convenience of explaining the
diffraction efficiencies in the needless orders, the stratified
multilayer type diffractive optical element of the invention is
assumed to have two layers in the structure as shown in FIG. 2.
Then, materials are selectively combined with grating thicknesses.
For the first diffraction grating 4, the material to be used is an
ultraviolet curable polymer C001 (nd=1.524, vd=50.8) made by
Dai-Nihon Ink Chemical Industry Co. Ltd. Another ultraviolet
curable polymer (nd=1.635, .nu.d=23.0) is used in the second
diffraction grating 5. The thickness of the first diffraction
grating 4 is taken at d1=9.5 .mu.m, the thickness of the second
diffraction grating 5 is taken at d2=6.9 .mu.m, the distance
between the two diffraction gratings 4 and 5 is taken at D1=1.0
.mu.m, and the grating pitch is taken at 140 .mu.m. Also, the
design order is the first order.
[0075] Here, the diffraction efficiency in the design or first
order is obtained as shown in FIG. 3. In this example, the design
wavelength .lambda.0 exits two in the visible region. In the order
from the shorter wavelength side, two design wavelengths .lambda.
are 438 nm and 588 nm. As is apparent from FIG. 3, the diffraction
efficiency is made to be 100% at the two design wavelengths 438 nm
and 588 nm. Also, in FIG. 4 are shown the diffraction efficiencies
in the needless orders. It is understandable that no needless
diffracted light exists at the design wavelengths 438 nm and 588 nm
and that, as light diffracts in the first order with lowered
efficiencies in regions between and beyond the two design
wavelengths, the diffraction efficiencies in each of the orders
other than the first order become high in those regions. In other
words, for the orders other than the design order, needless
diffracted light occurs. Further, from FIG. 4, it can be seen that,
concerning the higher orders: m+2, m+3, . . . and m-2, m-3, . . . ,
the farther the order goes away from the design order m, the lower
the diffraction efficiency becomes. Therefore, flare light becomes
progressively weaker, and comes to have lesser influence. For this
reason, of the needless orders, the diffraction orders next to the
design order, namely, the (m-1)st order and the (m+1)st order, can
be said to have a great influence on flare. Therefore, if flare
caused by the diffracted rays in the (m.+-.1)st order falls within
a tolerance, flare caused by the diffracted rays in even higher
ones of the needless orders will be acceptable as a matter of
course.
[0076] On this account, in the invention, of the needless orders,
particularly, the (m-1)st and (m+1)st orders are dealt with so that
the diffracted light in these orders has not to be appreciable as
color flare. For this purpose, as the diffractive optical element
is constructed with a plurality of diffraction gratings,
determination is made of all the design parameters (grating
pitches, refractive indices and dispersions of the materials, the
grating shapes in section, etc.) in such a way that the color flare
becomes white or a color near to white.
[0077] Additional three examples of diffractive optical elements
with variation of some of the parameters are shown in FIGS. 5 to
10. Here, the materials from which to make up the elements, the
grating pitches and the design orders remain the same with the
before-described or first example, but only the grating thicknesses
in each pair are made to change. In the table of FIG. 11, for every
example, the values of the grating thicknesses of the two
diffraction gratings are listed in combination. For every one of
these combinations, there are also listed the values of the two
design wavelengths, the interval between the design wavelengths,
and the amount of flare sensed when a color image is recorded by an
image pickup means to be described later. FIGS. 5 and 6 correspond
to the second example in FIG. 11, showing the diffraction
efficiencies in the design order and in the orders other than the
design order, respectively. Similarly, FIGS. 7 and 8 correspond to
the third example in FIG. 11, showing the diffraction efficiencies
in the design order and in the orders other than the design order,
respectively, and FIGS. 9 and 10 correspond to the fourth example
in FIG. 11, showing the diffraction efficiencies in the design
order and in the orders other than the design order, respectively.
As is understandable from these figures, the diffraction
efficiencies in the needless orders or those other than the design
order vary to large extent, depending on the design wavelengths in
the design order. So, as a whole, the color appearance of flare is
caused to change.
[0078] Before explaining the amount of flare, the spectral
characteristic of the image pickup means is described below. The
term "image pickup means" connotes a means for recording an image,
for example, silver-halide film or a CCD. Taking an example of the
common color film, the spectral sensitivity characteristic in the
visible spectrum is shown in FIG. 12. The image pickup means
comprises, usually, three light receiving means for respective
different wavelength regions. By mixing the outputs of the three
light receiving means, a color image is reproduced. The
silver-halide color film is constructed with three photo-sensitive
layers having peak sensitivities at blue, green and red,
respectively. The CCD, too, is constructed with three sensors
having peak sensitivities at blue, green and red, respectively.
Hereinafter, the three light receiving means whose sensitivities
reach peaks at blue, green and red are called respectively the
"first", "second" and "third" light receiving means.
[0079] On comparison of the two graphs of the spectral
characteristics of the image pickup means and the diffraction
efficiencies, in the case of FIGS. 5 and 6, the light in the
needless orders is rich in the blue component. Therefore, the color
flare is liable to be recorded on the first light receiving means.
In the case of FIGS. 7 and 8, a red component of the light in the
needless orders is stronger, so that the color flare is liable to
be recorded on the third light receiving means. In the case of
FIGS. 9 and 10, a green component of the light in the needless
orders is stronger, so that the color flare is liable to be
recorded on the second light receiving means.
[0080] Therefore, what causes the stratified multilayer DOE to
produce color flare can be said that the diffracted light in the
needless order, i.e., the (m+1)st order or the (m-1)st order (where
m is the design order), is increased in intensity as received by
each of the first, second and third light receiving means, and that
the color balance of the diffracted rays in the needless orders,
which are received by the three light receiving means,
collapse.
[0081] To solve this problem, in accordance with the present
invention, measure is taken so that the color flare becomes
inconspicuous by optimizing the combination of a plurality of
design wavelengths .lambda.0. For that purpose, the amount of the
diffracted light in the needless orders is defined on the basis of
the spectral characteristics. Then, determination of a plurality of
design wavelengths is made in such a way that the components of the
color flare received by the three light receiving means are taken
in good intensity balance. So, a while or nearly white flare
results.
[0082] The process for producing a most appropriate combination of
a plurality of design wavelengths .lambda. and the functions of the
thus-obtained arrangement are described below. First, explanation
is given to how the spectral characteristics of the photographic
optical system are changed by using the diffractive optical
element. Most of the conventional photographic optical systems have
their spectral characteristics determined from the light emission
spectrum of the light source, the spectral sensitivities of the
image pickup means and the spectral transmittance of the
photographic lens. Particularly, when the image pickup means is
divided into three light receiving means of respective different
wavelength regions, the spectral characteristics are defined as
follows:
L(.lambda.)F1(.lambda.)T(.lambda.) (7-1)
L(.lambda.)F2(.lambda.)T(.lambda.) (7-2)
L(.lambda.)F3(.lambda.)T(.lambda.) (7-3)
[0083] where L(.lambda.) represents the light emission spectrum of
the light source, being the energy of light of a wavelength
.lambda., F1(.lambda.), F2(.lambda.) and F3(.lambda.) represent
spectral sensitivity characteristics of the first, second and third
light receiving means of the image pickup means, each being the
sensitivity relative to the light of the wavelength .lambda., and
T(.lambda.) represents a spectral transmittance of the photographic
optical system, being the transmittance relative to the light of
the wavelength .lambda..
[0084] Further, these equations are related as follows:
.intg.L(.lambda.) F1(.lambda.) T(.lambda.)
d.lambda.=.intg.L(.lambda.) F2(.lambda.) T(.lambda.)
d.lambda.=.intg.L(.lambda.) F3(.lambda.) T(.lambda.) d.lambda.
(8)
[0085] The outputs of the three light receiving means are all made
equal to one another when the original colors are mixed. Thus, a
color image is formed in good color balance.
[0086] Here, the first embodiment of the invention operates under
the conditions that the light source is a white light source (D65)
having the spectral characteristic L(.lambda.) shown in FIG. 13,
the image pickup means is a common color film having the spectral
characteristics F1(.lambda.), F2(.lambda.) and F3(.lambda.) shown
in FIG. 12 and the lens has the transmittance T(.lambda.) shown in
FIG. 14. These conditions are factored into computation for the
spectral characteristics of the photographic optical system. The
spectral characteristics obtained as a result of the computation
are shown in FIG. 15.
[0087] Then, a diffractive optical element is used in such a
photographic optical system. Thereupon, another factor is added,
representing the diffraction efficiency D.sub.m(.lambda.) in the
design order m of the diffractive optical element. The diffraction
efficiency D.sub.m(.lambda.) varies as a function of the design
wavelengths .lambda.0 and any other wavelengths .lambda., so that
it can be described as D.sub.m(.lambda.0,.lambda.). Therefore, when
the diffractive optical element of the design wavelengths .lambda.0
is used, the spectral characteristics can be defined as
follows:
D.sub.m(.lambda.0,.lambda.) L(.lambda.) F1(.lambda.) T(.lambda.)
(9-1)
D.sub.m(.lambda.0,.lambda.) L(.lambda.) F2(.lambda.) T(.lambda.)
(9-2)
D.sub.m(.lambda.0,.lambda.) L(.lambda.) F3(.lambda.) T(.lambda.)
(9-3)
[0088] Of course, for the optical system, since the diffractive
optical element is incorporated therein, because the outputs of the
three light receiving means are equalized to reproduce colors in
good balance, equations similar to the equations (8) are
established.
.intg.D.sub.m(.lambda.0,.lambda.) L(.lambda.) F1(.lambda.)
T(.lambda.) d.lambda.=.intg.D.sub.m(.lambda.0,.lambda.) L(.lambda.)
F2(.lambda.) T(.lambda.)
d.lambda.=.intg.D.sub.m(.lambda.0,.lambda.) L(.lambda.)
F3(.lambda.) T(.lambda.) d.lambda. (10)
[0089] Using the above formulas, the spectral characteristics of
the diffracted light in the needless orders can be defined. Letting
the design order be denoted by m, the spectral characteristics of
the diffracted light in the (m-1)st order can be defined as
follows:
D.sub.m-1(.lambda.0,.lambda.) L(.lambda.) F1(.lambda.) T(.lambda.)
(11-1)
D.sub.m-1(.lambda.0,.lambda.) L(.lambda.) F2(.lambda.) T(.lambda.)
(11-2)
D.sub.m-1(.lambda.0,.lambda.) L(.lambda.) F3(.lambda.) T(.lambda.)
(11-3)
[0090] Similarly, the spectral characteristics of the diffracted
light in the (m+1)st order can be defined as follows:
D.sub.m+1(.lambda.0,.lambda.) L(.lambda.) F1(.lambda.) T(.lambda.)
(12-1)
D.sub.m+1(.lambda.0,.lambda.) L(.lambda.) F2(.lambda.) T(.lambda.)
(12-2)
D.sub.m+1(.lambda.0,.lambda.) L(.lambda.) F3(.lambda.) T(.lambda.)
(12-3)
[0091] The thus-defined spectral characteristics are used for
defining the amount of flare. Since the spectral characteristics
exhibit values of the energy of light of a wavelength .lambda.
which, after having passed through the optical system, are
recorded, in order to find the total energy of light contributing
to image formation, all what to do is to integrate each of the
spectral characteristics over all values of wavelengths
.lambda..
[0092] Therefore, each of the amounts of flare (total energy)
caused by the diffracted light in the needless orders can be
defined by integration of the formulas (11-1) to (11-3) and the
formulas (12-1) to (12-3). In such a manner, the amounts of flare
in the (m-1)st order and the (m+1)st order are defined. Taking the
sum of the amounts of flare and normalizing the results by the
integrated values of the spectral characteristics of the diffracted
light in the design order, or the total energy, (9-1) to (9-3), the
amounts of color flare E1(.lambda.0), E2(.lambda.0) and
E3(.lambda.0) for the first, second and third light receiving means
are respectively expressed by the following formulas:
[0093] The amount of color flare received by the first light
receiving means, E1(.lambda.0): 2 D m - 1 ( 0 , ) L ( ) F1 ( ) T (
) + D m + 1 ( 0 , ) L ( ) F1 ( ) T ( ) D m ( 0 , ) L ( ) F1 ( ) T (
) (13-1)
[0094] The amount of color flare received by the second light
receiving means, E2(.lambda.0): 3 D m - 1 ( 0 , ) L ( ) F2 ( ) T (
) + D m + 1 ( 0 , ) L ( ) F2 ( ) T ( ) D m ( 0 , ) L ( ) F2 ( ) T (
) (13-2)
[0095] The amount of color flare received by the third light
receiving means, E3(.lambda.0): 4 D m - 1 ( 0 , ) L ( ) F3 ( ) T (
) + D m + 1 ( 0 , ) L ( ) F3 ( ) T ( ) D m ( 0 , ) L ( ) F3 ( ) T (
) (13-3)
[0096] In the case of using the stratified multilayer DOE, the
amount of flare must be concerned about the magnitude of each of
the amounts of color flare received by the respective light
receiving means and the balance between any two of the amounts of
color flare received by the respective light receiving means. It
is, therefore, required that all the values of the formulas (13-1),
(13-2) and (13-3) become smaller, and that at least one of the
three light receiving means does not pick up an extremely large
amount of light relative to the others. Hence, for the stratified
multilayer DOE, the design wavelengths .lambda.0 have to be
determined so as to fulfill the above requirements.
[0097] Therefore, if color flare would otherwise be produced, the
color flare is made to become white or a color near to white, thus
making the color flare inconspicuous. For that purpose, letting the
formulas (13-1), (13-2) and (13-3) be denoted by E1(.lambda.0),
E2(.lambda.0) and E3(.lambda.0), respectively, all what to do is to
determine the design wavelengths .lambda.0 of the stratified
multilayer DOE so as to satisfy the following conditions (1) and
(2):
0<E1(.lambda.0)+E2(.lambda.0)+E3(.lambda.0)<0.04 (1)
0<max{E1(.lambda.0 ), E2(.lambda.0),
E3(.lambda.0)}-min{E1(.lambda.0), E2(.lambda.0),
E3(.lambda.0)}<0.02 (2)
[0098] where max{E1(.lambda.0), E2(.lambda.0), E3(.lambda.0)}
represents a maximum value among E1(.lambda.0), E2(.lambda.0) and
E3(.lambda.0), and min{E1(.lambda.0), E2(.lambda.0), E3(.lambda.0)}
represents a minimum value among E1(.lambda.0), E2(.lambda.0) and
E3(.lambda.0).
[0099] The factor in the inequalities (1) represents the sum of the
amounts of flare of blue, green and red colors at the design
wavelengths .lambda.0. Therefore, when the upper limit of 0.04 is
exceeded, the flare itself becomes conspicuous, thereby lowering
the image quality. The factor in the inequalities (2) represents
the difference between the maximum and minimum values of each of
the amounts of flare at the design wavelengths .lambda.0. When the
upper limit of 0.02 is exceeded, the flare appears in a color tint.
Therefore, however little it may be, the flare lowers the image
quality.
[0100] It will be appreciated from the foregoing that the
combination of a plurality of design wavelengths .lambda.0 is
optimized to reduce the amount of flare and to make the flare while
or nearly white. It is thus made possible to obtain a photographic
optical system which does not suffer color flare.
[0101] In actual practice, as applied to the silver-halide film
camera, electronic camera or video camera, the optimum values of
the design wavelengths will be described.
[0102] For the stratified multilayer DOE, it is preferred that, of
the plurality of design wavelengths, the shortest design wavelength
.lambda.01 satisfy one of the following conditions (14) and
(14a):
.lambda.01.ltoreq.455 nm (14)
[0103] preferably,
400 nm.ltoreq..lambda.01<455 nm (14a)
[0104] Referring to FIG. 11, the second example employs
".lambda.01=455 nm" as one of the combined design wavelengths. In
this case, the diffraction efficiencies in all the orders are shown
in FIGS. 5 and 6. On comparison of these figures, it can be seen
that the diffraction efficiencies in the needless orders are
increased in the shorter wavelength region.
[0105] Also, in the second example shown in FIG. 11, the amount of
flare for the first light receiving means is 1.94%, being far
larger than those for the other light receiving means. The above
condition (14) or (14a) is to determine the shortest acceptable one
of the plurality of design wavelengths .lambda.0 that maximize the
diffraction efficiency in the design order of the diffractive
optical element. When the upper limit is exceeded, as this means
that a longer wavelength than 455 nm is selected as the shortest
design wavelength, color flare of blue becomes conspicuous.
[0106] For the stratified multilayer DOE, it is further preferred
that, of the plurality of design wavelengths, the longest design
wavelength .lambda.0L satisfy one of the following conditions (15)
and (15a):
550 nm.ltoreq..lambda.0L (15)
[0107] preferably,
550 nm.ltoreq..lambda.0L.ltoreq.620 nm (15a)
[0108] Referring to FIG. 11, the third example employs
".lambda.0L=550 nm" as one of the combined design wavelengths. In
this case, the diffraction efficiencies are shown in FIGS. 7 and 8,
where it can be seen that the diffraction efficiencies in the
needless orders are increased in the longer wavelength region.
[0109] Also, in the third example shown in FIG. 11, the amount of
flare for the third light receiving means is 1.94%, being far
larger than those for the other light receiving means. The above
condition (15) or (15a) is to determine the longest acceptable one
of the plurality of design wavelengths .lambda.0 that maximize the
diffraction efficiency in the design order of the diffractive
optical element. When the lower limit of 550 nm is exceeded, color
flare of red becomes conspicuous.
[0110] For the stratified multilayer DOE, it is further preferred
that the interval .DELTA..lambda.0. a between adjacent two of the
plurality of design wavelengths satisfies the following
condition:
.DELTA..lambda.0.a.ltoreq.220 nm (16)
[0111] where .DELTA..lambda.0.a=.lambda.0. a+1-.lambda.0.a
1.ltoreq.a.ltoreq.L-1
[0112] where L is the number of the plurality of design
wavelengths.
[0113] Referring to FIG. 11, the fourth example employs
".DELTA..lambda.0.a 220 nm" between the combined design
wavelengths. In this case, the diffraction efficiencies are shown
in FIGS. 9 and 10, where it can be seen that the diffraction
efficiencies in the needless orders are increased in the region
between the two design wavelengths.
[0114] Also, in the fourth example shown in FIG. 11, the amount of
flare for the second light receiving means is 1.97%, being far
larger than those for the other light receiving means. The above
condition (16) is to determine the intervals between any adjacent
two of the plurality of design wavelengths .lambda.0 that maximize
the diffraction efficiency in the design order of the diffractive
optical element. When the upper limit of 220 nm is exceeded, color
flare of green becomes conspicuous.
[0115] The construction and arrangement described above has been
assumed that the design wavelength exists two in number. However,
the invention is established even in a case where there are three
or more design wavelengths.
[0116] The foregoing description has been directed to the
stratified multilayer DOE of a type in which the grating pitch
shown in FIG. 1 is constant. However, actually, the invention is
not confined thereto, being applicable to another type of
diffractive optical elements in which the grating pitch gradually
varies as shown in FIG. 16 to produce an effect like a spherical or
an aspherical lens.
[0117] Also, although the first embodiment has been described in
connection with the diffractive optical element having its
diffraction gratings formed on a parallel flat plate, it is to be
understood that the invention is applicable to the diffraction
gratings formed on a spherical surface of a lens. Even in this
case, similar improved results can be attained.
[0118] Although the first embodiment has been illustrated on the
assumption that the design order is the first order, the invention
is not confined to the limitation of the design order to the first
order. Even for the second or other higher orders than the first
order, if the combined optical path length difference is determined
to be equal to the desired design wavelength in the desired order,
and the amount of flare caused by the diffracted light in the other
orders than the design order satisfy the conditions of the
invention, similar improved results can be attained, except that,
in the case of using other than the first order as the design
order, the dependency of the diffraction efficiency on the
wavelength is intensified. In order to produce an optical system
which has reduced the amount of flare and made the flare
inconspicuous, it is preferred to choose the first order as the
design order.
[0119] A second embodiment of the invention is shown in FIG. 17,
where the first embodiment is applied to an optical apparatus such
as a camera. Referring to FIG. 17, a photographic lens 9 has a
plurality of lens members, an aperture stop 10 and the diffractive
optical element 1 of the invention. Film or a CCD is positioned on
an image plane 11. Incidentally, the diffractive optical element 1
is positive in refractive power and corrects the chromatic
aberrations of the lens.
[0120] By using the stratified multilayer structure and optimizing
the combination of design wavelengths, the dependency of the
diffraction efficiency on the wavelength is largely improved.
Therefore, it is possible to provide a photographic lens of lesser
flare and high resolving power in the low frequencies for high
performance. Also, the flare is made white or nearly while
according to the invention, becoming inconspicuous.
[0121] Although, in FIG. 17, the diffractive optical element of the
invention has been put on the glass surface of the parallel flat
plate near the stop, the invention is not confined thereto. It may
be provided on any one of the spherical surfaces of the lens
members. Two or more diffractive optical elements of the invention
may be used in the photographic lens.
[0122] Also, although the second embodiment has been described in
connection with the photographic lens of the film camera or video
camera, the invention is not confined thereto. It may be applied to
the office machines such as an image scanner, or a reader lens of a
digital copying machine or like image forming optical systems for
use in the visible or wide wavelength range.
[0123] FIG. 18 is a schematic diagram showing a third embodiment of
the invention as applied to an observation optical system such as a
binocular using the diffractive optical element of the invention.
Referring to FIG. 18, an objective lens 12 forms an object image on
an image plane 11. The erected image by a prism 13 is observed
through an eyepiece 14 by a pupil at an evaluation plane 15. The
diffractive optical element 1 of the invention is formed with an
aim to correct chromatic aberrations at the image plane 11 by the
objective lens 2.
[0124] Since the dependency of the diffraction efficiency on the
wavelength is greatly improved by using the stratified multilayer
structure, it is possible to provide an objective lens of lesser
flare and high resolving power in the low frequencies for high
performance. Also, the flare is made white or nearly while
according to the invention, becoming inconspicuous.
[0125] Although the third embodiment has been described as the
diffractive optical element formed in the objective lens section,
the invention is not confined thereto. Even if the diffractive
optical element formed on the surface of the prism, or in the
eyepiece, similar results are attained. However, when the position
of the diffractive optical element is on the object side of the
image plane 11, the chromatic aberrations of only the objective
lens can be reduced. Therefore, in the case of the observation
optical systems by the naked eye, it is preferred that the
diffractive optical element is positioned at least adjacent to the
objective lens.
[0126] Also, in the third embodiment, the case of the binocular is
illustrated. However, the invention is not confined thereto. The
invention may be applied to any other types of observation optical
apparatuses such as a terrestrial telescope and an astronomical
telescope. Also, the invention is applicable to optical type
viewfinders for the lens-shutter cameras or video cameras. Even in
this case, similar results can be attained.
[0127] According to the invention, it has been made possible to
achieve a diffractive optical element which makes inconspicuous the
color flare by changing its color to white or nearly white, and a
photographic optical system having the diffractive optical
element.
[0128] In particular, according to the invention, even when the
diffractive optical element is applied not only to the photographic
optical systems for use in normal light situation, but also to the
special photographic optical systems for use in special lighting
situations, it is possible to provide a photographic optical system
which produce no conspicuous color flare and, therefore, can form
images of good quality.
* * * * *