U.S. patent application number 10/422249 was filed with the patent office on 2004-07-22 for image display apparatus.
This patent application is currently assigned to MINOLTA CO., LTD.. Invention is credited to Endo, Takeshi, Kasai, Ichiro, Morimoto, Takashi, Noda, Tetsuya.
Application Number | 20040141217 10/422249 |
Document ID | / |
Family ID | 32709240 |
Filed Date | 2004-07-22 |
United States Patent
Application |
20040141217 |
Kind Code |
A1 |
Endo, Takeshi ; et
al. |
July 22, 2004 |
IMAGE DISPLAY APPARATUS
Abstract
A holographic element is a reflective holographic element formed
on a substrate and constituted by a composite hologram having a
plurality of patterns of interference fringes composed of
interference fringes nonparallel to the substrate. The holographic
element forms, out of image light having a predetermined wavelength
width emanating from an image display element, a plurality of
observation pupils at spatially different locations, and acts in
such a way as to fulfill prescribed conditions for an identical
incident ray over the entire area in which the image light is
incident on the holographic element.
Inventors: |
Endo, Takeshi; (Osaka-Shi,
JP) ; Kasai, Ichiro; (Toyonaka-Shi, JP) ;
Morimoto, Takashi; (Suita-Shi, JP) ; Noda,
Tetsuya; (Tenri-Shi, JP) |
Correspondence
Address: |
SIDLEY AUSTIN BROWN & WOOD LLP
717 NORTH HARWOOD
SUITE 3400
DALLAS
TX
75201
US
|
Assignee: |
MINOLTA CO., LTD.
|
Family ID: |
32709240 |
Appl. No.: |
10/422249 |
Filed: |
April 24, 2003 |
Current U.S.
Class: |
359/13 |
Current CPC
Class: |
G02B 5/203 20130101;
G02B 5/32 20130101; G03H 2270/55 20130101; G02B 2027/0174 20130101;
G03H 1/0408 20130101; G02B 2027/0178 20130101; G02B 27/0172
20130101 |
Class at
Publication: |
359/013 |
International
Class: |
G03H 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 22, 2003 |
JP |
2003-013451 |
Claims
What is claimed is:
1. A holographic element that is a reflective holographic element
formed on a substrate and constituted by a composite hologram
having a plurality of patterns of interference fringes composed of
interference fringes nonparallel to the substrate, the holographic
element forming, out of image light having a predetermined
wavelength width emanating from an image display element, a
plurality of observation pupils at spatially different locations,
wherein the holographic element acts in such a way as to fulfill
the following conditional formulae (I) and (II) for an identical
incident ray over an entire area in which the image light is
incident on the holographic element: .DELTA..theta.<2 (I)
.DELTA..eta.>50 (II) where .DELTA..eta. represents the
difference (.degree.) in angle of diffraction among the different
patterns of interference fingers; and .DELTA..eta. represents the
difference (%) in diffraction efficiency among the different
patterns of interference fringes.
2. A holographic element as claimed in claim 1, wherein the
composite hologram constituting the holographic element is a
multiple-layer hologram fabricated by laying on one another a
plurality of holograms each having a pattern of interference
fringes recorded thereon.
3. A holographic element as claimed in claim 1, wherein the
composite hologram constituting the holographic element is a
multiple-exposure hologram fabricated by recording a plurality of
patterns of interference fringes on a single photosensitive
material by multiple exposure.
4. A holographic element as claimed in claim 1, wherein the
holographic element has a nonaxisymmetric optical power.
5. A holographic element as claimed in claim 1, wherein the
holographic element has a plurality of diffraction wavelength
peaks.
6. A holographic element as claimed in claim 1, wherein the
composite hologram constituting the holographic element is formed
by using photopolymer as a photosensitive material.
7. An image display apparatus comprising: an image display element
for displaying a two-dimensional image; an enlargement optical
system for projecting, with enlargement, the image displayed on the
image display element; and a holographic element included in the
enlargement optical system, the holographic element being a
reflective holographic element formed on a substrate and
constituted by a composite hologram having a plurality of patterns
of interference fringes composed of interference fringes
nonparallel to the substrate, the holographic element forming, out
of image light having a predetermined wavelength width emanating
from the image display element, a plurality of observation pupils
at spatially different locations, wherein the holographic element
acts in such a way as to fulfill the following conditional formulae
(I) and (II) for an identical incident ray over an entire area in
which the image light is incident on the holographic element:
.DELTA..theta.<2 (I) .DELTA..eta.>50 (II) where
.DELTA..theta. represents the difference (.degree.) in angle of
diffraction among the different patterns of interference fringes;
and .DELTA..eta. represents the difference (%) in diffraction
efficiency among the different patterns of interference
fringes.
8. An image display apparatus as claimed in claim 7, wherein the
composite hologram constituting the holographic element is a
multiple-layer hologram fabricated by laying on one another a
plurality of holograms each having a pattern of interference
fringes recorded thereon.
9. An image display apparatus as claimed in claim 7, wherein the
composite hologram constituting the holographic element is a
multiple-exposure hologram fabricated by recording a plurality of
patterns of interference fringes on a single photosensitive
material by multiple exposure.
10. An image display apparatus as claimed in claim 7, wherein the
holographic element has a nonaxisymmetric optical power.
11. An image display apparatus as claimed in claim 7, wherein the
holographic element has a plurality of diffraction wavelength
peaks.
12. An image display apparatus as claimed in claim 7, wherein the
composite hologram constituting the holographic element is formed
by using photopolymer as a photosensitive material.
13. An image display apparatus comprising: an image display element
for displaying a two-dimensional image; an enlargement optical
system for reflecting, as a first image, the image displayed on the
image display element so as to project, with enlargement, the first
image onto an observer's pupil while transmitting, as a second
image, an image from an outside world so as to direct the second
image to the observer's pupil; and a holographic element included
in the enlargement optical system, the holographic element being a
reflective holographic element formed on a substrate and
constituted by a composite hologram having a plurality of patterns
of interference fringes composed of interference fringes
nonparallel to the substrate, the holographic element forming, out
of image light having a predetermined wavelength width emanating
from the image display element, a plurality of observation pupils
at spatially different locations, wherein the holographic element
acts in such a way as to fulfill the following conditional formulae
(I) and (II) for an identical incident ray over an entire area in
which the image light is incident on the holographic element:
.DELTA..theta.<2 (I) .DELTA..eta.>50 (II) where
.DELTA..theta. represents the difference (.degree.) in angle of
diffraction among the different patterns of interference fringes;
and .DELTA..eta. represents the difference (%) in diffraction
efficiency among the different patterns of interference
fringes.
14. An image display apparatus as claimed in claim 13, wherein the
composite hologram constituting the holographic element is a
multiple-layer hologram fabricated by laying on one another a
plurality of holograms each having a pattern of interference
fringes recorded thereon.
15. An image display apparatus as claimed in claim 13, wherein the
composite hologram constituting the holographic element is a
multiple-exposure hologram fabricated by recording a plurality of
patterns of interference fringes on a single photosensitive
material by multiple exposure.
Description
[0001] This application is based on Japanese Patent Application No.
2003-013451 filed on Jan. 22, 2003, the contents of which are
hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an image display apparatus,
more particularly to an image display apparatus such as an HAD
(head-mounted display) or HUD (head-up display) that projects a
two-dimensional image formed, for example, on a liquid crystal
display (LCD) onto an observer's eye by the use of a holographic
optical element (HOE) so as to permit the observer to observe the
image on a see-through basis.
[0004] 2. Description of the Prior Art
[0005] Japanese Patent Application Laid-Open No. H9-185009, U.S.
Pat. No. 4,655,540, and other publications propose image display
apparatuses of a see-through type that superimpose an image of an
object on an image of the outside world by the use of a combiner
realized with a hologram and then projects the resulting image for
display. Among these, Japanese Patent Application Laid-Open No.
H9-185009 proposes an image display apparatus in the shape of
eyeglasses that permits observation of a two-dimensional image on a
see-through basis by the use of a holographic optical element
having the function of an eyepiece lens. FIGS. 14A and 14B show an
outline of the optical construction of such an image display
apparatus. In FIG. 14A, broken lines represent the rays to which a
holographic optical element 22 is exposed when it is fabricated,
and solid and dotted lines represent the most off-axial rays of the
beam representing the reconstructed image when the displayed image
is observed.
[0006] The holographic optical element 22 is fabricated by the use
of a high-coherence light source such as a laser light source. The
holographic optical element 22 is fabricated by making a divergent
beam emanating from where the observation pupil E is located
interfere with a nonaxisymmetric beam with a complicated wavefront
and recording the resulting interference fringes as a pattern of
refractive index modulation on a photosensitive material. The
holographic optical element 22 thus obtained is a volume-phase
reflective hologram with good see-through characteristics and high
light-use efficiency. This holographic optical element 22 has a
phase function that converts the wavefront of incident light in
such a way that the light is reflected by diffraction in a desired
direction, and also functions as a nonaxisymmetric lens,
contributing to miniaturization. When an image is observed, the
holographic optical element 22 is reconstructed by the use of a
light source such as an LED (light-emitting diode), in particular a
light source of which the peak wavelength is roughly equal to the
wavelength of the exposure rays (with a difference in wavelength of
20 nm or less). When an image display element 21 is illuminated
with the light from such a light source, the image light emanating
from the image display element 21 is incident on the holographic
optical element 22, and the light reflected by diffraction it
exerts is directed to an observer's eye 23.
[0007] The holographic optical element 22 has narrow angle
selectivity, and therefore, when it is exposed and reconstructed in
the manners described above, its diffraction efficiency is highest
when the directions of the reconstruction rays (the solid and
dotted lines) are close to those of the exposure rays (broken
lines), and is low when the directions of the reconstruction and
exposure rays differ. Thus, the uppermost rays represented by the
dotted lines and the lower most rays represented by the solid lines
come to have a large angle difference .delta. relative to the
exposure rays represented by the broken lines. When photopolymer, a
common photosensitive material, is used, the resulting refractive
index modulation (in the vicinity of .DELTA.n=0.1) gives the
holographic optical element sharp angle selectivity in terms of
diffraction efficiency .eta.. This makes it impossible to obtain an
observation pupil E having the designed size. In FIG. 14B, the
designed observation pupil E is indicated with solid lines, and the
actually obtained observation pupil E is indicated with broken
lines. When photopolymer is used, the observation pupil E measures,
for example, 10 to 20 mm in the width direction and 1 mm (the
broken lines) or 3 to 5 mm (solid lines) in the height direction.
As a result, the displayed image goes out of sight when the
observer moves his or her eye 23 up or down even a little. Using a
photosensitive material such as silver halide or bichromated
gelatin instead of photopolymer results in larger values of An and
.eta., and thus helps obtain broader angle selectivity. This,
however, increases fabrication costs and leads to lower durability
under the influence of moisture absorption and temperature
variation.
SUMMARY OF THE INVENTION
[0008] An object of the present invention is to provide an
inexpensive image display apparatus that forms an observation pupil
that permits easy observation of the displayed image.
[0009] To achieve the above object, according to one aspect of the
present invention, a holographic element is a reflective
holographic element formed on a substrate and composed of a
composite hologram having a plurality of patterns of interference
fringes composed of interference fringes nonparallel to the
substrate. This holographic element forms, out of image light
having a predetermined wavelength width emanating from an image
display element, a plurality of observation pupils at spatially
different locations, and acts in such a way as to fulfill
conditional formulae (I) and (II) below for an identical incident
ray over the entire area in which the image light is incident on
the holographic element:
.DELTA..theta.<2 (I)
.DELTA..eta.>50 (II)
[0010] where
[0011] .DELTA..theta. represents the difference (.degree.) in angle
of diffraction among the different patterns of interference
fringes; and
[0012] .DELTA..eta. represents the difference (%) in diffraction
efficiency among the different patterns of interference
fringes.
[0013] According to another aspect of the present invention, an
image display apparatus is provided with an image display element
for displaying a two-dimensional image, an enlargement optical
system for projecting, with enlargement, the image displayed on the
image display element, and a holographic element as described above
included in the enlargement optical system.
[0014] According to still another aspect of the present invention,
an image display apparatus is provided with an image display
element for displaying a two-dimensional image, an enlargement
optical system for reflecting, as a first image, the image
displayed on the image display element so as to project, with
enlargement, the first image onto an observer's pupil while
transmitting, as a second image, an image from an outside world so
as to direct the second image to the observer's pupil, and a
holographic element as described above included in the enlargement
optical system
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] This and other objects and features of the present invention
will become clear from the following description, taken in
conjunction with the preferred embodiments with reference to the
accompanying drawings in which:
[0016] FIG. 1 is a sectional view showing an outline of the optical
construction of an image display apparatus embodying the
invention;
[0017] FIG. 2 is a perspective view showing the external appearance
of the image display apparatus embodying the invention;
[0018] FIGS. 3A and 3B are diagrams illustrating how different
patterns of interference fringes form observation pupils;
[0019] FIG. 4 is a graph showing the angle selectivity of the
holographic optical element;
[0020] FIG. 5 is an enlarged sectional view showing a portion of a
holographic optical element composed of a multiple-layer
hologram;
[0021] FIG. 6 is an optical construction diagram showing a two-beam
hologram exposure apparatus for fabricating a multiple-layer
hologram;
[0022] FIG. 7 is a diagram schematically showing an outline of the
overall construction of a two-beam hologram exposure apparatus for
fabricating a multiple-exposure hologram;
[0023] FIG. 8 is a diagram schematically showing the exposure unit
included in the two-beam hologram exposure apparatus shown in FIG.
7;
[0024] FIG. 9 is a diagram showing the optical construction, as
designed, of the two-beam hologram exposure apparatus shown in FIG.
7;
[0025] FIG. 10 is a diagram schematically showing an outline of the
construction of a single-beam hologram exposure apparatus for
fabricating a multiple-exposure hologram;
[0026] FIG. 11 is a diagram showing the optical construction, as
designed, of the single-beam hologram exposure apparatus shown in
FIG. 10;
[0027] FIG. 12 is a diagram schematically showing an example of how
the two-beam hologram exposure apparatus shown in FIG. 7 is
modified to cope with colors;
[0028] FIGS. 13A and 13B are graphs showing the angle selectivity
and wavelength selectivity of the holographic optical element;
and
[0029] FIGS. 14A and 14B are diagrams showing an outline of the
optical construction of a conventional image display apparatus.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0030] Hereinafter, an image display apparatus embodying the
invention will be described with reference to the drawings. FIG. 1
shows an outline of an image display apparatus, in the shape of
eyeglasses, embodying the invention, as seen in an optical section,
and FIG. 2 shows its external appearance. In FIG. 1, reference
numeral 1 represents an LED, reference numeral 2 represents a
condenser lens, reference numeral 3 represents a transmissive LCD
reference numeral 4 represents a holographic optical element
composed of a volume-phase reflective hologram (corresponding to an
enlargement optical system constituting part of an eyepiece optical
system), reference numeral 5 represents an observer's eye 5,
reference, numeral 6 represents a prism, and reference symbol E
represents an observation pupil. In FIG. 2, reference numeral 7
represents a cable, reference numeral 8 represents a bridge,
reference numerals 8R and 8L represent nose pads, reference
numerals 9R and 9L represent lenses, reference numeral 10
represents a display portion, reference numerals 11R and 11L
represent temples serving as a holding member that holds the image
display apparatus on the observer's head, and reference numeral 14
represents a casing.
[0031] The LED 1 is an illumination light source that emits
illumination light for illuminating the display surface of the LCD
3, and the condenser lens 2 is a collimator lens for illumination
that makes the light from the LED 1 into a parallel beam. The LCD 3
is a transmissive spatial modulation element that forms a
two-dimensional image on its display surface. Since this LCD 3 is
an image display element of the type that does not emit light by
itself, the two-dimensional image formed on it becomes visible when
its display surface is illuminated with the illumination light from
the LED 1. The image display element does not necessarily have to
be a transmissive spatial modulation element, but may be a
reflective spatial modulation element. For example, a reflective
LCD that operates fast permits color display on a time-division
basis, and is therefore suitable to achieve high-resolution color
display at low costs. Here, using the LED 1 as the illumination
light source and the LCD 3 as the image display element is
preferable to realize a compact, light-weight, and inexpensive
optical construction. However, it is also possible to use instead
an image display element of the self-illuminating type such as an
EL (electroluminescence) element. Using a self-illuminating image
display element eliminates the need for an illumination light
source and a condenser lens, and thus helps realize a more
light-weight, compact optical construction.
[0032] As described above, the LED 1, the condenser lens 2, and the
LCD 3 together constitute an image forming means for forming a
two-dimensional image. The image light emanating therefrom is
directed to the observer's eye 5 by an eyepiece optical system
constituted by the prism 6 and the reflective holographic optical
element 4. As a result, the two-dimensional image is projected onto
the observer's eye 5 on a see-through basis and with enlargement by
the holographic optical element 4 so as to be observed as a virtual
image by the observer. As shown in FIG. 2, the prism 6 is embedded
as part of the right-eye lens 9R, and the display portion 10 for
forming an image is fitted above the prism 6. The display portion
10 receives electric power and signals by way of the cable 7
connected thereto. The display portion 10 is housed in the casing
14, which is so fitted as to sandwich the prism 6. The LED 1, the
condenser lens 2, and the LCD 3 described above are, together with
other components, housed in the casing 14. With this construction,
it is possible to realize a light-weight, compact image display
apparatus. In the embodiment shown in FIG. 2, an image is displayed
for one eye only. It is also possible, however, to apply the same
construction not only to the right-eye lens but also to the
left-eye lens so that an image is displayed for both eyes.
Displaying an image for both eyes helps alleviate the eye strain
that the observer feels as a result of being forced to observe an
image with one eye.
[0033] The holographic optical element 4 is so arranged as to be
sandwiched between the prism 6 and the lens 9R, which thus serve as
hologram substrates. The holographic optical element 4 is composed
of a composite hologram, and has two patterns of interference
fringes composed of interference fringes nonparallel to the
hologram substrate surfaces. The diffraction exerted by the
patterns of interference fringes permits the holographic optical
element 4 to function as an eyepiece lens. Moreover, the wavelength
selectivity of the holographic optical element 4 permits it to
function as a suitable combiner. That is, while it is possible to
satisfactorily observe the two-dimensional image formed on the LCD
3, it is also possible to obtain a sufficient amount of light for
the outside world observed through the holographic optical element
4. Moreover, the holographic optical element 4 has an axisymmetric
optical power. Giving the holographic optical element 4 an
axisymmetric optical power permits observation of an image with
aberrations satisfactorily corrected for, and turning the optical
path by reflecting it by diffraction, i.e. at an angle of
reflection different from that achieved by regular reflection,
contributes to miniaturization of the image display apparatus. The
example being discussed here deals with a case where the
holographic optical element 4 has two patterns of interference
fringes, but the holographic optical element 4 may simply have more
than one pattern of interference fringes, for example three or
more.
[0034] Since the holographic optical element 4 has two patterns of
interference fringes, it causes imaging to take place in two ways.
That is, the two patterns of interference fringes of the
holographic optical element 4 form two observation pupils E1 and E2
as shown in FIGS. 3A an 3B. FIG. 3A shows the positional
relationship between the observer's eye 5 and the observation
pupils E1 and E2, and FIG. 3B shows the optical path to illustrate
how the holographic optical element 4 forms the observation pupils
E1 and E2. In this way, the holographic optical element 4 forms,
out of the image light having a predetermined wavelength width
emanating from the LCD 3, two observation pupils E1 and E2 at
spatially different locations. This, makes the size of the
observation pupils E1 and E2 as a whole larger, and thereby makes
the displayed image easier to observe. Moreover, the illumination
light emitted by the LED 1 has a wider wavelength width (than laser
light), and thus the image light has a predetermined wavelength
width. This gives the resulting observation pupils E1 and E2 a
suitable size.
[0035] Moreover, the holographic optical element 4 acts in such a
way that, over the entire area in which the image light is incident
thereon, conditions (I) and (II) below are fulfilled for an
identical ray.
.DELTA..theta.<2 (I)
.DELTA..eta.>50 (II)
[0036] where
[0037] .DELTA..theta. represents the difference (.degree.) in angle
of diffraction among the different patterns of interference
fringes; and
[0038] .DELTA..eta. represents the difference (%) in diffraction
efficiency among the different patterns of interference
fringes.
[0039] Condition (I) indicates that the two patterns of
interference fringes have roughly identical phase functions, and
condition (II) indicates that the two patterns of interference
fringes have different angle selectivity. Their having different
angle selectivity means that, as shown in a graph in FIG. 4 (where
.alpha. represents the angle of incidence of light and .eta.
represents diffraction efficiency), there is a difference in the
angle of incidence of light at which they exhibit high diffraction
efficiency. Thus, the holographic optical element 4, while
converting the wavefront with two roughly identical phase
functions, exhibits two different patterns of angle selectivity.
Accordingly, the two observation pupils E1 and E2 are formed at
spatially different locations with roughly equal optical
powers.
[0040] In the construction described above, when the observer moves
his or her eye 5 up or down, the observed image switches between
that of one of the two observation pupils E1 and E2 and that of the
other. The observer recognizes this switching as a change in the
size or position of the observed image the more distinctly the
greater the difference in location between the two observation
pupils E1 and E2 and the smaller the screen size of the LCD 3.
However, since the image produced by an MID typically has a viewing
angle of about 10.degree. to 40.degree., so long as condition (I)
is fulfilled, such a change is quite acceptable and does not unduly
degrade the quality of the observed image. On the other hand, with
respect to condition (II), how much different the angle selectivity
of the two patterns of interference fringes is from each other
affects how effectively the observation pupils E1 and E2 can be
enlarged. Specifically, if condition (II) is fulfilled, it is
possible to effectively enlarge the size of the observation pupils
E1 and E2 as a whole and thereby make the displayed image easier to
observe. Accordingly, fulfilling conditions (I) and (II) helps
enhance the quality of the displayed image and enlarge the
observation pupils E1 and E2 to make the displayed image easier to
observe.
[0041] Examples of the photosensitive material used to fabricate
the holographic optical element 4 include photopolymer, silver
halide, and bichromated gelatin. Among these materials,
photopolymer is particularly preferable, because it permits
fabrication by a dry process and thus at low costs, and because it
is excellently durable. Moreover, even without the use of silver
halide or bichromated gelatin, it is possible, as described above,
to enlarge the observation pupils E1 and E2 to make the displayed
image easier to observe. The composite hologram constituting the
holographic optical element 4 may be a multiple-layer hologram
fabricated by laying on each other two holograms each having a
pattern of interference fringes recorded thereon or a
multiple-exposure hologram fabricated by recording two patterns of
interference fringes on a single photosensitive material by
multiple exposure. Using a multiple-layer hologram fabricated by
laying on one another a plurality of holograms each having a
pattern of interference fringes recorded thereon results in high
diffraction efficiency. Accordingly, using a multiple-layer
hologram as the composite hologram in the holographic optical
element 4 helps make the displayed image brighter. On the other
hand, a multiple-exposure hologram fabricated by recording a
plurality of patterns of interference fringes on a single
photosensitive material by multiple exposure requires a smaller
amount of the photosensitive material and is easy to fabricate.
Accordingly, using a multiple-exposure hologram as the composite
hologram in the holographic optical element 4 helps reduce
costs.
[0042] Next, a method of fabricating a multiple-layer hologram for
use in the holographic optical element 4 will be described. As
shown in an enlarged view in FIG. 5, the multiple-layer hologram
described here has two holograms H1 and 12 laid on each other, and
the two holograms H1 and 12 have identical phase functions, which
represent how they convert the wavefront. FIG. 6 shows the optical
construction of a two-beam hologram exposure apparatus for
fabricating a multiple-layer hologram. Exposure is performed twice,
with virtual light source points arranged differently, namely P1
and Q1 as opposed to P2 and Q2, and with different types of
exposure optical system, namely G1 as opposed to G2, arranged
differently between when exposure is performed for the first and
second times. Specifically, the exposure optical systems G1 and G2
used here are so constructed that, even though exposure rays are
incident on the photopolymer at different angles of incidence
between when exposure is performed for the first and second times,
the exposure optical systems G1 and G2 produce patterns of
interference fringes having roughly identical phase functions.
Since exposure rays are incident at different angles of incidence
between when exposure is performed for the first and second times,
the holographic optical element 4 so fabricated exhibits two
different patterns of angle selectivity (i.e., two maximums in
diffraction efficiency) with respect to light of an equal
wavelength. The exposure optical systems G1 and G2 are separately
constructed to suit the respective sessions of exposure, and are
thus easy to design.
[0043] A multiple-layer hologram is fabricated in the following
manner. First, on the hologram substrate surface 6a of the prism 6,
a first photopolymer layer is fixed, By the use of the first
exposure optical system G1, a first session of exposure is
performed with two beams emanating from two virtual light source
points P1 and Q1. As a result, interference fringes are recorded as
a pattern of refractive index modulation on the first photopolymer
layer. The interference fringes recorded on the first photopolymer
layer are fixed by irradiation with UV (ultraviolet) rays and
baking to obtain the first hologram H1. Next, on the first hologram
H1, a second photopolymer layer is fixed. By the use of the second
optical system G2, a second session of exposure is performed with
two beams emanating from two virtual light source points P2 and Q2.
As a result, interference fringes are recorded as a pattern of
refractive index modulation on the second photopolymer layer. The
interference fringes recorded on the second photopolymer layer are
fixed by irradiation with UV rays and baking to obtain the second
hologram H2. In this way, the holographic optical element 4
composed of a multiple-layer hologram is fabricated (FIG. 5). Here,
the purpose of performing UV irradiation every time exposure is
performed is to irradiate each material with a sufficient amount of
UV radiation to perfectly fix the interference fringes. Thus, the
fabrication process may be simplified by omitting the UV
irradiation and baking after the first session of exposure and
performing them only once after the second session of exposure.
[0044] Next, a first method of fabricating a multiple-exposure
hologram for use in the holographic optical element 4 will be
described. FIG. 7 shows an outline of the overall construction of a
two-beam hologram exposure apparatus for fabricating a
multiple-exposure hologram. In FIG. 7, reference symbol S0
represents a laser light source, reference symbol m1 represents a
half mirror, reference symbols m2 and m3 are reflection mirrors,
reference symbols L1 and L2 represent objective lenses, and
reference numeral U1 represents an exposure unit. FIG. 8 shows the
exposure unit U1 constituting part of the exposure apparatus shown
in FIG. 7, and FIG. 9 shows the optical construction of the
exposure unit U1 as designed. In FIGS. 8 and 9, reference symbols
P, P1, P2, Q, Q1, and Q2 represent virtual light source points, and
reference symbols M1 and M2 represent axisymmetric exposure
mirrors.
[0045] As shown in FIG. 8, the prism 6 and the exposure mirrors M1
and M2 are fixed to the exposure unit U1 so that, as the exposure
unit U1 moves translationally, the prism 6 and the exposure mirrors
M1 and M2 move translationally together. The virtual light source
points P and Q formed by the objective lenses L1 and L2 are kept at
fixed locations, and therefore, as the exposure unit U1 moves
translationally, the relative positions of the virtual light source
points P and Q with respect to the prism 6 and the exposure mirrors
M1 and M2 vary. In FIG. 8, the optical paths indicated with solid
and broken lines respectively represent the exposure ray
arrangements before and after (i.e. at the times of the first and
second sessions of exposure) the prism 6 and the exposure mirrors
M1 and M2 actually move relative to the virtual light source points
P and Q, which are fixed. On the other hand, in FIG. 9, the optical
paths indicated with solid and broken lines respectively represent
the exposure ray arrangements before and after (i.e. at the times
of the first and second sessions of exposure) the virtual light
source points, namely P1 and Q1 as opposed to P2 and Q2, relatively
move with respect to the prism 6 and the exposure mirrors M1 and
M2, which are regarded as fixed here.
[0046] A multiple-exposure hologram is fabricated in the following
manner. First, a photopolymer layer is fixed on the hologram
substrate surface 6a of the prism 6, and the laser light source S0
shown in FIG. 7 is turned on. The exposure beam emanating from the
laser light source S0 is divided by the half mirror m1 into two
beam, of which one is incident on the objective lens L1 and the
other is reflected from the two reflection mirrors m2 and m3 and is
then incident on the objective lens L2. The objective lenses L1 and
L2 form, as shown in FIG. 8, virtual light source points P and Q,
respectively, inside the exposure unit U1. When a first session of
exposure is performed with two light beams (solid lines) emanating
from the two virtual light source points P and Q with the exposure
mirrors M1 and M2 placed at the locations indicated with solid
lines, interference fringes are recorded as a pattern of refractive
index modulation on the photopolymer layer. Next, the exposure unit
U1 is moved to the location indicated with broken lines. When a
second session of exposure is performed with two light beams
(broken lines) emanating from the two virtual light source points P
and Q with the exposure mirrors M1 and M2 placed at the locations
indicated with broken lines, interference fringes are recorded as a
pattern of refractive index modulation on the photopolymer layer.
The energy with which exposure is performed is set by appropriately
setting the output power of the laser light source and the duration
for which a shutter (not illustrated) is kept open. This applies
also to the other embodiments described later. When the patterns of
interference fringes recorded on the photopolymer layer are fixed
by UV radiation and baking, a composite hologram having two
patterns of interference fringes recorded thereon by exposure. i.e.
having interference fringe patterns doubly recorded thereon is
obtained. In the composite hologram thus obtained, the pattern of
interference fringes recorded in the first session of exposure and
the pattern of interference fringes recorded in the second session
of exposure have roughly identical phase functions.
[0047] In the above-described method of fabricating a
multiple-exposure hologram, the virtual light source points P and Q
do not move. This eliminates the need to use more than one exposure
optical system including exposure mirrors M1 and M2 and other
elements, and thus helps reduce fabrication costs. At the time of
designing, as shown in FIG. 9, the two points at which exposure
rays are incident are determined as the virtual light source points
P1 and P2. Then, the exposure mirrors M1 and M2 are so designed
that, for a given ray traveling between those two points, desired
imaging performance is obtained at two points corresponding to the
virtual light source points Q1 and Q2 and that the imaging
magnification is roughly unity. Since the two objective lenses L1
and L2 do not move relatively, the movement of the exposure unit U1
does not affect the imaging performance. Thus, the distance
traveled by the exposure unit U1 need not be controlled accurately,
and this makes it possible to perform multiple exposure with ease
and at low costs.
[0048] Next, a second method of fabricating a multiple-exposure
hologram for use in the holographic optical element 4 will be
described. FIG. 10 shows an outline of the overall construction of
a single-beam hologram exposure apparatus for fabricating a
multiple-exposure hologram, and FIG. 11 shows the optical
construction thereof as designed. In FIG. 10, reference symbol S0
represents a laser light source, reference symbol L3 represents an
objective lens, and reference symbol U2 represents an exposure
unit. In FIG. 11, reference numerals P1 and P2 represent virtual
light source points, and reference numerals N1, N2, and N3
represent axisymmetric exposure mirrors. The exposure apparatus
shown in FIGS. 10 and 11 is a modified version of the exposure
apparatus shown in FIGS. 7 to 9 which adopts single-beam exposure
recording. The three exposure mirror N1, N2, and N3 provided in the
exposure unit U2 makes single-beam exposure recording possible.
[0049] As shown in FIG. 10, the exposure beam emanating from the
laser light source S0 is incident on the objective lens L3. The
objective lens L3 forms, as shown in FIG. 11, a virtual light
source point P1. The exposure beam (solid lines) emanating from the
virtual light source point P1 passes through the prism 6,
irradiates the photopolymer layer on the hologram substrate 6a, and
is then reflected from the three exposure mirrors N1, N2, and N3
successively in this order. The beam so reflected then irradiates
the photopolymer layer on the hologram substrate 6a again. The beam
that irradiates the photopolymer layer first and the light that
irradiates it from the opposite direction after being reflected
from the exposure mirrors N1, N2, and N3 form interference fringes,
which are recorded as a pattern of refractive index modulation on
the photopolymer layer. In this way, the first session of exposure
is performed. Next, the exposure unit U2 is moved, and the second
session of exposure is performed, in a similar manner to the first
session of exposure, with the exposure beam (broken lines)
emanating from the virtual light source points P2. Here, as with
the first fabrication method, the exposure optical system is so
designed that the patterns of interference fringes recorded on the
composite hologram by exposure have roughly identical phase
functions. When the patterns of interference fringes recorded on
the photopolymer layer are fixed by UV irradiation and baking, a
composite hologram having two patterns of interference fringes
recorded thereon by exposure, i.e. having interference fringe
patterns doubly recorded thereon, is obtained. In this exposure
apparatus, the exposure optical system has a simple construction,
and therefore it is easy to make adjustments when exposure is
performed. Moreover, it is possible to alleviate the influence of
vibration wind and other factors that disturb the interference
fringes and thereby degrade optical performance.
[0050] Next, a third method of fabricating a multiple-exposure
hologram for use in the holographic optical element 4 will be
described. FIG. 12 shows an outline of the overall construction of
a two-beam hologram exposure apparatus for fabricating a
multiple-exposure color hologram. This exposure apparatus is a
modified version of the two-beam hologram exposure apparatus (FIG.
7) describe earlier which is adapted to cope with colors for
further enhanced information display. Thus, the exposure apparatus
here has the same construction as the two-beam hologram exposure
apparatus (FIG. 7) describe earlier except that laser light sources
S1, S2, and S3 for three primary colors, i.e., R (red), G (green),
and B (blue), are used instead of the laser light sources S0 and
that accordingly a reflection mirror m4 and half mirrors m5 and m6
are additionally provided. It is to be noted that full-color image
display is achieved by designing illumination and display to cope
with three primary colors, i.e., R, G, and B. However, depending on
what image to display, it is also possible to design illumination
and display to cope with two, or four or more, colors. In any such
case, exposure of a hologram is performed separately for each of
the plurality of colors that the hologram is designed to cope
with.
[0051] A multiple-exposure hologram is fabricated by performing the
first and second sessions of exposure, each separately for each of
the colors R, G, and B, by the use of the two-beam hologram
exposure apparatus (FIG. 7) described earlier. Specifically, the
fabrication process consists of, in order of occurrence, the first
session of exposure with an R exposure beam, the first session of
exposure with a G exposure beam, the first session of exposure with
a B exposure beam, moving the exposure unit U1, the second session
of exposure with an R exposure beam, the second session of exposure
with a G exposure beam, the second session of exposure with a B
exposure beam, UV irradiation, and baking. Exposure may be
performed simultaneously for all of the colors R, G, and B.
Specifically, the fabrication process then consists of, in order of
occurrence, the first session of exposure simultaneous for the
colors R, G, and B, moving the exposure unit U1, the second session
of exposure simultaneous for the colors R, G, and B, UV
irradiation, and baking. FIGS. 13A and 13B show the angle
selectivity and wavelength selectivity of the composite hologram
thus fabricated. FIG. 13A shows the angle selectivity (where a
represents the angle of incidence of light and .eta. represents
diffraction efficiency), and FIG. 13B shows the wavelength
selectivity (where .lambda. represents wavelength and .gamma.
represents the amount of reconstructed light).
[0052] As shown in FIG. 13B, the composite hologram obtained in
this way has three diffraction wavelength peaks corresponding to
the three primary color components R, G, and B. Thus, by using this
composite hologram as the holographic optical element 4, and in
addition using an LED 1 and an LCD 3 that can cope with three
colors R, G, and B, it is possible to realize a full-color image
display apparatus. For example, full-color display is achieved by
using as an illumination light source three LEDs 1 that
individually emit light corresponding to the three primary color
components R, G, and B, driving by a field sequential method an LCD
3 that forms images containing color information corresponding to
the three primary color components K, G, and B, and making the
three LEDs emit light one after another on a time division basis in
synchronism with the image signal of those images. When the second
session of exposure for a particular color is omitted, the
observation pupil E is not enlarged for the image light of that
color. This makes it possible to display an image with different
sizes of the observation pupil E for different colors.
[0053] The embodiments described above include the inventions (i)
to (v) having features as noted below, and, with those features, it
is possible to realize an inexpensive image display apparatus in
the shape of eyeglasses that forms an observation pupil that
permits easy observation of the displayed image.
[0054] (i) An image display apparatus provided with an image
display element for forming a two-dimensional image and an eyepiece
optical system including a reflective holographic optical element
for projecting the two-dimensional image with enlargement, wherein
the holographic optical element is a composite hologram having a
plurality of patterns of interference fringes composed of
interference fringes nonparallel to a hologram substrate surface,
has roughly identical phase functions and different angle
selectivity for at least two patterns of interference fringes, and
acts in such a way that image light having a predetermined
wavelength width emanating from the image display element forms a
plurality of observation pupils at spatially different
locations.
[0055] (ii) An image display apparatus as described in (i) above,
wherein the composite hologram is a multiple-layer hologram
fabricated by laying on one another a plurality of holograms each
having a pattern of interference fringes recorded thereon.
[0056] (iii) An image display apparatus as described in (i) above,
wherein the composite hologram is a multiple-exposure hologram
fabricated by recording a plurality of patterns of interference
fringes on a single photosensitive material by multiple
exposure.
[0057] (iv) An image display apparatus as described in one of (i),
(ii), and (iii) above, wherein the holographic optical element has
a nonaxisymmetric optical power.
[0058] (v) An image display apparatus as described in one of (i),
(ii), (iii), and (iv) above, wherein the image display element
forms images containing information of a plurality of colors, and
the holographic optical element has a plurality of diffraction
wavelength peaks corresponding to the different colors.
[0059] As described above, in an image display apparatus according
to the present invention, a holographic optical element forms a
plurality of observation pupils at spatially different locations,
and acts in such a way that, over the entire area in which image
light is incident, prescribed conditions are fulfilled for an
identical incident ray. This makes it possible to form at low costs
an observation pupil that permits easy observation of the displayed
image. Moreover, by using a multiple-layer hologram as the
holographic optical element, it is possible to make the displayed
image bright. Alternatively, by using a multiple-exposure hologram
as the holographic optical element, it is possible to reduce costs.
Moreover, by giving the holographic optical element an axisymmetric
optical power, it is possible to observe an image with aberrations
satisfactorily corrected for, and to contribute to miniaturization
of the image display apparatus. For further enhanced information
display, full-color display can be achieved by making an image
display element form images containing information of a plurality
of colors and giving the holographic optical element a plurality of
diffraction wavelength peaks corresponding to the different
colors.
* * * * *