U.S. patent number RE43,608 [Application Number 11/836,669] was granted by the patent office on 2012-08-28 for spatial light modulator imaging systems.
This patent grant is currently assigned to F. Poszat HU, LLC. Invention is credited to Jonathan R Hughes, Richard J Miller.
United States Patent |
RE43,608 |
Hughes , et al. |
August 28, 2012 |
**Please see images for:
( Certificate of Correction ) ** |
Spatial light modulator imaging systems
Abstract
A spatial light modulator imaging system comprises an
electrically addressed spatial light modulator (EASLM 4, 30) whose
optical image output is projected onto different areas of an
optically addressed spatial light modulator (OASLM, 6, 8, 31) in a
sequence. The OASLM carries electrodes which allow separate areas
to be selectively addressed by application of a voltage whilst
receiving light from the EASLM. The combined output from all areas
of the OASLM forms a visible image to an observer (11). When
illuminated by coherent light the OASLM may produce a holographic
image, otherwise incoherent light is used to provide a two
dimensional image. The OASLM in one example contains a layer of
nematic liquid crystal material between two cell walls both treated
with an alignment layer providing low tilt surface alignment that
is parallel in opposite direction; the product of layer thickness d
and material birefringence .DELTA.n approximately equals one
quarter of the wavelength .lamda. of read light (12, 37). Other
types of nematic devices may also be used, with cell parameters
arranged to give enhanced diffraction efficiency.
Inventors: |
Hughes; Jonathan R (Malvern,
GB), Miller; Richard J (Malvern, GB) |
Assignee: |
F. Poszat HU, LLC (Wilmington,
DE)
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Family
ID: |
9956276 |
Appl.
No.: |
11/836,669 |
Filed: |
August 9, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
10813299 |
Mar 31, 2004 |
6927748 |
Aug 9, 2005 |
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Foreign Application Priority Data
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Apr 5, 2003 [GB] |
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0307923.3 |
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Current U.S.
Class: |
345/32; 345/204;
349/25; 359/244; 359/259; 359/900; 345/87 |
Current CPC
Class: |
G02F
1/135 (20130101); G03H 1/2294 (20130101); G02F
1/1347 (20130101); G03H 2225/25 (20130101); G03H
2001/2271 (20130101); G03H 2225/60 (20130101); G03H
2001/2292 (20130101); G03H 2223/19 (20130101) |
Current International
Class: |
G09G
3/00 (20060101); G09G 5/00 (20060101); G09G
3/36 (20060101); G02F 1/03 (20060101); G02F
1/135 (20060101) |
Field of
Search: |
;359/15,17,22,244,245,259,272,292,900 ;353/30,31 ;349/25-30,249,201
;345/32,87,204 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2 330 471 |
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Apr 1999 |
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GB |
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2 350 962 |
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Dec 2000 |
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GB |
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2 350 963 |
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Dec 2000 |
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GB |
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WO 99/46768 |
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Sep 1999 |
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WO |
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00/40018 |
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Jul 2000 |
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WO |
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Other References
Cameron, C.D., et al.; "Computational Challenges of Emerging Novel
True 3D Holographic Displays"; SPIE Conference on Critical
Technologies for Future of Computer (San Diego, USA), Jul.-Aug.
2000; pp. 129-140. cited by other .
Stanley et al; "A Novel Electro-Optic Modulator System for the
Production of Dynamic Images From Giga-Pixel Computer Geneated
Holograms"; Proc SPIE, 2000, pp. 13-22, vol. 3956, Jan. 24-27,
2000. cited by other .
Jeon et al; "Image Tiling System Using Optically Addressed Spatial
Light Modulator for High-Resolution and Multiview 3-D Display";
Proc SPIE, 2000, pp. 165-176, vol. 3957, 42-27 Jan. 2000. cited by
other.
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Primary Examiner: Dinh; Jack
Attorney, Agent or Firm: Stolowitz Ford Cowger LLP
Claims
What is claimed is:
1. A spatial light modulator imaging system, comprising: at least
one electrically addressed spatial light modulator (EASLM); an
optically addressed spatial light modulator (OASLM) including a
layer of a nematic liquid crystal material contained between two
cell walls provided in parallel in opposite direction alignment,
the layer thickness d and the birefringence .DELTA.n at a
wavelength .lamda. approximately satisfy the equation
.DELTA.n.d=.lamda..[./4.]. .Iadd.4.Iaddend.; an optical system for
directing light from the EASLM onto the OASLM; a controller for
loading images on the EASLM then optically onto the OASLM; a
controller for applying write voltages to the OASLM simultaneously
with application of addressing light; and a read light source
providing coherent light of one or more wavelengths for
illuminating the OASLM to provide visible diffracted images,
wherein the OASLM has at least one electrode that is segmented into
a plurality of separately electrically addressable regions, and
wherein the controller is .[.adapted.]. .Iadd.configured
.Iaddend.to apply voltages to the electrode so as to address
different regions at different times, the arrangement being such
that a plurality of images are read into the EASLM and then onto
the OASLM at a rate sufficient to present a stable holographic
image to an observer.
2. A spatial light modulator imaging system comprising: at least
one electrically addressed spatial light modulator EASLM; a
monostable optically addressed spatial light modulator OASLM
arranged to receive addressing light from the EASLM through an
optical system; a controller for loading images onto the EASLM then
optically onto the OASLM; a controller for applying write voltages
to the OASLM simultaneously with application of addressing light;
and a read light for illuminating the OASLM to provide visible
images; wherein the OASLM has at least one electrode that is
segmented into a plurality of separately electrically addressable
regions, and wherein the controller is .[.adapted.].
.Iadd.configured .Iaddend.to apply write voltages to the electrode
so as to address different regions at different times, the
arrangement being such that a plurality of images are read into the
EASLM and thence onto the OASLM at a rate sufficient to present a
stable image to an observer.
3. The system of claim 2 wherein the read light is incoherent
light.
.[.4. The system of claim 2 wherein the read light is incoherent
light..].
5. The system of claim 2 wherein the OASLM comprises a layer of a
nematic liquid crystal material.
6. The system of claim 2 wherein the OASLM comprises a layer of
nematic liquid crystal material contained between two cell walls
provided with parallel in opposite direction alignment.
7. The system of claim 2 wherein the OASLM comprises a layer of a
nematic liquid crystal material contained between two cell walls
provided with parallel in opposite direction alignment with a
surface tilt of less than 10.degree..
8. The system of claim 2 wherein the OASLM comprises a layer of a
nematic liquid crystal material contained between two cell walls
provided with parallel in opposite direction alignment, the layer
thickness d and the birefringence .DELTA.n at a wavelength .lamda.
approximately satisfy the equation .DELTA.n.d=.lamda..[./4.].
.Iadd.4.Iaddend..
9. The system of claim 2 wherein the EASLM is a single EASLM whose
output is arranged to be scanned sequentially over different areas
of the OASLM.
10. The system of claim 2 wherein the OASLM is formed by a
plurality of single OASLMs arranged to be addressed in a sequence
by light from the EASLM.
11. The system of claim 2 wherein the controller .Iadd.for loading
images on the EASLM .Iaddend.contains computer generated
holographic images for providing a diffraction pattern to be loaded
into the EASLM and displayed to an observer as a three dimensional
image.
12. The system of claim 2 wherein the EASLM is an m by n array of
separately addressable EASLMs and the OASLM is an m-by-n array of
segments or separate OASLMs.
13. The system of claim 2 wherein the OASLM contains a layer of
nematic liquid crystal material arranged in a twisted configuration
(90.degree., 180.degree., 270.degree., 360.degree. of twist).
.Iadd.14. A spatial light modulator imaging system comprising: one
or more electrically addressed spatial light modulators (EASLM);
one or more optically addressed spatial light modulators (OASLM)
arranged to receive addressing light from the EASLM; and a
controller configured to apply write voltages to the OASLM
simultaneously with application of the addressing light wherein the
OASLM has at least one electrode that is segmented into a plurality
of separately electrically addressable regions, and wherein the
controller is further configured to apply write voltages to the
electrode so as to address different regions at different
times..Iaddend.
.Iadd.15. The spatial light modulator imaging system according to
claim 14 wherein the OASLM is configured to provide monostable
switching characteristics..Iaddend.
.Iadd.16. The spatial light modulator imaging system according to
claim 14 wherein the OASLM is configured to display two dimensional
images..Iaddend.
.Iadd.17. The spatial light modulator imaging system according to
claim 14 wherein the OASLM is configured to display three
dimensional computer generated holograms..Iaddend.
.Iadd.18. The spatial light modulator imaging system according to
claim 14 further comprising a relay lens array separating the EASLM
and the OASLM..Iaddend.
.Iadd.19. The spatial light modulator imaging system according to
claim 18 wherein the EASLM is comprised of an array of separate
EASLMs and the OASLM is comprised of an array of OASLM
segments..Iaddend.
.Iadd.20. The spatial light modulator imaging system according to
claim 14 wherein the OASLM is formed of separate OASLMs placed
together to form a large display..Iaddend.
.Iadd.21. A monostable optically addressed spatial light modulator
(OASLM) configured to receive addressing light from one or more
electrically addressed spatial light modulators (EASLM), wherein
the OASLM has at least one electrode that is segmented into a
plurality of separately electrically addressable regions, wherein a
controller is configured to apply voltages to the at least one
electrode so as to address different regions at different times,
and wherein the at least one electrode is configured to provide the
plurality of separately electrically addressable regions upon
receipt of the addressing light and according to an application of
the voltages to the at least one electrode..Iaddend.
.Iadd.22. The monostable optically addressed spatial light
modulator of claim 21 wherein the OASLM comprises a layer of a
nematic liquid crystal material..Iaddend.
.Iadd.23. The monostable optically addressed spatial light
modulator of claim 21 wherein the OASLM comprises a layer of
nematic liquid crystal material with alignment layers arranged in
anti-parallel alignment..Iaddend.
.Iadd.24. The monostable optically addressed spatial light
modulator of claim 23 wherein the layer of a nematic liquid crystal
material is contained between two cell walls..Iaddend.
.Iadd.25. The monostable optically addressed spatial light
modulator of claim 24 wherein the layer of a nematic liquid crystal
material includes a surface tilt of less than 10
degrees..Iaddend.
.Iadd.26. A method comprising: transmitting an addressing light
through an optical system; applying a write voltage, by a
controller, to at least one electrode provided in an optically
addressable spatial light modulator (OASLM), wherein the at least
one electrode is segmented into a plurality of separately
electrically addressable regions; addressing different regions of
the at least one electrode at different times according to the
application of the write voltage and a receipt of the addressing
light; and writing a successive array of images on the
OASLM..Iaddend.
.Iadd.27. The method according to claim 26 wherein the addressing
light is received by the OASLM simultaneously with the application
of the write voltage..Iaddend.
.Iadd.28. The method according to claim 26 further comprising:
transmitting a first addressing light from a first modulator that
alternately contains a positive image and a negative image; and
transmitting a second addressing light from a second modulator that
contains inverse images of the first modulator..Iaddend.
.Iadd.29. The method according to claim 26 further comprising
applying a balancing voltage to the OASLM when the write voltage is
not being applied to increase an amount of time the images may be
read..Iaddend.
.Iadd.30. The method according to claim 26 further comprising
alternately transmitting a positive image and a negative image onto
the OASLM..Iaddend.
Description
This application claims priority to GB Application No. 0307923.3,
filed 5 Apr. 2003. The entire contents of this application is
incorporated herein by reference.
TECHNICAL FIELD
This invention relates to spatial light modulator (SLM) imaging
systems that use both an electrically addressable spatial light
modulator (EASLM) and an optically addressable spatial light
modulator (OASLM) to provide visible images to an observer;
particularly systems displaying three dimensional images.
BACKGROUND
Re-configurable SLMs based on liquid crystal (and other types of)
devices are widely used for controlling and manipulating optical
beams. In diffractive mode they may be used for three dimensional
(3D) imaging [BROWN, C V and STANLEY, M, UK Patent Application
GB2330471, Production of Moving Images for Holography] and for
routing optical signals in telecommunications networks
The SLM modulates the complex amplitude of an incoming wave front
(i.e. changes its phase and/or amplitude), which causes it to
propagate in the desired manner. The SLM generally comprises a
liquid crystal panel containing a number of individually addressed
pixels, onto which a diffraction pattern or Computer Generated
Hologram (CGH) is written [CAMERON, C D et al, SPIE Conference on
Critical Technologies for the Future of Computing (San Diego, USA),
July-August 2000, Computational Challenges of Emerging Novel True
3D Holographic Displays].
CGH 3D display systems typically use a computer to generate and/or
store electronic copies of the hologram. This hologram is then
replayed on an SLM which is switched to modulate (in transmission
or reflection) light from a source which then passes through
suitable replay optics, thereby providing a visible
three-dimensional image to observers.
In one known system used in the production of three dimensional
(3D) images, a single EASLM is addressed to produce successive
different images which are imaged sequentially onto an OASLM
arranged in a matrix of segments which forms a complete display.
Once all the component images have been written to the OASLM a
complete image or pattern can be presented to an observer, e.g. by
illumination of the whole OASLM matrix by laser read light. This is
described in U.S. Pat. No. 6,437,919, WO-GB98/03097, GB2330471, and
has been described as Active Tiling.TM.. This system relies on high
speed switching in the EASLM and bistability in the OASLM material
to retain the switched image whilst the read light is applied, to
give a flicker free display.
Typically a SLM includes a layer of liquid crystal material
arranged between two electrode-bearing walls to form a liquid
crystal cell. The material is switched by application of an
electric field to the liquid crystal material, e.g. by electrical
waveforms applied to the electrodes.
A typical EASLM comprises a liquid crystal cell formed by two walls
enclosing a layer of nematic or smectic liquid crystal material.
Transparent electrode structures are formed as strips of row
electrodes on one wall and strips of column electrodes on the other
wall. Electrode intersections define pixels where the optical state
of the liquid crystal material is switched by application of an
electric voltage to appropriate row and column electrodes. The
electrodes receive electrical signals from driver circuits
controlled by a display controller. One known smectic EASLM uses an
integrated circuit backplane, and DC balance is achieved by
addressing to form a positive image followed by addressing to form
the inverse, i.e. a negative image.
A typical OASLM is basically similar to the EASLM but includes a
layer of a photosensitive material between electrodes on one wall
and a bistable ferroelectric liquid crystal material. In some
examples the electrodes are segmented so that electrical contact is
made separately to each segment; in this way an image may be
applied to more than one segment (commonly all segments) but a
voltage only applied to one segment to effect latching of the image
only at that one segment. The OASLM is addressed by application of
a voltage to the electrodes and simultaneous application of light
to selected parts of the photosensitive material. This combination
switches the liquid crystal material at illuminated parts whilst
other non-illuminated parts remain unswitched. A display is
viewable from the side of the OASLM remote from the photosensitive
layer.
A disadvantage of bistable ferro electric devices is their low
diffraction efficiency which results in a low level of image
brightness.
SUMMARY
The above problem is reduced, according to the present invention,
by using monostable nematic liquid crystal OASLM together with an
addressing technique that allows display of an image despite
switched pixels decaying back to an unswitched state.
According to this invention a spatial light modulator imaging
system comprises:
at least one electrically addressed spatial light modulator
EASLM;
a monostable optically addressed spatial light modulator OASLM;
arranged to receive addressing light from the EASLM through an
optical system;
a controller for loading images on the EASLM, then optically onto
the OASLM;
a controller for applying write voltages to the OASLM
simultaneously with application of addressing light;
a read light for illuminating the OASLM to provide visible
images;
the arrangement being such that a plurality of images are read into
the EASLM and thence onto the OASLM at a rate sufficient to present
a stable image to an observer.
The display is observed by reflection of light from the OASLM
directly or through a Fourier lens. The illuminating light may be
from a broadband source or a laser. In the latter case a
holographic display may be formed.
Preferably the OASLM is arranged to give maximum diffraction
efficiency; for example using properties of an anti parallel
aligned nematic layer with .DELTA.n.d=0.25.lamda. to give a good
diffraction grating (diffraction efficiency up to about 44%);
.DELTA.n is nematic birefringence, d is layer thickness, and
.lamda. is wavelength of read light, typically green (about 500 to
550 nano meters) for colour displays. Such an arrangement gives
enhanced holographic performance. In contrast a chiral smectic C
layer has a diffraction efficiency of less than 10%, typically
about 4%.
Other nematic alignments and .DELTA.n.d products may be used. For
example 180.degree. twist with parallel in the same direction
alignment and an amount of chiral dopant. Other nematic devices
that may be used include: twisted (90.degree.) nematic; super
twisted (270.degree.) nematic; n-cell (360.degree.) twist; hybrid
aligned nematic (HAN) cells with a planar alignment on one wall and
homeotropic alignment (90.degree. to wall) on the other wall.
Pleochroic dyes may be incorporated in small amounts to absorb
stray light.
The system may have a single EASLM addressing a single OASLM or
OASLM segment, or addressing a plurality of OASLMs or OASLM
segments. The system may have a plurality of EASLMs and OASLMs or
OASLM segments; alternatively a plurality of EASLMs addressing a
single OASLM or OASLM segment.
The system may have two EASLMs associated with a segmented OASLM
arranged so that one EASLM contains alternatively a positive image
and a negative image whilst the other EASLM contains the inverse.
This allows writing of successive positive images onto the OASLM
without dead periods during the time negative images are on the
EASLMs to give DC balance therein if such DC balance is
required.
The system may display monochrome information, or colour
information in a two or three or four frame sequential manner. For
a colour 3-D display, at least three different wavelength lasers
are used to illuminate the OASLM sequentially each wavelength being
associated with one of three different frames of images. For some
displays, two colours may provide sufficient information; this
requires only two frames of different colours.
A Fourier transform lens may be arranged between the OASLM and an
observer.
The OASLM may be a single large cell with segmented electrodes
whereby a voltage may be applied independently to any one area of
the liquid crystal material between opposing segment electrodes.
Alternatively, the OASLM may be a single large cell with a single
electrode in combination with an optical shutter. Another OASLM may
be formed by a mosaic of smaller OASLMs connected together. The
OASLM may be arranged in a matrix of m.times.n segments or
independent smaller OASLMs, where m and n are the number of
elements in rows and columns and where m and n may have the same or
different values.
When not receiving information with a WRITE voltage or light
pattern, the OASLM may receive a DC or AC balancing voltage which
may also assist in increasing the decay time of the nematic
material. For some displays, it is desirable to increase the decay
time of the nematic material then switch off rapidly, for example
between frames in frame sequential colour displays. Such effect may
be obtained by a holding voltage near a threshold to lengthen decay
followed by removal of voltage to affect a quick switch to OFF.
Alternatively, a two-frequency type of material may be used in a
twisted nematic arrangement. In such a material the dielectric
anisotropy is negative above a critical frequency and positive
below that frequency. This allows use of a high frequency signal to
obtain a quick turn OFF and lower frequency to turn ON as
normal.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described, by way of example only, with
reference to the accompanying drawings in which:
FIG. 1 shows schematically an imaging system having a single EASLM
imaged onto a large OASLM;
FIG. 2 shows schematically a cross section of an OASLM suitable for
use in the system of FIG. 1;
FIG. 3 is one timing diagram showing control of the system of FIG.
1; and
FIG. 4 is an alternative timing diagram for use in frame sequential
colour displays,
FIG. 5 shows an alternative system to FIG. 1 having an array of
EASLMs imaged onto a large OASLM having an array of segments, each
segment corresponding to one of the EASLMs;
DESCRIPTION OF EMBODIMENTS
Systems like that shown in FIG. 1 have been termed Active
Tiling.TM. and described in more detail in patent numbers U.S. Pat.
No. 6,437,919, WO-GB98/03097, and GB2330471. The system may be used
for large area two-dimensional displays, or for three dimensional
holographic image displays. Such holographic displays may be the
so-called computer generated holograms (CGHs). Prior art systems
employ bistable ferro electric OASLMs which have a low diffraction
efficiency but have advantages of high speed and bistability.
As shown in FIG. 1, a reconfigurable holographic display comprises
a light source 1 whose output 2 is directed through a lens 3 onto
an electrically addressable spatial light modulator (EASLM) 4. This
EASLM 4 may be a liquid crystal modulator in which a layer of
smectic liquid crystal material is held between two glass walls.
Column electrodes on one wall and row or line electrodes on the
other wall form a matrix of addressable elements or pixels at
electrode intersections. When a voltage is applied to a pixel, the
liquid crystal material rotates under the applied electric field to
modulate light transmission. One suitable EASLM uses known active
matrix addressing to obtain high switching speeds; such devices are
obtainable from e.g. DISPLAY-TECH INC or MICROVUE LTD. Other
devices using smectic, nematic, or cholesteric material may be
used. Other types of EASLM that may be used include silicon active
backplane devices, and micro mirror devices.
In front of the EASLM 4 is an optical arrangement 5, an optically
addressable spatial light modulator (OASLM) 6, and a further lens
7. The optical arrangement may be a micro lens array for focussing
multiple images of the EASLM 4 onto a corresponding part of the
OASLM 6, and may incorporate optional shutters for controlling
light onto a part of the OASLM 6 in a sequence. The OASLM 6
(described in more detail later) comprises a layer of a nematic
liquid crystal material between two glass walls. Both walls carry
transparent segment electrodes and one wall carries a layer of
photosensitive amorphous silicon. As shown the OASLM 6 is formed by
twenty five separate segments 8 each one being independently
addressable and arranged in a 5.times.5 matrix, other size matrices
can be formed.
In one embodiment, an image to be written into one segment 8 of the
OASLM is loaded into the EASLM and many copies are optically tiled
onto the complete OASLM. The segment 8 required to be addressed has
suitable voltage applied thereto which causes its photosensitive
layer to react to incident light. This incident light modulates the
applied voltage which in turn modulates locally the director
alignment of nematic material across the segment. As a result of
this modulated alignment the reflective (or transmissive)
properties of the nematic layer are also modulated. Thus when
illuminated by a read-light source 10 an image is visible at the
segment. Successive segments are illuminated by an appropriate
image from the EASLM.
By this means a large pattern formed of many separate sub images
can be formed on the OASLM 6. Images may be observed either by
reflection of light 12 from or by transmission of light through the
OASLM 6. Thus a pattern of light from the EASLM 4 is transmitted
from the OASLM 6 to an observer 11. For a monochrome display a
single light 10, 12 is used. For a colour display, three different
light sources may be used, or a single light source with three
different switchable colour filters used.
Alternatively the OASLM may be formed with a single continuous
sheet electrode on each cell wall. Such an arrangement requires the
optical arrangement 5 to incorporate an array of shutters which
direct light from the EASLM to one segment area 8 of the OASLM at a
time.
In another embodiment, the OASLM is formed of several separate
OASLMs placed together to form a large display.
A computer 13 controls the signals applied to the EASLM 4 and OASLM
6 and contains electronic copies of images to be displayed. The
images to be displayed may be two dimensional images, or be
diffraction patterns such as computer generated holograms (CGH) in
a known manner for providing three dimensional or quasi three
dimensional, or auto stereo or horizontal parallax displays. Such
CGH may be generated in the computer 13 or generated externally and
stored within the computer for display as required.
A description of EASLMs and OASLMs used with holographic displays
is described in patent application EP-1064651, PCT WO-00/2350472,
GB98/04996, and GB02/29154.0 (P7224).
The present invention uses particular mono stable switching
characteristics of the OASLM, namely birefringence (.DELTA.n) layer
thickness (d)=0.25.lamda. (.lamda.=wavelength) needed to give a
good diffraction grating. Therefore the construction of the OASLM
will be described in more detail as follows:
The structure of an OASLM is illustrated in FIG. 2. From left to
right in FIG. 2, the construction is as follows; a first glass
layer 15, an indium tin oxide layer 16 which forms a first
transparent electrode, a silicon photosensor layer 17, an optional
light blocking layer 18, an optional mirror 19, a first alignment
layer 20 which may be formed by brushing a polyimide layer, a
liquid crystal (LC) layer 21, a second alignment layer 22 a second
indium tin oxide electrode layer 23 which may be connected to
earth, a second glass layer 24, and an optional polariser 27. A
voltage source 25 is coupled to the two electrodes 16, 23 in order
to control the switching of the OASLM 6.
For an OASLM having enhanced diffraction efficiency a Freedericksz
type cell is used. This comprises a layer of nematic liquid crystal
material with alignment layers arranged in anti-parallel alignment
and with low tilt, e.g. less than 10.degree. surface tilt.
The thickness of the layer 21 is important; for maximum diffraction
efficiency the .DELTA.n.d product should be close to .lamda./4,
where .DELTA.n is the material birefringence, d layer thickness and
.lamda. the wavelength. The .lamda./4 figure arises because of a
double passage of read light through the liquid crystal layer 21.
For a monochrome display .lamda. is the wavelength of the read
light 12. For a full colour display .lamda. is usually about
500-550-nm i.e. in the mid range of the red, green and blue
wavelengths. Higher pretilts may be used, e.g. above about
10.degree., in which case the layer thickness may be increased to
compensate for the reduced retardation.
For an OASLM operating with a twisted nematic type cell effect (a
90.degree. twist), then the alignment layers 20, 22 are arranged
orthogonal and the polariser 27 aligned parallel (or orthogonal) to
the alignment on the adjacent layer 22. For other cell types, the
alignment is changed in an appropriate manner. For example a n-cell
may have parallel in the same direction alignment with a small
amount, <1% of a chiral material such as CB15 (Merck material).
Other types of liquid crystal devices may be used; for example STN
with 270.degree. of twist, and hybrid-aligned nematic (HAN) with
planar and homeotropic aligned surfaces, and other device
types.
One suitable liquid crystal material is ZLI-2293 (a Merck GmbH
material)
The junction between the silicon 17 and electrode layer 16 acts as
a diode 26; when a voltage of a first positive polarity is applied
between the electrodes this diode 26 is forward biased and most of
the voltage will be dropped across the LC layer 21, whilst when a
voltage of a second, negative polarity is applied to the
electrodes, most of the voltage will be dropped across the silicon
layer 17 unless write light 9 is applied in which case the voltage
will be dropped across the LC layer 21. The bias of the second
polarity is referred to as the "photosensitive directions". When
the bias is in the photosensitive direction and with no
illumination, the voltage appearing across the LC layer 7,
V.sub.lc, is given by the capacitive division of the total voltage
appearing across the OASLM 6: V.sub.lc=C.sub.Si/(C.sub.lc+C.sub.Si)
where C.sub.Si and C.sub.lc are the capacitances of the silicon and
LC layers respectively. As charge is generated in the Si layer, so
the voltage across the LC rises.
In the ideal case a Schottky barrier is formed in the OASLM by the
silicon and indium-tin-oxide (ITO) transparent electrode junction
26. This gives behaviour some way between that of a photodiode and
a photoconductor. If ohmic contacts are made then photoconductor
behaviour results. The major problem with a pure photoconductor is
the dark leakage current which is not sufficiently low to keep the
voltage from dropping across the LC in a non-illuminated addressed
state. A photodiode requires the deposition of p-doped, intrinsic
and n-doped Si and is a complicated process. For a photodiode 26
under reverse bias, when a photon is absorbed to produce an
electron-hole pair in the Si, the hole and electron are separated
and drift to the contacts. The blocking contacts stop the carriers
so that once they are collected the response is complete. The
photocurrent varies linearly with the light intensity over a wide
range of intensities because one electron-hole pair is collected
for each absorbed photon.
With the application of a positive applied voltage the photodiode
is forward biased so that all of the voltage should drop across the
LC. The presence of a write light 12 should not affect the state of
the LC 21 significantly, with a positive voltage applied. When a
negative applied voltage is applied, the photodiode 26 is reverse
biased, blocking the current, so that ideally the voltage across
the LC 21 is unchanged. When a write light 12 illuminates the
photodiode 26 a photocurrent charges the LC 21 to a negative
voltage and causes switching. This voltage is maintained across the
LC 21 until the drive voltage goes positive again.
The behaviour of the liquid crystal material 21 in response to an
electrical signal, the combination of received light induced
voltage and an applied voltage, is seen in FIG. 3. During
application of a voltage, visibility of the pattern increases up to
a maximum value representing a fully switched ON state. On removal
of the voltage, the material 21 returns to its unswitched or OFF
state where no pattern is visible. The time to switch to ON depends
upon material 21 characteristics and applied voltage level. The
time to decay OFF depends upon material 21 characteristics of
viscosity and layer thickness. For some materials the decay time
can be controlled by application of a small voltage.
FIG. 3 shows timing information for switching "N" different
segments of a layer 21 for a monochrome display. Illustrated are:
activity at the EASLM 4; voltage applied to the OASLM 6 by the
controller 13; optical response at segments 1, 2, and N in a first
frame time; write light onto the OASLM for each segment 1 to N;
read light for each segment 1 to N; time to address and read
segments 1 to N; time to address all segment, in a first frame time
and the start of frame 2. For this monochrome case, the addressing
continues without a break between addressing segment N at the end
of a first Frame n and addressing (for the second time) segment 1
at the beginning of a second frame Frame n+1.
The sequence of events shown in FIG. 3 is as follows:
First time period t1. Positive image information for displaying on
a first segment is loaded onto the EASLM 4 from the controller 13;
no other activity takes place except for decay of any previously
displayed information.
Time period t2. The positive image on the EASLM is projected onto
segment 1 of the OASLM 6 by operation of the write light; a
bi-polar voltage pulse is applied to segment 1 which causes
addressing of the liquid crystal material in segment 1 by the
combined action of applied voltage and light, this is shown by the
OASLM switch optical response rising from zero to its maximum
value. The voltages may be a series of bipolar pulses that
constitute a short square wave as long as the DC balance
requirement is met; other AC signals may also be used.
Time periods t3, t4. The read light 10,12 is applied during both
periods and an observer can see the positive image (or, for
holographic viewing, a diffraction pattern) written onto segment 1
of the OASLM; a negative image is loaded into the EASLM for t3 and
then held for t4 to provide a DC balance (net zero voltage) on the
EASLM. The end of period t4 marks the end of addressing segment
1.
The above activities are then repeated for segments 2 to N in turn.
The end of addressing segment N marks the end of frame 1 and the
whole of the OASLM has been addressed. The next frame is then
addressed with the same or amended information as necessary.
It can be seen that light from segment 1 has decayed to zero before
segment N is addressed. This is not a problem providing the refresh
rate (number of time that a given segment is addressed per second)
is above about 25 frames/second thanks to retinal persistence of
observers, and providing that the light output from each segment
(area under OASLM curve in FIG. 3) is large enough.
Colour displays may be achieved by the known technique of field
sequential display. In this a complete frame is formed by display
of a first sub-frame of a first colour, followed by a second and
then a third sub-frame of different colours.
FIG. 4 shows a timing information for such a colour system. This is
similar to that of FIG. 3 in that information is written to and
then read from segment 1 followed by segment 2 etc. A difference is
that a first colour is written to and read from segments 1 to N in
a first sub-frame, with a delay between sub-frame 1 ending and
sub-frame 2 starting. This delay is to allow all segments to decay
to their OFF state before a new colour is applied. The same delay
occurs between all sub-frames with their associated colour.
Another difference between the systems of FIGS. 3 and 4 is that
FIG. 4 uses a different EASLM. In this EASLM both positive and
negative images are written into a buffer in the EASLM and
projected alternately in adjacent time slots. Such loading takes
place whilst an earlier negative image is projected by the EASLM
(but not written to the OASLM). This means that e.g. second image
(image 2) is being loaded into the buffer of the EASLM while the
positive then the negative first image (image 1) is displayed on
the EASLM.
The EASLM used in FIG. 3 may also be used to display a frame
sequential colour display. Similarly the EASLM used in FIG. 4 may
be used to provide a monochrome display.
Calculations may be made for the relative light outputs from a
conventional bistable ferro electric OASLM output and that of a
field sequential display as in FIG. 4. Comparison has been made for
a FLCD and a Freedericksz(nematic) cell both of layer thickness 1.5
.mu.m, same frame time of 5555 .mu.s, same write time of 100 .mu.s,
same number of segment N=25. The calculated brightness of an FLCD
is 6 units and that of a Freedericksz cell is 73 units. This shows
that despite monostable operation the nematic cell's much improved
diffraction efficiency can give improved brightness for a
holographic display.
FIG. 5 shows a variation on the system of FIG. 1 and has been
termed Direct Tiling. This FIG. 5 system uses a 3.times.3 array of
EASLMs 30 and a 3.times.3 array of OASLM segments 31 separated by a
relay lens array 32; each EASLM 30 and the OASLM 31 are similar to
those in FIG. 4. Collimated light 33 from a light source 34 and
collimating lens 35 illuminates each EASLM 30. A read light source
36 illuminates 37 the OASLM 31 via a polarising beam splitter 38. A
Fourier lens 39 focuses images from the OASLM 31 to an observer 11.
Control of voltage and images to the EASLMs 30 is from a controller
40 which also applies voltages to the OASLM 31. The OASLM may have
segment electrodes as in FIGS. 1, 2, or a single electrode on each
side. A 3-D display is observed by reflection of coherent light 37
from the read light source 36 off the front face of the OASLM 31. A
benefit of the system of FIG. 5 is the much reduced frame time
compared with that in FIG. 3.
* * * * *