U.S. patent application number 10/134727 was filed with the patent office on 2002-11-21 for achromatic compound retarder.
This patent application is currently assigned to ColorLink, Inc.. Invention is credited to Chen, Jianmin, Johnson, Kristina M., Robinson, Michael G., Sharp, Gary D..
Application Number | 20020171793 10/134727 |
Document ID | / |
Family ID | 46276586 |
Filed Date | 2002-11-21 |
United States Patent
Application |
20020171793 |
Kind Code |
A1 |
Sharp, Gary D. ; et
al. |
November 21, 2002 |
Achromatic compound retarder
Abstract
This invention provides achromatic compound retarders,
achromatic polarization switches, and achromatic shutters using the
liquid crystal compound retarders. It further provides achromatic
variable retardance smectic and nematic liquid crystal retarders.
The achromatic compound retarders according to the invention are
used to create achromatic inverters for display applications. The
display comprises one or more retarders having in-plane retardance
and in-plane orientation, at least one of the retarders being an
actively controlled liquid crystal retarder, and a ferroelectric
liquid crystal display, wherein the one or more retarders work in
combination with the ferroelectric liquid crystal display to
provide four states of brightness.
Inventors: |
Sharp, Gary D.; (Boulder,
CO) ; Johnson, Kristina M.; (Longmont, CO) ;
Robinson, Michael G.; (Boulder, CO) ; Chen,
Jianmin; (Boulder, CO) |
Correspondence
Address: |
FLESHNER & KIM, LLP
P.O. Box 221200
Chantilly
VA
20153-1200
US
|
Assignee: |
ColorLink, Inc.
|
Family ID: |
46276586 |
Appl. No.: |
10/134727 |
Filed: |
April 30, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10134727 |
Apr 30, 2002 |
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09466053 |
Dec 17, 1999 |
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6380997 |
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09466053 |
Dec 17, 1999 |
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09215208 |
Dec 18, 1998 |
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6078374 |
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09215208 |
Dec 18, 1998 |
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08901837 |
Jul 28, 1997 |
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6046786 |
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08901837 |
Jul 28, 1997 |
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08419593 |
Apr 7, 1995 |
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5658490 |
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60113005 |
Dec 18, 1998 |
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60121494 |
Feb 24, 1999 |
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60134535 |
May 17, 1999 |
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Current U.S.
Class: |
349/117 |
Current CPC
Class: |
G02F 2413/02 20130101;
G02F 2413/03 20130101; G02F 2413/07 20130101; C09K 19/0225
20130101; G02F 2413/08 20130101; G02F 1/141 20130101; G02F 2413/01
20130101; G02F 2203/04 20130101; G02F 1/133638 20210101; G02F
2203/62 20130101; G02F 1/13471 20130101 |
Class at
Publication: |
349/117 |
International
Class: |
G02F 001/1335 |
Claims
What is claimed is:
1. A display, comprising: one or more retarders having in-plane
retardance and in-plane orientation, at least one of the retarders
being an actively controlled liquid crystal retarder; and a
ferroelectric liquid crystal display, wherein the one or more
retarders work in combination with the ferroelectric liquid crystal
display to provide four states of brightness.
2. The display according to claim 1, wherein the display is a
reflective display and the one or more retarders are disposed on a
light input side of the ferroelectric liquid crystal display.
3. The display according to claim 1, wherein the display is a
transmissive display and the one or more retarders are disposed on
a light input side of the ferroelectric liquid crystal display.
4. The display according to claim 3, further comprising one or more
retarders having in-plane retardance and in-plane orientation
disposed on a light output side of the ferroelectric liquid crystal
display, at least one of the retarders being an active
retarder.
5. The display according to claim 2, further comprising a linear
polarizer disposed on a light input side of the one or more
retarders.
6. The display according to claim 2, further comprising a
beamsplitter disposed on a light input side of the one or more
retarders.
7. The display according to claim 3, further comprising a linear
polarizer disposed on a light input side of the one or more
retarders.
8. The display according to claim 4, further comprising a linear
polarizer disposed on a light output side of the one or more
retarders disposed on a light output side of the ferroelectric
liquid crystal display.
9. The display according to claim 1, wherein the at least one
actively controlled liquid crystal retarder comprises at least one
smectic liquid crystal retarder.
10. The display according to claim 1, wherein the at least one
actively controlled liquid crystal retarder comprises at least one
nematic liquid crystal retarder.
11. The display according to claim 10, wherein the at least one
nematic liquid crystal retarder comprises at least one pi-cell.
12. The display according to claim 10, wherein the at least one
nematic liquid crystal retarder comprises two pi-celsl, the two
pi-cells having parallel orientations.
13. The display according to claim 1, wherein the one or more
retarders further comprises a passive retarder.
14. The display according to claim 2, wherein the one or more
retarders further comprises a passive retarder.
15. The display according to claim 3, wherein the one or more
retarders further comprises a passive retarder.
16. The display according to claim 4, wherein one or more retarders
disposed on a light output side of the ferroelectric liquid crystal
display further comprises a passive retarder.
17. The display according to claim 2, wherein the actively
controlled liquid crystal retarder is a half-wave retarder and the
ferroelectric liquid crystal display comprises a quarter-wave
retarders.
18. The display according to claim 4, wherein each of the actively
controlled liquid crystal retarders is a half-wave retarder and the
ferroelectric liquid crystal display comprises a half-wave
retarder.
19. The display according to claim 10, wherein the at least one
nematic liquid crystal retarder is a half-wave retarder and the
ferroelectric liquid crystal display comprises a quarter-wave
retarder.
20. The display according to claim 10, wherein a passive retarder
is disposed between the at least one nematic liquid crystal
retarder the ferroelectric liquid crystal display.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 5 09/215,208, filed Dec. 18, 1998, which is a
continuation-in-part of U.S. patent application Ser. No.
08/901,837, filed Jul. 28, 1997, which is a continuation of U.S.
patent application Ser. No. 08/419,593, filed Apr. 7, 1995 (U.S.
Pat. No. 5,658,490), both of which are herein incorporated by
reference in their entirety. This application also claims priority
from U.S Provisional Applications Nos. 60/113,005, filed Dec. 18,
1998, and 60/121,494, filed Feb. 24, 1999.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to compound
retarders. More specifically, the present invention is directed to
the use in display devices of achromatic compound retarders that
exhibit an achromatic composite optic axis orientation and/or an
achromatic composite compound retardance at each of at least two
composite retarder orientation states. Further, the present
invention is directed to the use of such achromatic compound
retarders to create achromatic inverters for display
applications.
[0004] 2. Background of the Related Art
[0005] Liquid crystal retarders are increasingly utilized within
optical devices such as tunable filters, amplitude modulators and
light shutters. Planar aligned smectic liquid crystal devices
function as rotative waveplates wherein application of an electric
field rotates the orientation of the optic axis but does not vary
the birefringence. In contrast, homeotropically aligned smectic
liquid crystals, homogeneous aligned nematic devices, and nematic
pi-cells function as variable retarders, wherein application of an
electric field varies the birefringence. Chromaticity is a property
of birefringent elements, both passive and active liquid crystals.
There are two main components to chromaticity: (1) dispersion,
which is the change in the birefringence (.DELTA.n) with wavelength
.lambda.; and (2) the explicit dependence of retardance on
1/.lambda. due to the wavelength dependent optical pathlength. Both
components contribute to increased birefringence with decreased
wavelength. A birefringent material having a particular retardance
at a design wavelength has higher retardance at shorter wavelengths
and lower retardance at longer wavelengths. Chromaticity places
limitations on the spectral operating range of birefringent optical
devices.
[0006] Chromaticity compensation for passive retarders was
addressed by S. Pancharatnam, Proc. Indian Acad. Sci. A41, 137
[1955], and by A. M. Title, Appl. Opt. 14, 229 [1975], both of
which are herein incorporated by reference in their entirety. The
wavelength dependence of passive birefringent materials can be
reduced by replacing single retarder compound retarders. The
principle behind an achromatic compound retarder is that a stack of
waveplates with proper retardance and relative orientation can be
selected to produce a structure which behaves as a pure retarder
with wavelength indensitive retardance. Pancharatnam showed, using
the Poincare sphere and spherical trigonometry, that such a device
can be implemented using a minimum of three films of identical
retarder material. A Jones calculus analysis by Title (supra)
verified the condition imposed on the structure in order to achieve
this result: (1) the requirement that the composite structure
behave as a pure retarder (no rotation) forces the input and output
retarders to be oriented parallel and to have equal retardance; and
(2) first-order stability of the compound retarder optic axis and
retardance with respect to wavelength requires that the central
retarder be a half-wave plate. These conditions yield design
equations that determine the retardance of the external elements
and their orientation relative to the central retarder for a
particular achromatic retardance. Because these design equations
specify a unique orientation of the central retarder and a unique
retardance for the external retarders, they have never been applied
to active liquid crystal devices and the problem of active retarder
chromaticity remains.
[0007] For the specific example of an achromatic half-wave
retarder, the design equations dictate that the external retarders
are also half-wave plates and that the orientation of the external
retarders relative to the central retarder is .pi./3. By
mechanically rotating the entire structure, wavelength insensitive
polarization modulation is feasible. Furthermore, Title showed that
the compound half-wave retarder can be halved, and one section
mechanically rotated with respect to the other half to achieve
achromatic variable retardance. Electromechanical rotation of such
compound half-wave retarders has been used extensively to tune
polarization interference filters for astronomical imaging
spectrometers.
[0008] The primary application of ferroelectric liquid crystals
(FLCs) has been shutters and arrays of shutters. In the current
art, on- and off-states of an FLC shutter (FIG. 1) are generated by
reorienting the optic axis of FLC retarder 10 between .pi./4 and 0
with respect to bounding crossed or parallel polarizers 20 and 22.
In the off-state, x-polarized light is not rotated by the liquid
crystal cell and is blocked by the exit polarizer. In the on-state,
the polarization is rotated 90.degree. and is therefore transmitted
by the exit polarizer.
[0009] For maximum intensity modulation, the cell gap is selected
to yield a half-wave retardance at the appropriate design
wavelength. The on-state transmission of x-polarized light is
theoretically unity at the design wavelength, neglecting
absorption, reflection and scattering losses. At other wavelengths
the transmission decreases. The ideal transmission function for an
FLC shutter as in FIG. 1 is given by 1 T = 1 - sin 2 / 2 ON ( = / 4
) 0 OFF ( = 0 ) ( 1 )
[0010] where .delta. is the deviation from half-wave retardance
with wavelength. This expression indicates a second-order
dependence of transmission loss on .delta.. The off-state
transmission is in principle zero, but in practice it is typically
limited to less than 1000:1 due to depolarization by defects, the
existence of multiple domains having different alignments, and
fluctuations in the tilt-angle with temperature.
[0011] High transmission through FLC shutters over broad wavelength
bands is feasible for devices of zero-order retardance, but it is
ultimately limited by the inverse-wavelength dependence of
retardation and the rather large birefringence dispersion of liquid
crystal materials. For instance, a visible FLC shutter device that
equalizes on-state loss at 400 nm and 700 nm requires a half-wave
retarder centered at 480 nm. A zero-order FLC device with this
retardance, using typical FLC birefringence data, has a thickness
of roughly 1.3 microns. The transmission loss at the extreme
wavelengths, due to the departure from half-wave retardance, is
approximately 40%. This significantly limits the brightness of FLC
displays and the operating band of FLC shutters and light
modulators. In systems incorporating multiple FLC devices, such as
tunable optical filters or field-sequential display color,
shutters, this source of light loss can have a devastating impact
on overall throughput and spectral purity.
[0012] The above references are incorporated by reference herein
where appropriate for appropriate teachings of additional or
alternative details, features and/or technical background.
SUMMARY OF THE INVENTION
[0013] This invention provides achromatic compound retarders,
achromatic polarization switches, and achromatic shutters using the
achromatic compound retarders. It further provides achromatic
variable retarders utilizing smectic liquid crystals. An achromatic
shutter according to this invention is demonstrated which provides
excellent on-state transmission over the entire visible,
.gtoreq.94% from 400 nm to 700 nm after normalization for polarizer
loss, and high contrast, 1000:1 from 450 nm to 650 nm.
[0014] One embodiment of the achromatic compound retarder of this
invention comprises a central rotatable smectic liquid crystal
half-wave retarder and two external passive retarders positioned in
series with and on either side of the liquid crystal retarder. The
external retarders are equal in retardance and oriented parallel to
each other. Design equations determine the retardance of the
external elements and their orientation relative to the central
retarder to obtain a particular retardance for the compound
structure. A reflective version of the achromatic compound retarder
described above is constructed with a smectic liquid crystal
quarter-wave retarder positioned between a single passive retarder
and a reflector.
[0015] In the achromatic compound retarders of this invention there
is, in general, an orientation of the central retarder for which
the structure has maximum achromaticity in both orientation and
retardance. Important aspects of this invention are the discoveries
that (1) the composite retardance at the design wavelength does not
change when the optic axis orientation of the central retarder is
changed and (2) there are optic axis orientations of the central
retarder for which the optic axis orientation of the compound
retarder is stable (achromatic) even though the composite
retardance is not achromatic.
[0016] The central retarder may comprise a liquid crystal retarder,
as described above. In the case of a smectic liquid crystal cell,
application of an electric field rotates the optic axis between two
or more orientations. One of the orientations provides maximum
achromaticity of the compound retardance. As discussed above, there
is also at least one other optic axis orientation for which the
optic axis of the compound retarder is achromatic, even though the
composite retardance is not. Furthermore, the composite retardance
at the design wavelength does not change when the optic axis
orientation of the smectic liquid crystal cell is changed.
[0017] The central retarder may also comprise a spatially switched
planar-aligned passive retarder, in which the orientation of the
optic axis varies as a function of position on the spatially
switched passive retarder. The spatially switched passive retarder
has at least two optic axis orientations states, with one of the
orientations causing the retardance of the compound retarder to be
substantially achromatic, and the second orientation causing the
optic axis orientation of the compound retarder to be substantially
achromatic, even though the composite retardance may not be.
[0018] The achromatic properties discussed above are utilized in
the achromatic polarization switch of this invention, comprising a
linear polarizer and the compound achromatic retarder, and in the
achromatic shutter of this invention, comprising the compound
achromatic retarder positioned between a pair of polarizers. In one
optic axis orientation state of the central retarder (the
"ON-state") the compound retarder is achromatic and in a second
optic axis orientation state of the central retarder (the
"OFF-state") the compound retarder is oriented parallel to one
polarizer and the light therefore does not "see" the compound
retarder. In the off-state, fixed retardance with wavelength is
therefore not necessary. Providing achromatic orientation of the
compound retarder in the off-state yields high contrast shutters.
Reflection-mode shutters are further provided in this
invention.
[0019] In alternative liquid crystal compound retarder embodiments,
the rotatable smectic liquid crystal half-wave retarder is replaced
by first and second liquid crystal variable birefringence
retarders. The first and second variable birefringence retarders
have first and second fixed optic axis orientations, respectively,
and retardances which can be switched between zero and half-wave.
In operation, when one retarder is switched to zero retardance, the
other is switched to half-wave, and vice-versa, so that the
composite retardance of the pair is a half-wave retardance with
orientation switchable between the first and second optic axis
orientations.
[0020] The achromatic variable retardance smectic liquid crystal
compound retarder of this invention comprises an active section
rotatable with respect to a passive section. The active section
comprises two liquid crystal retarders: a half-wave plate and a
quarter-wave plate oriented at angles .alpha..sub.2 and
.alpha..sub.2+.pi./3, respectively, where the angle .alpha..sub.2
is electronically switchable. The passive section comprises two
retarders: a quarter-wave plate and a half-wave plate oriented at
angles .alpha..sub.1 and .alpha..sub.1+.pi./3, respectively, where
the angle .alpha..sub.1 is fixed. The quarter-wave plates are
positioned between the half-wave plates. The composite retardance
of the compound structure is 2(.pi./2-.alpha..sub.2+.alpha..sub.1).
To vary the retardance, the liquid crystal retarders in the active
section are both rotated.
[0021] The planar-aligned smectic liquid crystal cells of this
invention have continuously or discretely electronically rotatable
optic axes. The smectic liquid crystal cells can utilize SmC* and
SmA* liquid crystals, as well as distorted helix ferroelectric
(DHF), antiferroelectric, and achiral ferroelectric liquid
crystals. The variable birefringence liquid crystal cells of this
invention can include homogeneously aligned nematic liquid
crystals, pi-cells, and homeotropically aligned smectic liquid
crystal cells.
[0022] The present invention may be achieved in whole or in part by
an achromatic compound retarder that exhibits a compound retardance
and a compound optic axis, comprising: (1) a first passive retarder
unit having a predetermined retardance at a design wavelength, and
having a predetermined optic axis orientation; (2) a second passive
retarder unit having the same retardance as the first passive
retarder unit at the design wavelength, and having substantially
the same optic axis orientation as the first passive retarder unit;
and (3) a central retarder unit positioned between the first and
second retarder units, the central retarder unit having a
retardance n at the design wavelength, and having an optic axis
orientation that varies as a function of position on the central
retarder unit, wherein the optic axis orientation varies between at
least a first orientation state, in which the compound retardance
is substantially achromatic, and a second orientation state.
[0023] The present invention may also be achieved in whole or in
part by a reflection mode achromatic compound retarder, comprising:
(1) a first passive retarder unit having a predetermined retardance
at a design wavelength, and having a predetermined optic axis
orientation; (2) a reflector; and (3) a spatially switched retarder
unit positioned between the first retarder unit and the reflector,
the spatially switched retarder unit having a retardance .pi./2 at
the design wavelength, and having an optic axis orientation that
varies as a function of position on the central retarder unit,
wherein the optic axis orientation varies between at least a first
orientation state, in which the compound retardance is
substantially achromatic, and a second orientation state.
[0024] The present invention may also be achieved in whole or in
part by an achromatic compound retarder that exhibits a composite
optic axis orientation and a composite retardance, comprising: (1)
a first passive retarder unit having a predetermined retardance at
a design wavelength, and having a predetermined optic axis
orientation; (2) a second passive retarder unit having the same
retardance as the first passive retarder unit at the design
wavelength, and having substantially the same optic axis
orientation as the first passive retarder unit; and (3) a central
retarder unit positioned between the first and second retarder
units, the central retarder unit having a retardance .pi. at the
design wavelength, and having an optic axis orientation that
switches between at least two orientation states as a function of
position on the central retarder unit, wherein the composite optic
axis orientation and/or the composite retardance is substantially
achromatic at two orientation states of the central retarder
unit.
[0025] The compound retarder according to the invention can also be
employed to provide a novel achromatic inverter in a reflective or
transmissive type display. The achromatic inverter works in
combination with a liquid crystal display panel to provide four
states of intensity or brightness, two high and two low, so that
the reflective or transmissive display is capable of displaying an
inverse image frame.
[0026] In particular, in accordance with one embodiment of the
invention, a reflective display comprises one or more retarders
having in-plane retardance and in-plane orientation, at least one
of the retarders being an active retarder, and a ferroelectric
liquid crystal display. The one or more retarders work in
combination with the ferroelectric liquid crystal display to
provide four states of brightness.
[0027] In accordance with another embodiment of the invention, a
reflective display comprises a linear polarizer, an actively
controlled liquid crystal retarder and a ferroelectric liquid
crystal display. In accordance with a further embodiment, a
reflective display comprises a polarizing beam splitter, an
actively controlled liquid crystal retarder and a ferroelectric
liquid crystal display. In both embodiments, the actively
controlled liquid crystal retarder and the ferroelectric liquid
crystal display are both switchable between at least two
orientations to provide four states of brightness. In accordance
with still another embodiment, a transmissive display comprises a
first linear polarizer, a first actively controlled liquid crystal
retarder and a ferroelectric liquid crystal display, a second
actively controlled liquid crystal retarded and a second linear
polarizer.
[0028] The active retarder can be either a smectic or a nematic
liquid crystal retarder. In the accordance with another embodiment
of the invention, a reflective display comprises a linear
polarizer, an actively controlled nematic liquid crystal retarder
and a ferroelectric liquid crystal display. In accordance with a
further embodiment, a reflective display comprises a polarizing
beam splitter, an actively controlled nematic liquid crystal
retarder and a ferroelectric liquid crystal display. In both
embodiments, the actively controlled nematic liquid crystal
retarder and the ferroelectric liquid crystal display are both
switchable between at least two orientations to provide in
combination four states of brightness. Additionally, a passive
retarder can be provided between the actively controlled nematic
liquid crystal retarder and a ferroelectric liquid crystal display.
Further, the actively controlled nematic liquid crystal retarder
can comprise one or more pi-cells. Where one or more pi-cells are
employed as the actively controlled nematic liquid crystal
retarder, a passive retarder can be located between the one or more
pi-cells and the ferroelectric liquid crystal display or between
adjacent pi-cells. Further, in addition to the one or more
pi-cells, the display may include additional actively controlled
liquid crystal retarders, arranged adjacent to the one or more
pi-cells or in between the one or more pi-cells.
[0029] Additional advantages, objects, and features of the
invention will be set forth in part in the description which
follows and in part will become apparent to those having ordinary
skill in the art upon examination of the following or may be
learned from practice of the invention. The objects and advantages
of the invention may be realized and attained as particularly
pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] The invention will be described in detail with reference to
the following drawings in which like reference numerals refer to
like elements wherein:
[0031] FIG. 1 is a light shutter comprising a ferroelectric liquid
crystal between crossed polarizers;
[0032] FIG. 2(a) illustrates a first embodiment of an achromatic
compound retarder, in accordance with the present invention;
[0033] FIG. 2(b) illustrates a second embodiment of an achromatic
compound retarder, in accordance with the present invention;
[0034] FIG. 2(c) illustrates a third embodiment of an achromatic
compound retarder, in accordance with the present invention;
[0035] FIG. 3(a) is a reflective achromatic compound retarder, in
accordance with the present invention;
[0036] FIG. 3(b) illustrates a second embodiment of a reflective
achromatic compound retarder, in accordance with the present
invention;
[0037] FIG. 4 illustrates an achromatic shutter utilizing the
achromatic compound retarder of the present invention;
[0038] FIGS. 5(a) and 5(b) are plots showing the calculated on- and
off-state transmission spectra of crossed polarizer shutters having
(a) the achromatic compound retarder of the present invention, and
(b) a single retarder;
[0039] FIG. 5(c) and 5(d) are plots showing the calculated on- and
off-state transmission spectra of parallel polarizer shutters
having (c) the achromatic compound retarder of the present
invention, and (d) a single retarder;
[0040] FIG. 6 is a plot showing measured on-state transmission
spectra of (a) a compound-retarder achromatic shutter, in
accordance with the present invention, and (b) a single-retarder
shutter;
[0041] FIG. 7 is the measured off-state transmission spectrum of a
compound-retarder achromatic shutter, in accordance with the
present invention;
[0042] FIG. 8 is a plot showing the calculated on-state
transmission, as a function of the deviation from half-wave
retardance .delta., of (a) a compound-retarder achromatic shutter,
in accordance with the present invention, and (b) a single-retarder
shutter;
[0043] FIG. 9 is a plot showing the calculated off-state
transmission, as function of .delta., of a compound-retarder
achromatic shutter, in accordance with the present invention;
[0044] FIG. 10 is a plot showing the calculated contrast ratio, of
a function of .delta., of a compound-retarder achromatic shutter,
in accordance with the present invention;
[0045] FIG. 11(a) is a plot showing the calculated on-state
transmission spectra of an achromatic shutter utilizing a compound
quarter-wave retarder, in accordance with the present
invention;.
[0046] FIG. 11(b) is a plot showing the calculated off-state
transmission spectra of an achromatic shutter utilizing a compound
quarter-wave retarder, in accordance with the present
invention;
[0047] FIG. 12(a) shows a multiple-pixel reflection-mode achromatic
shutter having parallel polarizers, in accordance with the present
invention;
[0048] FIG. 12(b) shows a multiple-pixel reflection-mode achromatic
shutter having crossed polarizers, in accordance with the present
invention;
[0049] FIG. 13 is multiple-pixel transmission-mode achromatic
shutter, in accordance with the present invention;
[0050] FIG. 14 is a compound achromatic variable retarder
comprising a pair of liquid crystal retarders and a pair of passive
retarders, in accordance with the present invention;
[0051] FIG. 15(a) shows an arrangement of a general reflective
display according to the invention;
[0052] FIG. 15(b) shows an unfolded revision of the reflective
display of FIG. 15(a);
[0053] FIG. 16 is a table that illustrates that the optimal
modulation of a conventional LCD panel is between an OFF-state
orientation of 0 (.pi./2) and an ON-state orientations of
.+-..pi./4;
[0054] FIG. 17 is a table illustrating that when a passive retarder
is oriented at 7.5.degree., the LCD panel rotates between
60.degree. (ON), and 105.degree. (OFF);
[0055] FIG. 18 is a table illustrating the performance for
half-wave retarders centered at 500 nm, where the dispersion of
polycarbonate is used for all elements;
[0056] FIG. 19(a) illustrates a first embodiment of a
reflection-mode achromatic FLC display that includes an achromatic
inverter, in accordance with the present invention;
[0057] FIG. 19(b) illustrates a second embodiment of a
reflection-mode achromatic FLC display that includes an achromatic
inverter, in accordance with the present invention;
[0058] FIG. 20 is a table that shows the output of one pixel of the
FLC display of FIG. 19(b) for different orientations of the LC
retarder and the FLC retarder;
[0059] FIG. 21 is a plot of the optical transmission of the FLC
display of FIG. 19(b) in the on-state as a function of wavelength
for different tilt angle combinations;
[0060] FIG. 22 illustrates a transmission-mode achromatic FLC
display that includes an achromatic inverter, in accordance with
the present invention;
[0061] FIGS. 23(a) and 23(b) show optical inverters according to
the invention implemented with a nematic liquid crystal variable
retarder;
[0062] FIG. 23 (c) shows an optical inverter implemented with a
pair of nematic liquid crystal variable retarders with improved
field of view (FOV) according to the invention;
[0063] FIG. 24 shows an optical inverter according to the invention
implemented with a nematic liquid crystal variable retarder and a
passive retarder;
[0064] FIG. 25 illustrates preferred orientations of the passive
retarder of the embodiment of FIG. 24;
[0065] FIG. 26 illustrates a preferred difference in angle between
the optic axes of the NLC and passive retarder of FIG. 24;
[0066] FIG. 27(a)-27(d) illustrate in diagrammatic form a preferred
polarization manipulation of all four states of brightness in the
embodiment of FIG. 24;
[0067] FIGS. 28-30 are plots of transmission versus wavelength for
preferred configurations according to the invention;
[0068] FIG. 31 shows another embodiment of a FLC display device
with improved FOV according to the invention;
[0069] FIG. 32 illustrates how off axis rays "see" a twisted liquid
crystal director profile;
[0070] FIG. 33 shows another embodiment of a FLC display device
according to the invention;
[0071] FIG. 33(a) illustrates preferred orientations of the various
wave plates in the embodiment of FIG. 33;
[0072] FIG. 34 shows another embodiment of a FLC display device
according to the invention;
[0073] FIG. 34(a) illustrates preferred orientations of the various
wave plates in the embodiment of FIG. 34;
[0074] FIG. 35 shows another embodiment of a FLC display device
according to the invention;
[0075] FIG. 35(a) illustrates preferred orientations of the various
wave plates in the embodiment of FIG. 35;
[0076] FIG. 36 shows the basic structure of another reflective
display according to the invention;
[0077] FIGS. 37(a)-37(b) show head-on spectra of four states of the
embodiment of FIG. 36;
[0078] FIGS. 38(a)-38(b) illustrate a total of four states of
intensity of the embodiment of FIG. 36;
[0079] FIG. 39 shows the basic structure of another reflective
display according to the invention;
[0080] FIGS. 40(a)-40(b) illustrate a total of four states of
intensity of the embodiment of FIG. 39;
[0081] FIG. 41 shows the basic structure of another reflective
display according to the invention;
[0082] FIG. 42(a)-42(b) illustrate a total of four states of
intensity of the embodiment of FIG. 41;
[0083] FIG. 43 shows the basic structure of another reflective
display according to the invention; and
[0084] FIGS. 44-47 show various display devices incorporating an
achromatic inverter according to the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0085] The elements in the devices of this invention are optically
coupled in series. The orientation of a polarizer refers to the
orientation of the transmitting axis, and the orientation of a
birefringent element refers to the orientation of the principal
optic axis of that element. Orientations are herein defined with
respect to an arbitrary axis in a plane perpendicular to the light
propagation axis z. This arbitrary axis is labeled the "x" axis in
the figures. In the illustrations of birefringent elements, the
orientation is shown by arrow-headed lines and the retardance is
labeled on the side of the element. When the retardance is
switchable between two values, the values are both labeled on the
side and are separated by a comma. The retardance refers to the
retardance at a design wavelength. Note that a TE retardance is
equal to a half-wave (.lambda./2) retardance.
[0086] The term fixed retarder refers to a birefringent element
wherein the orientation and retardance can not be electronically
modulated. The term active retarder refers to a birefringent
element wherein the orientation and/or the retardance can be
electronically modulated. Rotatable liquid crystal retarders of
this invention have electronically rotatable orientation and fixed
retardance at the design wavelength. Liquid crystal variable
retarders or, equivalently, liquid crystal variable birefringence
retarders have electronically variable retardance (birefringence)
and fixed orientation. The term compound retarder is used for a
group of two or more retarders which function as a single retarder.
The composite retardance of a compound retarder is characterized by
an orientation and a retardance.
[0087] A spatially switched retarder refers to an active or passive
retarder in which the orientation and/or the retardance varies as a
function of position on the retarder.
[0088] The terms design wavelength and design frequency
(.upsilon..sub.0) refer to the wavelength and frequency at which
the individual retarders within the compound retarder provide the
specified retardance. The term achromatic retarder refers to a
retarder with minimal first-order dependence of both the retardance
and the orientation on the deviation of the incident light from the
design frequency (.DELTA..upsilon./.upsilon..- sub.0). The term
achromatic orientation refers to an orientation of the optic axis
with minimal first-order dependence on the deviation of the
incident light from the design frequency.
[0089] A first embodiment of the achromatic compound retarder of
this invention (FIG. 2a) comprises planar-aligned smectic liquid
crystal retarder 30 having an orientation which is electronically
rotatable between angles .alpha..sub.2 and .alpha..sub.2'. These
orientations are herein termed the on-state and the off-state,
respectively. Retarder 30 provides a half-wave retardance
(.GAMMA..sub.2.sup.0=.pi.) at the design wavelength. Outer
retarders 40 and 42, with orientation .alpha..sub.1 and retardance
.GAMMA..sub.1.sup.0 at the design wavelength, are positioned on
either side of central retarder 30. In an alternative embodiment,
the outer retarders 40 and 42 are crossed instead of parallel. In
this application the design equations are derived for the case of
parallel retarders. Analogous equations can be derived for crossed
retarders.
[0090] In this embodiment, the central retarder is an FLC, but it
can be any material with an electronically rotatable optic axis,
including planar aligned SmC* and SmA* liquid crystals, as well as
distorted helix ferroelectric PHF), antiferroelectric, and achiral
ferroelectric liquid crystals. The retarder switches between at
least two orientations, .alpha..sub.2 and .alpha..sub.2'. It can,
depending on the liquid crystal employed and the electric field
applied, rotate continuously between a range of orientations
including .alpha..sub.2 and .alpha..sub.2', switch between bistable
states .alpha..sub.2 and .alpha..sub.2', or be switched between two
or more discreet but not necessarily stable orientations.
[0091] In a second embodiment of the achromatic compound retarder
FIG. 2(b)), rotatable retarder 30 is replaced by a spatially
switched retarder 100. The spatially switched retarder 100 is
prefereably a planar-aligned passive retarder with an optic axis
orientation that varies as a function of position on the spatially
switched retarder 100. In the embodiment shown in FIG. 2(b), the
spatially switched retarder 100 has a fixed optic axis orientation
s in one portion 100a of the retarder 100, and an optic axis
orientation .alpha..sub.2' in a second portion 100b of the
spatially switched retarder 100. The retardance of the spatially
switched retarder 100 at the design wavelength is preferably fixed
and the same in both the first and second retarder portions 100a
and 100b.
[0092] Similar to the embodiment shown in FIG. 2(a), the
orientations .alpha..sub.2 and .alpha..sub.2' are termed the
on-state and the off-state, respectively. The spatially switched
retarder 100 preferably provides a half-wave retardance
(.GAMMA..sub.2.sup.0=.pi.) at the design wavelength in both the
first portion 100a and the second portion 100b. The spatially
switched retarder 100 is divided into at least two portions 100a
and 100b, with respective optic axis orientations .alpha..sub.2 and
.alpha..sub.2'. However, the spatially switched retarder 100 can be
divided into additional portions that exhibit other optic axis
orientations.
[0093] The spatially switched retarder 100 can be any birefringent
material. Suitable materials include crystalline materials, such as
mica or quartz, stretched polymeric films, such as mylar or
polycarbonates, and polymer liquid crystal films.
[0094] In a third embodiment of the achromatic compound retarder
(FIG. 2(c)), rotatable retarder 30 is replaced by variable
retarders 31 and 33 having fixed orientations of .alpha..sub.2 and
.alpha..sub.2', respectively. The retardance of 31 and 33 can be
switched between zero and half-wave. The retardances are
synchronously switched which, as used herein, means that when one
has zero retardance the other has half-wave retardance and
vice-versa. Thus the composite retardance of 31 and 33 is always a
half-wave and the composite orientation is switchable between
.alpha..sub.2 and .alpha..sub.2'.
[0095] Liquid crystal variable retarders 31 and 33 can include, but
are not limited to, homogeneously aligned nematic cells, nematic
.pi.-cells, and homeotropically aligned smectic liquid crystal
retarders. As is known in the art, homogeneously aligned nematic
cells and nematic .pi.-cells are sometimes incapable of being
electrically driven to zero retardance. In this case, the liquid
crystal cell can be combined ("shimmed") with a passive retarder to
compensate for the residual retardance. The passive retarder is
oriented orthogonal to the liquid crystal retarder if the
birefringence has the same sign and parallel if the birefringence
has opposite sign. In the present invention, variable retarders 31
and 33 optionally include passive retarders to compensate for
non-zero residual retardance.
[0096] This invention is described herein with the rotatable liquid
crystal retarder (FIG. 2(a)) as the representative species of FIGS.
2(a)-2(c). It is to be understood that in all embodiments of the
present invention that utilize a tunable retarder, a liquid crystal
rotatable retarder can, in the manner of FIG. 2(c), be replaced by
a pair of liquid crystal variable retarders. The species of FIG.
2(a) is preferred over the species of FIG. 2(c) for several
reasons. The construction is simpler because it uses a single
liquid crystal cell instead of two active cells. In addition, the
switching speed of smectic liquid crystals is orders of magnitude
faster than nematics. Finally, the field of view is greater.
[0097] The passive outer retarders can be any birefringent
material. As discussed above, in connection with the spatially
switched retarder, suitable materials include crystalline
materials, such as mica or quartz, stretched polymeric films, such
as mylar or polycarbonates, and polymer liquid crystal films. In a
preferred embodiment, the dispersion of the passive outer retarders
is approximately matched to the dispersion of the central retarder.
Mylar, for example, has a similar dispersion to some FLCs.
[0098] The achromatic compound retarder of this invention is
designed to be achromatic in the on-state when the central retarder
is oriented at .alpha..sub.2. For achromaticity of the orientation
and retardance, one solution for the relative orientations of the
retarders is: 2 cos 2 = - .PI. 2 1 0 ( 2 )
[0099] where .DELTA.=.alpha..sub.2-.alpha..sub.1. In addition there
are isolated orientations for specific design frequencies that also
yield achromatic orientation and retardance. The retardance,
.GAMMA., of the compound retarder is obtained from 3 cos ( / 2 ) =
2 sin 1 0 1 0 ( 3 )
[0100] The orientation, .OMEGA.+.alpha..sub.1, of the compound
retarder is obtained from 4 tan 2 ( ) = tan 2 cos 1 0 ( 4 )
[0101] where .OMEGA. is the orientation of the compound retarder
with respect to the orientation of the outside passive
retarders.
[0102] Based on the above design equations, the retardance of the
outer retarders and the relative orientations of the retarders can
be chosen to provide the desired retardance of the compound
retarder and to ensure achromaticity. For example, for an
achromatic compound half-wave retarder (.GAMMA.=.pi.), Eq. 3
provides the solution .GAMMA..sub.1.sup.0=.pi., and Eq. 2 provides
the relative orientation of the retarders as .DELTA.=60.degree..
Eq. 4 gives the relative orientation of the compound retarder as
.OMEGA.=30.degree.. Therefore, to obtain an orientation of
.OMEGA.+.alpha..sub.1=45.degree. for the compound half-wave
retarder, the outer retarders are oriented at
.alpha..sub.1=15.degree.. Since .DELTA.=60.degree., the orientation
of the central retarder must then be .alpha..sub.2=75.degree..
Similarly, for an achromatic compound quarter-wave retarder
(.GAMMA.=.pi./2), the equations yield
.GAMMA..sub.1.sup.0=115.degree., .DELTA.=71.degree., and
.OMEGA.=31.degree.. Thus, for an orientation of
.OMEGA.+.alpha..sub.1=45.- degree., the outer retarders are
oriented .alpha..sub.1=14.degree. and the central retarder is at
.alpha..sub.2=85.degree..
[0103] In the achromatic compound retarder of FIG. 2(a), the liquid
crystal central retarder has an optic axis rotatable between
.alpha..sub.2 and .alpha..sub.2'. When the liquid crystal retarder
is at .alpha..sub.2', the orientation relative to the outer passive
retarders is .DELTA.'=.alpha..sub.2'-.alpha..sub.1 and the
orientation of the compound retarder relative to the outer
retarders is .OMEGA.'. Since Eq. 2 gives a unique solution for the
absolute value of .DELTA., at which the compound retarder is
achromatic, it teaches against changing the orientation of the
central retarder with respect to the outer retarders. An aspect of
the present invention is the discovery that (1) at orientations
.alpha..sub.2' of the central retarder which do not satisfy Eq. 2,
the composite retardance .GAMMA. is nevertheless unchanged at the
design wavelength and (2) there are orientations .alpha..sub.2' of
the central retarder for which, even though the composite retarder
is not achromatic, the optic axis orientation is stable with
respect to wavelength.
[0104] A further aspect of this invention is the realization that
in many devices the composite retardance does not affect device
output in certain switching states and, therefore it need not be
achromatic in those states. In particular, when the compound
retarder is oriented parallel to a polarizer, the polarized light
is not modulated by the retarder and hence any chromaticity of the
retardance is unimportant. Only stability of the orientation of the
optic axis is required so that the orientation remains parallel to
the polarizer throughout the operating wavelength range. These
properties lead to numerous useful devices utilizing the compound
retarder with a rotatable or spatially switched central
retarder.
[0105] In a preferred embodiment of the achromatic compound
retarder, the optic axis orientation of the compound retarder is
achromatic when the central retarder is oriented at .alpha..sub.2'.
The first order term of the frequency dependence of the orientation
of the retardation axis is 5 - - 2 1 0 tan 2 cos 2 2 sin 1 0 2 cos
2 ( 1 0 cos 2 + 2 ) = 0 ( 5 )
[0106] where .di-elect cons. is the relative frequency difference
.DELTA..nu./.nu..sub.0. Note that in the on-state, wherein Eq. 2 is
satisfied, Eq. 5 gives
.differential..OMEGA./.differential..di-elect cons.=0. This
confirms that the on-state orientation is achromatic. For off-state
orientations, .alpha..sub.2', Eq. 5 can be used to determine the
magnitude of .differential..OMEGA.'/.differential..di-elect cons..
For the special case of an achromatic half-wave retarder,
.GAMMA..sub.1.sup.0=.pi., and sin .GAMMA..sub.1.sup.0=0, so
.differential..OMEGA./.differential..di-elect cons.=0 for all
values of .alpha..sub.2', i.e., the optic axis orientation is
achromatic at all orientations.
[0107] Because of the symmetry of the achromatic retarder, it can
be implemented in reflection-mode, as illustrated in FIGS. 3(a) and
3(b). FIG. 3(a) is the reflection-mode embodiment of the retarder
of FIG. 2(a), and utilizes a single passive retarder 40, with
retardance .GAMMA..sub.1.sup.0 and orientation .alpha..sub.1,
liquid crystal quarter-wave retarder 32, with orientation
switchable between .alpha..sub.2 and .alpha..sub.2', and reflector
50. Because the reflector 50 creates a second pass through the
liquid crystal quarter-wave retarder 32, the net retardance of the
liquid crystal quarter-wave retarder 32 is a half wave. A forward
and return pass through the reflection-mode device is equivalent to
a single pass through the compound retarder of FIG. 2a. The
reflection-mode embodiment of the retarder of FIG. 2(c) (not shown)
uses a pair of variable retarders switchable between zero and
quarter-wave retardance in lieu of rotatable quarter-wave retarder
32 in FIG. 3(a). FIG. 3(b) illustrates a reflection mode embodiment
of the retarder of FIG. 2(b), and utilizes a spatially switched
quarter-wave retarder 110, with retarder portions 110a and 110b, in
lieu of the liquid crystal quarter-wave retarder 32 of FIG.
3(a).
[0108] The reflector in the embodiments shown in FIGS. 3(a) and
3(b) has R=1 but it can also have R<1. The reflector can
transmit an optical signal for addressing the liquid crystal
retarder of FIG. 3(a).
[0109] This invention further includes devices employing the
achromatic compound retarders described above. The polarization
switch of this invention comprises a linear polarizer in
combination with the achromatic compound retarder. The polarizer
can be neutral with wavelength or can be a pleochroic polarizer.
Light is linearly polarized by the polarizer and the polarization
is modulated by the achromatic compound retarder. For the case of a
half-wave achromatic compound retarder, the polarization remains
linear and the orientation is rotated. Other achromatic compound
retarder embodiments produce elliptically polarized light. The
polarization switch functions as a polarization receiver when light
is incident directly on the achromatic compound retarder rather
than on the polarizer.
[0110] In a preferred embodiment, the achromatic compound retarder
is achromatic in the on-state (.alpha..sub.2) and is oriented
parallel to the polarizer in the off-state (.alpha..sub.2'). With
this preferred off-state orientation, achromaticity of the
composite retardance is not needed because, with the orientation of
the achromatic compound retarder parallel to the polarizer, the
polarized light does not "see" the achromatic compound retarder and
is not modulated by it. In a more preferred embodiment, the
orientation of the achromatic compound retarder is stable in the
off-state, i.e., .differential..OMEGA.'/.differential..d- i-elect
cons. is small. In the most preferred embodiment, the orientation
of the achromatic compound retarder is achromatic, i.e.,
.differential..OMEGA.'/.differential..di-elect cons. is zero.
[0111] A particularly useful embodiment of the polarization switch
of the present invention is illustrated in FIG. 4. The polarization
switch 110 comprises polarizer 20, outer retarders 40 and 42, and
liquid crystal retarder 30. Outer retarders 40 and 42 are half-wave
retarders (.GAMMA..sub.1=.pi.) oriented at .alpha..sub.1=.pi./12.
The liquid crystal retarder 30 is a half-wave retarder, and is
switchable between on- and off-state orientations of
.alpha..sub.2=5.pi./12 and .alpha..sub.2'=8.pi./12, respectively.
This gives a compound retardance .GAMMA.=.lambda./2 and
orientations .OMEGA.+.alpha..sub.1=.pi./4 and
.OMEGA.'+.alpha..sub.1=0. In the off-state, light remains polarized
along the x-axis and in the on-state, light is oriented parallel to
the y-axis. Because the achromatic compound half-wave retarder has
an achromatic orientation for all values of .alpha..sub.2', it can
be used to achromatically rotate the polarization between the input
polarization state and any other linear polarization state.
[0112] The polarization switch 110 can be used in combination with
any polarization sensitive element. In combination with an exit
polarizer 22 it forms an achromatic shutter, as shown in FIG. 4. In
the embodiment of FIG. 4, the polarizers 20 and 22 are crossed, but
they can alternatively be parallel. The shutter shown in FIG. 4 is
analogous to the shutter shown FIG. 1 in that the achromatic
compound retarder has a half-wave retardance, and on- and off-state
composite retarder orientations of .pi./4 and 0, respectively. Like
the shutter of FIG. 1, the shutter of FIG. 4 requires only one
active retarder. One advantage is that the shutter of the present
invention is achromatic.
[0113] A mathematical analysis of the achromatic compound half-wave
retarder and the achromatic shutter demonstrates the wavelength
stability of the devices of this invention. The Jones matrix for
the compound half-wave retarder is the product of the matrices
representing the three linear retarders. The Jones matrix that
propagates the complex cartesian field amplitude is given by chain
multiplying the matrices representing the individual linear
retarders. For the on- and off-states these are given,
respectively, by the equations
W.sub.c (.PI./4
)=W(.PI.+.delta.,.PI./12)W(.PI.+.delta.,5.PI./12)W(.PI.+.d-
elta.,.PI./12) (6)
and
W.sub.c(0)=W(.PI.+.delta.,.PI./12)W(.PI.+.delta.,2.PI./3)W(.PI.+.delta.,.P-
I./12) (7)
[0114] where the general matrix for a linear retarder with
retardation .GAMMA. and orientation a is given by 6 W ( , ) = ( cos
/ 2 - i cos 2 sin / 2 - i sin 2 sin / 2 - i sin 2 sin / 2 cos / 2 +
i cos 2 sin / 2 ) ( 8 )
[0115] and the absolute phase of each retarder is omitted. For the
present analysis, each retarder is assumed identical in material
and retardance, with half-wave retardation at a specific design
wavelength. This wavelength is preferably selected to provide
optimum peak transmission and contrast over the desired operating
wavelength band. The retardance is represented here by the equation
.GAMMA.=(.pi.+.delta.), where .delta. is the wavelength dependent
departure from the half-wave retardance. For the present work, the
dispersion is modeled using a simple equation for birefringence
dispersion that is suitable for both FLC and the polymer retarders
used (Wu, S. T., Phys. Rev. (1986) A33:1270). Using a fit to
experimental FLC and polymer spectrometer data, a resonance
wavelength was selected that suitably models the dispersion of each
material.
[0116] Substituting the three matrices into Eqs. 6 and 7 produces
on- and off-state matrices that can be written in the general form
7 W c = ( t 11 - - t 12 - t 12 t 11 ) ( 9 )
[0117] where .vertline.t.sub.ij.vertline. denotes the magnitude and
.theta. the phase of the complex t.sub.ij matrix components of the
compound structure. The specific elements for the (achromatic)
on-state are given by: 8 t 11 = 3 2 sin 2 / 2 1 + 1 3 sin 2 / 2 , (
10 ) t 12 = 1 - 3 4 sin 4 / 2 ( 1 + 1 3 sin 2 / 2 ) , ( 11 ) = tan
- 1 [ 3 2 cot / 2 ] , ( 12 )
[0118] The components for the off-state are given by: 9 t 11 = 1 -
( 1 - 3 2 ) 2 sin 4 / 2 cos 2 / 2 , ( 13 ) t 12 = ( 1 - 3 2 ) sin 2
/ 2 cos / 2 , ( 14 ) = tan - 1 [ cos / 2 cos 2 / 2 + ( 3 - 1 / 2 )
sin 2 / 2 sin 2 / 2 + ( 3 - 1 ) cos 2 / 2 ] . ( 15 )
[0119] In the achromatic shutter device, the achromatic compound
retarder is placed between crossed polarizers. The Jones vector for
the transmitted field amplitude is given by the matrix equation
E(.lambda.)=P.sub.yW.sub.cP.sub.xE.sub.o(.lambda.). (16)
[0120] The polarizers are taken to be ideal 10 P x = ( 1 0 0 0 ) ,
( 17 ) P y = ( 0 0 0 1 ) , ( 18 )
[0121] and the input field spectral density, E.sub.o(.lambda.) is
taken to be {circumflex over (x)} polarized, with unity amplitude.
Under these conditions, the Jones vector for the transmitted field
is the off-diagonal component of W.sub.c. The component of the
output Jones vector gives the field transmittance of the
structure.
[0122] Since the components of W.sub.c are given above in terms of
their magnitudes, the intensity transmission of the on- and
off-states of the achromatic compound retarder are given by simply
squaring the off-diagonal terms of Eqs. 11 and 14, or
T=.vertline.t.sub.12.vertline..s- up.2. This gives the two
intensity transmission functions of the shutter 11 T = 1 - 3 4 sin
4 / 2 ( 1 + 1 3 sin 2 / 2 ) ON ( 2 = 5 / 12 ) ( 1 - 3 2 ) 2 sin 4 /
2 cos 2 / 2 OFF ( 2 = 2 / 3 ) ( 19 )
[0123] The above outputs illustrate the desirable result that the
second -order dependence of transmitted intensity on .delta.
vanishes. The loss in transmission in the on-state and the leakage
in the off-state have at most a fourth-order dependence on
.delta..
[0124] Like a simple FLC shutter, the mechanism for modulating
polarization with the smectic liquid crystal compound retarder is
by rotating the orientation of the compound retarder rather than by
varying the birefringence. This can clearly be seen by considering
wavelength bands sufficiently narrow that the second (and higher)
order terms of the Jones matrices in .delta. can be neglected. In
this instance the matrices representing on- and off-states reduce
respectively to 12 W c = ( 0 - i - i 0 ) , and ( 20 ) W c = ( - i 0
0 i ) . ( 21 )
[0125] The on-state matrix reduces, to this degree of
approximation, to an ideal achromatic half-wave retarder oriented
at .pi./4, while the off-state matrix reduces to an ideal linear
retarder oriented at 0, with retardation 2.theta.. Since only an
off-diagonal component is utilized in a shutter implementation, the
output is ideal to this degree of approximation.
[0126] The elimination of the second-order term is achieved using a
3-element structure that achieves ideal half-wave retardation at
two wavelengths, rather than a single wavelength for the simple FLC
shutter. This behavior can be seen by slightly varying the relative
orientation of the central and outer retarders in the on-state. The
two ideal transmission states, as well as the two null states, can
be further separated in this way, increasing the operating band but
producing a more pronounced dip Leakage) between maxima
(nulls).
[0127] Based on the above equations, comparisons can be drawn
between the achromatic compound retarder shutter and the
conventional FLC shutter. A 10% loss in transmission for a
conventional shutter occurs for a retardation deviation of
.delta.=37.degree., while the same loss for the achromatic shutter
occurs for .delta.=72.degree.. This is very nearly a factor of two
increase in .delta.. FIG. 5(a) shows a transmisstion spectrum,
created using a computer model for the structures, for an
achromatic shutter optimized for visible operation (400-700 nm).
The shutter has a 90% transmission bandwidth of 335 nm (409-744
nm). FIG. 5(b) shows the transmission spectrum for a conventional
shutter with a design wavelength of 480 nm. The conventional
shutter has a 90% bandwidth of 122 nm (433-555 nm). The use of an
achromatic compound retarder in the shutter results in a factor of
3.75 increase in bandwidth. Calculated spectra for parallel
polarizer shutters with a compound retarder, shown in FIG. 5(c),
and a single retarder, shown in FIG. 5(d), show the tremendous
improvement in the off-state provided by the achromatic compound
retarder of this invention.
[0128] The increase in operating bandwidth is accompanied by a
theoretical loss in contrast ratio. The first-order orientation
stability requirement of the optic axis allows off-state leakage
due to the presence of higher order terms. In practice, little if
any actual sacrifice is observed when incorporating the achromatic
compound retarder. An FLC optimized for visible operation
(half-wave retardance at 480 nm) gives a maximum departure in
retardance of .delta.=75.degree.. Using this value, and assuming
that the outer retarders have dispersion identical to the FLC, a
worst-case contrast ratio of 667:1 is found for operation in the
400-700 nm band. For most of this band, theory predicts contrast
far in excess of 1000:1.
[0129] The conventional and the achromatic shutters were
experimentally demonstrated to verify the performance predicted by
computer modeling. The FLC device was fabricated using ZLI-3654
material from E-Merck. The ITO coated substrates were spin coated
with nylon 6/6 and were rubbed unidirectionally after annealing.
Spacers with a diameter of 1.5 microns were dispersed uniformly
over the surface of one substrate and UW cure adhesive was
deposited on the inner surface of the other substrate. The
substrates were gapped by applying a uniform pressure with a vacuum
bag and subsequently UV cured. The FLC material was filled under
capillary action in the isotropic phase and slowly cooled into the
C* phase. After cooling, the leads were attached to the ITO and the
device was edge-sealed. The FLC cell had a half-wave retardance at
520 nm.
[0130] A conventional shutter, such as the one shown in FIG. 1, was
formed by placing the FLC cell with the optic axis oriented at
45.degree. between parallel polarizers. Polaroid HN22 polarizers
were used due to their high contrast throughout the visible
wavelength range. The structure was probed by illuminating it with
a 400 W Xenon arc lamp, and the transmitted light was analyzed
using a SPEX 0.5 m grating spectrometer system. The on-state
transmission of the conventional shutter is shown in plot (b) of
FIG. 6.
[0131] The achromatic shutter was assembled using the same FLC
device positioned between two Nitto NRF polycarbonate retarders
having half-wave retardance at 520 nm. Since the FLC device is not
dispersion matched to the polymer film, a loss in contrast ratio is
anticipated for the achromatic compound retarder due to increased
off-state leakage. The polycarbonate films were oriented at
15.degree. with respect to the input polarizer, which was closed
with the exit polarizer. The FLC was switched between orientations
of 5.pi./12 and 8.pi./12. The on-state spectra shown in plot (b) of
FIG. 6, and the off-state spectra, shown in FIG. 7, were measured.
Both of these spectra were appropriately normalized to remove
leakage due to non-ideal polarizers, depolarization by the
retarders, and the polarization dependence of the lamp
spectrum.
[0132] The measured transmission spectra indicate excellent
agreement with the model results, FIG. 6 is striking evidence of
the increased transmission over the visible spectrum provided by
the achromatic shutter of this invention.
[0133] The model was further used to calculate the on-state
transmission of a compound-retarder achromatic shutter (Eq. 19) and
a single retarder shutter (Eq. 1) as a function of the deviation
from half-wave retardance .delta.. The calculated transmission
spectra are shown in FIG. 8. FIG. 9 is the calculated off-state
transmission of a compound-retarder shutter as a function of
.delta., and FIG. 10 is the calculated contrast ratio.
[0134] Using the achromatic shutter at slightly longer center
wavelengths, where FLC dispersion is greatly reduced, enormous
operating bands are feasible. For instance, the calculated 95%
transmission bandwidth of a shutter centered at 600 nm is
approximately 400 nm (480 nm-880 nm), while that of a simple FLC
shutter is only 150 nm (540 nm -690 nm).
[0135] The achromatic polarization switches and shutters of this
invention can also utilize compound retarders with composite
retardances other than half-wave. For example, a polarization
switch can be fabricated using a linear polarizer and an achromatic
compound quarter-wave retarder. In one embodiment, the orientation
of the achromatic compound retarder switches between .pi./4 and 0
with respect to the input polarizer, i.e.
.OMEGA.+.alpha..sub.1=45.degree. and
.OMEGA.'+.alpha..sub.1=0.degree.. To achieve this, Eqs. 2-4 give
.GAMMA..sub.1.sup.0=115.degree., .DELTA.=71.degree.,
.alpha..sub.1=14.degree. and .alpha..sub.1=85.degree. in the
on-state, and in the off-state .DELTA.'=96.degree., and
.alpha..sub.2'=111.degree.. In the on state, the compound
quarter-wave retarder switches the linear light to circularly
polarized light, and in the off-state the linear polarization is
preserved. Addition of a second polarizer oriented perpendicular to
the first polarizer results in a shutter which switches between 50%
transmission in the on-state and zero transmission in the
off-state. The on-state transmission spectrum, shown in FIG. 11(a),
and the off-state transmission spectrum, shown in FIG. 11(b), were
calculated assuming no dispersion. Note that the off-state
transmission spectrum is shown on a logarithmic scale in FIG.
11(b).
[0136] The achromatic compound retarder, polarization switch and
shutter of this invention have been illustrated with FLCs having
two optic axis orientations. They can alternatively utilize more
than two optic axis orientations and can have a continuously
tunable optic axis.
[0137] The achromatic shutter of this invention can be utilized in
applications such as CCD cameras, eye protection systems, glasses
in virtual reality systems, three-color shutters in
field-sequential displays, beamsteerers, diffractive optics and for
increasing the brightness of LC flat-panel displays.
[0138] For many display applications the achromatic shutter can be
used in a multiple-pixel array, as shown in FIGS. 12 and 13. In
these figures, optical elements are shown in cross section and are
represented by rectangular boxes. The retardance of birefringent
elements is listed at the top of the respective box, and the
orientation is listed at the bottom. When elements can rotate
between two or more orientations, both orientations are listed in
the box and are separated by a comma.
[0139] Reflection-mode embodiments are shown in FIGS. 12(a) and
12(b). FLC retarder 32 has a quarter-wave retardance at the design
wavelength and the optic axis is rotatable between 5.pi./12 and
8.pi./12. The FLC cell is formed with substrates 90 and 92.
Voltages are applied to the FLC using transparent electrode 95 and
pixellated mirror electrodes 52. Each pixel can be separately
addressed to provide the desired display pattern. The compound
retarder is formed by the FLC in combination with passive half-wave
retarder 40, oriented at .pi./12.
[0140] In FIG. 12(a) the shutter array uses linear polarizer 20
oriented at 0.degree.. Since, in reflection-mode, polarizer 20 is
both the input and output polarizer, this is a parallel polarizer
embodiment. The array is illuminated by ambient light 100 and the
viewer is represented by an eye. In FIG. 12(b), the array uses
polarizing beam splitter 25 to create a crossed polarizer
embodiment. White light 101 illuminates the array and modulated
gray light is output to the viewer.
[0141] A transmission-mode array is illustrated in FIG. 13. In this
embodiment, the FLC has a half-wave retardance. Voltages are
applied using transparent electrode 95 and pixellated transparent
electrode 96. The compound retarder is formed by the FLC retarder
in combination with outer retarders 40 and 42. The shutter is
formed by polarizers 20 and 22 which, in this embodiment, are
crossed. The array is illuminated by backlight assembly 103, which
can be collimated by lens 104. The display is viewed in
transmission mode.
[0142] The achromatic compound retarder of this invention has been
demonstrated within an achromatic shutter. In addition, it can be
used in many other optical devices known in the art. In particular,
it is suited to devices in which the retarder needs to be
achromatic in only one orientation and in which slight
achromaticity in other retarder orientations can be tolerated.
Specific examples include polarization interference filters and
dye-type color polarizing filters.
[0143] Numerous previous devices by the inventors can be improved
by using the achromatic compound retarder of this invention. In the
polarization interference filters of U.S. Pat. Nos. 5,132,826,
5,243,455 and 5,231,521, all of which are herein incorporated by
reference in their entirety, a smectic liquid crystal rotatable
retarder and a passive birefringent element are positioned between
a pair of polarizers. In a preferred embodiment, the birefringent
element is oriented at .pi./4 with respect to a polarizer.
[0144] In the split-element polarization interference filters of
U.S. Pat. No. 5,528,393, which is herein incorporated by reference
in its entirety, a center retarder unit and a pair of split-element
retarder units are positioned between a pair of polarizers. The
retarder units can include a rotatable liquid crystal retarder. The
individual liquid crystal rotatable retarders of the
above-mentioned polarization interference filters can be replaced
with the achromatic compound retarders of the present
invention.
[0145] The liquid crystal handedness switch and color filters
described in U.S. Pat. No. 5,619,355, which is herein incorporated
by reference in its entirety, can also be improved by using the
achromatic compound retarders of the present invention. The
circular polarization handedness switch and the linear polarization
switch comprise a linear polarizer and a rotatable liquid crystal
retarder. The color filters use the polarization switch in
combination with a color polarizer, such as a cholesteric circular
polarizer or a pleochroic linear polarizer. The simple liquid
crystal rotatable retarders described in the handedness switch
patent can be replaced with the achromatic compound retarders of
the present invention.
[0146] The achromatic compound retarder can also be used to improve
other color filters known in the art, for example as described in
Handschy et al., U.S. Pat. No. 5,347,378, which is herein
incorporated by reference in its entirety. These color filters
comprise a linear polarizer and a rotatable liquid crystal
retarder. In some embodiments, they further comprise pleochroic
polarizers, and in other embodiments they further comprise a second
linear-polarizer and a passive birefringent element. The simple
liquid crystal rotatable retarder of the Handschy et al. invention
can be replaced with the achromatic compound retarders of the
present invention.
[0147] The color filters of this invention can be temporally
multiplexed, wherein the output color is switched on a timescale
which is rapid compared to a slow response time detector, such as
the human eye. The achromatic compound retarder of FIG. 2a,
employing a smectic liquid crystal cell, is particularly suited to
this application.
[0148] The criterion for replacing a single retarder with the
achromatic compound retarder of this invention is that the single
retarder must be rotatable between two or more orientations of the
optic axis. The achromatic compound retarder is especially suited
for use in devices wherein it is positioned adjacent to a linear
polarizer and wherein the orientation of the retarder is, in one of
its switching states, parallel to the linear polarizer. The
achromaticity of the compound retarder is particularly advantageous
in color filtering devices because it can increase the throughput
across the entire visible spectrum.
[0149] The achromatic compound retarder of this invention can also
be used in optical devices to replace a pair of variable retarders
in which the first and second variable retarders have first and
second fixed orientations, and have retardances switchable between
first and second valves, and wherein the retardances are
synchronously switched between opposite valves. In addition, since
the achromatic half-wave retarder can be used to rotate the
orientation of linearly polarized light, it can replace twisted
nematic cells in optical devices.
[0150] In addition to the achromatic compound retarder, this
invention provides an achromatic variable retarder, illustrated in
FIG. 14. An active section comprises smectic liquid crystal
half-wave retarder 60, oriented at .alpha..sub.2, and smectic
liquid crystal quarter-wave retarder 65, oriented at
.alpha..sub.2+.pi./3. Angle .alpha..sub.2 of retarders 60 and 65 is
electronically tuned, preferably synchronously. A passive section
comprises passive quarter-wave retarder 75, oriented at
.alpha..sub.1+.pi./3, and passive half-wave retarder 70, oriented
at 0.sub.1. Angle .alpha..sub.1 is fixed. The angle .alpha..sub.2
of the liquid crystal retarder orientation can be rotated
discreetly or continuously to at least one other angle
.alpha..sub.2'. The retardance of the compound structure is
2(.pi./2-.alpha..sub.2+.alpha..sub.1).
[0151] The achromatic compound retarders of the present invention
can be used to provide an achromatic inverter for an FLC display.
FLCs are generally binary electro-optic devices that are operated
in a one-bit mode, where (relative to the input polarizer) a
0.degree. orientation results in an off state (a black state) and a
45.degree. orientation results in an or. state (a white state).
[0152] Due to the ionic impurities in liquid crystal materials,
LCDs are operated A with zero net DC voltage drive schemes. This is
particularly important when making active matrix displays using
chiral smectic liquid crystals, such as FLC on silicon, as they are
generally two orders of magnitude less pure than their active
matrix compatible nematic counterparts. This means that if a
positive voltage is applied to the LC, then a voltage of equal and
opposite polarity must be applied, preferably immediately
following, and generally for the same amount of time. This is
called "DC balancing" the waveforms across the LC.
[0153] The problem with DC balancing an active matrix FLC display
is that, unlike a nematic LC, FLC's respond to the polarity of
applied voltage. That is, the optic axis rotates in-plane by twice
the molecular tilt angle, when the sign of an electric field
applied normally is reversed. When illuminated with polarized
light, the two optical frames will appear contrast reversed. What
is white becomes black and vice versa. In order to visually observe
the displayed data effectively, the inverse frame must be blanked
(amp turned off, or modulated with a shutter to emit no light)
resulting in loss of light through the optical system. For some
applications, such as head mounted displays, losses in brightness
are more tolerable than, for example, front data projection or rear
projection systems for computer monitors and televison systems,
where brightness is important.
[0154] Prior art inverters for FLC displays are single pixel FLC
devices that can be crossed with respect to the FLC display panel.
This method allows the display to recover light from the inverse
frame because inverting the voltages on both the FLC display panel
and the FLC inverter cell yields the same image. Prior art
inverter/FLC display panel combinations are limited by the fact
that both the FLC display panel and inverter are chromatic
devices.
[0155] FIG. 15(a) shows an arrangement of a general reflective
display according to the invention. In particular, the reflective
display of FIG. 15(a) comprises a stack of single-pixel retarder
devices 320a-n with in-plane retardances
.GAMMA..sub.1-.GAMMA..sub.N and in-line orientations
.alpha..sub.1-.alpha..sub.N, at least one of which may be active,
sandwiched between a polarizing beam splitter (PBS) 310 and a FLC
display panel (LCD panel) 370 comprising a FLC retarder 360 with
mirror 380. The LCD panel 370 may comprise, for example, an FLC
retarder 360 sandwiched between a transparent electrode (not shown)
and pixellated mirror electrodes (not shown) for applying voltages
across the FLC retarder 3 60, similar to the arrangement shown in
FIG. 19(a). The display is illuminated by white light 101 and the
viewer is represented by an eye 300.
[0156] To more clearly illustrate the path light takes through the
display of FIG. 15(a) an "unfolded" version of the display is shown
in FIG. 15(b). The arrows a and b show the direction of
polarization of the input and output light, respectively.
[0157] The following is a hierarchy of structures that fall within
the general case shown in FIGS. 15(a) and 15(b):
[0158] (1) N=0: A standard FLC panel with no inverter, previously
discussed in this application.
[0159] (2) N=1: A standard FLC display panel with the addition of a
passive retarder, as previously discussed in this application. This
type of structure still has two logic states but a compound
retarder is used to achromatize the on-state.
[0160] (3) N=1: A standard FLC display panel with the addition of
an active retarder. This is the simplest structure that can
implement an inverter according to the invention. There are
solutions using either a nematic or a smectic single pixel device,
as will be discussed below.
[0161] (4) N.gtoreq.2: A standard FLC display panel with one active
and one or more passive retarders. This structure improves the
overall performance of the display device relative to the structure
of case (3). This structure can have improved contrast ratio,
reduce flicker, or both.
[0162] A few assumptions are made about the various systems. First,
the display of FIG. 15(a) is a reflective (two pass) device. The
in-plane orientation of the molecular director of the FLC retarder
360 rotates when the polarity of applied voltage is switched. The
in-plane retardance is identical in both states and the FLC
retarder 360 has linear eigenstates (no twist). From this, we
conclude that optimum performance is achieved when the LC retarder
360 has a quarter-wave retardation in a single pass (half-wave in a
round trip), and the in-plane switching angle is .pi./4. Designs
are generated assuming this preferred arrangement, though it is
understood that a suitable adjustment in design can be made for
non-ideal FLC behavior.
[0163] The LCD panel 370 is preferably a chiral smectic liquid
crystal (CSLC) spatial light modulator or display. For example,
classes of CSLCs that can be used include SMA*, SMC* including
ferroelectric displays currently being commercialized by
Displaytech, Inc. in their Light Lasers series of products and
their alliance with Hewlett Packard, and distorted helix
ferroelectric displays.
[0164] In order to implement the inverter, two on-states and two
OFF-states are required (four logic states) given by an auxiliary
single-pixel switch. The single-pixel switch can either be a
nematic liquid crystal (NLC) or smectic (FLC) device.
[0165] For example, the single-pixel switch can be an
electronically controlled birefringence (ECB) cell, a pi-cell, a
hybrid aligned nematic cell, a vertically aligned nematic cell or
another LC device that allows switching between a non-zero
retardance and zero retardance. A NLC behaves as a zero-twist
retarder in the low-voltage state, and becomes isotropic (vanishes)
in the driven state. The term "vanishes", as used herein, refers to
a retarder state in which the polarization of input light is not
affected. Thus, the retarder is effectively not seen by the input
light (i.e., vanishes). Note that a double nematic (crossed cell)
solution is identical in design to a single nematic solution. The
second cell improves switching speed, but the combination can be
considered a single zero-twist retarder in any voltage state.
Because a nematic cell vanishes in the driven state, the scheme in
general modulates between structures with N values that differ by
unity.
[0166] The single-pixel FLC device switch is also taken to behave
as an in-plane switch, as described above. The tilt angle and
retardance can in principle be selected to accommodate the design.
Unlike nematic solutions, FLC solutions modulate between structures
with a fixed N value, because the in-plane retardance is fixed. The
only exceptions are designs in which the FLC is made to mimic the
behavior of a nematic. That is, the FLC device switch is directly
adjacent to the polarizer oriented along an eigenstate in one
voltage state. Therefore, FLC solutions can either have a fixed N
value, or modulate between solutions that differ by unity.
[0167] As discussed above, structures with NLC switches modulate
between structures that differ in N value by unity. Let M (2N+1)
represent the total number of retarders required in the unfolded
structure. First, consider the requirements placed on the structure
with the lower N value (NLC driven to high state). When no passive
retarders are used, the design reduces to the N=0 case. The LCD
panel optimally modulates between an OFF-state orientation of 0
(.pi./2) and on-state orientations of .+-..pi./4, as shown in FIG.
16. The OFF-state has unlimited contrast ratio in theory, while the
on-state is given by a zero-order half-wave plate.
[0168] Given this configuration, the insertion of the nematic
waveplate (by driving the NLC to the low state) produces the
additional states. The NLC orientation is preferably selected to
maximize the contrast ratio of the OFF-state. The angle between the
NLC retarder and the FLC retarder (in the LCD panel) in this state
is .DELTA.'=67.5.degree., per the M=1 requirements, which limits
the wavelength stability of the optic axis. The chrominance of the
on-state is fixed by the OFF-state requirements. For this case, the
on state is less chromatic than an LCD panel alone. The M=3
structure is a compound retarder with compound optic axis
switchable by the NLC only. This design methodology can be extended
to include modulation between higher order structures. When a NLC
is used in combination with a passive half-wave retarder (N=2) it
is possible to modulate between M=3 and M=5 structures. The passive
retarder can be placed either between the PBS and the NLC or
between the NLC and the LCD panel.
[0169] First, consider the M=3 structure (NLC driven high). The
symmetric structure forms a compound retarder, with compound
retardation determined by the retardation of the passive retarder.
Regardless of the passive retardation selected, orientations can be
selected for an OFF-state corresponding to an eigenstate of the
structure. Given this flexibility, one preferred on-state has
maximum transmission throughout the visible spectrum, as previously
described in this application This requires that the compound
retarder is an achromatic half-wave plate, which requires that the
passive retarder is also a half-wave plate. The OFF-state is
obtained as an eigenstate of the compound retarder, which is
produced by reorienting the optic axis of the NLC only.
[0170] Using this M=3 optimization, the passive retarder is
oriented at 15.degree. and the NLC rotates between orientations of
75.degree. and 120.degree.. What remains is to select the NLC
retardance and orientation.
[0171] The higher order structure can also be considered a
half-wave compound retarder, as required to optimize on-state
transmission. This forces the NLC to provide a half-wave of
retardation. The NLC must be used to determine the orientation of
the M=5 compound retarder optic axis. With the LCD panel oriented
at 75.degree., the NLC must generate an OFF-state, which is done by
orienting an eigenpolarization of the compound retarder along the
polarization of the input light.
[0172] With the NLC placed between the PBS and the passive
retarder, the highest density OFF-state occurs with the NLC optic
axis oriented at -67.5%. This M=5 OFF-state has significantly
better wavelength stability than the previous M=3 OFF-state.
Furthermore, the M=3 OFF-state of the present design is also
significantly more wavelength stable than the previous M=3
example.
[0173] With the NLC placed between the passive retarder and the LCD
panel, the process can be repeated. The highest density OFF-state
is obtained with the NLC optic axis oriented at -83.degree.. It
should be noted that the wavelength stability of the OFF-state is
better for the previous M=3 example.
[0174] The NLC switch design approach, according to the invention,
for modulating between M=3 and M=5 is discussed below. Again, the
performance of the M=3 structure is considered first for the case
where the NLC switch is modeled as an in-plane switch with a
45.degree. rotation angle. We are free to select .DELTA., the angle
between the passive retarder and the optic axis of the FLC in the
LCD panel that generates the OFF-state. The optimum optic axis
stability occurs when .DELTA.=90.degree., where the structure
degenerates to a single zero-order half-wave plate. This forces the
outside retarder(s) to be oriented along the input polarization,
the on-state being generated by the FLC retarder in the LCD panel
alone. This is not the most achromatic structure for retardance,
which occurs for .DELTA.=60.degree.. However, the optic axis
stability, and therefore the contrast ratio, degrades for any
.DELTA. either greater or less than 90.degree.. Therefore, one must
balance the contrast ratio against the on-state chrominance. For
the inverter where we modulate between M=3 and M=5 on-states, we
also balance on-state spectra for the purpose of minimizing
luminance modulation between true and inverse frames.
[0175] For a particular passive half-wave orientation, there is an
FLC orientation of the FLC retarder in the LCD panel that produces
an OFF-state (an eignepolarization of the M=3 structure). When the
FLC retarder in the LCD panel is switched by 45.degree., the
on-state is generated. This on-state is somewhat less chromatic
than a zero-order half-wave, but greater weight is assigned to
maximizing contrast and balancing transmission with the M=5
on-state. FIG. 17 shows that when the passive retarder is oriented
at 7.5.degree., the FLC switch rotates between 60.degree. (ON), and
105.degree. (OFF). When the NLC switch is inserted between the PBS
and the passive retarder at -67.5.degree., the output is
effectively inverted. Note that this angle is consistently used,
regardless of the details of the M=3 design (it was also used in
the design that modulated between M=1 and M=3 structures). This
angle is used because the M=3 structure is in general a compound
retarder with an optic axis either along the input polarization, or
at 45.degree. to the input polarization. As such, the requirement
for inverting the output is that the outside retarder apply an
additional 45.degree. change in orientation. In addition, the
specific angle is selected such that the chrominance of the NLC
compensates for the chrominance of the compound retarder. This is
particularly important for achieving stability of the optic axis in
the M=5 OFF-state.
[0176] The performance is given in FIG. 18 for half-wave retarders
centered at 500 nm, where the dispersion of polycarbonate is used
for all elements. The effects of dispersion, as well as non-ideal
tilt angles, require small adjustments to the angles. However, the
design methodology still applies. FIG. 18 shows that the M=5
structure produces an OFF-state transmission of <-27 db, while
the M=3 OFF-state has <-46 db transmission (theoretical). In
another optimization, these OFF-state leakages could be balanced.
This particular example has the desirable characteristic that
on-state and OFF-state transmission spectra are nearly identical.
The adjustment of the M=3 structure sacrifices some of the
potential achromatic behavior in order to improve the M=5
spectrum.
[0177] Mathematically, the methodology can be summarized as
follows:
[0178] 1) .alpha..sub.2: Select to control chrominance (typically
5.degree.-15.degree.)
[0179] 2) .alpha.'=90.degree.+2.alpha..sub.2
[0180] 3) .alpha.=.alpha.'-45' (or twice the tilt angle of the FLC
retarder in the LCD panel if less than 45.degree.)
[0181] 4) .alpha..sub.1=67.5.degree. (or the tilt angle of the FLC
retarder -90.degree. if less than 22.5.degree.)
[0182] If the tilt angle of the FLC retarder in the LCD panel is
less than 22.5.degree., for instance 18.6.degree., we have
.alpha.=.alpha.'-37.2.de- gree.. For this example, if we choose
.alpha..sub.2=6.5.degree. to balance the ON-state spectra, the
resulting design is:
[0183] .alpha..sub.2: 6.5.degree.
[0184] .alpha.': 103.degree.
[0185] .alpha.: 65.8.degree.
[0186] .alpha..sub.1=71.4.degree.
[0187] FIGS. 19(a) and 19(b) show specific configurations, of a
reflective achromatic display that utilizes an achromatic inverter
implemented with a compound retarder switch according to the
invention. The reflective display 500 of FIG. 19(a) comprises a
linear polarizer 510; an actively controlled liquid crystal FLC)
retarder 520 (switch), preferably a half-wave plate; transparent
substrates containing electrodes 530 and 540 for applying a voltage
across the FLC retarder 520; an FLC retarder 560, preferably a
quarter-wave plate; and a transparent substrate containing
electrode 570 and a transparent substrate containing pixellated
mirror electrodes 580 for applying voltages across the FLC retarder
560 in accordance with image data. The transparent substrate
containing electrode 570, the transparent substrate containing
pixellated mirror electrodes 580 and FLC retarder 560 collectively
make up an LCD panel 600.
[0188] In the embodiment of FIG. 19(a), the linear polarizer 510 is
oriented at 0.degree.. Since, in reflection-mode, polarizer 510 is
both the input and output polarizer, this embodiment is a parallel
polarizer embodiment. The display 500 is illuminated by ambient
light 100 and the viewer is represented by an eye 300. The LCD
panel 600 modulates the input light 100 in accordance with image
data.
[0189] In the reflective display 505 of FIG. 19(b), the linear
polarizer 510 is replaced with a polarizing beamsplitter 511, which
is used as both an input polarizer and an output polarizer for the
display 505. The polarizing beamsplitter 511 is illuminated by
white light 101, reflects light having a first polarization and
transmits light having a second polarization that is orthogonal to
the first polarization. Thus, the embodiment of FIG. 19(b) is a
crossed polarizer embodiment. In the embodiments of FIGS. 19(a) and
19(b), the achromatic display is formed by the LCD panel 600 in
combination with the actively controlled FLC retarder 520 (switch),
which functions as an achromatic inverter.
[0190] The FLC retarder 520 has an orientation that is
electronically switchable between at least two orientations,
+.alpha..sub.1 and -.alpha..sub.1, by applying a voltage across the
FLC retarder 520 with electrodes 530 and 540. The FLC retarder 560
has an orientation that is electronically switchable between at
least two orientations +.alpha..sub.2 and -.alpha..sub.2. The
orientation of sections or "pixels" of the FLC retarder 560 can be
independently switched by applying voltages to a corresponding
pixel in the pixellated mirror electrode 580. Thus, the LCD display
600 polarization modulates the input light 100 and 101 in
accordance with image data that drives the electrodes 570 and
580.
[0191] Because the embodiments of FIGS. 19(a) and 19(b) are
reflective, the light 100 and 101 makes two passes through the FLC
retarder 520 and the FLC retarder 560. The retardances provided by
the FLC retarder 520 and the FLC retarder 560 at the design
wavelength are preferably chosen so that the retardance provided by
the FLC retarder 560 after two passes is approximately half the
retardance provided by the FLC retarder 520 after two passes. In
the embodiments of FIGS. 19(a) and 19(b), the FLC retarder 520
preferably provides a half-wave retardance at the design wavelength
for a single pass (full-wave for two passes), and the FLC retarder
560 preferably provides a quarter-wave retardance at the design
wavelength for a single pass (half-wave for two passes).
[0192] In smectic liquid crystals, the angle between the liquid
crystal layer normal and the molecular director is generally
referred to as the tilt angle .theta. of the liquid crystal. FLCs,
which are a class of smectic liquid crystals, are typically
bistable in that the molecular director (the liquid crystal
orientation) can be switched between +.theta. and -.theta. either
side of the brushing direction.
[0193] In the embodiment of FIG. 19(b), the FLC retarder 520 and
the FLC retarder 560 are preferably positioned so that the rubbing
direction of the liquid crystals that make up the FLC retarder 520
is orthogonal to the rubbing direction of the liquid crystals that
make up the FLC retarder 560. Further, the FLC retarder 520 is
preferably positioned so that its rubbing direction is parallel or
perpendicular to the polarization direction of the input light 101,
which is the x-axis direction in the embodiment of FIG. 19(b).
[0194] In order to optimize the contrast ratio of the display, the
tilt angle of the FLC retarder 560 (.theta..sub.2) is preferably
approximately twice the tilt angle of the FLC retarder 520
(.theta..sub.1) in the embodiment if FIG. 19(b). In addition, the
achromaticity of the FLC display is preferably optimized, while
maintaining a symmetric switching arrangement. In view of these
preferences, the FLC retarder 560 tilt angle (.theta..sub.2) is
preferably chosen to be approximately 22.5.degree., and the FLC
retarder 520 tilt angle (.theta..sub.1) is preferably chosen to be
approximately 11.25.degree. in the embodiment of FIG. 19(b).
[0195] The operation of the achromatic inverter (the FLC retarder
520) in conjunction with the FLC retarder 560 will now be explained
in connection with the embodiment of FIG. 19(b). As explained
above, orientation angles are given with respect to an arbitrary
"x" axis. Because the rubbing direction of the FLC retarder 520 is
parallel to the x-axis in the embodiment of FIG. 19(b),
+.alpha..sub.1 is equal to +.theta..sub.1 (approximately
11.25.degree.) and -.alpha..sub.1 is equal to -.theta..sub.1
(approximately -11.25.degree.). However, because the rubbing
direction of the FLC retarder 560 is perpendicular to the x-axis,
+.alpha..sub.2 is equal to approximately 90.degree.-.theta..sub.2
(approximately 67.5.degree.) and -.alpha..sub.2 is equal to
90.degree.+.theta..sub.2 (approximately 112.5.degree., which is
equivalent to -67.5.degree.).
[0196] FIG. 20 is a table that shows the output of one pixel of the
display 505 of FIG. 19(b) for different orientations of the FLC
retarder 520 and the FLC retarder 560. As shown in FIG. 20, when
the sign of the orientation angle of the FLC retarder 560 is
changed in order to drive the FLC retarder 560 with the inverse
frame, the output of the display can remain the same by
simultaneously changing the sign of the orientation angle of the
FLC retarder 520.
[0197] In the embodiment of FIG. 19(b), the orientations of the FLC
retarder 520 and tile FLC retarder 560 are set to approximately
+11.25.degree. and approximately +67.5.degree., respectively, to
obtain an achromatic white state. The pixel of the FLC retarder 560
is driven with the inverse image frame by adjusting the pixel
driving voltage This reverses the polarity of the orientation angle
of the FLC retarder 560 (e.g., switches the orientation to
approximately -67.5.degree.). When the pixel of the FLC retarder
560 is driven with the inverse frame, the voltage driving the FLC
retarder 520 is adjusted to switch the orientation of the FLC
retarder 520 to approximately -11.25.degree., so that the output of
the LC display 505 remains an achromatic white state. This allows
the inverse frame to be viewed.
[0198] Similarly, the orientations of the FLC retarder 520 and the
FLC retarder 560 are set to approximately +11.25.degree. and
approximately -67.5.degree., respectively, to obtain a high
contrast black state. When the polarity of a pixel drving the FLC
retarder 560 is reversed (by setting the orientation to
approximately +67.5.degree.) to drive that portion of the FLC
retarder 560 with the inverse frame, the orientation of the FLC
retarder 520 is switched to approximately -11.25.degree. so that
the output of the display 505 remains a high contrast black
state.
[0199] As discussed above, the tilt angles of the FLC retarder 520
and the FLC retarder 560 are preferably chosen to optimize the
achromaticity of the display 505, while maintaining a symmetric
switching arrangement. FIG. 21, which is a plot of the optical
transmission of the display 505 of FIG. 19(b) in the ON-state as a
function of wavelength for different tilt angle combinations,
illustrates why a tilt angle of approximately 22.5.degree. is
preferred for the FLC retarder 560 and a tilt angle of
approximately 11.25.degree. is preferred for the FLC retarder 520.
As is shown, the best achromaticity is obtained when
.theta..sub.1=22.5.degree. and .theta..sub.2=11.25.degree..
[0200] A transmission-mode achromatic display 507 is shown in FIG.
22. The transmissive display 507 is similar to the reflective
displays, except that an FLC retarder 620, that preferably provides
a half-wave of retardance at the design wavelength, is used because
the input light only undergoes a single pass through the FLC
retarder 620. In addition, a transparent substrate containing a
transmissive pixellated electrode 630 is used with the FLC retarder
620, instead of a reflective electrode. The transparent substrate
containing electrode 570, transparent substrate containing
transmissive pixellated electrode 630, and FLC retarder 620
collectively make up a transmissive LCD panel 602.
[0201] Because the input light passes through the FLC retarder 620,
a second FLC retarder 640 that preferably provides a half-wave of
retardance at the design wavelength is positioned after the FLC
retarder 620. The transparent substrates containing electrodes 650
and 660 are used to apply a voltage across the second FLC retarder
640. An output polarizer 670 is positioned to analyze the output
light. In the embodiment of FIG. 22, the input and output
polarizers 510 and 670 are crossed. The display 507 is illuminated
by a light source 103, which can be collimated by lens 104.
Alternative materials such as diffusers or light control films can
also be inserted between the light source 103 and the display 507.
The display 507 is viewed in transmission mode.
[0202] In the transmissive display 507 of FIG. 22, the first and
second FLC retarders 520 and 640 operate together as the achromatic
inverter for the display 507. Thus, the orientations of FLC
retarders 520 and 640 are simultaneously switched to allow viewing
of the inverse frame.
[0203] In the embodiments of FIGS. 19(a), 19(b) and 22, the FLC
retarder and the FLC retarder are implemented with FLCs. However,
they can also be implemented with any other material that has an
electronically rotatable optic axis, including planar-aligned SmC*
or SmA* liquid crystals, as well as distorted helix ferroelectric
(DHF), antiferroelectric, and achiral ferroelectric liquid
crystals. In addition, the FLC retarder can also be implemented
with two retarders that have fixed orientations and retardances
that are electronically switchable between 0 and half-wave.
[0204] The achromatic inverter of the present invention can also be
implemented with a nematic liquid crystal variable retarder, such
as an electrically controlled birefringence (ECB) cell, pi-cell,
hybrid aligned nematic cell, vertically aligned nematic cell, or
any other liquid crystal device that allows switching between a
non-zero retardance, and zero retardance. Such a device is low in
cost and easily manufacturable. Examples of such devices are shown
in FIGS. 23(a)-(b).
[0205] The reflective display 712 of FIGS. 23(a) comprises a linear
polarizer 710; an in-line compensator, or shim 716; a switchable
nematic liquid crystal NLC) device 720 (single pixel device);
transparent substrates containing electrodes 730, 740; a FLC
retarder 760, preferably a quarter-wave plate; and a transparent
substrate containing electrode 770 and a transparent substrate
containing pixellated mirror electrodes 780 for applying voltages
across the FLC retarder 760 in accordance with image data. The
transparent substrate containing electrode 770, the transparent
substrate containing pixellated mirror electrode 780 and the FLC
retarder 760 collectively make up a reflective LCD panel 795. The
compensator 716, the NLC 720 and the transparent substrates
containing electrodes 730 and 740 collectively make up a NLC switch
785.
[0206] The display is illuminated by ambient light 100 and the
viewer is represented by an eye 300. In the embodiment of FIG.
23(b), the linear polarizer 710 is replaced with a polarizing
beamsplitter 711, which is used both as an input polarizer and an
output polarizer for the display 713. The polarizing beamsplitter
711 is illuminated by white light 101, reflects light having a
first polarization and transmits light having a second polarization
that is orthogonal to the first polarization. Thus, the embodiment
of FIG. 23(b) is a crossed polarizer embodiment.
[0207] The NLC 720 is preferably a half-wave plate in the visible
part of the spectrum, and is preferably oriented at approximately
-67.5.degree. to the incident polarized light. The optical axis of
the FLC retarder 760 is preferably switched between two states, one
that is parallel to the incident polarization, and one that is
approximately +45.degree. to the incident polarized light (or twice
the tilt angle when the tilt angle is approximately 22.5 degrees,
as is common for SmC* ferroelectric materials).
[0208] The structure operates as follows. During the normal view
frame, the NLC device 785 is "energized" (maximum voltage is
applied to the NLC 720 resulting in zero or near zero retardance,
and it is, ideally, as if the retarder has vanished). Incident
polarized light sees one of two states at the FLC retarder 760,
optic axis oriented parallel or at approximately 45.degree. to the
direction of incident polarization. The parallel orientation
results in no net rotation of polarization. When viewed in a
crossed polarization configuration (such as through beamsplitter
711 in the embodiment of FIG. 23 (b)) pixels in this state appear
black. Pixels with optic axis oriented at approximately 45.degree.
to the incident polarization undergo approximately 90.degree.
rotation of polarized light, which is transmitted by the crossed
polarizer (beamsplitter 711) and these pixels appear white. As
discussed below, a single passive retarder film can be placed
between the NLC device and the LCD panel 795 to make the LCD panel
795 appear like a half-wave plate in reflection for all wavelengths
in the visible spectrum (i.e. an achromatic compound retarder).
[0209] Now, when opposite voltages are applied to the LCD panel 795
during the inverse frame (for DC balancing the LCD panel 795), the
NLC device 785 is not energized such that it provides a half-wave
of retardance (ideally achromatic but in practice this is difficult
to do, so preferably half-wave at approximately 500 nm). Incident
polarized light sees the half-wave NLC 720, and its polarization is
rotated by approximately a net 135.degree. or +45.degree.. This new
polarization state sees the inverted ON pixel optic axes in the LCD
panel 795 oriented at approximately 0.degree. to the original
polarization direction, and hence is rotated by approximately a net
90.degree. to +45.degree.. The reflected polarized light now makes
a net angle of approximately 67.5.degree. with the NLC 720, and
rotates by approximately twice 67.5.degree. or to -90.degree. with
respect to the original polarization state of the incident light,
and is transmitted by the crossed polarizer (beamsplitter 711) and
again appears as an ON pixel with the correct optical polarity.
[0210] Inverted OFF pixels have optic axis orientations during the
DC balanced frame at approximately +45.degree. relative to the
original state of polarization of the incident light. The NLC 720
is oriented at approximately -67.5.degree. to the OFF pixel optical
axis orientation in the LCD panel 795. After traversing the NLC,
the polarization at the design wavelength is then oriented at
approximately 45.degree. parallel to the optic axis of the FLC in
this inverted OFF-state. The polarization is therefore left
unaltered to be rotated back to approximately 0.degree. by the NLC
on its return path. Thus, the pixel appears dark. The specific
orientation of the NLC/FLC retarders ensures a good achromatic
(i.e., black) OFF-states as it forms a compound retarder as per the
basic invention.
[0211] In order to achieve a good OFF state in the DC balanced
frame, the NLC 720 must have approximately the same retardance as
the FLC retarder 760, such that there is a good dispersion match
between the LC mixtures. This means that either the NLC 720 must
also be an achromatic retarder in the ON-state, or one may not want
to make the FLC retarder 760 achromatic in order to achieve a high
contrast ratio and a bright display using this optical inverter
method.
[0212] One drawback of using a nematic retarder instead of a FLC
retarder for the switch is that the NLC retarder 720 has an
asymmetrical response time to applied voltage. Switching to the
energized state is fast, but relaxing back to the non-energized
state is slow (approximately less than one millisecond). However,
if the LCD panel 795 is loaded with the view frames (for an 8 bit
display, this mean loading eight frames), and then loaded with the
inverse frames, then speed is not a problem because a dual ECB or
pi-cell configuration can be used, as discussed below.
[0213] The reflective display 714 of FIGS. 23(c) comprises a
polarizing beamsplitter 711; a first switchable nematic liquid
crystal (NLC) device 720 (single pixel device), preferably a
half-wave plate; transparent substrates containing electrodes 730,
740; a second switchable nematic liquid crystal (NLC) device 721
(single pixel device), preferably a half-wave plate; transparent
substrates containing electrodes 721a, 721b; a FLC retarder 760,
preferably a quarter-wave plate; and a transparent substrate
containing electrode 770 and a transparent substrate containing
pixellated mirror electrodes 780 for applying voltages across the
FLC retarder 760 in accordance with image data. The transparent
substrate containing electrode 770, the transparent substrate
containing pixellated mirror electrode 780 and the FLC retarder 760
collectively make up a reflective LCD panel 795. The NLC 720, the
transparent substrates containing electrodes 730 and 740; the NLC
721; and the transparent substrates containing electrodes 721a and
721b collectively make up a NLC switch 785.
[0214] The NLC 721 is aligned and switched as for the previous NLC
embodiments and is situated preferably nearest the FLC panel. The
NLC 720 is aligned at approximately 90.degree. to the NLC 721 and
acts as a dynamic compensator. During the view frame, both nematic
cells are energized, and effectively vanish. At an appropriate time
during the loading and displaying of the view frames (i.e., during
the approx. 1 millisecond from finishing loading the frames to
displaying the frames), the energizing voltages are removed from
the two nematic cells, and they relax back to the non-energized
state together. As they relax back to the non-energized state
together, their optic axes remain crossed such that they still
together exhibit a net zero retardance (i.e., they are invisible to
the normally incident light). When the inverse frames are viewed,
only the NLC 721 is energized, such that the incident polarization
sees only one half-wave plate, instead of two, and the structure
operates in the same manner as described above.
[0215] FIG. 24 shows another display embodiment employing an
inverter, according to the invention. The reflective display 815 of
FIG. 24 comprises a linear polarizer 810; an in-line compensator
816; a switchable nematic liquid crystal (NLC) device 818,
preferably a half-wave plate; a passive retarder 821, preferably a
half-wave plate; and an LCD panel 825. The LCD panel 825 comprises
a FLC retarder 860, and a transparent substrate containing
electrode 870 and a transparent substrate containing pixellated
electrodes 880 for applying voltages across the FLC retarder 860 in
accordance with image data. The NLC switch 818 comprises a nematic
liquid crystal retarder (NLC) 820 and transparent substrates
containing electrodes 830 and 840 for applying a voltage across the
NLC 820 along with the in-line compensator 816 and passive retarder
821. The display 815 is illuminated by ambient light 100 and the
viewer is represented by an eye 300. The display 815 exhibits good
contrast, better achromatic performance and twice throughput than
the bare LCD panel 825 and has the advantage of having near
optically equivalent high reflectivity states, thus avoiding
flicker.
[0216] The NLC 820 is, for example, an out of plane untwisted
nematic liquid crystal, such as, for example, a pi-cell or ECB.
However, other configurations may also be used. In one switched
state, the NLC 820 is relaxed such that light propagating through
the NLC 820 experiences a retardance, and in another switched state
the LC molecules in the NLC 820 are essentially normal to the light
propagation direction and impart little or no retardance to the
polarization of the light.
[0217] The approach is to have two high, and two low reflectivity
states corresponding to the four possible states of the combined
compound retarder. For accurate optical inversion, the two high
reflectivity states associated with the two electrically inverted
FLC retarder 860 states are preferably nearly optically equivalent
and as achromatic as possible, and the two low reflectivity states
preferably exhibit as low a reflectivity as possible over the
entire visible spectrum to give good contrast. The design of a
practical inversion scheme preferably ensure both these
properties.
[0218] The following methodology, used to design a compound
retarder inversion scheme, according to the invention, preferably
results in two good low reflectivity states; chooses the two optic
axes of the two FLC retarder 860 states to allow the most
achromatic high reflectivity performance; and alters the passive
retarder 821 alignment to equate (as best as possible) the spectra
of the two high reflectivity states. The following mathematical
approach is consistent with the previously discussed M=3 and M=5
analysis. With respect to the achromatic inverters according to the
invention, it is desirable to trade off achromatic ON-states with
good OFF-states. Mapping polarization onto the central retarder
gives good OFF-states, which is mathematically equivalent to
orienting the compound optic axis along the input polarization
direction.
[0219] As previously discussed, low reflectivity with a compound
retarder can be obtained between crossed polarizers. This is
achieved when the polarization at the design wavelength (at which
all retarders of the compound retarder are substantially half-wave)
is mapped onto, or at approximately 90.degree. to, the optic axis
of the central retarder in the display 815, the central retarder
corresponds to the FLC retarder 860. Assuming in one of its states
the FLC retarder 860 has an orientation .theta..sub.A to the input
polarization direction, and we wish to have a low reflectivity
state with the NLC 820 driven high (i.e. effectively vanishing),
then the passive retarder 821 must be oriented at either
.theta..sub.P=.theta..sub.A/2.+-.90 (22)
or
.theta..sub.P=.theta..sub.A/2.+-.45 (23)
or
.theta..sub.P=.theta..sub.A/2 (24)
[0220] degrees from the direction of input polarization, as shown
in FIG. 25.
[0221] These expressions can be understood from the fact that the
passive retarder 821 reflects the orientation of the input
polarization about its optic axis (at the design wavelength).
[0222] The second low reflectivity state is when the FLC retarder
860 is oriented in its other orientation .theta..sub.B, where:
.theta..sub.B=.theta..sub.A.+-..theta..sub.S
[0223] and .theta..sub.S is the switching angle of the FLC retarder
860. In this state, the NLC 820 is in its relaxed, non-driven mode
and acts like a retarder. In the same way as above, the orientation
.theta..sub.N of the NLC 820 is chosen such that the input
polarization is mapped on the new orientation of the FLC retarder
860. In general, the effect of having two retarders in sequence is
to rotate the design wavelength by the difference in angle between
the optic axes of the two elements, as shown in FIG. 26.
[0224] Therefore, by simple geometry
.theta..sub.P-.theta..sub.N=.theta..s- ub.B/2 or
.theta..sub.B/2.+-.90. Rearranging this expression and substituting
for .theta..sub.B, we get either: 13 N = P - A 2 S 2 90 or ( 25 ) N
= P - A 2 2 2 25 or ( 26 ) N = P - A 2 S ( 27 )
[0225] Substituting (22)-(24) into (25)-(27) yields the following
options for the NLC 820 orientation angle: 14 N = ( S 2 ) 90 or (
28 ) N = S 2 45 or ( 29 ) N = S 2 ( 30 )
[0226] The two high reflectivity states are the remaining two
options for the FLC retarder 860 and NLC 820 states, namely i) when
the FLC retarder 860 is at .theta..sub.A and the NLC 820 is
oriented at .theta..sub.N, and ii) when the FLC retarder 860 is at
.theta..sub.B and the NLC 820 is driven high and effectively
vanishes. Computer modeling indicates that the most achromatic high
(and low) reflectivities in the case of a switchable compound
retarder are obtained when successive retarders are oriented with
angles as close to 90.degree. as possible from each other, and when
the polarization impinging on the FLC retarder 860 in the low
reflectivity configurations is at 90.degree. to its optic axis. In
the case of small .theta..sub.P<10.degree., this yields the
following expressions for the most achromatic compound retarder
inverter system: 15 P = A 2 - 45 ( 31 ) B = A - S ( 32 ) N = S 2 90
( 33 )
[0227] For any given FLC retarder, .theta..sub.S is fixed which
implies .theta..sub.N is also fixed. This means there is only one
degree of freedom in the above set of defining equations that can
be altered to equate the high reflectivity spectra. Fortuitously,
by altering the by small angles (3-7.degree.) from the direction of
the input polarization the spectra can indeed be made near
equivalent. This is a key feature of this embodiment. The
polarization manipulation of all four states can be shown in
diagrammatic form as shown in FIGS. 27(a)-(d).
[0228] The design procedure above has been carried out for two
specific cases here. For .theta..sub.S=38.degree. the following
solution is close to optimum assuming typical LC dispersion and a
design wavelength of 550 nm.
[0229] .theta..sub.P=4.degree.
[0230] .theta..sub.A=98.degree.
[0231] .theta..sub.B=60.degree.
[0232] .theta..sub.N=109.degree.
[0233] The four output states yield the spectra shown in FIG. 28,
assuming typical dispersion and a design wavelength of 540 mn.
[0234] For .theta.=45.degree. switching
[0235] .theta..sub.A=100.degree.
[0236] .theta..sub.B=55.degree.
[0237] .theta..sub.N=112.5.degree.
[0238] The four outputs yield the spectra shown in FIG. 29,
assuming typical LC dispersion and a design wavelength of 550 nm.
Comparing these outputs with the inverter solution, using no
passive retarders, as follows:
[0239] .theta..sub.S=45.degree.
[0240] .theta..sub.P=45.degree.
[0241] .theta..sub.A=0.degree.
[0242] .theta..sub.N=112.5.degree.
[0243] clearly shows the improvement in the matching of high
reflectivity states. The four outputs yield the spectra shown in
FIG. 30.
[0244] As discussed above, one example suggested for the NLC 820 is
the pi-cell. While pi-cells (i.e. bent mode) have poor field of
view (FOV) characteristics due to the large amount of retardance
that is present in the device, it is nevertheless attractive due to
its fast switching speed and its use of a thicker (>41 .mu.m),
lower cost cell. Proposed here is a method that can be used to
increase the FOV of the pi-cell when used in a reflective type
device, for example, in an inverter.
[0245] FIG. 31 shows another example of a display device 915
according to the invention. The display device 915 comprises a
polarizing beamsplitter 911; an in-line compensator 916; a first
passive retarder 920, preferably a quarter-wave plate; a first
pi-cell retarder 921, preferably a half-wave plate; transparent
substrates containing electrodes 921a, 921b for applying voltages
across the first pi-cell retarder 921; a second passive retarder
922, preferably a quarter-wave plate; and LCD panel 925. LCD panel
995 comprises a FLC retarder 960, preferably a quarter-wave plate;
and transparent substrates containing electrodes 970, 980 for
applying voltages across the FLC retarder 960 in accordance with
image data.
[0246] The orientations of the quarter-wave FLC retarders 920, 960
are parallel and perpendicular to the pi-cells 921, 922,
respectively. That is, the quarter-wave retarder nearest between
the LCD panel 995 and the pi-cell has its optic axis at
approximately 90.degree. to the optic axis of the pi-cell, and the
other quarter-wave being approximately 90.degree. to this or
parallel to the pi-cell optic axis.
[0247] This approach, according to the invention, employs the fact
that rays that are off axis see a twisted liquid crystal director
profile. The effect of this twist is shown in FIG. 32. As can be
seen in this specific case of a half-wave pi-cell the effect of
going off-axis is to produce a polarization that is elliptical and
is oriented at an angle relative to the optic axis of the pi-cell.
By placing a quarter-wave retarder (half-wave in reflective type
devices) between the pi-cell and the LCD panel, this polarization
major axis is reflected about the optic axis of the quarter-wave
retarder and the ellipicity is reversed (i.e. left hand rotation to
right hand or visa versa). The effect then of passing through the
pi-cell for the second time is to undo this effect and the
resultant polarization becomes linear. The additional quarter-wave
layer is added to the other side of the pi-cell to negate the
additional in-plane retardance from the other half-wave layer.
[0248] This compensation scheme works particularly well for the
inverter device according to the invention, and is compatible with
the inverter in which an extra retardation film is added between
the active inverter cell and the FLC retarder.
[0249] FIG. 33 shows a display embodiment comprising a polarizer
1010; a in-line compensator 1016, preferably having an in-line
compensation .GAMMA. of approximately 30 nm; a first pi-cell
retarder 1020, preferably a quarter-wave plate; transparent
substrates containing electrodes 1020a, 1020b for applying voltages
across the first pi-cell retarder 1020; substrate 1050a; a second
pi-cell retarder 1021, preferably a quarter-wave plate; and FIX:
transparent substrates containing electrodes 1021a, 1021b for
applying voltages across the second pi-cell retarder 1021. The LCD
panel 1095 comprises a FLC retarder 1060, and transparent
substrates containing electrodes 1070, 1080 for applying voltages
across the FLC retarder 1060 in accordance with image data. The
orientations of the various plates are shown schematically in FIG.
33a. The orientation of the integrated display device is along the
rubbing direction of the two parallel half-wave pi-cells.
[0250] FIG. 34 shows another display embodiment comprising a
polarizing beamsplitter 1111; a in-line compensator 1116,
preferably having an in-line compensation .GAMMA. of approximately
30 nm; a first pi-cell retarder 1120, preferably a quarter-wave
plate; transparent substrate containing electrodes 1120a, 1120b for
applying voltages across the first pi-cell retarder 1120; a first
passive retarder 1121, preferably a half-wave plate; a second
pi-cell retarder 1122, preferably a quarter-wave plate; transparent
substrate containing electrodes 1122a, 1122b for applying voltages
across the second pi-cell retarder 1122; a second passive retarder
1123, preferably a half-wave plate; and LCD panel 1195. The LCD
panel 1195 comprises a FLC retarder 1160, and a transparent
substrate containing electrode 1170 and a transparent substrate
containing pixilated mirror electrode 1180 for applying voltages
across the FLC retarder 1160 in accordance with image data.
[0251] The orientations of the various plates are shown
schematically in FIG. 34a. The orientation of the integrated
display device is parallel to the orientation of the central
achromatic half-wave plate. The two pi-cell retarders 1120, 1122
are then oriented at approximately +45.degree. and -45.degree. to
this direction, respectively. Since the integrated display device
acts as a net half-wave plate with a defined optic axis, it can
also be used with further passive half-wave retarders to equalize
ON/STATES. It is, however, considered fast enough not to use the
dynamic relaxation compensation of the two pi-cell embodiment
previously discussed.
[0252] The display embodiments shown in FIGS. 35 and 35a overcome
the effect on contrast due to unwanted reflection in the display
device. FIG. 35 shows an example of a display device comprising a
polarizer 1210; a first pi-cell retarder 1220, preferably a
quarter-wave plate; transparent substrate containing electrodes
1220a, 1220b for applying voltages across the first pi-cell
retarder 1220; a passive retarder 1221, preferably a half-wave
plate having an in-line compensation .GAMMA. of approximately 50
nm; a second pi-cell retarder 1222, preferably a quarter-wave
plate; transparent substrates containing electrodes 1222a, 1222b
for applying voltages across the second pi-cell retarder 1222; and
LCD panel 1295. The LCD panel 1295 comprises a FLC retarder 1260,
preferably a quarter-wave plate; and a transparent substrate
containing electrode 1270 and a transparent substrate containing
pixilated mirror electrodes 1280 for applying voltages across the
FLC retarder 1260 in accordance with image data. The orientations
of the various plates are shown schematically in FIG. 35a.
[0253] Real devices have interfaces between layers that cause
unwanted reflection that compromises contrast. In particular, the
field of view compensation schemes so far discussed have
significant reflection deriving primarily from current methods of
LC cell fabrication. To overcome this problem and ensure adequate
FOV, there are approaches that utilize the fact that thinner cells
have inherently better FOV. So in using cells that are to thin to
switch a full half-wave at the design wavelength, a push-pull dual
cell embodiment can be used which has similar dynamic compensation
to the of the two pi-cell embodiment previously discussed, but
requires an additional small (preferably approximately 50 nm)
retarder aligned with the switching retarder. In its relaxed state,
the approximately 50 nm retardance adds to the switching element's
retardance and, in its driven state, the dynamic compensator cell
is not driven quite so high to negate the approximately 50 nm
retardance plus any residual retardance from the switching element.
That is, the cell furthest from the LCD panel is driven high (e.g.,
>24V) for one state of the inverter and not so high (e.g.,
.about.12V) in the other. These correspond to the high and low
driven states of the switching cell, respectively. Since the
dynamic compensator cell is effectively driven with high voltage
throughout, masking of the relaxation, and hence effective high
switching speed, can still be achieved.
[0254] FIGS. 36-43 show configurations for the various reflective
display embodiments shown in FIGS. 33-35(a). The basic structure of
the display is illustrated in FIG. 36. The FLC retarder has two
states, depending on the polarity of applied voltage. The NLC
switch has two states depending on high and low voltage applied.
Therefore, there are a total of four states, two of high brightness
and two of low brightness, as shown in FIGS. 38(a)-38(d). FIGS.
37(a)-37(b) show the head-on spectra of four states.
[0255] Additional reflective display embodiments are illustrated in
FIGS. 39 and 41, with the respective combined four states
illustrated in FIGS. 40(a)-40(d) and FIGS. 42(a)-42(d),
respectively. An additional display or embodiment is illustrated in
FIG. 43. The configurations of FIGS. 39, 41 and 43 provide decent
field of view for an f/2.5 application. The response time can also
be below 100 .mu.s at 50.degree. C., since birefringence of LC cell
is about 550 nm (green cell).
[0256] FIGS. 44-47 show various display devices incorporated an
inverter according to the invention. In particular, FIG. 44 shows a
full color sequential display implemented with a transmissive
liquid crystal display, and utilizing an achromatic inverter
according to the invention. The sequential display comprises a
light source 2500, a two-polarizer digital color sequencer 2455, a
first inverter 2555, a transmissive pixelized liquid crystal
display 2560, a second inverter 2565, a polarizer 2570, a
projection lens 2580 and a display screen 2590.
[0257] The light source 2500 is suitably a metal halide lamp and
preferably emits optical power in all three primary color bands.
Alternatively, the light source 2500 can be implemented with an
active lamp system or with a lamp/color wheel combination.
[0258] In operation, the light source 2500 and the sequencer 2455
sequentially illuminates the liquid crystal display 2560 with red,
green and blue light. The liquid crystal display 2560 is
sequentially driven with red, green and blue image information in
synchronism with the red, green and blue illumination from the
light source 2500 and the color sequencer 2455. The liquid display
2560, in combination with the polarizer 2570, modulates the
intensity of the light that is sent to the screen 2590, in
accordance with image information. The inverters 2555, 2565 in
combination with the liquid crystal display 2560 provide four
states of brightness, two high and two low. The inverters 2555,
2565 effectively double the brightness of the display, by allowing
the negative image frame to be viewed, as previously discussed.
[0259] The full color sequential display of FIG. 44 can be
implemented as a front projection display in which the screen 2590
is viewed from the same side as the projection optics, or as a rear
projection display, in which the screen 2590 is viewed from the
side opposite the projection optics.
[0260] FIG. 45 shows a full color sequential display using a
reflective liquid crystal display, and utilizing an achromatic
inverter according to the invention. The display of FIG. 45 is
similar to the display shown in FIG. 44, except that a reflective
liquid display 3600 is used instead of a transmissive liquid
crystal display. In this configuration, a polarizing beamsplitter
3610 is used as both the output polarizer for the digital color
sequencer 2455 and as the input/output polarizer for the reflective
liquid crystal display 2600. Thus, the polarizing beamsplitter 3610
reflects light whose polarization is crossed with respect to the
polarization axis of the input polarizer 2450. In operation, light
that passes through the color sequencer 2455 is reflected by the
polarizing beamsplitter 2610 to the reflective liquid crystal
display 3600. The reflective liquid crystal display 3600
polarization modulates the light in accordance with the image
information and reflects the polarization modulated light back
towards the polarizing beamsplitter 3610. The polarizing
beamsplitter 3610 passes components of the light reflected from the
liquid crystal display 3600 that are orthogonally polarized with
respect to the light that was reflected from the polarizing
beamsplitter 3610 towards the liquid display 3600. Accordingly,
image information is displayed on the screen 2590. The inverter
3565 in combination with the display 3600 provide four states of
brightness, two high and two low. The inverter 3565 effectively
doubles the brightness of the display 3600 by allowing the negative
image frame to be viewed, as previously discussed.
[0261] FIG. 46 shows another display using a reflective liquid
display and utilizing an achromatic inverter according to the
invention. FIG. 46 is similar to the display shown in FIG. 45
except the three reflective liquid displays 3601, 3602,3603 are
utilized. Beamsplitter 3610 divides white light into the primary
colors, red, green and blue, which are displayed at displays 3601,
3602, 3603. The inverter 3566 working in combination with the
displays 3601, 3602, 3603 provide four states of brightness, two
high and two low. The inverter 3566 effectively doubles the
brightness of the displays 3601, 3602, 3603 by allowing the
negative image frame to be viewed, as previously discussed.
[0262] FIG. 47 shows still another display using a reflective
liquid display and utilizing achromatic inverter according to the
invention. FIG. 47 is similar to the display shown in FIG. 45, with
the exception that achromatic inverters 3567, 3568, 3569 is
provided for each of the three liquid crystal displays 3601, 3602,
3603. The beamsplitter 3610 divides white light into the primary
colors, red, green and blue, which are displayed at displays 3601,
3602, 3603. The inverters 3567, 3568, 3569 work in combination with
the respective displays 3601, 3602, 3603 to provide four states of
brightness, two high and two low. The inverters 3567, 3568, 3569
effectively double the brightness of their corresponding displays
3601, 3602, 3603 by allowing the negative image frame to be viewed,
as previously discussed.
[0263] In the displays shown in FIG. 44 and 45, the digital color
sequencer 2455 is position between the light source 2500 and the
liquid crystal display (2560 in FIG. 44 and 3600 in FIG. 45).
However, the digital color sequencer 2455 can be positioned at
other locations in the display system, provided that it effectively
controls the illuminating color at the output, i.e., the screen
2590.
[0264] By placing the digital color sequencer 2455 between the
light source 2500 and the liquid crystal display 2560 or 3600, the
image at the screen 2590 is not sensitive to any wave-front
distortion caused by the digital color sequencer 2505.
[0265] The foregoing embodiments are merely exemplary and are not
to be construed as limiting the present invention. The present
teaching can be readily applied to other types of apparatuses. The
description of the present invention is intended to be
illustrative, and not to limit the scope of the claims. Many
alternatives, modifications, and variations will be apparent to
those skilled in the art. In the claims, means-plus-function
clauses are intended to cover the structures described herein as
performing the recited function and not only structural equivalents
but also equivalent structures. For example, although quartz and
mylar may not be structural equivalents in that quartz is a
crystalline material, whereas mylar is a polymeric material, in the
area of birefringent materials, quartz and mylar may be equivalent
structures.
* * * * *