Note: Descriptions are shown in the official language in which they were submitted.
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97/00567
High Efficiency Projection Displays
Having Thin Film Polarizing Beam-Splitters
Field of the Invention
This invention relates generally to projection displays, and more
particularly, to 2-
D and 3-D projection display systems.
Background of the Invention
Polarizers and polarizing beam-splitters (PBS) are optical components that are
currently widely used in optical instruments, lasers, electro-optic displays,
optical
recording and in the fabrication of other optical components. There are
several parameters
1o that can be used to describe the performance of a polarizer or polarizing
beam-splitter.
These parameters are: the wavelength range over which the polarizer or
polarizing beam-
splitter is effecti~~e, the angular field of the incident light in which the
pofarizer or
polarizing beam-sputter is effective and the extinction ratio of the desired
polarizLd light
to the unwanted polarized light after the light passes through a polarizer or
polarizing
15 beam-sputter.
Commonly available polarizers and polarizing beam-sputters can be divided into
several types that depend upon different physical principles: pile-af~-plates
polarizers,
refection poiarizers. dichroic sheet polarizers, polarizers based on
birefringent crystals,
metallic grid polarizers, and thin film interference palarizers.
2o Pile-of plates polarizers consist of a stack of parallel transparent plates
that are
placed in series. 'they are mainly used in the ultraviolet and infrared parts
of the spectrum.
Normally, light is incident at each interface at the Brewster angle such that
all thep-
polarized light and only some of the s-polarized light is transmitted. If a
sufficient number
of such plates are placed in series, the transmitted light can be highly
polarized anal have a
25 high extinction ratio. Although these poiarizers act over a very broad
spectral region, their
angular field is limited.
CA 02233597 1998-03-30
WO 98/07279 PCTICA97/00567
-2-
Reflection polarizers are based on a similar principle but use light reflected
from
one or more surfaces to polarize a light beam. One advantage of this polarizes
is that its
performance is independent of the wavelength. However, its performance is very
sensitive
to the angle of the incident beam. An additional complication is that the
reflected light .
propagates in a different direction from that of the incident light.
Dichroic poIarizers are both wide-angle and broad-band and are based on the
absorption of light of one polarization. They can be very thin and are
convenient to use.
They are made of plastic and can be produced in large sizes and at low cost.
However, at
least 50% of light is lost by absorption in these polarizers and the
extinction ratio is not
l0 very high. Therefore, these polarizers are typically used in low power
applications in
which damage to the device due to light absorption is not a concern.
Polarizers based on birefringent materials also perform well over a broad band
of
wavelengths and a wide range of angles. These polarizers are based on the
total internal
reflection of light in birefringent crystals. Normally these polarizers are
comprised of two
birefringent crystal prisms that are in contact with each other. 'The optical
axes of the two
prisms are arranged in such a way that the refractive indices at both sides of
the contact
surface are the same for the ordinary light (polarized in one direction) and
are higher in the
first prism and lower in the second prism for the extraordinary light
(polarized in the other
direction), or vice versa. When unpolarized light is incident upon the
interface between
2o the two prisms and if the angle of incidence is larger than the critical
angle for the
extraordinary light, the ordinary light will be substantially transmitted
while the
extraordinary light will be totally reflected. Therefore, a very high
extinction ratio is
achieved. Many different arrangements for such polarizers exist. However this
type of
polarizes is costly and cannot be made in large sizes because of the limited
availability of
35 birefringent crystal materials.
Metallic grid polarizers transmit light whose electric field vector is
parallel to the
grid lines and reflect light whose electric field vector is perpendicular to
the grid lines.
These polarizers are effective over a wide spectral region and a wide range of
angles.
However, because of the difficulty of making large grids of very small
spacings, their use
30 is confined to the infrared or longer wavelengths.
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97/00567
-3-
Multilayer thin film plate polarizers basically consist of quarterwave layers
and
generally have a high extinction ratio. Unfortunately, they operate over a
narrow spectral
' region and have a small angular field. Another type of thin film polarizers
invented by
MacNeille (US. Patent No. 2,403,73 I ), makes use of the Brewster angle.
MacNeille
polarizers are comprised of thin films embedded between two prisms and their
extinction
ratio increases with the number of layers. MacNeille polarizers operate over a
very broad
spectral region, but are only effective over a very narrow range of angles,
usually the
angular field measured in air is of the order of ~3°. In addition, the
extinction ratios for
the reflected and transmitted beams are different. There is some contamination
of s-
1 o polarized light in the transmitted p-polarized Light. Mouchart et. al. in
a paper entitled
"Modified MacNeille cube polarizers for a wide angular field," Appl. Opt. 28,
2847
( I 989), have shown that it is possible to broaden the angular field of
MacNeiIIe polarizers,
but only at the expense of reducing the width of the spectral region over
which they are
effective.
D. Lees and P. Baumeister, in a paper entitled "Versatile frustrated-total-
reflection
polarizer for the infrared," Opt. Lett. 4, 66 ( I 979) describe a thin film
infrared polarizer
based on the principle of frustrated total internal reflection Fabry-Perot
filters. In this
device a high refractive index spacer layer is sandwiched between two low
refractive index
layers to form an etalon that is deposited onto the surface of a germanium
prism. The light
is incident at an angle that is greater than the critical angle - one that is
chosen in such a
way that the phase difference between s- and p-polarized light is 180°.
Therefore, if the
Fabry-Perot etalon condition is satisfied for the p-polarization, all the p-
polarized light will
be transmitted while the s-polarized light is reflected. and vice versa. The
phase difference
between the s-polarized and p-polarized light depends on the refractive index
of the
substrate and the angle of incidence. Because of the limited range of
refractive indices of
materials for the visible part of the spectrum. such polarizers can only be
constructed for
the infrared. Furthermore, because these polarizers are essentially narrow
band filters, the
band-width is small. Another disadvantage of these polarizers is that they
also have a
' small angular field. This is because the phase change on reflection and the
optical
3o thickness of the spacer layer change with angle in opposite directions.
CA 02233597 1998-03-30
WO 98/07279 PCT1CA97/00567
-4
Polarizers are essential optical components in liquid crystal displays (LCDs).
Currently, most liquid crystal displays are based on twisted nematic liquid
crystals that
require the use of two polarizers. The principles and advantages/disadvantages
of LCDs
are described in the following prior art references. L. E. Tananas Jr., Flat
panel displays
and CRTs, Van Nostrand Reinhoid Company, New York, 1985. Terry J. Scheffer et
al,
"Supertwisted-Nematic (STN) LCDS." SID'95 Seminar Lecture Notes, 1995. Webster
E.
Iioward, "Supertwisted-Nematic (STN) LCDS," SID'95 Seminar Lecture Notes,
1995.
Twisted nematic (TN) liquid crystal displays can be passive or active matrix
addressed. A
basic TN LC cell or picture element (pixel) consists of a liquid crystal layer
and two
Io transparent electrodes, and they are is normally sandwiched by the first
polarizer and the
second poIarizer (also called analyzer). The transmitting axes of the two
polarizers are
perpendicular (or parallel} to each other. Unpolarized incident light passes
through the
first polarizer and becomes linearly polarized. When no voltage is applied to
the liquid
crystal layer, the liquid crystal acts as a halfwave plate and rotates the
polarization of the
f 5 incident polarized Light by 90°. The Light will therefore be
transmitted by the second
poiarizer; and, the pixel is said to in the"on" state. When a voltage is
applied to a liquid
crystal pixel, the liquid crystal molecules align themselves with the electric
field and the
light does not undergo rotation in the plane of polarization. In this
instance, the light is
blocked by the second polarizer and the pixel is in an "off' state.
2o Currently. dichroic sheet polarizers are the only choice for both flat
panel and
projection LCD displays due to their wide angular fields and broad band
widths. The
reason for this is that other currently available polarizers either have
limited angular field,
limited band width or are limited in size or are too expensive to use
(birefringence
crystals). however, dichroic sheet polarizers are based on the absorption of
light of one
25 polarization in order to obtain polarized light. This absorption causes two
problems: ~0-
54% of the incident light is absorbed by the first polarizer and this makes
the display less
energy efficient; and, the light absorbed by the first polarizer and by the
second polarizer
(from the "off' pixels) is transformed into heat which can cause the
polarizer's
performance to deteriorate. This problem can be very serious in projection
displays where
30 lugh power Light beams are applied.
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97/00567
-5-
Although LCDs are relatively more efficient than other types of flat panel
displays
such as those based on TFELs and on plasmas, the typical total transmission of
light in an
active matrix addressed direct-view flat panel LCD is still only about 3-7%.
The total
transmission of LCD projection displays is higher because of the use of three
panels. It
follows from the above that both problems mentioned are due to the absorption
of light in
dichroic sheet polarizers and so their elimination or replacement by
polarizers based on
other principles is highly desirable.
To avoid the use of dichroic sheet polarizers, other types of LCDs that do not
use
the polarization scheme have been proposed. For example, polymer dispersed
LCDs
I o (PDLCDs} which use light scattering to distinguish between an "on" or "off
pixel have
been demonstrated. However, PDLCDs do not have high contrast and are still
technically
immature compared to TN LCDs.
Another way to overcome the light loss problem is to use a polarization
recovery
scheme; such schemes are proposed by Noriji Oishi, "Polarization forming
optical device,"
is U.S. Patent No. 5,359,455, 1994., by Masso Imai, "LCD Projector," U.S.
Patent No.
5283600, 1994; and by GIueck, E. Ginter, E. Lueder and T. Kallfass,
"Reflective TFT-
Addressed LC Light-Valve TV Projectors with High Light effciency," SID'95,
235238( I 995). In these schemes, the polarization plane of the light with the
unwanted
polarization is rotated by 90° and the light is then redirected into
the optical image system.
20 Therefore, no light is Lost. However, only NEC has thus far offered a
commercial L,CD
projection display using a polarization recovery scheme. The above
polarization recovery
schemes require the use of thin film MacNeille polarizing beam-sputters which
have a
very small angular field (t2° in glass) and their extinction ratio is
good for only one
polarization and is not so good for the other polarization. Because of this,
extra dichroic
25 sheet poiarizers are needed to clean up the beam. Furthermore, the above
schemes do not
solve the problem of the light absorption by the second polarizers.
Fuad E. Doany et al describes a way to form high performance projection
display
with two light valves, U.S. Patent No. ~, 5I7, 340, issued May I4, 1996. Thev
use one
polarizing beam-splitter to split light into two polarized beams and then
direct the beams
30 to the corresponding light valves. The images from the light valves are
reflected and then
CA 02233597 1998-03-30
WO 98/07279 PCTICA97l00567
-6-
combined by the PBS to form a single image which is projected onto a screen.
In their
display, they use polarizing beam-splitters(PBS) in a cube form that are based
on
conventional thin film PBSs. According to the limited performance of the
conventional
PBSs, displays based on this approach will suffer poor image contrast and low
efficiency
and can not be made practical. This is because of the low extinction ratios
and limited
angular field of the conventional PBSs.
To summarize, all proposed approaches to overcome problems associated with the
use of dichroic poIarizers in LCDs will require the use of non-absorbing,
broad-band and
high extinction ratio polarizing beam-splatters. Unfortunately, none of the
current
to available polarizers or polarizing beam-splatters meet all the above
requirements.
Lt I,i and J. A. Dobrowolski have proposed a method of designing of broad-band
and wide-angle polarizing beam-splatters, see the paper "Visible broadband,
wide-angle
thin film multilayer polarizing beam splatter," Appl. Opt. Vol.
35,n.13,p2221(1996). The
proposed polarizing beam-splatters are better than the conventional MacNeille
polarizers;
t 5 however, extra dichroic sheet polarizers are still required.
Therefire an object of the invention is to provide a high efficiency
projection
display system that will work over a wide range of angles.
Su~ntnary of the Invention
According to the present invention there is provided a projection display
system
2o comprising a light source for generating an input beam of light; a
polarizing beam
discriminator employing frustrated total internal reflection and thin film
interference to
discriminate between s and p polarized light, said discriminator separating s
and p
polarized light beams from said input beam; spatial light modulator means for
encoding an
image onto said respective s and p polarized beams; means for combining said
encoded s
25 and p polarized beams into an output beam, said combining means employing
frustrated
total internal reflection and interference; and means for focussing the output
beam onto a
display means.
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97/00567
_7_
The means for combining the encoded beams is preferably the same discriminator
employed to separate the uncoded beams in to s- and p- polarized beams, in
which case the
encoded beams are reflected back into the device in accordance with a scheme
that
depends on the type of spatial modulators employed. However, this is not
necessary and it
is equally possible to combine the beams with a second similar polarizing beam
discriminator downstream of the first. Instead of reflecting the encoded beams
back into
the first device, the encoded beams are transmitted to a second device where
they are
combined into the output beam.
The invention makes use of a thin film polarizing device of the type claimed
per se
to in our co-pending application no. PCT/CA96/00545. This device has a thin
film
arrangement for separating s-polarized light and p-polarized light by
reflecting p-polarized
light and transmitting s-polarized light. The device comprises first and
second light
transmissive substrates; and, a plurality of thin film layers disposed between
the first and
second light transmissive substrates. The thin flm layers comprise high
refractive index
1 s layers and low refractive index layers, the high refractive index layers
having one or more
different refractive indices, and the low refractive index layers having one
or more
different refractive indices; the first and second light transmissive
substrates, each in the
form of a prism having a refractive index greater than the refractive index of
each of the
low refractive index layers; the prisms being shaped in such a manner as to
allow the
2o incident light to be incident upon the thin f lm layers at a plurality of
angles greater than or
equal to the critical angle for the highest refractive index of the low
refractive index layers;
the thickness of the low refractive index layers of the plurality of thin film
layers being
small enough so that light incident upon the thin film layers at an angle
greater than the
critical angle can be partially coupled out through the low refractive index
layers such that
...+....+na +..+..t :_ .r_~ -_n__ ~~__
muauavcu m.ai ltCtttal tGllCCaiiJil ULC;LIIS, InuS pertniiting interference to
take place
between the light reflected at the interfaces of all the thin film layers, and
in addition, the
thicknesses of the thin film layers being such that the equivalent optical
admittance of the
plurality of the thin film layers for s-polarized light is substantially the
same as the optical
admittance of the substrate for s-polarized light for a wide range of angles
of incidence and
30 a broad band of wavelengths when the incident light is incident upon the
low refractive
index layers at an angle greater that the critical angle, thereby allowing
substantially all the
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97/00567
-g_
incident s-polarized light to be substantially transmitted; the plurality of
the thin film
layers having an equivalent optical admittance forp-polarized light that is
substantially
different from the optical admittance of the substrate forp-polarized light
for a wide-range
of angles of incidence and a broad-band of wavelengths and thus substantially
reflecting
all the incident p-polarized light. This polarizing beam-sputter 30 lends
itself to use in
various embodiments of the novel 2-D and 3-D projection system claimed herein.
The invention makes provides a high efficiency projection display system that
employs a non-absorbing, broad-band, wide-angle and high extinction ratio thin
film
polarizing beam-sputter. The display system that utilizes both s-polarized
andp-polarized
to light and projects both two-dimensional(2D) and three-dimensional(3D)
images.
Aspects of this invention make use of a novel design of a very broad band
{wavelength ratios as large as 50:1 ), very wide angular field (up to ~6 l
° in the infrared)
thin film polarizing device. These thin film polarizing devices are based on
frustrated total
internal reflection. l Iowever, the design approach permits polarizers with
varying
15 performance specifications to be produced for the ultraviolet, visible,
infrared, far infrared
to the microwave spectral regions.
Brief Description of the Drawings
The present invention and exemplary embodiments of the invention will be
described in accordance to the following drawings in which:
20 Fig. I . is a schematic view showing the beams of light transmitted and
reflected by the
interfaces of a single thin film structure;
Fig. 2. is a three-dimensional view of the phase change on reflection as a
function of the
angle of incidence and of the refractive index ratio for s-polarized light;
Fig. 3. is a three-dimensional view of the phase change on reflection as a
function of the
'S angle of incidence and of the refractive index ratio fore-polarized Light,
the refractive
index ratio yis between 0.2 and 1;
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97/0056'7
-9
Fig. 4. is a three-dimensional view of the phase change on reflection as a
function of the
angle of incidence and of the refractive index ratio forp-polarized light, the
refractive
index ratio y is between 1 and 3;
' Fig. 5. shows another three-dimensional view of the phase change on
reflection forp-
polarized light as in Fig. 4;
Fig. 6. is a cross-sectional view of a basic thin film structure having low,
high and low
refractive index layers embedded between two substrates;
Fig. 7. is a cross-sectional view of another basic thin film structure having
high. low and high
refractive index Iavers embedded between two substrates;
1 o Fig. 8. shows the real and imaginary parts of the equivalent optical
admittance of a basic
syrrunetrical three-layer structure and the optical admittance of the
substrate as a
function of wavelength ratio for s-polarized light;
Fig. 9. shows the real and imaginary parts of the equivalent phase thickness
of the same basic
structure as in Fig. 8 for s-polarized Light;
Fig. I 0. shows the real and imaginary parts of the equivalent optical
admittance of the same
basic structure as in Fig. 8 and the optical admittance of the substrate as a
function of
wavelength ratio forp-polarized light;
Fig. 1 I . shows the real and imaginary parts of the equivalent phase
thickness of the same basic
structure as in Fig. 8 forp-polarized Light;
Fig. 12. shows the calculated reflectance curves for s-polarized light as a
function of
wavelength ratio for thin film systems having 1, 20, 40 and 60 periods of a
three-layer
basic structure as in Fig. 8. The angle of incidence 6b is 57.3°.
Fig. I 3. shows the calculated transmittance curves for s-polarized light as a
function of
wavelength ratio for the same thin film systems as shown in Fig. 12. The angle
of
incidence 6i~ is 57.3°.
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97/00567
- 10-
Fig. 14. shows the calculated reflectance curves for p-polarized light as a
fiznction of
wavelength ratio for the same thin flm systems as shown in Fig. 12. The angle
of
incidence 6~ is 57.3 °. '
Fig. 15. shows the calculated transmittance curves forp-polarized light as a
function of
s wavelength ratio for the same thin film systems as shown in Fig. I2. The
angle of
incidence 6b is 57.3°;
Fig. 16. shows the calculated reflectance curves for s-polarized light as a
function of
wavelength ratio for a thin film system having 40 periods of the basic
structure as in
Fig. 8. The angles of incidences are 50°, 55°, 60°,
65° and 70°;
1o Fig. I7. shows the calculated transmittance curves fore-polarized light as
a function of
wavelength ratio for the same thin film system as in Fig. 16. The angles of
incidences
are 50°, 55°, 60°, 65° and 70°;
Fig. l 8. is a cross-sectional view of an extended basic symmetrical thin film
structure having
(2L-I) layers, L is larger than I;
t 5 Fig. I 9. shows the calculated reflectance curves for s-polarized Light as
a function of
wavelength ratio for thin film systems having 1, 50 and 100 periods ofa five-
layer
basic structure. The angle of incidence f~ is 57.3°;
Fig. 20. shows the calculated transmittance curves forp-polarized Light as a
function of
wavelength ratio for the same thin film systems as shown in Fig. I 9. The
angle of
2o incidence b~ is 57.3°.
Fig. 21. shows the calculated reflectance curves for s-polarized light as a
function of
wavelength ratio for a thin film system having 100 periods of the basic
structure as in
1~ig. 19. The angles of incidences are 50°, 55° and 60°;
Fig. 22. shows the calculated transmittance curves forp-polarized light as a
function of
?s wavelength ratio for the same thin film system as in Fig. 19. The angles of
incidences
are 50°, 55°, and 60°;
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97/00567
-II
Fig. 23. is a cross-sectional view of a thin film system used in the thin film
polarizing device
in accordance with the present invention;
Fig. 24. is a cross-sectional view of one embodiment of a thin film polarizing
device in
accordance with the present invention. The device acts as a polarizer and only
transmitted s-polarized light is used;
Fig. 25. is a cross-sectional view of another embodiment of a thin film
polarizing device in
accordance with the present invention. The device acts also as a polarizes and
only
re#Iectedp-polarized light is used;
Fig. 26. is a cross-sectional view of another embodiment of a thin film
polarizing device in
accordance with the present invention. The device acts as a polarizing beam-
splitter
and both transmitted p-polarized light and reflected s-polarized light are
used;
Fig. 27. is a cross-sectional view of an embodiment of a thin film polarizing
beam-splitter in
accordance with the present invention. This thin film polarizing beam-splitter
is
designed intended for projection display application.
Fig. 28. shows the calculated reflectance curves for s-polarized light as a
function of
wavelength ratio for the embodiment VIS-1 at different angles of incidence;
Fig. 29. shows the calculated transmittance curves forp-polarized light as a
function of
wavelength ratio fox the embodiment VIS-1 at different angles of incidence;
Fig. 30. shows the calculated reflectance curves for s-polarized light as a
function of
2o wavelength ratio for the embodiment V1S=2 at different angles of incidence;
Fig. 3I. shows the calculated transmittance curves forp-polarized light as a
function of
wavelength ratio for the embodiment VIS-2 at different angles of incidence:
Fig. 32. shows a Light element I with a white lamp;
Fig. 33. shows a Light element 2 u~ith a rotating colour filter wheel;
Fig. 34. shows a Light element 3 with colour LEI~s;
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97/00567
-12-
Fig. 35. shows a Light element 4 with colour lasers;
Fig. 36. shows a Imaging element I that is a monochrome, reflective, twisted
nematic LC
panel;
Fig. 37. shows a Imaging element 2 that is a colour, reflective, twisted
nematic LC panel;
s Fig. 38. shows a Imaging element 3 that is a monochrome, transmissive,
twisted nematic LC
panel;
Fig. 39. shows a Imaging element 4 that is a colour, transmissive, twisted
nematic LC panel;
Fig. 40. shows a Imaging element 5 that is a monochrome, reflective, polymer
dispersive LC
panel with a quarterwave plate;
to Fig. 41. shows a Imaging element 6 that is a colour, reflective, polymer
dispersive LC panel
with a quarterwave plate;
Fig. 42. shows a Imaging element 7 that is a monochrome, transmissive, polymer
dispersive
LC panel;
Fig. 43. shows a Imaging element 8 that is a colour, transmissive, polymer
dispersive LC
15 panel;
Fig. 44, shows a Imaging element 9 that is a monochrome digital micro-mirror
device;
Fig. 45. shows a Imaging element l 0 that is a colour digital micro-mirror
device;
Fig. 46. shows a Imaging system 1 in accordance with the present invention
that uses one
single transmissive imaging element made of TNLCs or PDLCs with quarterwave
2o plates and are capable of displaying high efficiency 2D images.
Fig. 47. shows a imaging system 2 in accordance with the present invention
that uses two
reflective imaging elements made of TNLCs or PDLCs and are capable of
displaying
high efficiency 2D and 3D images.
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97/00567
-13-
Fig. 48. shows a Imaging system 3 in accordance with the present invention
that uses six
reflective imaging elements made of TNLCs or PDLCs with quarterwave plates and
' are capable of displaying super high efficiency 2D and 3D images.
Fig. 49. shows a Imaging system 4 in accordance with the present invention
that also uses six
reflective imaging elements made of TNLCs or PDLCs with quarterwave plates and
are capable of displaying super high efficiency 2D and 3D images.
Fig. 50. shows a Imaging system 5 in accordance with the present invention
that uses three
transmissive imaging element made of TNLCs or PDLCs and are capable of
displaying super high efficiency 2D images.
Figs. 5 l a to 51 c show respectively a cross-sectional view along the line A-
A in Figure 51 b, an
underneath view, and a cross-sectional view along the line B-B in Figure 51 b;
and
Fig. 52. is a pictorial view of a 3-D projection display system.
Detailed Description of the Invention
The thin film polarizing device employed in the present invention is based not
only on
t 5 the principle of the fnistrated total internal reflection but also on the
interference of light in
thin films. Because of this, the thin flm polarizing devices are more
versatile than devices
obtained by conventional thin f lm design methods. General references to the
design of thin
film coatings including thin film polarizers or polarizing beam-splitters, can
be found in the
book entitled "Optical interference filters," written by H. A. Macleod
(MacGraw HilI,1986,
New York). The phenomenon of frustrated total internal reflection is described
in the paper
"Some current developments in multilayer optical filters," by A. F. Turner in
J. Phys. Radium
1 l, 440(i950), and related applications can also be found in the paper
"Optical tunneling and
its applications in optical filters," by P. W. Baumeister in App. Opt. 6,
897(1967).
Clearly, the interference effect of light will not only depend on the
reflection
coefficients r~ and r2, but also on the phase difference ~tp between two
adjacent reflected or
transmitted beams. The phase difference Osa can be expressed by the following
equations:
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97/00567
-14-
d~P = ~Pz ' ~P~ + ~,
~ ' Z~c
~ n,dt cosB,
where ~, is the wavelength of the incident Light and ~, is the phase thickness
of the Layer 4.
In conventional thin ftIm designs, either the angles of incidence of light are
less than
the critical angle or no critical angles exist. This means that for non-
absorbing materials the
phase changes on reflection gyp, and ~ for both s- andp-polarized light are
either 0° or 180° as
shown in Fig. 2 and Fig. 3. This is because r/o, rat and rf2 are real numbers.
Thus, once the
coating materials have been chosen, phase changes on reflection at the
interfaces have little
effect on the thin film design. Only variations in the Layer thicknesses can
be used to obtain a
desired performance.
l0 However, when n~nl and when the angle of incidence 6~ is larger than the
critical
angle ~=siri ~(noln,), the'reflection coefficient r, at the interface 3 will
no longer be a real
number. As a consequence, the phase change on reflection will also be much
more
complicated. Three-dimensional diagrams of the phase changes on reflection for
both s- andp-
polarized light, respectively, at the interface 3, are plotted as a function
of the angle of
I5 incidence b6 and of the refractive index ratio y (y=nolnl ) for s-polarized
light (Fig. 2) and p-
poiarized light (Figs. 3, 4 and 5). These figures are particularly revealing
of phenomena that
are utilized in accordance with this invention:
1. As stated before, when the angle of incidence bb is less than the critical
angle ~, the phase
change on reflection for s-polarized light is 0° (Fig. 2). The phase
change on reflection for
2o p-polarized Light is 0° when the angle of incidence ~ Lies between
0° and the Brewster
angle ~=tan' (no/n1) (6~>BB), and it is 180° when the angle of
incidence is between the
Brewster angle ~ and the critical angle 6~ (Fig4).
2. When the angle of incidence 6~ is larger than the critical angle 6~, the
phase change on
reflection for s-polarized light changes from 0° to l 80° as the
angle of incidence ~
25 increases from the critical angle ~ to 90° (Fig_ 2). Forp-polarized
light, the phase change
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97/00567
-IS-
on reflection decreases from 180° to 0° as the angle of
incidence 6b increases from the
critical angle b~ to 90° (Figs. 4 and 5).
Some conclusions can be also drawn from the above diagrams. First, the phase
change
on reflection increases or decreases rapidly when the angle of incidence 6b is
close to the
s critical angle B° or when the refractive index ratio yis small. In
other words, under those
conditions, the dispersion of the phase change on reflection with the angle of
incidence is very
large. Second, if the thickness of the low refractive index layer 4 is small
compared to the
wavelength of the incident light, some light will leak out from the low
refractive index layer 4
and enter the exit medium 6 even though the total internal reflection
condition is satisfied.
I o This phenomenon is called frustrated total internal reflection. Third, and
most importantly,
the phase change on reflection characteristics for s- and p-polarized light
are very different.
The above phenomena make it possible to design thin film coatings that are
based not
only on the interference effect of light in thin Elms but also on the
frustrated total internal
reflection of light in thin films. Therefore, in a multilayer structure
consisting of alternating
15 low and high refractive index layers, phase changes on reflection at the
interfaces as well as
the Layer thicknesses contribute to the interference effects. This provides an
extra degree of
freedom for the design of optical thin film coatings. The thin film coatings
can thus have very
different requirements for s- and p-polarized light.
It is evident from Figs. 2, 4 and S that, when light passes from a high
refractive index
20 layer to a low refractive index layer and when the angle of incidence is
larger than the critical
angle, the phase changes on reflection for the s- and p-polarized light are no
longer 0° or 180°.
Since the phase changes on reflection are different, such multilayer
structures are useful for
the design of thin film polarizing devices in accordance with the present
invention.
In multilayer structures of this type, low and high refractive index layers
affect the
25 propagation of light very differently. When the angle of incidence is
larger than the critical
angle, for the low refractive index layer, the optical admittance rh and the
phase thickness &,
are imaginary. Hence, the layer thickness of the low refractive index
therefore purely affect
the amplitude of the reflected light. In other words, the low refractive index
layer acts as an
attenuator or behaves like a metal layer, but without the effect of Light
absorption. If the
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97/00567
- 16-
thickness of the low refractive index layer is small compared to the
wavelength of Iight,
frustrated total internal reflection will occur at all the nHln~ interfaces
and some of the light
will leak out from the low refractive index layers and enter the adjacent
layers. The amount of
light leakage depends on the thickness of the low refractive index layer and
also on the angle
of incidence. On the other hand, at ni/nH interfaces, the total internal
reflection condition of
light is not satisfied, and therefore, the high refractive index layers act as
phase adjusters.
Their layer thicknesses contribute to the phase changes on reflection and do
not attenuate the
light. All the light beams reflected from the nHlnL and nt/nH interfaces will
interfere with each
other and, as a result, modify the reflection and transmission of the light.
The above phenomena of the interference and frustrated total internal
reflection in thin
films is fully utilized in the thin film polarizing device in accordance with
the present
invention. Several approaches can be used for the design of such thin film
polarizing devices.
For example, one can use one of various optimization methods described in the
paper by J. A.
Dobrowolski and R. A. Kemp, Appl. Opt. 29, 2876(1990), or the needle design
method
1 s described by S. A. Furman and A. V. Tikhonrovov in the book entitled
"Optics of multilayer
system, "published by Edition Frontiers in 1992, Gif sur-Yvette, or the
equivalent optical
admittance method as described in the book by 1. Tang and Q. Zhen, entitled
"Applied thin
film optics, "published by Shanghai Publishing House of Sciences and
Technologies in 1984,
Shanghai. Of these methods, the equivalent optical admittance method, applied
to a
symmetrical periodic layer structure is the best for a good understanding of
the physics of the
thin film polarizing device in employed in the present invention.
Fn Fig. 6, a basic thin f lm symmetrical structure 64 is comprised of a low
refractive
index layer 61, a high refractive index layer 62 and a low refractive index
layer 63 and the
layers are embedded between two identical substrates 60 and 65. The two low
refractive index
layers 61 and 63 are identical. Alternatively, as shown in Fig. 7, a basic
thin film symmetrical
structure 74 consists of a high refractive index layer 71, a low refractive
index layer 72 and a
high refractive index layer 73. Layers 7I and 73 are identical and all three
layers are
embedded between two identical substrates 73 and 75. The analysis for both
structures 64 and
74 is similar. In order to simplify the explanation process, in the following
text, the analysis
for the structure 64 shown in Fig. 6 will be discussed in detail and the
analysis for structure 74
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97/00567
-17-
is similar. The equivalent optical admittance Sand the equivalent phase
thickness I"of the
basic thin film symmetrical structure 64 can be calculated from the equations
derived in the
book "Applied Thin Film Optics" by J. Tang.
r/; (sin 2~, cossx + 2 ( r/L l r/x + r/f, l r~L ) cos2~L sin 8x - 2 ( r/L. . l
~7x - rlx l ~lr. ) sin 8x )
E=
(sin 2S, cos8x + 2 ( r~L l r/x + r/x l r/, ) cos2s,, sin~x + 2 ( r/,, l r/x -
r/H l ~L ) sin8f, )
I-' = arccos(cos28, coscSx - ~ (r/L l r/x + r/x l r/, )sin28L sin8x)
s (6)
where rh, ~, r/o, ~, and ~ are given by:
~L - nG COSBL
r/x = nx cosBx, for s-polarized light (7)
~To = no cos Bo
r~L = nL / cos8,
r/x =nx /cos6x, forp-polarized light (8)
~lo =~o /COSBo
(8L = 2nrzLdL cosBL / R
jll8x = 2~cnxdx cos9x / /1.
l0 When the angle of incidence ~ is larger than the critical angle for the low
refractive index
layer, rh and cos8,_, will be imaginary, and r~i and cosBi, will be real.
In order to transmit all the s-polarized light, the equivalent optical
admittance E of the
basic structure 64 should be equal to the optical admittance of the substrate
r/o for the s-
polarized light, i.e,
Is Es =nocosBo (10)
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97/00567
-IS-
For a given angle of incidence 6b and a wavelength of incident light, it is
possible to
fnd solutions for the layer thicknesses d, and d2 that satisfy Eq. (10).
Because there are two
variables and only one equation to satisfy, there will be an infinite number
of solutions.
Therefore, it is possible to specify a second angle of incidence or a second
wavelength in order
to broaden the angular field or the band width of the thin film polarizing
device.
It can be shown that a multilayer structure consisting of S periods of the
above basic
structure can be replaced by a single layer with the same equivalent optical
admittance E and
an equivalent phase thickness of S*I: The reflectance and transmittance of
such muItilayer
structure for s- andp-polarized light are best calculated using a computer
program based on
lo the matrix method as described in the book "Optical interference filters"
by H. A. Macleod.
To demonstrate the performance ofthe above basic thin film structure, a thin
flm
system with a n0 / aL l bH l aL / n0 structure has been designed. Here, a and
b stand for the
optical thicknesses of the low and high refractive index layers respectively,
and no=2.35,
n~=1.45, nH=4.0_ For an angle of incidence bb=57.3° and a wavelength
~?,o =4 urn, the optical
1 5 thicknesses a and ,3 were then calculated to be 17.9 nm and 31.3 nrn. The
corresponding
calculated equivalent optical admittance E and equivalent phase change on
reflection hfor
both s-polarized light andp-polarized light are plotted in Figs. 8 and 9 and
Figs. 10 and I 1,
respectively. It is clear from the above equations that both the equivalent
optical admittance
and the equivalent phase thickness T could be complex. Hence, both the real
and the
20 imaginary parts of the two parameters are plotted in the above diagrams.
As shown in Fig. 8, the equivalent optical admittance for s-polarized light
has only real
part and is completely matched to the optical admittance of the substrate r/o
over a very broad
spectral region 0.2<g=,I,Q /~,<2. Therefore, very little s-polarized light is
reflected and most of
the s-polarized light is transmitted over this broad wavelength region. This
is conf rmed by
2s the calculated reflectance and transmittance of the s-polarized light shown
in Figs. 12 and 13.
As the number of periods of the basic structure S increases, the equivalent
optical admittance
is the same for s-polarized light. Therefore, the calculated reflectance does
not show any
significant change over the above broad wavelength region as is shown in Fig.
I2. Hence, the
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97/00567
_ 19-
performance for s-polarized light is essentially the same, regardless of the
number of periods
S.
As shown in Fig. 10, the equivalent optical admittance for the p-polarized
light has
only real part and it is different from the optical admittance of the
substrate. Therefore, some
of thep-polarized light will be reflected and the rest will be transmitted as
shown in Figs. 14
and I 5 respectively. The equivalent phase thickness T has only positive
imaginary part (Fig.
11 ), hence, the whole thin film structure acts like a amplitude attenuator
for the p-polarized
light. The transmittance or reflectance forp-polarized light will depend
strongly on the
number of periods S or the equivalent phase thickness of S*T, the imaginary
part of the
l o equivalent phase thickness Tas well as on the wavelength ~.. The larger S
or the shorter the
wavelength is, the smaller the transmittance forp-polarized light is . It is
possible to achieve
almost any degree of attenuation in transmission for the p-polarized light by
increasing S as
shown in Fig. 15.
The calculated reflectance for s-polarized light and transmittance forp-
polarized light
is are also plotted in Figs. 16 and 17, respectively, for different angles of
incidence 8a. in this
case, the number of periods S was fixed to 40.
Clearly, the above thin film system acts as a very good polarizing beam-
sputter over a
very broad spectral region 0.2<g=,Z,o /a,<2 ~d a very wide range of angles of
incidence from
50° to 70° measured within the substrate. The high to low
wavelength ratio is about 10 and
2o the equivalent angular field in air is f24°. By comparison, a
typical thin film MacNeille
polarizer has a wavelength ratio of 2 and an angular field of~3° in
air.
Thus, a thin film system having S periods of such a basic thin film structure
can be
used as a novel broad-band and wide-angle polarizing device. It can be used
for separating s-
polarized and p-polarized light by transmitting s-polarized light and
reflecting p-polarized
25 light. This is contrary to a conventional thin film plate polarizes or a
MacNeille polarizing
beam-splitter that reflects s-polarized light and transmits p-polarized light.
Although the above explanations are based on a simple three layer basic
structure,
clearly, without departing from the spirit and scope of the present invention,
the basic structure
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97/00567
-20-
can be extended to consist of more than three layers of alternate high and low
refractive index
layers as shown as in Fig. 18. The basic thin f Im structure 186 has (2L-I )
alternatively low
and high refractive index layers and the layers are embedded between two
identical substrates
180 and 189. Clearly, the analysis and equations described for the three-layer
basic structure
in the above section can also be equally applied to the thin film structure
described in Fig. 18.
Apparently, the center three layers 182 can be replaced by a single equivalent
layer EQi. This
equivalent layer EQ, together with the adjacent two layers marked as 184 in
turn can be
replaced by another equivalent layer EQ2. Repeating the same process, the
whole thin film
structure 186 can finally be replaced by a single equivalent layer EQL_j.
t o 'The apparent advantage having more than three layers in the basic
structure in
accordance with the present invention is that there are more parameters to
choose in order to
meet the requirements for different design wavelengths or angles of incidence.
According to
Eq. ( 10), if the equivalent optical admittance of the equivalent layer EQ~_~
is equal to the
optical admittance of the substrate, all the s-polarized light will be
transmitted while some of
t5 p-polarized light will be reflected. By increasing the number of periods in
the basic structure,
all the p-polarized Light can be reflected. If the refractive indices in the
basic structure are
fixed and the layer thicknesses are allowed to vary, there will be (2L-I)
variables and only one
equation to solve. Therefore, there will be much more freedom to choose the
layer thicknesses
to meet additional requirements. Thus, much wider band-width or wider angular
field thin
2o film polarizing device can be obtained.
To illustrate the above principle, a thin film polarizing device having five
layers in the
basic structure ( n0 / aL l bH l cL l bH 1 aL / np ) has been designed. Here,
a, b and c stand for
the optical thicknesses of the layers, and no=2.35, nL=I .45, nn=4Ø The
original design
wavelength is 4l.em and the angle of incidence is 57.3°. The optical
thicknesses a, b and c
25 were calculated to be I I .8 nm, I0.6 nm and 4I .8 nm, respectively. The
calculated reflectance
and transmittance curves are plotted as a function of the wavelength ratio in
Figs. I9 and 20
respectively. The thin film polarizing devices are shown with I, 50 or 100
periods of the basic
structure. The calculated reflectance and transmittance curves at different
angles of incidence
are also plotted in Figs. 21 and 22 for the case with S =100. Clearly, the
thin film polarizing
3o device is effective over a very broad band of wavelengths. For the thin
film polarizing device
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97/00567
-21 -
having 100 basic structures, the high to low wavelength ratio is as large as
S0:1. The highest
reflectance for s-polarized light is less than 3x10-5 and the highest
transmittance forp-
polarized light is about 1 x 10-I at X0.02 for angles of incidence between
50° to 60°. This
highest value of transmittance for p-polarized light can be reduced by
increasing the number of
periods while the reflectance for s-polarized light will be essentially the
same.
In addition, it is understood that each low refractive index layer in the
basic structure
can be replaced by a number of low refractive index layers having different
refractive indices
and each high index layer can be replaced by a number of high refractive index
layers
providing the critical angle condition is satisfied for each of the low
refractive index layers. In
addition, the performance of the initial thin film polarizing device based on
the above
symmetrical thin film structure can be further improved with the assistance of
a computer
optimization program wherein the layer thickness of each layer in the thin
film system will be
optimized. In the optimization process, the dispersion of the refractive
indices can be taken
into account. It is also possible to have two substrates having different
refractive indices
15 providing the critical angle condition is satisfied for the substrate
having the Lower refractive
index. Normally, after the optimization procedure, the thin f Im system will
not retain the
symmetrical structure any more unless special steps are taken.
In principle, it is always possible to design a thin film polarizing device
having a
multilayer shown in Fig. 18, providing that the angle fib is larger than the
critical angle for low
20 refractive index layers (nL<no<n~). However, the phase dispersion is very
large when the
angle of incidence is close to the critical angle. Thus, in practice, it is
easier to design wide
angle thin film polarizing devices in accordance with the present invention
when the angle of
incidence is not that close to the critical angle. In addition, the phase
dispersion is large when
the refractive index ratio nH/n~ is small. This is particularly the case in
the visible part of the
25 spectrum because there the highest available refractive index ratio is
about 1.75. In the
infrared the refractive index ratio is of the order of 4Ø For this reason
infrared broad band
and wide angle polarizers have a much better performance than corresponding
visible thin film
polarizing devices.
In general, a novel thin film polarizing device employed in the present
invention, as
3o shown in Fig. 23, is comprised of first and second light transmissive
substrates 230 and
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97/00567
-22-
231, and a plurality of thin film layers 238 disposed between the first and
second fight
transmissive substrates. The thin film layers 238 consist of alternating high
refractive
index layers 233, 235, etc., and low refractive index layers 232, 234, etc.
Each high
refractive index layer can include a number of high refractive index sub-
layers having one
or more different refractive indices, and each low refractive index layer can
include a
number of low refractive index sub-layers each having one or more different
refractive
indices. The first and second light transmissive substrates, each in the form
of a prism,
have a refractive index that is greater than the refractive index of each of
the low refractive
index layers. The thicknesses of the low refractive index layers of the
plurality of thin film
t o layers are small enough so that light incident upon the thin film layers
at an angle greater
than the critical angle can be partially coupled out through the low
refractive index layers
so that frustrated total internal reflection occurs. This permits interference
to take place
between the light reflected at the interfaces of the all thin film layers. In
addition, the
thicknesses of the thin film layers are such that the admittance of the
plurality of the thin
film layers for s-polarized light is substantially the same as the optical
admittance of the
substrate for s-polarized Light for a wide range of angles of incidence and a
broad band of
wavelengths when the incident light is incident upon the low refractive index
layers at an
angle greater that the critical angle. This permits substantially aII incident
s-polarized
light to be substantially transmitted. The plurality of the thin f lm layers
have an
2o admittance forp-polarized light that is substantially different from the
optical admittance
of the substrate for p-polarized light for a wide-range of angles of incidence
and a broad-
band of wavelengths and thus they substantially reflect incident p-polarized
light. The
prism is shaped in such a manner as to allow the incident Light to be incident
upon the thin
film layers at a plurality of angles greater than or equal to the critical
angle for the highest
refractive index of the low refractive index layers.
Figs. 24, 25, 26 and 27 represent cross-sectional views of four arrangements
for
suitable thin film polarizing devices for use in the present invention. The
multilayer thin film
coatings are embedded between two prisms. The shapes of the prisms are
selected in order to
make the angle of incidence of the incident light at the hypotenuse larger
than the critical angle
3o for the low index layers. The two prisms could be made of the same
material. In the
arrangement shown in Fig. 24, the shapes of the two prisms 240 and 244 are
identical and the
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97/00567
- 23
thin film coating 246 is deposited at the hypotenuse interface. The thin film
polarizing device
acts as a transmissive polarizes. Only the transmitted s-polarized light is
used and the
reflectedp-polarized light is absorbed by a light absorber 246. In the
arrangement shown in
Fig. 25, the shapes of the two prisms 250 and 254 are different. The thin film
coating is also
deposited at the hypotenuse interface. The thin film polarizing device is also
configured as a
reflective polarizes and only the reflected p-polarized light is used. The
transmitted s-
polarized light is absorbed by a light absorber 256. In the arrangements shown
in Figs. 26 and
27, both devices are configured as polarizing beam-splitters. Therefore, the
reflected p-
poiarized light and the transmitted s-polarized are both used. In the
.arrangement in Fig. 26,
1 o the two prisms 260 and 264 have different shapes while in the arrangement
in Fig. 27 they are
identical. The.advantage of this latter arrangement is that the unpolarized
beam can be
incident on either top-sides of the device. In fact, if a symmetrical layer
system solution is
found for the multiiayer 272, the light can be incident on any of the four
sides of the prism
arrangement.
Although the two prism substrates can have different refractive indices,
normally in
practice, they are made of the same material in order to reduce manufacturing
costs. The two
prisms may be joined together in various ways. For example, they can be joined
together with
optical cements that have refractive indices matching the refractive index of
the substrate.
They can also be joined with a liquid that has a refractive index matching the
refractive index
of the substrate and the out-most edges of the two contact faces of the two
prisms are then
sealed. The two prisms, with coatings on one or both prisms, can also be
brought together by
using optical contact. This technique has been successfully developed for the
construction of
high laser damage threshold polarizers.
Because the thin film polarizing devices in accordance with the present
invention are
based on frustrated total internal reflection, the layer thicknesses are a
fraction of the mean
wavelength of the designed spectral region. Hence, the layers are very thin
compared to those
of conventional thin film polarizers and thus it should be less costly to
manufacture such
systems. This is especially important in the case of far infrared polarizers
where normally the
total layer thicknesses are very thick and require a very long deposition
time.
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97/OOS67
-24-
The thin film coatings in the thin fim polarizing devices can be manufactured
by
conventional physical or chemical thin film deposition techniques, such as
thermal
evaporation, sputtering, ion-plating and plasma assisted evaporation. Those
process can
produce good quality thin films. Since no absorbing coating materials are used
in the thin fzlm
systems, the thin f lm polarizing devices are very durable both physically and
chemically.
The thin film polarizing devices employed in the present invention are very
broad-band
and are effective over a very wide range of angles. By controlling the layer
thiclcnesses, the
extinction ratio in transmitted light can assume almost any value. The
extinction ratio in
reflected light is also very high when compared to that of conventional thin
film polarizers. If
t o necessary, another polarizing device of the same type can be placed in
series to obtain an even
higher extinction ratio. The novel thin f hn polarizing devices can be used in
almost all
applications where current polarizers or polarizing beam-sputters are used.
For example, in
the visible and near infrared spectral regions, the novel thin film polarizing
devices in
accordance with this invention can be used to replace more expensive
polarizing devices based
15 on birefringent crystals. In the infrared and far infrared regions they can
replace metal grid
polarizers.
Two embodiments of the thin film polarizing beam-splatters or thin f lm
polarizing
devices, also referred to as polarization beam discriminators, that will be
used in the high
effciency 2D/3D projection displays in accordance with the present invention
have been
20 obtained. The layout of the thin film polarizing device is chosen as shown
in Fig. 27 having
two identical dove prisms and symmetrical thin film coatings. All four faces
of the thin film
polarizing device can be used to accept light. The two thin film coatings, VIS
1 and VIS 2,
were designed for visible light having wavelength region of 0.4-0.8 l,un. For
the visible thin
film polarizing devices, substrates with refractive indices of I .75 and I.85
and coating
25 materials having refractive indices of I.38 and 2.35 were selected. Plots
of the spectral
performances of these thin f lm coatings V1S=I and VIS 2 at dii~erent angles
of incidence are
given in Figs. 28 and 29 (VIS I), and Figs. 30 and 3I (VIS-2), for s-polarized
andp-polarized,
respectively.
Clearly, in terms of band-width, angular field and extinction ratios, all two
3o embodiments are signifcantly better than conventional thin film polarizers
or polarizing
CA 02233597 1998-03-30
WO 98/07279 PCT/CA97l00567
-as-
beam-splitters. The wavelength ratios of 2:1 and 1.5: l, the angular fields of
~11.4° and
115.8° in air, and the minimum extinction ratios of 1 x 104:1 and 1.5x
104:1 are obtained for
VIS-1 and VIS-2 respectively.
An error analysis has shown that random errors of the order of t 1 or f2% in
the
thicknesses of the layers of polarizers of this type will not unduly affect
the performance of the
devices. This, of course, relaxes the manufacturing tolerances for these
devices. Several
prototype of the above polarizing beam-sputters have been fabricated in
standard thin film
deposition systems. The measured performances showed very good agreements with
the
calculated performances. They exceed even the most strict requirements for
PBSs in very high
l0 performance projection displays.
Numerous other thin film polarizers based on interference and frustrated total
internal reflection may be employed without departing from the spirit and
scope of the
invention. Far example, other prism materials and coating materials can be
used also. In
the infrared, the prism material may be made of ZnS, ZnSe, Si, Ge, etc. In the
visible, the
15 substrates may be made of various glasses and various plastics. The coating
materials can
be selected from the common materials used in conventional thin film coatings,
such as
MgF2, ThF4, Si02, A1203, Zr02, Ti02, Ta~05, Nb205, Si, ZnS, ZnSe, Si, Ge, etc.
The preferred embodiments of this invention include the above novel thin film
polarizing beam-sputter described heretofore. Since the thin film polarizing
beam-splitter
2o works in a different manner from conventional beam-splitters, different
configurations for
high effciency 2-D and 3-D projection displays are provided.
A high e~ciency projection display system in accordance with the principles of
the
present invention generally comprises a lighting element, a novel thin film
polarizing
device, at least one imaging element, a projection lens, and an optional
screen. The
25 lighting element provides tight that is used to form images. The imaging
elements, also
referred to as spatial light modulators, consists of plurality of small pixels
that can be
addressed individually to form images. The thin film polarizing device as
designed above
splits the unpolarized incident light beam into two polarized beams and
project them onto
the corresponding imaging elements, and then combine the images from the
imaging
CA 02233597 2002-11-25
-2b-
elements. The projection Ions projects the combined image onto a screen which
then is
viewed
based on the type of imaging elements, the lighting element can take different
forms as shown in Figs- 32-35. In Fig. 32, the first lighting element shown in
Figure 32
comprises a lamp 300, a reflecting mirror 301, a cold mirror 302 to reraove
unwanted
infrared and ultraviolet light from the light beam and a collimating lens 303_
The lamp
emits white light that contains three primary colours of blue, green and red
in order to form
black arid white, or colour images. In Fi,g.33, the second lighting element
consists of a
lanop 300 which emits vvhitc light, a reflecting mirror 301, a cold mirror
302, a focusing
14 lens 313, a colour filter wheel 310 mounted with R, G, B colour filters,
and a collimating
lens 303. The colour filter wheel rotates at a fast speed that farms a
sequential-colour light
which is synchronized with the imaging eleraent. In Fig. 34, R, G and B colour
light
emitting diodes (LEDs) 312 are used in combination with the collimating lens.
The three
LEDs can be turned on sinoultaneously to give a white colour or sequentially
to forYn time-
sequential colour which is syncluonized with the imaging element. The lighting
element
shown in Fig. 35 is the same as the lighting element shown in Figure 34,
except that the
LEDs are replaced by three Rcd, Green, and Blue Iasezs.
The imaging elerraents shown in Figs. 36 to 39 are all based on twisted
nematic
liquid crystal displays_ In Fig. 36, the first imaging element is a monochrome
reflective
LCD. When no voltage is applied to the liquid crystal cell or pixel 320, the
,polarization of
an incident polarized light is rotated 90°. When a,roltage is applied
to the liquid cell or
pixel, no polarisation rotation occurs. The incident beam will retain is
original
polarization. To view an image, a second polarizes is needed. The second
imaging
element shown in Fig. 37 is similar to imaging element shown in Figure 34
except that R,
G and B colour filters 321, 322, 323 are added to eaclx individual liquid
crystal pixel to
only allow the intended colour light to be reflected. The imaging elements
shown in Figs.
38, 39, are the transmissive versions of imaging elements the imaging
elernents shown in
Figures 35 and 36 respectively. Incident light is transmitted, instead of
reflected by the
imagimg elements. The imaging elements shown in Figs. 40-43, are based on
polymer
3o dispersive liquid crystal displays (PALCs). 1n the imaging element shown in
Fig. 40,
r.. . ~- .~..
CA 02233597 2002-11-25
- 27 -
when no voltage is applied to the pixel 420, the incident light will be
scattered and very
little light will be collected by era optical system to form images, this
pixel is said to be
"off", l~Vhen voltage is applied to the cell or pixel, all the light will be
reflected. To use
PDLCs in combination with the thin film polarizing beam-splitters, an extra
duarterwave
s plate 330 is added. The puzpose of it is to rotate the polarization of the
"on" pixels by 90°
as in the case oftho imaging element shown in Figures 35, 36, 38 and 39. The
imaging
elenraent shown in Fig. 41 is the colour version of the imaging element shown
ixl Figure 39.
Additional R, Ki, and ~ flters are added to each individual liquid crystal
pixel to form
colour pixels. The imaging elements shown in Figures 42 and 43 are the
transmissive
versions of imaging elements shown in Figures 39 and 40. Instead of reflecting
light, these
imaging elements transmit light. The imaging elements shown in Figs. 44-4S are
based
an digital micro-mirror devices. Each individual pixel is a mirror that can
reflect the
iz~cidont beam and can be individually rotated by electronic means to a "on"
gosition in
which the reilactcd light can be collected by an optical system, or to a "off'
position in
which the reflected tight is out of the collecting angle of the optical system
and is absorbed
by an absorber. The imaging element shown in Fig. 44 consists of only
monochrome
mirrors while the imaging element shown in Fig. 4S consists of colour mirrors
as described
in T 3S patent no. 5,619,059 issued April $, 1992.
Different embodiments of high eff ciency display systems can be formed by
combining the above lighting elements, the thin filmy polarizing devices and
the different
imaging elemCntS.
In the first embodinmnt, shown in Fig. 46, the display system consists of a
thin flm
polarizing device 400 in accordance with the present invention, a lighting
elem~x~t 401, x
transmissive imaging element 402 that can be selected from the imaging
elements shown
in Figure 38 (TNLC) or Figure 42 (PDLC) to form monochrome images, and the
imaging
elements shown in Figure 39 (TNLC) or Figure 43 (PD)rC) to form colour images,
two
mirrors 403, 404, a projection lens 44S and an optional screen 406. in
operation,
unpolarized light front the lighting element x01 that an be selected $om
lighting elements
shaven in Figures 32 to 3~ is launched into the end face 407 of the hexagonal
thin film
3o polarizing device 400 at an angle of incidence greater than the critical
angle at the thin film
CA 02233597 2002-11-25
-28-
surface and is split into two polarized light beams by the thin film coating.
The p-
polarized light is reflected from the thin film layers to the end face 408;
the s-polarized
light is transmitted.through the thin film layers and exits the end face 409.
The reflectedp-polarized tight exiting the end face 408 is then incident upon
the
first mirror 403 and is directed to the transmissive imaging element 402. ror
the "on"
pixels, the polarization of the light is not changed by the imaging element;
therefore thep-
polarized light retains its polarization. After being reflected by the second
mirror 404, the
transmitted Iight from the "on" pixels hits the thin fllm polarizing device
440 through the
end face 409, it is reflected again by the thin film layers. The light then
exits tho and face
410 and is projected onto the screen by the projection lens 40$. For the "off'
pixels, the
polarization of thep-polarized light is rotated 90° by the imaging
element 402 and
becomes s-polarized- Therefore, the light is trFUismitted back off the thin
film layers
toward the lighting element 401 and is not proj ected onto the screen.
The transmitted s-polarized light exiting the ez~d face 409 is first incident
upon the
second mirror 404 and is directed to the transmissive imasing element 402_ For
the "on"
pixels, the polarization of the light is not changed and the s-polarised light
remains its
polarization and is subsequently transmitted again by the thin film polarizing
device 400
and projected onto the screen. The light from the "off' pixels is rotated
90° and is sent
back toward the lighting element 401.
2o In this embodiment, to form colour images, a white lighting element 1 in
combination with colour TNLC or PDLC can be used, yr the valour lighting
element
shown in Figures 33-35 in combination with monochrome TNIrC or PDLC can be
used.
Since both s-polarized and p-polarized light arc completely used for imaging,
no light loss
is present due to Choir polarization. Therefore, very high efficiency 2D
projection display
2s can be obtained. In addition, this embodiment is very cornpaet and can be
made in low
cast.
The display system shbwn in Fig. 47 consists of a thin film polarising device
400, a
lighting element 401, two identical reflective imaging elements 424 and 421
that can be
selected from the imaging elements shown in Figure 36 (TNL,C} or Figure 4U
(PDLC} to
CA 02233597 2002-11-25
_29_
farm monochrome images, and the imaging elements shown in Figure 37 (TNLC) or
Figure 41 (PDLC) to form colour images, a projection lens 405 and an optional
screen
406_ In operation., unpolarized light from the lighting element 401 is
lauanched into the end
face 407 of the thin film polarizing device 400 and is split into two
polazized light beams
by the thin film coating. Thep-polarized light is reflected fmm the thin film
layers to the
end face 408; the s-polarized light is transmitted through the thin film
Jayers and exits the
end face 409.
The reflected p-polarized light exiting the end face d08 is then incident upon
the
first reflective imaging element 420. For the "on" pixels, the polarization of
the light is
rotated 90° by the imaging element; therefore the p polarized Jight
becomes as s-polarized.
When the reflected light from the "on" pixels again hits the thin film
lrolarizing device 400
through the end face 408, it is transmitted through by the than film layers
and exits at the
end face 410 and then is projected onto the screen by the projection lens 405.
For the "off'
pixels, the polarization of the Jight is not changed by the imaging element
420 and,
l s therefore, the light is reflected Back off the thin film layers toward the
lighting element 401
and is not projected onto the screen- In this case, the project$d image coming
from the
imaging device 420 only consists of s-polarized light.
The transmitted s-polariz8d light exiting the end face 409 is then incident
upon the
second reflective imaging element 421. For the "on" pixels, the polarization
of the light is
2o rotated 90° by the imaging element, therefore the s-polarized light
becomes as p-polarized.
When the reflected ligkat from the "on" pixels again hits the thin iihn
polarizing device 400
through the end face 409, it is reflected by the thin film layers and then
projected onto the
screen by the projection leas 405. Far the "ofF' pixels, the polarization of
the light is not
changed by the imaging element 421 Gild, therefore, the light is transmitted
back off the
zs thin film layers toward the lighting element 401 and is not projected onto
the screen_ In
this case, the projected image only consists ofp-polarized light.
In this embodiment, to form colour images, a white lighting element of the
type
shown in Fig, 32 in combination with the colour TNLC or PDLC can be used, or
colour
lighting element of the type shown in Figures 33 to 35 in combination with
monochrome
3o TNLC or PDLC can be used. Clearly, if the images on the imaging elements
403 and 404
...._".~ _.... _
_._ e.....".~..
CA 02233597 2002-11-25
- 30 -
are identical and arc mirror images of each other, the combined images will be
identical
too, and a single 2-D image is seen on the screen- In addition, if the images
on the
imaging el4,ncnta 420 and 421 represent right-eye and Ieft-eye images (also
mirror images
in one case), the combined images can be viewed through a polarizing glass and
three-
s dimensional effects can be perceived. Since both s-polarized and p-polarized
light are
fully used for imaging, no light lost is present due to their polarization.
Therefore the
. display is very efficient and compact when compared to conventional >JCDs.
Therefore,
very high efficiency projection display that is capable of displaying 2D and
3D images can
be obtained. 2D and 3D display mode can be easily switched by just changing
the input
lo image signals to the 'imaging elements. No optical reconfiguration is
required for
displaying 3D images except that viewers have to wear polarizing glasses. This
embodiment are specially suitable to home theater.
The display system shown in shown in Figure 48 is a super high efficiency
2DI3D
projection display. 1t consists of one thin film polarizing device 400, a
lighting element
t5 401 that emits white light, six identical reflective imaging elements 440,
441, 442, 4.43,
444, and 44.5 that can be selected from monochrome ima,&ing elements shown in
Figure
3G ('TNarC) or Piguro 40 (PDL.C), two identical "X" shape colour separation
cubes or
plates d46, 447, aprojection lens 405 and an optional screen 40fi. In
operation,
unpolarized light from the lighting element 401 is launched into the end face
407 of the
20 thin film polarizing device 400 and is split into two polarized light beams
by the thin film
coatis. Thep polarized light is refleetcd from the thin :01m Layers to the end
face 408; the
s-polarized light is transmitted through the thin hlm layers and exits the end
face 409.
The refleetedn-polazi2ed light exiting the end face 408 is then incident upon
the
first "X" colour cube 446. The coating 450 reflects red light and transmits
groan ligbt_
2s The coating 451 reflects blue light and transmits green lift_ The red,
green and blue light
beams are separated by the first "X° colour cube 44.6 and are incident
upon the three
corresponding monochrome reflective imaging elements 4-40, 441 and 442
respectively.
These monochrome reflective imaging elements are fed with the corresponding
colour
images_ For "on" pixels, the polarization of the light is rotated 90°
by al l three ima~g
3o elements, therefore ihep-polarized light becomes as s-polarized. 'The three
colour images
CA 02233597 2002-11-25
_31 _
from the three imaging elements are combined by the "x" cuhe again, in the
same way as it
separates the colours, to form a full colour image. When the reflected light
from the "on"
pixels again hits the thin film polarizing device through the end face 408, it
is transmitted
through by the thin f lm layers and then projected onto the screen by the
projection lens
405. For the "off' pixels, the polarization of the light is not changed by the
imaging
element 440, 441, 442 and, therefore, the light is reflected buck off the thin
fl lm layers
toward the lighting element 401 and is not projected onto the screen. 1n this
case, the
projected image only consists ofs-polarized Light.
The transmitted s-polarized light exiting the end face 409 is then incident
upon the
to second "X" colour cube 447. The coating 46o reflects red light and
transmits green light.
The coating 461 reflects blue tight and transmits green light. The rod, green
and blue light
beams are separated by the seeond''X" colour cube 447 and are incidc-mt upon
the three
corresponding monochrome reflective imaging elements 443, 444, and 445
respectively.
These monochrome reflective imaging elcmcats are fed with the corresponding
colour
is images. For "on" pixels, the polarization of the light is rotated
90° by all throe imaging
elements, therefore the s polarized light becomes asp-polarized.. T'he three
colour images
from the th,~ee imaging elements are combined by the "X" cube again, in the
same way as it
separates the colours, to form a full colour image. When the reflected light
from the "on"
pixots again hits the thin film ~polariaing device 400 through the end face
409, it is
20 reftectcd by the thin filin layers and then projected onto the screen by
the projection lens
405. For the "ofF' pixels, the polarization of the light is not changed by the
imaging
element 443, 444, 445 and, therefore, the light is transmitted back off the
thin film Layers
toward the lighting element and is not projected onto the screen. In this
case, the projected
image only consists ofp-polarized light.
2s Clearly, if the colour images on atl the imaging elements 440, 445, 441 and
444,
442 and 443 are identical and are mirror image of their counter-parts, the
combined full
images will be identical too, and a single 2-D image will be seen on the
screen. In
addition, if the images on the imaging elements 440, 44t, 442 sand 443, 444,
445 represent
right-eye and left-eye images (also nc~irror images in one case), the
connbined images can
3t~ be viewed through a polarising glass and three-dimensional effects can be
perceived. 2D
CA 02233597 2002-11-25
-32-
and 3D display mode can be easily inter changed by just changing the input
image signals
to the imaging elements. No optical reconfiguration is required for displaying
3D images
except that viewers have to wear polarizing glasses.
In this embodiment, the vsrhite light is first separated into two polarized
beams, then
each polarized beam is split to three primary colours which are directed to
the
corresponding imaging elements. No light is lost at all either due to
polarization or due to
colour. Therefore, super high efficiency projection display can be obtained.
This
embodiment are specially suitable to entertainment applications such as movie
theaters.
The display system shown in Figure 49 is also a super high efficiency 2DI3D
to projection display_ It consists ofthree thin ~lrx~ polarizing devices 400',
400, and 4003, a
lighting element X101 that emits white light, six identical reflective imaging
elements 420,
421', 4202, 4212, 4203, and 421 ~ that can be selected from monochrome imaging
elements
shown in Figure 36 fTNLG) or Figure 40 (fDLC), two sets dichroic colour filter
470, 471,
a projection lens 405 and an optional screen 406. The operation of this
embodiment is
t S very similar to that of the display system shown in Fi~ue 4? except that
the light incident
upon the thin film polarized devices is already separated into colours. In
addition, the
operation of the display system shown in Figure 49 has some similarity to the
display
system shown in Figure 48. In this latter system, the sequence of operation is
like this;
white light ~ separating polarization --3 separating colow --~ foaming cblOUr
ilnage5 -~
20 combining colour images -~ combining polarizations. In the display system
shown it1
Figure 49, the order is Changed to the white Ixght --~ separating colours --~
separating
polarizationS --~ fOnniilg colour images ~ Combining polaTiZatlOns --~
combining GOICUrS.
It is clear, the advantages that the advantages of the display system shown in
>~igure 47
also apply the display system shown in Figure 49. Super high efficiency
projection
z5 displays that are capable ofdisplaying 2DI3p images can also be obtained in
this
embodiment.
An alternative embodiment of the above display system is shown in Figure S0.
In
operation, this embodiment is very similar to the display system shown in
1~igure 46 except
that the light incident upon the thin f lm polarized devices is already
separated into
CA 02233597 2002-11-25
colours, tt is also similar to the display system shown in Figure 49 except
that instead of
using six reflective imaging elements, it uses only three transmissive imaging
elements
402', ~022, 4Q2;. It is capable bf displaying super high efficiency 2D images.
Another display system, which is suitable for projection high eFhciency 2D and
3D
images with two AMDs, is shown in Figs. 51 a to 51 e. Fig. 5 Ib is a top view.
It consists of
two identical tlun film polarizing devices 400 and 4002 in accordance with the
present
invention, a lighting element 401, two identical imaging elements 490 and 491,
ono far
separating the ungolarized light and one for combining the polarized light,
made of DMDs
(Digital Micro Mirrors), a projection Lens 445 and an optional screen 406. In
operation,
t0 unpolarized tight from the Lighting element 401 is launched the first thin
film polarizing
device 400' and is split into two polarized light beams. The p-polarized light
is reflected
and the s-polarized light is transmitted.
The reflectedp-polarised light exiting the end face 408' is then incident upon
the
first DMD 490. For the "on" pixels, th,e light from DMDs is reflected at an
"on" angle
about 20° from the incident beam and is directed to the second
polarizing device 4002.
Since the polarization of the light is not changed by the DMD, the light is
reflected by the
second thin iilna polarizing device 400Z is projected onto a screen 406. For
"ofF' pixels,
the light from the DMD is reflected at an "off' angle about 60° from
the incident beam and
is out of the collecting angle of the projection lens 405 and is absorbed by
an absorber. In
2o this case, the projected image only consists. ofp-polarized light.
The transtnittod s-polarized light exiting from the first thin film polarizing
device -
400I is then incident upon the second DMD 491. For the "on" pixels, the light
From
DMDs is reflected at an "on" angle about ~0° frorra the incident beam
and is directed to the
second polarising device 400_ Sirtce the polarization is not changed by the
DMD and
2s Light is reflected again bytho second thin film polarizing device 4002 and
is projected to a
screen 406. For "ofF' pixels, the light from the DMD is reflected at an "off'
angle about
60° from the incident bum anal is out afthe collecting angle ofthc
projection lens and is
absorbed by an absorber, In this case, the projected image only consists ofs-
polarized
light.
CA 02233597 2002-11-25
Clearly, if the images on the imaging elements 490 and 491 are identical but
mirror
images of each other, the combined image is identical too and a 2D image is
formed. In
addition, if the images on the imaging elements represent right-eye and left-
eye images
(also mirror images in one case), the corobinad image is a 3D image and can be
viewed
through a polarizing glasses. in this embodiment, 2D and 3D diaplay mode carx
be easily
inter-changed by just changing the input image signalx to the imaging
elements. No
optical reconfiguration is required for displaying 3D images except that
viewers have to
WGaipOlaT7zing glasSCS.
To form full colour images, colour lighting systems in combination with
monochrome DMDs can be used. lior example, a colour Tier wheel with red,
green, and
blue filters as in the lighting system shown in Figure 33 can be used to form
a colour
image. In this case, only one colour is displayed at any given time. The
perceived colour
will bt the time-integrated result of the thrv;e primary colours.
.Alternatively, a white
lighting system in combination with colour DMDs can be used. For example, two
l7MDs
13 (digital micro mirrors) with integrated colour filters disclosed in US
patent no_ 5,619,0S9
dated April 8, 1997 by LiLi, J.A.Dobrowolslci, P.D. Grant and ~.T.Sullivan
entitled
"Colour Deformable Mirror Dcvico having Optioal Thin fihm Interference Colour
Coatings". In addition, more complicated and super high efficiency display
systems that
use more than three DMDs can be formed by expanding the display system shown
in
Figures 51a to 51 c in the same way as in the display systems shown in Figures
48 and 49.
As previously stated, in the above display systems shown in Figures 47, 48,
49, and
S I a to 51 c, the polarization of the light beams reriu'ning from the two
spatial light
modulators are different; vr~e beam isp polarized and the other is s-
polarized.
Advantageously, this is an ideal situation for forming 3-D images. To do so,
it is
z5 necessary for each of the images of the two panels to be recorded by each
of two cameras
representing the left and right eyes, respectively, as demonstrated in
pictorial view Fig. 52.
To provide a 3-D image, a scene is recorded by each of two cameras which are
disposed at
different, o$'set locations. Thus a first camera records a scene fr6m a first
location and a
second camera, offset from the first camera, records essentially the same
scene from a
different Location. Preferably, the offset is approximately a distance between
the eyes of an
CA 02233597 2002-11-25
-34II -
average person or a distance related thereto and to the nature of the camera
lenses
employed. In ardor for a viewer Io perceive 3-D effect the seine recorded by
the first
camera is viewed by only one of the viewer's eyes. Furthermore, the sosn~e
recorded by the
second camera is viewed by the other of the viewer's eyes. ~T'he configuration
shown and
described with reference to Fig. 52 lends itself to this ~-D recording and
viewing.
The extension of the invention from two dimensions to three dimensions is
possible in many display configurations. 'Therefore, the above configurations
can be used
equally well for both 2-D and 3-D images.
The invention uses both s and p-polarization to provide 2-I~ images which are
twice as bright as conventional systems that utilize only one polarization
state.
Furtl~orrnore, the invention provides a navel method of providing and
displaying 3D
images.
To summarize, in all tho described preferred high efficiency projection
display
embodiments, the thin film polarizing device acts both as the first and as the
second
~5 polarizer. It bas high transmittance and reflectance for the desired
polarization, broad
bandwidth, larger angular field and very high extinction ratios for both
transmitted and
reflected. light. All light is used for imaging and no light is lost due to
undesired
polarization and by the thin film polarizing device. Therefore, it solves the
problems
associated with the use of dichroic sheet polarizers in conventional liquid
crystal displays.
The preferred display systems are high efficient and can employ high power
light sources
because of the use of both planes of polarization and the non-absorbing nature
of the thin
film polarizing devices. In addition, they can also be made very compact since
the same
thin film polarizing device can act as both the first and socond polarizers_
Furthermore,
high contrast images can be obtained because of the high extinction ratio of
the thin film
polarizing device. Also, lame aperture lighting and projecting optics can be
used duo to the
wide angular field of the thin film polarizing device, which in turn, also
enhances the
efl~cieney of the display systems. Finally, most of tha display systems are
capable of
displaying high efficiency 2D and 3D images.
CA 02233597 1998-03-30
WO 98/07279 PCTlCA97/00567
-35
The invention uses both s and p-polarization to provide 2-D images which are
twice as bright as conventional systems that utilize only one polarization
state.
Furthermore, the invention provides a novel method of providing and displaying
3D
images.
To summarize, in all the described preferred high efficiency projection
display
embodiments, the thin film polarizing device acts both as the first and as the
second
polarizer. It has high transmittance and reflectance for the desired
polarization, broad
bandwidth, larger angular field and very high extinction ratios for both
transmitted and
reflected light. All light is used for imaging and no light is lost due to
undesired
l o polarization and by the thin film polarizing device. Therefore, it solves
the problems
associated with the use of dichroic sheet polarizers in conventional liquid
crystal displays.
The preferred display systems are high efficient and can employ high power
light sources
because of the use of both planes of polarization and the non-absorbing nature
of the thin
film polarizing devices. In addition, they can also be made very compact since
the same
15 thin film polarizing device can act as both the f rst and second
polarizers. Furthermore,
high contrast images can be obtained because of the high extinction ratio of
the thin film
polarizing device. Also, large aperture lighting and projecting optics can be
used due to the
wide angular field of the thin film polarizing device, which in turn, also
enhances the
eff ciency of the display systems. Finally, most of the display systems are
capable of
2o displaying high efficiency 2D and 3D images.
Numerous embodiments of high efficiency display systems that use different
configurations and other types of imaging elements can be envisaged without
departing
from the scope of the invention. As pointed out above, as one skilled in the
are would
appreciate, it is possible to employ a second thin film polarizer for
combining the encoded
25 beams, although the preferred embodiment employs a common device.
