Note: Descriptions are shown in the official language in which they were submitted.
W095117699 ~ pCT/US94/I4325
REFLECTIVE POLAR1ZER DISPLAY
' 5
Technical Field
The invention is an improved optical display.
Background
Optical displays are widely used for lap-top computers, hand-held
calculators, digital watches and the like. The familiar liquid crystal (LC)
display is a
common example of such an optical display. The conventional LC display locates
a
liquid crystal and an electrode matrix between a pair of absorptive
polarizers. In the
LC display, portions of the liquid crysf'el have their optical state altered
by the
application of an electric field. This process generates the contrast
necessary to
display "pixels" of information in polarized light.
For this reason the traditional LC display includes a front polarizes
and a rear polarizes. Typically, these polarizers use dichroic dyes which
absorb light
of one polarization orientation more strongly than the orthogonal polarization
orientation. In general, the transmission axis of the front polarizes is
"crossed" with
the transmission axis of the rear polarizes. The crossing angle can vary from
zero
degrees to ninety degrees. The liquid crystal, the front polarizes and rear
polarizes
together make up an LCD assembly.
LC displays can be classified based upon the source of illumination.
3 0 "Reflective" displays are illuminated by ambient light that enters the
display from the
"front." Typically a brushed aluminum reflector is placed "behind" the LCD
assembly. This reflective surface returns light to the LCD assembly while
pm"serving
the polarization orientation of the light incident on the reflective surface.
It is common to substitute a "backlight" assembly for the reflective
brushed aluminum surface in applications where the intensity of the ambient
light is
insufficient for viewing. The typical backlight assembly includes an optical
cavity
and a lamp or other structure that generates light. Displays intended to be
viewed
under both ambient light and backlit conditions are called "transflective."
One
CA 02177711 2004-09-07
60,557-5260
2
problem with transflective displays is that the typical
backlight is not as efficient a reflector as the traditional
brushed aluminum surface. Also the backlight randomizes the
polarization of the light and further reduces the amount of
light available to illuminate the LC display. Consequently,
the addition of the backlight to the LC display makes the
display less bright when viewed under ambient light.
Therefore, there is a need for a display which can
develop adequate brightness and contrast under both ambient
and backlight illumination.
Suunnary
The optical display of the present invention
comprises three basic elements. The first element is a
reflective polarizes. This reflective polarizes is located
between a liquid crystal display (LCD) assembly and an
optical cavity, which comprise the second and third elements
respectively.
According to one aspect the invention provides an
optical display comprising: a display module; an optical
cavity positioned to illuminate the display module, the
optical cavity adapted to randomize polarization orientation
of light incident upon a surface within the optical cavity;
and a reflective polarizes positioned between the display
module and the optical cavity, the reflective polarizes
adapted to transmit light from the optical cavity having a
first polarization orientation toward the display module,
and to reflect light having a different polarization
orientation toward the optical cavity; wherein the
reflective polarizes comprises a multiple layer stack of
alternating layers of at least two different materials,
wherein a refractive index difference between different
material layers in a first in-plane direction is greater
CA 02177711 2004-09-07
6057-5260
2a
than a refractive index difference between different
material layers in a second in-plane direction.
According to another aspect the invention provides
an optical display comprising: a display module; an optical
cavity positioned to illuminate the display module the
optical cavity adapted to randomize direction and randomize
polarization orientation of light incident upon a surface
within the optical cavity; and a brightness enhanced
reflective polarizes, the brightness enhanced reflective
polarizes comprising: a reflective polarizes adapted to
transmit light from the optical cavity having a first
polarization orientation toward the display module, and to
reflect light having a different polarization orientation
toward the optical cavity, wherein the reflective polarizes
comprises a multilayer stack of alternating layers of a
first material and a second material, wherein a refractive
index difference between layers of the first and second
materials is greater in a first in-plane direction than a
refractive index difference between layers of the first and
second materials in a second in-plane direction; and a
structured surface material adapted to reflect light from
the optical cavity within a first predetermined group of
angles toward the optical cavity, and adapted to direct
light from the optical cavity within a second predetermined
group of angles toward the display module; wherein at least
some of the reflected light having the different
polarization orientation is converted to the first
polarization orientation by the optical cavity, and further
wherein at least some of the light converted to the first
polarization orientation is transmitted by the multilayer
reflective polarizes toward the display module; and further
wherein at least some of the reflected light within the
first predetermined group of angles is converted to the
CA 02177711 2004-09-07
60557-5260
2b
second predetermined group of angles by the optical cavity,
and further wherein at least some of the light converted to
the second predetermined group of angles is directed by the
structured surface material toward the display module.
According to another aspect the invention provides
a display comprising: a light source having a reflective
optical cavity configured to randomize direction and
polarization of light reflected by the optical cavity; a
display module; a geometric structured surface material
disposed between the light source and the display module,
the structured surface material redirecting a portion of
light incident on the structured surface material from an
incidence angle to a transmission angle, the transmission
angle being less than the incidence angle measured from an
axis normal to the plane of the display module; a reflective
polarizes disposed between the light source and the display
module, the reflective polarizes having a birefringent
material arranged such that light of a first polarization is
substantially reflected toward the optical cavity while
light of a second polarization is substantially transmitted
through the reflective polarizes toward the display; and an
absorbing polarizes disposed between the reflective
polarizes and the display module.
According to another aspect the invention provides
a display comprising: a light source having a reflective
optical cavity configured to randomize direction and
polarization of light reflected by the optical cavity; a
display module; a geometric structured surface material
disposed between the light source and the display module,
the structured surfaced material redirecting a portion of
light incident on the structured surface material from an
incidence angle to a transmission angle, the transmission
angle being less than the incidence angle measured from an
CA 02177711 2004-09-07
60557-5260
2c
axis normal to the plane of the display module; and a
reflective polarizer disposed between the light source and
the display module, the reflective polarizer having a
birefringent material arranged such that light of a first
polarization encounters changes in refractive indices, as
the light of the first polarization propagates in the
reflective polarizer, sufficient to reflect the light of the
first polarization toward the optical cavity, while light of
a second polarization is substantially transmitted through
the reflective polarizer toward the display.
Brief Description of the Drawings
The drawings depict representative and
illustrative implementations of the invention. Identical
reference numerals refer to identical structure throughout
the several figures, wherein:
Figure 1 is a schematic cross section of an
optical display according to the invention;
Figure 2 is a schematic cross section of an
illustrative optical display according to the invention;
Figure 3 is a schematic cross section of an
illustrative optical display according to the invention;
Figure 4 is an exaggerated cross sectional view of
the reflective polarizer of the invention;
Figure 5 is a graph of the reflective polarizer
performance;
Figure 6 is a schematic diagram of an optical
display according to the invention with brightness
enhancement;
CA 02177711 2004-09-07
60$57-5260
2d
Figure 7 is a diagram illustrating the operation
of a brightness enhancer;
Figure 8 is a graph illustrating the operation of
a brightness enhancer;
Figure 9 is a schematic cross section of an
illustrative optical display;
Figure 10 is a schematic cross section of an
illustrative optical display;
WO 95/17699 2 ~ ~ ~ ~ ~ ~ PCTIUS94/14325
3
Figure 11 is a schematic cross section of an illustrative optical
display;
Figure 12 is a graph of test insults;
Figure 13 is a schematic cross section of an illustrative optical
display;
Figure 14 is a schematic cross section of a brightness enharuxd
reflective poLrrizer;
Figure I S shows a two layer stack of films forming a single
interface.
Figures 16 and 17 show reflectivity versus angle curves for a uniaxial
birefringent system in a medium of index 1.60.
Figure I8 shows reflectivity versus angle curves for a uniaxial
birefiingent system in a medium of index 1Ø
Figures 19, 20 and 21 show various relationships between in-plane
indices and z-index for a uniaxial birefiingent system.
Figure 22 shows off axis reflectivity versus wavelength for two different
uniaxial birefringent systems.
Figure 23 shows the effect of introducing a y-index difference in a
biaxial birefiingent film.
2 0 Figure 24 shows the effect of introducing a z-index difference in a
biaxial birefringent film.
Figure 25 shows a contour plot summarizing the information from
Figures 18 and 19;
Figures 26-3 I show optical performance of multilayer mirrors given in
the mirror Examples; and
Figures 32-35 show optical performance of multilayer polarizers given
in the polarizer Examples.
Detailed Description
Figure 1 is a schematic diagram of an illustrative optical display 10
that includes three principle components. These include the polarizing display
W095117699 ~~ P(.°1'/US94/14325
4
module shown as LCD assembly 16, a reflective polarizes 12, and an optical
cavity
24.
The LCD assembly 16 shown in this figure is illuminated by
polarized light provided by the reflective polarizes 12 and the optical cavity
24.
Ambient Iight incident on the display 10, depicted by ray 60 traverses
the LCD module 16, the reflective polarizes 12 and strikes the diffuse
reflective
surface 37 of the optical cavity 24. Ray 62 depicts this light as it is
reflected by the
diffusely reflective surface 37 toward the reflective polarizes 12;
Light originating from within the optical cavity 24 is depicted by ray
l0 64. This light is also directed toward the reflective polarizes 12 and
passes through
the diffusely reflective surface 37. Both ray 62 and ray 64 have light
exhibiting both
polarization states (a,b).
Figure 2 shows a schematic optical display I1 illustrated with a thr~
layer LCD assembly 15 that includes a front polarizes 18, a liquid crystal 20
and a
rear polarizes 23. In this embodiment the optical cavity 24 is an edge lit
backlight
which includes a lamp 30 in a reflective lamp housing 32. Light from the lamp
30 is
coupled to the light guide 34 where it propagates until it encounters a
diffuse
reflective structure such as spot 36. This discontinuous array of spots is
arranged to
extract lamp light and direct it toward the LCD module 15. Ambient light
entering
the optical cavity 24 may strike a spot or it may escape from the light guide
through
the interstitial areas between spots. The diffusely reflective layer 39 is
positioned
below the light guide 34 to intercept and reflect such rays. In general, all
the rays
that emerge from the optical cavity 24 are illustrated by ray bundle 38. This
ray
bundle is incident on the reflective polarizes 12 which transmits light having
a first
polarization orientation referred to as "(a)" and effectively reflects light
having the
orthogonal polarization orientation (b). Consequently, a certain amount of
light,
depicted by ray bundle 42, will be transmitted by the reflective polarizes 12
while a
substantial amount of the remaining light will be reflected as indicated by
ray bundle
40. The preferred reflective polarizes material is highly efficient and the
total losses
due to absorption within the reflective polarizes 12 are very low (on the
order of 1
percent). This lost light is depicted by ray bundle 44. The light having
polarization
state (b) reflected by the reflective polarizes 12 reenters the optical cavity
24 where it
strikes the diffusely reflective structures such as spot 36 or the diffusely
reflective
layer 39. The diffusely reflective surfaces serve to randomize the
polarization state
of the light reflected by the optical cavity 24. This recireulation and
randomization
process is depicted as path 48. The optical cavity 24 is not a perfect
reflector and the
WO 95/17699 PCT/US94/14325
21'~'~'~ 11
light losses in the cavity due to scattering and absorption are depicted by
ray bundle
46. These losses are also low (on the order of 20 percent). The multiple
recirculations effected by the combination of the optical cavity 24 and the
reflective
polarizes 12 form an efficient mechanism for converting light from state (b)
to state
5 (a) for ultimate transmission to the viewer.
The effectiveness of this process relies on the low absorption
exhibited by the reflective polarizes disclosed herein and the high
reflectivity and
randomizing properties exhibited by many diffusely reflective surfaces, In
Figure 2
both the discontinuous layer depicted by spot 36 and the diffusely reflective
continuous layer 39 may be formed of a titanium oxide pigmented material. It
should be appreciated that a diffuse reflective surface 37 (shown in Figure I)
can be
formed of transparent surface textured polycarbonate. This material could be
placed
above the light guide 34 to randomize incident light in the configuration
shown in
Figure 2. The specific and optimal configuration will depend on the particular
I5 application for the completed optical display.
In general, the gain of the system is dependent on the efficiency of
both the reflective polarizes body 12 and the optical cavity 24. Performance
is
maximized with a highly reflective optical cavity 24 consistent with the
requirement
of randomization of the polarization of incident light, and a very low loss
reflective
polarizes 12.
Figure 3 shows a schematic optical display 14 illustrated with a two
layer LCD assembly 17 that includes a front polarizes I8 and a liquid crystal
20. In
this embodiment the optical cavity 24 includes an electroluminescent panel 21.
The
traditional electroluminescent panel 21 is coated with a phosphor material 19
that
generates light when struck by electrons and that is also diffusely reflective
when
struck by incident light. Usually, electroluminescent displays are "grainy"
because
of the variations in efficiencies associated with the phosphor coating.
However, light
returned by the reflective polarizes 12 has a tendency to "homogenize" the
light
emissions and improve overall uniformity of illumination exhibited by the
optical
display 14. In the illustrative optical display 14 the LCD assembly 17 lacks a
rear
polarizes. In this optical display 14 the reflective polarizes 12 performs the
function
normally associated with the rear polarizes 23 shown in optical display 11 in
Figure
2.
Figure 4 is a schematic perspective diagram of a segment of the
reflective polarizes 12. The figure includes a coordinate system 13 that
defines X, Y
and Z directions that are referred to in the description of the reflective
polarizes 12.
W095/17699 ~ ~ ~ PC1YU$94114325
6
The illustrative reflective polarizer 12 is made of alternating layers
(ABABA...) of two different polymeric materials. These are referred to as
material
"(A)" and material "(B)" throughout the drawings and description. The two
materials are extruded together and the resulting multiple layer (ABABA...)
material -
is stretched (5:1) along one axis (3~, and is not stretched appreciably (I:1)
along the
other axis (I~. The X axis is referred to as the "stretched" direction while
the Y axis
is referred to as the "transverse" direction.
The (B) material has a nominal index of refraction (n=1.64 for
example) which is not substantially altered by the stretching process.
The (A) material has the property of having the index of refraction
altered by the stretching process. For example, a uniaxially stretched sheet
of the
(A) material will have one index of refraction (n=1.88 for example) associated
with
the stretched direction and a different index of refraction (n=1.64 for
example)
associated with the transverse direction. By way of definition, the index of
refraction
associated with an in-plane axis (an axis parallel to the surface of the film)
is the
effective index of refraction for plane-polarized incident light whose plane
of
polarization is parallel to that axis.
Thus, after stretching the multiple layer stack (ABABA...) of material
shows a large refractive index difference between layers (delta n=1.88-
1.64=0.24)
associated with the stretched direction. While in the transverse direction,
the
associated indices of refraction between layers are essentially the same
(delta
n=1.64-1.64=0.0). These optical characteristics cause the multiple layer
laminate
to act as a reflecting polatizer that will transmit the polarization component
of the
incident light that is correctly oriented with respect to the axis 22. This
axis is
defined as the transmission axis 22 and is shown in Figure 4. The light which
emerges from the reflective polarizer 12 is referred to as having a first
polarization
orientation (a).
The light that does not pass through the reflective polarizer 12 has a
polarization orientation (b) that differs from the ftrst orientation (a).
Light exhibiting
this polarization orientation (b) will encounter the index differences which
result in
reflecfion of this light. This defines a so-tailed "extinction" axis shown as
axis 25 in
Figure 4. In this fashion the reflective poiarizer I2 passes light having a
selected
polarization (a). ,
~pp~
R'O 95/17699 ~ ~ PC.'f1U594114325
7
The preferred "A" layer is a crystalline naphthalene dicarboxylic acid
polyester such as polyethylene naphthalate (PEN), and the preferred "B" layer
is a
copoIyester of naphthalene dicarboxylic acid and terephthalic or isothalic
acid
(coPEN). PEN and a 70 naphthalate/30 terephthalate copolyester (coPEN) can be
synthesized in standard polyester resin kettles using glycol as the diol. A
satisfactory
204-layered poiarizer was made by extruding PEN and coPEN in a 51-slot feed
block and then employing two layer doubling multipliers in series in the
extrusion.
The multipliers divide the extruded material exiting the feed block into two
half
width flow streams, then stack the half width flow streams on top of each
other.
Such multipliers are known in the art. The extrusion was performed at
approximately 295°C. The PEN exhibited an intrinsic viscosity of 0.50
dl/g and the
coPEN exhibited an intrinsic viscosity of 0.60 dl/g. The extrusion rate for
the PEN
material was 22.5 Ib/hr and the extrusion rate for the coPEN was 16.5 lb/hr.
The
cast web was approximately 0.0038 inches in thickness and was uniaxially
stretched
at a 5: I ratio in a longitudinal direction with the sides restrained at an
air temperature
of 140°C during stretching. Except for exterior skin layers, all layer
pairs were
designed to be 1/2 wavelength optical thickness for a design wavelength of 550
nm.
Two 204-layer polarizers made as described above were then
hand-laminated using an adhesive. Preferably the refractive index of the
adhesive
should match the index of the isotropic coPEN layer.
The optical performance of the reflective polarizes I2 depends in part
on the optical thicknesses of the various layers. Both thick film and thin
film
constructions are useful. If the layers exhibit optical paths that are many
wavelengths
of light long, then the optical properties of the reflective polarizes 12
WO 95117699 ~ ~ ~ ~ ~ , , , - . . PCTIUS94114325
8
are inherently broadband. If the layers have an optical thickness less than a
wavelength of light, then constructive interference can be exploited to
improve the
optical performance of the reflective polarizes 12 of selected wavelengths.
The manufacturing procedure described in the example can produce .
uniform layers that have an optical thickness that is less than the wavelength
of light
over the visible spectrum. Constructive interference occurs if the optical
thickness of ,
pairs of layers (A,B) add to one half of the wavelength of the incident light
(A+B=lambda/2). This half wavelength condition results in narrow band
constructive interference at the design wavelength. Broadband optical
performance
l0 can be achieved by laminating or otherwise coupling multiple narrow band
stacks.
For example, a first group 37 of layers having the same thickness
(A+B=lambda/2)
can be laminated to a second group 35 having a different thickness (A+B=lambda
prime/2). For the sake of clarity only a small number of layers are shown in
Figure
4, although typically hundreds of layers (ABAB...) may be stacked together to
achieve efficient broadband response. Preferably the reflective polarizes 12
should
be designed to reflect light at all angles and wavelengths of interest.
Although the reflective polarizes 12 has been discussed with an
exemplary multiple layer construction which includes alternating layers of
only two
materials it should be understood that the reflective polarizes 12 may take a
number
2 0 of forms. For example, additional types of layers may be included into the
multiple
layer construction. Also in a limiting case, the reflective polarizes may
include a
single pair of layers (AB) one of which is stretched. Furthermore, a dichroic
polarizes could be bonded directly to reflective polarizes 12.
Another important property of the optical cavity 24 is the fact that
polarization randomization process associated with the cavity will also alter
the
direction of the incident light. In general, a significant amount of light
exits the
optical cavity off axis. Consequently, the path of such light in the
reflective polarizes
is longer than the path length for near normal light. This effect must be
addressed to
optimize the optical performance of the system. The reflective polarizes body
12
described in the example is capable of broadband transmission into the longer
wavelengths which is desirable to accommodate off axis rays. Figure 5 shows
trace
31 which indicates a transmissivity of over 80 percent over a wide range of
wavelengths. Trace 33 shows efficient broadband reflectively over a large
portion of
the visible spectrom. The optimal reflectivity trace would extend into the
infrared
and extend from approximately 400nm to approximately SOOnm.
WO 95/17699 ~ ~ ~ PCTlUS94114325
In another embodiment, the apparent brightness of the display may be
increased by the use of a brightness enhancement film. Figure 6 shows an
optical
display 164 which has three primary components. These are the optical display
module 142, the brightness enhanced reflective polarizes 110 and the optical
cavity
140. Typically the complete optical display 164 will be planar and rectangular
in
. plan view as seen by observer 146 and will be relatively thin in cross
section with the
three primary components in close proximity to each other.
In use, the display module 142 is illuminated by light processed by
the brightness enhanced reflective polarizes I 10 and the optical cavity 140.
Together
1 o these two components direct polarized light into a viewing zone 136 shown
schematically as an angle. This light is directed through the display module
142
toward the observer 146. The display module 142 will typically display
information
as pixels. Polarized light transmission through a pixel is modulated by
electrical
control of the birefringence of the liquid crystal material. This modulates
the
polarization state of the light, affecting its relative absorption by a second
polarizes
layer that forms a part of the display module 142.
There are two sources for illumination shown in the figure. The first
is ambient light depicted by ray 162. This light passes through the display
module
142 and brightness enhanced reflective polarizes 110 and is incident on the
optical
2 4 cavity 140. The optical cavity reflects this light as indicated by ray
165. The second
source of light may be generated within the optical cavity itself as depicted
by ray
163. If the optical cavity 140 is a backlight then the principal source of
illumination
originates within the optical cavity 140 and the optical display is referred
to as
"backlit." If the principal source of illumination is ambient light
represented by ray
162 and ray 165 then the optical display is called "reflective" or "passive."
If the
display is to be viewed under both ambient and cavity generated light the
display is
called "transflective." The present invention is useful in each of these
display types.
Regardless of the origin of the light, the brightness enhanced
reflective polarizes 110 and the optical cavity 140 cooperate together to
"recirculate"
light so that the maximum amount of light is properly polarized and confined
to the
viewing zone 136.
In general, the brightness enhanced reflective polarizes 110 includes
two elements. The first is a reflective polarizes body 116 that transmits
light of a
particular polarization to the viewing zone 136. The second element is the
optically
structured layer 113 that defines the boundaries of the viewing zone 136.
2
WO 95117699 PC1YUS94114325
The optical cavity 140 serves several functions but with respect to its
interaction with the brightness enhanced reflective polarizes 110, the
important
parameters are a high reflectance value with respect to incident light and the
ability of
the optical cavity 40 to alter both the direction and the polarization of the
incident
5 light. Conventional optical cavities meet these requirements.
For any optical system, the sum of the reflectivity, losses and
transmissivity must equal 100 percent of the light. Absorbance can be a major
source of such losses. In the present invention the brightness enhanced
reflective
polarizes 110 has a very low absorbance and high reflectivity to certain
light.
10 Consequently light that is not passed directly into the viewing zone 136 is
efficiently
transferred to the optical cavity 140 where it is altered and may emerge from
the
cavity with the proper attributes to contribute to the light in the viewing
zone 136.
In the context of the optical display 164 the overall gain of the system
depends on the product of the reflectivity of the optical cavity 140 and the
reflectivity
of the brightness enhanced reflective polarizes 110. The invention is most
effective
when used with a low absorption optical cavity that has a high reflectivity
rear
surface consistent with its ability to alter the direction and polarization
state of the
incident Iight from the brightness enhanced reflective polarizes 110. For
these
purposes it should be noted that the optical cavity could be filled with a
transparent
dielectric material such as an acrylic.
Although the preferred structured surface 112 functions as a
geometric optic it is well known that diffractive or holographic optical
elements may
be designed to effectively mimic the light directing qualities exhibited by
geometric
optics. Therefore the term structured surface 112 should be understood to
describe
both geometric and diffractive optical systems that confine light to a
relatively narrow
viewing zone 136.
Figure 7 is an enlargement of structured surface material that will act
as a brightness enhances in the present invention. As described previously,
structured surface material 218 has a smooth side 220 and a structured side
222.
Structured side 222, in the preferred embodiment, includes a plurality of
triangular
prisms. In the preferred embodiment, such prisms are right isosceles prisms,
although prisms having peak angles in the range of 70 degrees to 110 degrees
will
work with varying degrees of effectiveness with the invention. Structured
surface
material 218 may be of any transparent material having an index of refraction
greater
than that of air, but, in general, the materials with higher indices of
refraction will
produce better results. Polycarbonate, which has an index of refraction of
1.586, has
WO 95/I7699 ~ ~ ~ ( ~ ~ ~ PC1'/US94I14325
11
proven to work very effectively. For purposes of description of the invention,
the
prisms on structured surface 222 will be assumed to have included angles of 90
degrees and structured surface material 218 will be assumed to be of
polycarbonate.
Alternatively other structured surface materials may be used. Symmetric cube
comer
sheeting has been shown to produce excellent results.
Figure 8 illustrates the operation of structured surface material 218.
Figure 8 is a graph having two axes 226 and 228. These axes represent the
angle
that a light ray makes to a normal to smooth surface 220. Specifically, axis
226
represents the angle that the light ray makes when the direction of the light
ray is
l 0 projected into a plane parallel to the linear extent of the structures on
structured
surface 222. Similarly axis 228 represents the angle that the light ray makes
to a
normal to smooth surface 220 when the direction of the light ray is projected
into a
plane perpendicular to the linear extent of the structures on structured
surface 222.
Thus a light ray striking perpendicular to smooth surface 220 would be
represented
by the origin, labeled 0 degrees, of the graph of Figure 8. As may be seen,
Figure 8
is divided into regions 230, 232, and 234. Light striking at angles that fall
within
region 230 will enter structured surface material 218 but be totally
internally reflected
by structured surface 222 so that they pass through smooth surface 220 a
second time
and reenter the optical cavity. Light rays striking smooth surface 220 at an
angle
such that they fall in region 232 or 234 will be transmitted but refracted to
a different
angle with respect to the normal. As may be seen from Figure 8, which
represents
the performance of polycarbonate, any light ray striking smooth surface 220 at
an
angle of less than 9.4 degrees to the normal, will 6e reflected.
Returning to Figure 7, four exemplary Light rays are shown. The
first, light ray 236, approaches smooth surface 220 at a grazing angle, i.e.,
an angle
to the normal approaching 90 degrees. If light ray 236 makes an angle of 89.9
degrees to the normal to surface 220 when it strikes structured surface
material 2I8,
it will be refracted such that it makes an angle of 39.1 degrees to the normal
as it
travels through structured surface material 2I8. Upon reaching structured
surface
222, it will be refracted again. Because of the structures on structured
surface 222, it
will be refracted so that again, it will make a smaller angle to the normal to
structured
surface 220. In the example it will make an angle of 35.6 degrees.
W~ 95117699 ~ PCTIIJS94/14325
Light ray 238 approaches smooth surface 220 at an angle much closer
to the cut off angle. It also is refracted as it passes through smooth surface
220, but
to a lesser extent. If light ray 238 approaches smooth surface 220 at an angle
of 10
degrees to the normal to smooth surface 220, it will emerge from structured
surface
222 at an angle of 37.7 degrees to the normal to smooth surface 220 but on the
opposite side of that normal.
Light ray 240 approaches at an angle less than the cut off angle and is
totally internally reflected twice by structured surface 222 and returned to
the interior
of the optical cavity.
Finally, light ray 242 approaches smooth surface 220 at an angle
similar to that of light ray 238, but in a location such that it is totally
internally
reflected by one side of a prism on structured surface 222 but not by the
second side.
As a result it emerges at a large angle to the normal to smooth surface 220.
Because
such a reflection only occurs to a light ray that is travelling in a direction
that forms a
-- high incidence angle to the side it strikes, the prisms provide a very
small cross
section to such rays. In addition many of those rays will reenter the next
prism and
be returned into display 210.
A fifth class of light ray is not shown in Figure 7. This is the set of
light rays that are reflected by smooth surface 220 and do not enter
structured surface
material 218. Such light rays simply join the others that are reflected back
into the
optical cavity. As may be seen from this discussion, light that, absent
stroctured
surface material 218, would have emerged from the display at a high angle to
the
axis of the display, where the axis of the display is taken to be the normal
to smooth
surface 220, is redirected into a direction closer to that axis. A small
amount of light
will be directed out at a large angle to the axis. Thus, we may say that light
that
enters structured surface material 218 through smooth surface 220 with an
angle of
incidence greater than a predetermined angle is directed into an output wedge
that is
narrower than the input wedge and the majority of the light that enters
structured
surface material 18 through smooth surface 220 at an angle of incidence of
less than
3 0 that predetermined angle will be reflected back into the optical cavity.
The light that is reflback into the optical cavity will strike the
diffuse reflector. The reflected light will travel back to structured surface
material
218, in general malang a different angle than it made the first time. The
process is
then repeated so that more of the light is redirected into the smaller wedge.
The key
aspect of the invention is that structured surface material 218 must be
capable of
reflecting light striking it in a first predetermined group of angles and
passing, but
WO 95/17699 ~ ~ ~ ~ PCT/DS94/14325
13
refracting, light striking it in a second predetermined group of angles
wherein the
angles in the second group of angles are greater than those in the first group
of angles
and wherein the light in the second group of angles is refiacted into an
output wedge
that is narrower than its input wedge. In this description the first and
second groups
of angles are relafive to an axis of the display perpendicular to the display
surface,
i.e. the liquid crystal.
Figure 9 shows a portion of the schematic optical display 164 without
the brightness enhanced reflective polarizes l I0 material to permit a
comparison of
performance without the brightness enhanced reflective polarizes 110. In
general,
the light emerging from a unit area of the optical cavity 140 depicted by ray
bundle
148 will be randomly polarized and have optical states (a), (b), (c), and (d)
present.
Approximately half of this Light, light of states (b) and (d), are absorbed by
the
dichroic absorptive polarizes 150 that forms a part of the display module 142.
The
remainder of the light, states (a) and (c), are passed through the dichroic
absorptive
potatizer 150. The light emerging from the display module 142, depicted by ray
bundle 152, thus contains light of states (a) and (c). Although the tight of
state (a) is
directed toward the observer 146, the light of state (c) is not. The remainder
of the
light having states (b) and (d) will be absorbed by the dichroic absorptive
polarizes
150. Thus, only approximately one quarter of the light provided by optical
cavity
2 0 140 actually contributes to the brightness of the display as viewed by
observer 146.
The brightness enhances operates to make more efficient use of the
light made available by optical cavity 140. If the same unit amount of light,
depicted
by ray bundle 154, is directed to the brightness enhanced reflective polarizes
110,
approximately a quarter of the light (light of state (a)) will pass through
the
2 5 brightness enhanced reflective polarizes 110 on the first pass. This light
will have the
correct polarization to match the transmission axis of the dichroic absorptive
polarizes
I50, and is depicted as ray bundle 161. However the remaining light having
states
(b), (c), and (d) will be reflected back into the optical cavity by the
brightness
enhanced reflective polarizes 110. Some portion of this light will be
randomized in
30 terms of direction and polarization to state (a) by the optical cavity 140.
Thus, this
light will emerge from the optical cavity with states (a), (b), (c), and (d)
as indicted
by ray bundle 157. The recircuiated light of state (a) will then be added to
the
originally transmitted light as depicted by ray bundle 160. Thus, the total
amount of
light depicted by ray bundle 160 and ray bundle 161 is increased by
"recirculation."
35 Because only light of the correct polarization to match the transmission
axis of the
dichroic absorptive polarizes I50 (state (a)) is passed through the brightness
enhanced
WO 95/17699 ~ ~ ~ ~ ~ ~. PCT/CTS94114325
14
reflective polarizes 110, much more of the light emitted from the display,
depicted
by ray bundle 63, is directed toward the observer 146. In addition, because
light of
states (b) and (d) is reflected by the brightness enhanced reflective
polarizes 110,
very little is absorbed by the dichroic absorptive polarizes 150. The result
is a
display in which the amount of light emerging from the display, depicted by
ray
bundle 163, may be 70 percent brighter than the amount of light indicated by
ray
bundle 152.
Figure 10 shows an optical display 170. The opfical display module
142 includes a liquid crystal matrix 147 placed between a front polarizes 149
and a
rear polarizes 150. In this embodiment the optically structured layer I 13 is
separated
from the refl~tive polarizes body 116 by gap 171. The gap 171 introduces
reflections for state (a) light rays which are not desirable. In the display
170 the
optical cavity 140 is a backlight which includes a lamp 172 within a lamp
reflector
173. Light from the lamp I72 enters the light guide 174 and travels until it
strikes a
diffuse reflective surface such as spot 176. Although a discontinuous array of
such
spots is required to effectively extract light from the light guide 174, the
intermittent
surface may not be sufficient to fully recirculate light. Therefore it is
preferred to
place a continuous diffuse reflective surface 175 below the discontinuous
surface to
aid in the recireulation process.
Figure 1 I shows an optical display 179 where the optically structured
layer 113 and structured surface 112 is a separate element proximate but not
directty
applied to the reflective polarizes body 116. Together these two components
along
with the gap 181 make up the brightness enhanced reflective polarizes 110. In
use,
the optical cavity 140 will provide light for the display and will also act to
reorient
the polarization and direction of light returned from the brightness enhanced
reflective polarizes 110. The optical cavity 140 includes an
eiectroluminescent panel
139 having a phosphor coating which acts as a diffuse reflective surface 137.
One
difference between this embodiment of the brightness enhanced reflective
polarizes
I 10 and that of Figure 10 is that light approaching the structured surface
112 at an
3 0 angle greater than the critical angle 134 is returned to the optical
cavity by total
internal reflection regardless of its state of polarisation. Another
difference is that
the light transmitted by optically structured layer 113 passes through the
reflective
polarizes body 116 at near normal angles. A further difference relates to the
presence of a front polarizes 149 and the absence of a rear polarizes in the
display
module 143. In embodiments where the
WO 95/17699 ~ ~ ~ PCTlIJS94/I4325
backlight is the dominant source of light, adequate contrast can be achieved
without
the use of an absorptive polarizes juxtaposed next to the brightness enhanced
reflective polarizes.
Figure 12 shows test results of a sample of brightness enhanced
5 reflective polarizes material taken with a standard electroluminescent
backlight. The
electroluminescent backlight met the requirements set forth above for the
optical
cavity in terms of randomizing the direction and the polarization orientation
of
incident light. To provide a basis for comparison, curve 162 shows light
transmission for a display having only a dichroic polarizes alone without a
brightness
l0 enhancement reflective polarizes body. Curve 164 represents the intensity
of light
versus the angular distribution of light for the Y-Z plane of a display which
includes
a brightness enhanced reflective polarizes body in a configuration v~ith the
reflective
polarizes body and structured surface as proximate layers, such as that shown
and
described above vrith respect to Figure 12. Curve 164 shows that an on-axis
15 brightness increase of about sixty percent as compared to the dichroic
polarizes alone
is achieved. Also, a brightness decrease of about 50 percent is observed at 60
degrees off axis.
In yet another example, using a standard backlight, a brightness
increase of 100 percent over a dichroic polarizes alone was measured along the
2 0 display normal to the viewing surface with a brightness enhanced
reflective polarizes
with the reflective polarizes body and structured surface as proximate layers
such as
shown and described above with respect to Figure 11. The reflective polarizes
alone
yielded a brightness increase of 30 percent, while the structured surface
alone yielded
a brightness increase of 70 percent, thus resulting in a total brightness
increase of 100
2 5 percent for on-axis viewing.
The difference in brightness increase between these two examples is
largely due to the different optical cavities used. The curve of Figure 12 was
taken
with an electroIuminescent backlight, while the latter example was taken with
a
standard backlight. The reflectance and losses of each type of optical cavity
effects
3 o the overall brightness increase that can be achieved.
Two dimensional control of the rays exiting the brightness enhanced
reflective polarizes body can be achieved using the alternate preferred
display
. configuration 192 shown in Figure 13. There, two optically structured layers
113
and 182, each having a structured surface 112 and 184, respectively, are
proximate
35 to each other and to a reflective polarizes body 116. These three elements
comprise
the brightness enhanced reflective polarizes body 110. Although in Figure I3
the
WO 95117699 ~ ~ ~ ~ ~ ~ ~ PGT/US94/14325
16
two optically structured layers are shown below the reflective polarizer body
116, it
shall be understood that the reflective polarizer body 116 could also be
placed
between or below the optically structured layers 112 and 182 without departing
from
the scope of the present invention. Two dimensional control is achieved by
crossing
the axes of orientation of the structured surfaces 112 and 184. The axes may
be
oriented at 90 degrees or at some other angle greater than 90 degrees
depending upon
the display application and associated polarization requirements.
In operation, the first optically structured layer results in a viewing
zone of approximately 70 degrees in the Y, Z plane and 110 degrees in the X, Z
l0 plane. The light exiting the first optically structured layer 182 then
becomes the
source for the second optically structured layer 113, whose strucd~red surface
I I2 has
a different axes of orientation than does the structured surface I84 of
optically
structured layer 182. If the axes of the two optically structured layers 113
and 184
are oriented at 90 degrees, for example, optically structured layer 182
operates on the
light within the 110 degree angle of the X, Z plane and compresses the viewing
angle
in the X, Z plane to a narrower field of something less than 70 degrees,
thereby
further increasing brightness.
Figure 14 is a schematic and perspective view of the brightness
enhanced reflective polarizes I10 shown in isolation. The figure is not drawn
to
2 0 scale to facilitate description of the structure of the invention. Figure
14 among
others include a coordinate system 118 that deftnes X, Y and Z directions that
are
referred to in the description of the invention.
As seen in Figure 14 the brightness enhanced reflective polarizes 110
includes an optically structured layer 113 that has a structured surface 112.
In
2 5 Figure 14 this optically structured layer 113 is replicated on a polymer
layer cast onto
the reflective polarizes body 116, resulting in a preferred unitary structure.
A unitary
structure such as the one shown in Figure 14 may be formed by various known
techniques of attaching two films, such as heat lamination or casting and
curing the
structured surface material on the reflective polarizes where the reflective
polarizes
30 acts as the substrate in a process such as is described in United States
Patent
5,175,030. For purposes hereof, the statement that the reflective polarizes
and the
brightness enhances are unitary shall be understood to mean that they are
bonded to
one another.
The preferred and illustrative structured surface 112 shown in
35 Figure 14, is an array of prisms, typified by prism 114. Each prism has an
apex
ridge that extends in the X direction. In the Y, Z plane each prism 114 has a
cross
W095/17699 ~ PCi'/ITS94I14325
17
section that is an isosceles triangle, with a prism apex angle 120 of ninety
degrees.
Although an array of prisms is preferred, the specific prism geometry and apex
angles 120 may be altered to meet the specific requirements of the
application. An
array of palms as shown in Figure 14 is especially useful where it is
desirable to
confine the light exiting the optical display 6o a relatively narrow viewing
zone 136
shown on Figure 6. However, where other viewing angles are desired, the
optically
structured layer 113 may take other forms. Although the preferred structured
surface
112 functions as a geometric optic it is well lmown that diffractive or
holographic
optical elements may be designed to effectively mimic the fight direcfing
qualities
exhibited by geometric opfics. Therefore the term structured surface I12
should be
understood to describe both geometric and diffractive optical systems which
confine
light to a relatively narrow viewing zone 136 (Figure 6). Due to the inherent
polarizing nature of an array of prisms, generally speaking, optimum
performance is
achieved when the axes of the prisms run paraiIel to the direction in which
the
I5 reflective polarizer was stretched.
R'O 95/17699 PCTIUS94114325
18
Optical Behavior of Multilayer Stacks
The optical behavior of a multilayer stack such as that shown above in Fig.
4 will now be described in more general terms. The multilayer stack can
include
hundreds or thousands of layers, and each layer can be made from any of a
number
of different materials. The characteristics which determine the choice of
materials
for a particular stack depend upon the desired optical performance of the
stack.
The stack can contain as many materials as there are layers in the stack. For
ease of manufacture, preferred optical thin film stacks contain only a few
different
materials. For purposes of illustration, the present discussion will describe
multilayer stacks including two materials.
The boundaries between the materials, or chemically identical materials with
different physical properties, can be abrupt or gradual. Except for some
simple
cases with analytical solutions, analysis of the latter type of stratified
media with
continuously varying index is usually treated as a much larger number of
thinner
uniform layers having abrupt boundaries but with only a small change in
properties
between adjacent layers.
The reflectance behavior at any angle of incidence, from any azimuthal
direction, is determined by the indices of refraction in each film layer of
the film
2 0 stack. If we assume that all layers in the film stack receive the same
process
conditions, then we need only look at a single interface of a two component
stack
to understand the behavior of the entire stack as a function of angle.
For simplicity of discussion, therefore, the optical behavior of a single
interface will be described. It shall be understood, however, that an actual
multilayer stack according to the principles described herein could be made of
hundreds or thousands of layers. To describe the optical behavior of a single
interface, such as the one shown in Fig. I5, the reflectivity as a function of
angle of
incidence for s and p polarized light for a plane of incidence including the z-
axis and
one in-plane optic axis will be plotted.
WO 95117699 ~ ~ ~. PCT/US94114325
Fig. I S shows two material film layers forming a single interface, with both
immersed in an isotropic medium of index no. For simplicity of illustration,
the
present discussion will be directed toward an orthogonal multflayer
birefringent
system with the optical axes of the two materials aligned, and with one optic
axis
(z) perpendicular to the film plane, and the other optic axes along the x and
y axis.
It shall be understood, however, that the optic axes need not be orthogonal,
and
that nonothorgonal systems are well within the spirit and scope of the present
invention. It shall be further understood that the optic axes also need not be
aligned
with the film axes to fall within the intended scope of the present invention.
The basic mathematical building blocks for calculating the optics of any
stack of films of any thickness, are the well known Fresnel reflection and
transmission coe~cients of the individual film interfaces. The Fresnel
coefficients
predict the magnitude of the reflectivity of a given interface, at any angle
of
incidence, with separate formulas for s and p-polarized light.
The reflectivity of a dielectric interface varies as a function of angle of
incidence, and for isotropic materials, is vastly different for p and s
polarized light.
The reflectivity minimum for p polarized light is due to the so called
Brewster
effect, and the angle at which the reflectance goes to zero is referred to as
Brewster's angle.
2 0 The reflectance behavior of any film stack, at any angle of incidence, is
determined by the dielectric tensors of all films involved. A general
theoretical
treatment of this topic is given in the text by R.M.A. Azzam and N.M. Bashara,
"Ellipsometry and Polarized Light", published by North-Holland, 1987. The
results
proceed directly from the universally well known Maxwell's equations.
The reflectivity for a single interface of a system is calculated by squaring
the absolute value of the reflection coefficients for p and s polarized light,
given by
equations 1 and 2, respectively. Equations 1 and 2 are valid for uniaxial
orthogonal
systems, with the axes of the two components aligned.
1) rPP = n2z * n2o ~fnlzZ-nozsin'0) - nlz * No ~fn2z2 no'sin~0)
n2z * n2o ~(nlzz-no2sin~0) + nlz * nlo'~(n2z2-no2sin=A)
~~'~~~5 ~~
WO 95!17699 PCT/US94114325
2) r" _ ~fnlo2 - nozsin=A) - ~fn2oz - no=sinZAl
~(nlo2 - noZSinze) + ~l(n2o2 - no2sin28)
where A is measured in the isotropic medium.
In a uniaxial birefringent system, n Ix = n I y = n 10, and n2x = n2y = n2o.
For a biaxial birefringent system, equations 1 and 2 are valid only for light
10 with its plane of polarization parallel to the x-z or y-z planes, as
defined in Fig. 15.
So, for a biaxial system, for light incident in the x-z plane, n I o = n lx
and n2o = n2x
in equation 1 (for p-polarized light), and n I o = n ly and n2o = n2y in
equation 2 (for
s-polarized light). For light incident in the y-z plane, n 10 = n ly and n2o =
n2y in
equation 1 (for p-polarized light), and n 10 = n 1 x and n2o = n2x in equation
2 (for
15 s-polarized light).
Equations I and 2 show that reflectivity depends upon the indices of
refraction in the x, y and z directions of each material in the stack. In an
isotropic
material, all three indices are equal, thus nx = ny = nz. The relationship
between nx,
ny and nz determine the optical characteristics of the material. Different
2 0 relationships between the three indices lead to three general categories
of materials:
isotropic, uniaxially birefringent, and biaxially birefringent.
A uniaxially birefringent material is defined as one in which the index of
refraction in one direction is different from the indices in the other two
directions.
For purposes of the present discussion, the convention for describing uniaxial
birefringent systems is for the condition nx = ny x nz. The x and y axes are
defined as the in-plane axes and the respective indices, nx and ny, will be
referred to
as the in-plane indices.
One method of creating a uniaxial birefringent system is to biaxially stretch
a polymeric multilayer stack (e.g., stretched along two dimensions). Biaxial
stretching of the multilayer stack results in differences between refractive
indices of
WO 95117699 ~ ~ PCTlUS94/14325
21
adjoining layers for planes parallel to both axes thus resulting in reflection
of light in
both planes ofpolarization.
A uniaxial birefringent material can have either positive or negative uniaxial
birefringence. Positive uniaxial birefringence occurs when the z-index is
greater
than the in-plane indices (nz > nx and ny). Negative uniaxial birefringence
occurs
when the z-index is less than the in-plane indices (nz < nx and ny).
A biaxial birefiingent material is defined as one in which the indices of
refraction in all three axes are different, e.g., nx * ny *nz. Again, the nx
and ny
indices will be referred to as the in-plane indices. A biaxial birefringent
system can
be made by stretching the multilayer stack in one direction. In other words
the
stack is uniaxially stretched. For purposes of the present discussion, the x
direction
will be referred to as the stretch direction for biaxial birefringent stacks.
WO 95/17699 ~ ~ ~ ~ PCTlUS94/14325
22
Uniaxial Birefrin eg nt ~vstems (Mirrorsl
The optical properties of uniaxial birefringent systems will now be
discussed. As discussed above, the general conditions for a uniaxial
birefringent
material are nx = ny x nz. Thus if each layer 102 and 104 in Fig. 15 is
uniaxially
birefringent, n 1 x = n 1 y and n2x = n2y. For purposes of the present
discussion,
assume that layer 102 has larger in-plane indices than layer 104, and that
thus nl >
n2 in both the x and y directions. The optical behavior of a uniaxial
birefringent
multilayer system can be adjusted by varying the values of n I z and n2z to
introduce
different levels of positive or negative birefringence.
Equation 1 described above can be used to determine the reflectivity of a
single interface in a uniaxial birefringent system composed of two layers such
as
that shown in Fig. 1 S. Equation 2, for s polarized light, is easily shown to
be
identical to that of the simple case of isotropic films (nx = ny = nz), so we
need only
examine equation I . For purposes of illustration, some specific, although
generic,
values for the film indices will be assigned. Let nlx = nly = 1.75, n lz =
variable,
n2x = n2y = 1.50, and n2z = variable. In order to illustrate various possible
Brewster angles in this system, no = 1.60 for the surrounding isotropic media.
Fig. 16 shows reflectivity versus angle curves for p-polarized light incident
from the isotropic medium to the birefringent layers, for cases where nlz is
numerically greater than or equal to n2z (nlz 2 n2z). The curves shown in Fig.
16
are for the following z-index values: a) n 1 z =1.75, n2z = I .50; b) n I z =
1.75, n2z
= 1.57; c) nlz= 1.70, n2z= 1.60; d) nlz= 1.65, n2z= 1.60; e) nlz= 1.61, n2z=
1.60; and ~ n 1 z = 1.60 = n2z. As n 1 z approaches n2z, the Brewster angle,
the
2 5 angle at which reflectivity goes to zero, increases. Curves a - a are
strongly angular
dependent. However, when n 1 z = n2z (curve f), there is no angular dependence
to
reflectivity. In other words, the reflectivity for curve f is constant for all
angles of
incidence. At that point, equation I reduces to the angular independent form:
(n2o - n 1 o)/(n2o + n l o). When n I z = n2z, there is no Brewster effect and
there is
3 0 constant reflectivity for all angles of incidence.
W0 95117699 ~ ~ pCT/US94/14325
Fig. 17 shows reflectivity versus angle of incidence curves for cases where
nlz is numerically less than or equal to n2z. Light is incident from isotropic
medium to the birefringent layers. For these cases, the reflectivity
monotonically
increases with angle of incidence. This is the behavior that would be observed
for
s-polarized light. Curve a in Fig. 17 shows the single case for s polarized
fight.
Curves b-a show cases for p polarized light for various values of nz, in the
following order: b) nlz =1.50, n2z = 1.60; c) nlz = I.SS, n2z = 1.60; d) nlz
=1.59, n2z = 1.60; and e) n 1 z = 1.60 = n2z. Again, when n I z = nZz (curve
e),
there is no Brewster effect, and there is constant reflectivity for all angles
of
incidence.
Fig. 18 shows the same cases as Fig. 16 and 17 bui for an incident medium
of index no =I .0 (air). The curves in Fig. 18 are plotted for p polarized
light at a
single interface of a positive uniaxial material of indices n2x = n2y = 1.50,
n2z =
I .60, and a negative uniaxially birefringent material with n I x = n ly =
1.75, and
values of n 1 z, in the following order, from top to bottom, of a) I .50; b)
1.55; c)
1.59; d) 1.60; f) 1.61; g) 1.65; h) 1.70; and i) 1.75. Again, as was shown in
Figs.
16 and I 7, when the values of n 1 z and n2z match (curve d), there is no
angular
dependence to reflectivity.
Figs. 16, 17 and 18 show that the cross-over from one type of behavior to
2 0 another occurs when the z-axis index of one film equals the z-axis index
of the other
film. This is true for several combinations of negative and positive
uniaxially
birefringent, and isotropic materials. Other situations occur in which the
Brewster
angle is shifted to larger or smaller angles.
Various possible relationships between in-plane indices and z-axis indices
are illustrated in Figs.19, 20 and 21. The vertical axes indicate relative
values of
indices and the horizontal axes are used to simply separate the various
conditions.
Each Figure begins at the left with two isotropic films, where the z-index
equals the
in-plane indices. As one proceeds to the right, the in-plane indices are held
constant
and the various z-axis indices increase or decrease, indicating the relative
amount of
3 0 positive or negative birefringence.
R'O 95117699 ~ ~ ~ ~ PCT/US94114325
24
The case described above with respect to Figs. 16, 17, and 18 is illustrated
in Fig. 19. The in-plane indices of material one are greater than the in-plane
indices
of material two, material 1 has negative birefringence (nlz less than in-plane
indices), and material two has positive birefringence (n2z greater than in-
plane
indices). The point at which the Brewster angle disappears and reflectivity is
constant for all angles of incidence is where the two z-axis indices are
equal. This
point corresponds to curve f in Fig. 16, curve a in Fig. 17 or curve d in Fig.
18.
In fig. 16, material one has higher in-plane indices than material two, but
material one has positive birefringence and material two has negative
birefringence.
In this case, the Brewster minimum can only shift to lower values of angle.
Both Figs. 19 and 20 are valid for the limiting cases where one of the two
films is isotropic. The two cases are where material one is isotropic and
material
two has positive birefringence, or material two is isotropic and material one
has
negative birefringence. The point at which there is no Brewster effect is
where the
z-axis index of the birefringent material equals the index of the isotropic
film.
Another case is where both films are of the same type, i.e., both negative or
both positive birefringent. Fig. 21 shows the case where both films have
negative
birefringence. However, it shall be understood that the case of two positive
birefringent layers is analogous to the case of two negative birefringent
layers
shown in Fig. 21. As before, the Brewster minimum is eliminated only if one z-
axis
index equals or crosses that of the other film.
Yet another case occurs where the in-plane indices of the two materials are
equal, but the z-axis indices differ. In this case, which is a subset of all
three cases
shown in Figs. 19 - 21, no reflection occurs for s polarized light at any
angle, and
the reflectivity for p polarized light increases monotonically with increasing
angle of
incidence. This type of article has increasing reflectivity for p-poiarized
light as
angle of incidence increases, and is transparent to s-polarized light. This
article can
be referred to, then, as a "p-poiarizer".
Those of skill in the art will readily recognize that the above described
3 0 principles describing the behavior of uniaxially birefringent systems can
be applied
WO 95/I7699 ~ ~ ~ PCT/US94/14325
to create the desired optical effects for a wide variety of circumstances. The
indices
of refraction of the layers in the multilayer stack can be manipulated and
tailored to
produce devices having the desired optical properties. Many negative and
positive
uniaxial birefringent systems can be created with a variety of in-plane and z-
axis
5 indices, and many useful devices can be designed and fabricated using the
principles
described here.
WO 95/17699 ~ ~'~ ~ PCT/US94114325
26
Biaxial Birefrin e~ nt Systems oIarizersl
Referring again to Fig. I5, two component orthogonal biaxiaI birefringent
systems will now be described. Again, the system can have many layers, but an
understanding of the optical behavior of the stack is achieved by examining
the
optical behavior at one interface.
A biaxial birefringent system can be designed to give high reflectivity for
tight with its plane of polarization parallel to one axis, for all angles of
incidence,
and simultaneously have low reflectivity for light with its plane of
polarization
parallel to the other axis at all angles of incidence. As a result, the
biaxial
birefringent system acts as a polarizer, transmitting light of one
polarization and
reflecting light of the other polarization. By controlling the three indices
of
refraction of each fllm, nx, ny and nz, the desired polarizer behavior can be
obtained.
The multilayer reflecting polarizer of PEN/coPEN described above is an
example of a biaxial birefringent system. It shall be understood, however,
that in
general the materials used to construct the multilayer stack need not be
polymeric.
Any materials falling within the general principles described herein could be
used to
construct the multilayer stack.
Referring again to Fig. 15, we assign the following values to the film indices
2 0 for purposes of illustration: n 1 x = I .88, n 1 y = 1.64, n lz =
variable, n2x = 1.65,
n2y = variable, and n2z = variable. The x direction is referred to as the
extinction
direction and the y direction as the transmission direction.
Equation 1 can be used to predict the angular behavior of the biaxial
birefringent system for two important cases of light with a plane of incidence
in
either the stretch or the non-stretch directions. The polarizer is a mirror in
one
polarization direction and a window in the other direction. In the stretch
direction,
the large index differential of 1.88 - 1.65 = 0.23 in a multilayer stack with
hundreds
of layers wilt yield very high reflectivities for s-polarized Light. For p-
polarized light
the reflectance at various angles depends on the nlzln2z index differential.
WO 95/17699 ~ ~ ~ PCTIUS94/14325
2T
In most applications, the ideal reflecting polarizer has high reflectance
along
one axis and zero reflectance along the other, at all angles of incidence. If
some
reflectivity occurs along the transmission axis, and if it is different for
various
wavelengths, the efficiency of the polarizer is reduced, and color is
introduced into
the transmitted light. Both effects are undesirable. This is caused by a large
z-index
mismatch, even if the in-plane y indices are matched. The resulting system
thus has
large reflectivity for p, and is highly transparent to s polarized light. This
case was
referred to above in the analysis of the mirror cases as a "p polarizer".
Fig. 22 shows the reflectivity (plotted as -Log[1-R]) at 75° for p
polarized
light with its plane of incidence in the non-stretch direction, for an 800
layer stack of
PEN/coPEN. The reflectivity is plotted as function of wavelength across the
visible
spectrum (400 - 700 nm). The relevant indices for curve a at 550 nm are n 1 y
=1.64,
nlz = 1.52, n2y = 1.64 and n2z= 1.G3. The model stack design is a simple
linear
thickness grade for quarterwave pairs, where each pair is 0.3% thicker than
the
previous pair. All layers were assigned a random thickness error with a
gaussian
distribution and a 5% standard deviation.
Curve a shows high off axis reflectivity across the visible spectrum along the
transmission axis (the y-axis) and that different wavelengths experience
different
levels of reflectivity. Since the spectrum is sensitive to layer thickness
errors and
spatial nonuniformities, such as film caliper, this gives a biaxial
birefringent system
with a very nonuniform and "colorful" appearance. Although a high degree of
color
may be desirable for certain applications, it is desirable to control the
degree of off
axis color, and minimize it for those applications requiring a uniform, low
color
appearance, such as LCD displays or other types of displays.
If the film stack were designed to provide the same reflectivity for all
visible
wavelengths, a uniform, neutral gray reflection would result. However, this
would
require almost perfect thickness contort. Instead, off axis reflectivity, and
off axis
color can be minimized by introducing an index mismatch to the non-stretch in-
plane
indices (nly and n2y) that create a Brewster condition offaxis, while keeping
the s-
polarization reflectivity to a minimum.
R'~ 95/17699 ~ ~ ~ ~ ~ PCTIUS94/14325
28
Fig. 23 explores the effect of introducing a y-index mismatch in reducing
off axis reflectivity along the transmission axis of a biaxial birefringent
system.
With n 1 z = 1.52 and n2z = 1.63 (~nz = 0.1 I), the following conditions are
plotted
for p polarized light: a) n I y = n2y = 1.64; b) n 1 y = 1.64, n2y = 1.62; c)
n 1 y =
1.64, n2y = 1.66. Curve a shows the reflectivity where the in-plane indices
nly and
n2y are equal. Curve a has a reflectance minimum at 0°, but rises
steeply after 20°.
For curve b, nly > n2y, and reflectivity increases rapidly. Curve c, where nly
<
n2y, has a reflectance minimum at 38°, but rises steeply thereafter.
Considerable
reflection occurs as well for s polarized light for n I y ~ n2y, as shown by
curve d.
Curves a -d of Fig. 23 indicate that the sign of the y-index mismatch (n 1 y -
n2y)
should be the same as the z-index mismatch (nlz- n2z) for a Brewster minimum
to
exist. For the case of n 1 y = n2y, reflectivity for s polarized light is zero
at all
angles.
By reducing the z-axis index difference between layers, the offaxis
reflectivity can be further reduced. If nlz is equal to n2z, Fig. 18 indicates
that the
extinction axis will still have a high reflectivity oft=angle as it does at
normal
incidence, and no reflection would occur along the nonstretch axis at any
angle
because both indices are matched (e.g., nly = n2y and nlz = n2z).
Exact matching of the two y indices and the two z indices may not be
2 0 possible in some polymer systems. If the z-axis indices are not matched in
a
polarizer construction, a slight mismatch may be required for in-plane indices
nly
and n2y. Another example is plotted in FIG. 24 assuming nlz = 1.56 and n2z =
1.60 (~nz = 0.04), with the following y indices a) nly= 1.64, n2y = 1.65; b)
nly=
1.64, n2y = 1.63. Curve c is for s-polarized light for either case. Curve a,
where the
sign of the y-index mismatch is the same as the z-index mismatch, results in
the
lowest off angle reflectivity.
The computed off axis reflectance of an 800 layer stack of ftlms at
75° angle
of incidence with the conditions of curve a in Fig. 24 is plotted as curve b
in Fig. 22.
Comparison of curve b with curve a in Fig. 22 shows that there is far less off
axis
3 0 reflectivity, and therefore lower perceived color, for the conditions
plotted in curve
WO 95/17699 ~ ~-~ ~ ~ pCTrt7S94/14315
29
b. The relevant indices for curve b at 550 nm are n ly = 1.64, nlz = 1.56, n2y
= 1.65
and n2z = 1.60.
Fig. 25 shows a contour plot of equation i which summarizes the ofFaxis
reflectivity discussed in relation to Fig. 15 for p-polarized light. The four
independent indices involved in the non-stretch direction have been reduced to
two
index mismatches, ~nz and any. The plot is an average of 6 plots at various
angles
of incidence from 0° to 75° in 15 degree increments. The
reflectivity ranges from
0.4 x 10'° for contour a, to 4.0 x 10'' for contour j, in constant
increments of 0.4 x
''. The plots indicate how high reflectivity caused by an index mismatch along
10 one optic axis can be offset by a mismatch along the other axis.
Thus, by reducing the z-index mismatch between layers of a biaxial
birefringent systems, and/or by introducing a y-index mismatch to produce a
Brewster effect, off axis reflectivity, and therefore off axis color, are
minimized
along the transmission axis of a multilayer reflecting polarizer.
It should also be noted that narrow band polarizers operating over a narrow
wavelength range can also be designed using the principles described herein.
These
can be made to produce polarizers in the red, green, blue, cyan, magenta, or
yellow
bands, for example.
With the above-described design considerations established, one of
ordinary skill will readily appreciate that a wide variety of materials can be
used
to form multilayer mirrors or poiarizers according to the invention when
processed under conditions selected to yield the desired refractive index
relationships. In general, all that is required is that one of the materials
have a
different index of refraction in a selected direction compared to the second
material. This differential can be achieved in a variety of ways, including
stretching during or after film formation (e.g., in the case of organic
polymers),
extruding (e.g., in the case of liquid crystalline materials), or coating. In
R'O 95117699 ~ ~ ~ PCTIUS94/t4325
addition, it is preferred that the two materials have similar rheologicai
properties
(e.g., melt viscosities) such that they can be co-extruded.
In general, appropriate combinations may be achieved by selecting, as the
first material, a crystalline or semi-crystalline organic polymer and an
organic
5 polymer for the second material as well. The second material, in tum, may be
crystalline, semi-crystalline, or amorphous, or may have a birefringence
opposite
that of the first material.
Specific examples of suitable materials include polyethylene naphthalate
(PEN) and isomers thereof (e.g., 2,6-, 1,4-, I,5-, 2,7-, and 2,3-PEN),
l0 polyalkylene terephthalates (e.g., polyethylene terephthalate, polybutylene
terephthalate, and poly-1,4-cyclohexanedimethylene terephthalate), polyimides
(e.g., polyacrylic imides), polyetherimides, atactic polystyrene,
polycarbonates,
polymethacryiates (e.g., polyisobutyl methacrylate, polypropylmethacrylate,
polyethylmethacrylate, and polymethylmethacrylate), polyacrylates (e.g.,
15 polybutylacrylate and polymethylacrylate), cellulose derivatives (e.g.,
ethyl
cellulose, cellulose acetate, cellulose propionate, cellulose acetate
butyrate, and
cellulose nitrate), polyalkylene polymers (e.g., polyethylene, polypropylene,
poIybutylene, polyisobutylene, and poly(4-methyl)pentene), fluorinated
polymers
(e.g., perfluoroalkoxy resins; polytetrafluoroethylene, fluorinated ethylene-
20 propylene copolymers, polyvinylidene fluoride, and
polychlorotrifluoroethylene),
chlorinated polymers (e.g., polyvinylidene chloride and polyvinylchloride),
polysulfones, polyethersulfones, polyacrylonitrile, polyamides, silicone
resins,
epoxy resins, polyvinylacetate, polyether-amides, ionomeric resins, elastomers
(e.g., polybutadiene, polyisoprene, and neoprene), and polyurethanes. Also
25 suitable are copolymers, e.g., copolymers of PEN (e.g., copolymers of 2,6-,
1,4-, 1,5-, 2,7-, and/or 2,3-naphthalene dicarboxylic acid, or esters thereof,
with
(a) terephthalic acid, or esters thereof; (b) isophthalic acid, or esters
thereof; (c)
phthalic acid, or esters thereof; (d) alkane glycols; (e) cycloalkane glycols
(e. g.,
cyclohexane dimethanol diol); (f) alkane dicarboxylic acids; and/or (g)
30 cycloalkane dicarboxylic acids (e.g., cycIohexane dicarboxylic acid)),
WO 95117699 PCTIUS94I1d325
3 ~ 2 .t '~'~'~ 1.1
copolymers of polyalkylene terephthalates (e.g., copolymers of terephthalic
acid,
or esters thereof, with (a) naphthalene dicarboxylic acid, or esters thereof;
(b)
isophthalic acid, or esters thereof; (c) phthalic acid, or esters thereof; (d)
alkane
glycols; (e) cycloaikane glycols (e.g., cyclohexane dimethanol diol); (f)
alkane
dicarboxylic acids; and/or (g) cycloalkane dicarboxyIic acids (e.g.,
cyclohexane
dicarboxylic acid)), and styrene copolymers (e.g., styrene-butadiene
copolymers
and styrene-acrylonitrile copolymers), 4, 4'-bibenmic acid and ethylene
glycol.
In addition, each individual layer may include blends of two or more of the
above-described polymers or copolymers (e.g., blends of SPS and atactic
polystyrene).
Particularly preferred combinations of layers in the case of polarizers
include PEN/co-PEN, polyethylene terephthalate (PET)/co-PEN, PEN/SPS,
PET/SPS, PEN/Eastair, and PET/Eastair, where "co-PEN" refers to a
copolymer or blend based upon naphthalene dicarboxylic acid (as described
above) and Eastair is polycyclohexanedimethylene terephthalate commercially
available from Eastman Chemical Co.
Particularly preferred combinations of layers in the case of mirrors
include PET/Ecdel, PEN/Ecdel, PEN/SPS, PEN/THV, PEN/co-PET, and
PET/SPS, where "co-PET" refers to a copolymer or blend based upon
terephthalic acid (as described above), Ecdel is a thermoplastic polyester
commercially available from Eastman Chemical Co., and THV is a
fluoropolymer commercially available from 3M Co.
The number of layers in the device is selected to achieve the desired
optical properties using the minimum number of layers for reasons of economy.
In the case of both polarizers and mirrors, the number of layers is preferably
less
than 10,000, more preferably less than 5,000, and (even more preferably) less
than 2,000.
As discussed above, the ability to achieve the desired relationships among
the various indices of refraction (and thus the optical properties of the
multilayer
device) is influenced by the processing conditions used to prepare the
multilayer
R'O 95117699 ~ ~ ~ ~ PCTIU594114325
32
device. In the case of organic polymers which can be oriented by stretching,
the
devices are generally prepared by co-extruding the individual polymers to form
a
multilayer film and then orienting the film by stretching at a selected
temperature, optionally followed by heat-setting at a selected temperature.
Alternatively, the extrusion and orientation steps may be performed
simultaneously. In the case of polarizers, the film is stretched substantially
in
one direction (uniaxial orientation), while in the case of mirrors the film is
stretched substantially in two directions (biaxial orientation).
The film may be allowed to dimensionally relax in the cross-stretch
direction from the natural reduction in cross-stretch (equal to the square
root of
the stretch ratio) to being constrained (i.e., no substantial change in cross-
stretch
dimensions). The film may be stretched in the machine direction, as with a
length orienter, in width using a center, or at diagonal angles.
The pre-stretch temperature, stretch temperature, stretch rate, stretch
ratio, heat set temperature, heat set time, heat set relaxation, and cross-
stretch
relaxation are selected to yield a multilayer device having the desired
refractive
index relationship. These variables are inter-dependent; thus, for example, a
relatively tow stretch rate could be used if coupled with, e.g., a relatively
low
stretch temperature. It will be apparent to one of ordinary skill how to
select the
2 0 appropriate combination of these variables to achieve the desired
multilayer
device. In general, however, a stretch ratio of 1:2-10 (more preferably 1:3-7)
is
preferred in the case of polarizers. In the case of mirrors, it is preferred
that the
stretch ratio along one axis be in the range of 1:2-10 (more preferably 1:2-8,
and
most preferably 1:3-7) and the stretch ratio along the second axis be in the
range
of 1:-0.5-10 (more preferably 1:1-7, and most preferably 1:3-6).
Suitable multilayer devices may also be prepared using techniques such as
spin coating (e.g., as described in Boese et al., 1. Polym. Sci.: Part B,
30:1321
(1992)) and vacuum deposition; the latter technique is particularly useful in
the
case of crystalline polymeric organic and inorganic materials.
w0 95/17699 ~ ~ ~ ~ ~ PCT/OS94/14325
3~
The invention will now be described by way of the following examples.
In the examples, because optical absorption is negligible, reflection equals I
minus tranmission (R = 1 - T).
Mirror Examples;
PET:Ecdel, 601 A coextruded film containing 601 layers was made on a
sequential flat-film-making line via a coextrusion process. Polyethylene
terephthalate (PET) with an Intrinsic Viscosity of 0.6 dI/g (60 wt. %
phenol/40
wt. R6 dichlorobenzene) was delivered by one extruder at a rate of 75 pounds
per
I 0 hour and Ecdel 9966 (a thermoplastic elastomer available from Eastman
Chemical) was delivered by another extruder at a rate of 65 pounds per hour.
PET was on the skin layers. The feedblock method (such as that described in
U.S. Patent 3,801,429) was used to generate 151 layers which was passed
through two multipliers producing an extrudate of 601 layers. U.S. Patent
3,565,985 describes exemplary coextmsion multipliers. The web was length
oriented to a draw ratio of about 3.6 with the web temperature at about
210°F.
The film was subsequently preheated to about 235°F in about 50 seconds
and
drawn in the transverse direction to a draw ratio of about 4.0 at a rate of
about
6Yb per second. The film was then relaxed about 5~ of its maximum width in a
heat-set oven set at 400°F. The finished film thickness was 2.5 mil.
The cast web produced was rough in texture on the air side, and provided
the transmission as shown in Figure 26. The 9'o transmission for p-polarized
light at a 60° angle (curve b) is similar the value at normal incidence
(curve a)
(with a wavelength shift).
For comparison, film made by Mearl Corporation, presumably of
isotropic materials (see Fig. 27) shows a noticeable loss in reflectivity for
p-
poiarized light at a 60° angle (curve b, compared to curve a for normal
incidence).
W~ 95117699 ~ ~ ~ PCT/US94114325
~3 4
PET:Ecdel, 151 A coextruded film containing 151 layers was made on a
sequential flat-film-making line via a coextrusion process. Polyethylene
terephthalate (PET) with an Intrinsic Viscosity of 0.6 dl/g (60 wt pheno1/40
wt.
Rb dichlorobenzene) was delivered by one extruder at a rate of 75 pounds per
hour and Ecdel 9966 (a thermoplastic elastomer available from Eastman
Chemical) was delivered by another extruder at a rate of 65 pounds per hour.
PET was on the skin layers. The feedblock method was used to generate 151
layers. The web was length oriented to a draw ratio of about 3.5 with the web
temperature at about 210°F. The film was subsequently preheated to
about
l0 215°F in about 12 seconds and drawn in the transverse direction to a
draw ratio
of about 4.0 at a rate of about 2530 per second. The film was then relaxed
about
596 of its maximum width in a heat-set oven set at 400°F in about 6
seconds.
The finished film thickness was about 0.6 mil.
The transmission of this film is shown in Figure 28. The % transmission
for p-polarized light at a 60° angle (curve b) is similar the value at
normal
incidence (curve a) with a wavelength shift. At the same extrusion conditions
the
web speed was slowed down to make an infrared reflecting film with a thickness
of about 0.8 mils. The transmission is shown in Fig. 29 (curve a at normal
incidence, curve b at 60 degrees).
PEN:Ecdel, 225 A coextruded film containing 225 layers was made by
extruding the cast web in one operation and later orienting the film in a
laboratory film-stretching apparatus. Polyethylene naphthalate (PEN) with an
Intrinsic Viscosity of 0.5 dl/g (60 wt. % phenol/40 wt. 9o dichlorobenzene)
was
delivered by one extruder at a rate of 18 pounds per hour and Ecdel 9966 (a
thermoplastic elastomer available from Eastman Chemical) was delivered by
another extruder at a rate of 17 pounds per hour. PEN was on the skin layers.
The feedblock method was used to generate 57 layers which was passed through
two multipliers producing an extrudate of 225 layers. The cast web was 12 mils
WO 95117699 ~ ~ ~ ~ ~ PCT/US94/14325
~5
thick and 12 inches wide. The web was later biaxially oriented using a
laboratory stretching device that uses a pantograph to grip a square section
of
film and simultaneously stretch it in both directions at a uniform rate. A
7.46 cm
square of web was loaded into the stretcher at about 100°C and heated
to 130°C
in 60 seconds. Stretching then commenced at 100~/sec (based on original
dimensions) until the sample was stretched to about 3.5x3.5. Immediately after
the stretching the sample was cooled by blowing room temperature air on it.
Figure 30 shows the optical response of this multilayer film (curve a at
normal incidence, curve b at 60 degrees). Note that the % transmission for p
polarized light at a 60° angle is similar to what it is at normal
incidence (with
some wavelength shift).
PEN:THV 500, 449 A coexwded film containing 449 layers was made by
extruding the cast web in one operation and later orienting the film in a
laboratory film-stretching apparatus. Polyethylene naphthalate (PEN) with an
Intrinsic Viscosity of 0.53 dl/g (60 wt. ~ phenol/40 wt. ! dichlorobenzene)
was
delivered by one extruder at a rate of 56 pounds per hour and THV 500 (a
fluoropolymer available from Minnesota Mining and Manufacturing Company)
2 0 was delivered by another extruder at a rate of 11 pounds per hour. PEN was
on
the skin layers and 50% of the PEN was present in the two skin layers. The
feedblock method was used to generate 57 layers which was passed through three
multipliers producing an extrudate of 449 layers. The cast web was 20 mils
thick and 12 inches wide. The web was later biaxially oriented using a
laboratory stretching device that uses a pantograph to grip a square section
of
film and simultaneously stretch it in both directions at a uniform rate. A
7.46 cm
square of web was loaded into the stretcher at about 100°C and heated
to 140°C
in 60 seconds. Stretching then commenced at 1096/sec (based on original
dimensions) until the sample was stretched to about 3.5x3.5. Immediately after
the stretching the sample was cooled by blowing room temperature air at it.
WO 9S/17699 ~, ~ ~ PCTlUS94134325
36
Figure 31 shows the transmission of this multilayer film. Again, curve a
shows the response at normal incidence, while curve b shows the response at 60
degrees.
Polarizer Examples:
PEN:CoPEN, 449-Low Color A coextruded film containing 449 layers was
made by extruding the cast web in one operation and later orienting the film
in a
l0 laboratory film-stretching apparatus. Polyethylene naphthalate (PEN) with
an
Intrinsic Viscosity of 0.56 dl/g (60 wt. 96 phenol/40 wt. ~ dichlorobenzene)
was
delivered by one extruder at a rate of 43 pounds per hour and a CoPEN (70
mol% 2,6 NDC and 30 mol% DMT) with an intrinsic viscosity of 0.52 (60 wt.
96 pheno1140 wt. Yo dichlorobenzene) was delivered by another extruder at a
rate
of 25 pounds per hour. PEN was on the skin layers and 40~ of the PEN was
present in the two skin layers. The feedblock method was used to generate 57
layers which was passed through three multipliers producing an extrudate of
449
layers. The cast web was 10 mils thick and I2 inches wide. The web was later
uniaxially oriented using a laboratory stretching device that uses a
pantograph to
grip a square section of film and stretch it in one direction while it is
constrained
in the other at a uniform rate. A 7.46 cm square of web was loaded into the
stretcher at about 100°C and heated to 140°C in 60 seconds.
Stretching then
commenced at 1036/sec (based on original dimensions) until the sample was
stretched to about S.Sx 1. Immediately after the stretching the sample was
cooled
by blowing room temperature air at it.
Figure 32 shows the transmission of this multilayer film. Curve a shows
transmission of p-polarized light at normal incidence, curve b shows
transmission
of p-polarized light at 60° incidence, and curve c shows transmission
of s-
polarized light at normal incidence. Note the very high transmission of p-
WO 95117699 PC1'/US94/14315
3z 217 ~~.~.
polarized light at both normal and 60° incidence (85-1000. Transmission
is
higher for p-polarized light at 60° incidence because the air/PEN
interface has a
Brewster angle near 60°, so the tranmission at 60° incidence is
nearly 100 % .
Also note the high extinction of s-polarized light in the visible range (400-
700nm) shown by curve c.
PEN:CoPEN, 601-High Color A coextruded film containing 601 layers was
produced by extruding the web and two days later orienting the film on a
different tenter than described in all the other examples. Polyethylene
Naphthalate (PEN) with an Intrinsic Viscosity of 0.5 dl/g (60 wt. % phenol/40
wt. % dichlorobenzene) was delivered by one extruder at a rate of 75 pounds
per
hour and CoPEN (70 mol ~ 2,6 NDC and 30 mol % DM't~ with an IV of 0.55
dl/g (60 wt. 96 phenol/40 wt. % dichlorobenzene) was delivered by another
extruder at a rate of 65 pounds per hour. PEN was on the skin layers. The
feedblock method was used to generate 151 layers which was passed through two
multipliers producing an extrudate of 601 layers. U.S. Patent 3,565,985
describes similar coextrusion multipliers. All stretching was done in the
tenter.
The film was preheated to about 280°F in about 20 seconds and drawn
in the
2 0 transverse direction to a draw ratio of about 4.4 at a rate of about 69&
per
second. The film was then relaxed about 2~ of its maximum width in a heat-set
oven set at 460°F. The finished film thickness was 1.8 mil.
The transmission of the film is shown in Figure 33. Curve a shows
transmission of p-polarized light at normal incidence, curve b shows
transmission
of p-polarized light at 60° incidence, and curve c shows transmission
of s-
polarized light at normal incidence. Note the nonuniform transmission of p-
polarized light at both normal and 60° incidence. Also note the non-
uniform
extinction of s-polarized light in the visible range (400-700nm) shown by
curve
c.
WO 95/17699 ~ ~ ~ ~ PCl'IUS94/14325
PET:CoPEN, 449 A coextruded film containing 449 layers was made by
extruding the cast web in one operation and later orienting the film in a
laboratory film-stretching apparatus. Polyethylene Terephthalate (PET) with an
Intrinsic Viscosity of 0.60 dl/g (60 wt. 35 phenol/40 wt. 96 dichlorobenzene)
was
delivered by one extruder at a rate of 26 pounds per hour and CoPEN (70 mol Y6
2,6 NDC and 30 mol3o DMT) with an intrinsic viscosity of 0.53 (60 wt. ~
phenol/40 wt. % dichlorobenzene) was delivered by another extruder at a rate
of
24 pounds per hour. PET was on the skin layers. The feedblock method was
used to generate 57 layers which was passed through three multipliers
producing
an extrudate of 449 layers. U.S. Patent 3,565,985 describes similar
coextrusion
multipliers. The cast web was 7.5 mils thick and 12 inches wide. The web was
later uniaxially oriented using a laboratory stretching device that uses a
pantograph to grip a square section of film and stretch it in one direction
while it
is constrained in the other at a uniform rate. A 7.46 cm square of web was
loaded into the stretcher at about 100°C and heated to 120°C in
60 seconds.
Stretching then commenced at IOSb/sec (based on original dimensions) until the
sample was stretched to about S.Ox 1. Immediately after the stretching the
sample
was cooled by blowing room temperature air at it. The finished film thickness
was about 1.4 mil. This film had sufficient adhesion to survive the
orientation
process with no delamination.
Figure 34 shows the transmission of this multilayer film. Curve a shows
transmission of p-polarized light at normal incidence, curve b shows
transmission
of p-polarized light at 60° incidence, and curve c shows transmission
of s-
polarized light at normal incidence. Note the very high transmission of p-
polarized light at both normal and 60° incidence (80-1006).
WO 95117699 ~ ~ ~ PCTIUS94/14325
39
PEN:coPEN, 601 A coextruded film containing 601 layers was made on a
sequential flat-film-making line via a coextrusion process. Polyethylene
naphthalate (PEN) with an intrinsic viscosity of 0.54 dl/g (60 wt 9& Phenol
plus
40 wt ~ dichlorobenzene) was delivered by on extruder at a rate of 75 pounds
per hour and the coPEN was delivered by another extruder at 65 pounds per
hour. The coPEN was a copolymer of 70 mole ~ 2,6 naphthalene dicarboxylate
methyl ester, 15 ~ dimethyl isophthalate and 1536 dimethyl terephthaiate with
ethylene glycol. The feedbIock method was used to generate 151 layers. The
feedblock was designed to produce a gradient distribution of layers with a
ration
of thickness of the optical layers of 1.22 for the PEN and 1.22 for the coPEN.
PEN skin layers were coextruded on the outside of the optical stack with a
total
thickness of 8 ~ of the coextruded layers. The optical stack was multiplied by
two sequential multipliers. The nominal multiplication ratio of the
multipliers
were 1.2 and 1.22, respectively. The film was subsequently preheated to
310°F
in about 40 seconds and drawn in the transverse direction to a draw ratio of
about 5.0 at a rate of 6% per second. The finished film thickness was about 2
mils.
Figure 35 shows the transmission for this multilayer film. Curve a shows
transmission of p-polarized light at normal incidence, curve b shows
transmission
2 o of p-polarized light at 60° incidence, and curve c shows
transmission of s
polarized light at normal incidence. Note the very high transmission of p-
polarized light at both normal and 60° incidence (80-100%). Also note
the very
high extinction of s-polarized light in the visible range (400-700nm) shown by
curve c. Extinction is nearly 10036 between S00 and 650nm.
For those examples using the 57 layer feedblock, all layers were designed
for only one optical thickness (I/4 of SSOnm), but the extrusion equipment
introduces deviations in the layer thicknesses throughout the stack resulting
in a
fairly broadband optical response. For examples made with the 151 layer
WO 95117699 ~ ~ ~ ~ ~ 4 O PCTIUS94/14325
feedblock, the feedblock is designed to create a distribution of layer
thicknesses
to cover a portion of the visible spectrum. Asymmetric multipliers were then
used to broaden the distribution of layer thicknesses to cover most of the
visible
spectrum as described in U.S. Patents 5,094,788 and 5,094,793.
10
WO 95!17699 ~ ~ ~ ~ p~~594/14325
41
The above described principles and examples regarding the optical behavior
of multilayer films can be applied to any of the display configurations shown
in
Figs. I-3, 6, 9 - I 1 or 13. In a display such as that shown in Figs. 1-3,
where the
reflective polarizer is located between an LCD panel and an optical cavity, a
high
color polarizer may be used. The high color polatizer does not uniformly
transmit
light at wide angles, which results in the nonuniform appearance and "color"
off
axis. However, for those applications where a highly collimated beam is
desirable,
then, the off axis performance of a high color reflective polarizer is less
important.
Alternatively, in applications where a diffuser is located between the
io reflective polarizer and the LCD panel, a wide angle, low color polarizer
is
desirable. In this configuration, the diffuser will operate to randomize the
direction
of light incident upon it from the reflective polarizer. If the reflective
polarizer
were high color, then some of the off axis color generated by the reflective
polarizer would be re-directed toward the normal by the diffuser. This is
highly
undesirable as it would lead to a display with a nonuniform appearance at
normal
viewing angles. Thus, for a display in which a diffuser is located between the
reflective polarizer and the LCD panel, a low color, wide angle polarizer is
preferred.
Another advantage of a low color, wide angle polarizer in the displays
2o shown in Figs. I-3 is that the undesired polarization is reflected not only
at normal
angles of incidence, but also at very high off axis angles. This allows even
further
randomization and recycling of tight to occur, thus resulting in further
brightness
gains for the display system.
For the display co~gurations shown in Figs. 9 and 10, a brightness
enhanced reflective polarizer is placed between the LCD panel and the optical
cavity. In these configurations, a low color and wide angle reflective
polarizer is
preferred. This is due to the beam turning effect of the structured surface
material.
The effect can be described with respect to Fig. 7. For a brightness enhanced
reflective polarizer, light first passes through the reflective polarizing
element.
3o Thus, a beam having a large off axis angle, such as beam 236 in Fig. 7,
will pass
through the reflective polarizing element and impinge upon the smooth side of
structured surface material 218. Fig. 7 shows that structured surface material
218
acts as a beam turning lens, redirecting beam 236 toward the normal as it
exits the
WO 95117699 ~ ~ ~ ~ 4 2 PCTIUS94114325
structured surface side of the material. A low color, wide angle reflective
polarizes
is therefore preferred for the brightness enhanced reflective polarizes
because
otherwise undesirable colored light is redirected toward the normal viewing
angles
of an observer. By using a wide angle, low color reflective polarizes, display
uniformity at normal viewing angles is maintained.
The brightness enhanced reflective polarizes can thus benefit from the
above discussion with respect to Figs. 23-25, and particularly Fig. 24, where
off
axis color is reduced by introducing a Brewster effect at some angle away from
the
normal. As described above this is achieved by introducing a y-index mismatch
to between layers ofthe multilayered reflective polarizes, and reducing the z-
index
mismatch between layers. Thus, any desired combination ofbrightness enhanced
reflective polarizes can be achieved by tuning the angle of the prisms of the
structured surface material (given its respective optical behavior, such as
shown in
Figs. 7 and 8 for the 90° structured surface material), to the desired
off angle color
15 performance tunable through introduction of a y-index mismatch and
reduction of
the z-index mismatch (since this behavior and can be tuned in accordance with
the
discussion shown and described above with respect to Figs. 23-25).
In a display configuration such as that shown in Fig. 11, the reflective
polarizes is located between a structured surface brightness enhancement film
and
2o the LCD panel. In this configuration, the restraints on the reflective
polarizes are
not as restrictive in terms of high or low color. This is due to the beam
turning
effect of the structured surface material. Since the structured surface
material
directs light toward the normal and does not transmit fight at very wide
angles (see
Fig. 8, for example) a low color, wide angle reflective polarizes is not
necessarily
25 required as the reflective polarizes will not see any wide angle light in
this
configuration. This effect is even more pronounced in the display of Fig. 13,
where
two crossed pieces of structured surface material are placed behind the
reflective
polarizes. This results in two-dimensional co3limation of light incident on
the
reflective polarizes.
30 The invention has been described with respect to illustrative examples, to
which various modifications may be made without departing from the spirit and
scope of the present invention as defined by the appended claims.
