U.S. patent application number 15/894723 was filed with the patent office on 2018-06-14 for hybrid polarizer.
This patent application is currently assigned to 3M INNOVATIVE PROPERTIES COMPANY. The applicant listed for this patent is 3M INNOVATIVE PROPERTIES COMPANY. Invention is credited to Timothy J. Nevitt, Roger J. Strharsky, Michael F. Weber.
Application Number | 20180164483 15/894723 |
Document ID | / |
Family ID | 39247748 |
Filed Date | 2018-06-14 |
United States Patent
Application |
20180164483 |
Kind Code |
A1 |
Weber; Michael F. ; et
al. |
June 14, 2018 |
HYBRID POLARIZER
Abstract
A hybrid polarizer includes an absorbing element having a first
major surface and a second major surface. The hybrid polarizer also
includes a first birefringent reflective polarizer disposed on the
first major surface of the absorbing element, the first
birefringent reflective polarizer having a first pass axis and a
first block axis. The hybrid polarizer further includes a second
birefringent reflective polarizer disposed on the second major
surface of the absorbing element, the second reflective polarizer
having a second pass axis and a second block axis.
Inventors: |
Weber; Michael F.;
(Shoreview, MN) ; Nevitt; Timothy J.; (Red Wing,
MN) ; Strharsky; Roger J.; (Woodbury, MN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
3M INNOVATIVE PROPERTIES COMPANY |
St. Paul |
MN |
US |
|
|
Assignee: |
3M INNOVATIVE PROPERTIES
COMPANY
|
Family ID: |
39247748 |
Appl. No.: |
15/894723 |
Filed: |
February 12, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14509704 |
Oct 8, 2014 |
9891362 |
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15894723 |
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13371025 |
Feb 10, 2012 |
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14509704 |
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12916838 |
Nov 1, 2010 |
8120730 |
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13371025 |
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11614494 |
Dec 21, 2006 |
7826009 |
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12916838 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 5/3083 20130101;
G02F 1/133536 20130101; G02B 27/281 20130101; G02B 5/3041 20130101;
G02F 2001/133545 20130101 |
International
Class: |
G02B 5/30 20060101
G02B005/30; G02F 1/1335 20060101 G02F001/1335; G02B 27/28 20060101
G02B027/28 |
Claims
1. A hybrid polarizer, comprising: an absorbing element having a
first major surface and a second major surface; a first
birefringent reflective polarizer disposed on the first major
surface of the absorbing element, the first birefringent reflective
polarizer having a first pass axis and a first block axis; a second
birefringent reflective polarizer disposed on the second major
surface of the absorbing element, the second reflective polarizer
having a second pass axis and a second block axis; and a second
absorbing element disposed on one of the first birefringent
reflective polarizer or the second birefringent reflective
polarizer; wherein the first and second pass axes of the first and
second reflective polarizers are substantially aligned.
2. The hybrid polarizer of claim 1, wherein the second absorbing
element is an absorbing polarizer.
3. The hybrid polarizer of claim 1, wherein at least one of the
first birefringent reflective polarizer or the second birefringent
reflective polarizer are nearly uniaxial.
4. The hybrid polarizer of claim 1, wherein the absorbing element
was coextruded with one or more reflective polarizers
5. A display device, comprising a display panel and the hybrid
polarizer of claim 1, where the second reflective polarizer is
disposed closer to the display panel than the first reflective
polarizer and the second absorbing element is disposed on the
second reflective polarizer.
6. The display device of claim 5, wherein the second reflective
polarizer comprises a plurality of layers characterized by a
varying optical thickness, and a majority of the layers having a
smaller optical thickness are disposed closer to the display panel
than the layers having a larger optical thickness.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 14/509,704, filed on Oct. 8, 2014, which is a continuation of
U.S. application Ser. No. 13/371,025, filed on Feb. 10, 2012, which
is a continuation of U.S. application Ser. No. 12/916,838, filed on
Nov. 1, 2010, issued as U.S. Pat. No. 8,120,730, which is a
continuation of U.S. application Ser. No. 11/614,494, filed on Dec.
21, 2006, issued as U.S. Pat. No. 7,826,009, and incorporated
herein by reference.
FIELD OF THE INVENTION
[0002] The present disclosure is directed to polarizers, and, for
example, hybrid polarizers and display devices using hybrid
polarizers.
BACKGROUND
[0003] Display devices, such as liquid crystal display (LCD)
devices, are used in a variety of applications including, for
example, televisions, hand-held devices, digital still cameras,
video cameras, and computer monitors. Because an LCD panel is not
self-illuminating, some display applications may require a
backlighting assembly or a "backlight." A backlight typically
couples light from one or more sources (e.g., a cold cathode
fluorescent tube (CCFT) or light emitting diodes (LEDs)) to the LCD
panel.
[0004] Common display devices usually include polarizers. The most
commonly used type of a polarizer is a dichroic polarizer. Dichroic
polarizers are made, for example, by incorporating a dye into a
polymer sheet that is then stretched in one direction. Dichroic
polarizers can also be made by uniaxially stretching a
semicrystalline polymer such as polyvinyl alcohol, then staining
the polymer with an iodine complex or dichroic dye, or by coating a
polymer with an oriented dichroic dye. Many commercial polarizers
typically use polyvinyl alcohol as the polymer matrix for the dye.
Dichroic polarizers normally have a large amount of absorption of
light.
[0005] Another common type of a polarizer used in displays is a
reflective polarizer. Reflective polarizers tend to be more
efficient in transmitting light of the high transmission
polarization. This is due to the use of a non-absorbing dielectric
stack for polarizing light. These types of polarizers sometimes
have defects, such as leakage of light through localized areas of
the sheet and incomplete reflectivity of the high extinction
polarization over the wavelength region of interest.
[0006] In some displays applications, reflective and dichroic
polarizers have been combined, as described, for example, in
Ouderkirk et. al. U.S. Pat. No. 6,096,375 and Weber et. al. in U.S.
Pat. No. 6,697,195, hereby incorporated by reference herein. The
combination of the two polarizers provides a high reflectivity of
one polarization and high transmission for the perpendicular
polarization for light incident on the reflective polarizer side of
the combined polarizer, and high absorption and transmission for
light of orthogonal polarizations incident on the dichroic
polarizer side.
SUMMARY
[0007] In one exemplary implementation of the present disclosure, a
hybrid polarizer includes an absorbing element having a first major
surface and a second major surface. The hybrid polarizer also
includes a first nearly uniaxial birefringent reflective polarizer
disposed on the first major surface of the absorbing element, the
first nearly uniaxial birefringent reflective polarizer having a
first pass axis and a first block axis. The hybrid polarizer
further includes a second birefringent reflective polarizer
disposed on the second major surface of the absorbing element, the
second reflective polarizer having a second pass axis and a second
block axis.
[0008] In another exemplary implementation, a hybrid polarizer
includes an absorbing polarizer having a pass axis and a block
axis, a first major surface and a second major surface. The hybrid
polarizer also includes a first nearly uniaxial birefringent
reflective polarizer disposed on the first major surface of the
absorbing polarizer, the first nearly uniaxial birefringent
reflective polarizer having a first pass axis and a first block
axis. The hybrid polarizer further includes a second birefringent
reflective polarizer disposed on the second major surface of the
absorbing polarizer, the second reflective polarizer having a
second pass axis and a second block axis.
[0009] In yet another exemplary implementation of the present
disclosure, a display device includes a display panel and a hybrid
polarizer. The hybrid polarizer includes an absorbing element
having a first major surface and a second major surface. The hybrid
polarizer also includes a first nearly uniaxial birefringent
reflective polarizer disposed on the first major surface of the
absorbing element, the first nearly uniaxial birefringent
reflective polarizer having a first pass axis and a first block
axis. The hybrid polarizer further includes a second birefringent
reflective polarizer disposed on the second major surface of the
absorbing element, the second reflective polarizer having a second
pass axis and a second block axis. The first and second pass axes
of the first and second reflective polarizers are substantially
aligned.
[0010] In yet another exemplary implementation of the present
disclosure, a display device includes a display panel and a hybrid
polarizer. The hybrid polarizer includes an absorbing element
having a first major surface and a second major surface. The hybrid
polarizer also includes a first birefringent reflective polarizer
disposed on the first major surface of the absorbing element, the
first nearly uniaxial birefringent reflective polarizer having a
first pass axis and a first block axis. The hybrid polarizer
further includes a second birefringent reflective polarizer
disposed on the second major surface of the absorbing element, the
second reflective polarizer having a second pass axis and a second
block axis. The first and second pass axes of the first and second
reflective polarizers are substantially aligned. The second
reflective polarizer is disposed closer to the display panel than
the first reflective polarizer and the second reflective polarizer
comprises a plurality of layers characterized by a varying optical
thickness and a majority of the layers having a smaller optical
thickness are disposed closer to the display panel than the layers
having a larger optical thickness.
[0011] These and other aspects of the polarizers and display
devices of the subject invention will become more readily apparent
to those having ordinary skill in the art from the following
detailed description together with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] So that those having ordinary skill in the art to which the
subject invention pertains will more readily understand how to make
and use the subject invention, exemplary embodiments thereof will
be described in detail below with reference to the drawings,
wherein:
[0013] FIG. 1 shows schematically a cross-section of an exemplary
hybrid polarizer of the present disclosure;
[0014] FIG. 2 shows schematically a cross-section of another
exemplary hybrid polarizer of the present disclosure;
[0015] FIG. 3 shows schematically a cross-section of yet another
exemplary hybrid polarizer of the present disclosure;
[0016] FIG. 4 is a schematic perspective view of a reflective
polarizer according to the present disclosure;
[0017] FIG. 5 is a schematic representation of light incident on a
biaxial reflective polarizer at non-zero polar angles (.theta.) and
at azimuth angles (.phi.) between 0 and 90 degrees;
[0018] FIG. 6 is a schematic representation of a display device
according to one exemplary embodiment of the present
disclosure;
[0019] FIG. 7 is a schematic representation of a display device
according to another exemplary embodiment of the present
disclosure;
[0020] FIG. 8 is a chart showing optical density vs. wavelength of
an exemplary hybrid polarizer of the present disclosure and its
components;
[0021] FIG. 9 is a chart showing optical density vs. wavelength of
another exemplary hybrid polarizer of the present disclosure and
its components;
[0022] FIG. 10 shows a plot of transmissivity of crossed absorbing
polarizers as a function of wavelength for angles of incidence from
0 to 75 degrees at the azimuth angle of 45 degrees;
[0023] FIG. 11 shows a plot of transmissivity of a D-plate between
crossed absorbing polarizers as a function of wavelength for angles
of incidence from 0 to 75 degrees at the azimuth angle of 45
degrees;
[0024] FIG. 12 shows a plot of transmissivity of a crossed
absorbing polarizer and a hybrid polarizer according to the present
disclosure as a function of wavelength for angles of incidence from
0 to 75 degrees at the azimuth angle of 45 degrees;
[0025] FIG. 13 shows a plot of transmissivity of a crossed
absorbing polarizer and another exemplary hybrid polarizer
according to the present disclosure as a function of wavelength for
angles of incidence from 0 to 75 degrees at the azimuth angle of 45
degrees;
[0026] FIG. 14 shows a plot of the same characteristics for the
same polarizers as in FIG. 12, except that a D-plate is inserted
between the polarizers;
[0027] FIG. 15 shows a plot of the same characteristics for the
same polarizers as in FIG. 13, except that a D-plate is inserted
between the polarizers;
[0028] FIG. 16 shows a plot of the same characteristics for the
same optical elements as in FIG. 15, with the layer profiles of the
reflective polarizers reversed such that the majority of the
optically thicker layers faced the analyzer;
[0029] FIG. 17 shows a plot of transmissivity of a crossed
absorbing polarizer and yet another exemplary hybrid polarizer
according to the present disclosure as a function of wavelength for
angles of incidence from 0 to 75 degrees at the azimuth angle of 25
degrees;
[0030] FIG. 18 shows a plot of the same characteristics for the
same polarizers as in FIG. 17, except that a D-plate is inserted
between the polarizers; and
[0031] FIG. 19 shows a plot of the same characteristics for the
same optical elements as in FIG. 18, with the layer profiles of the
reflective polarizers reversed such that the majority of the
optically thicker layers faced the analyzer.
DETAILED DESCRIPTION
[0032] The present disclosure is believed to be applicable to
hybrid polarizers, which may be suitable for use in display
devices. When used in display devices, such as LCDs, hybrid
polarizers according to the present disclosure may be used to
achieve higher contrast and lower color-distortion. The term
"hybrid polarizer" refers to a combination polarizer, including two
reflective polarizers and at least one absorbing element in a
(reflective polarizer)/(absorbing element)/(reflective polarizer)
stack. The hybrid polarizer can further include additional optical
elements. For example, additional absorbing elements, such as
absorbing or dichroic polarizers, may be provided on one or both
sides of the hybrid polarizer. Such constructions can provide high
contrast display polarizers for either the back, the front, or both
sides of a display panel, such as an LCD panel. For the purposes of
the present disclosure, contrast of a polarizer is defined as a
photopically averaged pass state transmission value divided by a
photopically averaged block state transmission value of the
polarizer. With an absorbing polarizer attached to a side of the
hybrid polarizer that faces the viewer, the hybrid polarizer can
serve the dual function of providing contrast to a display panel as
well as recycling polarization for brightness enhancement.
[0033] The drawings, which are not necessarily to scale, depict
selected illustrative embodiments and are not intended to limit the
scope of the disclosure. Although examples of construction,
dimensions, and materials are illustrated for the various elements,
those skilled in the art will recognize that many of the examples
provided have suitable alternatives that may be utilized.
[0034] Unless otherwise indicated, all numbers expressing feature
sizes, amounts, and physical properties used in the specification
and claims are to be understood as being modified in all instances
by the term "about." Accordingly, unless indicated to the contrary,
the numerical parameters set forth in the foregoing specification
and attached claims are approximations that can vary depending upon
the desired properties sought to be obtained by those skilled in
the art utilizing the teachings disclosed herein.
[0035] The recitation of numerical ranges by endpoints includes all
numbers subsumed within that range (e.g. 1 to 5 includes 1, 1.5, 2,
2.75, 3, 3.80, 4, and 5) and any range within that range.
[0036] As used in this specification and the appended claims, the
singular forms "a", "an", and "the" encompass embodiments having
plural referents, unless the content clearly dictates otherwise.
For example, reference to "a film" encompasses embodiments having
one, two or more films. As used in this specification and the
appended claims, the term "or" is generally employed in its sense
including "and/or" unless the content clearly dictates
otherwise.
[0037] FIG. 1 shows a hybrid polarizer 100 according to one
exemplary embodiment of the present disclosure, which includes an
absorbing element 140 having a first major surface and a second
major surface, a first reflective polarizer 120 disposed on the
first major surface of the absorbing element 140, and a second
reflective polarizer 160 disposed on the second major surface of
the absorbing element. The absorbing element 140 may be a layer of
any suitable material having non-zero absorption. The absorbing
element may be isotropic or birefringent. In some exemplary
embodiments, the absorbing element may be an absorbing polarizer
having a pass axis and block axis. Light polarized along the pass
axis of an absorbing polarizer is preferentially transmitted, while
light polarized along the block axis of an absorbing polarizer is
preferentially absorbed.
[0038] Each of the first and second reflective polarizers 120, 160
has a pass axis and a block axis (first and second, respectively).
Light polarized along the pass axis of a reflective polarizer is
preferentially transmitted, while light polarized along the block
axis of a reflective polarizer is preferentially reflected.
Preferably, the first and second pass axes of the first and second
reflective polarizers are aligned as closely as possible or
practicable. The degree of alignment will depend on a particular
application. For example, the first and second pass axes may be
aligned to within about +/-10 degrees, about +/-5 degrees, about
+/-1 degree, about +/-0.5 degree, or even about +/-0.2 degree. In
some exemplary embodiments including an absorbing polarizer, the
pass axis of the absorbing polarizer may be aligned with the first
pass axis, the second pass axis, or both, within the constraints
described above.
[0039] Not intending to be bound by a particular theory, it is
believed that the need for an absorbing element between the two
reflective polarizers is due to the fact that, in the absence of
any loss mechanism, half of the light that leaks through the first
reflective polarizer eventually also leaks through the second
reflective polarizer. This occurs due to the multiple reflections,
which are illustrated schematically in FIG. 1. For example, if the
first polarizer is 99% reflective and is non-absorbing, 1% of the
block state polarization is transmitted. Upon summing the infinite
number of multiple reflections with the binomial formula, it can be
shown that half of this light leaks through the second reflective
polarizer, if it is also lossless and 99% reflective. Thus, the
block state reflectivity of two lossless 99% reflective polarizers
is not 99.99%, but only 99.5%.
[0040] The equation for the overall transmission T of a
two-reflector system (representing the reflectivities of light
polarized along the block axes of two aligned reflective
polarizers) with an absorbing element between the two, derived by
summing the infinite series of multiple reflections between the two
reflectors is:
T = T 1 * T 2 * e - .alpha. d 1 - R 2 * R 2 * e - 2 .alpha. d
Equation 1 ##EQU00001##
where T1 and T2 are the transmissivity values of the two
reflectors, and R1 and R2 are their corresponding reflectivities.
The losses in the reflectors are assumed to be negligible. .alpha.
and d are the absorption coefficient and thickness, respectively,
of the absorbing element. Exp(-.alpha.d) is the internal
transmission of the absorbing element. To a first approximation,
small losses in the reflectors can be included in this term.
[0041] In one exemplary hybrid polarizer according to the present
disclosure, the first and second reflective polarizers each have a
reflectivity for light polarized along the first and second block
axes not higher than about 90%. In such exemplary embodiments,
absorption of the absorbing layer may be about 90% for light
polarized in the block direction. In other exemplary embodiments,
one or both reflective polarizers have a reflectivity for light
polarized along the first and second block axes not higher than
about 95, 96, 98 or 99%.
[0042] Another exemplary embodiment of a hybrid polarizer 200
constructed according to the present disclosure is shown in FIG. 2.
The hybrid polarizer 200 includes an absorbing element 240 having a
first major surface and a second major surface, a first reflective
polarizer 220 disposed on the first major surface of the absorbing
element 240, and a second reflective polarizer 260 disposed on the
second major surface of the absorbing element. In some exemplary
embodiments, the absorbing element 240 may be an absorbing
polarizer having a pass axis and block axis. In other exemplary
embodiments, the absorbing element 240 may be isotropic or nearly
isotropic.
[0043] The hybrid polarizer 200 further includes an anti-reflective
or anti-glare layer 230 disposed, for example, on the viewer side
200a of the hybrid polarizer 200 (i.e., the side of the hybrid
polarizer that is intended to face a viewer when the hybrid
polarizer is installed into a display device). The anti-reflective
layer 230 may be another absorbing element, such as an absorbing
polarizer having a pass axis and a block axis. In one exemplary
embodiment, the anti-glare layer 230 is a relatively low contrast
absorbing polarizer layer used to eliminate the glare from the
viewer side 200a of the hybrid polarizer 200. The absorbing
anti-glare layer also enables a contrast improvement. In such
exemplary embodiments, the absorbing element disposed between the
two reflective polarizers can have a lower contrast ratio than the
anti-glare layer, to the point of being isotropic. The anti-glare
or anti-reflective element 230 can be a layer including dyes
coextruded with one or more of the other elements of the hybrid
polarizer 200, or the anti-reflective element 230 may be coated or
laminated onto another element of the hybrid polarizer 200.
[0044] Each of the first and second reflective polarizers 220, 260
has a pass axis and a block axis (first and second, respectively).
Preferably, the first and second pass axes of the first and second
reflective polarizers are aligned as closely as possible or
practicable, as described above. In the exemplary embodiments where
one or both of the elements 240 and 230 are absorbing polarizers,
one or both of their pass axes may be aligned with the first pass
axis, the second pass axis, or both within the constraints provided
above.
[0045] Yet another exemplary embodiment of a hybrid polarizer 300
constructed according to the present disclosure is shown in FIG. 3.
The hybrid polarizer 300 includes an absorbing element 340 having a
first major surface and a second major surface, a first reflective
polarizer 320 disposed on the first major surface of the absorbing
element 340, and a second reflective polarizer 360 disposed on the
second major surface of the absorbing element. The hybrid polarizer
300 may or may not further include an anti-reflective or anti-glare
element 330 disposed on the viewer side 300a of the hybrid
polarizer 300. The hybrid polarizer 300 includes a rear absorbing
element 350, which may be an absorbing polarizer. The rear
absorbing element 350 is disposed on the back side 300b of the
hybrid polarizer 300, which is opposite to the viewer side 300a. In
some exemplary embodiments, one, two or all absorbing elements 330,
340 and 350 may be absorbing polarizers having a pass axis and
block axis. One or both of the anti-glare element 330 and the rear
absorbing element 350 may sometimes require a different absorber
type or concentration than the absorbing element 340 between the
reflecting polarizers.
[0046] The exemplary hybrid polarizer 300 may be used as the front
(viewer side) display polarizer of a display device. The rear
absorbing element 350 would minimize multiple reflections from
elements in the display device and the hybrid polarizer. Each of
the first and second reflective polarizers 320, 360 has a pass axis
and a block axis (first and second, respectively). Preferably, the
first and second pass axes of the first and second reflective
polarizers are aligned as closely as possible or practicable. In
the exemplary embodiments where one, two or all of the absorbing
elements 330, 340 and 350 are absorbing polarizers, one, two or all
of their pass axes may be aligned with the first pass axis, the
second pass axis, or both within the constraints stated above.
Reflective Polarizer
[0047] One or both reflective polarizers used in exemplary hybrid
polarizers according to the present disclosure may be birefringent
reflective polarizers. FIG. 4 illustrates one exemplary embodiment
of a reflective polarizer according to the present disclosure,
which is a multilayer optical film 111 that includes a first layer
of a first material 113 disposed (e.g., by coextrusion) on a second
layer of a second material 115. The depicted optical film 111 may
be described with reference to three mutually orthogonal axes x, y
and z. Two orthogonal axes x and y are in the plane of the film 111
(in-plane, or x and y axes) and a third axis (z-axis) extends in
the direction of the film thickness. One or both of the first and
second materials may be birefringent.
[0048] While only two layers are illustrated in FIG. 4 and
generally described herein, typical embodiments of the present
disclosure include two or more of the first layers interleaved with
two or more of the second layers. The total number of layers may be
hundreds or thousands or more. In some exemplary embodiments,
adjacent first and second layers may be referred to as an optical
repeating unit. Reflective polarizers suitable for use in exemplary
embodiments of the present disclosure are described in, for
example, U.S. Pat. Nos. 5,882,774, 6,498,683, 5,808,794, which are
incorporated herein by reference.
[0049] The optical film 111 may include additional layers. The
additional layers may be optical, e.g., performing an additional
optical function, or non-optical, e.g., selected for their
mechanical or chemical properties. As discussed in U.S. Pat. No.
6,179,948, incorporated herein by reference, these additional
layers may be orientable under the process conditions described
herein, and may contribute to the overall optical and/or mechanical
properties of the film, but for the purposes of clarity and
simplicity these layers will not be further discussed in this
application. For the purposes of the present disclosure, it is
preferred that thick biaxially birefringent outer layers are not
disposed on the side of the polarizer that faces a display panel.
If thick outer layers are needed on the side of the polarizer that
is intended to face the display once installed, such layers should
be removable or they should be made of isotropic or only weakly
biaxially birefringent materials.
[0050] In a birefringent reflective polarizer, the refractive
indices of the first layers 113 (n.sub.1x, n.sub.1y, n.sub.1z) and
those of the second layers 115 (n.sub.2x, n.sub.2y, n.sub.2z) are
substantially matched along one in-plane axis (y-axis) and are
substantially mismatched along another in-plane axis (x-axis). The
matched direction (y) forms a transmission (pass) axis or state of
the polarizer, such that light polarized along that direction is
preferentially transmitted, and the mismatched direction (x) forms
a reflection (block) axis or state of the polarizer, such that
light polarized along that direction is preferentially reflected.
Generally, the larger the mismatch in refractive indices along the
reflection direction and the closer the match in the transmission
direction, the better the performance of the polarizer.
[0051] To function well for wide angle viewing of a display device,
a display polarizer should maintain high block state contrast for
all angles of incidence and also maintain high pass transmission
over all angles of incidence. As it has been shown in the commonly
owned U.S. Pat. No. 5,882,774, pass state transmission increases
when the refractive indices of the alternating first and second
layers 113 and 115 are matched for light polarized along the z-axis
and for light polarized along the y-axis. The z-index matching also
ensures that the block state transmission does not degrade at high
angles of incidence.
[0052] Preferably, at least one reflective polarizer in a hybrid
polarizer according to the present disclosure is nearly uniaxial.
For the purposes of the present disclosure, "nearly uniaxial" is
defined as .DELTA.n.sub.yz=|n.sub.y-n.sub.z| being less than or
equal to about 0.05 at 633 nm for a particular birefringent
polarizer material. In some exemplary embodiments of nearly
uniaxial birefringent reflective polarizers, .DELTA.n.sub.yz may be
about 0.03 or less, about 0.02 or less, about 0.01 or less, or
about 0.005 or less. More preferably, the first and second
reflective polarizers of the hybrid polarizers constructed
according to the present disclosure are both nearly uniaxial. Even
more preferably, all components of the hybrid polarizer are either
nearly uniaxial or substantially isotropic.
[0053] In other exemplary embodiments, at least one reflective
polarizer in a hybrid polarizer may be biaxial, that is, having
.DELTA.n.sub.yz of more than about 0.05 for a particular
birefringent polarizer material. In other exemplary embodiments,
.DELTA.n.sub.yz can be at least 0.08 or another suitable value
depending on the application. In some exemplary embodiments,
.DELTA.n.sub.yz can be no more than about 0.1. All values of
refractive indices and refractive index differences are reported
for 633 nm.
[0054] Although biaxial reflective polarizers can have low
reflectivity for light polarized parallel to the pass axis (y) for
any angle of incidence, when light is incident onto the reflective
polarizer at non-zero polar angles (.theta.) and at azimuth angles
(.phi.) between 0 and 90 degrees (see FIG. 5), the magnitude of the
reflectivity can oscillate dramatically as a function of both angle
of incidence and wavelength of the incident light. This is believed
to be at least in part due to unequal conversion of s-polarization
to p-polarization and p-polarization to s-polarization as a
function of wavelength as light traverses the biaxial medium. This
phenomenon both increases appearance of undesirable color of the
display device and lowers the contrast of the reflective polarizer
when it is crossed with another reflective polarizer or with an
absorbing polarizer. When a polarizer used in a display is crossed
with another polarizer, a uniform extinction vs. wavelength
spectrum is desired for all azimuths (.phi.) from 0 to 360 degrees,
i.e., not just for the planes of incidence parallel to the block
and pass axes but for all azimuthal angles (.phi.) between these
axes.
[0055] The present disclosure provides a construction for a hybrid
polarizer including a biaxial reflective polarizer that yields
improved pass state transmission and improved contrast for all
azimuthal angles at non-zero polar angles of incidence. This
construction includes, for example, a biaxial reflective polarizer
constructed of alternating low and high index layers, e.g., first
and second layers 113 and 115, with the optical thickness of the
repeat unit d.sub.1*n.sub.x1+d.sub.2*n.sub.x2 at normal incidence
being of 1/2 .lamda. thickness and wherein the repeat units, as
well as the constituent layers, are arranged such that a majority
of the layers having a smaller optical thickness d*n (referred to
as "blue" layers) are disposed closer to a display panel than the
layers having a larger optical thickness (referred to as "red"
layers). When such a polarizing film is crossed with itself or with
an absorbing polarizer, the extinction is much better at more
angles around the azimuth, if the blue layers are closest to the
other polarizer.
[0056] Preferably, the profile of the optical thicknesses of the
layers in the thickness direction of the reflective polarizer is a
monotonic function, or at least, a majority of the layers
characterized by a varying optical thickness are disposed such that
their optical thicknesses decrease monotonically in the direction
toward the display panel. However, in some exemplary embodiments,
the function characterizing the profile of optical thicknesses of
the layers in a biaxial reflective polarizer may have local minima
and maxima. These minima and maxima can be disregarded, so long as
the majority of the layers having a smaller optical thickness are
disposed closer to the display panel than the layers having a
larger optical thickness, as described in commonly owned U.S.
patent application Ser. No. 11/614,409, 3M Docket No. 62631US002,
filed on even date herewith, the disclosure of which is
incorporated by reference herein.
[0057] Other exemplary reflective polarizers suitable for use in
hybrid polarizers according to the present disclosure are also
described in U.S. Pat. No. 6,697,195, hereby incorporated by
reference herein.
Absorbing Elements
[0058] An absorbing element can be stacked with reflective
polarizers, laminated to one or more reflective polarizers,
co-extruded with one or more reflective polarizers or coated onto
and oriented with one or more reflective polarizers. In some
exemplary embodiments, an entire (reflective polarizer)/(absorbing
element)/(reflective polarizer) combination may be coextruded as a
single film, or parts of it can be separately extruded and
laminated, or first oriented and then laminated.
[0059] Generally, any optically absorbing structure can be used as
the absorbing element depending, at least in part, on the desired
wavelengths of absorption and transmission. One example of an
absorbing element includes a layer of light absorbing material,
such as, for example, dye, pigment, or ink disposed in a supporting
matrix or on a supporting substrate.
[0060] Suitable absorbing elements include glass filters, such as
those obtainable from Schott Glass Technologies, Inc., Duryea, Pa.,
including the KG series of heat control filters which absorb
strongly in the near infrared but are relatively transparent in the
visible. Gentex Corporation (Carbondale, Pa.) makes plastic optical
filters under the trade name Filtron.TM. In addition, polycarbonate
or acrylic sheets loaded with dyes absorb at various wavelengths
across the visible and IR. Suitable IR and visible absorbing dyes
include dyes with good thermal stability that can be injection
molded with, for example, polycarbonate. Other suitable dyes have
broad solubility and are recommended for solution coating.
Alternative absorbing materials include pigments such as carbon
black and iron oxides, such as iron oxide-loaded glass.
[0061] The selection of the light absorbing material can be made
based on factors, such as, for example, the absorbance spectrum of
the light absorbing material, cost, processibility, stability, and
compatibility with other elements in the optical filter. A light
absorbing material may be selected with an average absorptance of
at least about 5%, 10%, 20%, 30%, or 50% over the wavelength range
that is to be reflected/absorbed. The light absorbing material may
have a relatively low average absorptance (e.g., no more than 40%,
20%, 10%, 5%, or 1%) over the wavelength range where transmission
is desired. It will be appreciated, however, that many light
absorbing materials suitable for broadband absorptive elements have
substantial absorbance over a relatively wide range of wavelengths
or a relatively constant absorptance value over portions of both
the transmission and reflection wavelength ranges. The use of the
combination of an absorptive element between two reflective
polarizers can allow the use of lower loadings of light absorbing
material than if the absorptive element was used alone or with a
single reflective element.
[0062] Many other types of lossy elements can be used, including,
for example, lossy elements that employ scattering or a combination
of scattering and absorption. For example, depending on particle
size, pigments or other particles used in the optical filters can
scatter light rays. Although this may introduce additional haze, a
scattering loss is typically equivalent to an absorptive loss.
Generally, scattering is only slowly wavelength dependent and is
typically stronger for shorter wavelengths. Scattering can be
polarization dependent based on the shape of the scattering
particles.
[0063] As mentioned above, absorbing polarizers are also suitable
for use in exemplary embodiments of the present disclosure. One
useful polarizing absorptive element is an oriented,
dye-containing, polyvinyl alcohol (PVA) film. Examples of such
films and their use as polarizing absorptive elements are
described, for example, in U.S. Pat. Nos. 4,895,769, and 4,659,523
and PCT Publication No. WO 95/17691, all of which are incorporated
herein by reference. To function as an absorbing polarizer, the
polyvinyl alcohol film is typically stretched to orient the film.
When stained with a polarizing dye or pigment, the orientation of
the film determines the optical properties (e.g., the axis of
extinction) of the film. Preferably, the absorbing element is such
that absorption of light polarized along the block axis does not
decrease (and sometimes increases) with increased angle of
incidence. One example of such absorbing elements are absorbing
polarizers including supra-molecular lyotropic liquid-crystalline
material, as described in Lazarev et al. article, entitled
"Low-leakage off-angle in E-polarizers, Journal of the SID 9/2, pp.
101-105 (2001), incorporated by reference herein.
[0064] Absorbing polarizers used in exemplary embodiments of the
present disclosure have a contrast ratio of less than 1000:1, thus
making the contribution of the reflective polarizers more
important. In some exemplary embodiments, the contrast ratios of
absorbing polarizers may be about 500:1 or less, about 100:1 or
less, about 10:1 or less, or about 5:1 or less. In some exemplary
embodiments, the absorbing polarizer may be characterized by a
contrast ratio of about 5:1 to about 100:1.
[0065] Where the absorbing polarizer of the
reflective/absorbing/reflective polarizer combination has a
contrast ratio of up to about 10:1, at least one of the reflective
polarizers preferably has a contrast of at least about 100:1. In
other exemplary embodiments, one or both of the biaxial reflective
polarizers may be characterized by a contrast ratio of at least
about 50:1, at least about 100:1 or at least about 200:1. The
reflective/absorbing/reflective polarizer combination according to
the present disclosure may have a total contrast ratio of about
500:1 or more or about 1000:1 or more. In some exemplary
embodiments, the contrast ratio of the
reflective/absorbing/reflective polarizer combination according to
the present disclosure may be as high as about 10,000:1.
Display Devices Including Hybrid Polarizers
[0066] Hybrid polarizers described above are believed to be useful
in several types of display devices, such as LCDs. They can
function as a replacement for high performance absorbing polarizers
on either the viewer side or the rear side of a display panel. When
the construction shown in FIG. 2 is used on the rear side of a
display panel, the hybrid polarizer can function as both a display
polarizer and a backlight polarization recycling film. The
construction of FIG. 2 can also be used as a viewer side polarizer
of a display panel to recycle unused light from off-state pixels to
the backlight in order to provide for increased brightness to the
on-state pixels.
[0067] With a hybrid polarizer construction, the reflective
polarizer of the three polarizer stack that is adjacent the display
should be arranged to have the blue to red layer construction just
described, with the blue layers closest to the display panel. The
construction of the other reflective polarizer, on the opposite
side of the absorbing layer, is not as important. An exemplary
display device 400 having a hybrid polarizer 440 disposed on the
rear side of a display panel 420 is illustrated in FIG. 6, where
the viewer is on the left. As shown, the hybrid polarizer 440
includes an absorbing element 444 having a first major surface and
a second major surface, a first birefringent reflective polarizer
446 disposed on the first major surface of the absorbing element
and a second birefringent reflective polarizer 442 disposed on the
second major surface of the absorbing element, and an
anti-reflective element 443 disposed on the viewer side of the
hybrid polarizer 440. Those of ordinary skill in the art will
readily appreciate that the hybrid polarizer 440 may have any of
the configurations shown in FIGS. 1-3.
[0068] In this exemplary embodiment, the second reflective
polarizer 442 is disposed closer to the display panel than the
first reflective polarizer 446, but in other exemplary embodiments,
the order may be reversed. Preferably, the reflective polarizer
that is disposed closer to the display panel includes a plurality
of layers characterized by a varying optical thickness, in which a
majority of the layers having a smaller optical thickness are
disposed closer to the display panel than the layers having a
larger optical thickness. The display device 400 further includes a
compensation film 470 and an additional display polarizer 410.
[0069] Another exemplary display device 500 having a hybrid
polarizer 540 disposed on the viewer side of a display panel 520 is
illustrated in FIG. 7, where the viewer is on the left. As shown,
the hybrid polarizer 540 includes an absorbing element 544 having a
first major surface and a second major surface, a first reflective
polarizer 546 disposed on the first major surface of the absorbing
element and a second reflective polarizer 542 disposed on the
second major surface of the absorbing element, and an
anti-reflective element 543 disposed on the viewer side of the
hybrid polarizer 540. Those of ordinary skill in the art will
readily appreciate that the hybrid polarizer 540 may have any of
the configurations shown in FIGS. 1-3.
[0070] In this exemplary embodiment, the second reflective
polarizer 542 is disposed closer to the display panel than the
first reflective polarizer 546, but in other exemplary embodiments,
the order may be reversed. Preferably, the reflective polarizer
that is disposed closer to the display panel includes a plurality
of layers characterized by a varying optical thickness, in which a
majority of the layers having a smaller optical thickness are
disposed closer to the display panel than the layers having a
larger optical thickness. The display device 500 further includes a
compensation film 570 and an additional display polarizer 510. The
additional polarizer 410 or 510 may also be a hybrid polarizer
according to the present disclosure.
[0071] Typically the compensation film 470, 570 may be disposed
between the display panel 420, 520 and the hybrid polarizer 440,
540, between the display panel 420, 520 and the additional
polarizer 410, 510, or both. One example of a suitable compensation
film is a biaxial birefringent film. One type of a biaxial
birefringent film is termed a "D-plate", an example of which is the
NRZ.TM. film available from Nitto Denko Corporation of Osaka,
Japan. Such a film has an out of plane retardation R.sub.th that is
approximately 0, where R.sub.th is given by
R.sub.th=[(n.sub.x+n.sub.y)/2-n.sub.z]*thickness. That is, the
z-index of the D-plate is approximately equal to the average of the
x and y indices of refraction of the film. A typical D-plate
compensation film also has an in-plane retardation
R.sub.0=(n.sub.x-n.sub.y)*thickness that is approximately equal to
1/2 .lamda., where .lamda. is in the wavelength range of interest.
The compensation film(s) also may be designed to correct for
angle-dependent retardation of the LC material. To this end,
additional retardance, in an amount equal but opposite in sign to
the LC material, is added to the compensation layer(s) to correct
for the retardance of the LC material in a complete LC display
panel.
[0072] Exemplary display devices according to the present
disclosure may include a backlight as known to those of skill in
the art. In the exemplary embodiments including a backlight, a
hybrid polarizer may be disposed between the backlight and the
display panel. The configuration of the backlight is not limited to
any specific construction. Any suitable structure capable of
providing light to the display panel may be used. Suitable examples
of backlights include, without limitation, edge-lit backlights
including one or more light sources optically coupled to one or
more edges of one or more lightguides, and direct-lit backlights
including one or more light sources disposed such that the display
panel is disposed between the one or more light sources and a
viewer, that is, directly behind the display panel in the field of
view of a viewer of the display. In the exemplary embodiments
including a back reflector and no backlight, the hybrid polarizer
may be disposed between the reflector and the display panel.
[0073] Other optical elements and films may be included into
display devices according to the present disclosure as would be
known to those of ordinary skill in the art. Exemplary suitable
additional optical elements include, without limitation, structured
surface films. Examples of structured surface films include films
having a plurality of liner prismatic surface structures, a
plurality of lenticular surface structures, a matrix or a random
array of surface structures, and others. Another type of an optical
film suitable for use in displays of the present disclosure are
optical films including a layer including beads dispersed in a
binder. Similarly, diffuser films used to increase the uniformity
of illumination could also be disposed at various locations such as
between the backlight and the biaxial reflecting polarizer film.
Such films may be disposed between the backlight and the biaxial
reflective polarizer or at another suitable location.
[0074] Exemplary hybrid polarizers of the present disclosure may be
capable of providing contrast of 10,000:1, which would be
exceptionally valuable in projection displays, either as a
polarizing beamsplitter or a prepolarizer. For such applications,
the preferred construction would be the one illustrated in FIG.
1.
Bandwidth
[0075] Although most LCD displays are broadband, i.e., they control
the transmission of most wavelengths of visible light, hybrid
polarizers of the present disclosure can be either broadband or
narrow band. For example, hybrid polarizers according to the
present disclosure and can operate in the UV, visible, or infrared
portions of the spectrum, or in any combination of the three. The
bandwidth of the absorbing elements and the anti-glare elements
typically need only absorb over the range of wavelengths to which
the reflective polarizers are tuned, but may be broader or narrower
than those ranges, depending on the application. The two reflective
polarizers would typically operate over the same range of
wavelengths, but may be wavelength shifted with respect to each
other as desired. For example, the two reflective polarizers may be
wavelength shifted so that identical spectral leaks do not
align.
EXAMPLES
[0076] The examples below explore some of the performance
characteristics of a hybrid polarizer including a dichroic dye
absorbing polarizer disposed between two multilayer reflective
polarizers.
Example 1
[0077] A construction shown in FIG. 1 was produced, using Advanced
Polarizer Film (APF), available from 3M Company, as reflective
polarizers. A suitable APF film has been described, for example in
the Invited Paper 45.1, authored by Denker et al., entitled
"Advanced Polarizer Film for Improved Performance of Liquid Crystal
Displays," presented at Society for Information Displays (SID)
International Conference in San Francisco, Calif., Jun. 4-9,
2006.
[0078] Each APF reflective polarizer was made of 275 alternating
layers of 90/10 coPEN, a polymer composed of 90% polyethylene
naphthalate (PEN) and 10% polyethylene therephthalate (PET), and a
low index isotropic layer, which was made with a blend of
polycarbonate and copolyesters such that the index is about 1.57
and remains substantially isotropic upon uniaxial orientation of
the coPEN. The absorbing layer was made with a magenta dichroic dye
mixed with PVA, which was then coated onto a PET cast web and then
uniaxially oriented in a batch stretcher. The dichroic coated PET
layer was then laminated between the two APF films with the block
axes of all three parallel to one another.
[0079] Using Equation 1, the predicted optical density (OD, here
defined as -Log(T)) was calculated and is shown in FIG. 8 as the
Theoretical OD. The OD of the laminate, measured in a Perkin Elmer
.lamda.-19 spectrophotometer against an integrating sphere, is
shown in the chart as the Measured OD. The samples were measured
with pre-polarized light from a Glan-Thompson crystal polarizer.
The transmission of each of the three polarizers in the hybrid
stack was also measured. The chart in FIG. 8 shows the measured
transmission spectrum of the two reflective polarizers, referred to
as APF1 and APF2, and of the absorbing layer, referred to as
Dichroic dye.
[0080] Several things can be noted from this chart. In the region
of longer wavelengths, the dye has essentially zero absorption, so
the OD of the combined polarizers is only a little more than that
of one reflective polarizer, as predicted by equation 1. The
measured OD is slightly higher than theoretical, which implies that
there is a small amount of loss in the laminate, which may be due
to either scattering or absorption, or both. At shorter
wavelengths, where the dye is absorbing and the reflective
polarizers have higher OD, the theoretical OD is quite high.
However, the measured OD is lower, although a peak OD of about 3.5
was obtained. The sample was then re-measured with a cleanup
polarizer oriented parallel to the pass state direction. The
measured OD then was equal to or above the theoretical values.
Since no measurable pass state light was incident on the laminate
from the crystal polarizer, this implies that some of the block
state light was converted to pass state light. This is indicative
of scattering in the films, which depolarizes some of the incident
light.
Example 2
[0081] For this sample, two slightly less reflective APF films were
laminated to a broadband absorbing polarizer, which had between 90%
and 95% absorption of block state light. The absorbing polarizer
was made by coating a mixture of red, green and blue lyotropic dyes
onto a polymethyl methacrylate (PMMA) film. The water soluble
lyotropic dyes were oriented by the shearing action in coating
process and then immediately dried. The OD of the absorbing
polarizer is shown in FIG. 9, labeled as "dichroic dye." As in the
previous example, the OD of the laminate was calculated using
equation 1 and is plotted as the theoretical OD. The OD of the
laminate was measured with a Perkin-Elmer .lamda.-950
spectrophotometer with light, which was pre-polarized by a
Glan-Thompson polarizer. Spectra were obtained with and without a
cleanup polarizer as in the previous example. The average OD is
about 3.7 without the clean-up polarizer and is about 5.0 with the
clean-up polarizer. The latter approximately matches the
theoretical value. The difference between the two can again be
ascribed to scattering and depolarization of the polarized
measurement beam. In order to minimize scattering, the material
should be made with as clean a resin as possible, and scattering
due to polymer crystallites should also be minimized. The latter
can be effected with the use of birefringent copolymers as
copolymers tend to have smaller crystal sizes than homopolymers. As
in known in the art, orientation conditions can also affect the
crystallite size in the polymer.
Comparative Example 3
[0082] The data in FIGS. 8 and 9 were obtained at near-normal
incidence. However, an LCD comprising crossed polarizers should
have a high contrast at all angles of incidence and azimuth. In
this Comparative Example, the transmission of crossed absorbing
polarizers was calculated as a function of wavelength for angles of
incidence from 0 to 75 degrees. Thick protective layers made from a
birefringent material such as cellulose triacetate (TAC) were not
included in the modeled polarizer construction. The plane of
incidence was at the azimuth angle of 45 degrees. These spectra are
shown in FIG. 10. Note that the average visible light transmission
at 60 degrees is about 1.5%, which is the value typically observed
with crossed absorbing polarizers.
[0083] The insertion of a D-plate between the two absorbing
polarizers dramatically reduces the transmission of light at the
higher angles of incidence. This result is illustrated in FIG. 11,
which has 10.times. expanded scale as compared to FIG. 10. The
photopic weighted visible light transmissivity at 60 degrees is now
only about 0.0005, which is much less than the value of 0.015
illustrated in FIG. 10 for the case with no compensation.
Approximately the same transmission, or lower, is obtained for
other azimuthal angles.
[0084] The D-plates used in this and the following modeled examples
were assumed to have an in-plane retardance of 1/2 .lamda. at 530
nm and were assumed to be made of a material with the nominal
indices and dispersion values of polycarbonate. The thickness of
the plate was assumed to be 10 microns and the in-plane indices
were assumed to be n.sub.x=1.592 and n.sub.y=1.566 at 633 nm. Given
these in-plane indices, the ideal D-plate has a value of
n.sub.z=1.579.
Example 4
[0085] Transmissivity of a hybrid polarizer having the construction
of FIG. 6, using APF for the two reflective polarizers, and crossed
with one of the absorbing polarizers of the previous comparative
example, was modeled at the azimuth angle of 45 degrees in the same
manner as described above. The high refractive index layers were
assumed to have a y-z index mismatch of about 0.015. Both the
absorbing layer between the APF structures, as well as the
anti-reflective (AR) top layer, are assumed to have the same real
indices as the high refractive index layers of the APF but have the
same imaginary (absorbing) indices as the absorbing polarizers. The
absorbing layer was assumed to have an absorption of about 50% of
block state polarization and the anti-reflective layer was assumed
to have an absorption of about 80% of light with the block state
polarization.
[0086] The spectra without a D-plate between the crossed polarizers
are shown in FIG. 12. Note that the spectra are similar to those in
FIG. 10 for absorbing polarizers, except for some ripple in the
spectra at high angles of incidence. This ripple was found to be
due to the biaxial nature of the anti-reflective layer. If this
anti-reflective layer is replaced with a nearly uniaxial layer such
as a PVA iodine polarizer layer with the same absorptivity, the
spectra of FIG. 13 are obtained. Note that these are also slightly
higher than the values in FIG. 10 for the crossed absorbing
polarizers. However, if all layers of the hybrid polarizer are
assumed to be true uniaxial, approximately the same result is
obtained as in FIG. 10.
[0087] When a D-plate is inserted between the polarizers, however,
the transmission, shown in FIGS. 14 and 15 for the two cases just
described, drops to about the values observed for the crossed
absorbing polarizers. Note that for the case with a uniaxial AR
layer, shown in FIG. 15, the spectra are almost identical to those
of crossed absorbing polarizers. As in the case above for absorbing
polarizers, approximately the same or lower transmission is
calculated for more azimuthal angles. This shows that a hybrid
polarizer constructed with a very low contrast AR layer can be used
in a display to produce high contrast over a wide range of viewing
angles similar to high performance absorbing polarizers.
Example 5
[0088] Example 4 illustrates that the hybrid polarizer does not
have to be constructed with purely uniaxial material layers. The
present Example 5 shows that it is desirable to have the thinnest
layers of such a construction facing the analyzer. When the layer
profiles of the reflective polarizers in Example 4 were reversed
such that the majority of the optically thicker layers faced the
analyzer, the spectra of FIG. 16 were obtained. Although the
average transmission is about the same, a higher frequency ripple
is introduced into the spectra, which can cause a higher incidence
of perceived color when the display is illuminated with narrow band
RGB light sources. Thus the construction of Example 4 is
preferred.
Example 6
[0089] Example 4, with the not perfectly uniaxial coPEN layers in
the APF (.DELTA.n.sub.yz=0.015) raises the question of how much
birefringence of the layers can depart from uniaxial and still
provide a high contrast, i.e. compensatable, display polarizer. To
test this limit, a hybrid polarizer was modeled using layers having
the indices of standard materials used for the reflective polarizer
referred to as Vikuiti.TM. Dual Brightness Enhancement Film (DBEF),
available from 3M Company, for which .DELTA.n.sub.yz 0.08 in the
high index layers. (n.sub.x.apprxeq.1.8, n.sub.y.apprxeq.1.62,
n.sub.z.apprxeq.1.54.) Since n.sub.y=1.62 the low index material
also has n.sub.x.apprxeq.1.62, giving .DELTA.n.sub.x.apprxeq.0.18
between the high and low index layers. To obtain a high contrast
construction, stacks of 550 layers in each reflective polarizer
were modeled as a laminate with biaxial absorbing and AR layers.
The latter two layers were assumed to be absorbing layers with real
index values of 1.8, 1.62, and 1.54. Thicknesses were 10 microns
and 15 microns respectively, as in Examples 4 and 5. The
construction of FIG. 6 was modeled for an azimuth of 25 degrees,
with blue layers facing the analyzer, resulting in the spectra of
FIG. 17. The irregular spectra and high transmission at 60 and 75
degrees result in rather low contrast at those angles.
[0090] The transmission of the spectra in FIG. 17 can be
substantially reduced if the dichroic AR and absorbing layers are
uniaxial. This may be possible if those layers are coated, as for a
PVA layer, but then the cost advantage of co-extrusion is not
realizable. An alternative is to use modified or additional
compensation films. For example if the D-plate (n.sub.x=1.5917,
n.sub.y=1.5655, n.sub.z=1.5794) is replaced with a film with
indices of n.sub.x=1.5917, n.sub.y=1.5655, n.sub.z=1.5731, the
spectra of FIG. 18 are obtained. This is only a slight reduction in
the z-index of the D-plate, which is rather surprising given that
the biaxial material in this hybrid polarizer construction has a
low z-index. The spectra of this example have transmissions almost
as low as for the uniaxial case (see FIG. 18).
[0091] For comparison, the same biaxial hybrid polarizer layers
were modeled with the red layers facing the analyzer. Using the
same modified D-plate, the spectra of FIG. 19 were obtained
[0092] Advantages of exemplary embodiments of the present
disclosure include the possibility of using only one high
extinction reflective polarizer in the hybrid polarizer. This can
result in reduction of visibility of optical defects in the
finished display polarizer. An optical defect is any film defect
that causes a noticeable leakage of light when the optical axes of
two polarizers are crossed and viewed above a backlight. Such
defects can be due to a local disruption of the optical layers by a
particle, or can be due to layer profile errors on a larger scale,
such as along a line or in an extended area, e.g., caused by
disruption of the laminar polymer flow during extrusion. Other
spectral leaks may be inherent in the film due to the design of the
extrusion hardware that generates the layers. When two reflective
polarizers are utilized, the probability of aligned defects is
small, the end result being that light leaking through a defect in
one polarizer is at least partially blocked by the second
polarizer.
[0093] Another advantage of exemplary embodiments of the present
disclosure is the use of reflective polarizers with lower layer
counts with a layer of low contrast absorbing dyes. In the
configuration shown in FIGS. 1-3, each of the reflective polarizers
needs only a modest level of reflectivity, for example, on the
order of or less than 99%, which is a performance level achievable
with only about 200 to 300 layers with currently available resin
materials. With relatively low polarizing demands on the absorbing
layer, a wide range of polarizing dyes can be used in this
construction, and these dyes can be chosen to maximize the
transmission of the pass state polarization.
[0094] Although the polarizers and devices of the present
disclosure have been described with reference to specific exemplary
embodiments, those of ordinary skill in the art will readily
appreciate that changes and modifications may be made thereto
without departing from the spirit and scope of the present
disclosure. In particular, although a specific display element has
not been specified in any of the preceeding examples it should be
appreciated that such examples approximately describe devices using
a liquid crystal display based on in-plane switching (IPS). As
mentioned previously, other types of displays can be employed which
may be compensated with additional or alternative elements other
than the D-plate.
* * * * *