U.S. patent application number 11/278346 was filed with the patent office on 2007-10-11 for reinforced optical films.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Olester JR. Benson, Patrick R. Fleming, Shandon D. Hart, Andrew J. Ouderkirk, Kristin L. Thunhorst.
Application Number | 20070237938 11/278346 |
Document ID | / |
Family ID | 38558475 |
Filed Date | 2007-10-11 |
United States Patent
Application |
20070237938 |
Kind Code |
A1 |
Ouderkirk; Andrew J. ; et
al. |
October 11, 2007 |
Reinforced Optical Films
Abstract
Optical films having structured surfaces are used, inter alia,
for managing the propagation of light within a display. As displays
become larger, it becomes more important that the film be
reinforced so as to maintain rigidity. An optical film of the
invention has a first layer comprising inorganic fibers embedded
within a polymer matrix. A second layer having a structured
surface, for providing an optical function to light passing
therethrough, is attached to the first layer. The film may have
various beneficial optical properties, for example, light that
propagates substantially perpendicularly through the first layer
may be subject to no more than a certain level of haze or light
incident on the film may be subject to a minimum value of
brightness gain. Various methods of manufacturing the films are
described.
Inventors: |
Ouderkirk; Andrew J.;
(Woodbury, MN) ; Hart; Shandon D.; (Maplewood,
MN) ; Benson; Olester JR.; (Woodbury, MN) ;
Fleming; Patrick R.; (Lake Elmo, MN) ; Thunhorst;
Kristin L.; (Stillwater, MN) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
38558475 |
Appl. No.: |
11/278346 |
Filed: |
March 31, 2006 |
Current U.S.
Class: |
428/292.1 |
Current CPC
Class: |
G02B 5/0278 20130101;
B29D 11/0073 20130101; Y10T 428/249924 20150401; B29D 11/00278
20130101; G02B 5/0221 20130101; G02B 5/0242 20130101; G03B 21/60
20130101; G02B 5/0257 20130101 |
Class at
Publication: |
428/292.1 |
International
Class: |
D04H 3/00 20060101
D04H003/00 |
Claims
1. An optical film, comprising: a first layer comprising inorganic
fibers embedded within a polymer matrix; and a second layer
attached to the first layer, the second layer having a structured
surface, wherein the optical film provides a brightness gain of at
least 10% to light that propagates through the optical film.
2. An optical film as recited in claim 1, wherein the brightness
gain is at least 50%.
3. An optical film as recited in claim 1, wherein the brightness
gain is at least 100%.
4. An optical film as recited in claim 1, wherein light that
propagates substantially perpendicularly through the first layer is
subject to a bulk haze of less than 30%.
5. An optical film as recited in claim 1, further comprising at
least one of inorganic nanoparticles, light diffusing particles or
hollow particles embedded within the polymer matrix.
6. An optical film as recited in claim 1, wherein the structured
surface comprises a brightness enhancing layer surface.
7. An optical film as recited in claim 1, wherein the structured
surface comprises a plurality of prismatic ribs.
8. An optical film as recited in claim 1, wherein the structured
surface comprises a plurality of retroreflecting elements.
9. An optical film as recited in claim 1, wherein the structured
surface comprises one or more lenses.
10. An optical film as recited in claim 9, wherein the one or more
lenses comprise at least one Fresnel lens.
11. An optical film as recited in claim 1, wherein the structured
surface comprises one of a diffractive surface and a light
collecting surface.
12. An optical film as recited in claim 1, further comprising a
third layer attached to one of the first and second optical
layers.
13. An optical film as recited in claim 12, wherein the third layer
comprises one of a reflective layer, a transmissive layer, a
diffusive layer and a layer having a structured surface.
14. An optical film as recited in claim 12, wherein the third layer
comprises a polarizer layer.
15. An optical film as recited in claim 14, wherein the polarizer
layer comprises a reflective polarizer layer.
16. An optical film as recited in claim 14, wherein the polarizer
layer comprises an absorbing polarizer layer.
17. An optical film as recited in claim 12, wherein the third layer
is attached to the structured surface.
18. An optical film as recited in claim 12, wherein the third layer
is attached to the first layer.
19. An optical film as recited in claim 12, wherein the third layer
is attached to the second layer and the third layer comprises a
polymer matrix having inorganic fibers embedded within a polymer
matrix.
20. An optical film as recited in claim 1, wherein the polymer
matrix comprises a thermosetting polymer.
21. An optical film as recited in claim 1, wherein the polymer
matrix comprises a thermoplastic polymer.
22. An optical film as recited in claim 1, wherein the polymer
matrix comprises a polymer having a value of T.sub.g less than
120.degree. C.
23. An optical film as recited in claim 1, wherein the single pass
transmission through the film for light directed substantially
normally to a surface of the film facing away from the structured
surface is less than 40%.
24. An optical film as recited in claim 23, wherein the single pass
transmission is less than 10%.
25. An optical film as recited in claim 1, wherein light directed
to the film having a principal ray at an angle of more 30.degree.
to a film normal is transmitted out of the film with the principal
ray propagating at an angle of less than 25.degree. to the film
normal.
26. An optical film as recited in claim 1, wherein when light is
incident on the optical film, the light having a principal ray
propagating in a first direction when incident on the optical film,
the light is transmitted out of the film with the principal ray
propagating in a second direction different from the first
direction by at least 5.degree..
27. An optical film, comprising: a first layer, comprising
inorganic fibers embedded within a polymer matrix; and a second
layer attached to the first layer, the second layer having a
structured surface wherein single pass transmission for light,
substantially normally incident on a side of the optical film
facing away from the structured surface, is less than 40%.
28. An optical film as recited in claim 27, wherein the single pass
transmission is less than 10%.
29. An optical film as recited in claim 27, wherein the single pass
transmission is less than 5%.
30. An optical film as recited in claim 27, wherein light that
propagates substantially perpendicularly through the first layer is
subject to a bulk haze of less than 30%.
31. An optical film as recited in claim 27, further comprising at
least one of inorganic nanoparticles, light diffusing particles or
hollow particles embedded within the polymer matrix.
32. An optical film as recited in claim 27, wherein the structured
surface comprises a brightness enhancing layer surface.
33. An optical film as recited in claim 27, wherein the structured
surface comprises a plurality of prismatic ribs.
34. An optical film as recited in claim 27, wherein the structured
surface comprises a plurality of retroreflecting elements.
35. An optical film as recited in claim 27, wherein the structured
surface comprises one or more lenses.
36. An optical film as recited in claim 35, wherein the one or more
lenses comprise at least one Fresnel lens.
37. An optical film as recited in claim 27, wherein the structured
surface comprises one of a diffractive surface and a light
collecting surface.
38. An optical film as recited in claim 27, further comprising a
third layer attached to one of the first and second layers.
39. An optical film as recited in claim 38, wherein the third layer
comprises one of a reflective layer, a transmissive layer, a
diffusive layer and a layer having a structured surface.
40. An optical film as recited in claim 38, wherein the third layer
comprises a polarizer layer.
41. An optical film as recited in claim 40, wherein the polarizer
layer comprises at least one of a reflective polarizer layer and an
absorbing polarizer layer.
42. An optical film as recited in claim 38, wherein the third layer
is attached to the structured surface.
43. An optical film as recited in claim 38, wherein the third layer
is attached to the second optical layer and the third optical layer
comprises a polymer matrix having inorganic fibers embedded within
the polymer matrix.
44. An optical film as recited in claim 27, wherein the polymer
matrix comprises a thermosetting polymer.
45. An optical film as recited in claim 27, wherein the polymer
matrix comprises a thermoplastic polymer.
46. An optical film as recited in claim 27, wherein the polymer
matrix comprises a polymer having a value of T.sub.g less than
120.degree. C.
47. An optical film as recited in claim 27, wherein light directed
to the film, having a principal ray at an angle of more 30.degree.
to a film normal, is transmitted out of the film with the principal
ray propagating at an angle of less than 25.degree. to the film
normal.
48. An optical film as recited in claim 27, wherein when light is
incident on the optical film, the light having a principal ray
propagating in a first direction when incident on the optical film,
the light is transmitted out of the film with the principal ray
propagating in a second direction different from the first
direction by at least 5.degree..
49. A display system, comprising: a display unit; a backlight; and
an optical film as recited in claim 1 disposed between the display
unit and the backlight.
50. A display system, comprising: a display unit; a backlight; and
an optical film as recited in claim 27 disposed between the display
unit and the backlight.
Description
FIELD OF THE INVENTION
[0001] The invention relates to optical films and more particularly
to optical films with structured surfaces that may be used in a
display, for example a liquid crystal display.
BACKGROUND
[0002] Optical films, such as films having a structured refractive
surface, are often used in displays, for example, for managing the
propagation of light from a light source to a display panel. For
example, a prismatic brightness enhancing film is often used to
increase the amount of on-axis light from a display.
[0003] As display systems increase in size, the area of the films
also becomes larger. Such surface structured films are thin,
typically tens or a few hundreds of microns thick and, therefore,
have little structural integrity, especially when used in larger
display systems. For example, while a film of a certain thickness
may be sufficiently rigid for use in a cell phone display, that
same film may well be insufficiently rigid for use in a larger
display such as a television or computer monitor, without some
additional means of support. Stiffer films should also make large
display system assembly processes less laborious and potentially
more automated, reducing the final assembled cost of the
display.
[0004] The surface structured film can be made to be thicker, in
order to provide additional rigidity, or may be laminated to a
thick polymer substrate to provide the support needed for use in a
large area film. The use of a thick film or a thick substrate,
however, increases the thickness of the display unit, and also
leads to increases in the weight and, possibly, in the optical
absorption. The use of a thicker film or substrate also increases
thermal insulation, reducing the ability to transfer heat out of
the display. Furthermore, there are continuing demands for displays
with increased brightness, which means that more heat is generated
with the display systems. This leads to an increase in the
distorting effects that are associated with higher heating, for
example film warping. In addition, the lamination of the surface
structured film to a substrate adds cost to the device, and makes
the device thicker and heavier. The added cost does not, however,
result in a significant improvement in the optical function of the
display.
SUMMARY OF THE INVENTION
[0005] One embodiment of the invention is directed to an optical
film that has a first layer comprising inorganic fibers embedded
within a polymer matrix and a second layer attached to the first
layer. The second layer has a structured surface. Light that
propagates substantially perpendicularly through the film is
subject to a bulk haze of less than 30%.
[0006] Another embodiment of the invention is directed to a display
system that has a display panel, a backlight and a reinforced film
positioned between the display panel and the backlight. The
reinforced film has a first layer formed of a polymer matrix with
inorganic fibers embedded within the polymer matrix. A second layer
is attached to the first layer and has a structured surface. Light
that propagates substantially perpendicularly through the
reinforced film is subject to a bulk haze of less than 30%.
[0007] Another embodiment of the invention is directed to a method
of manufacturing an optical film. The method includes providing a
first layer having a structured surface and providing a fiber
reinforced layer comprising inorganic fibers embedded within a
polymer matrix. Light propagating through the fiber reinforced
layer is subject to a bulk haze of less than 30%. The fiber
reinforced layer is attached to the first layer.
[0008] Another embodiment of the invention is directed to an
optical film that comprises a first layer. The first layer
comprises inorganic fibers embedded within a polymer matrix. A
second layer that is attached to the first layer has a structured
surface. The film provides a brightness gain of at least 10% to
light that propagates through the film.
[0009] Another embodiment of the invention is directed to an
optical film that includes a first layer having inorganic fibers
embedded within a polymer matrix, and a second layer. The second
layer has a structured surface. Single pass transmission for light,
substantially normally incident on a side of the film facing away
from the structured surface, is less than 40%.
[0010] The above summary of the present invention is not intended
to describe each illustrated embodiment or every implementation of
the present invention. The following figures and the detailed
description more particularly exemplify these embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The invention may be more completely understood in
consideration of the following detailed description of various
embodiments of the invention in connection with the accompanying
drawings, in which:
[0012] FIG. 1 schematically illustrates a display system that uses
a surface structured film according to principles of the present
invention;
[0013] FIG. 2A schematically illustrates an exemplary embodiment of
a fiber reinforced, surface structured film having a reinforced
layer attached directly to a surface structured layer, according to
principles of the present invention;
[0014] FIG. 2B schematically illustrates an exemplary embodiment of
a fiber reinforced surface structured film having a reinforced
layer attached to a surface structured layer via an adhesive layer,
according to principles of the present invention;
[0015] FIG. 3 schematically illustrates an embodiment of a system
for manufacturing a fiber reinforced surface structured film,
according to principles of the present invention;
[0016] FIG. 4 schematically illustrates another embodiment of a
system for manufacturing a fiber reinforced surface structured
film, according to principles of the present invention;
[0017] FIG. 5 schematically illustrates another embodiment of a
system for manufacturing a fiber reinforced surface structured
film, according to principles of the present invention;
[0018] FIG. 6 schematically illustrates an embodiment of a
reinforced surface structured film having two reinforced layers,
according to principles of the present invention;
[0019] FIGS. 7A-7F schematically illustrate different embodiments
of reinforced surface structured films, according to principles of
the present invention;
[0020] FIG. 8 schematically illustrates an embodiment of a
reinforced surface structured film that includes an attached
optical layer, according to principles of the present
invention;
[0021] FIG. 9 schematically illustrates an embodiment of a
reinforced surface structured film with an attached reflector,
according to principles of the present invention;
[0022] FIG. 10 schematically illustrates an embodiment of a
reinforced surface structured film with an attached polarizer
layer, according to principles of the present invention;
[0023] FIG. 11 schematically illustrates another embodiment of a
reinforced surface structured film, according to principles of the
present invention FIGS. 12A, 12B and 13 schematically illustrate
embodiments of reinforced surface structured films that include two
surface structured layers, according to principles of the present
invention.
[0024] While the invention is amenable to various modifications and
alternative forms, specifics thereof have been shown by way of
example in the drawings and will be described in detail. It should
be understood, however, that the intention is not to limit the
invention to the particular embodiments described. On the contrary,
the intention is to cover all modifications, equivalents, and
alternatives falling within the spirit and scope of the invention
as defined by the appended claims.
DETAILED DESCRIPTION
[0025] The present invention is applicable to optical systems and
is particularly applicable to optical display systems that use one
or more optical films. As optical displays, for example liquid
crystal displays (LCDs), become larger and brighter, the demands on
optical films within the displays become greater. Larger displays
require stiffer films, to prevent warping, bending and sagging.
Scaling a film's thickness up with its length and width, however,
leads to a thicker and heavier film. It is desirable, therefore,
that optical films be made stiffer so that they can be used in
large displays, without a concomitant increase in thickness. One
approach for increasing the stiffness of an optical film is to
include reinforcing fibers within the film. In some exemplary
embodiments, the fibers are matched in refractive index to the
surrounding material of the film so that there is little, or no,
scatter of the light passing through the film. Although it may be
desirable in many applications that the composite optical films are
thin, e.g. less than .about.0.2 mm, there is no particular
limitation to the thickness. In some embodiments it may be
desirable to combine the advantages of composite materials and
greater thickness, for example creating thick plates used in
LCD-TV's that could be 0.2-10 mm thick. For the purposes of this
application, the term `optical film` should be considered to
include these thicker optical plates or lightguides.
[0026] In some exemplary embodiments of the present invention, a
surface structured film includes a surface structured layer that is
attached to a fiber reinforced layer. This arrangement permits the
surface structured film to be made larger in area while maintaining
a rigid form that does not significantly deflect or warp under
operating conditions in larger displays.
[0027] A schematic exploded view of an exemplary embodiment of a
display system 100 that may include the invention is presented in
FIG. 1. Such a display system 100 may be used, for example, in a
liquid crystal display (LCD) monitor or LCD-TV. The display system
100 is based on the use of an LC panel 102, which typically
comprises a layer of liquid crystal (LC) 104 disposed between panel
plates 106. The plates 106 are often formed of glass, and may
include electrode structures and alignment layers on their inner
surfaces for controlling the orientation of the liquid crystals in
the LC layer 104. The electrode structures are commonly arranged so
as to define LC panel pixels, areas of the LC layer where the
orientation of the liquid crystals can be controlled independently
of adjacent areas. A color filter may also be included with one or
more of the plates 106 for imposing color on the image
displayed.
[0028] An upper absorbing polarizer 108 is positioned above the LC
layer 104 and a lower absorbing polarizer 110 is positioned below
the LC layer 104. In the illustrated embodiment, the upper and
lower absorbing polarizers 108, 110 are located outside the LC
panel 102. The absorbing polarizers 108, 110 and the LC panel 102
in combination control the transmission of light from a backlight
112 through the display system 100 to the viewer.
[0029] The backlight 112 includes a number of light sources 116
that generate the light that illuminates the LC panel 102. The
light sources 116 used in an LCD-TV or LCD monitor are often
linear, cold cathode, fluorescent tubes that extend across the
display device 100. Other types of light sources may be used,
however, such as filament or arc lamps, light emitting diodes
(LEDs), flat fluorescent panels or external fluorescent lamps. This
list of light sources is not intended to be limiting or exhaustive,
but only exemplary.
[0030] The backlight 112 may also include a reflector 118 for
reflecting light propagating downwards from the light sources 116,
in a direction away from the LC panel 102. The reflector 118 may
also be useful for recycling light within the display device 100,
as is explained below. The reflector 118 may be a specular
reflector or may be a diffuse reflector. One example of a specular
reflector that may be used as the reflector 118 is Vikuiti.TM.
Enhanced Specular Reflection (ESR) film available from 3M Company,
St. Paul, Minn. Examples of suitable diffuse reflectors include
polymers, such as polyethylene terephthalate (PET), polycarbonate
(PC), polypropylene, polystyrene and the like, loaded with
diffusely reflective particles, such as titanium dioxide, barium
sulphate, calcium carbonate and the like. Other examples of diffuse
reflectors, including microporous materials and fibril-containing
materials, are discussed in co-owned U.S. Patent Application
Publication 2003/0118805 A1, incorporated herein by reference.
[0031] An arrangement 120 of light management layers is positioned
between the backlight 112 and the LC panel 102. The light
management layers affect the light propagating from backlight 112
so as to improve the operation of the display device 100. For
example, the arrangement 120 of light management layers may include
a diffuser layer 122. The diffuser layer 122 is used to diffuse the
light received from the light sources, which results in an increase
in the uniformity of the illumination light incident on the LC
panel 102. Consequently, this results in an image perceived by the
viewer that is more uniformly bright.
[0032] The arrangement 120 of light management layers may also
include a reflective polarizer 124. The light sources 116 typically
produce unpolarized light but the lower absorbing polarizer 110
only transmits a single polarization state, and so about half of
the light generated by the light sources 116 is not transmitted
through to the LC layer 104. The reflecting polarizer 124, however,
may be used to reflect the light that would otherwise be absorbed
in the lower absorbing polarizer, and so this light may be recycled
by reflection between the reflecting polarizer 124 and the
reflector 118. At least some of the light reflected by the
reflecting polarizer 124 may be depolarized, and subsequently
returned to the reflecting polarizer 124 in a polarization state
that is transmitted through the reflecting polarizer 124 and the
lower absorbing polarizer 110 to the LC layer 104. In this manner,
the reflecting polarizer 124 may be used to increase the fraction
of light emitted by the light sources 116 that reaches the LC layer
104, and so the image produced by the display device 100 is
brighter.
[0033] Any suitable type of reflective polarizer may be used, for
example, multilayer optical film (MOF) reflective polarizers;
diffusely reflective polarizing film (DRPF), such as
continuous/disperse phase polarizers or cholesteric reflective
polarizers.
[0034] The MOF, cholesteric and continuous/disperse phase
reflective polarizers rely on the varying refractive index profile
within a material, usually polymeric material, to selectively
reflect light of one polarization state while transmitting light in
an orthogonal polarization state. Some examples of MOF reflective
polarizers are described in co-owned U.S. Pat. No. 5,882,774,
incorporated herein by reference. Commercially available examples
of MOF reflective polarizers include Vikuiti.TM. DBEF-II and
DBEF-D400 multilayer reflective polarizers that include diffusive
surfaces, available from 3M Company, St. Paul, Minn.
[0035] Examples of DRPF useful in connection with the present
invention include continuous/disperse phase reflective polarizers
as described in co-owned U.S. Pat. No. 5,825,543, incorporated
herein by reference, and diffusely reflecting multilayer polarizers
as described in, e.g. U.S. Pat. No. 5,867,316, also incorporated
herein by reference. Other suitable types of DRPF are described in
U.S. Pat. No. 5,751,388.
[0036] Some examples of cholesteric polarizer useful in connection
with the present invention include those described, for example, in
U.S. Pat. No. 5,793,456, and U.S. Patent Publication No.
2002/0159019. Cholesteric polarizers are often provided along with
a quarter wave retarding layer on the output side, so that the
light transmitted through the cholesteric polarizer is converted to
linear polarization.
[0037] The arrangement 120 of light management layers may also
include a prismatic brightness enhancing layer 128. A brightness
enhancing layer is one that includes a surface structure that
redirects off-axis light in a direction closer to the axis of the
display. This increases the amount of light propagating on-axis
through the LC layer 104, thus increasing the brightness of the
image seen by the viewer. One example is a prismatic brightness
enhancing layer, which has a number of prismatic elements that
redirect the illumination light, through refraction and reflection.
Examples of prismatic brightness enhancing layers that may be used
in the display device include the Vikuiti.TM. BEFII and BEFIII
family of prismatic films available from 3M Company, St. Paul,
Minn., including BEFII 90/24, BEFII 90/50, BEFIIIM 90/50, and
BEFIIIT. The prismatic elements may be formed as ridges that extend
across the width of the film, or as shorter elements.
[0038] An exemplary embodiment of a reinforced brightness enhancing
film 200 is schematically illustrated in FIG. 2A. The reinforced
film 200 includes a reinforcing layer 202 attached to a brightness
enhancing layer 208. The brightness enhancing layer 208 may include
any type of surface structured layer having structure for
redirecting light to propagate in a direction close to the display
axis. The reinforcing layer 202 comprises a composite arrangement
of inorganic fibers 204 disposed within a polymeric matrix 206.
[0039] The inorganic fibers 204 may be formed of glass, ceramic or
glass-ceramic materials, and may be arranged within the matrix 206
as individual fibers, in one or more tows or in one or more woven
layers. The fibers 204 may be arranged in a regular pattern or an
irregular pattern. Several different embodiments of reinforced
polymeric layers are discussed in greater detail in U.S. patent
application Ser. No. 11/125,580, filed on May 10, 2005,
incorporated herein by reference.
[0040] The refractive indices of the matrix 206 and the fibers 204
may be chosen to match or not match. In some exemplary embodiments,
it may be desirable to match the refractive indices so that the
resulting article is nearly, or completely, transparent to the
light from the light source. In other exemplary embodiments, it may
be desirable to have an intentional mismatch in the refractive
indices to create either specific color scattering effects or to
create diffuse transmission or reflection of the light incident on
the film. Refractive index matching can be achieved by selecting an
appropriate fiber 204 reinforcement that has an index close to the
same as that of the resin matrix 206, or by creating a resin matrix
that has a refractive index close to, or the same as, that of the
fibers 204.
[0041] The refractive indices in the x-, y-, and z-directions for
the material forming the polymer matrix 206 are referred to herein
as n.sub.1x, n.sub.1y and n.sub.1z. Where the polymer matrix
material 206 is isotropic, the x-, y-, and z-refractive indices are
all substantially matched. Where the matrix material is
birefringent, at least one of the x-, y- and z-refractive indices
is different from the others. The material of the fibers 204 is
typically isotropic. Accordingly, the refractive index of the
material forming the fibers is given as n.sub.2. The fibers 204
may, however, be birefringent.
[0042] In some embodiments, it may be desired that the polymer
matrix 206 be isotropic, i.e.
n.sub.1x.apprxeq.n.sub.1y.apprxeq.n.sub.1z.apprxeq.n.sub.1. Two
refractive indices are considered to be substantially the same if
the difference between the two indices is less than 0.05,
preferably less than 0.02 and more preferably less than 0.01. Thus,
the material is considered to be isotropic if no pair of refractive
indices differs by more than 0.05, preferably less than 0.02.
Furthermore, in some embodiments it is desirable that the
refractive indices of the matrix 206 and the fibers 204 be
substantially matched. Thus, the refractive index difference
between the matrix 206 and the fibers 204, the difference between
n.sub.1 and n.sub.2 should be small, at least less than 0.02,
preferably less than 0.01 and more preferably less than 0.002.
[0043] In other embodiments, it may be desired that the polymer
matrix be birefringent, in which case at least one of the matrix
refractive indices is different from the refractive index of the
fibers 204. In embodiments where the fibers 204 are isotropic, a
birefringent matrix results in light in at least one polarization
state being scattered by the reinforcing layer. The amount of
scattering depends on several factors, including the magnitude of
the refractive index difference for the polarization state being
scattered, the size of the fibers 204 and the density of the fibers
204 within the matrix 206. Furthermore, the light may be forward
scattered (diffuse transmission), backscattered (diffuse
reflection), or a combination of both. Scattering of light by a
fiber reinforced layer 202 is discussed in greater detail in U.S.
patent application Ser. No. 11/125,580.
[0044] Suitable materials for use in the polymer matrix 206 include
thermoplastic and thermosetting polymers that are transparent over
the desired range of light wavelengths. In some embodiments, it may
be particularly useful that the polymers be non-soluble in water,
the polymers may be hydrophobic or may have a low tendency for
water absorption. Further, suitable polymer materials may be
amorphous or semi-crystalline, and may include homopolymer,
copolymer or blends thereof. Example polymer materials include, but
are not limited to, poly(carbonate) (PC); syndiotactic and
isotactic poly(styrene) (PS); C1-C8 alkyl styrenes; alkyl,
aromatic, and aliphatic ring-containing (meth)acrylates, including
poly(methylmethacrylate) (PMMA) and PMMA copolymers; ethoxylated
and propoxylated (meth)acrylates; multifunctional (meth)acrylates;
acrylated epoxies; epoxies; and other ethylenically unsaturated
materials; cyclic olefins and cyclic olefinic copolymers;
acrylonitrile butadiene styrene (ABS); styrene acrylonitrile
copolymers (SAN); epoxies; poly(vinylcyclohexane);
PMMA/poly(vinylfluoride) blends; poly(phenylene oxide) alloys;
styrenic block copolymers; polyimide; polysulfone; poly(vinyl
chloride); poly(dimethyl siloxane) (PDMS); polyurethanes; saturated
polyesters; poly(ethylene), including low birefringence
polyethylene; poly(propylene) (PP); poly(alkane terephthalates),
such as poly(ethylene terephthalate) (PET); poly(alkane
napthalates), such as poly(ethylene naphthalate)(PEN); polyamide;
ionomers; vinyl acetate/polyethylene copolymers; cellulose acetate;
cellulose acetate butyrate; fluoropolymers;
poly(styrene)-poly(ethylene) copolymers; PET and PEN copolymers,
including polyolefinic PET and PEN; and poly(carbonate)/aliphatic
PET blends. The term (meth)acrylate is defined as being either the
corresponding methacrylate or acrylate compounds. These polymers
may be used in an optically isotropic form.
[0045] In some product applications, it is important that film
products and components exhibit low levels of fugitive species (low
molecular weight, unreacted, or unconverted molecules, dissolved
water molecules, or reaction byproducts). Fugitive species can be
absorbed from the end-use environment of the product or film, e.g.
water molecules can be present in the product or film from the
initial product manufacturing or can be produced as a result of a
chemical reaction (for example a condensation polymerization
reaction). An example of small molecule evolution from a
condensation polymerization reaction is the liberation of water
during the formation of polyamides from the reaction of diamines
and diacids. Fugitive species can also include low molecular weight
organic materials such as monomers, plasticizers, etc.
[0046] Fugitive species are generally lower molecular weight than
the majority of the material comprising the rest of the functional
product or film. Product use conditions might, for example, result
in thermal stress that is differentially greater on one side of the
product or film. In these cases, the fugitive species can migrate
through the film or volatilize from one surface of the film or
product causing concentration gradients, gross mechanical
deformation, surface alteration and, sometimes, undesirable
out-gassing. The out-gassing could lead to voids or bubbles in the
product, film or matrix, or problems with adhesion to other films.
Fugitive species can, potentially, also solvate, etch or
undesirably affect other components in product applications.
[0047] Several of these polymers may become birefringent when
oriented. In particular, PET, PEN, and copolymers thereof, and
liquid crystal polymers, manifest relatively large values of
birefringence when oriented. Polymers may be oriented using
different methods, including extrusion and stretching. Stretching
is a particularly useful method for orienting a polymer, because it
permits a high degree of orientation and may be controlled by a
number of easily controllable external parameters, such as
temperature and stretch ratio.
[0048] The matrix 206 may be provided with various additives to
provide desired properties to the film 200. For example, the
additives may include one or more of the following: an
anti-weathering agent, UV absorbers, a hindered amine light
stabilizer, an antioxidant, a dispersant, a lubricant, an
anti-static agent, a pigment or dye, a nucleating agent, a flame
retardant and a blowing agent.
[0049] Some exemplary embodiments may use a polymer matrix material
that is resistant to yellowing and clouding with age. For example,
some materials such as aromatic urethanes become unstable when
exposed long-term to UV light, and change color over time. It may
be desired to avoid such materials when it is important to maintain
the same color for a long term.
[0050] Other additives may be provided to the matrix 206 for
altering the refractive index of the polymer or increasing the
strength of the material. Such additives may include, for example,
organic additives such as polymeric beads or particles and
polymeric nanoparticles. In some embodiments, the matrix is formed
using a specific ratio of two or more different monomers, where
each monomer is associated with a different final refractive index
when polymerized. The ratios of the different monomers determine
the refractive index of the final resin 206.
[0051] In other embodiments, inorganic additives may be added to
the matrix 206 to adjust the refractive index of the matrix 206, or
to increase the strength and/or stiffness of the material. For
example, the inorganic material may be glass, ceramic,
glass-ceramic or a metal-oxide. Any suitable type of glass, ceramic
or glass-ceramic, discussed below with respect to the inorganic
fibers, may be used. Suitable types of metal oxides include, for
example, titania, alumina, tin oxides, antimony oxides, zirconia,
silica, mixtures thereof or mixed oxides thereof. Such inorganic
materials may be provided as nanoparticles, for example milled,
powdered, bead, flake or particulate in form, and distributed
within the matrix. Nanoparticles may be synthesized, for example,
using gas-phase or solution-based processing. The size of the
particles is preferably lower than about 200 nm, and may be less
than 100 nm or even 50 nm to reduce scattering of the light passing
through the matrix 206. The additives may have functionalized
surfaces to optimize the dispersion and/or the rheology and other
fluid properties of the suspension, or to react with the polymer
matrix. Other types of particles include hollow shells, for example
hollow glass shells.
[0052] Any suitable type of inorganic material may be used for the
fibers 204. The fibers 204 may be formed of a glass that is
substantially transparent to the light passing through the film.
Examples of suitable glasses include glasses often used in
fiberglass composites such as E, C, A, S, R, and D glasses. Higher
quality glass fibers may also be used, including, for example,
fibers of fused silica and BK7 glass. Suitable higher quality
glasses are available from several suppliers, such as Schott North
America Inc., Elmsford, N.Y. It may be desirable to use fibers made
of these higher quality glasses because they are purer and so have
a more uniform refractive index and have fewer inclusions, which
leads to less scattering and increased transmission. Also, the
mechanical properties of the fibers are more likely to be uniform.
Higher quality glass fibers are less likely to absorb moisture, and
thus the film becomes more stable for long term use. Furthermore,
it may be desirable to use a low alkali glass, since alkali content
in glass increases the absorption of water.
[0053] Discontinuous reinforcements, such as particles or chopped
fibers, may be preferred in polymers that need stretching or in
certain other forming processes. Extruded thermoplastics filled
with chopped glass, for example, as described in U.S. patent
application Ser. No. 11/323,726, incorporated herein by reference,
may be used as the fiber-filled reinforcing layer. For other
applications, continuous glass fiber reinforcements (i.e. weaves or
tows) may be preferred since these can lead to a larger reduction
in the coefficient of thermal expansion (CTE) and a greater
increase in modulus.
[0054] Another type of inorganic material that may be used for the
fiber 204 is a glass-ceramic material. Glass-ceramic materials
generally comprise 95%-98% vol. of very small crystals, with a size
smaller than 1 micron. Some glass-ceramic materials have a crystal
size as small as 50 nm, making them effectively transparent at
visible wavelengths, since the crystal size is so much smaller than
the wavelength of visible light that virtually no scattering takes
place. These glass-ceramics can also have very little, or no,
effective difference between the refractive index of the glassy and
crystalline regions, making them visually transparent. In addition
to the transparency, glass-ceramic materials can have a rupture
strength exceeding that of glass, and some types are known to have
coefficients of thermal expansion of zero or that are even negative
in value. Glass-ceramics of interest have compositions including,
but not limited to, Li.sub.2O--Al.sub.2O.sub.3--SiO.sub.2,
CaO--Al.sub.2O.sub.3--SiO.sub.2,
Li.sub.2O--MgO--ZnO--Al.sub.2O.sub.3--SiO.sub.2,
Al.sub.2O.sub.3--SiO.sub.2, and
ZnO--Al.sub.2O.sub.3--ZrO.sub.2--SiO.sub.2,
Li.sub.2O--Al.sub.2O.sub.3--SiO.sub.2, and
MgO--Al.sub.2O.sub.3--SiO.sub.2.
[0055] Some ceramics also have crystal sizes that are sufficiently
small that they can appear transparent if they are embedded in a
matrix polymer with an index of refraction appropriately matched.
The Nextel.TM. Ceramic fibers, available from 3M Company, St. Paul,
Minn., are examples of this type of material, and are available as
thread, yarn and woven mats. Suitable ceramic or glass-ceramic
materials are described further in Chemistry of Glasses, 2.sup.nd
Edition (A. Paul, Chapman and Hall, 1990) and Introduction to
Ceramics, 2.sup.nd Edition (W. D. Kingery, John Wiley and Sons,
1976), the relevant portions of both of which are incorporated
herein by reference.
[0056] In some exemplary embodiments, it may be desirable not to
have perfect refractive index matching between the matrix 206 and
the fibers 204, so that at least some of the light is diffused by
the fibers 204. In such embodiments, either or both of the matrix
206 and fibers 204 may be birefringent, or both the matrix and the
fibers may be isotropic. Depending on the size of the fibers 204,
the diffusion arises from scattering or from simple refraction.
Diffusion by a fiber is non-isotropic: light may be diffused in a
direction lateral to the axis of the fiber, but is not diffused in
an axial direction relative to the fiber. Accordingly, the nature
of the diffusion is dependent on the orientation of the fibers
within the matrix. If the fibers are arranged, for example,
parallel to the x-axis, then the light is diffused in directions
parallel to the y- and z-axes.
[0057] In addition, the matrix 206 may be loaded with diffusing
particles that isotropically scatter the light. Diffusing particles
are particles of a different refractive index than the matrix,
often a higher refractive index, having a diameter up to about 10
.mu.m. These can also provide structural reinforcement to the
composite material. The diffusing particles may be, for example,
metal oxides such as were described above for use as nanoparticles
for tuning the refractive index of the matrix. Other suitable types
of diffusing particles include polymeric particles, such as
polystyrene or polysiloxane particles, or a combination thereof.
The diffusing particles may also be hollow glass spheres such as
type S60HS Glass Bubbles, produced by 3M Company, St. Paul, Minn.
The diffusing particles may be used alone to diffuse the light, or
may be used along with non-index-matched fibers to diffuse the
light, or may be used in conjunction with the structured surface to
diffuse and re-direct light.
[0058] Some exemplary arrangements of fibers 204 within the matrix
206 include yarns, tows of fibers or yarns arranged in one
direction within the polymer matrix, a fiber weave, a non-woven,
chopped fiber, a chopped fiber mat (with random or ordered
formats), or combinations of these formats. The chopped fiber mat
or nonwoven may be stretched, stressed, or oriented to provide some
alignment of the fibers within the nonwoven or chopped fiber mat,
rather than having a random arrangement of fibers. Furthermore, the
matrix 206 may contain multiple layers of fibers 204: for example
the matrix 206 may include more layers of fibers in different tows,
weaves or the like. In the specific embodiment illustrated in FIG.
2A, the fibers 204 are arranged in two layers.
[0059] In another exemplary embodiment of a reinforced film 220,
schematically illustrated in FIG. 2B, a layer of adhesive 222 is
provided between the structured surface layer 208 and the fiber
reinforcement layer 202. The adhesive 222 may be any suitable type
of adhesive, for example a pressure sensitive adhesive or a curable
laminating adhesive.
[0060] One exemplary approach to manufacturing a reinforced surface
structured film is now described with reference to FIG. 3. In
general, this approach includes applying a matrix resin directly to
a pre-prepared surface structured layer. The manufacturing
arrangement 300 includes a roll of the fiber reinforcement 302,
which is passed through an impregnation bath 304 containing the
matrix resin 306. The resin 306 is impregnated into the fiber
reinforcement 302 using any suitable method, for example by passing
the fiber reinforcement 302 through a series of rollers 308.
[0061] Once the impregnated reinforcement 310 is extracted from the
bath 304, it is applied to a layer of surface structured film 312
and additional resin 318 may be added if necessary. The impregnated
fiber reinforcement 310 and the layer of surface structured film
312 are squeezed together in a pinch roller 316 to ensure good
physical contact between the two layers 310 and 312. Optionally,
the additional resin 318 may be applied over the reinforcement
layer 310, for example using a coater 320. The coater 320 may be
any suitable type of coater, for example a knife edge coater, comma
coater (illustrated), bar coater, die coater, spray coater, curtain
coater, high pressure injection, or the like. Among other
considerations, the viscosity of the resin at the application
conditions determines the appropriate coating method or methods.
The coating method and resin viscosity also affect the rate and
extent to which air bubbles are eliminated from the reinforcement
during the step where the reinforcement is impregnated with the
matrix resin.
[0062] Where it is desired that the finished film have low scatter,
it is important at this stage to ensure that the resin completely
fills the spaces between the fibers: voids or bubbles left in the
resin may act as scattering centers. Different approaches may be
used, individually or in combination, to reduce the occurrence of
bubbles. For example, the film may be mechanically vibrated to
encourage the dissemination of the resin 306 throughout the
reinforcement layer 310. The mechanical vibration may be applied
using, for example, an ultrasonic source. In addition, the film may
be subject to a vacuum that extracts the bubbles from the resin
306. This may be performed at the same time as coating or
afterwards, for example in an optional de-aeration unit 322.
[0063] The resin 306 in the film may then be solidified at a
solidification station 324. Solidification includes curing,
cooling, cross-linking and any other process that results in the
polymer matrix reaching a solid state. In the illustrated
embodiment, a radiation source 324 is used to apply radiation to
the resin 306. In other embodiments different forms of energy may
be applied to the resin 306 including, but not limited to, heat and
pressure, electron beam radiation and the like, in order to cure
the resin 306. In other embodiments, the resin 306 may be
solidified by cooling, polymerization or by cross-linking. In some
embodiments, the solidified film 326 is sufficiently supple as to
be collected and stored on a take-up roll 328. In other
embodiments, the solidified film 326 may be too rigid for rolling,
in which case it is stored some other way, for example the film 326
may be cut into sheets for storage.
[0064] Another approach to making a fiber reinforced surface
structured film is to first make the composite on a carrier film
from which it will later be separated. The composite can then be
used for supporting the surface structured film. In one exemplary
embodiment, the composite can be fed into a lamination process with
a laminating adhesive and the desired surface structured film. This
approach is schematically illustrated in FIG. 4. In this
manufacturing system 400, a layer of adhesive 404 is provided on a
surface structured film 402. The adhesive 404 may be any suitable
type of adhesive useful for laminating two films together. For
example, the adhesive may be a pressure sensitive adhesive or a
curable laminating adhesive. In the illustrated embodiment, the
adhesive 404 is applied as a liquid which is spread to a thin layer
using a coater 406. The adhesive layer may itself contain any of
the functional elements that could be added to the composite matrix
resin, such as UV absorbers or light-diffusing particles.
[0065] A pre-prepared, fiber reinforced, composite layer 408 is
then laid over the adhesive 406 and the fiber reinforced layer 408
is squeezed together with the surface structured film 402, for
example using a pressure roller 410, to form a reinforced laminate
412. If necessary, the adhesive 404 may then be cured, for example
though the application of radiation 414. The cured laminate 416 may
then be gathered on a roll 418 or cut into sheets for storage.
[0066] In a variation of this approach, the adhesive 404 may first
be applied to the fiber reinforced layer, and the surface-structure
film may then be pressed against the adhesive 404.
[0067] In another exemplary embodiment, a surface-structure film
may be cast onto a pre-prepared, fiber reinforced layer. This
approach is schematically illustrated in FIG. 5. In this
manufacturing system 500, a layer of polymer material 502 is spread
onto a fiber reinforced layer 504. The film is then guided to a
molding roll 506 by a guiding roll 508 and may optionally be
pressed against the molding roll 506 by a pressure roll 510. The
molding roll 506 has a shaped surface 512 that is impressed into
the coated material 502. The polymer material 502 may be hardened,
for example through the application of heat, radiation or the like,
while the monomer or polymer material 502 is in contact with the
molding roll 506. In the illustrated embodiment, radiation sources
514, such as heat lamps, are used to cure the surface structured
layer 516.
[0068] In some exemplary embodiments, a fiber reinforced layer may
be attached to each side of a surface structured film. FIG. 6
schematically illustrates an exemplary embodiment of a reinforced
surface structured film 600 that has a surface structured layer 602
sandwiched between two fiber reinforcement layers 604, 606. The
lower reinforcement layer 606 may be attached using any suitable
method, including the different methods discussed above.
[0069] The upper reinforcement layer 604 may be attached to the
structured surface 608 through the use of an adhesive layer 610
disposed on the lower surface 612 of the reinforcement layer 604.
The attachment of the structured surface of a prismatic brightness
enhancing layer to another optical film is discussed in greater
detail in U.S. Pat. No. 6,846,089, incorporated herein by
reference. Generally, the adhesive layer 610 is relatively thin
compared to the height of the surface structure. The structured
surface 608 is pressed into the adhesive layer to such a depth as
to leave a significant portion of the structured surface 608
interfaced with air. This maintains the relatively large refractive
index difference between the air and the layer 602, thus conserving
the refractive effects of the structured surface 612. It will be
appreciated that the structured surface of other types of surface
structured films, in addition to a brightness enhancing film, may
also be attached to a reinforced layer.
[0070] The figure also shows the optical path of one exemplary
light ray 614 that is redirected by the prismatic brightness
enhancing film in a direction more closely aligned with the axis
616. The axis 616 lies normal to the film 600. In some
configurations, the light ray 614 may be a principal ray. For the
purposes of this application, a principal ray is defined as the ray
propagating at the intensity-weighted, central direction of a
distributed light beam, where the distributed beam itself may
contain multiple rays propagating at different angles. The ray 614
is incident at the film 600 at an angle of more than 30.degree. to
the axis 614, and emerges from the film 600 with an angle of less
than 25.degree. to the axis 614. In some embodiments, the direction
of the principal ray 614 after being transmitted through the film
600 is more than 5.degree. different from the direction of the
principal ray 614 before entering the film 600, in other words the
film 600 has deviated the ray 614 through an angle of more than
5.degree., in some embodiments more than 10.degree. and in some
embodiments more than 20.degree..
[0071] The structured surface is not restricted to being a
brightness enhancing layer and may be any other type of surface.
For example, the structured surface may be a lensed surface, a
diffusing surface, a diffractive optical surface, a light-turning
surface (as used in commercially available "turning" films), or a
retroreflecting surface. For certain preferred transmissive
light-redirecting applications of the present invention, it is
desirable to use non-random structured surfaces that can
substantially re-direct a principal ray. For example, a film using
such a surface may redirect the principal ray through an angle of
5.degree. or more. Some exemplary structured surfaces are discussed
in greater detail below.
[0072] One exemplary type of structured surface is a lensed
surface, as is schematically illustrated in FIG. 7A. In this
embodiment, a structured surface layer 702 is attached to a
reinforced layer 704. The structured surface 706 includes a number
of lenses 708 that may be useful for adding optical power to light
passing therethrough. There may be any suitable number of lenses,
from one to a plurality of lenses. In addition, the lenses may
provide positive or negative optical power, and need not all
provide the same optical power.
[0073] Another type of a lens structured surface is a Fresnel lens.
In the exemplary embodiment of a reinforced film 710 schematically
illustrated in FIG. 7B, a surface structured layer 712 is attached
to a fiber reinforced layer 714. The surface structured layer 712
has a Fresnel surface 716 that focuses light 718 that passes
therethrough. In other embodiments, the surface structured layer
712 may include more than one Fresnel lens pattern.
[0074] Another type of a structured surface is a diffractive
optical surface. In the exemplary embodiment of a reinforced film
720 schematically illustrated in FIG. 7C, a surface structured
layer 722 is attached to a fiber reinforced layer 714. The surface
structured layer 722 has a diffractive optical surface 726 that
diffracts light 728 passing therethrough. It will be appreciated
that different types of diffraction may be imparted by the
diffractive optical surface 726. For example, in one embodiment,
the diffractive optical surface 726 may operate like a lens and
provide optical power to the light 728. In other embodiments, the
diffractive optical surface may diffract the light differently. For
example, the diffractive optical surface may be used to separate
light into differently colored components, form patterns such as
dot patterns, acts as lenses, or act as shaped diffusers.
[0075] Another exemplary embodiment of a reinforced structured
surface film is a reinforced turning film 730, schematically
illustrated in FIG. 7D. The reinforced turning film 730 includes a
turning layer 732 attached to a reinforced layer 734. The turning
layer 732 has a structured surface 736 that is directed towards the
source of light. Accordingly, light 738 that is incident on the
reinforced film 730 at a large angle is redirected by the
structured surface along a direction more parallel to the axis 740.
In the illustration, the light 738 enters a structural element 742
and is totally internally reflected within the element 742.
[0076] Another exemplary embodiment of a reinforced structured
surface film is a reinforced retroreflecting film 750,
schematically illustrated in FIG. 7E. The reinforced
retroreflecting film 750 includes a retroreflecting layer 752
attached to a reinforced layer 754. The retroreflecting layer 752
has a structured surface 756 that is directed away from the source
of light. Accordingly, at least some of the light 758 that is
incident on the reinforced film 750 may be totally internally
reflected by an element 760, which contains two surfaces where
total internal reflection takes place. Consequently, the light is
retroreflected by the surface 756.
[0077] Another exemplary embodiment of a reinforced structured
surface film is a reinforced light concentrator film. A light
concentrator is a reflective element, typically a non-imaging
element, that concentrates light from a larger area to a smaller
area. Examples of light concentrators include parabolic reflectors,
compound parabolic reflectors and the like. A light concentrator
film is a film that contains a number of light concentrators.
[0078] In the exemplary embodiment illustrated in FIG. 7F, a light
concentrator layer 772 is attached to a fiber reinforced layer 774.
The concentrator layer 772 includes a number of reflective
collectors 776 that have reflecting sidewalls 778. The light 780 is
concentrated at the output apertures 782 of the concentrator layer
772. This can function as a light collimator when operated in the
reverse direction, with light directed into the side with the
smaller apertures.
[0079] Other light management layers having a reinforced layer may
be included or attached for purposes other than brightness
enhancement. These uses include spatial mixing or color mixing of
light, light source hiding, and uniformity improvement. Films that
may be used for these purposes include diffusing films, diffusing
plates, partially reflective layers, color-mixing lightguides or
films, and diffusing systems in which the peak brightness ray of
the diffused light propagates in a direction that is not parallel
to the direction of the peak brightness ray of the input light.
[0080] Other layers may also be attached to a reinforced surface
structured layer, for example attached directly to the surface
structured layer itself, or to a fiber reinforced layer that is
attached to the surface structured layer. A general example of a
reinforced surface structured film 800 that includes an additional
optical layer is schematically illustrated in FIG. 8. In the
illustrated embodiment, the reinforced surface structured layer 800
has a surface structured layer 802 that is attached to a fiber
reinforced layer 804. In the illustrated embodiment, an additional
optical layer 806 is attached to the fiber reinforced layer 804.
The optical layer 806 may be any other type of optical layer that
is desired to be attached to the reinforced surface structured
layer 800. For example, the optical layer 806 may include an
optical layer that is transmissive, diffusive or reflective. A
diffusive layer may, for example, include optically diffusive
particles dispersed within a matrix. A reflective layer may be a
specularly reflective layer, for example a multi-layer film formed
from polymer or other dielectric materials. In other exemplary
embodiments, the optical layer 806 may be another optical layer
that includes a structured refracting surface. Different exemplary
types of optical layers with optically functional surfaces include
films with prismatic surfaces, films with lensed surfaces, films
with diffractive surfaces, diffusive surfaces, and films with
optically concentrating surfaces. In other embodiments, the
additional optical layer may be a surface structured layer or a
reflective polarizer layer.
[0081] Other light management layers may be included for purposes
other than brightness enhancement. These uses include spatial
mixing or color mixing of light, light source hiding, and
uniformity improvement. Films that may be used for these purposes
include diffusing films, diffusing plates, partially reflective
layers, color-mixing lightguides or films, and diffusing systems in
which the peak brightness ray of the diffused light propagates in a
direction that is not parallel to the direction of the peak
brightness ray of the input light.
[0082] One exemplary embodiment of a type of film that can be
attached to a reinforced surface structured film is a reflecting
layer. The reflecting layer may be, for example, a diffusive
reflecting layer, or may be a specularly reflecting layer. A
diffusive reflecting layer may be formed, for example, by loading a
film with a high density of diffusing particles. A specularly
reflecting layer may be formed, for example, using multiple
alternating layers of polymer materials of different refractive
indices. FIG. 9 schematically illustrates a reinforced structured
surface film 900 having a structured surface layer 902 attached to
one side of a reinforcing layer 904. A reflective layer 906 may be
attached to the other side of the reinforcing layer 904, as
illustrated, or between the reinforcing layer 904 and the
structured surface layer 902. Light 908 that passes through the
structured surface layer 902 is reflected by the reflective layer
906.
[0083] Another exemplary embodiment of a type of film that can be
attached to a reinforced surface structured film is a polarizing
layer. The polarizing layer may be, for example an absorbing
polarizer layer, in which the light in the block polarization state
is absorbed, or a reflecting polarizing layer, in which the light
in the block polarization state is reflected. One particular
embodiment of such a reinforced surface structured film 1000 is
schematically illustrated in FIG. 10. In this embodiment, a surface
structured layer 1002 is attached to a polarizer layer 1006, which
in turn is attached to a reinforced layer 1004. The surface
structured layer 1002 is illustrated as a brightness enhancing
layer, although other types of surface structured layer may be
used. In the illustrated embodiment, the polarizer layer 1006 is a
reflective polarizer layer, so that unpolarized light 1008 entering
the film 1000 is split into two orthogonally polarized components,
a first component 1008a that is transmitted through the film 1000
and a second, orthogonally polarized component 1008b that is
reflected from the film 1000. In other embodiments, the reinforced
layer 1004 may be positioned between the surface structured layer
1002 and the polarizer layer 1006.
[0084] Another embodiment of reinforced film 1100 is schematically
illustrated in FIG. 11, in which a surface structured layer 1102 is
attached to a fiber reinforced layer 1104 and a polarizing layer
1106 is attached to the structured surface of the surface
structured layer 1102.
[0085] Where the polarizing layer 1106 is an absorbing polarizer,
any suitable type of absorbing polarizer layer may be used,
including H type iodine based polarizers, K type intrinsic
absorbing polarizers, dye based polarizers and the like. Where the
polarizing layer 1106 is a reflecting polarizer, any suitable type
of reflecting polarizer may be used, including multilayer optical
film (MOF) polarizers, and diffusing polarizers such as DRPF
polarizers.
[0086] In some embodiments that include polarizers, it may be
desirable that the other layer(s) in the system exhibit low and
uniform birefringence so as not to disrupt the function of the
polarizer layer. An example of this is when a surface structured
layer is placed on top of a reflective polarizer, and this combined
element is used for brightness enhancement in an LCD display. In
this case it is generally desirable to maintain the dominant
polarization state that is passed through the reflective polarizer
upon transmission through the structured layer. This is one
advantage of glass-reinforced thermoset layers, which can be made
to have very low birefringence.
[0087] In some other embodiments, two or more surface structured
layers may be attached together with a fiber reinforced layer. The
surface structured layers may be the same or may be different. One
exemplary embodiment of a reinforced film that includes two of the
same type of surface structured layer is schematically illustrated
in FIG. 12A. A first brightness enhancing layer 1202 is attached to
a first reinforced layer 1204. A second brightness enhancing layer
1206 may be attached to the first brightness enhancing layer 1202
or to the first reinforced layer 1204. In some embodiments, the
ridges of the two brightness enhancing layers 1202,1206 may be
oriented perpendicularly to each other, for example if it is
desired that the brightness enhancing layers are used to change the
direction of the light for both the vertical and horizontal viewing
directions of a display system. In other embodiments, an optional
additional reinforcing layer 1208 may be included, as is
schematically illustrated in FIG. 12B for the reinforced brightness
enhancing layer 1210.
[0088] Other combinations of surface structured layers may be used.
For example, a brightness enhancing layer may be attached to a
layer structured with a diffractive surface pattern or to a layer
providing optical power.
[0089] A surface structured layer, attached to a reinforcement
layer, may also be attached to another surface structured layer
that itself includes fiber reinforcement. Reinforced surface
structured layers are discussed in greater detail in U.S. patent
application Ser. No. 11/125,580 and U.S. patent application Ser.
No. XX/XXX,XXX, "STRUCTURED COMPOSITE OPTICAL FILMS", filed on even
date herewith and having attorney docket no. 61102US002,
incorporated herein by reference. A reinforced surface structured
layer is an optical layer that includes inorganic fibers within the
polymer matrix for reinforcement and that also has at least one of
its surfaces structured. One exemplary embodiment of a reinforced
film 1300 having a surface structured layer attached to a
reinforced surface structured layer is illustrated in FIG. 13. A
brightness enhancing layer 1302 is attached to a fiber reinforced
layer 1304. A fiber reinforced diffractive surface layer 1306 is
attached to the brightness enhancing layer, for example through the
use of an adhesive layer 1308.
[0090] In the different embodiments of reinforced surface
structured film illustrated in FIGS. 6-13, it is important to
appreciate that the order of the different layers within the film
stack may be different from that illustrated. For example, in the
embodiment of film 1000 schematically illustrated in FIG. 10, the
reflector layer 1006 may be positioned between the reinforced layer
1004 and the surface structured layer 1002. Also, in all of the
examples showing the addition of another optical film, there may be
two or more fiber reinforced layers, instead of just a single
layer.
EXAMPLES
[0091] Select embodiments of this invention are described below.
These examples are not meant to be limiting, only illustrative of
some of the aspects of the invention.
[0092] All of the following examples of composite film used as the
inorganic fiber reinforcement a woven fiberglass produced by Hexcel
Reinforcements Corp., Anderson, S.C. The Hexcel 106 (H-106) fibers
were received from the vendor with finish applied to the fibers to
act as a coupling agent between the fiber and the resin matrix. In
the examples, all the H-106 glass fabrics used had a CS767 silane
finish. In other systems it may be desirable to add use a glass
reinforcement in the greige state that does not have a finish or
coupling agent applied to the glass fiber. The refractive index
(RI) of the fiber samples listed in Table I were measured with
Transmitted Single Polarized Light (TSP) with a 20.times./0.50
objective, and Transmitted Phase Contrast Zernike (PCZ) with a
20.times./0.50 objective. The fiber samples were prepared for
refractive index measurement by cutting portions of the fibers
using a razor blade. The fibers were mounted in various RI oils on
glass slides and covered with a glass coverslip. The samples were
analyzed using the Zeiss Axioplan (Carl Zeiss, Germany).
Calibration of the RI oils was performed on an ABBE-3L
Refractometer, manufactured by Milton Roy Inc., Rochester, N.Y.,
and values were adjusted accordingly. The Becke Line Method
accompanied with phase contrast was used to determine the RI of the
samples. The nominal RI results for the values of n.sub.D, the
refractive index at the wavelength of the sodium D-line, 589 nm,
had an accuracy of .+-.0.002 for each sample.
[0093] Summary information for various resins used in Examples 1-4
is provided in Table I. TABLE-US-00001 TABLE I Resin Components
Component Resin Refractive ID Manufacturer Component Index C1 Cytec
Surface Ebecryl 600 1.5553 Specialties C2 Sartomer Company TMPTA
1.4723 C3 Ciba Specialty Chemicals Darocur 1173 1.5286 Corp. C4
Cognis Corp. Photomer 6210 C5 Sartomer Company THFA (SR285) C6
Sartomer Company HDODA(SR238) C7 Ciba Specialty Chemicals Darocur
4265 Corp.
[0094] Darocur 1173 and Darocur 4265 are photoinitiators, while
THFA (tetrahydrofurfuryl acrylate) is a mono-functional acrylate
monomer. The remaining components in Table I are resins that
cross-link upon curing. Ebecryl 600 is a Bisphenol-A epoxy
diacrylate oligomer.
Example 1
BEF Attached to Reinforced Composite Layer
[0095] A light-directing, prismatic, brightness enhancing
microstructured film (Vikuiti.TM. Thin-BEF-90/24-II-T, available
from 3M Company, St. Paul, Minn.) was attached to a transparent
composite using a UV-cured resin acting as a laminating adhesive.
In this example, the flat side of the brightness enhancing film was
primed and laminated to a pre-made reinforced composite layer
containing glass fibers in a polymer matrix. The structure of the
finished article was, from bottom to top, i) reinforced composite
layer, ii) laminating adhesive and iii) brightness enhancing
layer.
[0096] The reinforced composite layer was formed using fiber
material F1 described above. The refractive index of the F1 glass
fibers, having a CS767 surface finish, was 1.551.+-.0.002.
[0097] The polymer resin used for the reinforcing layer was a per
weight mixture of the following components: TABLE-US-00002
Component % wt. C1 69.3 C2 29.7 C3 1.0
The refractive index of the cured composite resin mixture was
1.5517. Therefore, the difference in refractive index between the
fibers and the matrix was 0.0007.
[0098] The preparation of the transparent composite began by taping
a 12'.times.24' (30.5 cm.times.61 cm) sheet of PET to the leading
edge of a 12'.times.20'.times.1/4' (30.5 cm.times.50.8 cm.times.0.6
cm) sheet of aluminum. A sheet of the F1 fiberglass fabric was laid
on top of the PET. The fiberglass fabric was covered by another
sheet of 12'.times.24' PET and its leading edge taped to the
leading edge of the aluminum plate. The leading edge of the
aluminum plate was placed into hand-operated laminator. The top
sheet of PET and the fiberglass were peeled backwards to allow
access to the bottom sheet of PET. A bead of resin (6-8 mL) was
applied to the bottom sheet of PET near the edge closest to the
laminating rolls. The sandwich construction of the glass fiber
fabric between the layers of PET was fed through the laminator at a
steady rate forcing the resin up through the glass fiber fabric,
coating the fibers entirely.
[0099] The laminate, while still attached to the aluminum plate,
was placed in a vacuum oven and heated to a temperature between
60.degree. C. and 65.degree. C. The oven was evacuated to 27 inches
(68.6 cm) of Hg below atmospheric and the laminate degassed for
four minutes. The vacuum was released by introducing nitrogen into
the oven. The laminate was passed through the laminator once more.
The resin was then cured by passing the laminate beneath a UV
Fusion "D" lamp operating at 600 W/in (236 W/cm) at a speed of 30
fpm (15 cm/s).
[0100] A primer is used to improve the adhesion of the acrylate
resin to the bottom side of the brightness enhancing layer.
Radiation-graft primers for acrylic coatings are known. One primer
was formed of hexanediol diacrylate 97 wt. % and benzophenone
wt.3%. For priming sheets of film, three drops of the primer
solution were applied to the necessary side of the film and coated
using a tissue by wiping. Any excess primer solution was removed by
wiping with a clean tissue. The primer coating was cured using a
Fusion "D" lamp operating at 600 W/in (236 W/cm) at a line speed of
30 fpm (15 cm/s) in an air atmosphere.
[0101] The primed brightness enhancing layer is subsequently
attached to the pre-made transparent composite by coating and
curing the laminating adhesive between the primed brightness
enhancing layer and the reinforced composite layer. The laminating
adhesive was formed of a composition of: TABLE-US-00003 Component %
wt. C4 64.4 C5 24.7 C6 9.9 C7 1.0
[0102] In this example, the reinforced composite layer was attached
to the bottom side of the brightness enhancing layer using the
following procedure. First, a 12'.times.24' (30.5 cm.times.30.5 cm)
sheet of PET was taped to the leading edge of a
12'.times.20'.times.1/4' (30.5 cm.times.50.8 cm.times.0.6 cm) sheet
of aluminum. A primed brightness enhancing layer was laid onto the
PET with its primed surface facing upwards. The bottom sheet of PET
was carefully stripped away from the pre-made, reinforced composite
layer. The pre-made, reinforced composite layer was laid over the
brightness enhancing layer, with the exposed face of the composite
layer facing the primed face of the brightness enhancing layer. The
top PET layer of the reinforced composite layer was then taped to
the leading edge of the aluminum plate. The leading edge of the
aluminum plate was placed into a hand operated laminator. The
reinforced composite layer was pulled backwards to allow access to
the brightness enhancing layer. A bead of the laminating adhesive
resin, .about.5 mL, was applied to the edge of the brightness
enhancing layer closest to the laminating rolls. The sandwich
construction was fed through the laminator at a steady rate,
coating the brightness enhancing layer and the reinforced composite
with the laminating adhesive.
[0103] The laminate, still attached to the aluminum plate, was
placed in a vacuum oven heated to a temperature between 60.degree.
C. and 65.degree. C. The oven was evacuated to 27 inches (68.6 cm)
of Hg below atmospheric and the laminate was degassed for four
minutes. The vacuum was released by introducing nitrogen into the
oven. The laminate was then passed through the laminator again. The
laminating resin was cured by passing the laminate beneath a Fusion
"D" lamp operating at 600 W/in (236 W/cm) at a speed of 30 fpm (15
cm/s).
Example 2
BEF and RP Attached to Reinforced Composite Layer
[0104] A sample was prepared in the same manner as discussed above
in Example 1 except that the surface structured layer was
Vikuiti.TM. BEF-RP-II 90/24r, which is a brightness-enhanced,
reflective polarizer having a prismatic surface, available from 3M
Company, St. Paul, Minn. The reinforcing composite layer was made
from H-106 fiberglass with CS767 surface finish and 30/70
TMPTA/Ebecrly 600 resin. The composite layer was attached by
priming the flat side of the BEF-RP (with the HDODA/BP at 3%
solution) and coating and curing the composite layer directly onto
the BEF-RP, using similar techniques to those described in Example
1.
Example 3
RP+BEF Between Two Reinforced Composite Layers
[0105] A prismatically structured, brightness enhancing layer was
attached to a multilayer reflective polarizing layer (RP) and was
sandwiched between two reinforced composite layers. The
prismatically structured layer was a 5-mil (125 .mu.m) thick sheet
of monolithic polycarbonate brightness enhancing layer, Vikuiti.TM.
WBEF W818, available from 3M Company, St. Paul, Minn. The
reflective polarizing layer was a multilayer polymer reflective
polarizer having the same optical layer construction as a sheet of
Vikuiti.TM. DBEF-P2, available from 3M Company, although the skin
layers were slightly thinner than the commercial product.
[0106] In this example, each side of the RP layer and the
unstructured side of the WBEF layer were primed using the same
priming technique as described in Example 1. A pre-made fiber
reinforced composite layer was attached to each side of the RP
layer and the bottom of the WBEF layer was attached to the other
side of one of the reinforced composites using an UV-cured
laminating adhesive. The structure of the article was, therefore:
reinforced composite layer; laminating adhesive; primer; RP;
primer; laminating adhesive; reinforced composite; laminating
adhesive; primer; WBEF. The reinforced composite layers and the
laminating adhesive were the same as those described above for
Example 1.
[0107] The reinforced composite was attached to the WBEF film using
the same procedure as discussed in Example 1 for attaching the
reinforced composite layer to the BEF layer.
[0108] A different sheet of transparent composite was attached to
the RP layer using the following process. The leading edge of a
12'.times.24' (30.5 cm.times.61 cm) sheet of PET was taped to the
leading edge of a 12'.times.20'.times.1/4' (30.5 cm.times.50.8
cm.times.0.6 cm) sheet of aluminum. The sheet of RP was laid onto
the PET sheet. A sheet of reinforced composite, still laminated to
a single sheet of PET, was laid on top the RP and the leading edge
of the laminate taped to the leading edge of the aluminum plate.
The leading edge of the aluminum plate was placed into a hand
operated laminator. The top sheet of the reinforced composite was
peeled backwards to allow access to the layer of RP. A bead of
laminating resin (.about.5 mL) was applied to the edge of the RP
layer closest to the laminating rolls. The sandwich construction
was fed through the laminator at a steady rate forcing the
laminating adhesive resin between the reinforced composite and the
RP. The resin was cured by passing the laminate beneath a Fusion
"D" lamp operating at 600 W/in (236 W/cm) at a speed of 30 fpm (15
cm/s). The bottom sheet of PET was carefully stripped away from the
RP and set aside.
[0109] The PET sheet bearing the reinforced composite on the WBEF
layer was placed, with the exposed composite side up, on the
aluminum plate and its leading edge was taped down in the manner
previously described. The PET sheet bearing the reinforced
composite on the RP layer was placed, with the exposed RP layer
down, on top of the composite already on the aluminum sheet and its
leading edge taped down in the manner previously described. The
leading edge of the aluminum plate was placed into a hand operated
laminator. The top sheet of reinforced composite and the RP layer
were peeled backwards to allow access to the sheet of reinforced
composite. A bead of laminating adhesive resin (.about.5 mL) was
applied to the edge of the reinforced composite closest to the
laminating rolls. The sandwich construction was then fed through
the laminator at a steady rate forcing the laminating adhesive
resin between the reinforced composite and the RP. The laminating
adhesive resin was cured by passing the laminate beneath a UV
Fusion "D" lamp operating at 600 W/in (236 W/cm) at a speed of 30
fpm (15 cm/s). Both sheets of the PET were removed from the
composite reinforced laminated sandwich of films.
Example 4
Integrated BEF and RP with Reinforced Composite Layer
[0110] A sample was made as described in Example 1, except that the
surface structured layer was PC-BEF, a prismatic brightness
enhancing layer formed on a 250 .mu.m thick layer of polycarbonate
(PC) having a prism structure with pseudo-random height undulations
very similar to that found in Vikuiti-BEF-III 90/50, the only major
difference being that the prism tips were rounded to a radius of 7
microns. In addition, the PC-BEF layer had previously been attached
to a reflective polarizer layer. The RP layer was the same RP as
used in Example 3.
[0111] The PC-BEF layer and the reflective polarizer layer were
attached using the following procedure. Each side of the RP layer
and the unstructured side of the PC-BEF layer were primed using the
primer discussed above in Example 1. A pre-made reinforced
composite layer was attached to one side of the RP layer and the
unstructured side of a PC-BEF sheet was attached to the other side
of the RP layer, both using a UV-cured laminating adhesive. The
structure of the finished article, therefore, was: reinforced
composite layer; laminating adhesive; primer; RP; primer;
laminating adhesive; primer; PC-BEF.
[0112] The reinforced composite layer was prepared in the same
manner as discussed above in Example 1.
[0113] The PC-BEF layer was attached to the reflective polarizer
layer first by taping a 12'.times.24' (30.5 cm.times.61 cm) sheet
of PET to the leading edge of a 12'.times.20'.times. 1/4' (30.5
cm.times.50.8 cm.times.0.6 cm) sheet of aluminum. A layer of PC-BEF
was laid onto the PET sheet, with the prismatic structure facing
the PET sheet. A primed sheet of RP was laid on top of the PC-BEF
sheet. The RP sheet was covered by another sheet of 12'.times.24'
(30.5 cm.times.61 cm) PET and its leading edge was taped to the
leading edge of the aluminum plate. The leading edge of the
aluminum plate was then placed into a hand operated laminator. The
top PET sheet and the RP sheet were peeled backwards to allow
access to the sheet of PC-BEF. A bead of the laminating resin
(.about.5 mL) was applied to the PC-BEF sheet near the edge closest
to the laminating rolls. The sandwich construction was fed through
the laminator at a steady rate forcing the laminating adhesive
resin to evenly coat between the films.
[0114] The laminate, still attached to the aluminum plate, was
cured by passing the laminate beneath a UV Fusion "D" lamp
operating at 600 W/in (236 W/cm) at a speed of 30 fpm (15
cm/s).
[0115] The bottom PET sheet of a pre-made reinforced composite was
stripped away and the top PET sheet of the cured laminate sandwich
was stripped away to expose the underlying RP layer. The pre-made
reinforced composite was laid, composite side down, on top of the
exposed RP layer and the top layer of PET, on the composite, was
taped to the leading edge of the aluminum plate. The leading edge
of the aluminum plate was placed into a hand operated laminator.
The top sheet of reinforced composite and PET were pulled back to
allow access to the layer of RP. A bead of laminating adhesive,
.about.5 mL, was applied to the edge of the RP closest to the
laminating rolls. The laminating adhesive was the same as that
described in Example 1. The sandwich construction was fed through
the laminator at a steady rate coating the RP layer and the
pre-made reinforced composite layer. The resulting laminate, still
attached to the aluminum plate, was cured by passing the laminate
beneath a Fusion "D" lamp operating at 600 W/in (236 W/cm) at a
speed of 30 fpm (15 cm/s). Both remaining sheets of PET were
carefully stripped away.
Example 5
[0116] Example 5 was a single sheet of Vikuiti.TM.
Thin-BEF-90/24-II-T, available from 3M Company, St. Paul, Minn.,
and was used for comparison purposes. This was the same surface
structured layer as was used in Example 1.
Example 6
[0117] Example 6 was a single sheet of Vikuiti.TM. BEF-RP-II
90/24r, a brightness-enhanced reflective polarizer having a
prismatic surface available from 3M Company, St. Paul, Minn. This
example was used for comparison purposes.
Example 7
[0118] Example 7 was a single sheet of Vikuiti.TM. DBEF-DTV, a
second type of brightness-enhanced reflective polarizer having a
prismatic surface available from 3M Company, St. Paul, Minn. This
example was used for comparison purposes.
Sample Testing
[0119] Glass-resin composite layers similar to those included in
the examples here were evaluated under crossed polarizers and by
using a polarimeter with a spectral scanning source. The composite
samples were found to have low retardance and low birefringence.
The retardance (in nanometers) is defined here as d.times.
(|n.sub.o-n.sub.e|), where d is the thickness of the sample, and
the quantity (|n.sub.o-n.sub.e|) is equivalent to the birefringence
or the magnitude of the index difference between the ordinary and
extraordinary axes of the sample. Composite layers similar to those
made here were found to have retardance values below 2 nm (at 600
nm wavelength), corresponding to birefringence values below
0.0001.
[0120] The general relative gain test method used to quantify the
optical performance of the inventive optical films is now
described. Although specific details are given for completeness, it
should be readily recognized that similar results can be obtained
using modifications of the following approach. Optical performance
of the films was measured using a SpectraScan.TM. PR-650
SpectraColorimeter with an MS-75 lens, available from Photo
Research, Inc, Chatsworth, Calif. The films were placed on top of a
diffusely transmissive hollow light box. The diffuse transmission
and reflection of the light box can be described as Lambertian. The
light box was a six-sided hollow cube measuring approximately 12.5
cm.times.12.5 cm.times.11.5 cm (L.times.W.times.H) made from
diffuse PTFE plates of .about.6 mm thickness. One face of the box
is chosen as the sample surface. The hollow light box had a diffuse
reflectance of .about.0.83 measured at the sample surface (e.g.
.about.83%, averaged over the 400-700 nm wavelength range, box
reflectance measurement method described further below). During the
gain test, the box is illuminated from within through a .about.1 cm
circular hole in the bottom of the box (opposite the sample
surface, with the light directed towards the sample surface from
the inside). This illumination is provided using a stabilized
broadband incandescent light source attached to a fiber-optic
bundle used to direct the light (Fostec DCR-II with .about.1 cm
diam. fiber bundle extension from Schott-Fostec LLC, Marlborough
Mass. and Auburn, N.Y.). A standard linear absorbing polarizer
(such as Melles Griot 03 FPG 007) is placed between the sample box
and the camera. The camera is focused on the sample surface of the
light box at a distance of .about.34 cm and the absorbing polarizer
is placed .about.2.5 cm from the camera lens. The luminance of the
illuminated light box, measured with the polarizer in place and no
sample films, was >150 cd/m.sup.2. The sample luminance is
measured with the PR-650 at normal incidence to the plane of the
box sample surface when the sample films are placed parallel to the
box sample surface, the sample films being in general contact with
the box. The relative gain is calculated by comparing this sample
luminance to the luminance measured in the same fashion from the
light box alone. The entire measurement was carried out in a black
enclosure to eliminate stray light sources. When the relative gain
of film assemblies containing reflective polarizers was tested, the
pass axis of the reflective polarizer was aligned with the pass
axis of the absorbing polarizer of the test system.
[0121] The diffuse reflectance of the light box was measured using
a 15.25 cm (6 inch) diameter Spectralon-coated integrating sphere,
a stabilized broadband halogen light source, and a power supply for
the light source all supplied by Labsphere (Sutton, N.H.). The
integrating sphere had three opening ports, one port for the input
light (of 2.5 cm diameter), one at 90 degrees along a second axis
as the detector port (of 2.5 cm diameter), and the third at 90
degrees along a third axis (i.e. orthogonal to the first two axes)
as the sample port (of 5 cm diameter). A PR-650 Spectracolorimeter
(same as above) was focused on the detector port at a distance of
.about.38 cm. The reflective efficiency of the integrating sphere
was calculated using a calibrated reflectance standard from
Labsphere having .about.99% diffuse reflectance (SRT-99-050). The
standard was calibrated by Labsphere and traceable to a NIST
standard (SRS-99-020-REFL-51). The reflective efficiency of the
integrating sphere was calculated as follows: Sphere brightness
ratio=1/(1-R.sub.sphere*R.sub.standard) The sphere brightness ratio
in this case is the ratio of the luminance measured at the detector
port with the reference sample covering the sample port divided by
the luminance measured at the detector port with no sample covering
the sample port. Knowing this brightness ratio and the reflectance
of the calibrated standard (R.sub.standard), the reflective
efficiency of the integrating sphere, R.sub.sphere, can be
calculated. This value is then used again in a similar equation to
measure a sample's reflectance, in this case the PTFE light box:
Sphere brightness ratio=1/(1-R.sub.sphere*R.sub.sample)
[0122] Here the sphere brightness ratio is measured as the ratio of
the luminance at the detector with the sample at the sample port
divided by the thickness was measured using an EG-233 digital
linear gauge made by Ono Sokki (Yokohama, Japan). TABLE-US-00004
TABLE II Thickness, Relative Gain, and Chromaticity for Examples
1-6. Example Thickness Relative No. (.mu.m) gain, g x y .DELTA.x
.DELTA.y 1 113 1.593 0.4724 0.4255 0.0021 -0.0003 2 207 2.358
0.4731 0.4271 0.0014 -0.0019 3 380 2.022 0.4717 0.4270 0.0028
-0.0018 4 408 2.240 0.4708 0.4266 0.0037 -0.0014 5 63 1.592 0.4725
0.4255 0.0020 -0.0003 6 152 2.425 0.4723 0.4271 0.0022 -0.0019 7
638 2.118 0.4702 0.4268 -0.0043 0.0016 Blank n/a 1.000 0.4745
0.4252 0 0
[0123] In general, the relative gains of the reinforced brightness
enhancing films (Examples 1-4) are comparable to the examples of
commercially available, unreinforced, brightness enhancing films
(Examples 5 and 6) and no major color changes are evident. It is
worth noting the very small difference in relative gain between,
Examples 1 and 5. Example 1 uses the same surface structured layer
film as Example 5 but has the additional fiber reinforced layer.
The relative gains of these two Examples are comparable, indicating
that the reinforced composite layer has low light absorption and
scattering, which is advantageous for optical film applications
such as these where the light may be recycled through the film more
than once. Differences between some of the composite optical
products are due to varying haze levels and prism geometries.
[0124] A test that is commonly used to characterize the performance
of optical films is single-pass transmission. This type of
transmission measurement does not take into consideration the
effect of the film in a light-recycling cavity. Light that strikes
the detector in this test has passed through the film only once.
Further, the input light is typically directed at an angle that is
substantially normal to the plane of the film, and all transmitted
light is collected in an luminance measured without the sample.
Since R.sub.sphere is known from above, it is straightforward to
calculate R.sub.sample. These reflectances were calculated at 4 nm
wavelength intervals and reported as averages over the 400-700 nm
wavelength range.
[0125] The CIE (1931) chromaticity coordinates of the sample and
light box assembly are simultaneously recorded by the PR-650
SpectraColorimeter. The resulting chromaticity coordinates, x, y,
presented in Table III, give a quantitative measure of color of the
light transmitted through the different samples. The values of Ax
and Ay show the difference between the (x,y) co-ordinates measured
with and without the film present, i.e. show the color shift due to
the film.
[0126] The relative gain, g, is calculated by comparing the sample
luminance to the luminance measured in the same fashion from the
light box alone, i.e.: g=L.sub.f/L.sub.o where L.sub.f is the
measured luminance with the film in place and L.sub.o is the
measured luminance without the film. The measurements were carried
out in a black enclosure to eliminate stray light sources. When the
relative gain of film assemblies containing reflective polarizers
was tested, the pass axis of the reflective polarizer was aligned
with the pass axis of the absorbing polarizer of the test system.
The `blank` luminance measured from the light box alone, with the
absorbing polarizer of the test system in place and no samples
above the light box, was approximately 275 candelas m.sup.-2. The
measured values of relative gain, g, are presented in Table II. As
can be seen, the brightness gain in all cases is more than 10%
(equivalent to a relative gain of 1.1), is more than 50% (a
relative gain of 1.5), and in many cases is more than 100% (a
relative gain of 2).
[0127] The thickness of a sample was determined from the average of
four thickness measurements taken at different positions across the
film. The integrating sphere regardless of transmission angle. Many
common devices test this type of single-pass transmission,
including most commercially available haze-meters and UV-Vis
spectrometers.
[0128] Many efficient brightness-enhancing films and
light-redirecting films do not have high single-pass transmission.
In particular, when the brightness enhancing structure is directed
away from the light source, most brightness enhancing films have
low single-pass transmission. This is because the brightness
enhancing films are designed to efficiently create brightness
enhancement in a recycling backlight by re-directing off-axis light
towards the normal while recycling, through retroreflection, the
on-axis light that is measured in single pass transmission. The net
effect is efficient brightness enhancement in a display system.
Thus, when combined with other characterization tests such as the
relative gain test, single pass transmission can be used to
evaluate the light-recycling efficiency of a prismatic brightness
enhancing film. It is, therefore, desirable that brightness
enhancing films show low values of single pass transmission values,
when interpreted together with other measures, since they indicate
high efficiency of retroreflection. High single pass transmission
for certain brightness enhancing films is undesirable because it
indicates irregularity and light scattering, leading to less
efficient brightness enhancement in the completed display system.
In some embodiments it is desirable to have a single pass
transmission less than 40%, and in other embodiments less than
10%.
[0129] Exemplary optical films of the present invention were tested
for single-pass transmission (%T) using a Perkin Elmer Lambda 900
UV-Vis Spectrometer (using an approximate average from 450-650 nm).
The brightness enhancing structure was located on the side of the
film directed away from the light source. Results are shown in
Table III below. TABLE-US-00005 TABLE III Average single-pass
transmission from 450-650 nm wavelength Ave. % T Example (single
pass) Ex. 1 Thin BEF Composite 8.5 Ex. 5 Thin BEF Control 7.9
[0130] As can be seen, the composite brightness enhancing film
showed a very low single pass transmission, indicative of high
efficiency brightness enhancement in a display system.
[0131] For certain surface structured films, especially brightness
enhancing films, it is often desirable to limit the bulk diffusion
that occurs within the film. Bulk diffusion is defined as the light
scattering that takes place within the interior of an optical body
(as opposed to light scattering occurring at the surface of the
body). Bulk diffusion of a structured surface material can be
measured by wetting out the structured surface using index matching
oils and measuring the haze using a standard haze-meter. Haze can
be measured by many commercially available haze-meters and can be
defined according to ASTM D1003. Limiting the bulk diffusion
typically allows the structured surface to operate most efficiently
in re-directing light, brightness enhancement, etc. For some
embodiments of the current invention, it is desired that bulk
diffusion is low. In particular, in some embodiments the haze due
to bulk diffusion (Bulk haze) may be less than 30%, in other
embodiments less than 10% and in other embodiments less than
1%.
[0132] Bulk diffusion for Example 1 and certain other film samples
was measured by wetting out the structured surfaces using certified
refractive index matching oils made by Cargille (Series RF, Cat.
18005) and wetting out the films against a glass plate. The wet-out
films and the glass plate were then placed in the light path of a
BYK Gardner Haze-Gard Plus (Cat. No. 4725) and the haze recorded.
In this case, the haze is defined as the fraction of light
transmitted that is scattered outside an 8.degree. cone divided by
the total amount of light transmitted. The light is normally
incident on the film.
[0133] The measured values of bulk haze, i.e. the haze arising from
propagation within the bulk of the polymer matrix, rather than from
any diffusion occurring at the surface of the film, are shown below
in Table IV. The film of Example 1 was wet-out using an oil having
refractive index of 1.55. All other prism samples were wet-out
using oils of index 1.58. As can be seen the sample films showed a
haze of less than 30%, and less than 10%. TABLE-US-00006 TABLE IV
Bulk Haze Measurements Haze (due to bulk Sample diffusion) % Ex. 1
Thin BEF-II Composite 1.2 Ex. 5 Thin BEF-II-T 0.49 Blank (Glass
plate only) 0.2
Mechanical Properties
[0134] The glass transition temperature of a film sample was
measured using a TA Instruments Q800 series Dynamic Mechanical
Analyzer (DMA) with film tension geometry. Temperature sweep
experiments were performed in dynamic strain mode over the range of
-40.degree. C. up to 200.degree. C. at 2.degree. C./min. The
storage modulus and tan delta (loss factor) were reported as a
function of temperature. The peak of the tan delta curve was used
to identify the glass transition temperature, T.sub.g, for the
films. The T.sub.g was measured on a composite layer very similar
to that used in Example 1 and produced a value of 71.degree. C. The
measured T.sub.g on a corresponding sample of the same resin (with
no reinforcement) was 90.degree. C. Variability is due to
measurement factors. The resin materials used for the composite
layers had substantially the same T.sub.g for all of the examples
described here. In some embodiments it may be desirable for the
value of T.sub.g to be less than 120.degree. C.
[0135] The storage modulus and stiffness (in tension) were measured
with Dynamic Mechanical Analysis (DMA) using a TA instruments
model# Q800 DMA with film tension geometry. Terminology relating to
DMA testing can be defined according to ASTM D-4065 and ASTM
D-4092. Reported values are at room temperature (24.degree. C.).
The stiffness results are summarized in Table V. The measurements
were made at a temperature in the range 24.degree. C.-28.degree. C.
The table shows the marked increase in storage modulus which can be
obtained using the composite materials. Storage modulus is of
greater importance because it provides a thickness-independent
measure of the film properties. Some variability in these data is
to be expected both from the test method and the lab-scale
prototyping of the composite samples.
[0136] These high values of tensile modulus and stiffness can be
considered to correspond to potential bending stiffness as well,
depending on final article construction and geometry: proper
placement of the high-modulus layers results in an article having
high bending stiffness. Higher stiffness enables ease of handling,
thinner and lighter displays, and better display uniformity
(through less warp or bending of optical components of the
display). The actual performance of the final article will depend
on the arrangement of the fibers and the final geometry of the
article. For example, it may be desirable for some applications to
construct `balanced` articles, e.g. where there is either a single
central composite layer or two symmetrically opposed composite
layers, so that the material will not have a tendency to bend or
curl in a given direction upon curing or heating. The composite
samples tested here are substantially unbalanced in their
construction. In some applications, the `unbalanced` constructions
of the present invention also provide utility due to their
increased stiffness and modulus. There also may be processing,
cost, thickness, weight, and optical performance advantages to
using an unbalanced construction, which may require fewer composite
layers, depending on the details of the intended application and
article construction.
[0137] Table V lists the sample number along with a brief
description of the sample. The table also lists the orientation of
the measurements relative to the pass or block axes of the
polarizer, or to the direction relative to the web as is
manufactured on a machine. The direction "machine" corresponds to
the down-web direction while the direction "transverse" corresponds
to the direction across the web. The table also lists the average
storage modulus, the average stiffness, and the thickness.
TABLE-US-00007 TABLE V Storage Modulus and Stiffness values
measured for some representative samples. Polarizer Storage Thick-
Ex. or film Stiffness Modulus ness No. Brief Description
orientation (10.sup.4 N/m) (MPa) (.mu.m) 1 Reinforced Thin machine
23.2 7215 97 BEF 5 Thin BEF control machine 8.90 4512 62 1
Reinforced Thin transverse 28.4 7437 97 BEF 5 Thin BEF control
transverse 10.7 5296 62 2 Reinforced BEF-RP pass 24.3 5554 150 6
BEF-RP control pass 9.89 2677 120 2 Reinforced BEF-RP block 32.8
6746 150 6 BEF-RP control block 15.5 4171 120 3 Reinforced block
54.0 4427 360 WBEF/DBEF 4 Reinforced block 47.9 3670 410
PC-BEF/DBEF 7 DBEF-DTV control block 54.2 2537 630
Film Combinations
[0138] Spatially periodic patterns can sometimes create undesirable
Moire effects when combined with other periodic patterns at certain
specific spatial frequencies and angular relationships. Thus, in
some case it may be desirable to adjust the spacing, arrangement,
or angular bias of the reinforcing fibers in order to minimize
Moire patterns created between multiple composite layers, between
composite layers and any structured film surfaces (of the same or
adjacent films), or between composite layers and any display system
elements such as pixels, light-guide dot patterns, or LED sources.
Also, in cases where the index matching of the reinforcing fibers
is nearly perfect and the composite layers are nearly perfectly
smooth, Moire patterns should not occur.
[0139] Many of these composite optical articles can be
advantageously combined into assemblies. One example of an assembly
is "crossed-BEF" configuration, where two brightness enhancing
films are placed adjacent one another such that their prism grooves
are approximately orthogonal, with the prismatic surface of one
film adjacent the non-prismatic surface of the other. Many
different advantageous combinations of optical films may be
replicated using fiber reinforced optical films, combining the
enhanced mechanical properties of the composite films with the
advantageous optical properties of the film assemblies. An
inexhaustive list of exemplary embodiments of these assemblies
includes: [0140] 1. Reinforced brightness enhancing film (e.g.
Example 1) crossed with a reinforced brightness enhancing layer
integrated with a reflective polarizer layer (e.g. Examples 2-4).
[0141] 2. Unreinforced brightness enhancing film (e.g. Examples
5-6) crossed with a reinforced brightness enhancing integrated with
reflective polarizer layer (e.g. Examples 2-4). [0142] 3.
Reinforced brightness enhancing film (e.g. Example 1) crossed with
a reinforced brightness enhancing film (e.g. Example 1) [0143] 4.
Unreinforced brightness enhancing film (e.g. Examples 5-6) crossed
with a reinforced brightness enhancing film (e.g. Example 1).
[0144] 5. Reinforced brightness enhancing film (e.g. Example 1)
crossed with a reinforced brightness enhancing film (e.g. Example
1) and a reinforced reflective polarizer. [0145] 6. Unreinforced
brightness enhancing film (e.g. Examples 5-6) crossed with a
reinforced brightness enhancing film (e.g. Example 1) and a
reinforced reflective polarizer. [0146] 7. Reinforced brightness
enhancing film (e.g. Example 1) configured with a reinforced
reflective polarizer. [0147] 8. Reinforced brightness enhancing
film (e.g. Example 1) configured with an unreinforced reflective
polarizer. [0148] 9. Reinforced turning film configured with a
reinforced reflective polarizer.
[0149] For illustrative purposes, some of these film
combinations/assemblies were measured using the same relative gain
test method described previously. The tested combinations included
i) a reinforced BEF film crossed with a reinforced BEF layer, ii) a
reinforced BEF layer crossed with a reinforced brightness enhancing
layer integrated with a reflective polarizer layer and iii) an
unreinforced Thin BEF II layer crossed with a reinforced brightness
enhancing layer integrated with a reflective polarizer layer. These
exemplary combinations were compared to various combinations of the
commercially available layers. The results are presented in Table
VI below. TABLE-US-00008 TABLE VI Characteristics of Exemplary Film
Assemblies Film combinations Rel. CIE Chromaticity Bottom Film Top
Film gain, g x y Ex. 1 Ex. 1 2.368 0.4722 0.4271 Thin BEF II Thin
BEF II 2.3933 0.4707 0.4262 Thin BEF II BEF-RP 3.180 0.4716 0.4287
Ex. 1 Ex. 2 3.036 0.4734 0.4292 Thin BEF II Ex. 2 3.124 0.4720
0.4285 None None 1.000 0.4744 0.4252
[0150] In general, the relative gains of the composite examples are
approximately the same as the comparative examples and only small
color changes are evident. Also, it is worth noting the very small
differences in gain between, for example, crossed Example 1 films
and crossed Thin-BEF-II films. This indicates that the composite
substrate of Example 1 has very low light absorption and
scattering, which is advantageous for configurations in which the
light is recycled through the film multiple times.
[0151] The present invention should not be considered limited to
the particular examples described above, but rather should be
understood to cover all aspects of the invention as fairly set out
in the attached claims. Various modifications, equivalent
processes, as well as numerous structures to which the present
invention may be applicable will be readily apparent to those of
skill in the art to which the present invention is directed upon
review of the present specification. The claims are intended to
cover such modifications and devices.
* * * * *