U.S. patent application number 11/030708 was filed with the patent office on 2005-06-23 for method for stacking surface structured optical films.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Bianconi, Joseph J., Stevenson, James A..
Application Number | 20050134963 11/030708 |
Document ID | / |
Family ID | 33417803 |
Filed Date | 2005-06-23 |
United States Patent
Application |
20050134963 |
Kind Code |
A1 |
Stevenson, James A. ; et
al. |
June 23, 2005 |
Method for stacking surface structured optical films
Abstract
A display includes an optical film that has a surface structure,
such as a prismatically structured surface for increasing the
brightness of the display. The structured surface is bonded to an
opposing surface of a second film using a layer of adhesive, by
penetrating the structured surface into the adhesive layer to a
depth less than a feature height of the structured surface. The
bonded film structure provides additional strength to the films and
reduces the possibility of film damage during display assembly.
Inventors: |
Stevenson, James A.; (Saint
Paul, MN) ; Bianconi, Joseph J.; (Saint Paul,
MN) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
33417803 |
Appl. No.: |
11/030708 |
Filed: |
January 6, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11030708 |
Jan 6, 2005 |
|
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|
10439450 |
May 16, 2003 |
|
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6846089 |
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Current U.S.
Class: |
359/600 |
Current CPC
Class: |
G02B 6/0053 20130101;
G02F 2202/28 20130101; G02B 5/045 20130101; G02F 1/133606 20130101;
G02B 6/0065 20130101; G02F 1/133607 20210101 |
Class at
Publication: |
359/600 |
International
Class: |
B44C 001/00 |
Claims
We claim:
1. A method for stacking optical films, comprising: pressing
prismatic ribs of a prismatically ribbed, first optical film into a
first adhesive layer on a surface of a second optical film to a
depth so as to leave a gap between portions of the prismatic ribs
and the first adhesive layer.
2. A method as recited in claim 1, further comprising passing the
first and second optical films from respective rolls, overlapping
the first and second optical films, continuously pressing the first
and second optical films together, and cutting the first and second
optical films to a desired size in a continuous process.
3. A method as recited in claim 2, further including stripping
first and second liner layers from the respective first and second
optical films.
4. A method as recited in claim 2, wherein cutting the first and
second optical films includes cutting through the first and second
optical films to a depth that leaves a liner layer associated with
one of the optical films uncut.
5. A method as recited in claim 2, further including curing the
adhesive layer after pressing the prismatic ribs into the first
adhesive layer.
6. A method as recited in claim 2, wherein cutting the first and
second optical films includes passing the first and second optical
films through a die roller.
7. A method as recited in claim 1, wherein the second optical film
is a prismatically ribbed, second optical film having prismatic
ribs in a second orientation perpendicular to an orientation of the
prismatic ribs on the first optical film.
8. A method as recited in claim 1, further comprising stacking the
first and second optical films with at least a third optical film
to form a bonded stack.
9. A method as recited in claim 8, wherein the third optical film
is a reflective polarizer.
10. A method as recited in claim 1, further comprising feeding at
least one of the first and second optical films as a sheet from a
sheet feeder so as to overlap the first and second optical films
before pressing the prismatic ribs of the first optical film into
the first adhesive layer.
11. A method for stacking optical films, comprising: pressing
active refractive surfaces of refractive features of a first
optical film into a first adhesive layer on a surface of a second
optical film to a predetermined depth so as to leave a gap between
the first adhesive layer and portions of the active refractive
surfaces, other portions of the active refractive surfaces
contacting adhesive of the first adhesive layer.
12. A method as recited in claim 11, wherein the refractive
features of the first optical film are prismatic ribs.
13. A method as recited in claim 11, further comprising passing the
first and second optical films from respective rolls, overlapping
the first and second optical films, pressing the active refractive
surfaces of the first optical film into the first adhesive layer in
a continuous process, and cutting the first and second optical
films to a desired size in a continuous process.
14. A method as recited in claim 13, wherein cutting the first and
second optical films includes cutting through the first and second
optical films to a depth that leaves a liner layer associated with
the second optical film uncut.
15. A method as recited in claim 13, wherein cutting the first and
second optical films includes passing the first and second optical
films through a die roller.
16. A method as recited in claim 11, wherein the second optical
film is a prismatically ribbed, second optical film having
prismatic ribs in a second orientation perpendicular to an
orientation of the prismatic ribs on the first optical film.
17. A method as recited in claim 11, further including curing the
adhesive layer after pressing the prismatic ribs into the first
adhesive layer.
18. A method as recited in claim 11, wherein the second optical
film is a prismatically ribbed, second optical film having
prismatic ribs in a second orientation perpendicular to an
orientation of the prismatic ribs on the first optical film.
19. A method as recited in claim 11, further comprising stacking
the first and second optical films with at least a third optical
film to form a bonded stack.
20. A method as recited in claim 19, wherein the third optical film
is a reflective polarizer.
21. A method as recited in claim 11, further comprising feeding at
least one of the first and second optical films as a sheet from a
sheet feeder so as to overlap the first and second optical films
before pressing the prismatic ribs of the first optical film into
the first adhesive layer.
Description
[0001] This application is a divisional application of U.S. Ser.
No. 10/439,450, filed on May 16, 2003.
FIELD OF THE INVENTION
[0002] The present invention is directed to optical displays, and
more particularly to an approach for packaging light management
optical films used in optical displays.
BACKGROUND
[0003] Optical displays, such as liquid crystal displays (LCDs) are
becoming increasingly commonplace, finding use, for example, in
mobile telephones, hand-held computer devices ranging from personal
digital assistants (PDAS) to electronic games, to larger devices
such as laptop computers, and LCD monitors and television screens.
The incorporation of light management films into optical display
devices results in improved display performance. Different types of
films, including prismatically structured films, reflective
polarizers and diffuser films, are useful for improving display
parameters such as output luminance, illumination uniformity,
viewing angle, and overall system efficiency. Such improved
operating characteristics make the device easier to use, and the
concomitant reduction in battery requirements may allow the size of
the battery to be reduced, or for the time between battery
chargings to be increased. Even in displays that do not use
batteries, light management films are often useful for reducing the
complexity of the display, and can lead to breakthrough performance
in terms of luminance, uniformity, power efficiency, heat
management, and other characteristics.
[0004] The light management films are typically stacked, one by
one, into the display frame between a backlight assembly and the
flat panel display. The stack of films can be optimized to obtain a
particular desired optical performance. From a manufacturing
perspective, however, several issues can arise from the handling
and assembly of several discrete film pieces. These problems
include, inter alia, the excess time required to remove protective
liners from individual optical films, along with the increased
chance of damaging a film when removing the liner. In addition, the
insertion of individual sheets into the display frame to build the
stack of films is time consuming and provides further opportunity
for the film to be damaged. All of these problems can contribute to
diminished overall throughput or to reduced yield, which leads to
higher system cost.
SUMMARY OF THE INVENTION
[0005] In view of the problems listed above, the present invention
is directed to a new packaging method in which optical films are
bundled together before insertion into the display frame. This
bundling makes handling of the films easier, reduces the number of
steps required for assembly of the display device, reduces the
chance of damaging the films and increases yields.
[0006] Generally, the present invention relates to an approach to
bonding optical films that have a surface structure, such as
prismatically structured light directing films. The invention
includes bonding the structured surface of one film to an opposing
surface of a second film using a layer of adhesive, by penetrating
the surface features into the adhesive to a depth that is less than
the feature height.
[0007] In one particular embodiment, the invention is directed to a
light management film package for managing light within a display.
The package includes a first, brightness enhancing optical film
having a first surface structured with prismatic ribs, the ribs
having associated rib heights. A second optical film has a second
surface opposing the first surface of the first optical film. There
is a first layer of adhesive on the second surface. At least some
of the prismatic ribs of the first surface penetrate into the first
layer of adhesive. The first layer of adhesive has a thickness less
than the associated rib heights of the ribs penetrating into the
first adhesive layer.
[0008] Another embodiment of the invention is directed to a display
system, that comprises an illumination unit and a display unit. A
light management unit is disposed between the illumination unit and
the display unit to manage light passing from the illumination unit
source to the display unit. The light management unit comprises a
first optical film having a first surface structured with prismatic
ribs, the ribs having associated rib heights. A second optical film
has a second surface opposing the first surface of the first
optical film. There is a first layer of adhesive on the second
surface. At least some of the prismatic ribs of the first surface
penetrate into the first layer of adhesive. The first layer of
adhesive has a thickness less than the associated rib heights of
the ribs penetrating into the first adhesive layer.
[0009] Another embodiment of the invention is directed to a method
for stacking optical films. The method comprises pressing prismatic
ribs of a prismatically ribbed, first optical film into a first
adhesive layer on a surface of a second optical film to a depth so
as to leave a gap between portions of the prismatic ribs and the
first adhesive layer.
[0010] Another embodiment of the invention is directed to a light
management film package for managing light within a display. The
package comprises a first optical film having a first surface
structured with refractive features, the refractive features
including active refractive surfaces for light to pass
therethrough. A second optical film is disposed over the first
optical film, the second optical film having a second surface.
There is a first layer of adhesive on the second surface. At least
some of the refractive features penetrate partially into the first
layer of adhesive so as to leave a gap between the first adhesive
layer and portions of the first surface. Portions of the active
surfaces of the penetrating features interface with the gap and
other portions of the penetrating features interface with the first
adhesive layer.
[0011] Another embodiment of the invention is directed to a display
system that includes an illumination unit and a display unit. A
light management unit is disposed between the illumination unit and
the display unit to manage light passing from the illumination unit
source to the display unit. The light management unit comprises a
first optical film having a first surface structured with
refractive features. The refractive features include active
refractive surfaces for light to pass therethrough. A second
optical film is disposed over the first optical film and has a
second surface. There is a first layer of adhesive on the second
surface. At least some of the refractive features penetrate
partially into the first layer of adhesive so as to leave a gap
between the first adhesive layer and portions of the first surface.
Portions of the active surfaces of the penetrating features
interface with the gap and other portions of the penetrating
features interface with the first adhesive layer.
[0012] Another embodiment of the invention is directed to a method
for stacking optical films. The method comprises pressing active
refractive surfaces of refractive features of a first optical film
into a first adhesive layer on a surface of a second optical film
to a predetermined depth so as to leave a gap between the first
adhesive layer and portions of the active refractive surfaces.
Other portions of the active refractive surfaces contact adhesive
of the first adhesive layer.
[0013] The above summary of the present invention is not intended
to describe each illustrated embodiment or every implementation of
the present invention. The figures and the detailed description
which follow more particularly exemplify these embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] 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:
[0015] FIG. 1 schematically illustrates an embodiment of display
unit;
[0016] FIGS. 2A and 2B schematically illustrate light management
film units having at least one film with a structured surface,
according to different embodiments of the present invention;
[0017] FIGS. 2C and 2D schematically illustrate embodiments of
light management film units with different thicknesses of adhesive,
according to principles of the present invention;
[0018] FIG. 3 presents various scanning electron microscope (SEM)
cross-sections of a sample laminate construction assembled
according to principles of the present invention;
[0019] FIG. 4 presents various SEM cross-sections of another sample
laminate construction assembled according to principles of the
present invention;
[0020] FIG. 5 presents a graph showing the relationship between
relative brightness and adhesive thickness, for samples fabricated
according to principles of the present invention, using various
types of adhesives;
[0021] FIG. 6 presents a graph showing the relationship between
adhesive peel strength and adhesive thickness, for samples
fabricated according to principles of the present invention, using
various types of adhesives;
[0022] FIG. 7 presents a graph showing the relationship between
relative brightness and adhesive peel strength, for samples
fabricated according to principles of the present invention, using
various types of adhesives;
[0023] FIGS. 8A and 8B present graphs showing the brightness as a
function of viewing angle for samples fabricated according to
principles of the present invention, using various types of
adhesives;
[0024] FIG. 9 schematically presents a perspective view of an
embodiment of a display unit fabricated according to principles of
the present invention;
[0025] FIG. 10 schematically illustrates another embodiment of a
light management film stack according to principles of the present
invention;
[0026] FIG. 11 schematically illustrates another embodiment of a
light management film stack according to principles of the present
invention;
[0027] FIG. 12 schematically illustrates another embodiment of a
light management film stack according to principles of the present
invention;
[0028] FIG. 13 schematically illustrates an embodiment of a display
unit according to principles of the present invention
[0029] FIGS. 14A, 14B and 14C schematically illustrate additional
embodiments of a display unit according to principles of the
present invention; and
[0030] FIG. 15 schematically illustrates an embodiment of a method
of manufacturing a light management film stack according to
principles of the present invention.
[0031] 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
[0032] The present invention is applicable to displays, such as
liquid crystal displays, and is believed to be particularly useful
for reducing the number of steps required for making such a
display.
[0033] A display system 100 is schematically shown in FIG. 1. The
system includes an electronic display unit 102, such as a liquid
crystal display (LCD) panel, which is typically sandwiched between
two glass layers. Furthermore, the display unit 102 may include
absorbing polarizers above and below the LCD panel to provide
polarization contrast typically required for producing a
polarization-based image. The display unit 102 may be coupled to a
control unit 103 that controls the image displayed on the display
unit 102.
[0034] A backlight assembly 104 is typically used for providing
light through the display unit 102 when there is insufficient
ambient light for the user to view the image formed by the display
unit 102. In one particular embodiment, the backlight assembly 104
may include several elements such as a light source 106, a
lightguide 108, and one or more reflector layers 110. An important
feature of the display system 100 in many applications is that the
total thickness of the system 100 be small. Accordingly, the light
source 106 is commonly positioned to the side of the light guide
108, and the lightguide 108 directs the light from the light source
106 up through the system 100 towards the display element. The
light source 106 may be any suitable type of light source. In many
applications, it is desirable to illuminate the display 100 with
white light, in which case the light source 106 may be one or more
fluorescent lamps, an array of light emitting diodes whose colors
are mixed to produce white light, or the like. Some displays may
arrange light sources 106 along more than one edge of the light
guide 108.
[0035] In the illustrated embodiment, the light guide 108 is
provided with diffusely reflecting regions 112 that direct light
from the light guide 108 towards the display unit 102. The light
guide 108 may include other types of elements for directing light
towards the display unit 102, such as light extraction regions on
the upper surface of the light guide 108 facing the display
element.
[0036] Other embodiments of backlight assembly 104 may also be
used, for example, the backlight assembly 104 may be formed with an
array of lamps positioned in a suitable reflective cavity often
with a diffuser plate that covers the lamps. While there are
several other options for the design of backlight assembly 104, it
should be appreciated that the specific design of the backlight
assembly 104 is not important for the present invention.
[0037] A number of light management films are typically interposed
between the backlight assembly 104 and the display unit 102 in a
light management film stack 114. The light management film stack
114 typically contains a number of films to control various optical
characteristics of the light incident on the display unit 102. For
example, the light management film stack may include a first
diffuser film 116 for uniformizing the intensity of the light
passing up through the film stack 114.
[0038] Films 118 and 120 may be structured films, each having a row
of prism-shaped ribs 119 running across its upper surface. The
prism-shaped ribs help to direct the light towards the axis 121 of
the system 100. The ribs 119 of the film 118 redirect the light
within the plane of the figure. The ribs of the film 120 are
typically arranged perpendicular to those of the film 118 so that
the ribs of the film 120 redirect the light in a direction out of
the plane of the figure. This may be referred to as a
crossed-structure configuration. In another embodiment (not shown),
the layers 118 and 120 may be substituted with a single structured
optical film that redirects light received from the backlight
assembly 104.
[0039] The stack 114 may also include a reflective polarizer layer
122. This layer is useful for recycling light from the backlight
assembly 104 that is in the wrong polarization for transmission
through the display unit 102 as image light. The light reflected by
the reflective polarizer 122 may be diffusely reflected by the
reflector 110, with some polarization mixing, so that at least a
portion of the reflected light makes it through to the display unit
102 with the correct polarization for use as image light. Another
diffuser layer (not shown) may also be interposed between the
reflective polarizer 122 and the display unit 102.
[0040] It should be noted that, depending on the actual system
design, some of the elements represented by layers 116-122 may be
missing, added to, or substituted with other functional elements.
For example, one or more sheets may be added as cover sheets that
may or may not have diffusive properties. The cover sheets may have
a matte finish or other type of randomized surface structure for
reducing the appearance of defects in the image. Other light
control films may be included, for example diffusion sheets having
a light control function are often added in film stacks.
Additionally, backlights may include a conductive coating, such as
indium tin oxide (ITO), for electromagnetic shielding. Such a
conductive coating may be coated on a separate film or on one or
more of the light control films in the film stack.
[0041] With such a backlight assembly 104, each of the discrete
optical film layers 116-122 is conventionally inserted individually
to the display frame during manufacture. Since it is often
important to reduce the thickness of the films 116-122, to reduce
overall display thickness, the individual films 116-122 may be made
very thin. As a result, the individual film stiffness may be low,
which can result in increased difficulty in handling, processing,
and assembly during manufacture. Also, because these film layers
often have precise optical functionality, the introduction of
surface defects, such as scratches or debris may compromise total
system performance. Often, each film layer is provided by the
manufacturer with dual side protective liners, which must be
removed prior to insertion into the backlight assembly. The action
of liner removal and the resulting insertion into the backlight
assembly can expose the delicate film to a host of potential modes
of defect introduction. Examples of such defects include
scratching, and the attraction of lint and other debris to the film
surfaces due to the build up of static electricity. When multiple
film layers are incorporated into the backlight assembly, the
probability of creating/introducing a defect can grow ever higher,
which can result in slower manufacturing throughput due to
excessive re-work and higher unit costs.
[0042] This invention is directed to approaches to bundling various
optical film layers in order to improve handling and final
backlight/system assembly efficiency. In addition, the bundling of
films may improve stiffness and result in films that are more
mechanically stable.
[0043] One method of bundling multiple optical layers includes
inserting an adhesive layer between each of the films. The adhesive
layer may lie across the entire stack, from edge to edge, may be
positioned along one or more edges of the stack, or may be
patterned over the area of some or all of the film layers.
[0044] One approach to forming a bonded film stack 200 according to
the present invention is schematically illustrated in FIG. 2A.
Layers 218 and 220 represent different optical layers as may be
found in a light management film stack in a display system. For
example, layer 218 has a structured surface 222 on the side
opposing layer 220. The structured surface 222 contains features
that are typically used for refracting light passing through the
film 218. In the illustrated embodiment, the structured surface 222
is a prismatic surface having ribs 224 with a prismatic
cross-section. One example of such a film is BEF-type film
available from 3M Company, St. Paul, Minn. The second film 222 has
a layer of adhesive 226 on its lower surface 228. Parts of the
structured surface 222 penetrate into the adhesive layer 226, with
the result that the lower film 218 adheres to the upper layer
220.
[0045] It will be appreciated that the structured surface 222 need
not be limited to prisms having the cross-section of an isosceles
triangle, but may also include ribs having different types of
cross-sections. For example, the cross-sections of the ribs may
include other types of triangular prisms, truncated prisms, rounded
prisms, and curves such as sinusoids or paraboloids. In addition,
the structured surface 222 need not be limited to being structured
with ribs and may be structured with pyramids and/or posts.
[0046] The upper layer 220 may be any desired type of film, such as
another structured film, a reflective polarizer film, and absorbing
polarizer film, a diffuser layer, and the like. A reflective
polarizer film may include any suitable type type of reflective
polarizer, including, but not limited to a multiple layer polymeric
reflective polarizer, a wire grid reflective polarizer, a
cholesteric reflective polarizer, and a diffuse reflective
polarizer.
[0047] Furthermore, the layer 218 having the structured surface 222
may operate as a turning film, where light passes from the upper
layer 220 to the lower layer 218 at a large angle relative to a
film axis. The film axis lies perpendicular to the plane of the
film. Light may enter one of the prismatic ribs 224 through one
surface and then be totally internally reflected at the next
surface, so as to propagate in a direction closer to the film
axis.
[0048] One of the film layers 218 and 220 may be provided with a
conductive coating, such as a layer of indium tin oxide (ITO) or a
conducting polymer, or the adhesive 226 may be a conducting
adhesive layer. Provision of a conducting layer may be useful for
providing electrical shielding of electrical components from
electrical noise and interference. Furthermore, the adhesive layer
226 may be provided with pigments or dyes so as to adjust the
visible light spectrum of the light passing through the stack of
films.
[0049] It is desirable in some circumstances to build a packaged
stack of optical films where the approach of bonding the film
layers together provides the ability to maintain the desired
refractive and reflective properties of any surface structure and,
particularly in space-limited applications, adds little or no
thickness to the stack thickness. The adhesive layer 226 may be
used without adding significant thickness to the film stack, which
is advantageous in such applications where the space is limited.
For example, a thin layer of adhesive 226 may be used, with the
structured surface 222 penetrating all the way through the adhesive
to the surface 228 on the other side of the adhesive 226.
[0050] It may be desirable, under some circumstances, to use a
relatively thick layer of adhesive. The term "thick" under these
circumstances means that the thickness of the adhesive layer is a
significant fraction of the height of the structures of the
structured surface, for example one quarter or more. For example,
it may be desirable to provide some additional functionality to the
adhesive layer 226, in which case the optimum thickness of the
adhesive layer may be greater than that thickness needed only to
adhere the structured surface to the underlying layer. In such
cases, the lamination process may be controlled so as to ensure
that the structured surface 222 penetrates into the adhesive layer
226 to a controlled depth, see FIG. 2B, and does not penetrate all
the way to the surface 228 of the layer 220 on the other side of
the adhesive layer 226. Penetration of the structured surface 222
too far into the adhesive layer 226 results in an unacceptably high
reduction in the refractive and reflective properties of the
structured surface 222. Controlling the penetration depth, however,
reduces the possibility of such degradation in the refractive and
reflective properties of the structured layer, while providing
adhesion between the structured surface and the underlying layer
and providing a desired optical characteristic in the adhesive.
[0051] An expanded view of a surface feature 250 extending into the
adhesive layer 226 is schematically illustrated in FIG. 2C. The
height of the surface feature is h and the thickness of the
adhesive layer 226 is d. In the illustrated embodiment, the feature
250 has been pressed into the adhesive layer 226 so as to reach to
the surface 228 of the upper layer. The optical power, in other
words the reflective and refractive properties, of the structured
surface 222 is affected less when the layer of the adhesive 226 is
thinner. The thickness, d, should be less than the height, h, is
preferably less than 50% of h, and more preferably is less than 20%
of h. In other embodiments, not illustrated, the feature 250 may
not be pressed into the adhesive layer 226 so as to contact the
surface 228. Instead, the feature 250 may be pressed partially into
the adhesive layer 226. In such a case, it is preferable that the
depth of penetration into the adhesive layer is less than 50% of h,
and more preferably less than 20% of h.
[0052] Another feature of the invention is illustrated in FIG. 2D,
which shows a feature 260 penetrating into a layer of adhesive 262.
The adhesive close to the feature 260 does not necessarily lie
parallel to the surface 266 of the upper layer, but may wick along
the side of the feature 260. The extent of the wicking 264 depends
on several factors, such as the material used as the adhesive, the
(rheology) of the adhesive material, the degree of cross-linking in
the adhesive material, the surface energy of the adhesive material
and of the structure, the temperature at which the feature 260 is
made to penetrate the adhesive material, and the process conditions
of the laminating process, such as pressure, speed, and
temperature, of the lamination process.
[0053] An example of a light ray 270 is shown passing through the
feature 260, illustrating how the light is refracted by the portion
274 of the refractive surface 272 into the gap 276 between the
surface 272 and the adhesive 262. Another light ray 278 is shown
passing through the portion of the refractive surface 272
contacting the adhesive 262. The entire surface 272 may be
described as being active, in that the entire surface 272 may be
illuminated with light. Part 280 of the active surface 272 is
contacted to the adhesive 262 while the other part 274 is not
contacted to the adhesive 262. Since the reflective and refractive
characteristics of the active surface 272 are changed when the
surface 272 is in contact with adhesive, the changes in the
reflective and refractive characteristics of the active surface 272
are reduced when the layer of adhesive is thinner. However, it is
also important to provide sufficient adhesion to prevent the
adhered layers from peeling away from each other when being, for
example, handled in the manufacture of a display.
[0054] As was suggested above, the adhesive layer may provide more
functionality than simple adhesion. For example, diffusive
particles may be dispersed within the adhesive to obtain bulk
diffusive characteristics. In another example, the adhesive layer
may be provided with dyes and/or pigments so as to color the light
passing through the film stack. Diffusion characteristics may also
be added in other ways, such as including a component in the
adhesive whose phase separates from the remainder of the adhesive
mixture.
EXAMPLE 1
[0055] Samples of the construction illustrated in FIG. 2A were made
using a prismatically structured film that had a 50 .mu.m thick
substrate with prismatic structures 12 .mu.m in height. The apex
angle of the prismatic features was about 90.degree.. The samples
were made by applying a thin coating of adhesive to the planar side
of a sheet of 3M Thin Brightness Enhancement Film (TBEF). A second
sheet of TBEF was laminated to the adhesive with the prismatic
grooves oriented approximately 90.degree. to the grooves of the
first sheet.
[0056] Cross-sections through the structures, taken using a
scanning electron microscope (SEM), are illustrated in FIGS. 3 and
4 for two sets of laminates formed using different adhesives. The
adhesive used for the construction shown in FIG. 3 was a layer,
approximately 1 .mu.m thick, of iso-octyl acrylate/acrylic acid
(IOA/AA) in the ratio of 81%/19%, crosslinked at 0.15% Bisamide to
adhesive solids. The adhesive used for the construction shown in
FIG. 4 was a layer of UV curable urethane with a mixture of
iso-octyl acrylate/acrylic acid/methyl acrylate in the ratio
57.5%/35%/7.5%. This adhesive was not cross-linked.
[0057] One important feature for controlling the optical properties
of the laminated film construction is the depth of prism tip
penetration into the adhesive. Typically, as penetration increases,
the on-axis brightness of the light passing the films is reduced.
The depth of penetration includes the amount of wicking at the
tips. For example, a softer, less viscous adhesive can wick further
up the tips and can have a much higher effective depth of
penetration than a stiffer, more viscous adhesive with the same
thickness.
[0058] An important difference between the SEM images in FIGS. 3
and 4 is the amount of adhesive wicking up the tips. The softer,
more mobile, adhesive shown in FIG. 4 wicks much further than the
stiffer adhesive shown in FIG. 3. Thus, the brightness of light
passing through the construction is a good predictor of depth of
penetration. Also, brightness, given the same thickness and
refractive index, is a good predictor of the amount of wicking that
takes place. The depth of penetration may be inferred by measuring
the peak or on-axis brightness. This is an important feature when
comparing the properties of different adhesives. The different
adhesives should have similar wet out characteristics, such as
surface energy and mobility, or comparisons of factors such as
refractive index and adhesion will be confounded. While surface
energy is also involved with the depth of penetration, it becomes
less important as stiffness is increased. Thus, if the adhesive is
hardened to a sufficient degree, surface forces are insufficiently
strong to change the shape of the adhesive, and wicking can be
ignored.
EXAMPLE 2
[0059] Several laminated constructions of crossed TBEF film were
formed using different types of adhesives, over a variety of layer
thickness. The brightness of light passing through the
constructions was measured and normalized against a pair of crossed
TBEF films that had no adhesive: the results are presented in FIG.
5. Optical brightness of the laminate constructions was measured
using an Autronic Conoscope which measures the brightness
distribution versus angle that passes through the film.
[0060] Five different types of adhesives were used in the
experiments. Curve 502 corresponds to a layer of urethane acrylate
(UA) and iso-octyl acrylate (IOA) acrylic acid (AA) having a first
level of cross-linking. Curves 504 and 506 correspond also to
urethane acrylate, at respectively increased levels of
cross-linking of the IOA/AA component. Curve 508 corresponds to a
layer of iso-octyl acrylate/acrylic acid (IOA/AA) and curve 510
corresponds to a layer of epoxy acrylate (EA) and IOA/MA/AA without
cross-linking.
[0061] In general, the brightness decreases with increased adhesive
thickness. The plots indicate that the depth of penetration is
different for each of the adhesives. As the amount of cross-linking
in the UA layers increases, so does the brightness, thus
demonstrating that the brightness of the construction increases
with the adhesive stiffness. It may also be inferred that the
IOA/AA adhesive is stiffer than the UA/IOA/AA adhesives, which in
turn are stiffer than the EA/IOA/MA/AA adhesive.
[0062] The adhesive used in forming the laminated construction may
be partially cross-linked before lamination and/or partially or
fully cross-linked after lamination. Adhesives that offer the
possibility of partial cross-linking before and after lamination
provide the advantage of permitting the adhesive properties to have
different properties for the lamination process and in the finished
product. This permits the selection of a desirable lamination
rheology to improve the selection of the optical characteristics of
the laminated construction optics while later limiting the ability
of the adhesive to flow, thus providing increased long term
stability. The adhesives discussed above contain a UV curable
Urathane or Epoxy. These are chosen for their high adhesion to both
substrates, most particularly the prism tips, and their ability to
be post-cured. These UV curable adhesives are combined with a
standard pressure sensitive adhesive (PSA) whose chemistry does not
interfere with the ability to be postcrosslinked. It is benefical
that the chosen PSA additive has good adhesion to assist with
lamination and that its rheology can be easily altered to adjust
wet out. IOA/AA and IOA/MA/AA copolymers are candidate PSAs. It
should also be noted that this construction does not necessarily
require a PSA. Adhesive systems that are not typically termed PSAs
may also be used.
EXAMPLE 3
[0063] The environmental stability of the laminated construction
was tested by exposing samples of the laminated construction
fabricated using IOA/AA and a blend of IOA/AA or IOA/MA/AA and UV
curable epoxies or urethanes to temperatures of 100.degree. C.,
85.degree. C., and 65.degree. C. at a relative humidity of 95% for
1 week. All the samples passed visual inspection, and there were no
failures due to separation or bubbling. This is due to the ability
of gasses to move in and out of the laminate very easily. All
samples did curl to a certain degree. Curling is thought to occur
because of the difference in expansion coefficients between down
and cross web directions. The curl remains in the sample because
the adhesive can shift in the heated environment. The curling may
be reduced by using adhesives having a higher glass transition
temperature, by crosslinking to a higher degree, or by matching the
expansion coefficients. Curl is not an issue for assemblies, where
the film layers are held flat in the display.
[0064] The adhesives tested in environmental conditions were IOA/AA
@0.15 to 0.3% Bisamide crosslinker, UA & IOA/AA blends at 0.1
to 0.3% Bisamide crosslinking, and EA & IOA/MA/AA blends.
[0065] In relatively softer adhesives, the brightness of the
laminate construction reduced by as much as 9 percentage points,
while samples that were further cross-linked after lamination
showed a difference in brightness of .+-.1-2%. In general, the post
cross-linked samples improved in brightness slightly during
environmental exposure where the adhesive layer was thinner.
EXAMPLE 4
[0066] The adhesive properties of the different laminate
constructions discussed in Example 2 were tested for peel
strength.
[0067] It was found that post-curable adhesives increased peel
strengths. By adjusting the ratio of the post-curable to the
pre-curable components, increased peel strengths may be obtained.
The plots provided in FIG. 6 show the peel strengths as a function
of adhesive thickness. Curves 602, 604, and 606 respectively
correspond to the UA adhesive layers described above with regard to
curves 502, 504 and 506 in FIG. 5. Likewise, curve 608 corresponds
to the IOA/AA adhesive layer discussed above with respect to curve
508 in FIG. 5 and curve 610 corresponds to the EA adhesive layer
discussed above with respect to curve 510 in FIG. 5. Note that the
adhesion is provided in a logarithmic scale. The peel strengths are
all 180.degree. peels at 4"/min.
[0068] Another graph, presented in FIG. 7, shows the brightness,
normalized to crossed TBEF films without adhesive, as a function of
adhesion. Curves 702, 704 and 706 correspond respectively to the UA
adhesives discussed above with respect to curves 502/602, 504/604
and 506/606 in FIGS. 5 and 6. Curve 708 corresponds to the IOO/AA
adhesive discussed above with respect to curves 508 and 608 in
FIGS. 5 and 6. Curve 710 corresponds to the EA adhesive discussed
above with respect to curves 510 and 610 in FIGS. 5 and 6. This
graph presents useful information, since practical considerations
of laminate constructions suggest that the construction should have
relatively higher brightness and relatively higher adhesion. Thus,
those points lying in the upper right hand quadrant would appear to
provide the more desirable combinations of adhesive strength and
optical brightness. It is believed that points having an adhesion
of 30 g/in or more are sufficiently strong to be able to resist
delamination during cutting operations and the subsequent removal
of the protective liner, without regard to the liner removal
technique. The survival of constructions having an adhesion less
than about 30 g/in may depend on the liner removal process.
EXAMPLE 8
[0069] The optical characteristics of the laminate construction,
such as the optical gain (brightness), viewing angle, Moire
pattern, diffusion, and the ability to hide defects, may all be
affected by the adhesive and/or the lamination technique. The
horizontal and vertical viewing angles of the laminate
construction, fabricated with different types and thicknesses of
adhesive, are illustrated respectively in the graphs shown in FIGS.
8A and 8B. In each graph, the relative brightness for light passing
through the laminate construction is plotted as a function of angle
relative to the normal. The curves 802 and 852, corresponding to
the highest gain were obtained using crossed TBEF, without
adhesive. These curves also correspond to the narrowest viewing
angle. The other plots correspond to a variety of laminate
constructions using TBEF. The adhesives used in this plot are the
UA versions at different initial crosslinking levels. These plots
show the versatility of the laminate construction. The viewing
angle may be varied, a property that may be valuable in
applications requiring softer cutoffs and wider viewing angles. It
should also be noted that the larger viewing angle constructions
are mechanically more stable because adhesion is higher.
[0070] No reflective Moir pattern is observed in the laminate
construction by itself. Furthermore, the reflective Moire pattern
visible when used in a PDA was reduced when compared to unadhered
crossed TBEF construction.
[0071] The laminate construction also produces more diffusion than
the unadhered, crossed TBEF. The amount of diffusion increases with
an increased adhesive thickness. Diffusion can be used to tailor
the light profile for specific applications. Accordingly, the
laminate construction affords the possibility that a display may
not require a separate diffuser in the film management stack, thus
further reducing the height and cost of the film management stack.
One approach to increasing diffusion is to microreplicate patterns
into the adhesive layer. A linear pattern formed in the adhesive
layer that is parallel to the upper prisms may provide diffusion
without measurably changing the brightness or the viewing angle.
Particles or stress induced index changes may also be used to
increase the diffusion, The laminate construction provides the
ability to increase the uniformity of the appearance. This property
may arise from one of the following effects. One effect is the
elimination of surfaces that can be inspected. Defects that occur
in the center of the sheets may be hidden by the natural diffusion
of the laminate construction thus rendering these anomalies
non-functional. Another effect is index matching of the prism tips
by the adhesive layer. Damaged prism tips may be buried in the
adhesive. Thus, damaged tips that are buried within the adhesive
are less likely to cause significant defects, since the prism tips
are index matched, at least to some extent. Small defects at the
tips may disappear completely, while larger tip defects are reduced
in magnitude. Another effect is the added diffusion provided by the
laminate construction. This is believed to come from two distinct
light distributions. One distribution is the usual distribution of
light through the stack and the other, a new distribution that is
caused by the refractive and reflective differences in the area
that is in contact with the adhesive. These two different
distributions become a mix of light which makes an image harder to
resolve.
[0072] The laminate construction is typically manufactured using
lamination conditions that cause full penetration of the prism tips
into the adhesive. Under one set of lamination conditions, the
lamination temperature is set at 180.degree. F. with a lamination
speed of 1"/s (2.5 cm/s) or greater. Full penetration of the prism
tips into the adhesive layer maximizes adhesion, reduces sample
variation, and increases the likelihood that the resulting
laminated construction does not change with time and temperature.
All the samples discussed above were laminated under the same
lamination conditions, and so the lamination conditions were not
necessarily optimum for each sample.
[0073] An exploded view of a display 900 is presented in FIG. 9,
showing how the different parts of a display are assembled to form
the display. The display 900 uses a frame 902 to contain the other
components. The frame 902 may contain one or more slots 904, or
other alignment features, for example pins or the like, for
aligning the films in the light management film stack.
[0074] The backlight assembly 906 is the first component placed
within the frame 902. The backlight assembly 906 includes one or
more light sources 908 that illuminate the edge of the light guide
910. The light management film stack 912 is then positioned above
the backlight assembly 906. The light management film stack 912
includes two or more light management films 914 that are bonded
together in the manner described above for surface structured
films. The display element 920, for example a liquid crystal
display element, including polarizers, is positioned above the
light management film stack 912.
[0075] It will be appreciated that the light sources 908 and the
display element have electrical connections to receive electrical
power and control signals. The electrical connections are not
shown.
[0076] A cross-sectional view through an embodiment of the film
stack 912 is schematically presented in FIG. 10. In this particular
embodiment, the lower film is a prismatically structured film 1002
with its prismatic ribs oriented in a first orientation (out of the
plane of the figure). Above the lower film 1002 is a second
prismatic film 1004, with its prismatic ribs oriented perpendicular
(parallel to the plane of the figure) to those of the first film
1002. The second prismatic film 1004 has an adhesive layer 1006 on
its lower surface. At least some of the structure features, in this
case prismatic ribs, of the lower film 1002 penetrate into the
adhesive layer 1006.
[0077] The upper film 1008, which may be, for example, a reflective
polarizer, a diffuser film, or the like, has a second adhesive
layer 1010 on its lower surface. At least some of the structure
features of the second prismatic film 1004 penetrate into the
second adhesive layer 1010.
[0078] It will be appreciated that other optical film layers may be
added to the stack 912. Such additional film layers need not be
attached to the stack, or may be attached to the other film layers
of the stack using the same or different approaches. For example,
one or more film layers may be attached using a zero-thickness
bonding technique as discussed in co-owned U.S. patent application
Ser. No. 10/346,615, or may be attached using other techniques.
[0079] Different approaches may be used to reduce or prevent the
degradation of the optical quality of the stacked films. For
example, care is taken to reduce the presence of contaminant
particles between the films that may lead to Newton's rings.
Further, one or both of the surfaces that touch together may be
provided with small height variations to reduce wet-out, for
example as is discussed in U.S. Pat. No. 6,322,236, incorporated
herein by reference. This is shown schematically in FIG. 11, which
shows a first film 1102 having a structured surface 1104 with
features 1106 penetrating into the adhesive layer 1108 on the lower
surface 1110 of a second film 1112. The lower surface 1110 is not
flat, but is provided with random variations in height to prevent
wet-out along the prismatic ribs of the first film 1102.
[0080] In another approach, a prismatically structured film may be
provided with a variable height structured surface, as described in
U.S. Pat. No. 5,771,328, incorporated herein by reference. This is
illustrated schematically in FIG. 12, which shows a first film 1202
having a structured surface 1204. In this particular embodiment,
the structured surface 1204 includes prismatically ribbed features
1206 having different heights. The tallest features 1206 penetrate
farthest into the adhesive layer 1208 on the lower surface 1210 of
the upper layer 1212. In the illustrated embodiment, the tallest
features 1206 are pressed into the adhesive layer 1208 up to the
lower surface 1210 of the upper layer. Other features 1206 that are
not as tall either do not reach to the surface 1210 or may not even
penetrate the adhesive layer 1208.
[0081] The bonded, light management film stack may also be bonded
directly to another of the display components. One example of such
a display is schematically illustrated in FIG. 13. In this
particular embodiment, a bonded film stack 1314 is formed from a
bonded stack films 1318-1324. The display also includes a display
element 1302 and a backlight assembly 1304, having a light source
1306 and a light guide 1308. The bonded optical film stack 1314 may
have been bonded together previously using one of aforementioned
methods and then anchored to the chosen display element. In another
approach, the bonding process may be performed during the final
mounting to the chosen display element.
[0082] In the illustrated embodiment, the bonded film stack 1314 is
attached to the backlight assembly 1304, for example along an edge
of the backlight assembly 1304. In another embodiment, the optical
film stack 1314 may be anchored to the display element 1302 or to
the frame (not illustrated). This approach may be advantageous as
it may be performed mechanically, thereby avoiding manual insertion
of the optical film stack. In this way, the introduction of defects
may be minimized and manufacturing throughput and unit costs can be
improved.
[0083] Two embodiments of a display that may be particularly useful
for LCD television screens and other large displays are
schematically illustrated in FIGS. 14A, 14B and 14C. In the display
1400 illustrated in FIG. 14A, light 1402 is generated by one or
more light sources 1404. The light sources 1404 may be any suitable
type of light source, or combination of light sources, that
achieves the desired color in the illuminating light 1402. Examples
of light sources include cold cathode fluorescent tubes, light
emitting diodes and the like. A reflector 1405 may be positioned
behind the light sources 1404 to reflect light that is emitted away
from the display back towards the display. The reflector 1405 may
be a diffuse reflector so as to help make the illumination of the
display more uniform. The reflector 1405 may take one of several
different forms, including that of a sheet reflector placed below
the light sources 1404 and also that of a reflecting box or cavity
(illustrated) with reflecting surfaces along the side. The
reflector 1405 need not be flat, and may have a desired shape.
[0084] The light 1402 enters a diffusing plate 1406, which is used
to diffuse the light so that the viewer perceives a uniform image
brightness across the display 1400. The diffusing plate 1406 may be
a few millimeters thick to provide rigidity, and may contain
diffusing particles. The diffusing plate 1406 may be formed of any
suitable material, for example polycarbonate or poly methyl
methacrylate (PMMA).
[0085] After passing through the diffusing plate 1406, the light
has a wide viewing angle. Television screens typically use a wide
horizontal viewing angle so that viewers may be able to see the
image from a wide range of angles relative to the screen normal.
The vertical viewing angle, on the other hand is typically less
than the horizontal viewing angle, since the vertical position of
the viewers relative to the screen normal is usually spread over a
much smaller range than the horizontal spread. Therefore, it is
advantageous to reduce the vertical viewing angle relative to the
horizontal viewing angle, which results in the image becoming
brighter. A layer of prismatic brightness enhancing film 1408 may
be used to reduce the vertical viewing angle of the light that has
passed through the diffusing plate 1406. The prismatic brightness
enhancing film 1408 may be adhered to the diffusion plate 1406 In
another embodiment, there may be an air gap between the film 1408
and the diffusion plate 1406, or there may be intervening layers
between the film 1408 and the plate 1408.
[0086] The LCD 1416 usually includes a layer of liquid crystal 1418
sandwiched between first and second absorbing polarizers 1420 and
1422. The light 1402 from the light sources 1404 is typically
unpolarized, so a reflective polarizer 1412 may be inserted between
the brightness enhancing layer 1408 and the LCD 1416 to recycle the
light in the polarization state that would otherwise be absorbed in
the second absorbing polarizer 1422. The light reflected by the
reflective polarizer 1412 may subsequently have its polarization
rotated, at least partially, for example through diffuse reflection
or by passing through a polarization rotating element (not shown).
When it is returned to the reflective polarizer 1412, at least a
portion of the reflected light is in the polarization state that is
transmitted reflecting polarizer 1412 and the second absorbing
polarizer 1422. The reflective polarizer 1412 may be any suitable
type of reflective polarizer, for example wire grid polarizer, a
diffusely reflecting polarizer or a multiple polymer layer
reflective polarizer. In addition, the reflective polarizer may be
a cholosteric polarizer, and may include a retardation plate to
match the polarization of the transmitted light to the transmission
polarization state of the second absorbing polarizer 1422.
[0087] The prismatically structured surface 1410 of the prismatic
brightness enhancing film 1408 may be attached to the reflective
polarizer layer 1412 via a layer 1414 of adhesive. The peaks of the
prismatically structured surface penetrate at least part way into
the adhesive 1414, and may penetrate completely through the
adhesive 1414 to the lower surface of the reflective polarizer
layer 1412, in the manner described above. Furthermore, the depth
of penetration into the adhesive 1414 may be controlled in order to
tune the vertical viewing angle. A greater the depth of penetration
leads to a greater vertical viewing angle.
[0088] Light that has passed through the reflective polarizer 1412
is then directed to the LCD 1416, which imposes an image on the
light passing to the viewer. The second absorbing polarizer 1422
may remain separated from the reflective polarizer 1412, or may be
adhered to the reflective polarizer 1412. The outer surface 1424 of
the first absorbing polarizer 1420 may be treated with one or more
surface treatments. For example, the outer surface 1424 may be
provided with a matte finish or an anti-glare coating. The outer
surface 1424 may also be provided with a hard coating to provide
protection against scratching.
[0089] Additional diffusion may be provided within the screen 1400,
in addition to that provided in the diffusion plate 1406. For
example, the adhesive 1414 may include diffusing particles. Also,
there may be a diffusing layer provided on one or both sides of the
reflective polarizer 1412, for example a layer of diffusing
adhesive between the reflective polarizer 1412 and the second
absorbing polarizer 1422. The reflective polarizer 1412 itself may
be diffusive, for example by including diffusive particles in
reflective polarizer 1412.
[0090] In another embodiment of the display 1450, schematically
illustrated in FIG. 14B, a bulk diffuser layer 1452 is provided
between the reflective polarizer 1412 and the second absorbing
polarizer 1422. The bulk diffuser layer 1452 may be formed from any
suitable type of matrix material, including, but not limited to,
polycarbonate, PMMA, polyethylene and the like. Bulk diffusion is
provided by a plurality of diffusing particles 1453, typically a
few .mu.m in size, disposed throughout the matrix material. The
diffusing particles have a refractive index different from that of
the matrix material, and may be formed from, interalia, glass
beads, polystyrene beads, titanium dioxide particles or other
diffusing particles.
[0091] The display 1450 may also be provided with a structured
layer of adhesive 1454 between the prismatic brightness enhancing
film 1408 and the diffusion plate 1406. The structured layer of
adhesive 1454 provides one or more air gaps 1456 between the
prismatic brightness enhancing film 1408 and the diffusion plate
1406, which increases the ability of the brightness enhancing film
1408 to redirect light in a direction closer to the axis 1458 of
the display 1450. The structured layer 1454 of adhesive may include
ribs, parallel or non-parallel to the ribs of the prismatic
brightness enhancing film 1408, or may be formed using some other
pattern, for example a two dimensional pattern. The structured
layer 1454 of adhesive advantageously has a low fill factor, so
that a large fraction of the light passing from the diffuser plate
1406 into the film 1408 passes through the air gap 1456. It will be
appreciated that other bonding methods may be used to provide an
air gap at the lower surface of the prismatic brightness enhancing
film 1408. In one example, a structured layer may be bonded between
the diffusion plate 1406 and the film 1408, the structured layer
having recesses on its upper surface to form air gaps with the
lower surface of the film 1408.
[0092] It will be appreciated that additional layers and/or surface
treatments may be used in any of the displays described above. For
example, the upper surface 1413 of the reflective polarizer 1412
may be a matte surface so as to increase light diffusion and thus
increase the uniformity of the illumination of the light on the LCD
1416. One or more layers of the displays may be provided with an
antistatic coating, for example a thin layer of electrically
conductive material. One example of a suitable conductive material
is indium tin oxide (ITO), although other conductive materials,
such as conducting polymers, may be used.
[0093] Another useful embodiment of a display screen 1470 is
schematically illustrated in FIG. 14C. In this embodiment, the
prismatic brightness enhancement film 1408 and the reflective
polarizer layer 1412 are placed between two support sheets 1472 and
1474. The support sheets 1472 and 1474 may be formed of any
suitable material that is transmissive and, particularly for the
upper support sheet 1472, is polarization preserving. The support
sheets may be made from, for example, polycarbonate or other
suitably rigid, environmentally stable and mechanically robust
material. The support sheets 1472 and 1474 may have any required
thickness. For example, polycarbonate support sheets 1472 and 1474
may have a thickness in the range of approximately 2-20 thousandths
of an inch (0.05 mm-0.5 mm), and most preferably around 10
thousandths of an inch (0.25 mm), although the thickness may also
be outside this range.
[0094] The support sheets 1472 and 1474 are bonded to the
reflective polarizer 1412 and the prismatic brightness enhancement
film 1408 respectively, for example by using an adhesive or by
lamination. This construction provides added rigidity to the bonded
reflective polarizer/prismatic film combination 1476, thus
providing protection during handling and assembly of the display
1470. It is particularly advantageous that, where the two support
sheets 1472 and 1474 are formed of the same material, that the
thicknesses of the two support sheets 1472 and 1474 be the same.
This reduces the likelihood-that the combination 1476 warps when
exposed to different temperatures. In another embodiment, the
prismatic brightness enhancement film 1408 may be formed directly,
for example by replication, on a layer sufficiently thick to
provide the desired rigidity and mechanical performance, thus
obviating the need for a separate lower support sheet 1474.
[0095] It will be appreciated that the prismatic brightness
enhancement layers described herein may have any suitable prism
size and apex angle. While it is common that the apex angle of the
prisms is around 90.degree., there is no restriction on the apex
angle. There may, however, be preferred ranges of apex angle
depending on the particular light source used and the particular
application of the display, that provide enhanced brightness.
Furthermore, the length of the prism base may any value within a
wide range of values. The length of the prism base is typically
affected by such factors as the type of display, the allowable
thickness of the film stack, and the thickness of the adhesive. For
example, in hand-held displays, where the viewer is close to the
display and the film stack needs to be thin, then the prism base
length is shorter, in the range of a few 10's of microns. The
adhesive layer is, consequently, fairly thin. In LCD-TVs, on the
other hand, where the viewer is further away from the screen, and
the thickness of the display is less restricted, the size of the
prism may be larger, and may be in the range of a few 100's of
microns. Larger prisms enable the use of a thicker adhesive layer
without affecting the gain (on-axis brightness) of the light
passing through the display. This aids the stability of the
construction of the LCD-TV display, where the screen size may be up
to 60 inches (1.5 m) or more.
[0096] The use of a bonded film stacks, or bundled films, in such
devices as flat panel displays, offers several advantages. Many of
the light management films used in a display, particularly a
hand-held display, are very thin. For example the prismatically
structured films may each have a thickness of about 62 .mu.m, while
a reflective polarizer may have a thickness in the range 1 .mu.m to
100's of .mu.m. Such thin films are very flexible, which may cause
problems during assembly of the display. Bundling multiple thin,
flexible films, on the other hand, creates a stiffer film pack,
which can ease assembly issues. Eliminating the sequential stacking
of discrete layers when assembling the display also minimizes the
probability of defect introduction and ultimate yield loss.
Additionally, since films are usually delivered by the manufacturer
to the display integrator with protective linings on either side,
the number of protective liners that the display integrator has to
remove is reduced when the films are bundled. This further
optimizes yield and manufacturing unit costs.
[0097] Also, the bundling of optical films may improve final
inspection and quality yield versus the separate examination of
each discrete film layer. This can be easily considered when using
structured optical films, which may tend to distort and camouflage
defects in underlying or other film bundle layers that would
otherwise be detected if these layers were inspected individually.
Finally, bundling options, such as the method of attaching
structured films discussed above, can provide for bundled stacks
with very little increase in the stack thickness.
[0098] Optical films are often fabricated in large sheets, in some
cases on a roll. The individual film pieces that are assembled in a
display are cut from the large sheet, usually by a die. Several
different approaches may be used for bonding the films into bonded
stacks. For example, the films may be die-cut to the appropriate
shape, a layer of adhesive spread on the lower surface of a film
facing the structured surface of another film, and then the films
are aligned in a stack and bonded. In other approaches, the films
may be bonded first, before being cut, for example by a die.
Furthermore, films may be bonded two or more at a time. Therefore,
it should be appreciated that a stack that includes three or more
films may be formed using two or more bonding steps. For example,
the first two films may be bonded together to form the bonded stack
and then one or more additional films bonded to the stack in one or
more bonding steps.
[0099] One particular example of a method for bonding at least two
films together is now described with reference to FIG. 15. In this
particular embodiment, a first film roll 1502 contains a roll of
film that has a liner on at least one side. The liner 1504 is
stripped by a stripping roll 1506. A second film roll 1508 may also
contain a roll of film that has a liner on at least one side. The
liner 1510 is stripped by a second stripping roll 1512. The
stripped films 1514 and 1516 pass towards a pinch roller pair
1518.
[0100] A coater 1520 deposits a layer of adhesive 1522 of the
appropriate thickness on the surface of the lower film 1516 before
the pinch roller 1518 pair. On passing through the pinch roller
pair 1518, the structured surface 1524 of the upper film is pressed
into the adhesive layer 1522 to the desired depth.
[0101] After passing through the pinch roller pair 1518, the
adhesive 1522 between the two films 1514 and 1516 is cured, for
example by illumination with a UV lamp 1526. The layered film 1528
may then be passed to a die 1530 for cutting to the appropriate
shape. The die 1530 may be a rotary die for continuous cutting of
the layered film 1528. In one particular embodiment, the rotary die
1530 is formed from a die roller 1532 and an anvil roller 1534. The
separation between the die and anvil rollers 1532 and 1534 may be
set so that the die roller 1532 kiss cuts through the layered film
1528 to a controlled depth, to the lower liner of the lower film.
The peripheral weed 1536 may then be stripped away, leaving an
array of film stacks 1538 on the lower liner layer 1540. The cut
film 1540 may be collected on collection roll 1542.
[0102] The two films 1514 and 1516 may be any light management
films, although the upper film 1514 has a structured surface 1524
facing towards the lower film 1516. The structured surface 1524 may
be, for example, a prismatically structured surface with prismatic
ribs for a brightness enhancing film. In one embodiment, the upper
film 1514 may be a prismatically structured film with the ribs
oriented across the web of the film 1514, while the lower film 1516
is a prismatically structured film with the ribs oriented along the
web of the film 1514, or vice versa.
[0103] It will be appreciated that variations of the system
illustrated in FIG. 15 may be used. For example, the processes of
welding and cutting may be combined into a single pair of rollers
that make ultrasonic welds and die cut simultaneously. For example,
the raised portions of the die may transfer ultrasonic energy to
the film so as to weld the films at the die cuts.
[0104] In another method of stacking bonded sheets, the two rolls
contain prismatically structured film, each having the ribs
structured along the web. In such a situation, the films 1514 and
1516 from the two rolls 1502 and 1510 may cross at right angles to
each other, so that the stacked prismatic films are crossed in a
pinch/cutter roller. A coater may be used to coat one of the films
before entering the pinch/cutter roller so that so the two films
are pressed together and cut to form the desired film stack.
[0105] It will be appreciated that variations of the system
illustrated in FIG. 15 may be used. For example, a third film, or
additional films, may be added to the stack that is welded by the
ultrasonic welder. For example, a third film may be added to the
lower surface of the film 1516, along with a layer of adhesive, to
produce a structure as illustrated in FIG. 15. Additional film
layers may also be added, either at a time different from when the
first two films 1514 and 1516 are laminated together, or at the
same. Furthermore, instead of feeding continuous sheets, one or
more of the sheets may be fed as individual sections from
respective sheet feeders. Additional layers of adhesive may be
inserted between the additional sheets, or additional sheets may be
attached to the film stack using another approach, for example the
approach described in U.S. patent application Ser. No. 10/346,615,
incorporated herein by reference. It will be appreciated that other
approaches for stacking, bonding and cutting films be followed,
within the scope of the invention.
[0106] As noted above, the present invention is applicable to
optical displays and is believed to be particularly useful for
manufacturing optical display film units that are easier to handle,
that reduce the time and complexity of assembling a display unit,
and that permit the gain and viewing angle to be adjusted. 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.
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