U.S. patent application number 13/327919 was filed with the patent office on 2012-04-12 for patterned adhesives for reflectors.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Mieczyslaw H. Mazurek, Michael A. Meis, Audrey A. Sherman, Michael F. Weber.
Application Number | 20120087010 13/327919 |
Document ID | / |
Family ID | 42007076 |
Filed Date | 2012-04-12 |
United States Patent
Application |
20120087010 |
Kind Code |
A1 |
Meis; Michael A. ; et
al. |
April 12, 2012 |
PATTERNED ADHESIVES FOR REFLECTORS
Abstract
A reflector laminate, and backlights and displays incorporating
such reflector laminates are disclosed. The reflector laminate
includes a substrate and a reflector that are adhered together by a
plurality of adhesive protrusions. The adhesive protrusions provide
for a plurality of voids between the reflector and the substrate,
so that a free reflector surface is adjacent to an air gap between
the reflector and substrate. The reflector can include a multilayer
interference reflector, and the presence of the air gap ensures
high reflectivity for the laminate. The reflector can be adhered to
a solid lightguide, a frame enclosing a portion of a backlight, a
frame enclosing a hollow lightguide cavity, or an optical sheet
positioned at the output surface of a backlight.
Inventors: |
Meis; Michael A.;
(Stillwater, MN) ; Sherman; Audrey A.; (Saint
Paul, MN) ; Mazurek; Mieczyslaw H.; (Roseville,
MN) ; Weber; Michael F.; (Shoreview, MN) |
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
42007076 |
Appl. No.: |
13/327919 |
Filed: |
December 16, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12547576 |
Aug 26, 2009 |
|
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13327919 |
|
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61097724 |
Sep 17, 2008 |
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Current U.S.
Class: |
359/584 |
Current CPC
Class: |
G02F 2202/28 20130101;
G02B 6/0055 20130101; G02B 5/02 20130101; G02B 5/0841 20130101;
G02B 6/0031 20130101; G02F 2202/36 20130101; G02F 1/133615
20130101; G02F 2203/02 20130101 |
Class at
Publication: |
359/584 |
International
Class: |
G02B 1/10 20060101
G02B001/10 |
Claims
1. A reflective laminate, comprising: a substrate having a first
surface; an optically thick adhesive layer in contact with the
first surface of the substrate, the optically thick adhesive layer
comprising a plurality of protrusions; and an interference
reflector having a second surface disposed facing the first surface
of the substrate, wherein a portion of the second surface is in
contact with the protrusions.
2. The reflective laminate of claim 1, wherein a void is defined
between the second surface of the interference reflector, the
protrusions, and the first surface of the substrate.
3. The reflective laminate of claim 1, wherein the portion of the
second surface of the interference reflector in contact with the
protrusions comprises less than 10% of the second surface.
4. The reflective laminate of claim 1, wherein the portion of the
second surface of the interference reflector in contact with the
protrusions comprises less than 5% of the second surface.
5. The reflective laminate of claim 1, wherein the portion of the
second surface of the interference reflector in contact with the
protrusions comprises less than 1% of the second surface.
6. The reflective laminate of claim 1, wherein a refractive index
of the optically thick adhesive layer is less than a lowest
refractive index of the interference reflector over a wavelength
range of interest
7. The reflective laminate of claim 1, wherein the interference
reflector comprises a multilayer interference reflector.
8. The reflective laminate of claim 7, wherein the multilayer
interference reflector comprises a polymeric multilayer
interference reflector.
9. The reflective laminate of claim 8, wherein the polymeric
multilayer interference reflector comprises a broadband mirror for
visible light.
10. The reflective laminate of claim 1, wherein the optically thick
adhesive layer in contact with the first surface is a continuous
layer.
11. The reflective laminate of claim 1, wherein the optically thick
adhesive layer in contact with the first surface is a discontinuous
layer.
12. The reflective laminate of claim 11, wherein the discontinuous
layer is uniformly discontinuous.
13. The reflective laminate of claim 1, wherein the optically thick
adhesive layer in contact with the first surface comprises at least
one microstructured surface.
14. The reflective laminate of claim 1, wherein the protrusions
comprise a plurality of ridges.
15. The reflective laminate of claim 1, wherein the protrusions
comprise a plurality of islands.
16. The reflective laminate of claim 1, wherein the optically thick
adhesive layer comprises a plurality of particles.
17. The reflective laminate of claim 1, wherein the substrate
comprises a diffuser.
18. The reflective laminate of claim 17, wherein the interference
reflector includes a third surface opposite the second surface, and
further comprising: a second optically thick adhesive layer in
contact with the third surface of the interference reflector; and a
second substrate having a fourth surface in contact with the second
optically thick adhesive layer.
19. The reflective laminate of claim 18, wherein the second
optically thick adhesive layer comprises a second plurality of
protrusions projecting from the fourth surface of the second
substrate.
20. The reflective laminate of claim 18, wherein the second
optically thick adhesive layer comprises a second plurality of
ridges.
21. The reflective laminate of claim 18, wherein the second
substrate comprises an optically transparent plate.
22. The reflective laminate of claim 18, wherein the second
substrate comprises a diffuser.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. application Ser.
No. 12/547,576, filed Aug. 26, 2009, which claims the benefit of
U.S. Provisional Patent Application No. 61/097,724, filed Sep. 17,
2008, the disclosure of which is incorporated by reference herein
in their entirety.
BACKGROUND
[0002] Optical displays, such as liquid crystal displays (LCDs),
are becoming increasingly commonplace, finding use for example in
mobile telephones, portable computer devices ranging from hand held
personal digital assistants (PDAs) to laptop computers, portable
digital music players, LCD desktop computer monitors, and LCD
televisions. In addition to becoming more prevalent, LCDs are
becoming thinner as the manufacturers of electronic devices
incorporating LCDs strive for smaller package sizes.
[0003] Many LCDs use a backlight for illuminating the LCD's display
area. The backlight can include a solid lightguide in the form of a
slab or wedge. The solid lightguide is often made of an optically
transparent polymeric material produced by, for example, injection
molding. Many solid lightguides also include a reflector that is
used to more efficiently utilize light that may exit the bottom
surface of the solid lightguide. The backlight can instead include
a hollow cavity lightguide which has reflective surfaces
surrounding the interior of a hollow cavity. The output surface of
the hollow cavity lightguide is often a partially transmissive
surface.
[0004] In many applications, the backlight includes one or more
light sources that optically couple light into the lightguide from
one or more edges of the lightguide. The optically coupled light
typically travels through a solid lightguide by total internal
reflection (TIR) from the top and bottom surfaces. The optically
coupled light travels through the hollow cavity lightguide by
reflection from reflective surfaces. Eventually, the light
encounters some feature that causes a portion of the light to exit
the lightguide through an output surface. The feature can be an
extraction surface that re-directs light through the output
surface, or the output surface can be a partial reflector that
allows a portion of the light to leak from the lightguide with each
reflection.
[0005] Backlight reflectors used in both solid and hollow cavity
lightguides need to have a high reflectivity for efficient
transport of light. An efficient reflector is a multilayer
interference reflector available from 3M Company under the trade
designation Vikuiti ESR.TM. (Enhanced Specular Reflective) film.
High reflectivity is achieved using a suspended ESR film that has a
low index material (typically an air gap) adjacent to each surface
of the film. There is a need for a method to secure highly
reflective interference reflector films to surfaces, while
maintaining the air gap adjacent to each side of the film.
SUMMARY
[0006] Generally, the present disclosure relates to reflective
laminates. The present disclosure also relates to displays, and
backlights for displays, that use the reflective laminates. The
reflective laminates include adhesive protrusions that provide an
air gap adjacent the reflector.
[0007] In one aspect of the disclosure, a reflective laminate
includes a substrate, an optically thick adhesive layer in contact
with a first surface of the substrate, and an interference
reflector having a second surface disposed facing the first surface
of the substrate. The optically thick adhesive layer includes a
plurality of protrusions. The plurality of protrusions is in
contact with a portion of the second surface of the interference
reflector. A void is defined between the second surface of the
interference reflector, the protrusions, and the first surface of
the substrate.
[0008] In one embodiment, the portion of the second surface of the
interference reflector in contact with the protrusions can be less
than 10%, or less than 5%, or less than 1% of the second surface.
In another embodiment, the refractive index of the optically thick
adhesive layer can be less than a lowest refractive index of the
interference reflector over a wavelength range of interest. In one
embodiment, the optically thick adhesive layer in contact with the
first surface of the substrate can be a continuous layer or a
discontinuous layer, and the discontinuous layer can be uniformly
discontinuous. In one embodiment, the protrusions can include a
plurality of ridges or a plurality of islands, or a combination of
ridges and islands. In one embodiment, the optically thick adhesive
layer includes a plurality of particles. In one embodiment, the
substrate includes a diffuser.
[0009] In one aspect of the reflective laminate, the substrate is a
diffuser, the interference reflector includes a third surface
opposite the second surface, and the reflective laminate further
includes a second optically thick adhesive layer in contact with
the third surface of the interference reflector. A second substrate
having a fourth surface is in contact with the second optically
thick adhesive layer. In one embodiment, the second optically thick
adhesive layer includes a second plurality of protrusions
projecting from the fourth surface of the second substrate. In one
embodiment, the second optically thick adhesive layer includes a
second plurality of ridges. In one embodiment, the second substrate
includes an optically transparent plate. In another embodiment, the
second substrate comprises a diffuser.
[0010] In another aspect, a backlight assembly includes a
reflective laminate, an output element, and a light source. The
reflective laminate includes a substrate, an optically thick
adhesive layer in contact with a first surface of the substrate,
and an interference reflector having a second surface disposed
facing the first surface of the substrate. The optically thick
adhesive layer includes a plurality of protrusions. The plurality
of protrusions is in contact with a first portion of the second
surface of the interference reflector. A second portion of the
second surface is a free surface adjacent an optically thick void.
The output element is disposed facing the interference reflector,
defining a cavity between the output element and the interference
reflector, and the light source is capable of injecting light into
the cavity. In one embodiment, the ratio of the second portion of
the second surface to the first portion of the second surface is
greater than 10. In another embodiment, the output element includes
a partial reflector. In yet another embodiment, the partial
reflector is an Asymmetric Reflective Film. In one embodiment of
the backlight assembly, the partial reflector has an input surface
and an output surface opposite the input surface, and the output
element further includes an optically transparent plate, a third
optically thick adhesive layer, and a diffuser. The optically
transparent plate is adhered to the output surface of the partial
reflector with a second optically thick adhesive layer. The third
optically thick adhesive layer is in contact with the input
surface, and includes a second plurality of protrusions projecting
from the input surface. The diffuser is in contact with the second
plurality of protrusions, and faces the interference reflector.
[0011] These and other aspects of the present application will be
apparent from the detailed description below. In no event, however,
should the above summaries be construed as limitations of the
claimed subject matter, which subject matter is defined solely by
the attached claims and their equivalents, as may be amended during
prosecution.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Throughout the specification reference is made to the
appended drawings, where like reference numerals designate like or
similar elements, and wherein:
[0013] FIG. 1 is a schematic view of a backlit display;
[0014] FIG. 2 is a schematic view of backlit display; and
[0015] FIG. 3 is a schematic view of backlit display.
[0016] The figures are not necessarily to scale. Like numbers used
in the figures refer to like or similar components. However, it
will be understood that the use of a number to refer to a component
in a given figure is not intended to limit the component in another
figure labeled with the same number.
DETAILED DESCRIPTION
[0017] The present description discloses a reflective laminate that
can be used to reduce optical losses from a reflector in a display.
The optical losses are reduced by laminating the reflector to a
substrate member with an optically thick adhesive that includes
protrusions to maintain a separation between the reflector and the
substrate. The substrate member can be a solid lightguide, a frame
enclosing a portion of a backlight, a frame enclosing a hollow
lightguide cavity, or an optical sheet positioned at the output
surface of a backlight. The reflector can be a high efficiency
reflector, a partially transmissive reflector, or a diffuse
reflector. For a partially transmissive reflector, the substrate
member can be positioned at the output surface of a backlight.
[0018] For purposes of this disclosure, "optically thick" materials
refer to a material thickness that is generally greater than the
wavelength of light, preferably orders of magnitude greater, for
example at least 1 micrometer, and possibly hundreds of micrometers
or more. Geometrical optics can sufficiently predict or describe
optical properties of an optically thick film, such as its
reflective and transmissive properties. In contrast, interference
optics can be used to sufficiently describe the behavior of light
traveling in interference films, such as the thin film layers in
multilayer interference reflectors.
[0019] The optically thick adhesive includes a plurality of
protrusions which contact the reflector in a first portion of the
reflector surface area. The plurality of protrusions contacting the
reflector can be continuous across the surface of the reflector,
for example a series of ridges or crossed ridges. The plurality of
protrusions can instead be discontinuous, for example an array of
dots or islands of adhesive. The array of protrusions can be a
regular array, or the protrusions can be randomly dispersed over
the reflector surface. The protrusions are preferably uniformly
dispersed over the entire reflector surface, so that the reflector
can be firmly attached to the substrate. The optically thick
adhesive can be, for example, a dry-film hot melt adhesive, a
dry-film pressure sensitive adhesive, a radiation curable adhesive,
or a solvent based adhesive.
[0020] The adhesive protrusions can be deposited by selective
transfer, screen printing, ink jet printing, or any other
patterning techniques. The adhesive protrusions can be small,
widely spaced dots, lines or areas of adhesive. In one aspect, the
refractive index and the pattern of the adhesive can be adjusted to
control the reflectivity versus optical coupling through the
contact point, as described elsewhere. In another aspect, it may be
desirable to add diffusing particles to scatter some light in the
lightguide into a wide range of angles for extraction, while still
providing a reflector for recycling light. In another aspect, it
may be desirable to scatter some light into a random range of
angles for extraction from the lightguide, and also diffusely
reflect recycled light in the lightguide. In one aspect, a diffuse
reflector adhered to a lightguide by small adhesive dots can
produce a combination of randomly scattered light for extraction
and diffusely reflected light for recycling.
[0021] Contact of the first portion of the reflector with the
protrusions provides a second portion of the reflector which is in
contact with air, and inadvertent contact of the second portion
with any higher refractive index material is prevented. Inadvertent
contact can cause undesired light transmission through the
reflector (light leakage) in the region of contact. Selection of an
optical adhesive with an appropriate refractive index can reduce or
eliminate even the undesired light transmission through the
adhesive protrusions that contact the interference reflector.
[0022] Many optical products and devices that require a high
reflectivity mirror use a thin film interference stack for that
purpose. Such thin film interference reflectors can be made
economically, and can be designed to provide high reflectivity over
a desired wavelength band, such as the human visible wavelength
spectrum, the output spectrum of a specified light source, or the
sensitivity spectrum of a specified detector. The interference
reflectors can also provide reflectivity over a range of angles of
the incident light. Excellent reflectivity can usually be
achieved--at a particular wavelength, or even over the entire
wavelength range of interest--for normally incident light and for
moderate angles of incidence. This performance is usually adequate
for the intended end-use application. Examples of interference
reflectors, such as multilayer interference reflectors, include
those described in U.S. Pat. No. 6,208,466 (Liu et al.); U.S. Pat.
No. 5,825,543 (Ouderkirk et al.); U.S. Pat. No. 5,783,120
(Ouderkirk et al.); U.S. Pat. No. 5,882,774 (Jonza et al.); U.S.
Pat. No. 5,612,820 (Shrenk et al.) and U.S. Pat. No. 5,486,949
(Shrenk et al.). In some cases, wide angle mirror systems or
broadband mirrors can be used to improve the reflectivity over a
broad range of wavelengths and angles, as described for example in
U.S. Patent Publication No. US2008/0037127 (Weber), entitled "Wide
Angle Mirror System".
[0023] For some applications, a birefringent multilayer stack
adapted to reflect visible light can be used to reflect and
distribute some of the light that is injected into the edge of a
lightguide within a backlight. One such birefringent multilayer
stack is a multilayer interference reflector available from 3M
Company under the trade designation Vikuiti ESR.TM. (Enhanced
Specular Reflective) film. Acceptable performance of such
backlights is achieved by suspending the ESR film below a solid
lightguide such that the ESR film is immersed in a very low
refractive index medium such as air, for optimal performance.
However, optical losses can arise if the ESR film ceases to be
immersed in the low refractive index medium, for example, if the
ESR film comes in contact with the solid lightguide or another
portion of the display, particularly if this occurs on both sides
of the ESR film simultaneously.
[0024] According to one aspect of the disclosure, an interference
reflector such as an ESR film can be laminated to a lightguide
using an optically thick adhesive having an array of protrusions
that maintain a separation (air gap) over a portion of the ESR.
According to another aspect of the disclosure, an interference
reflector such as an ESR film can be laminated to a substrate
adjacent a lightguide, such as a back frame of the backlight, using
an optically thick adhesive having an array of protrusions that
maintain a separation (air gap) over a portion of the ESR.
[0025] In one embodiment, the optically thick adhesive can have an
index of refraction less than the lowest refractive index of the
ESR film. The refractive index difference can be chosen such that
substantially all injected light remains in the lightguide. In
another embodiment, light recycled back into the lightguide from
light management films disposed between the lightguide and display
can be incident on the ESR film, causing it to reflect back towards
the display. Various light management films are known, and include
prism films such as Vikuiti.TM. Brightness Enhancement Film "BEF"
or Thin Brightness Enhancement Film "TBEF", and reflective
polarizer film such as Vikuiti.TM. Dual Brightness Enhancement Film
"DBEF", available from 3M Company.
[0026] The lightguide can be of any desired size or shape, and can
be of uniform thickness such as a slab, or tapered such as a wedge.
The lightguide can, for example, be suitable for use in a backlight
for a liquid crystal display (LCD) in a mobile phone, laptop
computer, television, or other application. Extraction features can
be provided on a front surface or elsewhere on or in the
lightguide, to direct light out of the lightguide towards a liquid
crystal panel or other component to be illuminated.
[0027] The lightguide can include extraction features on the side
opposite of the laminated reflector, causing light to be directed
toward the viewer at predetermined angles. Examples of extraction
features can be found, for example, in U.S. Pat. No. 6,845,212
(Gardiner et al.) and U.S. Pat. No. 7,223,005 (Lamb et al.); and
also in U.S. Patent Publication No. 2007/0279935 (Gardiner et al.).
The extraction features can be grooves, lenslets, or other
microstructured features designed to extract light from the
lightguide. The extraction features can be imparted to the
lightguide using several methods, including but not limited to:
casting, embossing, microreplicating, printing, ablating, etching
and other methods known in the art.
[0028] The present disclosure also provides a lightguide and a
reflector as a single unit, such as a lightguide laminate, reducing
the backlight part count and cost to a backlight assembler.
Lamination of the lightguide to the reflector prevents debris,
which can cause defects in display uniformity, from entering
between the two surfaces. Appropriate selection of lightguide and
adhesive protrusion refractive indices can preserve lightguiding
and prevent light from entering the reflector at an angle greater
than the leak angle. The lamination of the two components may also
reduce warp of the individual components, resulting in improved
environmental performance and stability.
[0029] In another aspect of the disclosure, the backlight design
can be a hollow cavity lightguide having a recycling optical
cavity. In a hollow cavity lightguide, a large proportion of the
light undergoes multiple reflections between substantially
coextensive front and back reflectors before emerging from the
front reflector. The front reflector is partially transmissive and
partially reflective. Overall losses for light propagating in the
recycling cavity are kept extraordinarily low, for example, both by
providing a substantially enclosed cavity of low absorptive loss,
including low loss front and back reflectors as well as side
reflectors. The reflectors in a hollow cavity lightguide are
typically laminated to the interior surfaces of the lightguide. The
adhesive protrusions of the present disclosure contribute to
providing low loss reflectors by maintaining air contact with a
substantial portion of the surface of the reflectors.
[0030] In the case of a backlight designed to emit only light in a
particular (useable) polarization state, the front reflector can
have a high enough reflectivity for such useable light to support
lateral transport or spreading, and for light ray angle
randomization to achieve acceptable spatial uniformity of the
backlight output, but a high enough transmission into the
appropriate application-useable angles to ensure application
brightness of the backlight is acceptably high.
[0031] The backlight design can include a lightguide that has a
component or components that provide the cavity with a balance of
specular and diffuse characteristics, the component having
sufficient specularity to support significant lateral light
transport or mixing, but also having sufficient diffusivity to
substantially homogenize the angular distribution of steady state
light within the lightguide, even when injecting light only over a
narrow range of angles. Additionally, recycling within the
lightguide can result in a degree of randomization of reflected
light polarization relative to the incident light polarization
state. This allows for a mechanism by which unusable polarization
light can be converted by recycling into usable polarization
light.
[0032] The backlight design can include a front reflector that has
a reflectivity that generally increases with angle of incidence,
and a transmission that generally decreases with angle of
incidence, where the reflectivity and transmission are for
unpolarized visible light and for any plane of incidence, and/or
for light of a useable polarization state incident in a plane for
which oblique light of the useable polarization state is
p-polarized. Additionally, the front reflector has a high value of
hemispheric reflectivity, and simultaneously, a sufficiently high
value of transmission of application usable light.
[0033] The backlight design can include light injection optics that
partially collimate or confine light initially injected into the
lightguide to propagation directions close to a transverse plane
(the transverse plane being parallel to the output area of the
backlight), e.g., an injection beam having a full angle-width
(about the transverse plane) at half maximum power (FWHM) in a
range from 0 to 90 degrees, or 0 to 60 degrees, or 0 to 30 degrees.
In some instances it may be desirable for the maximum power of the
injection light to have a downward projection, below the transverse
plane, at an angle with the transverse plane of no greater than 40
degrees, and in other instances, to have the maximum power of the
injected light to have an upwards projection, above the transverse
plane towards the front reflector, at an angle with the transverse
plane of no greater than 40 degrees.
[0034] Backlights incorporating the design features discussed above
and disclosed in co-pending PCT Patent Application No.
US2008/064115 (Weber), entitled "Recycling Backlights with
Semi-specular Components" provide for efficient, uniform, and thin,
hollow backlights. Exemplary partial reflectors (front reflectors)
can be asymmetric reflective films (ARFs) as described in PCT
Patent Application No. US2008/064133 (Weber), entitled "Backlight
and Display System Using Same" and provide for low loss reflections
and also for better control of transmission and reflection of
polarized light than is possible with TIR in a solid light guide
alone.
[0035] A multilayer interference reflector such as an ESR film can
leak light incident to its surface, depending on the medium that
the film is immersed in. As the incidence angle increases from the
normal to the surface (0 degrees incidence angle) of the multilayer
interference reflector, a "leak angle" can be reached. For the
purposes of this description, the leak angle is the angle such that
most of the light incident on the multilayer interference reflector
surface, at or greater than the leak angle, is transmitted through
the film. At angles of incidence less than the leak angle, most of
the light is reflected from the multilayer interference reflector
surface. The leak angle can be dependent on the materials and layer
thicknesses in the multilayer interference reflector, the medium in
which the multilayer interference reflector is immersed, and the
wavelength of the incident light. The leak angle is significantly
reduced (i.e. the multilayer interference reflector leaks more
incident light) when the refractive index surrounding the
multilayer interference reflector is increased. It is to be
understood that the two outer surfaces of a multilayer interference
reflector can be immersed in different materials having different
refractive indices. The leak angle of the reflector can be
influenced by each of the different materials; however, if one of
the materials is air, light propagating within the multilayer
interference reflector and incident to the surface that is immersed
in air, undergoes reflection from a surface that has a high
relative leak angle. Interactions of light at a material interface
with an interference reflector are further discussed, for example,
in U.S. Patent Publication No. US2008/0037127 (Weber), entitled
"Wide Angle Mirror System".
[0036] Reduction in the leak angle of a multilayer interference
reflector (by immersion in a higher index material), can adversely
affect the brightness and uniformity of a display. For example,
when an ESR film comes into optical contact with the lightguide in
a backlight, light normally guided within the lightguide at large
angles undergoes frustrated total internal reflection (F-TIR), and
is optically coupled into the ESR film. This coupling is not
particularly detrimental unless another object also comes into
optical contact with the other surface of the ESR film in the
region of the coupling. The other object can be, for example, part
of the display frame, a piece of dust, a liquid, or any other
debris. When this occurs, the light optically coupled into the ESR
film via F-TIR transmits through the ESR film, and is lost out of
the backside of the display. This loss produces dark spots and
reduced uniformity in the display.
[0037] In some cases, the lightguide and ESR film reflector are
separated in the display backlight by an air gap, and a spacer
surrounding the lightguide can be used to maintain the air gap.
However, the spacer may not always protect the air gap, and debris
may enter the space between lightguide and reflector. This debris
can cause optical coupling and light leakage, resulting in display
dark spots and increased non-uniformity. Moreover, the lightguide
and reflector can move independently of each other, and can distort
or warp. Warping caused by, for example thermal changes, can cause
the reflector and lightguide to contact each other, resulting in
light leakage. In some cases, changes in humidity and static
electricity can also cause optical coupling and light leakage.
[0038] For purposes of this detailed description, a wavelength
range of interest can mean visible or near-visible light (e.g.,
400-700 nm wavelength), near infrared light (e.g., 700-1000 nm,
700-1400 nm or 700-5000 nm with the selection of one of these
ranges sometimes being dependent on the detector or transmission
medium employed), or both visible and near infrared light, or
portions thereof. Other ranges may also be used as the wavelength
range of interest. For example, if the reflective laminate is to be
used in a system with a narrow band emitter, such as an LED or a
laser, the wavelength range of interest may be relatively narrow
(e.g., 100 nm, 50 nm, 10 nm, or less). For reflective laminates
used in lighting systems such as backlights for liquid crystal
display (LCD) devices or other displays, the wavelength range of
interest may be broader (e.g., 400-800 nm, 400-900 nm, 400-1000 nm,
400-1200 nm, 400-1400 nm, 400-1600 nm or 400-1700 nm).
[0039] In some cases, alternating materials of suitable refractive
index, microlayer thickness profile across the stack, and total
number of microlayers can be selected to provide a stack having
characteristics such as: a reflection band extending throughout the
visible region and extending into the near infrared, having sharp
left- and right-band edges, and having a high average reflectivity
throughout at least the visible region (and for some applications
also throughout the near infrared) of at least 70%, 80%, or 90% or
more. Reference is made, for example, to Vikuiti.TM. Enhanced
Specular Reflector (ESR) film sold by 3M Company, which utilizes a
birefringent multilayer stack. ESR can have a reflectivity of
greater than 98% throughout the visible region.
[0040] The film stack can be entirely polymeric, and can be made by
a coextrusion process and a stretching process to induce an
appropriate amount of birefringence in the microlayers to enhance
reflectivity. In some cases, the film stack can include or be
limited to inorganic materials, and may be made by vacuum
evaporation techniques. Reference is made to U.S. Pat. No.
6,590,707 (Weber) for a birefringent thin film stack that utilizes
inorganic materials.
[0041] The optically thick adhesive can form a continuous or a
discontinuous layer between a substrate and a reflector. In some
cases, a continuous layer of the optically thick adhesive can
provide uniformity to the display appearance, and improve
performance of the backlight. For purposes of this detailed
description, "continuous layer" means that the layer covers
substantially the entire space between the substrate and the
reflector, and "discontinuous layer" means that at least some
portion of the space between the substrate and the reflector is not
covered by the layer. In some cases, the optically thick adhesive
can be made discontinuous, for example by depositing a segmented
adhesive pattern on either the substrate or the reflector. In some
cases, a uniformly discontinuous adhesive pattern can be used, such
as a plurality of adhesive segments uniformly distributed over the
substrate and/or reflector. In some cases, the discontinuous
coating is discontinuous on a small scale relative to the
dimensions of the lightguide, so that artifacts of the adhesive
pattern are not visible when the lightguide is being used.
[0042] The optically thick adhesive layer can be a continuous layer
having protrusions extending from one surface. In one aspect, the
optically thick adhesive layer can be a microstructured adhesive
that includes protrusions separated regions free of adhesive. The
microstructured adhesive can be an array of islands such as dots or
other structures, or it can be an array of ridges, or it can be an
array of intersecting ridges. The array can be a regular array of
protrusions or it can be a random array of protrusions. Examples of
microstructured adhesives, and methods of making microstructured
adhesives are described in, for example, U.S. Pat. Nos. 5,650,215
(Mazurek et al.), U.S. Pat. No. 6,123,890 (Mazurek et al.), U.S.
Pat. No. 6,315,851 (Mazurek et al.), U.S. Pat. No. 6,440,880
(Mazurek et al.), U.S. Pat. No. 7,250,210 (Mazurek et al.), and
U.S. Patent Publication Nos. 2007/0054133 (Sherman et al.),
2007/0054133 (Sherman et al.).
[0043] The adhesive material can be an optically clear or an
optically diffuse adhesive. An optically diffuse adhesive can
provide light scattering from the portion of the reflector that
contacts the protrusions, and can re-introduce some of the light
which may be lost from the cavity. The optically diffuse adhesive
can have particles dispersed within the adhesive to diffuse light,
as known in the art. The optically diffuse adhesive can instead be
a phase separated adhesive, or an adhesive having microdomains,
such as those described in co-pending U.S. patent application Ser.
No. 60/986,298 (Attorney Docket No. 63108US002) entitled, "Optical
Adhesive with Diffusive Properties" filed Nov. 8, 2007.
[0044] The protrusions contact the reflector in a first portion of
the reflector surface. The first portion of the reflector surface
is preferably a small portion of the entire surface, such as less
than 20%, 10%, 5%, 2%, 1%, or even less than 0.5% of the total
reflector surface. The second portion of the reflector surface
remains a free surface that is in contact with air, adjacent a void
defined between the second portion of the reflector surface, the
protrusions, and the substrate. The free surface adjacent the void
prevents light leakage through the reflector due to contact with
the adhesive.
[0045] We turn now to FIG. 1, which shows a backlit display 100
according to one aspect of the disclosure. Backlit display 100
includes a substrate 160, a lightguide assembly 110 affixed to
substrate 160 with an optically thick adhesive 150, optional light
management films 170, and an LCD module 180.
[0046] Lightguide assembly 110 includes a lightguide 120, a light
source 122 optically coupled to lightguide 120 with injection
optics 124, a back reflector 140 affixed to substrate 160 with
optically thick adhesive layer 150, and output element 130.
Lightguide 120 can be a solid lightguide, and output element 130
can be an optional partially transmissive front reflector.
Lightguide 120 can instead be a hollow cavity lightguide, and
output element 130 can be a partially transmissive front reflector,
as described elsewhere.
[0047] Back reflector 140 is affixed to substrate 160 with
optically thick adhesive layer 150. Optically thick adhesive layer
150 includes a plurality of protrusions 152, and voids 154 between
protrusions 152. In one embodiment, optically thick adhesive layer
150 covers substrate 160 as a continuous layer as shown in FIG. 1.
In another embodiment, optically thick adhesive layer 150 only
includes protrusions 152, and there is no adhesive between voids
154 and substrate 160 (not shown in FIG. 1).
[0048] A first light ray 125 within lightguide assembly 110 is
shown intercepting one of the plurality of protrusions 152 in the
contact portion 142 with back reflector 140. A portion 126 of light
ray 125 is shown to be lost from lightguide assembly 110 through
refraction in contact portion 142. In one embodiment (not shown),
the refractive index of optically thick adhesive layer 150 can be
lower than the lowest refractive index in back reflector 140, and
reflection rather than refraction is more prominent from contact
portion 142, and the light is re-directed back into lightguide
assembly 110. A second light ray 127 within lightguide assembly 110
is shown intercepting a free surface 144 of back reflector 140
adjacent voids 154. The second light ray 127 within lightguide
assembly 110 is shown to reflect by TIR from free surface 144, and
remains within lightguide assembly 110.
[0049] Optional light management films 170 can further condition
the light directed toward LCD module 180, and make more efficient
use of the light to improve the brightness and uniformity of
backlit display 100. Light leaves lightguide assembly 110, and
enters optional light management films 170 before passing through
LCD module 180 toward an observer. Optional light management films
170 can include a pair of crossed BEF prism films oriented with the
prisms facing LCD module 180. Optional light management films 170
can further include a diffuser and a DBEF reflective polarizer
positioned on opposite sides of crossed BEF prism films. Optional
diffuser can be positioned between crossed BEF prism films and
lightguide assembly 110. In some cases, optional light management
films 170 can additionally include other films for further
conditioning the light, such as diffusers, filters, and others as
known in the art. A portion of the light entering optional light
management films 170 passes through toward LCD module 180. Another
portion of light entering optional light management films 170 is
directed back into lightguide assembly 110 to be recycled.
[0050] FIG. 2 shows a backlit display 200 according to another
aspect of the disclosure. The description of like element numbers
in FIG. 2 correspond to the description of like element numbers in
FIG. 1, above. Backlit display 200 includes a substrate 160, a
lightguide assembly 210 affixed to substrate 160 with an optically
thick adhesive 250, optional light management films 170 and LCD
module 180. Lightguide assembly 210 includes a lightguide 120, a
light source 122 optically coupled to lightguide 120 with injection
optics 124, a back reflector 240 affixed to substrate 160 with
optically thick adhesive layer 250, a diffuser 290 affixed to back
reflector 240 with a plurality of adhesive protrusions 252, and
output element 130. Lightguide 120 can be a solid lightguide, and
output element 130 can be an optional partially transmissive front
reflector. Lightguide 120 can instead be a hollow cavity
lightguide, and output element 130 can be a partially transmissive
front reflector, as described elsewhere.
[0051] Back reflector 240 is affixed to substrate 160 with
optically thick adhesive layer 250. Optically thick adhesive layer
250 can include a plurality of protrusions (not shown) as described
elsewhere, for example as shown by optically thick adhesive layer
150 having protrusions 152 in FIG. 1. Optically thick adhesive
layer 250 can instead be a continuous layer in contact with back
reflector 240. Diffuser 290 is affixed to back reflector 240 with
adhesive protrusions 252. In this way, adhesive protrusions 252
contact back reflector 240 at a contact portion 242 and provide a
free back reflector surface 244 adjacent a void 254. Adhesive
protrusions 252 also contact diffuser 290 at a contact portion 292
and provide a free diffuser surface 294 adjacent void 254. In one
embodiment, diffuser 290 can be a surface diffuser; in another
embodiment, diffuser 290 can be a bulk diffuser.
[0052] A first light ray 225 within lightguide assembly 210 is
shown intercepting one of the plurality of adhesive protrusions 252
in the contact portion 292 with diffuser 290. In the embodiment
where the refractive index of adhesive protrusion 252 is lower than
the refractive index of diffuser 290, first light ray 225 reflects
by TIR (not shown) from contact portion 292. In the embodiment
where the refractive index of adhesive protrusion 252 is greater
than or equal to the refractive index of diffuser 290, a portion of
first light ray 225 is shown to be optically coupled to back
reflector 240 through contact portion 242 of adhesive protrusion
252.
[0053] Depending on the angle of propagation of first light ray
225, two different paths can be taken after intercepting back
reflector 240. An angle near normal incidence with back reflector
240 can cause first light ray 225 to be reflected from back
reflector 240 as second light ray 226 and re-enter the lightguide
assembly 210. An angle near grazing incidence with back reflector
240 can cause first light ray 225 to leak through back reflector
240, and become lost as third light ray 227 from lightguide
assembly 210 through optically thick adhesive layer 250. In the
embodiment where optically thick adhesive layer 250 includes
protrusions as described with reference to optically thick adhesive
layer 150 and protrusions 152 of FIG. 1, the leakage through back
reflector 240 can be further minimized or eliminated.
[0054] A fourth light ray 228 within lightguide assembly 210 is
shown intercepting the free diffuser surface 294 of diffuser 290,
adjacent voids 254. Fourth light ray 228 reflects by TIR at free
diffuser surface 294, and remains within lightguide assembly 210 as
light ray 229.
[0055] FIG. 3 shows a backlit display 300 according to another
aspect of the disclosure. The description of like element numbers
in FIG. 3 correspond to the description of like element numbers in
FIG. 1 and FIG. 2, above. Backlit display 300 includes substrate
160, lightguide assembly 310 attached to substrate 160 with
optically thick adhesive layer 250, optional light management films
170 and LCD module 180. In FIG. 3, output element 330 has been
expanded to show an embodiment of output element 130 of backlit
display 200 as shown in FIG. 2. In FIG. 3 the lightguide 120 is a
hollow cavity lightguide, and output element 330 includes a
partially transmissive front reflector 340, as described
elsewhere.
[0056] In FIG. 3, output element 330 includes partially
transmissive front reflector 340 which can be an Asymmetric
Reflective Film. Partially transmissive front reflector 340 can be
adhered to transparent plate 360 with optically thick adhesive
layer 350 to support partially transmissive front reflector 340
over the output area 370 of lightguide assembly 310. Optically
thick adhesive layer 350 can include protrusions (not shown) as
described with reference to optically thick adhesive layer 150 and
protrusions 152 in FIG. 1, or it can be a continuous layer as shown
in FIG. 3. Diffuser 390 is affixed to partially transmissive front
reflector 340 with adhesive protrusions 352. In this way, adhesive
protrusions 352 contact partially transmissive front reflector 340
at contact portion 342 and provide a free partially transmissive
front reflector surface 344 adjacent voids 354. Adhesive
protrusions 352 also contact diffuser 390 at contact portion 392
and provide a free diffuser surface 394 adjacent void 354. In one
embodiment, diffuser 390 can be a surface diffuser; in another
embodiment, diffuser 390 can be a bulk diffuser. Light rays
propagating through diffuser 390 can intercept free diffuser
surface 394 at a range of angles ranging from grazing incidence to
normal incidence.
[0057] A first light ray 325 within lightguide assembly 310 is
shown intercepting one of the plurality of adhesive protrusions 352
in the contact portion 392 with diffuser 390. In the embodiment
where the refractive index of adhesive protrusion 352 is lower than
the refractive index of diffuser 390, first light ray 325 reflects
by TIR (not shown) from contact portion 392. In the embodiment
where the refractive index of adhesive protrusion 352 is greater
than or equal to the refractive index of diffuser 390, a portion of
first light ray 325 is shown to be optically coupled to partially
transmissive front reflector 340 through refraction at contact
portion 342 of adhesive protrusion 352.
[0058] Depending on the angle of propagation of first light ray
325, two different paths can be taken after intercepting partially
transmissive front reflector 340. An angle near normal incidence
with partially transmissive front reflector 340 can cause first
light ray 325 to be reflected from partially transmissive front
reflector 340 as second light ray 326 and re-enter the lightguide
assembly 310. An angle near grazing incidence with partially
transmissive front reflector 340 can cause first light ray 325 to
leak through partially transmissive front reflector 340, and become
lost as third light ray 327 from lightguide assembly 310 through
optically thick adhesive layer 350 and transparent plate 360. Third
light ray 327 can then proceed through transparent plate 360, light
management film 170, and a portion of the light can then pass
through LCD module 180 to an observer. In the embodiment where
optically thick adhesive layer 350 includes protrusions as
described with reference to optically thick adhesive layer 150 of
FIG. 1, the leakage through partially transmissive front reflector
340 can be further controlled. A fourth light ray 328 within
lightguide assembly 310 is shown intercepting the free diffuser
surface 394 of diffuser 390, adjacent voids 354. Fourth light ray
328 reflects by TIR at free diffuser surface 394, and remains
within lightguide assembly 310 as light ray 329.
[0059] The embodiments described can be used anywhere that a
reflector is laminated to a substrate without compromising
reflectivity, TIR, or subsequent coupling of light out of a
lightguide. For some applications, it may be desirable to adhere
the reflective film layer to a surface securely and provide for
limited or controlled light leakage. For some other applications,
it may be further desirable to redirect light by a combination of
scattering and reflection. The embodiments described can be applied
anywhere that thin, optically transmissive, or reflective
structures are used, including displays such as TV, notebook and
monitors, and used for advertising, information display or
lighting. The present disclosure is also applicable to electronic
devices including laptop computers and handheld devices such as
Personal Data Assistants (PDAs), personal gaming devices,
cellphones, personal media players, handheld computers and the
like, which incorporate optical displays. The backlights of the
present disclosure have application in many other areas. For
example, backlit LCD systems, luminaires, task lights, light
sources, signs and point of purchase displays can be made using the
described embodiments.
[0060] Unless otherwise indicated, all numbers expressing feature
sizes, amounts, and physical properties used in the specification
and claims are to be understood as being modified by the term
"about." Accordingly, unless indicated to the contrary, the
numerical parameters set forth in the foregoing specification and
attached claims are approximations that can vary depending upon the
desired properties sought to be obtained by those skilled in the
art utilizing the teachings disclosed herein.
[0061] All references and publications cited herein are expressly
incorporated herein by reference in their entirety into this
disclosure. Although specific embodiments have been illustrated and
described herein, it will be appreciated by those of ordinary skill
in the art that a variety of alternate and/or equivalent
implementations can be substituted for the specific embodiments
shown and described without departing from the scope of the present
disclosure. This application is intended to cover any adaptations
or variations of the specific embodiments discussed herein.
Therefore, it is intended that this disclosure be limited only by
the claims and the equivalents thereof.
* * * * *