U.S. patent application number 16/706336 was filed with the patent office on 2020-04-16 for brightness enhancing film with embedded diffuser.
This patent application is currently assigned to 3M INNOVATIVE PROPERTIES COMPANY. The applicant listed for this patent is 3M INNOVATIVE PROPERTIES COMPANY. Invention is credited to Gary T. Boyd, Steven H. Kong, Tri D. Pham, Qingbing Wang.
Application Number | 20200116904 16/706336 |
Document ID | / |
Family ID | 49887250 |
Filed Date | 2020-04-16 |
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United States Patent
Application |
20200116904 |
Kind Code |
A1 |
Boyd; Gary T. ; et
al. |
April 16, 2020 |
BRIGHTNESS ENHANCING FILM WITH EMBEDDED DIFFUSER
Abstract
Brightness enhancing films with embedded diffusers are
described. More specifically, films including a birefringent
substrate, a prismatic layer carried by the substrate having linear
prisms, and an embedded structured surface disposed between the
substrate and the prismatic layer are disclosed. The embedded
structured surface may include closely-packed structures. Processes
for producing embedded structured surfaces having particular
topographies are also disclosed.
Inventors: |
Boyd; Gary T.; (Woodbury,
MN) ; Kong; Steven H.; (Woobury, MN) ; Pham;
Tri D.; (Woodbury, MN) ; Wang; Qingbing;
(Campbell, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
3M INNOVATIVE PROPERTIES COMPANY |
St. Paul |
MN |
US |
|
|
Assignee: |
3M INNOVATIVE PROPERTIES
COMPANY
|
Family ID: |
49887250 |
Appl. No.: |
16/706336 |
Filed: |
December 6, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14436770 |
Apr 17, 2015 |
10557973 |
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PCT/US2013/073276 |
Dec 5, 2013 |
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16706336 |
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61737220 |
Dec 14, 2012 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 5/3083 20130101;
G02B 5/0268 20130101; G02B 5/0221 20130101; G02B 5/0278 20130101;
G02B 5/045 20130101 |
International
Class: |
G02B 5/02 20060101
G02B005/02; G02B 5/30 20060101 G02B005/30; G02B 5/04 20060101
G02B005/04 |
Claims
1. An optical film, comprising: a birefringent substrate; a
prismatic layer carried by the substrate, the prismatic layer
having a major surface comprising a plurality of side by side
linear prisms extending along a same prism direction; and an
embedded structured surface disposed between the substrate and the
prismatic layer comprising larger first structures and smaller
second structures, the first and second structures both being
limited in size along two orthogonal in-plane directions; wherein
the first structures are non-uniformly arranged on the embedded
structured surface; wherein the second structures are closely
packed and non-uniformly dispersed between the first structures;
and wherein an average size of the first structures is greater than
15 microns and an average size of the second structures is less
than 15 microns.
2. The optical film of claim 1, wherein the embedded structured
surface is characterized by a bimodal distribution of equivalent
circular diameter (ECD) of structures of the embedded structured
surface, the bimodal distribution having a first and second peak,
the larger first structures corresponding to the first peak and the
smaller second structures corresponding to the second peak.
3. The optical film of claim 1, wherein the average size of the
first structures is in a range from 20 to 30 microns.
4. The optical film of claim 1, wherein the average size of the
second structures is in a range from 4 to 10 microns.
5. The optical film of claim 1, wherein the embedded structured
surface has a topography characterizable by a first and second
Fourier power spectrum associated with respective first and second
orthogonal in-plane directions, and wherein: the first Fourier
power spectrum includes one or more first frequency peak not
corresponding to zero frequency and being bounded by two adjacent
valleys that define a first baseline, each first frequency peak
having a first peak ratio of less than 0.8, the first peak ratio
being equal to an area between the first frequency peak and the
first baseline divided by an area beneath the first frequency peak;
and the second Fourier power spectrum includes one or more second
frequency peak not corresponding to zero frequency and being
bounded by two adjacent valleys that define a second baseline, each
second frequency peak having a second peak ratio of less than 0.8,
the second peak ratio being equal to an area between the second
frequency peak and the second baseline divided by an area beneath
the second frequency peak.
6. The optical film of claim 5, wherein the first and second peak
ratios are each less than 0.5.
7. An optical film, comprising: a birefringent substrate; a
prismatic layer carried by the substrate, the prismatic layer
having a major surface comprising a plurality of side by side
linear prisms extending along a same prism direction; and an
embedded structured surface disposed between the substrate and the
prismatic layer, wherein the embedded structured surface is made by
microreplication from a tool structured surface, the tool
structured surface being made by forming a first layer of a metal
by electrodepositing the metal using a first electroplating process
resulting in a major surface of the first layer having a first
average roughness, and forming a second layer of the metal on the
major surface of the first layer by electrodepositing the metal on
the first layer using a second electroplating process resulting in
a major surface of the second layer having a second average
roughness smaller than the first average roughness, the major
surface of the second layer corresponding to the tool structured
surface.
8. The optical film of claim 7, wherein the embedded structured
surface comprises larger first structures and smaller second
structures, the first and second structures both being limited in
size along two orthogonal in-plane directions; wherein the first
structures are non-uniformly arranged on the embedded structured
surface; wherein the second structures are closely packed and
non-uniformly dispersed between the first structures; and wherein
an average size of the first structures is greater than 15 microns
and an average size of the second structures is less than 15
microns.
9. The optical film of claim 8, wherein the embedded structured
surface is characterized by a bimodal distribution of equivalent
circular diameter (ECD) of structures of the embedded structured
surface, the bimodal distribution having a first and second peak,
the larger first structures corresponding to the first peak and the
smaller second structures corresponding to the second peak.
10. The optical film of claim 7, wherein the embedded structured
surface has a topography characterizable by a first and second
Fourier power spectrum associated with respective first and second
orthogonal in-plane directions, and wherein: the first Fourier
power spectrum includes one or more first frequency peak not
corresponding to zero frequency and being bounded by two adjacent
valleys that define a first baseline, each first frequency peak
having a first peak ratio of less than 0.8, the first peak ratio
being equal to an area between the first frequency peak and the
first baseline divided by an area beneath the first frequency peak;
and the second Fourier power spectrum includes one or more second
frequency peak not corresponding to zero frequency and being
bounded by two adjacent valleys that define a second baseline, each
second frequency peak having a second peak ratio of less than 0.8,
the second peak ratio being equal to an area between the second
frequency peak and the second baseline divided by an area beneath
the second frequency peak.
11. The optical film of claim 10, wherein the first and second peak
ratios are each less than 0.5.
12. An optical film, comprising: a birefringent substrate; a
prismatic layer carried by the substrate, the prismatic layer
having a major surface comprising a plurality of side by side
linear prisms extending along a same prism direction; and an
embedded structured surface disposed between the substrate and the
prismatic layer comprising larger first structures and smaller
second structures, the first and second structures both being
limited in size along two orthogonal in-plane directions; wherein
the first structures are non-uniformly arranged on the embedded
structured surface; wherein the second structures are closely
packed and non-uniformly dispersed between the first structures;
and wherein the embedded structured surface is characterized by a
bimodal distribution of equivalent circular diameter (ECD) of
structures of the embedded structured surface, the bimodal
distribution having a first and second peak, the larger first
structures corresponding to the first peak and the smaller second
structures corresponding to the second peak.
13. The optical film of claim 12, wherein the embedded structured
surface has a topography characterizable by a first and second
Fourier power spectrum associated with respective first and second
orthogonal in-plane directions, and wherein: the first Fourier
power spectrum includes one or more first frequency peak not
corresponding to zero frequency and being bounded by two adjacent
valleys that define a first baseline, each first frequency peak
having a first peak ratio of less than 0.8, the first peak ratio
being equal to an area between the first frequency peak and the
first baseline divided by an area beneath the first frequency peak;
and the second Fourier power spectrum includes one or more second
frequency peak not corresponding to zero frequency and being
bounded by two adjacent valleys that define a second baseline, each
second frequency peak having a second peak ratio of less than 0.8,
the second peak ratio being equal to an area between the second
frequency peak and the second baseline divided by an area beneath
the second frequency peak.
14. The optical film of claim 13, wherein the first and second peak
ratios are each less than 0.5.
15. The optical film of claim 12, wherein an average size of the
first structures is in a range from 20 to 30 microns, and an
average size of the second structures is in a range from 4 to 10
microns.
Description
BACKGROUND
[0001] Display systems, such as liquid crystal display (LCD)
systems, are used in a variety of applications and commercially
available devices such as, for example, computer monitors, personal
digital assistants (PDAs), mobile phones, miniature music players,
and thin LCD televisions. Most LCDs include a liquid crystal panel
and an extended area light source, often referred to as a
backlight, for illuminating the liquid crystal panel. Backlights
typically include one or more lamps and a number of light
management films such as, for example, light guides, mirror films,
light redirecting films (including brightness enhancement films),
retarder films, light polarizing films, and diffuser films.
Diffuser films are typically included to hide optical defects and
improve the brightness uniformity of the light emitted by the
backlight.
[0002] Some diffusing films use a beaded construction to provide
the light diffusion. For example, an optical film may have a layer
of microscopic beads adhered to one surface of the film, and the
refraction of light at the bead surfaces may operate to provide the
light diffusion characteristics of the film. Examples of beaded
diffusing films include: a linear prismatic brightness enhancement
film with a matte surface of sparsely distributed beads, sold under
the product designation TBEF2-GM by 3M Company, referred to herein
as a "sparsely distributed beaded diffuser" or "SDB diffuser"; a
reflective polarizing film with a beaded diffuser layer, sold under
the product designation DBEF-D3-340 by 3M Company, referred to
herein as a "densely-packed beaded diffuser" or "DPB diffuser"; and
a diffusing cover sheet included in a commercial display device,
referred to herein as a "commercial cover sheet diffuser" or "CCS
diffuser". FIG. 1 shows a scanning electron microscope (SEM) image
of a representative portion of the beaded surface of a CCS
diffuser, and FIG. 1A shows an SEM image of such surface in
cross-section. FIGS. 2 and 3 show SEM images of representative
portions of a DPB diffuser and a SDB diffuser, respectively.
[0003] Other diffusing films use a structured surface other than a
beaded layer to provide the light diffusion, where the structured
surface is made by microreplication from a structured tool.
Examples of such diffusing films include: films (referred to herein
as "Type I Microreplicated" diffusing films) with rounded or curved
structures microreplicated from a tool having corresponding
structures made by removing material from the tool with a cutter,
as described in US 2012/0113622 (Aronson et al.), US 2012/0147593
(Yapel et al.), WO 2011/056475 (Barbie), and WO 2012/0141261
(Aronson et al.); and films (referred to herein at "Type II
Microreplicated" diffusing films) with flat-faceted structures
microreplicated from a tool having corresponding structures made by
an electroplating process, as described in US 2010/0302479 (Aronson
et al.). An SEM image of a representative portion of the structured
surface of a Type I Microreplicated diffusing film is shown in FIG.
4, and a similar image of a Type II Microreplicated diffusing film
is shown in FIG. 5. Still other microreplicated diffusing films
include films in which a tool surface is made to be structured by a
sandblasting procedure, and the structured surface is then imparted
to the film by microreplication from the tool. See e.g. U.S. Pat.
No. 7,480,097 (Nagahama et al.).
SUMMARY
[0004] In one aspect, the present description relates to an optical
film. The optical film includes a birefringent substrate and a
prismatic layer carried by the substrate, the prismatic layer
having a major surface comprising a plurality of side by side
linear prisms extending along a same prism direction. The optical
film also includes an embedded structured surface disposed between
the substrate and the prismatic layer including closely-packed
structures arranged such that ridges are formed between adjacent
structures, the structures being limited in size along two
orthogonal in-plane directions. The embedded structured surface has
a topography characterizable by a first and second Fourier power
spectrum associated with respective first and second orthogonal
in-plane directions, and to the extent the first Fourier power
spectrum includes one or more first frequency peak not
corresponding to zero frequency and being bounded by two adjacent
valleys that define a first baseline, any such first frequency peak
has a first peak ratio of less than 0.9, the first peak ratio being
equal to an area between the first frequency peak and the first
baseline divided by an area beneath the first frequency peak.
Further, to the extent the second Fourier power spectrum includes
one or more second frequency peak not corresponding to zero
frequency and being bounded by two adjacent valleys that define a
second baseline, any such second frequency peak has a second peak
ratio of less than 0.8, the second peak ratio being equal to an
area between the second frequency peak and the second baseline
divided by an area beneath the second frequency peak. The embedded
structured surface is characterized by a total ridge length per
unit area in plan view of less than 200 mm/mm.sup.2.
[0005] In another aspect, the present description relates to an
optical film that includes a birefringent substrate and a prismatic
layer carried by the substrate, the prismatic layer having a major
surface comprising a plurality of side by side linear prisms
extending along a same prism direction. The optical film also
includes an embedded structured surface disposed between the
substrate and the prismatic layer including closely-packed
structures, the embedded structured surface defining a reference
plane and a thickness direction perpendicular to the reference
plane. The embedded structured surface has a topography
characterizable by a first and second Fourier power spectrum
associated with respective first and second orthogonal in-plane
directions, and to the extent the first Fourier power spectrum
includes one or more first frequency peak not corresponding to zero
frequency and being bounded by two adjacent valleys that define a
first baseline, any such first frequency peak has a first peak
ratio of less than 0.9, the first peak ratio being equal to an area
between the first frequency peak and the first baseline divided by
an area beneath the first frequency peak. Further, to the extent
the second Fourier power spectrum includes one or more second
frequency peak not corresponding to zero frequency and being
bounded by two adjacent valleys that define a second baseline, any
such second frequency peak has a second peak ratio of less than
0.8, the second peak ratio being equal to an area between the
second frequency peak and the second baseline divided by an area
beneath the second frequency peak. The closely-packed structures
are characterized by equivalent circular diameters (ECDs) in the
reference plane and mean heights along the thickness direction and
an average aspect ratio of each structure equals the mean height of
the structure divided by the ECD of the structure. An average
aspect ratio of the structures is less than 0.15.
[0006] In yet another aspect, the present disclosure relates to an
optical film including a birefringent substrate and a prismatic
layer carried by the substrate, the prismatic layer having a major
surface including a plurality of side by side linear prisms
extending along a same prism direction. The optical film also
includes an embedded structured surface disposed between the
substrate and the prismatic layer including closely-packed
structures having curved base surfaces. The embedded structured
surface has a topography characterizable by a first and second
Fourier power spectrum associated with respective first and second
orthogonal in-plane directions, and to the extent the first Fourier
power spectrum includes one or more first frequency peak not
corresponding to zero frequency and being bounded by two adjacent
valleys that define a first baseline, any such first frequency peak
has a first peak ratio of less than 0.9, the first peak ratio being
equal to an area between the first frequency peak and the first
baseline divided by an area beneath the first frequency peak.
Further, to the extent the second Fourier power spectrum includes
one or more second frequency peak not corresponding to zero
frequency and being bounded by two adjacent valleys that define a
second baseline, any such second frequency peak has a second peak
ratio of less than 0.8, the second peak ratio being equal to an
area between the second frequency peak and the second baseline
divided by an area beneath the second frequency peak. The embedded
structured surface provides an optical have of less than 95%.
[0007] In another aspect, the present disclosure relates to an
optical film including a birefringent substrate and a prismatic
layer carried by the substrate, the prismatic layer having a major
surface including a plurality of side by side linear prisms
extending along a same prism direction. The optical film also
includes an embedded structured surface disposed between the
substrate and the prismatic layer including closely-packed
structures. The embedded structured surface has a topography
characterizable by a first and second Fourier power spectrum
associated with respective first and second orthogonal in-plane
directions, and to the extent the first Fourier power spectrum
includes one or more first frequency peak not corresponding to zero
frequency and being bounded by two adjacent valleys that define a
first baseline, any such first frequency peak has a first peak
ratio of less than 0.9, the first peak ratio being equal to an area
between the first frequency peak and the first baseline divided by
an area beneath the first frequency peak. Further, to the extent
the second Fourier power spectrum includes one or more second
frequency peak not corresponding to zero frequency and being
bounded by two adjacent valleys that define a second baseline, any
such second frequency peak has a second peak ratio of less than
0.8, the second peak ratio being equal to an area between the
second frequency peak and the second baseline divided by an area
beneath the second frequency peak. The embedded structured surface
provides an optical haze in a range from 10 to 60% and an optical
clarity in a range from 10 to 40%.
[0008] In another aspect, the present disclosure relates to an
optical film including a birefringent substrate and a prismatic
layer carried by the substrate, the prismatic layer having a major
surface including a plurality of side by side linear prisms
extending along a same prism direction. The optical film also
includes an embedded structured surface disposed between the
substrate and the prismatic layer including larger first structures
and smaller second structures, the first and second structures both
being limited in size along two orthogonal in-plane directions. The
first structures are non-uniformly arranged on the embedded
structured surface and the second structures are closely packed and
non-uniformly dispersed between the first structures. An average
size of the first structures is greater than 15 microns and an
average size of the second structures is less than 15 microns.
[0009] In yet another aspect, the present disclosure related to an
optical film including a birefringent substrate and a prismatic
layer carried by the substrate, the prismatic layer having a major
surface including a plurality of side by side linear prisms
extending along a same prism direction. The embedded structured
surface is made by microreplication from a tool structured surface,
the tool structured surface being made by forming a first layer of
a metal by electrodepositing the metal using a first electroplating
process resulting in a major surface of the first layer having a
first average roughness, and forming a second layer of the metal on
the major surface of the first layer by electrodepositing the metal
on the first layer using a second electroplating process resulting
in a major surface of the second layer having a second average
roughness smaller than the first average roughness, the major
surface of the second layer corresponding to the tool structured
surface.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is an SEM image of a portion of the beaded surface of
a CCS diffuser (optical haze=72%, optical clarity=9.9%), and FIG.
1A is an SEM image of such surface in cross section.
[0011] FIG. 2 is an SEM image of a portion of the beaded surface of
a DPB diffuser (optical haze=97.5%, optical clarity=5%).
[0012] FIG. 3 is an SEM image of a portion of the beaded surface of
an SDB diffuser (optical haze=67%, optical clarity=30%).
[0013] FIG. 4 is an SEM image of a portion of the structured
surface of a Type I Microreplicated diffusing film (optical
haze=91.3%, optical clarity=1.9%).
[0014] FIG. 5 is an SEM image of a portion of the structured
surface of a Type II Microreplicated diffusing film (optical
haze=100%, optical clarity=1.3%).
[0015] FIG. 6 is a schematic side or sectional view of an optical
system that includes a microreplicated optical film having a
birefringent substrate.
[0016] FIG. 7 is a schematic perspective view of a microreplicated
optical film having an array of linear prisms, the figure
demonstrating various prism configurations that may be used.
[0017] FIG. 8 is a schematic side or sectional view of an optical
diffusing film having a structured surface.
[0018] FIG. 9 is a schematic flow diagram depicting steps used to
make structured surface articles, including structured surface
tools and structured surface optical films.
[0019] FIG. 10 is a schematic perspective view of a structured
surface tool in the form of a cylinder or drum.
[0020] FIG. 11A is a schematic side or sectional view of a portion
of the tool of FIG. 10;
[0021] FIG. 11B is a schematic side or sectional view of the tool
portion of FIG. 11A during a microreplication procedure in which it
is used to make the structured surface of an optical diffusing
film.
[0022] FIG. 11C is a schematic side or sectional view of a portion
of the optical diffusing film made which results from the
microreplication procedure depicted in FIG. 11B.
[0023] FIG. 12 is a graph of optical clarity vs. optical haze, each
point on the graph depicting a different optical diffusing film
sample made using a process in accordance with FIG. 9;
[0024] FIG. 13 is an SEM image of a representative portion of the
structured surface of an optical diffusing film sample referred to
as "502-1", and FIG. 13A is an SEM image of the 502-1 sample in
cross-section;
[0025] FIG. 14 is an SEM image of a representative portion of the
structured surface of an optical diffusing film sample referred to
as "594-1";
[0026] FIG. 15 is an SEM image of a representative portion of the
structured surface of an optical diffusing film sample referred to
as "599-1";
[0027] FIG. 16 is an SEM image of a representative portion of the
structured surface of an optical diffusing film sample referred to
as "502-2";
[0028] FIG. 17 is an SEM image of a representative portion of the
structured surface of an optical diffusing film sample referred to
as "RA22a";
[0029] FIG. 18 is an SEM image of a representative portion of the
structured surface of an optical diffusing film sample referred to
as "RA13a";
[0030] FIG. 19 is an SEM image of a representative portion of the
structured surface of an optical diffusing film sample referred to
as "N3";
[0031] FIG. 20 is an SEM image of a representative portion of the
structured surface of an optical diffusing film sample referred to
as "593-2";
[0032] FIG. 21 is an SEM image of a representative portion of the
structured surface of an optical diffusing film sample referred to
as "597-2";
[0033] FIG. 22 is a graph of power spectral density vs. spatial
frequency, the graph including a hypothetical curve used to
demonstrate how the degree of irregularity or randomness of a
structured surface along a given in-plane direction can be
characterized by a Fourier power spectrum associated with such
in-plane direction;
[0034] FIG. 23A is a graph of power spectral density vs. spatial
frequency in a downweb direction for a sample of the Type I
Microreplicated diffusing film (optical haze=91.3%, optical
clarity=1.9%), and FIG. 23B is a similar graph for the same sample
but in a perpendicular (crossweb) in-plane direction;
[0035] FIG. 24A is a graph of power spectral density vs. spatial
frequency in a downweb direction for the optical diffusing film
sample 502-1, and FIG. 24B is a similar graph for the same sample
but in the crossweb direction;
[0036] FIG. 25 is a schematic plan view of a portion of a
hypothetical structured surface with distinguishable structures,
demonstrating the concept of equivalent circular diameter
(ECD);
[0037] FIG. 26 is a composite image of a picture of the CCS
diffuser through a confocal microscope, on which dark shapes
representing the outer boundaries or edges of individual structures
of the structured surface are superimposed;
[0038] FIG. 27 is a composite image of a picture of a Type I
Microreplicated diffusing film sample (optical haze=91.3%, optical
clarity=1.9%) through a confocal microscope, on which dark shapes
representing the outer boundaries or edges of individual structures
of the structured surface are superimposed;
[0039] FIG. 28 is a composite image similar to FIGS. 26 and 27, but
for the optical diffusing film sample 594-1;
[0040] FIG. 29 is a composite image similar to FIGS. 26 through 28,
but for the optical diffusing film sample 502-1;
[0041] FIG. 30 is a graph of normalized count versus ECD for a
representative sampled area of the optical diffusing film sample
502-1;
[0042] FIG. 31 is a schematic side or sectional view of a portion
of a hypothetical structured surface with distinguishable
structures, demonstrating the concept of maximum height or
depth;
[0043] FIG. 32 is a schematic plan view of hypothetical individual
structures on a structured surface, demonstrating criterion used to
determine the presence of a ridge on the structured surface;
[0044] FIG. 33A is a composite image of a picture of the optical
diffusing film sample 594-1 through a confocal microscope, on which
dark line segments representing ridges that were detected on the
structured surface are superimposed;
[0045] FIG. 33B is an image that shows only the dark line segments
of FIG. 34a, i.e., only the detected ridges, in reverse printing
(dark/light reversed); and
[0046] FIGS. 34A and 34B are analogous to FIGS. 33A and 33B
respectively, but for the DPB diffuser.
DETAILED DESCRIPTION
[0047] In FIG. 6, an optical system 610 includes a microreplicated
optical film 619 disposed between an extended light source 602,
such as a planar light guide with an extended output surface that
emits white light, and a polarizer 604. The optical system 610 may
be an optical display, backlight, or similar system, and it may
include other components that are not shown in the figure, such as
a liquid crystal panel and additional polarizers, diffusers,
retarders, and/or other optical films or components. For purposes
of the present description, we ignore such other components for
ease of explanation. The optical film 619, which has a front major
surface 619a and a back or rear major surface 619b, is shown to be
constructed from a substrate 620 that carries a prismatic layer
650, although other layer configurations may also be used. The
substrate 620 may be said to carry the prismatic layer 650 even in
cases where one or more intervening layers physically connect the
substrate to the prismatic layer. The prismatic layer 650 may be
made by casting and curing a polymer composition onto a polymer
film substrate 620 using a micropatterned tool. The tool is
configured so that a first major surface 650a of the prismatic
layer 650, which coincides with the front major surface 619a of the
film 619, is microstructured replica of the tool, with distinct
faces or facets that form an array of linear prisms. Besides
casting-and-curing, other known manufacturing techniques can also
be used to form the microstructured surface 650a, such as
embossing, etching, and/or other known techniques. A second major
surface 650b of the prismatic layer 650 coincides with a first
major surface 620a of the substrate 620. A second major surface
620b of the substrate 620 coincides with the back major surface
619b of the film 619.
[0048] A Cartesian x-y-z coordinate system is included in the
figure for reference purposes. The film 619 extends generally
parallel to the x-y plane, and an optical axis of the system 610
may correspond to the z-axis. Each of the prisms of the structured
surface extends in a generally linear direction, at least in plan
view, parallel to the y-axis. The array of linear prisms refracts
light in such a way that the on-axis brightness or luminance of the
system is increased, compared to the same system without the film
619.
[0049] The substrate 620 that carries the prismatic layer 650 is
birefringent. The birefringence may be an intentional design
feature, or it may be unintentional. Films made from polyethylene
terephthalate (PET), for example, can be economically made to have
desirable mechanical and optical properties for use in optical film
applications, but films made from PET may exhibit non-negligible
amounts of birefringence. The birefringence may be substantially
spatially uniform, i.e., the birefringence at one position within
the substrate may be substantially the same as the birefringence at
other positions within the substrate. The birefringence is
typically characterized at least by an in-plane birefringence. That
is, if the substrate has refractive indices nx, ny, nz for light
polarized along the x-, y-, and z-axes, respectively, then a
significant difference exists between the in-plane refractive
indices nx and ny. The x- and y-directions may correspond, for
example, to cross-web and down-web directions of a polymer film.
The magnitude of nx-ny may typically be at least 0.01, or 0.02, or
0.03. The question of whether a particular refractive index
difference is significant can depend on the thickness of the
substrate: a small refractive index difference may be negligible
for a thin substrate, but significant for a thicker substrate.
[0050] In the figure, an arbitrary light ray 603 is shown traveling
from the light source 602 to an observer 601. Following this light
ray, we see that it is refracted at the major surface 620b (619b),
propagates through the substrate 620, is refracted again at the
major surface 620a (650b), propagates through the prismatic layer
650, is refracted again at the major surface 650a (619a), travels
to the polarizer 604, and one polarization component of the ray
passes through the polarizer and travels on to the observer 601.
The ray 603 is assumed to be unpolarized as it leaves the light
source 602 and before it strikes the film 619. When it strikes the
air/substrate interface at major surface 620b, it becomes partially
polarized because orthogonal s- and p-polarization states are in
general transmitted (and reflected) differently, depending on the
angle of incidence and the refractive indices of the substrate. The
reflected light components are not shown in FIG. 6 for ease of
explanation. A double-headed arrow is superimposed on the ray 603
near the surface 620b to indicate the partial polarization as the
light ray 603 begins its path through the substrate 620. As the ray
603 propagates through the substrate 620 toward the surface 620a,
its state of partial polarization is, in general, changed due to
the birefringence of the substrate 620. This change in polarization
state is dependent not only on the amount of birefringence (and the
thickness) of the substrate, but also on the angle of propagation
of the light ray and the wavelength of the light ray. The changed
polarization state is depicted in the drawing as a small ellipse
superimposed on the ray 603 near the surface 620a. The light ray
with its modified polarization state then is refracted by the prism
layer 650, and the polarization component that is aligned with the
pass axis of the polarizer 604 passes through the polarizer 604 and
to the observer 601.
[0051] As mentioned above, the change in polarization state
occurring within the substrate 620 depends on the wavelength of the
light. This is so even if the substrate material exhibits no
dispersion whatsoever. As a result, light rays of different
wavelengths that follow the same or nearly the same path through
the system 610, such as the path traced out by ray 603, will in
general be transmitted in different relative amounts to the
observer 601. The relative amounts will depend on the direction of
propagation of the light ray, and we assume that a range or cone of
propagation directions are present as a result of the source 602
emitting light over a significant angular range, e.g. in a
Lambertian distribution or in another suitable angular
distribution.
[0052] The prisms in FIG. 6 and in other figures below are shown as
having nominally the same geometry including height, width, and
apex angle. This is primarily for simplicity of illustration. In
general, unless otherwise stated, the prisms of the prismatic layer
may have any of a wide variety of configurations, as suggested by
FIG. 2.
[0053] In FIG. 7, a microreplicated optical film 719 is shown that
may function as a brightness enhancement film in a display,
backlight, or other system. The optical film 719 includes an array
of linear prisms or microstructures 751 for improving brightness.
The optical film 719 includes a first major or structured surface
719a that includes a plurality of microstructures or linear prisms
751 that extend along the y-direction. The film 719 includes a
second major surface 719b that is opposite the first major or
structured surface 719a.
[0054] The film 719 includes a substrate layer 720 that includes a
first major surface 720a and an opposing second major surface 720b,
which coincides with major surface 719b. Optical film 719 includes
a prismatic layer 750 that is carried by the substrate layer 720.
The prismatic layer 750 is disposed on the major surface 720a of
the substrate layer, which surface 720a coincides with a major
surface 750b of the layer 750, the layer 750 also including another
major surface 750a which coincides with major surface 719a of the
film 719.
[0055] The optical film 719 includes two layers: substrate layer
720, which for purposes of this description is assumed to be
birefringent, and prismatic layer 750. In general, the optical film
719 can have one or more layers. For example, in some cases, the
optical film 719 can have only a single layer that includes
respective first and second major surfaces 719a, 719b. As another
example, in some cases, the optical film 719 can have many layers.
For example, in some cases, the substrate 720 may be composed of
multiple distinct layers. When the optical film includes multiple
layers, the constituent layers are typically coextensive with each
other, and each pair of adjacent constituent layers comprise
tangible optical materials and have major surfaces that are
completely coincident with each other, or that physically contact
each other at least over 80%, or at least 90%, of their respective
surface areas.
[0056] Prisms 751 may be designed to redirect light that is
incident on major surface 719b of the optical film 719, along a
desired direction, such as along the positive z-direction. In the
exemplary optical film 719, prisms 751 are linear prismatic
structures. In general, the prisms 751 can be any type of prisms or
prism-like microstructures that are capable of redirecting light
by, for example, refracting a portion of incident light and
recycling a different portion of the incident light. For example,
the cross-sectional profiles of prisms 751 can be or include curved
and/or piece-wise linear portions.
[0057] Each of the prisms 751 includes an apex angle 752 and a
height measured from a common reference plane such as, for example,
major surface 750b. Individual prisms 751a, 751b, 751c, etc., are
shown with heights 753a, 753b, 753c, . . . , 753e, and so forth. In
some cases, e.g. when it is desirable to reduce optical coupling or
wet-out and/or improve durability of the light redirecting optical
film, the height of a given prism 751 can change along the
y-direction. For example, the prism height of linear prism 751a
varies along the y-direction. In such cases, prism 751a has a local
height 753a that varies along the y-direction, the varying height
defining a maximum height and an average height. In some cases, a
prism, such as linear prism 75k, has a constant height along the
y-direction. In such cases, the prism has a constant local height
753c that is equal to the prism's maximum height and average
height.
[0058] In some cases, such as when it is desirable to reduce
optical coupling or wet-out, some of the linear prisms are shorter
and some are taller. For example, height 753c of linear prism 75k
is smaller than height 753b of linear prism 751b.
[0059] The apex or dihedral angle 752 of each prism can have any
value that may be desirable in an application. For example, in some
cases, apex angle 752 can be in a range from about 70 degrees to
about 110 degrees, or from about 80 degrees to about 100 degrees,
or from about 85 degrees to about 95 degrees. In some cases, the
prisms 751 have equal apex angles which can, for example, be in a
range from about 88 or 89 degrees to about 92 or 91 degrees, such
as 90 degrees.
[0060] Prismatic layer 750 can be composed of any suitable
light-transmissive material and may have any suitable index of
refraction. For example, in some cases, the prismatic layer may
have an index of refraction in a range from about 1.4 to about 1.8,
or from about 1.5 to about 1.8, or from about 1.5 to about 1.7. In
some cases, the prismatic layer may have an index of refraction
that is not less than about 1.5, or not less than about 1.55, or
not less than about 1.6, or not less than about 1.65, or not less
than about 1.7. The prismatic layer may be entirely or partially
birefringent, and it may be entirely or partially (substantially)
isotropic.
[0061] In most cases, such as when the optical film 719 is used in
a liquid crystal display system, the optical film 719 increases the
on-axis brightness of the display, i.e., the brightness as measured
along the z-axis, when compared to the identical display without
the optical film 719. For purposes of quantifying the improvement
in axial luminance, the optical film 719 is said to have an
"effective transmission", or relative "gain", that is greater than
1. As used herein, "effective transmission" ("ET") refers to the
ratio of the on-axis luminance with the film in place to the
on-axis luminance of the display system without the film in place,
when the light source is a Lambertian or nearly Lambertian source
with a diffuse reflectivity >80%.
[0062] The ET of the optical film can be measured using an optical
system that includes a hollow Lambertian light box, a linear light
absorbing polarizer, and a photodetector centered on an optical
axis of the light box. The hollow light box may be illuminated by a
stabilized broadband light source connected to an interior of the
light box via an optical fiber, and the light emitted from an
emitting or exit surface of the light box may have a Lambertian
luminance distribution. The optical film or other test sample whose
ET is to be measured is placed at a location between the light box
and the absorbing linear polarizer. Dividing the photodetector
output with the optical film present in the system by the
photodetector output with the optical film absent from the system
yields the ET for the optical film.
[0063] A suitable photodetector for use in measuring ET is a
SpectraScan.TM. PR-650 SpectraColorimeter, available from Photo
Research, Inc, Chatsworth, Calif. A suitable light box for such
measurements is a Teflon cube having a total reflectance of about
85%.
[0064] The ET of the optical film 719 can be measured by placing
the optical film 719 at the specified location with the major
surface 719a (and the linear prisms 751) facing the photodetector
and the major surface 719b facing the light box. Next, the
spectrally weighted axial luminance I1 (the luminance along the
optical axis) is measured through the linear absorbing polarizer by
the photo detector. The optical film 719 is then removed and the
spectrally weighted luminance I2 is measured without the optical
film 719. ET is the ratio I1/I2. The ET may be specified in further
detail by specifying the orientation of the optical film relative
to the linear absorbing polarizer. For example, "ET0" refers to the
effective transmission when the optical film is oriented such that
each of the prisms 751 extends along a direction that is parallel
to the pass axis of linear absorbing polarizer, and "ET90" refers
to the effective transmission when the optical film is oriented
such that each of the prisms 751 extends along a direction that is
perpendicular to the pass axis of the linear absorbing polarizer.
Further in this regard, the "average effective transmission"
("ETA") is the average of ET0 and ET90. In view of this additional
terminology, the term "effective transmission" or "ET" referred to
earlier, without more, refers to the average effective transmission
of the optical film.
[0065] In exemplary cases, the disclosed microreplicated optical
films, including optical film 719, are configured to increase
system brightness, and the linear prisms have a refractive index of
at least about 1.6, and the average effective transmission (ETA) of
the optical film is at least about 1.3, or at least 1.5, or at
least 1.7, or at least 1.9, or at least 2.1.
[0066] Light diffusion or scattering can be expressed in terms of a
parameter called "optical haze" or simply "haze". For a film,
surface, or other object that is illuminated by a normally incident
light beam, the optical haze of the object refers to the ratio of
transmitted light that deviates from the normal direction by more
than 4 degrees to the total transmitted light. Haze can be
calculated in a simulation, and for actual samples it can be
measured using a Haze-Gard Plus haze meter (available from
BYK-Gardner, Columbia, Md.) according to the procedure described in
ASTM D1003, or with other suitable procedures. Related to optical
haze is optical clarity, which refers to the ratio
(T.sub.1-T.sub.2)/(T.sub.1+T.sub.2), where T.sub.1 is the
transmitted light that deviates from the normal direction between
1.6 and 2 degrees from the normal direction, and T.sub.2 is the
transmitted light that lies between zero and 0.7 degrees from the
normal direction. Clarity values may also be measured using the
Haze-Gard Plus haze meter from BYK-Gardiner.
[0067] In some embodiments, no air gap is provided between the
prismatic layer and the birefringent substrate, and the light
scattering or haze is provided by an embedded structured surface
rather than an exposed structured surface. The structured surface
can then be said to be buried or embedded, because it is bounded on
opposite sides by light-transmissive materials that are solid or
otherwise tangible, for example, suitable light-transmissive
polymer materials.
[0068] In some embodiments, structured surface is configured in
such a way that a substantial majority of the surface, for example,
at least 80% or at least 90% of the structured surface in plan
view, does not exhibit focusing properties. One way this can be
achieved is to configure the structured surface such that a
substantial majority of the surface is made up of portions that
curve in a same orientation, e.g., toward or away from the prisms
of the prismatic layer. Each such curved portion of the structured
surface can be referred to as a lenslet. In some embodiments, for
example, portions of a structured surface may all curve generally
away from the prism layer, and may be considered to be lenslets. In
some configurations, the lenslets will be defocusing, i.e. they
will each defocus incident collimated light due to a difference in
refractive index between layers. In some embodiments, at least 80%
of the structured surface is covered or occupied by the lenslets. A
substantial minority of the structured surface which preferably
cover or occupy less than 20% or less than 10% of the surface may
be curved in such a way as to have focusing properties.
[0069] Numerous design variations can be employed in the disclosed
optical films, including in particular the optical films that
incorporate an embedded structured surface. In addition to the
particular layer arrangements shown and described in connection
with the drawings, the films may include additional layers and/or
coatings to provide desired optical and/or mechanical
functionality. Any of the described layers may be constructed using
two or more distinct sub-layers. Similarly, any two or more
adjacent layers may be combined into, or replaced with, a single
unitary layer. Wide varieties of prism designs, film or layer
thicknesses, and refractive indices may be used. The prismatic
layer can have any suitable index of refraction, e.g., in a range
from about 1.4 to about 1.8, or from about 1.5 to about 1.8, or
from about 1.5 to about 1.7, or not less than about 1.5, or not
less than about 1.55, or not less than about 1.6, or not less than
about 1.65, or not less than about 1.7. The birefringent substrate
may have a typical birefringence, including an in-plane
birefringence, as discussed above. In some cases, dyes, pigments,
and/or particles (including scattering particles or other suitable
diffusing agents) can be included in one or more of the layers or
components of the optical films for desired functionality. Although
polymer materials are sometimes preferred for use in the disclosed
optical films for functionality and economy, other suitable
materials may also be used.
[0070] Nanovoided materials, including those having an ultra low
index (ULI), e.g. a refractive index of less than 1.4, or less than
1.3, or less than 1.2, or in a range from 1.15 to 1.35, may also be
used in the disclosed optical films. Many such ULI materials may be
described as porous materials or layers. When used in combination
with more common optical polymer materials that are not nanovoided,
and that have substantially higher refractive indices such as
greater than 1.5 or greater than 1.6, a relatively large refractive
index difference .DELTA.n can be provided across the embedded
structured surface. Suitable ULI materials are described e.g. in WO
2010/120864 (Hao et al.) and WO 2011/088161 (Wolk et al.), which
are incorporated herein by reference.
[0071] We have developed a process that can be used to form
structured surfaces that are well suited for making high
performance optical diffusing films, including embedded structured
surfaces used in conjunction with, for example, the configuration
of FIG. 6. The process can produce a structured surface in a
microreplication tool of considerable surface area, e.g., a surface
area at least as large as that of a typical desktop computer
display screen, in a period of time that is short compared to the
time it would take to produce a structured surface of equal area
and comparable feature size by cutting features in a substrate with
a cutting tool. This is because the process can employ
electroplating techniques rather than cutting techniques to produce
the structured surface. (However, in some cases described further
below, electroplating can be used in addition to cutting.) The
process can be tailored to produce a wide variety of structured
surfaces, including structured surfaces that provide very high haze
(and low clarity), structured surfaces that provide very low haze
(and high clarity), and structured surfaces in between these
extremes. The process can utilize a first electroplating procedure
in which a preliminary structured surface is produced, the
preliminary structured surface corresponding substantially to that
of a Type II Microreplicated diffusing film discussed above. Recall
in connection with FIG. 6 that Type II Microreplicated diffusing
films cover a general design space that has relatively high optical
clarity. We have found that by covering the preliminary structured
surface with a second electrodeposited layer using a second
electroplating procedure, a second structured surface is obtained,
and the second structured surface can produce diffusing films of
high, low, or intermediate haze, depending on process conditions;
however, diffusing films made from the second structured surface
are different from those made from the preliminary structured
surface. In particular, interestingly, diffusing films made from
the second structured surface fall within a general design space
having a substantially lower clarity (for intermediate values of
haze) than the design space for Type II Microreplicated diffusing
films. This will be shown in connection with optical diffusing
films made in accordance with the developed process. At least some
of the optical diffusing films are also shown to possess other
desirable characteristics, including a topography characterized by
little or no spatial periodicity, and average feature sizes less
than 15 microns, or less than 10 microns.
[0072] FIG. 8 depicts in schematic side or sectional view a portion
of a representative diffusing optical film 820 that can be made
with the disclosed processes. The film 820 is shown to have a first
major surface 820a and a second major surface 820b. Incident light
830 is shown impinging on the film 820 at the second surface 820b.
The light 830 passes through the film, and is scattered or diffused
as a result of refraction (and to some extent diffraction) at the
roughened or structured topography of the major surface 820a,
producing scattered or diffuse light 832. We may thus refer to the
major surface 820a alternatively as a structured surface 820a. The
orientation of the film 820 relative to the incident light 830 may
of course be changed such that the light 830 impinges initially on
the structured surface 820a, in which case refraction at the
structured surface again produces scattered or diffuse light.
[0073] The structured surface 820a extends generally along
orthogonal in-plane directions, which can be used to define a local
Cartesian x-y-z coordinate system. The topography of the structured
surface 820a can then be expressed in terms of deviations along a
thickness direction (z-axis), relative to a reference plane (the
x-y plane) lying parallel to the structured surface 820a. In many
cases, the topography of the structured surface 820a is such that
distinct individual structures can be identified. Such structures
may be in the form of protrusions, which are made from
corresponding cavities in the structured surface tool, or cavities,
which are made from corresponding protrusions in the structured
surface tool. The structures are typically limited in size along
two orthogonal in-plane directions, i.e., when the structured
surface 820a is seen in plan view, individual structures do not
typically extend indefinitely in a linear fashion along any
in-plane direction. Whether protrusions or cavities, the structures
may also in some cases be closely packed, i.e., arranged such that
at least portions of boundaries of many or most adjacent structures
substantially meet or coincide. The structures are also typically
irregularly or non-uniformly dispersed on the structured surface
820a. In some cases, some, most, or substantially all (e.g.,
>90%, or >95%, or >99%) of the structures may be curved or
comprise a rounded or otherwise curved base surface. In some cases,
at least some of the structures may be pyramidal in shape or
otherwise defined by substantially flat facets. The size of a given
structure may be expressed in terms of an equivalent circular
diameter (ECD) in plan view, and the structures of a structured
surface may have an average ECD of less than 15 microns, or less
than 10 microns, or in a range from 4 to 10 microns, for example.
The structured surface and structures can also be characterized
with other parameters as discussed elsewhere herein, e.g., by an
aspect ratio of the depth or height to a characteristic transverse
dimension such as ECD, or the total length of ridges on the surface
per unit area in plan view. The optical haze, optical clarity, and
other characteristics of the optical diffusing films can be
provided without the use of any beads at or on the structured
surface, or elsewhere within the optical film.
[0074] Among the various parameters that can be used to
characterize the optical behavior of a given optical diffusing
film, two key parameters are optical haze and optical clarity.
Light diffusion or scattering can be expressed in terms of "optical
haze", or simply "haze". For a film, surface, or other object that
is illuminated by a normally incident light beam, the optical haze
of the object refers essentially to the ratio of transmitted light
that deviates from the normal direction by more than 4 degrees to
the total transmitted light as measured, for example, using a
Haze-Gard Plus haze meter (available from BYK-Gardner, Columbia,
Md.) according to the procedure described in ASTM D1003, or with a
substantially similar instrument and procedure. Related to optical
haze is optical clarity, which is also measured by the Haze-Gard
Plus haze meter from BYK-Gardner, but where the instrument is
fitted with a dual sensor having a circular middle sensor centered
within an annular ring sensor, the optical clarity referring to the
ratio (T.sub.1-T.sub.2)/(T.sub.1+T.sub.2), where T.sub.1 is the
transmitted light sensed by the middle sensor and T.sub.2 is the
transmitted light sensed by the ring sensor, the middle sensor
subtending angles from zero to 0.7 degrees relative to an axis
normal to the sample and centered on the tested portion of the
sample, and the ring sensor subtending angles from 1.6 to 2 degrees
relative to such axis, and where the incident light beam, with no
sample present, overfills the middle sensor but does not illuminate
the ring sensor (underfills the ring sensor by a half angle of 0.2
degrees).
[0075] FIG. 9 shows an exemplary version 901 of the process. In a
step 902 of the process, a base or substrate is provided that can
serve as a foundation upon which metal layers can be electroplated.
The substrate can take one of numerous forms, e.g. a sheet, plate,
or cylinder. Circular cylinders are advantageous in that they can
be used to produce continuous roll goods. The substrate is
typically made of a metal, and exemplary metals include nickel,
copper, and brass. Other metals may however also be used. The
substrate has an exposed surface ("base surface") on which
electrodeposited layers will be formed in subsequent steps. The
base surface may be smooth and flat, or substantially flat. The
curved outer surface of a smooth polished cylinder may be
considered to be substantially flat, particularly when considering
a small local region in the vicinity of any given point on the
surface of the cylinder. The base surface may be characterized by a
base average roughness. In this regard, the surface "roughness" of
the base surface, or the "roughness" of other surfaces mentioned
herein, may be quantified using any generally accepted roughness
measure, such as average roughness R.sub.a or root mean squared
roughness R.sub.rms, and the roughness is assumed to be measured
over an area large enough to be fairly representative of the entire
relevant area of the surface at issue.
[0076] In a step 903 of the process 901, a first layer of a metal
is formed on the base surface of the substrate using a first
electroplating process. Before this step is initiated, the base
surface of the substrate may be primed or otherwise treated to
promote adhesion. The metal may be substantially the same as the
metal of which the base surface is composed. For example, if the
base surface comprises copper, the first electroplated layer formed
in step 903 may also be made of copper. To form the first layer of
the metal, the first electroplating process uses a first
electroplating solution. The composition of the first
electroplating solution, e.g., the type of metal salt used in the
solution, as well as other process parameters such as current
density, plating time, and substrate speed, are selected so that
the first electroplated layer is not formed smooth and flat, but
instead has a first major surface that is structured, and
characterized by irregular flat-faceted features. The size and
density of the irregular features are determined by the current
density, plating time, and substrate speed, while the type of metal
salt used in the first electroplating solution determines the
geometry of the features. Further teaching in this regard can be
found in patent application publication US 2010/0302479 (Aronson et
al.). The first plating process is carried out such that the first
major surface of the first electroplated layer has a first average
roughness that is greater than the base average roughness of the
substrate. The structured character and roughness of a
representative first major surface can be seen in the SEM image of
FIG. 5, which shows the structured surface of a Type II
Microreplicated diffusing film, the film being microreplicated from
the first major surface of a first electroplated layer made in
accordance with step 903.
[0077] After the first electroplated layer of the metal is made in
step 903, with its structured major surface of first average
roughness, a second electroplated layer of the metal is formed in
step 904 using a second electroplating process. The second layer of
the metal covers the first electroplated layer, and, since their
compositions may be substantially the same, the two electroplated
layers may no longer be distinguishable, and the first major
surface of the first layer may become substantially obliterated and
no longer detectable. Nevertheless, the second electroplating
process differs from the first electroplating process in such a way
that the exposed second major surface of the second electroplated
layer, although structured and non-flat, has a second average
roughness that is less than the first average roughness of the
first major surface. The second electroplating process may differ
from the first electroplating process in a number of respects in
order to provide the second major surface with a reduced roughness
relative to the first major surface.
[0078] In some cases, the second electroplating process of step 904
may use a second electroplating solution that differs from the
first electroplating solution in step 903 at least by the addition
of an organic leveler, as shown in box 904a. An organic leveler is
a material that introduces into a plating bath an ability to
produce deposits relatively thicker in small recesses and
relatively thinner on small protrusions with an ultimate decrease
in the depth or height of the small surface irregularities. With a
leveler, a plated part will have greater surface smoothness than
the basis metal. Exemplary organic levelers may include, but are
not limited to, sulfonated, sulfurized hydrocarbyl compounds; allyl
sulfonic acid; polyethylene glycols of various kinds; and
thiocarbamates, including bithiocarbamates or thiourea and their
derivatives. The first electroplating solution may contain, at
most, trace amounts of an organic leveler. The first electroplating
solution may have a total concentration of organic carbon less than
100, or 75, or 50 ppm. A ratio of a concentration of an organic
leveler in the second electroplating solution to a concentration of
any organic leveler in the first electroplating solution may be at
least 50, or 100, or 200, or 500, for example. The average
roughness of the second major surface can be tailored by adjusting
the amount of organic leveler in the second electroplating
solution.
[0079] The second electroplating process of step 904 may also or
alternatively differ from the first electroplating process of step
903 by including in the second step 904 at least one electroplating
technique or feature whose effect is to reduce the roughness of the
second major surface relative to the first major surface. Thieving
(box 904b) and shielding (box 904c) are examples of such
electroplating techniques or features. Furthermore, in addition to
or instead of an organic leveler, one or more organic grain
refiners (box 904d) may be added to the second electroplating
solution to reduce the average roughness of the second major
surface.
[0080] After step 904 is completed, the substrate with the first
and second electroplated layers may be used as an original tool
with which to form optical diffusing films. In some cases the
structured surface of the tool, i.e., the structured second major
surface of the second electroplated layer produced in step 904, may
be passivated or otherwise protected with a second metal or other
suitable material. For example, if the first and second
electroplated layers are composed of copper, the structured second
major surface can be electroplated with a thin coating of chromium.
The thin coating of chromium or other suitable material is
preferably thin enough to substantially preserve the topography and
the average roughness of the structured second major surface.
[0081] Rather than using the original tool itself in the
fabrication of optical diffusing films, one or more replica tools
may be made by microreplicating the structured second major surface
of the original tool, and the replica tool(s) may then be used to
fabricate the optical films. A first replica made from the original
tool will have a first replica structured surface which corresponds
to, but is an inverted form of, the structured second major
surface. For example, protrusions in the structured second major
surface correspond to cavities in the first replica structured
surface. A second replica may be made from the first replica. The
second replica will have a second replica structured surface which
corresponds to, and is a non-inverted form of, the structured
second major surface of the original too.
[0082] After step 904, after the structured surface tool is made,
optical diffusing films having the same structured surface (whether
inverted or non-inverted relative to the original tool) can be made
in step 906 by microreplication from the original or replica tool.
The optical diffusing film may be formed from the tool using any
suitable process, including e.g. embossing a pre-formed film, or
cast-and-curing a curable layer on a carrier film.
[0083] Turning now to FIG. 10, pictured there is a schematic view
of a structured surface tool 1010 in the form of a cylinder or
drum. The tool 1010 has a continuous major surface 1010a that we
assume has been processed in accordance with the method of FIG. 9
so that it has an appropriately structured surface. The tool has a
width w and a radius R. The tool can be used in a continuous film
manufacturing line to make optical diffusing film by
microreplication. A small portion P of the tool 1010, or of an
identical tool, is shown schematically in FIG. 11A.
[0084] In FIG. 11A, a structured surface tool 1110, assumed to be
identical to tool 1010, is shown in schematic cross-section. Having
been made by the process of FIG. 9, the tool 1110 is shown in the
figure as including a substrate 1112, a first electroplated layer
1114 of a metal having a structured first major surface 1114a, and
a second electroplated layer 1116 of the metal, the second layer
1116 having a structured second major surface 1116a which coincides
with the structured major surface 1110a of the tool 1110. In
accordance with the teachings of FIG. 9, the second major surface
1116a is structured or non-smooth, and it has an average roughness
less than that of the first major surface 1114a. The first major
surface 1114a, and the distinct layers 1114, 1116, are shown for
reference purposes in FIG. 11a, however, as noted above, the
formation of the second electroplated layer 1116 atop the first
electroplated layer 1114 may render the first major surface 1114a,
and the distinction between layers 1114 and 1116, undetectable.
[0085] In FIG. 11B, we show a schematic view of the tool 1110 of
FIG. 11A during a microreplication procedure in which it is used to
make the structured surface of an optical diffusing film 1120. Like
reference numerals from FIG. 11A designate like elements, and need
not be discussed further. During microreplication, the film 1120 is
pressed against the tool 1110 so that the structured surface of the
tool is transferred (in inverted form) with high fidelity to the
film. In this case, the film is shown to have a base film or
carrier film 1122 and a patterned layer 1124, but other film
constructions can also be used. The patterned layer may be for
example a curable material, or a thermoplastic material suitable
for embossing. The microreplication process causes the major
surface 1120a of the optical film 1120, which coincides with the
major surface 1124a of the patterned layer 1124, to be structured
or roughened in corresponding fashion to the structured major
surface 1110a of the tool.
[0086] In FIG. 11C, the optical film 1120 made in the
microreplication procedure of FIG. 11B is shown separated from the
tool 1110. The film 1120, which may be the same as or similar to
optical diffusing film 720 of FIG. 7, may now be used as an optical
diffusing film.
EXAMPLES
[0087] A number of optical diffusing film samples were made
according to methods as shown in FIG. 9. Thus, in each case, a
structured surface tool was made under a set of process conditions,
and then the structured surface of the tool was microreplicated to
form a corresponding structured surface (in inverted form) as a
major surface of the optical film. (The opposed major surface of
each optical film was flat and smooth.) The structured surface
provided each optical film with a given amount of optical haze and
optical clarity. The haze and clarity of each optical diffusing
film sample was measured with the Haze-Gard Plus haze meter from
BYK-Gardiner. The following table sets forth some of the chemical
solutions that were used during the fabrication of various samples,
as explained further below:
TABLE-US-00001 TABLE 1 Some Solutions Used Element Component
Supplier Quantity Alkaline cleaner 25% Sodium hydroxide Hawkins
Chemical 30% v/v (NaOH) (Minneapolis, MN) 16% Sodium carbonate
Hawkins Chemical 3.5% v/v Triton X-114 Dow Chemical 0.9% v/v
Company (Midland, MI) Mayoquest L-50 Vulcan Performance 0.9% v/v
Chemicals (Birmingham, AL) Dowfax C6L Dow Chemical 1.4% v/v Company
Deionized (DI) water Balance (15-18 megaohm) Citric acid solution
Citric acid 15% Hawkins Chemical 33% v/v solution DI water Balance
Sulfuric acid solution Sulfuric acid 96% Mallinckrodt Baker 1% v/v
reagent grade (Phillipsburg, NJ) DI water Balance First copper bath
Liquid copper sulfate Univertical (Angola, 53.5 g/L as copper (68.7
g/L copper) IN) Sulfuric acid 96% Mallinckrodt Baker 60 g/L as
H.sub.2SO.sub.4 reagent grade Hydrochloric acid 37% Mallinckrodt
Baker 60 mg/L as Cl.sup.- reagent DI water Balance Second copper
bath Liquid copper sulfate Univertical 53.5 g/L as copper (68.7 g/L
copper) Sulfuric acid 96% Mallinckrodt Baker 60 g/L as
H.sub.2SO.sub.4 reagent grade Hydrochloric acid 37% Mallinckrodt
Baker 60 mg/L as Cl.sup.- reagent Grain refiner Cuflex Atotech USA
(Rock 1.4% v/v 321 Hill, SC) DI water Balance Chrome bath Liquid
chromic acid Atotech USA 250 g/L as CrO.sub.3 (440 g/L CrO.sub.3)
Sulfuric acid 96% Mallinckrodt Baker 2.5 g/L reagent grade
Trivalent chromium 0-20 g/L byproduct DI water Balance
[0088] Preliminary Tool
[0089] A copper-coated cylinder, having a diameter of 16 inches and
a length of 40 inches, was used as a base for the construction of a
tool. The tool, which is referred to here as a preliminary tool
because it was made using only one of the electroplating steps
shown in FIG. 9, was first degreased with a mild alkaline cleaning
solution, deoxidized with a sulfuric acid solution, and then rinsed
with deionized water. The composition of the alkaline cleaner, as
well as the compositions of other relevant solutions, are shown in
Table 1. The preliminary tool was then transferred while wet to a
copper plating tank (Daetwyler Cu Master Junior 18). It was rinsed
with approximately 1 liter of the sulfuric acid solution at the
start of the plating cycle to remove surface oxide. The preliminary
tool was then immersed at a 50% level in the first copper bath. The
bath temperature was 25.degree. C. The copper bath was treated with
carbon-filled canisters to remove organic contamination.
Effectiveness of the treatment was verified both by using a 1000 mL
brass Hull Cell panel that is plated at 5 amps for 5 minutes and
evaluated for lack of brightness, and by TOC (total organic carbon)
analysis using a persulfate TOC analyzer. TOC levels were
determined to be below 45 parts per million (ppm). The preliminary
tool was DC-plated at a current density of 60 amps per square foot
(with a ramp up time at the start of 5 seconds) for 45 minutes
while being rotated at 20 rpm. The distance from the anode to the
nearest point on the tool during plating was approximately 45 mm.
When plating was completed, the thickness of the plated copper,
which we refer to as a first copper layer, was approximately 30
microns. The first copper layer had an exposed structured surface
that was roughened with a multitude of flat facets.
[0090] Rather than covering the first copper layer with an
electroplated second copper layer of lesser average roughness (in
accordance with FIG. 9), for reference purposes, this preliminary
tool, and in particular the structured surface of the first copper
layer, was used to make a Type II Microreplicated diffusing film.
This involved cleaning the preliminary tool and electroplating a
chromium coating on the structured surface of the first copper
layer. The chromium coating was thin enough to substantially
preserve the topography of the first copper layer structured
surface.
[0091] Accordingly, the preliminary tool, with the structured
surface of the first copper layer still exposed, was washed with
deionized water and a weak acid solution to prevent oxidation of
the copper surface. Next, the preliminary tool was moved to a Class
100 clean room, placed in a cleaning tank, and rotated at 20 rpm.
The preliminary tool was deoxidized using a citric acid solution,
and then washed with an alkaline cleaner. After that it was rinsed
with deionized water, deoxidized again with the citric acid
solution, and rinsed with deionized water.
[0092] The preliminary tool was transferred to a chrome plating
tank while wet and 50% immersed in the tank. The bath temperature
was 124.degree. F. The tool was DC-plated with chromium using a
current density of 25 amps per square decimeter while the
preliminary tool moved at a surface speed of 90 meters/minute. The
plating continued for 400 seconds. Upon completion of plating, the
preliminary tool was rinsed with deionized water to remove any
remaining chrome bath solution. The chromium coating serves to
protect the copper to prevent oxidation, and, as mentioned, it was
thin enough to substantially preserve the topography of the first
copper layer structured surface.
[0093] The preliminary tool was transferred to a cleaning tank
where it was rotated at 10 rpm, washed with 1 liter of deionized
water at ambient temperature, then washed with 1.5 liters of
denatured alcohol (SDA-3A, reagent grade at ambient temperature)
applied slowly to cover the entire tool surface. The tool rotation
speed was then increased to 20 rpm. It was then air dried.
[0094] Type II Microreplicated Optical Diffusing Film Once the
preliminary tool was dried, a hand-spread film was made from the
tool using a UV-curable acrylate resin coated on a primed PET film.
This procedure microreplicated the structured surface of the first
copper layer to produce a corresponding structured surface (but
inverted relative to that of the preliminary tool) on the cured
resin layer of the film. Due to its method of construction, the
film was a Type II Microreplicated optical diffusing film. A
scanning electron microscope (SEM) image of the film's structured
surface is shown in FIG. 5. The optical haze and clarity of the
film were measured with a Haze-Gard Plus system from BYK Gardner
(Columbia Md.), and found to be 100%, and 1.3%, respectively.
[0095] First Tool
[0096] Another structured surface tool, referred to here as the
first tool, was then made. Unlike the preliminary tool, the first
tool was made using both electroplating steps shown in FIG. 9, so
that the first copper layer was covered with an electroplated
second copper layer of lesser average roughness.
[0097] The first tool was prepared in the same way as the
preliminary tool, up to the chromium plating step. Then this first
tool, with its first copper layer whose structured surface was of
relatively high average roughness (substantially an inverted
version of FIG. 5), was transferred before drying to a copper
plating tank set up for additional plating. The first tool was
rinsed with approximately one liter of the sulfuric acid solution,
before the start of a second plating cycle, to remove surface oxide
generated during the loading of the tool into the tank. The first
tool was then 50% immersed in the second copper bath in a Daetwyler
Cu Master Junior 18 tank. The bath temperature was 25.degree. C.
The second copper bath was carbon treated to remove organic
contamination, as described above for the preliminary tool. After
the carbon treatment, the second copper bath was recharged with an
organic grain refiner (Cutflex 321 at a concentration of 14
milliliters/liter), such that the second copper bath had the
composition shown above in Table 1. The composition of the second
copper bath differed from that of the first copper bath by the
addition of the organic grain refiner. The anode was positioned at
a distance of approximately 45 mm from the first tool. The first
tool was then DC plated for 12 minutes in the second copper bath
using a current density of 60 amps per square foot while being
rotated at 20 rpm. The current ramp time was about 5 seconds. This
produced a second electroplated copper layer which covered the
first copper layer, the second copper layer having a structured
surface of lesser average roughness than that of the first copper
layer. The thickness of the second copper layer was 8 microns.
[0098] The first tool was then transferred to a cleaning tank. It
was rotated at 10-12 revolutions per minute while being washed with
approximately 1 liter of deionized water at ambient temperature
using a hose with a spray nozzle. A second wash was done using 1 to
2 liters of the citric acid solution at ambient temperature. Then
the first tool was washed with approximately 3 liters of deionized
water to remove excess citric acid using a hose with a spray
nozzle. Next the first tool was rinsed with approximately 2 liters
of denatured ethanol (SDA 3A of reagent grade) applied slowly at
ambient temperature to cover the entire tool surface in order to
aid in drying. The first tool was then air dried. Next, the first
tool was moved to a Class 100 clean room, cleaned, and chrome
plated, in the same way as was done with the preliminary tool. The
chromium plating substantially retained the topography of the
structured surface of the second copper layer.
[0099] Sample 502-1
[0100] After air drying, the first tool was used to make a film via
a hand spread. This too was done in the same way as was done with
the preliminary tool, and it produced an optical diffusing film
(referred to herein with the sample designation number 502-1)
having a microreplicated structured surface on the cured resin
layer of the film corresponding to (but inverted relative to) the
structured surface of the second copper layer. An SEM image of the
film's structured surface is shown in FIG. 14. Although the surface
is structured, one can see that the average roughness of the
surface is less than that of the structured surface of FIG. 5. An
SEM image of a cross-section of the 502-1 sample is shown in FIG.
14a. The optical haze and clarity of this optical diffusing film
sample 502-1 were measured with the Haze-Gard Plus system from BYK
Gardner (Columbia Md.), and found to be 92.8%, and 6.9%,
respectively. These values are listed in Table 2 below.
[0101] Second Tool
[0102] Another structured surface tool, referred to here as the
second tool, was made. The second tool was made in substantially
the same way as the first tool, except that the composition of the
second copper bath was different: two organic grain refiners were
used (Cutflex 321 at a concentration of 14 milliliters/liter, and
Cutflex 320H at a concentration of 70 millilters/liter), rather
than just one. The second copper plating step was, however, again
completed in 12 minutes, which produced a second electroplated
copper layer whose thickness was 8 microns. After chrome plating
the structured surface of the second copper layer, the second tool
was ready to be used for microreplication to an optical film.
[0103] Sample 594-1
[0104] The second tool was then used to make a film via a hand
spread. This was done in the same way as was done with the first
tool, and it produced an optical diffusing film (referred to herein
with the sample designation number 594-1) having a microreplicated
structured surface on the cured resin layer of the film
corresponding to (but inverted relative to) the structured surface
of the second copper layer. An SEM image of the film's structured
surface is shown in FIG. 15. Although the surface is structured,
one can see that the average roughness of the surface is less than
that of the structured surface of FIG. 5. The optical haze and
clarity of this optical diffusing film sample 594-1 were measured
with the Haze-Gard Plus system from BYK Gardner (Columbia Md.), and
found to be 87.9%, and 6.9%, respectively. These values are listed
in Table 2 below.
[0105] Third Tool
[0106] Another structured surface tool, referred to here as the
third tool, was made. The third tool was made in substantially the
same way as the second tool, except that the second copper plating
was completed in 18 minutes rather than 12 minutes, which produced
a second electroplated copper layer whose thickness was about 12
microns. After chrome plating the structured surface of the second
copper layer, the third tool was ready to be used for
microreplication to an optical film.
[0107] Sample 593-2
[0108] The third tool was then used to make a film via a hand
spread. This was done in the same way as was done with the first
and second tools, and it produced an optical diffusing film
(referred to herein with the sample designation number 593-2)
having a microreplicated structured surface on the cured resin
layer of the film corresponding to (but inverted relative to) the
structured surface of the second copper layer. An SEM image of the
film's structured surface is shown in FIG. 21. Although the surface
is structured, one can see that the average roughness of the
surface is less than that of the structured surface of FIG. 5. The
optical haze and clarity of this optical diffusing film sample
593-2 were measured with the Haze-Gard Plus system from BYK Gardner
(Columbia Md.), and found to be 17.1%, and 54.4%, respectively.
These values are listed in Table 2 below.
[0109] Fourth Tool
[0110] Another structured surface tool, referred to here as the
fourth tool, was made. In order to make this fourth tool, two
plating solutions were prepared. A first plating solution consisted
of 60 g/L of sulfuric acid (J. T. Baker Chemical Company,
Philipsburg, N.J.) and 217.5 g/L of copper sulfate (Univertical
Chemical Company, Angola, Ind.). A second plating solution
consisted of the contents of the first plating solution plus
additives CUPRACID HT leveler (0.05% by volume), CUPRACID HT fine
grainer (0.1% by volume), and CUPRACID HT wetting agent (0.3% by
volume), all available from Atotech USA. Both solutions were made
with deionized water. An 8 inch by 8 inch copper sheet was placed
in a tank holding the first plating solution. The tank size was 36
inches (length).times.24 inches (width).times.36 inches (depth).
The sheet was plated at 21.degree. C. for 24 hours using a current
density of 10 amps per square foot with a flow rate of 8 gallons
per minute created using a circulation pump. This first plating
step produced a first electrodeposited copper layer having a
relatively rough structured surface, the thickness of the
electrodeposited layer being about 330 microns. The plate was
removed from the first plating solution, rinsed, and dried. The
copper sheet with the first electroplated layer was then cut into a
1.5 inch.times.8 inch section. The backside of the section was
shielded with adhesive tape and placed in a four-liter beaker
containing the second plating solution, and plated at 25.degree. C.
for 35 minutes at a current density of 35 amps per square foot.
This second plating step produced a second electrodeposited copper
layer which covered the first copper layer, and the second copper
layer had a structured surface whose average roughness was less
than that of the first copper layer. The thickness of the second
copper layer was about 28 microns. After the second plating step,
the section, which is referred to as the fourth tool, was rinsed
and dried. Unlike the first, second, and third tools, the second
copper layer of the fourth tool was not plated with chromium.
Instead, the exposed structured surface of the second copper layer
was used directly for microreplication of an optical film.
[0111] It was discovered that, in contrast to the tools used to
make the other optical diffusing film samples disclosed herein, the
copper sheet used as a starting material to make the fourth tool
deviated significantly from flatness, in particular, it contained
substantially linear periodic undulations. These undulations were
carried over into the structured surfaces of the first and second
copper layers, such that the structured surface of the second
copper layer contained not only roughness attributable to the
electroplating steps, but also an undulation originating from the
base copper sheet upon which the electrodeposited copper layers
were formed.
[0112] Sample RA13a
[0113] The fourth tool was then used to make a film via a hand
spread. This was done by applying a polyester film substrate with a
uv-curable acrylate resin to the fourth tool. The resin was cured
using a uv-processor from RPC Industries (Plainfield, Ill.) with a
line speed of 50 feet per minute. The film was then removed from
the structured surface of the fourth tool. The film was an optical
diffusing film (referred to herein with the sample designation
number RA13a) having a microreplicated structured surface on the
cured resin layer of the film corresponding to (but inverted
relative to) the structured surface of the second copper layer. An
SEM image of the film's structured surface is shown in FIG. 19. The
faint periodic vertical lines seen in the figure are a result of
the periodic undulations in the copper sheet starting material, and
were not introduced by the two copper electroplating steps. The
optical haze and clarity of this optical diffusing film sample
RA13a were measured as with the other samples, and found to be
25.9%, and 19.4%, respectively. These values are listed in Table 2
below.
[0114] Samples 507-1, 600-1, 554-1, 597-1, 551-1, and 599-1
[0115] The tools used to make these optical diffusing film samples
were made in the same manner as the tools for samples 502-1 and
594-1 above, except that one or more of the following were varied
for the second electroplating step: the amount of organic leveler
used, the current density, and the plating time. The samples
themselves were then made from their respective tools via a hand
spread in the same manner as samples 502-1 and 594-1, and the haze
and clarity were measured as with the other samples. The measured
values are listed in Table 2 below. An SEM image of the structured
surface of film sample 599-1 is shown in FIG. 16.
[0116] Samples 502-2, 554-2, 551-2, 597-2, and 600-2
[0117] The tools used to make these optical diffusing film samples
were made in the same manner as the tool for sample 593-2 above,
except that one or more of the following were varied for the second
electroplating step: the amount of organic leveler used, the
current density, and the plating time. The samples themselves were
then made from their respective tools via a hand spread in the same
manner as sample 593-2, and the haze and clarity were measured as
with the other samples. The measured values are listed in Table 2
below. An SEM image of the structured surface of film sample 502-2
is shown in FIG. 17. An SEM image of the structured surface of film
sample 597-2 is shown in FIG. 22.
[0118] Samples RA13c, RA13b, RA22a, L27B, RA14b, RA24a, RA24b, N3,
and N2 The tools used to make these optical diffusing film samples
were made in the same manner as the tool for sample RA13a above
(i.e., the fourth tool), except that (i) the copper sheet used as a
starting material was flat and smooth and did not contain the
periodic undulations, and (ii) one or more of the following were
varied for the first or second electroplating step: the current
density, and the plating time. The samples themselves were then
made from their respective tools via a hand spread in the same
manner as sample RA13a, and the haze and clarity were measured as
with the other samples. The measured values are listed in Table 2
below. An SEM image of the structured surface of film sample RA22a
is shown in FIG. 18. An SEM image of the structured surface of film
sample N3 is shown in FIG. 20.
TABLE-US-00002 TABLE 2 Measured Optical Haze and Clarity Sample
Haze (%) Clarity (%) 600-2 1.57 88.3 597-2 2.5 83.1 551-2 5.3 72.5
RA24b 7.41 56.8 N2 8.2 76.6 554-2 11.7 41.1 RA24a 12.1 40.4 RA14b
13.9 57.8 L27B 14 51.1 593-2 17.1 54.4 N3 24.9 32.1 RA13a 25.9 19.4
RA22a 54.6 15.5 502-2 67.3 9 599-1 72.4 8.4 RA13b 72.5 9.1 551-1
79.4 10 RA13c 80 9.5 597-1 85.6 8.6 554-1 87.4 7.3 594-1 87.9 6.9
502-1 92.8 6.9 600-1 95 6.8 507-1 96.4 6.1
[0119] Each optical diffusing film sample listed in Table 2 was
made using a process in accordance with FIG. 9. The measured haze
and measured clarity values in this table are plotted in the
optical clarity vs. optical haze graph of FIG. 13. The points on
the graph are labeled according to the sample designation numbers
in Table 2. Of the samples listed in Table 2, SEM images of the
structured surfaces are provided for: sample 502-1 (FIGS. 14, 14A);
sample 594-1 (FIG. 15); sample 599-1 (FIG. 16); sample 502-2 (FIG.
17); sample RA22a (FIG. 18); sample RA13a (FIG. 19); sample N3
(FIG. 20); sample 593-2 (FIG. 21); and sample 597-2 (FIG. 22).
Inspection of these images reveals one or more of: [0120]
discernible individual structures (e.g. in the form of distinct
cavities and/or protrusions) that can be seen in the structured
surface; [0121] individual structures that are limited in size
along two orthogonal in-plane directions; [0122] individual
structures that are closely packed; [0123] individual structures
that are rounded or curved (crater-like or dome-like, with curved
base surfaces); [0124] individual structures that are pyramidal or
flat-faceted; and [0125] combinations of non-uniformly arranged
larger structures, and closely packed smaller structures
non-uniformly dispersed between the larger structures.
[0126] Further Discussion--Structured Surface Characterization
[0127] Further analysis work was performed to identify
characteristics of structured surfaces which, whether alone or in
combination with other characteristics, may be used to characterize
at least some of the structured surfaces made by the method of FIG.
9, and/or to distinguish at least some such structured surfaces
from those of other optical diffusing films, such as SDB diffusers,
DPB diffusers, CCS diffusers, Type I Microreplicated diffusing
films, and Type II Microreplicated diffusing films. Several
characterization parameters were studied in this regard, including:
[0128] power spectral density (PSD) of the topography along
orthogonal in-plane directions, as a measure of spatial
irregularity or randomness; [0129] identification of individual
structures (in plan view) that make up the structured surface, and
measurement of the in-plane size or transverse dimension (such as
ECD) of such structures; [0130] ratio of depth or height to
in-plane size of the structures; and [0131] identification of
ridges on the structured surface, and measurement of ridge length
(in plan view) per unit area. This further analysis work will now
be discussed.
[0132] Power Spectral Density (PSD) analysis
[0133] Part of the analysis work focused on the topography of the
structured surface, and sought to determine the degree of spatial
irregularity or randomness of the surface. The topography can be
defined relative to a reference plane along which the structured
surface extends. For example, the structured surface 820a of film
820 (see FIG. 8) lies generally in, or extends generally along, an
x-y plane. Using the x-y plane as a reference plane, the topography
of the structured surface 820a can then be described as the height
of the surface 820a relative to the reference plane as a function
of position in the reference plane, i.e., the z-coordinate of the
surface as a function of (x,y) position. If we measure the
topography of a structured surface in this manner, we can then
analyze the spatial frequency content of the topographical function
to determine the degree of spatial irregularity or randomness of
the surface (or to identify spatial periodicities present in the
structured surface).
[0134] Our general approach was to analyze the spatial frequency
content using Fast Fourier Transform (FFT) functions. Because the
topography provides height information along two orthogonal
in-plane directions (x and y), the spatial frequency content of the
surface is fully characterized by analyzing the spatial frequency
content along each of the in-plane directions. We determined the
spatial frequency content by measuring the topography over a
sufficiently large, and representative, portion of the structured
surface, and calculating a Fourier power spectrum for each in-plane
direction. The two resulting power spectra could then be plotted on
graphs of power spectral density (PSD) versus spatial frequency. To
the extent the resulting curves contain any local frequency peaks
(not corresponding to zero frequency), the magnitude of such a peak
can be expressed in terms of a "peak ratio" described further below
in connection with FIG. 22.
[0135] Having described our general approach, we now describe our
approach to the PSD analysis in more detail. For a given optical
diffusing film sample, a .about.1.times.1 cm piece of the sample
was cut from the central portion of the sample. The sample piece
was mounted on a microscope slide, and its structured surface was
Au--Pd sputter-coated. Two height profiles of the structured
surface were obtained using confocal scanning laser microscopy
(CSLM). Whenever possible, fields of view were chosen to give a
good sampling of the topography and any periodicity that was
present. The 2-dimensional (2D) power spectral density (PSD) was
calculated for each 2D height profile. The 2D PSD is the square of
the magnitude of the 2D spatial Fourier transform of the 2D height
profile. MATLAB was used to calculate the PSD using MATALB's Fast
Fourier Transform (FFT) function. Before using the FFT, a 2D
Hamming window was applied to the 2D height profile to help reduce
ringing in the FFT caused by the finite spatial dimensions of the
2D height profile. The 2D PSD was summed in the x-direction to give
the 1-dimensional (1D) PSD in the y-direction (downweb direction).
Likewise, the 2D PSD was summed in the y-direction to give the 1D
PSD in the x-direction (crossweb direction).
[0136] Analysis of the 1D PSDs with regard to spatial frequency
peaks will now be described in connection with FIG. 23. In that
figure, a hypothetical Fourier power spectrum curve is shown for
illustrative purposes. The curve, which may represent either of the
1D PSD functions (x or y) discussed above, appears on a graph of
power spectral density (PSD) versus spatial frequency. The vertical
axis (PSD) is assumed to be plotted on a linear scale starting at
zero. The curve is shown as having a frequency peak which (a) does
not correspond to zero frequency, and (b) is bounded by two
adjacent valleys that define a baseline. The two adjacent valleys
are identified by points p1, at spatial frequency f1, and p2, at
spatial frequency f2. The frequency f1 may be considered the
frequency at which the peak starts, and f2 may be considered the
frequency at which the peak ends. The baseline is the straight line
segment (dashed line) that connects p1 and p2. Keeping in mind that
the vertical axis (PSD) is on a linear scale starting at zero, the
magnitude of the peak can be expressed in terms of the areas A and
B on the graph. The area A is the area between the frequency peak
and the baseline. The area B is the area under or beneath the
baseline. That is, B=(PSD(f1)+PSD(f2))*(f241)/2. The sum A+B is the
area under or beneath the frequency peak. Given these definitions,
the magnitude of the peak can now be defined in terms of a relative
peak amplitude or "peak ratio" as follows:
peak ratio=A/(A+B).
[0137] In practice, we evaluated two 1D PSDs (two Fourier power
spectra--one for the x-direction, one for the y-direction) for each
sample that was evaluated, and we identified, to the extent the
Fourier power spectrum included any frequency peaks, the most
prominent peak for each curve. The above-described peak ratio was
then calculated for the most prominent peak for each curve. Since
the most prominent peak was measured, the calculated peak ratio is
an upper limit for all peaks that may be present in the given
Fourier power spectrum.
[0138] These PSD measurements were performed not only on optical
diffusing films made according to the method of FIG. 9, but also on
two Type I Microreplicated diffusing film samples. The two Type I
Microreplicated diffusing film samples were made in general
accordance with the teachings of the '622 Aronson et al., '593
Yapel et al., '475 Barbie, and '261 Aronson et al. references cited
above, these two samples referred to herein as "Type I Micro--1"
and "Type I Micro--4". These samples were made under differing
conditions, and had different haze values. In particular, the Type
I Micro--1 sample had a haze of 91.3% and clarity of 1.9%, and the
Type I Micro--4 sample had a haze of 79.1% and a clarity of 4.5%.
The SEM image in FIG. 4 is a picture of the Type I Micro--1
sample.
[0139] FIGS. 24A and 24B are graphs, for the downweb and crossweb
in-plane directions respectively, of power spectral density vs.
spatial frequency for the Type I Micro--1 sample. In each graph,
"f1" and "a" are the frequencies at which the most prominent peak
was determined to start and end, respectively. Although these
graphs use a logarithmic scale for the power spectral density
(PSD), the A and B values used for the calculation of peak ratio
were calculated based on a linear PSD scale, consistent with the
description above.
[0140] FIGS. 24A and 24B are graphs for the downweb and crossweb
directions respectively of power spectral density vs. spatial
frequency for the optical diffusing film sample 502-1. The labels
"f1" and "a" have the same meanings in these figures as in FIGS.
22, 23A, and 23B. The A and B values used to calculate peak ratio
were based on a linear PSD scale, even though a log scale is used
in FIGS. 24A, 24B.
[0141] The calculated PSD peak ratios for seven of the optical
diffusing films made in accordance with the method of FIG. 9, and
for the two Type I Microreplicated diffusing film samples, are
listed in Table 3.
TABLE-US-00003 TABLE 3 Measured PSD Peak Ratios Measured peak
Sample Measured peak ratio (downweb) ratio (crossweb) 502-1 0.24
0.15 594-1 0.12 0.23 502-2 0.10 0.17 593-2 0.19 0.12 RA22a 0.21
0.11 RA13a 0.14 0.76 N3 0.08 0.21 Type I Micro - 1 0.94 0.19 Type I
Micro - 4 0.99 0.84
[0142] In reviewing the results of Table 3, we see that for each of
the optical diffusing films made in accordance with FIG. 9, the
peak ratio for both in-plane directions (downweb and crossweb) is
less than 0.8, and, in most cases, much less than 0.8. In
comparison, although the Type I Micro--1 sample had a peak ratio of
0.19 in the crossweb direction, in all other cases the tested Type
I Microreplicated diffusing films had peak ratios greater than 0.8.
Thus, neither of the tested Type I Microreplicated diffusing films
satisfies the condition that the peak ratio for both in-plane
directions is less than 0.8.
[0143] In reviewing the results of Table 3, we also see that all
except one of the tested film samples made in accordance with FIG.
9 also satisfy a more stringent condition that the peak ratio for
both in-plane directions is less than 0.5, or 0.4, or 0.3. The
relatively small values for peak ratio in both in-plane directions
are suggestive of ultra-low spatial periodicity in the structured
surfaces. The sample RA13a, however, does not meet the more
stringent conditions. Out of all the tested film samples made in
accordance with FIG. 9, the RA13a sample has by far the highest
measured peak ratio, a ratio of 0.76 in the crossweb direction. In
the orthogonal in-plane direction, the RA13a sample has a much
smaller 0.14 peak ratio. Recall from the description above that the
RA13a sample was made with a copper sheet starting material that
contained periodic undulations, and these periodic undulations were
transferred to the structured major surface of the RA13a sample
during microreplication. In view of this, it is reasonable to
conclude that if the substrate for RA13a had been substantially
flat with no undulations, the peak ratio in the crossweb direction
would be much closer to the downweb peak ratio of 0.14. Stated
differently, to the extent a tool made in accordance with FIG. 9 is
made using a flat substrate that has no underlying structure, such
a tool (and any optical film made from the tool) is likely to have
PSD peak ratios in both in-plane directions of less than 0.8, or
0.5, or 0.4, or 0.3.
[0144] Similarly, to the extent a tool made in accordance with FIG.
9 is made using a substrate that has significant underlying
structure (whether periodic undulations, or more defined structure
such as a prismatic BEF structured surface), such a tool (and any
optical film made from the tool) is likely to exhibit a significant
or large peak in the power spectral density curve for at least one
in-plane direction, and is likely to have a significant or large
PSD peak ratio in such in-plane direction. In such cases, by
engaging in a more in-depth analysis of the PSD measurements,
particularly if information is available about the underlying
structure in the original substrate, one may distinguish between
peaks in the power spectral density curve that are due to the
underlying structure of the substrate used to form the tool, and
peaks that are due to the structures that were formed as a result
of the electroplating steps (see steps 903 and 904 in FIG. 9).
Making such a distinction may be complex, because the spatial
periodicit(ies) of the underlying structure need not be
significantly different than any spatial periodicit(ies) of the
electroplated structure, in fact, the spatial periodicities of
these different structures types may in at least some cases
substantially overlap. Nevertheless, if one succeeds in making such
a distinction, then the condition for a structured surface that the
PSD peak ratios in both in-plane directions be less than 0.8 (or
0.5, or 0.4, or 0.3) may still be satisfied by a structured surface
that was made in accordance with FIG. 9 using a substrate with
significant underlying structure, provided that any peaks in the
power spectral density curves that are due to the underlying
structure are disregarded.
[0145] The results given in Table 3 were obtained by identifying a
most prominent peak, if present, in the power spectral density
curve. And data for the power spectral density curves, as can be
seen in FIGS. 23A through 24B, extended over a spatial frequency
range from roughly 1 mm.sup.-1 to almost 2000 mm.sup.-1, hence, any
peaks that may be present throughout that range are candidates in
the determination of which peak is the most prominent, and they are
also candidates with respect to the criterion that the PSD peak
ratios in both in-plane directions are less than 0.8 (or 0.5, or
0.4, or 0.3). In practice, it may be advantageous to limit the
spatial frequency range over which peaks in the power spectral
density curves are considered for these analyses. For example, it
may be advantageous to limit the spatial frequency range over which
the PSD peak ratios in both in-plane directions are specified to be
less than 0.8 (or 0.5, or 0.4, or 0.3), to a frequency range whose
upper limit is 1000, or 500, or 100 mm.sup.-1, and whose lower
limit is 1, or 2, or 5 mm.sup.-1.
[0146] Transverse Dimension or Size (ECD) Analysis
[0147] For a structured surface in which distinct individual
structures can be identified, the structured surface can be
described in terms of a characteristic size, such as a transverse
or in-plane dimension, of the structures. Each structure may for
example be characterized as having a largest transverse dimension,
a smallest transverse dimension, and an average transverse
dimension. If the individual structures are limited in size along
two orthogonal in-plane directions, e.g., not extending
indefinitely in a linear fashion along any in-plane direction, each
structure may be characterized as having an equivalent circular
diameter "ECD". The ECD of a given structure may be defined as the
diameter of a circle whose area in plan view is the same as the
area in plan view of the structure. For example, with reference to
FIG. 25, a plan view of a hypothetical structured surface 2520a is
shown. The structured surface comprises distinguishable structures
2521a, 2521b, 252k, 2521d, which may be protrusions or cavities. A
circle 2523a is superimposed on the structure 2521a, the circle
allegedly having in this plan view an area equal to that of the
structure 2521a. The diameter (ECD) of the circle 2523a is the
equivalent circular diameter (ECD) of the structure 2521a. By
averaging the ECD values for all of the structures in a
representative portion of the structured surface, the structured
surface or structures thereof may then be said to have an average
equivalent circular diameter ECD.sub.avg.
[0148] We undertook a systematic analysis of structure size for a
number of optical diffusing films. For a given optical diffusing
film sample, a .about.1.times.1 cm piece of the sample was cut from
the central portion of the sample. The sample piece was mounted on
a microscope slide, and its structured surface was Au--Pd
sputter-coated. Two height profiles of the structured surface were
obtained using confocal scanning laser microscopy (CSLM). Whenever
possible, fields of view were chosen to give a good sampling of the
topography. Depending on what type of structure was predominant in
the sample, either peaks or valleys were sized. A consistent and
repeatable methodology was established for sizing the individual
structures that were identified on the structured surface. The
composite images of FIGS. 26-29 provide an indication of how this
was done. In these composite images, dark outline shapes are
superimposed on a picture of the structured surface through a
confocal microscope. The dark outline shapes are the computed outer
boundaries or edges of individual structures of the structured
surface. FIG. 26 is such a composite image for the CCS diffuser.
FIG. 27 is for the Type I Micro--1 sample discussed above. FIG. 28
is for the optical diffusing film sample 594-1. FIG. 39 is for the
optical diffusing film sample 502-1. Using such images and
techniques, the ECD of typically hundreds and in some cases
thousands of structures was calculated for a given structured
surface. The ECD measurements and measurement statistics are
summarized as follows:
TABLE-US-00004 TABLE 4 Measured ECD Statistics ECD Sample ECD mean
(um) ECD median (um) sigma (um) 502-1 10.3 9.7 3.6 594-1 6.1 6.1
2.6 593-2 5.8 5.5 2.5 RA13a 58.3 58.5 17.5 N3 6.3 6.0 3.3 Type I
Micro - 1 15.0 15.8 4.7 Type I Micro - 2 15.3 17.3 5.6 Type I Micro
- 3 16.5 17.8 4.6 Type I Micro - 4 16.8 17.5 3.5 Type I Micro - 5
17.6 18.1 3.5 Type I Micro - 6 17.5 18.3 4.2 Type II Micro 9.2 8.8
2.8 CCS Diffuser 3.6 3.0 2.0
[0149] The samples Type I Micro--2, Type I Micro--3, Type I
Micro--5, and Type I Micro--6 are additional Type I Microreplicated
diffusing film samples that were made in general accordance with
the teachings of the '622 Aronson et al., '593 Yapel et al., '475
Barbie, and '261 Aronson et al. references cited above. The Type I
Micro--2 sample had a haze of 90.7% and clarity of 2.9%, the Type I
Micro--3 sample had a haze of 84.8% and a clarity of 4.7%, the Type
I Micro--5 sample had a haze of 73.9% and a clarity of 5.5%, and
the Type I Micro--6 sample had a haze of 68.2% and a clarity of
4.9%. The Type II Micro sample in Table 4 was an optical diffusing
film similar to the Type II Microreplicated diffusing film shown in
FIG. 5, but the Type II Micro sample of Table 4 had a haze of 91.1%
and a clarity of 9.8%. In reviewing the results of Table 4, we see
that, except for the RA13a sample, each of the optical diffusing
films made in accordance with FIG. 9 had an average (mean) ECD of
less than 15 microns, and most had an average ECD of less than 10
microns, or in a range from 4 to 10 microns. This was in contrast
to the average ECD of the Type II Microreplicated diffusing film
samples, which was generally at least 15 microns or more. The RA13a
sample had a substantially higher average ECD than any of the other
films made in accordance with FIG. 9. The periodic undulations of
the RA13a sample discussed above are believed to be the reason for
this large difference. That is, it is reasonable to conclude that
if the substrate for RA13a had been substantially flat with no
undulations, the average ECD would have been much closer to that of
the other similarly fabricated films, e.g., less than 15 and less
than 10 microns.
[0150] The structured surfaces of some of the samples made by the
method of FIG. 9 were observed to contain a combination of
irregularly arranged larger pyramidal structures, between which
closely-packed smaller structures were irregularly dispersed. One
such sample was 502-1. An analysis of the structured surface was
done, and the results, shown as curve 3010 in the graph of FIG. 30,
demonstrate that the surface has a bimodal distribution of
structure sizes. The graph of FIG. 31 plots the normalized count
(in percent per bin) as a function of ECD in microns. Curve 3010 is
seen to have a larger peak 3010a and a smaller peak 3010b. The
larger peak 3010a is located at about ECD=8 microns, and
corresponds to the smaller structures on the structured surface.
The smaller peak 3010b is located at about ECD=24 microns, and
corresponds to the larger pyramidal structures. Thus, the average
size of the smaller structures is less than 15 microns, and less
than 10 microns, and the average size of the larger structures is
greater than 15 microns, and greater than 20 microns. Due to the
smaller population of the larger structures, the average ECD for
all structures (large and small) on the structured surface is 10.3
microns, as reported in Table 4.
[0151] Aspect Ratio of Height to Transverse Dimension (ECD)
Analysis
[0152] Some of the films made by the method of FIG. 9 had
structured surfaces in which individual structures were closely
packed and, in some cases, the structures were also curved or had
curved base surfaces. We decided to investigate the relationship
between the in-plane or transverse dimension (e.g. ECD) of the
structures and the mean height of the structures. In general, the
term "height" is broad enough to refer to the height of a
protrusion as well as to the depth of a cavity. For comparison
purposes we included in our investigation the DPB diffuser, which
has a densely-packed beaded surface.
[0153] The height of an exemplary structure is illustrated in the
drawing of a hypothetical structured surface in FIG. 31. In the
figure, an optical diffusing film 3120 includes a patterned layer
3122 with a structured major surface 3120a. The structured surface
3120a includes discernible individual structures 3121a, 3121b. The
structured surface extends along or defines an x-y plane. Three
reference planes parallel to the x-y plane are shown: RP1, RP2, and
RP3. The reference planes RP1, RP3 may be defined in terms of the
highest and lowest portions (respectively) of the structure 3121a.
The reference plane RP2 may be located at a position corresponding
to zero or near-zero curvature, i.e., the surface at that position
is neither curved inwardly, as at the top of a peak, nor curved
outwardly, as at the bottom of a cavity. Given these reference
planes, one can define a height h1 between RP1 and RP2, and a
height h2 between RP2 and RP3.
[0154] We undertook a systematic analysis of determining an aspect
ratio of structures on a given structured surface, the aspect ratio
being the height divided by the ECD of the structure. For the
height of the structure, we elected to use a value corresponding
substantially to h1 shown in FIG. 31. For a given optical diffusing
film sample, a .about.1.times.1 cm piece of the sample was cut from
the central portion of the sample. The sample piece was mounted on
a microscope slide, and its structured surface was Au--Pd
sputter-coated. Two height profiles of the structured surface were
obtained using confocal scanning laser microscopy (CSLM). Whenever
possible, fields of view were chosen to give a good sampling of the
topography. Valleys (cavities) in the structured surfaces were
sized; however, when evaluating the structured surface of the DPB
diffuser, the height profile of the structured surface was inverted
before sizing to convert peaks to valleys, for ease of calculation.
As was done with the ECD measurements described above, a consistent
and repeatable methodology was established for sizing the
individual structures that were identified on the structured
surface. The methodology was then modified to add the measurement
of the height to diameter aspect ratio (Hmean/ECD). The ratio was
calculated for each structure (valley region). The height Hmean was
the mean height on the perimeter of the structure (valley region)
minus the minimum height in the structure (valley region). The
height map in the valley region was tilt corrected using the data
points on the perimeter before the height was measured. The mean
aspect ratios for the tested samples were calculated, and are shown
in Table 5.
TABLE-US-00005 TABLE 5 Aspect Ratio Sample mean aspect ratio 502-1
0.078 594-1 0.069 597-2 0.006 DPB diffuser 0.210
[0155] In reviewing the results of Table 5, we see that the samples
made by the method of FIG. 9 can be readily distinguished from the
DPB diffuser on the basis of aspect ratio. For example, the average
aspect ratio of the former samples is less than 0.15, or less than
0.1.
[0156] Ridge Analysis
[0157] As mentioned above, some of the films made by the method of
FIG. 9 had structured surfaces in which individual structures were
closely packed. The closely packed structures tend to produce
ridge-like features, although ridge-like features may also occur in
the absence of closely packed structures. We decided to investigate
aspects of ridges on structured surfaces. In particular, we
investigated the extent to which ridges were present on the
structured surface. We quantified this by calculating the total
ridge length per unit area of structured surface in plan view. This
was done for many of the samples made according to the method of
FIG. 9, and, for comparison purposes, we also included several
beaded diffusers: the SDB diffuser, the CCS diffuser, and the DPB
diffuser.
[0158] A ridge is illustrated in the drawing of a hypothetical
structured surface in FIG. 32. In the figure, an optical diffusing
film includes a structured major surface 3220a. The structured
surface 3220a includes discernible individual structures 3221a,
3221b, 322k. The structured surface extends along or defines an x-y
plane. A ridge, which may be described as a long, sharp, peaked
region, is formed along at least a short segment at which the
boundaries of the structures 3221a, 3221b come together. The ridge
or segment includes points p1, p2, p3. The local slope and
curvature at each of these points, based on the known topography,
can be calculated along directions (see axes a1, a2, a3) that are
parallel to a gradient and perpendicular to the ridge, as well as
along directions (see axes b1, b2, b3) that are perpendicular to
the gradient and parallel to the ridge. Such curvatures and slopes
can be used to confirm that the points lie on a long, sharp peaked
region. For example, points on the ridge may be identified by: a
sufficiently different curvature along the two perpendicular
directions (e.g. a1, b1); a sharp curvature perpendicular to the
ridge (e.g. a1); a slope in the gradient direction (e.g. along the
ridge, see b1) that is less than the average slope; and a segment
length that is sufficiently long.
[0159] We undertook a systematic analysis of determining the ridge
length per unit area on a given structured surface using the
foregoing principles. For a given optical diffusing film sample, a
.about.1.times.1 cm piece of the sample was cut from the central
portion of the sample. The sample piece was mounted on a microscope
slide, and its structured surface was Au--Pd sputter-coated. Two
height profiles of the structured surface were obtained using
confocal scanning laser microscopy (CSLM). Whenever possible,
fields of view were chosen to give a good sampling of the
topography. Ridge analysis was used to analyze the height profiles
in accordance with the above principles.
[0160] The ridge analysis identified the peaks of ridges on a 2D
height map and calculated the total length of ridges per unit
sample area. Curvature along the gradient direction and transverse
to the gradient direction was calculated about each pixel.
Thresholding on the curvature and slope were carried out to
identify ridges.
[0161] The following is the definition of a ridge that was used in
the ridge analysis. [0162] 1. Curvature definitions: (a) gcurvature
is the curvature along the gradient direction; (b) tcurvature is
the curvature along the direction transverse (perpendicular) to the
gradient direction; (c) gcurvature is calculated by using three
points along the gradient and calculating the circle that
circumscribes the three points; the gcurvature=1/R, where R is the
radius of this circle; (d) tcurvature is calculated by using three
points along the direction transverse to the gradient and
calculating the circle that circumscribes the three points; the
gcurvature=1/R, where R is the radius of this circle; (e) the
curvature is assigned to the center point of these three points;
(f) the spacing of the three points is chosen to be large enough to
reduce the contribution by fine features that are not of interest
but small enough so that the contribution by features of interest
is preserved. [0163] 2. The curvature of a point on the ridge is
sufficiently different between two perpendicular directions. (a)
The gcurvature and tcurvature differ by at least a factor of 2
(either can be larger). [0164] 3. The ridge is sharper than most of
the valleys. (a) Curvature is greater than the absolute value of
the 1 percentile point of the gcurvature distribution (1% of the
gcurvature is lower than the 1 percentile point). [0165] 4. The
slope is lower than the mean slope. (a) gslope (slope along the
gradient) on ridge is less than the mean gslope of the surface. (b)
The slope on the top of a ridge is typically near zero unless it is
on a highly sloped surface. [0166] 5. The ridge is sufficiently
long. (a) A potential ridge is not considered a ridge if its total
length (including branches) is shorter than the mean radius of
curvature along the potential ridge top; (b) A potential ridge is
not considered a ridge if its total length is shorter than 3 times
the mean width of the potential ridge; (c) Note that these
dimensions are measured approximately. [0167] 6. Branches are
sufficiently long. (a) A branch from the midsection of a ridge is
considered a continuation of the ridge if it is longer than 1.5
times the mean width of the ridge. Otherwise, it is removed; (b)
Note that these dimensions are measured approximately.
[0168] The composite images of FIGS. 33A and 34A provide an
indication of how the systematic ridge identification was done. In
these composite images, dark line segments are superimposed on a
picture of the structured surface through a confocal microscope.
The dark line segments are areas of the structured surface
identified as ridges. FIG. 33A is such a composite image for the
594-1 sample. FIG. 34A is for the DPB diffuser. FIG. 33B
corresponds to FIG. 33A, but shows only the dark line segments
(i.e. the detected ridges) but in reverse printing so the ridges
can be more easily seen. FIG. 34B likewise corresponds to FIG. 34A,
but shows only the dark line segments and in reverse printing.
[0169] After identifying the ridges, the total length of all the
ridges in the height map was calculated and divided by the area of
the height map. This analysis was also repeated for identifying
valley ridges by inverting the height maps before running the
analysis. Note that the DPB sample was inverted to begin with.
Using such images and techniques, the ridge length per area was
calculated for the tested structured surfaces. The results of these
measurements are summarized as follows:
TABLE-US-00006 TABLE 6 Measured Ridge Length per Area Ridge Length
per Area Sample (mm/mm.sup.2) 502-1 47.3 507-1 48.3 551-1 29.7
554-1 111.8 594-1 109.5 597-1 44.2 599-1 89.3 600-1 116.8 502-2
32.3 551-2 18.8 554-2 35.2 593-2 36.4 597-2 1.1 600-2 0.1 N3 50.5
L27B 0.3 RA24a 0.2 RA13a 0.0 SDB diffuser 2.2 CCS diffuser 4.4 DPB
diffuser 244.8
[0170] In reviewing the results of Table 6, we see that all or most
of the non-beaded samples made by the method of FIG. 9 have
structured surfaces characterized by a total ridge length per unit
area in plan view of less than 200 mm/mm.sup.2, and less than 150
mm/mm.sup.2, and in a range from 10 to 150 mm/mm.sup.2.
[0171] Unless otherwise indicated, all numbers expressing
quantities, measurement of properties, and so forth 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 specification and claims
are approximations that can vary depending on the desired
properties sought to be obtained by those skilled in the art
utilizing the teachings of the present application. Not as an
attempt to limit the application of the doctrine of equivalents to
the scope of the claims, each numerical parameter should at least
be construed in light of the number of reported significant digits
and by applying ordinary rounding techniques. Notwithstanding that
the numerical ranges and parameters setting forth the broad scope
of the invention are approximations, to the extent any numerical
values are set forth in specific examples described herein, they
are reported as precisely as reasonably possible. Any numerical
value, however, may well contain errors associated with testing or
measurement limitations.
[0172] Various modifications and alterations of this invention will
be apparent to those skilled in the art without departing from the
spirit and scope of this invention, and it should be understood
that this invention is not limited to the illustrative embodiments
set forth herein. For example, the disclosed transparent conductive
articles may also include an anti-reflective coating and/or a
protective hard coat. The reader should assume that features of one
disclosed embodiment can also be applied to all other disclosed
embodiments unless otherwise indicated. It should also be
understood that all U.S. patents, patent application publications,
and other patent and non-patent documents referred to herein are
incorporated by reference, to the extent they do not contradict the
foregoing disclosure.
[0173] The following are exemplary embodiment according to the
present disclosure:
Item 1. An optical film, comprising: [0174] a birefringent
substrate; [0175] a prismatic layer carried by the substrate, the
prismatic layer having a major surface comprising a plurality of
side by side linear prisms extending along a same prism direction;
and [0176] an embedded structured surface disposed between the
substrate and the prismatic layer comprising closely-packed
structures arranged such that ridges are formed between adjacent
structures, the structures being limited in size along two
orthogonal in-plane directions; [0177] wherein the embedded
structured surface has a topography characterizable by a first and
second Fourier power spectrum associated with respective first and
second orthogonal in-plane directions, and wherein [0178] to the
extent the first Fourier power spectrum includes one or more first
frequency peak not corresponding to zero frequency and being
bounded by two adjacent valleys that define a first baseline, any
such first frequency peak has a first peak ratio of less than 0.8,
the first peak ratio being equal to an area between the first
frequency peak and the first baseline divided by an area beneath
the first frequency peak, and [0179] to the extent the second
Fourier power spectrum includes one or more second frequency peak
not corresponding to zero frequency and being bounded by two
adjacent valleys that define a second baseline, any such second
frequency peak has a second peak ratio of less than 0.8, the second
peak ratio being equal to an area between the second frequency peak
and the second baseline divided by an area beneath the second
frequency peak; and wherein the embedded structured surface is
characterized by a total ridge length per unit area in plan view of
less than 200 mm/mm.sup.2. Item 2. The film of item 1, wherein the
embedded structured surface separates two optical media that differ
in refractive index by at least 0.05. Item 3. The film of item 1,
wherein the total ridge length per unit area is less than 150
mm/mm.sup.2. Item 4. The film of item 1, wherein the first peak
ratio is less than 0.5 and the second peak ratio is less than 0.5.
Item 5. The film of item 1, wherein the closely-packed structures
are characterized by equivalent circular diameters (ECDs) in plan
view, and wherein the structures have an average ECD of less than
15 microns. Item 6. The film of item 5, wherein the structures have
an average ECD of less than 10 microns. Item 7. The film of item 1,
wherein the prism direction and one of the first and second
orthogonal in-plane directions are the same. Item 8. The film of
item 1, wherein at least some of the closely-packed structures
comprise curved base surfaces. Item 9. The film of item 8, wherein
most of the closely-packed structures comprise curved base
surfaces. Item 10. The film of item 9, wherein all of the
closely-packed structures comprise curved base surfaces. Item 11.
An optical film, comprising: [0180] a birefringent substrate;
[0181] a prismatic layer carried by the substrate, the prismatic
layer having a major surface comprising a plurality of side by side
linear prisms extending along a same prism direction; and [0182] an
embedded structured surface disposed between the substrate and the
prismatic layer comprising closely-packed structures, the embedded
structured surface defining a reference plane and a thickness
direction perpendicular to the reference plane; [0183] wherein the
embedded structured surface has a topography characterizable by a
first and second Fourier power spectrum associated with respective
first and second orthogonal in-plane directions, and wherein [0184]
to the extent the first Fourier power spectrum includes one or more
first frequency peak not corresponding to zero frequency and being
bounded by two adjacent valleys that define a first baseline, any
such first frequency peak has a first peak ratio of less than 0.8,
the first peak ratio being equal to an area between the first
frequency peak and the first baseline divided by an area beneath
the first frequency peak, and [0185] to the extent the second
Fourier power spectrum includes one or more second frequency peak
not corresponding to zero frequency and being bounded by two
adjacent valleys that define a second baseline, any such second
frequency peak has a second peak ratio of less than 0.8, the second
peak ratio being equal to an area between the second frequency peak
and the second baseline divided by an area beneath the second
frequency peak; and [0186] wherein the closely-packed structures
are characterized by equivalent circular diameters (ECDs) in the
reference plane and mean heights along the thickness direction and
wherein an aspect ratio of each structure equals the mean height of
the structure divided by the ECD of the structure; and [0187]
wherein an average aspect ratio of the structures is less than
0.15. Item 12. The film of item 11, wherein the embedded structured
surface is characterized by a total ridge length per unit area in
plan view of less than 200 mm/mm.sup.2. Item 13. The film of item
12, wherein the total ridge length per unit area is less than 150
mm/mm.sup.2. Item 14. The film of item 11, wherein the
closely-packed structures are characterized by equivalent circular
diameters (ECDs) in plan view, and wherein the structures have an
average ECD of less than 15 microns. Item 15. The film of item 14,
wherein the structures have an average ECD of less than 10 microns.
Item 16. The film of item 11, wherein at least some of the
closely-packed structures comprise curved base surfaces. Item 17.
The film of item 16, wherein most of the closely-packed structures
comprise curved base surfaces. Item 18. The film of item 17,
wherein all of the closely-packed structured comprise curved base
surfaces. Item 19. An optical film, comprising: [0188] a
birefringent substrate; [0189] a prismatic layer carried by the
substrate, the prismatic layer having a major surface comprising a
plurality of side by side linear prisms extending along a same
prism direction; and [0190] an embedded structured surface disposed
between the substrate and the prismatic layer comprising
closely-packed structures having curved base surfaces; [0191]
wherein the embedded structured surface has a topography
characterizable by a first and second Fourier power spectrum
associated with respective first and second orthogonal in-plane
directions, and wherein [0192] to the extent the first Fourier
power spectrum includes one or more first frequency peak not
corresponding to zero frequency and being bounded by two adjacent
valleys that define a first baseline, any such first frequency peak
has a first peak ratio of less than 0.8, the first peak ratio being
equal to an area between the first frequency peak and the first
baseline divided by an area beneath the first frequency peak; and
[0193] to the extent the second Fourier power spectrum includes one
or more second frequency peak not corresponding to zero frequency
and being bounded by two adjacent valleys that define a second
baseline, any such second frequency peak has a second peak ratio of
less than 0.8, the second peak ratio being equal to an area between
the second frequency peak and the second baseline divided by an
area beneath the second frequency peak; and [0194] wherein the
embedded structured surface provides an optical haze of less than
95%. Item 20. The film of item 19, wherein the embedded structured
surface provides an optical haze of less than 90% Item 21. The film
of item 20, wherein the embedded structured surface provides an
optical haze of less than 80% Item 22. The film of item 19, wherein
the embedded structured surface is characterized by a total ridge
length per unit area in plan view of less than 200 mm/mm.sup.2.
Item 23. The film of item 19, wherein the first peak ratio is less
than 0.5 and the second peak ratio is less than 0.5. Item 24. The
film of item 19, wherein the closely-packed structures are
characterized by equivalent circular diameters (ECDs) in plan view,
and wherein the structures have an average ECD of less than 15
microns. Item 25. The film of item 24, wherein the structures have
an average ECD of less than 10 microns. Item 26. An optical film,
comprising: [0195] a birefringent substrate; [0196] a prismatic
layer carried by the substrate, the prismatic layer having a major
surface comprising a plurality of side by side linear prisms
extending along a same prism direction; and [0197] an embedded
structured surface disposed between the substrate and the prismatic
layer comprising closely-packed structures; [0198] wherein the
embedded structured surface has a topography characterizable by a
first and second Fourier power spectrum associated with respective
first and second orthogonal in-plane directions, and wherein [0199]
to the extent the first Fourier power spectrum includes one or more
first frequency peak not corresponding to zero frequency and being
bounded by two adjacent valleys that define a first baseline, any
such first frequency peak has a first peak ratio of less than 0.8,
the first peak ratio being equal to an area between the first
frequency peak and the first baseline divided by an area beneath
the first frequency peak; and [0200] to the extent the second
Fourier power spectrum includes one or more second frequency peak
not corresponding to zero frequency and being bounded by two
adjacent valleys that define a second baseline, any such second
frequency peak has a second peak ratio of less than 0.8, the second
peak ratio being equal to an area between the second frequency peak
and the second baseline divided by an area beneath the second
frequency peak; and [0201] wherein the embedded structured surface
provides an optical haze in a range from 10 to 60% and an optical
clarity in a range from 10 to 40%. Item 27. The film of item 26,
wherein the embedded structured surface is characterized by a total
ridge length per unit area in plan view of less than 200
mm/mm.sup.2. Item 28. The film of item 26, wherein the first peak
ratio is less than 0.5 and the second peak ration is less than 0.5.
Item 29. The film of item 26, wherein the closely-packed structures
are characterized by equivalent circular diameters (ECDs) in plan
view, and wherein the structures have an average ECD of less than
15 microns. Item 30. The film of item 29, wherein the structures
have an average ECD of less than 10 microns. Item 31. An optical
film, comprising: [0202] a birefringent substrate; [0203] a
prismatic layer carried by the substrate, the prismatic layer
having a major surface comprising a plurality of side by side
linear prisms extending along a same prism direction; and [0204] an
embedded structured surface disposed between the substrate and the
prismatic layer comprising larger first structures and smaller
second structures, the first and second structures both being
limited in size along two orthogonal in-plane directions; [0205]
wherein the first structures are non-uniformly arranged on the
embedded structured surface; [0206] wherein the second structures
are closely packed and non-uniformly dispersed between the first
structures; and [0207] wherein an average size of the first
structures is greater than 15 microns and an average size of the
second structures is less than 15 microns. Item 32. The film of
item 31, wherein the average size of the first structures is in a
range from 20 to 30 microns. Item 33. The film of item 31, wherein
the average size of the second structures is in a range from 4 to
10 microns. Item 34. The film of item 31, wherein the embedded
structured surface is characterized by a bimodal distribution of
equivalent circular diameter (ECD) of structures of the embedded
structured surface, the bimodal distribution having a first and
second peak, the larger first structures corresponding to the first
peak and the smaller second structures corresponding to the second
peak. Item 35. An optical film, comprising: [0208] a birefringent
substrate; [0209] a prismatic layer carried by the substrate, the
prismatic layer having a major surface comprising a plurality of
side by side linear prisms extending along a same prism direction;
and [0210] an embedded structured surface disposed between the
substrate and the prismatic layer, wherein the embedded structured
surface is made by microreplication from a tool structured surface,
the tool structured surface being made by forming a first layer of
a metal by electrodepositing the metal using a first electroplating
process resulting in a major surface of the first layer having a
first average roughness, and forming a second layer of the metal on
the major surface of the first layer by electrodepositing the metal
on the first layer using a second electroplating process resulting
in a major surface of the second layer having a second average
roughness smaller than the first average roughness, the major
surface of the second layer corresponding to the tool structured
surface.
* * * * *