Note: Descriptions are shown in the official language in which they were submitted.
CA 02450932 2003-12-22
Optical Film
Bac>Fground of the Invention
S The use of a variety of structured surface films in backlit displays is well
known.
For example, U.S. Patent No. 5,161,041 ("Abileah") describes a prismatic film
to enhance
the apparent brightness of a backlit liquid crystal display. The Abileah
patent further
describes the use of two such structured surface films, preferably with the
structures
oriented perpendicular to one another, to further enhance the apparent
brightness of such a
display. In general, the increase in on-axis brightness produced by such a
structured
surface film is known as the "gain" of such a film. The on-axis gain of a film
refers to the
ratio of the intensity of light as measured in a direction perpendicular to
the backlight with
the film to the intensity observed in a direction perpendicular to the same
backlight without
the film.
Other structured surface films may be used to redirect light traveling in one
direction to a more desired output angle. United States patents 4,984,144
("Cobb et al.")
and 5,190,370 ("Miller et al."),
teach light fixtures utilizing such films. These light fixtures could be used
as backlights for
displays.
One problem with using films such as those described above in a display that
is
intended for close viewing, such as a computer display, is that the cosmetic
requirements
are very high. This is because, when such displays are studied very closely or
used for an
extended period of time, even very small defects may be visible and annoying.
Elimination
of such defects can be very costly both in inspection time and discarded
materials.
There are several approaches to the problems created by the visibility of such
small
defects and consequent low manufacturing yield. One solution is to simply
accept the
relatively high rejection rate of common manufacturing processes. A second
solution is to
provide more efficient, and expensive, clean rooms, use only ultra-clean
materials, use
extraordinary care in the preparation of manufacturing tooling, and employ
extremely rigid
quality control procedures. While this will reduce the waste, it can introduce
even more
expense in order to produce higher yield.
-1-
CA 02450932 2003-12-22
Another solution is to provide the film with a diffuser. This may be a matte
finish
on the smooth side, the structured side, or both, of the film or a bulk
diffuser provided in
the film. Such diffusers will hide many of the defects, malting them invisible
to the user.
This will significantly improve manufacturing yield, while only adding a small
increase in
cost to the manufactured part. The disadvantage of this approach is that the
diffuser will
scatter the light and thus decrease on-axis gain. Therefore, a diffuser will
increase yield but
at the expense of some performance.
Another problem that has been observed in prior art systems utilizing two
sheets of
brightness enhancing film, as described above, is known as "wet-out." Wet-out
occurs as a
result of optical coupling between the prisms of one sheet and the smooth
surface of the
other. The optical coupling prevents total internal reflection from occurring
along these
peaks, thus destroying the brightness enhancing effect desired. The result is
a mottled and
varying appearance to the backlight.
Summary of the Inv
According to the present invention an optical film has structures that vary in
height along
their length where the variations have a nominal period of less than forty
times the nominal
height of the stmctures. In some embodiments of the invention the variation is
random.
Brief Deacr~~tion of the Drawings
FIGURE 1 shows a display utilizing a brightness enhancing film;
FIGURE 2 shows a brightness enhancing film;
. FIGURE 3 is a graph showing the operation of a brightness enhancing film;
FIGURE 4 shows a brightness enhancing film of the prior art;
FIGURE 5 shows an optical film according to the invention;
FIGURE 6 is a photo-micrograph of an optical film according to the invention;
FIGURE 7 is a photo-micrograph of an optical film according to the invention;
FIGURE 8 shows a second optical film according to the invention;
-2-
CA 02450932 2003-12-22
FIGURE 9 shows a fast tool servo actuator for use in manufacturing a film
according to the
invention;
FIGURE 10 shows a third film according to the invention; and
FIGURE 11 shows a light fixture utilizing a light redirecting film according
to the
invention.
Detailed Description
Figure 1 illustrates a first embodiment of a display, 10, according to one
aspect of the
invention. Display 10 includes a case 12. Display 10 includes an area source
of light 16 and a
structured surface material 18. Generally, area light source 16 is a thin,
solid light guide that
receives light through one or more of its narrow edges from one or more line
light sources, such
as fluo»t tubes, although other area light sources such as electroluminescent
materials may
be used. Typically, if a solid light guide is used, it wi8 be of an acrylic
material and conduct light
by total internal reflection. Generally the light guide will have diffusely
reflective dots on the
back to extract light in the direction of structured surface material 18. The
operation of such
area light sources is well known in the art. A reflective material, 19, is
preferably positioned
behind area fight source 16. Reflective material 19 is preferably a diH'use
reflector.
StNChrred material 18 has a smooth surface 20 a~ a saucdrred s<rrfacx 22.
Smooth surfacx 20 lies toward the bade of display 10 and structured surface 22
lies toward the
front. Stnuxured surface material 18 and area light source 16 are separated by
an optical
diffuser 24. Structured surface 22 has a light exit wedge associated therewith
and smooth
surface 24 has a light entry wedge associated therewith, where the tight exit
wedge is narrower
that the tight entry wedge. Although surface 24 is descn'bed here as smooth,
it could also have
structures thereon. Structured surface material 18 and its operation in the
invention will be
desaibed in greater detail in conjunction with Figures 2 and 3.
Di~lay 10 further includes a light gating device 26. Typically light gating
device 26 is a
liquid crystal display, although other light gating devices may be used. As is
well known in the
art, a liquid crystal display may be made transparent or opaque, in the case
of a monochrome
display, or transparent or a variety of colors in the case of a color display
by the proper
-3-
CA 02450932 2003-12-22
application of electrical signals. This will form images that will be visible
when area light source
16 is illuminated. Display 10 further includes a transparent cover sheet 28.
Figure 2 is an enlargement of structured strr~ material 18 of Figure 1. As
descn'bed
previously, structured surface material 18 has a smooth side 20 and a
structured side 22.
S Structured side 22, in the preferred embodiment, includes a plurality of
triangular prisms. In the
preferred embodiment, such prisms are right isosceles prisms, although prisms
having other peak
angles may be used. Prisms with peak angles in the range of 70 degrees to 110
degrees have
been shown to work with varying degrees of effectiveness with the invention.
Furthermore the
peaks, valleys or both of the prism structures may be curved in cross section.
Although this will
decrease the gain provided by the structures, it will provide other effects
that are sometimes
desirable
Stnrcbared surface material 18 may be of any transparent material having an
index of
ion greater than that of air, but, in general, the materials with higher
indices of refraction
wiv produce better results. Polycarbonate, which has an index of refraction of
1.586, has
proven to work very effectively. For purposes of description of a preferred
embodiment of the
invention, the prisms on strut surface 22 will be assumed to have included
angles of 90
degroes and structurod surface material 18 will be assumed to be of
polycarbonate.
Figure 3 illustrates the operation of structured surface material 18. Figure 3
is a graph
having two axes 26 and 28. These axes represent the angle that a tight ray
makes to a nom~al to
smooth surface 20. Specifically, axis 26 r~resa~ts the angle that the light
ray makes why the
direction of the light ray is projected into a plane par allel to the linear
extent of the structures on
struc~ed stuface 22. Similarly axis 28 repr~aris the angle that the light ray
makes to a normal
to smooth surface 20 when the direction of the light ray is projected into a
plane perpendicular
to the linear extent of the sbudarres on strucx~rced surfa~x 22. Thus a light
ray striking
perpendicular to smooth surface 20 would be represented by the origin, labeled
0 degrees, of the
graph of Figure 3. As may be seen, Figure 3 is divided into regions 30, 32,
and 34. Light rays
striking at angles that fall within region 30 wiU enter structured surface
material 18 but be totally
inter naily reHa~d by strucxured surface 22 so that they pass through smooth
surface 20 a
second tune and reenter diffuser 24. Light rays saildng smooth surface 20 at
an angle such that
they fall in region 32 or 34 will be trancrrrittod but refracted to a
different angle with respect to
the normal. As may be seen from Figure 3, which represents the performance of
polycarba~nate,
CA 02450932 2003-12-22
any light ray striking smooth surface 20 at an angle of less than 9.4 degrees
to the normal, wiv
be reflected.
Returning to Figure 2, four exemplary light rays are shown. The first, Light
ray 36,
approaches smooth surface 20 at a grazing angle, i.e., an angle to the normal
approaching 90
degrees. If light ray 36 makes an angle of 89.9 degrees to the normal to
surface 20 when it
strikes structured surface material 18, it will be refracted such that it
makes an angle of 39.1
degrees to the nornnal as it travels through structured surface material 18.
Upon reaching
struchued surface 22, it wiU be refiacted again. Because of the shuctures on
structured surface
22, it will be refracted so that again it will make a smaller angle to the
normal to structured
surface 20. In the example it will make an angle of 35.6 degrees.
Light ray 38 approaches smooth surface 20 at an angle much closer to the
normal to
smooth surface 20. It also is refracted as it passes through smooth surface
20, but to a lesser
extent. If light ray 38 approaches smooth surface 20 at an angle of 10 degrees
to the normal to
smooth surface 20, it will ecc~rge from structured ~uface 22 at an angle of
37.7 degrees to the
1 S normal to smooth surface 20 but on the opposite side of that normal.
Light ray 40 approaches at an angle even closer to the normal to smooth
surface 20 than
did light ray 38 and is totally internally reflected twice by structured
surface 22 and returned to
the interior of display 10.
Finally, light ray 42 approaches smooth surface 20 at an angle similar to that
of light ray
38, but in a location such that it is totally internally reflected by one side
of a prism on structured
surface 22 but not by the second side. As a result it emerges at a large angle
to the normal to
smooth surface 20. Because such a reflection only occurs to a light ray that
is traveling in a
direction that forms a high incidence angle to the side it strikes, the prisms
provide a very small
cross section to such rays. In addition many of those rays will reenter tfu
next prism and be
retwned into display 10.
A fifth class of light ray is not shown in Figure 2. This is the set of light
rays that are
reflected by smooth surface 20 and do not enter structured surface material
18. Such light rays
simply join the others that are reflected back ir~o display 10.
As may be ~ from this discussion, light that, absent structured surface
material 18,
would have~emerged from the display at a high angle to the axis of the
display, where the axis of
the display is taken to be the normal to smooth surface 20, is redirected into
a direction closer to
-5-
CA 02450932 2003-12-22
that axis. A small amount of light will be directed out at a large angle to
the axis. Thus, we may
say that light that enters structured~surface material 18 through smooth
surface 20 with an angle
of incidence greater than a predetermined angle is directed into an output
wedge that is
narrower than the input wedge and the majority of the light that enters
structured surface
material 18 through smooth surface 20 at an angle of incidence of less than
that predetermined
angle will be refloctal back into display 10,
The light that is reflected back into display 10 will strike reflerxor 19 and
diffuser 24.
This light will be reflected and di$used and then will travel back to
structured surface material
18, in general at a different angle than it made the first time. The process
is then repeated so
that more of the light is redirected into the smaU~ wedge. Generally, a
brightness enhanang
film tech as structured surface material l 8 w~l be capable of reflecting
light striking it in a first
predetermined group of angles and passing, but rWacting, light striking it in
a second
predetermined group of angles ~ the angles in the second group of angles are
greater than
those in the first group of angles and wherein the light in the second group
of angles is refracted
into an output wedge that is narrower than its input wedge. Furthermore, the
system must be
capable of recycling the light that is reflected by structured surface film 18
so that the majority of
the light available to the system is eventually emitted in the narrower output
wedge.
FIGURE 4 shows a typical brightness enhanang film of the prior art, designated
genaaUy as 40. Brightness enhanang film 40 has a smooth surface 42 and a
structured
44. Structured s<rrface 44 includes a plurality of linear prisms such as prism
46. Each prism on
stn~ur~ed s<rrface 44 has a peak such as peak 48. Each peak such as peak 48
has associated
therewith a peak angle such as peak angle 50. Preferably, the peak angles such
as peak angle 50
are 90°, although deviation from 90° is possr'ble. Furthermore,
it is known that the peaks such
as peak 48 may be curved in doss section rather than sharply pointed. Using a
curved or
rounded peak, however, will reduce the gain of the brightness enhancing film.
Generally,
however, according to the prior art, the peaks such as peak 48 have been
straight lines that
individually maintained an essentially uniform distance from surface 42. In
some prior art
embodiments, surfacx 42 is not planar but has a structure thereon. In such
cases, there is a plane
that is ga~rally assoaated with surface 42 and peak line 48 nrns at a constant
distance from the
plane a~oclated with surface 42.
-6-
CA 02450932 2003-12-22
Figure 5 shows an optical film 60 according to the present invention. Optical
film
b0 a structured surface 64 and an opposing surface 62. Opposing surface 62
could be
optically smooth or could be relatively smooth but provided with a matte
surface or other
surface diffuser. Alternatively, various structures could also be formed on
opposing surface
62.
Structured surface 64 has a plurality of structures such as structure 66. For
a
brightness enhancing film structure 66 acts effectively like the prisms of
Figure 4 but the
peak does not form a straight line as do the peaks of the structures of Figure
4. Instead the
heights of the peaks of the prisms of the film shown in Figure 5 vary
continuously along
their lengths. Similarly the depths of the valleys between the peaks vary
continuously.
Alternatively stated, the distances from the peak lines and/or the valley
lines of the
structures on structured surface 64, or simply from the structures themselves,
to the plane
associated with opposing surface 62 are continuously varying. In general, the
actual
heights of the structures, or the distances from the structures to the plane
associated with
1 S opposing surface 62, vary between 2% and 12% and more preferably between
4% and 8%
of the nominal or average height of the structures. The nominal or average
period of the
variations preferably should be between four and forty times the height of the
structures.
More preferably, the nominal period of the variations should be between five
and sixteen
times the nominal height of the structures. Preferably, the actual height
varies by an
amount and with a nominal period sufficient to substantially mask the small
cosmetic
defects typically encountered in the manufacturing process. Preferably, the
actual height
varies by an amount and with a nominal period sufficient to substantially mask
cosmetic
point or spot defects having maximum dimensions equal to or less than eight
times or more
preferably equal to or less than ten times the nominal height of the
structures and most
scratch defects.
This variation in the heights of the prisms causes several unexpected results.
First
films according to the invention do not look like high performance,
transparent, optical
films. Instead they have a deceptive, almost hazy, appearance that masks some
of the small
defects in the prism sheets discussed above. This can considerably improve the
yield of the
manufacturing process. Second, it reduces the area where the structured
surface of a prism
sheet can contact the smooth surface of an adjacent sheet, thus reducing the
area where
CA 02450932 2003-12-22
optical coupling can occur. This significantly improves the visual quality of
the assembled
display. Films of the invention will also help to eliminate or hide moire
patterns resulting . "
from interference between the prisms and the pixel pattern of the LCD. The
most
surprising result, however, is that a brightness enhancing film according to
the invention
accomplishes all of this while still providing essentially the same gain as a
prior art film of
the same material and having the same prism spacing or pitch.
Brightness enhancing films according to the invention could be of any
substantially
transparent material. A bulk diffusing material could be incorporated in a
film according to
the invention, although in many cases this will degrade the performance of the
optical film.
Unitary, extruded films of acrylics and polycarbonates work well.
Alternatively, the film
could be a two part construction in which the structured surface according to
the invention
is cast and cured on a substrate. For example, ultraviolet-cured acrylics cast
on polyester
substrates may be used. Films of polyethylene terphthalate ("PET") have been
shown to
work well as substrates on which stnrctures of the invention may be cured.
Biaxially
oriented PET is often preferred for its mechanical and optical properties. A
smooth
polyester film that may be used as a substrate is commercially available from
ICI Americas
Inc. Hopcwell, Virginia under the tradename Melinext"" 617. A matte finish
coating that
may be applied on a film to be used as a substrate is commercially available
from Tekra
Corporation of New Berlin, Wisconsin under the tradename MarnotT"" 75 GU.
Other films
could be used as well. These films could be chosen for their optical,
mechanical, or other
properties. For example, a substrate could be a mufti-layer optical film as
described in
published PCT patent application WO-97/01774.
Examples of other films that could be used are wavelength selective
mufti-layer optical films and reflective polarizers. Reflective polarizers
could be mufti-layer
films, cholesteric materials, or materials of the type disclosed in published
PCT patent
application WO-97/32227..
For brightness enhancing films according to the invention, the included angles
of the
structures of structured surface may be any angle in the range of 70°
to 110° and more
preferably in the range of 80° to 100°. Most preferably an angle
of 90° is used to provide
the highest gain. If a lower gain with a softer transition at the boundary of
the output
wedge is desired, the peaks or valleys or both of the stnrctures could be
rounded. It is even
_g_
CA 02450932 2003-12-22
possible to use a continuously varying contour, such as a sinusoidal
structure, although the
gain will be significantly reduced. In another embodiment the structures do
not need to be
symmetric. For example they could be canted as shown in published PCT patent
application WO-97/28468.
The pitch of the structures of a brightness enhanang film according to the
invention is
preferably 10 Erm and 100 ltm and more preferably between 24 Lrm and 50 Vim. A
pitch of 50 Nm has been found to work quite well. The preferred pitch will
depend, in part, on
the pixel pitch of the liquid aystal display. The prism pitch should be chosen
to help minimize
moire interference.
Figures 6 and 7 are images of optical films according to the invention
produced by a
scanning electron microscope. Both of the films shown have prismatic
structures with 90°
included angles. The pitch of the prisms in each film is 50 p.m. Reference
numerals 68 and
68' indicate prism peaks and reference numerals 69 and 69' indicate prism
valleys. The variation
of the peaks and valleys of the prisms may be clearly seen in these images.
Masters for the tools used for manufacturing brightness enhancing films,
whether by
extrusion or by a cast and cure process, may be made by known diamond turning
techniques. Typically the tools are made by diamond turning on a cylindrical
blank known
as a roll. The surface of the roll is typically of hard copper, although other
materials may
be used. The prism structures are formed in continuous patterns around the
circumference
of the roll. In a preferred embodiment the grooves are produced by a technique
known as
thread cutting. In thread cutting, a single, continuous groove is cut on the
roU while the
diamond tool is moved in a direction transverse to the turning roll. If the
structures to be
produced have a constant pitch, the tool will move at a constant velocity. A
typical
diamond turning machine will pmvide independent control of the depth that the
tool
penetrates the roll, the horizontal and vertical angles that the tool makes to
the roll and the
transverse velocity of the tool.
Figure 8 shows an alternative embodiment of the invention in which the
structures
hay rounded peaks and valleys rather than the sharp peaks and valleys shown in
Figure 5.
In another alternative embodiment, the variation in the structures may have
sharp
discontinuities rather than be smoothly varying as shown in Figures 5 and 8.
-9-
CA 02450932 2003-12-22
In order to produce the structures of the invention a fast tool servo actuator
is
added to the diamond turning apparatus. A fast tool servo actuator, designated
generally
as 70, is shown in Figure 9. A diamond tool, ?2, extends from a case including
walls ?4
and back 76. Diamond tool 72 is supported by a piezoelectric stack 78. When
piezoelectric stack 72 is stimulated by a varying electrical signal, it will
cause diamond tool
72 to be moved such that the distance that it extends from the case changes.
It is possible
for the piezoelectric stack to be stimulated by a signal of constant or
programmed
frequency, but it is generally preferable to use a random or pseudo random
frequency. As
used herein, the term random will be understood to include pseudo random. The
tool so
produced may then be used in standard extrusion or cast and cure processes to
produce an
optical film.
In a test of the present invention a brightness enhancing film having a pitch
of 50
~tm and ~aip prism peaks and valleys, all having included angles of
90°, was made. The fast
tool servo actuator was set to allow the diamond tool to move in the depth of
cut direction with
an amplihrde of 2 Nm. Since the height of a right angle prism is half of its
width, this made the
variation equal to about 8% of the nominal height of the structure. The fast
tool servo actuator
was stir mlated with white noise filtered by a band pass filter that
transmitted 4KHz to S.6KlEiz.
The diamond tinning machine was set so that tt~e roll would turn at a speed
such that its surface
would have a velocity of approximately 0.8 m per second and a brightness
enhancing pattern
was thread cut on the roll. This produced a structured pattern with a
variation
according to the invention having a nominal period of approximately 145 Etm.
This pattern was
a continuous groove on tip roll with a nominal depth cording to the nominal
height of the
strucdrre on the film to be mane. The result was a brightness enhanang film
having
essa~ially the same gain as one without the strucdrre produced with the fast
tool servo actuator,
but with significantly fewer observable defects.
Figure 10 is a side view of an optical film, 80, according to the imrention.
Film 80
includes a substrate 82 and a structured surface portion 84. The prism peak,
86, varies in height
along its length. The prism valley, indicated by hidden line 88, has a similar
variation.
Figure 11 shows a display, 90, usipg a light redirecting film according to the
invention
Light firm lighting element 92 is directed by reflector 94 into light guide
96. Lighting elect
92 is typically a 8uarescent tube, although other lighting elements could be
used. As shown,
-10-
CA 02450932 2003-12-22
light guide 96 is a wedge, but other shapes such as pseudo wedges could be
used. Light guide
96 could be transparent or could include a buUc diffuser. Light emerging from
light guide 96 at a
iow or grazing angle will enter light redirecting film 98. Light redirecting
film 98 has a
structured surface side 100. Structured surface side 100 has a plurality of
linear prisms such as
linear prism 102. Linear prism 102 has a first side 104 and a second side 106.
Light from light
guide 96 will enter light redirecting film 98 through the first sides of the
linear prisms such as
first side 104 and be totally internally reflected by second side 106 and
emerge from light
redirecting film 98 through opposing surface 107 . The light will then pass
through a light gating
device I08. Light gating device 108 is typically a liquid cxystal.
As with~a brightness enhanang film, light redirecting film 98 could be
extruded or cast
and cured on a substrate. The shape and size of prisms such as prism 102 will
be dictated by the
design of light guide 96 and the nature of light gating device 108. Typically,
light emerging
from light redirecting film 96 should be traveling in a direction nom~al to
the surfaces of light
gating device 108. Generally, this will require that the sides of the linear
prisms, such as sides
104 and 106 , are substantially flat. However, if a greater angular spread of
light output is
desired, the sides of the prisms may be cawed in cross section. The linear
prisms, such as
prisms 102 may be symmetric or asymmetric. In general in symmetric designs the
prisms for a
light redirecting film wdl have peak angles in the range of 60° to
72° and asymmetric designs
will have smaller peak angles. The exact design, however, will always depend
on the backlight
and desired result.
_11_
