Wide angle optical retarder

In an acrylonitrile based retarder, relatively uniform performance is obtained as the angle of incidence varies over a wide range. A toughening material may be blended with an acrylonitrile based polymer to facilitate processing of a retarder and improve mechanical properties of the retarder without compromising the optical performance. A rubber-modified acrylonitrile based retarder which provides relatively uniform wide angle can be fabricated using conventional processing techniques. Such retarders are particularly suited for a number of specific applications which use relatively wide ranges of incidence angles.

BACKGROUND 
This invention relates generally to an optical retarder, and more 
particularly to an optical retarder operable over a wide range of 
incidence angles. 
Optical retarders are generally used in some manner to alter the relative 
phase of polarized light passing through the retarder. Optical retarders 
are particularly suited for use in applications where control over the 
polarization is required. Polarization of light generally refers to the 
restriction of electric (or magnetic) field vector vibrations to a single 
plane. The polarization direction of electromagnetic radiation is 
generally considered the direction in which the electric field vector 
oscillates back and forth. The polarization vector is orthogonal to the 
beam direction within the light plane. 
Polarized light can assume a number of different forms. Where a light beam 
oscillates in only one direction at a given point the beam is said to be 
linearly (or plane) polarized. The direction of oscillation is the 
polarization direction. If the light beam has two orthogonal polarization 
directions which vary in phase by 90.degree., the beam is said to be 
elliptical or circularly polarized. Circular polarization occurs when the 
magnitude of the two oscillations are equal (i.e., the tip of the electric 
field vector moves in a circle). Elliptical polarization occurs when the 
magnitudes are not equal (i.e., the tip of the electric field vector moves 
in a ellipse). In contrast, the orthogonal oscillations for unpolarized 
light are on average equal with a randomly varying phase difference. 
Linearly polarized light can be obtained by removing all waves from an 
unpolarized light beam except those whose electric field oscillate in a 
single plane. Optical retarders can be used, for example, to convert 
linearly polarized light to circular or elliptically polarized light. When 
used to control the polarization of light, retarders are commonly 
constructed to induce 1/2- and 1/4-wave retardations. Generally, such 
retarders are used to produce a desired relative phase delay between two 
linear components of the polarized light. 
One typical use of an optical retarder is a compensator which is used to 
introduce a phase delay in incident light to correct for phase differences 
between two components of polarized light introduced by mechanical or 
optical displacement of other optical components in a system. In a liquid 
crystal display (LCD), for example, birefringence of a liquid crystal cell 
may cause the linearly polarized light to become slightly elliptical. A 
retarder is used to convert the elliptically polarized light back to 
linearly polarized light. The compensating retarder is placed in the light 
path and is tuned to a particular phase difference introduced by the 
birefringence of the liquid crystal. 
Typical optical retarders are constructed of birefringent materials. The 
birefringent materials form a fast and slow path along two orthogonal 
in-plane axes of the retarder. When the axes of the birefringent retarder 
are aligned at 45.degree. degrees to the polarization direction of the 
incident light, the retarder can be used to introduce or compensate for 
phase differences between two polarization components. The fast and slow 
path of the birefringent retarder results from different refractive 
indices for light polarized along the in-plane axes of the retarder. 
Larger retardation differences between the two polarization axes are 
achieved by increasing the refractive indices difference between the two 
in-plane axes and/or increasing the thickness of the retarder. Thus, by 
controlling the thickness and refractive indices of the birefringent 
material in the retarder, the optical properties of the retarder can be 
controlled. 
In addition to refractive indices for light polarized along the in-plane 
axes of the retarder, the refractive index for light polarized in the 
thickness direction may influence the performance of the retarder in a 
given application. Compensators used in LCD display technology, for 
example, must provide relatively uniform retardation of light which is 
incident on the compensator over a relatively large angle range. It has 
been proposed that widened viewing angle ranges for LCD displays are 
obtainable by employing retardation films which have controlled refractive 
indices for light polarized in the thickness direction. 
Current attempts to produce retarders having uniform wide angle performance 
have proven to be expensive and difficult to manufacture and have only 
achieved limited success in obtaining uniform wide angle optical 
properties. Attempts to obtain uniform wide angle performance are varied 
and include, for example, shrinking the film in the direction 
perpendicular to the stretching direction at the time of stretching, 
controlling, by stretching, the birefringence of a raw film produced from 
a molten polymer or a polymer solution under an applied electric field, 
laminating a film produced under an electric field onto a conventional 
phase retarder, and the like. Such processes are typically quite complex 
and expensive and achieve only limited success. As the process and 
materials used in forming the birefringent portion of a retarder become 
more complex, it becomes increasingly difficult to incorporate such 
material into the retarder. 
SUMMARY 
Generally, the present invention relates to optical retarders. In one 
embodiment, an optical retarder is provided which uniformly retards light 
incident on the retarder over a relatively wide range of incidence angles 
varying from an angle normal to a plane of the retarder to a maximum angle 
of at least about 30 degrees. The optical retarder can include a substrate 
and a blended film of an acrylonitrile based polymer and elastomeric 
copolymer disposed on the substrate. The magnitude of retardation varies 
by less than about 25% of the normal angle incidence retardation as the 
angle of incidence varies from normal incidence to incidence at the 
maximum angle. In one embodiment the maximum angle may be greater than 
about 60 degrees. When the maximum angle is smaller the variance in 
retardation may be less. 
In another embodiment, an acrylonitrile based retarder mirror is provided. 
Linearly polarized light reflected by the retarder mirror is rotated to a 
substantially orthogonal linear polarization. The rotation of the 
polarization orientation is relatively uniform over a relatively wide 
range of incidence angles onto the retarder mirror. In another embodiment, 
an anti-reflective optical construct includes an acrylonitrile based 
retarder to improve off-normal angular performance of the anti-reflective 
construct. 
The above summary of the present invention is not intended to describe each 
illustrated embodiment or every implementation of the present invention. 
The figures and the detailed description which follow more particularly 
exemplify various embodiments.

DETAILED DESCRIPTION 
The present invention is applicable to a number of optical retarders. The 
present invention is particularly suited to optical retarders used in 
environments where the light to be retarded is incident on the retarder 
over a relatively wide range of incidence angles. Such a retarder is well 
suited for use as optical compensators, 1/2-wave and 1/4-wave retarders, 
and the like. To facilitate explanation of the invention, various examples 
of such retarders are provided below. 
An optical retarder in accordance with one particular embodiment of the 
invention will be described in connection with FIG. 1A. The optical 
retarder 101 in FIG. 1A is formed of an acrylonitrile based polymeric 
film. The film can be described by three mutually orthogonal axes, namely, 
two in-plane axes x and y and third axis z in the thickness direction of 
the film. As illustrated in FIG. 1B, the acrylonitrile based retardation 
film 101 illustrated in FIG. 1A can be disposed on a substrate 105. The 
substrate 105 may serve a variety of purposes. For example the substrate 
may be optically neutral, such as glass, and be used mainly for its 
mechanical properties and/or as a basis for affixing the retardation film 
101 to other optical elements. The substrate may also serve one or more 
optical functions. For example, the substrate may be a mirror, a 
polarizer, or the like, where the retardation film functions as an optical 
retarder in a larger optical construction. The substrate may also be a 
polymeric film. The polymeric film may be isotropic or may be birefringent 
(in-plane or out-of-plane) to act in cooperation with the optical 
performance of the acrylonitrile based retarder to obtain a desired 
overall optical performance. The film may also be combined with a 
compensator film to improve optical performance. 
Generally, the retardation film 101 can be used in connection with any 
suitable substrate 105. The retardation film 101 may be laminated to the 
substrate, affixed with an adhesive, or otherwise suitably disposed on the 
substrate. Care should be taken to ensure that the process and manner used 
to dispose the retardation film 101 on the substrate 105 does not 
adversely affect the optical performance of the ultimate retarder 
construction. 
As described more fully below, it has been determined in connection with 
the present invention that the acrylonitrile based retardation film is 
particularly suited for use in applications where it is desirable to have 
relatively uniform retardation of light incident on the retarder over a 
wide range of angles. Referring again to FIG. 1A, the retardation and 
angular performance of the retarder 101 are a function of the thickness d 
of the film and the relative refractive indices n.sub.x, n.sub.y and 
n.sub.z of the film for light polarized in the direction of the x, y and z 
axes, respectively. Birefringence along the in-plane axes, for example, 
creates short and long paths for polarized light incident on the film. 
Generally, the light is incident on the film with the polarization 
direction aligned at an angle of 45.degree. to the axes of the in-plane 
refractive indices. 
The retardation of the film is defined generally as the phase difference 
introduced between the linear components of polarized light E.sub.p and 
E.sub.s aligned along directions parallel (p) and perpendicularly (s) to 
the plane of incidence. In an ideal 1/4-wave retarder, for example, light 
polarized along one axis (i.e., the component of the polarized light along 
the axis) is delayed, relative to light polarized along the other in-plane 
axis, by one-quarter of its wavelength. When the polarized light is 
initially linearly polarized, the two components are either in-phase or 
180.degree. out-of-phase (i.e., the phase difference is equal to 0 or .pi. 
radians). When linearly polarized light passes through a 1/4-wave 
retarder, a phase difference of .pi./2 radians is introduced between the 
two components. The total phase difference between the two components 
E.sub.p and E.sub.s is now .pi./2 or 3.pi./2 radians. In this manner, a 
1/4-wave retarder can be used to convert between linearly polarized light 
and circularly polarized light. 
When light is incident on the retarder at an angle normal to the plane of 
the retarder, the retardation is a function of the thickness of the film 
and difference in the in-plane refractive indices n.sub.y and n.sub.x. As 
the angle of incidence deviates from normal incidence, the retardation of 
light passing through the retarder is also influenced by the refractive 
index n.sub.z for light polarized in the thickness direction z of the 
retarder. The off-normal performance of a given retarder can be considered 
by comparing the magnitude of retardation at normal incidence with the 
magnitude of retardation for incident light which varies from normal 
incidence. 
For a given retarder having known refractive indices, the relative 
magnitude of retardation at different angles can be examined using the 
relationship 
##EQU1## 
where .delta. is the magnitude of retardation between the s and p fields, 
d is the thickness of the film, n.sub.x, n.sub.y and n.sub.z are the 
respective refractive indices of the film for light of a given wavelength, 
and .phi. is the angle of incidence in the x-z plane (measured from an 
axis normal to the plane of the film). Thus, the magnitude of the 
retardation in equation (1) represents the difference in delay experienced 
by s- and p-polarized light components of the incident light, as the 
incident light passes through the retarder, as a function of incidence 
angle in the x-z plane. It should be appreciated that equation (1) is 
provided as one way of expressing the retardation. A similar expression 
can be derived to express the retardation for light as a function of light 
varying in other planes (e.g., y-z plane). 
In the above relationship, when light is incident on the film at a 
direction normal to the film (i.e., .phi.=0), the magnitude of retardation 
.delta. reduces to a function of the thickness d and the in-plane 
refractive indices difference described by the relationship 
EQU .delta.=d(n.sub.x -n.sub.y). (2) 
Thus, a desired retardation for light of normal angle incidence can be 
obtain by controlling the thickness of the film and the in-plane 
refractive indices. Higher retardation can be obtained by increasing the 
difference between n.sub.x and n.sub.y and/or by increasing the thickness. 
The amount of retardation desired depends generally on the particular 
application for which the retarder will be used and the wavelength of 
light to be retarded. Typical 1/4-wave retarders, for example, have 
retardation values ranging from about 115 nm to about 158 nm. Typical 
1/2-wave retarders have retardation values ranging from about 230 nm to 
about 316 nm. Full wave retarders could also be used to simply shift the 
phase of the two components by 2.pi. radians. There are a number of 
applications particularly suited for acrylonitrile based retarders having 
retardation values ranging from 115 nm to 158 nm. Unless otherwise noted, 
in the discussion set forth below light having a wavelength of about 550 
nm (the approximate center wavelength of visible light) is used to 
characterize the performance of the retarder. While such light is 
appropriately used as a means for characterizing the retarder, it should 
be appreciated that the retarder may be used to retard light over the full 
visible range or at any particular wavelengths or wavelength bands 
thereof. 
The difference between retardation of light incident at the normal angle 
and the retardation of light deviated from normal (off-normal) can be used 
to determine the appropriateness of the optical retarder for use in 
applications where light is incidence on the retarder over a wide range of 
angles varying from normal to a maximum off-normal angle. As described 
more fully below, the acrylonitrile based retarder provides exceptional 
off-normal optical performance. 
Generally, birefringence is induced in a polymeric material by stretching 
or drawing the material. As the material is stretched, molecules tend to 
align in the stretched direction. The induced molecular orientation 
creates refractive index differences for light polarized in the stretched 
and non-stretched directions. Stretching polymeric films not only induces 
a change in the refractive index for light polarized in the stretch 
direction but may also induce changes in the non-stretch and thickness 
directions. Under typical draw conditions using a tenter, for example, the 
changes in refractive indices for light polarized in the non-stretched and 
thickness direction are often quite different. As a result, as a film is 
stretched to obtain a desired in-plane refractive index mismatch, the 
thickness direction refractive index may not match either of the in-plane 
refractive indices. While such a change may not impact the performance of 
retarders used with normal incident light, the change can significantly 
impact the retarders performance when retarding off-normal incident light, 
especially where relatively large angles are used. 
From equation (1) it can be determined that improved off-normal performance 
is obtained when the refractive index n.sub.z for light polarized in the 
thickness direction is between the in-plane refractive indices n.sub.y and 
n.sub.x. Under typical drawing conditions, however, the thickness 
direction refractive index n.sub.z of a drawn polymeric film does not fall 
between the in-plane refractive indices n.sub.y and n.sub.x. In accordance 
with the present invention, when acrylonitrile based polymers are 
stretched a desired mismatch can be obtained between the in-plane 
refractive indices while maintaining substantially equal refractive 
indices for light polarized in the non-stretched and thickness directions. 
Moreover, closely matched n.sub.y and n.sub.z refractive indices are 
obtained even when the film is drawn in a manner which constrains 
dimensional reduction in the non-stretched direction (e.g., when the film 
is stretched using a conventional tenter process). 
As will be appreciated from equation (1), as the angle of incidence 
increases, the amount of retardation changes. In the acrylonitrile based 
retarders of the present invention, the magnitude of the change in 
retardation is significantly reduced as a result of the substantially 
equal refractive indices (e.g., n.sub.y and n.sub.z). In contrast, a 
typical birefringent polymer such as polypropylene, for example, when 
stretched in a conventional tenter exhibits a mismatch in the 
non-stretched and thickness direction refractive indices on the order of 
0.009. As a result of this mismatch, the off-normal performance of such a 
retarder is significantly impaired when compared to an acrylonitrile based 
retarder. 
While an acrylonitrile based retarding film can be used in retarder 
applications using normal and near normal incident light, such a retarder 
is particularly suited for use in applications where the incident light 
varies from normal incidence to an off-normal angle of incidence of at 
least about 30.degree.. In such applications, using an acrylonitrile based 
retarder permits one to obtain a difference in retardation between the 
normal and off-normal incident light which is less than 15% of the normal 
incidence retardation, and is more preferably less than 10% and even more 
preferably less than about 6%. As the off-normal angle of incidence 
increases the retardation difference also increases. However, at 
off-normal angles as high as 60.degree., the retardation difference of the 
acrylonitrile based retarder is less than 30% of the normal incidence 
retardation, and is more preferably less than about 25% and even more 
preferably less than about 20%. An acrylonitrile based retarder may also 
be used to get uniform retardance at lower angles of incidence. For 
example, advantages are obtained when the maximum off-normal angle is at 
least about 15.degree. or even less. Exemplary embodiments of 
acrylonitrile based retarders are provided below. 
As noted above, fabrication techniques suggested and used for the 
production of wide angle retarders are complex and expensive. Such 
techniques often involve steps of laminating multiple materials together, 
stretching birefringent materials using highly specialized equipment to 
artificially control the respective refractive indices, and the like. In 
contrast, in one embodiment of the invention, an acrylonitrile based 
retarder can be fabricated using standard processing equipment, such as a 
tenter for stretching, with little or no modification. Thus, significant 
cost savings can be achieved. Moreover, the process facilitates a high 
yield further reducing the costs of producing acrylonitrile based 
retarders. 
One acrylonitrile based film found to be particularly suited for optical 
retarders is a blend of an acrylonitrile phase and a toughening phase. An 
elastomeric (rubbery) copolymer, for example, may be used as a toughener 
in the blend. A number of advantages are obtained by the addition of a 
toughening phase. For example, the resultant film will have an increased 
resistance to impact and the film is rendered more flexible and exhibits 
enhanced resistance to cracking, splitting and tearing. The elastomeric 
phase may also enhance the drawability. 
The addition of a toughening phase, however, must also be taken into 
account in the formation of the optical retarder. As described more fully 
below, in accordance with one embodiment an acrylonitrile based polymer 
and elastomeric copolymer blend is uniaxially stretched to obtain the 
desired birefringence and thickness of the retarder. Because the 
acrylonitrile polymer and the elastomeric copolymer are oppositely 
birefringent in relation to the imposed strain, the strain-induced change 
in the refractive index of the elastomeric phase reduces the overall 
retardation of the stretched film. Thus, blended acrylonitrile based films 
including a elastomeric copolymer must be made thicker than acrylonitrile 
based films without the elastomeric copolymer in order to obtain the same 
overall retardation. Increased thickness, however, increases overall 
absorption and off-normal retardation. These can lead to reduced 
transmitted intensities and/or off-normal color variations, both of which 
may be detrimental to many retardation applications. Increased thickness 
may also be desirable in certain instances to improve film handling and 
processing (e.g., thicker film may be more easily laminated). 
Depending on the application, different amounts of the toughening copolymer 
may be added to the blend. In general, a balance must be struck between 
competing interests. For example, the amount of toughener used must be 
weighed against the increased thickness required to obtain a desired 
retardation. Generally, where an elastomeric copolymer is used, it is 
desirable that the elastomeric phase be less than about 18%-20% by weight. 
Where relatively high transmission is required it is desirable that the 
elastomeric phase be less than about 15% and even more preferably less 
than about 10%, and still even more preferably, less than about 5%. 
When using a toughener, other optical properties of the toughener must also 
be considered. It is generally desirable that the refractive indices of 
the acrylonitrile based polymer and the toughener be relatively close. 
This is important to minimize diffuse scattering and reflection of the 
light passing through the retarder as it interacts with the different 
phases. In the above example, relatively close matching can be obtained by 
matching the isotropic refractive indices prior to stretching of the 
acrylonitrile based polymer and the elastomeric copolymer. While this may 
not produce an exact match in the stretched film, due to different changes 
in the refractive indices during stretching, the indices are close enough 
for many applications. It also may be possible to select materials, 
composition and initial refractive indices such that in the process of 
orientation the refractive indices of the two phases approach one another 
further reducing or eliminating hazing in the stretched film. 
To facilitate an understanding of the present invention, exemplary 
retarders comprised of an acrylonitrile based polymer/elastomeric 
copolymer blend will be described. While the examples below describe a 
process in which a web is cast and then oriented in the transverse 
direction using a tenter, any of a number of other typical film processing 
techniques could be used. For example, the polymer may be extrusion-cast 
or solvent-cast. Webs may be cast on to an open-faced wheel or into a nip. 
Orientation may be affected in a variety of ways. For example, the film 
may be stretched uniaxially (machine or transverse direction) or biaxially 
using typical machine-direction orienters and/or tenters (e.g., mechanical 
and linear-motor). The film may also be oriented using a blown-film (e.g., 
single- and double-bubble) process, by calendering in a nip, by stretching 
the molten polymer into a web prior to cooling, and the like. 
In one particular embodiment, the retarders are fabricated using 
rubber-modified, acrylonitrile-methyl acrylate copolymers (72-99.5% 
copolymer, 18-0.5% elastomeric phase). The acrylonitrile-methyl acrylate 
copolymer composition ranges from 70-100% acrylonitrile and 30-0% methyl 
acrylate. The elastomeric phase contains from 70-90% butadiene with 30-10% 
acrylonitrile. Rubber-modified, acrylonitrile-methyl acrylate copolymers 
having 10% and 18% elastomeric phases are available from BP Chemicals 
(Barex.RTM. 210 and 218). 
While the example provided herein use acrylonitrile copolymerized with 
methyl acrylate, other types of acrylonitrile based polymers may be used. 
For example, suitable copolymers containing acrylonitrile can obtained by 
copolymerizing acrylonitrile with a variety of (meth)acrylate monomers 
which have glass transition temperatures (Tg) which is less than about 
20.degree. C. Such (meth)acrylate monomers include, for example, methyl 
acrylate, propyl acrylate, butyl acrylate, isooctyl acrylate, and 2-ethyl 
hexyl acrylate or a mixture of such monomers. 
In accordance with one embodiment of the invention, rubber-modified 
acrylonitrile based optical retarders were fabricated. The retarders were 
acrylonitrile based compositions having an elastomeric phase of either 10% 
or 18%. The copolymer phase composition was 75% acrylonitrile and 25% 
methyl acrylate. The elastomeric phase contained 70% butadiene and 30% 
acrylonitrile. As noted above, the inclusion of the elastomeric phase 
provides toughness to the composition. The compositions of the two phases 
(copolymer and butadiene-based elastomeric phase) are selected to obtain 
closely matched refractive indices. Such compositions are available from 
BP Chemicals (Barex.RTM. 210 and 218) in extrusion and injection molding 
grades. The Barex.RTM. family of acrylonitrile resins are typically used 
to form high gas barrier packing materials and the like. 
Webs of the above compositions were cast with an initial thickness ranging 
from 254-355 .mu.m. The cast webs were processed to obtain target 
retardation values of approximately 100-140 nm. The cast webs were drawn 
uniaxially in a tenter. Draw temperatures for such films generally range 
from about 25.degree. C. to 120.degree. C. The draw temperatures are more 
preferably between about 90.degree. C. and 110.degree. C., and even more 
preferably between about 90.degree. C. and 105.degree. C. Draw ratios for 
such a process range from approximately 1.5:1 to 5.0:1. The draw ratios 
are more preferably between about 2.0:1 and 5.0:1, and even more 
preferably between about 2.5:1 and 4.0:1. Appropriate draw rates range 
from about 1% to 3000% per second. The draw rates are more preferably 
between about 5% and 1000% per second , and even more preferably between 
about 10% and 200% per second. 
When using the 10% elastomeric composition, optical retardation films 
ranging in thickness from 63-115 .mu.m were produced which offered the 
targeted retardation ranges. The films also exhibited minimal off-normal 
coloration. The transmission intensity of such films exceeded 92%. 
When using the 18% elastomeric compositions, it was apparent that the 
target retardation values could only be obtained with significantly 
thicker initial webs to increase the drawn thickness (e.g., 254-635 
.mu.m). Thus, retarders comprised of an 18% elastomeric phase exhibit 
reduced transmission and worse off-normal performance. An optimum 
concentration of the elastomeric phase appears to be between 5-10%. Such 
concentrations are believed to strike an optimum balance where relatively 
high retardation values and light transmission were required. As described 
more fully below, a retarder manufactured from a composition including 
about 10% of the elastomeric phase can be fabricated in a relatively 
inexpensive manner and has relatively uniform performance over a wide 
range of incidence angles. 
As noted above, the inclusion of a toughener, such as an elastomeric phase, 
in the acrylonitrile based composition tends to reduce the ability to 
induce a desired birefringence in the film by stretching. In a typical 
tenter process, because the film must be stretched in many instances near 
its breaking point to obtain the desired retardation, it is desirable that 
the initial web be substantially free of orientation in the machine 
direction prior to stretching. This is because initial orientation in the 
non-stretched direction must be overcome during the tenter operation 
before the desired orientation in the stretched direction can be obtained. 
Drawing 10% rubber-modified acrylonitrile based webs in a tenter to obtain 
highly transmissive 1/4-wave retarders, for example, typically requires 
that the pre-stretched webs be free from any orientation in the machine 
direction. Thus, it is desirable that the cast webs must be initially cast 
in a manner which minimizes, or in certain instances eliminates, 
unintentional or residual molecular orientation in the machine direction. 
In certain instances, the film may be drawn in the direction of an initial 
orientation which relaxes the requirement for the casting process. For 
example, the film may be cast and then drawn in the machine direction 
using a length orienter (LO). Such an LO process may take advantage of the 
initial machine direction. In fact, in such a case a machine direction 
orientation may be purposefully induced during casting to assist the 
formation of the desired birefringence. Other orientation processes could 
also be used. For example, machine direction orientation may be induced in 
the molten polymer after it exits the die and prior to solidification. In 
general, it is desirable that prior to stretching the film have no 
substantial orientation in the non-stretch direction, regardless of the 
manner and/or direction in which the film is stretched. 
While cast webs of uniform thickness are described above, the thickness of 
the cast web may also be altered. As noted above, retardation is a 
function of the retarder thickness. Thus, retarder films having a varying 
retardation profile across the film may be produced by controlling the 
casting process to create thickness differences at different points on the 
web. 
The cast web of the present invention may be drawn in a direction 
orthogonal to the cast direction using a conventional tenter. The draw 
temperature, rate and ratio are selected to induce a desired refractive 
index differential between in-plane refractive indices of the drawn web. 
In this manner, a desired retardation .delta. is obtained according to the 
relationship .delta.=d(n.sub.x -n.sub.y), while substantially matching the 
refractive index of the drawn web for light polarized in the non-drawn and 
thickness directions. The off-normal retardation can be determined from 
Equation (1) (with n.sub.y and n.sub.z being substantially equal). 
FIGS. 2A-2C illustrate various optical properties of acrylonitrile based 
retarders in accordance with the present invention. Using the above 
general process, a transparent acrylonitrile based retarder film was 
obtained. A 10% rubber-modified acrylonitrile-methyl acrylate 312 .mu.m 
thick extruded web was used in which the initial isotropy was 
substantially preserved in its formation. The web was uniaxially stretched 
to 3 times its original width in the cross direction. The drawing 
temperature was about 90.degree. C. The resulting film was approximately 
88.5 .mu.m thick with refractive indices for 550 nm light polarized in the 
stretched direction of 1.5128 (n.sub.x) and 1.5142 for 550 nm light 
polarized in both the non-stretched and thickness directions (n.sub.y and 
n.sub.z, respectively). 
The retardation values for the above film were measured and compared with 
the retardation values determined using equation (1). FIG. 2A illustrates 
a comparison of the measured retardation values 201 and the retardation 
values derived from the measured refractive indices using equation (1) for 
the film at normal, 10.degree., 30.degree. and 40.degree. angles of 
incidence. The difference in retardation as the angle of incidence 
increases from normal to 40 degree is approximately 10% (13 nm) of normal 
incidence retardation. In contrast, a polypropylene film having a similar 
normal angle retardation will vary by approximately 50% (60 nm) at a 
40.degree. angle of incidence. Retardation of a polystyrene film drops by 
nearly 80% (100 nm) for light incident at 40.degree. off-normal, while 
having acceptable retardation performance at normal angle incidence. 
Using equation (1), the retardation difference as the incident light moves 
from normal incidence for the above film was determined. FIG. 2B lists the 
retardation values (nm) of the film at different angles of incidence 211, 
the change in retardation as the incident light moves off-normal 213, and 
the retardation at the respective incident angles as a percentage of 
retardation at the normal angle incidence. FIG. 2C illustrates a plot of 
the retardation values (nm) of the film as a function of incidence angles. 
A second acrylonitrile based retarder film was produced by uniaxially 
stretching a 317.5 .mu.m thick optically isotropic extruded film to 4 
times its original width in the cross direction at a temperature of 
95.degree. C. The resulting film was approximately 84 .mu.m. The 
refractive indices for light polarized along each direction, with n.sub.x 
being the refractive index for light polarized in the stretched direction, 
were measured for light at 488 nm, 550 nm and 700 nm as follows: 
______________________________________ 
488 nm 550 nm 700 nm 
______________________________________ 
n.sub.x 
1.5162 1.5124 1.5055 
n.sub.y 
1.5175 1.5139 1.5066 
n.sub.z 
1.5174 1.5139 1.5066 
______________________________________ 
FIG. 3A illustrate a Table which lists the retardation values (nm) 303 for 
light of 550 nm incident on the film at various angles 301. FIG. 3A also 
lists the difference in retardation (nm) 305 as the incident light moves 
from normal incidence. FIG. 3A further lists the retardation at off-normal 
angles of incidence as a percentage 307 of retardation values at normal 
angle incidence. FIG. 3B illustrates a plot of the retardation values (nm) 
311 as a function of the angle of incidence. FIG. 3C illustrates the 
retardation difference (nm) as a function of angle. As illustrated in 
these Figures, the off-normal performance of the acrylonitrile based 
retarder is relatively uniform compared to other single film retarders 
making the retarder well suited for a number of applications where uniform 
retardation is required for a wide range of incidence angles. 
As noted above, improved off-normal performance of the acrylonitrile-based 
retarder results from the matching of the n.sub.y and n.sub.z refractive 
indices. FIGS. 4A-4B illustrates how an increase in the difference between 
n.sub.y and n.sub.z would impact the off-normal performance of the 
retarder. In FIG. 4A, the retardation values 401 of the 
acrylonitrile-based retarder, as a function of incidence angle, are 
illustrated for the retarder described in connection with FIGS. 3A-3C. 
Columns 403, 405, and 407, illustrate the off-normal performance of 
retarders having the same normal axis retardation values as the difference 
in refractive indices n.sub.y and n.sub.z increases from 0.0003 to 0.0009, 
respectively. The retardation at wide angles of incidence changes 
significantly. 
As FIG. 4A illustrates, larger differences between n.sub.y and n.sub.z 
cause an increase in the drop in retardation at larger angles of 
incidence. In certain applications, it may be desirable that the overall 
retardation difference between normal incidence and incidence at 
60.degree. be less than about 20% (e.g., 20-30 nm) of the normal incidence 
retardation. This can be obtained using an acrylonitrile based retarder 
which has substantially equal n.sub.y and n.sub.z refractive indices. For 
example, as the above data illustrates, refractive indices equal to at 
least the fourth decimal places provide relatively uniform wide angle 
performance. FIG. 4B plots a comparison of the off-normal retardation 411 
of the above film with the those calculated from the refractive indices. 
As illustrated by the data in FIGS. 4A and 4B, even slight variations in 
n.sub.y and n.sub.z can significantly impact the off-normal performance of 
the retarder. This reinforces the particular suitability of the 
acrylonitrile-based optical retarders, particularly such retarders used in 
applications where uniform, wide-angle performance is desired. Moreover, 
such retarders can be fabricated using a process which permits production 
of relatively large retarders having uniform thickness and optical 
characteristics and which is relatively simple. 
While in the above examples, a butadiene elastomeric toughening material is 
added to the acrylonitrile-based retarder, it will be appreciated that 
other acrylonitrile-based retarders will have similar desirable optical 
properties. In general, other suitable materials may be added to the 
retarder, so long as the materials do not significantly impact the optical 
performance of the retarder. For example, isoprene based rubbers, natural 
rubbers and the like could be used. 
As noted above, acrylonitrile based retarders are particularly suited for 
applications requiring relatively uniform retardation over a wide range of 
incidence angles. More particular embodiments of the invention are 
described below in such applications. 
In accordance with one embodiment of the invention, the acrylonitrile based 
retarder is used as the basis of a retarder or polarization rotating 
mirror. By way of example, without intending to be limited to the example, 
a particular 1/4-wave mirror will be described. The 1/4-wave mirror is 
used to rotate the polarization direction of linearly polarize light, 
reflected from the mirror, by 90.degree.. One particular 1/4-wave mirror 
arrangement 500 is illustrated in FIG. 5A. An acrylonitrile based retarder 
501 is disposed in a plane parallel to the reflecting surface of a mirror 
503. A light source directs linearly polarized light 505 to the mirror at 
an incidence angle .phi.. The retarder 501 is oriented relative to the 
incident light such that light incident on the retarder at an angle normal 
to its surface (i.e., .phi.=0) is retarded by one quarter of its 
wavelength. In this construction, the linearly polarized light 505 is 
converted to circularly polarized light 505A, with a first rotation 
direction, as it passes through the retarder 501. 
The circularly polarized light 505A reflects off the surface of the mirror 
503. Light 505B reflected by mirror 503 is circularly polarized with an 
opposite rotation direction. The reflected light is directed back onto 
retarder 501 at an angle .phi.. As the reflected circularly polarized 
light 505B passes through the retarder 501 a second time, another 1/4-wave 
phase difference is introduced converting the circularly polarized light 
505B into linearly polarized light 507. The polarization direction of the 
reflected linearly polarized light 507 is substantially orthogonal to the 
initial polarization direction of the incident light 505. 
As will be appreciated, the above-description assumes normal incidence and 
precise 1/4-wave delays for each pass through the retarder 501. As the 
angle of incidence .phi. varies from normal, the relative phase shift will 
be impacted as a result of off-normal retardation deviations of the 
retarder 501. Thus, as the linearly polarized light 505 passes through the 
retarder at higher angles of incidence, the ellipticity introduced into 
the polarized light by the retarder 501 increases. As off-normal incident 
light is reflected, it also passes back through the retarder at an 
incidence angle .phi., assuming a substantially flat mirror. The 
ellipticity introduced by the off-normal retardation difference will add 
to the ellipticity introduced by the first pass. 
As the above discussion illustrates, the initial linearly polarized light 
505 passes through the retarder twice. An ellipticity introduced into the 
polarized light 507 reflected from the 1/4-wave mirror 500 will vary with 
the angle of incidence. Such ellipticity tends to degrade performance of 
those applications which rely on the linear polarization state of the 
reflected light. Accordingly, in applications using 1/4-wave mirrors and 
relatively wide angles of incidence, it is desirable to minimize the 
off-normal retardation difference so that the reflected light will be 
substantially linearly polarized, with the direction of polarization being 
rotated by 90.degree.. 
As will be appreciated from the above description, a 1/4-wave mirror 500 
constructed with an acrylonitrile based retarder provides relatively 
uniform off-normal performance in a form which can be constructed at 
relatively low cost and complexity. The construction allows rotation of 
the polarization direction of linearly polarized light without introducing 
substantial ellipiticity to the rotated linearly polarized light at 
relatively high angles of incidence. In general, it is desirable that any 
deviation from an ellipticity of 0 introduced at off-normal angles of 
incidence, be less than about 10%. It is more preferable that the 
deviations be less than about 5%. In certain incidences, it is necessary 
that the ellipiticity be less than 1% for all angles of incidence. As will 
be appreciated from the above description of the acrylonitrile based 
retarder, the above results can be obtained due to the particular 
wide-angle optical performance of the retarder. 
In FIG. 5A, the retarder 501 is illustrated as being separate from the 
mirror 503. In FIG. 5B, another embodiment of retarding mirror 510 is 
illustrated in which an acrylonitrile based retarder 511 is laminated or 
otherwise affixed to a mirror 513 by an adhesive 514. The optical 
performance of the mirror arrangement is generally the same as that 
described above in connection with FIG. 5A. Consideration, however, should 
be given to any additional components introduced by the construction. For 
example, lamination defects, refractive indices of adhesives, and the like 
must be considered. 
An optical system incorporating a retarder mirror, of the type illustrated 
in FIGS. 5A and 5B, for example, is illustrated in FIG. 6, The optical 
system of FIG. 6 is a projection display system 600 incorporating a folded 
light path. The general operation of the folded path projection display 
system is illustrated in FIG. 6. As will be described more fully below, 
the projection system 600 incorporates an acrylonitrile based 1/4-wave 
retarder/mirror arrangement 605 as a key element which must operate over a 
large range of incidence angles. The operation of the display system 600 
also requires that light reflected by the 1/4-wave retarder/mirror 
arrangement 605 be highly linearly polarized (e.g., exhibit minimal 
ellipticity). For a more detailed description of such systems, reference 
may be made to U.S. Pat. No. 5,557,343, entitled Optical System Including 
a Reflective Polarizer for a Rear Projection Picture Display Apparatus, 
and Published European Application EP 0,783,133 entitled Projecting 
Images. 
In the optical system of FIG. 6, light, representative of an image to be 
displayed, is projected from an image source 601 onto a screen assembly 
603. The light 602 from the source 601 is linearly polarized in a first 
direction. The rear surface of the screen assembly 603 includes a 
reflective polarizer. Reflective polarizing films are available from 
Minnesota Mining and Manufacturing Company under the name of the Dual 
Brightness Enhancement Film (DBEF). Other reflective polarizing films are 
described in U.S. patent applications Ser. No. 08/402,041, filed Mar. 10, 
1995 and entitled Optical Film, and Ser. No. 08/610,092, filed Feb. 29, 
1996, entitled An Optical Film, the contents of which are incorporated 
herein by reference. 
The reflective polarizer of the screen assembly reflects light of one 
particular linear polarization and transmits light of an opposite 
(orthogonal) linear polarization. The orientation of the reflective 
polarizer and the polarization direction of light initially incident on 
the reflective polarizer are such that the incident light is initially 
reflected by the reflective polarizer toward the acrylonitrile based 
retarding mirror assembly 605. The retarding mirror assembly 605 may be of 
the type illustrated in FIGS. 5A and 5B and serves to rotate the 
polarization direction of the linearly polarized light by 90.degree.. 
In operation, linearly polarized light reflects from the reflective 
polarizer and is incident on the retarding mirror 605. The light is 
reflected and the polarization direction is rotated by 90.degree. such 
that the polarization direction now aligns with the pass direction of the 
reflective polarizer. Thus, the light passes through the screen assembly 
603 for viewing. It is desirable that all of the light pass through the 
screen to increase viewing brightness. Any ellipticity in the light, 
however, will reduce the amount of light passing through the screen since 
the component of light still aligned in the direction of the original 
polarization will be reflected by the screen. 
As will be appreciated from the optical geometry illustrated in FIG. 6, 
light will be incident upon the retarder mirror 605 over a large range of 
incidence angles .phi..sub.1, .phi..sub.2, . . . .phi..sub.n. In such an 
application, the maximum angle of incidence may be quite high. As noted 
above, any ellipticity introduced into the light reflected from the 
retarder mirror 605 will degrade the overall performance of the display 
device. In the projecting device illustrated in FIG. 6, the retarder 
mirror 605 is formed of an acrylonitrile based retarder so as to minimizes 
the ellipiticity introduced into the reflected light as angles of 
incidence vary. It is generally desirable, that deviations from an 
ellipiticity of zero in such a system be less than 5%. It is more 
preferable, in certain instances to have the ellipiticity be even less 
than 1%. While the above discussion assumes an ellipticity of zero at 
normal angels of incidence, the preferred percentages for maximum 
ellipticity are appropriate if light incident on the retarder mirror at 
normal angles of incidence also reflects from the retarder mirror with 
some ellipticity. 
The acrylonitrile based retarders described above can be incorporated into 
a retarding mirror exhibiting ellipticity variation within the above 
tolerances. Thus, the projection device, incorporating an acrylonitrile 
based retarder mirror, will have improved performance over many typical 
retarders and can be manufactured relatively inexpensively. Moreover, an 
acrylonitrile retarder manufactured as described above, is well suited for 
lamination to mirror surfaces and other substrates. 
FIG. 7 illustrates an optical construction in accordance with still another 
particular embodiment of the invention. In the embodiment of FIG. 7, an 
acrylonitrile based retarder 701 is incorporated into an anti-glare 
optical construction. The anti-glare optical construction includes an 
absorptive polarizer 703, such as a dichroic polarizer, for example. The 
polarizer 703 linearly polarizes unpolarized light 705 incident on the 
polarizer. The acrylonitrile based retarder 701 is oriented relative to 
the absorbtive polarizer to convert the linearly polarized light 706 to 
circularly polarized light having a first rotation direction. If the 
circularly polarized light is reflected off the surface of an optical 
element 707, which is protected by the anti-glare construction, the light 
is reflected as circularly polarized light rotating in the opposite 
direction. The circularly polarized light 708 passes back through the 
retarder 701. Thus, as in the case of the retarding mirror above, the 
polarization direction of the light is now rotated by 90.degree.. The 
light, rotated by 90.degree., strikes the absorptive polarizer, this time 
linearly polarized in a direction of the absorption, to thereby inhibit or 
prevent light reflected from the surface of the optical element 707 from 
exiting the antiglare construction. 
The optical element 707 may be any type of reflective surface where it is 
desirable to reduce glare. For example, it may be the screen of a computer 
monitor. In such an application, the polarizer 703 and acrylonitrile based 
retarder 701 can be affixed or otherwise positioned in front of the 
monitor in any of a variety of ways conventionally known. As will be 
appreciated, when the optical element 707 is a monitor, light 721 exiting 
the monitor will pass through the acrylonitrile based retarder 701 and 
will be polarized by the absorptive polarizer 703. 
As in the above description of the retarder mirror, the off-normal 
performance of the acrylonitrile based retarder is important to ensure 
that light which is reflected from the optical element 707 is correctly 
polarized to be absorbed by the absorptive polarizer 703 upon reflection. 
Further, it will be appreciated that relatively large angles of incidence 
.phi. may be seen by the anti-reflective optical construction. For 
example, computer monitors are often used in environments where light 
sources causing reflection and glare are positioned at an angle relatively 
oblique with respect to the monitor. Thus, improved wide-angle performance 
of the retarder, serves to further reduce or eliminate the glare or 
reflection from the optical element 707 being protected. 
In one embodiment, the surface of the absorptive polarizer 703 facing the 
incident light may be A/R coated to reduce any reflection from the 
polarizer. The absorptive polarizer 703 may also be laminated or otherwise 
affixed to a substrate such as glass or other films. The substrate may 
also be A/R coated. The 1/4-wave film 701 may also be affixed by 
lamination or otherwise to a substrate. In certain instances this may be 
the same substrate to which the polarizer is affixed. The various elements 
may be laminated between glass. One or more of the glass surfaces may also 
be A/R coated. 
As noted above, the present invention is applicable to a number of optical 
retarders. It is believed to be particularly useful in applications where 
light is incident on the retarder over a wide range of angles. 
Accordingly, the present invention should not be considered limited to the 
particular examples described above, but rather should be understood to 
cover all aspects of the invention as fairly set out in the attached 
claims.