U.S. patent application number 11/173663 was filed with the patent office on 2005-11-03 for optical film with sharpened bandedge.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Ouderkirk, Andrew J., Weber, Michael F., Wheatley, John A..
Application Number | 20050243425 11/173663 |
Document ID | / |
Family ID | 21719234 |
Filed Date | 2005-11-03 |
United States Patent
Application |
20050243425 |
Kind Code |
A1 |
Wheatley, John A. ; et
al. |
November 3, 2005 |
Optical film with sharpened bandedge
Abstract
The present invention provides reflective films and other
optical bodies which exhibit sharp bandedges on one or both sides
of the main reflection bands. The optical bodies comprise
multilayer stacks M.sub.1 and M.sub.2, each having first order
reflections in a desired part of the spectrum and comprising
optical repeating units R.sub.1 and R.sub.2, respectively. At least
one of the optical repeating units R.sub.1 and R.sub.2 varies
monotonically in optical thickness along the thickness of the
associated multilayer stack.
Inventors: |
Wheatley, John A.; (Lake
Elmo, MN) ; Weber, Michael F.; (Shoreview, MN)
; Ouderkirk, Andrew J.; (Woodbury, MN) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
21719234 |
Appl. No.: |
11/173663 |
Filed: |
July 1, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11173663 |
Jul 1, 2005 |
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09634319 |
Aug 9, 2000 |
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09634319 |
Aug 9, 2000 |
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09006085 |
Jan 13, 1998 |
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6157490 |
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Current U.S.
Class: |
359/589 |
Current CPC
Class: |
G02B 5/287 20130101;
G02B 5/305 20130101; G02B 5/282 20130101 |
Class at
Publication: |
359/589 |
International
Class: |
G02B 001/10; G02B
005/28 |
Claims
What is claimed is:
1. A reflective film comprising a plurality of optical repeating
units having an optical thickness profile that defines a curve, the
curve having substantially no discontinuities in its first
derivative and shaped so that it, in order, monotonically decreases
from an initial optical thickness to a minimum optical thickness,
remains substantially constant at the minimum optical thickness,
monotonically increases from the minimum optical thickness to a
maximum optical thickness, remains substantially constant at the
maximum optical thickness, and monotonically decreases from the
maximum optical thickness to an end optical thickness, wherein the
optical repeating units monotonically increasing from the minimum
optical thickness to the maximum optical thickness form a
reflection band.
2. The reflective film of claim 1, wherein the reflection band
includes visible light.
3. The reflective film of claim 1, wherein the reflection band
includes infrared light.
4. The reflective film of claim 1, wherein the reflection band
includes ultraviolet light.
5. The reflective film of claim 1, wherein the optical repeating
units comprise polymer layers.
6. The reflective film of claim 1, wherein the optical repeating
units comprise a plurality of layers.
7. The reflective film of claim 5, wherein at least a portion of
the optical repeating units comprise two layers.
8. The reflective film of claim 5, wherein at least a portion of
the optical repeating units comprise three layers A, B, and C
arranged in an order ABC, the layers having respective indices of
refraction n.sup.a, n.sup.b, and n.sup.c that differ from each
other, index n.sup.b being intermediate to indices n.sup.a and
n.sup.c.
9. The reflective film of claim 8, wherein the three layers are
arranged in an order ABCB.
Description
[0001] This application is a continuation of U.S. Ser. No.
09/634319, filed Aug. 9, 2000, which is a continuation of U.S. Ser.
No. 09/006085, filed Jan. 13, 1998, now U.S. Pat. No.
6,157,490.
FIELD OF THE INVENTION
[0002] The present invention relates generally to multilayer
optical bodies, and in particular to multilayer films exhibiting a
sharpened reflective bandedge.
BACKGROUND OF THE INVENTION
[0003] The use of multilayer reflective films comprising
alternating layers of two or more polymers to reflect light is
known and is described, for example, in U.S. Pat. No. 3,711,176
(Alfrey, Jr. et al.), U.S. Pat. No. 5,103,337 (Schrenk et al.), WO
96/19347, and WO 95/17303. The reflection and transmission spectra
of a particular multilayer film depends primarily on the optical
thickness of the individual layers, which is defined as the product
of the actual thickness of a layer times its refractive index.
Accordingly, films can be designed to reflect infrared, visible or
ultraviolet wavelengths .lambda..sub.M of light by choice of the
appropriate optical thickness of the layers in accordance with the
following formula:
.lambda..sub.M=(2/M).multidot.D.sub.r (Formula I)
[0004] wherein M is an integer representing the particular order of
the reflected light and D.sub.r is the optical thickness of an
optical repeating unit (also called a multilayer stack) comprising
two or more polymeric layers. Accordingly, D.sub.r is the sum of
the optical thicknesses of the individual polymer layers that make
up the optical repeating unit. D.sub.r is always one half lambda in
thickness, where lambda is the wavelength of the first order
reflection peak. By varying the optical thickness of an optical
repeating unit along the thickness of the multilayer film, a
multilayer film can be designed that reflects light over a broad
band of wavelengths. This band is commonly referred to as the
reflection band or stop band.
[0005] It is desirable for a reflection band to have a sharp
spectral edge at the long wavelength (red) and/or short wavelength
(blue) side. However, the reflective films known to the art that
contain an optical repeating unit of varying optical thickness
typically have moderately sloped bandedges which cause reflections
outside of the desired wavelengths of interest. For example, if a
reflective film is designed to reflect infrared light while being
transparent over the visible spectrum, a sloped edge on the blue
side of the reflection band may encroach into the visible region of
the spectrum, thereby resulting in unwanted coloring of the
infrared reflective film body. Such coloring can be avoided by
designing the infrared film such that the infrared reflection band
is moved further into the infrared region, but this results in
substantial transmission of infrared light near the visible region
of the spectrum.
[0006] In other situations, it may be desirable to design a
reflective film or other optical body that reflects light over a
selected range in the visible region of the spectrum, e.g., a
reflective film that reflects only green light. In such a case, it
may be desirable to have sharp edges at both the red and blue sides
of the reflection band.
[0007] Many prior art reflective films comprising multilayer stacks
also show a number of small reflection peaks near the reflection
band. This so-called "ringing" also may introduce unwanted
reflections. It has been suggested in the art that, for multilayer
films that consist of an optical repeating unit of constant optical
thickness, such ringing might be suppressed by adding a number of
optical repeating units having an optical thickness of half that of
the other optical repeating units responsible for the reflection
band. However, while this approach may eliminate ringing, it does
not improve bandedge sharpness and, in fact, may worsen it.
Furthermore, this approach requires the presence of strippable
skins on multilayer extruded films, since it permits only thin
layers of specific optical thickness on the surface.
[0008] There is thus a need in the art for a reflective film, and a
method of making the same, that exhibits a sharp bandedge on one or
both sides of the main reflection band, and that avoids the
presence of ringing and other undesirable reflections. These and
other needs are met by the present invention, as hereinafter
described.
SUMMARY OF THE INVENTION
[0009] The present invention provides reflective films and other
optical bodies that exhibit sharp bandedges on one or both sides of
the main reflection bands. Optical bodies of the present invention
can comprise of reflective films having a plurality of optical
repeating units having an optical thickness profile that defines a
curve, the curve having substantially no discontinuities in its
first derivative and shaped so that it, in order, monotonically
decreases from an initial optical thickness to a minimum optical
thickness, remains substantially constant at the minimum optical
thickness, monotonically increases from the minimum optical
thickness to a maximum optical thickness, remains substantially
constant at the maximum optical thickness, and monotonically
decreases from the maximum optical thickness to an end optical
thickness, wherein the optical repeating units monotonically
increasing from the minimum optical thickness to the maximum
optical thickness form a reflection band.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The present invention is described in detail by reference to
the following drawings, without, however, the intention to limit
the invention thereto:
[0011] FIGS. 1a to 1e show the optical thickness variation of the
optical repeating units R.sub.1 and R.sub.2 in multilayer stacks
M.sub.1 and M.sub.2 to obtain bandedge sharpening at the red or
blue edge of the reflection band;
[0012] FIG. 2 shows the optical thickness variation of the optical
repeating units R.sub.1, R.sub.2 and R.sub.3 in multilayer films
M.sub.1, M.sub.2 and M.sub.3 to obtain bandedge sharpening at the
blue and red edges of the reflection band;
[0013] FIG. 3 shows the optical thickness variation of the optical
repeating units R.sub.1, R.sub.2 and R.sub.3 in multilayer films
M.sub.1, M.sub.2 and M.sub.3 in which M.sub.1 and M.sub.3 have
continuously changing slopes;
[0014] FIGS. 4a and 4b are schematic diagrams of a multilayer film
consisting of two alternating polymeric layers;
[0015] FIG. 5 is a 3-dimensional schematic diagram of an optical
repeating unit consisting of two alternating layers of polymeric
materials;
[0016] FIG. 6 is a 3-dimensional schematic diagram of an optical
repeating unit consisting of polymeric layers A, B and C arranged
in an ABCB pattern;
[0017] FIG. 7a is a layer thickness gradient profile showing a
combined layer thickness gradient of LTG1 and LTG2;
[0018] FIG. 7b is a computed spectrum illustrating the short
wavelength bandedge for the reflectance band created by layer
thickness gradient LTG1 and the effect of adding the reverse
gradient LTG2;
[0019] FIG. 8a is a layer thickness gradient profile of a stack
design having a reverse gradient with an f-ratio deviation;
[0020] FIG. 8b is a computational spectrum illustrating the
improvement in bandedge sharpness afforded by the combination of
LTG1 and LTG3;
[0021] FIG. 9a is a layer thickness gradient profile for the
combined stacks LTG1 and LTG4;
[0022] FIG. 9b is a computational spectrum illustrating the
improvement observed with the layer thickness gradient of FIG. 9a
compared to the LTG1 case;
[0023] FIG. 10a is a the layer thickness gradient profile in which
the low index layer is linear for the entire stack for LTG1 and
LTG5, but the high index component undergoes a gradient reversal in
the LTG5 section;
[0024] FIG. 10b is a computational spectrum illustrating the
improvement seen with the gradient of FIG. 10a vs. the LTG1
case;
[0025] FIG. 11a is a layer thickness gradient profile for a simple
band pass filter made by introducing a step discontinuity in the
layer thickness profile of a broad band reflecting stack;
[0026] FIG. 11b is the calculated spectrum for the gradient of FIG.
11a;
[0027] FIG. 12a is a layer thickness gradient profile made with two
graded linear thickness distributions and an additional non-graded
quarter wave stack;
[0028] FIG. 12b is the calculated spectrum for the gradient of FIG.
12a;
[0029] FIG. 13a is a layer thickness gradient profile illustrating
a curved layer thickness profile; and
[0030] FIG. 13b is the calculated spectrum for the gradient of FIG.
13a.
DETAILED DESCRIPTION OF THE INVENTION
[0031] The following definitions and conventions are used
throughout the disclosure:
[0032] Desired part of the spectrum: any continuous range of
wavelengths between 400 nm and 2500 nm, also called desired
reflection band.
[0033] Optical repeating unit (ORU): a stack of layers arranged in
a particular order, which arrangement is repeated across the
thickness of a multilayer film; the stack of layers have a first
order reflection at a wavelength according to Formula (I)
above.
[0034] Intrinsic bandwidth, or optical repeating unit (ORU)
bandwidth: The bandwidth that an infinite stack of ORU's of
identical thickness would exhibit, which is readily calculated from
the matrix elements of the characteristic matrix M as defined by
Born and Wolf, "Principles of Optics", Edition 5, page 67. For a
quarterwave stack of two materials with an index differential less
than about 0.3, it is given, to a good approximation, by the
absolute value of the Fresnel reflection coefficient for that
interface. Stopband: A reflectance band is defined in general as a
spectral band of reflection bounded on either side by wavelength
regions of low reflection. With dielectric stacks, the absorption
is typically low enough to be ignored for many applications, and
the definition is given in terms of transmission. In those terms, a
reflectance band or stop band is defined in general as a region of
low transmission bounded on both sides by regions of high
transmission.
[0035] In one preferred embodiment, a single reflectance band or
stop band for p-polarized light has a continuous spectrum between
any two successive wavelengths at which the transmission is greater
than 50 percent, and including such successive wavelengths as
endpoints, and where the average transmission from one endpoint to
the other is less than 20 percent. Such preferred reflectance band
or stop band is described in the same way for unpolarized light and
light of normal incidence. For s-polarized light, however, the
transmission values in the preceding description are calculated in
a way that excludes the portion of light reflected by an air
interface with the stack or the stack's skin layers or
coatings.
[0036] Bandwidth of stop band: For such a preferred embodiment as
described in the preceding paragraph, the bandwidth is defined to
be the distance, in nm, between the two wavelengths within the band
which are nearest each 50 percent transmission point, at which the
transmission is 10 percent. In commonly used terms, the bandwidths
are defined by the 10 percent transmission points. The respective
blue and red (i.e., short and long wavelength) bandedges are then
taken to be the wavelength at the above defined 10% transmission
points. The transmission of the preferred stop band is taken to be
the average transmission between the 10 percent transmission
points. If a reflectance band does not have high enough
reflectivity to satisfy the definitions of bandwidth and bandedge
slope for the preferred embodiment, then the bandwidth may be taken
simply to be the full width at half maximum (FWHM), where the
maximum is the peak reflectance value.
[0037] Bandedge slope of stop band: The slope of a band edge as
described in the preceding paragraph is taken from the 50 percent
and 10 percent transmission/wavelength points, and is given in
units of percent transmission per nm.
[0038] Pass band: A pass band is defined in general as a spectral
transmitting band bounded by spectral regions of relatively low
transmission. With the multilayer color shifting film, the passband
is bounded by reflective stopbands. The width of the pass band is
the Full Width at Half Maximum Transmission (FWHM) value.
[0039] Bandedge slope of pass band: Band edge slopes are calculated
from the two points on a given bandedge nearest the maximum
transmission point, the transmission values of which are 50 and 10
percent of the maximum transmission value. In one preferred
embodiment, the passband has low transmission regions on both sides
of the transmission peak with transmission minima of 10 percent or
less of the transmission value of the peak transmission point. For
example, in this preferred embodiment, a pass band having a 50
percent transmission maximum, would be bounded on both sides by
reflectance bands having 5 percent or lower transmission minima.
More preferably, the transmission minima on both sides of the
passband are less than 5 percent of the peak transmission value of
the passband.
[0040] Edge filter: reflectance filter having only one bandedge
within the wavelength range of interest.
[0041] Multilayer film: a film comprising an optical repeating unit
designed to reflect light over a particular range of wavelengths.
The multilayer film may contain additional layers between the
optical repeating units and which additional layers may or may not
be repeated throughout the multilayer film.
[0042] Monotonically varying layer thickness of an optical
repeating unit along a multilayer film: the thickness of the
optical repeating unit either shows a consistent trend of
decreasing or increasing along the thickness of the multilayer film
(e.g., the thickness of the optical repeating unit does not show an
increasing trend along part of the thickness of the multilayer film
and a decreasing trend along another part of the multilayer film
thickness). These trends are independent of layer-to-layer
thickness errors, which may have a statistical variance with a
1-sigma value as large as 5% or more. In addition, a local
variation in the optical repeating unit may cause ripples in the
layer thickness profile which is not strictly monotonic by the
mathematical definition, but the ripple should be relatively small
compared to the thickness difference between first and last optical
repeating unit.
[0043] Maximum optical thickness of an optical repeating unit: the
maximum of a statistical curve fit to the actual layer distribution
containing random errors in layer thickness.
[0044] Minimum optical thickness of an optical repeating unit: the
minimum of a statistical curve fit to the actual layer distribution
containing random errors in layer thickness.
[0045] In-plane axes: two mutually perpendicular axes that are in
the plane of the reflective film. For the sake of convenience, they
are denoted as the x-axis and the y-axis.
[0046] Transverse axis: an axis that is perpendicular to the plane
of the reflective film. For sake of convenience, this axis is
denoted the z-axis.
[0047] An index of refraction along a particular axis is referred
to as n.sub.i, wherein i indicates the particular axis, for
example, n.sub.x indicates an index of refraction along the
x-axis.
[0048] Negative birefringence: the index of refraction along the
transverse axis is less than or equal to the index of refraction
along both in-plane axes (n.sub.z<n.sub.x and n.sub.y)
[0049] Positive birefringence: the index of refraction along the
transverse axis is greater than the index of refraction along both
in-plane axes (n.sub.z>n.sub.x and n.sub.y).
[0050] Isotropic: the indices of refraction along x, y and z-axes
are substantially equal (e.g., n.sub.x=n.sub.y=n.sub.z)
[0051] Infrared region: 700 nm to 2500 nm
[0052] Visible region: 400 nm to 700 nm
[0053] The f-ratio is defined as: 1 f k = n k * d k m = 1 N n m d
m
[0054] wherein f.sub.k is the optical thickness of polymeric layer
k, 1 is the number of layers in the optical repeating unit, n.sub.k
is the refractive index of polymeric layer k, and d.sub.k is the
thickness of polymeric layer k. The optical thickness ratio of
polymeric layer k along an optical axis j is denoted as f.sub.jk
and is defined as above, but with replacement of n.sub.k with the
refractive index of polymer material k along axis j ().
[0055] Skin layer: a layer that is provided as an outermost layer
typically having a thickness between 10% and 20% of the sum the
physical thickness of all optical repeating units.
DETAILED DESCRIPTION
[0056] The construction of multilayer films in accordance with the
present invention can be used in a variety of ways to obtain
bandedge sharpening at the red or blue side of the band or on both
sides.
[0057] Bandedge Sharpening--Blue Edge
[0058] To obtain bandedge sharpening in accordance with the present
invention at the blue edge of the reflection band, a multilayer
stack M.sub.1 having an optical repeating unit R.sub.1 is combined
with a multilayer stack M.sub.2 having an optical repeating unit
R.sub.2. Both multilayer stacks are designed to have a first order
reflection band in a desired region of the spectrum, e.g., in the
infrared region. It is possible to produce a film or other optical
body having a first order reflection band in a particular region of
the spectrum by selecting polymeric materials with appropriate
indices of refraction and by manipulating the physical thickness of
each of the individual polymeric layers of an optical repeating
unit such that the optical thickness of the optical repeating unit
appears at the desired wavelength as predicted by Formula (I)
above. By varying the optical thickness of the optical repeating
unit in the multilayer film, the desired reflection over a
particular range in the spectrum can be obtained. Preferably, the
optical repeating unit R.sub.1 of multilayer stack M.sub.1 is
monotonically varied in optical thickness such that the desired
reflection band is obtained. However, it is also possible to use
several multilayer stacks comprising different optical repeating
units to cover a desired reflection band.
[0059] The optical thickness of optical repeating unit R.sub.1
preferably increases monotonically along the thickness of
multilayer stack M.sub.1. Multilayer stack M.sub.2 may comprise an
optical repeating unit R.sub.2 that is substantially constant in
optical thickness or the optical thickness of optical repeating
unit R.sub.2 may decrease monotonically along the thickness of
multilayer stack M.sub.2. If the optical thickness of optical
repeating unit R.sub.2 is substantially constant, the optical
thickness thereof should be approximately equal to the minimum
optical thickness of optical repeating unit R.sub.1 along the
thickness of multilayer stack M.sub.1. Preferably in this
embodiment, the optical thickness of optical repeating unit R.sub.2
is substantially equal to the minimum optical thickness of optical
repeating unit R.sub.1.
[0060] FIG. 1a depicts this embodiment and shows a plot of the
optical thickness of optical repeating units R.sub.1 and R.sub.2
versus the optical repeating unit number in a reflective film made
in connection with the present invention. In FIG. 1a, multilayer
stack M.sub.1 comprises optical repeating unit R.sub.1 of
increasing optical thickness, and multilayer stack M.sub.2
comprises optical repeating unit R.sub.2 of substantially constant
optical thickness. A reflective film designed in accordance with
FIG. 1a will have a sharpened bandedge on the blue side of the
reflection band.
[0061] FIG. 1b depicts another embodiment of the present invention
that also leads to sharpening of the reflection band on the blue
side. As shown in FIG. 1b, multilayer stack M.sub.2 in this
embodiment comprises an optical repeating unit R.sub.2 that
decreases monotonically in optical thickness along the thickness of
multilayer stack M.sub.2. The minimum optical thickness of optical
repeating unit R.sub.2 in this embodiment is such that it is
substantially equal to the minimum optical thickness of optical
repeating unit R.sub.1 along multilayer stack M.sub.1.
[0062] Bandedge Sharpening--Red Edge
[0063] To obtain bandedge sharpening in accordance with the present
invention at the red end of the reflection band, a multilayer stack
M.sub.1 having an optical repeating unit R.sub.1 is combined with a
multilayer stack M.sub.2 having an optical repeating unit R.sub.2.
Both multilayer films are designed to have a first order reflection
in a desired portion of the spectrum, e.g., a reflection band in
the green part of the visible spectrum.
[0064] The optical thickness of optical repeating unit R.sub.1
preferably increases monotonically along the thickness of
multilayer stack M.sub.1. Multilayer stack M.sub.2 may comprise an
optical repeating unit R.sub.2 that is substantially constant in
optical thickness, or else the optical thickness of optical
repeating unit R.sub.2 may decrease monotonically along the
thickness of multilayer stack M.sub.2. If the optical thickness of
optical repeating unit R.sub.2 is substantially constant, the
optical thickness thereof should be equal to the maximum optical
thickness of optical repeating unit R.sub.1 along the thickness of
multilayer stack M.sub.1. Preferably in this embodiment, the
optical thickness of optical repeating unit R.sub.2 is
substantially equal to the maximum optical thickness of optical
repeating unit R.sub.1.
[0065] FIG. 1c depicts this embodiment and shows a plot of the
optical thickness of optical repeating units R.sub.1 and R.sub.2
versus the optical repeating unit number in a reflective film body
in connection with the present invention. In FIG. 1c, multilayer
stack M.sub.1 comprises optical repeating unit R.sub.1 of
increasing optical thickness, and multilayer stack M.sub.2
comprises optical repeating units R.sub.2 of substantially constant
optical thickness. A reflective film body designed in accordance
with FIG. 1c will exhibit a sharpened bandedge at the red end of
the reflection band.
[0066] FIG. 1d depicts another embodiment of the present invention
that also leads to sharpening of the reflection band on the red
side. As shown in FIG. 1d, multilayer stack M.sub.2 now comprises
an optical repeating unit R.sub.2 that decreases monotonically in
optical thickness along the thickness of multilayer stack M.sub.2.
The maximum optical thickness of optical repeating unit R.sub.2 in
this embodiment is such that it is substantially equal to the
maximum optical thickness of optical repeating unit R.sub.1 along
multilayer stack M.sub.1.
[0067] Bandedge Sharpening--Both Edges
[0068] To obtain bandedge sharpening at both ends of the reflection
band, three multilayer stacks M.sub.1, M.sub.2 and M.sub.3 can be
combined as in the embodiment shown in FIG. 2. There, multilayer
stack M.sub.1 comprises an optical repeating unit R.sub.1 that
monotonically increases along the thickness of multilayer stack
M.sub.1. At the end of the stack, where R.sub.1 has the minimum
optical thickness, multilayer stack M.sub.1 is combined with
multilayer stack M.sub.2 that comprises optical repeating unit
R.sub.2 having a constant optical thickness. The optical thickness
of R.sub.2 is either substantially equal (as shown in FIG. 2) or is
less than the minimum optical thickness of optical repeating unit
R.sub.1. As already described above for obtaining bandedge
sharpening at the blue edge of the reflection band, optical
repeating unit R.sub.2 can also decrease monotonically along the
thickness of multilayer stack M.sub.2.
[0069] At the end of the stack where optical repeating unit R.sub.1
has its maximum optical thickness, there is combined a multilayer
film M.sub.3 comprising an optical repeating unit R.sub.3 that has
a substantially constant optical thickness. As shown in FIG. 2, the
optical thickness of R.sub.3 is equal to the maximum optical
thickness of optical repeating unit R.sub.1. As already described
above for obtaining bandedge sharpening at the red end, optical
repeating unit R.sub.3 can also decrease monotonically along the
thickness of multilayer film M.sub.3.
[0070] In each of the above described embodiments, the multilayer
stacks M.sub.1 and M.sub.2 and, optionally, M.sub.3 have been
described as being physically next to each other in the reflective
film. However, this is not a requirement. In particular, the
multilayer stacks may be spaced away from each other in the
reflective film body by additional multilayer stacks and/or
additional layers such as, for example, a layer which improves the
adherence between the multilayer stacks. For example, multilayer
stack M.sub.2 in FIG. 1a could equally well be present at the other
end of multilayer stack M.sub.1 as shown in FIG. 1e. Similarly, the
positions of multilayer stacks M.sub.2 and M.sub.3 in FIG. 2 can be
interchanged as well. However, the preferred spatial positions of
the multilayer stacks M.sub.1, M.sub.2 and, optionally, M.sub.3
relative to each other is that they join together such that
adjacent layers are of approximately equal optical thickness as
illustrated in FIGS. 1a-1d and 2, with no intervening material
layers or spaces.
[0071] Bandedge sharpening can be obtained even if the multilayer
stacks M.sub.1, M.sub.2, and M.sub.3 are not adjacent or in the
order illustrated in FIG. 1e. The materials and their indices may
even be different in each of the three multilayer stacks. However,
the most efficient use of optical layers will occur when repeat
units of the same or similar optical thickness (multilayer stacks
having overlapping reflection bands) are optically coupled to
enhance constructive interference between those layers. This
constraint also provides a guideline for the range of useful
thicknesses of repeat units R.sub.1, R.sub.2 and R.sub.3 in
multilayer stacks M.sub.1, M.sub.2 and M.sub.3. For example, in
FIG. 1d, as the repeat units in multilayer stack M.sub.2 get
progressively thinner to the right, with progressive deviation from
the thickness of the maximum repeat unit of M.sub.1, the optical
coupling for constructive interference is progressively weakened
between those layers at the extremities. If the minimum thickness
repeat unit of M.sub.2 is of optical thickness d that is outside of
the intrinsic bandwidth of the maximum thickness repeat unit in
M.sub.1, then that minimum thickness unit will not contribute
appreciably to bandedge sharpening on the red side of the
reflection band of multilayer M.sub.1.
[0072] A reflective film or other optical body made in accordance
with the present invention can be manufactured, for example, by
multilayer co-extrusion as described in more detail below.
Alternatively, the multilayer stacks forming the reflective films
or other optical bodies of the present invention may be
manufactured separately from each other (e.g., as separate,
free-standing films) and then laminated together to form the final
reflective film.
[0073] Optical Stack Designs
[0074] Layer thickness distributions for extended reflection bands
may take the form of a variety of exponentially or linearly
increasing functional forms. Such optical stacks create an extended
reflection band of pre-determined bandwidth and extinction. If the
same functional form is maintained from beginning to end (first to
last layer), then the slopes of the bandedges may not be as steep
as desired. To increase the slope of either the left or right
bandedge, the functional form of the layer thickness distribution
may change near the end points of the primary stack distribution
such that the slope of the layer thickness distribution approaches
zero.
[0075] To further sharpen the bandedges, additional layers with
zero or opposite sign slopes may be added. For example, combined
multilayer optical stacks M.sub.1, M.sub.2, and M.sub.3 can be
constructed as shown in FIG. 3 in which there are no
discontinuities in the first derivative of the (statistically
averaged) layer thickness profile. In FIG. 3, M.sub.2 itself has a
slight band sharpening profile in that the slope at the beginning
and end of M.sub.2 is equal to zero. Stacks M.sub.1 and M.sub.2 are
designed such that they also have zero slopes where they join
M.sub.2. The slopes of both M.sub.1 and M.sub.3 change continuously
until, at their endpoints, their slopes are equal and opposite to
that of the main stack M.sub.2. In FIG. 3, M.sub.1 consists of
repeat units 1 to 10, M.sub.2 of units 10 to 90, and M.sub.3 of
units 90 to 105. M.sub.2 itself consists of 3 regions: M.sub.21,
M.sub.22, and M.sub.23, similar to the profile in FIG. 2. M.sub.21
consists of units 10 to 20, M.sub.22 from 20 to 80, and M.sub.23
from 80 to 90. M.sub.22 is a linear thickness profile.
[0076] Furthermore, the combined distribution curve
M.sub.1+M.sub.2+M.sub.3 may be part of a larger optical stack and
can be in an interior or on the exterior position of a larger
stack. Thus, films and other optical bodies can be made in
accordance with the present invention whose total constructions
contain multiple reflecting bands created by multiple sets of layer
thickness gradients, all with their respective bandedge sharpening
layer groups.
[0077] Typically, the optical thickness variation of an optical
repeating unit in accordance with the present invention can be
obtained by varying the physical thickness of the polymeric layers
of the optical repeating unit. The optical thickness of a repeat
unit is selected according to the wavelengths selected to be
reflected. Any range of wavelengths outside of the intrinsic
bandwidth of the optical repeating unit can be selected by addition
of optical repeating units having the appropriate range of optical
thicknesses. According to one particular embodiment in connection
with the present invention, the physical thickness of all polymeric
layers constituting the optical repeating unit is varied at the
same rate. For example, all polymeric layers of the optical
repeating unit may be varied in thickness according to the same
linear function.
[0078] In an alternative embodiment of the present invention, the
physical thickness of the polymeric layers of the optical repeating
unit may be varied differently. This is particularly preferred
where it is desirable to obtain an optical thickness variation of
the optical repeating unit R.sub.2 or R.sub.3 of multilayer films
M.sub.2 and M.sub.3, respectively. For example, the optical
thickness of an optical repeating unit consisting of two
alternating polymeric layers may be monotonically varied in
accordance with the present invention by keeping the physical
thickness of one of the layers substantially constant while varying
the physical thickness of the other layer in accordance with, for
example, a linear function. Alternatively, both layers can be
varied in physical thickness but in accordance with different
functions, e.g., different linear functions or different subtile
power law functions.
[0079] Several preferred embodiments of the present invention are
illustrated in Table I and in the examples which follow. Table I
lists four separate layer thickness gradients. Each gradient is
comprised of repeating quarter wave layers of a high index material
(n=1.75) and a low index polymer (n=1.50). The starting thickness
and the thickness increment for each successive layer is provided.
A computer modeling program was used to investigate the effect of
several combinations of gradients on the bandedge steepness of the
primary reflectance band.
1TABLE I LTG1 LTG2 LTG3 LTG4 LTG5 Total number 170 30 30 30 30 of
layers High index 154.6 112.4 112.4 112.4 112.4 beginning layer
thickness (nm) High index -0.4965 0.726 0.726 0 0.726 layer
thickness increment (nm) Low index 183.3 133.3 133.3 133.3 133.3
beginning layer thickness (nm) High index -0.5882 0.8608 0 0
-0.5882 layer thickness increment (nm)
EXAMPLE 1
Reverse Gradient
[0080] An example of a reverse gradient is shown in FIG. 7a. This
figure shows the combined layer thickness gradient of LTG1 and
LTG2. In this case, the bandedge sharpening gradient, LTG2,
consists of 20 layers of alternating high and low index materials,
both of which increase in thickness to maintain an f-ratio of 0.5
from the first to last layer pair.
[0081] Another example of a reverse layer gradient is shown in FIG.
7b. This figure shows the short wavelength bandedge for the
reflectance band created by layer thickness gradient LTG1 and the
effect of adding the reverse gradient LTG2. The addition of LTG2
results in an increase to the edge slope. The bandedge slope
without the addition of LTG 2 is 1.1 percent/nm. When LTG 2 is
added, the slope increases to 1.9 percent/nm. The layer thickness
profiles are shown in FIG. 7a.
EXAMPLE 2
Reverse Gradient with f-ratio Deviation
[0082] An example of a stack design having a reverse gradient with
an f-ratio deviation is shown in FIG. 8a. This figure shows a film
stack design of only one material component with a reverse
thickness gradient while the other has a zero gradient in the added
band sharpening stack of LTG 3. This combination of LTG1 and LTG3
also shows an improvement in bandedge sharpness over the LTG1 case
as seen in FIG. 8b below. The bandedge slope with LTG 3 added is
7.3 percent/nm.
EXAMPLE 3
Zero Gradient
[0083] This example demonstrates bandedge sharpening for the case
of zero gradient stacks LTG4 for both materials. The stack design
of this example also produces a much sharper bandedge than LTG1
alone. The bandedge slope in this case is 3.6 percent/nm.
[0084] FIG. 9a shows the layer thickness gradient for the combined
stacks LTG1 and LTG4. LTG4 has a zero thickness gradient for both
materials, and maintains a constant ratio of thickness between the
high and low index layers. Again, as shown in FIG. 9b, substantial
improvement is seen compared to the LTG1 case, with a bandedge
slope of 3.6 percent/nm compared to the value of 1.1 percent per nm
for LTG 1.
EXAMPLE 4
Gradient Sign Change by Only One Component
[0085] In this case, the layer gradient for the low index layer is
linear for the entire stack for LTG1 and LTG5, but the high index
component undergoes a gradient reversal in the LTG5 section, as
shown in FIG. 10a below. The resulting spectra are shown in FIG.
10b, and a substantial improvement is seen vs. the LTG1 case, with
the bandedge slope increasing from 1.1 percent/nm to 3.6
percent/nm.
[0086] Band Pass Filters
[0087] The fabrication of narrow bandpass transmission filters,
sometimes referred to as notch filters, can be made by using two
broad reflection bands which cover most of the appropriate spectrum
except for a very narrow band between their adjacent bandedges. If
the band pass filter is to be of both narrow band and high
transmission, then nearly vertical bandedges are required. Typical
design techniques of the prior art, in which individual layer
thicknesses of each layer in the stack is assigned a unique value,
may be impractical for polymeric stacks involving hundreds of
layers. The edge sharpening techniques described herein are
particularly useful in this case.
[0088] One preferred embodiment involves the use of band sharpening
stacks having continuously varying gradients. The resulting band
pass filters have a higher transmission than filters made with
linear (constant gradient) layer thickness distributions. The
following computer modeled examples illustrate this
improvement.
[0089] FIG. 11a. A simple band pass filter can be made by
introducing a step discontinuity in the layer thickness profile of
a broad band reflecting stack, as illustrated in FIG. 11a. The
calculated spectrum of such a notch filter, made with the two
simple linear thickness distributions of FIG. 11a, is shown in FIG.
11b. Without the band sharpening techniques described above, the
bandedge slopes are not high enough to make a narrow band notch
filter. The bandedges slopes are about 1.2 percent/nm and 1.4
percent/nm for the blue and red edges, respectively. The Bandwidth
is 54 nm and the peak transmission value is 62 percent.
[0090] A notch filter can be made with two graded linear thickness
distributions and additional non-graded quarter wave stacks as
shown in FIG. 12a. The flat (zero gradient) sections are useful for
sharpening the respective bandedges of the adjacent reflecting
bands. With the additional layers concentrated at the two thickness
values on either side of the notch wavelength, a much sharper
transmission band can be made. The calculated spectrum for the
illustrated stack is given in FIG. 12b. The steepness of the
bandedges of the notch filter spectrum of FIG. 12b will increase
with the number of layers included in the band sharpening feature
of the stack, as illustrated in FIG. 12a. The bandedge slopes are
about 9 percent/nm for both the blue and red edges. The Bandwidth
is 13.8 nm and the peak transmission value is 55.9 percent.
[0091] The curved layer thickness profile of FIG. 13a was created
to improve upon a deficiency of the stack design and spectrum of
FIG. 12a and 12b. The side band ripples of the layer thickness
profiles of FIG. 12a overlap and limit the transmission of a notch
filter. Note that the peak transmission of the notch band in FIG.
12b is only about 50%. By introducing a curvature to the band
sharpening stack thickness profile, the ringing at the edge of the
spectrum of such a stack is reduced. Combining two such stacks will
then make a notch filter with steeper bandedges and higher peak
transmission, as illustrated by the results shown in FIG. 13b. The
bandedge slopes are about 12 percent/nm and 14 percent/nm for the
blue and red edges, respectively. The Bandwidth is 11 nm and the
peak transmission value is 76 percent. Note that, although the
bandwidth is narrower than in FIG. 12b, the maximum transmission is
significantly higher. The number of layers in the band sharpening
portion of the stack is 60 on each side of the thickness gap, which
is the same number of layers used in the zero gradient sections of
the layer distribution of FIG. 12a.
[0092] The curved profile can follow any number of functional
forms. The main purpose of the form is to break the exact
repetition of thickness present in a quarter wave stack with layers
tuned to only a single wavelength. The particular function used
here was an additive function of a linear profile (the same as used
on the remainder of the reflectance band) and a sinusoidal function
to curve the profile with the appropriate negative or positive
second derivative. An important feature is that the second
derivative of the layer thickness profile is positive for the red
bandedge of a reflectance stack and negative for the blue bandedge
of a reflectance stack. Note that the opposite sign is required if
one refers to the red and blue bandedges of a notch band. Other
embodiments of the same principle include layer profiles that have
multiple points with a zero value of the first derivative. In all
cases here, the derivatives refer to those of a best fit curve
fitted through the actual layer thickness profile which can contain
small statistical errors of less than 10% one sigma standard
deviation in layer thickness values.
[0093] As illustrated by the above examples, the band sharpening
profiles that are added to the layer thickness distribution can
have significant effects on the slope of the bandedges, for one or
both edges of a reflectance band, and for the edges of a pass band.
Sharp bandedges and high extinction are desirable in obtaining
color filters having saturated colors of high purity. Preferably
for reflectance bands, the slopes of the bandedges are at least
about 1 percent per nm, more preferably greater than about 2
percent per nm, and even more preferably greater than about 4
percent per nm. The same slopes are preferred for bandpass filters
having a bandwidth greater than or about 50 nm. For pass band
filters with a bandwidth of less than or about 50 nm, the edges are
preferably greater than about 2 percent per nm, more preferably
greater than about 5 percent per nm, and even more preferably,
greater than about 10 percent per nm.
[0094] Design of the Optical Repeating Units
[0095] The polymeric layers of an optical repeating unit in
accordance with the present invention can be isotropic or
anisotropic. An isotropic polymeric layer is a layer wherein the
index of refraction of the polymeric layer is the same independent
of the direction in the layer, whereas in case of an anisotropic
polymeric layer, the index of refraction will differ along at least
two different directions. The latter type of polymeric layer is
also called a birefringent layer. To describe an anisotropic
polymeric layer, an orthogonal set of axes x, y and z is used as
set out above in the definition section. Thus, an anisotropic
polymeric layer will have at least two of the indices of refraction
n.sub.x, n.sub.y and n.sub.z different from each other.
[0096] In one embodiment of the present invention, optical
repeating units R.sub.1, R.sub.2 and/or R.sub.3 consists of two
alternating isotropic polymeric layers that have an index of
refraction differing from each other, preferably by at least about
0.05 and more preferably by at least about 0.1. More preferably,
however, at least one of the two alternating polymeric layers is a
birefringent layer wherein at least one of the in-plane indices
n.sub.x and n.sub.y differs by at least 0.05 from the corresponding
in-plane index of refraction of the other layer. According to a
particular preferred embodiment in connection with the present
invention, the index of refraction along the transverse axes
(n.sub.z) of both layers is substantially matched, i.e., the
difference of the index of refraction along the z-axes between both
layers is preferably less than about 0.05. Optical repeating units
of this type are particularly suitable for reflecting light in the
visible region of the spectrum, but may also be used for reflecting
light in the infrared region of the spectrum. Optical repeating
units and multilayer films having this feature have been described
in detail in WO 96/19347 and WO 95/17303, which are incorporated
herein by reference. In another preferred embodiment of the present
invention, the transverse index of the polymer layer having the
highest in-plane index is lower than the in-plane indices of the
other polymer. This feature is also described in the above-cited
references.
[0097] FIGS. 4a and 4b illustrate these embodiments and show a
multilayer film 10 comprising an optical repeating unit consisting
of two alternating polymeric layers 12 and 14. Preferably, at least
one of the materials has the property of stress induced
birefringence, such that the index of refraction (n) of the
material is affected by the stretching process.
[0098] FIG. 4a shows an exemplary multilayer film before the
stretching process in which both materials have the same index of
refraction. Light ray 13 experiences relatively little change in
index of refraction and passes through the film. In FIG. 4b, the
same film has been stretched, thus increasing the index of
refraction of material 12 in the stretch direction (or directions).
The difference in refractive index at each boundary between layers
will cause part of ray 15 to be reflected. By stretching the
multilayer stack over a range of uniaxial to biaxial orientation, a
film is created with a range of reflectivities for differently
oriented plane-polarized incident light. The multilayer film can
thus be made useful as reflective polarizers or mirrors. If
stretched biaxially, the sheet can be stretched asymmetrically
along orthogonal in-plane axes or symmetrically along orthogonal
in-plane axes to obtain desired polarizing and reflecting
properties.
[0099] The optical properties and design considerations of
multilayer stacks comprising two alternating polymeric layers is
described most completely in copending and commonly assigned U.S.
patent application Ser. No. 08/402,041, filed on Mar. 10, 1995, the
disclosure of which is hereby incorporated herein by reference.
Very briefly, that application describes the construction of
multilayer films (mirrors and polarizers) for which the Brewster
angle (the angle at which reflectance goes to zero) is very large
or is nonexistent for the polymer layer interfaces. This feature
allows for the construction of mirrors and polarizers whose
reflectivity for p-polarized light decreases slowly with angle of
incidence, is independent of angle of incidence, or increases with
angle of incidence away from normality. As a result, multilayer
films having high reflectivity for both s- and p-polarized light
over a wide bandwidth, and over a wide range of angles, can be
achieved.
[0100] FIG. 5 shows an optical repeating unit consisting of two
polymeric layers, and indicates the three-dimensional indices of
refraction for each layer. The indices of refraction are n1x, n1y,
and n1z for layer 102, and n2x, n2y, and n2z for layer 104,
respectively. The relationships between the indices of refraction
in each film layer to each other and to those of the other layers
in the film stack determine the reflectance behavior of the
multilayer stack at any angle of incidence, from any azimuthal
direction.
[0101] The principles and design considerations described in U.S.
patent application Ser. No. 08/402,041 can be applied to create
multilayer films having the desired optical effects for a wide
variety of circumstances and applications. The indices of
refraction of the layers in the multilayer stack can be manipulated
and tailored to produce devices having the desired optical
properties. Many useful devices, such as mirrors and polarizers
having a wide range of performance characteristics, can be designed
and fabricated using the principles described therein.
[0102] In accordance with another embodiment of the present
invention, an optical repeating unit of a multilayer film in
accordance with the present invention comprises polymeric layers A,
B and C having different indices of refraction. Such type of
repeating unit is particularly suitable for designing an infrared
reflective multilayer film. In particular, by selecting polymeric
layers A, B and C such that polymeric layer B has an index of
refraction intermediate that of polymeric layers A and C, an
infrared reflective film can be designed for which at least two
successive higher order reflections are suppressed, thus allowing
the design of an infrared reflective film that is substantially
transparent in the visible. A multilayer film of this type is
described in detail in, e.g., U.S. Pat. No. 5,103,337, which is
also incorporated herein by reference.
[0103] In accordance with this embodiment of the invention,
multiple alternating substantially transparent polymeric layers A,
B and C having different indices of refraction niare arranged in
the order ABC. Additionally, the refractive index of polymeric
layer B is intermediate the respective refractive indices of the
polymeric layers A and C. In a particularly preferred embodiment
having an optical repeating unit comprising polymeric layers A, B
and C arranged in a pattern ABCB, and where multiple successive
higher order reflections are suppressed, the optical thickness
ratio f.sup.a of first material A is 1/3, the optical thickness
ratio f.sup.b of second material B is 1/6, the optical thickness
ratio f.sup.c of third material C is 1/3, and the index of
refraction of polymeric layer B equals the square root of the
product of the index of refraction of polymeric layers A and C.
This particular type of optical repeating unit can be used to
design a multilayer film in which reflections for the second,
third, and fourth order wavelengths will be suppressed.
[0104] In accordance with a further embodiment of the present
invention, the above multilayer film having an optical repeating
unit comprising polymeric layers A, B and C arranged in an ABCB
order can be designed using an anisotropic layer for at least one
of polymeric layers A, B and C. Thus, in accordance with one
embodiment of the present invention, a multilayer film that
reflects light in the infrared region of the spectrum while
transmitting light in the visible region of the spectrum may
comprise an optical repeating unit comprising polymeric layers A, B
and C arranged in an ABCB order, the polymeric layer A having
refractive indices n.sub.x.sup.a and n.sub.y.sup.a along in-plane
axes x and y, respectively, the polymeric layer B having refractive
indices n.sub.x.sup.b and n.sub.y.sup.b along in-plane axes x and
y, respectively, the polymeric layer C having refractive indices
n.sub.x.sup.c and n.sub.y.sup.c along in-plane axes x and y,
respectively, polymeric layers A, B and C having a refractive index
n.sub.z.sup.a, n.sub.z.sup.b and n.sub.z.sup.c, respectively, along
a transverse axis z perpendicular to the in-plane axes, wherein
n.sub.x.sup.b is intermediate n.sub.x.sup.a and n.sub.x.sup.c, with
n.sub.x.sup.a being larger than n.sub.x.sup.c and/or n.sub.y.sup.b
is intermediate to n.sub.y.sup.a and n.sub.y.sup.c with
n.sub.y.sup.a being larger than n.sub.y.sup.c and wherein
preferably at least one of the differences
n.sub.z.sup.a-n.sub.z.sup.b and n.sub.z.sup.b-n.sub.z.sup.c is less
than 0 or both said differences are substantially equal to 0.
[0105] By designing the optical repeating unit such that at least
one of the differences n.sub.z.sup.a-n.sub.z.sup.b and
n.sub.z.sup.b-n.sub.z.sup- .c is less than 0 and preferably less
than -0.05, or such that both said differences are substantially 0,
and while setting the index relationship along the in-plane axis
between the layers as set out above, at least second and third
higher order reflections can be suppressed without a substantial
decrease of the infrared reflection with angle of incidence of the
infrared light.
[0106] The polymeric layers A, B and C of the optical repeating
unit preferably form an ABCB optical repeating unit. A schematic
drawing of such a repeating unit is shown in FIG. 6. According to
this embodiment, the difference of the index of refraction between
layers A and B along the z-axis (n.sub.z.sup.a-n.sub.z.sup.b)
and/or the difference of the index of refraction between layers B
and C along the z-axis (n.sub.z.sup.b-n.sub.z.sup.c) is preferably
negative, i.e., has a value less than 0, more preferably less or
equal to -0.05, and most preferably less than or equal to -0.1. It
is particularly preferred to design the optical repeating unit such
that one of the differences is less than 0, more preferably less
than or equal to -0.05, and the other difference is either equal to
0 or less than 0. Most preferably, both difference are less than 0.
Such designs, wherein one of the difference is less than 0 and the
other is 0 or less than 0, yield an increase of the reflection with
the angle of incidence.
[0107] It is also possible to design an optical repeating unit in
accordance with the present embodiment wherein both differences are
substantially 0, i.e., wherein the absolute value of the
differences is preferably less than 0.03. When both differences are
substantially 0, there will be little or no decrease in the
infrared reflection with the angle of incidence.
[0108] According to a still further species of the present
embodiment, one of the differences in refraction index between
layers A and B across the z-axis is of opposite in sign to the
difference of the refraction index between layers B and C across
the z-axis. In the latter case, it is preferred that the difference
that is less than 0 has the largest absolute value or that the
absolute value of both differences is substantially equal.
[0109] By adjusting the optical thickness ratios along the
particular in-plane axis that has the index of refraction for
polymeric layer B intermediate that of polymeric layer A and
polymeric layer C, at least two higher order reflections for
infrared light having its plane of polarization parallel to that
particular in-plane axis can be suppressed. It is, however,
preferred that the index of refraction for polymeric layer B is
intermediate that of polymeric layers A and C along both in-plane
axes, and by adjusting the optical thickness ratios along both
in-plane axes, an infrared reflective mirror can be obtained for
which at least two successive higher order reflections are
suppressed. Such an infrared reflective mirror will be
substantially clear in the visible region and will be free of
color.
[0110] A particularly preferred optical repeating unit for
designing an infrared reflecting multilayer film in accordance with
the present invention comprises polymeric layers A, B and C
arranged in an ABCB pattern, with the refractive indices for
polymeric layers A, B and C such that
n.sub.x.sup.b=(n.sub.x.sup.an.sub.x.sup.c).sup.1/2 and/or
n.sub.y.sup.b=(n.sub.y.sup.an.sub.y.sup.c).sup.1/2 while keeping
the following optical thickness ratios: f.sub.x.sup.a=1/3,
f.sub.x.sup.b=1/6 and f.sub.x.sup.c=1/3 and/or f.sub.y.sup.a=1/3,
f.sub.y.sup.b=1/6 and f.sub.y.sup.c=1/3. Such an embodiment is
capable of suppressing second, third and fourth order reflections.
An infrared reflective multilayer film designed according to this
embodiment can be used to reflect infrared light up to about 2000nm
without introducing reflections in the visible part of the
spectrum.
[0111] Preferably, an optical repeating unit comprising polymeric
layers A, B and C has, along an in-plane axis, refractive indices
of polymers A, B and C different by at least 0.05. Thus, it is
preferred that n.sub.x.sup.a, n.sub.x.sup.b and n.sub.x.sup.c
differ from each other by at least 0.05 and/or that n.sub.y.sup.a,
n.sub.y.sup.b and n.sub.y.sup.c differ from each other by at least
0.05.
[0112] The above various embodiments describing different possible
designs of optical repeating units for use in the multilayer films
in accordance with the present invention is not intended to be
limiting to this invention. In particular, other designs of optical
repeating units can be used as well. Furthermore, multilayer films
comprising optical repeating units of different design can be used
in combination for forming a reflective film body in accordance
with the present invention. For example, a multilayer film
comprising an optical repeating unit consisting of only two
polymeric layers can be combined with a multilayer film comprising
an optical repeating unit comprising polymeric layers A, B and C
arranged in an ABC order in particular in an ABCB pattern.
[0113] One skilled in the art will readily appreciate that a wide
variety of materials can be used to form (infrared) mirrors or
polarizers according to the present invention when processed under
conditions selected to yield the desired refractive index
relationships. The desired refractive index relationships can be
achieved in a variety of ways, including stretching during or after
film formation (e.g., in the case of organic polymers), extruding
(e.g., in the case of liquid crystalline materials), or coating. In
addition, it is preferred that the two materials have similar
Theological properties (e.g., melt viscosities) so that they can be
co-extruded.
[0114] In general, appropriate combinations may be achieved by
selecting, for each of the layers, a crystalline, semi-crystalline,
or liquid crystalline material, or amorphous polymer. It should be
understood that, in the polymer art, it is generally recognized
that polymers are typically not entirely crystalline, and therefore
in the context of the present invention, crystalline or
semi-crystalline polymers refer to those polymers that are not
amorphous and includes any of those materials commonly referred to
as crystalline, partially crystalline, semi-crystalline, etc.
[0115] Specific examples of suitable materials for usew in the
present invention include polyethylene naphthalate (PEN) and
isomers thereof (e.g., 2,6-, 1,4-, 1,5-, 2,7-, and 2,3-PEN),
polyalkylene terephthalates (e.g., polyethylene terephthalate,
polybutylene terephthalate, and poly-1,4-cyclohexanedimethylene
terephthalate), polyimides (e.g., polyacrylic imides),
polyetherimides, atactic polystyrene, polycarbonates,
polymethacrylates (e.g., polyisobutyl methacrylate,
polypropylmethacrylate, polyethylmethacrylate, and
polymethylmethacrylate), polyacrylates (e.g., polybutylacrylate and
polymethylacrylate), syndiotactic polystyrene (sPS), syndiotactic
poly-alpha-methyl styrene, syndiotactic polydichlorostyrene,
copolymers and blends of any of these polystyrenes, cellulose
derivatives (e.g., ethyl cellulose, cellulose acetate, cellulose
propionate, cellulose acetate butyrate, and cellulose nitrate),
polyalkylene polymers (e.g., polyethylene, polypropylene,
polybutylene, polyisobutylene, and poly(4-methyl)pentene),
fluorinated polymers (e.g., perfluoroalkoxy resins,
polytetrafluoroethylene, fluorinated ethylene-propylene copolymers,
polyvinylidene fluoride, and polychlorotrifluoroethylene),
chlorinated polymers (e.g., polyvinylidene chloride and
polyvinylchloride), polysulfones, polyethersulfones,
polyacrylonitrile, polyamides, silicone resins, epoxy resins,
polyvinylacetate, polyether-amides, ionomeric resins, elastomers
(e.g., polybutadiene, polyisoprene, and neoprene), and
polyurethanes. Also suitable are copolymers, e.g., copolymers of
PEN (e.g., copolymers of 2,6-, 1,4-, 1,5-, 2,7-, and/or
2,3-naphthalene dicarboxylic acid, or esters thereof, with (a)
terephthalic acid, or esters thereof; (b) isophthalic acid, or
esters thereof; (c) phthalic acid, or esters thereof; (d) alkane
glycols; (e) cycloalkane glycols (e.g., cyclohexane dimethanol
diol); (f) alkane dicarboxylic acids; and/or (g) cycloalkane
dicarboxylic acids (e.g., cyclohexane dicarboxylic acid)),
copolymers of polyalkylene terephthalates (e.g., copolymers of
terephthalic acid, or esters thereof, with (a) naphthalene
dicarboxylic acid, or esters thereof; (b) isophthalic acid, or
esters thereof; (c) phthalic acid, or esters thereof; (d) alkane
glycols; (e) cycloalkane glycols (e.g., cyclohexane dimethane
diol); (f) alkane dicarboxylic acids; and/or (g) cycloalkane
dicarboxylic acids (e.g., cyclohexane dicarboxylic acid)), and
styrene copolymers (e.g., styrene-butadiene copolymers and
styrene-acrylonitrile copolymers), 4,4'-bibenzoic acid and ethylene
glycol. In addition, each individual layer may include blends of
two or more of the above-described polymers or copolymers (e.g.,
blends of SPS and atactic polystyrene).
[0116] Particularly preferred birefringent polymeric layers for use
in the present invention include layers containing a crystalline or
semi-crystalline polyethylenenaphthalate (PEN), inclusive of its
isomers (e.g. 2,6-; 1,4-; 1,5-; 2,7; and 2,3-PEN). A particularly
preferred isotropic polymeric layer for use in connection with this
invention is a layer containing a polymethylmethacrylate, and in
particular, polymethylmethacrylate itself.
[0117] It will further be understood by one skilled in the art that
each of the polymeric layers may be composed of blends of two or
more polymeric materials to obtain desired properties for a
specific layer.
[0118] The films and other optical devices made in accordance with
the invention may also include one or more anti-reflective layers
or coatings, such as, for example, conventional vacuum coated
dielectric metal oxide or metal/metal oxide optical films, silica
sol gel coatings, and coated or coextruded antireflective layers
such as those derived from low index fluoropolymers such as THV, an
extrudable fluoropolymer available from 3M Company (St. Paul,
Minn.). Such layers or coatings, which may or may not be
polarization sensitive, serve to increase transmission and to
reduce reflective glare, and may be imparted to the films and
optical devices of the present invention through appropriate
surface treatment, such as coating or sputter etching.
[0119] Both visible and near IR dyes and pigments are contemplated
for use in the films and other optical bodies of the present
invention, and include, for example, optical brighteners such as
dyes that absorb in the UV and fluoresce in the visible region of
the color spectrum. Other additional layers that may be added to
alter the appearance of the optical film include, for example,
opacifying (black) layers, diffusing layers, holographic images or
holographic diffusers, and metal layers. Each of these may be
applied directly to one or both surfaces of the optical film, or
may be a component of a second film or foil construction that is
laminated to the optical film. Alternately, some components such as
opacifying or diffusing agents, or colored pigments, may be
included in an adhesive layer which is used to laminate the optical
film to another surface.
[0120] It is preferred that the polymers have compatible rheologies
for coextrusion. That is, as a preferred method of forming the
reflective film bodies is the use of coextrusion techniques, the
melt viscosities of the polymers are preferably reasonably matched
to prevent layer instability or non-uniformity. The polymers used
also preferably have sufficient interfacial adhesion so that the
films will not delaminate.
[0121] The multilayer reflective film bodies of the present
invention can be readily manufactured in a cost effective way, and
they can be formed and shaped into a variety of useful
configurations after coextrusion. Multilayer reflective film bodies
in accordance with the present invention are most advantageously
prepared by employing a multilayered coextrusion device such as
those described in U.S. Pat. Nos. 3,773,882 and 3,884,606, the
disclosures of which are incorporated herein by reference. Such
devices provide a method for preparing multilayered, simultaneously
extruded thermoplastic materials, each of which are of a
substantially uniform layer thickness.
[0122] Preferably, a series of layer multiplying means as are
described in U.S. Pat. No. 3,759,647, the disclosure of which is
incorporated herein by reference, may be employed. The feedblock of
the coextrusion device receives streams of the diverse
thermoplastic polymeric materials from a source such as a heat
plastifying extruder. The streams of resinous materials are passed
to a mechanical manipulating section within the feedblock. This
section serves to rearrange the original streams into a
multilayered stream having the number of layers desired in the
final body. Optionally, this multilayered stream may be
subsequently passed through a series of layer multiplying means in
order to further increase the number of layers in the final
body.
[0123] The multilayered stream is then passed into an extrusion die
which is so constructed and arranged that stream-lined flow is
maintained therein. Such an extrusion device is described in U.S.
Pat. No. 3,557,265, the disclosure of which is incorporated by
reference herein. The resultant product is extruded to form a
multilayered body in which each layer is generally parallel to the
major surface of adjacent layers.
[0124] The configuration of the extrusion die can vary and can be
such as to reduce the thickness and dimensions of each of the
layers. The precise degree of reduction in thickness of the layers
delivered from the mechanical orienting section, the configuration
of the die, and the amount of mechanical working of the body after
extrusion are all factors which affect the thickness of the
individual layers in the final body.
[0125] The number of layers in the reflective film body can be
selected to achieve the desired optical properties using the
minimum number of layers for reasons of film thickness, flexibility
and economy. In the case of both reflective polarizers and
reflective mirrors, the number of layers is preferably less than
about 10,000, more preferably less than about 5,000, and (even more
preferably) less than about 2,000.
[0126] The desired relationship between refractive indices of
polymeric layers as desired in this invention can be achieved by
selection of appropriate processing conditions used to prepare the
reflective film body. In the case of organic polymers which can be
oriented by stretching, the multilayer films are generally prepared
by co-extruding the individual polymers to form a multilayer film
(e.g., as set out above) and then orienting the reflective film
body by stretching at a selected temperature, optionally followed
by heat-setting at a selected temperature. Alternatively, the
extrusion and orientation steps may be performed simultaneously. By
the orientation, the desired extent of birefringence (negative or
positive) is set in those polymeric layers that comprise a polymer
that can exhibit birefringence. Negative birefringence is obtained
with polymers that show a negative optical stress coefficient,
i.e., polymers for which the in-plane indices will decrease with
orientation, whereas positive birefringence is obtained with
polymers having a positive optical stress coefficient. This
terminology in the art of film orientation conflicts somewhat with
the standard optical definition of positive and negative
birefringence. In the art of optics, a uniaxially positive
birefringent film or layer is one in which the z-index of
refraction is higher than the in-plane index. A biaxially stretched
polymer film such as PET will have high in-plane indices, e.g.,
1.65, and a low out-of-plane or z-axis index of 1.50. In the film
making art, a material such as PET is said to be positively
birefringent because the index increases in the stretch direction,
but in the art of optics, the same material, after biaxially
stretching to film, is said to have uniaxial negative birefringence
because the z-index is lower than the in-plane indices which are
substantially equal. The term "positive birefringence" for a
material as used herein will be that of the polymer film art, and
will mean that the index of refraction increases in the stretch
direction. Similarly, the term "negative birefringence" for a
material will mean that the index of refraction of a film decreases
in the direction of stretch. The terms "uniaxially positive" or
"uniaxially negative", when used in reference to a birefringent
layer, will be taken to have the meaning in the optics sense.
[0127] In the case of polarizers, the reflective film body is
stretched substantially in one direction (uniaxial orientation),
while in the case of mirrors the film can be stretched
substantially in two directions (biaxial orientation). In the
latter case, the stretching may be asymmetric to introduce
specially desired features, but is preferably symmetric.
[0128] The reflective film body may be allowed to dimensionally
relax in the cross-stretch direction from the natural reduction in
cross-stretch (equal to the square root of the stretch ratio) or
may be constrained (i.e., no substantial change in cross-stretch
dimensions). The reflective film body may be stretched in the
machine direction, as with a length orienter, and/or in width using
a tenter.
[0129] The pre-stretch temperature, stretch temperature, stretch
rate, stretch ratio, heat set temperature, heat set time, heat set
relaxation, and cross-stretch relaxation are selected to yield a
multilayer device having the desired refractive index relationship.
These variables are inter-dependent; thus, for example, a
relatively low stretch rate could be used if coupled with, e.g., a
relatively low stretch temperature. It will be apparent to one
skilled in the art how to select the appropriate combination of
these variables to achieve the desired multilayer device. In
general, however, a stretch ratio in the range from about 1:2 to
about 1:10 (more preferably about 1:3 to about 1:7) in the stretch
direction and from about 1:0.2 to about 1:10 (more preferably from
about 1:0.2 to about 1:7) orthogonal to the stretch direction is
preferred.
[0130] Orientation of the extruded film can be accomplished by
stretching individual sheets of the material in heated air. For
economical production, stretching may be accomplished on a
continuous basis in a standard length orienter, tenter oven, or
both. Economies of scale and line speeds of standard polymer film
production may be achieved, thereby achieving manufacturing costs
that are substantially lower than costs associated with
commercially available absorptive polarizers.
[0131] Two or more multilayer films may also be laminated together
to obtain a reflective film body in accordance with the present
invention. Amorphous copolyesters, such as VITEL Brand 3000 and
3300 from the Goodyear Tire and Rubber Co. of Akron, Ohio, are
useful as laminating materials. The choice of laminating material
is broad, with adhesion to the multilayer films, optical clarity
and exclusion of air being the primary guiding principles.
[0132] It may be desirable to add to one or more of the layers, one
or more inorganic or organic adjuvants such as an antioxidant,
extrusion aid, heat stabilizer, ultraviolet ray absorber,
nucleator, surface projection forming agent, and the like in normal
quantities so long as the addition does not substantially interfere
with the performance of the present invention.
[0133] The preceding description of the present invention is merely
illustrative, and is not intended to be limiting. Therefore, the
scope of the present invention should be construed solely by
reference to the appended claims.
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