Two component infrared reflecting film

The present invention provides a two-component infrared reflecting film which reflects light in the infrared region of the spectrum while suppressing second, third and fourth order reflections in the visible spectrum. The film comprises alternating layers of first (A) and second (B) diverse polymeric materials having a six layer alternating repeat unit with relative optical thicknesses of about .778A.111B.111A.778B.111A.111B. The polymeric materials differ from each other in refractive index by at least about 0.03.

BACKGROUND OF THE INVENTION 
The present invention relates to a two component infrared reflecting film, 
and more particularly to two component films which reflect light in the 
infrared region of the spectrum while suppressing second, third and fourth 
order reflections in the visible region of the spectrum. 
Coextruded multilayer films have been made which comprise multiple 
alternating layers of two polymers with individual layer thicknesses of 
100 nanometers or less. Such multilayer films are described, for example, 
in Alfrey et al, U.S. Pat. No. 3,711,176. When polymers are selected to 
have a sufficient mismatch in refractive indices, these multilayer films 
cause constructive interference of light. This results in the film 
transmitting certain wavelengths of light through the film while 
reflecting other wavelengths. The multilayer films can be fabricated from 
relatively inexpensive and commercially available polymer resins having 
the desired refractive index differences. The films have the further 
advantage in that they may be shaped or formed into other objects. 
The reflection and transmission spectra for a particular two-component film 
are primarily dependent on the optical thickness of the individual layers, 
where optical thickness is defined as the product of the actual thickness 
of the layer times its refractive index. The intensity of light reflected 
from such films is a function of the number of layers and the differences 
in refractive indices of the polymers. Mathematically, the wavelength of 
the dominant, first order wavelength for reflected light (at normal 
incidence) is: 
##EQU1## 
where .lambda..sub.I is the first order wavelength, n is the refractive 
index of the polymer, and d is the layer thickness of the polymer, and k 
is the number of polymer components. Films can be designed to reflect 
infrared, visible, or ultraviolet wavelengths of light depending on the 
optical thickness of the layers. When designed to reflect infrared 
wavelengths of light, such prior art films also exhibit higher order 
appearance for the films. Mathematically, higher order reflections will 
appear at 
##EQU2## 
where m is the order of the reflection (e.g. 2, 3, 4, etc.) As can be 
seen, higher order reflections appear at fractions of the first order 
reflection. The films produced in accordance with the above mentioned 
Alfrey patent exhibit iridescence and changing colors as the angle of 
incident light on the film is changed. 
For some applications, while reflection of infrared wavelengths is 
desirable, higher order reflections of visible light are not. For example, 
infrared reflecting films can be laminated to glass in buildings and 
automobiles to reduce air conditioning loads. The films may also be 
laminated to other substantially transparent plastic materials to reflect 
infrared wavelengths. However, the films must be substantially transparent 
to visible light so that the vision of those looking through the glass or 
plastic is not impaired. 
It is possible to suppress some higher order reflections (i.e., reduce 
their intensity) by proper selection of the ratio of optical thicknesses 
in two component multilayer films. See, Radford et al, "Reflectivity of 
Iridescent Coextruded Multilayered Plastic Films", Polymer Engineering and 
Science, vol. 13, No. 3, May 1973. This ratio of optical thicknesses is 
termed "f-ratio", where f =n.sub.1 d.sub.1 /(n.sub.1 d.sub.1 +n.sub.2 
d.sub.2). However, such two component films do not suppress successive 
second, third and fourth order visible wavelengths. 
Other workers have designed optical coatings comprising layers of three or 
more materials which are able to suppress certain higher order 
reflections. For example, Thelen, U.S. Pat. No. 3,247,392, describes an 
optical coating used as a band pass filter reflecting in the infrared and 
ultraviolet regions of the spectrum. The coating is taught to suppress 
second and third order reflectance bands. However, the materials used in 
the fabrication of the coating are metal oxide and halide dielectric 
materials which must be deposited in separate steps using expensive vacuum 
deposition techniques. Also, once deposited, the coatings and the 
substrates to which they are adhered cannot be further shaped or formed. 
Further, the coatings are subject to chipping, scratching, and/or 
corrosion and must be protected. Finally, because vacuum deposition 
techniques must be used, it is both expensive and difficult to fabricate 
coatings which cover large surface areas. 
Rock, U.S. Pat. No. 3,432,225, teaches a two component, four layer 
antireflection coating which utilizes specified thicknesses of the first 
two layers of the coating to synthesize an equivalent layer having an 
effective index of refraction which is intermediate that of the first two 
layers. However, Rock also uses metal halides, oxides, sulfides, and 
selenides which must be deposited in separate processing steps using 
vacuum deposition techniques. 
Another technique has been suggested for a three-layer film comprised of 
two components which is equivalent in refractive index and optical 
thickness to a film comprised of three components. The third component is 
eliminated by synthesizing a three layer structure which has the same 
optical performance as a three component structure. See Ohmer, "Design of 
three-layer equivalent films", Journal of the Optical Society of 
AmericaVol. 68 (I), 137 (January 1978). However, Ohmer also uses vacuum 
deposition of metal oxides, halides, and selenides. Further, such a 
structure does not provide sufficient suppression of the fourth order 
reflectance band, thus hindering its optical performance. 
Rancourt et al, U.S. Pat. No. 4,229,066 teaches a visible light 
transmitting, infrared reflecting multilayer coating utilizing metal 
halide sulfides and selenides. The materials have either a high or low 
index of refraction and must be deposited in separate steps using vacuum 
deposition techniques. In addition, Rancourt requires 10 layers in the 
repeat unit. Further, the coatings of Rancourt et al cannot be further 
shaped or formed after deposition. 
Schrenk et al, U.S. Pat. No. 5,103,337, describes an all polymeric 
three-component optical interference film formed by coextrusion techniques 
which reflects infrared light while suppressing second, third and fourth 
order reflections in the visible region of the spectrum. However, the 
polymers in the film are required to have closely defined refractive 
indexes, which limits the choice of polymers which may be used. In 
addition, the production of the film requires separate extruders for each 
of the polymeric components. 
Accordingly, the need still exists in this art for a two-component film 
which reflects infrared light, successfully suppresses multiple successive 
higher order reflections to prevent unwanted reflections in the visible 
range, allows a wide choice of polymers, and does not require complicated 
extrusion equipment. 
SUMMARY OF THE INVENTION 
The present invention meets that need by providing a two-component 
polymeric infrared reflecting film which reflects wavelengths of light in 
the infrared region of the spectrum while suppressing second, third, and 
fourth order wavelengths which occur in the visible range. 
The terms "reflective", "reflectivity", "reflection", and "reflectance" as 
used herein refer to total reflectance (i.e., ratio of reflected wave 
energy to incident wave energy) of a sufficiently specular nature. The use 
of these terms is intended to encompass semi-specular or diffuse 
reflection as well. In general, reflectance measurement refers to 
reflectance of light rays into an emergent cone with a vertex angle of 15 
degrees centered around the specular angle. By the term "diverse" we mean 
that the polymeric materials need not differ in any respect except in 
terms of refractive index. Thus, while adjacent layers may be chemically 
diverse, if such materials have the same refractive index, then for 
purposes of the present invention they are not "diverse". 
A specific intensity of reflectance, when used herein, is the intensity of 
reflection which occurs at a wavelength where no substantial absorption 
occurs. For example, in a preferred embodiment of the invention, the film 
is designed to reflect infrared light having wavelengths in the range from 
about 770-2000 nm. Light of other wavelengths, such as in the visible 
range, pass through (i.e., are transmitted by) the film. It is at the 
infrared wavelengths to which the intensity of reflection is referring. 
According to one aspect of the present invention, a two component 
infrared-reflecting film is provided which reflects light in the infrared 
wavelength region of between about 770-2000 nm while suppressing second, 
third and fourth order reflections in the visible wavelength region of 
between about 380-770 run. The film comprises alternating layers of first 
(A) and second (B) diverse polymeric materials having a six layer 
alternating repeat unit with relative optical thicknesses of about 
.778A.111B.111A.778B.111A.111B. 
In an alternative embodiment of the invention, the two component film 
comprises a first portion of alternating layers comprising the six layer 
alternating layer repeating unit with relative optical thicknesses of 
about 778A.111B.111A.778B.111A.111B, and a second portion of alternating 
layers having a repeating unit AB. In one preferred form, the layers in 
the second portion of the film have substantially equal optical 
thicknesses. In this embodiment, the first portion of alternating layers 
reflects infrared light of wavelengths between about 1200-2000 run, while 
the second portion of alternating layers reflects infrared light of 
wavelengths between about 770-1200 nm. 
Preferably, at least 50% of visible light between about 380-770 nm incident 
on the film is transmitted and at least 50% of infrared light of 
wavelengths of between about 770-2000 nm is reflected. In other preferred 
embodiments of the invention, at least 80% of visible light incident on 
the film is transmitted and at least 80% of infrared light incident on the 
film is reflected. 
Preferably, the polymeric materials differ from each other in refractive 
index by at least about 0.03. In a preferred embodiment, the first 
polymeric material is polycarbonate and the second polymeric material is 
polymethyl methacrylate, the materials having a refractive index mismatch 
of about 0.1. 
The layers in the film preferably have a repeat unit gradient so that the 
film reflects a broad bandwidth of wavelengths in the infrared range. By 
repeat unit gradient, we mean a change in the thickness of the layers 
across the thickness of the body so that there is a difference in the 
thickness between the thinnest and thickest repeat unit in the multilayer 
stack. The gradient may be any regular function including, but not limited 
to, a linear function, a logarithmic function, a quartic function, or a 
quartic function superimposed on a linear gradient. 
In the embodiment of the invention which includes first and second portions 
of alternating layers, the first portion of alternating layers preferably 
has a repeat unit gradient of 5/3:1, and the second portion of alternating 
layers preferably has a repeat unit gradient of about 1.5:1. 
The two component infrared reflecting films of the present invention may 
find use in areas where infrared reflective properties are desired. For 
example, the films of the present invention may be laminated to glass used 
in buildings and automobiles to reflect infrared radiation, thus lowering 
the heating loads. Further, the films may also be laminated to other 
substantially transparent plastics to provide infrared reflective 
properties. For example, windshields and canopies on certain aircraft are 
fabricated from tough polymeric resins. Laminating the optical 
interference film of the present invention to, or incorporating the film 
into, such protective layers would provide protection from infrared 
radiation while still providing substantial transparency to light in the 
visible region of the spectrum. The films themselves, as well as the 
plastics to which they are laminated may be shaped or post-formed into a 
variety of useful objects. Because the films suppress successive higher 
order reflections in the visible region of the spectrum, the films have a 
high transmission in the visible region. 
Through the use of a broad-band reflective multilayer polymeric film or a 
suitable broad-band reflective metal oxide or halide coating in 
combination with the infrared-reflecting film of the present invention, 
the film is also capable of masking iridescent color reflected from the 
infrared-reflecting layers as taught in commonly assigned Wheatley et al 
U.S. patent application Ser. No. 07/888,705, filed May 27, 1992. now U.S. 
Pat. No. 5,233,456, issued Aug. 3, 1993. By "masking", it is meant that 
means are provided to reflect and/or refract light to interfere with the 
observance of iridescent visible color. For example, the color masking 
means may comprise a masking film which reflects light substantially 
uniformly over the visible portion of the spectrum. 
The color masking film may be located on one or both of the major surfaces 
of the polymeric film or between interior layers of the film. Preferably, 
the color masking film is laminated to the polymeric film. Alternatively, 
it may be coextruded therewith. The color masking means may also comprise 
a substantially colorless metal oxide or metal halide film having 
sufficient broad band reflectance in the visible range to mask the 
iridescent color effects of the infrared reflecting film. Such a metal 
oxide or halide film is preferably deposited on the film by conventional 
coating techniques such as pyrolysis, powder coating, chemical vapor 
deposition, vacuum coating, or cathode sputtering. The metal oxide or 
halide film may be located on one or both of the major surfaces of the 
polymeric film or between interior layers of the film. A preferred metal 
oxide film is tin oxide. 
Accordingly, it is an object of the present invention to provide a 
two-component infrared reflecting film which reflects light in the 
infrared region of the spectrum, suppresses successive higher order 
reflections of visible wavelengths, and is fabricated using relatively 
inexpensive materials. These, and other objects and advantages of the 
present invention, will become apparent from the following detailed 
description, the accompanying drawings, and the appended claims.

DETAINED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention provides improved two component infrared reflecting 
films with a number of desirable properties including broadband infrared 
reflectivity, the capability of suppressing successive second, third, and 
fourth order reflections, and the substantial absence of undesirable 
iridescence. The use of only two components in the film provides an 
advantage over three component films in that compatibility between 
polymers is more readily achieved as polymers do not have to be chosen 
based on their refractive indices, but rather on the difference in their 
refractive indices. Eliminating the need for a third polymer component 
greatly simplifies the selection of the polymers and also simplifies the 
extrusion apparatus which is needed. 
The optical theory of multiple reflections from layers having differing 
refractive indices demonstrates the dependency of the effect on both 
individual layer thickness and difference in the refractive indices of the 
materials. See, Radford et al, "Reflectivity of Iridescent Coextruded 
Multilayered Plastic Films", 13 Polymer Engineering and Science 216 
(1973). The primary or first order reflected wavelength (for normal 
incidence) for a two component multilayer film is given by the Equation 
below. 
EQU .lambda..sub.I =2 (n.sub.1 d.sub.1 +n.sub.2 d.sub.2) 
where, .lambda..sub.I is the wavelength of first order reflection in 
nanometers, n.sub.1 and n.sub.2 are the refractive indices of the two 
polymers, and d.sub.1 and d.sub.2 are the layer thicknesses of the two 
polymers, also in nanometers. 
As can be seen, the first order reflected wavelength is proportional to the 
sum of the optical thicknesses of the two polymers (where optical 
thickness, n.sub.i d.sub.i, is the product of layer thickness times 
refractive index). In addition to first order reflections, higher order 
reflections occur at integer fractions of the first order. 
Higher order reflections will occur at a wavelength 
EQU .lambda..sub.m 2/m (n.sub.1 d.sub.1 +n.sub.2 d.sub.2) 
where .lambda..sub.m is the mth order reflected wavelength and d is the 
layer thickness, both in nanometers, and m is the order of reflection 
(m=1,2,3,4,5, etc.). The relative intensity of these higher order 
reflections depends on the ratio of the optical thicknesses of the polymer 
components. As taught by Radford et al, by controlling the optical 
thickness ratios in a two component system, different order reflections 
may be enhanced while others are suppressed. However, prior art two 
component interference films have not been able to suppress successive 
second, third and fourth order visible wavelengths simultaneously. Such 
films which are designed to reflect strongly in the near infrared region 
of the spectrum will exhibit unwanted reflectivity in the visible region 
of the spectrum because of higher order reflections. 
However, in accordance with the present invention, a two component film 
having a six layer alternating repeating unit suppresses the unwanted 
second, third, and fourth order reflections in the visible wavelength 
region of between about 380-770 nm while reflecting light in the infrared 
wavelength region of between about 770-2000 nm. Reflections higher than 
fourth order will generally be in the ultraviolet, not visible, region of 
the spectrum or will be of such a low intensity as to be unobjectionable. 
The film comprises alternating layers of first (A) and second (B) diverse 
polymeric materials in which the six layer alternating repeat unit has 
relative optical thicknesses of about .778A.111B.111A.778B.111A.111B. The 
use of only six layers in the repeat unit results in more efficient use of 
material and simpler manufacture than previous designs. 
A repeat unit gradient may be introduced across the thickness of the film. 
Thus, in one embodiment of the invention, the repeat unit thicknesses will 
increase linearly across the thickness of the film. By linearly, it is 
meant that the repeat unit thicknesses increase at a constant rate across 
the thickness of the film. 
In some embodiments, it may be desirable to force the repeat unit optical 
thickness to double from one surface of the film to another. The ratio of 
repeat unit optical thicknesses can be greater or less than two as long as 
the short wavelength range of the reflectance band is above 770 nm and the 
long wavelength edge is about 2000 nm. 
Other repeat unit gradients may be introduced by using logarithmic and/or 
quartic functions. We have found that a logarithmic distribution of repeat 
unit thicknesses will provide nearly constant reflectance across the 
infrared band. 
In an alternative embodiment of the invention, the two component film may 
comprise a first portion of alternating layers comprising the six layer 
alternating layer repeating unit which reflects infrared light of 
wavelengths between about 1200-2000 nm. and a second portion of 
alternating layers having an AB repeat unit and substantially equal 
optical thicknesses which reflect infrared light of wavelengths between 
about 770-1200 nm. Such a combination of alternating layers results in 
reflection of light across the infrared wavelength region through 2000 nm. 
Preferably, the first portion of the alternating layers has a repeat unit 
gradient of about 5/3:1, and the second portion of alternating layers have 
a layer thickness gradient of about 1.5:1. 
FIG. 1 schematically illustrates a two component infrared reflective film 
10 having a six layer alternating repeat unit 
778A.111B.111A.778B.111A.111B in accordance with the present invention. 
The film 10 includes alternating layers of first polymer 12 having a 
refractive index, n.sub.l, and a second polymer 14 having a refractive 
index, n.sub.2. As previously described, the infrared reflecting film may 
be laminated to a substantially transparent substrate 17, such as a 
polymer or glass. In an alternative embodiment of the invention, substrate 
17 may be a color masking film such as that taught in copending, commonly 
assigned Wheatley et al, U.S. application Ser. No. 07/888,705, filed May 
27, 1992 now U.S. Pat. No. 5,233,465, issued Aug. 3, 1993, disclosure of 
which is incorporated by reference. The color masking film reflects light 
substantially uniformly over the visible portion of the spectrum, and may 
be located on one or both of the major surfaces of the polymeric film or 
between interior layers of the film. Preferably, the color masking film is 
laminated to the polymeric film as shown in FIG. 1. Alternatively, it may 
be coextruded therewith. The color masking film may also comprise a 
substantially colorless metal oxide or metal halide film having sufficient 
broad band reflectance in the visible range to mask the iridescent color 
effects of the infrared reflecting film. Such a metal oxide or halide film 
is preferably deposited on the film by conventional coating techniques 
such as pyrolysis, powder coating, chemical vapor deposition, vacuum 
coating, or cathode sputtering. The metal oxide or halide film may be 
located on one or both of the major surfaces of the polymeric film or 
between interior layers of the film. A preferred metal oxide film is tin 
oxide. 
FIG. 2 illustrates an alternative embodiment of the invention including a 
first portion of alternating layers 18 comprising the six layer 
alternating repeat unit and a second portion of alternating layers 20 
having a repeating unit AB. It should be appreciated that the order of the 
first and second portions of alternating layers may be varied as desired. 
Preferably, the polymers chosen have a refractive index mismatch of at 
least 0.03. A preferred two component film includes polycarbonate as the 
first polymeric material and polymethyl methacrylate as the second 
polymeric material. It is preferred that the polymers selected have 
compatible rheologies for coextrusion. That is, as a preferred method of 
forming the two component infrared reflecting films is the use of 
coextrusion techniques, the melt viscosities of the polymers must be 
reasonably matched to prevent layer instability or nonuniformity. The 
polymers used also should have sufficient interfacial adhesion so that the 
films will not delaminate. 
Multilayer bodies in accordance with the present invention are most 
advantageously prepared by employing a multilayered coextrusion device as 
described in U.S. Pat. Nos. 3,773,882 and 3,884,606, the disclosures of 
which are incorporated herein by reference. 
One method of providing the necessary 7:1 ratio of layer thicknesses for 
the six layer optical repeat unit is to group a series of repeating 
eighteen feedslot feedports in the device of Schrenk, U.S. Pat. No. 
3,884,606. Thus, seven slots may be used for the A polymer, one slot for 
the B polymer, one slot for A, seven slots for B, one slot for A, and one 
slot for B, with the pattern repeating around the feed ring. Because of 
the large number of feedslots allocated for a repeating unit, additional 
layer manipulation may be required to increase the total number of layers 
as will be explained below. 
Another method for providing the necessary 7:1 ratio of layer thicknesses 
is to use only six feed slots per optical repeat unit, but control the 
volumetric ratios of the polymer melt streams entering those feed slots 
through the use of precisely controlled gear pumps. Gear pump speed may be 
controlled to feed a 7:1 pumping ratio into separate manifolds in the feed 
block. 
Protective boundary layers may be added to the multilayer bodies by an 
apparatus as described in commonly-assigned copending U.S. patent 
application Serial No. 07/955,788 filed Oct. 2, 1992, now U.S. Pat. No. 
5,269,995, issued Dec. 14, 1993, to Ramanathan et al entitled, "Improved 
Control of Protective Boundary Layer", the subject matter of which is 
hereby incorporated by reference. Such coextrusion devices provide a 
method for preparing multilayered, simultaneously extruded thermoplastic 
materials, each of which are of a substantially uniform layer thickness. 
To increase the total number of layers in the multilayer body, preferably 
a series of layer multiplying means as are described in U.S. Pat. Nos. 
5,094,793 and 5,094,788, the disclosures of which are incorporated herein 
by reference may be employed. The layer multiplying means are termed 
interfacial surface generators, or ISG's. 
Layer thickness gradients may be introduced into the two component film by 
controlling the volume of heat plastified polymers passing through the 
feed ports of the co-extrusion device as taught in Schrenk, U.S. Pat. No. 
3,687,589. Alternatively, the layer thickness gradients may be introduced 
upstream or downstream of the interfacial surface generators by the use of 
adjustable valves to control the amount of heat plastified polymer 
introduced at the various feed slots to the ISG's. In yet another 
alternative method for introducing a layer thickness gradient into the two 
component film, a temperature gradient may be imposed on the feedblock to 
the co-extrusion device. 
In operation, 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 film. Optionally, this multilayered 
stream may be subsequently passed through a series of layer multiplying 
means (i.e., ISG's) in order to further increase the number of layers in 
the final film. 
The multilayered stream is then passed into an extrusion die which is so 
constructed and arranged that streamlined 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 film in which each layer is 
generally parallel to the major surface of adjacent layers. 
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 film after extrusion are all factors which 
affect the thickness of the individual layers in the final film. 
The feedblock of the coextrusion device delivers a designed thickness 
gradient of repeat units to the ISG's to achieve substantially uniform 
broadband reflectance for the film. The feedblocks may be designed, as 
taught in the above-mentioned patents, to deliver layer thickness 
distributions which fit a quartic function. Such a quartic function, when 
superimposed on a linear gradient distribution produces a redundancy in 
layers having substantially the same thickness. This redundancy is 
desirable as it compensates for any flaws or inconsistencies in the layers 
by placing groups of layers at different locations within the reflective 
film which reflect in the same wavelength region. 
The two-component infrared reflecting films of the present invention find a 
number of uses. For example, they may find use in areas where infrared 
reflective properties are desired. The films of the present invention may 
be laminated to glass used in buildings and automobiles to reflect 
infrared radiation, thus lowering the heating loads. Further, the films 
may also be laminated to other substantially transparent plastics to 
provide infrared reflective properties. For example, windshields and 
canopies on certain aircraft are fabricated from tough polymeric resins. 
Laminating the optical interference film of the present invention to, or 
incorporating the film into, such protective layers would provide 
protection from infrared radiation while still providing substantial 
transparency to light in the visible region of the spectrum. 
The films themselves, as well as the plastics to which they are laminated 
may be shaped or post-formed into a variety of useful objects. Because the 
films suppress successive higher order reflections in the visible region 
of the spectrum, no iridescence or other undesirable color effects are 
present. 
In order that the invention may be more readily understood, reference is 
made to the following example, which is intended to be illustrative of the 
invention, but is not intended to be limiting in scope. 
EXAMPLE 
To demonstrate the infrared reflecting capabilities of the film of the 
present invention, a computer simulation was run to predict the 
reflectance characteristics of a two-component polymethyl 
methacrylate/polycarbonate multilayer film having 2496 layers. The 
simulation used a software program entitled "Macleod Thin Film Optics" 
available from Kidger Optics, Sussex, England. A refractive index mismatch 
of 0.1 was assumed based on the actual mismatch of the two polymers 
(refractive indices of 1.59 and 1.49, respectively) when measured at 
visible wavelengths. 
As can be seen from FIG. 3, the 2496 layer design produced a reflectance in 
the infrared range which varied from about 60% at 770 nm to 90% at about 
2000 nm. This high reflectance over such a wide range of wavelengths is 
attributable to the repeat unit gradient which was imposed. Again, the 
film was essentially transparent to visible light. 
While certain representative embodiments and details have been shown for 
purposes of illustrating the invention, it will be apparent to those 
skilled in the art that various changes in the methods and apparatus 
disclosed herein may be made without departing from the scope of the 
invention, which is defined in the appended claims.