Multilayer optical film

Abstract

A multilayer optical film includes a plurality of alternating polymeric first and second layers numbering 10 to 120 in total. Each of the first and second layers having an average thickness of less than about 500 nm. The first layers include polyethylene naphthalate, and the second layers include a copolyester comprising at least one of cyclohexane dicarboxylic acid groups, adipic acid groups, or tetramethyl cyclobutanediol groups, such that for substantially normally incident light and for at least one polarization state, the multilayer optical film has an average optical reflectance of at least about 70% in a wavelength range at least about 200 nm wide.

Description

[0001] MULTILAYER OPTICAL FILM

[0002] TECHNICAL FIELD

[0003] The present description relates generally to multilayer optical films.

[0004] BACKGROUND

[0005] A multilayer optical film can include a plurality of alternating first and second layers that provide optical reflectance.

[0006] SUMMARY

[0007] In some aspects, the present description provides a multilayer optical film including a plurality of alternating polymeric first and second layers numbering 10 to 120 in total, where each of the first and second layers have an average thickness of less than about 500 nm. The first layers can comprise polyethylene naphthalate, and the second layers comprise a copolyester comprising at least one of cyclohexane dicarboxylic acid groups, adipic acid groups, or tetramethyl cyclobutanediol groups, such that for substantially normally incident light and for at least one polarization state, the multilayer optical film has an average optical reflectance of at least about 70 percent in a wavelength range at least about 200 nm wide.

[0008] In some aspects, the present description provides a multilayer optical film including a plurality of alternating polymeric first and second optical layers disposed between polymeric first and second outermost layers. Each of the first and second optical layers can have an average thickness of less than about 500 nm, and each of the first and second outermost layers can have an average thickness greater than about 500 nm. The first optical layers are biaxially oriented and can comprise polyethylene naphthalate, and the second optical layers and each of the first and second outermost layers are substantially optically isotropic and comprise a copolyester comprising cyclohexane dicarboxylic acid groups and tetramethyl cyclobutanediol groups.

[0009] In some aspects, the present description provides a multilayer optical film including a plurality of alternating polymeric first and second optical layers disposed between polymeric first and second outermost layers, where each layer of the alternating polymeric first and second optical layers can have an average thickness of less than 500 nm, and each of the first and second outermost layers can have an average thickness greater than about 500 nm, such that for substantially normally incident light and for at least one polarization state, the plurality of alternating polymeric first and second optical layers has an average optical reflectance in a wavelength range at least about 200 nm wide of at least about 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, or 1.5 percent times a total number of the alternating polymeric first and second optical layers.

[0010] In some aspects, the present description provides a display system including a display configured to form an image for viewing by a viewer; and a multilayer optical film disposed on the display between the display and the viewer. The multilayer optical film can have an average total thickness less than about 15 micrometers. For substantially normally incident light and for each of orthogonal first and second polarization states, the multilayer optical film has a photopically weighted visible light transmission Tvis of at least about 85%; and an AMI.5 weighted solar reflectance Rsolar of at least about 20%.

[0011] These and other aspects will be apparent from the following detailed description. In no event, however, should this brief summary be construed to limit the claimable subject matter.

[0012] BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 is a schematic cross-sectional view of a multilayer optical film, according to some embodiments.

[0014] FIG. 2 is a plot of transmittance versus wavelength for substantially normally incident light for various multilayer optical films, according to some embodiments.

[0015] FIG. 3 is a plot of the layer thickness profiles for the optical films of FIG. 2.

[0016] FIG. 4 is a schematic cross-sectional view of a display system, according to some embodiments.

[0017] DETAILED DESCRIPTION

[0018] In the following description, reference is made to the accompanying drawings that form a part hereof and in which various embodiments are shown by way of illustration. The drawings are not necessarily to scale. It is to be understood that other embodiments are contemplated and may be made without departing from the scope or spirit of the present description. The following detailed description, therefore, is not to be taken in a limiting sense.

[0019] Display devices (e.g., mobile devices such as cell phones and tablets) may be exposed to direct sunlight in some use cases. For example, outdoor displays or dash-mounted mobile devices used for navigation may be exposed to direct sunlight. A broadband mirror may be used on the back of the device to reject incident sunlight. However, it is difficult to incorporate a solar rejection film in front of the display (e.g., over the display surface) since it would typically be desired that any such film be non- metallic for low electromagnetic interference and have a high visible light transmission (e.g., a photopically weighted visible light transmission Tvis of at least about 85%), a significant solar reflectance (e.g., an AMI.5 weighted solar reflectance Rsolar of at least 20%) and a low thickness (e.g., less than about 15 micrometers). Furthermore, it would typically be desired that such a film would have a high surface flatness (e.g., so surface roughness does not undesirably scatter light) and be compatible with typical display technologies including organic light emitting diode (OLED), micro or mini light emitting diode (LED), liquid crystal (LCD), and / or foldable displays. Typical polymeric thermal management films are undesirably thick (e.g., about 50 micrometers) for front surface applications. However, making traditional thermal management films thinner by reducing the number of layers, for example, typically results in poor surface flatness (e.g., due to the difficulty of processing conventional polymeric materials at such reduced total thicknesses), undesirably low Rsolar (e.g., due to limited index contrast between higher and lower index optical layers of conventional materials), or both.

[0020] According to some embodiments of the present description, it has been found that a thermal management fdm having the desired thickness, Tvis, and Rsolar, for example, can be made by using a relatively low number of alternating first and second layers (e.g., no more than 120 layers in total) when polyethylene naphthalate (PEN), for example, is used for the higher index (e.g., first) layers and a copolyester described further elsewhere herein is used for the lower index (e.g., second) layers. It has been found that the copolyester allows the first and second layers to be coextruded and (e.g., biaxially) stretched and have resulting high flatness major surfaces (e.g., resulting in a low optical haze) even when a low total number of layers is utilized. For example, the copolyester can have a suitable melt flow viscosity for coprocessing with PEN layers or other higher index polyester layers. Further, it has been found that the copolyester and PEN, for example, have a suitably high index difference between them that a suitably high reflectance (e.g., in the near infrared) can be achieved with a low total number of layers. The copolyester, according to some embodiments, can have a refractive index lower than that of other copolyesters (e.g., 1.5 or less compared to 1.57 for glycol-modified polyethylene terephthalate) and can provide better interlayer bonding to PEN or other birefringent polyesters compared to polymers such as polymethylmethacrylate (PMMA) commonly used with PEN.

[0021] As is known in the art, multilayer optical films including alternating first and second polymeric layers can be used to provide desired reflection and transmission in desired wavelength ranges by suitable selection of layer thicknesses and refractive index differences. Multilayer optical films and methods of making multilayer optical films are described in U.S. Pat. Nos. 5,882,774 (Jonza et al.); 6,783,349 (Neavin et al.); 6,949,212 (Merrill et al.); 6,967,778 (Wheatley et al.); 9,162,406 (Neavin et al.); and 11,493,677 (Haag et al.), for example. The alternating first and second layers may be referred to as optical layers. Optical layers are layers sufficiently thin (e.g., less than about 500 nm) to reflect or transmit light primarily by optical interference. Optical layers, which may be referred to as interference layers, may be described as reflecting or transmitting light primarily by optical interference when the reflectance and transmittance of the layers can be reasonably described by optical interference or reasonably accurately modeled as resulting from optical interference.

[0022] FIG. 1 is a schematic cross-sectional view of a multilayer optical film 200, according to some embodiments. The optical film 200 includes a plurality of alternating first and second layers 10 and 11 that can number at least 4, 6, 8, 10, 15, 20, 30, 40, or 50 in total. The plurality of alternating first and second layers 10 and 11 can number up to 120, 110, 100, 95, 90, 85, 80, 75, 70, 65, or 60 in total. Each layer of the first and second polymeric layers can have an average thickness (e.g., unweighted mean of thickness over an area of the layer) less than about 500, 400, 300, 250, or 200 nm, for example. The average thicknesses can be at least about 20, 30, 40, 50, or 60 nm, for example. For example, in some embodiments, multilayer optical film 200 includes a plurality of alternating polymeric first and second layers 10 and 11 numbering 10 to 120 in total where each of the first and second layers 10 and 11 has an average thickness of less than about 500 nm.

[0023] In some embodiments, the plurality of alternating polymeric first and second (e.g., optical) layers 10 and 11 is disposed between first and second outermost (or skin) layers 124 and 126. In some embodiments, each of the first and second outermost layers 124 and 126 has an average thickness of greater than about 500, 600, 700, 800, or 900 nm. The average thickness of each of the outermost layers can be up to about 3, 2.5, 2, 1.5, or 1 micrometers, for example. In some embodiments, the multilayer optical film 200 has an average total thickness T of less than about 15, 14, 13, 12, or 11 micrometers. In some embodiments, the average total thickness T is at least about 5, 6, 7, 8, or 9 micrometers. In some embodiments, each layer disposed between the first and second outermost layers 124 and 126 is a layer of the plurality of alternating polymeric first and second optical layers 10 and 11.

[0024] In some embodiments, the first (e.g., optical) layers 10 are biaxially oriented and can comprise polyethylene naphthalate (PEN), polyethylene terephthalate (PET), or copolymers thereof (e.g., copolyethylene naphthalate terephthalate copolymer), for example, and the second (e.g., optical) layers 11 and each of the first and second outermost layers 124 and 126 are substantially optically isotropic and can comprise a copolyester (e.g., comprising cyclohexane dicarboxylic acid groups and tetramethyl cyclobutanediol groups) as described further elsewhere herein. Substantially optically isotropic layers can have a largest birefringence of no more than about 0.04, 0.03, 0.02, or 0.01 over a wavelength range of about 450 nm to about 650 nm. Biaxially oriented layers can have an out-of-plane birefringence (average in-plane refractive index minus out-of-plane refractive index) of at least about 0.05, 0.075, 0.1, 0. 125, 0.15, 0.175, 0.2, or 0.225 for at least one wavelength in a range of about 450 nm to about 650 nm. The average out-of-plane birefringence can be up to about 0.3, 0.275, or 0.25, for example. In some embodiments, the difference in the average in-plane refractive indices for the biaxially oriented layers (e.g., biaxially oriented PEN layers) and the substantially isotropic copolyester layers is at least about 0.05, 0.075, 0.1, 0.125, 0.15, 0.175, 0.2, 0.225, or 0.25 for at least one wavelength in a range of about 450 nm to about 650 nm. The difference can be up to about 0.35, 0.3, or 0.275, for example.

[0025] FIG. 2 is a plot of transmittance versus wavelength for substantially normally incident light (e.g., within about 25, 20, 15, or 10 degrees of normal incidence) for various multilayer optical films, according to some embodiments. Optical absorption is typically negligible, so reflectance is typically 100% minus transmittance to a good approximation. The transmittance and reflectance can be the same to a good approximation for each of two orthogonal polarization states 141 and 142 (e.g., the film can be an infrared mirror), or the reflectance can be substantially higher for one polarization state than for the other polarization state (e.g., the film can be an infrared reflective polarizer). Typically, it is desired that thermal management films reflect each of orthogonal first and second polarization states at infrared wavelengths to block solar heating. The transmittances shown in FIG. 2 were calculated using standard optical modeling techniques. Results are shown for a comparative thermal management film (TMF) including alternating layers of polyethylene terephthalate (PET) and co-polymethyhnethacrylate (coPMMA) numbering 224 layers in total (PET 224), and exemplary films including alternating layers of polyethylene naphthalate (PEN) and a copolyester numbering 56 or 72 layers in total (PEN 56 and PEN 72, respectively). The comparative TMF included 5 micrometers thick coPMMA skin layers on each side of the optical layers. The exemplary films included 1 micrometer thick layers of the copolyester on each side of the optical layers. The copolyester was modeled as having similar refractive indices as PMMA and the films were modeled as having been biaxially stretched. FIG. 3 shows the layer thickness profiles (thickness of optical layers versus layer number) for the optical films of FIG. 2.

[0026] In some embodiments, the optical film has an average optical reflectance R of at least about 70, 75, 80, or 85% in a wavelength range (e.g., about 900 nm to about 1100 nm) at least about 200 nm wide. The wavelength range can be at least about 210, 220, 230, 240, or 250 nm wide, for example. The wavelength range can be up about 400, 350, 300, or 250 nm wide, for example. The wavelength range can be disposed between about 800 nm and about 1200 nm, or between about 850 nm and about 1150 nm. In some embodiments, the wavelength range extends at least from about 900 nm to about 1100 nm. For example, the wavelength range can be from about 900 nm to about 1100 nm.

[0027] In some embodiments, for substantially normally incident unpolarized light, or for substantially normally incident light and for at least one polarization state, or for substantially normally incident light and for each of orthogonal first and second polarization states, the multilayer optical film has a photopically weighted visible light transmission Tvis of at least about 85% and an AM 1.5 weighted solar reflectance Rsolar of at least about 20%. In some such embodiments, or in other embodiments, Tvis is at least about 86, 87, 88, 89, 90, 91, or 92 percent. In some such embodiments, or in other embodiments, Rsolar is at least about 21, 22, 23, 24, or 25 percent. R, Tvis, and Rsolar for the optical film are determined for the film in air unless otherwise specified. Photopically weighted quantities can be determined using standard photometric techniques where the International Commission on Illumination (CIE) 1931 tristimulus y function may be used as the photopic luminous efficiency function for converting radiant quantities to luminous (photopically weighted) quantities. Similarly, Air Mass 1.5 (AMI.5) is a standard spectrum described in ASTM GI73-03(2020) and can be used to determine the AMI.5 weighted solar reflectance Rsolar using standard photometric techniques. Results for modeled optical films are provided in the following table. In some embodiments, for substantially normally incident light 12 and for at least one polarization state (e.g., for each of orthogonal first and second polarization states 141 and 142), the multilayer optical film 200, or the plurality of alternating polymeric first and second (e.g., optical) layers 10 and 11, has an average optical reflectance R of at least about 70% in a wavelength range at least about 200 nm wide. In some such embodiments, or in other embodiments, the average optical reflectance R divided by a total number of the alternating polymeric first and second layers 10 and 11 is at least about 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, or 1.5 percent. This ratio was about 0.4% for the comparative TMF PET 224 and was about 1.3% and 1.56% for the respective exemplary films PEN 72 and PEN 56. In some such embodiments, or in other embodiments, for substantially normally incident light and for each of orthogonal first and second polarization states 141 and 142, the multilayer optical film 200 has a photopically weighted visible light transmission Tvis of at least about 85%; and an AM 1.5 weighted solar reflectance Rsolar of at least about 20%.

[0028] In some embodiments, the first layers 10 comprise polyethylene naphthalate (PEN). In some such embodiments, or in other embodiments, the second layers 11 comprise a copolyester comprising at least one of cyclohexane dicarboxylic acid groups, adipic acid groups, or tetramethyl cyclobutanediol groups. In this context, a specified group refers to the group as incorporated into the polymer (e.g., ethylene diol groups in a polyester refers to -CH2-CH2- after the end -OH groups are removed in a condensation reaction). The first layers 10 may include the PEN at no less than 80, 82, 84, 86, 88, 90, 92, 94, 96, 98, 99, or 100 weight percent. The second layers 11 may include the copolyester at no less than 88, 90, 92, 94, 96, 98, 99, or 100 weight percent. The first layers 10 may include the PEN at no less than 80, 82, 84, 86, 88, 90, 92, 94, 96, 98, 99, or 100 mole percent. For example, the first layers may be formed from a coPEN that is about 90 mol % PEN and about 10 mol % PET.

[0029] Copolyesters can be prepared via a condensation reaction of diols and diacids. In some embodiments, the copolyester comprises: 50 to 30 mole percent cyclohexane dicarboxylic acid groups; 0 to 20 mole percent adipic acid groups; 50 to 25, 30, 35, 40, or 45 mole percent tetramethyl cyclobutanediol groups; 0 to 17 mole percent cyclohexane dimethonal groups; and 0 to 8 mole percent ethylene glycol groups. In some embodiments, the copolyester comprises no more than 5 mole percent ethylene glycol groups. In some embodiments, the copolyester comprises at least one mole percent ethylene glycol groups. In some embodiments, the copolyester comprises no more than 5 mole percent cyclohexane dimethonal groups. In some embodiments, the copolyester comprises at least one mole percent cyclohexane dimethonal groups. In various embodiments, the copolyester comprises the cyclohexane dimethonal groups at 0 to 15 mole percent, 0 to 10, 5, or 3 mole percent, or 5 to 15 mole percent, for example. In some embodiments, the copolyester comprises the adipic acid groups at 5 to 15 mole percent. In some embodiments, the copolyester comprises the cyclohexane dicarboxylic acid groups at about 50 mole percent. In some embodiments, the copolyester comprises the cyclohexane dicarboxylic acid groups, adipic acid groups, and tetramethyl cyclobutanediol groups at about 40, 10, and 50 mole percent, respectively. The copolyesters of the present description have been found to have flow properties, for example, such that layers of the copolyester can be coextruded and co-stretched with PEN layers, for example, to provide an optical fdm with a low (e.g., no more than 120) total number of optical layers and with substantially smooth surfaces (e.g., no micro-wrinking) and with a desired reflectivity. In some embodiments, the copolyester has a melt viscosity of about 7000 to 13000 poise, or about 8000 to 12000 poise at about 250 deg. C. at a shear rate of about 10 1 / s. In some embodiments, the melt viscosity is about 10000 poise at these conditions. The copolyester can have a low (e.g., about 90 deg. C or lower) glass transition temperature (Tg) which has been found to result in improved co-stretching with PEN, for example. In some embodiments, the Tg is in a range of about 50 to 90 deg. C or about 60 to 85 deg. C, for example.

[0030] The copolyester can have a substantially lower refractive index than PEN so that a suitable reflectance can result with a low total number of alternating PEN and copolyester layers. In some embodiments, the copolyester has a refractive index at a wavelength of about 633 nm of no more than 1.51, 1.50, or 1.49, for example. The refractive index is typically at least 1.44, 1.45, 1.46, or 1.47, for example. The refractive index can be about 1.49, for example. The copolyester can be substantially isotropic so that the refractive index is substantially the same in each direction, and / or the refractive index can be an average of refractive indices along each of three orthogonal directions or the average of inplane (in the plane of the fdm, which is the xy plane in FIG. 1 referring to the illustrated x-y-z coordinate system) refractive indices, for example.

[0031] The Tg, melt flow viscosity, and refractive index can be adjusted by adjusting the relative amounts of cyclohexane dicarboxylic acid monomer, adipic acid monomer, or tetramethyl cyclobutanediol monomer, for example, or other monomers used in formulating the copolyester. For example, increasing the relative amount of cyclohexane dicarboxylic acid monomer results in increased Tg but also increased refractive index, while increasing the relative amount of adipic acid can decrease the refractive index but also decreases Tg and melt flow stability at elevated temperatures. In addition, the melt flow viscosity generally increases with increasing molecular weight which can be adjusted based on process conditions (e.g., reaction time in forming the copolyester).

[0032] An exemplary copolyester was made that included cyclohexane dicarboxylic acid groups, adipic acid groups, and tetramethyl cyclobutanediol groups at about 40, 10, and 50 mole percent, respectively. This copolyester had a melt viscosity of about 10000 poise at about 250 deg. C. at a shear rate of about 10 1 / s, a refractive index of about 1.49 at about 633 nm, and a glass transition temperature (Tg) of about 60 to 85 deg. C. In comparison, glycol-modified PET (PETg) having a Tg of about 85 deg. C had a similar melt flow viscosity but a significantly larger refractive index of about 1.57 at about 633 nm. As another comparison, PMMA has a Tg of about 100 deg. C, a melt viscosity of about 10000 poise at about 250 deg. C. at a shear rate of about 10 1 / s, and a refractive index of about 1.49 at about 633 nm. As still another comparison, coPMMA has a Tg of about 65 deg. C, a melt viscosity of about 10000 poise at about 250 deg. C. at a shear rate of about 10 1 / s, and a refractive index of about 1.49 at about 633 nm. CoPMMA can be coextruded and co-stretched with PEN, but the resulting interlayer bonding is low (< 40 g / in), while of a fdm made with layers of PEN and the exemplary copolyester had an interlayer bonding of greater than 100 g / in with the copolyester providing improved index contrast with PEN compared to PETg, for example.

[0033] In some embodiments, ultraviolet (UV) protection (e.g., for protecting PEN layers) is included in or on the multilayer optical film 200. For example, UV absorbers can be included in the skin layer(s) or a thin inorganic stack, or other UV reflector, can be added as a UV mirror to protect the PEN layers, for example. Techniques for incorporating UV protection in a multilayer optical film are described in U.S. Pat. No. 9,945,994 (Hebrink et al.), for example.

[0034] FIG. 4 is a schematic cross-sectional view of a display system 500, according to some embodiments. The display system 500 includes a multilayer optical film 200 which can be any of the optical films described elsewhere herein. For example, in some embodiments, the multilayer optical film 200 includes a plurality of alternating polymeric first and second layers 10 and 11 numbering 10 to 120 in total (or in a range described elsewhere herein), where each of the first and second layers 10 and 11 having an average thickness of less than about 500 nm (or in a range described elsewhere herein). In some embodiments, the first layers 10 comprise polyethylene naphthalate, and the second layers 11 include a copolyester comprising at least one of cyclohexane dicarboxylic acid groups, adipic acid groups, or tetramethyl cyclobutanediol groups, or the compositions of the first and / or second layers 10, 11 can be as described further elsewhere herein.

[0035] In some embodiments a display system 500 includes a display 100 configured to form an image 110 for viewing by a viewer 120; and a multilayer optical film 200 disposed on the display 100 between the display 100 and the viewer 120. The multilayer optical film 200 can have an average total thickness T less than about 15, 14, 13, 12, or 11 micrometers or the average total thickness can be in any range described elsewhere herein. In some embodiments, for substantially normally incident light 12 and for each of orthogonal first and second polarization states 141 and 142, the multilayer optical film 200 has a photopically weighted visible light transmission Tvis of at least about 85%; and an AM 1.5 weighted solar reflectance Rsolar of at least about 20%. The multilayer optical film 200 used in display system 500 can have any of the properties described elsewhere herein. For example, the multilayer optical film 200 can have any of the ranges for Tvis and / or Rsolar described elsewhere herein. As another example, in some embodiments, for substantially normally incident light 12 and for each of orthogonal first and second polarization states 141 and 142, the multilayer optical film 200 has an average optical reflectance of at least about 70, 75, 80, or 85% in a wavelength range at least about 200 nm wide.

[0036] Terms such as “about” will be understood in the context in which they are used and described in the present description by one of ordinary skill in the art. If the use of “about” as applied to quantities expressing feature sizes, amounts, and physical properties is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, “about” will be understood to mean within 10 percent of the specified value. A quantity given as about a specified value can be precisely the specified value. For example, if it is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, a quantity having a value of about 1, means that the quantity has a value between 0.9 and 1. 1, and that the value could be 1.

[0037] Terms such as “substantially” will be understood in the context in which they are used and described in the present description by one of ordinary skill in the art. If the use of “substantially” with reference to a property or characteristic is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description and when it would be clear to one of ordinary skill in the art what is meant by an opposite of that property or characteristic, the term “substantially” will be understood to mean that the property or characteristic is exhibited to a greater extent than the opposite of that property or characteristic is exhibited.

[0038] All references, patents, and patent applications referenced in the foregoing are hereby incorporated herein by reference in their entirety in a consistent manner. In the event of inconsistencies or contradictions between portions of the incorporated references and this application, the information in the preceding description shall control.

[0039] Descriptions for elements in figures should be understood to apply equally to corresponding elements in other figures, unless indicated otherwise. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and / or equivalent implementations can be substituted for the specific embodiments shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations, or variations, or combinations of the specific embodiments discussed herein. Therefore, it is intended that this disclosure be limited only by the claims and the equivalents thereof.

Claims

What is claimed is:

1. A multilayer optical film comprising a plurality of alternating polymeric first and second layers numbering 10 to 120 in total, each of the first and second layers having an average thickness of less than about 500 nm, the first layers comprising polyethylene naphthalate, the second layers comprising a copolyester comprising at least one of cyclohexane dicarboxylic acid groups, adipic acid groups, or tetramethyl cyclobutanediol groups, such that for substantially normally incident light and for at least one polarization state, the multilayer optical film has an average optical reflectance of at least about 70 percent in a wavelength range at least about 200 nm wide.

2. The multilayer optical film of claim 1 having an average total thickness less than about 15 micrometers.

3. The multilayer optical film of claim 1, wherein the average optical reflectance divided by a total number of the alternating polymeric first and second layers is at least about 0.6 percent.

4. The multilayer optical film of claim 1, wherein the copolyester comprises:50 to 30 mole percent cyclohexane dicarboxylic acid groups;0 to 20 mole percent adipic acid groups;50 to 25 mole percent tetramethyl cyclobutanediol groups; 0 to 17 mole percent cyclohexane dimethonal groups; and 0 to 8 mole percent ethylene glycol groups.

5. The multilayer optical film of claim 4, wherein the copolyester comprises no more than 5 mole percent ethylene glycol groups.

6. The multilayer optical film of claim 4, wherein the copolyester comprises the adipic acid groups at 5 to 15 mole percent.

7. The multilayer optical film of claim 4, wherein the copolyester comprises the cyclohexane dicarboxylic acid groups at about 50 mole percent.

8. The multilayer optical film of claim 1, wherein the wavelength range extends at least from about 900 nm to about 1100 nm.

9. The multilayer optical film of claim 1, wherein for substantially normally incident unpolarized light, the multilayer optical film has a photopically weighted visible light transmission Tvis of at least about 85% and an AMI .5 weighted solar reflectance Rsolar of at least about 20%.

10. A multilayer optical film comprising a plurality of alternating polymeric first and second optical layers disposed between polymeric first and second outermost layers, each of the first and second optical layers having an average thickness of less than about 500 nm, each of the first and second outermost layers having an average thickness greater than about 500 nm, the first optical layers being biaxially oriented and comprising polyethylene naphthalate, the second optical layers and each of the first and second outermost layers being substantially optically isotropic and comprising a copolyester comprising cyclohexane dicarboxylic acid groups and tetramethyl cyclobutanediol groups.

11. The multilayer optical film of claim 10, wherein for substantially normally incident light and for each of orthogonal first and second polarization states, the plurality of alternating polymeric first and second optical layers has an average optical reflectance in a wavelength range at least about 200 nm wide of at least about 0.6 percent times a total number of the alternating polymeric first and second optical layers.

12. A multilayer optical film comprising a plurality of alternating polymeric first and second optical layers disposed between polymeric first and second outermost layers, each layer of the alternating polymeric first and second optical layers having an average thickness of less than 500 nm, each of the first and second outermost layers having an average thickness greater than about 500 nm, such that for substantially normally incident light and for at least one polarization state, the plurality of alternating polymeric first and second optical layers has an average optical reflectance in a wavelength range at least about 200 nm wide of at least about 0.8 percent times a total number of the alternating polymeric first and second optical layers.

13. The display of claim 12, wherein the first optical layers comprise polyethylene naphthalate, and the second optical layers comprise a copolyester comprising at least one of cyclohexane dicarboxylic acid groups, adipic acid groups, or tetramethyl cyclobutanediol groups.

14. A display system comprising: a display configured to form an image for viewing by a viewer; and a multilayer optical film disposed on the display between the display and the viewer, the multilayer optical film having an average total thickness less than about 15 micrometers, wherein for substantially normally incident light and for each of orthogonal first and second polarization states, the multilayer optical film has a photopically weighted visible light transmission Tvis of at least about 85%; andan AMI .5 weighted solar reflectance Rsolar of at least about 20%.

15. The display of claim 14, wherein the multilayer optical film comprises a plurality of alternating polymeric first and second layers numbering 10 to 120 in total, each of the first and second layers having an average thickness of less than about 500 nm, the first layers comprising polyethylene naphthalate, the second layers comprising a copolyester comprising at least one of cyclohexane dicarboxylic acid groups, adipic acid groups, or tetramethyl cyclobutanediol groups.

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