Multilayer Optical Films

Abstract

The multilayer optical film includes a plurality of optical repeating units. Each of the optical repeating units includes at least four sequentially arranged first to fourth layers, including first and third layers having respective f-ratios f1 and f3. The optical film has a primary reflection band having a reflection peak height Rp1, and any secondary reflection band has a reflection peak height Rp2, where Rp1 / Rp2≧5. Rp1 / Rp2 may remain 5 or greater when at least one of the f-ratios f1 and f3 is continuously varied by 0.2. The thickness of each of the second and fourth layers may be at least two times less than the thickness of each of the first and third layers, or the thickness of the same one of the first and third layers may differ from the thickness of each of the other layers by at least 2.5 times.

Description

[Background technology]

[0001] A multilayer optical film can include a stack of polymer layers arranged into an optical repeating unit. A multilayer optical film can be a reflective polarizer or mirror. Summary of the Invention

[0002] This specification generally relates to a multilayer optical film having a first reflection band located at a wavelength greater than about 600 nm or greater than about 700 nm. The optical film can include a plurality of optical repeat units, each optical repeat unit including at least four individual layers. The optical repeat units can be configured such that the second harmonic of the first reflection band is suppressed.

[0003] In some embodiments herein, a multilayer optical film is provided that includes a plurality of optical repeat units. The number of optical repeat units in the plurality of optical repeat units is greater than about 10 in total. Each of the optical repeat units may include at least four sequentially arranged first to fourth individual layers. For each of the optical repeat units, the first and third individual layers may have respective f-ratios f1 and f3 for the same in-plane x-direction respective refractive indices nx1 and nx3, such that for substantially normally incident light polarized along the x-direction, the multilayer optical film has a primary reflection band located at a wavelength greater than about 600 nm. The primary reflection band has a first reflection peak having a peak height Rp1, and any secondary reflection band of the multilayer optical film has a reflection peak having a peak height Rp2. The ratio Rp1 / Rp2 may be at least 5 and may remain greater than or equal to 5 when at least one of the f-ratios f1 and f3 is continuously changed by 0.2. In some embodiments, for each of the optical repeat units, the average thickness of each of the second and fourth individual layers is at least a factor of 2 less than the average thickness of each of the first and third individual layers, In some embodiments, the average thickness of the same one of the first and third individual layers for each of the optical repeat units differs from the average thickness of each other of the first through fourth individual layers by a factor of at least 2.5.

[0004] In some aspects of the present disclosure, a multilayer optical film is provided that includes a plurality of optical repeat units. The number of optical repeat units in the plurality of optical repeat units is greater than about 10 in total. Each of the optical repeat units includes at least four individual layers. The first and second individual layers of the at least four individual layers of each of the optical repeat units have respective f-ratios f1 and f2 for the same in-plane x-direction respective refractive index. In some embodiments, 0.5≦f1≦0.8 and 0.02≦f2≦0.3, such that for substantially normally incident light polarized along the x-direction, the multilayer optical film has a first reflection band located at a wavelength greater than about 700 nm and has at least no second reflection band. At least a third layer of the at least four individual layers of each of the optical repeat units can have an average thickness of less than about 75 nm.

[0005] These and other aspects will become apparent from the following detailed description, but in no way should this brief summary be construed as limiting the claimed subject matter. [Brief description of the drawings]

[0006] [Figure 1A] 1 is a schematic cross-sectional view of an optical film according to some embodiments. [Figure 1B] 1 is a schematic cross-sectional view of a portion of an optical film according to some embodiments. [Figure 1C] 1 is a schematic cross-sectional view of an optical repeating unit of an optical film according to some embodiments. [Figure 1D] 1 is a schematic cross-sectional view of an optical repeating unit of an optical film according to some embodiments. [Figure 1E] 1 is a schematic cross-sectional view of an optical repeating unit of an optical film according to some embodiments. [Figure 1F] 1 is a schematic cross-sectional view of an optical repeating unit of an optical film according to some embodiments. [Diagram 2]1 is a schematic perspective view of exemplary layers of an optical film. [Diagram 3] 1 is a schematic plot of light reflectance versus wavelength of an optical film, according to some embodiments. [Figure 4] 1 is a schematic plot of light transmittance versus wavelength of an optical film, according to some embodiments. [Diagram 5] 1 is a schematic plot of the ratio of the first and second reflective peaks versus changes in f-ratio in accordance with some embodiments; [Figure 6] 1 is a layer thickness profile of a first exemplary multilayer optical film. [Figure 7] 1 is a plot of the optical transmission of a first exemplary multilayer optical film. [Figure 8] 4 is a layer thickness profile of a second exemplary multilayer optical film. [Figure 9] 4 is a plot of the optical transmission of a second exemplary multilayer optical film. [Figure 10A] 4 is a contour plot illustrating the relative reflection intensities of respective primary and secondary reflection bands for optical films corresponding to first and second exemplary multilayer optical films. [Figure 10B] 4 is a contour plot illustrating the relative reflection intensities of respective primary and secondary reflection bands for optical films corresponding to first and second exemplary multilayer optical films. [Figure 11] 4 is a layer thickness profile of a third exemplary multilayer optical film. [Figure 12] 11 is a plot of the optical transmission of a third exemplary multilayer optical film. [Figure 13] 4 is a layer thickness profile of a fourth exemplary multilayer optical film. [Figure 14] 11 is a plot of the optical transmission of a fourth exemplary multilayer optical film. [Figure 15A] 11 is a contour plot showing the relative reflection intensities of the respective primary and secondary reflection bands for optical films corresponding to the third and fourth exemplary multilayer optical films. [Figure 15B]11 is a contour plot showing the relative reflection intensities of the respective primary and secondary reflection bands for optical films corresponding to the third and fourth exemplary multilayer optical films. [Figure 16] 11 is a layer thickness profile of a fifth exemplary multilayer optical film. [Figure 17] 13 is a plot of the optical transmission of a fifth exemplary multilayer optical film. [Figure 18] 11 is a layer thickness profile of a sixth exemplary multilayer optical film. [Figure 19] 13 is a plot of the optical transmission of a sixth exemplary multilayer optical film. [Figure 20A] 11 is a contour plot showing the relative reflection intensities of the respective primary and secondary reflection bands for optical films corresponding to the fifth and sixth exemplary multilayer optical films. [Figure 20B] 11 is a contour plot showing the relative reflection intensities of the respective primary and secondary reflection bands for optical films corresponding to the fifth and sixth exemplary multilayer optical films. [Figure 21] FIG. 1 is a schematic diagram of testing a multilayer optical film for interlayer adhesion. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] In the following description, reference is made to the accompanying drawings, which 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 disclosure. Accordingly, the following detailed description is not to be taken in a limiting sense.

[0008] Multilayer optical films comprising alternating polymer layers can be used to provide desired reflection and transmission in desired wavelength ranges by appropriately selecting layer thicknesses and refractive index differences, as generally described, for example, in U.S. Pat. Nos. 5,882,774 (Jonza et al.), 6,179,948 (Merrill et al.), 6,783,349 (Neavin et al.), 6,967,778 (Wheatley et al.), and 9,162,406 (Neavin et al.). The alternating polymer layers typically comprise alternating high and low refractive index layers, which can be described as optical layers that transmit and reflect light primarily by optical interference. Multilayer optical films comprising alternating high and low refractive index layers can be described as comprising a plurality of optical repeat units, each optical repeat unit comprising a high and low refractive index layer. An optical repeat unit is generally the smallest discrete unit of an optical layer that repeats along the thickness direction of an optical film. Each optical repeat unit may include one or more layers in addition to the high and low refractive index layers, for example, as described in U.S. Pat. Nos. 5,103,337 (Schrenk et al.), 5,540,978 (Schrenk), and 6,207,260 (Wheatley et al.).

[0009] In some cases, it may be desirable for an optical film to have reflection bands located at wavelengths greater than about 600 nm or greater than about 700 nm, possibly up to about 1300 nm or up to about 1200 nm, while providing high light transmission for wavelengths in the visible wavelength range, for example, from about 420 nm to at least about 600 nm. Such optical films may be useful, for example, as window films to reduce infrared heating. However, when a main (primary) near-infrared reflection band is provided, second and higher harmonics of the reflection band present in the visible wavelength range may be present. Thus, it may be desirable to suppress or eliminate the second and sometimes higher order reflection bands. The intensity of the second and higher harmonics is a function of the f-ratio(s) of the optical film, as generally described, for example, in U.S. Pat. No. 9,678,252 (Kivel et al.) for a two-layer optical repeat unit. The f-ratio of a layer of an optical repeat unit is the optical thickness of the layer divided by the optical thickness of the optical repeat unit, the optical thickness of each layer of the optical repeat unit is the thickness of the layer multiplied by the refractive index of the layer along the same in-plane direction, and the optical thickness of the optical repeat unit is the sum of the optical thicknesses of each of the layers. Unless otherwise specified, the wavelength used in determining the refractive index is about 633 nm. For an optical film utilizing n optical repeat units, the sum of the f-ratios of the layers of the optical repeat unit is 1, so there are n-1 independent f-ratios that can be specified. An optical film having a near-infrared reflection band and including multiple optical repeat units, each optical repeat unit including four layers, in order, an A layer, a B layer, a C layer, and another B layer, can have suppressed second and third harmonics of the infrared reflection band if the refractive index of the B layer is the geometric mean of the refractive indexes of the A layer and the C layer, and the f-ratios of the A layer and the C layer are each 1 / 3, and the f-ratio of each of the B layers is 1 / 6. However, using a B layer with a refractive index that is the geometric average of the refractive indices of the A and C layers can undesirably limit the materials that can be selected for the various layers.According to some embodiments herein, multilayer optical films are described that provide robust suppression of at least the second harmonic utilizing an f-ratio of the A and C layers substantially different from 1 / 3 and / or utilizing a refractive index of the A and C layers substantially different from the geometric mean.

[0010] According to some aspects herein, in conventional multilayer optical films that attempt to suppress second harmonics, it has been found that the suppression is sensitive to variations in the f-ratio that may occur across the width and / or length of the optical film, for example, due to normal manufacturing variations. This can result in some areas of the optical film having undesirably low suppression of second and / or higher harmonics. According to some embodiments herein, the optical film has a specific f-ratio and provides substantial suppression (e.g., the ratio of the first reflective peak height to any second reflective peak height may be at least 5) of at least the second harmonics (e.g., the second harmonic or each of the second and third harmonics) at the specific f-ratio and when at least one of the f-ratios varies continuously by 0.2 or more. This can substantially reduce or eliminate areas of the optical film having undesirably low suppression of second and / or higher harmonics, for example, due to manufacturing variations. In some embodiments, this is accomplished, for example, by using an f-ratio in the range of 0.5 to 0.8, or another range described elsewhere herein, for one of the A and C layers, and an f-ratio in the range of 0.02 to 0.3, or another range described elsewhere herein, for the other of the A and C layers. Further, according to some embodiments herein, it has been found that the materials of the layers of each optical repeating unit can be selected to provide the desired robust suppression of second and / or higher harmonics, and to provide the desired interlayer adhesion (e.g., at least about 14 grams per inch, measured at a 90 degree peel angle). For example, it has been found that A and C layers that otherwise have poor bonding to each other can be used in the optical film if the B layer is selected to have good bonding to each of the A and C layers to provide an optical film with the desired interlayer adhesion.

[0011] FIG. 1A is a schematic cross-sectional view of a multilayer optical film 300 according to some embodiments. The optical film 300 can be adapted to selectively reflect and transmit light based on wavelength and / or polarization state. The optical film 300 can be, for example, a reflective polarizer or an optical mirror. The optical film 300 includes a plurality of optical repeat units 10, each of which includes at least four individual layers. In the illustrated embodiment, the at least four individual layers include an A layer, a B1 layer, a C layer, and a B2 layer. The optical film 300 can include more optical repeat units 10 than are shown diagrammatically in FIG. 1A. FIG. 1B is a schematic cross-sectional view of a portion of a multilayer optical film 300 according to some embodiments, showing a number of optical repeat units 10. Each of the B1 and B2 layers can be substantially thinner than each of the A and C layers, as shown diagrammatically in FIGS. 1A-1B. In other embodiments, one of the A and C layers may be substantially thinner than each of the other layers in the optical repeating unit 10, as shown generally in Figures 1C-1D, or one of the A and C layers may be substantially thicker than each of the other layers in the optical repeating unit 10, as shown generally in Figures 1E-1F, or one of the A and C layers may be substantially thinner than the other of the A and C layers and substantially thicker than the B1 and B2 layers (e.g., the C layer in Figure 1E may be substantially thinner than the A layer and substantially thicker than the B1 and B2 layers).

[0012] In some embodiments, the multilayer optical film 300 includes a plurality of optical repeat units 10 disposed between the first skin layer 20 and the second skin layer 21, each optical repeat unit including at least one polymer A layer, at least two B layers (e.g., at least B1 and B2 layers), and at least one polymer C layer. Each pair of adjacent A and C layers can have at least one of the two B layers disposed between the adjacent A and C layers. In some embodiments, each of the B layers is a polymer layer. The total number of A and C layers in the plurality of optical repeat units disposed between the first skin layer 20 and the second skin layer 21 can be less than about 400. For example, the total number of A and C layers can be in the range of about 10 or about 20 to about 400 or about 350. The total number of optical repeat units in the plurality of optical repeat units refers to the total number of distinct optical repeat units (no layer of the film is in more than one distinct optical repeat unit). In some embodiments, the multilayer optical film 300 includes a plurality of optical repeat units, where the number of optical repeat units in the plurality of optical repeat units is, for example, less than about 200 in total, or less than about 175, or less than about 150, or less than about 130, or less than about 120, or less than about 115, or less than about 110, or less than about 105, or less than about 100, or less than about 95, or less than about 90. The total number of optical repeat units in the plurality of optical repeat units can be, for example, at least about 10, or at least about 15, or at least about 20, or at least about 25, or at least about 30, or at least about 40, or at least about 50, or at least about 60, or at least about 70, or at least about 75. The plurality of optical repeat units can have a total average thickness of, for example, less than about 22 micrometers, or less than about 20 micrometers, or less than about 19 micrometers, or less than about 18 micrometers, or less than about 17 micrometers, or less than about 16 micrometers. The total average thickness of the optical repeating units may be, for example, greater than about 5 micrometers, or greater than about 10 micrometers.

[0013] In some embodiments, each optical repeat unit 10 is defined by four distinct layers (e.g., A, B1, C, B2). In other words, in some embodiments, each optical repeat unit 10 includes only four distinct layers. In other embodiments, each optical repeat unit 10 includes more than four distinct layers.

[0014] In some embodiments, each of the A, B1, B2, and C layers can have an average thickness of at least about 0.5 nm, or at least about 1 nm, or at least about 2 nm, or at least about 3 nm, or at least about 5 nm, or at least about 10 nm, and about 500 nm or less, or about 400 nm or less, or about 300 nm or less. For example, each of the A, B, and C layers can have an average thickness of about 1 nm to about 500 nm, or about 5 nm to about 400 nm, or about 10 nm to about 300 nm. In some embodiments, at least one layer of the at least four individual layers of each of the optical repeating units (e.g., at least one of the B1 or B2 layers) has an average thickness of less than about 75 nm, or less than about 60 nm, or less than about 50 nm, or less than about 40 nm, or less than about 30 nm, or less than about 20 nm, or less than about 15 nm, or less than about 10 nm, or less than about 7.5 nm. In some such embodiments, or in other embodiments, each layer of the at least one layer has an average thickness of at least about 0.5 nm, or at least about 1 nm, or at least about 2 nm, or at least about 3 nm.

[0015] In some embodiments, the multilayer optical film 300 includes a plurality of optical repeat units 10, where the number of optical repeat units in the plurality of optical repeat units may total more than about 10, or may be within any range described elsewhere herein, and each of the optical repeat units includes at least four sequentially arranged first through fourth individual layers. The first through fourth individual layers may be labeled sequentially, for example, in the positive or negative z direction of FIG. 1B, and the starting (first) layer may be, for example, layer A or layer C. For example, A, B1, C, B2 may be identified as an optical repeating unit comprising first through fourth individual layers arranged in sequence, or A, B2, C, B1 may be identified as an optical repeating unit comprising first through fourth individual layers arranged in sequence, or C, B2, A, B1 may be identified as an optical repeating unit comprising first through fourth individual layers arranged in sequence, or C, B1, A, B2 may be identified as an optical repeating unit comprising first through fourth individual layers arranged in sequence. In some embodiments (see, e.g., FIG. 8), the average thickness of each of the second (e.g., B1) and fourth (e.g., B2) individual layers for each optical repeat unit 10 is at least 2 times, or at least about 2.25 times, or at least about 2.5 times, or at least about 2.75 times, or at least about 3 times, or at least about 3.25 times, or at least about 3.5 times less than the average thickness of each of the first (e.g., one of the A and C layers) and third (e.g., the other of the A and C layers) individual layers. In some such embodiments, or in other embodiments, the average thickness of each of the second (e.g., B1) and fourth (e.g., B2) individual layers for each optical repeat unit 10 is, for example, up to about 20 times, or up to about 15 times, or up to about 10 times, or up to about 8 times, or up to about 6 times, or up to about 5 times less than the average thickness of each of the first (e.g., one of the A and C layers) and third (e.g., the other of the A and C layers) individual layers.In some embodiments (see, e.g., Figures 6, 11, 13, 16, and 18), for each optical repeat unit 10, the average thickness of the same one of the first and third distinct layers (e.g., one of the A and C layers) differs from the average thickness of each other of the first through fourth distinct layers (e.g., the other of the A and C layers, and each of the B1 and B2 layers) by at least 2 times, or at least about 2.25 times, or at least about 2.5 times, or at least about 2.75 times, or at least about 3 times, or at least about 3.25 times, or at least about 3.5 times. In some such embodiments, or in other embodiments, for each of the optical repeating units 10, the average thickness of the same one of the first and third individual layers (e.g., one of the A and C layers) differs from the average thickness of each of the other layers of the first through fourth individual layers (e.g., the other of the A and C layers, and each of the B1 and B2 layers) by, for example, up to about 20 times, or up to about 15 times, or up to about 10 times, or up to about 8 times, or up to about 6 times, or up to about 5 times. The same one of the first and third individual layers may have a greater average thickness than each of the other layers of the first through fourth individual layers, or the same one of the first and third individual layers may have a lesser average thickness than each of the other layers of the first through fourth individual layers, or the same one of the first and third individual layers may have an average thickness less than some of the other layer(s) of the first through fourth individual layers and greater than the remaining layer(s) of the first through fourth individual layers.

[0016] In some embodiments, the first skin layer 20 and the second skin layer 21 each have an average thickness greater than about 500 nm, or greater than about 750 nm, or greater than about 1000 nm, or greater than about 1250 nm, or greater than about 1500 nm. In some such embodiments, or in other embodiments, the first skin layer 20 and the second skin layer 21 each have an average thickness less than about 8 micrometers, or less than about 5 micrometers, or less than about 4 micrometers. The optical film 300 may include additional layers, such as protective boundary layers 120, 121 used to protect packets of optical repeat units, or additional layers 129, 131 disposed between the protective boundary layers 120, 121, or additional layer(s) 132 between the optical repeat units, as known in the art. The additional layers 129, 131, 132, if included, can each have an average thickness less than about 500 nm, or the average thickness can be within any of the ranges described for any of the A, B1, B2, or C layers. The protective boundary layers 120, 121, if included, can each have an average thickness ranging from, for example, about 500 nm or about 750 nm to about 2 micrometers. In some embodiments, the optical film 300 has an average thickness t that is, for example, less than about 28 micrometers, or less than about 26 micrometers, or less than about 24 micrometers, or less than about 22 micrometers, or less than about 20 micrometers, or less than about 19 micrometers, or less than about 18 micrometers, or less than about 17 micrometers. In some such embodiments, or in other embodiments, the average thickness t is, for example, greater than about 5 micrometers, or greater than about 10 micrometers, or greater than about 12 micrometers. The average thickness t can be described as the total average thickness of the optical repeating units plus the average thickness of the skin layers 20, 21 plus the average thickness of any additional layers that may be included (eg, 120, 121, 129, 131, 132).

[0017] In some embodiments, the second and fourth individual layers (e.g., B1 and B2 layers) have the same composition. In other embodiments, the second and fourth individual layers have different compositions. In some embodiments, for each optical repeat unit in the optical film 300, or for each optical repeat unit in at least a majority of the optical repeat units of the optical film 300, the second and fourth individual layers (e.g., B1 and B2 layers) in the optical repeat unit have substantially the same thickness (e.g., the B1 and B2 layers in the optical repeat unit can have thicknesses within 10% of each other or within 5% of each other). The substantially same average thickness of the second and fourth individual layers in a first optical repeat unit may be substantially different from the substantially same average thickness of the second and fourth individual layers in a different second optical repeat unit. In some embodiments, for each optical repeat unit in at least a majority of the optical repeat units of the optical film 300, the second and fourth individual layers in the optical repeat unit have different thicknesses. The first and third individual layers (e.g., A and C layers) can have the same composition or different compositions. In some embodiments, for each optical repeat unit in optical film 300, or for each optical repeat unit in at least a majority of the optical repeat units of optical film 300, the A and C layers have the same composition and have substantially different thicknesses (e.g., differ in thickness by more than 10%). In some embodiments, the A and C layers have different compositions.

[0018] In some embodiments, the optical film 300 is integrally formed. As used herein, a first element "integrally formed" with a second element means that the first and second elements are manufactured together, rather than being manufactured separately and then bonded. Integral forming includes manufacturing the first element and then manufacturing the second element on the first element. An optical film including multiple layers is integrally formed when the layers are manufactured together (e.g., combined as a melt stream and then cast on a chill roll to form a cast film having each layer, and then orienting the cast film) rather than being manufactured separately and then bonded.

[0019] Light 130 and 230 are shown generally in FIG. 1A at substantially normal incidence (e.g., at 30, or 20, or 10, or 5 degrees normal incidence) incident on optical film 300. Light 130 has a polarization state 31 (polarized along the in-plane x-direction) and light 230 has an orthogonal polarization state 32 (polarized along the in-plane y-direction). In some embodiments, multilayer optical film 300 has a reflection band within a predetermined wavelength range (e.g., any of the infrared wavelength ranges described elsewhere herein) for substantially normally incident light 130 polarized along the x-direction. In some embodiments, multilayer optical film 300 is an optical mirror that also has a reflection band within a predetermined wavelength range for substantially normally incident light 230 polarized along the y-direction. In some embodiments, multilayer optical film 300 is a reflective polarizer that has an average light transmission within a predetermined wavelength range of at least about 70% or at least about 80% for substantially normally incident light 230 polarized along the y-direction.

[0020] Suitable materials for the various layers of the multilayer optical film 300 include, for example, polyethylene naphthalate (PEN), coPEN (copolyethylene naphthalate terephthalate copolymer), polyethylene terephthalate (PET), polyhexylethylene naphthalate copolymer (PHEN), glycol modified PET (PETG), glycol modified PEN (PENG), syndiotactic polystyrene (sPS), THV (terpolymer of tetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride), polymethyl methacrylate (PMMA), coPMMA (methyl methacrylate), and the like. Examples of the polymerizable composition include styrene block copolymers (block copolymers containing a styrene block), such as linear triblock copolymers based on styrene and ethylene / butylene, acrylic block copolymers (block copolymers containing an acrylate or methacrylate block), such as linear triblock copolymers based on methyl methacrylate and n-butyl acrylate, anhydride modified ethylene vinyl acetate polymers, ketone ethylene ester terpolymers, polyolefin-based thermoplastic elastomers, or blends thereof. For example, in some embodiments, each A layer comprises PEN, each B layer comprises PETG, and each C layer comprises PMMA or coPMMA. As another example, in some embodiments, each A layer comprises PETG, each B layer comprises PEN, and each C layer comprises PMMA or coPMMA. As yet another example, in some embodiments, each A layer comprises PMMA or coPMMA, each B layer comprises PEN, and each C layer comprises PMMA or coPMMA. As yet another example, in some embodiments, each A layer comprises PEN, each B layer comprises coPEN, and each C layer comprises PMMA or coPMMA.

[0021] Atactic polystyrene (aPS) can optionally be blended with sPS (e.g., at about 5 to about 30 weight percent aPS) to adjust the refractive index of the resulting layer and / or reduce the haze of the layer (e.g., by reducing the crystallinity of the layer). Suitable THV polymers include those described, for example, in U.S. Patent Application Publication No. 2019 / 0369314 (Hebrink et al.) and available under the tradename DYNEON THV from 3M Company (St. Paul, MN). In some embodiments, the THV can contain about 35 to about 75 mole percent tetrafluoroethylene, about 5 to about 20 mole percent hexafluoropropylene, and about 15 to about 55 mole percent vinylidene fluoride. Suitable styrenic block copolymers include KRATON G1645 and KRATON G1657 available from KRATON Polymers (Houston, TX). Suitable acrylic block copolymers include those available under the trade name KURARITY from Kuraray Co., Ltd. (Tokyo, JP). PETG can be described as PET in which some of the glycol units of the polymer have been replaced with different monomer units, typically monomer units derived from cyclohexanedimethanol. PETG can be produced, for example, by replacing a portion of the ethylene glycol used in the transesterification reaction to produce the polyester with cyclohexanedimethanol. Suitable PETG copolyesters include GN071 available from Eastman Chemical Company (Kingsport, TN). PEN and coPEN can be produced, for example, as described in U.S. Pat. No. 10,001,587 (Liu). A low melting PEN is a coPEN containing about 90 mole percent naphthalene dicarboxylate groups based on the total carboxylate groups, also known as coPEN 90 / 10.Another useful coPEN is coPEN 70 / 30, which contains about 70 mole percent naphthalene dicarboxylate groups and about 30 mole percent terephthalate dicarboxylate groups based on the total carboxylate groups. More generally, coPEN Z / 100-Z may be used, where coPEN Z / 100-Z contains Z mole percent naphthalene dicarboxylate groups (typically greater than 50 mole percent and up to about 90 mole percent) and 100-Z mole percent terephthalate dicarboxylate groups based on the total carboxylate groups. Glycol-modified polyethylene naphthalates (PENGs) can be described as PENs in which a portion of the glycol units of the polymer are replaced with different monomer units, and can be prepared, for example, by replacing a portion of the ethylene glycol used in the transesterification reaction to produce the polyester with cyclohexanedimethanol. PHENs can be prepared, for example, as described for PENs in U.S. Pat. No. 10,001,587 (Liu), except that a portion (e.g., about 40 mole percent) of the ethylene glycol used in the transesterification reaction is replaced with hexanediol. Suitable PET can be obtained, for example, from Nan Ya Plastics Corporation, America (Lake City, SC). Suitable sPS can be obtained, for example, from Idemitsu Kosan Co., Ltd. (Tokyo, Japan). Suitable PMMA can be obtained, for example, from Arkema Inc. (Philadelphia, PA). Suitable anhydride modified ethylene vinyl acetate polymers include, for example, those available under the trade name BYNEL from Dow Chemical (Midland, MI). Suitable ketone ethylene ester terpolymers include, for example, those available under the trade name BYNEL from Dow Chemical (Midland, MI). Suitable polyolefin-based thermoplastic elastomers include those available under the trade name ADMER from Mitsui Chemicals (Tokyo, Japan).

[0022] In some embodiments, each optical repeat unit 10 includes at least one fluoropolymer layer. Fluoropolymers (e.g., THV) may be used in near infrared reflective films, for example, for their stability to ultraviolet light and / or their low refractive index, as described, for example, in U.S. Patent Application Publication Nos. 2019 / 0369314 (Hebrink et al.) and 2019 / 0111666 (Hebrink et al.). In some embodiments, at least one layer of each optical repeat unit has a refractive index of 1.4 or less at a wavelength of 633 nm in at least one direction. In some embodiments, at least one layer of each optical repeat unit has a refractive index of 1.4 or less at a wavelength of 633 nm in each of three mutually orthogonal directions. In other embodiments, the optical repeat unit 10 does not include a fluoropolymer layer. In some embodiments, each layer of each optical repeat unit has a refractive index of at least 1.45 at a wavelength of 633 nm in at least one direction. In some embodiments, each layer of each optical repeating unit has a refractive index of at least 1.45 at a wavelength of 633 nm in each of three mutually orthogonal directions.

[0023] In some embodiments, each optical repeat unit 10 includes at least one birefringent thermoplastic polymer layer. In some such embodiments, or in other embodiments, each optical repeat unit 10 includes at least one substantially isotropic (e.g., the maximum difference in refractive index in any two mutually orthogonal directions is less than about 0.03, or less than about 0.02, or less than about 0.01) thermoplastic polymer layer. The at least one birefringent thermoplastic polymer layer may be biaxially oriented or uniaxially oriented. In some embodiments, the at least one birefringent thermoplastic polymer layer includes at least one positively birefringent (birefringence increases along the stretch direction) thermoplastic polymer layer. In some such embodiments, or in other embodiments, the at least one birefringent thermoplastic polymer includes at least one negatively birefringent (birefringence decreases along the stretch direction) thermoplastic polymer layer. PEN, PET, and PHEN are examples of positively birefringent thermoplastic polymers, and sPS is an example of a negatively birefringent thermoplastic polymer. Whether a polymer exhibits positive or negative birefringence may depend on the geometry of the crystallites formed when the polymer is oriented, as described, for example, in U.S. Pat. No. 9,069,136 (Weber et al.). Suitable positively birefringent thermoplastic polymers include those that form crystallites with an axis of symmetry substantially aligned with the stretch direction, while suitable negatively birefringent thermoplastic polymers include those that form crystallites with a disk-shaped unit cell structure with the minimum unit cell dimension substantially aligned with the stretch direction. Styrene block copolymers, PMMA, coPMMA, THV, acrylic block copolymers, coPEN, and PETG are examples of thermoplastic polymers that may be substantially isotropic after orientation. Substantially isotropic polymers typically either do not substantially form crystallites when oriented, or form crystallites that melt away when a film containing the polymer is heat set. Further examples of positively and negatively birefringent thermoplastic polymers and isotropic thermoplastic polymers are described, for example, in U.S. Pat. No. 9,069,136 (Weber et al.).Other suitable materials for the various layers in the multilayer optical film 300 include those described, for example, in U.S. Pat. Nos. 5,103,337 (Schrenk et al.), 5,540,978 (Schrenk), 5,882,774 (Jonza et al.), 6,179,948 (Merrill et al.), 6,207,260 (Wheatley et al.), 6,783,349 (Neavin et al.), 6,967,778 (Wheatley et al.), 9,069,136 (Weber et al.), and 9,162,406 (Neavin et al.).

[0024] The various layers of the optical film 300 can be characterized by their refractive indices along a first in-plane direction (e.g., x-direction), an orthogonal second in-plane direction (e.g., y-direction), and / or a thickness direction (z-direction) orthogonal to the in-plane direction. In embodiments where the refractive indices of multiple layers are specified, the refractive indices along the x-, y-, and z-directions may be represented as nxi, nyi, and nzi, respectively, where "i" is the layer number (e.g., 1, 2, etc.), which may depend on how the layers are numbered (e.g., sequentially or otherwise). FIG. 2 is a schematic perspective view of the layers of a multilayer optical film. The refractive indices nxi, nyi, and nzi along the x-, y-, and z-directions are shown for layer "i," which may correspond, for example, to any of layer A, layer B1, layer C, and layer B2 shown in FIGS. 1A-1F. For example, in some embodiments, the multilayer optical film 300 includes a plurality of optical repeat units 10, each of which includes at least four individual layers (e.g., A, B1, C, B2), and at least a first (e.g., one of A and C) and a second (e.g., the other of A and C) individual layer of the at least four individual layers of each of the optical repeat units have the same respective refractive indices nx1 and nx2 along an in-plane x-direction, respective refractive indices ny1 and ny2 along an in-plane y-direction perpendicular to the x-direction, and respective refractive indices nz1 and nz2 along a z-direction perpendicular to the x-direction and y-direction. The optical thicknesses and f-ratios of the various layers can be determined from the refractive indices, as further described elsewhere herein. The f-ratios of the various layers in the optical repeat unit can be represented as fi, where the subscript "i" is the layer number. The f-ratio of a particular layer can be, for example, f for each A and C layer. A or f CIn some embodiments, each of the optical repeating units 10 includes at least four individual layers (e.g., A, B1, C, B2), and a first (e.g., one of A and C) and a second (e.g., the other of A and C) individual layer of the at least four individual layers of each of the optical repeating units have respective f-ratios f1 and f2 to respective refractive indices in the same in-plane x-direction. In some embodiments, each of the optical repeating units includes at least four sequentially arranged first through fourth individual layers. In some embodiments, for each of the optical repeating units 10, a first (e.g., one of A and C) and a third (e.g., the other of A and C) individual layer have respective f-ratios f1 and f3 to respective refractive indices nx1 and nx3 in the same in-plane x-direction.

[0025] The following table lists the refractive index at a wavelength of about 633 nm for various exemplary materials that can be used for the layers of the optical film 300 according to some embodiments. Birefringent materials are typically uniaxially oriented for reflective polarizers and biaxially oriented for optical mirrors. Materials such as styrene block copolymers, THV, PMMA, coPMMA, acrylic block copolymers, coPEN, and PETG are typically isotropic, regardless of whether other layers in the multilayer optical film are biaxially or uniaxially oriented. For example, multilayer optical films including layers of such materials can be heat set at temperatures above the glass transition temperatures of the materials used in these layers to obtain isotropic layers. Layers of some materials (e.g., PHEN) can be uniaxially oriented, biaxially oriented, or isotropic, depending on the stretching conditions and heat setting temperatures. For example, PHEN layers can be biaxially or uniaxially oriented, or PHEN layers can be heat set at temperatures above the glass transition temperature of PHEN to obtain isotropic layers even after the layers are uniaxially or biaxially stretched.

[0026] [Table 1]

[0027] In some embodiments, each individual layer of each optical repeating unit has a refractive index in the range of about 1.3 to about 1.9 in each of the x, y, and z directions. In some embodiments, each individual layer of each optical repeating unit has a maximum refractive index difference in any two of the x, y, and z directions that is less than about 0.4, or less than about 0.36. In some embodiments, the maximum refractive index difference of the individual layers in the optical repeating unit along each of the x, y, and z directions is less than about 0.55, or less than about 0.5, or less than about 0.45. In some embodiments, the maximum refractive index difference of the individual layers in the optical repeating unit along the same direction (e.g., one of the x, y, and z directions) is greater than about 0.05, or greater than about 0.07, or greater than about 0.1, or greater than about 0.15, or greater than about 0.2, or greater than about 0.22, or greater than about 0.25.

[0028] FIG. 3 is a schematic plot of the reflectivity of a multilayer optical film 300 for substantially normally incident light 130 in at least one polarization state 31, according to some embodiments. The reflectivity has a primary reflection band 278 in the wavelength range of λa to λb, where, for example, λa may range from about 600 nm to about 1000 nm and λb may range from about 1100 nm to about 1300 nm. In some embodiments, the multilayer optical film 300 is an optical mirror that has substantially the same reflectivity for substantially normally incident light 130, 230 in mutually orthogonal polarization states 31 and 32. In some embodiments, the multilayer optical film 300 is a reflective polarizer that has a reflectivity of less than about 20% for substantially normally incident light 230 in a second polarization state 32, for example, across the entire visible wavelength range of 450 nm to 680 nm and / or across the infrared wavelength range of 800 nm to 1200 nm.

[0029] 4 is a schematic plot of the transmittance of a multilayer optical film 300 for substantially normally incident light 130, 230 according to some embodiments. The transmittance 342 is for substantially normally incident light 130 polarized along the x-direction. In the case of an optical mirror, the transmittance 342 may also be for substantially normally incident light 230 polarized along the y-direction. In the case of a reflective polarizer, the transmittance 344 may be for substantially normally incident light 230 polarized along the y-direction. The average optical transmittance T1 in the wavelength range from λ1 to λ2 is shown for the transmittance 342, and the average optical transmittances T2 and T3 in the wavelength range from λ3 to λ4 are shown for the respective transmittances 344 and 342. In some embodiments, for substantially normally incident light 130 polarized along the x-direction, the multilayer optical film 300 has an average light transmittance (e.g., T1) of at least about 70% in the visible wavelength range of about 450 nm to about 680 nm, and an average light reflectance (e.g., Ravg; see, e.g., FIG. 3) of at least about 40% in the infrared wavelength range of about 1000 nm to about 1150 nm. For example, in FIG. 4, λ1-λ4 may be about 450 nm, about 680 nm, about 1000 nm, and about 1150 nm, respectively, and λa and λb in FIG. 3 may be about 1000 nm and about 1150 nm, respectively. The average light reflectance Ravg may be about 100% minus the average light transmittance (e.g., T3) in the same wavelength range (ignoring light absorption, which is typically less than 2% or less than 1%). In some embodiments, the average light transmittance T1 is at least about 75%, or at least about 80%, or at least about 85%. In some embodiments, the average light reflectance Ravg is at least about 50%, or at least about 60%, or at least about 70%, or at least about 80%. In some embodiments, for substantially normally incident light 230 polarized along an in-plane y-direction orthogonal to the x-direction, the multilayer optical film 300 has an average light transmittance (e.g., T1) in the visible wavelength range of at least about 70%, and an average light reflectance (e.g., Ravg) in the infrared wavelength range of at least about 40%.The average optical transmittance and reflectance for substantially normally incident light 230 polarized along an in-plane y-direction may be in any of the ranges described for the average optical transmittance and reflectance, respectively, for substantially normally incident light 130 polarized along an in-plane x-direction. In some embodiments, for substantially normally incident light 230 polarized along an in-plane y-direction, orthogonal to the x-direction, the multilayer optical film 300 has an average optical transmittance in the visible wavelength range (e.g., T1) of at least about 70%, and an average optical transmittance in the infrared wavelength range (e.g., T2) of at least about 60%, or at least about 70%, or at least about 80%.

[0030] In some embodiments, for substantially normally incident light 130 polarized along the x-direction, the multilayer optical film 300 has a primary reflection band 278 disposed at a wavelength greater than about 600 nm or greater than about 700 nm. For example, the wavelength λa shown in FIG. 3 can be greater than about 600 nm or greater than about 700 nm. The primary reflection band 278 can be disposed at a wavelength less than about 2000 nm, or less than about 1500 nm, or less than about 1400 nm, or less than about 1300 nm, for example. For example, the wavelength λb shown in FIG. 3 can be less than about 2000 nm, or less than about 1500 nm, or less than about 1400 nm, or less than about 1300 nm. In some embodiments, the primary reflection band 278 can be disposed within a wavelength range of, for example, 600 nm or 700 nm to 2000 nm or 1400 nm. In some embodiments, the primary reflection band 278 has a first reflection peak with a peak height Rp1, and any secondary reflection band 282 of the multilayer optical film 300 has a reflection peak with a peak height Rp2. The peak height of a reflection peak refers to the maximum reflectance in the reflection band relative to a baseline reflectance, which can be considered to be the average of the reflectance in the regions on either side of the reflection band (see, e.g., FIG. 7). The primary reflection band 278 has a first reflection peak with a peak height Rp1, and may have additional smaller secondary reflection peak(s) (e.g., due to oscillations in reflectance; see, e.g., FIG. 7). The baseline reflectance may be primarily due to Fresnel reflectance from the outermost major surface(s) of the optical film. In some embodiments, Rp1 / Rp2 > 5, or Rp1 / Rp2 > 8, or Rp1 / Rp2 > 10, or Rp1 / Rp2 > 15, or Rp1 / Rp2 > 20, or Rp1 / Rp2 > 30, or Rp1 / Rp2 > 40, or Rp1 / Rp2 > 50. In some embodiments, Rp1 is at least about 50%, or at least about 60%, or at least about 70%, or at least about 80%, or at least about 85%. In some embodiments, Rp2 is less than about 10%, or less than about 5%, or less than about 3%, or less than about 2%.

[0031] The optical film 300 can be characterized by how Rp1 / Rp2 changes when the f-ratios of the optical film are changed from their actual values. In some embodiments, the ratio Rp1 / Rp2 remains 5 or greater, or 8 or greater, or 10 or greater, or 15 or greater, or 20 or greater, when at least one of the f-ratios f1 and f3 is continuously changed by 0.2 (e.g., f1 is continuously changed from its value in the optical film to its value plus 0.2 or its value minus 0.2). In some embodiments, the ratio Rp1 / Rp2 remains 5 or greater when at least one of the f-ratios f1 and f3 is continuously changed by 0.3, or 0.4, or 0.5, or 0.6. In some embodiments, the ratio Rp1 / Rp2 remains 8 or more, or 10 or more, or 15 or more, or 20 or more when at least one of the f ratios f1 and f3 is continuously changed by 0.3, or 0.4, or 0.5, or 0.6. In some embodiments, the ratio Rp1 / Rp2 remains 5 or more, or 8 or more, or 10 or more, or 15 or more, or 20 or more when at least one of the f ratios f1 and f3 is continuously changed by at least 0.2, or at least 0.3, or at least 0.4, or at least 0.5, or at least 0.6. The ratio Rp1 / Rp2 may, for example, remain up to about 500, or up to about 200, or up to about 100, or up to about 50. In some such embodiments, at least one of the f-ratios f1 and f3 can be continuously changed, for example, by up to 0.9, or up to 0.8, or up to 0.7, while Rp1 / Rp2 remains in any of these ranges. Unless otherwise indicated, changing at least one of the f-ratios f1 and f3 means changing at least one of the f-ratios f1 and f3 while keeping the optical thickness of each optical repeat unit constant, where the optical thickness of each optical repeat unit is the sum of the optical thicknesses of each of the individual layers of the optical repeat unit, and the optical thickness of each of the individual layers is the thickness of the individual layer times the refractive index of the individual layer along the same in-plane x-direction. The f-ratios of the second and fourth layers (B1 and B2 layers) can be maintained at the same ratio to each other when changing at least one of the f-ratios f1 and f3.For example, in some embodiments, each of the optical repeating units includes at least four sequentially arranged first through fourth discrete layers having respective f-ratios f1 through f4 for the same in-plane x-direction respective refractive indices nx1 through nx4, where f2 / f4 is held constant when at least one of the f-ratios f1 and f3 is continuously varied. In some embodiments, f2 / f4 is about 1.

[0032] The change in Rp1 / Rp2 when varying at least one of the f-ratios f1 and f3 can be determined, for example, by measuring the thickness of the various layers of the optical film using atomic force microscopy (AFM) and determining, for example, the materials used for the various layers by standard chemical analysis techniques. The refractive index of the layer can be determined from the material of the layer, and the orientation of the film (e.g., uniaxial or biaxial) can be determined from the transmission spectrum of the film. A series of new films can then be physically created or optically modeled using standard optical modeling techniques, each new film differing from the original film by changing at least one of f1 and f3. This allows a plot (e.g., one-dimensional plot or contour plot) of the change in Rp1 / Rp2 versus at least one of the f-ratios to be generated, from which, for example, the maximum shift in the f-ratio for a given minimum desired ratio of Rp1 / Rp2 can be determined. The plot can be generated using standard optical modeling techniques from measurements of the layer thickness profile, the material properties of the layers, and the transmission spectrum of the optical film, so that the maximum continuous change in f-ratio for a given minimum desired ratio of Rp1 / Rp2 of the optical film can be calculated based on these measurements. Suitable optical modeling techniques can include, for example, using the 4x4 stack code of Berreman and Scheffer, Phys. Rev. Lett. 25, 577 (1970) to determine the transmission and reflection spectra. A description of this method is given in the book "Ellipsometry and Polarized Light" written by Azzam and Bashara and published by Elsevier Science (Holland).

[0033] FIG. 5 is a schematic plot of the change in Rp1 / Rp2 versus the f-ratio according to some embodiments. A change of 0 in the f-ratio is shown, corresponding to the f-ratio of the optical film 300. A minimum desired ratio (Rmin) of Rp1 / Rp2 is shown. Rmin may be, for example, at least 5, at least 8, or at least 10, or at least 15, or at least 20. A first change Δ1 in the f-ratio is shown. Δ1 may be, for example, a typical maximum shift in the f-ratio expected due to manufacturing variations. The ratio Rp1 / Rp2 is at least Rmin for a continuous change in the f-ratio up to and including a second change Δ2 in the f-ratio that is greater than the first change Δ1 in the f-ratio. In some embodiments, Δ2 is, for example, at least 0.2, or at least 0.3, or at least 0.4, or at least 0.5, or at least 0.6. In some embodiments, continuously changing at least one of the f-ratios f1 and f3 by at least Δ2 includes monotonically increasing or decreasing at least one of the f-ratios by at least Δ2. In some cases, the ratio Rp1 / Rp2 may increase above Rmin for changes in the f-ratio that are greater than a third change in the f-ratio Δ3>Δ2. However, any successive change in the f-ratio that reaches Δ3 passes through a range of f-ratios from Δ2 to Δ3, so that the ratio Rp1 / Rp2 does not remain above Rmin when the f-ratio is continuously changed by at least Δ3. A contour plot of Rp1 / Rp2 versus the change in the at least two f-ratios, or separate contour plots of the primary and secondary reflection band intensities versus the change in the at least two f-ratios, can make it possible to identify, for example, a desired range of the at least two f-ratios that results in robust suppression of the secondary reflection band.

[0034] The reflection band can be understood to include a reflectance with respect to at least 5% of the baseline. Therefore, when Rp2 is less than 5%, it can be explained that the multilayer optical film does not have a secondary reflection band. The optical multilayer optical film can be explained as having at least no secondary reflection band when the optical multilayer optical film has no secondary reflection band and optionally no reflection bands such as tertiary and quaternary. In some embodiments, for substantially perpendicularly incident light 130 polarized along the x direction, the multilayer optical film 300 has a primary reflection band 278 disposed at a wavelength greater than about 700 nm and has at least no secondary reflection band.

[0035] In some embodiments, each of the optical repeating units 10 includes at least four sequentially arranged first to fourth individual layers, and the first and third individual layers (e.g., A layer and C layer) have respective f ratios f1 and f3 with respect to their respective refractive indices nx1 and nx3 in the in-plane x direction. In some embodiments, the second and fourth individual layers (e.g., B1 layer and B2 layer) have respective refractive indices nx2 and nx4 along the in-plane x direction, and nx1 < nx2 < nx3 and nx1 < nx4 < nx3. In some embodiments, each of nx2 and nx4 is between one of nx1 and nx3 and the geometric mean of nx1 and nx3. For example, in some embodiments, nx1 < nx2 < (nx3 × nx1) 1 / 2 , and nx1 < nx4 < (nx3 × nx1) 1 / 2 is true. In some embodiments, nx1 + 0.02 < nx2 < (nx3 × nx1) 1 / 2 - 0.03, and nx1 + 0.02 < nx4 < (nx3 × nx1) 1 / 2 - 0.03. In some embodiments, nx1 + 0.03 < nx2 < (nx3 × nx1) 1 / 2 - 0.05, and nx1 + 0.03 < nx4 < (nx3 × nx1) 1 / 2 - 0.05. In some embodiments, |nx2 - (nx3 × nx1) 1 / 2 | and |nx4 - (nx3 × nx1) 1 / 2| is greater than 0.03, or greater than 0.04, or greater than 0.05, respectively. Suitable f-ratios for optical films having refractive indices in any of these ranges include those in which f1 is at least 0.44 and those in which f1 is 0.1 or less. In some embodiments, 0.44≦f1≦0.55, and 0.02≦f3≦0.55, or 0.45≦f1≦0.55, and 0.02≦f3≦0.45, or 0.46≦f1≦0.54, and 0.02≦f3≦0.4. In some embodiments, 0.02≦f1≦0.1, and 0.02≦f3≦0.65, or 0.02≦f1≦0.1, and 0.1≦f3≦0.5, or 0.02≦f1≦0.1, and 0.1≦f3≦0.45. Suitable materials for layers having refractive indices in any of these ranges include those described elsewhere herein. In some embodiments, the first individual layer comprises polyethylene naphthalate (PEN), the third individual layer comprises polymethyl methacrylate (PMMA) or a copolymer of methyl methacrylate and ethyl acrylate (coPMMA), and the second and fourth individual layers each comprise glycol-modified polyethylene terephthalate (PETG) or a copolyethylene naphthalate terephthalate copolymer (coPEN). Any layer described as comprising a polymeric material may be a layer of that polymeric material. For example, a layer comprising PEN may be a PEN layer.

[0036] The thickness of the optical repeat unit determines the wavelengths reflected by the optical repeat unit, and the refractive index difference between the layers in the optical repeat unit determines the intensity of the reflection. Thus, the optical reflectances and transmittances of Figures 3-4 can be achieved, for example, by appropriate selection of layer materials to define the refractive index difference, and appropriate selection of the layer thickness profile. Figure 6 is an exemplary plot of layer thickness versus optical repeat unit number for various layers in an optical repeat unit. Given the refractive indices and layer thicknesses of the layers of a multilayer optical film, the optical transmission spectrum of the optical film can be calculated using conventional optical modeling techniques. Figure 7 is a plot of the calculated optical transmittance of multiple optical repeat units as a function of wavelength determined from the layer thickness profile of Figure 6, where the A layer is a biaxially oriented PEN layer having refractive indices in the x, y, and z directions of about 1.766, 1.74, and 1.49, respectively, the B1 and B2 layers are PETG layers having a refractive index of about 1.56 in each direction, and the C layer is a coPMMA layer having a refractive index of about 1.49 in each direction. Similar light transmittances can be obtained, for example, when the C layer is a PMMA layer, a styrene block copolymer layer, or an acrylic block copolymer layer. The calculated transmittances in FIG. 7 and in FIGS. 9, 12, 14, 17, and 19 are for substantially normally incident light 130 polarized along the x-direction. In these cases, the optical film is biaxially oriented, so the reflectances for substantially normally incident light 230 polarized along the y-direction are approximately the same as those shown in these figures. The refractive indices along the x-direction of the A, B, and C layers can be represented as nA, nB, and nC, respectively. The reflection peak heights Rp1 and Rp2 are shown diagrammatically in FIG. 7, assuming that the reflectance is 100% minus the transmittance since the light absorption is negligible. In this case, the A layer has an f-ratio (e.g., one of f1 and f3) of 0.08, the B1 and B2 layers each have an f-ratio of 0.355, and the C layer has an f-ratio (e.g., the other of f1 and f3) of 0.25.

[0037] Figure 8 is another exemplary plot of layer thickness versus optical repeat unit number for various layers in an optical repeat unit. Figure 9 is a plot of calculated optical transmittance of multiple optical repeat units as a function of wavelength determined from the layer thickness profile of Figure 8, where the layer materials are as described for Figures 6-7. In this case, the A layer has an f-ratio (e.g., one of f3 and f1) of 0.45, the B1 and B2 layers each have an f-ratio of 0.1, and the C layer has an f-ratio (e.g., the other of f3 and f1) of 0.35.

[0038] 10A-10B are contour plots showing the relative reflection intensities of the first and second order reflection bands for the layer materials described in FIGS. 6-9. The optical thickness of the optical repeat unit is kept constant and the f-ratios of the two B layers are kept equal to each other, while the f-ratios of the A and C layers are varied. FIG. 10B shows two regions of high second harmonic suppression. One of these regions is 0 <f A ≦0.1 and 0 <f C ≦0.65, and the other of these regions may be characterized by 0.44≦f A ≦0.55 and 0 <f C ≦0.55. Typically, it is preferred that the minimum f ratio of any layer in the optical repeat unit is at least 0.005, or at least 0.01, or at least 0.02, or at least 0.03. In some embodiments, 0.02≦f A ≦0.1 and 0.02≦f C ≦0.65 or 0.44≦f A ≦0.55 and 0.02≦f C ≦0.55.

[0039] In some embodiments, each of the optical repeating units 10 includes at least four individual layers, and a first (e.g., one of the A and C layers) and a second (e.g., the other of the A and C layers) individual layer of each of the at least four individual layers of the optical repeating unit 10 have respective f-ratios f1 and f2 for the same in-plane x-direction respective refractive index. In some embodiments, 0.5≦f1≦0.8 and 0.02≦f2≦0.3, such that for substantially normally incident light 130 polarized along the x-direction, the multilayer optical film 300 has a primary reflection band 278 located at wavelengths greater than about 700 nm and has at least no secondary reflection bands. In some embodiments, 0.02≦f2≦0.25. In some embodiments, 0.55≦f1≦0.8, or 0.55≦f1≦0.75, or 0.55≦f1≦0.7. In some embodiments, at least a third layer (e.g., B1 and / or B2) of the at least four individual layers of each of the optical repeat units has an average thickness less than about 75 nm, or the average thickness may be within any range described elsewhere herein. In some embodiments, the at least a third layer has a refractive index along the x-direction that is greater than each of the refractive indices along the x-direction of the first and second individual layers. As further described elsewhere herein, the first and second individual layers can have the same composition or can have different compositions. In some embodiments, for example, each of the first and second individual layers comprises a copolymer of methyl methacrylate and ethyl acrylate (coPMMA) and at least a third layer comprises polyethylene naphthalate (PEN).

[0040] In some embodiments, each of the optical repeating units 10 includes at least four sequentially arranged first through fourth discrete layers, the first and third discrete layers (e.g., A and C layers) having respective f-ratios f1 and f3 relative to respective refractive indices nx1 and nx3 along the same in-plane x-direction. In some embodiments, the second and fourth discrete layers have respective refractive indices nx2 and nx4 along the same in-plane x-direction, each of nx2 and nx4 being greater than each of nx1 and nx3. In some embodiments, each of nx2 and nx4 is at least 0.05, or at least 0.1, or at least 0.15 greater than each of nx1 and nx3. In some such embodiments, or in other embodiments, 0.45≦f1≦0.8 and 0.02≦f3≦0.4, or 0.5≦f1≦0.8 and 0.02≦f3≦0.4, or 0.5≦f1≦0.75 and 0.02≦f3≦0.3, or 0.5≦f1≦0.75 and 0.02≦f3≦0.25, or 0.55≦f1≦0.7 and 0.02≦f3≦0.2. The sequentially arranged first to fourth individual layers can be considered to be, in order, A, B1, C, B2 (e.g., numbered from layer A down in FIG. 1B) or C, B1, A, B2 (e.g., numbered from layer C up in FIG. 1B to define an optical repeat unit of C, B1, A, B2), so that the values ​​of f1 and f3 can be interchanged for the same film. For example, an embodiment where 0.45≦f1≦0.8 and 0.02≦f3≦0.4 can alternatively be described by 0.02≦f1≦0.4 and 0.45≦f3≦0.8.

[0041] In some embodiments, the first individual layer (e.g., layer A) comprises glycol-modified polyethylene terephthalate (PETG), copolyethylene naphthalate terephthalate copolymer (coPEN), polymethyl methacrylate (PMMA), copolymer of methyl methacrylate and ethyl acrylate (coPMMA), styrene block copolymer, or acrylic block copolymer, the third individual layer (e.g., layer C) comprises polymethyl methacrylate (PMMA), copolymer of methyl methacrylate and ethyl acrylate (coPMMA), styrene block copolymer, or acrylic block copolymer, and each of the second and fourth individual layers (e.g., layers B1 and B2) comprises polyethylene naphthalate (PEN). In some such embodiments, the first individual layer (e.g., layer A) comprises glycol-modified polyethylene terephthalate (PETG) or copolyethylene naphthalate terephthalate copolymer (coPEN). In some such embodiments, or in other embodiments, 0.02≦f1≦0.4 and 0.45≦f3≦0.65, or 0.02≦f1≦0.3 and 0.5≦f3≦0.65, or 0.45≦f1≦0.8 and 0.02≦f3≦0.25, or 0.5≦f1≦0.8 and 0.1≦f3≦0.25.

[0042] In some embodiments, the first and third individual layers (e.g., A and C layers) each comprise polymethyl methacrylate (PMMA), a copolymer of methyl methacrylate and ethyl acrylate (coPMMA), a styrene block copolymer, or an acrylic block copolymer, and the second and fourth individual layers each comprise polyethylene naphthalate (PEN). In some embodiments utilizing these layer materials, or in other embodiments, 0.02≦f1≦0.4, or 0.02≦f1≦0.3, or 0.02≦f1≦0.25, or 0.02≦f1≦0.2. In some such embodiments, or in other embodiments, 0.45≦f3≦0.8, or 0.5≦f3≦0.8, or 0.5≦f3≦0.75, or 0.55≦f3≦0.7. In some embodiments utilizing these layer materials, or in other embodiments, 0.02≦f3≦0.4, or 0.02≦f3≦0.3, or 0.02≦f3≦0.25, or 0.02≦f3≦0.2. In some such embodiments, or in other embodiments, 0.45≦f1≦0.8, or 0.5≦f1≦0.8, or 0.5≦f1≦0.75, or 0.55≦f1≦0.7. Figure 11 is an exemplary plot of layer thickness versus optical repeat unit number for various layers in an optical repeat unit. 12 is a plot of the calculated optical transmittance of multiple optical repeat units as a function of wavelength determined from the layer thickness profile of FIG. 11, where the A layer is a PETG layer having a refractive index of about 1.56 in each direction, the B1 and B2 layers are biaxially oriented PEN layers having refractive indices of about 1.766, 1.74, and 1.49 in the x, y, and z directions, respectively, and the C layer is a coPMMA layer having a refractive index of about 1.49 in each direction. In this case, the A layer has an f-ratio (e.g., one of f1 and f3) of 0.08, the B1 and B2 layers each have an f-ratio of 0.18, and the C layer has an f-ratio (e.g., the other of f1 and f3) of 0.56. Similar optical transmittance can be obtained, for example, when the C layer is a PMMA layer, a styrene block copolymer layer, or an acrylic block copolymer layer.

[0043] Figure 13 is an exemplary plot of layer thickness versus optical repeat unit number for various layers in an optical repeat unit. Figure 14 is a plot of calculated optical transmittance of multiple optical repeat units as a function of wavelength determined from the layer thickness profile of Figure 13, where the layer materials are as described for Figures 11-12. In this case, the A layer has an f-ratio (e.g., one of f1 and f3) of 0.7, the B1 and B2 layers each have an f-ratio of 0.09, and the C layer has an f-ratio (e.g., the other of f1 and f3) of 0.12.

[0044] 15A-15B are contour plots showing the relative reflection intensities of the first and second order reflection bands for the layer materials described in connection with FIGS. 11-14. The optical thickness of the optical repeat unit is kept constant and the f-ratios of the two B layers are kept equal to each other, while the f-ratios of the A and C layers are varied. FIG. 15B shows two regions of high second harmonic suppression. One of these regions is 0 <f A ≦0.4 and 0.45≦f C ≦0.65, and the other of these regions may be characterized by 0.45≦f A ≦0.8 and 0 <f C It may be characterized by ≦0.25.

[0045] Figure 16 is an exemplary plot of layer thickness versus optical repeat unit number for various layers in an optical repeat unit. Figure 17 is a plot of calculated optical transmittance of multiple optical repeat units as a function of wavelength determined from the layer thickness profile of Figure 16, where the B1 and B2 layers are biaxially oriented PEN layers each having a refractive index of about 1.766, 1.74, and 1.49 in the x, y, and z directions, respectively, and the A and C layers are each coPMMA layers having a refractive index of about 1.49 in each direction. In this case, the A layer has an f-ratio (e.g., one of f1 and f3) of 0.08, the B1 and B2 layers each have an f-ratio of 0.17, and the C layer has an f-ratio (e.g., the other of f1 and f3) of 0.58. Similar optical transmittance can be obtained, for example, when the A and C layers are each PMMA layers, styrene block copolymer layers, or acrylic block copolymer layers.

[0046] FIG. 18 is an exemplary plot of layer thickness versus optical repeat unit number for various layers in an optical repeat unit. In this case, for each of the optical repeat units, the average thickness of the A layer is at least 2.5 times greater than the average thickness of each of the other layers in the first through fourth individual layers, and the average thickness of the C layer differs from the average thickness of each of the other layers in the first through fourth individual layers by at least 2.5 times, because the C layer is at least 2.5 times thicker than each of B1 and B2, and the C layer is at least 2.5 times thinner than the A layer. FIG. 19 is a plot of the calculated optical transmittance of a plurality of optical repeat units as a function of wavelength determined from the layer thickness profile of FIG. 18, where the layer materials are as described for FIGS. 16-17. In this case, the A layer has an f-ratio (e.g., one of f1 and f3) of 0.68, each of the B1 and B2 layers has an f-ratio of 0.075, and the C layer has an f-ratio (e.g., the other of f1 and f3) of 0.17.

[0047] 20A-20B are contour plots showing the relative reflection intensities of the first and second order reflection bands for the layer materials described in FIGS. 16-19. The optical thickness of the optical repeat unit is kept constant and the f-ratios of the two B layers are kept equal to each other, while the f-ratios of the A and C layers are varied. FIG. 20B shows two regions of high second harmonic suppression. One of these regions is 0 <f A ≦0.4 and 0.45≦f C ≦0.8, and the other of these regions may be characterized by 0.45≦f A ≦0.8 and 0 <f C It may be characterized by ≦0.4.

[0048] For example, from Figures 7, 9, and 10A-B, or from Figures 12, 14, and 15A-B, or from Figures 17, 19, and 20A-B, it can be seen that the intensity of the primary band can be adjusted while keeping any secondary bands strongly suppressed, thereby achieving a desired reflectance in the near infrared wavelength range while maintaining a high transmittance in the visible wavelength range.

[0049] In some embodiments, the multilayer optical film 300 includes a plurality of optical repeating units 10, and the total number of optical repeating units in the plurality of optical repeating units is greater than about 10. Each of the optical repeating units in the plurality of optical repeating units includes at least four sequentially arranged first to fourth individual layers. For each of the optical repeating units in the plurality of optical repeating units, the average thickness of each of the second and fourth individual layers is at least twice smaller than the average thickness of each of the first and third individual layers, or the same one of the average thicknesses of the first and third individual layers is at least 2.5 times different from the average thicknesses of the other layers of the first to fourth individual layers. The first and third individual layers have respective f ratios f1 and f3 with respect to respective refractive indices nx1 and nx3 in the same in-plane x direction, where 0.44 ≦ f1 ≦ 0.55 and 0.02 ≦ f3 ≦ 0.55. The second and fourth individual layers have respective refractive indices nx2 and nx4 along the same in-plane x direction, where nx1 < nx2 < nx3 and nx1 < nx4 < nx3. For substantially perpendicularly incident light polarized along the x direction, the multilayer optical film has a primary reflection band disposed at a wavelength greater than about 600 nm. The primary reflection band has a first reflection peak with a peak height Rp1. Any secondary reflection band of the multilayer optical film 300 has a reflection peak with a peak height Rp2, and Rp1 / Rp2 ≧ 5. In some embodiments, |nx2 - (nx3 × nx1) 1 / 2 | and |nx4 - (nx3 × nx1) 1 / 2 | are each greater than 0.03. In some such embodiments, or in other embodiments, nx1 < nx2 < (nx3 × nx1) 1 / 2 and nx1 < nx4 < (nx3 × nx1) 1 / 2 is satisfied.

[0050] In some embodiments, the multilayer optical film 300 includes a plurality of optical repeat units 10, the number of optical repeat units in the plurality of optical repeat units being greater than about 10 in total. Each of the optical repeat units in the plurality of optical repeat units includes at least four sequentially arranged first through fourth individual layers. For each of the optical repeat units in the plurality of optical repeat units, the average thickness of each of the second and fourth individual layers is at least two times less than the average thickness of each of the first and third individual layers, or the average thickness of the same one of the first and third individual layers differs from the average thickness of each other of the first through fourth individual layers by at least a factor of 2.5, the first and third individual layers have respective f-ratios f1 and f3 for the same in-plane x-direction respective refractive indices nx1 and nx3, where 0.45≦f1≦0.8 and 0.02≦f3≦0.4, and the second and The fourth discrete layer has respective refractive indices nx2 and nx4 along the same in-plane x-direction, each of nx2 and nx4 being greater than each of nx1 and nx3, such that for substantially normally incident light polarized along the x-direction, the multilayer optical film 300 has a primary reflection band located at wavelengths greater than about 600 nm, the primary reflection band having a first reflection peak having a peak height Rp1, and any secondary reflection bands of the multilayer optical film having reflection peaks having peak heights Rp2, such that Rp1 / Rp2≧5. In some embodiments, each of nx2 and nx4 is at least 0.05 greater than each of nx1 and nx3.

[0051] In some embodiments, the multilayer optical film 300 includes a plurality of optical repeating units 10, and the number of optical repeating units in the plurality of optical repeating units is more than about 10 in total. Each of the optical repeating units in the plurality of optical repeating units includes at least four sequentially arranged first to fourth individual layers. For each of the optical repeating units in the plurality of optical repeating units, the same one average thickness of the first and third individual layers is at least 2.5 times different from the average thickness of each of the other layers of the first to fourth individual layers, the first and third individual layers have respective f ratios f1 and f3 with respect to respective refractive indices nx1 and nx3 in the same in-plane x direction, 0.02 ≦ f1 ≦ 0.1 and 0.02 ≦ f3 ≦ 0.65, the second and fourth individual layers have respective refractive indices nx2 and nx4 along the same in-plane x direction, nx1 < nx2 < nx3 and nx1 < nx4 < nx3, and for substantially perpendicularly incident light polarized along the x direction, the multilayer optical film has a primary reflection band disposed at a wavelength of more than about 600 nm, the primary reflection band has a first reflection peak having a peak height Rp1, any secondary reflection band of the multilayer optical film has a reflection peak having a peak height Rp2, and Rp1 / Rp2 ≧ 5. In some embodiments, |nx2 - (nx3 × nx1) 1 / 2 | and |nx4 - (nx3 × nx1) 1 / 2 | are each greater than 0.03. In some such embodiments, or in other embodiments, nx1 < nx2 < (nx3 × nx1) 1 / 2 and nx1 < nx4 < (nx3 × nx1) 1 / 2 is true.

[0052] Any of the multilayer optical films described herein can have an interlayer adhesion of the individual layers in the optical repeating units of at least about 14 grams / inch when measured at a 90 degree peel angle. In some embodiments, the interlayer adhesion of the individual layers in the optical repeating units is greater than about 16, 18, 20, 22, 25, 27, 30, 32, 35, 37, 40, 45, 50, or 55 grams / inch when measured at a 90 degree peel angle. In some embodiments, the interlayer adhesion can be, for example, up to about 500 grams / inch, or up to about 300 grams / inch, or up to about 200 grams / inch, or up to about 150 grams / inch, or up to about 120 grams / inch. In some embodiments, the interlayer adhesion is, for example, in the range of about 14 grams / inch to about 500 grams, or about 30 grams / inch to about 300 grams / inch. The peel force can be increased by appropriate selection of materials for the A, B1, C, and B2 layers. For example, the B1 and B2 layers can be selected to have adequate bonding to the A and C layers even if the A and C layers have poor direct bonding to each other. In some embodiments, the B1 and B2 layers can be copolymers of the polymers used for the A and C layers. According to some embodiments, layers formed from polymers with low glass transition temperatures, or block copolymers containing (e.g., soft) polymer blocks with low glass transition temperatures, or blends thereof, have been found to provide improved bonding with other layers described herein. Suitable materials include, for example, styrene block copolymers, acrylic block copolymers, PMMA, coPMMA, or blends of any of the latter three. The glass transition temperature (of layer B or the soft block of layer B) may be, for example, less than 105°C, or less than 100°C, or less than 90°C, or less than 80°C, or less than 70°C, or less than 60°C, or less than 50°C, or less than 40°C, or less than 30°C, or less than 20°C, or less than 10°C, or less than 0°C, or less than -10°C, or less than -20°C, or less than -30°C, or less than -40°C, or less than -50°C.The glass transition temperature of a polymer block of a copolymer can be determined as the glass transition temperature of a homopolymer of the monomeric unit of the polymer block. The block copolymer may also include other (e.g., hard) blocks, for example, for mechanical properties (e.g., for material handling and / or low creep). The (e.g., hard) blocks for mechanical properties can have a glass transition temperature of, for example, above 50°C, or above 60°C, or above 70°C, or above 80°C, or above 90°C, or above 100°C, or above 105°C. In some embodiments, each B layer is a chemically inert or substantially chemically inert polymer layer. That is, in some embodiments, the polymer of the B layer does not form covalent bonds with the material of the A layer or the material of the C layer, or forms only covalent bonds that have a negligible effect on the bond with any of the adjacent layers.

[0053] Other suitable materials for the A, B1, C, and B2 layers are further described elsewhere herein. As a specific example, an optical mirror made with directly adjacent sPS and low melting point PEN (coPEN 90 / 10) layers had an interlayer adhesion of the individual layers in the optical repeat units of about 11 grams / inch when measured at a 90 degree peel angle, while an optical mirror made similarly but including a styrene block copolymer layer between each sPS and low melting point PEN layer had an interlayer adhesion of the individual layers in the optical repeat units of about 33 grams / inch when measured at a 90 degree peel angle.

[0054] FIG. 21 is a schematic diagram of testing a multilayer optical film 300 for interlayer adhesion. The multilayer optical film 300 is bonded to a substrate 360 ​​(e.g., a rigid glass substrate) via double-sided tape 151. A cut 170 is then formed in the optical film 300, for example, using a razor blade. Another piece of tape 152 is then used to peel an end 333 of a portion 331 of the optical film 300 adjacent the cut 170 from the remaining portion 332 of the optical film 300. The peel force can then be measured by pulling the tape 152 in a direction (z-direction) away from the substrate 360 ​​that defines a peel angle φ of 90 degrees. The peel force per unit width (dimension in the x-direction) of the optical film 300 is the interlayer adhesion force. In some embodiments, the peel force is determined by using a pull speed (also called crosshead speed) of 12 inches / minute and averaging the measured force over a time period of at least about 5 seconds. In some embodiments, the peel force per unit width is the average peel resistance strength determined according to ASTM D6862-11 (reapproved 2016) testing standard, except that a crosshead speed of 12 inches / minute is used. In some embodiments, the interlayer adhesion of the individual layers in the plurality of optical repeating units is at least about 14 grams / inch, or the interlayer adhesion may be in any range described elsewhere herein, and the interlayer adhesion is determined as the average peel resistance strength measured according to ASTM D6862-11 (reapproved 2016) testing standard, except that a crosshead speed of 12 inches / minute is used to pull the top portion 331 of the multilayer optical film 300 from the bottom portion 332 of the multilayer optical film 300 at a 90 degree peel angle while the bottom portion 332 is bonded to the substrate 360.

[0055] Terms such as "about" will be understood by those of skill in the art in the context in which they are used and described herein. Unless otherwise clear to one of skill in the art in the context in which the use of "about" as applied to quantities describing feature sizes, quantities, and physical properties is used and described herein, "about" will be understood to mean within 10 percent of a particular value. A quantity given as about a particular value may be exactly that particular value. For example, unless otherwise clear to one of skill in the art in the context in which it is used and described herein, an amount having a value of about 1 means that the amount has a value between 0.9 and 1.1, and may even be 1.

[0056] All references, patents, or patent applications referenced above are hereby incorporated by reference in their entirety. In the event of any inconsistency or contradiction between any of the incorporated references and this application, the information in the foregoing description shall prevail.

[0057] Descriptions of elements in the drawings should be understood to apply equally to corresponding elements in other drawings unless otherwise indicated. Although specific embodiments have been illustrated and described herein, those skilled in the art will appreciate that the specific embodiments illustrated and described may be replaced by various alternative and / or equivalent embodiments without departing from the scope of the present disclosure. The present application is intended to cover any adaptations, or variations, or combinations of the specific embodiments discussed herein. Accordingly, the present disclosure is intended to be limited only by the claims and the equivalents thereof.

Claims

1. 1. A multilayer optical film comprising a plurality of optical repeating units, the number of said optical repeating units in said plurality of optical repeating units being greater than about 10 in total, each of said optical repeating units comprising at least four sequentially arranged first through fourth individual layers, and for each of said optical repeating units: an average thickness of each of the second and fourth discrete layers is at least two times less than an average thickness of each of the first and third discrete layers; the first and third discrete layers have respective f-ratios f1 and f3 for the same in-plane x-direction respective refractive indices nx1 and nx3; and for substantially normally incident light polarized along the x-direction, the multilayer optical film has a primary reflection band located at wavelengths greater than about 600 nm, the primary reflection band having a first reflection peak having a peak height Rp1, any secondary reflection bands of the multilayer optical film having a reflection peak having a peak height Rp2, Rp1 / Rp2≧5, and such that the ratio Rp1 / Rp2 remains greater than or equal to 5 when at least one of the f-ratios f1 and f3 is continuously varied by 0.

2. Multilayer optical films.

2. 2. The multilayer optical film of claim 1, wherein the second and fourth discrete layers have respective refractive indices nx2 and nx4 along the same in-plane x-direction, where nx1<nx2<nx3 and nx1<nx4<nx3.

3. 3. The multilayer optical film of claim 2, wherein 0.44≦f1≦0.55 and 0.02≦f3≦0.

55.

4. 2. The multilayer optical film of claim 1, wherein the second and fourth discrete layers have respective refractive indices nx2 and nx4 along the same in-plane x-direction, each of nx2 and nx4 being greater than each of nx1 and nx3.

5. The multilayer optical film of claim 4 , wherein 0.45≦f1≦0.8 and 0.02≦f3≦0.

4.

6. 1. A multilayer optical film comprising a plurality of optical repeating units, the number of said optical repeating units in said plurality of optical repeating units being greater than about 10 in total, each of said optical repeating units comprising at least four sequentially arranged first through fourth individual layers, and for each of said optical repeating units: the average thickness of the same one of the first and third distinct layers differs from the average thickness of each other of the first through fourth distinct layers by a factor of at least 2.5; the first and third discrete layers have respective f-ratios f1 and f3 for the same in-plane x-direction respective refractive indices nx1 and nx3; and for substantially normally incident light polarized along the x-direction, the multilayer optical film has a primary reflection band located at wavelengths greater than about 600 nm, the primary reflection band having a first reflection peak having a peak height Rp1, any secondary reflection bands of the multilayer optical film having a reflection peak having a peak height Rp2, Rp1 / Rp2≧5, and such that the ratio Rp1 / Rp2 remains greater than or equal to 5 when at least one of the f-ratios f1 and f3 is continuously varied by 0.

2. Multilayer optical films.

7. 7. The multilayer optical film of claim 6, wherein the second and fourth discrete layers have respective refractive indices nx2 and nx4 along the same in-plane x-direction, where nx1<nx2<nx3 and nx1<nx4<nx3.

8. 8. The multilayer optical film of claim 7, wherein 0.02≦f1≦0.1 and 0.02≦f3≦0.

65.

9. 8. The multilayer optical film of claim 7, wherein 0.44≦f1≦0.55 and 0.02≦f3≦0.

55.

10. 7. The multilayer optical film of claim 6, wherein the second and fourth discrete layers have respective refractive indices nx2 and nx4 along the same in-plane x-direction, each of nx2 and nx4 being greater than each of nx1 and nx3.

11. 11. The multilayer optical film of claim 10, wherein 0.45≦f1≦0.8 and 0.02≦f3≦0.

4.

12. 1. A multilayer optical film comprising a plurality of optical repeating units, the number of said optical repeating units in said plurality of optical repeating units being greater than about 10 in total, each of said optical repeating units comprising at least four individual layers, a first and a second individual layer of said at least four individual layers of each of said optical repeating units having respective f-ratios f1 and f2 to respective refractive indices in the same in-plane x-direction, where 0.5≦f1≦0.8, 0.02≦f2≦0.3; for substantially normally incident light polarized along the x-direction, the multilayer optical film has a first order reflection band located at wavelengths greater than about 700 nm and no second order reflection bands; such that at least a third layer of the at least four individual layers of each of the optical repeating units has an average thickness of less than about 75 nm. Multilayer optical films.

13. 13. The multilayer optical film of claim 12, wherein the at least the third layer has a refractive index along the x-direction that is greater than each of the refractive indices along the x-direction of the first and second individual layers.

14. 14. The multilayer optical film of claim 12 or 13, wherein 0.02≦f2≦0.

25.

15. 15. The multilayer optical film of claim 14, wherein 0.55≦f1≦0.8.

Images