Reflective polarizer and optical system including same

Abstract

A reflective polarizer includes a plurality of alternating first and second polymeric layers. For different first and second wavelengths in a predetermined wavelength range and for first and second pairs of adjacent first and second polymeric layers where the first and second pairs are spaced apart along a thickness direction of the reflective polarizer: the first and second polymeric layers of the first pair, but not the second pair, have refractive indices along an in-plane axis that differ by less than about 0.002 for the first wavelength; and the first and second layers of the second pair, but not the first pair, have refractive indices along the in-plane axis that differ by less than about 0.002 for the second wavelength. An optical system can include the reflective polarizer.

Description

[0001] REFLECTIVE POLARIZER AND OPTICAL SYSTEM INCLUDING SAME

[0002] TECHNICAL FIELD

[0003] The present description relates generally to reflective polarizers, and more specifically to reflective polarizers including polymeric layers for reflecting polarized light.

[0004] BACKGROUND

[0005] A reflective polarizer can include polymeric interference layers for providing reflection for a block polarization state in a desired wavelength range. An optical system can include the reflective polarizer.

[0006] SUMMARY

[0007] In some aspects of the present description, a reflective polarizer including a plurality of alternating first and second polymeric layers numbering at least 10 in total is provided. Each layer of the first and second polymeric layers has an average thickness of less than about 500 nm, such that for substantially normally incident light and for a predetermined wavelength range at least 50 nm wide, the plurality of alternating first and second polymenc layers has an average optical reflectance of greater than about 60 percent when the incident light is polarized along a first axis and an average optical reflectance of less than about 2 percent when the incident light is polarized along a second axis orthogonal to the first axis. The first layers can have a refractive index along the first axis greater than a refractive index of the second layers along the first axis by at least about 0.05 for at least one wavelength in the predetermined wavelength range. For different first and second wavelengths in the predetermined wavelength range and for first and second pairs of adjacent first and second polymeric layers where the first and second pairs are spaced apart along a thickness direction of the reflective polarizer: the first and second polymeric layers of the first pair, but not the second pair, have refractive indices along the second axis that differ by less than about 0.002 for the first wavelength; and the first and second layers of the second pair, but not the first pair, have refractive indices along the second axis that differ by less than about 0.002 for the second wavelength.

[0008] In some aspects of the present description, a reflective polarizer including a plurality of optical repeat units is provided. The optical repeat units in the plurality of optical repeat units can number greater than 5 in total. Each optical repeat unit includes a first polymeric layer and at least one second polymeric layer where each of the first and second polymeric layers of each optical repeat unit has an average thickness of less than about 500 nm, such that for substantially normally incident light and for a predetermined wavelength range at least 50 nm wide, the plurality of optical repeat units has an average optical reflectance of greater than about 60 percent when the incident light is polarized along a first axis and an average optical reflectance of less than about 2 percent when the incident light is polarized along a second axis orthogonal to the first axis. For first and second optical repeat units spaced apart along a thickness direction of the reflective polarizer, and for different first and second wavelengths in the predetermined wavelength range: for each of the first and second optical repeat units, the first layer has refractive indices nix and nly along the respective first and second axes and the at least one second layer has thickness-weighted average refractive indices n2x and n2y along the respective first and second axes; for the first optical repeat unit, but not the second optical repeat unit, nly and n2y differ by less than about 0.002 for the first wavelength; for the second optical repeat unit, but not the first optical repeat unit, nly and n2y differ by less than about 0.002 for the second wavelength; and for each of the first and second optical repeat units and for each of the first and second wavelengths, nix and n2x differ by at least about 0.05.

[0009] In some aspects of the present description, a reflective polarizer including a plurality of optical repeat units is provided. The optical repeat units in the plurality of optical repeat units can number greater than 5 in total. Each optical repeat unit includes at least one birefringent first polymeric layer and at least one substantially optically isotropic second polymeric layer where each of the first and second polymeric layers of each optical repeat unit has an average thickness of less than about 500 nm, such that for substantially normally incident light and for a predetermined wavelength range at least 50 nm wide, the plurality of optical repeat units has an average optical reflectance of greater than about 60 percent when the incident light is polarized along a first axis and an average optical reflectance of less than about 2 percent when the incident light is polarized along a second axis orthogonal to the first axis. For first and second optical repeat units spaced apart along a thickness direction of the reflective polarizer, and for different first and second wavelengths in the predetermined wavelength range: for each of the first and second optical repeat units, the at least one birefringent first polymeric layer and the at least one substantially optically isotropic second polymeric layer have respective thickness-weighted average refractive indices nix and n2x along the first axis and respective thickness-weighted average refractive indices nly and n2y along the second axis; for the first optical repeat unit, but not the second optical repeat unit, nly and n2y differ by less than about 0.002 for the first wavelength; for the second optical repeat unit, but not the first optical repeat unit, nly and n2y differ by less than about 0.002 for the second wavelength; and for each of the first and second optical repeat units and for each of the first and second wavelengths, nix is greater than n2x by at least about 0.05.

[0010] In some aspects of the present description, an optical system is provided. The optical system includes a reflective polarizer of the present description, a partial reflector, and a retarder layer. The partial reflector and the reflective polarizer define an optical cavity therebetween. The retarder layer is disposed in the optical cavity.

[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. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is an illustrative plot of refractive index versus wavelength for a higher index polymer and two lower index polymers, according to some embodiments.

[0013] FIGS. 2-4 are schematic cross-sectional views of reflective polarizers, according to some embodiments.

[0014] FIGS. 5-6 are schematic flow diagrams for forming reflective polarizers, according to some embodiments.

[0015] FIGS. 7-9 are illustrative layer thickness profdes for reflective polarizers, according to some embodiments.

[0016] FIG. 10 is a plot of reflectance of optical layers of a reflective polarizer versus wavelength for substantially normally incident light for two orthogonal polarization states, according to some embodiments.

[0017] FIGS. 11-13 are plots of pass state reflectance of optical layers reflective polarizers versus wavelength for substantially normally incident light, according to some embodiments.

[0018] FIG. 14 is a schematic cross-sectional view of an optical construction, according to some embodiments.

[0019] FIG. 15 is a schematic cross-sectional view of an optical system, according to some embodiments.

[0020] DETAILED DESCRIPTION

[0021] 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.

[0022] As is known in the art, multilayer optical films including alternating polymeric layers can be used to provide desired reflection and transmission in desired wavelength ranges and polarization states 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. A multilayer optical film can be a reflective polarizer substantially reflecting a first polarization state (block state) and substantially transmitting a second polarization state (pass state).

[0023] An optical system can utilize a reflective polarizer and a partial reflector with a retarder therebetween to provide a folded optical path as generally described in U.S. Pat. No. 10,678,052 (Ouderkirk, et al.), for example. In such optical systems, it can be desired to minimize pass state reflectance of the reflective polarizer. It has been found that even a small pass state reflectance can result in undesired ghost images, for example. Further, it has been found that such problems caused by undesired pass state reflectance is particularly problematic when the reflective polarizer is disposed between bonded lenses. However, it has been found that it is difficult to achieve a very low pass state reflectance throughout a visible wavelength range, for example, due to a difference in dispersion of the refractive index with wavelength of the first and second layers of a plurality of alternating first and second layers of a reflective polarizer when conventional material choices are made for the first and second layers. The first layers can be higher index (higher refractive index along the block axis) optical (HIO) layers and the second layers can be lower index (lower index along the block axis) optical (LIO) layers. HIO and LIO layers may be referred to as interference layers. Interference layers (or optical layers) may be described as reflecting or transmitting light primarily by optical interference when the reflectance and transmittance of the interference layers can be reasonably described by optical interference or reasonably accurately modeled as resulting from optical interference.

[0024] According to some embodiments of the present description, it has been found that using different materials for different LIO layers can result in a closer index match to the HIO layers over a predetermined wavelength range compared to using a single LIO material. Alternatively, or in addition, according to some embodiments of the present description, it has been found that using different materials for different HIO layers can results in a closer index match to the LIO layers over a predetermined wavelength range compared to using a single HIO material.

[0025] FIG. 1 is a plot of refractive index along a pass axis versus wavelength of a HIO material and two LIO materials, according to some embodiments. LIO1 provides a close index match to the HIO material in a red wavelength range but provides a poorer match in a blue wavelength range. LIO2 provides a close index match to the HIO material in a blue wavelength range but provides a poorer match in a red wavelength range. It has been found, according to some embodiments, that combining both LIO1 and LIO2 can provide a close index match throughout the visible range (e.g., about 400 nm to about 700 nm), for example. The LIO materials can be combined by using different LIO materials in different packets of alternating HIO and LIO layers; by blending the different LIO material such that the composition of the blend varies continuously along the thickness direction of the reflective polarizer over at least a portion of the thickness of the reflective polarizer; and / or by splitting each of at least some of the LIO layers into a plurality of LIO layers (or sub-layers) which includes at least one LIO1 layer and at least one LIO2 layer where the relative thickness of the at least one LIO 1 layer and the at least one LIO2 layer varies along the thickness direction of the reflective polarizer over at least a portion of the thickness of the reflective polarizer. Alternatively, or in addition, different HIO materials can similarly be combined to improve the index match to the LIO layers over a predetermined wavelength range. At least one HIO and an adjacent at least one LIO layer (e.g., an HIO and LIO layer pair, or an HIO layer and a plurality of LIO sub-layers, or an LIO layer and a plurality of HIO sub-layers, or pluralities of HIO and LIO sublayers) may define an optical repeat unit. An optical repeat unit is generally the smallest unit of optical layers that repeat in a thickness direction of the reflective polarizer over at least a portion of the thickness. Generally, an optical repeat unit has a first order reflection at a wavelength of twice the optical thickness of the optical repeat unit. In order to reduce or minimize the reflectance in the pass state, the materials for the HIO and LIO layers can be selected so that for the pass state, the indices (or the effective indices in the case of sublayers) of the HIO and LIO layers in each optical repeat unit are suitably closely matched at a wavelength of twice the optical thickness of the optical repeat unit.

[0026] FIGS. 2-4 are schematic cross-sectional views of reflective polarizers 200, 200’, and 200”, according to some embodiments. In some embodiments, the reflective polarizer includes a plurality of optical repeat units (10, 11 and 10’, 11’ for reflective polarizer 200; 10 and 11 for reflective polarizer 200’; and 10, 11, 11’ and 11” for reflective polarizer 200”). Each optical repeat unit generally includes at least one first polymeric layer (e.g., 10) and at least one second polymeric layer (e.g., 11, 11’, 11”). An optical repeat unit can include more than two layers as described in U.S. Pat. Nos. 5,103,337 (Schrenk et al.); 5,540,978 (Schrenk) and 6,207,260 (Wheatley et al.), for example. In some embodiments, each of the at least one first polymeric layer is a birefringent layer (e.g., having a maximum birefringence greater than 0.05, 0.06, 0.07, 0.08, 0.9, or 0.1 for at least one wavelength in a range of about 420 nm to about 680 nm) and each of the at least one second polymeric layer is a substantially optically isotopic layer (e.g., having a maximum birefringence less than 0.04, 0.03, 0.02, 0.015, or 0.01 for the at least one wavelength). The maximum birefringence of a layer for a given wavelength is generally the refractive index for that wavelength along a direction where the refractive index is a maximum (e g., a stretch direction) minus the refractive index for that wavelength along a direction where the refractive index is a minimum (e.g., a thickness direction or an in-plane direction orthogonal to the stretch direction). The maximum birefringence can be up to about 0.35, 0.3, 0.25, or 0.2, for example.

[0027] The total number of optical repeat units and / or the total number of alternating first and second layers may be substantially larger than schematically illustrated. The optical repeat units can number at least 5, 10, 20, 50, or 100 in total. In some embodiments, optical repeat units can number up to 1000, 800, 600, or 500, for example. In some embodiments, the reflective polarizer includes a plurality of alternating first and second polymeric layers 10, 11 numbering at least 10, 20, 40, 100, or 200 in total. In some embodiments, the alternating first and second polymeric layers 10, 11 can number up to 2000, 1600, 1200, or 1000, for example.

[0028] In some embodiments, each layer of each optical repeat unit or each layer of the plurality of alternating first and second polymeric layers has an average thickness less than about 500, 400, 300, 250, 200, 175, or 150 nm. The average thickness can be greater than about 1, 2, 3, 4, or 5 nm, for example.

[0029] The reflective polarizer 200, 200’, 200” can include at least one (e.g., polymeric) layer (e.g., a protective boundary layer or a skin layer) having an average thickness greater than about 500, 600, 700, 800, 900, 1000, 1200, 1500, or 2000 nm. The average thickness can be up to about 20, 15, 10, or 5 microns, for example. Reflective polarizers 200, 200’, 200” include (e.g., skin) layers 124 and 126 that can each have a thickness in any of these ranges, where the plurality of alternating first and second polymeric layer and / or the plurality of optical repeat units are disposed between the layers 124 and 126. In some embodiments, the plurality of alternating first and second polymeric layers (or the plurality of optical repeat units) includes first and second pluralities 110 and 111 of alternating first and second polymeric layers (or first and second pluralities 110 and 111 of optical repeat units) separated along the thickness direction (z-direction) of the reflective polarizer by at least one layer 125 having an average thickness greater than about 500 nm (or in any of the ranges described above). The reflective polarizer can optionally include additional pluralities of optical repeat units or alternating first and second polymeric layers.

[0030] In some embodiments, the first and / or second layers have a composition and / or refractive index that varies along a thickness direction (z-direction) of the reflective polarizer over at least a portion of the reflective polarizer. The portion of the reflective polarizer is a portion along the thickness direction that includes a plurality of the alternating first and second layers and / or a plurality of the optical repeat units.

[0031] For example, the reflective polarizer 200 can include first and second pluralities 110 and 111 of optical layers where each first layer 10, 10’ of the first and second pluralities have a same first composition, the second layers 11 of the first plurality 110 have a same second composition, and the second layers 11’ of the second plurality 111 have a same third composition, where the first, second and third compositions are different compositions. Alternatively, each second layer 11, 11’ of the first and second pluralities can have a same first composition, the first layers 10 of the first plurality 110 can have a same second composition, and the first layers 10’ of the second plurality 111 have a same third composition, where the first, second and third compositions are different compositions. Alternatively, each first layer 10 of the first plurality 110 can have a same first composition, each first layer 10’ of the second plurality 111 can have a same second composition, each second layer 11 of the first plurality 110 can have a same third composition, and the second layers 11’ of the second plurality 111 can have a same fourth composition, where the first through fourth compositions are different compositions.

[0032] As another example, the reflective polarizer 200’ can include first layers 10 having a same composition and second layers 11 having a composition that varies continuously over at least a portion of the reflective polarizer (as schematically illustrated in FIG. 3), or first layers 10 having a composition that varies continuously over at least a portion of the reflective polarizer and second layers 11 having a same composition, or first and second layer 10, 11 having compositions that each varies continuously over at least a portion of the reflective polarizer.

[0033] As still another example, the reflective polarizer 200” can include optical repeat units that each include at least one first layer (e.g., 10) and at least one second layer (e.g., 11, 11’, 11”) where at least one of the at least one first layer and the at least one second layer includes a plurality of layers that have a continuous relative thickness variation over at least a portion of the reflective polarizer resulting in a continuous variation in refractive index. For example, in FIG. 4, the at least one second layer 11, 11’, 11” includes three second layers where the middle second layer has a thickness relative to a total thickness of the three second layers that continuously decreases along the z-direction. The at least one first layer 10 can alternatively, or in addition, optionally include more than one layer (e.g., as schematically indicated by the horizontal dashed lines within layer 10).

[0034] In some embodiments, each optical repeat unit includes at least one first polymeric layer and at least one second polymeric layer, where each first polymeric layer has a higher refractive index along a block axis than each second polymeric layer and / or each first polymeric layer has a higher birefringence than each second polymeric layers. In some embodiments, the first layers have a refractive index along the first axis (e.g., block axis) greater than a refractive index of the second layers along the first axis by at least about 0.05, 0.06, 0.07, 0.08, 0.09, or 0.1 for at least one wavelength in a predetermined wavelength range. This difference may be up to about 0.4, 0.35, 0.3, 0.25, or 0.2 for example.

[0035] In some embodiments, for different first and second wavelengths (e.g., wavelengths I and X2 illustrated in FIG. 1) in a predetermined wavelength range and for first and second pairs (e g., pairs 15 and 16 schematically illustrated in FIG. 2) of adjacent first and second polymeric layers where the first and second pairs are spaced apart along a thickness direction of the reflective polarizer: the first and second polymeric layers of the first pair, but not the second pair, have refractive indices along the second axis that differ by less than about 0.002 for the first wavelength; and the first and second layers of the second pair, but not the first pair, have refractive indices along the second axis that differ by less than about 0.002 for the second wavelength. The first and second polymeric layers of the first pair can have refractive indices along the second axis that differ by less than about 0.002, 0.0015, 0.001, or 0.0005 for the first wavelength XI (e.g., the indices can be substantially matched at XI for a pair including HIO and LIO2 as schematically illustrated in FIG. 1). The first and second polymeric layers of the first pair can have refractive indices along the second axis that differ (e.g., corresponding to the difference D2 schematically illustrated in FIG. 1) by greater than about 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, or 0.01 for the second wavelength X2. The first and second layers of the second pair can have refractive indices along the second axis that differ by less than about 0.002, 0.0015, 0.001, or 0.0005 for the second wavelength X2 (e.g., the indices can be substantially matched at X2 for a pair including HIO and LIO1 as schematically illustrated in FIG. 1). The first and second polymeric layers of the second pair can have refractive indices along the second axis that differ (e.g., corresponding to the difference DI schematically illustrated in FIG. 1) by greater than about 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, or 0.01 for the first wavelength XI. The differences DI and D2 may each be up to about 0.04, 0.03, 0.02, or 0.015, for example.

[0036] In some embodiments, the predetermined wavelength range extends at least from about 420 nm to about 680 nm or at least from about 400 nm to about 700 nm. For example, the predetermined wavelength range can extend from about 380 nm to about 1200 nm or from about 400 nm to about 1000 nm. In some embodiments, the first and second wavelengths XI and X2 differ by at least about 100, 150, 200, or 250 nm, for example. In some embodiments, for the first and second pairs (e.g., pairs 15 and 16 schematically illustrated in FIG. 2) of adjacent first and second polymeric layers: the first polymeric layers of each of the first and second pairs have a same composition; and the second polymeric layers of the first and second pairs have different compositions. In some embodiments, for the first and second pairs of adjacent first and second polymeric layers: the first polymeric layers of each of the first and second pairs have different compositions; and the second polymeric layers of the first and second pairs have a same composition. In some embodiments, the plurality of alternating first and second polymeric layers includes first and second pluralities 110 and 111 (see, e.g., FIG. 2) of alternating first and second polymeric layers where each of the first and second pluralities includes at least 10 (or at least 20, 40, 60, 80, 100 or a range of 10 to 800 or 20 to 500) layers in total. In some embodiments, for each of the first and second pluralities 110 and 111, the first layers have a same first composition and the second layers have a same second composition different than the first composition. In some embodiments, the first composition for the first and second pluralities 110 and 111 are a same composition and the second composition for the first and second pluralities 110 and 111 are different compositions. In some embodiments, the first composition for the first and second pluralities 110 and 111 are different compositions and the second composition for the first and second pluralities 110 and 111 are a same composition. In some embodiments, the first composition for the first and second pluralities 110 and 111 are different compositions and the second composition for the first and second pluralities 110 and 111 are different compositions. In some embodiments, the first and second pluralities 110 and 111 are separated along a thickness direction of the reflective polarizer by at least one layer 125 having a thickness greater than about 500 nm (or in a range described elsewhere herein).

[0037] In some embodiments, for first and second optical repeat units (e g., optical repeat units 15 and 16 schematically illustrated in FIG. 2; or optical repeat units 15’ and 16’ schematically illustrated in FIG. 3; or optical repeat units 15” and 16” schematically illustrated in FIG. 4) spaced apart along a thickness direction of the reflective polarizer where each optical repeat unit includes a first polymeric layer and at least one second polymeric layer, and for different first and second wavelengths in the predetermined wavelength range: for each of the first and second optical repeat units, the first layer has refractive indices nix and nly along the respective first and second axes and the at least one second layer has thickness- weighted average refractive indices n2x and n2y along the respective first and second axes; for the first optical repeat unit, but not the second optical repeat unit, nly and n2y differ by less than about 0.002 for the first wavelength; for the second optical repeat unit, but not the first optical repeat unit, nly and n2y differ by less than about 0.002 for the second wavelength; and for each of the first and second optical repeat units and for each of the first and second wavelengths, nix and n2x differ by at least about 0.05, 0.06, 0.07, 0.08, 0.09, or 0.1. The difference (|nlx - n2x|) can be up to about 0.4, 0.35, 0.3, 0.25, or 0.2, for example. In some embodiments, for each of the first and second optical repeat units and for each of the first and second wavelengths, nix is greater than n2x by at least about 0.05, 0.06, 0.07, 0.08, 0.09, or 0.1 (e.g., the first layers can be HIO layers). In some embodiments, for each of the first and second optical repeat units and for each of the first and second wavelengths, n2x is greater than nix by at least about 0.05, 0.06, 0.07, 0.08, 0.09, or 0.1 (e.g., the second layers can be HIO layers).

[0038] Similarly, in some embodiments, for first and second optical repeat units (e.g., optical repeat units 15 and 16 schematically illustrated in FIG. 2; or optical repeat units 15’ and 16’ schematically illustrated in FIG. 3; or optical repeat units 15” and 16” schematically illustrated in FIG. 4) spaced apart along a thickness direction of the reflective polarizer where each optical repeat unit includes at least one birefringent first polymeric layer and at least one substantially optically isotropic second polymeric layer, and for different first and second wavelengths in the predetermined wavelength range: for each of the first and second optical repeat units, the at least one birefringent first polymeric layer and the at least one substantially optically isotropic second polymeric layer have respective thickness-weighted average refractive indices nix and n2x along the first axis and respective thickness-weighted average refractive indices nly and n2y along the second axis; for the first optical repeat unit, but not the second optical repeat unit, nly and n2y differ by less than about 0.002 for the first wavelength; for the second optical repeat unit, but not the first optical repeat unit, nly and n2y differ by less than about 0.002 for the second wavelength; and for each of the first and second optical repeat units and for each of the first and second wavelengths, nix is greater than n2x by at least about 0.05, 0.06, 0.07, 0.08, 0.09, or 0. 1. In some embodiments, for the first optical repeat unit, nly and n2y differ by less than about 0.002, 0.0015, 0.001, or 0.0005 for the first wavelength XI (e.g., the indices can be substantially matched at I for an optical repeat unit including HIO and LIO2 as schematically illustrated in FIG. 1). In some embodiments, for the first optical repeat unit, nly and n2y differ (e.g., corresponding to the difference D2 schematically illustrated in FIG. 1) by greater than about 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, or 0.01 for the second wavelength X2. In some embodiments, for the second optical repeat unit, nly and n2y differ by less than about 0.002, 0.0015, 0.001, or 0.0005 for the second wavelength X2 (e.g., the indices can be substantially matched at X2 for an optical repeat unit including HIO and LIO 1 as schematically illustrated in FIG. 1). In some embodiments, for the second optical repeat unit, nly and n2y differ (e g., corresponding to the difference DI schematically illustrated in FIG. 1) by greater than about 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, or 0.01 for the first wavelength XL The differences DI and D2 can be in any range described elsewhere herein.

[0039] The thickness-weighted average index may be used as an effective refractive index, for example, when a HIO or LIO layer is divided into multiple HIO or LIO layers, respectively. For example, referring to FIG. 4, if the layers 11, 11’, and 11” have respective refractive indices ny, ny ’, and ny ’ ’ along the y- axis and respective average thicknesses t, t’, and t”, then the thickness-weighted average index along the y-axis is given by n2y = (txny + t’xny’ + t”xny”) / (t + 1’ + 1”) and this may be regarded as an effective refractive index along the y-axis of the three layers 11, 11’, and 11”.

[0040] FIGS. 5-6 are schematic flow diagrams for forming reflective polarizers, according to some embodiments. FIG. 5 schematically illustrates mixing (e.g., via a static mixer) flow streams 521 and 522 to form second (or first) layers (e.g., corresponding to layers 11 of FIG. 3) while flow stream 511 forms first (or second) layers (e g., corresponding to layers 10 of FIG. 3). The composition of the layers formed from flow streams 521 and 522 varies along the thickness direction (z -direction) as schematically illustrated by the relative thicknesses of the flow streams. FIG. 6 schematically illustrates forming first (or second) layers (e.g., corresponding to layers 10 of FIG. 4) from flow stream 511 and forming a plurality of second (or first) layers (e.g., corresponding to layers 11, 11’, and 11 ” of FIG. 5) from flow streams 521, 522, and 523. The relative thicknesses of the layers formed from flow streams 521, 522, and 523 vary along the thickness direction (z-direction) as schematically illustrated by the relative thicknesses of the flow streams.

[0041] FIGS. 7-9 are illustrative layer thickness profiles for reflective polarizers, according to some embodiments.

[0042] In FIG. 7, the reflective polarized includes a plurality of layers 310 (e.g., corresponding to layers lO ofFIG. 3) and a plurality of layers 311 (e.g., corresponding to layers 11 ofFIG. 3). The layers of the reflective polarizer are divided into regions 221, 222 and 223. In regions 222 and 223 each of the layers 310 and 311 have fixed compositions. Layers 311 have different compositions in regions 222 and 223 while layers 310 have a same composition in each of regions 221, 222, and 223. For example, the compositions of layers 311 in regions 222 and 223 can be selected to be suitably well refractive index matched to the composition of the layers 310 along the pass state in respective blue and red wavelength ranges. Layers 311 have variable composition in region 221. For example, layers 311 can have a composition in region 221 varying continuously along the thickness direction from the composition of layers 311 in region 222 to the composition of layers 311 in region 223. The continuous variation can be achieved as schematically illustrated in FIG. 5, for example. A layer thickness profile can be said to be continuous over a region when the layer thickness can be described as a continuous function of the (discrete) layer number that varies gradually in the region (e.g., as illustrated in FIGS. 7-9). Similarly, a composition can be said to vary continuously over a region when the composition can be described as a continuous function of the (discrete) layer number that varies gradually in the region.

[0043] In some embodiments, the plurality of alternating first and second polymeric layers includes a first plurality (e.g., in region 221 of FIG. 7) of alternating first and second polymeric layers including at least 10, 20, 50, 80 or 100 layers in total (or in a range described elsewhere herein for the reflective polarizer), such that for at least one for the first polymeric layers of the first plurality or the second polymeric layers of the first plurality, the polymeric layers have a composition that varies continuously along the thickness direction (z-direction) of the reflective polarizer. In some embodiments, each first polymeric layer of the first plurality has a same composition, and the composition of the second polymeric layers varies continuously along the thickness direction. In some embodiments, each second polymeric layer of the first plurality has a same composition, and the composition of the first polymeric layers varies continuously along the thickness direction. In some embodiments, the composition of the first polymeric layers varies continuously along the thickness direction and the composition of the second polymeric layers varies continuously along the thickness direction. In some embodiments, the plurality of alternating first and second polymeric layers includes second and third pluralities (e.g., in respective regions 222 and 223 of FIG. 7) of alternating first and second polymeric layers where each of the first and second pluralities includes at least 5 (or at least 10, 20, 30, 50, 70) layers in total with the first plurality disposed between the second and third pluralities. In some embodiments, each of the first and second pluralities includes a range of 10 to 400, 20 to 300, or 30 to 250 layers in total. In some embodiments, the first layers of the second plurality have a same first compositions, the first layers of the third plurality have a same second composition, the second layers of the second plurality have a same third composition, and the second layers of the third plurality have a same fourth composition. In some embodiments, the first and second compositions are a same composition and the third and fourth composition are different compositions. In some embodiments, each second polymeric layer of the first plurality is a blend of the third and fourth compositions. In some embodiments, the first and second compositions are different compositions and the third and fourth composition are a same composition. In some embodiments, each first polymeric layer of the first plurality is a blend of the first and second compositions. In some embodiments, the first through fourth compositions are different compositions, each first polymeric layer of the first plurality is a blend of the first and second compositions, and each second polymeric layer of the first plurality is a blend of the third and fourth compositions.

[0044] In FIG. 8, the reflective polarized includes a plurality of layers 310’ (e.g., corresponding to layers 10 of FIG. 4) and a plurality of layers 311’ (e.g., corresponding to two of layers 11, 11’, 11’ of FIG. 4). The layers of the reflective polarizer are divided into regions 221’, 222’ and 223’. In regions 222’ and 223 ’ , each optical repeat unit include s one layer 310’ and one layer 311’ while in region 221’, each optical repeat unit includes one layer 310’ and two layers 311’ (one layer 31 la’ and one layer 31 lb’). The layers 311b’ can have a same composition as that of layers 311’ in region 223’ and the layers 31 la’ can have a same composition as that of layers 311’ in region 222 ’ . The layers 311’ can have different compositions in regions 222’ and 223’ while the layers 310’ can have a same composition in each of regions 221’, 222’, and 223’. Similarly, in FIG. 9, the reflective polarized includes a plurality of layers 310” (e.g., corresponding to layers 10 of FIG. 4) and a plurality of layers 311” (e.g., corresponding to layers 11, 11’, 11’ ofFIG. 4). The layers of the reflective polarizer are divided into regions 221”, 222” and 223”. In regions 222” and 223”, each optical repeat unit include one layer 310” and one layer 311” while in region 221 ”, each optical repeat unit includes one layer 310’ and three layers 311’ (two layers 311a’ and one layer 311b’). The layers 311b” can have a same composition as that of layers 311” in region 223 ” and the layers 311a” can have a same composition as that of layers 311 ” in region 222 ’ ’ . The layers 311” can have different compositions in regions 222 ’ ’ and 223 ” while the layers 310” can have a same composition in each of regions 221”, 222”, and 223”. For example, the compositions of layers 311’ (or 311”) in regions 222’ and 223’ (or 222” and 223”) can be selected to be suitably well refractive index matched to the composition of the layers 310’ (or 310”) along the pass state in respective blue and red wavelength ranges. The layer thickness profiles of FIGS. 8-9 can be achieved as schematically illustrated in FIG 6, for example.

[0045] In some embodiments, for each of the first and second optical repeat units (e.g., optical repeat units 15” and 16” schematically illustrated in FIG. 4), the first layer comprises a first composition and the at least one second layer comprises a plurality of second layers comprising at least two different compositions where each of the at least two different compositions is different from the first composition. In some embodiments, for each of the first and second optical repeat units, the at least one second layer comprises at least three second layers. In some embodiments, the at least three second layers include at least two layers comprising a same second composition and at least one layer comprising a same third composition different from the second composition, where the at least two different compositions comprise the second and third compositions.

[0046] In some embodiments, the plurality of optical repeat units comprises first, second, and third pluralities of optical repeat units (e g., corresponding to regions 222’, 221’, and 223’; or regions 222”, 221”, and 223”) where the second plurality is disposed between the first and third pluralities. In some embodiments, the at least one second polymeric layer of each optical repeat unit of the second plurality includes at least two second polymeric layers, and the at least one second polymeric layer of each optical repeat unit of each of the first and third pluralities is a single second polymeric layer. In some embodiments, each second layer of each optical repeat unit of the first plurality has a same first composition and each second layer of each optical repeat unit of the third plurality has a same second composition different from the first composition. In some embodiments, the at least two second polymeric layers of each optical repeat unit of the second plurality includes at least one layer having the first composition and at least one layer having the second composition.

[0047] In some embodiments, a corresponding layer of the at least one second polymeric layer has a thickness that varies continuously along the thickness direction (z-direction) of the reflective polarizer over at least a portion (e.g., corresponding to region 221’ or 221 ”) of the reflective polarizer where the portion includes at least 5 of the optical repeat units. In some embodiments, a ratio of a thickness of a corresponding layer of the at least one second polymeric layer to a thickness of the first layer varies continuously along the thickness direction of the reflective polarizer over at least a portion of the reflective polarizer where the portion includes at least 5 of the optical repeat units. In some embodiments, the at least one second polymeric layer comprises at least two second polymeric layers, and a ratio of a thickness of a corresponding layer of the at least two second polymeric layers to a total thickness of the at least two second polymeric layers varies continuously along the thickness direction of the reflective polarizer over at least a portion of the reflective polarizer, where the portion includes at least 5 of the optical repeat units. In some embodiments, a corresponding layer (e.g., layers 31 la’ or 31 la”) of the at least one second polymeric layer of the optical repeat units has a thickness that continuously decreases along the thickness direction over at least a portion of the reflective polarizer where the portion includes at least 5 of the optical repeat units, and were the first polymeric layer (e.g., layers 311’ or 311”) of the optical repeat units having a thickness that continuously increases along the thickness direction over the portion of the reflective polarizer. In any of these embodiments, the portion (e.g., corresponding to region 221’ or 221”) may include at least 5, 10, 20, 30, 50, 70, 90, or 100 of the optical repeat units.

[0048] In some embodiments, each optical repeat unit includes at least one birefringent first polymeric layer and at least one substantially optically isotropic second polymeric layer. In some embodiments, for each optical repeat unit and for at least one wavelength in the predetermined wavelength range, each layer of the at least one birefringent first polymeric layer has an in-plane birefringence of greater than 0.05 and each layer of the at least one substantially optically isotropic second polymeric layer has an in-plane birefringence of less than 0.04. In some embodiments, the in-plane birefringence of the first polymeric layers is greater than 0.06, 0.07, 0.08, 0.09, or 0.1. The in-plane birefringence of the first polymeric layers can be up to 0.35, 0.3, 0.25, or 0.2, for example. In some embodiments, the in-plane birefringence of the second polymeric layers is less than 0.04, 0.03, 0.025, 0.02, 0.015, or 0.01.

[0049] In some embodiments, for each optical repeat unit in at least a sub-plurality (e.g., corresponding to regions 222’ or 222”) of the optical repeat units, the at least one birefringent first polymeric layer is a single first polymeric layer and the at least one substantially optically isotropic second polymeric layer includes a plurality of second polymeric layers. In some embodiments, for each optical repeat unit in at least a sub-plurality (e.g., corresponding to regions 222’ or 222”), the at least one birefringent first polymeric layer includes a plurality of first polymeric layers and the at least one substantially optically isotropic second polymeric layer is a single second polymeric layer. In some embodiments, for each optical repeat unit in at least a sub-plurality (e.g., corresponding to regions 222’ or 222”), the at least one birefringent first polymeric layer includes a plurality of first polymeric layers and the at least one substantially optically isotropic second polymeric layer includes a plurality of second polymeric layers. The sub-plurality of the optical repeat units at least 5, 10, 20, 30, 50, 70, 90, 100, 120, or 140 of the optical repeat units. In some embodiments, the plurality of optical repeat units comprises first, second, and third pluralities (e.g., corresponding to regions 222’, 221’, and 223’; or regions 222”, 221”, and 223”) of optical repeat units where the second plurality is disposed between the first and third pluralities. In some embodiments, at least one of the at least one birefringent first polymeric layer and the at least one substantially optically isotropic second polymeric layer of each optical repeat unit of the second plurality comprising at least two layers, and each of the at least one birefringent first polymeric layer and the at least one substantially optically isotropic second polymeric layer of each optical repeat unit of each of the first and third pluralities is a single layer. Each of the first, second, and third pluralities at least 5, 10, 20, 30, 50, 70, 90, or 100 of the optical repeat units.

[0050] Suitable materials for the various layers of the reflective polarizer 200, 200’, 200” include polyethylene naphthalate (PEN), polyethylene terephthalate (PET), polycarbonate, copolyesters, and blends or copolymers thereof. Other suitable materials are described in the multilayer optical film references provided elsewhere herein. In some embodiments, each of the HIO layers can be or include coPEN 90 / 10 (also referred to as Low-melt PEN or LmPEN) as described in U.S. Pat. No. 6,946,188 (Hebrink et al.), for example, and each of the LIO layers can be a blend of coPEN 90 / 10 and at least one other polyester or other polymer (e g., different blends in different LIO layers), for example. The at least one other polyester or other polymer can include a glycol-modified PET. As is known in the art, polyesters and copolyesters can be made by polycondensation reactions of dicarboxylic acids (or diesters thereof) with diols. For example, polyethylene terephthalate can be made by reacting ethylene glycol with terephthalic acid or dimethyl terephthalate. Glycol-modified PET can be made similarly, but with at least some of the ethylene glycol replaced with different diol(s) such as cyclohexanedimethanol. Generally, the units of the chain of the polyester or copolyester derived from dicarboxylic acids or diesters thereof are referred to as dicarboxylate units and the units of the chain of the polyester or copolyester derived from diols are referred to as diol units.

[0051] Suitable glycol-modified PETs include those available from SK Chemicals (Gyeonggi-do, South Korea) under the tradename ECOZEN, such as the ECOZEN T-series which includes ECOZEN T120, and include PCTg and EASTAR GN071 (both available from Eastman Chemical Company, Knoxville, TN), for example. Glycol-modified PET can be described as PET with at least some of the glycol units replaced with different units. For example, for PCTg and GN071 the different units can be derived from cyclohexanedimethanol and for ECOZEN T 120 the different units can be derived from isosorbide and cyclohexanedimethanol. Glycol-modified PETs that include isosorbide units typically have different dispersions (refractive index as a function of wavelength) than those of glycol-modified PETs that do not include isosorbide units. In some embodiments, including isosorbide units in glycol-modified PET of the LIO layers can result in improved index matching to the HIO layers which may be PEN or coPEN layers, for example.

[0052] In some embodiments, a reflective polarizer includes a plurality of alternating first (e.g., HIO layers) and second (e.g., LIO layers) polymeric layers where at least some of the second polymeric layers comprise a glycol-modified polyester terephthalate comprising isosorbide units. In some embodiments, a reflective polarizer includes a plurality of optical repeat units where each optical repeat unit includes a first polymeric layer (e.g., HIO layer) and at least one second polymeric layer (e.g., LIO layer) and where for at least some of the optical repeat units, the at least one second polymeric layer includes at least one layer comprising glycol-modified polyester terephthalate comprising isosorbide units. In some embodiments, a reflective polarizer includes a plurality of optical repeat units where each optical repeat unit includes at least one birefringent first polymeric layer (e g., HIO layer) and at least one substantially optically isotropic second polymeric layer (e g., LIO layer) and where for at least some of the optical repeat units, the at least one substantially optically isotropic second polymeric layer includes at least one layer comprising glycol-modified polyester terephthalate comprising isosorbide units. The glycol- modified polyester terephthalate can comprise the isosorbide units at greater than 10, 15, 20, or 25 mole percent based on total moles of diol units. The glycol-modified polyester terephthalate may further comprise cyclohexanedimethanol units at greaterthan 15, 20, 25, or 30 mole percent based on total moles of diol units. The glycol-modified polyester terephthalate may further comprise ethylene glycol units at greater than 5, 10, 15, or 20 mole percent based on total moles of diol units. In any of these cases, the first polymeric layers (e.g., HIO layers), or at least some of the first polymeric layers, can comprise a polyester or copolyester comprising naphthalate units (e.g., PEN or a coPEN described elsewhere herein). In some embodiments, at least some of the first polymeric layers comprise a copolyester comprising naphthalate and terephthalate units.

[0053] CoPEN 90 / 10 can be described a copolyester where carboxylate units of the copolyester comprises 90 mole percent naphthalate units and 10 mole percent terephthalate units. More generally, the copolyester can include other ratios of naphthalate and terephthalate units. For example, in some embodiments, carboxylate units of the copolyester can include about 80 to 95 mole percent naphthalate units and about 5 to 20 mole percent terephthalate units. Such copolyesters may be denoted coPEN N / 100-N where N is the mole percent naphthalate units and 100-N is the mole percent terephthalate units. In some embodiments, each HIO layer includes coPEN N / 100-N or a blend (e.g., different blends in different HIO layers) of coPEN N / 100-N and at least one other polyester or other polymer. In some embodiments, each LIO layer includes glycol-modified PET, a blend of different glycol-modified PETs, or a blend of at least one glycol-modified PET and at least one coPEN N / 100-N. For example, in some embodiments, each HIO layer includes at least one coPEN N / 100-N, and each LIO layer includes at least one glycol-modified PET and at least some of the LIO layers includes a blend of at least one glycol- modified PET and at least one coPEN N / 100-N. In some embodiments, the at least one glycol-modified PET includes isosorbide units as described further elsewhere herein.

[0054] In other embodiments, other polymers are used for the HIO and LIO layers. For example, the HIO layers can include any of the birefringent polymers described in the multilayer optical film references provided elsewhere herein and the LIO layers can include any of the substantially isotropic polymers described in these references where the LIO material is chosen to approximately index match the HIO material along the pass axis and at least two different LIO materials can be used in at least two different LIO layers (and / or at least two different HIO materials can be used in at least two different HIO layers) in order to achieve improved pass state index matching for at least two different wavelengths as described further elsewhere herein.

[0055] In some embodiments, for a substantially normally (e.g., within 25, 20, 15, 10, or 5 degrees of normal) incident light 12 and for a predetermined wavelength range at least about 50, 75, 100, 125, 150, 175, or 200 nm wide, the plurality of optical repeat units (or the plurality of alternating first and second polymeric layers) has an average optical reflectance of greater than about 60 percent when the incident light is polarized along a first axis (e.g., corresponding to polarization state 141 polarized along the x- axis) and an average optical reflectance of less than about 2 percent when the incident light is polarized along a second axis (e.g., corresponding to polarization state 142 polarized along the y-axis) orthogonal to the first axis. In some embodiments, the average optical reflectance is greater than about 70, 80, 90, 95, 96, or 97 percent when the incident light is polarized along the first axis. In some embodiments, the average optical reflectance is less than about 1.5, 1, 0.5, 0.4, 0.3, 0.2, 0.1, 0.075, 0.05, 0.04, 0.03, 0.025, or 0.02 percent when the incident light is polarized along the second axis. The predetermined wavelength range can extend at least from about 420 nm to about 680 nm or can be in another range described elsewhere herein. In some embodiments, for the substantially normally incident light 12 and for each wavelength in a wavelength range from about 450 nm to about 600 nm, the plurality of optical repeat units (or the plurality of alternating first and second polymeric layers) has an optical reflectance of less than about 0.1, 0.075, 0.05, 0.04, 0.03, 0.025, or 0.02 percent when the incident light is polarized along the second axis. The first and second axes are typically in-plane (in the plane of the reflective polarizer) axes. The reflectance and transmittance of the plurality of optical layers (the plurality of optical repeat units or the plurality of alternating first and second polymeric layers) of the reflective polarizer generally corresponds to immersed reflectance and transmittance (reflectance and transmittance determined with the reflective polarizer immersed in a medium having an index matching that of the outer layers of the reflective polarizer). The reflectance and transmittance measured with the reflective polarizer immersed between layers (e.g., glass layers) having a refractive index of 1.51 (e.g., at a wavelength of 633 nm) is typically a good approximation for the reflectance and transmittance of the plurality of optical layers or the reflective polarizer.

[0056] FIG. 10 is a plot of reflectance of the plurality of optical layers of a reflective polarizer versus wavelength for normally incident light for two orthogonal polarization states, according to some embodiments. FIGS. 11-13 are plots of pass state reflectance of the plurality of optical layers of reflective polarizers versus wavelength for substantially normally incident light, according to some embodiments. For comparison, the pass state reflectance of the plurality of optical layers of a conventional reflective polarizer film that utilizes a single LIO material is shown in FIGS. 11-13.

[0057] FIGS. 10-11 show results for reflectance of the optical layers of a reflective polarizer including two different LIO compositions in two different packets (e.g., as schematically illustrated in FIG. 2 for reflective polarizer 200) determined via standard optical modeling techniques. The HIO layers of both packets were modeled as LmPEN and the LIO layers of the different packets were different blends of ECOZEN T120 and LmPEN (85 wt. % ECOZEN T120 and 15 wt. % LmPEN for one packet and 90 wt. % ECOZEN T120 and 10 wt. % LmPEN for the other packet) resulting in the LIO1 and LIO2 refractive indices of FIG. 1.

[0058] FIG. 12 show results for reflectance of the optical layers of a reflective polarizer including LIO layers having varying compositions (e g., as schematically illustrated in FIG. 3 for reflective polarizer 200’ with layers 11 having variable compositions which can include blends of ECOZEN T120 and LmPEN that vary in the z-direction). The layer thickness profile of the optical layers of the reflective polarizer of FIG. 12 is shown in FIG. 7.

[0059] FIG. 13 show results for reflectance of the optical layers of a reflective polarizer including three different LIO layers comprising two different LIO materials in each unit cell (e.g., as schematically illustrated in FIG. 4 for reflective polarizer 200” with layers 11 and 11’ having a same composition and layers 11” having a different composition such as a different blend of ECOZEN T120 and LmPEN) determined via standard optical modeling techniques. The layer thickness profile of the optical layers of the reflective polarizer of FIG. 13 is shown in FIG. 9.

[0060] The reflective polarizers of the present description may be 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 manufactured separately and then subsequently joined. Integrally formed includes manufacturing a first element followed by manufacturing the second element on the first element. An optical film including a plurality of layers is integrally formed if the layers are manufactured together (e.g., combined as melt streams and then cast onto a chill roll to form a cast film having each of the layers, and then orienting the cast film) rather than manufactured separately and then subsequently joined.

[0061] The reflective polarizers of the present description may be used in an optical construction including at least one optical lens and / or may be used in an optical system, for example. For example, the reflective polarizer can be used in an optical system where a very low pass state reflectance (e.g., less than about 0.1, 0.075, 0.05, 0.04, 0.03, 0.025, or 0.02 percent) is desired.

[0062] FIG. 14 is a schematic cross-sectional view of an optical construction 301, according to some embodiments. The optical construction 301 includes at least first and second lenses 40 and 41 and a reflective polarizer 60 and a retarder layer 80 disposed therebetween. The reflective polarizer 60 can correspond to any of the reflective polarizers described herein. The retarder layer 80 can be a quarterwave retarder for at least one wavelength in a wavelength range from about 400 nm to about 700 nm, for example. The optical construction 301 can be a bonded optical construction where each layer of the construction is bonded to at least one adjacent layer. For example, an (e.g., optically clear) adhesive 45 may bond (e.g., indirectly through other layers) the first and second lenses 40 and 41 together. The optical construction 301 can be considered to be an optical system or can be a portion of an optical system.

[0063] FIG. 15 is a schematic cross-sectional view of an optical system 300, according to some embodiments. In some embodiments, the optical system 300 includes a partial reflector 50 and a reflective polarizer 60 and can include at least one lens 40, 41. The partial reflector 50 and reflective polarizer 60 define an optical cavity 70 therebetween. The optical system 300 includes a retarder layer 80 disposed in the optical cavity. The reflective polarizer 60 can correspond to any of the reflective polarizers described elsewhere herein. The optical system 300 can include a display 30 and can be configured to form a virtual image 32 of an image 31 emitted by the display 30 for viewing by a viewer 43 (e.g., as generally described in in U.S. Pat. No. 10,678,052 (Ouderkirk, et al.), for example). The display 30, the at least one lens 40, 41, the partial reflector 50, the reflective polarizer 60, and the retarder layer 80 can be disposed along an optical axis 211 of the optical system 300. In some embodiments, the at least one lens includes first and second lenses 40 and 41, and the reflective polarizer is disposed between, and bonded to, the first and second lens. The at least one lens, the reflective polarizer 60, the retarder layer 80 and optionally the partial reflector 50 can be included in a bonded optical construction (e.g., corresponding to optical construction 301, optionally with partial reflector 50 disposed on, and attached to, a major surface of lens 40 opposite lens 41). In some embodiments, the partial reflector 50 has an average optical reflectance in a range of about 20% to about 80%, or about 30% to about 70%, or about 40% to about 60% in a wavelength range extending from about 420 nm to about 680 nm (e.g., the partial reflector can be a half-silvered mirror). In some embodiments, the optical system 300 is substantially centered on an optical axis 211.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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 reflective polarizer comprising: a plurality of alternating first and second polymeric layers numbering at least 10 in total, each layer of the first and second polymeric layers having an average thickness of less than about 500 nm, such that for substantially normally incident light and for a predetermined wavelength range at least 0 nm wide, the plurality of alternating first and second polymeric layers has an average optical reflectance of greater than about 60 percent when the incident light is polarized along a first axis and an average optical reflectance of less than about 2 percent when the incident light is polarized along a second axis orthogonal to the first axis, the first layers having a refractive index along the first axis greater than a refractive index of the second layers along the first axis by at least about 0.05 for at least one wavelength in the predetermined wavelength range, wherein for different first and second wavelengths in the predetermined wavelength range and for first and second pairs of adjacent first and second polymeric layers, the first and second pairs spaced apart along a thickness direction of the reflective polarizer: the first and second polymeric layers of the first pair, but not the second pair, have refractive indices along the second axis that differ by less than about 0.002 for the first wavelength; and the first and second layers of the second pair, but not the first pair, have refractive indices along the second axis that differ by less than about 0.002 for the second wavelength.

2. The reflective polarizer of claim 1, wherein for the first and second pairs of adjacent first and second polymeric layers: the first polymeric layers of each of the first and second pairs have a same composition; and the second polymeric layers of the first and second pairs have different compositions.

3. The reflective polarizer of claim 1, wherein for the first and second pairs of adjacent first and second polymeric layers: the first polymeric layers of each of the first and second pairs have different compositions; and the second polymeric layers of the first and second pairs have a same composition.

4. The reflective polarizer of claim 1, wherein the plurality of alternating first and second polymeric layers comprises first and second pluralities of alternating first and second polymeric layers, each of the first and second pluralities comprising at least 10 layers in total, wherein for each of the first and second pluralities, the first layers have a same first composition and the second layers have a same second composition different than the first composition.

5. The reflective polarizer of claim 4, wherein the first composition for the first and second pluralities are a same composition and the second composition for the first and second pluralities are different compositions.

6. The reflective polarizer of claim 4, wherein the first composition for the first and second pluralities are different compositions and the second composition for the first and second pluralities are a same composition.

7. The reflective polarizer of claim 1, wherein the plurality of alternating first and second polymeric layers comprises a first plurality of alternating first and second polymeric layers comprising at least 10 layers in total, such that for at least one for the first polymeric layers of the first plurality or the second polymeric layers of the first plurality, the polymeric layers have a composition that varies continuously along the thickness direction of the reflective polarizer.

8. A reflective polarizer comprising: a plurality of optical repeat units, the optical repeat units in the plurality of optical repeat units numbering greater than 5 in total, each optical repeat unit compnsing a first polymeric layer and at least one second polymeric layer, each of the first and second polymeric layers of each optical repeat unit having an average thickness of less than about 500 nm, such that for substantially normally incident light and for a predetermined wavelength range at least 50 nm wide, the plurality of optical repeat units has an average optical reflectance of greater than about 60 percent when the incident light is polarized along a first axis and an average optical reflectance of less than about 2 percent when the incident light is polarized along a second axis orthogonal to the first axis, wherein for first and second optical repeat units spaced apart along a thickness direction of the reflective polarizer, and for different first and second wavelengths in the predetermined wavelength range: for each of the first and second optical repeat units, the first layer has refractive indices nix and nly along the respective first and second axes and the at least one second layer has thickness-weighted average refractive indices n2x and n2y along the respective first and second axes; for the first optical repeat unit, but not the second optical repeat unit, nly and n2y differ by less than about 0.002 for the first wavelength; for the second optical repeat unit, but not the first optical repeat unit, nly and n2y differ by less than about 0.002 for the second wavelength; and for each of the first and second optical repeat units and for each of the first and second wavelengths, nix and n2x differ by at least about 0.05.

9. The reflective polarizer of claim 8, wherein for each of the first and second optical repeat units, the first layer comprises a first composition and the at least one second layer comprises a plurality of secondlayers comprising at least two different compositions, each of the at least two different compositions being different from the first composition.

10. The reflective polarizer of claim 9, wherein for each of the first and second optical repeat units, the at least one second layer comprises at least three second layers.

11. The reflective polarizer of claim 10, wherein the at least three second layers comprise at least two layers comprising a same second composition and at least one layer comprising a same third composition different from the second composition, the at least two different compositions comprising the second and third compositions.

12. The reflective polarizer of claim 8, wherein the at least one second polymeric layer comprises at least two second polymeric layers, a ratio of a thickness of a corresponding layer of the at least two second polymeric layers to a total thickness of the at least two second polymeric layers varies continuously along the thickness direction of the reflective polarizer over at least a portion of the reflective polarizer, the portion comprising at least 5 of the optical repeat units.

13. The reflective polarizer of claim 8, wherein a corresponding layer of the at least one second polymeric layer of the optical repeat units has a thickness that continuously decreases along the thickness direction over at least a portion of the reflective polarizer, the portion comprising at least 5 of the optical repeat units, the first polymeric layer of the optical repeat units having a thickness that continuously increases along the thickness direction over the portion of the reflective polarizer.

14. A reflective polarizer comprising: a plurality of optical repeat units, the optical repeat units in the plurality of optical repeat units numbering greater than 5 in total, each optical repeat unit comprising at least one birefringent first polymeric layer and at least one substantially optically isotropic second polymeric layer, each of the first and second polymeric layers of each optical repeat unit having an average thickness of less than about 500 nm, such that for substantially normally incident light and for a predetermined wavelength range at least 50 nm wide, the plurality of optical repeat units has an average optical reflectance of greater than about 60 percent when the incident light is polarized along a first axis and an average optical reflectance of less than about 2 percent when the incident light is polarized along a second axis orthogonal to the first axis, wherein for first and second optical repeat units spaced apart along a thickness direction of the reflective polarizer, and for different first and second wavelengths in the predetermined wavelength range: for each of the first and second optical repeat units, the at least one birefringent first polymeric layer and the at least one substantially optically isotropic second polymeric layer have respectivethickness-weighted average refractive indices nix and n2x along the first axis and respective thickness- weighted average refractive indices nly and n2y along the second axis; for the first optical repeat unit, but not the second optical repeat unit, nly and n2y differ by less than about 0.002 for the first wavelength; for the second optical repeat unit, but not the first optical repeat unit, nly and n2y differ by less than about 0.002 for the second wavelength; and for each of the first and second optical repeat units and for each of the first and second wavelengths, nix is greater than n2x by at least about 0.

05.

15. An optical system comprising : the reflective polarizer of any one of claims 1 to 14; a partial reflector, the partial reflector and the reflective polarizer defining an optical cavity therebetween; and a retarder layer disposed in the optical cavity.

Images