Multilayer partial mirror, backlight, and display system

Abstract

The multilayer partial mirror includes a plurality of alternating first and second polymer layers, totaling at least 50, disposed between and integrally formed with opposing first and second polymer skin layers. For incident light propagating in a visible wavelength range spanning from about 420 nm to about 680 nm and in a plane of incidence including the x-direction, for s-polarized incident light, the multilayer partial mirror has an average reflectivity Rsl for a first angle of incidence less than about 10 degrees and an average reflectivity Rs2 for a second angle of incidence greater than about 45 degrees, and for p-polarized incident light, the multilayer partial mirror has an average reflectivity Rpl for the first angle of incidence and an average reflectivity Rp2 for the second angle of incidence. Each of Rs2 / Rsl and Rp2 / Rpl is greater than about 1.15.

Description

[Technical field]

[0001] The present disclosure relates generally to multi-layer partial mirrors, and more particularly to multi-layer partial mirrors for backlighting display panels. [Background technology]

[0002] Typically, collimating reflective polarizers are used in display devices to eliminate optical artifacts such as reflective moiré, however, such collimating reflective polarizers can reduce the axial brightness of the display device. Summary of the Invention

[0003] In a first aspect, the present disclosure provides a multi-layer partial mirror. The multi-layer partial mirror includes a plurality of alternating first and second polymer layers totaling at least 50. The plurality of alternating first and second layers are disposed between and integrally formed with opposing first and second polymer skin layers. Each of the first and second polymer layers has an average thickness greater than about 0.5 microns. The first and second polymer layers have the same respective refractive indices nx1 and nx2 along an in-plane x-direction, ny1 and ny2 along an in-plane y-direction orthogonal to the x-direction, and nz1 and nz2 along a z-direction of the polymer layers orthogonal to the x-direction and y-direction. For at least one wavelength in the visible wavelength range spanning from about 420 nanometers (nm) to about 680 nm, nx1 and ny1 are within about 0.05 of each other. For at least one wavelength in the visible wavelength range, nz1 is at least 0.06 less than each of nx1 and nx2. For at least one wavelength in the visible wavelength range and for each of the first and second polymer skin layers, the corresponding refractive indices of one of the polymer skin layers and the first and second polymer layers along each of the x-, y-, and z-directions are within about 0.05 of each other. For incident light propagating in a plane of incidence that includes the visible wavelength range and the x-direction, the plurality of alternating first and second polymer layers have an average reflectivity Rs1 for a first angle of incidence less than about 10 degrees, and an average reflectivity Rs2 for a second angle of incidence greater than about 45 degrees, for s-polarized incident light. Furthermore, for incident light propagating in a plane of incidence that includes the visible wavelength range and the x-direction, the plurality of alternating first and second polymer layers have an average reflectivity Rp1 for a first angle of incidence, and an average reflectivity Rp2 for a second angle of incidence, for p-polarized incident light. Rs2 / Rs1 and Rp2 / Rp1 are each greater than about 1.15.

[0004] In a second aspect, the present disclosure provides a backlight for providing illumination to a display panel configured to form an image. The backlight includes an extended light source configured to emit light through an emission surface thereof. The extended light source further includes a back reflector. The backlight further includes a multi-layer partial mirror of the first aspect. The multi-layer partial mirror is disposed on the emission surface and configured to receive the emitted light and transmit a portion of the received light. The multi-layer partial mirror and the back reflector form a recycling optical cavity therebetween.

[0005] In a third aspect, the present disclosure provides a display system including a display panel disposed on the backlight of the second aspect.

[0006] In a fourth aspect, the present disclosure provides a multi-layer partial mirror. The multi-layer partial mirror includes a plurality of alternating first and second polymer layers totaling at least 50. Each of the first and second polymer layers has an average thickness less than about 500 nm. Furthermore, the first and second polymer layers have respective refractive indices nx1 and nx2 along the same in-plane x-direction, ny1 and ny2 along the in-plane y-direction perpendicular to the x-direction, and nz1 and nz2 along the z-direction of the polymer layers perpendicular to the x-direction and y-direction. For at least one wavelength in the visible wavelength range spanning from about 420 nm to at least about 680 nm, nz1 is at least 0.06 less than each of nx1 and nx2. For at least one wavelength in the visible wavelength range, the magnitude of the difference between nx2 and ((nx1+nz1) / 2) is less than about 0.05. Further, for at least one wavelength in the visible wavelength range, nx2 and nz2 are within about 0.05 of each other. In the visible wavelength range, in the infrared wavelength range of about 800 nm to about 1300 nm, for incident light propagating in a plane of incidence that includes the x direction, and at a first angle of incidence less than about 10 degrees, the plurality of alternating first and second polymer layers have an average reflectance Rp1 for incident light polarized along the x direction and an average reflectance Rs1 for incident light polarized along the y direction. Each of Rp1 and Rs1 is less than about 85% in the visible wavelength range and less than about 50% in the infrared wavelength range. Further, in the visible wavelength range, in the infrared wavelength range, for incident light propagating in a plane of incidence, and at a second angle of incidence greater than about 45 degrees, the plurality of alternating first and second polymer layers have average reflectances Rp2 and Rs2 for p-polarized incident light and s-polarized incident light, respectively. Each of Rp2 and Rs2 is greater than about 70% in the visible wavelength range.

[0007] In a fifth aspect, the present disclosure provides a multi-layer partial mirror. The multi-layer partial mirror includes a plurality of alternating first and second polymer layers totaling about 50-600. The plurality of alternating first and second polymer layers are disposed between opposed integrally formed first and second polymer skin layers. Each polymer layer disposed between the first and second polymer skin layers has an average thickness less than about 500 nm. For at least one wavelength in the visible wavelength range spanning from about 420 nm to about 680 nm, each of the first and second polymer layers has an in-plane birefringence less than about 0.05. For at least one wavelength in the visible wavelength range, the first polymer layer has an out-of-plane birefringence greater than about 0.06. Additionally, for at least one wavelength in the visible wavelength range, the second polymer layer has an out-of-plane birefringence less than about 0.02. For incident light propagating in a plane of incidence that includes a visible wavelength range and a first direction in the same plane, the multilayer partial mirror has an average reflectance Rs1 for s-polarized incident light and an average reflectance Rp1 for p-polarized incident light at a first angle of incidence less than about 10 degrees, where Rp1 / Rs1 is from about 0.8 to about 1.2. The optical reflectance versus wavelength of the multilayer partial mirror for the first angle of incidence and each of the p-polarized and s-polarized incident light includes reflectance band edges from about 760 nm to about 980 nm. [Brief description of the drawings]

[0008] Exemplary embodiments disclosed herein will be more fully understood upon consideration of the following Detailed Description in conjunction with the following figures. The figures are not necessarily drawn to scale. Like numbers used in the figures refer to like components. However, it will be understood that the use of a number to refer to a component in a particular figure is not intended to limit the component in another figure bearing the same number.

[0009] [Figure 1] FIG. 2 is a detailed schematic cross-sectional view of a multi-layer partial mirror according to one embodiment of the present disclosure. [Figure 2A]1 is a plot illustrating the variation in thickness of multiple alternating first and second polymer layers of a multi-layer partial mirror, according to one embodiment of the present disclosure. [Figure 2B] FIG. 2B is an enlarged view of a portion of FIG. 2A according to one embodiment of the present disclosure. [Figure 3A] 1 is a schematic cross-sectional view of a multi-layer partial mirror according to an embodiment of the present disclosure. [Figure 3B] 1 is a schematic cross-sectional view of a multi-layer partial mirror according to an embodiment of the present disclosure. [Figure 4A] 1 is a plot showing optical reflectivity versus wavelength for a multi-layer partial mirror for s-polarized incident light incident at a first incidence angle in accordance with an embodiment of the present disclosure. [Figure 4B] 1 is a plot showing optical reflectivity versus wavelength for a multi-layer partial mirror for p-polarized incident light incident at a first incidence angle in accordance with an embodiment of the present disclosure. [Figure 4C] 11 is a plot showing optical reflectivity versus wavelength for a multi-layer partial mirror for s-polarized incident light incident at a second incidence angle in accordance with an embodiment of the present disclosure. [Figure 4D] 11 is a plot showing optical reflectivity versus wavelength for a multi-layer partial mirror for p-polarized incident light incident at a second incidence angle in accordance with an embodiment of the present disclosure. [Figure 5A] 1 is a detailed schematic cross-sectional view of a display system including a multi-layer partial mirror according to one embodiment of the present disclosure. [Figure 5B] 1 is a schematic diagram of a reflective polarizer of a display system according to one embodiment of the present disclosure. [Figure 5C] 5C is a plot showing the optical properties of the reflective polarizer of FIG. 5B according to one embodiment of the present disclosure. [Figure 6A] FIG. 2 is a detailed schematic cross-sectional view of an optical system including a multi-layer partial mirror, according to one embodiment of the present disclosure. [Figure 6B] FIG. 6B is a schematic diagram of a reflective polarizer of the optical system of FIG. 6A according to one embodiment of the present disclosure. [Figure 6C] FIG. 6B illustrates an emission distribution of light exiting the optical system of FIG. 6A according to one embodiment of the present disclosure. [Figure 6D]6B is a plot of the relative intensity of light exiting the optical system of FIG. 6A according to one embodiment of the present disclosure. [Figure 7A] 1 is an exemplary plot of layer thicknesses for a conventional collimating multilayer optical film (CMOF) and samples A, B, and C. [Figure 7B] FIG. 7B is an enlarged view of a portion of the plot of FIG. 7A. [Figure 8A] 1 is an exemplary plot showing optical reflectance versus wavelength for Sample A and CMOF for s-polarized incident light incident at a first angle of incidence. [Figure 8B] 1 is an exemplary plot showing optical reflectance versus wavelength for Samples B and C for s-polarized incident light incident at a first angle of incidence. [Figure 9A] 1 is another exemplary plot showing optical reflectance versus wavelength for Sample A and CMOF for p-polarized incident light incident at a first angle of incidence. [Figure 9B] 11 is another exemplary plot showing optical reflectance versus wavelength for Samples B and C for wavelengths of p-polarized incident light incident at a first angle of incidence. [Figure 10A] 13 is another exemplary plot showing optical reflectance versus wavelength for Sample A and CMOF for wavelength of s-polarized incident light incident at a second angle of incidence. [Figure 10B] 13 is another exemplary plot showing optical reflectance versus wavelength for Samples B and C for s-polarized incident light incident at a second angle of incidence. [Figure 11A] 13 is another exemplary plot showing optical reflectance versus wavelength for Sample A and CMOF for p-polarized incident light incident at a second angle of incidence. [Figure 11B] 13 is another exemplary plot showing optical reflectance versus wavelength for Samples B and C for p-polarized incident light incident at a second angle of incidence. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which are shown by way of illustration various embodiments. 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.

[0011] In the disclosure that follows, the following definitions apply:

[0012] All numbers recited herein are intended to be modified by the term "about." As used herein, "a," "an," "the," "at least one," and "one or more" are used interchangeably.

[0013] The term "generally," when used herein as a modifier to a characteristic or attribute, unless specifically defined otherwise, means that the characteristic or attribute is one that would be readily recognized by one of ordinary skill in the art, but does not require absolute precision or perfect agreement (e.g., within ±20% for quantifiable characteristics).

[0014] The term "substantially," unless specifically defined otherwise, means a close degree of approximation (e.g., within ±10% for quantifiable characteristics), but again does not require absolute precision or perfect agreement.

[0015] The term "about," unless specifically defined otherwise, means a high degree of approximation (e.g., within ±5% for quantifiable properties), but again does not require absolute precision or exact correspondence.

[0016] As used herein, the terms "first" and "second" are used as identifiers. Thus, such terms should not be interpreted as limiting the present disclosure. When used in conjunction with features or elements, the terms "first" and "second" can be interchanged throughout the embodiments of the present disclosure.

[0017] As used herein, when a first material is said to be "similar" to a second material, at least 90% by weight of the first and second materials are identical, and if there is a variation between the first and second materials, the variation is less than about 10% by weight of each of the first and second materials.

[0018] As used herein, "at least one of A and B" should be understood to mean "A only, B only, or both A and B."

[0019] As used herein, the term "film" generally refers to a material with a very high ratio of length or width to thickness. A film has two major surfaces defined by a length and a width. A film typically has good flexibility and can be used in a wide variety of applications, including displays. A film can also be of a thickness or material composition such that it is semi-rigid or rigid. The films described in this disclosure can be constructed from a variety of polymeric materials. A film can be a single layer, a multilayer, or a blend of different polymers.

[0020] As used herein, the term "layer" generally refers to a thickness of material having a relatively consistent chemical composition within a film. A layer may be any type of material, including polymeric, cellulosic, metallic, or blends thereof. A given polymer layer may include a single polymer type or a blend of polymers, and may include additives. A given layer may be bonded or connected to other layers to form a film. A layer may be partially or completely continuous compared to an adjacent layer or film. A given layer may be partially or completely coextensive with an adjacent layer. A layer may include sub-layers.

[0021] As used herein, the term "between about" generally refers to an inclusive or closed range unless specifically defined otherwise. For example, if a parameter X is between about A and B, then A≦X≦B.

[0022] As used herein, the term "index" generally refers to the refractive index of a material or layer unless specifically defined otherwise. Similarly, the term "refractive index" generally refers to the refractive index of multiple materials or layers unless specifically defined otherwise.

[0023] The present disclosure relates to a multi-layer partial mirror and a backlight including the multi-layer partial mirror. The backlight including the multi-layer partial mirror may be used in a display system or an optical system. For example, the multi-layer partial mirror may be used to backlight a display device. The display device may be incorporated into an electronic device including a computer monitor, a television, a mobile phone, a personal digital assistant (PDA), a wearable device, and other portable devices. Some other examples may include an optical biometric scanner, such as a fingerprint scanner or a retina scanner.

[0024] In some examples, a display device including a liquid crystal display (LCD) panel further includes a backlight, since the LCD panel is not self-illuminating. Light emitted by the backlight passes through the LCD panel to reach the viewer. In some other examples, the display device may include an organic light-emitting display (OLED). In still other examples, the display device may include a display, such as a liquid crystal on silicon (LCoS) display or a micro light-emitting diode (LED) display. In some examples, the display device may be used for augmented reality (AR) or virtual reality (VR) applications. Typically, the light emitted by the display is recycled to reduce the power consumption of the display device. To recycle the light, a collimating reflective polarizer, such as a collimating multilayer optical film (CMOF), may be used.

[0025] Display devices may also be susceptible to optical artifacts such as reflective moiré. A collimating reflective polarizer may further be used in the display device to eliminate such optical artifacts. Specifically, a collimating reflective polarizer may be placed between a diffuser and a display panel of the display device to eliminate such optical artifacts. However, the diffuser may depolarize the light polarized by the collimating reflective polarizer. In such cases, an additional non-collimating reflective polarizer (APF) is generally used to repolarize the light. However, this may reduce the axial brightness of the display device. Furthermore, CMOFs typically collimate light polarized along one direction and substantially reflect light polarized along one other direction orthogonal to the one direction. As a result, light polarized along one other direction is not collimated, resulting in reduced axial brightness. In some cases, the power consumption of the display device may be increased to increase the axial brightness. This may have a negative impact on the battery life of electronic devices that include the display device.

[0026] In one aspect, the present disclosure provides a multi-layer partial mirror. The multi-layer partial mirror includes a plurality of alternating first and second polymer layers totaling at least 50. The plurality of alternating first and second layers are disposed between and integrally formed with opposing first and second polymer skin layers. Each of the first and second polymer layers has an average thickness greater than about 0.5 microns. The first and second polymer layers have the same respective refractive indices nx1 and nx2 along an in-plane x-direction, ny1 and ny2 along an in-plane y-direction orthogonal to the x-direction, and nz1 and nz2 along a z-direction of the polymer layers orthogonal to the x-direction and y-direction. For at least one wavelength in the visible wavelength range spanning from about 420 nanometers (nm) to about 680 nm, nx1 and ny1 are within about 0.05 of each other. For at least one wavelength in the visible wavelength range, nz1 is at least 0.06 less than each of nx1 and nx2. For at least one wavelength in the visible wavelength range, for each of the first and second polymer skin layers, the corresponding refractive indices of the polymer skin layers and one of the first and second polymer layers along each of the x-, y-, and z-directions are within about 0.05 of each other. For incident light propagating in a visible wavelength range and in a plane of incidence that includes the x-direction, the plurality of alternating first and second polymer layers have an average reflectivity Rs1 for a first angle of incidence less than about 10 degrees, and an average reflectivity Rs2 for a second angle of incidence greater than about 45 degrees, for s-polarized incident light. Furthermore, for incident light propagating in a visible wavelength range and in a plane of incidence, the plurality of alternating first and second polymer layers have an average reflectivity Rp1 for a first angle of incidence, and an average reflectivity Rp2 for a second angle of incidence, for p-polarized incident light. Rs2 / Rs1 and Rp2 / Rp1 are each greater than about 1.15.

[0027] Thus, for the visible wavelength range and for each of s-polarized light and p-polarized light, the multilayer partial mirror has a higher average light reflectance for light incident at a second angle of incidence greater than about 45 degrees (i.e., off-axis light) than for light incident at a first angle of incidence less than about 10 degrees (i.e., substantially normal incidence or on-axis light). In other words, for the visible wavelength range and for each of s-polarized light and p-polarized light, the multilayer partial mirror has a higher light transmittance for on-axis light than for off-axis light. In the visible wavelength range, the multilayer partial mirror may substantially collimate the off-axis light for both s-polarized light and p-polarized light, so the multilayer partial mirror may provide improved collimation over conventional collimating films that typically reflect only one of s-polarized light and p-polarized light incident at the second angle of incidence. Furthermore, in the visible wavelength range, the multilayer partial mirror may have a lower average light reflectance for on-axis light compared to conventional collimating films that substantially block even on-axis light polarized along one direction (one of s-polarized light and p-polarized light). Optical reflectance for on-axis light is generally undesirable as it can reduce on-axis brightness. Thus, in the visible wavelength range, a lower average optical reflectance for on-axis light can improve the axial brightness of a display system that includes a multi-layer partial mirror.

[0028] Furthermore, the reflection band edges of the multi-layer partial mirror may be tuned to pass light in the infrared wavelength range, thereby enabling display devices including the multi-layer partial mirror for fingerprint sensing applications.

[0029] Referring to the drawings, FIG. 1 is a detailed schematic diagram of a multi-layer partial mirror 10 according to an embodiment of the present disclosure. The multi-layer partial mirror 10 may be interchangeably referred to as "mirror 10". The mirror 10 defines mutually orthogonal x-, y- and z-directions. The x- and y-directions are in-plane axes of the mirror 10, while the z-direction is a transverse axis disposed along the thickness of the mirror 10. In other words, the x- and y-directions are along the plane of the mirror 10 (i.e., the xy-plane), and the z-direction is perpendicular to the plane of the mirror 10, i.e., along the thickness of the mirror 10. The z-direction may be interchangeably referred to as "thickness direction". The mirror 10 further defines mutually orthogonal first and second directions. In some embodiments, the first and second directions are along the in-plane axes of the mirror 10. In other words, the first direction corresponds to the x-direction and the second direction corresponds to the y-direction.

[0030] In some embodiments, mirror 10 includes opposing first and second major surfaces 101, 102. In some embodiments, first and second major surfaces 101, 102 of mirror 10 are exposed to an external environment. In such embodiments, first and second major surfaces 101, 102 of mirror 10 may form a first mirror-environment interface and a second mirror-environment interface, respectively, with respect to the external environment. In some embodiments, the external environment includes air.

[0031] Reflective polarizer 10 includes a plurality of alternating first and second polymer layers 11 and 12. The plurality of alternating first and second polymer layers 11 and 12 may be referred to interchangeably as "alternating first and second polymer layers 11 and 12," or "first and second polymer layers 11 and 12," or "polymer layers 11, 12."

[0032] In some embodiments, the plurality of alternating first polymer layers 11 and second polymer layers 12 totals at least 50. In some other embodiments, the plurality of alternating first polymer layers 11 and second polymer layers 12 totals at least 100, at least 200, at least 300, or at least 400. In some embodiments, the plurality of alternating first polymer layers 11 and second polymer layers 12 totals at least 50 to 600. In some embodiments, the plurality of alternating first polymer layers 11 and second polymer layers 12 totals about 100 to about 500, about 200 to about 450, or about 300 to about 450. As shown in FIG. 1, the plurality of alternating first polymer layers 11 and second polymer layers 12 are stacked in an alternating configuration along the thickness direction of the mirror 10.

[0033] In some embodiments, the multilayer partial mirror (such as mirror 10) may be a multilayer optical film (MOF) as previously demonstrated by coextrusion of alternating polymer layers. See, for example, U.S. Pat. Nos. 3,610,729 (Rogers), 4,446,305 (Rogers et al.), 4,540,623 (Im et al.), 5,448,404 (Schrenk et al.), and 5,882,774 (Jonza et al.). In these polymeric multilayer optical films, the individual layers (e.g., first polymer layer 11 and second polymer layer 12) are fabricated mostly or exclusively using polymeric materials. Moreover, such multilayer optical films may be compatible with mass production processes and may be manufactured and stored as large sheets and roll products.

[0034] A multilayer optical film includes layers having different refractive index characteristics, such that at least a portion of light incident on a layer is reflected at the interfaces between adjacent layers. The layers are generally thin enough that the light reflected at the interfaces between adjacent layers undergoes constructive or destructive interference to give the multilayer optical film the desired reflective or transmissive properties. For multilayer optical films designed to reflect ultraviolet, visible, or near infrared wavelength light, each of the layers generally has an optical thickness (physical thickness of the layer times the corresponding refractive index) of less than about 1 μm. Layers having a greater optical thickness may be included, such as skin layers on the outer surfaces of the multilayer optical film, or protective boundary layers (PBLs) disposed within the multilayer optical film that separate coherent groups (hereinafter referred to as "packets") of layers.

[0035] For polarizing applications, e.g., reflective polarizers, at least some of the layers may comprise a birefringent polymer, in which the refractive indices of the layers have different values ​​along the axes of the polymer's Cartesian coordinate system. Birefringent polymers can also be used in non-polarizing applications.

[0036] In some cases, the layers have thicknesses and refractive indices corresponding to a quarter-wave stack, i.e., arranged in an optical repeat unit or unit cell, with each optical repeat unit or unit cell having two adjacent layers with substantially equal optical thickness (f ratio=50%). Such an optical repeat unit reflects light whose wavelength λ is twice the total optical thickness of the optical repeat unit by constructive interference. Other layer configurations are also known, such as multilayer optical films having two optical repeat units with f ratios different from 50%, or multilayer optical films in which the optical repeat unit includes three or more layers. Multilayer optical films having such optical repeat units can be configured to reduce or increase certain high-order reflections, see, for example, U.S. Pat. Nos. 5,360,659 (Arend et al.) and 5,103,337 (Schrenk et al.). A thickness gradient along the thickness direction (e.g., z-direction) of a multilayer optical film can be used to provide an extended reflection band, such as a reflection band that extends over a significant portion of the visible wavelength range, and may include the near infrared wavelength range, such that the layer has a reflectance across the visible wavelength range for light incident at oblique angles of incidence as the reflection band shifts to shorter wavelengths. Thickness gradients tailored to sharpen the band edges, i.e., the wavelength transition between high reflection and high transmission, are described in U.S. Patent No. 6,157,490 (Wheatley et al.).

[0037] Further details of the multilayer optical films and related designs and structures may be as described in U.S. Pat. Nos. 5,882,774 (Jonza et al.) and 6,531,230 (Weber et al.), PCT Publication Nos. WO 95 / 17303 (Ouderkirk et al.) and WO 99 / 39224 (Ouderkirk et al.), and the publication entitled "Giant Birefringent Optics in Multilayer Polymer Mirrors", Science, Vol. 287, March 2000 (Weber et al.). The multilayer optical films and related articles may include additional layers and coatings selected for their optical, mechanical, and / or chemical properties. For example, an ultraviolet (UV) absorbing layer may be added to the light-incident side of the multilayer optical film to protect the components of the multilayer optical film from degradation caused by UV light. The multilayer optical film may be attached to the mechanical reinforcement layer using a UV-curable acrylate adhesive or other suitable material. Such reinforcing layers may include polymers such as polyethylene terephthalate (PET) or polycarbonate, and may also include structured surfaces that provide optical functionality such as light diffusion or collimation, for example, by using beads or prisms. Multilayer optical films may further include additional layers and coatings such as scratch-resistant layers, tear-resistant layers, and hardeners, see, for example, U.S. Patent No. 6,368,699 (Golbert et al.). Methods and apparatus for making multilayer optical films are described in U.S. Patent No. 6,783,349 (Neavin et al.).

[0038] In general, the reflective and transmissive properties of a multilayer optical film (such as mirror 10) can be a function of the refractive index (e.g., nx1, ny1, nz1, nx2, ny2, nz2, etc.) of each layer (e.g., first polymer layer 11 and second polymer layer 12), the average thickness (e.g., average thickness t), and the layer thickness variation (e.g., as described in Figures 2A and 2B). Each layer can be characterized, at least at a local location within the multilayer optical film, by an in-plane index of refraction (e.g., nx1, ny1, nx2, ny2) and an index of refraction associated with the thickness direction of the multilayer optical film (e.g., nz1, nz2). These indices can represent the corresponding indices of refraction of the layers of the multilayer optical film for light polarized along the mutually orthogonal x and y directions, respectively.

[0039] In some cases, the layers of the multilayer optical film may be oriented along the x-direction of the multilayer optical film, so that the magnitude of the difference between nx1 and nx2 (Δnx) has a maximum value. For ease of explanation in this patent application, unless otherwise specified, the x-direction, y-direction, and z-direction are axes of a local Cartesian coordinate system applicable to any point of interest on the multilayer optical film, the layers extend parallel to the xy plane, and the x-axis is oriented in the plane of the multilayer optical film to maximize the magnitude of Δnx. In this case, the magnitude of the difference between ny1 and ny2 (Δny) may be equal to or smaller than Δnx, but does not exceed Δnx. Furthermore, the material of the first layer of the multilayer optical film may be defined by requiring that Δnx is non-negative. In other words, the material of the layer may be selected such that nx1 is equal to or greater than nx2, and therefore Δnx≧0. In practice, the refractive index of the layers of the multilayer optical film may be controlled by appropriate material selection and processing conditions.

[0040] Multilayer optical films may be made by coextruding multiple (e.g., tens or hundreds) of alternating layers of polymers A, B, optionally followed by passing the multilayer extrudate of alternating polymers A, B through one or more intensifiers and then stretching or otherwise orienting the multilayer extrudate to form the final film. The resulting film is typically composed of multiple alternating layers whose thicknesses and refractive indices are tailored to provide one or more reflection bands in a desired region of the electromagnetic spectrum, such as the visible or near infrared.

[0041] In some cases, to achieve a target reflectivity for light polarized along one direction (such as the x-direction), the alternating layers may be tailored to exhibit a difference in their refractive index (e.g., Δnx) for light polarized along one direction of at least about 0.04. In such cases, to further achieve a target reflectivity for light polarized along a direction orthogonal to one direction (such as the y-direction), the alternating layers may be tailored to exhibit a difference in their refractive index (e.g., Δny) for light polarized along a direction orthogonal to one direction of at least about 0.05. The above-referenced '774 (Jonza et al.) patent describes, among other things, how the difference between nz1 and nz2 (Δnz) can be tailored to achieve a desired reflectivity for obliquely incident p-polarized light. In some cases, to maintain a high reflectivity for obliquely incident p-polarized light, the alternating layers may be prepared such that Δnz can be substantially less than the maximum in-plane refractive index difference (i.e., Δnz). In some cases, Δnz≦0.5*Δnx, or Δnz≦0.25*Δnx. In some cases, Δnz may have a value of 0 or close to 0, which may result in interfaces between alternating layers whose reflectivity for p-polarized light may remain substantially constant as a function of the angle of incidence of p-polarized light. In some cases, Δnz may be less than 0, which may result in interfaces whose reflectivity for p-polarized light increases with increasing angle of incidence of p-polarized light. In some cases, the values ​​of Δnx, Δny, and Δnz may be similarly adjusted to achieve a target reflectivity for s-polarized light.

[0042] The '774 (Jonza et al.) patent further describes certain design considerations for multilayer optical films that can be configured as polarizers, called multilayer reflective or reflective polarizers. In many applications, an ideal reflective polarizer may have substantially high reflectance along one axis (the "extinction" or "block" axis) and substantially zero reflectance along the other axis (the "transmission" or "pass" axis). For purposes of this application, light whose polarization state is substantially aligned with the pass or transmission axis is referred to as passed light, and light whose polarization state is substantially aligned with the block or extinction axis is referred to as blocked light. Unless otherwise specified, passed light at an incidence angle of 60° is measured with p-polarized passed light. If any reflectance occurs along the transmission axis, the efficiency of the polarizer at off-normal angles is reduced, and color may be introduced into the transmitted light if the reflectance for various wavelengths is different. Furthermore, exact matching of the two y and two z indices may not be possible in some multilayer systems, and it may be desirable to introduce a slight mismatch in the in-plane indices n1y and n2y when the z-axis indices are mismatched. In particular, by configuring the y-index mismatch to have the same sign as the z-index mismatch, a Brewster effect occurs at the microlayer interfaces to minimize off-axis reflectance along the transmission axis of the multilayer reflective polarizer, and therefore off-axis color.

[0043] In some examples, a method for making multiple layers includes: (a) providing at least a first stream and a second stream of resin corresponding to a first polymer A and a second polymer B to be used in the finished film; and (b) splitting the first and second streams into multiple layers using a suitable feedblock, the feedblock including: (i) a gradient plate including a first flow channel and a second flow channel, the first channel having a cross-sectional area that varies from a first location to a second location along the flow channel; and (ii) a feed tube plate having a first plurality of conduits in fluid communication with the first flow channel and a second plurality of conduits in fluid communication with the second flow channel, the first plurality of conduits being in fluid communication with the second flow channel. (c) passing the composite stream through an extrusion die to form a multilayer web, each layer being generally parallel to a major surface of an adjacent layer; and (d) casting the multilayer web onto a chill roll, sometimes referred to as a casting wheel or casting drum, to form a cast multilayer film. The cast film may have the same number of layers as the finished film, although the layers of the cast film are typically much thicker than those of the finished film.

[0044] After cooling, the multilayer web can be reheated and stretched or drawn to produce a near-finished multilayer optical film. The stretching or drawing accomplishes two objectives: thinning the layers to the desired final thickness profile and orienting the layers so that at least some of them are birefringent. The directing or drawing can be accomplished simultaneously or sequentially along the cross-web direction (e.g., by a tenter), along the down-web direction (e.g., by a length orienter), or any combination thereof. If drawn in only one direction, the drawing can be "unconstrained" (the film can relax dimensionally in the in-plane direction perpendicular to the drawing direction) or "constrained" (the film is constrained and therefore cannot relax dimensionally in the in-plane direction perpendicular to the drawing direction). If drawn along both in-plane directions, the drawing can be symmetric, i.e., equal along the orthogonal in-plane directions, or asymmetric. Alternatively, the film can be drawn in a batch process. In either case, subsequent or simultaneous draw reduction, stress or strain balancing, heat setting, and other processing operations may also be applied to the film.

[0045] The polymers of the alternating layers may be selected to have similar rheological properties, e.g., melt viscosities, so that they can be coextruded without significant flow disturbances. Extrusion conditions may be selected to adequately feed, melt, mix, and pump the respective polymers as feed and melt streams in a continuous and stable manner. The temperatures used to form and maintain each of the melt streams may be selected within a range that avoids freezing, crystallization, or excessive pressure drop at the lower end of the temperature range, and material degradation at the upper end of the temperature range.

[0046] Multilayer optical films can be manufactured using any suitable light-transmitting material, but it can often be beneficial to use low-absorption polymeric materials. Such materials can provide low or negligible absorption of layers across visible and infrared wavelengths, so that the sum of the reflectance (R) and transmittance (T) of the multilayer optical film is approximately 100%, i.e., R+T≈100%, or R≈100%-T, for any given wavelength and any particular angle of incidence and polarization of incident light incident on the multilayer optical film. Exemplary multilayer optical films can be constructed of polymeric materials and manufactured using coextrusion, casting, and orientation processes. Reference is made to U.S. Pat. No. 5,882,774 (Jonza et al.) "Optical Film," U.S. Pat. No. 6,179,948 (Merrill et al.) "Optical Film and Process for Manufacture Thereof," U.S. Pat. No. 6,783,349 (Neavin et al.) "Apparatus for Making Multilayer Optical Films," and U.S. Patent Application Publication No. 2011 / 0272849 (Neavin et al.) "Feedblock for Manufacturing Multilayer Polymeric Films."

[0047] In some embodiments, the polymer layers 11, 12 may comprise one or more polymeric materials, such as polyhexylethylene naphthalate (PHEN), polyethylene naphthalate (PEN), copolymers containing PHEN, PEN, and / or other polyesters (e.g., polyethylene terephthalate (PET) or polyesters containing dibenzoic acid), glycol-modified polyethylene terephthalate, polycarbonate (PC), poly(methyl methacrylate) (PMMA), or blends of materials from these classes.

[0048] In some embodiments, the first polymer layer 11 may comprise a high index optical (HIO) layer. In some examples, the first polymer layer 11 may comprise a PHEN copolymer having a resulting percentage of hexanediol of about 30 mol % to about 70 mol % based on the diol in the final polymer.

[0049] In some embodiments, the second polymer layer 12 may comprise a low index optical (LIO) layer. In some examples, the second polymer layer 12 may comprise a PHEN copolymer with a resulting percentage of hexanediol of about 30 mol % to 70 mol % based on diol in the final polymer. In some examples, the second polymer layer may comprise a blend of a PHEN copolymer and PET, where the level of PET is 30% to 70% by weight of the final polymer.

[0050] The first polymer layer 11 and the second polymer layer 12 have respective refractive indices nx1 and nx2 along the same in-plane x-direction. Specifically, the first polymer layer 11 includes a refractive index nx1 along the in-plane x-direction, and the second polymer layer 12 includes a refractive index nx2 along the same in-plane x-direction. In other words, each first polymer layer 11 includes a refractive index nx1 along the x-direction. Each second polymer layer 12 includes a refractive index nx2 along the x-direction.

[0051] In some embodiments, the refractive index nx1 is about 1.6 to about 1.76, i.e., 1.6≦nx1≦1.76. In some examples, the refractive index nx1 can be about 1.690, about 1.739, or about 1.740.

[0052] In some embodiments, the refractive index nx2 is about 1.55 to about 1.75, for example, 1.55≦nx2≦1.75. In some examples, the refractive index nx2 can be about 1.620, about 1.628, or about 1.629.

[0053] The first polymer layer 11 and the second polymer layer 12 have respective refractive indices ny1 and ny2 along an in-plane y direction perpendicular to the x direction. Specifically, the first polymer layer 11 includes a refractive index ny1 along the in-plane y direction, and the second polymer layer 12 includes a refractive index ny2 along the in-plane y direction. In other words, each first polymer layer 11 includes a refractive index ny1 along the y direction. Each second polymer layer 12 includes a refractive index ny2 along the y direction.

[0054] In some embodiments, the refractive index ny1 is about 1.6 to about 1.76, i.e., 1.6≦ny1≦1.76 In some examples, the refractive index ny1 can be about 1.695, about 1.739, or about 1.740.

[0055] In some embodiments, the refractive index ny2 is from about 1.55 to about 1.75, i.e., 1.55≦ny2≦1.75. In some examples, the refractive index ny2 can be about 1.620, 1.628, or about 1.629.

[0056] The first polymer layer 11 and the second polymer layer 12 further include respective refractive indices nz1 and nz2 along a thickness direction of the polymer layers 11, 12 perpendicular to the x-direction and the y-direction. Specifically, the first polymer layer 11 includes a refractive index nz1 along the thickness direction, and the second polymer layer 12 includes a refractive index nz2 along the thickness direction. In other words, each first polymer layer 11 includes a refractive index nz1 along the z-direction. Each second polymer layer 12 includes a refractive index nz2 along the z-direction.

[0057] In some embodiments, the refractive index nz1 is about 1.5 to about 1.55, i.e., 1.5≦nz1≦1.55. In some examples, the refractive index nz1 can be about 1.510, about 1.513, or about 1.514.

[0058] In some embodiments, the refractive index nz2 is about 1.6 to about 1.7, i.e., 1.6≦nz2≦1.7 In some examples, the refractive index nz2 can be about 1.620, about 1.623, or about 1.624.

[0059] In some embodiments, for at least one wavelength in the visible wavelength range 95 (shown in FIGS. 4A-4D ) ranging from about 420 nanometers (nm) to about 680 nm, nx1 and ny1 are within about 0.05 of each other. In some embodiments, the at least one wavelength in the visible wavelength range 95 is a red wavelength. In some embodiments, the at least one wavelength in the visible wavelength range 95 is about 633 nm. In other words, for at least one wavelength in the visible wavelength range 95, the magnitude of the difference between nx1 and ny1 may be about 0.05 or less, i.e., |nx1-ny1|≦0.05. Thus, in some cases, nx1 and ny1 may be substantially equal. In other words, the first polymer layer 11 may have an in-plane birefringence of less than about 0.05. In general, birefringence refers to a measure of optical anisotropy in a material. Furthermore, birefringence is measured as the difference between two refractive indices of a material along two mutually perpendicular directions (e.g., the x-direction and the y-direction). In some embodiments, for at least one wavelength within the visible wavelength range 95, nx1 and ny1 are within about 0.02, within about 0.015, within about 0.01, within about 0.0075, within about 0.005, within about 0.004, within about 0.003, or within about 0.002 of each other.

[0060] In some embodiments, for at least one wavelength in the visible wavelength range 95, nz1 is at least 0.06 less than each of nx1 and nx2. In other words, the difference between nx1 and nz1 may be 0.06 or more, i.e., (nx1-nz1) ≧ 0.06, and the difference between nx2 and nz1 may be 0.06 or more, i.e., (nx2-nz1) ≧ 0.06. In some embodiments, for at least one wavelength in the visible wavelength range 95, nz1 is at least 0.08, at least 0.1, at least 0.12, at least 0.14, at least 0.16, at least 0.18, at least 0.20, at least 0.21, or at least 0.22 less than each of nx1 and nx2. Thus, for at least one wavelength in the visible wavelength range 95, the first polymer layer 11 has an out-of-plane birefringence of greater than about 0.06.

[0061] In some embodiments, for at least one wavelength in the visible wavelength range 95, nx2, ny2, and nz2 are within about 0.05 of each other. In other words, the magnitude of the difference between nx2 and ny2 may be about 0.05 or less, i.e., |nx2-ny2|≦0.05, the magnitude of the difference between ny2 and nz2 may be about 0.05 or less, i.e., |ny2-nz2|≦0.05, and the magnitude of the difference between nz2 and nx2 may be about 0.05 or less, i.e., |nz2-nx2|≦0.05. In some embodiments, for at least one wavelength in the visible wavelength range 95, nx2, ny2, and nz2 are within about 0.02, about 0.015, about 0.01, about 0.0075, about 0.005, or about 0.0025 of each other. Thus, in some cases, nx2, ny2, and nz2 may be substantially equal to each other. Additionally, in some instances, the second polymer layer 12 may be isotropic.

[0062] In some embodiments, for at least one wavelength in the visible wavelength range 95, the magnitude of the difference between nx2 and ((nx1+nz1) / 2) is less than about 0.05. In other words, the magnitude of the difference between nx2 and the average of nx1 and nz1 is less than about 0.05, i.e., |nx2-((nx1+nz1) / 2)|<0.05. In some embodiments, for at least one wavelength in the visible wavelength range 95, the magnitude of the difference between nx2 and ((nx1+nz1) / 2) is less than about 0.045, less than about 0.04, less than about 0.035, less than about 0.03, less than about 0.025, less than about 0.02, less than about 0.015, or less than about 0.01. Thus, in some cases, nx2 may be substantially equal to the average of nx1 and nz1.

[0063] In some embodiments, for at least one wavelength in the visible wavelength range 95, each of the first polymer layer 11 and the second polymer layer 12 has an in-plane birefringence of less than about 0.05. Specifically, for at least one wavelength in the visible wavelength range 95, each of the first polymer layer 11 has an in-plane birefringence of less than about 0.05, i.e., |nx1-ny1|<0.05, and each of the second polymer layers 12 has an in-plane birefringence of less than about 0.05, i.e., |nx2-ny2|<0.05. In some embodiments, for at least one wavelength in the visible wavelength range 95, each of the first polymer layer 11 and the second polymer layer 12 has an in-plane birefringence of less than about 0.4, less than about 0.03, less than about 0.02, or less than about 0.01. In some examples, |nx1-ny1| may be about 0.005. In some examples, |nx2-ny2| may be about 0.

[0064] In some embodiments, for at least one wavelength in the visible wavelength range 95, the first polymer layer 11 has an out-of-plane birefringence greater than about 0.06. Specifically, for at least one wavelength in the visible wavelength range 95, the first polymer layer 11 has an out-of-plane birefringence greater than about 0.06 in the z direction, i.e., |nx1-nz1|>0.6 and |ny1-nz1|>0.6. In some embodiments, for at least one wavelength in the visible wavelength range 95, the first polymer layer 11 has an out-of-plane birefringence greater than about 0.08, greater than about 0.12, greater than about 0.14, or greater than about 0.16. In some examples, |nx1-nz1| may be about 0.18. In some examples, |ny1-nz1| may be about 0.185.

[0065] In some embodiments, for at least one wavelength in the visible wavelength range 95, the second polymer layer 12 has an out-of-plane birefringence of less than about 0.02. Specifically, for at least one wavelength in the visible wavelength range 95, the second polymer layer 12 has an out-of-plane birefringence of less than about 0.02 in the z direction, i.e., |nx2-nz2|<0.02 and |ny2-nz2|<0.02. In some embodiments, for at least one wavelength in the visible wavelength range 95, the second polymer layer 11 has an out-of-plane birefringence of less than about 0.015, less than about 0.01, less than about 0.005, or less than about 0.002. In some examples, |nx2-nz2| may be about 0. In some examples, |ny2-nz2| may be about 0. In some examples, the second polymer layer 12 may be isotropic.

[0066] The mirror 10 further includes opposing first and second polymer skin layers 13, 14. The first and second polymer skin layers 13, 14 may be collectively referred to as "polymer skin layers 13, 14." Each of the first and second polymer layers 11, 12 is disposed between the first and second polymer skin layers 13, 14. In other words, the first and second polymer skin layers 13, 14 comprise opposing major surfaces of the mirror 10. In some embodiments, the first and second polymer skin layers 13, 14 comprise respective first and second major surfaces 101, 102. Thus, in some embodiments, the first and second polymer skin layers 13, 14 form respective first and second mirror-to-environment interfaces with respect to the external environment. In some cases, the mirror 10 includes integrally formed opposing first and second polymer skin layers 13, 14. A plurality of alternating first and second polymer layers 11, 12 are disposed between the integrally formed opposing first and second polymer skin layers 13, 14. In some cases, a plurality of alternating first and second polymer layers 11, 12 are disposed between and integrally formed with the opposing first and second polymer skin layers 13, 14.

[0067] Each of the polymeric skin layers 13, 14 has an average thickness "ts". Each of the polymeric skin layers 13, 14 defines an average thickness "ts" along the z-direction. As used herein, the term "average thickness" refers to the average thickness along the plane of the layer (i.e., the xy-plane). Each of the polymeric skin layers 13, 14 has an average thickness "ts" greater than about 0.5 microns. In some embodiments, each of the polymeric skin layers 13, 14 has an average thickness "ts" greater than about 0.75 microns, greater than about 1 micron, greater than about 2 microns, greater than about 5 microns, greater than about 6 microns, greater than about 7 microns, greater than about 8 microns, greater than about 9 microns, greater than about 10 microns, greater than about 12 microns, or greater than about 15 microns.

[0068] In some embodiments, the first polymer skin layer 13 and the second polymer skin layer 14 have substantially the same material composition. In some embodiments, the first polymer layer 11 and at least one of the first polymer skin layer 13 and the second polymer skin layer 14 have substantially the same material composition. In some embodiments, the second polymer layer 12 and at least one of the first polymer skin layer 13 and the second polymer skin layer 14 have substantially the same material composition.

[0069] In some embodiments, the first polymer skin layer 13 and the second polymer skin layer 14 may comprise one or more polymeric materials, such as PHEN, PEN, copolymers containing PHEN, PEN, and / or other polyesters (e.g., PET or polyesters containing dibenzoic acid), glycol modified polyethylene terephthalate, PC, PMMA, or blends of these classes of materials.

[0070] In some embodiments, for at least one wavelength in the visible wavelength range 95, the corresponding refractive indices of the polymer skin layers 13, 14 and one of the first polymer layer 11 and second polymer layer 12 along each of the x-, y-, and z-directions for each of the first polymer skin layer 13 and second polymer skin layer 14 are within about 0.05 of each other. In some embodiments, for at least one wavelength in the visible wavelength range 95, the corresponding refractive indices of the polymer skin layers 13, 14 and one of the first polymer layer 11 and second polymer layer 12 along each of the x-, y-, and z-directions for each of the first polymer skin layer 13 and second polymer skin layer 14 are within about 0.02, within about 0.015, within about 0.01, within about 0.0075, within about 0.005, or within about 0.0025 of each other.

[0071] As discussed above, in some embodiments, the first polymer layer 11 and at least one of the first polymer skin layer 13 and the second polymer skin layer 14 have substantially the same material composition. Thus, in some embodiments, for each of the first polymer skin layer 13 and the second polymer skin layer 14, the corresponding refractive indices of the polymer skin layers 13, 14 and the first polymer layer 11 along each of the x-, y-, and z-directions are substantially similar. Specifically, for each of the first polymer skin layer 13 and the second polymer skin layer 14, the corresponding refractive indices of the polymer skin layers 13, 14 and the first polymer layer 11 along each of the x-, y-, and z-directions are within about 0.05 of each other. In some embodiments, for each of the first polymer skin layer 13 and the second polymer skin layer 14, the corresponding refractive indices of the polymer skin layers 13, 14 and the first polymer layer 11 along each of the x-, y-, and z-directions are within about 0.02, within about 0.015, within about 0.01, within about 0.0075, within about 0.005, or within about 0.0025 of each other.

[0072] As discussed above, in some embodiments, the second polymer layer 12 and at least one of the first polymer skin layer 13 and the second polymer skin layer 14 have substantially the same material composition. Thus, in some embodiments, for each of the first polymer skin layer 13 and the second polymer skin layer 14, the corresponding refractive indices of the polymer skin layers 13, 14 and the second polymer layer 12 along each of the x-, y-, and z-directions are substantially similar. Specifically, for each of the first polymer skin layer 13 and the second polymer skin layer 14, the corresponding refractive indices of the polymer skin layers 13, 14 and the second polymer layer 12 along each of the x-, y-, and z-directions are within about 0.05 of each other. In some embodiments, for each of the first polymer skin layer 13 and the second polymer skin layer 14, the corresponding refractive indices of the polymer skin layers 13, 14 and the second polymer layer 12 along each of the x-, y-, and z-directions are within about 0.02, within about 0.015, within about 0.01, within about 0.0075, within about 0.005, or within about 0.0025 of each other.

[0073] In some embodiments, the mirror 10 may include at least one intermediate layer 16. In some embodiments, the at least one intermediate layer 16 has an average thickness of more than about 500 nm. In some embodiments, the at least one intermediate layer 16 is disposed between the first layer 11 and the second layer 12 of a plurality of polymers. In particular, the at least one intermediate layer 16 may be disposed between two stacks of the first layer 11 and the second layer 12 of polymers. In the illustrated embodiment of FIG. 1, the at least one intermediate layer 16 is disposed between the first polymer layer 11a and the second polymer layer 12a. In some embodiments, the material composition of the at least one intermediate layer 16 may be substantially similar to the material composition of at least one of the first polymer skin layer 13 and the second polymer skin layer 14.

[0074] In some embodiments, the polymer skin layers 13, 14, the polymer layers 11, 12, and the at least one intermediate layer 16 may be substantially coextensive with one another or may have similar in-plane dimensions (i.e., length and width). In other words, the polymer skin layers 13, 14, the polymer layers 11, 12, and the at least one intermediate layer 16 may be substantially coextensive with one another in the xy plane.

[0075] In some embodiments, mirror 10 may include additional or intermediate layers, such as adhesive layers, substrate layers, etc. Mirror 10 may have any suitable thickness based on desired application attributes.

[0076] FIG. 2A shows a plot 150 illustrating the variation in thickness of the multiple alternating first and second polymer layers 11, 12 of a mirror 10, according to one embodiment of the present disclosure. The thicknesses of the multiple alternating first and second polymer layers 11, 12 are shown in nanometers (nm) on the ordinate and the layer number is shown on the abscissa. Specifically, the plot 150 shows the average layer thickness in nm on the ordinate and the layer number on the abscissa. In the illustrated embodiment of FIG. 2A, the plot 150 shows the variation in thickness of the multiple alternating first and second polymer layers 11, 12 of layers 1-420 of the mirror 10. The plot 150 includes a thickness curve 155 illustrating the variation in thickness of the multiple alternating first and second polymer layers 11, 12 of the mirror 10. FIG. 2B is an expanded view of portion A of plot 150 illustrating the variation in thickness of multiple alternating first polymer layers 11 and second polymer layers 12 of layers 100 - 150 of mirror 10 .

[0077] 1, 2A and 2B, each of the first polymer layer 11 and the second polymer layer 12 has an average thickness "t". Each of the first polymer layer 11 and the second polymer layer 12 defines an average thickness "t" along the z-direction. Each of the first polymer layer 11 and the second polymer layer 12 may be interchangeably referred to as "each polymer layer 11, 12". Each of the first polymer layer 11 and the second polymer layer 12 has an average thickness "t" that is less than about 500 nm. Specifically, each of the polymer layers 11, 12 disposed between the first polymer skin layer 13 and the second polymer skin layer 14 has an average thickness "t" that is less than about 500 nm. In some embodiments, each of the first layer 11 and the second layer 12 has an average thickness "t" that is less than about 400 nm, or less than about 300 nm, or less than about 200 nm.

[0078] As is evident from plot 150, in some embodiments, the difference between the maximum and minimum thicknesses of the alternating first and second polymer layers 11 and 12 is about 45% to 65%. In some embodiments, the difference between the maximum and minimum thicknesses of the alternating first and second polymer layers 11 and 12 may be about 50% to about 60%.

[0079] In the illustrated embodiment of Figure 2A, the maximum and minimum thicknesses of the alternating first and second polymer layers 11 and 12 are 126.7 nm and 57.6 nm, respectively. Thus, the difference between the maximum and minimum thicknesses of the alternating first and second polymer layers 11 and 12 is about 54.5%.

[0080] Further, in some embodiments, the thicknesses of the first polymer layer 11 and the second polymer layer 12 generally increase with layer number up to a threshold layer number of 350 to 400. Further, the thicknesses of the first polymer layer 11 and the second polymer layer 12 generally decrease with layer number after the threshold layer number.

[0081] In some embodiments, the maximum difference between the thicknesses of adjacent polymer layers in the alternating first and second polymer layers 11, 12 is less than about 25%. In some embodiments, the maximum difference between the thicknesses of adjacent polymer layers in the alternating first and second polymer layers 11, 12 is less than about 20%, less than about 15%, or less than about 10%. In the illustrated embodiment of Figures 2A and 2B, the maximum difference between the thicknesses of adjacent polymer layers in the alternating first and second polymer layers 11, 12 is about 10%.

[0082] The difference in thickness between adjacent polymer layers 11, 12 is best seen in Figure 2B. Specifically, thickness curve 155 has multiple V-shaped segments that indicate respective values ​​of the thickness of alternating first polymer layer 11 and second polymer layer 12.

[0083] The thickness of each of the alternating first and second polymer layers 11 and 12 is a distinct value indicated by the peaks or valleys of the respective V-shaped segments. Figure 2B shows the thicknesses of exemplary polymer layers 211, 212, 213, 214, and 215 (211-215). Polymer layers 211, 212, 213, 214, and 215 may correspond to layer numbers 105, 106, 107, 108, and 109 shown in Figure 2B. Polymer layers 211-215 have corresponding thicknesses 201, 202, 203, 204, and 205 (201-205). In this example, polymer layers 211, 213, 215 having respective thicknesses 201, 203, 205 may be first polymer layers 11, and polymer layers 212, 214 having respective thicknesses 202, 204 may be second polymer layers 12, such that the thickness of each of the second polymer layers 12 is less than the thickness of the adjacent subsequent first polymer layer 11.

[0084] However, in some examples, polymer layers 211, 213, 215 having respective thicknesses 201, 203, 205 may be second polymer layers 12, and polymer layers 212, 214 having respective thicknesses 202, 204 may be first polymer layers 11, such that each thickness of the first polymer layers 11 is less than the thickness of the adjacent subsequent second polymer layers 12.

[0085] 3A and 3B are schematic cross-sectional views of a mirror 10 according to one embodiment of the present disclosure, showing incident light 20, 21, respectively, propagating within an input surface 22.

[0086] The incidence surface 22 includes an x-direction. The incidence surface 22 may further include a normal N to the mirror 10. The normal N is substantially perpendicular to the plane of the mirror 10 (i.e., the xy-plane). In other words, the normal N is substantially along the z-direction of the mirror 10. The incidence surface 22 substantially corresponds to the xz-plane of the mirror 10. In some embodiments, the incidence surface 22 includes a first direction in the same plane. In some embodiments, the same plane is the xz-plane of the mirror 10, and the same first direction in the same plane is the x-direction.

[0087] 3A and 3B, each of the incident lights 20, 21 is incident on the mirror 10 at an angle of incidence θ with respect to the normal N. Specifically, each of the incident lights 20, 21 is incident on the mirror 10 at a first major surface 101 (i.e., a first interface between the mirror and the environment). However, in some embodiments, at least one of the incident lights 20, 21 may be incident on the mirror 10 at a second major surface 102 (i.e., a second interface between the mirror and the environment).

[0088] In some cases, the angle of incidence θ may be a first angle of incidence that is less than about 10 degrees. In some embodiments, the first angle of incidence is less than about 9 degrees, less than about 8 degrees, less than about 7 degrees, less than about 6 degrees, less than about 5 degrees, less than about 4 degrees, less than about 3 degrees, less than about 2 degrees, or less than about 1 degree. In some examples, the first angle of incidence is about 0 degrees.

[0089] In some cases, the angle of incidence θ may be a second angle of incidence that is greater than about 45 degrees. In some embodiments, the second angle of incidence is greater than about 50 degrees, or greater than about 55 degrees. In some embodiments, the second angle of incidence is about 60 degrees.

[0090] 3A, the incident light 20 is s-polarized. The incident light 20 may also be interchangeably referred to as "s-polarized incident light 20" or "s-polarized incident light 20." The incident light 20 is polarized along a second direction. In some embodiments, the second direction may be along the y direction. Thus, the incident light 20 is polarized along the y direction. The incident light 20 may also be interchangeably referred to as "incident light 20 polarized along the y direction."

[0091] 3B, the incident light 21 is p-polarized. The incident light 21 may also be interchangeably referred to as "p-polarized incident light 21" or "p-polarized incident light 21". Specifically, when the incident light 21 is incident at a first angle of incidence, the incident light 21 is polarized along a first direction. In some embodiments, the first direction may be along the x-direction. Thus, when the incident light 21 is incident at a first angle of incidence, the incident light 21 may be polarized along the x-direction. The incident light 21 may also be interchangeably referred to as "incident light 21 polarized along the x-direction".

[0092] 4A illustrates a plot 400 showing optical reflectance versus wavelength of a multilayer partial mirror 10 (shown in FIG. 1 ) for s-polarized incident light 20 (shown in FIG. 3A ) incident at a first angle of incidence, according to one embodiment of the present disclosure. Specifically, plot 400 illustrates optical reflectance versus wavelength of a plurality of alternating first polymer layers 11 and second polymer layers 12 for s-polarized incident light 20 incident at a first angle of incidence less than about 10 degrees. Wavelength is represented in nanometers (nm) on the abscissa. Optical reflectance is represented as percent reflectance on the left ordinate. Optical transmittance is represented as percent transmittance on the right ordinate. Percent transmittance is approximated to be complementary to percent reflectance, i.e., transmittance=(100-percent reflectance).

[0093] Plot 400 includes a curve 30 corresponding to optical reflectance versus wavelength for multilayer partial mirror 10 for s-polarized incident light 20 incident at a first angle of incidence. In some cases, in plot 400, the first angle of incidence is about 0 degrees. Curve 30 may be interchangeably referred to as "optical reflectance versus wavelength 30." Optical reflectance may be interchangeably referred to as "reflectance."

[0094] 1, 3A, 3B, and 4A, for incident light 20 propagating within an incident surface 22 that includes a visible wavelength range 95, an infrared wavelength range 96 of about 800 nm to about 1300 nm, and an x-direction, and at a first angle of incidence less than about 10 degrees, the multiple alternating first polymer layers 11 and second polymer layers 12 have an average reflectivity Rs1 for incident light 20 polarized along the y-direction. Specifically, for incident light 20 propagating within an incident surface 22 that includes a visible wavelength range 95, an infrared wavelength range 96, and the same in-plane first direction (i.e., the x-direction), at a first angle of incidence less than about 10 degrees, the mirror 10 has an average reflectivity Rs1 for s-polarized incident light 20. In other words, for incident light 20 propagating within an incidence plane 22 that includes the visible wavelength range 95, the infrared wavelength range 96, and the x-direction, and for s-polarized incident light 20, the multiple alternating first polymer layers 11 and second polymer layers 12 have an average reflectivity Rs1 for a first angle of incidence that is less than about 10 degrees.

[0095] In some embodiments, for the visible wavelength range 95, Rs1 is less than about 85%. In some embodiments, for the visible wavelength range 95, Rs1 is less than about 80%, less than about 75%, less than about 70%, or less than about 65%.

[0096] Further, in some embodiments, relative to the visible wavelength range 95, Rs1 is greater than about 50%. In some embodiments, relative to the visible wavelength range 95, Rs1 is greater than about 55%, greater than about 60%, greater than about 65%, or greater than about 70%. Thus, in some embodiments, relative to the visible wavelength range 95, Rs1 is between about 50% and about 85%. In some examples, relative to the visible wavelength range 95, Rs1 is about 64% or about 71.7%.

[0097] Further, in some embodiments, for s-polarized incident light 20 propagating in a plane of incidence 22 that includes an infrared wavelength range 96 and an x-direction, at a first angle of incidence, the multiple alternating first and second polymer layers 11 and 12 have an average transmittance Ts1 that is greater than about 50%. In some embodiments, for s-polarized incident light 20 propagating in a plane of incidence 22 that includes an infrared wavelength range 96 and an x-direction, at a first angle of incidence, the multiple alternating first and second polymer layers 11 and 12 have an average transmittance Ts1 that is greater than about 60%, greater than about 70%, greater than about 75%, or greater than about 80%. In some examples, Ts1 is about 81% or about 83.7%.

[0098] Further, as shown in plot 400, for the first angle of incidence and s-polarized incident light 20, the optical reflectance versus wavelength 30 includes a reflection band edge 32 from about 760 nm to about 980 nm. In some embodiments, for the first angle of incidence and s-polarized incident light 20, the optical reflectance versus wavelength 30 includes a reflection band edge 32 from about 780 nm to about 960 nm, or from about 800 nm to about 940 nm.

[0099] 4B illustrates a plot 410 showing optical reflectance versus wavelength of a multilayer partial mirror 10 (shown in FIG. 1 ) for p-polarized incident light 21 (shown in FIG. 3B ) incident at a first angle of incidence, according to one embodiment of the present disclosure. Specifically, plot 410 illustrates optical reflectance versus wavelength of a plurality of alternating first polymer layers 11 and second polymer layers 12 for p-polarized incident light 21 incident at a first angle of incidence less than about 10 degrees. Wavelength is represented in nanometers (nm) on the abscissa. Optical reflectance is represented as percent reflectance on the left ordinate. Optical transmittance is represented as percent transmittance on the right ordinate. Percent transmittance is approximated to be complementary to percent reflectance, i.e., transmittance=(100-percent reflectance).

[0100] Plot 410 includes a curve 31 corresponding to optical reflectance versus wavelength for multilayer partial mirror 10 for p-polarized incident light 21 incident at a first angle of incidence. In some cases, in plot 410, the first angle of incidence is about 0 degrees. Curve 31 may be interchangeably referred to as "optical reflectance versus wavelength 31."

[0101] 1, 3A, 3B, and 4B, for incident light 21 propagating in a plane of incidence 22 that includes a visible wavelength range 95, an infrared wavelength range 96, and an x-direction, at a first angle of incidence less than about 10 degrees, the multiple alternating first and second polymer layers 11, 12 have an average reflectivity Rp1 for incident light 21 polarized along the x-direction. Specifically, for incident light 21 propagating in a plane of incidence 22 that includes a visible wavelength range 95, an infrared wavelength range 96, and the same in-plane first direction, at a first angle of incidence less than about 10 degrees, the mirror 10 has an average reflectivity Rp1 for p-polarized incident light 21. In other words, for incident light 21 propagating within an incidence plane 22 that includes a visible wavelength range 95, an infrared wavelength range 96, and an x-direction, for p-polarized incident light 21, the multiple alternating first polymer layers 11 and second polymer layers 12 have an average reflectivity Rp1 for a first angle of incidence.

[0102] In some embodiments, for the visible wavelength range 95, Rp1 is less than about 85%. In some embodiments, for the visible wavelength range 95, Rp1 is less than about 80%, less than about 75%, less than about 70%, or less than about 65%.

[0103] Further, in some embodiments, for the visible wavelength range 95, Rp1 is greater than about 50%. In some embodiments, for the visible wavelength range 95, Rp1 is greater than about 55%, greater than about 60%, greater than about 65%, or greater than about 70%. Thus, in some embodiments, for the visible wavelength range 95, Rp1 is between about 50% and about 85%. In some examples, for the visible wavelength range 95, Rp1 is about 64% or about 71.7%.

[0104] Further, in some embodiments, for p-polarized incident light 21 propagating in an infrared wavelength range 96 and in an incidence plane 22 including the x-direction and at a first incidence angle, the multiple alternating first polymer layers 11 and second polymer layers 12 have an average transmittance Tp1 that is greater than about 50%. In some embodiments, for p-polarized incident light 21 propagating in an infrared wavelength range 96 and in an incidence plane 22 including the x-direction and at a first incidence angle, the multiple alternating first polymer layers 11 and second polymer layers 12 have an average transmittance Tp1 that is greater than about 60%, greater than about 70%, greater than about 75%, or greater than about 80%. In some examples, Tp1 is about 81% or about 83.7%.

[0105] Additionally, as shown in plot 410, the optical reflectance versus wavelength 31 for the first angle of incidence and p-polarized incident light 21 includes a reflection band edge 33 from about 760 nm to about 980 nm. In some embodiments, the optical reflectance versus wavelength 31 for the first angle of incidence and p-polarized incident light 21 includes a reflection band edge 33 from about 780 nm to about 960 nm, or from about 800 nm to about 940 nm.

[0106] 4A and 4B, each of Rp1 and Rs1 is less than about 85% for the visible wavelength range 95. In some embodiments, each of Rp1 and Rs1 is less than about 80%, less than about 75%, less than about 70%, or less than about 65% for the visible wavelength range 95. Further, each of Rp1 and Rs1 is less than about 50% for the infrared wavelength range 96. In some embodiments, each of Rp1 and Rs1 is less than about 45%, less than about 35%, less than about 30%, or less than about 25% for the infrared wavelength range 96.

[0107] Thus, the multiple alternating first and second polymer layers 11, 12 may be substantially transparent at a first angle of incidence for an infrared wavelength range 96 and for each of s-polarized incident light 20 and p-polarized incident light 21. Thus, the multi-layer partial mirror 10 may be suitable for IR fingerprint sensing applications.

[0108] In some embodiments, for the 95 visible wavelength range, Rp1 / Rs1 is about 0.8 to about 1.2. In other words, for the 95 visible wavelength range, the ratio of Rp1 to Rs1 is about 0.8 to about 1.2. In some embodiments, for the 95 visible wavelength range, Rp1 / Rs1 is about 0.85 to about 1.15, about 0.90 to about 1.10, or about 0.95 to about 1.05. In some examples, for the 95 visible wavelength range, Rp1 / Rs1 is about 1.

[0109] In some embodiments, Rp1 and Rs1 are within 10% of each other over the visible wavelength range 95. In some embodiments, Rp1 and Rs1 are within 9%, 8%, 7%, 6%, 5%, 4%, 3%, or 2% of each other over the visible wavelength range 95.

[0110] Thus, for the visible wavelength range 95, the multiple alternating first polymer layers 11 and second polymer layers 12 may have substantially similar average optical reflectivities for each of s-polarized incident light 20 and p-polarized incident light 21 and for a first angle of incidence.

[0111] The optical reflectance versus wavelength 31, 30 for p-polarized incident light 21 and s-polarized incident light 20 includes reflection band edges 33, 31 between about 760 nm and about 980 nm.

[0112] The reflection band edges 32, 33 of the optical reflectance versus wavelength 30, 31 of each of the multi-layer partial mirrors 10 may be tailored based on desired application attributes of the IR fingerprint sensing application.

[0113] 4C illustrates a plot 420 showing optical reflectance versus wavelength of a multilayer partial mirror 10 (shown in FIG. 1 ) for s-polarized incident light 20 (shown in FIG. 3A ) incident on the mirror 10 at a second angle of incidence, according to one embodiment of the present disclosure. Specifically, the plot 420 illustrates optical reflectance versus wavelength of a plurality of alternating first polymer layers 11 and second polymer layers 12 for s-polarized incident light 20 incident at a second angle of incidence greater than about 45 degrees. Wavelength is represented in nanometers (nm) on the abscissa. Optical reflectance is represented as percent reflectance on the left ordinate. Optical transmittance is represented as percent transmittance on the right ordinate. Percent transmittance is approximated to be complementary to percent reflectance, i.e., transmittance=(100-percent reflectance).

[0114] Plot 420 includes a curve 35 corresponding to optical reflectance versus wavelength for multilayer partial mirror 10 for s-polarized incident light 20 incident at a second angle of incidence. In some cases, in plot 420, the second angle of incidence is about 60 degrees. Curve 35 may be interchangeably referred to as "optical reflectance versus wavelength 35."

[0115] 1, 3A, 3B, and 4C, for incident light 20 propagating in an incidence plane 22 including a visible wavelength range 95, an infrared wavelength range 96, and an x-direction, at a second incidence angle greater than about 45 degrees, the multiple alternating first polymer layers 11 and second polymer layers 12 have an average reflectivity Rs2 for incident light 20 polarized along the y-direction. Specifically, for incident light 20 propagating in an incidence plane 22 including a visible wavelength range 95, an infrared wavelength range 96, and an s-polarized incident light 20, the multiple alternating first polymer layers 11 and second polymer layers 12 have an average reflectivity Rs2 for a second incidence angle greater than about 45 degrees.

[0116] In some embodiments, over the visible wavelength range 95, Rs2 is less than about 97%. In some embodiments, over the visible wavelength range 95, Rs2 is less than about 96%, less than about 95%, or less than about 94%.

[0117] Further, in some embodiments, over the visible wavelength range 95, Rs2 is greater than about 70%. In some embodiments, over the visible wavelength range 95, Rs2 is greater than about 75%, greater than about 80%, greater than about 85%, or greater than about 90%. Thus, in some embodiments, over the visible wavelength range 95, Rs2 is between about 70% and about 97%. In some examples, Rs2 is about 92.4% or about 94.8%.

[0118] Further, in some embodiments, for s-polarized incident light 20 propagating in the infrared wavelength range 96 and in the plane of incidence 22 that includes the x-direction, at the second angle of incidence, the plurality of alternating first and second polymer layers 11, 12 have an average transmittance Ts2 of greater than about 40%. In some embodiments, for s-polarized incident light 20 propagating in the infrared wavelength range 96 and in the plane of incidence 22, at the second angle of incidence, the plurality of alternating first and second polymer layers 11, 12 have an average transmittance Ts2 of greater than about 45%, greater than about 50%, greater than about 55%, or greater than about 60%. In some examples, Ts2 is about 64.2% or about 64.6%.

[0119] 4D illustrates a plot 430 showing optical reflectance versus wavelength for a multilayer partial mirror 10 (shown in FIG. 1 ) for p-polarized incident light 21 (shown in FIG. 3B ) incident on the mirror 10 at a second angle of incidence, according to one embodiment of the present disclosure. Specifically, plot 430 illustrates optical reflectance versus wavelength for a plurality of alternating first polymer layers 11 and second polymer layers 12 for p-polarized incident light 21 incident at a second angle of incidence greater than about 45 degrees. Wavelength is represented in nanometers (nm) on the abscissa. Optical reflectance is represented as percent reflectance on the left ordinate. Optical transmittance is represented as percent transmittance on the right ordinate. Percent transmittance is approximated to be complementary to percent reflectance, i.e., transmittance=(100-percent reflectance).

[0120] Plot 430 includes a curve 36 that corresponds to the optical reflectance versus wavelength of the multilayer partial mirror 10 for p-polarized incident light 21 incident at a second angle of incidence. In some cases, in plot 430, the second angle of incidence is about 60 degrees. Curve 36 may be interchangeably referred to as "optical reflectance versus wavelength 36."

[0121] 1, 3A, 3B, and 4D, for incident light 21 propagating in a plane of incidence 22 that includes a visible wavelength range 95, an infrared wavelength range 96, and an x-direction, for p-polarized incident light 21, the multiple alternating first polymer layers 11 and second polymer layers 12 have an average reflectivity Rp2 for a second angle of incidence.

[0122] In some embodiments, for the visible wavelength range 95, Rp2 is less than about 98%. In some embodiments, for the visible wavelength range 95, Rp2 is less than about 97%, less than about 96%, less than about 95%, less than about 94%, or less than about 93%.

[0123] Further, in some embodiments, for the visible wavelength range 95, Rp2 is greater than about 70%. In some embodiments, for the visible wavelength range 95, Rp2 is greater than about 75%, greater than about 80%, greater than about 85%, or greater than about 90%. Thus, in some embodiments, for the visible wavelength range 95, Rp2 is between about 70% and about 98%. In some examples, for the visible wavelength range 95, Rp2 is about 93.3% or about 94.8%.

[0124] Further, in some embodiments, for p-polarized incident light 21 propagating in an incident plane 22 that includes an infrared wavelength range 96 and an x-direction, at the second incident angle, the multiple alternating first polymer layers 11 and second polymer layers 12 have an average transmittance Tp2 of greater than about 70%. In some embodiments, for p-polarized incident light 21 propagating in an infrared wavelength range 96 and an incident plane 22 that includes an x-direction, at the second incident angle, the multiple alternating first polymer layers 11 and second polymer layers 12 have an average transmittance Tp2 of greater than about 75%, greater than about 80%, greater than about 85%, greater than about 90%, or greater than about 95%. In some examples, Tp2 is about 99.5% or about 99.7%.

[0125] 4C and 4D, for incident light 20, 21 propagating in an incidence plane 22 that includes a visible wavelength range 95, an infrared wavelength range 96, and an x-direction, at a second incidence angle greater than about 45 degrees, the multiple alternating first polymer layers 11 and second polymer layers 12 have average reflectivities Rp2 and Rs2 for p- and s-polarized incident light 20, 21, respectively. Further, each of Rp2 and Rs2 is greater than about 70% for the visible wavelength range 95. In some embodiments, each of Rp2 and Rs2 is greater than about 75%, greater than about 80%, greater than about 85%, or greater than about 90% for the visible wavelength range 95.

[0126] Thus, for each of Rp2 and Rs2 and for the visible wavelength range 95, the mirror 10 may substantially reflect off-axis light incident at an angle of incidence greater than about 45 degrees. Specifically, for the visible wavelength range 95, the mirror 10 may substantially reflect each of the s-polarized incident light 20 and the p-polarized incident light 21 at a second angle of incidence.

[0127] 4A-4D, for the visible wavelength range 95, Rs2 / Rs1 and Rp2 / Rp1 are each greater than about 1.15. In some embodiments, for the visible wavelength range 95, Rs2 / Rs1 and Rp2 / Rp1 are each greater than about 1.2, greater than about 1.25, greater than about 1.3, greater than about 1.35, greater than about 1.4, or greater than about 1.45.

[0128] Thus, for the visible wavelength range 95, for each of the s-polarized incident light 20 and the p-polarized incident light 21, the mirror 10 has a higher average optical reflectance for light incident at the second angle of incidence (i.e., off-axis light) than for light incident at the first angle of incidence (i.e., substantially perpendicularly incident or on-axis light). In other words, for each of the s-polarized incident light 20 and the p-polarized incident light 21, for the visible wavelength range 95, the mirror 10 has a higher optical transmittance for on-axis light than for off-axis light. Because the mirror 10 can substantially collimate the off-axis light for both the s-polarized incident light 20 and the p-polarized incident light 21 for the visible wavelength range 95, the mirror 10 may provide improved collimation over conventional collimating films that typically reflect only one of the s-polarized and p-polarized light incident at the second angle of incidence. Furthermore, for the visible wavelength range 95, the mirror 10 may have a lower average light reflectance for on-axis light compared to a conventional collimating film that substantially blocks on-axis light polarized along even one direction (one of s-polarized light 20 and p-polarized light 21). Thus, for the visible wavelength range 95, the lower average light reflectance for on-axis light may provide improved axial brightness.

[0129] Moreover, for an infrared wavelength range 96, the multiple alternating first and second polymer layers 11, 12 are substantially transmissive to s-polarized incident light 20 and p-polarized incident light 21 incident at a first and second angle of incidence, respectively. In other words, the mirror 10 may be substantially transparent to the infrared wavelength range 96. Thus, the mirror 10 may be suitable for IR fingerprint sensing applications.

[0130] In some cases, a display system (not shown) for sensing a user's finger may include a mirror 10 (shown in FIG. 1). An embodiment of such a display system may be described herein. The display system may include a display panel configured to generate an image for a user to view, and a backlight including a light guide for providing illumination to the display panel. The backlight may further include a mirror 10 disposed adjacent to the light guide on an opposite side of the display panel. The mirror 10 may act as a rear reflector for the backlight and may function to recycle light from the light guide toward the display panel. The display system may further include an optical structure disposed between the display panel and the light guide, the optical structure including a reflective polarizer including a plurality of polymer layers and having a first structured major surface disposed between the display panel and the plurality of polymer layers. The display system may further include an infrared sensor disposed opposite the reflective polarizer in proximity to the backlight. The display system may further include an infrared light source configured to emit infrared light toward the display panel. A user's finger may be placed on the display panel, a portion of the infrared light may be reflected from the finger and transmitted through the backlight and mirror 10, and an infrared sensor may detect at least a portion of the infrared light transmitted through the mirror 10. In some cases, the display system may further include a structured mirror disposed between the mirror 10 and the infrared sensor, the structured mirror including an optical mirror, and an array of discrete, spaced apart optical ridges formed on the optical mirror and facing the light guide. For substantially normally incident light, the optical mirror may have an average optical reflectance of greater than about 30% in the visible wavelength range for at least a first polarization state, and a specular transmittance of greater than about 20% at at least one wavelength in the infrared wavelength range for each of the first polarization state and an orthogonal second polarization state.

[0131] 5A is a detailed schematic cross-sectional view of a display system 60 including a mirror 10 according to one embodiment of the present disclosure. Specifically, the display system 60 includes a display panel 41 disposed on a backlight 40. The backlight 40 provides illumination to the display panel 41. The display panel 41 is configured to form an image 42.

[0132] In some embodiments, the display panel 41 includes a liquid crystal display (LCD) panel. The backlight 40 includes an extended light source 43 configured to emit light 44 through its emitting surface 45. The light 44 may be interchangeably referred to as "emitted light 44."

[0133] The backlight 40 further includes a mirror 10. The mirror 10 is disposed on an emitting surface 45. The mirror 10 and a back reflector 46 form a recycling optical cavity 47 therebetween.

[0134] The extended light source 43 further includes a back reflector 46. In some embodiments, the back reflector 46 may be highly reflective. For example, the back reflector 46 may have an on-axis average reflectance of at least 90%, 95%, 98%, 99%, or more. Such reflectance values ​​may reduce the amount of losses in the recycling optical cavity 47. Furthermore, such reflectance values ​​may include both specular and diffuse reflection. In some embodiments, the back reflector 46 may be a predominantly specular reflector, a diffuse reflector, or a combination specular / diffuse reflector, whether spatially uniform or patterned. In some embodiments, the back reflector 46 may be a semi-specular reflector. In some cases, the back reflector 46 may include a rigid metal substrate with a high reflectance coating, or a high reflectance film laminated to a supporting substrate. In some embodiments, the back reflector 46 may include one or more elements such as silver, aluminum, a white coating, a non-conductive coating, and the like.

[0135] The mirror 10 is configured to receive the emitted light 44. The mirror 10 is further configured to transmit a portion of the received light. In other words, the mirror 10 is configured to receive the emitted light 44 that is incident on the mirror 10, and the light exiting the mirror 10 includes a portion of the emitted light 44 that is transmitted through the mirror 10.

[0136] In some embodiments, the backlight 40 further includes a first optical diffuser 48 disposed between the mirror 10 and the emitting surface 45. The first optical diffuser 48 diffuses the light 44 emitted by the extended light source 43.

[0137] In some embodiments, the backlight 40 further includes a second optical diffuser 49. The second optical diffuser 49 is disposed on the mirror 10 opposite the emitting surface 45. The second optical diffuser 49 is configured to diffuse the light transmitted by the mirror 10.

[0138] Each of the first optical diffuser 48 and the second optical diffuser 49 may include any suitable diffusing film or plate configured to diffuse or scatter light. For example, the first optical diffuser 48 and the second optical diffuser 49 may diffuse light through the use of a textured surface of the substrate or through other means such as the incorporation of light diffusing particles within the matrix of the film.

[0139] In some embodiments, the backlight 40 further includes a reflective polarizer 50 disposed on the mirror 10 opposite the emission surface 45. In the illustrated embodiment of Figure 5A, the reflective polarizer 50 is disposed adjacent to a second light diffuser 49, which is located between the mirror 10 and the reflective polarizer 50. Thus, in the illustrated embodiment of Figure 5A, the reflective polarizer 50 is positioned to accept transmitted light that is diffused by the second light diffuser 49.

[0140] In some embodiments, the reflective polarizer 50 may be a collimating multilayer optical film (CMOF). However, the reflective polarizer 50 may be any suitable reflective polarizer. In some embodiments, the reflective polarizer 50 may include one or more of a multilayer polymeric reflective polarizer, a wire grid reflective polarizer, and a diffuse reflective polarizer. Reflective polarizers rely on the difference in refractive index between at least two materials, usually polymeric materials, to selectively reflect light of one polarization state while transmitting light of the orthogonal polarization state.

[0141] Figure 5B is a schematic diagram of the reflective polarizer 50 of the display system 60 shown in Figure 5A, according to one embodiment of the present disclosure. Figure 5B further illustrates substantially normally incident light 501 incident on the reflective polarizer 50, i.e., the substantially normally incident light 501 is incident at an angle of about 0 degrees with respect to the normal N1 to the reflective polarizer 50. In some embodiments, the normal N1 may be substantially parallel to the normal N (shown in Figures 3A and 3B). The substantially normally incident light 501 may be interchangeably referred to as "incident light 501."

[0142] In some cases, the incident light 501 may be polarized along a first direction. In some embodiments, the first direction may be the x-direction. Additionally, in some other cases, the incident light 501 may be polarized along a second direction that is orthogonal to the first direction. In some embodiments, the second direction may be the y-direction.

[0143] 5C shows a plot 510 illustrating the optical properties of a reflective polarizer 50 according to one embodiment of the present disclosure. Specifically, plot 510 shows the optical reflectance and optical transmittance of the reflective polarizer 50. Wavelength is represented in nanometers (nm) on the abscissa. Optical reflectance is represented as percent reflectance on the left ordinate. Optical transmittance is represented as percent transmittance on the right ordinate. Percent transmittance is approximated to be complementary to percent reflectance, i.e., transmittance=(100-percent reflectance).

[0144] 5B and 5C, plot 510 includes a reflectance curve 505. Reflectance curve 505 shows the light reflectance of reflective polarizer 50 for substantially normally incident light 501 that is polarized along a first direction.

[0145] Plot 510 further includes a reflectance curve 506. Reflectance curve 506 shows the optical transmission of reflective polarizer 50 for substantially normally incident light 501 that is polarized along a second direction that is orthogonal to the first direction.

[0146] As shown by reflectance curve 505, for the visible wavelength range 95 and substantially normally incident light 501, the reflective polarizer 50 has an average light reflectance of greater than about 60% when the incident light 501 is polarized along a first direction. In some embodiments, for the visible wavelength range 95 and substantially normally incident light 501, the reflective polarizer 50 has an average light reflectance of greater than about 70%, greater than about 80%, or greater than about 90% when the incident light 501 is polarized along the first direction.

[0147] As shown by reflectance curve 506, for the visible wavelength range 95 and substantially normally incident light 501, the reflective polarizer 50 has an average light transmission of greater than about 60% when the incident light 501 is polarized along a second direction that is orthogonal to the first direction. In some embodiments, for the visible wavelength range 95 and substantially normally incident light 501, the reflective polarizer 50 has an average light transmission of greater than about 70%, greater than about 80%, or greater than about 90% when the incident light 501 is polarized along a second direction that is orthogonal to the first direction.

[0148] 6A shows a detailed schematic cross-sectional view of an optical system 70 according to one embodiment of the present disclosure. In some embodiments, the optical system 70 is formed by providing a substantially Lambertian light source 71, which includes a back reflector 72 and emits substantially unpolarized light 73 through its extended emission surface 74.

[0149] A Lambertian light source is a light source that follows Lambert's cosine law I=I0cosα, where α is the viewing angle, and I0 is the light intensity at viewing angle α, which is about 0 degrees, i.e., perpendicular to the light source. For Lambertian light sources, the light intensity at viewing angles α close to 90 degrees is very low. Light emitting diodes (LEDs) approximate Lambertian sources because they tend to have large beam divergences and radiation patterns that approximate a sphere. In some cases, a Lambertian light source may be achieved by providing a non-Lambertian light source with an out-coupling photometric diffuser (e.g., of or including acetal, silicon dioxide, etc.) in close proximity to the non-Lambertian light source to achieve a Lambertian or Lambertian-like effect.

[0150] In some embodiments, the back reflector 72 has a total reflectance of greater than about 90% at at least one wavelength in the visible wavelength range 95. In some embodiments, the back reflector 72 has a total reflectance of greater than about 92%, greater than about 94%, greater than about 96%, greater than about 98%, or greater than about 99% at at least one wavelength in the visible wavelength range 95. The optical system 70 is further formed by disposing a mirror 10 between the reflective polarizer 75 and the emitting surface 74.

[0151] Light 78 exits optical system 70. Light 78 may exit optical system 70 through reflective polarizer 75.

[0152] In some embodiments, the rear reflector 72 may be substantially similar to the rear reflector 46 of FIG. 5A.

[0153] 6B is a schematic diagram of a reflective polarizer 75 of an optical system 70 according to one embodiment of the present disclosure. FIG. 6B further illustrates substantially normally incident light 76 incident on the reflective polarizer 75, i.e., substantially normally incident light 76 incident at an angle of about 0 degrees with respect to a normal N2 to the reflective polarizer 75. In some embodiments, the normal N2 may be substantially parallel to the normal N (shown in FIGS. 3A and 3B). The substantially normally incident light 76 may be interchangeably referred to as "incident light 76."

[0154] In some cases, the incident light 76 may be polarized along a first direction. In some embodiments, the first direction may be the x-direction. Further, in some cases, the incident light 76 may be polarized along a second direction that is orthogonal to the first direction. In some embodiments, the second direction may be the y-direction. In some cases, the incident light 76 may be partially polarized. Further, in some cases, the incident light 76 may be unpolarized, having an unknown or arbitrary polarization state or distribution of polarization states.

[0155] In some embodiments, the reflective polarizer 75 may be substantially similar to the reflective polarizer 50 (shown in FIGS. 5A and 5B). In particular, the optical properties of the reflective polarizer 75 may be substantially similar to the optical properties of the reflective polarizer 50, as shown in plot 510 (shown in FIG. 5C). Thus, in some embodiments, for the visible wavelength range 95 and substantially normally incident light 76, the reflective polarizer 75 has an average optical reflectance of greater than about 60% when the incident light 76 is polarized along the x-direction. In some embodiments, for the visible wavelength range 95 and substantially normally incident light 76, the reflective polarizer 75 has an average optical reflectance of greater than about 70%, greater than about 80%, or greater than about 90% when the incident light 76 is polarized along the x-direction.

[0156] Furthermore, for the visible wavelength range 95 and substantially normally incident light 76, the reflective polarizer 75 has an average light transmission of greater than about 60% when the incident light 76 is polarized along the y-direction. In some embodiments, for the visible wavelength range 95 and substantially normally incident light 76, the reflective polarizer 75 has an average light transmission of greater than about 70%, greater than about 80%, or greater than about 90% when the incident light 76 is polarized along the y-direction.

[0157] FIG. 6C illustrates an emission distribution 77 of light 78 (shown in FIG. 6A) exiting optical system 70 (shown in FIG. 6A) according to one embodiment of the present disclosure.

[0158] 6D illustrates a plot 600 of the relative intensity of light 78 (shown in FIG. 6A ) exiting optical system 70 (shown in FIG. 6A ) when viewed at different angles relative to the z-direction, according to one embodiment of the present disclosure. Specifically, plot 600 illustrates the relative intensity of light 78 on the left ordinate and different angles (in degrees) relative to the z-direction on the abscissa. Plot 600 includes intensity curve 601, intensity curve 602, and intensity curve 603, which illustrate the relative intensity of light 78 exiting optical system 70.

[0159] 6A-6D, the emission distribution 77 of the light 78 exiting the optical system 70 through the reflective polarizer 75 includes at least one global peak at an angle 79 with a line perpendicular to the reflective polarizer 75 (i.e., normal N2). In some embodiments, the at least one global peak is at an angle 79 greater than about 5 degrees relative to a line perpendicular to the reflective polarizer 75. In some embodiments, the line perpendicular to the reflective polarizer 75 is along the z-direction. Thus, the emission distribution 77 of the light 78 exiting the optical system 70 through the reflective polarizer 75 includes at least one global peak at an angle 79 greater than about 5 degrees relative to the z-direction of the reflective polarizer 75. In the illustrated embodiment of FIG. 6D, the angle 79 is about 13 degrees. In some embodiments, the emission distribution 77 of the light 78 exiting the optical system 70 through the reflective polarizer 75 includes at least one global peak at an angle 79 greater than about 7 degrees, greater than about 10 degrees, or greater than about 12 degrees relative to a line perpendicular to the reflective polarizer 75.

[0160] In some embodiments, at least one global peak lies in a plane that makes an oblique angle β with the x-direction. In some embodiments, the oblique angle β is between about 30 degrees and about 60 degrees. In some embodiments, the oblique angle β is between about 35 degrees and about 55 degrees, or between about 40 degrees and about 50 degrees. In the illustrated embodiment of FIG. 6C, the oblique angle β is about 45 degrees. The emission distribution 77 of the light 78 includes four global peaks at an angle 79 of more than about 5 degrees with respect to a line perpendicular to the reflective polarizer 75. Specifically, the emission distribution 77 includes a first global peak P1, a second global peak P2, a third global peak P3, and a fourth global peak P4. The first global peak P1 and the third global peak P3 lie on a plane 80 that makes an oblique angle β with the x-direction. The second global peak P2 and the fourth global peak P4 lie on a plane 81 that makes an oblique angle β with the x-direction.

[0161] 6A-6D, intensity curves 601, 602, and 603 show the variation or distribution of light 78 exiting optical system 70 at oblique angles β of 0 degrees, 45 degrees, and 90 degrees, respectively. From plot 600 in FIG. 6D, it can be observed that curve 602 includes a peak value of the relative intensity of light 78 exiting optical system 70. Further, it can be observed that the peak value is at an angle 79 of greater than about 5 degrees relative to the z-direction. This corresponds to the location of at least one global peak in emission distribution 77.

[0162] In some embodiments, the first global peak P1, the second global peak P2, the third global peak P3, and the fourth global peak P4 may have substantially similar relative intensities.

[0163] Examples and Data A multilayer partial mirror of the present disclosure ("Mirror 10") was constructed and compared to a conventional collimating multilayer optical film (CMOF). The CMOF included multiple first and second layers, and first and second skin layers. The CMOF included 650 layers.

[0164] Table 1 below lists the refractive indices of the different layers of the CMOF. [Table 1]

[0165] The mirror was constructed similarly to the construction of mirror 10 described in Figure 1. The mirror contained 420 layers.

[0166] Table 2 below lists exemplary refractive indices for the different layers of the mirror. [Table 2]

[0167] The mirrors were constructed using alternating high index optical (HIO) and low index optical (LIO) films. The HIO and LIO films included polymeric materials including polyhexylethylene naphthalate (PHEN) copolymer, polyethylene terephthalate (PET), and blends thereof. The materials for the HIO and LIO films were selected according to the desired refractive indices along the x, y, and z directions. A series of PHEN copolymers were produced using the following ingredients: dimethylnaphthalene dicarboxylate (NDC), hexanediol (HD), ethylene glycol (EG), cobalt acetate (CoAc), and tetrabutyl titanate (TBT). The ingredients were added to a room temperature stainless steel 10 gallon reactor equipped with a hot oil temperature control, an overhead separation column, and a vacuum pump, and heated and mixed at about 125 revolutions per minute (rpm) under a nitrogen atmosphere at a pressure of about 138 kilopascals (kPa). The transesterification reaction was carried out at a temperature of about 257 degrees Celsius (°C) for about 2 hours. Methanol produced during the reaction was driven off through a separation column and collected in a receiver. Once the reaction was complete, the pressure in the reactor was gradually returned to atmospheric pressure, and then the reactor was evacuated to allow the batch viscosity to increase to an acceptable level. Excess EG was removed. After about 2 hours at a temperature of about 285° C. and a vacuum of about 1 millimeter (mm) mercury (Hg), the reaction had proceeded to the desired end point. The desired end point may correspond to a viscosity of about 0.48 deciliters per gram (dL / g) for PEN. The reactor was then emptied and the resin was cooled and then further crushed into small pieces. The mole % of HD was determined. Five batches of exemplary PHEN copolymers (PHEN30, PHEN35, PHEN40, PHEN50, and PHEN70) were prepared by varying the proportions of the components used in the reaction. For example, PHEN30 had 30 mole % HD.

[0168] Table 3 below lists exemplary ingredient ratios used for five batches of PHEN. [Table 3]

[0169] A series of bilayer films were produced using the example PHEN copolymers. A first extruder was made to feed a series of PHEN copolymers and a second extruder was made to feed 0.64 IV polyester resin. The material was extruded and cast onto a chill roll to produce a 24 mil cast web film. The cast web film was biaxially oriented at a ratio of 350% x 350% of the original film dimensions. Stretching was performed at about 100°C to about 105°C with a preheat time of about 45 seconds (s). The oriented films were then annealed at about 225°C for about 15 seconds. The refractive index of the top layer of each of the oriented films was measured.

[0170] Table 4 below lists the refractive indices of the oriented films along the x, y, and z directions (nx, ny, nz, respectively). [Table 4]

[0171] The results suggested that a HD of 40% or more was suitable for isotropic LIO films.

[0172] The PHEN copolymer was further blended with different levels of PET. They were cast into films, stretched, oriented, and annealed. The refractive index of the top layer of each of the oriented films was measured.

[0173] Table 5 below lists the refractive indices of oriented films along the x, y, and z directions (nx, ny, nz, respectively) for blends of PHEN40 with different levels of PET. [Table 5]

[0174] PHEN or PHEN blended with PET copolymer was selected based on the desired refractive index of the HIO and LIO layers (eg, the films in Table 2).

[0175] Three samples A, B, C of the mirror of the present disclosure were constructed, each with a different thickness profile.

[0176] FIG. 7A shows an exemplary plot 700 illustrating the variation in layer thickness of different layers of a CMOF and samples A, B, and C. Sample A is substantially similar to "mirror 10" of the present disclosure. The ordinate shows thickness in nanometers (nm) and the abscissa shows layer number. Plot 700 includes thickness curve 701 illustrating the variation in thickness of adjacent layers of the CMOF. Plot 700 further includes thickness curves 705, 706, and 707 illustrating the variation in thickness of adjacent layers of samples A, B, and C, respectively. FIG. 7B further shows an expanded view of portion B of plot 700.

[0177] Table 6 lists exemplary thickness profiles of the CMOF and samples A, B, and C. [Table 6]

[0178] 7A and 7B, for the CMOF, the maximum and minimum thicknesses of the first and second layers were 131.2 nm and 47.1 nm, respectively. Thus, for the CMOF, the difference between the maximum and minimum thicknesses of the first and second layers was about 64.1%. Also, for the CMOF, the maximum difference in thickness between the first and second layers was about 31.1%. The difference in thickness between the first and second layers of the CMOF is best shown in FIG. 7B.

[0179] In sample A, the maximum and minimum thicknesses of the first and second layers were 126.7 nm and 57.6 nm, respectively. Thus, in sample A, the difference between the maximum and minimum thicknesses of the first and second layers was about 54.5%. Also, in sample A, the maximum difference between the thickness of the first layer and the thickness of the second layer was about 10.1%. The difference between the thicknesses of the first and second layers of sample A is best shown in FIG. 7B.

[0180] In sample B, the maximum and minimum thicknesses of the first and second layers were 131.1 nm and 55.6 nm, respectively. Thus, the difference between the maximum and minimum thicknesses of the first and second layers in sample B was about 57.6%. Also, the maximum difference between the thickness of the first layer and the thickness of the second layer in sample B was about 6.6%. The difference in thickness between the first and second layers in sample B is best shown in FIG. 7B.

[0181] In sample C, the maximum and minimum thicknesses of the first and second layers were 132.2 nm and 56.3 nm, respectively. Thus, the difference between the maximum and minimum thicknesses of the first and second layers in sample C was about 57.4%. Also, the maximum difference between the thickness of the first layer and the thickness of the second layer in sample C was about 6.6%. The difference in thickness between the first and second layers in sample C is best shown in FIG. 7B.

[0182] 7A and 7B, it has been observed that each of the curves 701, 705, 706, 707 has a number of V-shaped segments that indicate respective values ​​of the thickness of the first and second layers, with the peaks and valleys of the V-shaped segments representing the thicknesses of the individual layers.

[0183] It was observed that the variations in thickness curves 705, 706, and 707 (corresponding to samples A, B, and C, respectively) were comparable to each other, but the variation in thickness curve 701 (corresponding to CMOF) was larger than the variations in thickness curves 705, 706, and 707. This implied that the thickness variation between adjacent layers of CMOF was larger than the thickness variation between adjacent layers of samples A, B, and C.

[0184] FIG. 8A shows an exemplary plot 800 of the optical properties of sample A and the CMOF. With reference to FIGS. 1, 3A, 3B, and 8A, the plot 800 includes a reflectance curve 805 showing the optical reflectance versus wavelength of sample A for s-polarized light incident at a first angle of incidence less than about 10 degrees. Additionally, the plot 800 includes a reflectance curve 806 showing the optical reflectance versus wavelength of the CMOF for s-polarized light incident at a first angle of incidence less than about 10 degrees. The s-polarized light may be light polarized in a second direction (e.g., y-direction) that is orthogonal to the first direction (e.g., x-direction). In this example, the s-polarized incident light may be incident on the mirror-environment interface of each of the CMOF and sample A, and the external environment may include air. The wavelength is represented in nanometers (nm) on the abscissa. The optical reflectance is represented as a percentage reflectance on the left ordinate. The optical transmittance is represented as a percentage transmittance on the right ordinate. Percent transmittance is approximated as the complement of percent reflectance, ie, transmittance=(100-percent reflectance).

[0185] As shown by reflectance curve 805, for s-polarized incident light propagating within an incident plane 22 (shown in FIG. 3A ) comprising a first direction, at a first incidence angle of less than about 10 degrees, the multiple alternating first and second polymer layers of Sample A had an average optical reflectance of about 64% in the visible wavelength range 95 and an average optical transmittance of about 84% in the infrared wavelength range 96.

[0186] As shown by reflectance curve 806, for s-polarized incident light propagating in the same incident plane 22 including a first direction, at a first incidence angle of less than about 10 degrees, the multiple alternating first and second layers of CMOF had an average optical reflectance of about 48% in the visible wavelength range 95 and an average transmittance of about 88% in the infrared wavelength range 96.

[0187] FIG. 8B shows an example plot 810 of the optical properties of samples B and C. The plot 810 includes reflectance curves 815, 816 showing the optical reflectance versus wavelength for samples B and C, respectively, for s-polarized light incident at a first angle of incidence less than about 10 degrees. In this example, s-polarized incident light may be incident on the mirror-to-environment interface of samples B and C, respectively. Wavelength is represented in nanometers (nm) on the abscissa. Optical reflectance is represented as percent reflectance on the left ordinate. Optical transmittance is represented as percent transmittance on the right ordinate. Percent transmittance is approximated to be complementary to percent reflectance, i.e., transmittance=(100-percent reflectance).

[0188] As shown by reflectance curve 815, for s-polarized incident light propagating within an incident plane 22 (shown in FIG. 1 ) comprising a first direction, at a first incidence angle of less than about 10 degrees, the multiple alternating first and second polymer layers of Sample B had an average optical reflectance of about 71% in the visible wavelength range 95 and an average optical transmittance of about 82% in the infrared wavelength range 96.

[0189] As shown by reflectance curve 816, for s-polarized incident light propagating in an incident plane 22 including a first direction, and at a first incident angle of less than about 10 degrees, the multiple alternating first and second polymer layers of Sample C had an average optical reflectance of about 72% in the visible wavelength range 95 and an average optical transmittance of about 81% in the infrared wavelength range 96.

[0190] FIG. 9A shows an exemplary plot 900 of the optical properties of sample A and the CMOF. The plot 900 includes a reflectance curve 905 showing the optical reflectance versus wavelength of sample A for p-polarized light incident at a first angle of incidence less than about 10 degrees. Additionally, the plot 900 includes a reflectance curve 906 showing the optical reflectance versus wavelength of the CMOF for p-polarized light incident at a first angle of incidence less than about 10 degrees. The p-polarized light incident at a first angle of incidence less than about 10 degrees may be light polarized along a first direction. In this example, the p-polarized incident light may be incident at an interface between the mirror and the environment of each of the CMOF and sample A, and the external environment may include air. The wavelength is represented in nanometers (nm) on the abscissa. The optical reflectance is represented as a percentage reflectance on the left ordinate. The optical transmittance is represented as a percentage transmittance on the right ordinate. The percentage transmittance is approximated to be complementary to the percentage reflectance, i.e., transmittance=(100-percent reflectance).

[0191] As shown by reflectance curve 905, for p-polarized incident light propagating within an incident plane 22 (shown in FIG. 3B ) comprising a first direction, at a first incidence angle of less than about 10 degrees, the multiple alternating first and second polymer layers of Sample A had an average optical reflectance of about 64% in the visible wavelength range 95 and an average optical transmittance of greater than about 84% in the infrared wavelength range 96.

[0192] As shown by reflectance curve 906, for p-polarized incident light propagating in the same incident plane 22 including a first direction, at a first incident angle of less than about 10 degrees, the multiple alternating first and second layers of CMOF had an average optical reflectance of about 98% in the visible wavelength range 95 and an average transmittance of about 71% in the infrared wavelength range 96.

[0193] FIG. 9B shows an example plot 910 of the optical properties of samples B and C. The plot 910 includes reflectance curves 915, 916 showing the optical reflectance versus wavelength for samples B and C, respectively, for p-polarized light incident at a first angle of incidence less than about 10 degrees. In this example, p-polarized incident light may be incident on the mirror-environment interface of each of samples B and C, where the external environment may include air. The wavelength is represented in nanometers (nm) on the abscissa. The optical reflectance is represented as a percent reflectance on the left ordinate. The optical transmittance is represented as a percent transmittance on the right ordinate. The percent transmittance is approximated to be complementary to the percent reflectance, i.e., transmittance=(100-percent reflectance).

[0194] As shown by reflectance curve 915, for p-polarized incident light propagating within an incident plane 22 (shown in FIG. 3B ) comprising a first direction, at a first incidence angle of less than about 10 degrees, the multiple alternating first and second polymer layers of Sample B had an average optical reflectance of about 71% in the visible wavelength range 95 and an average optical transmittance of about 82% in the infrared wavelength range 96.

[0195] As shown by reflectance curve 916, for p-polarized incident light propagating within the incident plane 22 including a first direction, at a first incidence angle of less than about 10 degrees, the multiple alternating first and second polymer layers of Sample C had an average optical reflectance of about 72% in the visible wavelength range 95 and an average optical transmittance of about 81% in the infrared wavelength range 96.

[0196] As is apparent from Figures 8A to 9B, in the visible wavelength range, the CMOF substantially transmitted s-polarized light incident at the first angle of incidence and substantially blocked p-polarized light incident at the first angle of incidence. Meanwhile, each of Samples A, B, and C substantially transmitted s-polarized and p-polarized light incident at the first angle of incidence. Thus, in the visible wavelength range, each of Samples A, B, and C had a lower average optical reflectance for axial light compared to the CMOF. In the visible wavelength range, the smaller the average optical reflectance for axial light, the higher the axial brightness.

[0197] FIG. 10A shows an exemplary plot 1000 of the optical properties of sample A and the CMOF. The plot 1000 includes a reflectance curve 1005 showing the optical reflectance versus wavelength of sample A for s-polarized light incident at a second angle of incidence greater than about 45 degrees. Additionally, the plot 1000 includes a reflectance curve 1006 showing the optical reflectance versus wavelength of the CMOF for s-polarized light incident at a second angle of incidence greater than about 45 degrees. The s-polarized light may be light polarized along a second direction. In this example, the s-polarized incident light may be incident on the mirror-to-environment interface of each of the CMOF and sample A. The wavelength is represented in nanometers (nm) on the abscissa. The optical reflectance is represented as a percentage reflectance on the left ordinate. The optical transmittance is represented as a percentage transmittance on the right ordinate. The percentage transmittance is approximated to be complementary to the percentage reflectance, i.e., transmittance=(100-percent reflectance).

[0198] As shown by reflectance curve 1005, for s-polarized incident light propagating within an incident plane 22 (shown in FIG. 3A ) comprising a first direction, at a second incident angle of greater than about 45 degrees, the multiple alternating first and second polymer layers of Sample A had an average optical reflectance of about 92% in the visible wavelength range 95 and an average optical transmittance of about 65% in the infrared wavelength range 96.

[0199] As shown by reflectance curve 1006, for s-polarized incident light propagating at an incident surface 22 including a first direction, at a second incident angle of greater than about 45 degrees, the multiple alternating first and second layers of CMOF had an average optical reflectance of about 86% in the visible wavelength range 95 and an average optical transmittance of about 67% in the infrared wavelength range 96.

[0200] FIG. 10B shows an example plot 1010 of the optical properties of samples B and C. The plot 910 includes reflectance curves 1015, 1016 showing the optical reflectance versus wavelength for samples B and C, respectively, for s-polarized light incident at a second angle of incidence greater than about 45 degrees. In this example, s-polarized incident light may be incident on the mirror-environment interface of samples B and C, respectively. Wavelength is represented in nanometers (nm) on the abscissa. Optical reflectance is represented as percent reflectance on the left ordinate. Optical transmittance is represented as percent transmittance on the right ordinate. Percent transmittance is approximated to be complementary to percent reflectance, i.e., transmittance=(100-percent reflectance).

[0201] As shown by reflectance curve 1015, for s-polarized incident light propagating at an incident surface 22 (shown in FIG. 3A ) comprising a first direction, at a second incident angle of greater than about 45 degrees, the multiple alternating first and second polymer layers of Sample B had an average light reflectance of about 94% in the visible wavelength range 95 and an average light transmittance of about 64% in the infrared wavelength range 96.

[0202] As shown by reflectance curve 1016, for s-polarized incident light propagating at an incident surface 22 including a first direction, at a second incident angle of greater than about 45 degrees, the multiple alternating first and second polymer layers of Sample C had an average light reflectance of about 93% in the visible wavelength range 95 and an average light transmittance of about 64% in the infrared wavelength range 96.

[0203] FIG. 11A shows an exemplary plot 1100 of the optical properties of sample A and the CMOF. The plot 1100 includes a reflectance curve 1105 showing the optical reflectance versus wavelength of sample A for p-polarized light incident at a second angle of incidence greater than about 45 degrees. Additionally, the plot 1100 includes a reflectance curve 1106 showing the optical reflectance versus wavelength of the CMOF for p-polarized light incident at a second angle of incidence greater than about 45 degrees. In this example, p-polarized incident light may be incident at the mirror-to-environment interface of each of the CMOF and sample A. The wavelength is represented in nanometers (nm) on the abscissa. The optical reflectance is represented as a percentage reflectance on the left ordinate. The optical transmittance is represented as a percentage transmittance on the right ordinate. The percentage transmittance is approximated to be complementary to the percentage reflectance, i.e., transmittance=(100-percent reflectance).

[0204] As shown by reflectance curve 1105, for p-polarized incident light propagating at an incident surface 22 including a first direction, at a second incident angle of greater than about 45 degrees, the multiple alternating first and second polymer layers of Sample A had an average light reflectance of about 93% in the visible wavelength range 95 and an average light transmittance of about 99% in the infrared wavelength range 96.

[0205] As shown by reflectance curve 1106, for p-polarized incident light propagating at an incident surface 22 including a first direction, at a second incident angle of greater than about 45 degrees, the multiple alternating first and second layers of CMOF had an average optical reflectance of about 99% in the visible wavelength range 95 and an average optical transmittance of about 99% in the infrared wavelength range 96.

[0206] FIG. 11B shows an example plot 1110 of the optical properties of samples B and C. The plot 1110 includes reflectance curves 1115, 1116 showing the optical reflectance versus wavelength for samples B and C, respectively, for p-polarized light incident at a second angle of incidence greater than about 45 degrees. In this example, p-polarized incident light may be incident on the mirror-environment interface of samples B and C, respectively. Wavelength is represented in nanometers (nm) on the abscissa. Optical reflectance is represented as percent reflectance on the left ordinate. Optical transmittance is represented as percent transmittance on the right ordinate. Percent transmittance is approximated to be complementary to percent reflectance, i.e., transmittance=(100-percent reflectance).

[0207] As shown by reflectance curve 1115, for p-polarized incident light propagating within an incident surface 22 (shown in FIG. 3B ) comprising a first direction, at a second incident angle of greater than about 45 degrees, the multiple alternating first and second polymer layers of Sample B had an average optical reflectance of about 95% in the visible wavelength range 95 and an average optical transmittance of about 99% in the infrared wavelength range 96.

[0208] As shown by reflectance curve 1116, for p-polarized incident light propagating at an incident surface 22 including a first direction, at a second incident angle of greater than about 45 degrees, the multiple alternating first and second polymer layers of Sample C had an average light reflectance of about 94% in the visible wavelength range 95 and an average light transmittance of greater than about 99% in the infrared wavelength range 96.

[0209] As is evident from FIG. 8A-FIG. 11B, in the visible wavelength range, for both s-polarized and p-polarized light, Samples A, B, and C had higher average optical reflectance for light incident at a second angle of incidence greater than about 45 degrees (i.e., off-axis light) than for light incident at a first angle of incidence less than about 10 degrees (i.e., substantially perpendicularly incident or on-axis light). In other words, Samples A, B, and C had higher optical transmittance for on-axis light than for off-axis light in the visible wavelength range for both s-polarized and p-polarized light. In the visible wavelength range, Samples A, B, and C substantially collimated off-axis light for both s-polarized and p-polarized light, thus providing improved collimation over the CMOF, which substantially reflected on-axis p-polarized light. Thus, in the visible wavelength range, Samples A, B, and C had lower average optical reflectance for on-axis light compared to the CMOF. Optical reflectance for on-axis light is generally undesirable as it can reduce on-axis brightness. Thus, in the visible wavelength range, the lower the average optical reflectance for on-axis light, the improved the axial brightness provided by Samples A, B, and C.

[0210] Unless otherwise specified, all numbers expressing feature sizes, quantities and physical properties used in the specification and claims are to be understood as being modified by the term "about." Accordingly, unless specifically indicated to the contrary, the numerical parameters set forth in the above specification and appended claims are approximations that may vary depending upon the desired properties one of ordinary skill in the art would seek to obtain using the teachings disclosed herein.

[0211] While specific embodiments have been illustrated and described herein, those skilled in the art will recognize 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. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Accordingly, it is intended that the present disclosure be limited only by the claims and the equivalents thereof.

Claims

1. 1. A multi-layer partial mirror comprising a plurality of alternating first and second polymer layers totaling at least 50, disposed between and integrally formed with opposing first and second polymer skin layers, each of the first and second polymer layers having an average thickness less than about 500 nm, each of the polymer skin layers having an average thickness greater than about 0.5 microns, the first and second polymer layers having the same respective refractive indices nx1 and nx2 along an in-plane x-direction, ny1 and ny2 respectively along an in-plane y-direction orthogonal to the x-direction, and nz1 and nz2 respectively along a z-direction of the polymer layers orthogonal to the x-direction and the y-direction, for at least one wavelength in a visible wavelength range spanning from about 420 nm to about 680 nm; nx1 and ny1 are within about 0.05 of each other; nz1 is at least 0.06 smaller than each of nx1 and nx2; nx2, ny2, and nz2 are within about 0.05 of each other; for each of the first polymer skin layer and the second polymer skin layer, corresponding refractive indices of the polymer skin layer and one of the first polymer layer and the second polymer layer along each of the x-direction, the y-direction, and the z-direction are within about 0.05 of each other; For incident light propagating in a plane of incidence that includes the visible wavelength range and the x-direction, For s-polarized incident light, the plurality of alternating first and second polymer layers have an average reflectivity Rs1 for a first angle of incidence less than about 10 degrees and an average reflectivity Rs2 for a second angle of incidence greater than about 45 degrees; 1. A multilayer partial mirror comprising: a first polymer layer having a first thickness and a second polymer layer having a first thickness and a second thickness, the first thickness being greater than about 1.15; and a second thickness being greater than about 1.55;

2. 2. The multi-layer partial mirror of claim 1, wherein for each of the first polymer skin layer and the second polymer skin layer, corresponding refractive indices of the polymer skin layer and the first polymer layer along each of the x-direction, the y-direction, and the z-direction are within about 0.05 of each other.

3. The multi-layer partial mirror of claim 1 , wherein the second polymer layer and at least one of the first polymer skin layer and the second polymer skin layer have substantially the same material composition.

4. a difference between a maximum thickness and a minimum thickness of the alternating first and second polymer layers is from about 45% to about 65%; 10. The multi-layer partial mirror of claim 1, wherein a maximum difference between thicknesses of adjacent polymer layers in the alternating first and second polymer layers is less than about 25%.

5. The multi-layer partial mirror of claim 1 , wherein Rp2 is less than about 98% and greater than about 70%.

6. 2. The multi-layer partial mirror of claim 1, wherein Rp1 and Rs1 are within 10% of each other.

7. 2. The multilayer partial mirror of claim 1, wherein for the s-polarized incident light propagating in the plane of incidence that includes an infrared wavelength range of about 800 nm to about 1300 nm and the x-direction, at the first angle of incidence, the plurality of alternating first and second polymer layers have an average transmission Ts1 that is greater than about 50%.

8. 2. The multilayer partial mirror of claim 1, wherein for the s-polarized incident light propagating in the plane of incidence that includes an infrared wavelength range of about 800 nm to about 1300 nm and the x-direction, at the second angle of incidence, the plurality of alternating first and second polymer layers have an average transmittance Ts2 that is greater than about 40%.

9. 2. The multilayer partial mirror of claim 1, wherein for the p-polarized incident light propagating in the plane of incidence that includes an infrared wavelength range of about 800 nm to about 1300 nm and the x-direction, at the first angle of incidence, the plurality of alternating first and second polymer layers have an average transmission Tp1 that is greater than about 50%.

10. 2. The multilayer partial mirror of claim 1, wherein for the p-polarized incident light propagating in the plane of incidence that includes an infrared wavelength range of about 800 nm to about 1300 nm and the x-direction, at the second angle of incidence, the plurality of alternating first and second polymer layers have an average transmission Tp2 that is greater than about 70%.

11. 1. A backlight for providing illumination to a display panel configured to form an image, comprising: an extended light source configured to emit light through an emission surface and including a back reflector; a multi-layer partial mirror according to claim 1 disposed on the emitting surface and configured to receive the emitted light and transmit a portion of the received light, wherein the multi-layer partial mirror and the back reflector form a recycling optical cavity therebetween.

12. The optical system providing a substantially Lambertian light source that includes a back reflector and emits substantially unpolarized light through its extended emitting surface, the back reflector having a total reflectance of greater than about 90% at at least one wavelength within the visible wavelength range; disposing the multilayer partial mirror of claim 1 between a reflective polarizer and the emission surface such that, for the visible wavelength range and for substantially normally incident light, the reflective polarizer has an average optical reflectance of greater than about 60% when the incident light is polarized along the x-direction and an average optical transmission of greater than about 60% when the incident light is polarized along the y-direction; and an emission distribution of light exiting the optical system through the reflective polarizer includes at least one global peak at an angle of greater than about 5 degrees relative to a line normal to the reflective polarizer.

13. The multi-layer partial mirror according to claim 12 , wherein the at least one global peak lies in a plane that is at an oblique angle to the x-direction.

14. 1. A multi-layer partial mirror comprising a plurality of alternating first and second polymer layers totaling at least 50, each of the first and second polymer layers having an average thickness less than about 500 nm, the first and second polymer layers having the same respective refractive indices nx1 and nx2 along an in-plane x-direction, ny1 and ny2 respectively along an in-plane y-direction orthogonal to the x-direction, and nz1 and nz2 respectively along a z-direction of the polymer layers orthogonal to the x-direction and the y-direction, for at least one wavelength in a visible wavelength range spanning from about 420 nm to about 680 nm; nz1 is at least 0.06 smaller than each of nx1 and nx2; the magnitude of the difference between nx2 and ((nx1+nz1) / 2) is less than about 0.05; nx2, ny2, and nz2 are within about 0.05 of each other; For incident light propagating in a plane of incidence that includes the visible wavelength range, an infrared wavelength range of about 800 nm to about 1300 nm, and the x-direction, At a first angle of incidence less than about 10 degrees, the plurality of alternating first and second polymer layers have an average reflectivity Rp1 for the incident light polarized along the x-direction and an average reflectivity Rs1 for the incident light polarized along the y-direction, each of Rp1 and Rs1 being less than about 85% in the visible wavelength range and less than about 50% in the infrared wavelength range; a first polymer layer having a first reflectance Rp2 and a second polymer layer having a second reflectance Rs2 for p-polarized and s-polarized incident light, respectively, at a second angle of incidence greater than about 45 degrees, wherein each of Rp2 and Rs2 is greater than about 70% in the visible wavelength range.

15. 1. A multi-layer partial mirror comprising a plurality of alternating first and second polymer layers totaling about 50 to about 600 and disposed between integrally formed opposing first and second polymer skin layers, each polymer layer disposed between the first and second polymer skin layers having an average thickness of less than about 500 nm, and for at least one wavelength in a visible wavelength range spanning from about 420 nm to about 680 nm: each of the first polymer layer and the second polymer layer has an in-plane birefringence of less than about 0.05; the first polymer layer has an out-of-plane birefringence greater than about 0.06; the second polymer layer has an out-of-plane birefringence of less than about 0.02; for incident light propagating in a plane of incidence that includes the visible wavelength range and a first direction in the same plane, at a first angle of incidence that is less than about 10 degrees, the multilayer partial mirror has an average reflectivity Rs1 for the incident light that is s-polarized and an average reflectivity Rp1 for the incident light that is p-polarized, where Rp1 / Rs1 is between about 0.8 and about 1.2; the multilayer partial mirror having an optical reflectivity versus wavelength for the first angle of incidence and for each of the p-polarized incident light and the s-polarized incident light includes a reflection band edge from about 760 nm to about 980 nm.

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