Optical system including light control film and Fresnel lens

Abstract

The present invention provides an optical system, which includes a light source, an optical film curved around a first axis, and a light control film curved around the first axis and disposed between the light source and the optical film. The optical film includes a microstructured first major surface and an opposing second major surface. The microstructured first major surface defines a linear Fresnel lens, which includes a plurality of Fresnel elements extending longitudinally along the first axis. The first major surface of the optical film faces the light control film. The light control film includes a plurality of alternating optically transmissive regions and optically absorptive regions extending longitudinally along the first axis, such that in a cross section orthogonal to the first axis, for at least most of the optically transmissive regions, a centerline between adjacent optically absorptive regions is substantially perpendicular to the major surface of the light control film.

Description

BACKGROUND OF THE INVENTION

[0001] The light control film may include alternating optically transmissive regions and optically absorptive regions and may be used to adjust the direction of light transmitted through the film.

[0002] The display surface may be curved. SUMMARY OF THE INVENTION

[0003] In some aspects, the present disclosure provides an optical system that includes a light source, an optical film curved about a first axis, and a light control film curved about the first axis and disposed between the light source and the optical film. The optical film includes a microstructured first major surface and an opposite second major surface. The microstructured first major surface defines a linear Fresnel lens that includes a plurality of Fresnel elements longitudinally extending along the first axis. The first major surface of the optical film faces the light control film. The light control film includes a plurality of alternating optically transmissive regions and optically absorptive regions longitudinally extending along the first axis such that, in a cross-section orthogonal to the first axis, for at least most of these optically transmissive regions, a centerline between adjacent optically absorptive regions is substantially perpendicular to the major surface of the light control film.

[0004] In some aspects, the present disclosure provides an optical system that includes an optical film curved about a first axis and a light control film curved about the first axis and disposed adjacent to the optical film. The optical film includes a microstructured first major surface and an opposite second major surface. The microstructured first major surface defines a linear Fresnel lens that includes a plurality of Fresnel elements longitudinally extending along the first axis. Each Fresnel element includes an optical facet and a sidewall intersecting the optical facet at a ridge of the Fresnel element. For each of at least most of the Fresnel elements, an optically absorptive layer may be disposed on the sidewall and substantially cover the sidewall. The optically absorptive layer may have an average thickness t, where 100 nm < t < 1 micron. The first major surface of the optical film faces the light control film. The light control film includes a plurality of alternating optically transmissive regions and optically absorptive regions longitudinally extending along the first axis such that, in a cross-section orthogonal to the first axis, for at least most of these optically transmissive regions, a centerline between adjacent optically absorptive regions is substantially perpendicular to the major surface of the light control film. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Figures 1 to 3 is a schematic cross-sectional view of an optical system;

[0006] Figures 4 to 6 is a schematic cross-sectional view of an optical stack including an optical film and a light control film;

[0007] Figures 7 and 8 is a schematic cross-sectional view of an optical system schematically showing the shape of an optical film and a light control film according to some embodiments;

[0008] Fig. 9 is a schematic top view of a linear Fresnel lens;

[0009] Fig.10 is a schematic top view of a light control film;

[0010] Fig.11 is a schematic cross-sectional view of an optical film including a plurality of Fresnel elements;

[0011] Figures 12 to 17 is a schematic cross-sectional view of a portion of an optical film including a plurality of Fresnel elements;

[0012] Figures 18 to 20 is a schematic cross-sectional view of a Fresnel element;

[0013] Fig.21 is a schematic diagram of the mean deviation surface roughness; and

[0014] Figure 22 to Figure 24 It is a graph of relative brightness and position at various viewing angles. DETAILED DESCRIPTION

[0015] In the following description, reference is made to the accompanying drawings, which form a part of the present invention and in which various embodiments are shown by way of example. The drawings are not necessarily drawn to scale. It should be understood that other embodiments may be conceived and carried out without departing from the scope or essence of the present description. Therefore, the following specific embodiments should not be considered to have a limiting meaning.

[0016] A light control film, which may also be referred to as a collimating film or privacy filter, may be used to adjust the orientation of light transmitted through the film. The light control film may be placed on a curved display, for example, to filter the light output of the display. However, it is generally desirable to change the directionality of the transmitted light compared to using a conventional light control film placed on a curved display surface (e.g., so that the light is directed primarily toward the observer rather than primarily along the normal of the display surface). This can be achieved by changing the inclination of the grid (e.g., optical absorption area) of the light control film, as described in U.S. patent application 62 / 865,510 filed on June 24, 2019 and entitled "Curved Display with Light Control Film". Alternatively, a linear Fresnel lens may be placed on a conventional light control film to change the directionality of the transmitted light. However, it has been found that this may result in undesirable bright bands at higher viewing angles. According to some embodiments of the present specification, it has been found that the Fresnel lens may be positioned in the optical system so that the Fresnel element faces the light control film, and this may reduce or substantially eliminate bright bands at higher viewing angles.

[0017] In addition, it has been discovered that according to some embodiments, an optically absorbing layer can be disposed on the sidewalls of the Fresnel element to reduce undesired redirection of light caused by the sidewalls. It has been discovered that utilizing a thin optically absorbing layer reduces undesired effects of the optically absorbing layer (e.g., blocking light that is incident on the edge of the layer that is intended to be transmitted). When the optically absorbing layer is thin, it preferably has a relatively high extinction coefficient in order to absorb light efficiently. In some cases, the high extinction coefficient can interact with the sidewalls to produce a reflective metal-like interface. It has been discovered that adding a coating (e.g., a layer having an extinction coefficient less than the extinction coefficient of the optically absorbing layer and greater than the light extinction coefficient of the sidewall material) between the high extinction coefficient absorbing layer and the sidewalls can reduce reflectivity, thereby reducing undesired redirection of light caused by the sidewalls.

[0018] Figures 1 to 3 Schematic cross-sectional views of optical systems 100, 200 and 300, respectively. Optical systems 100, 200, 300 include an optical film 110 curved about a first axis (z-axis) and a light control film 120 curved about the first axis. Optical film 110 includes a microstructured first major surface 112 and an opposing second major surface 114. The microstructured first major surface 112 defines a linear Fresnel lens 111, which includes a plurality of Fresnel elements 115 extending longitudinally along the first axis. Light control film 120 can be disposed proximate to optical film 110, wherein the first major surface 112 of optical film 110 faces light control film 120. In some embodiments, the optical system includes a light source 130. Light control film 120 can be disposed between light source 130 and optical film 110, wherein the first major surface 112 of optical film 110 faces light control film 120. The light control film 120 includes a plurality of alternating optically transmissive regions 122 and optically absorptive regions 124 extending longitudinally along a first axis such that in a cross-section (xy cross-section) orthogonal to the first axis, a centerline 126 between adjacent optically absorptive regions is substantially perpendicular to the major surfaces 128 and / or 129 of the light control film 120 (e.g., within 20 degrees, or within 10 degrees, or within 5 degrees of the normal) for at least a majority of the optically transmissive regions 122. The Fresnel lens 111 can be configured such that light 866 emitted from the light source 130 perpendicular to the light source emission surface is redirected by the Fresnel lens to substantially the same redirection direction (y direction).

[0019] The microstructured surface typically includes a microstructure (e.g., the Fresnel element 115 can be a microstructure), wherein the microstructure is a structure having at least two orthogonal dimensions, such as a width and a height less than 1 mm and greater than 100 nm). The microstructure can be formed in a polymer layer. A polymer layer is a layer comprising a continuous phase of an organic polymer. For example, a polymer layer can also include inorganic nanoparticles dispersed in a continuous phase of an organic polymer. As is known in the art, a polymer microstructured layer can be formed on a substrate using a casting and curing process (see, for example, U.S. Patent Application Publication 2006 / 0114569 (Capaldo et al.) and U.S. Patents 5,175,030 (Lu et al.) and 5,183,597 (Lu)). Other methods for preparing a polymer structured layer include extrusion, machining, and embossing. Any suitable material can be used. For example, an acrylate (e.g., polymethyl methacrylate) can be cast and cured onto a polyester (e.g., polyethylene terephthalate) substrate.

[0020] The light control film 120 can be any suitable light control film, including, for example, those described in U.S. Patents 8,213,082 (Gaides et al.), 9,335,449 (Gaides et al.), and 9,063,284 (Jones et al.), and, for example, International Patent Application Publication No. WO 2019 / 118685 (Schmidt et al.).

[0021] In some embodiments (e.g., as shown for optical system 100), light source 130 is or includes an organic light emitting diode (OLED) display 132 that is bent around a first axis. Display 132 may alternatively be another type of emissive display, such as a micro light emitting diode (microLED) display. In some embodiments (e.g., as shown for optical systems 200, 300), light source 130 includes backlight 134. In some embodiments, backlight 134 includes a light guide 136 that is bent around a first axis. In some embodiments, light source 130 also includes one or more light emitting elements 135 (e.g., light emitting diodes). For example, backlight 134 may be a direct-lit backlight or an edge-lit backlight. In some embodiments (e.g., as shown for optical system 200), the optical system may include a display panel 139 disposed between backlight 134 and light control film 120. In some embodiments (e.g., as shown for optical system 300), the optical system may include display panel 139, wherein optical film 110 is disposed between display panel 139 and light control film 120.

[0022] In some embodiments, air gap 178 separates at least a majority of first major surface 112 of optical film 110 from light management film 120. For example, the air gap can be provided by not bonding optical film 110 and light management film 120 together, by bonding the films together only at one or more edges, or by attaching the films together using elongated spacer members.

[0023] Figure 4 is a schematic cross-sectional view of an optical system or portion of an optical system including an optical stack 400, which includes an optical film 110 and a light management film 120. In some embodiments, the optical film 110 and the light management film 120 are adhered to each other (e.g., with tape 176 or 177) along at least one edge 171 or 172 of the optical film 110 and the corresponding edge 181 or 182 of the light management film, respectively. In some embodiments, the optical film 110 and the light management film 120 are adhered to each other along at least two edges 171 and 172 of the optical film 110 and the corresponding edges 181 and 182 of the light management film 120, respectively. Adhering together along the edges is understood to include applying tape close to the edges along at least a portion (e.g., at least a majority) of the length of the edges.

[0024] Figure 5 is a schematic cross-sectional view of an optical system or a portion of an optical system including an optical stack 500, which includes an optical film 210 and a light control film 120. Optical film 210 can correspond to optical film 110, except that a first major surface 212 of optical film 210 includes a plurality of elongated spacer members 142. In some embodiments, first major surface 212 of optical film 210 includes a plurality of elongated spacer members 142 extending toward light control film 120 and received by adhesive layer 144 disposed on light control film 120. Elongated spacer members 142 can be formed with Fresnel elements 115, or can be deposited after Fresnel elements 115 have been formed. Elongated spacer members 142 can also be referred to as optical decoupling structures, and can have any of the geometric shapes of optical decoupling structures described, for example, in International Application Publication No. WO2019 / 135190 (Pham et al.).

[0025] Figure 66 is a schematic cross-sectional view of an optical system or portion of an optical system including an optical stack 600, the optical stack including an optical film 110 and a light control film 120. The optical system or optical stack 600 also includes a low refractive index layer 148 disposed on and in contact with at least a majority of the first major surface 112 of the optical film 110, and an optical adhesive 144 that bonds the low refractive index layer 148 to the light control film 120. In some embodiments, the low refractive index layer 148 has a refractive index of no more than 1.45, or no more than 1.4, or no more than 1.35, or no more than 1.25, or no more than 1.2. For example, the refractive index of the low refractive index layer 148 can be in the range of 1.05 to 1.45, or in the range of 1.1 to 1.35, or in the range of 1.1 to 1.25, or in the range of 1.05 to 1.2. The low refractive index layer 148 can be a nanovoided polymeric layer as described in U.S. Pat. No. 8,808,811 (Kolb et al.). In some embodiments, low refractive index layer 148 is an adhesive and adhesive layer 144 is omitted.

[0026] For example, the optical film 110, or a layer of the optical film 110 including the Fresnel elements 115, or the Fresnel elements can have a refractive index greater than 1.5, or greater than 1.6, or greater than 1.7.

[0027] In some embodiments, optical film 110 and light management film 120 are substantially concentric. For example, optical film 110 and light management film 120 can have substantially similar shapes and can be substantially coextensive with each other.

[0028] In some embodiments, the optical system (e.g., 100, 200, or 300) is adapted to display an image 888 to an observer 870 (see, e.g., Figure 1 ), wherein in a cross section orthogonal to the first axis, optical film 110 and light control film 120 are each convex toward observer 870. Optical film 110 and light control film 120 may alternatively have a Figures 7 and 8 Different shapes are schematically shown in FIG. Figure 7 is a schematic cross-sectional view of an optical system 700 that includes an optical film 810 (e.g., corresponding to optical film 110) and a light control film 820 (e.g., corresponding to light control film 120) and is suitable for displaying an image 888 to an observer 870, wherein in a cross-section orthogonal to a first axis (z-axis), the optical film 810 and the light control film 820 are each concave toward the observer 870. Figure 8is a schematic cross-sectional view of an optical system 800 that includes an optical film 910 (e.g., corresponding to optical film 110) and a light control film 920 (e.g., corresponding to light control film 120), and is suitable for displaying an image 888 to an observer 870, wherein in a cross-section orthogonal to a first axis, a first portion (910a and 920a, respectively) of each of the optical film 910 and the light control film 920 is concave toward the observer 870, and a second portion (910b and 920b, respectively) of each of the optical film 910 and the light control film 920 is convex toward the observer 870.

[0029] exist Figures 1 to 10 In the embodiments schematically shown in , the Fresnel elements of the optical film and / or the optically transmissive and absorptive regions of the light control film can extend along a first axis (z-axis). In some embodiments, the first major surface 112 of the optical film 110 defines a linear Fresnel lens 111 that includes Fresnel elements 115 extending longitudinally along the first axis (z-axis). Fig. 9 is a schematic top view (a view facing the first major surface 112 ) of the linear Fresnel lens 111 when the optical film 110 is laid flat, according to some embodiments. Fig.10 is a schematic top view of a light control film 120 when the light control film 120 is laid flat according to some embodiments. In some embodiments, the light control film 120 includes a plurality of alternating optically transmissive regions 122 and optically absorptive regions 124 extending longitudinally along a first axis (z-axis).

[0030] In some embodiments, each Fresnel element 115 of the optical film 110 includes an optical facet 117 and a sidewall 118 that intersects the optical facet 117 at a ridge 119 of the Fresnel element. In some embodiments, each Fresnel element in at least a majority of the Fresnel elements 115 includes one or more of at least a partial coating (e.g., a conformal coating) of an inorganic optically transparent layer, an optically absorbing layer, a first layer disposed between the optically absorbing layer and the sidewall, and a second layer disposed on the optically absorbing layer opposite the sidewall, wherein the first layer may be less optically absorbing than the optically absorbing layer, wherein the second layer may be less optically absorbing than the optically absorbing layer and may be one or more of a protective layer or an anti-reflective layer.

[0031] Fig.11FIG. 310 is a schematic cross-sectional view of an optical film 310 that can correspond to the optical film 110. In some embodiments, each Fresnel element 115 includes an optical facet 117 and a sidewall 118 that intersects the optical facet 117 at a ridge 119 of the Fresnel element. In some embodiments, for each of at least most of the Fresnel elements, an optical absorption layer 152 is disposed on the sidewall 118 and substantially covers the sidewall. In some embodiments, the plurality of Fresnel elements includes a plurality of polymeric Fresnel elements 334 (e.g., see Fig.13 ) that are at least partially coated (e.g., conformally coated) with an inorganic optically transparent layer 157. In some embodiments, for each of at least most of the Fresnel elements, the optical absorption layer 152 is disposed on the inorganic optically transparent layer 157 disposed on the sidewall 118 and substantially covers the inorganic optically transparent layer.

[0032] As further described elsewhere herein, the inorganic optically transparent layer can be used as an etch stop layer during the process of removing the optical absorption layer and / or other layers from the optical facet. Suitable materials for the inorganic optically transparent layer include a-Si, SiOx, SiAlOx, SiCyOx, TiO2, and AlOx. The layer can be thin enough such that, for example, greater than 60% of the vertically incident visible light passes through the layer. A layer can be considered optically transparent if its average optical transmittance (unweighted average over wavelength for non-polarized light) for visible light (wavelength in the range of 400 nm to 700 nm) vertically incident on the layer in air is at least 60%. In some embodiments, the average optical transmittance of the inorganic optically transparent layer is greater than 60%, or greater than 70%, or greater than 80%, or greater than 85%, or greater than 90%.

[0033] In some embodiments, the optical absorption layer 152 has an average thickness t, where 100 nm < t < 2 microns or 100 nm < t < 1 micron. In some embodiments, the average thickness t is less than 500 nm. In some embodiments, for each of at least most of the Fresnel elements 115a of the Fresnel elements 115, the sidewall 118 of the Fresnel element 115a has a height H from the bottom 185 of an adjacent Fresnel element 115b to the ridge 119 of the Fresnel element 115a, where H / t > 10, or H / t > 15, or H / t > 18, or H / t > 20.

[0034] In some embodiments (e.g., optical films 110, 210, or 310), for each Fresnel element of at least most of the Fresnel elements, the sidewall 118 of the Fresnel element is substantially perpendicular to the second major surface 114 (e.g., within 30 degrees, or within 20 degrees, or within 10 degrees, or within 5 degrees of the normal). In some embodiments, the optical films 110, 210, or 310 include a substrate 164, on which a plurality of Fresnel elements 115 are formed on the major surface 162 of the substrate 164. In some embodiments, for each Fresnel element of at least most of the Fresnel elements, the sidewall 118 of the Fresnel element is substantially perpendicular to the major surface 162 of the substrate 164. In some embodiments, the sidewall 118 forms an angle within the range of 85 degrees to 90 degrees or other ranges described elsewhere herein with the major surface 162 or with the major surface 114 or with the bottom of the Fresnel element.

[0035] Fig.12 is a schematic cross-sectional view of a portion of an optical film (e.g., corresponding to optical films 110, 210, or 310). In some embodiments, for each Fresnel element 115 of at least most of the Fresnel elements, a first layer 154 is disposed between the sidewall 118 and the optical absorption layer 152. The first layer 154, which may be referred to as an overcoat, can be included to reduce light reflection at the interface between the sidewall and the optical absorption layer 152. In some embodiments, the first layer 154 has an extinction coefficient k1, and the optical absorption layer 152 has an extinction coefficient k2. In some embodiments, k2 > k1. In some embodiments, the extinction coefficient k0 of the plurality of Fresnel elements is < 0.05. In some embodiments, k2 > k1 > k0. The extinction coefficient k0 can be understood to define the extinction coefficient of the material at the first major surface 112 of the Fresnel element. In some embodiments, k2 - k1 is greater than 0.05, 0.1, 0.15, or 0.2. In some embodiments, k2 - k1 is less than 1, or less than 0.8, or less than 0.5. In some embodiments, k1 - k0 is greater than 0.001, 0.005, or 0.01. In some embodiments, k1 - k0 is less than 0.15, or less than 0.1, or less than 0.05. In some embodiments, k0 is less than 0.01, or less than 0.005, or less than 0.002. In some embodiments, k1 is in the range of 0.005 to 0.15, or 0.01 to 0.1. In some embodiments, k2 is in the range of 0.1 to 0.5, or 0.2 to 0.4. In some embodiments, k1 < 0.5k2. In some embodiments, 0.005 < k1 < 0.5k2. Instead of or in addition to describing the first layer 154 and the optical absorption layer 152 in terms of the extinction coefficient, the layers can be described in terms of the concentration of the optical absorption material as further described elsewhere herein.

[0036] In some embodiments, the optically absorptive layer 152 has a refractive index n1 and the first layer 154 has a refractive index n2, wherein |n1-n2| is less than 0.5, or less than 0.3, or less than 0.2, or less than 0.1.

[0037] The extinction coefficient may be expressed as the imaginary part of the complex refractive index, and the refractive index may be expressed as the real part of the complex refractive index.Except where otherwise indicated elsewhere, the extinction coefficient and refractive index may be understood to be evaluated at a wavelength of 550 nm.

[0038] For example, the extinction coefficient k of a thin film coating can be determined, for example, by ellipsometry or spectroscopy. k can be expressed as αλ / (4π), where α is the absorption coefficient and λ is the wavelength. For a thin film coating on a transparent substrate, spectroscopy can be used to measure the absorbance A as 100%–T–R, where T is the transmittance and R is the reflectance. The measured A can be appropriately corrected for the A of the substrate to obtain the A of the film itself. The resulting A can then be used to determine α according to the equation α=-ln[(100%-A) / 100%] / h, where h is the thickness of the thin film coating; when R is relatively small and A is relatively large, this formula for α is an approximation used. The thickness h can be measured, for example, by stylus profilometry or cross-sectional scanning electron microscopy.

[0039] The light absorbing material for the optically absorbing layer 152 and / or the first layer 154 can be one or more of a dye, a pigment, or a particle (e.g., a nanoparticle). Suitable light absorbing materials include carbon black nanopigments and other nanopigments, such as those available from Cabot Corporation (Boston, MA) under the CAB-O-JET trade name. Other suitable light absorbing materials include those described in International Patent Application Publication No. WO 2019 / 118685 (Schmidt et al.).

[0040] In some embodiments, first layer 154 has a concentration C1 of light absorbing material 184, and optically absorptive layer 152 has a concentration C2 of light absorbing material 182, where C2>C1. In some embodiments, C1<0.7C2, or C1<0.5C2.

[0041] The concentration of the light absorbing material (e.g., light absorbing nanoparticles) in the optical absorbing layer can be at least 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, or 50 wt% of the optical absorbing layer. In some embodiments, the concentration of the light absorbing material in the optical absorbing layer is at least 55 wt%, 60 wt%, 65 wt%, 70 wt%, 75 wt%, 80 wt%, 85 wt%, 90 wt%, or 95 wt% of the optical absorbing layer. In some embodiments, the concentration of the light absorbing material in the optical absorbing layer is 100 wt%. In some embodiments, the concentration of the light absorbing material in the optical absorbing layer is 30 wt%-100 wt% or 75 wt%-90 wt% of the optical absorbing layer.

[0042] The concentration of the light absorbing material in the first layer (covering) is preferably less than the concentration of the light absorbing material in the optically absorptive layer. The concentration of the light absorbing material in the first layer is typically at least 0.5%, 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, or 45% by weight of the first layer. In some embodiments, the concentration of the light absorbing material in the first layer does not exceed 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 70%, or 75% by weight of the first layer. In some embodiments, the concentration of the light absorbing material in the first layer is 0.5% to 50% or 25% to 45% by weight of the first layer.

[0043] In some embodiments, the light control film 120 is as generally described, for example, in International Patent Application Publication WO 2019 / 118685 (Schmidt et al.). In some such embodiments, the optically absorptive layer of the grid includes an optically absorptive core layer and a cladding on each side of the optically absorptive core layer. The optically absorptive core layer may be as described for the optically absorptive layer 152, and the cladding layer may be as described for the first layer 154. The use of a cladding layer may reduce the reflectivity of the grid, thereby reducing high-angle light leakage and ghosting. Light control films including a core layer and a cladding layer are described in International Patent Application IB2019 / 056489 (Schmidt et al.).

[0044] Figures 13 to 17 is a schematic cross-sectional view of a portion of an optical film (e.g., corresponding to optical film 110, 210, or 310). In some embodiments, as Fig.13, for example, the plurality of Fresnel elements 115 include a microstructured polymer layer 333 conformally coated with an inorganic optically transparent layer 157. The inorganic optically transparent layer 157 may include opposing first major surfaces 167 and second major surfaces 166, wherein the second major surface 166 faces the polymer layer 333. The polymer layer 333 has a major surface 113 facing the substrate layer 164. In some embodiments, as Fig.14 As schematically shown in FIG. 1 , for example, for each Fresnel element in at least a majority of the Fresnel elements 115 , the optically absorptive layer 152 is disposed on and substantially covers the inorganic optically transparent layer 157 disposed on the sidewall 118 . In some embodiments, as Fig.15 As schematically shown in FIG. 1 , for example, for each Fresnel element in at least a majority of the Fresnel elements 115 , the first layer 154 is disposed between the sidewall 118 and the optically absorptive layer 152 . In some embodiments, as Fig.14 Schematically shown in FIG. 1 , for example, the first layer 154 is omitted. In some embodiments, as shown in FIG. Fig.16 154, for example, for each of at least a majority of the Fresnel elements 115, a second layer 156 is disposed on the optically absorptive layer 152 opposite the sidewall 118 or opposite the first layer 154. For example, the second layer 156 can be a protective layer and / or an anti-reflective layer. The second layer can be as generally described for the first layer 154 (e.g., having an average thickness and / or extinction coefficient and / or concentration of light absorbing material within the ranges described for the first layer 154). The inorganic optically transparent layer 157 can be optionally omitted.

[0045] In some embodiments, such as Fig.17 , for example, the microstructured surface 168 is only partially coated with the inorganic optically transparent layer 157 (e.g., only on the sidewalls). For example, the inorganic optically transparent layer can be conformally coated onto the microstructured surface 168 and then subsequently removed (or at least partially removed) from the optical facets. Figures 13 to 16In any of the embodiments shown, the inorganic optically transparent layer 157 can optionally be removed or partially removed from the optical facets (e.g., a thin layer can remain). In some embodiments, the first major surface 112 of the optical film can include a major surface of the inorganic optically transparent layer 157 (e.g., surface 167 of the plurality of sidewalls 118). In some embodiments, the at least partially coated microstructured surface Fresnel elements 115 can include optical facets that are part of the microstructured surface 168 of the polymer layer 333, and can include sidewalls that are part of the major surface of the inorganic optically transparent layer 157. Alternatively, the Fresnel elements 115 can be considered to include optical facets and sidewalls that are part of the microstructured surface 168 of the polymer layer 333, and the inorganic optically transparent layer 157 can be considered to be a coating on the polymer microstructure sidewalls.

[0046] In some embodiments, the optical film includes a polymer layer 333 including a microstructured surface 168 at least partially coated with an inorganic optically transparent layer 157. The at least partially coated microstructured surface 177 (which may include optical facets 117 and a surface 167 of the inorganic optically transparent layer 157 on the sidewalls) includes a plurality of Fresnel elements 115. Each Fresnel element 115 includes an optical facet 117 and a sidewall 118 that intersects the optical facet 117 at a ridge 119 of the Fresnel element 115. The optical facet 117 and the sidewall 118 may define an oblique angle therebetween. For each Fresnel element 115 in at least a majority of the Fresnel elements, an optically absorptive layer 152 is disposed on and substantially covers the sidewall 118.

[0047] In some embodiments, the optical film includes a polymer layer 333 that includes a major surface 168 that includes a plurality of Fresnel elements. Each Fresnel element includes an optical facet (e.g., 117) and a sidewall (e.g., a portion of the major surface 168 that faces a sidewall portion of the inorganic optically transparent layer 157) that intersects the optical facet at a ridge of the Fresnel element. The optical facet and the sidewall may define a tilt angle therebetween. For each Fresnel element in at least a majority of the Fresnel elements: the inorganic optically transparent layer 157 is disposed on the sidewall and substantially covers the sidewall; and the optically absorptive layer 152 is disposed on the inorganic optically transparent layer 157, opposite the sidewall where the optically absorptive layer 152 substantially covers the inorganic optically transparent layer 157.

[0048] In some embodiments, the inorganic optically transparent layer 157 can be disposed on the sidewalls along with other layers, such as an optically absorbing layer as follows. The inorganic optically transparent layer can be conformally coated onto the microstructured polymer layer, the optically absorbing layer (and optionally other layers) can be conformally coated onto the inorganic optically transparent layer, the optically absorbing layer (and optionally other layers) can be removed from the optical facets in a first etching step, and then the inorganic optically transparent layer can be removed or partially removed from the optical facets in a second etching step. For example, the first etching step and the second etching step can utilize different etchants (e.g., oxygen plasma only for the first etching step and fluorinated gas or other halogenated gas for the second etching step) so as to remove the desired layers in the corresponding etching steps.

[0049] In some embodiments, for each Fresnel element in at least a majority of the Fresnel elements 115, the first layer 154 has an average thickness t1, and the optically absorptive layer 152 has an average thickness t2. In some embodiments, t1 and t2 are each greater than 100 nm and less than 2 microns or less than 1 micron or less than 500 nm. In some embodiments, t1 and t2 are each less than 500 nm. In some embodiments, for each Fresnel element in at least a majority of the Fresnel elements 115, the sidewall 118 of the Fresnel element has a height H from the bottom of an adjacent Fresnel element to a ridge 119 of the Fresnel element. In some embodiments, H / t1>10, or H / t1>12, or H / t1>15, or H / t1>18, or H / t1>20. In some embodiments, H / t1<100. In some embodiments, H / t2>10, or H / t2>12, or H / t2>15, or H / t2>18, or H / t2>20. In some embodiments, H / t2<100. For example, in some embodiments, H / t1>15 and H / t2>15.

[0050] In some embodiments, for each Fresnel element in at least a majority of the Fresnel elements, the optical facet 117 is planar (eg, as in a typical conventional Fresnel lens). Fig.18 is a schematic diagram of a Fresnel element having planar optical facets 217, wherein the sidewalls 218 of the Fresnel element are at an angle to the bottom 285 of the Fresnel element In other embodiments, for each Fresnel element in at least a majority of the Fresnel elements 115, the optical facets 117 are curved. For example, the facets can be curved to adjust various optical properties, as described, for example, in U.S. Statutory Invention Registration No. H423 (Mohon et al.). For example, the optical facets 117 can be curved, such as Fig.19 Schematically depicted for curved optical facets 317 (recessed toward sidewall 218 and / or toward bottom 285 and / or toward major surface 114 (eg, see Figure 1 )), or Fig. 20 Schematically depicted in FIG. 1 for a curved optical facet 417 (convex toward the side wall 218 and / or toward the bottom 285 and / or toward the main surface 114). Figures 18 to 20 Angle in For example, it may be in the range of 80 to 90 degrees or 85 to 90 degrees. In some embodiments, for example, due to manufacturing limitations, Less than 90 degrees (e.g., 85 to 89.5 degrees).

[0051] Fig.21FIG. 517 is a schematic illustration of the arithmetic mean deviation surface roughness Ra of a surface, which may be the surface of an optical facet. The arithmetic mean deviation surface roughness Ra is the average of the absolute values of the deviations of the surface heights from the mean surface height (e.g., the average over the length of a cross-section through the facet (e.g., a cross-section orthogonal to the length of the facet)). In the case of a curved facet, the deviation of the surface height may be defined relative to the mean or nominal curved surface. In some embodiments, for each of at least most of the Fresnel elements 115, the optical facet 117 has an arithmetic mean deviation surface roughness Ra of <250 nm. In some embodiments, Ra is less than 200 nm, 150 nm, 100 nm, 70 nm, 50 nm, 30 nm, or 20 nm. In some embodiments, Ra is greater than 50 nm, 70 nm, 90 nm. For example, in some embodiments, 50 nm < Ra < 200 nm or 70 nm < Ra < 200 nm. For some applications, it is desirable for the optical facet 117 to be optically smooth (e.g., Ra < 50 nm). For other applications, a certain degree of surface roughness may be required (e.g., 50 nm < Ra < 250 nm or 90 nm < Ra < 200 nm). For example, having a certain degree of surface roughness for anti-reflection may be useful, for providing (e.g., weak) optical diffusion and / or for improving adhesion. It has been found that depositing and removing an optical absorption layer from an inclined facet using the method described in International Patent Application WO 2019 / 118685 (Schmidt et al.) results in a surface roughness higher than desired (e.g., Ra greater than 300 nm or greater than 400 nm) for many applications, even though the same method applied to a horizontal facet results in a low surface roughness. However, it has been found that, for example, by carefully controlling the time the optical facet is exposed to the plasma in a reactive ion etching process, the surface roughness (e.g., Ra) can be reduced (e.g., reduced to less than 250 nm or less than 200 nm). Additionally, it has been found that applying an inorganic optically transparent layer as an etch barrier layer before applying the optical absorption layer (and / or other layers) can further reduce the surface roughness and can produce an optically smooth optical facet (e.g., Ra < 50 nm), even with a longer etch time. The root mean square surface roughness Rq can also be determined and can be within any of the ranges described for Ra.

[0052] The surface roughness can be determined from the surface profile measured, for example, by atomic force microscopy (AFM), stylus profilometry, or optical profilometry. For small surface roughness (e.g., less than 50 nm), AFM is generally preferred.

[0053] The inorganic optically transparent layer, the first layer (e.g., a capping layer), the optically absorbing layer, and / or the second layer (e.g., a protective layer) can be deposited using any suitable deposition technique (e.g., as a conformal coating over the entire structured surface). Various coating methods that can be used include, for example, layer-by-layer (LbL) coating, chemical vapor deposition (CVD), sputtering, reactive sputtering, and atomic layer deposition (ALD). At least some of the deposited layers (e.g., all of the layers, or all of the layers except the optional inorganic optically transparent layer) can then be substantially removed from the optical facets (e.g., at least to the extent that any material of at least some of the deposited layers remaining on the optical facets does not significantly affect optical performance).

[0054] In some embodiments, at least one of the first layer and the optical absorption layer includes at least two bilayers deposited by what is commonly referred to as a "layer-by-layer self-assembly process". This process is typically used to electrostatically assemble films or coatings of oppositely charged polymer electrolytes, but other functions such as hydrogen bond donors / acceptors, metal ions / ligands, and covalent bond moieties can be driving forces for film assembly. Some examples of suitable processes include those described in U.S. Patents 8,234,998 (Krogman et al.) and 8,313,798 (Nogueira et al.), U.S. Patent Application Publication 2011 / 0064936 (Hammond-Cunningham et al.), and International Patent Application WO 2019 / 118685 (Schmidt et al.). For example, layer-by-layer dip coating can be performed using a StratoSequence VI (nanoStrata Inc., Tallahassee, FL) dip coating robot.

[0055] In some embodiments, the plurality of bilayers deposited by layer-by-layer self-assembly is a polymer electrolyte stack comprising an organic polymer polyion (e.g., cation) and a counterion (e.g., anion) containing a light absorbing material (e.g., pigment). At least a portion of the cationic layer, at least a portion of the anionic layer, or at least a portion of a combination thereof may include a light absorbing material (e.g., pigment) ionically bonded to the polymer electrolyte. The light absorbing compound may be dispersed within at least a portion of the polymer electrolyte layer. A variety of polymer electrolytes may be utilized, including inorganic compounds such as silica or silicates, and various phosphonocarboxylic acids and salts thereof (some of which are described in U.S. Pat. No. 10,365,415 (Schmidt)).

[0056] The thickness of the bilayer and the number of bilayers can be selected to achieve the desired optical properties (e.g., light absorption in the case of an optically absorbing layer, or reduced reflection between the sidewall and the optically absorbing layer in the case of a first layer). In some embodiments, the thickness of the bilayer and / or the number of bilayers are selected to achieve the desired optical properties using the self-assembled layer with the minimum total thickness and / or the minimum number of layer-by-layer deposition steps. The thickness of each bilayer is typically in the range of about 5 nm to 350 nm. The number of bilayers is typically at least 5, 6, 7, 8, 9, or 10. In some embodiments, the number of bilayers per stack is no more than 150 or 100. It should be understood that the individual bilayers in the final product may not be distinguishable from each other by common methods in the art such as scanning electron microscopy (SEM) or transmission electron microscopy (TEM). In some embodiments, the thickness of the bilayer and the total number of bilayers are selected so that the total thickness of the first layer and the optically absorbing layer is less than 2 microns.

[0057] After the optically absorbing layer and the optional first and / or second layer and / or the inorganic optically transparent layer are applied to the (e.g., entire) microstructured surface of the film and dried, the optically absorbing layer and the first and second layers (when present) can be removed from the optical facets and also from the terrace regions (if any) between adjacent Fresnel elements. The inorganic optically transparent layer, which can serve as an etch stop, is typically not removed, but can optionally be removed in a subsequent etching step.

[0058] Any suitable method can be used to selectively remove the layer from the optical facet. In some embodiments, the layer is removed by reactive ion etching. Reactive ion etching (RIE) is a directional etching process that utilizes ion bombardment to remove materials. RIE systems are used to remove organic or inorganic materials by etching the surface in the direction of ion bombardment. The most significant difference between reactive ion etching and isotropic plasma etching is the etching direction. Reactive ion etching is characterized in that the ratio of the vertical etching rate to the lateral etching rate is greater than 1. Systems for reactive ion etching are usually built around a durable vacuum chamber. Before starting the etching process, the chamber is usually evacuated to a base pressure below 1 Torr, 100mTorr, 20mTorr, 10mTorr or 1mTorr. Typically, the electrode holds the material to be processed and is electrically isolated from the vacuum chamber. The electrode may be a rotatable electrode in the shape of a cylinder. The counter electrode is also usually arranged in the chamber and may include a vacuum reactor wall. A gas containing an etchant usually enters the chamber through a control valve. The chamber gas can be continuously evacuated by a vacuum pump to maintain the process pressure. The type of gas used depends on the etching process. For example, the etchant may include one or more of oxygen, fluorinated gases, or other halogenated gases. Carbon tetrafluoride (CF4), sulfur hexafluoride (SF6), octafluoropropane (C3F8), perfluorohexane (C6F 14), trifluoromethane (CHF3), boron trichloride (BCl3), hydrogen bromide (HBr), nitrogen trifluoride (NF3), chlorine, argon and oxygen are commonly used for etching. Radio frequency (RF) power is applied to the electrodes to generate a plasma. Reactive ion etching is known in the art and is further described in U.S. Pat. No. 8,460,568 (David et al.) and International Patent Application WO 2019 / 118685 (Schmidt et al.).

[0059] The sample can be passed through the plasma over the electrode for a controlled period of time to achieve a specific etch depth or to limit etching so that the etching does not penetrate below the layer desired to be removed. For example, if it is desired to remove an optically absorbing layer, and no inorganic optically transparent layer is included, the period of time can be adjusted to minimize the time that the optical facets can be exposed to the plasma. As described elsewhere, this can reduce the surface roughness of the optical facets (e.g., to Ra < 250 nm).

[0060] In some embodiments, the optically absorbing layer (and / or other layers) is removed by laser ablation (e.g., pulsed laser ablation). Pulsed laser ablation (PLA) is a directional ablation process that utilizes a photon flux to generate sufficient photon energy density to remove material by oxidation and / or sputtering. PLA systems are used to remove organic or inorganic materials by etching the surface in the direction of the photon energy density, which can evaporate the target when a plasma plume is generated. PLA depends on the wavelength of the laser, and the amount of absorption present at that wavelength in the light-absorbing material. The absorption of the laser pulse generates energy for thermal, chemical or mechanical evaporation, ablation and / or plasma formation. The presence of an oxidizing gas in the laser pulse position may change the amount of chemical oxidation that occurs during the PLA process. The energy density can be varied by optical configuration to change the size and position of the focus, and can be varied by the power setting of the laser system. The relative orientation of the laser energy density and the sample can allow a person skilled in the art to ablate an optically absorbing layer coated on a structured surface at an oblique angle.

[0061] U.S. application 62 / 932,578, filed on November 8, 2019 and entitled “OPTICAL FILM,” describes an example of an optical film including microstructures, each microstructure having an optical facet and sidewalls that intersect the optical facets at a ridge of the microstructure, wherein an optically absorbing layer is disposed on the sidewalls and not on the optical facets.

[0062] The following is a list of exemplary embodiments of the present specification.

[0063] A first embodiment is an optical system comprising:

[0064] light source;

[0065] an optical film curved about a first axis and comprising a microstructured first major surface and an opposing second major surface, the microstructured first major surface defining a linear Fresnel lens comprising a plurality of Fresnel elements extending longitudinally along the first axis; and

[0066] A light control film is bent around the first axis and disposed between the light source and the optical film, the first major surface of the optical film faces the light control film, the light control film includes a plurality of alternating optically transmissive regions and optically absorptive regions extending longitudinally along the first axis, such that in a cross section orthogonal to the first axis, for at least most of the optically transmissive regions, a center line between adjacent optically absorptive regions is substantially perpendicular to the major surface of the light control film.

[0067] A second embodiment is the optical system according to the first embodiment, wherein the light source includes a backlight.

[0068] A third embodiment is the optical system according to the second embodiment, wherein the backlight comprises a light guide that is curved about the first axis.

[0069] A fourth embodiment is the optical system according to the second embodiment or the third embodiment, further comprising a display panel disposed between the backlight and the light control film.

[0070] A fifth embodiment is the optical system according to any one of the first to third embodiments, further comprising a display panel, wherein the optical film is disposed between the display panel and the light control film.

[0071] A sixth embodiment is the optical system according to the first embodiment, wherein the light source comprises an organic light emitting diode (OLED) display curved about the first axis.

[0072] A seventh embodiment is an optical system according to any one of the first to sixth embodiments, wherein the plurality of Fresnel elements comprises a plurality of polymer Fresnel elements at least partially coated with an inorganic optically transparent layer.

[0073] An eighth embodiment is an optical system according to the seventh embodiment, wherein each Fresnel element comprises an optical facet and a sidewall intersecting the optical facet at a ridge of the Fresnel element, wherein for each Fresnel element in at least a majority of the Fresnel elements, an optically absorptive layer is disposed on and substantially covers the inorganic optically transparent layer disposed on the sidewall.

[0074] The ninth embodiment is the optical system according to any one of the first to eighth embodiments, wherein an air gap separates at least most of the first major surface of the optical film from the light control film.

[0075] The tenth embodiment is the optical system according to any one of the first to ninth embodiments, wherein the optical film and the light control film are adhesively bonded to each other along at least an edge of the optical film and a corresponding edge of the light control film.

[0076] The eleventh embodiment is the optical system according to any one of the first to ninth embodiments, wherein the first major surface of the optical film includes a plurality of elongated spaced members extending toward the light control film and received by an adhesive layer provided on the light control film.

[0077] The twelfth embodiment is the optical system according to any one of the first to ninth embodiments, further comprising:

[0078] A low refractive index layer provided on and in contact with at least most of the first major surface of the optical film, the low refractive index layer having a refractive index of no more than 1.45; and

[0079] An optical adhesive that bonds the low refractive index layer to the light control film.

[0080] The thirteenth embodiment is an optical system comprising:

[0081] An optical film that is bent about a first axis and includes a microstructured first major surface and an opposite second major surface, the microstructured first major surface defining a linear Fresnel lens, the linear Fresnel lens including a plurality of Fresnel elements longitudinally extending along the first axis, each Fresnel element including an optical facet and a sidewall intersecting the optical facet at a ridge of the Fresnel element, wherein for each of at least most of the Fresnel elements, an optical absorption layer is provided on the sidewall and substantially covers the sidewall, the optical absorption layer having an average thickness t, 100 nm < t < 1 μm;

[0082] A light control film that is bent about the first axis and is disposed adjacent to the optical film, the first major surface of the optical film facing the light control film, the light control film including a plurality of alternating optical transmission regions and optical absorption regions longitudinally extending along the first axis, such that in a cross-section orthogonal to the first axis, for at least most of the optical transmission regions, a centerline between adjacent optical absorption regions is substantially perpendicular to the major surface of the light control film.

[0083] The fourteenth embodiment is an optical system according to the thirteenth embodiment, wherein for each Fresnel element of at least most of the Fresnel elements, the sidewall of the Fresnel element has a height H from the bottom of an adjacent Fresnel element to the ridge of the Fresnel element, H / t>15.

[0084] The fifteenth embodiment is an optical system according to the thirteenth embodiment or the fourteenth embodiment, wherein the extinction coefficient of the multiple Fresnel elements k0<0.05, and wherein for each Fresnel element of at least most of the Fresnel elements, a first layer is arranged between the side wall and the optical absorption layer, the extinction coefficient of the first layer k1>k0, and the optical absorption layer has an extinction coefficient k2, wherein k2-k1>0.05.

[0085] Example

[0086] For example Figure 1 The optical system generally described in and the comparative optical system are optically modeled, wherein the Fresnel lens is disposed on the light control film, wherein the Fresnel facets face away from the light source 130. The light source surface is taken to have a parabolic shape in a cross section orthogonal to the z-axis, wherein the reference Figure 1 The xyz coordinate system of the shape is y = 1 / 20.00645x 2 Given as , where y and x are in mm. The radius of curvature at x=0 is about 155 mm. The Fresnel lens is designed so that light emitted perpendicular to the light source surface is refracted by the Fresnel lens into the y direction. The brightness distribution is calculated for various viewing angles (90 degrees on axis) and various positions along the light source surface (x coordinate). Fig. 22 Results for a comparative optical system are shown. Fig.23 shows that there is no optical absorption layer on the sidewalls Figure 1 The result of the optical system. Fig.24 An optically absorbing layer (e.g., Fig.11 shown) Figure 1 The comparison optical system shows a bright band for a viewing angle of about 60 degrees and an x-coordinate of about -40 mm. Figure 1 and Fig.23 In an optical system with a resolution of 1000 x 2000 pixels, the bright band is substantially eliminated. However, there is light leakage for a viewing angle of about 50 degrees and an x-coordinate of about 55 mm, which may be objectionable in some applications. Fig.24 The inclusion of an optically absorptive layer on the sidewalls substantially eliminates this light leakage.

[0087] All references, patents, and patent applications cited above are hereby incorporated by reference in their entirety in a consistent manner. In the event of inconsistencies or conflicts between an incorporated reference portion and the present application, the information in the foregoing description shall prevail.

[0088] Unless otherwise indicated, the description of the elements in the drawings should be understood to apply equally to the corresponding elements in the other drawings. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that the specific embodiments shown and described may be replaced with a variety of alternative and / or equivalent implementations without departing from the scope of the present disclosure. The present application is intended to cover any modification or variation of the specific embodiments discussed herein. Therefore, the present disclosure is intended to be limited only by the claims and their equivalents.

Claims

1. An optical system, comprising: Light source; An optical film that is bent around a first axis and includes a microstructured first major surface and an opposite second major surface, the microstructured first major surface defining a linear Fresnel lens that includes a plurality of Fresnel elements longitudinally extending along the first axis; and A light control film that is bent around the first axis and disposed between the light source and the optical film, with the first major surface of the optical film facing the light control film, the light control film including a plurality of alternating optically transmissive regions and optically absorptive regions longitudinally extending along the first axis, such that in a cross-section orthogonal to the first axis, for at least most of the optically transmissive regions, the centerline between adjacent optically absorptive regions is substantially perpendicular to the major surface of the light control film. The optical system of claim 1 , wherein the light source comprises a backlight.

3. The optical system of claim 2, wherein the backlight comprises a light guide that is curved about the first axis. 4 . The optical system of claim 2 , further comprising a display panel disposed between the backlight and the light control film. 5 . The optical system of claim 1 , further comprising a display panel, the optical film being disposed between the display panel and the light control film.

6. The optical system of claim 1, wherein the light source comprises an organic light emitting diode (OLED) display that is curved about the first axis.

7. The optical system of claim 1, wherein the plurality of Fresnel elements comprises a plurality of polymer Fresnel elements at least partially coated with an inorganic optically transparent layer.

8. The optical system of claim 7, wherein each Fresnel element comprises an optical facet and a sidewall intersecting the optical facet at a ridge of the Fresnel element, wherein for each Fresnel element in at least a majority of the Fresnel elements, an optically absorptive layer is disposed on and substantially covers the inorganic optically transparent layer disposed on the sidewall.

9. The optical system of claim 1, wherein an air gap separates at least a majority of the first major surface of the optical film from the light management film.

10. The optical system of any one of claims 1 to 9, wherein the optical film and the light management film are adhered to each other along at least an edge of the optical film and a corresponding edge of the light management film.

11. The optical system of claim 1 , wherein the first major surface of the optical film comprises a plurality of elongated spacing members extending toward the light management film and received by an adhesive layer disposed on the light management film.

12. The optical system of claim 1 , further comprising: A low refractive index layer that is disposed on and in contact with at least most of the first major surface of the optical film, the low refractive index layer having a refractive index of no more than 1.45; and An optical adhesive that bonds the low refractive index layer to the light control film.

13. An optical system comprising: Light source; An optical film that is bent around a first axis and includes a microstructured first major surface and an opposite second major surface, the microstructured first major surface defining a linear Fresnel lens that includes a plurality of Fresnel elements longitudinally extending along the first axis, each Fresnel element including an optical facet and a sidewall intersecting the optical facet at a ridge of the Fresnel element, wherein for at least most of the Fresnel elements, an optical absorption layer is disposed on and substantially covers the sidewall, the optical absorption layer having an average thickness t, where 100 nm < t < 1 μm; A light control film that is bent around the first axis, disposed between the light source and the optical film, and disposed close to the optical film, with the first major surface of the optical film facing the light control film, the light control film including a plurality of alternating optically transmissive regions and optically absorptive regions longitudinally extending along the first axis, such that in a cross-section orthogonal to the first axis, for at least most of the optically transmissive regions, the centerline between adjacent optically absorptive regions is substantially perpendicular to the major surface of the light control film.

14. The optical system of claim 13, wherein for each Fresnel element of at least most of the Fresnel elements, the sidewall of the Fresnel element has a height H from the bottom of an adjacent Fresnel element to a ridge of the Fresnel element, H / t>15.

15. The optical system of claim 13, wherein the extinction coefficient of the plurality of Fresnel elements is k0<0.05, and wherein for each Fresnel element of at least most of the Fresnel elements, a first layer is disposed between the sidewall and the optical absorption layer, the first layer having an extinction coefficient k1>k0, the optical absorption layer having an extinction coefficient k2, wherein k2-k1>0.05.

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