Optical film

Abstract

To provide an optical film or a mirror film that is reflective up to a near-infrared band edge and permeable in loner wavelengths.SOLUTION: The skin layer has an average thickness greater than about 2 microns. A transmittance of the film for substantially normally incident light includes a band edge 120 separating first and second wavelength ranges 122, 126, where each range is at least 250 nm wide. A reflectance of the film is greater than about 95% for each wavelength in the first wavelength range 122; an average transmittance of the film is greater than about 80% in the second wavelength range 126; and a difference between maximum and minimum values of the optical transmittance of the film in the second wavelength range 126 is less than about 30%. The band edge 120 may have a slope that is greater than about 2% / nm. The transmittance may increase monotonically at least from about 10% to about 70% with increasing wavelength.SELECTED DRAWING: Figure 2

Description

Background Art

[0001] Optical films are used in various display applications. For example, a mirror film can be used for light recycling in the backlight unit of a liquid crystal display panel.

Summary of the Invention

[0002] In some aspects of the present description, an optical film is provided that includes a plurality of alternating first polymer layers and second polymer layers disposed on a skin layer. The first layer and the second layer each have an average thickness of less than about 250 nm. The skin layer has an average thickness of more than about 2 micrometers. The first layer, the second layer, and the skin layer are integrally formed with each other. The light transmittance of the optical film for light incident substantially perpendicularly has a band edge that separates a first wavelength range and a second wavelength range. The first wavelength range extends from at least about 400 nm to about 700 nm, and the second wavelength range extends from at least about 950 nm to about 1300 nm. For light incident substantially perpendicularly in air, the light reflectance of the optical film is greater than about 95% for each wavelength in the first wavelength range, the average light transmittance of the optical film is greater than about 80% in the second wavelength range, and the difference between the maximum value and the minimum value of the light transmittance of the optical film in the second wavelength range is less than about 25%. The best linear approximation to the band edge, which correlates the light transmittance with the wavelength over at least a wavelength range where the light transmittance increases from about 10% to about 70%, has a slope greater than about 2% / nm.

[0003] In some aspects of the present description, an optical film is provided that includes a plurality of alternating first and second polymer layers of numbers from 50 to 800. Each of the first and second layers has an average thickness of less than about 500 nm. The light transmittance of the optical film for light incident substantially perpendicularly has a band edge region that separates a first wavelength range and a second wavelength range, each range having a width of at least 250 nm. The difference between the maximum and minimum values of the light transmittance of the optical film in each wavelength range is less than about 30%. In the plane of the first and second layers, the first and second layers have respective refractive indices, namely, n1x and n2x along a first polarization state, n1y and n2y along a second polarization state orthogonal to the first polarization state, and n1z and n2z along a z-axis orthogonal to the first and second polarization states. For at least one wavelength that falls into at least one of the first wavelength range and the second wavelength range, each of n1x and n1y is at least 0.2 greater than n1z, the difference between n1x and n1y is less than about 0.04, the maximum difference among n2x, n2y, and n2z is less than about 0.01, and the difference between n1x and n2x is greater than about 0.2. The light transmittance of the optical film for light incident substantially perpendicularly within the band edge region increases monotonically from at least about 10% to about 70% as the wavelength increases.

[0004] In some aspects of the present description, there is provided a display including a display panel for displaying a visible image in a first wavelength range of at least about 400 nm to about 700 nm to an observer, an infrared light source for emitting light having an infrared wavelength beyond the first wavelength range toward the observer through the display panel, and a mirror film disposed between the display panel and the infrared light source. For light incident substantially vertically, and for each of the first and second polarization states orthogonal to each other, for each wavelength in the first wavelength range, the mirror film reflects at least 90% of the light, at the infrared wavelength, the mirror film transmits at least 70% of the light, and at a first wavelength between the first wavelength range and the infrared wavelength, the mirror film transmits 40% to 60% of the light. The first wavelength is within about 50 nm from the infrared wavelength.

Brief Description of the Drawings

[0005]

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Mode for Carrying Out the Invention

[0006] In the following description, reference is made to the accompanying drawings, which form a part of this specification and in which various embodiments are shown by way of example. The drawings are not necessarily to scale. It is to be understood that other embodiments may be contemplated and can be practiced without departing from the scope or spirit of this specification. Accordingly, the embodiments for carrying out the invention below are not to be construed in a limiting sense.

[0007] In some displays, a mirror film, such as an Enhanced Specular Reflector (ESR) film (available from 3M Company (St. Paul, MN)), is disposed below a liquid crystal display (LCD) panel (on the side opposite the light output side) in a recycled backlight unit. In some applications, the display includes one or more sensors. For example, a smartphone may include one or more of a proximity sensor, an ambient light sensor, or a fingerprint sensor. Such sensors have conventionally been disposed outside the active area of the display. To reduce the bezel around the display, it may be desirable to include one or more sensors disposed in the active area of the display. The sensor is preferably disposed below the mirror film. In some cases, it is desirable to include an infrared (IR) light source below the mirror film. For example, a fingerprint sensor may include an IR light source that emits IR light through the display panel, and may include a sensor that receives the IR light after being reflected from a finger and returning through the display panel. To operate the light source and / or sensor as desired, the mirror film preferably has a significant transmittance (e.g., at least 70%) for light of a wavelength adapted such that the light source and / or sensor functions. Typically, for an IR light source and sensor, this wavelength is 850 nm or 940 nm. Currently, mirror films used in displays of smartphones and other consumer electronics do not transmit light of these wavelengths.

[0008] In some embodiments of the present description, an optical film or a mirror film is provided that is reflective up to a near-infrared band edge and transmissive at longer wavelengths. The band edge can have a large (e.g., greater than about 2% / nm) slope. This can be desirable because the optical film provides recycling across the visible range for obliquely incident light (at least up to the desired angle of incidence) and is transmissive at infrared wavelengths close to the band edge at which the light source / sensor operates. This infrared wavelength can be, for example, within about 50 nm from the band edge. The optical film of the present description is also useful, for example, in automotive display applications where an IR source is included in the display behind a mirror film of the display to provide IR illumination inside the vehicle.

[0009] Multilayer optical films having a reflection band with a sharp band edge are described, for example, in U.S. Patent No. 6,157,490 (Wheatley et al.). In some cases, the band sharpening techniques described in this reference result in a sharp slope of the band edge measured between 10% and 50% transmittance, but ringing of the transmittance spectrum that can be present at transmittances less than 70% in the case of near-infrared band edges, resulting in a non-monotonic increase in transmittance from 10% to 70%, and as a result, the slope of the band edge measured between 10% and 70% transmittance can be substantially smaller due to ringing of the transmittance spectrum that can suppress the slope of the band edge. Such ringing can be caused, at least in part, by interference with light reflected by Fresnel reflection at the outermost major surface of the skin layer. It may also be desirable to suppress ringing of the transmittance in the infrared transmission region. According to some embodiments, by suitably selecting the thickness of the skin layer (e.g., according to some embodiments, using a skin layer having a thickness greater than about 2 micrometers), ringing of the transmittance can be suppressed in the infrared transmission region and can be suppressed or eliminated at least in the band edge region within the range where the transmittance changes from about 10% to about 70% or more. Other techniques for increasing the slope of the band edge at transmittances of 10% to 70% or more and / or suppressing unwanted ringing as described herein include increasing the number of layers in the optical film having a thickness that provides reflection at wavelengths near the band edge. For example, layer pairs (optical repeating units) in alternating first and second layers of the optical film can have a maximum optical thickness, and the alternating first and second layers can include at least 20, or at least 25, or at least 30 separate layer pairs having an optical thickness within about 30 nm, or about 25 nm, or about 20 nm, or about 15 nm, or about 12 nm, or about 10 nm, or about 8 nm, or about 7 nm, or about 6 nm of the maximum optical thickness.By using a suitable skin thickness and a suitable layer thickness profile, a monotonic increase in transmittance of at least about 10% to about 70% (e.g., from about 5% to about 80% or more) can be achieved, and the best linear approximation to the band edge that correlates the light transmittance to wavelength over at least the wavelength range where the light transmittance increases from about 10% to about 70% can result in a slope exceeding, for example, about 2% / nm.

[0010] Figure 1 is a schematic view of an optical film 100 including a plurality of alternating first polymer layers 101 and second polymer layers 102. In the illustrated embodiment, the plurality of alternating first layers 101 and second layers 102 are disposed on a skin layer 110. A second skin layer may be disposed on the opposite side of the plurality of alternating first layers 101 and second layers 102. The plurality of alternating first layers 101 and second layers 102 mainly reflect and transmit light by light interference and may be referred to as an optical layer or an interference layer. An interference layer may be described as mainly reflecting and transmitting light by light interference if the reflectivity and transmittance of the interference layer can be reasonably explained by light interference or can be reasonably accurately modeled as a result of light interference. Adjacent pairs of interference layers having different refractive indices reflect light by light interference when the pair has a combined optical thickness (physical thickness multiplied by the refractive index) of 1 / 2 of the wavelength of light. The refractive index used to determine the optical thickness can be a certain reference wavelength (e.g., 532 nm or 633 nm). The interference layer typically has a physical thickness of less than about 500 nanometers or less than about 250 nm. The skin layer typically has an optical thickness that is too large to mainly reflect and transmit light by light interference and may be referred to as a non-interference layer, a non-optical layer, or an optically thick layer. However, Fresnel reflection from the main surface of the skin layer (e.g., the outermost main surface) can affect the transmission spectrum of the optical film, as further described elsewhere in this specification.

[0011] The thicknesses of the first and second layers that interact can be selected to provide a desired reflection band, and the thickness profile can be selected to include a number of layer pairs having an optical thickness corresponding to the band edge, as further described elsewhere in this specification. The appropriate thickness of the skin layer can be determined by optical modeling, which can, for example, determine the transmission spectrum for a range of skin thicknesses. It has been found that the skin thickness that results in suppression of optical ringing can be selected.

[0012] The average thickness ti of the i-th layer is shown. In some embodiments, the first and second layers each have an average thickness of less than about 500 nm, or less than about 250 nm, or less than about 200 nm, or less than about 180 nm, or less than about 200 nm, or within the range of 20 nm to 250 nm, or within the range of 25 nm to 200 nm, or within the range of about 30 nm to about 180 nm. The average thickness refers to the simple average of the thicknesses across a region of the optical film 100. Since the thickness of the layer can be substantially constant (e.g., varying by 10% or less, or 5% or less, or 3% or less), the average thickness is the substantially constant thickness of the layer. In some embodiments, the skin layer 110 has an average thickness of greater than about 2 micrometers, or greater than about 3 micrometers, or greater than about 4 micrometers, or greater than about 5 micrometers, or greater than about 6 micrometers, or greater than about 7 micrometers. In some embodiments, the skin layer 110 has a thickness of less than about 30 micrometers, or less than about 20 micrometers, or less than about 15 micrometers, or less than about 10 micrometers. In some embodiments, the skin layer 110 has an average thickness within the range of, for example, about 2 micrometers to about 15 micrometers, or within the range of about 3 micrometers to about 10 micrometers. In some embodiments, an optional second skin layer is included, which has an average thickness within about 20% or less, or within about 15% or less, or within about 10% or less of the average thickness of the skin layer 110.

[0013] As used herein, a first element that is "formed integrally" with a second element means that the first and second elements are manufactured together, rather than being joined after being separately manufactured. Forming integrally includes manufacturing the first element and subsequently manufacturing the second element on the first element. An optical film including a plurality of layers is formed integrally when the layers are manufactured together (e.g., combined as a molten stream and then cast onto a chill roll to form a cast film having each layer, and subsequently orienting the cast film), rather than being joined after being separately manufactured. In some embodiments, the first layer 101, the second layer 102, and the skin layer 110 are formed integrally with each other. In some embodiments, the first layer 101, the second layer 102, the first skin layer 110, and the second skin layer on the opposite side are formed integrally with each other.

[0014] In some embodiments, the optical film 100 includes a plurality of alternating first polymer layers 101 and second polymer layers 102 in a number of 50 to 800, or 400 to 800, or 500 to 800. In some embodiments, only the layers having a thickness of less than about 500 nm or less than about 250 nm in the optical film are the first layer 101 and the second layer 102, and the total number of the alternating first polymer layers 101 and second polymer layers 102 is within the range of 50 to 800, or within the range of 400 to 800, or within the range of 500 to 800.

[0015] The optical films of this description can be made using any suitable light-transmissive material, but in many cases it is beneficial to use a low-absorbing polymer material. Such materials can reduce or make negligible the absorption rate of the micro-layer laminate at visible and infrared wavelengths, such that the sum of the reflectance and transmittance of the laminate (or the optical film of which the laminate forms a part) at any given wavelength and for any particular angle of incidence and polarization state is approximately 100%, i.e., R + T ≈ 100% or R ≈ 100% - T.

[0016] Suitable materials for the interacting first layer 101 and second layer 102, and the skin layer 110 include polyethylene naphthalate (PEN), copolymers containing PEN and polyester (e.g., polyethylene terephthalate (PET) or dibenzoic acid), glycol-modified polyethylene terephthalate, polycarbonate (PC), poly(methyl methacrylate) (PMMA), or blends of materials of these classes. In some embodiments, the first layer 101 includes PEN and the second layer 102 includes PMMA.

[0017] Exemplary optical films are composed of polymeric materials and can be fabricated using coextrusion processes, casting processes, and orientation processes. For methods of making such films, see U.S. Patent No. 5,882,774 (Jonza et al.), "Optical Film"; U.S. Patent No. 6,179,948 (Merrill et al.), "Optical Film and Process for Manufacture Thereof"; U.S. Patent No. 6,783,349 (Neavin et al.), "Apparatus for Making Multilayer Optical Films"; and Patent Application Publication No. 2011 / 0272849 (Neavin et al.), "Feedblock for Manufacturing Multilayer Polymeric Films". Multilayer optical films can be formed by coextrusion of the polymers described in any of the foregoing references. The polymers of the various layers can be selected to have similar rheological properties, e.g., to have a melt viscosity, such that these polymers can be coextruded without significant flow disturbance. The extrusion conditions are selected to properly feed, melt, mix, and pump each of the polymers as a continuous and stable feed stream or melt stream. The temperature used to form and maintain each of the melt streams can be selected within a range that avoids freezing, crystallization, or excessive pressure drop at the lower limit of the temperature range and avoids material degradation at the upper limit of the temperature range.

[0018] In some embodiments, the optical film 100 is reflective (e.g., having a light reflectance of greater than about 90% or greater than about 95%) within a first wavelength range (e.g., spanning at least about 430 nm to about 680 nm or at least about 400 nm to about 700 nm). Such an optical film may be described as a mirror film or a visible light mirror film. In some embodiments, the optical film 100 or mirror film is transmissive (e.g., having an average light transmittance of greater than about 75% or greater than about 80%) within a second wavelength range (e.g., spanning at least about 1000 nm to about 1250 nm or at least about 950 nm to about 1300 nm).

[0019] In some embodiments, the main reflection band provides reflection within the first wavelength range. The main reflection band or the fundamental harmonic reflection band is a reflection band in which wavelengths within the reflection band are reflected by a layer pair or an optical repeating unit having an optical thickness of half the wavelength. The layer pair or the optical repeating unit may also reflect higher order harmonics that are wavelengths that are inverse integer multiples of this fundamental wavelength.

[0020] In some embodiments, in a plane of the first layer 101 and the second layer 102 (parallel to the x-y plane with reference to the illustrated x-y-z coordinate system), the first layer 101 and the second layer 102 each have respective refractive indices, namely, n1x and n2x along the first polarization state (a polarization state with an electric field parallel to the x-axis), n1y and n2y along the second polarization state orthogonal to the first polarization state (a polarization state with an electric field parallel to the y-axis), and n1z and n2z along the z-axis orthogonal to the first polarization state and the second polarization state. For at least one wavelength (e.g., 532 nm or 633 nm) falling within at least one of the first wavelength range and the second wavelength range, each of n1x and n1y is at least 0.2 greater than n1z, the difference between n1x and n1y is less than about 0.04, the maximum difference among n2x, n2y, and n2z is less than about 0.01, and the difference between n1x and n2x is greater than about 0.2. For example, in some embodiments, at a wavelength of 633 nm, n1x is about 1.737, n1y is about 1.763, n1z is about 1.496, and n2x, n2y, and n2z are each about 1.495.

[0021] Figure 2 is a schematic graph of the light transmittance of an optical film for light incident substantially vertically. The light transmittance of the optical film includes a band edge 120 that separates a first wavelength range 122 and a second wavelength range 126. The band edge region 124 includes at least one wavelength range in which the light transmittance increases from about 10% to about 70% as the wavelength increases. In some embodiments, the light transmittance of the optical film for light incident substantially vertically within the band edge region 124 increases monotonically from at least about 10% to about 70%, or from at least about 10% to about 75%, or from at least about 10% to about 80%, or from at least about 5% to about 80% as the wavelength increases. In some embodiments, the first wavelength range 122 extends over at least about 400 nm to about 700 nm. In some embodiments, the second wavelength range 126 extends over at least about 950 nm to about 1300 nm. In some embodiments, the first wavelength range and the second wavelength range each have a width of at least 250 nm or at least 300 nm. In some embodiments, the band edge region 124 has a width of 30 nm or less, 20 nm or less, or 15 nm or less.

[0022] Light incident substantially vertically is light that is close enough to being incident perpendicular to the optical film that the transmittance and reflectivity of the light incident substantially vertically differ only negligibly from the transmittance and reflectivity of the light incident perpendicular to the optical film. In some embodiments, the light incident substantially vertically may be within 20 degrees, 10 degrees, or 5 degrees of normal incidence, or may be normal incidence or nominally normal incidence.

[0023] The transmittance of an optical element (e.g., an optical film or a mirror film) generally refers to the intensity of transmitted light divided by the intensity of incident light (for light with a given wavelength, incident direction, etc.), and may also be expressed by terms such as "external transmittance" or "internal transmittance". The external transmittance of an optical element is the transmittance of that optical element when the surroundings are air and no correction is made regarding the Fresnel reflection at the air / element interface in front of the element or regarding the Fresnel reflection at the element / air interface behind the element. The internal transmittance of an optical element is the transmittance of that element when the Fresnel reflections at its front and rear surfaces are removed. Removing the front and rear Fresnel reflections can be done either computationally (e.g., by subtracting an appropriate function from the external transmittance spectrum) or experimentally. For many types of polymer and glass materials, the Fresnel reflection is approximately 4 - 6% at each of the two external surfaces (for normal or near-normal angles of incidence), which causes the external transmittance to shift approximately 10% downward compared to the internal transmittance. When referring to the transmittance without specifying internal or external in this specification, the transmittance may be regarded as referring to the external transmittance, unless otherwise specified or particularly indicated by the context.

[0024] In some embodiments, for light incident substantially vertically in air, the light reflectance of the optical film is greater than about 95% (e.g., greater than 93%, or greater than 94%, or greater than 95%, or greater than 96%) for each wavelength in a first wavelength range 122, the average light transmittance of the optical film is greater than about 80% in a second wavelength range 126, and the difference between the maximum value 132 and the minimum value 134 of the light transmittance of the optical film in the second wavelength range 126 is less than about 30% or less than about 25% (e.g., the maximum value 132 of the light transmittance in the second wavelength range 126 may be about 95% and the minimum value 134 may be about 75%, whereby the difference is about 20%). The average light transmittance in the second wavelength range 126 is the simple average of the polarization states over the wavelengths in the second wavelength range 126.

[0025] In some embodiments, the display includes an optical film and an infrared light source adapted to emit light at an infrared wavelength 127. Optionally, the optical film is desirably transmissive to the infrared wavelength 127 and reflective to wavelengths near the infrared wavelength 127 (e.g., about 50 nm shorter). In some embodiments, at the infrared wavelength 127, the optical film transmits at least 70% of the light incident substantially perpendicularly, and at a first wavelength 123 between the first wavelength range 122 and the infrared wavelength 127, the optical film transmits 40% - 60% of the light incident substantially perpendicularly. In some embodiments, the first wavelength 123 is within about 60 nm, or about 50 nm, or about 40 nm, or about 30 nm, or about 20 nm of the infrared wavelength 127.

[0026] In some embodiments, the best linear approximation to the band edge that correlates light transmittance to wavelength over at least a wavelength range in which the light transmittance increases from about 10% to about 70% (e.g., band edge region 124) has a slope greater than about 2% / nm, or greater than about 3% / nm, or greater than about 4% / nm, or greater than about 5% / nm. The best linear approximation 136 is schematically shown in FIG. 2. The best linear approximation 136 can be determined as the linear least squares approximation to the transmittance as a function of wavelength over at least a wavelength region in which the transmittance increases from about 10% to about 70% (e.g., over a wavelength range in which the transmittance increases from about 10% to about 70%, or from about 10% to about 75%, or from about 10% to about 80%). In some embodiments, the best linear approximation to the band edge that correlates light transmittance to wavelength extends over at least a wavelength range in which the light transmittance increases from about 10% to about 75% or from about 10% to about 80%. In some embodiments, the best linear approximation to the band edge that correlates light transmittance to wavelength over at least a wavelength range in which the light transmittance increases from about 10% to about 75% has a slope greater than about 2% / nm, or greater than about 3% / nm, or greater than about 4% / nm, or greater than about 5% / nm. In some embodiments, the best linear approximation to the band edge that correlates light transmittance to wavelength over at least a wavelength range in which the light transmittance increases from about 10% to about 80% has a slope greater than about 2% / nm, or greater than about 3% / nm, or greater than about 4% / nm, or greater than about 5% / nm.

[0027] FIG. 3 is a schematic diagram of an optical thickness profile of adjacent layer pairs of an optical film according to some embodiments. The optical thickness profile is the average optical thickness of each layer pair as a function of the layer pair number that refers to the consecutive numbers of the alternating first and second layer pairs included in the film. The optical thickness profile is defined by the optical thicknesses of distinct pairs (optical repeating units) of adjacent first layer 101 and second layer 102 among the plurality of alternating first polymer layers 101 and second polymer layers 102. Different layer pairs are distinct (i.e., different layer pairs do not include a common layer). Each layer pair includes one first layer and one second layer and does not include an additional first or second layer between the one first layer and the one second layer (e.g., the one first layer and the one second layer can be directly adjacent). The optical thickness of each layer is the product of the average thickness of the layer and the in-plane refractive index of the layer. In the illustrated embodiment, the optical film includes a first packet 141 and a second packet 143, and each packet has an optically thickness that varies substantially continuously with the layer pairs. An optically thick (e.g., having an optical thickness of at least 2 micrometers) protective boundary layer can be optionally disposed between the first packet 141 and the second packet 143. The optical film has a layer pair 140 having a maximum optical thickness Tm (no other layer pairs among the plurality of alternating first layers 101 and second layers 102 have a greater optical thickness). In some embodiments, the optical thickness profile increases from the outermost layer pair 142 among the plurality of alternating first polymer layers 101 and second polymer layers 102 to the layer pair 140 having the maximum optical thickness Tm, and decreases from the layer pair 140 having the maximum optical thickness Tm in a direction away from the outermost layer pair 142. In some embodiments, the maximum optical thickness Tm is at least 15 nm greater than the optical thickness of the outermost layer pair 142. In some embodiments, the maximum optical thickness Tm is in the range of about 330 nm to about 480 nm or in the range of about 360 nm to about 460 nm.

[0028] In some embodiments, the optical film includes a significant number of successive layer pairs 144 having an optical thickness close to the maximum optical thickness Tm. In some embodiments, the plurality of alternating first polymer layers 101 and second polymer layers 102 have an optical thickness within about 20 nm, or within about 15 nm, or within about 12 nm, or within about 10 nm, or within about 8 nm, or within about 7 nm, or within about 6 nm of the maximum optical thickness Tm and include at least 20 distinct layer pairs (e.g., layer pair 144). In some embodiments, the plurality of alternating first layers 101 and second layers 102 have an optical thickness within about 25 nm, or within about 20 nm, or within about 15 nm, or within about 12 nm, or within about 10 nm, or within about 8 nm, or within about 7 nm, or within about 6 nm of the maximum optical thickness Tm and include at least 25 distinct layer pairs. In some embodiments, the plurality of alternating first layers 101 and second layers 102 have an optical thickness within about 30 nm, or within about 25 nm, or within about 20 nm, or within about 15 nm, or within about 12 nm, or within about 10 nm, or within about 8 nm, or within about 7 nm, or within about 6 nm of the maximum optical thickness Tm and include at least 30 distinct layer pairs.

[0029] In some embodiments, a pair of adjacent first and second polymer layers among the plurality of alternating first and second polymer layers has a maximum optical thickness (e.g., layer pair 140), and the plurality of alternating first and second polymer layers includes at least 20 distinct layer pairs (e.g., layer pair 144) having an optical thickness within about 20 nm, or within about 15 nm, or within about 12 nm, or within about 10 nm, or within about 8 nm, or within about 7 nm, or within about 6 nm of the maximum optical thickness. In some embodiments, a pair of adjacent first and second polymer layers among the plurality of alternating first and second polymer layers has a maximum optical thickness (e.g., layer pair 140), and the plurality of alternating first and second polymer layers includes at least 25 distinct layer pairs (e.g., layer pair 144) having an optical thickness within about 25 nm, or within about 20 nm, or within about 15 nm, or within about 12 nm, or within about 10 nm, or within about 8 nm, or within about 7 nm, or within about 6 nm of the maximum optical thickness. In some embodiments, a pair of adjacent first and second polymer layers among the plurality of alternating first and second polymer layers has a maximum optical thickness (e.g., layer pair 140), and the plurality of alternating first and second polymer layers includes at least 30 distinct layer pairs (e.g., layer pair 144) having an optical thickness within about 30 nm, or within about 25 nm, or within about 20 nm, or within about 15 nm, or within about 12 nm, or within about 10 nm, or within about 8 nm, or within about 7 nm, or within about 6 nm of the maximum optical thickness. In some embodiments, the optical thickness profile may differ from that shown in FIG. 3. For example, the optical thickness may decrease non-monotonically from the layer pair 140 having the maximum optical thickness to the outermost layer pair 142, or may have a substantially constant optical thickness from the layer pair 140 to the outermost layer pair 142. In some embodiments, the plurality of layer pairs may have the same maximum optical thickness. In this case, any of these layer pairs can be the layer pair 140 having the maximum optical thickness (e.g., the layer pair farthest from the outermost layer pair 142 among these pairs).

[0030] In some embodiments, a display is provided that includes any of the optical films described elsewhere herein. FIG. 4 is a schematic diagram of a display 401 that includes an optical film 400, a display panel 450 for displaying a visible image 455 within a first wavelength range (e.g., wavelength range 122) to an observer 460, and at least one of an infrared sensor 454 or an infrared light source 452 disposed adjacent to the optical film 400 on a side opposite the display panel 450. The display panel 450 is disposed between the optical film 400 and the observer 460. In the illustrated embodiment, both an infrared sensor 454 and an infrared light source 452 are included. In other embodiments, one of the infrared sensor 454 and the infrared light source 452 may be omitted. The optical film 400 can be a mirror film.

[0031] In some embodiments, the display 401 includes a display panel 450 for displaying a visible image 455 to an observer 460 within a first wavelength range of at least about 400 nm to about 700 nm, an infrared light source 452 for radiating light having an infrared wavelength (e.g., infrared wavelength 127 shown in FIG. 2) beyond the first wavelength range through the display panel 450 toward the observer 460, and a mirror film 400 disposed between the display panel 450 and the infrared light source 452, the mirror film 400 reflecting at least 90% of the light for each wavelength in the first wavelength range for light incident substantially perpendicularly and for each of a first polarization state and a second polarization state that are orthogonal, transmitting at least 70% of the light at the infrared wavelength, and transmitting 40% - 60% or 40% - 50% of the light at a first wavelength (e.g., wavelength 123 shown in FIG. 2) between the first wavelength range and the infrared wavelength. In some embodiments, the first wavelength is within about 60 nm, about 50 nm, about 40 nm, about 30 nm, or about 20 nm from the infrared wavelength.

[0032] In some embodiments, the infrared light source 452 is or includes a laser diode or a light emitting diode (LED). In some embodiments, the infrared light source 452 has a peak at an infrared wavelength and has an emission spectrum with a full width at half maximum of about 50 nm or less, about 40 nm or less, about 30 nm or less, about 20 nm or less, or about 10 nm or less. FIG. 5 is a schematic diagram of the emission spectrum of the infrared light source 452 having a peak at a wavelength λI corresponding to an infrared wavelength and a full width at half maximum of 459. In some embodiments, the infrared wavelength λI is about 850 nm or about 940 nm.

[0033] The mirror film 400 may be any of the optical films described elsewhere herein. In some embodiments, the light transmittance of the mirror film for light incident substantially perpendicularly includes a band edge that separates a first wavelength range and a second wavelength range spanning at least about 950 nm to about 1300 nm, and for light incident substantially perpendicularly in air, the light reflectivity of the mirror film is greater than about 95% for each wavelength in the first wavelength range, the average light transmittance of the mirror film is greater than about 80% in the second wavelength range, and the difference between the maximum and minimum values of the light transmittance of the mirror film in the second wavelength range is less than about 25%. In some embodiments, the best linear approximation to the band edge, which correlates the light transmittance with wavelength over at least a wavelength range where the light transmittance increases from about 10% to about 70%, or to about 75%, or to about 80%, has a slope greater than about 2% / nm, or greater than about 3% / nm, or greater than about 4% / nm, or greater than about 5% / nm. In some embodiments, the mirror film 400 includes a band edge region that separates the first wavelength range from a second wavelength range that is at least 250 nm wide, and the difference between the maximum and minimum values of the light transmittance of the optical film in each of the first wavelength range and the second wavelength range is less than about 30%. In some embodiments, the light transmittance of the mirror film 400 for light incident substantially perpendicularly within the band edge region increases monotonically with increasing wavelength, from at least about 10% to about 70%, or from at least about 10% to about 75%, or from at least about 10% to about 80%.

[0034] Example Transmission / Reflection Spectrum Test Method All spectral transmission or reflection characteristics were measured using a PerkinElmer LAMBDA 950 (available from PerkinElmer (Waltham, Mass.)) having a wavelength range of 350 nm to 1500 nm.

[0035] Comparative Example C1 (ESR-80v2) A visible light mirror film with the product name ESR-80v2 was obtained from 3M Company (St. Paul, MN). The film included alternating first and second optical layers, where the first layer was a polyethylene naphthalate (PEN) homopolymer (100 mol% of naphthalene dicarboxylate with 100 mol% of ethylene glycol), and the second layer was poly(methyl methacrylate) or PMMA. The film included skin layers on the outer surfaces of the alternating first and second layers. The polymer used for the skin layer was formed of the same material as that used for the first layer. Including the skin layers, the optical film had 656 layers. The refractive indices of the alternating first and second optical layers were approximately the same as those reported in Example 1. The thickness profiles of the alternating first and second layers were determined by atomic force microscopy (AFM) and are shown in FIG. 6. FIG. 6 shows the average physical thicknesses of the first and second optical layers in separate pairs of directly adjacent first and second layers. The skin thicknesses were determined by AFM to be 6.1 and 5 micrometers. The transmission spectrum was determined and is shown in FIG. 7. The slope of the band edge was determined to be 1.11% / nm from the best linear approximation to the band edge that correlates the light transmittance with the wavelength over the wavelength range in which the light transmittance increased from about 10% to about 70%. The film had a physical thickness of approximately 84 micrometers as measured by AFM.

[0036] Comparative Example C2 (ESR2) Except for the following exceptions, a visible light mirror film comprising alternating first and second optical layers was prepared by coextrusion and biaxial orientation as described in U.S. Patent Application Publication No. 2001 / 0013668 (Neavin et al.). The first optical layer was an ethylene naphthalate (PEN) homopolymer (100 mol% naphthalene dicarboxylate with 100 mol% ethylene glycol) having a Tg of 121 - 123 °C. The second optical layer was poly(methyl methacrylate), i.e., PMMA. Examples of PMMA are available from Arkema (Pasadena, TX, USA) and have a Tg of 100 °C. Skin layers were formed on the outer surfaces of the alternating first and second layers. The polymer used for the skin layer was formed of the same material as that used for the first optical layer. The skin thickness was determined by AFM to be 3.1 and 3.6 micrometers. The film had a physical thickness of approximately 32 micrometers as measured by AFM. The refractive indices of the alternating first and second optical layers were approximately the same as those reported in Example 1.

[0037] The materials were fed from separate extruders into a multilayer coextrusion feed block where they were assembled into alternating optical layers. The skin layers were added to the structure in a manifold specialized for that purpose, resulting in a final structure having 269 layers. This multilayer melt was then cast onto a chill roll through a film die and quenched in a conventional manner for polyester films. Next, the cast web was stretched in a commercial-scale biaxial tenter at temperature and draw profiles similar to those described in U.S. Patent Application Publication No. 2001 / 001366.

[0038] The transmission spectrum was determined and is shown in Figure 8. The slope of the band edge was determined to be 1.02% / nm from the best linear approximation to the band edge, which correlates the light transmittance to the wavelength over the wavelength range where the light transmittance increased from about 10% to about 70%.

[0039] Example 1 An optical film (visible light mirror film) including a first optical layer and a second optical layer that interact with each other and including a skin layer was fabricated in substantially the same manner as described for Comparative Example 2, except that the thickness profile and the thickness of the skin layer were changed. The thickness profiles of the alternating first and second layers determined by AFM are shown in FIG. 6. The thickness of the skin layer facing the chill roll during film processing was measured to be 7.13 micrometers, and the skin layer on the opposite side had a measured thickness of 7.20 micrometers. The transmittance spectrum was determined and is shown in FIG. 7. The slope of the band edge was determined to be 5.17% / nm from the best linear approximation to the band edge that correlates the light transmittance with wavelength over the wavelength range in which the light transmittance increased from about 10% to about 70%.

[0040] The refractive index of the first layer was determined by measuring the refractive index of the skin layer since the skin layer was formed of the same material as the first layer and oriented under the same conditions. The refractive indices were determined using a Metricon 2010 / M prism coupler at a wavelength of 633 nm and were found to be n1x = 1.737, n1y = 1.763, and n1z = 1.496. The refractive index of the second layer was determined as the refractive index of the isotropic PMMA material used for the second layer at 633 nm. The results were n2x ≈ n2y ≈ n2z ≈ 1.495. The physical thickness of the film was measured using an Ono-Sokki DG-925 Micrometer and was measured to be approximately 77 micrometers by a capacitance meter.

[0041] Example 2 An optical film including a first layer and a second layer that interact and including a skin layer was fabricated generally as described for Example 1, except that the thickness of the skin layer was changed and the layer thickness profile was changed to shift the band edge to a shorter wavelength. The skin thickness was determined to be 5.1 and 6.7 micrometers by AFM. The film had a physical thickness of approximately 69 micrometers as measured by AFM. The transmission spectrum was obtained and is shown in FIG. 8. The slope of the band edge was determined to be 6.25% / nm from the best linear approximation to the band edge that correlates the light transmittance to the wavelength over the wavelength range in which the light transmittance increased from about 10% to about 70%. The refractive indices of the first layer and the second layer that interact were approximately the same as the refractive indices reported in Example 1.

[0042] Example 3 An optical film including a first layer and a second layer that interact and including a skin layer was fabricated generally as described for Example 1, except that the number, thickness profile, and thickness of the skin layer of the first layer and the second layer that interact were changed. Including the skin layer, the optical film had 536 layers. The thickness of the skin layer facing the chill roll during film processing was measured to be 3.73 micrometers, and the skin layer on the opposite side had a measured thickness of 4.43 micrometers. The film had a physical thickness of approximately 59 micrometers as measured by AFM. The layer thickness profile was chosen to generate the transmission spectrum shown in FIG. 9. The slope of the band edge was determined to be 6.30% / nm by the best linear approximation to the band edge that correlates the light transmittance to the wavelength over the wavelength range in which the light transmittance increased from 8.7% to 71.9%, and was determined to be 5.84% / nm by the best linear approximation to the band edge that correlates the light transmittance to the wavelength over the wavelength range in which the light transmittance increased from 8.7% to 75.1%. The refractive indices of the first optical layer and the second optical layer that interact were approximately the same as the refractive indices reported in Example 1.

[0043] Example 4 An optical film including a first layer and a second layer that interact and including a skin layer was fabricated generally as described for Example 4 except that the thickness of the skin layer was varied. The thickness of the skin layer facing the chill roll during film processing was measured to be 5.11 micrometers and the skin layer on the opposite side had a measured thickness of 5.62 micrometers. The film had a physical thickness of approximately 61 micrometers as measured by AFM. The transmission spectrum was obtained and is shown in FIG. 9. The slope of the band edge was determined to be 3.71% / nm by the best linear approximation to the band edge correlating the optical transmittance to the wavelength over the wavelength range in which the optical transmittance increased from 9.8% to 71.2%, and was determined to be 3.62% / nm by the best linear approximation to the band edge correlating the optical transmittance to the wavelength over the wavelength range in which the optical transmittance increased from 9.8% to 81.5%. The refractive indices of the alternating first and second optical layers were approximately the same as those reported in Example 1.

[0044] Terms such as "about" will be understood by those skilled in the art in the context in which they are used and described herein. Where the use of "about" as applied to a feature size, amount, and quantity representing a physical property is not apparent to those skilled in the art in the context in which it is used and described herein, "about" is understood to mean within 10 percent of the specified amount, including the exactly specified amount. For example, where not apparent to those skilled in the art in the context in which it is used and described herein, an amount having a value of about 1 means having a value between 0.9 and 1.1, including exactly 1.

[0045] Any of the foregoing references, patents, or patent applications are hereby incorporated by reference in their entirety in a consistent manner. In the event of a discrepancy or conflict between a portion of the incorporated reference and the present application, the information in the foregoing description shall control.

[0046] It should be understood that the descriptions of the elements in the figures apply equally to the corresponding elements in other figures, unless otherwise indicated. Although specific embodiments have been illustrated and described herein, those skilled in the art will understand that various alternative and / or equivalent implementations can replace the specific embodiments shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations or variations of the specific embodiments described herein. Accordingly, the present disclosure is to be limited only by the claims and their equivalents. Exemplary embodiments are shown below. [Item 1] An optical film including a plurality of alternating first polymer layers and second polymer layers disposed on a skin layer, wherein the first layer and the second layer each have an average thickness of less than about 250 nm, the skin layer has an average thickness of more than about 2 micrometers, and the light transmittance of the optical film for light incident substantially vertically includes a band edge that separates a first wavelength range and a second wavelength range, the first wavelength range extending from at least about 400 nm to about 700 nm, the second wavelength range extending from at least about 950 nm to about 1300 nm, for light incident substantially vertically in air, the light reflectance of the optical film is greater than about 95% for each wavelength in the first wavelength range, the average light transmittance of the optical film is greater than about 80% in the second wavelength range, the difference between the maximum value and the minimum value of the light transmittance of the optical film in the second wavelength range is less than about 25%, the best linear approximation to the band edge that correlates the light transmittance with wavelength over at least the wavelength range in which the light transmittance increases from about 10% to about 70% has a slope greater than about 2% / nm, and the first layer, the second layer, and the skin layer are integrally formed with each other. An optical film. [Item 2] The optical film according to Item 1, wherein the light transmittance of the optical film for light incident substantially vertically increases monotonically from at least about 10% to about 70% as the wavelength increases. [Item 3] The optical film according to item 1 or 2, wherein the slope of the best linear approximation exceeds about 3% / nm. [Item 4] The optical film according to any one of items 1 to 3, wherein a pair of adjacent first and second layers among the plurality of alternating first and second polymer layers has a maximum optical thickness, and the plurality of alternating first and second polymer layers includes at least 20 separate layer pairs having an optical thickness within about 20 nm from the maximum optical thickness. [Item 5] In the plane of the first layer and the second layer, the first layer and the second layer each have respective refractive indices, namely, n1x and n2x along a first polarization state, n1y and n2y along a second polarization state orthogonal to the first polarization state, and n1z and n2z along a z-axis orthogonal to the first polarization state and the second polarization state, and for at least one wavelength falling within at least one of the first wavelength range and the second wavelength range, each of n1x and n1y is at least 0.2 greater than n1z, the difference between n1x and n1y is less than about 0.04, the maximum difference among n2x, n2y, and n2z is less than about 0.01, the difference between n1x and n2x is greater than about 0.2, the optical film according to any one of items 1 to 4. [Item 6] An optical film comprising a plurality of alternating first and second polymer layers of a number from 50 to 800, each first and second layer having an average thickness of less than about 500 nm, the light transmittance of the optical film for light incident substantially perpendicularly including a band edge region separating a first wavelength range and a second wavelength range, each range having a width of at least 250 nm, the difference between the maximum and minimum values of the light transmittance of the optical film in each wavelength range being less than about 30%, in the plane of the first and second layers, the first and second layers having respective refractive indices, namely, n1x and n2x along a first polarization state, n1y and n2y along a second polarization state orthogonal to the first polarization state, and n1z and n2z along a z-axis orthogonal to the first and second polarization states, for at least one wavelength falling into at least one of the first wavelength range and the second wavelength range, each of n1x and n1y is at least 0.2 greater than n1z, the difference between n1x and n1y is less than about 0.04, the maximum difference among n2x, n2y and n2z is less than about 0.01, the difference between n1x and n2x is greater than about 0.2, the light transmittance of the optical film for light incident substantially perpendicularly within the band edge region increases monotonically from at least about 10% to about 70% as the wavelength increases. An optical film. [Item 7] The best linear approximation to the band edge, correlating the light transmittance with wavelength over at least the wavelength range in which the light transmittance increases from about 10% to about 70%, has a slope greater than about 2% / nm. The optical film according to Item 6. [Item 8] The optical film according to Item 6 or 7, wherein the first wavelength range extends over at least about 400 nm to about 700 nm and the second wavelength range extends over at least about 950 nm to about 1300 nm. [Item 9] The optical film according to any one of items 6 to 8, wherein the difference between the maximum value and the minimum value of the light transmittance of the optical film in each wavelength range is less than about 25%. [Item 10] The optical film according to any one of items 6 to 9, wherein each of the first layers and each of the second layers has an average thickness of less than about 250 nm. [Item 11] The optical film according to any one of items 6 to 10, wherein a pair of adjacent first and second layers among the plurality of alternating first and second polymer layers has a maximum optical thickness, and the plurality of alternating first and second polymer layers includes at least 20 separate layer pairs having an optical thickness within about 20 nm from the maximum optical thickness. [Item 12] A display, wherein the display a display panel for displaying a visible image to an observer within a first wavelength range of at least about 400 nm to about 700 nm, an infrared light source for radiating light having an infrared wavelength beyond the first wavelength range toward the observer through the display panel, a mirror film disposed between the display panel and the infrared light source, and for each of light incident substantially perpendicularly and each of a first polarization state and a second polarization state orthogonal thereto, for each wavelength in the first wavelength range, the mirror film reflects at least 90% of the light, at the infrared wavelength, the mirror film transmits at least 70% of the light, a mirror film that is between the first wavelength range and the infrared wavelength and transmits 40% to 60% of the light at a first wavelength within about 50 nm from the infrared wavelength, comprising the display. [Item 13] The display according to item 12, wherein the first wavelength is within about 40 nm from the infrared wavelength. [Item 14] The light transmittance of the mirror film for light incident substantially vertically includes a band edge that separates the first wavelength range and a second wavelength range spanning at least about 950 nm to about 1300 nm, and for light incident substantially vertically in air, the light reflectivity of the mirror film is greater than about 95% for each wavelength in the first wavelength range, the average light transmittance of the mirror film is greater than about 80% in the second wavelength range, the difference between the maximum value and the minimum value of the light transmittance of the mirror film in the second wavelength range is less than about 25%, The best linear approximation to the band edge, which correlates the light transmittance with wavelength over at least a wavelength range where the light transmittance increases from about 10% to about 70%, has a slope greater than about 2% / nm. The display according to item 12 or 13. [Item 15] The light transmittance of the mirror film has a band edge region that separates the first wavelength range from a second wavelength range having a width of at least 250 nm. The difference between the maximum value and the minimum value of the light transmittance of the mirror film in each of the first wavelength range and the second wavelength range is less than about 30%. The light transmittance of the mirror film for light incident substantially vertically within the band edge region increases monotonically from at least about 10% to about 70% as the wavelength increases. The display according to any one of items 12 to 14.

Claims

1. 1. An optical film comprising a plurality of alternating first and second polymer layers, the number of which is between 50 and 800, each of the first and second polymer layers having an average thickness of less than 500 nm, the optical transmittance of the optical film for normally incident light comprising a band edge region separating a first wavelength range and a second wavelength range, the first wavelength range spanning at least 400 nm to 700 nm, the second wavelength range spanning at least 950 nm to 1300 nm, the band edge region being 30 nm or less in width, and the optical transmittance of the optical film for normally incident light comprising a band edge region separating a first wavelength range and a second wavelength range, the first wavelength range spanning at least 400 nm to 700 nm, the second wavelength range spanning at least 950 nm to 1300 nm, the band edge region being 30 nm or less in width, a difference between the maximum and minimum values ​​of the light transmission of the film is less than 30%, and in the plane of the first and second polymer layers, the first and second polymer layers have respective refractive indices, n1x and n2x along a first polarization state, n1y and n2y along a second polarization state orthogonal to the first polarization state, and n1z and n2z along a z-axis orthogonal to the first and second polarization states, for at least one wavelength in at least one of the first and second wavelength ranges; Each of n1x and n1y is at least 0.2 greater than n1z; the difference between n1x and n1y is less than 0.04; the maximum difference between n2x, n2y, and n2z is less than 0.01; the difference between n1x and n2x is greater than 0.2; An optical film, wherein the optical transmittance of the optical film for normally incident light within the bandedge region increases monotonically with increasing wavelength from at least 10% to 70%.

2. 2. The optical film of claim 1, wherein a best linear approximation for the band edge region correlating the optical transmittance to wavelength over at least the wavelength range in which the optical transmittance increases from 10% to 70% has a slope of greater than 2% / nm.

3. 3. The optical film of claim 1, wherein the difference between the maximum and minimum light transmittance of the optical film in each wavelength range is less than 25%.

4. The optical film of any one of claims 1 to 3, wherein each first polymer layer and each second polymer layer has an average thickness of less than 250 nm.

5. 5. The optical film of claim 1, wherein a pair of adjacent first and second polymer layers of the plurality of alternating first and second polymer layers has a maximum optical thickness, and the plurality of alternating first and second polymer layers includes at least 20 distinct layer pairs having optical thicknesses within 20 nm of the maximum optical thickness.

6. A display, the display comprising: a display panel for displaying to a viewer a visible image within a first wavelength range spanning at least 400 nm to 700 nm; an infrared light source for emitting light at infrared wavelengths above the first wavelength range through the display panel toward the viewer; The optical film according to claim 1 , which is disposed between the display panel and the infrared light source; and A display comprising:

Images