Optical diffuser with high infrared clarity

Abstract

The present invention discloses an optical stack (100), which includes an optical diffuser (10) and a first reflective polarizer (40) disposed on the optical diffuser (10). For light incident substantially perpendicularly and for a first wavelength range and a second wavelength range that are non-overlapping, the optical diffuser (10) has a first scattering rate R1 for at least one wavelength in the first wavelength range and a second scattering rate R2 for at least one wavelength in the second wavelength range, such that R1 / R2 is greater than or equal to 2. The first reflective polarizer (40) can transmit at least 40% of light in a first polarization state for each wavelength in the first wavelength range, can reflect at least 70% of light in an orthogonal second polarization state for each wavelength in the first wavelength range, and can transmit at least 40% of light in each of the first polarization state and the second polarization state for each wavelength in the second wavelength range.

Description

Summary of the invention

[0001] In some aspects of the present specification, an optical stack is provided, the optical stack including an optical diffuser and a first reflective polarizer disposed on the optical diffuser. For light at substantially normal incidence and for a first wavelength range and a second wavelength range that are non-overlapping, the first wavelength range being at least from about 450 nm to about 600 nm, and the second wavelength range being at least from about 800 nm to about 1200 nm: the optical diffuser has a first scattering efficiency R1 for at least one wavelength in the first wavelength range, and a second scattering efficiency R2 for at least one wavelength in the second wavelength range, such that R1 / R2 is greater than or equal to about 2. The first reflective polarizer can transmit at least 40% of light in a first polarization state for each wavelength in the first wavelength range, can reflect at least 70% of light in an orthogonal second polarization state for each wavelength in the first wavelength range, and can transmit at least 40% of light in each of the first polarization state and the second polarization state for each wavelength in the second wavelength range.

[0002] In some aspects of the specification, a backlight is provided, the backlight comprising a back reflector, an optical stack disposed on the back reflector, and a light guide disposed between the back reflector and the optical stack. The optical diffuser has a first scattering rate R1 for at least one wavelength in a first wavelength range and a second scattering rate R2 for at least one wavelength in a second wavelength range, as described herein. For light incident substantially perpendicularly and for each of a first polarization state and a second polarization state, the back reflector reflects at least 70% of the light for each wavelength in the first wavelength range and transmits at least 70% of the light for each wavelength in the second wavelength range.

[0003] In some aspects of the present specification, an optical stack is provided, the optical stack including an optical diffuser; and an optical film disposed on the optical diffuser and including a plurality of alternating polymeric first and second interference layers greater than about 50, each interference layer having an average thickness less than about 250 nm, such that for non-overlapping first and second wavelength ranges, the first wavelength range is at least from about 450 nm to about 600 nm, and the second wavelength range is at least from about 800 nm to about 1200 nm. The optical diffuser may have a first scattering rate R1 for at least one wavelength in the first wavelength range, and may have a second scattering rate R2 for at least one wavelength in the second wavelength range, such that R1 / R2 is greater than or equal to 2. For light incident at an incident angle relative to a direction perpendicular to the optical film, the optical film may have an average optical transmittance T0 in a first wavelength range when the incident angle is about zero degrees, an average optical transmittance T60 at an incident angle of about 60 degrees, and an average optical transmittance T1 in a second wavelength range when the incident angle is about zero degrees, such that T60 / T0 is less than about 0.8 and T1 is greater than about 40%.

[0004] In some aspects of the present specification, an optical stack is provided, the optical stack comprising an optical diffuser having an average total transmittance T1 and an average diffuse transmittance T2 in a first wavelength range from about 450 nm to about 600 nm, such that T2 / T1 is greater than about 0.4; a multilayer optical film disposed on the optical diffuser and comprising a plurality of alternating first and second polymer layers in number of at least 30, each first polymer layer and each second polymer layer having an average thickness of less than about 500 nm; and an optical reflector disposed on the multilayer optical film and reflecting at least 70% of light in each of the orthogonal first and second polarization states for each wavelength in the first wavelength range, wherein for at least one wavelength in the second wavelength range from about 800 nm to about 1200 nm, the optical stack has a modulation transfer function (MTF) greater than about 0.4 at 2.2 line pairs per mm. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Figure 1 is a cross-sectional view of an optical stack including a diffuser having high infrared clarity according to one embodiment described herein;

[0006] Figure 2 is an exploded cross-sectional view of an optical stack including a diffuser having high infrared clarity according to one embodiment described herein;

[0007] Figure 3A and Figure 3BCross-sectional views of optical stacks including diffusers with high infrared clarity according to embodiments described herein are provided;

[0008] Figure 4 is a cross-sectional view of an optical diffuser with high infrared clarity according to one embodiment described herein;

[0009] Figure 5 shows how visible light can be diffused by an optical diffuser with high infrared clarity according to one embodiment described herein;

[0010] Figure 6 illustrates the operation of a collimating multilayer optical film according to one embodiment described herein;

[0011] Figure 7 is a front view of a display having a sensing device disposed behind a display surface according to an embodiment described herein;

[0012] Figure 8 shows an example of how a fingerprint may be scanned by a sensing device disposed behind a display according to one embodiment described herein;

[0013] Fig. 9A and Fig. 9B shows respectively a measured transmission spectrum and a layer thickness profile of a first exemplary multilayer optical film according to one embodiment described herein;

[0014] Fig. 10A and Fig. 10B Measured transmission spectra and layer thickness profiles, respectively, are shown for a second exemplary multilayer optical film according to embodiments described herein;

[0015] Fig.11A and Fig. 11B Measured transmission spectrum and layer thickness profile of a third exemplary multilayer optical film according to one embodiment described herein are shown, respectively;

[0016] Fig. 12A and Fig. 12B Measured transmission spectra and layer thickness profiles, respectively, of a fourth exemplary multilayer optical film according to one embodiment described herein;

[0017] Fig.13 shows measured modulation transfer functions for several exemplary films and film stacks according to one embodiment described herein;

[0018] Fig.14 A method for measuring the Fig.13 An exemplary test setup for modulation transfer function values ​​of

[0019] Fig.15 Coordinate system reference diagrams are provided for exemplary films and measurement results discussed herein. DETAILED DESCRIPTION

[0020] 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.

[0021] According to some aspects of the present specification, an optical stack is provided, the optical stack including an optical diffuser and a first reflective polarizer disposed on the optical diffuser. For light at substantially normal incidence and for a first wavelength range and a second wavelength range that are non-overlapping, the first wavelength range being at least from about 450 nm to about 600 nm, and the second wavelength range being at least from about 800 nm to about 1200 nm: the optical diffuser has a first scattering efficiency R1 for at least one wavelength in the first wavelength range (such as, in some embodiments, about 500 nm), and a second scattering efficiency R2 for at least one wavelength in the second wavelength range (such as, in some embodiments, about 840 nm or about 940 nm), such that R1 / R2 is greater than or equal to about 2, or greater than about 2.5, or greater than about 3.0, or greater than about 3.5, or greater than about 4.0, or greater than about 4.5, or greater than about 5.0. In some embodiments, the optical diffuser may have a first average scattering index, R11, in a first wavelength range and a second average scattering index, R22, in a second wavelength range such that the ratio R11 / R22 is greater than about 2.0, or greater than about 2.5, or greater than about 3.0, or greater than about 3.5, or greater than about 4.0, or greater than about 4.5, or greater than about 5.0.

[0022] In some embodiments, the first wavelength range can be from about 420 nm to about 650 nm. In some embodiments, the second wavelength range can be from about 800 nm to about 1550 nm, or from about 800 nm to about 2000 nm.

[0023] In some embodiments, for a specified wavelength or wavelength range and for light entering the diffuser at an angle of incidence normal to the surface of the diffuser, the scattering rate can be defined as the ratio of the amount of diffusely transmitted light leaving the diffuser to the total amount of light (including specularly transmitted light) leaving the diffuser. That is, the total amount of light T leaving the diffuser is total It can be expressed as:

[0024] T total =T diff +T spec

[0025] Where T spec is the total amount of light transmitted from the specular surface (leaving the diffuser at angles less than about 5 degrees from normal), and T diff is the total amount of light that comes from diffuse transmission (light that leaves the diffuser at an angle greater than about 5 degrees from the normal). The scattering rate R can then be defined as follows:

[0026] R=T diff / T total

[0027] In some embodiments, the optical diffuser may include a plurality of particles (e.g., beads) substantially uniformly dispersed in a binder. In some embodiments, precise control of the size of the particles may determine which wavelengths of light are scattered and to what extent they are scattered. In some embodiments, the particle size may be selected such that relatively low scattering (e.g., diffusion) of light occurs for light in the near infrared range (e.g., about 800 nm to about 1200 nm). In some embodiments, the optical diffuser may include a binder defining a plurality of interconnected voids therein such that for at least one cross section of the optical diffuser along its thickness, the voids cover at least 20% of the cross section, the optical diffuser and the binder having respective refractive indices nd and nb at at least one wavelength within a first wavelength range such that nd is less than nb. In some embodiments, the value of nb may be greater than about 1.45, or greater than about 1.5, or greater than about 1.55. In some embodiments, the value of nd may be less than about 1.4, or less than about 1.3, or less than about 1.25, or less than about 1.2, or less than about 1.15, or less than about 1.1. In some embodiments, the optical diffuser may include a plurality of particles dispersed in a binder, in a set of interconnected voids, or in both a binder and interconnected voids. In some embodiments, the plurality of interconnected voids may include a plurality of surface voids disposed at at least one major surface of the binder and a plurality of internal voids disposed within the interior of the binder such that at least one hollow channel connects at least one internal void to at least one surface void. In some embodiments, the average thickness of the binder is less than about 1.5 microns, or less than about 1.0 microns, or less than about 0.75 microns, or less than about 0.5 microns.

[0028] In some embodiments, a diffuser that provides a higher amount of diffusion in a first wavelength range (e.g., wavelengths of light visible to humans) and a lower amount of diffusion (or substantially no diffusion) in a second wavelength range (e.g., wavelengths of near infrared light) may be useful for certain applications. Such an optical diffuser having high transmittance (high definition) in one or more infrared wavelengths may be suitable for use in a backlight of a display to diffuse human visible light transmitted from the backlight to a display (such as a liquid crystal display or LCD), thereby providing a flatter, more uniform light source for the display while allowing, for example, near infrared wavelengths to pass through substantially unchanged. In some embodiments, this may allow an infrared sensor (such as, for example, a CMOS / TFT camera sensitive to infrared wavelengths) to be placed behind the surface of the display. Typical optical diffusers in the prior art will cause diffusion in both human visible and infrared wavelengths. While diffusion is generally beneficial for providing a more uniform illumination of the display (e.g., smoothing out imperfections and non-uniformities from point sources of light), light entering the display from the outside, such as light detected by a camera or sensor behind the display, will also be diffused. This means that the camera or sensor cannot detect enough detail to form a clear image. However, by using an optical diffuser that has relatively high scattering in the visible wavelengths and relatively low scattering in the infrared wavelengths, it is possible to achieve both uniform display illumination and image clarity as seen at the sensor. That is, visible light from the backlight will be diffused while allowing infrared light to pass through the diffuser to the sensor with little or no diffusion.

[0029] In some embodiments, the optical stack can receive light from one or more light sources such that the one or more light sources emit light within each of a first wavelength range and a second wavelength range (e.g., within both the human visible range and the infrared range). For example, in some embodiments, the optical stack can receive light emitted by a light source (e.g., a light emitting diode or a laser) and directed by internal reflection through a light guide plate.

[0030] The first reflective polarizer may transmit at least 40%, or at least 50%, or at least 60%, or at least 70%, or at least 80% of the light in the first polarization state for each wavelength in the first wavelength range. The first reflective polarizer may reflect at least 70%, or at least 80%, or at least 90% of the light in the orthogonal second polarization state for each wavelength in the first wavelength range. The first reflective polarizer may transmit at least 40%, or at least 50% of the light in each of the first polarization state and the second polarization state for each wavelength in the second wavelength range. In some embodiments, the reflective polarizer may be combined with one or more light redirecting films (such as brightness enhancement films or BEFs), collimating multilayer optical films (CMOFs), or combinations thereof. In some embodiments, the reflective polarizer itself may be a polarizing CMOF (i.e., a CMOF that acts as a reflective polarizer).

[0031] For example, in some embodiments, the optical stack can include a first light redirecting film disposed between a first reflective polarizer and an optical diffuser, and a second light redirecting film disposed between the first reflective polarizer and the optical diffuser. The first light redirecting film can include a plurality of substantially parallel first microstructures extending along a first direction, and the second light redirecting film can include a plurality of substantially parallel second microstructures extending along a second direction different from the first direction. In some embodiments, for light incident substantially perpendicularly, each of the first light redirecting film and the second light redirecting film can absorb less than about 10% of the incident light in each of the first wavelength range and the second wavelength range.

[0032] In some embodiments, the reflective polarizer may include a plurality of alternating first and second polymer layers in a number between 50 and 1000, each first polymer layer and each second polymer layer having an average thickness of less than about 500 nm, wherein each first polymer layer is substantially uniaxially oriented and each second polymer layer is substantially biaxially oriented. In some embodiments, within the plane of the first polymer layer, the first polymer layer may have refractive indices n1x, n1y, and n1z along the first polarization state, the second polarization state, and the z-axis orthogonal to the first polarization state and the second polarization state, respectively, such that for at least one wavelength within the first wavelength range, the maximum difference between n1x and n1z is less than about 0.02, and the absolute value between n1x and n1y is greater than about 0.05.

[0033] In some embodiments, when the reflective polarizer is a CMOF, for a first wavelength range and for light incident at an incident angle relative to a direction perpendicular to the first reflective polarizer, the first reflective polarizer has an average optical transmittance T0 at an incident angle of about zero degrees and an average optical transmittance T60 at an incident angle of about 60 degrees, such that the ratio T60 / T0 is less than about 0.8 or about 0.75 or about 0.7 or about 0.65 or about 0.6 or about 0.55 or about 0.5.

[0034] In some embodiments, the optical stack may have a first average optical haze H1 within a first wavelength range and a second average optical haze H2 within a second wavelength range such that the ratio H1 / H2 is greater than about 1.5, or greater than about 2.0, or greater than about 2.5, or greater than about 3.0, or greater than about 3.5, or greater than about 4.0, or greater than about 4.5, or greater than about 5.0. As used herein, optical haze refers to transmission haze, where light passing through a material (such as a diffuser or optical stack) interacts with and / or is affected by irregularities in the material (e.g., suspended particles, contaminants, voids, and / or air spaces). The light is scattered at angles determined by the refractive index of the material (including the irregularities) and the illumination angle, thereby producing optical haze.

[0035] According to some aspects of the present specification, a backlight is provided, the backlight including a back reflector, an optical stack disposed on the back reflector, and a light guide disposed between the back reflector and the optical stack. The optical diffuser has a first scattering rate R1 for at least one wavelength in a first wavelength range (e.g., human visible light) and a second scattering rate R2 for at least one wavelength in a second wavelength range (e.g., near infrared light), as described elsewhere herein. For substantially perpendicular incident light and for each of a first polarization state and a second polarization state, the back reflector reflects at least 70% of the light of each wavelength in the first wavelength range and transmits at least 70% of the light of each wavelength in the second wavelength range. In some embodiments, a display may be formed, the display including a backlight such that the backlight is disposed between a liquid crystal panel or module and an infrared-sensitive detector. In some embodiments, when an infrared emission source emitting light in the second wavelength range is disposed proximate to the liquid crystal panel, the infrared-sensitive detector detects at least some of the light in the second wavelength range emitted by the infrared emission source. In some embodiments, the display may be configured to form a first image in the first wavelength range for viewing by a viewer, and a second image in the second wavelength range detectable by an infrared-sensitive camera.

[0036] According to some aspects of the present specification, an optical stack is provided, the optical stack including an optical diffuser; and an optical film disposed on the optical diffuser and including a plurality of alternating polymeric first and second interference layers greater than about 50, each interference layer having an average thickness less than about 250 nm, such that for non-overlapping first and second wavelength ranges, the first wavelength range is at least from about 450 nm to about 600 nm in some embodiments, and the second wavelength range is at least from about 800 nm to about 1200 nm in some embodiments. The optical diffuser may have a first scattering rate R1 for at least one wavelength in the first wavelength range, and may have a second scattering rate R2 for at least one wavelength in the second wavelength range, such that R1 / R2 is greater than or equal to 2. For light incident at an incident angle relative to a direction perpendicular to the optical film, the optical film may have an average optical transmittance T0 in a first wavelength range when the incident angle is about zero degrees, an average optical transmittance T60 at an incident angle of about 60 degrees, and an average optical transmittance T1 in a second wavelength range when the incident angle is about zero degrees, such that T60 / T0 is less than about 0.8 and T1 is greater than about 40%.

[0037] According to some aspects of the present specification, an optical stack is provided, which includes an optical diffuser, a multilayer optical film disposed on the optical diffuser, and an optical reflector disposed on the multilayer optical film. In some embodiments, the optical diffuser may have an average total transmittance T1 and an average diffuse transmittance T2 in a first wavelength range from about 450nm to about 600nm, such that the ratio T2 / T1 is greater than about 0.4, or greater than about 0.5, or greater than about 0.6. In some embodiments, the optical diffuser may have an average total transmittance T1' and an average diffuse transmittance T2' in a second wavelength range, such that the ratio T2' / T1' is less than about 0.3, or less than about 0.2. In some embodiments, the second wavelength range may be from about 800nm ​​to about 1200nm. In some embodiments, the optical reflector may transmit at least 70% of light in each of the first polarization state and the second polarization state for each wavelength in the second wavelength range.

[0038] In some embodiments, the optical reflector can reflect at least 70% of light in each of the orthogonal first and second polarization states for each wavelength in the first wavelength range, wherein for at least one wavelength in the second wavelength range from about 800 nm to about 1200 nm (e.g., about 940 nm), the optical stack has a modulation transfer function (MTF) greater than about 0.4 at 2.2 line pairs per mm. In some embodiments, the optical reflector includes a plurality of alternating lower refractive index polymer layers and higher refractive index polymer layers in number of at least 30, each lower refractive index polymer layer and each higher refractive index polymer layer having an average thickness of less than about 500 nm.

[0039] In some embodiments, the multilayer optical film may include a plurality of alternating first and second polymer layers in number of at least 30, each first polymer layer and each second polymer layer having an average thickness of less than about 500 nm. In some embodiments, the multilayer optical film may be a reflective polarizer that transmits at least 40% of light in a first polarization state for each wavelength in a first wavelength range and reflects at least 70% of light in a second polarization state for each wavelength in the first wavelength range. In some embodiments, the reflective polarizer transmits at least 40% of light in each of the first and second polarization states for each wavelength in a second wavelength range.

[0040] In some embodiments, for the first wavelength range and for light incident at an angle of incidence relative to a direction perpendicular to the multilayer optical film, the multilayer optical film may have an average optical transmittance T0 at an angle of incidence of about zero degrees and an average optical transmittance T60 at an angle of incidence of about 60 degrees, such that the ratio T60 / T0 is less than about 0.8, or less than about 0.75, or less than about 0.7, or less than about 0.65, or less than about 0.6, or less than about 0.55, or less than about 0.5.

[0041] In some embodiments, the optical stack may have a modulation transfer function (MTF) greater than about 0.5 at 2.2 line pairs per mm for the at least one wavelength in the second wavelength range. In some embodiments, the optical stack may have an MTF greater than about 0.3 at 3.1 line pairs per mm for the at least one wavelength in the second wavelength range. In some embodiments, the optical stack may have an MTF greater than about 0.2 at 3.9 line pairs per mm for the at least one wavelength in the second wavelength range.

[0042] Now turning to the attached figure, Figure 11 is a cross-sectional view of one embodiment of an optical stack including a diffuser with high infrared clarity. In some embodiments, optical stack 100 includes an optical diffuser 10 and a reflective polarizer 40 disposed on optical diffuser 10. As described elsewhere herein, optical diffuser 10 may have a first scattering rate R1 for light in a first wavelength range (e.g., human visible light) and a second scattering rate R2 for light in a second wavelength range (e.g., near infrared light), such that R1 is greater than R2. That is, in some embodiments, optical diffuser 10 will scatter more light in the first wavelength range than light in the second wavelength range. Optical stack 100 may also include a light guide plate 20. In some embodiments, light guide plate 20 may receive input light from one or more light sources 60. In some embodiments, light source 60 may be disposed on an edge of light guide plate 20 such that light from light source 60 enters light guide plate 20 and is directed via internal reflection such that it exits light guide plate 20 and enters optical diffuser 10. In some embodiments, the optical stack 100 may also include a reflector 30 disposed on a side of the light guide plate 20 opposite the optical diffuser 10, such that light within the first wavelength range that escapes the light guide 20 toward the reflector 30 will be reflected back into the light guide 20 so as to have another opportunity to be directed into the diffuser 10.

[0043] In some embodiments, reflector 30 may substantially reflect light within a first wavelength range and may substantially transmit light within a second wavelength range. In some embodiments, light emitted by light source 60 may include wavelengths of light within both the first wavelength range and the second wavelength range. In some embodiments, light emitted by light source 60 may include light of both a first polarization state and a second polarization state (e.g., s-polarized light and p-polarized light). In other words, light emitted by light source 60 may initially be unpolarized (i.e., include light of multiple polarization states simultaneously).

[0044] In some embodiments, light passes through the optical diffuser 10 and enters the reflective polarizer 40. In some embodiments, the reflective polarizer 40 may substantially transmit light of a first polarization state and substantially reflect light of a second polarization state. In some embodiments, the light exiting the optical diffuser 10 may be unpolarized. When the light enters the reflective polarizer 40, the light of the second polarization state may be substantially reflected back into the diffuser 10, and the light of the first polarization state may be substantially transmitted. The light transmitted through the reflective polarizer 40 (i.e., light of substantially the first polarization state) may then pass into the display 50, which may selectively transmit or block the light to form an image on the display. In some embodiments, the display 50 may be a liquid crystal display, but any suitable type of display or light modulation device may be used. In some embodiments, the display 50 is designed to transmit or block light of a single polarization state, but may not be used for light of different polarization states. Therefore, in order to prevent undesired light of the second polarization state that has leaked from the reflective polarizer 40 (because the reflective polarizer 40 may not be 100% effective) from passing through the display 50, one or more absorptive polarizers 55 may be disposed on one or more sides of the display 50. The intent of absorptive polarizer 55 is to substantially absorb light of the second polarization state that may have leaked from reflective polarizer 40 and / or display 50. In some implementations, each of reflective polarizer 40, absorptive polarizer 55, light guide plate 20, and reflector 30 may substantially allow transmission of infrared (IR) light.

[0045] It should be noted that reference to a first polarization state and a second polarization state is not intended to be limiting. In one embodiment, the first polarization state may be s-polarized light and the second polarization state may be p-polarized light, but in other embodiments, these states may be swapped. In some embodiments, the first polarization state may be linearly polarized light and the second polarization state may be circularly polarized light, or vice versa. In some embodiments, the first polarization state may be circularly polarized light in one direction (e.g., right-handed circular polarization), and the second polarization state may be circularly polarized light in the opposite direction (e.g., left-handed circular polarization). Any suitable type of polarization may be used for the first polarization state and the second polarization state, as long as the two types are different from each other.

[0046] In some embodiments, optical stack 100 may include infrared sensor 80 (e.g., an infrared sensitive device such as an IR camera). IR sensor 80 may detect IR light that has passed through the various layers of the optical stack. IR sensor 80 placed below optical stack 100 may remain substantially hidden from view by an observer viewing display 50, but may receive and process infrared light from something in front of (outside) display 50. For example, IR sensor 80 may be able to receive infrared light reflected from a fingerprint pressed to or held near the surface of display 50, allowing a fingerprint sensor to be placed below the display in some devices (e.g., smartphones).

[0047] Figure 2 yes Figure 1 An exploded cross-sectional view of an optical stack 100 of FIG. 1 is shown illustrating how light of various wavelengths and polarizations may interact with the layers of the stack in some embodiments. Figure 1 Shared Figure 2 Components in the drawings will have similarly numbered reference numerals and should function the same as previously described. Light source 60 emits unpolarized light 70a, which in some embodiments may include light at both human visible wavelengths and infrared wavelengths. For clarity, only the human visible portion of light 70a is shown using solid arrows, and any infrared portion of the emitted light is not shown.

[0048] Unpolarized light 70a enters the light guide plate 20 where it travels through the length of the light guide plate 20 via internal reflection before exiting the light guide plate 20. Any unpolarized light 70a that exits through the bottom side of the light guide plate 20 (the side adjacent to the reflector 30) will fall on the reflector 30. Portions of the light 70a in the human visible wavelength range will be substantially reflected by the reflector 30, while portions of the light 70a in the near infrared range (not shown) will be substantially transmitted through the reflector 30. However, a majority of the unpolarized light 70a will be transmitted through the top side of the light guide plate 20 (i.e., the side adjacent to the optical diffuser 10) and will pass into the optical diffuser 10. The optical diffuser 10 will cause the light to diffuse, thereby forming diffused light 70b. The diffused light 70b then passes into the reflective polarizer 40, and portions of the light 70b in the first polarization state are substantially transmitted through the reflective polarizer 40 to become transmitted polarized light 70c, and portions of the light 70b in the second polarization state are substantially reflected back as reflected polarized light 70d. Note that transmitted polarized light 70c and reflected polarized light 70d are in different (e.g., opposite) polarization states. Transmitted polarized light 70c is allowed to pass into display 50 (and, in some embodiments, absorbing polarizer 55) to form an image on display 50. Reflected polarized light 70d passes through light guide plate 20, is incident on reflector 30, and is reflected back to be recycled into the optical stack. In some cases, part of the reflected light may change polarization state due to reflection and may become unpolarized light 70a again.

[0049] In some embodiments, Figure 2 Light within the second wavelength range (i.e., infrared light) 75, shown by dashed arrows in FIG. 1 , can be substantially transmitted through each layer of optical stack 100 without being diffused by diffuser 10 or reflected by any of the layers. Infrared light 75 is thus allowed to reach IR sensor 80 where it is detected and processed.

[0050] Figure 3A and Figure 3B Cross-sectional views of alternative embodiments of optical stacks that include diffusers with high infrared clarity are provided. Figure 3A and Figure 3B Parts in will have like numbered reference numerals and shall function the same as previously described. Figure 3A, an alternative embodiment of an optical stack 100a is provided. In the optical stack 100a, two additional layers have been added, which are disposed between the optical diffuser 10 and the reflective polarizer 40. These layers include a first light redirecting film and a second light redirecting film 90. In some embodiments, each light redirecting film 90 includes a plurality of substantially parallel microstructures (e.g., parallel transparent prisms). The microstructures of the first light redirecting film 90 may extend along a first direction, and the microstructures of the second light redirecting film 90 may extend along a second direction that is different from (e.g., orthogonal to) the first direction. In some embodiments, these crossed light redirecting films 90 tend to focus and redirect the light passing through them so that the light output from the pair of films 90 is substantially collimated and coaxial. In some embodiments, only a single light redirecting film 90 may be used.

[0051] exist Figure 3B In the optical stack 100b, Figure 3A The dual light redirecting film 90 of the optical stack 100b can be replaced with a single collimating multilayer optical film (CMOF). In some embodiments, the CMOF is a single film that can provide multiple functions, including serving as an optical diffuser, a light redirecting (prism) film, and / or a reflective polarizer. In some embodiments, a single CMOF film can replace other layers in the optical stack 100b, including the reflective polarizer 40 and the optical diffuser 10. In some embodiments, the reflector 30 at the bottom of the optical stack 100b can also be replaced with an infrared transmission enhanced specular reflector (ESR). The ESR is a non-metallic mirror film that can be designed to substantially reflect human visible light and substantially transmit light in the near infrared wavelengths.

[0052] Figure 4 1 is a cross-sectional view of one embodiment of an optical diffuser 10 having high infrared clarity according to one embodiment described herein. In some embodiments, optical diffuser 10 can be constructed using two layers, namely, base layer 18 and coating 14. Base layer 18 can be any suitable polymer substrate, such as, for example, polyethylene terephthalate (PET). Coating 14 is a layer placed over base layer 18 and contains particles that can change the path of light traveling through the layer. The size of the particles can be selected so that only certain wavelengths of light are scattered (i.e., diffused) while substantially allowing other wavelengths to pass through the layer.

[0053] Figure 5 It is shown how visible light can be diffused by an optical diffuser with high infrared clarity according to one embodiment described herein. Figure 51 shows how some light 70a that enters the diffuser at an angle of incidence that is substantially perpendicular to the diffuser 10 passes through the diffuser 10 in the form of diffuse light 70b. A portion of the diffuse light 70b will include specular transmission (i.e., light that passes through and leaves the diffuser in a manner that is substantially perpendicular to the surface of the diffuser) and diffuse transmission (i.e., light that leaves the diffuser at an angle that deviates from the vertical, such as 5 degrees or more from the normal). As previously described herein, for a given wavelength, the total amount of light contained in the specular transmission and the total amount of light contained in the diffuse transmission can be used to determine the scattering rate of the diffuser for that wavelength. In some embodiments, the scattering rate calculated at one wavelength of light can be significantly different from the scattering rate for light of a different wavelength. In some embodiments of the present specification, the diffuser can have a first scattering rate R1 for human visible light that is significantly higher than a second scattering rate R2 for infrared light.

[0054] Figure 6 The operation of a collimating multilayer optical film or CMOF according to one embodiment described herein is shown. The purpose of the CMOF is to receive light at various angles on one surface and redirect at least a portion of that light so that an increased amount of light passing through the CMOF will exit the other side of the CMOF at an angle substantially perpendicular to the surface of the CMOF. Figure 6 , light is shown incident on the bottom surface of the CMOF at two separate angles of incidence: 0 degrees (i.e., perpendicular to the surface of the CMOF) and 60 degrees (i.e., 60 degrees off the perpendicular). In reality, light will enter the CMOF at several different angles, but for purposes of illustration and discussion, these two angles are shown. Additionally, for purposes of discussion, it should be assumed that the light shown represents light in a first wavelength range (e.g., light visible to humans).

[0055] Light with an incident angle of 0 degrees (i.e., directly incident on the surface of the CMOF) is shown as I 0 . Light with an incident angle of 60 degrees is shown as I 60 I 60 A portion of the light will be R 60 The form of reflection leaves CMOF, while I 60 Some of the I will be passed into CMOF. 0Light will be transmitted into the CMOF. Again, the purpose of the CMOF is to increase the total percentage of light that is transmitted through the CMOF and leaves the CMOF at an incident angle of 0 degrees. In other words, the purpose of the CMOF is to increase the collimation of the outgoing light rather than to increase the collimation of the light entering the CMOF. The light leaving the CMOF is shown as T0 (light leaving at an incident angle of about zero degrees) and T60 (light leaving at an incident angle of about 60 degrees). As before, some light can leave the CMOF at any number of exit angles, but for discussion purposes, 0 degrees and 60 degrees are shown. Some portion of all light entering the CMOF at all incident angles can be collimated by the CMOF, thereby increasing the amount of light in the form of T0 and reducing the amount of light transmitted in the form of T60. In other words, regardless of the relative sizes of I0 and I60, T0 should be greater than T60, so that the ratio T60 / T0 can be less than about 0.8, or less than about 0.75, or less than about 0.7, or less than about 0.65, or less than about 0.6, or less than about 0.55, or less than about 0.5.

[0056] Figure 1 The optical stack 100 or alternative embodiments thereof may be useful in many end applications. For example, as discussed elsewhere herein, the use of an optical diffuser that scatters infrared light significantly less than human visible light allows the IR sensor to be embedded below the surface of the display, thereby allowing the size of the display (the portion of the bezel covered by the display) to be increased. Figure 7 is a front view of one embodiment of a display having a sensing device disposed behind the display surface. In this case, an IR sensor 80 (shown in phantom) can be placed below a display 210 and an optical backlight stack (not shown) of a user device 200 (e.g., a smartphone). This allows the sensor 80 to be completely hidden from view by the user, allows the display 210 to extend closer to the edge of the bezel, and provides the option of removing other user interface devices (e.g., such as a visible fingerprint sensor or control buttons) from the device 200. The display 210 can be Figure 1 The optical stack 100 or any similar optical stack as discussed herein.

[0057] For example, Figure 8 shows how a fingerprint can be scanned by a sensing device placed behind a display. Figure 185 is an optical stack 100 that is located below the front glass 85 of a smartphone or similar application. In this example, the IR sensor 80 may be an infrared sensitive CMOS / TFT camera or any other suitable IR sensing device. A user 400 (represented here as a finger) presses his fingerprint 410 against the glass 85. Light 70 that leaves the optical stack 100 and any ambient light around the fingerprint 410 is incident on the fingerprint 410 and is reflected. Any infrared component 75 of the light that reflects off the ridges of the fingerprint 410 is able to pass through the optical stack 100 and be incident on the IR sensor 80. Because the optical diffuser 10 is designed to have a low scattering rate (i.e., low diffusion) for infrared light, the infrared light 75 passes through the optical stack 100 with little impact, thereby allowing the IR sensor 80 to see (i.e., detect) a high-definition image.

[0058] In some embodiments, light source 60 can emit light in both a first wavelength range and a second wavelength range (e.g., human visible light and infrared light) such that infrared light that leaves the display as part of exit light 70 can be used to illuminate fingerprint 410 using infrared wavelengths. It should be noted that human visible wavelengths can also be reflected from fingerprint 410, but because they are diffused and / or partially absorbed by the layers of optical stack 100, they are not as useful to IR sensor 80 as the infrared components, and therefore they are not reflected in the IR sensor 80. Figure 8 Shown in.

[0059] Example

[0060] Several exemplary membrane stacks were prepared according to embodiments of the present specification. FIG. 9A to FIG. 15 These films and the resulting transmission spectra and layer thickness profiles are described in . Fig.15 The coordinate system reference used for all membrane tests and results is provided in .

[0061] Exemplary Film 1. A multilayer optical film was made using two sequential (stacked) packets of microlayers, with a layer of 325 individual microlayers in each packet. The microlayers in each packet were arranged as alternating layers of material A and material B. Material A was a birefringent polyester PEN (polyethylene naphthalate), and material B was an amorphous polyester PETg GN071. The two microlayer packets were each designed to have reflection bands at two separate, slightly overlapping regions of visible and near IR wavelengths.

[0062] The process conditions chosen for the fabrication of this film resulted in wavelength dependent refractive index values ​​as shown in Table 1:

[0063]

[0064] Additionally, the extrusion settings used in the manufacture of this film were set to provide a phase thickness ratio of the PEN microlayer relative to the sum of the phase thicknesses of the same PEN microlayer plus its PETg microlayer pair that was 64% when calculated using the refractive index set at 633 nm on the x-axis (transverse to the machine axis) from Table 1.

[0065] A representative metrology spectrum of exemplary film 1 was measured and Fig. 9A The metric spectrum is shown in . Fig.15 A coordinate system reference diagram is presented in . Fig. 9B The layer thickness profile (ΣThickness-A, Thickness-B) of the microlayer pairs in each of the two layer groups is shown in .

[0066] Exemplary Film 2. A multilayer optical film was made using two sequential (stacked) layer groups of microlayers, with a layer of 275 individual microlayers in each layer group. The microlayers in each layer group were arranged as alternating layers of material A and material B for layer group 1, and alternating layers of material A and material C for layer group 2. For layer group 1, material A was a birefringent polyester low-melting point PEN (90% polyethylene naphthalate, 10% polyethylene terephthalate), and material B was an amorphous CoPEN (55% polyethylene naphthalate, 45% polyethylene terephthalate, and containing a hexanediol crystallization inhibitor). For Layer Set 2, Material A is a birefringent polyester low-melting PEN (90% polyethylene naphthalate, 10% polyethylene terephthalate), and Material C is an amorphous cycloaliphatic copolyester Neostar FN007 supplied by Eastman Chemical Co. The two microlayer layer sets were each designed to have reflection bands at two separate, slightly overlapping regions of visible and near IR wavelengths.

[0067] The process conditions chosen for the fabrication of this film resulted in wavelength dependent refractive index values ​​as shown in Table 2:

[0068]

[0069] Additionally, the extrusion settings used in the manufacture of this film were set to provide a phase thickness ratio of the LmPEN microlayer relative to the sum of the phase thicknesses of the same LmPEN microlayer plus its amorphous microlayer pair that was 50% when calculated using the refractive index set for the x-axis (transverse to the machine axis) at 633 nm from Table 2. This was true for the materials in both Layer Set 1 and Layer Set 2.

[0070] A representative metrology spectrum of Example Film 2 was measured and Fig. 10A The metric spectrum is shown in . Fig.15 A coordinate system reference diagram is presented in . Fig. 10B The layer thickness profile of the microlayer pairs in each of the two layer groups is shown in (layer group 1: Σthickness-A, thickness-B, layer group 2: Σthickness-A, thickness-C).

[0071] Exemplary Film 3. A multilayer optical film was made using a layer group of microlayers, wherein there were layers of 275 individual microlayers in the layer group. The microlayers in each layer group were arranged as alternating layers of material A and material B. Material A was a birefringent polyester low melting point PEN (90% polyethylene naphthalate, 10% polyethylene terephthalate), and material B was an amorphous blend of polycarbonate (PC) and amorphous polyester PCTg in a ratio of 80:20, called Hebron. Hebron was then blended with PETg (GN017) in a ratio of 85:15. The microlayer layer group was designed to have a reflection band spanning the visible and near IR wavelength regions.

[0072] The process conditions selected for the fabrication of this film resulted in the wavelength-dependent refractive index values ​​shown in Table 3.

[0073]

[0074] The extrusion settings used in the manufacture of this film were set to provide a phase thickness ratio of the LmPEN microlayer relative to the sum of the phase thicknesses of the same LmPEN microlayer plus its Hebron / PETg microlayer pair, which was 50% when calculated using the refractive index set at 633 nm on the x-axis (transverse to the machine axis) from Table 3.

[0075] A representative metrology spectrum of exemplary film 3 was measured and Fig.11A The metric spectrum is shown in . Fig.15 A coordinate system reference diagram is presented in . Fig. 11B The layer thickness profile (ΣThickness-A, Thickness-B) of a microlayer pair of a single layer group is shown in .

[0076] Exemplary Film 4. A multilayer optical film was made using a single layer group of microlayers, wherein there were 184 individual microlayer layers in the layer group. The microlayers in the layer group were arranged as alternating layers of material A and material B. Material A was a birefringent polyester low melting point PEN (90% polyethylene naphthalate, 10% polyethylene terephthalate), and material B was an amorphous blend of polycarbonate (PC) and amorphous polyester PCTg in a ratio of 80:20, called Hebron. Hebron was then blended with PETg (GN017) in a ratio of 85:15. The microlayer layer group was designed to have a reflection band spanning the visible and near IR wavelength regions.

[0077] The process conditions selected for the fabrication of this film resulted in the wavelength-dependent refractive index values ​​shown in Table 4.

[0078]

[0079] The extrusion settings used in the manufacture of this film were set to provide a phase thickness ratio of the LmPEN microlayer relative to the sum of the phase thicknesses of the same LmPEN microlayer plus its Hebron / PETg microlayer pair, which was 50% when calculated using the refractive index set at 633 nm on the x-axis (transverse to the machine axis) from Table 4.

[0080] A representative metrology spectrum of exemplary film 4 was measured and Fig. 12A The metric spectrum is shown in . Fig.15 A coordinate system reference diagram is presented in . Fig. 12B The layer thickness profile (ΣThickness-A, Thickness-B) of a microlayer pair of a single layer group is shown in .

[0081] Table 5 below shows measured modulation transfer function values ​​for several exemplary films and film stacks under different conditions and illuminated by near infrared light. Modulation transfer functions can be used to characterize the resolution and performance of optical systems (such as the optical stacks described herein). The modulation transfer function (MTF) provides a measure of the ability of a film or optical stack to transfer image contrast information (or image detail) from an original image to a target plane (e.g., the surface of an imaging sensor such as an infrared camera) in the form of a virtual image of a specific resolution.

[0082] The MTF values ​​of several films and optical stacks were determined as follows. The measurement system was set up in a laboratory under controlled conditions such as Fig.14 It should be noted that Fig.14 The test setup shown is intended for illustrative purposes and is not intended to be limiting. Fig.14 In the illustrated embodiment, a test fixture 1445 is provided with an infrared camera 1440. Various line resolution test charts 1410 are placed over the aperture at the top of the test fixture 1445. In some examples, the line resolution test chart 1410 is placed alone over the aperture to determine a base reading without any films or stacks. In other examples, additional layers are placed over the aperture along with the line resolution test chart 1410. Fig.14 In the exemplary setup shown, a liquid crystal module (LCM) 1420 and a cover glass 1430 are added to the test stack. A description of the stack components for each test performed is given in Table 5.

[0083] Each line resolution test chart 1410 has a sequence of line pairs (alternating black and white "stripes") of different spatial frequencies. For illustration purposes, Fig.1414. Three exemplary line resolution test charts 1410 are shown in FIG. 14. For example, line resolution test chart 1450 has a spatial frequency of 2.2 line pairs per millimeter (mm), line resolution test chart 1460 has a spatial frequency of 3.1 line pairs per mm, and line resolution test chart 1470 has a spatial frequency of 3.9 line pairs per mm. These are the specific line resolution test charts 1410 used for the testing documented herein.

[0084] The MTF measurement is determined as follows. First, use Fig.14 The test fixture 1445 is used to obtain the minimum (min) and maximum (max) grayscale value readings for each stack tested. The value of the modulation M of the baseline (for the line resolution test chart 1410 only without the stack) is then calculated for each spatial frequency using the formula M = (max-min) / (max+min). The modulation value M' is then calculated in the same manner for each stack tested. The MTF value is then determined as the ratio M' / M. Table 5 shows the measurement results for several films / film stacks at each of the three spatial resolutions.

[0085]

[0086] The terms used in Table 5 are generally defined as follows: "CMOF004" is an exemplary collimating multilayer optical film, "APFv3" (and "APF" in general) is an exemplary reflective polarizer, "pseudo diffuser" is an exemplary optical diffuser with low scattering rate for near-infrared wavelengths, "NSR" is an exemplary reflector or enhanced specular reflector, "BEF" (including TBEF) is an exemplary light redirecting film (such as "brightness enhancement film"), "LCM" is an exemplary liquid crystal module, and "UDF35" is a diffuser unique to the prior art that does not exhibit low scattering (low diffusion) in the infrared wavelengths.

[0087] Fig.13 A graph of the MTF values ​​for the tested films and film stacks shown in Table 5 is shown. Fig.13 The graph of FIG. 5 shows a graph of MTF curves representing four separate groups of Table 5, which are a single film solution (according to an embodiment of the present specification) 1300, a prior art single film value 1325, a membrane stack 1350, and a membrane stack paired with an LCM 1375. Each of these groups is represented by Fig.13 In general, Table 5 and Fig.13It was confirmed that single film 1300 (as described herein) exhibited the highest MTF value for infrared image transfer, and single film 1325 from the prior art had the lowest MTF value. That is, the data indicates that the use of diffusers and other film layers that exhibit high infrared clarity (i.e., relatively high levels of infrared transmittance and low infrared scattering, such as single film 1300 according to embodiments of the present specification) provides better image transfer (higher resolution images in the infrared wavelengths) than existing single films used in the prior art.

[0088] Fig.13 Line 1308 represents the MTF values ​​of the reflective polarizer (APFv3) across the depicted spatial frequencies, line 1306 represents the MTF values ​​of the camouflaged diffuser, line 1304 represents the MTF values ​​of the specular reflector (NSR), and line 1302 represents the MTF values ​​of the collimating multilayer optical film (CMOF004).

[0089] Fig.13 Line 1333 represents the MTF values ​​of the brightness enhancing film (TBEF2-DT) across the depicted spatial frequencies, line 1331 represents the MTF values ​​of the standard prior art diffuser (UDF35), line 1329 represents the MTF values ​​of the second brightness enhancing film (BEF4-DML), and line 1327 represents the MTF values ​​of the third brightness enhancing film (BEF4-DMH).

[0090] Terms such as "about" will be understood by one of ordinary skill in the art in the context of use and description in this specification. If it is not clear to one of ordinary skill in the art in the context of use and description in this specification that "about" should be used to express the amount of feature size, quantity, and physical properties, then "about" will be understood to mean within 10% of the specified value. A quantity given as about a specified value may be exactly the specified value. For example, if it is not clear to one of ordinary skill in the art in the context of use and description in this specification, a quantity having a value of about 1 means that the quantity has a value between 0.9 and 1.1, and the value may be 1.

[0091] One of ordinary skill in the art will understand terms such as "substantially" in the context of use and description in this specification. If the use of "substantially equal" in the context of use and description in this specification is unclear to one of ordinary skill in the art, "substantially equal" will refer to approximately the situation of approximately as described above. If the use of "substantially parallel" in the context of use and description in this specification is unclear to one of ordinary skill in the art, "substantially parallel" will refer to within 30 degrees of parallel. In some embodiments, directions or surfaces described as being substantially parallel to each other may be within 20 degrees or within 10 degrees of parallel, or may be parallel or nominally parallel. If the use of "substantially aligned" in the context of use and description in this specification is unclear to one of ordinary skill in the art, "substantially aligned" will refer to alignment within 20% of the width of the object of alignment. In some embodiments, the objects described as being substantially aligned may be aligned within 10% or within 5% of the width of the object of alignment.

[0092] 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.

[0093] 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 stack, comprising: Optical diffusers; as well as a first reflective polarizer disposed on a first side of the optical diffuser, Such that for light of substantially normal incidence and for non-overlapping first and second wavelength ranges, the first wavelength range is at least from 450 nm to 600 nm, and the second wavelength range is at least from 800 nm to 1200 nm: The optical diffuser has a first scattering rate R1 for at least one wavelength in the first wavelength range and a second scattering rate R2 for at least one wavelength in the second wavelength range, R1 / R2 ≥ 2; and the first reflective polarizer transmits at least 40% of light in a first polarization state for each wavelength in the first wavelength range, reflects at least 70% of light in an orthogonal second polarization state for each wavelength in the first wavelength range, and transmits at least 40% of light in each of the first polarization state and the second polarization state for each wavelength in the second wavelength range, Wherein, the optical stack further comprises: a first light redirecting film disposed between the first reflective polarizer and the optical diffuser and comprising a plurality of first substantially parallel microstructures extending along a first direction; and a second light redirecting film disposed between the first reflective polarizer and the optical diffuser and comprising a plurality of second substantially parallel microstructures extending in a second direction different from the first direction; such that for substantially normally incident light, each of the first light redirecting film and the second light redirecting film absorbs less than 10% of the substantially normally incident light in each of the first wavelength range and the second wavelength range, The optical stack also includes a light guide and a reflector disposed on a second side of the optical diffuser opposite the first side, the light guide disposed between the reflector and the optical diffuser such that, for substantially normally incident light and for each of the first polarization state and the second polarization state, the reflector reflects at least 70% of the light at each wavelength in the first wavelength range and transmits at least 70% of the light at each wavelength in the second wavelength range.

2. The optical stack of claim 1 , wherein the optical diffuser comprises a binder defining a plurality of interconnected voids therein such that, for at least one cross-section of the optical diffuser along a thickness thereof, the voids cover at least 20% of the at least one cross-section, the optical diffuser and the binder having respective refractive indices nd and nb at at least one wavelength within the first wavelength range, nd<nb.

3. The optical stack of claim 1, wherein the first wavelength ranges from 420 nm to 650 nm.

4. The optical stack of claim 1, wherein the second wavelength ranges from 800 nm to 1550 nm.

5. The optical stack of claim 1 , wherein the optical diffuser has a first average scattering index R11 in the first wavelength range and a second average scattering index R22 in the second wavelength range, R11 / R22>2, or R11 / R22>2.5, or R11 / R22>3.0, or R11 / R22>3.5, or R11 / R22>4.0, or R11 / R22>4.5, or R11 / R22>5.

0.

6. The optical stack of claim 1, wherein the first reflective polarizer transmits at least 70% of light in the first polarization state at each wavelength in the first wavelength range.

7. The optical stack of claim 1, wherein the first reflective polarizer transmits at least 50% of light in each of the first polarization state and the second polarization state for each wavelength in the second wavelength range.

8. The optical stack according to claim 1, wherein the optical stack has a first average optical haze H1 in the first wavelength range and a second average optical haze H2 in the second wavelength range, H1 / H2 >1.5, or H1 / H2 >2.0, or H1 / H2 >2.5, or H1 / H2 >3.0, or H1 / H2 >3.5, or H1 / H2 >4.0, or H1 / H2 >4.5, or H1 / H2 >5.

0.

9. The optical stack of claim 1 , wherein for the first wavelength range and for light incident at an angle of incidence relative to a direction perpendicular to the first reflective polarizer, the first reflective polarizer has an average optical transmittance T0 when the angle of incidence is about zero degrees, and an average optical transmittance T60 when the angle of incidence is about 60 degrees, T60 / T0 <0.8, or T60 / T0 <0.75, or T60 / T0 <0.7, or T60 / T0 <0.65, or T60 / T0 <0.6, or T60 / T0 <0.55, or T60 / T0 <0.

5.

10. A display comprising an optical stack according to claim 1, wherein the optical stack is arranged between a liquid crystal panel and an infrared sensitive detector so that when an infrared emission source emitting light within the second wavelength range is set close to the liquid crystal panel, the infrared sensitive detector detects at least some of the light within the second wavelength range emitted by the infrared emission source.

11. A display comprising the optical stack of claim 1 and configured to form a first image in the first wavelength range for viewing by a viewer and a second image in the second wavelength range detectable by an infrared-sensitive camera.

12. An optical stack, comprising: An optical diffuser having an average total transmittance T1 and an average diffuse transmittance T2 in a first wavelength range from 450 nm to 600 nm, T2 / T1>0.4, and having an average total transmittance T1' and an average diffuse transmittance T2' in a second wavelength range from 800 nm to 1200 nm, T2' / T1'<0.3; a collimating multilayer optical film disposed on a first side of the optical diffuser and comprising a plurality of alternating first and second polymer layers numbering at least 30, each first and second polymer layer having an average thickness of less than 500 nm; a reflective polarizer disposed on a side of the collimating multilayer optical film distal from the optical diffuser, wherein for light incident substantially normally, the reflective polarizer transmits at least 40% of light in a first polarization state for each wavelength in the first wavelength range, reflects at least 70% of light in an orthogonal second polarization state for each wavelength in the first wavelength range, and transmits at least 40% of light in each of the first polarization state and the second polarization state for each wavelength in the second wavelength range; a light guide disposed on a second side of the optical diffuser opposite the first side; an optical reflector disposed on a side of the light guide remote from the optical diffuser, wherein, for substantially normally incident light, the optical reflector reflects at least 70% of each of an orthogonal first polarization state and a second polarization state for each wavelength in the first wavelength range, and transmits at least 70% of each of the first polarization state and the second polarization state for each wavelength in the second wavelength range, Wherein for at least one wavelength from within the second wavelength range, the optical stack has a modulation transfer function (MTF) greater than 0.4 at 2.2 line pairs per mm.

13. The optical stack of claim 12, wherein the collimating multilayer optical film transmits at least 40% of light in each of the first polarization state and the second polarization state for each wavelength in the second wavelength range.

14. The optical stack of claim 12, wherein for the first wavelength range and for light incident at an incident angle relative to a direction perpendicular to the collimating multilayer optical film, the collimating multilayer optical film has an average optical transmittance T0 when the incident angle is about zero degrees, and an average optical transmittance T60 when the incident angle is about 60 degrees, T60 / T0 <0.8, or T60 / T0 <0.75, or T60 / T0 <0.7, or T60 / T0 <0.65, or T60 / T0 <0.6, or T60 / T0 <0.55, or T60 / T0 <0.

5.

15. The optical stack of claim 12, wherein for the at least one wavelength in the second wavelength range, the optical stack has a modulation transfer function (MTF) greater than 0.5 at 2.2 line pairs per mm.

Images