Edge-lit back light unit with improved efficiency

Abstract

To provide an edge-lit back light unit for a backlit display having improved efficiency that allows increased viewing brightness without increasing the electric power supplied to the display.SOLUTION: An edge-lit back light unit for a backlit display includes a reflector, an edge-lit light guide film, a diffuser film, and a pair of crossed brightness enhancement films with increased efficiency. The diffuser film has an angular light distribution output that is matched to light acceptance angles of the pair of crossed brightness enhancement films to provide increased on-axis brightness without having to increase power to the edge-lit back light unit.SELECTED DRAWING: None

Description

[Technical field]

[0001] (CROSS REFERENCE TO RELATED APPLICATIONS) This application claims the benefit of priority to U.S. Provisional Patent Application No. 63 / 127,325, filed December 18, 2020, and U.S. Provisional Patent Application No. 63 / 214,730, filed June 24, 2021, the entire contents of which are incorporated herein by reference.

[0002] The present invention is directed to edge-lit backlight units for backlit displays with improved efficiency that allows for increased perceived brightness without increasing power to the display. [Background technology]

[0003] An edge-lit backlight unit (BLU) typically uses a light guiding film 100 with multiple light emitting diodes (LEDs) 110 positioned along one side of the light guiding film 100 as illustrated in FIG. 1. The light guiding film 100 has a width W and a length L, and the LEDs 110 are positioned at one edge along the width W of the light guiding film 100 and are configured to emit light into the light guiding film 100 such that the light travels along the length L of the light guiding film 100 as illustrated in FIG. 2, which is a cross section taken along line 2-2 in FIG. 1. The light guiding film 100 typically has small structures on the top and / or bottom surfaces to couple light out into the light guiding film 100. Ideally, the structures are apodized so that the outcoupled light of the light guiding film 100 has a spatially uniform intensity. Generally, light being coupled into the light guiding film 100 does so with an angular distribution that is directed away from the light input edge of the light guiding film 100, as illustrated in Figure 2. This general direction away from the light guiding film 100 nominally extends along the length L of the light guiding film 100 and is referred to as the light direction of the light guiding film 100.

[0004] It is desirable for backlight units for edge-lit displays to maximize light efficiency, ie, increase the perceived brightness without having to increase the power provided to the backlight unit. Summary of the Invention [Means for solving the problem]

[0005] The present invention achieves increased efficiency by replacing conventional diffuser films in a backlight unit with an improved light management diffuser film that has an angular light dispersion output that is matched to the acceptance angle of the crossed brightness enhancement films.

[0006] According to one aspect of the present invention, an edge-lit backlight unit for a backlit display is provided. The edge-lit backlight unit includes a specular reflector and an edge-lit light guiding film positioned above the specular reflector. The edge-lit light guiding film has a length and a width. The combination of the edge-lit light guiding film and the specular reflector is configured to provide a peak optical dispersion of 15°-20° and a full-width half-maximum diffusion angle of 25°-45°. A diffuser film is positioned above the edge-lit light guiding film. The diffuser film has a plurality of parallel prismatic microstructures on one side thereof facing the edge-lit light guiding film and a plurality of diffuser microstructures on the other side thereof. Each of the plurality of parallel prismatic microstructures has an apex angle of 78°-92° and a refractive index of 1.49-1.58. The diffuser microstructures have a full-width half-maximum diffusion angle of 30°-60°. A pair of crossed brightness enhancement films are positioned above the diffuser film. Each brightness enhancement film has a plurality of parallel microprisms on one side thereof facing away from the diffuser film. The parallel microprisms of one of the brightness enhancement films are oriented perpendicular to the parallel microprisms of the other brightness enhancement film. The prism microstructures of the diffuser film are substantially aligned with the parallel microprisms that are closest to those that are aligned with the length of the light guiding film.

[0007] In some embodiments, the diffuser microstructures of the diffuser film are circular diffuser microstructures.

[0008] In some embodiments, the diffuser microstructures of the diffuser film are conical microstructures, hi some embodiments, the conical microstructures have an apex angle of about 110°.

[0009] In some embodiments, the diffuser microstructures of the diffuser film are square pyramidal microstructures. In some embodiments, the square pyramidal microstructures have an apex angle of about 110°. In some embodiments, the faces of the square pyramidal microstructures are aligned parallel or perpendicular to the prismatic microstructures of the diffuser film. In some embodiments, the faces of the square pyramidal microstructures are oriented at about 45° to the prismatic microstructures of the diffuser film.

[0010] In some embodiments, the edge-lit backlight unit also includes a number of LEDs positioned along the width of the light guiding film.

[0011] In some embodiments, the diffuser microstructures are angle bending microstructures oriented to bend light in a direction aligned with the multiple prismatic microstructures and away from the multiple LEDs.

[0012] According to one aspect of the present invention, an edge-lit backlight unit for a backlit display is provided. The edge-lit backlight unit includes a diffuse reflector and an edge-lit light guiding film positioned above the diffuse reflector. The edge-lit light guiding film has a length and a width. The combination of the edge-lit light guiding film and the diffuse reflector is configured to provide a peak optical dispersion of 30°-50° and a full-width half-maximum diffusion angle of 55°-85°. A diffuser film is positioned above the edge-lit light guiding film. The diffuser film has a plurality of parallel prismatic microstructures on one side thereof facing the edge-lit light guiding film and a plurality of diffuser microstructures on the other side thereof. Each of the plurality of parallel prismatic microstructures has an apex angle of 75°-85° and a refractive index of 1.59-1.67. The diffuser microstructures have a full-width half-maximum diffusion angle of less than 20°. A pair of crossed brightness enhancement films is positioned above the diffuser film. Each of the brightness enhancement films has a plurality of parallel microprisms on one side thereof facing away from the diffuser film, the plurality of parallel microprisms of one of the brightness enhancement films are oriented perpendicular to the plurality of microprisms of the other brightness enhancement film, and the plurality of prism microstructures of the diffuser film are substantially aligned with the plurality of closest parallel microprisms so as to be aligned with the length of the light guiding film.

[0013] In some embodiments, the diffuser microstructure of the diffuser film is a circular diffuser microstructure. In some embodiments, the diffuser microstructure has a full width half maximum diffusion angle less than or equal to 10°. In some embodiments, the diffuser microstructure has a full width half maximum diffusion angle less than or equal to 5°.

[0014] These and other aspects, features, and characteristics of the present invention, and the method and function of operation of the associated elements of structure, combination of parts and economies of manufacture will become more apparent upon consideration of the following description and the appended claims, with reference to the accompanying drawings, all of which form a part of this specification. It is to be expressly understood, however, that the drawings are for purposes of illustration and description only and are not intended as a definition of the limits of the invention. As used in this specification and claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. The present invention provides, for example, the following: (Item 1) 1. An edge-lit backlight unit for a backlit display, the edge-lit backlight unit comprising: A specular reflector; an edge-lit light guiding film positioned above the specular reflector, the edge-lit light guiding film having a length and a width, the combination of the edge-lit light guiding film and the specular reflector configured to provide a peak optical dispersion of between 15° and 20° and a full width half maximum diffusion angle of between 25° and 45°; a diffuser film positioned above the edge-lit light guiding film, the diffuser film having on one side thereof facing the edge-lit light guiding film a plurality of parallel prism microstructures, at least some of the parallel prism microstructures having apex angles between 78° and 92° and formed from a material having a refractive index between 1.49 and 1.58, and on the other side thereof a plurality of diffuser microstructures, the diffuser microstructures having a full width half maximum diffusion angle between 30° and 60°; a pair of crossed brightness enhancement films positioned above the diffuser film, at least one of the brightness enhancement films having a plurality of parallel microprisms on one side thereof facing away from the diffuser film, the plurality of parallel microprisms of one of the brightness enhancement films being oriented perpendicular to the plurality of microprisms of the other brightness enhancement film; Equipped with The edge-lit backlight unit, wherein the plurality of prismatic microstructures of the diffuser film are substantially aligned with the plurality of parallel microprismatic structures of at least one of the brightness enhancement films. (Item 2) Item 2. The edge-lit backlight unit of item 1, wherein at least one of the brightness enhancement films comprises the plurality of parallel microprisms oriented to be most closely aligned with the light direction. (Item 3) Item 3. The edge-lit backlight unit of item 2, wherein at least one of the brightness enhancement films is positioned on top of the edge-lit backlight unit. (Item 4) Item 2. The edge-lit backlight unit of item 1, wherein the diffuser microstructures are formed in a circular shape. (Item 5) Item 2. The edge-lit backlight unit of item 1, wherein the diffuser microstructures are formed in a cone shape. (Item 6) Item 6. The edge-lit backlight unit of item 5, wherein the conical shape of the diffuser microstructures has an apex angle of about 110°. (Item 7) Item 2. The edge-lit backlight unit of item 1, wherein the diffuser microstructures are formed in a square pyramid shape. (Item 8) Item 8. The edge-lit backlight unit of item 7, wherein the square pyramidal microstructures have an apex angle of about 110°. (Item 9) Item 8. The edge-lit backlight unit of item 7, wherein the square pyramidal microstructures have an apex angle of about 90°. (Item 10) 8. The edge-lit backlight unit of item 7, wherein the square pyramidal microstructures are formed from a material having a refractive index of about 1.57. (Item 11) Item 8. The edge-lit backlight unit of item 7, wherein at least some faces of the pyramidal microstructures are aligned to be parallel to the prismatic microstructures of the diffuser film. (Item 12) Item 8. The edge-lit backlight unit of item 7, wherein at least some faces of the square pyramidal microstructures are aligned to be within less than or equal to 10 degrees of parallel to the prismatic microstructures of the diffuser film. (Item 13) Item 8. The edge-lit backlight unit of item 7, wherein at least some faces of the square pyramidal microstructures are aligned to be within less than or equal to 20 degrees of parallel to the prismatic microstructures of the diffuser film. (Item 14) Item 14. The edge-lit backlight unit of item 13, wherein at least one of the brightness enhancement films comprises the plurality of parallel microprisms oriented to be most closely aligned with a light direction. (Item 15) Item 8. The edge-lit backlight unit of item 7, wherein at least some faces of the square pyramidal microstructures are oriented at about 45° to the prismatic microstructures of the diffuser film. (Item 16) Item 16. The edge-lit backlight unit of item 15, wherein at least one of the brightness enhancement films comprises the plurality of parallel microprisms oriented to be most closely aligned with a light direction. (Item 17) Item 1. The edge-lit backlight unit of item 1, further comprising a plurality of LEDs positioned along the width of the light guiding film. (Item 18) Item 16. The edge-lit backlight unit of item 15, wherein the diffuser microstructure is an angle bending microstructure that is aligned with the plurality of prismatic microstructures and oriented in a direction away from the plurality of LEDs for bending light. (Item 19) 1. An edge-lit backlight unit for a backlit display, the edge-lit backlight unit comprising: A diffuse reflector; an edge-lit light guiding film positioned above the diffusive reflector, the edge-lit light guiding film having a length and a width, the combination of the edge-lit light guiding film and the diffusive reflector configured to provide a peak optical dispersion of between 30° and 50° and a full width half maximum diffusion angle of between 55° and 85°; a diffuser film positioned above the edge-lit light guiding film, the diffuser film having on one side thereof facing the edge-lit light guiding film a plurality of parallel prism microstructures, at least some of the parallel prism microstructures having an apex angle of 75°-85° and a refractive index of 1.59-1.67, and on an opposite side thereof a plurality of diffuser microstructures, the diffuser microstructures having a full width half maximum diffusion angle of less than 20°; a pair of crossed brightness enhancement films positioned above the diffuser film, at least one of the brightness enhancement films having a plurality of parallel microprisms on one side thereof facing away from the diffuser film, the plurality of parallel microprisms of one of the brightness enhancement films being oriented perpendicular to the plurality of microprisms of the other brightness enhancement film; Equipped with An edge-lit backlight unit, wherein the plurality of prismatic microstructures of the diffuser film are substantially aligned with the plurality of parallel microprismatic structures of at least one of the pair of crossed brightness enhancement films. (Item 20) 20. The edge-lit backlight unit of claim 19, wherein the parallel microprisms of at least one of the pair of crossed brightness enhancement films are oriented to be most closely aligned with the light direction. (Item 21) Item 21. The edge-lit backlight unit of item 20, wherein at least one of the pair of crossed brightness enhancement films is positioned on top of the edge-lit backlight unit. (Item 22) 20. The edge-lit backlight unit of claim 19, wherein the diffuser microstructures are formed in a circular shape. (Item 23) 20. The edge-lit backlight unit of item 19, wherein the diffuser microstructures are configured to have a full width half maximum diffusion angle less than or equal to 10°. (Item 24) 20. The edge-lit backlight unit of item 19, wherein the diffuser microstructures are configured to have a full width half maximum diffusion angle less than or equal to 5°. (Item 25) A backlight unit, A specular reflector; a light guiding film positioned above the specular reflector, the light guiding film configured to guide light along a light direction; a diffuser film positioned above the light guiding film, the diffuser film having on one side thereof facing the light guiding film a plurality of parallel prism microstructures, at least some of the parallel prism microstructures having a prism apex direction, and further having on the other side thereof a plurality of pyramidal microstructures, at least some of the pyramidal microstructures having a common pyramidal apex direction; a first brightness enhancement film positioned above the diffuser film, the first brightness enhancement film having a plurality of parallel prism microstructures on one side thereof facing away from the diffuser, at least some of the plurality of parallel prism microstructures of the first brightness enhancement film having a common prism apex direction; a second brightness enhancement film positioned above the first brightness enhancement film having a plurality of parallel prism microstructures on one side thereof facing away from the first brightness enhancement film, at least some of the plurality of parallel prism microstructures of the second brightness enhancement film having a common prism apex direction; Equipped with A backlight unit, wherein a common prism apex direction of at least one of the first and second brightness enhancement films is oriented substantially along the light direction, a common prism apex direction of the diffuser film is oriented substantially along the common prism apex direction of at least one of the first and second brightness enhancement films, and a common pyramid apex direction of the diffuser film is oriented at a desired angle relative to the common prism apex direction of the diffuser film. (Item 26) 26. The backlight unit of claim 25, wherein at least one of the first and second brightness enhancement films having a common prism apex direction oriented substantially along the light direction has a prism apex direction that is less than 10 degrees from the light direction. (Item 27) 26. The backlight unit of claim 25, wherein at least one of the first and second brightness enhancement films having a common prism apex direction oriented substantially along the light direction has a prism apex direction that is less than 20 degrees from the light direction. (Item 28) Item 26. The backlight unit of item 25, wherein the desired angle is substantially zero degrees. (Item 29) Item 26. The backlight unit according to item 25, wherein the desired angle is substantially 45 degrees. (Item 30) 26. The backlight unit of item 25, wherein the other of at least one of the first and second brightness enhancement films has a prism apex direction oriented approximately perpendicular to the prism apex direction of at least one of the first and second brightness enhancement films. (Item 31) Item 26. The backlight unit of item 25, wherein at least one of the first and second brightness enhancement films having the prism apex direction oriented substantially along the light direction is positioned at the top of the backlight unit. (Item 32) Item 26. The backlight unit according to item 25, wherein at least some of the plurality of parallel prism microstructures of the diffuser film have an apex angle of 78° to 94°. (Item 33) Item 26. The backlight unit according to item 25, wherein at least some of the parallel prism microstructures of the diffuser film are formed from a material having a refractive index of 1.5 to 1.66. (Item 34) Item 26. The backlight unit of item 25, wherein at least some of the plurality of pyramidal microstructures of the diffuser film have an apex angle of 84° to 114°. (Item 35) Item 26. The backlight unit of item 25, wherein at least some of the plurality of pyramidal microstructures of the diffuser film are formed from a material having a refractive index of 1.5 to 1.65. (Item 36) 26. The backlight unit of claim 25, wherein at least some of the plurality of parallel prismatic microstructures of at least one of the first and second brightness enhancement films have an apex angle of 86° to 94°. (Item 37) 26. The backlight unit of claim 25, wherein at least some of the parallel prismatic microstructures of at least one of the first and second brightness enhancement films are formed from a material having a refractive index of 1.57 to 1.7. [Brief description of the drawings]

[0015] Components in the following figures are illustrated to emphasize the general principles of the present disclosure and are not necessarily drawn to scale, although at least one of the figures may be drawn to scale. Reference characters designating corresponding components are repeated as necessary throughout the figures for consistency and clarity.

[0016] [Figure 1]FIG. 1 is a schematic top view of an edge-lit light guide film with multiple light emitting diode (LED) light sources for use in an edge-lit backlight unit for a backlit display.

[0017] [Diagram 2] FIG. 2 is a schematic cross-sectional view of FIG. 1 taken along line 2-2.

[0018] [Figure 3A] FIG. 3A is a schematic exploded cross-sectional view of a backlight unit for a backlit display including the light guiding film of FIGS. 1 and 2 and an LED.

[0019] [Figure 3B] FIG. 3B is a schematic diagram of a brightness enhancement film for a backlit display having prismatic microstructures, including details of the apex and base angles.

[0020] [Figure 3C] FIG. 3C is a schematic diagram of a stacked pair of brightness enhancement films for a backlit display with prismatic microstructures illustrating the orthogonal orientation of the two films.

[0021] [Figure 3D] FIG. 3D is a schematic diagram of a gain-enhancement film for a backlit display with prismatic microstructures on one side, including details of the apex and base angles.

[0022] [Figure 3E] FIG. 3E is a schematic diagram of a gain-enhancement film for backlit displays with prismatic microstructures on one side, where the prismatic columns are formed at a desired angle relative to the long axis of the film.

[0023] [Figure 3F]FIG. 3F is a schematic diagram of a gain-enhancement film for a backlit display having an inverted pyramidal microstructure on one side and a pillar on the other side, including details of the apex and base angles of the inverted pyramidal structure.

[0024] [Figure 3G] FIG. 3G is a schematic diagram of a gain-enhancement film for a backlit display having an inverted pyramidal microstructure on one side and a prism on the opposite side, with the pyramid axis formed at a desired angle relative to the long axis of the film and the prism on the opposite side aligned with the diagonal of the pyramid base.

[0025] [Figure 3H] Figure 3H is a schematic top view of an edge-lit light-guiding film with multiple light-emitting diode (LED) light sources, showing the orientation of the light direction, the desired orientation of the pyramids at the top of the first brightness enhancement film, the desired orientation of the prisms at the top of the second brightness enhancement film, and the desired orientation of the prisms on the bottom of the gain enhancement film.

[0026] [Figure 4] FIG. 4 is a two-dimensional plot of the dispersion of light output from an LED as a function of angle as measured by a light distribution meter and after the light has passed through a pair of crossed brightness enhancement films, as measured by the light distribution meter.

[0027] [Figure 5A] FIG. 5A is a three-dimensional representation of the defined angles at which light approaching a pair of crossed brightness enhancement films may be transmitted through the pair of crossed brightness enhancement films.

[0028] [Figure 5B] FIG. 5B is a two-dimensional representation of FIG. 5A.

[0029] [Figure 6]FIG. 6 is a two-dimensional representation of FIG. 5A of the defined angles at which light approaching the pair of crossed brightness enhancement films may transmit on-axis through the pair of crossed brightness enhancement films, normal to the top surface of the light guiding film.

[0030] [Figure 7] FIG. 7 is a two-dimensional plot of the distribution of light output from the top surface of an edge-lit light guiding film having a narrow distribution with a specular reflector positioned on the bottom surface of the edge-lit light guiding film, as measured by a light distribution meter.

[0031] [Figure 8] FIG. 8 is a two-dimensional plot of the distribution of light output from the top surface of an edge-lit light guiding film having a wide distribution with a diffuse reflector positioned on the bottom surface of the edge-lit light guiding film, as measured by a light distribution meter.

[0032] [Figure 9] FIG. 9 is a two-dimensional plot of the distribution of light output from the top surface of a circular diffuser positioned on the top surface of an edge-lit light guiding film having narrow distribution, with a specular reflector positioned on the bottom surface of the edge-lit light guiding film, as measured by a light distribution meter.

[0033] [Figure 10] FIG. 10 is a two-dimensional plot of the distribution of light output from the top surface of a circular diffuser positioned on the top surface of an edge-lit light guiding film having a wide distribution, with a diffuse reflector positioned on the bottom surface of the edge-lit light guiding film, as measured by a light distribution meter.

[0034] [Figure 11]FIG. 11 is a two-dimensional plot of the distribution of light output from the top surface of a diffuser positioned on the top surface of an edge-lit light guiding film having narrow distribution, with a specular reflector positioned on the bottom surface of the edge-lit light guiding film, as measured by a light distribution meter in accordance with one embodiment of the present invention.

[0035] [Figure 12] FIG. 12 is a schematic two-dimensional representation of the defined angles at which light approaching a pair of crossed brightness enhancement films may be transmitted on-axis through the pair of crossed brightness enhancement films, normal to the top surface of the edge-lit light guiding film.

[0036] [Figure 13] FIG. 13 is a combination of FIG. 11 and FIG.

[0037] [Figure 14] FIG. 14 is a two-dimensional plot of the distribution of light output from a top surface of a diffuser positioned on the top surface of an edge-lit light guiding film having a wide distribution, with a diffuse reflector positioned on the bottom surface of the edge-lit light guiding film, as measured by a light distribution meter in accordance with one embodiment of the present invention.

[0038] [Figure 15] FIG. 15 is a schematic two-dimensional representation of the defined angles at which light approaching a pair of crossed brightness enhancement films may be transmitted on-axis through the pair of crossed brightness enhancement films, normal to the top surface of the edge-lit light guiding film.

[0039] [Figure 16] FIG. 16 is a combination of FIG. 14 and FIG.

[0040] [Figure 17] FIG. 17 is a graph of relative luminance as a function of top brightness enhancement film prism angle relative to light direction for an edge-lit backlight unit of the present teachings. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] Detailed Description The luminous intensity distribution of the light guide film 100 and the light guide film 100 with the diffuser film 320 can be used to understand how such films would transfer the inherent output of the LED 110 to a light distribution well matched to the acceptance angle of the crossed brightness enhancement film pair 330, 340. A light distribution meter is generally used to measure their light distribution. The equipment includes a mechanical light distribution meter with a horizontal axis for rotating the test sample and a vertical axis, and a photometer for measuring the luminous intensity over a given distance. The photometer is positioned at a much longer distance from the test sample than the light emitting surface dimension of the test sample so that the measured results are not related to the size of the test sample. This process is often referred to as a "far field" dispersion measurement. The optical dispersion data described herein was collected using a light distribution meter with the aforementioned apparatus.

[0042] FIG. 3A diagrammatically illustrates the light guiding film 100 of FIGS. 1 and 2 in a backlight unit 300. Specifically, FIG. 3A is a schematic exploded cross-sectional view of a backlight unit for a backlit display including the light guiding film of FIGS. 1 and 2 and LEDs. As shown, a reflector 310 is positioned directly below the light guiding film 100. The reflector 310 may be a specular reflector, a diffuse reflector, or a combination thereof, and is configured to reflect the outcoupled light below the light guiding film 100 back toward the light guiding film 100. Positioned above the light guiding film 100 is a diffuser film 320, which may also be referred to as a gain enhancement film, and is typically circularly symmetric and configured to improve the uniformity of the outcoupled light above the light guiding film 100.

[0043] Positioned above the diffuser film 320 are two brightness enhancement films (BEFs) 330, 340. The brightness enhancement films 330, 340 in some embodiments have a plurality of parallel microprisms with a 90° apex angle on one side thereof, and the refractive index of the prisms is typically between 1.55 and 1.7. In the backlight unit 300, the brightness enhancement films 330, 340 are positioned with the prisms facing away from the light guiding film 100, the prisms being positioned on the top surface of the BEFs, and the prisms in the top film 340 are oriented perpendicular to the bottom film 330. The parallel microprisms of one of the brightness enhancement films 330, 340 are generally aligned (i.e., within about 20°) with the length L of the light guiding film 100. The parallel microprisms of the other of the brightness enhancement films 330, 340 are generally aligned (i.e., within about 20°) with the width W of the light guiding film 100. That is, the alignment of the directions of the prism apexes is generally along the light direction of the light guiding film 100. In other embodiments, the alignment of the directions of the prism apexes is closer than 20 degrees. In some embodiments, the crossed brightness enhancement films 330, 340 increase the on-axis brightness of the light exiting the backlight unit 300.

[0044] It is desirable for a backlight unit 300 for an edge-lit display to maximize light efficiency, i.e., increase the perceived brightness, without having to increase the power provided to the backlight unit 300. One feature of the present teachings is the recognition that the direction of light propagating through a backlit display, i.e., the orientation of the prisms and / or the orientation of the films relative to the light direction, can be selected to produce high on-axis brightness for the display.

[0045] For example, in some embodiments, the orientation of the prisms of at least one brightness enhancement film 330, 340 is oriented along the direction of light propagating through the light guiding film. In some embodiments, this is the direction of the apex of the prism on the top brightness enhancement film 340 that is nominally along the direction of light propagating through the light guiding film (i.e., less than 10 degrees or less than 20 degrees). In some of these embodiments, the direction of the apex of the prism on the second brightness enhancement film 330 is nominally perpendicular to the direction of the apex of the prism on the top brightness enhancement film 340. In some embodiments, the direction of the apex of the prism on the bottom of the diffuser film 320 is aligned with the direction of the apex of the prism on the top brightness enhancement film 340. In some embodiments, the direction of at least some of the pyramid faces located on the top side of the diffuser film 320 is aligned parallel to the direction of the apex of the prism on the bottom of the diffuser film 320. That is, the apex directions of the pyramids in the diffuser film 320 are nominally aligned parallel to the direction of the prism apex. As used herein, the term "nominal parallel" means that the angle between the two directions is substantially zero degrees. In some embodiments, the directions of at least some of the pyramid faces located on the top side of the diffuser film 320 are aligned at 45 degrees with the direction of the apex of the prism on the bottom of the diffuser film 320. These various embodiments of microstructure alignment result in high brightness from the backlight unit 300.

[0046] FIG. 3B is a schematic diagram of a brightness enhancement film 350 for a backlit display having a prismatic microstructure, including details of the apex and base angles of the prismatic faces. This structure 350 can be used for one or both of the brightness enhancement films 340, 330 described in connection with FIG. 3A. Multiple prismatic columns are oriented parallel to each other along one direction of the film. The direction of the line through the apex of the prismatic columns is referred to as the prismatic apex direction. In some embodiments, multiple prismatic columns extend along a straight line. In some embodiments, the prismatic columns do not follow a perfectly straight line, but follow a slightly wavy path. In embodiments with a wavy line of prismatic apex columns, the prismatic columns are still nominally parallel and can be considered to have a pronounced prismatic apex alignment direction. The prismatic columns in some embodiments are located on the top side of the film.

[0047] The facet of one embodiment of the triangular prism in view exhibits an apex angle of 90°±4°. The base angle for this embodiment of the facet of the prism is 45°±2°. For example, the refractive index of the prism can be nominally in the range of 1.57 to 1.7.

[0048] 3C is a schematic diagram of a stacked pair of brightness enhancement films 355 for a backlit display with prismatic microstructures illustrating the orthogonal orientation of the two films. This stacked pair of brightness enhancement films 355 can be, for example, the two brightness enhancement films 340, 330 described in connection with FIG. 3A. The films are positioned such that the directions of the apexes of the parallel prisms in the two films are oriented at right angles to each other. In some embodiments, the direction of the apexes of the prisms on the top film (e.g., film 340 of FIG. 3A) is oriented along the direction of light propagating through the light-guiding film (e.g., film 100) of FIG. 3A.

[0049] In an orthogonal orientation of the prism apex directions of stacked brightness enhancement films 355, as will be clear to one skilled in the art, the orientation of one of the two films will generally be more aligned along the direction of light propagating through the light guiding film than the other. This is because the films are less likely to be oriented strictly diagonally. Thus, in general, when there are two stacked brightness enhancement films 355, it is possible to define the direction of the apex of the prism in the stack that is most closely aligned with the direction of light propagating through the light guiding film. In some embodiments, this will be the top film direction, and in other embodiments, this will be the bottom film direction, depending on which is most closely aligned along the direction of light propagating through the light guiding film.

[0050] FIG. 3D is a schematic diagram of a gain enhancement film 360 for a backlit display having prismatic microstructures on one side, including details of the apex and base angles of the prismatic faces. The terms "gain enhancement film" and "diffuser film" are used interchangeably herein. The gain enhancement film 360 can be, for example, the diffuser film 320 described in connection with FIG. 3A. In some embodiments, with reference to the orientation of the backlit display in FIG. 3A, the gain enhancement film 360, 320 has prismatic columns extending along the bottom side of the film. That is, the prismatic microstructures are positioned on the side of the film 360, 320 that is closest to the light guiding film 100. In some embodiments, the direction of the apex of the prismatic microstructures along the film 360, 320 is oriented to be nominally aligned along the direction of light propagating through the light guiding film. In some embodiments, the alignment is within 10 degrees, and in other embodiments, the alignment is within 20 degrees.

[0051] A face on view of one embodiment of a triangular prism shows that the apex angle can be in the range of 78° to 94°. The corresponding base angle for this embodiment of the prism face is 43° to 51°. The refractive index of the prism can nominally be in the range of 1.5 to 1.66.

[0052] FIG. 3E is a schematic diagram of a gain enhancement film 365 for a backlit display having prismatic microstructures on one side, where the prismatic microstructures are formed at a desired angle with respect to the long axis of the film. The angle can be any angle. The ability to fabricate the apex of the parallel prismatic microstructures with a different direction from the long axis of the film is one way to create various relative alignments of the microstructure direction and light direction in the various films as described herein. That is, for example, referring to FIG. 3A, the long axes of two or more of the various films 100, 320, 330, 340 can be aligned in a stack of films that produces a backlight unit 300. Then, for example, the angle between the direction of the apex prismatic microstructures of at least one brightness enhancement film 330, 340 and the direction of the light propagating through the light guide film 100 is based on the angle between the direction of the apex of the parallel prismatic microstructures and the direction of the long axis of the film 330, 340.

[0053] 3F is a schematic diagram of a gain-enhancement film 370 for a backlit display having inverted pyramidal microstructures on one side and prisms on the other side, including details of the apex and base angles of the inverted pyramidal structures. In this embodiment, the direction passing along the aligned apexes of the pyramidal structures located on one side of the film, which is the direction passing through the two opposing faces of the pyramidal structures, is referred to as the pyramid apex direction. The pyramid apex direction in some embodiments is aligned parallel to the apex direction of the parallel prism structures located on the other side of the film.

[0054] In some embodiments, the gain enhancement film 370 is configured as the diffuser film 320 described in connection with Figure 3A. In some of these embodiments, the prismatic microstructures of the gain enhancement film 370, 320 are on the bottom of the film 370, 320 facing toward the light directing film 100, and the pyramidal structures are on the top of the film 370, 320. In some embodiments, the pyramids are inverted and face toward the inside of the film 370, as illustrated in Figure 3F. In other embodiments, the pyramids face upward and out from the top side of the film 370.

[0055] 3A , in some embodiments, the orientation of the apexes of the parallel prism structures on the bottom of the films 370, 320 is aligned with at least one of the orientations of the apexes of the parallel prism structures on the top brightness enhancement films 330, 340. In some embodiments, this alignment is with the film 330, 340 that has the orientation of the apexes of the parallel prism structures most closely aligned with the direction of light propagating through the light guiding film 100. In other embodiments, the film with the orientation of the apexes of the parallel prism structures most closely aligned with the direction of light propagating through the light guiding film 100 is the top film 340.

[0056] A face on view of one embodiment of a pyramidal cross section of film 370 shows that the apex angle can be in the range of 84° to 114°. The corresponding base angle for this embodiment of the prism face is 33° to 48°. For example, the refractive index of the prism can nominally be in the range of 1.5 to 1.65.

[0057] One feature of the gain enhancement films of the present teachings, also referred to as diffusion films, is that embodiments having pyramidal microstructures on one side of the film and prismatic structures on the other side of the film can utilize various angles between the directions passing along the aligned apexes of the pyramidal structures and the directions passing along the aligned apexes of the prismatic structures. For example, in various embodiments, these directions can be parallel or the same as those described in connection with FIG. 3F. These directions can also be at a 45 degree angle. The prismatic structures can be configured with various angles between these directions to achieve various performance metrics.

[0058] 3G is a schematic diagram of a gain-enhancement film 375 for a backlit display having an inverted pyramidal microstructure on one side and a prism on the other side, where the pyramid axis is formed at a desired angle relative to the long axis of the film and the prism on the opposite side is aligned with the diagonal of the pyramid base. In some embodiments, either of the orientations can also be at any angle relative to the long axis of the film.

[0059] FIG. 3H is a schematic top view of an edge-lit light guiding film 380 with multiple light emitting diode (LED) light sources 110 showing the orientation of the light direction and the desired orientation of the prisms on the top of the first brightness enhancement film, and also showing the desired orientation of the prisms on the top of the second brightness enhancement film and the desired orientation of the prisms on the bottom of the gain enhancement film. For example, the light guiding film 380 can be the light guiding film 100 described in connection with FIG. 3A. An exemplary configuration of the orientations described herein with respect to the microstructures on the other films (not shown in FIG. 3H) is shown. Referring again to FIG. 3A, in this embodiment, the orientation of the prism apex on the top BEF film 340 is perpendicular to the orientation of the prism apex on the bottom BEF film 330. Thus, the orientation of the prism apex on the top BEF film 340 is most closely aligned with the light direction in the light guiding film 380. In this embodiment, the directions of the apexes of the prisms on the bottom side of the diffuser film 320 are aligned in the same direction as the directions of the apexes of the prisms on the top BEF film 340. In some embodiments, these directions are not along the long sides of the individual films.

[0060] 3A-H, the description of various embodiments associated with the present teachings has focused on prismatic and / or pyramidal microstructures, however, as will be apparent to one of ordinary skill in the art, the present teachings are not limited to these shapes. For example, conical and / or angled bent shapes may also be used in various embodiments in accordance with the present teachings.

[0061] 4 shows the extent to which the Lambertian dispersion 400 emitted from the LED 110 is transformed into a narrower dispersion 410 with increased on-axis brightness by the crossed brightness enhancement film pair 330, 340. FIG. 5A illustrates the extent to which the crossed brightness enhancement films 330, 340 come into play and is a three-dimensional representation of the defined angles at which light approaching the crossed brightness enhancement film pair 330, 340 may transmit through the pair of brightness enhancement films 330, 340. Only light approaching the crossed brightness enhancement films 330, 340 from defined angles, represented by the four lobes 510, 520, 530, and 540, may transmit through the crossed brightness enhancement films 330, 340 and exit on-axis with the narrow dispersion 410 of FIG. 4, while most other light is reflected back downward toward the light guide film 100. A portion of the light that is reflected back downward toward the light guiding film 100 is lost due to absorption of the diffuser film 320, the light guiding film 100, and the reflector 310. Light that is reflected downward and recirculates back upward toward the crossed brightness enhancement film pair 330, 340 may exit the crossed brightness enhancement films 330, 340 in the second or third attempt. Figure 5B is a two-dimensional representation of Figure 5A. Figure 6 illustrates light receiving locations 610, 620, 630, 640 where light exits on-axis from the crossed brightness enhancement film pair 330, 340.

[0062] Different light guiding films 100 and reflectors 310 can have very different angular output dispersions, and the properties of both the light guiding film 100 and the reflector 310 define the output dispersion of the two combinations. Figures 7 and 8 illustrate the measured angular output dispersion for two different combinations of light guiding film 100 and reflector 310. Figure 7 illustrates the measured angular output dispersion 700 of a combination of a light guiding film 100 with a narrow output dispersion and a specular reflector 310 positioned directly below the light guiding film 100, with an area 710 representing the highest intensity of light. The combination of a light guiding film 100 with a narrow output dispersion and a specular reflector 310 is configured to provide a peak optical dispersion of 15°-20° and a full width at half maximum (FWHM) diffusion angle of 25°-45°. Figure 8 illustrates the measured angular output dispersion 800 of a light guiding film 100 of an embodiment of a backlight unit 300, described in relation to Figure 3A, having a wide dispersion output, in combination with the use of a more diffusive (diffuse) reflector 310 compared to a more specular reflector 310 positioned directly below the light guiding film 100. The area 810 represents the highest intensity of light. The combination of the light guiding film 100 with a wide dispersion output and the diffuse reflector 310 is configured to provide a peak optical dispersion of 30° to 50° and a full width at half maximum (FWHM) diffusion angle of 55° to 85°.

[0063] The addition of the diffuser film 320 further modifies the angular light output distribution. Figures 9 and 10 illustrate the extent to which a circular diffuser film modifies the optical dispersion for each light guiding film 100 and reflector 310 combination represented by Figures 7 and 8, respectively. Figure 9 illustrates the measured angular output distribution 900 of a combination of a circular diffuser on top of the light guiding film 100 with a narrow dispersion output and a specular reflector 310 positioned directly below the light guiding film 100, with an area 910 representing the highest intensity of light. Figure 10 illustrates the measured angular output distribution 1000 of a combination of a circular diffuser on top of the light guiding film 100 with a wide dispersion output and a diffuse reflector 310 positioned directly below the light guiding film 100, with an area 1010 representing the highest intensity of light. It has been found that the optical dispersion of any light-guiding film 100 with its individual reflectors 310, with or without a circular diffuser, is often not well matched to the input dispersion required for on-axis emission from the backlight unit 300 by a crossed brightness enhancement film pair 330, 340, as discussed above with respect to Figures 5A, 5B, and 6.

[0064] In particular, in light of the fact that some portion of the light reflected and recycled within the backlight unit 300 will be lost due to the absorption of the reflector 310 and a reflectivity of less than 100%, it is desirable for the optical output distribution of the diffuser film 320 / light guiding film 100 / reflector 310 combination to meet the light acceptance criteria of the crossed brightness enhancement film pair 330, 340 for as much on-axis transmission as possible so that the on-axis brightness exiting the backlight unit 300 can be maximized. As described in more detail below, the relative on-axis brightness of each of the above-described combinations of light guiding film 100 and reflector 310 and crossed brightness enhancement film pair 330, 340 was measured using various different embodiments of the diffuser film 320. Backlight unit with narrow dispersion light guiding film and specular reflector

[0065] Comparative Example A: The on-axis luminance of a light guiding film 100 having a narrow dispersion output with a specular reflector 310 as described above, a typically 50° full width at half maximum (FWHM) circular volume diffuser 320 used in conjunction with such a light guiding film 100 and reflector 310, and a crossed brightness enhancement film pair 330, 340 was measured as a baseline and set to 100.0% for comparison purposes.

[0066] A series of diffuser films 320 with circular microstructured diffusers having full width at half maximum (FWHM) diffusion angles ranging from 20° to 90° were each substituted for the circular volume diffuser film 320 used for Comparative Example A in the backlight unit 300, and the on-axis luminance of the backlight unit 300 was measured for Comparative Example A. Specifically, Example 1 included a diffuser film 320 with 20° FWHM circular diffuser microstructures, Example 2 included a diffuser film 320 with 40° FWHM circular diffuser microstructures, Example 3 included a diffuser film 320 with 55° FWHM circular diffuser microstructures, Example 4 included a diffuser film 320 with 80° FWHM circular diffuser microstructures, and Example 5 included a diffuser film 320 with 90° FWHM circular diffuser microstructures. The results of the on-axis luminance test for Comparative Example A are listed in Table I below.

[0067] Table I: Relative On-Axis Brightness Using Diffuser Film with Circular Diffuser Microstructures in a Backlight Unit [Table 1]

[0068] The results in Table I show that the diffuser film 320 with the circular diffuser microstructures used in Examples 1-5 provides very similar on-axis luminance to the circular volume diffuser film used in Comparative Example A.

[0069] Three diffuser films 320 were then tested in the backlight unit 300, each with a plurality of parallel prismatic microstructures on one side of the diffuser film 320 oriented toward the light guiding film 100 and aligned in the same direction as the micropillars of the brightness enhancement films 330, 340 closest to those aligned along the length L of the light guiding film 100. The other side of the diffuser film 320 facing the brightness enhancement films 330, 340 was smooth. Example 6 included a diffuser film 320 with a plurality of parallel prismatic microstructures, each having an apex angle of 90° and a refractive index of 1.5. Example 7 included a diffuser film 320 with a plurality of prismatic microstructures, each having an apex angle of 90° and a refractive index of 1.57. Example 8 included a diffuser film 320 with a plurality of prismatic microstructures, each having an apex angle of 90° and a refractive index of 1.7. The results of the on-axis luminance testing of Examples 6-8 versus Comparative Example A are listed in Table II below.

[0070] Table II: Relative On-Axis Luminance Using a Diffuser Film with a 90° Corner Prism on One Side in a Backlight Unit [Table 2]

[0071] The results show that none of the diffuser films 320 with 90° pillars on one side used in Examples 6-8 performed as well as the circular volume diffuser film used in Comparative Example A or the diffuser films 320 with various circular diffuser microstructures used in Examples 1-5 described above and listed in Table I.

[0072] Circular diffuser microstructures were then added to the smooth side of the diffuser film 320 used at various full width at half maximum (FWHM) diffusion angles in Example 8. Example 9 included a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.7 on one side facing the light guiding film 100, and a plurality of circular diffuser microstructures with a 20° FWHM on the other side. Example 10 included a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.7 on one side facing the light guiding film 100, and a plurality of circular diffuser microstructures with a 30° FWHM on the other side. Example 11 included a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.7 on one side facing the light guiding film, and a plurality of circular diffuser microstructures with a 40° FWHM on the other side. The results of the on-axis luminance testing of Examples 8-11 versus Comparative Example A are listed in Table III below.

[0073] Table III: Relative on-axis brightness using a diffuser film with a refractive index of 1.7 and a 90° apex prism on one side and a circular diffuser on the other side in a backlight unit. [Table 3]

[0074] The results show that adding circular diffuser microstructures to the opposite side of a diffuser film 320, each having multiple prismatic microstructures on one side with a 90° apex angle and a refractive index of 1.7, significantly improves the performance of the diffuser film 320 in a backlight unit 300.

[0075] Circular diffuser microstructures with various full width at half maximum (FWHM) diffusion angles were then added to the smooth side of the diffuser film used in Examples 6 and 7. Specifically, Example 12 included a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.5 on one side facing the light guiding film 100, and a plurality of circular diffuser microstructures with a 20° FWHM on the other side. Example 13 included a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.5 on one side facing the light guiding film 100, and a plurality of circular diffuser microstructures with a 30° FWHM on the other side. Example 14 included a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.5 on one side facing the light guiding film 100, and a plurality of circular diffuser microstructures with a 40° FWHM on the other side. Example 15 included a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.5 on one side facing the light guiding film 100, and a plurality of circular diffuser microstructures, each having a 55° FWHM on the other side. Example 16 included a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.57 on one side facing the light guiding film 100, and a plurality of circular diffuser microstructures, each having a 40° FWHM on the other side. Example 17 included a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.57 on one side facing the light guiding film, and a plurality of circular diffuser microstructures, each having a 55° FWHM on the other side. The results of the on-axis brightness test of Examples 12-17 against Comparative Example A are listed in Table IV below.

[0076] Table IV: Relative on-axis brightness using diffuser films with refractive index of 1.50 or 1.57 and 90° apex prisms on one side and circular diffuser microstructures on the other side in a backlight unit. [Table 4]

[0077] Surprisingly, even though a diffuser film 320 (Example 6) with a plurality of prismatic microstructures, each having a 90° apex angle on one side and a refractive index of 1.5, and a smooth opposite side performed less well than a diffuser film 320 (Example 8) with a plurality of prismatic microstructures, each having a 90° apex angle on one side and a refractive index of 1.7, and a smooth opposite side, it was found that adding a circular diffuser microstructure to the opposite side of a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle on one side and a refractive index of 1.5, had an even greater increase in relative on-axis luminance than adding a circular microstructure diffuser to the opposite side of a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle on one side and a refractive index of 1.7. It was also found that diffuser films 320 (Examples 16 and 17, respectively), each with a plurality of prismatic microstructures having a 90° apex angle and a refractive index of 1.57 on one side and a plurality of circular diffuser microstructures having either a 40° FWHM or a 55° FWHM on the other side, had slightly lower brightness than the corresponding diffuser films 320 (Examples 14 and 15, respectively) with prismatic microstructures having a refractive index of 1.5.

[0078] To illustrate the extent to which a diffuser film 320 of one embodiment of the present invention, specifically the diffuser film 320 used in Example 15 with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.5 on one side, and a 55° FWHM circular diffuser microstructure on the other side, performs within a backlight unit 300, the angular light dispersion of the combination of the diffuser film 320 of Example 15, the light guiding film 100 with narrow dispersion, and the specular reflector 310 was measured and compared to the light acceptance angle criteria for crossed brightness enhancement films 330, 340. Figure 11 illustrates the measured angular light dispersion 1100 of the combination of this diffuser film 320 of Example 15, the light guiding film 100 with narrow dispersion, and the specular reflector 310, along with areas 1110 and 1120 in Figure 11 that show the highest intensity of light passing through. Figure 12 illustrates the light acceptance angle criterion 1200 of the crossed brightness enhancement films 330, 340 along with areas 1210, 1220, 1230, 1240 showing where light would pass on-axis through the crossed brightness enhancement films 330, 340. The combination of Figures 11 and 12 is represented by 1300 in Figure 13, showing an excellent match between the areas 1110, 1120 of the highest intensity of light output by the diffuser film 320, each with a plurality of prismatic microstructures having 90° apexes and a refractive index of 1.5 on one side and a 55° FWHM circular diffuser microstructure on the other side, the light directing film 100 with narrow dispersion, and the specular reflector 320, and the input criterion represented by the areas 1210, 1220, 1230, 1240 of the crossed brightness enhancement films 330, 340 of the backlight unit 300.

[0079] The effect of the apex angle of the prismatic microstructures on the diffuser film was then investigated. Example 18 included a diffuser film 320 with a plurality of prismatic microstructures, each having an 80° apex angle and a refractive index of 1.5 on one side facing the light guiding film 100, and a plurality of circular diffuser microstructures with a 20° FWHM on the other side. Example 19 included a diffuser film 320 with a plurality of prismatic microstructures, each having an 80° apex angle and a refractive index of 1.5 on one side facing the light guiding film 100, and a plurality of circular diffuser microstructures with a 30° FWHM on the other side. Example 20 included a diffuser film 320 with a plurality of prismatic microstructures, each having an 80° apex angle and a refractive index of 1.5 on one side facing the light guiding film 100, and a plurality of circular diffuser microstructures with a 40° FWHM on the other side. The results of the on-axis luminance testing of Examples 18-20 versus Comparative Example A are listed in Table V below.

[0080] Table V: Relative On-Axis Brightness in a Backlight Unit Using a Diffuser Film with a 1.50 Index of Refraction and 80° Apex Prism on One Side and Circular Diffuser Microstructures on the Other Side [Table 5]

[0081] The results show that the relative on-axis luminance performance of the diffuser film 320 (Examples 18-20) with a plurality of prismatic microstructures, each having an 80° apex angle and a refractive index of 1.5 on one side, and a circular diffuser microstructure on the other side, is very similar to that of the diffuser film 320 (Examples 12-14) with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.5 on one side, and a circular diffuser microstructure on the other side.

[0082] Then, instead of circular diffuser microstructures, conical and square pyramidal microstructures were investigated. Conical microstructures diverge a collimated beam of light into a circular ring. A conical structure with an apex angle of 110° diverges the light into a ring with a FWHM of about 40°. Inverted square pyramidal microstructures with an apex angle of about 110° were also investigated. The square pyramidal microstructures were aligned in two orientations, i.e., the face of the pyramidal microstructure was either parallel (and perpendicular) to the opposite prism or at 45° to the opposite prism. Example 21 included a diffuser film 320 with a plurality of prism microstructures, each with a 90° apex angle and a refractive index of 1.5 on one side facing the light-guiding film 100, and a plurality of conical microstructures with 110° on the opposite side. Example 22 included a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.57 on one side facing the light guiding film 100, and a plurality of conical microstructures, each having a 110° apex angle on the other side. Example 23 included a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.5 on one side facing the light guiding film 100, and a plurality of square pyramidal microstructures, each having a 110° apex angle and a face oriented parallel (and perpendicular) to the plurality of prismatic microstructures. Example 24 included a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.5 on one side facing the light guiding film 100, and a plurality of square pyramidal microstructures (refractive index of 1.5) with a 110° apex angle and a face oriented 45° to the plurality of prismatic microstructures. Example 25 included a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.5 on one side facing the light guiding film 100, and a plurality of square pyramidal microstructures (refractive index of 1.5) with a 110° apex angle and a face oriented at 45° to the plurality of prismatic microstructures. The square pyramidal structures may be of either polarity, either "bulge" or "dimple". The results of on-axis luminance testing of Examples 21-25 against Comparative Example A are listed in Table VI below.

[0083] Table VI: Relative on-axis brightness in a backlight unit using a diffuser film with a prism of refractive index of 1.50 and a 90° apex on one side and conical and pyramidal microstructures on the other side. [Table 6]

[0084] The results show that a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.5 on one side, and both 110° apex angle conical microstructures and 110° apex angle pyramidal microstructures on the other side, performed well and increased on-axis luminance compared to Comparative Example A. In addition, the results show that increasing the refractive index of the prismatic microstructures from 1.50 to 1.57 reduces the relative on-axis luminance for the diffuser film with conical and pyramidal microstructures by 3-4%.

[0085] The apex angle and refractive index of the pyramidal structures were investigated. Changing the apex angle of the pyramids from 110 degrees to 90 degrees very significantly and unexpectedly increased the brightness from 115% to 127.5%. In addition, the refractive index of the pyramidal microstructures was increased from 1.5 to 1.57, which resulted in an increase in brightness from 127.5% to 129%.

[0086] One feature of the present teachings is that the orientation of the pillars in the top BEF is important for diffuser films with approximately 90 degree apex pillars (oriented parallel to the pillars in the top BEF) with a refractive index of 1.5 and 90 degree apex square pyramid structures with a backside refractive index of 1.57 or 1.5. This importance of this feature is illustrated in connection with FIG. 17 for a pyramid with a refractive index of 1.57. FIG. 17 is a graph of relative luminance as a function of top brightness enhancement film pillar angle with respect to the light direction for an edge-lit backlight unit according to the present teachings. It can be seen that maximum luminance is achieved when the top BEF pillars are within + / - 20 degrees of the light direction, especially when the degree of the light direction is within + / - 10 degrees. When the bottom pillars of the BEF and the bottom pillars of the diffuser are aligned with the light direction, the performance is nearly good. The BEF orientation affects other diffuser films, but not significantly.

[0087] Then, instead of the circular diffuser microstructures described above, angular bend microstructures were investigated. Such angular bend microstructures are described in commonly owned U.S. Patent Application No. 16 / 625,830, filed on December 23, 2019 as a U.S. national stage application of International Patent Application No. PCT / US2018 / 040268, filed on June 29, 2018, and published on January 3, 2019 as International Publication No. WO 2018 / 006288 A1, the entire contents of which are hereby incorporated by reference. Specifically, the diffuser film 320 included a plurality of angular bend microstructures having the form of an array of microprisms, as illustrated in FIG. 8 of WO 2018 / 006288 A1, included on the side of the diffuser film 320 opposite the plurality of parallel prism microstructures. Both the angle bending microstructures and the parallel prismatic microstructures had a refractive index of 1.5. The angle bending microstructures were oriented such that when placed in a backlight unit 300 with a light guiding film 100 having a narrow dispersion output and a specular reflector 310 as described above, the angle bending microstructures aligned the light with the prismatic microstructures and bent the light away from the LED 110. The backlight unit 300 with the diffuser film 320 having the angle bending microstructures had a performance improvement over Comparative Example A that was equal to or better than the examples listed in Tables III-V.

[0088] It is envisioned that other microstructures having different shapes and configurations than those disclosed herein may also be used on the side of the diffuser film 320 that faces away from the light guiding film 100 toward the crossed brightness enhancement film pair 330, 340. The embodiments described herein are not intended to be limiting in any way. Backlight unit with broad dispersion light guiding film and diffuse reflector - Patents.com

[0089] The relative on-axis luminance of a light guiding film having a broad dispersion, a diffusive reflector as described above, a 35° full width at half maximum (FWHM) circular volume diffuser 320 (referred to herein as Comparative Example B) typically used in conjunction with such light guiding films and reflectors, and a pair of crossed brightness enhancement films was measured as a baseline such that the relative on-axis luminance was set to 100.0%.

[0090] A series of circular diffuser films 320 with full width at half maximum diffusion angles ranging from 20° to 90° were each substituted for the 35° full width at half maximum (FWHM) circular volume diffuser film 320 of Comparative Example B, and the relative on-axis luminance was measured. Specifically, Example 26 included a 20° FWHM circular microstructured diffuser film 320, Example 27 included a 30° FWHM circular microstructured diffuser film 320, Example 28 included a 40° FWHM circular microstructured diffuser film 320, Example 29 included a 55° FWHM circular microstructured diffuser film, and Example 30 included a 90° FWHM circular microstructured diffuser film 320. The results of the on-axis luminance testing of Examples 26-30 versus Comparative Example B are listed in Table VII below.

[0091] Table VII: Relative On-Axis Brightness Using Diffuser Film with Circular Diffuser Microstructures in a Backlight Unit [Table 7]

[0092] Similar to the results with the light guiding film with narrow dispersion and the specular reflector listed in Table I above, the diffuser film 320 with the different circular diffuser microstructures is not believed to have any significant effect on the on-axis brightness of the backlight unit 300, which includes the light guiding film 100 with wide dispersion and the diffuse reflector 310.

[0093] Four different diffuser films 320 were then tested in the backlight unit 300, each with a plurality of parallel prism microstructures on one side of the diffuser film 320 oriented toward the light guiding film 100 and aligned in the same direction as the microprisms of the brightness enhancement film closest to those aligned along the length L of the light guiding film 100. The other side of the diffuser film 320 facing the bottom brightness enhancement film 330, 340 was smooth. Example 31 included a diffuser film 320 with a plurality of parallel prism microstructures, each having an apex angle of 90° and a refractive index of 1.5. Example 32 included a diffuser film 320 with a plurality of prism microstructures, each having an apex angle of 90° and a refractive index of 1.57. Example 33 included a diffuser film 320 with a plurality of prism microstructures, each having an apex angle of 90° and a refractive index of 1.65. Example 34 included a diffuser film 320 with a plurality of prismatic microstructures, each having an apex angle of 90° and a refractive index of 1.7. The results of the on-axis luminance testing of Examples 31-34 against Comparative Example B are listed in Table VIII.

[0094] Table VIII: Relative On-Axis Luminance Using a Diffuser Film with a 90° Corner Pillar on One Side in a Backlight Unit [Table 8]

[0095] The results show that all of the diffuser films 320 having multiple pillar microstructures on one side thereof increase the relative on-axis luminance of the backlight unit 300, and that the on-axis luminance increases with increasing refractive index of the pillars. The results listed in Table VIII are in contrast to the results listed in Table II for the backlight unit 300 with the narrow dispersion light directing film 100 and specular reflector 310 described above, which showed that the pillars caused a decrease in on-axis luminance.

[0096] Circular diffuser microstructures with various degrees of full width at half maximum (FWHM) diffusion angles were then added to the smooth side of the diffuser film 320 of Example 31. Example 35 included a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.50 on one side facing the light guiding film 100, and a plurality of circular diffuser microstructures with a 10° FWHM on the other side. Example 36 included a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.50 on one side facing the light guiding film 100, and a plurality of circular diffuser microstructures with a 20° FWHM on the other side. Example 37 included a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.50 on one side facing the light guiding film 100, and a plurality of circular diffuser microstructures with a 30° FWHM on the other side. Example 38 included a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.50 on one side facing the light guiding film 100, and a plurality of circular diffuser microstructures with a 40° FWHM on the other side. Example 39 included a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.50 on one side facing the light guiding film 100, and a plurality of circular diffuser microstructures with a 55° FWHM on the other side. In addition, circular diffuser microstructures having a 20° full width at half maximum (FWHM) diffusion angle were added to the smooth side of the diffuser film 320 of Example 34. Specifically, Example 40 included a diffuser film 320 with a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.7 on one side facing the light guiding film 100, and a plurality of circular diffuser microstructures having a 20° FWHM on the opposite side. The results of the on-axis luminance tests of Examples 35-40 against Comparative Example B are listed in Table IX.

[0097] Table IX: Relative on-axis brightness of diffuser films with refractive index of 1.50 or 1.57 and 90° apex prisms on one side and circular diffuser microstructures on the other side in a backlight unit. [Table 9]

[0098] The results show that having a circular diffuser microstructure on the opposite side of the diffuser film 320 with multiple prismatic microstructures causes a reduction in the on-axis brightness of the backlight unit 300, but in some embodiments, a small amount of diffusion on the back side of the diffuser film 320 may be desirable to achieve sufficient hiding of scattering points from the light-guiding film 100. The results also show that although backlight unit 300 with diffuser film 320 with prismatic microstructures of 1.7 refractive index on one side and a smooth opposite side (Example 34) is much brighter than backlight unit 300 with diffuser film 320 with prismatic microstructures of 1.5 refractive index on one side and a smooth opposite side (Example 31), when 20° FWHM circular diffuser microstructures are added to the opposite side of diffuser film 320 with prismatic microstructures, backlight unit 300 with diffuser film with prismatic microstructures of 1.5 refractive index (Example 36) is slightly brighter than backlight unit 300 with diffuser film 320 with prismatic microstructures of 1.7 (Example 40) when 20° FWHM circular diffuser microstructures are added to the opposite side of diffuser film 320 with prismatic microstructures.

[0099] The effect of the apex angle of the prisms was then investigated. Example 41 included a diffuser film 320 with a plurality of prism microstructures, each having an 80° apex angle and a refractive index of 1.5, on one side facing the light guiding film 100, and a smooth opposite side. Example 42 included a diffuser film with a plurality of prism microstructures, each having an 80° apex angle and a refractive index of 1.57, on one side facing the light guiding film 100, and a smooth opposite side. Example 43 included a diffuser film with a plurality of prism microstructures, each having an 80° apex angle and a refractive index of 1.65, on one side facing the light guiding film 100, and a smooth opposite side. The results of the on-axis brightness test of Examples 41-43 against Comparative Example B are listed in Table X.

[0100] Table X: Relative On-Axis Luminance Using a Diffuser Film with a Prism with an 80° Apex on One Side [Table 10]

[0101] The results show that for each backlight unit 300 with a diffuser film 320 with multiple prismatic microstructures, each having an 80° apex angle, the on-axis brightness was increased over the backlight unit 300 with a diffuser film 320 with multiple prismatic microstructures, each having a 90° apex angle for the same refractive index.

[0102] Additional testing of diffuser films with a plurality of prismatic microstructures, each with an 80° apex angle, with a different refractive index, was completed. Example 44 included a diffuser film 320 with a plurality of prismatic microstructures, each with an 80° apex angle and a refractive index of 1.61, on one side facing the light guiding film 100, and a smooth opposite side. Example 45 included a diffuser film 320 with a plurality of prismatic microstructures, each with an 80° apex angle and a refractive index of 1.62, on one side facing the light guiding film 100, and a smooth opposite side. Example 46 included a diffuser film 320 with a plurality of prismatic microstructures, each with an 80° apex angle and a refractive index of 1.63, on one side facing the light guiding film 100, and a smooth opposite side. Example 47 included a diffuser film 320 with a plurality of prismatic microstructures, each having an 80° apex angle and a refractive index of 1.64, on one side facing the light guiding film 100 and a smooth opposite side. The results of the on-axis brightness tests of Examples 44-47 against Comparative Example B are listed in Table XI.

[0103] Table XI: Relative On-Axis Luminance Using a Diffuser Film with a Prism with an 80° Apex on One Side [Table 11]

[0104] The results show that in combination with a light guiding film 100 having a wide dispersion and a diffusive reflector 310, the optimal refractive index of the prismatic microstructures with an apex angle of 80° for the diffuser film 320 is about 1.63 to 1.65.

[0105] As explained above, some amount of diffusion on the opposite side of the diffuser film 320 with multiple prismatic microstructures on one side may be desirable to improve the optical uniformity of the light exiting the backlight unit 300, even at the expense of some reduction in luminance. The addition of some diffusion provided by the opposite side of the diffuser film 320 with multiple prismatic microstructures with an 80° apex angle and a refractive index of 1.65 was investigated. Example 48 included a diffuser film 320 with multiple prismatic microstructures, each with an 80° apex angle and a refractive index of 1.65 on one side, and multiple circular diffuser microstructures with a 3° FWHM on the opposite side. Example 49 included a diffuser film 320 with multiple prismatic microstructures, each with an 80° apex angle and a refractive index of 1.65 on one side, and multiple circular diffuser microstructures with a 10° FWHM on the opposite side. The results of the on-axis brightness testing of Examples 48 and 49 versus Comparative Example B are listed in Table XII.

[0106] Table XII: Relative On-Axis Brightness of Diffuser Film with 80° Apex Prism on One Side and Circular Diffuser Microstructures on the Other Side in a Backlight Unit [Table 12]

[0107] As shown by the results listed in Table XII, adding a small amount of diffusion (by using a plurality of circular diffuser microstructures with 3° FWHM or 10° FWHM) to the opposite side of a diffuser film 320 with a plurality of prismatic microstructures, each having an 80° apex angle and a refractive index of 1.65 on one side, reduced the on-axis luminance of the backlight unit 300 when used in conjunction with a light guiding film 100 with a wide dispersion and a diffuse reflector 310. This result contrasts with using a diffuser film 320 with a plurality of prismatic microstructures on one side and adding a significant amount of diffusion to the opposite side in a backlight unit 300 with a light guiding film 100 with a narrow dispersion and a specular reflector 310, which increased the on-axis luminance, as shown above in Tables III-VI.

[0108] To illustrate the extent to which the diffuser film 320 of one embodiment of the present invention, specifically the diffuser film used in Example 48 with a plurality of prismatic microstructures, each having an 80° apex angle and a refractive index of 1.65 on one side facing the light guiding film 100, and a plurality of circular diffuser microstructures with 3° FWHM on the other side, performs within the backlight unit 300, the angular light dispersion of the combination of the diffuser film 320 of Example 48, the light guiding film 100 with broad dispersion, and the diffuse reflector 310 was measured and compared to the light acceptance angle criteria for the crossed brightness enhancement films 330, 340. Figure 14 illustrates the measured angular light dispersion 1400 of the combination of this diffuser film 320 of Example 48, the light guiding film 100 with broad dispersion, and the diffuse reflector 310, along with areas 1410 and 1420 in Figure 14 that show the highest intensity of light passing through. 15 illustrates the light acceptance angle criterion 1500 of the crossed brightness enhancement films 330, 340 used in the backlight unit 300, along with areas 1510, 1520, 1530, 1540 showing where light would pass on-axis through the crossed brightness enhancement films 330, 340. A combination of FIGS. 14 and 15 is represented by 1600 in FIG. 16, showing an excellent match between the diffuser film 320 with multiple prismatic microstructures, each having an 80° apex and a refractive index of 1.65 on one side, and multiple circular diffuser microstructures with 3° FWHM on the other side, the light directing film 100 and specular reflector 320 with wide dispersion, and the input criterion of the crossed brightness enhancement films 330, 340 of the backlight unit 300.

[0109] The inventors discovered that one of the best diffuser films 320 for use with a light guiding film 100 with a narrow dispersion output with a specular reflector 310 included a plurality of prismatic microstructures, each having a 90° apex angle and a refractive index of 1.5 on one side, and a circular diffuser microstructure with a 55° FWHM diffusion angle (used in Example 15). In contrast, one of the best diffuser films 320 for use with a light guiding film 100 with a diffuse reflector included a plurality of prismatic microstructures, each having an 80° apex angle and a refractive index of 1.64 on one side, and a smooth opposite side (used in Example 47). Additional testing was completed to understand how well these diffuser films 320 would each perform in a backlight unit 300 that included another light guiding film 100 and a reflector 310.

[0110] Example 50 included a diffuser film 320, each with a plurality of prismatic microstructures having an 80° apex angle and a refractive index of 1.64 on one side facing the light guiding film 100 and a smooth opposite side, a light guiding film 100 with a narrow dispersion output, a specular reflector 310, and a crossed brightness enhancement film pair 330, 340. A comparison of the on-axis brightness results of Example 50 to Comparative Example A, along with Example 15 and Comparative Example A, are listed in Table XIII below.

[0111] Table XIII: Relative On-Axis Brightness of Diffuser Films with Narrow Dispersion Light Guide Films and Specular Reflectors in Backlight Units [Table 13]

[0112] The results show that Example 15 provided much higher on-axis brightness than Example 50, but Example 50 was also a slight improvement over Comparative Example A.

[0113] Example 51 included a diffuser film 320 with a plurality of prismatic microstructures with a 90° apex angle and a refractive index of 1.5, each on one side facing the light guiding film 100, and a circular diffuser microstructure with a 55° FWHM diffusion angle on the other side, a light guiding film 100 with a wide dispersion output, a diffuse reflector 310, and a crossed brightness enhancement film pair 330, 340. A comparison of the on-axis brightness results of Example 51 to Comparative Example B, along with Example 47 and Comparative Example B, are listed in Table XIV below.

[0114] Table XIV: Relative On-Axis Brightness of Diffuser Films with Wide Dispersion Light Guide Films and Diffuse Reflectors in Backlight Units [Table 14]

[0115] The results show that Example 47 provided much higher on-axis brightness than Example 51, which was also a slight improvement over Comparative Example B.

[0116] It is apparent from Tables XIII and XIV that the best diffuser film 320 for use with one light guiding film / reflector combination in the backlight unit 300 performs poorly for other light guiding film / reflector combinations.

[0117] The embodiments illustrated and described above are not intended to be limiting in any way, and any such modifications to the embodiments described herein are intended to be within the spirit and scope of this disclosure and protected by the following claims.

Claims

[Claim 1] The invention described in this specification.

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