Edge-lit backlight unit with improved efficiency

Abstract

The present invention is directed to an edge-lit backlight unit for a backlit display with improved efficiency that allows for increased perceived brightness without increasing power to the display. The edge-lit backlight 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 the acceptance angle of the pair of crossed brightness enhancement films to provide increased on-axis brightness without the need to increase the output to the edge-lit backlight unit.

Description

Technical Field

[0004] ,

[0001] (Cross - reference to related applications) This application claims the benefit of priority of U.S. Provisional Patent Application No. 63 / 127,325, filed on December 18, 2020, and U.S. Provisional Patent Application No. 63 / 214,730, filed on June 24, 2021. The entire contents of both are hereby incorporated by reference herein.

[0002] The present invention is directed to an edge - lit backlight unit for a backlight - type display having improved efficiency that enables an increase in visual brightness without increasing the power to the display.

Background Art

[0003] An edge - lit backlight unit (BLU) typically uses a light - guiding film 100 with a plurality of light - emitting diodes (LEDs) 110 positioned along one side of the light - guiding film 100, as shown 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 shown in the cross - section obtained along line 2 - 2 of FIG. 1 and shown in FIG. 2. The light - guiding film 100 typically has small structures on the upper surface and / or bottom surface for externally coupling light to the light - guiding film 100. Ideally, the structures are apodized such that the externally coupled light of the light - guiding film 100 has a spatially uniform intensity. Generally, the light externally coupled to the light - guiding film 100 undergoes angular dispersion and external coupling such that it is directed away from the light - input edge of the light - guiding film 100, as shown in FIG. 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] For backlight units used in edge-lit displays, it is desirable to maximize light efficiency, that is, to increase visible brightness without increasing the power supplied to the backlight unit. [Overview of the project] [Means for solving the problem]

[0005] The present invention achieves increased efficiency by replacing the conventional diffuser film in the backlight unit with an improved light management diffuser film having an angular light dispersion output that matches the light-receiving angle of the crossed brightness-enhancing film.

[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 guide film positioned above the specular reflector. The edge-lit light guide film has a certain length and a certain width. The combination of the edge-lit light guide film and the specular reflector is configured to provide a peak optical dispersion of 15° to 20° and a full-width half-maximal diffusion angle of 25° to 45°. A diffuser film is positioned above the edge-lit light guide film. The diffuser film has a plurality of parallel prism microstructures on one side facing the edge-lit light guide film and a plurality of diffuser microstructures on the opposite side. Each of the plurality of parallel prism microstructures has an apex angle of 78° to 92° and a refractive index of 1.49 to 1.58. The diffuser microstructures have a full-width half-maximal diffusion angle of 30° to 60°. A pair of intersecting luminance-enhancing films is positioned above the diffuser film. Each luminance-enhancing film has multiple parallel microprisms on one side facing away from the diffuser film. The multiple parallel microprisms of one luminance-enhancing film are oriented perpendicular to the multiple microprisms of the other luminance-enhancing film. The multiple prism microstructures of the diffuser film are substantially aligned with the multiple parallel microprisms closest to the one aligned with the length of the light guide film.

[0007] In one embodiment, the diffuser microstructure of the diffuser film is a circular diffuser microstructure.

[0008] In one embodiment, the diffuser microstructure of the diffuser film is a conical microstructure. In one embodiment, the conical microstructure has an apex angle of approximately 110°.

[0009] In one embodiment, the diffuser microstructure of the diffuser film is a square pyramidal microstructure. In one embodiment, the square pyramidal microstructure has an apex angle of about 110°. In one embodiment, the faces of the square pyramidal microstructure are aligned to be parallel to or perpendicular to the prismatic microstructure of the diffuser film. In one embodiment, the faces of the square pyramidal microstructure are oriented at about 45° to the prismatic microstructure of the diffuser film.

[0010] In one embodiment, the edge-lit backlight unit also includes a plurality of LEDs positioned along the width of the light guide film.

[0011] In one embodiment, the diffuser microstructure is an angle-bent microstructure that is oriented to align light with multiple prismatic microstructures and bend it in a direction away from 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 guide film positioned above the diffuse reflector. The edge-lit light guide film has a certain length and a certain width. The combination of the edge-lit light guide film and the diffuse reflector is configured to provide a peak optical dispersion of 30° to 50° and a full-width half-maximal diffusion angle of 55° to 85°. A diffuser film is positioned above the edge-lit light guide film. The diffuser film has a plurality of parallel prism microstructures on one side facing the edge-lit light guide film and a plurality of diffuser microstructures on the opposite side. Each of the plurality of parallel prism microstructures has an apex angle of 75° to 85° and a refractive index of 1.59 to 1.67. The diffuser microstructures have a full-width half-maximal diffusion angle of less than 20°. A pair of crossed luminance-enhancing films is positioned above the diffuser film. Each luminance-enhancing film has a plurality of parallel microprismatics on one side facing away from the diffuser film. The plurality of parallel microprismatics on one of the luminance-enhancing films are oriented perpendicular to the plurality of microprismatics on the other luminance-enhancing film. The plurality of prismatic microstructures on the diffuser film are substantially aligned with the plurality of parallel microprismatics that are closest to each other so as to match the length of the light guide film.

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

[0014] These and other aspects, features, and characteristics of the present invention, as well as the manner of operation and function of the related elements of the structure, and the combination of the parts and the economics of manufacture, all of which form a part of this specification, will become more apparent upon consideration of the following description and the appended claims, with reference to the accompanying drawings. However, it should be clearly understood that the drawings are for purposes of illustration and explanation only and are not intended as a definition of the limits of the present invention. As used in this specification and the 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) An edge-lit backlight unit for a backlit display, wherein the edge-lit backlight unit is A specular reflector, An edge-lit light guide film positioned above the specular reflector, wherein the edge-lit light guide film has a length and a width, and the combination of the edge-lit light guide film and the specular reflector is configured to provide a peak optical dispersion of 15° to 20° and a full-width half-maximum diffusion angle of 25° to 45°, A diffuser film positioned above the edge-lit light guide film, wherein the diffuser film has, on one side facing the edge-lit light guide film, a plurality of parallel prism microstructures, at least some of which have an apex angle of 78° to 92° and are formed from a material having a refractive index of 1.49 to 1.58, and on the opposite side, a plurality of diffuser microstructures, each having a total width half-maximal diffusion angle of 30° to 60°. [[ID=I4]] A pair of intersecting brightness-enhancing films positioned above the diffuser film, wherein at least one of the brightness-enhancing films has a plurality of parallel minute prisms on one side facing away from the diffuser film, and the plurality of parallel minute prisms of one of the brightness-enhancing films are oriented perpendicular to the plurality of minute prisms of the other brightness-enhancing film, and Equipped with, An edge-lit backlight unit in which a plurality of prismatic microstructures of the diffuser film are substantially aligned with a plurality of parallel microprismatics of at least one of the brightness-enhancing films. (Item 2) The edge-lit backlight unit according to item 1, wherein at least one of the brightness-enhancing films comprises a plurality of parallel minute prisms oriented to be most closely aligned with the light direction. (Item 3) The edge-lit backlight unit according to item 2, wherein at least one of the brightness-enhancing films is positioned on top of the edge-lit backlight unit. (Item 4) The aforementioned diffuser microstructure is formed in a circular shape, as described in item 1, for the edge-lit backlight unit. (Item 5) The aforementioned diffuser microstructure is formed in a conical shape, as described in item 1, for the edge-lit backlight unit. (Item 6) The edge-lit backlight unit described in item 5, wherein the conical shape of the diffuser microstructure has an apex angle of approximately 110°. (Item 7) The aforementioned diffuser microstructure is formed in a square pyramidal shape, as described in item 1, for the edge-lit backlight unit. ] (Item 8) The aforementioned square pyramidal microstructure has an apex angle of approximately 110°, as described in item 7, for the edge-lit backlight unit. (Item 9) The aforementioned square pyramidal microstructure has an apex angle of approximately 90°, as described in item 7, for the edge-lit backlight unit. (Item 10) The edge-lit backlight unit according to item 7, wherein the square pyramidal microstructure is formed from a material having a refractive index of approximately 1.57. (Item 11) The edge-lit backlight unit according to item 7, wherein at least some faces of the square pyramidal microstructure are aligned to be parallel to the prismatic microstructure of the diffuser film. (Item 12) The edge-lit backlight unit according to item 7, wherein at least some faces of the square pyramidal microstructure are aligned to be within a range of less than 10 degrees from parallel to or equal to that of the prismatic microstructure of the diffuser film. (Item 13) The edge-lit backlight unit according to item 7, wherein at least some faces of the square pyramidal microstructure are aligned to be within an angle of less than or equal to 20 degrees from parallel to the prismatic microstructure of the diffuser film. (Item 14) The edge-lit backlight unit according to item 13, wherein at least one of the brightness-enhancing films comprises a plurality of parallel minute prisms oriented to be most closely aligned with the light direction. (Item 15) The edge-lit backlight unit according to item 7, wherein at least some faces of the square pyramidal microstructure are oriented at approximately 45° with respect to the prismatic microstructure of the diffuser film. (Item 16) The edge-lit backlight unit according to item 15, wherein at least one of the brightness-enhancing films comprises a plurality of parallel minute prisms oriented to be most closely aligned with the light direction. (Item 17) The edge-lit backlight unit according to item 1, further comprising a plurality of LEDs positioned along the width of the light guide film. (Item 18) The edge-lit backlight unit according to item 15, wherein the diffuser microstructure is an angularly bent microstructure that is aligned with the plurality of prismatic microstructures with respect to refracted light and oriented in a direction away from the plurality of LEDs. (Item 19) An edge-lit backlight unit for a backlit display, wherein the edge-lit backlight unit is Diffuse reflector, An edge-light type light guide film positioned above the diffuse reflector, wherein the edge-light type light guide film has a length and a width, and the combination of the edge-light type light guide film and the diffuse reflector is configured to provide a peak optical dispersion of 30° to 50° and a full-width half-maximum diffusion angle of 55° to 85°, A diffuser film positioned above the edge-lit light guide film, wherein the diffuser film has, on one side facing the edge-lit light guide film, a plurality of parallel prism microstructures, at least some of which have an apex angle of 75° to 85° and a refractive index of 1.59 to 1.67, and on the opposite side, a plurality of diffuser microstructures, each having a total width half-maximal diffusion angle of less than 20°. A pair of intersecting brightness-enhancing films positioned above the diffuser film, wherein at least one of the brightness-enhancing films has a plurality of parallel minute prisms on one side facing away from the diffuser film, and the plurality of parallel minute prisms of one of the brightness-enhancing films are oriented perpendicular to the plurality of minute prisms of the other brightness-enhancing film, and Equipped with, An edge-lit backlight unit in which the plurality of prismatic microstructures of the diffuser film are substantially aligned with the plurality of parallel microprismatics of at least one of the pairs of intersecting luminance-enhancing films. (Item 20) An edge-lit backlight unit according to item 19, wherein at least one of the pairs of intersecting brightness-enhancing films has a plurality of parallel minute prisms oriented to be aligned in the closest proximity to the light direction. (Item 21) The edge-lit backlight unit according to item 20, wherein at least one of the pair of crossed brightness-enhancing films is positioned on top of the edge-lit backlight unit. (Item 22) The aforementioned diffuser microstructure is formed in a circular shape, as described in item 19, for the edge-lit backlight unit. (Item 23) The edge-lit backlight unit according to item 19, wherein the diffuser microstructure is configured to have a full-width half-maximal diffusion angle of less than or equal to 10°. (Item 24) The edge-lit backlight unit according to item 19, wherein the diffuser microstructure is configured to have a full-width half-maximal diffusion angle of less than or equal to 5°. (Item 25) It is a backlight unit, A specular reflector, A light guide film positioned above the aforementioned specular reflector, wherein the light guide film is configured to guide light along the direction of light, A diffuser film positioned above the light guide film, wherein the diffuser film has a plurality of parallel prism microstructures on one side facing the light guide film, where at least some of the plurality of parallel prism microstructures have a prism vertex direction, and further, the diffuser film has a plurality of pyramidal microstructures on the opposite side, where at least some of the plurality of pyramidal microstructures have a common pyramidal vertex direction. A first brightness-enhancing film positioned above the diffuser film, having a plurality of parallel prism microstructures on one side facing away from the diffuser, wherein at least some of the plurality of parallel prism microstructures of the first brightness-enhancing film have a common prism vertex direction, and the first brightness-enhancing film, A second brightness-enhancing film is positioned above the first brightness-enhancing film, on one side opposite to the first brightness-enhancing film, and having a plurality of parallel prism microstructures, wherein at least some of the plurality of parallel prism microstructures of the second brightness-enhancing film have a common prism vertex direction. Equipped with, A backlight unit in which the common prism vertex direction of at least one of the first and second brightness-enhancing films is substantially oriented along the light direction, the common prism vertex direction of the diffuser film is substantially oriented along the common prism vertex direction of at least one of the first and second brightness-enhancing films, and the common pyramidal vertex direction of the diffuser film is oriented at a desired angle with respect to the common prism vertex direction of the diffuser film. (Item 26) The backlight unit according to item 25, wherein the common prism vertex direction of at least one of the first and second brightness-enhancing films having prism vertex directions substantially oriented along the light direction has a prism vertex direction that is less than 10 degrees from the light direction. (Item 27) The backlight unit according to item 25, wherein the common prism vertex direction of at least one of the first and second brightness-enhancing films having prism vertex directions substantially oriented along the light direction has a prism vertex direction that is less than 20 degrees from the light direction. (Item 28) The backlight unit according to item 25, wherein the desired angle is substantially zero degrees. (Item 29) The backlight unit according to item 25, wherein the desired angle is substantially 45 degrees. (Item 30) The backlight unit according to item 25, wherein at least one of the first and second brightness-enhancing films has a prism vertex direction that is oriented substantially perpendicular to the prism vertex direction of at least one of the first and second brightness-enhancing films. (Item 31) The backlight unit according to item 25, wherein at least one of the first and second brightness-enhancing films having the prism vertex direction substantially oriented along the light direction is positioned on top of the backlight unit. (Item 32) The backlight unit according to item 25, wherein at least some of the plurality of parallel prism microstructures of the diffuser film have apex angles of 78° to 94°. (Item 33) The backlight unit according to item 25, wherein at least some of the plurality of parallel prism microstructures of the diffuser film are formed from a material having a refractive index of 1.5 to 1.66. (Item 34) The backlight unit according to item 25, wherein at least some of the multiple pyramidal microstructures of the diffuser film have apex angles of 84° to 114°. (Item 35) The backlight unit according to 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) The backlight unit according to item 25, wherein at least some of the plurality of parallel prism microstructures of at least one of the first and second brightness-enhancing films have apex angles of 86° to 94°. (Item 37) The backlight unit according to item 25, wherein at least some of the plurality of parallel prism microstructures of at least one of the first and second brightness-enhancing films are formed from a material having a refractive index of 1.57 to 1.7. [Brief explanation of the drawing]

[0015] The components of the following figures are not necessarily drawn to scale, although at least one of them may be drawn to scale. They are illustrated to emphasize the general principles of this disclosure. Reference numerals designating the corresponding components are repeated throughout the figures as needed for consistency and clarity.

[0016] [Figure 1]Figure 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] [Figure 2] Figure 2 is a schematic cross-sectional view of Figure 1 obtained along line 2-2.

[0018] [Figure 3A] Figure 3A is a schematic exploded cross-sectional view of a backlight unit for a backlit display, including the light guide film shown in Figures 1 and 2 and an LED.

[0019] [Figure 3B] Figure 3B is a schematic diagram of a brightness-enhancing film for a backlit display, which has a prismatic microstructure and includes details of the apex and base angles.

[0020] [Figure 3C] Figure 3C is a schematic diagram of a stacked pair of brightness-enhancing films for a backlit display having a prismatic microstructure, illustrating the right-angle orientation of the two films.

[0021] [Figure 3D] Figure 3D is a schematic diagram of a gain-enhancing film for a backlit display, which has a prismatic microstructure on one side and includes details of the apex and base angles.

[0022] [Figure 3E] Figure 3E is a schematic diagram of a gain-enhancing film for a backlit display, having a prismatic microstructure on one side, where the prismatics are formed at a desired angle with respect to the long axis of the film.

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

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

[0025] [Figure 3H] Figure 3H is a schematic top view of an edge-lit light guide film with multiple light-emitting diode (LED) light sources, showing the orientation in the optical direction, the desired orientation of the pyramid on the top of the first brightness-enhancing film, the desired orientation of the prism on the top of the second brightness-enhancing film, and the desired orientation of the prism on the bottom of the gain-enhancing film.

[0026] [Figure 4] Figure 4 shows a two-dimensional plot of the dispersion of light emitted from an LED as a function of angle, as measured by a light distribution meter, and a two-dimensional plot of the dispersion of light emitted from an LED as a function of angle, as measured by a light distribution meter, after the light has passed through a pair of intersecting brightness-enhancing films.

[0027] [Figure 5A] Figure 5A is a three-dimensional representation of defined angles through which light approaching a pair of intersecting brightness-enhancing films can be transmitted.

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

[0029] [Figure 6]Figure 6 is a two-dimensional representation of Figure 5A of the defined angles at which light approaching a pair of intersecting brightness-enhancing films can be transmitted through the pair of intersecting brightness-enhancing films axially and in the direction normal to the upper surface of the light guide film.

[0030] [Figure 7] Figure 7 is a two-dimensional plot of the dispersion of light emitted from the upper surface of an edge-lit light guide film, with a narrow dispersion, accompanied by a specular reflector positioned on the bottom surface of the edge-lit light guide film, as measured by a light distribution meter.

[0031] [Figure 8] Figure 8 is a two-dimensional plot of the dispersion of light emitted from the upper surface of an edge-lit light guide film, with broad dispersion, accompanied by a diffuse reflector positioned on the bottom surface of the edge-lit light guide film, as measured by a light distribution meter.

[0032] [Figure 9] Figure 9 is a two-dimensional plot of the dispersion of light emitted from the upper surface of a circular diffuser located on the upper surface of an edge-light type light guide film, with a narrow dispersion, accompanied by a specular reflector located on the bottom surface of the edge-light type light guide film, as measured by a light distribution meter.

[0033] [Figure 10] Figure 10 is a two-dimensional plot of the dispersion of light emitted from the upper surface of a circular diffuser located on the upper surface of an edge-light light guide film, with broad dispersion, accompanied by a diffuse reflector located on the bottom surface of the edge-light light guide film, as measured by a light distribution meter.

[0034] [Figure 11]Figure 11 is a two-dimensional plot of the dispersion of light emitted from the upper surface of a diffuser located on the upper surface of an edge-light type light guide film, having a narrow dispersion, with a specular reflector located on the bottom surface of the edge-light type light guide film, as measured by a light distribution meter, according to one embodiment of the present invention.

[0035] [Figure 12] Figure 12 is a schematic two-dimensional representation of the defined angles at which light approaching a pair of intersecting brightness-enhancing films can be transmitted axially through the pair of intersecting brightness-enhancing films in the direction normal to the upper surface of the edge-light type light guide film.

[0036] [Figure 13] Figure 13 is a combination of Figures 11 and 12.

[0037] [Figure 14] Figure 14 is a two-dimensional plot of the dispersion of light emitted from the upper surface of a diffuser located on the upper surface of an edge-light type light guide film, having broad dispersion, with a diffuse reflector located on the bottom surface of the edge-light type light guide film, as measured by a light distribution meter, according to one embodiment of the present invention.

[0038] [Figure 15] Figure 15 is a schematic two-dimensional representation of the defined angles at which light approaching a pair of intersecting brightness-enhancing films can be transmitted axially through the pair of intersecting brightness-enhancing films in the direction normal to the upper surface of the edge-light type light guide film.

[0039] [Figure 16] Figure 16 is a combination of Figures 14 and 15.

[0040] [Figure 17] Figure 17 is a graph of relative luminance as a function of the angle of the upper luminance enhancement film prism with respect to the light direction for the edge-lit backlight unit of this teaching. [Modes for carrying out the invention]

[0041] Detailed explanation The luminous dispersion of the light guide film 100, along with the light guide film 100 and the diffuser film 320, can be used to understand how such a film would transfer the intrinsic output of the LED 110 to a luminous dispersion that is well matched to the light-receiving angles of the crossed luminance-enhancing films 330, 340. A photometer is generally used to measure their luminous dispersions. The equipment includes a mechanical photometer with a horizontal axis for rotating the test sample and a vertical axis, and a photometer for measuring luminosity over a given distance. The photometer is positioned at a distance from the test sample that is much longer than the dimensions of the light-emitting surface of the test sample, so that the measured result is not dependent on the size of the test sample. This process is often referred to as "far-field" dispersion measurement. The optical dispersion data described herein were collected using a photometer with the aforementioned equipment.

[0042] Figure 3A schematically illustrates the light guide film 100 of Figures 1 and 2 within the backlight unit 300. Specifically, Figure 3A is a schematic exploded cross-sectional view of a backlight unit for a backlit display, including the light guide film of Figures 1 and 2 and LEDs. As shown, a reflector 310 is positioned directly below the light guide film 100. The reflector 310 may be a specular reflector, a diffuse reflector, or a combination thereof, and is configured to reflect the externally coupled light on the underside of the light guide film 100 back towards the light guide film 100. Positioned above the light guide film 100 may also be called a gain-enhancing film, and is typically a diffuser film 320, which is circularly symmetrical and configured to improve the uniformity of the externally coupled light on the upper side of the light guide film 100.

[0043] Positioned above the diffuser film 320 are two brightness-enhancing films (BEFs) 330, 340. In some embodiments, the brightness-enhancing films 330, 340 have a plurality of parallel minute prisms with a 90° apex angle on one side, and the refractive index of the prisms is typically 1.55 to 1.7. Within the backlight unit 300, the brightness-enhancing films 330, 340 are positioned so that the prisms are oriented away from the light guide film 100, the prisms are positioned on the upper surface of the BEFs, and the prisms in the upper film 340 are oriented perpendicular to the lower film 330. The plurality of parallel minute prisms of one of the brightness-enhancing films 330, 340 are generally aligned with the length L of the light guide film 100 (i.e., within about 20°). The other set of parallel minute prisms in the brightness-enhancing films 330, 340 are generally aligned with the width W of the light guide film 100 (i.e., within approximately 20°). That is, the alignment of the prism vertices is generally along the optical direction of the light guide film 100. In other embodiments, the alignment of the prism vertices is closer than 20°. In some embodiments, the intersecting brightness-enhancing films 330, 340 increase the on-axial brightness of the light emitted from the backlight unit 300.

[0044] For an edge-lit display, it is desirable for the backlight unit 300 to maximize light efficiency, that is, to increase visible brightness without increasing the power supplied to the backlight unit 300. One feature of this teaching is the recognition that the direction of light propagating through the backlight display, i.e., the orientation of the prism and / or the film relative to the direction of light, 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 luminance-enhancing film 330, 340 is oriented along the direction of light propagating through the light guide film. In some embodiments, this is the direction of the vertex of the prism on the upper luminance-enhancing film 340 which is nominally aligned with the direction of light propagating through the light guide film (i.e., less than 10 degrees or less than 20 degrees). In some of these embodiments, the direction of the vertex of the prism on the second luminance-enhancing film 330 is nominally perpendicular to the direction of the vertex of the prism on the upper luminance-enhancing film 340. In some embodiments, the direction of the vertex of the prism on the bottom of the diffuser film 320 is aligned with the direction of the vertex of the prism on the upper luminance-enhancing film 340. In some embodiments, the direction of at least some of the faces of the pyramid located on the upper side of the diffuser film 320 is aligned parallel to the direction of the vertex of the prism on the bottom of the diffuser film 320. In other words, the direction of the pyramidal vertex within the diffuser film 320 is nominally aligned parallel to the direction of the prism vertex. The term “nominally parallel” as used herein means that the angle between the two directions is substantially zero degrees. In some embodiments, at least some of the directions of the pyramidal faces located on the upper side of the diffuser film 320 are aligned at a 45-degree angle to the direction of the prism vertex on the bottom of the diffuser film 320. These various embodiments of microstructure alignment result in high brightness from the backlight unit 300.

[0046] Figure 3B is a schematic diagram of a brightness-enhancing film 350 for a backlit display, having a prismatic microstructure and including details of the apex and base angles of the prismatic faces. This structure 350 may be used for one or both of the brightness-enhancing films 340, 330, which are described in relation to Figure 3A. Multiple prismatics are oriented parallel to each other along one direction of the film. The direction of the lines passing through the vertices of the prismatics is called the prismatic vertex direction. In some embodiments, multiple prismatics extend along a straight line. In some embodiments, the prismatics do not follow a perfectly straight line but follow a slightly wavy path. In embodiments with wavy lines at the prismatic vertices, the prismatics may still be considered nominally parallel and have a prominent prismatic vertex alignment direction. In some embodiments, the prismatics are located on the upper side of the film.

[0047] In one embodiment of the triangular prism, the faces in view exhibit a vertex angle of 90° ± 4°. The base angles for the faces of the prism in this embodiment are 45° ± 2°. For example, the refractive index of the prism can nominally be in the range of 1.57 to 1.7.

[0048] Figure 3C is a schematic diagram of a stacked pair of brightness-enhancing films 355 for a backlit display having a prismatic microstructure, illustrating the right-angle orientation of the two films. This stacked pair of brightness-enhancing films 355 may be, for example, two brightness-enhancing films 340, 330, as described in relation to Figure 3A. These films are positioned such that the directions of the vertices of the parallel prismatics in the two films are oriented perpendicular to each other. In some embodiments, the directions of the vertices of the prismatics on the upper film (e.g., film 340 in Figure 3A) are oriented along the direction of light propagating through the light guide film (e.g., film 100) in Figure 3A.

[0049] In the right-angle orientation of the prism vertices of the stacked luminance-enhancing films 355, as will be obvious to those skilled in the art, the orientation of one of the two films will generally be aligned with the direction of light propagating through the light guide film more than the other. This is because it is unlikely that the films will be oriented strictly along the diagonals. Therefore, generally, when two stacked luminance-enhancing films 355 are present, it is possible to define the orientation of the prism vertices in the stack that is most closely aligned with the direction of light propagating through the light guide film. In some embodiments, depending on which is most closely aligned with the direction of light propagating through the light guide film, this will be the upper film direction, and in other embodiments, this will be the bottom film direction.

[0050] Figure 3D is a schematic diagram of a gain-enhancing film 360 for a backlit display, having a prismatic microstructure on one side and including details of the apex and base angles of the prismatic faces. The terms “gain-enhancing film” and “diffuser film” are used synonymously herein. The gain-enhancing film 360 may be, for example, a diffuser film 320, as described in relation to Figure 3A. In some embodiments, referring to the orientation of the backlit display in Figure 3A, the gain-enhancing films 360, 320 have prismatics extending along the bottom side of the film. That is, the prismatic microstructure is located on the side of the film 360, 320 closest to the light guide film 100. In some embodiments, the orientation of the apex of the prismatic microstructure along the film 360, 320 is oriented so as to nominally align with the direction of light propagating through the light guide film. In some embodiments, the alignment is within 10 degrees, and in other embodiments, the alignment is within 20 degrees.

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

[0052] Figure 3E is a schematic diagram of a gain-enhancing film 365 for a backlit display, having a prismatic microstructure on one side, where the prismatic is formed at a desired angle with respect to the long axis of the film. The angle can be arbitrary. The ability to process the vertices of parallel prismatics with a direction different from the long axis of the film is one method for producing various relative alignments between the direction of microstructures in various films and the direction of light, as described herein. That is, for example, referring to Figure 3A, the long axes of two or more of the various films 100, 320, 330, 340 can be aligned within a stack of films producing a backlight unit 300. Then, for example, the angle between the direction of the vertex prismatics of at least one of the brightness-enhancing films 330, 340 and the direction of light propagating through the light guide film 100 is based on the angle between the direction of the vertex of the parallel prismatic and the direction of the long axis of the films 330, 340.

[0053] Figure 3F is a schematic diagram of a gain-enhancing film 370 for a backlit display, having an inverted pyramidal microstructure on one side and a prism on the opposite side, and including details of the apex and base angles of the inverted pyramidal structure. In this embodiment, the direction passing through the two opposite faces of the pyramidal structure, along the aligned vertices of the pyramidal structure located on one side of the film, is referred to as the pyramidal vertex direction. In some embodiments, the pyramidal vertex direction is aligned parallel to the direction of the vertices of the parallel prism structure located on the other side of the film.

[0054] In some embodiments, the gain-enhancing film 370 is configured as a diffuser film 320, as described in relation to Figure 3A. In some of these embodiments, the prismatic microstructures of the gain-enhancing films 370, 320 are located on the bottom of the films 370, 320 facing toward the light guide film 100, and the pyramidal structures are located on the top of the films 370, 320. In some embodiments, the pyramidal structures are inverted and face toward the inside of the film 370, as shown in Figure 3F. In other embodiments, the pyramidal structures face upward, outward from the top side of the film 370.

[0055] Referring again to Figure 3A, in some embodiments, the orientation of the vertices of the parallel prism structures on the bottom of films 370, 320 is aligned with at least one of the orientations of the vertices of the parallel prism structures on the upper brightness-enhancing films 330, 340. In some embodiments, this alignment is with films 330, 340 having orientations of the vertices of the parallel prism structures that are most closely aligned with the orientation of the light propagating through the light guide film 100. In other embodiments, the upper film 340 has orientations of the vertices of the parallel prism structures that are most closely aligned with the orientation of the light propagating through the light guide film 100.

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

[0057] One feature of the gain-enhancing film described herein, also known as a diffusion film, is that embodiments having pyramidal microstructures on one side of the film and prismatic structures on the opposite side can utilize various angles between the directions passing along the aligned vertices of the pyramidal structures and the directions passing along the aligned vertices of the prismatic structures. For example, in various embodiments, these directions can be parallel or identical to those described in relation to Figure 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] Figure 3G is a schematic diagram of a gain-enhancing film 375 for a backlit display, having a pyramidal microstructure on one side and a prism on the opposite side, where the pyramidal axis is formed at a desired angle with respect to the major axis of the film, and the opposite prism is aligned with the diagonal of the base of the pyramid. In some embodiments, either direction can also be at any angle with respect to the long side of the film.

[0059] Figure 3H is a schematic top view of an edge-lit light guide film 380 with multiple light-emitting diode (LED) light sources 110, showing the orientation of the light direction and the desired orientation of the prism on the upper part of the first brightness-enhancing film, as well as the desired orientation of the prism on the upper part of the second brightness-enhancing film and the desired orientation of the prism on the bottom of the gain-enhancing film. For example, the light guide film 380 can be the light guide film 100, which is described in relation to Figure 3A. Exemplary configurations of the orientations described herein with respect to microstructures on other films (not shown in Figure 3H) are shown. Referring again to Figure 3A, in this embodiment, the orientation of the prism vertices on the upper BEF film 340 is perpendicular to the orientation of the prism vertices on the bottom BEF film 330. Thus, the orientation of the prism vertices on the upper BEF film 340 is most closely aligned with the light direction within the light guide film 380. In this embodiment, the direction of the vertex of the prism on the bottom side of the diffuser film 320 is aligned with the direction of the vertex of the prism on the upper BEF film 340. In some embodiments, these directions do not align with the long side of the individual films.

[0060] While the descriptions of various embodiments associated with Figures 3A-H focus on prism and / or pyramidal microstructures, this teaching is not limited to these shapes, as will be obvious to those skilled in the art. For example, conical and / or angularly bent shapes can also be used in various embodiments in accordance with this teaching.

[0061] Figure 4 shows the extent to which the Lambertsian dispersion 400 emitted from LED 110 is converted to a narrower dispersion 410, accompanied by increased on-axial luminance due to the crossed luminance enhancement film pair 330, 340. Figure 5A illustrates the extent to which the crossed luminance enhancement films 330, 340 act, and is a three-dimensional representation of defined angles to which light approaching the crossed luminance enhancement film pair 330, 340 can be transmitted through the pair of luminance enhancement films 330, 340. Only light approaching the crossed luminance enhancement films 330, 340 from defined angles, represented by the four lobes 510, 520, 530, and 540, can be transmitted through the crossed luminance enhancement films 330, 340 and emitted on-axially with the narrow dispersion 410 in Figure 4, while most of the other light is reflected back downward toward the light guide film 100. A portion of the light reflected downward toward the light guide film 100 is lost due to absorption by the diffuser film 320, the light guide film 100, and the reflector 310. The light reflected downward and recirculating upward toward the crossed pair of luminance-enhancing films 330, 340 may be emitted from the crossed luminance-enhancing films 330, 340 in a second or third trial. Figure 5B is a two-dimensional representation of Figure 5A. Figure 6 illustrates the light receiving locations 610, 620, 630, 640 from which light is emitted axially from the crossed pair of luminance-enhancing films 330, 340.

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

[0063] The addition of the diffuser film 320 further modifies the angular light output dispersion. Figures 9 and 10 illustrate the extent to which the circular diffuser film modifies the optical dispersion for each combination of the light guide film 100 and reflector 310, as shown in Figures 7 and 8, respectively. Figure 9 illustrates the measured angular output dispersion 900 for a combination of a circular diffuser above the light guide film 100 and a specular reflector 310 positioned directly below the light guide film 100, which has a narrow dispersion output, along with an area 910 representing the peak light intensity. Figure 10 illustrates the measured angular output dispersion 1000 for a combination of a circular diffuser above the light guide film 100 and a diffuse reflector 310 positioned directly below the light guide film 100, which has a wide dispersion output, along with an area 1010 representing the peak light intensity. It was found that, with or without the circular diffuser, the optical dispersion of any of the light guide films 100 with their individual reflectors 310 often does not match well the input dispersion required to be emitted axially from the backlight unit 300 by the crossed luminance enhancement film pairs 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 recirculated within the backlight unit 300 will be lost due to the absorbance and less than 100% reflectivity of the reflector 310, it is desirable for the optical output dispersion of the diffuser film 320 / light guide film 100 / reflector 310 combination to match the light receiving standard of the crossed luminance enhancement film pairs 330, 340 for as much axial transmission as possible, so that the on-axial luminance emanating from the backlight unit 300 can be maximized. As will be described in more detail below, the relative on-axial luminance of each combination of the light guide film 100 and reflector 310 and the crossed luminance enhancement film pairs 330, 340 described above was measured using various different embodiments of the diffuser film 320. A backlight unit comprising a light guide film with narrow dispersion and a specular reflector.

[0065] Comparative Example A: The on-axial luminance of a light guide film 100 having a narrow dispersion output, accompanied by a specular reflector 310 as described above, a 50° full-width half-value (FWHM) circular volume diffuser 320 typically used in conjunction with such a light guide film 100 and reflector 310, and a crossed luminance-enhancing film 330, 340 was measured as a baseline and set to 100.0% for comparative purposes.

[0066] A series of diffuser films 320, each containing circular microstructure diffusers with a full-width half-maximum (FWHM) diffusion angle ranging from 20° to 90°, were used in place of the circular volume diffuser film 320 used in Comparative Example A within the backlight unit 300, and the on-axial luminance of the backlight unit 300 was measured relative to Comparative Example A. Specifically, Example 1 included a diffuser film 320 with a 20° FWHM circular diffuser microstructure, Example 2 included a diffuser film 320 with a 40° FWHM circular diffuser microstructure, Example 3 included a diffuser film 320 with a 55° FWHM circular diffuser microstructure, Example 4 included a diffuser film 320 with an 80° FWHM circular diffuser microstructure, and Example 5 included a diffuser film 320 with a 90° FWHM circular diffuser microstructure. The results of the on-axial luminance tests relative to Comparative Example A are listed in Table I below.

[0067] Table I: Relative on-axial brightness in backlight units using diffuser films with circular diffuser microstructures. [Table 1]

[0068] The results in Table I show that the diffuser film 320 with circular diffuser microstructure used in Examples 1-5 provides axial brightness very similar to that of the circular volume diffuser film used in Comparative Example A.

[0069] Next, three diffuser films 320 were inspected within the backlight unit 300, each having multiple parallel prism microstructures on one side of the diffuser film 320 facing the light guide film 100, aligned in the same direction as the microprisms of the brightness-enhancing films 330 and 340, which were closest to the one aligned along the length L of the light guide film 100. The opposite side of the diffuser film 320 facing the brightness-enhancing films 330 and 340 was smooth. Example 6 included a diffuser film 320, each having multiple parallel prism microstructures with an apex angle of 90° and a refractive index of 1.5. Example 7 included a diffuser film 320, each having multiple prism microstructures with an apex angle of 90° and a refractive index of 1.57. Example 8 included a diffuser film 320, each having multiple prism microstructures with an apex angle of 90° and a refractive index of 1.7. The results of the on-axis luminance inspection for Example 6-8 compared to Comparative Example A are listed in Table II below.

[0070] Table II: Relative on-axial brightness in a backlight unit using a diffuser film with a 90° vertex prism on one side. [Table 2]

[0071] The results show that none of the diffuser films 320 with 90° prisms 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] Next, circular diffuser microstructures were added to the smooth side of the diffuser film 320 used in Example 8 at various full-width half-maximum (FWHM) diffusion angles. Example 9 included a diffuser film 320, each having a plurality of prismatic microstructures on one side facing the light guide film 100, with a 90° apex angle and a refractive index of 1.7, and a plurality of circular diffuser microstructures on the opposite side, with an FWHM of 20°. Example 10 included a diffuser film 320, each having a plurality of prismatic microstructures on one side facing the light guide film 100, with a 90° apex angle and a refractive index of 1.7, and a plurality of circular diffuser microstructures on the opposite side, with an FWHM of 30°. Example 11 included a diffuser film 320, each having a plurality of prismatic microstructures on one side facing the light guide film, with a 90° apex angle and a refractive index of 1.7, and a plurality of circular diffuser microstructures on the opposite side, with an FWHM of 40°. The results of the axial brightness inspection for Example 8-11 compared to Comparative Example A are listed in Table III below.

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

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

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

[0076] Table IV: Relative on-axial brightness in a backlight unit using a diffuser film with a refractive index of 1.50 or 1.57 and a 90° vertex prism on one side and a circular diffuser microstructure on the opposite side. [Table 4]

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

[0078] To illustrate the extent to which the diffuser film 320 of one embodiment of the present invention, specifically the diffuser film 320 used in Example 15, which has a plurality of prismatic microstructures on one side having a 90° apex angle and a refractive index of 1.5, and a 55° FWHM circular diffuser microstructure on the opposite side, performs within the backlight unit 300, the angular light dispersion of the combination of the diffuser film 320 of Example 15, a light guide film 100 having narrow dispersion, and a specular reflector 310 was measured and compared to a light receiving angle reference for crossed luminance enhancement films 330, 340. Figure 11 illustrates the measured angular light dispersion 1100 of the combination of the diffuser film 320 of Example 15, the light guide film 100 having narrow dispersion, and the specular reflector 310, along with areas 1110 and 1120 in Figure 11, which indicate the maximum intensity of light passing through. Figure 12 illustrates the light receiving angle reference 1200 of the intersecting brightness enhancement films 330, 340, along with areas 1210, 1220, 1230, and 1240 indicating where light would pass axially through the intersecting brightness enhancement films 330, 340. The combination of Figures 11 and 12 is represented by 1300 in Figure 13, each showing an excellent match between the areas 1110, 1120 of the highest intensity of light output by the diffuser film 320 with multiple prismatic microstructures having 90° vertices and a refractive index of 1.5 on one side and a 55° FWHM circular diffuser microstructure on the opposite side, the light guide film 100 having narrow dispersion, and the specular reflector 320, and the areas 1210, 1220, 1230, and 1240 of the intersecting brightness enhancement films 330, 340 of the backlight unit 300.

[0079] Next, the effect of the apex angles of multiple prismatic microstructures on the diffuser film was investigated. Example 18 included a diffuser film 320, each having multiple prismatic microstructures with an apex angle of 80° and a refractive index of 1.5 on one side facing the light guide film 100, and multiple circular diffuser microstructures with an FWHM of 20° on the opposite side. Example 19 included a diffuser film 320, each having multiple prismatic microstructures with an apex angle of 80° and a refractive index of 1.5 on one side facing the light guide film 100, and multiple circular diffuser microstructures with an FWHM of 30° on the opposite side. Example 20 included a diffuser film 320, each having multiple prismatic microstructures with an apex angle of 80° and a refractive index of 1.5 on one side facing the light guide film 100, and multiple circular diffuser microstructures with an FWHM of 40° on the opposite side. The results of the on-axis luminance inspection for Examples 18-20 compared to Comparative Example A are listed in Table V below.

[0080] Table V: Relative on-axial brightness in a backlight unit using a diffuser film having a refractive index of 1.50 and a prism with an 80° vertex on one side and a circular diffuser microstructure on the opposite side. [Table 5]

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

[0082] Next, conical and pyramidal microstructures were investigated instead of circular diffuser microstructures. Conical microstructures diverge the collimated beam of light into a circular ring. Conical structures with a 110° apex angle diverge the light into a ring with an FWHM of approximately 40°. Inverted pyramidal microstructures with a 110° apex angle were also examined. The pyramidal microstructures were aligned in two orientations, namely, that the faces of the pyramidal microstructures were either parallel (and perpendicular) to the opposite prism, or at a 45° angle to the opposite prism. Example 21 included a diffuser film 320, each with a plurality of prism microstructures having a 90° apex angle and a refractive index of 1.5 on one side facing the light guide film 100, and a plurality of conical microstructures having a 110° apex angle on the opposite side. Example 22 included a diffuser film 320, each having a plurality of prismatic microstructures on one side facing the light guide film 100, with a 90° apex angle and a refractive index of 1.57, and a plurality of conical microstructures on the opposite side, with a 110° apex angle. Example 23 included a diffuser film 320, each having a plurality of prismatic microstructures on one side facing the light guide film 100, with a 90° apex angle and a refractive index of 1.5, and a plurality of square pyramidal microstructures having a 110° apex angle and a surface oriented parallel (and perpendicular) to the plurality of prismatic microstructures. Example 24 included a diffuser film 320, each having a plurality of prismatic microstructures on one side facing the light guide film 100, with a 90° apex angle and a refractive index of 1.5, and a plurality of square pyramidal microstructures (refractive index of 1.5) having a 110° apex angle and a surface oriented at 45° to the plurality of prismatic microstructures. Example 25 included a diffuser film 320, each having a plurality of prismatic microstructures with a 90° apex angle and a refractive index of 1.5 on one side facing the light guide film 100, and a plurality of square pyramidal microstructures (refractive index of 1.5) with a 110° apex angle and a surface oriented at 45° relative to the plurality of prismatic microstructures. The square pyramidal structures may have either a "bulge" or a "depression," or any other polarity. The results of the on-axial luminance inspection of Examples 21-25 compared to Comparative Example A are listed in Table VI below.

[0083] Table VI: Relative on-axial brightness in a backlight unit using a diffuser film having a refractive index of 1.50 and a prism with a 90° vertex on one side, and a conical microstructure and a pyramidal microstructure on the opposite side. [Table 6]

[0084] The results show that diffuser film 320, each having multiple prismatic microstructures with a 90° apex angle and a refractive index of 1.5 on one side, and both a conical microstructure with a 110° apex angle and a pyramidal microstructure with a 110° apex angle on the opposite side, performed well and increased on-axial brightness 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-axial brightness of diffuser films 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 pyramid from 110 degrees to 90 degrees resulted in a very significant and unexpected increase in luminance from 115% to 127.5%. In addition, increasing the refractive index of the pyramidal microstructure from 1.5 to 1.57 resulted in an increase in luminance from 127.5% to 129%.

[0086] One feature of this teaching is that the orientation of the prisms within the upper BEF is important for the diffuser film, with prisms with a refractive index of 1.5 (oriented parallel to the prisms within the upper BEF) having an apex angle of approximately 90 degrees and a square pyramidal structure with a 90-degree apex angle on the back side having a refractive index of 1.57 or 1.5. The importance of this feature is illustrated in relation to Figure 17 with respect to the pyramidal structure with a refractive index of 1.57. Figure 17 is a graph of relative luminance as a function of the angle of the upper luminance enhancement film prism with respect to the optical direction for an edge-lit backlight unit according to this teaching. It can be seen that maximum luminance is achieved when the upper BEF prism is within + / -20 degrees of the optical direction, and especially when the degree of optical direction is within + / -10 degrees. When the bottom prism of the BEF and the bottom prism of the diffuser are aligned with the optical direction, the performance is generally good. The BEF orientation affects the other diffuser films, but not significantly.

[0087] Next, instead of the circular diffuser microstructures described above, angular bent microstructures were investigated. Such angular bent microstructures are described in the jointly owned U.S. Patent Application No. 16 / 625,830 (its entire contents are incorporated herein by reference), filed on December 23, 2019, as a U.S. national phase 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. Specifically, the diffuser film 320 contained a plurality of angular bent microstructures having the form of an array of microprismatics, as illustrated in Figure 8 of WO 2018 / 006288 A1, contained on the side of the diffuser film 320 opposite the plurality of parallel prismatic microstructures. Both the multiple angle-bent microstructures and the multiple parallel prism microstructures had a refractive index of 1.5. The multiple angle-bent microstructures were oriented such that, when installed in a backlight unit 300 with a light guide film 100 having a narrow dispersion output as described above and a specular reflector 310, the light was aligned with the multiple prism microstructures and bent away from the LED 110. The backlight unit 300 with the diffuser film 320 having multiple angle-bent microstructures exhibited performance improvements superior to comparative example A, equal to or better than the examples listed in Table III-V.

[0088] Other microstructures having shapes and configurations different from those disclosed herein may also be used on the sides of the diffuser film 320 facing away from the light guide film 100 toward the crossed luminance-enhancing film pairs 330, 340. The embodiments described herein are not intended to limit in any way. A backlight unit comprising a light guide film with broad dispersion and a diffuse reflector.

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

[0090] A series of circular diffuser films 320 having a full-width half-width diffusion angle (FWHM) ranging from 20° to 90° were used in place of the 35° full-width half-width (FWHM) circular volume diffuser film 320 of Comparative Example B, and the relative on-axial luminance was measured. Specifically, Example 26 included a 20° FWHM circular microstructure diffuser film 320, Example 27 included a 30° FWHM circular microstructure diffuser film 320, Example 28 included a 40° FWHM circular microstructure diffuser film 320, Example 29 included a 55° FWHM circular microstructure diffuser film, and Example 30 included a 90° FWHM circular microstructure diffuser film 320. The results of the on-axial luminance tests for Examples 26-30 compared to Comparative Example B are listed in Table VII below.

[0091] Table VII: Relative on-axial brightness in backlight units using diffuser film with circular diffuser microstructures. [Table 7]

[0092] Similar to the results with the light guide film having a narrow dispersion and the specular reflector listed in Table I above, the diffuser film 320 with a different circular diffuser microstructure is considered to have no significant effect on the on-axial brightness of the backlight unit 300, which includes the light guide film 100 having a wide dispersion and the diffuse reflector 310.

[0093] Next, four different diffuser films 320 were inspected within the backlight unit 300, each having multiple parallel prism microstructures on one side of the diffuser film 320 oriented toward the light guide film 100, and aligned in the same direction as the prism microstructures of the brightness enhancement film that are closest to the one aligned along the length L of the light guide film 100. The opposite side of the diffuser film 320 oriented toward the bottom brightness enhancement films 330, 340 was smooth. Example 31 included a diffuser film 320, each with multiple parallel prism microstructures having an apex angle of 90° and a refractive index of 1.5. Example 32 included a diffuser film 320, each with multiple prism microstructures having an apex angle of 90° and a refractive index of 1.57. Example 33 included a diffuser film 320, each with multiple prism microstructures having an apex angle of 90° and a refractive index of 1.65. Example 34 included a diffuser film 320, each containing multiple prismatic microstructures, each having an apex angle of 90° and a refractive index of 1.7. The results of the on-axial luminance tests of Examples 31-34 compared to Comparative Example B are listed in Table VIII.

[0094] Table VIII: Relative on-axial brightness in a backlight unit using a diffuser film with a 90° vertex on one side. [Table 8]

[0095] The results show that all diffuser films 320 having multiple prismatic microstructures on one side increased the relative on-axial brightness of the backlight unit 300, and that the on-axial brightness increased with increasing refractive index of the prismatics. The results listed in Table VIII are in contrast to the results listed in Table II for the backlight unit 300 with the light guide film 100 having narrow dispersion and the specular reflector 310 described above, which showed that the prismatics caused a decrease in on-axial brightness.

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

[0097] Table IX: Relative on-axial brightness of a diffuser film in a backlight unit, with a prism on one side having a refractive index of 1.50 or 1.57 and a 90° vertex, and a circular diffuser microstructure on the opposite side. [Table 9]

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

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

[0100] Table X: Relative on-axial brightness using a diffuser film with a prism having an 80° vertex on one side. [Table 10]

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

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

[0103] Table XI: Relative on-axial brightness using a diffuser film with a prism having an 80° vertex on one side. [Table 11]

[0104] The results show that, in the combination of the light guide film 100 having broad dispersion and the diffuse reflector 310, the optimal refractive index for the prismatic microstructure with an apex angle of 80° for the diffuser film 320 is approximately 1.63 to 1.65.

[0105] As described above, a certain amount of diffusion on the opposite side of the diffuser film 320, which has multiple prismatic microstructures on one side, may be desirable to improve the optical uniformity of the light emitted from the backlight unit 300, even at the expense of a certain degree of brightness reduction. The additional amount of diffusion provided by the opposite side of the diffuser film 320, which has multiple prismatic microstructures with an apex angle of 80° and a refractive index of 1.65, has been investigated. Example 48 each included a diffuser film 320 with multiple prismatic microstructures having an apex angle of 80° and a refractive index of 1.65 on one side and multiple circular diffuser microstructures having a 3° FWHM on the opposite side. Example 49 each included a diffuser film 320 with multiple prismatic microstructures having an apex angle of 80° and a refractive index of 1.65 on one side and multiple circular diffuser microstructures having a 10° FWHM on the opposite side. The results of the on-axis luminance inspection for Examples 48 and 49 compared to Comparative Example B are listed in Table XII.

[0106] Table XII: Relative on-axial brightness of a diffuser film in a backlight unit, with a prism with an 80° vertex on one side and a circular diffuser microstructure on the opposite side. [Table 12]

[0107] As shown by the results enumerated in Table XII, adding a small amount of diffusion (by using multiple circular diffuser microstructures having 3°FWHM or 10°FWHM) to the opposite side of the diffuser film 320, which has multiple prismatic microstructures, each having an 80° apex angle and a refractive index of 1.65 on one side, when used in conjunction with the light guide film 100 and diffuse reflector 310, which have broad dispersion, reduces the on-axial brightness of the backlight unit 300. This result is in contrast to adding a significant amount of diffusion to the opposite side of the backlight unit 300, which uses the diffuser film 320 with multiple prismatic microstructures on one side, which has increased on-axial brightness, as shown above in Table III-VI, and is combined with the light guide film 100, which has narrow dispersion, and the specular reflector 310.

[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, which has a plurality of prismatic microstructures having an 80° apex angle and a refractive index of 1.65 on one side facing the light guide film 100 and a plurality of circular diffuser microstructures having a 3° FWHM on the opposite side, performs within the backlight unit 300, the angular light dispersion of the combination of the diffuser film 320 of Example 48, the light guide film 100 having broad dispersion, and the diffuse reflector 310 was measured and compared to a light receiving angle reference for the crossed luminance enhancement films 330, 340. Figure 14 illustrates the measured angular light dispersion 1400 of the combination of the diffuser film 320 of Example 48, the light guide film 100 having broad dispersion, and the diffuse reflector 310, along with areas 1410 and 1420 in Figure 14, which indicate the maximum intensity of light passing through. Figure 15 illustrates the light receiving angle reference 1500 of the crossed luminance enhancement films 330, 340 used within the backlight unit 300, along with areas 1510, 1520, 1530, and 1540 indicating the locations where light will pass axially through the crossed luminance enhancement films 330, 340. The combination of Figures 14 and 15 is represented by 1600 in Figure 16, each demonstrating an excellent match between the diffuser film 320 with multiple prismatic microstructures having an 80° vertex and a refractive index of 1.65 on one side and multiple circular diffuser microstructures having a 3° FWHM on the opposite side, the light guide film 100 and specular reflector 320 having broad dispersion, and the input reference of the crossed luminance 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 guide film 100 having a narrow dispersion output and a specular reflector 310 comprises 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 guide film 100 having a wide dispersion output and a diffuse reflector comprises 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 tests were completed to determine the extent to which each of these diffuser films 320 performs within a backlight unit 300, which included the other light guide film 100 and the reflector 310.

[0110] Example 50 included a diffuser film 320, each having a plurality of prismatic microstructures with an 80° apex angle and a refractive index of 1.64 on one side facing the light guide film 100 and a smooth opposite side; a light guide film 100 with a narrow dispersion output; a specular reflector 310; and pairs of crossed luminance-enhancing films 330 and 340. A comparison of the on-axial luminance results of Example 50 with those of Comparative Example A is listed in Table XIII below, along with Example 15 and Comparative Example A.

[0111] Table XIII: Relative on-axial luminance of a diffuser film in a backlight unit with a light guide film having narrow dispersion and a specular reflector. [Table 13]

[0112] The results show that Example 15 provided significantly higher on-axial brightness than Example 50, but Example 50 was still a slight improvement over comparative Example A.

[0113] Example 51 included a diffuser film 320, each having a plurality of prismatic microstructures with a 90° apex angle and a refractive index of 1.5 on one side facing the light guide film 100, and a circular diffuser microstructure with a 55° FWHM diffusion angle on the opposite side; a light guide film 100 with a wide dispersion output; a diffuse reflector 310; and pairs of crossed luminance-enhancing films 330 and 340. A comparison of the on-axial luminance results of Example 51 with those of Comparative Example B, along with Example 47 and Comparative Example B, is listed in Table XIV below.

[0114] Table XIV: Relative on-axial brightness of diffuser film in a backlight unit with a light guide film having broad dispersion and a diffuse reflector. [Table 14]

[0115] The results show that Example 47 provided significantly higher on-axial brightness than Example 51, but Example 51 was still a slight improvement over comparative Example B.

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

[0117] The embodiments illustrated and described above are not intended to limit 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 to be protected by the subsequent claims.

Claims

1. An edge-lit backlight unit for a backlit display, wherein the edge-lit backlight unit is Reflector, An edge-light type light guide film positioned above the reflector, wherein the edge-light type light guide film has a length and a width, and the combination of the edge-light type light guide film and the reflector is configured to provide a peak optical dispersion of 15° to 20° and a full-width half-maximum diffusion angle of 25° to 45°, A diffuser film positioned above the edge-lit light guide film, wherein the diffuser film has a plurality of parallel prism microstructures on one side facing the edge-lit light guide film, and a plurality of diffuser microstructures on the opposite side of the diffuser film, at least some of the plurality of parallel prism microstructures having an apex angle of 78° to 92° and being formed from a material having a refractive index of 1.49 to 1.58, and the plurality of diffuser microstructures having a total width half-maximal diffusion angle of 30° to 60°, and A pair of crossed luminance-enhancing films positioned above the diffuser film, wherein at least one of the pair of crossed luminance-enhancing films has a plurality of parallel minute prisms on one side facing away from the diffuser film, and the plurality of parallel minute prisms of one of the pair of crossed luminance-enhancing films is oriented perpendicular to the plurality of parallel minute prisms of the other of the pair of crossed luminance-enhancing films. Equipped with, The plurality of parallel prism microstructures of the diffuser film are substantially aligned with the plurality of parallel microprisms of at least one of the pair of intersecting brightness-enhancing films. An edge-lit backlight unit in which each of the plurality of diffuser microstructures is formed in a square pyramidal shape, and at least some faces of the square pyramidal shape of each of the plurality of diffuser microstructures are oriented at approximately 45° with respect to the plurality of parallel prism microstructures of the diffuser film.

2. The edge-lit backlight unit according to claim 1, wherein at least one of the pair of intersecting brightness-enhancing films comprises a plurality of parallel minute prisms oriented to be most closely aligned with the light direction.

3. The edge-light type backlight unit according to claim 2, wherein at least one of the pair of crossed brightness-enhancing films is positioned on top of the edge-light type backlight unit.

4. The edge-lit backlight unit according to claim 1, wherein the square pyramidal shape has an apex angle of approximately 110°.

5. The edge-lit backlight unit according to claim 1, wherein the square pyramidal shape has an apex angle of approximately 90°.

6. The edge-lit backlight unit according to claim 1, wherein each of the plurality of diffuser microstructures is formed from a material having a refractive index of about 1.

57.

7. The edge-lit backlight unit according to claim 1, further comprising a plurality of LEDs positioned along the width of the edge-lit light guide film.

8. An edge-lit backlight unit for a backlit display, wherein the edge-lit backlight unit is Diffuse reflector, An edge-light type light guide film positioned above the diffuse reflector, wherein the edge-light type light guide film has a length and a width, and the combination of the edge-light type light guide film and the diffuse reflector is configured to provide a peak optical dispersion of 30° to 50° and a full-width half-maximum diffusion angle of 55° to 85°, A diffuser film positioned above the edge-lit light guide film, wherein the diffuser film has a plurality of parallel prism microstructures on one side facing the edge-lit light guide film, and a plurality of diffuser microstructures on the opposite side of the diffuser film, at least some of the plurality of parallel prism microstructures having an apex angle of 75° to 85° and a refractive index of 1.59 to 1.67, and the plurality of diffuser microstructures having a total width half-maximum diffusion angle of less than 20°, A pair of crossed luminance-enhancing films positioned above the diffuser film, wherein at least one of the pair of crossed luminance-enhancing films has a plurality of parallel minute prisms on one side facing away from the diffuser film, and the plurality of parallel minute prisms of one of the pair of crossed luminance-enhancing films is oriented perpendicular to the plurality of parallel minute prisms of the other of the pair of crossed luminance-enhancing films. Equipped with, The plurality of parallel prism microstructures of the diffuser film are substantially aligned with the plurality of parallel microprisms of at least one of the pair of intersecting brightness-enhancing films. An edge-lit backlight unit in which each of the plurality of diffuser microstructures is formed in a square pyramidal shape, and at least some faces of the square pyramidal shape of each of the plurality of diffuser microstructures are oriented at approximately 45° with respect to the plurality of parallel prism microstructures of the diffuser film.

9. The edge-lit backlight unit according to claim 8, wherein the plurality of parallel minute prisms of at least one of the pair of intersecting brightness-enhancing films are oriented to be most closely aligned with the light direction.

10. The edge-light type backlight unit according to claim 9, wherein at least one of the pair of crossed brightness-enhancing films is positioned on top of the edge-light type backlight unit.

11. The edge-lit backlight unit according to claim 8, wherein each of the plurality of diffuser microstructures is configured to have a total width half-maximal diffusion angle of less than 10° or equal to 10°.

12. The edge-lit backlight unit according to claim 8, wherein each of the plurality of diffuser microstructures is configured to have a total width half-maximal diffusion angle of less than 5° or equal to 5°.

13. A backlight unit, wherein the backlight unit is Reflector, A light guide film positioned above the reflector, wherein the light guide film is configured to guide light along the direction of light, A diffuser film positioned above the light guide film, wherein the diffuser film has a plurality of parallel prism microstructures on one side facing the light guide film, at least some of the plurality of parallel prism microstructures having a prism vertex direction, and the diffuser film further has a plurality of pyramidal microstructures on the opposite side, at least some of the plurality of pyramidal microstructures having a common pyramidal vertex direction, A first brightness-enhancing film positioned above the diffuser film, wherein the first brightness-enhancing film has a plurality of parallel prism microstructures on one side of the first brightness-enhancing film facing away from the diffuser, and at least some of the plurality of parallel prism microstructures of the first brightness-enhancing film have a common prism vertex direction, A second brightness-enhancing film positioned above the first brightness-enhancing film, wherein the second brightness-enhancing film has a plurality of parallel prism microstructures on one side of the second brightness-enhancing film facing away from the first brightness-enhancing film, and at least some of the plurality of parallel prism microstructures of the second brightness-enhancing film have a common prism vertex direction. Equipped with, A backlight unit in which the common prism vertex direction of at least one of the first brightness-enhancing film and the second brightness-enhancing film is substantially oriented along the light direction, the common prism vertex direction of the diffuser film is substantially oriented along the common prism vertex direction of at least one of the first brightness-enhancing film and the second brightness-enhancing film, and the common pyramidal vertex direction of the diffuser film is oriented at a desired angle with respect to the common prism vertex direction of the diffuser film, the desired angle being substantially 45°.

14. The backlight unit according to claim 13, wherein the common prism vertex direction of at least one of the first brightness-enhancing film and the second brightness-enhancing film, which have a prism vertex direction substantially oriented along the light direction, has a prism vertex direction that is less than 10 degrees from the light direction.

15. The backlight unit according to claim 13, wherein the common prism vertex direction of at least one of the first brightness-enhancing film and the second brightness-enhancing film, which have a prism vertex direction substantially oriented along the light direction, has a prism vertex direction that is less than 20 degrees from the light direction.

16. The backlight unit according to claim 13, wherein the other of the first brightness-enhancing film and the second brightness-enhancing film has a prism vertex direction that is oriented substantially perpendicular to the prism vertex direction of the at least one of the first brightness-enhancing film and the second brightness-enhancing film.

17. The backlight unit according to claim 13, wherein at least one of the first brightness-enhancing film and the second brightness-enhancing film, which have a prism vertex direction substantially oriented along the light direction, is positioned on top of the backlight unit.

18. The backlight unit according to claim 13, wherein at least some of the plurality of parallel prism microstructures of the diffuser film have apex angles of 78° to 94°.

19. The backlight unit according to claim 13, wherein at least some of the plurality of parallel prism microstructures of the diffuser film are formed from a material having a refractive index of 1.5 to 1.

66.

20. The backlight unit according to claim 13, wherein at least some of the plurality of pyramidal microstructures of the diffuser film have an apex angle of 84° to 114°.

21. The backlight unit according to claim 13, 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.

57.

22. The backlight unit according to claim 13, wherein at least some of the plurality of parallel prism microstructures of at least one of the first brightness-enhancing film and the second brightness-enhancing film have apex angles of 86° to 94°.

23. The backlight unit according to claim 13, wherein at least some of the plurality of parallel prism microstructures of at least one of the first brightness-enhancing film and the second brightness-enhancing film are formed from a material having a refractive index of 1.57 to 1.7.

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