WO2019046223A1

WO2019046223A1 - Light guides including gratings

Info

Publication number: WO2019046223A1
Application number: PCT/US2018/048219
Authority: WO
Inventors: Xiang-Dong Mi
Original assignee: Corning Incorporated
Priority date: 2017-08-29
Filing date: 2018-08-28
Publication date: 2019-03-07
Also published as: KR20200037870A; JP2020532823A; TW201921041A; CN111183316A

Abstract

A light guide includes a glass plate, a pattern of first gratings, a pattern of second gratings, and a pattern of light extractors. The glass plate has a first surface and a second surface opposite to the first surface. The pattern of first gratings is on the first surface of the glass plate. The pattern of second gratings is on the second surface of the glass plate where each second grating is aligned with a first grating. The pattern of light extractors is on the first or second surface of the glass plate.

Description

LIGHT GUIDES INCLUDING GRATINGS

[0001] This application claims the benefit of priority under 35 U.S.C. § 119 of U.S. Provisional Application Serial No. 62/551,375 filed on August 29, 2017, the content of which is relied upon and incorporated herein by reference in its entirety.

BACKGROUND

Field

[0002] The present disclosure relates generally to apparatuses and methods for spreading light via light guides. More particularly, the present disclosure relates to spreading light via patterned light guides, for example in backlights for displays.

Technical Background

[0003] Liquid crystal displays (LCDs) are light-valve based displays in which the display panel includes an array of individually addressable light valves. A backlight is used to produce an emissive image in the LCD displays. Backlights are either edge-lit or direct-lit. An edge-lit backlight, for example, includes a light emitting diode (LED) array edge -coupled to a light guide plate that emits light from its surface. A direct-lit backlight, for example, includes a two-dimensional (2D) array of LEDs directly behind the LCD panel. Edge-lit backlights are typically thinner than direct-lit backlights, while direct-lit backlights enable improved dynamic contrast since LEDs in dark regions of the display may be turned off.

[0004] Direct-lit backlights may suffer from a large amount of light loss due to multiple reflections within the backlight. Accordingly, apparatuses and methods for spreading light via a light guide with reduced light loss are disclosed herein.

SUMMARY

[0005] Some embodiments of the present disclosure relate to a light guide. The light guide includes a glass plate, a pattern of first gratings, a pattern of second gratings, and a pattern of light extractors. The glass plate has a first surface and a second surface opposite to the first surface. The pattern of first gratings is on the first surface of the glass plate. The pattern of second gratings is on the second surface of the glass plate where each of said second gratings is aligned with a first grating. The pattern of light extractors is on the first or second surface of the glass plate. [0006] Yet other embodiments of the present disclosure relate to a backlight. The backlight includes a glass light guide, a bottom reflector, and a plurality of light sources. The glass light guide includes a bottom surface and a top surface, a pattern of light extractors on the bottom surface or the top surface, and a pattern of first gratings on the bottom surface or the top surface. The plurality of light sources is between the bottom reflector and the glass light guide. Light from each light source is coupled into the glass light guide by a corresponding first grating such that a first portion of the light travels laterally in the glass light guide and is extracted out of the glass light guide by the light extractors.

[0007] Yet other embodiments of the present disclosure relate to a method for fabricating a display. The method includes attaching a plurality of light emitting diodes (LEDs) to a printed circuit board (PCB). The method further includes applying a bottom reflector to the PCB between the plurality of LEDs. The method further includes applying a light guide plate over the plurality of LEDs, the light guide plate including a bottom surface and a top surface, a pattern of light extractors on the bottom surface or the top surface, and a pattern of gratings on the bottom surface or the top surface. The method further includes applying a patterned reflector over the light guide plate.

[0008] The apparatuses and methods disclosed herein provide light guides having reduced light loss and thin and efficient backlights having the two-dimensional (2D) local dimming capability of direct-lit backlights.

[0009] Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments as described herein, including the detailed description which follows, the claims, as well as the appended drawings.

[0010] It is to be understood that both the foregoing general description and the following detailed description describe various embodiments and are intended to provide an overview or framework for understanding the nature and character of the claimed subject matter. The accompanying drawings are included to provide a further understanding of the various embodiments, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments described herein, and together with the description serve to explain the principles and operations of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS [0011] FIGS. 1A-1B schematically depict one example of a light guide including a transmitting grating;

[0012] FIGS. 2A-2B schematically depict one example of a light guide including a reflecting grating;

[0013] FIGS. 3A-3C schematically depict one example of a light guide including a transmitting grating and a reflecting grating;

[0014] FIGS. 4A-4C schematically depict another example of a light guide including a transmitting grating and a reflecting grating;

[0015] FIG. 5 illustrates one example of a pattern of light extractors;

[0016] FIG. 6 schematically depicts one example of a backlight;

[0017] FIGS. 7A-7B schematically depict one example of a first subassembly for fabricating a display;

[0018] FIGS. 8A-8B schematically depict one example of a second subassembly for fabricating a display;

[0019] FIGS. 9A-9B schematically depict one example of a third subassembly for fabricating a display;

[0020] FIGS. 10A-10B schematically depict one example of a fourth subassembly for fabricating a display;

[0021] FIG. 11 schematically depicts one example of a display;

[0022] FIGS. 12A-12B illustrate examples of gratings;

[0023] FIG. 13A schematically depicts one example of a grating;

[0024] FIG. 13B illustrates one example of the response of the grating of FIG. 13A to a ray of light;

[0025] FIG. 14 illustrates diffraction efficiency versus grating thickness for one example of a transmitting grating;

[0026] FIG. 15 illustrates diffraction efficiency versus grating thickness for one example of a reflecting grating;

[0027] FIGS. 16A-16B schematically depict one example of a light guide including a transmitting grating where the azimuthal angle φ for incident light equals 90°;

[0028] FIG. 16C illustrates diffraction efficiency versus incident polar angle Θ where azimuthal angle φ equals 90° for the example transmitting grating of FIGS 16A-16B;

[0029] FIGS. 17A-17B schematically depict one example of a light guide including a transmitting grating where the azimuthal angle φ for incident light equals 0°; [0030] FIG. 17C illustrates diffraction efficiency versus incident polar angle Θ where azimuthal angle φ equals 0° for the example transmitting grating of FIGS. 17A-17B;

[0031] FIGS. 18A-18B schematically depict one example of a light guide including a reflecting grating where the azimuthal angle φ for incident light equals 90°;

[0032] FIG. 18C illustrates diffraction efficiency versus incident polar angle Θ where azimuthal angle φ equals 90° for the example reflecting grating of FIGS. 18A-18B;

[0033] FIGS. 19A-19B schematically depict one example of a light guide including a reflecting grating where the azimuthal angle φ for incident light equals 0°;

[0034] FIG. 19C illustrates diffraction efficiency versus incident polar angle Θ where azimuthal angle φ equals 0° for the example reflecting grating of FIGS. 19A-19B.

DETAILED DESCRIPTION

[0035] Reference will now be made in detail to embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. However, this disclosure may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

[0036] Ranges can be expressed herein as from "about" one particular value, and/or to "about" another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent "about," it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

[0037] Directional terms as used herein - for example up, down, right, left, front, back, top, bottom, vertical, horizontal - are made only with reference to the figures as drawn and are not intended to imply absolute orientation.

[0038] Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order, nor that with any apparatus, specific orientations be required. Accordingly, where a method claim does not actually recite an order to be followed by its steps, or that any apparatus claim does not actually recite an order or orientation to individual components, or it is not otherwise specifically stated in the claims or description that the steps are to be limited to a specific order, or that a specific order or orientation to components of an apparatus is not recited, it is in no way intended that an order or orientation be inferred, in any respect. This holds for any possible non-express basis for interpretation, including: matters of logic with respect to arrangement of steps, operational flow, order of components, or orientation of components; plain meaning derived from grammatical organization or punctuation, and; the number or type of embodiments described in the specification.

[0039] As used herein, the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise. Thus, for example, reference to "a" component includes aspects having two or more such components, unless the context clearly indicates otherwise.

[0040] Referring now to FIGS. 1A-1 B, an exemplary a light guide 100 including a transmitting grating 106 is schematically depicted. FIG. 1A is a side view of the light guide 100, and FIG. IB is a bottom view of the transmitting grating 106. Light guide 100 includes a glass plate 101 having a first surface 102 and a second surface 104 opposite to the first surface 102. Light guide 100 includes the transmitting grating 106 on the first surface 102 of the glass plate 101. In other examples, light guide 100 includes a pattern of transmitting gratings 106, such as a two-dimensional (2D) array of transmitting gratings 106 on the first surface 102 of the glass plate 101. Light guide 100 also includes a pattern of light extractors (not shown), which will be described below, for example, with reference to FIG. 5. The pattern of light extractors is on the first surface 102 or the second surface 104 of the glass plate 101.

[0041] As shown in FIG. IB, transmitting grating 106 is a linear grating including a plurality of lines parallel to the y-axis. Transmitting grating 106 couples light 108 incident on transmitting grating 106 into the light guide 100 such that a portion of the light, i.e., the transmitted first order (Tl) light indicated at 112, travels laterally in the light guide in the xz- plane due to total internal reflection. The light that travels laterally in the light guide 100 due to total internal reflection is eventually extracted out of the light guide 100 by the light extractors. The remaining portion of the light including the transmitted zero order (TO) light indicated at 110 passes through the second surface 104 of the glass plate 101.

[0042] FIGS. 2A-2B schematically depict an exemplary light guide 120 including a reflecting grating 126. FIG. 2A is a side view of the light guide 120, and FIG. 2B is a top view of the reflecting grating 126. Light guide 120 includes a glass plate 121 having a first surface 122 and a second surface 124 opposite to the first surface 122. Light guide 120 includes the reflecting grating 126 on the second surface 124 of the glass plate 121. In other examples, light guide 120 includes a pattern of reflecting gratings 126, such as a 2D array of reflecting gratings 106 on the second surface 124 of the glass plate 121. Light guide 120 also includes a pattern of light extractors (not shown), which will be described below, for example, with reference to FIG. 5. The pattern of light extractors is on the first surface 122 or the second surface 124 of the glass plate 121.

[0043] As shown in FIG. 2B, reflecting grating 126 is a linear grating including a plurality of lines parallel to the y-axis. Reflecting grating 126 couples light 128 incident on reflecting grating 126 into the light guide 120 such that a portion of the light, i.e., the reflected first order (Rl) light indicated at 132, travels laterally in the light guide in the xz-plane due to total internal reflection. The light that travels laterally in the light guide 120 due to total internal reflection is eventually extracted out of the light guide 120 by the light extractors. The remaining portion of the light includes the reflected zero order (R0) light indicated at 130 and the TO light indicated at 134. The TO light 134 passes through second surface 124 of the glass plate 121 while the R0 light 130 is reflected back towards the first surface 122 of the glass plate 121.

[0044] FIGS. 3A-3C schematically depict an exemplary light guide 140 including a transmitting grating 146 and a reflecting grating 148. FIG. 3A is a side view of the light guide 140, FIG. 3B a top view of the reflecting grating 148, and FIG. 3C is a bottom view of the transmitting grating 146. Light guide 140 includes a glass plate 141 having a first surface 142 and a second surface 144 opposite to the first surface 142. Light guide 140 includes the transmitting grating 146 on the first surface 142 of the glass plate 141. Light guide 140 includes the reflecting grating 148 on the second surface 144 of the glass plate 141. The transmitting grating 146 is aligned with the reflecting grating 148 in the z-axis direction. In other examples, light guide 140 includes a pattern of transmitting gratings 146 and a pattern of reflecting gratings 148, such as a 2D array of transmitting gratings 146 on the first surface 142 of the glass plate 141 and a 2D array of reflecting gratings 148 on the second surface 144 of the glass plate 141, where each transmitting grating is aligned with a reflecting grating in the z-axis direction. Light guide 140 also includes a pattern of light extractors (not shown), which will be described below, for example, with reference to FIG. 5. The pattern of light extractors is on the first surface 142 or the second surface 144 of the glass plate 141.

[0045] As shown in FIG. 3B, reflecting grating 148 is a linear grating including a plurality of lines parallel to the y-axis. As shown in FIG. 3C, transmitting grating 146 is also a linear grating including a plurality of lines parallel to the y-axis. In one example, the lines of transmitting grating 146 and the lines of reflecting grating 148 are parallel to each other within about 10°. Transmitting grating 146 couples light 150 incident on transmitting grating 146 into the light guide 140 such that a portion of the light, i.e., Tl light indicated at 158, travels laterally in the light guide in the xz-plane due to total internal reflection. Reflecting grating 148 couples light 150 incident on reflecting grating 148 into the light guide 140 such that a portion of the light, i.e., the Rl light indicated at 154, travels laterally in the light guide in the xz-plane due to total internal reflection. The light that travels laterally in the light guide 140 due to total internal reflection is eventually extracted out of the light guide 140 by the light extractors. The remaining portion of the light includes the R0 light indicated at 152 and the TO light indicated at 156. The TO light 156 passes through the second surface 144 of the glass plate 141 while the R0 light 152 is reflected back towards the first surface 142 of the glass plate 141.

[0046] FIGS. 4A-4C schematically depict another example of a light guide 160 including a transmitting grating 166 and a reflecting grating 168. FIG. 4A is a side view of the light guide 160, FIG. 4B is a top view of the reflecting grating 168, and FIG. 4C is a bottom view of the transmitting grating 166. Light guide 160 includes a glass plate 161 having a first surface 162 and a second surface 164 opposite to the first surface 162. Light guide 160 includes the transmitting grating 166 on the first surface 162 of the glass plate 161. Light guide 160 includes the reflecting grating 168 on the second surface 164 of the glass plate 161. The transmitting grating 166 is aligned with the reflecting grating 168 in the z-axis direction. In other examples, light guide 160 includes a pattern of transmitting gratings 166 and a pattern of reflecting grating 168, such as a 2D array of transmitting gratings 166 on the first surface 162 of the glass plate 161 and a 2D array of reflecting gratings 168 on the second surface 164 of the glass plate 161, where each transmitting grating is aligned with a reflecting grating in the z-axis direction. Light guide 160 also includes a pattern of light extractors (not shown), which will be described below, for example, with reference to FIG. 5. The pattern of light extractors is on the first surface 162 or the second surface 164 of the glass plate 161.

[0047] As shown in FIG. 4B, reflecting grating 168 is a linear grating including a plurality of lines parallel to the x-axis. As shown in FIG. 4C, transmitting grating 166 is a linear grating including a plurality of lines parallel to the y-axis. In one example, the lines of transmitting grating 166 and the lines of reflecting grating 168 are orthogonal to each other within about 10°. Transmitting grating 166 couples light 170 incident on transmitting grating 166 into the light guide 160 such that a portion of the light, i.e., the Tl light indicated at 178, travels laterally in the light guide in the xz-plane due to total internal reflection. Reflecting grating 168 cou les light 170 incident on reflecting grating 168 into the light guide 160 such that a portion of the light, i.e., the Rl light indicated at 174, travels laterally in the light guide in the yz-plane (indicated by dashed lines) due to total internal reflection. The light that travels laterally in the light guide 160 due to total internal reflection is eventually extracted out of the light guide 160 by the light extractors. The remaining portion of the light includes the R0 light indicated at 172 and the TO light indicated at 176. The TO light 176 passes through the second surface 164 of the glass plate 161 while the R0 light 172 is reflected back towards the first surface 162 of the glass plate 161.

[0048] FIG. 5 illustrates an exemplary pattern of light extractors 220. Pattern of light extractors 220 is for an individual unit block having a unit length Lo as indicated at 222 and a unit width Wo as indicated at 224. The center X of the depicted unit block corresponds to the area under which a light source is placed. As will be described with reference to FIG. 7A below, each light source may include the depicted unit block. In general, the density of light extraction features increases with distance from the center X. In some examples, the light extraction features may be patterned to form a grid, in which the central region X and the regions Y extending orthogonally from the center are more sparsely populated with light extraction features, while the corner regions Z are more densely populated. In other examples, others light extraction feature patterns may be used as appropriate to create a desired light output distribution.

[0049] The light guide plate may be treated to create the light extraction features using any suitable method. In one example, the light extraction features may be areas of the surface coated with a highly scattering paint, laser damaged areas, and/or micro-optic features such as prisms or lenses formed on the glass surface out of plastic or another glass. In another example, the light extraction features may be formed by etching the surface of the light guide plate. Parameters for the etching may be varied to achieve the desired light extraction features.

[0050] FIG. 6 schematically depicts one example of a backlight 300. In one example, backlight 300 is a direct-lit 2D local dimming capable backlight. Backlight 300 includes a bottom reflector 302, a plurality of light sources 304, a glass light guide 310, and a patterned reflector 320. Bottom reflector 302 includes a reflective material, such as a metal or another suitable material. The plurality of light sources 304 are arranged between the bottom reflector 302 and the glass light guide 310 in a 2D array and have a pitch indicated at 305. In one example, each light source 304 includes a LED or another suitable light source. [0051] Light guide 310 includes a glass plate 311 having a first surface 312 and a second surface 314 opposite to the first surface 312. Light guide 310 includes a pattern of first (e.g., transmitting) gratings 316 on the first surface 312 of the glass plate 311, such as a 2D array of first gratings 3 16. Light guide 310 includes a pattern of second (e.g., reflecting) gratings 318 on the second surface 314 of the glass plate 311, such as a 2D array of second gratings 318. Each light source 304 is aligned with a first grating 316 in the vertical direction, and each first grating 316 is aligned with a corresponding second grating 318 in the vertical direction. Light guide 310 also includes a pattern of light extractors (not shown), such as described above, for example, with reference to FIG. 5. The pattern of light extractors is on the first surface 312 or the second surface 314 of the glass plate 311.

[0052] Patterned reflector 320 includes a first (i.e., reflective) area 325 (indicated by white areas within patterned reflector 320) and a second (i.e., transparent) area 326a, 326b, and 326c (collectively referred to as second area 326 and indicated by black areas within patterned reflector 320) aligned with each light source 304. The first area 325 is more reflective than the second area 326, and the second area 326 is more transmissive than the first area 325. The second area 326 varies in size in the x-axis direction as indicated by a smaller 326a subarea, larger 326b subareas, and yet larger 326c subareas. In this example, there is an air gap between light guide 310 and patterned reflector 320. In other examples, patterned reflector 320 may be monolithically integrated with the light guide 310, for example, by using a patterned metallic layer or a multilayer dielectric coating on the second surface 314 of the glass plate 311.

[0053] At least a portion of light emitted from each light source 304 is coupled into the light guide 310 by the gratings 316 and 318 and spread laterally by total internal reflection.

Another portion of light emitted from each light source 304 is spread laterally between the bottom reflector 302 and the patterned reflector 320 due to reflections at the bottom reflector 302 and the patterned reflector 320. A ray Rl indicated at 330 is transmitted through the transparent area 326a, while a ray R2 indicated at 332 is first reflected by the reflective area 325, then reflected by the bottom reflector 302, and finally passes through the transparent area 326b of the patterned reflector 320. A ray R3 indicated at 334 is spread laterally within light guide 310 due to total internal reflection and is then extracted from light guide 310 by the pattern of light extractors, and finally passes through the transparent area 326c of the patterned reflector 320.

[0054] Rays like R3 334 can travel a longer distance laterally without suffering the loss during the multiple reflections between the patterned reflector 320 and the bottom reflector 302. While rays like R3 334 experience the internal absorption of light guide 310, the internal absorption is relatively small since the LED pitch 305 in backlight 300 is in one example less than about 150 mm, and in another example is less than about 80 mm. Rays like R3 334 travel about half of the LED pitch 305 before they are extracted out of the light guide 310. In one example, a suitable light guide 310 has an internal transmission over 75 mm not less than 98% at 450 nm, 550 nm, and 650 nm. One such suitable light guide may be fabricated using Coming's Iris™ glass. A glass light guide may have a higher thermal stability and higher mechanical stability compared to a light guide made of polymethyl methacrylate (PMMA).

[0055] The optical distance between the bottom reflector 302 and the patterned reflector 320 is indicated by 340. By using light guide 310 with the light from each LED 304 coupled into the light guide through a grating, the optical distance 340 may be as small as the thickness of the light guide 310. The thickness of light guide 310 may be, for example, within a range between about 0.1 mm and 2 mm.

[0056] FIGS. 7A-11 illustrate an exemplary method for fabricating a display. FIG. 7A is a top view and FIG. 7B is a side view of an exemplary first subassembly 400 for fabricating a display. First subassembly 400 is fabricated by attaching a plurality of LEDs 404 to a printed circuit board (PCB) 402. The LEDs 404 are arranged in a 2D array with each LED 404 having an individual unit block 410 having a unit length Lo as indicated at 412 and a unit width Wo as indicated at 414. In one example, each unit block 410 defines where each pattern of light extractors of a light guide, such as a light extractor pattern 220 previously described and illustrated with reference to FIG. 5, will be vertically aligned with each LED 404.

[0057] FIG. 8A is a top view and FIG. 8B is a side view of an exemplary second subassembly 420 for fabricating a display. Second subassembly 420 is fabricated by applying a bottom reflector 422 to the PCB 402 between the plurality of LEDs 404 of first subassembly 400 previously described and illustrated with reference to FIGS. 7A-7B. The bottom reflector 422 may include a metallic material or another suitable reflective material.

[0058] FIG. 9A is a top view and FIG. 9B is a side view of an exemplary third subassembly 430 for fabricating a display. Third subassembly 430 is fabricated by applying a light guide plate 432 over the plurality of LEDs 404 of second subassembly 420 previously described and illustrated with reference to FIGS. 8A-8B. The light guide plate 432 includes a bottom surface and a top surface, a pattern of light extractors on the bottom surface or the top surface, and a pattern of gratings on the bottom surface and/or the top surface as previously described herein, for example, with reference to FIGS. 1 A-5.

[0059] FIG. 10A is a top view and FIG. 1 OB is a side view of an exemplary fourth subassembly 440 for fabricating a display. Fourth subassembly 440 is fabricated by applying a patterned reflector 442 over the light guide plate 432 of third subassembly 430 previously described and illustrated with reference to FIGS. 9A-9B. Patterned reflector 442 includes a first area 444 indicated by black dots and a second area 446 indicated by continuous white space between the black dots. In one example, the pattern provided by the first area 444 and the second area 446 is divided into individual unit blocks 410 previously described and illustrated with reference to FIG. 7A. The first area 444 is more transparent than the second area 446, and the second area 446 is more reflective than the first area 444. The areal density of the first area 444, such as the dots, varies in the plane of the patterned reflector 442. The first area 444 has the lowest areal density right above the LEDs 404, and the highest areal density at the corners between unit blocks. The areal density of the first area 444 may be varied by the number of dots, by the size of the dots, or by a combination thereof. The variation in the areal density of the first area 444 may be designed to provide a uniform illuminance distribution after the light goes through the patterned reflector 442. The dots may be placed irregularly to minimize the pattern visibility. In one example, the size of the dots can be in a range between about 5 μιη and 5000 μιη. The dots may be circular, rectangular, or any other suitable shape.

[0060] FIG. 11 schematically depicts an exemplary display 450. Display 450 is fabricated by applying a diffuser plate 452 over the patterned reflector 442 of the fourth subassembly 440 previously described and illustrated with reference to FIGS. 10A-10B and applying a quantum dot film 454 over the diffuser plate 452. The fabrication of display 450 further includes applying a prismatic film 456 over the quantum dot film 454, applying a reflective polarizer 458 over the prismatic film 456, and applying a display panel 460 over the reflective polarizer 458.

[0061] FIG. 12A illustrates an exemplary circular grating 500. Circular grating 500 includes a plurality of nested circles 502 including partial circles at the edges of the grating. In one example, each of the transmitting gratings and each of the reflecting gratings of FIGS. 1A, 2A, 3A, and 4A may be circular gratings.

[0062] FIG. 12B illustrates an exemplary elliptical grating 510. Elliptical grating 510 includes a plurality of nested ellipses 512 including partial ellipses at the edges of the grating. In one example, each of the transmitting gratings and each of the reflecting gratings of FIGS. 1A, 2A, 3A, and 4A may be elliptical gratings. In other examples, each of the transmitting gratings and each of the reflecting gratings of FIGS. 1 A, 2A, 3A, and 4A may have other suitable shapes.

[0063] A suitable grating has a pitch smaller than the wavelength λ of the light source and greater than (ng-\), where ng is the refractive index of the glass light guide plate. The pitch of the grating is greater than (ng-\) so that the first order of the diffraction exists. The pitch of the grating is less than λ so that the first order of the diffraction for the incident angle equal to 0° satisfies the condition of total internal reflection. Note that under these conditions, for the light that is incident in a plane that is parallel to the lines of the grating, the first order reflected or transmitted light meets the condition of total internal reflection independent of the incident angle. For light that is incident in a plane that is perpendicular to the lines of the grating, one of the first order rays (Tl and T-l for a transmitting grating or Rl and R-l for a reflecting grating) either meet the condition of total internal reflection, or disappear, dependent on the incident angle. The other of the first order rays may or may not meet the condition of total internal reflection. Even when rays do not undergo total internal reflection, the rays are still redirected laterally away from the light source. This portion of light can travel laterally due to the multiple reflections between the bottom reflector and the patterned reflector as shown if FIG. 6. For a transmitting grating, the lateral coupling efficiency from the first order diffraction is about 50% for the incident angle equal to 0°. For a reflecting grating, the lateral coupling efficiency from the first order diffraction is about 67% for the incident angle equal to 0°. To enhance the lateral diffraction efficiency, two grating, one on the bottom of the light guide and one on the top of the light guide, may be used.

[0064] FIG. 13A schematically depicts an exemplary grating 600. In this example, grating 600 is a linear binary grating including a substrate 602 and a plurality of parallel lines 604 extending from the substrate 602. Each line 604 of grating 600 has a pitch indicated at 606, a width indicated at 608, and a thickness indicated at 610. While four lines 604 are illustrated in FIG. 13 A, grating 600 may include any suitable number of parallel lines 604. The grating duty cycle is defined as the grating width 608 divided by the grating pitch 606. The surface profile of grating 600 may be square or rectangular. If a zone in which the grating is to be used has a length different from the width, a rectangular surface profile may spread light more uniformly.

[0065] FIG. 13B illustrates an example of the response 620 of the grating 600 of FIG. 13A to a ray of light. A ray of light with a wave vector k, P-polarization electric field El, and S- polarization electric field E2, impinges on the grating with an incident polar angle Θ and an azimuthal angle φ. When φ = 0°, the plane of incidence is perpendicular to the lines of the grating. When φ = 90°, the plane of incidence is parallel to the lines of the grating. When a = 0°, the ray has S-polarization (TE). When a = 90°, the ray has P-polarization (TM). When β = 0°, the ray is linearly polarized. When β = ±45°, the ray is circularly polarized.

[0066] FIG. 14 illustrates diffraction efficiency versus grating thickness for an example of a transmitting grating for unpolarized normal incidence light. In this example, the transmitting grating is designed for a wavelength of 476 nm with a pitch of 0.47 μιη and a grating duty cycle of 50%. The diffraction efficiency is maximized by selecting an optimal grating thickness and duty cycle. As shown in FIG. 14, in this example the maximum diffraction efficiency for Tl+T-1 is about 50% when the grating thickness is about 0.30 μιη. A similar maximum diffraction efficiency for Tl+T-1 is maintained for duty cycles within the range between about 40% and 60%. As the duty cycle is decreased below about 40%, the diffraction efficiency for Tl+T-1 is reduced such that at a duty cycle of 20%, the diffraction efficiency for Tl+T-1 is less than about 40%. As the duty cycle is increased above about 60%, the diffraction efficiency for Tl+T-1 is reduced such that at a duty cycle of 80%, the diffraction efficiency Tl+T-1 is less than about 30%.

[0067] FIG. 15 illustrates diffraction efficiency versus grating thickness for an example of a reflecting grating for unpolarized normal incidence light. In this example, the reflecting grating is made of aluminum and is designed for a wavelength of 476 nm with a pitch of 0.47 μιη and a grating duty cycle of 50%. In other examples, the reflecting grating may be made of a stack of dielectric materials with alternating low and high indices of refraction, such as S1O2 and T1O2. The diffraction efficiency is maximized by selecting an optimal grating thickness and duty cycle. As shown in FIG. 15, in this example the maximum diffraction efficiency for Rl+R-1 is about 70% when the grating thickness is about 0.15 μιη. A similar maximum diffraction efficiency for Rl+R-1 is maintained for duty cycles within the range between about 40% and 60%. As the duty cycle is decreased below about 40%, the diffraction efficiency for Rl+R-1 is reduced such that at a duty cycle of 20%, the diffraction efficiency for Rl+R-1 is less than about 30%. As the duty cycle is increased above about 60%, the diffraction efficiency for Rl+R-1 is reduced such that at a duty cycle of 80%, the diffraction efficiency for Rl+R-1 is less than about 40%.

[0068] When the duty cycle is too high, R0 is undesirably too high. For example, when the duty cycle is about 80%, R0 is greater than about 80%. When the duty cycle is too low, TO is undesirably too high. For example, when the duty cycle is about 20%, TO is greater than about 50%. When the duty cycle is within the range between about 40% and 60%, both TO and R0 are low, and the desirable Rl +R-1 is high. For example, when the duty cycle is 55%, Rl+R-1 is greater than 60% for a grating with a thickness greater than about 0.23 μηι. When the duty cycle is 53%, Rl+R-1 is greater than 60% for a grating with a thickness within a range between about 0.21 μηι and 0.60 μηι.

[0069] FIGS. 16A-16B schematically depict an exemplary light guide 700 including a transmitting grating 702 where the azimuthal angle φ for incident light equals 90°. FIG. 16A is a side view of the light guide 700, and FIG. 16B is a bottom view of the transmitting grating 702. In this example, the transmitting grating 702 is designed for a wavelength of 476 nm with a pitch of 0.47 μηι, a thickness of 0.29 μηι, and a duty cycle of 45%. As shown in FIG. 16A, when the azimuthal angle φ = 90°, both Tl and T-1 have the same diffraction efficiency and meet the condition of total internal reflection regardless of the incident polar angle Θ. Thus both Tl and T-1 may travel for a long distance.

[0070] FIG. 16C illustrates diffraction efficiency versus incident polar angle Θ where azimuthal angle φ equals 90° for the example transmitting grating of FIGS 16A-16B. In this example, the maximum diffraction efficiency for Tl+T-1 is about 60% when Θ is about 15°.

[0071] FIGS. 17A-17B schematically depict an exemplary light guide 720 including a transmitting grating 722 where the azimuthal angle φ for incident light equals 0°. FIG. 17A is a side view of the light guide 720, and FIG. 17B is a bottom view of the transmitting grating 722. In this example, the transmitting grating 722 is designed for a wavelength of 476 nm with a pitch of 0.47 μηι, a thickness of 0.29 μηι, and a duty cycle of 45%. When the azimuthal angle φ = 0°, Tl either meets the condition of total internal reflection or passes through light guide 720 depending on the incident polar angle Θ. T-1 may also not meet the condition of total internal reflection. In any case, T and T-1 are redirected laterally away from the light source.

[0072] FIG. 17C illustrates diffraction efficiency versus incident polar angle Θ where azimuthal angle φ equals 0° for the example transmitting grating of FIGS. 17A-17B. In this example, the maximum diffraction efficiency for Tl+T+1 is about 50% when Θ is about 5°. When comparing FIG. 17C to FIG. 16C, note that when the polar angle Θ is less than 60°, TO is greater than 40% regardless of whether φ = 0° or 90°. This effect may be suppressed by including a reflecting grating on the top of the light guide. Also note that when the polar angle Θ is less than 30°, TO is greater with the azimuthal angle φ = 0° than with the azimuthal angle φ = 90°. This effect may be suppressed by using a reflecting grating with its lines orthogonal to the lines of the transmitting grating.

[0073] FIGS. 18A-18B schematically depict an exemplary light guide 740 including a reflecting grating 742 where the azimuthal angle φ for incident light equals 90°. FIG. 18A is a side view of the light guide 740, and FIG. 17B is a top view of the reflecting grating 742. In this example, the reflecting grating 742 is designed for a wavelength of 476 nm with a pitch of 0.47 μιη, a thickness of 0.16 μιη, and a duty cycle of 50%.

[0074] FIG. 18C illustrates diffraction efficiency versus incident polar angle Θ where azimuthal angle φ equals 90° for the example reflecting grating of FIGS. 18A-18B. In this example, the maximum diffraction efficiency for Rl+R-1 is about 70% when Θ is about 0°.

[0075] FIGS. 19A-19B schematically depict an exemplary light guide 760 including a reflecting grating 762 where the azimuthal angle φ for incident light equals 0°. FIG. 19A is a side view of the light guide 760, and FIG. 19B is a top view of the reflecting grating 762. In this example, the reflecting grating 762 is designed for a wavelength of 476 nm with a pitch of 0.47 μιη, a thickness of 0.16 μιη, and a duty cycle of 50%.

[0076] FIG. 19C illustrates diffraction efficiency versus incident polar angle Θ where azimuthal angle φ equals 0° for the example reflecting grating of FIGS. 19A-19B. In this example, the maximum diffraction efficiency for Rl+R-1 is about 70% when Θ is about 0°.

[0077] The direct-lit backlight disclosed herein provides improved light efficiency compared to direct-lit backlights not including a light guide. The improved light efficiency is achieved by the glass light guide that is placed above the LEDs. At least a portion of the light from the LEDs is spread laterally in the glass light guide by total internal reflection. The total internal reflection is enabled by gratings on the glass light guide that couple the normal incident light emitted from the LEDs into the light guide at an angle greater than the total internal reflection critical angle.

[0078] It will be apparent to those skilled in the art that various modifications and variations can be made to embodiments of the present disclosure without departing from the spirit and scope of the disclosure. Thus it is intended that the present disclosure cover such modifications and variations provided they come within the scope of the appended claims and their equivalents.

Claims

What is claimed is:

1. A light guide comprising:

a glass plate having a first surface and a second surface opposite to the first surface; a pattern of first gratings on the first surface of the glass plate;

a pattern of second gratings on the second surface of the glass plate, each of said second gratings aligned with a first grating; and

a pattern of light extractors on the first or second surface of the glass plate.

2. The light guide of claim 1, wherein each of said first gratings and each of said second gratings couples light into the light guide such that a portion of the light travels laterally in the light guide and is extracted out of the light guide by the light extractors.

3. The light guide of claim 1 , wherein each of said first gratings is a linear grating comprising first lines, and

wherein each of said second gratings is a linear grating comprising second lines.

4. The light guide of claim 3, wherein the first lines are parallel to the second lines.

5. The light guide of claim 3, wherein the first lines are orthogonal to the second lines.

6. The light guide of claim 1 , wherein each of said first gratings is a circular grating or an elliptical grating, and

wherein each of said second gratings is a circular grating or an elliptical grating.

7. The light guide of claim 1 , wherein the pattern of light extractors comprises a lower density of light extractors nearer to each of said gratings and a higher density of light extractors farther from each of said gratings.

8. A backlight comprising:

a glass light guide comprising a bottom surface and a top surface, a pattern of light extractors on the bottom surface or the top surface, and a pattern of first gratings on the bottom surface or the top surface; a bottom reflector; and

a plurality of light sources between the bottom reflector and the glass light guide, wherein light from each light source is coupled into the glass light guide by a corresponding first grating such that a first portion of the light travels laterally in the glass light guide and is extracted out of the glass light guide by the light extractors.

9. The backlight of claim 8, further comprising:

a patterned reflector comprising a first area and a second area, the first area being more reflective than the second area, and the second area being more transmissive than the first area,

wherein the glass light guide is between the patterned reflector and the plurality of light sources, and

wherein a second portion of the light from each light source travels laterally between the bottom reflector and the patterned reflector due to reflections at the bottom reflector and the patterned reflector.

10. The backlight of claim 8, wherein the pattern of first gratings are on the top surface of the glass light guide and a first order of reflected light from each first grating meets the condition of the total internal reflection.

11. The backlight of claim 8, wherein the pattern of first gratings are on the bottom surface of the glass light guide and a first order of transmitted light through each first grating meets the condition of the total internal reflection.

12. The backlight of claim 8, wherein the glass light guide comprises a pattern of second gratings, and

wherein the pattern of second gratings and the pattern of first gratings are on opposing surfaces of the glass light guide.

13. The backlight of claim 12, wherein each second grating comprises lines orthogonal to lines of each first grating.

14. The backlight of claim 8, wherein each first grating comprises aluminum.

15. The backlight of claim 8, wherein a duty cycle of each first grating is between 40% and 60%.

16. The backlight of claim 8, wherein a pitch of each first grating is shorter than a wavelength of light generated by the plurality of light sources.

17. The backlight of claim 8, wherein each first grating comprises a linear grating, a circular grating, or an elliptical grating.

18. The backlight of claim 8, wherein the glass light guide has a thickness between 0.1 mm and 2 mm.

19. A method for fabricating a display, the method comprising:

attaching a plurality of light emitting diodes (LEDs) to a printed circuit board (PCB); applying a bottom reflector to the PCB between the plurality of LEDs;

applying a light guide plate over the plurality of LEDs, the light guide plate comprising a bottom surface and a top surface, a pattern of light extractors on the bottom surface or the top surface, and a pattern of gratings on the bottom surface or the top surface; and applying a patterned reflector over the light guide plate.

20. The method of claim 19, further comprising:

applying a diffuser plate over the patterned reflector;

applying a quantum dot film over the diffuser plate;

applying a prismatic film over the quantum dot film;

applying a reflective polarizer over the prismatic film; and

applying a display panel over the reflective polarizer.