WO2019231901A1

WO2019231901A1 - Display device, backlight unit and light coupling device therefor

Info

Publication number: WO2019231901A1
Application number: PCT/US2019/034156
Authority: WO
Inventors: Aleksandra Boskovic; Shenping Li; Natesan Venkataraman
Original assignee: Corning Incorporated
Priority date: 2018-05-30
Filing date: 2019-05-28
Publication date: 2019-12-05
Also published as: TW202004289A

Abstract

A display device including a backlight unit with a light coupling device for coupling light into a light guide plate of the backlight unit is disclosed. The light coupling device includes a first section with plurality of light guiding projections and a second section including a curved portion optically coupled to the first section. The curved portion extends through an angle in a range from about 90 degrees to about 180 degrees.

Description

DISPLAY DEVICE, BACKLIGHT UNIT AND LIGHT COUPLING DEVICE THEREFOR

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U.S. Provisional Application Serial No. 62/677,946 filed on May 30, 2018 the contents of which are relied upon and incorporated herein by reference in their entirety as if fully set forth below.

BACKGROUND

FIELD

[0002] The present disclosure relates to a display device configured for local dimming, and in particular a display device comprising a backlight unit (BLU) including a light coupling device for coupling light into a light guide plate of the BLU.

TECHNICAL BACKGROUND

[0003] While organic light emitting diode display devices are gaining in popularity, costs are high, and liquid crystal display (LCD) devices still represent most display devices sold, particularly large panel size devices, such as television sets and other large-format devices such as commercial signs. Unlike OLED display panels, LCD panels do not themselves emit light, and are therefore dependent on a backlight unit (BLU) positioned behind the LCD panel to provide transmissive light to the LCD panel. Light from the BLU illuminates the LCD panel and the LCD panel functions as a light valve that selectively allows light to pass through pixels of the LCD panel or be blocked, thereby forming a viewable image.

[0004] Without augmentation, the native contrast ratio achievable with an LCD display is the ratio of the brightest portion of an image to the darkest portion of the image. The simplest contrast augmentation occurs by increasing the overall illumination for a bright image, and decreasing the overall illumination for a dark image. Unfortunately, this leads to muted brights in a dark image, and washed out darks in a bright image. To overcome this limitation, manufacturers can incorporate active local dimming of the image, wherein the illumination within predefined regions of the display can be locally dimmed relative to other regions of the display panel, depending on the image being displayed. Such local dimming can be relatively easily incorporated when the light source is positioned directly behind the LCD panel, for example a two-dimensional array of light emitting diodes (LEDs). Local dimming is more difficult to incorporate with an edge lighted BLU, wherein an array of LEDs is arranged along an edge of a light guide plate incorporated into the BLU.

[0005] Typical light guide plates incorporate a polymer light guide, such as poly methyl methacrylate (PMMA). PMMA is easily formed, and can be molded or machined to facilitate local dimming. However, PMMA can suffer from thermal degradation, comprises a relatively large coefficient of thermal expansion, suffers from moisture absorption and is easily deformed. On the other hand, glass is dimensionally stable (comprises a relatively low coefficient of thermal expansion), and can be produced in large thin sheets suitable for the growing popularity of large, thin TVs. But what is commercially true for either material is that it would be beneficial to either reduce or eliminate the bezel typically surrounding the display panel about a peripheral edge portion while simultaneously forming a thinner display and improving the contrast ratio of the image.

SUMMARY

[0006] In accordance with the present disclosure an optical coupling device is described that can enable 1D and 2D local dimming in an edge-lighted light guide plate (LGP), while providing effective light coupling from LEDs to the LGP. The optical coupling device comprises two sections. The first section comprises a plurality of light guiding projections with a width that increases along a light propagation direction. The second section comprises a bend through an angle in a range from about 90 degrees to about 180 degrees. An output of the optical coupling device can be introduced by bending the second section, although in other embodiments both sections can include a bend. The shape of the plurality of light guiding projections provides efficient confinement of light in a predetermined width zone for achieving local dimming of the lighted display panel. A small radius bend facilitates narrow bezel designs, or even bezel-free designs, and slimmer overall LCD displays. In addition, by tapering the plurality of light guiding projections of the first section in a thickness direction, efficient light coupling from thicker LEDs to thinner LGP can be achieved.

[0007] Embodiments of the light coupling device can be integrated into a LGP, for example as a single, monolithic piece, or made separately and joined to the LGP such as with an adhesive. The light coupling device can be formed from a polymer or an inorganic material that exhibits high transparency at visible wavelengths. Various methods can be used to make such light coupling devices, such as injection molding, 3D printing, and extraction from a bulk material (e.g., sheet of material) can be performed by laser or mechanical cutting. [0008] Accordingly, a display device is disclosed, comprising a display panel and a backlight unit positioned adjacent the display panel, the backlight unit comprising a light guide plate (for example a glass light guide plate), a light coupling device comprising a curved portion optically coupled to a first edge of the light guide plate, and a plurality of light guiding projections optically coupled to the curved portion, each light guiding projection including an input facet positioned at a distal end of each light guiding projection, and a plurality of light emitting diodes optically coupled to the respective input facets of the light guiding projections. Each input facet intersects with two opposing side edges of the respective light guiding projection. In some embodiments, the two opposing side edges can be curved, while in other embodiments, the two opposing side edges can be linear. Each side edge can subtend an angle in a range from about 1 degree to about 7 degrees relative to a normal to a respective input facet.

[0009] The curved portion can extend through an angle in a range from about 90 degrees to about 180 degrees. A radius of curvature R of the curved portion can be equal to or greater than about 2 millimeters (mm), for example in a range from about 2 mm to about 20 mm.

[0010] In some embodiments, the plurality of light guiding projections can be arranged periodically along a length of the curved portion.

[0011] In some embodiments, the light coupling device is a first light coupling device, the backlight unit further comprising a second light coupling device optically coupled to a second edge of the light guide plate orthogonal to the first edge.

[0012] In other embodiments, a backlight unit is described, comprising a light guide plate, a light coupling device comprising a curved portion optically coupled to a first edge of the light guide plate, and a plurality of light guiding projections optically coupled to the curved portion, each light guiding projection including an input facet positioned at a distal end of each light guiding projection, and a plurality of light emitting diodes optically coupled to the respective input facets of the light guiding projections.

[0013] In various embodiments the curved portion can extend through an angle in a range from about 90 degrees to about 180 degrees. A radius of curvature R of the curved portion can be equal to or greater than about 2 mm, for example in a range from about 2 mm to about 20 mm.

[0014] In some embodiments, the plurality of light guiding projections can be arranged periodically along a length of the curved portion. [0015] Each input facet intersects with opposing side edges of the respective light guiding projection. Each side edge subtends an angle in a range from about 1 degree to about 7 degrees relative to a normal to the respective input facet.

[0016] In some embodiments, the light coupling device is a first light coupling device, the backlight unit further comprising a second light coupling device optically coupled to a second edge of the light guide plate orthogonal to the first edge.

[0017] In still other embodiments, a light coupling device is disclosed, comprising a first section comprising a plurality of light guiding projections, each light guiding projection including an input facet positioned at a distal end of each light guiding projection, and a second section including a curved portion optically coupled to the plurality of light guiding projections.

[0018] In some embodiments, the plurality of light guiding projections can be arranged along a length of the light coupling device with a period in a range from about 5 mm to about 100 mm .

[0019] In some embodiments, a radius of curvature of the curved portion can be equal to or greater than about 2 mm, for example in a range from about 2 mm to about 20 mm.

[0020] In some embodiments, the curved portion extends through an angle in a range from about 90 degrees to about 180 degrees.

[0021] Each light guiding projection of the plurality of light guiding projections comprises side edges extending between a proximal end and a distal end of the light guiding projection, and an angle Q subtended by each side edge relative to a normal to the input facet can be in a range from about 1 degree to 7 degrees.

[0022] Additional features and advantages of the embodiments disclosed herein will be set forth in the detailed description that follows, and in part will be clear 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.

[0023] It is to be understood that both the foregoing general description and the following detailed description present embodiments intended to provide an overview or framework for understanding the nature and character of the embodiments disclosed herein. The

accompanying drawings are included to provide further understanding, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the disclosure, and together with the description explain the principles and operations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG. l is a cross sectional view of an exemplary display device comprising a display panel and a backlight unit;

[0025] FIG. 2 is a top view of an exemplary light guide plate;

[0026] FIG. 3 is a top view of an exemplary light coupling device according to embodiments of the present disclosure illustrating a plurality of light guiding projections;

[0027] FIG. 4 is an edge (side) view of the light coupling device of FIG. 3;

[0028] FIG. 5 is a top view of a single light guiding projection of the light coupling device of FIG. 3 and illustrating linear side edges;

[0029] FIG. 6 is a top view of a single light guiding projection of the light coupling device of FIG. 3 and illustrating curved side edges;

[0030] FIG. 7 is an edge view of another exemplary light coupling device according to the present disclosure and including a variable thickness;

[0031] FIG. 8 is an edge view of the light coupling device or FIG 4 coupled to the light guide plate of FIG. 2 to form at least a portion of a backlight unit;

[0032] FIG. 9 is a bottom view of the backlight unit of FIG. 8;

[0033] FIG. 10 is a bottom view of another backlight unit comprising several light coupling devices in accordance with embodiments of the present disclosure;

[0034] FIG. 11 is a view of modeled light distribution from a single light source coupled to a light coupling device of the present disclosure in turn coupled to a light guide plate to form a backlight unit, and showing lateral confinement of the light from the light source within the light guide plate;

[0035] FIG. 12 is a plot illustrating the modeled efficiency with which light from the light coupling device of FIG. 11 is confined within the light guide plate;

[0036] FIG. 13 is a plot illustrating the percent of light confined within predetermined width zones of the light guide plate of FIG. 11;

[0037] FIG. 14 is a modeled contour map of light confined within a 150 mm wide zone at a 500 mm distance from the LGP input edge surface (normalized to the total light power coupled into the LGP and expressed as a percent) as a function of angle Q and light guiding projection length L_t; [0038] FIG. 15 is a contour map of output width W₀ as a function of angle Q and light guiding projection length L_t when the input width Wi is 4 mm;

[0039] FIG. 16 a view of modeled light distribution from seven light sources coupled to a light coupling device of the present disclosure in turn coupled to a light guide plate to form a backlight unit, and showing lateral confinement of the light from the light source within the light guide plate;

[0040] FIG. 17 is a plot showing the modeled efficiency of light coupled into the LGP of FIG. 16, and light confined within a 150 mm wide zone at distances of 350 mm and 500 mm from the LGP input edge surface as a function of bend radius R of the curved portion of the light coupling device;

[0041] FIG. 18 is a plot showing the modeled ratio of light power within the 150 mm wide zone of FIG. 17 at distances of 350 mm and 500 mm from the input LGP edge surface to the total light coupled into the LGP as a function of bend radius R of curved portion of the light coupling device;

[0042] FIG. 19 is a contour map of modeled light confined within a 150 mm wide zone of FIG. 16 at a distance of 350 mm from the LGP input edge surface (normalized to the total light power coupled into the input edge of the LGP) as a function of angle Q and input width

Wi;

[0043] FIG. 20 is a modeled contour map of the length Lt as a function of angle Q and the input width Wi when the output width W₀ of the trimmed waveguide is 21.4 mm for the backlight unit of FIG. 16;

[0044] FIG. 21 is a modeled contour map of light confined in a 150 mm wide zone at 500 mm distance from LGP input edge (normalized to the total light power coupled into the input edge of the LGP) as a function of angle Q and input width Wi of the light guiding projections for the backlight unit of FIG. 16; and

[0045] FIG. 22 depicts light distribution in the LGP (BLU) of FIG. 16 when two light coupling devices are used to couple light from each light coupling device into two adjacent and orthogonal LGP edges.

DETAILED DESCRIPTION

[0046] 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.

[0047] 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 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.

[0048] Directional terms as used herein - for example up, down, right, left, front, back, top, bottom - are made only with reference to the figures as drawn and are not intended to imply absolute orientation.

[0049] 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.

[0050] 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.

[0051] The word“exemplary,”“example,” or various forms thereof are used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “exemplary” or as an“example” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Furthermore, examples are provided solely for purposes of clarity and understanding and are not meant to limit or restrict the disclosed subject matter or relevant portions of this disclosure in any manner. It is to be appreciated that a myriad of additional or alternate examples of varying scope could have been presented, but have been omitted for purposes of brevity.

[0052] Current light guide plates used in LCD backlight applications are typically formed with PMMA, as PMMA exhibits reduced optical absorption compared to many alternative materials. However, PMMA can present certain mechanical drawbacks that make the mechanical design of large size (e.g., 32 inch diagonal and greater) displays challenging.

Such drawbacks include poor rigidity, high moisture absorption, and a relatively large coefficient of thermal expansion (CTE).

[0053] For example, conventional LCD panels are made of two pieces of thin glass (color filter substrate and TFT backplane), with the BLU comprising a PMMA light guide and a plurality of thin plastic films (diffusers, dual brightness enhancement films (DBEF) films, etc.) positioned behind the LCD panel. Due to the poor elastic modulus of PMMA, the overall structure of the LCD panel exhibits insufficient rigidity, and additional mechanical structure may be necessary to provide stiffness for the LCD panel, thereby adding mass to the display device. It should be noted that a Young's modulus of PMMA is generally about 2 GPa, while certain exemplary glasses can comprise a Young's modulus ranging from about 60 GPa to 90 GPa or more.

[0054] Humidity testing shows that PMMA is sensitive to moisture and can undergo dimensional changes by up to about 0.5%. Thus, for a PMMA panel with a length of one meter, a 0.5% change can increase the panel length by up to 5 mm, which is significant and makes the mechanical design of a corresponding BLU challenging. Conventional approaches to solve this problem include leaving an air gap between the mechanical frame and the PMMA LGP to allow the PMMA LGP to expand. Meanwhile, because the melting temperature of PMMA is low, to avoid deforming the PMMA LGP due to heating by the LEDs an air gap is formed between the PMMA LGP and the LEDs. However, light coupling between the LEDs and the LGP is extremely sensitive to the distance from the LEDs to the LGP. The greater the distance between LED and LGP, the less efficient the light coupling between the LED and LGP. Moreover, lateral misalignment can cause display brightness to change, for example as a function of humidity.

[0055] Still further, PMMA comprises a CTE of about 75E-6/°C, and comprises a relatively low thermal conductivity (approximately 0.2 W/m/K). In comparison, some glasses suitable for use as an LGP can comprise a CTE less than 8E-6/°C with a thermal conductivity of 0.8 W/m/K or more. [0056] Accordingly, glass as a light guiding medium for BLUs offers superior qualities not found in polymer (e.g., PMMA) LGPs, but PMMA LGPs remain popular, particularly in small, low cost devices due to fabrication ease and cost.

[0057] Whether an LGP is formed from glass or polymer, a growing commercial trend is toward thin display devices and narrow or non-existent (not visible) bezels. This is particularly true for large size display devices (e.g., wall mounted displays, signage, etc.).

The trend toward thin displays is best satisfied by providing the backside illumination typically used by LCD display devices with an edge-lighted BLU. On the other hand, edge lighting for BLUs typically demands a sufficiently wide bezel to accommodate the presence of LEDs along the edge(s) of the BLU’s LGP in direct contrast to the continuing trend for a thin bezel. The situation is exacerbated by the practice of locally dimming sections of the display to increase contrast ratio, which is more easily accomplished with a back-lighted LGP than an edge-lighted LGP. Unfortunately, lighting the LGP from the back side tends to increase the thickness of the BLU and hence the display device. Light coupling devices described herein can provide edge lighting that occupies very little space at the edge of the LGP and behind the LGP.

[0058] Shown in FIG. 1 is an exemplary LCD display device 10 comprising an LCD panel 12 formed from a first substrate 14 and a second substrate 16 joined by an adhesive material 18 positioned between and around a peripheral edge portion of the first and second substrates. First and second substrates 14, 16 and adhesive material 18 form a gap 20 therebetween containing liquid crystal material. Spacers (not shown) may also be used at various locations within the gap to maintain consistent spacing of the gap. First substrate 14 may include color filter material. Accordingly, first substrate 14 may be referred to as the color filter substrate. On the other hand, second substrate 16 includes thin film transistors (TFTs) for controlling the polarization state of the liquid crystal material, and may be referred to as the backplane. LCD panel 12 may further include one or more polarizing filters 22 positioned on a surface thereof.

[0059] LCD display device 10 further comprises BLU 24 arranged to illuminate LCD panel 12 from behind, i.e., from the backplane side of the LCD panel. In some embodiments, the BLU may be spaced apart from the LCD panel, although in further embodiments, the BLU may be in contact with or coupled to the LCD panel, such as with a transparent adhesive. BLU 24 comprises an LGP 26 including a first major surface 30, a second major surface 32, and a plurality of edge surfaces extending between the first and second major surfaces. In some embodiments, LGP 26 may be a glass light guide plate, although in further embodiments, LGP 26 may be formed from a polymer material such as PMMA. [0060] In embodiments, LGP 26 may be a parallelogram, for example a square or rectangle comprising four edge surfaces 34a, 34b, 34c and 34d as shown in FIG. 2 extending between the first and second major surfaces. For example, edge surface 34a may be opposite edge surface 34c, and edge surface 34b may be positioned opposite edge surface 34d. Edge surface 34a may be parallel with opposing edge surface 34c, and edge surface 34b may be parallel with opposing edge surface 34d. Edge surfaces 34a and 34c may be orthogonal to edge surfaces 34b and 34d. The edge surfaces 34a - 34d may be planar and orthogonal to, or substantially orthogonal (e.g., 90 +/- 1 degree, for example 90 +/- 0.1 degree) to first and second major surfaces 30, 32, although in further embodiments, the edge surfaces may include chamfers, for example a planar center portion orthogonal to, or substantially orthogonal to first and second major surfaces 30, 32 and joined to the first and second major surfaces by two adjacent angled surface portions.

[0061] First and/or second major surfaces 30, 32 may include an average roughness (Ra) equal to or less than about 0.6 nanometer (nm), equal to or less than about 0.5 nm, equal to or less than about 0.4 nm, equal to or less than about 0.3 nm, equal to or less than about 0.2 nm, or equal to or less than about 0.1 nm when measured by white light interferometry with a commercial system such as those manufactured by Zygo. An average roughness (Ra) of the edge surfaces may be equal to or less than about 0.05 micrometers (pm), for example in a range from about 0.005 micrometers to about 0.05 micrometers.

[0062] For a glass LGP, the foregoing level of major surface roughness can be achieved, for example, by using a fusion draw process or a float glass process followed by polishing. Edge roughness can be achieved by grinding and/or polishing.

[0063] LGP 26 further comprises an average thickness t_g in a direction orthogonal to first major surface 30 and second major surface 32 (see FIG. 8). In some embodiments, thickness t_g may be equal to or less than about 3 millimeters (mm), for example equal to or less than about 2 mm, or equal to or less than about 1 mm, although in further embodiments, thickness t may be in a range from about 0.1 mm to about 3 mm, for example in a range from about 0.1 mm to about 2.5 mm, in a range from about 0.3 mm to about 2.1 mm, in a range from about 0.5 mm to about 2.1 mm, in a range from about 0.6 to about 2.1, or in a range from about 0.6 mm to about 1.1 mm, including all ranges and subranges therebetween. In embodiments, first major surface 30 may be substantially parallel with second major surface 32.

[0064] LGP 26 may have dimensions, such as length L and width W, which can vary depending on the application. In some embodiments, the length L can range from about 0.01 meters (m) to about 10 m, such as from about 0.1 m to about 5 m, from about 0.5 m to about 2.5 m, or from about 1 m to about 2 m, including all ranges and subranges therebetween. Similarly, the width W can range from about 0.01 m to about 10 m, such as from about 0.1 m to about 5 m, from about 0.5 m to about 2.5 m, or from about 1 m to about 2 m, including all ranges and subranges therebetween.

[0065] While a rectangular LGP 26 is illustrated in FIG. 2, it is to be understood that LGP 26 can have any regular or irregular geometric shape as appropriate to produce a desired light distribution for a chosen application. For example, LGP 26 may comprise four edges as illustrated in FIG. 2, or may comprise more than four edges. In other embodiments, the LGP 26 may comprise less than four edges, e.g., a triangle. By way of a non-limiting example, the LGP may comprise a rectangular, square, or rhomboid sheet having four edges, although other shapes and configurations are intended to fall within the scope of the disclosure including those having one or more curvilinear portions or edges.

[0066] According to various embodiments, LGP 26 may comprise any transparent material used in the art for lighting and display applications. As used herein, the term“transparent” is intended to denote that the LGP material exhibits an optical transmission of greater than about 80% over a length of 500 mm in the visible region of the spectrum (e.g., in a range from about 420 nm to about 750 nm). For instance, an exemplary transparent material may have greater than about 85% transmittance in the visible light range over a length of 500 mm, such as equal to or greater than about 90%, equal to or greater than about 95%, or equal to or greater than about 99% transmittance, including all ranges and subranges therebetween.

[0067] In certain embodiments, an exemplary transparent material can exhibit an optical transmittance of greater than about 50% in the ultraviolet (UV) region (e.g., in a range from about 100 nanometers (nm) to about 400 nm) over a length of 500 mm, such as equal to or greater than about 55%, equal to or greater than about 60%, equal to or greater than about 65%, equal to or greater than about 70%, equal to or greater than about 75%, equal to or greater than about 80%, equal to or greater than about 85%, equal to or greater than about 90%, equal to or greater than about 95%, or equal to or greater than about 99% transmittance, including all ranges and subranges therebetween.

[0068] The optical properties of LGP 26 may be affected by the refractive index of the transparent material from which it is formed. According to various embodiments, LGP 26 may possess a refractive index in a range from about 1.3 to about 1.8, such as from about 1.35 to about 1.7, from about 1.4 to about 1.65, from about 1.45 to about 1.6, or from about 1.5 to about 1.55, including all ranges and subranges therebetween. [0069] In embodiments, LGP 26 may exhibit a relatively low level of light attenuation (e.g., due to absorption and/or scattering). The light attenuation a of the LGP may, for example, be equal to or less than about 5 decibels per meter (dB/m) for wavelengths in a range from about 420 nm to about 750 nm, for example equal to or less than about 3 dB/m, equal to or less than about 2 dB/m, equal to or less than about 1 dB/m, equal to or less than about 0.5 dB/m, e.g., in a range from about 0.2 dB/m to about 5 dB/m.

[0070] LGP 26 may comprise one or more polymeric materials, such as plastics, e.g., polymethyl methacrylate (PMMA), methylmethacrylate styrene (MS), polydimethylsiloxane (PDMS), or other similar materials. LGP 26 can also comprise a glass material, such as aluminosilicate, alkali-aluminosilicate, borosilicate, alkali-borosilicate, aluminoborosilicate, alkali-aluminoborosilicate, soda lime, or other suitable glasses. Non-limiting examples of commercially available glasses suitable for use as a glass light guide plate include, for instance, EAGLE XG^®, Lotus™, Willow^®, Iris™, and Gorilla^® glasses from Corning Incorporated.

[0071] LGP 26 may, in some embodiments, comprise glass that has been chemically strengthened, e.g., ion exchanged. During the ion exchange process, ions within a glass sheet at or near the surface of the glass sheet may be exchanged for larger metal ions, for example, from a salt bath. The incorporation of the larger ions into the glass can strengthen the glass by creating a compressive stress in a near surface region. A corresponding tensile stress can be induced within a central region of the glass sheet to balance the compressive stress.

[0072] Ion exchange may be carried out, for example, by immersing the glass in a molten salt bath for a predetermined time. Exemplary salt baths include, but are not limited to, KNCh, LiNCh, NaNCh, RbNCh, and combinations thereof. The temperature of the molten salt bath and treatment time can vary. It is within the ability of one skilled in the art to determine the time and temperature according to the desired application. By way of a non-limiting example, the temperature of the molten salt bath may range from about 400°C to about 800°C, such as from about 400°C to about 500°C, and the predetermined time may range from about 4 to about 24 hours, such as from about 4 hours to about 10 hours, although other temperature and time combinations are envisioned. By way of a non-limiting example, the glass can be submerged in a KNCh bath, for example, at about 450°C for about 6 hours to obtain a K-enriched layer which imparts a surface compressive stress.

[0073] FIGS. 3 and 4 depict a bottom view and a side (edge) view, respectively, of an exemplary light coupling device 100 in accordance with various embodiments. Light coupling device 100 comprises a first section 102 with a thickness tf comprising a plurality of light guiding projections 104. In some embodiments, first section 102 can be planar. Each light guiding projection 104 includes an input facet 106 positioned at a distal end 107 of the light guiding projection and defining an input width Wi, and a base 108 positioned at a proximal end 109 of the light guiding projection opposite distal end 107 and defining an output width Wo. A length Lt of the light guiding projections is defined between input facet 106 and base 108.

[0074] In addition to input facet 106 and base 108, the plurality of light guiding projections 104 include side edges 110, 112 extending between the proximal and distal ends of each light guiding projection of the plurality of light guiding projections. Side edges 110, 112 can be linear side edges, as shown in FIG. 5, or side edges 110, 112 can be curved, as shown in FIG. 6. In the instance of linear side edges, the linear side edges can be of the same length and angled relative to a normal to input facet 106 by an angle Q. In the instance where the side edges are curved, angle Q is defined between a line extending through the end points of a side edge at the input facet and the base (see FIG. 6) and the normal to input facet 106. Angle Q can be in a range from about 1 degree to about 7 degrees. In various embodiments angle Q between a side edge and a normal to input facet 106 is the same for both sides edges of a light guiding projection and each input facet is centered relative to the respective base (such that a median line 113 normal to and extending through a middle of input facet 106 also passes through the middle of base 108 and light guiding projection 104 is symmetric about median line 113). The light guiding projections 104 may be arranged periodically along a length of the light guiding device 100 with a period P defined between a distal end of one side edge of a first light guiding projection and the distal end of a corresponding side edge of an adjacent light guiding projection, as shown in FIG. 3. The period P can be in a range from about 5 mm to about 100 mm.

[0075] In some embodiments, such as depicted in FIG. 7, thickness tf of first section 102 can decrease in a direction from input facet 106 toward base 108 such the thickness of a light guiding projection 104 at input facet 106 is greater than the thickness of the light guiding projection at base 108. In certain embodiments, a thickness of second section 114 can be generally equal to the thickness of LGP 26. Because a thickness of a commercially available LED may be greater than a thickness of the light coupling device 100 at input facet 106, an increased thickness dimension at the input facet can provide improved light coupling with such large LEDs.

[0076] Light coupling device 100 further comprises a second section 114, best seen in FIG.

4, including a curved portion 116 comprising a radius of curvature R. In various embodiments, R can be equal to or greater than about 2 mm, for example in a range from about 2 mm to about 20 mm. Curved portion 116 can be parabolic, elliptical, circular, or the like. In various embodiments, curved portion 116 can comprise a 180 degree bend, although in further embodiments other angular curves are possible, such as in a range from about 90 degrees to about 180 degrees.

[0077] Second section 114 can be optically coupled with first section 102 such that the plurality of light guiding projections are optically coupled to the curved portion 116. A used herein, optically coupled is intended to mean that a first component is arranged such that light propagating in or emitted from the first component is injected into the second component.

The first component and the second component may be spaced apart and need not be in direct contact. In other embodiments, the first component and the second component may be optically coupled through an intermediate component, for example an index-matching gel, although in further embodiments the first component and the second component can be rigidly optically coupled such as with an appropriate adhesive (e.g., an index-matching epoxy). In some embodiments, second section 114 and first section 102 can form one monolithic member formed, for example, by injection molding so that the first section 102 is one with second section 114. However, in other embodiments, first section 102 may be separately formed and joined to second section 114, for example with a suitable adhesive, such that light guiding device 100 is an assembly of parts. Second section 114 may further comprise an output facet 120 that forms a light output surface from curved portion 116.

[0078] FIG. 8 is an edge view of a light coupling device 100 as shown in FIG. 7 optically coupled to an LGP 26. In accordance with FIG. 8, output facet 120 of second section 114 (e.g., curved portion 116) is optically coupled to edge surface 34a of LGP 26 at interface 122. For example, output facet 120 can be optically coupled to edge surface 34a using an index- matching fluid such as an index-matching gel. However, in some embodiments, output facet 120 may be optically coupled to edge surface 34a using an optical adhesive (e.g., an adhesive material, such as an epoxy, with an index of refraction approximately equal to the index of refraction of LGP 26). Curved portion 116 of second section 114 forms a transition that optically couples first section 102, which in the embodiment illustrated in FIG. 8 extends through an angle of about 180 degrees, with LGP 26 that extends generally parallel with first section 102. In addition, at least one LED 124 is optically coupled to an input facet of each light guiding projection 104 projecting from first section 102. The input thickness of light guiding projections 104 should be equal to or greater than a thickness of LEDs (LED). On the other hand, the thickness to of second section 114 should be equal to or less than the thickness tLGP of LGP 26.

[0079] While in some embodiments a single light coupling device 100 can be optically coupled to an edge surface of LGP 26 to provide 1D dimming, as depicted in FIG. 9, in other embodiments a second light coupling device can be optically coupled to another edge surface of LGP 26 to provide 2D dimming to LGP 26. For example, FIG. 10 illustrates an exemplary LGP 26 comprising a first light coupling device lOOa optically coupled to a first edge surface, e.g., edge surface 34a, and a second light coupling device lOOb optically coupled to an adjacent edge surface, e.g., edge surface 34b.

Examples

[0080] Three cases were simulated using ray tracing computing methods. In all three cases, at least one LED is optically coupled to at least one light guiding projection in the first section of the light coupling device. The first two cases include a LGP with 1D local dimming by using one light coupling device. The third case is an example of 2D local dimming with one LED being switched on for each of the two light coupling devices. For 1D local dimming cases, one LED is switched on in the first case, and seven LEDs of seven light guiding projections are switched on in the second case.

[0081] Case 1

[0082] In one example a single LED with a thickness of LED = 1.0 mm and a length of 3.6 mm is coupled to a light guiding projection in first section 102 of a light coupling device 100. The LED output is a Lambertian angular distribution. A gap between the LED and the input facet of the light coupling device is 0.1 mm. A thickness of the light guiding projections is 1.1 mm and uniform. A thickness of the LGP is 1.1 mm. The width Wi of each input facet 106 is 4 mm. The refractive index of the light coupling device is 1.5, which is the same as that of the LGP. The length x width of the LGP is 600 mm x 600 mm. Bend radius R of curved portion 116 is 3.85 mm.

[0083] FIG. 11 depicts the modeled light distribution across an LGP when one LED optically coupled to one (center) light guiding projection of a light coupling device 100 is switched on. The bend radius R of the curved portion of the light coupling device is 3.85 mm. In the present configuration, 87% of the total LED light output is coupled into the LGP, and 94% of the light coupled into the LGP is confined within a 150 mm width zone at a distance of 500 mm from the LGP input edge. [0084] FIG. 12 is a graph showing the efficiency of light coupling into the LGP of FIG. 11, and efficiency of light confinement in both 100 mm and 150 mm wide zones at a distance of 500 mm from the input LGP edge surface (e.g., edge surface 34a) as a function of bend radius R of curved portion 116. Angle Q of the side edges of the light guiding projections is assumed to be 3 degrees, and the length Lt and input width Wi of the light guiding projections are 170 mm and 4 mm, respectively. Here, for light coupled into the LGP, the coupling efficiency is defined as the ratio of the total light power coupled into the LGP to the total output power of the LED. More particularly, for light confined in 100 mm, or 150 mm, width zones, the efficiency is defined as the ratio of the total light power confined in 100 mm or 150 mm width zones at a distance of 500 mm from the input LGP edge to the total output power of the LED. The data show that coupling efficiency equal to or greater than about 80% can be achieved when bend radius R is equal to or greater than about 2.7 mm, for example in a range from about 2.7 mm to about 8.2 mm, such as in a range from about 4 mm to about 8.2 mm, such as in a range from about 5 mm to about 8.2 mm, including all ranges and sub-ranges therebetween.

[0085] FIG. 13 plots the ratio of light power confined in 100 mm and 150 mm wide zones centered about the lighted LED at the input edge of the LGP at a distance of 500 mm from the LGP input edge to the total light coupled into the LGP as a function of bend radius R.

The modeling parameters are the same as for FIG. 11. FIG. 13 indicates that good local dimming performance, that is, the amount of light retained within the width zone as a percentage of the total light injected into the LGP by the single LED, in this instance greater than 75%, can be achieved over a range of bend radii between about 1.2 mm and about 8.2 mm for both 100 mm and 150 mm width (confinement) zones.

[0086] FIG. 14 shows a contour map of light confined within a 150 mm wide zone at a 500 mm distance from the LGP input edge surface (normalized to the total light power coupled into the LGP and expressed as a percent) as a function of angle Q and light guiding projection length L_t. As shown, greater than about 90% light confinement can be achieved with angle Q in a range from about 1.5 degrees to about 6.2 degrees and a light guiding projection length L_t equal to or greater than 100 mm.

[0087] FIG. 15 shows a contour map of output width W₀ as a function of angle Q and light guiding projection length L_t when the input width Wi is 4 mm. This chart is useful for design of the light guiding projections. For example, if the input width Wi is 4 mm and the period of the light guiding projections is 20 mm, the output width W₀ should be equal to or less than 20 mm, and the angle Q and Lt can be determined for a given percentage light confinement by considering FIGS. 14 and 15.

[0088] Case 2

[0089] In another example illustrated in FIG. 16, light distribution across a LGP when 1D local dimming was modeled with seven LEDs switched on is depicted. The LED and LGP parameters are the same as the preceding example (Case 1). The gap between the LEDs and the input facet of the light guiding projections is 0.1 mm. The thickness of the coupling device is 1.1 mm and uniform, which is the same as the thickness of the LGP. The input width Wi of each light guiding projection is 4 mm. The refractive index of the coupling device is 1.5, which is the same as that of the LGP.

[0090] FIG. 17 shows the efficiency of light coupled into the LGP of FIG. 16, and light confined within a 150 mm wide zone at distances of 350 mm and 500 mm from the LGP input edge surface (e.g., edge surface 34a) as a function of bend radius R of curved portion 116. Angle Q is assumed to be 2 degrees, and the input width Wi and output width W₀ are 4 mm and 21.4 mm, respectively. As shown, the efficiency with which light can be coupled into the LGP can be equal to or greater than about 80% when bend radius R is equal to or greater than about 2.7 mm, for example in a range from about 2.7 mm to about 9.6 mm, such as in a range from about 4 mm to about 9.6 mm, such as in a range from about 5 mm to about 9.6 mm, such as in a range from about 2.7 mm to about 8.2 mm, such as in a range from about 4 mm to about 8.2 mm, such as in a range from about 5 mm to about 8.2 mm including all ranges an sub-ranges therebetween.

[0091] FIG. 18 plots the ratio of light power within the 150 mm wide zone at distances of 350 mm and 500 mm from the input LGP edge surface (e.g., edge surface 34a) to the total light coupled into the LGP as a function of bend radius R of curved portion 116. The data indicate that good local (1D) dimming performance can be achieved for a bend radius in a range from about 1.2 mm to about 8.9 mm. The modeling parameters are the same as for FIG. 16.

[0092] FIG. 19 is a contour map of light confined within a 150 mm wide zone at a distance of 350 mm from the LGP input edge surface (normalized to the total light power coupled into the input edge of the LGP) as a function of angle Q and input width Wi. As shown, light confinement equal to or greater than about 80% can be achieved when angle Q is in a range from about 1.0 to about 7.2 degrees and the input width Wi is equal to or less than about 6 mm. Again, the modeling parameters for FIG. 19 are the same as for FIG. 16. [0093] FIG. 20 is a contour map of the length Lt as a function of angle Q and input width Wi when the output width W₀ of the trimmed waveguide is 21.4 mm. The modeling parameters are the same as for FIG. 16.

[0094] FIG. 21 shows a contour map of light confined in a 150 mm width zone at 500 mm distance from LGP input edge (normalized to the total coupled light power) as a function of the length of the light guiding projections and angle Q.

[0095] Case 3

[0096] FIG. 22 depicts light distribution in a LGP when two light coupling devices 100 (e.g., see FIG. 10) are used to couple light from each light coupling device 100 into two adjacent and orthogonal LGP edges. The LED and LGP parameters are the same as in the first example (Case 1). The gap between LEDs and the input facets of the light guiding projections for both light coupling devices is 0.1 mm. The thickness of the light coupling devices is 1.1 mm and uniform, which is the same as the thickness of the LGP. The input width Wi of each light guiding projection input facet for both light coupling devices is 4 mm. The refractive index of the coupling devices is 1.5, which is the same as that of the LGP. Only a single LED is lighted for each light coupling device. The bend radius R of the curved portion 116, angle Q of the straight side edges, and the length Lt and input width Wi of the light guiding projections of the coupling device at the LGP left edge are R = 4 mm, 0 = 3° degree, Lt = 170 mm, and Wi = 4 mm. The light coupling device at the LGP bottom edge differs from the light coupling device at the left edge in that R = 2.75 mm. It is clearly seen that 2D dimming functionality can be enabled by using two light coupling devices according to embodiments described herein.

[0097] 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. For example, light couplers described herein can be utilized in a variety of light guide plate designs to form a backlight unit. It should be further understood that such coupler-configured backlight units can be incorporated into various LCD devices including without limitation, computer displays, hand-held device displays (e.g., cell phones, tablets), television displays, and signage displays. 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 display device, comprising:

a display panel and a backlight unit positioned adjacent the display panel, the backlight unit comprising:

a light guide plate;

a light coupling device comprising a curved portion optically coupled to a first edge of the light guide plate and a plurality of light guiding projections optically coupled to the curved portion, each light guiding projection of the plurality of light guiding projections comprising an input facet and a light emitting diodes optically coupled thereto.

2. The display device according to claim 1, wherein the curved portion extends through an angle in a range from about 90 degrees to about 180 degrees.

3. The display device according to claim 1, wherein a radius of curvature R of the curved portion is equal to or greater than about 2 mm.

4. The display device according to claim 3, wherein R is in a range from about 2 mm to about 20 mm.

5. The display device according to claim 1, wherein the plurality of light guiding projections are arranged periodically along a length of the curved portion.

6. The display device according to claim 1, wherein each input facet intersects with two opposing linear side edges of the respective light guiding projection.

7. The display device according to claim 6, wherein each side edge subtends an angle in a range from about 1 degree to about 7 degrees relative to a normal to the respective input facet.

8. The display device according to claim 1, wherein each input facet intersects with two opposing curved side edges of the respective light guiding projection.

9. The display device according to claim 1, wherein the light coupling device is a first light coupling device, the backlight unit further comprising a second light coupling device optically coupled to a second edge of the light guide plate orthogonal to the first edge.

10. The display device according to claim 1, wherein the light guide plate comprises glass.

11. A backlight unit, comprising:

a light guide plate;

a light coupling device comprising a curved portion optically coupled to a first edge of the light guide plate, and a plurality of light guiding projections optically coupled to the curved portion, each light guiding projection including an input facet positioned at a distal end of each light guiding projection; and

a plurality of light emitting diodes optically coupled to the respective input facets of the light guiding projections.

12. The backlight unit according to claim 11, wherein the curved portion extends through an angle in a range from about 90 degrees to about 180 degrees.

13. The backlight unit according to claim 11, wherein a radius of curvature R of the curved portion is equal to or greater than about 2 mm.

14. The backlight unit according to claim 13, wherein R is in a range from about 2 mm to about 20 mm.

15. The backlight unit according to claim 11, wherein the plurality of light guiding projections are arranged periodically along a length of the curved portion.

16. The backlight unit according to claim 11, wherein each input facet intersects with opposing side edges of the respective light guiding projection.

17. The backlight unit according to claim 16, wherein each side edge subtends an angle in a range from about 1 degree to about 7 degrees relative to a normal to the respective input facet.

18. The backlight unit according to claim 11, wherein the light coupling device is a first light coupling device, the backlight unit further comprising a second light coupling device optically coupled to a second edge of the light guide plate orthogonal to the first edge.

19. A light coupling device, comprising:

a first section comprising a plurality of light guiding projections, each light guiding projection including an input facet positioned at a distal end of each light guiding projection; and

a second section including a curved portion optically coupled to the plurality of light guiding projections.

20. The light coupling device according to claim 19, wherein a radius of curvature of the curved portion is in a range from about 2 mm to about 20 mm.

21. The light coupling device according to claim 19, wherein the plurality of light guiding projections are arranged along a length of the light coupling device with a period in a range from about 5 mm to about 100 mm.

22. The light coupling device according to claim 19, wherein the curved portion extends through an angle in a range from about 90 degrees to about 180 degrees.

23. The light coupling device according to claim 19, wherein each light guiding projection of the plurality of light guiding projections comprises side edges extending between a proximal end and the distal end of the light guiding projection, and wherein an angle Q subtended by each side edge relative to a normal to the input facet is in a range from about 1 degree to 7 degrees.