LED BACKLIGHTING FOR DISPLAYS

INVENTOR

Ban P. LOH

BACKGROUND

[0001] The present invention relates to the field of display devices, and more particularly to lighting of flat panel display devices such as liquid crystal display (LCD) devices.

[0002] LCD devices are widely used in flat panel -displays for computer and display monitors, televisions, bill boards, road signs, and other applications. In some applications, for example as a computer display for notebook computers, LCD devices are desirable because they are much thinner and lighter than alternatives such as, for example, CRT (cathode ray tube) displays.

[0003] An LCD panel is generally (but not necessarily) configured as a thin rectangular panel having a first major surface (its "front") and a second major surface (its "back ^" ). As a rectangular panel, it has four edges, or sides, defining the boundaries of the LCD panel. The LCD panel includes a number of sub-layers such as a liquid crystal, layer and a plurality of optical filter layers. An LCD screen can consist of thousands or millions of pixels covering the screen; manipulation of these pixels, in combination of a lighting subsystem, results in the displaying effect.

[0004] As is known to those skilled in the art. an LCD does not emit light; rather, the displaying effect is produce by control of light from a side or backside lighting subsystem that provides light to the LCD panel. Thus, the quality of the displayed image depends on, in part, the quality and the wavelengths of light from the lighting subsystem (typically on lhe backside of the LCD panel).

[0005] A common backlit LCD device includes an LCD panel with one or more fluorescent tubes running proximal to one or more edges of the LCD panel. Light pipes guide light from the fluorescent tubes or any light source to the back surface of the LCD panel. Thus, the LCD panel is backlit allowing a viewer to more easily view, from the front surface, the information on the LCD panel. Unfortunately, such edge lighting system is

inefficient with up to 50% or more of the light being lost See, for example, U.S. Patent No. 6,672,734 B2 ₅ U.S. Patent No. 6,768,525 B2, and U.S. Patent No. 6,844,903 B2.

[0006] In an alternative design, an array of fluorescent tubes are placed neaT the back surface of the LCD panel allowing direct illumination of the back surface of the LCD panel thereby eliminating the need for the light pipes and increasing the efficiency of the lighting system. However, fluorescent light does not deliver all the primarily colors - Red, Green and Blue - which can then be combined to yield millions of secondary colors for the display. That is. the images displayed are not truly natural and brilliant as desired for some applications.

[0007] Consequently, there remains a need for an improved system for illuminating flat panel displays that overcomes or alleviates the shortcomings of the prior art devices. Semiconductor light emitting devices, such as Light Emitting Diode (LED) devices, also may be used for edge illumination of a planar array of LCD devices. For example, U.S. Patent Application Serial No. 10/898.608, filed July 23. 2004. entitled Reflective Optical Elements for Semiconductor Light Emitting Devices _> to co-inventor Negley, and assigned to the assignee of the present invention., the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein, describes side emission . LEDs that may be used for large area LCD and/or television backlighting.

SUMMARY

[0008] The need is met by the present invention. In one embodiment of the. present invention, a display system includes a display ^; panel having a first major surface and a second major surface and a lighting system adapted to provide light proximal to the second major surface of tile display- panel. The lighting system includes an illuminating substrate defining an array of mounting holes and an array of light emitting diode (LED) modules, each module occupying a mounting hole of the array of mounting holes.

[0009] The illuminating substrate includes a substrate core having a first major surface and a second major surface opposite the first major surface and a light reflective layer covering at least a portion of the first major surface of the substrate core. The light reflective layer includes at least one member from a group consisting of silver, gold, aluminum, and reflective paint or coating, for example. ^' Barium Sulfate or Titanium Dioxide paint. The

light reflective layer can also incorporate optical gratings or textures of its surface to scatter light rays that arc reflected from it. The illmiώxating substrate further includes circuit elements on the second major surface of the substrate core.

[0010] The display system may also include a diffusant layer between the display panel and the illuminating substrate. The diffusant layer receives light emitted by individual emitters of the LED modules and distributes il uniformly with minimal loss. It is optically coupled with the illuminating substrate via optical gel to minimize losses of light at the interface. Outer edges of the diffusant layer are made to a bevel angle with specular surface finishes to minimize light loss from the edges. The bevel angle can be close to critical refractive angle of the medium to promote light rays being reflected by TIR (Total Internal Refraction). Alternatively, or in combination, the edges can have reflective coating such as Barium Sulfate or Titanium Oxide paint to prevent light rays escaping through the edges

[0011] Each of the LED modules includes one or more LED chips and conductive traces connected to the LED chips. The circuit elements are adapted to connect to the conductive traces of the LED module. The LED module may also include heat sink for improved heat dispersion.

[0012] The display panel, depending on technology and implementation, can include a plurality of sub-layers such as, for example, a first filter sub-layer, a thin film transistor sub-layer, a liquid crystal sub-layer, and a second filter sub-layer.

[0013J Other aspects and advantages of the present invention will become apparent from the following detailed description., taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure IA is an exploded perspective view of an apparatus according to one embodiment of the present invention;

[0015] Figure IB is a portion of the view of Figure 1 in greater detail; [0016] Figure 2A is view of the apparatus of Figure 1 from front of the apparatus; [0017] Figure 2B is view of the apparatus of Figure 1 from rear of the apparatus;

[0018] Figure 2C is a cutaway side view of the apparatus of Figures 2A and 2B cut along line

2C—2C of Figures 2A and 2B;

[0019] Figure 2D is a more detailed view of a portion of the cutaway side view of figure 2C; [0020] Figure 2E is a simplified diagram of another portion of the display system 100 of Figures

2A through 2D; and [0023] Figure 3 is a perspective view of a LED module of Figures I A through 2D.

DETAILED DESCRIPTION

[0022] The present invention will now be described with reference to the Figures IA through 4. which illustrate various embodiments of the present invention. As illustrated in the Figures, some sizes of structures or portions are exaggerated relative to other structures or portions for illustrative purposes and. thus, are provided to illustrate the general structures of the present invention. Furthermore, various aspects of the present invention are described with reference lo a structure or a portion being formed on oilier structures; portions, or both. As will be appreciated by those of skill in the art. references to a structure being formed "on ^" or "above" another structure or portion contemplates that additional structure, portion, or both may intervene. References to a structure or a portion being formed "on" another structure or portion without an intervening structure or portion are described herein as being formed "directly on" the structure or portion. [0023] Furthermore, relative terms such as "on" or "above" are used herein to describe one structure's or portion's relationship to another structure or portion as illustrated in the Figures. It will be understood that relative terms such as "on" or "above" are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. For example, if the device in the Figures is turned over, structure or portion described as "above" other structures or portions would now be oriented "below" the other structures or portions. Likewise, if the device in the Figures is rotated along an axis. structure ^~ or portion described ^" as "abσve" ^~ όthef structures ^" όTp ^" ortions ^' wdϋrdlϊδw ^" l5e ^" oriented "next to" or "left of the other structures or portions. Like numbers refer to like elements throughout.

[0024] As shown in the figures for the purposes of illustration, embodiments of the present invention are exemplified by a display system includes a display panel having a first

major surface and a second major surface and a lighting system adapted to provide light proximal to the second major surface of the display panel. The lighting system includes an illuminating substrate defining an array of mounting holes and an array of light emitting diode (LED) modules, each module occupying a mounting hole of the array of mounting holes. The lighting system can also include a diffusant layer which is placed between the display panel and the illuminating substrate to achieve high luminous efficiency and uniform luminance.

[0025] In the present invention, LEDs (light emitting diodes) of the primary colors (Red, Green and Blue) are used to illuminate a display panel with a complete spectrum of colors by varying the electrical currents that drive the LED's. Inside the lighting system, an optical gel (rather than an air gap) couples the illuminating substrate to the diffusant layer to minimize light loss at the interface. Further, the illuminating substrate incorporates a highly reflective, grated or textured surface facing the diffusant layer to diffuse and reflect light rays to achieve high luminous efficiency and a uniform luminance. In addition, the edges of the diffusant layer are beveled at an angle and made to specular finish or coated with reflective material such that light rays that hit the edge surface are less likely to escape from the medium of the- diffusant layer but is more likely to be reflected and directed towards the front side of the diffusant layer.

Details

[0026J Figure I A is a partially exploded perspective view of a display system 100 according to one embodiment of the present invention. Figure I B is a portion 102 of the display system 100 in greater detail. The display system 100 is, for example, a flat-panel computer monitor. Referring to Figures IA and IB ₅ the display system 100 has a first major surface 101 (front) and a second major surface 103 (rear). The display system 100 is viewed from its front 101.

[0027] ^" Figτπ-e ^" 2A ^~ is ^" £rvtew ^~ όf1tø ^" displa ^" S' systeiir lOO frorrrits froiϊf lOl ^~ Figure-2Bis ^~ a ^~ viewof the display system 100 from its rear 103. Figure 2C is a cutaway side view of the display system 100 cut along line 2C — 2C of Figures 2A and 2B. Figure 2D is a portion 104 of the cutaway side view of the display system 100 as illustrated in Figure 2D in greater detail.

[0028] Referring to Figures IA through 2D, the display system 100 includes a lighting system 110 adapted to provide light for a display panel 140. In the illustrated embodiment, a diffusant layer 130 which is a component of lighting system is placed between the illuminating substrate 110 and the display panel 140 to diffuse the light from the illuminating substrate 110 before the light reaches the display panel 140. The display panel 140 includes a plurality of sub-layers to polarized, filter, and otherwise operates on the diffused light whereby a desired pattern is lit and discemable from the front 101 of the display system 100.

[0029] The display system 100 can have a wide range of lateral dimensions 106, 108 depending on application and desired characteristics of the display system 100. For example, the lateral dimensions 106, 108 can be in the order of millimeters or in the order of meters or even beyond. .

Lighting System

[0030] Continuing to refer to Figures IA through 2D, the lighting system 1 10 includes an illuminating substrate 1 12. The illuminating substrate 112 has a first major surface 111 and a second major surface 113 and defines an array of mounting holes 114. To avoid clutter only two of eleven mounting holes 114 illustrated in Figure 1 B are pointed to with reference numeral 114. An array of light emitting diode (LED) modules 150 occupy the mounting holes 1 14, one LED module 150 per mounting hole 114. That is. the array of LED modules 150 is affixed to the illuminating substrate 112. Again, for clarity of discussion, only one LED module 150 is illustrated in Figure IB thus more clearly illustrating the mounting holes 1 14: however, in actual application, each mounting hole 1 14 is occupied by an LED module 150. In Figure IB. the LED module 150 is illustrated "floating ^" under the lighting system 110 to avoid obscuring the illustrations of the mounting holes 1 14.

Illuminating Substrate

[0031] The illuminating substrate 112 includes a substrate core 118 which is, for example, an insulating layer of printed circuit board (PCB). The substrate core 118 has a first major surface 1 17 and a second major surface 119 opposite the first major surface 117. A light

reflective layer 120 covers a major portion of the first major surface 117 of the substrate coτe 118 forming an optically reflective surface 111 as the first major surface 11 1 of the illuminating substrate 112. The light reflective layer 120 can be realized by application of optically reflective material such as, for example, metal silver (Ag), gold (Au), anodized aluminum (Al) ₅ or reflective material such as Barium Sulfate. Titanium Dioxide paint. The reflective surface 11 ] can be polished to a specular finish. Alternatively, the reflective surface 111 can incorporate diffusing grating to promote dispersion and diffusion of light reflecting on the reflective surface 111.

[0032] The illuminating substrate 112 includes circuit elements 124 on the second major surface 119 of the substrate core 11 8. The circuit elements 124 can include, for example, electrically conductive traces for delivery of electrical power, ground,, and control signals to the LED modules 150. The circuit elements 124 can be made of copper or other electrically conductive material.

[0033] In the illustrated embodiment, the substrate core 1 18, the reflective layer 120. and the circuit elements 124 have thickness in the order of a few microns to a few millimeters depending on application. Here, the circuit elements 124 can be connection traces having thickness in the order of several microns. Neither the reflective layer 120 nor the circuit elements 124 are allowed to cover-up the mounting holes 114 thus allowing mounting of the LED modules 1 50 through the illuminating substrate 112.

[0034] The illuminating substrate 112 defines the array of mounting holes 114. The size of the array (thus the. number of the mounting holes.114), the shape and the dimensions of each mounting hole 1 14. and the spacing distance 115 between the mounting holes 114 depend on the desired application and properties of the lighting system 100. In the illustrated example embodiment each mounting hole 114 is generally circular in shape with diameter ranging in the order of a few to several millimeters and is placed in a predetermined, pattern. The shape, the size, and the spacing of the mounting hole 114 can vary widely depending on application. For example, the mounting holes 114 may have other shapes (rectangular, elliptical, or other suitable shapes), sizes ranging in the order of millimeters or less to meters or more. The spacing of the mounting holes 114 are determined by a plurality of factors such as, for example only, size of the mounting holes 1 14, heat dissipation requirements, and desired luminous intensity and total luminous

flux. In the illustrated embodiment, the mounting holes 114 are arranged within several millimeters from each other. Much, if not the entire lighting system 110 may be populated with these mounting holes 114.

LED modules

[0035] Each mounting hole 114 is occupied by the LED module 150. Figure 3 is a perspective view of one embodiment of the LED module 150. Figure.2D illustrates, inter alia, a cutaway side view of the LED module 150 of Figure 3. Referring to Figures 2D and 3, the LED module 150 includes a LED module body 152, a reflector cup portion 154, and a heat sink portion 156. The module body 152 provides a supporting substrate for one or a cluster of LED chips 160 which may be of different colors, for example, red, green and blue, attached to the module body 152. For convenience only one LED chip 160 is illustrated. The LED chip 160 represents one or more LED chips.

[0036] A reflecior cup portion 154 of the LED module 150 surrounds and defines a cavity 155 within which the LED chip 160 is mounted to the module body 152. The reflector cup portion 154 defines an outer shape and outer dimensions that are substantially similar to the shape and the dimensions of the mounting holes 114. In the illustrated embodiment, the ouier shape of reflector cup portion 154 is generally circular, and its diameter 157 is in the order of millimeters.

[0037] Electrically conductive traces 158 are provided on the module body 152 to allow for electrical connection of the LED chip 160 to the circuit elements 124 of the illuminating substrate 112 when the LED module 150 is mounted at a mounting hole 114. When the LED module 150 is mounted at a particular mounting hole 114, one or more conductive traces 158 of the LED module 150 connect with the circuit elements 124 of the illuminating substrate 1 12. The conductive traces 158 are connected to the bottom side LED chip 160, top side of the LED chip 160 via bond wire 162, or both. The LED chip 160-can_ be._for example only,-. from chemicals GaN. for. blue, and gr.een_lights and. AlGaAsP for Red.

[0038] After the placement of the LED chip 160, the bond wire 162. oi both, the cavity 155 can be filled with encapsulanl material such as, for example, soft thermo-set plastics such as, for example, silicone, epoxy, or a combination of these, ^' The encapsulant material can

include additional material such as, for example, wavelength shifting material such as phosphors to absorb light having a first wavelength and reemit the light at a second wavelength. Further, the encapsulant material can include material to diffuse the light from the LED chip 160 for example, glass, titanium dioxide, barium sulfate, calcium carbonate, and other suitable material.

[0039] The LED modules 150 are mounted in the mounting holes 114 to allow light to launch, toward the diffusant layer 130, and eventually to the display panel 140. The LED modules 150 can be mounted in the mounting holes 114 using solders, or any electrically conductive adhesive, such as, for example, Ag-epoxy.

Heat Sink

[0040] When the LED chip 160 is activated, light and heat are generated by the LED chip 160. The heat spreads throughout the LED module 150. Much, of the heat is dissipated by the heat sink 156 which can be an integral portion of, or attached to the module body 152 to increase the thermal dissipation capacity of the LED module 150 to an extend that no additional heatsink is needed to dissipate heat generated by LEDs.

Diffusant Layer

[0041] Referring again to Figures IA through 2D, in the illustrated embodiment, the diffusant layer 130, a component of lighting system is placed between the illuminating substrate 1 12 and the display panel 140 and though this is not necessary in all embodiments. The diffusant layer 130 can be made of transparent material such as glass or transparent plastics such as polycarbonate or Polymethylmethacrylate (PMMA) that is filled with small particles of light reflective material, for example only. Barium Sulfate, Tatinum Dioxide, or other suitable material. When an LED module 150 emits light, its light rays cover only a smaller area of the illuminating substrate. After passing through the diffusant layer,- the light rays will cover a much-larger area, hence- delivering a uniform luminance on the front face of the diffusant layer for illuminating a display panel . In additional, phosphors may be added in the diffusant layer to absorb certain wavelengths of light and re-emit oϊher wavelengths.

[0042] Figure 2E is a simplified diagram of another portion of the display system 100 of Figures 2A through 2D. Referring to Figure 2E, the LED module 150 emits light in a wide range of angles relative to the normal axis of second major surface (back) 143 of the display panel 140. A first portion of the emitted light illustrated by ray 131 is emitted at a first angle 133 relative to normal (perpendicular) to the back 143 of the display panel 140 such that the light 131 is transmitted directly through the diffusant layer 130 into and beyond the display panel 140. This is when the first angle 133. is less than the THR (Total Internal Refraction) angle presented by the back surface 143 of the display panel 140. [0043] A second portion of the emitted light illustrated by ray 134 is emitted at a second angle 135 relative to the back 143 of the display panel 140. The second angle 135 can be, in the present example, greater than the TIR (Total Internal Refraction) angle presented by the back surface 143 of the display panel 140. In the illustrated embodiment, the second portion 134 of the emitted light reflects off of a diffusant 137, then off the optically reflective surface 1 11 toward the back surface 1.43 of the display panel 140 at a third angle 138 less than the TIR angle. If, for example, the diffusant 137 were not present, the light ray 134 would have incident on the back surface 134 at an angle greater than the critical angle, hence totally reflected from it; subsequently, ray 134 may be trapped between the two surfaces and its luminous intensity be reduced by a small percentage at each reflection and weaker ray 134 may eventually exit from the medium. As illustrated, the diffusant 137 reduces loss of light. Further, the diffusant 137 allows light from an LED module to be scattered and spread out thus covering a larger area. This has the effect of mixing light from multiple LED modules. Accordingly, more uniform light is provided to the LED panel 140.

[0044] Consequently, light from a plurality of LED modules 150 are mixed in the diffusant layer

13.0 as it interacts with the light scattering particles and layer interfaces. Light reflected back toward the illuminating substrate 1 12 is reflected back toward the display panel 140 by -the -reflective- surface 111 of the illuminating- substrate 11-2, hence conserving the

' photons emitted by the light sources.

[0045] Referring to Figures 2A through 2E, the diffusant layer 130 is placed in direct contact with the front face, of illuminating substrate 1 12. Alternatively, as illustrated in the Figures, the diffusant layer 130 can be Optically coupled with the lighting system 110 by

optical gel 132 which can be Nye's soft silicone gel - which may be filled with phosphors or a diffusants. The optical gel 132 can have refractive index that matches the refractive index of the diffusion layer to minimize interface loss.

[0046] To minimize loss of light from the side edges of the diffusant layer 130, the edges of the diffusant layer 130 may be made to specular finished and beveled at an angle such that light rays that hit at the edges are reflected at TTR (Total Internal Reflection) angle. Alternatively, the surface 136 of the fedges of the diffusant layer 130 can include a reflective coating 136 realized by application of optically reflective material such as, for example, silver (Ag), gold (Au), aluminum (Al), or any other reflective material such as Barium Sulfate, Titanium Dioxide.

Display Panel

[0047] Referring again to Figures IA through 2D, in the illustrated embodiment, the display panel 140 has a first major surface 141 first major surface (its "front") and a second major surface 143 (its "back"). The display panel 140- is a controllable and partially translucent display panel known in the art. For example, the display panel 140 can be a TFT (Thin Film Transistor) LCD (Liquid Crystal Display) type panel designed to work with backlight provided by the lighting system 110. The display panel 140, as a TFT LCD panel, includes a number of sub-layers as illustrated in Figure 4. Referring to Figures IA. IB, and 4. the display panel 140 includes a first polarizing filter sub-layer 140a. a thin film transistor sub-layer 140b. a liquid crystal sub-layer 140c, a color filter sub-layer 14Od. and a second polarizing filter sub-layer 140e. Design and operations of these sub-layers and the TFT LCD panel 140 is known to those skilled in the art.

[0048] The lighting system 110 provides light proximal to the second major surface 143 of the display panel 140. The diffused light is provided to the display panel 140. The sublayers of the display panel 140 are connected to, powered by, and controlled with various circuits (hot illustrated) outside the display system 100 to operate on the. light entering the display panel 140 selectively allowing varying portions of the diffused light through the display panel 140. The result is that a desirable, viewable pattern or other information, appears on the surface 101 of the- display system 100.

[0049] The display system 100 is applicable in a wide range of application such as, fox example only, displays of instruments and watches, computer monitors, television, bill boards, road signs, and commercial advertisement displays.

Conclusion

[0050J From the foregoing, it will be apparent that the present invention is novel and offers advantages over the current art. Although specific embodiments of the invention are described and illustrated above, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. For example, differing configurations, sizes, or materials may be used to practice the present invention. The invention is limited by the claims that follow.