PATENT DOCUMENT

Publication Number: US-10067283-B2
Application Number: US-201514848721-A
Country: US
Kind Code: B2

Title: Display backlight with patterned backlight extraction ridges

Abstract:
A display may have a backlight unit with a row of light-emitting diodes that emit light into the edge of a light guide plate. The light guide plate may have opposing upper and lower surfaces. Backlight may be extracted from the light guide plate using an array of bumps on the lower surface and ridges on the upper surface. Ridge density may vary as a function of location across the display. Some of the ridges may be terminated along a meandering border between regions of differing ridge density. Ridge length and endpoint location can be dithered along borders between regions and ridge widths and thicknesses may be tapered down towards the endpoints. Ridges may be patterned to reduce the density of the ridges immediately adjacent the light-emitting diodes and thereby avoid over-extraction of the light at the light-emitting diodes.

Claims:
What is claimed is: 
     
       1. A display, comprising:
 display layers that form an array of pixels; and 
 a backlight that illuminates the array of pixels, wherein the backlight includes a light guide plate having ridges, wherein the ridges have different densities in different portions of the light guide plate, wherein the light guide plate has an edge, wherein the backlight includes an array of light-emitting diodes along the edge that emit light into the light guide plate through the edge, wherein each ridge has a respective length and extends along a longitudinal axis perpendicular to the edge, wherein each ridge has a uniform cross-section along the respective length, and wherein the number of ridges per unit area is selectively decreased adjacent to the light-emitting diodes. 
 
     
     
       2. The display defined in  claim 1  wherein the light guide plate has at least a first region in which the ridges have a first density and a second region in which the ridges have a second density that is greater than the first density, wherein the first density comprises a first number of ridges per unit area, and wherein the second density comprises a second number of ridges per unit area. 
     
     
       3. The display defined in  claim 2  wherein the first and second regions are separated by a border and wherein the ridges have end points with positions that are dithered about the border. 
     
     
       4. The display defined in  claim 2  wherein the first and second regions are separated by a border, wherein at least some of the ridges have endpoints at the border, and wherein at least some of the ridges with endpoints at the border have thicknesses that taper down towards the endpoints. 
     
     
       5. The display defined in  claim 2 , wherein the first and second regions are separated by a curved border. 
     
     
       6. The display defined in  claim 2 , wherein the first region has a plurality of portions each of which is aligned with a respective one of the light-emitting diodes. 
     
     
       7. The display defined in  claim 1  wherein the light guide plate has protrusions on the edge. 
     
     
       8. The display defined in  claim 1  wherein the light guide plate has a tapered thickness along the edge. 
     
     
       9. The display defined in  claim 1  wherein the light guide plate has a symmetrical tapered thickness having an upper tapered portion on an upper surface of the light guide plate and a symmetrical lower tapered portion on an opposing lower surface of the light guide plate. 
     
     
       10. The display defined in  claim 1  wherein the display layers include a liquid crystal layer sandwiched between first and second substrates. 
     
     
       11. The display defined in  claim 10  wherein the light guide plate has opposing first and second surfaces, wherein the ridges are on the first surface, and wherein the display further comprises bumps on the second surface. 
     
     
       12. The display defined in  claim 1 , wherein the ridges are formed on an upper surface of the light guide plate. 
     
     
       13. A display, comprising:
 display layers that form an array of pixels; and 
 a backlight that illuminates the array of pixels, wherein the backlight includes a light guide plate having an edge between an upper surface and a lower surface and ridges on the upper surface, wherein the upper surface is interposed between the display layers and the lower surface, wherein the backlight includes an array of light-emitting diodes along the edge that emit light into the light guide plate through the edge, wherein the light guide plate has at least a first region in which the ridges have a first density and a second region in which the ridges have a second density that is greater than the first density, wherein the first and second regions are separated by a curved border, and wherein the first region has a plurality of portions each of which is aligned with a respective one of the light-emitting diodes. 
 
     
     
       14. The display defined in  claim 13  wherein the light guide plate has a symmetrically tapered thickness along the edge. 
     
     
       15. The display defined in  claim 14  further comprising protrusions on the edge, wherein the protrusions are adjacent to the light-emitting diodes and receive light from the light emitting diodes. 
     
     
       16. The display defined in  claim 13 , wherein each ridge extends along an axis perpendicular to the edge. 
     
     
       17. A liquid crystal display, comprising:
 first and second transparent substrates; 
 a liquid crystal layer between the first and second substrates; and 
 backlight structures that produce backlight that passes through the first and second substrates and the liquid crystal layer, wherein the backlight structures include a light guide plate, wherein the light guide plate has a first pair of opposing edges that extend along a first dimension and a second pair of opposing edges that extend along a second dimension perpendicular to the first dimension, wherein the backlight structures include light-emitting diodes that emit light into one of the first pair of edges, wherein the light guide plate has a plurality of parallel ridges that run parallel to the second pair of edges, wherein the light guide plate has a first region in which the ridges have a first density of the number of ridges, a second region adjacent the first region in which the ridges have a second density of the number of ridges that is greater than the first density, and a third region adjacent the second region in which the ridges have a third density of the number of ridges that is greater than the second density, and wherein the first region is interposed between the light-emitting diodes and the second region. 
 
     
     
       18. The liquid crystal display defined in  claim 17  wherein the ridges include ridges of different lengths and wherein the light guide plate has a symmetrically tapered profile along the edge into which the light is emitted. 
     
     
       19. The liquid crystal display defined in  claim 17 , wherein the first and second regions are separated by a first curved border and the second and third regions are separated by a second curved border. 
     
     
       20. The liquid crystal display defined in  claim 17 , wherein the light guide plate has an upper surface and a lower surface, wherein the upper surface is interposed between the lower surface and the liquid crystal layer, and wherein the plurality of parallel ridges are formed on the upper surface of the light guide plate.

Description:
This application claims the benefit of provisional patent application No. 62/096,065 filed on Dec. 23, 2014, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates generally to electronic devices with displays, and, more particularly, to displays with backlights. 
     Electronic devices such as computers and cellular telephones have displays. Some displays such as plasma displays and organic light-emitting diode displays have arrays of pixels that generate light. In displays of this type, backlighting is not necessary because the pixels themselves produce light. Other displays contain passive pixels that can alter the amount of light that is transmitted through the display to display information for a user. Passive pixels do not produce light themselves, so it is often desirable to provide backlight for a display with passive pixels. 
     In a typical backlight assembly for a display, a light guide plate is used to distribute backlight generated by a light source such as a light-emitting diode light source. Optical films such as a diffuser layer and prism films may be placed on top of the light guide plate. A reflector may be formed under the light guide plate to improve backlight efficiency. 
     A strip of light-emitting diodes may provide light to an edge of a light guide plate. Light scattering features such as bumps may be provided on the light guide plate. Light from the light-emitting diodes that is traveling within the light guide plate may be scattered upwards by the bumps to form backlight for a display. Light guide plates may also sometimes be provided with elongated ridges (sometimes referred to as lenticular features) that help extract backlight from the light guide plate. The ridges are provided in a uniform rectangular region on the light guide plate. 
     Light from the strip of light-emitting diodes is initially concentrated in the vicinity of the outputs of the light-emitting diodes. The light must travel a sufficient distance into the light guide plate to mix enough to be used as backlight illumination. Backlight units that require large mixing distances may consume more volume within a display than desired. At the same time, reducing the mixing distance in a backlight too much may lead to undesired backlight hotspots. 
     It would therefore be desirable to be able to provide displays with improved backlights. 
     SUMMARY 
     A display may have an array of pixels for displaying images for a viewer. The array of pixels may be formed from display layers such as a color filter layer, a liquid crystal layer, a thin-film transistor layer, and polarizer layers. 
     A backlight unit may be used to produce backlight illumination for the display. The backlight illumination may pass through the polarizers, the thin-film transistor layer, the liquid crystal layer, and the color filter layer. The backlight unit may have a row of light-emitting diodes that emit light into the edge of a light guide plate. 
     The light guide plate may have opposing upper and lower surfaces. Backlight may be extracted from the light guide plate using an array of bumps on the lower surface and ridges on the upper surface. The density of the ridges may vary as a function of location across the display to avoid creating backlight hotspots. 
     If desired, different regions of the light guide plate may have different densities of ridges. Some of the ridges may be terminated along a meandering border between regions with different ridge densities. Ridge lengths and endpoint locations can be dithered about the border to help smooth out the transition in density between the different regions. Ridge widths and thicknesses may also be tapered down towards ridge endpoints to smooth out transitions in ridge density. Ridges may be patterned to locally reduce the density of ridges. For example, ridges may be patterned to reduce the density of ridges immediately adjacent to the light-emitting diodes and thereby avoid over-extraction of the light at the light-emitting diodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an illustrative electronic device having a display in accordance with an embodiment. 
         FIG. 2  is a cross-sectional side view of an illustrative display in an electronic device in accordance with an embodiment. 
         FIG. 3  is a top view of an illustrative display in accordance with an embodiment. 
         FIG. 4  is cross-sectional side view of an illustrative symmetrically tapered portion of a light guide plate for a display backlight in accordance with an embodiment. 
         FIG. 5  is a cross-sectional side view of an illustrative light guide plate with light-scattering features such as bumps on its lower surface in accordance with an embodiment. 
         FIG. 6  is graph in which bump density has been plotted as a function of position along the length of a light guide plate in accordance with an embodiment. 
         FIG. 7  is a graph in which bump density has been plotted as a function of distance along the width of a light guide plate in accordance with embodiment. 
         FIG. 8  is a perspective view of illustrative light guide plate edge protrusions in accordance with embodiment. 
         FIG. 9  is a cross-sectional side view of a light guide plate showing how different light rays interact with the surfaces of the light guide plate by different amounts in accordance with an embodiment. 
         FIG. 10  is a cross-sectional perspective view of a light guide plate showing how rounded ridges may extend along the upper surface of the light guide plate in accordance with an embodiment. 
         FIG. 11  is a diagram of an illustrative variable density ridge pattern that may be used in a light guide plate in accordance with an embodiment. 
         FIG. 12  is a top view of a portion of a light guide plate showing how the endpoint locations of ridges may be randomized along a border between two different areas of different ridge density to help reduce visible variations in backlight intensity along the border in accordance with an embodiment. 
         FIG. 13  is a top view of an illustrative tapered ridge in a light guide plate in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An illustrative electronic device of the type that may be provided with a display is shown in  FIG. 1 . As shown in  FIG. 1 , electronic device  10  may have control circuitry  16 . Control circuitry  16  may include storage and processing circuitry for supporting the operation of device  10 . The storage and processing circuitry may include storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  16  may be used to control the operation of device  10 . The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, application specific integrated circuits, etc. 
     Input-output circuitry in device  10  such as input-output devices  12  may be used to allow data to be supplied to device  10  and to allow data to be provided from device  10  to external devices. Input-output devices  12  may include buttons, joysticks, scrolling wheels, touch pads, key pads, keyboards, microphones, speakers, tone generators, vibrators, cameras, sensors, light-emitting diodes and other status indicators, data ports, etc. A user can control the operation of device  10  by supplying commands through input-output devices  12  and may receive status information and other output from device  10  using the output resources of input-output devices  12 . 
     Input-output devices  12  may include one or more displays such as display  14 . Display  14  may be a touch screen display that includes a touch sensor for gathering touch input from a user or display  14  may be insensitive to touch. A touch sensor for display  14  may be based on an array of capacitive touch sensor electrodes, acoustic touch sensor structures, resistive touch components, force-based touch sensor structures, a light-based touch sensor, or other suitable touch sensor arrangements. 
     Control circuitry  16  may be used to run software on device  10  such as operating system code and applications. During operation of device  10 , the software running on control circuitry  16  may display images on display  14 . 
     Device  10  may be a tablet computer, laptop computer, a desktop computer, a television, a cellular telephone, a media player, a wristwatch device or other wearable electronic equipment, or other suitable electronic device. 
     Display  14  for device  10  includes an array of pixels. The array of pixels may be formed from liquid crystal display (LCD) components or other suitable display structures. Configurations based on liquid crystal display structures are sometimes described herein as an example. 
     A display cover layer may cover the surface of display  14  or a display layer such as a color filter layer, thin-film transistor layer, or other portion of a display may be used as the outermost (or nearly outermost) layer in display  14 . The outermost display layer may be formed from a transparent glass sheet, a clear plastic layer, or other transparent member. 
     A cross-sectional side view of an illustrative configuration for display  14  of device  10  is shown in  FIG. 2 . As shown in  FIG. 2 , display  14  may include a backlight unit such as backlight unit  42  (sometimes referred to as a backlight or backlight structures) for producing backlight  44 . During operation, backlight  44  travels outwards (vertically upwards in dimension Z in the orientation of  FIG. 2 ) and passes through pixel structures in display layers  46 . This illuminates any images that are being produced by the pixels for viewing by a user. For example, backlight  44  may illuminate images on display layers  46  that are being viewed by viewer  48  in direction  50 . 
     Display layers  46  may be mounted in chassis structures such as a plastic chassis structure and/or a metal chassis structure to form a display module for mounting in a housing in device  10  or display layers  46  may be mounted directly in an electronic device housing for device  10  (e.g., by stacking display layers  46  into a recessed portion in a metal or plastic housing). Display layers  46  may form a liquid crystal display or may be used in forming displays of other types. 
     In a configuration in which display layers  46  are used in forming a liquid crystal display, display layers  46  may include a liquid crystal layer such a liquid crystal layer  52 . Liquid crystal layer  52  may be sandwiched between display layers such as display layers  58  and  56 . Layers  56  and  58  may be interposed between lower polarizer layer  60  and upper polarizer layer  54 . 
     Layers  58  and  56  may be formed from transparent substrate layers such as clear layers of glass or plastic. Layers  56  and  58  may be layers such as a thin-film transistor layer and/or a color filter layer. Conductive traces, color filter elements, transistors, and other circuits and structures may be formed on the substrates of layers  58  and  56  (e.g., to form a thin-film transistor layer and/or a color filter layer). Touch sensor electrodes may also be incorporated into layers such as layers  58  and  56  and/or touch sensor electrodes may be formed on other substrates. 
     With one illustrative configuration, layer  58  may be a thin-film transistor layer that includes an array of pixel circuits based on thin-film transistors and associated electrodes (pixel electrodes) for applying electric fields to liquid crystal layer  52  and thereby displaying images on display  14 . Layer  56  may be a color filter layer that includes an array of color filter elements for providing display  14  with the ability to display color images. If desired, layer  58  may be a color filter layer and layer  56  may be a thin-film transistor layer. Configurations in which color filter elements are combined with thin-film transistor structures on a common substrate layer may also be used. 
     During operation of display  14  in device  10 , control circuitry (e.g., one or more integrated circuits on a printed circuit) may be used to generate information to be displayed on display  14  (e.g., display data). The information to be displayed may be conveyed to a display driver integrated circuit such as circuit  62 A or  62 B using a signal path such as a signal path formed from conductive metal traces in a rigid or flexible printed circuit such as printed circuit  64  (as an example). Integrated circuits such as integrated circuit  62 A and/or flexible printed circuits such as flexible printed circuit  64  may be attached to substrate  58  in ledge region  66  (as an example). 
     Backlight structures  42  may include a light guide plate such as light guide plate  78 . Light guide plate  78  may be formed from a transparent material such as clear glass or plastic. During operation of backlight structures  42 , a light source such as light source  72  may generate light  74 . Light source  72  may be, for example, an array of light-emitting diodes (e.g., a series of light-emitting diodes that are arranged in a row that extends into the page in the orientation of  FIG. 2 ). 
     Light  74  from light source  72  may be coupled into edge surface  76  of light guide plate  78  and may be distributed in dimensions X and Y throughout light guide plate  78  due to the principal of total internal reflection. Light guide plate  78  may include light-scattering features such as pits, bumps, grooves, or ridges that help light exit light guide plate  78  for use as backlight  44 . These features may be located on an upper surface and/or on an opposing lower surface of light guide plate  78 . With one illustrative configuration, which is described herein as an example, a first surface such as the lower surface of light guide plate  78  has a pattern of bumps and an opposing second surface such as the upper surface of light guide plate  78  has a pattern of ridges (sometimes referred to as lenticules, lenticular structures, or lenticular ridges). Light source  72  may be located at the left of light guide plate  78  as shown in  FIG. 2  or may be located along the right edge of plate  78  and/or other edges of plate  78 . 
     Light  74  that scatters upwards in direction Z from light guide plate  78  may serve as backlight  44  for display  14 . Light  74  that scatters downwards may be reflected back in the upward direction by reflector  80 . Reflector  80  may be formed from a reflective structure such as a substrate layer of plastic coated with a dielectric mirror formed from alternating high-index-of-refraction and low-index-of-refraction inorganic or organic layers. 
     To enhance backlight performance for backlight structures  42 , backlight structures  42  may include optical films  70 . Optical films  70  may include diffuser layers for helping to homogenize backlight  44  and thereby reduce hotspots. Optical films  70  may also include prism films (sometimes referred to as turning films) for collimating backlight  44 . Optical films  70  may overlap the other structures in backlight unit  42  such as light guide plate  78  and reflector  80 . For example, if light guide plate  78  has a rectangular footprint in the X-Y plane of  FIG. 2 , optical films  70  and reflector  80  may each have a matching rectangular footprint. Optical films  70  may include compensation films for enhancing off-axis viewing or compensation films may be formed within the polarizer layers of display  14  or elsewhere in display  14 . 
       FIG. 3  is a top view of a portion of display  14  showing how display  14  may have an array of pixels  90  formed within display layers  46 . Pixels  90  may have color filter elements of different colors such as red color filter elements R, green color filter elements G, and blue color filter elements B. Pixels  90  may be arranged in rows and columns and may form active area AA of display  14 . The borders of active area AA may be slightly inboard of the borders of light-guide plate  78  to ensure that there are no visible hotspots in display  14  (i.e., no areas in which the backlight illumination for display  14  is noticeably brighter than surrounding areas). For example, border  92  of active area AA may be offset by a distance  82  from lower edge  76  of light guide plate. It is generally desirable to minimize the size of distance  82  so that display  14  is as compact as possible for a given active area size. Nevertheless, distance  82  should not be too small to ensure that there is adequate light mixing. In particular, distance  82  should be sufficiently large to allow light  74  that is emitted from light-emitting diodes  72  to homogenize enough to serve as backlight illumination. When light  74  is initially emitted from individual light-emitting diodes  72 , light  74  is concentrated at the exits of light-emitting diodes  72  and is absent in the spaces between light-emitting diodes  72 . After light  74  has propagated sufficiently far within light-guide plate  78  (i.e., after light  74  has traversed a sufficiently large mixing distance  82 ), light  74  will be smoothly distributed along dimension X and will no longer be concentrated near the exits of respective individual light-emitting diodes  72 . 
     To enhance the efficiency with which light  74  is coupled into edge  76  of light guide plate from light-emitting diodes  72  without overly thickening light-guide plate  78 , it may be desirable to provide light-guide plate  78  with an outwardly tapered (flared) edge. Conventional edge tapers are formed by creating a taper in the upper surface of a light guide plate adjacent to the light-emitting diodes and leaving the opposing planar lower surface of the light guide plate untouched. If care is not taken, however, this type of taper may have an angle that is too steep, raising the potential for excessive light leakage due to the loss of total internal reflection conditions in the taper region. With the illustrative taper configuration shown in the cross-sectional side view of illustrative light guide plate  78  of  FIG. 4 , excessive light losses are avoided by providing light guide plate  78  with both upper and lower taper structures  100  and  102 , respectively. Tapers  102  and  100  may be symmetrical or tapers  102  and  100  may have different shapes. In region  96 , light-guide plate  78  is planar and has planar parallel opposing upper and lower surfaces  106  and  108 , respectively. In taper region  98 , light guide plate  78  has a thickness that varies from the thickness of region  96  (T 2 ) to enlarged thickness T 1  at edge  76 , so taper structure surfaces  112  and  104  are angled at non-zero angles with respect to planar upper and lower light guide plate surfaces  106  and  108 . Thickness T 2  may be about 400 microns 300-500 microns, less than 600 microns, more than 200 microns, or other suitable thickness. The enlarged size of dimension T 1  helps light guide plate  78  receive light  74  from light-emitting diodes  72 . The taper in light guide plate  78  formed by taper structures  100  and  102  helps concentrate light  74  into region  96  of light guide plate for use in forming backlight  44 . 
     As shown in  FIG. 5 , lower surface  96  of light guide plate  78  may be provided with light scattering features such as bumps (protrusions)  114 . Bumps  114  may help redirect light  74  that is traveling within the interior of light guide plate  78  upwards in direction Z to serve as backlight  44  for display  14 . 
     As light  74  that is traveling within light guide plate  78  is directed upwards in direction Z to serve as backlight  44 , the intensity of the light  74  that remains in light guide plate  78  decreases. As a result, the intensity of light  74  is greatest at edge  76  of light guide plate  78  adjacent to light-emitting diodes  72  and decreases with increasing distance along axis Y away from edge  76 . It is generally desirable for the intensity of backlight  44  to be evenly distributed across the surface of light guide plate  78  in dimensions X and Y. To ensure that backlight  44  is not too dim at large values of Y, the density of bumps  114  can be increased as a function of increasing value of Y, as shown in  FIG. 6 . The increase in the density of bumps  114  at larger Y values offsets the decrease in the intensity of light  74  within light guide plate at larger Y values and thereby ensures that backlight  44  has a uniform intensity as a function of dimension Y. 
     Bumps  114  can be distributed unevenly in dimension X to help counteract the hotspot that would otherwise be associated with each light-emitting diode. An illustrative lateral bump density distribution is shown in  FIG. 7  for multiple locations in dimension Y. In the example of  FIG. 7 , a first light-emitting diode is emitting light  74  into edge  76  at location X 1  and a second light-emitting diode is emitting light  74  into edge  76  at location X 2 . There is therefore a natural tendency for backlight  44  to be concentrated about locations X 1  and X 2 , particularly at low Y values immediately adjacent to edge  76 . This can be counteracted by locally reducing the density of bumps  114  at the exits of light-emitting diodes  72  (i.e., by reducing the density of bumps  114  at positions X 1  and X 2 ). An illustrative bump density reduction scheme of this type is illustrated by curves  120 ,  122 ,  124 , and  126  of  FIG. 7 . Curve  120  corresponds to the density of bumps  114  adjacent to edge  76  (dimension Y 1 ). The decrease in bump density is greatest at this Y location, because light  74  tends to be most concentrated just after being emitted into edge  76  of light guide plate  78 . At slightly larger Y values such as Y value Y 2 , it is not necessary to locally reduce the bump density as much, so the localized reductions in bump density at Y 2  are lower than at Y 1 , as shown by curve  122 . Curve  124  illustrate how the density of bumps  114  may be reduced by a further decreased amount at larger Y value Y 3 . At Y locations greater than Y 4  (e.g., about 0.5 to 1 mm, 0.5 to 10 mm, 1-5 mm, more than 1 mm, less than 10 mm, etc.) the density of bumps  114  may be constant as a function of lateral dimension X (and may vary in dimension Y as shown in  FIG. 6 ). 
     To enhance light mixing as light  74  is emitted into edge  76  of light guide plate  78 , edge  76  may be provided with locally raised features such as protrusions  128  of  FIG. 8 . Protrusions  128  may have triangular profiles (in the XY plane of  FIG. 8 ), may have semicircular profiles, or may have other shapes. Angle A may be about 140-160° or other suitable value to help refract light  74  at relatively steep angles in plate  78 , as shown by illustrative light ray  74  of  FIG. 8 , thereby enhancing light mixing and helping to reduce mixing distance  82  ( FIG. 3 ). Protrusions  128  may have widths (in dimension X) of about 75-125 microns or other suitable widths. Protrusions  128  may be spaced apart by about 250 microns, 200-300 microns, less than 320 microns, or more than 150 microns (as examples). Protrusions  128  may be spread evenly along edge  76  or may be clustered adjacent to respective light-emitting diodes  72 . 
     Light-emitting diodes  72  emit light  74  in a cone. This cone of light includes highly angled off-axis light rays. As shown in the cross-sectional side view of light guide plate  78  of  FIG. 9 , some of the highly angled light rays such as light ray  74 - 1  lie primarily in the YZ plane. These light rays interact strongly with upper surface  106  and lower surface  108  of light guide plate and therefore tend to be heavily extracted by bumps  114  on lower surface  108 . Other highly angled light rays in the cone of emitted light  74  such as illustrative light ray  74 - 2  in  FIG. 9  lie primarily in the XY plane. These rays are angled more along dimension X than dimension Z and therefore interact with surfaces  106  and  108  less frequently than ray  74 - 1 . To ensure that light rays such as light ray  74 - 2  are adequately extracted and can serve as backlight  44 , light guide plate  78  may be provided with lenticular ridges such as ridges  130  of  FIG. 10 . Ridges  130  may be formed on upper surface  106  of light guide plate  78  (as an example). As shown in  FIG. 10 , ridges  130  may run parallel to dimension Y (i.e., the direction in which the exit faces of light-emitting diodes  72  are oriented and the direction in which light  74  is emitted into edge  76  of light guide plate  78 ). Ridges  130  may have semicircular cross-sectional shapes or may have other suitable shapes (triangular, etc.). As shown in  FIG. 10 , the presence of ridges  130  may help extract highly angled light rays such as light ray  74 - 2  that are propagating close to the XY plane to produce corresponding backlight  44 . 
     A conventional light guide plate may have a rectangular pattern of parallel ridges that evenly covers the surface of the light guide plate. This type of uniform ridge pattern tends to over-extract light that has just been injected from the light-emitting diodes, leading to hotspots along the edge of the light guide plate. The tendency of conventional ridge arrangements to locally over-extract light can be exacerbated when using highly angled protrusions such as protrusions on the edge of light guide plate that is receiving the light from the light-emitting diodes (see, e.g., protrusions  128  of  FIG. 8 ). 
     To reduce or eliminate these hotspots and therefore allow mixing distance  82  to be minimized, ridges  130  on light guide plate  78  can be implemented using a non-uniform pattern. Consider, as an example, the illustrative pattern of ridges  130  shown in  FIG. 11 . Different areas in light guide plate  78  of  FIG. 11  each have a different density of ridges  130 . Borders  134  and  132  separate regions with different respective ridge densities. Border  136  separates ridges  130  from taper region  98 . 
     Ridges  130  include ridges  130 - 1  that terminate at endpoints along border  132 . Ridges  130 - 2  extend past border  132  and terminate at border  134 . Ridges  130 - 2  therefore tend to be longer than ridges  130 - 1 . Ridges  130 - 3  extend to border  136  between tapered region  98  and planar region  96 . Ridges  130 - 1 ,  130 - 2 , and  130 - 3  may all extend continuously in direction Y until reaching the edge of light guide plate  78  opposing edge  76 . 
     Because different ridges  130  cover different portions of light guide plate  78  with different densities, the amount of light extraction produced by ridges  130  varies as a function of lateral position on light guide plate  78  (i.e., the density of ridges  130  varies as a function of position in the X-Y plane of  FIG. 11 ). This allows the amount of light extraction produced by ridges  130  to be selectively reduced in the vicinity of light-emitting diodes  72 . For example, the relative scarcity of ridges  130  in region  138  helps prevent over-extraction of light  74  in region  138 . Region  140  extends between light emitting diodes  72  and does not receive as much of light  74  as region  138 . There is therefore a risk that region  140  will become too dark. With the illustrative pattern of  FIG. 11 , region  140  has been provided with more ridges (i.e., ridges  130 - 2 ) than region  138  and therefore exhibits more light extraction than region  138 . This helps balance the amount of light extracted in region  138  (where there is ample light and therefore fewer light extraction ridges) and region  140  (where there is less light and therefore more light extraction ridges to compensate). 
     In general, light guide plate  78  may be provided with a continuously varying ridge density, two or more areas with two or more respective ridge densities, or other patterns of ridges. In the example of  FIG. 11 , there are three different areas each of which has a different respective density of ridges. Area  138  contains only ridges  130 - 1  and therefore has a relatively low ridge density. Area  138  may be located near light-emitting diodes  72  to prevent over-extraction of light  74 . Area  140  contains both ridges  130 - 1  and ridges  130 - 2  and therefore has a higher ridge density than area  138 . Area  142  contains ridges  130 - 1 ,  130 - 2 , and  130 - 3  and therefore has a higher ridge density than area  140 . The presence of third area  142  (i.e., use of three different ridge density regions) helps smooth out the intensity of extracted backlight. If desired, additional areas with different respective ridge densities may be provided (e.g., four or more regions, continuously varying density portions, etc.). The use of three distinct ridge densities in the illustrative light guide plate of  FIG. 11  is merely an example. 
     The homogeneity of backlight  44  can be enhanced by using smooth shapes for borders  134  and  132  (e.g., curved paths in the example of  FIG. 11 ). Backlight homogeneity may also be enhanced by imposing pseudorandom variations on the locations of the ends of the ridges. As shown in  FIG. 12 , for example, the locations of endpoints  144  of ridges  130 - 1  need not terminate precisely along border  132 . Imposing slight variations in the lengths of ridges  130 - 1  (i.e., dithering the lengths of the ridges) allows these ridges to be aligned with a desired border (e.g., border  132  in the  FIG. 12  example) while helping to ensure that the ridge density transition associated with border  132  does not produce backlight intensity variations that are visible to viewer  48 . 
     Another way to smooth the transition between regions of differing ridge density involves imposing a tapering width and/or thickness on the ends of ridges  130 . This type of arrangement is shown in  FIG. 13 . As shown in  FIG. 13 , ridge  130  may be characterized by a thickness TB and a width WB in main region  148 . The width of ridge  130  and the thickness of ridge  130  may be smoothly reduced towards endpoint  144  (i.e., the ridge may taper to smaller thicknesses towards the endpoint). For example, in end region  146 , ridge  130  may have reduced thicknesses such as thickness TS (TS&lt;TB) and reduced widths such as width TS (WS&lt;WB). Tapering the width, thickness, or other characteristic along the length of ridge  130  towards the end of ridge  130  helps make the location of endpoint  144  visually indistinct and therefore helps to ensure that transitions between light guide plate regions with different ridge densities are not noticeable to a viewer. 
     Light guide plate  78  may be formed by shaping a polymer layer using a metal blank. Grooves with varying depths and widths may be formed in the metal blank by varying the operating height of a grinding bit during groove formation. A metal blank with grooves of varying depth and width may be used in forming a light guide plate with corresponding ridges of varying thickness and width (i.e., ridges with thicknesses that taper down to smaller values as the ridges extend towards their endpoints). 
     In general, any one or more of these schemes for reducing the visibility of ridge density transitions on light guide plate  78  and for reducing hotspots may be used (e.g., selective variation of ridge density, use of multiple regions with distinct ridge densities, imposing small variations on the locations of ridge endpoints while still aligning the ridge endpoints with a desired ridge density region border, varying thickness and width of ridges, etc. 
     The foregoing is merely illustrative and various modifications can be made by those skilled in the art without departing from the scope and spirit of the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20150909
Publication Date: 20180904
Grant Date: 20180904
Priority Date: 20141223
Inventors: BROWN, MICHAEL J.
Assignee: APPLE INC
CPC Classifications: [{"code": "G02B6/002", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0038", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/002", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/0061", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/0036", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133615", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/0061", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/0016", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/0038", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0036", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0038", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0016", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0016", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/002", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/0061", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/002", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/0061", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/0036", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/0016", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/0038", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133607", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 56129178