Patent Publication Number: US-2017371086-A1

Title: Display Backlights with Reduced Mixing Distances

Description:
This application claims the benefit of provisional patent application No. 62/353,965, filed Jun. 23, 2016, 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 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 images 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 on the upper and/or lower surfaces of the light guide plate may scatter light out of the light guide plate so that the scattered light may serve as backlight illumination for the display. 
     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 mixing distance into the light guide plate to become homogenized enough to be used as backlight illumination. Backlight units with large mixing distances may consume more volume within a display than desired and may give rise to unsightly display borders. On the other hand, reducing the mixing distance in a backlight too much may lead to undesired hotspots. 
     SUMMARY 
     A display may have a backlight that provides backlight illumination for an array of pixels. The array of pixels may be an array of liquid crystal display pixels or other pixels for displaying images for a user. 
     The backlight may have a light guide layer that distributes backlight across the display. The light guide layer may have edge surfaces. A light source such as a row of light-emitting diodes that extends along an edge surface of the light guide layer may emit light into the edge surface of the light guide layer. 
     The light guide layer may have opposing planar surfaces. Light-scattering structures such as cylindrical laser-drilled light-scattering holes (through holes) that extend between the planar surfaces may be used to redirect rays of light from the light source by refraction and/or diffraction. In this way, the light-scattering holes can homogenize light from the light-emitting diodes within a reduced mixing distance. The homogenized light may be extracted from the light guide layer using light extraction features on one or both of the planar surfaces. The extracted light may serve as the backlight illumination for the array of pixels. 
     To enhance homogenization of the light from the light-emitting diodes, the edge surface(s) of the light guide layer may be provided with light-scattering structures such as grooves, pits, bumps, and other structures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an illustrative electronic device having a display with a backlight 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 perspective view of laser processing equipment of the type that may be used for forming light scattering structures on an edge surface of a light guide layer in accordance with an embodiment. 
         FIG. 4  is a side view of additional laser processing equipment that may be used for forming light scattering structures for a light guide layer in accordance with an embodiment. 
         FIG. 5  is a diagram of illustrative operations and equipment for forming an electronic device having a display with a backlight in accordance with an embodiment. 
         FIG. 6  is a top view of an illustrative light guide layer with light-scattering holes such as cylindrical laser-drilled light-scattering holes in accordance with an embodiment. 
         FIG. 7  is a top view of a portion of an illustrative light guide layer with light-scattering structures that include multiple rows of light-scattering holes in accordance with an embodiment. 
         FIG. 8  is a perspective view of an edge surface of a light guide layer with illustrative light-scattering structures in accordance with an embodiment. 
         FIG. 9  is a cross-sectional view of an illustrative light scattering pit of the type that may serve as a light-scattering structure on the edge surface of a light guide layer in accordance with an embodiment. 
         FIG. 10  is a cross-sectional side view of an illustrative light scattering bump of the type that may serve as a light-scattering structure on the edge surface of a light guide layer in accordance with an embodiment. 
         FIG. 11  is a perspective view of an edge surface of a light guide layer with illustrative light scattering grooves in accordance with an embodiment. 
         FIGS. 12, 13, and 14  are top views of the edges of illustrative light guide layers with vertically extending light-scattering structures such serrated grooves in accordance with embodiments. 
     
    
    
     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., 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 received by device  10  from external equipment or a user and to allow data to be provided from device  10  to external equipment or a user. 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 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, electrophoretic display components, or other suitable display structures. Configurations based on liquid crystal display pixels 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 such as backlight  42  (sometimes referred to as a backlight unit, backlight system, or backlight structures) for producing backlight illumination (backlight)  44 . During operation, backlight illumination  44  travels outwards (vertically upwards in dimension Z in the orientation of  FIG. 2 ) and passes through an array of pixels P in one or more display layers  46 . The array of pixels P forms an active area AA for display  14 . Backlight illumination  44  illuminates any images that are being produced by pixels P for viewing by a user in active area AA. For example, backlight illumination  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. The liquid crystal layer may be sandwiched between a thin-film transistor layer and a color filter layer or other substrates. These layers may, in turn, be sandwiched between an upper polarizer and a lower polarizer. Touch sensor electrodes may be formed from a layer that overlaps layer(s)  46  or may be incorporated into layer(s)  46 . 
     Backlight  42  may include a light guide layer such as light guide layer  78 . Light guide layer  78  may be formed from a transparent material such as clear glass or plastic. Layer  78  may be a molded plastic plate or may be a flexible light guide film. Light guide layer  78  may, as an example, have a thickness of 0.25-0.4 mm, more than 0.2 mm, less than 0.4 mm, or other suitable thickness. During operation of backlight  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 layer  78  and may be distributed in dimensions X and Y throughout light guide layer  78  due to the principal of total internal reflection. The upper and/or lower planar surfaces of light guide layer  78  in active area AA may include light-scattering features such as pits, bumps, grooves, or ridges that help light exit light guide layer  78  for use as backlight illumination  44 . Layer  78  may be otherwise solid and free of holes (through holes) in area AA. Light source  72  may be located at the left of light guide layer  78  as shown in  FIG. 2  or may be located along the right edge of layer  78  and/or other edges of layer  78 . 
     Light  74  that scatters upwards in direction Z from light guide layer  78  may serve as backlight illumination  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  42 , backlight  42  may include optical films  70 . Optical films  70  may include diffuser layers for helping to homogenize backlight illumination  44  and thereby reduce hotspots. Optical films  70  may also include prism films (sometimes referred to as turning films) for collimating backlight illumination  44 . 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 . 
     Optical films  70  may overlap the other structures in display  14 . For example, if the array of pixels P in layer(s)  46  forms an active area AA for display  14  with a rectangular footprint in the X-Y plane of  FIG. 2 , optical films  70  may have a matching rectangular footprint. Light guide layer  78  and reflector  80  may also have rectangular footprints. 
     Light  74  that exits light source  72  is initially concentrated next to the outputs of the light-emitting diodes in light source  72 . Light  74  traverses a non-zero mixing distance MD through a light mixing region running along the edge of layer  78  before light  74  has spread out sufficiently in the X and Y dimensions to be homogenized enough to serve as backlight  44  for active area AA of display  14 . Pixels P of active area AA overlap a corresponding portion of layer  78 . Display  14  is free of pixels P over the mixing region of layer  78 . 
     Mixing distance MD (i.e., the distance between edge surface  76  and the adjacent edge of active area AA) may, in general, have any suitable value. With one illustrative configuration, light guide holes and other light-scattering structures are formed along the edge of light guide layer  78  (e.g., on edge surface  76  and/or within the border portion of light guide layer  78  associated with mixing distance MD). The presence of these light-scattering structures may help reduce mixing distance MD to about 2.5-3 mm, less than 3 mm, less than 2.5 mm, less than 2.0 mm, less than 1.5 mm, 1-2 mm, 1-2.5 mm, or other suitable distance. Displays with minimized mixing distances MD may fit within relatively compact housing structures and allow the borders of display  14  to be minimized to enhance the appearance of device  10 . 
     Light-scattering structures that help reduce mixing distance MD in light guide layer  78  may be formed using any suitable technique (e.g., laser processing, mechanical drilling, water jet cutting, knife cutting, die cutting, punching, molding, etc.). With one illustrative configuration, laser processing techniques are used to pattern light-scattering structure into light guide layer  78 . Consider, as an example, the laser processing arrangement of  FIG. 3  in which laser processing equipment  100  is being used to process edge surface  76  of light guide layer  78 . In the example of  FIG. 3 , edge surface  76  of light guide layer  78  is being exposed to laser light  104  from laser  102 . Laser  102  may be an excimer laser that produces pulses of ultraviolet light that ablate material from edge surface  76  or may be any other suitable laser that can remove material from edge surface  76  (e.g., a visible light laser, an infrared laser, etc.). Light  104  may be patterned using mask  106  (e.g., a transparent substrate formed from a material such as fused silica with an opaque pattern formed from chromium or other metal). The pattern of light  104  after light  104  passes through mask  106  may form an array of light dots surrounded by dark regions (e.g., when equipment  100  is forming an array of pits in surface  76 ) or may form an array of dark dots surrounded by light regions (e.g., when equipment  100  is forming an array of bumps on edge surface  76 ). Other patterns of light  104  may be created by mask  106 , if desired (e.g., patterns for forming grooves or other recesses, ridges or other protrusions, etc.). An electrically controlled positioner such as positioner  108  that is coupled to layer  78  and/or to equipment  100  may be used to move layer  78  and equipment  100  relative to each other during processing (e.g., to smoothly scan patterned laser light  104  along the edge of layer  78 , to repeatedly step equipment  100  to different locations along the edge of layer  78 , etc.). 
     In the example of  FIG. 3 , laser light  104  is applied to edge surface  76  in a direction that is parallel to surface normal n of edge surface  76 . If desired, laser processing equipment such as equipment  110  of  FIG. 4  may be used to apply layer light  104  in a direction that is perpendicular to surface normal n of edge surface  76  and that is parallel to surface normal np of the planar upper surface of layer  78 . Equipment  110  may have a laser such as laser  102  for generating laser light  104 . An electrically controlled positioner such as positioner  108  may be used to control the position of laser  102  and therefore laser light  104  and/or may be used to position light guide layer  78  relative to laser  102 . During operation, laser light  104  may be applied to portions of layer  78  near to edge surface  76  to form through-holes such as holes  112  that pass through layer  78  and/or to cut or otherwise pattern a desired shape into edge surface  76  (e.g., to create a serrated or grooved edge surface  76 , etc.). Laser  102  may remove portions of layer  78  while laser  102  is stationary or laser  102  may be scanned along the edge of layer  78  using positioning equipment such as equipment  108 . If, as an example, it is desired to form holes such as holes  112 , laser  102  may be stepped between each of a number of different hole locations. At each hole location, laser  102  may produce light  104  to drill a corresponding hole  112 . If desired, holes  112  may be drilled by moving laser  102  and applying a series of pulses of light  104  to layer  78  while laser  102  is moving. Cutting operations and other operations that involve application of light  104  from laser  102  to layer  78  may be performed by repeatedly stepping laser  102  along the edge of layer  78  (e.g., to each of a number of different laser processing positions) and/or may be performed by supplying constant or pulsed laser light  102  while laser  102  is being moved relative to layer  78  by positioning equipment  108 . 
     Illustrative operations and equipment of the type that may be used in forming light-scattering structures for light guide layer  78  are shown in  FIG. 5 . 
     As shown in  FIG. 5 , a singulation tool such as singulation tool  116  may be used to divide a large sheet of light guide material such as sheet  114  into multiple individual light guide layers such as light guide layer  78 . Tool  116  may include die cutting equipment (e.g., stamping equipment), knife cutting equipment, laser cutting equipment, and/or other tools for cutting light guide layers such as layer  78  from sheet  114 . After forming light guide layer  78 , a laser processing tool such as laser processing tool  118  may be used to apply laser light to layer  76  to form light-scattering structures, as described in connection with laser processing tools  100  and  110  of  FIGS. 3 and 4 . 
     Laser light may, for example, be applied to edge surface  76  of layer  78  or other portions of layer  78  (e.g., the upper and/or lower planar surfaces of layer  78  along the edge of layer  78  on which edge surface  76  is formed and/or other portions of the upper and/or lower surface of layer  78 ). Applied laser light may selectively remove portions of layer  78  (e.g., by ablation, thermal decomposition, etc.). The light scattering features that are formed in light guide layer  78  (see, e.g., light-scattering features  120  of  FIG. 5 ) may help scatter and thereby homogenize light  74  that is propagating in layer  78  within a relatively short mixing distance MD. The homogenization of light  74  within a short mixing distance MD in layer  78  helps avoid undesired visible hotspots along the edge of display  14  and allows the inactive border area of display  14  to be minimized. If desired, laser processing equipment and/or other equipment may be used in creating protrusions and/or recesses in the upper and/or lower surfaces of layer  78  in the portion of layer  78  that lies under active area AA of display  14  (e.g., to create an array of pits or other recesses and/or bumps or other protrusions). These structures may also be formed using mechanical embossing techniques or other light guide patterning techniques. 
     Following formation of light guide layer  78  with light-scattering structures  120  on edge surface  76  and/or adjacent to edge surface  76  (e.g., within the border of light guide layer  78  that is less than mixing distance MD from edge surface  76 ), assembly equipment  128  may be used to assemble display  14  from light guide layer  78  and other components and may be used to mount display  14  within housing  124  of electronic device  10 . Assembly equipment  128  may include electrically controlled positioners, machine vision equipment, and/or other equipment for placing the layers of display  14  into housing  124  of device  10 , for mounting light source  72  along edge surface  76  of light guide layer  78 , and for performing other device assembly operations. 
     To reduce mixing distance MD, light guide layer  78  may be provided with light-scattering features  120  that are formed from one or more holes through light guide layer  78  such as holes  112  of  FIG. 6 . As shown in the example of  FIG. 6 , light source  74  may include an array of light-emitting diodes  72 D extending along the edge of light guide layer  78  parallel to edge surface  76 . Each light-emitting diode  72 D may emit a corresponding beam of light  74  into an adjacent portion of edge surface  76 . 
     There may be one or more rows of light-scattering holes  112  in border mixing region (border portion) EP of layer  78  (i.e., in the strip of layer  78  that runs along the left edge of layer  78  and that is associated with mixing distance MD in the example of  FIG. 6 ). In the illustrative configuration of  FIG. 6 , a single row of holes  112  has been formed in layer  78  to serve as light-scattering features  120  in border region EP (sometimes referred to as a light mixing region or border light mixing region). The array of pixels P of display  14  overlaps only the portion of layer  78  in active area AA that is free of holes  112 . No pixels P in active area AA of display  14  overlap light mixing region EP and holes  112  along the edge of light guide layer  78 . Holes  112  may extend in an uninterrupted line across layer  78  (as shown in the example of  FIG. 6 ) or may be arranged in clusters (e.g., sets of one or more rows) that are positioned at the exits of respective light-emitting diodes  74 D. The example of  FIG. 6  is merely illustrative.  FIG. 7  shows how layer  78  may be provided with light-scattering features  120  formed from multiple rows of holes  112  in region EP. In the example of  FIG. 7 , there are three row of light-scattering holes  112 . There may, in general, be one or more row of holes  112 . 
     Holes  112  may serve as lens elements that refract light  72 . If desired, holes  112  may have sizes and shape that diffract light  72  in addition to or instead of refracting light  72 . In general, holes  112  and/or other light-scattering structures formed in border portion EP of light-guide layer  78  may homogenize light (i.e., distribute light  72  evenly within the X-Y plane of  FIG. 6 ) using any suitable technique. 
     Holes  112  may have circular outlines (i.e., holes  112  may form cylindrical openings through layer  78 ) or may have outlines of other suitable shapes (e.g., rectangular, triangular, hexagonal, other shapes with of cured and/or straight edges, etc.). Holes  112  may have diameters D of 25-50 microns, 10-80 microns, more than 15 microns, more than 20 microns, less than 100 microns, less than 75 microns, or other suitable sizes. The hole-to-hole spacing (pitch) of holes  112  in light-scattering features  120  may be 30-11 microns, more than 10 microns, more than 20 microns, more than 50 microns, less than 75 microns, less than 80 microns, less than 120 microns, or other suitable pitch. Holes  112  may be organized in an array having one or more rows and/or columns, may be formed in a pseudorandom pattern, or may have other suitable configurations. In configurations of the type shown in  FIG. 7  there are three rows of holes  112 . If desired, there may be a single row of holes  112  in light guide layer  78 , two or more rows of holes  112 , three or more rows of holes  112 , 1-4 rows of holes  112 , fewer than five rows of holes  112 , or other suitable number of rows of holes. 
     As shown in  FIG. 8 , light-scattering structures  120  in border region EP may be formed from light-scattering structures  122  on edge surface  76 . Structures  122  may be formed in an array with rows and columns, may be arranged in a pseudorandom pattern, or may be provided on edge surface  76  in other suitable patterns. Structures  122  may help distribute light  74  evenly within light guide plate  78  (e.g., structures  122  may help ensure that light  74  has been homogenized after traveling mixing distance MD from edge surface  76 ). Structures  122  may be used in combination with other light-scattering structures such as the light-scattering holes  112  of  FIGS. 6 and 7  (as an example). 
     Structures  122  on edge surface  76  may have any suitable shapes that distribute light  74  by diffraction and/or refraction. As an example, structures  122  may be recesses such as semispherical pits or other pits in surface  76 , as shown by the cross-sectional view of illustrative pit-shaped structure  122  of  FIG. 9 . Some or all of structures  122  on edge surface  76  may be protrusions such as bumps or other localized protruding structures (see, e.g., illustrative bump  122  in the cross-sectional view of  FIG. 10 ). 
     As shown in the example of  FIG. 11 , surface  76  may be provided with light-scattering structures  120  based on vertically extending structures  122 . In the illustrative configuration of  FIG. 11 , structures  122  have the shape of grooves with rectangular cross-sections that run vertically across the thickness of layer  78  parallel to vertical dimension Z. If desired, vertically extending ribs may protrude from layer  76 . The grooves of  FIG. 11  have rectangular cross-sectional shapes, but grooves and ribs (e.g., vertically extending grooves and/or ribs) for forming structures  120  may, in general, have any suitable profiles (e.g., semicircular, triangular, etc.). The pitch of structures  122  of  FIGS. 8, 9, 10, and 11  and other light-scattering structures  120  on surface  76  may be 30-11 microns, more than 10 microns, more than 20 microns, more than 50 microns, less than 75 microns, less than 80 microns, less than 120 microns, or other suitable pitch. 
       FIGS. 12, 13, and 14  are top views of illustrative edge regions of light guide layer  78  showing how light-scattering structures  120  may be formed from vertically extending scalloped grooves  112 . Illustrative grooves  112  of  FIG. 12  overlap with each other along their edges to form a serrated edge surface for surface  76  of layer  78 . In the example of  FIG. 13 , edge surface  76  has a double serrated profile created by nesting narrow scalloped recesses  122 T within wide scalloped recesses  122 B. In the example of  FIG. 14 , edge surface  76  has an undulating shape created by a series of vertically extending protrusions such as ribs  122 A that alternate with adjacent vertically extending recesses such as grooves  122 B. The profiles of ribs  122 A and  122 B of  FIG. 14  are semicircular, but other rib and/or groove shapes may be used, if desired (e.g., triangular shapes, rectangular shapes, shapes with combinations of curved and straight edges, 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.