PATENT DOCUMENT

Publication Number: US-9081230-B2
Application Number: US-201313887904-A
Country: US
Kind Code: B2

Title: Liquid crystal displays with reduced light leakage

Abstract:
A display is provided that has upper and lower polarizers, a color filter layer, a liquid crystal layer, and a thin-film transistor layer. The color filter layer and thin-film transistor layer may be formed from materials such as glass that are subject to stress-induced birefringence. To reduce light leakage that reduces display performance, one or more birefringence compensation layers may be incorporated into the display to help compensate for any birefringence effects. The compensation layers may include a birefringence compensation layer attached to the color filter layer or the thin-film transistor layer. A display may include an upper compensation layer attached to the color filter layer and a lower compensation layer attached to the thin-film transistor layer. The compensation layer may be formed from glass or polymer materials that have a negative photo-elastic constant.

Claims:
What is claimed is: 
     
       1. A display, comprising:
 an upper polarizer; 
 a lower polarizer; 
 a liquid crystal layer; 
 a first glass layer interposed between the upper polarizer and the liquid crystal layer; 
 a second glass layer interposed between the lower polarizer and the liquid crystal layer; and 
 a third glass layer located between the upper polarizer and the first glass layer that is configured to compensate for stress-induced birefringence in the first and second transparent layers. 
 
     
     
       2. The display defined in  claim 1  wherein the first glass layer comprises a color filter layer. 
     
     
       3. The display defined in  claim 2  wherein the second glass layer comprises a thin-film transistor layer. 
     
     
       4. The display defined in  claim 3  wherein the third glass layer is attached to the color filter layer. 
     
     
       5. The display defined in  claim 4 , further comprising a fourth glass layer attached to the thin-film transistor layer. 
     
     
       6. The display defined in  claim 1 , further comprising a fourth glass layer located between the upper polarizer and lower polarizer. 
     
     
       7. The display defined in  claim 1  wherein the third glass layer has a negative photo-elastic constant. 
     
     
       8. The display defined in  claim 7  wherein the negative photo-elastic constant has a value of less than −3.0 Brewster. 
     
     
       9. A display, comprising:
 an upper polarizer; 
 a lower polarizer; 
 a liquid crystal layer; 
 a first glass layer having a positive photo-elastic constant interposed between the upper polarizer and the liquid crystal layer; 
 a second glass layer interposed between the lower polarizer and the liquid crystal layer; 
 a first polymer layer having a negative photo-elastic constant interposed between the upper polarizer and the first glass layer; and 
 a second polymer layer interposed between the lower polarizer and the second glass layer. 
 
     
     
       10. The display defined in  claim 9  wherein the first polymer layer comprises a polymer material selected from the group consisting of: polystyrene, polysulfone, and polymethyl methacrylate. 
     
     
       11. The display defined in  claim 9  wherein the second polymer layer has a negative photo-elastic constant. 
     
     
       12. The display defined in  claim 11  wherein the second glass layer has a positive photo-elastic constant. 
     
     
       13. The display defined in  claim 11  wherein the negative photo-elastic constant of each of the first and second polymer layers is less than −10 Brewster. 
     
     
       14. The display defined in  claim 13  wherein the first and second polymer layers each have a Young&#39;s modulus that is less than 30 GPa. 
     
     
       15. The display defined in  claim 11  wherein the first glass layer comprises a color filter layer. 
     
     
       16. The display defined in  claim 15  wherein the first polymer layer is formed on the color filter layer. 
     
     
       17. A display, comprising:
 a first polarizer; 
 a second polarizer; 
 a liquid crystal layer; 
 a first transparent layer interposed between the first polarizer and the liquid crystal layer; 
 a second transparent layer interposed between the second polarizer and the liquid crystal layer; 
 a birefringence compensation layer located between the second transparent layer and the second polarizer that is configured to compensate for stress-induced birefringence in the first and second transparent layers; and 
 adhesive material that attaches the birefringence compensation layer to the second transparent layer, wherein the adhesive material forms a rectangular ring of adhesive between the birefringence compensation layer and the second transparent layer. 
 
     
     
       18. The display defined in  claim 17  wherein the first transparent layer comprises a glass color filter layer and wherein the second transparent layer comprises a glass thin-film transistor layer. 
     
     
       19. The display defined in  claim 18  wherein the birefringence compensation layer comprises glass. 
     
     
       20. The display defined in  claim 19 , wherein the adhesive material comprises a layer of optically clear adhesive material that attaches the birefringence compensation layer to the glass thin-film transistor layer. 
     
     
       21. The display defined in  claim 18  wherein the birefringence compensation layer comprises polymer material. 
     
     
       22. The display defined in  claim 17 , further comprising an additional birefringence compensation layer located between the first transparent layer and the first polarizer.

Description:
BACKGROUND 
     This relates generally to electronic devices, and more particularly, to electronic devices with displays. 
     Electronic devices often include displays. For example, cellular telephones and portable computers often include displays for presenting information to a user. An electronic device may have a housing such as a housing formed from plastic or metal. Components for the electronic device such as display components may be mounted in the housing. 
     It can be challenging to incorporate a display into the housing of an electronic device. Size and weight are often important considerations in designing electronic devices. In some mounting configurations, standoffs, housing walls, display bezels and other structures may press against a display, leading to bending. If care is not taken, optical effects such as stress-induced birefringence may cause a display to exhibit undesired light leakage when the display is in a dark state. 
     It would therefore be desirable to be able to provide improved displays for electronic devices. 
     SUMMARY 
     An electronic device may be provided with a display. The display may have upper and lower polarizers. A color filter layer, a liquid crystal layer, and a thin-film transistor layer may be interposed between the upper and lower polarizers. A backlight unit may provide backlight that passes through the layers of the display. 
     The color filter layer and thin-film transistor layer may be formed from materials such as glass that are subject to stress-induced birefringence when the display is mounted in a housing for the electronic device. Light leakage may be reduced by incorporating one or more internal layers into the display between the upper and lower polarizers to help ensure that linearly polarized backlight that passes through the display is not undesirably converted into elliptically polarized light. 
     The internal layers of the display may include one or more birefringence compensation layers. A birefringence compensation layer may be a transparent display layer that generates a change in polarization that compensates for corresponding changes in polarization that are generated by other layers of the display. 
     The birefringence compensation layer may be a glass layer having a photo-elastic constant that is opposite in sign to the photo-elastic constant of the thin-film transistor layer and/or the photo-elastic constant of the color filter layer. A birefringence compensation layer of this type may be attached to the thin-film transistor layer or the color filter layer using adhesive such as optically clear adhesive. However, this is merely illustrative. 
     In another embodiment, the birefringence compensation layer may be formed from a polymer material such as a polystyrene film. The polystyrene film may be bonded to or coated onto one or both of the color filter layer and the thin-film transistor layer. 
     Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an illustrative electronic device such as a laptop computer with a display in accordance with an embodiment of the present invention. 
         FIG. 2  is a perspective view of an illustrative electronic device such as a handheld electronic device with a display in accordance with an embodiment of the present invention. 
         FIG. 3  is a perspective view of an illustrative electronic device such as a tablet computer with a display in accordance with an embodiment of the present invention. 
         FIG. 4  is a schematic diagram of an illustrative electronic device with a display in accordance with an embodiment of the present invention. 
         FIG. 5  is a cross-sectional side view of an illustrative display in accordance with an embodiment of the present invention. 
         FIG. 6  is a cross-sectional side view of a display layer such as a layer of glass in a thin-film transistor layer or color filter layer showing how stress-induced birefringence may be generated upon application of tensile stress to the layer of glass. 
         FIG. 7  is a cross-sectional side view of a display layer such as a layer of glass in a thin-film transistor layer of color filter layer showing how stress-induced birefringence may be generated upon application of compressive stress to the layer of glass. 
         FIG. 8  is a cross-sectional side view of a layer of material such as a layer of glass that has been subjected to bending and that exhibits stress-induced birefringence in a display. 
         FIG. 9  is a cross-sectional side view of a display with layers of glass that have been subjected to bending due to forces imparted by mounting the display in a device housing in accordance with an embodiment of the present invention. 
         FIG. 10A  is a cross-sectional diagram of display layers in a conventional liquid crystal display. 
         FIG. 10B  is a Poincare sphere showing how the polarization of backlight may vary when passing through the conventional display layers of  FIG. 10A  when the layers are subject to stress-induced birefringence. 
         FIG. 11A  is a cross-sectional diagram of display layers in a liquid crystal display with a birefringence compensation layer that helps to reduce light leakage due to stress-induced birefringence in accordance with an embodiment of the present invention. 
         FIG. 11B  is a Poincare sphere showing how the polarization of backlight may vary when passing through the display layers of  FIG. 11A  in the presence of stress-induced birefringence in some of the layers in accordance with an embodiment of the present invention. 
         FIG. 12  is a cross-sectional diagram of display layers in a liquid crystal display with a birefringence compensation layer that is attached to a color filter layer in accordance with an embodiment of the present invention. 
         FIG. 13A  is a cross-sectional diagram of display layers in a liquid crystal display with a birefringence compensation layer that is attached to a thin-film transistor layer using a layer of optically clear adhesive in accordance with an embodiment of the present invention. 
         FIG. 13B  is a Poincare sphere showing how the polarization of backlight may vary when passing through the display layers of  FIG. 13A  in the presence of stress-induced birefringence in some of the layers in accordance with an embodiment of the present invention. 
         FIG. 14  is a cross-sectional diagram of display layers in a liquid crystal display with a birefringence compensation layer that is attached to a color filter layer using a layer of optically clear adhesive in accordance with an embodiment of the present invention. 
         FIG. 15A  is a cross-sectional diagram of display layers in a liquid crystal display with multiple birefringence compensation layers in accordance with an embodiment of the present invention. 
         FIG. 15B  is a Poincare sphere showing how the polarization of backlight may vary when passing through the display layers of  FIG. 15A  in the presence of stress-induced birefringence in some of the layers in accordance with an embodiment of the present invention. 
         FIG. 16  is a cross-sectional diagram of display layers in a liquid crystal display with birefringence compensation layers that are formed from polymer films on surfaces of a thin-film transistor layer and a color filter layer in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices may include displays. The displays may be used to display images to a user. Illustrative electronic devices that may be provided with displays are shown in  FIGS. 1 ,  2 , and  3 . 
       FIG. 1  shows how electronic device  10  may have the shape of a laptop computer having upper housing  12 A and lower housing  12 B with components such as keyboard  16  and touchpad  18 . Device  10  may have hinge structures  20  that allow upper housing  12 A to rotate in directions  22  about rotational axis  24  relative to lower housing  12 B. Display  14  may be mounted in upper housing  12 A. Upper housing  12 A, which may sometimes referred to as a display housing or lid, may be placed in a closed position by rotating upper housing  12 A towards lower housing  12 B about rotational axis  24 . 
       FIG. 2  shows how electronic device  10  may be a handheld device such as a cellular telephone, music player, gaming device, navigation unit, or other compact device. In this type of configuration for device  10 , housing  12  may have opposing front and rear surfaces. Display  14  may be mounted on a front face of housing  12 . Display  14  may, if desired, have a display cover layer or other exterior layer that includes openings for components such as button  26 . Openings may also be formed in a display cover layer or other display layer to accommodate a speaker port (see, e.g., speaker port  29  of  FIG. 2 ). 
       FIG. 3  shows how electronic device  10  may be a tablet computer. In electronic device  10  of  FIG. 3 , housing  12  may have opposing planar front and rear surfaces. Display  14  may be mounted on the front surface of housing  12 . As shown in  FIG. 3 , display  14  may have a cover layer or other external layer with an opening to accommodate button  26  (as an example). 
     The illustrative configurations for device  10  that are shown in  FIGS. 1 ,  2 , and  3  are merely illustrative. In general, electronic device  10  may be a laptop computer, a computer monitor containing an embedded computer, a tablet computer, a cellular telephone, a media player, or other handheld or portable electronic device, a smaller device such as a wrist-watch device, a pendant device, a headphone or earpiece device, or other wearable or miniature device, a television, a computer display that does not contain an embedded computer, a gaming device, a navigation device, an embedded system such as a system in which electronic equipment with a display is mounted in a kiosk or automobile, equipment that implements the functionality of two or more of these devices, or other electronic equipment. 
     Housing  12  of device  10 , which is sometimes referred to as a case, may be formed of materials such as plastic, glass, ceramics, carbon-fiber composites and other fiber-based composites, metal (e.g., machined aluminum, stainless steel, or other metals), other materials, or a combination of these materials. Device  10  may be formed using a unibody construction in which most or all of housing  12  is formed from a single structural element (e.g., a piece of machined metal or a piece of molded plastic) or may be formed from multiple housing structures (e.g., outer housing structures that have been mounted to internal frame elements or other internal housing structures). 
     Display  14  may be a touch sensitive display that includes a touch sensor or may be insensitive to touch. Touch sensors for display  14  may be formed from an array of capacitive touch sensor electrodes, a resistive touch array, touch sensor structures based on acoustic touch, optical touch, or force-based touch technologies, or other suitable touch sensor components. 
     Displays for device  10  may, in general, include image pixels formed from light-emitting diodes (LEDs), organic LEDs (OLEDs), plasma cells, electrowetting pixels, electrophoretic pixels, liquid crystal display (LCD) components, or other suitable image pixel structures. In some situations, it may be desirable to use LCD components to form display  14 , so configurations for display  14  in which display  14  is a liquid crystal display are sometimes described herein as an example. It may also be desirable to provide displays such as display  14  with backlight structures, so configurations for display  14  that include a backlight unit may sometimes be 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 or other portion of a display may be used as the outermost (or nearly outermost) layer in display  14 . A display cover layer or other outer display layer may be formed from a transparent glass sheet, a clear plastic layer, or other transparent member. 
     Touch sensor components such as an array of capacitive touch sensor electrodes formed from transparent materials such as indium tin oxide may be formed on the underside of a display cover layer, may be formed on a separate display layer such as a glass or polymer touch sensor substrate, or may be integrated into other display layers (e.g., substrate layers such as a thin-film transistor layer). 
     A schematic diagram of an illustrative configuration that may be used for electronic device  10  is shown in  FIG. 4 . As shown in  FIG. 4 , electronic device  10  may include control circuitry  28 . Control circuitry  28  may include storage and processing circuitry for controlling the operation of device  10 . Control circuitry  28  may, for example, 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. Control circuitry  28  may include processing circuitry based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio codec chips, application specific integrated circuits, etc. 
     Control circuitry  28  may be used to run software on device  10 , such as operating system software and application software. Using this software, control circuitry  28  may present information to a user of electronic device  10  on display  14 . When presenting information to a user on display  14 , sensor signals and other information may be used by control circuitry  28  in making adjustments to the strength of backlight illumination that is used for display  14 . 
     Input-output circuitry  30  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 circuitry  30  may include communications circuitry  32 . Communications circuitry  32  may include wired communications circuitry for supporting communications using data ports in device  10 . Communications circuitry  32  may also include wireless communications circuits (e.g., circuitry for transmitting and receiving wireless radio-frequency signals using antennas). 
     Input-output circuitry  30  may also include input-output devices  34 . A user can control the operation of device  10  by supplying commands through input-output devices  34  and may receive status information and other output from device  10  using the output resources of input-output devices  34 . 
     Input-output devices  34  may include sensors and status indicators  36  such as an ambient light sensor, a proximity sensor, a temperature sensor, a pressure sensor, a magnetic sensor, an accelerometer, and light-emitting diodes and other components for gathering information about the environment in which device  10  is operating and providing information to a user of device  10  about the status of device  10 . 
     Audio components  38  may include speakers and tone generators for presenting sound to a user of device  10  and microphones for gathering user audio input. 
     Display  14  may be used to present images for a user such as text, video, and still images. Sensors  36  may include a touch sensor array that is formed as one of the layers in display  14 . 
     User input may be gathered using buttons and other input-output components  40  such as touch pad sensors, buttons, joysticks, click wheels, scrolling wheels, touch sensors such as sensors  36  in display  14 , key pads, keyboards, vibrators, cameras, and other input-output components. 
     A cross-sectional side view of an illustrative configuration that may be used for display  14  of device  10  (e.g., for display  14  of the devices of  FIG. 1 ,  FIG. 2 , or  FIG. 3  or other suitable electronic devices) is shown in  FIG. 5 . As shown in  FIG. 5 , display  14  may include backlight structures such as backlight unit  42  for producing backlight  44 . During operation, backlight  44  travels outwards (vertically upwards in dimension Z in the orientation of  FIG. 5 ) and passes through display pixel structures in display layers  46 . This illuminates any images that are being produced by the display 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 housing  12  or display layers  46  may be mounted directly in housing  12  (e.g., by stacking display layers  46  into a recessed portion in housing  12 ). 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 thin-film transistors and associated electrodes (display 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. 
     During operation of display  14  in device  10 , control circuitry  28  (e.g., one or more integrated circuits such as components  68  on printed circuit  66  of  FIG. 5 ) may be used to generate information to be displayed on display (e.g., display data). The information to be displayed may be conveyed from circuitry  68  to display driver integrated circuit  62  using a signal path such as a signal path formed from conductive metal traces in flexible printed circuit  64  (as an example). 
     Display driver integrated circuit  62  may be mounted on thin-film transistor layer driver ledge  82  or elsewhere in device  10 . A flexible printed circuit cable such as flexible printed circuit  64  may be used in routing signals between printed circuit  66  and thin-film transistor layer  60 . If desired, display driver integrated circuit  62  may be mounted on printed circuit  66  or flexible printed circuit  64 . Printed circuit  66  may be formed from a rigid printed circuit board (e.g., a layer of fiberglass-filled epoxy) or a flexible printed circuit (e.g., a flexible sheet of polyimide or other flexible polymer layer). 
     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. 
     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 or bumps. The light-scattering features may be located on an upper surface and/or on an opposing lower surface of light guide 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 upwards direction by reflector  80 . Reflector  80  may be formed from a reflective material such as a layer of white plastic or other shiny materials. 
     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, compensation films for enhancing off-axis viewing, and brightness enhancement films (also 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. 5 , optical films  70  and reflector  80  may have a matching rectangular footprint. 
     When display  14  is mounted in a housing, the layers of display  14  such as thin-film transistor layer  58  and color filter layer  56  (e.g., the glass layers of the display) may be subjected to stresses. Stress may be imparted by bending the layers of display  14  when display  14  is mounted within housing  12  (e.g., using standoffs, housing walls, internal frame structures, display bezels, adhesive, and other mounting and support structures). The optical behavior of the layers of display  14  when bent depends on the type of materials used in forming the display layers. 
     As shown in  FIG. 6 , when a glass layer such as glass layer  92  is subjected to tensile stress by pulling ends  90  of glass layer  92  in opposing outward directions  94 , the glass layer may exhibit birefringence so that the optical axis (extraordinary axis) of the glass runs parallel to the direction of stress (i.e., horizontally within the page in the orientation of  FIG. 6 ). The ordinary axis of the glass layer may run perpendicular to the optical axis in the plane of glass layer  92 . 
     As shown in  FIG. 7 , when a glass layer such as glass layer  92  is subjected to compressive stress by pushing ends  90  of glass layer  92  in opposing inward directions  96 , the glass layer may exhibit birefringence so that the optical axis (extraordinary axis) of the glass runs parallel to the direction of stress (i.e., into the page in the orientation of  FIG. 6 ). The ordinary axis may run perpendicular to the extraordinary axis in the plane of layer  92 . 
     A bent layer of glass such as glass layer  92  of  FIG. 8  may exhibit compressive stress along top surface  98  (e.g., near the edge of glass layer  92 ) and may exhibit tensile stress along lower surface  100 . As a result, glass layer  92  may be characterized by an optical axis such as optical axis  102  that extends into the page of  FIG. 8  along upper surface  98  and an optical axis such as optical axis  104  that extends parallel to the page of  FIG. 8  along lower surface  100 . 
     As shown in  FIG. 9 , when the layers of display  14  such as color filter layer  56  and thin-film transistor layer  58  are mounted in device housing  12 , these layers may become bent (e.g., from forces introduced when mounting display  14  in housing  12 ). The bending of layers  56  and  58  may give rise to stress-induced birefringence. If care is not taken, this birefringence can adversely affect the performance of a display by causing light leakage in a display dark state when the display is viewed by a user. 
     Layers  56  and  58  may be glass layers or layers of other material with optical characteristics of the type described in connection with  FIGS. 6 ,  7 , and  8 . A layer of sealant such as sealant  106  (e.g., a bead of adhesive) may be interposed between color filter layer  56  and thin-film transistor layer  58 . Sealant  106  may run around the periphery of display  14  in a rectangular ring and may surround and enclose liquid crystal material  52 . The presence of sealant  106  may bond layers  52  and  58 , so that layers  52  and  58  act as a single body in the bending. Since the neutral axis of layer  52  and  58  is at the center of sealant  106 , when bent as shown, layer  52  has a net compressive stress while layer  56  has a net tensile stress. These net stresses may cause optical axis  108  of layer  56  near the edge of layer  56  (e.g., in edge region ER) to point into the page of  FIG. 9  and may cause optical axis  110  of layer  58  near the edge of layer  58  (e.g., in edge region ER) to lie in the plane of layer  58  (lying within the page and pointing to the right in the example of  FIG. 9 ). The perpendicular optical axes of layers  58  and  56  (particularly prevalent in edge regions ER) may lead to changes in the polarization state of backlight passing through these layers that cause light leakage in conventional displays. 
       FIG. 10A  is a cross-sectional side view of a conventional display having glass layers that may be subject to stress-induced birefringence. As shown in  FIG. 10 , the display of  FIG. 10A  is a liquid crystal display in which liquid crystal layer  116  is sandwiched between thin-film transistor layer  114  and color filter layer  118 . The display has upper and lower polarizers located respectively above and below the layers of  FIG. 10 . A backlight may generate backlight  112  that travels vertically upwards through the display in direction Z. Upon passing through the lower polarizer (i.e., at point A of  FIG. 10 ), backlight  112  may be linearly polarized (i.e., the lower polarizer may impart a linear polarization on backlight  112 ). The polarization of light  112  may then be affected by passing from point A to point B through thin-film transistor layer  114  (which is exhibiting stress-induced birefringence), by passing from point B to point C through liquid crystal  116  (which is birefringent), and by passing from point C to point D through color filter layer  118  (which is exhibiting stress-induced birefringence). The upper and lower polarizers in the display do not typically exhibit stress-induced birefringence and are not shown in  FIG. 10A . 
     The polarization state of backlight  112  as backlight  112  travels through the layers of the conventional display of  FIG. 10A  is illustrated in the Poincare sphere of  FIG. 10B . In a Poincare sphere, linear polarization states are represented by points on equatorial line  130 . Point  132  represents right-hand circularly polarized light. Point  134  represents left-hand circularly polarized light. Intermediate points on the Poincare sphere represent various types of elliptically polarized light. 
     Each of the layers of  FIG. 10A  has an optical axis that is aligned in a different direction. In the Poincare sphere representation of  FIG. 10B  representing portions of the display in which light leakage occurs, thin-film transistor layer  114  is characterized by optical axis  122 , color filter layer  118  is characterized by optical axis  124 , and liquid crystal layer  116  is characterized by optical axis  120 . However, this is merely illustrative. Optical axis  120  may represent an e-mode optical axis for a liquid crystal display such as an in-plane-switching (IPS) LCD display or a fringe-filed-switching (FFS) LCD display. However, other LCD modes may be used. The LCD optical axis may be different for other LCD modes. 
     On the Poincare sphere, azimuthal angle α of a vector to each point P on equatorial line  130  is equal to 2θ, where θ is equal to an actual physical angle (e.g., an azimuthal angle in real space that is associated with the orientation of an optical axis for a display layer or an angle associated with the polarization of light such as light  112  that is passing through the display). As a result, a pair of axes such as thin-film transistor axis  122  and liquid crystal layer axis  120  that appear to be perpendicular to each other in the Poincare sphere representation of  FIG. 10B  are, within the real-life coordinate system of the display, oriented at a 45° angle with respect to each other. Similarly, a pair of axes such as thin-film transistor axis  122  and color filter layer axis  124  that appear to be separated by 180° in the Poincare sphere representation of  FIG. 10B  are, within the real-life coordinate system of the display, oriented at a 90° angle with respect to each other (i.e., axis  124  is perpendicular to axis  122 ). 
     The behavior of the polarization of light  112  is affected by the orientation of each optical axis and the thickness of each layer in the display of  FIG. 10A . As shown in  FIG. 10B , light  112  is initially linearly polarized (point A). Following passage through layer  114 , the polarization of light  112  is represented by point B on the Poincare sphere of  FIG. 10B  (i.e., light  112  is transformed from linearly polarized light to elliptically polarized light due to the stress-induced birefringence of layer  114 ). Visually, the transition from point A to point B along line  140  on the surface of the Poincare sphere is associated with rotation of point A about thin-film transistor layer optical axis  122  on the surface of the sphere. Following passage of light  112  through layer  114 , light  112  passes through liquid crystal layer  116 . Layer  116  causes the polarization of light  112  to move from point B to point C along line  142  on the Poincare sphere of  FIG. 10B  (rotating about liquid crystal layer optical axis  120 ). 
     After traveling through liquid crystal layer  116 , light  112  passes through layer  118 . The birefringence of layer  118  causes the polarization of light  112  to change from the polarization state represented by point C to the polarization state represented by point D along line  144  of the Poincare sphere of  FIG. 10B  (rotating about color filter layer optical axis  124 ). 
     If liquid crystal layer  116  had not been present, the polarization state changes associated with lines  144  and  140  would have canceled each other out, resulting in minimal changes to the linear polarization of light  112  (i.e., light  112  would have remain linearly polarized with a polarization state represented by point A and the display would have operated satisfactorily). Because of the presence of liquid crystal layer  116  and the associated transition of the polarization state of light  112  from point B to point C, however, light  112  at point D (i.e., light  112  exiting the upper surface of color filter layer  118  of  FIG. 10A ) is substantially elliptically polarized, rather than being linearly polarized as desired. When this elliptically polarized light passes through the upper polarizer whose transmission axis is perpendicular to the lower polarizer, the fact that the light is not linearly polarized in a direction perpendicular to the upper polarizer, as expected, allows some of the light to leak out from the upper surface of the display through the upper polarizer, even when the electric field being applied to liquid crystal layer  116  is attempting to display a black display pixel. Display performance in conventional displays is therefore limited by the inability of conventional displays to satisfactorily display black images in the presence of stress-induced birefringence in the layers of the display. 
     Illustrative display configurations with designs that address the shortcomings of conventional displays in handling stress-induced birefringence are shown in  FIGS. 11A ,  12 ,  13 A,  14 ,  15 A, and  16 . 
     As shown in the example of  FIG. 11A , display  14  may be provided with a birefringence compensation layer such as layer  170 . Birefringence compensation layer  170  may be formed from a material having a photo-elastic constant that is opposite in sign to the photo-elastic constants of TFT glass  58  and color filter (CF) glass  56 . Compensation layer materials having relatively higher magnitude photo-elastic constants may be used in order to reduce the thickness of compensation layer  170 . 
     For example, TFT glass  58  and color filter glass  56  may have a photo-elastic constant of between 3.0 and 3.6 Brewster. In this type of configuration, birefringence compensation layer  170  may be formed from glass having a photo-elastic constant between −3.0 and −3.6 Brewster (as an example). Compensation layer  170  may be a glass layer having a thickness of between 0.1 mm and 0.9 mm, between 0.2 mm and 0.4 mm, between 0.1 mm and 0.4 mm, less than 1 mm, or greater than 0.1 mm (as examples). TFT glass  58  and color filter glass  56  may each have a thickness of between 0.1 mm and 0.9 mm, between 0.5 mm and 0.9 mm, between 0.6 mm and 0.8 mm, less than 1 mm, or greater than 0.1 mm (as examples). 
     Compensation layer  170  may be a glass layer having a thickness that is substantially equal to the thickness of layer  56  and/or substantially equal to the thickness of layer  58 . However, this is merely illustrative. If desired, compensation layer  170  may be a glass layer having a thickness that is different from the thickness of layers  56  and/or  58  or compensation layer  170  may be formed from a material other than glass such as a polymer material (e.g., a polystyrene film, a polysulfone material, or a thermoplastic polymer such as polymethyl methacrylate (PMMA)). 
     In configurations in which layer  170  is formed from a material other than glass, layer  170  be a polymer birefringence compensation layer having a thickness between 0.001 microns and 300 microns, between 0.001 microns and 10 microns, between 0.001 microns and 100 microns, between 10 microns and 300 microns, greater than 0.001 microns, less than 1000 microns, or between 100 microns and 300 microns (as examples). The thickness of layer  170  may be chosen based on the photo-elastic constant of the polymer material and the Young&#39;s modulus of the polymer material as well as the thickness of TFT layer  58  and color filter layer  56 . 
     Birefringence compensation layer  170  may have other properties such as a Young&#39;s modulus. Birefringence compensation layer  170  may have a thickness, a Young&#39;s modulus, and a photo-elastic constant (sometimes referred to as a stress-optic constant) that are chosen so that, under stress, layer  170  causes a change in polarization of light such as light  44  that compensates for changes in the polarization of light  44  by layer  56  and/or layer  58 . 
     The layers of display  14  that are shown in  FIG. 11A  may be sandwiched between upper and lower polarizers (not shown in  FIG. 11A ) such as upper polarizer  54  and lower polarizer  60 . In the example of  FIG. 11A , compensation layer  170  is attached to a surface TFT substrate  58  using adhesive  172 . Adhesive  172  may be formed from the same material as adhesive sealant  106  or may be formed from another adhesive material. Adhesive  172  may run around the periphery of display  14  in a rectangular ring between compensation layer  170  and thin-film transistor layer  58 . The presence of adhesive  172  may cause tensile stress on compensation layer  170  to be similar to any tensile stress on layer  58 . In this way, birefringence effects generated by compensation layer  170  may at least partially counteract birefringence effects generated by thin-film transistor layer  58 . 
     The behavior of the polarization of display backlight such as light  44  is affected by the orientation of each optical axis, the photo-elastic constant, and the thickness of each layer in display  14 . Optical axis  150  of  FIG. 11B  may be associated with thin-film transistor layer (TFT glass)  58 , which may exhibit stress-induced birefringence. Optical axis  152  of  FIG. 11B  may be associated with the liquid crystal (LC) layer  52 . Optical axis  151  of  FIG. 11B  may be associated with birefringence compensation layer  170 , which may exhibit stress-induced birefringence. Optical axis  154  may be associated with color filter layer  56 , which may exhibit stress-induced birefringence. 
     As shown in  FIG. 11B , light  44  is initially linearly polarized (point P 1 ). Following passage through birefringence compensation layer  170 , the polarization of light  44  may be represented by point P 2  on the Poincare sphere of  FIG. 11B  (i.e., light  44  may be transformed from linearly polarized light to elliptically polarized light due to the stress-induced birefringence of layer  170 ). The transition from point P 1  to point P 2  along line  156  on the surface of the Poincare sphere of  FIG. 11B  is associated with rotation about compensation layer (Comp glass) optical axis  151 . Following passage of light  44  through layer  58 , the polarization of light  44  may be represented by point P 3 . Because layer  170  has a photo-elastic constant that is substantially opposite to the photo-elastic constant of layer  58 , the change in polarization of light  44  due to birefringence in layer  58  may undo some or all of the polarization change that occurred when light  44  passed through layer  170 . The transition of the polarization state of light  44  from point P 2  to P 3  through layer  58  of  FIG. 11A  is represented by the transition from polarization state P 2  to polarization state P 3  in  FIG. 11B  along line  158 . 
     Due to the presence of compensation layer  170 , light  44  in polarization state P 3  will be less elliptically polarized (more linearly polarized) upon passing through layers  52  and  56  than in conventional display arrangements. As shown in  FIG. 11B , the transition of the polarization state of light  44  when traveling from point P 3  to point P 4  through liquid crystal layer  52  of  FIG. 11A  may be represented by line  160  and the transition of the polarization state of light  44  when traveling from point P 4  to point P 5  through color filter layer  56  may be represented by line  162 . Although light  44  is elliptically polarized at point P 5 , light  44  at point P 5  is more linearly polarized than conventional light at point D of  FIGS. 10A and 10B , thereby reducing light leakage and improving the performance of display  14 . 
     The example of  FIG. 11A  in which birefringence compensation layer  170  is attached to thin-film transistor layer  58  is merely illustrative. If desired, layer  170  may be attached to color filter layer  56  as shown in  FIG. 12 . As shown in  FIG. 12 , layer  170  may be attached to color filter substrate  56  using adhesive  172 . In this type of configuration, adhesive  172  may run around the periphery of display  14  in a rectangular ring between compensation layer  170  and color filter layer  56 . The presence of adhesive  172  may cause tensile stress on compensation layer  170  to be similar to any tensile stress on layer  56 . In this way, birefringence effects generated by compensation layer  170  may at least partially counteract birefringence effects generated by layer  56  (as well as effects generated by layers  52  and  58 ). 
       FIG. 13A  is a diagram of another illustrative configuration that may be used for the middle layers of display  14  between upper polarizer  54  and lower polarizer  60 . As shown in  FIG. 13A , compensation layer  170  may be attached to layer  58  using a layer of optically clear adhesive such as adhesive layer  174 . Adhesive layer  174  may substantially fill the gap between layer  170  and layer  58  so that substantially all of the surface of layer  170  is attached to layer  58 . 
     Adhesive layer  174  may, for example, be formed from a pressure sensitive adhesive. Adhesive  174  may be formed from a material having a strength of greater than 100 MPa or other suitable adhesive strength for transferring tensile stresses on layer  58  efficiently into matching stresses on layer  170 . 
     As shown in  FIG. 13B , light  44  is initially linearly polarized (as indicated by point P 1  of  FIG. 13B ). Following passage through birefringence compensation layer  170 , the polarization of light  44  may be represented by point P 2  on the Poincare sphere of  FIG. 13B . The transition from point P 1  to point P 2  along line  176  on the surface of the Poincare sphere of  FIG. 13B  is associated with rotation about compensation layer (Comp glass) optical axis  151 . 
     Following passage of light  44  through adhesive layer  174  and thin-film transistor layer  58 , the polarization of light  44  may be represented by point P 3 . The transition of the polarization state of light  44  when from point P 2  to P 3  through layer  58  of  FIG. 13A  is represented by the transition from polarization state P 2  to polarization state P 3  in  FIG. 13B  along line  178 . Because layer  170  of  FIG. 13A  is attached to layer  58  by extended adhesive layer  174 , the change in polarization of light  44  due to birefringence in layer  58  may be more closely compensated by the polarization change that occurred when light  44  passed through layer  170  than the compensation described above in connection with  FIG. 11B . 
     As shown in  FIG. 13B , the transition of the polarization state of light  44  when traveling from point P 3  to point P 4  through liquid crystal layer  52  of  FIG. 13A  may be represented by line  180  and the transition of the polarization state of light  44  when traveling from point P 4  to point P 5  through color filter layer  56  may be represented by line  182 . Although light  44  may still be elliptically polarized at point P 5 , because layer  170  is attached to layer  58  along substantially the entire surface of layer  58 , light  44  at point P 5  of  FIG. 13B  may be more linearly polarized than light  44  at point P 5  of  FIG. 11B . 
     The example of  FIG. 13A  in which birefringence compensation layer  170  is attached to thin-film transistor layer  58  using adhesive layer  174  is merely illustrative. If desired, adhesive layer  174  may attach layer  170  to color filter layer  56  as shown in  FIG. 14 . As shown in  FIG. 14 , adhesive layer  174  may substantially fill the space between compensation layer  170  and color filter layer  56 , thereby attaching substantially all of the surface of layer  170  to the surface of layer  56 . The presence of adhesive layer  174  may be more efficient at transferring tensile stress on layer  56  to compensation layer  170  than adhesive  172  of  FIG. 12 . The configuration of  FIG. 14  for display  14  may therefore be more efficient at reducing light leakage from display  14  than conventional displays and may be more efficient at reducing light leakage than displays of the type shown in  FIG. 12 . 
     If desired, display  14  may be provided with multiple birefringence compensation layers, as shown in  FIG. 15A . In the example of  FIG. 15A , display  14  includes lower birefringence compensation layer  170 L and upper birefringence compensation layer  170 U. Lower birefringence compensation layer  170 L may be attached to thin-film transistor layer  58  using a lower adhesive layer such as adhesive layer  174 L. Upper birefringence compensation layer  170 U may be attached to color filter layer  56  using an upper adhesive layer such as adhesive layer  174 U. 
     Birefringence compensation layers  174 U and  174 L may be formed from glass having a negative photo-elastic constant such as a photo-elastic constant between −3.0 and −3.6 Brewster, between 0 Brewster and −3.6 Brewster, between, −0.1 Brewster and −0.3 Brewster, between −0.1 Brewster and 3.0 Brewster, or less than zero Brewster (as examples). In general, the photo-elastic constant and the thickness of compensation layers  170 L and  170 U may be chosen in any suitable combination for compensating for birefringence effects due to stresses in layers  56  and  58 . The layers of display  14  that are shown in  FIG. 15A  may be sandwiched between upper and lower polarizers (not shown in  FIG. 15A ) such as upper polarizer  54  and lower polarizer  60 . 
     Lower adhesive layer  174 L may substantially fill the gap between layer  170 L and layer  58 . Upper adhesive layer  174 U may substantially fill the gap between layer  170 U and layer  56 . Adhesive layers  174 U and  174 L may, for example, be formed from adhesive material such as pressure sensitive adhesive. As examples, adhesive layers  174 U and  174 L may be pressure sensitive adhesives having a strength of between 190 MPa and 210 MPa, between 150 MPa and 250 MPa, greater than 150 MPa, greater than 190 MPa or other suitable adhesive strength. However, these examples are merely illustrative. If desired, any suitable adhesive material may be used to attach compensation layers  170 L and  170 U respectively to layers  58  and  56 . 
     As shown in  FIG. 15B , light  44  is initially linearly polarized (as indicated by point P 1  of  FIG. 15B ). Following passage through lower birefringence compensation layer  170 L, the polarization of light  44  may be represented by point P 2  on the Poincare sphere of  FIG. 15B . The transition from point P 1  to point P 2  along line  196  on the surface of the Poincare sphere of  FIG. 15B  is associated with rotation about bottom compensation layer (bottom Comp glass) optical axis  151 ′. 
     Following passage of light  44  through lower adhesive layer  174 L and thin-film transistor layer  58 , the polarization of light  44  may be represented by point P 3 . The transition of the polarization state of light  44  from point P 2  to P 3  through layer  58  of  FIG. 15A  is represented by the transition from polarization state P 2  to polarization state P 3  in  FIG. 15B  along line  198 . 
     Because light at point P 3  has passed through both thin-film transistor layer  58  and lower compensation layer  170 L, light  44  may experience a minimal or negligible rotation when passing through liquid crystal layer  52 . The transition of the polarization state of light  44  when traveling from point P 3  to point P 4  through color filter layer  56  of  FIG. 15A  may be represented by line  200 . The transition of the polarization state of light  44  when traveling from point P 4  to point P 5  through upper birefringence compensation layer  170 U may be represented by line  202  about top compensation layer (top Comp glass) optical axis  151 ″. Because display  14  of  FIG. 15A  is provided with upper and lower birefringence compensation layers, light  44  at point P 5  may be substantially linearly polarized, thereby reducing light leakage to negligible levels and improving the performance of display  14 . 
     The examples of  FIGS. 11A ,  12 ,  13 A,  14 , and  15 A in which birefringence compensation layers  170  are attached to layers  56  and/or  58  using adhesive are merely illustrative. If desired, birefringence compensation layers such as birefringence compensation layers  170 U and  170 L of  FIG. 16  may be formed from a polymer coating or a bonded polymer material such as a polystyrene film that is coated or laminated to layers  56  and/or  58 . 
     In the example of  FIG. 16 , compensation layers  170 L and  170 U are formed from a polymer material (e.g., a polystyrene film, a polysulfone material, a thermoplastic polymer such as polymethyl methacrylate (PMMA) or other suitable polymer materials). Polymer birefringence compensation layers of this type may have a thickness between 0.1 mm and 0.6 mm, between 0.05 mm and 0.3 mm, between 0.05 mm and 0.4 mm, between 0.3 mm and 0.6 mm, less than 1 mm, or greater than 1 micron (as examples). 
     Polymer birefringence compensation layers of the type shown in  FIG. 16  may, if desired, have a stress-optic coefficient, a thickness, and a Young&#39;s modulus configured to generate changes in polarization of light  44  that compensate for changes in the polarization of light  44  by layer  56  and/or layer  58  when layer  56  and/or layer  58  are stressed. 
     As examples, layers  56  and  58  may have a stress-optic coefficient of between 3.0 Brewster and 3.6 Brewster and a Young&#39;s modulus of between 65 gigapascal (GPa) and 85 GPa. In one suitable example, polymer compensation layers  170 U and  170 L may be formed from polystyrene layers having a stress-optic coefficient of between −45 Brewster and −65 Brewster and a Young&#39;s modulus of between 3.0 GPa and 3.6 GPa. In another example, polymer compensation layers  170 U and  170 L may be formed from polysulfone layers having a stress-optic coefficient of between −45 Brewster and −65 Brewster and a Young&#39;s modulus of between 2.0 GPa and 10 GPa. In a third example, polymer compensation layers  170 U and  170 L may be formed from PMMA materials having a stress-optic coefficient of between −1 Brewster and −4.5 Brewster and a Young&#39;s modulus of between 1.0 GPa and 4.0 GPa. In general, polymer compensation layers such as polymer compensation layers  170 U and  170 L may have a negative photo-elastic constant that is less than −10 Brewster and a Young&#39;s modulus that is smaller than 40 GPa. 
     The example of  FIG. 16  in which display  14  includes two polymer birefringence compensation layers is merely illustrative. If desired, display  14  may be provided with a single polymer birefringence compensation layer such as a selected one of polymer compensation layers  170 U and  170 L of  FIG. 16 . 
     The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention.

Metadata:
Filing Date: 20130506
Publication Date: 20150714
Grant Date: 20150714
Priority Date: 20130506
Inventors: HUANG YI
TOWASHIRAPORN PONGPINIT
ZWEIGLE ERIK A.
GU MINGXIA
CHEN WEI
CHEN CHENG
YANG YOUNG CHEOL
KAUVAR ISAAC V.
Assignee: APPLE INC
CPC Classifications: [{"code": "G02F2202/40", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/1333", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133528", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133634", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/0131", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/13363", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F1/133305", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/0131", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133528", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/133305", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F2202/40", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/13363", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F1/133634", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02F1/1333", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 50588938