PATENT DOCUMENT

Publication Number: US-9804316-B2
Application Number: US-201414476610-A
Country: US
Kind Code: B2

Title: Display having backlight with narrowband collimated light sources

Abstract:
A display has an array of display pixels formed from display layers such as one or more polarizer layers, a substrate on which an array of display pixel elements such as color filter elements and downconverter elements are formed, a liquid crystal layer, and a thin-film transistor layer that includes display pixel electrodes and display pixel thin-film transistors for driving control signals onto the display pixel electrodes to modulate light passing through the display pixels. A light source such as one or more laser diodes or light-emitting diodes may be used to generate light for the display. The light may be launched into the edge of a polymer layer or other light guide plate structure. A light guide plate ma include phase-matched structures such as holographically recorded gratings or photonic lattices that direct the light upwards through the array of display pixels.

Claims:
What is claimed is: 
     
       1. A display, comprising:
 an array of display pixels formed from a layer of liquid crystal material, a polarizer layer, and a thin-film transistor layer;
 a light source that emits exclusively blue light and green light; and 
 a light guide plate having phase-matched structures that direct the blue and green light from the light source through the array of display pixels, wherein the light guide plate has opposing upper and lower surfaces connected by an edge, wherein the layer of liquid crystal material overlaps the upper surface of the light guide plate, wherein the thin-film transistor layer is interposed between the light guide plate and the layer of liquid crystal material, wherein the light source emits light into the edge of the light guide plate, wherein the array of display pixels comprises a first subarray of display pixels having red downconverters that convert the blue light to red light, a second subarray of display pixels through which the green light passes, and a third subarray of display pixels through which the blue light passes. 
 
 
     
     
       2. The display defined in  claim 1  wherein the light source comprises a laser. 
     
     
       3. The display defined in  claim 1  wherein the light source comprises a light-emitting diode. 
     
     
       4. The display defined in  claim 1  wherein the phase-matched structures comprise gratings. 
     
     
       5. The display defined in  claim 4  wherein the light guide plate comprises a layer of polymer and wherein the gratings comprise holographically recorded gratings in the layer of polymer. 
     
     
       6. The display defined in  claim 1  wherein the display pixel elements that pass the blue light have light spreading features. 
     
     
       7. The display defined in  claim 6  wherein the light spreading features comprise microlenses. 
     
     
       8. The display defined in  claim 6  wherein the display pixel elements that pass the blue light are formed from a polymer and wherein the light spreading features include structures embedded within the polymer. 
     
     
       9. The display defined in  claim 1  wherein the red downconverters comprise polymer in which quantum dots are embedded and wherein the quantum dots comprise semiconductor particles. 
     
     
       10. The display defined in  claim 9  further comprising dichroic filters adjacent to the red downconverters. 
     
     
       11. A display, comprising:
 display layers that includes a liquid crystal layer and at least one layer containing light spreading features, wherein the display layers are configured to form an array of display pixels to display images; 
 a light source that produces exclusively blue light and green light; and 
 a light guide plate having holographically recorded gratings that direct the blue and green light from the light source through the display pixels, wherein the light guide plate has at least a first holographically recorded grating that directs only the green light from the light source through the display pixels, wherein the light guide plate has at least a second holographically recorded grating that directs only the blue light from the light source through the display pixels, and wherein the liquid crystal layer is interposed between the layer containing light spreading features and the light guide plate. 
 
     
     
       12. The display defined in  claim 11  wherein the display pixels include red downconverters, wherein the at least second holographically recorded grating directs a first portion of the blue light from the light source through the red downconverters, and wherein the red downconverters convert the blue light to red light. 
     
     
       13. The display defined in  claim 12 , wherein the display pixels comprise blue color filter elements and wherein the at least second holographically recorded grating directs a second portion of the blue light from the light source through the blue color filter elements. 
     
     
       14. The display defined in  claim 13 , wherein the display pixels comprise green color filter elements and wherein the at least first holographically recorded grating directs the green light from the light source through the green color filter elements.

Description:
This application claims the benefit of provisional patent application No. 61/919,070, filed Dec. 20, 2013, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates generally to electronic devices and, more particularly, to electronic devices with displays. 
     Electronic devices often include displays. For example, cellular telephones, computers, and televisions have displays. 
     Displays such as liquid crystal displays have arrays of display pixels. To enhance the visibility of images that are displayed on an array of display pixels, a display may be provided with a backlight. In a typical configuration, an array of light-emitting diodes emits light into the edges of a clear light guide plate. The light guide plate distributes the light laterally across the display. Light scattering features in the light guide plate help scatter the light outwards through the array of display pixels. 
     Conventional backlight arrangements such as these include additional layers to enhance performance such as a reflector to reflect inwardly scattered light outward through the display pixel array, diffuser layers for homogenizing backlight, compensation films to enhance off-axis viewing, and prism films that help collimate light from the backlight. These layers and other layers in a display may add undesired bulkiness, cost, and complexity. Efficiency losses may also arise due to the presence of black matrix structures in the color filter layer of a display that separate adjacent color filter elements. 
     It would therefore be desirable to be able to provide a backlight configuration that overcomes these issues. 
     SUMMARY 
     A display may have an array of display pixels. The display pixels may be formed from display layers such as one or more polarizer layers, a substrate that supports an array of display pixel elements such as color filter elements formed from colored polymer clear polymer elements, and downconverter elements, a liquid crystal layer, and a thin-film transistor layer. The thin-film transistor layer may include display pixel electrodes and display pixel thin-film transistors for driving control signals onto the display pixel electrodes to modulate light passing through the display pixels. 
     A light source such as one or more laser diodes or light-emitting diodes may be used to generate light for the display. The light may be launched into the edge of a polymer layer or other planar light guide plate structure. The light guide plate may include phase-matched structures such as holographically recorded gratings or photonic lattices that direct the light upwards through the array of display pixels in the form of narrow collimated beams. Display pixel elements may be provided with microlenses or other light spreading features to ensure that light is distributed over a desired angle. 
     The light source may include red, green, and blue light sources that are configured to create corresponding red, green, and blue image frames that are displayed sequentially on the array of display pixels in a field sequential display arrangement. If desired, the light source may have fewer colors and the display pixel array may be provided with downconverters to produce other colors. For example, a blue light source may produce blue light that is downconverted to red light by a subarray of red downconverters in the array of display pixels. With this type of arrangement, the light source can be used in providing light for all display pixels simultaneously. 
    
    
     
       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. 
         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. 
         FIG. 3  is a perspective view of an illustrative electronic device such as a tablet computer with a display in accordance with an embodiment. 
         FIG. 4  is a perspective view of an illustrative electronic device such as a display for a computer or television with display structures in accordance with an embodiment. 
         FIG. 5  is a cross-sectional side view of an illustrative display in accordance with an embodiment. 
         FIG. 6  is a diagram of an illustrative frequency downconverter element of the type that may be used in a display in accordance with an embodiment. 
         FIG. 7  is a graph showing how light with a short wavelength can be downconverted to produce light at longer wavelengths in a backlight in accordance with an embodiment. 
         FIG. 8  is a diagram of an illustrative diffuser based on light scattering particles for spreading light in a display in accordance with an embodiment. 
         FIG. 9  is a diagram of an illustrative diffuser that has an array of microlenses for spreading light in a display in accordance with an embodiment. 
         FIG. 10  is a diagram of an illustrative diffuser that has bumps and pits for spreading light in a display in accordance with an embodiment. 
         FIG. 11  is a diagram of an illustrative color filter element showing how light scattering features such as embedded particles and surface features such as microlenses can be incorporated into a color filter element to help angularly spread light in a display in accordance with an embodiment. 
         FIG. 12  is a cross-sectional side view of an illustrative display having an array of display pixel elements including downconverters fed by a single-wavelength light source and color filter elements in accordance with an embodiment. 
         FIG. 13  is a cross-sectional side view of an illustrative display having an array of display pixel elements including downconverters fed by a multiple-wavelength light source and color filter elements in accordance with an embodiment. 
         FIG. 14  is a cross-sectional side view of a portion of an illustrative display in which a filter is located on the underside of a downconverter to prevent leakage of downconverted light in accordance with an embodiment. 
         FIG. 15  is a cross-sectional side view of an illustrative display having a multiwavelength light source that is used to sequentially illuminate an array of display pixels using different respective colors in accordance with an embodiment. 
         FIG. 16  is a top view of an illustrative light guide plate in a display showing how a fiber with grating structures may be used to emit light into the edge of the light guide plate in accordance with an embodiment. 
         FIG. 17  is a top view plan illustrative light guide plate being edge lit by an array of light sources such as an array of light-emitting diodes in accordance with an embodiment. 
         FIG. 18  is a top view of an illustrative light guide plate in which a grating structure is being used to distribute light from a light source along the edge of a display active area in accordance with an embodiment. 
         FIG. 19  is a cross-sectional side view of an illustrative display backlight structure in which light is being routed from a light source in one plane to a light guide plate in a parallel plane using light guiding structures in accordance with an embodiment. 
         FIG. 20  is a cross-sectional side view of an illustrative display backlight structure having a reflective polarizer in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices may be provided with displays having backlights. Light for a backlight may be provided using a narrowband collimated light source such as a laser diode or nanowire light-emitting diode. The light can be fanned out to the pixel array efficiently using a light guide plate with phase-matched structures such as holographically recorded gratings or photonic lattices. The phase-matched structures may redirect light outwards (e.g., upwards) through an array of display pixels (i.e., the light may be redirected 90°) in the form of a corresponding array of narrow collimated beams. This scheme is able to retain the low etendue of the light source better than the light scattering surfaces on conventional backlight light guide plates. Well-controlled and high-efficiency light redirection techniques are used in place of random scattering and reflection events. Outwardly (upwardly) redirected light front the phase-matched structures in the light guide plate can be accurately aligned with the display pixels&#39; modulating aperture, reducing or eliminating wasteful illumination of black-mask areas. 
     The display pixel array may be based on liquid crystal display pixel structures. Because the upwardly directed backlight that is incident on each liquid crystal display pixel is narrowband and collimated the liquid crystal display pixels can be optimized to exhibit high contrast and need not be configured to form conventional wide-angle-of-view liquid crystal structures. Rather, the liquid crystal display pixels can be optimized to exhibit high contrast, high transmission, high speed, desirable image retention properties, and other desirable display properties. High speed display pixels may be used for example, to support field sequential color displays. Use of narrowband collimated light sources and associated phase-matched structures for redirecting light through the array of display pixels may allow display thickness to be minimized and may help simplify display backlight structures (e.g., diffuser films and prism films for collimating backlight before the backlight passes through the display pixels may be eliminated). 
     Electronic devices  10  of the type that may be provided with displays having backlights with narrowband collimated light sources and light-guide plates with holographic gratings or other phase-matched structures for directing light through an array of liquid crystal display pixel structures are shown in  FIGS. 1, 2, 3, and 4 . 
     Electronic device  10  of  FIG. 1  has the shape of a laptop computer and has upper housing  12 A and lower housing  12 B with components such as keyboard  16  and touchpad  18 . Device  10  has hinge structures  20  (sometimes referred to as a clutch barrel) to allow upper housing  12 A to rotate in directions  22  about rotational axis  24  relative to lower housing  12 B. Display  14  is mounted in housing  12 A. Upper housing  12 A, which may sometimes be referred to as a display housing or lid, is placed in a closed position by rotating upper housing  12 A towards lower housing  12 B about rotational axis  24 . 
       FIG. 2  shows an illustrative configuration for electronic device  10  based on 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  has opposing front and rear surfaces. Display  14  is mounted on a front face of housing  12 . Display  14  may have an exterior layer that includes openings for components such as button  26  and speaker port  28 . Device  10  may, if desired, be a compact device such as a wrist-mounted device or pendant device (as examples). 
     In the example of  FIG. 3 , electronic device  10  is a tablet computer. In electronic device  10  of  FIG. 3 , housing  12  has opposing planar front and rear surfaces. Display  14  is mounted on the front surface of housing  12 . As shown in  FIG. 3 , display  14  has an opening to accommodate button  26 . 
       FIG. 4  shows an illustrative configuration for electronic device  10  in which device  10  is a computer display, a computer that has an integrated computer display, or a television. Display  14  is mounted on a front face of housing  12 . With this type of arrangement, housing  12  for device  10  may be mounted on a wall or may have an optional structure such as support stand  30  to support device  10  on a flat surface such as a tabletop or desk. 
     Display  14  may be a liquid crystal display or a display using other types of display technology. Examples in which display  14  use liquid crystal display technology are sometimes described herein as an example. 
     A cross-sectional side view of an illustrative configuration for display  14  of device  10  (e.g., a liquid crystal display for the devices of  FIG. 1 ,  FIG. 2 ,  FIG. 3 ,  FIG. 4  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 pixels  80  in display layers  46 . This illuminates any images that are being produced by display pixels  80  for viewing by a user. For example, backlight  44  may illuminate images on an array of display pixels formed from display layers  46  that are being viewed by viewer  48  in direction  50 . Display pixels  80  may be arranged in rows and columns to from a rectangular array (e.g., an array that lies in the X-Y plane of  FIG. 5 ). 
     Display layers  46  and the layer(s) of backlight structures  42  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  and/or structures  42  may be mounted directly in housing  12  (e.g., by stacking display layers  46  and/or structures  42  into a recessed portion in housing  12 ). 
     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 upper (outer) display layer(s)  56  and lower (inner) display layer(s)  58 . 
     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, downconverter elements, light spreading structures, 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. The electrodes, which are sometimes referred to as display pixel electrodes, may each be associated with a respective display pixel  80 . Dating operation of display  14 , the display pixel electrodes of the thin-film transistor layer apply electric fields to liquid crystal layer  52  and thereby displaying images on display  14 . Layer  56  may be a layer (sometimes referred to as a color filter layer) that includes an array of color filter elements and/or frequency downconverters 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. 
     Display  46  may contain one or more polarizer layers. For example, layers  56  may include an upper polarizer. Layer  58  may include an optional lower polarizer and/or light  44  may be polarized upon exiting backlight  42  (in which case the lower polarizer may be omitted to minimize display thickness). During operation of display  14 , the relative orientations between the polarization of light  44  entering liquid crystal layer  52  from below and the polarization of the upper polarizer serve to create a normally on or normally of array of display pixels. The thin-film transistors and electrodes in the thin-film transistor layer are used to adjust the liquid crystal material of layer  52  in each display pixel, thereby displaying images on display  14 . 
     As shown in  FIG. 5 , display backlight structures  42  may include light source  82 . Light source  82  may include narrowband light emitters such as one or more laser diodes and/or one or more narrowband light-emitting diodes (e.g., nanowire light-emitting diodes). Light source  82  may emit light  86  at one wavelength blue) or multiple wavelengths (e.g., blue and green or blue, green, and red). Backlight structures  42  may include a light guide plate such as light guide plate  84 . Light guide plate  84  may be a planar structure that lies in the X-Y plane of  FIG. 5 , parallel to display layers  46 . 
     Light source  82  may emit light  86  into one or more edges of light guide plate  84 . Light  86  may be distributed within light guide plate  84  due to the principal of total internal reflection. Phase-matched structures  88  in light guide plate  84  may be used to redirect light  86  that is traveling horizontally in the X-Y plane of light guide plate  84  in upwards (outwards) direction Z (i.e., structures  88  may direct light  86  upwards at a 90° angle to serve as backlight  44  for display  14 ). Light guide plate  84  may be formed from a transparent material such as a clear polymer, nonlinear crystal, or other transparent material. In a rectangular display, light guide plate  84  may have a rectangular footprint matched to a rectangular display active area. Light redirecting structures  88  may be formed from holographically recorded gratings (e.g., in scenarios in which light guide plate  84  is formed from a photosensitive polymer) or other suitable phase-matched structures. Configurations in which structures  88  are implemented using holographically recorded gratings are sometimes described herein as an example. 
     Gratings  88  may be uniformly distributed throughout light guide plate  84  to ease alignment tolerances between plate  84  and the pixel structures of layers  46 . If desired, gratings  88  may be pixelated so that a grating (or concentrated region of a distributed grating) is located in alignment with each of the pixels  80  of display  14 . For example, gratings  88  may be configured to form an array of light directing structures that are aligned with corresponding display pixels  80 . The array of light directing (redirecting) structures may create a corresponding array of narrow collimated outwardly directed light beams. In particular, the light directing structures may be configured to accurately direct an array of beams of light  44  into the center of each display pixel  80  in the array of display pixels for display  14 . The narrow collimated nature of the light beams produced by gratings  88  may make it possible to reduce or eliminate the use of a grid (matrix) of black masking material of the type that is used to isolate adjacent display pixels from each other in the color filter layers of conventional displays. 
     By using gratings  88  and a narrowband light source  82 , light  86  can be efficiently directed upwards through the portions of liquid crystal layer  52  and other display layers  46  of corresponding display pixels  80 . 
     If desired, light source  82  may be configured to emit polarized light (e.g., linearly polarized light from a laser diode source) and gratings  88  may be configured to preserve the polarization of this light (i.e., light  44  exiting gratings  88  in light guide plate  84  may be linearly polarized). In this type of arrangement, it is not necessary to incorporate a lower polarizer into layers  58  to linearly polarize light  44 , thereby saving space in display  14 . Gratings  88  can also be configured to efficiently direct light vertically upwards in direction Z without substantial light leakage in downwards direction −Z. If desired, a reflector may be placed below light guide plate  84  to help redirect any downwardly directed light back in upwards direction Z or, due to the inherence efficiency of the grating structures of light guide plate  84  in directing light upwards, the reflector can be omitted, thereby helping to reduce display bulk. 
     It may be desirable to provide display  14  with the ability to display color images (e.g., images formed from an array of display pixels  80  of different colors such as an array of red, green and blue display pixels). Display pixels  80  may be provided with the ability to display color images by using light sources  82  of three different wavelengths of light (e.g., red, green, and blue light  86 ). Alternatively, or in addition to using light sources that produce three different wavelengths of light, layer  56  or other structures in display  14  may be provided with color filter structures, color downconverter structures, or other display pixel elements for creating tri-color display pixels from monochromatic light  86  or light go of two different colors. 
     In a color filter element for a display pixel, a display pixel is provided with a colored polymer element (sometimes referred to as a color filter element or color filter) that imparts a desired color to light that is passing through the color filter structure. Color may be imparted to a polymer element using dye that is dissolved within the polymer or using particles of pigment dispersed throughout the polymer. For example, a blue color filter element may have a blue polymer layer formed by dissolving blue the into the polymer layer or formed by incorporating blue pigment into the polymer layer. The blue color filter element will convert white light to blue light, will allow blue light to pass, and will block red and green light. Clear display pixel element structures may also be used. For example, a clear display pixel element may be used to pass blue light that has been directed upwards through liquid crystal layer  52  and other layers  46  from grating structures  88  in light guide plate  84 . In a field sequential color arrangement for display  14  in which light source  82  contains red, green, and blue sources that operate in series, clear display pixels may be used to pass red, green, and blue light in respective frames of image data. 
     Downconverter display pixels are used to change the wavelength of light that is being passed to the viewer. Downconverter structures may be formed by incorporating quantum dots into a clear binder material such as clear polymer. Quantum dots may be formed from semiconductor particles such as cadmium selenide particles. The size of the semiconductor particles (typically on the order of nanometers in diameter) may be selected to tune the band gap of the quantum dots to a desired value. When an energetic wavelength of light (e.g., blue light) illuminates a downconverter, the quantum dots become excited and emit light at a less energetic wavelength associated with the band gap of the quantum dots (e.g., red light). Using this type of arrangement, light  86  of one color can be converted to another (longer wavelength) color. For example, blue light can be converted to red light using a red downconverter, blue light can be converted to green light using a green downconverter, and green light can be converted to a red light using a red downconverter. 
     In configurations in which display  14  has a light source that emits fewer than three colors of light, downconverters can be used to create additional colors. For example, if light source  82  emits only blue light for an array of display pixels, subarrays of red and green downconverters can be used to create red tight for a subarray of red display pixels and green light for a subarray green display pixels. Some of the blue light can be passed through a subarray of blue color filter elements or clear pixels to create a subarray of blue display pixels. 
     If light source  82  emits two colors of light, fewer downconverters can be used. For example, if light source  82  emits blue light and green light, the blue (or green) light can be converted to red light using red downconverters. 
     In configurations in which display  14  has a light source that emits three colors of light, display  14  can be configured to display successive frames of display data in different respective colors. Displays that display red, green, and blue image frames in sequence are sometimes referred to as field sequential color displays. A field sequential color display may use display pixels  80  to display a full frame of red image data using a red light source, followed by respective full green and blue frames using green and blue light sources. Because a single array of display pixels  80  is used for displaying images of three different colors when field sequential color arrangements are used, it is not necessary for the display pixel array to include three respective subarrays of display pixels (e.g., a red subpixel array, a green subpixel array, and a blue subpixel array). Rather, all of the display pixels in the array of display pixels can be used in displaying images. 
     Display pixels preferably switch relatively fast in a field sequential color arrangement to accommodate the process of displaying fames of three different colors in a frame time that would otherwise be used for displaying all three colors at once. To accommodate fast switching speed requirements in field sequential color arrangements, it may be desirable to form display pixels  80  using liquid crystal electrode configurations that switch relatively quickly (e.g., twisted nematic effect configurations). Other types of display pixels arrangements can be used if desired (e.g., in-plane switching, fringe field switching, etc.). 
       FIG. 6  is a cross-sectional side view of an illustrative downconverter. As shown in  FIG. 6 , downconverter  90  may have a layer of polymer or other binder material  92  in which quantum dot structures  94  have been embedded. Material  92  may be, for example a polymer such as a clear polymer or a colored polymer. Quantum dots  94  may be formed from semiconductor particles or other nanostructures. Incident light at a first wavelength λ1 is absorbed by structures  94  and is remitted. This process converts incoming light at the first wavelength λ1 to emitted light at a second wavelength λ2 that is longer than λ1 (i.e., the second wavelength is at a lower and therefore downconverted frequency). 
     Light that is emitted from downconverter quantum dot structures  94  tends to have a wider angular spread than incoming collimated light from light guide plate  84 . If desired, microlenses, light scattering features such as pits and/or bumps, non-semiconducting embedded particles or voids for diffusing light, or other light spreading optical structures may be incorporated into a downconverter element to further increase the angular spread of the emitted light. The illustrative display pixel downconverter structure of  FIG. 6  is merely illustrative. 
       FIG. 7  is a graph showing how light may be downconverted from color C1 (e.g., blue) to other colors C2 (e.g., green) and C3 (e.g., red) using a downconverter such as downconverter  90 . In the graph of  FIG. 7 , light intensity has been plotted as a function of wavelength. Peak  96  corresponds to an illuminating light source color that is at a shorter wavelength (higher frequency) than desired target colors C2 and C3. Peak  96  may, as an example, correspond to blue light. A green downconverter may convert blue light  96  to green light associated with green peak  100 . A red downconverter may convert blue light  96  to red light (e.g., peak  102 ). Peak  96  is narrower than illustrative peak  98 . Peak shapes such as illustrative peak  96  may be produced by light sources such as laser diodes (which may have, as an example, a full width at half maximum value of a few nm). Peak shapes such as illustrative peak  98  may be produced by light sources such as light-emitting diodes (which may have, as an example, a full width at half maximum value of 15-20 nm). Use of narrowband light sources may help enhance light redirection efficiency by gratings  88  and display pixel switching performance for the liquid crystal display pixel structures formed using liquid crystal layer  52 . 
       FIGS. 8, 9, and 10  are cross-sectional side views of illustrative display pixel elements such as red, green, and blue color filter elements, clear display pixel (filter) elements, and/or downconverters. The display pixel elements of  FIGS. 8, 9, and 10  may be formed from polymer or other binder material  92  (e.g., clear and/or colored polymer). 
     As shown in  FIG. 8 , display pixel element  104  may include embedded structures such as structures  106 . Structures  106  may be quantum dots (e.g., for downconverters), particles of materials such as glass, ceramic, or plastic (e.g., inorganic dielectric particles such as titania particles or organic dielectric particles that are not index-matched to the material of binder  92  and that may therefore serve to scatter light), voids such as bubbles, or other embedded light scattering or wavelength converting structures. These embedded structures may be used for diffusing light (e.g., to serve as light spreading structures that increase the etendue of emitted light from a display pixel), may be used for converting the wavelength of light passing through the display pixel, or may otherwise be used in adjusting the light passing through the display pixel. 
       FIG. 9  shows how display pixel element  104  may be provided with light spreading structures such as microlenses  108 . Microlenses  108  may be provided on the upper and/or lower surfaces of element  104 . 
       FIG. 10  shows how display pixel element  104  may be provided with light scattering features  110  such as pits and/or bumps. Light scattering features  110  and/or other light spreading features such as microlenses may be provided on the upper and/or lower surfaces of each display pixel elements. If desired, light scattering features that are provided on one surface may have a different feature size than the light scattering features provided on an opposing surface. Light scattering features and/or microlenses may also be mixed and/or provided using structures of a uniform size and/or a mixture of sizes. Structures of the type shown in  FIGS. 9 and 10  that include one or more surfaces covered with microlenses, light scattering pits or bumps, or other surface features may also be combined with structures of the type shown in  FIG. 8  that include embedded structures  106 . For example, a downconverter or a clear, red, green, or blue display pixel element may be provided with embedded structures  106 , microlenses  108 , and/or light scattering features  110 . The examples of  FIGS. 8, 9, and 10  are merely illustrative. 
     As shown in the illustrative display pixel element of  FIG. 11 , a display pixel element may include embedded structures  106  (e.g., quantum dots in a downconverter and/or light scattering particles such as dielectric particles and/or voids filled with air or another gas) and may include surface features such as microlenses  108  and/or pits or bumps. Light  44  that is directed upwards in direction Z by light guide plate  84  ( FIG. 2 ) can be spread over an angle A (i.e., the etendue of light  44  that is passing through a display pixel can be increased to enhance off-axis viewing performance). 
       FIG. 12  is a cross-sectional side view of display  14  in a configuration in which upper display lasers  56  include an array of display pixel elements  56 - 2  having red downconverters RDC, green downconverters GDC, and blue color filter elements BCF. Layers  56  may include upper polarizer layer  56 - 1 . Display pixel elements  56 - 2  may be formed on a substrate such as substrate  56 - 3  (sometimes referred to as a color filter layer substrate or display pixel element substrate). Substrate  56 - 3  may be formed from a clear glass or plastic layer (as examples). 
     In the illustrative configuration of  FIG. 12 , light source  82  emits light  86  that is blue (B). Light guide plate  84  may have gratings  88  (e.g., an array of gratings) that direct blue light B upwards in direction Z. Blue light B may, for example, be directed upwards in a rectangular pattern of collimated beams each of which is aligned with the center of a corresponding display pixel  80 . The display pixel elements of layer  56 - 2  may include three subarrays. A first subarray may be made up of red downconverter elements (red downconverters RDC). Each red downconverter RDC receives a beam of blue light B and converts that blue light beam into a corresponding red light beam R. A second subarray may be made up of green downconverter elements (green downconverters GDC). Each green downconverter GDC receives a beam of blue light B and converts that blue light beam into a corresponding green light beam G. A third subarray in display pixel element array  56 - 2  may be made up of blue color filter elements BCF (i.e. blue color filter elements formed from clear polymer colored with a blue dye or pigment). Each blue element BCF may receive a beam of blue light B and may pass that blue beam without significant attenuation. If desired, clear display pixel elements may be used in place of blue color filter elements BCF. 
     Gratings  88  may direct light upwards in relatively narrow collimated blue light beams. The liquid crystal structures of display pixels  80  such as the electrodes and other pixel structures that are used in adjusting the liquid crystal material of layer  52  may be optimized to modulate highly collimated monochromatic light (i.e., blue light in the  FIG. 12  example) and may therefore exhibit superior performance (e.g., modulation depth, modulation speed, and/or power efficiency) when compared with structures that are designed to switch light of multiple colors. The narrow collimated light beams from light guide plate  84  may not, however, have sufficient angular spread (angle A) to satisfy off-angle viewing requirements for display  14 . The quantum dots in downconverters such as red downconverters RDC and green downconverters GDC may increase the angular spread (etendue) of emitted light to satisfy off-angle viewing requirements. Blue color filters or clear display pixel elements (and, if desired, downconverters RDC and GDC) may also incorporate light scattering features, microlenses, and light scattering embedded structures that form integrated light spreading (diffusing) structures. These light spreading features may also increase the etendue of the upwardly propagating light beams to enhance off-axis viewing performance for display  14 . 
     In the illustrative configuration of  FIG. 13 , light source  82  emits light  86  of two colors. In particular, light source  82  emits blue light B and green light G. Display pixel elements  56 - 2  include red downconverters RDC, green color filter elements GCF (i.e., pixels with clear polymer that has been colored green by incorporation of green dye or pigment or alternatively, clear elements), and blue color filter elements BET (e.g., pixels with clear polymer that has been colored blue by incorporation of blue dye or pigment or, alternatively, clear elements). 
     Light guide plate  84  may have holographically recorded gratings  88  that produce an array of blue and green upwardly propagating collimated light beams each of which is aligned with a respective display pixel  80  in display  14 . Green tight beams G are aligned with green color filters GCF and pass through the green color filters. Some of the blue light beams B pass through corresponding aligned blue color filters BCF. Other blue light beams B (or, if desired, some of the green light beams) may be downconverted into red light beams R by corresponding aligned red downconverters RDC. Light spreading features such as microlenses, embedded structures, and/or pits and bumps may be incorporated into the display pixel elements to increase etendue. For example, blue color filters B and green color filters GCF may be provided with microlenses or other light spreading features. Red downconverters RDC may emit red light that has more angular spread than the blue light beams B that are applied to the red downconverters and may, if desired, include optional light spreading features such as microlenses, pits, bumps, etc. 
     As shown in illustrative display pixel  80  of  FIG. 14 , layers  56  may be provided with filters (e.g., dichroic filters) that help prevent light leakage between adjacent display pixels of different colors. Optional lower polarizer  58 ′ may be provided between layer  58  and liquid crystal layer  52 . Upper polarizer layer  56 - 1  may supported by substrate layer  56 - 3 . Display pixel element  56 - 2  (e.g., a dog in the example of  FIG. 14 ) may be formed on the underside of substrate  56 - 3 . Display pixel element  56 - 2  may, as an example, be a red downconverter that converts incoming blue light B into red light R for display pixel  80 . Filter  56 F may be a filter that passes blue light while blocking light of other colors. For example, filter  56 F may be a filter formed from a dielectric stack that contains layers of dielectric with different respective index of refraction values (e.g., alternating high and low index materials, stacks with three or more or four or more types of layers with different index values, etc.). The layers of material that form filter  56 F may be inorganic materials such as silicon oxide, aluminum oxide, titanium oxide, nitrides, oxynitrides, metal oxides other than aluminum oxide and titanium oxide, etc. As shown in the cross-sectional side view of  FIG. 14 , red light R that is emitted in downwards direction −Z by quantum dot  94  may be reflected upwards by filter  56 F, while upwardly propagating blue light B is allowed to pass into red downconverter  56 - 2 . Green light that has leaked towards the red downconverter from an adjacent green pixel will be blocked by filter  56 F before reaching quantum dots  94  (i.e., filter  56 F will reduce green crosstalk). Crosstalk can also be reduced by incorporating a green filter on the adjacent green pixel (which will prevent leakage of green light from the green pixel toward the red pixel). 
     As shown in  FIG. 15 , light source  82  may be used to produce light of three colors such as red light R, green light G, and blue light B. Source  82  may produce each different color in sequence (i.e., display  14  of  FIG. 15  may be a field sequential color display that uses a field sequential color scheme to sequentially display image frames of different colors). With this type of arrangement, light guide plate  84  may have gratings  88  that are configured to directly light of all three colors upwards to display pixels  80 . In particular, gratings  88  may be used to direct red light R upwards into each of display pixels  80  during image frames in which light source  82  is emitting red light R, gratings  88  may be used to direct green light G upwards into each of display pixels  80  during image frames in which light source  82  is emitting green light G, and gratings  88  may be used to direct blue light B upwards into each of display pixels  80  during image frames in which light source  82  is emitting blue light B. The color filter elements and downconverter elements of layer  56 - 2  may be omitted from layer  56 , because colors are imparted into the images on display  14  by producing sequential red, green, and blue frames produced by corresponding red, green, and blue sources in source  82  rather than using simultaneously displayed red, green, and blue light patterns associated with respective red, green, and blue subpixel arrays. To ensure that the liquid crystal display pixel switching structures of display  14  switch sufficiently fast to support field sequential color operation, display pixels  80  may be implemented using twisted nematic structures or other structures that switch rapidly (as an example). 
     If desired, light  86  may be distributed into light guide plate  84  using gratings  120  that run along an optical fiber such as optical fiber  122  of  FIG. 16 . With this type of arrangement, light source  82  emits light into fiber  122 . Gratings  120  in fiber  122  are configured to direct light  86  from inside fiber  122  into one or more of the edges of light guide plate  84 . Emitted light  86  may then propagate in the X-Y plane of light guide plate  84  before gratings  88  direct light  86  upwards in direction Z though display pixels  80 . 
     In the illustrative backlight configuration of  FIG. 17 , light  86  is emitted into an edge of light guide plate  84  from an array of light sources  82 - 1 ,  82 - 2 ,  82 - 3 ,  82 - 4 , . . .  82 -N. The light sources of  FIG. 17  may be laser diodes or light-emitting diodes and may emit light  86  of one or more colors. 
     In the illustrative arrangement of  FIG. 18 , light guide plate  84  has an active area AA that overlaps a rectangular array of display pixels  80  and has an inactive mixing area  84 ′. Light sources  82  emit light  86  into edge  84 E 1  of light guide plate  84 . Gratings  88 ′ or other light scattering features (pits, bumps, embedded structures, etc.) in region  84 ′ help mix light  86  so that light  86  evenly illuminates edge  84 E 2  of active area AA. 
     The lateral dimensions of light guide plate  84  can be minimized by performing mixing using light mixing structures  84 ′ that are located in a different X-Y plane than light guide plate  84 . This type of arrangement is shown in the cross-sectional side view of  FIG. 19 . As shown in  FIG. 19 , light source  82  emits light  86  that is mixed (e.g., laterally distributed as shown in  FIG. 18 ) using gratings  88 ′ or other structures in mixing structures  84 ′. Vertical light guide structures  130  (e.g., clear polymer or glass structures) may include light reflectors  132  (e.g., gratings, mirrors, prims, etc.) that distribute light  86  upwards in direction Z. Structures  84 ′,  130 , and  84  may be formed from a single layer of polymer that has been bent or otherwise formed into the shape shown in  FIG. 19  or may be formed from separate optical structures that are assembled together in housing  12  of device  10  or a display chassis. If desired, a bent section of light guide plate material or other C-shaped optical path (e.g., a curved light guide without internal mirrors) may be used to form structures  130 . For example, a strip of bent clear plastic or glass may be used to route light from a lower plane associated with light source  82  to an upper plane including light guide plate  84 . Mixing structures  84 ′ may also be integrated into structures  130 , if desired. The curved light guide structure can be provided with a relatively large bend radius to allow the light guide structure to confine light due to total internal reflection or may be provided with a metal coating to help confine the light. 
       FIG. 20  shows how a reflective polarizer may be used in producing linearly polarized light for display  14 . As shown in  FIG. 20 , light source  82  may produce unpolarized light  86 . Unpolarized light  86  may be emitted into the edge of light guide plate  84 . Light guide plate  84  may include gratings  88 . Gratings  88  may be configured to redirect linearly polarized light upwards in direction Z. In particular, beams of light such as illustrative polarized beam B 1  may be directed upwards to reflective polarizer  142 . Beam B 1  may have a first linear polarization state (e.g., a linear polarization that is oriented perpendicular to the page in the example of  FIG. 20 ). Reflective polarizer  142  may be configured to pass light with this polarization and to reflect any light that has an orthogonal polarization (see, e.g., reflected beam B 2 , which has a second linear polarization state that is parallel to the page in the example of  FIG. 20 ). Reflected beam B 2  may pass through the gratings of light guide plate as beam B 3  without reflection because the gratings are configured to reflect perpendicularly polarized light rather than parallel polarized light (in this example). A quarter wave plate such as quarter wave plate  146  may be mounted below light guide plate  84 . A reflector such as reflector  144  may be mounted below quarter wave plate  146 . Beam B 3  passes through quarter wave plate  146  in downwards direction −Z and is reflected hack upwards from reflector  144  as beam B 4 . Due to the presence of quarter wave plate  146 , upwardly propagating beam B 4  will have a linear polarization that is orthogonal to that of beam B 3  (i.e., beam B 4  will be linearly polarized in an orientation that is perpendicular to the page of  FIG. 20  in this example). This beam, which forms beam  135  of  FIG. 20  may pass through light guide plate  84  and reflective polarizer  142  to serve as additional backlight for display  14 . 
     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: 20140903
Publication Date: 20171031
Grant Date: 20171031
Priority Date: 20131220
Inventors: DROLET JEAN-JACQUES
CHEN WEI
Assignee: APPLE INC
CPC Classifications: [{"code": "G02B6/0056", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/0035", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/0035", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/0056", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 53399792