# Patent

## 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 may include phase-matched structures such as holographically recorded gratings or photonic lattices that direct the light upwards through the array of display pixels.

## Background
This application is a continuation-in-part of patent application Ser. No. 14/476,610, filed Sep. 3, 2014, which is hereby incorporated by reference herein in its entirety and which 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 of an 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.
FIG. 21 is a cross-sectional side view of an illustrative display 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 from the phase-matched structures in the light guide plate can be accurately aligned with the display pixels' 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 12A and lower housing 12B 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 12A to rotate in directions 22 about rotational axis 24 relative to lower housing 12B. Display 14 is mounted in housing 12A. Upper housing 12A, which may sometimes be referred to as a display housing or lid, is placed in a closed position by rotating upper housing 12A towards lower housing 12B 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 uses 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. During 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 off 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 (e.g., 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 86 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 dye 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 light 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 frames 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 reemitted. 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 layers 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 BCF (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 light 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 downconverter 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 56F may be a filter that passes blue light while blocking light of other colors. For example, filter 56F 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 56F 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 56F, 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 56F before reaching quantum dots 94 (i.e., filter 56F 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 84E1 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 84E2 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 B1 may be directed upwards to reflective polarizer 142. Beam B1 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 B2, which has a second linear polarization state that is parallel to the page in the example of FIG. 20). Reflected beam B2 may pass through the gratings of light guide plate as beam B3 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 B3 passes through quarter wave plate 146 in downwards direction −Z and is reflected back upwards from reflector 144 as beam B4. Due to the presence of quarter wave plate 146, upwardly propagating beam B4 will have a linear polarization that is orthogonal to that of beam B3 (i.e., beam B4 will be linearly polarized in an orientation that is perpendicular to the page of FIG. 20 in this example). This beam, which forms beam B5 of FIG. 20 may pass through light guide plate 84 and reflective polarizer 142 to serve as additional backlight for display 14.
FIG. 21 is a cross-sectional side view of an illustrative display having an array of pixel elements including quantum dot pixel elements that serve as downconverters. As shown in FIG. 21, backlight unit 42 has light source 82. Light source 82 may be a blue light source that produces light 86 that is blue (B). Light source 82 may include an array of blue light-emitting diodes or other blue light source. Blue light 86 may be emitted into one or more edges of light guide plate 84. Light guide plate 84 may be formed from a clear layer of polymer or other transparent material and may be used to laterally distribute light 86 in dimensions X and Y in accordance with the principal of total internal reflection. The upper and/or lower surfaces of light guide plate 84 may have light scattering features such as dots, ridges, lines, grooves, pits (e.g., pits such as pits 204), bumps (e.g., bumps such as bumps 202), or other features that help scatter light 86 upwards in direction Z from backlight unit 42 through display layers 46 as backlight 44 (i.e., blue backlight).
Pixels 80 may include red pixels, green pixels, and blue pixels and/or pixels of other colors. An array of pixel elements 56-2 may be used to ensure that each pixel in display 14 has a desired color. Elements 56-2 may include red downconverters RDC (i.e., red quantum dots in a polymer binder or other material) and green downconverters GDC (i.e., green quantum dots in a polymer binder or other material). Blue color filter (BCF) pixel elements and/or clear blue pixel elements may be associated with blue pixels. In the red pixels, red downconverters RDC convert blue backlight 44 into red emitted light R. In the green pixels, green downconverters GDC convert blue backlight 44 into green emitted light G. In the blue pixels, blue backlight 44 is allowed to pass through clear or blue pixel elements.
Display 46 may have a lower polarizer such as polarizer 58′ that polarizes blue backlight 44. Thin-film transistor layer 58 may have an array of pixel circuits with electrodes that impart electric fields to each pixel. The applied electric fields control the orientation of liquid crystals in an associated portion of liquid crystal layer 52. Upper polarizer 56-1 serves as an analyzer layer. As blue backlight 44 passes through the portions of pixels 80 associated with polarizer layer 58′, thin-film transistor layer 58, liquid crystal layer 52, and upper polarizer 56-1, the pattern of electric fields produced by the electrodes in thin-film transistor layer will cause layers 58′, 58, 52, and 56-1 to modulate the intensity of the blue backlight that is passing upwards to produce a desired image pattern for display 14 (e.g., a blue monochromatic image). The layers of material above upper polarizer layer 56-1 convert the blue monochromatic image into a color image (e.g., a red, blue, green image).
Blue pixels 80 may have blue color filters BCF or clear elements. Each blue color filter BCF may contain a binder (e.g., a polymer or other material) and features for scattering blue light. The light scattering features may be 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 the binder and that may therefore serve to scatter light), voids such as bubbles, or other embedded light scattering structures. The presence of light-scattering features in the blue pixel elements may help ensure that blue light is spread angularly by the same amount as the red and green light produced by the isotropic emission of the quantum dots in the red and green downconverters. During operation, the blue backlight 44 that reaches each blue pixel 80 is passed vertically upwards in direction Z as blue pixel light B.
Red pixels 80 include red downconverters RDC (red quantum dot elements) that convert blue backlight 44 in the red pixels into red light R. Green pixels 80 include green downconverters GDC (green quantum dot elements) that convert blue backlight 44 in the green pixels into green light G. Not all blue light 44 is converted into red and green light by the quantum dots in the red and green pixel elements. Accordingly, red pixels 80 and green pixels 80 preferably include a yellow dichroic filter 56C to ensure that only red and green light passes through the red and green pixel elements, respectively.
Filter 56C is a blue-light blocking filter. As shown in FIG. 21, filter 56C overlaps red downconverters RDC and green downconverters GDC but does not overlap the blue pixel elements. Filter 56C blocks residual blue backlight 44 that has not been converted into red and green light, while allowing red and green light to pass freely upwards towards viewer 48 in direction Z.
Filter 56C may be the same for both the red and green pixels or different filters may be used for the red and green pixels. For example, each red pixel may have a filter with a red pass band (e.g., a red filter) and each green pixel may have a filter with a green pass band (e.g., a green filter). The use of a yellow dichroic filter (i.e., a filter with high transmission for both red and green light) may help reduce process complexity, because the same type of filter structure may be provided for both red and green pixels.
Blue pixels 80 pass blue light 44 without converting the blue light into light of a different color. The pixel elements for the blue pixels may be formed from clear material or, if desired, may incorporate a blue filter structure (e.g., a blue dye or pigment, a blue dichroic filter layer, etc.). The use of a blue color filter structure in the blue pixel elements may help prevent stray red or green light from passing through the blue pixel elements.
Substrate 56-3 may be formed from a layer of clear glass or plastic. During fabrication of layer 56, filter 56C may be patterned onto the lower surface of color filter substrate 56-3. Filter 56C may be patterned by depositing filter 56C through a shadow mask, by depositing a global filter layer and patterning the global filter layer using photolithography, or using other processing techniques. Filter 56C may be formed by alternating high-and-low index of refraction materials (e.g., inorganic layers such as silicon oxide, metal oxides, etc.). If desired, filter 56C may be formed using organic materials or other filter structures suitable for blocking blue light.
Black matrix 200 may have a grid shape with openings for respective pixels 80 and may be deposited on filter layer 56C and substrate layer 56-3. The presence of black matrix 200 may help prevent undesired color mixing between adjacent pixels. Black matrix 200 may be formed from a black photoimageable polymer or other opaque material.
Pixel elements 56-2 may be formed on substrate 56-3 after black matrix 200 has been formed. Pixel elements 56-2 may be formed from photoimageable polymers that include quantum dots and/or light scattering features as appropriate, may be formed by depositing polymers or other materials with quantum dots and/or light scattering features through a shadow mask, or may be formed using other deposition techniques (ink-jet printing, screen printing, etc.).
There is a potential for moisture intrusion into pixel elements 56-2 that could degrade elements 56-2. If desired, a moisture barrier layer such as layer 56B may be formed on (under) pixel elements 56-2. Moisture barrier layer 56B may be formed from multiple layers of organic and/or inorganic materials (e.g., polymer layers, silicon oxide layers, silicon nitride layers, etc.). In the presence of moisture barrier layer 56B, moisture may be prevented from reaching pixel elements 56-2.
Blue dichroic filter layer 56A may be formed on (under) moisture barrier layer 56B. Blue dichroic filter 56A may be formed from alternating high and low index of refraction materials and may be used to enhance the efficiency of display 14. Blue dichroic filter 56A reflects red and green light while allowing blue light to pass. Because blue dichroic filter 56A is transparent to blue light, blue backlight 44 may pass upwards through filter 56A to pixel elements 56-2. Because blue dichroic filter 56A reflects red and green light, red light that is produced by red downconverters RDC and green light that is produced by green downconverters GDC that is traveling downwards into display 14 away from viewer 48 may be reflected back upwards in direction Z. The ability of filter 56A to reflect red and green light therefore helps recycle red and green light within the red and green pixel elements and thereby improves efficiency.
Upper polarizer 56-1 may be formed on (under) blue dichroic filter layer 56A. Liquid crystal layer 52 may be sandwiched between layer 56 and layer 58.
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.

## Claims

1. A display, comprising:
a backlight unit that produces blue light;
an array of pixels having pixel elements including red downconverters having red quantum dots that convert the blue light into red light, pixel elements that pass the blue light, and green downconverters having green quantum dots that convert the blue light into green light;
a yellow dichroic filter that overlaps the red downconverters and the green downconverters, wherein the yellow dichroic filter blocks the blue light while passing the red light and the green light; and
a blue dichroic filter that overlaps the red and green downconverters and that overlaps the elements that pass the blue light.

2. The display defined in claim 1 further comprising:
a thin-film transistor layer; and
a lower polarizer layer between the backlight unit and the thin-film transistor layer.

3. The display defined in claim 2 further comprising:
an upper polarizer layer; and
a layer of liquid crystal material between the thin-film transistor layer and the upper polarizer layer.

4. The display defined in claim 3 further comprising a moisture barrier layer between the pixel elements and the upper polarizer layer.

5. The display defined in claim 4 wherein the blue dichroic filter is interposed between the moisture barrier layer and the upper polarizer layer.

6. The display defined in claim 5 wherein the pixel elements that pass the blue light comprise blue color filter pixel elements.

7. A display, comprising:
an upper polarizer;
a lower polarizer;
a liquid crystal layer that is interposed between the upper polarizer and the lower polarizer;
a backlight unit that produces blue light that passes through the lower polarizer, the liquid crystal layer, and the upper polarizer;
an array of pixel elements including red quantum dot pixel elements, green quantum dot pixel elements, and blue pixel elements that pass blue light and that do not have any quantum dots, wherein the array of pixel elements receive blue light from the backlight unit that has passed through the upper polarizer and wherein the blue pixel elements include light scattering features; and
a blue-light blocking filter that overlaps the red quantum dot pixel elements and the green quantum dot pixel elements and that does not overlap the blue pixel elements.

8. The display defined in claim 7 wherein the backlight unit has blue light-emitting diodes that produce the blue light.

9. The display defined in claim 7 further comprising a blue dichroic filter that is interposed between the red and green quantum dot pixel elements and the upper polarizer.

10. The display defined in claim 7 further comprising a blue dichroic filter that is interposed between the array of pixel elements and the upper polarizer.

11. A display, comprising:
display layers that modulate blue backlight to produce a blue monochromatic image;
an array of pixel elements including red quantum dot elements that convert some of the blue monochromatic image to red light and green quantum dot elements that convert some of the blue monochromatic image to green light;
a yellow dichroic filter that overlaps the red quantum dot elements and the green quantum dot elements; and
a blue dichroic filter that is interposed between the display layers and the array of pixel elements.