PATENT DOCUMENT

Publication Number: US-11221512-B2
Application Number: US-201916294731-A
Country: US
Kind Code: B2

Title: Displays with direct-lit backlight units and color conversion layers

Abstract:
A display may have a pixel array such as a liquid crystal pixel array. The pixel array may be illuminated with backlight illumination from a backlight unit. The backlight unit may include a printed circuit board, a plurality of light-emitting diodes mounted on the printed circuit board, at least one light spreading layer formed over the printed circuit board that spreads light received from the plurality of light-emitting diodes, a partially reflective layer formed over the at least one light spreading layer, a color conversion layer formed over the partially reflective layer, a collimating layer formed over the color conversion layer, a brightness enhancement film formed over the collimating layer, and a diffuser formed over the brightness enhancement film. The at least one light spreading layer may include two light spreading layers with elongated protrusions that are rotated relative to each other.

Claims:
What is claimed is: 
     
       1. A display, comprising:
 a plurality of pixels; and 
 a backlight configured to produce backlight illumination for the plurality of pixels, wherein the backlight comprises:
 a printed circuit board; 
 a plurality of light-emitting diodes mounted on the printed circuit board; 
 at least one light spreading layer formed over the printed circuit board that spreads light received from the plurality of light-emitting diodes; and 
 a color conversion layer above the at least one light spreading layer, wherein the color conversion layer includes:
 a phosphor layer having a first upper surface and a first lower surface; and 
 reflective structures, wherein the phosphor layer comprises a first plurality of quantum dots that convert light of a first color from the plurality of light-emitting diodes to light of a second color, wherein the phosphor layer comprises a second plurality of quantum dots that convert light of the first color from the plurality of light-emitting diodes to light of a third color, wherein the reflective structures have a reflectance greater than 40%, wherein the reflective structures have a second upper surface and a second lower surface, wherein the second upper surface is coplanar with the first upper surface, and wherein the second lower surface is interposed between the first lower surface and the first upper surface. 
 
 
 
     
     
       2. The display defined in  claim 1 , wherein the reflective structures do not include any of the first and second pluralities of quantum dots. 
     
     
       3. The display defined in  claim 1 , wherein the reflective structures have hexagonal cross-sections. 
     
     
       4. The display defined in  claim 1 , wherein the reflective structures are arranged in a honeycomb pattern. 
     
     
       5. A display, comprising:
 a plurality of pixels; and 
 a backlight configured to produce backlight illumination for the plurality of pixels, wherein the backlight comprises:
 a printed circuit board; 
 a plurality of light-emitting diodes mounted on the printed circuit board; 
 at least one light spreading layer formed over the printed circuit board that spreads light received from the plurality of light-emitting diodes; and 
 a color conversion layer above the at least one light spreading layer, wherein the color conversion layer includes a phosphor layer and reflective structures, wherein the phosphor layer comprises a first plurality of quantum dots that convert light of a first color from the plurality of light-emitting diodes to light of a second color, wherein the phosphor layer comprises a second plurality of quantum dots that convert light of the first color from the plurality of light-emitting diodes to light of a third color, wherein the reflective structures have a reflectance greater than 40%, and wherein the reflective structures are formed in an interconnected web that defines a plurality of cells for a phosphor material of the phosphor layer. 
 
 
     
     
       6. The display defined in  claim 5 , wherein the interconnected web includes a plurality of openings to allow flow between the cells. 
     
     
       7. A display, comprising:
 a plurality of pixels; and 
 a backlight configured to produce backlight illumination for the plurality of pixels, wherein the backlight comprises:
 a printed circuit board; 
 a plurality of light-emitting diodes mounted on the printed circuit board; 
 at least one light spreading layer formed over the printed circuit board that spreads light received from the plurality of light-emitting diodes; and 
 a color conversion layer above the at least one light spreading layer, wherein the color conversion layer includes a phosphor layer and reflective structures having a reflectance greater than 30%, wherein the reflective structures are formed in an interconnected web that defines a plurality of cells for a phosphor material of the phosphor layer, and wherein the interconnected web includes a plurality of openings to allow flow between the cells. 
 
 
     
     
       8. The display defined in  claim 7 , wherein the reflective structures are arranged in a honeycomb pattern.

Description:
This application claims the benefit of provisional patent application No. 62/642,539, filed Mar. 13, 2018, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates generally to displays, and, more particularly, to backlit displays. 
     Electronic devices often include displays. For example, computers and cellular telephones are sometimes provided with backlit liquid crystal displays. Edge-lit backlight units have light-emitting diodes that emit light into an edge surface of a light guide plate. The light guide plate then distributes the emitted light laterally across the display to serve as backlight illumination. 
     Direct-lit backlight units have arrays of light-emitting diodes that emit light vertically through the display. If care is not taken, however, a direct-lit backlight may be bulky or may produce non-uniform backlight illumination. 
     SUMMARY 
     A display may have a pixel array such as a liquid crystal pixel array. The pixel array may be illuminated with backlight illumination from a backlight unit. The backlight unit may include an array of light-emitting diodes and a light reflector that helps reflect light from the light-emitting diodes through the pixel array. Each light-emitting diode may be placed in a respective cell. 
     The backlight unit may include a printed circuit board, a plurality of light-emitting diodes mounted on the printed circuit board, at least one light spreading layer formed over the printed circuit board that spreads light received from the plurality of light-emitting diodes, a partially reflective layer formed over the at least one light spreading layer, a color conversion layer formed over the partially reflective layer, a collimating layer formed over the color conversion layer, a brightness enhancement film formed over the collimating layer, and a diffuser formed over the brightness enhancement film. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an illustrative electronic device having a display in accordance with an embodiment. 
         FIG. 2  is a cross-sectional side view of an illustrative display in accordance with an embodiment. 
         FIG. 3  is a top view of an illustrative light-emitting diode array for a direct-lit backlight unit in accordance with an embodiment. 
         FIG. 4  is a cross-sectional side view of an illustrative light-emitting diode array with a thermally conductive layer adhered to a printed circuit board with light-emitting diodes in accordance with an embodiment. 
         FIG. 5  is a cross-sectional side view of an illustrative light-emitting diode array with a radiative cooling coating on a printed circuit board with light-emitting diodes in accordance with an embodiment. 
         FIG. 6  is a top view of an illustrative light-emitting diode array for a direct-lit backlight unit having four rectangular light-emitting diodes in each cell in accordance with an embodiment. 
         FIG. 7  is a top view of an illustrative light-emitting diode array for a direct-lit backlight unit having light-emitting diodes with different properties in each cell in accordance with an embodiment. 
         FIG. 8  is a cross-sectional side view of an illustrative light-emitting diode array having a light redirecting layer between light-emitting diodes in accordance with an embodiment. 
         FIGS. 9 and 10  are cross-sectional side views of illustrative light-emitting diodes with and without a reflector layer such as distributed Bragg-reflector in accordance with an embodiment. 
         FIG. 11  is a cross-sectional side view of an illustrative light-emitting diode array having a reflective layer in accordance with an embodiment. 
         FIG. 12  is a cross-sectional side view of an illustrative light-emitting diode array having encapsulant with a planar upper surface over light-emitting diodes in accordance with an embodiment. 
         FIG. 13  is a cross-sectional side view of an illustrative light-emitting diode array having encapsulant with a curved upper surface over each light-emitting diode in accordance with an embodiment. 
         FIG. 14  is a cross-sectional side view of an illustrative light-emitting diode array having encapsulant with a curved upper surface and a recessed portion over each light-emitting diode in accordance with an embodiment. 
         FIG. 15  is a cross-sectional side view of an illustrative light-emitting diode array having encapsulant with dopants evenly distributed throughout the encapsulant material in accordance with an embodiment. 
         FIG. 16  is a cross-sectional side view of an illustrative light-emitting diode array having encapsulant with dopants at the upper surface of the encapsulant material in accordance with an embodiment. 
         FIG. 17  is a cross-sectional side view of an illustrative light-emitting diode array having encapsulant with dopants at the lower surface of the encapsulant material in accordance with an embodiment. 
         FIG. 18  is a cross-sectional side view of an illustrative dopant with portions having different densities in accordance with an embodiment. 
         FIG. 19  is a cross-sectional side view of an illustrative dopant having a shape to control orientation of the dopant in encapsulant material in accordance with an embodiment. 
         FIG. 20  is a cross-sectional side view of an illustrative backlight unit having a patterned layer formed on the encapsulant material in accordance with an embodiment. 
         FIG. 21  is a cross-sectional side view of an illustrative light-emitting diode array having encapsulant with a textured upper surface over light-emitting diodes in accordance with an embodiment. 
         FIG. 22  is a cross-sectional side view showing a method for heating solder in a backlight unit in accordance with an embodiment. 
         FIG. 23  is a cross-sectional side view of an illustrative backlight unit having a light spreading layer coupled to the encapsulant in accordance with an embodiment. 
         FIG. 24  is a cross-sectional side view of an illustrative backlight unit having a first light spreading layer below a second light spreading layer in accordance with an embodiment. 
         FIG. 25  is a cross-sectional side view of an illustrative backlight unit having a first light spreading layer below a second light spreading layer with an intervening wave guide layer in accordance with an embodiment. 
         FIG. 26  is a cross-sectional side view of an illustrative backlight unit having encapsulant with respective portions having a parabolic upper surface over each light-emitting diode in accordance with an embodiment. 
         FIG. 27  is a cross-sectional side view of an illustrative backlight unit having light leakage promotion structures between each light-emitting diode in accordance with an embodiment. 
         FIG. 28  is a cross-sectional side view of an illustrative display having a light spreading layer with microlenses on the upper surface and the lower surface of the light spreading layer in accordance with an embodiment. 
         FIG. 29  is a cross-sectional side view of an illustrative display having two light spreading layers that are rotated relative to each other in accordance with an embodiment. 
         FIG. 30  is a cross-sectional side view of an illustrative light spreading layer having protrusions in accordance with an embodiment. 
         FIG. 31  is a cross-sectional side view of an illustrative light spreading layer having recesses in accordance with an embodiment. 
         FIG. 32  is a top view of an illustrative light spreading layer having protrusions or recesses in accordance with an embodiment. 
         FIGS. 33 and 34  are top views of illustrative light spreading layers having protrusions formed by partial-cube structures in accordance with an embodiment. 
         FIG. 35  is a perspective view of an illustrative protrusion for a light spreading layer that is in the shape of a tapered pyramid in accordance with an embodiment. 
         FIG. 36  is a cross-sectional side view of an illustrative backlight unit having microlenses on a lower surface of a partial reflective layer and an encapsulant with a planar upper surface over the light-emitting diodes in accordance with an embodiment. 
         FIG. 37  is a cross-sectional side view of an illustrative backlight unit having microlenses on a lower surface of a partial reflective layer and encapsulant portions with planar upper surfaces over each light-emitting diode in accordance with an embodiment. 
         FIG. 38  is a cross-sectional side view of an illustrative backlight unit having microlenses on a lower surface of a partial reflective layer and encapsulant portions with curved upper surfaces over each light-emitting diode in accordance with an embodiment. 
         FIG. 39  is a cross-sectional side view of an illustrative backlight unit having microlenses on a substrate that is separated from the light-emitting diodes by an air-gap in accordance with an embodiment. 
         FIG. 40  is a cross-sectional side view of an illustrative separately formed backlight enhancement film and collimating layer in accordance with an embodiment. 
         FIG. 41  is a cross-sectional side view of an illustrative optical film that serves as both a backlight enhancement film and a collimating layer in accordance with an embodiment. 
         FIG. 42  is a cross-sectional side view of an illustrative display having edge coatings in accordance with an embodiment. 
         FIG. 43  is a diagram showing how the positions of light-emitting diodes may be dithered to improve performance of the display in accordance with an embodiment. 
         FIG. 44  is a top view of an illustrative display having light-emitting diodes arranged in positions that are dithered in accordance with an embodiment. 
         FIG. 45  is a top view of an illustrative display where each light-emitting diode is covered by encapsulant that is offset relative to the light-emitting diode in accordance with an embodiment. 
         FIG. 46  is a cross-sectional side view of an illustrative display showing how a slab of encapsulant with a tuned thickness may be formed over light-emitting diodes in accordance with an embodiment. 
         FIG. 47  is a cross-sectional side view of an illustrative display showing how support structures may be included to preserve the structural integrity of a slab of encapsulant in accordance with an embodiment. 
         FIG. 48  is a cross-sectional side view of an illustrative display having support structures that both preserve the structural integrity of a slab of encapsulant and serve as light-leakage promotion structures in accordance with an embodiment. 
         FIG. 49  is a cross-sectional side view of an illustrative color conversion layer having a phosphor layer in accordance with an embodiment. 
         FIGS. 50A-50C  are cross-sectional side views of an illustrative color conversion layer having a phosphor layer with reflective structures in accordance with an embodiment. 
         FIGS. 51A and 51B  are top views of an illustrative phosphor layer with reflective structures in accordance with an embodiment. 
         FIG. 52  is a cross-sectional side view of an illustrative color conversion layer having a phosphor layer with Rayleigh scattering dopants in accordance with an embodiment. 
         FIG. 53  is a cross-sectional side view of an illustrative color conversion layer having an optical film with prisms over a phosphor layer in accordance with an embodiment. 
         FIG. 54  is a cross-sectional side view of an illustrative color conversion layer having a phosphor layer that is patterned to reduce the path length of off-axis light in accordance with an embodiment. 
         FIG. 55  is a cross-sectional side view of an illustrative color conversion layer having an optical film with prisms over a phosphor layer that is patterned to reduce the path length of off-axis light in accordance with an embodiment. 
         FIG. 56  is a diagram showing illustrative steps for attaching a pre-soldered light-emitting diode to a printed circuit board in accordance with an embodiment. 
         FIG. 57  is a cross-sectional side view of an illustrative backlight that includes an optical film with a low-index layer laminated between a collimating layer and brightness-enhancement film in accordance with an embodiment. 
         FIG. 58  is a cross-sectional side view of an illustrative backlight with a light spreading layer that has concave light spreading features on an upper surface and a lower surface in accordance with an embodiment. 
         FIG. 59  is a cross-sectional side view of an illustrative backlight with a light spreading layer that has concave light spreading features on a lower surface and convex light spreading features on an upper surface in accordance with an embodiment. 
         FIG. 60  is a cross-sectional side view of an illustrative backlight with a light spreading layer that has first and second layers having elongated protrusions that are rotated relative to each other in accordance with an embodiment. 
         FIG. 61  is a cross-sectional side view of an illustrative backlight with a color conversion layer that includes prisms on an upper surface in accordance with an embodiment. 
         FIG. 62  is a cross-sectional side view of an illustrative backlight with light-emitting diodes covered by a dome of encapsulant in accordance with an embodiment. 
         FIG. 63  is a cross-sectional side view of an illustrative backlight with light-emitting diodes covered by a dome of encapsulant and interposed between reflective structures in accordance with an embodiment. 
         FIG. 64  is a cross-sectional side view of an illustrative backlight with light-emitting diodes covered by a dome of encapsulant and interposed between reflective structures having curved upper surfaces in accordance with an embodiment. 
         FIG. 65  is a cross-sectional side view of an illustrative backlight with light-emitting diodes interposed between reflective structures and covered by a slab of encapsulant in accordance with an embodiment. 
         FIG. 66  is a cross-sectional side view of an illustrative backlight with light-emitting diodes covered by a slap of encapsulant in accordance with an embodiment. 
         FIG. 67  is a cross-sectional side view of an illustrative backlight with light-emitting diodes covered by a slab of encapsulant and reflective material on the encapsulant in accordance with an embodiment. 
         FIG. 68  is a cross-sectional side view of an illustrative backlight with light-emitting diodes that interposed between reflective structures and that are covered by a slab of encapsulant and reflective material on the encapsulant in accordance with an embodiment. 
         FIG. 69  is a cross-sectional side view of an illustrative backlight with light-emitting diodes surrounded by specular reflective material in accordance with an embodiment. 
         FIG. 70  is a cross-sectional side view of an illustrative backlight in which a partially reflective layer is omitted and a light spreading layer with protrusions on an upper surface and a lower surface is included in accordance with an embodiment. 
         FIG. 71  is a cross-sectional side view of an illustrative backlight in which a partially reflective layer is omitted and a light spreading layer includes first and second layers that are laminated together in accordance with an embodiment. 
         FIG. 72  is a cross-sectional side view of an illustrative backlight in which a color conversion layer includes reflective structures in accordance with an embodiment. 
         FIG. 73  is a cross-sectional side view of an illustrative backlight with light-emitting diodes that are dithered in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices may be provided with backlit displays. The backlit displays may include liquid crystal pixel arrays or other display structures that are backlit by light from a direct-lit backlight unit. A perspective view of an illustrative electronic device of the type that may be provided with a display having a direct-lit backlight unit is shown in  FIG. 1 . Electronic device  10  of  FIG. 1  may be a computing device such as a laptop computer, a computer monitor containing an embedded computer, a tablet computer, a cellular telephone, a media player, or other handheld or portable electronic device, a smaller device such as a wrist-watch device, a pendant device, a headphone or earpiece device, a device embedded in eyeglasses or other equipment worn on a user&#39;s head, or other wearable or miniature device, a television, a computer display that does not contain an embedded computer, a gaming device, a navigation device, an embedded system such as a system in which electronic equipment with a display is mounted in a kiosk or automobile, equipment that implements the functionality of two or more of these devices, or other electronic equipment. 
     As shown in  FIG. 1 , device  10  may have a display such as display  14 . Display  14  may be mounted in housing  12 . Housing  12 , which may sometimes be referred to as an enclosure or case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc.), other suitable materials, or a combination of any two or more of these materials. Housing  12  may be formed using a unibody configuration in which some or all of housing  12  is machined or molded as a single structure or may be formed using multiple structures (e.g., an internal frame structure, one or more structures that form exterior housing surfaces, etc.). 
     Housing  12  may have a stand, may have multiple parts (e.g., housing portions that move relative to each other to form a laptop computer or other device with movable parts), may have the shape of a cellular telephone or tablet computer, and/or may have other suitable configurations. The arrangement for housing  12  that is shown in  FIG. 1  is illustrative. 
     Display  14  may be a touch screen display that incorporates a layer of conductive capacitive touch sensor electrodes or other touch sensor components (e.g., resistive touch sensor components, acoustic touch sensor components, force-based touch sensor components, light-based touch sensor components, etc.) or may be a display that is not touch-sensitive. Capacitive touch screen electrodes may be formed from an array of indium tin oxide pads or other transparent conductive structures. 
     Display  14  may include an array of pixels  16  formed from liquid crystal display (LCD) components or may have an array of pixels based on other display technologies. A cross-sectional side view of display  14  is shown in  FIG. 2 . 
     As shown in  FIG. 2 , display  14  may include a pixel array such as pixel array  24 . Pixel array  24  may include an array of pixels such as pixels  16  of  FIG. 1  (e.g., an array of pixels having rows and columns of pixels  16 ). Pixel array  24  may be formed from a liquid crystal display module (sometimes referred to as a liquid crystal display or liquid crystal layers) or other suitable pixel array structures. A liquid crystal display for forming pixel array  24  may, as an example, include upper and lower polarizers, a color filter layer and a thin-film transistor layer interposed between the upper and lower polarizers, and a layer of liquid crystal material interposed between the color filter layer and the thin-film transistor layer. Liquid crystal display structures of other types may be used in forming pixel array  24 , if desired. 
     During operation of display  14 , images may be displayed on pixel array  24 . Backlight unit  42  (which may sometimes be referred to as a backlight, backlight layers, backlight structures, a backlight module, a backlight system, etc.) may be used in producing backlight illumination  45  that passes through pixel array  24 . This illuminates any images on pixel array  24  for viewing by a viewer such as viewer  20  who is viewing display  14  in direction  22 . 
     Backlight unit  42  may include a plurality of optical films  26  formed over light-emitting diode array  36 . Light-emitting diode array  36  may contain a two-dimensional array of light sources such as light-emitting diodes  38  that produce backlight illumination  45 . Light-emitting diodes  38  may, as an example, be arranged in rows and columns and may lie in the X-Y plane of  FIG. 2 . Light-emitting diodes  38  may be mounted on printed circuit board  50  (sometimes referred to as substrate  50 ) and may be encapsulated by encapsulant  52  (sometimes referred to as transparent encapsulant  52 ). 
     Light-emitting diodes  38  may be controlled in unison by control circuitry in device  10  or may be individually controlled (e.g., to implement a local dimming scheme that helps improve the dynamic range of images displayed on pixel array  24 ). The light produced by each light-emitting diode  38  may travel upwardly along dimension Z through optical films  26  before passing through pixel array  24 . 
     Optical films  26  may include films such as light spreading layer  28 , partially reflective layer  30 , color conversion layer  34  (which may include phosphor layer  40  and partially reflective layer  41 ), collimating layer  44 , brightness enhancement film  46 , diffuser layer  48 , and/or other optical films. 
     Light-emitting diodes  38  may emit light of any suitable color (e.g., blue, red, green, white, etc.). With one illustrative configuration described herein, light-emitting diodes  38  emit blue light. To help provide uniform backlight across backlight unit  42 , light from light-emitting diodes  38  may be spread by light spreading layer  28 . After passing through light spreading layer  28 , light from light-emitting diodes  38  may pass through partially reflective layer  30 . Partially reflective layer  30  (sometimes referred to as dichroic layer  30  or dichroic filter layer  30 ) may be configured to reflect some light from the LEDs and transmit some light from the LEDs. Partially reflective layer may include a multi-Bragg reflector and a diffuser layer in one possible embodiment. Light that is reflected off of partially reflective layer  30  may be recycled (e.g., the reflected light will reflect off of other layers such as substrate  50  before reaching partially reflective layer  30  again). Light that is transmitted through partially reflective layer  30  then passes through color conversion layer  34  (which may sometimes be referred to as a photoluminescent layer). 
     The transmission of partially reflective layer  30  may be selected to maximize the efficiency of display  14 . Lowering the transmission of blue light (e.g., from the light-emitting diodes) through the partially reflective layer increases the amount of blue light that is recycled. However, recycling more light may cause more light to be absorbed by printed circuit board  50  (or other layers below partially reflective layer  30 ). Increasing the transmission of blue light may cause more visible artifacts, however. Therefore, the transmission of the partially reflective layer may be selected to optimize efficiency and uniformity of the display. The reflectance of printed circuit board  50  may influence the optimum transmission level of partially reflective layer  30 . In one illustrative embodiment, printed circuit board  50  may have a reflectance of about 90% and partially reflective layer  30  may reflect 50% of blue light from light-emitting diodes  38 . Increasing the reflectance of printed circuit board  50  would increase the optimum reflectance of partially reflective layer  30 . 
     Color conversion layer  34  may convert the light from LEDs  38  from a first color to a different color. For example, when the LEDs emit blue light, color conversion layer  34  may include a phosphor layer  40  (e.g., a layer of white phosphor material or other photoluminescent material) that converts blue light into white light. If desired, other photoluminescent materials may be used to convert blue light to light of different colors (e.g., red light, green light, white light, etc.). For example, one layer  34  may have a phosphor layer  40  that includes quantum dots that convert blue light into red and green light (e.g., to produce white backlight illumination that includes, red, green, and blue components, etc.). Configurations in which light-emitting diodes  38  emit white light (e.g., so that layer  34  may be omitted, if desired) may also be used. In addition to phosphor layer  40 , color conversion layer  34  may include a partially reflective layer  41 . Partially reflective layer  41  (sometimes referred to as a dichroic layer or dichroic filter layer) may reflect all red and green light and partially reflect blue light, for example. 
     If desired, color conversion layer  34  and partially reflective layer  30  may be formed as a single integral layer. This may reduce the thickness of the optical film stack-up. 
     By the time light from light-emitting diodes  38  reaches collimating layer  44 , the light has been converted from blue to white and has been homogenized (e.g., by the light spreading layer). Collimating layer  44  (sometimes referred to as microlens layer  44  or microlens array diffuser  44 ) may collimate off-axis light. One or more brightness enhancement films  46  may be included to further help collimate light  45  and thereby increase the brightness of display  14  for user  20 . Finally, backlight unit  42  may include diffuser layer  48  to homogenize light from the array of light-emitting diodes. 
       FIG. 3  is a top view of an illustrative light-emitting diode array for backlight  42 . As shown in  FIG. 3 , light-emitting diode array  36  may contain row and columns of light-emitting diodes  38 . Each light-emitting diode  38  may be associated with a respective cell (tile area)  38 C. The length D of the edges of cells  38 C may be 2 mm, 18 mm, 1-10 mm, 1-4 mm, 10-30 mm, more than 5 mm, more than 10 mm, more than 15 mm, more than 20 mm, less than 25 mm, less than 20 mm, less than 15 mm, less than 10 mm, less than 1 mm, less than 0.1 mm, greater than 0.01 mm, greater than 0.1 mm, or any other desired size. If desired, hexagonally tiled arrays and arrays with light-emitting diodes  38  that are organized in other suitable array patterns may be used. In arrays with rectangular cells, each cell may have sides of equal length (e.g., each cell may have a square outline in which four equal-length cell edges surround a respective light-emitting diode) or each cell may have sides of different lengths (e.g., a non-square rectangular shape). The configuration of  FIG. 3  in which light-emitting diode array  36  has rows and columns of square light-emitting diode regions such as cells  38 C is merely illustrative. 
     If desired, each cell  38 C may have a light source that is formed form an array of light-emitting diode dies (e.g., multiple individual light-emitting diodes  38  arranged in an array such as a 2×2 cluster of light-emitting diodes at the center of each cell  38 C). For example, light source  38 ′ in the leftmost and lowermost cell  38 C of  FIG. 3  has been formed from a 2×2 array of light-emitting diodes  38  (e.g., four separate light-emitting diode dies). In general, each cell  38 C may include a light source  38 ′ with a single light-emitting diode  38 , a pair of light-emitting diodes  38 , 2-10 light-emitting diodes  38 , at least two light-emitting diodes  38 , at least 4 light-emitting diodes  38 , at least eight light-emitting diodes  38 , fewer than five light-emitting diodes  38 , or other suitable number of light-emitting diodes. Illustrative configurations in which each cell  38 C has a single light-emitting diode  38  may sometimes be described herein as an example. This is, however, merely illustrative. Each cell  38 C may have a light source  38  with any suitable number of one or more light-emitting diodes  38 . The diodes  38  in light-emitting diode array  36  may be mounted on a printed circuit board substrate that extends across array  36  or may be mounted in array  36  using other suitable arrangements. 
     Light-emitting diodes  38  in light-emitting diode array  36  may generate heat. If care is not taken, the resulting heat gradient (e.g., with areas of the array closer to the light-emitting diodes being hotter than areas of the array between the light-emitting diodes) may cause a heat gradient in pixel array  24 . The color of light emitted by pixel array  24  may be dependent on temperature. Therefore, a high thermal gradient in the pixel array can negatively affect display performance. 
       FIGS. 4 and 5  are cross-sectional side views of illustrative light-emitting diode arrays with layers that help distribute heat from light-emitting diodes  38  (and prevent a thermal gradient in pixel array  24 ). As shown in  FIG. 4 , light-emitting diodes  38  may be attached to conductive pads  54  (e.g., solder pads) of printed circuit board  50  using conductive material (e.g., solder)  56 . A thermally conductive layer  60  may be attached to the bottom surface of printed circuit board  50  using adhesive layer  58 . Adhesive layer  58  may conduct heat in the negative Z-direction (e.g., from printed circuit board  50 ) to thermally conductive layer  60 . Thermally conductive layer  60  may then distribute heat within the XY-plane. 
     Thermally conductive layer  60  may be formed from any desired material. For example, thermally conductive layer  60  may be formed from aluminum, graphite, a carbon-fiber reinforced sheet, carbon nanotubes, or metal particles. Thermally conductive layer  60  may have a thermal conductivity of greater than 200 W/mK, greater than 300 W/mK, greater than 400 W/mK, between 200 W/mK and 400 W/mK, or another desired thermal conductivity. Thermally conductive layer  60  may have a thickness  62  of less than 0.1 millimeters, less than 0.2 millimeters, about 55 microns, greater than 20 microns, or another desired thickness. 
     Adhesive layer  58  may be a pressure sensitive adhesive layer. If desired, adhesive layer  58  may include additive  64  (e.g., metal particles) to increase thermal conductivity of the adhesive layer. 
     In another embodiment, shown in  FIG. 5 , a radiative cooling coating may be attached to printed circuit board  50 . As shown in  FIG. 5 , layer  66  (sometimes referred to as a coating layer, radiative cooling layer or radiative cooling coating) may be attached to a bottom surface of the printed circuit board  50  (e.g., using adhesive  58 ). Layer  66  may include metal particles that are embedded in a polymer sheet. The metal particles may, for example, emit infrared light, thereby cooling the adjacent layers. 
     As shown in  FIGS. 4 and 5 , light-emitting diodes  38  are attached to solder pads on printed circuit board  50  using solder. If the overlap area between the solder and the light-emitting diodes is low, the strength of the bond between the solder and the light-emitting didoes may be low. To increase the overlap area between the solder and the light-emitting diodes (and accordingly, the bonding strength and SMT yield), the light-emitting diodes may have a rectangular design, as shown in  FIG. 6 . 
       FIG. 6  is a top view of illustrative light-emitting diodes having a rectangular (non-square) shape. As shown, each light-emitting diode in light-emitting diode array  36  may have a width (W) and a length (L) that is longer than the width (e.g., greater than the width, at least twice as long as the width, at least 1.5 times as long as the width, at least 3 times as long as the width, less than 3 times long than the width, etc.). In other words, each light-emitting diode may have a non-square rectangular shape. Each cell (tile area)  38 C may have an associated four light-emitting diodes  38 - 1 ,  38 - 2 ,  38 - 3 , and  38 - 4 . As shown in  FIG. 6 , light-emitting diodes  38 - 1 ,  38 - 2 ,  38 - 3 , and  38 - 4  are arranged in a 2×2 grid within cell  38 C. Light-emitting diode  38 - 1  is in the upper-left of the grid, light-emitting diode  38 - 2  is in the upper-right of the grid, light-emitting diode  38 - 3  is in the lower-left of the grid, and light-emitting diode  38 - 3  is in the lower-right of the grid. Light-emitting diodes  38 - 1  and  38 - 4  may have lengths that are parallel to the Y-axis, whereas light-emitting diodes  38 - 2  and  38 - 3  may have lengths that are parallel to the X-axis (and rotated 90° relative to the lengths of  38 - 1  and  38 - 4 ). With this arrangement, each light-emitting diode may have a non-square rectangular outline (for increased bonding strength) while ensuring uniform backlight (because the four light-emitting diodes in each tile will together emit uniform backlight). The light-emitting diodes in each tile may be controlled together. Each light-emitting diode in a given cell may have any desired positioning and orientation. Each light-emitting diode in a given cell may have any desired pattern of solder. Each light-emitting diode in a given cell may have any desired encapsulant additives of any desired particle size and distribution. 
     Another arrangement for light-emitting diodes in a light-emitting diode array  36  is shown in  FIG. 7 . As shown in  FIG. 7 , each tile  38 C may have an associated plurality (e.g., two, three, more than three, etc.) of light-emitting diodes. In the example of  FIG. 7 , each tile  38 C includes light-emitting diodes  38 - 1 ,  38 - 2 , and  38 - 3 . These light-emitting diodes may have different properties to help tune the light emitted from the cell. In the example of  FIG. 7 , light-emitting diode  38 - 2  is larger than light-emitting diode  38 - 1 , and light-emitting diode  38 - 1  is larger than light-emitting diode  38 - 3 . Each light-emitting diode may have the same or different sizes. The example of  FIG. 7  in which each light-emitting diode has a non-square rectangular shape is merely illustrative, and each light-emitting diode may have any desired shape. In another possible embodiment, the light-emitting diodes may have different peak wavelength emissions. Additionally, as shown in  FIG. 2  each light-emitting diode may be covered by encapsulant  52 . Each light-emitting diode in tile  38 C may be covered by encapsulant of different shapes or encapsulant with different dopants. 
     In general, multiple light-emitting diodes in a given tile may have different properties to tune the light that is emitted from that tile. These properties may include positioning, orientation, properties of the solder attached to the light-emitting diodes, particle size of encapsulant additives, and distribution of encapsulant additives. In one illustrative example, light emitting diodes  38 - 1 ,  38 - 2 , and  38 - 3  may be red, blue, and green light-emitting diodes respectively (and tile  38 C emits white light). 
       FIG. 2  shows light spreading layer  28  that is used to spread light from light-emitting diodes  38  in light-emitting diode array  36 .  FIG. 8  is a cross-sectional side view of an illustrative light-emitting diode array with a light redirecting layer on the printed circuit board adjacent the light-emitting diodes. As shown in  FIG. 8 , light-emitting diodes  38  are mounted on an upper surface of printed circuit board  50 . Light redirecting layer  68  is also mounted on the upper surface of printed circuit board  50 . Light redirecting layer  68  may be a prismatic reflector that helps redirect light emitted from the edges of light-emitting diodes  38 . Layer  68  may be a separate film that is laminated to printed circuit board  50  or may be embossed on a UV curable or thermal curable coating on the printed circuit board. 
     Layer  68  may be designed to guide light in a particular direction to improve optical uniformity and efficiency. The layer can include diffusive, refractive, and/or diffractive optical properties. Layer  68  may include a micro structure, a parabolic reflector, an embossed structure that diffracts a certain wavelength, or a prismatic structure that emits light at a defined cone angle. Layer  68  may work in combination with light spreading layer  28  or may replace light spreading layer  28 . In the example of  FIG. 8 , layer  68  includes two protrusions between each adjacent light-emitting diode  38 . 
       FIGS. 9 and 10  are cross-sectional side views showing how light-emitting diodes  38  in light-emitting diode array  36  may or may not include a reflector layer. As shown in  FIG. 9 , light-emitting diode may include a reflector layer  70  (e.g., a distributed Bragg-reflector) to help direct emitted light from light-emitting diode  38  sideways. This can reduce the on-axis intensity of light emitted by light-emitting diode  38 . However, this increases the amount of light that hits the upper surface of printed circuit board  50 , solder pads  54 , and solder  56 . Printed circuit board  50 , solder pads  54 , and solder  56  may not have a high reflectance. Therefore, the presence of reflector layer  70  may result in high light loss. To reduce light loss, the light-emitting diode may not include reflector layer  70  (as shown in  FIG. 10 ). 
       FIG. 10  additionally shows various ways to reduce the solder thickness and corresponding height of the light-emitting diode relative to the printed circuit board and improve solder quality by reducing air enclosed under the light-emitting diode. As shown in  FIG. 10 , solder pads  54  may have recesses  53 B (e.g., in the upper surface of the solder pads) that receive portions of solder  56 . Recesses  53 B allow solder  56  to better attach to solder pad  54  due to capillarity and gravity. Similarly, the lower surface of light-emitting diode  38  may have one or more recesses  53 A that receive portions of solder  56 . Again, recesses  53 A may allow solder  56  to better attach to light-emitting diode  38 . One or more of the recesses may be coated with a conductive coating if desired. 
     Additionally, a coating  57  may be included on printed circuit board  50  underneath light-emitting diode  38 . Coating  57  may be a dielectric (e.g., non-conductive) coating. Coating  57  may be hydrophobic, allowing solder  56  to flow across the coating. When solder  56  is deposited, excess solder may flow across coating  57  into recesses  55  in printed circuit board  50 . Recesses  55  may extend partially through printed circuit board  50  or may pass entirely (e.g., from an upper surface to a lower surface) through the circuit board. Vacuum suction may also be applied (e.g., during solder deposition) to align the die and remove excess air. Having excess solder flow into recesses  55  as described may improve solder quality and placement reliability, reduce die tilting, and reduce air under the die. 
       FIG. 11  shows ways to reduce light loss by increasing the reflectivity of the printed circuit board and other adjacent layers. As shown in  FIG. 11 , solder mask  72  (sometimes referred to as a solder mask reflector or a reflective layer) may be included in light-emitting diode array  36 . Solder mask  72  may be formed from a highly reflective material to reduce losses when light from light-emitting diode  38  hits solder mask  72 . The solder mask may include titanium dioxide (TiO 2 ) (e.g., titanium dioxide particles dispersed in a polymer) or other desired materials to increase reflectivity. The particle size of the titanium dioxide in the solder mask as well as the polymer refraction index may be tuned to optimize reflectivity. In one embodiment, the solder mask may be directly laminated onto the upper surface of the printed circuit board. In another embodiment, the solder mask may be a thermally curable coating or an ultraviolet light curable coating. The solder mask may have any desired thickness (e.g., 50 microns, greater than 50 microns, less than 100 microns, less than 50 microns, etc.). 
     Properties of printed circuit board  50  may also be optimized for high reflectivity. For example, glass fiber material or white polyimide polymer may form the core of printed circuit board  50 . The reflectivity of the solder mask and printed circuit board may be the same or may be different (e.g., greater than 80%, greater than 50%, greater than 90%, greater than 92%, greater than 94%, between 85% and 95%, less than 90%, less than 99%, between 80% and 95%, etc.). Thermally conductive fillers may also be added to printed circuit board  50  to tune the thermal conductivity of the printed circuit board. 
     There are numerous possible embodiments for the encapsulant that encapsulates (e.g., conforms to) the light-emitting diodes on printed circuit board  50 .  FIG. 12  is a cross-sectional side view of an illustrative light-emitting diode array with encapsulant  52  that has a planar upper surface. As shown in  FIG. 12 , encapsulant  52  may have a planar upper surface  74  that extends over the light-emitting diode. The encapsulant  52  (sometimes referred to as a slab of encapsulant in this embodiment) provides additional structural integrity for light-emitting diode  38 . Additionally, to reduce total internal reflection of light emitted from light-emitting diode  38  off of the interface between encapsulant  52  and adjacent material  76  (e.g., air), encapsulant  52  may have an index of refraction that is between the index of refraction of light-emitting diode  38  and material  76 . For example, encapsulant  52  may have an index of refraction of between 1.2 and 1.5, between 1.3 and 1.4, less than 1.4, less than 1.5, greater than 1.1, greater than 1.2, greater than 1.3, about 1.35, etc. The slab of encapsulant  52  shown in  FIG. 12  may cover all of the light-emitting diodes in light-emitting diode array  36 . Encapsulant  52  may have any desired thickness  78  (e.g., between 0.1 and 0.4 millimeters, less than 0.5 millimeters, less than 0.3 millimeters, between 0.2 and 0.4 millimeters, between 0.15 and 0.25 millimeters, between 0.25 millimeters and 0.35 millimeters, about 0.2 millimeters, about 0.3 millimeters, etc.). 
       FIG. 13  is a cross-sectional side view of an illustrative light-emitting diode array with encapsulant  52  that has a curved upper surface. As shown in  FIG. 13 , encapsulant  52  may have a curved upper surface  80  (e.g., a convex upper surface). The encapsulant may form a dome shape over the light-emitting diode. In this embodiment, the encapsulant may be referred to as a droplet lens. The encapsulant  52  provides additional structural integrity for light-emitting diode  38 . Additionally, to reduce total internal reflection of light emitted from light-emitting diode  38  off of the interface between encapsulant  52  and adjacent material  76  (e.g., air), encapsulant  52  may have an index of refraction that is between the index of refraction of light-emitting diode  38  and material  76 . For example, encapsulant  52  may have an index of refraction of between 1.2 and 1.5, between 1.3 and 1.4, less than 1.4, less than 1.5, greater than 1.1, greater than 1.2, greater than 1.3, about 1.35, etc. Each light-emitting diode may have a corresponding encapsulant portion (e.g., one dome of encapsulant for each light-emitting diode). Encapsulant  52  may have any desired thickness  82  (e.g., between 0.1 and 0.4 millimeters, less than 0.5 millimeters, less than 0.3 millimeters, between 0.2 and 0.4 millimeters, between 0.15 and 0.25 millimeters, between 0.25 millimeters and 0.35 millimeters, about 0.2 millimeters, about 0.3 millimeters, etc.) and any desired width  84  (e.g., between 0.3 and 2.5 millimeters, between 0.3 and 0.7 millimeters, between 0.8 and 0.9 millimeters, greater than 0.5 millimeters, greater than 1.0 millimeters, greater than 2.0 millimeters, less than 2.0 millimeters etc.). The ratio of width to thickness may be greater than 5-to-1, greater than 3-to-1, between 4-to-1 and 6-to-1, less than 10-to-1, etc. 
       FIG. 14  is a cross-sectional side view of an illustrative light-emitting diode array with encapsulant  52  that has a curved upper surface and a recess. The encapsulant in  FIG. 14  is similar to the encapsulant of  FIG. 13  (e.g., with a dome shape). However, in  FIG. 14  encapsulant  52  additional has a recess  86  formed over light-emitting diode  38 . Recess  86  may also be a referred to as a dip. Recess  86  may have any desired shape (e.g., a pyramid shape). Recess  86  may further increase uniformity compared to the embodiment of  FIG. 13  (without the recess) by reducing the intensity of on-axis (e.g., zero order) light. 
       FIGS. 15-17  show examples of a dopant being included in encapsulant  52 .  FIG. 15  is a cross-sectional side view showing a light-emitting diode  38  covered by encapsulant  52  that includes dopant  88 . Dopant  88  (sometimes referred to as additive  88 ) may be a scattering dopant. The presence of dopant  88  may increase the light-emitting area of light-emitting diode  38 . Without the scattering dopant, light emitted from light-emitting diode  38  will be concentrated directly over light-emitting diode  38 . The scattering dopant, however, may cause light from light-emitting diode  38  to be spread throughout encapsulant  52 . Therefore, the entire area of encapsulant  52  emits light from light-emitting diode  38 . This spreading of light may work in combination with or replace light spreading layer  28  in  FIG. 2 , for example. 
     Any desired dopants may be used for dopant  88 . For example, dopant  88  may include polymer nanoparticles, inorganic nanoparticles, voids (e.g., air bubbles), or any other desired dopant. Distribution of the dopants in encapsulation  52  can be controlled via buoyancy of the dopant in the encapsulation material (e.g., encapsulation resin). For example,  FIG. 15  shows a neutral buoyancy example. Dopants  88  in  FIG. 15  are therefore distributed evenly throughout encapsulation  52  (e.g., dopants may be suspended anywhere in the encapsulation material).  FIG. 16  shows an example where dopants  88  have high buoyancy and therefore tend to be distributed along the upper surface of the encapsulant (e.g., because the dopants float to the top of the encapsulant). In contrast,  FIG. 17  shows an example where dopants  88  have a low buoyancy and therefore tend to be distributed along the lower surface of the encapsulant (e.g., because the dopants sink to the bottom of the encapsulant). 
     The location of dopants  88  may be controlled if desired. For example, dopants may be placed directly on top of light-emitting diode  38 . Dopants  88  may also be designed such that the orientation of dopants  88  in encapsulant  52  can be controlled. In one example, shown in  FIG. 18 , dopant  88  (e.g., a particular particle) may have two portions with two different densities. As shown in  FIG. 18 , dopant  88  may include portion  88 - 1  and portion  88 - 2 . Portion  88 - 2  may have a different density (e.g., higher) than portion  88 - 1 . In another example, shown in  FIG. 19 , the shape of dopant  88  may be designed to control the orientation of dopant  88 . The shape shown in  FIG. 19  (sometimes referred to as a mushroom shape) may allow control of the orientation of dopant  88  in encapsulant  52 . 
     The aforementioned examples showing dopants  88  in a dome-shaped encapsulant region are merely illustrative. In general, any desired encapsulant (e.g., encapsulant with a planar upper surface as in  FIG. 12  or a recessed portion as in  FIG. 14 ) may include the dopants. 
       FIG. 20  is a cross-sectional side view of an illustrative backlight unit showing how light-emitting diode array  36  may include a patterned layer  90  on encapsulation  52 . Patterned layer  90  may help spread light from light-emitting diode  38  (e.g., working in combination with or replacing light spreading layer  28  in  FIG. 2 ). Patterned layer  90  may also help maintain gap  92  (e.g., an air gap) between optical films  26  and encapsulant  52 . Patterned layer  90  may be a separate film that is attached to an upper surface of encapsulant  52 . Alternatively, patterned layer  90  may be formed from a patterned portion of the encapsulant material (e.g., patterned layer  90  may be part of encapsulant  52 ). 
       FIG. 21  is a cross-sectional side view of an illustrative backlight unit showing how light-emitting diode array  36  may include encapsulation  52  having a textured surface. As shown in  FIG. 21 , encapsulation  52  has a textured upper surface  94 . Textured upper surface  94  may be generally planar (e.g., the encapsulation is formed as a slab). However, the upper surface may be rough (instead of smooth). The textured upper surface may prevent total internal reflection of light emitted from light-emitting diode  38 , increasing efficiency. The texture of the upper surface may be random. 
     To increase the reflectivity of the printed circuit board in a backlight unit, a reflective layer may be used. For example, a reflective material may be laminated on the printed circuit board. However, care must be taken to ensure that the reflective material is not damaged during solder reflow. The reflective material may have a melting point less than or equal to that of the solder. Reflowing the solder may therefore melt (and damage) the reflective material. 
     To prevent damage of the reflective layer, the reflective layer may be laminated to the circuit board after solder reflow. Alternatively, the reflective layer may be attached to the printed circuit board before solder reflow. The solder mask may then be used to protect the reflective layer from damage during solder reflow. An arrangement of this type is shown in  FIG. 22 . 
     As shown in  FIG. 22 , reflective layer  96  may be formed on printed circuit board  50  (e.g., on an upper surface of printed circuit board  50 ). The reflective layer may be formed from any desired material. The reflective layer may be formed from one or more layers. The reflective layer may have a high reflectance of light at the wavelength emitted by the light-emitting diodes. For example, if the light-emitting diodes emit blue light, reflective layer  96  may have a high reflectance (e.g., greater than 80%, greater than 90%, greater than 92%, greater than 94%, greater than 96%, less than 99%, etc.) of the blue light. Solder  56  may also be formed on printed circuit board  50  (e.g., on conductive pads on an upper surface of the printed circuit board that may not be covered by the reflective layer). Heat source  100  may be used to heat solder  56  for reflow. Heat source  100  may heat solder  56  by emitting energy (e.g., infrared light) through solder mask  98 . Solder mask  98  may have openings that overlap solder  56 . Portions of printed circuit board  50  that are not covered by solder  56  may be covered by solder mask  98 . In this way, solder mask  98  may prevent reflective layer  96  from being damaged during heating of solder  56 . Solder mask  98  may be formed from any material that is opaque or resistant to the energy ( 102 ) emitted by heat source  100 . Heat source  100  may, for example, be a laser that emits infrared light. 
     In certain embodiments, backlight unit  42  may include a polarization grating that helps increase the point spread function (PSF) width of the light emitted by light-emitting diodes  38 . Increasing the PSF width may reduce reliance on the reflectance of the printed circuit board for recycling.  FIGS. 23-25  are cross-sectional side views of illustrative backlight units with a polarization grating layer (sometimes referred to as a diffractive layer, light spreading layer, polarization phase grating layer, polarization grating, or polarization phase grating). As shown in  FIG. 23 , backlight unit  42  includes light-emitting diodes  38  on printed circuit board  50  that are covered by encapsulation  52 . The backlight unit may also have optical films  26  that include light spreading layer  28 . However, the backlight unit may include additional light spreading layer  104 . Polarization grating layer  104  (sometimes referred to as a diffractive layer, light spreading layer, polarization phase grating layer, polarization grating, or polarization phase grating) may split light received from light-emitting diodes  38  into two different polarizations. Light spreading layer  104  may reduce the on-axis intensity peak associated with the light-emitting diodes. 
     As shown in  FIG. 23 , light spreading layer  104  may be coupled to encapsulation  52  that covers the light-emitting diodes (e.g., light spreading layer  104  may directly contact encapsulation  52 ). This example is merely illustrative. In another example shown in  FIG. 24 , light spreading layer  104  is formed on a lower surface of light spreading layer  28 . Light spreading layer  28  and light spreading layer  104  may be attached together in any desired manner. Light spreading layer  28  may have evenly patterned protrusions on an upper surface of the light spreading layer. In yet another embodiment, shown in  FIG. 25 , a wave guide layer  106  (sometimes referred to as a light guiding layer) may be interposed between light spreading layer  28  and light spreading layer  104 . Light spreading layer  28  may have protrusions on an upper surface of the light spreading layer that are unevenly spaced (e.g., statistically spaced to control wave guide spreading) in this embodiment. 
     Other structures may be used to increase the point spread function (PSF) width of the light emitted by light-emitting diodes  38 .  FIGS. 26 and 27  are cross-sectional side views of backlight units that include structures to increase the point spread function (PSF) width of the light emitted by light-emitting diodes  38  in combination with a highly reflective printed circuit board. As shown in  FIG. 26 , light-emitting diodes  38  may be positioned on printed circuit board  50  and may be covered by encapsulant  52 . Each light-emitting diode  38  may be covered by a respective portion of encapsulant that has a respective parabolic upper surface  108 . The parabolic upper surfaces of adjacent encapsulant portions may meet at vertex  110 . This portion may be referred to as a dimpled or recessed portion of encapsulant  52 . 
     Light  112  emitted from light-emitting diodes  38  may be reflected towards vertex  110  by total internal reflection (TIR). When light  112  reaches vertex  110  (sometimes referred to as light leakage promotion structure  110 ), the light may be more likely to exit encapsulant  52  in the direction of the viewer. In this way, light from the light-emitting diodes may be directed to and emitted from the area between adjacent light-emitting diodes (thereby increasing the PSF width). This type of arrangement relies on the light reflecting off of printed circuit board  50  (e.g., light is emitted from the light-emitting diode, reflects off of the parabolic upper surface of the encapsulant due to total internal reflection, and then reflects off of the upper surface of the printed circuit board). Therefore, if the reflectance of the printed circuit board is low, the efficiency of a backlight unit with the arrangement of  FIG. 26  may be low. The arrangement of  FIG. 26  may therefore be suited to embodiments where the printed circuit board has a high reflectance (e.g., greater than 90%, greater than 94%, greater than 96%, etc.). 
     In another embodiment, shown in  FIG. 27 , light-emitting diodes  38  are again positioned on printed circuit board  50  and may be covered by encapsulant  52 . In this embodiment, the light-emitting diodes are covered by encapsulant with a planar upper surface (e.g., a slab of encapsulant). A light leakage promotion structure  114  may be interposed between each light-emitting diode. Light leakage promotion structure  114  in  FIG. 27  may be formed from an opaque white structure. In another embodiment, light leakage promotion structure  114  may be an air gap (sometimes referred to as a void) in encapsulant  52 . 
     Light emitted from light-emitting diodes  38  may be reflected towards light leakage promotion structure  114  by total internal reflection (TIR). When light reaches light leakage promotion structure  114 , the light may be more likely to exit encapsulant  52  in the direction of the viewer. In this way, light from the light-emitting diodes may be directed to and emitted from the area between adjacent light-emitting diodes (thereby increasing the PSF width). This type of arrangement relies on the light reflecting off of printed circuit board  50  (e.g., light is emitted from the light-emitting diode, reflects off of the upper surface of the encapsulant due to total internal reflection, and then reflects off of the upper surface of the printed circuit board). The arrangement of  FIG. 27  may therefore be suited to embodiments where the printed circuit board has a high reflectance (e.g., greater than 90%, greater than 94%, greater than 96%, etc.). 
     There are many possible ways for light spreading layer  28  (shown in  FIG. 2 , for example) to spread light.  FIG. 28  is a cross-sectional side view of an illustrative display with a light spreading layer that has microlens structures on both the upper surface and lower surface. As shown in  FIG. 28 , light spreading layer  28  has an upper surface  116  and a lower surface  118 . Upper surface  116  has a plurality of microlenses  120  that are formed from recesses in the upper surface of the light spreading layer. Microlenses  120  may be formed from spherically shaped recesses or recesses of any other desired shape in upper surface  116 . Lower surface  118  also has a plurality of microlenses  122  that are formed from recesses in the lower surface of the light spreading layer. Microlenses  122  may be formed from spherically shaped recesses or recesses of any other desired shape in lower surface  118 . Microlenses  122  may be larger than microlenses  120  (as shown in  FIG. 28 ). However, this example is merely illustrative and microlenses  122  may also be the same size or smaller than microlenses  120 . Additionally, each microlens in microlens array  120  may have the same size or a different size, and each microlens in microlens array  122  may have the same size or a different size. 
     Another arrangement for light spreading layer  28  is shown in  FIG. 29 . As shown in  FIG. 29 , the light spreading layer may include two films (layers). In the example of  FIG. 29 , light spreading layer  28 - 1  is positioned above light spreading layer  28 - 2 . Light spreading layer  28 - 1  and light spreading layer  28 - 2  may be attached using adhesive  128  (e.g., pressure sensitive adhesive), for example. Adhesive  128  may have diffusive properties if desired. Alternatively, light spreading layers  28 - 1  and  28 - 2  may be laminated together to form an integral film. 
     As shown in  FIG. 29 , light spreading layer  28 - 2  has an upper surface with microlenses  132 . Microlenses  132  may be formed from a plurality of recesses in the upper surface of light spreading layer  28 - 2 . Light spreading layer  28 - 2  may also include a plurality of protrusions  130 . Protrusions  130  may protrude towards the light-emitting diodes. Protrusions  130  may be elongated protrusions (sometimes referred to as ridges) that extend along a longitudinal axis across the layer (e.g., parallel to the Y-axis in  FIG. 29 ). Light spreading layer  28 - 1  may be the same as light spreading layer  28 - 2 , except for being rotated 90° relative to light spreading layer  28 - 2 . For example, light spreading layer  28 - 1  also has a plurality of protrusions that extend along a longitudinal axis across the layer. However, the protrusions of light spreading layer  28 - 1  may extend parallel to the X-axis (perpendicular to the protrusions of light spreading layer  28 - 2 ). Light spreading layer  28 - 1  also includes microlenses  126 . Microlenses  126  may be formed from a plurality of recesses in the upper surface  124  of light spreading layer  28 - 1 . 
     Light emitted from light-emitting diodes  38  will be spread relative to a first axis (e.g., from a point source to two points) upon reaching light-spreading layer  28 - 2 . Microlenses  132  may reduce total internal reflection to promote light passing to light spreading layer  28 - 1 . Light spreading layer  28 - 1  will spread the incoming light relative to a second axis (e.g., from the two points to four points) that is perpendicular to the first axis (because the protrusions of layer  28 - 1  are perpendicular to the protrusions of layer  28 - 2 ). Microlenses  126  may reduce total internal reflection to promote leakage of light out of layer  28 - 1  towards layer  30 . Layers  28 - 1  and  28 - 2  may each have any desired thickness (e.g., between 20 and 30 microns, between 20 and 25 microns, less than 50 microns, less than 25 microns, about 22 microns, greater than 15 microns, less than 100 microns, etc.). The thickness of layers  28 - 1  and  28 - 2  may be the same. 
     The example of light spreading layer  28  having elongated protrusions that extend along a longitudinal axis (and that split an incoming point light source into two points) is merely illustrative. If desired, light spreading layer  28  may instead include an array of protrusions or recesses. Each protrusion may split an incoming point light source into three or more points.  FIG. 30  is a cross-sectional side view of a light spreading layer  28  that includes protrusions  134 . Protrusions  134  may have any desired shape. For example, the protrusions may have a pyramidal shape (e.g., with a square base and four triangular faces that meet at vertex  136 ) or a triangular pyramidal shape (e.g., with a triangular base and three triangular faces that meet at vertex  136 ). The pyramidal protrusions may split a point light source into four points, whereas the triangular pyramidal protrusions may split a point light source into three points. In another embodiment, shown in  FIG. 31 , light spreading layer  28  may include a plurality of recesses  138 . Recesses  138  may have any desired shape. For example, the recesses may have a pyramidal shape (e.g., with a square base and four triangular faces that meet at vertex  140 ) or a triangular pyramidal shape (e.g., with a triangular base and three triangular faces that meet at vertex  140 ). The pyramidal recesses may split a point light source into four points, whereas the triangular pyramidal recesses may split a point light source into three points.  FIG. 32  is a top view of light spreading layer  28  showing how protrusions  134  (or recesses  138 ) may be arranged in an array. The protrusions and/or recesses may have any desired shapes and may be arranged in any desired type of array. 
       FIG. 33  is a top view of a light spreading layer with protrusions  134  formed from partial-cube structures (sometimes referred to as corner-cubes). As shown, each protrusion (or recess) may be formed by three square faces (e.g., a corner cube) that meet at a vertex  136 . In  FIG. 34 , each protrusion  134  is formed from two partial-cube structures that are off-set from each other. Each protrusion  134  in  FIG. 34  therefore has six faces (and splits an incoming point light source into six points). 
     In yet another embodiment shown in  FIG. 35 , protrusion  134  may be formed from a tapered pyramid structure. As shown in  FIG. 35 , the protrusion may have four faces  142  that meet at vertex  148 . Each face may have a lower portion  144  and an upper portion  146  that are at an angle  150  relative to each other. Angle  150  may be selected to optimize the light spreading functionality of the light spreading layer. Two films with any of the light spreading features shown in  FIGS. 30-35  may be laminated together if desired (e.g., a first film with protrusions on an upper surface may be laminated to a second film with protrusions on a lower surface). 
     Additional embodiments for spreading light from light-emitting diodes  38  are shown in  FIG. 36-39 .  FIG. 36  is a cross-sectional side view of an illustrative backlight unit with a microlens array formed on a lower surface of partially reflective layer  30 . Microlenses  152  may protrude from the lower surface of partially reflective layer  30 . The microlenses may help spread light received from light-emitting diodes  38 . Microlenses  152  may have an associated focal length (f). The thickness  154  of encapsulant  52  may be a function of the focal length (e.g., t=n×f where t is thickness  154  and n is the refractive index of the encapsulant material). 
       FIG. 37  shows an arrangement similar to that of  FIG. 36 . However, in  FIG. 36  encapsulant  52  is formed across all of the light-emitting diodes in the array (e.g., encapsulant slab). In  FIG. 37 , each light-emitting diode has a respective portion of encapsulant  52 . In  FIG. 37 , the thickness  154  of each encapsulant portion may still be selected based on the focal length of microlenses  152  and the refractive index of the encapsulant material. 
       FIG. 38  shows yet another embodiment with microlenses  152  on the lower surface of partially reflective layer  30 . In  FIG. 38 , each light-emitting diode may be covered by an encapsulant portion  52  having a curved upper surface (instead of a planar upper surface as in  FIG. 37 ). In  FIG. 38 , the thickness  154  of each encapsulant portion may be selected based on the focal length of microlenses  152  (e.g., t=f where t is thickness  154  and f is the focal length of microlenses  152 ). 
     The example in  FIGS. 36-38  of microlenses  152  being formed on a lower surface of partially reflective layer  30  is merely illustrative. Microlenses  152  may be formed in other locations in the display if desired.  FIG. 39  shows an embodiment where microlenses  152  are formed on substrate  156  (separate from partially reflective layer  30 ). There may be an air gap  158  between light-emitting diodes  38  and the substrate  156  for microlenses  152 . 
     In the embodiments shown in  FIGS. 36-39 , light spreading layer  28  is omitted (because microlenses  152  spread the light from the light-emitting diodes). However, this is merely illustrative. If desired, light spreading layer  28  may be included in any of the embodiments of  FIGS. 36-39  (and spread light in combination with microlenses  152 ). 
       FIG. 2  showed an example where backlight unit  42  includes a brightness enhancement film  46  and a collimating layer  44 . Examples of these layers are again shown in  FIG. 40 . As shown in  FIG. 40 , brightness enhancement film  46  may include protrusions  160  on an upper surface of the film. Collimating layer  44  may include an array of microlenses  162  on a lower surface of the film. To reduce the number of layers in backlight unit  42 , brightness enhancement film  46  and collimating layer  44  may be combined into a single film.  FIG. 41  is a cross-sectional side view of an illustrative optical layer  164  having protrusions  160  on an upper surface (serving the function of the brightness enhancement film) and microlenses  162  on a lower surface (serving the function of the collimating layer). 
     Light leaking from the edge of optical films  26  may cause the edge of the display to have an undesirable blue tint (due to the pixel array  24  receiving an excess of blue light from light-emitting diodes  38 ). To mitigate this problem, one or more coatings may be included in the edge of the display. As shown in  FIG. 42 , display  14  may include a coating  168  on a lower layer of pixel array  24 . The display may include a chassis  166  (e.g., a plastic chassis sometimes referred to as a p-chassis) that supports one or more other layers in display  14 . One or more surfaces of the chassis (e.g., an edge surface  176 ) is coated by coating  174 . An upper surface of the optical layers is coated with coating  170 . An edge surface of the optical layers is coated with coating  172 . Coatings  168 ,  170 ,  172 , and  174  (which are all in the edge of the display) may be formed from a black material, a gray material, a yellow material, or a phosphor material. The black material, gray material, and yellow material may adjust or eliminate reflectance of blue light (thereby reducing the excess blue light in the edge of the display). The phosphor material may convert blue light to white light (thereby reducing the excess blue light in the edge of the display). 
     In some displays (e.g., as shown in  FIG. 3 ), the light-emitting diodes for the display may be arranged in a grid. The light-emitting diodes may be arranged in evenly spaced rows and columns that extend along the entire display. This type of arrangement may be referred to as a square grid or a rectangular grid. However, having the light-emitting diodes arranged in a rectangular grid may result in visible artifacts (sometimes referred to as grid mura) when operating the display. To mitigate visible artifacts associated with the positioning of the light-emitting diodes in a grid, the positions of the light-emitting diodes may be dithered. In other words, the actual position of each light-emitting diode may be adjusted relative to the rectangular-grid position for that light-emitting diode. 
       FIG. 43  is a diagram showing how the positions of light-emitting diodes may be dithered to improve performance of the display.  FIG. 43  shows rectangular grid positions  202 - 1 ,  202 - 2 ,  202 - 3 , and  202 - 4 . The rectangular grid positions may be positions associated with a regularly spaced grid that extends across the entire display (e.g., each rectangular grid position is part of a row of rectangular grid positions that extends across the entire display and each rectangular grid position is part of a column of rectangular grid positions that extends across the entire display). Light-emitting diodes may optionally be mounted at each rectangular grid position (similar to as in  FIG. 3 ). Alternatively, to mitigate visible artifacts caused by this arrangement the light-emitting diodes may be offset from the rectangular grid positions as shown in  FIG. 43 . 
     In  FIG. 43 , each light-emitting diode may be positioned at a position that is offset relative to its corresponding rectangular grid position. For example, a light-emitting diode may be located at position  206 - 1  that is moved by offset distance  204 - 1  away from rectangular grid position  202 - 1 . Offset distance  204 - 1  is at an angle  208 - 1  relative to the X-axis. Similarly, a light-emitting diode may be located at position  206 - 2  that is moved by offset distance  204 - 2  away from rectangular grid position  202 - 2 . Offset distance  204 - 2  is at an angle  208 - 2  relative to the X-axis. A light-emitting diode may be located at position  206 - 3  that is moved by offset distance  204 - 3  away from rectangular grid position  202 - 3 . Offset distance  204 - 3  is at an angle  208 - 3  relative to the X-axis. A light-emitting diode may be located at position  206 - 4  that is moved by offset distance  204 - 4  away from rectangular grid position  202 - 4 . Offset distance  204 - 4  is at an angle  208 - 4  relative to the X-axis. 
     Each offset distance may have any desired magnitude. In one example, the offset distance may be greater than 0, may be shorter than a distance between adjacent rectangular grid positions in the same row (e.g., distance  210 ), and may be shorter than a distance between adjacent rectangular grid positions in the same column (e.g., distance  212 ). Each offset distance may be the same (e.g., offset distances  204 - 1 ,  204 - 2 ,  204 - 3 , and  204 - 4  may all have the same magnitude) or one or more of the offset distances may be different (e.g., offset distances  204 - 1 ,  204 - 2 ,  204 - 3 , and  204 - 4  may all have different magnitudes). In some cases, the offset distances may be random or pseudo-random. 
     Similarly, each offset distance may be moved away from its corresponding rectangular grid position by any desired angle. In one illustrative example, each angle is 90 degrees offset from the angle associated with the light-emitting diode in an adjacent row or column. As shown in  FIG. 43 , angle  204 - 4  may be a given angle (e.g., θ) between 0 degrees and 90 degrees, angle  204 - 2  may be offset by 90 degrees relative to the given angle (e.g., angle  204 - 2 =θ+90 degrees), angle  204 - 1  may be offset by 180 degrees relative to the given angle (e.g., angle  204 - 1 =θ+180 degrees), and angle  204 - 3  may be offset by 270 degrees relative to the given angle (e.g., angle  204 - 3 =θ+270 degrees). Effectively, the 2×2 group of light-emitting diodes is rotated relative to the rectangular grid positions. Each 2×2 group of light-emitting diodes within the display may be independently rotated. The example of using a 2×2 group is merely illustrative. Light-emitting diode groups of any desired size may be rotated across the display. 
     Each light-emitting diode position  206  may be offset from its rectangular grid position  202  by an angle that is 180 degrees different than the light-emitting diode position of a diagonally adjacent light-emitting diode. Each light-emitting diode position  206  may be offset from its rectangular grid position  202  by an angle that is 90 degrees different than the light-emitting diode position of a horizontally adjacent light-emitting diode (e.g., a light-emitting diode in the same row). Each light-emitting diode position  206  may be offset from its rectangular grid position  202  by an angle that is 90 degrees different than the light-emitting diode position of a vertically adjacent light-emitting diode (e.g., a light-emitting diode in the same column). 
     This example is merely illustrative. In general, the location of each light-emitting diode and the rectangular grid position associated with that light-emitting diode may be offset by any desired angle. In some cases, the angles may be random or pseudo-random. 
       FIG. 44  is a top view of an illustrative display with illustrative light-emitting diodes that are dithered. As shown, each light-emitting diode  38  is dithered with respect to a rectangular grid position (as shown in  FIG. 43 ). In particular, each 2×2 group of light-emitting diodes are rotated relative to their rectangular grid positions. Therefore, the light-emitting diodes are not arranged in uniform rows and columns that extend across the entire display. Dithering the location of the light-emitting diodes in this way mitigates grid mura associated with the light-emitting diodes in a uniform rectangular grid. 
     Each light-emitting diode is covered by a respective encapsulant  52  (e.g., a slab or dome of encapsulant). In  FIG. 44 , each light-emitting diode is centered underneath its respective encapsulant layer  52 . However, this example is merely illustrative. If desired, each light-emitting diode may be offset relative to the center of the overlying encapsulant layer. 
       FIG. 45  is a top view of an illustrative display with light-emitting diodes that are offset relative to the center of the overlying encapsulant layer. Similar to how the positions of the light-emitting diodes are shifted relative to the rectangular grid positions in  FIG. 43 , the positions of the encapsulant are shifted relative to the positions of the light-emitting diodes in  FIG. 45 . As shown, each encapsulant layer  52  has a center that is shifted relative to the light-emitting diode  38 . The shifting of the center of the encapsulant layer in  FIG. 45  may follow the same offset scheme as shown in  FIGS. 43 and 44  in connection with the position of the light-emitting diodes. 
     When the encapsulant layers are shifted relative to the light-emitting diodes, the light-emitting diodes may be arranged in a rectangular grid (as shown in  FIG. 45 ). Alternatively, the positions of the light-emitting diodes may be dithered (as in  FIG. 44 ) and the encapsulant layers may be offset relative to the light-emitting diode positions. 
     In another embodiment, shown in  FIG. 46 , light-emitting diodes  38  are positioned on printed circuit board  50  and may be covered by encapsulant  52 . In this embodiment, the light-emitting diodes include a reflector layer  70  (e.g., a distributed Bragg-reflector) to help direct emitted light from light-emitting diode  38  sideways. This can reduce the on-axis intensity of light emitted by light-emitting diode  38 . As shown in  FIG. 46 , due to the presence of reflector layer  70 , the cone of light emitted by each light-emitting diode  38  may have a width  222 . The width of the cone of high intensity light may be relatively narrow (e.g., width  224 ), which helps promote uniform light distribution across the display. 
       FIG. 46  also shows how each light-emitting diode is covered by encapsulant with a planar upper surface (e.g., a slab of encapsulant). In one arrangement, the thickness  226  of the encapsulant may be selected based on the thickness  228  of the light-emitting diode  38 . In other words, a ratio between the thickness of the encapsulant and the thickness of the light-emitting diode may be selected to optimize display performance. The ratio between the thickness of the encapsulant and the thickness of the light-emitting diode may be used to help spread light evenly across the display. For example, if thickness  226  of encapsulant  52  is larger, the emission area associated with light-emitting diode  38  will tend be larger. If thickness  226  of encapsulant  52  is smaller, the emission area associated with light-emitting diode  38  will tend to be smaller. The thickness may therefore be selected such that the emission areas of adjacent light-emitting diodes meet at an interface without overlapping and without having an emission-free gap between them. The ratio of thickness  226  to thickness  228  may be between 3:1 and 10:1, between 2:1 and 20:1, between 5:1 and 15:1, less than 10:1, more than 10:1, less than 5:1, more than 5:1, more than 20:1, less than 8:1, etc. 
     In cases where encapsulant is formed as a slab over the light-emitting diodes (as in  FIG. 46 ), the slab may be susceptible to warping. The light-emitting diodes generate heat that may heat the encapsulant slab. The encapsulant slab may expand and contract with temperature variations associated with operation of the light-emitting diodes due to thermal expansion. To maintain the structural integrity of the encapsulant slab, a support structure may be incorporated into the display. 
       FIG. 47  is a cross-sectional side view of an illustrative display that includes a support structure  232  to maintain the structural integrity of encapsulant slab  52 . Support structure  232  may have a low coefficient of thermal expansion (e.g., lower than the encapsulant) to ensure it is not adversely affected by temperature changes associated with operation of the light-emitting diodes. Support structure  232  may not affect the light emitted from the light-emitting diodes. In other words, support structure  232  may be formed from a transparent material that has the same index-of-refraction as the surrounding encapsulant slab  52  (e.g., support structure  232  is index-matched to slab  52 ). Support structure  232  will therefore not influence the path or intensity of the light emitted from light-emitting diodes  38 . Support structure  232  may therefore have any desired shape (since the shape will not affect the optical performance of the display). 
       FIG. 48  is a cross-sectional side view of an illustrative display with a support structure that also serves as a light-leakage promotion structure. Similar to as in  FIG. 47 , support structure  234  in  FIG. 48  may be used to maintain the structural integrity of encapsulant slab  52 . Support structure  234  may have a low coefficient of thermal expansion (e.g., lower than the encapsulant) to ensure it is not adversely affected by temperature changes associated with operation of the light-emitting diodes. However, unlike in  FIG. 47 , the support structure  234  in  FIG. 48  may reflect light from light-emitting diodes  38 . Support structure  234  may be formed from a reflective material (e.g., a white structure, metal structure, etc.). Alternatively, support structure  234  may be formed from a transparent material that has an index-of-refraction that is different than the index-of-refraction of encapsulant layer  52 . The index-of-refraction difference may be sufficient for total internal reflection (TIR) to occur when light from light-emitting diodes  238  reaches support structure  234 . Support structure  234  may have a shape that is selected to redirect light upwards towards the viewer. The example of  FIG. 48  of support structure  234  having a triangular cross-sectional shape is merely illustrative. Support structure  234  may have a pyramidal shape, cone shape, dome shape, etc. 
     As previously mentioned, display  14  may include a color conversion layer  34 . Color conversion layer  34  may convert the light from LEDs  38  from a first color to a different color. For example, when the LEDs emit blue light, color conversion layer  34  may include a phosphor layer  40  (e.g., a layer of white phosphor material or other photoluminescent material) that converts blue light into white light. If desired, other photoluminescent materials may be used to convert blue light to light of different colors (e.g., red light, green light, white light, etc.). For example, one layer  34  may have a phosphor layer  40  that includes quantum dots that convert blue light into red and green light (e.g., to produce white backlight illumination that includes, red, green, and blue components).  FIG. 49  is a cross-sectional side view of an illustrative color conversion layer. 
     As shown in  FIG. 49 , color conversion layer  34  includes phosphor layer  40  with red quantum dots  242  and green quantum dots  244 . The red quantum dots may convert blue light from the light-emitting diodes of the display into red light whereas the green quantum dots may convert blue light from the light-emitting diodes of the display into green light. A partially reflective layer  41  (sometimes referred to as a dichroic layer or dichroic filter layer) may optionally be included in the color conversion layer. The dichroic filter  41  may reflect all red and green light and partially reflect blue light, for example. An additional film such as film  246  may also be included in the color conversion layer. The additional film  246  (sometimes referred to as an optical film) may be formed from a polymer material (e.g., polyethylene terephthalate). 
     As blue light from the light-emitting diodes passes through phosphor layer, the blue light is converted to red and green light by quantum dots  242  and  244 . The longer the distance the blue light travels through phosphor layer  40 , the more likely it is that the blue light will be converted to red and green light. This may result in undesirable display performance. As shown in  FIG. 49 , on-axis light  248  (e.g., light that is parallel or close to parallel to the Z-axis) has an optical path with a length  252  in phosphor layer  40 . In contrast, off-axis light  250  (e.g., light at a relatively large angle relative to the Z-axis) has an optical path with a length  254  in phosphor layer  40 . As shown, the optical path  254  is longer than optical path  252 . Accordingly, the off-axis light will include more red and green light and less blue light (because more blue light is converted while in the phosphor layer for a longer time) than the on-axis light. This may lead to non-uniformity in the display. 
     There are numerous ways to prevent non-uniformity in the display due to a narrower angular profile of blue light than red and green light. In one illustrative embodiment, shown in  FIG. 50A , reflective structures may be introduced in the phosphor layer to reduce the length of the optical path of off-axis light. Structures  256  (sometimes referred to as microstructures or path-length-reducing structures) may not include any quantum dots  242  or  244 . Structures  256  may be formed from any desired reflective materials. Structures  256  may have a reflectance that is greater than 40%, may have a reflectance that is greater than 30%, may have a reflectance that is greater than 50%, or may have any other desired reflectance. Structures  256  reduce the optical pathlength of off-axis light through phosphor layer  40 , making the on-axis and off-axis light more uniform. The smallest dimension of each structure  256  may be less than 100 microns, less than 10 microns, less than 5 microns, less than 2 microns, less than 1 micron, less than 0.1 micron, greater than 0.1 micron, greater than 1 micron, etc. The thickness  249  of structures  256  may be less than the thickness of phosphor layer  40  (as in  FIG. 50A ) or may be equal to the thickness of phosphor layer  40 . A distance  247  may separate structures  256 . Distance  247  may be greater than 15 microns, greater than 10 microns, greater than 5 microns, greater than 25 microns, less than 50 microns, or any other desired distance. 
     In  FIG. 50A , reflective structures  256  are formed in phosphor layer  40 . In other words, the reflective structures  256  are embedded in and in direct contact with phosphor layer  40 . However, this example is merely illustrative. Reflective structures  256  may instead be immersed in a transparent coating layer and then laminated to the phosphor layer, as shown in  FIG. 50B . 
       FIG. 50B  shows a cross-sectional side view of color conversion layer  34  in which reflective structures  256  are formed in an additional layer  245 . Additional layer  245  is interposed between phosphor layer and film  246 . Additional layer  245  may include reflective structures  256  embedded in a transparent coating  251  (sometimes referred to as filler material). Alternatively, filler material  251  may be air. In embodiments where filler material  251  is air, an additional transparent coating  253  may be interposed between the reflective structures and the phosphor layer, as shown in  FIG. 50C . 
       FIG. 51A  is a top view of a phosphor layer with reflective structures such as the phosphor layer of  FIG. 50A . As shown in  FIG. 51A , structures  256  may have a hexagonal cross-sectional shape when viewed from above. This example is merely illustrative. Structures  256  may have any desired cross-sectional shape when viewed from above (e.g., circular, square, non-square rectangular, octagonal, etc.). Structures  256  may also optionally be cones, pyramids, or other shapes having a varying cross-section along the Z-axis. 
     In another illustrative embodiment, shown in  FIG. 51B , the reflective structures may be arranged in a honeycomb pattern. In other words, the reflective structures may be arranged in an interconnected web that defines hexagonal openings (filled by phosphor layer  40 , for example). This example is merely illustrative. The reflective structures may be arranged in an interconnected web that defines a plurality of triangular openings, a plurality of circular openings, a plurality of square openings, a plurality of octagonal openings, a plurality of non-square rectangular openings, etc. In one embodiment, the reflective structures (sometimes referred to as reflective walls) may have openings to allow the phosphor layer to flow between cavities. Optional openings  265  in the reflective walls are shown in  FIG. 51B . Each reflective wall portion may form one of six sides of the hexagonal shape around a respective cell of phosphor material. Each reflective wall portion may have one respective opening, each reflective wall portion may have two or more respective openings, or only some of the reflective wall portions may have openings. 
       FIG. 52  is a cross-sectional side view of an illustrative color conversion layer with Rayleigh scattering dopants for increasing the amount of off-axis blue light. As shown in the inset portion of  FIG. 52 , red quantum dots  242  output light in a random direction (e.g., the direction that red light is output is not correlated to the direction that blue light is received). Similarly, green quantum dots  244  output light in a random direction (e.g., the direction that green light is output is not correlated to the direction that blue light is received). To make the emission direction of blue light more random (and therefore equalize the off-axis emission of blue light to the off-axis emission of red and green light), Rayleigh scattering dopants  262  may be included in the phosphor layer. Rayleigh scattering dopants  262  may elastically scatter blue light. This means that no energy is lost when the Rayleigh scattering dopants  262  receive blue light and that the wavelength of the light is not changed by the Rayleigh scattering dopants. However, the Rayleigh scattering dopants randomize the direction of the blue light. The blue light will be scattered by the Rayleigh scattering dopants while the red and green light will tend not to be scattered by the Rayleigh scattering dopants. Consequently, the distribution of red, blue, and green light may be equalized both on-axis and off-axis. 
     The average diameter of the Rayleigh scattering dopants may be between 5 and 20 nanometers, less than 100 nanometers, less than 50 nanometers, less than 20 nanometers, more than 5 nanometers, more than 1 nanometer, or any other desired diameter. The average diameter of quantum dots  242  and  244  may be more than 1 micron, more than 2 microns, between 1 and 3 microns, less than 5 microns, or any other desired diameter. 
     Uniformity of the display may also be improved by patterning the optical film over the phosphor layer in the color conversion layer.  FIG. 53  is a cross-sectional side view of an illustrative color conversion layer  34  showing how film  246  may be patterned to include protrusions  272 . Film  246  may be laminated directly to phosphor layer  40  or formed integrally with phosphor layer  40 . The presence of protrusions  272  on the upper surface of film  246  may broaden the light that passes through color conversion layer  34 . The protrusions may spread the blue light from the light-emitting diodes (e.g., by recycling more on-axis blue light thus creating a broader profile for the blue light). Protrusions  272  may be pyramidal shaped protrusions, cone shaped protrusions, hemispherical shaped protrusions, or protrusions of any other desired shape. Film  246  may sometimes be referred to as a prism film and protrusions  272  may sometimes be referred to as prisms. 
       FIG. 54  is a cross-sectional side view of an illustrative color conversion layer  34  showing another embodiment for improving uniformity of the display. In  FIG. 54 , the upper surface of phosphor layer  40  is patterned to have protrusions  274 . The presence of protrusions  274  may result in on-axis light and off-axis light having a similar path length within phosphor layer  40  (similar to as shown and discussed in connection with  FIGS. 49 and 50 ). Protrusions  274  may be pyramidal shaped protrusions, cone shaped protrusions, hemispherical shaped protrusions, or protrusions of any other desired shape. The lower surface of film  246  may conform to the upper surface of phosphor layer  40  (e.g., film  246  has recesses that receive the protrusions  274 ). 
     If desired, the concepts from  FIGS. 53 and 54  may be combined into a single color conversion layer.  FIG. 55  shows a cross-sectional side view of a color conversion layer of this type. As shown, film  246  may be patterned to include protrusions  272 . Film  246  may be laminated directly to phosphor layer  40  or formed integrally with phosphor layer  40 . The presence of protrusions  272  on the upper surface of film  246  may broaden the light that passes through color conversion layer  34  (e.g., the protrusions may spread the blue light from the light-emitting diodes). Protrusions  272  may be pyramidal shaped protrusions, cone shaped protrusions, hemispherical shaped protrusions, or protrusions of any other desired shape. The upper surface of phosphor layer  40  may also be patterned to have protrusions such as protrusions  274  in  FIG. 55 . The presence of protrusions  274  may result in on-axis light and off-axis light having a similar path length within phosphor layer  40 . Protrusions  274  may be pyramidal shaped protrusions, cone shaped protrusions, hemispherical shaped protrusions, or protrusions of any other desired shape. 
       FIG. 56  is a diagram showing an illustrative method for attaching light-emitting diodes to a printed circuit board. One option for attaching light-emitting diodes to a printed circuit board is to deposit solder balls on the printed circuit board. Then, the light-emitting diode may be placed on the solder balls and the solder may be reflowed to attach the light-emitting diode to the printed circuit board. This method requires two placement steps (e.g., one for depositing the solder and one for placing the light-emitting diode). This may undesirably increase the possible manufacturing error and increase the time it takes to attach the light-emitting diodes to the printed circuit board. 
       FIG. 56  shows an alternate method for attaching light-emitting diodes to the printed circuit board in which the light-emitting diodes are pre-soldered before being placed on the printed circuit board. As shown in  FIG. 56 , at step  302  an adhesive layer  282  may be deposited over the printed circuit board. Adhesive layer  282  may be any desired type of adhesive. Next at step  304 , a pre-soldered light-emitting diode  38  that includes solder  284  on solder pad  286  may be attached to printed circuit board  50 . The light-emitting diode may be biased in direction  288  towards adhesive  282  such that adhesive  282  secures the light-emitting diode to the printed circuit board. After securing the light-emitting diode to the printed circuit board using adhesive  282 , reflow may be performed at step  306 . Adhesive  282  may evaporate during the reflow process (e.g., adhesive  282  may have a boiling point lower than the melting point of solder  284 ). Adhesive  282  may have a melting point that is lower than the melting point of solder  284 . During reflow, solder  284  may form an electrical contact with printed circuit board  50 . The light-emitting diode  38  is therefore attached to printed circuit board  50  using solder  284 . Attaching light-emitting diodes using the method of  FIG. 56  (with pre-soldered light-emitting diodes) may result in better light-emitting diode alignment and may be faster than attaching light-emitting diodes by separately depositing solder. 
       FIGS. 57-74  show cross-sectional side views of backlight units incorporating the aforementioned features. As shown in  FIG. 57 , backlight unit  42  may include a plurality of optical films formed over light-emitting diode array  36 . Light-emitting diode array  36  may contain a two-dimensional array of light sources such as light-emitting diodes  38  that produce backlight illumination. Light-emitting diodes  38  may, as an example, be arranged in rows and columns and may lie in the X-Y plane of  FIG. 57 . Light-emitting diodes  38  may be mounted on printed circuit board  50  (sometimes referred to as substrate  50 ). A reflective layer  96  may be formed on an upper surface of printed circuit board  50  to increase efficiency of the backlight unit (similar to as shown in  FIG. 22 ). The reflective layer may be formed from any desired material. In one possible arrangement, the reflective layer may also serve as a solder mask layer during attachment of the light-emitting diodes to the substrate (as with reflective layer  72  in  FIG. 11 ). 
     Light-emitting diodes  38  may be controlled in unison by control circuitry in device  10  or may be individually controlled (e.g., to implement a local dimming scheme that helps improve the dynamic range of images displayed on pixel array  24 ). The light produced by each light-emitting diode  38  may travel upwardly along dimension Z through the optical films before passing through a pixel array. 
     Optical films within the backlight unit may include films such as light spreading layer  28 , partially reflective layer  30 , color conversion layer  34  (which may include phosphor layer  40  and partially reflective layer  41 ), collimating layer  44  (sometimes referred to as microlens array  44 ), brightness enhancement films  46 - 1  and  46 - 2 , diffuser layer  48 , and/or other optical films. 
     Light-emitting diodes  38  may emit light of any suitable color (e.g., blue, red, green, white, etc.). With one illustrative configuration described herein, light-emitting diodes  38  emit blue light. To help provide uniform backlight across backlight unit  42 , light from light-emitting diodes  38  may be spread by light spreading layer  28 . As shown in  FIG. 57 , light spreading layer  28  includes first light spreading features  402  on the upper surface of light spreading layer  28  and light spreading features  404  on the lower surface of light spreading layer  28 . In the example of  FIG. 57 , light spreading features  402  (sometimes referred to as light spreading structures  402 , prisms  402 , lenses  402 , etc.) are convex microlenses and light spreading features  404  (sometimes referred to as light spreading structures  404 , prisms  404 , lenses  404 , etc.) are concave microlenses. This example is merely illustrative and both sets of light spreading features may have any desired shape and may be convex features (e.g., protrusions) or concave features (e.g., recesses). 
     After passing through light spreading layer  28 , light from light-emitting diodes  38  may pass through partially reflective layer  30 . Partially reflective layer  30  (sometimes referred to as dichroic layer  30  or dichroic filter layer  30 ) may be configured to reflect some light from the LEDs and transmit some light from the LEDs. As shown in  FIG. 57 , partially reflective layer  30  may include a multi-Bragg reflector  406  and a diffuser layer  408  that are laminated together. Light that is reflected off multi-Bragg reflector  406  may be recycled (e.g., the reflected light will reflect off of other layers such as substrate  50  before reaching multi-Bragg reflector  406  again). Light that is transmitted through multi-Bragg reflector  406  then passes through color conversion layer  34  (which may sometimes be referred to as a photoluminescent layer). 
     The transmission of multi-Bragg reflector  406  may be selected to maximize the efficiency of display  14 . Lowering the transmission of blue light (e.g., from the light-emitting diodes) through the partially reflective layer increases the amount of blue light that is recycled. However, recycling more light may cause more light to be absorbed by printed circuit board  50  (or other layers below multi-Bragg reflector  406 ). Increasing the transmission of blue light may cause more visible artifacts. Therefore, the transmission of the partially reflective layer may be selected to optimize efficiency and uniformity of the display. The reflectance of printed circuit board  50  may influence the optimum transmission level of partially reflective layer  30 . In one illustrative embodiment, printed circuit board  50  may have a reflectance of about 90% and multi-Bragg reflector  406  may reflect 50% of blue light from light-emitting diodes  38 . Increasing the reflectance of printed circuit board  50  increases the optimum reflectance of partially reflective layer  30 . 
     Color conversion layer  34  may convert the light from LEDs  38  from a first color to a different color. For example, when the LEDs emit blue light, color conversion layer  34  may include a phosphor layer  40  (e.g., a layer of white phosphor material or other photoluminescent material) that converts blue light into white light. If desired, other photoluminescent materials may be used to convert blue light to light of different colors (e.g., red light, green light, white light, etc.). For example, one layer  34  may have a phosphor layer  40  that includes quantum dots that convert blue light into red and green light (e.g., to produce white backlight illumination that includes, red, green, and blue components, etc.). Configurations in which light-emitting diodes  38  emit white light (e.g., so that layer  34  may be omitted, if desired) may also be used. In addition to phosphor layer  40 , color conversion layer  34  may include a partially reflective layer  41 . Partially reflective layer  41  (sometimes referred to as a dichroic layer or dichroic filter layer) may reflect all red and green light and partially reflect blue light, for example. Phosphor layer  40  and partially reflective layer  41  may be laminated together to form a single integral color conversion layer  34 . 
     Collimating layer  44  (sometimes referred to as microlens layer  44  or microlens array diffuser  44 ) may collimate off-axis light. As shown in  FIG. 57 , collimating layer  44  may include an array of microlenses  162  on the lower surface of the film. Backlight unit  42  also includes brightness enhancement films such as brightness enhancement film  46 - 1  and brightness enhancement film  46 - 2  to further collimate the light. As shown, brightness enhancement film  46 - 1  may include protrusions  160  on an upper surface of the film. An additional optical layer  410  may be interposed between brightness enhancement film  46 - 1  and collimating layer  44 . The additional optical layer  410  may be formed from a transparent material that has a low index-of-refraction. Optical layer  410  may therefore sometimes be referred to as low-index layer  410 . As shown in  FIG. 57 , brightness enhancement film  46 - 1 , low-index layer  410 , and collimating layer  44  may be laminated together to form a combined optical layer  412 . 
     An additional brightness enhancement film  46 - 2  may be included over brightness enhancement film  46 - 1  to further help collimate the backlight and thereby increase the brightness of the display. Brightness enhancement films  46 - 1  and  46 - 2  may optionally have protrusions that extend along longitudinal axes that are rotated 90° relative to each other if desired. Finally, backlight unit  42  may include diffuser layer  48  to homogenize light from the array of light-emitting diodes. 
     The example of  FIG. 57  of light spreading layer  28  including convex light spreading features on an upper surface and concave light spreading features on a lower surface is merely illustrative. In general, both sets of light spreading features may have any desired shape and may be convex or concave.  FIG. 58  is a cross-sectional side view of an illustrative backlight unit with concave light spreading features  402  on an upper surface and concave light spreading features  404  on a lower surface. 
     Also in  FIG. 58 , collimating layer  44  includes microlenses  163  on the upper surface of the film in addition to microlenses  162  on the lower surface of the film. Microlenses  162  and  163  (sometimes referred to as recesses  162  and  163  or surface features  162  and  163 ) may have any desired shape. Collimating layer  44  and brightness enhancement film  46 - 1  are formed as separate films in  FIG. 58  (instead of being laminated together with a low-index layer as in  FIG. 57 ). In general, in all of the embodiments herein collimating layer  44  and brightness enhancement film  46 - 1  may be formed separately or may be laminated together. 
     Additional layers within the backlight unit may also be laminated together if desired.  FIG. 59  is a cross-sectional side view of a backlight unit similar to the backlight unit of  FIG. 57 . However, in  FIG. 59  the phosphor layer  40 , partially reflective layer  41 , multi-Bragg reflector  406 , and diffuser layer  408  may be laminated together to form an integral conversion film  414 . 
       FIG. 60  is a cross-sectional side view of an illustrative backlight unit with first and second light spreading layers. As shown, light spreading layer  28 - 1  is positioned above light spreading layer  28 - 2 . Light spreading layer  28 - 1  and light spreading layer  28 - 2  may be attached using adhesive. Alternatively, light spreading layers  28 - 1  and  28 - 2  may be laminated together to form an integral film. As shown in  FIG. 60 , light spreading layer  28 - 2  has an upper surface with microlenses  132 . Microlenses  132  may be formed from a plurality of recesses in the upper surface of light spreading layer  28 - 2 . Light spreading layer  28 - 2  may also include a plurality of protrusions  130 . Protrusions  130  may protrude towards the light-emitting diodes. Protrusions  130  may be elongated protrusions (sometimes referred to as ridges) that extend along a longitudinal axis across the layer (e.g., parallel to the Y-axis in  FIG. 60 ). Light spreading layer  28 - 1  may be the same as light spreading layer  28 - 2 , except for being rotated 90° relative to light spreading layer  28 - 2 . Light spreading layer  28 - 1  also includes microlenses  126 . Microlenses  126  may be formed from a plurality of recesses in the upper surface of light spreading layer  28 - 1 . Microlenses  126  and  132  may optionally be protrusions instead of recesses if desired. 
     Light emitted from light-emitting diodes  38  will be spread relative to a first axis (e.g., from a point source to two points) upon reaching light-spreading layer  28 - 2 . Microlenses  132  may reduce total internal reflection to promote light passing to light spreading layer  28 - 1 . Light spreading layer  28 - 1  will spread the incoming light relative to a second axis (e.g., from the two points to four points) that is perpendicular to the first axis (because the protrusions of layer  28 - 1  are perpendicular to the protrusions of layer  28 - 2 ). Microlenses  126  may reduce total internal reflection to promote leakage of light out of layer  28 - 1  towards layer  30 . Layers  28 - 1  and  28 - 2  may each have any desired thickness (e.g., between 20 and 30 microns, between 20 and 25 microns, less than 50 microns, less than 25 microns, about 22 microns, greater than 15 microns, less than 100 microns, etc.). The thickness of layers  28 - 1  and  28 - 2  may be the same. 
       FIG. 61  is a cross-sectional side view of a backlight having protrusions  272  formed over phosphor layer  40  in color conversion layer  34 . The presence of protrusions  272  may broaden the light that passes through color conversion layer  34  (e.g., the protrusions may spread the blue light from the light-emitting diodes). Protrusions  272  may be pyramidal shaped protrusions, cone shaped protrusions, hemispherical shaped protrusions, or protrusions of any other desired shape. Protrusions  272  may sometimes be referred to as prisms and may be formed in a film (prism film). Instead of protrusions, an optical film laminated to phosphor layer  40  may include recesses having any desired shape. Due to the presence of protrusions  272 , collimating layer  44  may optionally be omitted from the backlight as shown in  FIG. 61 . Collimating layer  44  may optionally be omitted from any of the embodiments herein. 
     Any of the color conversion layers shown in  FIGS. 53-55  may be used in the backlight units described herein. The example of  FIG. 61  of the color conversion layer having protrusions  272  (similar to  FIG. 53 ) is merely illustrative. The color conversion layer may instead have a patterned phosphor layer (as in  FIG. 54 ) or may have both a patterned phosphor layer and patterned film over the phosphor layer (as in  FIG. 55 ). The features of the patterned phosphor layer and patterned film over the phosphor layer may have any desired shapes. 
     The example in  FIG. 61  of light spreading layer  28  including two layers ( 28 - 1  and  28 - 2 ) having elongated protrusions that extend along a longitudinal axis is merely illustrative. If desired, light spreading layer  28  may instead include an array of protrusions or recesses, as shown in  FIG. 62 . As shown in  FIG. 62 , light spreading layer  28  includes light spreading features  422  on an upper surface and light spreading features  424  on a lower surface. Light spreading features  422  and  424  may have any desired shape. For example, the light spreading features (sometimes referred to as light spreading structures  422  or light spreading structures  424 ) may be protrusions having a pyramidal shape (e.g., with a square base and four triangular faces that meet at a vertex) or a triangular pyramidal shape (e.g., with a triangular base and three triangular faces that meet at a vertex). The pyramidal protrusions may split a point light source into four points, whereas the triangular pyramidal protrusions may split a point light source into three points. In another embodiment, the light spreading structures may be recesses having a pyramidal shape or a triangular pyramidal shape. Any of the light spreading structures of  FIGS. 33-35  may be included in the light spreading layer. The shapes of the structures on the upper surface of layer  28  may be the same or may be different than the shapes of the structures on the lower surface of layer  28 . Layer  28  may be formed from a single optical film with light spreading structures patterned on the upper and lower surface. Layer  28  may also be formed from first and second optical films that are laminated together and that have respective light spreading structures. 
       FIG. 62  also shows an example of an encapsulant arrangement for the light-emitting diodes  38  on printed circuit board  50 . As shown in inset portion  426 , light-emitting diode  38  is mounted to printed circuit board  50 . Reflective layer  96  is formed on printed circuit board  50  around the light-emitting diode. Each light-emitting diode  38  may include a reflector layer  70  (e.g., a distributed Bragg-reflector) formed over the light-emitting diode. The light-emitting diode is also covered by encapsulant such as dome-shaped encapsulant  52 . Reflector layer  70  and reflective layer  96  may help spread the light from light-emitting diode  38  to avoid hotspots. 
     The arrangement for the encapsulant and light-emitting diode in  FIG. 62  is merely illustrative.  FIG. 63  is cross-sectional side view of another backlight with a different possible LED arrangement depicted in inset portion  426 . As shown in  FIG. 63 , structures  428  (sometimes referred to as support structures, reflective structures, or light leakage promotion structures) may be formed on substrate  50 . Structures  428  may be transparent structures configured to provide mechanical support for the backlight unit (similar to structures  232  in  FIG. 47 ). Structures  428  may alternatively be reflective structures configured to redirect light from light-emitting diode  38  towards the user (as shown in  FIG. 63 ). 
     In  FIG. 63 , structures  428  are depicted as having a triangular cross-sectional shape. However, this example is merely illustrative and structures  428  may have any desired shape. Structures  428  may have a trapezoidal cross-sectional shape or a cross-sectional shape with a curved upper surface.  FIG. 64  shows an example where structures  428  have a curved upper surface. 
       FIG. 65  shows yet another possible arrangement for the light-emitting diodes on printed circuit board  50  in inset portion  426 . As shown in  FIG. 65 , encapsulant  52  may be formed as a slab between structures  428  (as opposed to a dome of encapsulant as in  FIG. 64 ). The encapsulant may have a planar upper surface or an upper surface with a slight convex curve. 
     Inset portion  426  of  FIG. 66  shows another arrangement for light-emitting diodes  38  and encapsulant  52 . In  FIG. 66 , structures  428  (from  FIG. 65 ) are omitted and encapsulant  52  is formed as a slab over the light-emitting diodes. The encapsulant may have a planar upper surface and may have a thickness that is tuned for optimal spreading of light from the light-emitting diodes. 
     In another embodiment, shown in inset portion  426  of  FIG. 67 , additional reflective material  430  may be incorporated on the slab of encapsulant  52 . Additional reflective material  430  may be translucent and may cause specular reflection (in which the angle of incidence of a ray of light equals the angle of output of the reflected ray of light) ad/or diffusive reflection (in which an incident ray of light may be reflected in any direction). Reflective material  430  may be patterned to have portions formed directly over corresponding light-emitting diodes. The reflective material may be printed directly on encapsulant  52  or may be formed on a separate film that is aligned over the light-emitting diodes. Reflective material  430  may reflect at least 10% of light, at least 25% of light, at least 50% of light, at least 75% of light, less than 90% of light, or any other desired amount of light. 
     Additional reflective material  430  may be included in embodiments where additional structures  428  are present.  FIG. 68  shows an embodiment of this type. As shown, encapsulant  52  is formed between reflective structures  428  and reflective material  430  is formed on the encapsulant. Reflective material  430  may reflect at least 10% of light, at least 25% of light, at least 50% of light, at least 75% of light, less than 90% of light, or any other desired amount of light. 
     In  FIGS. 62-68 , reflective layer  96  is depicted on the upper surface of printed circuit board  50 . Reflective layer  96  may cause diffusive reflection, meaning an incident ray of light may be reflected in any direction. This helps direct light towards the viewer in areas between the light-emitting diodes. However, in some embodiments, additional reflective materials may be included that cause specular reflection.  FIG. 69  is a cross-sectional side view of a backlight unit that includes reflective material  446  formed in a ring around light-emitting diode  70 . Reflective material  446  may cause specular reflection, meaning an incident ray of light will be reflected at the same angle it strikes the reflective material. Reflective material  446  (sometimes referred to as specular reflective material  446 ) may help spread light further away from light-emitting diode  38  before the light is directed towards the viewer by reflective material  96  (sometimes referred to as diffusive reflective material  96 ). 
       FIGS. 62-69  show various arrangements for light-emitting diode  38  and encapsulant  52  on substrate  50 . It should be understood that for all of the embodiments herein (e.g. in  FIGS. 57-61  and  FIGS. 70-73 ), any of the arrangements of  FIGS. 62-69  may optionally be used or no encapsulant may be included. 
       FIGS. 70 and 71  are cross-sectional side views of an illustrative backlight unit showing how the design of light-emitting diodes  38 , encapsulant  52 , and structures  428  may allow partially reflective layer  30  to be omitted from the backlight unit. Omitting the partially reflective layer  30  may allow for the thickness of the backlight unit to be reduced.  FIG. 70  shows an example where light spreading layer  28  includes light spreading features on the upper and lower surface and the partially reflective layer is omitted.  FIG. 71  shows an example where light spreading layer  28  includes layers  28 - 1  and  28 - 2  rotated 90° relative to each other (as in  FIG. 60 ) and the partially reflective layer is omitted. Layers  28 - 1  and  28 - 2  in  FIG. 71  are laminated together. 
     It should be noted that, in any of the embodiments described herein, partially reflective layer  41  may optionally be omitted. The light spreading provided by the patterned film on phosphor layer  40  with protrusions  272  may provide sufficient light spreading such that the reflective properties of partially reflective layer  41  are not required for satisfactory display performance. In this case, partially reflective layer  41  may be replaced by a transparent optical layer without partially reflective properties. The transparent optical layer may still provide mechanical support for phosphor layer  40 . 
       FIG. 72  is a cross-sectional side view of an illustrative backlight unit with a color conversion layer that includes reflective structures. As shown in  FIG. 72 , reflective structures  256  may be formed in phosphor layer. In other words, the color conversion layer of  FIG. 50A  may be used. Other color conversion layers with reflective structures may be used. For example, color conversion layers with reflective structures formed in a film over the phosphor layer (as in  FIG. 50B or 50C ) may also be used if desired. The reflective structures may have any desired arrangement when viewed from above.  FIGS. 51A and 51B  show illustrative arrangements for the reflective structures that may be used in  FIG. 72 . Structures  256  may optionally be cylinders, cones, pyramids, or other shapes. The reflective structures may be arranged in an interconnected web that defines a plurality of triangular openings, a plurality of circular openings, a plurality of square openings, a plurality of hexagonal openings, a plurality of octagonal openings, a plurality of non-square rectangular openings, etc. In one embodiment, the reflective structures (sometimes referred to as reflective walls) may have openings to allow the phosphor material to flow between cavities. 
       FIG. 73  is a cross-sectional side view of an illustrative backlight similar to the backlight of  FIG. 72 . However, in  FIG. 73  light-emitting diodes  38  have been dithered, as shown by inset portion  442 . The light-emitting diodes may be dithered similar to as shown in connection with  FIGS. 43-45 . Any of the embodiments herein may optionally include dithered light-emitting diodes of the type shown in  FIG. 73 . 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20190306
Publication Date: 20220111
Grant Date: 20220111
Priority Date: 20180313
Inventors: LIU, RONG
Xu, Daming
QI, JUN
HAN, LING
SON, Mookyung
YOU, CHENHUA
YIN, VICTOR H.
ZHU, XINYU
SUH, HEESANG
HE, Juan
SPECHLER, JOSHUA A.
LUO, Zhenyue
GORKHALI, SURAJ P.
Assignee: APPLE INC
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