Light Diffusers for Backlit Displays

An array of pixels in a display may be illuminated by a backlight having an array of light-emitting diodes in an array of respective light-emitting diode cells. A cavity reflector in each cell may help distribute blue light emitted from the light-emitting diode of that cell toward edges of the cell. Optical films in the backlight may include a photoluminescent layer such as a phosphor layer that converts blue light from the light-emitting diode array into white light and may include a dichroic filter for reflecting white light away from the diode array towards the pixel array. A light diffuser layer for the backlight may have printed white ink pads, recesses filled with a high index of refraction material, protrusions for light scattering, light-scattering particles, thin-film interference filters, partially reflective mirrors, and other structures for diffusing the light from the light-emitting diodes.

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.

Direct-lit backlights may have locally dimmable light-emitting diodes that allow dynamic range to be enhanced. If care is not taken, however, a direct-lit backlight may be bulky or may produce non-uniform backlight illumination.

It would therefore be desirable to be able to provide improved backlighting arrangements for electronic device displays.

SUMMARY

A display may be provided with an array of pixels for displaying images for a viewer. The array of pixels may be provided with backlight illumination from a direct-lit backlight. The backlight may have an array of light-emitting diodes in an array of respective light-emitting diode cells. Each light-emitting diode cell may have a center at which a light-emitting diode is located and edges. A cavity reflector in each cell may help distribute blue light emitted from a light-emitting diode at the center of that cell toward the edges of the cell and outwards through the array of pixels.

The backlight may include optical films. The optical films may include a photoluminescent layer such as a white phosphor layer that converts blue light from the light-emitting diode array into white light and may include a dichroic filter for reflecting white light away from the diode array towards the pixel array.

A light diffuser layer for the backlight may have printed white ink pads, recesses filled with a material having an elevated index of refraction, protrusions for light scattering, light-scattering particles, thin-film interference filters, partially reflective mirrors, and other structures for diffusing and recycling the light from the light-emitting diodes. The light diffuser may preferentially reflect on-axis light and light emitted from the light-emitting diode towards portions of the light diffuser at the center of each cell to help reduce hotspots in the diffused light.

DETAILED DESCRIPTION

Electronic devices may be provided with backlit displays. The backlit displays may include liquid crystal display modules or other display structures that are backlit by light from a direct-lit backlight. A perspective view of an illustrative electronic device of the type that may be provided with a display having a direct-lit backlight is shown inFIG. 1. Electronic device10ofFIG. 1may 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'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 inFIG. 1, device10may have a display such as display14. Display14may be mounted in housing12. Housing12, 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. Housing12may be formed using a unibody configuration in which some or all of housing12is 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.).

Housing12may have a stand such as optional stand18, 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 (e.g., in arrangements in which stand18is omitted), and/or may have other suitable configurations. The arrangement for housing12that is shown inFIG. 1is illustrative.

Display14may 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.

Display14may include an array of pixels16formed from liquid crystal display (LCD) components or may have an array of pixels based on other display technologies. A cross-sectional side view of display14is shown inFIG. 2.

As shown inFIG. 2, display14may include a pixel array such as pixel array24. Pixel array24may include an array of pixels such as pixels16ofFIG. 1(e.g., an array of pixels having rows and columns of pixels26). Pixel array24may 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 array24may, 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 array24, if desired.

During operation of14, images may be displayed on pixel array24. Backlight42(which may sometimes be referred to as a backlight, backlight layers, backlight structures, a backlight module, a backlight unit, etc.) may be used in producing backlight illumination44that passes through pixel array24. This illuminates any images on pixel array24for viewing by a viewer such as viewer20who is viewing display14in direction22.

Backlight unit42may have optical films26, a light diffuser such as light diffuser (light diffuser layer)34, and light-emitting diode array36. Light-emitting diode array36may contain a two-dimensional array of light-emitting diodes38that produce backlight illumination44. Light-emitting diodes38may, as an example, be arranged in rows and columns and may lie in the X-Y plane ofFIG. 2.

Light-emitting diodes38may be controlled in unison by control circuitry in device10or may be individually controlled (e.g., to implement a local dimming scheme that helps improve the dynamic range of images displayed on pixel array24). The light produced by each light-emitting diode38may travel upwardly along dimension Z through light diffuser34and optical films26before passing through pixel array24. Light diffuser34may contain light-scattering structures that diffuse the light from light-emitting diode array36and thereby help provide uniform backlight illumination44. Optical films26may include films such as dichroic filter32, phosphor layer30, and films28. Films28may include brightness enhancement films that help to collimate light44and thereby enhance the brightness of display14for user20and/or other optical films (e.g., compensation films, etc.).

Light-emitting diodes38may emit light of any suitable color. With one illustrative configuration, light-emitting diodes38emit blue light. Dichroic filter layer32may be configured to pass blue light from light-emitting diodes38while reflecting light at other colors. Blue light from light-emitting diodes38may be converted into white light by a photoluminescent material such as phosphor layer30(e.g., a layer of white phosphor material or other photoluminescent material that converts blue light into white light). White light that is emitted from layer30in the downwards (−Z) direction may be reflected back up through pixel array24as backlight illumination by dichroic filter layer32(i.e., layer32may help reflect backlight outwardly away from array36). By placing the photoluminescent material of backlight42(e.g., the material of layer30) above diffuser layer34, light-emitting diodes38may be configured to emit more light towards the edges of the light-emitting diode cells (tiles) of array36than at the centers of these cells, thereby helping enhance backlight illumination uniformity.

FIG. 3is a top view of an illustrative light-emitting diode array for backlight42. As shown inFIG. 3, light-emitting diode array36may contain row and columns of light-emitting diodes38. Each light-emitting diode38may be associated with a respective cell (tile area)38C. The length D of the edges of cells38C 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, or other suitable size. If desired, hexagonally tiled arrays and arrays with light-emitting diodes38that are organized in other suitable array patterns may be used. The configuration ofFIG. 3in which light-emitting diode array36has rows and columns of rectangular (e.g., square) light-emitting diode regions such as cells38C is merely illustrative.

FIG. 4is a cross-sectional side view of illustrative light-emitting diode. Light-emitting diodes such as light-emitting diode38ofFIG. 4may have terminals such as contacts58. Contacts58may be soldered to a printed circuit or other substrate (e.g., so that light-emitting diodes38may be mounted in an array such as array36ofFIG. 3). Light-emitting diode38may have n-type region54and p-type region56. Regions54and56may be formed on substrate52from a crystalline semiconductor material such as gallium nitride. Substrate52may be formed from a transparent crystalline material such as sapphire or other suitable substrate material. Reflector layer50(e.g., a distributed Bragg reflector) may be formed on substrate52to help direct emitted light from diode38sideways.

FIG. 5is a cross-sectional side view of an illustrative light-emitting diode cell. As shown inFIG. 5, each light-emitting diode cell (tile)38C in light-emitting diode array36may have a reflector such as cavity reflector68. Reflector68may have a square outline (i.e., a square footprint when viewed from above) or may have other suitable shapes and may be formed from sheet metal (e.g., stamped sheet metal), metallized polymer film, a thin-film metal on a plastic carrier, a dielectric thin-film stack that forms a dielectric mirror on a polymer film or molded plastic carrier, or other suitable reflector structure. Light-emitting diode38may be soldered or otherwise mounted to metal traces in printed circuit60. An opening in the center of reflector68may receive light-emitting diode38. A transparent structure such as transparent dome structure70may be formed over light-emitting diode38. Dome structure70may be formed from a bead of clear silicone or other transparent polymer (as an example). During operation, light-emitting diode38emits light that is refracted outwardly (away from the Z axis) by dome structure70as shown by emitted light ray62and refracted light ray64. Rays of light such as ray64may then be reflected upwardly (in the Z direction) off of the surface of the curved walls of cavity reflector68(see, e.g., illustrative reflected light ray66), thereby producing backlight illumination44.

As shown inFIG. 6, emitted light rays from light-emitting diode38such as ray72, may be characterized by an angle A with respect to surface normal n of light-emitting diode38. Light72that is traveling parallel to the Z dimension is parallel to surface normal n (angle A=0°). Light72that is traveling parallel to the X-Y plane is traveling perpendicular to the Z dimension and surface normal n (i.e., A=90°). Light72that is traveling at other orientations relative to surface normal n is characterized by an intermediate value of angle A.

If light-emitting diode38were to include a white phosphor, light-emitting diode38might emit light with a Lambertian intensity profile as illustrated by Lambertian curve76ofFIG. 7. This type of light intensity profile is characterized with a concentration of light about dimension Z (A=0°) and has a tendency to generate hotspots (areas of enhanced light output intensity) aligned with the centers of the light-emitting diode cells. This could lead to dark regions at the borders between adjacent cells.

Ideally, emitted light from light-emitting diodes would have an angular profile such as profile74ofFIG. 7(e.g., a profile that varies as a function of cos(A)−3). In practice, the intensity of light emitted from diodes38differs somewhat from the ideal angular profile of curve74. As a result, there is still a risk that hotspots will develop directly above diodes38(i.e., in the centers of cells38C). This could create undesirable visible artifacts in backlight illumination44(e.g., dark borders between cells38C, etc.).

To help ensure that backlight44is uniform, light diffuser34and/or other structures in backlight42may be provided with patterned ink, patterns of reflecting protrusions, angularly-dependent thin-film interference filters, and/or other light reflecting and light scattering structures that help reflect on-axis emitted light at the center of cells38C back towards diodes38while allowing light (e.g., obliquely angled light) at the edges of cells38C to be passed upwardly towards films26. This helps reduce hotspots in the middle of cells38C and smooths out light intensity variations that might otherwise arise as light from array36is diffused by light diffuser34.

Consider, as an example, the scenario ofFIG. 8. As shown inFIG. 8, light diffuser34may be formed from a layer of transparent material such as such as transparent layer92. Transparent layer92may be formed from a material such as glass, polymer (e.g., polystyrene, polycarbonate, acrylic, etc.), ceramic, or other suitable material, Transparent layer92may have a planar shape with a thickness of 0.05 to 2 mm, more than 0.1 mm, more than 0.2 mm, more than 0.5 mm, less than 1.5 mm, less than 1 mm, less than 0.5 mm, or other suitable thickness. Transparent layer92in light diffuser34may have opposing upper and lower surfaces such as upper surface88(facing pixel array24) and opposing lower surface90(facing array36).

To help homogenize backlight illumination44being emitted by backlight42, backlight42may be provided with light homogenizing structures such as structures78. In the example ofFIG. 8, structures78are formed from a patterned coating of white ink or other reflective material on lower surface90of light diffuser34. If desired, light homogenizing structures for backlight42may be embedded within material92, may be formed on upper surface88, may be formed from one or more layers of material that are separate from light diffuser34, and/or may be formed from other suitable structures in backlight42.

Structures78may be formed from opaque light reflecting material (e.g., metal or a dielectric stack that forms a dielectric mirror), and/or may be formed from material that is at least somewhat transparent (e.g., translucent material such as white ink). Translucent materials such as white ink may be formed from polymer that includes light-scattering particles (e.g., particles of titanium dioxide, etc.) and may help diffuse and scatter light as well as reflecting light in the center of cells38C back towards diodes38. Structures78may be patterned by depositing blanket coating layer(s) and patterning the coating using photolithography, by depositing white ink or other material using inkjet printing, screen printing, pad printing, spraying through a shadow mask, or, by using other suitable patterning techniques. In sonic configurations, light homogenizing structures for backlight42may be formed by creating recesses and/or protrusions in diffuser34(e.g., pyramidal or conical recesses filled with polymer of a different index of refraction, etc.).

With one illustrative configuration, structures78ofFIG. 8may be formed by printing an array of pads (dots) of white ink (e.g., circular pads, square pads, dots of other shapes, etc.) on lower surface90. Each white ink pad may be aligned with a respective light-emitting diode38(as shown inFIG. 8) or there may be multiple pads for each diode38.

During operation, at least some of the light from light-emitting diode38that is emitted directly upwards in the center of cell38C (e.g., light80ofFIG. 8) will be reflected downwards, as shown by reflected light82. Reflected light82will be spread out laterally (e.g., by reflecting from cavity reflector68). Other light, such as light84that is emitted from light-emitting diode38sideways, may reflect off of cavity reflector68without reflecting off of structure78and will pass upwards through diffuser34to serve as backlight44. By recycling light near the center of each cell38C while allowing light near the edges of each cell38C to pass directly through diffuser34, the intensity of light near the edges of each cell38C may be increased relative to the intensity of light near the center of each cell38C. This helps ensure that backlight44will be uniform across the surface of light diffuser34and backlight42. If desired, light-scattering particles86(e.g., microbeads, hollow microspheres, bubbles, and/or other light-scattering particles) may be embedded within material92to further diffuse emitted light. Light-scattering particles86may have an index of refraction that differs from that of material92. For example, the refractive index of particles86may be larger than the refractive index of material92or may be lower than the refractive index of material92.

In the illustrative configuration ofFIG. 8, a single structure78(e.g., a single pad) has been provided above the light-emitting diode38in each cell38. If desired, a cluster of pads (circular pads, square pads, or pads of other shapes) may be formed above each light-emitting diode, as illustrated by the cluster of pads78D forming structure78inFIG. 9. If desired, the density of pads78D in each cluster (e.g., the number of pads per unit area and/or the area consumed by the pads per unit area) may be varied as a function of position. For example, each pad cluster may have more pads and/or larger pads near the center of that pad cluster than near the edges of that pad cluster. The use of graded structures such as pad clusters with graded pad densities (e.g., pads concentrated over diodes38) may help smoothly reduce hotspots in cells38C.

FIG. 10shows how recesses such as recess96may be formed on lower surface90. Recesses96may each be aligned with a respective light-emitting diode (e.g., each recess96may be aligned with the center of a respective cell38C) or multiple recesses96may overlap each cell38C. Recesses96may be filled with a filler material that has an index of refraction that is different than the index of refraction of material92of light diffuser34. For example, recesses96may be filled with a polymer, inorganic material, or other material that has a refractive index that is greater than the refractive index of material92to promote downwards light reflection of rays such as illustrative light ray98due to total internal reflection. Particles86may have a refractive index that is different than (e.g., lower than) the refractive index of material92to promote light scattering as light passes through diffuser34. Recesses96may have the shape of pyramids, grooves, cones, pits with curved sidewall profiles, and/or other suitable shapes. These shapes may help reflect on-axis light (e.g., light traveling directly upwards from diodes38at the centers of cells38C) while allowing off-axis light (e.g., light reaching the edge of cells38C at oblique angles) to pass. Recesses96may also be placed directly above light-emitting diodes38to help reduce hotspots in the center of cells38C.

As shown inFIG. 11, particles100may be included in the material that fills recesses96. Particles100may be, for example, silver microspheres that enhance reflectivity for light from light-emitting diodes38or may be other reflectivity enhancement particles. Using pyramidal or conical structures such as illustrative recesses96ofFIGS. 10 and 11, on-axis light of the type that is emitted directly upwards from light-emitting diodes38in the center of cells38C can be recycled whereas off-axis light may escape and pass through diffuser34, thereby helping to homogenize light44. The concentration of pyramidal or conical reflectors or other reflecting structures such as recesses96ofFIGS. 10 and 11may also, if desired, be larger in the portions of cell38C directly above light-emitting diodes38than at the edges of each cell38C. As shown inFIG. 12, for example, the concentration of recesses96may be varied gradually from a high density near the center of cell38C to a low density near the edge of cell38C. Recesses96may also be arranged so that a single recess96overlaps each diode38or so that recesses96are uniformly distributed across surface90of diffuser34(and so that there are equal densities of recesses96over the centers of cells38C and over the edges of cells38C).

FIG. 13shows how protrusions such as protrusions102may be formed on upper surface88of material92in light-diffuser34. Recesses96may be formed in lower surface90and may be filled with high index of refraction material, as described in connection withFIG. 10. The presence of protrusions102may help further diffuse light emitted from array36and thereby homogenize backlight illumination44. Protrusions102may be ridges, bumps, pyramidal protrusions, conical protrusions, and/or other light-scattering protrusions.

If desired, protrusions102may be formed on upper surface88of layer92in a light diffuser having printed white ink light-diffusing structures (pads) such as light homogenizing structures78of light diffuser34ofFIG. 14. Protrusions102, recesses96, and/or light homogenizing structures such as printed white ink structures78ofFIG. 14may be provided in uniform patterns (e.g., arrays of rows and columns or uniformly dispersed random patterns) across surfaces88and/or90or may be graded in density (e.g., so that enhanced concentrations of light homogenizing structures are located above respective diodes38), or a single light-homogenizing structure (a single ink pad, a single light-reflecting recess filled with high refractive index material, etc.) may be formed above each respective light-emitting diode38.

In the illustrative configuration ofFIG. 15, light diffuser34has a partially reflective film (e.g., a thin metal layer, a stack of dielectric thin-film layers, etc.) such as partially reflective film104. Film104may be formed on the lower surface of layer92, may be embedded in layer92(see, e.g., illustrative embedded film location104″), and/or may be separate from layer92(see, e.g., illustrative film location104′). Light that is reflected downwardly from film104may be reflected back in the upwards direction by cavity reflector68. The presence of film104thereby helps to enhance the number or reflections for each light ray and therefore enhances the homogenization of emitted light from light-emitting diodes array36before this light passes through all of layer92. If desired, additional diffusion may he provided by diffusive coating106. Coating106may be formed from a polymer layer on the upper surface of diffuser34with embedded light-scattering particles86.

FIG. 16shows how the density (concentration) of light-scattering particles86(e.g., the number of particles86per unit area of diffuser34) may be graded. For example, the density of light-scattering particles86may be greater in the center M of each cell38C than at the edges E of each cell38C. This variation in the density of light-scattering particles86as a function of lateral distance across diffuser34may help increase scattering near the center M and thereby reduce hotspots for light44. Particles86may be concentrated as shown inFIG. 16in light diffusers34with patterned white ink pads, reflective recesses, and/or other light homogenizing structures.

FIG. 17is a cross-sectional side view of light diffuser34in an illustrative configuration in which light diffuser34has been provided with an interference filter120formed from a stack of thin-film dielectric layers122. Layers122may be, for example, inorganic layers of differing refractive indices (e.g., alternating high and low index-of-refraction layers formed from materials such as aluminum oxide, silicon oxide, silicon nitride, titanium oxide, other metal oxides, nitrides, and/or oxynitrides, etc.). Filter120may be formed on the surface of layer92(e.g., on the lower surface) or may be embedded in material92(see, e.g., location120′).

Layers122may be configured so that filter120blocks shorter wavelength light and passes longer wavelength light. The transmission spectrum of the filter may vary as a function of angle of incidence. This causes the transmission of light at a given wavelength such as wavelength λb ofFIG. 18, which may be associated with the blue light emitted from diode38, to vary depending on the angle-of-incidence of that light with respect to filter120.

As shown inFIG. 18, filter120may exhibit transmission spectrum150when exposed to light from diode38at angle A1(e.g., close to 0° and parallel to surface normal n ofFIG. 6). Filter120may exhibit transmission spectrum152for light that is traveling at angles near angle A2(e.g., 45°). At larger angles (e.g., angles A3above 60°), filter120may be characterized by transmission spectrum154. Due to the variation in the transmission spectrum of filter120as a function of angle of incidence, blue light at λb will be reflected (e.g., transmission T will be close to 0%) when characterized by an angle-of-incidence of A1, will be partly reflected and partly transmitted (e.g., transmission T will be close to 50%) when characterized by an angle-of-incidence of A2that is greater than A1, and will be transmitted (e.g., transmission T will be close to 100%) when characterized by an angle-of-incidence of A3that is greater than A2. As the curves ofFIG. 18demonstrate, blue light emitted from light-emitting diodes38will be reflected and therefore recycled effectively when emitted directly upwards (parallel to surface normal n), but will be allowed to pass when characterized by more oblique angles. This may help reduce hotspots for emitted light in the centers of cells38C.