Display backlight with patterned backlight extraction ridges

A display may have a backlight unit with a row of light-emitting diodes that emit light into the edge of a light guide plate. The light guide plate may have opposing upper and lower surfaces. Backlight may be extracted from the light guide plate using an array of bumps on the lower surface and ridges on the upper surface. Ridge density may vary as a function of location across the display. Some of the ridges may be terminated along a meandering border between regions of differing ridge density. Ridge length and endpoint location can be dithered along borders between regions and ridge widths and thicknesses may be tapered down towards the endpoints. Ridges may be patterned to reduce the density of the ridges immediately adjacent the light-emitting diodes and thereby avoid over-extraction of the light at the light-emitting diodes.

BACKGROUND

This relates generally to electronic devices with displays, and, more particularly, to displays with backlights.

Electronic devices such as computers and cellular telephones have displays. Some displays such as plasma displays and organic light-emitting diode displays have arrays of pixels that generate light. In displays of this type, backlighting is not necessary because the pixels themselves produce light. Other displays contain passive pixels that can alter the amount of light that is transmitted through the display to display information for a user. Passive pixels do not produce light themselves, so it is often desirable to provide backlight for a display with passive pixels.

In a typical backlight assembly for a display, a light guide plate is used to distribute backlight generated by a light source such as a light-emitting diode light source. Optical films such as a diffuser layer and prism films may be placed on top of the light guide plate. A reflector may be formed under the light guide plate to improve backlight efficiency.

A strip of light-emitting diodes may provide light to an edge of a light guide plate. Light scattering features such as bumps may be provided on the light guide plate. Light from the light-emitting diodes that is traveling within the light guide plate may be scattered upwards by the bumps to form backlight for a display. Light guide plates may also sometimes be provided with elongated ridges (sometimes referred to as lenticular features) that help extract backlight from the light guide plate. The ridges are provided in a uniform rectangular region on the light guide plate.

Light from the strip of light-emitting diodes is initially concentrated in the vicinity of the outputs of the light-emitting diodes. The light must travel a sufficient distance into the light guide plate to mix enough to be used as backlight illumination. Backlight units that require large mixing distances may consume more volume within a display than desired. At the same time, reducing the mixing distance in a backlight too much may lead to undesired backlight hotspots.

It would therefore be desirable to be able to provide displays with improved backlights.

SUMMARY

A display may have an array of pixels for displaying images for a viewer. The array of pixels may be formed from display layers such as a color filter layer, a liquid crystal layer, a thin-film transistor layer, and polarizer layers.

A backlight unit may be used to produce backlight illumination for the display. The backlight illumination may pass through the polarizers, the thin-film transistor layer, the liquid crystal layer, and the color filter layer. The backlight unit may have a row of light-emitting diodes that emit light into the edge of a light guide plate.

The light guide plate may have opposing upper and lower surfaces. Backlight may be extracted from the light guide plate using an array of bumps on the lower surface and ridges on the upper surface. The density of the ridges may vary as a function of location across the display to avoid creating backlight hotspots.

If desired, different regions of the light guide plate may have different densities of ridges. Some of the ridges may be terminated along a meandering border between regions with different ridge densities. Ridge lengths and endpoint locations can be dithered about the border to help smooth out the transition in density between the different regions. Ridge widths and thicknesses may also be tapered down towards ridge endpoints to smooth out transitions in ridge density. Ridges may be patterned to locally reduce the density of ridges. For example, ridges may be patterned to reduce the density of ridges immediately adjacent to the light-emitting diodes and thereby avoid over-extraction of the light at the light-emitting diodes.

DETAILED DESCRIPTION

Input-output circuitry in device10such as input-output devices12may be used to allow data to be supplied to device10and to allow data to be provided from device10to external devices. Input-output devices12may include buttons, joysticks, scrolling wheels, touch pads, key pads, keyboards, microphones, speakers, tone generators, vibrators, cameras, sensors, light-emitting diodes and other status indicators, data ports, etc. A user can control the operation of device10by supplying commands through input-output devices12and may receive status information and other output from device10using the output resources of input-output devices12.

Input-output devices12may include one or more displays such as display14. Display14may be a touch screen display that includes a touch sensor for gathering touch input from a user or display14may be insensitive to touch. A touch sensor for display14may be based on an array of capacitive touch sensor electrodes, acoustic touch sensor structures, resistive touch components, force-based touch sensor structures, a light-based touch sensor, or other suitable touch sensor arrangements.

Control circuitry16may be used to run software on device10such as operating system code and applications. During operation of device10, the software running on control circuitry16may display images on display14.

Device10may be a tablet computer, laptop computer, a desktop computer, a television, a cellular telephone, a media player, a wristwatch device or other wearable electronic equipment, or other suitable electronic device.

Display14for device10includes an array of pixels. The array of pixels may be formed from liquid crystal display (LCD) components or other suitable display structures. Configurations based on liquid crystal display structures are sometimes described herein as an example.

A display cover layer may cover the surface of display14or a display layer such as a color filter layer, thin-film transistor layer, or other portion of a display may be used as the outermost (or nearly outermost) layer in display14. The outermost display layer may be formed from a transparent glass sheet, a clear plastic layer, or other transparent member.

A cross-sectional side view of an illustrative configuration for display14of device10is shown inFIG. 2. As shown inFIG. 2, display14may include a backlight unit such as backlight unit42(sometimes referred to as a backlight or backlight structures) for producing backlight44. During operation, backlight44travels outwards (vertically upwards in dimension Z in the orientation ofFIG. 2) and passes through pixel structures in display layers46. This illuminates any images that are being produced by the pixels for viewing by a user. For example, backlight44may illuminate images on display layers46that are being viewed by viewer48in direction50.

Display layers46may be mounted in chassis structures such as a plastic chassis structure and/or a metal chassis structure to form a display module for mounting in a housing in device10or display layers46may be mounted directly in an electronic device housing for device10(e.g., by stacking display layers46into a recessed portion in a metal or plastic housing). Display layers46may form a liquid crystal display or may be used in forming displays of other types.

In a configuration in which display layers46are used in forming a liquid crystal display, display layers46may include a liquid crystal layer such a liquid crystal layer52. Liquid crystal layer52may be sandwiched between display layers such as display layers58and56. Layers56and58may be interposed between lower polarizer layer60and upper polarizer layer54.

Layers58and56may be formed from transparent substrate layers such as clear layers of glass or plastic. Layers56and58may be layers such as a thin-film transistor layer and/or a color filter layer. Conductive traces, color filter elements, transistors, and other circuits and structures may be formed on the substrates of layers58and56(e.g., to form a thin-film transistor layer and/or a color filter layer). Touch sensor electrodes may also be incorporated into layers such as layers58and56and/or touch sensor electrodes may be formed on other substrates.

With one illustrative configuration, layer58may be a thin-film transistor layer that includes an array of pixel circuits based on thin-film transistors and associated electrodes (pixel electrodes) for applying electric fields to liquid crystal layer52and thereby displaying images on display14. Layer56may be a color filter layer that includes an array of color filter elements for providing display14with the ability to display color images. If desired, layer58may be a color filter layer and layer56may be a thin-film transistor layer. Configurations in which color filter elements are combined with thin-film transistor structures on a common substrate layer may also be used.

During operation of display14in device10, control circuitry (e.g., one or more integrated circuits on a printed circuit) may be used to generate information to be displayed on display14(e.g., display data). The information to be displayed may be conveyed to a display driver integrated circuit such as circuit62A or62B using a signal path such as a signal path formed from conductive metal traces in a rigid or flexible printed circuit such as printed circuit64(as an example). Integrated circuits such as integrated circuit62A and/or flexible printed circuits such as flexible printed circuit64may be attached to substrate58in ledge region66(as an example).

Backlight structures42may include a light guide plate such as light guide plate78. Light guide plate78may be formed from a transparent material such as clear glass or plastic. During operation of backlight structures42, a light source such as light source72may generate light74. Light source72may be, for example, an array of light-emitting diodes (e.g., a series of light-emitting diodes that are arranged in a row that extends into the page in the orientation ofFIG. 2).

Light74from light source72may be coupled into edge surface76of light guide plate78and may be distributed in dimensions X and Y throughout light guide plate78due to the principal of total internal reflection. Light guide plate78may include light-scattering features such as pits, bumps, grooves, or ridges that help light exit light guide plate78for use as backlight44. These features may be located on an upper surface and/or on an opposing lower surface of light guide plate78. With one illustrative configuration, which is described herein as an example, a first surface such as the lower surface of light guide plate78has a pattern of bumps and an opposing second surface such as the upper surface of light guide plate78has a pattern of ridges (sometimes referred to as lenticules, lenticular structures, or lenticular ridges). Light source72may be located at the left of light guide plate78as shown inFIG. 2or may be located along the right edge of plate78and/or other edges of plate78.

Light74that scatters upwards in direction Z from light guide plate78may serve as backlight44for display14. Light74that scatters downwards may be reflected back in the upward direction by reflector80. Reflector80may be formed from a reflective structure such as a substrate layer of plastic coated with a dielectric mirror formed from alternating high-index-of-refraction and low-index-of-refraction inorganic or organic layers.

To enhance backlight performance for backlight structures42, backlight structures42may include optical films70. Optical films70may include diffuser layers for helping to homogenize backlight44and thereby reduce hotspots. Optical films70may also include prism films (sometimes referred to as turning films) for collimating backlight44. Optical films70may overlap the other structures in backlight unit42such as light guide plate78and reflector80. For example, if light guide plate78has a rectangular footprint in the X-Y plane ofFIG. 2, optical films70and reflector80may each have a matching rectangular footprint. Optical films70may include compensation films for enhancing off-axis viewing or compensation films may be formed within the polarizer layers of display14or elsewhere in display14.

FIG. 3is a top view of a portion of display14showing how display14may have an array of pixels90formed within display layers46. Pixels90may have color filter elements of different colors such as red color filter elements R, green color filter elements G, and blue color filter elements B. Pixels90may be arranged in rows and columns and may form active area AA of display14. The borders of active area AA may be slightly inboard of the borders of light-guide plate78to ensure that there are no visible hotspots in display14(i.e., no areas in which the backlight illumination for display14is noticeably brighter than surrounding areas). For example, border92of active area AA may be offset by a distance82from lower edge76of light guide plate. It is generally desirable to minimize the size of distance82so that display14is as compact as possible for a given active area size. Nevertheless, distance82should not be too small to ensure that there is adequate light mixing. In particular, distance82should be sufficiently large to allow light74that is emitted from light-emitting diodes72to homogenize enough to serve as backlight illumination. When light74is initially emitted from individual light-emitting diodes72, light74is concentrated at the exits of light-emitting diodes72and is absent in the spaces between light-emitting diodes72. After light74has propagated sufficiently far within light-guide plate78(i.e., after light74has traversed a sufficiently large mixing distance82), light74will be smoothly distributed along dimension X and will no longer be concentrated near the exits of respective individual light-emitting diodes72.

To enhance the efficiency with which light74is coupled into edge76of light guide plate from light-emitting diodes72without overly thickening light-guide plate78, it may be desirable to provide light-guide plate78with an outwardly tapered (flared) edge. Conventional edge tapers are formed by creating a taper in the upper surface of a light guide plate adjacent to the light-emitting diodes and leaving the opposing planar lower surface of the light guide plate untouched. If care is not taken, however, this type of taper may have an angle that is too steep, raising the potential for excessive light leakage due to the loss of total internal reflection conditions in the taper region. With the illustrative taper configuration shown in the cross-sectional side view of illustrative light guide plate78ofFIG. 4, excessive light losses are avoided by providing light guide plate78with both upper and lower taper structures100and102, respectively. Tapers102and100may be symmetrical or tapers102and100may have different shapes. In region96, light-guide plate78is planar and has planar parallel opposing upper and lower surfaces106and108, respectively. In taper region98, light guide plate78has a thickness that varies from the thickness of region96(T2) to enlarged thickness T1at edge76, so taper structure surfaces112and104are angled at non-zero angles with respect to planar upper and lower light guide plate surfaces106and108. Thickness T2may be about 400 microns 300-500 microns, less than 600 microns, more than 200 microns, or other suitable thickness. The enlarged size of dimension T1helps light guide plate78receive light74from light-emitting diodes72. The taper in light guide plate78formed by taper structures100and102helps concentrate light74into region96of light guide plate for use in forming backlight44.

As shown inFIG. 5, lower surface96of light guide plate78may be provided with light scattering features such as bumps (protrusions)114. Bumps114may help redirect light74that is traveling within the interior of light guide plate78upwards in direction Z to serve as backlight44for display14.

As light74that is traveling within light guide plate78is directed upwards in direction Z to serve as backlight44, the intensity of the light74that remains in light guide plate78decreases. As a result, the intensity of light74is greatest at edge76of light guide plate78adjacent to light-emitting diodes72and decreases with increasing distance along axis Y away from edge76. It is generally desirable for the intensity of backlight44to be evenly distributed across the surface of light guide plate78in dimensions X and Y. To ensure that backlight44is not too dim at large values of Y, the density of bumps114can be increased as a function of increasing value of Y, as shown inFIG. 6. The increase in the density of bumps114at larger Y values offsets the decrease in the intensity of light74within light guide plate at larger Y values and thereby ensures that backlight44has a uniform intensity as a function of dimension Y.

Bumps114can be distributed unevenly in dimension X to help counteract the hotspot that would otherwise be associated with each light-emitting diode. An illustrative lateral bump density distribution is shown inFIG. 7for multiple locations in dimension Y. In the example ofFIG. 7, a first light-emitting diode is emitting light74into edge76at location X1and a second light-emitting diode is emitting light74into edge76at location X2. There is therefore a natural tendency for backlight44to be concentrated about locations X1and X2, particularly at low Y values immediately adjacent to edge76. This can be counteracted by locally reducing the density of bumps114at the exits of light-emitting diodes72(i.e., by reducing the density of bumps114at positions X1and X2). An illustrative bump density reduction scheme of this type is illustrated by curves120,122,124, and126ofFIG. 7. Curve120corresponds to the density of bumps114adjacent to edge76(dimension Y1). The decrease in bump density is greatest at this Y location, because light74tends to be most concentrated just after being emitted into edge76of light guide plate78. At slightly larger Y values such as Y value Y2, it is not necessary to locally reduce the bump density as much, so the localized reductions in bump density at Y2are lower than at Y1, as shown by curve122. Curve124illustrate how the density of bumps114may be reduced by a further decreased amount at larger Y value Y3. At Y locations greater than Y4(e.g., about 0.5 to 1 mm, 0.5 to 10 mm, 1-5 mm, more than 1 mm, less than 10 mm, etc.) the density of bumps114may be constant as a function of lateral dimension X (and may vary in dimension Y as shown inFIG. 6).

To enhance light mixing as light74is emitted into edge76of light guide plate78, edge76may be provided with locally raised features such as protrusions128ofFIG. 8. Protrusions128may have triangular profiles (in the XY plane ofFIG. 8), may have semicircular profiles, or may have other shapes. Angle A may be about 140-160° or other suitable value to help refract light74at relatively steep angles in plate78, as shown by illustrative light ray74ofFIG. 8, thereby enhancing light mixing and helping to reduce mixing distance82(FIG. 3). Protrusions128may have widths (in dimension X) of about 75-125 microns or other suitable widths. Protrusions128may be spaced apart by about 250 microns, 200-300 microns, less than 320 microns, or more than 150 microns (as examples). Protrusions128may be spread evenly along edge76or may be clustered adjacent to respective light-emitting diodes72.

Light-emitting diodes72emit light74in a cone. This cone of light includes highly angled off-axis light rays. As shown in the cross-sectional side view of light guide plate78ofFIG. 9, some of the highly angled light rays such as light ray74-1lie primarily in the YZ plane. These light rays interact strongly with upper surface106and lower surface108of light guide plate and therefore tend to be heavily extracted by bumps114on lower surface108. Other highly angled light rays in the cone of emitted light74such as illustrative light ray74-2inFIG. 9lie primarily in the XY plane. These rays are angled more along dimension X than dimension Z and therefore interact with surfaces106and108less frequently than ray74-1. To ensure that light rays such as light ray74-2are adequately extracted and can serve as backlight44, light guide plate78may be provided with lenticular ridges such as ridges130ofFIG. 10. Ridges130may be formed on upper surface106of light guide plate78(as an example). As shown inFIG. 10, ridges130may run parallel to dimension Y (i.e., the direction in which the exit faces of light-emitting diodes72are oriented and the direction in which light74is emitted into edge76of light guide plate78). Ridges130may have semicircular cross-sectional shapes or may have other suitable shapes (triangular, etc.). As shown inFIG. 10, the presence of ridges130may help extract highly angled light rays such as light ray74-2that are propagating close to the XY plane to produce corresponding backlight44.

A conventional light guide plate may have a rectangular pattern of parallel ridges that evenly covers the surface of the light guide plate. This type of uniform ridge pattern tends to over-extract light that has just been injected from the light-emitting diodes, leading to hotspots along the edge of the light guide plate. The tendency of conventional ridge arrangements to locally over-extract light can be exacerbated when using highly angled protrusions such as protrusions on the edge of light guide plate that is receiving the light from the light-emitting diodes (see, e.g., protrusions128ofFIG. 8).

To reduce or eliminate these hotspots and therefore allow mixing distance82to be minimized, ridges130on light guide plate78can be implemented using a non-uniform pattern. Consider, as an example, the illustrative pattern of ridges130shown inFIG. 11. Different areas in light guide plate78ofFIG. 11each have a different density of ridges130. Borders134and132separate regions with different respective ridge densities. Border136separates ridges130from taper region98.

Ridges130include ridges130-1that terminate at endpoints along border132. Ridges130-2extend past border132and terminate at border134. Ridges130-2therefore tend to be longer than ridges130-1. Ridges130-3extend to border136between tapered region98and planar region96. Ridges130-1,130-2, and130-3may all extend continuously in direction Y until reaching the edge of light guide plate78opposing edge76.

Because different ridges130cover different portions of light guide plate78with different densities, the amount of light extraction produced by ridges130varies as a function of lateral position on light guide plate78(i.e., the density of ridges130varies as a function of position in the X-Y plane ofFIG. 11). This allows the amount of light extraction produced by ridges130to be selectively reduced in the vicinity of light-emitting diodes72. For example, the relative scarcity of ridges130in region138helps prevent over-extraction of light74in region138. Region140extends between light emitting diodes72and does not receive as much of light74as region138. There is therefore a risk that region140will become too dark. With the illustrative pattern ofFIG. 11, region140has been provided with more ridges (i.e., ridges130-2) than region138and therefore exhibits more light extraction than region138. This helps balance the amount of light extracted in region138(where there is ample light and therefore fewer light extraction ridges) and region140(where there is less light and therefore more light extraction ridges to compensate).

In general, light guide plate78may be provided with a continuously varying ridge density, two or more areas with two or more respective ridge densities, or other patterns of ridges. In the example ofFIG. 11, there are three different areas each of which has a different respective density of ridges. Area138contains only ridges130-1and therefore has a relatively low ridge density. Area138may be located near light-emitting diodes72to prevent over-extraction of light74. Area140contains both ridges130-1and ridges130-2and therefore has a higher ridge density than area138. Area142contains ridges130-1,130-2, and130-3and therefore has a higher ridge density than area140. The presence of third area142(i.e., use of three different ridge density regions) helps smooth out the intensity of extracted backlight. If desired, additional areas with different respective ridge densities may be provided (e.g., four or more regions, continuously varying density portions, etc.). The use of three distinct ridge densities in the illustrative light guide plate ofFIG. 11is merely an example.

The homogeneity of backlight44can be enhanced by using smooth shapes for borders134and132(e.g., curved paths in the example ofFIG. 11). Backlight homogeneity may also be enhanced by imposing pseudorandom variations on the locations of the ends of the ridges. As shown inFIG. 12, for example, the locations of endpoints144of ridges130-1need not terminate precisely along border132. Imposing slight variations in the lengths of ridges130-1(i.e., dithering the lengths of the ridges) allows these ridges to be aligned with a desired border (e.g., border132in theFIG. 12example) while helping to ensure that the ridge density transition associated with border132does not produce backlight intensity variations that are visible to viewer48.

Another way to smooth the transition between regions of differing ridge density involves imposing a tapering width and/or thickness on the ends of ridges130. This type of arrangement is shown inFIG. 13. As shown inFIG. 13, ridge130may be characterized by a thickness TB and a width WB in main region148. The width of ridge130and the thickness of ridge130may be smoothly reduced towards endpoint144(i.e., the ridge may taper to smaller thicknesses towards the endpoint). For example, in end region146, ridge130may have reduced thicknesses such as thickness TS (TS<TB) and reduced widths such as width TS (WS<WB). Tapering the width, thickness, or other characteristic along the length of ridge130towards the end of ridge130helps make the location of endpoint144visually indistinct and therefore helps to ensure that transitions between light guide plate regions with different ridge densities are not noticeable to a viewer.

Light guide plate78may be formed by shaping a polymer layer using a metal blank. Grooves with varying depths and widths may be formed in the metal blank by varying the operating height of a grinding bit during groove formation. A metal blank with grooves of varying depth and width may be used in forming a light guide plate with corresponding ridges of varying thickness and width (i.e., ridges with thicknesses that taper down to smaller values as the ridges extend towards their endpoints).

In general, any one or more of these schemes for reducing the visibility of ridge density transitions on light guide plate78and for reducing hotspots may be used (e.g., selective variation of ridge density, use of multiple regions with distinct ridge densities, imposing small variations on the locations of ridge endpoints while still aligning the ridge endpoints with a desired ridge density region border, varying thickness and width of ridges, etc.