PATENT DOCUMENT

Publication Number: US-11804187-B2
Application Number: US-202217848929-A
Country: US
Kind Code: B2

Title: Displays with reduced color non-uniformity

Abstract:
An electronic device may include a display having an array of pixels and a backlight that provides backlight illumination for the array of pixels. The backlight may be a direct-lit backlight with a two-dimensional array of light-emitting diodes operable in a local dimming scheme. The electronic device may include control circuitry that provides pixel signals to the array of pixels and backlight signals to the backlight. The control circuitry may adjust the pixel signals and the backlight signals to compensate for brightness and color non-uniformity in the backlight. To compensate for image-dependent backlight non-uniformity, the control circuitry may simulate artificial backlight data based on the target image to be displayed and stored point spread information. To compensate for white-point-dependent backlight non-uniformity, the control circuitry may use measured actual backlight data that describes color variations across the backlight for a given target white point.

Claims:
What is claimed is: 
     
       1. An electronic device comprising:
 an array of pixels; 
 a backlight having an array of light-emitting diodes that produce backlight illumination for the array of pixels; and 
 control circuitry that provides pixel signals to the array of pixels and backlight signals to the backlight, wherein the pixel signals and the backlight signals are adjusted based on simulated artificial backlight data and measured actual backlight data, and wherein the pixel signals and the backlight signals are adjusted to compensate for a predicted backlight non-uniformity. 
 
     
     
       2. The electronic device defined in  claim 1  wherein the control circuitry stores point spread function information for the array of light-emitting diodes. 
     
     
       3. The electronic device defined in  claim 2  wherein the control circuitry:
 determines backlight brightness and color values based on a target image to be displayed; and 
 calculates the simulated artificial backlight data based on the backlight brightness and color values and the point spread function information. 
 
     
     
       4. The electronic device defined in  claim 3  wherein the point spread function information describes a brightness spread associated with a zone of light-emitting diodes in the array of light-emitting diodes. 
     
     
       5. The electronic device defined in  claim 1  wherein the measured actual backlight data indicates an amount of color non-uniformity across the backlight for a given target white point. 
     
     
       6. The electronic device defined in  claim 5  wherein the measured actual backlight data is measured during manufacturing and stored in the electronic device. 
     
     
       7. The electronic device defined in  claim 1  wherein the predicted backlight non-uniformity comprises image-dependent backlight non-uniformity and wherein the measured actual backlight data compensates for white-point-dependent backlight non-uniformity. 
     
     
       8. The electronic device defined in  claim 1  wherein the array of light-emitting diodes comprises blue light-emitting diodes that emit blue light and the backlight comprises a phosphor layer that converts the blue light into white light. 
     
     
       9. The electronic device defined in  claim 1  wherein the backlight comprises quantum dots. 
     
     
       10. The electronic device defined in  claim 1  wherein the array of light-emitting diodes comprises red, green, and blue light-emitting diodes. 
     
     
       11. An electronic device, comprising:
 a two-dimensional array of pixels; 
 a two-dimensional array of light sources that provides backlight illumination for the two-dimensional array of pixels using a local dimming scheme; and 
 control circuitry that determines brightness and color values for the two-dimensional array of light sources based on a target image to be displayed, wherein the control circuitry adjusts the brightness and color values to compensate for predicted image-dependent backlight non-uniformity and measured white-point-dependent backlight non-uniformity. 
 
     
     
       12. The electronic device defined in  claim 11  wherein the electronic device stores point spread function information for the two-dimensional array of light sources and stores measured color non-uniformity information for the two-dimensional array of light sources. 
     
     
       13. The electronic device defined in  claim 12  wherein the control circuitry simulates artificial backlight data based on the point spread information and the brightness and color values associated with the target image to be displayed and wherein the brightness and color values are adjusted based on the artificial backlight data to compensate for the predicted image-dependent backlight non-uniformity. 
     
     
       14. The electronic device defined in  claim 13  wherein the control circuitry adjusts the brightness and color values based on the measured color non-uniformity information to compensate for the measured white-point-dependent backlight non-uniformity. 
     
     
       15. The electronic device defined in  claim 11  wherein the control circuitry determines pixel values for the two-dimensional array of pixels based on the target image to be displayed and wherein the control circuitry compensates the pixel values based on the brightness and color values. 
     
     
       16. An electronic device, comprising:
 a display having an array of pixels and an array of light sources that provide illumination for the array of pixels using a local dimming scheme; and 
 control circuitry that provides compensated signals to the array of pixels and the array of light sources, wherein the compensated signals are based on a predicted amount of brightness non-uniformity in the illumination and a measured amount of color non-uniformity in the illumination. 
 
     
     
       17. The electronic device defined in  claim 16  wherein the predicted amount of brightness non-uniformity in the illumination is based on a target image to be displayed and stored point spread function information for the array of light sources. 
     
     
       18. The electronic device defined in  claim 16  wherein the measured amount of color non-uniformity in the illumination describes color variation across the array of light sources for a given target white point. 
     
     
       19. The electronic device defined in  claim 16  wherein the array of light sources comprises blue light-emitting diodes that emit blue light, the electronic device further comprising a phosphor layer that converts the blue light into white light. 
     
     
       20. The electronic device defined in  claim 19  wherein the phosphor layer comprises red and green phosphors.

Description:
This application claims the benefit of provisional patent application No. 63/215,023, filed Jun. 25, 2021, which is hereby incorporated by reference herein in its entirety. 
    
    
     FIELD 
     This relates generally to electronic devices with displays and, more particularly, to displays with backlights. 
     BACKGROUND 
     Electronic devices such as computers and cellular telephones have displays. Some displays such 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 such as liquid crystal displays include pixels that do not produce light but instead are used to adjust the amount of light transmitted from a backlight through the display. 
     A backlight may be an edge-lit type backlight or a direct-lit type backlight. In an edge-lit type backlight, one or more light sources emit light into an edge of a light guide plate that distributes the light across the array of pixels. A direct-lit backlight may include an array of light sources that provide light directly to the array of pixels. Direct-lit backlights may be used to implement a local dimming scheme in a display. 
     If care is not taken, backlights may exhibit brightness and color non-uniformities which can lead to undesirable artifacts in the displayed images. 
     SUMMARY 
     A display may have an array of pixels for displaying images for a viewer. The display may be a liquid crystal display having display layers such as a color filter layer, a liquid crystal layer, a thin-film transistor layer, an upper polarizer layer, and a lower polarizer layer. 
     The pixel array may be illuminated with backlight illumination from a backlight. The backlight may be a direct-lit backlight with a two-dimensional array of light-emitting diodes operable in a local dimming scheme. In other arrangements, the pixel array may be a front-lit pixel array that is illuminated with a front light. 
     The electronic device may include control circuitry that provides pixel signals to the array of pixels and backlight signals to the backlight. The control circuitry may adjust the pixel signals and the backlight signals to compensate for brightness and color non-uniformity in the backlight. To compensate for image-dependent backlight non-uniformity, the control circuitry may simulate artificial backlight data based on the target image to be displayed and stored point spread function information associated with the array of light-emitting diodes. To compensate for white-point-dependent backlight non-uniformity, the control circuitry may use measured actual backlight data that describes color variations across the backlight for a given target white point. The measured actual backlight data may be gathered during manufacturing and stored in the electronic device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of an illustrative electronic device having a display in accordance with an embodiment. 
         FIG.  2    is a top view of an illustrative display in accordance with an embodiment. 
         FIG.  3    is a cross-sectional side view of an illustrative display in an electronic device that has a backlight and a pixel array in accordance with an embodiment. 
         FIG.  4    is a top view of an illustrative backlight having light-emitting diodes arranged in respective cells in accordance with an embodiment. 
         FIG.  5    is a top view of an illustrative display showing how different portions of the display may have different target brightness levels in accordance with an embodiment. 
         FIG.  6    is a diagram illustrating how compensated pixel signals and compensated backlight signals may be used to display a desired image in accordance with an embodiment. 
         FIG.  7    is a diagram illustrating how simulated artificial backlight data and measured actual backlight data may be used to generate compensated backlight signals in accordance with an embodiment. 
         FIG.  8    is a flow chart of illustrative steps involved in gathering backlight calibration data in accordance with an embodiment. 
         FIG.  9    is a flow chart of illustrative steps involved in displaying an image using compensated backlight signals and compensated pixel signals in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An illustrative electronic device of the type that may be provided with a display is shown in  FIG.  1   . Electronic device  10  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 display, a computer display that contains an embedded computer, 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, or other electronic equipment. Electronic device  10  may have the shape of a pair of eyeglasses (e.g., supporting frames), may form a housing having a helmet shape, or may have other configurations to help in mounting and securing the components of one or more displays on the head or near the eye of a user. 
     As shown in  FIG.  1   , electronic device  10  may include control circuitry  16  for supporting the operation of device  10 . Control circuitry  16  may include storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid-state drive), volatile memory (e.g., static or dynamic random-access memory), etc. Processing circuitry in control circuitry  16  may be used to control the operation of device  10 . The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, application-specific integrated circuits, etc. 
     Input-output circuitry in device  10  such as input-output devices  12  may be used to allow data to be supplied to device  10  and to allow data to be provided from device  10  to external devices. Input-output devices  12  may 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 device  10  by supplying commands through input resources of input-output devices  12  and may receive status information and other output from device  10  using the output resources of input-output devices  12 . 
     Input-output devices  12  may include one or more displays such as display  14 . Display  14  may be a touch screen display that includes a touch sensor for gathering touch input from a user or display  14  may be insensitive to touch. A touch sensor for display  14  may 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. A touch sensor for display  14  may be formed from electrodes formed on a common display substrate with the display pixels of display  14  or may be formed from a separate touch sensor panel that overlaps the pixels of display  14 . If desired, display  14  may be insensitive to touch (i.e., the touch sensor may be omitted). Display  14  in electronic device  10  may be a head-up display that can be viewed without requiring users to look away from a typical viewpoint or may be a head-mounted display that is incorporated into a device that is worn on a user&#39;s head. If desired, display  14  may also be a holographic display used to display holograms. 
     Control circuitry  16  may be used to run software on device  10  such as operating system code and applications. During operation of device  10 , the software running on control circuitry  16  may display images on display  14 . 
     Input-output devices  12  may also include one or more sensors  18  such as force sensors (e.g., strain gauges, capacitive force sensors, resistive force sensors, etc.), audio sensors such as microphones, touch and/or proximity sensors such as capacitive sensors (e.g., a two-dimensional capacitive touch sensor associated with a display and/or a touch sensor that forms a button, trackpad, or other input device not associated with a display), and other sensors. In accordance with some embodiments, sensors  18  may include optical sensors such as optical sensors that emit and detect light (e.g., optical proximity sensors such as transreflective optical proximity structures), ultrasonic sensors, and/or other touch and/or proximity sensors, monochromatic and color ambient light sensors, image sensors, fingerprint sensors, temperature sensors, proximity sensors and other sensors for measuring three-dimensional non-contact gestures (“air gestures”), pressure sensors, sensors for detecting position, orientation, and/or motion (e.g., accelerometers, magnetic sensors such as compass sensors, gyroscopes, and/or inertial measurement units that contain some or all of these sensors), health sensors, radio-frequency sensors, depth sensors (e.g., structured light sensors and/or depth sensors based on stereo imaging devices), optical sensors such as self-mixing sensors and light detection and ranging (lidar) sensors that gather time-of-flight measurements, humidity sensors, moisture sensors, gaze tracking sensors, and/or other sensors. In some arrangements, device  10  may use sensors  18  and/or other input-output devices to gather user input (e.g., buttons may be used to gather button press input, touch sensors overlapping displays can be used for gathering user touch screen input, touch pads may be used in gathering touch input, microphones may be used for gathering audio input, accelerometers may be used in monitoring when a finger contacts an input surface and may therefore be used to gather finger press input, etc.). 
     Display  14  may be a liquid crystal display or may be a display based on other types of display technology (e.g., organic light-emitting diode displays). Device configurations in which display  14  is a liquid crystal display are sometimes described herein as an example. This is, however, merely illustrative. Any suitable type of display may be used, if desired. In general, display  14  may have a rectangular shape (i.e., display  14  may have a rectangular footprint and a rectangular peripheral edge that runs around the rectangular footprint) or may have other suitable shapes. Display  14  may be planar or may have a curved profile. 
       FIG.  2    is a top view of a portion of display  14  showing how display  14  may have an array of pixels  22 . Pixels  22  may have color filter elements of different colors such as red color filter elements, green color filter elements, and blue color filter elements. Pixels  22  may be arranged in rows and columns and may form active area AA of display  14 . Pixels  22  may be liquid crystal display pixels, if desired. The rectangular shape of display  14  and active area AA in  FIG.  2    is merely illustrative. If desired, the active area AA may have a non-rectangular shape (e.g., a shape with one or more curved portions, one or more rounded corners, one or more recesses for accommodating input-output components, etc.). 
     A cross-sectional side view of display  14  is shown in  FIG.  3   . As shown in  FIG.  3   , display  14  may include a pixel array such as pixel array  24 . Pixel array  24  may include an array of pixels such as pixels  22  of  FIG.  2    (e.g., an array of pixels having rows and columns of pixels). Pixel array  24  may include liquid crystal display pixels or other suitable pixel elements. 
     During operation of display  14 , images may be displayed using pixel array  24 . Backlight unit  42  (which may sometimes be referred to as a backlight, a direct-lit backlight, backlight layers, backlight structures, a backlight module, a backlight system, etc.) may be used in producing backlight illumination  44  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  21 . 
     Backlight  42  may have optical films  26 , a light diffuser such as light diffuser (light diffuser layer)  34 , and 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  44 . Light-emitting diodes  38  may, as an example, be arranged in rows and columns and may lie in the X-Y plane of  FIG.  3   . 
     The light produced by each light-emitting diode  38  may travel upwardly along dimension Z through light diffuser  34  and optical films  26  before passing through pixel array  24 . Light diffuser  34  may contain light-scattering structures that diffuse the light from light-emitting diode array  36  and thereby help provide uniform backlight illumination  44 . Optical films  26  may include films such as dichroic filter  32 , phosphor layer  30 , and films  28 . Films  28  may include brightness enhancement films that help to collimate light  44  and thereby enhance the brightness of display  14  for user  20  and/or other optical films (e.g., compensation films, etc.). 
     Light-emitting diodes  38  may emit light of any suitable color. With one illustrative configuration, light-emitting diodes  38  emit blue light. Dichroic filter layer  32  may be configured to pass blue light from light-emitting diodes  38  while reflecting light at other colors. Blue light from light-emitting diodes  38  may be converted into white light by a photoluminescent material such as phosphor layer  30  (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  30  (which may sometimes be referred to as a photoluminescent layer or color conversion layer) may include 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  include red, green, and blue light-emitting diodes and/or light-emitting diodes that emit white light (e.g., so that layer  30  may be omitted, if desired) may also be used. 
     In configurations in which layer  30  emits white light such as white light produced by phosphorescent material in layer  30 , white light that is emitted from layer  30  in the downwards (−Z) direction may be reflected back up through pixel array  24  as backlight illumination by dichroic filter layer  32  (i.e., layer  32  may help reflect backlight outwardly away from array  36 ). In configurations in which layer  30  includes, for example, red and green quantum dots, dichroic filter  32  may be configured to reflect red and green light from the red and green quantum dots, respectively to help reflect backlight outwardly away from array  36 . By placing the photoluminescent material of backlight  42  (e.g., the material of layer  30 ) above diffuser layer  34 , light-emitting diodes  38  may be configured to emit more light towards the edges of the light-emitting diode cells (tiles) of array  36  than at the centers of these cells, thereby helping enhance backlight illumination uniformity. The use of backlight  42  is merely illustrative. If desired, pixel array  24  may be a front-lit pixel array that is illuminated with a front light. Arrangements in which pixel array  24  is illuminated with a backlight are sometimes described herein as an illustrative example. 
     In a configuration in which pixel array  24  is formed using a liquid crystal display, pixel array  24  may include a liquid crystal layer such a liquid crystal layer  52 . Liquid crystal layer  52  may be sandwiched between display layers such as display layers  58  and  56 . Layers  56  and  58  may be interposed between lower polarizer layer  60  and upper polarizer layer  54 . Liquid crystal display structures of other types may be used in forming pixel array  24 , if desired. 
     Layers  56  and  58  may be formed from transparent substrate layers such as clear layers of glass or plastic. Layers  56  and  58  may be layers such as a thin-film transistor layer and/or a color filter layer. Conductive traces, color filter elements, transistors, and other circuits and structures may be formed on the substrates of layers  58  and  56  (e.g., to form a thin-film transistor layer and/or a color filter layer). Touch sensor electrodes may also be incorporated into layers such as layers  58  and  56  and/or touch sensor electrodes may be formed on other substrates. 
     With one illustrative configuration, layer  58  may be a thin-film transistor layer that includes an array of pixel circuits based on thin-film transistors and associated electrodes (pixel electrodes) for applying electric fields to liquid crystal layer  52  and thereby displaying images on display  14 . Layer  56  may be a color filter layer that includes an array of color filter elements for providing display  14  with the ability to display color images. If desired, layer  58  may be a color filter layer and layer  56  may be a thin-film transistor layer. 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 display  14  in device  10 , control circuitry  16  (e.g., one or more integrated circuits on a printed circuit) may be used to generate information to be displayed on display  14  (e.g., display data). The information to be displayed may be conveyed to a display driver integrated circuit such as circuit  62 A or  62 B using a signal path such as a signal path formed from conductive metal traces in a rigid or flexible printed circuit such as printed circuit  64  (as an example). Integrated circuits such as integrated circuit  62 A and/or flexible printed circuits such as flexible printed circuit  64  may be attached to substrate  58  in ledge region  66  (as an example). 
       FIG.  4    is a top view of an illustrative light-emitting diode array for backlight  42 . As shown in  FIG.  4   , light-emitting diode array  36  may contain rows and columns of light-emitting diodes  38 . Each light-emitting diode  38  may be associated with a respective cell  38 C (sometimes referred to as a tile area, zone, etc.). 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, or other suitable 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 cells may have sides of different lengths (e.g., a non-square rectangular shape). The configuration of  FIG.  4    in which light-emitting diode array  36  has rows and columns of square light-emitting diode regions such as cells  38 C (e.g., a two-dimensional array of cells  38 C) is merely illustrative. 
     In some cases, each cell  38 C may include a single light-emitting diode. Alternatively, each cell  38 C may have a light source  38  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 two-by-two group of light-emitting diodes or a three-by-three group of light-emitting diodes in each cell  38 C). The multiple diodes in each cell  38 C may be mounted on a common substrate, may be mounted on a printed circuit substrate that extends across array  36 , may be mounted on a glass substrate that extends across array  36 , or may be mounted in array  36  using other desired arrangements. In general, each cell  38 C may include a single light-emitting diode, a pair of light-emitting diodes, 2-20 light-emitting diodes, at least 2 light-emitting diodes, at least 4 light-emitting diodes, at least 8 light-emitting diodes, fewer than 5 light-emitting diodes, between 4 and 12 light-emitting diodes, between 8 and 12 light-emitting diodes, between 8 and 10 light-emitting diodes, 9 light-emitting diodes, or other desired number of light-emitting diodes. 
     Light-emitting diodes  38  may be controlled in unison by control circuitry in device  10  or may be individually controlled. Controlling the light-emitting diodes  38  in unison may be used to provide backlight illumination with uniform brightness across display  14 . Controlling the light-emitting diodes individually may enable the electronic device to implement a local dimming scheme that helps improve the dynamic range of images displayed on pixel array  24  and that potentially reduces the power consumption of the backlight. The dynamic range of a display may be considered the ratio between the light of the highest intensity (e.g., the brightest light) that the display is capable of emitting and the light of the lowest intensity (e.g., the dimmest light) that the display is capable of emitting. 
     Consider the example depicted in  FIG.  5   . In  FIG.  5   , objects such as objects  72 - 1  and  72 - 2  are displayed on display  14  (sometimes referred to as screen  14 ). In this example, object  72 - 1  may have a high brightness level. Object  72 - 2  may have an intermediate brightness level. The background of the display may have a low brightness level. If the light-emitting diodes providing backlight for display  14  in  FIG.  5    are controlled in unison (e.g., to produce uniform backlight brightness across display  14 ), all of the light-emitting diodes may be set to a brightness that is optimized for object  72 - 1 . In this scenario, object  72 - 1  may be displayed with its intended brightness. However, the background of the display is also receiving backlight with a high brightness optimized for object  72 - 1 . Therefore, the background of the display may appear brighter than desired due to display limitations such as light leakage through the pixels or other limitations, and the dynamic range of the display is lower than desired. Alternatively, all of the light-emitting diodes may be set to a brightness that is optimized for the background of the display. In this scenario, the background may be displayed with its intended brightness. However, object  72 - 1  is also receiving backlight with a low brightness optimized for the background. Therefore, object  72 - 1  will appear dimmer than desired and the dynamic range of the display will be lower than desired. In yet another embodiment, the brightness of all of the light-emitting diodes may be set to a brightness that is optimized for object  72 - 2 . In this scenario, object  72 - 1  will appear dimmer than desired and the background will appear brighter than desired. 
     Additionally, controlling all of the light-emitting diodes in backlight unit  42  in unison may introduce power consumption limitations. The maximum allowable power consumption of the backlight unit may prevent all of the light-emitting diodes from being operated at a peak brightness level. For example, all of the light-emitting diodes may not be able to emit light with a desired brightness for bright object  72 - 1  while meeting power consumption requirements. 
     To increase the dynamic range of the display (and to allow for peak brightness levels without exceeding power consumption requirements), the light-emitting diodes in backlight unit  42  may be controlled individually. For example, light-emitting diodes in region  14 - 1  of the display may have a high brightness optimized for the high brightness of object  72 - 1 , light-emitting diodes in region  14 - 2  of the display may have a brightness optimized for the intermediate brightness of object  72 - 2 , and light-emitting diodes in region  14 - 3  of the display may have a low brightness optimized for the low brightness of the background of the display. In one example, the light-emitting diodes in region  14 - 1  may operate at a maximum brightness whereas the light-emitting diodes in background region  14 - 3  may be turned off (e.g., operate at a minimum brightness). Varying the brightness of the light-emitting diodes across the display in this manner increases the dynamic range of the display. 
     Having a two-dimensional array of independently controllable light sources such as light-emitting diodes  38  for producing backlight illumination  44  therefore may increase the dynamic range of the display. Backlights with two-dimensional arrays of light-emitting diodes may sometimes be referred to as two-dimensional backlights. These types of backlights may also sometimes be referred to as direct-lit backlights. The direct-lit backlights emit light vertically towards the pixel array, as opposed to backlights with edge-lit light guide plates (where light is emitted parallel to the plane of the pixel array and redirected vertically towards the pixel array by the light guide plate). 
     Driving circuitry may be included in display  14  to controlling the light-emitting diodes in backlight  42 . Driving circuitry for the light-emitting diodes may be formed from integrated circuits, thin-film transistor circuits, and/or other suitable circuitry. In one example, driving circuitry may be incorporated as thin-film transistor circuitry on a rigid printed circuit board (e.g., a printed circuit board with a plurality of layers of dielectric material such as polyimide and conductive layers). However, the costs associated with such an arrangement may be high, particularly in backlights with a high number of light-emitting diodes. An alternative arrangement for the light-emitting diode driving circuitry is for driver integrated circuits (sometimes referred to as driver integrated circuits) to be included in backlight  42 . Each driver integrated circuit may control one or more corresponding light-emitting diodes. In this way, the light-emitting diodes may be controlled to have varying brightness magnitudes across the backlight. The driver integrated circuits may also be used in combination with a glass substrate in one example. In other words, instead of the light-emitting diodes and driver integrated circuits being mounted on a printed circuit board, the light-emitting diodes and driver integrated circuits may be mounted on a glass substrate. The glass substrate may have conductive traces (e.g., copper traces) to allow signals to be transferred between components as necessary. 
     If care is not taken, backlights may sometimes exhibit color non-uniformity issues that can lead to undesirable artifacts in the displayed images. For example, manufacturing variations among light-emitting diodes  38  may cause brightness and/or color variations in the light emitted across backlight  42 . This type of non-uniformity can be compensated for by storing calibration data in device  10  and using the calibration data to adjust pixel values to account for the brightness and/or color variations in backlight  42 . For example, one or more point spread functions for each zone of light-emitting diodes  38  in backlight  42  may be stored in device  10 . During operation, control circuitry  16  may use this point spread function information to simulate an artificial backlight based on a target image to be displayed. Control circuitry  16  can then adjust pixel values to compensate for the variations in the simulated artificial backlight. However, if the calibration data does not have sufficient resolution (e.g., due to limited memory space), some grid mura may remain in the displayed image. 
     In addition to manufacturing variations among light-emitting diodes, other aspects of backlight  42  may lead to non-uniformity issues. For example, light-emitting diodes  38  may emit blue light that is converted into white light by a phosphor layer. Light-emitting diodes  38  at the edge of backlight  42  (e.g., along the four sides of backlight  42 ) may emit a slightly different color than the rest of light-emitting diodes  38  due to a lack of mixing with yellow light from neighboring light-emitting diodes. This can lead to a bluish edge appearing along the four sides of backlight  42 . Additionally, non-uniformity in the red and green phosphors of phosphor layer  30  may cause blotchy color mura in backlight  42 . 
     To account for all of the different types of non-uniformity that may arise in backlight  42 , control circuitry  16  may compensate pixel signals and/or backlight signals based on simulated artificial backlight data and measured actual backlight data. The simulated artificial backlight data may be used to remove image-dependent variations (e.g., grid mura) that can occur from image-to-image due to manufacturing variations among light-emitting diodes  38 . The measured actual backlight data may be used to remove white-point-dependent variations (e.g., bluish edge and blotchy color mura) that can occur for a given target white point due to light-emitting diode location and non-uniformities in phosphor layer  30 . 
       FIG.  6    is a diagram showing how control circuitry  16  may adjust pixel signals and/or backlight signals to compensate for variations in backlight  42 . During operation of display  14 , control circuitry  16  may provide compensated pixel signals  74  to pixels  22  and may provide compensated backlight signals  76  to backlight  42  to generate displayed image  78 . If desired, pixel signals  74  and backlight signals  76  may be compensated based on simulated artificial backlight data (e.g., a predicted amount of image-dependent backlight non-uniformity) and measured actual backlight (e.g., a measured amount of white-point-dependent backlight non-uniformity). 
       FIG.  7    is a diagram showing how simulated artificial backlight data and measured actual backlight data may be combined to generate compensated backlight data (sometimes referred to as compensated backlight signals, compensated backlight brightness and/or color values, etc.). In the graphs shown in  FIG.  7   , the x-axis represents pixel location (e.g., a pixel location aligned with an associated backlight location), whereas the y-axis may be backlight brightness, may be backlight color, or may be a parameter that expresses both brightness and color (e.g., a ratio of tristimulus vales Z/Y) of backlight  42 . 
     During operation of display  14 , control circuitry  16  may receive image data associated with a target image to be displayed. For each new target image, control circuitry  16  may determine an optimal brightness and/or color value for each light-emitting diode  38  in backlight  42  (e.g., to implement a local dimming scheme as described in connection with  FIG.  5   ). Since the optimal brightness and/or color value s for backlight  42  may change from image-to-image, the non-uniformity in backlight  42  that arises from variations among light-emitting diodes  38  (e.g., grid mura) may also change from image-to-image. To account for this changing non-uniformity, control circuitry  16  may simulate (e.g., predict or reconstruct) artificial backlight data  80  for each new target image to be displayed. Simulated backlight data  80  may be generated based on the backlight brightness and/or color value s associated with the target image to be displayed and point spread function information that is stored in device  10 . The point spread function information may be calibration data that describes the brightness and/or color spread of each individual light-emitting diode  38  or of each group of light-emitting diodes  38  in a given cell of backlight  42 . For example, device  10  may store one or more point spread functions for each cell  38 C of light-emitting diodes  38  and/or may store a global mean point spread function for the entire array of light-emitting diodes  38 . A point spread function may express the distribution of any suitable variable. For example, a point spread function stored in device  10  may express a distribution of a particular tristimulus value such as X, Y, or Z, may express a distribution of a ratio of tristimulus values (e.g., X/Y, Z/Y, etc.), may express a distribution of luminance values, may express a distribution of chromaticity values, etc. Simulated backlight data  80  may describe how the brightness and/or color of backlight  42  is predicted to vary with pixel location. As shown in  FIG.  7   , simulated backlight data  80  for a given target image exhibits grid-like variations across the array of pixels  22 . 
     To account for other variations in backlight  42  such as a bluish edge and blotchy color mura, control circuitry  16  may use measured actual backlight data  82  that is stored in device  10 . Measured actual backlight data  82  may be stored calibration data that describes how the brightness and/or color of backlight  42  actually varies with pixel location for a given target white point (e.g., D65 or other suitable target white point). As shown in  FIG.  7   , measured backlight data  82  exhibits brightness and/or color variations across the array of pixels  22 . 
     Control circuitry  16  may combine simulated artificial backlight data  80  and measured actual backlight data  82  to produce compensated backlight data  84 . Using artificial backlight data  80  in compensated backlight data  84  removes image-dependent non-uniformities such as grid mura, while using measured actual backlight data  82  in compensated backlight data  84  removes white-point-dependent non-uniformities such as a bluish edge and blotchy color mura. This is, however, merely illustrative. If desired, control circuitry  16  may use only measured actual backlight data  82  and/or simulated artificial backlight data  80  to produce compensated backlight data  84 . 
     If desired, device  10  may store measured actual backlight data  82  for a single white point such as D65, or may store measured actual backlight data  82  for multiple target white points (e.g., D65, D110, etc.). Control circuitry  16  may then select which measured backlight data  82  to use to produce compensated backlight data  84  based on the target white point of display  14 . 
       FIG.  8    is a flow chart of illustrative steps involved in gathering and storing calibration data such as measured actual backlight data  82  of  FIG.  7   . 
     During the operations of block  86 , a calibration system may be used to gather measurements of backlight  42 . This may include, for example, turning backlight  42  on to produce a given target white point (e.g., D65, D110, etc.) and using one or more light sensors to measure the brightness and/or color across backlight  42 . For example, the measured data may indicate how the brightness and/or color of backlight  42  changes depending on location (e.g., pixel location, light-emitting diode location, etc.). If desired, the calibration system may gather data for multiple target white points during the operations of block  86 . 
     During the operations of block  88 , the data measured during the operations of block  86  may be stored in device  10 . The stored data (e.g., measured actual backlight data  82  of  FIG.  7   ) may indicate how the color and/or brightness of backlight  42  actually varies with pixel location for a given target white point, which in turn can be compensated for during operation of device  10 . 
       FIG.  9    is a flow chart of illustrative steps involved in displaying images during operation of device  10 . 
     During the operations of block  90 , control circuitry  16  may simulate artificial backlight data  80  based on the target image to be displayed. This may first include determining optimal brightness and/or color value s for each light-emitting diode  38  in backlight  42  based on the target image (e.g., using a local dimming scheme as described in connection with  FIG.  5   ). Based on these optimal brightness and/or color value s, control circuitry  16  may reconstruct (e.g., predict) artificial backlight data  80  using stored point spread function information for light-emitting diodes  38 . 
     During the operations of block  92 , control circuitry  16  may combine the artificial backlight data  80  (calculated during the operations of block  90 ) with the measured actual backlight data  82  (gathered during the calibration operations of  FIG.  8    and stored in device  10 ) to produce compensated backlight signal  84 . If different sets of measured actual backlight data  82  corresponding to different target white points are stored in device  10 , control circuitry  16  may select which set of measured actual backlight data  82  to use based on the target white point of the displayed image. 
     During the operations of block  94 , control circuitry  16  may determine compensated pixel values for pixels  22  based on the combined artificial backlight data  80  and the measured actual backlight data  82 . If desired, control circuitry  16  may compensate pixel values based on the compensated backlight signal  84  after compensated backlight signal  84  is determined in step  92 , or control circuitry  16  may use artificial backlight data  80  and measured actual backlight data  82  to compensate both backlight values and pixel values in parallel. 
     During the operations of block  96 , control circuitry  16  may provide compensated pixel values to pixels  22  and compensated backlight values to light-emitting diodes  38  to display the target image. 
     The foregoing is merely illustrative and various modifications can be made by those skilled in the art without departing from the scope and spirit of the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20220624
Publication Date: 20231031
Grant Date: 20231031
Priority Date: 20210625
Inventors: LIN, FANG-CHENG
LE, CHENGRUI
HUANG, YI-PAI
GORKHALI, SURAJ P
JUNG, TOBIAS
LI, HE
LIU, YANSONG
Assignee: APPLE INC
CPC Classifications: [{"code": "G09G3/3413", "inventive": true, "first": true, "tree": "[]"}, {"code": "G09G3/3426", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/36", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2320/0233", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0242", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0693", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2360/16", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02F1/133603", "inventive": true, "first": true, "tree": "[]"}, {"code": "G09G3/3413", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02F1/133611", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/3426", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2320/0242", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0233", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0693", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2360/16", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0666", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0233", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0693", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0242", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2360/16", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/3426", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/36", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 84542416