PATENT DOCUMENT

Publication Number: US-9940879-B2
Application Number: US-201113253739-A
Country: US
Kind Code: B2

Title: White point uniformity techniques for displays

Abstract:
The present disclosure generally relates to systems and techniques for calibrating displays to improve the white point uniformity between similar type devices. In one embodiment, a backlight includes multiple strings of LEDs, where each string is driven by a separate driver, or driver channel. Each string may be separately tested at a base current to determine its emitted chromaticity, and values indicative of the emitted chromaticities may be stored within the backlight as calibration values. The calibration values may then be used to determine the driving strength for each string that allows the display to produce the target white point when the light from the strings is mixed. Further, in certain embodiments, adjustments also may be made to the LCD panel based on the emitted chromaticities at the base current.

Claims:
What is claimed is: 
     
       1. A display, comprising:
 a first string of first light emitting diodes; 
 a second string of second light emitting diodes; 
 a storage containing calibration values representing a first emitted chromaticity of the first string when driven at a base current in isolation and a second emitted chromaticity of the second string when driven at the base current in isolation; and 
 a controller configured to determine a first driving strength for the first string and a second driving strength for the second string based on the calibration values, wherein the calibration values comprise a first set of chromaticity coordinates representing the first emitted chromaticity and a second set of chromaticity coordinates representing the second emitted chromaticity. 
 
     
     
       2. A display, comprising:
 a first string of first light emitting diodes; 
 a second string of second light emitting diodes; 
 a storage containing calibration values representing a first emitted chromaticity of the first string when driven at a base current in isolation and a second emitted chromaticity of the second string when driven at the base current in isolation; and 
 a controller configured to determine a first driving strength for the first string and a second driving strength for the second string based on the calibration values, wherein the calibration values comprise a set of chromaticity coordinates representing a mixed chromaticity of the first emitted chromaticity and the second emitted chromaticity. 
 
     
     
       3. A method, comprising:
 storing, in a storage of an electronic device comprising a backlight, calibration values representing emitted chromaticities for each of a plurality of strings of light emitting diodes driven at a base current in isolation; and 
 configuring a controller of the electronic device to determine individual driving strengths for each of the plurality of strings based on the calibration values, wherein the individual driving strengths are configured to align a mixed chromaticity for the plurality of strings with a target white point, wherein storing calibration values comprises storing sets of chromaticity coordinates and brightness values for each of the emitted chromaticities.

Description:
BACKGROUND 
     The present disclosure relates generally to displays, and more particularly to displays employing light emitting diode based backlights. 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     Liquid crystal displays (LCDs) are commonly used as screens or displays for a wide variety of electronic devices, including portable and desktop computers, televisions, and handheld devices, such as cellular telephones, personal data assistants, and media players. Traditionally, LCDs have employed cold cathode fluorescent light (CCFL) light sources as backlights. However, advances in light emitting diode (LED) technology, such as improvements in brightness, energy efficiency, color range, life expectancy, durability, robustness, and continual reductions in cost, have made LED backlights a popular choice for replacing CCFL light sources. However, while a single CCFL can light an entire display; multiple LEDs are typically used to light comparable displays. 
     Numerous white LEDs may be employed within a backlight. Depending on manufacturing precision, the light produced by the individual white LEDs may have a broad color or chromaticity distribution, for example, ranging from a blue tint to a yellow tint or from a green tint to a purple tint. During manufacturing, the LEDs may be classified into bins with each bin representing a small range of chromaticity values emitted by the LEDs. Within each backlight, LEDs may be selected to produce the target white point. However, due to the range of chromaticity values emitted by LEDs, even by those within the same bin, the white points emitted by different displays may vary. Further, other display components, such as the diffuser plate and thin film transistor layers, can magnify variations in the chromaticity values emitted by the LEDs, and further, can shift the white points emitted by displays. Accordingly, users may perceive variations in the color of different displays. These variations may be particularly noticeable in the displays of handheld devices, such as portable media players and cellular phones, which are frequently exchanged between users or viewed in close proximity to one another. 
     SUMMARY 
     A summary of certain embodiments disclosed herein is set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of these certain embodiments and that these aspects are not intended to limit the scope of this disclosure. Indeed, this disclosure may encompass a variety of aspects that may not be set forth below. 
     The present disclosure relates generally to techniques for calibrating displays to produce a target white point. Displays used in similar devices each may be calibrated to the target white point to promote uniformity in the appearance of device displays. In accordance with disclosed embodiments, a display may include an LED backlight that has multiple strings of LEDs, with each string including LEDs from a different bin. Each of the strings may be separately tested at a base current, such as 20 mA, to determine the emitted chromaticity of the string. The emitted chromaticity values for each string may be stored as calibration values within the display, and then subsequently used to determine driving strengths for the LED strings. For example, an LED controller for the backlight may compare the calibration values to the target white point and then determine the driving strength for each string that allows the display to produce the target white point when the light from the strings is mixed. 
     Further, in certain embodiments, one or more adjustments also may be made to the LCD panel included in the display. For example, in certain embodiments, the driving strength adjustments may not be sufficient to align the emitted white point with the target white point. In these embodiments, hardware and/or software adjustments may be employed in the LCD panel to compensate for the deviation between the emitted white point and the target white point. For example, the pixels may be adjusted, or a color mask may be shaped, to shift the overall chromaticity emitted by the display in the green, blue, and/or red direction. In another example, the voltages provided to certain pixels may be adjusted to shift the overall chromaticity emitted by the display in the green, blue, and/or red direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of this disclosure may be better understood upon reading the following detailed description and upon reference to the drawings in which: 
         FIG. 1  is a front view of an example of an electronic device employing an LCD display with an LED backlight, in accordance with aspects of the present disclosure; 
         FIG. 2  is a block diagram of an example of components of the electronic device of  FIG. 1 , in accordance with aspects of the present disclosure; 
         FIG. 3  is an exploded view of the LCD display of  FIG. 2 , in accordance with aspects of the present disclosure; 
         FIG. 4  is a block diagram of an example of components of an LCD display, in accordance with aspects of the present disclosure; 
         FIG. 5  is a diagram illustrating LED bins, in accordance with aspects of the present disclosure; 
         FIG. 6  is a schematic diagram of an example of LED strings that may be employed in an LED backlight, in accordance with aspects of the present disclosure; 
         FIG. 7  is a chart depicting the base chromaticity values of the LED strings of  FIG. 6 , as well as the target white point, in accordance with aspects of the present disclosure; 
         FIG. 8  is a flowchart depicting a method for setting calibration values for an LCD display, in accordance with aspects of the present disclosure; 
         FIG. 9  is a schematic diagram illustrating operation of an embodiment of the LED backlight of  FIG. 3 , in accordance with aspects of the present disclosure; 
         FIG. 10  is a flowchart depicting a method for calibrating the display of  FIG. 3  to a target white point, in accordance with aspects of the present disclosure; 
         FIG. 11  is a schematic diagram illustrating operation of another embodiment of the LED backlight of  FIG. 3 , in accordance with aspects of the present disclosure; 
         FIG. 12  is a chart depicting the base chromaticity values of the LED strings employed in the backlight of  FIG. 11 , in accordance with aspects of the present disclosure; 
         FIG. 13  is a chart depicting the base chromaticity values of embodiments of LED strings that may be employed in an LED backlight, in accordance with aspects of the present disclosure; 
         FIG. 14  is a flowchart depicting a method for calibrating a display employing the LED strings of  FIG. 13 , in accordance with aspects of the present disclosure; and 
         FIG. 15  is a flowchart depicting a method for assembling a display employing the LED strings of  FIG. 13 , in accordance with aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     One or more specific embodiments will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     The present disclosure is directed to techniques for producing a consistent white point on displays used in different devices. In particular, the present techniques are designed to enable displays on similar devices (e.g., devices of the same model or type) to emit a consistent white point so that the displays appear to have an identical, or substantially identical color and brightness, as observed by a user. According to certain embodiments, the uniform white point may be determined and then set as the target white point for displays used in similar devices. 
     The displays may each include an LED backlight that illuminates the display using multiple strings of LEDs, with each string including LEDs from a different color bin. Accordingly, each string within an LED backlight may have a different chromaticity. The strings may be selected to have complementary chromaticities, so that when light from the strings is mixed together, a white point that is fairly close to the target white point is emitted. Each of the strings may be separately tested at a base current, such as 20 mA, to determine the emitted chromaticity of the string. Values indicative of the emitted chromaticities may then be stored within the display as calibration values. For example, in certain embodiments, the chromaticity coordinates for each string may be stored as calibration values. The calibration values can then be used during operation of the backlight to determine driving strengths for the LED strings. Each string may be controlled independently by separate driver, or driver channel, which in turn allows each string to be operated at a separate driving strength to fine-tune the white point of the display to the target white point. In particular, control logic within the display may be used to determine the driving strength for each string that aligns the emitted white point with the target white point. 
     In certain embodiments, the driving strength adjustments may not be sufficient to align the emitted white point with the target white point. In these embodiments, adjustments also may be made to the LCD panel to compensate for the deviation from the target white point so that the overall chromaticity emitted by the display matches a target chromaticity. For example, in certain embodiments, the voltage applied to pixels in the LCD panel may be adjusted to shift the overall chromaticity in the green, blue, and/or red direction. In another example, hardware modifications, such as shaping a color mask or adjusting the number or size of pixels, may be employed to shift the overall chromaticity. 
       FIG. 1  illustrates an electronic device  10  that may make use of the white point adjustment techniques described above. It should be noted that while the techniques will be described below in reference to illustrated electronic device  10  (which may be a mobile phone), the techniques described herein are usable with any electronic device employing an LED backlight. For example, other electronic devices may include a desktop computer, a laptop computer, a tablet computer, a viewable media player, a personal data organizer, a workstation, a standalone display, or the like. In certain embodiments, the electronic device may include a model of an iPod® or iPhone® available from available from Apple Inc. of Cupertino, Calif. In other embodiments, the electronic device may include other models and/or types of electronic devices employing LED backlights, available from any manufacturer. 
     As illustrated in  FIG. 1 , electronic device  10  includes a housing  12  that supports and protects interior components, such as processors, circuitry, and controllers, among others, that may be used to generate images to display on display  14 . Housing  12  also allows access to user input structures  16 ,  18 ,  20 , and  22  that may be used to interact with electronic device  10 . User input structures  40 ,  42 ,  44 , and  46 , in combination with the display  18 , may allow a user to control the handheld device  34 . For example, input structure  16  may activate or deactivate the handheld device  34 ; input structure  42  may activate a home screen, a user-configurable application screen, or a voice-recognition feature; input structures  20  may provide volume control, and input structure  22  may toggle between vibrate and ring modes. Electronic device  10  also includes a microphone  48  that receives voice data from a user, and a speaker  50  that enables audio playback or certain phone capabilities. 
     Further, user input structures  16 ,  18 ,  20 , and  22  may be manipulated by a user to operate a graphical user interface (GUI) and/or applications running on electronic device  10 . Moreover, in certain embodiments, electronic device  10  may include a touch screen, located in front of display  14 , that allows the user to interact with electronic device  10 . Electronic device  10  also may include input and output (I/O) ports  28  and  30  that allow connection of device  10  to external devices, such as headphones, external speakers, a power source, or other electronic device. 
       FIG. 2  is a block diagram illustrating various components and features of device  10 . In addition to display  14 , input structures  16 ,  18 ,  20 , and  22 , and I/O ports  28  and  30  discussed above, device  10  includes a processor  32  that may control operation of device  10 . Processor  32  may use data from a storage  34  to execute the operating system, programs, GUI, and any other functions of device  10 . Storage  24  may include non-transitory, computer readable media that stores instructions, programs, and/or code for execution by processor  32 . Further, storage  24  may represent random-access memory, read-only memory, rewritable flash memory, hard drives, and optical discs, among others. Processor  32  also may receive data through I/O port  30  or through a network device  36 , which may represent, for example, one or more network interface cards (NIC) or a network controller. 
     Information received through network device  36  and I/O port  30 , as well as information contained in storage  34 , may be displayed on display  14 . Display  14  may generally include an LED backlight  38  that functions as a light source for an LCD panel  40  within display  14 . As noted above, a user may select information to display by manipulating a GUI through user input structures  16 ,  18 ,  20 , and  22 , and a touch screen. In certain embodiments, a user may adjust properties of LED backlight  38 , such as the color and/or brightness of the white point, by manipulating a GUI through user input structures  16 ,  18 ,  20 , and  22  and the touch screen. An input/output (I/O) controller  42  may provide the infrastructure for exchanging data between input structures  16 ,  18 ,  20 , and  22 , I/O ports  28  and  30 , display  14 , and processor  32 . 
       FIG. 3  is an exploded view of an embodiment of display  14  employing an edge-lit LED backlight  38 . Display  14  includes backlight  38  and LCD panel  40 , which may be assembled within a frame  44 . LCD panel  40  may include an array of pixels configured to selectively modulate the amount and color of light passing from backlight  38  through LCD panel  40 . For example, LCD panel  40  may include a liquid crystal layer, one or more thin film transistor (TFT) layers configured to control orientation of liquid crystals of the liquid crystal layer via an electric field, and polarizing films, which cooperate to enable LCD panel  40  to control the amount of light emitted by each pixel. LCD panel  40  may be a twisted nematic (TN) panel, an in-plane switching (IPS) panel, a fringe-field switching (FFS) panel, variants of the foregoing types of panels, or any other suitable panel. 
     Backlight  38  includes a light guide  46 , such as a light guiding plate, one or more optical films  48 , such as one or more brightness enhancement films, and a light source  50  that includes LEDs  52 . Light from LEDs  52  is directed through light guide  46  and optical films  48  and generally emitted toward LCD panel  40 . As shown in  FIG. 3 , backlight  38  is an edge-lit backlight that includes one light source  50  located at an edge of display  14 . However, in other embodiments, multiple light sources  50  may be disposed around the edges of display  14 . Further, in certain embodiments, instead of an edge-lit backlight, the backlight may be a direct-light backlight that has an array of LEDs mounted on an array tray behind the LCD panel. 
     LEDs  52  may be any type of LEDs designed to emit a white light. In certain embodiments, LEDs  52  may include phosphor based white LEDs, such as single color LEDs coated with a phosphor material, or other wavelength conversion material, to convert monochromatic light to broad-spectrum white light. For example, a blue die may be coated with a yellow phosphor material. In another example, a blue die may be coated with both a red phosphor material and a green phosphor material. The monochromatic light, for example, from the blue die, may excite the phosphor material to produce a complementary colored light that yields a white light upon mixing with the monochromatic light. LEDs  52  also may include multicolored dies packaged together in a single LED device to generate white light. For example, a red die, a green die, and a blue die may be packaged together, and the light outputs may be mixed to produce a white light. Further, LEDs  52  may include ultraviolet (UV) dies with a mix of red, green, blue, or yellow phosphor material. 
     Additional details of illustrative display  14  may be better understood through reference to  FIG. 4 , which is a block diagram illustrating various components and features of display  14 . Display  14  includes LCD panel  40  and LED backlight  38 . LCD panel  40  includes an LCD controller  54  that governs operation of the LCD panel. For example, LCD controller  54  may include one or more driver integrated circuits that receive image data, for example, from a graphics card or controller of device  10 , and output control signals to change the transmissive state of pixels  56  within LCD panel  40 . According to certain embodiments, LCD controller  54  may be located on a driver ledge within the LCD panel  40 , while the pixels  56  may be located within an active area of the LCD panel  40  that is visible to a user. Further, in certain embodiments, a flexible circuit (i.e. a flex cable) may be used to connect LCD controller  54  to the I/O controller  42  ( FIG. 1 ) of electronic device  10 . 
     LED backlight  38  includes an LED controller  58  that governs operation of light source  50 . In particular, LED controller  58  includes one or more drivers  60  that power and drive strings  62  of LEDs  52  mounted within backlight  38 . Each string  62  includes LEDs  52  that emit light of a similar color and/or brightness. Specifically, LEDs  52  may include groups of LEDs selected from different bins defining properties of the LEDs, such as color or chromaticity, flux, and/or forward voltage. LEDs  52  from the same bin may be joined together in one or more strings  62 , with each string being independently driven by a separate driver  60  or driver channel. Each display  14  may have a target white point, represented by a set of chromaticity coordinates, tristimulus values, or the like. The same target white point may be used across similar devices, and each device may be calibrated to emit the target white point so that similar devices all emit a uniform white point. 
     Drivers  60  may include one or more integrated circuits that may be mounted on a printed circuit board and controlled by LED controller  58 . In certain embodiments, drivers  60  may include multiple channels for independently driving multiple strings  52  of LEDs with one driver  60 . Drivers  60  may include a current source, such as a transistor, that provides current to LEDs  62 , for example, to the cathode end of each LED string. Further, the drivers  60  may include components, such as resistors, amplifiers, and field effect transistors, for regulating the current provided to LEDs  62 . Drivers  60  also may include voltage regulators. In certain embodiments, the voltage regulators may be switching regulators, such as pulse width modulation (PWM) regulators. 
     LED controller  58  may set the driving strengths of drivers  60  to certain driving strengths that enable display  14  to emit the target white point. Specifically, LED controller  58  may send control signals to drivers  60  to vary the current and/or the duty cycle to LEDs  52 . For example, LED controller  58  may provide forward current reference signals (e.g., in the form of control voltages) to drivers  60  to adjust the amount of current passing through strings  62 . In another example, LED control  58  may vary the PWM duty cycle of drivers  60 . 
     LED controller  58  may determine the driving strengths at which to set drivers  60  using information stored in memory  64 . For example, LED controller  58  may use calibration values  66  stored in memory  64  in conjunction with calibration logic  68  to determine the driving strength for each driver  60 , or driver channel. Calibration values  66  describe chromaticity and/or brightness properties of LED strings  62  that can be used to determine the driving strengths for producing the target white point. For example, according to certain embodiments, calibration values  66  may represent the chromaticities and/or brightness of each LED string  62  included within backlight  38 . In another example, calibration values  66  may represent the chromaticity and/or brightness of mixed light emitted by the combination of LED strings  62 . In yet another example, calibration values  66  may represent the deviation in each string from the target white point, or the deviation in the mixed light from the LED strings  62  from the target white point. 
     The calibration values  66  may be determined by independently testing the LED strings  62  prior to, or after, assembly of LED strings  62  within display  14 , as discussed further below with respect to  FIGS. 6-12 . The chromaticities, or values based on the chromaticities, may then be stored in memory  64  as calibration values  66  that can be employed by LED controller  58  to calibrate the display  14  to emit the target white point. For example, in certain embodiments, a user may program the calibration values  66  into memory  64  during assembly of display  14 . However, in other embodiments, a user may enter the calibration values  66  through a user interface of device  10 , through an I/O port  30 , or through a network connection. 
     LED controller  58  may then employ the calibration values  66  to determine the appropriate driving strengths for each LED string  62 . For example, LED controller  58  may execute calibration logic  64  stored within memory  64  to determine the driving strengths, as discussed further below with respect to  FIGS. 10 and 14 . According to certain embodiments, calibration logic  64  may include hardware and/or software control algorithms or instructions that can be executed by LED controller  58  to determine the driving strengths based on calibration values  66 . Further, in certain embodiments, LED controller  58  may employ calibration curves or tables stored in memory  64  in conjunction with calibration logic  64  to determine the driving strengths. 
     According to certain embodiments, memory  64  may be an EEPROM, flash memory, or other suitable optical, magnetic, or solid-state computer readable media. As shown in  FIG. 4 , memory  64  is included within backlight  38  as part of LED controller  58 . However, in other embodiments, memory  64  may be a standalone component included within backlight  38 . Further, in other embodiments, the calibration values  66  and calibration logic  68  may be stored within a memory of LCD panel  40 , such as within a memory of LCD controller  54 , or within a memory of electronic device  10 , such as storage  34  ( FIG. 2 ). 
     After determining the driving strengths, LED controller  58  may then adjust drivers  60  to operate at the determined driving strengths. According to certain embodiments, LED controller  58  may store the determined driving strengths in memory  64 , as base driving strengths that can be employed throughout the operation life of backlight  38 . For example, the chromaticity and brightness of the LEDs  52  may shift over time due to aging or changes in temperature. In certain embodiments, LED controller  58  may be designed to compensate for these shifts by adjusting the driving strength of drivers  60 . In these embodiments, LED controller  58  may use the base driving strengths as a starting point for future driving strength adjustments. 
     As described above with respect to  FIG. 4 , LEDs  52  may be selected from multiple bins, with each bin defining color and/or brightness properties of the LEDs, such as color, brightness, forward voltage, flux, and tint, among others.  FIG. 5  illustrates a representative LED bin chart  70 , such as from a commercial LED manufacturer, that may be used to group LEDs into bins  72 , with each bin of LEDs exhibiting a different white point. Bin chart  70  may generally plot chromaticity values, describing color as seen by a standard observer, on x and y axes  74  and  76 . For example, bin chart  70  may use chromaticity coordinates corresponding to the CIE 1976 UCS chromaticity diagram developed by the International Commission on Illumination (CIE). On bin chart  70 , x-axis  74  may plot the u′ chromaticity coordinates, which may generally progress from blue to red along x-axis  74 , and y-axis  76  may plot the v′ chromaticity values, which may generally progress from blue to green along y-axis  76 . However, in other embodiments, LEDs  52  may be selected from bins represented by other chromaticity diagrams, such as the CIE 1931 chromaticity diagram, which plots the x and y chromaticity coordinates. 
     Each bin represents different chromaticities, and LEDs may be selected from different bins so that when light from the LEDs mixes, a chromaticity close to the target white point is produced. The center bin W may encompass chromaticity values corresponding to the target white point, while the surrounding bins N 1-17  may encompass chromaticity values which are further from the target white point. According to certain embodiments, LEDs may be selected from the neighboring bins N 1-17  on opposite sides of center bin W so that when the light from each of the LEDs  52  is mixed, the emitted light may closely match the target white point. For example, as shown on chart  70 , bin W may encompass the target white point. A backlight employing all bin W LEDs may substantially match the target white point. However, manufacturing costs may be reduced if a larger number of bins are used within a backlight. Accordingly, LEDs from neighboring bins N 1-17 , for example, may be employed within the backlight. The LEDs from the neighboring bins N 1-17  may be selectively positioned within the backlight to produce an output close to the target white point. For example, the LEDs from neighboring bins may be staggered or arranged sequentially throughout backlight  38 . The LEDs from the same bin may be joined on separate strings, so that the driving strength of LEDs from different bins may be independently adjusted to align the emitted light with the target white point. 
     In certain embodiments, LEDs from two or more neighboring bins N 1-17  may be selected and mixed within an LED backlight. For example, a backlight may employ LEDs from complementary bins N 2  and N 6 ; complementary bins N 1  and N 5 ; or complementary bins N 5 , N 3 , and N 8 . Moreover, LEDs from the target white point bin W and from the neighboring bins N 1-12  may be mixed to yield the desired white point. For example, a backlight may employ LEDs from bins W, N 6 , and N 2 . In another example, a backlight may employ multiple strings of LEDs selected from bin W. As may be appreciated, any suitable combination of bins may be employed within a backlight. Further, a wider range of bins than is shown may be employed. 
       FIG. 6  depicts two LED strings  62 A and  62 B that may be employed in backlight  38 . String  62 A includes LEDs  52 A from bin N 1 , and string  62 B includes LEDs  52 B from bin N 5 . As shown, the strings  62 A and  62 B are arranged in parallel, extend from a shared anode, and terminate at separate cathodes  80 A and  80 B. However, in other embodiments, strings  62 A and  62 B may each have separate anodes and cathodes. Further, as shown in  FIG. 6 , each string  62 A and  62 B includes four LEDS  52 A and  52 B, respectively. However, in other embodiments, any number of LEDs may be included on each string. 
     Each string  62 A and  62 B may be tested separately to determine its chromaticity. For example, string  62 A may be driven at a base current, such as 20 mA, while no current is directed to string  62 B. Similarly, string  62 B may be driven at the base current, while not current is direct to string  62 A. Optical sensors, such as phototransistors, photodiodes, or photoresistors, among others, can then be employed to detect the chromaticity of each string  62 A and  62 B. Further, in certain embodiments, optical sensors may be employed to detect the chromaticity of the mixed light produced by operating both strings  62 A and  62 B. However, in other embodiments, the chromaticity of the mixed light from strings  62 A and  62 B, referred to as the “mixed chromaticity,” may be calculated from the individual chromaticities of strings  62 A and  62 B. 
       FIG. 7  is a chart  82  depicting the chromaticities  84 A and  84 B of strings  62 A and  62 B, respectively. As discussed above, the chromaticities  84 A and  84 B may be determined by driving strings  62 A and  62 B, respectively, at the base current and measuring the emitted chromaticity with optical sensors. The chromaticities  84 A and  84 B may be represented by the u′ and v′ coordinates, shown on the x and y axes  74  and  76 , respectively. The target white point  88  lies generally on a line  91  between chromaticities  84 A and  84 B. The chromaticity  86  of the mixed light from strings  62 A and  62 B also lies generally on line  91 . As may be appreciated, the chromaticity of the mixed light may be adjusted to any chromaticity on line  91  by varying the driving strengths of the LED strings. 
     As shown by chart  82 , the mixed chromaticity  86  deviates from the target white point  88  by an amount  90 . However, as discussed further below with respect to  FIGS. 9-10 , the driving strengths of strings  62 A and  62 B can be adjusted to align mixed chromaticity  86  with the target white point  88 . For example, since chromaticity  84 A is closer to the target white point  88 , the driving strength of string  62 A may be increased, relative to the driving strength of string  62 B, to bring the mixed chromaticity  86  closer to the target white point  88 . As the current through the strings  62 A and  62 B increases, the overall brightness of backlight  38  also may increase. Accordingly, the ratio of the driving strengths may be adjusted, rather than just increasing the driving strength of one string  62 A or  62 B, to align the mixed chromaticity  86  with the target white point  88  while maintaining a relatively constant brightness. 
       FIG. 8  depicts a flowchart of a method  92  for calibrating a display to emit the target white point. Method  92  may begin by testing (block  94 ) each LED string in isolation that may be included in the backlight. For example, as described above with respect to  FIG. 6 , the base current may be applied to each LED string  62  in a sequential manner to individually drive each string  62 , while no current is provided to the other strings. As each string is tested, the chromaticity of each string may be measured (block  96 ) using one or more optical sensors. According to certain embodiments, each string  62  may be tested after the string is installed within display  14 . Accordingly, the measured chromaticities may account for white point shifts that may be introduced by display components, such as the backlight diffuser and thin film transistor layers included within LCD panel  40 . However, in other embodiments, the strings  62  may be tested prior to installation in the display. 
     The measured chromaticity values may then be used to determine (block  98 ) the calibration values. According to certain embodiments, the calibration values may correspond to the measured chromaticity values. For example, as shown in  FIG. 7 , the u′ and v′ coordinates of chromaticity values  84 A and  84 B may be used as the calibration values. In another example, the u′ and v′ coordinates of the mixed chromaticity  86  may be used as the calibration values. In this example, additional information, such as the LED bins used in each string may be included as part of the calibration values. In a further example, the magnitude and direction of the amount  90  of deviation from the target white point  88  may be used as the calibration values. Further, any combination of the preceding information may be used as the calibration values. 
     The calibration values may then be stored (block  100 ) within the display. For example, as described above with respect to  FIG. 4 , the calibration values  66  may be stored within a memory  64  of the LED controller  58  for the backlight  38 . Further, calibration logic  64  for using the calibration values  66  to produce the target white point also may be stored within the memory  64 . Moreover, in other embodiments, the calibration values  66  may be stored within other parts of device  10 , such as within LCD panel  40  or storage  34  ( FIG. 1 ). 
       FIG. 9  is a schematic diagram illustrating operation of LED backlight  38 . The LEDs  52 A and  52 B from strings  62 A and  62 B, respectively, are alternated between one another. Each string  62 A and  62 B is driven by a separate driver  60 A and  60 B, each of which is communicatively coupled to LED controller  58 . As discussed further below with respect to  FIG. 10 , LED controller  58  may employ calibration logic  68  to determine the driving strength for each driver  60 A and  60 B based on the calibration values  66 . LED controller  58  may then transmit control signals to set the driving strength of each driver  60 A and  60 B to the determined driving strength. For example, LED controller  58  may transmit control voltages to drivers  60 A and  60 B to vary the forward current applied to each LED string  62 A and  62 B. In another example, LED controller  58  may vary the duty cycles of drivers  60 A and  60 B. 
       FIG. 10  is a flowchart depicting a method  102  for determining and setting the driving strength of each driver  60 A and  60 B to produce the target white point. Method  102  may begin by retrieving (block  104 ) the calibration values. For example, LED controller  58  may retrieve the calibration values  66  from memory  64 . In another example, LED controller  58  may retrieve the calibration values from storage  34  ( FIG. 1 ) or from LCD controller  54 . The LED controller may then determine (block  106 ) the target white point. In certain embodiments, the target white point may be stored within memory  64  as part of the calibration values  66 . In these embodiments, the LED controller  58  may retrieve the target white point as part of the calibration values  66 . However, in other embodiments, the LED controller  58  may retrieve the target white point from the storage  34  ( FIG. 1 ) or from the LCD controller  54  ( FIG. 4 ). 
     LED controller  58  may then determine (block  108 ) the driving strengths for the LED strings included within the backlight. In particular, LED controller  58  may use the calibration logic  68  to calculate the driving strengths based on the calibration values  66 . For example, in embodiments where the calibration values  66  represent the chromaticities of each string of LEDs, LED controller  58  may employ the calibration logic  68  to determine the ratios that should exist between the driving strengths to produce the target white point. According to certain embodiments, LED controller  58  may determine the deviation in the chromaticity for each string of LEDs from the target white point and calculate the driving strength ratios based on the deviations. After determining the ratios, LED controller  58  may scale the driving strengths for each string of LEDs to produce the desired ratios. 
     In another example, in embodiments where the calibration values  66  represent the mixed chromaticity, LED controller  58  may employ the calibration logic  68  to compare the mixed chromaticity to the target white point and determine the amount of driving strength adjustment that should produce the target white point. In a further example, in embodiments where the calibration values  66  represent the magnitude and direction of deviation in the mixed chromaticity from the target white point, LED controller  58  also may employ the calibration logic to determine the driving strength adjustments that should produce the target white point. LED controller  58  may then apply the driving strength adjustments to the default driving strength settings for each driver  60  to determine the specific driving strengths. 
     After determining the driving strengths, the LED controller  58  may then set (block  110 ) the drivers  60  to the determined driving strengths. For example, the LED controller  58  may transmit control signals to the drivers  60  to adjust the amount of forward current applied to the LED strings. In another embodiment, LED controller  58  may transmit control signals to drivers  60  to vary the PWM duty cycle. 
     Although methods  92  and  102 , shown in  FIGS. 8 and 10 , respectively, have been described above in the context of a backlight that employs two strings of LEDs, these methods also may be employed in backlights employing three or more strings of LED.  FIG. 11  depicts an embodiment of a backlight that employs three LED strings  62 C,  62 D, and  62 E. Each string  62 C,  62 D, and  62 E employs LEDs  52 C,  52 D, and  52 E from a different bin. For example, LEDs  52 C may be from bin N 5 , LEDs  52 D may be from bin N 2 , and LEDs  52 E may be from bin N 8 . The LEDs  52 C,  52 D, and  52 E are alternated sequentially along backlight  38 . 
     Each string  62 C,  62 D, and  62  is driven by a separate driver  60 C,  60 D, and  60 E, each of which is communicatively coupled to LED controller  58 . The backlight may be assembled using method  92  described above with respect to  FIG. 8 , and calibration values representing the chromaticity of each string  62 C,  62 D, and  62 E may be stored within memory  64 . As discussed above with respect to  FIG. 10 , LED controller  58  may employ calibration logic  68  to determine the driving strength for each driver  60 C,  60 D, and  60 E based on the calibration values  66 . LED controller  58  may then transmit control signals to set the driving strength of each driver  60 C,  60 D, and  60 E to the determined driving strength. 
       FIG. 12  is a chart  111  depicting the chromaticities  84 C,  84 D, and  84 E of strings  62 C,  62 D, and  62 E, respectively, when the strings are driven at the base current. As shown by chart  111 , the target white point  88  lies within a triangle  113  formed by the chromaticities  84 C,  84 D, and  84 E. By varying the driving strengths of strings  62 C,  62 D, and  62 E, the mixed chromaticity may be adjusted to any chromaticity encompassed by triangle  113 . Accordingly, employing three strings of LEDs may allow a greater range of adjustment in the mixed chromaticity. At the base current, the mixed chromaticity  112  lies slightly above and to the right of the target white point  88 . However, method  102  may be employed as described above with respect to  FIG. 10  to adjust the driving strengths so that the mixed chromaticity matches the target white point. 
       FIGS. 7-12  depict embodiments where the driving strengths of LED strings within the backlight may be adjusted to produce the target white point. However, in certain embodiments, the LCD panel also may be adjusted to produce the target white point for the display. In particular,  FIGS. 13-15  depict embodiments where LCD panel adjustments may be employed in addition to driving strengths adjustments in the backlight. The LCD panel adjustments may be particularly beneficial where additional adjustments are desired in addition to those that can be achieved by varying the LED driving strengths. The LCD panel adjustments also may be beneficial where the LED driving strength adjustments by themselves may produce less desirable results, such as a display that may be too dim. 
       FIG. 13  is a chart  115  depicting the chromaticities  84 F and  84 G of two different LED strings, when the strings are driven at the base current. As shown by chart  115 , the mixed chromaticity  114  lies on a line  117  that extends between chromaticities  84 F and  84 G. As may be appreciated, the mixed chromaticity may be adjusted to any chromaticity that lies generally along line  117  by adjusting the driving strengths of the LED strings. However, target white point  88  lies above line  117  by a distance  116 . Accordingly, additional adjustments may be desired to produce the target white point on display  14 . As discussed further below, the additional adjustment may be provided through hardware and/or software modifications to LCD panel  44 . 
       FIG. 14  is a flowchart of a method  118  for adjusting the emitted white point by modifying operation of the LCD panel  44 . Method  118  may begin by retrieving (block  120 ) calibration values and by determining (block  122 ) the target white point, in a manner as described above with respect to blocks  104  and  106  of  FIG. 10 . For example, the calibration values and the target white point may be retrieved from memory  64  ( FIG. 11 ), storage  34  ( FIG. 1 ), or from LCD panel  44 . The LCD controller  58  may then determine (block  124 ) the deviation of the mixed chromaticity from the target white point. For example, as shown in  FIG. 13 , LED controller  58  may employ calibration logic  68  to determine the chromaticity difference between the mixed chromaticity  114  and the target white point  88 . 
     LED controller  58  may then determine (block  126 ) the driving strengths for the respective LED strings that will more closely align the mixed chromaticity  114  with the target white point  88 . The driving strengths may generally be determined as described above with respect to block  108  of  FIG. 10 . However, rather than determining the driving strengths that will align the mixed chromaticity with the target white point  88 , the LED controller  58  may determine the driving strengths that will bring the mixed chromaticity closes to the target white point  88 . In other words, in this embodiment, while the driving strength adjustments may allow the mixed chromaticity to approach the target white point, further adjustment may be desired to align the mixed chromaticity with the target white point. LCD controller  58  may then set (block  128 ) the drivers  60  to the determined driving strengths. For example, the LED controller  58  may transmit control signals to the drivers  60  to adjust the current or duty cycles of the drivers  60 , as described above with respect to block  110  of  FIG. 10 . 
     LED controller  58  may then determine ( 130 ) the LCD adjustment required to align the mixed chromaticity with the target white point. For example, LED controller  58  may determine a gamma correction that should be applied to pixels  56  ( FIG. 4 ) of LCD panel  14 . In particular, LED controller  58  may determine the amount and type of gamma correction. In the illustrated embodiment, the target white point  88  lies above the mixed chromaticity in the green direction, as shown in  FIG. 13 . Accordingly, LED controller  58  may employ the calibration logic  64  to determine that LCD panel  40  should be shifted in the green direction. However, in other embodiments, depending on the difference between the mixed chromaticity and the target white point, the LCD panel  40  may be shifted in the red or blue direction. 
     LED controller  58  may then set (block  132 ) the LCD adjustment. For example, LED controller  58  may transmit a control signal to LCD controller  54  ( FIG. 4 ) that indicates the type and amount of gamma correction. LCD controller  54  may then perform the gamma correction. For example, in the illustrated embodiment, LCD controller  54  may increase the voltage for the green pixels. However, in other embodiments, LCD controller  54  may adjust the voltage for the green pixels, the red pixels, and/or the blue pixels depending on the type of adjustment that is desired. For example, the voltage of the green pixels may be increased so that these pixels are brighter than the red and blue pixels, which in turn shifts the white point in the green direction. Further, in certain embodiments, the ratios of the voltages between the red, green, and blue pixels may be adjusted to shift the white point while maintaining a constant brightness. Moreover, in other embodiments, LED controller  58  may operate in conjunction with LCD controller  54  to determine (block  130 ) the LCD adjustment. For example, in certain embodiments, LCD controller  54  may determine the type and amount of gamma correction that should be employed based on data received from LED controller  58 . 
       FIG. 15  is a flowchart depicting a method  134  for assembling a backlight where a hardware adjustment may be made to the LCD panel to allow the backlight to be calibrated to the target white point. Method  134  may begin by testing (block  136 ) each string separately, measuring (block  138 ) the chromaticity for each string, and determining (block  140 ) the calibration values, in a manner as described above with respect to blocks  94 ,  96 , and  98  of  FIG. 8 . Method  134  may then continue by determining (block  142 ) an LCD hardware adjustment that will allow the mixed chromaticity to align with the target white point. For example, a technician may determine the range of chromaticity adjustments that can be achieved by varying the driving strengths of the LED strings to allow the mixed chromaticity to approach the target white point. The technician may then determine a direction and amount of additional adjustment that is needed to allow the mixed chromaticity to align with the target white point. For example, as shown in  FIG. 13 , a technician may determine that the line  117  connecting the measured chromaticities lies a distance  116  below the target white point. The technician may then identify an LCD adjustment that can compensate for the distance  116  from the target white point. According to certain embodiments, the LCD adjustment may include shaping a color mask around red, green, or blue pixels, including a more reflective layer around red, green, or blue pixels, including a greater number of red, green, or blue pixels in LCD panel  40 , or applying a voltage setting. The calibration values may then be stored (block  146 ), in a manner similar to that described above with respect to block  100  of  FIG. 8 . 
     The specific embodiments described above have been shown by way of example, and it should be understood that these embodiments may be susceptible to various modifications and alternative forms. It should be further understood that the claims are not intended to be limited to the particular forms disclosed, but rather to cover all modifications, equivalents, and alternatives falling within the spirit and scope of this disclosure.

Metadata:
Filing Date: 20111005
Publication Date: 20180410
Grant Date: 20180410
Priority Date: 20111005
Inventors: GETTEMY SHAWN ROBERT
WURZEL JOSHUA GREY
GUILLOU JEAN-PIERRE SIMON
XU MING
DOYLE DAVID ANDREW
Assignee: APPLE INC
CPC Classifications: [{"code": "G09G2360/145", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0666", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/3406", "inventive": true, "first": true, "tree": "[]"}, {"code": "G09G2320/0646", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0666", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/3406", "inventive": true, "first": true, "tree": "[]"}, {"code": "G09G2360/145", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0646", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 47008684