Patent Publication Number: US-2011063214-A1

Title: Display and optical pointer systems and related methods

Description:
RELATED APPLICATIONS 
     This application claims priority to the following co-pending provisional applications: U.S. Provisional Patent Application Ser. No. 61/273,518 filed Aug. 5, 2009 by David J. Knapp and entitled “Display and Optical Pointer Systems and Related Methods;” U.S. Provisional Patent Application Ser. No. 61/273,536 filed Aug. 5, 2009 by David J. Knapp and entitled “Display Calibration Systems and Related Methods;” U.S. Provisional Patent Application Ser. No. 61/277,871 filed Sep. 30, 2009 by David J. Knapp and entitled “LED Calibration Systems and Related Methods;” U.S. Provisional Patent Application Ser. No. 61/281,046 filed Nov. 12, 2009 by David J. Knapp and entitled “LED Calibration Systems and Related Methods;” U.S. Provisional Patent Application Ser. No. 61/336,242 filed Jan. 19, 2010 by David J. Knapp and entitled “Illumination Devices and Related Systems and Methods;” and U.S. Provisional Patent Application Ser. No. 61/339,273 filed Mar. 2, 2010 by David J. Knapp, et al., and entitled “Systems and Methods for Visible Light Communication;” each of which is hereby incorporated by reference in its entirety. 
     This application is also a continuation-in-part application of the following co-pending application: U.S. patent application Ser. No. 12/803,805 filed on Jul. 7, 2010 by David J. Knapp and entitled “Intelligent Illumination Device;” which in turn claims priority to U.S. Provisional Patent Application Ser. No. 61/224,904 filed on Jul. 12, 2009 by David J. Knapp and entitled “Intelligent Illumination Device;” each of which is hereby incorporated by reference in its entirety. This application is also a continuation-in-part application of the following co-pending patent applications: U.S. patent application Ser. No. 12/360,467 filed Jan. 27, 2009 by David J. Knapp and entitled “Fault Tolerant Network Utilizing Bi-Directional Point-to-Point Communications Links Between Nodes;” and U.S. patent application Ser. No. 12/584,143, filed Sep. 1, 2009 by David J. Knapp and entitled “Optical Communication Device, Method and System;” which in turn claims priority to U.S. Provisional Patent Application Ser. No. 61/094,595 filed on Sep. 5, 2008 by David J. Knapp and entitled “Optical Communication Device, Method and System;” each of which is hereby incorporated by reference in its entirety. 
     This application is also related to the following concurrently filed patent applications: U.S. patent application Ser. No. ______ filed Aug. ______, 2010 by David J. Knapp and entitled “Display Calibration Systems and Related Methods;” U.S. patent application Ser. No. ______ filed Aug. ______, 2010 by David J. Knapp and entitled “LED Calibration Systems and Related Methods;” U.S. patent application Ser. No. ______ filed Aug. ______, 2010 by David J. Knapp and entitled “Illumination Devices and Related Systems and Methods;” U.S. patent application Ser. No. ______ filed Aug. ______, 2010 by David J. Knapp and entitled “Broad Spectrum Light Source Calibration Systems and Related Methods;” and U.S. patent application Ser. No. ______ filed Aug. ______, 2010 by David J. Knapp, et al., and entitled “Systems and Methods for Visible Light Communication;” each of which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The inventions relate to light emitting diodes (LEDs) and systems and methods that utilize LEDs. 
     BACKGROUND 
     Since the invention of the computer this past century and now, user interfaces have continually improved. The currently accepting mainstream approach is different for different types of displays, devices, and applications. Handheld devices and kiosks for applications, such as ATM machines and grocery store checkouts, have migrated to touch screens. The ubiquitous mouse in all its variations dominates desktop computing while the touch pad has become most popular for laptop computers. The common theme amongst all these devices and applications is the ability of the user to interact directly with on screen displays and graphical user interfaces. 
     While computer GUIs continue to evolve and improve, the home television state of the art remains with a handheld remote with many confusing buttons and an infrared unidirectional communication link. As TVs get smarter, more and more are implementing graphical interfaces, however, the user must scroll through menus using the remote instead of pointing at a particular item and clicking. This makes using the remote more complicated and confusing, and can result in the user being frustrated. 
     With the current revolution in LED technology, which is dramatically improving optical output power and reducing costs, displays made from large arrays of red, green, and blue LEDs are becoming more and more popular for applications such as billboards, stadium score boards, and general signage. Such displays, however, typically have no interactivity with a user or an observer, such as a potential customer. 
     TVs and PC LCD displays are transitioning from Cold Cathode Fluorescent Lights (CCFL) to LED backlighting. PCs with relatively smaller displays have LEDs arranged along one side to shine light into a diffusing layer that produces uniform illumination across the back of the LCD. New large screen TVs have arrays of LEDs for uniform lighting and for local dimming. Local dimming provides darker shades which improves the contrast ratio. Some TVs (Samsung) use white LEDs while others (Sony) use combinations of red, green, and blue. The user interface is still a remote controller that cannot control the position of a cursor to select items from an on screen display. The buttons on the remote move a highlighted box around the screen in a clumsy way. A need exists to provide a solution. 
     Billboards and other types of advertising signs are transitioning to array of LEDs. Such displays are becoming popular to provide moving images and to timeshare multiple advertisements on one display. Full motion high definition video displays at sports arenas are also be coming more popular for instant replays and advertising. Such displays have no interactivity with fans or potential customers. With a device that could point at an advertisement to get more information or to order something or provide feedback on a bad call, advertisers could increase their revenues and sign owners can charge more for their sign. A need exists for a means to communicate with such displays. 
     SUMMARY OF THE INVENTION 
     Display and optical pointer systems and related methods are disclosed that utilize LEDs in a display device to respond to optical signals from optical pointing devices. Various embodiments are described with respect to the drawings below. Other features and variations can also be implemented, if desired, and related systems and methods can be utilized, as well. 
     In part, the disclosed embodiments relate to displays with arrays of LEDs and associated pointing devices that communicate with individual LEDs in the arrays using visible light. The LED arrays can produce images directly as in LED billboards and sports arena scoreboards or can produce the backlight for LCD screens for instance. The pointing devices communicate with individual pixels or groups of pixels using a beam of light that may or may not be modulated with data, which is detected by the LEDs in the array that are exposed to the beam. Periodically, the LEDs in an array stop producing light and are configured with an associated driver device to detect light from the pointing device. Such a configuration enables the user to point and click at on screen displays much like a computer mouse. 
     One improved system, as described herein, uses an optical pointing device, such as a laser pointer or flashlight, to control a graphical user interface for instance, on an LCD display with LED backlights or a display made from an array of organic or conventional LEDs. As an image is scanned across such a display, there are times every frame when the LEDs are not producing light. During such light off times, the LEDs are used to detect the presence or absence of light from the optical pointing device. The graphics controller processes such information over a series of frames to detect a pattern of light from the pointing device illuminating a particular location on the display and takes the appropriate action. Such action could be among other things selecting an item in a menu, dragging and dropping an item, or popping up a menu. 
     The simplest pointing device could be a laser pointer or flashlight with a single on/off button. With a display playing a video or a television broadcast for instance, the display could pop up a main menu over part of the screen in response to a bright spot of light detected anywhere on the display. Once the spot is positioned over a particular item in the menu, such as change channel for a television; and then turned off and on again, the appropriate action could be taken. With a display, such as a billboard, advertising merchandise for instance, patterns of light on and off from a pointing device could cause the display to provide more information about a particular item. These are just some examples of interactions between a display and a pointing device, with many more possible. 
     With more sophisticated pointing devices and displays, data could be communicated from the pointing device to the display and potentially even from the display to pointing device. For instance, a laser pointer specially modified to produce light modulated with data could transmit personal information, such as an email address, to a display, such as a billboard. A user could instruct the display to send more information to an email address in this example. Again, this example is just one of many possible data communication applications. 
     The types of displays addressed herein include any that use LEDs for illumination, but typically can be divided into three categories; Organic LED (OLED) displays, conventional LED displays, and liquid crystal displays (LCDs) with LED backlighting. OLED displays typically comprise a piece of glass with thin film transistors and LEDs made from organic compounds grown on one side to produce an array of pixels typically comprising red, green, blue, and white sub-pixels. Each sub-pixel typically has a current source made from the thin film transistors that is controlled by column and row drivers typically situated on two sides of the perimeter of the glass. The row drivers typically produce a logic level write signal to a row of pixels or sub-pixels while the column drivers produce an analog voltage corresponding to the desired sub-pixel current. Such voltage is typically stored on a capacitor in each pixel or sub-pixel. 
     Video images are typically displayed one row at a time as the row drivers sequence the write signals to the OLED array typically from the top to bottom of the array. Moving images are produced a series of still images or frames displayed over time. As one image is displayed one row at a time, the previous image is removed one row at a time. To prevent the well known visual effect called “motion blur”. Every row of LEDs is turned off for a period of time, which removes the previous frame, before displaying a line of the current frame. A high speed snapshot of an OLED display properly designed to reduce motion blur will show a band of LED rows illuminated with the rest of the display is dark. The row drivers typically write to each row of pixels or sub-pixels twice per frame in order to turn the LEDs on and then off. 
     The spot on the display illuminated by the pointing device is detected one row at a time. According to one embodiment, the row drivers produce sense signals sequentially to each pixel or sub-pixel row at some offset from the rows producing light to prevent optical crosstalk from the rows producing light to the row detecting light. When a sense signal is active, each sub-pixel in the row can produce a current if the voltage induced across the associated LED by incident light is greater than a certain level, which can then be detected across the columns by current sense circuitry associated with the column drivers. The graphics controller monitors the current sense circuitry output for each row over a frame to determine the location of the illumination from the pointing device, and over many frames to determine the action to take. 
     Although OLED pixels typically comprise a number of different colored LEDs, such as red, green, blue, and white, typically only one color is used to detect the illumination from the pointing device. For instance, if a red laser pointer is used as the pointing device, the red sub-pixels in the display are used to detect the illumination. If a flashlight producing white light is the pointing device, the red or green sub-pixels in the display may be used to detect the illumination depending on the spectrum of the white light. 
     Displays made from conventional LEDs, which typically comprise of the element Gallium and are individually packaged, typically are very large and are used for billboards or video displays in sports stadiums. As with small OLED displays, each pixel typically comprises red, green, and blue sub-pixels, but typically do not have white sub-pixels. Each sub-pixel LED typically is driven by a current source from an LED driver IC (integrated circuit), which typically comprises a number of current sources associated with a number of sub-pixels. Such ICs can be serially connected together and through a network interface IC to a graphics controller, which produces the pixel data, receives the location of the illumination from the pointing device, and takes the appropriate action. 
     Each driver IC comprises a current source controlled by a pulse width modulator to produce light from each associated LED, and a comparator to detect light incident on each LED. Unlike the OLED display, each LED is driven with a fixed current for a variable amount time, instead of a variable current for fixed amount of time. The pulse width modulator associated with each LED receives a digital value from the graphics controller each frame and turns on the associated current source for a proportional amount of time. The maximum digital value corresponds to a maximum amount of time the current source can be on, which should be less than a frame period to prevent motion blur. 
     During the time between frames when the current source associated with a particular LED is guaranteed to be off, the illumination from the pointing device can be detected. If the voltage induced across the LED by incident light is greater than a certain value, the associated comparator output goes high indicating the presence of light from the pointing device. If the induced voltage is less than the certain amount, the comparator output is low indicating the absence of light. The state of all the comparator outputs is communicated back to the graphics controller for processing. 
     Like the OLED display, a conventional LED display is typically scanned one row or column at a time, which at any one time produces a band of illuminated LEDs across the display. The rest of the display is dark. To prevent optical crosstalk from LEDs producing light to LEDs detecting light, each LED driver IC typically samples the comparator outputs when the associated LEDs are located near the middle of the dark region. 
     Liquid crystal displays modulate the amount of light produced by a backlight to create an image on the screen. Backlights comprising LEDs typically come in one of two versions. For smaller displays on a laptop computer for instance, LEDs situated along one side of the display inject light into a diffuser that produces uniform white light across the display. For large screen televisions using LED backlights, the LEDs are typically arranged in an array, like the conventional LED display, behind the liquid crystal pixel array. The amount of light produced by each LED or group of LEDs can be adjusted per frame to increase the contrast ratio in a manner called “local dimming”, which not possible for LCDs with fluorescent backlights or LED backlights situated along one side of the display. 
     LED backlights for LCDs typically comprise of either white LEDs, which are made from blue LEDs with a yellow phosphor coating, or a combination of red, green, and blue LEDs, for instance. One embodiment uses colored LEDs configured in an array, like a conventional LED display, for LCD backlighting. 
     A liquid crystal pixel array typically comprises a thin film transistor and capacitor associated with each liquid crystal sub-pixel. The transparency of the liquid crystal sub-pixel is determined by the voltage held on the capacitor and is controlled by the associated row and column drivers. Like the OLED display, the liquid crystal array is typically written one row at a time when the associated logic level write signal goes active. The analog voltages from the column drivers are then transferred on to the capacitors through the transistors in each pixel element in the row. Typically, this analog voltage is held for one frame period, until that row is programmed with data for the next frame. 
     To reduce motion blur, the backlight array can be scanned so that the display only produces light from any given row for a portion of a frame period. A band of light produced by the LED backlight array follows the updating of the liquid crystal rows by a fixed offset to allow the liquid crystal element time to settle. The LEDs in the backlight array can be connected to the same driver ICs described for conventional LED displays, which produce a fixed current for a variable amount of time to produce light from the LEDs and monitor the voltage induced across the LEDs by incident light to detect the illumination from the pointing device. 
     Just like the conventional LED display, the LED backlight array could detect light from the pointing device when each row of LEDs is not producing light. However, if the image being displayed is very dark, then the liquid crystal elements will block light both from and to the backlight. During such scenes, the LED array may not be able to detect the light from the pointing device. To improve this sensitivity, each liquid crystal row could be set to fully transparent for some period of time prior to being programmed with data for the next image, which would create a band of transparent liquid crystal following the band of light from the backlight with some fixed offset. Behind this transparent band, the LEDs, which are not producing light, could detect light from the pointing device. Such a system typically requires the liquid crystal array to be written twice as often or requires additional circuitry and signals in each pixel element, and could degrade the contrast ratio due to light leakage from the backlight through the transparent band. 
     One embodiment maintains high contrast ratio and lower liquid crystal update rates, prevents motion blur, and detects signals from the pointing device by inserting a short dark frame between image frames. At the end of each frame, the entire backlight is first turned off, and then the entire liquid crystal array is set to be fully transparent by enabling all row write signals simultaneously and holding all column data signals to the voltage associated with transparency. While the liquid crystal is transparent, the driver ICs monitor the voltage induced across the connected LEDs to detect illumination from the pointing device, and report the results to the graphics controller. Finally, the entire liquid crystal array is set to be opaque, by enabling all row write signals simultaneously and holding all column data signals to the voltage associated with opaque, just prior to scanning the next frame. 
     The improved display and pointer systems described herein address issues with displays using LEDs directly or as backlights for illumination. Bulky and confusing television remote controllers can be replaced by a simple laser pointer or flashlight, and advertiser&#39;s effectiveness can be improved by providing audiences an interactive experience. 
     In one embodiment, the invention is a display device that includes an array of LEDs configured to emit light wherein the array of LEDs is further configured to be responsive to light produced by a pointing device. In a further embodiment, the light produced by individual LEDs in the array of LEDs can be configured to be momentarily turned off to detect the light produced by the pointing device. In a further embodiment, the light produced by the pointing device can be detected by measuring a voltage induced across an individual LED in the array by the light from the pointing device. In a further embodiment, light produced by the pointing device can be detected by measuring a voltage induced across an individual LED in the array by the light from the pointing device. 
     In a further embodiment, the array of LEDs can be configured to provide a backlight for an LCD display. In a further embodiment, the array of LEDs within the display can be made from organic LEDs. In a further embodiment, the array of LEDs can also be made from discreet LED components. In a further embodiment, a menu can be produced on the display device in response to light from the pointing device. In a further embodiment, the array of LEDs can be responsive to light produced by the pointing device that is modulated with data. Still further, the array of LEDs can be responsive to on and off sequences of light from the pointing device. Still further, the array of LEDs is responsive to a laser pointer as the pointing device. 
     In another embodiment, the invention is a method for operating a display device that includes providing a display device comprising an array of LEDs that is responsive to light produced by a pointing device, utilizing a pointing device to cause light to be incident on the array of LEDs within the display device, and responding to the incident light with the display device. In a further embodiment, the method can also include turning off the light produced by individual LEDs in the array of LEDs momentarily to detect the light produced by the pointing device. In a further embodiment, the method can also include detecting the light produced by the pointing device by measuring a voltage induced across an individual LED in the array by the light from the pointing device. In a further embodiment, the method can also include detecting the light produced by the pointing device by measuring a voltage induced across an individual LED in the array by the light from the pointing device. 
     In a further embodiment, the method can also include using the LED array to provide a backlight for an LCD display. Still further, the LED array can be made from organic LEDs. Still further, the LED array is made from conventional discreet LED components. In a further embodiment, the method can also include producing a menu on the display device in response to light from the pointing device. In a further embodiment, the method can also include producing with the pointing device light that is modulated with data. In a further embodiment, the method can also include responding to on and off sequences of light from the pointing device. Still further, the method can also include utilizing a laser pointer as the pointing device is a laser pointer. 
     In another embodiment, the invention is a system that includes a pointing device configured to output light, and a display device comprising an array of LEDs that is responsive to the light produced by the pointing device. In a further embodiment, the light produced by individual LEDs in the array of LED are momentarily turned off to detect the light produced by the pointing device. In a further embodiment, light produced by the pointing device is detected by measuring a voltage induced across an individual LED in the array by the light from the pointing device. In a further embodiment, light produced by the pointing device is detected by measuring a voltage induced across an individual LED in the array by the light from the pointing device. In a further embodiment, the display device is configured to produce a menu in response to light from the pointing device. Further, the pointing device can be configured to output that is modulated with data. Still further, the display device can be configured to be responsive to on and off sequences of light from the pointing device. Still further, the pointing device is a laser pointer. 
     As described herein, other embodiments and variations can also be implemented, if desired, and related systems and methods can be utilized, as well. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Other objects and advantages will become apparent upon reading the following detailed descriptions of the different related embodiments and upon reference to the accompanying drawings. It is noted that a number of different related embodiments are described with respect to the drawings. 
         FIG. 1  (Pointer and Display System) is an exemplary system diagram of the display and Pointer. 
         FIG. 2  (System Communication Protocol) is an exemplary system communication protocol. 
         FIG. 3  (OLED Display Block Diagram) is an exemplary block diagram of an Organic LED (OLED) display. 
         FIG. 4  (OLED Pixel Block Diagram) is an exemplary block diagram of an OLED pixel. 
         FIG. 5  (OLED Sub-pixel and Current Sense Circuit Diagrams) is an exemplary circuit diagram of the OLED sub-pixel and current sense circuits. 
         FIG. 6  (OLED Display Timing) is an exemplary OLED display timing diagram. 
         FIG. 7  (LED Display Architecture) is an exemplary LED display architecture. 
         FIG. 8  (Driver IC Block Diagram) is an exemplary LED driver IC block diagram. 
         FIG. 9  (LED Display Timing) is an exemplary LED display timing diagram. 
         FIG. 10  (LED Driver IC Timing) is an exemplary LED driver IC timing diagram. 
         FIG. 11  (LCD Display with LED Backlight Block Diagram) is an exemplary LCD with LED backlight block diagram. 
         FIG. 12  (LCD Pixel and Driver Circuit Diagram) is an exemplary LCD pixel and driver circuit diagram. 
         FIG. 13  (LCD and Backlight Timing) is an exemplary LCD and backlight timing illustration. 
         FIG. 14  (Display Calibration System) is an exemplary system diagram of the display calibration system. 
         FIG. 15  (OLED Display Block Diagram) is an exemplary block diagram of an OLED display. 
         FIG. 16  (OLED Pixel Block Diagram) is an exemplary block diagram of an OLED pixel. 
         FIG. 17  (OLED Sub-pixel and Current Sense Circuit Diagrams) illustrates exemplary OLED sub-pixel and current sense circuit diagrams. 
         FIG. 18  (LED Display Architecture) is an exemplary LED display architecture. 
         FIG. 19  (Driver IC Block Diagram) is an exemplary driver IC block diagram. 
         FIG. 20  (Intensity Correction Matrix Block Diagram) is an exemplary intensity correction matrix block diagram. 
         FIG. 21  (Intensity and Wavelength Correction Matrix Block Diagram) is an exemplary intensity and wavelength correction matrix block diagram. 
         FIG. 22  (IV Sense Block Diagram) is an exemplary current and voltage sense block diagram. 
         FIG. 23  (LCD Display with LED Backlight Block Diagram) is an exemplary LCD display with LED backlight block diagram. 
         FIG. 24  (LCD Pixel and Driver Circuit Diagram) is an exemplary LCD pixel and driver circuit diagram. 
         FIGS. 25A-D  illustrate a first step in an exemplary method for determining the optical power emitted from a group of LEDs using the photo-sensitivity of such LEDs and an additional light source. 
         FIG. 26C-D  illustrate a second step in an exemplary method for determining the optical power emitted from a group of LEDs using the photo-sensitivity of such LEDs and an additional light source. 
         FIG. 27A-D  illustrate a first step in an exemplary method for determining the relative optical power emitted from a group of LEDs using the photo-sensitivity of such LEDs without an additional light source. 
         FIG. 28A-D  illustrate a second step an exemplary method for determining the relative optical power emitted from a group of LEDs using the photo-sensitivity of such LEDs without an additional light source. 
         FIG. 29  is an exemplary block diagram for circuitry to implement the methods illustrated in  FIGS. 25A-D ,  26 A-D,  27 A-D and  28 A-D. 
         FIG. 30  is an exemplary block diagram a color correction matrix that compensates for LED intensity variations. 
         FIG. 31A-C  illustrate an exemplary method to determine the peak emission wavelength of light produced by an LED by measuring the photo-sensitivity of the LED. 
         FIG. 32  is an exemplary block diagram for a color correction matrix that compensates for LED intensity and wavelength variations. 
         FIG. 33  is a simplified example block diagram for a typical LCD. 
         FIG. 34  is a simplified example block diagram for a Field Sequential Color (FSC) LCD. 
         FIG. 35  is a mechanical drawing of an illumination device that uses a silicon photodiode, or other light detecting device, integrated into an LED controller to measure the light produced by red, green, and blue LEDs. 
         FIG. 36  is a block diagram of an exemplary LED controller with integrated photodiode. 
         FIG. 37  is a block diagram of exemplary temperature and photodiode current measurement circuitry using an integrated photodiode. 
         FIG. 38  is an exemplary connection diagram for multiple illumination devices with integrated photodiodes in a display backlight. 
         FIG. 39  depicts a timing diagram for the power supplies to and the light output from an illumination device with an integrated photodiode. 
         FIG. 40  is a mechanical drawing of an illumination device that uses a discreet silicon photodiode, or other light detecting device, to measure the light produced by red, green, and blue LEDs. 
         FIG. 41  is a block diagram of an exemplary LED controller that uses a discreet photodiode to measure the light from LEDs. 
         FIG. 42  is a block diagram of exemplary temperature and photodiode current measurement circuitry using a discreet photodiode. 
         FIG. 43  is an exemplary connection diagram for multiple illumination devices with discreet photodiodes in a display backlight. 
         FIG. 44  depicts a timing diagram for the power supplies to and the light output from an illumination device with a discreet photodiode. 
         FIG. 45  is a block diagram for exemplary color adjustment circuitry. 
         FIG. 46  is a block diagram for exemplary matrix multiplication circuitry. 
         FIG. 47  is a simplified example block diagram for a typical LCD. 
         FIG. 48  is a simplified example block diagram for a Field Sequential Color (FSC) LCD. 
         FIG. 49  an exemplary system diagram of an illumination device and a remote controller. 
         FIG. 50  is an exemplary list of functions performable by an exemplary illumination device. 
         FIG. 51  is an exemplary timing diagram of data communication between the illumination device and the remote controller. 
         FIG. 52  is an exemplary timing diagram of the bit timing and coding scheme for transferring data between the illumination device and the remote controller. 
         FIG. 53  is an exemplary illumination device block diagram. 
         FIG. 54  is an exemplary diagram of a lighting system comprising illumination devices and remote controller. 
         FIG. 55  is an exemplary timing diagram for communication within the light system. 
         FIG. 56  is a diagram of an exemplary data frame for communicating data with the lighting system. 
         FIG. 57  is an exemplary block diagram of an illumination device. 
         FIG. 58  is an exemplary block diagram for a receiver module within an illumination device. 
         FIG. 59  is an exemplary block diagram for a PLL and timing module within an illumination device. 
         FIG. 60  is an exemplary detailed receive timing diagram. 
         FIG. 61  is an exemplary block diagram for color calibration circuitry to set and maintain a precise color emitted by red, green, blue, and white LEDs. 
         FIG. 62  is an exemplary block diagram for circuitry to sense photocurrents from the LEDs. 
         FIG. 63  illustrates exemplary emission spectra of red, green, blue, and white LEDs. 
         FIG. 64  illustrates exemplary differences in white LED emission spectrum. 
         FIG. 65  illustrates exemplary spectral characteristics of red, green, and blue LEDs when operating as light detectors. 
         FIG. 66A-D  illustrate an exemplary first step in an exemplary method to set and maintain a precise color emitted by red, green, blue, and white LEDs. 
         FIG. 67A-D  illustrate an alternative exemplary first step in an exemplary method to set and maintain a precise color emitted by red, green, blue, and white LEDs. 
         FIG. 68A-D  illustrate an alternative exemplary second step in an exemplary method to set and maintain a precise color emitted by red, green, blue, and white LEDs. 
         FIG. 69A-D  illustrates an alternative exemplary third step in an exemplary method to set and maintain a precise color emitted by red, green, blue, and white LEDs. 
         FIG. 70  is a color diagram illustrating an exemplary color adjustment step in the exemplary method to set and maintain a precise color emitted by red, green, blue, and white LEDs. 
         FIG. 71  is an exemplary block diagram for measuring optical power emitted from an LED. 
         FIG. 72  is an exemplary circuit diagram for measuring optical power emitted from an LED with another LED. 
         FIG. 73A-C  illustrates an exemplary method for approximately determining the optical power emitted from a group of LEDs using the photo-sensitivity of such LEDs. 
         FIG. 74A-D  illustrate an exemplary method determining the optical power emitted from a group of LEDs using a light source as a reference. 
         FIGS. 75A-F  illustrate exemplary methods to improve the accuracy of the method illustrated in  FIG. 3 . 
         FIG. 76A-D  illustrate an exemplary method to determine the optical power emitted from a group of LEDs relative to each other. 
         FIG. 77  is an exemplary block diagram for circuitry to implement the methods illustrated in  FIGS. 73A-C ,  74 A-D,  75 A-F, and  76 A-D. 
         FIG. 78  is an exemplary block diagram a color correction matrix that compensated for LED intensity variations. 
         FIG. 79A-C  illustrates an exemplary method to determine the peak emission wavelength of light produced by an LED by measuring the photo-sensitivity of the LED. 
         FIG. 80  is an exemplary block diagram for a color correction matrix that compensates for LED intensity and wavelength variations. 
     
    
    
     While the embodiments are susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the inventions to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present embodiments. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Various embodiments are described herein that utilize light emitting diodes (LEDs) for emitting light, for receiving light from light sources, for detecting light emissions and for various other purposes and applications. While the following eight embodiments describe different aspects for the use of LEDs, they are also related. As such, the disclosed embodiments can be combined and utilized with respect to each other as desired. For example, the calibration and detection systems and methods described with respect to the second, third, seventh and eighth embodiments can be utilized with the various illumination devices described herein with respect to all embodiments. It is also noted that the various disclosed embodiments can be utilized in a variety of applications, including liquid crystal displays (LCDs), LCD backlights, digital billboards, organic LED displays, AMOLED (Active Matrix OLED) displays, LED lamps, lighting systems, lights within conventional socket connections, projection systems, portable projectors and/or other display, light or lighting related applications. It is also noted that as used herein “r” designations and subscripts typically refer to the color red, “g” designations and subscripts typically refer to the color green, “b” designations and subscripts typically refer to the color blue, and “w” designations and subscripts typically refer to the color white. 
     It is further noted that as used herein an illumination device is generally intended to include any of a wide variety of devices, systems or other apparatus or assemblies that produce light using one or more light sources, including light sources that are implemented using one or more LEDs. LEDs that can be utilized in the embodiments described herein include conventional LEDs, organic LEDs (OLEDs), and any other desired LED. The illumination devices can be implemented in any desired form and/or application including being used within display devices, LCDs, LCD backlights, digital billboards, organic LED displays, AMOLED (Active Matrix OLED) displays, LED lamps, lighting systems, lights within conventional socket connections, projection systems, portable projectors and/or any other desired application that utilizes light sources, including LED light sources, and LEDs and/or other light detectors to detect emitted light. As such, it should be understood that the embodiments described below provide example applications and implementations and should not be considered as limiting. Rather, the techniques, methods and structures described herein for emitting light, detecting light emissions and adjusting light emissions can be used in any desired device, system or application where light is emitted, detected or adjusted, particularly in combination with the use of LEDs for emitting light, detecting light emissions and/or adjusting light emissions. Further, integrated circuits and/or combinations of integrated circuits and other circuitry, whether discreet or integrated, can be used to implement the techniques, methods and structures described herein, as desired. The integrated circuits and/or other circuitry can be combined with light sources, such as LEDs, to form illumination devices for use with the techniques, methods and structures described herein for emitting light, detecting light emissions and adjusting light emissions. It is further noted that as described herein, an LED can be implemented as a discreet LED, an integrated LED, a set of serially connected LEDs, parallel sets of serially connected LEDs or other combinations of LEDs, as desired, depending upon the application and implementation desired. 
     It is further noted that an illumination device as used herein is generally intended to include any device or apparatus that emits light to illuminate an area or another object with visible light, for example, for purposes of being viewed or seen by human eyes, such as would be provided in or by a lamp, lighting system, display system, OLED panel, LCD panel, projector, billboard and/or any other device or apparatus that produces visible light for purposes of being viewed by human eyes or by some other viewing system as visible light. In this sense, a device or apparatus that uses light solely for communication purposes would likely not be an illumination device as generally used herein. 
     Example embodiments will now be described with respect to the drawings. The first embodiment describes the use of the techniques, methods and structures described herein with respect to display devices and optical pointing systems. The second embodiment describes the use of the techniques, methods and structures described herein with respect to calibration of display systems. The third embodiment describes the use of the techniques, methods and structures described herein with respect to LED calibration. The fourth embodiment describes the use of the techniques, methods and structures described herein with respect to various illumination devices. The fifth embodiment describes the use of the techniques, methods and structures described herein with respect to intelligent LED lights. The sixth embodiment describes the use of the techniques, methods and structures described herein with respect to synchronization of visible light communications. The seventh embodiment describes the use of the techniques, methods and structures described herein with respect to calibration of broad spectrum light emitters including white light emitters. And the eighth embodiment provides a alternative description of techniques, methods and structures for LED calibration. As noted above, these embodiments can be used alone or in combination with each other, as desired, to take advantage of the techniques, methods and structures described herein for emitting light, detecting light emissions, and adjusting light emissions, particularly using LEDs. 
     It is further noted that the operational blocks and circuitry shown and described with respect to the block diagrams depicted herein, for example, in  FIGS. 3 ,  7 ,  8 ,  11 ,  15 ,  18 ,  19 ,  20 ,  21 ,  22 ,  23 ,  29 ,  30 ,  32 ,  36 ,  37 ,  38 ,  41 ,  42 ,  43   45 ,  46 ,  53 ,  57 ,  58 ,  59 ,  61 ,  62 ,  77 ,  78  and  80 , can be implemented using any desired circuitry including integrated circuitry, non-integrated circuitry or a combination of integrated and non-integrated circuitry, as desired. Further, it is noted that programmable or programmed circuitry, such as digital signal processors (DSPs), microprocessors, microcontrollers and/or other programmable or programmed circuitry, can also be used with respect to these blocks. Further, software, firmware or other code can be utilized along with this circuitry to implement the functionality as described herein, if desired. 
     First Embodiment 
     Display and optical pointer systems and related methods are disclosed that utilize LEDs in a display device to respond to optical signals from optical pointing devices. Various embodiments are described with respect to the drawings below. Other features and variations can also be implemented, if desired, and related systems and methods can be utilized, as well. 
     In part, the disclosed embodiments relate to displays with arrays of LEDs and associated pointing devices that communicate with individual LEDs in the arrays using visible light. The LED arrays can produce images directly as in LED billboards and sports arena scoreboards or can produce the backlight for LCD screens for instance. The pointing devices communicate with individual pixels or groups of pixels using a beam of light that may or may not be modulated with data, which is detected by the LEDs in the array that are exposed to the beam. Periodically, the LEDs in an array stop producing light and are configured with an associated driver device to detect light from the pointing device. Such a configuration enables the user to point and click at on screen displays much like a computer mouse. 
     One improved system, as described herein, uses an optical pointing device, such as a laser pointer or flashlight, to control a graphical user interface for instance, on an LCD display with LED backlights or a display made from an array of organic or conventional LEDs. As an image is scanned across such a display, there are times every frame when the LEDs are not producing light. During such light off times, the LEDs are used to detect the presence or absence of light from the optical pointing device. The graphics controller processes such information over a series of frames to detect a pattern of light from the pointing device illuminating a particular location on the display and takes the appropriate action. Such action could be among other things selecting an item in a menu, dragging and dropping an item, or popping up a menu. 
     The simplest pointing device could be a laser pointer or flashlight with a single on/off button. With a display playing a video or a television broadcast for instance, the display could pop up a main menu over part of the screen in response to a bright spot of light detected anywhere on the display. Once the spot is positioned over a particular item in the menu, such as change channel for a television, and then turned off and on again, the appropriate action could be taken. With a display, such as a billboard, advertising merchandise for instance, patterns of light on and off from a pointing device could cause the display to provide more information about a particular item. These are just some examples of interactions between a display and a pointing device, with many more possible. 
     With more sophisticated pointing devices and displays, data could be communicated from the pointing device to the display and potentially even from the display to pointing device. For instance, a laser pointer specially modified to produce light modulated with data could transmit personal information, such as an email address, to a display, such as a billboard. A user could instruct the display to send more information to an email address in this example. Again, this example is just one of many possible data communication applications. 
     The types of displays addressed herein include any that use LEDs for illumination, but typically can be divided into three categories, Organic LED (OLED) displays, conventional LED displays, and liquid crystal displays (LCDs) with LED backlighting. OLED displays typically comprise a piece of glass with thin film transistors and LEDs made from organic compounds grown on one side to produce an array of pixels typically comprising red, green, blue, and white sub-pixels. Each sub-pixel typically has a current source made from the thin film transistors that is controlled by column and row drivers typically situated on two sides of the perimeter of the glass. The row drivers typically produce a logic level write signal to a row of pixels or sub-pixels while the column drivers produce an analog voltage corresponding to the desired sub-pixel current. Such voltage is typically stored on a capacitor in each pixel or sub-pixel. 
     Video images are typically displayed one row at a time as the row drivers sequence the write signals to the OLED array typically from the top to bottom of the array. Moving images are produced a series of still images or frames displayed over time. As one image is displayed one row at a time, the previous image is removed one row at a time. To prevent the well known visual effect called “motion blur”. Every row of LEDs is turned off for a period of time, which removes the previous frame, before displaying a line of the current frame. A high speed snapshot of an OLED display properly designed to reduce motion blur will show a band of LED rows illuminated with the rest of the display is dark. The row drivers typically write to each row of pixels or sub-pixels twice per frame in order to turn the LEDs on and, then off. 
     The spot on the display illuminated by the pointing device is detected one row at a time. According to one embodiment, the row drivers produce sense signals sequentially to each pixel or sub-pixel row at some offset from the rows producing light to prevent optical crosstalk from the rows producing light to the row detecting light. When a sense signal is active, each sub-pixel in the row can produce a current if the voltage induced across the associated LED by incident light is greater than a certain level, which can then be detected across the columns by current sense circuitry associated with the column drivers. The graphics controller monitors the current sense circuitry output for each row over a frame to determine the location of the illumination from the pointing device, and over many frames to determine the action to take. 
     Although OLED pixels typically comprise a number of different colored LEDs, such as red, green, blue, and white, typically only one color is used to detect the illumination from the pointing device. For instance, if a red laser pointer is used as the pointing device, the red sub-pixels in the display are used to detect the illumination. If a flashlight producing white light is the pointing device, the red or green sub-pixels in the display may be used to detect the illumination depending on the spectrum of the white light. 
     Displays made from conventional LEDs, which typically comprise of the element Gallium and are individually packaged, typically are very large and are used for billboards or video displays in sports stadiums. As with small OLED displays, each pixel typically comprises red, green, and blue sub-pixels, but typically do not have white sub-pixels. Each sub-pixel LED typically is driven by a current source from an LED driver IC (integrated circuit), which typically comprises a number of current sources associated with a number of sub-pixels. Such ICs can be serially connected together and through a network interface IC to a graphics controller, which produces the pixel data, receives the location of the illumination from the pointing device, and takes the appropriate action. 
     Each driver IC comprises a current source controlled by a pulse width modulator to produce light from each associated LED, and a comparator to detect light incident on each LED. Unlike the OLED display, each LED is driven with a fixed current for a variable amount time, instead of a variable current for fixed amount of time. The pulse width modulator associated with each LED receives a digital value from the graphics controller each frame and turns on the associated current source for a proportional amount of time. The maximum digital value corresponds to a maximum amount of time the current source can be on, which should be less than a frame period to prevent motion blur. 
     During the time between frames when the current source associated with a particular LED is guaranteed to be off, the illumination from the pointing device can be detected. If the voltage induced across the LED by incident light is greater than a certain value, the associated comparator output goes high indicating the presence of light from the pointing device. If the induced voltage is less than the certain amount, the comparator output is low indicating the absence of light. The state of all the comparator outputs is communicated back to the graphics controller for processing. 
     Like the OLED display, a conventional LED display is typically scanned one row or column at a time, which at any one time produces a band of illuminated LEDs across the display. The rest of the display is dark. To prevent optical crosstalk from LEDs producing light to LEDs detecting light, each LED driver IC typically samples the comparator outputs when the associated LEDs are located near the middle of the dark region. 
     Liquid crystal displays modulate the amount of light produced by a backlight to create an image on the screen. Backlights comprising LEDs typically come in one of two versions. For smaller displays on a laptop computer for instance, LEDs situated along one side of the display inject light into a diffuser that produces uniform white light across the display. For large screen televisions using LED backlights, the LEDs are typically arranged in an array, like the conventional LED display, behind the liquid crystal pixel array. The amount of light produced by each LED or group of LEDs can be adjusted per frame to increase the contrast ratio in a manner called “local dimming”, which not possible for LCDs with fluorescent backlights or LED backlights situated along one side of the display. 
     LED backlights for LCDs typically comprise of either white LEDs, which are made from blue LEDs with a yellow phosphor coating, or a combination of red, green, and blue LEDs, for instance. One embodiment uses colored LEDs configured in an array, like a conventional LED display, for LCD backlighting. 
     A liquid crystal pixel array typically comprises a thin film transistor and capacitor associated with each liquid crystal sub-pixel. The transparency of the liquid crystal sub-pixel is determined by the voltage held on the capacitor and is controlled by the associated row and column drivers. Like the OLED display, the liquid crystal array is typically written one row at a time when the associated logic level write signal goes active. The analog voltages from the column drivers are then transferred on to the capacitors through the transistors in each pixel element in the row. Typically, this analog voltage is held for one frame period, until that row is programmed with data for the next frame. 
     To reduce motion blur, the backlight array can be scanned so that the display only produces light from any given row for a portion of a frame period. A band of light produced by the LED backlight array follows the updating of the liquid crystal rows by a fixed offset to allow the liquid crystal element time to settle. The LEDs in the backlight array can be connected to the same driver ICs described for conventional LED displays, which produce a fixed current for a variable amount of time to produce light from the LEDs and monitor the voltage induced across the LEDs by incident light to detect the illumination from the pointing device. 
     Just like the conventional LED display, the LED backlight array could detect light from the pointing device when each row of LEDs is not producing light. However, if the image being displayed is very dark, then the liquid crystal elements will block light both from and to the backlight. During such scenes, the LED array may not be able to detect the light from the pointing device. To improve this sensitivity, each liquid crystal row could be set to fully transparent for some period of time prior to being programmed with data for the next image, which would create a band of transparent liquid crystal following the band of light from the backlight with some fixed offset. Behind this transparent band, the LEDs, which are not producing light, could detect light from the pointing device. Such a system typically requires the liquid crystal array to be written twice as often or requires additional circuitry and signals in each pixel element, and could degrade the contrast ratio due to light leakage from the backlight through the transparent band. 
     One embodiment maintains high contrast ratio and lower liquid crystal update rates, prevents motion blur, and detects signals from the pointing device by inserting a short dark frame between image frames. At the end of each frame, the entire backlight is first turned off, and then the entire liquid crystal array is set to be fully transparent by enabling all row write signals simultaneously and holding all column data signals to the voltage associated with transparency. While the liquid crystal is transparent, the driver ICs monitor the voltage induced across the connected LEDs to detect illumination from the pointing device, and report the results to the graphics controller. Finally, the entire liquid crystal array is set to be opaque, by enabling all row write signals simultaneously and holding all column data signals to the voltage associated with opaque, just prior to scanning the next frame. 
     The improved display and pointer systems described herein address issues with displays using LEDs directly or as backlights for illumination. Bulky and confusing television remote controllers can be replaced by a simple laser pointer or flashlight, and advertiser&#39;s effectiveness can be improved by providing audiences an interactive experience. 
     As stated above, this first embodiment can also be used with the techniques, methods and structures described with respect to the other embodiments described herein. For example, the calibration and detection systems and methods described with respect to the second, third, seventh and eighth embodiments can be used with respect to the display systems and methods described in this first embodiment, as desired. Further, the various illumination devices, light sources, light detectors, displays, and applications and related systems and methods described herein can be used with respect to display systems and methods described in this first embodiment, as desired. Further, as stated above, the structures, techniques, systems and methods described with respect to this first embodiment can be used in the other embodiments described herein, and can be used in any desired lighting related application, including liquid crystal displays (LCDs), LCD backlights, digital billboards, organic LED displays, AMOLED (Active Matrix OLED) displays, LED lamps, lighting systems, lights within conventional socket connections, projection systems, portable projectors and/or other display, light or lighting related applications. 
     Turning now to the drawings,  FIG. 1  is one example of pointer and display system  10  that comprises the display  11  and pointer  12 . Display  11  comprises light emitting diodes (LEDs) for image illumination either directly in the case of OLED or conventional LED displays, or backlighting in the case of Liquid Crystal Displays (LCDs). LEDs of preferentially different colors, for instance red, green, and blue, produce the wide gamut of colors typically necessary for accurate representation of images either directly in the case of OLED or LED displays, or modulated by an LCD. 
     Pointer  12  preferentially comprises a button  15  that when depressed causes pointer  12  to produce beam  16  and when released removes beam  16 . Beam  16  is preferentially produced by a red laser pointer, but could be any color or combinations of colors including white. Also beam  16  could be produced by an LED or multiple LEDs, an incandescent flashlight, or any other possible source of light. When pointer  12  is aimed at display  11  and button  15  is depressed, beam  16  produces spot  14  on display  11 . Display  11  detects spot  14  and preferentially produces interactive menu  13 . By moving spot  14  around display  11  and depressing and releasing button  15  at appropriate times, system  10  can operate much a computer and a computer mouse. 
     Spot  14  is detected by display  11  preferentially during visually imperceptible times when the LEDs comprising the pixels or backlights are turned off. Beam  16  induces a voltage on those LEDs that are illuminated under spot  14  with the appropriate wavelength, which is detected and processed by the display. Sequences of button  15  clicks in combination with the location of spot  14  enable a user to pop up menus, navigate through a graphical user interface, and drag and drop items among many other things. 
       FIG. 1  is one example of many possible display and pointer systems  10 . For example, pointer  12  can have multiple buttons or no buttons. Beam  16  could be computer generated and controlled for instance, and could be modulated with data to communicate more information to display  11 . Display  11  could modulate light from individual pixels to communicate back to pointer  15 , to another display  11 , or some other electronic device. 
       FIG. 2  is an example of a simple communication protocol for system  10 , which shows the button state  26  of button  15 , the optical output state  27  of beam  16 , and the states  28  of display  11  as a function of time. The high state of button  15  represents the button released, while the low state represents the button depressed. The high state of beam  16  represents light being produced by pointer  12 , while the low state represents no light. Display  11  states S 0  through S 6  represent one of many possible temporal and spatial combinations of spot  14  to select an item from main menu  20 . 
     Display  11  state S 0  represents normal operation of the display, for instance, when displaying a video or a television broadcast. State S 1  is entered time Ton 1  after button  15  is depressed, which produces beam  16  and spot  14 . In state S 1 , main menu  20  is overlaid on the video for instance, which is being played. State S 2  illustrates when spot  14  is positioned by the user over the appropriate main menu  20  item to be selected. Display  11  enters state S 3  when button  15  is released and beam  16  turns off. Provided button  15  is depressed and beam  16  is produced within time Tsel, display  11  enters state S 4 . Display  11  detects the short off time of beam  16  and responds time Ton 2  later with sub-menu  21  for instance in state S 5 . In this example, items from sub-menu  21  are not needed and state S 6  is entered when button  15  is released and beam  16  turns off. Time Toff later, display  11  returns to the normal operating state S 0 . 
     The example protocol illustrated in  FIG. 2  is one of many possible different means to communicate or control display  11 . For instance, button  15  could be double clicked to drag and drop items or different buttons could produce different codes or colors of light to indicate different things. As another possibility, pointer  12 , another display  11 , or another electronic devices could synchronize to the periodic light off periods and communicate high bandwidth data across display  11 . 
       FIG. 3  is an example block diagram of OLED display  11  comprising LED array  33  with R rows and C columns of sub-pixel LEDs typically arranged in pixels of one red, one green, one blue, and one white sub-pixel LED. LED array  33  comprises R/2 rows and C/2 columns of such sub-pixels. Each sub-pixel LED is configured to produce a certain amount of light by a combination of voltages on a particular WR (write) signal produced by row driver  32  and DATA signal produced by column driver  31 . When a WR signal is high, the analog voltage on each DATA signal is programmed into the row of LEDs activated by the particular WR signal. 
     Power supply  35  produces the main power Vdd for LED array  33  and the reference voltages Vr and Vc for detecting spot  14  preferentially on red sub-pixels. When one of the SNS (sense) signals from row driver  32  goes high, the IOUT signals from the LED array  33  source current into current sense  34  for red sub-pixels in the row activated by a particular SNS signal when illuminated by spot  14 . No current is present on the IOUT signals associated with red sub-pixels not illuminated by spot  14 . Current sense  34  produces an SOUT logic level signal in response to each IOUT input, which are detected and processed by graphics and timing control circuitry  30 . Graphics and timing control circuitry  30 , which also produces the timing for row driver  32  and the data for column driver  31 , combines the SOUT inputs with timing to determine precisely which sub-pixels are illuminated by spot  14 . 
       FIG. 3  is just one of many possible block diagrams for display  11 , which could be built using any one of a wide range of technologies including but not limited to discreet inorganic LED arrays or liquid crystals. Likewise, the block diagram for display  11  built with OLEDs could be substantially different. For instance, if LED array  33  comprised more complex pixel and sub-pixel circuitry, such sub-pixels could be calibrated by additional external circuitry to eliminate variations in LED light output and drive current, or multiplexed by a set of enable signals to reduce the sub-pixel circuitry. The block diagrams of such an LED display  11  would be substantially different. 
       FIG. 4  is an example block diagram of OLED pixel  40  in LED array  33  referenced by row coordinates I and I+1, and column coordinates J and J+1, and comprising red sub-pixel  41 , green sub-pixel  42 , blue sub-pixel  43 , white sub-pixel  44 , and comparator  45 . The circuitry in all sub-pixels is the same except the color of the included LED. Red sub-pixel  41  is different only in that the Vled signal is connected to comparator  45 , which compares the voltage on the anode of the red LED to Vr and sources current to IOUT(j/2) when red sub-pixel  41  is illuminated by spot  14  and SNS(i/2) is active. 
     Signals WR(i) and DATA(j) program the light produced by red sub-pixel  41 , signals WR(i) and DATA(j+1) program the light produced by white sub-pixel  44 , signals WR(i+1) and DATA(j) program the light produced by green sub-pixel  42 , and signals WR(i+1) and DATA(j+1) program the light produced by blue sub-pixel  43 . All sub-pixels are powered by VDD. 
       FIG. 4  is just one of many possible pixel  40  block diagrams. For instance, any combinations of colors or just one color could be used. Additionally, LEDs of any or all colors could be used to detect one or more instances of spot  14 , or one or more data communication light channels. All sub-pixels could be accessed by one WR signal and one DATA signal if two enable signals select between the sub-pixels. 
       FIG. 5  is an example circuit diagram for red sub-pixel  41 , comparator  45 , and an individual current sense element in current sense  34  referenced by coordinate J. When producing light, LED  56  is driven by the current through transistor  50 , which is set by the voltage stored on capacitor  55  and the gate of transistor  50 . The voltage on capacitor  55  is set to the voltage on DATA(j) signal when WR(i) signal is high. When WR(i) goes low, capacitor  55  holds the voltage so that DATA(j) can be used to program the current in other rows of sub-pixels when other WR signals go high. All the sub-pixels connected to WR(i) are programmed simultaneously by all the DATA signals when WR(i) is high. 
     To detect light from spot  14 , transistor  50  is first turned off by programming the voltage across capacitor  50  to zero volts or some value less than transistor  50  threshold voltage. Then SNS(i/2) signal goes low to produce a current through transistor  52 , which is steered to ground through transistor  54  when the voltage across LED  56  is less than reference voltage Vr and to IOUT(j/2) through transistor  53  when the voltage across LED  56  is greater than Vr. SNS(i/2) is connected to transistor  52  in all red sub-pixels  41  in the 112 row of LED array  33 . VDD is connected to all sub-pixels and Vr is connected to comparator  45   s  in LED array  33 . All components in pixel  40  are typically processed using thin film technology. 
     Current sourced by red sub-pixel  41  into current sense  34  element J is converted to a voltage by resistor  57  and amplifier  58 , and such voltage is compared to reference voltage Vc through comparator  59 . The voltage induced on LED  56  by spot  14  can vary from a few millivolts to a couple volts. Reference voltage Vr is set to a value high enough to prevent ambient light from causing comparator  45  to source current on IOUT, but low enough for display  11  to detect a spot  14  with low optical power. Voltage settings for Vr could be adjusted dynamically based on the ambient light level incident on display  11 , but typically would reside in the range of 500 mV to 1V. Since the signal Vr is connected to the positive input terminal of amplifier  58 , the voltage of the IOUT is held very close to Vr through feedback resistor  57 . The output of amplifier  58  drops below reference voltage Vr when current is sourced by comparator  45 . Reference voltage Vc is connected to the positive terminal of comparator  59 . When the output of amplifier  58  drops below Vc, current sense  34  output SOUT(j) goes high. The reference voltage Vc should be set to be less than the reference voltage, Vr by an amount sufficient to reject noise. Vc is typically about half Vr. 
       FIG. 5  is one of many possible circuit diagrams for sub-pixels and spot  14  detection. For instance, the sub-pixel circuitry could include the capability to calibrate out variations in transistor  50  threshold voltage or in LED  56  output light. Comparator  45  could include additional transistors to output a voltage instead of a current, or photo generated current instead of voltage from LED  56  could be detected. An additionally signal could be used to turn off the current into LED  56  instead of using the WR(i) signal. Many other circuit configurations are possible. 
       FIG. 6  is an example illustration of display  11  timing for a High Definition (HD) TV with 1080 rows of pixels that shows how images are scanned and spot  14  is detected.  FIG. 6  includes four snapshots  60  of display  11  at times T 0 , T 1 , T 2 , and T 3  within one frame period. A frame is a single image in a sequence that produces a video or motion picture and a frame period is the time from the start of presentation of a first frame to the start of presentation of a second frame. Below the snapshots  60  are detailed timing diagrams  61  for the input and output signals for a red sub-pixel  41  that is illuminated with coordinates (1080,j) and not illuminated with coordinates (1082,j). 
     At time T 0 , frame N begins to be displayed with WR(0) going high and DATA(j) signals containing the analog voltages corresponding to the desired output light power from each sub-pixel in the first row of sub-pixels in LED array  33 . Just prior to WR(0) going high, WR(200) went high with all DATA(j) signals shorted to VDD to turn off all LEDs in all sub-pixels in row  200 . The box labeled “black” and shown in snapshot  60  at T 0  between WR(0) and WR(200), at T 1  between WR(540) and WR(740), at T 2  between WR(1080) and WR(1280) and at T 3  between WR(1620) and WR(1820), represent a region of display  11  that is emitting no light. It is in this region, which repetitively travels down display  11  as shown, that spot  14  is detected. At T 0 , frame N−1 is still displayed below the dark region starting with row  201 . 
     Time T 1  occurs one quarter of a frame period after the start of frame N at which time only the top 25% of frame N is displayed. WR(740) went high to clear another line of frame N−1 and WR(540) went high to display another line of frame N. At time T 2 , the top half frame N is displayed with WR(1080) going high to display another line of frame N and with WR(1280) going high to clear another line of frame N−1. At time T 3 , three quarters of frame N is displayed with WR(1620) going high to display another line of frame N and with WR (1820) going high to clear another line of frame N−1. 
     The timing diagram  61  illustrates the state of the write and sense signal pairs WR(0) and SNS(0), WR(540) and SNS(270), WR(1080) and SNS(540), and WR(1280) and SNS(640) as a function of time over two frame periods, N and N+1. As shown in  FIG. 4 , each pixel  40  has 2 input write signals WR(i) and WR(i+1) and one input sense signal SNS(i/2). Detailed timing diagram  62  expands the region in time from T 2  when WR(1280) clears a line of frame N−1 to the time when WR(1280) goes low again to display another line of frame N. 
     At time T 4  in detailed timing diagram  62 , WR(1280) goes low while all DATA(j) signals are high, which turns the light off from any sub-pixel in row  1280  by discharging capacitor  55  and turning transistor  50  off. The voltages across the red LEDs in the red sub-pixels  41  connected to WR(1280) prior to WR(1280) going high is determined by the currents sourced by transistor  50  in each of the red sub-pixels  41  and can be anywhere from 0 to 2 or 3 volts. Detailed timing diagram  62  illustrates the voltage across one particular red LED that is illuminated by spot  14 . Prior to WR(1280) going high, Vled(1280,j) can be anywhere from 0 to 2 or 3v. When WR(1280) goes high, the voltage relatively slowly drifts towards and intermediate value determined by the optical power of spot  14 . 
     At time T 5 , WR(1280) returns high and WR(1080) goes low with the DATA(j) being driven by column driver  31  with the analog voltages to be programmed into the sub-pixels in row  1080 . At time T 6 , WR(1282) goes low with all DATA(j) signals high, which turns off the current to all of the red sub-pixels  41 , in the next row of pixels  40  below the row connected to WR(1280). Detailed timing diagram  62  also illustrates the voltage across the red LED in a particular red sub-pixel  41  connected to WR(1282) that is not illuminated by spot  14 . Vled(1282,j) goes low shortly after WR(1282) goes high. 
     At time T 7 , sense signal SNS(640), which is connected to the same row of pixels  40  as WR(1280), goes low. This turns comparator  45  on, which compares Vled(1280,j) to the reference voltage Vr. Since Vled(1280,j) is at an intermediate voltage and assuming Vr is properly set below this intermediate voltage, SOUT(j) from current sense  34  goes high. At time T 8 , SNS(640) goes high and SNS(641) goes low, which turns comparator  45  off in the pixel  40  row connected to WR(1280) and on in the pixel row  40  connected to WR(1282). Vled(1282,j) is compared to Vr and since Vled(1282,j) is low, SOUT(j) will go low. 
     At time T 9 , WR(1280) goes low again, but this time with the DATA(j) signals driven to levels by column driver  31 , appropriate to display the red sub-pixel  41  and the white sub-pixel  44  in the 640 th  line of the image in frame N. Vled(1280,j) changes accordingly. At time T 10 , WR(1282) goes low with the DATA(j) signals driven to levels by column driver  31  appropriate to display the red sub-pixel  41  and the white sub-pixel  44  in the 641 st  line of the image in frame N. Vled(1282,j) changes accordingly. 
     The time between WR(1280) going low the first time at T 4  and the second time at T 9  is equal to the time it takes to display 106 pixel  40  rows of the image in frame N. Since this example illustrates the timing for an HD display with 1080 rows, the time from T 4  to T 9  is equal to about 10% of the frame period. At a 60 Hz frame rate, this time is about 1.7 mSec, which is sufficient for Vled(1280,K) to reach its final value. 
     Timing diagram  61  and detailed timing diagram  62  only show a small subset of the signals in an OLED display  11  since there are thousands of such signals. In particular WR(1281) is not shown since it is not connected to a red sub-pixel  41  and therefore not involved is detecting spot  14 . 
       FIG. 6  illustrates one of many possibilities for OLED display  11  timing. Since the block and circuit diagrams could be substantially different from  FIGS. 3 ,  4 , and  5 , the associated signals could be substantially different from those shown in  FIG. 6  and consequently the timing diagrams would be completely different. For the block and circuit diagrams shown in  FIGS. 3 ,  4 , and  5 , the timing shown in FIG. could also be significantly different. For instance, the time from T 4  to T 9  could shorter or much longer, or the sequencing of the WR(i) signals could clear multiple lines of the previous frame and then write multiple lines of the current frame. 
       FIG. 7  is an example architectural diagram for display  11  that uses conventional discreet semiconductor LEDs, which comprises an array LED driver ICs  70  with associated LEDs  71  connected serially to each other and to a network interface (I/F) IC  72 . Network interface IC  72  connects to graphics controller  73  through control and data busses. The array in this example has N columns and M rows of driver ICs  70  each connected to P LEDs  71 . With P equal to 16 and three LEDs per pixel, N and M would equal 120 and 3240 respectively for an HD display with 1920×1080 resolution. For a standard 48 foot by 14 foot bill board with 3 LEDs per pixel, and P equal to 16, N would equal 48 and M would equal 672. 
     LED&#39;s  71  could all be the same color or could be divided between red, green, and blue for instance. For an RGB display, the different colors could be arranged in different ways. One example is to organize the display in groups of 3 rows with each row in each group being a different color. 
     Graphics controller  73  produces the data to be displayed digitally, which is forwarded to network interface IC  72 . Network interface IC  72  serializes the data, which is sent through the chain driver ICs  70  in a time division multiplexed data frame. Each driver IC is assigned specific time slots from which image data is received and information about spot  14  is sent. The data frame repeats at the video frame rate, which enables each driver IC  70  to update the drive current to each LED  71  and to report the presence of spot  14  to graphics controller  73  every video frame. Graphics controller  73  processes the responses from all driver ICs  72  to determine the precise spot  14  location and takes the appropriate action. 
       FIG. 7  is one of many possible architectural diagrams. For instance, each driver IC  70  could be connected directly to graphics controller  73  through a multiplexer either serially or in parallel. The LED drivers could be made from discreet components instead of driver IC  70 . The data for the LED drivers could even be communicated with analog voltages instead of digital values. 
       FIG. 8  is an example block diagram for driver IC  70 , which in this example drives sixteen LEDs  71  and comprises network interface  81 , timing and control circuitry  82 , sixteen output drivers  84 , digital to analog converter (DAC)  85 , buffer amplifier  86 , and current bias  87 . Timing and control circuitry  82  further comprises register  83 . Output driver  84  further comprises pulse width modulator  89 , current source  90 , and comparator  88 . 
     Network interface  81  accepts serial input data from upstream and produces serial data for downstream driver ICs  70  as shown in  FIG. 7 . Network interface  81  further recovers the clock (CK) from the data, and detects and synchronizes to the input data frame timing. Most received serial data is retransmitted, however, data in the assigned timeslots are forwarded to timing and control circuitry  82 . Information about the presence or absence of spot  14  among other things is produced by timing and control circuitry  82  and forwarded to network interface  81  for transmission in the assigned timeslots from which LED  71  illumination data was removed. 
     Timing and control circuitry  82  manages the functionality of driver IC  70 . Illumination data for LEDs  71  is buffered, processed, delayed, and forwarded at the appropriate time to the sixteen output drivers  84 . Timing and control circuitry  82  also provides the appropriate digital values at the appropriate times for DAC  85  to produce, together with buffer  86  and ibias  87 , the voltage reference signal VREF and the bias current IBIAS used by comparator  88  and current source  90  respectively. Register  83  is also clocked at the appropriate time by the capture (CAP) signal to store the sixteen comparator  88  outputs (CMP). 
     Output driver  84  produces pulse width modulated current to LED  71  and monitors the LED  71  voltage induced by incident light from spot  14  for instance. Modulator  89  receives a digital number from timing and control circuitry  82  and produces a logic level signal (PWM) that turns current source  90  on and off. The frequency of PWM is typically equal to the serial data frame and the video frame rate with the duty cycle related to the digital value from timing and control circuitry  82 . Current source  90  produces current proportional to IBIAS during the time that PWM is high that is drawn through LED  71  to produce light. 
     The maximum duty cycle of PWM is set by the maximum value of the number from timing and control circuitry  82 , and is typically some fraction of a video frame period, for instance one quarter. Once this maximum amount of time has passed from the start of a pulse on PWM, timing and control circuitry  82  changes the value provided to DAC  85  to produce VREF and generates a pulse on CAP to store the sixteen comparator  88  outputs in register  83  some time later. If spot  14 , for instance, is illuminating one of the LEDs  71 , that LED  71  will generate a voltage that is greater than VREF, which causes the CMP output from the associated comparator  88  to go high. An LED  71  that is not illuminated will not generate a voltage greater than VREF, which will cause the CMP output from the associated comparator  88  to be low. 
       FIG. 8  is just one example of many possible driver IC  70  block diagrams. For instance, network interface  81  would not be needed if each driver IC  70  in  FIG. 7  were directly connected to graphics controller  73 . With the serial configuration shown in  FIG. 7 , network interface  81  would not need to recover a clock from data if another input was used to accept a clock input. Likewise, if a frame clock input was provided, network interface  81  would not need to synchronize to the serial input frame timing. Additionally, each output driver  84  could include a current DAC instead of modulator  89  and current source  90 . Such a DAC would provide a variable amount of current for a fixed amount of time instead of a fixed current for a variable amount of time. Also spot  14  could be detected by measuring the LED  71  current induced by spot  14  instead of LED  71  voltage. 
       FIG. 9  illustrates an example for the timing of an LED display  11  using conventional discreet semiconductor LEDs, which includes snapshots  91  and timing diagram  92 . Snapshots  91  illustrate the state of display  11  at four different times, T 0 , T 1 , T 2 , and T 3  within one video frame N. The region labeled “frame n” of each snapshot represents the image and the region labeled “black” of each snapshot represents rows that are not producing light. For example, at T 0  only rows  1  to M/4 are producing light; at T 1  only rows M/4 to M/2 are producing light, at T 2  only rows M/2 to 3M/ 4  are producing light, and at T 3  only rows 3M/4 to m are producing light. 
     Time T 0  occurs one quarter of the way through frame N with the top one quarter of the image displayed. At T 0  all the PWM signals in all driver ICs  70  in the M/4 th  row are just turning on and all the PWM signals in all the driver ICs  70  in the first row are guaranteed to be off. Most of the PWM signals in the first row will be off before T 0  due to modulated brightness, but T 0  is the first time all PWM signals in such row are guaranteed to be off. 
     Time T 1  occurs one half of the way through frame N with the second quarter of the image displayed from row M/4 to M/2. Time T 2  occurs three quarters of the way through frame N with the third quarter of the image displayed. Time T 3  occurs at the end of frame N with the bottom quarter of the image displayed. At times between those that the snapshots  91  represent, one quarter of the image will be displayed in this example, but will be located at different positions on the display  11 . The quarter displayed progresses from the top of the display to the bottom during a frame period. 
     Timing diagram  92  illustrates possible timing of PWM and CAP signals in driver ICs  70  in four different rows,  1 , M/4, M/2, and 3M/4, which are located at the top, and one quarter, one half, and three quarters of the way down display  11 . The index J indicates all columns in such row. At time T 4 , which is the beginning of frame N, the PWM signals in the first row of driver ICs  70  turn on. By T 0  all such signals are guaranteed to be off. At time T 5 , which is equally far apart from T 0  and the end of frame N at T 3 , the CAP signals in all driver ICs  70  in the first row are pulsed to capture the CMP signals output from comparators  88 . Such timing of CAP relative to PWM minimizes optical coupling from LEDs that are on from interfering with spot  14  detection. 
     Times T 6 , T 7 , and T 8  illustrate possible times to pulse CAP in driver ICs  70  one quarter, one half, and three quarters of the way down display  11 . The pulse on the CAP signals progresses down display  11  following the section of the image being displayed by three eighths of the display. 
       FIG. 9  illustrates one of many possible LED display diagrams. For instance, the amount of time LEDs  71  in any one are off can be substantially shorter or longer, and the time when LEDs  71  are sampled for spot  14  detection can vary as well. Rows as well columns can also be scanned so that only one driver IC  70  turns on at a time, instead of an entire row. Display  11  can be scanned on a column basis instead of a row basis, or not at all. The entire image can be flashed on and then off. If driver IC  70  uses variable current for fixed amounts of time instead of fixed current for variable amounts of time the PWM signals that enable the current to LEDs  71  would all be high for a fixed amount of time instead of a variable amount as shown in timing diagram  92 . 
       FIG. 10  illustrates an example timing diagram for the signals within one driver IC  70  located in a row near the top of display  11 , which is partially illuminated by spot  14 . In this example driver IC  70  has 16 output drivers  84  connected to sixteen LEDs  71 . The first LED  71  is illuminated by spot  14  and the sixteenth is not. At time T 0 , frame N begins. At time T 1 , the PWM signals go active. At time T 2 , all PWM signals are guaranteed to be low and the current sources  90  are guaranteed to be off. The VLED(1) signal associated with the first LED  71  and output driver  84  in driver IC  70 , which is illuminated by spot  14  moves towards the voltage induced by the incident light. VLED(16) simply goes high since the associated LED  71  is not illuminated. 
     At time T 3 , timing and control circuitry  82  loads DAC  85  with the appropriate value for VREF. By the time T 4 , all VLED signals and VREF have stabilized. CAP is pulsed by timing and control circuitry  82  and comparator  88  outputs CMP are sampled. Such information is communicated to graphics controller  73 , which determines spot  14  location and takes the appropriate action. 
       FIG. 10  is just one example of many possible driver IC  70  timing diagrams. Output driver  84  may not have a pulse width modulator, so the PWM signals would be different. The time that CAP is pulsed could be different and does not necessarily need to exist. If comparator  88  is replaced by analog to digital converter (ADC), the stream of digital sample values can be analyzed by a processor. VREF could be a fixed value or a variable value controlled by a dedicated DAC. 
       FIG. 11  is an example block diagram of display  11  implemented with a liquid crystal display (LCD) and an LED backlight, which comprises LCD array  100 , LED array  101 , graphics and timing controller  102 , row driver  103 , column driver  104 , and backlight driver network  105 . In this example, LCD array  100  has R rows and C columns of elements with row driver  103  producing R number of WR signals and column driver  104  producing C number of DATA signals. Graphics and timing control circuitry  102  provide data and timing to both row driver  103  and column driver  104  in a similar manner to an OLED display as described in  FIG. 3 . 
     In this example, LED array  101  comprises M rows and N columns of LEDs driven by backlight driver network  105 , which comprises a number of LED driver ICs connected together as in the LED display illustrated in  FIG. 7 . LCD array  100  comprises pixel elements that control the amount of light that can pass through. LED array  101  produces the light that is selectively passed through LCD array  100 . Both LCD array  100  and LED array  101  can be scanned to minimize motion blur. Between frames, all elements of LED array  101  are turned off and all elements of LCD array  100  are made transparent so that spot  14  can be detected by LED array  101  in combination with backlight driver network  105  and graphics and timing control circuitry  102 . 
       FIG. 11  is just one of many possible block diagrams for display  11  based on LCD and LED backlighting technology. For instance, all LED elements in LED array  101  could be directly connected to graphics and timing control circuitry  102  through a multiplexer instead of backlight driver network  105 . 
       FIG. 12  is an example circuit diagram for the LCD pixel element in LCD array  100  and the associated row driver  103  and column driver  104 , which comprises transistor  120 , capacitor  121 , liquid crystal  122 , buffer amplifier  123 , and inverter  124 . Such pixel element is repeated horizontally C times and vertically R times to produce LCD array  100 , with each row of pixel elements controlled by a WR signal from an inverter  124  in row driver  103  and each column of pixel elements connected to a single DATA signal from buffer amplifier  123  in column driver  104 . 
     The transparency of liquid crystal  122  is controlled by the voltage across capacitor  121 , which is set by driving DATA(j) with the desired voltage and then pulsing WR(i) high to make transistor  120  conductive. When WR(i) is high, capacitor  121  is charged to the voltage on DATA(J), which is driven by buffer amplifier  123 . 
       FIG. 12  is just one of many possible LCD array  100 , row driver  103 , and column driver  104  circuit diagrams. For instance, some pixel elements contain multiple transistor to compensate for transistor  120  variations and speed up the write process. 
       FIG. 13  is an example illustration of display  11  timing for a 60 Hz High Definition (HD) TV with 1080 rows of pixels, which shows how the image and backlight are scanned, and spot  14  is detected. The backlight in this example comprises 64 rows of LEDs  71 .  FIG. 13  includes seven snapshots  130  of display  11  at times T 1 , T 2 , T 3 , T 4 , T 5 , T 6 , and T 7  within one frame period. Below the snapshots  130  is timing diagram  131  for the WR signals to LCD array  100  and the PWM signals in driver IC  70  in backlight driver network  105 . Below timing diagram  131  is detailed timing diagram  132  that illustrates the last ten percent of a frame, which is when spot  14  is detected, in expanded detail. Detailed timing diagram  132  illustrates the signals inside driver IC  70  for an LED  71  that is illuminated by spot  14 , VLED(1), and for an LED  71  that is not illuminated, VLED(16). 
     A frame starts at time T 0 , with image data written to the top row of LCD array  100  by WR(1) pulsed high. At time T 1 , as shown in snapshots  130 , a top portion of display  11  represented by the region labeled “loaded” has been loaded with image data, and a bottom portion represented by the region labeled “black” has not been loaded with image data. At times T 2  through T 5 , the regions labeled “loaded” also represent regions that have been loaded with image data, and the regions labeled “black” also represent regions that have not been loaded with data. At time T 1 , the first row of the LED array  101  is also turned on by PWM(1,j) going high. The index J represents all the PWM signals in a row, which in this case is the first row. The time from T 0  to T 1  represented by Tdly is 3.3 mSec in this example and is typically necessary for liquid crystal  122  to stabilize after being written and before being illuminated by LEDs  71 . 
     At time T 2 , WR(540) is pulsed high, which indicates that the top half of the image has been loaded into LCD array  100 . At this time the LEDs  71  in the first row of LED array  101  are also turned off as PWM(1,j) goes low. The offset non-labeled box in snapshots  130  at times T 2  through t 5  represents the region of the LED array  101  that is emitting light. The box is offset to represent that these rows are also loaded with image data. The time from T 1  to T 2  represented by Tbl is 1.7 mSec in this example and is the length of time each row of LED array  101  is turned on. 
     At time T 3 , the illuminated region of LED array  101  reaches the center of display  11  with PWM(32,j) going high. At time T 4 , the last row of LCD array  100  is loaded with data completing the image scan, which began at time T 0 . The time between T 0  and T 4  represented by Tscn is 10 mSec in this example. After an additional Tdly of 3.3 mSec, the illuminated region of LED array  100  reaches the bottom of display  11  with PWM(64,j) going high at time T 5 . After another Tbl time of 1.7 mSec, LED array  101  is completely turned off with PWM(64,j) going low at time T 6 . 
     At time T 6 , all pixel elements of LCD array  100  are configured to be transparent by setting all DATA signals to the level that makes liquid crystal  122  transparent, which is high in this example, and simultaneously pulsing all WR signals. While LCD array  100  is clear, spot  14  can be detected by backlight driver network  105 . After sufficient time for such detection, LCD array  100  is made opaque at time T 7  by setting all DATA signals to the level that makes liquid crystal  122  opaque, which is low in this example, and simultaneously pulsing all WR signals a second time. 
     Detailed timing diagram  132  is an expanded version of the time from T 6  to T 7  and shows the relevant signals of driver IC  70  for detecting spot  14 . Just prior to T 6 , PWM(64,j) goes low, which turns LED array  100  completely off. At T 6 , all WR signals represented by WR(1:1080) pulses while all DATA signals represented by DATA(1:5760) are high, which clears LCD array  100 . There are 5760 DATA signals in this example, which provides 1920 signals for each color component. At time T 6 , the voltage across the LED  71  that is illuminated by spot  14 , which is represented by signal VLED(1) begins to drift toward an intermediate level, while the voltage of signal VLED connected to an LED  71  that is not illuminated, which is represented by VLED(16), goes high since LED  71  is connected to VDD. 
     At time T 8 , timing and control circuitry  82  in every driver IC  70  in backlight driver network  105  updates DAC  85  with the appropriate value to generate a proper Vref. Vref in all driver ICs  70  is represented by Vref(i,j). Some time after Vref is properly set, CMP(16) stabilizes at a low level indicating no spot  14  and CMP(1) stabilizes at a high level indicating the presence of spot  14 . At time T 9 , the CAP signal in all driver ICs  70  in backlight driver network  105  represented by CAP(i,j) pulses, which stores the state of the CMP signals in register  83 . Such spot  14  information is communicated to graphics and timing control circuitry  102 , which takes the appropriate action. At time T 7 , all WR signals represented by WR(1:1080) pulse while all DATA signals represented by DATA(1:5760) are low, which makes LCD array  100  opaque in this example. The time from T 6  to the end of the frame can be an additional Tsns of 1.7 mSec. 
       FIG. 13  illustrates just one of many possible timing diagrams for display  11  built using LCD with LED backlighting technology. LCD array  100  and LED array  101  can be scanned many different ways. Additionally, LED array  101  may be flashed instead of scanned, with all flashes being the same color or sequenced through the color components, such as red, green, and blue. The timing diagrams for the different scanning or flashing methods could be substantially different from  FIG. 13 . 
     Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. 
     Second Embodiment 
     Display calibration systems and related methods are also disclosed that use the photo-sensitivity of LEDs to correct for variations between LEDs during initial production and over lifetime for display systems. Various embodiments are described with respect to the drawings below. Other features and variations can also be implemented, if desired, and related systems and methods can be utilized, as well. 
     In part, the disclosed embodiments relate to displays including arrays of LEDs that use the photo-sensitivity of the LEDs to correct for variations between LEDs during initial production and over lifetime of such a display. Such LED arrays can produce images directly as in LED billboards and sports arena scoreboards, and smaller Organic LED (OLED) displays, or can produce the backlight for LCD screens for instance. Variations in LED brightness and color can be compensated for in order for such a display to have uniform color and brightness. Such compensation, which is typically done in prior systems by measuring the optical output power of each individual LED or purchasing specially tested LEDs; is performed in the embodiments described below by simply measuring the signal induced on each LED by uniform incident light. 
     In one improved embodiment, the system infers the optical output power and optionally also the peak wavelength produced by each LED in an LED array for LED billboards and stadium displays for instance, or LCD backlighting, by measuring the photo-sensitivity of each such LED, comparing such sensitivity to the photo-sensitivity of the other LEDs in such array, and adjusting such LED drive current correction factors accordingly. Such correction factors can be initially generated during production of such LED array by measuring each such LED optical output power and peak wavelength directly, for instance, or by inferring each such LED optical output power and peak wavelength from photo-sensitivity and other measurements. 
     LEDs not only produce light with a specific peak wavelength when forward biased, but also forward bias when illuminated with light at or above such peak wavelength. The electrical power produced by a fixed incident optical power decreases with decreasing incident wavelength with the maximum power produced by incident light with a wavelength near such peak emission wavelength. Incident wavelengths above such peak emission wavelength produce roughly no electrical power in such LED. At a specific temperature, the relationship between voltage and current induced across a properly illuminated LED depends on the amount of illumination, the bandgap voltage of the semiconductor, and the resistive load placed across the LED. As the bandgap voltage of the semiconductor increases, the open circuit voltage (Voc) increases and the short circuit current (Isc) decreases. Since peak emission wavelength decreases with increasing bandgap voltage, the ratio of Voc to Isc can be measured to get an indication of wavelength variations between LEDs in an LED array. 
     The amount of light produced by different LEDs within a manufacturing lot or between lots when driven with a fixed current varies primarily due to differences in the optical path, such as transparency or alignment, and differences in the extent of imperfections in the structure of the light emitting region of the LED. Likewise, such differences similarly affect the photo-sensitivity of such LED when properly illuminated. Consequently, photo-sensitivity parameters, such as Voc and Isc, can be monitored to infer the amount of light that such LED will produce when driven with current. 
     Wavelength and output power from individual LEDs in an LED array can be compensated by correction coefficients to produce uniform intensity and color across such an array. Such correction coefficients determined during manufacturing of such an LED array by the methods described above, by directly measuring the intensity and wavelength of the light produced by each LED, or any other method, can be stored in memory in such a display. Likewise, photo-sensitivity parameters, such as Voc and Isc, produced in response to a light source with fixed parameters, can also be stored in such memory. Periodically, during the life of such a display, the LED array can be illuminated with a light source with the same or different parameters as the initial light source, the photo-sensitivity parameters can be measured, and differences between the initial and new photo-sensitivity parameter values can be used to modify the correction coefficients to correct for any additional shift in illumination from LEDs in such an LED array. 
     The light source used to calibrate an LED array during initial production can be direct or diffuse sunlight, a lamp that mimics the spectrum of sunlight, or any light source with a spectrum sufficient to generate reliably measurable photo-sensitivity parameters from LEDs of each color. To re-calibrate a large LED billboard or stadium display, for instance, the same light source with the same intensity can be used to measure the photo-sensitivity parameters under the exact same condition as when such a display was manufactured. Any shift in any photo-sensitivity parameter can be used directly to update corresponding correction coefficients. If precisely controlling the light source intensity is not possible, then comparing changes in one LED relative to the others enables uniform display intensity and color to be recreated. The user could simply manually adjust overall brightness. 
     For consumer devices such as an LCD television, calibration with a precise light source may not be possible. A close approximation could be diffuse sunlight, but the spectrum of sunlight varies with time day and year, and location. Additionally, such a device could be in an enclosed room with artificial lighting. In such a case, uniformity across LEDs of each color component can be produced, but the relative intensity between color components may not. The user in this case could manually adjust both overall brightness and hue to the desired levels. 
     The improved display calibration systems and related methods described herein address calibration issues for displays using arrays of LEDs directly or as backlights for illumination. And the calibration systems and related methods described herein greatly reduce or eliminate the need for teams of specially trained and equipped people to keep LED billboards and stadium displays calibrated during operation over time. 
     As stated above, this second embodiment can also be used with the techniques, methods and structures described with respect to the other embodiments described herein. For example, the calibration and detection systems and methods described with respect to this embodiment can be used within the other described embodiments, as desired. Further, the various illumination devices, light sources, light detectors, displays, and applications and related systems and methods described herein can be used with respect to calibration and detection systems and methods described in this second embodiment, as desired. Further, as stated above, the structures, techniques, systems and methods described with respect to this second embodiment can be used in the other embodiments described herein, and can be used in any desired lighting related application, including liquid crystal displays (LCDs), LCD backlights, digital billboards, organic LED displays, AMOLED (Active Matrix OLED) displays, LED lamps, lighting systems, lights within conventional socket connections, projection systems, portable projectors and/or other display, light or lighting related applications. 
     Turning now to the drawings,  FIG. 14  is one example of display calibration system  1410  that comprises the display  1411  and light source  1414 . Display  1411  comprises an array of light emitting diodes (LEDs) arranged as pixels  1412  for image illumination either directly in the case of OLED or conventional LED displays, or backlighting in the case of Liquid Crystal Displays (LCDs). Pixel  1412  preferentially comprises different color sub-pixels  1413 , for instance red, green, and blue, to produce the wide gamut of colors typically necessary for accurate representation of images either directly in the case of OLED or LED displays, or modulated as in an LCD. Sub-pixel  1413  comprises an LED. 
     Light source  1414  can be direct or diffuse sunlight, or artificial light from a lamp with a precise emitted light spectrum. During the manufacturing of display  1411 , light source  1414  illuminates display  1411  uniformly to calibrate the intensity and wavelength of light emitted from each pixel  1412  and to measure and store photo-sensitivity parameters such as Voc and Isc, or to simply measure and store photo-sensitivity parameters in which case the intensity and wavelength of all pixels  1412  are calibrated by some other means such as measuring the light produced by each such pixel and adjusting some compensation coefficients accordingly. After some period of use, preferentially the same light source  1414  again illuminates display  1411  and the photo-sensitivity parameters of the LED comprising each sub-pixel  1413 , such as Voc and Isc, are again measured and preferentially compared to those stored during the manufacturing of such display  1411 . Any shift in such photo-sensitivity parameters or preferentially any difference in shift of such parameters in one pixel  1412  relative to preferentially the average shift in all pixels  1412  causes such compensation coefficients to be adjusted inversely proportional in such one pixel  1412 . 
     If the Isc of the LED comprising a red sub-pixel  1413  for instance, decreases by more than the average decrease of all red sub-pixels  1413 , such red sub-pixel compensation coefficients are increased to produce more current to such red sub-pixel  1413  by an amount preferentially inversely proportional to the percentage difference in the Isc change between such red sub-pixel  1413  and the average Isc change from all red sub-pixels  1413  in display  1411 . Since the intensity of illumination on display  1411  from light source  1414  is relatively difficult to control from manufacturing time to such re-calibration time, any change in compensation coefficients for red sub-pixels  1413  for instance, is preferentially normalized to the average Isc from all red sub-pixels  1413 . 
       FIG. 14  is one example of many possible display calibration systems  1410 . For example, pixel  1412  could comprise more or less sub-pixels  1413  and such sub-pixels  1413  could comprise more or less different colored LEDs including just one color. Display  1411  could be an LCD, an OLED display, or a conventional LED display or just portions of such displays. Light source  1414  could be a single light source or many light sources with the same or different spectrums. 
       FIG. 15  is an example block diagram of OLED display  1411  comprising LED array  1523  with R rows and C columns of sub-pixels  1413  typically arranged in pixels  1412  of one red, one green, one blue, and one white sub-pixel LED. LED array  1523  comprises R/2 rows and C/2 columns of such sub-pixels  1413 . Each sub-pixel  1413  is configured to produce a certain amount of light by a combination of voltages on a particular WR (write) signal produced by row driver  1522  and DATA signal produced by column driver  1521 . When a WR signal is high, the analog voltage on each DATA signal is programmed into the row of sub-pixels  1413  activated by the particular WR signal. 
     Power supply  1525  produces the main power Vdd and the ground Vg for LED array  1523 . The voltage on such Vg signal is equal to zero volts during normal operation and during the Voc measurement of each sub-pixel  1413 , and is elevated slightly above display  1411  ground during Isc measurements. 
     During calibration, graphics and timing control circuitry  1520  sequences row driver  1522  through rows of LED array  1523  by pulsing each SNS (sense) signal high. When one of the SNS signals from row driver  1522  goes high, the IVOUT signals from the LED array  1523  source current or voltage into IV sense  1524  for sub-pixels  1413  in the row activated by a particular SNS signal. Depending on the state of the voltage mode enable signal Ven, N sense  1524  either will pass the voltages on the IVOUT signals to ADC  1526  or will short the IVOUT signals to Vg, convert the resulting currents to voltages, and forward the resulting voltages to ADC  1526 . ADC  1526  together with timing information from graphics and timing control circuitry  1520  sequentially converts the voltages forwarded by IV sense  1524  to digital values, which are forwarded to graphics and timing control circuitry  1520  for processing. 
     Graphics and timing control circuitry  1520  can receive Voc and Isc, and other calibration information from sub-pixels  1413 , and can compare such information with previously stored such values to determine any changes necessary to correction coefficients. Graphics and timing control circuitry  1520  can use such correction coefficients to adjust the voltages programmed into sub-pixels  1413  to compensate for variations in light output from each sub-pixel  1413  relative to other sub-pixels  1413 . 
       FIG. 15  is just one of many possible block diagrams for display  1411 , which could be built using any one of a wide range of technologies including but not limited to discreet inorganic LED arrays or liquid crystals. Likewise, the block diagram for display  1411  built with OLEDs could be substantially different. For instance, with additionally circuitry in sub-pixels  1413 , the SNS signals or the IVOUT signals could be eliminated, by using the WR and DATA signals during calibration. Additionally, the Vg could simply be system ground provided IV sense  1524  circuitry was different. 
       FIG. 16  is an example block diagram of OLED pixel  1412  in LED array  1523  referenced by row coordinates i and i+1, and column coordinates J and J+1, and comprising red, green, blue, and white sub-pixels  1413 . The circuitry in all sub-pixels is the same except the color of the included LED. 
     Signals WR(i) and DATA(j) program the light produced by red sub-pixel  1413 , signals WR(i) and DATA(j+1) program the light produced by white sub-pixel  1413 , signals WR(i+1) and DATA(j) program the light produced by green sub-pixel  1413 , and signals WR(i+1) and DATA(j+1) program the light produced by blue sub-pixel  1413 . All sub-pixels are powered by the voltage difference between Vdd and Vg. 
       FIG. 16  is just one of many possible pixel  1412  block diagrams. For instance, any combinations of colors or just one color could be used. Additionally, all sub-pixels could be accessed by one WR signal and one DATA signal if two enable signals select between the sub-pixels. 
       FIG. 17  is an example circuit diagram for sub-pixel  1413  and an individual current and voltage sense element in IV sense  1524  referenced by coordinate J. When producing light, LED  1744  is driven by the current through transistor  1740 , which is set by the voltage stored on capacitor  1743  and the gate of transistor  1740 . The voltage on capacitor  1743  is set to the voltage on DATA(j) signal when WR(i) signal is low. When WR(i) goes high, capacitor  1743  holds the voltage so that DATA(j) can be used to program the current in other rows of sub-pixels  1413  when other WR signals go low. All the sub-pixels  1413  connected to WR(i) are programmed simultaneously by all the DATA signals when WR(i) is low. 
     When SNS(i) goes high, the Voc and Isc induced across LED  1744  by incident light can be measured by IV sense  1524 , after capacitor  1743  is discharged by setting WR(i) low and DATA(j) high. Voc is measured when graphics and timing control circuitry  1520  sets the Ven signal high, which tri-states the output of amplifier  1746  and causes power supply  1525  to hold Vg at zero volts. The voltage on IVOUT(j) passes through resistor  1745  and to the high impedance input of ADC  1526 , which converts such voltage to a digital value and forwards such value to graphics and timing control circuitry  1520 . 
     Isc is measured when graphics and timing control circuitry  1520  sets the Ven signal low, which enables amplifier  1746  and forces the voltage on IVOUT(j) to the voltage on Vg. The resulting current flows through resistor  1745  producing a voltage on Sout(j) proportional to the Isc induced on LED  1744  by incident light. Since the voltage on Sout(j) is lower than that on Vg and IVOUT(j), the negative supply for IV sense  1524  and ADC  1526  is set to be lower than Vg. Power supply  1525  can raise the voltage on Vg to some small voltage, such as one volt above the negative supply, for instance ground, for display  1411 . 
     Although not associated with photo-sensitivity of LED  1744 , characteristics of transistor  1740  can be measured by such sub-pixel  1413 , IV sense  1524 , and ADC  1526  circuitry, and compensated by graphics and timing control circuitry  1520 . After a voltage is programmed across capacitor  1743 , the corresponding current produced by transistor  1740  can be measured when SNS(i) high and Ven is low. The voltage on IVOUT(j) is forced to the voltage on Vg by amplifier  1746  and resistor  1745  with the resulting current flowing through resistor  1745 , which produces a voltage on SOUT(j) proportional to transistor  1740  current. Such voltage can be digitized by ADC  1526  and processed by graphics and timing control circuitry  1520 , which can compensate for variations between transistors  1740  in all sub-pixels  1413 . 
       FIG. 17  is one of many possible circuit diagrams for sub-pixels  1413  and IV sense  1524 . For instance, sub-pixel  1413  could include additional circuitry to compensate for transistor  1740  variations without involving graphics and timing control circuitry  1520 . Additionally, to detect LED  1744  Voc and Isc in response to incident light, sub-pixel  1413  could include more complex circuitry to buffer such signals prior to leaving such sub-pixel  1413 . 
       FIG. 18  is an example architectural diagram for display  1411  that uses conventional discreet semiconductor LEDs, which comprises an array of LED driver ICs  1850  with associated LEDs  1851  connected serially to each other and to a network interface (I/F) IC  1852 . Network interface IC  1852  connects to graphics controller  1853  through control and data busses. The array in this example has N columns and M rows of driver ICs  1850  each connected to P LEDs  1851 . With P equal to 16 and three LEDs per pixel, N and M would equal 120 and 3240 respectively for an HD display with 1920×1080 resolution. For a standard 48 foot by 14 foot bill board with 3 LEDs per pixel, and P equal to 16, N would equal 48 and M would equal 672. 
     LED&#39;s  1851  could all be the same color or could be divided between red, green, and blue for instance. For an RGB display, the different colors could be arranged in different ways. One example is to organize the display in groups of 3 rows with each row in each group being a different color. 
     Graphics controller  1853  produces the data to be displayed digitally, which is forwarded to network interface IC  1852 . Network interface IC  1852  serializes the data, which is sent through the chain of driver ICs  1850  in a time division multiplexed data frame. Each driver IC  1850  is assigned specific time slots from which image data is received and calibration information can be sent. The data frame repeats at the video frame rate, which enables each driver IC  1850  to update the drive current to each LED  1851 . 
     Driver IC  1850  can further process the data to be displayed with correction coefficients that adjust the drive current to each LED  1851  such that brightness and color are uniform across display  1411 . Such correction coefficients can be stored in graphics controller  1853 , downloaded through network interface IC  1852  to driver ICs  1850  each time display  1411  is turned on, and updated periodically by graphics controller  1853 . Such correction coefficients can be created and updated periodically over the life of display  1411  by graphics controller  1853  using individual LED photo-sensitivity parameters such Voc and Isc measured by driver ICs  1850  on commands from graphics controller  1853 , for instance. 
       FIG. 18  is one of many possible architectural diagrams. For instance, each driver IC  1850  could be connected directly to graphics controller  1853  through a multiplexer either serially or in parallel. The LED drivers could be made from discreet components instead of driver IC  1850 . The data for the LED drivers could even be communicated with analog voltages instead of digital values. Additionally, the creation and updating of correction coefficients could be performed by driver IC  1850 , or processing of the data to be displayed with correction coefficients could be performed by graphics controller  1853  for instance. 
       FIG. 19  is an example block diagram for driver IC  1850 , which in this example drives sixteen LEDs  1851  and comprises network interface  1960 , timing and control circuitry  1961 , and sixteen output drivers  1964 . Timing and control circuitry  1961  further comprises IV sense block  1962  and correction matrix  1963 . Output driver  1964  further comprises pulse width modulator  1965 , and current source  1966 . 
     Network interface  1960  accepts serial input data from upstream and produces serial data for downstream driver ICs  1850  as shown in  FIG. 18 . Network interface  1960  further recovers the clock (CK) from the data, and detects and synchronizes to the input data frame timing. Most received serial data is retransmitted, however, data in the assigned timeslots are forwarded to timing and control circuitry  1961 . Calibration information, such Voc and Isc, among other things is produced by timing and control circuitry  1961  and forwarded to network interface  1960  for transmission in the assigned timeslots from which LED  1851  illumination data was removed. 
     Timing and control circuitry  1961  manages the functionality of driver IC  1850 . Illumination data for LEDs  1851  is buffered, processed, delayed, and forwarded at the appropriate time to the sixteen output drivers  1964 . Such processing can include among other things adjustment of the illumination data to compensate for variations between LEDs to produce uniform brightness and color across display  1411 . Matrix  1963  can comprise correction coefficients that when combined with the illumination data produce the data forwarded to output drivers  1964 , which have pulse width modulators  1965  that produce logic level signals that turn current sources  1966  on and off to LEDs  1851 . The frequency of such PWM signals is typically equal to the serial data frame rate and the video frame rate with the duty cycle related to the digital value from matrix  1963 . 
     Timing and control circuitry  1961  has access to both terminals of all 16, in this example, LEDs connected to driver IC  1850  through IV sense block  1962 , which among other things can measure. Voc and Isc produced across LEDs  1851  in response to incident light. The anodes of all sixteen LEDs in this example can be tied together to a single supply voltage Vd, or can be connected to different supply voltages. In the case all sixteen LEDs  1851  are of one color, all anodes preferentially would be connected together. In the case such sixteen LEDs  1851  are of different colors, each such different color LED  1851  would preferentially be connected to each such different supply voltage. 
       FIG. 19  is just one example of many possible driver IC  1850  block diagrams. For instance, network interface  1960  would not be needed if each driver IC  1850  in  FIG. 18  were directly connected to graphics controller  1853 . With the serial configuration shown in  FIG. 18 , network interface  1960  would not need to recover a clock from data if another input was used to accept a clock input. Likewise, if a frame clock input was provided, network interface  1960  would not need to synchronize to the serial input frame timing. Additionally, the function of matrix  1963  could be performed by graphics controller  53 , which would eliminate the need for such matrix  1963  in driver IC  1850 . Modulator  1965  would not be needed if LEDs  1851  were driven with variable current for fixed amount of times, for instance. 
       FIG. 20  is an example block diagram of correction matrix  1963  that can correct for variations in light intensity produced by a pixel  1412  comprising red, green, and blue LEDs  1851  to produce relatively uniform brightness and color across a display  1411 . Matrix  1963  comprises memory  2070  that can store correction coefficients Cr, Cg, and Cb, which are combined by multipliers  2071  with the red, green, and blue illumination data respectively from graphics controller  1853  to produce the illumination data forwarded to modulators  1965  controlling red, green, and blue LEDs  1851  respectively. Such correction coefficients are typically relatively large, which produce adjustments in the illumination data to compensate for variations between LEDs  1851 . 
     Memory  2070  can be made from SRAM, DRAM, FLASH, registers, or any other form of read-writable semiconductor memory. Such correction coefficients periodically can be modified by graphics controller  1853 , driver IC  1850 , or any other processing element in display  1411  to adjust for changes in LED  1851  characteristics for instance over temperature or lifetime. Typically, such correction coefficients are downloaded into memory  2070  from graphics processor  1853  every time display  1411  is turned on. Such correction coefficients are typically modified to compensate for LED  1851  aging effects by graphics controller  1853  or driver IC  1850  after some fixed number of hours of use, after every use, or on demand. 
     Multipliers  2071  scale the illumination data from graphics controller  1853  by multiplying each color component by the corresponding correction coefficient. Such multiplication can be performed by discreet hardware in bit parallel or bit serial form, in an embedded microcontroller, or by any other means. Preferentially, one hardware multiplier comprising a shifter and an adder performs all three multiplications each video frame. As such,  FIG. 20  is just one of many possible block diagrams for correction matrix  1963 . 
       FIG. 21  is an example block diagram for correction matrix  1963  that can correct for variations in both light intensity and wavelength produced by a pixel  1412  comprising red, green, and blue LEDs  1851  to produce uniform brightness and color across a display  1411 . Matrix  1963  comprises memory  2070  that can store nine correction coefficients with three such coefficients for each color component produced. Coefficients Crr, Cgg, and Cbb would typically be effectively the same as Cr, Cg, and Cb from  FIG. 20  to adjust for intensity variations in LEDs  1851 , while the remaining coefficients (Crg, Crb, Cgr, Cgb, Cbr, Cbg) compensate for wavelength variations. 
     For instance, if the red illumination data from graphics controller  1853  was intended for an LED  1851  with a wavelength of 650 nm and the connected LED  1851  wavelength was exactly 650 nm, coefficients Cgr and Cbr would be zero and Crr would be close to one. If such connected LED  1851  wavelength was 640 nm and had the same intensity as the just previous example, Crr would be slightly smaller than in the just previous example and Cgr and Cbr would be non-zero, which would produce some light from such green and blue LEDs  1851 . The wavelength of the combination of light from such red, green, and blue LEDs  1851  would be perceived the same as mono-chromatic light from a single red LED  1851  emitting at precisely 650 nm. 
     Memory  2070  and multipliers  2071  can operate and be implemented as described for  FIG. 20 . Adder  2180  sums the multiplication results from the three connected multipliers  2071  to produce the illumination data forwarded to modulators  1965 . Such adders  2080  can be implemented in hardware or software, or be performed bit parallel or bit serial. Preferentially, such three adders  2080  are implemented with common bit serial hardware that performs such three additions sequentially each video frame. As such  FIG. 21  is just one of many possible intensity and wavelength correction matrix  1963  block diagrams. 
       FIG. 22  is an example block diagram of IV sense block  1962  in timing and control block  1961  of driver IC  1850 , which can measure LED  1851  photo-sensitivity parameters, such as Voc and Isc, when current source  1966  is off. Following the example in  FIG. 18 , the anodes of sixteen LEDs  1851  represented by the signals Vled(1:16) are connected to multiplexer  2294  with one such Vled signal selected to pass through. Such output of multiplexer  2294  is connected to an input of multiplexer  2291  and to the negative terminal of amplifier  2292  and resistor  2293 . Such multiplexers  2291  and  2294  comprise switches that connect input to output and allow current to flow in both directions for the selected input. The output of amplifier  2292  is also connected to multiplexer  2291  the output of which is connected to analog to digital converter (ADC)  2290 . 
     Amplifier  2292  and resistor  2293  form a trans-impedance amplifier which forces the anode of the LED  1851  selected by multiplexer  2294  to the same voltage as signal Vd, which is connected to the positive terminal of amplifier  2292 . The resulting current flows through resistor  2293  producing a voltage proportional to the selected LED  1851  short circuit current Isc, which can be digitized by ADC  2290  if selected by multiplexer  2291 . Alternatively, the open circuit voltage Voc of the LED  1851  selected by multiplexer  2294  can be digitized by ADC  2290  if multiplexer  2291  selects such signal. The power supply for IV sense block  1962  is made to be higher than Vd since the output of amplifier  2292  may go higher than Vd. 
       FIG. 22  is just one of many possible block diagrams for circuitry to measure photo-sensitivity parameters of LEDs  1851 . For instance, a range of LED  1851  current and voltage characteristic could be measured by controlling the positive input to amplifier  2292  with a digital to analog converter (DAC). If each LED  1851  had a dedicated IV sense block  1962 , no multiplexer  2294  would be needed. Additionally, Voc could be measured by adjusting the voltage on the positive terminal of amplifier  2292  until no current flows through resistor  2293 . Further, switched capacitor and sample and hold techniques could be implemented which would have a completely different architecture. 
       FIG. 23  is an example block diagram of display  1411  implemented with a liquid crystal display (LCD) and an LED backlight, which comprises LCD array  2300 , LED array  2301 , graphics and timing control circuitry  2302 , row driver  2303 , column driver  2304 , and backlight driver network  2305 . In this example, LCD array  2300  has R rows and C columns of elements with row driver  2303  producing R number of WR signals and column driver  2304  producing C number of DATA signals. Graphics and timing control circuitry  2302  provide data and timing to both row driver  2303  and column driver  2304  in a similar manner to an OLED display as described in  FIG. 15 . 
     In this example, LED array  2301  comprises M rows and N columns of LEDs driven by backlight driver network  2305 , which comprises a number of LED driver ICs connected together as in the LED display illustrated in  FIG. 18 . LCD array  2300  comprises pixel elements that control the amount of light that can pass through. LED array  2301  produces the light that is selectively passed through LCD array  2300 . When photo-sensitivity parameters, such as Voc and Isc of LEDs  1851  in LED array  2301  are measured, row driver  2303  and column driver  2304  configure LCD array  2300  to be transparent. 
       FIG. 23  is just one of many possible block diagrams for display  1411  based on LCD and LED backlighting technology. For instance, all LED elements in LED array  2301  could be directly connected to graphics and timing control circuitry  2302  through a multiplexer instead of backlight driver network  2305 . 
       FIG. 24  is an example circuit diagram for the LCD pixel element in LCD array  2300  and the associated row driver  2303  and column driver  2304 , which comprises transistor  2410 , capacitor  2411 , liquid crystal  2412 , buffer amplifier  2413 , and inverter  2414 . Such pixel element is repeated horizontally C times and vertically R times to produce LCD array  2300 , with each row of pixel elements controlled by a WR signal from an inverter  2414  in row driver  2303  and each column of pixel elements connected to a single DATA signal from buffer amplifier  2413  in column driver  2304 . 
     The transparency of liquid crystal  2412  is controlled by the voltage across capacitor  2411 , which is set by driving DATA(j) with the desired voltage and then pulsing WR(i) high to make transistor  2410  conductive. When WR(i) is high, capacitor  2411  is charged to the voltage on DATA(J), which is driven by buffer amplifier  2413 . When photo-sensitivity parameters, such as Voc and Isc of LEDs  1851  in LED array  2301  are measured, liquid crystal  2412  for every pixel element in LCD array  2300  is made transparent by preferentially setting all WR signals high simultaneously and setting the voltage on all DATA signals to the value that makes liquid crystal  2412  transparent. 
       FIG. 24  is just one of many possible LCD array  2300 , row driver  2303 , and column driver  2304  circuit diagrams. For instance, some pixel elements contain multiple transistors to compensate for transistor  2410  variations and speed up the write process. 
     Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. 
     Third Embodiment 
     LED calibration systems and related methods are also disclosed that use the photo-sensitivity of LEDs to correct for variations between LEDs during initial production and over the lifetime of systems using LEDs. Various embodiments are described with respect to the drawings below. Other features and variations can also be implemented, if desired, and related systems and methods can be utilized, as well. 
     In part, the disclosed embodiments relate to using the photo-sensitivity of an LED to determine emission parameters such as intensity and wavelength. Applications for the disclosed embodiments include solid state lamps, LCD backlights, and LED displays for instance. Variations in LED brightness and wavelength should be compensated for in order for such devices to have uniform color and brightness. Such compensation, which is typically done by measuring the optical output of each individual LED with a camera or purchasing specially tested LEDs, is performed by simply measuring the signal induced on each LED by light from other LEDs in the device or from an additional light source. 
     The disclosed embodiments include methods to set the color or color temperature produced by a group of LEDs during the manufacturing of a device such as a lamp, an LED display, or an LCD backlight, and maintaining such color or color temperature over the operating life of such a device. The methods involve measuring the intensity and wavelength of light produced by each LED within a group of LEDs and adjusting the amount of light generated by each LED to produce precise color and intensity from the group of LEDs. 
     Two methods that operate some of the LEDs in photovoltaic or photoconductive mode to measure the light intensity produced by other LEDs in the group are presented. The first method that uses an additional light source as a reference determines the light intensity emitted from each LED relative to such reference, while the second method determines the light intensity emitted from each LED relative to each other. As such, the first method can produce a precise color and intensity from each group of LEDs, while the second method can only produce a precise color. 
     Both intensity measurement methods typically comprise two steps and can be used to calibrate devices during both manufacturing and over lifetime. The first step of the first method illustrated in  FIGS. 25A-D  and the first step of the second method illustrated in  FIG. 27A-D  can be performed in a manufacturing environment on a special control device that has all LEDs manually adjusted to produce the desired light intensities. The results of the first step on such control device are then used in the second step of the first method illustrated in  FIGS. 26A-D  and the second step of the second method illustrated in  FIGS. 28A-D  on production devices to determine the actual emitted light intensities. 
     Both intensity measurement methods can also be used to maintain a precise color produced by a group of LEDs and uniform intensity from an array of groups of LEDs, for instance pixels in an LED display, LCD backlight, or LED lamp, over time. The first step of both methods is typically performed on a device after such device has been calibrated during manufacturing and the second step is performed in the field at periodic intervals. The reference light source for the first intensity measurement method can be ambient light. 
     Emission intensity is measured in all cases by measuring the photocurrent produced in the longest wavelength LED within the group of LEDs by light from the other LEDs and in the first method from an additional reference light source. For instance, in an LED array for an LED display or backlight, according to the example shown for the first method, the red LED in a pixel measures the light from the blue and green LEDs in the same pixel, and from the reference light source. Next, the red LED in an adjacent pixel measures the light from the first red LED and the reference light source. Such measured light can be reflected off a mirror for an LED display during manufacturing or off the waveguide or diffuser in an LED backlight for instance. In the field, such light can be scattered by the LED packages or enclosures or by any other means. 
     In the example shown for the second method, which includes two red LEDs and one white LED in an LED lamp for instance, a first red LED measures the light from a second red LED and from the white LED. Next, light from the second LED measures the light from the first LED and from the white LED. Both the first and second intensity measurement methods can be used for any groups of LEDs in any types of products with the difference between the methods being the presence or absence of a reference light source. The second method could be used in an LED display or backlight to produce precise color and uniform intensity from all pixels by daisy chaining the measurements sequentially across such LED array. 
     The example intensity measurement methods are divided into two steps which measure the differences in relative intensity between a known good measurement and the unknown measurement. For instance, during manufacturing, a control device with the desired output intensities is measured to determine what the relative photocurrents should be. Using the first method, the ratios of the photocurrent in a first LED produced by the light from the other LEDs over the photocurrent produced by the reference light source generate coefficients used for the second step. Provided the ratio of intensities from the reference light source in the first and second steps is known, the unknown LED intensities in the second step can be determined. Likewise, using the second method, the ratios of photocurrents induced on one LED by the two other LEDs in a calibrated control device generate coefficients used in the second step. In the second step, the difference in the ratios from the first step determines the difference in relative unknown intensities between the two LEDs. 
     When the first or second method is used to calibrate a device over time, the first step determines what ratios of photocurrents should be and over time the second step determines what they are. The change in such ratios determines the change in actual emission intensity. Since only ratios currents measured at one time are compared to ratios of currents at another time, any changes in operating conditions cancel out. For instance, such measurements are independent of temperature differences. 
     The method presented to measure emission wavelength illuminates each LED with two different wavelengths of lights, such as a light wavelength slightly above and below the anticipated peak emission wavelength range, and measures the resulting photocurrent. Since the responsivity of an LED drops off dramatically for incident wavelengths longer than the peak emission wavelength, the difference in induced photocurrents is directly related to the peak emission wavelength.  FIGS. 31A-C  provide a graphical illustration of LED responsivity as a function of incident wavelength and the resulting photocurrent differences. 
     Since LED emission wavelength does not vary significantly over time, such wavelength measurements can just be performed on a production line. As in the two emission intensity methods, such wavelength measurement should first be done on a control device with known emission intensity to calibrate the production test setup. Subsequent measurements of devices with unknown emission wavelengths, will be relative to the control device results. 
     Once the emission wavelengths and the emission intensity or relative emission intensity between a group of LEDs is known, color correction coefficients can be determined that adjust the emission intensity of light from each LED within a group of LEDs to produce a precise color and optionally a precise intensity from such group of LEDs.  FIG. 29  illustrates hardware to implement the calibration methods.  FIG. 30  illustrates color correction coefficients and hardware to correct for emission intensity variations, while  FIG. 32  illustrates such coefficients and hardware to correct for both emission intensity and wavelength variations between the red, green, and blue LEDs associated with a pixel in an LED display or a triplet in an LCD backlight. 
     Although such calibration methods are appropriate for any devices that contain groups of LEDs, of particular interest are LCDs that use Field Sequential Color (FSC).  FIG. 33  illustrates a simplified block diagram of a conventional LCD, while  FIG. 34  illustrates such a diagram for a FSC LCD. While a conventional LCD has white backlight that is filtered into red, green, and blue components by special color filters, a FSC LCD eliminates the costly color filters and sequences each color component at three times the conventional frame rate or more. Such FSC LCDs require red, green, and blue backlights and as such are a primary application for the color calibration methods described herein. 
     The improved methods herein address problems associated with devices using groups of different colored LEDs directly or as backlights for illumination. Such calibration methods reduce the need for specially binned LEDs for the production of lamps, displays, or backlights, and maintain the color or color temperature of the light produced over the operating life of the device. 
     As stated above, this third embodiment can also be used with the techniques, methods and structures described with respect to the other embodiments described herein. For example, the calibration and detection systems and methods described with respect to this embodiment can be used within the other described embodiments, as desired. Further, the various illumination devices, light sources, light detectors, displays, and applications and related systems and methods described herein can be used with respect to calibration and detection systems and methods described in this third embodiment, as desired. Further, as stated above, the structures, techniques, systems and methods described with respect to this third embodiment can be used in the other embodiments described herein, and can be used in any desired lighting related application, including liquid crystal displays (LCDs), LCD backlights, digital billboards, organic LED displays, AMOLED (Active Matrix OLED) displays, LED lamps, lighting systems, lights within conventional socket connections, projection systems, portable projectors and/or other display, light or lighting related applications. 
     Turning now to the drawings,  FIGS. 25A-D  in association with  FIGS. 26A-D  illustrate one possible method for calibrating the intensity of light produced by each LED within a group of LEDs to produce a specific blended color. Such group of LEDs could be any combination of colors, but as an example comprise red, green, and blue LEDs. Specifically, in such example, LEDs  2510 ,  2520 , and  2530  could comprise the red, green, and blue light sources respectively in an LED display pixel or LCD backlight triplet. LED  2540  comprises the red light source in an adjacent LED display pixel or backlight triplet. 
       FIGS. 25A-D  illustrate the first step in such calibration method, which can be performed on one special device comprising such group of LEDs that is representative of many such devices produced on a manufacturing line for instance. Alternatively, such first step could be performed on a device that is to be re-calibrated some time later using the second step in such calibration method illustrated in  FIGS. 26A-D . 
     The following equations are associated with  FIGS. 25A-D . In particular, equations 1 and 2 are associated with  FIG. 25A . Equations 3A and 3B are associated with  FIG. 25B . Equations 4A and 4B are associated with  FIG. 25C . And equations 5A and 5B are associated with  FIG. 25D . 
       V r0n =E 0 R r0   [EQ. 1]
 
       V r1n =E 0 R r1   [EQ. 2]
 
         V   r0gn   =E   gd   R   r0   C   r0g   =E   gd ( V   r0n   /E   0 ) C   r0g   [EQ. 3A]
 
         C   r0g =( V   r0gn   /V   r0n )( E   0   /E   gd )  [EQ. 3B]
 
         V   r0bn   =E   bd   R   r0   C   r0b   =E   bd ( V   r0n   E   0 ) C   r0b   [EQ. 4A]
 
         C   r0b =( V   r0bn   /V   r0n )( E   0   /E   bd )  [EQ. 4B]
 
         V   r1r0n   =E   r0d   R   r1   C   r1r0   =E   r0d ( V   r1n   /E   0 ) C   r1r0   [EQ. 5A]
 
         C   r1r0 =( V   r1r0n   /V   r1n )( E   0   /E   r0d )  [EQ. 5B]
 
     The light emitted from LEDs  2510  (red),  2520  (green), and  2530  (blue) is adjusted by varying current sources  2511 ,  2521 , and  2531  to produce the desired light intensities E gd , E bd , and E r0d  from the green, blue, and red LEDs  2520 ,  2530 , and  2510  respectively as shown in  FIGS. 25B-D . Light source  2550  is adjusted to produce a fixed intensity E 0 , which illuminates LEDs  2510  and  2540 , induces photo-currents proportional to incident light intensity, and produces the nominal voltages V r0n  and V r1n  across resistors  2512  and  2542  respectively as shown in  FIG. 25A . Likewise, LEDs  2520  and  2530  illuminate LED  2510  as shown in  FIGS. 25B-C  and LED  2510  illuminates LED  2540  as shown in  FIG. 25D  to produce the nominal voltages V r0gn , V r0bn , and V r1r0n  respectively across resistors  2512  and  2542 . Preferably, the resistance of resistors  2512  and  2542  should be small enough such that the induced voltages do not significantly forward bias LEDs  2510  and  2540 . 
     The responsivities R r0  and R r1  of LEDs  2510  and  2540  to incident light from light source  2550  as shown in equations 1 and 2 respectively are equal to the ratios of the induced voltages V r0n  and V r1n  over the incident light intensity EQ. As shown in equations 3A-B, the voltage V r0gn  induced across resistor  2512  by light from LED  2520  is equal to the light intensity E gd  times such responsivity R r0  times a correction coefficient C r0g . Likewise, as shown in equations 4A-B, the voltage V r0bn  induced across resistor  2512  by light from LED  2530  is equal to the light intensity E bd  times such responsivity R r0  times the correction factor C r0b . Such correction factors C r0g  and C r0b  take into account differences in emitted light wavelength and optical attenuation between light from light source  2550 , LED  2520 , and LED  2530  and incident on LED  2510 . For instance, light source  2550  could produce red light while LED  2520  and  2530  produce green and blue respectively. Additionally, light source  2550  could shine directly on LED  2510  while light from LEDs  2520  and  2530  could be indirect since such LEDs could be mounted adjacent to each other. Alternatively, light from LEDs  2510 ,  2520 , and  2530  could be reflected by a mirror in the case of an LED display or by a light diffusion film in the case of an LCD backlight. As shown in equation 3B, when equation 3A is combined with equation 1, such correction coefficient C r0g  is equal to the ratio of the measured voltages V r0gn  over V r0n  times the ratio of the known light intensities E 0  over E gd . Likewise, when equation 1 is substituted into equation 4A, C r0b  is expressed as a function of measured voltages and known light intensities as shown in equation 4B. 
     As in equations 3A-B and 4A-B, equations 5A-B relate the nominal voltage V r1r0n  induced across resistor  2542  by light from LED  2510  with the desired intensity E r0d &#39;. Substituting equation 2 into equation 5A results in the correction factor C r1r0  being expressed as a function of measured voltages and known light intensities as shown in equation 5B. 
     With known values for such correction coefficients from a device, such as an LED display or LCD backlight, with emitted intensities adjusted to the desired values, the color point of such devices can be adjusted to a fixed point on a manufacturing line and maintained in the field following the second step in the calibration process illustrated in  FIGS. 26A-D . The procedure illustrated in  FIGS. 26A-D  is performed on a different device with LEDs  2510 ,  2520 , and  2530  emitting unknown intensities E r0 , E g , and E b  respectively and light source  2550  emitting either the same or known intensity on a manufacturing line for instance or an unknown intensity in the field from ambient light for instance. 
     The following equations are associated with  FIGS. 26A-D . In particular, equations 6 and 7 are associated with  FIG. 26A . Equations 8A, 8B and 8C are associated with  FIG. 26B . Equations 9A, 9B and  9 C are associated with  FIG. 26C . And equations 10A, 10B and 10C are associated with  FIG. 26D . 
       V r0 =E 1 R r0   [EQ. 6]
 
       V r1 =E 1 R r1   [EQ. 7]
 
         V   r0g   =E   g   R   r0   C   r0g   =E   g ( V   r0   /E   1 ) C   r0g   [EQ. 8A]
 
         V   r0g   =E   g ( V   r0   /E   1 )( V   r0g   /V   r0n )( E   0   /E   gd )  [EQ. 8B]
 
         E   g   /E   gd =( V   r0g   /V   r0 )( V   r0n   /V   r0gn )( E   1   /E   0 )  [EQ. 8C]
 
         V   r0b   =E   b   R   r0   C   r0b   =E   b ( V   r0   /E   1 ) C   r0b   [EQ. 9A]
 
         V   r0b   =E   b ( V   r0   /E   1 )( V   r0gbn   /V   r0n )( E   0   /E   bd )  [EQ. 9B]
 
         E   b   /E   bd =( V   r0b   N   r0 )( V   r0n   /V   r0bn )( E   1   /E   0 )  [EQ. 9C]
 
         V   r1r0   =E   r0   R   r1   C   r1r0   =E   r0 ( V   r1   /E   1 ) C   r1r0   [EQ. 10A]
 
         V   r1r0   =E   r0 ( V   r1   E   1 )( V   r1r0n   /V   r1n )( E   0   /E   r0d )  [EQ. 10B]
 
         E   r0   /E   r0d =( V   r1r0   /V   r1 )( V   r1n   /V   r1r0n )( E   1   /E   0 )  [EQ. 10C]
 
     Equations 6 and 7 relate the responsivities R r0  and R r1  of LEDs  2510  and  2540  respectively to the intensity E 1  emitted by light source  2550 . Equation 8A shows the voltage V r0g  induced across resistor  2512  by the unknown light intensity E g  from LED  2520  being equal to E g  times R r0  times the correction coefficient C r0g . Substituting equation 6 into equation 8A and replacing C r0g  with equation 3 results in the ratio of the actual emitted intensity E g  over the desired emitted intensity E gd  equal to the ratio of the measured voltages V r0g  over V r0  times the ratio of the nominal voltages V r0n  over V r0gn  measured as illustrated in  FIGS. 25A-D  times the ratio of intensities E 1  over E 0  emitted by light source  2550 , as shown in equations 8B an 8C. Likewise, equations 9A-C and 10A-C express the ratios of the unknown intensities E b  and E r0  over the desired intensities E bd  and E r0d  respectively as a function of measured voltages and known light intensities emitted from light source  2550 . 
     Since the light intensity produced by an LED changes over time, a device such as an LED display or an LCD backlight with red, green, and blue LEDs, should be re-calibrated after some time to maintain the precise color calibrated during production of such device. In such a field re-calibration light source  2550  may be daylight or office ambient light of unknown intensity. In such cases, the ratio E 1  over E 0  is unknown but is the same for equations 8A-C, 9A-C, and 10A-C so the relative intensity of light produced by LEDs  2510 ,  2520 , and  2530 , and consequently the color can be maintained. Likewise, the intensity of light produced by all such pixels or backlight triplets can be kept uniform since the ratio of E 1  over E 0  should be the same for all such pixels or triplets. 
       FIGS. 27A-D  in association with  FIGS. 28A-D  illustrate a method of calibrating the light produced by a group of LEDs  2510 ,  2520 , and  2540  to produce a fixed color similar to such method illustrated in  FIGS. 25A-D  and  26 A-D but without the reference light source  2550 . In the method illustrated in  FIGS. 27A-D  and  28 A-D the relative intensity of light produced by each LED can be controlled but not the absolute intensity of the group of LEDs  2510 ,  2520 , and  2540 . In this example, LEDs  2510  and  2540  are shown to be red, while LED  2520  is shown to be a white LED. An example application for such a group of LEDs is a lamp emitting white light with a low color temperature similar to that of an incandescent light bulb. 
     The following equations are associated with  FIGS. 27A-D . In particular, equations 11, 12, 13A and 13B are associated with  FIGS. 27A-B . And equations 14, 15, 16A and 16B are associated with  FIGS. 27C-D . 
       V r0wn =E wd R r0 C r0w   [EQ. 11]
 
       V r0r1n =E r1d R r0 C r0r1   [EQ. 12]
 
         V   r0wn   /V   r0r1n =( E   wd   /E   r1d )( C   r0w   /C   r0r1 )  [EQ. 13A]
 
         C   r0w   /C   r0r1 =( V   r0wn   /V   r0r1n )( E   r1d   /E   wd )  [EQ. 13B]
 
       V r1wn =E wd R r1 C r1w   [EQ. 14]
 
       V r1r0n =E r0d R r1 C r1r0   [EQ. 15]
 
         V   r1wn   /V   r1r0n =( E   wd   /E   r0d )( C   r1w   /C   r1r0 )  [EQ. 16A]
 
         C   r1w   /C   r1r0 =( V   r1wn   /V   r1r0n )( E   r0d   /E   wd )  [EQ. 16B]
 
     The following equations are associated with  FIGS. 28A-D . In particular, equations 17, 18, 19A and 19B are associated with  FIGS. 28A-B . And equations 20, 21, 22A, 22B and  23  are associated with  FIGS. 28C-D . 
       V r0w =E w R r0 C r0w   [EQ. 17]
 
       V r0r1 =E r1 R r0 C r0r1   [EQ. 18]
 
         V   r0r1   /V   r0w =( E   r1   /E   w )( C   r0r1   /C   r0w )  [EQ. 19A]
 
         E   r1   /E   w =( V   r0r1   /V   r0w )( C   r0w   /C   r0r1 )=( V   r0r1   /V   r0w )( V   r0wn   /V   r0r1n )( E   r1d   /E   wd )  [EQ. 19B]
 
       V r1w =E w R r1 C r1w   [EQ. 20]
 
       V r1r0 =E r0 R r1 C r1r0   [EQ. 21]
 
         V   r1r0   /V   r1w =( E   r0   /E   w )( C   r1r0   /C   r1w )  [EQ. 22A]
 
         E   r0   /E   w =( V   r1r0   /V   r1w )( C   r1w   /C   r1r0 )=( V   r0r0   /V   r1w )( V   r1wn   /V   r1r0n )( E   r0d   /E   wd )  [EQ. 22B]
 
         E   r0   /E   r1 =( E   r0   /E   w )/( E   r1   /E   w )  [EQ. 23]
 
     The first step in such calibration method as shown in  FIGS. 27A-D  is to adjust current sources  2511 ,  2521 , and  2541  to produce the desired light intensities E r0d , E wd , and E r1d  from LEDs  2510 ,  2520 , and  2540  respectively as shown in  FIGS. 27A-D . Then the light from LED  2520  and LED  2540  are measured by LED  2510 , which produce the nominal voltages V r0wn  and V r0r1n  respectively across resistor  2512  as shown in  FIGS. 27A and 27B . Equations 11 and 12 illustrate the relationship between such voltages, emitted powers, responsivity, and correction factors. Equations 13A-B take the ratio of equation 11 over 12 to produce the ratio of correction coefficients C r0w  over C r0r1  expressed as a function of the ratio of nominal voltages V r0wn  over V r0r1n  times the ratio of desired emission intensities E r1d  over E wd . 
     Next the light from LEDs  2520  and  2510  are measured by LED  2540 , which produce the nominal voltages V r1wn  and V r1r0n  respectively across resistor  2542  as shown in  FIGS. 27C and 27D . Equations 14 and 15 relate such voltages to emitted powers, responsivity, and correction factors. Equations 16A-B take the ratio of equation 14 over 15 to produce the ratio of correction coefficients C r1w  over C r1r0 . Once such ratios of correction coefficients are known, the relative intensities of light produced by similar such devices on a manufacturing line can be determined and adjusted to produce a desired color. Likewise, such devices can be re-calibrated in the field after use to maintain the desired color. 
       FIGS. 28A-D  illustrate the second step in the method to calibrate color without a reference light source. In such second step, LEDs  2520  and  2540  sequentially illuminate LED  2510  with unknown light intensities E w  and E r1  respectively, which produce voltages V r0w  and V r0r1  respectively across resistor  2512  as shown in  FIGS. 28A and 28B . Equations 17 and 18 relate the induced voltages V r0w  and V r0r1  to the emitted powers E w  and E r1 , the responsivity R r0 , and the correction coefficients C r0w  and C r0r1  respectively. Equations 19A-B take the ratio of equation 18 over 17 and substitutes equation 13B for the ratio of correction coefficients C r0w  over C r0r1  to express the ratio of emitted intensities E r1  over E w  as a function of measured voltages and desired emitted intensities. 
     Next, LEDs  2520  and  2510  sequentially illuminate LED  2540  with unknown light intensities E w  and E r0  respectively, which produce voltages V r1w  and V r1r0  respectively across resistor  2542  as shown in  FIGS. 28A-D . Equations 20 and 21 relate the induced voltages V r1w  and V r1r0  to the emitted powers E w  and E r0 , the responsivity R r1 , and the correction coefficients C r1w  and C r1r0  respectively. Equations 22A-B take the ratio of equation 21 over 20 and substitutes equation 16B for the ratio of correction coefficients C r1w  over C r1r0  to express the ratio of emitted intensities E r0  over E w  as a function of measured voltages and desired emitted intensities. Equation 23 expresses the ratio of E r0  over E r1  as the ratio of equation 22B over equation 19B. Once such relative intensities emitted from each LED  2510 ,  2520 , and  2540  are known, such intensities can be adjusted to produce the desired color. 
       FIGS. 25A-D ,  26 A-D,  27 A-D and  28 A-D illustrate just two of many possible methods for calibrating the color point emitted from a group of different colored LEDs using such LEDs as photo-detectors. Any number of LEDs in some cases from two to many more can be calibrated using such methods or other methods. Any color LEDs can be used provided the LEDs used as photo-detectors measure the light produced by LEDs with roughly equal or shorter wavelengths. Although  FIGS. 25A-D  and  26 A-D use red, green, and blue LEDs common in LED panels and increasingly in LED backlights, such method is equally appropriate for a lamp or any other type of illumination or display device including Organic LEDs (OLEDs). Although  FIGS. 27A-D  and  28 A-D use white and red LEDs in a lamp as an example, such calibration method is equally appropriate for an LED display, backlight, or any other type of illumination device including OLEDs. Such methods could be performed on a manufacturing line to ensure consistent color of devices or could be performed on the same device over time to maintain color. 
       FIG. 29  is an example block diagram for circuitry that can implement the methods illustrated in  FIGS. 25A-D ,  26 A-D,  27 A-D and  28 A-D which comprises integrated circuit (IC)  2980 , LEDs  2510 ,  2520 ,  2540 , and optionally  2530 , and resistors  2512  and  2542 . Integrated circuit (IC)  2980  further comprises timing and control circuitry  2981 , coefficient matrix  2982 , digital to analog converter (DAC)  2983 , analog to digital converter (ADC)  2984 , and three or four output drivers  2985  for producing currents for LEDs  2510 ,  2520 ,  2530  (optional) and  2540 , depending upon whether optional LED  2530  is included. Output drivers  2985  further comprise of pulse width modulators  2987  and current sources  2986 . 
     Timing and control circuitry  2981  manages the functionality of driver IC  2980 . Illumination data for LEDs  2510 ,  2520 ,  2530 , and  2540  is either hardwired into timing and control circuitry  2981  or is communicated to timing and control circuitry  2981  through some means, and is forwarded at the appropriate time to the color correction matrix  2982 . Color correction matrix  2982  can, among other things, adjust the illumination data for LEDs  2510 ,  2520 ,  2530 , and  2540  to compensate for variations between LEDs to produce uniform brightness and color across a display or from a lamp. Matrix  2982  can comprise correction coefficients that when combined with the illumination data produce the data forwarded to output drivers  2985 , which have pulse width modulators  2987  that produce logic level signals that turn current sources  2986  on and off to LEDs  2510 ,  2520 ,  2530 , and  2540 . 
     ADC  2984  has access to both terminals of LEDs  2510  and  2540  and can, among other things, measure the voltage produced across resistors  2512  and  2542  in response to light incident on LEDs  2510  and  2540 . The anodes of all three or four LEDs in this example, depending upon whether optional LED  2530  is used, can be tied together to a single supply voltage Vd  2988 , or can be connected to different supply voltages. In the case all LEDs  2510 ,  2520 ,  2530  (optional), and  2540  are of one color, all anodes preferentially would be connected together. In the case such LEDs  2510 ,  2520 ,  2530  (optional), and  2540  are of different colors, each such different color LED  2510 ,  2520 ,  2530  (optional), and  2540  would preferentially be connected to each such different supply voltage. 
       FIG. 29  is just one example of many possible driver IC  2980  block diagrams. For instance PWM  2987  would not be needed if LEDs  2510 ,  2520 ,  2530 , and  2540  were driven with variable current for fixed amount of times. Resistors  2512  and  2542  would not be needed if ADC  2984  measured open circuit voltage, short circuit current, or some other combination of current and voltage from LEDs  2510  and  2540 . DAC  2983  could be a fixed current source if variable currents were not desired. Color correction matrix  2982  could reside elsewhere in a device. 
       FIG. 30  is an example block diagram of correction matrix  2982  that can correct for variations in light intensity produced by a combination of red, green, and blue LEDs  2510 ,  2520 , and  2530  to produce relatively uniform brightness and color across a display for from a lamp. Matrix  82  comprises memory  3090  that can store correction coefficients C r , C g , and C b , which are combined by multipliers  3091  with the red, green, and blue, for instance; illumination data respectively from timing and control circuitry  2981  to produce the illumination data forwarded to modulators  2987  controlling red, green, and blue LEDs  2510 ,  2520 , and  2530  respectively. Such correction coefficients are typically relatively large, which produce adjustments in the illumination data to compensate for variations between LEDs  2510 ,  2520 , and  2530 . 
     Memory  3090  can be made from SRAM, DRAM, FLASH, registers, or any other form of read-writable semiconductor memory. Such correction coefficients periodically can be modified by driver IC  2980  or any other processing element in a display or lamp for instance to adjust for changes in LEDs  2510 ,  2520 , and  2530  characteristics for instance over temperature or lifetime. 
     Multipliers  3091  scale the illumination data from timing and control circuitry  2981  by multiplying each color component by the corresponding correction coefficient. Such multiplication can be performed by discreet hardware in bit parallel or bit serial form, in an embedded microcontroller, or by any other means. Preferentially, one hardware multiplier comprising a shifter and an adder performs all three multiplications. As such,  FIG. 30  is just one of many possible block diagrams for correction matrix  2982 . Likewise, correction matrix  2982  could reside elsewhere in a device, such as software in a graphics controller. 
       FIGS. 31A-C  illustrate one possible method to determine the peak emission wavelength from an LED by determining such LED&#39;s photosensitivity as a function of the wavelength of light incident on such LED. Such measurement system could comprise light source  2550 , LED  2510  and resistor  2512  as illustrated in  FIGS. 25A-D , with the wavelength of light emitted by light source  2550  switched between wavelengths λ −  and λ +  that are slightly shorter and longer respectively than the expected peak emission wavelength λ p  of LED  2510 . 
     Plot  3100  in  FIG. 31A  represents the photosensitivity of LED  2510  with a nominal peak emission wavelength λ pn  as a function of incident wavelength with the vertical axis representing the voltage induced across resistor  2512 . At wavelengths longer than λ pn , the photosensitivity reduces significantly, while at wavelengths shorter than λ pn , the photosensitivity reduces linearly with wavelength. Also shown is incident light with wavelength λ −  producing voltage V −  across resistor  2512  and incident light with wavelength λ +  producing voltage V +  across resistor  2512 . Line  3103  connecting the points (λ − , V − ) and (λ + ,V + ) has a slope M=(V−−V+)/(λ − −λ + ). 
     Plot  3101  in  FIG. 31B  illustrates the photosensitivity of an LED  2510  with a peak emission wavelength λ p−  that is slightly shorter than the nominal peak emission wavelength λ pn . When such an LED  2510  is illuminated by light source  2550  with wavelengths λ −  and λ + , voltages V −  and V +  respectively are generated across resistor  2512 . The difference in voltage between such V −  and V +  is greater for such LED  2510  with peak emission wavelength λ p−  that is slightly shorter than the nominal peak emission wavelength λ pn  than for such LED  2510  with the nominal peak emission wavelength λ pn . Additionally, the slope M of line  3104  is more negative for the LED  2510  emitting the peak wavelength λ − , than for the LED  2510  emitting the nominal peak wavelength λ pn . 
     Plot  3102  in  FIG. 31C  illustrates the photosensitivity of an LED  2510  with a peak emission wavelength λ p+  that is slightly longer than the nominal peak emission wavelength λ pn . When such an LED  2510  is illuminated by light source  2550  with wavelengths λ −  and λ − , voltages V −  and V +  respectively are generated across resistor  2512 . The difference in voltage between such V −  and V +  is smaller for such LED  2510  with peak emission wavelength λ p+  that is slightly longer than the nominal peak emission wavelength λ pn  than for such LED  2510  with the nominal peak emission wavelength λ pn . Additionally, the slope M of line  3105  is less negative for the LED  2510  emitting the peak wavelength λ + , than for the LED  2510  emitting the nominal peak wavelength λ pn . 
     Since the slopes of lines  3103 ,  3104 , and  3105  in  FIGS. 31A ,  31 B and  31 C are directly related to the peak emission wavelength of LED  2510 , such slopes can be used to determine such peak emission wavelengths. For instance, such relationship could be linear.  FIGS. 31A-C  illustrate one of many possible methods to determine the peak emission wavelength of light produced by an LED by measuring the photosensitivity of such LED. For instance, LED light induced current could be measured instead of voltage or some other combination of current and voltage could be measured. Additionally, light with broader spectrums of light could induce such voltages or currents instead of the mono-chromatic sources illustrated in  FIGS. 31A-C . 
       FIG. 32  is an example block diagram for correction matrix  2982  that can correct for variations in both light intensity and wavelength produced by a combination of red, green, and blue LEDs  2510 ,  2520 , and  2530  for instance to produce uniform brightness and color from an array of LEDs. Matrix  2982  comprises memory  3090  that can store nine correction coefficients with three such coefficients for each color component produced. Coefficients C rr , C gg , and C bb  would typically be effectively the same as C r , C g , and C b  from  FIG. 30  to adjust for intensity variations in LEDs  2510 ,  2520 , and  2530 , while the remaining coefficients (Crg, Crb, Cgr, Cgb, Cbr, Cbg) compensate for wavelength variations. 
     For instance, if the red illumination data from timing and control circuitry  2981  was intended for an LED  2510  with a wavelength of 650 nm and the connected LED  2510  wavelength was exactly 650 nm, coefficients C gr  and C br  would be zero and C rr  would be close to one. If such connected LED  2510  wavelength was 660 nm and had the same intensity as the just previous example, C rr  would be slightly smaller than in the just previous example and C gr  and C br  would be non-zero, which would produce some light from such green and blue LEDs  2520  and  2530  respectively. The combination of light from such red, green, and blue LEDs  2510 ,  2520 , and  2530  would be perceived the same as if the red LED  2510  emitted at 650 nm. 
     Memory  3090  and multipliers  3091  can operate and be implemented as described for  FIG. 6 . Adder  3210  sums the multiplication results from the three connected multipliers  3091  to produce the illumination data forwarded to modulators  2987 . Such adders  3210  can be implemented in hardware or software, or be performed bit parallel or bit serial.  FIG. 32  is just one of many possible intensity and wavelength correction matrix  2982  block diagrams. 
       FIG. 33  is an example simplified block diagram of an LCD display comprising of backlight  3321 , diffuser  3322 , polarizers  3323  and  3326 , color filter  3324 , and liquid crystal array  3325 . Image pixel  3330  is expanded to illustrate liquid crystal sub-pixel elements  3331 , which modulate the amount of red, green, and blue light from color filter pixel element  3332 , to produce a particular color and intensity from such image pixel  3330 . The backlight  3321  produces white light from one or many light sources, such as LED  3333 , that is made uniform across the display by diffuser  3322 . Polarizer  3323  only lets a particular polarization of light through to color filter  3324 , which produces red, green, and blue light. Liquid crystal array  3325  selectively rotates the polarization of such light, which is then filtered by polarizer  3326  to produce a color image of pixels  3330 . Backlight  3321  typically comprises one or more white LEDs  3333 , but could comprise a color calibrated combination of red, green, and blue LEDs. 
       FIG. 34  is an example simplified block diagram of LCD  3440  that eliminates color filter  3324  by sequencing the red, green, and blue colors through a single liquid crystal pixel element  3331  three times as fast as LCD  3320 . Such a display is commonly called a Field Sequential Color (FSC) LCD, which costs significantly less than and consumes much less power than LCD  3320 , because the color filter is eliminated. Since the red, green, and blue colors are typically sequenced, white LED  3333  is replaced by red, green, and blue LEDs  2510 ,  2520 , and  2530 . Current source  3334  is replaced with driver IC  2980  that sequentially enables LEDs  2510 ,  2520 , and  2530  by sequentially sinking current through the enable signals enr  3441 , eng  3442 , and enb  3443  respectively. To establish and maintain a precise average color produced by the combination of light from LEDs  2510 ,  2520 , and  2530 , the methods illustrated in  FIGS. 25A-D ,  26 A-D,  27 A-D,  28 A-D, and  31 A-C can be performed by driver IC  2980  or other circuitry. 
     Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. 
     Fourth Embodiment 
     Illumination devices and related systems and methods are disclosed that can be used for LCD (Liquid Crystal Display) backlights, LED lamps, or other applications. The illumination devices can include a photo detector, such as a photodiode or an LED or other light detecting device, and one or more LEDs of different colors. A related method can be implemented using these illumination devices to maintain precise color produced by the blended emissions from such LEDs. Other methods, systems and applications for these illumination devices can also be implemented, as desired. One application for the illumination devices is backlighting for FSC (Field Sequential Color) LCDs (Liquid Crystal Displays). FSC LCDs temporally mix the colors in an image by sequentially loading the red, green, and blue pixel data of an image in the panel and flashing the different colors of an RGB backlight. Precise and uniform color temperature across such a display can be advantageously maintained by continually monitoring ratios of photo currents induced by the different colored LEDs in each illumination device as each color is flashed. Various embodiments are described with respect to the drawings below. Other features and variations can also be implemented, if desired, and related systems and methods can be utilized, as well. 
     As described further below, example embodiments for illumination devices are disclosed that include LEDs with different emission wavelengths and a photo detector. In addition, a method is disclosed to maintain a precise color and intensity emitted from the combination of LEDs in the illumination device. The disclosed embodiments, for example, can be used for LCDs using FSC in which typically only one color LED from a group of red, green, and blue LEDs emit light at any one time. Such embodiments can also be used for conventional LCD backlights and LED lamps in which all the LEDs typically emit at the same time, but periodically sequence the colors for measurement. The embodiments can also be used in other systems and applications, if desired. 
     In one embodiment, as further described below, a photo detector in a illumination device including red, green, and blue LEDs can be used to monitor (e.g., continually, periodically, etc.) the intensity of light produced by each color LED. A controller, such as a controller integrated circuit (IC), for example, can then use the intensity measurements to maintain the fixed blended color and intensity produced by the LEDs. One method that can be performed by the controller IC to control color includes comparing ratios of signals induced in the photo detector by the different colored LEDs to desired ratios, for example, as described herein with respect to the third and seventh embodiments. Desired ratios can be determined, for example, during manufacturing of the illumination device or the display. It is noted that the photo detector can be any light detecting device including but not limited to a silicon photodiode, a discreet LED, a light detecting LED or a light detecting LED integrated on the same die as one of the light emitting LEDs. As such, in the discussions below addressing the use of photodiodes, it is understood that other light detectors can be used instead of the photodiode, including a discreet LED, a light detecting LED, a light detecting LED integrated on the same die as one of the light emitting LEDs or some other light detecting device. 
     Although the intensity control process could be performed continually as in the color control process, preferentially intensity control can also be performed periodically in response to a user command or power up. Other control timing could also be applied if desired. Because the human eye is much more sensitive to variations in color than in intensity, small intensity variations can typically be tolerated by the human eye. 
     Although one primary application for the invention is backlights for FSC LCDs, many other applications such as solid state lighting and conventional LCDs could also benefit from the disclosed embodiments. For example, combining a photo detector, such as a photodiode or an LED or other light detecting device, with different colored LEDs, including white, in the same package enables the light produced by each such LED to be accurately measured even in the presence of significant ambient light or light from LEDs in adjacent packages. In one embodiment, a photodiode enables the temperature of the package and consequently the LEDs to be easily and accurately measured using well known techniques that inject currents into such photodiode, measure forward voltages, and calculate temperature from the results. With such measurements, the color and intensity of the light produced by such an illumination device can be accurately controlled using the methods described herein for any application. The ratio of photo currents can be used to control the relative intensity and consequently the color of light produced by the device and the absolute photo current compensated for temperature can be used to control the total intensity produced by the device. 
     While the embodiments described herein are applicable to a broad range of applications, it is noted, however, that the disclosed embodiments are particularly useful for FSC LCD backlights, because the colors are sequenced and as such the photo detector (e.g. photodiode, LED, etc.) can monitor the light produced by each LED in the illumination device without requiring modifications to the display timing or optics. 
     As stated above, this fourth embodiment can also be used with the techniques, methods and structures described with respect to the other embodiments described herein. For example, the calibration and detection systems and methods described with respect to the second, third, seventh and eighth embodiments can be used with respect to the systems and methods described in this fourth embodiment, as desired. Further, the various illumination devices, light sources, light detectors, displays, and applications and related systems and methods described herein can be used with respect to systems and methods described in this fourth embodiment, as desired. Further, as stated above, the structures, techniques, systems and methods described with respect to this fourth embodiment can be used in the other embodiments described herein, and can be used in any desired lighting related application, including liquid crystal displays (LCDs), LCD backlights, digital billboards, organic LED displays, AMOLED (Active Matrix OLED) displays, LED lamps, lighting systems, lights within conventional socket connections, projection systems, portable projectors and/or other display, light or lighting related applications. 
     As described below, in some embodiments, the illumination device can include one or more colored LEDs, such as a red LED, a green LED, and a blue LED, and a silicon photodiode or other photo detector (e.g., LED, etc.) packaged together as shown in  FIGS. 35 and 40 .  FIG. 35  illustrates the preferential illumination device which includes a photo detector, such as a silicon photodiode or other light detecting device, integrated on the controller IC illustrated in  FIG. 36  that measures such LED output light and temperature, and performs a method to maintain precise color and intensity produced by such LEDs.  FIG. 40  illustrates an alternative illumination device including a photo detector, such as a discreet silicon photodiode or other light detecting device, that is used to measure LED output light and illumination device temperature. The external controller IC illustrated in  FIG. 41  can be used to implement a color and intensity control method for any number of illumination devices. Although the discussions below primarily use a silicon photodiode as the photo detector, it is again noted that the photo detector can be any light detecting device including but not limited to a silicon photodiode, a discreet LED, a light detecting LED or a light detecting LED integrated on the same die as one of the light emitting LEDs. 
       FIGS. 37 ,  38 , and  39  illustrate possible photodiode current and temperature measurement circuitry, simplified system connection diagram, and timing diagram respectively for the preferential illumination device comprising the photodiode integrated on the controller IC. Likewise,  FIGS. 42 ,  43 , and  44  illustrate the same for the illumination device comprising the discreet photodiode. Because an LCD backlight typically needs many illumination devices to provide uniform and sufficient brightness across the display, the system connection diagrams illustrate how such illumination devices can be connected together and to the controller IC in the case of the illumination device comprising a discreet photodiode. 
     Because an LCD backlight typically has many illumination devices, packaging the photodiode with each set of red, green, and blue LEDs helps to minimize the affect light from adjacent LEDs has on the photodiode current induced in a first illumination device by LEDs in such first illumination device. Further, the illumination device package can include an opaque body to block the direct light between adjacent illumination devices and clear plastic fill to allow light to be emitted directly into a display waveguide or diffuser. Some light from adjacent illumination devices can scatter from such waveguide or diffuser into such first illumination device, but provided the photodiode resides in the illumination device the amount of such scattered light is typically sufficiently small to not affect the measurement. 
     As shown in the example timing diagrams illustrated in  FIGS. 39 and 44 , only one color of the red, green, and blue LEDs are emitting at one time in the backlight for an FSC LCD, which enables the photodiode in each illumination device to continually measure the light produced by each such LED. Additionally because scattered light from adjacent illumination devices is sufficiently small, the light produced by all the LEDs in all the illumination devices in an FSC LCD backlight can be measured simultaneously without requiring modifications to the display timing and without special waveguides necessary with conventional RGB backlights. Timing diagrams for other applications, such as conventional LCD backlighting or LED lamps for general illumination, are not shown, but would preferentially have all LEDs emitting simultaneously most of the time. Periodically, each color LED would emit independently for measurement. 
       FIGS. 39 and 44  also illustrate two different approaches for driving the LEDs in the illumination devices, which reduce the number of package pins required when using an integrated photodiode and a discreet photodiode respectively. As shown in  FIG. 36 , the power supply for the red LED also provides power for the controller IC. Because the forward voltages for green and blue LEDs are typically similar, the power supply for both such LEDs is shown to be the same. Additionally, as shown in  FIG. 39 , such green and blue LED power supply preferentially goes high after such red LED power supply, which generates a reset pulse on the controller IC. 
     As shown in  FIG. 41 , the power supplies for the red, green, and blue LEDs are separate, but all three cathodes are connected to one pin on the controller IC. As shown in  FIG. 44 , such LED power supplies sequentially turn on with only one being high at one time. As such, one LED driver on the controller IC can be used to drive all three LEDs in one illumination device. 
       FIG. 45  illustrates possible circuitry in a controller IC to implement the LED color and intensity control method for the illumination device, which can include three steps or processes that include factory calibration, color control, and/or intensity control, if desired. 
     During factory calibration, which would occur at the time an illumination device or a backlight or display is manufactured, the intensity and wavelength of the red, green, and blue LEDs of each illumination device can be measured, coefficients to compensate for such variations can be generated, and the temperature and the photodiode current induced by each LED when producing the desired amount of light can be measured. Such correction coefficients, photodiode currents, and temperature measurements can then be stored in a corresponding controller IC, if such IC has non-volatile memory, and used directly, or they can be stored in some common memory for all illumination devices in a display and loaded each time such display powers up for instance. 
     During normal operation, the color produced by the combination of light from the red, green, and blue LEDs can be precisely maintained by comparing the ratios of photodiode currents induced by the LEDs in an illumination device to the ratio of the desired photodiode currents measured during factory calibration. Because the intensity of light produced by the blue LED remains relatively constant over temperature, the color control process can use the photocurrent induced by the blue LED as a reference. The photodiode currents induced by the red and green LEDs can be divided by the photodiode current induced by the blue LEDs to produce both the actual measured ratios of red over blue and green over blue and the factory desired ratios of red over blue and green over blue. The differences between the actual ratios and the desired ratios can then be low pass filtered before adjusting the average drive current to the red and green LEDs. The color control process can then compare ratios of photodiode currents to cancel any measurement variations that occur over operating conditions such as temperature and power supply voltage and over lifetime. Because the color of light produced by a combination of different colored LEDs is determined by the relative intensity produced by each such LED, comparing ratios of photodiode currents is well suited for the color control process. 
     To maintain a relatively precise intensity produced by the illumination device, the intensity control process can be configured to compare the measured photodiode current induced by the blue LED during operation to the desired photodiode current induced by the blue LED measured during factory calibration. Because such measured photodiode current can vary with temperature, the temperature of the photodiode, which should be nearly the same as the LEDs in the same package, is also measured and the measured photodiode current can be compensated appropriately before being compared to the desired photodiode current. The difference between the temperature compensated and the desired photodiode currents can be stored in a register, which adjusts the average blue LED drive current accordingly. 
     Turning now to the drawings,  FIG. 35  illustrates an illumination device  3510  that comprises an integrated circuit  3511  with a photo detector  3512  and three LEDs, one for each of the colors red  3513 , green  3514 , and blue  3515 . The photo detector  3512  can be, for example, a silicon photodiode, and the discussions below primarily use a photodiode as the photo detector. However, as indicated above, the photo detector  3512  can also be any other light detecting device, as desired. The package encapsulating the IC and LEDs comprises a four pin leadframe  3516 , an opaque plastic body  3517 , and a clear plastic fill  3518  that allows light from the LEDs to emit vertically from the package. The leadframe  3516  comprises four pins for the signals Vr  3519 , Vbg  3520 , Din  3521 , and Dout  3522 . The signal Vr  3519  provides the power to the red LED  3513  and the controller IC  3511 , the signal Vbg  3520  provides the power to the green LED  3514  and the blue LED  3515 , and the Din  21  and Dout  22  signals communicate data and control information into and status from the controller IC  11 . The backside of the illumination device is a commonly used exposed pad that provides good thermal conduction to a printed circuit board and an electrical ground connection. It is also noted that on an integrated circuit, if desired, the silicon photodiode can be implemented as a diffused junction between a P-type substrate and an N-type diffusion layer. Further, if desired, the silicon photodiode can also be implemented as a diffused junction between an N-type substrate and a P-type diffusion later. 
     During calibration for some applications and during normal operation for FSC LCD backlighting applications, the integrated circuit  3511  sequentially provides current to the different colored LEDs, which results in only one LED producing light at a time. The silicon photodiode  3512  and associated detection circuitry continually monitor the light produced by each LED. Control circuitry adjusts the average current provided to each LED to maintain a precise illumination intensity and color. The blue LED  3515  and green LED  3514  typically have two surface contacts and are shown to be flip chip mounted to the integrated circuit  3511 . The red LED  3513  typically has one surface contact and one backside contact and is shown to be attached directly to the integrated circuit  3511  with the top surface contact wire bonded to Vr  3519 . 
       FIG. 35  is one of many possible illumination devices that combine an LED controller IC  3511  with an integrated photo detector, such as a silicon photodiode or other light detecting device, and a set of different colored LEDs in the same package. The example illustrated in  FIG. 35  shows a combination of red, green, and blue LEDs, but such illumination device could comprise any color LEDs including the combination of white and red LEDs for general lighting or conventional LCD backlighting applications. The illumination device  3510  is also shown to have four pins and a backside contact for ground, but could have a wide variety of pin combinations. The LEDs are also shown to be attached directly to the integrated circuit  3511 , but could be attached in a variety of ways including being mounted to the leadframe  3516  or to some other form of substrate and wire bonding to the IC  3511 . Further, as noted above, the photo detector depicted as a photodiode can be any light detecting device, as desired, including but not limited to a silicon photodiode, a discreet LED, a light detecting LED or a light detecting LED integrated on the same die as one of the light emitting LEDs. 
       FIG. 36  illustrates a possible LED controller IC  3511  that provides the drive current to the red  3513 , green  3514 , and blue  3515  LEDs and monitors the light produced by such LEDs and measures the illumination device  3510  temperature using the silicon photodiode  3512  and measurement block  3630 . Network interface  3634  receives illumination and control data from signal Din  3521  and produces status information on signal Dout  3522 . Such illumination data can adjust the intensity and optionally the color of light produced by each illumination device  3510  to support local dimming. Oscillator  3635  provides a reference clock to network interface  3634 , which can recover a clock from the data received on Din  3521  that can be used to clock the rest of integrated circuit  3511 . Timing and control circuitry  3633  uses the recovered clock to manage the operation and functionality of integrated circuit  3511 . 
     The color adjustment circuitry  3636  performs the tasks necessary to maintain precise LED illumination intensity and color produced by the LEDs. Such tasks include monitoring the current produced by the photodiode and adjusting the digital values forwarded to the pulse width modulators  3638 ,  3639 , and  3640  that control the amount of time that current sources  3641 ,  3642 , and  3643  draw current through signals PWMr (red)  3647 , PWMg (green)  3648 , and PWMb (blue)  3649  that are connected to LEDs  3513 ,  3514 , and  3515  respectively. Also shown is low dropout (LDO) regulator  3637  producing the power supply VDD  3645  for IC  3511  from Vr  3519  and reset circuitry  3644  producing the master reset signal/RST  3646  for IC  3511  from Vbg  3520 . Example voltage values for Vr  3519 , Vbg  3520 , and VDD  3645  could be 2.5v, 3.5v, and 1.8v respectively. 
       FIG. 36  is one of many possible block diagrams for a controller IC comprising a photodiode or other light detecting device, such as an LED, and producing the drive current to any number of LEDs. For instance, the illumination intensity produced by such LEDs could be controlled by adjusting the current produced by current sources instead of controlling the amount of time such current sources are producing current using pulse width modulators. Additionally, the control and data signals into and out of the controller IC could be completely different. For instance, illumination data and control data could have separate input pins. Likewise, a clock could be input with data instead of recovering a clock from the data. 
       FIG. 37  is illustrates a possible block diagram for measurement block  3630  contained within control IC  3511 . In this example, amplifier  3750  is configured as a trans-impedance amplifier that forces the current produced by photodiode  3512  through resistor  3751  with amplifier  3750  maintaining a relative fixed voltage on the photodiode cathode. The voltage developed across resistor  3751  is forwarded to mux  3756  and ADC  3757 . 
     The temperature sensor comprises current sources  3752  and  3753  sourcing current I 0  into diodes  3754  and  3755 . Diode  3755  comprises ten diodes with the same physical and electrical characteristics at diode  3754  connected in parallel to produce a diode  3755  with ten times the area as diode  3754 . The voltage difference between the anodes of diodes  3754  and  3755  is proportional to absolute temperature and is forwarded to ADC  3757  through mux  3756 . 
       FIG. 37  is one of many possible block diagrams for measurement block  3630 . For instance, temperature could be measured by forcing two different currents in the forward biasing direction through photodiode  3512  and measuring the differences in the resulting two voltages. The polarity of photodiode  3512  could be reversed and an amplifier could be configured to force zero volts across photodiode  3512 . Additionally, amplifier  3750  could be eliminated and resistor  3751  could be connected across photodiode  3512  to produce a voltage proportional to the current produced by photodiode  3512 . As such,  FIG. 37  is just one example of many possible block diagrams for photocurrent and temperature measurement. 
       FIG. 38  illustrates a possible connection diagram for multiple illumination devices with integrated photodiodes in a display backlight. Illumination devices  3510  illustrate a group of any number of instances of illumination device  3510  that are serially connected together by connecting the Dout signal  3522  of one illumination device  3510  to the Din signal  3521  of the next serially connected illumination device  3510 . The Dout signal  3522  of the last illumination device  3510  is connected to video controller  3861 , which also provides the Din signal  3521  to the first illumination device  3510  and completes a communication ring between video controller  3861  and all the illumination devices. Video controller  3861  can produce the illumination intensity and color data for each illumination device  3510  and can control and monitor the functionality of all such devices. 
     Power supply  3860  provides the Vr  3519  and Vbg  3520  power supplies to the red, and the green and blue LEDs respectively for all the illumination devices  3510 . Such power supplies can be static or can be switched as illustrated in  FIG. 39 . Power supply  3860  is also shown to provide power to video controller  3861 , which typically would be a fixed voltage. 
     In a display backlight, illumination devices  3510  can be connected serially along one or more edges of a liquid crystal panel in a so called edge lit LCD or in an array behind the liquid crystal panel in a so called direct lit LCD. In edge lit LCDs and in some direct lit LCDs, the illumination devices should provide uniform intensity and color behind the liquid crystal panel, with the pixels in the panel producing the image by letting more or less different colored light through. Some direct lit LCDs implement local dimming in which the brightness and sometimes color of each illumination device  3510  or groups of illumination devices can be controlled uniquely for each image frame. 
       FIG. 38  is one of many possible connection diagrams for illumination devices  3510  in a display. For instance, video controller  3861  could connect to multiple chains of illumination devices  3510 . Additionally video controller  3861  could be a graphics or an I/O controller for instance. Chains of illumination devices could be any number including just one. The Vr  3519  and Vbg  3520  power supplies could be connected or separated as shown or could be completely different with different illumination device pinouts. Likewise, connections diagrams in LED lamp or other applications could be implemented differently, as desired. 
       FIG. 39  illustrates one of many possible timing diagrams for the power supplies Vr  3519  and Vbg  3520 , and the LED current source outputs PWMr (red)  3647 , PWMg (green)  3648 , and PWMb (blue)  3649  in an FSC LCD backlight. During start up, Vr  3519  goes high first and then Vbg  3520  goes high, which enables reset generator  3644  in controller IC  3511  to produce a valid /RST signal  3646  to start controller IC  3511  operating from a known state. Controller IC  3511  then drives the red  3513 , green  3514 , and blue  3515  LEDs sequentially by enabling each corresponding PWM and current source. When such PWM signal is shown to be high, no current is drawn through the corresponding LED and no light is produced by that LED. When such PWM signal is shown to be enabled (labeled EN in drawing), the corresponding PWM is enabled and pulsing current through each such. LED. 
     Because only one color LED is emitting at one time, photodiode  3512  can monitor the light produced by each such LED on a continual basis and controller IC  3511  can continually adjust the drive current produced for each such LED to maintain a precise color point and intensity. 
     The sequencing of the light colors shown in  FIG. 39  are appropriate for FSC LCDs among other applications, which mix the red, green, and blue pixel data in time as opposed to in space. Conventional LCDs have a white backlight and colors filters that produce the red, green, and blue light for each pixel, which comprises liquid crystal sub-pixel elements for each color. The red, green, and blue pixel data then allows different amounts of light through each red, green, and blue sub-pixel. FSC LCDs have one liquid crystal pixel element that operates at least three times as fast to allow each color to be presented sequentially, which is mixed temporally by the eye. 
       FIG. 39  is one of many possible timing diagrams for power supplies and LED drive signals. For conventional displays and lamps, for instance, all LEDs would typically produce light at the same time to generate the necessary white light. A different timing diagram could be used to enable the photodiode  3512  to monitor the light produced by each such LEDs. For FSC LEDs, to reduce visual artifacts such as color breakup, the sequence of the different colored LEDs could be different. For instance, the color sequence could repeat over a number of video frames instead of just one as shown. Additionally, methods such as so called stenciling reduce color breakup by inserting a fourth field with all three colors illuminated between each set of red, green, and blue fields. For both conventional and FSC displays, the timing of both the power supply and LED drive signals could be significantly different.  FIG. 39  is just one example. 
       FIG. 40  illustrates an illumination device  4080  that includes a photo detector  4081  and three LEDs, one for each of the colors red  4082 , green  4083 , and blue  4084  as in illumination device  3510  but does not include a controller IC  3511 . The photo detector  4081  can be, for example, a silicon photodiode, and the discussions below primarily use a photodiode as the photo detector. However, as indicated above, the photo detector  4081  can also be any light detecting device including but not limited to a silicon photodiode, a discreet LED, a light detecting LED or a light detecting LED integrated on the same die as one of the light emitting LEDs. As such, in some embodiments, the photo detector  4081  can be implemented as a light detecting LED integrated on the same die as one or more of the light emitting LEDs, if desired. The package encapsulating such photo detector and LEDs comprises a six pin leadframe  4085 , an opaque plastic body  4086 , and a clear plastic fill  4087  that allows light from the LEDs to emit vertically from the package. The leadframe  4085  comprises six pins for the signals Vr  4088 , Vg  4089 , and Vb  4090  that connect to the anodes of such red, green, and blue LEDs respectively, and for the signals LC  4091 , PDC  4092 , and PDA  4093 . The LC signal  4091  connects to the cathodes of all such LEDs, and PDC  4092  and PDA  4093  connect to the photodiode  4081  cathode and anode respectively. The backside of the illumination device  4080  should remain electrically isolated in this example. 
     The photodiode  4081 , and the green  4083  and blue  4084  LEDs can have both contacts on the top side of each such die with all anodes wire bonded directly to the corresponding pins. The LED cathodes are down bonded to the lead frame which is then wire bonded to the LC  4091  pin. The surface anode connection on photodiode  4081  is wire bonded to the PDC  4093  pin. The red LED  4082  is shown to have a surface contact for the anode that is wire bonded to the Vr  4088  pin and a backside contact for the cathode that is electrically and mechanically connected to the lead frame. 
       FIG. 40  is one of many possible illumination devices  4080  that comprise LEDs and a photo detector to monitor the relative output power of each LED to maintain a precise color point and intensity. For instance, illumination device could comprise more or less LEDs or additional photo detectors. The cathodes of the LEDs could have dedicated pins instead of being connected together as shown or all the anodes could be common with the cathodes pinned out separately. The photodiode cathode or anode could share a common connection with one or more LEDs. The package could be mechanically completely different. The pins could be surface mount or through hole for instance. Further, as noted above, the photo detector depicted as a photodiode can be any light detecting device, as desired, including but not limited to a silicon photodiode, a discreet LED, a light detecting LED or a light detecting LED integrated on the same die as one of the light emitting LEDs. 
       FIG. 41  illustrates a possible LED controller IC  4100  that resides outside the illumination device and provides the drive current to the red  4082 , green  4083 , and blue  4084  LEDs and monitors the light produced by such LEDs and the temperature using the photodiode  4081  and measurement block  4101 . Such controller IC  4100  connects to N number of illumination devices  4080  and maintains the proper illumination color and intensity produced by all connected illumination devices  4080  over operating conditions and lifetime. In this example, all the PDA  4093  signals from illumination devices  4080  are tied together to the PDA  4114  pin of controller IC  4100 . Each illumination device PDC  4092  pin is connected to a unique PDC pin on controller IC  4100  labeled PDC 1   4115  through PDCn  4116 . 
     Network interface  4102 , timing and control circuitry  4103 , oscillator  4104 , and color adjust block  4105  should be very similar or identical to such blocks comprising controller IC  3511 . Likewise, PWM blocks  4106  through  4107  and current sources  4108  through  4109  should be very similar or identical to such blocks comprising controller IC  3511 . The primary differences between controller IC  3511  and controller IC  4100  include measurement block  4101 , the number of LEDs that can be driven, and the timing of how the LEDs are driven. The cathodes of all three LEDs  4082 ,  4083 , and  4084  of an illumination device  4080  are connected to one current source  4108  through signal PWM 1   4110 . Up to N illumination devices  4080  can be connected to the N current sources identical to current source  4108  with current source  4109  connected to signal PWMn  4111  representing the connection to the Nth illumination device  4080 . The LDO  3637  and reset circuitry  3644  shown in controller IC  3511  are replaced with input pins VDD  4112  and /RST  4113 . Din  4117  and Dout  4118  have similar functionality to Din  3521  and Dout  3522  on Controller IC  3511 . 
       FIG. 41  is one of many possible block diagrams for a controller IC connecting to and controlling an illumination device  4080  comprising a photodiode or other light detecting device, such as an LED, and LEDs in which the photodiode or other light detecting device, such as an LED, monitors the amount of light produced by such LEDs. For instance the controller IC could have LED driver circuitry for each color LED individually instead of controlling all three LEDs with one driver. If illumination device  4080  comprised a different number of LEDs, then controller IC  4100  would connect to that number of LEDs in each illumination device. 
       FIG. 42  is one possible block diagram for the measurement block  4101  that measures the photodiode  4081  current produced by LEDs  4082 ,  4083 , and  4084  in all connected illumination devices  4100  and that uses such photodiodes  4081  to measure the temperature in each such connected illumination device  4080 . When LightEn signal  4228 , which is controls switch  4220 , is high and TempEn signal  4229 , which controls switch  4221 , is low, measurement block  4101  is configured to measure current in photodiodes  4081  by shorting the photodiode  4081  anodes to ground and letting the selected photodiode  4081  cathode be controlled by amplifier  4224 . The current in the selected photodiode  4081  is forced through resistor  4225  producing a voltage which is forwarded through mux  4226  to ADC  4227 . 
     When LightEn signal  4228  is low and TempEn signal  4229  is high, measurement block  4101  is configured to measure the temperature of the photodiode in the selected illumination device  4080  by shorting the cathode of the selected photodiode  4081  to ground and forcing different currents from current source  4223  through the selected photodiode. Current source  4223  supplies two different currents, I 0  and ten times I 0 , through the photodiode  4081  selected by mux  4222  and mux  4226  forwards the resulting voltages to ADC  4227 . The difference in the two resulting voltages is proportional to absolute temperature. 
       FIG. 42  is one of many possible block diagrams for measurement block  4101 . As described for  FIG. 37 , the current induced in, the photodiodes by the LEDs can be measured in a variety of ways. Likewise, the temperature can be measured in a variety ways. For instance, two photodiodes of different sizes could reside in the illumination device  4080  and the voltage difference when applying the same current to both photodiodes can be measured. 
       FIG. 43  illustrates a possible connection diagram for multiple illumination devices with discreet photodiodes in a display backlight. Multiple instances of illumination device  4080  are connected to the N number of LED drivers and photodiode measurement blocks on one controller IC  4100 . Power Supply  4331  provides the Vr  4088 , Vg  4089 , and Vb  4090  to all the illumination devices  4080  and provides a fixed power supply to the video controller  4330  and the VDD  4112  power supply for the controller IC  4100 . The video controller  4330  provides the Din  4117  and /RST  4113  signals to and accepts the Dout  4118  signal from the controller IC  4100 . The PDA  4093  signals from all the illumination devices  4080  are connected to the PDA  4114  pin of the controller IC  4100 . 
     As described for  FIG. 38 , illumination devices  4080  can reside along the edges of an edge lit LCD or in an array behind the liquid crystal panel in a direct lit LCD. The video controller  4130  can communicate illumination data for each illumination device and can manage the controller IC  4100 . Since the cathode of all the LEDs  4082 ,  4083 , and  4084  are connected together to one driver on controller IC  4100 , such connection diagram only allows the intensity of light from color LED to be controlled. Each color component is controlled individually by enabling the Vr  4088 , Vg  4089 , and Vb  4090  power supplies one at a time. 
       FIG. 43  is one of many possible connection diagrams for illumination devices  4080  and controller IC  4100 . In displays that require more illumination devices than one controller IC  4100  can support, multiple controller ICs  4100  can be serially connected through network interface  4102  or video controller  4330  can connect directly to multiple controller ICs  4100 . For conventional displays, illumination device  4080  and controller IC  4100  could be configured to enable all LED colors to be emitting simultaneously by tripling the number of drivers and the connection diagram would differ accordingly. 
       FIG. 44  illustrates one of many possible timing diagrams for the power supplies VDD  4112 , Vr  4088 , Vg  4089 , and Vb  4090 , the /RST signal  4113 , and the LED current source output PWM 1   4110  in an FSC LCD backlight. During start up, VDD  4112  goes high first and then /RST  4113  goes high, which starts controller IC  4100  operating from a known state. Controller IC  4100  then signals to video controller  4330  and power supply  4331  to begin sequencing the LED power supplies Vr  4088 , Vg  4089 , and Vb  4090 . While each such LED power supply is high, controller IC  4100  drives the appropriate average current through each such LED. For instance, when Vr  4088  is high, controller IC  4100  forwards the appropriate illumination information for the red LED  4082  to the PWM  4106  and current source  4108  to produce that appropriate light intensity from the red LED. Since the power supplies, Vg  4089  and Vb  4090 , to the green  4083  and blue  4084  LEDs are low during this time, no current flows through such LEDs and no light is produced. 
     Because only one color LED is emitting at one time, photodiode  4081  or other light detecting device, such as an LED, can monitor the light produced by each such LED on a continual basis and controller IC  4100  can continually adjust the drive current produced for each such LED to maintain a precise color point and intensity. Measurement block  4101  sequentially and repetitively monitors the photodiodes  4081  connected to pins PDC 1   4115  through PDCn  4116 . 
       FIG. 44  is one of many possible timing diagrams for power supplies and LED drive signals in an FSC display. For instance, the color sequence could be different for different video frames and repeat over a number of video frames instead of just one as shown.  FIG. 44  is just one example. Likewise, timing diagrams in conventional displays or LED lamps could be implemented similarly or in a different manner, if desired. In such applications, the illumination device package could provide independent pins for the LED cathodes so that all LEDs could be emit simultaneously for some period of time and independently when emitted power is measured. Other techniques could also be implemented if desired. 
       FIG. 45  is illustrates a possible block diagram for the color adjustment block  3636  in controller IC  3511 , which is essentially repeated N times in the color adjust block  4105  in controller IC  4100 . For simplicity the remainder of this discussion will reference only illumination device  3510  and not illumination device  4080  and controller IC  4100 ; however, the discussion is also applicable to these other embodiments. Further, for simplicity, this discussion assumes that a photodiode is being used as the photo detector. However, as indicated above, the photo detector could be any light detecting device including but not limited to a silicon photodiode, a discreet LED, a light detecting LED or a light detecting LED integrated on the same die as one of the light emitting LEDs, as desired. 
     Color adjustment block  3636  receives the intensity data for the red  3513 , green  3514 , and blue  3515  LEDs from timing and control circuitry  3633 , adjusts such values in matrix  4540 , and forwards them to PWMs  3638 ,  3639 , and  3640 . Matrix  4540  comprises coefficients determined during manufacturing of illumination device  3510  that are used to compensate for variations in LED intensity and wavelength to produce the desired color and intensity from the combination of red  3513 , green  3514 , and blue  3515  LEDs at one temperature. The proper light color is maintained during normal operation by continually comparing the ratio of currents induced in photodiode  3512  by the red  3513  and green  3514  LEDs over the current induced in photodiode  3512  by the blue  3515  LED to the desired ratios of such current determined during manufacturing, and adjusting the values forwarded to PWMs  3638  and  3639  through feedback loops that include multipliers  4541  and  4543 . The proper average intensity of light produced by the combination of red  3513 , green  3514 , and blue  3515  LEDs is controlled periodically by comparing the temperature adjusted current induced in photodiode  3512  by blue  3515  LED to the desired such current determined during manufacturing, and adjusting the value forwarded to PWM  3640  with multiplier  4542 . 
     During the manufacturing of illumination device  3510  when the coefficients for matrix  4540  are determined, the currents induced in photodiode  3512  by each LED  3513 ,  3514 , and  3515  are measured and stored in registers  4544 ,  4546 , and  4545  respectively. Likewise, the temperature is measured and saved. During operation, the photodiode currents induced by the red  3513 , green  3514 , and blue  3515  LEDs are continually measured, digitized and stored in registers  4547 ,  4549 , and  4548  respectively. The ratios of actual photodiode currents induced by the red LED  3513  and green LED  3514  over such current induced by the blue LED  3515  are determined by dividers  4550  and  4551  respectively. Such ratios of actual photodiode currents are compared to the ratios of such desired photodiode currents determined during manufacturing and produced by dividers  4552  and  4553 . Since the photodiode currents measured during manufacturing corresponded to particular intensity data from timing and control circuitry  3633 , dividers  4554  and  4555 , and multipliers  4556  and  4558  adjust the ratio of such desired photodiode currents prior to be compared to the output of dividers  4550  and  4551  by adders  4559  and  4561 . The differences between the desired photodiode current and actual photodiode current ratios determined by adders  4559  and  4561  are filtered by low pass filters  4562  and  4564  respectively prior to being applied to multipliers  4541  and  4543  respectively. Low pass filters (LPFs)  4562  and  4564  are configured to ensure that the feedback loop is stable. 
     Color adjustment block  3636  references the photodiode current induced by the red LED  3513  and the green LED  3514  to the blue LED  3515  because the intensity of light produced by blue typically varies very little over temperature. Color adjustment block  3636  compares ratios of photodiode currents instead of individual photodiode currents because the photodiode response varies over temperature and other conditions. By comparing ratios of photodiode currents measured at the same time, any such variations cancel out and precise color can be maintained. 
     As blue LED  3515  ages, the light intensity produced for a given average drive current changes. Color adjustment block  3636  typically compensates for such changes in the blue LED once in a while, for instance during power up or on command, but could continually compensate. During such compensation, the actual photodiode current induced by blue LED  3515  is measured by measurement unit  3630  and the results are stored in register  4548 . Measurement unit  3630  also measures the temperature, the results of which temperature compensation block  4565  uses to scale the actual photodiode current to the temperature during manufacturing when the desired photodiode current stored in register  4545  was measured. Multiplier  4557  scales the output from register  4545  by the blue data from timing and control circuitry  3633 , divider  4560  produces the ratio of multiplier  4557  output over the output from temperature compensation block  4565 , and the result of which is stored in register  4563 . Multiplier  4542  adjusts the blue output from matrix  4540  prior to forwarding to PWM  3640 . 
       FIG. 45  is one of many block diagrams for a color adjustment block  3636  that maintains precise color and intensity of light produced by an illumination device  3510 . Although color is preferentially controlled by comparing a ratio of photodiode currents induced by different colored LEDs at one time, for instance during manufacturing, to the ratio of photodiode currents induced by the same LEDs at a different time, color could be controlled by comparing actual photodiode currents to desired photocurrents. Such preferential color and intensity control circuitry could be implemented in many different forms including software. The functionality illustrated in  FIG. 45  does not compensate for variations in LED emission wavelength; which could be done by adjusting the coefficients in matrix  4540 . Likewise, the intensity adjustment performed by multipliers  4541 ,  4542 , and  4543  could be done by adjusting the coefficients in matrix  4540 . 
       FIG. 46  is an example block diagram for matrix  4540  that can correct for variations in both light intensity and wavelength produced by a combination of red, green, and blue LEDs  3513 ,  3514 , and  3515  for instance to produce uniform brightness and color from an array of LEDs. Matrix  4540  comprises memory  4670  that can store nine correction coefficients with three such coefficients for each color component produced. Coefficients Crr, Cgg, and Cbb would typically adjust for intensity variations in LEDs  3513 ,  3514 , and  3515 , while the remaining coefficients (Crg, Crb, Cgr, Cgb, Cbr, Cbg) compensate for wavelength variations. 
     Memory  4670  can comprise SRAM, DRAM, FLASH, registers, or any other form of read-writable semiconductor memory. Such correction coefficients are typically determined during manufacturing and remain unchanged during operation, however, such coefficients could be periodically modified by controller IC  3511  or any other processing element in a display or lamp for instance to adjust for changes in LEDs  3513 ,  3514 , and  3515  characteristics for instance over temperature or lifetime. If memory  4670  does not comprise non-volatile memory such as FLASH, the correction coefficients should be loaded into such memory when powered up. 
     Multipliers  4671  scale the illumination data from timing and control circuitry  3633  by multiplying each color component by the corresponding correction coefficient. Such multiplication can be performed by discreet hardware in bit parallel or bit serial form, in an embedded microcontroller, or by any other means. Preferentially, one hardware multiplier comprising, a shifter and an adder performs all nine multiplications. Adder  4672  sums the multiplication results from the three connected multipliers  4671  to produce the illumination data forwarded to modulators  3638 ,  3639 , and  3640 . Such adders  4672  can be implemented in hardware or software, or be performed bit parallel or bit serial. 
       FIG. 46  is just one of many possible block diagrams for correction matrix  4540 . Likewise, correction matrix  4540  could reside elsewhere in a display, such as software in a graphics controller. 
       FIG. 47  is an example simplified block diagram of an LCD display  4780  comprising of backlight  4781 , diffuser  4782 , polarizers  4783  and  4786 , color filter  4784 , and liquid crystal array  4785 . Image pixel  4790  is expanded to illustrate liquid crystal sub-pixel elements  4791 , which modulate the amount of red, green, and blue light from color filter pixel element  4792 , to produce a particular color and intensity from such image pixel  4790 . The backlight  4791  produces white light from one or many light sources, such as LED  4793 , that is made uniform across the display by diffuser  4782 . Polarizer  4783  only lets a particular polarization of light through to color filter  4784 , which produces red, green, and blue light. Liquid crystal array  4785  selectively rotates the polarization of such light, which is then filtered by polarizer  4786  to produce a color image of pixels  4790 . Backlight  4781  typically comprises one or more white LEDs  4793 , but could comprise a color calibrated combination of red, green, and blue LEDs. 
       FIG. 48  is an example simplified block diagram of FSC LCD  4800  that eliminates color filter  4784  by sequencing the red, green, and blue colors through a single liquid crystal pixel element  4791  typically three times as fast as LCD  4780 . Such a display typically costs significantly less than and consumes much less power than LCD  4780 , because the color filter is eliminated. Since the red, green, and blue colors must be sequenced, white LED  4793  is replaced by red, green, and blue LEDs  3513 ,  3514 , and  3515  in illumination device  3510 . Current source  4794  is replaced with driver IC  3511  that sequentially enables LEDs  3513 ,  3514 , and  3515  by sequentially sinking current through the PWM signals PWMr (red)  3647 , PWMg (green)  3648 , and PWMb (blue)  3649  respectively. To establish and maintain a precise average color and intensity produced by the combination of light from LEDs  3513 ,  3514 , and  3515 , illumination device  3510  can comprise the circuitry and implement the methods described herein. Illumination device  3510  illustrated in  FIG. 48  illuminates many pixels  4791 . 
     Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. 
     Fifth Embodiment 
     In certain exemplary embodiments, an improved illumination device uses the components in an LED lamp to perform a variety of functions for very low cost. The LEDs that produce light can be periodically turned off momentarily, for example, for a duration that the human eye cannot perceive, in order for the lamp to receive commands optically. The optically transmitted commands can be sent to the lamp, for example, using a remote control device. The illumination device can use the LEDs that are currently off to receive the data and then configure the light accordingly, or to measure light. Such light can be ambient light for a photosensor function, or light from other LEDs in the illumination device to adjust the color mix. Various embodiments are described with respect to the drawings below. Other features and variations can also be implemented, if desired, and related systems and methods can be utilized, as well. 
     In certain exemplary embodiments, an illumination device uses LEDs to produce light and to provide bi-directional communication to a controller that implements power saving features not possible with conventional lighting. The illumination device, for example, can be programmed with modulated light from a remote controller to turn on and off, to adjust brightness or color, and to turn on or off in response to changes in ambient light or timer count values. The LEDs that produce the illumination during normal operation are periodically used to receive modulated light from a controller during short intervals undetectable by the human eye. In response to a command from the remote controller, the illumination device can produce light modulated with data. Additionally, when the remote controller is turned off and is exposed to sunlight, the LEDs in the controller can provide a trickle charge current to maintain full battery power. 
     In certain aspects, the invention provides a system of an intelligent illumination device and, in some cases, a remote controller. The illumination device, which is typically connected to an AC mains power supply, can receive commands from the remote controller, which is typically battery powered, via light. The remote controller then programs the lamp for timer or photosensitive operation. For instance, at dusk the lamp could turn on and then go off, the light could come on when power is switched on and goes off a fixed time later, the light could come on and go off at fixed times, or the light could come on at dusk and off at dawn. Dimming could also be enabled or disabled, or could be automatically adjusted based ambient light. 
     When turned on, the illumination device periodically turns off the LEDs to determine if any commands are being sent or to measure ambient light. The remote control synchronizes to these momentary “light off” periods and sends a command if directed by the user. The commands can be on/off, dim, timer, photo cell, color, etc. When the light is turned off by the remote, ac power is still active. The device goes into a low power mode. When the remote turns the light on, the incident light can power the LEDs and enable the light to turn on. The light can also be turned off by removing AC power and turned on by turning AC power on. Cycling power in a certain sequence can reset the light to a default state. 
     In certain embodiments, the illumination device uses the photosensitive LEDs (i.e., the red LEDs) to detect received data or DC light during the intervals when the light output is momentarily turned off. For multi-colored light, the illumination device can use a chain of the longest wavelength LEDs (i.e., the red LEDs) to detect the output power of the other colors. With two chains of the longest wavelength LEDs, each chain can measure the output power of the other, thereby enabling a feedback loop to control the output power of each color and the blended color mix. 
     Once the illumination device (i.e., the “lamp”) is installed in an existing socket that may or may not be connected to a dimming switch, the illumination device can be dimmed by the remote controller. The remote controller sends commands to increment or decrement the output light level during the short “off” periods. The dimming function can be performed by pulse width modulating the LED drive current at a switching frequency preferably locked to the switching regulator frequency or by simply adjusting the LED drive current. 
     If photosensing is enabled, during the short light off periods, the longest wavelength LED chain can be used to measure ambient light. To do so, the LEDs may be configured in photovoltaic mode, and produce a voltage proportional to incident light. If the voltage is above a level specified through a command, the lamp can turn off in response. If the voltage drops back below the specified level, the lamp can turn on. Such a mechanism enables the light to turn on at night and off during the day. In combination with a timer, the light can turn on at dusk and off after a specified amount of time. 
     When the timer is enabled, the lamp can turn on and off at different times of day or turn off after a specified amount of time after being turned on. The lamp can be turned on by remote control, by power being applied through a switch, or by the photosensor function. In a mains connected application, the timer is synchronized to the AC frequency for a precise frequency reference. 
     When powered by a battery, the photosensitive LED chains can provide trickle current to re-charge the battery. A chain of 30 red LEDs (e.g., in the CREE lamp) can produce nearly 1 mW of power that can keep a re-chargeable battery charged in applications, such as emergency lights, that are not used often. For applications such as solar-powered, off-grid systems that are common in the developing world, the charging capability of the lamp can augment that of the solar panel. 
     As stated above, this fifth embodiment can also be used with the techniques, methods and structures described with respect to the other embodiments described herein. For example, the calibration and detection systems and methods described with respect to the second, third, seventh and eighth embodiments can be used with respect to the intelligent LED lights described in this fifth embodiment. Further, the communication techniques described with respect to this fifth embodiment can be used with respect to the other embodiments, if desired. 
     Turning now to the drawings,  FIG. 49  is one example of an intelligent illumination device system  4910  that comprises the illumination device  4911  and the remote controller  4912 . The remote controller  4912  is preferably battery powered like a flashlight or TV remote control and is used to program the illumination device  4911  with modulated light. When the illumination device  4911  is powered preferably by the AC mains of an electrical socket (e.g., an Edison base socket), the illumination device  4911  can be controlled by the remote controller  4912 . When the illumination device  4911  is enabled to produce light (i.e., “turned on” or “producing light”), the illumination device  4911  briefly and periodically stops emitting light to detect commands from the remote controller  4912  or ambient light from the environment, or to calibrate colors in a multi-colored illumination device  4911 . When the illumination device  4911  is powered by the AC mains, but is not enabled to produce light (i.e., “turned off”), the illumination device enters a low power state. Commands from the remote controller  4912  can still be detected by the illumination device  4911  in this state. The illumination device  4911  responds to the remote controller  4912  by momentarily producing light modulated with data. To reset the illumination device  4911  to a default state, power to the illumination device  4911  is cycled in a specific sequence. 
       FIG. 49  is just one example of many possible intelligent illumination device systems. For example, the illumination device  4911  could be powered with a battery or the remote controller  4912  could be powered by the AC mains. In another example, if the illumination device is programmed when it is designed or produced, no remote controller  4912  is needed. Examples of pre-programmed devices include pre-configured night lights, and lights that automatically turn of perhaps 1 hour (or other delay) after being turned on. In such case, the functionality of the illumination device may be reduced. 
     In another example, light from the remote controller  4912  could power an un-powered illumination device  4911  with light while programming. For instance, a consumer could buy a light bulb replacement including this remote controller. The consumer could then hold the bulb to the remote and configure it to turn off 35 minutes after being turned on, then take the programmed bulb and screw in a socket somewhere. Without this self-powered variant, the bulb would need to be screwed into an energized socket in order to program it, which may be possible, but still perhaps less convenient. 
     In a further example, the remote controller battery could be charged by sunlight or ambient light when not in use. Additionally, multiple illumination devices  4911  could communicate with each other. For example, various governments have recently introduced mandates that certain buildings must have intelligent lights that automatically turn on and off based on whether or not people are present. Some large lighting companies provide systems consisting of lamps with motion detectors and 900 MHz RF transceivers. When one lamp in a room detects motion, it tells the rest of the lights to turn on. The two main issues with this approach are: (1) the lights are expensive, and (2) the RF signal passes through walls to other rooms with no people. The devices described herein could communicate with each other via light which: (1) does not require the expense of the RF circuitry, and (2) does not go through walls. Additionally, functions like dimming or color control could benefit from lamps communicating with each other. For example, a user could program one lamp, and that lamp then reconfigures the other lamps. Additional applications could be security where two lamps constantly communicate with each other. If an intruder passes between them and momentarily blocks the light, the lamps detect this and broadcast info to other lamps in the building in sort of a daisy chain way to a central security system. 
       FIG. 50  provides Table 2 that includes an example list of commands  5014  for the illumination device  4911  that enable the remote controller  4912  to turn the illumination device  4911  on and off, adjust the output power, and change the color to one of three different settings. Additionally, the illumination device  4911  can be configured to automatically turn on in response to a time of day counter reaching a particular count or ambient light dropping below a certain level, and to automatically turn off after a timer reaching a particular count from when the illumination device  4911  is turned on or ambient light rising above certain level. In this example, the color mix is always automatically measured and adjusted to a specific setting. The example set of commands  5014  can use 4 bits to produce hex codes  5013 . 
     Preferably, the hex codes  5013  are preceded by a synchronization pattern and followed by parity to produce an 8 bit transfer sequence. Additionally, the commands that set a time must be followed with the actual time. Since there are 1440 minutes in a day, a time with one minute resolution requires 11 bits, which could be sent in two successive transfers after the command. 
     Table 2 is just one example of many possible sets of commands  5014  and hex codes  5013 . For instance, in a multi-color light each individual component could be dimmed or color calibration could be enabled and disabled. As another example the time of day counter could count days of the week as well. The illumination device  4911  could have a subset of these functions or could have a variety of other functions such as strobing or continuous color variation. Additionally, illumination device  4911  status and register contents could be read. Further, the assignment of hex codes  5013  to commands  5014  could be completely different and could contain more or less bits depending on the number of commands  5014 . 
       FIG. 51  is an example timing diagram for communicating commands  5014  between the illumination device  4911  and the remote controller  4912  when the illumination device  4911  is producing light. Pulse width modulated light PWM  5120  from the illumination device  4911  is periodically interrupted by gaps  5121  when no light is produced. The gap period  5122  in this example is one second. The gap time  5123  is equal to one half the mains period or 8.33 mSec at 60 Hz. The remote controller  4912  synchronizes to gaps  5121  in the PWM  5120  light from the illumination device  4911  and can send commands CMD  5124  during gaps  5121 . When a CMD  5124  is sent from the remote controller  4912  and is properly received by the illumination device  4911 , the illumination device  4911  provides a response RSP  5125  immediately after CMD  5124 . The remote controller  4912  may preferably be narrowly focused (much like a flashlight) to assist a user in directing the remote commands to a particular illumination device in a room with multiple such illumination devices. The user could see the light beam and shine it directly on one light. This would focus light from the remote on the illumination device and light from the illumination device on the detector in the remote. 
     In this example, the light from the illumination device  4911  is pulse width modulated at 16 times the mains frequency or 960 Hz for 60 Hz AC, to enable dimming without changing LED wavelengths. At full brightness, the off time is very short or non-existent and at low light levels, the on time is short. The frequency of the pulses stays fixed. To prevent the remote controller  4912  from losing synchronization with the illumination device  4911 , the last pulse from the illumination device  4911  before a gap  5121  is preferably not reduced below a minimum width that the remote controller  4912  can detect. 
     In another example, the one second gap period  5122  can be shortened to 200 msec for instance, after the illumination device  4911  and remote controller  4912  communicate a first CMD  5124  so that successive commands can be communicated faster. This may be important for dimming since there may be many power level steps between low and high power. Once the remote controller  4912  stops sending commands, the gap period  5122  widens back to one second intervals. 
     When the illumination device  4911  is not producing light, the remote controller  4912  does not detect gaps  5121  and can send commands CMD  5124  at any time. The protocol shown in  FIG. 51  remains the same except that the illumination device  4911  is not outputting PWM  5120  light before and after the transaction. 
     During gaps  5121  when commands CMD  5124  are not sent or when the illumination device  4911  is not producing light, the illumination device  4911  can measure ambient light. The ambient light level is subtracted from the received light when commands CMD  5124  are sent and is used to determine when to turn the illumination device  4911  on or off when photo-sensor functionality is enabled. More specifically, when the illumination device is receiving commands, the background or ambient light produces a DC offset in the optically induced voltage across the LEDs (or photodiode). This DC offset can be eliminated by measuring the optically induced voltage during gaps  5121  when no commands are sent, and subtracting it from the induced voltage when receiving commands. Alternatively, the receiver in the illumination device can high pass filter the induced voltage to remove the DC offset. Since the data rate is low, the receiver may use a digital filter for DC blocking (and equalization). If the DC offset is known prior to receiving a command, the initial state of the digital filter can be set accordingly, and reduce the settling time. When photosensor functionality is enabled, ambient light is measured during gaps  5121  when the illumination device is producing light, and measured all the time when not producing light. 
     Additionally, in a multi-color illumination device  4911 , the intensity of each individual color can be measured during gaps  5121  or when the illumination device  4911  is not producing light. For instance, when the illumination device  4911  is turned on, the illumination device  4911  can briefly measure the intensity of each color before producing the desired light. Then periodically as the illumination device warms up for instance, the color components can be measured during gaps  5121 . 
       FIG. 51  is just one example of many possible timing diagrams. The gap period  5122  and gap time  5123  could be substantially different depending on the applications. The response RSP  5125  can be sent at different times or not at all. The commands CMD  5124  could even be sent during the off times of the PWM cycle and responses RSP  5125  could be variations in PWM duty cycle. To provide additional error protection, commands CMD  5124  could be repeated one or more times before taking affect. Many different timing diagrams and communication protocols could be implemented. For an illumination device  4911  that is powered by the light from the remote controller  4912  instead of a battery or AC mains, the protocol can include significant illumination durations in order to store sufficient charge on a capacitor for instance to power the illumination device  4911  and to communicate data. 
       FIG. 52  is an example timing diagram illustrating the bit level communication between the illumination device  4911  and the remote controller  4912  when the illumination device  4911  is producing light. Communication begins with the illumination device  4911  stopping the PWM  5120  output. The illumination device synchronization IDSYNC  5230  pulse is the last PWM pulse produced by the illumination device  4911  prior to a gap  5121 . The width of IDSYNC  5230  is greater than the minimum pulse width detectable by the remote controller  4912 . Other synchronization sequences, such as short series of pulses, may also be produced before each gap  5121 . The CMD  5124  from the remote controller  4912  comprises a synchronization pattern SYNC  5231  of 3 ones, a hex code  5013 , and an even parity bit P  5232  that are biphase encoded. In this example, the command  5014  is “light off”. If the illumination device  4911  receives the CMD  5124  properly, the response RSP  5125  comprises the same biphase encoded SYNC  5231 , hex code  5013 , and parity P  5232  that comprised the CMD  5124 . 
     When the illumination device  4911  is not producing light, the protocol shown in  FIG. 52  remains the same except that the illumination device is not outputting PWM  5120  light (nor IDSYNC  5230 ) before and after the transaction. 
       FIG. 52  is just one example of many possible bit timing diagrams. Instead of biphase encoding, the protocol could use any one of many well known coding schemes such 4b5b, 8b10b, or NRZ. The SYNC  5231  could have a wide variety of lengths and sequences including none at all. The hex codes  5013  could have more or less bits and parity P  5232  could be even or odd, more than one bit, or none at all. CRC codes could be used for error detection. For an illumination device  4911  that is powered by light from the remote controller  4912 , the protocol could be substantially different. In particular, it may be necessary to transmit data one bit at a time from the illumination device  4911  to the remote controller  4912  with the remote controller  4912  emitting light to re-charge a capacitor on the illumination device  4911  for instance between bits sent from the illumination device  4911 . Useful transceiver techniques for so doing are described in U.S. patent application Ser. No. 12/360,467 filed Jan. 27, 2009 by David J. Knapp and entitled “Fault Tolerant Network Utilizing Bi-Directional Point-to-Point Communications Links Between Nodes,” and in U.S. patent application Ser. No. 12/584,143, filed Sep. 1, 2009 by David J. Knapp and entitled “Optical Communication Device, Method and System,” each of which is hereby incorporated by reference in its entirety. 
       FIG. 53  is an example block diagram for an exemplary illumination device  4911  that comprises an EMI filter and rectifier  5341 , an AC to DC converter, a voltage divider, an integrated circuit IC  5354 , and the LED chain  5353 . The EMI filter and rectifier  5341  produces a full wave rectified version of the AC mains VAC  5340 , and minimizes both transient disturbances on the mains from affecting the rectified power, and switching noise in the illumination device  4911  from affecting the mains. The voltage divider comprises resistors R  5342  and R  5343  and produces signal S  5357  that is a reduced voltage version of the rectified mains signal for IC  5354 . The AC to DC converter includes inductors  5344  and  5345  (also referred to herein as inductors L  5344  and L  5345 ), capacitors  5346  and  5347  (also “capacitors C  5346  and C  5347 ”), diode  5348  (also “diode D  5348 ”), the N-channel switch transistor  5349  (also “switch N  5349 ”), and the power controller  5362  on integrated circuit  5354  (IC  5354 ). This example shows LED chain  5353  comprising of LED  5350 , LED  5351 , and LEDn  5352 , with the dashed line between LED  5352  and LEDn  5353  indicating that LED chain  5353  can include many LEDs. This architecture is typical for monochrome light or white light produced by blue LEDs with a phosphor coating. A multi-color illumination device typically would have separate LED chains for each color. 
     IC  5354  includes memory and control  5360 , PLL and timing  5361 , power control  5362 , receiver  5363 , and output driver  5364 . Memory and control  5360  includes non-volatile memory for storing configuration information, such as enabling the timer or photo-sensor, and volatile (or non-volatile) memory for settings such as dimming. Memory and control  5360  also includes logic that manages the transfer of data with the remote controller  4912 , produces the pulse width modulated (PWM) LED drive signal S  5359 , and implements the timers and state machines that control the overall function of IC  5354  and the illumination device  4911 . 
     PLL and timing  5361  includes a phase locked loop that produces a high frequency clock that is phase locked to S  5357  when the illumination device is powered. The voltage divider comprising of R  5342  and R  5343  provides a low voltage version of the rectified mains voltage S  5357  that does not exceed the voltage rating of IC  5354  and that the PLL locks to. All other circuitry on IC  5354  is synchronized to the PLL and timing  5361  outputs (not shown). 
     PLL and timing  5361  enables the illumination device  4911  to maintain a precise time base for time of day timer functionality by locking to the mains frequency. Likewise, gap period  5122  and gap time  5123  can be precisely aligned to VAC  5340  timing. Such timing could enable multiple illumination devices  4911  to synchronize and communicate directly between each other with light. For example, multiple illumination devices (i.e., “IDs”) can sync to each other by first looking for GAPS (e.g., gaps  5121 ) just before producing light. If proper GAPs are found, the illumination device syncs to them. If no gaps are found, there is nothing to sync to and the illumination device effectively becomes a timing master that other illumination devices lock to when turned on. Such an illumination device preferably should also be able to detect if sync is lost and to re-lock. It is further noted that additional embodiments for illumination devices and systems as well as for visible light communication systems and methods are also described with respect to the fourth and sixth embodiments described herein. It is further noted that display related systems and methods, display calibration systems and methods, and LED calibration systems and methods are also described with respect to the first, second, third, seventh and eighth embodiments described herein. 
     When VAC  5340  is turned off, capacitor C  5347  can maintain power to IC  5354  for some period of time. If VAC  5340  is turned off and on within this time, IC  5354  can remain powered. To reset the illumination device  11  to a default state, VAC  5340  can be turned off and on a number of times for specified amounts of time. For instance, the reset sequence could be 3 short off and on intervals, followed by 3 longer off and on intervals, and followed finally by 3 more short off and on intervals. PLL and timing  5361  monitors signal S  5357 , signals IC  5354  to enter a low power state when signal S  5357  stays low, and measures the time between short VAC  5340  off and on periods. When PLL and timing  5361  detects the proper VAC  5340  off and on sequence, IC  5354  is reset to a default state. 
     Power control  5362 , together with the external components inductors L  5344  and L  5345 , capacitors C  5346  and C  5347 , diode D  5348 , and switch N  5349 , and current sensing feedback from output driver  5364 , implement the AC-to-DC converter function. The configuration implemented is the well known Single Ended Primary Inductor Converter (SEPIC). Switch N  5349  is turned on and off by power control  5362  at a relatively high frequency such as 1 MHz, with the duty cycle varying to produce the desired current through LED chain  5353 . When switch N  5349  is closed, the current from L  5344  and L  5345  is pulled through switch N  5349  and charge stored on the capacitor C  5346  provides current to LED chain  5353 . When switch N  5349  is open, the current through inductors L  5344  and L  5345  flows through the diode D  5348  and to LED chain  5353  and C  5347 . 
     Power control  5362  compares voltage feedback signal Vfb  5365  from output driver  5364  to an internal reference voltage to produce an error signal that adjusts the duty cycle of the control signal S  5358  that is coupled to switch N  5349 . The signal Vfb  5365  is produced by LED chain  5353  current flowing through a small resistor in output driver  5364  (not shown). When LED chain  5353  is turned off, Vfb  5365  becomes a divided down version of V+  5355 , which occurs when receiving data and during the PWM dimming off times. A control loop adjusts the feedback divider to maintain V+  5355  at the same voltage as when LED chain  5353  is on. 
     When output driver  5364  turns the current to LED chain  5353  on or off, large voltage transients can occur on V+  5355  before the power control  5362  can adjust to the new duty cycle of signal S  5358 . When the LED chain  5353  current is turned off, V+  5355  will go high until the duty cycle of S  5358  is reduced, and when the LED chain  5353  current is turned on, V+  5355  will go low until the duty cycle of S  5358  is increased. To minimize such transients, power control  5362  receives information from memory and control  5360  in advance of when such changes will occur and adjusts S  5358  duty cycle the instant such a change is needed. Just prior to the output driver  5364  turning the LED chain  5353  current off, power control  5362  measures S  5358  duty cycle and stores the result. This duty cycle is restored instantly the next time LED chain  5353  current is turned off to prevent V+  5355  from spiking high. Likewise, the S  5358  duty cycle is measured when the LED current is turned on, and the result is stored, and then restored to prevent V+  5355  from spiking low. 
     Output driver  5364  turns LED chain  5353  current on and off with a switch connected to ground (not shown). Current flows from V+  5355  to ground through LED chain  5353  and the switch, when the switch is on, and no current flows when the switch is off. A small resistor in series with the switch produces Vfb  5365  when the switch is on. When the switch is on, a control loop compares the output of a variable voltage divider from V+  5355  to Vfb  5365  and adjusts the divider until the output equals Vfb  5365 . When the LED chain  5353  current is turned off, the V+  5355  voltage divider loop is also turned off and the voltage divider remains fixed. While the LED chain  5353  current is off, this divided version of V+  5355  is forwarded to power control  5362  through Vfb  5365 . 
     Receiver  5363  can receive data from the remote controller  4912 , when the LED chain  5353  current is turned off by output driver  5364 . Modulated light from remote controller  4912  is converted to a voltage signal S  5359  by LED chain  5353 , which operates in photo-voltaic mode as in a solar panel. Receiver  5363  high pass filters S  5359  to block the DC content from ambient light and to cancel the low bandwidth of the photo-voltaic LED chain  5353 . Such bandwidth typically supports up to 1 k bits per second (1 kbps), but with the proper equalization filter the data rate can be increased by 10 times or more. To support the protocol in  FIGS. 51 and 52 , 2 kbps are needed. Receiver  5363  comprises an A/D converter and a digital filter to equalize signal S  5359 . Timing recovery is not needed since the data is sent from the remote controller  4912  synchronously to the AC mains frequency that IC  5354  is locked to. The output of the digital filter is simply sampled at the appropriate times. 
     When the illumination device  4911  is not producing light, the remote controller  4912  detects the absence of gaps  5121 . Since the remote controller  4912  is not synchronized to the gaps  5121  from the illumination device  4911 , and since the remote controller  4912  is battery powered, data from the remote controller  4912  is asynchronous to the timing in the illumination device  4911 . Provided the remote controller  4912  has a precise oscillator, such as a quartz crystal, the remote controller  4912  and the illumination device reference clocks will typically be within a couple hundred parts per million of each other. The illumination device  4911  resets a timer clocked at high frequency on the falling edge of the third SYNC  5231  pulse and uses this timer to sample received data and produce transmitted data. The drift between the two reference clocks over the 16 msec period of one transfer is insignificant. 
     The illumination device  4911  measures ambient light during gaps  5121 , and also when the illumination device  4911  is not producing light, by measuring the average voltage of signal S  5359  with the A/D converter in receiver  5363 . The A/D converter should be architected to have small DC errors, such as the well known chopper stabilization architecture, to measure very low light levels. 
       FIG. 53  is just one example of many possible illumination device  4911  block diagrams. For example, an illumination device  4911  architecture for multi-colored light could comprise of an LED chain  5353  and output driver  5364  for each component color. Example color combinations could comprise of red, green, and blue, or of red, yellow, green, and blue, or of red and White. During gaps  5121 , and also when the illumination device  4911  is not producing light, the lower light frequency LEDs can measure the light intensity of each other and of the higher light frequency LEDs. For instance, in a red and white illumination device, during gaps  5121  for instance, the white LED chain could produce light and the red LED chain could be connected to the receiver and could measure the light power. If the red LEDs are organized in two separate chains with separate output drivers, during gaps  5121  for instance, one red LED chain could measure the light power of the other. By measuring the light power from each LED chain, the illumination device could adjust the current to the different LED chains to maintain a specific color point for instance over LED variations, temperature variations, and LED lifetime. A single receiver  5363  could be shared and connected at different times to different LED chains, or multiple receivers  5363  could be implemented. 
     Another example illumination device  4911  block diagram for an illumination device that can be powered by the remote controller  4912  during configuration could comprise a second very low power receiver. The second receiver could be powered by an LED chain receiving modulated light and could store configuration information in non-volatile memory. The average voltage induced across the LED chain by light is typically significantly lower than the voltage necessary to produce light from the same LED chain. The induced voltage could be stored across capacitor C  5347  and a smaller segment of the LED chain  5353  could be connected to output driver  5364  to emit responses to the remote controller  4912 . The communication protocol to configure an illumination device  4911  when not powered could be different from  FIG. 51  to enable capacitor C  5347  to be re-charged after each emitted light pulse. Useful techniques for so doing are described in the aforementioned U.S. application Ser. No. 12/360,467 and No. 12/584,143. 
     The block diagram for an illumination device  4911  that is powered by a battery instead of the AC mains would have a battery and potentially a different type of switching power supply such as the well known buck, boost, boost buck, or flyback. With a re-chargeable battery, ambient light or sunlight incident on the LEDs could produce power to re-charge the battery. A block diagram for such an illumination device  4911  could have a second power control  5362  that manages the battery charger. An illumination device powered by the AC mains could also have any of a wide variety of different AC-DC converters, such as the boost buck or flyback. Such an illumination device could also have a back up re-chargeable battery that enables the illumination device to maintain the time of day counter when power goes off. The timing for the illumination device  4911  could also be based on a local crystal oscillator instead the mains frequency for instance. 
     As a further example, the block diagram for an illumination device that uses a silicon photodiode instead of LEDs for instance for receiving data would have the receiver  5363  connected to the photodiode instead of LED chain  5353 . Such architectures would be particularly useful for illumination devices that only use phosphor coated white LEDs that do not operate well in photo-voltaic mode. The silicon photodiode could receive commands  5124  from the remote controller  4912 , measure ambient light, and measure emitted light from the LED chain. 
     Multiple illumination devices could also communicate with each other. In this example, an illumination device  4911  could execute a protocol to synchronize to other illumination devices and to arbitrate for transmission bandwidth. When turned on, an illumination device  4911  could monitor the ambient light, search for gaps  5121  with the proper gap period  5122  and gap time  5123 , and synchronize to the gaps  5121  if found. If all the illumination devices are connected to the AC mains, then very precise synchronization is possible. Illumination devices could arbitrate for bandwidth according any one of many well known arbitration protocols. For instance, if two illumination devices transmit at the same time, both illumination devices detect the collision and wait a random amount of time before trying to communicate again. As another possibility, a CMD  5124  could include a priority code that indicates in the case of a collision, which illumination device stops transmitting. 
     As used herein, an illumination device is assumed to produce a general light, usually of a human-perceivable nature, but possibly infrared or some other wavelength. An illumination device enabled to produce light (i.e., “turned on”) may be thought of as being set to an “on-state” (i.e., having its illumination state set to an on-state), even though, as described above, there may be very short periods of time during which the light source is momentarily turned “off” and is not actually emitting light, such as during the gaps, and during the off-times in a PWM signal. The on-state and off-state of the illumination device should be clear in the context described above and not confused with the on and off status of the actual light source. 
     An illumination device may be set to an on-state or off-state by any of several events, such as application/removal of power to the illumination device (such as by energizing the light socket into which the illumination device is inserted), by a timer event, by ambient light control, and by a remote command. 
     Exemplary block diagrams are depicted herein. However, other block partitionings of an illumination device may be provided. As used herein, an illumination device attribute may represent an operational state or a configuration parameter of the illumination device. Examples include the illumination state, timer settings, delay settings, color settings for each of one or more light sources within the illumination device, photosensing mode settings, dimmer settings, time-of-day, etc. 
     Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. 
     Sixth Embodiment 
     Systems and methods for visible light communication are disclosed. In part, illumination devices and related systems and methods are disclosed that can be used for general illumination, lighting control systems, or other applications. The illumination devices utilize one or more synchronized timing signals to synchronize, preferentially to the AC mains, so as to produce time division multiplexed channels in which control information can be communicated optically by the same light source that is producing illumination. Such illumination devices preferentially comprise LEDs for producing illumination, transmitting data, detecting ambient light, and receiving data, however, other light sources and detectors can be used. The physical layer for such communication can be used with a variety of protocols, such as ZigBee, from the Media ACcess (MAC) layer and higher. Various embodiments are described with respect to the drawings below. Other features and variations can also be implemented, if desired, and related systems and methods can be utilized, as well. 
     In certain embodiments, the visible light communication techniques described herein can be used in combination with existing electronics for LED lights to implement a variety of advantageous lighting control systems and features, such as remote control, daylight harvesting, scheduling, and occupancy sensing in the light are possible at very little additional cost. These lighting control systems further allow a plurality of illumination devices to communicate with each other, remote controllers, and a central controller. Further, the techniques described herein could also be used by a single illumination device and controller, or other devices and applications, as desired. In particular, an AC mains powered controller with a light source that is normally off could communicate information, such as dimming level and color settings, to one or more LED lamps. In contrast with the techniques described herein, control of conventional lighting is typically performed by separate electronic units that communicate with each other over wires or radios, which add cost and complexity. 
     Illumination devices described herein preferentially comprise phase locked loops (PLLs) that phase lock to the AC mains and produce the synchronized timing signals for operating the devices. Since other illumination devices in the lighting systems for instance phase lock to the same AC mains signal, all such devices have precisely the same internal timing. With such synchronized timing, communication channels can be formed during which all devices can communicate. Likewise, since the bit level timing of data communication within such channels is precisely synchronized, data recovery within a receiver is substantially easier since the received data timing is known. 
     A communication channel is a timeslot that preferentially spans a fraction of an AC mains period (16.67 mSec for 60 Hz) during which all the members of a group of devices stop producing illumination. Higher layers in a communication protocol, such as ZigBee, can dynamically assign individual devices to communicate on different channels. During such timeslots information can be communicated optically between such members when one member produces light modulated with data. During such timeslots when data is not being communicated, ambient light can be measured for daylight harvesting applications and for improving receiver sensitivity. 
     Preferentially, the illumination devices comprise LEDs for illumination and for transmitting and receiving data to minimize cost and maximize receiver sensitivity. Because white LEDs that comprise a blue LED covered with a phosphor have poor sensitivity to received light, preferentially the illumination devices comprise LEDs with different colors to produce the desired white light. Possible combinations include white and red, or red, yellow, green, and blue, but could include any combination or even a single color provided at least one LED in the illumination device is preferably not phosphor coated. Preferentially, the illumination devices comprise red LEDs for best receiver sensitivity. The additional cost of controlling multicolored LEDs can be reduced or eliminated in lamps that combine the systems and methods described herein with those described in additional embodiments as described herein for calibrating devices using LEDs such as those described herein with respect to the second embodiment, the third embodiment, the seventh embodiment and the eighth embodiment. These embodiments describe in part techniques to precisely control the color of light produced by combinations of different colored LEDs, such as white and red, or red, yellow, green, and blue, and can do so without the need for additional photo-detectors or temperature sensors thereby making such implementations more cost effective. 
     The messages in a communication channel are preferentially sent a few bytes at a time in successive timeslots over a complete physical layer data frame. Such a data frame comprises a MAC layer data frame superseded by additional physical layer information with most of the physical layer data frame scrambled by well known methods to remove DC content. The MAC layer data frame can conform to any protocol including ZigBee. 
     The systems and methods disclosed herein address problems with prior systems in part by providing physical layers for lighting control systems for reduced costs and/or relatively insignificant additional costs. Advantageously, the illumination devices and other devices in the lighting system described herein can communicate using the devices already needed for illumination. 
     As stated above, this sixth embodiment can also be used with the techniques, methods and structures described with respect to the other embodiments described herein. For example, the calibration and detection systems and methods described with respect to the second, third, seventh and eighth embodiments can be used with respect to the visible light communication systems and methods described in this sixth embodiment. Further, the communication and synchronization techniques described with respect to this sixth embodiment can be used with respect to the other embodiments, if desired. 
     Turning now to the drawings,  FIG. 54  is one example of a lighting system  5410  comprising illumination devices  5411 , AC mains  5413 , and optionally remote controller  5412  that uses visible light for both illumination and communication. The illumination devices  5411  preferentially comprise LEDs to produce light for both lighting and communicating, and are preferentially connected and synchronized to the AC mains  5413 . Timing circuits in the illumination devices  5411  lock to the AC mains  5413  frequency and produce periodic intervals during which all illumination devices  5411  do not emit light for illumination and may communicate data. The periodic interval rate is sufficiently high for humans to simply perceive continuously light. The data communicated preferentially comprises information to control the illumination devices, but could comprise any digital information. 
     Optional remote controller  5412  can be AC mains  5413  or battery powered and preferentially comprises at least one LED for producing visible light to communicate with the illumination devices  5411 . If remote controller  5412  is AC mains powered, timing circuits lock to the AC mains  5413 , which synchronizes remote controller  5412  with the illumination devices  5411  and enables optical communication. If remote controller  5412  is not connected to the AC mains  5413 , remote controller  5412  monitors the light produced by illumination devices  5411  and locks to the periodic light off intervals to enable communication. If the illumination devices are turned off and are not producing light, remote controller  5412  can communicate with illumination devices  5411  anytime. 
     The network protocol stack for communicating information, with the exception of the physical layer, preferentially follows the well known Zigbee standard, but could follow many different protocols. While the Zigbee physical layer can use multiple different radio frequency communication channels, the embodiments described herein can communicate over multiple visible light communication channels that are shifted in time relative to each other and the AC mains signal. Both physical layers can interface to the Zigbee Media ACcess or MAC layer. 
       FIG. 54  is just one example of a lighting system using visible light for synchronous communication. For instance, any number of illumination devices  5411  could be supported. Some illumination devices  5411  could be AC mains  5413  powered, while others are battery powered. More or less remote controllers  5412  could be supported. A variety of other AC mains  5413  or battery powered devices such as switches, dimmers, appliances, and even computers could communicate under the techniques described herein. Likewise, illumination and other devices could synchronize in many different ways. For instance a dedicated wire, RF channel, or some other communication channel could provide such synchronization signal. Additionally, devices could synchronize to other devices already communicating by monitoring the light being produced by such other devices, and locking to communication gaps in such light. 
       FIG. 55  is an example timing diagram for communicating between illumination devices  5411  in lighting system  5410  that illustrates the relationship between the AC mains  5413  timing that is typically 50 or 60 Hz, four different communication channels  5524  labeled Channel  0  through Channel  3  that comprise PWM time  5520  and communication gap time  5521 , and the gap timing that comprises four data bytes  5522  labeled BYTE  0  through BYTE  3 . During PWM time  5520 , illumination devices  5411  can produce light for illumination and during gap time  5521  illumination devices  5411  can communicate. In this example, channels  0  through  3  provide gap times  5521  that have different non-overlapping phases relative to the AC mains timing, which provide four independent communication channels. 
     The gap period for each channel  0  through  3  in this example is equal to one over the AC mains frequency and comprise alternating PWM  5520  and gap  5521  times. During PWM  5520  times, light from an illumination device  5411  can be on continually to produce a maximum brightness or Pulse Width Modulated (PWM) to produce less brightness. During the repetitive gap  5521  times, data can be sent from any device to any or all other devices. In this example, the gap time is one quarter of the AC mains  5413  period and enables four data bytes  5522  to be communicated at an instantaneous bit rate of 60 Hz times four times 32 or 7.68K bits per second and an average bit rate of 1.92K bits per second. 
     Higher layers in the Zigbee or other protocol stack select which channel is used for communication between which devices. For instance, a group of illumination devices  5411  that are physically located over a sufficiently wide area such that some illumination devices  5411  cannot communicate directly with each other can be divided into multiple groups of illumination devices  5411  with each group configured to communicate on a different communication channel. Communication between such groups could pass through an illumination device that communicates on two channels for instance. 
       FIG. 55  is just one of many possible timing diagrams for lighting system  5410 . For instance, gaps  5521  could occur multiple times per AC mains  5413  period or once per multiple AC mains  5413  periods. The gap  5521  time could comprise a larger or smaller percentage of the AC mains  5413  period, and the number of bytes  5522  communicated within a gap  5521  time could be more or less than 4 including fractions of bytes. The number of channels could be more or less than 4 depending on the relationship between the gap  5521  time and period and the AC mains  5413  timing. 
       FIG. 56  illustrates the contents of data frame  5630  comprising a four byte preamble  5631 , a start byte  5632 , a frame length byte  5633 , and up to 128 bytes of MAC frame  5634 . Data frame  5630  conforms to the Zigbee physical layer specification and can be used to communicate information between devices in lighting system  5410 . Data frame  5630  is sent according to the timing illustrated in  FIG. 55  four bytes at a time in each gap  5521  until the entire data frame  5630  has been transmitted. 
     Preamble  5631  comprises four bytes of alternating ones and zeros that the receivers in all devices detect and adjust receiver parameters, such as gain, accordingly. The start byte  5632  is a unique code that all receivers detect and synchronize byte boundary timing to. The length byte  5633  identifies the length of the MAC frame  5634  in bytes. MAC frame  5634  contains data as defined by the Zigbee MAC layer specification. 
       FIG. 56  is one of many possible physical layer data frame  5630  formats. The preamble  5631 , start  5632 , and length  5633  could be completely different and still remain compatible with the Zigbee MAC layer specification provided that MAC frame  5634  is properly communicated from the MAC layer of a transmitting device to the MAC layer of receiving devices. To support different higher layer protocols even MAC frame  5634  could be completely different. Preamble  5631  may or may not be necessary with any MAC layer protocol depending on the capabilities of the receive circuitry. 
       FIG. 57  is an example block diagram for an illumination device  5411  that comprises power supply  5741 , controller IC  5742 , and LED chain  5743 . LED chain  5743  preferentially comprises red LEDs  5754  connected in series with resistors  5755  connected in parallel with each red LED  5754 . Typically, an illumination device  5411  would comprise additional chains of white LEDs for instance, but such chains are not shown for simplicity. As such,  FIG. 57  as drawn is a block diagram for an illumination device  5411  that produces red light for instance. 
     Power supply  5741  accepts the AC mains  5413  and produces DC voltage  5744  that provides power for controller IC  5742  and LED chain  5743 . The magnitude of DC voltage  5744  depends on the number of LEDs  5754  in LED chain  5743 . Power supply  5741  also produces synchronization signal  5745  for controller IC  5742  to synchronize to. Signal  5745  preferentially is a low voltage version of the AC mains  5413  voltage that PLL and timing circuitry  5748  can accept and phase lock to. 
     Controller IC  5742  comprises PLL and timing circuitry  5748 , control circuitry  5749 , PLI (physical layer interface)  5750 , receiver  5751 , PWM  5752 , mux  5753 , and current source  5756 . PLL and timing circuitry  5748  locks to the AC mains  5413  frequency and phase and produces the timing illustrated in  FIG. 55 . During PWM time  5520 , mux  5753  enables PWM  5752  to control current source  5756 , which can cause LED chain  5743  to produce illumination depending on the state of the PWM  5752  output. During gap time  5521 , mux  5753  enables PLI  5750  to control current source  5756 . When transmitting data  5522 , PLI  5750  modulates current source  5756  with preferentially scrambled non return to zero (NRZ) data, and when not transmitting data  5522 , PLI  5750  disables current source  5756 . 
     Receiver  5751  monitors LED chain  5743  during gap times  5521  when PLI  5750  is not transmitting data and forwards recovered data if present to PLI  5750 . PLI  5750  interfaces to control circuitry  5749 , which implements the MAC layer protocol and higher layers used for illumination devices  5411  to communicate properly. When transmitting, PLI  5750  accept MAC frames  5634  from control circuitry  5749 , generates preambles  5631 , start bytes  5632 , and length bytes  5633 , scrambles the length bytes  5633  and MAC frames  5634  with well known techniques; and forwards the resulting data to current source  5756 . Likewise when receiving, PLI  5750  accepts serial received data from receiver  5751 , unscrambles the length bytes  5633  and MAC frames  5634 , removes preambles  5631 , start bytes  5632 , and length bytes  5633 , and forwards MAC frames  5634  to control circuitry  5749 . 
       FIG. 57  is just one example of many possible illumination device  5411  block diagrams. For instance, preferentially illumination devices  5411  should comprise additional chains of different color LEDs such as white, or green and blue to produce white light.  FIG. 57  does not show such chains for simplicity. Such additional chains would be enabled during PWM times  5520  and disabled during gap times  5521  when receiving data. When transmitting data, such additional chains would preferentially be modulated with the same data as LED chain  5743 . 
     Additionally, receiver  5751  could be connected to a silicon photodiode or other optical sensing device for receiving data.  FIG. 57  preferentially illustrates LED chain  5743  sensing received light since such LEDs are used to produce illumination as well. PWM  5752  can be removed if illumination device  5411  does not need to be dimmable. Control circuitry  5749  could reside somewhere else, for instance in an external microcontroller. Controller IC  5742  functionality could be implemented with various electronic components instead of a completely integrated solution. 
       FIG. 58  is one of many possible block diagrams for receiver  5751  that comprises switch  5860 , amplifier  5861 , low pass filter  5862 , ADC  5863 , and DSP  5864 , and that interfaces with LED chain  5743 . Light modulated with data and incident on LEDs  5754  induce current in each LED  5754  that flows in a loop through each resistor  5755 . The voltages consequently induced across each resistor  5755  substantially sums to produce a larger voltage across signals  5744  and  5747 , which is then further gained by amplifier  5861 . Low pass filter  5862  substantially eliminates any noise or interference near and above the A/D Clock  5867  frequency that could alias into the signal bandwidth. 
     ADC  5863  preferentially has an over-sampling delta sigma architecture that samples the analog input at a high frequency, which is 9.44 MHz in this example, and low resolution, and then digitally low pass filters the result to produce high resolution samples at a substantially lower frequency, which is 7.68 kHz in this example. The high resolution ADC  5863  output preferentially is further processed by DSP  5864  to increase channel bandwidth using well known decision feedback equalization techniques. 
     Switch  5860  is turned on when CLR  5865  goes low momentarily at the beginning of each gap period  5521  to short signals  5744  and  5747  together just prior to receiving data, which produces a low frequency affect on the received signal. DSP  5864  eliminates this and other low frequency affects such as from ambient light and 60 Hz interference, by preferentially monitoring and storing ADC  5863  output samples when no data is being received and subtracting average values of such samples from ADC  5863  results when receiving data. DSP  5864  alternatively could implement a high pass filter to remove such affects. 
       FIG. 58  is just one of many possible block diagrams for receiver  5751 , which could receive data using a silicon photodiode instead of LED chain  5743 . Amplifier  5861  could be configured as a trans-impedance amplifier to detect current instead of voltage from LED chain  5743 . The ADC  5863  architecture could be well known FLASH or SAR or could be completely eliminated depending on the quality of the amplifier  5861  output. Likewise, DSP  5864  may or may not be needed depending on performance. Additionally, a variety of different channel equalization techniques could be implemented instead of decision feedback. As such  FIG. 58  is just an example. 
       FIG. 59  is an example block diagram for PLL and timing circuitry  5748  that comprises comparator  5970 , PLL  5971 , dividers  5972  and  5973 , and divider/decode  5974  and that produces the clocks synchronized to the AC mains  5413  used for controller IC  5442  to operate. Signal  5745  from power supply  5741  is converted to AC mains clock  5975  with a frequency and phase equal to the AC mains  5413  by comparator  5970 . AC mains  5413  frequency is assumed to be 60 Hz in this example. AC mains clock  5975  frequency is multiplied by 1,572,864 by PLL  5971  to produce the A/D Clock  5867  with a frequency approximately equal to 9.44 MHz. A/D Clock  5867  is divided by 12288 by divider  5972  to produce bit clock  5866  with a frequency equal to 7.68 kHz, which is also precisely equal to the instantaneous bit rate of bytes  5522 . Divider  5973  divides bit clock  5866  by 32 and also produces the pulsed signal CLR  5865 . The output of divider  5973  is further divided by four and decoded by divider/decode  5974  to produce channel clocks  5524 . 
       FIG. 59  illustrates one of many possible PLL and timing circuitry  5748  block diagrams that synchronizes the timing of controller IC  5742  to the AC mains  5413  frequency and phase. Depending on the architecture of controller IC  5742  as described previously PLL and timing circuitry  5748  could be completely different. For example, PLL  5971  could lock directly to the AC mains  5413  without comparator  5970 . 
       FIG. 60  is an example diagram illustrating the timing of data being received at the beginning of a gap  5521  on channel one (1)  5524 . In this example, the gap  5521  period is one bit clock  5866  longer than the total number of bits to be communicated within such gap  5521 . The gap  5521  time starts at time  6080  when bit clock  5866  and CLR  5865  go high. Also at time  6080  mux  5753  switches control of current source  5756  to PLI  5750 , which is configured to receive data in this example and consequently ensures that current source  5756  is disabled. Data bytes  5522  can begin to be communicated one bit at a time starting at time  6081  when AC mains clock  5413  and channel one  5524  clock go high. 
     Traces  6082  and  6083  represent the voltages on Vled  5747 , which is the bottom of LED chain  5743 , when received data is not present (Vled  5747  Ambient) and is present (Vled  5747  Receiving) respectively. When CLR  5865  is high, Vled  5747  is shorted to the power supply  5744 . Just prior to CLR  5865  going high, current source  5756  may be enabled or disabled so the voltage on Vled  5747  is unknown. After CLR  5865  goes low ambient light induces a voltage across LED chain  5743 , which causes traces  6082  and  6083  to drop exponentially. As shown on trace  6082 , if no data is being received, the voltage on Vled  5747  may settle after many bit clock  5866  periods and could cause data errors when receiving data as shown on trace  6083 . Consequently, DSP  5864  preferentially subtracts an averaged version of trace  6082  from Vled  5747  during gap  5521  times when data is being received. 
     Since illumination devices  5411  are synchronized to the same AC mains  5413  signal, the bit clocks  5866  in all such devices are also synchronized in both frequency and phase. Trace  6083  illustrates the voltage on Vied  5747  in a receiving illumination device  5411  when a second illumination device  5411  is sending the sequence beginning with one, zero, and one during the three bit clock  5866  periods after time  6081 . A one in this example is represented by light being on which produces a lower voltage on Vled  5747  relative to power supply  5744 . ADC  5863  samples Vled  5747  when bit clock  5866  goes high, which is precisely at the times  6084 ,  6085 , and  6086  when Vled  5747  has the largest signal. If illumination devices  5411  were not synchronized to each other through the AC mains  5413  or other means, receiver  5751  in a receiving illumination device  5411  would need to recover a clock from the received signal Vled  5747  prior to recovering data, which would substantially increase the complexity of receiver  5751  and potentially degrade performance. 
       FIG. 60  is just one example of many possible timing diagrams. For instance, if receiver  5751  was connected to a dedicated silicon photodiode, CLR  5865  and switch  5860  would not be needed. Likewise, the gap  5521  period could have started at time  6081  instead of  6080  with or without a dedicated silicon photodiode. Depending on the data rate and sensitivity required for the application, traces  6082  and  6083  could be completely different. As such  FIG. 60  is just one example. 
     Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. 
     Seventh Embodiment 
     Systems and methods are also disclosed for light sources that use the photo-sensitivity of one or more colored LEDs to determine at least a portion of the emission spectrum of a white light source or other broad spectrum light emitter. As described herein, the white LED or other broad spectrum light emitter can be used as the light source, if desired, and the same one or more colored LEDs or different LEDs, if desired, can be used to emit light and to adjust a color point produced by the light source. Applications for the disclosed embodiments include but are not limited to general lighting, LCD backlighting, projectors, and direct emission displays such as OLEDs and digital billboards. Various embodiments are described with respect to the drawings below. Other features and variations can also be implemented, if desired, and related systems and methods can be utilized, as well. 
     One embodiment includes a method and system to set a precise color temperature produced by a substantially white light source, such as a phosphor coated blue LED, or some other broad spectrum light emitter, in combination with one or more colored or substantially mono-chromatic LEDs during the manufacturing of a device, such as an LED lamp, a display backlight, a projector, a digital billboard, or AMOLED (Active Matrix OLED) display, and to maintain such color temperature over the operating life of such a device. The method involves analyzing a portion of the spectrum of the white light source or broad spectrum light emitter using one or more colored LEDs as wavelength selective light sensors and then using such colored LEDs (or one or more additional different LEDs if desired) to emit light whereby adjusting the color of light produced by the combination of the white light source and the colored LEDs. These LEDs allow for the color point of the light source to be adjusted, as desired, based upon the measurements made with respect to portions of the spectrum of the broad spectrum light emitter. Embodiments further include a light source comprising a white light source and colored LEDs, which could be a pixel in a digital billboard or AMOLED, or the entire light source for a lamp, backlight, or projector, for instance. 
     The disclosed embodiments apply to any broad spectrum light emitter and/or substantially white light source and any number of colored LEDs. Of particular interest, however, and as described in more detail below, is the combination of red, green, blue, and white LEDs. In such example illustration, the red, green, and blue LEDs analyze the spectrum of light produced by the white LED by each LED operating as a different wavelength selective light detector. The blue LED measures the blue part of the spectrum, the green LED measures the green plus blue parts of the spectrum, and the red LED measures substantially the entire spectrum with emphasis on the red and green portions, of the white LED light source. Subsequent to such spectral analysis, the red, green, and blue LEDs emit light with intensities adjusted to produce a desired color point when combined with light produced by the white LED. 
     To reduce optical measurement errors due in particular to variations in LED responsiveness to incident light, further embodiments create ratios of signals induced on each LED by the white LED and other LEDs that are used to determine relative brightness of each LED to produce the desired color point. For instance and as described in one example herein, the brightness of the spectrum of the white LED filtered by the red, green, and blue LEDs is determined relative to the brightness of the blue LED. Additionally, the brightness of the green and red LEDs are determined relative to the brightness of the blue LED. All such relative brightness levels can then be compared to the desired relative brightness levels between the red, green, and blue LEDs and the three different spectral bands produced by the white LED, and the brightness of the red, green, and blue LEDs, and can be adjusted to produce the desired color point from all four LEDs. 
     Methods for using measured ratios of light are disclosed herein and also with respect to additional embodiments described herein, for example, with respect to the third embodiment. The disclosed embodiments include spectral analysis of a substantially white light to compensate for spectral variations in the emissions of such white light source. The methods described herein are associated with measuring ratios of emitted light, further comparing such ratios to desired ratios, and further adjusting the brightness produced by the LEDs in response to such ratios. 
     Specifically related to phosphor coated white LEDs, another aspect of the disclosed embodiments compensate for common variations between white LEDs during manufacturing and variations that occur over time in a particular LED. The amount of blue light produced by the blue LED that does not get absorbed by the phosphor relative to the amount of light emitted by the phosphor varies with phosphor thickness and uniformity during manufacturing and with phosphor degradation over time. With the methods described herein, the amount of blue light relative to the amount of phosphor converted light produced by the white LED can be determined and the amount of light produced by associated red, green, and blue LEDs or just red and green LEDs for instance can be adjusted to compensate for the difference in such ratio from a desired ratio. 
     The calibration methods and apparatus described herein address issues for devices using groups of different colored LEDs directly or as backlights for illumination. Such calibration methods reduce the need for specially binned LEDs for the production of lamps, displays, or backlights, and maintain the color or color temperature of the light produced over the operating life of the device. 
     As stated above, this seventh embodiment can also be used with the techniques, methods and structures described with respect to the other embodiments described herein. For example, the calibration and detection systems and methods described with respect to this embodiment can be used within the other described embodiments, as desired. Further, the various illumination devices, light sources, light detectors, displays, and applications and related systems and methods described herein can be used with respect to calibration and detection systems and methods described in this seventh embodiment, as desired. Further, as stated above, the structures, techniques, systems and methods described with respect to this seventh embodiment can be used in the other embodiments described herein, and can be used in any desired lighting related application, including liquid crystal displays (LCDs), LCD backlights, digital billboards, organic LED displays, AMOLED (Active Matrix OLED) displays, LED lamps, lighting systems, lights within conventional socket connections, projection systems, portable projectors and/or other display, light or lighting related applications. 
     Turning now to the drawings,  FIG. 61  is an example block diagram for light source  6110  that uses a broad spectrum light emitter and multi-colored LEDs to produce a fixed blended color emitted by such light source. In this example, such broad spectrum light emitters are white LEDs  6124  and  6128  and the multi-colored LEDs are red LEDs  6121  and  6125 , green LEDs  6122  and  6126 , and blue LEDs  6123  and  6127 , however, any type of emitter that produces substantially white light and any combination of different colored LEDs can be used. Such light source  6110  can be used in any application including but not limited to general lighting, LCD backlighting, projectors, and direct emission displays such as OLEDs and digital billboards. As such, a broad spectrum light emitter as used herein is generally meant to include any light emitter that includes one or more light sources that alone or together emit a spectrum of light across multiple color regions, such as two or more different colored light sources and/or a white light source. For example, a white LED that operates to produce white light would be a broad spectrum light emitter as used herein. As another example, a blue LED and a green LED that operate together to produce mixed blue/green light would also be a broad spectrum light emitter as used herein. Other combinations of different colored LEDs could also be used to simultaneously produce light across multiple color regions so as to operate as a broad spectrum light emitter. In short, a broad spectrum light emitter is one that simultaneously produces or emits light in multiple color regions. 
     In this example  FIG. 61 , light source  6110  comprises controller  6111  and RGBW (red, green, blue, white) LED packages  6112  and  6113 . LED package  6112  comprises red LED  6121 , green LED  6122 , blue LED  6123 , and white LED  6124 . LED package  6113  comprises red LED  6125 , green LED  6126 , blue LED  6127 , and white LED  6128 . Such red, green, blue, and white LEDs are not necessarily combined in RGBW LED packages  6112  and  6113 , however, since such packages are commonly available, for some applications such packages may be preferred. 
     Controller  6111  comprises eight output drivers  6118 , control circuitry  6116 , and current sense  6117  in this example  FIG. 61 . Each output driver  6118  comprises a current source  6120  and modulator  6119  that control the current to each LED  6121  through  6128  and optionally the duty cycle of such current to control the intensity of light produced by each such LED  6121  through  6128 . Current sense  6117  can measure the photo-current induced in red LEDs  6121  and  6125 , green LEDs  6122  and  6126 , and blue LEDs  6123  and  6127  by the other LEDs  6121  through  6128  as described in  FIGS. 66A-D ,  67 A-D,  68 A-D and  69 -A-D. The anodes of LEDs  6121  through  6128  are shown to be tied together and to the power supply Vd  6114 . The cathodes of LEDs  6121  through  6128  are shown connected to signal bus VC[7:0]  6115 , which connects each cathode to a different output driver  6118  and current sense  6117  input in controller  6111 . 
       FIG. 61  is just one example of many possible block diagrams for light source  6110 . For instance, any broad spectrum emitter in combination with any combination of different colored LEDs can be used. A broad spectrum emitter is generally meant to include an emitter that emits a spectrum of light across multiple color regions, such as a white light source. Additionally, any number of broad spectrum emitters and LEDs can be combined to produce light source  6110  with any emission power. When the broad spectrum light source is a white LED, any number of such white and colored LEDs can be connected in series or parallel, with any number of driver circuits. Controller  6111  can comprise a single or many integrated circuits and discreet components. Driver  6118  may or may not comprise modulator  6119 , in which case, current sources  6120  would be adjusted to vary the intensity of light produced by each attached LED. Likewise, current sense  6117  could measure voltage instead of current or some combination of both. As such  FIG. 61  is simply an example block diagram for light source  6110 . 
       FIG. 62  is an example block diagram for current sense  6117 , which comprises ADC (analog to digital converter)  6232 , resistor (R)  6231 , and multiplexer (mux)  6230 . As shown in both  FIG. 61  and  FIG. 62 , the inputs to current sense  6117  comprise Vd  6114 , which is connected to the anodes of LEDs  6121  through  6128  in this example, and VC[7:0]  6115  signals that are connected to the cathodes of each LED  6121  through  6128 . The output of ADC  6232  is forwarded to control circuitry  6116 , which processes the information and controls drivers  6118 . To measure the photocurrent induced on any LED  6121  through  6128 , multiplexer  6230  passes the selected signal from VC[7:0]  6115  from the cathode of the selected LED to resistor  6231  and ADC  6232 . Since Vd  6114  is connected to the opposite side of resistor  6231  and to the anode of the selected LED, any current induced in the selected LED passes through resistor  6231  and induces a small voltage, which is measured by ADC  6232 . Preferably, the resistance of resistor  6231  should be selected to never produce a sufficiently high voltage when measuring photocurrent to forward bias the LED. For instance, a typical resistor value of 100 k ohms would produce a typical ADC  6231  input voltage of 10-100 mV. 
       FIG. 62  is just one example of many possible block diagrams for current sense  6117 . For example, if resistor  6231  is removed, ADC  6232  could measure the open circuit voltage induced across each LED  6121  through  6128 . Multiplexer  6230  is shown to select between all 8 LEDs, however, white LEDs  6124  and  6128  are typically not measured and consequently do not need to be connected to current sense  6117 . Multiplexer  6230  is not needed at all if an ADC  6232  is connected directly to each LED cathode. As such,  FIG. 62  is just one example of many possible current sense  6117  block diagrams. 
       FIG. 63  is an illustration of exemplary emission spectra produced by red, green, blue, and white LEDs  6121  through  6128  in light source  6110 . Emission spectrum  6340  illustrates one possible spectrum emitted by white LEDs  6124  and  6128 . Since white LEDs typically comprise blue LEDs with a phosphor coating, the emission spectrum  6340  shows a peak around 450 nm, which is produced by the blue LED, and a much broader peak around 550 nm, which is produced by the phosphor. The blended spectrum appears as white light, however, the color or color temperature of such white light can vary substantially. 
     Emission spectrums  6341  (blue),  6342  (green), and  6343  (red) represent typical spectrums produced by blue LEDs  6123  and  6127 , green LEDs  6122  and  6126 , and red LEDs  6121  and  6125  respectively. Typical peak emission wavelengths are 450 nm for blue, 530 nm for green, and 625 nm for red, which are represented by the highest intensity points in emission spectra  6341  (blue),  6342  (green), and  6343  (red) respectively. The peak emission wavelength for green LEDs  6122  and  6126  typically is just shorter than the peak emission wavelength produced by the phosphor in white LEDs  6124  and  6128 , while the peak emission wavelength for red LEDs  6121  and  6125  typically is longer than most of the optical power produced by white LEDs  6124  and  6128 . 
       FIG. 63  is one example of many possible emission spectrums from the individual lighting elements in light source  6110 . For instance, light source  6110  can have a broad spectrum emitter to typically produce white light that is not an LED. In such case, the emission spectrum typically would be substantially different from emission spectrum  6340 . Likewise, light source  6110  may comprise more or less colored LEDs and such LEDs could be any color. As such the LEDs in light source  6110  could have more or less emission spectrums than spectrums  6341 ,  6342 , and  6343  shown in  FIG. 63 , and each spectrum could have substantially different peak emission wavelengths and other spectral characteristics.  FIG. 63  is just one example of many possible spectral plots. 
       FIG. 64  is illustrates two example emission spectrums from a white LED  6124  or  6128  that produce two different white Color temperatures. In this example both emission spectrums are produced by a white LED  6124  or  6128  comprising of a phosphor converted blue LED. Such phosphor could be in contact with such blue LED or separated by some distance. The difference in the two spectrums could be produced by different phosphor thickness for two different LEDs at the end of a manufacturing line or could be produced by the same LED at two different times or at two different temperatures. LED phosphor coatings are well known to change characteristics over temperature and to degrade over time. Likewise, the optical power emitted by the blue LED in white LED  6124  or  6128  is also well known to change over operating conditions and lifetime.  FIG. 64  is just one example of many possible differences in spectral emissions from two white LEDs or the same LED under different conditions. 
       FIG. 64  illustrates the spectral peak  6446  produced by such blue LED in white LED  6124  or  6128 , and substantially broader spectral peaks  6444  and  6445  produced by such phosphors in white LED  6124  or  6128 . In this example  FIG. 64 , spectral peak  6444  could represent the emissions produce by such phosphor in white LED  6124  or  6128  at the time it was manufactured, and lower spectral peak  6445  could represent the emissions produced by such phosphor in white LED  6124  or  6128  after some time. As such, the white color temperature of the light produced by white LED  6124  or  6128  is shown to change over time in this example. 
       FIG. 65  is one example of the spectral responsiveness  6552  (red),  6551  (green), and  6550  (blue) of the red  6121  and  6125 , green  6122  and  6126 , and blue  6123  and  6127  LEDs in light source  6110  respectively. Spectral responsiveness is the relative amount of current induced on an LED by a fixed incident optical power as function of incident wavelength. As shown in this example  FIG. 65 , spectral responsiveness  6550  (blue),  6551  (green), and  6552  (red) illustrates that blue  6123  and  6127 , green  6122  and  6126 , and red  6121  and  6125  LEDs produce current in response to light with incident wavelengths roughly equal to or shorter than such blue, green, and red LED peak emission wavelengths as shown in emissions spectrums  6341  (blue),  6342  (green), and  6343  (red). As such, such red LEDs can detect light from such red, green, and blue LEDs, such green LEDs can detect light from such green and blue LEDs, and such blue LEDs can detect light from such blue LEDs. Likewise, such red, green, and blue LEDs can detect light from different parts or portions of a spectrum emitted by a broad spectrum light emitter as filtered by these LEDs, such as, for example, filtered portions of the spectrum  6340  from white LED  6124  or  6128  as shown in  FIG. 63 . 
     Since light source  6110  may comprise a different number of different colored LEDs,  FIG. 65  illustrates the spectral responsiveness of just one example set of LEDs in light source  6110 . Likewise, responsiveness  6550  (blue),  6551  (green), and  6552  (red) are just rough approximations for the spectral responsiveness of such blue, green, and red LEDs respectively. Actual responsiveness may vary substantially. As such,  FIG. 65  is just one example. 
     The following equations are associated with  FIGS. 66A-D . In particular, equation 24 is associated with  FIGS. 66A-B . Equation 25 is associated with  FIGS. 66C-D . And equation 26 provides a ratio using equations 24 and 25. 
       V b1w0 =E w0b R b1 C b1w0   [EQ. 24]
 
       V b1b0 =E b0 R b1 C b1b0   [EQ. 25]
 
         E   w0b   /E   b0 =( V   b1w0   /V   b1b0 ) C   0   [EQ. 26]
 
       FIGS. 66A-D  provide an exemplary first step in an exemplary method to set and maintain a precise color temperature produced by a combination of red, green, blue, and white LEDs  6121  through  6128  in light source  6110 . In such first step, current induced in blue LED  6127  by white LEDs  6124  as shown in  FIGS. 66A-B  is compared to current induced in blue LED  6127  by blue LED  6123  as shown in  FIGS. 66C-D . White LED  6124  is illuminated by producing current I w0  in a current source  6120 , and the current induced in blue LED  6127  is measured by connecting resistor  6231  across LED  6127  and measuring the resulting voltage V b1w0  by ADC  6232 . Equation 24 illustrates that the induced voltage V b1w0  is equal to the power emitted by LED  6124  E w0b  times the responsiveness R b1  of LED  6127  times a constant Cb 1   w   0 . Spectral plot  6660  illustrates the responsiveness  6550  of blue LED  6127  superimposed on the spectrum  6340  of white LED  6124  along with the alternative phosphor produced spectrum  6445  that could result from white LED  6124  aging. As shown in spectral plot  6660  of  FIG. 66B , the resulting current induced in blue LED  6127  by white LED  6124  should be independent of variations in the light produced by such phosphor. 
     Subsequently, blue LED  6123  is illuminated by producing current Ib 0  in a current source  6120 , and the current induced in blue LED  6127  is again measured by connecting resistor  6231  across LED  6127  and measuring the resulting voltage. Equation 25 illustrates that the induced voltage Vb 1   b   0  is equal to the power emitted by LED  6123  Eb 0  times the responsiveness Rb 1  of LED  6127  times a constant Cb 1   b   0 . Spectral plot  6661  as shown in  FIG. 66D  illustrates the responsiveness  6550  of blue LED  6127  superimposed on the spectrum  6341  of blue LED  6123 . As shown, a significant portion of the emitted power Eb 0  induces current in LED  6127 . 
     Equation 26 illustrates the result of dividing equation 24 by equation 25 and combining Cb 1   w   0  and Cb 1   b   0  to produce the constant C 0 . As shown in equation 26 and spectral plots  6660  and  6661 , the ratio of the emitted powers Ew 0   b  over Eb 0  with wavelengths roughly shorter than 450 nm in this example is proportional to the ratios of the induced voltages Vb 1   w   0  over Vb 1   b   0 . The responsiveness of blue LED  6127  cancels out. 
     The following equations are associated with  FIGS. 67A-D . In particular, equation 27 is associated with  FIGS. 67A-B . Equation 28 is associated with  FIGS. 67C-D . And equation 29 provides a ratio using equations 27 and 28. 
       V g0w0 =E w0g R g0 C g0w0   [EQ. 27]
 
       V g0b0 =E b0 R g0 C g0b0   [EQ. 28]
 
         E   w0g   /E   b0 =( V   g0w0   /V   g0b0 ) C   1   [EQ. 29]
 
       FIGS. 67A-D  provide an exemplary second step in an exemplary method to set and maintain a precise color temperature produced by a combination of red, green, blue, and white LEDs  6121  through  6128 . Such second step is identical to such first step illustrated in  FIGS. 66A-D  except that green LED  6122  is used to measure the light produced by white  6124  and blue  6123  LEDs. White  6124  and blue  6123  LEDs are again illuminated by producing currents Iw 0  and Ib 0  respectively in current sources  6120  respectively as shown in  FIGS. 67A-B  and  67 C-D. The resulting voltages Vg 0   w   0  and Vg 0   b   0  induced by white  6124  and blue  6123  LEDs respectively on green LED  6122  are equal to the white LED  6124  emitted power Ew 0   g  and the blue LED  6123  emitted power Eb 0  times the green LED  6122  responsiveness times the constants as shown in equations 27 and 28. 
     Equation 29 illustrates the result of dividing equation 27 by equation 28 which shows the ratio of the emitted powers Ew 0   g  over Eb 0  with wavelengths roughly shorter than 550 nm in this example is proportional to the ratios of the induced voltages Vg 0   w   0  over Vg 0   b   0 . The responsiveness of green LED  6122  cancels out. Spectral plot  6770  as shown in  FIG. 67B  illustrates that light from the blue peak and some of the light from the phosphor converted peak induce current in green LED  6122 , while spectral plot  6771  as shown in  FIG. 67D  illustrates that all the light from blue LED  6123  induces current in green LED  6122 . 
     The following equations are associated with  FIGS. 68A-F . In particular, equation 30 is associated with  FIGS. 68A-B . Equation 31 is associated with  FIGS. 67C-D . Equation 32 is associated with  FIGS. 68E-F . And equations 33 and 34 provide ratios using equations 30, 31 and 32. 
       V r0w0 =E w0r R r0 C r0w0   [EQ. 30]
 
       V r0b0 =E b0 R r0 C r0b0   [EQ. 31]
 
       V r0g0 =E g0 R r0 C r0g0   [EQ. 32]
 
         E   w0r   /E   b0 =( V   r0w0   /V   r0b0 ) C   2   [EQ. 33]
 
         E   g0   /E   b0 =( V   r0g0   /V   r0b0 ) C   3   [EQ. 34]
 
       FIGS. 68A-F  illustrate an exemplary third step in an exemplary method to set and maintain a precise color temperature produced by a combination of red, green, blue, and white LEDs  6121  through  6128 . While the first step illustrated in  FIGS. 66A-D  determined the ratio of light produced by white LED  6124  over blue LED  6123  as filtered by blue LED  6127  and the second step illustrated in  FIGS. 67A-D  determined the ratio of light produced by the white LED  6124  over the blue LED  6123  as filtered by green LED  6122 , the third step determines the ratio of light produced by white LED  6124  over blue LED  6123  as filtered by red LED  6121 . Additionally in the third step, the ratio of light produced by green LED  6122  over blue LED  6123  is also determined by red LED  6121 . White  6124 , blue  6123 , and green  6122  LEDs are illuminated by current sources  6120  producing currents Iw 0 , Ib 0 , and Ig 0  as shown in  FIGS. 68A ,  68 C and  68 E, respectively. The voltages induced across red LED  6121  by white  6124 , blue  6123 , and green  6122  LEDs are Vr 0   w   0 , Vr 0   b   0 , and Vr 0   g   0  respectively. Equations 30, 31, and 33 illustrate that such induced voltages Vr 0   w   0 , Vr 0   b   0 , and Vr 0   g   0  are equal to the optical powers Ew 0   r , Eb 0 , and Eg 0  produced by the white  6124 , blue  6123 , and green  6122  LEDs respectively times the responsiveness of red LED  6121  times constants. Equations 33 and 34 illustrate the division of equations 30 and 32 by equation 31 respectively and show that the ratio of optical power emitted by the white LED  6124  over the blue LED  6123  as filtered by the red LED  6121  is proportional to the ratio of Vr 0   w   0  over Vr 0   b   0 , and that the ratio of optical power emitted by the green LED  6122  over the blue LED  6123  as filtered by the red LED  6121  is proportional to the ratio of Vr 0   g   0  over Vr 0   b   0 . 
     Spectral plot  6880  shown in  FIG. 68B  illustrates that red LED  6121  is responsive to nearly the complete emission spectrum of white LED  24  including nearly the complete spectral emission peak due to phosphor conversion, which means any change or degradation in phosphor efficiency will affect the current induced in red LED  6121  by white LED  6124 . Spectral plot  6881  shown in  FIG. 68D  and spectral plot  6882  shown in  FIG. 68F  illustrate that red LED  6121  is also responsive to substantially the complete spectral emissions from blue  6123  and green  6122  LEDs, however, the responsiveness of red LED  6121  to blue LED  6123  is reduced. 
     The following equations are associated with  FIGS. 69A-D . In particular, equation 35 is associated with  FIGS. 69A-B . Equation 36 is associated with  FIGS. 69C-D . And equation 37 provides a ratio using equations 35 and 36. 
       V r1g0 =E g0 R r1 C r1g0   [EQ. 35]
 
       V r1r0 =E r0 E r1 C r1r0   [EQ. 36]
 
         E   g0   /E   r0 =( V   r1g0   /V   r1r0 ) C   4   [EQ. 37]
 
     The fourth exemplary step in the exemplary method to set and maintain a precise color temperature produced by a combination of red, green, blue, and white LEDs  6121  through  6128 , is illustrated in  FIGS. 9A-D  in which the ratio of currents induced in red LED  6125  by green LED  6122  over red LED  6121  is determined. Current sources  6120  illuminate green  6122  and red  6121  LEDs by producing currents Ig 0  and Ir 0  respectively, which induce voltages Vr 1   g   0  and Vr 1   r   0  respectively across red LED  6125 , as shown in  FIGS. 69A and 69C . Equations 35 and 36 illustrate that the voltages Vr 1   g   0  and Vr 1   r   0  are proportional to the green LED  6122  emitted power Eg 0  and the red LED  6121  emitted power Er 0  times the responsiveness of red LED  6125  Rr 1  respectively. Equation 37 illustrates equation 35 divided by 36, which shows the ratio of Eg 0  over Er 0  to be proportional to the ratio of induced voltages Vr 1   g   0  over Vr 1   g   0 . The responsiveness of red LED  6125  cancels out. 
     The following equations described with respect to  FIGS. 66A-D ,  67 A-D,  68 A-F and  69 A-D can be used to represent how color matching can be achieved. 
         E   w0b   /E   b0 =( V   b1w0   /V   b1b0 ) C   0   [EQ. 26]
 
         E   w0g   /E   b0 =( V   g0w0   /V   g0b0 ) C   1   [EQ. 29]
 
         E   w0r   /E   b0 =( V   r0w0   /V   r0b0 ) C   2   [EQ. 33]
 
         E   g0   /E   b0 =( V   r0g0   /V   r0b0 ) C   3   [EQ. 34]
 
         E   g0   /E   r0 =( V   r1g0   /V   r1r0 ) C   4   [EQ. 37]
 
         E   r0   /E   b0 =( V   r0g0   /V   r0b0 )( V   r1r0   /V   r1g0 ) C   3   /C   4 )  [EQ. 38]
 
         E   w0b   /E   w0r =( V   b1w0   /V   b1b0 )( V   r0b0   /V   r0w0 )( C   0   /C   2 )  [EQ. 39]
 
         E   w0g   /E   w0r =( V   g0w0   /V   g0b0 )( V   r0b0   /V   r0w0 )( C   1   /C   2 )  [EQ. 40]
 
     In particular, equations 26, 29, 33, 34 and 37 that were described with respect to  FIGS. 66A-D ,  67 A-D,  68 A-F and  69 A-D can be used to generate equations 38, 39 and 40. These equations provide an exemplary set of equations describing the exemplary method to set and maintain a precise color emitted by red, green, blue, and white LEDs. Equations 26, 29, 33, and 34 relate the optical power emitted by blue LED  6123  to the optical power emitted by white LED  6124  as filtered by blue  6127 , green  6122 , and red  6121  LEDs and by green LED  6122  respectively. Equation 38 divides equation 34 by equation 37 to relate the optical power emitted by blue LED  6123  to the optical power emitted by red LED  6121 . Equations 26, 29, 33, 34, and 38 relate the emitted power of red  6121 , green  6122 , and three different filtered versions of white LED  6124  to the emitted power of blue LED  6123 . Such ratios of optical power can be compared to desired ratios as described herein, for example, as described with respect to the third embodiment, and enable a precise color temperature light to be set and maintained by the combination of such red, green, blue, and white LEDs illustrated in this example. Switching LEDs  6121  through  6124  with LEDs  6125  through  6128  and repeating the steps illustrated in  FIGS. 66A-D ,  67 A-D,  68 A-F and  69 A-D provides the ratios of optical power emitted by LEDs  6125  through  6128  to balance the color produced by all the LEDs in light source  6110  illustrated in this example light source and calibration method. 
     Equation 39 illustrates the ratio of equation 26 over equation 33, which shows the ratio of light produced by white LED  6124  over the spectrum detectable by blue LED  6127  divided by the light produced by white LED  6124  over the spectrum detectable by red LED  6121 . Likewise, equation 40 illustrates the ratio of equation 29 over equation 33, which shows the ratio of light produced by white LED  6124  over the spectrum detectable by green LED  6122  divided by the light produced by white LED  6124  over the spectrum detectable by red LED  6121 . Such ratios illustrated by equations 39 and 40 can also be compared to desired ratios as described herein, for example, as described with respect to the third embodiment, and the intensities of the red, green, and blue LEDs can be adjusted relative to the white LEDs to compensate for changes in the spectrum of the white LEDs at the end of a manufacturing line, and over operation conditions and lifetime. 
       FIG. 70  illustrates the well known CIE  1931  Color Space diagram  7010  for the XY color space. The range of theoretically producible colors lie within the boundary  7011 , and the range of actual colors producible by a combination of red, green, blue, and white LEDs  6121  through  6128  lie within the triangle  7012 . In this example, the color points produced the red, green, and blue LEDs independently are the corners of the triangle labeled  7013 ,  7014 , and  7015  respectively. In this example, the desired color point produced by the combination of the red, green, blue, and white LEDs  6121  through  6128  is identified as  7016 , while in this example the actual color point at the time of calibration is identified at  7017 . 
     Such difference in color points  7016  and  7017  represents the change that can occur as a phosphor based white LED ages for instance. As the phosphor degrades and converts less blue light to other wavelengths, such phosphor converted peak  6444  changes to peak  6445  from  FIG. 64  relative to blue peak  6446  and the color point shifts from  7016  to  7017  as an example. Equation 39 then provides the actual ratio of the optical power in blue peak  6446  over approximately the optical power in phosphor converted peak  6445  at the time of calibration. Equation 39 also provides the desired ratio of the optical power in the blue peak  6446  over the same approximation of the optical power in phosphor converted peak  6444  at the time the device was manufactured. The ratio of such actual ratio divided by such desired ratio specifies how much relative optical power has shifted from the phosphor converted range of the white LED spectrum to the blue LED range of the white LED spectrum. Increasing the optical power produced by red LEDs  6121  and  6125  and green LEDs  6122  and  6126  in the proportion that produces color point  7018  by an amount determined by such ratio of such actual ratio over such desired ratio adjusts the color point produced by light source  6110  from the actual color  7017  back to the desired color point  7016 . 
     Since typically the responsiveness of red LEDs  6121  and  6125  drops off rapidly with decreasing incident wavelength near typical blue LED emission wavelength, the current induced in red LEDs  6121  and  6125  by white LEDs  6124  and  6128  is dominated by the optical power in the phosphor converted range of a typical white LED. The small amount of current induced in red LEDs  6121  and  6125  by the blue LED range of the white LED spectrum is effectively removed from the calculation results by taking the ratio of such actual ratio over such desired ratio. 
       FIG. 70  and the associated preceding description illustrate just one example of how the systems and methods herein can be applied. There are many other possible applications including but not limited to calibrating to the color of light source  6110  at the end of a manufacturing line. In such case, such desired ratios can be measured from a control device that produces the desired color point and compared to the actual ratios measured on production devices. In such case, the actual color point could be in any relation to the desired color point with different combinations of additional red, green, and blue LED light needed to produce the desired color point. In another example, the color point of the white LED could be deliberately shifted to the green region of the color diagram with light source  10  comprising such white LEDs along with only blue and red LEDs. As such, green LEDs would not be needed since the blue and red LEDs could always measure and adjust the combined light from the greenish white LEDs, and blue and red LEDs to the desired color point. Likewise, the substantially white light source could be something other than an LED, in which case different combinations of LEDs may be needed. Further, more than three different colored LEDs, such red, green, blue, and amber, or red, green, blue, cyan, and magenta, or any other combination of colors could be used to analyze the spectral emissions of the broadband light source and compensate for variations to set and maintain a precise color point. As such  FIGS. 61 through 70  illustrate just one example. 
     Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. 
     Eighth Embodiment 
     It is noted that detailed discussions have been provided above for LED calibration systems and methods with respect to the third embodiment and the seventh embodiment. The following discussion with respect to LED calibration systems and methods is also provided as an alternative but related discussion of calibration systems and methods. This alternative discussion is not intended to change or alter the discussions above but is merely included as an additional discussion of possible calibration techniques, systems and methods. 
     LED calibration systems and related methods are disclosed that use the photo-sensitivity of LEDs to correct for variations between LEDs during initial production and over the lifetime of systems using LEDs. Various embodiments are described with respect to the drawings below. Other features and variations can also be implemented, if desired, and related systems and methods can be utilized, as well. 
     In part, the disclosed embodiments relate to groups of LEDs that use the photo-sensitivity of each other and an optional light source to determine the light intensity produced by each LED in such group and to adjust such intensity to create and maintain a precise color produced by the group of LEDs. Applications for the LED calibration systems and methods include solid state lamps, LCD backlights, and LED displays for instance. Variations in LED brightness and color should be compensated for in order for such devices to have uniform color and brightness. Such compensation, which is typically done by measuring the optical output power of each individual LED or purchasing specially tested LEDs, is performed by simply measuring the signal induced on each LED by light from other LEDs in the device or optionally from an additional light source. 
     The disclosed embodiments include a number of methods to set the color or color temperature produced by a group of LEDs during the manufacturing of a device such as a lamp, an LED display, or an LCD backlight, and maintaining such color or color temperature over the operating life of such a device. The methods operate some of the LEDs in photovoltaic or photoconductive mode to measure the light produced by other LEDs in the group and optionally from a light source, and adjust the light produced by each LED in such group of LEDs to produce a precise color or color temperature from such group. 
     The first method illustrated in  FIG. 73A-C  relies on the correlation between the light produced by an LED from a fixed current and the photocurrent produced by such LED from a fixed light intensity. Since such correlation is not perfect, such first method is an approximation. However, such first method is the simplest and although not limited to such is intended to enable a device with a combination of red, green, and blue LEDs with large intensity variations to self calibrate relatively close to a desired color or color temperature and output intensity during production and over operating life. 
     The second method illustrated in  FIG. 74A-D  uses the same basic mechanisms as the first method to produce a fixed color or color temperature and output intensity, but uses a light source with typically a known intensity as a reference and typically requires two LEDs in such group of LEDs to emit light of the same color. Such second method introduces an error factor between the light produced by an LED from a fixed current and the photocurrent produced from a fixed light intensity, and mathematically illustrates that such second method is independent of such error factor and as such is significantly more accurate than such first method. Although not limited to such, the second method is intended to be used during the manufacturing of a device to create a precise color or color temperature and overall light intensity produced by a group of different colored LEDs in a device, but can also be used over operating life provided a light source is available. 
     The third method illustrated in  FIGS. 75A-F , although not limited to such, is intended to combine the results of the second method used during the manufacturing of a device comprising such group of LEDs with the first, second, or fourth methods used over the operating life of such device. Initial errors in the correlation between light produced by an LED from a fixed current and the photocurrent produced from a fixed light intensity are removed. Only changes in such errors over operating life introduce affect the color, color temperature, and light intensity produced by such group of LEDs in a device. 
     The fourth method illustrated in  FIG. 76A-D  uses the same basic mechanisms as the first and second methods, but is only capable of maintaining the color or color temperature of the light produced by a group of LEDs. Only the ratios of the emitted intensities can be determined so the overall emitted intensity is not precisely controlled. Although not limited to such, the fourth method is intended to be used over the operating life of such group of LEDs to maintain the color or color temperature of the light produced by such device. No external light source is needed, but typically two LEDs within the group of LEDs emit the same color light. 
     The fifth method illustrated in  FIG. 79A-C  uses the photosensitivity of an LED as a function of incident wavelength to determine the peak emission wavelength of such LED. A light source produces light with at least two different wavelengths typically just above and just below the expected peak emission wavelength range of such LED with the difference in induced photocurrents being directly related to the actual peak emission wavelength of such LED. 
     Once the relative intensity or both the relative intensity and wavelength of light produced by LEDs within a group of different colored LEDs are known, the color or color temperature of the combined light produced by such group of LEDs can be fixed, adjusted, and maintained over the operating life of such group of LEDs by adjusting the relative intensity of light produced by each such LED. A color correction matrix with coefficients determined by the calibration methods described herein can provide the compensated intensities to the driver circuitry for each such LED. 
     The second and fourth methods described in  FIGS. 74A-D  and  76 A-D respectively typically require two LEDs within such group of LEDs to emit the same color light. For instance, a lamp or LCD backlight comprising red, green, and blue LEDs would have at least two independently controlled red LEDs or serially connected strings of red LEDs. As another example, a lamp with white and red LEDs, would also have two independently controlled red LEDs or serially connected strings of red LEDs. In an LED display or LCD backlight comprising arrays of pixels of red, green, and blue LEDs, such group of LEDs could comprise two red LEDs from two adjacent pixels, for instance. Such two red LEDs could be successively grouped together with each of the remaining two blue and two green LEDs, for instance, to determine the intensity or relative intensity produced by all the LEDs in both pixels. Additionally, a uniform light source, such as sunlight, could illuminate such arrays of pixels to enable the second method illustrated in  FIG. 74A-D  to produce uniform intensity across the array. 
     The methods address problems associated with devices using groups of different colored LEDs directly or as backlights for illumination. Such calibration methods reduce the need for specially binned LEDs for the production of lamps, displays, or backlights, and maintain the color or color temperature of the light produced over the operating life of the device. 
     As stated above, this eighth embodiment can also be used with the techniques, methods and structures described with respect to the other embodiments described herein. For example, the calibration and detection systems and methods described with respect to this embodiment can be used within the other described embodiments, if desired. Further, the various illumination devices, light sources, light detectors, displays, and applications and related systems and methods described herein can be used with respect to calibration and detection systems and methods described in this eighth embodiment, as desired. Further, as stated above, the structures, techniques, systems and methods described with respect to this eighth embodiment can be used in the other embodiments described herein, and can be used in any desired lighting related application, including liquid crystal displays (LCDs), LCD backlights, digital billboards, organic LED displays, AMOLED (Active Matrix OLED) displays, LED lamps, lighting systems, lights within conventional socket connections, projection systems, portable projectors and/or other display, light or lighting related applications. 
     Turning now to the drawings,  FIG. 71  is an example circuit for measuring the actual emitted optical power labeled E a20  produced by LED  7120 , which is driven by constant current source  7121  with the nominal current of I 0  amps. The actual emitted power is measured by optical power meter  7128 . The following equation is associated with  FIG. 71 . 
       Actual Emitted Power/Nominal Emitted Power= E   a20   /E   n20   =E   20   [EQ. 41]
 
     Equation 41 relates the actual optical power E an  emitted by LED  7120  to the nominal optical power E n20  emitted by a group of LEDs representative of LED  7120 . The nominal or desired emitted optical power E n  can be any optical power but is typically the average or mean optical power produced by the group of LEDs representative of LED  7120 . The ratio of the actual power emitted by LED  7120  over the nominal power emitted by the group of LEDs representative of LED  7120  is the result of equation 41 labeled E 20 , which is independent of the current produced by current source  7121  and the optical losses between LED  7120  and optical power meter  7128  provided such conditions are the same during the optical power measurements for all LEDs within the group of LEDs representative of LED  7120  and including LED  7120 . 
     The group of LEDs representative of LED  7120  can be a so called characterization lot for the LED  7120  design that is specifically manufactured to produce LEDs with emission characteristics representative of the range of emission characteristics anticipated during mass production of the LED  7120  design. 
       FIG. 72  is an example circuit that produces a voltage V a30  induced across LED  7230  by the nominal optical power E n20  described with  FIG. 71  and emitted by LED  7120 . LED  7120  is configured to emit the nominal optical power E n20  as measured by optical power meter  7128  by adjusting the current produced by current source  7121  to an amount I 1 . Provided the peak emission wavelength of LED  7230  is roughly equal to or longer than the peak emission wavelength of LED  7120 , the light from LED  7120  induces a current in LED  7230  that produces the voltage V a30  across resistor  7232  between the anode  7233  and cathode  7234  of LED  7230 . The following equation is associated with  FIG. 72 . 
       Actual Voltage/Nominal Voltage= V   a30   /V   n3020   =V   3020   ˜=E   a30   /E   n30   =E   30   [EQ. 42]
 
     Equation 42 relates the actual voltage V a30  produced by LED  7230  in response to the nominal emitted power E n20  from LED  7120  to the nominal voltage V a3020  produced by a group of LEDs representative of LED  7230  in response to the nominal emitted power E n20 . Among other things, since variations in the optical path between the LED active region and the surface of the LED package approximately equally affect light entering and leaving the LED and since variations in the quantum efficiency of the active region approximately equally affect the conversion of electric current to light and of light to electrical current, such ratio of voltages is approximately equal to the ratio of actual optical power E a30  emitted by LED  7230  over the nominal optical power E n30  emitted by a group of LEDs representative of LED  7230 . Such ratio, which is the result of equation  7238  is called E 30  for LED  7230 . 
       FIG. 73A-C  illustrates a method using the relationship illustrated in  FIG. 72  to determine the actual optical power produced by LEDs  7120 ,  7230 , and  7340  relative to the nominal optical power produced by groups of LEDs representative of such LEDs  7120 ,  7230 , and  7340 . LEDs  7120 ,  7230 , and  7340  could be any combination of colors or could be one color. Two common configurations include red, green, and blue, and red, red, and white for LEDs  7340 ,  7230 , and  7120  respectively. In such method, LED  7120  illuminates both LED  7230  and LED  7340 , and then LED  7230  illuminates LED  7340 . From measurements of induced voltages V a30  and V a40 , the ratio of actual optical power emitted to nominal optical power emitted can be calculated for each LED  7120 ,  7230 , and  7340 . 
     The following equations are associated with  FIGS. 73A-C . In particular, equations 43A-B are associated with  FIG. 73A . Equations 44A-B are associated with  FIG. 73B . Equations 45A-B are associated with  FIG. 73C . And equations 46-51 utilize the other equations. 
         V   a30 ˜=( V   n3020 )( E   30 )( E   20 )  [EQ. 43A]
 
         V   a30   /V   n3020   =V   3020 ˜=( E   30 )( E   20 )  [EQ. 43B]
 
         V   a40 ˜=( V   n4020 )( E   40 )( E   20 )  [EQ. 44A]
 
         V   a40   /V   n4020   =V   4020 ˜=( E   40 )( E   20 )  [EQ. 44B]
 
         V   a40 ˜=( V   n4030 )( E   30 )( E   40 )  [EQ. 45A]
 
         V   a40   /V   n4030 )= V   4030 ˜=( E   30 )( E   40 )  [EQ. 45B]
 
     Rearranging 43B Provides 
         E   20 ˜=( V   3020 )( E   30 )  [EQ. 46]
 
     Substituting 46 into 44B Provides 
         V   4020 ˜=( E   40 )( V   3020 )( E   30 )
 
         E   30 ˜=( E   40 )( V   3020 )/( V   4020 )  [EQ. 47]
 
     Substituting 47 into 45B Provides 
         V   4030 ˜=( E   40 )( E   40 )( V   3020 )/( V   4020 )
 
       ( V   4030 )( V   4020 )/( V   3020 )˜=( E   40 ) 2   [EQ. 48]
 
         E   40 ˜=square root[( V   4030 )( V   4020 )/( V   3020 )]  [EQ. 49]
 
     From 45B 
         E   30 ˜=( V   4030 )/( E   40 )  [EQ. 50]
 
     From 44B 
         E   20 ˜=( V   4020 )/( E   40 )  [EQ. 51]
 
     Current source  7121  produces the nominal current I 0 , which causes LED  7120  to emit optical power E a20 , which induces voltages V a30  and V a40  across resistors  7232  and  7342  respectively. Equations 43A-B relate V a30  to the voltage V n3020  induced on a group of LEDs representative of LED  7230  by the nominal power emitted an LED  7120  as shown in  FIG. 73A . The actual voltage V a30  approximately equals the nominal voltage V n3020  scaled by the ratios of actual emitted power over nominal emitted power E 30  and E 20  for LED  7230  and LED  7120  respectively. The parameter V 3020  is defined as the ratio of the actual voltage V a30  over the nominal voltage V n3020 . 
     Equations 44A-B and 45A-B are the same as equation 43A-B except that equations 44A-B are for light from LED  7120  incident on LED  7340  as shown in  FIG. 73B  and equations 45A-B are for light from LED  7230  produced by current source  7331  incident on LED  7340  as shown in  FIG. 73C . Such three equations contain the three independent variables E 20 , E 30 , and E 40 , which are solved for through equations 46-51. Equation 49 relates E 40  to the known parameters V 4030 , V 4020 , and V 3020 . The calculated value for E 40  is then applied to equations 45 and 44 to form equations 50 and 51 that determine E 30  and E 20  respectively. 
       FIGS. 73A-C  provide one of many possible methods to determine the intensity of light produced by a group of LEDs by measuring LED photosensitivity. For instance light induced current instead of voltage can be measured or some combination of current and voltage can be measured. When measuring light induced current, an LED can be reverse biased or short circuited for instance. The number of LEDs used to determine emitted power can be 2 provided both LEDs peak emission wavelengths are similar or more than 3. The color of the LEDs can be any combination of colors or one single color. The LEDs can be arranged side by side with scattered light detected by adjacent LEDs or with light reflected by a mirror for instance. The product comprising the LEDs can a lamp, a display, or display backlight for instance. 
       FIGS. 74A-D  illustrate a more precise method to determine the intensity of light produced by LEDs  20 ,  30 , and  40  which uses a fixed light source  60  as a known reference that eliminates variations in the relationship between LED emitted power and photosensitivity. In this example LEDs  30  and  40  have approximately equal peak emission wavelengths and LED  20  has a shorter peak emission wavelength. 
     The following equations are associated with  FIGS. 74A-D . In particular, equations 52A-B and 53A-B are associated with  FIG. 74A . Equations 54A-B are associated with  FIG. 74B . Equations 55A-B are associated with  FIG. 74C . Equations 58A-B are associated with  FIG. 74D . And equations 56, 57 and 59 utilize the other equations. 
         V/V   n30   =V   30 =( C   30 )( E   a30   /E   n30 )= C   30   E   30   [EQ. 52A]
 
         E   30   =V   30   /C   30   [EQ. 52B]
 
         V   a40   /V   n40   =V   40 =( C   40 )( E   a40   /E   n40 )= C   40   E   40   [EQ. 53A]
 
         E   40   =V   40   /C   40   [EQ. 53B]
 
         V   a30 =( V   n3040 )( C   30 )( E   30 )( E   40 )  [EQ. 54A]
 
         V   a30   /V   n3040   =V   3040 =( C   30 )( E   30 )( E   40 )  [EQ. 54B]
 
         V   a40 =( V   n4030 )( C   40 )( E   40 )( E   30 )  [EQ. 55A]
 
         V   a40   /V   n4030   =V   4030 =( C   40 )( E   40 )( E   30 )  [EQ. 55B]
 
     Substituting 52B into 54B Provides 
         V   3040 =( C   30 )( V   30   /C   30 )( E   40 ) 
         E   40 =( V   30 )( V   3040 )  [EQ. 56]
 
     Substituting 53B into 55B Provides 
         V   4030 =( C   40 )( V   40   /C   40 )( E   30 ) 
         E   30   =V   40 )/( V   4030 )  [EQ. 57]
 
         V   a30 =( V   n3020 )( C   30 )( E   30 )( E   20 )  [EQ. 58A]
 
         V   a30   /V   n3020   =V   3020 =( C   30 )( E   30 )( E   20 )  [EQ. 58B]
 
     Substituting 52B into 58B Provides 
         V   3020 =( V   30 )( E   20 ) 
         E   20   =V   3020   /V   30   [EQ. 59]
 
     Light source  7460  emits a fixed and known amount of light E 1  on LEDs  7230  and  7340 , which induces voltages V a30  and V a60  across resistors  7232  and  7342  respectively. Equations 52A-B relate the ratio of actual voltage V a30  over the nominal voltage V n30  generated by a group of LEDs representative of LED  7230  in response to light source  7460  to the ratio of actual optical power E a30  emitted by LED  7230  when driven with a fixed current over the nominal optical power E n30  emitted by a group of LEDs representative of LED  7230  when driven with a fixed current as shown in  FIG. 74A . Since the light power emitted by an LED is not perfectly correlated with the photosensitivity of such LED, equations 52A-B introduce a correction coefficient C 30  that precisely defines the relationship between such ratios. Equations 53A-B are the same as equations 52A-B except for LED  7340  instead of LED  7230  as shown in  FIG. 74A . 
     After light source  7460  illuminates LEDs  7230  and  7340  for sufficient time to measure voltages V a30  and V a40 , light source  7460  is turned off. Subsequently, LED  7340  is turned on using current source  7441  and illuminates LED  7230  with actual optical power E a40 , which induces the voltage V a30  across resistor  7232  as shown in  FIG. 74B . Equations 54A-B relate the ratio V 3040  of V a30  over the nominal voltage V n3040  generated by a group of LEDs representative of LED  7230  in response to a nominal optical power E n40  emitted by a group of LEDs representative of LED  7340  to the ratios E 30  and E 40  of the actual optical power emitted by LEDs  7230  and  7340  when driven by fixed currents over the nominal optical power emitted by groups of LEDs representative of LEDs  7230  and  7340  when driven by the same such fixed current respectively. As in equations 52A-B, the constant C 30  determines the relationship between V 3040  and the product of E 30  and E 40 . 
     Subsequent to LED  7340  illuminating LED  7230 , LED  7230  then illuminates LED  7340  with actual output power E a30 , which induces the voltage V a40  across resistor  7342  as shown in  FIG. 74C . Equations 55A-B are the same as equations 54A-B except with LED  7230  and  7340  reversed. Substituting equation 52B into equation 54B yields equation 56 with E 40  expressed as a function of the measured values V 30  and V 3040 . Likewise, substituting equation 53B into equation 55B yields equation 57 with E 30  expressed as a function of the measured values V 40  and V 4030 . 
     Subsequent to determining the actual optical power emitted by LEDs  7230  and  7340 , the actual optical power E a20  emitted by LED  7120  can be determined by illuminating LED  7230  with light from LED  7120 , which produces the voltage V a30  across resistor  7232  as shown in  FIG. 74D . Equations 58A-D are the same as equations 54A-B and 55A-B except with LEDs  7120  illuminating LED  7230  instead of LED  7340  illuminating LED  7230  and LED  7230  illuminating LED  7340  respectively. Substituting equation 52B into equation 58B yields equation 59 with E 20  determined by the ratio of V 3020  over V 30 . 
     As in  FIGS. 73A-C ,  FIGS. 74A-D  represent one of many possible methods to determine the intensity of light produced by a group of LEDs by measuring LED photosensitivity. Light induced current instead of voltage can be measured or any combination of current and voltage can be measured to determine output intensity. The number of LEDs can be two or any number more than 2. The LEDs can be any combination of colors or any single color provided that two LEDs in the group have approximately the same peak emission wavelength that is approximately equal to or longer than the peak emission wavelength of the light source if monochromatic. Monochromatic or broad spectrum light sources can be used and multiple light sources with different spectrums can be used. The two LEDs with approximately equal peak emission wavelengths (the same color) can be two red LEDs from adjacent pixels in an RGB display or two strings of red LEDs in a lamp for instance. 
       FIGS. 75A-C  and  FIGS. 75D-F  in combination with the method illustrated in  FIGS. 73A-C  represent two possible methods of approximately determining the actual output power emitted by LEDs  7120 ,  7230 , and  7340  over lifetime. Subsequent to intensity calibration during the manufacturing of a device comprising LEDs  7120 ,  7230 , and  7340  using the method described in  FIGS. 74A-D  for instance, the voltages V n3020 , V n4020 , and V n4030  are measured and stored in some form of non-volatile memory. According to the method illustrated in  FIG. 75A-C , current sources  7121  and  7331  produce the nominal current I 0  used during the calibration method described in  FIGS. 74A-D  for instance, and according to the method illustrated in  FIG. 75A-C , the current sources  7121  and  7331  are adjusted to output the nominal optical power. The color point and intensity of light produced by LEDs  7120 ,  7230 , and  7340  are adjusted by turning such LEDs in  FIG. 75A-C  off for different percentages of time using commonly known pulse width modulating (PWM) techniques, while the color point and intensity of light produced by such LEDs in  FIG. 75D-F  are adjusted by changing the current produced by current sources  7121 ,  7331 , and  7441 . 
     After operating a device comprising LEDs  7120 ,  7230 , and  7340  for some time, the actual optical output intensity from each such LED  7120 ,  7230  and  7340  may change and can be re-measured according to the method illustrated in  FIG. 73A-C  using the stored open circuit voltages V n3020 , V n4020 , and V n4030  as the nominal voltages. Such method determines the change in emitted output intensity from the change in voltage, which is approximately proportional. Such measurements ideally should be performed when ambient light is small in comparison to the intensity of light produced by LEDs  7120  and  7230  and incident on LEDs  7230  and  7340 . The intensity of such ambient light can be determined by measuring the open circuit voltage across any LED  7120 ,  7230 , or  7340  when all LEDs  7120 ,  7230 , and  7340  are turned off. In the presence of ambient light, the effects of such light can be removed by calculating the current induced by such ambient light and removing such current&#39;s affect on the measurements of V 3020 , V 4020 , and V 4030  illustrated in  FIGS. 73A-C . 
       FIGS. 75A-C  and  75 D-F illustrate two of many possible methods of determining the change in optical power emitted by LEDs over lifetime by measuring the photosensitivity of such LEDs. For instance, the current induced by incident light can be measured instead of voltage. The number and configuration of such LEDs can be different from the three illustrated in  FIGS. 75A-C  and  75 D-F, which represents a possible optical power measurement method for a combination of three red, green, and blue LEDs. For instance, two LEDs with approximately the same peak emission wavelength can measure each other&#39;s change in emission intensity. Additionally, a fixed intensity light could illuminate the LEDs and the LED emission intensity could be determined according to the method illustrated in  FIGS. 74A-D  for instance. 
       FIGS. 76A-D  illustrate an example method of determining the relative intensity of light emitted by LEDs  7120 ,  7230 , and  7340  where two such LEDs  7230  and  7340  have approximately equal peak emission wavelength and LED  7120  has a peak emission wavelength approximately equal to or shorter than that of LEDs  7230  and  7340 . As an example, LEDs  7230  and  7340  could be red and LED  7120  could be white, green, or blue. As another example, in an array of red, green, and blue LED groups or pixels, the red LEDs of two adjacent groups or pixels of a red, green, and blue LED could be used as LEDs  7340  and  7230 , and LED  7120  can sequentially be the two green and the two blue LEDs in such two adjacent groups or pixels used one at a time as LED  7120 . 
     The following equations are associated with  FIGS. 76A-D . In particular, equations 60 and 61 are associated with  FIG. 76A . Equation 62 is associated with  FIG. 76B . Equation 63 is associated with  FIG. 76C . Equation 64 is associated with  FIG. 76D . And equations 65, 66 and 67 utilize the other equations. 
         R   x =( C   x )( E   ax   /E   nx )  [EQ. 60]
 
         V   a30   /V   a3040   =V   3040 =( R   30 )( E   40 )  [EQ. 61]
 
         V   a40   /V   n4030   =V   4030 =( R   40 )( E   30 )  [EQ. 62]
 
         V   a30   /V   n3020   =V   3020 =( R   30 )( E   20 )  [EQ. 63]
 
         V   a40   /V   n4020   =V   4020 =( R   40 )( E   20 )  [EQ. 64]
 
     Ratio of 61 over 63 Provides 
         V   3040   /V   3020 =( R   30 )( E   40 )/( R   30 )( E   20 )=( E   40 )( E   20 )  [EQ. 65]
 
     Ratio of 62 over 64 Provides 
         V   4030   /V   4020   =R   40 )( E   30 )/( R   40 )( E   20 )=( E   30 )( E   20 )  [EQ. 66]
 
     Ratio of 65 over 66 Provides 
       ( V   3040   /V   3020 )/( V   4030   /V   4020 )=( E   40   /E   20 )( E   30   /E   20 ) 
       ( V   3040 )( V   4020 )/( V   4030 )( V   3020 )= E   40   /E   30   [EQ. 67]
 
     In such method, LED  7340  first illuminates LED  7230  as shown in  FIG. 76A  to create equation 61, which relates the ratio V 3040  of voltage Va 30  over the nominal Vn 3040  produced when LED  7230  is illuminated with the nominal optical power to the ratio E 40  of the actual emitted optical power Ea 40  over such nominal optical power. The proportionality factor between V 3040  and E 40  is the normalized responsivity of LED  7230  defined as R 30  using the general responsivity equation set forth as equation 60. Subsequently, LED  7230  illuminates LED  7340  as shown in  FIG. 76B  and then LED  7120  illuminates both LEDs  7230  and  7340  as shown in  FIGS. 76C and 76D  to form equations 62, 63 and 64 respectively. 
     As shown in equation 65, the ratio of equation 61 over 63 provides the relative emitted power between LED  7340  and  7120 . Likewise, equations 66 and 67 provide the relative power emitted power between LED  7230  and  7120 , and between LED  7340  and  7230  respectively. Such equations provide the relative optical power emitted by all three LEDs from measurements of induced voltages so that compensation circuits can adjust the emitted intensity from each LED to produce a precise color or to maintain a fixed color over the lifetime of the LEDs. 
       FIG. 77  is an example block diagram for circuitry that can implement the methods illustrated in  FIGS. 73A-C ,  74 A-D,  75 A-C,  75 D-F, and  76 A-D which comprises driver integrated circuit  7780 , LEDs  7120 ,  7230 , and  7340 , and resistors  7232  and  7342 . Integrated circuit  7780  further comprises timing and control circuitry  7781 , coefficient matrix  7782 , digital to analog converter (DAC)  7783 , analog to digital converter (ADC)  7784 , and three output drivers  7785  for producing currents for LEDs  7120 ,  7230  and  7340 . Output drivers  7785  further comprise of pulse width modulators  7787  and current sources  7786 . 
     Timing and control circuitry  7781  manages the functionality of driver IC  7780 . Illumination data for LEDs  7120 ,  7230 , and  7340  is either hardwired into timing and control circuitry  7781  or is communicated to timing and control circuitry  7781  through some means, and is forwarded at the appropriate time to the color correction matrix  7782 . Color correction matrix  7782  can, among other things, adjust the illumination data for LEDs  7120 ,  7230 , and  7340  to compensate for variations between LEDs to produce uniform brightness and color across a display or from a lamp. Matrix  7782  can comprise correction coefficients that when combined with the illumination data produce the data forwarded to output drivers  7785 , which have pulse width modulators  7787  that produce logic level signals that turn current sources  7786  on and off to LEDs  7120 ,  7230 , and  7340 . 
     ADC  7784  has access to terminals of all 3, in this example, LEDs connected to driver IC  7780  and can, among other things, measure the voltage produced across resistors  7232  and  7342  in response to light incident on LEDs  7230  and  7340 . The anodes of all three LEDs in this example can be tied together to a single supply voltage Vd  7788 , or can be connected to different supply voltages. In the case all three LEDs  7120 ,  7230 , and  7340  are of one color, all anodes preferentially would be connected together. In the case, such three LEDs  7120 ,  7230 , and  7340  are of different colors, each such different color LED  7120 ,  7230 , and  7340  would preferentially be connected to each such different supply voltage. 
       FIG. 77  is just one example of many possible driver IC  7780  block diagrams. For instance PWM  7787  would not be needed if LEDs  7120 ,  7230 , and  7340  were driven with variable current for fixed amount of times. Resistors  7232  and  7342  would not be needed if ADC  7784  measured open circuit voltage, short circuit current, or some other combination of current and voltage from LEDs  7120 ,  7230 , and  7340 . DAC  7783  could be a fixed current source if variable currents were not desired. 
       FIG. 78  is an example block diagram of correction matrix  7782  that can correct for variations in light intensity produced by a combination of red, green, and blue LEDs  7120 ,  7230 , and  7340  to produce relatively uniform brightness and color across a display or from a lamp. Matrix  82  comprises memory  7890  that can store correction coefficients Cr, Cg, and Cb, which are combined by multipliers  7891  with the red, green, and blue, for instance, illumination data respectively from timing and control circuitry  7781  to produce the illumination data forwarded to modulators  7787  controlling red, green, and blue LEDs  7120 ,  7230 , and  7340  respectively. Such correction coefficients are typically relatively large, which produce adjustments in the illumination data to compensate for variations between LEDs  7120 ,  7230 , and  7340 . 
     Memory  7890  can be made from SRAM, DRAM, FLASH, registers, or any other form of read-writable semiconductor memory. Such correction coefficients periodically can be modified by driver IC  7780  or any other processing element in a display or lamp for instance to adjust for changes in LEDs  7120 ,  7230 , and  7340  characteristics for instance over temperature or lifetime. 
     Multipliers  7891  scale the illumination data from timing and control circuitry  7781  by multiplying each color component by the corresponding correction coefficient. Such multiplication can be performed by discreet hardware in bit parallel or bit serial form, in an embedded microcontroller, or by any other means. Preferentially, one hardware multiplier comprising a shifter and an adder performs all three multiplications. As such,  FIG. 78  is just one of many possible block diagrams for correction matrix  7782 . 
       FIGS. 79A-C  illustrate one possible method to determine the peak emission wavelength λ p  from an LED by determining such LED&#39;s photosensitivity as a function of the wavelength of light incident on such LED. Such measurement system could comprise light source  7460 , LED  7230  and resistor  7232  as illustrated in  FIG. 74A , with the wavelength of light emitted by light source  7460  switched between wavelengths λ −  and λ +  that are slightly shorter and longer respectively than the expected peak emission wavelength λ p  of LED  7230 . 
     Plot  7900  in  FIG. 79A  represents the photosensitivity of LED  7230  with a nominal peak emission wavelength λ pn  as a function of incident wavelength with the vertical axis representing the voltage induced across resistor  7232 . At wavelengths longer than λ pn , the photosensitivity reduces significantly, while at wavelengths shorter than λ pn , the photosensitivity reduces linearly with wavelength. Also shown is incident light with wavelength λ −  producing voltage V −  across resistor  7232  and incident light with wavelength λ +  producing voltage V +  across resistor  7232 . Line  7903  connecting the points (λ − , V − ) and (λ + ,V + ) has a slope M=(V−−V+)/(λ − −λ + ). 
     Plot  7901  in  FIG. 79B  illustrates the photosensitivity of an LED  7230  with a peak emission wavelength λ p−  that is slightly shorter than the nominal peak emission wavelength λ pn . When such an LED  7230  is illuminated by light source  7460  with wavelengths λ −  and λ_, voltages V −  and V +  respectively are generated across resistor  7232 . The difference in voltage between such V −  and V +  is greater for such LED  7230  with peak emission wavelength λ p−  that is slightly shorter than the nominal peak emission wavelength λ pn  than for such LED  7230  with the nominal peak emission wavelength λ pn . Additionally, the slope M of line  7904  is more negative for the LED  7230  emitting the peak wavelength λ − , than for the LED  7230  emitting the nominal peak wavelength λ pn . 
     Plot  7902  in  FIG. 79C  illustrates the photosensitivity of an LED  7230  with a peak emission wavelength λ +  that is slightly longer than the nominal peak emission wavelength λ pn . When such an LED  7230  is illuminated by light source  7460  with wavelengths λ −  and λ − , voltages V −  and V +  respectively are generated across resistor  32 . The difference in voltage between such V −  and V +  is smaller for such LED  7230  with peak emission wavelength λ p−  that is slightly longer than the nominal peak emission wavelength λ pn  than for such LED  7230  with the nominal peak emission wavelength λ pn . Additionally, the slope M of line  7905  is less negative for the LED  7230  emitting the peak wavelength λ − , than for the LED  7230  emitting the nominal peak wavelength λ pn . 
     Since the slopes of lines  7903 ,  7904 , and  7905  are directly related to the peak emission wavelength of LED  7230 , such slopes can be used to determine such peak emission wavelengths.  FIGS. 79A-C  illustrate one of many possible methods to determine the peak emission wavelength of light produced by an LED by measuring the photosensitivity of such LED. For instance, LED light induced current could be measured instead of voltage or some other combination of current and voltage could be measured. Additionally, light with broader spectrums of light could induce such voltages or currents instead of the mono-chromatic sources illustrated in  FIG. 79 . 
       FIG. 80  is an example block diagram for correction matrix  7782  that can correct for variations in both light intensity and wavelength produced by a combination of red, green, and blue LEDs  7340 ,  7230 , and  7120  for instance to produce uniform brightness and color from an array of LEDs. Matrix  7782  comprises memory  7890  that can store nine correction coefficients with three such coefficients for each color component produced. Coefficients Crr, Cgg, and Cbb would typically be effectively the same as Cr, Cg, and Cb from  FIG. 78  to adjust for intensity variations in LEDs  7120 ,  7230 , and  7340 , while the remaining coefficients (Crg, Crb, Cgr, Cgb, Cbr, Cbg) compensate for wavelength variations. 
     For instance, if the red illumination data from timing and control circuitry  7781  was intended for an LED  7340  with a wavelength of 650 nm and the connected LED  7340  wavelength was exactly 650 nm, coefficients Cgr and Cbr would be zero and Crr would be close to one. If such connected LED  7340  wavelength was 640 nm and had the same intensity as the just previous example, Crr would be slightly smaller than in the just previous example and Cgr and Cbr would be non-zero, which would produce some light from such green and blue LEDs  7230  and  7120  respectively. The wavelength of the combination of light from such red, green, and blue LEDs  7340 ,  7230 , and  7120  would be perceived the same as mono-chromatic light from a single red LED  7340  emitting at precisely 650 nm. 
     Memory  7890  and multipliers  7891  can operate and be implemented as described for  FIG. 78 . Adder  8010  sums the multiplication results from the three connected multipliers  7891  to produce the illumination data forwarded to modulators  7887 . Such adders  8010  can be implemented in hardware or software, or be performed bit parallel or bit serial.  FIG. 80  is just one of many possible intensity and wavelength correction matrix  7782  block diagrams. 
     Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated.