Systems and methods for calibrating solid state lighting panels

[0001] The present application claims priority to U.S. Provisional Patent Application No. 60/738,305, filed Nov. 18, 2005, entitled System and Method for Interconnection and Integration of LED Backlighting Modules, the disclosure of which is hereby incorporated herein by reference in its entirety. The present application is a continuation in part of U.S. patent application Ser. No. 11/368,976, filed Mar. 6, 2006, entitled Adaptive Adjustment of Light Output of Solid State Lighting Panels, the disclosure of which is hereby incorporated herein by reference in its entirety.

[0002] The present invention relates to solid state lighting, and more particularly to adjustable solid state lighting panels and to systems and methods for adjusting the light output of solid state lighting panels.

[0003] Solid state lighting arrays are used for a number of lighting applications. For example, solid state lighting panels including arrays of solid state lighting devices have been used as direct illumination sources, for example, in architectural and/or accent lighting. A solid state lighting device may include, for example, a packaged light emitting device including one or more light emitting diodes (LEDs). Inorganic LEDs typically include semiconductor layers forming p-n junctions. Organic LEDs (OLEDs), which include organic light emission layers, are another type of solid state light emitting device. Typically, a solid state light emitting device generates light through the recombination of electronic carriers, i.e. electrons and holes, in a light emitting layer or region.

[0004] Solid state lighting panels are commonly used as backlights for small liquid crystal display (LCD) display screens, such as LCD display screens used in portable electronic devices. In addition, there has been increased interest in the use of solid state lighting panels as backlights for larger displays, such as LCD television displays.

[0005] For smaller LCD screens, backlight assemblies typically employ white LED lighting devices that include a blue-emitting LED coated with a wavelength conversion phosphor that converts some of the blue light emitted by the LED into yellow light. The resulting light, which is a combination of blue light and yellow light, may appear white to an observer. However, while light generated by such an arrangement may appear white, objects illuminated by such light may not appear to have a natural coloring, because of the limited spectrum of the light. For example, because the light may have little energy in the red portion of the visible spectrum, red colors in an object may not be illuminated well by such light. As a result, the object may appear to have an unnatural coloring when viewed under such a light source.

[0006] The color rendering index of a light source is an objective measure of the ability of the light generated by the source to accurately illuminate a broad range of colors. The color rendering index ranges from essentially zero for monochromatic sources to nearly 100 for incandescent sources. Light generated from a phosphor-based solid state light source may have a relatively low color rendering index.

[0007] For large-scale backlight and illumination applications, it is often desirable to provide a lighting source that generates a white light having a high color rendering index, so that objects and/or display screens illuminated by the lighting panel may appear more natural. Accordingly, such lighting sources may typically include an array of solid state lighting devices including red, green and blue light emitting devices. When red, green and blue light emitting devices are energized simultaneously, the resulting combined light may appear white, or nearly white, depending on the relative intensities of the red, green and blue sources. There are many different hues of light that may be considered “white.” For example, some “white” light, such as light generated by sodium vapor lighting devices, may appear yellowish in color, while other “white” light, such as light generated by some fluorescent lighting devices, may appear more bluish in color.

[0008] The chromaticity of a particular light source may be referred to as the “color point” of the source. For a white light source, the chromaticity may be referred to as the “white point” of the source. The white point of a white light source may fall along a locus of chromaticity points corresponding to the color of light emitted by a black-body radiator heated to a given temperature. Accordingly, a white point may be identified by a correlated color temperature (CCT) of the light source, which is the temperature at which the heated black-body radiator matches the hue of the light source. White light typically has a CCT of between about 4000 and 8000K. White light with a CCT of 4000 has a yellowish color, while light with a CCT of 8000K is more bluish in color.

[0009] For larger display and/or illumination applications, multiple solid state lighting tiles may be connected together, for example, in a two dimensional array, to form a larger lighting panel. Unfortunately, however, the hue of white light generated may vary from tile to tile, and/or even from lighting device to lighting device. Such variations may result from a number of factors, including variations of intensity of emission from different LEDs, and/or variations in placement of LEDs in a lighting device and/or on a tile. Accordingly, in order to construct a multi-tile display panel that produces a consistent hue of white light from tile to tile, it may be desirable to measure the hue and saturation, or chromaticity, of light generated by a large number of tiles, and to select a subset of tiles having a relatively close chromaticity for use in the multi-tile display. This may result in decreased yields and/or increased inventory costs for a manufacturing process.

[0010] Moreover, even if a solid state display/lighting tile has a consistent, desired hue of light when it is first manufactured, the hue and/or brightness of solid state devices within the tile may vary non-uniformly over time and/or as a result of temperature variations, which may cause the overall color point of the panel to change over time and/or may result in non-uniformity of color across the panel. In addition, a user may wish to change the light output characteristics of a display panel in order to provide a desired hue and/or brightness level.

[0011] Some embodiments of the invention provide methods of calibrating a lighting panel including a plurality of segments, each of said segments configured to emit a first color light and a second color light in response to pulse width modulation control signals having respective duty cycles. The methods include determining an average segment luminance for the lighting panel, determining a luminance variation of each segment to the average segment luminance, comparing the luminance variation of each segment to a threshold, and adjusting the duty cycle of at least one color of at least one segment to reduce the luminance variation in response to the luminance variation of a segment exceeding the threshold.

[0012] Determining the average segment luminance may include sequentially illuminating a plurality of segments, measuring a display luminance from the illuminated segments at a measurement location, and averaging the display luminance measurements. The measurement location may include a location at about the center of the lighting panel.

[0013] Sequentially illuminating a plurality of segments may include applying a pulse width modulation control signal having an adjusted duty cycle to at least one of the plurality of segments.

[0014] The luminance variation to the average segment luminance may be calculated according to the equation ΔY_mn=[Y_mn−max(Y_mn)]/Y_center, where Y_mn represents the luminance of an mth segment measured at an nth measurement location, and Y_center represents the average segment luminance.

[0015] Adjusting the duty cycle of at least one color of at least one segment may include determining a maximum duty cycle for all colors/segments, and dividing the duty cycle of the at least one color of the at least one segment by the maximum duty cycle.

[0016] The methods may further include determining a uniformity coefficient for the at least one segment, and adjusting the duty cycles of each color of the at least one segment using the uniformity coefficient before determining the maximum duty cycle.

[0017] The uniformity coefficient may be determined according to the equation C_m=[1−min(ΔY_m1, . . . , ΔY_mn)]/1.1, where ΔY_mn represents the luminance variation of the mth segment at the nth location to the average segment luminance and C_m represents the uniformity coefficient for the at least one segment.

[0018] Determining the luminance variation of each segment to the average segment luminance may include determining the luminance variation of each segment to the average segment luminance for each color.

[0019] Adjusting the duty cycle of at least one color of at least one segment may include for each color, determining a maximum duty cycle for all segments, and dividing the duty cycle of the at least one color of the at least one segment by the maximum duty cycle for the at least one color. Each segment may include a group of tiles and/or a bar of tiles.

[0020] The methods may further include adjusting the duty cycles of a segment to reduce the maximum color variation of the segment. Adjusting the duty cycles of a segment to reduce the maximum color variation for the segment may include, for each color, measuring a luminance of each segment at a first duty cycle, and determining a nominal luminance ratio including a ratio of a total luminance of each color divided by a total luminance of the lighting panel. A luminance ratio is determined for each color including a ratio of a total luminance of a color of the segment to a total luminance of the segment, and a variation of luminance ratios for each color of the segment from the nominal luminance ratio is determined. A duty cycle of at least one color of the segment is adjusted to reduce the at least one variation of luminance ratios from the nominal luminance ratio in response to at least one variation of luminance ratios from the nominal luminance ratio exceeding a second threshold. The first duty cycle may include a maximum duty cycle.

[0021] Determining a variation of luminance ratios from the nominal luminance ratio for each color may include determining a maximum variation of luminance ratios from the nominal luminance ratio for each color.

[0022] Determining a luminance ratio for each color may include determining a total luminance for each segment for each color.

[0023] Adjusting a duty cycle of at least one color of the segment may include selecting a color with a lowest relative luminance, and multiplying a duty cycle by a coefficient generated based on the luminance of the selected color.

[0024] Some embodiments of the invention provide a calibration system for calibrating a lighting panel including a plurality of segments, each of said segments configured to emit a first color light and a second color light in response to pulse width modulation control signals having respective duty cycles. The calibration system includes a calibration controller configured to be coupled to the lighting panel, and a calibration unit coupled to the calibration controller and including a calorimeter. The calibration controller is configured to determine an average segment luminance for the lighting panel, to determine a luminance variation of each segment to the average segment luminance, to compare the luminance variation of each segment to the average segment luminance to a threshold, and, in response to the luminance variation of a segment exceeding the threshold, to adjust the duty cycle of at least one color of at least one segment to reduce the luminance variation.

[0025] The calibration controller may be further configured to cause the lighting panel to individually illuminate a plurality of segments, to measure a display luminance from the illuminated segments at a measurement location, and to average the display luminance measurements.

[0026] The measurement location may include a location at about the center of the lighting panel.

[0027] The calibration controller may be configured to calculate the luminance variation to the average segment luminance according to the equation ΔY_mn=[Y_mn−max(Y_mn)]/Y_center, where Y_mn represents the luminance of an mth segment measured at an nth measurement location, and Y_center represents the average segment luminance.

[0028] The calibration controller may be further configured to adjust the duty cycle of at least one color of at least one segment by determining a maximum duty cycle for all colors/segments and dividing the duty cycle of the at least one color of the at least one segment by the maximum duty cycle.

[0029] The calibration controller may be further configured to determine a uniformity coefficient for the at least one segment, and to adjust the duty cycles of each color of the at least one segment using the uniformity coefficient before determining the maximum duty cycle.

[0030] The calibration controller may be configured to determine the uniformity coefficient according to the equation Cm=[1−min(ΔY_m1, . . . , ΔY_mn)]/1.1, where ΔY_mn represents the luminance variation of the mth segment at the nth location to the average segment luminance and Cm represents the uniformity coefficient for the at least one segment.

[0031] The calibration controller may be configured to determine the luminance variation of each segment to the average segment luminance by determining the luminance variation of each segment to the average segment luminance for each color.

[0032] The calibration controller may be configured to adjust the duty cycle of at least one color of at least one segment may include by, for each color, determining a maximum duty cycle for all segments, and by dividing the duty cycle of the at least one color of the at least one segment by the maximum duty cycle for the at least one color.

[0033] The calibration controller may be further configured to adjust the duty cycles of a segment to reduce the maximum color variation of the segment.

[0034] The calibration controller may be further configured to measure a luminance of each segment at a first duty cycle for each color, to determine a nominal luminance ratio for each color including a ratio of a total luminance of each color divided by a total luminance of the lighting panel, to determine a luminance ratio for each color of the segment including a ratio of a total luminance of a color of the segment to a total luminance of the segment, and to determine a variation of luminance ratios for each color of the segment from the nominal luminance ratios. The calibration controller is further configured to adjust a duty cycle of at least one color of the segment to reduce the at least one variation of luminance ratios from the nominal luminance ratio in response to at least one variation of a luminance ratio from the nominal luminance ratio exceeding a second threshold. The first duty cycle may include a maximum duty cycle.

[0035] The calibration controller may be configured to determine the variation of luminance ratios from the nominal luminance ratio for each color by determining a maximum variation of luminance ratios from the nominal luminance ratio for each color.

[0036] The calibration controller may be configured to calculate the luminance ratio for each color by determining a total luminance for each segment for each color.

[0037] The calibration controller may be configured to adjust the duty cycle of at least one color of at least one segment by selecting a color with a lowest relative luminance, and by multiplying a duty cycle by a coefficient generated based on the luminance of the selected color.

[0038] The calibration unit may further include an XZ positioner connected to the colorimeter and configured to move the colorimeter in two dimensions.

[0039] The calibration unit may further include an enclosure having an entrance, a conveyor extending from outside the enclosure to inside the enclosure through the entrance, and a pallet on the conveyor and configured to hold the lighting panel during calibration. The conveyor and the pallet are configured to bring the lighting panel into enclosure, and the calorimeter is positioned within the enclosure so as to detect light emitted by the lighting panel.

[0040] Methods of calibrating a lighting panel according to further embodiments of the invention include selectively energizing one of the plurality of strings, measuring a dominant wavelength of the light emitted by the energized string, comparing the dominant wavelength of the light emitted by the energized string to a desired dominant wavelength, and adjusting an on-state current level of a pulse width modulation control signal for the energized string to reduce a difference of the dominant wavelength emitted by the energized string to the desired dominant wavelength.

[0041] Adjusting the on-state current level of the pulse width modulation control signal may include increasing the on-state current level of the pulse width modulation control signal if the dominant wavelength of the light emitted by the energized string is greater than the desired dominant wavelength.

[0042] Adjusting the on-state current level of the pulse width modulation control signal may include reducing the on-state current level of the pulse width modulation control signal if the dominant wavelength of the light emitted by the energized string is less than the desired dominant wavelength.

[0043] The lighting panel may include a plurality of strings configured to emit light of a first color, and the methods may further include measuring the dominant wavelength of each of the strings configured to emit light of the first color, and determining an average of the dominant wavelengths of each of the strings configured to emit light of the first color. Comparing the dominant wavelength of the light emitted by the energized string to a desired dominant wavelength may include comparing the dominant wavelength of-the light emitted by the energized string to the average dominant wavelength.

[0044] The methods may further include determining a variance of the dominant wavelengths of each of the strings configured to emit light of the first color, and adjusting the on-state current level of the pulse width modulation control signal for at least one string to reduce the variance of the dominant wavelengths emitted by the strings.

[0045] The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate certain embodiment(s) of the invention. In the drawings:

[0046] FIG. 1 is a front view of a solid state lighting tile in accordance with some embodiments of the invention;

[0047] FIG. 2 is a top view of a packaged solid state lighting device including a plurality of LEDs in accordance with some embodiments of the invention;

[0048] FIG. 3 is a schematic circuit diagram illustrating the electrical interconnection of LEDs in a solid state lighting tile in accordance with some embodiments of the invention;

[0049] FIG. 4A is a front view of a bar assembly including multiple solid state lighting tiles in accordance with some embodiments of the invention;

[0050] FIG. 4B is a front view of a lighting panel in accordance with some embodiments of the invention including multiple bar assemblies;

[0051] FIG. 5 is a schematic block diagram illustrating a lighting panel system in accordance with some embodiments of the invention;

[0052] FIGS. 6A-6D are a schematic diagrams illustrating possible configurations of photosensors on a lighting panel in accordance with some embodiments of the invention;

[0053] FIGS. 7-8 are schematic diagrams illustrating elements of a lighting panel system according to some embodiments of the invention;

[0054] FIG. 9 is a flowchart illustrating calibration methods according to some embodiments of the invention;

[0055] FIGS. 10-12 are schematic diagrams illustrating calibration systems according to some embodiments of the invention; and

[0056] FIGS. 13, 14, 15A and 15B are flowchart diagrams illustrating calibration operations according to some embodiments of the invention.

[0057] Embodiments of the present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.

[0058] It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present invention. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

[0059] It will be understood that when an element such as a layer, region or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.

[0060] Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “vertical” may be used herein to describe a relationship of one element, layer or region to another element, layer or region as illustrated in the figures. It will be understood that these terms are intended to encompass different orientations of the device in addition to the orientation depicted in the figures.

[0061] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” “comprising,” “includes” and/or “including” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

[0062] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0063] The present invention is described below with reference to flowchart illustrations and/or block diagrams of methods, systems and computer program products according to embodiments of the invention. It will be understood that some blocks of the flowchart illustrations and/or block diagrams, and combinations of some blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be stored or implemented in a microcontroller, microprocessor, digital signal processor (DSP), field programmable gate array (FPGA), a state machine, programmable logic controller (PLC) or other processing circuit, general purpose computer, special purpose computer, or other programmable data processing apparatus such as to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

[0064] These computer program instructions may also be stored in a computer readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer readable memory produce an article of manufacture including instruction means which implement the function/act specified in the flowchart and/or block diagram block or blocks.

[0065] The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. It is to be understood that the functions/acts noted in the blocks may occur out of the order noted in the operational illustrations. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality/acts involved. Although some of the diagrams include arrows on communication paths to show a primary direction of communication, it is to be understood that communication may occur in the opposite direction to the depicted arrows.

[0066] Referring now to FIG. 1, a solid state lighting tile 10 may include thereon a number of solid state lighting elements 12 arranged in a regular and/or irregular two dimensional array. The tile 10 may include, for example, a printed circuit board (PCB) on which one or more circuit elements may be mounted. In particular, a tile 10 may include a metal core PCB (MCPCB) including a metal core having thereon a polymer coating on which patterned metal traces (not shown) may be formed. MCPCB material, and material similar thereto, is commercially available from, for example, The Bergquist Company. The PCB may further include heavy clad (4 oz. copper or more) and/or conventional FR-4 PCB material with thermal vias. MCPCB material may provide improved thermal performance compared to conventional PCB material. However, MCPCB material may also be heavier than conventional PCB material, which may not include a metal core.

[0067] In the embodiments illustrated in FIG. 1, the lighting elements 12 are multi-chip clusters of four solid state emitting devices per cluster. In the tile 10, four lighting elements 12 are serially arranged in a first path 20, while four lighting elements 12 are serially arranged in a second path 21. The lighting elements 12 of the first path 20 are connected, for example via printed circuits, to a set of four anode contacts 22 arranged at a first end of the tile 10, and a set of four cathode contacts 24 arranged at a second end of the tile 10. The lighting elements 12 of the second path 21 are connected to a set of four anode contacts 26 arranged at the second end of the tile 10, and a set of four cathode contacts 28 arranged at the first end of the tile 10.

[0068] The solid state lighting elements 12 may include, for example, organic and/or inorganic light emitting devices. An example of a solid state lighting element 12′ for high power illumination applications is illustrated in FIG. 2. A solid state lighting element 12′ may comprise a packaged discrete electronic component including a carrier substrate 13 on which a plurality of LED chips 16A-16D are mounted. In other embodiments, one or more solid state lighting elements 12 may comprise LED chips 16A-16D mounted directly onto electrical traces on the surface of the tile 10, forming a multi-chip module or chip on board assembly. Suitable tiles are disclosed in commonly assigned U.S. Provisional Patent Application Ser. No. ______ entitled “SOLID STATE BACKLIGHTING UNIT ASSEMBLY AND METHODS” filed Dec. 9, 2005 (Attorney Docket 5308-634PR).

[0069] The LED chips 16A-16D may include at least a red LED′ 16A, a green LED 16B and a blue LED 16C. The blue and/or green LEDs may be InGaN-based blue and/or green LED chips available from Cree, Inc., the assignee of the present invention. The red LEDs may be, for example, AlInGaP LED chips available from Epistar, Osram and others. The lighting device 12 may include an additional green LED 16D in order to make more green light available.

[0070] In some embodiments, the LEDs 16 may have a square or rectangular periphery with an edge length of about 900 μm or greater (i.e. so-called “power chips.” However, in other embodiments, the LED chips 16 may have an edge length of 500 μm or less (i.e. so-called “small chips”). In particular, small LED chips may operate with better electrical conversion efficiency than power chips. For example, green LED chips with a maximum edge dimension less than 500 microns and as small as 260 microns, commonly have a higher electrical conversion efficiency than 900 micron chips, and are known to typically produce 55 lumens of luminous flux per Watt of dissipated electrical power and as much as 90 lumens of luminous flux per Watt of dissipated electrical power.

[0071] As further illustrated in FIG. 2, the LEDs 16A-16D may be covered by an encapsulant 14, which may be clear and/or may include light scattering particles, phosphors, and/or other elements to achieve a desired emission pattern, color and/or intensity. While not illustrated in FIG. 2, the lighting device 12 may further include a reflector cup surrounding the LEDs 16A-16D, a lens mounted above the LEDs 16A-16D, one or more heat sinks for removing heat from the lighting device, an electrostatic discharge protection chip, and/or other elements.

[0072] LED chips 16A-16D of the lighting elements 12 in the tile 10 may be electrically interconnected as shown in the schematic circuit diagram in FIG. 3. As shown therein, the LEDs may be interconnected such that the blue LEDs 16A in the first path 20 are connected in series to form a string 20A. Likewise, the first green LEDs 16B in the first path 20 may be arranged in series to form a string 20B, while the second green LEDs 16D may be arranged in series to form a separate string 20D. The red LEDs 16C may be arranged in series to form a string 20C. Each string 20A-20D may be connected to an anode contact 22A-22D arranged at a first end of the tile 10 and a cathode contact 24A-24D arranged at the second end of the tile 10, respectively.

[0073] A string 20A-20D may include all, or less than all, of the corresponding LEDs in the first path 20 or the second path 21. For example, the string 20A may include all of the blue LEDs from all of the lighting elements 12 in the first path 20. Alternatively, a string 20A may include only a subset of the corresponding LEDs in the first path 20. Accordingly the first path 20 may include four serial strings 20A-20D arranged in parallel on the tile 10.

[0074] The second path 21 on the tile 10 may include four serial strings 21A, 21B, 21C, 21D arranged in parallel. The strings 21A to 21D are connected to anode contacts 26A to 26D, which are arranged at the second end of the tile 10 and to cathode contacts 28A to 28D, which are arranged at the first end of the tile 10, respectively.

[0075] It will be appreciated that, while the embodiments illustrated in FIGS. 1-3 include four LED chips 16 per lighting device 12 which are electrically connected to form at least four strings of LEDs 16 per path 20, 21, more and/or fewer than four LED chips 16 may be provided per lighting device 12, and more and/or fewer than four LED strings may be provided per path 20, 21 on the tile 10. For example, a lighting device 12 may include only one green LED chip 16B, in which case the LEDs may be connected to form three strings per path 20, 21. Likewise, in some embodiments, the two green LED chips in a lighting device 12 may be connected in serial to one another, in which case there may only be a single string of green LED chips per path 20, 22. Further, a tile 10 may include only a single path 20 instead of plural paths 20, 21 and/or more than two paths 20, 21 may be provided on a single tile 10.

[0076] Multiple tiles 10 may be assembled to form a larger lighting bar assembly 30 as illustrated in FIG. 4A. As shown therein, a bar assembly 30 may include two or more tiles 10, 10′, 10″ connected end-to-end. Accordingly, referring to FIGS. 3 and 4, the cathode contacts 24 of the first path 20 of the leftmost tile 10 may be electrically connected to the anode contacts 22 of the first path 20 of the central tile 10′, and the cathode contacts 24 of the first path 20 of the central tile 10′ may be electrically connected to the anode contacts 22 of the first path 20 of the rightmost tile 10″, respectively. Similarly, the anode contacts 26 of the second path 21 of the leftmost tile 10 may be electrically connected to the cathode contacts 28 of the second path 21 of the central tile 10′, and the anode contacts 26 of the second path 21 of the central tile 10′ may be electrically connected to the cathode contacts 28 of the second path 21 of the rightmost tile 10″, respectively.

[0077] Furthermore, the cathode contacts 24 of the first path 20 of the rightmost tile 10″ may be electrically connected to the anode contacts 26 of the second path 21 of the rightmost tile 10″ by a loopback connector 35. For example, the loopback connector 35 may electrically connect the cathode 24A of the string 20A of blue LED chips 16A of the first path 20 of the rightmost tile 10″ with the anode 26A of the string 21A of blue LED chips of the second path 21 of the rightmost tile 10″. In this manner, the string 20A of the first path 20 may be connected in serial with the string 21A of the second path 21 by a conductor 35A of the loopback connector 35 to form a single string 23A of blue LED chips 16. The other strings of the paths 20, 21 of the tiles 10, 10′, 10″ may be connected in a similar manner.

[0078] The loopback connector 35 may include an edge connector, a flexible wiring board, or any other suitable connector. In addition, the loop connector may include printed traces formed on/in the tile 10.

[0079] While the bar assembly 30 shown in FIG. 4A is a one dimensional array of tiles 10, other configurations are possible. For example, the tiles 10 could be connected in a two-dimensional array in which the tiles 10 are all located in the same plane, or in a three dimensional configuration in which the tiles 10 are not all arranged in the same plane. Furthermore the tiles 10 need not be rectangular or square, but could, for example, be hexagonal, triangular, or the like.

[0080] Referring to FIG. 4B, in some embodiments, a plurality of bar assemblies 30 may be combined to form a lighting panel 40, which may be used, for example, as a backlighting unit (BLU) for an LCD display. As shown in FIG. 4B, a lighting panel 40 may include four bar assemblies 30, each of which includes six tiles 10. The rightmost tile 10 of each bar assembly 30 includes a loopback connector 35. Accordingly, each bar assembly 30 may include four strings 23 of LEDs (i.e. one red, two green and one blue).

[0081] In some embodiments, a bar assembly 30 may include four LED strings 23 (one red, two green and one blue). Thus, a lighting panel 40 including nine bar assemblies may have 36 separate strings of LEDs. Moreover, in a bar assembly 30 including six tiles 10 with eight solid state lighting elements 12 each, an LED string 23 may include 48 LEDs connected in serial.

[0082] For some types of LEDs, in particular blue and/or green LEDs, the forward voltage (Vf) may vary by as much as ±0.75V from a nominal value from chip to chip at a standard drive current of 20 mA. A typical blue or green LED may have a Vf of 3.2 Volts. Thus, the forward voltage of such chips may vary by as much as 25%. For a string of LEDs containing 48 LEDs, the total Vf required to operate the string at 20 mA may vary by as much as ±36V.

[0083] Accordingly, depending on the particular characteristics of the LEDs in a bar assembly, a string of one light bar assembly (e.g. the blue string) may require significantly different operating power compared to a corresponding string of another bar assembly. These variations may significantly affect the color and/or brightness uniformity of a lighting panel that includes multiple tiles 10 and/or bar assemblies 30, as such Vf variations may lead to variations in brightness and/or hue from tile to tile and/or from bar to bar. For example, current differences from string to string may result in large differences in the flux, peak wavelength, and/or dominant wavelength output by a string. Variations in LED drive current on the order of 5% or more may result in unacceptable variations in light output from string to string and/or from tile to tile. Such variations may significantly affect the overall color gamut, or range of displayable colors, of a lighting panel.

[0084] In addition, the light output characteristics of LED chips may change during their operational lifetime. For example, the light output by an LED may change over time and/or with ambient temperature.

[0085] In order to provide consistent, controllable light output characteristics for a lighting panel, some embodiments of the invention provide a lighting panel having two or more serial strings of LED chips. An independent current control circuit is provided for each of the strings of LED chips. Furthermore, current to each of the strings may be individually controlled, for example, by means of pulse width modulation (PWM) and/or pulse frequency modulation (PFM). The width of pulses applied to a particular string in a PWM scheme (or the frequency of pulses in a PFM scheme) may be based on a pre-stored pulse width (frequency) value that may be modified during operation based, for example, on a user input and/or a sensor input.

[0086] Accordingly, referring to FIG. 5, a lighting panel system 200 is shown. The lighting panel system 200, which may be a backlight for an LCD display panel, includes a lighting panel 40. The lighting panel 40 may include, for example, a plurality of bar assemblies 30, which, as described above, may include a plurality of tiles 10. However, it will be appreciated that embodiments of the invention may be employed in conjunction with lighting panels formed in other configurations. For example, some embodiments of the invention may be employed with solid state backlight panels that include a single, large area tile.

[0087] In particular embodiments, however, a lighting panel 40 may include a plurality of bar assemblies 30, each of which may have four cathode connectors and four anode connectors corresponding to the anodes and cathodes of four independent strings 23 of LEDs each having the same dominant wavelength. For example, each bar assembly 23 may have a red string 23A, two green strings 23B, 23D, and a blue string 23C, each with a corresponding pair of anode/cathode contacts on one side of the bar assembly 30. In particular embodiments, a lighting panel 40 may include nine bar assemblies 30. Thus, a lighting panel 40 may include 36 separate LED strings.

[0088] A current driver 220 provides independent current control for each of the LED strings 23 of the lighting panel 40. For example, the current driver 220 may provide independent current control for 36 separate LED strings in the lighting panel 40. The current driver 220 may provide a constant current source for each of the 36 separate LED strings of the lighting panel 40 under the control of a controller 230. In some embodiments, the controller 230 may be implemented using an 8-bit microcontroller such as a PIC18F8722 from Microchip Technology Inc., which may be programmed to provide pulse width modulation (PWM) control of 36 separate current supply blocks within the driver 220 for the 36 LED strings 23.

[0089] Pulse width information for each of the 36 LED strings may be obtained by the controller 230 from a color management unit 260, which may in some embodiments include a color management controller such as the Agilent HDJD-J822-SCR00 color management controller.

[0090] The color management unit 260 may be connected to the controller 230 through an I2C (Inter-Integrated Circuit) communication link 235. The color management unit 260 may be configured as a slave device on an I2C communication link 235, while the controller 230 may be configured as a master device on the link 235. I2C communication links provide a low-speed signaling protocol for communication between integrated circuit devices. The controller 230, the color management unit 260 and the communication link 235 may together form a feedback control system configured to control the light output from the lighting panel 40. The registers R1-R9, etc., may correspond to internal registers in the controller 230 and/or may correspond to memory locations in a memory device (not shown) accessible by the controller 230.

[0091] The controller 230 may include a register, e.g. registers R1-R9, G1A-G9A, B1-B9, G1B-G9B, for each LED string 23, i.e. for a lighting unit with 36 LED strings 23, the color management unit 260 may include at least 36 registers. Each of the registers is configured to store pulse width information for one of the LED strings 23. The initial values in the registers may be determined by an initialization/calibration process. However, the register values may be adaptively changed over time based on user input 250 and/or input from one or more sensors 240 coupled to the lighting panel 40.

[0092] The sensors 240 may include, for example, a temperature sensor 240A, one or more photosensors 240B, and/or one or more other sensors 240C. In particular embodiments, a lighting panel 40 may include one photosensor 240B for each bar assembly 30 in the lighting panel. However, in other embodiments, one photosensor 240B could be provided for each LED string 30 in the lighting panel. In other embodiments, each tile 10 in the lighting panel 40 may include one or more photosensors 240B.

[0093] In some embodiments, the photosensor 240B may include photo-sensitive regions that are configured to be preferentially responsive to light having different dominant wavelengths. Thus, wavelengths of light generated by different LED strings 23, for example a red LED string 23A and a blue LED string 23C, may generate separate outputs from the photosensor 240B. In some embodiments, the photosensor 240B may be configured to independently sense light having dominant wavelengths in the red, green and blue portions of the visible spectrum. The photosensor 240B may include one or more photosensitive devices, such as photodiodes. The photosensor 240B may include, for example, an Agilent HDJD-S831-QT333 tricolor photo sensor.

[0094] Sensor outputs from the photosensors 240B may be provided to the color management unit 260, which may be configured to sample such outputs and to provide the sampled values to the controller 230 in order to adjust the register values for corresponding LED strings 23 in order to correct variations in light output on a string-by-string basis. In some embodiments, an application specific integrated circuit (ASIC) may be provided on each tile 10 along with one or more photosensors 240B in order to pre-process sensor data before it is provided to the color management unit 260. Furthermore, in some embodiments, the sensor output and/or ASIC output may be sampled directly by the controller 230.

[0095] The photosensors 240B may be arranged at various locations within the lighting panel 40 in order to obtain representative sample data. Alternatively and/or additionally, light guides such as optical fibers may be provided in the lighting panel 40 to collect light from desired locations. In that case, the photosensors 240B need not be arranged within an optical display region of the lighting panel 40, but could be provided, for example, on the back side of the lighting panel 40. Further, an optical switch may be provided to switch light from different light guides which collect light from different areas of the lighting panel 40 to a photosensor 240B. Thus, a single photosensor 240B may be used to sequentially collect light from various locations on the lighting panel 40.

[0096] The user input 250 may be configured to permit a user to selectively adjust attributes of the lighting panel 40, such as color temperature, brightness, hue, etc., by means of user controls such as input controls on an LCD panel.

[0097] The temperature sensor 240A may provide temperature information to the color management unit 260 and/or the controller 230, which may adjust the light output from the lighting panel on a string-to-string and/or color-to-color basis based on known/predicted brightness vs. temperature operating characteristics of the LED chips 16 in the strings 23.

[0098] Various configurations of photosensors 240B are shown in FIGS. 6A-6D. For example, in the embodiments of FIG. 6A, a single photosensor 240B is provided in the lighting panel 40. The photosensor 240B may be provided at a location where it may receive an average amount of light from more than one tile/string in the lighting panel.

[0099] In order to provide more extensive data regarding light output characteristics of the lighting panel 40, more than one photosensor 240B may be used. For example, as shown in FIG. 6B, there may be one photosensor 240B per bar assembly 30. In that case, the photosensors 240B may be located at ends of the bar assemblies 30 and may be arranged to receive an average/combined amount of light emitted from the bar assembly 30 with which they are associated.

[0100] As shown in FIG. 6C, photosensors 240B may be arranged at one or more locations within a periphery of the light emitting region of the lighting panel 40. However in some embodiments, the photosensors 240B may be located away from the light emitting region of the lighting panel 40, and light from various locations within the light emitting region of the lighting panel 40 may be transmitted to the sensors 240B through one or more light guides. For example, as shown in FIG. 6D, light from one or more locations 249 within the light emitting region of the lighting panel 40 is transmitted away from the light emitting region via light guides 247, which may be optical fibers that may extend through and/or across the tiles 10. In the embodiments illustrated in FIG. 6D, the light guides 247 terminate at an optical switch 245, which selects a particular guide 247 to connect to the photosensor 240B based on control signals from the controller 230 and/or from the color management unit 260. It will be appreciated, however, that the optical switch 245 is optional, and that each of the light guides 245 may terminate at a photosensor 240B. In further embodiments, instead of an optical switch 245, the light guides 247 may terminate at a light combiner, which combines the light received over the light guides 247 and provides the combined light to a photosensor 240B. The light guides 247 may extend across partially across, and/or through the tiles 10. For example, in some embodiments, the light guides 247 may run behind the panel 40 to various light collection locations and then run through the panel at such locations. Furthermore, the photosensor 240B may be mounted on a front side of the panel (i.e. on the side of the panel 40 on which the lighting devices 16 are mounted) or on a reverse side of the panel 40 and/or a tile 10 and/or bar assembly 30.

[0101] Referring now to FIG. 7, a current driver 220 may include a plurality of bar driver circuits 320A-320D. One bar driver circuit 320A-320D may be provided for each bar assembly 30 in a lighting panel 40. In the embodiments shown in FIG. 7, the lighting panel 40 includes four bar assemblies 30. However, in some embodiments the lighting panel 40 may include nine bar assemblies 30, in which case the current driver 220 may include nine bar driver circuits 320. As shown in FIG. 8, in some embodiments, each bar driver circuit 320 may include four current supply circuits 340A-340D, i.e., one current supply circuit 340A-340D for each LED string 23A-23D of the corresponding bar assembly 30. Operation of the current supply circuits 340A-340B may be controlled by control signals 342 from the controller 230.

[0102] The current supply circuits 340A-340B are configured to supply current to the corresponding LED strings 13 while a pulse width modulation signal PWM for the respective strings 13 is a logic HIGH. Accordingly, for each timing loop, the PWM input of each current supply circuit 340 in the driver 220 is set to logic HIGH at the first clock cycle of the timing loop. The PWM input of a particular current supply circuit 340 is set to logic LOW, thereby turning off current to the corresponding LED string 23, when a counter in the controller 230 reaches the value stored in a register of the controller 230 corresponding to the LED string 23. Thus, while each LED string 23 in the lighting panel 40 may be turned on simultaneously, the strings may be turned off at different times during a given timing loop, which would give the LED strings different pulse widths within the timing loop. The apparent brightness of an LED string 23 may be approximately proportional to the duty cycle of the LED string 23,. i.e., the fraction of the timing loop in which the LED string 23 is being supplied with current.

[0103] An LED string 23 may be supplied with a substantially constant current during the period in which it is turned on. By manipulating the pulse width of the current signal, the average current passing through the LED string 23 may be altered even while maintaining the on-state current at a substantially constant value. Thus, the dominant wavelength of the LEDs 16 in the LED string 23, which may vary with applied current, may remain substantially stable even though the average current passing through the LEDs 16 is being altered. Similarly, the luminous flux per unit power dissipated by the LED string 23 may remain more constant at various average current levels than, for example, if the average current of the LED string 23 was being manipulated using a variable current source.

[0104] The value stored in a register of the controller 230 corresponding to a particular LED string may be based on a value received from the color management unit 260 over the communication link 235. Alternatively and/or additionally, the register value may be based on a value and/or voltage level directly sampled by the controller 230 from a sensor 240.

[0105] In some embodiments, the color management unit 260 may provide a value corresponding to a duty cycle (i.e. a value from 0 to 100), which may be translated by the controller 230 into a register value based on the number of cycles in a timing loop. For example, the color management unit 260 indicates to the controller 230 via the communication link 235 that a particular LED string 23 should have a duty cycle of 50%. If a timing loop includes 10,000 clock cycles, then assuming the controller increments the counter with each clock cycle, the controller 230 may store a value of 5000 in the register corresponding to the LED string in question. Thus, in a particular timing loop, the counter is reset to zero at the beginning of the loop and the LED string 23 is turned on by sending an appropriate PWM signal to the current supply circuit 340 serving the LED string 23. When the counter has counted to a value of 5000, the PWM signal for the current supply circuit 340 is reset, turning the LED string off.

[0106] In some embodiments, the pulse repetition frequency (i.e. pulse repetition rate) of the PWM signal may be in excess of 60 Hz. In particular embodiments, the PWM period may be 5 ms or less, for an overall PWM pulse repetition frequency of 200 Hz or greater. A delay may be included in the loop, such that the counter may be incremented only 100 times in a single timing loop. Thus, the register value for a given LED string 23 may correspond directly to the duty cycle for the LED string 23. However, any suitable counting process may be used provided that the brightness of the LED string 23 is appropriately controlled.

[0107] The register values of the controller 230 may be updated from time to time to take into account changing sensor values. In some embodiments, updated register values may be obtained from the color management unit 260 multiple times per second.

[0108] Furthermore, the data read from the color management unit 260 by the controller 230 may be filtered to limit the amount of change that occurs in a given cycle. For example, when a changed value is read from the color management unit 260, an error value may be calculated and scaled to provide proportional control (“P”), as in a conventional PID (Proportional-Integral-Derivative) feedback controller. Further, the error signal may be scaled in an integral and/or derivative manner as in a PID feedback loop. Filtering and/or scaling of the changed values may be performed in the color management unit 260 and/or in the controller 230.

[0109] In some embodiments, calibration of a display system 200 may be performed by the display system itself (i.e. self-calibration), for example, using signals from photosensors 240B. However, in some embodiments of the invention, calibration of a display system 200 may be performed by an external calibration system.

[0110] Some aspects of self-calibration of the display system 200 are illustrated in FIG. 9. In some embodiments, the controller 230 may cause the color management unit 260 to sample a photosensor 240B when the lighting panel 40 is momentarily dark (i.e. when all of the light sources within the unit are momentarily switched off) in order to obtain a measure of ambient light (e.g. a dark signal value). The controller 230 may also cause the color management unit 260 to sample the photosensor 240B during a time interval in which the display is lighted for at least a portion of the interval in order to obtain a measure of the display brightness (e.g. a light signal value). For example, the controller 230 may cause the color management unit 260 to obtain a value from the photosensor that represents an average over an entire timing loop.

[0111] For example, referring to FIG. 9, all LED strings in the lighting panel 40 are turned off (block 910), and the photosensor 240B output is sampled to obtain a dark signal value (block 920). The LED strings are then energized (block 930), and the display output is integrated over an entire pulse period and sampled (block 940) to obtain a light signal value. The output of the lighting panel 40 is then adjusted based on the dark signal value and/or the light signal value (block 950)

[0112] The brightness of the lighting panel 40 may be adjusted to account for differences in ambient light. For example, in situations in which the level of ambient light is high, the brightness of the lighting panel 40 may be increased via a positive feedback signal in order to maintain a substantially consistent contrast ratio. In other situations in which the level of ambient light is low, a sufficient contrast ratio may be maintained with a lower brightness, so the display brightness may be decreased by a negative feedback signal.

[0113] As explained above, the brightness of the lighting panel 40 may be adjusted by adjusting the pulse widths of the current pulses for one or more (or all) of the LED strings 23 in the lighting panel 40. In some embodiments, the pulse widths may be adjusted based on a difference between the sensed display brightness and the sensed ambient brightness. In other embodiments, the pulse widths may be adjusted based on a ratio of the sensed display brightness (the light signal value) to the sensed ambient brightness (the dark signal value).

[0114] Accordingly, in some embodiments, the feedback loop formed by the lighting panel 40, the photosensor 240B, the color management unit 260 and the controller 230 may tend to maintain the average luminosity of the lighting panel 40 independent of ambient illumination. In other embodiments, the feedback loop may be configured to maintain a desired relationship between the average luminosity of the lighting panel 40 and the level of ambient illumination. *

[0115] In some embodiments, the feedback loop may employ digital incremental logic. The digital incremental logic of the feedback loop may reference indices of a lookup table including a list of values such as duty cycle values.

[0116] Same colored LED strings in a lighting panel need not be driven with the same pulse width. For example, a backlight panel 40 may include a plurality of red LED strings 23, each of which may be driven with a different pulse width, resulting in a different average current level. Accordingly, some embodiments of the invention provide a closed loop digital control system for a lighting panel, such as an LCD backlight, that includes first and second LED strings 23 that include a plurality of LED chips 16 therein that emit narrow band optical radiation having a first dominant wavelength when energized, and third and fourth LED strings 23 that include a plurality of LED chips 16 that emit narrow band optical radiation having a second dominant wavelength, different from the first dominant wavelength.

[0117] In some embodiments, the first and second LED strings 23 are maintained at a different average current level than one another yet are driven at substantially the same on-state current. Likewise, the third and fourth LED strings are maintained at different average current levels than one another yet are driven at substantially the same on-state current.

[0118] The on-state current of the first and second LED strings 23 may be different than the on-state current of the third and fourth LED strings. For example, the on-state current used to drive red LED strings 23 may be different than the on-state current used to drive green and/or blue LED strings. The average current of a string 23 is proportional to the pulse width of the current through the string 23. The ratio of average current between the first and second LED strings 23 may be maintained relatively constant, and/or the ratio of average current between the third and fourth LED strings 23 may be maintained relatively constant. Furthermore, the ratio of average current between the first and second LED strings 23 compared to the average current of the third and fourth LED strings 23 may be allowed to change as part of the closed loop control in order to maintain a desired display white point.

[0119] In some embodiments, the on-state current level provided to a given LED string 23 may be adjusted by the current supply circuit 340 in response to commands from the controller 230. In that case, a particular LED string may be driven at an on-state current level selected to adjust a dominant wavelength of a particular LED string 23. For example, due to chip-to-chip variations in dominant wavelength, a particular LED string 23 may have an average dominant wavelength that is higher than an average dominant wavelength of other LED strings 23 of the same color within a lighting panel 40. In that case, it may be possible to drive the higher-wavelength LED string at a slightly higher on-state current, which may cause the dominant wavelength of the LED string 23 to drop and better match that of the shorter-wavelength LED strings 23.

[0120] In some embodiments, the initial on-state drive currents of each of the LED strings 23 may be calibrated by a calibration process in which each of the LED strings is individually energized and the light output from each string is detected. The dominant wavelength of each string may be measured, and an appropriate drive current may be calculated for each LED string in order to adjust the dominant wavelength as necessary. For example, the dominant wavelengths of each of the LED strings 23 of a particular color may be measured and the variance of the dominant wavelengths for a particular color may be calculated. If the variance of the dominant wavelengths for the color is greater than a predetermined threshold, or if the dominant wavelength of a particular LED string 23 is higher or lower than the average dominant wavelength of the LED strings 23 by a predetermined number of standard deviations, then the on-state drive current of one or more of the LED strings 23 may be adjusted in order to reduce the variance of dominant wavelengths. Other methods/algorithms may be used in order to correct/account for differences in dominant wavelength from string to string.

[0121] Referring to FIG. 10, an external calibration system 400 may be coupled to a lighting system 200 so that the calibration system 400 can control certain operations of the lighting system 200 in order to calibrate the lighting system 200. For example, the calibration system 200 may cause the lighting system 200 to selectively illuminate one or more LED strings 23 for a desired time at a desired duty cycle in order to measure light output by the lighting system 200.

[0122] Referring to FIG. 11, a calibration system 400 may include a calibration controller 410 that is coupled to the lighting system 200 and that is configured to control certain operations of the lighting system 200 as well as other elements of the calibration system 400. The calibration system 400 further includes a stand 420 on which an XZ positioner 430 is mounted, and a calorimeter 440 mounted on the XZ positioner. The XZ positioner 430 is configured to move the calorimeter 440 in two dimensions (e.g. horizontally and vertically) in order to position the calorimeter 440 at a desired location relative to a lighting panel being calibrated. The XZ positioning system 430 may include a linear positioning system manufactured by Techno, Inc. The colorimeter 440 may include a PR-650 SpectraScan® Colorimeter from Photo Research Inc.

[0123] Referring to FIG. 12, the colorimeter 440 and XZ positioning system 430 may be located within a darkened enclosure 450 that includes an entrance 455 that may be shrouded by vertical black cloth strips to reduce/prevent external light from entering the enclosure 450. A conveyor 460 extends from outside the enclosure 450 to the interior of the enclosure 450 through the entrance 455. A lighting panel 210 of a lighting system 200 is carried into the enclosure 450 on a pallet 470 by the conveyor 460, where the calorimeter 440 can measure light output by the lighting panel 210 in response to commands from the calibration controller 410.

[0124] FIGS. 13, 14 and 15A-B are flowchart diagrams that illustrate further operations according to some embodiments of the invention associated with calibrating a lighting panel 40 having M segments, such as bars 30, each of which may include a group of tiles 10. The lighting panel 40 may be calibrated by measuring the light output by the bars 30 from N different locations. In some embodiments, the number of bars 30 may be 9 (i.e. M=9), and/or the number of measurement locations N may be 3.

[0125] Referring to FIG. 13, calibration of a lighting panel 40 may include adjusting the duty cycles of the LED strings 23 on the bars 30 to reduce the maximum color luminance variation for each bar 30 to below a first threshold variation (block 1310) and adjusting the duty cycles of the LED strings 23 to reduce a maximum luminance variation to the center of the lighting panel to below a second threshold value (block 1320).

[0126] Adjusting duty cycles of the bars 30 to reduce the maximum color luminance variation for each bar is illustrated in FIG. 14. As shown therein, the luminance of all bars is measured at maximum duty cycle for each color (block 1410). That is, the red LEDs of each bar 30 are sequentially energized at a 100% duty cycle, and N measurements are taken for each bar. The process is then repeated for the blue and green LEDs. The measurements may include measurement of total luminance Y of each bar m ε [1 . . . M] for each color (R, G, B) and each measurement location n ε [1 . . . N]. The CIE chromaticity (x, y) may also be measured for each bar/color/location. Measurements may be taken using, for example, a PR-650 SpectraScan® Colorimeter from Photo Research Inc., which can be used to make direct measurements of luminance, CIE Chromaticity (1931 xy and 1976 u′v′) and/or correlated color temperature.

[0127] Next, nominal luminance ratios are calculated for each color (block 1420). In order to calculate nominal luminance ratios, total luminance values for each color Y_R,total; Y_G,total, and Y_B,total are calculated as follows: $\begin{array}{cc}{Y}_{R,\mathrm{total}}=\sum _{m,n}{Y}_{\mathrm{Rmn}}& \left(1a\right)\\ Y_{G,\mathrm{total}}=\sum _{m,n}{Y}_{\mathrm{Gmn}}& \left(1b\right)\\ Y_{B,\mathrm{total}}=\sum _{m,n}{Y}_{\mathrm{Bmn}}& \left(1c\right)\end{array}$

[0128] The nominal RGB luminance ratios may then be calculated for each color as a ratio of the total luminance of-a color to the total luminance of all colors as follows:
Y_R|_ratio=Y_R,total/(Y_R,total+Y_G,total+Y_B,total)   (2a)
Y_G|_ratio=Y_G,total/(Y_R,total+Y_G,total+Y_B,total)   (2b)
Y_B|_ratio=Y_B,total(Y_R,total+Y_G,total+Y_B,total)   (2c)

[0129] Next, for each bar, luminance ratios are calculated for each color (block 1430), as follows. First, a total luminance is calculated for each bar as follows: $\begin{array}{cc}{Y}_{\mathrm{Rm},\mathrm{total}}=\sum _n{Y}_{\mathrm{Rmn}}& \left(3a\right)\\ Y_{\mathrm{Gm},\mathrm{total}}=\sum _n{Y}_{\mathrm{Gmn}}& \left(3b\right)\\ Y_{\mathrm{Bm},\mathrm{total}}=\sum _n{Y}_{\mathrm{Bmn}}& \left(3c\right)\end{array}$
Then, for each bar, a luminance ratio for each color is calculated as a ratio of the total luminance of a color emitted by a bar to the total luminance of all colors emitted by the bar, as follows:
Y_Rm|_ratio=Y_Rm,total/(Y_Rm,total+Y_Gm,total+Y_Bm,total)   (4a)
Y_Gm|_ratio=Y_Gm,total/(Y_Rm,total+Y_Gm,total+Y_Bm,total)   (4b)
Y_Bm|_ratio=Y_Bm,total/(Y_Rm,total+Y_Gm,total+Y_Bm,total)   (4c)

[0130] A maximum variation from the nominal luminance ratio for each bar may then be obtained (block 1440) by calculating a variation from the nominal luminance ratio for each color and for each bar as follows:
ΔY_Rm|_ratio=(Y_Rm|_ratio−Y_R|_ratio)/Y_R|_ratio   (5a)
ΔY_Gm|_ratio=(Y_Gm|_ratio−Y_G|_ratio)/Y_G|_ratio   (5a)
ΔY_Bm|_ratio=(Y_Bm|_ratio−Y_B|_ratio)/Y_B|_ratio   (5a)
The maximum variation from the nominal luminance ratio may then be obtained for each bar as follows:
ΔY_m|_ratio,max=max(ΔY_Rm|_ratio, ΔY_Gm|_ratio, ΔY_Bm|_ratio)   (6)

[0131] If in block 1450 it is determined that the maximum variation from the nominal luminance ratio for a bar is greater than a first threshold THRESH1, then the duty cycles of the colors of the bar are adjusted to reduce the maximum variation from the nominal luminance ratio (block 1460) to below the first threshold THRESH1. The first threshold THRESH1 may be less than 1%. For example, the first threshold THRESH1 may be 0.4% in some embodiments.

[0132] The duty cycles of the colors of a bar may be adjusted by first selecting the color with the lowest relative luminance as follows:
ΔY_Km|_ratio,min=min(ΔY_Rm|_ratio, ΔY_Gm|_ratio, ΔY_Bm|_ratio)   (7)
where K=R, G or B; color K has the lowest relative luminance. A duty cycle coefficient for each color is then calculated for each bar to provide color uniformity as follows:
C_Km=Y_Km|_ratio/Y_K|_ratio   (8)
where K=R, G or B; color K has the lowest relative luminance.

[0133] The duty cycles (DC) for each color are then adjusted for color balance as follows:
DC_Rm=C_Km*Y_R|_ratio/Y_Rm|_ratio   (9a)
DC_Gm=C_Km*Y_G|_ratio/Y_Gm|_ratio   (9b)
DC_Bm=C_Km*Y_B|_ratio/Y_Bm|_ratio   (9c)

[0134] Referring now to FIG. 15A, the calibration process is continued by determining the luminance variation to center points of the display (block 1470). First, the luminance after color balance (duty cycle adjustment) for each bar/color/measurement point is calculated as follows:
Y_Rmn′=DC_Rm*Y_Rmn   (10a)
Y_Gmn′=DC_Gm*Y_Gmn   (10b)
Y_Bmn′=DC_Bm*Y_Bmn   (10c)

[0135] The RGB mixed luminance is then calculated for each position as follows:
Y_mn′=Y_Rmn′+Y_Gmn′+Y_Bmn′  (11)
for each of M bars (m ε [1 . . . M]) and N measurement positions (n ε [1 . . . N]).

[0136] Assuming M=9 and N=3, a center luminance average may be calculated as follows:
Y_center=(Y₅₂′+Y₇₂′+Y₃₂′)/3   (12)

[0137] A luminance variation to the center luminance average may then be calculated for each bar/measurement position as follows:
ΔY_mn=[Y_mn′−max(Y_mn′)]/Y_center   (13)

[0138] The maximum variation to the center luminance is then compared in block 1480 to a second threshold THRESH2, which may be, for example, 10%. If the maximum variation to the center luminance exceeds the second threshold THRESH2, then the duty cycles are again adjusted to reduce the maximum variation to the center luminance (block 1490). First, a uniformity coefficient is calculated for each bar as follows:
C_m=[1−min(ΔY_m1, . . . , ΔY_mn)]/1.1   (14)

[0139] A new duty cycle is then calculated as follows:
DC_Rm′=C_m*DC_Rm   (15a)
DC_Gm′=C_m*DC_Gm   (15b)
DC_Bm′=C_m*DC_Bm   (15c)

[0140] The maximum duty cycle of all bars/colors is then determined as follows:
DC_max=max(DC_Km′)   (16)
where K=R, G or B, and m ε [1 . . . M].

[0141] The duty cycles may then be re-normalized such that the maximum duty cycle is 100% as follows:
DC_Rm″=DC_Rm′/DC_max   (17a)
DC_Gm″=DC_Gm′/DC_max   (17b)
DC_Bm″=DC_Bm′/DC_max   (17c)

[0142] In some embodiments of the present invention illustrated in FIG. 15B, in adjusting the luminance variation to the center luminance, a maximum duty cycle for each color is determined, and the duty cycles of the bars/colors are normalized to the maximum duty cycle for each respective color. That is, the duty cycles of the red strings are normalized to the maximum duty cycle of red strings, the duty cycles of the blue strings are normalized to the maximum duty cycle of blue strings, etc.

[0143] Referring now to FIG. 15B, the luminance variation to center points of the display is determined (block 1470B). First, the luminance after color balance (duty cycle adjustment) for each bar/color/measurement point is calculated as follows:
Y_Rmn′=DC_Rm*Y_Rmn   (18a)
Y_Gmn′=DC_Gm*Y_Gmn   (18b)
Y_Bmn′=DC_Bm*Y_Bmn   (18c)

[0144] The RGB mixed luminance is then calculated for each position as follows:
Y_mn′=Y_Rmn′+Y_Gmn′+Y_Bmn′  (19)
for each of M bars (m 0 [1 . . . M]) and N measurement positions (n 0 [1 . . . N]).

[0145] Assuming M=9 and N=3, a center luminance average-may be calculated as follows:
Y_center=(Y₅₂′+Y₇₂′+Y₃₂′)/3   (20)

[0146] A luminance variation to the center luminance average may then be calculated for each bar/measurement position as follows:
ΔY_mn=[Y_mn′−max(Y_mn′)]/Y_center   (21)

[0147] The maximum variation to the center luminance is then compared in block 1480B to a second threshold THRESH2, which may be, for example, 10%. If the maximum variation to the center luminance exceeds the second threshold THRESH2, then the duty cycles are again adjusted to reduce the maximum variation to the center luminance (block 1490B). First, a uniformity coefficient is calculated for each bar as follows:
C_m=[1−min(ΔY_m1, . . . , ΔY_mn)]/1.1   (22)

[0148] A new duty cycle is then calculated as follows:
DC_Rm′=C_m*DC_Rm   (23a)
DC_Gm′=C_m*DC_Gm   (23b)
DC_Bm′=C_m*DC_Bm   (23c)

[0149] The maximum duty cycle of all bars for each color is then determined as follows:
DC_Rmax=max(DC_Rm′)   (24a)
DC_Gmax=max(DC_Gm′)   (24b)
DC_Bmax=max(DC_Bm′)   (24c)
where m 0 [1 . . . M].

[0150] The duty cycles may then be re-normalized such that the maximum duty cycle is 100% as follows:
DC_Rm″=DC_Rm′/DC_Rmax   (25a)
DC_Gm″=DC_Gm′/DC_Gmax   (25b)
DC_Bm″=DC_Bm′/DC_Bmax   (25c)

[0151] In the drawings and specification, there have been disclosed typical embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.