Patent Publication Number: US-11032884-B2

Title: Method for making tunable multi-led emitter module

Description:
CLAIM OF PRIORITY 
     The present application is a continuation application of, and claims benefit and priority to U.S. application Ser. No. 15/144,747, filed May 2, 2016, entitled “METHOD FOR MAKING TUNABLE MULTI-EMITTER MODULE” (now allowed), which is a divisional of, and claims benefit and priority to U.S. application Ser. No. 13/781,162, filed Feb. 28, 2013, entitled “TUNABLE MULTI-LED EMITTER MODULE”, now U.S. Pat. No. 9,345,095, which claims the benefit and priority under 35 U.S.C. 119(e) of U.S. Provisional Application No. 61/606,351, filed Mar. 2, 2012, the contents of which are hereby incorporated by reference for all purposes. 
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is also related to U.S. patent application Ser. No. 12/756,861, filed Apr. 8, 2010, entitled “PACKAGE FOR MULTIPLE LIGHT EMITTING DIODES,” now U.S. Pat. No. 8,384,097, and U.S. patent application Ser. No. 13/106,808, filed May 12, 2011, entitled “TUNING OF EMITTER WITH MULTIPLE LEDS TO A SINGLE COLOR BIN,” now U.S. Pat. No. 8,598,793, the disclosures of both of which are incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates in general to lighting devices based on light-emitting diodes (LEDs) and in particular to tunable emitter modules that include multiple LEDs. 
     LEDs are a promising technology more energy efficient than incandescent light bulbs and are already widely deployed for specific purposes, such as traffic signals and flashlights. However, the development of LED-based lamps for general illumination has run into various difficulties. Among these is the difficulty of mass-producing lamps that provide a consistent color temperature. 
     As is known in the art, not all white light is the same. The quality of white light can be characterized by a color temperature, which ranges from the warm (slightly reddish or yellowish) glow of standard tungsten-filament light bulbs to the cool (bluish) starkness of fluorescent lights. Given existing processes for LED manufacture, mass-producing white LEDs with a consistent color temperature has proven to be a challenge. 
     Various solutions have been tried. For example, white LEDs can be binned according to color temperature and the LEDs for a particular lamp can be selected from the desired bin. However, the human eye is sensitive enough to color-temperature variation that a large number of bins is required, with the yield in any particular bin being relatively low. Another solution relies on mixing different colors of light to produce a desired temperature. However, this approach can be expensive and not reliable. 
     Therefore, there is a need for a multiple-LED emitter module that can be tuned to provide desired light colors. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments of the present invention relate to emitter modules tunable emitter modules that include multiple LEDs and embedded information for tuning the color of light. Particular embodiments are adapted for use with emitter modules that include two or more independently addressable groups of LEDs that each produce light of a different color or color temperature. The uniform color or color temperature output from the emitter module is tuned by varying input current to each of the groups of LEDs. In some embodiments, the emitter module also includes a memory device. The LEDs are pre-tested, and information relating the electrical current for each group of LEDs to the output light color is stored in the memory device. A controller can access this information and provides the correct amount of current to allow the emitter module to provide the desired light color. 
     Depending on the embodiments, one or more of the following advantages can be realized by embodiments of the invention. For example, the utilization of LED dies can be increased, because the LED dies that would otherwise be excluded by color binning can now be combined to produce the desired light color. For lamps using LEDs whose color is stable over time, the tuning can be performed once, e.g., during manufacture and/or factory testing of the lamp, and the lamp can thereafter operate at a stable color temperature without requiring active feedback components. In another example, output light of the emitter module can be varied to provide lighting for different occasions. The emitter module can be adapted by lamp manufacturers in many different applications. 
     According to an embodiment of the present invention, a light-emitting diode (LED) emitter module includes a substrate having a plurality of base layers of an electrically insulating material, a plurality of electrical contacts disposed on a top one of the base layer, and a plurality of electrical paths coupled to the electrical contacts, wherein at least a portion of the plurality of electrical paths is disposed between the base layers. The emitter module also includes two or more groups of light-emitting diodes (LEDs), each group having one or more LEDs, and each of the LEDs is coupled to an electrical contact. The electrical paths are configured for feeding separate electrical currents to the two or more groups of LEDs. The emitter module also includes a memory device containing information associating a plurality of output light colors with a corresponding plurality of combinations of electrical currents, each combination specifying an electric current for each of the two or more groups of LEDs. The emitter module further includes a circuit for accessing the information in the memory device. 
     In an embodiment of the above emitter module, the memory device is a non-volatile memory device. In an embodiment, the output light color is specified by a target wavelength within a wavelength range of no more than 10 nm. In an embodiment, the emitter module further includes a circuit for wired communication. In a different embodiment, the emitter module further includes a circuit for wireless communication. In some embodiments, the emitter module also includes a processor. In another embodiment, the emitter module also includes a processor and a PWM (pulse with modulation) control circuit. In yet another embodiment, the emitter module also includes a processor and an analog current splitter circuit. 
     In embodiments of the invention, the emitter module also includes a substrate on which the two or more groups of light-emitter diodes (LEDs) are disposed. In some embodiments, the memory device is disposed on the substrate. In some embodiments, the emitter module also includes a metal core printed circuit board (MCPCB) on which the substrate is disposed. In an embodiment, the memory device is disposed on the MCPCB. In some embodiments, the two or more groups of light-emitter diodes (LEDs) are configured as a single emitter having a single substrate and a single primary lens. 
     According to another embodiment of the invention, a light-emitting diode (LED) emitter module includes two or more groups of light-emitter diodes (LEDs), each group having one or more LEDs. The emitter module has connections for feeding electric current to each of the two or more groups of LEDs. The emitter module also includes a memory device containing at least information associating a plurality output light colors with a corresponding plurality of combinations of electrical current values, each combination specifying an electric current for each of the groups of LEDs. The emitter module also has a circuit for accessing to the information in the memory device, thereby allowing selection of output light colors. 
     Embodiments of the invention provides various lighting systems that include the emitter modules described above. For example, in an embodiment, a lighting system includes one of the emitter module described above and a driver module configured to access information stored in the memory device and to provide electrical current to the groups of LEDs. In another embodiment, a lighting system includes one of the emitter module described above, a controller configured to access information stored in the memory device, and a driver module configured to provide electrical current to the groups of LEDs based on information provided by the controller. In yet another embodiment, a lighting system includes one of the emitter module described above and a driver module configured to provide electrical current to the groups of LEDs. Here, the emitter module also has a processor configured to access information stored in the memory device and a control circuit configured to control the driver module. In still another embodiment, the control circuit further comprising a PWM (pulse with modulation) control circuit. In an alternative embodiment, the control circuit further comprising an analog current splitter circuit. 
     According to another embodiment of the invention, a method is provided for producing a target color using an LED emitter module having an LED emitter with two or more groups of LEDs and a memory device. The method includes reading, from the memory device, electrical current values for each of the two groups of LEDs for producing the target color and providing current to each of the two groups of LEDs based on the current values from the memory device. 
     According to yet another embodiment of the invention, a method for making an LED (light-emitter diode) emitter module includes providing an LED emitter having two or more groups of LEDs and a memory device, each group having one or more LEDs. The method also includes testing the two or more groups of LEDs to determine required current for each group for the emitter to output a target color. The method further includes storing information about the required current into the memory device. 
     A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view illustrating a tunable multi-LED emitter module according to an embodiment of the present invention; 
         FIG. 2A  is a simplified cross-sectional view illustrating a multi-LED tunable emitter that can be used in the tunable emitter module of  FIG. 1  according to an embodiment of the present invention; 
         FIG. 2B  is a simplified cross-sectional view illustrating a substrate for an LED emitter package according to an embodiment of the present invention; 
         FIG. 2C  is a simplified top view of a substrate holding LEDs that may be used in the tunable multi-LED emitter module of  FIG. 1 ; 
         FIGS. 2D and 2E  illustrate examples of electrical connectivity that can be used to provide independent addressability of warm white and cool white LEDs according to an embodiment of the present invention; 
         FIG. 3A  is a plot illustrating operating characteristics of LED lamps usable in some embodiments of the present invention; 
         FIG. 3B  illustrates an operating principle for tuning an LED lamp according to an embodiment of the present invention; 
         FIGS. 4A-4D  illustrate a 12-LED package according to an embodiment of the present invention. More specifically,  FIG. 4A  is a simplified top view of a substrate;  FIG. 4B  is a simplified cutaway top view of the substrate of  FIG. 4A ;  FIG. 4C  is a simplified schematic illustration of another configuration of electrical connections among certain components shown in  FIG. 4B ; and  FIG. 4D  is a bottom view of the substrate of  FIG. 4A ; 
         FIG. 5  illustrates a lighting system including a tunable multi-LED emitter module according to an embodiment of the present invention; 
         FIG. 6  illustrates another lighting system including a tunable multi-LED emitter module according to another embodiment of the present invention; 
         FIG. 7  illustrates yet another lighting system including a tunable multi-LED emitter module according to an alternative embodiment of the present invention; 
         FIG. 8  illustrates yet another lighting system including a tunable multi-LED emitter module according to an alternative embodiment of the present invention; and 
         FIG. 9  illustrates yet another lighting system including a tunable multi-LED emitter module according to an alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The description below is presented with reference to a series of drawing figures enumerated above. These diagrams are merely examples, and should not unduly limit the scope of the claims herein. In connection with the various aspects illustrated and described, one of ordinary skill in the art would recognize other variations, modifications, and alternatives. 
       FIG. 1  is a perspective view illustrating a tunable multi-LED emitter module according to an embodiment of the present invention. As shown in  FIG. 1 , emitter module  100  includes a multiple-LED emitter  120  overlying a circuit board  130 . Emitter  120  has two or more groups of light-emitter diodes (LEDs), with each group including one or more LEDs. As will be described below with reference to  FIGS. 2A-4D , the groups of LEDs are individually addressable. In other words, each group can receive a different amount of electric current for adjusting its brightness. Emitter  120  also includes a plurality of contacts  170  for feeding electric current to the two or more groups of LEDs. By supplying different electrical currents to the groups of LEDs, emitter  120  can be configured to produce different output light colors. 
     Moreover, emitter  120  includes a memory device  140  that contains information about the characteristics of the emitter module. For example, memory device  140  can include at least information associating light colors with electrical currents. For example, memory device  140  may include information associating two or more output light colors with two or more corresponding combinations of electrical current values, each combination specifying an electric current for each of the two or more groups of LEDs. In some embodiments, the output light color is specified by a target wavelength within a range of, for example, 10 nm or 20 nm. The output light color can be varied according to the demand of the environment. For example, the output of emitter module  100  can be changed from warm white or cool white, or vice versa. Alternatively, by varying the current provided to difference groups of LEDs, emitter module  100  can provide light of any desirable color, or even patterns of different colors. Emitter module  100  can include circuits  160  for accessing the information in the memory device, thereby allowing tuning of output light colors. 
     Embodiments of the invention provides methods for producing a target color using a tunable LED emitter. In a specific embodiment, the emitter has two groups of LEDs, and the method includes reading electric current values that are stored in the memory device for each of the two groups of LEDs for producing the target color. The required current values are then provided to two or more drivers to cause the drivers to provide the required currents. In other embodiments, the emitter can have more than two groups of LEDs, the required current for a target color can be read from the memory device in the emitter module. More information about tuning the multi-LED emitter light color is described below with reference to  FIGS. 2-3 . Lighting systems incorporating the multi-emitter tunable LED module are described with reference to  FIGS. 5-7 . 
     In some embodiments, the memory device is a non-volatile memory device. For example, the memory device can include read-only memory (ROM), Flash memory, electrically-programmable memory (EPROM), or erasable electrically-programmable memory (EEPROM), etc. 
     In some embodiments, emitter  120  includes a substrate on which the two or more groups of light-emitter diodes (LEDs) are disposed. In an embodiment, the substrate has a plurality of base layers of an electrically insulating material, a plurality of electrical contacts disposed on a top one of the base layer, and a plurality of electrical paths coupled to the electrical contacts. At least a portion of the plurality of electrical paths is disposed between the base layers. Each of the LEDs being coupled to an electrical contact, and the electrical paths are configures for feeding separate electrical currents to the two or more groups of LEDs. Emitter module  100  can also include a circuit board  130 , e.g., a metal core printed circuit board (MCPCB), on which the substrate is located. More details about the substrate and the circuit board are described below with reference to  FIGS. 2-4 . 
     Depending on the embodiment, memory device  140  can be disposed on the substrate or on the MCPCB  130 . In some embodiments, emitter  120  has the two or more groups of light-emitter diodes (LEDs) configured as a single emitter having a single substrate and a single primary lens, as illustrated below in  FIG. 2A . 
     In some embodiment, emitter module  100  also includes contact pads  150  coupled to circuits for communication, which enable access to information stored in memory device  140  and which enables control information to be provided to emitter module  100 . Depending on the embodiment, the communication circuit can include wired interface circuits implementing the SPI (Serial Peripheral Interface) or i2C (Inter-Integrated Circuit, or two-wire interface) protocols. In alternative embodiments, the communication circuit can include wireless interface circuits, including antenna, for example, for communication in the infrared (IR) or radio frequency (RF). 
     Embodiments for tuning lamps with two independently addressable groups of LEDs are described below, and it is understood that the techniques can be extended to lamps with larger numbers of groups. As used herein, a “group” of LEDs refers to any set of one or more LEDs that occupies a defined region in color space; the regions are defined such that regions occupied by different groups in the same lamp do not overlap. The lamp is advantageously designed such that the current supplied to each group of LEDs can be controlled independently of the current supplied to other LEDs, and the groups are thus said to be “independently addressable.” 
       FIG. 2A  illustrates a simplified cross-sectional side view of a multi-LED tunable emitter  120  that can be used in the tunable emitter module  100  of  FIG. 1  according to an embodiment of the present invention. Emitter  120 , which can be symmetric about an axis  101  (other shapes can also be used) and includes a substrate  106  on which are mounted individual LEDs  108 . Each LED  108  can be a separate semiconductor die structure fabricated to produce light of a particular color in response to electrical current. In some embodiments, each LED  108  is coated with a material containing a color-shifting phosphor so that LED  108  produces light of a desired color. For example, a blue-emitting LED die can be coated with a material containing a yellow phosphor; the emerging mixture of blue and yellow light is perceived as white light having a particular color temperature. 
     In some embodiments, Emitter  120  also includes a control circuit  116  that controls, among other things, the power provided from an external power source (not shown) to LEDs  108 . As described below, control circuit  116  advantageously allows different amounts of power to be supplied to different LEDs  108 . 
     A primary lens  110 , which can be made of glass, plastic, or other optically transparent material, is positioned to direct light emitted from LEDs  108  to the desired direction. In some embodiments, a secondary optics  112  (shown in dotted line) is disposed over primary lens. Secondary optics  112  advantageously include a total-internal-reflection (TIR) lens that also provides mixing of the colors of light emitted from LEDs  108  such that the light beam exiting through front face  114  has a uniform color. Examples of suitable lenses are described in U.S. Patent Application Pub. No. 2010/0091491; other color-mixing lens designs may also be used. In some embodiments, primary lens and secondary optics can be combined into one mixing lens structure. Tuning is advantageously performed based on the color of light exiting through front face  114  of TIR lens  112  or the front face of another mixing lens. 
     Embodiments of the present invention provide substrates and packages for LED-based light devices that can significantly improve thermal performance, allowing the LEDs to operate at higher current and therefore higher brightness. In addition, some embodiments provide improved electrical properties by providing separate electrical and thermal paths through the substrate. The separation of electrical and thermal paths further allows different operating current to be supplied to different LEDs, enhancing the ability to control the light output of the device. 
       FIG. 2B  is a simplified side view of a substrate  200  for a multi-LED emitter according to an embodiment of the present invention. Substrate  200  is formed as a series of layers  201 - 205  of a ceramic material (e.g., alumina or aluminum nitride (AlN)). Layers  201 - 205  have different thicknesses that can be optimized to control thermal expansion and thermal stress. For example, layers  201  and  202  can be 0.15 millimeters (mm) each, layer  203  can be 0.10 mm, layer  204  can be 0.50 mm, and layer  205  can be 0.15 mm, for a total thickness of 1.05 mm. 
     Upper layers  204  and  205  define a recess  210  within which one or more LEDs (not shown) can be placed. In one embodiment, recess  210  has the shape of a truncated cone; sidewall  211  is circular and slanted inward, e.g., at an angle of about 20° with respect to the vertical axis. Sidewall  211  of recess  210  can be coated with a reflective material (e.g., silver) to increase light output of the device. 
     Upper layer  205  can provide a circular opening, allowing light to escape from recess  210 . In this embodiment, the edge of layer  205  is set back from the edge of layer  204  at the periphery of recess  210 , thereby forming a ledge  212  upon which a primary lens can be placed. 
     Layers  201 - 203  provide a base for the package. A patterned metal layer  214  is deposited on top-most base layer  203  within recess  210 . Patterned metal layer  214  provides various bond pads (e.g., pad  220 ) for electrical contacts to LEDs disposed within recess  210 . (These are referred to herein as “top-side” bond pads because they are on the topmost one of the base layers.) Specific examples are described below, but it will be appreciated that the present invention is not limited to any particular configuration of bond pads or of metal layer  214 . 
     External electrical contacts  216 ,  218  are provided at a peripheral edge of substrate  200 . In one embodiment, external contacts  216 ,  218  include metal coatings that extend vertically along the entire thickness of substrate  200 . Any number of external contacts can be provided. Each top-side bond pad of patterned metal layer  214  can be connected to one (or more) of the external electrical contacts, e.g., using metal lines disposed between ceramic layers and metal vias passing through the ceramic layers. By way of illustration,  FIG. 2B  shows top-side bond pad  220  connected to external contact  216  by interlayer metal lines  222 ,  224  and vias  226 ,  228 . Any configuration of connections may be used. Further, in some embodiments, some of the top-side bond pads can be connected to each other by interlayer metal lines and vias that do not connect to external electrical contacts. In some embodiments, as described below, the electrical connections are arranged such that power can be supplied separately to different LEDs or groups of LEDs. In some embodiments, external contacts  216 ,  218  may also extend inward on the bottom surface of substrate  200 , e.g., as bottom metal pads  232 ,  234 . 
     A metal plate  230  is disposed on the bottom surface of bottom layer  201 . Metal plate  230 , which is advantageously circular and as large as possible in some embodiments, provides a metallic surface for attaching a heat sink. Metal plate  230  is also advantageously electrically isolated from the various electrical paths and pads that may be present on, within, and/or under substrate  200 . 
     Substrate  200  can be used to support any number and arrangement of LEDs. Specific examples include 4-LED, 12-LED, and 16-LED configurations. An example is illustrated in  FIG. 2C , which is a top view of substrate  106  in  FIG. 2A  according to an embodiment of the present invention. Those skilled in the art with access to the present teachings will understand that many other configurations are also possible. 
     In some embodiments LEDs  108  advantageously include both “warm” and “cool” white LEDs. As shown in  FIG. 2C , twelve LEDs  108   a - l  are arranged within a recess  156  on substrate  106 . In this example, six of the LEDs are cool white (“CW”) LEDs  108   a - f ; the other six are warm white (“WW”) LEDs  108   g - l . “Cool” white and “warm” white, as used herein, refer to the color temperature of the light produced. Cool white, for example, can correspond to a color temperature above, e.g., about 4000 K, while warm white can correspond to a color temperature below, e.g., about 3000 K. In some embodiments, it is desirable that cool white LEDs  108   a - f  have a color temperature cooler than a target color temperature for lamp  100  while warm white LEDs  108   g - l  have a color temperature warmer than the target color temperature. When light from cool white LEDs  108   a - f  and warm white LEDs  108   g - l  is mixed by mixing lens  112 , the target temperature can be achieved. More generally, for purposes of providing a tunable emitter, the emitter can include LEDs belonging to any number of “groups,” with each group being defined as producing light within a different color or color temperature range (or “bin”); the ranges associated with different groups advantageously do not overlap, and the desired color or color temperature to which the lamp will be tuned is somewhere between the colors or color temperatures associated with the groups of LEDs. 
     To facilitate achieving a desired color temperature, the LEDs  108  of emitter  120  are advantageously connected such that cool white LEDs  108   a - f  and warm white LEDs  108   g - l  are independently addressable, i.e., different currents can be supplied to different LEDs.  FIGS. 2D and 2E  are simplified schematics illustrating examples of electrical connectivity that can be used to provide independent addressability of warm white and cool white LEDs. These electrical connections can be implemented, e.g., using traces disposed on the surface of substrate  106  and/or between electrically insulating layers of substrate  106 . Examples of substrates that provide independent addressability for groups of LEDs are described in U.S. patent application Ser. No. 12/756,86, U.S. Patent App. Pub. No. 2010/0259930; other substrates can also be used. 
     In  FIG. 2D , cool white LEDs  108   a - f  are connected in series between a first input node  252  and a first output node  254 ; warm white LEDs  108   g - l  are connected in series between a second input node  256  and a second output node  258 . Consequently, one current (I C ) can be delivered to cool white LEDs  108   a - f  while a different current (I W ) is delivered to warm white LEDs  108   g - l . The currents I C  and I W  can be independently controlled, thereby allowing the relative brightness of cool white LEDs  108   a - f  and warm white LEDs  108   g - l  to be controlled; this provides control over the color temperature of light produced by emitter  120 . For example, control circuit  116  ( FIG. 2A ) can be connected to nodes  252  and  254  and to nodes  256  and  258  to deliver the desired currents I C  and I W . 
       FIG. 2E  illustrates one specific technique for implementing per-group current control. As in  FIG. 2D , cool white LEDs  108   a - f  are connected in series, and warm white LEDs  108   g - l  are also connected in series. In  FIG. 2E , the last LEDs in each series (LEDs  108   f  and  108   l ) are connected to a common output node  268 . A common input node  262  receives a total current I TOT , which is divided between cool white LEDs  108   a - f  and warm white LEDs  108   g - l  using potentiometers (or variable resistors)  264  and  266 . Potentiometer  264  can be set to a resistance R C  while potentiometer  266  can be independently set to a resistance R W ; as a result, a current I C  is delivered to cool white LEDs  108   a - g  while a current I W  is delivered to warm white LEDs  108   g - l . By controlling R W  and R C , I TOT  can be divided between I W  and I C  in a controllable proportion according to the property that I W /I C =R C /R W . Thus, as in  FIG. 2D , the relative brightness of cool white LEDs  108   a - f  and warm white LEDs  108   g - l  can be controlled, thereby providing control over the color temperature of light produced by lamp  100 . In one embodiment, control circuit  116  can be connected to nodes  262  and  268  to supply current I TOT , and further connected to control resistances R C  and R W . 
     Other addressing schemes can also be used; for example, each of the LEDS  108   a - l  can be independently addressable. 
     It will be appreciated that emitter  120  described herein is illustrative and that variations and modifications are possible. In one embodiment, emitter  120  can be similar to the emitter in a LuxSpot™ lamp, manufactured and sold by LedEngin Inc., assignee of the present invention. Those skilled in the art with access to the present teachings will recognize that any lamp that has independently addressable warm white and cool white LEDs can also be used; thus, details of the lamp are not critical to understanding the present invention. 
     In accordance with some embodiments of the present invention, the currents I C  and I W  (shown in  FIGS. 2D and 2E ) can be efficiently tuned so that the light output from emitter  120  has a desired color temperature. The tuning process advantageously requires only a small number (e.g., three or four) of measurements and does not rely on trial-and-error. The process can also be automated to allow tuning of a large number of lamps in a mass-production environment; thus, color tuning can be incorporated into lamp production, e.g., as a stage in an assembly line. 
     As described below, emitter  120  can be placed into a tuning apparatus and color-tuned during production. Thereafter, emitter  120  can be configured to operate at the desired color temperature simply by maintaining the division (or distribution) of current determined in the tuning process. Provided that the LEDs in emitter  120  can maintain a stable color temperature over time, no further tuning or active feedback is needed during normal emitter operation. Since active feedback is not needed, the cost of manufacture can be reduced as compared to emitters that require active feedback to maintain a stable color temperature. 
     The tuning process can also be used to identify different current distributions for different target colors. A tunable multi-LED emitter module (e.g., emitter module  100  of  FIG. 1 ) can include a memory device containing at least information associating two or more output light colors with two or more corresponding combinations of electrical current values, each combination specifying an electric current for each of the two or more groups of LEDs. A tuning process according to embodiments of the present invention is described in more detail below with reference to  FIGS. 3A and 3B . 
       FIG. 3A  is a plot illustrating operating characteristics of emitters usable in some embodiments of the present invention. The graph  300  represents a portion of CIE color space, which characterizes light in terms of luminance (CIE y) and chromaticity (CIE x) coordinates. The portion of the CIE color space represented encompasses much of the range associated with white light. The various data points (black diamonds) represent colors measured from a number of LED-based emitters having independently addressable warm white and cool white LED groups, e.g., as described above with reference to emitter  120 , under various operating conditions. 
     More specifically, for purposes of these measurements, a total current I TOT  of 1000 mA was supplied to the emitter, and the constraint I C +I W =I TOT  was maintained. “Cool white” data, represented by points  302 , was measured for each emitter by setting I C =I TOT  and I W =0. “Warm white” data, represented by points  304 , was measured for each emitter by setting I C =0 and I W =I TOT . “Balanced” data, represented by points  306 , was measured by setting I C =I W =0.5*I TOT . 
     A target color is represented by circle  308 , and the goal is to produce colors as close to this target as possible. As can be seen, merely applying equal current to the warm white and cool white LEDs results in balanced data points  306  being scattered about target  308 . While the balanced colors are more consistent across different emitters than can readily be obtained by using LEDs of a single white color, further improvement in color consistency can be achieved by tuning the relative currents I C  and I W  (and consequently the color) on a per-emitter basis. Such tuning in a typical case results in unequal currents being supplied to the warm white and cool white LEDs, with the currents being selected to reduce the lamp-to-lamp variation by bringing the light from each emitter closer to target  308 . 
       FIG. 3B  illustrates an operating principle for tuning an emitter according to an embodiment of the present invention. Point  402 , at coordinates (x C , y C ) in CIE color space, represents the location of a “cool white” data point for a particular emitter (e.g., one of data points  302  in  FIG. 3A ). Similarly, point  404 , at coordinates (x W , y W ) in CIE color space, represents the location of a “warm white” data point for the same emitter (e.g., one of data points  304  in  FIG. 3A ). Point  406 , at coordinates (x B , y B ) represents the balanced data for that emitter (e.g., one of data points  306 ). Point  408 , at coordinates (x s , y s ), represents a single-color point to which it is desirable to tune the emitter. (This point, which can correspond to target  308  in  FIG. 3A , may be specified by the manufacturer of the lamp or any other entity who may be performing the tuning process.) 
     Blending light of the colors corresponding to points  402  and  404  results in a color somewhere along line  410 . Thus, it may not be possible to produce blended light with a color corresponding exactly to single-color point  408 . Accordingly, the aim instead is to reach the closest point to point  408  that is on line  410 , i.e., “tuned” point  412  at coordinates (x t , y t ). In a typical case (x t , y t ) and (x B , y B ) are not the same, and (x t , y t ) may be different for different lamps; thus, tuning on a per-emitter basis is desired. 
     In general, the relationship between a change in the relative currents (measured, e.g., as I W /I C ) supplied to the warm and cool LEDs and the resulting shift in color temperature is nonlinear. Further, the magnitude of the shift in color temperature resulting from a given change in relative current varies from one lamp to another. However, according to embodiment of the invention, over a sufficiently narrow range of color space, the relationship can be approximated as linear. Examples of tuning techniques based on this property are described in U.S. patent application Ser. No. 13/106,808, filed May 12, 2011, entitled “Tuning Of Emitter With Multiple LEDS To A Single Color Bin,” now U.S. Pat. No. 8,598,793. 
     In embodiments of the invention, the tuning is facilitated by arranging the substrate to provide individual access and control of the LED dies.  FIG. 4A  is a simplified top view of a substrate  400  for a 12-LED package according to another embodiment of the present invention. Substrate  400 , viewed from the top, can be a square of any size desired, e.g., about 0.7-5 cm on a side; in one embodiment, the square is about 9 mm on a side. Its thickness can be, e.g., about 0.5-2 mm or other thickness as desired; in one embodiment, the thickness is between 0.7-1 mm. Like substrate  200  of  FIG. 2B , substrate  400  is formed as a set of ceramic layers. A recess  402  is defined by an angled sidewall  404  and optionally surrounded by a ledge (not explicitly shown), similar to recess  210  and ledge  212  of  FIG. 2B . Top-side bond pads, which include LED bond pads  410   a - l  and wire bond pads  412   a - l , are disposed within the recess. In this embodiment, twenty-four peripheral bond pads  420   a - x  are provided for external electrical contacts. Twelve LEDs (not shown in  FIG. 4A ) can be bonded to LED bond pads  410   a - l  and connected, e.g., via wire bonds, to wire bond pads  412   a - l , as shown in  FIG. 4B  and described below. 
     Depending on how the LEDs are connected and how top-side bond pads  410   a - l ,  412   a - l  are electrically coupled to peripheral bond pads  420   a - x , a number of electrical configurations are possible. 
     For example,  FIG. 4B  is a cutaway top view of substrate  400 , with the upper layers forming sidewall  404  (e.g., corresponding to layers  204  and  205  of  FIG. 2B ) removed. The inner periphery of recess  402  is indicated by broken line  403 . As can be seen, some of top-side bond pads  410   a - l ,  412   a - l  can extend outward beyond the boundary of recess  402 , further spreading heat across more of the upper surface of ceramic substrate  400 . In addition, some of top-side bond pads  410   a - l ,  412   a - l  can connect to some of peripheral bond pads  420   a - x  without the use of vias or metal interconnects between other layers. Connections to the remaining bond pads are not explicitly shown in  FIG. 4B . It is to be understood that paths not shown in  FIG. 4B  may be implemented using vias and metal interconnect between the ceramic layers (e.g., as illustrated in  FIG. 2B ). For example, metal interconnects may be created on a layer below the layer seen in  FIG. 4B  and connected to the various pads by vias. 
       FIG. 4B  also shows how twelve LEDs  430   a - l  can be placed and electrically connected to substrate  400  according to an embodiment of the present invention. In this embodiment, each of LEDs  430   a - l  has an electrical contact on its bottom surface (not explicitly shown) and a top pad (also not explicitly shown) for a wire bond  440   a - l.    
     The pad configuration of  FIG. 4B  can provide a separate, independently controllable, electrical connection path for each of the twelve LEDs  430   a - l . (Herein, LEDs or groups of LEDs with a separate electrical connection path are referred to as being “independently addressable.”) For example, peripheral bond pad  420   b  connects to LED bond pad  410   a . LED  430   a  is connected between LED bond pad  410   a  and wire bond pad  412   a  by wire bond  440   a . Wire bond pad  412   a  connects to peripheral bond pad  420   c . Likewise, peripheral bond pad  420   w  connects to LED bond pad  410   c . LED  430   c  is connected between bond pad  410   c  and bond pad  412   c  by wire bond  440   c . Bond pad  412   c  is connected to peripheral bond pad  420   v . Further, peripheral bond pad  420   x  connects to LED bond pad  410   d  (the connection is not explicitly shown). LED  430   d  is connected between LED bond pad  410   d  and wire bond pad  412   d  by wire bond  440   d . Wire bond pad  440   d  connects to peripheral bond pad  420   a  (again, the connection is not explicitly shown). Similarly, each other LED  430  is electrically coupled between a different pair of peripheral bond pads. 
     Thus, LEDs  430   a - l  are each individually addressable; this is also illustrated schematically in  FIG. 4C . In this configuration, applying a potential difference across the appropriate pair of peripheral bond pads will provide power to one of the twelve LEDs  430   a - l . The individually addressable connections to the LEDs provide flexibility to make connections outside of the package and thereby connect the LEDs together in different groups. For example, LEDs  430   a - l  could be connected into four groups of three LEDs each or two groups of six LEDs each. The LEDs within a group can be connected in series or in parallel as desired. For example,  FIG. 4C  is a schematic diagram illustrating a configuration with two groups of six LEDs  430   a - l  connected in series according to an embodiment of the present invention. 
     In still other embodiments, series or parallel connections of multiple LEDs can be “built in” to the substrate. For example, if a wire bond pad (e.g., pad  412   d ) were electrically connected to an LED bond pad (e.g., pad  410   c ), a serial connection would be permanently defined for LEDs  430   c ,  430   d . Such a connection can be made directly between the pads, or indirectly using vias and metal interconnects between base layers of substrate  400 . 
     Referring again to  FIG. 4B , it should be noted that LED bond pads  410   a - l  are advantageously made as large as possible and can be substantially larger than LEDs  430   a - l . The large area of the LED bond pads allows heat generated by LEDs  430   a - l  to spread quickly across the upper surface of the ceramic substrate, increasing the amount of heat that can be transferred vertically through the substrate. 
       FIG. 4D  is a bottom view of substrate  400  of  FIG. 4A . A metal region  470 , which is advantageously circular and as large as possible is centered relative to recess  402  ( FIG. 4A ). Metal region  470  acts as a heat dissipation plate. A heat sink can be placed in thermal contact with metal region  470  to further dissipate heat. 
     Peripheral bond pads  420   a - x  can extend along the entire vertical thickness of substrate  400  (similar to substrate  200  of  FIG. 2A ) and can be connected to bottom pads  460   a - x . External electrodes (e.g., wires) can be connected directly to peripheral bond pads  420   a - x  and/or to bottom pads  460   a - x  as desired. 
     It should be noted that metal region  470  is not electrically coupled to any of peripheral bond pads  420   a - x , bottom pads  460   a - x , or top-side bond pads  410   a - l ,  412   a - l . Thus, metal region  470 , in conjunction with the thermally conductive ceramic body of substrate  400 , provides a thermal path that is separate from the electrical path. 
       FIG. 5  illustrates a lighting system  500  including a tunable multi-LED emitter module according to an embodiment of the present invention. As shown in  FIG. 5 , lighting system  500  includes a tunable multi-LED emitter module  510  and a driver module  520 . In an embodiment, tunable multi-LED emitter module  510  is similar to tunable multi-LED emitter module  100  of  FIG. 1 . In particular, emitter module  510  includes two or more groups of LEDs, memory device. Driver module  520  includes a power supply, e.g., a switch mode power supply (SMPS), that has multiple channels for providing a separate current to each groups of LEDs. Driver module  520  also includes control circuits configured for accessing the information in the memory device in emitter module  510  and adjusting the current output in each channel for tuning the emitter module to provide the target light color. 
       FIG. 6  illustrates another lighting system  600  including a tunable multi-LED emitter module according to an embodiment of the present invention. As shown in  FIG. 6 , lighting system  600  includes a tunable multi-LED emitter module  610 , a driver module  620 , and a controller  630 . Tunable multi-LED emitter module  610  is similar to tunable multi-LED emitter module  100  of  FIG. 1 . In an embodiment, controller  630  includes a processor that is configured to read information stored in the memory device in emitter module  610  and determine the output for each channel of driver module  620  for tuning the light output of emitter module  610 . 
     In some embodiments, controller  630  is coupled to emitter module  610  and driver module  620  through wire connections. In some other embodiments, controller  630  can be coupled to emitter module  610  and driver module  620  through wireless communications. 
       FIG. 7  illustrates yet another lighting system  700  including a tunable multi-LED emitter module according to an embodiment of the present invention. As shown in  FIG. 7 , lighting system  700  includes a tunable multi-LED emitter module  710  and a driver module  720 . In some embodiments, tunable multi-LED emitter module  710  is similar to tunable multi-LED emitter module  100  of  FIG. 1 . In addition, emitter module  710  also includes a processor  712  and a current control circuit  714 . In some embodiments, driver module  720  can be a conventional driver circuit with a power supply. In this case, the currents to each group of LEDs can be adjusted by current control circuit in emitter module  710 . For example, processor  712  can access the information from the memory device in emitter module  710  and direct control circuit  714  to distribute the current received from driver module  720  to each group of LEDs in emitter module  710 . In some embodiments, emitter module  710  can receive information to be written into the memory device through wired or wireless communication with an external system. For example, emitter module  710  can receive, from an external test system, electrical current and light color information that is calibrated to the groups of LEDs in this emitter module for color tuning. 
       FIG. 8  illustrates a lighting system  800  including a tunable multi-LED emitter module according to an embodiment of the present invention. As shown in  FIG. 8 , lighting system  800  includes a tunable multi-LED emitter module  810  and a driver module  820 . In some embodiments, emitter module  810  is similar to emitter module  710  in lighting system  700 , and includes a processor  812  and a control circuit  814 . In  FIG. 8 , control circuit  814  includes a PWM (Pulse Width Modulation) controller for controlling driver module  810 . In this case, driver module  820  includes the necessary components of a power supply, such as a transformer, power transistor, output rectifier, etc. 
       FIG. 9  illustrates another lighting system  900  including a tunable multi-LED emitter module according to an embodiment of the present invention. As shown in  FIG. 9 , lighting system  900  includes a tunable multi-LED emitter module  910  and a driver module  920 . In some embodiments, emitter module  910  is similar to emitter module  710  in lighting system  700 , and includes a processor  912  and a control circuit  914 . Driver module  920  can be a conventional power supply, e.g., an SMPS. Here, control circuit  914  includes an analog current splitter circuit that can distribute the current received from driver module  920  for tuning light output of emitter module  910 . 
     In the above description, specific circuits and examples are used to illustrate the embodiments, it is understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this invention.