Patent Application
20130075769
The present invention relates in general to lamps based on light-emitting diodes (LEDs) and in particular to selection of phosphors and LED dies for a multi-die emitter to support tuning of the emitter to a single white color bin.
With the incandescent light bulb producing more heat than light, the world is eager for more efficient sources of artificial light. LEDs are a promising technology 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. For example, an LED lamp can include a number of white LEDs plus some red LEDs. The brightness of the red LEDs can be increased to warm the light to the desired color temperature. Such lamps generally require an active feedback mechanism to maintain the color temperature, in part because the LEDs used are not stable in their color characteristics over time. The active feedback mechanism requires a sensor to detect the light being produced, an analyzer to determine whether the light is at the desired color, and an adjustment mechanism to adjust the relative brightness of the white and red LEDs as needed to maintain the desired color. These feedback-loop elements can be a weak point in the system; for example, if the light sensor drifts over time (as most do), so will the color of the light. In addition, incorporating active feedback components into a lamp drives up the cost of manufacturing (and operating) the lamp.
Embodiments of the present invention relate to selection of LED dies and phosphors to be used in multi-die emitters to produce light that can be tuned to a single white color bin. The emitter is advantageously designed such that different groups of the LEDs are independently addressable, that is, the current delivered to each group is separately controllable. Such an emitter allows the net color of light produced by a lamp in which the emitter is included to be tuned to a color intermediate between the colors produced by the different groups of LEDs, e.g., by adjusting the relative current delivered to different groups.
To produce a desired white color (e.g., white light having a particular color temperature), the LED dies and a phosphor-containing element for each die can be selected such that one of the independently-addressable groups of LEDs produces a warmer white light than the desired color while another of the groups produces a cooler white light than the desired color, with the desired white color being intermediate between the warmer and cooler color temperatures so that the lamp can be tuned to the desired white color. In some embodiments, individual LED dies of a specified color, e.g., blue, are binned according to a particular spectral characteristic (e.g., peak wavelength) of the light produced by the LED die. Phosphor-containing elements can be provided as “chips” made of a solid matrix material in which are dispersed phosphorescent chemicals (e.g., conventional yellow, green, and/or red phosphors) at a known concentration; a variety of different “types” of phosphor chip can be provided, with each type being characterized, e.g., by thickness, combination (mixture) of phosphorescent chemicals, and/or concentration (density) of phosphorescent chemicals relative to matrix material. Phosphor chips can be made by producing a large sheet of phosphor-containing material and cutting the sheet into chips, with each chip having an area approximately equal to the top surface area of an LED die.
A blue LED die, when covered by a phosphor chip containing predominantly yellow phosphors, can produce white light. The particular color (or color temperature) of the white light depends, in a reproducible manner, on the initial wavelength (and/or other spectral characteristics) of the blue light and the particular characteristics, or chip type, of the phosphor chip. Given a finite number of types of phosphor chips and blue LED dies sorted into die types based on spectral properties (e.g., peak wavelength) into a finite number of color bins, a finite number of die-type/phosphor-type combinations are possible. The color produced by each combination can be determined, e.g., by measuring samples of each. Using this color data, it is possible to select a set of LED dies for a multi-die emitter and a phosphor chip for each LED die such that the white color bin intended for a lamp containing the emitter is intermediate between a “warm white” color produced by one group of the LEDs and a “cool white” color produced by another group of the LEDs. Thus, the emitter can be designed such that the lamp will be tunable to a specific desired white color bin. The white color bin can be defined as a relatively small region in color space, such that variations in color between lamps in the bin are imperceptible or nearly so.
The selection of the LEDs and phosphors for a multi-die emitter can be automated. For example, based on a color measurement of light produced by a number of test samples of various combinations of LED die type and phosphor-chip type, a look-up table can be constructed that associates each combination with the color of light produced. Once the table is constructed, it can be used to select, for a given emitter with LED dies of known peak wavelengths (or other spectral characteristics), an appropriate phosphor chip for each LED die such that each group of independently addressable LEDs produces light of the color temperature desired for that group.
Certain aspects of the present invention relate to light-emitting devices. Such devices can include a substrate with multiple LED dies arranged on a surface of the substrate and multiple phosphor chips disposed such that each phosphor chip covers a light-emitting surface of a different one of the LED dies. The LED dies are electrically connected to form at least two independently addressable groups of LEDs, including a first group to produce light whose color is in a first region of a color space (e.g., a warm white region) and a second group to produce light whose color is in a second region of the color space (e.g., a cool white region) that does not overlap with the first region. Some embodiments may include additional groups (e.g., a red group, a green group, or a second warm white or cool white group); all groups advantageously produce light in non-overlapping regions in color space. The phosphor chips covering each of the LED dies is individually selected based on a known spectral characteristic (e.g., peak wavelength) of light produced by the given LED die and the color of the light to be produced by the group in which the given LED die is included.
In some embodiments, the LED dies are blue LED dies (emitting light primarily or entirely in a blue region of the electromagnetic spectrum) and the phosphor chips contain at least a yellow phosphor. The phosphor chips advantageously differ from each other in at least one of chip thickness, concentration of phosphorescent material contained therein, or mixture (or composition) of phosphorescent materials contained therein.
Certain aspects of the invention relate to methods for making a light-emitting device. For example, a color bin for the light-emitting device can be determined. An emitter substrate can be provided; the substrate advantageously has multiple locations for connecting light-emitting diodes (LEDs) and provides electrical connections between the locations such that the LED locations are arranged into at least two independently addressable groups. A source region in a color space is selected to be associated with the independently addressable group such that the color bin for the device is intermediate between the source regions in the color space. For each LED location, an LED die and a phosphor chip are selected such that the LED die and the phosphor chip together produce light in the source region associated with the independently addressable group to which the LED location belongs. The selected LED dies and phosphor chips are mounted on the emitter substrate at the LED locations. For instance, a pick-and-place system can be operated to place the selected LED dies and phosphor chips onto the emitter substrate. In some embodiments, the selected LED dies are first placed on and electrically connected to the substrate, after which the phosphor chips are attached to the LED dies, e.g., by applying an adhesive to the top surfaces of the LED dies and placing the phosphor chips on the adhesive.
In some embodiments, selecting the LED die and the phosphor chips includes selecting a phosphor chip type from a number of options; the various phosphor chip types can differ from each other in one or more of chip thickness, concentration of phosphorescent material, and/or mixture (or composition) of phosphorescent materials. Selecting the LED die and the phosphor chips may include selecting a wavelength bin for the LED die.
Certain aspects of the invention relate to apparatus for assembling a light-emitting device. Such an apparatus can include a robotic pick-and-place system to select LED dies and phosphor chips from a component tray and place the selected LED dies and phosphor chips on an emitter substrate; the component tray advantageously holds LED dies sorted into a plurality of wavelength bins and phosphor chips sorted into a plurality of phosphor chip types. A control system to control the pick-and-place system can include an interface, a memory, and a processor coupled to the interface. The interface is operable to receive input specifying an emitter configuration and a color bin to be associated with the emitter. The emitter configuration can specify a number of LED locations on an emitter substrate, with each LED location being associated with one of a number of independently addressable groups of LEDs. The memory is operable to store data associating each of a plurality of source regions in a color space with a selection of an LED wavelength and a phosphor chip type. The processor is operable to establish, based on the input, an association of each of the independently addressable groups of LEDs with one of the source regions such that different independently addressable groups are associated with different and non-overlapping source regions; to determine, based on the data stored in the memory, an LED wavelength and a phosphor chip type for each of the locations on the emitter substrate; and to direct operation of the pick-and-place system such that the pick-and-place system selects, for each location, an LED having the determined wavelength and a phosphor chip having the determined phosphor chip type. In some embodiments, the control system can also control additional tools to electrically connect the LEDs to the emitter substrate.
The following detailed description together with the accompanying drawings will provide a better understanding of the nature and advantages of the present invention.
Embodiments of the present invention relate to an emitter for an LED-based lighting device that has multiple groups of LEDs that are independently addressable, allowing the emitter to be tuned to a desired color bin (e.g., a specific white color) by adjusting the relative current supplied to different groups. Improved control over tunability of the emitter is achieved by selecting LED dies and a phosphor chip for each LED die such that each die/phosphor combination produces light in a desired source region associated with the group to which the LED belongs. The source regions are selected such that they do not overlap with each other in color space and such that the desired color bin is intermediate between the source regions. Robotic pick-and-place systems can be used to automate assembly of the emitters by selecting LED dies based on known spectral properties (e.g., peak wavelength) and phosphor chips from a number of distinct phosphor chip types.
Multi-LED tunable emitters can be incorporated into a variety of lamps.
Within housing 102 is an LED package 104. Package 104 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 covered 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 covered 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.
The surface area of each phosphor chip 109 is advantageously large enough to cover the light-emitting surface (in this example, the top surface) of a single LED die 107 but not large enough to extend over other LED dies 107. This allows the phosphor chip 109 for each LED die 107 to be selected independently of all others. In some embodiments, phosphor chips 109 can be secured to LED dies 107, e.g., using a thin layer or film of an optically transparent adhesive. Mechanical attachments can also be used; for example, a phosphor chip 109 can be formed so as to fit as a cap over an LED die 107.
In operation, each LED die 107 emits light in the blue region of the electromagnetic spectrum (approximately 450-500 nm), and some of that light is shifted to a lower wavelength by interaction with phosphors in phosphor chip 109. With an appropriate choice of phosphor chip 109, the combination of shifted and unshifted light exiting phosphor chip 109 appears to the human eye as white. As described below, phosphor chips 109 and LED dies 107 are advantageously selected such that each die/phosphor produces light in a desired “source” region in color space; different dies/phosphors advantageously produce light in different source regions. As described below, this selection advantageously allows the overall color of light emitted by lamps 100 to be tuned to a desired small region, or “bin,” in color space, reliably and with high yield, even if LED dies 108 themselves vary significantly in their light output properties.
Referring again to
In some embodiments, lamp 100 also includes a control circuit 116 that controls 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.
In some embodiments LEDs 108 advantageously include both “warm” and “cool” white LEDs. (The classification of an LED as “warm white” or “cool white” should be understood to include the effect of the phosphor chip 109 covering the LED die 107.) An example is illustrated in
To allow the lamp to be tuned to the desired color bin, the LEDs 108 of lamp 100 are advantageously connected such that cool white LEDs 108a-f and warm white LEDs 108g-l are independently addressable, i.e., different currents can be supplied to different groups of LEDs.
In some embodiments, such as those shown in
In
Other addressing schemes can also be used; for example, each of the LEDS 108a-l can be independently addressable. In some embodiments, LEDs 108 can be electrically connected to form three or more independently addressable groups. (Examples of configurations with more than two groups are described below.)
It will be appreciated that lamp 100 described herein is illustrative and that variations and modifications are possible. In one embodiment, lamp 100 can be similar to 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 embodiments described herein can be implemented in a variety of lamps; thus, details of the lamp are not critical to understanding the present invention.
A lamp with a tunable emitter (such as that shown in
Constraining the individual LEDs in an emitter to a particular source region can improve the ability to tune a lamp incorporating the emitter to a small color bin.
The particular source regions selected depend in part on the desired properties of the lamps to be produced. In general, the source regions for a lamp (e.g., regions 322, 324) can be selected based on the desired color bin (e.g., bin 328). The source regions are advantageously selected such that they do not overlap with each other or with the desired color bin, which should be intermediate between them. The source regions advantageously have the property that intermediate between any two points in the source regions is a point within the desired color bin; this is indicated by dotted lines 326. In the example of
In some embodiments, this principle of source-region selection can be extended to produce different lamps that are tunable to multiple different color bins, with each bin being regarded as a single white color. By way of illustration,
Selection of the individual LED dies and phosphor chips for an emitter can be highly automated because the effect of a particular type of phosphor chip on a particular LED die is predictable from the LED die's spectral characteristics and the particular properties of the phosphor chip that define its type (e.g., thickness, phosphor mix, phosphor concentration).
In some embodiments, for example, phosphor chips are made of a solid matrix material in which phosphors (which can include any phosphorescent chemical) are dispersed. Different chips can include different mixtures and concentrations of phosphors. In some embodiments, where blue LED dies are used, the phosphor mixture can include predominantly yellow phosphor such as YAG (yttrium aluminum garnet) or the like. Smaller amounts of non-yellow phosphors can be added. For example, red phosphor (e.g., CaAlSiN3-based (CASN) phosphors) can be added for a warmer output light or green phosphor can be added for a cooler output light. Batches of phosphor chips may be made in N different thicknesses, with M different mixtures of phosphors at each thickness (different mixtures might be, e.g., yellow only, 95% yellow plus 5% red, 90% yellow plus 10% red, 95% yellow plus 5% green, etc.) and C different concentrations (relative to the matrix material, e.g., by weight) for each phosphor mixture, for a total of T=N*M*C types of phosphor chip. It is not required that all phosphor mixtures be available at all concentrations or at all thicknesses, as long a number of different phosphor-chip types (with different effects on light) are available.
LED dies may be binned according to their spectral characteristics. For example, a given blue LED die produces light over a relatively narrow region, e.g., about 25 nm FWHM, with a peak intensity at a particular wavelength. Due to process variations, the peak wavelength for different LED dies from the same process (e.g., from different parts of a wafer) can vary by 20 nm or more. In some embodiments, the LED dies are binned according to peak wavelength (also referred to herein simply as “the wavelength” of the die). Thus, for example blue LED dies may be binned at 460 nm, 462.5 nm, 464 nm, etc., up to a longest wavelength of, e.g., 480 nm. A sampling of LED dies from each bin can be tested with each of the available types of phosphor chips (e.g., T different types) to measure the color-space components of the resulting light.
The measured color (column 506) is then compared to the source regions for the desired lamp color bins, e.g., the six regions 412, 414, 416, 418, 420, 422 as shown in
Based on data of the kind shown in
Lookup table 550 can be used to automate the selection and assembly process for an emitter. For example, a computer-controlled robotic assembly system can be programmed to produce emitters for lamps in a given color bin, e.g., bin 404 of
Control system 602 can be a computer system that controls operations of apparatus 600.
Processor 702 can include any type of microprocessor, microcontroller, or application-specific integrated circuit capable of performing control operations as described herein in response to input.
Memory 704 can include any computer-readable storage media, including magnetic disk, optical disk, SRAM, DRAM, flash memory, etc. Memory 704 can be used to store program code for execution by processor 702 as well as data such as emitter specifications 710 and look-up table 712 (which can be an implementation of lookup table 550 of
User interface 706 can include standard interface components, such as a keyboard, mouse, track ball, track pad, touch pad, display screen, printer, etc., along with associated software executed by processor 702 to control and communicate with the interface components. Via user interface 706, a user can communicate with control system 602 to control operation thereof. For example, the user can control starting and stopping of an assembly process, for example selecting emitter configurations and source regions in color space for the LED groups; examples are described below.
Pick-and-place controller 708 can include a combination of hardware and/or software components that provide a control interface between processor 702 and pick-and-place system 608 of
Referring again to
Component tray 606 holds components, including LED dies and/or phosphor chips, that are available for placement onto emitter substrates. In some embodiments, component tray 606 can be segmented into sections 612, with each section holding a specific type of component. Thus, for example, section 612a may hold LED dies in the 460-nm (peak) wavelength bin, section 612b may hold LED dies in the 462.5-nm bin; section 612e may hold phosphor chips of a first type, section 612f may hold phosphor chips of a second type, and so on. The number of sections 612 may be considerably larger than shown, enough to accommodate the total number of distinct die wavelength bins and phosphor-chip types. In some embodiments, the LED dies and phosphor chips may be held in separate trays.
Pick-and-place system 608 can incorporate a robotic assembly that includes a movable arm 614. Conventional robotic systems can be used. Movable arm 614 is advantageously capable of selecting a component from one of the sections 612 of tray 606 and placing that component in a desired location (e.g., one of positions 616) on emitter substrate 610. Operation of movable arm 614 is advantageously directed by control system 602 based on lookup table 712, emitter specifications 710, and instructions from the user as to the desired color bin of the lamp(s) in which the emitters will be included.
It will be appreciated that the assembly apparatus described herein is illustrative and that variations and modifications are possible. Additional automated components can be provided. For example, an automated robotic assembly can place an emitter substrate 610 on stage 604 for LED die and phosphor chip attachment and remove substrate 610 after LED die and phosphor chip attachment are completed. Wire-bonding apparatus or the like can be provided to establish electrical connections between the LEDs and emitter substrate 610. Additional apparatus can apply adhesive to the top surfaces of the LED dies prior to placement of the phosphor chips thereon. In some embodiments, the emitter assembly process can be fully automated, facilitating high-volume production of emitters for a single color bin. Further, emitter assembly apparatus 600 or the like can be implemented as part of a larger production line for lamps, and the production line can be fully or partially automated as desired. In some embodiments, control system 602 may provide additional functionality, such as tracking inventory based on parts selected and used; notifying a user of low inventory, mechanical problems, or other error conditions; and so on.
At block 802, input is received indicating a desired color bin. This input can refer to the color of light to be produced by a lamp in which the emitter is intended ultimately to be included. For example, with reference to
At block 804, a source region for each LED group is determined. As described above, for a given color bin and emitter configuration (in particular the number of independently-addressable groups of LEDs), a source region in color space for each group can be determined. Thus, for example, if bin 404 in
In some embodiments, emitter specifications 710 (
At block 806, an LED-die wavelength and phosphor-chip type for each location on the emitter can be selected, e.g., using lookup table 712 (
In some cases, multiple combinations of LED-die wavelength and phosphor-chip type may produce light in the desired source region (as is the case for region WW-2 in
At block 808, pick-and-place system 608 is controlled to assemble the LED dies and phosphor chips onto the emitter substrate. For example, pick-and-place controller 708 of
In other embodiments, selection of LED-die wavelengths and phosphor-chip types can be separated into different process stages.
At block 902, input is received indicating a desired color bin, and at block 904, a source region for each independently-addressable LED group in the emitter is determined based on the desired color bin. These blocks can be similar or identical to blocks 802 and 804 of process 800 described above.
At block 906, an LED die is selected for each location on the emitter based on the source regions. In this embodiment, a lookup table may be provided that specifies an LED-die wavelength to use for each source region. In some instances, multiple LED-die wavelengths may be associated with a single source region, and selection rules (e.g., as described above) can be used to determine which LED-die wavelength(s) will be used in a given instance.
At block 908 the selected LED dies are placed on the emitter substrate. Block 908 may include establishing electrical connections to the emitter substrate, e.g., via wire-bonding.
At block 910, a phosphor-chip type for each LED die is determined based the known source region and LED-die wavelength; a lookup table can be used. As long as the LED-die wavelength for each LED die on the emitter is known, an appropriate phosphor-chip type can be selected based on the desired source region.
At block 912, pick-and-place system 608 is controlled to place a phosphor chips of appropriate type onto each LED die. As noted above, this block can include applying adhesives or otherwise securing a selected phosphor chip to each LED die.
It will be appreciated that the assembly process described herein is illustrative and that variations and modifications are possible. Steps described as sequential may be executed in parallel, order of steps may be varied, and steps may be modified, combined, added or omitted.
In some embodiments, phosphor chips can be attached to the LED dies before the LED dies are attached to the emitter. Thus, for example, each section 612 of component tray 606 may contain LED/phosphor subassemblies (i.e., LED dies with phosphor chips already attached); each section can contain LED dies of a particular wavelength with attached phosphor chips of a particular type. An appropriate selection can be made using a lookup table similar to table 550 of
In the foregoing description, reference is made to embodiments with two groups of LEDs (warm white and cool white), but those skilled in the art with access to the present disclosure will appreciate that the invention is not limited to this specific case. As described in above-referenced U.S. patent application Ser. No. 13/106,808, tuning can be performed for emitters with more than two independently-addressable groups of LEDs. Where there are more than two groups, each group can be associated with a source region for purposes of selecting LED dies and phosphor chips as described above. For reliable tuning behavior, the source regions associated with different groups advantageously do not overlap, and the lamp color bin is intermediate in color space between the source regions.
In some embodiments, the groups may include two or more different white groups plus one or more non-white (e.g., red or green) groups. By way of illustration,
Red LED 1104m and green LEDs 1104n-q can be conventional, single-color LEDs. As is known in the art, red and green LEDs can be produced with relatively small dispersion in color space. Accordingly, selection of phosphors to produce particular red and green colors need not be done for these groups. Selection of LEDs and phosphors, e.g., using techniques described above, is advantageously performed to control the color-space regions occupied by cool white LEDs 1104a-f and warm white LEDs 1104g-l.
In other embodiments, the independently addressable groups can include more than two groups of white LEDs occupying different source regions in color space, e.g., two different cool white groups and one warm white group.
In this example, the two cool white groups can be chosen to lie on opposite sides of the blackbody curve.
With these source region selections, it is possible to tune to any point within triangle 1308. For example, point 1310 on blackbody curve 1300 can be the lamp color bin. The source regions can be chosen such that the desired tuning point is where the color contributions from different groups are approximately balanced; this will generally produce the brightest light possible for a particular lamp but is not required.
As described above, selection of the individual phosphor chips and LED dies for an emitter can improve control over the source regions in color space in which the LEDs produce light. This in turn reduces the size of the color bin to which a lamp containing the emitter is tuned, allowing greater uniformity in the color of light from one lamp to the next.
In addition, active selection of individual phosphor chips and LED dies can reduce manufacturing costs as compared to standard techniques where the same phosphor mixture, thickness, and concentration is used for all LEDs. For example, given that blue LED dies in particular can vary significantly in wavelength (due to the manufacturing process), the ability to adjust an LED to a desired color by selecting a suitable phosphor on a per-die basis allows a larger fraction of manufactured LED dies to be used; this in turn can reduce production costs to the extent that fewer LED dies are discarded as unusable.
The use of multiple source regions in a single lamp allows source regions to be larger (in color space) than the single color bins to which the lamps are ultimately tuned. This can also help to expand the fraction of LED dies that can be used for further savings in production cost. For typical blue LED production processes, sufficient color control to allow all LED wavelengths to “hit” at least one useful source region can be achieved with a manageable number of phosphor-chip types, e.g., a dozen or so.
The phosphor chips used can be manufactured using existing processes that provide sufficient control over thickness and phosphor density to provide a predictable response for phosphor chips of a given type. Thus, the emitter manufacturing process need not include a color-test of the light. Instead, once it has been determined that a particular LED-die wavelength and phosphor-chip type produce a given light color, that knowledge can be relied on during manufacture. Quality-control testing of emitters can accordingly be limited to merely testing that the LEDs turn on when power is supplied. (Color testing may still be performed at a later stage, e.g., as part of color tuning of the lamp.)
Phosphor chips are also well suited for use with existing pick-and-place systems, thus allowing the manufacturing process to be highly automated. Further, the use of discrete phosphor chips is results in highly controllable application of a desired quantity and concentration of phosphors on each LED die so that light of a desired color can reliably be produced. The deliberate variation between different phosphor-chip types can advantageously be used to control for the variation in LED-die wavelength that occurs as a result of LED die manufacturing processes; with a suitable range of phosphor-chip options many LED dies with different wavelengths can be tuned to a desired region in color space.
While the invention has been described with respect to specific embodiments, one skilled in the art will recognize that numerous modifications are possible. For example, the invention is not limited to a particular lamp geometry or form factor or as to the number and type of LEDs. The number of groups of LEDs, number of LEDs in any group, and/or the color of a group can be varied. The LEDs can be arranged as desired; the examples shown in
In general, an emitter for a tunable lamp will include at least two groups of LEDs, with each group producing light in a different source region in color space. The size of the source regions can be adjusted to provide a desired degree of control over the color bin of the resulting lamp. The source regions associated with different groups are advantageously chosen such that the regions do not overlap and the desired (tuned) color for light produced by the lamp is intermediate between the source regions. In some embodiments, the source regions are selected such that tuning to the desired color bin is achieved when nearly equal currents are supplied to the various groups, as this tends to result in maximizing brightness of the lamp.
Embodiments described above use peak wavelength to characterize the light produced by an LED die. Other embodiments may use other parameters, such as a combination of peak wavelength and a spectral width parameter (e.g., full width at half maximum intensity (FWHM)). Any spectral characteristic or combination of spectral characteristics can be used to sort the LED dies into bins, and the LED dies may be binned coarsely or finely as desired. Embodiments are not limited to any particular number of LED-die bins or phosphor-chip types.
Although the present description specifically refers to combinations of blue LED dies with phosphor chips containing predominantly yellow phosphors to produce white light, those skilled in the art will appreciate that other color combinations may be used. Phosphor chips can be made of various materials, with any mixture and concentration of phosphorescent material, and used together with a variety of LED dies of different wavelengths or colors.
Thus, although the invention has been described with respect to specific embodiments, it will be appreciated that the invention is intended to cover all modifications and equivalents within the scope of the following claims.
