Abstract:
An adjustable adjustable color illumination source comprises: a first color channel including at least first and second sub-channels independently selectively switchable on or off to generate illumination of a first color with at least three different selectable intensity levels not including zero intensity; a second color channel including at least first and second sub-channels independently selectively switchable on or off to generate illumination of a second color with at least three different selectable intensity levels not including zero intensity; a third color channel including at least first and second sub-channels independently selectively switchable on or off to generate illumination of a third color with at least three different selectable intensity levels not including zero intensity; the first, second, and third color channels arranged such that the illumination of the first, second, and third colors combine to generate a source illumination; and a controller communicating with the first, second, and third color channels to selectively switch on or off the sub-channels of the first, second, and third color channels to adjust the source illumination to a selected one of at least sixty four different colors. light source comprises a light source having input channels for generating illumination of different channel colors, and an electrical power supply selectively energizing the input channels in a time division multiplexed fashion to generate a illumination of a selected color.

Description:
BACKGROUND 
       [0001]    The following relates to the illumination arts, lighting arts, and related arts. 
         [0002]    In solid state lighting devices including a plurality of LEDs of different colors, control of both intensity and color is typically achieved using pulse width modulation (PWM). For example, Chliwnyj et al., U.S. Pat. No. 5,924,784 discloses independent microprocessor-based PWM control of two or more different light emitting diode sources of different colors to generate light simulating a flame. Such PWM control is well known, and indeed commercial PWM controllers have long been available specifically for driving LEDs. See, e.g., Motorola Semiconductor Technical Data Sheet for MC68HCO5D9 8-bit microcomputer with PWM outputs and LED drive (Motorola Ltd., 1990). In PWM, a train of pulses is applied at a fixed frequency, and the pulse width is modulated to control the time-integrated power applied to the light emitting diode. Accordingly, the time-integrated applied power is directly proportional to the pulse width, which can range between 0% duty cycle (no power applied) to 100% duty cycle (power applied for the entire time interval). 
         [0003]    Existing PWM illumination control has certain disadvantages. For a typical red/green/blue type system. Full color PWM control entails providing three independent power supplies, one for each of the red, green, and blue channels, each of which must be a high-speed switching power supply capable of operating at switching speeds corresponding to the pulse frequency. The pulse frequency must be faster than the flicker fusion threshold, which the frequency above which flickering caused by the light color switching becomes substantially visually imperceptible. This frequency is preferably of order about 30 Hz or higher. The power supply for each color channel must also include high-precision control of the pulse width. These complex characteristics of PWM controllers increase manufacturing cost. 
         [0004]    The fundamental or harmonic frequency components entailed in performing PWM control also have the potential to generate radio frequency interference (RFI), which can be problematic in residential and commercial environments. 
         [0005]    Another concern with PWM illumination control is that the pulsating operation of the LEDs may have the potential to shorten LED operational lifetime. 
         [0006]    PWM has become a common approach for adjustable color control of illumination sources including red, green, and blue channels (or other sets of channels providing time-averaged illumination of a selected color or other characteristics). However, other approaches have also been used, typically employing variant pulse modulation schemes. For example, in pulse frequency modulation, pulses of a fixed width are used, with the frequency of pulse repetition varied to achieve adjustable color control. These variant pulse modulation schemes typically exhibit some of the disadvantages of PWM, such as complex and costly high speed switchable power supplies, possible RFI generation, and possibly adverse impact of continuous high-speed switching on LED operational lifetime. 
       BRIEF SUMMARY 
       [0007]    The illustrative claims appended at the end provide a non-exhaustive summary of some disclosed embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The invention may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. 
           [0009]      FIG. 1  diagrammatically illustrates an illumination system. 
           [0010]      FIG. 2  diagrammatically shows a look-up table for determining switch settings for different colors at a selected constant intensity level. 
           [0011]      FIG. 3  diagrammatically illustrates the red power supply of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0012]    With reference to  FIG. 1 , a solid state lighting system includes an illumination source  10  having a plurality of red, green, and blue light emitting diodes (LEDs). The red LEDs include small red LEDs  141 , medium sized red LEDs R 2 , and large red LEDs R 3 . The green LEDs include small green LEDs G 1 , medium sized green LEDs G 2 , and large green LEDs G 3 . The blue LEDs include small blue LEDs B 1 , medium sized blue LEDs B 2 , and large blue LEDs B 3 . In some instances, the plural sets of red LEDs are referred to as a red channel, and each set of small, medium, and large red LEDs R 1 , R 2 , R 3  is referred to as a sub-channel of the red channel, with analogous phraseology for green and blue channels and sub-channels. 
         [0013]    The various types of LEDs R 1 , R 2 , R 3 , G 1 , G 2 , G 3 , B 1 , B 2 , B 3  across a light-emitting surface or area  10 . In the illustrated embodiment, the red LEDs are grouped into LED groups each including one small red LED R 1 , one medium red LED R 2 , and one large red LED R 3 . Similarly, the green LEDs are grouped into LED groups each including one small green LED G 1 , one medium green LED G 2 , and one large green LED G 3 ; and the blue LEDs are grouped into LED groups each including one small blue LED B 1 , one medium blue LED B 2 , and one large blue LED B 3 . However, this arrangement is optional, and other arrangements can be used for distributing the various types of LEDs R 1 , R 2 , R 3 , G 1 , G 2 , G 3 , B 1 , B 2 , B 3  across the light-emitting surface or area  10 . 
         [0014]    The small red LEDs R 1  are electrically interconnected (circuitry not shown) such that a drive electrical current I R1  can be flowed through the small red LEDs R 1 . In one approach, all small red LEDs R 1  are suitably connected in electrical series such that the drive electrical current I R1  can be flowed through the series. In another approach, sub-groups of N small red LEDs can be connected in parallel and the sub-groups connected in series such that an input drive current of magnitude N times I R1  input to the series causes the current I R1  to flow through the individual small red LEDs R 1 . This latter arrangement, referred to herein as a series-parallel arrangement with a parallel factor N, enhances robustness against an open-circuit or other high-resistance failure of one of the small red LEDs. 
         [0015]    In analogous fashion, the medium red LEDs R 2  are electrically interconnected such that a drive electrical current I R2  can be flowed through the medium red LEDs R 2 . The large red LEDs R 3  are electrically interconnected such that a drive electrical current I R3  can be flowed through the large red LEDs R 2 . The small green LEDs G 1  are electrically interconnected such that a drive electrical current I G1  can be flowed through the small green LEDs G 1 . The medium green LEDs G 2  are electrically interconnected such that a drive electrical current I G2  can be flowed through the medium green LEDs G 2 . The large green LEDs G 3  are electrically interconnected such that a drive electrical current I G3  can be flowed through the large green LEDs G 3 . The small blue LEDs B 1  are electrically interconnected such that a drive electrical current I B1  can be flowed through the small blue LEDs B 1 . The medium blue LEDs B 2  are electrically interconnected such that a drive electrical current I B2  can be flowed through the medium blue LEDs B 2 . The large blue LEDs B 3  are electrically interconnected such that a drive electrical current I B3  can be flowed through the large blue LEDs B 3 . 
         [0016]    An adjustable color controller includes red, green, and blue power supplies  12 ,  14 ,  16 . The red power supply  12  includes a small red LED driver switch  20  that switches on or off a constant root mean square (rms) current I R1S  that is input to the small red LEDs R 1 . If the small red LEDs R 1  are interconnected in series, then the constant rms current I R1S  is suitably equal to the drive electrical current I R1  to be flowed through the small red LEDs R 1 . On the other hand, if the small red LEDs R 1  are interconnected in a series-parallel configuration with parallel factor N, then the constant rms current I R1S  is suitably equal to N times the drive electrical current I R1  to be flowed through the small red LEDs R 1 , that is, I R1S =N×I R1 . 
         [0017]    Thus, when the small red LED driver switch  20  is off, there is no drive current flowing through the small red LEDs R 1  and they do not emit light. When the small red LED driver switch  20  is on, the drive current I R1  flows through the small red LEDs R 1  and they do emit light. 
         [0018]    In similar fashion, the red power supply  12  includes a medium red LED driver switch  22  that switches on or off a constant rms current I R2S  that is input to the medium red LEDs R 2 . For a purely serial interconnection of the medium red LEDs R 2 , I R2S =I R2 ; whereas, for a series-parallel interconnection of parallel factor N the current I R2S =N×I R2 . Again, by switching the medium red LED driver switch  22  the medium red LEDs R 2  can be turned on or off Still further, the red power supply  12  includes a large red LED driver switch  24  that switches on or off a constant rms current I R3S  that is input to the large red LEDs R 3 . For a purely serial interconnection of the large red LEDs R 3 , I R3S =I R3 ; whereas, for a series-parallel interconnection of parallel factor N the current I R3S =N×I R3 . Again, by switching the large red LED driver switch  24  the large red LEDs R 3  can be turned on or off 
         [0019]    The green power supply  14  includes a small green LED driver switch  30  that switches on or off a constant rms current I G1S  that is input to the small green LEDs G 1 . If the small green LEDs G 1  are interconnected in series, then the constant rms current I G1S  is suitably equal to the drive electrical current I G1  to be flowed through the small green LEDs G 1 . On the other hand, if the small green LEDs G 1  are interconnected in a series-parallel configuration with parallel factor N, then the constant rms current I G1S  is suitably equal to N times the drive electrical current I G1  to be flowed through the small green LEDs G 1 , that is, I G1S =N×I G1 . The green power supply  14  also includes a medium green LED driver switch  32  that switches on or off a constant rms current I G2S  that is input to the medium green LEDs G 2 . If the medium green LEDs G 2  are interconnected in series, then the constant rms current I G2S  is suitably equal to the drive electrical current I G2  to be flowed through the medium green LEDs G 2 . On the other hand, if the medium green LEDs G 2  are interconnected in a series-parallel configuration with parallel factor N, then the constant rms current I G2S  is suitably equal to N times the drive electrical current I G2  to be flowed through the medium green LEDs G 2 , that is, I G2S =N×I G2 . The green power supply  14  also includes a large green LED driver switch  34  that switches on or off a constant rms current I G3S  that is input to the large green LEDs G 3 . If the large green LEDs G 3  are interconnected in series, then the constant rms current I G3S  is suitably equal to the drive electrical current I G3  to be flowed through the large green LEDs G 3 . On the other hand, if the large green LEDs G 3  are interconnected in a series-parallel configuration with parallel factor N, then the constant rms current I G3S  is suitably equal to N times the drive electrical current I G3  to be flowed through the large green LEDs G 3 , that is, I G3S =N×I G3 . 
         [0020]    The blue power supply  16  includes a small blue LED driver switch  40  that switches on or off a constant rms current I B1S  that is input to the small blue LEDs B 1 . If the small blue LEDs B 1  are interconnected in series, then the constant rms current I B1S  is suitably equal to the drive electrical current I B1  to be flowed through the small blue LEDs B 1 . On the other hand, if the small blue LEDs B 1  are interconnected in a series-parallel configuration with parallel factor N, then the constant rms current I B1S  is suitably equal to N times the drive electrical current I B1  to be flowed through the small blue LEDs B 1 , that is, I B1S =N×I B1 . The blue power supply  14  also includes a medium blue LED driver switch  42  that switches on or off a constant Has current I B2S  that is input to the medium blue LEDs B 2 . If the medium blue LEDs B 2  are interconnected in series, then the constant rms current I B2S  is suitably equal to the drive electrical current I B2  to be flowed through the medium blue LEDs B 2 . On the other hand, if the medium blue LEDs B 2  are interconnected in a series-parallel configuration with parallel factor N, then the constant rms current I B2S  is suitably equal to N times the drive electrical current I B2  to be flowed through the medium blue LEDs B 2 , that is, I B2S =N×I B2 . The blue power supply  14  also includes a large blue LED driver switch  44  that switches on or off a constant rms current I B3S  that is input to the large blue LEDs B 3 . If the large blue LEDs B 3  are interconnected in series, then the constant rms current I B3S  is suitably equal to the drive electrical current I B3  to be flowed through the large blue LEDs B 3 . On the other hand, if the large blue LEDs B 3  are interconnected in a series-parallel configuration with parallel factor N, then the constant rms current I B3S  is suitably equal to N times the drive electrical current I B3  to be flowed through the large blue LEDs B 3 , that is, I B3S =N×I B3 . 
         [0021]    To understand how the system of  FIG. 1  provides versatile adjustable color control without the complexity of pulse modulation and the corresponding potential for RFI, consider a system in which the red LED currents I R1 , I R2 , I R3  applied to the respective sets of small, medium, and large red LEDs R 1 , R 2 , R 3  provide red light of three corresponding respective optical power levels P 1 , 2×P 1 , and 4×P 1 ; and where similarly the green LED currents I G1 , I G2 , I G3  applied to the respective sets of small, medium, and large green LEDs G 1 , G 2 , G 3  provide green light of the three corresponding respective optical power levels P 1 , 2×P 1 , and 4×P 1 ; and where the blue LED currents I B1 , I B2 , I B3  applied to the respective sets of small, medium, and large blue LEDs B 1 , B 2 , B 3  provide blue light of the three corresponding respective optical power levels P 1 , 2×P 1 , and 4×P 1 . Table 1 shows the power levels attainable for a given color channel (for example, either the red channel, or the green channel, or the blue channel) by illuminating various combinations of the small, medium, and large sets of LEDs of the given color channel. For three color channels, this corresponds to eight possible levels (including zero power, i.e. off; corresponds to seven possible levels without counting zero power). 
         [0000]                                TABLE 1                   Set of medium   Set of large   Total       Set of small LEDs   LEDs   LEDs   Power                   Off   Off   Off   0       On (power = P)   Off   Off   P       Off   On (power = 2 × P)   Off   2P       On (power = P)   On (power = 2 × P)   Off   3P       Off   Off   On (power = 4 × P)   4P       On (power = P)   Off   On (power = 4 × P)   5P       Off   On (power = 2 × P)   On (power = 4 × P)   6P       On (power = P)   On (power = 2 × P)   On (power = 4 × P)   7P                    
For three color channels, this provides 8×8×8=512 possible combinations of color and intensity. Each combination has (i) an illumination color defined by the relative intensity ratios of the three channels and (ii) an illumination intensity defined by the sum of the intensities of the three channels. For example, the total visually perceived optical power can be represented as:
 
         [0000]        P   total   =A   R   P   R   +A   G   P   G   +A   B   P   B    (1), 
         [0000]    where P R , P G , and P R  are the optical power output by the red, green, and blue channels and the constants A R , A G , and A B  adjust for relative visual sensitivity differences between the red, green, and blue colors. The color can be represented as: 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0000]    where each of the coordinates u R , v G , and w B  lie in the range [0,1]. The color representation of Equation (2) can readily be converted to other color coordinate systems using known conversion formulae. The combinations do not provide every achievable color at every achievable intensity, or vice versa. The most color/intensity flexibility is achieved for intermediate intensity levels. For example, assuming A R =A G =A B =1 and each channel power being selectable as per Table 1, there are between 46 and 48 different attainable colors for each of the intermediate intensities P total =9P, P total =10P, P total =11P, and P total =12P. On the other hand, there is only one attainable color for the maximum power level of P total =21P, namely the color (⅓,⅓,⅓); and only three attainable colors for the minimum (non-zero) total power level of P total =P, namely (1,0,0), (0,1,0), and (0,0,1). The available 46-48 colors for power levels in the intermediate range is sufficient for typical adjustable color illumination applications. For example, 46 available colors provides sufficient color resolution to perform smooth transitions from one color to another at a constant intensity level. It is also contemplated to further add a fourth, fifth or more sub-channels to each color channel provide larger numbers of color and intensity combinations. Going the other direction, it is contemplated to include only two different sub-channels of LEDs of a given color, which can provide up to 4 power levels (including zero power; three power levels not including zero power), and if this is done for all three color channels the adjustable color illumination source can provide 4 3 =64 combinations of color and intensity. 
         [0022]    With reference to  FIGS. 1 and 2 , color control is suitably implemented using a lookup table  50  relating the switches  20 ,  22 ,  24 ,  30 ,  32 ,  34 ,  40 ,  42 ,  44  or equivalent information to the desired color and intensity. For example  FIG. 2  shows a lookup table for various colors represented using the (u R ,v G ,w B ) representation of Equation (2), assuming A R =A G =A B =1 and each channel power being selectable as per Table 1, for an intensity level total power P total =10P. The saturation colors of pure red, pure green, or pure blue colors are not attainable for this power level. More saturated colors than those shown in  FIG. 2  are attainable at the cost of a slight change in total power (completely saturated colors are attainable at P total =7P or lower, for example). A high level of color flexibility is obtained at intermediate intensity levels for colors near white. Thus, a constant intensity adjustable color illumination source intended to output white light of various characteristics (e.g., cold white or warm white) is readily implemented. 
         [0023]    With reference to  FIG. 3 , the simplicity of the power supplies  12 ,  14 ,  16  is illustrated by depicting an electrical schematic for one suitable embodiment of the red power supply  12 . (The green and blue power supplies  14 ,  16  can be analogously constructed). The illustrated red power supply  12  employs a constant current source I cc  powering a simple voltage divider formed by resistors R 1 , R 2 , and R 3 . In the described operation, each of the resistors R 1 , R 2 , and R 3  is assumed to have a much lower resistance value than output resistors R cc1 , R cc2 , and R cc3 , and the output resistors R cc1 , R cc2 , and R cc3  are assumed to have much larger impedance than the driven set of LEDs. Under these assumptions, voltages V 1 , V 2 , and V 3  are given by: 
         [0000]        V   1   =I   cc ·( R   1   +R   2   +R   3 )   (3), 
         [0000]        V   2   =I   cc ·( R   2   +R   3 )   (4), 
         [0000]      and 
         [0000]        V   3   =I   cc   ·R   3    (5), 
         [0000]    and the currents I R1S , I R2S , and I R3S  each have substantially constant rms value given by: 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0000]    If the output resistors R cc1 , R cc2 , and R cc3  are variable resistors, then the magnitudes of the currents I R1S , I R2S , and I R3S  can also be adjusted in a continuous fashion in accordance with Equations (6)-(8). For example, such adjustment can be used in the previous example to achieve more saturated colors at total power P total =10P. 
         [0024]    The power supply circuit of  FIG. 3  is an illustrative example. Other circuits can be used to generate the constant rms currents I R1S , I R1S , and I R3S , such as transistor-based power supply circuits, switching power supplies, and so forth. In the case of a switching power supply, the output currents I R1S , I R2S , and I R3S  can be d.c. or substantially d.c. (e.g., perhaps with some ripple) and the high frequency components of the power supply disposed in a shielded box so that RFI is minimized. Moreover, it is contemplated for the output currents I R1S , I R2S , and I R3S  to have a constant rms level but to be other than d.c. For example, the output currents I R1S , I R1S , and I R3S  can be sinusoidal a.c. currents of constant rms value. As already noted, “constant” rms level is to be broadly construed as allowing some adjustment of the current level, for example by trimming or adjusting the output resistors R cc1 , R cc2 , and R cc3 . 
         [0025]    Heretofore, adjustable color operation of illumination sources including red, green, and blue channels has typically been performed using pulse modulation techniques such as PWM. The skilled artisan may find it surprising that the approach described herein can provide practical adjustable color operation, even up to and including full color operation with white light as an available output, without the concomitant complexity, RFI concerns, and other disadvantages entailed in pulse modulation control techniques. 
         [0026]    One factor enabling the presently disclosed approach is the recognition that an adjustable color illumination source typically does not require the high color resolution that is typically desired for a full-color display. It is further recognized herein that an adjustable color illumination source also does not typically require complete independence of intensity and color. For example, the inability to achieve all color combinations at precisely P total =10P (see  FIG. 2 ) is not problematic for an adjustable color illumination source. 
         [0027]    Heretofore, designers of adjustable color illumination sources have typically constructed illumination systems using substantially the same PWM control as is typically used in full color LED displays. It is recognized herein that an adjustable color illumination device is very different from a full-color display, and accordingly color and intensity control techniques appropriate for a full-color display may be less than optimal for controlling an adjustable color illumination device. By taking a fundamentally different approach that recognizes the less stringent requirements for a typical adjustable color illumination device, substantially less complex and yet operatively satisfactory devices are contemplated and disclosed herein. 
         [0028]    The illumination device or source  10  is an illustrative example; in general the illumination source can be any multi-color illumination source having sets of solid state light sources electrically interconnected to define different color channels. In some embodiments, for example, the red, green, and blue LEDs are arranged as red, green, and blue LED strings. Moreover, the different colors can be other than red, green, and blue, and there can be more or fewer than three different color channels. For example, in some embodiments a blue channel and a yellow channel are provided, which enables generation of various different colors that span a color range less than that of a full-color RGB light source, but including a “whitish” color achievable by suitable blending of the blue and yellow channels. The individual LEDs are diagrammatically shown as black, gray, and white dots in the light source  10  of  FIG. 1 . The LEDs can be semiconductor-based LEDs (optionally including integral phosphor), organic LEDs (sometimes represented in the art by the acronym OLED), semiconductor laser diodes, or so forth. The different sets of LEDs of a given color do not need to have different sizes or different power outputs. For example, the red LED sets can all have the same size and power output, optionally even using the same type of LED chips for each red LED set. As already mentioned, the illustrative example of three sets of LEDs per color channel can be replaced by two, four, or more sets per color channel. Moreover, different color channels can have different numbers of sets of LEDs. Still further, the device need not be a full color device including three primary colors. For example, an adjustable color device intended to achieve white light of adjustable color characteristics (e.g., adjustable color temperature providing varying degrees of warm or cold white, adjustable color rendering, or so forth) may use color channels other than red, green, and blue. For example, red, green, amber, and blue color channels may be provided, with the blue color channel having a substantially lower maximum optical output compared with other color channels. Still further, although series and series-parallel interconnections are described for the sets of LED chips, other interconnection topologies are also contemplated. Likewise, the illustrated switches switches  20 ,  22 ,  24 ,  30 ,  32 ,  34 ,  40 ,  42 ,  44  or are incorporated with the power supplies  12 ,  14 ,  16 , but in other contemplated embodiments the switches may form a separate control unit or be otherwise arranged respective to the power supplies and the illumination device. 
         [0029]    Appended claims follow. These appended claims are representative, and it is to be understood that the invention further encompasses other novel and nonobvious aspects not expressly set forth in these claims.