Patent Publication Number: US-7916296-B2

Title: Method and apparatus for adjusting a color point of a light source

Description:
This application is a national stage application under 35, U.S.C. §371, of international Application No. PCT/IB2007/052194, filed on Jun. 11, 2007, and published in the English language on Dec. 27, 2007, as international Publication No. WO/2007/148254, which claims priority to European Application No. 06115800.2, filed on Jun. 21, 2006, incorporated herein by reference. 
     TECHNICAL FIELD 
     The present patent application relates in general to adjusting a color point of a lamp. 
     BACKGROUND 
     Solid-state lighting (SSL) devices, such as light-emitting diodes (LEDs), are used in current lighting systems. These LEDs provide light at a certain wavelength. in order to operate the lamp with different colors, differently colored LEDs are assembled within one single lamp. The LEDs, when operated, provide the primary colors, which are superposed to create the output color of the lamp. 
     The LEDs may define an area within the color space indicating the color that can be realized by the lamp through linear combinations of the fluxes of the LEDs. Adjusting the flux of each LED so as to contribute to the overall color of the lamp is known in the art for lamps comprising three LEDs. It is known that there is an exact relation between a target color point, for example defined by tristimulus values (X t , Y t , Z t ), and the average flux  Φ  of three primary colors. It is also known that the average flux  Φ  can be defined by the duration a LED is operated. For example, in pulse width modulation (PWM) based systems, the average flux  Φ   i , of the i th , color can be given by  Φ i   ={circumflex over (Φ)} i p i , with p i , representing the duty cycle (duration of operation in one period divided by total length of period) of the i th , primary color and {circumflex over (Φ)} i , the peak value of the flux. Within a three-color system, i.e. lamps with three different primary colors, the duty cycle for each of the primary colors required for obtaining one given color point can thus be calculated. 
     However, it is not possible in the art to calculate the contribution of a primary color to obtain a target color point if more than three primary colors are used in a lamp. 
     Therefore, it is a first object of the present patent application to provide an easy color point calculation. Another object of the present patent application is to provide a lamp, which emits light with a high color-rendering index. Another object of the present patent application is to provide an easy implementation of color point calculation. Another object of the present patent application is to provide a color point calculation using pulse width modulated systems. 
     SUMMARY 
     These and other objects of the patent application are achieved by a method of adjusting a color point of a lamp, which method comprises: defining a target color point, operating N different light sources within the lamp, which light sources emit light of different wavelengths, and adjusting the contributions of M out of the N light sources to the overall light to be emitted by the lamp so as to meet said target color point, wherein said adjusting comprises calculating 
                   (         N           M         )           
permutations of combinations of light sources, calculating for at least two of the calculated permutation the contributions of the light sources to the overall light emitted by the lamp to meet the color point, adding up the calculated contributions of each light source from at least two of the calculated permutations into an overall contribution of each light source separately, and operating the lamp with the resulting overall contributions of all light sources.
 
     It has been found that a target color point can be met with lamps having a plurality of different light sources by adding the contributions of the light sources for at least two permutations to an overall contribution, and operating the light sources according to their own overall contributions. It is particularly useful to define the target color point in the CIE xyz color space, also known as the CIE 1931, color space. According to this definition, the human eye has receptors for different wavelengths, for example blue, green, and red. These three parameters can thus be sufficient in principle to describe a color sensation. To describe a color point that gives a good color sensation to a human eye, tristimulus values, denoted X, Y, and Z values, may be used, which describe a triplet of red, green, and blue, respectively. It has been found that colors perceptible to human eye are to be found within the color triangle defined by the X, Y, Z values. 
     Even though a target color point may be met by adding the contributions of one possible combination of light sources, it has further been found that the color appearance of such a lamp may have a low color-rendering index. The color-rendering index may be used as a quality distinction between light sources emitting light of the same color. A reference source, such as a black body radiator, may be defined as having a color-rendering index of 100. The test source with the same color temperature is compared with this, and the perceived colors under the reference source and under the test illumination, i.e. measured in the CIE color space, may be compared using a standard formula. Therefore, the present patent application provides a calculation of possible permutations of combinations of light sources to meet the target color point. With N different light sources, and using M of these to meet a color point, it will be possible to calculate 
               (         N           M         )     ,         
preferably
 
                   (         N           3         )           
permutations of combinations of light sources. Calculating permutations of combinations may comprise calculating
 
               (         N           3         )     =       N   !           (     N   -   3     )     !     ⁢     3   !               
permutations.
 
     For each of these permutations, the contributions of M of the N different light sources to meet the target color point can be calculated. Then, after having calculated the contribution of each of the N light sources for the permutations, the contributions of all light sources from the different permutations are summed to obtain an overall contribution. This overall contribution accounts for all contributions of a light source within the different permutations. Having obtained the overall contributions of each of the light sources separately, the lamp may be operated by operating each of the light sources with its overall contribution. This leads to an emission of light meeting the target color point and having a high color-rendering index. 
     The method according to the invention provides a color point calculation for lamps with N being more than 2, preferably 3. light sources, in particular solid-state light sources, such as LEDs. The calculation of the contribution of each of the light sources is easy and fast. The algorithm allows easy implementation. 
     It has been found that a lamp with N=4, different light sources provides good color rendering index results. Preferably, N=5, light sources provide an even better color rendering index. For example, a light source having the colors red, green, blue, amber, and yellow may be used. 
     The calculation of the contributions is preferred with M, being equal to or greater than 3, out of N light sources contributing to the overall light within one permutation. 
     The contribution of a light source to the overall light may be understood as the flux of each light source. Adding the fluxes of the different light sources of each of the permutations may result in meeting the target color point. 
     in particular in pulse width modulated systems, the contribution of each light source may be understood as the duration of activation of the light source within a time frame. It is particularly preferred to calculate a pulse width duty cycle for each light source within each of the permutations to meet the target color point. 
     As was indicated above, the color triangle of the tristimulus values defines the area of target color points which are perceptible to humans. Therefore, it is preferred to consider the visibility of each of the permutations, i.e. whether the superposed contributions of M light sources found within a permutation are within the color triangle, and thus can be seen by viewers. In this respect, it is preferred that the calculated duty cycle of each of the light sources is between 0, and 1. 
     After a check as to which solutions are capable of providing a target color point which is within the color triangle and which requires duty cycles in the range between 0, and 1, a number of possible solutions m is obtained. With this number m a virtual pulse width modulation cycle 
               T   i     =     T   m           
can be assumed, where T represents a period of the PWM system. The contributions of each of the light sources may be scaled to this virtual pulse width modulated cycle T i . The fixed, identical time frame for each permutation may thus be T i , and the duty-cycle for each of the light sources may be p i . This may result in a weighted superposition of the overall contributions of the light source being calculated to
 
     
       
         
           
             
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     To obtain a good color-rendering index, the lamp may then be operated with the overall contributions of all of the light sources within each of the fixed pulses of the pulse width modulation cycles T. The color rendering may also be improved in that a pulse width duty cycle is calculated for each light source within a non-identical time frame for each permutation. 
     Another aspect of the present patent application is a device for operating a lamp, comprising a target color point definition unit, adjusting means arranged for adjusting the contribution of M out of N different light sources to the overall light emitted by the lamp so as to meet the target color point, permutation means arranged for calculating 
                   (         N           M         )           
permutations of combinations of light sources, calculation means arranged for calculating for at least two of the calculated permutation the contributions of the light sources to the overall light emitted from the lamp to meet the target color point, and adding means arranged for adding the calculated contributions of each light source from at least two of the calculated permutations into an overall contribution of each light source separately.
 
     A further aspect of the present patent application is system comprising a lamp with N different light sources emitting light of different wavelengths, and a device of claim  12 . 
     These and other aspects of the present patent application will be apparent from and elucidated with reference to the following Figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       in the Figures. 
         FIG. 1  is a schematic view of a system according to embodiments; 
         FIG. 2  is a flowchart of a method according to embodiments; 
         FIG. 3  is a Table illustrating permutations according to embodiments; 
         FIG. 4   a , is a chart illustrating the duty cycles of light sources according to permutations; 
         FIG. 4   b , is a chart illustrating the cumulated duty-cycles. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a system  100  arranged for operating a lamp  116  according to embodiments of the present invention. 
     The system  100  comprises a device  102  for adjusting a color point of the lamp  116 . The device  102  is comprised of a target color point definition unit  104 , and an adjusting unit  106 . The adjusting unit  106  comprises a permutation unit  108 , a calculation unit  110 , and an adding unit  112 , as well as a control unit  114 . The units  104 - 114  can cooperate with each other. 
     The control unit  114  can be arranged to control light sources  118   a - e , in the lamp  116 . The light sources  118   a - e , may be solid-state light sources, such as LEDs. The light sources  118  are located within a housing of the lamp  116  and emit light through an optical system  120 , such that the light emitted from the light sources  118   a - e  is superposed to provide an overall lighting impression of the lamp  116 . 
     The operation of the system  100  will be described in more detail with reference to  FIG. 2 . 
       FIG. 2  illustrates a method  200  for operating the system  100  according to embodiments. In a first step  202 , a color point is defined within the color point definition unit  104 . This color point is the color which is desired for the lamp  116  and may be defined by its tristimulus values X, Y, Z. Other color point definition methods may also be within the scope of this patent application. 
     The defined color point is forwarded to adjusting unit  106 . Permutation unit  108  calculates permutations in step  204 . For example, as illustrated, five different light sources  118   a - e , are provided in lamp  116 , and the color point is defined by three tristimulus values. In this case, the permutation unit  108  may calculate 
                   (         5           3         )           
permutations. This results in 10, possible triplets of light sources, as is illustrated in  FIG. 3 .
 
     As can be seen in  FIG. 3 , ten permutation T 1 -T 10 , are provided, each of which comprises the contributions D A , D B , D C , D D , D E . of the light sources  118   a - e . For meeting the target color point, the average flux of each of the light sources  118   a - e for each permutation can be calculated in step  206 . In PWM based systems, for example, the average flux may be a function of the duty cycle of the light sources  118   a - e within a certain time frame. For each permutation T 1 -T 10 , the duty cycles D of the three primary colors involved can be calculated in step  206 . 
     The permutation unit  118  transfers the possible permutations to calculation unit  110 . Calculation unit  110  calculates the contributions of each light source  118   a - e , involved within a permutation T 1 -T 10 , in step  206 . Calculation unit  110  further checks in step  208  whether the calculated duty cycle of the respective light source  118   a - e , lies between 0, and 1. Only in this case can the target color point be met by operating the system  100  in a pulse width modulation with a fixed time frame. 
     In addition, calculation unit  110  checks whether the calculated duty cycles for each permutation cause the target color point to be emitted within the allowed color triangle defined by the tristimulus values in step  208 . Only those permutations which meet both of the above requirements are considered for further processing. 
       FIG. 4   a , illustrates an example of a different duty cycle for different permutations. As can be seen, calculation unit  110  has calculated for permutation T 1 , the duty-cycles D A1 , D B1 , and D C1 , in a first virtual pulse width modulation cycle T/m. In a second cycle T/m, the contributions D A4 , D C4 , and D D4 . of permutation T 4 . are calculated, and in a third cycle, the contributions D A6 , D D6 , D E6 . of permutation T 6 . In the illustrated example, the calculation unit  110  has calculated in step  208  that only the permutations T 1 , T 4 , and T 6 . provided a feasible target color point with feasible duty-cycles. Only these three permutations T 1 , T 4 , T 6 , are accordingly considered for creating the target color point. These m (equal to three) permutations account for a virtual pulse width modulation cycle T/3, of the overall pulse width modulation cycle T. 
     The calculated permutations and their duty cycles are further processed in adding unit  112  in step  210 . Adding unit  112  sums the duty-cycle D A1 , D A4 , D A6 . for light source  118   a . For light source  118   b , the overall contribution is D B1 . For light source  118   c , the overall contribution is D C1 +D C4 . For light source  118   d , the overall contribution is D D4+D   D6 . For light source  118   e , the overall contribution is D E6 . The overall contribution can be calculated by 
     
       
         
           
             
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     With p i . the overall contribution of a light source  118   i , and m the number of feasible permutations. As illustrated in  FIG. 4   b , the overall contributions can be summed into one overall duty cycle. 
     It may also be possible to use a weighting value w k , to weight the calculate the contributions for the colors (0&lt;=, p ik &lt;=1) and to add the weighted contributions. This would result in: 
     
       
         
           
             
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     The color rendering index may be improved by means of this weighting. 
     These summed duty cycles are transferred to control unit  114  in step  212 . Control unit  114  operates the light sources  118   a - e , in accordance with the respective summed contributions within one pulse width modulation cycle T. It is preferred that the overall contributions account for the operation time of a light source  118  within a cycle T. 
     It should be noted that the contribution p i , may be used to calculate a constant current that may be applied to the light sources  118   a - e . The flux-current characteristics render it possible to obtain a current value for driving the light sources  118   a - e . It is not necessary to drive the light sources  118   a - e , with pulse width modulation, a constant drive with the current value may also be possible. It is also possible to supply the light sources with the calculated contributions with a current which is varied around the required average flux. 
     The light output through optical system  120  of lamp  116  meets the target color point and has a high color-rendering index. The calculation of the contributions of more than three light sources  118  is easy to implement, and the method renders it possible to calculate the target color points with more than three primary light sources. This may be done using a pulse width modulated system. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. 
     Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.