Patent Publication Number: US-7718942-B2

Title: Illumination and color management system

Description:
This application is related to application Ser. No. 11/565,540, entitled LIGHT SOURCE HAVING MORE THAN THREE LEDs IN WHICH THE COLOR POINTS ARE MAINTAINED USING A THREE CHANNEL COLOR SENSOR, filed on Nov. 30, 2006, which is hereby incorporated by reference for all that is disclosed therein. 
   BACKGROUND 
   In order to generate a wide spectrum of colors using an illumination system, a few different colors are mixed or combined in different ratios. The different colors are monitored and, based on their intensity, are modified to achieve a desired color or chromaticity. This system is referred to herein as an illumination and color management (ICM) system. The ICM system serves to maintain a desired color point stable. 
   A typical illumination system uses three primary colors, such as red, green, and blue to generate desired colors. Three sensors are used to monitor the three primary colors in order to assure that the desired color is generated. In an illumination system, additional parameters can to be monitored in order to achieve better colors. Monitoring these parameters and performing corrections based on the parameters yields better results when more color sources are used to generate the desired color. However, when more color sources are used, more sensors are required to monitor the color sources, which increases the complexity and cost of the illumination system. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of an embodiment of an illumination and color management system. 
       FIG. 2  is a flowchart of an embodiment of using fewer detectors than light sources to set at least one optical parameter. 
   

   DESCRIPTION 
   An embodiment of an illumination and color management (ICM) system  100  is schematically shown in  FIG. 1 . The ICM system  100  includes an LED driver  110  that drives a plurality of LEDs  112 . In the embodiment of the ICM system  100  described herein, the LED driver  110  drives four colors of LEDs  112 . The four colors of LEDs  112  are referred to individually as an amber LED  116 , a red LED  118 , a green LED  120 , and a Blue LED  122 . It is noted that the LED driver  110  is shown driving different colored LEDs, however, the LED driver  110  may drive a plurality of LEDs having the same color. It is also noted that colors other than amber, red, green, and blue may be used with the ICM system  100 . While the system described herein emits light using LEDs  112 , it is to be understood that light emission via means other than LEDs may be used. Therefore, the term LED may refer to light sources other than light emitting diodes. 
   The ICM system  100  includes a plurality of color sensors  130  that monitor certain colors of light emitted by the LEDs  112 . In the embodiment of the ICM system  100  described herein, three color sensors  130  are used and are referred to individually as a red sensor  132 , a green sensor  134 , and a blue sensor  136 . Systems and methods are described herein that enable color point control and control of other parameters using fewer sensors than colors of LEDs or colors of other light emitters. The color point control described herein enables the color rendering index to be maximized. 
   Each of the color sensors  130  includes an amplifier, a detector, and a low pass RC filter or sample circuit, which are sometimes referred to as filters. The amplifiers are referred to individually as reference numerals  140 , 142 , and  144  for the red amplifier, the green amplifier, and the blue amplifier, respectively. In the embodiment described herein, the filters are resistor-capacitor networks, and are referred to individually as the red filter  148 , the green filter  150 , and the blue filter  152 . The resistors are referred to individually as R 1 , R 2 , and R 3  and the capacitors are referred to individually as C 1 , C 2 , and C 3 . In one embodiment, the resistors R 1 , R 2 , and R 3  have values of approximately 68 k ohms and the capacitors C 1 , C 2 , and C 3  have values of approximately 1.0 μF. 
   The color sensors  130  may include LED detectors with filters located thereon so as to receive certain bandwidths of light. The red sensor  132  has a detector  160  that is adapted to receive a bandwidth of light centered around red light. The green sensor  134  has a detector  162  that is adapted to receive a bandwidth of light centered around green light. The blue sensor  136  has a detector  164  that is adapted to receive a bandwidth of light centered around blue light. The sensors detect a spectrum of light and the spectrum of light will be referred to as a single color herein. For example, when the red sensor  132  detects or senses red light, it is to be understood that a spectrum of light centered or including red is detected or sensed. It is noted that colors may overlap. Thus, the red sensor  132  may detect light having blue or green components. The intensity of light received by individual sensors  130  is proportional to a voltage output by the respective sensors  130 . 
   The outputs of the color sensors  130  are connected to the input of an analog to digital converter (ADC)  170 . The ADC  170  outputs a digital representation of the colors sensed by the sensors  130 . In one embodiment, the ADC  170  converts the output of a single sensor to a binary number and repeats this process periodically for the different sensors  130 . For example, the ADC  170  may output a binary number representative of the intensity of the sensed red light. Subsequently, the ADC  170  may output a binary number representative of the sensed green light. This process may continue during operation of the ICM system  100 . 
   A color generator  174  generates binary numbers or the like that are representative of the colors that are supposed to be sensed by the color sensors  130 . For example, if the LED driver  110  is instructed to output a specific color having specific color components, these color components are measured by the color sensors  130  and binary or digital representations of the colors are output by the ADC  170 . 
   The outputs from the ADC  170  and the color generator  174  are compared by a comparator  176 . An error signal is output by the comparator  176 , wherein the error signal is representative of the difference between the output of the ADC  170  and the color generator  174 . Thus, if the magnitude of the error signal exceeds a predefined threshold, the difference between the color emitted by the combination of LEDs  112  and the color that was supposed to be emitted is great. Likewise, if the magnitude of the error signal below a predefined threshold, then the difference between the color emitted by the combination of LEDs  112  and the color that was supposed to be emitted is minimal. 
   The feed back of the ICM  100  described above can be explained with the following example of a system using three LEDs and three detectors. In these embodiments, there is a strict 1:1 map between color output by the LEDs  112  and voltages output by the color sensors  130 . In this example, the color of 4000 degrees Kelvin is desired to be output. There is a CIE x,y coordinate that maps to this specific color temperature and may be represented by 1.2 volts, 1.1 volts and 0.4 volts from the red, green, and blue sensor outputs respectively. No other voltage set can map to this color temperature. The sensors  130  detect the combined color from the LEDs  112 . If that detected color combination is not 4000 degrees Kelvin, the outputs of the sensors  130  will be in error compared to the 1.2, 1.1 and 0.4 volts described above. This generates a set of three error signals, one for red, one for green, and one for blue. A feedback system such as a PID system can be used to minimize the error by manipulating the three pulse width modulation (PWM) signals input to the LED driver  110 . The LED driver  110 , in turn, manipulates the intensity of each primary color output (red, green, blue) of the LEDs  112 . This process continues until the voltages output by the color sensors  130  and the color generator  174  are the same. 
   As briefly described above, the error signal provides feed back for a controller  180  that sends control signals to the LED driver  110 . The embodiment of the controller  180  described herein uses four colors and three sensors and includes a color rendering index (CRI) optimization look up table  182 , and a feedback controller  184 . The controller  180  serves to control the intensity of the different colors of LEDs  112  in order to have the LEDs  112  produce the correct color, while maximizing the color rendering index. In the embodiment provided herein, the intensities of the LEDs  112  are varied by varying the duty cycle of pulse width modulation (PWM) signals transmitted to the LED driver  110 . 
   In operation, the controller  180  transmits signals to the LED driver  110  indicating the intensities of the outputs of the LEDs  112 . As stated above, the intensities may be controlled using the duty cycle of pulse width modulated signals. The LED driver  110  causes the LEDs  112  to emit light based on the signals from the controller  180 . 
   The three color detectors  156  monitor the intensities of the red, green, and blue spectral components of the light emitted by the LEDs  112 . Using the red sensor  132  as an example, the detector  160  receives red light and outputs a voltage proportional to the intensity of red light. The voltage is amplified by the amplifier  140  and is held for a short period by the filter  148 , which allows the voltage to be sampled by the ADC  170 . The same process applies to the green sensor  134  and the blue sensor  136 . It is noted that the light incident on the sensors  130  is pulsing due to the pulse width modulation signals driving the LEDs  112 . Therefore, the outputs from the sensors  130  are pulsing; the purpose of the RC filters is to provide a time average signal to the ADC  170 . 
   The ADC  170  outputs signals are representative of the emitted colors to the comparator  176 . The color generator  174  outputs a signal representative of the desired colors to the comparator  176 . An error signal is generated by the comparator  176  based on the differences between the signals from the ADC  170  and the color generator  174 . This error signal is transmitted to the generator  180 , which modifies the signals to the LED driver  110  in order to have the LEDs  112  emit the correct colors or the correct intensities that combine for the correct color. 
   Having described the ICM system  100 , its operation will now be described. More specifically, the use of three sensors to determine colors using four emitters will be described. It is noted that the following description is for exemplary purposes and that other numbers of sensors and emitters may be used in other embodiments. However, the methods described herein apply to ICM systems wherein there are more emitters than sensors. The following methods described herein may be performed using computer code in a computer readable medium, such as magnetic storage, optical storage, firmware, or other hardware devices. 
   In summary, synthetic sources are created and sampled during a calibration phase. The synthetic sources are combinations of the actual sources. For example, one synthetic source may be a combination of the green LED  120  and the blue LED  122 . It is noted that several synthetic sources may be used herein. Analysis of the combinations are stored in the look up table  182  and are compared to various operating parameters. A specific combination is used based on specific operating parameters. 
   An example of the above-described method is provided in  FIG. 2 , which is a flowchart  200  of an embodiment of using fewer detectors than light sources to set at least one optical parameter in the ICM system of  FIG. 1 . In step  210 , a plurality of synthetic source sets are created. Synthetic sources are combinations of light emitters or LEDs  112 . In the embodiment of the ICM system  100  of  FIG. 1 , there are four sources, the amber LED  116 , the red LED  118 , the green LED  120 , and the blue LED  122 , and three color sensors  130 . Therefore, two sources need to be combined in order to yield three sources, the combined sources constitute a synthetic source. The synthetic source space may have the following six combinations: blue-green, blue-amber, blue-red, green-amber, green-red, and amber-red. The combinations can have varying intensities of their constituent sources, which constitute a plurality of different synthetic sources. For example, each combination may have nine different intensities, wherein the intensities are based on ten percent increment steps, which yields the nine different intensities. Accordingly, each combination has a possibility of nine synthetic sources. Because there are six combinations, there are fifty-four sample points for the synthetic source space. 
   With regard to the above-described example, there are six combinations: blue/green, blue/amber, blue/red, green/amber, green/red, and amber/red, and each combination has nine different intensities. Using the blue/green combination as an example, there are nine different intensities of: blue 10% and green 90%; blue 20% and green 80%; blue 30% and green 70%, etcetera. Therefore, there are 54 synthetic source sets. It is noted that increments other than ten percent may be used, which may yield more or less than 54 synthetic sources. 
   In step  212  the target space is sampled. In the example described herein, the possible target color points are the chromaticity coordinates of Black Body sources with color temperatures of 2500K, 4000K, 6500K, and 9300K. In other embodiments, other color temperatures may be used. It is noted that the target space denotes different desired colors. 
   At step  214 , the ICM system  100  is simulated for each of the fifty-four sets of synthetic sources with respect to the four target color points. This yields  216  simulations; 54 synthetic source sets with four color temperatures. For example, each synthetic source is used with the actual sources to achieve the target color temperatures. In an example of a red/green synthetic source, each of the nine combinations of red/green is used with blue and amber to achieve the different color temperatures. 
   At step  216 , the synthetic sources that generate optimal results for each target color point are stored in the look up table  182  or the like. In the example provided herein, the results with optimal color rendering index (CRI) are stored in the look up table  182 . However, parameters other than CRI may be used as criteria for storing the synthetic source combinations that generate optimal results. 
   In one example, synthetic source combinations that yield optimal CRI are stored. The optimal CRI may be as follows for each target color point, which constitutes the target look up table: 
   
     
       
         
             
             
           
             
                 
             
             
               Target color point 
               Synthetic source with optimal CRI 
             
             
                 
             
           
          
             
               2500K 
               B-50% A-50% 
             
             
               4000K 
               B-30% R-40% 
             
             
               6500K 
               G-10% A-90% 
             
             
               9300K 
               A-40% R-60% 
             
             
                 
             
          
         
       
     
   
   During use, a user selects a target color point, or a desired color, by selecting a color temperature. At step  218 , the ICM system  100  selects the color temperature stored in the look up table  182  that is closest to the target color point. In step  220 , the synthetic source values of the selected color temperature from step  218  from the lookup table are used in the feed back of the ICM system  100  to maintain consistent colors with optimal CRI or other parameter. 
   With regard to the above-described example, a user sends a target color point to the ICM  100 . For example, the user may send a color temperature of 9000K. The ICM  100  will select the closest color temperature to the target color point from the look up table  182 . In this example, the closest color temperature/color point is 9300K. Because 9300K is the closest color temperature, the system will use the synthetic source of Amber 40% and red 60% for the ICM  100  to maintain consistent color. As noted above, this ratio has the optimal CRI from step  214 . 
   The ICM  100  has been described herein as using a combination of two light sources to generate one synthetic source. However, several light sources may be combined to generate several synthetic sources. For example, in a situation of five light sources and three detectors, two pairs of light sources may be combined to generate two synthetic sources. Likewise, three sources may be combined to make a single synthetic source. 
   Having described portions of the operation of the ICM system  100 , calibration of the ICM system  100  will now be described. 
   Conventional ICM systems require the user to acquire the responses of the sensors to each source (S matrix) and the chromaticity coordinates of each source (C matrix). The ICM system  100  described herein may be calibrated using several different methods as described below. 
   In the first method, the user collects spectral information of each source or LEDs  112 . The above-described lookup table uses the spectra collected from the LEDs  112 . This method provides very accurate calibration. However, this procedure must be done for each ICM system  100 . 
   In a second method, a user obtains the spectral information for each lot or bin of LEDs  112  or other light sources. More specifically, a vendor of light sources may obtain the spectral information of a lot or bin of sources. This spectral information may then be used by the ICM system  100 . The disadvantage is that the individual light sources may emit spectrums that are slightly different from the lot or bin information. The advantage is that the ICM system  100  does not need to be calibrated by measuring the spectra of each of the LEDs  112  that are from the same lot or bin. 
   The third method requires a user to perform a one time calibration using a typical set of RGBA LEDs. The look up table generated by this one set of RGBA LEDs will represent all other sets of RGBA LEDs used in the production. Alternatively, a user can send RGBA LEDs spectral information to a manufacturer, which will generate a look up table based on that the LED spectral information. In a similar embodiment pre-generated look up tables that are stored within the ICM system  100  can be used based on standard RGBA LEDs spectral information provided by LEDs suppliers. The spectral information is retrieved and used in the feed back system of the ICM system  100 . This calibration method is the least costly. However, this calibration method is also the least precise in that the spectral information of the LEDs  112  or light sources is not precisely known. 
   The fourth method involves measuring the spectral information for each of the LEDs  112  in addition to the corresponding XYZ tristimulus values. This information is used to generate a matrix that can be multiplied by a user specified target color point to yield the drive level of each of the LEDs  112 . The matrix will serve to maximize the CRI of the LEDs  112  in addition to controlling their color points. In this embodiment, the CRI of the LEDs  112  is inversely proportional to the difference in color of surfaces rendered by a test light source to those rendered by a reference light source of similar correlated color temperature (CCT). Thus, minimizing the spectral difference between the test and the reference light sources will maximize the CRI, while maintaining the desired color point. This process involves minimizing: 
   
     
       
         
           
             1 
             2 
           
           ⁢ 
           
              
             
               Ax 
               - 
               
                 b 
                 ⁡ 
                 
                   ( 
                   d 
                   ) 
                 
               
             
              
           
         
       
     
       
       
         
           subject to Cx−d=0 and 
           wherein:
           A is the LED spectra at maximum drive in the matrix column;   C is the corresponding XYZ tristimulus values in matrix columns;   d is the XYZ tristimulus value of the desired color point as a column vector; and   x is the LED drive levels from zero to one as a column vector.   
         
         
       
     
  
   In practice, each of the LEDs  112  is driven at their maximum and their spectra are measured. The measuring of the spectra are performed at predetermined intervals, such as 1.0 nm intervals and stored as the columns of matrix A. The equation is solved giving x in terms of a matrix equation as a function of d. 
   When computing CRI, different function for b apply to CCTs above and below 5000K. However, using only the b function for CCTs above 5000K may be suitable even at low CCTs. It is noted that the CRI may only be meaningful for colors close to the black body locus. Therefore, b may be a legitimate argument for the function d.