Abstract:
A method for calibrating an illumination system is disclosed herein. The illumination system comprises at least one light emitter and at least one color sensor. The method comprises emitting a first wavelength of light from the a light emitter and measuring the color of light emitted by the light emitter using the color sensors. A second wavelength of light is emitted from the a light emitter and the color of light emitted by the light emitter is measured using the color sensors. The shift in color space based on the measurements is calculated. At least one calibration matrix based on the shift in colors space is calculated.

Description:
BACKGROUND 
       [0001]    Color illumination systems use a plurality of different colors emitters to display a wide range of colors. Some illumination systems use red, green, and blue emitters. Others illumination systems use different colors, such as red, green, blue, and amber. Combinations of these colors are used to emit the wide range of colors emitted by the illumination systems. 
         [0002]    In order to be sure the correct colors are emitted by the illumination system, a plurality of color sensors or detectors are located proximate the emitters. The sensors measure the intensity of light emitted by their associated emitters, which enables the ratio of light to be calculated for specific areas of the illumination system. As noted above, the ratio of light determines the color of light emitted by the illumination system. 
         [0003]    The calculated ratio of light is compared to the ratio of light that is supposed to be emitted by an illumination system and corrections are made to correct the color. For example, a specific color having a 20% red, 20% green, and 60% blue is supposed to be emitted. If the color sensors detect other ratios, the ratios will be corrected by an illumination color management system or the like associated with the illumination system. 
         [0004]    In order to provide accurate feedback as to the actual colors being emitted, the color sensors have to be precisely calibrated. As the gamut of colors that is able to be emitted increases, the accuracy of the calibration needs to be increased. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a schematic diagram of an embodiment of an illumination system. 
           [0006]      FIG. 2  is a flowchart describing an embodiment of a calibration method applied to the illumination system of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0007]    A simplified version of a color illumination system  100  is shown in  FIG. 1 . The illumination system  100  of  FIG. 1  includes a plurality of light emitters  106 , which are described herein as being light emitting diodes (LEDs)  106 . It is noted that emitters other than LEDs may be used in the illumination system  100 . It is also noted that the term light emitter means a single source of light and not a combination of light sources. A combination of light sources refers to the illumination system  100 . The LEDs  106  are referred to individually as the red LED  108 , the green LED  110 , and the blue LED  112 . The color designations of the LEDs  106  refer to the general color appearance of the LEDs  106 , which corresponds to the peak wavelength of light emitted by the LEDs  106 . However, colors other than the above-described colors may be emitted by the LEDs  106 . 
         [0008]    Located proximate the LEDs  106  are a plurality of color sensors  116 . The sensors may be located in area where the emission from the light emitters is well mixed. The color sensors  116  are referred to individually as a red sensor  118 , a green sensor  120 , and a blue sensor  122 . Each of the sensors  116  detects a band of light or a bandwidth of wavelengths emitted by the corresponding LEDs  106 . For example, the red sensor  118  detects a bandwidth of light centered around a wavelength of red light. The bandwidth may extend to the green and blue wavelengths. 
         [0009]    The sensors  116  output a voltage, number, current, or other indicator, that is proportional to the intensity of light they receive. In the embodiment where the sensors  116  output voltages, a high voltage may be indicative of detection of a high intensity of light. Likewise a low voltage will be indicative of detection of a low intensity of light. Thus, when the sensors  116  are illuminated with a red light, the red sensor  118  will output a high voltage and the green sensor  120  and the blue sensor  122  will output relatively low voltages. 
         [0010]    The sensors  116  are connected to a processor  130  that processes the data from the sensors  116 . For example, the processor  130  may convert analog outputs from the sensors  116  to digital representations of the intensity of light received by the sensors  116 . The processor  130  outputs this data to an illumination color management system (ICM)  134 . As described in greater detail below, the ICM  134  analyzes the data from the processor  130  to correct the colors emitted by the illumination system  100 . The ICM  134  receives data from user input, which may be digital data from a computer or processor indicative of the color desired to be output by the illumination system  100 . 
         [0011]    The ICM  134  outputs instructions or data to an LED driver  138 . The instructions include the intensities of light to be emitted by the LEDs  106  in order to produce a desired color. As described in greater detail below, the ICM  134  analyzes or uses calibration data and the like in order to determine the intensities of light to be emitted by the LEDs  106 . In some embodiments, the LED driver  138  uses pulse width modulation (PWM) signals to drive the LEDs  106 . Therefore, the intensities of light emitted by the LEDs  106  are varied by varying the duty cycles of the PWM signals. 
         [0012]    The illumination system  100  is required to be calibrated so that corrections applied by the ICM  134  are accurate. The ICM  134  corrects the colors per a calibration matrix. Reference is made to the CIE color space, which is used as a calibration standard for color. It is noted that other color standards may be used herein. Some conventional illumination systems use a single calibration matrix that is applied to colors within the entire CIE color space. As described herein, the illumination system  100  may use several different calibration matrices depending on the location of the desired color in the CIE color space. As described in greater detail below, slight wavelength shifts are induced into the emitters  106  and measured by the sensors  116 , which provide for more accurate calibration matrices. 
         [0013]    The below-described calibration techniques may be performed during manufacture of the illumination system  100  subsequent to mounting of the LEDs  106  and the sensors  116 . After the illumination system  100  is manufactured, individual LEDs  106  are illuminated and their colors are measured using the sensors  116 . With regard to the illumination system  100  of  FIG. 1 , there are three LEDs  106 , therefore, three color points in the CIE space will have the above-described matrices applied thereto. 
         [0014]    The calibration methods described herein may be performed by a computer or the like operating by way of a computer-readable medium, which includes firmware, magnetic media, optical media, and any other form of data storage. Computer code to perform the calibration may be stored on the computer-readable medium. The computer may be a component of the illumination system  100  or it may be associated with manufacturing devices. Values for the calibration matrices described herein may be transmitted to the illumination device  100  and stored therein. 
         [0015]    The LEDs  106 , and other light sources that may be used by the illumination system  100  do not always emit a single and constant wavelength of light. For example, the bandwidth of light emitted by the red LED  108  may be centered around a specific peak wavelength that is referred to as the wavelength for that specific red LED  108 . For example, the wavelengths of the LEDs  106  will change slightly depending on the current passing through the LEDs  106 . In addition, the wavelengths of the LEDs  106  will change as the temperature of the LEDs  106  change and with the age of the LEDs  106 . These changes in the wavelengths emitted by the LEDs  106  may be known. For example, the data may be supplied by the manufacturer of the LEDs  106 . 
         [0016]    In one embodiment, the peak wavelength for the red LED  108  at an ambient temperature of 50.0 degrees Celsius is 639.1 nanometers. Values detected by the sensors  116  are 264.0 for the red sensor  118 , 19.0 for the green sensor  120 , and 4.0 for the blue sensor  122 . When the ambient temperature is lowered to 30.0 degrees Celsius, the peak wavelength moves to 636.5 nanometers. At this wavelength, the red sensor  118  outputs a value of 288.0, the green sensor outputs a value of 20, and the blue sensor outputs a value of 5.0. More specifically, the wavelength of the peak output of the red led  108  was lowered by approximately 2.2 nanometers, which was detected by the sensors  116 . 
         [0017]    The peak wavelength output by the red sensor  108  may be increased by heating the red LED  108 . When the red LED  108  is heated to 80.0 degrees Celsius, the peak wavelength moves to 641.5 nanometers. At this wavelength, the red sensor  118  outputs a value of 240.0, the green sensor outputs a value of 17.0, and the blue sensor outputs a value of 4.0. Accordingly, the peak wavelength increased 2.4 nanometers, which was detected by the sensors  116 . 
         [0018]    Similar changes to the peak wavelengths can be accomplished with regard to the green LED  110  and the blue LED  112 . Accordingly, the peak wavelengths of all the LEDs  106  can be varied by varying the temperatures of the LEDs  106 . 
         [0019]    Another method of changing the peak wavelength of the LEDs  106  is by changing the drive current of the LEDs  106 . In one embodiment, the red LED  108  outputs a peak wavelength of 635.9 nanometers with 20.0 mA drive current. At this wavelength, the red sensor  118  outputs a value of 289.0, the green sensor  120  outputs a value of 20.0, and the blue sensor outputs a value of 4.0. When the drive current is dropped to 5.0 mA, the peak wavelength drops to 654.6 nanometers, which is a change of 1.3 nanometers. At this current, the red sensor  118  outputs a value of 67.0, the green sensor  120  outputs a value of 8.0, and the blue sensor  122  outputs a value of 4.0. Thus, the difference in peak wavelength can be detected by the sensors  116 . 
         [0020]    At least one of the above-described techniques is applied to each of the LEDs  106  during manufacture in order to obtain calibration matrices. For example, the responses of the sensors  116  when the red LED  108  is illuminated may be obtained at several different and known wavelengths. Based on the sensor responses and the peak wavelengths, correction matrices, such as three by three matrices, may be calculated. The ICM  134  may use the matrices during operation in order to cause the illumination system  100  to output very precise colors. More specifically, the ICM  134  may use the correction matrices to correct values received by the sensors  116 . 
         [0021]    In some embodiments, the corrections may be applied based on operating conditions of the illumination system. For example, if the peak wavelength of a specific LED changes as the LED ages, the correction matrices may apply a correction based on the known peak wavelength changes. In other examples, the matrices may be applied based on temperature, wherein the peak wavelengths of the LEDs  106  change with temperature and the correction matrices have been calculated based on these temperatures. 
         [0022]    Having described the illumination system  100  and the operation of the illumination system  100 , the calibration will now be described with additional reference to the flow chart  200  of  FIG. 2 . At step  210 , the sensor responses are measured while illuminating individual LEDs  106 . For example, the red LED  108  may be illuminated while the green LED  110  and the blue LED  112  are turned off. The sensors  116  measure the light emitted by the red LED  108 . 
         [0023]    At step  212 , a different response on the sensors  116  is induced by shifting the wavelengths of the individual LEDs  106  as described above. In addition, the wavelength shift can be simulated by using the known changes in the peak wavelengths depending on various operating conditions, such as drive current and temperature. In some embodiments, the peak wavelengths are both increased and decreased. With reference to the example of the red LED  108 , the peak wavelength of the red LED  108  is shifted and measured again by the sensors  116 . 
         [0024]    In step  214 , the corresponding shift in the CIE color space is calculated. More specifically, the shift in peak wavelength detected by the sensors  116  corresponds to a shift in the CIE color space, which is calculated. In step  216 , the calibration matrices are calculated based on the CIE color space and the like. During operation of the illumination system  100 , the correction matrices are applied in order to correct the colors sensed by the sensors  116  so that the feedback to the LED driver  138  is precise. 
         [0025]    The calibration matrices may be three by three matrices. The three rows may represent the colors, red, green, and blue. The columns represent the values obtained by the shifts in wavelengths. For example, the middle column may be representations of values obtained at the prescribed wavelengths. The left column may be representations of values obtained using shorter wavelengths and the right column may be representations of values obtained using longer wavelengths. Similar matrices may be calculated based on CIE space. The matrices may be combined in order to calculate the correction matrix. 
         [0026]    The calibration described herein may be applied to different locations in color space. More specifically, calibration matrices are calculated for all the sensors. Therefore, the calibration described herein calculates calibration matrices for a plurality of different color space locations. Thus, precise color control can be achieved. Conventional calibration techniques only generate one calibration matrix for all the color space, which does not provide for precise color management.