Patent Publication Number: US-2006018118-A1

Title: Spectrum matching

Description:
BACKGROUND  
      The sun is the dominant source of light for all living things on earth. Additional currently available sources of light include fire, incandescent lamps, fluorescent lamps, solid-state light emitting devices and so on. Light sources other than the sun are often referred to as artificial light sources. Artificial light sources are sometimes considered deficient in one form or another compared to sunlight.  
      For many applications, sunlight is the preferred light source. This can be, for example, because sunlight is better able to show true colors for objects of interest, such as paintings, fabric, ink, paper, cotton and so on. Additionally, sunlight can benefit health. For example, in cases of vitamin D deficiency and jaundice, sunlight exposure is recommended. In addition, recent medical findings have shown that the human body heals faster when exposed to sunlight in general and to specific light colors in particular.  
      However, in many applications, a particular place and/or time make it unfeasible to use sunlight as a light source. Therefore, it is desirable to make available an artificial light source that approximates the same light composition as sunlight.  
      Commission Internationale de l&#39;Eclairage (CIE) standard illuminant D65 has been recommended to represent average daylight with a color temperature of 6500 Kelvin. The spectral power distribution (SPD) for CIE standard illuminant D65 is very wide ranging from ultraviolet (UV) to infrared (IR) and includes all wavelengths of the visible spectrum in relatively equal amounts.  
      A typical SPD of an incandescent lamp is mostly in the red and IR range. When incandescent lamps are used as an artificial light source to approximate sunlight, the resulting light has a high color rendering index (CRI). However, since the SPD of incandescent lights is lower at lower light wavelengths, incandescent lamps do not render blues very well. Additionally, incandescent lamps typically have relatively low power efficiency and a short lifetime in the range of 1,000 hours.  
      A typical SPD of a fluorescent lamp exhibits sharp and narrow spikes corresponding to the type of phosphor used in the lighting lamp. Typically red, green and blue phosphors are used when fluorescent lamps are used as an artificial light source to approximate sunlight. Fluorescent lamps have moderate to high CRI at blue and green regions but low CRI at yellow and red regions. Typical fluorescent lamps have moderate lifetimes in the range of 10,000 hours.  
      Even though both incandescent and fluorescent lamps can generate lights of different color temperature or SPD, a single incandescent or fluorescent lamp can only generate light source with a single and fixed SPD curve.  
      In an LED based light source system, different SPD curves can be achieved by adjusting the amount of light output from LED of different colors. LED light sources have high CRI if a substantial number of LEDs of different colors are used. Also, LED based light source systems typically have much longer operating lifetimes than incandescent or fluorescent lamps. Further, LED based light source systems are generally more power efficient than incandescent based light systems.  
      The optical performance of LEDs can vary with temperature, drive current and aging. LED characteristics also vary from batch to batch for the same fabrication process. Drift of LEDs optical characteristics during operation is not acceptable for many applications because the drift can affect color consistency. Therefore there is a need to control and maintain color consistency dynamically. U.S. Pat. No. 6,344,641 B1, U.S. Pat. No. 6,448,550 B1 and U.S. Pat. No. 6,507,159 B2 provide examples of the management of feedback systems used to protect against drift in LED light systems.  
     SUMMARY OF THE INVENTION  
      In accordance with an embodiment of the present invention, light is generated in accordance with a desired spectral power distribution curve. A spectrum of light is generated with a plurality of different light sources. An optical measurement device measures the spectrum of light generated by the plurality of different light sources. The optical measurement device is able to detect light within the entire spectrum of light generated by the plurality of different light sources. The measured spectrum of light is used as feedback to vary the spectrum of light generated with the plurality of different light sources to approximate the desired spectral power distribution curve.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a simplified block diagram of a light source in accordance with an embodiment of the present invention.  
       FIG. 2  shows an SPD curve that represent sunlight as well as additional individual SPD curve that each measure color output of a color LED within the light source shown in  FIG. 1 .  
       FIG. 3  shows an SPD curve that represent sunlight as well as an additional SPD curve that represents the light source shown in  FIG. 1 .  
       FIG. 4  is a simplified block diagram of a light source in accordance with another embodiment of the present invention.  
       FIG. 5  is a simplified block diagram of an optical measurement device in accordance with an embodiment of the present invention. 
    
    
     DESCRIPTION OF THE EMBODIMENT  
       FIG. 1  is a simplified block diagram of a light source  14 . The light source includes a red LED  21 , a green LED  22 , a blue LED  23 , an amber LED  24 , a cyan LED  25  and a deep red LED  26 . An LED driver  13  controls forward current amplitude, and thus brightness for each of red LED  21 , green LED  22 , blue LED  23 , amber LED  24 , cyan LED  25  and deep red LED  26 . Alternatively, when pulse width modulation is used, LED driver  13  controls signal duty cycle, and thus brightness for each of red LED  21 , green LED  22 , blue LED  23 , amber LED  24 , cyan LED  25  and deep red LED  26 .  
      An optical measurement device  27  measures the spectrum of light generated by red LED  21 , green LED  22 , blue LED  23 , amber LED  24 , cyan LED  25  and deep red LED  26 . Optical measurement device  27  provides feedback on the spectrum of light generated by red LED  21 , green LED  22 , blue LED  23 , amber LED  24 , cyan LED  25  and deep red LED  26  to a feedback controller  12 . Feedback controller  12  controls LED driver  13  so that the spectrum of light generated by red LED  21 , green LED  22 , blue LED  23 , amber LED  24 , cyan LED  25  and deep red LED  26  matches a spectrum of light requested by user input  11 . For example, a desired spectral power distribution curve for the requested spectrum of light gives a white color with color temperature that lies close to the black body curve.  
      For example, in order to match the SPD of sunlight, optical measurement device  27  measures light intensity at a broad spectrum that includes all the light generated by red LED  21 , green LED  22 , blue LED  23 , amber LED  24 , cyan LED  25  and deep red LED  26 . For example, to accomplish this, optical measurement device  27  is implemented using a spectrometer or by multiple optical sensors that have spectral responses at different wavelength ranges. For example, optical measurement device  27  is implemented by the combination of a photosensor with a red color filter, a photosensor with a green color filter, a photosensor with a blue color filter, a photosensor with an amber color filter, a photosensor with a cyan color filter and a photosensor with a deep red color filter.  
      The capability of matching a broad spectrum of color allows optical measurement device  27  to control light source  14  to match the SPD of a target spectrum. Matching the target spectrum gives light source  14  the flexibility to generate different metamers of the same color. Metamers are colors that have different SPDs but the same visual appearance or tristimulus values.  
       FIG. 2  and  FIG. 3  illustrate how light source  14  can be used to generate light with an SPD curve that represents sunlight. In  FIG. 2 , an axis  38  represents wavelength in nanometers. An axis  39  represents normalized relative power. A trace  37  represents an SPD curve for CIE standard illuminant D65. A trace  31  represents an individual SPD curve for blue LED  23 . A trace  32  represents an individual SPD curve for cyan LED  25 . A trace  33  represents an individual SPD curve for green LED  22 . A trace  34  represents an individual SPD curve for amber LED  24 . A trace  35  represents an individual SPD curve for red LED  21 . A trace  36  represents an individual SPD curve for deep red LED  26 .  
      In  FIG. 3 , axis  38  represents wavelength in nanometers. Axis  39  represents normalized relative power. Trace  37  represents an SPD curve for CIE standard illuminant D65. A trace  41  represents a combined SPD curve for blue LED  23 , cyan LED  25 , green LED  22 , amber LED  24 , red LED  21  and deep red LED  26 .  
      Embodiments of the invention can also extend the light spectrum generated by a light source to include non-visible spectral range. For example, both IR and UV light emitters can be added to the LED light source to produce a particular SPD curve for application that requires infrared (IR) and ultraviolet (UV) components. This is illustrated by  FIG. 4 .  
       FIG. 4  is a simplified block diagram of a light source  114 . The light source includes a red LED  121 , a green LED  122 , a blue LED  123 , an amber LED  124 , a cyan LED  125 , a deep red LED  126 , a UV light emitter  128  and an IR light emitter  129 . For example, UV light emitter  128  is a UV LED; and, IR light emitter  129  is an IR LED. An LED driver  113  controls forward current amplitude or signal duty cycle, and thus brightness for each of red LED  121 , green LED  122 , blue LED  123 , amber LED  124 , cyan LED  125 , deep red LED  126 , UV light emitter  128  and IR light emitter  129 .  
      An optical measurement device  127  measures the spectrum of light generated by red LED  121 , green LED  122 , blue LED  123 , amber LED  124 , cyan LED  125 , deep red LED  126 , UV light emitter  128  and IR light emitter  129 . Optical measurement device  127  provides feedback on the spectrum of light generated by red LED  121 , green LED  122 , blue LED  123 , amber LED  124 , cyan LED  125 , deep red LED  126 , UV light emitter  128  and IR light emitter  129  to a feedback controller  112 . Feedback controller  112  controls LED driver  113  so that the spectrum of light generated by red LED  121 , green LED  122 , blue LED  123 , amber LED  124 , cyan LED  125 , deep red LED  126 , UV light emitter  128  and IR light emitter  129  matches a spectrum of light requested by user input  
      For example, in order to match the SPD of sunlight, optical measurement device  127  measures light intensity at a broad spectrum that includes all the light generated by red LED  121 , green LED  122 , blue LED  123 , amber LED  124 , cyan LED  125 , deep red LED  126 , UV light emitter  128  and IR light emitter  129 . For example, to accomplish this, optical measurement device  127  is implemented by a spectrometer. Alternatively, optical measurement device  127  is implemented by a combination of optical sensors. This is illustrated by  FIG. 5 .  
       FIG. 5  shows optical measurement device implemented by a combination of optical sensors that have spectral responses at different wavelength ranges. A red color optical sensor is implemented by a red color filter  61  and a photosensor  51 . A red color filter only allows red component of light generated by a light source to pass through. A green color optical sensor is implemented by a green color filter  62  and a photosensor  52 . A blue color optical sensor is implemented by a blue color filter  63  and a photosensor  53 . An amber color optical sensor is implemented by an amber color filter  64  and a photosensor  54 . A cyan color optical sensor is implemented by a cyan color filter  65  and a photosensor  55 . A deep red color optical sensor is implemented by a deep red color filter  66  and a photosensor  56 . A UV optical sensor is implemented by a UV filter  67  and a photosensor  57 . An IR optical sensor is implemented by an IR filter  68  and a photosensor  58 . An interface  60 , that includes, for example, analog-to-digital converters (ADCs), generates feedback signals sent to feedback controller  112 .  
      While light sources with six and eight different spectrals are described above, the spectrals chosen and number of spectrals used are meant to be illustrative only. The particular spectrals used and the number of different spectrals is determined by the desired light spectrum and the accuracy desired for a particular generated SPD as compared to the target SPD. In general, increasing the number of spectrals allows better SPD matching with fewer gaps.  
      The foregoing discussion discloses and describes merely exemplary methods and embodiments of the present invention. As will be understood by those familiar with the art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.