Abstract:
A feedback method on occasion independently senses a characteristic of light produced by each of several light sources in a lighting apparatus. The sensed value of that characteristic is compared to a reference value for the respective light source and that light source&#39;s operation is adjusted accordingly. This method has particular application in a lighting apparatus that produces different lighting effects by varying the intensity of different colors of light produced by the various light sources. The feedback method compensates for light emission variation as the sources age, thus ensuring that the lighting apparatus continues to produce the desired lighting effects. This enables multiple lighting apparatus in an area to be calibrated to the same standard so that uniform illumination is provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not Applicable 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to lighting apparatus which produce white light that is variable within a predefined range of correlated color temperatures, and more particularly to such lighting apparatus that employ a plurality of light sources each emitting light of a different color which blend together to produce the white light. 
   2. Description of the Related Art 
   The interior spaces, such as those of buildings and vehicles, historically were illuminated by incandescent or fluorescent lighting devices. More recently lighting systems have been developed that utilize groups of a light emitting diodes (LED&#39;s). For example U.S. Pat. No. 6,158,882 describes a vehicle lighting system which employs a plurality of LED&#39;s mounted in a linear array to form a lighting strip. By varying the voltage applied to the lighting device, the intensity of the illumination can be varied to produce a desired environmental effect. For example, it is desirable to control the illumination intensity and color of the passenger cabin of executive aircraft and custom motor coaches to accent or emphasize the cabin decor and to set different environmental moods for the occupants. Subtle changes in the shade of white light can have a dramatic effect on the interior environment of those vehicles. 
   One technique for characterizing white light is correlated color temperature based on the temperature in degrees Kelvin of a black body that radiates the same color light. An ideal model of a white light source is referred to as a “Planckian radiator”. The loci of the chromaticities of different Planckian radiators form a curve on the chromaticity chart of the Commission Internationale de l&#39;Eclairage (CIE) in Vienna, Austria, which characterizes colors by a luminance parameter and two color coordinates x and y. 
   Another characterizing technique measures the color rendering properties of a light source based on the degree to which reference colors are shifted by light from that source. The result of this characterization is a numerical Color Rendering Index (CRI) having a scale from 0 to 100, with 100 being a perfect source spectrally equal to sunlight or full spectrum white light. In general, light sources with a CRI between 80 and 100 make people and objects look better and tend to provide a safer environment than light sources with lower CRI values. Typical cool white fluorescent lamps have a CRI of 65 while rare-earth phosphor lamps have a CRI of 80 and above. 
   Some prior variable lighting systems contain several emitters that create light of different colors which mix to produce an resultant illumination color. The most common of these systems utilize red, green, and blue light sources driven at specific excitation levels to create an equivalent “white” light balance point. However, it is difficult with prior lighting systems to create white light that adheres to the Planckian radiator curve on the CIE chromaticity chart and has a CRI greater than 80. 
   Other variable lighting systems in common use utilize a broad spectrum “white” light source, along with individual red, green and blue light sources. The “white” light spectrum is then shifted on the color chart by amounts related to the contributions of the individual red, green, and blue light levels with respect to the level of the broad spectrum light source level and to each other. Although this type of lighting apparatus can replicate the Planckian radiator over a range in the visible spectrum of light, it has a poor Color Rendering Index over most of that range. 
   In order to illuminate an entire room or the passenger cabin of an aircraft, the lighting system must employ numerous light sources and different areas may be illuminated by different lighting systems. Even where all the sources are commonly controlled, various ones may produce different shades of white light. Thus it is difficult to provide a uniform color of light throughout the interior space. 
   Therefore, it is desirable to provide a lighting system which permits the color temperature of a broad spectrum light to be varied within a predefined range in a controlled manner. It is further desirable to provide a mechanism that automatically calibrates each light source to consistently produce light at a predefined correlated color temperature, thereby compensating for changes that occur as the source ages over time. 
   SUMMARY OF THE INVENTION 
   A lighting apparatus has a plurality of light sources each producing different colored light which combine to produce a resultant color of light from the apparatus. For example, the lighting apparatus may include a white light source, a monochromatic light source and a polychromatic light source. A method is provided to occasionally adjust the operation of each light source to ensure that the desired resultant color is produced as the sources age. 
   That method comprises defining a separate reference value for a characteristic of the light produced by each light source. For example, the characteristic may be light luminance, although a different characteristic may be used for each light source. The characteristic of the light produced by each light source is sensed independently, which produces a sensed value for each light source. Then, each sensed value is compared to the associated reference value and the operation of respective light source is adjusted, if necessary, based on the comparing. Preferably, a given light source&#39;s operation is adjusted until its sensed value substantially equals the respective reference value. That adjustment may involve altering the amount of electric current that flows to the respective light source, for example. 
   In a preferred embodiment of the method, the reference values are defined by first setting the luminance of the white light source to a predefined level. Then operation of the other light sources are independently adjusted until the resultant color of light has a predefined correlated color temperature. At that time, the characteristic of the light produced by each light source is sensed, thereby producing the reference values for the light sources. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an isometric view of an LED lighting strip that is part of a lighting system according to the present invention; 
       FIG. 2  is a schematic circuit diagram of the lighting system in which several LED lighting strips are connected to a controller and a power supply; 
       FIG. 3  is a schematic circuit diagram of the lighting strip; 
       FIG. 4  is a schematic circuit diagram of a current controller in  FIG. 3 ; 
       FIG. 5  is a flowchart of a process performed in the factory to calibrate the lighting strip to produce white light at a predefined correlated color temperature; 
       FIG. 6  is the CIE chromaticity chart for the lighting strip; 
       FIG. 7  is a graph depicting the color rendering index throughout the spectrum of the combined light produced by the lighting strip; and 
       FIG. 8  is a flowchart of a recalibration process performed by each lighting strip. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With initial reference to  FIG. 1 , a lighting strip  10  includes a housing  12  in with a U-shaped channel which supports longitudinal edges of a printed circuit board  11 . A plurality of light emitting diodes (LED&#39;s)  13 ,  14 ,  15  and  16  are mounted along a row that extends longitudinally on the printed circuit board  11 . The first type of LED&#39;s  13 , which preferably emit red light, collectively form a monochromatic light source  17 . As used herein a monochromatic light source emits light in which 90% of the energy is concentrated within a spectral wavelength width of a few angstroms. The second type of LED&#39;s  14  emit white light and create a broad spectrum light source  18 . For example, each second type of LED  14  emits blue light that strikes a phosphor coating which produces white light of a correlated color temperature greater than 6500° Kelvin. The third type of LED&#39;s  15  preferably emits amber light and fourth type of LED&#39;s  16  preferably emits green light. The third and fourth types LED&#39;s  15  and  16  combine to form a polychromatic light source  18  which is defined herein as a source that emits light having at least two distinct wavelengths. As will be described, the third and fourth types of LED&#39;s  15  and  16  are driven in unison, i.e. identically, and thus form a single light source. The different types of LED&#39;s are arranged in an alternating pattern in which the second type of LED  14 , that emits broad spectrum light, is located between the other types of LED&#39;s. In the embodiment shown in  FIG. 1 , a red first type of LED  13  is followed by a white second type of LED  14  going along the row. Next there is an amber third type of LED  15 , then another white second type of LED  14  followed by a green fourth type of LED  16 , with the series concluding with yet another white second type of LED  14 . The series pattern of six LED&#39;s repeats over and over again along the length of the a lighting strip  10 . Other repeating patterns of the six LED&#39;s may be used. Although the present invention is being described in the context of a system that uses light emitting diodes, other types of emitters can be utilized as the monochromatic, broad spectrum and polychromatic light sources. 
   The lighting strip  10  has a first electrical connector  21  at one end and a mating second electrical connector  22  at the opposite end. Thus a plurality of lighting strips  10  can be connected in a daisy chain  24  by inserting the first electrical connector  21  of one lighting strip into the second electrical connector  22  of a another lighting strip and so on to create a lighting system  20  as illustrated in  FIG. 2 . The connectors  21  and  22  carry control data and power between the lighting strips  10  connected in this manner. This chain of multiple lighting strips  10  can be used to illuminate a large space, such as by installing the lighting strips along the length of the passenger cabin of an airplane, for example. 
   An exposed electrical connector  21  of the lighting strip  10   a  at one end of the daisy chain  24  receives a mating connector on a cable  23  that carries electrical power from a power supply  26  and control commands on a communication bus  25  from a system controller  28 . A first pair of pushbutton switches  27  is connected to the system controller  28  by which a user is able to increase and decrease shade of the white light produced by the chain  24  of lighting strips  10 . A second pair of pushbutton switches  29  enables the user to increase and decrease the luminance (brightness) of the light. The system controller  28  includes a microcomputer that executes a software program which supervises the operation of the lighting system  20  and sends control commands to the lighting strips  10 , as will be described. 
   Within a given lighting strip  10 , the LED&#39;s of each light source are electrically connected together in a separate circuit branch from the other sources as shown in  FIG. 3 . Specifically all the first type of LED&#39;s  13  are coupled in series to form a circuit branch for the monochromatic light source  17  and all the second type of LED&#39;s  14  are serially connected in a circuit branch of the broad spectrum light source  18 . The third and fourth types of LED&#39;s similarly are connected in series with one another to form a common circuit branch for the polychromatic light source  19 . This interconnection enables each of the three light sources  17 – 19  to be controlled independently, as will be described. 
   Application of electricity to the light sources  17 – 19  is governed by a microcomputer based, light source controller  30  that responds to the control commands received from the system controller  28 . Operation of the lighting strip  10  is controlled by a software program that is stored in a memory and executed by the light source controller  30 . The light source controller  30  operates first, second and third current circuits  31 ,  32  and  33  which supply electric current to the first, second and third light sources  17 ,  18  and  19 , respectively. The details of one of the current circuits  31 – 33  is shown in  FIG. 4  and has a voltage divider  35  connected between circuit ground and a power conductor  34  to which the power supply  26  attaches. The voltage divider  35  includes a digitally controlled potentiometer  36  that adjusts a variable voltage level which is applied to an input of a voltage-to-current converter  37 . The voltage divider  35  and the voltage-to-current converter  37  form a variable current source  38 . The digitally controlled potentiometer  36  and thus the variable voltage level are controlled by a frst signal from the light source controller  30 . The variable voltage level results in a variable output current being produced by the voltage-to-current converter  37 . That output current is fed to a controlled current mirror  39  that acts as a driver which switches the electric current to the respective light source  17 ,  18  or  19  and its LED&#39;s. Switching of the current mirror  39  is controlled by a pulse width modulated (PWM) second signal from the light source controller  30 . The duty cycle of the PWM second signal determines the effective magnitude of the electric current that is applied to the respective LED light source and thus controls the luminance of the light output. 
   Referring again to  FIG. 3 , a light sensor  40  is located at a position on the light strip  10  so as to receive light from all four types of LED&#39;s  13 – 16 . The light sensor  40  produces an output signal indicating the intensity of the light that impinges thereon. That signal is processed by an automatic gain control (AGC) circuit  42  to provide an amplified sensor signal to an analog input of the light source controller  30 . In a calibration mode to be described, each light source  17 – 19  is activated individually and the resultant light is sensed. Because the different types of LED&#39;s inherently produce light at different intensity levels when driven by the same magnitude of current, the gain of the AGC circuit  42  is varied depending upon which source  17 – 19  is being calibrated. Specifically the gain is increased for the types of LED&#39;s that generate lower intensity light levels. 
   The operation of the lighting strip  10  is initially calibrated at the factory by connecting one lighting strip to a power supply  26  and a system controller  28  similar to that illustrated in  FIG. 2 . A spectrophotometer (not shown) is positioned to receive light emitted by all the light sources  17 – 19 . The calibration process is depicted by the flowchart of  FIG. 5  and commences at step  52  by the system controller  28  activating only the broad spectrum light source  17  that produces white light. Specifically the system controller  28  sends a command via the communication bus  25  to the light source controller  30  within the lighting strip  10  being calibrated. The command instructs the light source controller  30  to operate the broad spectrum light source  17  (i.e. white LED&#39;s  14 ) at a default current level and PWM duty cycle (e.g. 50%). At step  54 , current from the second current circuit  32  for that light source  17  is adjusted until the spectrophotometer indicates a predefined reference luminance level. That current level variation is accomplished by a technician adjusting a corresponding one of three system controller calibration potentiometers  44 . The system controller  28  responds a change of the calibration potentiometer by sending another current level command to the light source controller  30  in the lighting strip  10 . The light source controller  30  carries out the command by changing operation of the digital potentiometer  36  in the second current circuit  32  to vary the current magnitude accordingly. 
   After the luminance level of the broad spectrum light source  17  (i.e. white LED&#39;s  14 ) has been set to the reference level, the system controller  28  activates all the light sources  17 – 19  at step  56 . The light sources are driven by PWM signals which initially have equal duty cycles (e.g. 50%). The spectrophotometer then is observed while manually adjusting the operation of the current circuits  31  and  33  for the first and third light sources  17  and  19 , i.e. the red LED&#39;s  13 , and the combination of green and amber LED&#39;s  15  and  16 . The current levels of the first and third current circuits  31  and  33  are varied until the spectrophotometer indicates that the light which results from the mixture of light from the three sources  17 – 19  has a predefined correlated color temperature. Specifically, a calibration reference point is chosen on the curve  65  which corresponds to a Planckian radiator on the standard CIE chromaticity chart as illustrated in  FIG. 6 . The current levels of the first and third current circuits  31  and  33  are varied by the technician adjusting the other two calibration potentiometers  44  in  FIG. 2 . The system controller  32  responds by sending the appropriate current level commands over the communication bus  25  to the light source controller  30 , which alters the operation of the digital potentiometer  36  within the respective current circuit  31  or  33 . Adjustment of the first light source  17 , the red LED&#39;s, varies the chromaticity along the X axis of the CIE chromaticity chart, while adjustment of the third light source  17 , the amber and green LED&#39;s, varies the chromaticity along the Y axis. Thus, the system controller  32  enables orthogonal control of the light emitted by the lighting strip. 
   Once the lighting strip has been calibrated to produce light at the predefined white correlated color temperature at step  58 , the current level settings for the current circuits  31 – 33  are stored at step  60  in the memory of the light source controller  30 . These settings define the color temperatures of the three light sources  17 – 19 . With reference to the CIE chromaticity chart in  FIG. 6 , the chromaticity of the red light from the monochromatic light source  17  and the first type of LED&#39;s  13  is denoted by point  66  and the shade of white light produced by the broad spectrum light source  18  and the second type of LED&#39;s  14  is indicated by point  67 . Point  68  represents the chromaticity of the polychromatic light source  19  comprising the third and fourth types of LED&#39;s  15  and  16  and represents an averaging of the individual wavelengths of the light from those LED types. If more that two types of emitters are used for the polychromatic light source, the resultant chromaticity point also will be an average of their individual wavelengths. Point  69  indicates the chromaticity of the resultant light from the mixture of light from the three light sources  17 – 19 . 
   Then at step  61 , each LED light source  17 ,  18  and  19  is activated to full luminance one at a time and the output of sensor  40  is stored within the memory of the light source controller  30  at step  62 . This process stores reference sensor values for each light source for use subsequently during recalibration of the lighting strip  10 , as will be described. A determination is made at step  63  whether all three light sources have been sensed. If not the next light source is selected at step  64  and the process returns to step  61  to sense and store that light source&#39;s light output level. After a light output level has been stored for each light source, the factory calibration process terminates. 
     FIG. 2  depicts a typical a lighting system  20  in which a plurality of individual lighting strips  10  are connected together and controlled in unison. The communication bus  25  passes through every strip and each of their respective light source controllers  30  listens and responds to the commands transmitted by the system controller  28 . Those commands instruct every light source controller  30  how to adjust the relative intensity of each light source  17 ,  18  and  19 . 
   This command transmittal process enables the user to vary the shades of white light produced by the combination of light from each light source  17 – 19  within every strip. By activating one of the pushbutton switches  27  in  FIG. 2 , the user is able to increase or decrease the correlated color temperature of the combined light along the curve  65  for a Planckian radiator on the CIE chromaticity chart in  FIG. 6 . A look-up table correlates locii on the Planckian radiator curve  65  to the relative intensities of the light produced by each source  17 ,  18  and  19  of the lighting strip  10 , i.e. the intensities of the monochromatic light, the broad spectrum light and the polychromatic light. Those relative light intensities are defined by PWM duty cycles for each of the three light sources. Changing the duty cycle of the PWM signals that are applied to the current mirrors  39  in one or two current circuits  31 – 33 , alters the relative intensity of light from the LED light sources thereby varying the correlated color temperature of the combined light produced from the lighting strip  10 . For example, increasing the PWM duty cycle of the monochromatic light source  17  in the exemplary system, increases the intensity of the red light without affecting the intensity of light from the other two sources  18  and  19 . The addition of more red light yields warmer combined light. 
   The user also can vary the overall brightness of the combined light by operating one of the other pair of pushbutton switches  29  which increases or decreases the PWM duty cycles for each current circuit  31 – 33  by the same amount. Thus the intensity relationship of the light from the light sources  17 – 18  is maintained constant, that is change in color occurs while the combined luminance varies. 
   The light from the three sources  17 – 19  mix to produce a resultant shade of white light having a correlated color temperature that can be adjusted along the Planckian radiator curve  65 . Proper control of the relative intensity of the light from each source  17 – 19 , enables the lighting strip to replicate the light from Planckian radiators through a substantially continuous range of color temperatures, from 2700° K to 6500° K, for example. The degree to which the variation of the color temperature is continuous is a function of the resolution at which the relative intensity of the light  17 – 19  can be varied. 
     FIG. 7  graphically depicts the color rendering index (CRI) of the resultant shade of white light, produced when the light from the three light sources mix. A substantial amount of the visible spectrum produced by the lighting strip, at least 80% the 2700° K to 6500° K range of color temperatures, has a color rendering index of at least 80. This results from the use of a broad spectrum light source  18  that produces white light the of which is shifted by the monochromatic and polychromatic light from the other two light sources  17  and  19 . 
   Over time, the light emitting diodes age causing a change in the color temperature of the produced light. Therefore, the combined light deviates from the locii of correlated color temperatures along the Planckian radiator curve  65  on the CIE chromaticity chart. Change of individual light sources also alters the correlated color temperature of the combined light from each lighting strip  10 . As a consequence, the shade of the white combined light produced varies from lighting strip to lighting strip in a lighting system  20  and no longer uniformly illuminates the adjacent area. 
   The present lighting system  20  provides a mechanism by which the individual lighting strips  10  are automatically recalibrated. Such recalibration can occur either whenever power is initially applied to the lighting strip, in response to a command from the system controller  28 , or upon the occurrence of another trigger event. 
   The light source controller  30  within each lighting strip  10  responds to the occurrence of the trigger event by executing a recalibration software routine  70  depicted in  FIG. 8 . The recalibration process commences at step  72  where one light source, the monochromatic source  17  for example, is selected and then activated at step  73 . At this time, only the LED&#39;s  13  in the selected light source emit light and those LED&#39;s are driven to their full intensity. Then, at step  74 , the light source controller  30  reads the input signal from the automatic gain control circuit  42  which represents the light level detected by the sensor  40 . The sensed light level is compared to the reference level for the selected light source that was stored in memory during the factory calibration of the lighting strip. If at step  76 , the determination is made that the two light levels are not equal, the program execution branches to step  78  where a decision is made whether or not the sensed light level is greater than the reference light level. If not, the program execution branches to step  80  where the current produced by the first current circuit  31 , in this case, is increased an incremental amount in an attempt to equalize the sensed level to the reference level. Alternatively, if at step  78 , the sensed light level is greater than the reference light level, the program execution branches to step  82  where the magnitude of current from the first current circuit  31  is reduced. The program execution then returns to step  74  to once again sense the actual light level produced by the first selected light source. This procedure continues to loop through steps  74 – 82  until the sense level of light equals the reference light level at step  76 . 
   Upon that occurrence, the program execution branches to step  84  where a determination is made whether another light source needs to be recalibrated. If so, the program execution branches through step  86  where the next light source is selected and then the program returns to step  73  to energize the LED&#39;s of that light source. When all three light sources  17 – 19  have been recalibrated, the program execution saves the new current magnitude settings at step  86  before terminating. 
   The recalibration method restores the lighting strip  10  to the operational level and performance that existed upon its manufacture so that the entire lighting system  20 . will uniformly illuminate the area with a desired shade of white light. In other words, all the individual lighting strips  10  will produce the same shade of white combined light. 
   The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. For example, although light emitting diodes are used in the preferred embodiment, other types of light emitters could be used. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.