Abstract:
Control systems and methods that directly measure the actual junction temperature of LEDs utilize internal electrical measurements, thereby dispensing with external sensors and/or wires. In various embodiments, the actual junction LED temperature is obtained based on the measured electrical properties, such as the voltage across and/or current passing through the LEDs, during operation. The measured junction temperature may be used in a closed-loop feedback configuration to control the power applied to the LED in order to avoid overheating.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefit of, and incorporates herein by reference in its entirety, U.S. Provisional Patent Application No. 61/490,279, which was filed on May 26, 2011. 
     
    
     FIELD OF THE INVENTION 
       [0002]    Embodiments of the present invention relate, in general, to light-emitting diodes (LEDs), and more specifically to a control system and method for measuring the temperature of LEDs. 
       BACKGROUND 
       [0003]    An increasing number of light fixtures utilize light emitting diodes (LEDs) as light sources due to their lower energy consumption, smaller size, improved robustness, and longer operational lifetime relative to conventional incandescent light sources. Furthermore, LEDs operate at a relatively low constant temperature in comparison to incandescent light sources. A typical operating temperature of an incandescent filament is over 2000° C., whereas an LED may have a maximum operating temperature of approximately 150° C.; indeed, operation above this temperature can decrease the operational lifetime of the LED. At high temperatures the carrier recombination processes and a decrease in the effective optical band gap of the LED decrease the light output of the LED. Therefore, a typical operating temperature of a LED is controlled below 100° C. in order to preserve operational lifetime while maintaining acceptable light output. 
         [0004]    In addition, high-power LEDs used for room lighting require more precise current and heat management than compact fluorescent lamp sources of comparable output. LEDs that use from 500 milliwatts to as much as 10 watts in a single package have become standard, and even higher-power LEDs are expected to be used in the future. Some of the electricity in any LED becomes heat rather than light, and particularly in the case of high-power LEDs, it is essential to remove enough of that heat to prevent the LED from running at high temperatures. Thus, thermal monitoring of LEDs is desirable and, in high-power applications, critical. 
         [0005]    Conventionally, LED lighting systems use sensors, e.g., thermocouples or thermistors to measure and monitor temperatures of LEDs. These sensors are located near the LED and connected to a temperature-monitoring system, typically using a separate dedicated set of wires. These temperature-detection sensors generally cannot directly measure the actual junction temperature of the LED itself, since they are spaced apart from the LED due to optical and connectivity considerations. This can result in measurement inaccuracies. Inaccurate measurements of the LED temperature may cause poor performance and reduce the lifetime of the LED. Additionally, an extra set of wires between the thermistor and the monitoring system can be inconvenient, especially if the monitoring system is far from the thermistor. Finally, the extra cost of the sensors and wires, and their placement within the circuit, represent another disadvantage of utilizing external sensors. 
         [0006]    Consequently, there is a need for an approach to directly measure the LED temperature and adjust the temperature accordingly for optimizing the performance and lifetime of the LED. 
       SUMMARY 
       [0007]    In various embodiments, the present invention relates to control systems and methods that directly measure the actual junction temperature of LEDs utilizing internal electrical measurements, thereby dispensing with external sensors and/or wires. The actual junction LED temperature is obtained based on the measured electrical properties, such as the voltage across and/or current passing through the LEDs, during operation. The measured junction temperature may be used in a closed-loop feedback configuration to control the power applied to the LED in order to avoid overheating. This approach provides a fast, easily implemented, and inexpensive way to directly and accurately measure and control the junction temperature of LEDs in a lighting system, thereby optimizing the performance and lifetime of the LEDs. 
         [0008]    Accordingly, in one aspect, the invention pertains to a system including an LED, a constant-current source switchably connectable to the LED, and a controller for determining the junction temperature of the LED based at least in part on a temperature coefficient and a measured voltage across the LED with the constant-current source connected thereto. In various embodiments, the system includes a power supply and an LED power controller for controlling, based on the temperature coefficient, a load current supplied by the power supply to the LED to maintain a temperature of the LED during operation within a fixed range. The system may further include a switch for switching a power source of the LED between the power supply and the constant-current source; the LED power controller is then switchably connectable to the LED so as to disconnect the power supply from the LED when the constant-current source is connected thereto. 
         [0009]    In some embodiments, the controller computes the temperature coefficient based at least in part on multiple temperatures at which the LED is operated and multiple voltages, each associated with one of the multiple temperatures, measured across the LED. A memory may be included in the system for storing the temperature coefficient and/or the multiple temperatures at which the LED is operated and the multiple voltages, each associated with one of the multiple temperatures, measured across the LED. The temperature coefficient may satisfy the equation: 
         [0000]    
       
         
           
             
               
                 C 
                 T 
               
               = 
               
                 
                   
                     V 
                     
                       
                         f 
                          
                         
                             
                         
                          
                         2 
                       
                        
                       
                           
                       
                     
                   
                   - 
                   
                     V 
                     
                       f 
                        
                       
                           
                       
                        
                       1 
                     
                   
                 
                 
                   
                     T 
                     2 
                   
                   - 
                   
                     T 
                     1 
                   
                 
               
             
             , 
           
         
       
     
         [0000]    where C T  denotes the temperature coefficient, V f1  and V f2  are two of the plurality of voltages measured across the LED, and T 1  and T 2  are two of the plurality of temperatures at which the LED is operated. 
         [0010]    The system may include a detecting sensor for detecting a luminous intensity of LED light in an environment; the LED power controller may be responsive to the sensor to control the load current based on the temperature coefficient and the detected luminous intensity. 
         [0011]    In a second aspect, the invention relates to a method of operating an LED within a fixed temperature range. In various embodiments, the method includes: (i) measuring an actual junction temperature of the LED in real time; (ii) based on the measured real-time junction temperature and a load current of the LED, determining an operational current corresponding to a target operating temperature; and (iii) adjusting the load current to the determined operational current to maintain the LED at the target temperature. The method may include repeating steps (i), (ii), and (iii). In one embodiment, the method further includes detecting a luminous intensity of LED light in an environment and adjusting the load current to maintain a value of LED brightness. 
         [0012]    In some embodiments, measuring an actual junction temperature of the LED includes establishing a temperature coefficient of the LED; operating the LED at a constant current and measuring the voltage thereacross; and based on the measured voltage and the temperature coefficient, determining the actual junction temperature of the LED. In one implementation, determining the actual junction temperature includes calculating the temperature coefficient of the LED. Further, calculating the temperature coefficient may include operating the LED at a constant current at multiple temperatures and measuring a voltage thereacross at each of the temperatures. The temperature coefficient may then be calculated by establishing a relationship between the multiple temperatures at which the LED is operated and multiple voltages, each associated with one of the multiple temperatures, measured across the LED. For example, the temperature coefficient may satisfy an equation: 
         [0000]    
       
         
           
             
               
                 C 
                 T 
               
               = 
               
                 
                   
                     V 
                     
                       
                         f 
                          
                         
                             
                         
                          
                         2 
                       
                        
                       
                           
                       
                     
                   
                   - 
                   
                     V 
                     
                       f 
                        
                       
                           
                       
                        
                       1 
                     
                   
                 
                 
                   
                     T 
                     2 
                   
                   - 
                   
                     T 
                     1 
                   
                 
               
             
             , 
           
         
       
     
         [0000]    where C T  denotes the temperature coefficient, V f1  and V f2  are two of the plurality of voltages measured across the LED, and T 1  and T 2  are two of the plurality of temperatures at which the LED is operated. 
         [0013]    As used herein, the term “approximately” means ±10%, and in some embodiments, ±5%. Reference throughout this specification to “one example,” “an example,” “one embodiment,” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the example is included in at least one example of the present technology. Thus, the occurrences of the phrases “in one example,” “in an example,” “one embodiment,” or “an embodiment” in various places throughout this specification are not necessarily all referring to the same example. Furthermore, the particular features, structures, routines, steps, or characteristics may be combined in any suitable manner in one or more examples of the technology. The headings provided herein are for convenience only and are not intended to limit or interpret the scope or meaning of the claimed technology. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, with an emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
           [0015]      FIG. 1A  depicts a depletion region in a semiconductor diode and the charge density, electric field, and built-in potential across the depletion region; 
           [0016]      FIG. 1B  is a current-voltage (I-V) curve of semiconductor diodes; 
           [0017]      FIG. 2  illustrates an equivalent circuit diagram of an LED; 
           [0018]      FIGS. 3A and 3B  depict characteristic curves of an LED operating at temperatures from 0° C. to 80° C. on a linear plot and a semi-logarithmic plot, respectively; 
           [0019]      FIG. 4  depicts characteristic I-V curves of six LEDs connected in series at temperatures from 0° C. to 80° C. on a semi-logarithmic plot; 
           [0020]      FIG. 5  depicts temperature coefficients of six LEDs connected in series at operating currents of 1 mA and 100 μA; 
           [0021]      FIG. 6  is an implementation of an LED thermometry system in accordance with an embodiment of the invention; and 
           [0022]      FIG. 7  is a method for directly measuring the temperature of LEDs in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Refer first to  FIG. 1A , which schematically illustrates a modern semiconductor diode  100  composed of a crystalline material, e.g., silicon, that has added impurities to create an n-type semiconductor  110  (which contains negative charge carriers, i.e., electrons) or a p-type semiconductor  112  (which contains positive charge carriers, i.e., holes). After joining the n-type and p-type semiconductors  110 ,  112  together, electrons near the resulting p-n junction  114  tend to diffuse into the p-type region  116 ; likewise, holes near the p-n junction  114  diffuse into the n-type region  118 . Following such movement, the diffused electrons and holes in p-type region  116  and n-type region  118 , respectively, are eliminated due to recombination with the complementary charge carriers; this creates a depletion region  120  in which charge carriers are not mobile. The uncompensated positive and negative charge carriers left on the n-type side and p-type side, respectively, create an electric field E and a “built-in” potential V across the depletion zone  120 ; the created electric field E causes electrons to drift from the p-type side to the n-type side and holes to drift in the opposite direction.  FIG. 1A  illustrates the charge density, Q, the electric field E the built-in potential V diffused electrons and holes, and charge drift across the depletion region  120 . The depletion region  120  reaches equilibrium at a given temperature when the electric field E prevents further drift and diffusion of electrons and holes. 
         [0024]    Upon applying an external voltage  122  whose polarity opposes the “built-in” potential (i.e., a forward voltage), the crystal conducts electrons from the n-type side  110  to the p-type side  112  across the p-n junction  114  and thereby generates a substantial electric current (i.e., a forward current) through the p-n junction  114 . Referring to  FIG. 1B , a measured current-voltage (I-V) curve  130  can be used to characterize the behavior of semiconductor diodes in a circuit. For example, the shape of the curve is determined by the transport of charge carriers through the depletion region  120  near the p-n junction  114 . Typically, an approximate forward voltage (V f ) versus forward current (I f ) model of an LED operating at temperatures between 0° C. and 80° C. may be given by the equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     f 
                   
                   = 
                   
                     
                       
                         nV 
                         T 
                       
                        
                       
                         ln 
                          
                         
                           [ 
                           
                             
                               I 
                               f 
                             
                             
                               I 
                               s 
                             
                           
                           ] 
                         
                       
                     
                     + 
                     
                       
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                         s 
                       
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                         I 
                         f 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where n is the diode ideality factor which has a value between 1 and 2, R s  is the series resistance, I s  is the reverse saturation current, and V T  is the thermal voltage. The thermal voltage V T  depends on the absolute operating temperature T, and is given as: 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     T 
                   
                   = 
                   
                     
                       kT 
                       q 
                     
                     ≈ 
                     
                       
                         
                           T 
                           C 
                         
                         + 
                         273.15 
                       
                       11604.51 
                     
                   
                 
               
               
                 
                   ( 
                   2 
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         [0000]    where q is the magnitude of the electrical charge on the electron, k is Boltzmann&#39;s constant, and T C  is temperature in ° C. Based on equations (1) and (2), the thermal voltage is computed; a typical value is approximately 26 mV at a room temperature of 300 K (27° C.). 
         [0025]    Referring to  FIG. 2 , the actual diode voltage upon applying an external voltage can be deduced from the total operating voltage based on the equivalent circuit diagram  200  of an LED; the diagram  200  includes a diode junction  210 , a series resistance R s , and a shunt resistance, R sh . The operating voltage, V f , at a measured current, I, is divided across the two circuit elements: R s , and the diode  210  as: 
         [0000]        V   f   =IR   S   +V   d   (3)
 
         [0000]    where R s  is the series resistance and V d  is the voltage across the diode. At relatively low voltages, typically below 1.5 V to 2 V, the shunt resistance R sh  of the equivalent circuit  200  dominates and the LED remains cool and produces little useful light. At relatively high voltage, typically above 2.5 V, the series resistance R s  dominates and the high voltage is near the limit of LED operation. Thus, a typical operating voltage is above where the shunt resistance R sh  dominates and below where the series resistance R s  dominates. To determine the series resistance R s  the LED should be operated so that the series resistance dominates. The voltage across the series resistance R s  at high current is much larger than the voltage drop V d  across the diode  210 . An approximate value for R s  can then be obtained from the exponential curve, shown in  FIG. 1B , by graphically determining the final slope of the curve at a high current (i.e., by calculating the ratio of voltage to current). For example, for LEDs that have characteristic curves as illustrated in  FIG. 1B , the series resistance is given by R s ≈0.41Ω. 
         [0026]    With reference to  FIG. 3A , because the characteristic I-V curve of, for example, a REBEL LED is highly temperature-dependent, the temperature of the LED can be properly determined by manipulating and measuring its voltage and current if the values of the other parameters in Equation (1) are available. The characteristic I-V curve in a linear plot  310 , however, has an exponential shape; this indicates that a small increase in the forward voltage V f  results in a much larger increase in the forward current I f . In other words, the current I f  covers a large range of values while the voltage V f  has only a restricted range of values. A semi-logarithmic plot  320 , as depicted in  FIG. 3B , may be utilized to improve the resolution of the current I f  in the diagram and thus bring out features in the data that would not easily be seen when both V f  and I f  are plotted linearly. The characteristic features of V f  and I f , especially in the range of small voltages or currents (e.g., V f &lt;3 V or I f &lt;0.1 A) are clarified in the semi-logarithmic plot  320  compared with those presented in a linear plot  310 . Note that the operating current starts to bend to the right at a forward voltage V f  of about 3.2V in the top region because as current rises, resistance begins to dominate the exponential characteristics of the diode string. 
         [0027]    Multiple LEDs connected in series will require a larger voltage to operate at the same current as a single LED.  FIG. 4  depicts a semi-logarithmic plot  410  of the I f -V f  characteristic curves of six series-connected LEDs at temperatures between 0° C. and 80° C. with 20° C. increments. The individual curves are equally spaced since their temperature values are 20° C. apart. Experimentally, with renewed reference to  FIG. 3B , the forward voltage V f  of a single LED varies from 2.474 V at 0° C. to 2.289 V at 80° C. at an operating current of 100 μA. This means there is a 185 mV change in the forward voltage over an 80° C. temperature range. This change, in turn, corresponds to a temperature coefficient C T  (where 
         [0000]    
       
         
           
             
               
                 C 
                 T 
               
               = 
               
                 
                   Δ 
                    
                   
                       
                   
                    
                   
                     V 
                     f 
                   
                 
                 
                   Δ 
                    
                   
                       
                   
                    
                   T 
                 
               
             
             ) 
           
         
       
     
         [0000]    of approximately −2.3 mV/° C. for a single LED. As shown in  FIG. 4 , with the same forward current of 100 μA, the total forward voltage V a  varies between 14.82 V at 0° C. and 13.734 V at 80° C., i.e., a change of 1.086 V over an 80° C. temperature range. This corresponds to a temperature coefficient of approximately −13.575 mV/° C. for six series-connected LEDs; thus, the temperature coefficient of six series-connected LEDs is approximately six times that of the single LED operating at the same current. 
         [0028]    In addition, the curves in  FIG. 4  are steeper than those in  FIG. 3B  because the effective series resistance R s , of six series-connected LEDs is larger than that of one LED. In theory, if m LEDs are connected in series, the total applied voltage V a  is m times the forward voltage V f  of each LED because the forward current I f  flowing through them is the same. By regarding the series string of LEDs as a single LED device, the total applied voltage is given by: 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     a 
                   
                   = 
                   
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         m 
                       
                        
                       
                         V 
                         fi 
                       
                     
                     = 
                     
                       
                         
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                           T 
                         
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                                 ln 
                                  
                                 
                                   ( 
                                   
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                                     si 
                                   
                                 
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                           ] 
                         
                       
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                               si 
                             
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                   4 
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         [0000]    Assuming that the characteristic curve of a series string of LEDs is similar to that of a single LED, the composite string may be modeled using the equation: 
         [0000]    
       
         
           
             
               
                 
                   V 
                   = 
                   
                     
                       
                         nV 
                         T 
                       
                        
                       
                         ln 
                          
                         
                           [ 
                           
                             
                               
                                 I 
                                 ~ 
                               
                               f 
                             
                             
                               
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                         st 
                       
                     
                   
                 
               
               
                 
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         [0000]    which is of the same form as Equation (1), with 
         [0000]    
       
         
           
             
               
                 
                   I 
                   ~ 
                 
                 f 
               
               = 
               
                 
                   ( 
                   
                     I 
                     f 
                   
                   ) 
                 
                 m 
               
             
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         [0000]    For m identical LEDs: 
         [0000]        V   a   =m[nV   T  ln( I   f   /m )+ E   g   +I   f   R   s ]  (6)
 
         [0000]    where E g  is a value of the effective optical band gap. Equation (6) thus indicates that the total applied voltage V a  of m identical LEDs in series is equal to m times the forward voltage V f  of an individual LED when the LEDs are operated at the same forward current I f . 
         [0029]    Equation (6) also indicates that, theoretically, a relatively bigger drop of the forward voltage due to temperature increase—i.e., a larger temperature coefficient—should occur at a smaller LED operating current.  FIG. 5  depicts the relationship between the forward voltage V f  and temperature T for two values of constant forward current, i.e., I f =1 mA (line  510 ) and I f =100 μA (line  520 ). If these two lines  510 ,  520  are extended to the left, they will eventually meet at a temperature of absolute zero. Experimentally, the temperature coefficients are given by the slopes of the lines  510 ,  520 , showing that the coefficient is larger for smaller values of the operating current (i.e., 100 μA) as expected in theory. This effect can also be observed from the curves in  FIGS. 3B and 4 , where it is evident that the horizontal voltage difference between adjacent curves decreases as the vertical operating current increases. Thus, both theoretically and experimentally, for multiple LEDs (e.g., m LEDs) connected in series, a temperature coefficient approximately m times as large as that for a single LED operating at the same forward current is to be expected. Accordingly, in some embodiments, m series-connected LEDs provide a larger corresponding voltage increase in temperature resolution. 
         [0030]    Referring to  FIG. 6 , in various embodiments, a thermometer  600  is utilized to directly measure the junction temperature of LEDs  610  utilizing the temperature coefficient. A fixed DC forward current I f  is passed through the LEDs  610 , and the corresponding forward voltage V f  across the LEDs  610  is measured. Because the temperature coefficient of a semiconductor device, such as an LED, is constant when the device is operated at a constant forward current, the junction temperature of the LEDs  610  can be calculated if the temperature coefficient of the LEDs  610  at this operating current is known: the junction temperature T is proportional to the forward voltage V f  at the fixed forward current I f . The temperature coefficient is larger for a smaller operating current, and therefore, it is advantageous to choose a smaller operating current so that a larger voltage difference is produced for a given temperature change. This facilitates accurate measurement of the voltage V f . 
         [0031]    In one embodiment of the invention, the value of the temperature coefficient of the LED(s)  610  is determined using an offline calibration procedure. The value of the temperature coefficient and the calibration temperature are then stored, for example, in an area of non-volatile memory  612  in a monitoring and control module. Referring back to  FIG. 4 , the S-shaped I-V characteristic curves on the semi-logarithmic plot  410  can be split into three distinct regions  412 ,  414 ,  416 : (i) the “dark” low current region  412  located at the bottom of the “S” shape of the curve (below approximately 10 −5  A), (ii) the middle “linear” constant-slope region  414  where the LEDs begin to emit low-intensity light (between approximately 10 −5  A and 10 −2  A), and (iii) the operating current region  416  located at the top of the set of curves where it bends to the right (above approximately 10 −2  A). Since the temperature coefficient is larger at smaller current values and a reasonably large current has to flow through LEDs to cause light emission, a proper choice for the calibration current thus would be around 10 −4  A=100 μA. Additionally, the choice of this small current can reduce internal heating of the LEDs. 
         [0032]    Referring again to  FIG. 6 , in various embodiments, to determine the junction temperature of the LED(s)  610  at any given time, the power  614  to the LED(s)  610  is temporarily disconnected and a constant current  616  is applied to the LED(s)  610  for a short time duration t; the time duration t is sufficient for measuring the voltage across the LEDs  610  but insufficient to be detected by the human eye, thereby imposing at most a negligible impact on normal LED operation. The applied current  616  is not critical and reflects an engineering tradeoff: typically, the current will lie in the linear region  414  of the S-shaped characteristic curve of the LED being monitored, as described above, and should produce a large enough voltage signal to be measured with adequately low error—that is, if the chosen current  616  is too small, then the voltage across the LEDs  610  at that current will also be small and the measurement resolution will be reduced. If the chosen current  616  is too large, on the other hand, internal heating will cause errors (even though the voltage signal will be large and thereby aid resolution). The optimal current, therefore, reflects the characteristic curve for a particular manufacturer&#39;s LED (i.e., the voltage range produced over the temperature range being measured), as well as the complexity of the voltage measurement circuitry being employed. 
         [0033]    In various embodiments, while the constant current  616  is flowing through the LED(s)  610 , the voltage across the LED(s)  610  is measured and the junction temperature is calculated by the controller  618  (e.g., by firmware in the controller&#39;s microprocessor). The controller  618  schedules a time for a temperature measurement to take place and, at the appointed time, the electronically controlled switch  620  is flipped to connect the constant current source  616  to the LED(s)  610 . While the switch  620  is in this position, the power controller module  622  is temporarily disabled and the voltage measurement  624  of the LED(s)  610  is taken. Once the measurement is complete, the switch  620  is restored to its original position and the LED power control resumes. The measured voltage is then processed by the controller  618  to calculate the junction temperature and, based thereon, an operational current and temperature that optimizes the performance and lifetime of the LED can be calculated by the controller  618 . Values for the optimal load current and the associated temperature are sent to the LED power controller  622  and appropriate actions can be taken—e.g., adjustment of the load current and the associated temperature to optimize the lifetime of the LED or shutdown the circuit due to overheating or any other fault conditions. In one embodiment, the thermometer  600  includes a detecting sensor  626 ; upon detecting a luminous intensity of light in the environment below a predetermined threshold, the sensor transmits a signal to the controller  618 , automatically triggering a larger load current to flow through the LEDs  610 , thus increasing the brightness of the LEDs  610 . The temperature increase resulting from the current increase is measured and monitored by the controller  618 ; the controller  618  adjusts the load current again to prevent overheating of the LEDs  610 . This process may be repeated until an optimal combination (e.g., in terms of performance and LED lifetime) of LED brightness and operating temperature is achieved. Systems and methods based on this approach provide a fast, easily implemented, and inexpensive way to directly measure the actual junction temperature of the LEDs and optimize the performance and lifetime of the LEDs. A temperature coefficient can be determined by simply measuring the LED voltage at various temperatures while the LED is driven at a constant current. The resulting straight line provides the temperature coefficient per Equation (7) below. In general, a single coefficient is determined from the slope of the line. If multiple lines are obtained due to errors in the measurements, a curve fit, such as a regression analysis, may be employed and the average slope obtained. However, this is rarely necessary as the physical behavior of the LEDs is well controlled by the manufacturer and by the physics of semiconductors. 
         [0034]    The controller  618  and/or the LED power controller  622  may be provided as either software, hardware, or some combination thereof. For example, the system may be implemented on one or more server-class computers, such as a PC having a CPU board containing one or more processors such as the CORE PENTIUM or CELERON family of processors manufactured by Intel Corporation of Santa Clara, Calif. and POWER PC family of processors manufactured by Motorola Corporation of Schaumburg, Ill., and/or the ATHLON line of processors manufactured by Advanced Micro Devices, Inc., of Sunnyvale, Calif. The controller  618  and/or the LED power controller  622  may also include a main memory unit for storing programs and/or data relating to the methods described above. The memory may include random access memory (RAM), read only memory (ROM), and/or FLASH memory residing on commonly available hardware such as one or more application specific integrated circuits (ASIC), field programmable gate arrays (FPGA), electrically erasable programmable read-only memories (EEPROM), programmable read-only memories (PROM), or programmable logic devices (PLD). In some embodiments, the programs are provided using external RAM and/or ROM such as optical disks, magnetic disks, as well as other commonly used storage devices. 
         [0035]    For embodiments in which the controller  618  and/or the LED power controller  622  are provided as a software program, the program may be written in any one of a number of high level languages such as FORTRAN, PASCAL, JAVA, C, C++, C#, LISP, PERL, BASIC, PYTHON or any suitable programming language. Additionally, the software can be implemented in an assembly language and/or machine language directed to the microprocessor resident on a target device. 
         [0036]    In some embodiments, a constant current is passed through the LEDs and the voltage across them is measured at a plurality of temperatures (at least two: the maximum and minimum expected operating temperatures). Then a straight line is drawn between the temperature-voltage pairs and the coefficient is determined as the slope of the line of the resulting graph in volts per ° C. (or mV/° C.). Referring to  FIG. 7 , in some embodiments, the following steps are used to calibrate the thermometer, measure an actual junction temperature of the LED(s) during operation, and adjust the temperature accordingly: 
         [0037]    (A) choosing a fixed operating current, such as 100 μA as previously discussed, for the constant current source (step  710 ); 
         [0038]    (B) passing the fixed current through the LED(s) at a temperature, T 1 , and recording the value of the forward voltage, V f1 , across the LED(s) (step  720 ); 
         [0039]    (C) passing the fixed current through the LED(s) at a temperature T 2  and recording the value of the forward voltage V f2  (step  730 ). A reasonably large range of temperatures between T 1  and T 2  should be used as is feasible; 
         [0040]    (D) calculating the temperature coefficient (step  740 ) using the following formula: 
         [0000]    
       
         
           
             
               
                 
                   
                     C 
                     T 
                   
                   = 
                   
                     
                       
                         
                           V 
                           
                             
                               f 
                                
                               
                                   
                               
                                
                               2 
                             
                              
                             
                                 
                             
                           
                         
                         - 
                         
                           V 
                           
                             f 
                              
                             
                                 
                             
                              
                             1 
                           
                         
                       
                       
                         
                           T 
                           2 
                         
                         - 
                         
                           T 
                           1 
                         
                       
                     
                      
                     
                         
                     
                      
                     mV 
                      
                     
                       / 
                     
                      
                     
                       ° 
                        
                       
                           
                       
                        
                       C 
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0041]    (E) determining the temperature, T m , of the LED(s) operated under a normal condition (step  750 ) as: 
         [0000]    
       
         
           
             
               
                 
                   
                     T 
                     m 
                   
                   = 
                   
                     
                       T 
                       2 
                     
                     + 
                     
                       
                         
                           
                             V 
                             m 
                           
                           - 
                           
                             V 
                             
                               f 
                                
                               
                                   
                               
                                
                               2 
                             
                           
                         
                         
                           C 
                           T 
                         
                       
                        
                       
                           
                       
                        
                       ° 
                        
                       
                           
                       
                        
                       
                         C 
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where V m  is the measured forward voltage across the m LED(s) at the same fixed current that was used for the calibration. As an example, assume that T 2 =85° C., V f2 =15.50 V, C T =−14 mV/° C., and the voltage measured across the LED(s) is V m =15.22 V, we can calculate the temperature of the LED(s) as: 
         [0000]    
       
         
           
             
               T 
               m 
             
             = 
             
               
                 85 
                 + 
                 
                   
                     15.22 
                     - 
                     15.50 
                   
                   
                     
                       - 
                       14 
                     
                     × 
                     
                       10 
                       
                         - 
                         3 
                       
                     
                   
                 
               
               = 
               
                 105 
                  
                 ° 
                  
                 
                     
                 
                  
                 
                   C 
                   . 
                 
               
             
           
         
       
     
         [0042]    (F) sending the information about the computed temperature to the LED power controller (step  760 ); and 
         [0043]    (G) adjusting the load current passing through the LEDs to change the LED temperature (step  770 ). 
         [0044]    In one embodiment, steps  750 - 770  are iteratively implemented until the measured temperature of the LED(s) is optimized for LED performance and lifetime; the temperature is then maintained within a fixed range (e.g., within ±10% of the recommended operating temperature) during LED operation. This approach thus provides a fast and inexpensive way to directly measure the actual junction temperature of LEDs and adjust the temperature accordingly. 
         [0045]    In some embodiments, the luminous intensity in the environment is detected (step  780 ). If the intensity is below a threshold, a larger load current is adjusted to flow through the LEDs to increase the brightness (step  790 ). The temperature increase resulting from the current increase is then measured and this temperature information is sent to the controller to further adjust the load current to prevent overheating of the LEDs, if necessary. This process may be repeated until an optimal combination (e.g., in terms of performance and LED lifetime) of LED brightness and operating temperature is achieved. 
         [0046]    In accordance with the approach disclosed herein, LED manufacturers may publish a table of temperature coefficients versus current. The lighting designer may then choose a measurement current based on the considerations outline above, and obtain the corresponding coefficient. The coefficient may be multiplied by the number of LEDs in the circuit to derive the overall coefficient for that current. The selected number of LEDs may then be connected in series and voltage measured at even a single selected temperature. This information (the coefficient and the one temperature-voltage point, as well as the measurement current value chosen) may be stored in memory, and firmware in the lighting module or luminaire can then determine the temperature of the LEDs during operation. The same data obtained from the single measurement could be stored in all lighting devices that use the same type and number of LEDs. 
         [0047]    The terms and expressions employed herein are used as terms and expressions of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof. In addition, having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. Accordingly, the described embodiments are to be considered in all respects as only illustrative and not restrictive.