Abstract:
A high brightness LED ( 102 ) is precisely controlled. The temperature of the LED ( 102 ) is controlled via controlled thermal resistance ( 300 ), measurement of the base temperature ( 302 ) and careful power monitoring of the LED ( 102 ).

Description:
RELATED APPLICATION DATA 
     This application claims priority from U.S. provisional application Ser. No. 61/178,634 filed May 15, 2009, that we incorporate by reference. 
    
    
     FEDERALLY SPONSORED RESEARCH 
     Not Applicable 
     SEQUENCE LISTING OR PROGRAM 
     Not Applicable 
     FIELD OF INVENTION 
     This invention relates to calibration standards, specifically calibration standards for High Brightness LEDs (HBLEDs). The invention provides a stabilized light output, including both a stabilized light intensity and a stabilized light wavelength. 
     BACKGROUND OF THE INVENTION 
     LED calibration standards today use an LED which uses 20 mA of current. An example of a calibration standard today is the Inphora IPR-DES2 unit. This unit supplies about 20 mA at approximately 3V to the LED. A key attribute in a stable calibration standard is the temperature of the LED is controlled. In the Inphora IPR-DES2 unit this is done by sensing the LED voltage. The LED voltage will change depending on temperature approximately −2 mV per degree C. This is fed back to the circuit to stabilize the temperature. 
     LEDs will change properties over time. Observations have been made where the voltage across the LED will increase over time, even though the temperature and current are controlled. The variation over time is likely due to changes in the optically active band gap of the semiconductor. Thus a feedback system using the LED voltage will vary the controlled temperature. The rate of change of this property varies on how long the LED has been in operation. One technique to minimize this effect is to age the LED until the rate of change is low enough to tolerate. 
     In the Inphora IPR-DES2 unit, the LED has a power dissipation of 60 mW. The low power dissipated is adequately cooled with the surrounding environment. Since the LED temperature is above ambient, heating is the main issue, not cooling. For HBLED units the power dissipation is 3.2 W or higher; therefore more power needs to be dissipated. Thus the cooling and dissipation of the heat would be a problem with the current scheme. 
     The LED junction is maintained at a higher temperature than the surrounding environment. Nominally 60 degrees C. is the target temperature in the Inphora IPR-DES2 unit. By picking a temperature higher than ambient, only a heater is needed to control the temperature of the LED. The Inphora IPR-DES2 unit currently has a heater, but has no need for heat removal given the low power dissipated. 
     Therefore what is needed is an LED calibration standard that can dissipate the greater amount of heat generated by HBLED and provide a controlled HBLED light output that is independent of the age of the HBLED used in the standard. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present disclosure solves many of the disadvantages associated with existing LED calibration standards. Several of the many objects and advantages of the present invention are: 
     (a) High power LED drive, 920 mA at 3.2V 
     (b) Precise current control, 920 mA controlled to +/1 mA 
     (c) LED aging does not effect temperature control 
     (d) High power (approximately 3.2 W) heat is removed effectively from the LED to allow precise control of the temperature. 
     (e) Cooling of the LED and moving the heat to an outside radiator without appreciably increasing temperature in the vicinity of the LED, 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is the front view of the HBLED calibration standard 
         FIG. 2  is the side view of the HBLED calibration standard 
         FIG. 3  shows the copper bar, the LED on the LED board, and the heater board. 
         FIG. 4  is the schematic of the control system, implementing the correction equation 
     
    
    
     DRAWING 
     Reference Numbers 
     
         
           100  LED board. The LED mounts to this board 
           102  LED 
           104  tube 
           106  mounting screw 
           200  2 pin connector, supplies power to LED 
           202  Heater board 
           204  Power board, supplies LED power and Heater power 
           206  Control board 
           208  Copper bar 
           300  controlled thermal resistance feature in copper bar 
           302  mounting hole for thermistor to measure base temperature of copper bar 
       
    
     DETAILED DESCRIPTION 
     A preferred embodiment of the HBLED calibration standard is illustrated in  FIGS. 1-3 . A tube  104  with a diameter of 25 mm is used. The current calibration standards and test fixtures are designed for 25 mm Outside Diameter (OD) tubes. The HBLED  102  is centered on a board  100 . The board  100  is held to a copper bar  208  by plastic screws  106 . 
     In the preferred embodiment the copper bar  208  is used. The bar could alternately be made of other materials such as Aluminum, but with increased thermal resistance. 
     In the preferred embodiment, a heater board  202  is used to generate heat. The board is composed of 8 1 ohm resistors in series. The LED board  100  is connected to the power board  204  using a 2 pin connector  200 . The power board  204  provides 920 mA of current to the LED+/−2%. The power board  204  also provides the power to drive the heater board  202 . The power board  204  is controlled by the control board  206 . The copper bar  208  provides the thermal pathway for cooling the system. Not shown is the heatsink at the end of the copper bar providing cooling to the environment. 
     The copper bar  208  has a narrow feature  300  to provide controlled thermal resistance. In the preferred embodiment, the copper bar  208  has dimensions of about 0.1″.times.0.177″.times.0.328″ in the preferred embodiment. The feature  300  provides a controlled thermal resistance of .about.4 degree C./W. This feature&#39;s dimensions are calibrated to the amount of power dissipated by the combined LED  102  and heater board  202  to provide a controlled temperature to the LED based on the variation in room temperature through the heat sink. 
     The hole  302  is where a precision thermistor is placed to measure the temperature. 
     The mass of a copper used provides a nice damper on the system. 
     To control the temperature of the system, the power to the heater board  202  is controlled. The control is based on the equation:
 
 P heater=( T set− T cool)* K 1+( P set− P led)*K2
 
     Pheater is the power to the heater board  202 . Tset is the desired temperature at the LED  102 . Tset is set to be 20 to 30 degrees higher than the maximum temperature in the room of operation. In the preferred embodiment, Tset is 60 degrees C. Tcool is the temperature measured at the hole  302 . Pset is set to the maximum power the LED  102  will ever consume in any situation. The intent of (Pset-Pled) part of the equation is to cause the system to provide constant power through controlled thermal resistance feature  300 . K1 is the coefficient needed to convert temperature to power. K1 is determined by the thermal resistance from the heater to the cooling base. K2 is the coefficient needed to convert the difference in power to the LED  102  and the maximum power. The coefficient K2 is only a scaling factor and the thermal resistance does not come into play. 
     In the preferred embodiment,  FIG. 4  is an electronic control used. The circuit implements the control as described in the above equation. The inputs are the voltage corresponding to the temperature at the hole  302  (Vtherm) and the voltage across the LED  102 . The voltage across the LED  102  can be converted to the power knowing the current is set at 920 mA. The output voltage of the control system, Vpwr, varies between 0V and 1.25V. The maximum power of the heater board  202  is 8 W when Vpwr is 1.25V. 
     ADVANTAGES 
     From the description above, a number of advantages of the stabilized HBLED system become evident: 
     (a) The aging of the LED does not change the operational points 
     (b) The power from the LED is removed from the area of the LED in a controlled manner 
     (c) The unit fits into a small 25 mm tube package allowing for the continued use of current test systems. 
     (d) The control systems avoid temperature oscillations 
     (e) The control system is stable 
     CONCLUSIONS, RAMIFICATIONS, AND SCOPE 
     Thus the reader will see that the stabilized HBLED unit provides a highly stabilized reliable high brightness LED useful as a laboratory standard. This invention allows the continued use of standard text fixtures with high power dissipation in the LED. Also the invention controls the temperature precisely allowing the LED to operate in a consistent manner. 
     Although the description above contains many specificities, these should not be construed as limiting the scope of the invention but merely providing illustration of some of the presently preferred embodiments of this invention. For example the control system could be implemented in a microprocessor. As another example the shape and size of the thermal resistance feature could be changed if the power consumed in the LED is changed. The size of the thermal resistance feature could be varied if the set point is changed, etc. 
     Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.