Software determination of LED brightness and exposure

Abstract

The junction temperature of an LED is determined. This result is used to determine an initial brightness value of the LED. The initial brightness value is derated by a long-term degradation amount that is determined using a stored indication of the lifetime total on time. The derated brightness value is used to determine an exposure for a capture system. The junction temperature is determined by measuring the ambient temperature and then keeping a running junction temperature determined from the on and off times.

What is claimed is:  
     
         1 . An image capture device, comprising: 
 an illumination source;    a thermal model of said illumination source that determines a temperature of said illumination source;    a light output model of said illumination source that determines a light output of said illumination source from said temperature; and,    an exposure adjustment that is changed to compensate for changes in said illumination source as indicated by said model output.    
     
     
         2 . The image capture device of  claim 1  wherein said thermal model has a model input and said model input is an indication of the on times and the off times of said illumination source.  
     
     
         3 . The image capture device of  claim 2 , further comprising: 
 an ambient temperature sensor producing a sensed ambient temperature wherein said temperature is affected by said sensed ambient temperature.    
     
     
         4 . The image capture device of  claim 3  wherein said illumination source is at least one light emitting diode.  
     
     
         5 . The image capture device of  claim 4  wherein said thermal model of said illumination source comprises software executing on a computer.  
     
     
         6 . The image capture device of  claim 4  wherein said illumination model of said illumination source comprises software executing on a computer.  
     
     
         7 . The image capture device of  claim 4  wherein said exposure adjustment changes said on times of said illumination source.  
     
     
         8 . A method of compensating for changes in an illumination source, comprising: 
 determining a temperature of said illumination source;    determining a light output of said illumination source from said temperature; and,    adjusting an exposure to compensate for changes in said illumination source as indicated by said light output.    
     
     
         9 . The method of  claim 8  wherein said determining a temperature is done using an indication of the on times and the off times of said illumination source.  
     
     
         10 . The method of  claim 9  further comprising: 
 sensing an ambient temperature.  
 
     
     
         11 . The method of  claim 10  wherein said illumination source is at least one light emitting diode.  
     
     
         12 . The method of  claim 11  wherein said step of determining a light output includes derating said light output by an amount determined in part by a long-term total on time of said illumination source.  
     
     
         13 . An article of manufacture comprising a program storage medium having computer readable program code means embodied therein for causing the adjustment of an exposure, the computer readable program code means in said article of manufacture comprising: 
 computer readable program code means for causing a computer to determine an indication of a temperature of an illumination source;    computer readable program code means for causing a computer to determine an indication of brightness of said illumination source from said indication of said temperature; and,    computer readable program code means for causing said computer to adjust said exposure based on said indication of said illumination sources brightness.    
     
     
         14 . The article of manufacture of  claim 13  further comprising: 
 computer readable program code means for causing said computer to turn on and turn off said illumination source.  
 
     
     
         15 . The article of manufacture of  claim 14  further comprising: 
 computer readable program code means for causing said computer to determine the on times and off times of said illumination source.  
 
     
     
         16 . The article of manufacture of  claim 15  further comprising: 
 computer readable program code means for causing said computer to obtain an indication of an ambient temperature; and,  
 computer readable program code means for causing said computer to adjust said temperature based on said indication of said ambient temperature.  
 
     
     
         17 . The article of manufacture of  claim 16  wherein said illumination source is at least one light emitting diode.  
     
     
         18 . The article of manufacture of  claim 17  further comprising: 
 computer readable program code means for causing said computer to obtain an indication of a lifetime on time of said illumination source; and,  
 computer readable program code means for causing said computer to adjust said indication of brightness of said illumination source based on said indication of said lifetime on time.  
 
     
     
         19 . The article of manufacture of  claim 18  wherein said indication of said lifetime on time is stored in a non-volatile storage device.  
     
     
         20 . An image capture device, comprising: 
 illumination means;    thermal modeling means, said thermal modeling means producing a thermal modeling means output that is indicative of said illumination means temperature;    brightness modeling means, said brightness modeling means producing a brightness modeling means output that is indicative of said illumination means brightness; and,    exposure adjustment means for changing and exposure to compensate for changes in said brightness of said illumination means as indicated by said brightness modeling means output.    
     
     
         21 . The image capture device of  claim 20  wherein said thermal modeling means has a thermal modeling means input and said thermal modeling means input is an indication of the on times and the off times of said illumination means.  
     
     
         22 . The image capture device of  claim 21 , further comprising: 
 ambient temperature sensor means for producing a sensed ambient temperature wherein said thermal modeling means output is changed to compensate for said sensed ambient temperature.    
     
     
         23 . The image capture device of  claim 22  wherein said illumination means is at least one light emitting diode.  
     
     
         24 . The image capture device of  claim 23  wherein said brightness modeling means output is affected by an indication of a lifetime on time of said illumination means.  
     
     
         25 . The image capture device of  claim 24  wherein said indication of said lifetime on time of said illumination means is stored in a non-volatile memory means.  
     
     
         26 . The image capture device of  claim 25  wherein said exposure is adjusted by changing said on times of said illumination source.