Hardware modeling of LED relative brightness

Abstract

Via simple electronic circuitry, an analog voltage that tracks the LED light output is produced. This analog voltage is read by an A/D converter to ascertain an approximate relative light output of the LED so that light output compensation can be quickly calculated. A resistor-capacitor circuit is used to approximate the behavior of the LED light output. The output voltage from this circuit is sampled and used along with a sensed ambient temperature to adjust the exposure time of an image capture system.

1 . An image capture device, comprising: 
 an illumination source connected to a power source;    a model of said illumination source, said model being a circuit, said model comprising a circuit model output and a model input, wherein said circuit input is connected to said power source; and    an exposure adjustment device coupled to said model output, wherein said exposure adjustment device compensates for changes in said illumination source as indicated by said model output.    
   
   
       2 . The image capture device of  claim 1  wherein said model provides 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 exposure adjustment is also changed to compensate for 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 model of said illumination source comprises a capacitor and a resistor.  
   
   
       6 . The image capture device of  claim 4  wherein said model of said illumination source comprises an inductor and a resistor.  
   
   
       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, said method comprising: 
 modeling said illumination source using a circuit, said circuit comprising an input and an output;    applying a potential to said illumination source and the input of said circuit;    monitoring the potential of the output of said circuit; and    adjusting an exposure to compensate for changes in said illumination source based on said potential of the output of said circuit.    
   
   
       9 . The method of  claim 8  wherein said circuit provides 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; and    adjusting said exposure to compensate for said 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 circuit comprises at least one capacitor that is charged and discharged.  
   
   
       13 . The method of  claim 12  wherein the charging and discharging of said at least one capacitor is done through at least one resistor.  
   
   
       14 . The method of  claim 11  wherein said circuit comprises at least one inductor that is energized and de-energized.  
   
   
       15 . The method of  claim 14  wherein the rate of energizing and de-energizing is determined by at least one resistor.  
   
   
       16 - 29 . (canceled)