12-7] IMPORTANT CHARACTERISTICS OF THE HUMAN OPERATOR 641 



In practical cases the detection of a spot is strongly dependent upon the 

 brightness of the spot relative to the background. Intuitively, contrast is 

 usually defined as the difference in brightness or intensity between the spot 

 and the background. In vision experiments, contrast is defined as the ratio 

 of the diff^erence in energy between the spot and the background to the 

 energy of the background. Conventionally the measure of energy that is 

 used in determining contrast is brightness: 



Bs- Bb ^ A5 

 Bb B 



Contrast 



where Bs — spot brightness 



Bb = background brightness. 



(12-5) 



100 



0.10 



0.01 



0.001 



0.01 0.1 



EXPOSURE TIME (sec) 



Fig. 12-22 Brightness of Spot X Exposure Time as a Function of Exposure Time, 

 Background Brightness, and Spot Size for a Detection Probabihty of 50%. 



