for the Measurement of Visibility of Objects. 103 



is just perceptible to the eye when adapted to the field 

 brightness B 2 . The sensibility is inversely proportional 

 to /\B and hence to 



AB 

 fix ■ 



Fig. 2. 



SENSIBILITY -OF-THE- EYE • fo 

 • BRIGHTNESS- CONTRAST- 



— 1 









— t 







A 



a 



-J 



( LQ< 



I B,J 





■ 











i 





















































































































i 



































































































































































































































































































































































S 

































































i 







! 









s- 



LOG BRIGHTNESS, LOG B(™0 



The Sensibility of the Eye to Brightness Contrast. 



The curve shown is therefore the reciprocal of the sensibility 

 curve and is given in this form for the sake of convenience 

 in application to this problem. It will be noted that the 

 sensibility at low values of B is relatively low (AB large) 

 increasing to a maximum (&B small), which remains sensibly 

 constant over a considerable range of brightness values and 

 then again decreasing for very high values of B x . 



The term k, which will be called the "contrast constant" 

 of the eye for any given value of B, is defined by the equation, 



* = 1 + 



AB 



