42 



PHYSICAL ASPECTS OF VISION 



Simple model 



Fluctuation model 



Threshold intensity 



/_ 



Intensity 



Fig. 17. A schematic representation of the frequency with which a flash of 

 each intensity is seen, for two models. The simple model assumes that each flash 

 has the same number of photons. The fluctuation model takes into account the 

 fact that a given average number of photons (plotted on the abscissa) will have 

 a fluctuating number of photons in each individual flash, for experimental 

 reasons. 



Intensity of the light flashes 

 (on a logarithmic scale) 



Fig. IS. Calculated curves of the fraction of times a light flash will contain 

 at least n photons for the average intensity plotted on the abscissa. 



a fluctuating number of photons, and only flashes containing at least the 

 threshold number will be seen. As the intensity increases, a steadily in- 

 creasing proportion of flashes will exceed the threshold number. Finally, 

 intensities will be reached for which, despite the fluctuations, all flashes 

 will exceed the threshold number of photons. 



The simple model and the fluctuation model are presented schemati- 

 cally in Fig. 17. A flash of a given intensity is presented repeatedly to 

 the subject, and the fraction of times he sees the flash is recorded. 

 Then, in succeeding experiments, using a series of intensities, similar 

 data are recorded. The data are then plotted to show the fraction of 

 times the flash is seen for each incident intensity. 



