Rod and Cone Contributions to Mesopic Vision

Mesopic light levels span approximately 3-4 log units in the natural viewing environment, including many indoor lighting settings, and most twilight or nighttime outdoor and traffic lighting environments. Traditional studies of the contribution of the different photoreceptor types to mesopic vision have been hampered by an inability to modulate the rod or cone systems independently. Our approach differs from the traditional studies in employing a four-primary method that allows independent control of the stimulation of the four receptor types in a stimulus field. We developed a general method for adjusting the radiances of four lights to silence up to three photoreceptor classes and change the excitation of the nonsilenced classes in a specified manner. Here, we include an overview of rod anatomy and physiology and a description of the logic and implementation of the four-primary methodology. Data and analysis are given for two functional areas that may impact lighting solutions; dark-adapted rods suppress cone vision, and how rod and cone reaction times vary at scotopic, mesopic and photopic adaptation levels.