Marc Green Phd.
Night vision is an important factor in understanding the cause of accidents that occur under low visibility. Here, I briefly outline some basics, roughly what I would expect my students to know at the end of an introductory perception course. [See related articles The Invisible Pedestrian and Police Shootings.]
Photopic, Mesopic and Scotopic Vision
Humans can see over a light intensity range of several million to one. In order to achieve this extraordinary feat while maintaining good contrast sensitivity, the eye adjusts to the prevailing conditions and changes its mode of operation as light levels decline from day to night.Every beginner’s textbook discusses rods and cones, so amateurs pick up on this these terms and focus too heavily on them. Photoreceptors alone are insufficient to explain night vision. Moreover, rod vision and night vision are not synonymous. The more important concept is „receptive field,“ which is fundamental to all visual processing. Anyone who claims to be an expert in vision/perception must have a thorough understanding of receptive fields, their various types, how they operate, how they change with conditions and how they determine visual capability. I won’t go into a full explanation of receptive fields because it is too large a topic. However, I will mention two of their properties, inhibition and convergence.
Individual cones and rods have very similar sensitivity to light. Both respond to a single quantum of light, although rods produce a bigger response. A major difference between day and night vision is inhibition and convergence, the way the photoreceptors are wired together, and the amount of light-sensitive photopigment available. Moreover, most „night vision“ occurs in a mixed rod/cone mode. The overall operation of the eye in diminishing light levels is better described in terms of three operating modes, photopic, mesopic and scotopic. Photopic vision occurs at high light levels and is characterized by 1) cone photoreceptors, 2) low light sensitivity, 3) high acuity and 4) color vision. Scotopic vision occurs at very low light levels and exhibits 1) use of rod photoreceptors, 2) high light sensitivity, 3) poor acuity and 4) no color vision.
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