Black and White and Colour

Abstract

What is it like to be a rod achromat? At night, or more concisely under low lighting conditions, the ordinary trichromat experiences the visual world by means of signals from the rods alone. This experience of the night ime world is commonly described as `seeing in black and white'. But if `seeing in black and white' describes the trichromat's visual experience under conditions of weak illumination, and if the visual experience of the rod achromat is a function of rod input under all lighting conditions, then the rod achromat must also see ``in black and white.'' Despite the intuitive nature of this line of reasoning, this conclusion is probably not correct. Through an exploration of the nature of luminance vision and albedo perception (the ability to see object surfaces as inherently light or dark), this paper explains why the dichotomy of `black and white or colour' is not a distinction that sits easily with the processes of human vision–and hence cannot be used to explain either the achromat's visual phenomenology or the trichromat's visual experience of the world at night.