Visual Mechanism

Abstract

A visual sensation is the result of processes in the eye and brain. Light entering the eye is projected on the back of the inner part of the eye, the retina. The retina contains photoreceptor cells: cones and rods. Photopigments in these receptor cells absorb light, resulting in a chemical-electrical signal which travels down a nerve into the visual cortex part of the brain where the visual sensation is invoked. A small area of the retina around the axis of the eye, the fovea, only contains cone cells. The other, peripheral, areas have few cone and many rod cells. The cone cells in the fovea have a one-to-one nerve connection to the brain. Rod photoreceptor cells are located in the periphery of the retina. Many of them converge on a single ganglion cell. Consequently, foveal vision is sharp and peripheral vision is not sharp. The set of rods converging on the same ganglion (the receptive field of that cell) are processed through an opponent mechanism. Colour vision is possible because there are three types of cones, one with sensitivity for reddish, one for greenish and one for bluish light. A colour opponent mechanism processes their signals. Since we have just one type of rod cell, colour vision with rods is impossible. Cones are mainly active at lighting levels larger than some 5 cd/m ² . Vision is then referred to as photopic. The spectral eye sensitivity curve V(λ) defined for photopic vision is the basis for all photometric units.