The Eye and Vision

The visual process consists of Human vision is remarkable in many respects: it accommodates to a range in brightness of at least 1,000,000,000,000 through pupilary, photochemical, and neurological changes in the eye; there is a significant amount of image processing in the neural network of the eye; there is a physiological feedback mechanism affecting behavior.

The rear of the eye is lined by the retina; it contains the photosensitive elements that convert the incident light into nerve pulses. There are several types of detector cells.

Under bright conditions the cone cell receptors of the retina operate (photopic vision); they are concentrated near the center of the retina. and there are three types primarily sensitive to red (580 nm), green (550 nm), and blue (450 nm) light. The detective threshold for cones is ~0.001 cd/m^2.

Under conditions of very very low light levels, the rods are the photon receptors (scotopic vision); there are more away from the center of the retina; the rods do not see colors. Rods require 60 min or longer to fully adapt to darkness. The scotopic rod threshold is ~0.000001 cd/m^2.

At intermediate luminance levels the visual response is somewhere in between. This mesopic transition region is usually taken to be from 0.034 to 3.4 cd/m^2.

Retina (analog to front-illuminated 11K by 11k CCD, 2 micron pixels, QE~20%)

Adaptation Processes

shift the operating range of the eye; they are not instantaneous.

The dynamic range for the brightness sensation ranges over about 3 decades; it is non-linear, and we tend to see things in ratios; usually scarcely notice a factor of two change.

Yet the eye accommodates a luminance range of as large as 14 decades; there are at least four major mechanisms

  • Transient adaptation is the phenomenon associated with reduced visibility after viewing a high or low luminance (eg walking into a dark movie theater from the outside on a sunny afternoon). Photopigments are involved in all changes taking more than a second.

    Photobiology

    There is a optic neural pathway to the hypothalamus (then on to other parts of the brain) anatomical separate from the pathway to the visual cortex, with various consequences.

    Circadian rhythms, Vitamin D and Calcium metabolism, SAD
    UV in the 220-300 nm band is germicidal (DNA absorption peak)
    Age effects
    Other effects on flora and fauna


    p.a. ianna - 1 November 1997