The Eye and Vision
The visual process consists of
- Formation of the optical image on the retina
- Conversion of the image into nerve impulses
- Interaction between these pulses and their transmission to the brain
- Interpretation in the brain
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%)
- 120 million rods (diameters 1-2.5 microns)
- 6.8 million cones (diam. 1-3 microns; ~million in central 1 deg.)
- fovea is only area where detailed vision is possible (1.5 mm=>5 deg)
- there are less than 1 million fibers in the optic nerve
implying considerable preprocessing
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
- Change in Pupil Diameter
Approximate range is 2-8 mm (less with age) for a factor ~15 in aperture.
Decreases in seconds, dilation over minutes.
pupil size is smaller with more blue content in the light
- Neural Adaptation
Less than 1 sec sensitivity adjustments by synaptic interactions,
mostly at moderate levels ~500 cd/m^2
- Photochemical Adaptation
- rods: rhodopsin (or visual purple) is bleached by light
when illumination is removed, the retina regenerates the rhodopsin,
requiring about an hour after strong bleaching
rod saturation starts ~10 cd/m^2, total ~300 cd/m^2
regeneration time constant is about 7.5 min
- cones: red (erythrolab), green (chlorolab), blue (cyanolab)
bleaching reduces sensitivity
regeneration of cone pigments is also exponential but with a time
constant of 2 minutes.
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