a. Photopic Vision. During periods of sunlight or high illumination we
find the aperture of the eye (pupil) closing to accommodate the brightness.
Discriminations during these periods require only the use of cones. Since
the focal point on the retina is the fovea, this set of conditions gives a
clear, crisp picture with good color discrimination and depth of view (depth
b. Mesopic Vision. During periods of reduced illumination the pupil
opens to accommodate the reduction in illumination. Discriminations during
these periods require the use of both rods and cones. The focal point on
the retina is still the fovea, but since the discrimination must be made on
the periphery of the fovea, the picture transmitted to the brain will be
different. Pictures will not be as sharp and colors will change.
c. Scotopic Vision. During periods of darkness or low illumination the
pupil is completely open to accommodate the low illumination.
Discriminations during these periods require the use of rods only. During
these periods you will have a central vision blind spot (Figure 6), be
unable to distinguish the color of objects and peripheral vision will be the
only means of seeing very dim objects.
Night blind spot.
NIGHT VISION LIMITATIONS AND CAPABILITIES
There are four limitations to vision created by reductions in the
amount of illumination. These are the existence of the night blind spot,
the inability to detect objects while the eye is in motion, the degradation
of depth perception and a reduction in visual acuity.