Figure 6. Night blind spot.

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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.

Figure 6.

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.