Page 180 - Modern Optical Engineering The Design of Optical Systems

Characteristics of the Human Eye 163

can repeat vernier settings to better than 5 seconds of arc and that he
or she will be accurate to about 10 seconds of arc. Exceptional individ-
uals may do as well as 1 or 2 seconds. Thus, the vernier acuity is 5 or
10 times the visual acuity. Vernier acuity is best when setting one line
abutting between two, next best setting a line on cross hairs or align-
ing two butting lines, and less effective in superimposing two lines.
The narrowest black line on a bright field that the eye can detect
1
subtends an angle of from
to 1 second of arc. In conditions of
2
reversed contrast, i.e., a bright line or bright spot, the size of the line
is not as important as its brightness. The governing factor is the
amount of energy which reaches and triggers the retinal cell into
responding. The minimum level seems to be 50 to 100 quanta incident
on the cornea (only a few percent of the energy incident on the cornea
actually reaches the cell).
The eye is capable of detecting angular motion to the order of
10 seconds of arc. The slowest motion that the eye will detect is 1 or
2 minutes of arc per second of time. At the other extreme, a point mov-
ing faster than 200° per second will blur into a streak.
The eyes judge distance from a number of clues. Accommodation,
convergence (the turning in of the eyes to view a near object), haze,
perspective, experience, etc., each play a part. Three-dimensional, or
stereo, vision results from the separation of the two eyes, which causes
each eye to see a slightly different picture of an object. The amount of
stereo parallax which can be detected is as small as 2 to 4 seconds. In
a clueless surround, a test subject can adjust two rods to be equidis-
tant to within about 1 in when the rods are 20 ft away. The detectable

D in millimeters is approximately the square of the distance in
meters (D ).
2
The ability of the eye to detect flicker is a function of the brightness
of the scene. The critical flicker fusion frequency is the frequency at
which the flicker ceases to be detected. At a low scene brightness (e.g.,
0.001 to 0.01 cd/m 2) the flicker fusion frequency (FFF) is about 10 or
11 cycles per second. At a high brightness (e.g., 10 cd/m 2 ) the FFF
rises to 40 to 50 cycles per second. The FFF is lower in the outer por-
tions of the field of view. The FFF is what determines the minimum
acceptable shutter frequency in movie projection and the refresh rate
for television.

Sensitivity
The lowest level of brightness which can be seen or detected is deter-
mined by the light level to which the eye has become accustomed. When
the illumination level is reduced, the pupil of the eye expands, admit-
ting more light, and the retina becomes more sensitive (by switching