Optical system
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Visual acuity
There are different forms of visual acuity and various ways to measure them.
Minimum separable acuity is the ability to see two lines or points as being separate.
This is the type of visual acuity that is most frequently referred to.
Some visual acuity targets; A) Landolt C, B)grating
and C)vernler acuity
Figure 19 shows three types of visual acuity targets; Landolt C, grating
and vernler acuity. The proportions of the Landolt C are are kept constant
and the minimum sized opening of the target that may be detected is a measure
of resolution. The grating may be reduced in size until the lines are just
visible as separate. The results of acuity measures using gratings are often
reported in terms of cycles per degree, a cycle being the width of one dark
and one light bar. Vernier acuity is measured by aligning the two bars until
they appear to be once continuous line.
Patterns such as A and B in Figure 19 may be used to test minimum separable
acuity. The ability to resolve a one minute of arc wide separation is often
considered "normal" acuity. Under reasonable conditions, however,
a person with good vision should be able to resolve a separation of about half
that size.
Vernier acuity is another form of acuity as shown in C of Figure 19. Vernier
acuity is the ability to detect a discontinuity in a line or the misalignment
of a segment of a line. This ability is about 10 times greater than minimum
separable acuity. The ability to detect a difference between a D and an O is
more a function
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