Biomedical Engineering Reference
In-Depth Information
which point the astigmatism is corrected). If the examiner adds too much minus
power to the vertical meridian, the patient will report that position 1 is clearer
than position 2. (As discussed in the next section, during crossed-cylinder testing,
the doctor adds plus spherical power to keep the circle of least confusion posi-
tioned on the retina.)
SPHERICAL EQUIVALENCY
The eye in Figure 9-15A has simple hyperopic with-the-rule astigmatism. The
vertical meridian has a power of 60.00 D and is focused on the retina. The prescrip-
tion is
090
Suppose that you do not have access to cylindrical lenses and can prescribe
only a spherical lens to correct this eye. What would be the best spherical lens to
prescribe?
To minimize the amount of distortion that the patient experiences, the circle of
least confusion should generally be located on the retina. This can be accomplished
by prescribing a
spherical equivalent
lens whose power is determined with the
following relationship:
pl
+
5.00
×
A
No corrective lens in place
60.00 D
55.00 D
B
+ 2.50 DS
Spherical equivalent lens in place
62.50 D
57.50 D
Figure 9-15.
When a spherical equivalent lens centers the interval of Sturm dioptically
(not linearly) on the retina, the circle of least confusion falls upon the retina. In this
figure, the spherical equivalent is
+
2.50 D.
