Biomedical Engineering Reference
In-Depth Information
cylindrical correction
spherical equivalent
=
spherical correction
+
2
For the current example, the spherical equivalent power is determined as follows:
5.00 D
spherical equivalent
=
0.00 D
+
2
spherical equivalent
= +
2.50 D
2.50 DS lens causes the interval of Sturm to
be
dioptrically
centered on the retina, which means that the circle of least confusion
is located in the plane of the retina.
For the JCC test to produce accurate results, the circle of least confusion must
remain on the retina. For each
As can be seen in Figure 9-15B, the
+
0.50 of cylindrical power that is added during the
testing procedure, the spherical equivalent power changes by
−
0.25 DS. That is, if
the circle of least confusion initially falls on the retina, the addition of
−
−
0.50 D of
cylinder will cause the circle to fall
0.25 DS behind the retina. To keep the circle
of least confusion on the retina during the JCC test, the examiner can increase the
spherical power by
−
+
0.25 DS each time he or she increases the cylinder power
by
−
0.50 D.
WHAT DOES A PERSON
WITH ASTIGMATISM SEE?
A patient with uncorrected myopia or uncompensated hyperopia experiences uni-
form blur—the image is equally blurred in all orientations: up and down, left and
right, and every orientation in between. For example, when an uncorrected myope
views a distant point source, he or she perceives a blurred circle (Fig. 9-16A).
A
B
C
Figure 9-16.
A.
Appearance of a point source to a patient with uncorrected myopia.
B.
Appearance of a point source to a patient with uncorrected with-the-rule simple
myopic astigmatism.
C.
Appearance of the optotype
F
to a patient with uncorrected
with-the-rule simple myopic astigmatism.
