Biomedical Engineering Reference
In-Depth Information
What does a patient with the following uncorrected astigmatic correction per-
ceive when viewing a distant point source?
pl
−
2.00
×
180
This patient has simple with-the-rule myopic astigmatism. The horizontal
meridian of the eye requires no correction. This meridian focuses the point source's
horizontally diverging rays onto the retina. Now consider the stronger vertical
meridian. This meridian focuses the vertically diverging rays in front of the ret-
ina. These rays subsequently diverge vertically and fall upon the retina to form a
smeared vertical line. The patient reports seeing a vertical line with sharp lateral
edges and fuzzy vertical edges (Fig. 9-16B).
Suppose we ask this patient to read a visual acuity chart.
Are there any optotypes
that the patient will find particularly difficult to resolve?
Consider the
F
optotype. To correctly identify this optotype (which is an
extended object), it is necessary to resolve the gap between the two horizontal line
segments. The vertically diverging light rays emanating from the horizontal lines
are focused by the vertical meridian
in front
of the retina. The rays then diverge
vertically to form vertically smeared horizontal lines on the retina. As illustrated in
Figure 9-16C, this makes the gap difficult to see; consequently, patients with myo-
pic with-the-rule astigmatism often mistakenly identify an
F
as a
P
.
This astigmatic patient will have less difficulty identifying the vertical line in
the
F
. The horizontal meridian of the eye focuses the vertical line on the retina. Its
lateral edges will be sharply focused, but its vertical edges will be smeared because
they are formed by the out-of-focus vertical meridian.
SUMMARY
Image formation by a converging spherocylindrical lens can be understood by
visualizing a point source as producing light rays that have vertical and horizontal
divergence. Rays with vertical divergence are focused by the vertical lens meridian
as a horizontal line, while rays with horizontal vergence are focused as a vertical line
by the horizontal lens meridian. For an extended source, such as a cross, the vertical
lens meridian focuses the horizontal line and the horizontal lens meridian focuses
the vertical line.
An astigmatic eye has two focal points separated by the interval of Sturm. Diop-
trically centered between these two points is the circle of least confusion. The
Jackson crossed cylinder test is commonly used to determine the amount of ocular
astigmatism.
