Biomedical Engineering Reference
In-Depth Information
The light rays that form the retinal image exist in aqueous. Therefore, the image
distance is
n
′
′ =
L
′
l
1.333
+
59.00 D
=
′
l
′
= +
l
0.2259 m, or
+
22.59 mm
0.37 mm)
causes 1.00 D of myopia. This points to the exquisite coordination of growth that
is required for eyes to become emmetropic, a process referred to as
emmetropiza-
tion.
A useful rule of thumb is that for every 1/3-mm increase in the eye's
axial length, the eye becomes 1.00 D more myopic.
Consequently, an eye that
is 1.00 mm too long is approximately 3.00 D myopic.
An increase in the axial length of only 0.37 mm (22.59
−
22.22 mm
=
HYPEROPIA
In hyperopia (sometimes called hypermetropia), an infinitely distant object is
focused posterior to the retina.
Relative to the reduced eye, the eye can be too short
(axial length less than 22.22 mm), too weak (refractive power less than
60.00 D),
or a combination of these two factors. Consider an eye that is too short—it has an
axial length of 21.22 mm and a power of
+
60.00 D. As can be seen in Figure 7-6,
an infinitely distant object is focused 22.22 mm behind the refracting surface, or
1.00 mm posterior to the retina of this eye.
+
0.00 D
+60.00 D
n
= 1.000
n
′
= 1.333
+60.00 D
+21.22 mm
+22.22 mm
Figure 7-6.
In hyperopia, an infinitely distant object is imaged behind the retina.
