Biomedical Engineering Reference
In-Depth Information
What is the image distance? The key is to realize that the rays forming the image
exist in aqueous, the secondary medium. This can be seen in Figure 7-2. The image
distance is calculated as
n
′
′
=
L
′
l
1.333
+
60.00 D
=
′
l
′
= +
l
0.02222 m, or
+
22.22 mm
This calculation confirms what we already know—the distance from the refrac-
tive surface to the retina in the reduced eye is 22.22 mm.
To summarize, an infinitely distant object is focused on the retina of the reduced
eye. Such an eye is said to be
emmetropic
(or have the condition of emmetropia).
Another way of saying this is that in emmetropia, the retina is conjugate with infinity.
In
ametropia
, an infinitely distant image is not focused on the retina. The
remainder of this chapter discusses the quantification of two common forms of
ametropia—myopia and hyperopia—and their correction.
MYOPIA
In the myopic eye, an infinitely distant object is focused anterior to the retina.
3
Relative to the reduced eye, the eye can be too long (an axial length greater than
22.22 mm), too strong (a refractive power greater than
60.00 D) or a combina-
tion thereof.
4
Consider an eye that is too long—it has an axial length of 23.22 mm
rather than 22.22 mm. The eye's power is
+
60.00 D. As can be seen in Figure 7-3A,
an infinitely distant object is focused 22.22 mm behind the refracting surface, or
1.00 mm anterior to the retina of this eye.
+
A large part of our practices as eye doctors is to quantify a patient's myopia so
that we can prescribe appropriate corrective lenses. How would we quantify the
myopia in the previous paragraph? Answering the following question will help us
to do this:
Where must an object be located for it to be imaged on the retina of
an eye that has an axial length of 23.22 mm and power of
+
60.00 D?
Figure 7-3B guides us through the solution of this problem. First, we determine
the vergence required for an image to be focused on the retina. Since the eye has
3. A patient who is myopic has the condition of
myopia
.
4. In Chapter 13, we'll learn about axial and refractive myopia. Clinically, these terms are typically
used when comparing the refractive states of a patient's two eyes. If, for example, one eye is more
myopic than the other and this is due to a difference in axial length between the two eyes, the
myopia is labeled as axial. Likewise, if the difference in myopia is due to a difference in corneal
power, the myopia is refractive.
