Biomedical Engineering Reference
In-Depth Information
7
Ametropia
The eye is a sophisticated optical system constituted of multiple refracting surfaces,
and to solve certain optical problems it is necessary to consider the optical proper-
ties of the eye in all their complexity. For many problems and clinical cases, how-
ever, a satisfactory solution can be obtained by using a simplified optical model of
the eye, referred to as a
schematic eye
.
There are various schematic eyes. For our purposes, we will work with what is
called the
reduced eye
, which is given in Figure 7-1. It consists of a single spheri-
cal refracting surface
1
with a radius of curvature of 5.55 mm that separates air
from aqueous, which is assumed to have an index of refraction of 1.333.
There is
a single nodal point located at the center of curvature of the refracting surface. As
is the case for all spherical refracting surfaces, the principal planes are coincident
with the surface. The distance from the surface to the retina—the
axial length
—is
22.00 mm.
The reduced eye is
emmetropic
, meaning that as illustrated in Figure 7-2, light
rays originating at infinity are focused on the retina.
2
Let's look at this in more
detail. First, we'll determine the power of the reduced eye. Since we know its radius
of curvature and the relevant refractive indices, the surface's refractive power can
be calculated as follows:
n
′
−
n
F
=
r
1.333
1.000
0.00555
−
F
=
F
=
+
60.00 D
1. The refracting surface of the reduced eye is located 1.67 mm posterior to the cornea. We ignore
this small distance in this topic and use the terms “anterior surface of the eye” and “cornea”
interchangeably.
2. Both the cornea and relaxed crystalline lens contribute to the reduced eye's refractive power.
Accommodation is discussed in the next chapter.
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