Biomedical Engineering Reference
In-Depth Information
But before delving into this important issue, we first need to discuss retinal image
size in uncorrected ametropia.
RETINAL IMAGE SIZE IN
UNCORRECTED AMETROPIA
As we learned in Chapter 7, ametropia is due to a mismatch between the eye's
refractive power and its axial length. The retinal image size in uncorrected ametro-
pia depends on whether the ametropia is axial or refractive in nature.
The reduced eye has a length of 22.22 mm. When the eye is longer than this, it
is said to have
axial myopia
, and when it is shorter,
axial hyperopia
.
Most myopia
is axial in nature.
If the eye is more powerful than the reduced eye, which has a
power of
60.00 D, the condition is called
refractive myopia
, and when the eye
is weaker than
+
60.00 D, the condition is
refractive hyperopia.
Either myopia or
hyperopia can have a mixture of axial and refractive components.
Let's first look at axial ametropia. Figure 13-1 shows three axial lengths that
would be expected in hyperopia, emmetropia, and myopia. Suppose the eye is view-
ing an arrow. A light ray emerging from the tip of the arrow passes undeviated
through the eye's nodal point and contributes to the retinal image. Note that as
the eye's axial length increases, the size of the retinal image also increases.
This
tells us that in uncorrected axial myopia, retinal image size is larger than
in emmetropia and that in uncorrected axial hyperopia, it is smaller than in
emmetropia.
Now let's see what happens to retinal image size in uncorrected refractive
ametropia. As illustrated in Figure 13-2A, the eye has a fixed length. When the
refractive power of the eye increases (as in myopia) or decreases (as in hypero-
pia), the image becomes blurred, but its size does not change. Think of a projec-
tor that focuses an arrow on a screen. If we adjust the focus so that the image
is blurred, as in Figure 13-2B, the blurred image, as measured from the centers
of the blur circles, is the same size as the focused image. It doesn't matter if
+
Hyperopia Emmetropia Myopia
N
Figure 13-1.
In axial ametropia, the retinal image size increases as the eye's axial
length increases.
