Biomedical Engineering Reference
In-Depth Information
There are three main ametropias (see Figure 10.17):
1. Myopia, in which the image is formed before the retina. The rays of light coming
from objects at a proximal distance are focused correctly. Myopia is generally due
to an excessive length of the bulb.
2. In hypermetropia, the image is placed behind the retina and there is no point in
space where the rays are focused on the retina. Hypermetropia is generally due to
a small length of the bulb.
3. In astigmatism, the images look doubled, and they can be placed both before and
behind the retina. This also implies that, even in this case, there is no point in
space where all rays are focused on the retina.
Hypermetropia is a physiologic condition at birth. It is spontaneously compensated for
by means of an accommodating system. Hypermetropic patients can therefore have a nor-
mal visual acuity at all distances, until they are supported by an adequate accommodation
system, although this requires a continuous muscular contraction. Accommodation also
triggers convergence movements and this often implies esophoria. The perfect visual acu-
ity of a hypermetropic patient therefore requires a higher muscular effort, especially in
proximal vision.
Ametropias can be compensated for through different ophthalmic lenses for each
ametropia. The positive, convex, converging lenses are used to compensate for hyperme-
tropia, whereas negative, concave, diverging lenses are used for myopia (Figure 10.17).
Compensating astigmatism requires toric lenses.
It is also important to note that the unit of measurement of the dioptric power of the
lenses is the diopter (
D
), which can be defined as the reciprocal of the focal distance (
f
) in
Hyperopia, or far-sightedness, occurs when the focal point falls behind the retina
Hyperopia (corrected
with a convex lens)
Myopia, or near-sightedness, occurs when the focal point falls in front of the retina
Myopia (corrected
with a concave lens)
FIgUre 10.17
Ametropie. From the top: far-sighted eye and near-sighted eye.
