Biomedical Engineering Reference
In-Depth Information
0.00 D
-2.59 D
-2.59 D
+60.00 D
n
= 1.000
n
′
= 1.333
FP
+57.41 D
+23.22 mm
-38.61 cm
-2.59 D
Figure 7-5.
A corrective contact lens images an infinitely distant object at its secondary
focal point, which is coincident with the eye's far point. Since the far point is conjugate
with the retina, a focused image is formed on the retina. Note that the image vergence
emerging from the corrective lens (
2.59 D) serves as the object vergence for the myo-
pic eye. (At the top of both the contact lens and eye surface are the object vergence and
refractive power, respectively. Image vergence is given below these refracting elements.)
−
A contact lens,
7
which we'll assume rests on the reduced eye's anterior surface,
must have a power of
2.59 D. Figure 7-5 shows that when light rays from an infi-
nitely distant object strike this lens, an image is formed at its secondary focal point,
which is coincident with the far point of the eye; this image serves as an object for
the eye and is focused on the retina.
Note that the contact lens power (
−
−
2.59 D)
is equal to the eye's far point vergence.
In summary, to correct myopia, the secondary focal point of a
minus correcting lens must be coincident with the far point of the
eye. When this is the case, an infinitely distant object is imaged at
the far point. Since the far point is conjugate with the retina, an
image is focused on the retina.
Just a slight increase in axial length can cause clinically significant myopia.
What
is the axial length of a 1.00 D myopic eye that has a refractive power of
+
60.00 D?
1.00 D (the far point vergence) will be focused on the
retina. The vergence relationship can be used to determine the axial length as follows:
L
An object with a vergence of
−
′
=
L
+
F
L
′
= − 1.00
+
60.00 D
L
′
= +
59.00 D
7. We'll discuss correction with spectacle lenses when we talk about lens effectivity later in this chapter.
