Biomedical Engineering Reference
In-Depth Information
where
F
1
is the power of the front surface,
F
2
is the power of the second surface,
t
is
the thickness of the lens in meters, and
n
is the lens's index of refraction.
As is the case for all the thick lens formulae in this chapter, thickness (
t
) is in
meters and the index of refraction (
n
) is that of the lens.
In clinical practice, the nominal power,
F
T
(which does not take into account lens
thickness), and the back vertex power,
F
V
,
are sometimes very similar to each
other.
Take, for example, a polycarbonate lens that has a front surface power of
+
8.00 DS. Assume a central thickness of
2.00 mm. What is the nominal power of this lens? How does this compare to the
back vertex power, which is more accurate because it takes thickness into account?
2.00 DS and a back surface power of
+
10.00 D, the sum of the two surfaces. This
value is an estimate of the lens power that ignores its thickness
.
To determine the
back vertex power, we substitute as follows:
The nominal power is, of course,
+
F
1
F
v
=
+
F
2
t
⎛
⎞
⎠
1
−
F
1
⎝
+
2.00 D
F
v
=
+
8.00 D
=
+
10.01 D
0.002 m
1.586
⎛
⎝
⎞
⎠
1
−
(
+
2.00 D)
In this case, the nominal and back vertex powers are essentially the same. Now,
let's look at a lens that has the same nominal power (
+
10.00 D) but is made with
a much more curved front surface (
F
1
= +
15.00 D) and a back surface power of
−
5.00 D. Substituting into the front vertex power formula, we have
+
15.00 D
F
v
=
+
(
−
5.00 D)
=
+
10.28 D
⎛
⎝
0.002 m
1.586
⎞
⎠
1
−
(
+
15.00 D)
In this case, there is a difference of 0.28 D between the estimate provided
by adding the two surfaces together (
F
T
= +
10.00 D) and the actual lens power
(
F
V
= +
10.28 D). This difference could be clinically significant.
By examining the denominator of the back vertex power formula, we can arrive at
a general rule that will help us to know when we should worry about the effect of
lens thickness.
As the curvature of the front surface and/or thickness
of the lens increases, the back vertex power will become increas-
ingly different than the nominal power. For plus lens, the back ver-
tex power will become more positive and for minus lenses, it will
become less minus.
High plus lenses, which generally have highly curved
front surfaces, are especially likely to have nominal and back vertex powers that
are significantly different from each other.
