Biomedical Engineering Reference
In-Depth Information
c
c
F
′
f
′
Figure 10-5.
According to Prentice's rule, a lens's prismatic effect is equal to the dis-
tance in centimeters from the optical axis (
c
) multiplied by the dioptric power of the lens.
where
P
is the prism power in prism diopters,
1
c
is the distance
in centimeters
from
the optical axis to the light ray, and
F
is the power of the lens. This handy relation-
ship, which allows us to calculate the prismatic power of an ophthalmic lens, is
referred to as
Prentice's rule
.
Figure 10-6 illustrates two important manifestations of Prentice's rule. First,
as the distance from the optical axis (
c
) increases, the prismatic power increases.
Second, as the power of the lens increases (
F
), the prismatic power increases.
Let's look at some examples.
A
6.00 D ophthalmic lens is decentered so that its
optical center is 3.0 mm nasal to a patient's pupil. What is the prismatic effect at
the point where the patient is looking through the lens?
+
Figure 10-7A schematically illustrates this from the perspective of looking down
onto the patient's head. Applying Prentice's rule, we have
P
=
(
c
)(
F
)
P
=
(0.3 cm)(6.00 D)
1.8
Δ
Since the base of the prism is facing nasally, the prismatic power is designated
as 1.8
Δ
base in
. If a
P
=
6.00 DS the lens were decentered so that its optical center was
3.00 mm temporal to a patient's pupil, as in Figure 10-7B, the prismatic power
would now be 1.8
Δ
base out
because the prism base is temporal.
+
1. The symbol “
Δ
” is sometimes used instead of “
P
.”
