Biomedical Engineering Reference
In-Depth Information
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Figure 10-4.
Lenses can be conceptualized as a series of prisms whose prismatic
power increases from the center of the lens to its edge.
To summarize, when looking along the optical axis of a lens, there is no prism
power, but when viewing through more peripheral regions of the lens, there is.
Prismatic power increases as the distance from the optical axis increases. This
has important clinical implications in management of conditions that require the
prescription of prisms.
Figure 10-5 shows how the prismatic power of a lens can be quantified. A light ray,
originating from infinity, strikes a lens at a distance
c
from its optical axis. After refrac-
tion, this ray intersects the optical axis at the secondary focal point of the lens,
F
. Let's
assume the distances are in centimeters. From the definition of a prism diopter, we have
′
c
f
′
⎛
⎝
⎞
⎠
P
=
(100)
Since the distances are in centimeters, we know that
100
′
=
f
F
Substituting, we have
P
=
(
c
)(
F
)
