Biomedical Engineering Reference
In-Depth Information
H
H
′
A
1
H
A
2
H
′
F
F
′
f
n
f
v
f
e
f
e
′
Figure 6-2.
Equivalent lens showing the front vertex (
f
n
), back vertex (
f
v
), and equiva-
lent focal lengths (
f
e
and
f
e
′
).
Also
shown are distances from the front surface apex to
the pri
mary
principal plane (
A
1
H
) and back surface apex to the secondary principal
plane (
A
2
H
′
).
focal length
(
f
v
)
is measured from the back
surface
of the lens to the secondary
focal point, and the
front vertex focal length
(
f
n
)
is measured from the front
sur-
face
of the lens to the primary focal point.
For each of these focal lengths, there is a corresponding power, which is sim-
ply the reciprocal of the focal length in meters.
1
These three powers—equivalent
power (
F
e
), back vertex power (
F
v
), and front vertex power (
F
n
)—are discussed in
the following sections.
BACK AND FRONT VERTEX POWER
For most clinical applications, we are interested in the power of a lens with respect
to its front or back surface, not its equivalent planes. The power specified with
respect to the back surface (which is called the ocular surface in an ophthalmic lens),
is called the
back vertex power
, abbreviated
F
v
.
When using a lensometer to
neutralize the distance prescription in a patient's spectacle lenses, we typically
measure the back vertex power.
It can be calculated with the following formula:
F
1
F
v
=
+
F
2
t
⎛
⎞
⎠
1
−
F
1
⎝
1. This assumes the lens is situated in air, which has a refractive index of 1.00.
