Biomedical Engineering Reference
In-Depth Information
4
Thin Lenses
A lens is made up of two surfaces, both of which are often, but not always, spheri-
cal.
1
The back surface of an
ophthalmic lens
—the term we use to designate lenses
used in clinical practice—is sometimes referred to as the ocular surface.
Common
lens forms are given in Figure 4-1. Most ophthalmic lenses have a meniscus shape,
which tends to minimize aberrations that can reduce vision.
As illustrated in Figure 4-2, each surface may contribute to the lens's total diop-
tric power. The thickness of the lens also contributes to its power, but in many
ophthalmic applications we can ignore lens thickness because its contribution to
lens power is very small. When we ignore the effect of thickness, we call the sim-
plified lens a
thin lens
. (Chapters 5 and 6 provide a detailed discussion of lens
thickness.)
Since we are ignoring thickness, the power of a thin lens is simply the sum of the
powers of the front and back (ocular) surfaces. We refer to this as the lens's
nomi-
nal or approximate power
. Expressed as an equation, we have
F
T
=
F
1
+
F
2
where
F
T
is the nominal (or approximate) power,
F
1
is the power of the front sur-
face, and
F
2
is the power of the back (ocular) surface.
As illustrated in Figure 4-3, thin lenses are represented as vertical lines. Figures 4-4
and 4-5 show that the secondary (
F
) and primary (
F
) focal points are located in the
same manner as for a spherical refracting surface (Chapter 2). When parallel light
rays travel from left to right, the secondary focal point is (1) the point to which the
light rays converge for a plus lens or (2) the point from which they appear to origi-
nate for a minus lens (Fig. 4-4). The primary focal point is located by reversing the
′
1. Certain ophthalmic lenses have aspheric surfaces that are intended to reduce aberrations, and in the
case of plus lenses, to also decrease center thickness.
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