Biomedical Engineering Reference
In-Depth Information
+2.00 +5.00
×
180
+2.00 D
+ 5.00 D
+ 2.00 D +
pl =
+7.00 D
+2.00 D
+7.00 D
pl
+7.00 D +
- 5.00 D =
+7.00 - 5.00
×
090
Figure 9-5.
The spherocylindrical lens on the right can be conceptualized as a
combination of the two lenses on the top (plus-cylinder form) or the two on the bottom
(minus-cylinder form).
this lens. The lens can be considered as a combination of lenses whose powers are
+
7.00 DS and pl
−
5.00
×
090. When conceptualized this way the lens formula is
090
This is the
minus-cylinder form
of the prescription (because the cylinder power
is minus).
It is important to keep in mind that the plus- and minus-cylinder forms of
the prescription represent the same lens—namely, the spherocylindrical lens in
Figure 9-5, which has a power of
+
7.00
−
5.00
×
2.00 D in
the horizontal meridian.
Plus-cylinder and minus-cylinder prescription forms
are simply different ways of designating the same lens
. Figure 9-6 provides
another example. Traditionally, optometrists tend to write lens prescriptions in
minus-cylinder form, while ophthalmologists tend to write lens prescriptions in
plus-cylinder form.
Students and practitioners alike often confuse the written form of a prescription
with the shape of the lens. With respect to shape, one of the surfaces of a sphero-
cylindrical lens is toric, meaning that it has two curves that are at right angles to
+
7.00 D in the vertical meridian and
+
