Biomedical Engineering Reference
In-Depth Information
-3.50 +6.00
×
180
-3.50 D
+6.00 D
pl =
-3.50 D +
+ 2.50 D
- 3.50 D
+2.50 D
pl
+2.50 D +
- 6.00 D =
+2.50 - 6.00
×
090
Figure 9-6.
The plus-cylinder (top) and minus-cylinder forms of the spherocylindrical
lens (on the right).
each other. Another way of saying this is that a spherocylindrical lens can be ground
with the cylinder on either the front or the back surface. The former is called a
front toric lens, while the latter is a back toric lenses. Almost all modern lenses are
back toric designs. It is sometimes
mistakenly
assumed that a plus cylinder prescrip-
tion is synonymous with a front toric design and a minus cylinder prescription is
synonymous with a back toric design. This is not the case!
It's relatively straightforward to transpose a plus-cylinder prescription into minus-
cylinder form and vice versa. The rule is as follows:
• Keeping the signs, add the sphere and cylindrical powers to arrive at the new
sphere value.
• Reverse the cylinder sign to arrive at the new cylinder power.
• Change the axis by 90 degrees to arrive at the new axis. (Remember that the
axis is never greater than 180 degrees.)
You should confirm that these rules apply to the examples in Figures 9-5 and 9-6.
