Biomedical Engineering Reference
In-Depth Information
Sphere
Cylinder
Spherocylindrical
+ 2.00 D
+ 5.00 D
+ 7.00 D
+ 2.00 D +
pl
=
+ 2.00 D
Figure 9-3.
A spherocylindrical lens can be conceptualized as a combination of
spherical and cylindrical lenses.
meridians
. For the lens in Figure 9-3, the meridians whose powers are
+
7.00 D
and
2.00 (i.e., the vertical and horizontal meridians) are the principal meridians.
By convention, lens meridians are specified in degrees measured
counterclockwise
from the 3 o'clock position when facing the patient (Fig. 9-4).
3
Horizontal is always
labeled as 180 degrees, not 0 degrees. Using this convention, the power of the
+
90°
135°
45°
180°
Figure 9-4.
Lens meridians (and axes) are specified in degrees as measured
counterclockwise from the 3 o'clock position. The horizontal should be referred to as
180 degrees—not 0.00 degrees.
3. When the doctor is facing a patient, the 3 o'clock position is on the doctor's right.
