Biomedical Engineering Reference
In-Depth Information
Oblique
meridian
-3.00 D
pl
70°
Power
meridian
Figure 9-10.
Lens cross used to determine power in the vertical meridian of a
pl
−
3.00
×
160 lens.
and axis meridian (160 degrees) make an angle 70 degrees, this value is substituted
into the sine-squared formula as follows:
F
=
(
F
cyl
)sin
2
θ
θ
F
=
(
−
3.00 D)sin
2
(70)
θ
F
=
−
2.65 D
θ
Combining
−
2.65 D with the
+
2.00 D sphere component gives a power of
−
0.65 D in the vertical meridian.
ASTIGMATISM: DEFINITIONS
AND CLASSIFICATIONS
The power of an
astigmatic eye
is spherocylindrical.
5
In
regular astigmatism
, the
two major meridians of the eye are perpendicular to each other. This form of
astigmatism, which is by far the more common form, can be corrected with sphe-
rocylindrical spectacle lenses.
Irregular astigmatism
is generally secondary to
5. The Greek term
stigma
refers to a point or small mark. Astigmatism is a refractive error that results
in an image that is not a point.
