Jackson Cross Cylinder
Believe it or not, the Jackson Cross Cylinder has made our jobs as
optometrists easier. Here are some important points to remember about the
JCC:
The JCC lens is always a spherocylinder lens such that:
-one meridian is plus power and the other meridian is of equal minus
power.
If the JCC is a +0.25/-0.25 lens, a prescription for the lens will be:
+0.25-0.50X(depends on orientation).
The red dots identify the AXIS of the minus power. The power
in the meridian of the red dots is PLUS power.
The JCC lens in a plus cylinder phoropter is identical to that of a minus
cylinder phoropter.
When you use a plus cylinder phoropter you "chase" the white dots instead
of the red dots.
When you perform the JCC power test the length of the astigmatic interval
changes. Flipping the lens causes a change in interval length (a change in
image quality)between the two meridians if cylinder is not fully corrected
.
The end of the JCC power test is when both images appear
equal or are
equally blurred. This occurs because with the correct cylinder power in
place and the JCC lens in place, the astigmatic interval is the same
length in each position.
The end of the JCC axis test occurs when the resultant cylinder, caused by
obliquely crossing two cylinders, has been neutralized.