Monday, May 22, 2006

Jackson Cross Cylinder (JCC)

Cross cylinders are combinations of two cylinders whose powers are numerically equal and of opposite sign and whose axes are perpendicular to each other. The Jackson Cross Cylinder is usually mounted in a ring with a handle at 45 degrees from the axis so that a twirl of the handle changes the cross cylinder to a second position.

Example: +0.25x90/–0.25x180 to –0.25x90/+0.25x180

When the JCC is placed in contact with a spherocylinder, it displaces both focal lines simultaneously in opposite directions, expanding the initial Interval of Sturm in the first position and contracting it in the second. However, there will be no displacement of the Circle of Least Confusion, only the diameter of the circle will increase in the first and decrease in the second position of the JCC.

When a +/-0.50 JCC is placed on a lensometer, with the red axis at 0 and 180 degrees, the lensometer will read the power as -0.50 +1.00 x 090. But, remembering that the JCC has no spherical power, only cylindrical power. For this reason, we can more accurately write the power of the JCC as –0.50 x 180 combined with +0.50 x 90

Question: When the Jackson cross cylinder is used to define the astigmatic axis, is the handle of the lens parallel to the axis or 45 degrees from it?

Answer: Parallel. To define the astigmatic power, the handle is rotated 45 degrees to the axis. Normally, you should define the axis before the power.