Jackson Cross Cylinder

The Jackson Cross Cylinder (JCC) is a combination of two cylinders of equal strength but opposite signs, placed with their axes at 90 degrees to each other and mounted on handle.Also known as the flip cross technique.

Principle of the Jackson Cross Cylinder

 When light rays pass through an optical system that does not have similar curvature in two principal meridians, it causes the rays to focus at two focal points instead of forming a single focal point. Similarly, when light passes through an astigmatic eye, it causes the formation of two focal points. The distance between the two focal points is known as the interval or strum conoid. The best image in this type of optical system lies somewhere in the middle of the strum interval, where there is an equal amount of convergence from one meridian and an equal amount of divergence from another meridian, known as the circle of least confusion. This provides a compromised but the best possible image produced by this system.

Optics of JCC

Cross cylinder is spherocylindrical lens with zero spherical equivalent.

There are following variety of JCC available in market.

±0.25 JCC

±0.50 JCC

±1.00 JCC

±2.00 JCC .

Having the advantage of a spherical equivalent of zero when the JCC is placed in front of an astigmatic eye, the Jackson Cross Cylinder will not move the circle of least confusion. It increases or decreases the interval of blur.

Working of the Jackson Cross Cylinder

Step 1: Astigmatism Discovery

Position the Jackson Cross Cylinder so that the axis is at 90 or 180 degrees. Ask the patient which position is clear. Look for the negative axis. If the patient indicates position 2, where the negative cylinder axis was pointing towards 180 degrees, note down this axis.

Place the Jackson Cross Cylinder obliquely now so that the axis is at 45 and 135 degrees. Ask the patient which position is clear. Look for the negative axis. If the patient indicates position 2, where the negative cylinder axis was pointing towards 135 degrees, note down this axis.

Now, place the cylindrical power lens in the trial frame so that the axis is in the middle between 180 and 135, which is 165.

Step 2: Axis Refinement

Remember to refine the patient's axis before refining the power. The correct axis can be found if the power is wrong, but you cannot find the correct power if the axis is wrong.

For axis refinement, position your Jackson Cross Cylinder in such a way that its handle is parallel to an axis of the cylinder that was placed in the trial frame during the astigmatism discovery.

Ask the patient which position is clear. If the patient chooses one position over the other, rotate the cylindrical axis towards the chosen one. The amount of rotation depends on the amount of cylinder. The rotation of the cylindrical axis should be kept minimal, around 5 degrees. If there is less power of the cylinder, the rotation can be around 10 to 20 degrees

Step 3: Power Refinement

For power refinement, position the transverse axis of the Jackson Cross Cylinder parallel to the axis of the cylindrical lens placed in the trial frame. Ask the patient which position is clear. If the patient prefers a positive axis over the negative cylindrical power placed in the trial frame, decrease the cylindrical power in the trial frame. If the patient prefers a negative axis over the negative cylindrical power placed in the trial frame, increase the cylindrical power in the trial frame. Don't forget to add half of the opposite power of the cylindrical lens to the spherical side in the trial frame. Repeat this procedure until the patient reports both positions as blurry. Here, it is the final power.

Hurrah! You have done it.