The Power Cross
Any spherical, toric (spherocylindrical), or purely cylindrical lens can be represented graphically by a power cross. A spherical lens has the same power at every meridian. A toric lens has a different power at every meridian that varies between a maximum and a minimum value. The maximum and minimum powers are always in meridians that are 90° apart. The power cross shows the maximum and minimum powers and their meridians.
A pure cylinder has zero power along its axis and either highest positive or lowest negative power perpendicular to its axis. The rule is “the power of a cylinder is perpendicular to its axis.” For instance, a +2.00 × 015 cylinder has zero power in the 15° meridian and +2.00 D power in the 105° meridian (Figure 1-30).
As discussed in the Quick-Start Guide, a −3.00 +2.00 × 015 toric lens has a −1.00 D power in the 105° meridian and −3.00 D in the 15° meridian (Figure 1-31). Using spherocylinder notation, we can always represent the same toric lens in 2 ways—a plus cylinder form and a minus cylinder form—which can lead to confusion. To convert from one form to the other (“transposition” of the spherocylindrical specification):
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The new sphere is the algebraic sum of the old sphere and cylinder.
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The new cylinder has the same value as the old cylinder but with opposite sign.
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The axis needs to be changed by 90°.
One advantage of the power cross is that any toric lens has only 1 power-cross representation.
However, we can use a power cross to combine spherocylinders only when the orientations of their meridians are identical. For instance, what is the result of combining a +2.00 +1.00 × 080 lens with a +3.00 −2.00 × 080 lens? Using a power cross to represent each lens, we can simply add the powers in the corresponding meridians (Figure 1-32).
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.