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Astigmatic Lenses 37

Fig. 4.9: Pencil of light results from retraction at an astigmatic lens

Figure 4.10 illustrates the toroidal surface of the sphero-cylinder lens.
The lower power is usually referred to as the base curve of the surface and
the higher power as the cross curve. In the plano-cylindrical surface, the
base curve is along the axis meridian which is zero and the cross curve is
simply the power of the cylindrical surface. In case of the toroidal surface,
the “axis meridian” is curved and the cylindrical power of the surface is
the difference between the cross curve and the base curve.

Fig. 4.10: Toroidal surface of the sphero-cylinder lens

DETECTION OF CYLINDRICAL LENS
Cylinder lenses are often referred to as cylinder or toric lens because of
their out of round surfaces. In order to determine the cylinder lens and to
detect its dioptric strength, hold the lens a few inches away from your
eyes; sight a straight lined object, such as window or the door frame. Rotate
the lens slowly as you would turn a steering wheel – first to the right