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New Development in Spectacle Lenses 207

Fig. 15.3: Enhanced near vision lens fitted to near PD on Datum line

ATORIC LENSES
The rotationally symmetric aspheric lens provides excellent imaging
properties for any lens prescription if it is in spherical. But in case of
astigmatism prescription the two principal meridians requires two different
eccentricity. For example, in case the lens prescription is + 4.00/+ 2.00 ×
90, the two principal meridians has + 4.00 D and + 6.00 D, which may need
two different aspheric curves for the respective meridians. Such a surface
is employed for the Zeiss Hypal series of lenses when dispensed for
astigmatic prescription.
Advances in lens design have provided the lens designers with the
ability to produce surfaces that are even more complex than aspheric. Just
as aspheric denotes a surface that departs from being completely spherical,

Fig. 15.4: Optical performance comparisons between the three lens designs for
plus and minus astigmatic lenses manufactured in a polycarbonate material of
refractive index 1.586. The white area in the frame represents the field of clear
vision while the shaded area represents regions of reduced vision