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130 Ophthalmic Lenses

portion to a maximum value in the lower near portion. Unlike bifocal or
trifocal lenses, progressive lenses ensure that the presbyopic spectacle
wearer finds the right dioptric power for every distance, guaranteeing
smooth and uninterrupted vision without any visible line of demarcation.
The power increase is achieved by constantly decreasing the radii of
curvature in the vertical and horizontal directions. Figure 11.2 demonstrates
the gradual increase in curvature and surface power towards the lower,
near portion of the lens.

Fig. 11.2: Cross-sectional view of progressive addition lens surface. The shorter
radius of curvature in near portion provides a stronger surface power than the
longer radius of curvature in the distance portion
In the most used zones of vision, virtually aberration-free vision is
possible, as here the radii of curvature are almost identical in the vertical
and horizontal directions.
A typical, general-purpose progressive lens will have three district zones
of vision as shown in Figure 11.3.

Fig. 11.3: Distance, intermediate and near zones of a typical progressive lens