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Progressive Addition Lenses 143

There are two types of contour plots:
• Isosphere contour plots
• Isocylinder contour plots

Isosphere Contour Plots

This is a two dimensional map of the lens representing the distribution of
spherical power across the lens. This form of graphical representation
divides the lens along lines of equal dioptric values. Each contour line/
shade represents an increasing level of power at given interval. The value
of this line/shade is chosen arbitrarily, usually at the increments of 0.25D
or 0.50D to 1.00D. Between two consecutive lines, sphere value varies by a
relatively constant rate. Figure 11.20 shows isosphere contour plot of a
progressive addition lens. In addition to describing sphere distribution the
lens designer may communicate information about the location and size of
the near vision zone using an isosphere contour plot.

Fig. 11.20: Power contour plot of a PAL (+ 2.00 with a addition + 2.50D)

Isocylinder Contour Plots
Isocylinder contour plots are two-dimensional map that divides the lens
design into ranges of cylinder levels through out the lens. Most of the time
isocylinder plot is misused by the lens designer in the promotion of their
progressive addition lenses. It is routinely misapplied by some lens marketer
seeking to establish competitive claims such as “a larger reading zone” or
“less unwanted astigmatism”. Since induced cylinder is disturbing but