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142 Ophthalmic Lenses

linear as assumed in Figure 11.18 or it may be more complex to provide a
greater or less increase in power at the start of the progression as assumed
in the Figure 11.19.
The curve so drawn represents the power progression of the lens along
its meridional line from distance to near vision. The power progression is
the result of a continuous shortening of the radius of curvature of the front
surface.

Fig. 11.18: Power profile for a progressive lens with a linear power law

Fig. 11.19: Comparison of isocylinder lines for hard and soft design of power,
plano add + 2.00D

Contour Plots
The contour plot is the most common method used to represent progressive
addition lens designs. It describes the surface power of the front surface of
the lens and identifies zones of constant surface power indicated by contour
lines. Contour Plots also attempt to show the size of the distance,
intermediate and near zones as well as the extent and gradient of peripheral
aberrations.