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Progressive Addition Lenses 149

orientation of the vertical horopter linked to the natural tilting of reading
material (Fig. 11.26).

Horizontal (lateral) Location of the Near Vision Area
Once the power profile has been defined, its lateral positioning on the lens
must be adapted to the natural convergence of the eyes and the value of
the addition. With the advanced presbyopia, the reading distance becomes
closer, the meridional line, therefore, must be shifted nasally as the addition
increases.
Besides, balancing of vertical prism between right and left lens is very
important to respect retinal image fusion in binocular vision. This is
achieved by an asymmetrical design of the progressive addition lens surface
coupled with proper positioning of the meridional line. In the lens
periphery, image quality constraints are less demanding, while the control
of prismatic effects is of utmost importance for motion perception.
All of the above optical requirements are introduced in the Merit
function and are then integrated into the lens design optimization software.
The merit function evaluates the numerous points of the lens and also
overall performance of the lens by the weighted sum of the found merit
function values and then the lens design is optimized. Numerous lens
prototypes of each design are then produced and tested through rigorous
clinical trials. Comparative lens evaluations are made after in depth analysis
and patients comments, leading to a final selection of the progressive
addition lens design.

PRISM THINNING
The progressive addition lenses are designed by increasing the curvature
of the progressive zone in the near vision area. As a result of this, the
progressive addition lens is thinner at the bottom and thicker at the top. To

Fig. 11.27: Thinning prism