What type of curvature is needed to create an increasing plus power on PALS?

• A. Spherical curvature
• B. Cylindrical curvature
• C. Plano curvature
• D. Aspheric curvature

Answer: (D)

In progressive addition lenses, the near addition power increases gradually from the distance portion toward the bottom of the lens. To produce that smooth, increasing plus power across the lens surface, the curvature must vary with position. An aspheric surface changes its curvature as you move away from the center, so the local optical power increases toward the lower part of the lens. Spherical curvature has the same curvature everywhere, so it can’t create a progressive increase in power. Cylindrical curvature changes power only along one meridian and mainly introduces astigmatism, not a monotonic add. Plano curvature is flat in one direction and doesn’t add power at all. Therefore, aspheric curvature is needed to create the increasing plus power on PALS.