A progressive addition lens (PAL) is a type of multifocal lens that employs a surface with a continuously smooth increase in addition (Plus) power. The curvature of the surface increases from its minimum value in the distance zone to its maximum value in the near zone (Figure 1). The total increase in surface power between these two zones is equal to the specified Add power of the lens. This gradual increase in power also results in a variable focus intermediate zone.

Fundamentals of Progressive Lenses (pdf)

by Darryl Meister