 The optical center of the add segment may be placed on the lens surface or may hang off into empty space near the lens surface.Although the surface profile of a bifocal segment is spherical, it is often trimmed to have straight edges so that it is contained within a small region of the overall lens surface.The progressive addition lens (PAL, also commonly called a no-line or varifocal lens) eliminates the line in bi/tri-focals and is very complex in its profile.