The glass trifocal lens features a curved segment top (the "C" in the product name stands for "curved"). The plastic trifocal lens is available with a straight segment top and is denoted by an "S" (= straight) in the product name. The first figure after the letter indicates the depth of the intermediate portion in millimetres, while the following two figures specify the segment width in millimetres.
Alternatives for Trifocal Lenses
To an even greater extent than bifocals, trifocals have now been almost completely replaced by progressive lenses. Progressive spectacles provide comfortable vision at all distances, as the dioptric power increases continuously from the distance to the near zones, providing optimum vision for every range without any irritating image jumps.