For over 40 years, progressive addition lenses have increasingly become the solution for optical correction of presbyopic and even some non-presbyopic people. Before this, the catalogues of glassmakers contained a wide variety of bifocals and trifocals of all forms, to fit to all kinds of visual needs. Fabrication of multifocal lenses is complex. Bifocals and trifocals are made by several consecutive steps, which need to be very precise to control the dimensions of the segments. Irregular surfaces of progressive lenses have to be generated under computer control. The next step of polishing must be done very carefully to preserve the geometry of the design. To maintain acceptable prices, these operations are made serially on the front surface of great quantities of lenses, which are stored as semi-finished blanks; the other side is then finished to the individual prescription with conventional, sphero-cylindrical generators.
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