Progressive lens

Progressive spectacle lenses, also called progressive addition lenses (PAL), progressive power lenses, graduated prescription lenses, and varifocal or multifocal lenses, are corrective lenses used in eyeglasses to correct presbyopia and other disorders of accommodation. They are characterised by a gradient of increasing lens power, added to the wearer's correction for the other refractive errors. The gradient starts at the wearer's distance prescription, at the top of the lens and reaches a maximum addition power, or the full reading addition, at the bottom of the lens. The length of the progressive power gradient on the lens surface depends on the design of the lens, with a final addition power between 0.75 and 3.50 dioptres. The addition value prescribed depends on the level of presbyopia of the patient. In general the older the patient, the higher the addition.

The first patent for a PAL was British Patent 15,735, granted to Owen Aves with a 1907 priority date. Aves' patent included the progressive lens design and the manufacturing process. However this was unlike modern PALs. It consisted of a conical back surface and a cylindrical front with opposing axis in order to create a power progression. This design was never commercialized.

While there were several intermediate steps (H. Newbold appears to have designed a similar lens to Aves around 1913), there is evidence to suggest that Duke Elder in 1922 developed the world's first commercially available PAL (Ultrifo) sold by "Gowlland of Montreal". This was based on an arrangement of aspherical surfaces.

Irving Rips at Younger Optics developed the first commercially viable blended lens in 1955 called the Younger Seamless Bifocal.

The Varilux & Carl Zeiss lenses were the first PAL of modern design. It was developed by Bernard Maitenaz, patented in 1953, and introduced by the Société des Lunetiers (which later became part of Essilor) in 1959. The breakthrough for the adaptation and the comfort of the progressive lens occurred in 1972 with the market introduction of Varilux 2. Bernard Maitenaz discovered the importance of the design periphery for the peripheral and dynamic vision. So while for Varilux the surface structure was close to the characteristics of the bifocal lens, with an upper aberration-free half of the surface for far vision and a rather large "segment" for clear near vision, Varilux 2 was distinguished by a totally aspheric design.

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