Progression of bifocal and trifocal lenses

Eyeglass lenses have advanced to offer more options for various vision problems. One convincing example is the invention and progression of multifocal lenses for presbyopia correction.

Before the first design of multifocal lenses named Franklin bifocal, people with presbyopia had to frequently switch between two different pairs of eyeglasses, one for close reading and the other for distance vision. Also called Executive bifocal, this design of bifocal lenses had a line extending across the entire lens width. A later version of Franklin design involved a smaller, half-moon near vision segment in the lens. Today, multifocal lenses with different prescriptions and various lens segments designs are available. And intermediate vision can also be offered by special multifocal lenses.

A bifocal lens has a power for distance vision and another power for close vision. Modern designs of bifocal lenses always contain only a small lens segment for near vision. The early 1900s witnessed the invention of fused bifocal, which had a near vision segment attached to a full distance lens. Currently, fused bifocals with D-shaped near segments at the bottom of the lenses are the most popular. Other shapes of the near vision segment include a round segment and a narrow horizontal rectangle. Bifocal lenses are limited in providing intermediate vision needed by certain activities such as computer use.

A trifocal lens provides an additional segment above the near segment for intermediate vision. The intermediate segment properly magnifies objects in the intermediate zone, such as a computer screen, a speedometer and other dashboard gauges. Trifocal lenses are especially helpful for people above 50, who lack the ability to cope with intermediate vision. Eye doctors always fit trifocal lenses at a higher position than bifocal lenses. The top line of the intermediate segment should be even with the lower margin of the pupil. In similar to bifocal lenses, a wearer of trifocal lenses focuses through the intermediate segment for objects in distances between 18 and 24 inches and looks over the distance segments to see things in the distance.

Most of the bifocal and trifocal lenses have visible or slightly visible lines at borders of different lens segments. Progressive lenses are preferred by many youngsters because these lenses have no segment lines. Progressive lenses also provide a comfortable power transition, rather than two or three separate visual zones.

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Article Tags: bifocal lenses, trifocal lenses