Progression of bifocal and trifocal lenses
During the long history, eyeglass lenses have advanced a lot and now can offer more options for people with various vision problems. A significant breakthrough in the eyeglass industry is the change from single-vision lenses to multifocal lenses. Researchers have successfully incorporated more than one power into a single lens. The benefit brought by this breakthrough for patients is that several vision problems can be corrected by one pair. Yet another convincing breakthrough in the eyewear lens industry is the progression of multifocal lenses for presbyopia correction. More specifically, progressive lenses have been developed to overcome some significant shortcomings of regular bifocals and trifocals.
From single-vision to multifocal lenses
Before the initial version 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 designs with different prescriptions and various lens segments are available. And intermediate vision can also be offered by special multifocal lenses.
Popular designs of bifocal 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 which is needed by certain activities such as computer use.
A typical design of trifocal lenses
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 users, a wearer of trifocal lenses focuses through the intermediate segment for objects in distances between 18 and 24 inches and looks over the distance segment to see things in the distance.
Aesthetical improvement made by progressive lenses
Most of the bifocal and trifocal lenses have visible or slightly visible lines at borders of different lens segments. In detail, a bifocal lens has a visible line separating the distance section from the near section. And a trifocal lens has two visible lines separating the intermediate vision part from the distance vision part and the near section respectively. The vision of people who use properly fitted bifocals and trifocals will not be affected these visible lines. But they really affect eyewear appearance and tell the public secret that you are suffering from presbyopia. The first improvement made by progressive lenses lies in that these lenses have no segment lines and appear exactly the same as regular optical lenses.
Visual improvement made by progressive lenses
The second improvement marked by progressive lenses is that they provide a comfortable power transition, rather than two or three separate powers. This is exactly better visual aid. People who regularly wear bifocal or trifocal glasses have probably experienced abrupt vision transition when they switch their vision from the near to the distance or the like. The exact reason is the sudden power separation at the power boundary. In contrast to bifocals or trifocals, progressive lenses are designed without sudden power separation, but with a gradient of increasing lens powers. Power transition will be seamless and smooth.
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