Bifocals

From Encyclopedia Jr, free information reference for Kids

Bifocals are eyeglasses that have corrective lenses containing two different optical powers. They are most commonly prescribed for people with presbyopia, and who also require a distance correction for myopia, hyperopia, and/or astigmatism.

Taking advantage of that fact that when a person views something at close range, they usually look down and the reverse is true when they view at a distance, the early split 'Franklin' bifocals were designed with the lenses for close viewing in the lower half of the frame and the distance viewing lenses on the upper. Originally the lenses were simply cut in half and combined together in the rim of the frame. Later examples involved the cementing of a bifocal segment onto a larger lens, but now most bifocals are made by molding a small reading segment into the lens.

Bifocal lenses are available with the reading segments in a variety of shapes and widths. The most popular is the flat-top or D-shaped segment, 28 mm wide.

With the advent of computers, bifocal users have experienced some problems. Although most printed reading materials are easily viewed with bifocals, computer monitors are generally placed directly in front of users, but still close enough to require corrective lenses, requiring bifocal wearers to tilt their heads up to view the screen.

A new next generation bifocal is being developed at the University of Arizona by Nasser Peyghambarian and his team. Instead of the old double lenses, the new lens has an ultrathin liquid crystal layer sandwiched between the two layers of glass. When an electric current is turned on, the liquid crystal molecules rotate bending the incoming light like a lens.

[edit] See also

• Progressive lenses
• Benjamin Franklin, the inventor of bifocals.