Bifocals

A bifocal lens is a spectacle lens, the two (bi 'two'; focal, focus " focal point " ) achieved visual effects and therefore can be used for two different distances. Visible are such lenses at the dividing line between the two lenses. A pair of glasses with bifocal lenses are called accordingly bifocals.

As the inventor of bifocals is true Benjamin Franklin. He was annoying, constantly exchanging his distance glasses to reading glasses. At about 1770, he came up with the idea, per side (R / L) two spectacle lenses with appropriate visual impact to assemble, the so-called Franklin glass. The bifocals was born. The first patent was awarded in 1909 the American Orford. Today bifocals are prepared with two different types of glass: in the base glass, which is held in the distance power, a glass portion is melted with a higher refractive index. This enables a smooth surface without tangible transition between distance and near portions.

Bifocal lenses are also available in plastic materials. The optically different effects of distance and near portions can this not be achieved through different materials, but only by the surface design. This creates a protruding, tactile edge between the two starch components.

At higher difference between long-distance and Nahwert can also be incorporated to complement the viewing zones, a third strength. In this case, the near segment is divided into two areas, there arises a Trifokalglas.

Reading part of the bifocal lenses are produced in different widths. Common dimensions are 25 mm, 28 mm and up to 40 mm for specific needs (the wider the more specialized and more expensive ).

An alternative to bifocal lenses are so-called progressive lenses, which are becoming increasingly common.

A Bifocals is the treatment of choice in the clinical picture of hypoakkommodativen convergence excess, and the other, related accommodative strabismus with large convergent Nahwinkel.

Swell

• Glasses
• Benjamin Franklin