Bifocals are eyeglasses whose corrective lenses each contain regions with two distinct optical powers. Bifocals are most commonly prescribed to people with presbyopia who also require a correction for myopia, hypermetropia, and/or astigmatism. Image File history File linksMetadata Download high-resolution version (2096x1920, 1109 KB) Bildinhalt: Bifokalbrille, stark angeschnitten Fotograf: Frank C. Müller Aufnahmedatum: 08. ... Image File history File linksMetadata Download high-resolution version (2096x1920, 1109 KB) Bildinhalt: Bifokalbrille, stark angeschnitten Fotograf: Frank C. Müller Aufnahmedatum: 08. ... Glasses, spectacles, or eyeglasses are frames bearing lenses worn in front of the eyes, sometimes for purely aesthetic reasons but normally for vision correction or eye protection. ... A bifocal corrective eyeglasses lens A corrective lens is a lens worn on or before the eye, used to treat myopia, hyperopia, astigmatism, and presbyopia. ... Optical power or dioptric power or refractive power is the degree to which a lens or mirror converges or diverges light. ... Presbyopia (Greek word presbyteros (πρεσβύτερος), meaning elder) is the eyes diminished ability to focus that occurs with aging. ... Normal vision. ... Hyperopia (or more rarely, hypermetropia), also known as farsightedness or longsightedness, is a defect of vision caused by an imperfection in the eye (often when the eyeball is too short), causing inability to focus on near objects, and in extreme cases causing a sufferer to be unable to focus on... Astigmatism is an affliction of the eye, where vision is blurred by an irregularly shaped cornea. ...

History

Benjamin Franklin is usually credited with the creation of the first pair of bifocals in the early 1760's, though the first indication of his double spectacles comes from a political cartoon printed in 1764. A great number of letters and publications from that time period refer to Dr. Franklin's double spectacles, including his first reference to them in a letter dated August 21, 1784. Benjamin Franklin (January 17 [O.S. January 6] 1706 – April 17, 1790) was one of the most well known Founding Fathers of the United States. ... Glasses, spectacles, or eyeglasses are frames bearing lenses worn in front of the eyes, sometimes for purely aesthetic reasons but normally for vision correction or eye protection. ...

John Isaac Hawkins, the inventor of trifocals, coined the term bifocals in 1824 and credited Dr. Franklin with their invention. Trifocals are eyeglasses where the lenses have 3 regions to correct for distance, intermediate (arms length), and near vision. ...

Construction

Franklin's original bifocals were designed with the most convex lenses (for close viewing) in the lower half of the frame and the least convex lenses on the upper. Up until the beginning of the 20th century two separate lenses were cut in half and combined together in the rim of the frame. The mounting of two half lenses into a single frame led to a number of early complications and rendered such spectacles quite fragile. A method for fusing the sections of the lenses together was developed by Louis de Wecker at the end of the 19th century and patented by Dr. John L. Borsch, Jr. in 1908. This article is about the optical device. ...

Today most bifocals are created by molding a reading segment into a primary lens and are available with the reading segments in a variety of shapes and sizes. The most popular is the flat-top (a.k.a. straight-top or D-segment), 28 mm wide.

Problems

Bifocals' division of the field of vision has been known to cause headaches and even dizziness in some users. Acclimation to the small field of view offered by the reading segment of bifocals can take some time, as the user learns to move either the head or the reading material rather than the eyes. Computer monitors are generally placed directly in front of users and can lead to muscle fatigue due to the unusual angle and constant movement of the head. This trouble is mitigated by the use of trifocal lenses. bamf For the same phenomenon in photography see Angle of view. ... Nineteen inch (48 cm) CRT computer monitor A computer display, monitor or screen is a computer peripheral device capable of showing still or moving images generated by a computer and processed by a graphics card. ... Trifocals are eyeglasses where the lenses have 3 regions to correct for distance, intermediate (arms length), and near vision. ...

Future

Research continues in an attempt to eliminate the limited field of vision present in current bifocals. New materials and technologies may provide a method which can selectively adjust the optical power of a lens. Researchers at the University of Arizona have constructed such a switchable lens using a liquid crystal layer sandwiched between two glass substrates. When electricity is applied the effective refractive index changes, varying the optical power of the lens. The University of Arizona (UA or U of A) is a land-grant and space-grant public institution of higher education and research located in Tucson, Arizona, United States. ... Schlieren texture of Liquid Crystal nematic phase Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional liquid, and those of a solid crystal. ...

References

• Letocha, Charles E., M.D. (1990). "The Invention and Early Manufacture of Bifocals". Survey of Ophthalmology 35: 226-235. DOI:10.1016/0039-6257(90)90092-A. 
• G. Li, et al (April 2006). "Switchable electro-optic diffractive lens with high efficiency for ophthalmic applications". Proc. Natl. Acad. Sci. USA 103: 6100-6104. 

A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ...

See also

• Progressive lenses
• Trifocals