Bifocals

Overview
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.

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.

John Isaac Hawkins, the inventor of trifocals, coined the term bifocals in 1824 and credited Dr. Franklin with their invention.

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.

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.

They are available at most eye doctors.

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.

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.