Bifocals are glasses with two different optical strengths. Bifocal is commonly prescribed for people with presbyopia who also require correction for myopia, hyperopia, and/or astigmatism.
Video Bifocals
Histori
Benjamin Franklin is generally credited with the invention of bifocals. Historians have produced some evidence to suggest that others may have come before him in this discovery; however, the correspondence between George Whatley and John Fenno, editor of The Gazette of the United States, shows that Franklin indeed created glasses, and probably 50 years earlier than originally thought. But the College of Optometry concludes:
- Unless further evidence we can say is that Franklin was one of the first to wear separate glasses and the act of wearing this caused his name to be associated with the type from the beginning of the date. This undoubtedly contributed greatly to its popularization. However, the evidence implies that when he tried to order this type of lens, London optical experts were already familiar with them. Other members of the Franklin circle of British friends may have worn it even earlier, from the 1760s, but the most uncertain (and arguably impossible?) That separates the bifocal lens has a famous male inventor.
Since many discoveries were developed independently by more than one person, perhaps bifocal invention may be such a case.
John Isaac Hawkins, the inventor of the trifocal lens, coined the term "bifocal" in 1824 and praised Dr. Franklin.
In 1955, Irving Rips of Younger Optics created the first invisible bifocal or "invisible", a precursor to all progressive lenses.
Construction
The original lens is designed with the most convex lens (for close view) at the bottom of the frame and the convex lens at least at the top. Until the early 20th century, two separate lenses were cut in half and combined together on the fringes of the frame. The installation of two half-lenses into a single frame causes a number of early complications and makes such glasses quite fragile. A method for incorporating lens parts together was developed by Louis de Wecker in the late 19th century and patented by Dr. John L. Borsch, Jr. in 1908. Today most bifocals are created by forming the reading segment into the main lens and available with segment readings in various shapes and sizes. The most popular is the D-segment, 28 mm wide. While bifocal d-segments offer superior optics, more and more people are opting for progressive bifocal lenses.
Maps Bifocals
Problem
Bifocals can cause headaches and even dizziness in some users. Acclimation to the small field of view offered by reading reading segments can be time consuming, as the user learns to move the head or reading material rather than the eye. Computer monitors are generally placed directly in front of the user and can cause muscle fatigue due to irregular and constantly straight head movement. This problem is reduced by the use of trifocal lenses or by the use of monofocal lenses for computer users.
In an interesting legal case reported in England in 1969, the plaintiff's ability to use bifocals was disrupted by accident.
Future
Research continues to be done in an effort to remove the limited field of vision in today's eyeglasses. New materials and technologies can provide methods that can selectively adjust the optical power of the lens. Researchers have built such lenses using a liquid crystal layer flanked between two glass substrates.
Bifocals in animal world
The aquatic larvae stage of the diving beetle Thermonectus marmoratus has, in the primary eye, two retinas and two distinct focal plane which are substantially separated (by bifocal mode) to shift their view from near by. for distance, to capture their prey easily and efficiently, mostly mosquito larvae. This is the first use of bifocal technology recorded in the animal world.
References
Source
- Letocha, Charles E. (1990). "Bifokal Early Discovery and Creation". Ophthalmology Survey . 35 (3): 226-235. doi: 10.1016/0039-6257 (90) 90092-A. PMID 2274850.
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G. Li; et al. (April 2006). "Electro-optic lenses are switchable with high efficiency for ophthalmic applications". Proc. Natl. Acad. Sci. USA . 103 (16): 6100-6104. doi: 10.1073/pnas.0600850103. PMCÃ, 1458838 . PMIDÃ, 16597675
External links
- Franklin's letter to Whatley about double glasses.
Source of the article : Wikipedia
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