Eyeglasses & Eye Care Community

Eyeglass Prescription: Lens Strength

The values indicated in the “sphere” and “cylinder” columns of an eyeglass prescription specify the optical power of the lenses in diopters, abbreviated D. The higher the number of diopters, the more the lens refracts or bends light. A diopter is the reciprocal of the focal length in meters.

If a lens has a focal length of 1/3 meters, it is a 3 diopter lens.A +10 diopter lens, which has a focal length of 10 centimeters, would make a good magnifying glass. Eyeglass lenses are usually much weaker, because eyeglasses do not work by magnifying; they work by correcting focus.

The eye itself has a refractive power of 60 diopters.Stacking lenses combines their strength. A +1 diopter lens combined with a +2 diopter lens forms a +3 diopter system.

Lenses come in positive (plus) and negative (minus) powers. Given that a positive power lens will magnify an object and a negative power lens will reduce it, it is often possible to tell whether a lens is positive or negative by looking through it.
Positive eyeglass lenses can concentrate sunlight.

This series of pictures shows the shadow cast by a pair of 1 diopter drugstore “reading glasses” outdoors in sunlight as we hold it farther and farther away from a wall. As the distance from the wall increases, the shadow of the frame seems to thicken and the bright area in the center gets smaller and brighter. It slowly changes from being “eyeglass-shaped” to circular.

Negative lenses spread sunlight instead of concentrating it.

A negative lens combined with a positive lens removes some of its strength. A -2 lens combined with a +5 lens forms a +3 diopter system.

A -3 lens stacked on top of a +3 lens looks almost like flat glass, because the combined strength is 0.

In science textbooks, positive lenses are usually diagrammed as convex on both sides; negative lenses are usually diagrammed as concave on both sides. In a real optical system, the best optical quality is usually achieved where most rays of light are roughly normal (i.e., at a right angle) to the lens surface.

In the case of an eyeglass lens, this means that the lens should be roughly shaped like a cup with the hollow side toward the eye, so most eyeglass lenses are menisci in shape

Popularity: 14% [?]

Other Names of Sunglasses

There are also various words referring to eyepieces with darkened lenses:

.Glares is a term popular in India if the glass is dark. If it is light then Coolers

.Sun spectacles is a term used by some opticians.

.Spekkies is a term used predominantly in southern Australia.

.Sun specs (also sunspecs) is the shortened form of the above term.

.Sunglasses is a term in common usage in Britain and North America, and it is also used when preceded by “pair of”.

.Sun-shades can also refer to the sun-shading eyepiece-type, although the term is not exclusive to these.

Also in use is the

.derivative abbreviation, shades.

.Dark glasses (also preceded by pair of) – generic term in common usage.

.Sunnies is Australian and New Zealand Slang

.Specs is a common name for sunglasses in North America.

.Smoked spectacles usually refers to the darkened eyepieces worn by blind people.

.Solar Shields Usually refers to the models of sunglasses with large lenses.

.Stunna shades Used as a slang term in the hyphy movement, usually referring to sunglasses with oversized lenses.

.Shades

.Hater blockers

.Locs (also maddoggers) is a term for very dark lensed sunglasses.

.Cheaters

.Sang Gleezies (from sunglasses)

.Eyewear is a term used by opticians

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Popularity: 12% [?]

History of Sunglasses

Precursors

It is said that the Roman emperor Nero liked to watch gladiator fights with emeralds. These, however, appear to have worked rather like mirrors.Flat panes of smoky quartz which offered no corrective powers but did protect the eyes from glare were used in China in the 12th century or possibly earlier. Contemporary documents describe the use of such crystals by judges in Chinese courts to conceal their facial expressions while questioning witnesses.

James Ayscough began experimenting with tinted lenses in spectacles in the mid-18th century. These were not “sunglasses” as such; Ayscough believed blue- or green-tinted glass could correct for specific vision impairments. Protection from the sun’s rays was not a concern of his.

Yellow/Amber and brown-tinted spectacles were also a commonly-prescribed item for people with syphilis in the 19th and early 20th centuries because of the sensitivity to light that was one of the symptoms of the disease.

Modern developments

In the early 1900s, the use of sunglasses started to become more widespread, especially among the pioneering stars of silent movies. It is commonly believed that this was to avoid recognition by fans, but the real reason was they often had perennially sore eyes from the powerful arc lights that were needed due to the extremely slow speed film stocks used.

The stereotype persisted long after improvements in film quality and the introduction of ultraviolet filters had eliminated this problem.

Inexpensive mass-produced sunglasses were introduced to America by Sam Foster in 1929. Foster found a ready market on the beaches of Atlantic City, New Jersey, where he began selling sunglasses under the name Foster Grant from a Woolworth on the Boardwalk.

Sunglasses first became polarized in 1936, when Edwin H. Land began experimenting with making lenses with his patented Polaroid filter.

In 2004, Oakley developed the THUMP, sunglasses with built-in digital audio player. This design has been copied by a number of smaller companies.

Popularity: 15% [?]