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What type of lenses are available to fill prescriptions?

The simplest form of spectacle or contact lens is the single vision lens, be it concave or convex.

This means that the lens is made to a single prescription to correct a particular vision deficiency,

be it short or long-sightedness.

 

Convex Lens

A convex lens has either one or both of its surfaces curving outwards, that is, wider divergence

from the plan at the centre. Theses lenses are used to correct long-sightedness (hypermetropia).

The more long sighted a person is, the more convex the lens has to be to correct the vision

deficiency by bringing the rays of light into focus.

 

A long sighted person's focus is actually some way behind the back of the eye and the convex

lens pulls the rays of light together sooner than they normally would be.

 

Concave Lens

A concave lens is the opposite of a convex lens. Here one or both of the lenses surfaces are

curved inwards. That is, the centre of the lens is closer to the plane than the edge. A concave

lens is used to correct short-sightedness (myopia). A short sighted person's focus is focusing

before the back of the eyeball. The concave lens pushes the rays of light further apart so that

they arrive together in proper focus at the back of the eye.

 

Photochromic Lenses

These are lenses which react to light, so that in dull conditions they have a soft neutral tint, and

in bright light react with a tint appropriate to the strength of that light. Unlike glass, the latest

plastic photochromic lenses give maximum protection against harmful ultra violet light and are

especially useful against harsh glare or fluorescent lights and VDU screens. These lenses can

be supplied in glass or lightweight plastic to suit most prescriptions.

 

Bifocals

The bifocal lens is seen frequently, if decreasingly. It contains two optical corrections. The most

common use of a bifocal is for people who have become presbyopic, as we all do with age, and

need assistance with both close work and long distance vision. The upper part of the lens is used

to correct the shortsightedness, whilst the lower half assists with reading or other close work.

Between the two parts, there is a distinctive line.


Trifocals

These can be distinguished from bifocals by the fact that they have three sections, incorporating

a correction for middle distance sight. They can be seen visually by the fact that they have two

lines separating the different parts of the lens. Some people find these useful once they have been

able to adjust to the lines on the lens, but many find them distracting. Few of these lenses are

prescribed today.


Varifocal

The varifocal or progressive lens is the modern day equivalent of the bi and trifocal lenses.

These lenses have no distracting lines between the different prescription areas, but have a

graduated section that moves smoothly from one prescription to the other. These are the

lenses of the future and are much more heavily used in many countries already. They provide

the dual benefit of being better cosmetically; they do not draw attention to the ageing process,

that Presbyopic is in all of us, and don't have the distracting lines across the lens for the wearer.

 

Reflection-free coating

Spectacle lenses these days can be provided with reflection-free coatings which virtually

eliminate distracting reflections - from lights or windows for example - on the back or the front of

the lens, helping people to see better and to be visually more satisfying. A very fashionable and

expensive spectacle frame can be ruined by the glare coming back from reflections on people's

lenses.

 

Scratch resistant / hardening coating

People tend not to have the time to treat their spectacles as well as they might. Beyond that

there are every day accidents. Scratched lenses can at lest be an irritant to the wearer and

look unsightly, and at worst become impossible to use. However, lenses can now be dispensed

with a scratch resistant coating, which prolongs their lives.

Types of Spectacle lenses and their coatings
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