Prescription lenses

Bi-focals
These are lenses that incorporate two powers in one lens. Usually, the power for distance in the upper portion of the lens and the power for near vision in the lower portion. These are available in various designs but the most commonly used are the moon shaped, D shaped and straight line ones. The choice of the design is often guided by the function they are needed for. They are a practical option for people over forty years of age who require different powers for distance and near. They are available in both glass and plastic, in different tints and can be coated with ARC. They are also available in Photochromic glass and plastic.

The major disadvantage of ARC is its vulnerability to scratches. (How to care for your ARC Lenses.) Often marketed as antiglare lenses these are sold as computer lenses and night vision lenses. For people wearing low power prescriptions to relieve stress caused reflections produced by the lens surfaces , ARC is definitely useful. For people who need no prescription, ARC lenses act, if at all they do, as nothing more than psychological security. Any glare problem for a computer user has to be addressed at the screen itself by placing a filter on the screen or by simply by changing the settings of the monitor. For most computer users the problem is that the lights in the conventional offices are placed behind the person. These are reflected from the screen and cause glare. ARC is not your total anti-glare solution. It only reduces Glare by about 8 to 10 %.

In addition to these the ARC increases transmittance of light, from 92% in an uncoated lens to 98% in a coated lens. Thus the intensity of the image increases and so does the perception of color contrast. More than anything else it gives a psychological edge to the wearer.

Anti-reflecting Coatings
Anti-reflecting Coatings (ARC) are superficially applied coats which decrease the amount of reflections from the lens surfaces thus, reducing the portion of glare produced within the lens. These coatings are useful in following cases:

Photochromic lenses (Sunglasses)
Photo-chromatic lenses have been a preferred choice of many people, especially those who have to move in and out a lot. But these lenses lose their efficiency when the temperatures reach the mid 40s. Also these lenses retain a 5% residual tint even in the dark which may interfere with visibility in dark. Photo chromatic lenses are available in glass and plastic. The photo chromatic effect is achieved by the addition of silver halide crystals to the lens material which change color only when exposed to UV light. Therefore, they are ineffective while you are driving a car as the wind-screen itself will absorb the UV rays and there will be no photochromic action. Photo chromatic lenses are generally referred to as "sunglasses" by the layman and not an ideal substitute for dark glasses to beat the tropical summer.

Tints (Lens Colors)
To add a tinge of variety to your lenses, you can have them in various tints. In glass, for prescription purposes the most widely used tints are Pink (SP2), Calobar (Dark Green), B2 (Dark Grey), B15 (Dark Brown), Light Brown (SP10), Light Blue (Alpha) and Light Grey. Out of these the darker shades are basically to provide protection against the dazzle of the sun, the light pink and blue are used for specific purposes to give extra protection against ultra-violet rays at the same time making the intensity of light slightly less without compromising on the quality of vision. In plastic, any shade is possible. You can virtually create your own shade in any prescription you want. Lighter colors as fashion tints serve the dual purpose of braving moderate amounts of sun and acting as a shield to the eyes from the wind and dust at night.

Refractive Index
Refractive index is that characteristic of the material of the lens that determines the thickness of the lens. Higher the refractive index, thinner the lens will be. Normal glass has a refractive index of 1.523 while normal plastic CR39 has refractive index of 1.49. Therefore, plastic lens, although lighter in weight, is thicker than a glass lens of equivalent power. High refractive glass is available in indices 1.60, 1.70 and 1.80 while high refractive index plastic is available in 1.52, 1.56, and 1.67 indices. To give you an approximation of the effect of refractive index on thickness of lens, a 1.70 index lens is about 33% thinner than normal (1.523) index glass lens.

( How to care for your plastic lenses)
The biggest advantage of plastic is that it is very light in weight. Also, plastic is shatter-proof, so a much safer choice than glass. At this juncture let us broaden our perspective on plastic lenses. The plastic lenses used for optical purposes are basically acrylic, CR39, and polycarbonate. Acrylic is the cheapest of all and not usually used for prescription purposes. It is more prone to scratches and needs frequent replacement. CR39 is the most frequently used and features like scratch resistance and UV protection can be incorporated in them which meet the US FDA standards. Polycarbonate is a superior material but very expensive and not readily available. The biggest advantage of these lenses is their higher impact resistance which makes them an option for the sportspersons.But the most important thing to remember is that if a UV blocker is incorporated in a plastic lens, which usually is in a lens from a reputed dispensing optician, they do not cause harm to the eyes. Plastic is the material of choice in most cases.

Material
The basic choice of material is between glass and plastic.
Glass is the conventional material used for prescription lenses. They are less expensive and less prone to scratches. However, glass is heavy and brittle i.e. it can break easily. Plastic is relatively expensive and more prone to scratches. Application of scratch-resistant coatings helps making these lenses less vulnerable to scratches.

Prescription lenses
Selection of the right kind of prescription lenses is an aspect often taken for granted. Ignorance on part of the consumer is probably the basic reason for that. Today, with easy import facilities and several MNCs entering the Indian market, you can access the best in lens technology from world over. Consideration of the following factors may help you select the appropriate lens to suit your prescription, utility, and budget.