All you need to know about Spectacle Lenses

All you need to know about Spectacle Lenses - Mr. Laquee Ansari Types of Spectacle Lenses Single vision lenses Single vision has the same optical co...
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All you need to know about Spectacle Lenses -

Mr. Laquee Ansari

Types of Spectacle Lenses Single vision lenses Single vision has the same optical correction over the entire area of the lens. Bifocal lenses In Bifocals the upper part of the lens is generally used for distance vision, while the lower part is used for near vision. Usually, a segment line separates the two focal areas. Trifocal lenses Trifocal lenses are similar to bifocals, except that the two focal areas are separated by a third middle area with intermediate focus correction, used for intermediate vision e.g. computer distance. This lens type has two segment lines, dividing the three different correcting segments. Progressive lenses Progressive or varifocal or multifocal lenses provide a smooth transition from distance correction to near correction, eliminating segment lines and allowing the viewing of all intermediate distances. The progressive lens eliminates the line in bi/tri-focals and are very complex in their profile. Aphakic lenses Aphakic lenses are high power “Plus” lenses used generally in patients without intraocular lens implants in cataract surgery.

Types of Materials used in Spectacle Lenses Standard Plastic (cr- 39 Lenses - Columbia Resin) CR-39 Plastic lenses are currently the most commonly prescribed lens, due to their relative safety, low weight, low cost, low dispersion, ease of production, and outstanding optical quality. The main drawbacks are the ease, by which a lens can be scratched and the

costs of producing higher index lenses. They can be treated

with scratch resistance coating. Standard Glass Lenses (optical Crown Glass) Glass lenses have become less common in recent years due to the danger of shattering and their relatively higher weight compared to plastic lenses. They still remain in use for specialized circumstances, for example in extremely high prescriptions (currently, glass lenses can be manufactured up to a refractive index of 1.9) and therefore can be made thinner.They are recommended in certain occupations where the hard surface of glass offers more protection from sparks or shards of material. Hi-Index Lenses Hi-index lenses are made of a special plastic material that refracts light in a different way than regular plastic lenses. With hi-index lenses, vision can be corrected with a much thinner lens. Plastics are graded in numbers, such as 1.50 or 1.67. The higher the number, the thinner the lens. Because they are thinner, hi-index lenses are also lighter, making them more comfortable to wear. This is important to patients with high prescriptions, as their glasses can be made cosmetically attractive and appealing. Hi-index lens materials tend to cost more than standard plastic lenses. Polycarbonate Lenses

Polycarbonate lenses are made of a type of plastic that is more impact-resistant than standard plastic lenses. Polycarbonate lenses are also much thinner and lighter in weight. They are considered a high index plastic. Polycarbonate lenses also have built-in ultraviolet protection. Because of these properties, it is the lens material of choice for children's lenses, sport lenses and safety lenses. However, some people complain that polycarbonate lenses fail to give them the crispest, clearest vision. Trivex Lenses Trivex is a relatively new material that is similar to polycarbonate lenses but with higher quality optics, and thus provides clearer vision. Trivex is lighter in weight than standard plastic but not quite as thin as polycarbonate. Trivex is a more rigid material, making it a better selection for rimless or drill mount frames. Trivex is just as impact resistant as polycarbonate and may also be prescribed for children's lenses, sport lenses and safety lenses.

Special Coatings Anti- reflective Coating Anti-reflective coating is applied to eyeglass lenses to reduce the amount of internal and external reflections on a lens. This increases the amount of light transmitted through the lens, which improves quality of vision. Anti-reflective coating also decreases unwanted glare and halos at night time. It also makes the lenses appear some what invisible and very thin. While everyone could benefit from an antireflective coating, it is especially beneficial for people with high prescriptions, people who have a decrease in vision at night, and professions in which cosmetic appearance is important. Scratch-Resistant Coating No eyeglass lenses — not even glass lenses — are scratch-proof. However, lenses that are treated front and back with a clear, scratch-resistant coating have a much

harder surface that is more resistant to scratching, whether it's from dropping your glasses on the floor or occasionally cleaning them with a paper towel. Kids' lenses, especially, benefit from a scratch-resistant hard coat. Ultraviolet Treatment Another beneficial lens treatment is an invisible dye that blocks ultraviolet (UV) light. Just as sunscreen keeps the sun's UV rays from harming your skin, UVprotective treatments for eyeglass lenses block those same rays from damaging your eyes. Overexposure to ultraviolet light is thought to be a cause of cataracts, retinal damage and other eye problems. Regular plastic eyeglass lenses block most UV light, but adding a UV-blocking dye boosts UV protection to 100 percent for added safety. Other eyeglass lens materials, including polycarbonate and most high-index plastics, have 100 percent UV protection built-in, so an extra lens treatment is not required for these lenses. Photochromatic lenses These are lenses that darken on exposure to ultraviolet (UV) radiation. Once the UV is removed (for example by walking indoors), the lenses will gradually return to their clear state. Photochromatic lenses may be made of glass, polycarbonate or another plastic. The main advantage is that they will darken into a sunglass when exposed to U.V. light, thus obviating the need to carry a separate pair of sunglasses for protection against harmful UV rays from the sun. The main disadvantage of photochromic lenses is they do not adjust immediately. It could take up to two minutes for the lenses to completely change from light to dark or vice versa. Another possible disadvantage is that they will not darken when worn inside vehicles with glass that blocks UV light. Polarized Lenses

Polarized lenses are used in sunglasses to reduce glare from reflective surfaces such as the surface of a lake or the hood of a car. They accomplish this feat through a process called polarization, much like a venetian blind controls sunlight through a window. Polarized lenses have a laminated surface containing vertical stripes. These stripes only allow vertically-polarized light to enter the wearer's eyes. Glare is eliminated because the horizontally-polarized light waves cannot bypass the polarized filter.