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Transitions Optical has produced an Eyewear Guide for consumers. It is an excellent resource for lens style definitions, features and benefits, and availability. The guide can be downloaded at eyeglassguide.com. This website also provides an interactive Personal Eye Quiz to provide eyewear recommendations.
Progressive Lenses:
Progressive Addition Lenses (PALs) are a type of eyeglass lens designed so that power for near distances increase gradually from the center of the lens down to the bottom. Essilor introduced the first commercially successful PALs in the late 1970's. Today they make up more than 50% of all lenses prescribed for those afflicted with presbyopia. They are available in virtually all lens materials.
Photochromic Lenses:
Lenses that change from light to dark when exposed to UV light (or sunlight) and lighten when not in sunlight. This type of lens has been available since the early 1970's but early versions were plagued with performance issues. Transitions, the market leader, introduced the first plastic photochromic lens in the early 1990's. Today's photochromic lenses darken very quickly when exposed to sunlight, and lighten almost as quickly when indoors. They are available in virtually all lens designs and materials including single vision, progressive addition lenses, bifocals, and trifocals. They also provide 100% UV protection and protect the eyes from harmful UV light. IMPORTANT NOTE: Because photochromic lenses depend on UV light to darken, they are not suitable for use as sunglasses while driving, as UV light does not penetrate car windshields.
Polarized Lenses:
Polarized lenses are designed specifically for glare elimination. Here's how they work: Light reflected from smooth surfaces like a flat road, or water is polarized (concentrated) horizontally and can be very intense and bright. By utilizing a vertically oriented polarized filter, the bright glare is eliminated. Imagine vertical blinds - when you look directly through the open blind your vision is good. However, when you get down at an angle, you can no longer see through the blinds. The intent is to eliminate the unwanted light and allow the desired light to pass through.
High Index Lenses:
The term index refers to the index of refraction of a given material. The higher the index of refraction, the thinner the lens. For example, regular plastic lenses have an index of refraction of 1.498. High index plastic ranges from an index of 1.54 up to 1.74. The highest index lenses result in the thinnest lenses. And because they are thinner, they are also lighter in weight. AR coating is highly recommended because although thinner and lighter, light is reflected and very distracting to the wearer as well as those looking at the wearer. It's also important that high index lenses are aspheric in design to ensure the best optics possible.
Polycarbonate:
Most impact resistant of all lens materials used today, polycarbonate is the best choice for safety eyewear, children's eyewear, as well as sports eyewear. It is also a natural UV filter, protecting against virtually all UVA and UVB rays. Polycarbonate is also considered a high index lens and is thin and lightweight. As with all high index lenses, AR coating is highly recommended.
Trilogy:
Trilogy is a material with "tri-performance" attributes for eyeglass lenses - Clarity, Safety and Light Weight. They are very strong so are ideal for three piece and rimless frames, as well children's eyewear.
Computer Lenses:
The primary purpose of computer lenses is to provide intermediate (arm's length) in the upper portion of the lens and near (reading) power in the bottom. The most popular are progressive in design and can be used for any activity that requires intermediate and near vision - examples include playing the piano, computer work, shooting pool, mechanics, etc.
Aphakic Lenses:
Better known as cataract lenses, aphakics are very high power lenses that were once the only way to restore sight when the patient's own crystalline lens clouded (cataracts) and was surgically removed from the eye. Today, surgeons implant a substitute lens when surgery is performed, and as a result aphakic lenses are rarely required.
Bifocals:
Bifocals were invented by Ben Franklin, and provide two distinct viewing areas separated by a line, distance and near. The distance area is in the top part of the lens, and the near or reading area is typically in the bottom part of the lens called the "segment". Near or reading segments are available in different widths to best meet individual needs, either 28mm, 35mm, or width of the lens (Executive).
Trifocals:
Same as a bifocal but with a third segment for intermediate viewing.
Occupational Lenses:
Lenses designed to serve special purposes, this type of lens has an additional segment at the top to provide near vision above as well as below. This type of lens is called a "double seg". If the bottom portion is a trifocal the lens it is called a quadrafocal.
Aspheric Lenses:
Aspheric lenses are a recent development in the design and manufacturing of lenses. The curves change over the lens surface, rather than remaining constant like a sphere. The gradual change, although not noticeable to the naked eye, helps cut down the weight and thickness of lenses while enhancing the overall optical performance.
Atoric Lenses:
Atoric lenses utilize aspheric design on both sides of the lens to produce the best acuity by eliminating distortion caused primarily by astigmatism.
Scratch Resistant Coatings:
Nothing is 100% scratch proof, but today's super-tough scratch coatings perform exceptionally well under normal use.
Anti-Reflective (AR) Lenses:
These lenses reduce reflections inside the lens, enhancing colors, clarity and light coming through to the eyes. Non-AR lenses lose from 8%-18% of the light because of these reflections depending on the index of refraction. Higher index lenses generate more reflections so AR is highly recommended. There are many AR lenses to choose from with different performance levels that your eye care professional can explain and demonstrate to you.
Mirror Coatings:
Often seen in plano sunglasses, solid mirror coatings make the front of lenses reflective so that the eyes cannot be seen. There are also "flash mirror coatings" that are more transparent and available in more colors. Mirror coatings work especially well when the backside is AR coated because it eliminates glare for increased visual comfort and clarity.
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