High-Index Lenses: Thin is Most Definitely In
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High Index lenses are made from material that is of much greater density with a higher index of refraction than standard plastic lenses. Because of these characteristics, less material is required to provide the prescribed amount of vision correction and results in thinner, lighter lenses. These lenses are especially beneficial for patients with moderate to strong prescriptions who wish to avoid thick, heavy, and unsightly lenses
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| Lens materials with a higher index of refraction (IOR) allow light to pass more quickly through the lens, so less of the actual lens material needs to be used. That's why lenses with a higher index of refraction, or high index lenses, can be thinner than traditional lenses.
High index lenses are available in either glass or plastic (CR-39, for example). Different manufacturers make different high index lenses, and what sets each lens apart from the others is its index of refraction. The higher the IOR, the denser the material. All things being equal, a 1.66 IOR material will result in a thinner and lighter-weight lens than a 1.57 IOR material will.
Best Candidates
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High index lenses are traditionally used for myopes. High index materials don't provide as much benefit to patients wearing plus corrections as minus. Plus lenses made in high index will be thinner, but the thinning takes place in the center of the lens where it is not visible and does little to improve the cosmetic appearance of the eyewear. For plus patients, aspheric lenses with their flatter, slimmer profile offer the greatest cosmetic benefits for plus lenses. Most aspheric lenses are made of higher index materials, and they provide improved cosmetic benefits and better vision for plus patients.
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Pros and Cons
The advantages of high index lenses are:
- They are thinner, flatter and lighter than conventional plastic or glass
- They can fit into neat and fashionable frames
- They have a higher scratch resistance than normal plastic and higher impact resistance than glass.
- Built-in UV protection
The disadvantages are:
- They can be expensive
- They are not as scratch resistant as glass.
Ultra-High Index
Without question, "ultra-high index" lenses are designed for people who want the latest in thin and light lens technology. Depending on the design (e.g., aspheric) and the final center thickness, lenses with refractive indices of 1.67 and above can be as much as twice as thin as polycarbonate lenses after processing. For higher prescriptions?say, +/-10.00D and above?this can be quite significant.
Generally, the higher the IOR, the higher the cost of the lenses will be. The thinnest, most lightweight lenses are typically the most expensive.
Ultra-high index lenses provide their best benefits when they are matched with aspheric curves. These lenses have flatter front curves than their lower index counterparts, resulting in thinner and lighter lenses?clearly a plus for patients. Aspheric curves are especially helpful. The asphericity reduces spatial distortion, reduces the eye's magnification or minification, and maintains a thin and flat lens profile.
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