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PLEASE READ THIS: The information contained herein is not intended as a substitute for medical advice and care from qualified, licensed health care providers. The reader should regularly consult a physician in matters relating to his or her health and particularly with respect to any symptoms that may require diagnosis or medical attention. The information is presented here to educate and inform and to guide you to an understanding of cause, prevention as well as treatment.
There are a number of different materials from which ophthalmic lenses are made. Lenses can be made of plastic (and there are a number of different types, including hard resins, acrylics, polyurethenes, polycarbonates) or glass (of difference densities).
Lenses materials have different properties, including weight and density(specific gravity), index of refraction and Abbe distortion. Some materials can be ground thinner than others, another factor influencing overall weight and thickness for a given power.
It depends upon what you are looking for and the power of your prescription lenses. To some degree, there is a trade off---quality of optics opposed to lightest weight and thickness. But this issue is not always critical for some spectacle lens wearers. Stronger powered lenses are more subject to aberrations and distortion in the optics than lower powered lenses. Under 2 diopters, there is little benefit from high index materials. From 2 to about 3.5 diopters, itÕs a nice touch to reduce weight but thickness reduction is still minimal. Above 4 is where high index lenses really benefit both weight and thickness reduction. Powers above 7 can appreciate even further reductions using even higher index materials.
If you are looking for the best combination of cosmetic appearance and optics, there are a number of variables that are under your control.
1. Frame size and shape: the smaller the frame, the thinner the lens will be and appear. Smaller means less mass and less weight. More closely matching the frame size to the distance between your eyes (so that the center of the lens is aligned with the centers of your eyes,) the less the observable distortion. The closer to round the frame eye shape, the more likely that the overall appearance will be comsmetically appealing
2. Lens material: Glass and plastic lenses are available in varying refractive abilities called index of refraction. The higher the index, the more the material bends light for a given curvature and thickness. Glass is heavier but more dense and resistant to scratching and warpage. Plastic is lighter and more resistant to breakage, but more readily scratched. One plastic, polycarbonate, is so impact resistant that it is used as a safety lens. Many of the "premium" high index lenses come with built-in scratch resistant coating and UV blocking.
It is also important to remember that all ophthalmic lenses filter ultraviolet light. Polycarbonate and most plastic lenses block essentially all UV, both long and short wavelengths (the "tanning"and "burning" rays respectively). Glass blocks all UV-B rays and most of the UV-A rays.
Other factors influencing thickness and weight are aspheric curve design and center thickness. Aspherics tend to reduce edge thickness. Some materials can be manufactured with less center thickness than others, while maintaining stability of the curvatures.
3.Abbe value and index of refraction. The Abbe number represents the relative degree of distortion generated while looking through off-center areas of the lens. The higher the number, the lower the aberration. The higher the index of refraction, the thinner the lens. Generally, the higher the index of refraction, the lower the Abbe, but this is not a linear proportion. Lenses of the same index can have somewhat different Abbe values. Abbe of 30 is considered to have a high potential aberration and Abbe of 60 is considered the most minimal distortion for consumer level lenses.
You can choose a lens that is the thinnest, the lightest or one that has the best optics. But not all in one package. You need to weigh the benefits of these attributes and choose the lens that best suits your personal needs.
(Writer's opinion follows:) If you are looking for the thinnest glass lens with least distortion, there is no superior lens to Zeiss Lantal Glass. In plastic, once again Zeiss wins with their Claret Hard Resin lens. The best resin lenses for optics, but not quite as thin or light, are Sola Spectralite and Rodenstock Cosmolite. The Essilor Thin will give the best optics in a resin lens, still very light, but the somewhat thicker for the same lens power than the former.
Please remember that this list below is far from complete. There are other brands available. Not all brands are available in all powers or lens designs, and some are more readily available than others. The listing is presented to allow an understanding of the differences between lenses of different materials and designs. You should consult with your optical dispensing professional for guidance as to the best lens material and design for your prescription.
polycarbonate, 1.586, 30 hardcoat
Optima HyperIndex 166, 1.66, 32 aspheric, hardcoat
Pentax UltraThin 1.66, 32 hardcoat,AR coat
Seiko Super 16, 1.6, 33 hardcoat
Pentax 1.6, 1.6, 36 hardcoat
Zeiss Claret 1.6, 36 AR, hardcoat
Signet/Armorlite1.56, 36 hardcoat
Essilor Thin&Lite, 1.6, 37 aspheric, hardcoat
Optima 160 1.6, 37 aspheric, hardcoat
Rodenstock Cosmolite 1.6, 1.6 , 37 hardcoat
Sola Spectralite 1.537, 47 hardcoat
Rodenstock Cosmolite 1.5, 47 hardcoat
Essilor Thin 1.498, 58 hardcoat
Vision Ease 1.8 index, 2.5 Abbe
X-Cel 1.8 ,25 (other similar lenses from X-Cel, Phillips)
Zeiss Lantal 1.8, 39.3 AR avail.
Zeiss Tital 1.7, 39.3 AR avail. (other similar lenses from X-Cel, VisionEase and Phillips, Abbe around 32)
Zeiss, VisionEase, X-Cel 1.6, 42
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