Vision Care Product News

SEP 2017

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VCPN VisionCareProducts.com September 2017 75 than +6.00D and minus lens- es need a base curve lesser than +6.00D. at's fine for "general use" ophthalmic lenses, which tend to be relatively flat, but wrap lenses use a base curve of about +8.00D to +9.00D. Using a base curve in this range for most Rx's and tilting it dramatically in front of the eyes (as wrap sunglasses do) would result in very poor optics. Today, free-form design and production techniques en- able lenses to have steep base curves but retain the visual clarity that Von Rohr and Tscherning in- tended by correcting for errors ob- tained by using the "wrong" VisionCareTechnology WE REINVENT SIMPLICITY FOR YOU The All-In-One sysTem wOrkIng wITh mOre c O mfOrT, speed, prec I s IO n A nd vers ATIl IT y. Learn More Today ! 855-393-4647 www.essilorinstrumentsusa.com info@essilorinstrumentsusa.com A L L W R A P P E D U P A L L W R A P P E D U P A L L W R A P P E By Ed De Gennaro, MEd, ABOM Wrap lenses with steep base curves upset the apple cart of lens design. e fundamental spher- ical base curve design concepts we use today originally came from research in the late 1800s and early 1900s by people such as Moritz Von Rohr and Mar- ius Tscherning. But today, if it weren't for free-form lens design and surfacing techniques in con- cert with advanced edging meth- ods, wrap lenses with good optics wouldn't be possible for most prescriptions. Von Rohr's work focused on improving visual clarity by re- ducing lens aberrations, most notably marginal astigmatism. is aberration causes objects to become increasingly blurred the farther a wearer looks from the lens's optical center. Tscherning taught us that there is only one ideal base curve for every lens power and using the wrong one causes reduced clarity through the lens. He plotted the ideal base curve for each power on a graph (known as Tscherning's Ellipse), which guides spherical base curve lens designers to this day. In simple terms, Tscherning tells us that plano lenses need a +6.00D base curve while plus lenses need a base curve steeper TECHNOLOGY SOMETIMES NEEDS TO CATCH UP WITH STYLE, AS WAS THE CASE WHEN WRAP EYEWEAR HIT THE MARKET, BUT NOW LENSES CAN HANDLE THE CURVES. Photo courtesy of Coburn Photo courtesy of Briot

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