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.
Wrap lenses with steep base curves upset the apple cart of lens design. The fundamental spherical 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 Marius Tscherning. But today, if it weren’t for free-form lens design and surfacing techniques in concert with advanced edging methods, wrap lenses with good optics wouldn’t be possible for most prescriptions.
Von Rohr’s work focused on improving visual clarity by reducing lens aberrations, most notably marginal astigmatism. This 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 than +6.00D and minus lenses need a base curve lesser than +6.00D. That’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 enable lenses to have steep base curves but retain the visual clarity that Von Rohr and Tscherning intended by correcting for errors obtained by using the “wrong” base curve—a steep one instead of a flatter one.
Nearly every Rx lens caster offers a wrap lens solution. Some clearly market this option as a brand. For example, Shamir’s Attitude III is available in a wrap sunwear version. Other companies include a steep lens option inside selected brands. For example, Younger Optics offers the Image WRAP NuPolar polarized polycarbonate progressive design with an effective blank size of 83mm, and Carl Zeiss Vision labels its newest wrap lenses lenses “Sport” within selected lens brands.
Upper end sunwear companies such as Bollé, Oakley, Wiley X, Rudy Project, Smith Optics, Costa, SPY Optics and others have all learned the value of premium wrap lenses that have been compensated for their steep base, tilt, prism and other parameters. Their lenses have evolved into a highly complex array of films and treatments combined with compensated optics to deliver features that enhance the wearer’s experiences, including scratch resistance, anti-reflective, mirrors, anti-fog, hydrophobic, oleophobic, polarization, photochromic properties, UV, blue light and more.
While higher prescriptions on a steeper base curve call for a lens thicker than what is normally available, both lens casters and surfacing labs have found solutions for this. For example, FEA Industries uses a special process of blending lenticularization into the lens. Carl Zeiss Vision has developed a software program that uses free-form lens calculations to make the lens aspheric and reduce its thickness. The benefit of this technique is that the lens comes out of the generator already thinned and with optimal optics; no other treatment is needed. All ZEISS Sport lenses use this feature.
One of the difficulties of producing wrap eyewear is that it’s challenging to keep a steep base curve lens in a frame. Most lenses use their rim to encircle the lens’s bevel, which on a conventional edger would be cut at the wrong angle because of the grinding wheel’s bevel shape and location.
Equipment manufacturers such as Briot (Luneau Technology USA), Coburn Technologies, Essilor Instruments, Santinelli International, MEI, Optek and others offer edger systems that can adjust a lens’s bevel for high base curve frames and create dramatic and custom edge bevels and configurations.
The wrap lens market is fairly small, but its technology has become very advanced. Considering the low percentage of Rx sunwear that’s sold to patients each year, it’s clear this category has plenty of room to grow.
Ed De Gennaro is editor emeritus of First Vision Media Group.
WHERE TO FIND IT: Bollé 800.222.6553 • Bolle.com // Briot USA 800.292.7468 • BriotUSA.com • Contact@BriotUSA.com // Carl Zeiss Vision, Inc. 800.358.8258 • ZEISS.com/Lenses // Coburn Technologies, Inc. 800.262.8761 • CoburnTechnologies.com • CustomerCareCenter@CoburnTechnologies.com // Costa 800.447.3700 • CostaDelMar.com • Sales@CostaDelMar.com // Essilor Instruments USA 855.393.4647 EssilorInstrumentsUSA.com • Info@EssilorInstrumentsUSA.com // FEA Industries, Inc. 800.327.2002 • FEAInd.com • Sales@FEAInd.com // MEI S.r.l. 847.357.0323 • MEISystem.com • Info@MEISystem.com // Oakley, Inc. 800.733.6255 • Oakley.com // Rudy Project North America 888.860.7597 • RudyProjectUSA.com • SRiding@GoRace.com // Santinelli International, Inc. 800.644.3343 • Santinelli.com • Sales@Santinelli.com // Shamir Insight, Inc. 877.514.8330 • ShamirLens.com • Info@ShamirLens.com // SMITH 888.206.2995 • SmithOptics.com // SPY 800.779.3937 • SpyOptic.com // Wiley X, Inc. 800.776.7842 • WileyX.com • Info@WileyX.com // Younger Optics 800.366.5367 • YoungerOptics.com • Marketing@YoungerOptics.com