|Smith’s Frontman with Blue Mirror ChromaPop are Rxable in a 8.00D-base curve.|
|There’s a need to find a lens with a sufficient front curve at a given thickness, such as this 6.00D-base 1.67 lens in polarized brown from FEA Industries.|
|BETTER RESULTS Correct frame measurements are paramount. Face form (wrap), vertex distance, and pantoscopic tilt are vital. Most lens manufacturers have simple charts and tools to assist ECPs in taking these. Others have ones that are more complex. Most opticians can be quite accurate measuring wrap angle with a simple protractor. Some lens manufacturers have specialized lenses for wrap frames that use default measurements if you don’t supply position-of-wear measurements. Individualized measurements will produce better results. Inexperienced opticians should ask their labs for assistance.|
|IOT offers a measuring device for wrap frames.|
|Putting an Rx lens, like Seiko’s, into a wrap sunwear frame requires precise measurements.|
|PRECISE CENTER Before free-form surfacing, lenses were generated with either a spherical or toric curve backside curve. This does a good job of delivering the Rx at the optical center, but becomes less accurate across the breadth of the lens. Free-form design and processing changed this. The placement of the optical center is precise, as well as the powers as they extend outward because they are compensated. The entire lens surface represents a point-map focus.|
Wrap sun lens processing requires compensation to correct optical problems.
Wraparound sunglasses became a huge fashion statement in the late ’80s. Their popularity continues to grow as many Rx eyeglass wearers desire that look as well. While not all these designs were originally made for fashion purposes (e.g., various sports and athletic safety sunglasses), a growing number of companies now design wrap sunglasses for both sports enthusiasts and fashion-conscious consumers looking for eye protection, quality, fashion, and fit. Putting Rx lenses into a wrap sunwear frame poses some optical issues that must be overcome, otherwise the wearer will have lenses that deliver the wrong power.
Two simultaneous issues must be considered when producing wraparound eyewear: compensating for both the frame’s curvature and its wrap angle. Normally, the front base curve of the lens is dictated by the patient’s Rx. With wrap sunwear, however, the front base curve of the lens must conform to the frame’s curvature, which typically has a nominal front curve of 8.25D. What this means is that an Rx requiring a 4.00D-base curve must be ground on an 8.00D-base curve lens to accommodate the wrap frame. This will result in optical aberrations like peripheral distortion.
Another problem concerns the wrap angle of the frame, which generally ranges from 12Ëš to 23Ëš. This angle rotates the optical axis of the lens toward the temporal area of the lens, resulting in power errors and unwanted prism. This means that wrap sun lens processing requires compensation to correct the optical problems caused by using lenses with steeper base curves and frames with high wrap angles.
Digitally processed free-form lenses provide the optimal solution for both aberrations associated with sun wrap lenses. Labs that specialize in fabricating wrap sunwear deal with the special challenges of this product every day. Greg Ruden, president of Expert Optics, Inc., suggests the key to working with wrap frames is to design decentration and compensation for position-of-wear measurements. This solution assumes you choose a design that can be decentered and that the lens is compensated to minimize the effects of induced prism and oblique aberration.
Bill Heffner, marketing & IT director at FEA Industries, Inc., agrees. He feels that free-form lenses are better because they take the wrap angle into account when creating the lens. Traditionally surfaced lenses have not really been able to do that, unless the calculations were done by hand to change the prescription. Even if you did this, however, it would only account for the vision when the patient looks straight ahead, and not really do anything for optimizing peripheral vision. Consequently, free-form lenses are the optimal design for wraparound sunwear.
Jason Kvam, supervisor of technical services at Seiko Optical Products of America, Inc., advises that conventional high-base wrap eyewear designs create asymmetry between the nasal and temporal sides of the lens. This results in power error and astigmatism across the entire lens surface, which causes uncomfortable, distorted, and over- or undercorrected vision for both eyes. Prism error is also created due to the angle between the line of vision and the lens optical axis, resulting in eye fatigue and visual discomfort.
All the experts I spoke with agree that the more information you provide the lab when ordering wrap sun lenses (in terms of frame measurements), the more accurate they will be. Based on how the frame sits on the patient’s face, the calculations will adjust the lens’ back surface to ensure that light is bent correctly so that it enters the eye at the correct angle. This is especially important when calculating the same prescription on a flat lens vs. a curved one, as the angle where light enters the lens will be drastically different.
Ruden suggests not all free-form processed lenses reduce or eliminate aberrations. Those that do use a series of complex calculations to minimize the impact of the obliquity in high-wrap frames. Heffner, however, offers that all prescriptions will benefit, but generally the effects become more noticeable above +/- 2.00D. This is simply because before that there’s not enough power in the lens to cause noticeable deviations. Generally speaking, the higher the prescription, the larger the amount of unwanted power, as the eye moves from the optical center. This makes it noticeable more quickly in higher powered lenses. The frame type will also affect this, as a frame with even a relatively low Rx but a high-wrap angle will receive benefits from compensation.
According to Heffner, there are three things that limit the possibilities for sunwear: lens availability, machinery, and frames. The technology for designing a free-form lens is just math. The problem usually arises when trying to make this calculated lens a reality.
In terms of lens availability, there’s still a need to find a lens with a sufficient front curve at a given thickness. For higher prescriptions, especially in large wrap frames, it can often be an issue where the lens blanks are not thick enough to create a lens large enough to fit.
While free-form allows lenses to be decentered to help with cut-out, it’s still possible to have issues with lens blank size. This is especially obvious when the patient is in a large frame with a very narrow PD. This results in a lot of prism needed to be ground into the lens, which can often result in surface defects of the lens.
Free-form machinery can also have problems when trying to polish higher-curve lenses. Since free-form processing uses a soft, spongy polishing tool, it may not ideally fit well into high curves. Since the curves can be so steep, often an air pocket will form and cause problems polishing the lens’ center. This is something the lab needs to address to ensure a quality lens is produced.
While the future holds solutions for these issues, free-form processing cannot completely eliminate base curve and thickness issues found with wrap sunwear. However, the advantages far outweigh the disadvantages.
Steven Warfield is a lab technician and freelance writer based in Harrisburg, PA.
WHERE TO FIND IT:
Expert Optics Inc.
800-892-0097 • expertoptics.net
FEA Industries, Inc.
800-327-2002 • feaind.com
Seiko Optical Products of America, Inc.
800-235-5367 • seikoeyewear.com