HANDLING THE CURVES

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Putting Rx lenses into a wrap frame poses optical issues that must be overcome. Here’s how ECPs can address this steep-base dilemma.

Two issues must be considered when producing lenses for wrap 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. This means that an Rx requiring a 4.00D-base curve must be ground on an 8.00D-base curve lens to accommodate the wrap frame, resulting in optical aberrations such as peripheral distortion.

Another problem concerns the wrap angle of the frame, which generally ranges from 12° to 23°. This angle rotates the optical axis of the lens toward the temporal area of the lens, causing power errors and unwanted prism, so 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.

Based on how the frame sits on the patient’s face, the calculations will adjust the back surface of the lens to ensure that light is bent correctly to enter the eye at the correct angle. This is especially important when calculating the same prescription on a flat lens versus a curved one, as the angle where light enters the lens will be drastically different.

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, causing 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 center of the lens. This is something the lab needs to address to ensure a quality lens is produced.

High-Base Edging
Another difficulty of producing wrap eyewear is the challenge to keep a steep base curve lens in a frame. Most lenses use their rim to encircle the bevel of the lens, which on a conventional edger would be cut at the wrong angle because of the grinding wheel’s bevel shape and location.However, cutting wrap lenses in-house allows for your lab to shine and your office to be known as the go-to place for edging quality. Here are some edgers to consider in the high-base wrap-edging market:

NATIONAL OPTRONICS: 7EX AND QM-X3
The 7Ex multi-cutter offers several high wrap-style blades. While it is a three-axis edger, changing the bevel angle and profile can simulate the characteristics of a four-axis edger at a substantially lower price, according to the company. The QM-X3 is also a three-axis edger but incorporates a separate cutting unit to provide a fourth axis for cutting specialty shapes.
800.800.1550     NationalOptronics.com     cs.na@dactechnologies.com

SANTINELLI INTERNATIONAL: LEX-1200
This edger contains a high-curve tracer and a mechanical, multi-axis stylus for a smooth and accurate trace that can match “virtually any frame curve,” according to Santinelli. The LEX-1200 also offers a tailored mini-bevel and multiple choices for high curves and asymmetrical lens bevel profiles plus a semi-step bevel for insertion of lenses into non-Rxable eyewear.
800.644.3343     Santinelli.com     Sales@Santinelli.com

ESSILOR INSTRUMENTS: MR. BLUE SUN & SPORT EDGING SYSTEM
Mr. Blue Sun & Sport also enables sport wrap frame coverage for endurance and extreme sports with the partial step bevel in addition to full high curve, asymmetric bevel and step bevel. The Sun & Sport edition and its special features are available for every Mr. Blue Sun & Sport Edition edger order associated with a new or already owned Mr. Blue Tracer or Mr. Orange Tracer.
855.393.4647     EssilorInstrumentsUSA.com     Info@EssilorInstrumentsUSA.com

COBURN TECHNOLOGIES: EXXPERT HPE-8000X
With 11 different lens edge finishes and step-bevel cutting, Coburn’s EXXPERT HPE-8000X edger creates edges to precisely fit into high-wrap frames, even with special shape cuttings such as ventilation holes. The HPE-8000X also features “axial roughing,” which prevents the lens from rotating on the wheel until the thickest part of the lens is removed, an adaptive swivel chuck to reduce slippage, plus six edging positions, frame shape modification and a high-performance wheel.
800.262.8761     CoburnTechnologies.com     CustomerCareCenter@CoburnTechnologies.com

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