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.
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
SANTINELLI INTERNATIONAL: LEX-1200
ESSILOR INSTRUMENTS: MR. BLUE SUN & SPORT EDGING SYSTEM
COBURN TECHNOLOGIES: EXXPERT HPE-8000X