lens tips

The Possibilities of Wraps

by Karlen McLean, ABOC, NCLC

Prescription wrap sunwear has blossomed into a healthy market segment with full frame and lens packages offered by a number of manufacturers. But what about wrap eyewear without the umbrella of a branded company, frame, and lens system?

Are there guidelines for wraps? In a word—yes! Here we provide key wrap pointers.

THE POSSIBILITIES

Wraps are typically 12 to 18 degrees of facial wrap (face form). Beyond the 18-degree mark, viewed images can be displaced.

While it is possible to go beyond 18 degrees, you should always consult with the laboratory first and check pricing, as going outside the 12- to 18-degree range can quickly ramp up costs.

Vertex distance, the distance between the back of the lens and the front of the eye, can be measured using a PD ruler or manufacturer-issued tool. Photo courtesy of Shamir Insight

Also, it's important to consider the following factors.

■ Rx RANGE: The average Rx range for wraps is around +2.00D to -4.00, with 2.00D cylinder maximum, because in this range the final product offers the best visual acuity and wearing comfort.

High-minus prescriptions can have too-thick edges in large-wrap sizes. In addition, steep back curves can result with high-minus Rx's, making surfacing difficult.

Prism and magnification are more pronounced in high corrections with wraps. Even plano lenses have induced prism when wrapped; quality plano sunlenses feature prism-corrected (decentered) lenses. Always refer to the manufacturer's recommended power chart before suggesting wraps to patients.

■ PANTOSCOPIC ANGLE: Most frames come with eight degrees of pantoscopic angle. A “panto average” is from eight to 12 degrees.

A wrap angle protractor/gauge is a tool that can help you read the angle of nasal-temporal edges. These are available from certain lens or tool manufacturers. If necessary, a standard protractor can work in a pinch.

■ FITTING ANGLE: The fitting angle on most wraps falls somewhere between 13 and 20 degrees, which causes the optical axis to rotate. If no frame-wrap angle is specified, a default angle will be used—typically around five degrees. Some companies supply a frame-wrap tool to help accurately measure wrap angle.

■ VERTEX DISTANCE: This can be measured by using a PD ruler, or ideally by the use of a manufacturer's specialized vertex measurement tool. Vertex distance can be found by measuring the distance from the back of the lens to the front of the eye. If vertex distance isn't specified in the order, a default or average measurement—one is 12mm—is used.

Wrap sunwear is 12 to 18 degrees of facial wrap (face form). Images viewed beyond the 18-degree mark can be displaced. Photo courtesy of Shamir Insight

■ FRAMES: It's important to know which wrap frames in your dispensary will work well with Rx's and which won't, so that you can make the right recommendations to your patients.

Before purchasing a wrap frame or wrap-frame line from any manufacturer, make certain that each frame is Rx-able; or if not, be sure to earmark, separate, and show the plano wrap frames separately to avoid confusion. This is the easiest dispensing protocol. Always pre-adjust the frame before taking measurements.

■ LENS MATERIALS: While these lenses are available in a variety of lens materials, including standard plastic (CR39), most wrap styles are worn for outdoor activity/sports use.

In that case, high-impact-resistant lenses are the best recommendation. It's best not to use polycarbonate material for dark tints unless polarized polycarbonate is ordered, as poly can be difficult to tint uniformly in dark hues. EB