HANDS ON - Fix and Fit
Curve Control
by Alex Yoho, ABOM

Optical scientists have given us some outstanding new lens options over the last several years. Using aspheric and atoric curves, they've made lenses markedly thinner and flatter, while at the same time reducing lens aberrations significantly.

When it comes to wrapped frames which require more curvature, additional attention to detail is required by lens designers and dispensers.

Versus style VR 6007 for Luxottica

Peripheral blur. The sample lenses in an average wrapped frame are generally a +9.00 diopter base curve, while the average ophthalmic lens has a +6.00 base curve. When a prescription lens is curved more than the "corrected curve" calculated by the lens designers, light no longer travels properly through the lens and into the eye, resulting in visual discomfort. This is known as "off axis," and it translates to peripheral blur.

Aspheric issues. Though there have been some inroads made to aspherically correct highly curved lenses for optical distortions using freeform lens manufacturing, this technology is still in its infancy for ECPs and patients.

Tilt trauma. Not only is the curvature of the lenses a problem from an optical standpoint, but the tilt of the lenses—which can be close to 30 degrees of wrap—actually changes the effective prescription that the lenses produce.

Unwanted astigmatism and prism are the culprits here. Lens prescriptions can be compensated for this lens tilt using Martin's formula (see Quick Tips), but this correction is most often ignored in the real world.

We should look at real world circumstances; when the focus is on the cosmetics of the eyewear, special materials and edging are required.

THE SOLUTIONS

Follow the curves. Avoid forcing the lens to accept the frame curve by heating the lenses. Use a steeper base curve, even if it is the wrong base curve. While highly motivated patients can get used to anything, typical patient complaints will be that their peripheral vision is not clear.

Edge wisely. The traditional "1/3, 2/3" bevel doesn't stay in wrapped frames well, so ophthalmic lenses require special edging considerations.

Use an edger capable of a specially profiled bevel, or hand-edge the bevel after edging. The beveling on most wrapped frames has virtually no bevel on the front and a sharp bevel on the back surface.

Mounting issues. Using Trivex or polycarbonate lenses to prevent lenses from chipping can help. Be aware that some wrapped frames have painted finishes and can be prone to scratching.

Mount lenses without heat by inserting the entire top of the lens into the frame from the front and finally tucking the bottom nasal in last. The Rx power range for wrapped frames using normal ophthalmic lenses is +4.00 to -2.00.

Outside help. Some sunwear companies and specialty lens manufacturers offer complete eyewear packages and lens options that address the high-wrap and base curve issue. The Rx is optically compensated and optimized for the individual prescription and frame used.

The final product will not have the doctor's original prescription but rather a compensated one that will have the same effect as the original Rx. This is sometimes referred to as an "as-worn prescription" and is addressed in ANSI standards.

The bottom line: Take the time to learn about this technology in-depth. Educate your patients and ask lifestyle questions. Then design your best frame/lens solution.

 Comments or questions can be sent to the author at ayoho@cox.net.