There is high demand for sunwear in wrap form; it is estimated that as many as 25% of the prescription market is dispensed in highly curved lenses. Besides looking cool, wrap sunwear also provides better wind, dust, and sun protection than other lenses since they fit close and reduce light leak. However, the lens position and shape creates a problem if not considered. So how do you offer your patients this same product when you know you are probably limiting the Rx range or do you just say “no?”
First, the base curve of the frame requires a lens often steeper than the prescription would usually use. Wrap sunwear lenses are usually 8 or 9 diopters of curvature and this curve is incorrect for most prescriptions and will reduce the clear field of view. Second, lenses are tilted around the vertical axis; some wrap frames tilt lenses more than 18-20 degrees. For example, an Oakley Eye Jacket has about 23 degrees of tilt. Prescriptions change when tilted and this creates prism and a power change.
The issues that bother patients with wrap prescription eyewear in non-compensated eyewear are horizontal prism that affects binocularity, wrong effective power centrally and poorer vision peripherally than their standard glasses. The results are typically rejected lenses and refunds.
Make the glasses wearable. Order the lenses with a compensating prism; after all, the most important goal of the prescription is to provide binocularity. Then, improve on center vision by compensating the prescription. In low Rx’s this has little effect but prism is always a problem. Lastly, use lenses that are compensated peripherally for the extreme wrap angles.
The How-To When Ordering from Your Lab
1. Order a lens that is the same curve as the Plano lens being replaced. This is usually an 8 base lens. However, I have found that a lens one base curve flatter will work and the effects on peripheral vision are improved. So, if an 8 base lens is needed, order a 6 or 7 base.
2. Compensate the prism (for powers -6 to +4 at an average wrap angle of 18-20 degrees). This neutralizes the base out prismatic effect of horizontal lens tilt so the result is no prism in the “as worn” position. If this is done most of the problems are resolved. The rule of thumb is:
For powers <±2.50D, order with 0.25ΔBI in each eye
For powers >± 2.50D, order with 0.50ΔBI in each eye
3. A better solution – compensate the prism and the on center power for the as worn position. There is a terrific tool available from Darryl Meister of Carl Zeiss Vision on his website. This clever program allows the input of the prescription, frame and fitting requirements. It displays the compensated prescription to order so that the patient “sees” the prescription written by the doctor. Using these tools allows virtually any prescription to be made in a wrap frame. Download the Rx Compensator* at www.opticampus.com/tools/tilt.php.
Since frame wrap angles need to be measured, you can use 20 degrees as an average.
Wrap frames typically fit closer than standard frames so in high minus Rx’s compensating for vertex may be important, especially in progressives since the effective add power will be weak if not compensated.