L&T: Lens Choices

Jan
2008

Composing the Perfect Wrapsody

Because of the technical demands of fabricating an Rx wrap, dispensers must take special care when ordering sun lenses. That means knowing the parameters of the product and providing precise information to the wholesale lab so the sunglass can be customized.

To help you compose the perfect “wrapsody” for your patients, L&T has collected some of the most valuable technical advice we have published on this timely topic.

—Andrew Karp

Compensating for Optical Problems
The goal of wraparound sun lens processing is to compensate for the optical problems caused by the steeper base curve and wrap angle. In order to minimize unwanted prism and power error in the patient’s straight-ahead vision and reduce peripheral distortions, optical labs must take into account prescription power, the patient’s monocular PD and the measurements of the frame when selecting and processing the wraparound sun lens.

The optical effects of steeper base curves and wrap lenses can be minimized if the dispenser conveys exact prescription and frame information to the optical lab, then has an in-depth discussion with the optical lab about lens options for individual patients. Dispensers have the responsibility to advise patients their eyes may not accommodate to this minimal peripheral blur immediately.

Base Curve Selection
Hyperopes have fewer issues than myopes because plus powers typically use front base curves of six and higher, so the base curve switch is not as severe. Most dispensers are hesitant to fit moderate-to-high hyperopes with wrap sun lenses—edges can be too thin to mount—but Rxs of +2.00D and below have few problems. The point to remember is to order a spherical, not aspherical product.

For myopes however, the base curve change is dramatic. But in general, base curve is determined by a prescription. In the case of wraps, base curve is determined by the cosmetics of the frame.

There is a range of prescriptions optimum for each base curve. The philosophy is different with wrap sunwear because you are ordering a base curve that takes into account the curvature of the frame. The idea is to try and get a lens curve that is close to the one normally used for that prescription.

Lens manufacturers offer front base curves in 1 to 2 diopter steps. The higher the value, the steeper the curve. Manufacturers differ in how they specify their base curves. Dispensers should review the seven to nine base availability of the lenses they prefer for the patient. In general, curves within 0.5 diopters of the wraparound frame curve will work fine.

Most wrap sunwear requires an eight base curve. But there are six base curve wraps and even some 10 base frames. These are typically plano although more ambitious dispensers and labs have had success with mounting prescriptions in these “severe wrap” frames. Use a lens clock to measure the front base curve of the frames’ demo lens and, with your lab, find a lens that will best match that curve and still be able to contain the patient’s prescription.

The Rx Factor
Due to the need for steeper curves, there are general prescription limitations for wrap sunwear: +2.00D to a -4.00D sphere and cylinder powers up to -2.00D. But these are more rules of thumb than written in stone. Other factors, such as eye size, frame design and patient PD play significant roles in determining if a wrap lens can be processed. It requires discussing the prescription—and frame selection—with the lab.
In lens processing, the lab grinds a back curve on the lens and the prescription is essentially a combination of the front and back curves. Some eight base blanks will not be thick enough for higher minus prescriptions or the lens edges will be too thick for mounting.

Even powers on the high end or just outside recommended prescription range can be used for wraparounds, as long as such things as patient PD, frame measurements and the direction of lens power is taken into consideration. If the cylinder power is too high in the 90 degree meridian, consider using a lower curve than an eight base.

Lens Selection
Sometimes it’s necessary to choose a different type of lens. For example, polarized lenses, which are typically in an eight base curve for wraps, may not be the best lens choice for patients with prescriptions above a -3.00D with a cylinder power of -2.00D. For these patients, try going to polycarbonate or high-index material, generally 1.60.

There are some high-index manufacturers that produce a 7.25 base curve, which will fit nicely in most wraps and still accommodate the Rx. The thinner material will mean less steep curves, edges are thinner and the lens won’t pop out of the frame.

The Prism Effect
Choosing a base curve to match the frame curve is only part one of the ordering process. In wrap styles, the optical axis of the lens rotates more temporally than in other frame styles because of the frame’s wrap angle. This causes unwanted prism and shifts decentration. In a wrap design, the eye is looking through the lens differently than in other lenses and prescriptions have to be adapted to these changes. For most myopic prescriptions, a compensating prism—base-in prism—must be ground into the lens to enable the patient to see through his or her wraparound sunwear just like they do through flatter more typical styles.

Base-in prism, which is ordered in 1/4 diopter steps, reduces the prismatic effect that comes from rotating lenses wrap sunwear. Prism is added so the optical axis of the lens matches the straight ahead visual axis of the eye. The rule of thumb is, for powers less than 2.50 diopters, use a 0.25 prism diopter base in, for powers 2.50 and greater order 0.50 prism diopters base in.

Check the PD
In addition to the prescription power, the patient’s PD is a factor in determining wraparound sunwear. In general, a 58 PD or narrower can pose problems, but those problems can be mitigated by prescription power and/or wraparound frame dimensions.

A wider PD won’t be as much of a problem with a lower prescription. Opticians should suggest a smaller eye size, which can also compensate for the narrow PD as well as the Rx. Too narrow a PD and a large frame may cause problems such as too thick edges, not enough lens diameter or not enough blank thickness. Too large a PD and a large frame could mean not enough blank diameter or thickness, and the inside bevel of the frame may be unable to secure the lens. Certain edge thicknesses in minus Rxs may not allow the temples to close.

 

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