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Getting the Angle on Aspheric and Atoric Lenses
New designs in these premium lenses have resulted in better performance and comfort in SVs and PALs
By Karlen McLean, ABOC, NCLC

Aspheric and atoric lenses are available in many materials. Shown: Younger Optics' Trilogy

From their beginning as post-cataract lenses, aspheric lens designs have improved overall lens performance and comfort, and given ECPs premium plus, minus, or PAL lens designs.

While aspheric and atoric designs can be created in any lens material, manufacturers tend to produce their new releases in these forms. They are a large part of premium lens lines in mid-index, high-index, polycarbonate, and Trivex materials.

Aspheric and atoric lenses are also the basis of many single-vision and progressive addition lenses. They are the building blocks of freeform-designed PALs and are thin and lightweight; plus, they offer improved optics.

SINGLE VISION

Corrected curve or best form lens designs change base curves for optimal vision with less distortion and the best cosmetics. Due to freeform generating, it's now possible to produce single-vision aspheric and atoric lenses. The range of powers is extensive and growing.

Layout charts ensure lenses will cut out

Typically, aspheric lenses are flatter. Front curves of plus aspheric lenses flatten toward the edge of the lens and the back surface is brought closer to lens' front with these results:

►Magnification is reduced for a more natural eye appearance.

►Decreased center thickness or bulging means less material is used, so the lenses are lightweight.

►Aspherized high-plus lenses minimize off-center errors, so patients have better clarity when the eye moves away from center.

►While flattening is less obvious with minus powers, asphericity does help reduce edge thickness. The front curves of a minus aspheric lens get steeper toward the lens edge, which thins it, reduces weight, and offers a more natural appearance.

Select frames that center the patient's eyes

►Occasionally, because aspheric lenses are flatter, they may cause a patient's eyelashes to brush against the backside lens surface. A concave curve on the backside helps position the lens further from the eye and provides a wider field of vision.

When patients change from traditional lenses to aspheric/atoric lenses, they may notice some changes:

►High-plus lens wearers may feel like their surroundings are smaller.

►High-minus lens wearers may observe objects as smaller and farther away. These patients are getting their vision back to a more normal size level, since asphericity

reduces viewed magnification. Encourage patients to make a full conversion to aspheric/atoric for visual comfort.

Also, supply a complete pair of semi-finished or finished lenses, since semi-finished lenses may have a steeper front curve and finished lenses may use flatter curves in plus powers.

PROGRESSIVE PROGRESS

Many premium PALs use aspheric/ atoric design on the front, back, or front and backside of lenses, allowing better visual acuity in far, intermediate, and near zones. They include the following traits:

►Aspherized PALs utilize aspheric curves on the frontside. The aspheric curve is averaged for a wide range of curves that could be used on the back surface. With a cylinder Rx, one meridian might be improved while the other may be less so. Backside aspheric curves move asphericity to the back of a lens, where each meridian of the cylinder is totally aspherized.

►Atoric designs are the basis for many personalized/ freeform PALs, since each meridian of the back surface of a lens is aspherized to match the front base curve.

►Glass molds cast PAL curves that change the add power in the progressive channel. Rather than using a traditional diamond wheel, a freeform generator cuts complex curves using a precise diamond tool that cuts curves with a single point, eliminating elliptical error. A single-point generator can reproduce virtually any complex concave surface.

Full change to atoric/aspheric aids adaptation

►Better PAL vision is possible due to software, or point files, which guide computer-run freeform generators. Each point file is programmed to the patient's prescription and specialized measurements.

Freeform equipment has the capability to produce lenses with accuracy to .05D, while traditional equipment is accurate to +/- 0.12D. As a result, aspheric and atoric lenses with their complicated curves can now be produced and customized with ease.