Beyond Good, Better, & Best

Variable corridor PALs

By Karlen McLean, ABOC, NCLC

As technology improves, a wider array of eyewear can be fitted with Rx lenses. Image courtesy of Shamir

Fabricating conventional PALs takes several steps: software design, direct mold cutting, semi-finished casting, then lens cutting. Creating free-form PALs is streamlined and simply requires software design and lens cutting.

When lenses are made straight from the software design, errors are eliminated allowing for more precise Rx's within 1/100th of a diopter. Free-form design also eliminates compromises that are inherent in semi-finished lenses created from a mold by making lenses individually with no molds required.

Software is the driver for these lenses, and proprietary designs offer various benefits inherent to the designer's goals. That's why it's important to work closely with manufacturers and labs to deeply understand the free-form technologies you choose to work with and have all the lens information and tools updated and at your fingertips to use at all times.

• LEVEL ONE. The first level of free-form production produces designs that optimize sphere, cylinder, axis, and add powers. This helps eliminate power errors and compromises in the Rx and can increase fields of clear vision by up to 50 percent.
• LEVEL TWO. The second level takes into account frame shape and height. This is where variable corridors make impact by automatically adjusting to continuously deliver the optimal corridor length for virtually any frame. This can significantly increase fields of view.
• LEVEL THREE. The third level of optimization takes face shape and the position of wear into consideration, which minimizes power distortion such as unwanted cylinder while maximizing accurate real-life optics.
  All these factors combined deliver PALs with the least possible distortion and unwanted aberration for the most precise optics possible today.
  With three levels of PAL optimization and price points, ECPs can enter the customized PAL arena at a level that's comfortable for the practice, staff, and patients. Rank products by the design and technology and recommend to patients based on price, features, and benefits.

Good PALs deliver less distortion and aberration and more precise optics. Image courtesy of Signet Armorlite

VARIABLE CORRIDOR

Many dispensers have a go-to standard corridor and short corridor PAL, selecting frame and lens designs based on what corridor lengths are appropriate.

What if one PAL design did the duty of both, delivering a corridor that self-adjusts to virtually any Rx, frame, or face shape?

That's what variable corridor PALs can do. A continuously variable corridor ensures reading vision with the best possible intermediate for a specific frame.

In any scenario, shortening the corridor entails some compromise, but proprietary free-form-designed PALs help keep those compromises to the absolute minimum while maximizing optics by fine-tuning them to frame shape. Benefits include wider intermediate, usable reading area, overall clearer vision for the wearer, and easier dispensing protocols for ECPs.

• FIXED. Variable corridor (fixed) PALs automatically adjust corridor lengths from a corridor height selection to meet various ranges of fitting heights.
• FLEXIBLE. Continuously variable (flexible) corridors automatically adjust corridor length for physiological comfort. Most recommended fitting heights in this category range from 13mm to 35mm.
  For example, one manufacturer's flagship PAL has a corridor that's continuously variable from 10 to 16mm in 1/10th mm increments. Their research shows that 16mm is the maximum corridor length to maintain physiological comfort.
  Visual performance is maximized for every frame. In short, variable corridor lengths are not too short or too long, and add power builds at a usable, comfortable rate. This ensures the reading area is not too shallow, the intermediate area is precisely placed and the distance area is not too high, because computerized software automatically adjusts them.

Precise measurements are critical in dispensing the highest quality variable corridor PALs. Image provided by Vision-Ease Lens

MEASUREMENTS

In all cases, a monocular pupillary distance using a pupilometer and fitting heights must be taken. Frame measurements are necessary to allow software to calculate the optimum positioning of the corridor and other design attributes.

How patients will wear their glasses should drive lens choice. Image by Ophthonix

Vertex distance, pantoscopic tilt, and panoramic angle measurements are also necessary with many proprietary premium designs. If complete measurements aren't supplied default measurements using averaged parameters are used.

Software adjusts the corridor power profile to better serve the needs of the wearer for a specified distance. But accurate measurements must be taken—and patient eye usage notes made—to ensure the best customized lens results for each patient.

For example, if a dispenser notes that the patient works as a proofreader and holds copy closely from 8cm to 20cm during the majority of her workday, the dispenser should write this on the order and the power profile is adapted to that working near distance. In this case, standard measurements are taken, as well as a distance measurement from the patient's eyes to their work area.

Because these lenses are enhanced for individualized use, two Rx's are typically issued by the fabricating lab: the original doctor-prescribed Rx and the as-worn Rx, which notes the compensations made for real-life wear. Dispensers should verify the lenses using the as-worn Rx.

The most important management strategy, in addition to the correct Rx and measurements, is managing lifestyle questions. Knowing how patients use their eyes in every situation is crucial to the success of the lens customization process.

The now and near future of customized PALs includes portfolios of free-form produced lenses, typically in low, mid and high price points.

In addition to three levels of customized PALs, there are three levels of customized single vision lenses, with free-form produced single vision lenses optimized in the same way PALs are: Rx, frame, and position of wear.

Today, there is a plethora of beyond-basic brand name and private label free-form produced PALs, including lenses by lifestyle. There are also more dispensing instruments and tools to achieve precise measurements on free-form products as well. EB