NOV 2015

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Your monthly guide to staff training outside the box

Eyes / Lenses / Fitting Lenses / Free-Form / Frames / Sunwear / Patient Solutions / In-office / Standards

LENSES: Customization


Although frames and faces may come in a wide variety of styles and sizes, engineers working with traditional single vision and progressive designs had little choice in deciding what distance, tilt and wrap angle values a frame would place the lenses. Therefore, they settled on averaged values calculated from vast amounts of real-world fitting data. Examining this data, they found that on average, most lenses would be positioned approximately 13 mm in front of the eyes for vertex distance, 10 degrees for Pantoscopic lens tilt and 5 degrees for frame wrap angle, otherwise called position of wear, or POW.

For years, engineers employed these “default” values in their calculations. But with the arrival of free-form processing, manufacturers recognized that significantly higher levels of optimization would be possible if individual position of wear values were included in their calculations.

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To begin the choice of a corridor length, start with your fitting height, and subtract 4 mm to 5 mm, which allocates enough height for a minimum useful reading area. The difference left is a good approximate number to begin determining your target corridor length.

Here’s an example: For a fitting height of 18 mm, corridor length roughly calculates as follows: 18 mm minus 4 mm (minimum reading area height) = a 14 mm Corridor Length Value. Tip: Your calculated corridor length value may not always be obtainable in a specific progressive design. Therefore, always keep the minimum useful reading area in mind as you select the next closest corridor available.

In the above example, if the design selected only provides a choice between an 8 mm or 10 mm corridor, the final value chosen will depend upon Total Frame Height and Contour, Vertex Distance and Pantoscopic Angle.

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The reading zone is located using the frame’s “B” size and fitting height (pupil center). Considering minimum fitting height for lenses, i.e., ensuring 3 or 4 mm of vertical reading availability, we choose lenses that fit frames, often without thinking about changes to corridor length as we switch patients from one size to another. Most optimized digital lenses make a variety of corridor lengths available. However, if not specified and recorded, from eyeglass to eyeglass, the corridor length supplied can vary.

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Not to be confused with Pantoscopic Angle, which is the angular measure between the frame front and the temple plane, Pantoscopic Tilt describes the vertical angle between a wearer’s primary gaze and the intersection of the same with the plane of the lenses.

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Remake reduction starts at the time of the sale. Accurate measurements, proper frame fit and correct lens choices all go into creating eyeglasses perfectly suited for your patient. Knowledge of the patient’s current habitual pair of glasses can greatly impact your success in producing a new pair they love. And they’ll love you for it.

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Knowing when to use a variable corridor versus a fixed corridor design is both a powerful and useful tool. The main distinction is that variable designs are often used to prioritize the quality of the distance/intermediate area, and fixed designs are used for prioritizing the reading area.

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When delivering a finished progressive lens to a patient, the most important factor is not the work of the lens designer or the laboratory, but you—the dispenser. That’s because the decisions you make to construct the finished product affect the patient’s satisfaction. We assume the critical factors of a proper refraction (correct prescription, fitting height and PD measurements) have been done correctly. Of course, if these three items are not correct, it doesn’t matter how good some of the other options might be. The eyewear will not be successful.

Next in importance are base curve selection, progressive design, corridor length, primary uses and lens material. I often get asked, “Which progressive design is best?” The answer is not in the design, but rather in all the parameters required to build those glasses. That’s what makes patients happy. The design of the progressive, while important, is not any more significant than the other items in the list.

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