L&T: In the Lens Lab

Mar
2002

Computer Compromise





Computer Compromise

The design details of contemporary
occupational lenses




by John Young

If your patients are wearing bifocals or standard progressives while doing computer work, they can be victims of the “Now-You-See-It, Now-You-Don’t” phenomenon.

Standard progressive lenses have a limited field of view (this problem is less pronounced with bifocal lenses, but it is still an issue). Generally speaking, computer keyboards are positioned a little farther away from the eye than normal reading distance, rendering a progressive or bifocal near segment less effective. Similarly, a computer screen is usually positioned much closer to the eye than the distance prescription these lenses are designed for. The result is a visual and ergonomic, issue.

figure 1a figure 1b
figure 2a figure 2b

Until recently, dispensers had few options for computer-using patients. That began to change in the late 1990s, however, with the advent of “occupational” or “indoor” progressives.

Occupational progressive lenses (or OPLs) are designed specifically for a computer user’s needs. In addition to computer users, wearers in other occupations may benefit from their use, such as architectural drawing and drafting. In general, the lower portion of the OPL (the normal reading area of a progressive lens) is widened from the 8mm to 12mm range typical in standard progressive lenses (see Figure 1a) to as much as the 15mm to 40mm (see Figure 2a). The add power is also reduced slightly so the wearer can have good visual performance as well as a wider field of vision. As the eye moves upward—looking toward the computer screen—the width of the reading area or “keyboard” area of the OPL is reduced gradually—and not as much as in standard progressive lenses (see Figures 1b and 2b). The lens power is again reduced slightly to allow for the closer proximity of the computer screen, giving the wearer “intermediate” as opposed to “distance” vision.

Why aren’t all progressive lenses like this? When a lens designer attempts to create wide areas of good visual performance while changing power, a variety of lens distortions arise. It’s the reason current progressive lenses have distinct areas of “use” (distance and near) and “disuse” (in the periphery). These latter areas were what caused adaptation issues for new wearers with earlier progressive designs. Thanks to aggressive research and design efforts at the manufacturer level, these concerns have been greatly reduced in newer generation PALs, but compensating for them is still a factor in any design process.

When looking at the design of the OPL the same optical rules apply. If we want a wider “reading” area and a wider intermediate area we are going to sacrifice the distance area. Of all the OPLs on the market only one has a distance area and it is minimal. It is this compromise that makes the OPL task specific. As dispensers, you need to be sure to follow manufacturer recommendations with these products because each makes different compromises and each design is different.

John Young is an ophthalmic lens expert with more than 25 years experience in the optical industry. He has worked for several lens manufacturers, including American Optical and Essilor, and is the former technical director of the Optical Industry Association. His company, COLTS Laboratories, is a Clearwater, Fla.-based independent lens testing facility designed to provide thorough and accurate quality and performance evaluations of spectacle lens products. His clients include lens manufacturers, wholesale labs, independent research organizations, large retailers and independent dispensers. The lab was the first U.S. ophthalmic testing laboratory accredited by the American Association for Laboratory Accreditation. It is also a Safety Equipment Institute-accredited eye protection/safety test lab. Young can be reached by phone at (727) 725-2323 and by email at
john@colts-laboratories.com.

 

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