Hands On - FRAME TIPS
B Aware
by Karlen McLean, ABOC, NCLC
When fitting multifocals, the B (vertical) frame measurement is critical. And when it comes to specialty multifocals, the B measurement is definitely the "B-all, end all." Consider the following multifocal measurement challenges.
Ribbon segments. Providing unimpaired distance vision above and below the near-vision segment, these lenses suit carpet installers, tractor drivers, and machinery operators.
The 9mm- to 14mm-wide ribbon seg should be measured higher than normal, and the B measurement must be deep enough for clear distance vision below the segment. To maximize the horizontal viewing area and ensure sufficient B measurement, try a modified rectangle bottom eyewire.
Double-D segments and quadrifocals. With segments for near-vision viewing below and above eye level, these work for painters, dentists, mechanics, and airline pilots. Have the patient model their visual tasks, to find where to fit the bottom segment.
Plastic double-D segs commonly have a center area for distance viewing that's 14mm high, while glass double-D segs have either 13mm or 15mm center viewing areas. The frame B measurement must be large enough for the patient to easily use both top and bottom near segments.
The frames should be large enough to avoid edging away portions of the top and bottom segs. When gazing at a distance, the wearer's pupil should be as close to the frame's mechanical center as possible, and the frame pupillary distance should approximate the wearer's PD.
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quick tips |
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Although some bifocals may have existed previously, Benjamin Franklin popularized their use with the creation of what is similar to an executive bifocal. He cut distance and near lenses in half, placing the distance lens on top of the near lens. |
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Specialty trifocals. Created with intermediate segments that offer more power than the standard 50 percent of near add power, patients with specialty trifocals get enhanced near visual acuity at intermediate range for long use with hobbies, computers, machining, and industrial applications.
Specialty trifocals segments are often set higher to enhance the intermediate viewing area, so select frames with a large vertical measurement. An appropriate B measurement helps reduce the likelihood that the reading portion of the segment will be edged away.
Executive bifocals and trifocals. Limited in availability, the preferred alternatives are D-35 bifocal or 7x35 or 8x35 trifocal.
Before ordering any specialty multifocal, be sure to check with your lab or lens supplier to make sure that you can get the add powers you want, and check on the turnaround timeframe: Specialty lenses often take longer to order, process, and deliver.
wearer's guide
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wearer's guide |
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| MULTIFOCAL STYLE | CANDIDATES |
| Ribbon segments | Carpet installers, tractor drivers, and machinery operators |
| Double-D segments and quadrifocals | Painters, mechanics, dentists, and airline pilots |
| Specialty trifocals | Computer users, machinists, hobbyists, and industrial workers |
| Executive bifocals and trifocals | Office workers |
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glossary of terms |
| Bifocals. A lens with two points of focus, usually one for distance and one for near. Flattop bifocals are half-moon shaped and can be measured for various heights according to patient preference or specific task.
Double-D. The bottom of the lens is a half-moon-shaped flattop bifocal. The top is an upside-down flattop. The remaining lens area is used for distance correction. Segments are set so that wearer has near vision both overhead and below. Executive trifocals. An E-D or executive trifocal lens contains distance correction in the top half and intermediate correction beneath it. At the bottom is a D half-moon segment that carries the near correction. Ribbon or bar segments. A narrow strip-type bifocal segment that has a 9mm or 14mm depth. The segment is set so that the wearer has distance vision both above and below the segment. Quadrifocals. A flattop bifocal segment is in the top of the lens, with the lens center set for distance viewing. The bottom is set up like an executive trifocal, with an executive bifocal for intermediate vision all the way across and a flattop bifocal for near vision in the bottom. It is recommended when near vision acuity is needed above and below eye level, along with distance and intermediate vision. Trifocals. Lenses with three distinct areas--usually near, intermediate, and distance. |
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