The straight-top, or flat-top, D segment is the most popular lined multifocal. It is named for its shape: a capital letter D lying on its side, with the flat edge on top. It is made in several widths, so it can give a wide enough reading field for desk chores and other near work.
A multifocal lens corrects vision for two or more distances. A bifocal has two powers, one for far and one for near. The near part is called the segment, or add. The big advantage of a lined bifocal is that the visible line tells the patient where to look. The patient looks above the line for distance and below it for reading. The visible line also makes the lens the easiest multifocal to fit. A D segment can be ordered in bifocal or trifocal form. A trifocal adds an intermediate zone for arm's length work, which a bifocal wearer misses as reading power climbs with age. Lined lenses do have a drawback. The line is visible, and many patients link it with getting older. That is one reason progressive addition lenses, which have no line at all, are now the most popular multifocal of any kind.
When a question asks for the usual lined bifocal in practice today, answer the flat-top D segment. Round-top and executive styles are older designs you may still see. A tradesperson who must see overhead, far, and near at work may instead be sent a double-D occupational lens. Knowing spectacle lens styles by name belongs to the COA optics and spectacles content area.
Incorrect Answers
- A. Round-top segments are circles ground or fused into the distance lens. They are seldom used anymore, though old pairs still come in.
- C. An executive bifocal divides the whole width of the lens into a top distance band and a bottom near band. It is rarely used today.
- D. A blended or invisible bifocal hides the line with a smooth zone. That zone is only cosmetic, gives no intermediate power, and its optics are poor.
Testing Pearls
- The flat-top D segment is the most popular lined bifocal and comes in several widths.
- Segment, add, and near add all name the near part of a multifocal lens.
- A trifocal has three zones: distance, intermediate, and near.
- Progressive addition lenses have no line and are the most popular multifocal overall.
- A double-D occupational lens serves trades needing near, far, and overhead vision.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 12, Fundamentals of Practical Opticianry.
