Progressive spectacle lenses, also called progressive addition lenses (PAL), progressive power lenses, graduated prescription lenses, and varifocal or multifocal lenses, are corrective lenses used in eyeglasses to correct presbyopia and other disorders of accommodation. They are characterised by a gradient of increasing lens power, added to the wearer's correction for the other refractive errors.
The gradient starts at the wearer's distance prescription, at the top of the lens and reaches a maximum addition power, or the full reading addition, at the bottom of the lens. The length of the progressive power gradient on the lens surface depends on the design of the lens, with a final addition power between 1.00 and 3.50 dioptres. The addition value prescribed depends on the level of presbyopia of the patient. In general the older the patient, the higher the addition.
The visible lines in bifocals and trifocals are points where there's an abrupt change in lens power. When a bifocal or trifocal wearer's line of sight moves across these lines, images suddenly move, or "jump." The discomfort caused by this "image jump" can range from being mildly annoying to creating nausea.
Progressive lenses are line-free and have a smooth transition in lens power for clear vision at all distances.
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1. Wider zones of clear vision.
In today's progressive lenses, the size of the zones for computer use and reading has been increased. And for computer users, special occupational designs greatly expand the intermediate zone for enhanced comfort at the computer.
2. More comfort for active wear.
With early progressive lens designs, first-time wearers frequently noticed soft blur and other peripheral aberrations that could give the sensation of movement or "swim" during quick head turns.
3. Compatibility with smaller frames.
In early progressive lens designs, the power changes within lenses required them to be relatively large. This limited frame selection to larger styles.