Progressive lenses are corrective lenses used for presbyopia and other eye problems. They are special because of their gradient of increasing lens power. Progressive lenses have several names, such as progressive addition lenses (PAL), progressive power lenses, graduated lenses, no-line bifocals, and varifocal lenses. All of these names can reflect some of the features of progressive lens.

People look down for close reading so that the gradient of progressive lenses starts at the top of the lens, at a minimum power and reaches a maximum addition power at the bottom of the lens. This structure of progressive lenses provides the most comfortable and convenient reading effect. The power gradient length of progressive lens is usually between 15 and 20mm. The power addition gap of progressive lens is between 1.0 to 2.5 which depends on the level of presbyopia of the patient.

Just as the name no-line bifocals suggests, progressive lenses have relationship with bifocal lenses, which have visible lines on the lenses. Progressive lens just eliminates the lines on bifocals and trifocals, so that others will not find that you need reading glasses. This is one advantage of progressive lenses.

Bifocal lenses and trifocal lenses provide two or three lens power on the lens, while progressive lenses provide a seamless progression of many lens power. Progressive lens performs well in more situations, including intermediate zone computer reading. In contrast to several lens power, progressive lenses just provide a more natural correction.

Progressive lens is not perfect. As the lenses combine a range of powers in a single surface, geometric distortions may be caused to visual field, which in turn bring visual discomfort.

• Previous Entry: Powers of reading eye glasses
• Next Entry: English scientist Roger Bacon and eye glasses