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Bifocal contact lens

Bifocal contact lenses are lenses comprised of two or more areas with different optical powers, often referred to as zones. A far-power zone provides the optical power for a wearer's distance vision and a near-power zone, sometimes called a segment, provides the optical power for a wearer's near vision. Bifocal lenses provide vision correction for both distance vision and up-close or near vision. As the name bifocal suggests, bifocal lenses provide the two different types of vision correction with two separate lens sections, each having a different focal length. In bifocal contact lenses, each lens must remain properly oriented on the wearer's cornea in use with the distance vision section at the top of the cornea and the near vision section at the bottom of the cornea. Bifocal contact lenses generally are classified into two types, segmented and concentric. Segmented bifocal contact lenses have two or more vertically separated power zones. Concentric bifocal contact lenses have two or more annular power zones. Concentric bifocal contact lenses have a central power zone and one or more annular power zones that function usually, but not always, by the simultaneous vision principle. Concentric bifocal contact lenses have an advantage over segmented lenses in that they are usually more comfortable for the wearer and usually do not rely on a shifting action, although some shifting may be desirable. Many designs of concentric bifocal contact lenses exist, all of which have at least two power zones. They differ in terms of whether the power zones are spherical or aspherical and whether the far or near power zone of the lens occurs at the lens center. In addition, they differ in the design of the transition between the power zones. Segmented bifocal contact lenses have two or more vertically separated power zones that function usually. The far and near-power zones, together with optional transition curves, comprise the bifocal area. The remainder of the lens surface is comprised of one or more options that are used to complete the lens shape, such as lenticular curves, truncations, slab off, and various edge contours. There are several subtypes of segmented bifocal contact lenses, with dividing lines of different shapes between the power zones, including round, D shaped, flat, crescent, and others. The near power zone is placed at the lower portion of the lens and maintained in that position by one of various features that can be incorporated into the lens to stabilize the meridional rotation of the lens as much as possible. This is commonly accomplished by adding a "prism" to the lens, which thickens the lower area of the lens and serves to maintain the desired orientation and keep the near power zone of the lens downward relative to the pupil, making it ineffective during distance viewing.

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