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Advanced Techniques
Presbyopia

Bifocal contact lenses may be divided into two broad categories of design. These include the Concentric or Target Bifocal and the Segment Bifocal.

The Concentric, Target, or Annular Bifocal.
The three terms mean essentially the same thing and are often used interchangeably. In this design the distance prescription is located in the central portion of the lens while the surrounding optical portion contains the near vision prescription. While the gaze is directed straight ahead, the patient looks through the central distance portion of the lens. When gazing downward to read, the lens naturally raises upward permitting the patient to look through near vision periphery.

One form of the concentric bifocal lens contains two distinct powers, one for distance and one near vision. The added plus power for near vision may be achieved either by grinding different curves, or by fusing a portion with a higher index of refraction along the periphery of the main or carrier portion of the lens.

Another form of concentric bifocal may contain a progressive addition where the front surface of the lens is aspherical in shape rather than spherical. It results in a gradual or progressive change of power as the lens periphery is approached.

The simultaneous bifocal may also be considered to be a variety of the concentric design. Here, the reading and distance portion come into the pupillary area at the same time. It results in the patient looking through the distance and near portion of the lens simultaneously presenting two images at once. The image selection is made by the wearer at a subliminal level. To ensure the distance portion remains properly positioned in front of the pupil, the lens must be fit relatively tight.

Figure A above illustrates a back surface design of a concentric spherical bifocal contact lens. The two distinct powers of the lens are indicated by the abrupt junction of the two posterior curves.

Figure B above illustrates a front surface design of a concentric aspherical bifocal contact lens. The front surface gradually steepens peripherally resulting in the needed plus power.

 

 


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