Abstract

There is a growing population of presbyopic contact lens wearers. Many of these individuals would like to retain the comfort, convenience and cosmesis of soft lenses and avoid the use of spectacle lenses to achieve adequate vision at distance and near. This demand has led to the development of a wide range of soft bifocal contact lens (SBCL) designs. The majority rely on "simultaneous- vision" where the in-focus image of the distant or near object formed by the appropriate portion of the lens is always accompanied by a superimposed out-of-focus image formed by the remainder of the lens. Simultaneous-vision may be achieved by various techniques. The traditional approach is the concentric design where a small (~2.5 mm) central near portion is surrounded by the distance portion (e.g. the Alges lens). Alternatively the distance portion may be placed at the lens center (e.g. the Bi-soft lens). A second approach is the aspheric design where a gradual transition in lens power between the distance and near portions is achieved by producing and back or front lens surface (e.g. the PA1 lens). Increases in asphericity will produce a more pronounced variation in surface power, thereby increasing the range of distances over which the lens is focussed but this will be at the expense of image quality (Charman and Walsh, 1988). A third approach is the diffractive design as employed in the Echelon lens (Klein and Ho, 1986). In this lens, images of distance objects are formed by refraction, while images of near objects are formed by diffraction. Diffraction is achieved by annular grooves on the back surface of the lens.

© 1992 Optical Society of America