Soft Lens Optics

Introduction

Single-vision soft contact lenses have a number of optically attractive features. They centre well on the cornea with only small amounts of lateral movement and hence introduce little additional asymmetric aberration into the lens–eye system. The diameter of their optic zone normally exceeds that of the entrance pupil of the eye under all lighting conditions; thus the ‘haloes’ around light sources that are observed at night following excimer refractive surgery or during wear of some rigid lenses are avoided. Their large overall diameter, greater than that of the cornea, ensures the absence of the discontinuities, flare and stray light effects that can arise with smaller-diameter rigid lenses in the peripheral visual field, due to refractive and scattering effects at the lens edges. Their refractive index (about 1.37–1.48) is quite close to that of the cornea, so that Fresnel reflection losses are comparable to those in the natural eye. They shape themselves so that their back surface conforms closely to the anterior surface of the cornea, thus minimizing fitting problems.

On the other hand, the tendency to drape to conform to the corneal surface and the consequent near-elimination of tear lens effects mean that, unlike rigid lenses, soft lenses cannot compensate for modest amounts of corneal astigmatism. Only by using a well-stabilized toric lens can the latter be corrected (see Chapter 9 ). Other possible disadvantages from the optical point of view include lens flexure and hydration variations, which may result in on-eye power changes. This chapter will primarily be concerned with such on-eye power changes and aberration effects, insofar as they apply to the optic zone of spherical corrections.

Appendix A details conventions for specifying the essential design features of contact lenses (soft or rigid) as well as the terms, symbols and abbreviations used to describe these features.