Longitudinal spherical aberration and mass of bi-aspheric aphakic rigid contact lenses.

The purpose of this study is to calculate the longitudinal spherical aberration and thickness, and consequently the mass of aphakic corneal contact lenses in air to determine optimum lens designs. The longitudinal spherical aberration has been calculated by the ray tracing method through the aspheric surfaces. The volume of the lens was computed by solving integrations of rotationally symmetric bodies. The results show that longitudinal spherical aberration can be made zero in ellipsoid-ellipsoid designs for back vertex powers ranging from +8 to +20 dioptres (for a 4 mm marginal axial ray height). A reduction in mass of about 23% was obtained when the anterior surface became ellipsoid and posterior surface spherical for back vertex power +20 dioptres.