Printed in Great Britain 0 1993 British Contact Lens Association CONCENTRIC-DESIGN RIGID BIFOCAL LENSES, PART I: OPTICAL PEFWORMANCE”

Abstract- Experimental rigid bifocal contact lenses of a concentric design with varying central optic zone diameter (COZD) and of both centre-distance (CD) and centre-near (CN) format were investigated, Modulation transfer&actions (MTF) of the bifocal contact lenses were measured on a solid-state EROS MTF Analyser. The MTF varied with COZD, aperture size, and decentration. CD lenses were shown to give slightly better optical performance with one of the two optical performance indices. Differences may have been due to optical aberrations inherent in the back-surface design of these contact lenses. As expected, with increased COZD, optical pe$ormance improved when the central optic zone (COZ) formed the focus, and reduced when the peripheral optic zone formed the&us. With the bifocal contact lenses centred over the aperture the optimal aperture coverage of the COZ (equal distance and near optical performance) was found to remain at 37 % of the aperture area for apertures of 3-6mm diameter. Decentration of the bifocal contact lens over the aperture caused marked changes in both image form and the MTF, which were only partly explained by changes in the proportion of the aperture covered by the COZ. The MTF was shown to be a usefil measure of changes in the optical performance which influence visual per$ormance. KEY WORDS: Bifocal contact lens, optical performance, modulation transfer function