The validity of determining the hydrogel lens thickness from the swell factor.

It is well established that the thickness of a hydrogel contact lens is a significant parameter both in terms of fitting (Mertz et al., 1979; Killpatrick, 1980) and oxygen transmissibility (Hill, 1975). It is acknowledged that the average thickness is clinically significant with respect to gas transmission and the edge thickness relates primarily to comfort and peripheral corneal anoxia. In practice however one is generally concerned with the centre thickness, because it is more easily measured and can be checked in air using a conventional radiuscope (Harris et al., 1973; Paramore and Wechsler, 1979; Wechsler and Paramore, 1978). A tolerance better than ± 0.05 mm (wet) as recommended by the British Standards Institute (BSI) in 1978 may readily be achieved (see Table 1).