The measurement of ultraviolet radiation at the surface of the eye

There appears to be a clear role for ultraviolet radiation in the pathogenesis of some ocular disorders. However the relative amount of UV radiation which reaches the eye and its relationship to the site and incidence of UV-induced ocular pathology is less clear. This is due to the difficulty in acquiring true ocular UV dose values because of modulation by various factors, such as: interpalpebral aperture (squint mechanism); facial anatomy; head posture; surface reflectivity; and protection (e.g. hats, sunglasses, spectacles, and how these items are worn).

To account for these dose modulating factors, a contact lens UV dosimeter was required at the surface of the eye. Therefore, the aim of this thesis was to develop a contact lens dosimeter for the assessment of ocular UV exposure.

Two UV-sensitive plastics were manufactured into contact lenses: CR-39 and polysulphone. Since both CR-39 and polysulphone have never been used as contact lens materials, trials were initially conducted to assess the cytotoxicity and wearability of these materials as contact lenses.

Cytotoxicity testing of CR-39 and polysulphone suggested that these materials were non-toxic using the agarose diffusion test. Contact lenses made from CR-39 and polysulphone contact lenses were found to be wearable. However CR-39 and polysulphone have no significant gas transmission properties and induced mild, but reversible corneal oedema after about two hours of lens wear. Therefore, CR- 39 and polysulphone contact lenses were considered wearable provided that the lenses were worn for relatively short periods.

After several UV exposure trials, CR-39 proved to be an unreliable UV dosimeter when exposed to low doses of solar UV radiation. On the other hand, similarly-exposed polysulphone possessed a reliable dose response for a similar low dose range. Subsequently, CR-39 contact lens dosimetry was abandoned in favour of polysulphone contact lens.

Average corneal dose could be registered accurately with a conventional polysulphone rigid contact lens design. Several polysulphone contact lens designs were then developed to counteract lens movement to better record the central corneal dose. Solutions to these problems eventually led to the design of a stabilised large-diameter polysulphone monocurve lens inlayed in a PMMA scleral carrier. This design also offered the potential for regional dose assessment. Consequently, an imaging system was developed to analyse the dose distribution across the polysulphone monocurve lens. In conclusion, two polysulphone contact lens designs are proposed for ocular UV dosimetry which will provide a new research tool for ocular UV dose assessment.