Corneal solar ultraviolet radiation (UVR) dosimetry and computerised quantification of UVR-protective effects of soft contact lenses.
Document Type Article
Aims and objectives This study was undertaken to develop a corneal dosimetry model implemented in a MATLAB (Mathworks, Inc) program. This program was written to calculate protection factors (PFs) and corneal tissue safe exposure durations (SEDS) associated with the wearing of commercially available soft contact lenses in natural solar ultraviolet radiation (UVR) conditions. Methods A MATLAB program was written to compile and calculate mean results from previous publications in order to generate a corneal tissues threshold and action spectrum encompassing the entire UVR spectrum. Methodology for the spectrophotometric measurement of the UVR transmittance curves of contact lenses using the Perkin-Elmer Lambda 900 dual beam spectrophotometer was developed, applied and verified. This included the design and construction of a contact lens holder for the mounting of the soft contact lens under investigation. The UVR transmittance spectra were measured for a range of commercially available contact lenses. A MATLAB program was written to calculate mean UVR spectra and PFs for the contact lenses measured. A further MATLAB program was developed and written to generate direct, diffuse and global solar UVR spectral data incident at the earth’s surface for a defined time of day and terrain (surface of participation) as chosen by the user. This program was based on an adaptation of the model published by Bird (1984). Data generated by the earlier sub programs were included in this corneal dosimetry model for the calculation of PFs and SEDs for the contact lens selected. Results The PFs calculated provide a rating of the relative effectiveness of soft contact lenses in attenuating UVR incident at the corneal surface. The SEDs calculated indicate the maximum duration that corneal tissue may be exposed to natural solar UVR before the onset of photokeratitis. The corneal tissue thresholds and associated action spectra UVR transmittance characteristics of soft contact lenses and the spectral quantity and nature of the solar UVR incident at the earth’s surface are required for the calculation of PFs and SEDs. These are calculated for a range of typical exposure conditions and contact lens brands and give the user and indication of the relative effectiveness of soft contact lenses in providing UVR protection to corneal tissue. Significance of findings The facilitation of user inputs pertaining to the surface of participation, time of day of participation and contact lens worn, allow the calculation of an individual photokeratitis risk assessment for the individual exposed to natural solar UVR. This corneal dosimetric model may be used as a tool by the eyecare practitioner and contact lens manufacturer for recommending SEDs for different soft contact lenses to be worn in outdoor environment