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Poly(methyl methacrylate) model study of optical surface quality after excimer laser photorefractive keratectomy
E Hauge1, S A Naroo, W N Charman
1Department of Optometry and Neuroscience, UMIST, Manchester, United Kingdom.
Purpose:
To evaluate lenses produced by excimer laser ablation of poly(methyl methacrylate) (PMMA) plates.
Setting:
University research laboratory.
Methods:
Two Nidek EC-5000 scanning-slit excimer laser systems were used to ablate plane-parallel plates of PMMA. The ablated lenses were examined by focimetry, interferometry, and mechanical surface profiling.
Results:
The spherical optical powers of the lenses matched the expected values, but the cylindrical powers were generally lower than intended. Interferometry revealed marked irregularity in the surface of negative corrections, which often had a positive "island" at their center. Positive corrections were generally smoother. These findings were supported by the results of mechanical profiling. Contrast sensitivity measurements carried out when observing through ablated lenses whose power had been neutralized with a suitable spectacle lens of opposite sign confirmed that the surface irregularities of the ablated lenses markedly reduced contrast sensitivity over a range of spatial frequencies.
Conclusion:
Improvements in beam delivery systems seem desirable.