Poly(methyl methacrylate) model study of optical surface quality after excimer laser photorefractive keratectomy

Purpose: To evaluate lenses produced by excimer laser ablation of poly(meth yl 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 va lues, but the cylindrical powers were generally lower than intended. Interf erometry revealed marked irregularity in the surface of negative correction s, which often had a positive "island" at their center. Positive correction s were generally smoother. These findings were supported by the results of mechanical profiling. Contrast sensitivity measurements carried out when ob serving through ablated lenses whose power had been neutralized with a suit able spectacle lens of opposite sign confirmed that the surface irregularit ies of the ablated lenses markedly reduced contrast sensitivity over a rang e of spatial frequencies. Conclusion: Improvements in beam delivery systems seem desirable. (C) 2001 ASCPS and ESCPS.