Surface Roughness in PMMA is Linearly Related to the Amount of Excimer Laser Ablation

To determine if surface roughness after excimer laser ablation is a function of the amount of ablation and to identify a standard unit for ablation roughness.We used a VISX 20/20 excimer laser to perform a series of single zone 6-mm diameter ablations (photorefractive keratectomy [PRK]) in polymethylmethacrylate (PMMA). Corrections ranged from -1.00 diopter (D) to -15.00 D. A scanning white light interferometry microscope (Zygo Corp, Middlefield, Conn) was used to quantify the surface roughness at the center of each ablation.We found a linear increase in surface roughness as the refractive correction increased. Each diopter increment resulted in an approximately 300 nm increased peak-to-valley measurement. This represented an increase of 25 nm roughness per micron of ablation in PMMA.Surface irregularities in PMMA increase with ablation depth. We propose a unit of measure of roughness, the "ablation," expressed as the peak-to-valley distance in nm/divided by m of ablation.