Advanced Surface Ablation (ASA), also called PRK (Photo-Refractive Keratectomy), is the process of using the excimer laser to reshape the cornea so light is focused naturally on the retina. ASA was the first procedure to use the excimer laser to treat many types of refractive errors and is still the procedure of choice for certain patients. During ASA, the excimer laser uses a cool ultraviolet light beam to precisely remove very tiny bits of tissue from the surface of the cornea in order to reshape it. Once the cornea is reshaped, light is more easily focused in the eye and onto the retina, resulting in increased clarity in vision. Clarity in vision will continue to increase as the cornea heals, which can take up to three months.