PRK stands for photo refractive keratotomy. ASA, advanced surface ablation, is the modern version of PRK with Customvue technology. It is an all laser vision correction procedure that is used for people who are not candidates for Lasik (those with thin corneas, some members of the military or who people who enjoy contact sports). Instead of creating a flap, the laser is used to reshape the cornea, just like in LASIK. The main difference is that the flap consists of only the outer layer of cell, epithelium, rather than about 1/3 thickness of the front surface, known as the cornea.