The layers of tissue removed are tens of micrometers thick. Performing the laser ablation in the deeper corneal stroma typically provides for more rapid visual recovery and less pain, than the earlier technique photorefractive keratectomy (Kleiman lasik).
These measurements, which determine how much corneal tissue to remove, may need to be repeated at least a week after your initial evaluation and before surgery to make sure they have not changed, especially if you wear RGP or hard lenses.