a method for separating lamellae in the stroma of an eye includes establishing a focal depth that will be located in relatively weaker tissue at an interface layer between lamellae in the stroma . a laser beam can then be focused to photoablate stromal tissue and create a photoablative response thereto . this photoablative response is then compared to a reference value using wavefront analysis techniques to determine an effective minimum energy level for the laser beam . maintenance of a proper focal depth can be periodically verified by maintaining a birefringent reference using an ellipsometer . once the lamellae are separated , a flap of corneal tissue can be created that can be lifted to expose underlying stromal tissue for further surgical photoablation .