How it works, like the real thing: the excimer laser (193 nm) removes ≈0.25 µm of tissue per pulse — a −6.25 D correction over a 6.5 mm zone needs ≈88 µm by the Munnerlyn formula. The cornea starts at 540 µm; the flap takes 110 µm, and the residual stromal bed must never drop below 250 µm or the eye is in real trouble. The tracker loses lock when the eye drifts — firing then is how patients get hurt.