If there is an emergency and you cannot use the sector 19 staircase, there are emergency exits at the sector 20 alcove and through to LCLS. You can pass safely through the curtains to make an emergency exit even if the laser is operational.
Opening the laser curtains shuts off the laser transport.
Passing through the curtains in Class 4 (laser operation) mode will close and disable T1/T2 transport safety shutters. There is no hazard- you are safe to proceed without eyewear.
This is part of the laser safety system. Opening the curtains disables the hazard and you can proceed to the nearest emergency exit.