A theater's house lights are located in the auditorium and typically will be switched on at all times other than during a performance. The house lights are sufficiently bright that audience members are able to find their seats before the performance begins and can read any theater program provided for the production. House lights are turned up during intermission and at the end of the production so that the audience members can find their way in the auditorium.
During a performance, house lights may be dimmed to a low setting or turned off completely. Typically, house lights are left on at a dim level so that audience members can see their surroundings. Also, dimmed house lights allow theater staff to monitor the audience and be aware of any medical emergency which may occur during a performance. Dimmed house lights also make it possible for a person to leave the auditorium during a performance if necessary.
Theater staff typically carry a flashlight when moving around the auditorium during a production. Because the house lights are usually dimmed, a flashlight allows the staff member to safely navigate the auditorium, including any steps or stairs. When seating latecomers once a performance has already started, theater ushers will use a flashlight to check the seat numbers on that person's ticket and then to locate the assigned seat in the auditorium.