Single work light left burning at night on stage or set — safety for crew entering next day, prevents accidents. Theater tradition, occasionally used in film production.
The ghost light burns at night on an empty stage or in a dark studio — a single work light, usually mounted on a mobile stand in the center of the stage. It provides just enough illumination so that technicians and actors don't trip in the dark the next morning, and dispels the feeling of absolute emptiness in a soundstage after closing. Practically, it prevents accidents when entering the set before the full lighting is operational.
Where the name comes from is theater folklore — supposedly the light drives the "ghosts" out of the scenery and prevents unpleasant surprises. In filmmaking, the device is less sacred but certainly established. We turn it on before we leave so that the production assistant doesn't run into equipment in the morning and the first AD doesn't stumble blindly through the fog room. A single work spotlight — no dimmer, no color — just on/off — is completely sufficient. Some gaffers discreetly mount the light high on a riser or grip, others simply place it on a piece of plywood in the center.
What's interesting for the DP: The ghost light has nothing to do with lighting design. It's purely a work light, a safety instrument. Nevertheless, cameras can be left overnight without worrying about the equipment — the light signals: "There's still life here, work is still being done here." Some old film people also swear that the small light "guards" the film set overnight, but that's superstition. Practically speaking: it costs a 16-amp outlet and saves you half an hour of confusion in the morning upon arrival.
On a modern set, the ghost light is more at home in the theater than in film — there it is tradition and almost ritual. In filmmaking, we use it when production operations plan night shifts or the shooting location remains set up for several days. Simple rule: If the ghost light is still on, shooting is happening today. Basically, an analog-ascetic communication code between departments.