Wikipedia: Ghost Light (theatre)
A ghost light is an electric light that is left energized on the stage of a theater when the theater is unoccupied and would otherwise be completely dark. It typically consists of an exposed incandescent bulb, CFL lamp, or LED lamp mounted in a wire cage on a portable light stand. It is usually placed near center stage. Ghost lights are also sometimes known as equity lights or equity lamps, possibly indicating their use was originally mandated by the Actors’ Equity Association.
 
Origin and superstitions
The practical use of a ghost light is for safety. A ghost light enables one to navigate the theater to find the lighting control console and to avoid accidents such as falling into the orchestra pit and stepping on or tripping over set pieces. There is an unsubstantiated story of a burglar who tripped on a dark stage, broke his leg and sued the theater for damages.
   
Altas Obscura
The Story Behind the Ritual that Still Haunts Broadway
BY ANDY WRIGHT
OCTOBER 1, 2015
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“The superstition around it is that theaters tend to be inhabited by ghosts,” says (stage manager Matt—ed.) Stern. “Whether it’s the ghost of old actors or people who used to work in the building, and ghost lights are supposed to keep those ghosts away so that they don’t get mischievous while everyone else is gone.”
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Some claim that the tradition began in the days of gas-lit theaters when dim gas lights were left burning to relieve pressure on the gas valves.
 
Playbill
Why Do Broadway Theatres Keep a ‘Ghost Light’ Burning on the Stage?
Going inside the long-standing tradition.
BY LINDSEY WILSON
SEPTEMBER 19, 2008
Theatrical folk are a superstitious lot. Wishing actors a broken leg (don’t ever tell them “good luck”), avoiding whistling backstage (it used to confuse the stagehands), and being careful not to say Mac. . .—I mean, “The Scottish Play” (it’s cursed, you know) are just a sampling of the multitude of time-honored theatre traditions. But the ghost light is one superstition that’s also proven itself a modern theatre necessity.
 
A ghost light is a single bulb left burning whenever a theatre is dark. Some argue that its function is to chase away mischievous spirits; others insist it lights the way for the ghosts that are said to inhabit virtually every theatre, keeping them happy and contented. Either way, that light ensures that no one takes an accidental tumble off the stage.
     
The ghost light is also sometimes called the “Equity Lamp” which implies that it was once required by the Actor’s Equity Association but Stern says he does not know of any official mandate requiring ghost lights today.
     
MinnPost.com
With the Ghostlight Project, theaters aim to create a light for dark times
By Pamela Espeland | Columnist
Jan. 13, 2017
Did you know that theaters in America are never completely dark? A single bare bulb called a “ghost light” is left burning when everyone leaves. It’s there for any and all of these reasons: tradition; safety (so no one falls off the stage or into the pit); ghosts (every theater has ghosts); because (maybe) it was once required by the Actor’s Equity Association, so it’s also sometimes called the Equity Lamp.
     
Theatre Geekery
Wednesday, September 19, 2018
Ghost Light - Theatre Etymology - Part 7
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The ghost light is also known as the Equity Light or Equity Lamp. It is thought that this is because leaving a light out and on was a requirement by Actors Equity, though it seems that nobody can find this rule. However, there are currently rules from OSHA that require lights to be on in a theatre. Sometimes ghost lights aren’t the traditional bulb on a stand. The New Amsterdam Theater in New York has lights (in the grid) that basically stay on all the time due to OSHA’s rules for the type of building. However, they still function in the same way as a ghost light. The lights are said to appease Olive Thomas, the ghost of a former Ziegfeld show girl.
 
On Stage Blog
March 25, 2020
The History of the Ghost Light
Cristina D’Almeida
It’s usually known as an end of night procedure. You might hear people say, “hey don’t forget to put the ghost light on” before they leave the theater for the night. A night would not be complete without lighting it. So many have asked through the years what is the purpose of the ghost light in theaters? Why do we need to do this? I’ve often asked that question myself in my early theatre days, and it wasn’t until more recently that I grasped its true purpose other than it is there for safety reasons.
 
A ghost light is a small, single bulbed light, usually a floor lamp of some sort, that shines on the dark stage throughout the night when the theater is closed and unoccupied. The more logical reason as to why we light this at night is because it’s there in case someone is in the theater working late. It allows them to see where they’re going. Stages have so many hidden places, bumps, and holes. There’s always the orchestra pit that you want to stay clear of or any working equipment if the set is being built and of course the set itself. The light helps to avoid any of these obstacles.
 
YouTube
Theatre Lore: All About the GHOST LIGHT
Nicholas Acciani
Oct 17, 2020
The Ghost Light is an ancient theatre tradition steeped in superstition, but based in safety. This video digs into it’s history and the supernatural attributes that have been given this symbol that perseveres still, today.