Pepper's ghost is an illusion technique used in theatre, haunted houses, dark rides and in some magic tricks. Using plate glass, Plexiglas or plastic film and special lighting techniques, it can make objects seem to appear or disappear, to become transparent, or to make one object morph into another. It is named after John Henry Pepper, who popularized the effect.
In order for the illusion to work, the viewer must be able to see into the main room, but not into the hidden or "Blue Room." The edge of the glass is sometimes hidden by a cleverly designed pattern in the floor.
The hidden room may be an identical mirror-image of the main room, so that its reflected image matches the main rooms; this approach is useful in making objects seem to appear or disappear. This illusion can also be used to make one object or person reflected in the mirror appear to morph into another behind the glass (or vice versa). This is the principle behind the Girl-to-Gorilla trick found in old carnival sideshows and in the James Bond movie Diamonds Are Forever.
The hidden room may instead be painted black, with only light-colored objects in it. In this case when light is cast on the room, only the light objects reflect the light and appear as ghostly translucent images superimposed in the visible room.
In the Haunted Mansion at Disneyland/Walt Disney World and Disneyland Tokyo, the glass is vertical to the viewer as opposed to the normal angled position, reflecting animated props below and above the viewer that create the appearance of three-dimensional, translucent "ghosts" which appear to be dancing through the ballroom and interacting with props in the physical ballroom. The apparitions appear and disappear when the lights on the animations are turned on and then off.
Giambattista della Porta
Giambattista della Porta was a 16th-century Neapolitan scientist and scholar who is credited with a number of scientific innovations, including the camera obscura. His 1584 work Magia Naturalis (Natural Magic) includes a description of an illusion, titled "How we may see in a Chamber things that are not" that is believed to be the first known description of the Pepper's Ghost effect.
Porta's description, from the 1658 English language translation, is as follows.
Let there be a chamber wherein no other light comes, unless by the door or window where the spectator looks in. Let the whole window or part of it be of glass, as we used to do to keep out the cold. But let one part be polished, that there may be a Looking-glass on bothe sides, whence the spectator must look in. For the rest do nothing. Let pictures be set over against this window, marble statues and suchlike. For what is without will seem to be within, and what is behind the spectator's back, he will think to be in the middle of the house, as far from the glass inward, as they stand from it outwardly, and clearly and certainly, that he will think he sees nothing but truth. But lest the skill should be known, let the part be made so where the ornament is, that the spectator may not see it, as above his head, that a pavement may come between above his head. And if an ingenious man do this, it is impossible that he should suppose that he is deceived.
John Pepper and Henry Dircks
The Royal Polytechnic Institute London was a permanent science-related institution, first opened in 1838. With a degree in chemistry, John Henry Pepper joined the institution as a lecturer in 1848. The Polytechnic awarded him the title of Professor. In 1854, he became the director and sole lessee of the Royal Polytechnic.
In 1862, inventor Henry Dircks developed the Dircksian Phantasmagoria, his version of the long-established phantasmagoria performances. This technique was used to make a ghost appear on-stage. He tried unsuccessfully to sell his idea to theatres; it required them to be completely rebuilt just to support the effect, which proved too costly for them to consider. Later in the year, Dircks set up a booth at the Royal Polytechnic, where it was seen by John Pepper.
Pepper realized that the method could be modified to make it easy to incorporate into existing theatres. Pepper first showed the effect during a scene of Charles Dickens's The Haunted Man, to great success. Pepper's implementation of the effect tied his name to it permanently. Though he tried many times to give credit to Dircks, the title "Pepper's ghost" endured.
The relationship between Dircks and Pepper was summarised in an 1863 article from Spectator:
"This admirable ghost is the offspring of two fathers, of a learned member of the Society of Civil Engineers, Henry Dircks, Esq., and of Professor Pepper, of the Polytechnic. To Mr. Dircks belongs the honour of having invented him, or as the disciplines of Hegel would express it, evolved him from out of the depths of his own consciousness; and Professor Pepper has the merit of having improved him considerably, fitting him for the intercourse of mundane society, and even educating him for the stage."
Political Speech-World Record
Narendra Modi, Chief Minister of Gujarat, India transmitted and delivered his 3D speech during the 2012 assembly elections using Pepper's Ghost illusion at 53 locations which entered the Guinness Book of World Records.The Guinness World Record for most simultaneous shows of the Pepper's Ghost Illusion (a technology) is now held by Raj Kasu Reddy and Mani Shankar of NChant 3D, which telecast, live, a 55 minute speech by Modi to 53 locations across Gujarat on December 10, 2012. Nchant 3D used 3D 'holographic projection technology' for this purpose.   
The world's largest implementation of this illusion can be found at the Haunted Mansion and Phantom Manor attractions at several Walt Disney Parks and Resorts theme parks. There, a 90-foot (27 m)-long scene features multiple Pepper's ghost effects, brought together in one scene. Guests travel along an elevated mezzanine, looking through a 30-foot (9.1 m)-tall pane of glass into an empty ballroom. Animatronic ghosts move in hidden black rooms beneath and above the mezzanine.
The walk-through attraction Turbidite Manor employs variations of the classic technique, enabling guests to see various spirits that also interact with the physical environment, viewable at a much closer proximity. The House at Haunted Hill, a halloween attraction in Woodland Hills, CA, employs a similar variation in their front window to display characters from their storyline.
An example that combines the Pepper's ghost effect with a live actor and film projection can be seen in the Mystery Lodge exhibit at the Knott's Berry Farm theme park in Buena Park, California and the Ghosts of the Library exhibit at the Abraham Lincoln Presidential Library and Museum in Springfield, Illinois, as well as the depiction of Maori legends called A Millennium Ago at the Museum of Wellington City & Sea.
Pepper's ghost exhibits are beginning to be more widely used in museums, as they attempt to create livelier attractions that will appeal to visitors. In the mid-70s James Gardener designed the Changing Office installation in the London Science Museum, consisting of a 1970s-style office that transforms into an 1870s-style office as the audience watches. It was designed and built by Will Wilson and Simon Beer of Integrated Circles. Another particularly intricate Pepper's ghost display is the Eight Stage Ghost built for the British Telecom Showcase Exhibition in London in 1978. This display follows the history of electronics in a number of discrete transitions.
More modern examples of Pepper's ghost effects can be found in various museums in the United Kingdom and Europe. Examples of these in the United Kingdom are the ghost of Annie McLeod at the New Lanark World Heritage Site, the ghost of John McEnroe at the Wimbledon Lawn Tennis Museum, which reopened in new premises in 2006, and one of Sir Alex Ferguson, which opened at the Manchester United Museum in 2007. Other examples include the ghost of Sarah (who picks up a candle and walks through the wall) and also the ghost of the Eighth Duke at Blenheim Palace.
In October 2008 a life-sized Pepper's ghost of Shane Warne was opened at the National Sports Museum in Melbourne, Australia. The effect is also used at the Dickens World attraction at Chatham Maritime, Kent, United Kingdom. Both the York Dungeon and the Edinburgh Dungeon use the effect in the context of their 'Ghosts' shows.
The latest example can be found at Our Planet Centre in Castries, St Lucia, which opened in May 2011, where a life-size Prince Charles and Governor general of the island appear on stage talking about climate change.
Television and video
The South African Jewish Museum in Cape Town uses elaborate Pepper's ghost video technology in their permanent exhibit. The Artist Group PXNG.LI is showing evolutionary processes in a Pepper's ghost box at the Natural Science Museum in Karlsruhe, Germany.
In the episode of The Magic School Bus that deals with light, Gets a Bright Idea, Arnold's mischievous cousin Janet uses a Pepper's ghost illusion to convince the class a theater is haunted. It ends up helping the class learn more about how light and reflections behave.
At the 2006 Grammy Awards, the Pepper's ghost technique was used to project Madonna with the virtual members of the band Gorillaz onto the stage in a "live" performance. This type of system consists of a projector (usually DLP) or LED screen, with a very high resolution (1280×1024 or higher) and very high brightness (at least 5,000 lumens), a high-definition video player, a stretched film between the audience and the acting area, a 3D set/drawing that encloses three sides, plus lighting, audio, and show control.
During Dr. Dre and Snoop Dogg's performance at the 2012 Coachella Valley Music and Arts Festival, a projection of deceased rapper Tupac Shakur appeared and performed "Hail Mary" and "2 of Amerikaz Most Wanted". The use of this approach was repeated in 2013 at west coast Rock the Bells dates, featuring projections of Eazy-E and Ol' Dirty Bastard.
For Halloween of 2012, Hallmark began marketing "My Pet Ghost", a series of three Pepper's Ghost collectibles. Each is shaped like a brown milk bottle, with an opening in the front where one can see a vignette inside (haunted house, mad scientist's lab, girl's room). When the cap of the bottle is pressed, a transparent ghost appears, appropriate to the scene (standard ghost, scientist ghost and little girl ghost, respectively) and will say one of three recorded sayings.
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