A teleprompter is based on the theatrical concept called ‘Pepper’s Ghost’. This concept is whereby a two-way mirror is used to reflect a ghostly apparition to the audience, while also allowing the audience to see the actor simultaneously.
Two-way mirrors are often used in police stations, where victims stand behind a two-way mirror in the dark, and the accused stand in the light. The victim can see through the glass to the accused, but the accused can’t see the victim. A similar system was used in the Tupac “hologram” experiment, and was even used in the Ghostbusters Live Show at Universal Studios.
A teleprompter has two main components to it. The first is a piece of partially silvered glass, and the second is a cue card or computer with scrolling words on it. The purpose of a teleprompter is so that the talent can have eye-to-eye contact with the camera, building trust and credibility.
By only allowing light to enter the camera in one direction, the camera can see the talent, whereas the talent can see the camera and the words, effectively creating a ‘ghost’ image of the text the talent has to read.
A computer operator or voice recognition controll the speed of the words as they scroll, keeping in pace with the speaking talent.
This image courtesy of Wikipedia.