Illusion-O
As with most of his productions, Castle used a gimmick to promote the movie. For 13 Ghosts, audience members were given a choice: the "brave" ones could watch the film and see the ghosts, while the apprehensive among them would be able to opt out of the horror and watch without the stress of having to see the ghosts. The choice came via the special viewer, supposedly "left by Dr. Zorba."
In the theatres, scenes involving ghosts were shown in a "process" dubbed Illusion-O: the filmed elements of the actors and the sets — everything except the ghosts — were displayed in regular black-and-white, while the ghost elements were tinted a pale blue and superimposed over the frame. Audiences received viewers with red and blue cellophane filters. Choosing to look through the red filter intensified the images of the ghosts, while the blue filter "removed" them. Despite Castle's claims to the contrary, not many heart attacks or nervous breakdowns were averted by the Illusion-O process; although the blue filter did screen out the ghostly images, the ghosts were visible with the naked eye, without the red filter.
Because the ghosts were indeed viewable by the naked eye, the movie ran for years on television with no viewer needed to see the ghosts. DVD editions have varied in their preservation of the Illusion-O effect, with the latest DVD release including versions with and without the ghost outlines and a set of special viewers.
Read more about this topic: 13 Ghosts