Illusion-O
As with several of his more famous productions, Castle used a gimmick to promote the movie. For 13 Ghosts, audience members were given a choice: the "brave" ones could watch the movie 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, most scenes were black and white, but scenes involving ghosts were shown in a "process" dubbed Illusion-O: the filmed elements of the actors and the sets — everything except the ghosts — had a blue filter applied to the footage, while the ghost elements had a red filter and were superimposed over the frame.[4] Audiences received viewers with red and blue cellophane filters. Unlike early 3D glasses where one eye is red and the other is cyan or blue, the Illusion-O viewer required people to look through a single color with both eyes. Choosing to look through the red filter intensified the images of the ghosts, while the blue filter "removed" them.[5] Despite Castle's claims to the contrary, not many heart failures 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 was broadcast for years by television without a viewer needed to see the ghosts. DVD editions have varied in their preservation of the Illusion-O effect, with the first DVD release including versions with and without the ghost outlines and a set of special viewers. Later releases after the initial DVD release, such as Sony's The William Castle Film Collection and all DVD and Blu-ray releases from Mill Creek Entertainment are of the Illusion-O version, but do not include Ghost Viewers. Mill Creek has removed the "Use Viewer" and "Remove Viewer" on screen prompts from their releases of the film.