In the theaters 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. 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.