Many consider the “golden era” of 3D to be late 1952 with the release of the first color stereoscopic feature, Bwana Devil. The film was projected dual-strip, with Polaroid filters. Viewers used disposable anaglyph glasses made of cardboard.
Although panned by critics, Bwana Devil was nevertheless highly successful with audiences due to the novelty of the technique, which increased Hollywood interest in 3D during a period of declining box-office admissions.
Because 3D features utilized two projectors, the capacity limit of film being loaded onto each projector (~ 6,000 feet) was an hour’s worth of film. Therefore an intermission was necessary for every feature-length film. Intermission points were often written into the script at a major plot point.
Source: Wikipedia