Bipack

In cinematography, bipacking, or a bipack, is the process of loading two reels of film into a camera, so that they both pass through the camera gate together. It was used both for in-camera effects (effects that are nowadays mainly achieved via optical printing) and as an early subtractive colour process.

Eastman, Agfa, Gevaert, and DuPont all manufactured bipack film stocks for use in colour processes from the 1920s onwards. Two strips of film, one orthochromatic and having a very thin and superficial red dye layer on its emulsion, and one panchromatic, would be exposed together with their emulsions pressed into close contact, the orthochromatic one nearest the lens. The orthochromatic negative ended up reversed from the normal handedness, but as the two negatives were often contact-printed onto one duplitized film for subsequent colour-toning, as in the Prizma process, this often worked to the advantage of the laboratory.

Early colour processes such as Prizmacolor, Multicolor, Cinecolor, and Trucolor all used bipack film.

The most famous version of Technicolor, the full-colour three-strip Technicolor Process 4 used from 1932 to 1955, exposed two of the three strips—the blue and red images—in bipack. The green record, the highest definition record, was exposed directly.

Alas, certain early colour TV transfers were exposed without respect to whether the film was wound conventionally on the reel (A-wind, i.e. emulsion facing toward the hub) or whether the wind was reversed (B-wind) rendering the resulting colour image as somewhat faulty, i.e. due to the thickness of the film itself, one primary colour was out-of-focus. Later transfers corrected this error.

To achieve the in-camera effect, a reel would be made up of pre-exposed and developed film, and unexposed raw film, which would then be loaded into the camera. The exposed film would sit in front of the unexposed film, with the emulsion of both films touching each other, causing the images on the exposed film to be contact-printed onto the unexposed stock, along with the image from the camera lens. This method, in conjunction with a static matte placed in front of the camera, could be used to print angry storm clouds into a background on a studio set. The process differs from Optical Printing in that no optical elements (lenses, field lenses, etc.) separate the two films. Both films are sandwiched together in the same camera and make use of a phenomenon known as contact printing.

...
Wikipedia