Film and Cartridge
The film is on a polyethylene naphthalate (PEN) base, and is housed in a single-spool 39 mm long plastic cartridge. The basic diameter is 21 mm, while it measures 30 mm at the slot where the film exits. The slot is protected by a lightproof door. It is available in 40, 25 and 15 exposure lengths. The film surface has a transparent magnetic coating, and the camera uses this information exchange (IX) system for recording information about each exposure. The camera handles winding and rewinding automatically, to the extent that partially exposed films can, in certain cameras, be removed and used later. Numbered symbols (called 'visual indicators') on the cartridge end indicate the status:
- Full circle: Unexposed
- Half circle: Partly exposed
- Cross sign: Fully exposed but not processed
- Rectangle: Processed
Additionally, a tab on this end of the cartridge indicates that the cartridge has been processed.
Read more about this topic: Advanced Photo System
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—David Thomson, U.S. film historian. America in the Dark: The Impact of Hollywood Films on American Culture, ch. 8, William Morrow (1977)