Film Base
The transparent support that carries the emulsion, historically nitrate or acetate and now usually polyester.
Measured in
µm
In depth
The base must be dimensionally stable, chemically inert and optically clear. Cellulose nitrate, used until the 1950s, was dangerously flammable and decays autocatalytically, which is why so much early cinema is lost. Polyester is far more durable but so strong that it can damage a camera's mechanism if the film jams.
Examples in practice
Archives store surviving nitrate negatives in cold vaults because the base degrades and can ignite spontaneously.