What Is Polyester Film?
Photographic technology has evolved significantly over the past 100 years. The types of camera bodies, lenses, lighting rigs, and photographic film bases that are used continues to change at a rapid pace today.
When it comes to photographic film bases, one of the biggest changes in the photographic and movie industries was the move to polyester film. This material has far greater strength and stability compared to other film bases available at the time. It is also safer to handle than some types of older film. This article will explain what a film base is, and identify a few of the advantages of polyester film bases.
What is a film base?
A film base is the transparent material that supports the photosensitive emulsion used to capture a photographic image. The base accounts for most of the thickness of the film. The quality of the film base will determine how strong, stable, and durable a photographic film is.
The three most common film bases are:
- Nitrocellulose(cellulose nitrate)
- Cellulose acetate (cellulose diacetate, cellulose acetate propionate, cellulose triacetate and cellulose acetate butyrate)
- Polyester (polyethylene terephthalate (PET))
These film bases have been used in a wide range of film products, including still photography film, motion picture film, and microfilm. Here is a little more information about each film base:
Nitrocellulose (cellulose nitrate)
Nitrate film base was the first transparent flexible base to be made available commercially. It was invented by Eastman Kodak, John Carbutt, and Hannibal Goodwin in the 1880’s and sold to the general public in 1889.
Unfortunately, this film base was extremely flammable. It also decomposes fairly quickly over time, releasing a flammable gas and degrading the quality of the images on the film.
There have been many cases of nitrate films igniting in movie theatres due to the film being too hot while in the projector. The fires caused by nitrate film were also difficult to extinguish because cellulose nitrate contains oxygen. This worsened the severity of many fires.
In 1978, the film archives at the United States National Archives and Records Administration and George Eastman House lost hundreds of films due to auto-igniting fires. Most nitrate films are now stored separately to prevent such events from occurring again. Nitrate film was widely in use until 1952.
By 1952, Kodak had moved away from cellulose nitrate and began using cellulose acetate film base for all of their products. Cellulose acetate was developed in 1909 and initially sold as a “safety film”. It was often used for formats smaller than 35-mm so the general public wasn’t forced to use the more dangerous cellulose nitrate films. By 1952, it was available in all formats including large format movie camera film.
Polyester film was first used for specialised applications in the 1950’s, however it was not widely adopted until the 1990s. In the 1990’s it became the most popular choice for motion picture prints because of its flexibility, image stability, and strength. These characteristics make it very useful for post-production work, exhibition, and archiving.
However, the strength of polyester film base can be a drawback in certain situations. Most film makers won’t use polyester film when shooting the original camera negative, as a breakage is more likely to damage the film equipment. Instead, they will still use cellulose acetate film.
Identifying a polyester film base
If you have some film and are unsure about the material the film base it is made from, here are a few things you can check:
- Look at the printing on the edge of the film
Nitrate films will often say “nitrate” alone the edge of the film, while cellulose acetate might say “safety”. Newer films will often have an emulsion number that identifies the stock. The edge of the film might also have a date which can help you determine the base. No cellulose acetate films were made after 1951 and no polyester films were made before 1955.
- Look for deterioration artefacts
Cellulose nitrate film does not age well. It will often have an amber discolouration, a sticky texture, and powdery film on it. Cellulose acetate films may develop a red or blue discolouration, bubbles or crystals, and brittleness.
- View the film with cross-polarised filters
When you look at a film with a polyester film base using cross-polarised filters, you will see red and green interference colours.
- Check light penetration
When you aim light at the side of a roll of polyester film, it will shine through. That is not the case for acetate films.
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