Photographic film is a thin layer of plastic coated with photo sensitive emulsion layers. Each of these layers will have a very specific role in the photographic process.
In general, photographic film contains the following layers:
- Overcoat
- UV Absorbing Layer
- Imaging Layers/Emulsion
- Support/Film Base
- Backing
Overcoat
The top layer of the film is called the overcoat. It is a protective layer.
It may contain microscopic polystyrene matte beads, some of which protrude from the emulsion. This provides a slight roughness to help prevent sticking (known as ferro-typing) and the formation of Newton’s Rings during contact printing.
The overcoat mostly protects the underlying imaging layers from scratching and assists in transport of the film through the camera.
- Protective Layer on Emulsion Side of Film
- Helps Prevent Sticking (Ferro-typing) of Film Wound in Rolls
- Scratch Resistance
- Aids in Transport Through Camera
That is why when we look at the surface of a film strip through reflected light, the emulsion side looks matte while the film base side looks glossy.
UV Absorbing Layer
The ultraviolet absorbing layer, located above the imaging layers, filters out any UV light.
All the emulsion layers have some sensitivity to UV light and must be protected from it, otherwise all the layers would be exposed identically, with no color discrimination.
When taking photos in locations where there is a lot of UV light, as in mountains or in the tropics, additional UV protection may be required and a UV filter can be fitted to the camera lens.
Support/Film Base
The base is the backbone of film providing a support medium for the imaging emulsion layer. It constitutes over 80% of the thickness of film.
Backing
- Prevents secondary exposure by light reflected from the base (“anti-halation”)
- Anti-static function
- Lubricant for camera transport
- Scratch protection
So I have already covered 4 out of the 5 layers in film structure. The most important and sophisticated layer, the imaging layer/emulsion, will be a much bigger topic so I will leave it for the next several posts.
To be continued……
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