Lesson 15 : Processing of Film in Conventional Photography
Structure
Film is constructed in layers. These layers consist of, from top to bottom, an anti-scratch layer, the gelatin and silver layer, an adhesive layer, the film base, another adhesive layer, then an anti-halation layer:
In detail, the anti-scratch layer is just that, a coating on the image side of the film to help protect against scratches.
The gelatin/silver layer is of particular importance, as this is where the image happens. Physically, the tiny particles of silver are suspended in gelatin, which is coated on the base. The gelatin is just that, a natural emulsion. It is the characteristics of this gelatin that it can be dried and form an flexible, resistent layer. When soaked in water, the gelatin becomes permeable, like a sponge, so that chemistry can enter and react with the silver. It can then be dried and will close up again.
The silver at this point is silver bromide. When light hits it, it forms a latent image, which is made visible and intensified during the development process.
The film base is polyester, which has replaced glass and celluloid. Polyester is flexible but very dimensionally stable (menaing it doesn't expand or contract much with moisture and temperature changes).
The anti-halation layer is a light-absorbing coating on the back of the film which prevents halos from forming in the image by absorbing light which may otherwise bounce back up into the silver layer.
The adhesive layers simply help the other stuff stick to the polyester.
