These are not light-sensitive and serve to complement the layer structure of films and photographic papers.
All photographic materials require a solid base.
In a film this consists of transparent cellulose triacetate or PEN resp. Other photographic papers have a thin layer of paper as a base, coated on both sides with white polyethylene to prevent chemicals and water from penetrating the base. As their bases are resistant to water and chemicals, they have particularly short processing times. The most important layers are the light-sensitive emulsions, i.e. the actual carriers of photographic information. The remaining layers affect the final result more indirectly.