Film speed (S) is defined as the reciprocal of the radiation dose required to produce a density of 1.0 above base and __________ densities.

• A. Fog
• B. Primary
• C. Secondary
• D. Mid

Answer: (A)

Film speed measures how sensitive a radiographic film is to exposure. It is defined as the reciprocal of the radiation dose required to produce a density of 1.0 above the film’s base plus fog. The base plus fog account for the inherent density present in the unexposed film and any fog from processing, so the speed reflects how the film responds to exposure above that starting point. This standardizes comparisons of speed across different films and processing conditions. Other densities aren’t used in this definition, so they don’t describe how quickly the film reaches the specified density.