Why couldn’t the first film badge be used for more than a month?

• A. Because it would fog
• B. Because it would vanish
• C. Because it would overexpose
• D. Because it would degrade

Answer: (A)

Film badges work by the photographic emulsion darkening in proportion to the radiation dose received. If the badge is worn too long, the film can develop a background darkening called fog. This fog comes from natural background radiation, heat, humidity, and imperfect storage or development, and it adds a uniform density to the film that isn’t related to the actual exposure during the wear period. That extra darkness hides the true signal, making the dose reading unreliable. By limiting use to about a month, the fog remains manageable and the measured density more accurately reflects the worker’s exposure during that specific period. The other ideas—vanishing, overexposure, or simple degradation—don’t explain the need for a shorter, defined usage interval the way fog does.