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How does a film badge show the cumulative amount of radiation received by its wearer?

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detector output connected to a measuring circuit — radiation measurement Figure 1: (A) A simple equivalent circuit for the development of a voltage pulse at the output of a detector. R represents the resistance and C the capacitance of the circuit; V(t) is the time (t)-dependent voltage produced. (B) A representative current pulse due to the interaction of a single quantum in the detector. The total charge Q is obtained by integrating the area of the current, i(t), over the collection time, t_c. (C) The resulting voltage pulse that is developed across the circuit of (A) for the case of a long circuit time constant. The amplitude (V_max) of the pulse is equal to the charge Q divided by the capacitance C.

A film badge dosimeter monitors cumulative radiation exposure through the use of photographic emulsion. When the wearer is exposed to ionizing radiation, such as X-rays or gamma rays, the radiation interacts with silver halide crystals within the emulsion. This interaction triggers a photochemical change that darkens the film in proportion to the amount of radiation received.123

To determine the total dose, the developed film is compared against an identical film that has been exposed to a known radiation dose, which helps account for variations in film properties or development procedures. Because photographic film has a limited dynamic range, the badge holder often includes metallic filters of varying thicknesses. These filters cover different regions of the film, allowing for a wider range of measurable exposure levels and helping to distinguish between weakly penetrating radiation, such as beta particles, and more penetrating radiation, such as gamma rays.1