Holder
The holder may contain a number of filters that attenuate certain types of radiation, such that only the target radiation is monitored. To monitor gamma rays or x-rays, the filters are metal, usually aluminum,or copper. To monitor beta particle emission, the filters use various densities of plastic. It is typical for a single badge to contain a series of filters of different thicknesses and of different materials, the precise choice may be determined by the environment to be monitored. The use of several different thicknesses allows an estimation of the energy/wavelength of the incident radiation.
Filters are usually placed on both the back and front of the holder, to ensure operation regardless of orientation. Additionally, the filters need to be sufficiently large (typically 5 mm or more) to minimize the effect of radiation incident at oblique angles causing exposure of the film under an adjacent filter. This places a minimum useful size on the holder and badge.
Read more about this topic: Film Badge Dosimeter