Working Principle
Similar to photographic film, radiochromic film changes its optical density when exposed to ionizing radiation. In contrast to photographic film, radiochromatic film is not sensitive to visible light and does not need to be developed.
Radiation exposure leads to a polymerization reaction inside of a active layer, leading to a color change of the film. There are different sensitivities available for doses ranging from 0.001 Gy to 1000 Gy. For each different film, the usable dose range comprises up to three orders of magnitude, i.e. the optical density increases linearly with the dose received. The most commonly used film for dosimetry applications, especially in the context of x-ray computed tomography, has the name of Gafchromic marketed by ISP Technology. This film is almost transparent before exposure and turns blue after contact with ionizing radiation.
Two different pieces of unexposed radiochromic film:
Radiochromic film after exposure to Na-22 beta+ radiation.
A distinct absorption maximum in the region of 650 - 700 nm is present in the coloring of the film. This can be used for quantitatively evaluating the dose with an accuracy of 2 % by the use of a transmission flatbed scanner. The illumination from behind is necessary to measure high optical densities reliably: For an optical density of d = 3 , only 1 in 1000 photons are transmitted. If only a qualitative assessment of the beam position is wanted a common document scanner can be also used. Only using the red channel of the color image is advisable, as the absorption maximum corresponds to this wavelength.