Polarization properties of EBT3, EBT4, EBT-XD, MD-V3, and HD-V2 radiochromic films

<i>Objective</i>. Orientation dependency of radiochromic film dosimeters has been connected to their ability to modulate the polarization of the incident light during the film scanning. We investigate the dose-dependent and color-dependent polarization properties of EBT3, EBT4, EBT-XD, MD-V3, and HD-V2 radiochromic films.<i>Approach</i>. Film samples were irradiated at different doses by a 6 MV photon beam. The films were scanned at 16 orientations by a flatbed scanner. The pixel value (PV) of the scanned images at each orientation and color channel (red, green, and blue) was measured. The modulation of PVs, defined as the ratio of PV_min/PV_max, was calculated for each model and dose level.<i>Main results</i>. In EBT3, EBT4, EBT-XD, and MD-V3 models, a cosinusoidal curve was fitted to the measured PVs in accordance with Malus's law indicating that an ensemble polarization axis exists in the films because of the elongation of the polymer chains in the active layer of the films. The modulation of PVs showed an increasing trend with dose in all film models indicating that the cross-linking and polymerization strengthened the polarization properties of the films.<i>Significance</i>. Our results provide valuable information about the nature of the orientation dependency of radiochromic films. The impact of orientation dependency is non negligible and to minimize any systematic inaccuracies, one must maintain the polarization between calibration and measurement film.