  Purpose : To relate the physical transmittance parameters of the water
equivalent Gafchromic EBT 2 Film with the delivered dose in a transparent
absolute calibration protocol. The protocol should be easy to understand, easy
to perform, and should be able to predict the residual dose error.
  Conclussions : The gafchromic EBT2 Films are properly calibrated with an
accessible robust calibration protocol. The protocol largely deals with the
uniformity problems of the Film. The proposed method allowed to relate the dose
with the red channel transmittance using only T0, T_inf, and a dose scaling
factor. Based on the local and global uniformity the red channel dose errors
could be predicted to be smaller than 5%.
