Autoradiography technique generally provides an image in cps cm−2, the conversion in Bq g−1 remaining a challenge. The Geant4 tool has been used to establish a conversion procedure based on the estimation of a key parameter, the emission fraction FE. The conversion method was applied on three samples containing 3H, 14C and 238U respectively, allowing then to assess the detection efficiency of a digital radioimager, BeaQuant™. Particular attention was paid to matrix effect of contaminated material that could affect the detection efficiency. This article is coupled with a companion paper, providing support to apply the conversion method to decommissioning investigations.