https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1594875 (Publisher version)
Schwaninger, A., Michel, S., & Bolfing, A. (2005). Towards a model for estimating image difficulty in x-ray screening. In 39th Annual 2005 International Carnahan Conference on Security Technology (pp. 185-188). Piscataway, NJ, USA: IEEE.