The PROBADO Project - Approach and Lessons Learned in Building a Digital Library System for Heterogeneous Non-textual Documents. - René Berndt; Ina Blümel; Michael Clausen; David Damm; Jürgen Diet; Dieter W. Fellner; Christian Fremerey; Reinhard Klein; Frank Krahl; Maximilian Scherer (ECDL, 2010)