1. Faculty of Hydrotechnics, Department of Physics, University of Architecture, Civil Engineering and Geodesy, Hristo Smirnenski Blvd. 1; 1046 Sofia Bulgaria
2. Faculty of Forest Industry, University of Forestry, Kl. Ohridsky Blvd. 10; 1756 Sofia Bulgaria
3. Institute of Physics, Martin-Luther-University, Von-Seckendorff-Platz 1; 06099 Halle Germany
4. Department of Physics, University of Sofia, Blvd. J. Bouchier 5; 1164 Sofia Bulgaria