Author(s):
1. Plamen Tsankov, Technical University of Gabrovo , Bulgaria
2. Veselina Stefanova, Technical University of Gabrovo, Bulgaria
3. Milko Yovchev, Technical University of Gabrovo, Department of Physics, Chemistry and Ecology 4 Hadzhy Dimitar str., Bulgaria
4. Ivaylo Stoyanov, Technical University of Gabrovo, Bulgaria

Final paper