1. Institute of Electronics, Bulgarian Academy of Sciences, Tzarigradsko Chaussee 72, 1784 Sofia, Bulgaria
2. Department of Industrial Design and Textile Engineering, Technical University of Gabrovo, 4 Hadzhi Dimitar Str., 5300 Gabrovo, Bulgaria
3. Department of Mathematics, Informatics and Natural Sciences, Technical University of Gabrovo, 4 Hadzhi Dimitar Str., 5300 Gabrovo, Bulgaria