1. Department of Basic Scientific Education, Higher Technical Teachers Training College, University of Ebolowa 1 , PO Box 886, Ebolowa, Cameroon
2. Department of Physics, Faculty of Science, University of Yaoundé I 2 , PO Box 812 Yaoundé, Cameroon
3. ZHO, Optoelektronik, Universität Duisburg-Essen 3 , D-47048 Duisburg, Germany