have been calculated. The visible absorption spectra of Co2+ glass are shown in Fig. 15 and an absorption band at 590 nm and a broad absorption band centered at about 1466 nm could be noticed, and these are assigned to spin-forbidden transitions 4A2 ( 4F)- 4T1 ( 4P) and 4A2 ( 4F)- 4T1 ( 4F) of tetrahedrally coordinated Co2+ ions [20,29]. The spectra of cobalt materials, their optical properties in the near infrared (1.3–1.6 mm) [ 4A2 ( 4F) - 4T1 ( 4F)] region of tetrahedrally co-ordinated Co2+ ions are considered more important. The excitation spectra of Co2+ glasses are shown in Fig. 16, with lemis ¼ 625 nm assigned to4A2- 4T1 ( 4P) transition of tetrahedrally co-ordinated Co2+ ions. The metastable 2E ( 2G) energy level is assigned to the 4A2 one of the tetrahedrally co-ordinated Co2+ ion. Broad luminescence bands at 581 nm (0.2 mol%) and at 582 nm (0.5 mol%) have been attributed to the electric dipole allowed transition from the