[en] A systematic series of Se_xGe_25-xTe₇₅ (x=0, 5, 10, 15, 20 at%) far infrared transmitting glasses were prepared by traditional melt-quenching method. Physical, thermal and optical properties of the glass system were analyzed. The allowed indirect transition of samples was calculated according to the classical Tauc equation. The results show the density increases with the substitution of Ge by Se and a maximum Τ_g value of 175 ^oC was obtained for the Se₁₅Ge₁₀Te₇₅ glass. The energy band gap of Se₁₅Ge₁₀Te₇₅ glass is 1.25 eV, possessing the largest metallization criterion value (0.25) and the lowest refractive index (3.16). When the dissolved amount of Se increased from 0 to 15 at%, the values of indirect optical band gap were in the range from 0.573 to 0.679 eV. A wide optical transparent window with a cut-off wavelength beyond 18 μm was shown.