The current research demonstrates the effectiveness of both silicon and germanium as transmissive materials for use within the far infrared wavelength range of 20 to 160 microns. This study involves samples with a wide range of resistivities and temperatures including: n-type Si of 4000, 2000, 160, 65, 12, and 2.6 ohm-cm and p-type Si of 500 and 60 ohm-cm within a temperature range of -100 degree(s)C to 250 degree(s)C and n-type Ge of 39, 25, 14.5, 5.0, 2.5, and 0.5 ohm-cm within a temperature range of -100 degree(s)C to 100 degree(s)C. Far infrared absorption mechanisms are briefly discussed. The experimental absorption data are used to discuss the interaction between absorption by lattice resonance and free carrier absorption. Highly resistive germanium and silicon are both found to be excellent transmissive materials in the far infrared. These studies may be used to develop the feasibility of silicon and germanium as optical windows or lenses within an extraterrestrial environment.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.