The current research demonstrates the effectiveness of silicon as a transmissive material 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°C to 250°C, as well as n-type Ge of 39, 25, 14.5, 5.0, 2.5, and 0.5 ohm-cm within a temperature range of −100°C to 100°C. Far infrared absorption mechanisms are briefly discussed. The experimental transmission data are used to discuss the interaction between absorption by lattice resonance and free carrier mechanisms. The effect of room temperature resistivity on silicon’s far infrared transmission characteristics is shown. The primary free carrier scattering mechanism, at elevated temperature, is shown to be acoustic phonons. Highly resistive silicon is found to be an excellent transmissive material in the far infrared. These results may be used to develop silicon and germanium optical systems in the far infrared range. © 1999 Society of Photo-Optical Instrumentation Engineers.