Silicon is used extensively as a transmissive optic in the mid infrared (IR) region of the spectrum. It has not been used in the far IR primarily due to an absorption band at about nine microns. This absorption is the result of both intrinsic lattice absorption and an impurity band absorption due to oxygen (O$-2$/). The absorption can be minimized by reducing or eliminating the impurity band component. In this work we report results obtained from transmission scans for low oxygen 'O$-2$/ Free' silicon. We compare these results with those obtained for typical Czochralski (CZ) silicon. Absorption coefficients are calculated from the transmission data. Tabular data and graphical representations of the data are shown in the eight to twelve micron region of the spectrum.
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