Information concerning the effect of irradiation on the optical properties of materials in the infrared, sub-mm-and mm-wavelength regions is of increasing importance in fusion plasma diagnostics. The radiation induced transmission loss of a number of materials has therefore been investigated at wavelengths in the ranges 200 nm to 40 μm and 0.23 to 2.0 mm. The samples were irradiated with doses of up to 1010rad in a nuclear reactor. While germanium shows considerable transmission loss at doses as low as 106rad, and the transmission of TPX decreases at 109rad, other materials, e.g. fused quartz and possibly ZnSe, exhibit sufficient radiation hardness for use in fusion plasma diagnostics.