"We have used a nonresonant cavity to measure, at room temperature, the angle averaged absorptance spectra P(ω) of aluminum, molybdenum, tantalum, titanium, tungsten, and iron in the 30-300-cm-1 wavenumber region. The real part of the normalized surface impedance spectrum, z(ω) = r(ω) + ix(ω), was determined from P(ω). Measurements were also made on iron from 400 to 4000 cm-1 using standard reflectance techniques. The r(ω) spectrum was combined with previous measurements by others at higher frequencies and Kramers-Kronig analyses of the resultant combined r(ω) spectra provided ε(ω) = ε1(ω) + iε2(ω) and N(ω) = n(o) + ik(ω)."
M. A. Ordal et al. Optical properties of Al, Fe, Ti, Ta, W, and Mo at submillimeter wavelengths, Appl. Opt. 27, 1203-1209 (1988)