Interferometric spectroscopy in the far infrared

Michelson’s visibility technique for obtaining spectral distributions interferometrically in the visible region is compared with the new far-infrared interferogram method. The scanning function for the lamellar-grating type interferometric modulator that is used for the far-infrared region has been derived. Its theoretical resolution limit is compared with the spectrum estimate obtained from experimental results with a microwave source. Discrepancies between the derived spectrum estimate, obtained by a finite Fourier series expansion, and the true spectrum are discussed. A simple technique, due to Rayleigh, is invoked to avoid distortion of the spectrum due to the so-called Gibbs phenomenon. A spectrum of H2O extending from 8 to 60 cm−1 (λ = 1250 to 167 μ) illustrates the application of Rayleigh’s method.