A fast and sensitive method for measurement of spectral dependence of the optical absorption coefficient α(E) in thin films of photosensitive materials is introduced. A Fourier transform infrared (FTIR) spectrometer is used with a photoconductive sample as an external detector. Experimental conditions and procedures to obtain α(E) from normalized FTIR signal are described and results are compared to the standard measurements of transmittance and reflectance, and the constant photocurrent method. The FTIR method is applied to thin microcrystalline silicon layers and the resulting optical data are discussed in terms of optical absorption connected with defects and disorder. The measurement of α(E) is extended down to very low photon energy and reveals the threshold energy for the dangling bond optical absorption in microcrystalline silicon. Photoionization cross section of the silicon dangling bonds is measured over several orders of magnitude and full dynamical range of α(E) exceeds nine orders of magnitude. © 2002 Elsevier Science B.V. All rights reserved.