Hydrogenated microcrystalline silicon (mu c-Si:H) thin films with different silane concentration (SC) have been prepared using the HWCVD technique. Dual beam photoconductivity (DBP), photothermal deflection spectroscopy (PDS), and transmission measurements have been used to investigate the optical properties of the mu c-Si:H films. Two different sub-bandgap absorption, alpha(hv), methods have been applied and analyzed to obtain a better insight into the electronic states involved. A good agreement has been obtained in the absorption spectrum obtained from the PDS and DBP measurements at energies above the bandgap. Differences between PDS and DBP spectra exist below the bandgap energy where DBP spectra always give lower alpha(hv) values and show a dependence on the SC. For some films, differences exist in the alpha(hv) spectra when the DBP measurements are carried out through the film and substrate side. In addition, for some films, there remains fringe pattern left on the spectrum after the calculation of the fringe-free absorption spectrum, which indicates structural inhomogeneities present throughout the film. (c) 2005 Elsevier B.V. All rights reserved.