Optical absorption measurements have been derived from photothermal deflection spectroscopy (PDS) and the constant photocurrent method (CPM) in the photon energy range from 06 to 1.9eV for undoped hydrogenated amorphous silicon films with thicknesses between 1.0 and 3.2 μm deposited on either Corning 7059 or quartz substrates. For glass-deposited samples the absorption curves derived from each method diverge considerably for photon energies less than 1 eV and the PDS results exhibit a marked dependence on thickness. By depositing on quartz substrates, both PDS and CPM results are in close agreement at all energies and the thickness dependence in the PDS measurements disappears. These results are explained in terms of absorption in the 7059 glass substrate which can lead to serious overestimates of the film absorption data from PDS experiments.