Sub-bandgap optical absorption in the low photon energy range (0.7-1.5 eV) has been measured on the same undoped hydrogenated amorphous silicon alloys, using both CPM and DBT techniques in gap-cell and Schottky barrier (direct and reverse bias) electrical contacts configurations. Significant differences in results, sometimes larger than those noticed in the literature, are observed between spectra. These discrepancies are interpreted in terms of photocurrent equations theory, density of gap states model and light-induced or Staebler-Wronski effect. (C) 2002 Elsevier Science B.V. All rights reserved.