The optical properties of undoped &agr;‐Si:H films prepared using magnetron sputtering at different deposition conditions were studied by measuring their transmittance and reflectance between &lgr;=0.25 and &lgr;=1.5 &mgr;m and their thickness. The extracted optical constants are interpreted to give values of the band gap. Values of dark conductivity and activation energy are also obtained. The study has been extended to structures SnO2/&agr;‐Si:H/substrate. From measurements of transmittance and reflectance of the system optical constants of the components can be extracted.