The thermal conductivity of amorphous silicon thin films is determined by using the non-intrusive, in situ optical transmission measurement as well as by the 3omega method. The temperature dependence of the film complex refractive index is determined by spectroscopic ellipsometry. The acquired transmission signal is fitted with predictions obtained by coupling conductive heat transfer with multi-layer thin film optics in the optical transmission measurement. The results of the two independent methods are in close agreement.