We have used the photoluminescence (PL) generated in a thin‐film sample of a‐Si:H to probe low absorption levels by measuring the absorption of the PL as it travels down the length of the film in a waveguide mode. This technique, which we have called PL absorption spectroscopy of PLAS, allows the measurement of values of the absorption coefficient &agr; down to about 0.1 cm−1. Because this technique probes the top and bottom surfaces of the a‐Si:H sample, it is important to separate surface from bulk absorption mechanisms. An improved sample geometry has been employed to facilitate this separation. One sample consisted of an a‐Si1−xNix:H/a‐Si:H/ a‐Si1−xNx:H/NiCr layered structure where the silicon nitride layers served as the cladding layers for the waveguide. In a second sample the a‐Si:H layer was interrupted near the middle for two separate, thin (100 A˚) layers of a‐Si1−xNx:H in order to check for the importance of the absorption at the silicon/silicon nitride interfaces in these PLAS measurements. Changes in the below‐gap absorption on light soaking were examined using irradiation from an Ar+laser (5145 A˚, ∼200 mW/cm2for 5.5 hours at 300 K). The silicon/silicon nitride interface is responsible for an absorption which has a shoulder near 1.2 eV while the bulk a‐Si:H absorption exhibits no such shoulder. The metastable, optically‐induced increase in the below gap absorption appears to come entirely from the bulk of the a‐Si:H. These low temperature PLAS measurements are compared with those obtained at 300 K by photothermal deflection spectroscopy.