In this work we have compared the sub-bandgap absorptance of nominally undoped GaN samples grown Under different conditions. The absorptance is measured using standard transmission measurements and transverse photothermal deflection spectroscopy (PDS). The sub-bandgap absorptance is dominated by an exponential Urbach tail (3.0-3.42 eV) and a defect absorptance (energy range <3.0 eV). By measuring the thickness dependence of the absorptance we show that the defect absorptance has both a surface and a bulk contribution. The defect absorptance decreases by capping the sample with a thin layer of AlGaN. This Suggests that the capping layer passivates the contribution of the Surface states to the defect absorptance. We show that sub-bandgap absorptance is a sensitive indicator Of bulk and surface defects introduced due to changes in carrier gas purity, making it a valuable tool for characterization of nitride semiconductors. (C) 2008 Elsevier B.V. All rights reserved.