The quantification of disorder and the effects of clustering in the sp(2) phase of amorphous carbon thin films are discussed. The sp(2) phase is described in terms of disordered nanometer-sized conductive sp(2) clusters embedded in a less conductive sp(3) matrix. Quantification of the clustering of the sp(2) phase is estimated from optical as well as from electron and nuclear magnetic resonance methods. Unlike in other disordered group IV thin film semiconductors, we show that care must be exercised in attributing a meaning to the Urbach energy extracted from absorption measurements in the disordered carbon system. The influence of structural disorder, associated with sp(2) clusters of similar size, and topological disorder due to undistorted clusters of different sizes is also discussed. Extensions of this description to other systems are also presented. (c) 2006 Elsevier B.V. All rights reserved.