A photorefractive imaging technique, the principle of which is earlier described by Bylsma et al. , is reconsidered. This technique allows an automatic contactless characterization of semi-conductor substrates. Here, we are specifically dealing with measurements of Fe concentrations in InP wafers. The Fe2+ and Fe3+ densities are deduced from the graph of the grating decay time constant as a function of the read-out beam intensity by considering a single defect model and two types of carriers. This model seems to be more adequate to describe the experiments performed at large wavelengths such as lambda = 1.32 mum. We finally propose a modified photorefractive imaging geometry which uses successively two gratings written at two different wavelengths. Although not yet implemented, this geometry is based on the association of two types of measurements that concern the decay time constants and the diffraction efficiencies resulting from the photoexcitations at lambda = 1.32 mum.