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Summary form only given. Spectral interferometry allows to probe the coherence of resonantly excited secondary emission and confirms clearly that resonant Rayleigh scattering (RRS) dominates the early part of the signal. More specifically for quantum wells, the relation between disorder and the different phases of the localized excitons still remains to be clarified. This requires the spectral analysis of a single speckle and comparison with a refined theoretical model. We have set up a Mach-Zehnder-type interferometer (MZI) with an active stabilization of the path difference. The QW samples are held at 18 K in a cold finger cryostat and the optical excitation is very low such as to minimize the contribution of temporally incoherent luminescence.