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Marks on writable and rewritable optical data storage media are typically created by a thermal process, that is initiated by a focused laser pulse. While for phase-change (PC) media the local heating induces a physical change between the amorphous and the crystalline phase, for magneto-optical (MO) media heating above the Curie temperature allows the reversal of the magnetic moment. These changes result in a change of the intensity or the polarization state of the reflected beam. For development or optimisation of such writable or rewritable layers a system is helpful, that allows to irradiate the media with a pulsed and diffraction limited laser spot, whose pulse power and duration can be modified. Fast photodetectors allow to measure the change of intensity and/or polarization during and after the heating process initiated by the laser, which gives indications for the dynamics of the media.