A new kind of Real Time Spectroscopic Ellipsometer (RTSE) system is presented in details. A multichannel analyser with photointensifier allows to get spectroscopic measurements over the 0.25-0.85 mu m wavelength range with resolution lambda/Delta lambda better than 500 and a good signal to noise ratio even for samples with poor reflectance. Precision and reproducibility of the system are around 0.3 and 0.1% for the two ellipsometric parameters. The measurement speed can be increased up to 15 spectra/s but must be adapted accurately to the process under control. Two examples of application concerning the field of semiconductors are presented: a heating process of GaAs substrate measured in-situ by RTSE has been performed and the temperature and native oxide thickness deduced; another example concerns the crystallization of amorphous silicon by excimer laser annealing for Active Matrix Liquid Crystal Display. In this latter case, RTSE allows to measure at the same time the thickness of the polysilicon layer, its crystallinity and the top surface roughness of the stack.