A new spectroscopic phase modulated ellipsometer (SPME) is presented. As compared to other ellipsometric techniques like rotating analyzer ellipsometry (RAE), the phase modulation uses a high frequency modulation (50 kHz) provided by a photoelastic modulator. Then SPME allows at least two orders of magnitude faster real-time mesurements than RAE. Thus, SPME is particularly suitable for in situ real-time applications. New insights on phase modulated ellipsometry are given. In particular, it is shown that an optical model, taking into account the presence of higher harmonics in the modulation, leads to an improvement of the precision measurement. Therefore, it can be inferred that both RAE and SPME provide comparable high precision measurements. Moreover SPME can be combined with numerical data processing systems. A new Fourier analysis of the signal, based on the use of a high precision analog digital converter and a fast digital processor, is presented. The adaptation of the SPME to a deposition chamber is illustrated. In particular, the use of optical fibers in both optical arms allows an increase of the compactness of the ellipsometer. Four detectors can be used simultaneously providing the spectroscopic capability for real-time applications. On-line connexions between the data acquisition system and external analog signals and triggers can also be used. Thus phase modulated ellipsometry appears a powerful technique for in situ control processing applications.