A novel ellipsometer was developed for real time analysis of the growth of thin films; its construction and testing is reported. It is based on a four-detector photopolarimeter, a polarization detection system where the light is successively reflected from the first to the fourth detector in such a way that the incidence plane is changed at each reflection. The four measured currents are linearly related to the Stokes polarization vector of the incombing light through a matrix that can be worked out at each wavelength by calibration. As UV enhanced silicon detectors are employed, the instrument is able to operate in the 300-1100 nm optical range and the time resolution depends only on the specific detector and electronic speeds. Besides the photopolarimeter, only a linear polarizer is required to set up the ellipsometer, therefore it is a very compact and inexpensive instrument. The main drawbacks are related to the amplitude division and to the very small optical aperture. A laser is required as light source and at present the effective wavelength range is limited. The ellipsometer was calibrated and tested at 632.8 nm (HeNe laser). The averaged error for the measurement of PSI and DELTA were respectively 0.13-degrees and 0.20-degrees. The ability to characterize the surface was checked by comparing results with those obtained using a null ellipsometer.