Ellipsometers based on four-detector photopolarimeter (FDP), where the polarization of the reflected beam is characterized in terms of the Stokes vector, have been recently developed and employed as in situ diagnostic of thin film growth. Two main problems are still open. The first is the accuracy of alignment of the FDP with respect to the incident beam, the other concerns the extension of FDP to spectral applications. The base of the set-up proposed in this work is a grating photopolarimeter where a diffraction grating is use data normal incidence and two couples of detectors are arranged in the direction of the diffracted beams corresponding to the +1 and -1 orders. The working spectral range can be chosen by selecting a proper grating. The zero order is used for the autocollimation allowing accurate alignment. For this preliminary study we used a 2 mW HeNe laser as light source. The results of analytical study of the device, its computer simulation, and the examples of application to the ellipsometric measurements of thin films and bare surfaces are presented.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.