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Imaging polarimetry is a powerful tool for detecting and characterizingexoplanets and circumstellar environments. Polarimetry allows a separation ofthe light coming from an unpolarized source such as a star and the polarizedsource such as a planet or a protoplanetary disk. Future facilities like SPHEREat the VLT or EPICS at the E-ELT will incorporate imaging polarimetry to detectexoplanets. The Extreme Polarimeter (ExPo) is a dual-beam imaging polarimeterthat currently can reach contrast ratios of 10^5, enough to characterizecircumstellar environments. We present the data reduction steps for a dual-beamimaging polarimeter that can reach contrast ratios of 10^5. The data obtainedwith ExPo at the William Herschel Telescope (WHT) are analyzed. Instrumentalartifacts and noise sources are discussed for an unpolarized star and for aprotoplanetary disk (AB Aurigae). The combination of fast modulation anddual-beam techniques allow us to minimize instrumental artifacts. A proper dataprocessing and alignment of the images is fundamental when dealing with largecontrasts. Imaging polarimetry proves to be a powerful method to resolvecircumstellar environments even without a coronagraph mask or an AdaptiveOptics system.

Data Reduction Techniques for High Contrast Imaging Polarimetry. Applications to ExPo