We present the design and prototype laboratory results of ExPo, an imaging polarimeter for the study of circumstellar disks and possibly exoplanet detection currently under development at the University of Utrecht. The instrument is designed to achieve a contrast ratio of 10$^-9$ between the unpolarized starlight and the polarized source. First light is scheduled for the second half of 2008 at the 4.2 m William Herschel telescope at La Palma. The instrument is based on the dual beam-exchange technique, simultaneously imaging the two orthogonal polarization states. It employs a ferro-electric liquid crystal retarder and a single electron-multiplying camera for fast modulation of the polarization. The instrument operates in the visible and has a field of view of (20″ × 20″).