We report a sensitive surface and feature measurement technique that uses a novel imaging ellipsometer. Polarization signatures from unresolved subwavelength structures are utilized as a sensitive measure of linewidth. A focused beam rigorous coupled wave analysis method is developed to simulate the polarization effects from isolated subwavelength structures. Experimental results show that this technique can accurately measure linewidths down to 100 nm with an imaging system whose diffraction-limited resolution is 500 nm. The accuracy of our measurements is ∼10 nm for lines that are broader than 100 nm.