An improved real-time optical image subtraction method based on the work of Zhao and Liu ["Real-time optical interferometric image subtraction by wave polarization," Appl. Opt. 21, 3864 (1982)] is proposed. A hologram of the beams from a Wollaston prism is recorded and used as an optical element in the system. It is shown that a dark field over a larger area for image subtraction can be obtained with such a system.