We propose an optical method for detecting the displacement of an object in stereo images. Two images are displayed on two liquid crystal displays (LCDs), respectively, which are placed at a constant distance. Several plane waves of different propagating directions are illuminated to the LCDs. We can measure the displacement of the object by detecting which plane wave presents the strongest intensity after the plane waves pass through the LCDs. The method is applied to measuring the distance between cars from stereo images of a car.