Until now, we have been working with detectors that respond only to motion in one direction.  By combining these detectors, one in each direction, and comparing the results through subtraction, we can obtain a result that indicates motion in either direction.


This will also ensure that responses are to motion detected, not screen flicker or areas of intense illumination, both of which would produce constant output for a single detector.  Because of this, the single motion detector is not useful for applications.