1. An intrusion detection system comprising: a first camera configured to generate first images of a monitored area; a second camera configured to generate second images of the monitored area; a detection device configured to compare the first images with a background image of the monitored area, the detection device marking differences between the first images and the background image as a potential intruder; and a tracking device configured to evaluate each of the first images relative to each of the second images to determine three-dimensional characteristics associated with the potential intruder, the tracking device comprising a threshold comparator configured to allow a user of the intrusion detection system to designate at least one three-dimensional space within the monitored area, the at least one three-dimensional space being one of a threshold space and a null zone, the tracking device activating the indicator in response to one of motion into and motion toward the threshold space, and the tracking device not activating the indicator in response to differences between the first images and the background image in the null zone.