A robotic vision system capable of detecting intruders and abnormalities in a gantry robot work space has been constructed to ensure the safe operation of the robot. The system uses two CCD video cameras mounted on the frame of the gantry for monitoring the work space and a novel four-point ranging algorithm for determining the relationship between the camera reference frame and the robot base frame. Intruder/abnormality location is found through a triangulation scheme based on features extracted from the two camera images. Without compensating for lens distortion, accuracy achieved for locating intruders is between 1/60 to 1/100 of depth.>