A robot obstacle detection system comprising: A Robot housing which navigates with respect to a surface; a sensor subsystem having a defined relationship with respect to the housing and aimed at the surface for detecting the surface. The sensor subsystem including: An optical emitter which emits a directed beam having a defined field of emission, and a photon detector having a defined field of view which intersects the field of emission of the emitter at a finite region; and a circuit in communication with the detector for redirecting the robot when the surface does not occupy the region to avoid obstacles.