An obstruction detection system (10) for a vehicle window (30) includes a shade and/or a filter, and/or a dual channel monitoring system to prevent interference with the monitoring beam (18) by ambient noise; a transducer (48, 50 and 52) at either end and one at an intermediate apex monitors obstructions relative to nonlinear, rectilinear or curvilinear edges (40); a wide angle transducer, closely aligned pair of transducers, or a mechanical interrupter is used to ensure tripping the beam (18) by even small obstacles (86), and a camming surface (72) guides small obstacles into the path of the beam driven by the monitored window edge.