An infrared laser beam (B1) is irradiated from a transmitter (100) toward a window pane (410) of a building (400), and a reflected laser beam (B2) is received by a receiver (200). A laser spot (Q) picked up by a CCD camera is displayed on respective target display units (113, 213). Within the receiver,...http://www.google.de/patents/US6317237?utm_source=gb-gplus-sharePatent US6317237 - Voice monitoring system using laser beam
