Abstract: Optical system for detecting and signaling a ball out-of-bounds condition on a tennis court. An optical shape plane interfered by a tennis player disables an underlying optical timed plane. The timed plane discriminates between an interference caused by a player's foot or a tennis ball. The optical planes are produced by emitter and receptor sensor units which surround the tennis court. The sensor units may be configured around the tennis court in sets of units referred to as optical lattices. Each optical lattice provides intersecting light beams allowing for continuity in court coverage when a player disables a timed plane. An out-of-bounds ball is signalled by an audiovisual device.