Design methods of a light barrier consisting of a reciprocally placed transmitter-receiver system with a continuous directional light beam are considered. An optical diagram is designed. Usage of multiple lasers in the transmitter is proposed to form a rectangular detection area. A sequential pulsed driving method is used. A detection method to find the detectors blocked by an object existing in the detection area is proposed. Light barrier detection area determination, line laser placements, photodetector array design principles, a common operation principle of sequentially pulsed lasers, reaction time obtainment, a simplified diagram, the structure of the light barrier using two line lasers, and the time diagrams explaining the operation principle are given. (c) 2008 Optical Society of America.