OSI-R-SS is a 4-wire conventional reflector-type linear optical beam smoke detector for use in fire alarm systems. The beam operates primarily on the principle of light obscuration using an infrared beam. Optical beam smoke detectors are uniquely suited to protecting buildings with large open areas with high ceilings such as a warehouse or atrium. The OSI-R-SS detector is a combined transmitter/receiver unit that can be directly connected to a conventional detector circuit.