The OSI-RE-SS is really a reflector- style linear optical beam smoke detector built to function to be a part of fire alarm programs. The beam operates primarily on the basic principle of light obscuration utilising an Infra-Purple beam. OSI-R-SS is usually a 4-wire conventional reflector-type linear optical beam smoke detector https://www.firesense.co.uk/type/conventional-fire-system/conventional-beam-detectors/