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