The OSI-RE-SS is often a reflector- style linear optical beam smoke detector designed to operate as being a component of fire alarm systems. The beam operates primarily on the basic principle of sunshine obscuration utilising an Infra-Red beam. OSI-R-SS is actually a 4-wire conventional reflector-form linear optical beam smoke detector https://www.firesense.co.uk/type/conventional-fire-system/conventional-beam-detectors/