The OSI-RE-SS is really a reflector- variety linear optical beam smoke detector meant to operate like a part of fireplace alarm systems. The beam operates primarily on the principle of light obscuration utilising an Infra-Crimson beam. Optical beam smoke detectors are uniquely suited to shielding structures with significant open areas https://www.firesense.co.uk/type/conventional-fire-system/conventional-beam-detectors/