The Replacement Obscuration Filter is used for UL sensitivity testing of both projected and reflective Beam Smoke Detectors, including Fireray models 50R, 100R, and 3000EExd. Optical Beam Smoke Detectors operate on the principle of light obscuration: in normal conditions, the photosensitive element receives light from the Transmitter, and the Receiver is calibrated to a pre-set sensitivity based on a percentage of light obscured. The type of detector—projected or reflective—is chosen according to the installation environment and specific site constraints.