Hochiki SPC-24 Projected Beam Smoke DetectorThe Hochiki SPC 24 Projected Beam Smoke Detector consists of an emitter and receiver. The SPC 24 detector should be placed so that smoke generated by a fire will likely rise into the path of the beam. The receiver is constantly monitoring and measuring the intensity of the beam transmitted by the emitter. Should the smoke from a fire cause a decrease in the signal strength of a magnitude that exceeds the programmed obscuration setting, an alarm signal