Patent ID: 7940190

Claim:
A self-monitoring smoke detector comprising: a housing defining an optical chamber; an optical transmitter mounted within the housing so as to direct light into the optical chamber; an optical receiver mounted in the housing and in optical communication with the optical chamber, the optical transmitter and the optical receiver being so positioned that light from the transmitter cannot directly reach the optical receiver, but that in the presence of reflective particles in the optical chamber, light from the transmitter reaches the optical receiver after scattering on the reflective particles, and generates an increased output signal indicative of smoke detection; and first and second light-scattering devices positioned respectively in alignment with the optical transmitter and the optical receiver, the positioning of the first and second light-scattering devices being such that: in the absence of reflective particles in the optical chamber, a portion of the light from the transmitter can reach the optical receiver only after scattering at the first light-scattering device and then after scattering at the second light-scattering device so that the receiver emits an output signal even when no smoke is present thereby indicating that the detector is operational.