A fire alarm system comprises a first light emitting device (11), a first polarizing filter (31), a first light receiving device (21), a second light emitting device (12), a second polarizing filter (32), and a second light receiving device (22). With the above arrangement, the amount of the parallel...http://www.google.es/patents/US5576697?utm_source=gb-gplus-sharePatente US5576697 - Fire alarm system
