A smoke detector has a housing apertured to permit introduction of smoke. A photodiode 10 projects a beam of light along a projection axis through the interior of the housing to a light-absorbing catcher 16. A multiplicity of light receptor zones are formed about the projections axis to receive light deflected from the projection axis by smoke particles. In one form, these receptor zones are the inlet ends of fiber-optic light guides LG which receive and direct the deflected light onto the active surfaces of a photodector 18 whereby producing a signal indicative of the cumulative intensity of the deflected light. In another form Fig 3 (not shown) a sheet of flexible photoresponsive material (52) is shaped into generally cylindrical structure about the projection axis and once again produces a signal indicative of the cumulative intensity of the deflected light. In either form, the light intensity signal is compared with a detection threshold, an alarm signal being generated when the threshold is exceeded. The detection threshold can consequently be raised to reduce the incidence of false alarms. <IMAGE>
