A smoke detector, comprising: a housing defining a chamber receptive of ambient materials; a receiver disposed to receive light reflected from the chamber along a receiving axis; first, second and third emitters (16, 17, 18) disposed to emit light of first, second and first wavelengths, respectively, into the chamber at first, first and second angles relative to the receiving axis, respectively; and a controller configured to determine whether a current condition of the chamber should trigger an alarm based on output signals generated by the receiver resulting from light emitted into the chamber being reflected toward the receiver by the ambient materials; characterised in that the controller is configured to: activate the receiver and cause the first light emitter to emit light into the chamber; receive a first output signal from the receiver corresponding to the emission time of the first light emitter, filter the first output signal, and determine whether the filtered output signal is above a trigger level; and, if so: cause the second light emitter and third light emitter to emit light into the chamber, receive and filter second and third output signals from the receiver corresponding to the emission times of the second and third light emitters respectively, calculate first, second and third output signal ratios, and determine whether the current condition should trigger the alarm based on respective durations of the first, second and third output signal ratios. a housing defining a chamber receptive of ambient materials; a receiver disposed to receive light reflected from the chamber along a receiving axis; first, second and third emitters (16, 17, 18) disposed to emit light of first, second and first wavelengths, respectively, into the chamber at first, first and second angles relative to the receiving axis, respectively; and a controller configured to determine whether a current condition of the chamber should trigger an alarm based on output signals generated by the receiver resulting from light emitted into the chamber being reflected toward the receiver by the ambient materials; characterised in that the controller is configured to: activate the receiver and cause the first light emitter to emit light into the chamber; receive a first output signal from the receiver corresponding to the emission time of the first light emitter, filter the first output signal, and determine whether the filtered output signal is above a trigger level; and, if so: cause the second light emitter and third light emitter to emit light into the chamber, receive and filter second and third output signals from the receiver corresponding to the emission times of the second and third light emitters respectively, calculate first, second and third output signal ratios, and determine whether the current condition should trigger the alarm based on respective durations of the first, second and third output signal ratios. activate the receiver and cause the first light emitter to emit light into the chamber; receive a first output signal from the receiver corresponding to the emission time of the first light emitter, filter the first output signal, and determine whether the filtered output signal is above a trigger level; and, if so: cause the second light emitter and third light emitter to emit light into the chamber, receive and filter second and third output signals from the receiver corresponding to the emission times of the second and third light emitters respectively, calculate first, second and third output signal ratios, and determine whether the current condition should trigger the alarm based on respective durations of the first, second and third output signal ratios.