A fluorometer for measuring a phase shift induced in a light signal by fluorescence emission by a sample, the fluorometer comprising: a first light source for emitting a fluorescence excitation light signal along a measurement optical path, wherein the measurement optical path includes a sample location; a second light source for emitting a light signal along a dummy measurement optical path, means (29, 3) for switching the first and second light sources (5, 7) on and off alternately so that light is emitted from the first and second light sources (5, 7) alternately for a plurality of periods; a measurement electronic circuit for receiving the light signals from the measurement and dummy measurement optical paths (11, 9), the measurement electronic circuit being arranged to provide outputs separated in time which are respectively indicative of the phase of the light signals received from each of the measurement and dummy measurement optical paths (11, 9), wherein in use a phase shift is induced in light in the measurement optical path by fluorescence emission by the sample in said sample location such that the phase of light received from the measurement optical path is different to the phase of light emitted from the first light source; a reference electronic circuit for receiving a signal indicative of the phase of the light signals emitted by the first and second light sources (5, 7); circuitry arranged to compare the phase of light indicated by the output of the measurement electronic circuit responsive to the first light source with the phase of light responsive to the first light source indicated by the reference electronic circuit to provide an output indicative of a first measured phase difference and arranged to compare the phase of light indicated by the output of the measurement electronic circuit responsive to the second light source with the phase of light responsive to the second light source indicated by the reference electronic circuit to provide an output indicative of a second measured phase difference, and circuitry arranged to apply a correction to the first measured phase difference on the basis of the second measured phase difference to correct for errors in said first measured phase difference due to phase changes induced by said measurement and reference electronic circuits (15, 51) so as to obtain an improved measurement of the shift in phase induced in the light of the first light source by the sample. a first light source for emitting a fluorescence excitation light signal along a measurement optical path, wherein the measurement optical path includes a sample location; a second light source for emitting a light signal along a dummy measurement optical path, means (29, 3) for switching the first and second light sources (5, 7) on and off alternately so that light is emitted from the first and second light sources (5, 7) alternately for a plurality of periods; a measurement electronic circuit for receiving the light signals from the measurement and dummy measurement optical paths (11, 9), the measurement electronic circuit being arranged to provide outputs separated in time which are respectively indicative of the phase of the light signals received from each of the measurement and dummy measurement optical paths (11, 9), wherein in use a phase shift is induced in light in the measurement optical path by fluorescence emission by the sample in said sample location such that the phase of light received from the measurement optical path is different to the phase of light emitted from the first light source; a reference electronic circuit for receiving a signal indicative of the phase of the light signals emitted by the first and second light sources (5, 7); circuitry arranged to compare the phase of light indicated by the output of the measurement electronic circuit responsive to the first light source with the phase of light responsive to the first light source indicated by the reference electronic circuit to provide an output indicative of a first measured phase difference and arranged to compare the phase of light indicated by the output of the measurement electronic circuit responsive to the second light source with the phase of light responsive to the second light source indicated by the reference electronic circuit to provide an output indicative of a second measured phase difference, and circuitry arranged to apply a correction to the first measured phase difference on the basis of the second measured phase difference to correct for errors in said first measured phase difference due to phase changes induced by said measurement and reference electronic circuits (15, 51) so as to obtain an improved measurement of the shift in phase induced in the light of the first light source by the sample.