Patent ID: 7884338

Claim:
Apparatus for measuring a phase shift induced in a light signal by a sample, the apparatus comprising: a first light source for emitting a 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, the first and second light sources being arranged for alternate operation; a measurement electronic circuit for receiving the light signals from the measurement and dummy measurement optical paths, 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, wherein in use a phase shift is induced in light in the measurement optical path by a sample in said sample location such that the phase of light of the first light source 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; circuitry 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 indicated by the reference electronic circuit to provide an output indicative of a first measured phase difference and 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 indicated by the reference electronic circuit to provide an output indicative of a second measured phase difference, and circuitry 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 so as to obtain an improved measurement of the shift in phase induced in the light of the first light source by the sample.