A first light source emits a light signal along a measurement optical path that includes a sample and a second light source emits a light signal along a dummy measurement optical path. A measurement circuit receives the light signals and provides outputs separated in time which are indicative of the phase of the respective light signals. A phase shift is induced in light in the measurement optical path by the sample. A reference circuit receives a signal indicative of the phase of the light signals emitted by the first and second light sources. Circuitry compares the phases of light output from the two circuits to provide output indicative of a first measured phase difference during operation of the first light source. Correction is applied to this measurement by taking a similar phase difference measurement during operation of the second light source and comparing the two phase differences.