A laser beam is splitted by a cubic beamsplitter into two beams with equal intensity. The two beams are reflected by two mirrors and superposed on a photodiode. Scanning one of the mirrors allows the extraction of the phase information and makes the instrument insensitive to intensity drifts of the light source. The sample cell is mounted in the path of one of the optical arms.
Variation of the optical density in the sample cell, e.g. replacing a solvent by a polymer solution leads to a phase shift which is recorded by the photodiode.
The accuracy of refractive index increment determination is of the order 10^{-5}