The paper describes an interference refractometer for liquids and gases which operates automatically and reads out in digital or analog form. A compensating technique using white light is used for measurement. Zero adjustment is achieved by rotating the compensator and capturing the zero-order white light fringe by photoelectric means. Measurement of the path difference compensated by the compensator is based on electronic interpolation and counting of interference fringes by optointerferometric means, a time division multiplex technique with pulse amplitude modulation being used to obtain the electrical fringe signals.
W. Kinder, J. Neumann, H. Plesse, and R. Torge, "Automatic Interferometer with Digital Readout for Refractometric Analysis," Appl. Opt. 7, 341-342 (1968)