With an updated, flexible, highly efficient and easily installed system we obtained accurate refractivity ( n−1) values. This system is a refractometer based on a Fabry–Perot interferometer and was used to monitor the refractivity of DELPHI RICH Cherenkov radiators near the VUV region. By using a Pt–Ne spectral lamp and improved alignment and temperature control, the refractivities of C 5F 12 and C 4F 10 have been monitored since 1996. With this light source, selected to have large coherence lengths, we can extract the refractivity at several wavelengths from one data set only. The estimated errors of the refractivity measurements are less than 1.2%, and depend on wavelength and the type of gas used. The various parameters affecting the accuracy of the refractometer are also discussed. Finally, results from special sample refractivity measurements of the liquid radiator (C 6F 14) in its gas phase, are presented.