Abstract. The construction of a cheap but very stable and accurate Michelson interferometer is described. To avoid (expensive) lock-in techniques a DC feedback loop has been implemented. A major advantage of this system is that the signal is proportional to the optical path length change itself, and not to the sine thereof. The frequency stabilization of the He-Ne laser used-necessary to attain the desired high resolution-is outlined. the Abel inversion procedure, used in a cylindrically symmetrical situation to convert lateral measurements into radial profiles, is discussed. A length-equivalent accuracy of about 0.1 nm has been accomplished. The system has been used to measure the radial profile of the refractive index of RF plasmas in CF4 and Ar.