a method is disclosed for determining the index of refraction profile of a stepped or graded index type optical fiber having a core encased within a surrounding cladding . an interferogram is formed with a beam of radiant energy passed transversely through the optical fiber . fringe line shift is measured at a series of points corresponding to a series of mutually parallel ray chord paths extending through the core at mutually diverse minimum radial distances from the core axis . that portion of the measured fringe line shift attributable to discrete differences in indices of refraction of cylindrical core ring portions of thicknesses defined by successive minimum radial distances in the series of ray paths and that of the cladding is sequentially calculated from the outermost core ring portion inwardly towards the core center .