The sample to be measured is processed into a wedge slab. The interference pattern, which is related to the wedge angle, obtained by this method is a group of eccentric ring fringes. The advantage of this technique is that the absolute refractive index profile of a sample can be obtained from the interference film using a standard interference instrument. The principle is analysed and its measuring errors are disoussod. The index profiles for some samples have been measured and their eccentric interference patterns and the results from data processing are given.