Anomalous dispersion measurement by interference fringe shift

The shift of interference fringes near an absorption line is measured. In an advanced evaluation procedure values of Nfl from isolated lines as well as from line groups can be determined over a large dynamic range. Criteria are presented for estimating the sensitivity of the method and for calculating and adjusting the optimum fringe system. In comparison to the conventional hook technique the overall accuracy is improved, a rms error can be given for every single measurement, and an overestimation of small hook distances by an observer is excluded. However, this is at the expense of more measurements and computation. Comparison is made with other dispersion methods. variations are demonstrated.