(a) Difference interferogram (black) versus optical path difference . The data set includes = 788 values, and the noise level for each reads = 132.29 μV. The values of the spatial grid are determined by the standard model with , the constant increment = 40 μm, and the origin . This origin is estimated as −5.118 mm by a fit of a quadratic polynomial to the maximum and its neighbouring values to either side. Clearly, deviates from the baseline (red). (b) Difference quotient for different subsets of . Since the zig-zag-pattern in the point-to-point variation appears only for the first case (black), the standard model = const. seems to be inadequate. Instead, a model seems appropriate for which = holds.