Principle of measurement of birefringence with channeled spectrum analysis, in the general case of birefringence dispersion. The free spectral range (FSR_σ), i.e., the spatial frequency difference between two successive bright channels, depends on group birefringence B_g: it is equal to 1/(B_g · L_f). Theoretically, the position of the bright channels does depend on phase birefringence B, but there is no mean to measure it, since the channel order m cannot be known precisely without access to all orders down to zero frequency (grey line channel).