Comments on papers on quantitative phase determination from three-beam diffraction by Chang and Tang (1988)

In two papers [Chang & Tang (1988). Acta Cryst. A44, 1065-1072 and Tang & Chang (1988). Acta Cryst. A44, 1073-1078] the authors are confused with respect to the rotation sense of the crystal lattice during a Renninger ψ-scan experiment. This leads to wrong phase determination. We show that the definition of the triple phase sum involved in a three-beam case used by Chang & Tang is not valid if strong anomalous-dispersion effects must be taken into account.