  The measurement of the silicon lattice parameter by a separate-crystal
triple-Laue x-ray interferometer is a key step for the kilogram realisation by
counting atoms. Since the measurement accuracy is approaching nine significant
digits, a reliable model of the interferometer operation is demanded to
quantify or exclude systematic errors. This paper investigates both
analytically and experimentally the effect of defocus (a difference between the
splitter-to-mirror distance on the one hand and the analyser-to-mirror one on
the other) on the phase of the interference fringes and the measurement of the
lattice parameter.
