  To implement dual phase grating x-ray interferometry with x-ray tubes, one
needs to incorporate an absorbing source grating. In order to attain good
fringe visibility, the period of a source grating should be subject to a
stringent condition. In literature some authors claim that the Lau-condition in
Talbot-Lau interferometry can be literally transferred to dual phase grating
interferometry. In this work we show that this statement in literature is
incorrect. Instead, through an intuitive geometrical analysis of fringe
formation, we derived a new generalized Lau-condition that provides a useful
design tool for implementation of dual phase grating interferometry.
