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Variation of effective periodicity, 
, as a function of the actual periodicity, 
, of the multi-grating crystal, for different position of the object satisfying 4f imaging condition, 
, 70 mm and 40 mm. The solid lines are the theoretical results derived using the experimental parameters in Eq. 3. The black dash line and black arrows are used to identify the measurement resolution. The error bar is calculated using the measurement error of 
in Eq. 5.
