The arbitrary and position dependent (APD) phase stepping algorithm improves phase retrieval.

<p>(<b>A</b>) In an example of phase contrast imaging with electromagnetic phase stepping, the measured phase increment in the first of 6 steps is shown. Considerable variation can be seen over the oval area covered by the gratings. A profile across the center of the area (<b>B</b>) revealed a 20% gradual decrease of the phase increment. (<b>C</b>) For comparison, the differential phase contrast image of two horizontal polyacetal rods was retrieved with both the APD and the previous globally uniform algorithms. In the area outlined by the small square, the previous algorithm resulted in vertical fringe artifacts (<b>D</b>) which indicate incomplete demodulation of the moiré fringes, while the APD algorithm removed the artifacts (<b>E</b>). The scalebar in (<b>C</b>) is 3 mm long.