Figure 2. Analyser-based phase contrast imaging schematic showing the geometrical optics construction for imaging a cylindrical object. The inset shows the analyser rocking curve for the Si(333) Bragg reflection as measured experimentally and calculated from theory (full width at half maximum 5 x 10^–6 radians). The theoretical curve incorporates the double-bounce Si(311) monochromator with the Si(333) analyser reflections and the known beam divergence for the beamline. _L and _H show the low and high angle points where the reflected intensity is at half of the peak intensity found at the Bragg angle _B. Images are often acquired with the analyser tilted to either _L or _H since the intensity gradient with angle is a maximum, which provides large intensity changes for small angular deviations of X-rays.