Bragg columnar phases

The columnar approximation implies that the object is thin, because the depletion of energy of the incident wave is neglected. This means that the amplitude of the incident beam Aq is kept constant, whereas only the phase is modified by the object crossing (phase object). To approach this condition, even at the exact Bragg angle, the object must be sufficiently thin to provide scattered beams that are very faint relative to the transmitted beam (yveak phase object). The resulting contrast is the object CTF (contrast transfer function). 

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