Contrast mechanisms in scanning transmission x-ray microscopy

We present a derivation of the main contrast mechanisms accessible in scanning transmission x-ray microscopy with a pixel-array detector. We consider the effect of the probe defocus and show that it can produce strong differential absorption contrast. The effect of noise is derived and used to compare the relative merits of absorption and differential contrast imaging in various experimental conditions. We illustrate the main results with an experiment that combines a through-focus series with the near-edge signal of a cerium oxide sample. The measurements are seen to follow closely the derived contrast expressions, including the defocus-dependent differential absorption contrast. The analysis includes additional considerations about the application of principal component analysis on a through-focus image series.