Section 11.3.3.2. Background

The region around a spot is assumed to have been chosen to be large enough to include a sufficient number of pixels which can be used for determination of the background. Background determination, as implemented in XDS, begins by sorting all pixels belonging to a reflection by increasing intensity. For weak or absent reflections, these values should represent a random sample drawn from a normal distribution. If this is not the case, the pixel with the largest intensity is removed until the sampling distribution of the remaining smaller items satisfies the expected distribution. This method will also exclude pixels with unexpectedly high values, such as ice reflections. The background, determined as the mean value of the accepted pixels, is systematically overestimated for strong spots because of some residual intensity extending into the accepted background pixels. This residual intensity is estimated from the expected distribution defined in Section 11.3.2.3 and removed from the final background value.