Region of Interest (ROI)

Nexus event files contain Processing Variables (PV) as metadata. The following PV’s define two rectangular regions of interest (ROI) in the detector to limit the ranges of the peak and background regions in the detector:

  • ROI1StartX, ROI1SizeX, ROI1StartY, and ROI1SizeY define the region enclosing the peak

  • ROI2StartX, ROI2SizeX, ROI2StartY, and ROI2SizeY define the background region for the peak

There are two valid ROI configurations, as shown in the following picture:

valid configurations for ROI1 and ROI2

In configuration (a), the peak region is defined by ROI1 and the background region is defined by ROI2. The regions don’t overlap. In configuration (b), the background region used for reduction will be the region of ROI2 that doesn’t overlap with ROI1 (the region shaded in green). This type of background is termed as “side background”.

When the experiments contains multiple reflected peaks, additional PV’s are used:

  • ROI3StartX, ROI3SizeX, ROI3StartY, and ROI3SizeY define the region enclosing the second peak

  • ROI4StartX, ROI4SizeX, ROI4StartY, and ROI4SizeY define the background region for the second peak

  • ROI5StartX, ROI5SizeX, ROI5StartY, and ROI5SizeY define the region enclosing the third peak

  • ROI6StartX, ROI6SizeX, ROI6StartY, and ROI6SizeY define the background region for the third peak