Fig. 2

Working principle of the diaphragm position extraction. The diaphragm positions used for the scaling factor-based normalization are extracted by first selecting a ROI (A) to get the line profile of the intensity for each frame in the image series. Each line profile (blue) is then fitted using a sigmoid function (red) as shown in (B). To determine the actual position of the diaphragm, the maximum of the derivative of the fitted sigmoid function is calculated, which is indicated by the black dashed line