Creating a dimming characterization mask. Base difference images (BD) and percentage base difference images (PBD) are transformed into dimming masks. First the BD and PBD images are thresholded and the intersection of these initial masks is obtained. Then only those clusters that overlap with the union of all previous PDMs (see Fig. 4) are kept, creating the total dimming mask. The largest dimming area is extracted and stored as the maximum dimming mask. Both masks are used to extract dimming characteristics by overlaying these with the original BDs. As an example, this procedure is shown for October 23, 2012, 08:28 UTC.