Hi Ebi,

The illuminated area depends on how you define illuminated. Generally, the detector pixels have recorded varying levels of flux. By using ZOS-API and a supported programming language it would be possible to read the detector data as an array of pixels, and then define an illuminated pixel to be any pixel with recorded flux above some threshold. Then the illuminated area could be determined by just counting the illuminated pixels. It would also be possible to employ more sophisticated methods such as clustering to identify regions.

 For illuminated regions that have reasonable symmetry, the RMS spot sizes calculated by the NSDD merit function operand might suffice. That operand can calculate the x and y RMS spot radii. This can be used to determine the flux-weighted 2nd moment area of a Gaussian or an astigmatic Gaussian where the astigmatism is oriented along the x or y axis. I attach a zar with an example.