The AEC philosophy described here is probably not the best for all observations, but it is intended to be broadly applicable and makes no assumptions about the intensity distribution of the image falling on the detector.
Exposure time is specified by an integer index to the exposure table.
The AEC algorithm adjusts the exposure index based on a 128-bin histogram of the previous CCD image, which is taken after the removal of bad pixels. This camera offset is subtracted from the pixel values prior to creating the histogram. Put another way, PAL is the percentage of pixels that must not be underexposed in order for the image not to be underexposed. The greater resolution of PAH is intended to help the AEC with EUV images, where the bright loops may comprise a small percentage of pixels. The behavior of the AEC algorithm is implicit in the above parameter definitions, except for one special case.