Digital cameras capture and embed EXIF data in the photographs they make. The way the panoptican is set up there are only three EXIF values that differ from frame to frame. They are date and time created, aperture and file size. Statistically speaking, a comparison of aperture and file size will reveal trends in spacial conditions. An overall dark frame or light frame will have similar file sizes but their aperture value will reveal which was dark and which was light.
This is a quick comparison of 1000 images from the panopticon library. These two graphs show a period of one afternoon. The flat line to the left represents the time when the courtyard was too dark to make a clear photograph.
This is the file size and aperture comparison for the night pinup: