I'm trying to figure out how auto-brightness works. First, is it based on an ambient light sensor, time of day, what? And second, I notice that it doesn't disable the brightness slider. It seems to work in conjunction with it, but I can't figure out the exact design intent. If I move the slider to the very low end, then enabling auto-brightness raises brightness a bit. If I move the slider to the very high end, then enabling auto-brightness dims the lighting it a bit. But for the majority of the slider positions in the middle, auto-brightness seems to have no effect at all.
As you surmise, there is a photosensor involved. It is attached to the purple SisBotBoard (motor-control circuit board) which you can see if you look under your table (it is on its own tiny circuit board, and looks like a tiny, clear hockey puck. It "looks" at the floor under your table, and takes a light reading about 20 times a second. These are then averaged / filtered, and a "guess" is made about the ambient lighting in the room. This works pretty well in most situations, but has limitations (people's floors vary). It is a nice feature, in that usually the lighting level you want during the day will be far brighter than desired once the sun goes down.
Since conditions vary from site to site, the brightness slider is still active - but instead of it being the only control (as it is when auto-dim is disabled), its setting is combined with the photosensor's readings. This allows you to tweak the autodim's "gain."