Visibility at ILS minima..
Visibility determination at the minima is determined by how thepilot transitions from instrument flying to visual flying.
When 1.5km vis is required the pilot should see all of the HIALs along with the PAPI / VASIS...
When 1.2km vis is required all of the HIALs and the beginning of the threshold should be visible...
When 0.8km vis is required..one should see HIALs / no need to see beginning of threshold..
Thinking deeper about this rather than dogmatically it will make sense...i.e threshold to VASIS or PAPI is 300m! Etc etc (1.5 versus 1.2 requirements)
0.8km used with aid of FD or coupled AP...to assist in persisting with the approach without the visual slope guidance of PAPI / VASIS offers.
The stories going around about light spacing etc...seriously...who counts rows of lights when ~300ft AGL travelling at speed at the DA!!! (Not to mention if OEI!)
Hope this helps...and makes visibility determination easier for some!