I think that the minimum light required is going to vary from person to person and tend to increase as we age. I used to always read Tom Block's columns in "Flying". In one article he mentioned how much more light he needed in the cockpit before he retired as a US Airways captain than in his earlier days. His vision would've had to be good or he couldn't have kept his 1st Class Medical, but he still needed more light on the instruments.
Joe
