These are alternate and I think superior rules governing illumination. The goal is to have darkness be dangerous without being completely deadly to characters without darkvision, and still have Darkness spells be useful to dark dwellers.
I operate with three levels of illumination: Light (no concealment), Dim Light (concealment 20% mischance), and Darkness (total concealment 50% mischance).
I do not give "Bright Light" its own illumination category, because it is has the same concealment properties as Light, and that is what illumination is about. Whether the light is hurtful to a creature's eyes is a separate issue.
We all have a pretty clear idea of what happens when we light a torch in the dark, and the standard rules work fine for mundane light. A torch creates 20 feet radius of Light and an additional 20 feet of Dim Light. The problem occurs when we introduce magical light and darkness.
