How does the human eye (in ArM5) perceive light or shadow?
I contend it never perceives light or shadow per se. It perceives lighted or shadowed objects. If there's a lot of light hitting an object, a lot of species will be carried to the eye, providing a sense the object is highly illuminated. If there's little light hitting an object, few species will be carried to the eye, providing a sense the object is in shadow. This has subtle consequences on PeIm spells.
Almost any object under a source of light has a "shadowed" side. Not pitch black, but visibly darker. If you assume that shadows can be perceived directly, then it's quite difficult to make stuff Invisible - a shadowy residue will be left in almost all circumstances. But if you assume that in the absence of species you can't see an object at all, whether it's lighted or shadowed... then an invisible object in shadow stays invisible. This is good.
However, if shadow is "lack of species" (and the fewer the species, the deeper the shadow) this means that if you destroy all the optic species reaching someone's eyes, that person should see perfect blackness; if you destroy all species in a Container (e.g.a Room, or a Circle) every person in that Container should see perfect blackness. I think this does not contradict any published effect, but I may well be wrong.