Light here is obhāsa:
and yes, I believe it refers to what later texts refer to as the nimitta. Perhaps confusingly, I don’t believe the word nimitta,
here means the nimitta, instead it means the “reason”. As I pointed out long ago in my A Swift Pair of Messengers, nimitta never means “vision” in the suttas, but it does frequently mean “cause”. This sutta is about discovering the reason why meditation progresses and declines.
“Forms” (rūpā) is similar in meaning here, although the plural suggests a diversity that is absent from plain old light. Perhaps it refers to a kind of psychic vision, perhaps of past lives or something of the sort. But in any case, it is definitely a visionary experience happening in subtle meditation. Unlike most Western terms for the physical world, rūpa includes purely mental experiences, which is one reason it is so hard to translate. Older translations sometimes used “aura” for this, which is not too bad, actually.