I think it is perfectly alright to "create" light with imaginem.
You might remember the "Phantasmal Fire"-Spell, CrIm20? It does NOT have an Ig-Requisite. (well, at least in 4th Ed.)
So you can create the image of a torch, will shed light, but you probably couldn't create an unseen source of light as you could with Ignem.
Imaginem and the other arts overlap.
Your Imaginem-creations come from the world of ideas, not the world of facts.
A phantasmal fire is as realistic as the casting magus imagines it. It will hurt to put your hand in it, tough your hand wont be burnt. The quality of the pain will depend on the magus 'hands-on-experience'. Ouch!
You could create the scent of flowers with Imaginem instead of Auram, but if you were allergic to those flowers, the scent wouldn't make you sneeze.
Illusionary food certainly tastes, but won't satiate.
And so on...
As Imaginem does never even touch the essential nature of its target, you can do almost anything. Such as disguising a man as a woman, which wouldn't be feasible with Corpus.
There is no need to make imaginem spells physically correct, they just need to be believeable. This is the one big cutting edge of Imaginem. And its one and only reason to exist.
Sometimes you will need to enhance Imaginem-Spells with a Mentem-Requisite, to make them more believeable. e.g. to give the image of a peace of chainmail some weight.
Other requisites may be helpful to add realism to your effects where necessary, or to make up for a lack in finesse.
Requisites might allow for "half-real" imaginem effects.
Imaginem overlaps with most of the other arts. That's sometimes troublesome, but necessary if you want Imaginem to be of any use in the game.
I guess I slightly broadend the topic...
Hope i was any help.