Saves to disbelieve illusions benefit from any wisdom-based mental save modifier.

Illusions do not cast light (or darkness) in and of themselves. The creator of the illusion must have the illusion depict the effect of light. They can't use an illusion of light to actually illuminate a room and reveal what's in the room.

Imagine an illusionist is in a completely dark hallway and they create an illusion of a flame. If the illusionist doesn't know what's in that hallway, the best they can do is invest an illusion of an illuminated hallway. If the illusionist passed that way before with a real light, the illusionist could manifest an illusion of an illuminated hallway that looks like the real hallway. But in no case can the illusion truly illuminate the hallway. The illusory hallway and its contents lit with illusory light will be visible, but the real hallway and its contents will not be visible.