Is an illusion a trick of the mind, making someone see something that is not actually there? (In which case, only the target actually sees the illusion, correct?)
If not, it is something visible to anyone who comes across it. This then brings up the question: What is it made of? Does light strike it, thus causing you to see it, or does it produce its own image, thus making it visible? How does it affect the world around it?
I used the 'can an illusionary light produce true light to see by', and a number of people said 'no'. Well, then. Can an illusionary sound produce an echo?
I'm thinking, for my game world, if a sense can detect the illusion, the illusion is 'apparent' to that sense. If you make an illusionary flame, it will produce light. If you add the sensation of 'touch' to it, it will hurt like all hell, but not actually cause burns. If you produce an illusionary sound, you will get an echo (if conditions are appropriate). This means, I think, that in my game world, phantasms can bend and alter light, thus allowing things such as telescopes and invisibility.
Just some food for thought. :)