You need an ignem requite in your fire to make it appear real in dark places, then. 
Otherwise the fire glows, but does not illuminate anything around it.
Xavi
With all due respect, I think you're failing to see it because you tend to think of illusions as holograms, whereas they're more like 3-dimensional pictures. Like I said before, a picture doesn't emit light.
Imagine a high-resolution photography on an entire wall, picturing a very sunny exterior. It could very conceivably fool people, especially if they're not accustomed to it (Like people at the beginning of cinema were frightened by it). Shut down the light, it won't do anything, especially not light up the room.
Does this helps you?
Your illusions, on the other hand? Create an illusion of someone in a room. Shut down the light (or even dim it). The person will glow, thus making the illusion very easy to detect.
In some ways, this make illusions harder: A stable illusion of a solid reacts "normally" to light unders Ars 5 rules, whereas yours would appear as such whereas ambiant light would dim.
So, as you see, your imaginem is not more logical, just different.
Think of it this way, it adds options to the game...
And noone can see the glow because it doesnt produce light, ANY light at all. Which is why it gets silly.
If it cant generate light, it cant glow visibly either without an Ig requisite. You cant have it both ways.
Not really. As i´ve said before, start with real physics and then add magic a´la AM to it. Wether that includes species or not, meh i dont really care as its just a way of bending reality to work for the game, i skip that extra, rather troublesome step and add the magic directly into real world physics instead.
And of course, there are ways to create illusions using real world physics as well, so no killing of Imagonem even then.
I fail to see the difference. I can just reword your argument and get:
"Illusions arent meant to be bright for one matter. You can make it LOOK bright, but you cant MAKE it bright anyway. But an illusion which is obviously fake just from a momentary poke at it, thats a failed illusion."
I tried explaining it below. If I failed, well, I really can't think of a much better way to do it.
It depends on the circumstances. An illusionary bridge that does not support people standing on it is very obvious. An illusionary fire burning at the bottom of a tree without the tree eventually catching fire is also very obvious.
I disagree. An illusionary fire burning under a bright midday sun would be almost impossible to detect as fake at a glance.
That's right; and that's the reason why one has to be careful with illusions. An illusion is like a lie. The main problem with lying is that you are editing only a portion of reality, which will almost inevitably create inconsistencies at the "frontier" between the lie and the truth. "You said you could not come to mass because you were helping Jacob out at the farm. But Jacob was supposed to be gone to town that day. What? Oh, so he did not go because his sister was ill? But I saw her perfectly well that very afternoon! What, her twin???" A good lier minimizes the possible "interactions" between the lie and the rest of reality. So does a good illusionist, who should minimize the "side-effects" of the illusion on the outside world that would create obvious inconsistencies.
If you want to make an illusion of a light source, avoid situations where its light should make its surroundings considerably brighter than they already are. If you want to make an illusion of something solid, avoid anything against which people are likely to exert considerable force. And so on.
But an illusion isnt a picture... Or a hologram for that matter.
Huh???? 
How the **** did you come up with THAT? What I am saying is that if you create an illusion it is a perfect look-a-like.
An illusion of a person in a dark room wont glow, thats a totally insane idea and how ever you came up with it i just cant figure out.
An illusion of a torch in a dark room will look like a torch and give off light like a torch, an illusion of fluorescent fungi will look like it was real, including the faint glow, an illusion of anything that if it was real wouldnt give off any light would be invisible because the illusion OBVIOUSLY wouldnt give off any more light than the real thing.
Otherwise it would be an even WORSE fail then that of the "cant emit any light at all" camp.
An illusion isnt a photo. You´re trying to limit illusions to be like something already known. A photo, picture or painting etc, none will be an exact replica of the real thing. An illusion IS an exact replica of the real thing, except it only exists to one or more senses.
An illusion is an object without physical presence to simplify it. An illusion will look exactly like the real thing.
Only from falling through it and logical deduction respectively.
Both can still be perfect illusions.
Its still not an illusion, its just a very pretty and fancy moving picture. Why does people have to limit themself to what you know of for real to emulate a concept instead of actually going with the concept itself?
An illusion looks like the real thing, its not a picture, a movie or a mirage.
And what you get then is complete rubbish.
DW75: I think part of the problem is that by the medival physics, you can't see light. No more so than you can see heat. What you do is that you see the effects of light. It would perhaps be easier for the modern mindset to consider the medival visual species to be equalent to modern light, and medival light as unseen light. You need there to be medival light for the species to be generated, but if the light is there then for all intents and purposes, the fire glows.
This is kinda like it is easier to see a fire fully the more light there is in an area no matter what generates the light.
Image from the Wizard Torn, ArM5 p147
"Further, it appears as if illuminated by whatever light is falling on you, no matter what the level of illumination is at the image."
Yes, this is ReIm while you are talking about CrIm. Still, an illusion has no shadow and therefore in my mind should not react to light level. You can keep concentration and use Finesse, or use an extra magnitude to adjust it.
I'm sorry, I really can't parse this sentence 
Let me try one last time. Then I'll give up.
Think of an illusionary fire - a small one like those we use in the kitchen for cooking - and one affecting all five senses. In particular, it appears to shed light (you look at it and it looks really bright), and it appears to shed heat (you extend a hand and feel the heat).
Now, there's a pot over the fire, and you are touching the pot. If I understand correctly, you are saying that the illusionary fire would look bright to you, and "therefore" illuminate the pot. On the other hand, you are saying that the illusionary fire would feel warm to you, but would not heat the pot. I find these two views inconsistent, both in terms of modern physics, but especially in term of medieval physics. If you fail to see the inconsistency, I am sorry, there's not much more I can communicate to you.
What would happen according to Ars Magica physics is that, even if the fire looks bright and feel hot to you directly, it does not heat, nor illuminate the pot (though if the pot could see and feel and talk, it would certainly say that the fire is bright and hot to its senses). Neither does it heat or illuminate you.
Do you mind if I take the bold part as a signature ? 
Go for it
By that logic, if i make an illusion of a huge bonfire, and then that is in a dark room except for a candle... Then you have a bonfire that gives of just the reflected light of the candle. I call that a damned lousy illusion.
Again, thats just a pretty picture, not an illusion.
Also, by this logic you create a number of other fun problems. If you cant create anything "real" with Im, then how do you affect taste by casting CrIm on food? And audible illusions cant be heard. And olfactory illusions cant be smelled.
Unless you cast it directly on the target.
And very simply, i think that sucks bigtime.
The brightness is PART OF THE ILLUSION. Without it, the illusion doesnt actually look as its meant.
Since you added the ability for it to affect touch, yes. But that is illusionary as well, so it only feels warm.
Brightness is a part of how it looks, and if an illusion doesnt copy that part as well, its not a functional illusion.
Sorry, sometimes i manage to produce horribly complex structuring...
What im saying is that you´re so busy trying to have a "real world" comparative as a model for illusions that you refuse to simply use the concept of illusions without changing it to fit such a model.
And as i said in my previous post, few illusions will work at all unless they´re directly targeted at those meant to notice them, if you go by the logic that "bright cant give off light".
I know you are working from modern physics, but I dont agree with this paradigm. Species do not beget species. When species from your nice illusionary fire hit a wall, that wall does not shed bright species. This way, Ignem and Imaginem are separate concepts.
EDIT: Just to be clear: your eyes don't see light but species
Look, Phantasmal Fire (page 144) has an Ignem requisite in order to illuminate random physical stuff around it. Without it, it wouldn't, and would look very strange glimmering on its own on a dark night.
An illusion in ars magica creates a ... "source of sensory impressions", not "a thing that isn't that acts as if it was". There's a subtle difference. With requisites, you can add some reality to this source of images - like illumination and heat.
Very well, let me ask a not quite as silly question as at first it may seem then...
If person A uses a spell to see in the dark, this person looks at a visual illusion, does the person see the illusion?
And if the illusion is changing person B:s appearance, does A see the real appearance of B?
With my rule interpretation, A sees the illusion. By opposing view, A sees the real appearance of B or a blank spot, a "shadow".
Depends on what spell he uses.
If he uses eyes of the bat (seeing borders of the air), then the illusion wouldn't be seen - but then, it wouldn't work very well against him even with lots of light.
Using eyes of the cat - this is slightly open to interpretation, since it doesn't say anything about HOW you see in the dark. However, I'd judge that the eyes provide their own light, and as such he would see the illusion as normal.
IMO Eyes of the cat mean you can see as in bright day while in the night with the light of the moon.
So the illusion would been seen.
I agree that without requisite ignem, an imaginem candle is very bad .
Antoher reason for the Animal is that a Bjornaer can use it to increaze her Charateristics... A Inteligent (and Sobrenatural) Animal needs Creo Mentem in all theirs "Mental Characteristics" or only in Intelligence?
Sigh...
I fail how you can understand how the RAW works, if you even refuse to consider simple exemples 
One last, wasted time, let's try again
I don't care what it is. I was trying to provide you for an analogy as to how they work in the RAW. Your ideas on what an illusion is or should be are irrelevant in this regard, although, of course, they are perfectly fine in your game.
So I invite you to read again, and consider the picture analogy.
Come on and READ what I wrote.
Create an illusion in a room. Say, a man lighted by the sun. It is lighted, ok? So, your illusion emits light on par with the other items in the room, no? It just doesn't show up, since it is the perfect, appropriate light.
If you shut down the light in the room, you illusion should glow. Unless it is intelligent enough to detect the changes and act accordingly. Which opens its own bag of worms.
The alternative is how the RAW works as far as I understand it: you create species, that interact with light as normal species do.
Unless your illusions do everything you want them to do? Some glow, some don't? Sometimes they're RAW, sometimes they aren't?
Of course it isn't a photo. Again: This is an exemple. An analogy.
And are you so sure that an illusion is a perfect replica in the RAW? Maybe you're right, but then, what good is Finesse when trying to create an illusion of someone? Isn't there something on this in the Jerbiton Chapter (Just asking, I really don't remember)?
And if the magus wants to create an illusion of, say, an elephant, having just heard the name, will it be accurate? What about a Bilburgate? Could you create a perfect illusion of one? What if all he knows about elephant is that they're big, grey rocks with a tentacle and a mouth?
Oh, a last word: Ars 5 went to great lengths to have each Technique/Form defined and special. Sure, you can skin a cat in different ways sometimes, but the ways show themselves. You've just rendered the entire "Light" part of Ignem redondant and useless to anyone with Imaginem. This may be fine for you. But, then again, your HR are not the RAW, and no, they're not nescessarily better (unless working on handwavium)
As you say, this is ReIm, you deplace your image: The species are excited by your light, then displaced (or something like it, I'm no longer up to date with species).
Note that, in a dark room, such a deplaced image would glow if you're in sunlight. The reverse holds true.
And shadows have to do with physical bodies, not species. That's why invisible persons still cast shadows.
Exactly.
DW, you may find it stupid, and change it in your game. Fine. It's just not RAW, that's all. Nothing good/bad with it.
My take on this, too.
Any "vision" spell that would depend on senses not affected by the illusion (say, see the smells) would cause it to fail. Otherwise, it could work.