That would fit with many of them and makes perfect sense with what's written about Florae and Fauni.
Among the others I mentioned are Eidolons, which "appear in human form, a simulacrum of the spirit’s “owner,” but expressing a caricature of the emotion on its face," and Jinn (one type of Genii Loci), which are described as having their own distinct appearances in multiple places in canon.
Well, you can in canon. While the only guidelines are for "Recovery Rolls," we have this spell:
This second sight stretches well beyond the capabilities of this spell, doing things like spotting regios, and somehow functioning without penetrating magic resistance.
But something that hermetic magic can do is make a mind or spirit visible (level 15 Muto Mentem guideline from the core book), more or less the same final effect as this spell.
This model feels right to me, but..
How then would a MuMe(Im) spell make a spirit visible? I suppose because it actually changes the spirit itself the spirit is "Muto-ed" into having a shape, something that Illumination of the Airy Spirits wouldn't do because it targets a fire not a spirit.
I looked at a bit of RoP: Faerie and I didn't see much that indicated that they as a rule maintained any sort of shape while immaterial.
Ghosts however, they've generally got a shape don't they? What about other spirits, how about elementals?
Ah good point. I forgot that everyone in my game with Second Sight also has Strong Faerie Blood.
So, making the spirits directly visible is a MuMe 15 - I don't think doing it with MuIg should have a lower base than that. If we starthent with MuMe base 15, +1 for useful Ignem requisite, +1 touch, +1 sun duration, +0 Imaginem requisite, ending with a MuMe(IgIm)30, that seems more reasonable for what you're looking for. Its way higher level than the InVi or InMe to detect spirits.
I don't think there should be a discount on the spell's magnitude just because you're transforming a fire to do it, heh. If the goal is just to know the spirit is there, rather than to make it visible, you may be able to argue that altering a fire to react to spirits might cause the spirits to give shadow, or a haze similar to a heat wave as the firelight interacts with the immaterial.
I don't believe that a spirit-ghost would 'give off' species naturally, and I don't think Second Sight should be species-dependant. YSMV of course, but that's what 'feels' right to me. MuMe(Im) transforms the spirit into a spirit that gives off species (via Imaginem). If you cast a MuMe(Co) to turn an immaterial ghost into a body, you don't need to add Imaginem, becuase the body gives off species naturally.
Nightwalkers interact with spirits that have real shapes. Lots of them.
So here's a rewrite boosting the level so that it doesn't outshine the MuMe option, pointing out that many spirits don't have shapes and would this be unaffected, making magic realm specific, and adding shadows cast by spirits.
Earlier I said:
The answer to this query is that magical light is itself doing something magical, while venomous drool is doing something mundane. It's more akin to something like the drool growing wool on things that it touches.
I'm not making a breakthrough required. I'm not yet sold on the intellego requisite but more open to being convinced on that issue.
Shadows do bring up another difficulty however. With the reflected light only version it was pretty clear that magic resistance applied (at least it was to me). While one could alter a fire to create light that reflected off of immaterial spirits light that reflected off of magic resistance that it couldn't penetrate seemed silly on the face of it and was never seriously considered. However it seems to me that whether or not my enchanted light penetrates magic resistance it is still unlikely to get through the spirit and will still leave a shadow. How would you judge that?
How's that?
The section about immaterial faeries ROP:F p.48 "Immaterial faeries... can pass through solid objects. Some can interact with humans simply by shedding specied with glamour." This implies a surface to shed pecies from and gives the impression that glamour is like aether- immaterial but posessing volume and shape- a shape which their bodies fill in when they gather incidental matter to themselves. Note that faeries appear to have an even less material form which defines them as they take on or shed roles which are defined by its glamour...
This is a much cleaner setup, mechanically, and doesn't feel like it overshadows (hah, Ignem joke) the 'normal' methods of it. The fact it doesn't 'define' individual spirits clearly (just reflects) and the complexity magnitudes makes it feel on par with MuMe(Im); The fact it requires a spirit to have a form to be illuminated means it doesn't beat out InVi. I like the shadows aspect, but I'm biased on that...and spirits may be able to hide their 'shadow' in many circumstances with torchlight, et cetera. You may also be able to make a MuIg(Me) to reveal all different realm ghosts but not other spirits... hmm. Possibilities.
I find the spell incredibly cool, and thematically appropriate (fire/light should reveal hidden things!).
I would say Base 5, to keep it in line with InVi and InMe creature/spirit locators guidelines. And then you could eliminate 1 complexity magnitude, keeping it at the same level, so same magnitude.
Now I wonder if this an Unnatural Fire spell (Yes Focus 8) ) or an Unnatural Light spell (no Focus 
)? It seems that the Unnatural fires Focus is extending into Light Focus territory? But perhaps there's no issue with an overlap, and it could be both.
I'm wondering if it should be Mentem or Auram, not Vim. Here is my thought process:
versus
So maybe it should have been Auram, not Mentem in the effect mentioned in #1.
That gives me MuIg(Au or Me) base 4, +1 Touch, +2 Sun, +1 requisite for MuIg(Au or Me) 20. Are extra magnitudes needed for complexity? I would think so if you want it to treat the spirit as reflective. So maybe MuIg(Au or Me) 25. Still have to figure out the Au v. Me bit, though.
Jusr as Ward Against Faeries of the Waters ReAq, and Circular Ward Against Faeries ReVi are both valid spells, I don't think that a mentem ghost specific version invalidates this spell.
I chose base 4 because it is "change a fire so it is completely unnatural" When someone reads the spell they'll go to the MuIg guidelines first I think that this will be clearer. As you said, the level doesn't change.
Now that I've got a few ok's I'm going to copy it back to the other thread here:
edit: I wanted to add how that I appreciae how great you all are. Every post here is foused on making a better spell. thanks
I have an additional question sort of related to this so necro'ing the thread. As the MuMe(Im) guidline allows spirits to be made visible (in raw), and the spell discussed here uses MuIg(Vi) (or arguable Mentem) to make spirits detectable.
Is it also plausible to CrIm a ghost if you can target it with magic?
If so, I assume it does not need a Me or Vi requisite? As most CrIm spells don't need the form they are affecting.
Something like this?
The spirit would have to have a shape, which is an issue that we've been confronting with the fire spell, but otherwise I can't see why it wouldn't work. I don't see a complexity modifier for a moving image being appropriate, the image is moving itself. It's more or less spraying magical paint on your ghost.
That’s awesome Erik.
The use I have in mind is cast a CrIm spell targeting Intangible Tunnels in a room to make them visible, in the same way a PeVi can target hidden demons in a room. Once painted the people in the room can target the tunnel.
Hmm, I get the sense that it shouldn't work based on my understanding of Creo and tunnels - intangible tunnels don't have a visible species, and using creo to create species doesn't mean they're from the intangible tunnel. Creo shouldn't be able to give the intangible tunnel the species. This spell definitely makes the tunnels 'visible', but I believe knowledge that they are there isn't enough to use them to cast spells through them.
EDIT halfway through:
This could just me being off base on interpretations though.
That's a nice question. Filling a room with "paint all of the arcane tunnels in this room with the fur of a golden gopher" effect doesn't have the same simplicity of "destroy all of the spirits in this room". There is some point of complexity where the spell is apparently thinking for itself, at that point the answer becomes - no you can't do this. Filling a room with creo imaginem-ness that only "sticks" to arcane tunnels is right on the edge.
It would be easy for me to say yes if the spell had intellego and vim requisites, or if it's just painting the tunnels that the caster is already aware of, but without something like that I'm thinking no you can't do it. It's tough for me to to form a convincing description of the action of the spell where the spell isn't deciding what is and isn't an arcane tunnel.
(In the process of writing this post, my opinion changed from "yes why would that be a problem?", through "I guess that's kind of questionable", to "no, you can't do that".)
Edit: With regard to Racoonmask's issue of, to paraphrase,-you're not sensing the tunnel, you're sensing the spell that give's the tunnel an image, I say this strikes me as adding unnecessary and unwanted complication. My guess is that an interpretation like this will make players frustrated and I don't see how being very restrictive about what constitutes "magically recognizing" adds to the experience. If someone is covered head to toe in bodypaint, can't a magus cast the spell Words of the Unbroken Silence on them even if they can't directly see the target's body, much less their mind?
For the most part, I actually agree with you. The limit of Arcane Connection (paraphrase because I've closed down my pdf for the core book) is that you have to be able to perceive something to target them. Covering someone in paint is a way to perceive them. The only reason I bring up the issue for Intangible Tunnel is the description stating you have to be able to 'magically recognize' the tunnel to use it. Painting it red with CrIm is enough to see it, to dispell it, and to possibly cast magic at it. but to utilize it as a conduit for the spells, do you need to be able some sort of magical perception of it.
Also to clarify, this is more a thought about it, if my SG said that perceiving it via CrIm is enough, I would be happy, because it makes it easier. If they said that you need magical awareness of its workings to use it, I'd say that's fine.. really, I don't have a strong opinion, I just noticed a point where my group would probably argue over it. 
Hi,
If the casting magus can see the spirit, even though I objected to the other spell I don't object to this one. The magus can see the spirit, which means he can target it and which means the spirit has a location and which means that the spirit is not intrinsically undetectable. So CrIm targeting the spirit seems ok. CrIg targeting the spirit is also ok, but the fire won't do anything.
(This is very different from MuIg to modify a fire to burn something intrinsically unburnable, or to modify a fire into a fire that knows what the local bishop is thinking and whose crackling flames speak those thoughts.)
Anyway,
Ken
I’d be ok with needing the requisites, it seems incongruous to the Target and also to other CrIm spells though.
I’m not convinced that the requisites are needed yet though, as I think Target: Room spells already seem to have the ability to differentiate without In requisites due to the way Room works.
How is that PeVi spell able to hurt the demons and not the fae in the room? Or the Magi in the room?
How does the spell tell a lamp from a demon?
One way could be to suggest that the wording from T:Room is involved (paraphrasing) as “the room and all its contents”. If so, then everything in the room must be momentarily affected, and creatures with MR will know a spell was cast as it tries to penetrate their MR.
previously I would have said that the fae and magi in the room wouldn’t know the PeVi demon spell was cast, but with this view they must have
This changes usage for Room spells (well my understanding anyway). Probably also changes T: Boundary and T: Structure spells too.
I said I was torn on this. Here's me trying to interpret what my gut is saying :
It is easier to imagine filing a room with a force that destroys infernal might it drains infernal might from everything in the room; lamps, faeries, everything, a force that only creates an image on a specific sort of magical effect seems more specific, it doesn't create the image on everything.
Detecting the spells in the room and only marking them (even if our spell targets any magic not just arcane tunnels) seems to be an intellego vim action -a creo imaginem effect could even be cosmetic to an InVi spell that does this. That's the big thing the spell is doing - detecting tunnels not creating images.
The spell discussed earlier in this thread that alters the output of a fire so as to illuminate spirits is working with a fire which is already producing light and it has a vim requisite. It is a somewhat different situation.