For a long time I have allowed the InIm spell discerning the images of Truth and Falsehood to see through , PeIm invisibility spells. I recently discovered that my interpretation was not universal 
How do other people interpret these spell interactions and why?
It is not an altered image, it is a supressed one. there is a difference here. IMS you need Second Sight to see invisible things. However an InIm or InVI spell can detect them using the quasi-loophole of "what would be there if there was no magic?" that discovers a dark shape where there used to be nothing when parma stops the effect. That is one way of doing it. The other is to simply throw True Sight of the Air and then casting something like a dust whirlwind in the area. Pinging dust in your body, mate. 
. The rain of ink has also been discussed in my group, but nobody is that proficient with aquam to make it a workable spell.
Cheers,
Xavi
I share Xavi's thoughts. My personal favorite is Eyes of the Bat... well, Second Sight ranks up there, too.
Chris
HoH: S contains information on invisibility in combat and how it might be defeated (see pp. 32-33).
Alternatively, you could try a Creo Herbam spell to create a rain of flour in the area, which would mark out the invisible character with a light dusting (hmmm, would magically created flour "rest on" the invisible character's Parma, or "slide off"?). Likewise, using magic to cover the floor of the area with flour would enable you to spot footprints.
I imagine that the most powerful (or paranoid) covenants would employ a number of non-Hermetic or non-magical devices and employees to defeat invisibility. The following spring to mind off the top of my head:
Floors covered with flour, as suggested above.
Squeaky floorboards, or other things on the floor to create noise when they are stepped on.
Grogs with Second Sight, Magic Sensitivity, etc.
Hounds, especially pointers and sniffers.
Buckets of paint, flour, etc. in corners, within easy reach for people to throw around to pick invisible characters out.
I hope that this helps.
Eyes of the Bat is my favorite zanshin spell. No fine detail but you know of everything in the room.
Hum... Wouldn't this be a CrIm? I'd like to see your reasoning.
As per InVi, I'd rather say any spell to detect active magics will detect you easily.
You're also vulnerable to mundane detection, through the shadow you cast, the traces you may make... You opponent gets a Perception + Awareness contest vs your Dex + Stealth + Variable bonus (it's easier being invisible in a castle than in the mud, for exemple 
)
In that case, sounds like a parma variant blocking InVi from seeing your personal active spells would be an active topic of research. I agree that stock ArM5 implies it doesn't.
I think that Eyes of the bat altered to have target sight rather than hearing is pretty much the way to go. I suppose that you could go one better and develop an even higher level spell where you perceived all of the air currents in the room in such a way that an area of magic resistance that you couldn't sense might stick out like a hole a three dimensional volume of absolute darkness in that way it wouldn't matter whether or not you penetrated.
It does indeed appear that I am in a tuny minority in beleiving InIm can detect people using PeIm invisibility , which means I am incorrect.
Thanks for the alternate ways to detect illusion , that was not the problem it was the fact that I believed InIm spells would defeat the invisibilty spell and have been playing that way for years
I'm going to be the fly in the ointment here again, but Eyes of the Bat as an InAu has always grated my nerve; ignore the next few sentences if you've changed it to MuAn in 5th. My argument is this: I believe it violates the medieval paradigm, and not just a little. There is no such thing as "sonar" or "pinging" in the Middle Ages. Bats can see in the dark...with their eyes. Medieval folk knew this from observation, but I don't recall ever reading any medieval scholars that made the connection between bat squeaks and their ability to see in the dark--and I have read extensively on medieval history. Now I could be wrong and just never read the right book, but if I'm right then Eyes of the Bat must be a MuAn (req-Au) spell.
This idea is creative, and interesting, but I have serious problems using InAu as a sort of sonar. Along this line I feel very strongly that this is a modern way of thinking; it's a magical ultrasound mapper, and it strikes me as wrong in the paradigm. I can see using it to detect very broad things (like the walls of a room) but allowing an InAu that paints a contour map of sufficient detail to pick out a person strikes me as too much 20th century. I would make you cast a very high level spell to get the kind of detail needed to locate an invisible person if I was SG.
I like the CrHe to create a cloud of flour, or any variant (CrTe to make dust, even a CrAq to spray water and look for a silhouette of droplets). I think it's eventually going to be easier to use clever but mundane methods rather than direct magic, unless one has Sight of the Active Magics or Second Sight.
The name has always annoyed me as well for a target hearing spell, but the spell isn't sonar. The spell allows the character to literally hear the edges of the winds , the places where air currents end make a noise detectable to the caster. In its current incarnation as an InAu spell the spell is fine, the name is stupid. there is already a MuCo(an) spell that lets folks see in the dark its called Eyes of the Cat.
Ignore the silly name, my suggestion has nothing to do with sonar does that make a difference to you? It's a spell that makes the air currents visible to the caster. This makes an area where he can't see the currents stick out like a sore thumb. It also means that his distance vision is going to be totally obscured by the moving air between himself and the distance and yes I'm thinking of giving it a ten level bump for giving the caster sufficient detail to discern holes caused by folks with magic resistance.
In fact let me show you what I'm thinking (hopefully the fifth edition format won't throw you too much)
Eyes of the Butterfly Navigator
InAu 25 (edited, thanks Xavi)
R: Personal, D: Concentration, T: Vision
All of the wind currents within the sight of the caster even the most minute and weak appear to the caster as softly glowing translucent lines. The caster is able distinguish where the currents begin and end. This makes invisible objects clearly visible to the caster as patches without any air movement. Likewise the caster is similarly unable to see any air currents within a volume of space protected by magic resistance that the spell does not penetrate. The translucent lines of air currents will obscure other objects within the caster's vision. In a very calm area such as a tightly sealed room the caster sees as if he were in a light fog. Outdoors on a still day the caster sees as if he were in an area of fairly heavy fog and has difficulty making out objects beyond 30 paces, is a moderate wind the caster will be able to see no further then 10 paces. In a fairly strong wind no further then five paces and in areas of exceptionally strong winds such the caster will be completely blinded.
(base 2, +1 Concentration, + 4 Vision, +2 magnitudes exceptional detail)
That's lower level than I thought (sensing one property of air is only base 2), it may need to be subject to another magnitude bump on "central rule" grounds.
I've used CrCo to make a cloud of dandruff in a game before (it was the character's best form behind vim and mentem). The problem is that magical material (dust, dandruff, flour, what have you) is going to be kept from touching a magic resistant target so the effect is only temporary (of course the protected creature will leave clear footprints in the magical material). So I agree, non-magical methods are going to be one of the better ways to go.
Nice one. Way less powerful than Eyes of the Bat as written. Also, it is level 25, not level 20, but hey 
Given the range of the effect, T:hearing should be enough and drops the level to the actual level 20 Erik listed. Dunno if this will grate you, but sensing the HEAT of the target (that is a species and so not blocked by MR) is what we generally use. Combine eyes of the Cat (no flying blood suckers for us!) with detecting heat spells for the invisible dudes. Works great for humans and animals, even if some faeries that are basically cold trump us on a regular basis.
Cheers!
Xavi
If sensing the heat is really sensing species, then it would still be an InIm spell right? Not InIg? Seems a viable method of seeing invisible living beings. Is this in accordance with the medieval paradigm? It would seem reasonable that natural philosophers do know that humans and animals are quite hot to the touch, so sensing the heat would be ok. But how would it work in practice, do you need differences in temperatures for this to work? And if so, how much? Or is this too much "D&D Infravision"?
About using InAu, what if the spell doesn't sense air currents but the presence of air? The air is present in all empty spaces, and objects, walls, people are seen as 'holes' in this air. In this case it would work regardless of wind speed. Perhaps the spell even shows different air types as different colurs, so smoke, poisonous gas etc. would show up differently than ordinary, clean air. And air with lots of dust or debris would have lots of specks of 'empty' showing up.
Being able to hear something in my opinion is not the same as being able to see something. I can't imagine being in the open and having sound coming from all directions that weren't inside of the trees, ground, or one human sized shape 7 yards away and being able to exactly point out that shape.
I don't think that eyes of the bat or even a target vision version will spot an invisible person when you don't penetrate their resistance. With eyes of the bat you're hearing the borders of the air, these borders are (as far as I see it) clearly within the parma.
You'll have to convince me that heat is a species.
While it seems sensible I think that using auram to effect generic air rather than only phenomena such as winds, air currents, smoke, poisonous gases. Is eventually going to lead to parma issues that I won't like.
Well a rare few blind people have managed to get to a level roughly thereabouts. So at least its possible. Considering the extreme rarity(and that many more have actively tried to learn it than have succeeded), its likely exceptionally difficult though...
I'd rather not try, since I don't think so either. Heat is a natrural property of a fire, and therefore covered by Ignem. Just look at "Visions of Lights Heat". IDHMBWM but I have the 5th ed spell list as well as the 4th ed core book pdf. Both list the spell as InIg20. 4th ed has the spell give the target the ability to see targets of human temperature or higher. I'm guessing 5th ed version is about the same. Not good enough to navigate safely in darkness, as most inanimate objects, terrain, walls etc. won't show up. But good enough for spotting an invisible magus. Penetrate? Sure, it is as invasive as any other Intellego spell trying to sense the target.
Just my two mythic cents.
Correct me if I'm mistaken but Eyes of the Cat requires some ambient light, but I really don't know for sure about the medieval perception of feline night vision. If someone knew of a medieval bestiary that described cats as having night vision that worked in total darkness then that changes everything. We've always ruled this spell requires ambient light, but the medieval paradigm always trumps our modern knowledge. As far as making InAu to make the "edges of air" hearable to the caster I would think the result would be very crude and insufficient to detect a man-sized object unless it were very close.
More promising line of thought here, but again I think it would be very hard to get much "resolution". Not impossible, but crude, and no detail.
Explained better now, yes it does make sense.
This is actually a very beautiful, elegant approach, and I like it. It doesn't offend my medieval sensibilities at all, falls perfectly within the range of magical power and such, and avoids the "magic ultrasound" problem with artistic flair. Bravo!
You only need a moment to glimpse a "shadow" of the obstruction in order to see it, know its there, and react. True, it won't "paint" the intruder with any persistence, but just knowing an invisible intruder is there is often enough to spoil a lot of their advantage. (Your mage may have used blood instead of dandruff, but maybe he wasn't so good at Aq...). And no, the 5th ed format is still understandable in this Cro Magnon's bulgy, sloped head.
Another possibility not mentioned is using dogs. A past saga I was in was subject to a rash of invisible intruders and we tried multiple things, from strategically-placed mirrors (many invisibility spells revealed invisible people in reflections, though not all) to bells on the doors and we also invested in a litter of puppies and our grogs raised dogs. Not suitable for use in close proximity to magi but our watchmen patrolled the covenant with them, and obviously they can smell invisible people...of course, a mage who cases your covenant will just PeIm his smell, but it only has to work once.
My troupe and I have since time immemorial vetoed the Vision of Heat's Light spell on "aesthetic" grounds, i.e., it is too much D&D infravision, but this discussion has made it more difficult to justify this position. You are correct that ancient natural philosophers were aware of body heat and all my research on the subject hasn't turned up any definitive argument for or against the concept of sensing heat. I think sensing heat is definitely possible, but how well and by what mechanic is a big problem. Like, does the heat appear as a glowing color to the caster's vision? If so, how fine-tuned will it be?
I think it may be possible to create a scale of temperature differentials that could be detected, in ascending orders of magnitude for each level of difference detected. But it would have to be fairly large differences, IMO, such as the difference between a candle flame and a torch, a torch to a campfire, campfire to a bonfire, etc., at least at the lower levels. In no way, shape or form would I allow a magic spell in my saga that allowed a mage to see the temperature gradient of rocks, trees, earth, etc. and so have an infrared map of his surroundings. But a good-sized spell might be allowed to detect the heat of a person in a cold room, but not a warm one. I just can't abide "infravision".
I too hate the idea of D&D infravision spell, it just doesn't fit Ars magica IMHO. Neiter does a spell showing subtle temperature differences by different colours, like a modern thermographic device.
But I can accept this spell as letting you see things as hot as a human or hotter, as red pots, in an otherwise dark area. Not good for navigating or seeing properly. But fine for detecting an invisible target.
But what does the medieval paradigm say about light cover in from of this heat source? Modern thermo-vision easily sees heat sources through light bush or scrub, or even sees the hot foorprints of someone who just walked by. IMHO the spell is LOS based, so if light or regular vision is blocked by the intervening terrain, so is the VIsion of Heat's Light.