How would you create a spell to see through certain matter?
For instance, a spell to see through plant matter so that you can track people under the tree canopy while flying above, or a spell to see through stone so that you can see any secret passages or compartments otherwise concealed in a cunning fashion?
Opacity is a property of objects, so the art should be Muto. I saw a character with a MuTe spell to look through walls, with a base of 5.
The problems are the targets of these spells you suggest: turning all the stone walls of a chamber or specially the trees in a forest trasnsparent may require some serious size modifiers.
An alternative to see people in forest while flying over could be heat vision, Vision of Heat's Light should do the trick. And to look for secret passages you may design a InTe spell to get an idea of the map of the room, which would help to know where are the hidden passages.
Or you could just crumble the walls to look behind them and burn the forest to remove these disturbing trees!
Intellego (form) spells, such as True Sight of the Niad (its in InAq in core), to see clearly in water.
This seems like the best match to your description and has a sample spell in core to support it.
Perdo Imagonem, with target Part to destroy the visual image of the barrier or object. Problematic for large areas but very good for small visual windows. I think there is a Window based spell on the HOHTL book(?).
maybe the Muto as said above, but I'd like to see a similar cannon spell to verify and I can't think of one. I'm dubious.
Vision of the Forest Without the Trees
Intellego Herbam 15, R: Personal, D: Concentration, T: Vision
This spell allows the caster’s vision to be unaffected by both living and dead Herbam based materials. The effects extend to trees, shrubs, fences, building, or weapon hafts, which all appear translucent and ghost-like.
(Base 2, +1 Conc, +4 Vision)
InAu - Unhindered by air is base 1.
InAq - Unhindered by water is base 1.
InIg - Unhindered by a raging fire is base 4 (with an Auram requisite if smoke is present).
InTe - Unhindered by earth is base 20.
InIm - Enhance your senses in one way is base 3 (see far, see small, see in the dark are examples).
A similar "unhindered" InHe guideline should probably be closer to the InTe in level than the other 3, maybe base 10 or 15?
Hard question, I'm not sure. I chose base 2 considering InTe "the Miners Keen Eye" which uses base 2, as Herbam doesn't have a base. I can't explain how that spell and the Base InTe20 line up in core, something isn't right.
I would err towards Base 10-15 for the following reasons:
Seeing through air (mist/fog) is naturally doable, except in case of thick fog/clouds, etc; you could even say that seeing through air is the most common state (ok, it depends where you live )
Seeing through water is also naturally doable and in optimal condition you can see quite far, but never as good as in air (in "normal average conditions"), but there is far more occasions where vision is limited or even obstructed in water compared to air;
Seeing through flames is doable, but starts to be more tricky as the "normal average conditions" of a fire tend to at least partially obstruct vision;
Seeing through earth/stone/metal is never possible under "normal average conditions", thus a significantly higher base level seems fair;
Seeing through wood is never possible under "normal average conditions", but seeing through leaves is somewhat doable, so it should be slightly easier than seeing through earth/stone - so InHe base of 15 sounds appropriate - three magnitudes above the InIg, one below InTe.
By the way, if you use a MuHe or PeHe to change the property of the woods, it works both ways, whereas an InHe will only target the mage.