Thanks for all the advice and comments, guys. I like the idea of the illumination area of effect.
And I definitely won't be putting any penetration in it! 
Cheers,
Mark
Target illumination sounds really cool. Nevertheless for me it seems pretty the same as a circle/ring combination. The only difference is that the magus does not need to draw a circle or to pace off the circle.
Salvete
Widewitt
There are two traditional definitions of a "pace":
- the distance between the right foot-fall and the left foot-fall that immediately follows, and
- the distance between one right foot-fall and the next right foot-fall.
(One could swap right and left, of course.)
The (1) definition is about ¾ meter, and is the traditional Byzantine pace; the (2) definition is about 1.5 meter, and is the traditional Roman pace, or five Roman feet.
There are at least three additional paces one might reasonably use:
3. one modern yard (91.44 cm),
4. one modern meter, and
5. one modern marching step (about ¾ meter*).
Which do we use? I'm not convinced it's terribly necessary to define which, except to be sure that everyone playing at a given table is using the same definition. Even then, it's reasonable for it to be a fairly squishy unit, since magic is flexible and a lot of other units in Ars Magica have fairly squishy definitions too.
- According to a school friend who was in the ROTC in the early 1980s, a marching step was precisely 30 inches (76.2 cm). Since precision marching is such a critical military skill , it was a big part of the grading in his ROTC classes. It's possible that the US military has redefined the marching pace as part of its shift to metric units, so it may be 75 cm now. On the other hand, maybe it's defined as 76.2 cm now, changing the defining units but not the distance. (Nothing in this footnote has anything to do with Ars Magica, of course. The only point is to indicate that something closely related to the Roman pace remains in use in the modern world, just as some other Roman units remain in use.)