Penumbra wedges are exactly what they say they are, wedges of the penumbra (if you want to know more look it up . Modifier volumes can tell you if you are in such a wedge, not how deep.
Are you talking about #1? That one's got serious problems in trying to construct the wedges, as adjacent wedges must have identical side planes, and that's hard to enforce with concave geometry. Even concave objects run into problems once wedges from opposite sides overlap (i.e. when the umbra starts to disappear).
Read up on paper #3. It's got a really cool way of determining how much of the light is visible.
Ah, so that is #1. From Eurographics Workshop, right? Check out #3 on the page I linked to above. #1 is a very flaky solution not suitable for general scenes, as stated in #3.
. Modifier volumes can tell you if you are in such a wedge, not how deep.
