My modest entry on the weekly.
This is what I saw first, despite the keyhole shape being generally the imediatelly obvious. Just wanted to play with the idea a bit.
Just to be a little clear on the rules here, if you take the peak frame of the lighthouse (maximum light), and paste it over the original keyhole shape, it'll fit the exact limits. Even though I don't like this "halo" light (I'd prefer making it "beam" based, like real light would do), I made it so the keyhole shape can be obvious.
EDIT: Polished the light a little into something more realistic.