No dark rings at all. Just pure bright white light. Are all Luxeon's this way? 25 hours runtime on 3 AAA. Seems so far to be true.
Luxeons typically have a smooth beam, even with a smooth reflector. The only high-flux emitter known to commonly shows artifacts in the beam is the Cree XR-E, and that shows up as a dark ring between the spot & spill. This can be somewhat smoothed out with the use of a textured reflector, which also defocuses the spot slightly, reducing throw.
Cree has remedied this more recently with the XP-E and XP-G emitters which are more current technology. The current tech in emitters is much more efficient than a Luxeon, which can increase output, runtime, or both. The Luxeon Rebel is competitive with the Cree XR-E and Seoul Semiconductor P4 in efficiency, but the Cree XP-E and XP-G emitters have surpassed Luxeon's current offerings.