I have an older Union headlight (battery powered) that takes a 6V halogen. Glad to see the links above! However, I did, at one point, try an LED. The only down side is some higher power LEDs get hot. My Union headlight doesn't have any vents or heat sinks and I ended up melting the LED bulb on my 3rd ride to work.

I'm sure you already know this...but be careful not to touch the glass halogen bulb with your bare hands when installing it. The oils from your fingers may cause the bulb to burn out. I also did that on a $7 replacement halogen bulb for my Union. Some bulbs are made better than others.

Jerry H