I'm actually looking at LED which limits my options unless I get a AC to DC converter. The LED bulbs I found that are automotive and they are AC/DC where they work either way the brightest is just .4 to .7 watts and puts out just 25 lumens. Hardly enough for a headlight not even for a tail light.
I can get a plain old T10 wedge based LED bulb that is 350 lumens but its a 12v DC bulb. The 350 lumen one has a comparable wattage of 35 to 40 watts. This would be better for a headlight. But have to work around the whole AC/DC issue. I wonder why he doesn't design his to have a rectifier to convert it to DC. I mean it seems strange to offer a generator that is AC. No automotive vehicle out there produces AC current its all DC and while yes conventional bulbs can run off AC or DC, why would you create the issue in the first place.
Ok, time for some good ol' "mansplaining"...
All modern automotive charging systems are a 3 phase AC "alternator".
Their output is rectified to DC (usually internally) to suit the battery and other components in the system.
3 phase is used because of its efficiency and it produces a very smooth (ripple free) DC current.
Alternators (which produce AC) are used rather than DC generators because for a given output they are lighter,cooler and more maintenance free.
Small motorcycles like my KTM offroad bike sometimes use a single phase alternator because they are simple, cheap and light. Because it has no battery, horn or turn signals, it can use the AC voltage to efficiently work the few incandescent lights it has. Because the voltage goes up with rpm it has a voltage regulator which is nothing more than a voltage limiter which shorts the current to ground if it goes over the 14.8v that the bulbs can withstand. To convert to DC is 1) not needed, 2) takes extra components, 3) reduces available wattage (power output) which is in short supply on small motorcycles and bicycles.
LED bulbs typically come in 1) a DC only configuration or 2) some have an AC capability.
You gotta test them with a meter to find which they are.
The AC bulbs have a fullwave rectifier circuit built into them, so they are usually more expensive and generate some heat. No free rides. LED bulbs will tolerate a very wide voltage range (typically 4v-32v on an automotive LED) but if you exceed that voltage even ever so briefly, it is gone. Unfortunately AC tends to have voltage peaks that exceed its average voltage, so 12v AC can have 40v spikes in it if you don't have a voltage regulator in the system.
I'd agree that about 350 lumens (which is about 35w incandescent) is about a minimum for a vehicle going 20-30mph at night. Still quite dim.