Peter White says we don't have to worry about blowing out our lights with too much voltage because there is voltage cutoff circuitry. Is that in the light? I hope so. My 30- or 40-year old German-made Schwinn Approved™ bottle dynamo puts out 15 volts at fairly low speed. Is this typical? I've run my modern B&M headlight on it without anything blowing.
I dunno specifically, but the dynamos put out AC so it's somewhat more complicated than DC. Depending on your meter and if the load, you may be reading a no-load peak voltage rather than running RMS voltage. It is possible that the dynamo has a protective Zener that will short it out above some level, like 15V but that would be wasteful of you power if you reached that level in use. If the headlight is LED, it almost certainly has its own regulator and protection, but again I don't know specifically.
I generally discount what peter white says but in this case he's correct, led runs on dc current and requires a regulator circuit. However, if your dynamo does put out 15v, you need the proper regulator circuit to drive the led, otherwise the circuit will fry.
OK, then I take it that the regulation is in the light, not the dynamo. As I've said, I've run the light with no problems. And Loogi, I forgot about load. You're right that I measured it without a load. I'll put a load in and see what I get. As long as I have enough light, I'm OK, but I wonder if producing too much power is why it's so much draggier than a hub dynamo.
most high power LED's can withstand over 1 amp of current, voltage doesn't matter as long as it's DC. Most dynos put out less than 500mA of current. Therefore, no regulation is required, but lights need to ensure that the LED does not see reverse voltage.