You may find some theory at Peter White's website and at the Audax U.K. webstier (some references haven't been updated in the last 2 years).

Essentially, your generator always produces the same intensity, 0,5 A. Without a bulb, you'll measure up very high voltages, and some indeed have measured up to 110 or 120 V, because the generator is still trying to produce its 0,5 A. Now I suspect that if you had measured the voltage (at the same speed) with a functionning headlight, you would have measured 10 or 15 V. Quite less, but still way too much!

So the real solution is to put some kind of voltage-limiting device on the lamp. You either design your own, or you buy a controlled headlamp such as the Lumotec which has a zener diode to protect the bulb from over voltage.