You could be more specific in your request. Is this a daytime running light or used to illuminate your path at night. You don't need a 500 lumen light for a daytime running light.

The only helmet light I have seen up close and one that gets a lot of recommendations is the Lights and Motion Vis 360 ($100) or Vis+ ($180). They are 120 lumens with a 2 hour run time and 250 lumens with a 3 hour run time. That's more than I would want to pay for a running light but it certainly is effective on a fellow rider who uses one. Having a wire dangling from your head to a battery pack is a PIA in my opinion and the battery pack for a 500 lumen light with a decent run time may be heavier than what you would want mounted on a helmet

I don't find a flashing light "not useful". That's what I always use for a running light.