Originally Posted by
Caliper
Easy. Just switch from a simple reflector to a projector lens. Same way those super-bright movie projector bulbs have been aimed for decades and the current trend in cars as well. In a car it's like any other projector and behind the lens is nothing more than a parabolic reflector and a piece of metal that shapes the cutoff line. So, instead of projecting an image of Godzilla it projects the cutoff, giving lots of light below the line and very little above. Some cars have a small solenoid that pivots that piece of metal out of the way for the high beams. Beam width is controlled by the specs of the lens. Cars use this technology to control the light from a 3200-3500 lumen (per side) HID light as well as less powerful halogens.
Substitute a LED for the headlight bulb and add on a lever to switch high/low beams (hopefully with a little blue indicator LED to remind the rider their brights are on?) and it could all work just fine on a bike.
Actually, it's cheap enough to buy projector headlight assemblies off ebay and high output LED's... I've got a project idea now.
I don't think it'll be as easy as you think.
Halogen and xenon HID capsules throw out light in all directions except for the bulb base. LED's have a much tighter angle of light output. Since the halogen/HID light is coming out all around, the projector housing uses a parabolic reflector to focus the light forward, and a shield in front of the lower half to prevent the light from bouncing off the bottom of the reflector and up into the air above the horizon. With an LED there won't be any light (or not much light) going down onto a reflector, so a shield is not going to give a sharp cutoff line.