After the rather disappointing results of the truck side marker LED light, this seems more promising:
The 1 watt amber LED is soldered to an aluminum heat spreader stuck to a scrap heatsink with thermally conductive tape. The PCB is a buck converter build around an MP1584 regulator. The lens on the left claims a 5° beam angle, which seems aspirational at best.
Not counting the heatsink, you’re looking at less than three bucks of parts; living in the future is great.
Fitting the lens over the LED produces a shatteringly bright beam, at least in the Basement Laboratory:
The lens has a conical cavity surrounding the LED lens to capture the light and redirect it to the beam forming reflector. It’s done with total internal reflection, there are no coatings, and it’s a wonder to behold: one-shot molded aspheric optics at work.
Not seating the lens firmly against the LED produces a dark spot in the middle of the beam. I soldered the leads directly to the LED and cut out the sides of the black lens holder, as soldering them to the convenient side pads would prevent the lens from seating properly.
The LED drops about 2.5 V at 430 mA (1.08 W). The bench supply delivered 6.3 V at 190 mA (1.2 W) to simulate the headlight output of the Bafang motor controller.
The headlight output is good for 6-ish V and 3 W = 500-ish mA, so burning half the power in a simple dropping resistor or linear current regulator is a Bad Idea™. You can get constant current LED drivers, but apparently not with 6 V input and 1 W output, so stepping the voltage down makes more sense. You’d want at least a little ballast resistor in there to soak up small forward drop changes with temperature variations.
The regulator can handle up to 28 V input and the tiny trimpot must cover nearly that range of output voltages, so the 2.5 V output jams it near the minimum end of its rotation (which is, of course, backwards). This calls for a fixed resistor to eliminate the effects of vibration on a trimpot at 10% of its range.