The "projector" headlamps you find on eBay are total junk. The light control is very limited, the included bulb is trash, and they are not technically
road legal anyways.
light control is where the light goes, does it scatter or is it aimed. DOT is usually unaimed (newer ones can kinda be aimed), scatters light which is "okay" with halogen bulbs, but complete trash once you drop a HID unit into it...
Euro codes (if you look at the pic) have lenses designed to send most of it onto the street with a slight patch of light upwards to illuminate signs and pedestrians at night...
Check out: http://www.danielsternlighting.com/tech/aim/aim.html
He's forgotten more about it than I've learned
DOT Approved DOES NOT
indicate a specific way the light is shaped. All the DOT Approved stamp indicates is that the lamp won't kill you or harm you or others in any way when used as directed or intended.
All lenses aim the light, just some do it a lot better than others. Lenses are designed to focus around 95% of the light on the road in front of you, whereas the other 5% is direct upward at road signs off to the right side of the road, as well as straight above for overhead signage.
When you install HID bulbs into a lamp designed for a Halogen bulb, you change the optics completely. HID bulbs create an arch of light around the center of the bulb, whereas Halogen bulbs do not. This arch of light radiates off of the bulb differently than the light coming off of a filament. Ontop of that, the length of the HID bulb is longer than a halogen bulb, so again, the optics are changed (right into a massive flood light that limits your viewing field and blinds oncoming traffic- too bad all of the idiots around here don't realize that, maybe if they spend just a little
time researching before they did it then we wouldn't have issues). The light source is radiating differently, and the point of origin is further out from the lens than designed. Try messing with a Maglite and you will understand. Lenses are PRECISELY engineered for the type of bulb that will be shining inside of them. If you find a fluted lens or aimed optical reflector designed for HID bulbs, then it won't be any worse than looking at every other car in oncoming traffic.
Offroad lights are not as affected, as they typically uses a symmetrical fluted lens, or a symmetrical reflector. Those fluted lenses on our headlights (or all of those fancy bends inside of the reflector if you have a clear lens) are what focus the light on the road ahead and create the optical light pattern. That's why they look a bit funky and have all sorts of different patterns on them.
FWIW, I have never credited DSL as being entirely accurate, as his information is relatively outdated.
So, now that we are all up to speed on the technicalities of light sources and types, maybe we can get back on topic