As an MoT tester, I get to see quite a few aftermarket HiD setups. Some have great beam patterns, some just acceptable and some plain unacceptable. The fact is, a headlamp reflector is designed to focus light from a finite spot in front of it, which is where the filament inside your standard bulb is positioned when correctly installed. If the globe of the discharge lamp is positioned on the same point, it will produce an acceptable beam pattern, otherwise the light will scatter everywhere. This is even the case if you install a standard bulb incorrectly. I'm forever re installing wonky bulbs in order to get cars through tests.
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