The ability of the lens and reflector to focus light into a usable pattern depends on the light source being in exactly the correct, designed location. Even though the HID replacement bulbs are the same size and shape, the exact location of the light source may be slightly different. As a result, the light pattern won't be as carefully focused as designed. The intense light generated by the HID bulbs makes it even worse. The bulb, reflector and lens are designed to work together. Upgrading to a higher wattage halogen bulb works because the design of the bulb is the same for any wattage.

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