The Germans have always been big fans of using the latest and greatest technologies, and that definitely shows in their choice of lighting instruments. Before Audi rocked the world with high-intensity LED lights, BMW welded its place into lighting history with Xenon arc light bulbs like those on the X5 SUV.
A Xenon arc light is so-called for the inert gas inside the bulb. It functions not unlike a plasma welding torch or MIG welder, which passes a high-intensity electrical arc between two electrodes in a noble gas atmosphere -- in the welder's case, usually argon. The arc's color is often largely determined by the gas around it and the intensity of the electricity flowing through. As with most electrical arcs within a given gas, the exact color of the light emitted is determined by its temperature; higher temperatures yield more intense colors. For Xenon bulbs, that rating is in degrees Kelvin
A stock BMW X5 Phillips D1S bulb -- used through the BMW line and in dozens of Porsches, Audis, Volkswagens, Minis, Cadillacs and even the Chevy Corvette C6 -- is rated at 4,300 Kelvin. At this intensity, the light it casts is decidedly in the "white" spectrum, with a very slight blue tint. More powerful aftermarket bulbs in the 6,000k range are a more noticeable blue-white, and super-powerful aftermarket bulbs in the 8,000k to 9,000k range are definitively blue. Theoretically, you could install a 15,000k bulb, which would glow bright purple under full power and give folks in front of you tans; however, the BMW's electrical system won't put out anywhere near enough power to run 15,000k bulbs.
