Why Do Headlights Go Bad When You Touch Them?by Doug Leenhouts
Even the cleanest hands are dirty things -- at least as far as your headlight bulbs are concerned. Mother Nature may have seen fit to enhance your paws with a few essential oils, but your headlights definitely won't appreciate them.
HID and Halogen Bulbs
Many automobiles use halogen light bulbs in their headlights, which can get extremely hot. The heat is transferred uniformly through the glass casing of the bulb, through the headlight casing, and finally into the atmosphere. However, if there is any oil on the glass bulb, the heat transfer will not be uniform, causing the glass to expand at nonuniform rates. This is especially critical for HID bulbs, which run much hotter than typical halogens -- and halogens are already extremely hot compared to incandescent bulbs.
Due to the dirt and oil associated with working under the hood, precautions must be taken to keep the bulbs as clean as possible. The bulb should be kept in its packaging until the headlight casing and bulb holder are disassembled. New, disposable rubber gloves should be worn when screwing the bulb into place. Use new gloves for each bulb change, since dirt accumulates on them.
Even if you have just washed your hands with soap and water, the skin's natural oils can be enough to break a bulb under high temperatures. If you do not have any gloves handy, a clean paper towel may suffice, though you should minimize the bulb's exposure to the elements between its journey from the packaging to the sealed headlight casing. If you do accidentally touch the bulb, you can remove the fingertip oils with some rubbing alcohol and a clean cloth.
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