How Your Headlights Work
I’m willing to bet that drivers only think about headlights during one of two events: when for some reason they can’t see at night, or when an oncoming car blinds them. Like the alternator, these critical pieces are overlooked—until they don’t work. And that’s a shame, because for the gearhead, there’s a lot of interesting tech behind the glass. Like, for example, did you know that an HID headlight is like a lightning bolt in that a glowing arc of electricity illuminates the fixture? Plus, automakers are calling on headlights to increase not only safety but also fuel efficiency.
Knowing how your lights work will better prepare you to keep them maintained, so you’re more likely to see that deer before it runs into your path. Besides, the rapid evolution of headlight tech is interesting in itself. Here’s a primer.
The Old-Fashioned Way
The first cars used crude lamps fueled by either kerosene or (gasp) acetylene. About 100 years ago, the open flames were replaced with a small electric bulb housed between a polished reflector and a lens. These lights weren’t sealed well, so the reflector corroded quickly, making the already insufficient lights even dimmer—and worse, they provided plenty of glare to oncoming traffic. These types of lights were made illegal in 1941, a scant year after the introduction of the sealed beam.
A sealed-beam headlamp is nothing different from a giant household bulb, a tungsten filament housed in a glass enclosure that’s sealed and filled with inert gases. The reflector is inside the glass envelope. Like household bulbs, these gradually lose brightness as the tungsten evaporates from the filament and deposits on the reflector. Dippable high-and-low beams didn’t come along until the ’20s. Brightness and beam control in this era were inconsistent because of poor manufacturing tolerances. And the inside of the poorly sealed glass lens easily corroded, further reducing the brightness. Sealed-beam lights were cheap partly because they came in only three sizes—5- and 7-inch round and one square size—but more important, the standardized sizes limited styling differences among cars. Automakers started replacing sealed-beam lights with quartz-iodine technology in 1973.
Sylvania Silverstar H13 1. Quartz Glass 2. Support Strut 3. High Beam 4. Low Beam
QI is the predominant automotive lighting technology today. Legal in the U.S. since 1984, it uses a small bulb that resides inside a reflector/lens assembly. Thanks to modern sealing materials and techniques, the reflector is far less likely to corrode from moisture intrusion. The high-temperature quartz glass envelope allows the filament to remain at a much higher temperature, for a light that’s closer to natural daylight. The higher temperature means a lot more light for the power consumed, but also makes the tungsten filament evaporate and redeposit on the glass, gradually reducing light output. To combat this, halogen bulbs are filled with iodine or bromine rather than the customary inert gas. The halogen combines with the tungsten vapor coating the cooler glass, then disassociates when it touches the hot filament, basically redepositing the evaporated tungsten back where it started.
Manufacturing these cylindrical bulbs is very high-tech. After the filaments are sealed to the glass at the bottom, most of the air is evacuated from the top. While a propane flame heats the neck in the top of the bulb to a semiliquid state, a jet of liquid nitrogen cools the base to minus 321 degrees F. Then a pellet of frozen gases is dropped in. Instantly, the hot, soft glass at the top is crimped, sealing the envelope. When the temperatures equalize and the gaseous pellet boils, the pressure inside rises to 4 to 5 atmospheres. The H13 high–low beam lamp shown in the lead photo is the industry’s latest version. Computer vision systems carefully tweak the position of the filaments in each bulb as it’s assembled, maintaining tolerances within 0.004 inch—which means replacing a lamp shouldn’t require reaiming the headlamp. The high-beam filament sits at the precise focal point of the reflector, providing the best illumination up the road. The low-beam filament sits slightly off the focal point to spread the beam and establish a cutoff pattern to keep glare out of oncoming drivers’ eyes. Some quartz-lamp systems rely on the use of a metal shield to provide the cutoff pattern.
The color of bulbs is expressed as a function of the temperature of the light emitted. A QI bulb is around 3400 degrees K, as compared to natural sunlight, which is considered to be around 6000 K. Recently, we’ve started to see QI lights that have a blue-white color, not the usual warm yellow light. These are aftermarket bulbs with different filaments and glass coatings that attempt to emulate the blue hue of expensive high-intensity-discharge (HID) lights (I’ll get to HID lights in a minute). While these bulbs do raise the color temperature, they don’t get close to an HID’s 5000-plus K color. Oh, and they don’t necessarily raise the light output either. So what’s the point? Style—it’s one way to spend $20 and acquire the cachet of $2000 HID lamps.
Headlights: The Next Generation >>>