Headlight tech

  • Headlight tech Picture by Stuart Tyson
  • This tail/brake lamp from Sylvania is due to become industry-standard.
  • This 50 mm lamp, with those five tiny high-brightness LEDs in the centre, could soon replace incandescent or HID headlights.
Date:1 June 2011 Tags:,

I’m willing to bet that drivers think about headlights only 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. 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, manufacturers 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 wild animal 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 fuelled by either paraffin 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.

Sealed beams

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 (though admittedly these are being phased out domestically in favour of more energy-efficient equivalents). 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, but more important, the standardised sizes limited styling differences among cars. Manufacturers started replacing sealed-beam lights with quartziodine technology in 1973.

Quartz-Iodine

QI is the predominant automotive lighting technology today. 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 vapour 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 semi-liquid state, a jet of liquid nitrogen cools the base to minus 196 degrees. 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 equalise 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 0,1 mm – 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 lowbeam filament sits slightly off the focal point to spread the beam and establish a cut-off pattern to keep glare out of oncoming drivers’ eyes. Some quartz-lamp systems rely on the use of a metal shield to provide the cut-off pattern.

The colour of bulbs is expressed as a function of the temperature of the light emitted. A QI bulb is around 3 400 degrees K, as compared with natural sunlight, which is considered to be around 6 000 K. Recently, we’ve started to see QI lights that have a blue-white colour, 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-intensitydischarge (HID) lights (I’ll get to HID lights in a minute). While these bulbs do raise the colour temperature, they don’t get close to an HID’s 5 000-plus K colour. Oh, and they don’t necessarily raise the light output either. So what’s the point? Style – it’s one way to spend a couple of hundred bucks and acquire the cachet of HID lamps costing thousands.

HID

HID bulbs don’t use a tungsten filament. Instead, they pass a high-voltage electrical arc through a partially evacuated chamber that’s filled with noble gases and mineral salts. A ballast/igniter is necessary to provide the high voltage and high initial current. However, once the bulb is started, it uses far less energy and gives off more light than the QI lamp. Plus, the arc projects a crisp beam, with an easy-tomanage pattern.

One disadvantage: it takes a few seconds for the arc to warm up to reach full output – that’s a problem for using these bulbs as high beams, which need full brightness instantly. Some vehicles use HID low beams for their sharp cut-off and glareless light, and a conventional QI bulb for the high beams. Other high-end cars move a mechanical shroud inside the reflector to change the pattern of a single HID lamp to suit both patterns. Those work-arounds make these lights expensive. Despite all that, Sylvania’s director of aftermarket sales and lighting, Jim Sanford, says HID lamps will soon represent 40 per cent of the OEM market share in the next three to five years, up from only a few per cent.

The future belongs to LEDs, which are already in use in some brake lights. Manufacturers are just now figuring out how to use these devices for headlights (only the Audi A8 uses them so far). Since there has been no standardised LED bulb to plug into a reflector/lens assembly, car manufacturers have had to design a unique PC board for every model, an expensive proposition. No more. Sylvania has designed a universal tail/brake light bulb that’s already used in the Ford Mustang. When someone succeeds in doing the same for headlights, the LED’s benefits – lightweight, vibration tolerance, long life and ultralow power drain – will quickly make even HIDs obsolete. But that future is at least five years away.

Next up
LEDS

We already use LEDs for taillights. Aside from giving engineers the ability to design lights with substantially different styling, LEDs consume very little energy and illuminate 400 to 500 milliseconds faster than an incandescent lamp, which provides that cellphone- chattin’ texter behind you an additional 12 to 13 metres of warning (at 100 km/h) that you’re on the brakes.

While we wait for LED headlights to become commonplace, some DIYers might opt to switch their taillights to LEDs. Be warned: they draw so little juice that a conventional turn indicator signal switch won’t cycle – so if you choose to convert to LEDs, you’ll need a different flasher relay that’s not dependent on load, or to add ballast resistors to draw sufficient current.

 

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