LED headlights in pole position
Two years ago, LED headlamps were in demand at motor shows, but xenon high-intensity discharge (HID) lights were brighter, and 99.9% of car buyers still got good old-fashioned halogen bulbs.
Now LED headlamp technology has matured and caught up, although, thanks to banking crisis, almost everyone still gets halogen bulbs, writes Steve Bush.
“There is not a big difference on the street between xenon and LED. LEDs have the same performance as a good xenon system, not just a low-end xenon system,” Dr Wolfgang Pohlmann of German automotive lighting firm Hella told Electronics Weekly.
So why are car makers still using HID in their top models, despite their limited lifespan and the need for high-voltage inverters?
“It is a question of time, LED technology is very new,” he said. “Maybe in four to five years we will see more LED headlamps than xenon headlamps.”
Halogen and HID lamps are both essentially omni-directional infra-red-cooled point light sources so they can share similar optical designs. LEDs emit light in less than a hemisphere and need cooling from the back, so they needed a complete re-think to fit into even outwardly similar designs. However, the work has been done and car lighting firms have a portfolio to pick from when they get a design brief.
“There are options: to keep the classical system used for bulbs and HID with one light source with a reflector, projector or a lens; or the new option is to have more than one light source: for example the Audi A8 has a couple of projectors and also reflectors,” said Pohlmann: “You don’t need one big reflector, you can spread it over several small reflectors.” And even though the light comes from LEDs, the laws of physics bring limitations that even the all-powerful automotive stylists have to obey.
“In general, you need nearly the same space so you cannot have a flat headlight system for example,” said Pohlmann. And if the stylists want tiny headlights, physics once more has something to say.
“Some car makers want to have designs with small reflectors and lenses, but you don’t get so much light from the LED to the street,” said Pohlmann.
With generous proportions, over 70% of light leaving the LED can be delivered to the street. Narrow or short optical paths can cut this to 20%.
“If you want xenon performance, which is 1,000 lm or more, you need an LED with 2,000-2,500 lm if the optics have efficiency of 50%.”
Styling constraints do not only affect headlights. According to Hella, a daytime running light can be implemented with 150 lm, but sometimes 1,000 or more lumens are required if the optics are constrained. The actual devices, the headlight LEDs, have diverged from their general lighting cousins become a breed on their own. Osram’s Ostar Headlamp, for example, comes in five chip configurations, with: 1×1, 1×2, 1×3, 1×4 or 1×5 die in a row.
“Typical light values achieved for each die are 160 lm at 700mA,” said Osram. “Depending on the variant and operating current, values between 125 and 1,100 lm are achievable.
Lumileds has something similar with its Altilon range, which in 1×4 form can produce 850lm at 1A.
The row format is intended to ease the design of dip-beam optics, where anti-dazzle legislation puts a strict limit on light spilled above the horizontal. In the Ostar, Osram goes so far as to paint the metal around the die black to cut local reflections which will otherwise put more light up in the air.
“For some functions, it is good to have a four or five die,” said Pohlmann. “If you want a one LED solution, a four or five die device has the right geometry, especially for low beam. A 2×2 or 1×3 does not.”
Although lighting class LEDs have increased dramatically in efficiency – first appearing at 20 lm/W, which is little better than a halogen lamp, to better than 100 lm/W – there is still too much heat for passive heatsinking in most headlights. This is one of the Achilles heals of LED lighting.
Halogen lamps are inefficient and waste a lot of heat, but they waste it as infra-red which passes out through the lens system and off into the night – pausing only to defrost the front lens if it is icy. HID are far better than halogen lamps as they waste less heat, and it still emerges as infra-red.
LEDs are even more efficient, so have less heat to loose, but they have to run far cooler than either of the other technologies and any heat has to be removed by conduction. In practice, this means adding a fan – the sort of rotating device that can give automotive reliability engineers sleepless nights.
“You can realise headlamps without a fan. If you don’t use a fan, you need a big heavy heatsink,” said Pohlmann. “If you use a fan, you can have tiny heatsink.”
Keeping the fan inside the sealed environment of the headlight prevents its blades from filling up with fluff, and keeps grit out of the bearings, both of which ease anxiety in the reliability department. It also allows warm air to be directed over the inside of the cool front lens, which it can usefully defrost in the winter.
“LED efficiency will be better in future, so the effort of thermal management will reduce,” added Pohlmann.
At least headlight designers can use efficient cool white LEDs and not worry unduly about colour rendering index or producing the warm white so beloved by architects.
“Consistent colour over the beam is important. Colour rendering index is not important,” said Pohlmann.
Hella was one of the first major car lighting firms to work on LED headlights, along with Japanese firms Koito and Stanley Electric. Since then they have been joined by several others including Ichikoh and Visteon. With Valeo, Ichikoh developed an LED front lighting systems for the Nissan Leaf electric vehicle. It puts over 500 lumens on to the road at a cool white colour temperature of 5,500K.
“This low beam module is the first on the market with consumption as low as 50W per vehicle using only two LEDs per headlamp,” said Valeo, comparing it with existing 70W three LED designs, 90W for xenon and 130W for halogen.
Fig 1. Hella Luminator
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