LED headlamps are getting better and more cars are getting them as industry finds its feet with the technology.
Right now there are some 20 car models on the road with LED headlamps - some all-LED and some with LED low (dip) beams and halogen high (main) beams, known as hybrids.
Why just the dip?
Price, reliability and style are the answers.
"At low end of the market the low beams are LED because they are on all the time, even when main beam is on. It is also for styling," Steffen Pietzonka, v-p of car lighting maker Hella, told Electronics Weekly. "Car makers will accept this hybrid solution for the next few years, and the majority in compact segment over next few years will be hybrid because it is much more cost-effective."
Hella is a major producer of LED headlamps, and provides them for Audi's A8, A6 and A3, Cadillac's Escalade, the Mercedes E-Class and a DAF truck. "The next one will be out later this year," said Pietzonka. "We have talked about LED systems for even small cars. These are under development.
Other companies offering LED headlamps to car makers include: AL-Automotive Lighting (part of Magneti Marelli), Hella, Ichikoh, Koito, Valeo and Visteon.
The technology with LED headlamps is settling down.
Premium cars - the likes of Audi's A8 and A6 - get projector headlamps.
In the mid-range, where cars are starting to get LEDs, less-costly reflector designs are more likely.
"In the new Mercedes C-Class you have an LED reflector low beam. Whenever you need to be cost-effective, the choice is reflector," said Pietzonka. "As a generalisation, reflectors are used for the basic model of premium cars and as options on mid-range cars. This is not always true, you also find projector headlamps in mid-range cars."
Projector headlamps need less frontal area, allowing the bonnet line of a car to be lowered. However, expense is not their only disadvantage, particularly in a world where styling is everything.
"With reflectors and LEDs, you can create totally different styles, everyone can see it is different," said Pietzonka. "LED projector headlamps look similar to any other projector, like a xenon projector, it is just a 60mm lens. You can style the lens, with LEDs you can do a rectangular lenses, but they are still just lenses."
Everyone is out to create a light shape that is as distinctive as the radiator grilles of old. Audi's A8 and A6 have 8-10 unique lenses to get their 'wing' headlamp shape,
"Some are looking for a simple reflector, others use expensive lens systems, and some using crazy systems to create signatures," said Pietzonka.
Unlike bulb-based reflector designs, which are constrained by their omni-directional sources where a significant fraction of output has to go straight from the filament to the outside world, LED headlamps can take advantage of their directional sources and reflect every part of the output at least once.
"The LED position is at the side or pointing back into the reflector, and not forward facing. You can control the beam better, and create more interesting reflector shapes. It depends on styling," said Pietzonka.
Something that is not being seen is the funnel-shaped 'collimators' or 'TIRs' found in many lower power LED lighting applications.
One issue is that moulding them without cracks much above 40mm is exceedingly difficult.
Another is that "they are close to projectors", said Pietzonka.
Despite the marketing, early LED headlamps were actually dimmer than lamps based on older xenon high-intensity discharge (HID) technology. Now the difference is minimal, allowing advance features to be incorporated.
"In general, LED lighting features all the stuff of other high-end solutions like xenon, for example glare-free high-beam where a camera detects on-coming traffic," said Pietzonka.
Glare-free high-beams automatically put a dark patch in the beam where the camera identifies other vehicles - the Mercedes-Benz CLS has glare-free LED headlamps.
At the moment, the beam both xenon and LED designs are shaped by a variable shape shutter part way along the optical path within a projector set-up - a shutter which throws away a lot of light, which is why plenty of optical power is needed.
This will be simplified in future by replacing the single light source with an array of LEDs - something that is impossible with HID.
Array beam forming is only practical in LED headlamps
With the array approach, there are no moving parts. An image of the array is projected out of the front of the car.
"You can dim each individual die, put them on and off to create a huge number of patterns. With array system there are up to 80 die and one research project is discussing 3,000 LED chips," said Pietzonka.
A change in the way cars are developed is affecting headlamp makers.
"It used to be that new designs were used on high-end cars, then they were introduced to lower cars after two years. Now, we are developing products for the low and high-ends in parallel, concentrating on cost and energy efficiency at the low end."
The actual LEDs used still largely have the same arrangement that they had in the first LED headlamps - a linear array of 1mm2 die.
Horizontal linear arrays aids optics in creating the legally-regulated horizontal light to dark beam transition in dip beams that prevents on-comers being dazzled.
Single die packages are being used where stylists want to put multiple smaller sub-headlamps together to shape the overall lamp into a line of some sort.
"We have Ostar Headlamp, a metal-core PCB package with a row of chips - two, three, four or five in a row," Juliana Baron, of Osram's automotive marketing team told Electronics Weekly. "There is a new trend: a lot of single-chip LEDs are being used for forward lighting. They need more flexibility on compact designs to support different designs."
Osram is one of the semiconductor companies that are making LEDs for cars. Others include Lumileds and Nichia.
According to Baron, the 1mm2 die is still the standard emitter, with Osram also offering 750x750μm die a 1.5x1.9mm package called Oslon Compact.
Oslon Compact is Osram's single die offering for building LED headlamp arrays
"With this we are especially targeting matrix sources," she said.
Why not rectangular die?
"Standard square chips have most flexibility," said Baron.
The number of die needed in package depends on the application.
"These days we are challenging with LEDs to reach HID performance. We are realising his with packages that can be driven harder. Normally we recommend five chips, although you can use four chip if you drive harder," said Baron. "Our Ostar Headlamp Pro can handle 1.2A, the next generation will be 1.4A. The target is always higher current and lower forward voltage."
What colour temperature do car makers want?
"At the moment, most people request over 5,000K. It really depends on the application. If you have a light guide, the colour temperature shifts. You have to remain within regulations," said Baron, pointing out that her firm offers colour temperature matching across packages so all lights on a car look the same.
An annoying thing that doesn't want to go away is the fan.
Unlike traditional headlamps, which throw heat forward and away as infra-red radiation, LEDs need to loose heat by conduction from their rear by semiconductor-style heatsinking.
With no heat at the front, LED headlamps c an freeze over in icy weather.
The solution is to use a fan, allowing a smaller heatsink, and get it to blow heat over the front glass.
Sealing the system keeps grit out of the fan and makes the whole thing more reliable, but only as reliable as the fan.
"For very energy-efficient vehicles, electric cars for example, we try to avoid a fan. For future systems, with more efficient LEDs, we don't have to drive them so hard, so no fan. For high-performance systems, we will still have fan," said Pietzonka. "The topic of freezing is still an issue. It can be avoided with a fan, or we can defrost it like rear window, with lines or a heating film. There are several systems under development."
And what about the rest of the lights
While signature headlamps are being used broadcast the fact that LEDs dwell within, car companies have not been slow to realise that it is not headlights but thousands of tail lights that drivers see at night.
So they are trying to develop brand awareness with signature rear lights
"People are looking for really stylist rear lights. This started in Europe and is going to the US and other places. The US was not so in-favour of unique styling," said Hella's Pietzonka. "The first steps for LEDs were dotted designs to show it can't be a light bulb. Then the car makers closed the gaps between the dots to make lines, and now they make shapes and letters. These signatures should look very homogeneous, this is a real demand."
Having a homogeneous look to an illuminated surface, at the same time as meeting lighting regulations which effectively require rear lights to emit a broad, shaped, beam, is quite a challenge.
"This is a double feature, and the same for direction indicators. It needs new materials and some micro-optics: not lenses but micro-optical structured prisms," said Pietzonka.
Lighting innovation is a two-way activity. While the car stylists are dreaming up ideas which are put to the lamp companies as challenges, the lamp companies are offering devising new technology for stylist to dream with.
"We are developing a lot of demonstrators for opportunities for car makers," said Pietzonka. "For example, we are using existing rear lamp housings and creating new looks with light guides, micro-optics and optical fabrics."
Even the red and amber LED technology used in cars is changing.
AlInGaP, also called InGaAlP, materials have dominated indicator and rear/stop lights. However, the temperature-dependence of their colour, however slight, can take there output outside the regulations.
"For turn indicators, especially the front ones which are close to the engine, need a more stable light output," said Osram's Baron. "What we are using is blue chips with phosphor to convert to yellow. This is much more temperature stable that InGaAlP, so it is easier to stay within colour and intensity specifications."
CAP - InGaN yellow is Osram's Oslon Black Flat package...
Called 'InGaN yellow', brightness is higher for a given electrical power, and they can be driven harder.
"They are more expensive, so in the back we use less expensive InGaAlP," said Baron. "Some customers use InGaN yellow for rear as well. It depends how much light they need, there can be light guides and multiple lens."
