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01/06/2011
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Lighting systems are becoming increasingly complex, but at the same time they need to be more efficient to help minimise CO2 emissions, as Keith Howard discovers
Night driving is an issue that concerns many motorists, whether or not they know about the statistical evidence suggesting that accidents are about twice as likely to occur at night as during daylight hours, despite the lighter traffic.
When automotive lighting company Sylvania polled drivers in the US, almost a fifth reported sometimes avoiding night driving, because of visibility issues.
It's not a concern that is lost on car makers or their lighting component suppliers. The more precise control of beam pattern provided by complex surface reflector design and the introduction of high intensity discharge (HID) xenon headlights, which generate about 30% higher light intensity than conventional halogen alternatives, have been steps in the right direction – although, 20 years after its introduction, HID technology remains too costly for standard fitment on cars of all classes.
Further substantial improvement rests with the development of a new generation of adaptive headlight technologies, such as the Highbeam Assistant system supplied by Automotive Lighting GmbH for the Mercedes-Benz S-Class, Hella's Dynamic Light Assist (Audi A8, Mercedes-Benz S-Class and Volkswagen Touareg) and Valeo's BeamAtic Premium system, premiered on the Volkswagen Phaeton.
Whereas first-generation adaptive lighting improved visibility by directing illumination to follow the road more closely, these new systems take the more radical step of using as extensive a beam as possible, for as much of the time as possible. They do this by detecting when other cars are ahead or oncoming and automatically protecting them from glare.
According to research commissioned by Valeo, average high beam usage with its base BeamAtic system – which controls the switching of conventional dipped and main beams automatically – jumps from 8% of the time (with manual control) to almost 40%. Automotive Lighting's Highbeam Assistant is more complex, in that it is able to vary the beam cut-off in discrete steps, according to the distance of other vehicles, by using actuator motors to vary the angle of the projector module.
This facility is also used to implement a motorway light above 90km/h, with raised light-dark boundary and output of the xenon module increased from 35W to 38W.
With BeamAtic Premium and Hella Light Assist – examples of the most sophisticated such systems currently available – main beam is used continuously, with a cylindrical screen within the headlight, positioned between the light source and projection lens, being automatically adjusted by a stepper motor to ensure that other vehicles are selectively protected from glare, their drivers seeing what appears to be a dipped beam.
Cost is again an issue with these technologies, since a camera is required, together with an ECU running sophisticated image processing software. But duty-sharing offers a measure of relief, in that the camera can also be used for other functions.
The next significant development will be the adoption of LED light sources in these systems. LED technology's attractions are well established and the pace of development of high-output LED modules has been surprisingly rapid. LEDs last the lifetime of the car – a bonus not just in that it removes the need for bulb replacement, but also as it avoids progressive fall-off in performance. Moreover, because LEDs generate 'cold light', without accompanying heat, they are also more efficient. The two-LED low-beam modules in the Nissan Leaf EV, for example, consume 50W, compared to 90W and 130W respectively, for comparable xenon and halogen units.
By 2015, Valeo reveals, it fully expects that ongoing development will reduce LED power consumption by 25%.
This high efficiency also makes LEDs the obvious choice for meeting the EU running light regulation that comes into force in Europe later this year. Using dipped beam headlights as running lights is doubly inefficient, as the beam is both too powerful and directed away from other road users. Low-power LEDs that are not bright enough to cause glare can be directed towards other road users, ensuring maximum efficacy and minimum impact on fuel consumption and CO2 generation.
LEDs have already been used in adaptive lighting systems as a supplement to xenon main units. Automotive Lighting's aforementioned S-Class installation uses twin LEDs with rotating reflectors for the cornering light function that improves visibility when negotiating tight bends or junctions, or turning into drives.
But LEDs also offer the potential for what Valeo describes as "bending light systems without mechanical movement" – in other words, adaptive lighting systems comprising an array of LEDs, each of which contributes a different part of the beam pattern.
"The DLA system can use the same mechanical components, together with LEDs or a matrix system, switching on or off the single arrays of an LED chip," says Dr Ing Gunnar Koether from the lighting and vision department of Volkswagen AG. "The challenge with the second concept," Koether points out, "is to achieve an homogeneous light pattern, without the limits between the arrays being visible."
If this problem can be cracked, though, a beam pattern even more adaptable than that provided by a moving screen should become achievable.
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Author
Keith Howard
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