LED features match application needs
The precise nature of the point of light from the LED has led to increased adoption of LEDs for exterior applications such as daytime running lights (DRLs) — the front lights on a vehicle that automatically switch on when the vehicle is moving forward. The transition to LEDs, however, has required LED-centric participants to adapt to the auto environment and the automotive engineers to adapt to the world of solid-state lighting (SSL).
LED-based vehicle lights are usually configured in the form of a string, which ensures that the current is the same across the series-connected LEDs. A single string may include 6, 8, 10, 15, or as many as 20 LEDs. The series configuration is preferable to parallel LEDs, explained Bryan Legates, director of design engineering for power products at Linear Technology. The alternative, parallel formation does not inherently share current equally, resulting in some lights being brighter than others. In the case of parallel LEDs, a filter is needed to make all lights equal, adding to cost and complexity. The constant current afforded by LED strings maintains brightness regardless of the input voltage or LED forward voltage variations.
European DRL legislation
While you can find a number of reasons why LEDs are a good match for automotive applications, legislation will also provide strong influence for the adoption of LEDs in some cases, and rejection in others. The decision by the European Commission (EC) for all new passenger cars to be fitted with DRLs in February 2011, followed by a mandate for the same on all trucks and buses from August last year, has seen the use of LEDs increase. The low energy budget of LEDs makes them the obvious choice for a light source that is on all the time.
Some parts of the motor industry question the eco-friendliness of having a vehicle’s lights on during daylight hours. Supporters counter the energy consumption objection with statistics that daytime running lights use only 25–35% of the energy that driving lights consume. When the DRLs use LEDs, this number further reduces to just 10% of the energy that driving lights use.
There are also question marks over the distractions that a light from a car can cause in daylight and claims that the glare of running lamps can mask other road users, such as pedestrians or cyclists. There are also claims from dissenters that the glare in daylight can distort distance perception, making it difficult for road users to discern how far away a vehicle is. Whatever the doubts, the EC has pressed ahead and DRLs are now to be fitted in all new vehicles. The EC says it has reduced accident rates in Scandinavian countries where the daylight hours in winter months are limited to just two to four hours in the north.
Although LEDs have been used in brake lights, reverse lights, signal indicators, and interior lighting for years, the conventional headlight is only now yielding ground. The headlamp application is moving at a slow pace, largely due to the cost penalty associated with LED headlights in favor of a conventional bulb and its metal surround. At present only high-end cars from luxury automakers use LED headlights in place of halogen or incandescent bulbs. Current examples include the Audi A8 and R8, the Lexus LS600h and RX450h, and the Porsche Cayenne.
In Europe, LEDs are seen as part of the aesthetics of a vehicle, with the distinctive blue light of LEDs being a distinguishing, attractive feature. Every industry observer remarks on the curve of the headlights and the aesthetic appeal — with accompanying price tag — that they represent.
LEDs also require less energy and that ultimately leads to better fuel economy. Legates estimates that the differential between traditional bulbs and LED lighting is a factor of 8 — a 12W LED is roughly equivalent to a 100W bulb and the typical headlamp is approximately 200W for an incandescent bulb or 30W for an LED. This energy reduction is beneficial for electric vehicles. Unlike gas vehicles, the electricity consumption in an electric vehicle directly relates to the engine battery and thus the range of a journey.
However, there are practical constraints to using LEDs, especially in forward lighting in both DRLs and headlamps. They have to operate in the harsh automotive electrical environment and must operate at relatively high power levels, typically 15–75W, all in the confines of the headlamp enclosure space. DRLs typically account for ~15W of LED power, with low beams at ~20W and high beams at ~30W of LED power. A Matrix system can have 75W of total LED power, although typically this is 30-45W as not all LEDs will be on at the same time.
Highlighting a divide between Europe and North America and Japan, Adzan, who is based in Germany, praised LED headlights for their reactivity. “LED headlights will provide added driver safety by adapting the beam brightness based on the proximity of other vehicles. In other words, an oncoming driver should not be blinded if your high beams are on. Cars with this capability will sense this happening and adjust the beam angle and intensity around the oncoming car.”
The intelligent dimming of headlights is hotly contested, with European car manufacturers advocating the benefits for safe driving, whereas North American, Japanese, and Korean car manufacturers remain largely unimpressed.
The LED headlamp market was estimated to be worth $1 billion in 2012 and set to double to $2 billion by 2014. Such a fast-growing market is bound to excite companies, but it seems that a US law has also agitated many.
The future of OLEDs
OLEDs in vehicle lighting are still at the conceptual phase, confirms Shai Dewan, Philips Lighting. Philips, Audi, and Merck are working on a project with the University of Cologne, Germany, where OLED panels can be used in the curve of the car exterior for seamless lighting design.
Today, OLEDs are commonly used in phones and tablets but are finding their way into vehicles in headrest screens or dashboard consoles. Tesla has used a 17-inch display as its central console, replacing dials and knobs with a sleek screen. Vital for an electric vehicle, this reduces the energy budget, which is sourced from the engine battery.
The common goal of car manufacturers is to reduce weight, energy consumption, and bill of materials cost, and this has been preserved in the pursuit of LEDs used in vehicles. The industry-leading technology will become economically viable lower down the range in time, and small improvements, such as adjustable interior light colors to match a mood or paintwork, can continue to advance the driving experience.