All About LED DRL and Headlights

Summary :

• You’ve probably heard / read that LEDs are more energy efficient, longer lasting and are brighter.
• Except that it’s not entirely true. Read on to find out more.

According to estimates by McKinsey, automotive lighting market is expected to grow at around 4% per annum between 2010 and 2020. But when you look at the data in detail, one sub-sector stands out – LED lighting. Automotive LED market is expected to outgrow the automotive lighting industry by 5 times, expanding at 22% per annum for the same period. Contrary to popular perception, the use of LEDs in automotive applications is not new. LEDs have been in use in mainstream series production cars for easily 15 years now. Most people don’t realize their existence as they are “hidden” away from view. Backlighting for instrument clusters, climate and audio control switches, and sometimes even ambient lighting are some common automotive applications for LEDs. Today, close to 50% of automotive interior lighting market is contributed by LEDs.

What is new however, is adoption of LEDs for automotive exterior applications. The reason why LEDs took such a long time to penetrate the exterior lighting market is because of the harsher operating conditions and greater demands placed on them (lighting is considered a primary safety feature). Critical applications like headlamps and tail lamps must be robust enough to operate across a wide range of temperature, vibration, maintain a reasonable long lifespan and yet remain cheap enough to manufacture. Much progress have been made since. LEDs started to become cheap and robust enough to be first used as third brake lamps, and then tail lamps and turn signals. The last frontier for LEDs left to conquer are front headlamps. Due to their close proximity to the engine (high temperature, vibration), subjected to government regulations, providing critical safety function, headlamps are the most expensive and complicated exterior lighting component.

LEDs broke new ground with the Audi R8, featuring the world’s first full LED headlamp. Lexus would later introduce the LS600h, with LEDs for low beam. Main beam (high beam) function is still performed by regular halogen lamps. In contrast, the Audi R8 relies fully on LEDs for both main beam, low beam and turn signal functions. The Cadillac Escalade became first SUV to feature full LED headlamps. The most recently launched car with full LED headlamps is the Audi A6.

• Audi – LEDs by Philips Lumileds and headlamp design by Automotive Lighting
• Lexus LS600h – By Koito Manufacturing (official materials did not state separate suppliers for LED / headlamp design, other than Fuji and Renesas supplying the necessary MOSFET and LED-driver circuits)
• Cadillac Escalde – LEDs from Osram and headlamp design by Hella

Lexus LS600h

Cadillac Escalade

Exploded view, Audi Q5 full LED tail lamp

Exploded view, Audi Q5 headlamp with LED DRL

Cross section of automotive LED.

On 7-February 2011, a new regulation on dayime running lights (DRL), Directive 2008/89/EC, took effect in European Union member states, requiring all new types of passenger cars and small delivery vans to be equipped with DRL. DRL regulations are not new. In Scandinavian countries where daytime light is often insufficient, DRL regulation have been in place since the late 70s. You may remember seeing Volvo cars with side mounted red / amber lights on the bumpers. Even Malaysia have an existing DRL regulation for motorcycles. Many European manufacturers, especially premium brands, are now interpreting the regulation as a way to differentiate their cars, by using light pattern, a long overlooked attribute, as a unique design element for their brand. This is why we are now seeing LED DRL strips on the latest Audi, Mercedes, BMW, Porsche and Range Rover. LEDs have finally penetrated their final frontier.

You’ve probably heard or read enough about LEDs to know that three attributes are commonly associated with them – brightness, efficiency, long lasting (which also means low maintenance cost). What if we tell you’ve either been wrongly informed or that what you’ve been told / read isn’t entirely true? Surprised? Read on.

Myth No.1 : LEDs are super bright.
Below is a chart made by Automotive Lighting, the same guys who manufacture the full LED headlamps for the Audi R8.

As you can see from the chart above, the brightness of LEDs (high intensity LEDs) are still nowhere near that of traditional halogen light bulbs, let alone HIDs that still dominate the luxury segment. Presently, manufacturers compensate this shortfall in brightness by packing multiple LEDs closely in an array. Look at the table below, 17 LEDs are required to generate the equivalent brightness of one HID (Xenon). Of course, the number will drop as technology progresses.

The close proximity of LEDs brings a secondary problem, heat (read below).

Myth No.2 : LEDs are more efficient, consuming less power.
Again, the notion that LEDs are more energy efficient is only true when compared against halogen bulbs. But against HIDs (Xenons) that still dominate the luxury segment, LEDs have a far lower efficiency level.

While LEDs are still more energy efficient than halogens, they still produce a lot of heat for every unit of brightness generated. And the fact that the multiple LEDs need to be arranged closely in an array to be bright enough means that heat management is a major challenge.

Both halogen and HIDs generate a lot of heat too. But the unique thing about LEDs is that all the heat is concentrated at the back, towards the engine bay, whereas halogen and HIDs radiate their heat outwards, so cooling is not a major issue. This lack of heat generated in front creates another secondary problem – defrosting, which is important in four-season countries. In traditional headlamps, defrosting is not an issue because heat is radiated outwards. Frost will melt once the headlamps are turned on. For LED headlamps, cooling fans are required to redirect heat from the back to the front for defrosting, adding another level of design complexity.

At the moment, all full LED or LED dominated headlamps generate too much heat for conventional packaging designs which relies on passive cooling. Some form of active cooling is still required for LED headlamps. The Audi R8 and Cadillac Escalde feature cooling fans (and fins) while the Lexus LS600h uses a heat pipe filled with distilled water. DRL only units however, do not require active cooling but they still need heat sinks.

Myth No.3 : They last longer.
It is true, LEDs do have a far longer lifespan compared to halogen bulbs. But LEDs are soldered directly to a circuit board and thus individual replacement is not possible. So if a single (or a few) LED is fail, you will have to live with that and at the moment, there is still no established legal guideline stating at which is point is the owner legally compelled to replace his LED tail lamp / headlamp. With conventional units, this is a non-issue. The unit either turns on or doesn’t. With LED units, there may be dead spots scattered.

One of our associate does failure analysis on automotive LED applications. He commented that yes LEDs lasts longer and individually, their failure rate is much lower than halogen lighting. But you can’t look at it as an individual unit because the LEDs are configured in an array. So your actual failure rate is much higher than in theory. Having said that, they still last longer than conventional light bulbs. But at some point they will still fail, and replacement is going to be far more complicated than normal bulbs, which you can replace without any special tools or assistance. High temperature may also damage the bonding / soldering and their diode junctions. DRL / LED replacement is more complicated for Audi headlamps, which integrates the DRL array within the headlamp unit.

Mercedes-Benz models, which have the DRL as a separate unit, are less complicated to replace. But the downside in Merc models is that foglamps are no longer offered on the latest DRL equipped models (DRL is mounted where the foglamps used to be). There have been quite a bit of confusion between foglamps and LED DRLs in recent Mercedes models. Even some Mercedes- Benz salesman are telling their customers the the DRLs also serve as foglamps, which is of course not true (DRL’s don’t have focusing function). The truth is that Mercedes-Benz have integrated the foglamp function within the adaptive front lighting (AFL) feature. So you still see the front foglamp switch in the W212 E-class, but there are no foglamps fitted to the car. Front Foglamp feature is now provided by the intelligent adaptive front lighting (AFL), which can now replicate a light pattern similar to that of a front foglamp.

Notice there are no foglamps.

Manufacturers prolong the lifespan of LEDs, and also reduce glare to other road users, by varying intensity of the LEDs depending on ambient conditions. In the case of regular Audis, using information from a light sensor, the DRL operate on full intensity on a bright day (which require more contrast), powering down when the headlamps are turned on / or ambient condition is dark (which require less contrast). It will also power down on the side where a turn signal is active.

But we know there are some Audi cocks who reprogrammed their lighting computer to set the DRLs on full power all the time, causing great discomfort to other road users, especially at night. And because the DRLs dim whenever a side indicator is used, they don’t use indicators at all.

Despite the many technical challenges, adoption of LEDs can only increase. We can point various advantages of LEDs to you but there is little point in repeating the mantra chanted by car manufacturers. But none of them are the real reasons for the rise in adoption. The real reason is because consumers like them as a design element. Everything else is just to create rational justification for their high price, which manufacturers know you want them bad enough to pay extra for it.

Watch the videos below from Audi, explaining the LED design features in their cars.

Reference sources :
Philips Lumileds application briefs and technical data sheets
OSRAM application notes
Automotive Lighting – Integration of LEDs in forward lighting
Nikkei Electronics Asia
McKinsey & C0. – Freeing the light : LED Auto Headlamps