More safety and more economy in one: LED headlamps (LED = light-emitting diode) reconcile apparent opposites. They are now optionally available for the premium touring coaches Setra ComfortClass 500 and TopClass 500 and for the world’s top-selling urban bus, the Mercedes-Benz Citaro. This involves a world premiere: the first integrated LED headlamps on buses. Drivers benefit from a light that has a less tiring effect on the eyes, while companies enjoy noticeably lower maintenance costs. Good visibility means safety, and with LED lamps good light also means efficiency as a result of reduced power consumption and long service lives.

Daimler Buses Lighting Technology Workshop

Safety, an essential value of Mercedes-Benz and Setra
Safety is an essential value of the two leading bus brands Mercedes-Benz and Setra. For decades now, both brands have played a decisive role in advancing the development of safety engineering and technology. The central focus here is on active safety, that is, preventing accidents. This is of pre-eminent importance in particular for buses, with their large dimensions and gross vehicles weights, and above all in view of the numerous passengers on board. From anti-lock braking system through Electronic Stability Program to emergency braking systems – again and again, Mercedes-Benz and Setra have led the way. The same applies to the area of vehicle lighting.

Light means safety, LED means perfect headlamp light
Light means safety: seeing and being seen is vital in road traffic. Particularly in bad weather conditions and at night. The best possible light and the attendant safety are of outstanding importance to bus drivers, who are responsible for ensuring that dozens of passengers arrive safely at their destination – at night-time, too.

Many drivers however dislike being on the road in the dark. This is borne out by objective figures: according to the German Federal Statistical Office, for example, roughly one in five traffic accidents occurs in the dark.

And the severity of accidents is substantially higher. Almost 40 percent of all accidents involving fatalities on motorways in 2014 took place in the dark. The logical conclusion to be drawn from these statistics is that good light and good visibility can save lives.
As such, Setra and Mercedes-Benz are committed as a matter of course to ensuring maximum visibility in the interest of maximum safety at night. LED lamps currently represent the state of the art in lighting technology for headlamps, to the benefit of drivers, companies and other road users alike.

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The colour temperature of the light from LED headlamps corresponds roughly to daylight, as a result of which they have a substantially less pronounced tiring effect on the driver’s eyes. In contrast to other lamps, the light flux of an LED lamp remains at a virtually constant high level throughout the lamp’s entire service life. And this service life is many times longer than that of halogen or xenon lamps. This also noticeably lowers costs and avoids the danger of „one-eyed“ buses with poor visibility on the road.

Mercedes-Benz and Setra lead the way with xenon lamps
At its premiere in 1999, the Mercedes-Benz Travego was the first touring coach in the world to be optionally available with xenon main headlamps – also known as Litronic headlamps. Only two years later, the Setra TopClass 400 was unveiled – the first bus featuring xenon headlamps as standard. As an additional first in the bus world, Setra also premiered LED front indicator lamps. These were installed in six rows above the headlamps.

Xenon headlamps marked a revolution in lighting technology at the time. A wide light cone resulted in more extensive illumination of the roadway, in addition to which the light was brighter and had a substantially larger range than the halogen headlamps which remain in common use to this day. The new lighting technology was gradually phased in for the exterior and interior lights on the touring coaches and urban buses of both brands.

Bi-xenon lamps for the Mercedes-Benz Citaro urban bus as well
Xenon light has since become bi-xenon, which means xenon light for both high and low beam in a single lamp. This applies not only to touring coaches, but also to the top-selling Mercedes-Benz Citaro urban regular-service bus, the current generation of which is the first urban bus to be available with this cutting-edge technology. At the same time, the LED daytime running lamps have been integrated into the new headlamp housing.

The Citaro, the Setra TopClass 500 and the ComfortClass 500 have also been setting standards in interior lighting with LED lamps since their premiere.

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LED headlamps for Setra touring coaches and Mercedes-Benz Citaro
As the next step in this evolutionary process, Setra TopClass 500 and ComfortClass 500 as well as the Mercedes-Benz Citaro are now optionally available with LED headlamps. These were unveiled – after a development period of around 1.5 years – at the Busworld Kortrijk show last autumn.This also marks another world premiere: the first integrated LED headlamps on buses.

Integrated LED headlamps: form and function combined to perfection
For the two premium brands, the developers do not simply resort to the suppliers‘ ranges of basic round spherical-cap headlamps for fitting in a mask. Rather, the LEDs are integrated in the headlamps of the model series. This means that the individual face of the brands and models is retained in full, with form and function combined to perfection. A Setra and a Mercedes-Benz are immediately recognizable in the dark, too.

The developers benefit here from the two brands‘ ingenious modular system. Despite the different lenses and headlamp housings, the concealed technical installations for the headlamps of Setra TopClass 500 and ComfortClass 500 and the Citaro are identical and are accommodated in the same basic housing.

From the outside, only experts can distinguish the new LED headlamps from xenon headlamps, as the switch from xenon to LED involves no change to the clear lens or the lamp layout. Only the frame around the lamp is different. In addition, there is no need for the additional high beam which has been employed to date in xenon headlamps – it is replaced by a dark screen.

Five LEDs for low beam, three additional LEDs for high beam
The new lamps are bi-LEDs for low and high beam in a single housing. In each lamp, five LEDs generate low beam, while three additional lamps produce high beam. Each LED has its own reflector. They shine forwards into a lens. As with xenon lamps, this lens directs the light onto the road.

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The rear-mounted engine configuration for buses clearly favours the use of LED headlamps: LED lamps are sensitive to heat, as a result of which LED lamps on passenger cars sometimes require complex cooling systems employing fans to counteract the waste engine heat. No such cooling is required on buses.

Overheating of the LED tail lights on the Setra TopClass 500 and ComfortClass 500 touring coaches is counteracted by air-gap insulation and double casing for the lamps.

LED lamps: reduced power consumption, maximum efficiency
Xenon headlamps already lower power consumption substantially in comparison to conventional halogen headlamps, achieving a reduction from 70 W to 35 W, despite the higher luminous power of xenon lamps. LED lamps are markedly more efficient yet again, despite their comparable luminous power. On bi-xenon lamps, low beam and high beam are simultaneously generated at all times, and the actual light output is controlled by a screen. As a result, light generation results in continuous power consumption of 35 W per headlamp. When the control units required for the right- and left-hand lamps respectively is additionally taken into account, the total power consumption rises to 42 W per headlamp.

Not so with LED headlamps. Here, the power consumption for low beam amounts to only 20 W per lamp. While the power consumption together with high beam again rises to 35 W, the overall reduction in power consumption is tangible, as high beam is generated separately and is only used for three percent of the time the vehicle is in use on average.

Although no savings are achieved on a scale that might affect fuel consumption, the batteries are saved from unnecessary wear when the bus has to remain stationary for some time with its lights on and the engine off. Exterior lighting consisting exclusively of LED lamps roughly halves the power required for lighting.

LED technology in detail
Light-emitting diodes (LED) were invented in 1962. They consist of several layers of semi-conductor compounds, such as silicon. The type and composition of these semi-conductors determine the LED’s light colour and light output. The LED is encased in plastic, which defines the radiating characteristic. When current flows in the direction from the anode to the cathode, light is generated.

As individual LEDs generate a comparatively low light flux, they are deployed in bundles for vehicle headlamps, for example. A further advantage is that there is no legal requirement for a headlamp cleaning system, because the total generated light flux is below 2000 lumen. Subjectively, the light output appears to correspond to that of a xenon headlamp, however.

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Subjective impression of improved visibility and de facto enhanced safety with LED
To the human eye, the performance of LED lamps is comparable to that of xenon lamps, which have defined the state of the art to date. A direct comparison between the lamps reveals virtually no discernible differences with regard to range, illumination and light/dark boundary. Halogen headlamps attain a maximum range of around 120 m, while xenon and LED headlamps each have a range in the order of 150 m. The headlamps on touring coaches reveal a greater range than those on urban buses when set to the same angle, because they are installed at a higher level. This represents a substantial advantage in view of the generally much higher speeds at which touring coaches travel.

A decisive advantage of LED light is that it is subjectively perceived to be more pleasant – the light distribution is more homogeneous and the light colour more pleasant. While halogen light, with a colour temperature of 3200 kelvin, is perceived as being warm, xenon light, at around 4000 kelvin, appears neutral white and cool. At 5500 kelvin, LED light appears cooler again and comes very close to daylight (6500 kelvin). It is perceived as being brighter than xenon and halogen light.

This gives rise to a substantial safety factor, as studies show that the closer the colour of artificial light is to daylight, the less strain it imposes on the human eye. The gentle light/dark boundary of the LED headlamps for Setra and Mercedes-Benz have a positive effect – the range subjectively appears greater and the driver does not have the impression of driving towards a black wall.

Full luminous intensity directly after switching on
A further advantage of LED light is that the full luminous intensity is available directly after switching on the lamps. By contrast, xenon light or conventional bulbs take several seconds to attain full luminous intensity. This factor plays an even more important role with regard to brake lights: the quickly intensifying light signal warns other road users more effectively and is able to shorten their reaction times.

Other road users also benefit from LED headlamps, as their glare characteristics are perceived as being markedly more acceptable.

Safety pays: LED headlamps are economical
With LED headlamps, even greater safety also means greater economy. Throughout its service life, a bus runs for around 15 000 hours on low beam. The halogen H7 lamps which are in standard use only achieve a service life of some 400 to 800 hours, however. This means that the lamps on either side of the vehicle need to be replaced around 19 times throughout the service life of a bus.

Even with lamps designed for simple replacement, as on Setra and Mercedes-Benz buses, these costs add up. 38 lamp replacements in all mean 38 times costs for lamps, downtimes, mechanics‘ time. In addition, a lamp failure while a vehicle is in use leads to a safety deficit: the driver’s visibility is restricted, other road users have greater difficulty recognizing the vehicle.

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The figures for xenon lamps are different, but not fundamentally better. While these lamps run for around 2500 to 3000 hours, xenon bulbs are several times more expensive than halogen bulbs.

LED lamps outlive the bus
At around 20 000 hours, the service life of LED lamps actually exceeds the customary vehicle life-span. Even frequently switching the LEDs on and off fails to shorten their service life. No costly lamp replacements are necessary, and the bus never runs in „one-eyed“ mode. This additional safety and the economical factor represent a crucial argument in favour of introducing LED headlamps for the Mercedes-Benz Citaro urban regular-service bus as well. Even with only a medium-sized fleet, urban buses fitted with halogen lamps require in excess of a hundred lamp changes annually, entailing corresponding costs in terms of materials and labour.

LED lighting also offers an additional long-term safety effect: the lamps‘ performance deteriorates so slowly over the course of their service lives that the user does not notice any change. The degradation – deterioration in performance – of a xenon lamp towards the end of its service life of 2500 to 3000 hours amounts to more than a third. In other words, it only attains around two thirds of its original light flux.

The degradation of LED lamps, on the other hand, amounts to only three to seven percent after a life-span of some 15 000 hours. This means that virtually the full luminous power is retained throughout the service life of a bus.

Availability and retrofitting
The new LED lamps are available as optional equipment for the Setra ComfortClass 500 and Mercedes-Benz Citaro. Alternatively, customers can continue to order xenon headlamps. For the Setra TopClass 500, xenon headlamps are to remain standard for the time being, with LED available as an option.

For the stated model series, the Omniplus service brand is additionally preparing a retrofit option for vehicles with state-of-the-art electronics architecture. This is recommendable above all for younger buses. The retrofit also involves a modification of the software, as the law requires a lamp failure indicator in the cockpit for LED main headlamps.

Let there be light: the evolution of vehicle headlamps
Electric headlamps have been used on automobiles since 1908. While, in the very earliest days of the automobile, lighting took the form of candle lanterns comprising a wax candle in a sheet metal housing, these nevertheless featured an automatic ejection device employing a coil spring for spent candles. Carbide lamps employing a gaseous fuel then followed. These were familiar as miners‘ lamps.

The asymmetric low-beam headlamp was introduced in Germany in 1957. This reduces glare for oncoming traffic by means of differently shaped light cones for the vehicle’s own lane and the oncoming lane respectively.

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Halogen headlamps became established in the 1960s and 1970s. Xenon headlamps followed for passenger cars in 1992, while bi-xenon headlamps appeared in 1999. LEDs were first used at the beginning of the 1990s as a third brake light, and finally as daytime driving lights as of 2004. The first LED High Performance headlamps went into series production for passenger cars in 2008. The first passenger cars – including Mercedes-Benz models – are currently being fitted with so-called Multibeam LED headlamps, in which single LEDs are controlled individually. The technical advantages of LED headlamps are also accompanied by design merits: manufacturers are able to depart from classic headlamp designs and develop their own individual variants.

For many years, headlamps consisted of a lamp in front of a reflector. Headlamp glasses in the form of scattering lenses attended to light distribution. Today, these have been succeeded by clear-lens headlamps in which free-form reflectors behind the lamp focus and distribute the light.

The latest xenon headlamps in particular, as well as the new LED headlamps, are designed as projection-beam headlamps. Here, a lens in front of the lamp is responsible for light distribution.

Source: Daimler Buses