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LED in headlamps

In rear lamps, applications with red and yellow LEDs already are state of the art in lighting technology. In headlamps, especially white LEDs are necessary that for main functions must provide a better light performance. White LEDs have already been used for signal functions like position or daytime running light in some serial projects.

With the development of the first Full-LED headlamp for Audi R8, Automotive Lighting set up a world premiere. For the first time, all light functions of a serial headlamp are being realized in LED technology, these are low and high beam, daytime running light, turn indicator and position light. LED technology offers new opportunities for the future concerning styling, technology and energy consumption.

Advantages of LED

LEDs especially stand out for the following characteristics:

- the light temperature of the LEDs nearly achieve daylight quality

- an extremely high lifetime lasting more than a whole vehicle life

- a significantly lower energy consumption

- more effective opportunities for the use of installation space in headlamps

- unique freedom and variety in styling for the development of brand characteristic styling elements within headlamps

Function of LED

LEDs are based on semiconductor technology. The light is emitted directly out of the semiconductor substrate when a forward voltage is applied. The colour depends on the band-gap energy of valence and conduction band for the materials forming the semiconductor. The amber colour is directly generated by the LED by Aluminium Indium Gallium Phosphide (AlInGaP) semiconductor material. Through the luminescence conversion process, white light is generated. A blue diode in combination with an applied converter material, the colour blue and yellow, the combination of which is perceived as white light.

Technical implementation in headlamps

To achieve the targeted light patterns with LED applications in headlamps, two conventional opportunities are available that are already implemented in filament and also in gas discharge lamps. Either the light of the LED is redirected through a reflector in order to achieve the light pattern on the road. Or, alternatively compact systems can be applied that project the light in the street using a lens. Due to their small sizes, LEDs offer the opportunity to combine both light systems for the first time.

Temperature management

In contrast to Halogen or Xenon systems, LEDs emit "cold" light meaning that no infrared radiation is being produced. Due to the high efficiency, 20% of the input energy are transformed into visible light (for comparison a filament bulb only transforms 5%) - the rest of the energy generates heat within the semiconductor chip. Luminous flux, color and forward voltage are dependant on the temperature. As soon as the allowed temperature is exceeded, the lifetime of the LED will be badly affected or in worst case will be destroyed. To protect the hottest part, the chip, cooling elements with a controlled air transportation system have been developed. The systematic direction of the warm air to the bezel of the headlamp is so simultaneously being used for de-icing and decondensation.

Reduction of energy consumption

The application of LED technology significantly contributes to the avoidance of CO2 emission and the reduction of fuel consumption. This aspect especially gains importance with the implementation of daytime running light (DRL). DRL with LED needs 14W of energy, whereas the use of conventional vehicle lighting at day (low beam, rear lights, position lights) consumes 300W.

Outlook and trend

LED technology within the external vehicle lighting is still in its infancy. Future fields of activity are to be found especially in the implementation of adaptive light systems and in the further optimization of energy consumption.

Wikipedia - Light emitting diodes (LED)

LEDs are being used with increasing frequency in automotive lamps. They offer very long service life, extreme vibration resistance, and can permit considerably shallower packaging compared to most bulb-type assemblies. LED also offers a significant safety performance benefit when employed in brake lights, for when power is applied they rise to full intensity approximately 200 milliseconds (0.2 seconds) faster than incandescent bulbs. This fast rise time not only improves the attentional conspicuity of the brake lamp, but also provides following drivers with increased time in which to react to the appearance of the brake lamps.

LEDs were first applied to automotive lighting in Centre High Mount Stop Lamps (CHMSL), beginning in the early 1990s. Adoption of LED for other signal functions on passenger cars has been slow, but is beginning to increase with demand for the technology and related styling updates. The 2007 Audi R8 sports car uses two strips of optically-focused high-intensity LED for its Daytime Running Lamps. Optional on the R8 in ECE markets is the world's first LED headlamp made by AL-Automotive Lighting.[70] The low and high beams along with the position (parking) lamp and front turn signal are all realized with LED. The Lexus LS 600h features LED low beam, position and sidemarker lamps in North America, and the 2009 Cadillac Escalade Platinum uses LED for the low and high beams, as well as for the position and sidemarker lamps.

The commercial vehicle industry has rapidly adopted LED for virtually all signaling and marking functions on trucks and buses, because in addition to the fast rise time and concomitant safety benefit, LEDs' extremely long service life reduces vehicle downtime. Almost all commercial vehicles use exterior lighting devices of standardised format and fitment, which has cost-reduced and sped the changeover.

LED light sources

Automotive headlamp applications using light-emitting diodes (LEDs) have been undergoing very active development since 2004. The first series-production LED headlamps were factory-installed on the Lexus LS 600h / LS 600h L starting with the 2008 models. Low beam, front position light and sidemarker functions are performed by LEDs; high beam and turn signal functions use filament bulbs. The headlamp is supplied by Koito. Full-LED headlamps supplied by AL-Automotive Lighting were fitted on the 2008 V10 Audi R8 sports car except in North America. The Hella headlamps on the 2009 Cadillac Escalade Platinum became the first U.S. market all-LED headlamps. Present designs give performance between halogen and HID headlamps, with system power consumption slightly lower than other headlamps, longer lifespans and more flexible design possibilities. As LED technology continues to evolve, the performance of LED headlamps is predicted to improve to approach, meet, and perhaps one day surpass that of HID headlamps.

The limiting factors with LED headlamps presently include high system expense, regulatory delays and uncertainty, and logistical issues created by LED operating characteristics. LEDs are commonly considered to be low-heat devices due to the public's familiarity with small, low-output LED used for electronic control panels and other applications requiring only modest amounts of light. However, LED actually produce a significant amount of heat per unit of light output. Rather than being emitted together with the light as is the case with conventional light sources, an LED's heat is produced at the rear of the emitters. The cumulative heat of numerous high-output LEDs operating for prolonged periods poses thermal-management challenges for plastic headlamp housings. In addition, this heat buildup materially reduces the light output of the emitters themselves. LEDs are quite temperature sensitive, with many types producing at 30 °C (86 °F) only 60% of the rated light output they produce at an emitter junction temperature 16 °C (61 °F). Prolonged operation above the maximum junction temperature will permanently degrade the LEDs and ultimately shorten the device's life. The need to keep LED junction temperatures low at high power levels always requires additional thermal management measures such as heatsinks and exhaust fans which are typically quite expensive.

Additional facets of the thermal issues with LED headlamps reveal themselves in cold ambient temperatures. Many types of LEDs produce at −12 °C (10 °F) up to 160% of their 16 °C (61 °F) rated output. The temperature-dependency of LED's light output creates serious challenges for the engineering and regulation of automotive lighting devices, which are in some cases required to produce intensities within a range smaller than the variation in LED output with temperatures normally experienced in automotive service.

Cold weather also brings another thermal-management conundrum: Not only must heat be removed from the rear of the headlamp so that the housing does not deform or melt and the emitters' output does not drop excessively, but heat must in addition be effectively applied to thaw snow and ice from the front lenses, which are not heated by the comparatively small amount of infared radiation emitted forward with the light from LED.