LEDs (light emitting diodes) are currently used as light sources in tail lamps, and also increasingly at the front of a vehicle, for signal functions.
The first full-LED headlamps, which will generate both low and high beam, will be on the market in 2007 and 2008. They open up new, far-reaching styling options and therefore great differentiation potential for automakers. At Hella, a new, future generation of LED headlamp is already under development. Beginning in 2012, full-functioning LED arrays (assemblies) will make new, active lighting possible.
Extremely long service life (up to 10,000 hours), a high level of reliability, low package space, rapid switch-on behavior and, depending on the function, up to 80 percent less power consumption, these advantages have now secured LEDs a firm position on the vehicle rear.
Consistently, these advantages are being used especially in the case of central high mounted stop lamps, but also increasingly in combination tail lamps. For at least six years, LEDs, which generate white light, have been available. The first applications in the front-end area are the position light and daytime running light. The rapid development of their performance now makes them usable for low and high beam as well.
Compared to more traditional automotive lighting, the light color of the white LEDs (color temperature approx. 5,500 kelvin) approaches that of daylight (approx. 6,000 kelvin), or considerably more than xenon light (approx. 4,000 kelvin). This resembles what humans are used to seeing. Drivers do not tire as quickly and are more relaxed when driving. In addition, the white LEDs also are a convincing choice due to their fast response time, and it is possible to cover two functions (such as position light and daytime running light) using a single light source. Their luminous efficiency is increasing rapidly. Today, it is over 40 lm/W and there are already prototypes in research institutes with approximately 130 lm/W (in comparison with xenon light: approx. 90 lm/W, halogen light 20 lm/W).
LED Headlamps have a Modular Design
As high luminous power is required for low and high beam, Hella chooses multi-chip LEDs for the LED headlamp. Special chip arrangements and housing geometries make it possible to generate the cut-off line without additional mechanical shading. This allows compact headlamp systems to be used. The light distribution is designed in an almost modular manner:
– A symmetrical basic light distribution with large dispersion provides homogeneous illumination of the close range in front of the vehicle.
– In the central area close to the cut-off line, a field with high illuminance overlaps this basic light distribution and generates the range of the low beam on the left-hand side of the road (oncoming traffic).
– The long-range asymmetrical “light finger” on the right-hand side of the road provides the necessary long-range orientation for the driver.
– The addition of further optical modules makes it possible to realize AFS lighting functions, such as town, country, freeway and adverse weather light. Triggering takes place by means of electronic control units which then switch on or switch off further light sources according to the current driving situation.
The different light distributions are generated by means of different optical modules that are matched to each other. Variation possibilities with regard to the arrangement and number of the modules open up new, far-reaching styling options and therefore great potential for automaker differentiation.
LED low beam is close to the xenon values
To date, the full-LED headlamp prototype from Hella has been used in an Opel Signum to demonstrate different transmissive optical elements. Six optical units generate the low beam. Three further units are added for the high beam, a special module is sufficient for the cornering light. Daytime running light/position light and direction indicator with LEDs complete the full LED headlamp, which, in the case of low beam, is already very close to the xenon values. These and other prototypes, designed with luminous power similar to a xenon headlamp can be expected by 2008. With regard to this, the Hella lighting specialists take into consideration the continuously increasing performance of white LEDs, which will be used by then.
Hella is working with LED arrays (assemblies) as light sources for future headlamps. They consist of several individually-customized LED chips which are arranged in accordance with a predefined pattern onto a carrier substrate. The individual chips are triggered by means of pulse-width modulation. This allows, on the one hand, the switching-on and switching-off of individual LED chips and therefore the generation of different cut-off line geometries. On the other hand, it also allows an intensity modulation in the light distribution. In addition to the implementation of AFS lighting functions without mechanisms, LED arrays make possible, in conjunction with forward-looking vehicle sensor technology and intelligent triggering electronics, the realization of active lighting functions such as marker light or glare-free high beam.
Approval situation for LEDs in headlamps:
In the USA, headlamps with LEDs for the main lighting functions are already permitted according to SAE standards. In Europe, or in the scope of application of the ECE regulations, corresponding approval can be expected by 2008. Signal functions in the headlamp (direction indicator, side marker light, position light and daytime running light, as well as cornering light) with LEDs are today permitted both in the ECE and SAE scope of application.