Overview
Automotive lighting includes headlamps, daytime running lights, turn signals, brake lights and tail lights. Headlamps provide a high beam for long-range illumination and a low beam whose upper part is cut off to limit glare to oncoming drivers. A good headlamp gives enough brightness and reach without dazzling other road users.
Headlamp structure
- Light source: the part that emits light (filament bulb, discharge tube, LED chip or laser diode).
- Optics: reflectors and projector lenses that collect the light and shape it into the required beam pattern.
- Cooling and control: LEDs and lasers need heat sinks, driver circuits and controllers to change the beam. Semiconductor sources lose efficiency and life as they heat up, so heat transfer design matters.
Evolution of light sources
- Halogen: an incandescent bulb whose tungsten filament is sealed with halogen gas to limit filament evaporation. It appeared first on European cars in the 1960s and long remained the cheap, simple standard. Earlier, from 1940, the United States required standardized round sealed-beam headlamps.
- HID (xenon): light comes from an arc between two electrodes in xenon gas rather than a filament. It is brighter and more efficient than halogen but causes more glare, so automatic levelling and headlamp washers are required. It first reached production on the low beam of the BMW 7 Series (E32) in 1991.
- LED: semiconductors emit light efficiently, last long, respond quickly and can be arranged as many small chips, giving designers freedom. The Lexus LS 600h used LEDs for its low beam in 2007, and in 2008 the Audi R8 became the first production car with all-LED low and high beams.
- Laser: laser diodes a few hundred micrometres across emit blue light (about 450 nm) onto a phosphor that converts it to white light; the laser does not shine on the road directly. The tiny, intense source greatly extends range, so it is used mainly to boost the high beam. It entered production on the Audi R8 LMX and the BMW i8 in 2014.
Smart lighting: matrix LED and ADB
Once LEDs could be split into segments switched individually, a camera could detect preceding and oncoming vehicles and dim only those parts of the beam while keeping the rest on high beam. This is the adaptive driving beam (ADB). Audi introduced such matrix LED headlamps on the A8 in 2013. In the United States, Federal Motor Vehicle Safety Standard No. 108 long prevented ADB, until NHTSA amended it in February 2022 in response to a petition from Toyota. Adaptive front-lighting systems that swivel with steering, automatic high beams and road-projection lighting are also being developed.
Challenges
Brighter sources make glare control more important, and LED and laser headlamps are costly to buy and repair. Because human vision depends on colour temperature and contrast as well as brightness, lighting design has to combine engineering with vision science.
References
- Wördenweber, B., Wallaschek, J., Boyce, P., & Hoffman, D. D. (2007). Automotive Lighting and Human Vision. Berlin: Springer.
- National Highway Traffic Safety Administration. (2022, February 22). Federal Motor Vehicle Safety Standards; Lamps, Reflective Devices, and Associated Equipment, Adaptive Driving Beam Headlamps (Final rule). Federal Register, 87(35), 9916.