"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,[40] 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 LEDs used for electronic control panels and other applications requiring only modest amounts of light. However, LEDs 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 LEDs.
LEDs are increasingly being adopted for signalling functions such as parking lamps, brake lamps and turn signals as well as daytime running lamps, as in those applications they offer significant advantages over filament bulbs with fewer engineering challenges than headlamps pose."
Source: Wikipedia
