29.7.
LED Lighting Systems
Light emitting diode (LED) displays were first introduced commercially in 1968 by Hewlett-Packed. Although LEDs have been used for dashboard displays in vehicles, but until recently legislation has prevented the use of LEDs for exterior lighting. A simple change in the legislative language from incandescent lamp to light source has allowed using lighting devices other than filament bulbs. Figure 29.36 shows a light unit containing LEDs. The LED lighting is highly reliable and has a typical rated life of over 50,000 hours, compared to just a few thousand for the incandescent lamps.
Fig. 29.36. Light unit containing LEDs.
Although LEDs are more expensive than bulbs, the potential savings in design costs due to sealed units and greater freedom of design could outweigh the extra cost. An additional advantage is that LEDs glow faster than ordinary bulbs, the turn on time being approximately 130 ms for the LEDs whereas 200 ms for bulbs. If this time difference is related to a vehicle brake light at motorway speeds, then the increased reaction time equates to about a car length. This future of LED is potentially a major contribution to road safety.
The inherent reliability of the LED allows the designers to integrate the lights into the vehicle body, which has not been possible so far. The colour of light emitted by an LED is red, orange, amber, yellow or green. Developments are under way to produce a blue LED, which when combined with red and green light will allow white light from a solid state device. This will work in much the same way as the combinations of pixels (RGB), on a colour television screen. The type of lights used and the possible positions of the lights on the vehicle are countless.
The research is undertaken for the use of LED lighting specifically as the high level brake lights. This is because of their shock resistance; they can be mounted on the boot lid. In convertible cars, which have no rear screen as such, this application may be ideal. Efforts are being put to design rear spoilers with lights built in. This would not only be a good development in terms of safety, but also be aesthetically pleasing. Heavy vehicle side markers are another application being examined. Side indicator repeaters are also being investigated.
Ford in the USA is using LEDs on the new Thunderbird model for cosmetic purposes. This illustrates the many possibilities for the future use of LEDs. BMW 5 series cars in America use a strip of 25 LEDs for a high mounted brake light. Hella has produced a rear lamp assembly using LEDs. Several firms are producing after market replacement lamps using LEDs as the light source for LGVs.
