First used as status and indicator lamps, and more recently in under-shelf illumination, accent lighting, and directional marking applications, high-brightness LEDs have emerged within the last six years. But only recently have they been seriously looked upon as a feasible option in general purpose or automotive lighting applications. This is an overview of the basic technology which these devices are based.
Light-emitting diodes (LEDs) are solid-state devices that convert electric energy directly into light of a single color. Because they employ “cold” light generation technology, in which most of the energy is delivered in the visible spectrum, LEDs don’t waste energy in the form of non-light producing heat like halogen bulbs.
In addition to producing cold light, LEDs:
- Can be powered from a portable battery pack or even a solar array.
- Can be integrated into a control system.
- Are small in size and resistant to vibration and shock.
- Have a very fast “on-time” (60 nano-seconds vs 10 mili-seconds for an incandescent lamp).
- Have good color resolution and present low, or no, shock hazard.
The centerpiece of a typical LED is a diode that is chip-mounted in a reflector cup and held in place by a mild steel lead frame connected to a pair of electrical wires. The entire arrangement is then encapsulated in epoxy. The diode chip is generally about 0.25 mm square. When current flows across the junction of two different materials, light is produced from within the solid crystal chip. The shape, or width, of the emitted light beam is determined by a variety of factors: the shape of the reflector cup, the size of the LED chip, the shape of the epoxy lens and the distance between the LED chip and the epoxy lens. The composition of the materials determines the wavelength and color of light. The picture displayed to the right is an example of a traditional LED apparatus which is not very usable in automotive lighting except for indicators inside the vehicle. Today LEDs have evolved into a more usable product that can create massive amounts of light output by creating new types of Light Emitting Diodes. High Power Car LEDs are now available as bulb replacements and universal applications.