13.2 Active circuit elements

Led applications

LEDs have many uses. Some of these are given here.

1. thin, lightweight message displays, e.g. in public information signs (at airports and railway stations, among other places)
2. status indicators, e.g. on/off lights on professional instruments and consumers audio/video equipment
3. infrared LEDs in remote controls (for TVs, VCRs, etc.)
4. clusters of LEDs are used in traffic signals, replacing ordinary bulbs behind coloured glass
5. car indicator lights and bicycle lighting
6. calculator and measurement instrument displays (seven segment displays), although now mostly replaced by LCDs
7. red or yellow LEDs are used in indicator and [alpha]numeric displays in environments where night vision must be retained: aircraft cockpits, submarine and ship bridges, astronomy observatories, and in the field, e.g. night time animal watching and military field use
8. red or yellow LEDs are also used in photographic darkrooms, for providing lighting which does not lead to unwanted exposure of the film
9. illumination, e.g. flashlights (a.k.a. torches, UK), and backlighting for LCD screens
10. signaling/emergency beacons and strobes
11. movement sensors, e.g. in mechanical and optical computer mice and trackballs
12. in LED printers, e.g. high-end colour printers

LEDs offer benefits in terms of maintenance and safety.

1. The typical working lifetime of a device, including the bulb, is ten years, which is much longer than the lifetimes of most other light sources.
2. LEDs fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs.
3. LEDs give off less heat than incandescent light bulbs and are less fragile than fluorescent lamps.
4. Since an individual device is smaller than a centimetre in length, LED-based light sources used for illumination and outdoor signals are built using clusters of tens of devices.

Because they are monochromatic, LED lights have great power advantages over white lights where a specific colour is required. Unlike the white lights, the LED does not need a filter that absorbs most of the emitted white light. Coloured fluorescent lights are made, but they are not widely available. LED lights are inherently coloured, and are available in a wide range of colours. One of the most recently introduced colours is the emerald green (bluish green, wavelength of about 500 nm) that meets the legal requirements for traffic signals and navigation lights.

There are applications that specifically require light that does not contain any blue component. Examples are photographic darkroom safe lights, illumination in laboratories where certain photo-sensitive chemicals are used, and situations where dark adaptation (night vision) must be preserved, such as cockpit and bridge illumination, observatories, etc. Yellow LED lights are a good choice to meet these special requirements because the human eye is more sensitive to yellow light.

The light emitting diode

1. What is an LED?
2. List 5 applications of LEDs.