Sending right signals

LIGHT-EMITTING diodes (LEDs) have come a long way since they became known to the public 20 years ago with the introduction of the first digital wrist watches. Now red LEDs can be used in traffic lights to reduce the energy consumption of traffic signals by 60 per cent and to bring down the maintenance and replacement costs by 90 per cent. Their use can also diminish traffic jams and improve visibility.

Although LEDs soon disappeared from wrist watches and calculators, the technology began popping up on high- fidelity amplifier volume displays, car dashboards, telephones, TV sets, etc. In less visible applications, such as fibreoptic cable telecommunications and fax machines, they have become the poor person's laser.

One of the reasons for the great interest in LEDs is that they give a lot of lumens per watt (w) input and their efficacy keeps increasing. High efficiency means decreased operating temperature which translates into a longer life for the diode. The light is monochromatic so here is no colour drift. LEDs are robust and reliable. However, the light emitted from LEDs has its limitations. They have the characteristics of a signal - they are meant to be seen directly and not to illuminate things.

According to the us electronics company, Hewlett-Packard, red LEDs based on the most recent technology, have an efficacy of 24 lumens watt. In per contrast, an incandescent lamp with a red filter has an efficacy of less than 10 lumens per watt. The optical efficiency of the LED in a signal is higher than that of incandescents, which means that with LEDs, much fewer watts are needed to produce the same signal effect.

In a conventional red traffic light, the monochromatic appearance is achieved by deleting all the non-red wavelengths. First, the lamp produces white light i.e. the full-spectrum light. The green, yellow and blue wavelengths are absorbed by the red lens and converted into heat. In the case of arrow traffic lights, still more light is wasted by the screen that blocks out all light except that escaping through the arrow-shaped exposure. An LED, on the other hand, produces monochromatic light of a given colour. Thus a red, 12-inch arrow traffic light, which is equipped with a 150 iv incandescent lamp, can be replaced by a nine W LED array resulting in a 94 per cent saving in power.

For a circular traffic light the difference is smaller, but the savings potential is still impressive. A conventional red traffic light typically uses a 70 w incandescent lamp or an equivalent 30 w halogen lamp. These lights can be replaced by a 10 W LED package.

Red LEDs are now being tested in a number of places in Canada and the us. In Europe, Sweden and France are leading the way. Sweden could save at least three mw-or up to two million KW of energy on red lights alone for its 50,000 traffic lights. in the us, nearly 300 mw energy can be saved on the 260,000 traffic intersections. Even though the LED package is expensive, the energy and power savings alone can pay for the extra cost of installing LEDs in many countries.

The maintenance costs can also be reduced substantially. For example, light bulbs in the red signals are presently replaced three times a year, whereas, the estimated life of the LED packet is not less than 10 years. Besides, since LED disks do not seem to be prone to light depreciation which is caused due to dust clinging to the lens, there may be less need for cleaning.

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