Optics and lighting of Traffic Lights

In the mid 1990s, cost-effective traffic light lamps using light-emitting diodes (LEDs) were developed; prior to this date traffic lights were designed using incandescent or halogen light bulbs. Unlike the incandescent-based lamps, which use a single large bulb, the LED-based lamps consist of an array of LED elements, arranged in various patterns. When viewed from a distance, the array appears as a continuous light source.

LED-based lamps (or ‘lenses’) have numerous advantages over incandescent lamps; among them are:

  • Much greater energy efficiency (can be solar-powered).
  • Much longer lifetime between replacement, measured in years rather than months. Part of the longer lifetime is due to the fact that some light is still displayed even if some of the LEDs in the array are dead.
  • Brighter illumination with better contrast against direct sunlight, also called ‘phantom light’.
  • The ability to display multiple colors and patterns from the same lamp. Individual LED elements can be enabled or disabled, and different color LEDs can be mixed in the same lamp
  • Much faster switching.
  • Instead of sudden burn-out like incandescent-based lights, LEDs start to gradually dim when they wear out, warning transportation maintenance departments well in advance as to when to change the light. Occasionally, particularly in green LED units, segments prone to failure will flicker rapidly beforehand.[citation needed]

The operational expenses of LED-based signals are far lower than equivalent incandescent-based lights. As a result, most new traffic light deployments in the United States, Canada and elsewhere have been implemented using LED-based lamps; in addition many existing deployments of incandescent traffic lights are being replaced. Sometimes, only the red and green lamps are replaced, leaving the yellow lamp as an incandescent, since yellow lamps are rarely on compared to the red and green lamps. In 2006, Edmonton, Alberta, Canada completed a total refit to LED-based lamps in the city’s over 12,000 intersections and all pedestrian crosswalks. Many of the more exotic traffic signals discussed on this page would not be possible to construct without using LED technology. However, color-changing LEDs are in their infancy and may surpass the multi-color array technology.

Another new LED technology is the use of CLS (Central Light Source) optics. These comprise around 7 high-output LEDs (sometimes 1 watt) at the rear of the lens, with a diffuser to even out and enlarge the light. This gives a uniform appearance, more like traditional halogen or incandescent luminaries.

Replacing halogen or incandescent reflector and bulb assemblies behind the lens with an LED array can give the same effect. This also has its benefits: minimal disruption, minimal work, minimal cost and the reduced need to replace the entire signal head (housing).