Summary of LED and Traffic Signal Technology

Solid state LED light sources are known as p-n semiconductor devices. By doping a substrate material with different materials, a p-n junction is formed within the semiconductor crystal. The dopant in the n region provides mobile negative charge carriers (electrons), while the dopant in the p region provides mobile positive charge carriers (holes). Within a semiconductor crystal, when a forward voltage is applied to the p-n junction from the p region to the n region, the charge carriers inject across the junction into a zone where they recombine and convert their excess energy into light. The materials used at the junction determine the wavelength of the emitted light. A clear or diffuse epoxy lens covers the semiconductor chip and seals the LED. It also provides some optical control to the emitted light; LEDs come in a variety of angular distributions.

Click here for a table comparing various international traffic signal standards and specifications.

Materials used in early LEDs to generate specific colors are listed as follows, with the peak spectral wavelength generated by the LED and the material used (IESNA, 1993):

• Deep blue; 470 nm; SiC
• Blue; 490 nm; GaN
• Green; 565 nm; GaP:N
• Yellow; 590 nm; GaAs.15P.85:N
• Amber; 610 nm; GaAs.5P.N
• Orange; 630 nm; GaAs.35P.65:N
• Red; 660 nm; GaAs.6P.4
• Red (super bright); 650 nm; AlGaAs
• Deep red; 690 nm; GaP:Zn

More recently, a promising material for red, amber and yellow LEDs has been identified. Aluminum indium gallium phosphide (AlInGaP: commonly pronounced like "allen gap") is used to develop long visible wavelength - yellow, amber and red - LEDs. This material results in a much lower degradation in light output over the life of the LED. For shorter wavelength LEDs - green and blue - a promising material is indium gallium nitride (InGaN). Using these materials, LEDs have been developed that have a luminous efficacy (lumens per watt) exceeding that of incandescent lamps. However, the relatively small lumen package that is produced by a single LED still means that dozens, if not hundreds, of LEDs must be used together to produce even a modest amount of light. LEDs are low-voltage, low-current devices.

The spectral power distribution of an LED is fairly narrow, with half-bandwidths of around 20 to 50 nm, depending upon the substrate material. This means that LEDs produce highly saturated, nearly monochromatic light. White LEDs are a recent development, constructed by adding a phosphor to a blue LED. Some of the blue light is converted by the phosphor into broader- spectrum yellow which results in light that has a bluish-white appearance. White light can also be generated by mixing the light generated by blue, green and red LEDs.

LED traffic signals. Several companies manufacture traffic signals using LEDs. The usual strategy with these signals is to package hundreds of LEDs together with reflectors or lenses to create the high-luminance signal face that is required by specifications of the Institute for Transportation Engineers (ITE). Of course, signals such as directional arrows require fewer LEDs, in contrast to a stencil that is typically used with incandescent traffic signal arrow heads.