Lighting up the night

LEDs are rapidly taking over areas traditionally served by incandescent and fluorescent lights. And there's even brighter technology in the wings. These LED lamps replaced the original incandescent lamps used in the Bardavon Theater marquee and blade sign. Each lamp is made from a cluster of LED emitters surrounded by a polycarbonate globe for weather and UV protection. Should one emitter in the cluster fail, others continue to provide light. Integral electronics creates a self-contained screw-in replacement for the original bulb. This cutaway diagram of a typical LED shows basic components. The actual light-emitting portion is the small rectangular chip atop the cathode lead. Seven electronic billboards in Cleveland use daylight-visible red, green, and blue LEDs to form the picture elements, or pixels, in their display screens. The pixels are spaced on 20-mm centers across the 48 14 ft displays. The boards display 720 208 pixel images in .jpg, .bmp, or .psd file formats. Workers assemble the top and bottom sections for the right side of an LED billboard before lifting it to join its left-side twin beside the interstate. Interchangeable LED panels ease board construction, maintenance, and repair. The incandescent light bulb that resulted from Thomas Edison's thousands of Menlo Park experiments might soon be a thing of the past. Light-emitting diodes, or LEDs, are starting to move into traditional lighting areas now dominated by incandescent and fluorescent lights. These solid-state lamps promise high efficiency, low heat, and a major boost in life expectancy over their traditional cousins. The reason LEDs can be efficient light sources comes out of their principle of operation. They typically are composed of two types of silicon material similar to an ordinary solid-state diode. As with a common diode, the material is classified as N-type and P-type. When placed in direct contact, the two different materials create a region called the PN junction. When an electric current of sufficient voltage and polarity is applied to this junction, electrons in the N-type material combine with holes in the P-type material. The electric current supplies additional electrons to the N-type material to replace those combined with holes, while positive voltage pulls electrons away from the P-type material creating new holes. The process continues as long as electric current flow is present. Light emission comes when electrons combine with holes. The process generates