How Electric Lighting Works

Electric lighting converts highly versatile electricity into usable light. Very different technologies can be employed to do this.

In incandescent lighting—the standard light bulb invented 130 years ago by Thomas Edison—electric current passes through a thin wire (filament) with high electric resistance. The heated filament glows, emitting a soft, high-quality light. Most incandescent lamps have a relatively short lifespan, ending when the filament breaks, cutting the electric circuit. Halogen lamps are a version of incandescents in which the filament is inside an inner tube that includes a halogen gas, such as iodine or bromine; this increases filament life and improves energy performance. Incandescent lamps convert as much as 90% of the electricity into heat, with only 10% delivered as visible light.

Fluorescent lighting involves establishing an electric discharge or arc between two electrodes. A ballast regulates the electric current to produce the arc, which excites mercury vapor in the sealed glass tube (which can be straight or twisted into a compact fluorescent lamp, or CFL) in a process that emits ultraviolet (UV) light. A coating of phosphor molecules on the inside of the tube absorbs this purplish UV light, and the molecules are excited and fluoresce, emitting white light. By adjusting the composition of the phosphors, different color temperatures and light quality (as measured by the color rendering index or CRI) can be achieved. Fluorescent lamps last much longer than incandescent lamps—typically 10,000 hours for CFLs and 20,000 hours or more for straight-tube fluorescent lamps, compared with 1,000 hours or less for incandescent.

Various types of high-intensity discharge or HID light sources work much like fluorescent lamps, except that the electric discharge usually takes place in a bulb within a bulb, and the light output is much more concentrated. All of these rely on mercury vapor. High-pressure and low-pressure sodium lamps incorporate sodium compounds and very efficiently produce light at the yellow or orange band of the spectrum. Metal halide lamps typically use sodium iodide or scandium iodide along with the mercury. Most mercury vapor lamps (now banned in the U.S.) use a phosphor—like fluorescent lamps—to convert the purplish UV light into whiter light.

The other major category of lighting today relies on the light-emitting diode (LED), a semiconductor (or “solid state”) device that emits light when direct current is applied to it. Multiple LEDs are typically used in an LED light fixture to produce the required light output. LEDs of different colors can be combined to collectively produce white light (“RGB” approach), or phosphors can be used to convert the bluish LEDs into white. LED lighting today has higher efficacy than incandescent and is likely soon to surpass even the best fluorescent lighting. LEDs, which don’t use mercury, generate conductive heat that is usually removed with finned aluminum heat sinks; the emitted light contains little heat. Well-designed LED lights can last 50,000 hours or longer.

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IMAGE CREDITS:
1. Source: Drawn partly from “Electric Lighting: Focus on Lamp

DISCUSSIONS Other light sources Patricia Cornelison Jan 7, 2011 More comments RELATED GREEN DESIGN Lighting

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