Over the past decade, light-emitting diodes (LEDs) have gained much in brightness and color fidelity and the image-processing systems empowering their displays have become very sophisticated. So much so, that today large-format LED displays are regularly seen presenting moving video images in stadiums, shopping malls, and on roadsides. Essentially, an LED is a positiveegative-junction semiconductor diode that emits a monochromatic light. (See Diagram 1 on page 56.) The first commercially viable LEDs were developed in the 1960s by combining three primary elements: gallium, arsenic, and phosphorus (GaAsP) to obtain a 655nm red light source that by today's standards would be considered fairly low in intensity. Improvements were made throughout the '70s, and a ma-jor development came in the late '80s with the introduction of indium gallium aluminum phosphide (InGaAlP) as the luminescent material. This allowed colors including yellow, green, and orange to be produced with greater brightness using the same basic technology, simply by adjusting the size of the energy band gap. In the mid-'90s, the most difficult hue of the color spectrum was achieved-blue. Today, blue LEDs are constructed from gallium nitride (GaN) and silicon carbide (SiC). This breakthrough opened the door for the new generation of large-format, full-color LED video screens that has become so prevalent.
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