LED Panels and Displays
From Chempedia
Light Emitting Diodes
LED is the acronym for "light emitting diode." An LED is a small conductor which is similar in appearance to a Christmas tree bulb, while a diode is an electronic device that forces current to flow in only one direction. LEDs come in a wide variety of sizes, and unlike light bulbs, they do not use filaments to create light (Figure 1). As technology improves, there is a continuous desire to improve light intensity and the efficiency of light generation, and LEDs have been up to the task. LEDs are important in the manufacture and operation of display devices that are essential for everyday life. LEDs also have the unique ability to operate at low voltages: at less than 5 V they are compatible with integrated-circuit devices (Pankove, 11).
The main function of an LED is the production of light. Light consists of many small energy particles that have no mass, referred to as photons. When electrons shift to a lower orbital, energy is released in the form of photons, and light is emitted (Figure 2). LEDs consist of a negatively charged layer that electrons occupy, and a positively charged layer that has positively charged holes. When an electrical current flows through the diode layers, the electrons and the holes are forced to move (See Figure 3).
 Figure 2. from http://electronics.howstuffworks.com/led1.htm |  Figure 3. from http://electronics.howstuffworks.com/led1.htm |
The opposite charge causes the electrons to become attracted to the holes, which forces the electrons to drop down to attach to the holes. The energy that electrons lose in this attachment is emitted as photons or light (Figure 2). The wavelength of light emitted depends on the size of the reduction in energy as the electrons attach and gain stability. Properties of light such as color and intensity usually depend on the element(s) being used in the LED. The appearance of the light emitted may also be altered by using bulb coverings of different color, material and transparency.
Semiconductors are an important aspect in the construction of LED panels. Semiconductors are poor conductors that have had other materials, or impurities, added to them. Varying kinds have been developed to provide emissions ranging from red to blue for multiple function displays (Pankove, 11). The addition of these impurities is accomplished through a doping process. A common conductor used in LEDs is Aluminum-gallium-arsenide, which bonds fully to neighboring Aluminum-gallium-arsenide atoms ensuring that there are no free electrons to conduct a current.
While singular LEDs are used in a few devices, it is more common to find several LEDs grouped together to form display panels. A display is a screen that lights up to display pictures or inscriptions. Today, LED displays are widely used in various industries(Figure 4).
The first step in creating an LED display is to gather thousands of LEDs together. The number of LEDs required depends on the size of panel to be created. LEDs are arranged in rectangular units called pixels, which are made of 3 to 12 LEDs. By collecting enough of these pixels one can evenly disperse them to create the images seen on the displays. However, energy is required to light the individual LEDs. This energy is acquired from an energy source located behind the panel, and a computer program directs the flow of energy through only certain LEDs at a time. This selective flow of energy is used to create the image seen on the panel.
LEDs are not just seeing action in such novelties as jumbo television screens, but also in many simpler items. In television or stereo remotes, when you press a command button you are completing a connection that is then relayed to a chip that produces a signal specific to that button. The remote transistor then amplifies the signal and passes it on to the LED, which will break the signal down into streams of infrared light. A sensor located in the television then receives the infrared light and the command is issued by the remote is carried out. In small digital displays, such as those used in alarm clocks and cell phones, LEDs are used to produce the illuminated numbers on the screen. At sporting events, the scoreboards and video screens utilize LEDs to produce scores and replays.
Most recently, LEDs have seen increasing use in traffic lights. They provide numerous advantages over the more traditional incandescent halogen bulbs. For starters, LED bulbs are brighter, last for tens of thousand of hours, and are impervious to heat, cold, shock and vibration. LED bulbs are also extremely energy efficient, using on average nearly eighty to eighty-five percent less energy than halogen bulbs. This translates to huge fiscal savings for cities. For example, the city of Syracuse, NY, projected that the installation of 10,044 LED traffic lights at 299 intersections will save the city approximately $225,000 annually.
With the current energy crisis, the continual development of energy efficient forms of light is a much needed step in the right direction to saving money and limited resources for humans and the environment, and in conserving the constantly decreasing energy supply. LEDs have improved the appearance of images, making products that equip LEDs increasingly attractive to consumers shopping for many things such as televisions, cell phones, and PDAs.
Bibliography
Pankove, J. I et al: Display Devices, Springer-Verleg (1980)
Matsumoto, Shoichi: Electronic Display Devices, John Wiley and Sons (1990)
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[1] Accessed November 6th2005
[2] Accessed October 23rd 2005
This article was researched and written by:
Samuel Cole, Olivia Trittipo, Jeremiah Johnson, Clayton Gripp, and Ali Al-Saadoon.
