17LED(Light Emitting Diode) A light emitting diode is a semiconductor light source. Working principle of a LED is electroluminescence that is when the LED switched on, electrons are able to recombine with holes within the device, releasing energy in the form of photons.
18Commercial Development The first commercial LEDs were commonly used as replacements for incandescent and neon indicator lamps, and in seven-segment displays, first in expensive equipment such as laboratory and electronics test equipment, then later in such appliances as TVs, radios, telephones, calculators, and even watches. Until 1968, visible and infrared LEDs were extremely costly, in the order of US$ 200 per unit, and so had little practical use.
19Commercial Development The Monsanto Company was the first organization to mass-produce visible LEDs, using gallium arsenide phosphide (GaAsP) in 1968 to produce red LEDs suitable for indicators. Later, other colors became widely available and appeared in appliances and equipment. In the 1970s commercially successful LED devices at less than five cents each were produced by Fairchild Optoelectronics.
20Commercial Development As LED materials technology grew more advanced, light output rose, while maintaining efficiency and reliability at acceptable levels.
21Lifetime and FailureThe most common symptom of LED failure is the gradual lowering of light output and loss of efficiency. Sudden failures, although rare, can occur as well.LED performance is temperature dependent. Most manufacturers' published ratings of LEDs are for an operating temperature of 25 °C. LEDs used outdoors, such as traffic signals, and that are utilized in climates where the temperature within the light fixture gets very hot, could result in low signal intensities or even failure.
22Lifetime and FailureLED light output rises at lower temperatures, leveling off, depending on type, at around −30 °C. Thus, LED technology may be a good replacement in uses such as supermarket freezer lighting and will last longer than other technologies. However, because they emit little heat, ice and snow may build up on the LED light fixture in colder climates. This lack of waste heat generation has been observed to sometimes cause significant problems with airport runway lighting in snow-prone areas.
23Applications Indicators and Signs Because of their long life, fast switching times andtheir ability to be seen in broad daylight, LEDs have been used in ;-Brake lights and rear light clusters for vehicles-Traffic lights
24ApplicationsIndicators and Signs Red or yellow LEDs are used in indicator and alphanumeric displays in environments where night vision must be retained: aircraft cockpits, submarine ship bridges and astronomy observatories.
25Applications Lightining LEDs are used increasingly in aquarium lights. Especially for reef aquariums, LED lights provide an efficient light source with less heat output to help maintain optimal aquarium temperatures.
26ApplicationsLightingLEDs are the ideal solution for street lighting due to their long life, directional light, uniform brightness and illumination.
29A liquid-crystal display (LCD) is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals. Liquid crystals do not emit light directly.
30Liquid crystals were first discovered in 1888 Liquid crystals were first discovered in By 2008, annual sales of televisions with LCD screens exceeded sales of CRT units worldwide; the CRT became obsolescent for most purposes.
31They are used in a wide range of applications including Computer monitorsTelevisionsInstrument panelsAircraft cockpit displaysSignageThey are common in consumer devices such as video players, gaming devices, clocks, watches, calculators, and telephones, and have replaced cathode ray tube (CRT) displays in most applications.Also they are available in a wider range of screen sizes than CRT and plasma displays.
32The LCD screen is more energy efficient and can be disposed of more safely than a CRT. Its low electrical power consumption enables it to be used in battery-powered electronic equipment.
33Each pixel of an LCD typically consists of a layer of molecules aligned between two transparent electrodes, and two polarizing filters, the axes of transmission of which are perpendicular to each other. Without the liquid crystal between the polarizing filters, light passing through the first filter would be blocked by the second polarizer.
35Since LCD panels produce no light of their own, they require external light to produce a visible image. In a "transmissive" type of LCD, this light is provided at the back of the glass "stack" and is called the backlight.
36Specifications There are several factors when evaluating an LCD Resolution versus rangeSpatial performanceTemporal/timing performanceColor performanceColor depth or color supportBrightness and contrast ratio
37Plasma DisplayThese televisions are light-weight and save a lot of space
38made of 2 sheets of glass with 2 gases stored between the sheets xenon and neon gasesred, blue and green phosphors (substances that give off light when struck by light)
39 plasma display panel (PDP) small cellscontaining electrically charged ionized gases, or what are in essence chambers more commonly known as fluorescent lamps
40*displays are bright (1,000 lux or higher for the module), have a wide color gamut *can be produced in fairly large sizes—up to 3.8 metres (150 in) diagonally*very low-luminance "dark-room" black level* life time is 100,000 hours of actual display time, or 27 years at 10 hours per day.
41native plasma resolutions *The most common native resolutions for plasma display panels are 853×480 (EDTV), 1,366×768 or 1,920×1,080 (HDTV). *upscaling and downscaling algorithms used by each display manufacturer.
42enhanced-definition plasma television *Early plasma televisions were enhanced-definition (ED) with a native resolution of 840×480 (discontinued) or 853×480, and down-scaled their incoming High-definition video signals to match their native display resolution.
43ED Resolutions *Following ED resolutions were common prior to the introduction of HD displays, but have long been phased out in favor of HD displays.
44High-definition plasma television Early high-definition (HD) plasma displays had a resolution of 1024x1024 and were alternate lighting of surfaces (ALiS) panels made by Fujitsu/Hitachi. These were interlaced displays, with non-square pixels.HD Resolutions1024×1024 (discontinued)1024×7681280×7681366×7681280×10801920×1080
45How plasma display works *millions of tiny cells in between two panels of glass *noble gases and a minuscule amount of mercury gas or plasma
47LCD Advantages Slim profile DisadvantagesPoor black level: Some light passes through even when liquid crystals completely untwist, so the best black color that can be achieved is varying shades of dark gray, resulting in worse contrast ratios and detail in the image. This can be mitigated by the use of a matrix of LEDs as the illuminator to provide nearly true black performance.Narrower viewing angles than competing technologies. It is nearly impossible to use an LCD without some image warping occurring.LCDs rely heavily on thin-film transistors, which can be damaged, resulting in a defective pixel.Typically have slower response times than Plasmas, which can cause ghosting and blurring during the display of fast-moving images. This is also improving by increasing the refresh rate of LCD displaysAdvantagesSlim profileLighter and less bulky than rear-projection televisionsIs less susceptible to burn-in: Burn-in refers to the television displaying a permanent ghost-like image due to constant, prolonged display of the image. Light-emitting phosphors lose their luminosity over time and, when frequently used, the low-luminosity areas become permanently visible.LCDs reflect very little light, allowing them to maintain contrast levels in well-lit rooms and not be affected by glare.Slightly lower power usage than equivalent sized Plasma displays.Can be wall-mounted.
48Plasma display Disadvantages Susceptible to Screen burn-in and image retention (however, newer models have built-in technologies to prevent this such as pixel shifting)Phosphors lose luminosity over time, resulting in gradual decline of absolute image brightness (newer models are less susceptible to this, having lifespans exceeding 60,000 hours, far longer than older CRT technology)Generally do not come in sizes smaller than 42 inchesSusceptible to reflection glare in bright roomsHigh power consumptionHeavier than LCDs due to the requirement of a glass screen to hold the gasesDamage to the glass screen can be permanent and far more difficult to repair than an LCDAdvantagesSlim profileCan be wall mountedLighter and less bulky than rear-projection televisionsAchieves better and more-accurate color reproduction than LCDs (68 billion (236) versus 16.7 million (224)) colors[Produces deep, true blacks allowing for superior contrast ratios (up to 1:1,000,000)Far wider viewing angles than those of LCD (up to 178°), images do not suffer from degradation at high angles unlike LCD‘sAbsence of motion blur, because of very high refresh rates and faster response times (as fast as one microsecond) make plasmas ideal for fast motion video (films or sports viewing)