Summary
1. When there is an Ad that says LED TV, it is still LCD TV but the backlight comprises LED light sources. 2. LED TV is a not a conventional terminology but appears to be a marketing strategy. 3. LED video displays are seen in outdoor bill boards and sports stadium and LED TV has nothing to do with this type of LED video displays.
Analysis
Traditionally the name of TV screen technology is used before the name TV. For example, CRT TV is the TV that contains Cathode Ray Tube (CRT) technology in the display screen. Similarly Plasma TV is the TV that employs plasma display technology in the display screen. Laser TV means the TV that employs Laser technology for displaying images on the screen. Projection TV means the TV that projects images on an ordinary white screen. The images in this case are generated by any one of the technologies like, Digital Light Processing (DLP), LCD, Laser, CRT etc,. In the same way LCD TV means the TV that employs Liquid Crystal Display (LCD) technology for its display screen. Since LCD does not generate light, it needs a backlight for the images to be seen on the screen. All along- and even now- Cold Cathode Fluorescent Lamps (CCFLs) have been employed in LCD TV for backlighting LCD. Since CCFLs are not flat light sources, they require a 'light guide' to convert the linear CCFL light sources to flat light source. CCFL in combination with light guides and some optical films is called CCFL backlight. Currently CCFL backlight is the dominant backlight technology and the price is low. Recently, in the last three years LEDs have started replacing CCFL as the light source in Notebook and LCD TV. LEDs have substantial advantages over CCFL and the most attractive features for the end-user are (1) low power consumption (2) high quality images (3) low weight (4) slimness (5) long life. The recent advertisement that calls LED TV is nothing but LCD TV whose backlight technology is based on LED technology. The display screen still employs LCD technology. (The display screen is NOT made of LED technology). There are two types of LED backlight. One type is called the 'edge-lit' version and the other is called 'direct-lit' version. Edge-lit version means the LED light sources are assembled at the edge of light guide, either on two edges or on one edge. Generally white LEDs (a discrete LED emitting white light) are employed in edge-lit mode. The overall thickness of the light guide can be as low as 2 mm. These white LEDs are nothing but blue LEDs coated with a phosphor that converts blue light to yellow light. The blue light from the LED and the yellow light from the phosphor combine to give white light. Since the white light is not rich in red and green, the color images seen on the LCD screen is not of high quality and can be at best the same as in CCFL back-lit LCD screen. But the substantial advantage is in the low power consumption and slimness together with long life of over 50,000hrs (recently 70,000 hrs). The direct-lit LED backlight consists of Red, Blue and Green LEDs assembled inside a reflective 'light box' with unique lenses on top of LEDs and a few optical films and diffuser at the top of the 'light box'. In this case, the white light is produced by mixing red light, blue light and green light from LEDs inside the 'light box' and directing the mixed white to the back of the LCD. This white light has a rich red, blue and green spectrum coupled with low values of Full Width at Half Maximum (FWHM) of the peak wavelength emitted by the RGB LEDs. This property in combination with the color filters employed in LCD gives the viewer a superb color quality of images. The color quality is expressed in terms of U'-V' domain area which is more than 120%. In addition, the 'direct-lit' LED backlight lends itself for ease of dimming certain area of the backlight (local dimming). For example when the user is viewing dark scenes on the LCD screen, there is no need for the Backlight to waste its power to illuminate dark areas of the screen. The backlight needs to illuminate only the bright objects contained in the dark scene. This means the backlight needs to adapt to the incoming video information. This is also termed as 'adaptive dimming'. In this mode of dimming, the power consumption is further reduced. Through this description, one will be tempted to think that the bright portion of the scene under view will also be dimmed. This is not true because there is a compensation provided for bright pixels not to become dim. In spite of this there are extreme image situations where the compensation also is not adequate. However the performance of 'Adaptive dimming' or 'local dimming' provides a quantum jump in the enhancement of images on LCD screen in terms of color, contrast, absence of motion blur and grey level. Color quality of images will be superior to plasma screen and power consumption can also be low. The slimness of ‘direct-lit’ backlight is not as good as ‘edge-lit backlight because of the ‘light box’ and mixing of colors. For large area of LCD TV (46”-65” diagonal) 100ds of LEDs are needed for both ‘edge-lit’ and ‘direct-lit’ modes. This involves ‘binning problems’ in the selection of LEDs and thermal problems in running at high brightness. White LEDs are cheaper than RGB LEDs. Direct-lit backlight employing RGB LEDs increase the cost of LED-lit LCD TV (LEDL LCD TV). White LEDs can also be employed in ‘direct-lit’ mode for the purpose of increasing the brightness and not color quality. This can reduce the price of LEDL-LCD-TV. LCD TV with screen sizes in the range of 40”-65” have been introduced with LED backlight. Sony and Samsung are the leaders in LEDL-LCD TV. Sony-JVC joint venture, Samsung, Vizio, Sharp and others are having their LCD TV models with LED backlight. The penetration of LEDL-LCD TV will also give a boost to LED backlight manufacturers like, Stanley Electric. Omron. Citizen, Nippon Leiz,,Toshiba Matsushita Displays,. WooYong,. Welly Power,. Several Chinese companies, Several small companies Forhouse. Taiwan Nano Electro-Optical Technology,.Kenmos Technology. Radiant Opto-Electronics Coretronic, Lumex, Forward Electronics. LED manufacturers like Lumiled/Philips, Osram, Toyoda Gosei, Cree, Epistar, Everlight, Seoul Semiconductors, Nichia and many others will be beenfited by this likely explosive growth. Going back to the chain, suppliers of MOCVD equipment for the manufacture of LED chips, like Aixtron and VEECO will gain increased orders for MOCVD equipment. This in turn will go back to the saphhire substrate and SiC substrate suppliers to the LED industry. Many small LED packaging houses stand to benefit in LEDL-LCD TV.
