Hi friends, now a days a trend started all over the world LED TV. How many of LED TV users know that they are actually having LCD TV’s with LED back-light. Yes it is, unfortunately this is true. In this article we are going to understand the technology behind it.
It is a flat panel display which uses LED back-lighting instead of the CCFL (cold cathode fluorescent) backlighting used by most other LCDs. LED-backlit LCD TVs use the same TFT LCD (thin film transistor liquid crystal display) technologies as CCFL-backlit LCD TVs. Please not that picture quality is primarily based on TFT LCD technology, independent of backlight type. While not an LED display, a television using this display is called an “LED TV” by some manufacturers and suppliers,
As we knew there are different type s of LED’s are available in market, following LED’s may be used as backlight display:
- White-edge LEDs around the rim of the screen, using a special diffusion panel to spread the light evenly behind the screen (the most common use)
- LED array behind the screen, whose brightness is not controlled individually
- Dynamic “local dimming” array of LEDs, controlled individually (or in clusters) to achieve a modulated backlight pattern
What is backlight?
A typical LCD consists of liquid crystal material with transparent electrodes and polarizing filters. Applying voltage across the liquid crystal layer allows light to pass through in varying amounts. Therefore, to illuminate a visible display the majority of LCDs employ an external light source, or backlighting.
As LCDs do not produce light themselves as CRT (cathode ray tube), they need illumination (ambient light or a special light source) to produce a visible image. Backlights illuminate the LCD from the side or back of the display panel, unlike front lights, which are placed in front of the LCD.
Here we have source of backlight as follows:
- Several Light Emitting Diodes (LEDs)
- An Electroluminescent panel (ELP)
- One or more Cold Cathode Fluorescent Lamps (CCFLs)
- One or more Hot Cathode Fluorescent Lamps (HCFLs)
- One or more External Electrode Fluorescent Lamps (EEFLs)
- One or more Incandescent light bulbs
Now the question arise why LED backlight instead of well proved CCFL technology, It is just not only about power saving or cost parity but It is also the ability for LEDs to deliver superior picture quality with greater contrast and reduced motion blur that is increasingly important, especially as larger panel sizes and 3D TVs become more and more popular. This article will look at the demands these new features place on different LED backlighting schemes and the solutions provided by the sophisticated timing and control circuits designed into the latest LED driver chips.
Trend of LCD’s started in around 1990, was quite expensive initially. Those was the era of Cathode ray tube TV’s. Price different between these two was quite big enough to shoo away the consumer. It actually made a very less impact on sales irrespective of being much thinner, lighter and lower power consumption.
What is LED Backlight Technology?
LED-backlit LCDs are not self-illuminating. There are different methods of backlighting an LCD panel using LEDs, including the use of either white LED or RGB (Red, Green, and Blue) LED arrays behind the panel and edge-LED lighting (which uses white LEDs around the inside frame of the TV and a light-diffusion panel to spread the light evenly behind the LCD panel).
LED backlighting using “white” LEDs produces a broader spectrum source feeding the individual LCD panel filters (similar to CCFL sources), resulting in a more limited display gamut than RGB LEDs at lower cost. White LEDs usually employ a constant-current drive using a pulse-width modulation (PWM) for dimming effects. You can drive the LEDs in either parallel or series. The series connection needs a relatively high-voltage boost converter to produce enough voltage to illuminate a large LED string.
The emission spectrum of the backlight source and the transmission spectrum of its color filters determine the color gamut of the LCD display (i.e., the range of colors it produces). The problem with white LEDs is that their spectrum is not ideal for photographic reproduction because they are basically blue LEDs with a yellow phosphor on top. Their color spectrum has two peaks, one at blue and another at yellow.
LED-backlit LCDs have longer life and better energy efficiency than plasma and CCFL LCD TVs. LED’s don’t use mercury unlike CCFL backlight. LEDs can be switched on and off faster than CCFLs and can offer a higher light output, it is theoretically possible to offer very high contrast ratios. They can produce deep blacks (LEDs off) and high brightness (LEDs on). However, measurements made from pure-black and pure-white outputs are complicated by the fact that edge-LED lighting does not allow these outputs to be reproduced simultaneously on screen.
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