LCD: Liquid Crystal Display
A LCD TV panel uses pixels made of liquid crystals that produce light and color when electric current is applied to them and a backlight passes through. The liquid crystals don’t generate their own light, which is where a backlight becomes necessary.
There are different types of backlight offering different image quality:
CCFL (cold cathode fluorescent): Early flat panel LCD TVs used CCFL lamps for backlighting, which were cheap and allowed for flat TVs to be affordable. However, the fluorescent bulb tended to reduce the quality of dark, deep blacks to dingy grays thus making contrast an issue.
LED (Light-emitting diode): More recent flat panel TVs use small LED backlighting to improve the problems with contrast. Therefore, LED TVs do not use a different display technology, they are in fact LCD TVs that use LEDs for illumination and backlighting. There are three main types of LED backlighting used in current LCD panels:
Edge-lit: Strips of LEDs lightning along one or more edges (sides, and/or top-bottom) of the display, using a special guide to spread the light evenly across the screen.
Pros: Allows for ultra-thin TVs
Cons: Edge lit screens can have problems with light visibly leaking from the edge of the screen, mostly a concern for the lower end LED TVs.
Direct-lit: A grid of LEDs across the back of the display.
Pros: Lower manufacturing costs
Cons: LEDs cannot be independently dimmed, which can limit picture contrast. Thicker TVs than edge-lit.
Full Array with local dimming: Similar to Direct-lit, a grid of LEDs across the back of the display, but with much higher number of LEDs which can be intelligently dimmed for darker areas of the screen and be bright when necessary
Pros: Golden standard for LCD backlighting, exceptional contrast generated by local dimming technology.
Cons: More expensive. Thicker than edge-lit TVs.
OLED: Organic Light Emitting Diode
Organic LED displays are a bit different than their liquid crystal competitor. Every individual pixel on the screen is capable of providing its own illumination, resulting in a number of picture quality benefits without the need for backlighting.
Most significant would the better black levels than a typical flat panel and a majority of LED TVs. These organic displays have the ability to turn off individual pixels resulting in complete black, as opposed to a dim gray with the best of LED displays
Pros: Self-illuminating pixels, no need for backlighting. Exceptional contrast and color. Better viewing angle. Thin TVs.
Cons: Difficult to build large screen TVs, expensive manufacturing even for mid-size TVs.
QLED: Quantum Dot Light Emitting Diode
A QLED TV is an LCD TV with Quantom Dots. These are very small particles that vary in size, ranging between 2 and 10 nanometers and when hit by light they are able to emit different colors according to their size. So, unlike OLED pixels that can emit their own light, QLED dots rely on a LED backlight.
Pros: Very good color definition, brighter and more vibrant colors, more cost-efficient than OLED technology
Cons: Contrast levels not in par with OLED, relies on LCD backlight
Plasmas use UV light created by igniting pockets of gas to excite red, green, and blue phosphors that produce the colored image. At launch, Plasma was the HDTV powerhouse for larger screen TVs. Incredibly deep blacks and bright, crisp displays were an extreme selling point. LCD had problems with consistent quality at large sizes, which made the larger screens prohibitively expensive by comparison
However, with the cost of OLED panels coming down, LED backlit screens already low and the backlighting, refresh rate and off-axis viewing enhancements of the past decade, LED panels have now officially pushed Plasma technology out of the market.