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Pros: OLEDs enable a greater range of colors, brightness, and viewing angle than LCDs because OLED pixels directly emit light. OLED pixel colors appear correct and unshifted, even as the viewing angle approaches 90 degrees from normal. LCDs use a backlight and cannot show true black, while an "off" OLED element produces no light and consumes no power. Energy is also wasted in LCDs because they require polarizers which filter out about half of the light emitted by the backlight. Additionally, color filters in most color LCDs filter out two-thirds of the light; technology to separate backlight colors by diffraction has not been widely adopted.
OLEDs also have a faster response time than standard LCD screens. Whereas a standard LCD currently has an average of 4-8 millisecond response time, an OLED can have less than 0.01ms response time
Cons: The biggest technical problem for OLEDs is the limited lifetime of the organic materials. In particular, blue OLEDs historically have had a lifetime of around 14,000 hours (5 years at 8 hours a day) when used for flat-panel displays, which is lower than typical lifetime of LCD, LED or PDP technology—each currently rated for about 60,000 hours, depending on manufacturer and model. Toshiba and Panasonic have come up with a way to solve this problem with a new technology that can double the lifespan of OLED displays, pushing its expected life past that of LCD displays. A metal membrane helps deliver light from polymers in the substrate throughout the glass surface more efficiently than current OLEDs. The result is the same picture quality with half the brightness and a doubling of the screen's expected life.[35]
In 2007, experimental PLEDs were created which can sustain 400 cd/m² of luminance for over 198,000 hours for green OLEDs and 62,000 hours for blue OLEDs.[36]
The intrusion of water into displays can damage or destroy the organic materials. Therefore, improved sealing processes are important for practical manufacturing and may limit the longevity of more flexible displays.
Sufferers of some eye conditions including nystagmus are unable to use OLED displays for an extended period of time due to how the LEDS emit a non-constant stream of light. Nystagmus affects approximately 1 in every 5,000-10,000 people.
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The quality of the current iMac's screen compared to a LED screen would probably not be that noticeable. If you want a comparison, go to the Apple store and look at the current 24" iMac and the new Cinema Display. I doubt there will be a huge difference except maybe in terms of brightness.