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Robbed this from WIKI.. Dont kill me xD
Response time is the amount of time a pixel in an LCD monitor takes to go from black to white and back to black again. It is measured in milliseconds (ms). Lower numbers mean faster transitions and therefore fewer visible image artifacts.
Older monitors with long response times would create a smear or blur pattern around moving objects, making them unacceptable for moving video. Long response times can be annoying to a viewer depending on the type of data being displayed and how rapidly the image is changing or moving. Many current LCDs' monitor models have improved to the point that this is rarely seen.
A figure of 8 to 16 ms for rise + fall times is typical. The response time was traditionally recorded at the full black > white transition which became the ISO standard for this specification on LCDs. Grey transitions are far more common in practice but in terms of pixel latency, they remained significantly behind the ISO transition. In recent years there have been a wide range of Response Time Compensation (RTC) / overdrive technologies introduced which have allowed panel manufacturers to significantly reduce grey transitions. Response times are now commonly quoted in "G2G" (alternately "GTG," meaning: "grey-to-grey") figures and specs of 6ms, 4ms and 2ms G2G are widely available. There are various names used for RTC technologies, and these vary from one manufacturer to another. Terms such as ClearMotiv (Viewsonic), AMA (BenQ), MagicSpeed (Samsung) and ODC (LG.Philips) are widely used to identify RTC enabled displays.
In comparison, a CRT displaying a picture with an update frequency of 60 to 80 Hz could be said to have a response time of 12.5 ms and upwards. However, as the picture is updated completely (and virtually instantly) each time the electron beam passes over the screen, CRTs do not have the same problems with smearing or ghosting. The same is true for plasma displays (however, both CRTs and plasma displays can have problems with flicker).
LCD screens with a high response time value are often unsuitable to play fast paced computer games. The pixel response time is often confused with the LCD input lag which adds another form of latency to pictures displayed by LCD screens. An LCD screen with high response time and significant input lag will not give satisfactory results when playing fast paced computer games or performing fast high accuracy operations on the screen (e.g. CAD). Manufacturers only state the response time of their displays and do not inform customers of the input lag value