* Turn the room lights on. Dell monitors are not validated in a dark room. All monitors are inspected per industry standard, which is with 150lux ambient lighting (similar to typical office lighting)
* Reset the monitor to Factory Defaults, page 43. The Brightness and Contrast should return to 75 for both
* Post pictures of the standalone BID (Built-in Diagnostics), page 50, and I will tell you if it meets our manufacturer specifications for overall uniformity

Thank you for reply, I did what you said.

This is  an Image after Factory Rest and Built in Diagnostic, 

Image from Built in Dignostic with room light on

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@Prasanna00 wrote:
Sometimes when I watch a movie I notice something like a wave cooming from the bottom, like tearing or shuttering.

 

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 - you mean something like this?

https://www.dell.com/community/Monitors/U2518D-tearing-stuttering/m-p/5169435/highlight/true#M112576

it's similar, but I update the firmware and now I do not see any 'wave'