Check the screen for existence of so-called "dead pixels". So called bad and

defective pixels are defects of the electronic device which is perceiving or

reproducing the image and having pixel structure. A more detailed explanation is

<a href="https://en.wikipedia.org/wiki/Defective_pixel">in Wikipedia</a>. Also see the

<a href="https://en.wikipedia.org/wiki/ISO_13406-2">ISO_13406-2</a> standard for more

information.

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