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.