Dead Pixel Tester

Screens have thousands of pixels, on a 1024x768 monitor, there are three cells for each pixel - one each for red, green, and blue - which amounts to nearly 2.4 million cells (1024x768x 3 = 2,359,296).

There's only a slim chance that all of these will be perfect; more likely, some will be stuck on (creating a "bright" defect) or dead (resulting in a "dark" defect).