1. Apparatus for checking the integrity of visual display information wherein the visual display information represents an image comprising a plurality of pixels, the apparatus being arranged to; segment at least a part of the image into a plurality of segmented areas, for each segmented area, compare a at least one attribute of each pixel therein with at least one predetermined value, for each segmented area, determine a number of pixels which meets one or more predetermined criteria based on the comparison with said at least one predetermined value, for each segment, compare the determined number of pixels which meets one or more predetermined criteria with one or more reference values, and generate an error signal if the determined number of pixels which meets the one or more predetermined criteria in one or more segmented areas, fails to match one or more of said reference values.