There is provided an integrity monitor for a display system, the display system comprising an image source and a display. The integrity monitor comprises a controller configured to analyse sensed characteristics at one or more predetermined positions in an image area of the display system and to relate the sensed characteristics to predetermined characteristics of a stimulus, when injected into the system, thereby to determine the presence of a fault in the system. The sensed characteristics may be sensed optically or non-optically. The stimulus may be optical or non-optical.