1. A system for testing a display, comprising a test generator arranged to display a series of test patterns, each at a different luminance or colour, and with a predetermined minimum difference of luminance or colour from their background, each pattern being unpredictable to a user, and having a test evaluator arranged to determine and record if the user has correctly identified each of the patterns, wherein the test generator is arranged to display alongside each displayed test pattern a selection of candidate patterns for the user to choose a matching pattern.