Once a right to light exists, the owner of the right is entitled to "sufficient light according to the ordinary notions of mankind": "Colls v. Home & Colonial Stores Ltd" (1904). Courts rely on expert witnesses to define this term. Since the 1920s, experts have used a method proposed by Percy Waldram to assist them with this. Waldram suggested that ordinary people require one foot-candle of illuminance (approximately ten lux) for reading and other work involving visual discrimination. This equates to a sky factor (similar to the daylight factor) of 0.2%. Today, Waldram's methods are increasingly subject to criticism and the future of expert evidence in rights to light cases is currently the subject of much debate within the surveying profession.