This paper describes some work, mainly of a theoretical nature, done at the Royal Aircraft Establishment, Farnborough, during the years 1946-1947. The immediate object of this work was to establish some general principles for the guidance of the Ministry of Civil Aviation Airfield Lighting Committee, which is the body charged with the task of deciding what visual aids shall be installed on the civil airports of this country. The work is far from completion and from the scientific point of view, this account of it must necessarily leave some loose ends, but it is felt that the method of attack and the underlying ideas may prove useful to other workers in this field. If this should prove to be the case, then international standardisation will have been brought a little nearer, and as will be made clear in this paper, this is almost as necessary for the visual aids as for the radio aids.