Early use of sodium light for the crossbars of the Calvert runway approach lighting system and experiments at the Royal Air Force station at Gutersloh during the Berlin Airlift gave rise to a strong preference among pilots for this type of light. However, controlled dimming is not practicable with the 140W lamp and its use was limited. When the linear-type lamp became available in 1959, the Air Ministry raised the question of its suitability, and as a result a special form of the lamp was developed in which a lamp oven was used instead of the vacuum jacket to maintain the arc-tube temperature.After experiments with other methods, a system of thyratron brightness control was chosen, and equipment was made up for trial at Stansted Airport, Essex, in 1961. The projectors each gave 20kcd intensity over ±4° vertically on their direction of aim, with a wide fan horizontally. Six stages of dimming were provided, each reducing the intensity to one-third that of the previous stage. It was possible to flash any or all of the lamps at any desired rate from 30 to 240 per minute.The trials confirmed the eminent suitability of the linear sodium lamp for approach lighting on all-weather runways, providing as it does a non-dazzling, distinctive, high- and low-intensity all-purpose light, with brilliancy control adjustable within an acceptable time response to suit the prevailing meteorological visibility. Two aerodromes are now to be equipped in Scotland.The technical and economic advantages of the development are of great interest to airport authorities throughout the world, not only in latitudes subject to fog and mist but also wherever dust storms and other disturbances impair a pilot's vision.