Airfield lighting differs from the other lighting systems described in this book in that, typically, the lights are not used to illiminate surfaces but are used as signals; that is, the luminaires are viewed directly. Perhaps the nearest case to this so far described is in road lighting where the road lighting luminaires are used to give visual guidance, but even here there main function is to provide a bright road surface. The main reason for not lighting the surface of the runway is that no part of the luminaires is allowed to protrude more than 150 mm from the runway, so as not to obstruct the aircraft. As a consequence of this low mounting height, the light would be incident at grazing angles. This would tend to exaggerate the appearance of small irregularities in the runway and, in any case, would give a very patchy appearance. Moreover, the lit surface would not have the high luminance needed for the airfield to stand out a distance from other lit surfaces, such as roads.