||Following the introduction of AEI's Linear Sodium lamp, it was quick to find an application in airfield lighting. The combination of high output and narrow diameter made it eminently suited to use in a parabolic mirror to produce a narrow beam of high intensity in runway approach lights, and the monochromatic yellow output was highly effective in penetrating rain and fog compared to other light sources. Rather inconveniently though it could not be used at night time because it caused too much glare to pilots, and LPS lamps cannot realistically be dimmed because the sodium vapour begins to condense out of the discharge.
This lamp was created to solve that problem, and allows the sodium lamp to be dimmed to suit the prevailing weather conditions and daylight level. A dimmable low pressure sodium lamp requires a varying level of thermal insulation, and that is provided here by a heater coil of nichrome resistance wire wound around a glass sleeve surrounding the discharge tube. The spacing between the turns must be kept constant so as to maintain a flat thermal profile along the lamp length, and this is ingeniously ensured by sandwiching a sheet of mica between the wire and the glass tube. The mica, originally a flat sheet, is held in its circular shape under compression of the resistance wire and as the wire elongates when heated up, the mica maintains enough force on the wire to prevent changes in the pitch of its coils.
The discharge tube is the same as that employed in the standard 60W general lighting lamp. End caps are machined from BTH's Sindanyo asbestos-substitute, and brass binding posts are used for the electrical terminals. These special Airfield lamps were made in both 60W and 200W ratings.