||The low pressure sodium discharge is a very convenient source of near-monochromatic light, which has a multitude of scientific and industrial applications. The actual radiation is almost totally radiated in the Sodium Resonance D-lines at wavelengths of 589.0 and 589.6nm but for most practical purposes this can be considered monochromatic light.
Many special optical phenomena occur under such conditions, for instance interference patterns can easily be observed such as in the classic Young's Slits experiment. The lamp also had applications in producing interference fringes which could be viewed to determine the flatness of a surface, the flow of liquids etc.
The GE "Lab-Arc" NA-1 lamp here is a scaled-down version of the 180W 10,000 lumen NA-9 equivalent which was once popular for street lighting in America. It is intended for operation inside a separate dewar vacuum jacket which is a part of the luminaire rather than attached to the lamp. The lamp is a short-arc low voltage type, very similar in construction to Philips' first DC operated sodium lamps. This lamp also requires preheating of the cathodes at each end of the bulb, before the discharge is struck in a neon-argon mixture. The glass is ordinary borosilicate which has had a sodium-resistant borate coating applied to the inner surface by liquid deposition, and is then fired into the glass. The seal area is in a lower bulb, separated by a mica disc to prevent sodium condensation in the other bulb half. Sodium dosing in all of the GE lamps was effected by exploding a small glass 'bomb' inside the bulb during the exhaust stage. This had been previously filled with sodium under vacuum and was exploded by heating with an RF induction oscillator.