||Spectral lamps find a home in laboratory applications where they are commonly employed as stable, high quality sources of discrete spectral lines. Atomic spectra were originally produced either by creating an arc between electrodes fabricated of the metal to be studied, or by sprinkling a powdered salt into an ordinary gas flame. Both methods produce somewhat unstable results and require constant attention. In the 1940s a range of electric discharge lamps was developed to supersede these crude methods, and could guarantee superior results by virtue of the high purity of the metals contained within the discharge tube.
Lamps containing the alkali metals feature a discharge tube constructed from 2-ply glass tubing with an inner layer of silica-free borate glass, essentially the same as is found in modern low pressure sodium lamps. Such glasses are especially resistant to the highly corrosive nature of this family of metals. Simple electrodes of thoriated tungsten wire are employed, with a triple carbonate style emitter impregnated within the coils. The electrical feedthroughs are made of a special iron-nickel-cobalt alloy which resists attack from the metal dose. An argon gasfilling is employed for easy ignition, and two auxiliary starting electrodes are provided, one at each end of the discharge tube.
The construction of the sodium lamp is slightly different owing to the fact that a very high light intensity is desirable. Here the electrodes take the form of a coil of tungsten wire around a thoria pellet which has superior emissivity and can endure a higher loading with longer lamp life. A neon-argon gas filling is employed and ignition is accomplished with an internal glowbottle starter which preheats the electrodes.