||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 transition metal elements feature a discharge tube fabricated of quartz. This is because these lamps must be operated at much higher temperatures on account of the relatively low vapour pressure of the metals they contain (with the exception of mercury). The spectral output of these lamps is also enhanced at high pressure, because the proportion of invisible ultraviolet radiation is decreased, this increasing the relative intensity of the visible lines and making a more useful lamp. The thallium vapour lamp has a special construction on account of its very low vapour pressure and low power dissipation. The arc tube is enclosed in a metal cylinder for heat conservation, equipped with a tiny observation window also made of quartz.
Simple electrodes of thoriated tungsten wire are employed, with a triple carbonate style emitter impregnated within the coils. Electrical feedthroughs take the form of conventional molybdenum foil pinch-seals. An argon gasfilling is employed for easy ignition, and two auxiliary starting electrodes are provided, one at each end of the tube.