There are 4 important properties for the substance used in a gas-discharge lamp.
1. For a sufficient gas pressure in the lamp, the substance must be either a gas or a metal with low boiling temperature, so that it will be vaporized by an electrical discharge.
2. The gas must not react chemically with the enclosure and with the electrodes, which prevents the use of most gases except noble gases and metallic vapors. Except for noble gases and metallic vapors, the lamps using other substances must not have electrodes, so they need a more complex and less efficient electronic system for producing a high-frequency AC discharge, e.g. using a magnetron from microwave ovens.
3. The ionization potential must be low for a good energy efficiency. Alkaline metals have low ionization potentials and low boiling temperatures, so they are better than noble gases and other metals.
4. The color of the light must be one where the sensitivity and the visual acuity are high. This narrows the choice to yellow light, i.e. to sodium, between the alkaline metals.
1. For a sufficient gas pressure in the lamp, the substance must be either a gas or a metal with low boiling temperature, so that it will be vaporized by an electrical discharge.
2. The gas must not react chemically with the enclosure and with the electrodes, which prevents the use of most gases except noble gases and metallic vapors. Except for noble gases and metallic vapors, the lamps using other substances must not have electrodes, so they need a more complex and less efficient electronic system for producing a high-frequency AC discharge, e.g. using a magnetron from microwave ovens.
3. The ionization potential must be low for a good energy efficiency. Alkaline metals have low ionization potentials and low boiling temperatures, so they are better than noble gases and other metals.
4. The color of the light must be one where the sensitivity and the visual acuity are high. This narrows the choice to yellow light, i.e. to sodium, between the alkaline metals.