What do high-pressure sodium lamps have?
One thing these lamps have is a mixture of mercury and sodium, rather than just sodium. If only sodium was in these, the voltage across the lamp would be excessively low. Making the arc tube longer to increase voltage drop would also increase the watt-per-centimeter loss (explained below in section 8). A higher sodium vapor pressure would also increase the voltage drop, but would broaden the sodium’s emission band to the point that much of the spectral output is nearly infrared. This detracts from maximum most-visible light output. Also, a mercury-sodium mixture conducts heat less than pure sodium vapor. This reduces thermal conduction of energy away from the arc (The watt-per-centimeter loss). Another thing: Hot sodium is very highly chemically reactive. Some of the sodium is lost as the lamp ages, either permeating through the arc tube or chemically becoming part of it. Therefore, a surplus of sodium is included in the arc tube. The sodium vapor pressure is controlled by the temperat
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