Temperature rise characteristic
The low pressure sodium lamp works by low-pressure sodium vapor discharge luminescence. At the beginning of the period after the lamp is activated, the discharge occurs only in the rare gas enthalpy, which can be seen from the magenta characteristic radiation emitted during discharge. The heat generated by the discharge of the rare gas causes the solid sodium in the discharge tube to be melted and partially vaporized, so that the sodium atoms participate in the discharge. Due to the lower excitation potential and ionization potential of sodium, the radiation emitted by sodium far exceeds that of strontium. When the discharge tube is hot enough, the color of the light radiation emitted by the lamp becomes a yellow color showing sodium characteristics.
It is known from experiments that as the supply voltage changes, the changes in lamp current and lamp voltage tend to cancel each other out, with the result that the lamp power is somewhat constant over a wide range of lumen output.
Restart feature after voltage interruption
Unlike a high-pressure discharge lamp, the same as a fluorescent lamp, the hot low-pressure sodium lamp in operation will immediately emit a full-light or a reduced luminous flux corresponding to the cooling time after a short interruption of current. This feature makes low pressure sodium lamps suitable for street lighting. If the interruption time is short, the lamp will immediately emit nearly all of the light after it is ignited. For high-pressure discharge lamps, after the current is interrupted, the discharge tube must be sufficiently cooled before it can be ignited again. This is a defect that high-pressure discharge lamps use for illumination.
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