A sodium vapor lamp is a gas discharge lamp that uses sodium in an excited state to produce light. There are two varieties of such lamps low pressure and high pressure. Because sodium vapor lamps cause less light pollution than mercury-vapor and other types of lamps.
Sodium discharge lamps have a similar action to mercury lamps, but the filling used is sodium instead of mercury. Lamps designated SOX and SLI work at low pressure and their luminance is low.
They therefore have to be very long to give a good light output; in order to reduce the overall length the tube of an SOX lamp is bent into the shape of a U .The SLI lamp has a straight tube and is therefore double ended.
To withstand the sodium vapor the inner tube is made of ordinary glass with a thin coating of special glass fused onto its inner surface. The inner tube is enclosed in a double-walled vacuum flask. Each electrode consists of a coated spiral whose ends are twisted together; there is no flow of heating current through the electrode as there is in a mercury lamp. Neon is contained within the inner tube with the sodium and starts the discharge. When the lamp is cold the sodium condenses and exists as small globules along the length of the tube. It is important that they should be fairly evenly distributed along the tube, and, therefore, the lamp must be kept nearly horizontal. At the same time, the sodium must not be allowed to condense on the electrodes. To satisfy this requirement luminaries for sodium lamps are arranged to hold the lamp tilted slightly above the horizontal.
The operating pressure is very low, being in the region of 1mm mercury, although the vapor pressure of the sodium alone is of the order of 0.001mm mercury, the rest being due to the neon. To start an arc through the neon when the lamp is cold requires a voltage higher than normal mains voltage (about 450V). The necessary striking voltage is obtained from an auto transformer which is specially designed to have poor regulation, that is to say the voltage when current flows drops greatly below the no-load voltage. Consequently, as soon as the discharge starts the voltage drops to that required to keep the discharge going. The transformer thus performs the functions of the ballast and no separate choke is required. A capacitor for power factor correction is, however, needed. The discharge which starts in the neon is of a red color. This warms the tube and gradually vaporizes the sodium. After about twenty minutes, the sodium is fully vaporized and gives its characteristic yellow color. The sodium discharge lamp is the most efficient means so far known of converting electrical into light energy, but because of its peculiar color the low pressure sodium lamp is limited to street lighting and similar applications. The control gear, consisting of auto-transformer and power factor capacitor, is usually accommodated within the column which supports the luminaire.
Alternatively, it can be housed within the luminaire and this is done in floodlights and other luminaries intended for mounting at low level. High-pressure sodium lamps give a rather sunny yellow light. For this reason when first introduced they were called solar colour lamps, but they are now more generally referred to by the designation SON. They are suitable for factories and warehouses and are now also widely used for street lighting and floodlighting. The pressure within the arc tube when the lamp is fully warmed up is between 30 and 60kPa. The lamp runs at a temperature of 1300°C and to withstand the corrosive properties of sodium at this temperature alumina ceramic is used in the manufacture of the tube
High pressure sodium lamps can work either vertically or horizontally. They may be mounted at any angle below 20° above the horizontal. The tube does not have to be as long as the low-pressure sodium tube and the construction and shape are similar to those of the mercury discharge lamp. High pressure sodium lamps have a very high efficiency and a long life. Most high pressure sodium lamps have rated lives of more than 24,000 hours. Both light output and the power consumed increase very rapidly with an increase in the supply voltage and the design of the ballast has to be such as to limit variation in the applied voltage in order to preserve lamp life. Figure 7.10 shows the variation of light output with voltage.
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