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Pt.7: The High-Pressure Sodium Vapour Lamp Electric Lighting The high pressure sodium vapour lamp is widely used in industrial and commercial applications and in road lighting. Unlike the monochromatic yellow low pressure sodium vapour lamp discussed in Pt.6, the high pressure version produces light across a wide spectrum. By JULIAN EDGAR It was recognised quite early in the development of the Low Pressure Sodium (LPS) lamp that its colour appearance and render­ing would be improved with little loss of luminous efficacy if the internal pressure could be greatly increased. But before this could occur, a suitable material had to found for the arc tube. It had to 40  Silicon Chip transmit light, be resistant to the highly reactive sodium and be stable at high temperatures. The degree of difficulty in developing this material was enormous. LPS lamps were widely used by the 1930s but it was another 25 years before research into High Pressure Sodium (HPS) lamp yielded good results! The breakthrough came in 1959 with the development of a special ceramic material, polycrystalline translucent alumina (PCA), which transmits 92% of light and lacks the minute pores that would allow active sodium to pass through. The PCA material is also chemically resistant to sodium and can withstand the central arc temperature of 1500K. The first commercial lamp appeared in 1965 and was rated at 400 watts, 42,000 lumens and had a life of 6,000 hours. Today, a typical 400 watt HPS lamp has a luminous flux of 47,000 lumens and a life of 24,000 hours. Construction Fig.1 shows the construction of an HPS lamp. The inner PCA tube is translucent (not transparent) and is Fig.1 (left): a high pressure sodium vapour lamp uses an arc discharge tube made from polycrystalline translucent alumina. The tube contains sodium, mercury and xenon and is mounted within a glass envelope. (Murdoch, B; Illumination Engineering). Fig.2: the luminous efficacy of sodium vapour lamps varies with the internal pressure. At the left of the diagram is a low pres­sure sodium vapour (SOX) lamp while the SON plus, standard SON, SON Comfort and White SON are all high pressure sodium vapour lamps. (Philips Lighting Manual). held in place by a system of springs and support wires. The end nearer to the lamp cap is a sliding fit over the tube support, with a flexible electrical connector allowing the tube to expand when hot. The discharge tube contains an excess of sodium to give saturated vapour conditions when the lamp is running. Some mer­cury is present within the tube to act as a buffer gas. The tube also contains xenon gas to aid starting and to limit heat conduc­tion from the discharge arc to the tube wall. Feed conductors are made from niobium, which has a coeffi­cient of expansion close to PCA. The electrodes consist of rods of tungsten with tungsten coils wound around them. These are mounted at each end of the discharge tube, which is in turn housed within an evacuated protective glass bulb. The bulb is evacuated to reduce heat loss from the discharge tube and to eliminate corrosion of the niobium by air. Where the lamp is to be used with specially designed opti­ cal systems (eg, in a floodlight), the outer bulb Fig.3: as sodium vapour pressure increases, the colour rendering index (Ra) improves. It’s unfortunate, because as Fig.2 shows, luminous efficacy is reduced at higher pressures. (Philips Light­ing Manual). is tubular in shape. General purpose HPS lamps use an ovoid bulb. Some ovoid lamps have a diffusing coating of calcium pyrophosphate on the inside of the bulb which is designed to reduce glare. Note that this coating does not flu- oresce like the coating on a mercury lamp. As Fig.4 shows, the output from an HPS discharge tube con­tains almost no UV radiation. Lamp performance The performance of a HPS lamp is These are Sylvania High Pressure Sodium vapour lamps. The coating used on the inside of some of the bulbs is for diffusing purposes only. (Sylvania). June 1998  41 Fig.4: this piechart shows the energy balance of a typical 400W high pressure sodium vapour lamp. Of the 400 watts input power, 118 watts of visible radiation is produced. (Philips Lighting Manual). very dependent on the sodium vapour pressure in the discharge tube. Fig.2 shows the variation in luminous efficacy at various sodium vapour pressures, with the performance of four different Philips lamps indicat- Fig.5: initial current (I) is high while lamp power (P), lamp voltage (V) and luminous flux (φ) take around nine minutes to reach normal operating values. (Philips Lighting Manual). ed. The SOX lamp is a Low Pressure Sodium lamp and as can be seen, its luminous efficacy is very high. The four High Pressure Sodium lamps shown on the diagram are the standard SON and SON Plus, the SON Comfort Fig.6: on start up, the spectral output of the lamp is very red. This changes to the yellow of a low pressure sodium vapour lamp after about 10 seconds, then changes to the golden-yellow of a high pressure lamp. (de Groot, J & van Vliet, J; The High Pres­sure Sodium Lamp). 42  Silicon Chip and the White SON. It can be seen that the White SON has a lower luminous efficacy than the standard SON. As an example, the standard Philips SON50 (50W) High Pressure Sodium lamp has a luminous flux of 3300 lumens, while the 50W White SON has a luminous flux of just 2300 lumens. So why would anyone specify a White SON rather than a standard SON lamp? The answer is that the colour rendering of the White SON at Ra 83 is far better than the Ra 20 of the standard SON. High Pressure Sodium lamps that use lower pressure (are you following?) have a “golden yellow” appearance that correlates to a colour temperature of 1950K. The higher pressure lamps have a warm-white colour appearance, correlating to a colour temperature of 2500K. The relationship between sodium vapour pressure and colour rendering index (Ra) can be seen in Fig.3. Since luminous efficacy decreases with improved colour rendering, this must be taken into account when selecting the most appropriate HPS lamp for a given application. Is colour rendering or luminous efficacy more important? Fig.4 shows the energy balance of a 400W Philips SON-T lamp. Of the input power of 400 watts, 118 watts of visible radiation is produced. Interestingly, the spectral power distribution of a HPS coincides well with the plant sensitivity curve for photosynthe­ sis, meaning that there are horticultural applications for the lamp. Starting The HPS lamp is ignited by a high voltage pulse of 1.8 - 5kV, depending on the lamp type and wattage. Once ignition has occurred, it takes about 9 minutes before the lamp reaches stable operating conditions. Fig.5 shows the changes that take place in lamp current, power, voltage and luminous flux in the 12 minutes following ignition. What this diagram doesn’t show is the changing spectral output during this period. Fig.6 shows the characteristic changes in the spectrum with the increase in sodium vapour pressure that follows ignition. Initially, the lamp exhibits the red spectrum of xenon, the starting gas. This is followed within 10 seconds by the characteristic yellow spectrum of an LPS vapour lamp, which then gradually changes into an HPS discharge spectrum. If the mains supply is broken, the lamp has to cool down before re-ignition can occur. This takes about a This Philips floodlight is fitted with a tubular 150W high pressure sodium lamp and uses integral control gear. (Philips). minute. Where constant lamp operation is crucial for safety, a HPS lamp con­ taining two identical discharge tubes can be used. When one tube is operating, the other is off. If a mo- mentary power failure extinguishes the lamp, the non-operating tube will be ignited as soon as power returns, avoiding the normal one-minute cooldown delay. Some types of High Pressure Sodium vapour lamps have sufficiently good colour rendering to be used indoors in commer­cial lighting. (Philips). June 1998  43 Fig.7: to overcome the problem of delayed re-ignition of the lamp, the Sylvania 250 Standby (dotted line) uses two arc tubes within the one envelope. Only one tube is used at a time, allow­ing immediate ignition following a power cut. (Sylvania Lighting Solutions). Fig.8: a typical HPS lamp starter circuit (inside dotted lines). It uses a semiconductor switch to close a resonance circuit which generates a train of ignition pulses. These pulses are stepped up to the desired amplitude by a transformer which also forms part of the starter. Once the lamp has ignited, the starter automati­cally stops functioning. (Philips Lighting Manual). Fig.9: in this circuit, the electronic starter is connected to a tapping point on the ballast which acts as a step-up auto-transformer. (Philips Lighting Manual). This luminaire is suitable for mounting on low ceilings and can be used to illuminate food preparation areas, loading docks and the like. It can be fitted with High Pressure Sodium vapour lamps ranging from 150 to 400W. (Sylvania). Fig.7 shows the operation of this type of dual tube lamp. Control circuits Most HPS lamps are operated with a choke ballast and have an external 44  Silicon Chip starter. A series type of circuit is shown in Fig.8. In this circuit, a semiconductor switch closes a resonance circuit which generates a train of ignition pulses. These pulses are stepped up to the desired amplitude by a transformer which forms part of the start­er. Once the lamp has ignited, the starter automatically stops functioning. Note that the starter circuit must be located with 0.5 metres of the lamp otherwise the ignition pulses will be absorbed due to capacitive losses in the wiring. A so-called semi-parallel control circuit is shown in Fig.9. Here the electronic starter is connected to a tapping point on the ballast which acts as a step-up auto-transformer. HPS lamps with good colour rendering use a stabilisation unit that prevents colour shifts occurring as a result of mains voltage fluctuations or lamp aging. The distance between this type of control unit and the lamp must be kept to less than 0.3 metres. Next month, we’ll take a look at SC metal halide lamps