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Pt.6: The Low-Pressure Sodium Vapour Lamp Electric Lighting The low-pressure sodium vapour lamp can be instantly recognised by its monochromatic yellow light. Widely used in road and security lighting, it is the most efficient light source manufactured. By JULIAN EDGAR The invention of a whole family of low-pressure and high-pressure mercury discharge tubes as possible light sources oc­curred in the period between 1890-1910. However, it took until 1920 for a discharge in low-pressure sodium vapour to be ob­tained, the main stumbling block being the required development of sodium-resistant glass. Even then, it wasn’t until 12  Silicon Chip the 1930s that such lamps began to have a commercial impact. In 1932, Giles Holst developed a low-voltage, low-pressure sodium vapour lamp. Working in Holland, he perfected a special glass that could withstand the highly alkaline affects of vapor­ised sodium. The lamp became widely used for street lighting in Europe and was introduced to the US in 1933 and in Australia in the late 1930s. Construction A low-pressure sodium lamp is similar to a fluorescent lamp in many ways. However, unlike a fluorescent lamp, a low-pressure sodium vapour lamp does not use the excitation of a fluorescent powder to produce the light. Instead, the sodium discharge itself produces the light. The lamp consists of an evacuated glass envelope which contains a U-shaped discharge tube. The outer glass tube is coated on its inner surface with indium oxide. This coating re­ flects most of the heat (infrared) radiation back to the dis­charge tube while still allowing the transmission of visible radiation. This helps keep Fig.2 (below): the luminous efficacy of the low-pressure sodium vapour lamp is better than any other common form of electric lighting - and has been for a very long time! (de Groot, J & van Vliet, J; The High Pressure Sodium Lamp). Fig.1: because of the use of a U-shaped discharge tube, the luminous intensity distribution of a low pressure sodium vapour lamp is not uniform perpendicular to its axis (Phil­ips Lighting Manual). Fig.3: the spectral distribution of a low-pressure sodium vapour lamp is dominated by two very close wavelengths - 589nm and 589.6nm. This gives the lamp no colour rendering properties (Philips Light Sources). the discharge tube at its required 260°C operating temperature. The discharge tube is made of soda-lime glass and is coated on its inner surface with borate glass. This ply-glass construc­ tion protects the soda-lime glass from the corrosive effects of the sodium vapour. The inner surface of the tube contains a number of small dimples, where the sodium condenses as the lamp cools after being switched off. If the dimples were not present, the sodium would condense during operation to form mirrors which would intercept the light and reduce the lamp’s output. The discharge tube contains metallic sodium of high purity. It is also filled with a mixture of neon and argon, which acts as a starting and buffer gas. In a similar way to fluorescent lamps, low-pressure sodium lamps have coiled tungsten wire electrodes positioned at each end of the discharge tube. These are coated with a mixture of oxides of barium, strontium and calcium. Most single-ended sodium lamps use a bayonet mount so that accurate positioning of the lamp automatically occurs when the lamp is placed in the luminaire. This is required because the light output of a single-ended sodium lamp varies around its perpendicular axis. Fig.1 shows this variation in the luminous intensity distribution perpendicular to the longitudinal axis of the lamp. Lamp performance The greatest advantage of the low-pressure sodium vapour lamp over other types is its luminous efficacy. Fig.2 shows the luminous efficacies of a number of different lamp types over the last century or so. It can be seen that the sodium lamp has an efficacy much higher than that of other commonly-used lamps. One of the reasons for this is the fact that low-pressure sodium lamps radiate almost entirely at two very close wave­lengths - 589.0nm and 589.6nm. This can be clearly seen from the spectral distribution curve of a Philips SOX lamp (Fig.3). Although this monochromatic output provides little or no colour rendering, the wavelengths of light produced are close to the peak sensitivity of the human eye - see Fig.4. In fact, although only about 35-40% of the input power is radiated at these wavelengths (compared with 65% at 253.7nm for a fluorescent lamp), the luminous efficacy of a sodium lamp is about twice that of a fluorescent lamp (see Fig.2). In addition to its high efficacy and long life, another advantage of the low-pressure sodium vapour lamp is that its monochromatic light gives better visual acuity than multi-spec­tral light. This means that the eye can better differentiate objects that are close together. This occurs because there is no chromatic aberration within the eye when viewing an object under a monochromatic light. The complete energy balance of a 180W low-pressure sodium lamp is shown in Fig.5. Of the 180W input, April 1998  13 Fig.4: the near monochromatic output may be poor for colour rendering but its output is very close to the wavelengths to which the eye is most sensitive. This factor is largely responsi­ble for the high efficacy of low-pressure sodium vapour lamps (Philips Lighting Manual). Fig.5: the energy balance of a typical 180W low pressure sodium vapour lamp: visible radiation - 63W; total IR radiation - 62W; convection and conduction - 55W (Philips Lighting Manual). Fig.6: a basic choke and starter circuit for a low-powered low-pressure sodium vapour lamp. The dotted components are used to correct the power factor and block high frequency switching signals (Philips Lighting Manual). Fig.7: a constant wattage ballast circuit, as the name suggests, keeps the power consumption of the lamp approximately constant during the lamp’s life (Philips Lighting Manual). 55W is lost by convec­tion and conduction, 62W is converted to infrared radiation, 63 watts of visible radiation is produced After switch-on, the lamp takes approximately 10 minutes to reach its stable operating condition. During start-up, it has a red appearance, the result of the neon gas discharge that ini­ tially occurs. This is short-lived because the sodium discharge soon takes over. A life of up to 18,000 hours is quoted for common low-pres­sure sodium lamps - about 18 times that of a normal general-service incandescent lamp. A life of 18,000 hours is the equival­ent of running continuously for about two years. Unlike a fluorescent lamp, temperature fluctuations have little affect on lamp performance. This is primarily 14  Silicon Chip because of the good thermal insulation of the discharge tube provided by the outer glass envelope. The lamp is also able to be used in very cold conditions - down to as low as -30°C when fitted with an electronic starter. Mains voltage fluctuations within the range of +6% to -8% also have very little affect on lamp performance. In fact, the change in lamp voltage is almost entirely balanced by a simulta­ neous change in lamp current, meaning that lamp wattage (and to a certain extent the luminous flux) remain nearly constant over a wide range of supply voltages. Control circuits As with other discharge lamps, a ballast is needed to prev­ent current runaway. Two main types of ballasts are used: (1) choke ballasts with or without a separate starter and (2) con­ stant wattage transformer ballasts with a separate starter. Sodium vapour lamps are quite short when compared with a fluorescent tube. Consequently, lamp voltages are relatively low and allow the lamp to be operated by a simple circuit such as the one shown in Fig.6. Here, a choke is wired in series with the lamp and an electronic starter is fitted in parallel with the lamp. The dotted components indicate a parallel capacitor for power factor correction and a filter coil which is fitted when high-frequency signalling via the mains is used. Ballasts of this type can be used with conventional sodium vapour lamps of up to 90 watts. Constant wattage ballasts maintain lamp power at the same value during the life of the lamp. Fig.7 shows a hybrid constant wattage circuit. It consists of a ballast, a series capacitor for power factor correction and an electronic starter. Street lighting A long lamp life, high efficacy and resulting low running costs makes sodium vapour lamps very suitable for road lighting. In addition, tests have shown that, as mentioned above, sodium lighting gives excellent visual acuity. In fact, if high-pressure mercury vapour lighting is used instead, the road surface lumi­nance has to be approximately 1.5 times greater than for low- pressure sodium vapour lighting to give the same visual acuity. Furthermore, compared to other types of road lighting, sodium vapour lamps give a greater speed of perception, less discomfort, less glare and a shorter recovery time after glare has occurred. While fluorescent, metal halide and high pressure sodium vapour lamps are also widely used for street lighting, low-pres­ sure sodium vapour lamps reign supreme on main highways. Road lighting luminaires are designed to direct light along the road length, with minimal lighting of houses lining the sides of the road. Their Downwards Light Output Ratio (DLOR) must be high - although one wouldn’t always believe this to be the case when viewing a city at night from an aeroplane! However, a road lighting luminaire with a very high DLOR often has poor light distribution, necessitating the use of closer pole spacing. Fig.8 shows an isolux diagram for a typical road lighting luminaire. The spacing of the poles, their height and their location are all vital parts of road lighting design. Fig.9 shows four different pole arrangements. A single sided arrangement (Fig.9a) is used only when the width of the road is equal to (or less than) the mounting height of the luminaire. However, this arrangement inevitably results in a lower level of luminance of the side furthest from the poles. A staggered arrangement (Fig.9b) is used mainly when the width of the road is 1-1.5 times the mounting height of the luminaires. This, however, can result in a zig-zag pattern of light and dark along the road. Placing the poles opposite one another down both sides of the road (Fig.9c) is used mainly when the width Fig.8: an isolux diagram for a typical street light. The lamps must be positioned such that the lighting is acceptably even along the road (Philips Commercial Lighting). Fig.9: typical lighting arrangements for two-way roads: (a) single-sided, (b) staggered, (c) opposite, (d) span wire. Each approach has particular costs and benefits (Philips Lighting Manual). of the road is greater than 1.5 times the mounting height of the luminaires. Finally, there is the rare approach of using a span wire (Fig.10d), where the luminaires are suspended from a wire hung along the central axis of the road. This gives excellent lumi­nance uniformity and less glare because drivers see only the blank ends of the luminaires. Next month: the high pressure soSC dium vapour lamp. April 1998  15