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Since its commercial introduction in 1932, the Low Pressure Sodium lamp has consistently maintained its enviable position as the most efficient light source available Present-style LPS lamps are known as the SOX type, and the entire global supply of approximately four million pieces per year is made exclusively in the UK by Philips at Hamilton, and by GE at Leicester Until June 2000, Osram also manufactured this light source at its Shaw factory near Manchester, UK An ancillary factory in India used to assemble lamps from British-made discharge tubes until recently Despite increasing competition from High Pressure Sodium and Metal Halide lamps, the market for this light source remains quite stable and it continues to be specified for new installations The construction of a typical SOX lamp is illustrated in Figure 1 below.
Figure 1 - A Typical SOX Lamp
SOX lamps aregenerally employed in streetlighting applications, primarily because they are the most efficient light sources available This means that they deliver more lumens of light for each watt of power than any other type of lamp SOX installations therefore have the lowest energy consumption costs which is of crucial importance when thousands of miles of roads must be lit and the electricity bills must be kept as low as possible The principal reason for the high efficacy is because the colour of the light is close to the maximum sensitivity of the human eye in normal viewing conditions. (See Figure 2).
Figure 2 - Sodium Light and the Human Eye Response
Low Pressure Sodium light has a number of very unique properties arising from this spectral output, many of which make it technically the most suitable light source for road lighting and these are covered in detail on the next page In addition,the large physical size of the lamp means that it has a low luminance so it is less likely to give rise to glare, and the low operating temperature permits the use of compact optical systems and lightweight plastic lanterns They are the favoured light sources for tunnel illumination, particularly in Japan and Korea where underground roads extending 10 miles or more are not unusual The long lamps may be aligned end-to-end to produce a continuous line of light and this almost totally eliminates the stroboscopic effect of driving past high brightness lights at speed Driver fatigue is drastically reduced there is a well proven link between low pressure sodium lighting and reduced accident rates in tunnels. Fluorescent lamps also lend themselves well to this application and are sometimes encountered, but SOX offers a longer-lived and more energy efficient solution.
Furthermore, the lamp itself is relatively inexpensive and can be operated on low cost electrical control gear Of increasing significance is the fact that it contains zero mercury, and can be easily disposed of as non-toxic waste without incurring extra expense at its end of life Most high pressure sodium and all other light sources employed in street lighting contain poisonous mercury and special restrictions apply to the disposal of used lamps A final advantage is that being a low pressure discharge lamp, its striking voltage is not sensitive to temperature as is the case for other discharge lamps Thus in the case of a momentary power supply interruption, the lamp will restrike as soon as the power is restored and no cooling down time is required.
The burning position is generally confined to the horizontal position±20°. Greater inclinations can result in the liquid sodium running down to the lower end of the lamp with the result that the upper part of the lamp becomes depleted of sodium vapour and efficacy is lost Vertical burning is permitted only for the low wattage lamps, and here they must be run cap up Illumination with the cap down would cause an accumulation of sodium behind the electrodes, and the glass-to-metal seals here are a weak point which could fail in the presence of excess sodium.
In addition to these advantages, SOX does have two major drawbacks No colour rendering is possible under this light source as will be explained in the next section, but of primary importance is the fact that its rated life is shorter than other types of discharge lamps Typical installations have to be re-lamped every two or three years whereas the expensive maintenance schedule can be extended to three or four years with high pressure sodium, and this reduced maintenance cost can offset the energy savings of low pressure sodium for certain installations The long life SOX-Plus lamp was introduced in 1994 to counteract this change, but has only been partially successful because the lamp was essentially unchanged, it just had a longer guarantee period The SOX-PSG lamp introduced in 2003 is a much more satisfactory solution in which premature failures before 6000 hours, the first 18 months of service, have been completely eliminated A three year maintenance cycle is perfectly feasible with this lamp since it attains a survival rate of 92% after 12,000 burning hours.
The lamp has been subjected to continual improvements in materials and manufacturing technology over the years, which has allowed it to unfailingly maintain its position as the most efficient light source available Figure 3 illustrates the elevating efficacy of several light source technologies with time, and there is no reason why low pressure sodium should stop here It is interesting to note from this chart that since the inception of each lamp technology, it generally maintains its position in the league of efficacies at all times - rarely overtaking or falling behind a competitive light source The sole exception to this is the semiconductor light emitting diode, which has only witnessed serious development in very recent times There is no doubt that this unique light source will overtake many of its competitors in its efficacy, however it remains to be seen whether or not LED's can be developed in other areas, particularly luminous flux and price, which would see them begin to threaten other light sources The prime position which Low Pressure Sodium holds at the top of this chart is expected to be maintained for many years into the future!
Figure 3 - Effects of Technological Progressions on the Efficacy of Light Sources
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