The Low Pressure Sodium Lamp

Since its commercial introduction in 1932, the Low Pressure Sodium lamp has consistently maintained its enviable position as the most efficient light source available. In recent years, increasing competition from other discharge lamps is resulting in a slow decline in its market, but it continues to be specified for new installations, particularly in Belgium, the Netherlands, and other global regions in the vicinity of important astronomical observation sites in view of the greatly reduced light pollution it causes. The construction of a typical LPS lamp is illustrated in Figure S1.

For reasons of optimal efficacy the sodium vapour is kept at low pressure, which calls for a discharge tube of large dimensions having a relatively low operating temperature. This permits the use of ordinary glass types such as soda-lime silicate or borosilicate, however a protective layer of special borate glass is blown onto the inner surface of the glass tube so as to reduce the rate of attack by the chemically corrosive sodium vapour. To reduce the length of the long discharge tube it is customary to fold it into a U-shape, although linear designs also exist. The discharge tube is dosed with metallic sodium and also contains a rare gas filling, usually neon-based, which facilitates starting. The electric current is supplied via thermionic electrodes at either end, which are similar in construction to those of the low pressure mercury fluorescent lamp, but of somewhat heavier construction on account of the higher lamp currents. The discharge tube requires thermal insulation to ensure a high lamp efficacy, and this is provided by mounting it inside a secondary outer bulb. It is evacuated to minimise thermal conduction and convection losses, and in the more modern lamps is coated on its inner surface with an infra-red reflective film to minimise radiated heat loss. The outer bulb is equipped with either a bayonet or pin-type cap to ensure the correct alignment of the discharge tube in the optical system of the luminaire.

The reason for the remarkably high efficacy of the low pressure sodium discharge is not so much because its discharge converts electrical energy into visible light particularly efficiently, moreover it is due to the fact that the wavelength of light it radiates happens to be very close to the peak sensitivity of the human eye under normal viewing conditions. Figure S2 shows the energy balance of a typical low pressure sodium lamp, which reveals that in fact only about 30% of the input power is converted into visible light. This percentage is comparable with other modern discharge lamps. Figure S3 meanwhile illustrates its spectrum superimposed on the sensitivity curve of the human eye, which shows the proximity of its radiation to the most effective wavelengths.

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 S4 illustrates the elevating efficacy of each of the principal light source technologies over time, and there is no technical reason why low pressure sodium should stop here. Whether or not manufacturers find it commercially interesting to make the necessary investments in further improvements is another matter however! That fact alone is responsible for the flattening of the LPS line since the 1990s, when all further research on this technology effectively ceased.

It is interesting to note from this chart that since the inception of each light source technology, it generally maintains its relative 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 cost, 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.

Low Pressure Sodium light has a number of very unique properties arising from its 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 surface 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, but SOX offers a 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 mercury and special restrictions apply to the disposal of used lamps. A final advantage is that in the case of a momentary power supply interruption, the lamp will restrike as soon as the power is restored and no cooling down period 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, but only with the cap uppermost. Illumination with the cap down would cause an accumulation of sodium behind the electrodes, and the glass-to-metal seals in this region are a weak point which can fail in the presence of excess sodium.

No light source offers a full set of advantages, and the SOX lamp is certainly no different. One of its principal weaknesses is that no colour rendering is possible under its monochromatic light. Althogh that is not necessarily a limitation in outdoor lighting, and can even be beneficial in street lighting, its principal weakness is that its rated life is shorter than other types of discharge lamps. Typical installations have to be re-lamped every two years whereas the expensive maintenance schedule can be extended to three or four years with high pressure sodium, and the reduced maintenance cost of the latter can totally offset the cost of energy savings with low pressure sodium.

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 simply 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, i.e. the first 18 months of normal streetlighting service, have been completely eliminated. A three year maintenance cycle is perfectly feasible with this lamp which attains a survival rate of 92% after 12,000 burning hours. Even though there may be room in low pressure sodium technology to extend its lifetime further, manufacturers can earn greater revenues with more modern light source technologies and for commercial reasons it is no longer interesting to invest effort in the further improvement of the low pressure sodium lamp.

The monochromatic output of the low pressure sodium lamp precludes its use from all indoor general lighting applications, and all outdoor applications where colour rendering is required. This confines its use almost exclusively to the lighting of major trunk roads, where the high luminous efficacy often makes it the most economical light source - so long as maintenance costs are not so high that the longer-lived high pressure sodium becomes a more viable alternative. Security perimeter lighting around buildings is another important application, in which the monochromatic radiation is outstanding for allowing enhanced visiual detection of movement, changes in contrast etc. Owing to the fact that its radiation is monochromatic, it can easily be filtered out with a single wavelength filter plate, and this makes LPS the favoured source to reduce light pollution in the vicinity of astronomical observation sites. By placing a narrow bandwidth filter over the analysis equipment, the sodium radiation can be removed from the sky-glow while allowing all other wavelengths to pass. Perhaps their only important indoor application is in the general illumination of photographic laboratories producting black-and-white prints. The monochromatic yellow radiation is at a wavelength which will not cause the photographic papers to fog, and permits high illumination levels in the working environment