High- and Low-Pressure Sodium Lamp
Lamp that produces light as a result of an electrical discharge, generated between two electrodes, in a sodium vapor that is contained in a transparent bulb.
Sodium Gas Discharge Lamps
Sodium gas discharge lamps can be distinguished in two main groups: low-pressure sodium lamps in which the gas pressure is very low (0.7 Pa or 7.10−6 atm) and high-pressure sodium lamps in which the gas pressure is a factor 10,000–100,000 times higher (10–100 kPa); [2, 3, 4]. Because their construction and performance is in many ways different, they are dealt with in two different sections of this entry.
Low-Pressure Sodium Lamps
Low-pressure sodium lamps belong to the group of high-intensity discharge (HID) lamps, because they are available in high-light output (and thus high-luminous-intensity) versions. All low-pressure gas discharge lamps have in common the fact that they are long. Low-pressure sodium lamps are highly efficient lamps with a good lifetime but no color rendition at all. Their application is therefore restricted to those situations where color rendering is of no importance, as, for example, on motorways, on railway-marshaling yards, and in some security-lighting situations. Low-pressure sodium lamps are sometimes also referred to as LPS lamps.
Materials and Lamp Construction
Outer bulb with inner coating
Just as with the low-pressure mercury, fluorescent tube, and lamp, the power dissipated in the low-pressure sodium lamp largely determines the length of the discharge tube. Especially for the higher wattages (and thus lumen packages), the unfolded length has to be really long. To reduce the actual length of the lamp, the discharge tube of low-pressure sodium lamps is therefore always U shaped. Nevertheless, the highest lumen packages still require a lamp length of about 1.2 m. The U-shaped discharge tube is made of sodium-resistant glass and contains a number of small dimples, or hollows, where the sodium is deposited as a liquid during manufacture. After ignition, the discharge first takes place through the inert gas mixture. As the temperature in the tube gradually increases, some of the sodium in the dimples vaporizes and takes over the discharge, which then emits the monochromatic radiation. At switch-off, the sodium condenses and again collects at the dimples, these being the coldest spots in the tube. Without the dimples the sodium would, after some switch-on switch-off cycles, gradually condense along the whole inner tube wall, decreasing light transmission considerably.
Most modern low-pressure sodium lamps have cold-start electrodes. These consist of a triple-coiled tungsten wire, so that they can hold a large quantity of emitter material.
Apart from the dimples, the bend of the U-shaped discharge tube also forms a cold spot where sodium can condense and accumulate. The bend is therefore insulated by a heat-reflecting metal cap.
All low-pressure sodium lamps are provided with a bayonet-type lamp cap. This allows the discharge tube to be accurately positioned. This is critical, because the light distribution of a low-pressure sodium luminaire is dependent on the position of the U-shaped discharge tube.
A low-pressure sodium lamp emits approximately 40 % of the input power in the form of visible radiation. This is the highest percentage of all gas discharge lamps. The remaining part of the input power is lost in the form of heat.
System Luminous Efficacy
The luminous efficacy of the system is strongly dependent on the wattage of the lamp and ranges from 70 to 190 lm/W, for low and high wattages, respectively.
Low-pressure sodium lamps are available in the range from approximately 2,000 to 30,000 lm (corresponding wattage range, 18–180 W).
Apart from the normal cause of failure in gas discharge lamps (viz., electrode emitter exhaustion), low-pressure sodium lamps may also fail because of cracks or leaks in the long discharge tube or outer bulb. This may especially be the case in environments where there are strong vibrations as, for example, may occur in poorly designed road-lighting luminaires during strong winds. Leakage in the outer bulb disrupts the thermal isolation, which in turn means that not enough sodium will vaporize. As a consequence, the discharge will take place in the starting-gas mixture, so emitting only the corresponding deep-red light. Economic lamp life is around 12,000 h (based on a 20 % mortality rate). Some versions make use of special getter material to maintain a high vacuum, resulting in fewer failures during the economic lifetime of the lamp: the economic lamp lifetime is about 15,000 h (again, mortality rate of 20 %).
Lumen depreciation occurs through blackening of the discharge tube by scattering of the emitter material of the electrodes and by discoloration of the glass caused by the sodium. Depending on the type of control gear used, these effects are partially counteracted by a slow and gradual increase in the power dissipated in the lamp.
Electrodes and lead-in wires coming into contact with condensed sodium can eventually suffer damage. To prevent this from happening, low-pressure sodium lamps have restrictions as to their burning position. Base-down burning positions in particular have to be avoided. The restrictions are less critical for lower-wattage (viz., smaller) lamps, simply because they contain less sodium.
Run-Up and Reignition
As has already been explained, the sodium needs time to vaporize while the discharge takes place in the starting-gas mixture. The warming-up process takes about 10 min. Nearly all low-pressure sodium lamps reignite immediately. The exceptions are the highest wattage lamps (131 and 180 W), which restrike after 10 min.
Low-pressure sodium lamps cannot be dimmed. Dimming would decrease the lamp temperature so that not enough sodium would remain in the vapor state to maintain the sodium discharge.
The good thermal insulation afforded by the outer bulb ensures that lamp performance is almost independent of ambient temperature. Also, thanks to the starting-gas mixture, starting too is almost independent of ambient temperature.
The variations in lamp current and lamp voltage as a consequence of a change in mains supply voltage tend to cancel each other out, the net result being that the lamp wattage, and, to a certain extent, the luminous flux remain practically constant over a wide range.
Low-pressure sodium lamps are available in two versions, each with a different balance between efficacy and lumen output. There is a high-lumen-output version with a 10–15 % lower efficacy and a high-efficacy version with a 20 % lower lumen output, respectively. The high-efficacy lamps can be operated on HF electronic control gear, which further increases their efficacy by between 15 % and 35%. The length of low-pressure sodium lamp increases considerably with wattage: the 18 W version (the lowest wattage available) has a length of 22 cm, while the longest lamps (131 and 180 W versions) have a length of 112 cm.
High-Pressure Sodium Lamps
High-pressure sodium gas discharge lamps belong as well as low-pressure sodium lamps to the group of HID lamps, because they are available in high-light output (and thus high-luminous-intensity) versions. High-pressure sodium lamps are also referred to as HPS lamps. High-pressure sodium lamps, in common with all high-pressure discharge lamps, are relatively compact. By increasing the vapor pressure in a sodium lamp, the color rendering improves and the color appearance changes from yellow to yellow-white, albeit at the cost of a decrease in efficacy. However, the resulting efficacy is more than double that of a high-pressure mercury lamp. At its introduction in the late 1960s, a very efficient alternative was thus obtained for the many high-pressure mercury lamps employed at that time in road lighting. Today, road-lighting installations all over the world very often use high-pressure sodium lamps, although for some installations LED solutions have become an alternative.
By further increasing the sodium pressure, the color quality of the light improves to such an extent that the light becomes real white. These so-called white high-pressure sodium lamps have a lower efficacy but sometimes offer an acceptable alternative to halogen and compact metal halide lamps for accent lighting.
Materials and Construction
Sodium at high temperatures has the tendency to migrate slowly through quartz. Quartz can therefore not be used as material for discharge tubes of high-pressure sodium lamps. Discharge tubes made of ceramic material withstand sodium at very high operating temperatures. This is why, right from their introduction in 1966, high-pressure sodium lamps have translucent, tubular-shaped, ceramic discharge tubes. Since the ceramic tube is produced by sintering minuscule aluminum-oxide particles, one cannot look straight through the wall, although the light transmittance is more than 90 % (translucent instead of transparent material).
Sodium is introduced into the gas discharge tube as a sodium-mercury amalgam composition, which partially vaporizes when the lamp reaches its operating temperature. The sodium vapor is responsible for excitation and subsequent light radiation, while the mercury gas acts to regulate the voltage of the lamp and reduces thermal losses (buffer gas). The starter gas normally added is xenon. An exception to this is found in the case of low-wattage lamps, which have a built-in igniter in the form of a bimetal switch and a neon-argon mixture – the Penning mixture also used in fluorescent lamps – as starting gas. By increasing the pressure of the starting-gas xenon, the luminous efficacy of the lamp increases by about 20 %. However, to ensure proper ignition at this higher starting-gas pressure, an auxiliary ignition wire has to be added very close to the discharge tube. Some versions of high-pressure sodium lamps therefore have an internal ignition strip (see Fig. 7). From an environmental point of view, it can be desirable to have high-pressure sodium lamps that do not contain mercury. Such lamps are, in fact, available. In these mercury-free high-pressure sodium lamps, xenon is used not only as a starting gas but also as the buffer gas that regulates the voltage and reduces thermal losses.
The electrodes employed in high-pressure sodium lamps are basically the same as those found in high-pressure mercury lamps. They consist of a core of tungsten rod with a tungsten coil (impregnated with emissive material) wound around it.
The lamp caps employed for normal high-pressure sodium lamps are of the Edison screw type. The white high-pressure sodium lamps have a special bi-pin cap to ensure exact positioning in a luminaire.
During the process in which the lamp is evacuated, it is impossible to remove all traces of air and water vapor. During the operation of the lamp, minuscule particles evaporate from the glass and metals in the tube. All these traces would lead to an unacceptably short life. To remove them a getter is added that absorbs these traces. The getter is usually in the form of a small piece of solid material (see Fig. 7).
Some 30 % of the input power of a middle-range type of high-pressure sodium lamp is emitted in the form of visible radiation. Compare this with the 40 % of low-pressure sodium lamps and the 17 % of high-pressure mercury lamps.
System Luminous Efficacy
The efficacy of the compact white high-pressure sodium lamp varies between approximately 30 and 45 lm/W. The high-pressure sodium lamp with color-rendering index of 60 has an efficacy between 75 and 90 lm/W, and the normal high-pressure sodium lamps (with Ra around 25) an efficacy of between some 80 and 140 lm/W. The higher the wattage, the higher the efficacy.
Compact white high-pressure sodium lamps are available in lumen packages from some 1,500 to 5,000 lm (corresponding wattage range of 35–100 W). Normal high-pressure sodium lamps are being produced in the approximate range of 4,000–150,000 lm (corresponding wattage range of 50–1,000 W).
The lamp voltage of a high-pressure sodium lamp increases gradually with life. The chief cause of lamp failure is that the lamp voltage rises higher than the voltage output of the ballast, causing the lamp to extinguish. When this happens, the lamp cools down and the pressure in the lamp decreases so that the igniter can ignite the lamp again. After some minutes, the lamp voltage again increases too much and the lamp extinguishes again. So, the normal end of life of a high-pressure sodium lamp is accompanied by this so-called cycling effect. For those situations where this on-off cycling might be disturbing or would damage the ballast, special “self-stopping” gear is available that stops igniting the lamp once the high voltage at end of life is reached.
Normal high-pressure sodium lamps have an economic life of up to some 20,000 h (20 % mortality). The lifetime of white high-pressure sodium lamps is not determined by the moment of actual failure of the lamps but by the onset of too large a color shift of the light, which is caused by the gradual increase of lamp voltage. To greatly increase their lifetime, white high-pressure sodium lamps therefore employ an electronic voltage stabilizer integrated into their control gear. The economic lifetime of compact white high-pressure sodium lamps, depending on type, lies between some 8,000 and 12,000 h (based on a 20 % too-large color shift).
For the compact white lamps, lumen-depreciation values vary between some 20 % and 25 % (after 10,000 h). The normal high-pressure sodium lamps have a much smaller lumen depreciation of between approximately 5 % and 10 % (after 20,000 h).
Run-Up and Reignition
The high-pressure sodium lamp must be ignited by a high-voltage pulse, typically 1.8–5 kV. After ignition, the color of the light is initially white (discharge in the starting gas), changing to yellowish after some 20 s as the sodium amalgam gradually vaporizes and the vapor pressure rises; until after some 3–5 min, the nominal pressure and full light output are reached. Reignition of the hot lamp requires the lamp to cool down for about 1 min to allow the pressure to decrease to a point where the ignition pulse can again ionize the sodium atoms.
All high-pressure sodium lamps can be dimmed to a certain extent, depending on the type of dimming equipment used. Lower wattages (100–150 W) can be dimmed with special electronic gear, which allows for dimming to 20 %. Higher lamp wattages can be dimmed by including an extra inductive coil (ballast) in the ballast circuit. Lamp color remains virtually constant and lifetime is not affected.
The behavior of high-pressure sodium lamps during temperature variations differs from that of other discharge lamps because of the excess of amalgam used in the lamp. Although the outer bulb offers some degree of thermal isolation, the lamp manufacturer’s specifications should be followed in the design of luminaires as far as its effect on lamp temperature is concerned.
A 5 % mains-voltage variation has a 15 % effect on the light output of high-pressure sodium lamps. The same 5 % mains-voltage variation has a 5 % effect on lamp voltage.
As shown, high-pressure sodium lamps are available in three basic types: standard high-pressure sodium, an improved color version of high-pressure sodium, and a compact, white-light, high-pressure sodium lamp. The standard high-pressure sodium lamp is available in two forms: the standard one and an extra-high-efficacy version (with internal ignition strip). Most high-pressure sodium lamps contain a very small quantity of mercury, but there is also a version that is completely free of mercury.
For ease of replacement of high-pressure mercury lamps in existing installations with more efficient high-pressure sodium types, a special type of high-pressure sodium lamp has been developed. This type uses a neon-argon (Penning) mixture as starting gas and is fitted with an ignition coil surrounding the discharge tube. These features allow the lamp to be operated on a standard high-pressure mercury-lamp ballast.
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