High-Pressure Sodium Lighting

GENERAL INFORMATION

GE introduced the first commercially available high-pressure sodium (HPS) lamps in the United States in 1965 and in the 400-watt size. Initially, and for some years, they were costly, and it was not until 1970 before 100 and 250-watt sizes came on the scene, offering more flexibility among installations. The primary advantage of HPS lighting was (and still is) very high light efficacy - twice that of mercury lamps and relatively long life. In addition they provide fairly good color rendition, superior lumen maintenance compared to mercury lamps and small arc tube size. The first lamps had an estimated life of 6,000 hours and could be operated only in specified positions for many years, such as base up only or base down. During the 1970s, particularly during the 1974 Arab Oil Embargo which dramatically increased electricity costs, their acceptance caught on and lamp costs gradually were becoming more affordable and as more of them were being produced. In addition, as time progressed during the 1970s some types were developed that offered more universal burning positions and all had life spans that were gradually lengthened to 12,000 hours and more. By 1980, lamp sizes had ranged from 35 to 1,000 watts; with most types attaining 24,000 hour estimated operating hours life, creating much competition with mercury lamps of the time. Beyond the early 1980s, lamp costs continued to dwindle as demand, and therefore the production of HPS lamps continued to increase. Luminarie costs also have been on the decline for a number of years. As of the late 1990s, HPS is generally the light source of choice among newly installed streetlighting applications in the United States, and has been gradually replacing former mercury and incandescent luminaries. Efficacies as of the late 1990s range from 64 lumens per watt for the small 35-watt lamp to 140 lumens per watt for the 1000-watt size. Common HPS lamp wattages are 35, 50, 70, 100, 150, 200, 250, 400 and 1,000. Medium base lamps are available through 150 watts; all are otherwise produced with mogul screw threads.

This chapter will focus upon several specific aspects of HPS lamps in some detail; in particular, how they are made and their operating characteristics. It should be kept in mind that the numerous manufacturers of these lamps have and have had differing methods of constructing methods, so the material that follows should be used only as a guide.

Materials and Construction

The requirement for a special glass tubing enclosure to resist the extreme chemical activity of sodium becomes increasingly acute as the lamp operating temperatures rise; even the best types of glass will discolor significantly under temperatures of 400 degrees C. It is necessary with HPS lamps to have a minimum temperature of 700 degrees C in order to obtain a proper pressure of sodium in the lamp's arc tube. The HPS lamp therefore had to await the development of a translucent ceramic material before experimental lamps could be made. There are now a number of translucent metal oxide ceramics with varying characteristics, and single crystal materials such as sapphire that are transparent, at least at low temperatures, and can be grown in tubing form.

The ceramic that has found satisfactory for the arc tubes within HPS lamps contains over 99 percent aluminum oxide. There were several basic problems concerning the material, as follows: First, initially, material costs were very high but have been significantly reduced in past years. Aluminum oxide is very abundant in the earth's crust but the processing problem had been quite a severe one. Arc tube ceramics now used are completely different than glass in that it cannot be softened or worked. These materials have a very high melting point. In addition, they have a moderately high expansion coefficient and can be readily cracked by thermal shock. Being translucent and not transparent, strain and its effect on lamp life cannot be assessed by polarized light. Finally, arc tube materials are very hard and can only be cut with diamond tools.

Manufacturing Techniques

In order to produce a reliable lamp from this material, lengthy research was required and a new manufacturing technique had to be developed. To this date, all the major companies manufacturing HPS lamps are making their arc tubes in different ways and each has its own advantages.

A 400 watt arc tube requires a piece of ceramic tubing 115 mm long with an eight millimeter bore and 75 millimeter wall, with an electrode support and lead in wire hermetically sealed into each end. The seal must be designed to withstand differences between ambient temperature and 700 degrees C and the seal materials must not be attacked by sodium at any extent. The manufacturing process must include the ability to exhaust unwanted gases and vapors and insert within the arc tube quantities of sodium, mercury and xenon. Sodium is added in considerable excess, to give saturated vapor conditions when the lamp is operating and allow for loss by absorption on the internal surfaces of the arc tube. Excess of mercury is also present, providing a buffer gas. Xenon at a low pressure is required to allow the lamp to start.

It is possible to use various sealing materials in order to attach end closures to the poly-crystalline alumina tube: glasses, other ceramics and metals have all been used. It has been general practice to use niobium for exhaust tubes and metal parts sealed to the ceramic tube, for niobium has an expansion near that of the ceramic employed.

The sealing of the arc tube has taken various methods:

1.During the manufacture of the ceramic tube it is possible to place close fitting ceramic plugs in each end so that during the final sintering process, crystal growth seals plug to the tube.

This process provides ready-made end caps but metallic leads and exhaust tubes have still to be sealed to the holes in the caps. It is known that some lamps used this type of end closure with niobium tubes in order to support the electrode assembly. The tubes are sealed in with a glassy material; the process ensuring that a minimum of sealant is left on the sealing surfaces and very little penetrates into the arc tube. The exhaust of unwanted gases and final filling of the arc tube with sodium, mercury and xenon is performed during this sealing process.

2.A second method of manufacture used a completely different approach. The ends of a plain ceramic tube were enclosed in a disc of niobium, which may have a small annular indentation for strain relief and location. At one end of the disc carries a tab to hold the electrode while at the other end it holds both an electrode and exhaust tube.

This lamp was assembled using a brazing material composed of rare metals. These metals were highly reactive at high temperature and etch the tube as they seal, producing a very strong joint. This tube was exhausted separately.

3. A third method of arc tube construction used a ceramic tube closed by metal end caps, which in this case were turned up at the edges. The cap to ceramic tube seal is again made with a brazing material similar to that in the above example but with lower melting point and reduced reaction with the tube's components. This type of seal has been designed so that only compressive stresses exist in the seal region providing satisfactory life expectancy.

There are obviously other types of end closures that have been and are employed among manufacturers of high pressure sodium lamps. Most HPS lamps produced in recent years have reservoirs within either sealing end (typically the lower) which stores the amalgam mixture of starting and operating materials inserted in the arc tube during the manufacturing process.

The use of niobium seems to be a common metal used for all metal seal parts. Its high melting point is a valuable asset and it has a suitable expansion coefficient; it is not attacked by sodium and can be easily made in the required shapes. A drawback, however, is that if it absorbs even small quantities of gas, particularly hydrogen, it will become brittle and unsuitable.

The seal manufacturing processes that a manufacturer employs are determined by the protection required for the electrode assembly and the niobium parts; the enclosing atmosphere being limited to pure inert gases or high vacuum. Seals may be made and in some cases the exhaust of the lamp performed in small enclosures; alternatively, seals may be made in bulk in large vacuum furnaces. When exhaust is performed, a separate machine seals the arc tube by cold welding then the exhaust tube is pinched with extremely high pressure applied by suitably shaped jaws.

The completed arc tube has external niobium surfaces and must be protected from the atmosphere. Therefore it is mounted and sealed in an outer envelope similar to that of mercury lamps. Mounting differs only slightly, but the arc tube with a temperature of over 1,100 degrees C expands considerably at its maximum temperature and therefore requires one end to be free for axial movement. There is no striking electrode but a suitable piece of special material that absorbs gaseous contaminants which otherwise would cause interior bulb darkening is sometimes mounted and fired. This "getter" as it is called also assures the maintenance of a high vacuum in the outer envelope during lamp life.

Outer envelopes generally are elliptical or ellipsoidal, or of the familiar bulged tubular design. All are made of heat resistant hard glass. Although the majority are clear, a diffusing layer of phosphor is occasionally employed upon the inside of the outer bulb to reduce glare, at a slight sacrifice in luminous output.

Operating Characteristics

The operation of the electrodes within a HPS lamp is basically similar to that of a mercury vapor lamp. The vapor pressure of the mercury-sodium insertion in the arc tube is very low at ambient temperatures, so as a result xenon is necessary to start the discharge and to operate it to full efficiency. It is noted there is no mercury radiation in the light output of the lamp despite the fact that the weight of mercury used is about four times that of the sodium. Some light emission occurs at temperatures as low as 1,250 degrees Kelvin (K). Raising the tube temperature increases the radiation until at about 3,000 degrees K; any further increased power input into the arc results in an equal increase in radiation and a gas temperature limit is reached. Mercury vapor with its high excitation and ionization potentials requires at least 4,000 degrees K in order to emit light and in a high-pressure sodium lamp this temperature is not achieved in the arc. Mercury vapor therefore only acts as a buffer gas within the arc tube.

Ambient temperature can affect lamp operation only to a small extent with more noticeable effects within an enclosed luminarie. The outer envelope of the lamp absorbs the long wave radiation from the arc tube and remains at a fairly steady temperature (275 degrees C to 465 degrees C, according to its size). If the radiated luminous energy is partially returned to the arc tube by reflecting surfaces it may raise the temperature of the seals, resulting in the rise of sodium vapor pressure and rise of arc voltage. This process will eventually extinguish the lamp. Luminaries with reflectors must take this into utmost consideration during their design.

As mentioned earlier, a starting electrode is not included in the HPS lamp. It would otherwise raise considerable design problems on account of possible electrolytic effects with sodium, and the risk of short circuit to the starting electrode probe by liquid metal would cause difficulties. Furthermore, for a third electrode to be completely effective, a special gas mixture would be required in the arc tube in place of Xenon. As the mercury vapor pressure is greatly decreased when the lamp is cold a neon-argon mixture as used in low-pressure sodium lamps would be necessary. This mixture would result in considerable losses in the discharge tube and lamp efficacy could be impaired as much as 20 percent.

It is generally accepted that in order to obtain the best life and light output the lamps should be operated with a starting pulse of 2,000 volts or more, applied for a few milliseconds to ionize the xenon filling and allow the arc to start from the voltage supplied directly from the ballast. The use of semiconductor devices to provide the high voltage "kick" to start HPS lamps is standard practice. Such devices are known as starting aids, igniters or starters and are wired as part of the ballast circuit. For this reason all HPS lamps must be installed in sockets that are able to withstand such high voltages. Like mercury lamps, there are several differing electrical configurations of HPS ballasts, each depending upon user requirements and lamp wattage. HPS lamps will only operate on ballasts assigned to their respective wattage. In addition, HPS lamps will not operate on mercury ballasts (except for special retrofit HPS lamps,) and mercury lamps will not operate as designed on HPS ballasts.

Upon power interruption a HPS lamp must cool for approximately one minute before it will restart. Under these conditions the ballast can only re-ignite the lamp once the lamp's restarting voltage has dropped to the point where the ballast can provide re-striking. Some HPS lamps are available among manufacturers that will instantly "hot restart" upon a voltage dip or brief power extinction, eliminating most of the otherwise momentary warm up time.

As a HPS lamp ages its operating voltage requirement slowly increases throughout its life until the point is reached where the ballast cannot provide enough voltage for full warm-up. Under this condition, the lamp starts and continues to brighten, while the necessary voltage from the ballast is supplied to the lamp to the point it no longer can do so, since the lamp requires more than the ballast can provide. As a result, the lamp extinguishes. As it does so, it cools and re-ignites again in a minute or so to the point where it cannot get enough ballast voltage, then drops out again. This is called HPS lamp cycling and is indicative of end of lamp life. Cycling HPS lamps have been known as a nuisance and technology has been developed whereby some specially designed HPS lamps automatically "quit" once they sense that the ballast can no longer provide full operating voltage. In addition, some manufacturers are producing HPS lamps with two arc tubes; the benefit derived is continued lamp operation after the first one has failed. Lamp cycling can produce undue wear on their starting aids so it is important for users to promptly re-lamp when this condition is reached.

The HPS lamp is by no means in its final form and developments are possible in several directions. Innovations during the last decade have provided lamps that have integral starting circuits, thereby eliminating the necessity of a starting aid within the ballast circuitry wiring. Also, HPS lamps that are providing more of a white light source are available. Although improvements among the latter are still forthcoming, the light emitted is considerably more attractive to the eye than standard yellowish HPS color radiation. While "white" HPS lamps with greatly improved color rendering qualities compared to standard lamps do not have the life expectancies of their regular counterparts, developments in coming years may lengthen their average rated lifetimes and provide improved whiteness of their light with even better color rendition as well.

HPS Retrofit Lamps

These lamps are designed for and limited for use with mercury lamp ballasts of some specified types, particularly most "lag" type mercury; 240 and 277 volt mercury reactor ballasts; and 6.6 ampere street series isolation transformers. Their operation is similar to the electrical characteristics of the mercury lamps they were intended to replace. Retrofit HPS lamps have an arc tube basically identical to standard HPS lamps and require no starting aid. Rather, to initiate the arc, the retrofit lamp has a carefully balanced mixture of rare gases in its arc tube, which operate in conjunction with a wire that is wound around the tube, assuring that the lamp will start at mercury lamp ballast open circuit voltages. HPS retrofit lamps run at a slightly lower wattage than the mercury lamp replaced, i.e., 150 watts on a 175 watt ballast and 360 watts on a 400 watt ballast, and so forth. These lamps are very efficient, producing almost twice the illumination at a wattage savings of 10 to 15 percent. Other advantages of these retrofit lamps include simplified maintenance since no starting aid is required, a re-strike time of about only a minute in the event of a power interruption, and the fact that HPS operation can be achieved by only changing the lamp and not the fixture.

HPS retrofit lamps require about three to four minutes until they reach full brilliancy, a little less time required compared to a mercury or metal halide lamp. Upon starting the arc tube emits a rather strong red glow, characteristic of the neon utilized as a starting gas. As the warm up continues, the arc tube turns to a bright blue and then to a yellowish color, similar to that of a low-pressure sodium source. As lamp temperature and pressure increase within the arc tube the light emitted gradually attains full brightness with the color appearance of a standard high-pressure sodium lamp.

HPS retrofit lamps are manufactured with a hard glass envelope in both the bulged tubular and elliptical bulb shapes, identical to those employed with mercury lamps. They are also very similar in construction to high-pressure sodium and mercury lamps and can be operated in any position.

These lamps have been available since about 1975 and owing to their much higher cost compared to mercury lamps, they only occasionally served in streetlighting applications through the years. As mercury luminaries became older and high-pressure sodium lamps and their luminaries became less expensive through the 1980s, it became more economically feasible to convert the luminarie instead of just the lamp.

Typical wattages for HPS retrofit lamps are 150, 215, 360 and 750, replacing 175, 250, 400 and 1,000 watt mercury lamps, respectively. Their estimated life is about 12,000 to 13,000 hours, except for the 360-watt size, which has a life expectancy of 24,000 hours.

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