Sodium vapor lamp

The vapors of Sodium are used in several type of luminescent Gas-discharge lamp under high pressure, the lamps with High pressure and low pressure .

History

The use of the sodium vapor as source of light goes back to the use of peat combustible where the orange light of the flame was wrongfully allotted to the Soufre. It is only in the middle of the 19th century, with the advent of the tubes with discharge under low pressure and of the arcs to the carbon, which one studies the use of sodium and his salts for lighting. It is however only about the years 1930 that the first sodium metal vapor lamps will see the day thanks to the development by Arthur Compton of glass to the borate resistant to the alkaline but as the steam pressure and the temperature of discharge are increased, the lamp worsened irremediably at the end of a few seconds. In 1932, Philips and Osram, respectively with the Netherlands and in Germany, market the first lamps of this type which will be used immediately for road lighting. With an apparent brightness of 55 lm/W, these sources were most economic at the time. The technology of these lamps enormously evolved/moved until in the years 1950, where their morphology changed then little. Their very bad returned rather broad colors and their dimensions limits their applications to the lighting of public highways. So it was very early planned to increase the steam pressure in order to dissipate more power per unit of length, and to enrich the spectrum emitted in order to make the light more pleasant to the eye.

A crucial aspect of these sources, which was the subject of considerable efforts of research and development, is the thermo isolation of the tube with discharge. The first lamps used a tube with discharge coupled with a Vase of Dewar transparent, similar to those present in the Thermos bottles. Although the thermo isolation was satisfactory at the time, these external bottles had the disadvantage of dirtying interior quickly. This problem was solved in the middle of the Années 1950 with a design monopièce where the tube with discharge is enclosed in a vacuum drawn enclosure. The thermo isolation was improved with the use of sheaths of glasses which made these lamps rather heavy and fragile. A major improvement was the replacement of these sheaths by a transparent film of Or, Argent or Bismuth deposited on the internal surface of the external bulb, thinking the radiation Infrarouge towards the tube of discharge. It is only at the end of years 1950 that one discovers that synthetic sapphire is resistant to the sodium vapors. Thus a first lamp with high pressure is manufactured in 1958 in the laboratories of Thorn, in Great Britain. However, it is only with development of the polycrystalline alumina tubes, and adequate sealings, that a commercial lamp will be born in 1964. Although the barrier of the 100 lm/W was reached with this technology, the use of these thin films posed the problem of the absorption of the light emitted by the electric shock. This problem was partly solved with the use of Oxyde of tin for film, then of Oxyde of indium and tin which made it possible to reach with the beginning of the year the 1980 200 lm/W, a limit which is to date unequalled.

The first lamps made in laboratory had a sodium and xenon filling, but for practical reasons of mercury was added.

Technology

Low pressure

The sodium vapor lamps under low pressure (LPS) are made up of a tube with discharge folded U-shaped and enclosed in a vacuum drawn external bulb. The tube with discharge is filled with a mixture Néon (99%) Argon (1%) under low pressure allowing the starting of the discharge and the heating of sodium up to 260 °C. The tube is manufactured containing glass sodocalcic covered with a thin layer of glass to borate, resistant to the vapor of the alkaline Métal. This tube is equipped at its ends with electrodes covered with oxides with Rare earths for a good electron emission.

The external bulb has a vacuum whose quality is maintained thanks to mirrors of Baryum located close to the casing. A pastille of Zirconium is often employed for to crack the vapors of Hydrocarbure S which can be present. Tin and an indium oxide film, a thickness of 0,3 micrometer recovers the interior of the external bulb. This coating is designed to think the infra-red radiations towards the tube of discharge.

High pressure

The sodium vapor lamps under high pressure (SOX) use others chemical compounds like the mercury for practical reasons; however only sodium is responsible for the light output, the xenon and mercury being used only to make it possible the lamp to start, and to fix the good electric properties of the arc. The sources under low pressure are characterized by a radiation quasi monochromatic orange, which is not the case of the lamps under high pressure where the interaction between the various elements gives a superimposed spectrum of bands and discrete lines. Thus the light of these lamps has a better quality because it contains other colors that the orange. However, IRC remains poor owing to the fact that the prevalent color relatively remains an yellow-orange saturated. It is this characteristic which gives to these lamps an excellent apparent brightness. For these two reasons, these sources light the vast majority of the roads and industries of the world.

Two types of lamps whose light is whiter were developed in the Eighties. The first type of lamp has a CRY of Ra65, compared with Ra20 for the traditional lamps, and a temperature of color of 2200 K instead of 2000 K. These changes are obtained by the increase in the temperature and the steam pressure, of which the side effects are a less good output and a fall lifespan. In spite of this change, the quality of the light is still not sufficient with a regular commercial practice. These lamps find their employment of center town and in the residential districts.

The second type of lamp has a pressure and a temperature more raised even, giving a light to which the color is close to that of an incandescent lamp. It is thus quite naturally, with a CRY of Ra80 and a temperature of color of 2500-2700 K, that these white lamps with sodium found an application in the commercial lighting, where one seeks to obtain a cordial environment. However, their output and their less lifespan do not give them a reliability for urban lighting.

The family of the lamps sodium vapor standard extends from 35 Watts to 1000 W, with an apparent brightness of 90 lm/W up to 140 lm/W, which in fact a source of choice for an economic lighting.

In the powers, there exist multiple different models:

In the clear tubular lamps standards, it there a:

50 W
70 W
100 W
150 W
250 W
400 W
600 W
1000 W

Power supplies

Except for the lamps of 18 Watt, all the models up to 180 Watt have a Tension starting higher than 250 Volt. So the majority of these lamps are fed by an auto-transformer with dispersion whose tension with the secondary in open circuit is of 450 V. Since the years 1980 there exist feeding systems says hybrid made up of a self inductance and an igniter high voltage. The coil is designed in such a way that the third harmonic of the current is important. The resulting wave of current is squarer than sinusoidal, property which increases the output of these lamps. These lamps are very practical to make push plants in any kind in interior.

Theory of operation

Ecological impact

See too

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