How do SOX lights work?

Railway View, Clitheroe, Lancashire. November 1992.

Why do they shine yellow? When electric current is passed through vaporised sodium, light is produced. This light will always be yellow if sodium is used, provided that the pressure is normal and not raised to several times atmospheric pressure.

The SOX lamp is red when switched on because of neon, which is there to warm up the lamp from cold until the solid lumps of sodium have vaporised.

The above is a summary. Below are further details of the science and the engineering behind low-pressure sodium lamps. Further down on this page is a guide to how the lamps warm up in the first few minutes.

The science

The yellow light given off by a low-pressure sodium lamp comes from a tube of sodium vapour (at a temperature of around 260°C) which is excited by electrical energy. This sodium vapour contains millions of individual atoms. Each sodium atom contains a central nucleus of 11 protons (particles with a small positive electric charge) and 12 neutrons (particles with no charge); this nucleus is surrounded by orbiting electrons (negatively-charged particles — see Figure 1 ). The electrons of each atom are arranged in three shells as shown: 2 electrons in the innermost shell, 8 in the second shell, and a single electron in the outermost shell.

The outermost electron is the one that is the most loosely attached to the nucleus. When a sodium lamp is lit, an electric current is flowing through the lamp tube; this electric current can be thought of as a stream of high-energy electrons. When one of these electrons collides with an atom of vaporised sodium, the outermost electron of the sodium atom absorbs the energy that the electron that collided with the atom previously had. This excess energy allows the outermost electron of the sodium atom to orbit further away from the nucleus. In scientific terms, this is “excitation”, or “ionisation” if the excess energy causes the electron to leave the sodium atom altogether. Whether the result is excitation or ionisation depends on how much energy is received into the atom.

The electron then relinquishes the extra energy in the form of the characteristic yellow/orange light and returns to its normal state. Because sodium atoms are all identical in structure, the light given off is (more or less) all the same wavelength, or colour. This wavelength is 589 nanometres, where a nanometre is a millionth of a millimetre (strictly speaking, there are two wavelengths of light — 589.0 and 589.6 nanometres). 

The passage of electricity through a gas, especially where light is produced as a result, is referred to in physics as a discharge (or an arc); hence the sodium lamp is a discharge lamp. The light at 589.0 and 589.6 nanometres, together with infra-red (heat) radiation given off, make up 99% of the total radiation emitted by the lamp. (Both visible and infra-red light are types of “electromagnetic radiation”, which exists as several different types. In order of wavelength, from longest to shortest, electromagnetic radiation includes electric waves, radio waves, infra-red, visible light, ultraviolet, X-rays, gamma rays, and cosmic rays.) 

The sodium must be in vapour form. If you pass an electrical current through solid sodium, as is the case at room temperature, it will become heated and a magnetic field will be generated, but no light will be produced. The optimum temperature for producing light from sodium vapour is 260°C, and this is because the vapour pressure of the sodium is the best for light output (i.e. the efficacy is highest) at this temperature. Even a small variation in temperature has a large effect on the light output; therefore it is very important to accurately control the temperature inside the lamp. (ref: “Lamps and Lighting”, Hewitt & Vause; and James Hooker)

The parts of a lamp

Below: The parts of a side-entry Thorn Beta 5, the most common type of 35-watt lamp in the late 20th century. (Thorn datasheet)

Why are they called SOX lamps? “SOX” is part of a three-letter coding system used to indicate types of streetlights: 

• “SO” simply stands for sodium.

• “X” indicates a type of low-pressure sodium lamp that uses metal oxide as a coating on the inside of the lamp tube in order to reflect infra-red (heat) radiation back inside the lamp and hence reduce heat losses.

One of my Beta 5s in same orientation as diagram above. It dates from the 1970s or 1980s.	Inside one of my Beta 5 lanterns.

Different types of streetlight are often referred to using a two- or three-letter code. Low-pressure sodium lights can include:

SO - old design of lamp with U-shaped tube and a detachable Dewar jacket (a double-walled glass flask with a vacuum between the walls)

SOI - old design of lamp with U-shaped tube in an integral style, i.e. the inner arc tube and outer jacket are combined into one unit. These came out after SO, in the 1950s.

SOX - U-shaped tube with metal oxide coating to reflect infra-red radiation

SLI - old design of lamp with the tube in one straight length (i.e. not U-shaped). They were popular from the 1960s to the 1980s and were made by Osram, GEC and Mazda/AEI (later Thorn). See the section “Lamp data: sizes, current, voltage & efficacy" for more information about SLI lamps. (ref: Colin Grimes)

Table 1: Sizes and weights of current models of SOX lanterns (from company websites)

The SOX lamp tube

Here we look at the bits and pieces inside the SOX lamp tube.

The tube is U-shaped, as seen earlier, and contained in an outer glass envelope, with an electrode at each end of the ‘U’. This U-shaped tube contains sodium (in solid form at room temperature, looking like silver-coloured blobs) with a gas mixture containing neon and 5% argon, all in carefully-measured quantities. This assembly of tube and outer envelope is held in position at one end by a lampholder (see the “Cap” section below), and in the middle by the ‘lamp support’ which consists of a ring of coiled metal; this is all housed inside an outer glass bowl with the metal casing, or canopy, over it.

A Philips 35W SOX tube

More detail on the construction of a typical SOX lamp is shown below (reproduced by permission from James Hooker).

The parts of the SOX tube are described below:

Cap
 
The lamps are fitted with a bayonet cap similar to those used with domestic light bulbs.

Electrodes and emitter

The electrodes are a key part of the SOX tube and considerably affect the performance and life of the lamp. Electrodes can be subdivided into the anode (positive electrode) and cathode (negative electrode).

The electrode contains emitter which is applied during manufacture.  This emitter is a chemical substance, such as an oxide or carbonate, which normally has a white furry appearance on the electrode and which enables the current to enter the discharge area (by emitting electrons) with a minimum loss of energy, again to make the lamp more effective in light output.

The electrodes become hot when the lamp is started, in order for them to emit electrons more easily, and they remain hot while the lamp is running because of the temperature inside the tube (260°C). Because the electrodes are hot throughout their operating life, their emitter coating is continually evaporating and being ‘sputtered’ away (sputtering being the dislodging of atoms of material from the electrode).

When the coating on one electrode runs out, the ballast can no longer supply enough energy to operate the lamp, and this is the end of lamp life (electrode failure). Even if there is still emitter remaining on the other electrode, the lamp will only last for a matter of days! (ref: Allan Court, James Hooker)

Getter
 
The getter is there to maintain the vacuum in the outer envelope. It usually consists of barium, which is a soft, white, reactive metal; it appears as a dark film. This barium getter absorbs any trace gases, such as nitrogen and hydrogen, that may exist in the vacuum between the inner U-tube and outer jacket. By maintaining a good vacuum in this way, it helps to properly maintain the correct U-tube temperature and maximize light output, keeping to a minimum the power dissipated in the lamp. (ref: Philips website)

Philips have developed a SOX lamp using a “PSG” or Philips Solid-State Getter, which consists of a small tablet mounted on a stalk inside the outer jacket near the electrode end. It has been developed to overcome the problem of blackening in SOX lamps.

Right: The full range of SOX tubes from 180W down to 10W. (Colin Grimes)

Indium (tin) oxide coating
 
This is a film on the inner surface of the outer jacket which helps to reduce heat losses from the lamp by reflecting outgoing infra-red (i.e. heat) radiation back inside the lamp, while allowing the outgoing yellow light to pass through.
 
• SOX-E lamps: These have a film made up of slightly different substances and increased thickness, and they also have improved heat insulation around the U-bend (ref: James Hooker)

Dimples
 
These are a feature of Philips SOX lamps. They are there to collect the condensing sodium when the lamp is cooling down in order to try and maintain an even distribution of sodium along the tube, as this improves lamp performance.

Glass: Inner U-tube
 
The glass used for the tube is two-ply. The outer glass is soda lime; the inner glass is borate glass, especially used to withstand the sodium vapour which is highly reactive with other substances and which would attack most types of glass. Sodium-resisting glass can discolour slightly during the life of the lamp, causing some visible staining in old lamps.

The reason a U-shaped tube is used is that it simply means the overall length can be halved, and as the higher wattages of SOX lamps in particular are already very elongated, this is no bad thing!

Right: A Thorn Beta 8 in Wrexham (2004).

Switching on
 
Street lamps can be switched on either manually, through a timeswitch, or using a photocell. The photocell on SOX lamps usually looks like a little cone on top of the lantern canopy — the photocell detects when the light level has fallen below a certain value. This value is normally quoted as 50 lumens per square metre (or lux) to 70 lux for switching on at dusk, and 110 lux for switching off again at dawn; however, the actual behaviour of photocells can vary enormously, especially over time. The photocell can be regarded as being part of the control gear (see later section). There is also a delay mechanism in the lamp of around 60 seconds to allow for sudden changes of light level, e.g. lightning or headlights from a passing vehicle.

The neon/argon gas mixture

However, there is a problem before a sodium lamp can be switched on. At normal air temperature, sodium is solid and there is so little vapour present in the lamp it could not be started without the initial voltage being ridiculously high. (All substances which are solid or liquid can produce vapour — which is why you can smell certain substances when the vapour enters the nose — but the ‘vapour pressure’ is only high if the temperature is close to the boiling point of a substance. For example, you will get very little vapour (“steam”) coming from a basin of cold water, but plenty from a hot pan of water which is nearly boiling.)

Left: A Revo "Dalek" 90W SOX lamp warming up (Ian Young, West Wales Street Lighting)

The solution to this is the neon/argon gas mixture, as briefly mentioned earlier. Filling the discharge tube with a gas mixture greatly lowers the starting voltage required. “Rare gases” are what is needed — these are gases such as neon, argon, krypton and xenon, which are present in the air in very small percentages. If the lamps contained a pure rare gas, the initial voltage (‘striking voltage’) would still be quite high (several thousand volts) as gases do not conduct electricity easily, but this can be further reduced by using a mixture of two rare gases which are known as Penning mixtures. The SOX lamp is therefore filled with neon gas that has a small percentage of argon added, at a pressure slightly below normal air pressure. (ref. Mike Docherty)

Warming up

So when a cold lamp is switched on, the control gear provides a high voltage kick, which is required to overcome the initial high resistance of the gas mixture in the lamp's discharge tube, so that the production of light caused by the discharge process can be started by the neon/argon gas mixture. This discharge is shown by the neon red colour of the light immediately after switch-on.

Because of this high initial voltage, at the instant after switch-on, the SOX lamp consumes a much higher power than that normally quoted for when the lamp is in normal (stable) operation. For example, a 90W SOX would draw about 110W immediately after ignition, rise to a peak of about 125W and then gradually diminish down to 90W by the time it has fully warmed up. Once the discharge has been started inside the lamp, the resistance of the gases falls, which means that the required voltage for the discharge also falls. (ref: James Hooker, Don Klipstein, Simon Cornwell)

The initial colour of the streetlight depends on how old the SOX tube is. A new lamp will briefly (for the first second or two) give a pinky-purply colour when switched on because of an abundance of emitter on the lamp electrodes (potassium on the emitter causes the purple colour). An old lamp, in contrast, shines a steady deep red at switch-on, as the emitter coating is running out. (ref: Allan Court)

After the first instant the electrical discharge starts in the neon and argon, causing the familiar reddish glow. This discharge of neon and argon generates some heat and after a few minutes (when the temperature inside the tube has reached 120°C) the sodium metal begins to melt. Some of the sodium vaporises, and the light emitted by the lamp begins to change colour to orange. The colour change happens first in the area nearest the electrodes, as this area is the hottest, and the orange area then creeps along the tube. This starts to take place around 2 minutes after switch-on, although a lot of SOX lamps remain at an overall red colour for the first 3 minutes. After around 3½ to 4 minutes, the overall lamp colour is orange.

Left: "red-orange" - sodium light is starting to be emitted at the electrode end of the lantern. Bwlchgwyn, Wrexham, 2003.

The final deep yellow sodium light colour will form along the outside of the U-tube first, again because this area is hotter. After around 4½ minutes after switch-on, the overall colour is yellow-orange. By this time, the slight flickering will have finished.

The lamp will reach its final yellow colour typically after 5 to 5½ minutes. At this time there will still be some red/orange colour visible at close range in the centre of the U-tube, but this will be fading, and will all have gone within another minute. The tube will now be glowing yellow throughout apart from a small orange glow (again only visible at close range) at the electrodes, which will also have disappeared after around 7½ minutes. Finally, full brightness is reached after 8½ to 9 minutes. The actual timespan will vary a little from one lamp to the next.

There is actually only 0.1 to 1.0% sodium vapour in the tube compared with 99.0 to 99.9% of neon/argon. But the reason why sodium is ionised/excited more readily than neon and argon (once the sodium is hot enough in the lamp for ionisation/excitation to be possible) is that in a hot lamp it requires less input energy to ionise the sodium than the neon/argon. There is simply not enough energy to excite the atoms of gas. So even though the ratio is 1%:99%, the light emitted is almost entirely due to the sodium vapour. (ref: Don Klipstein, J. W. Denneman)

Gear
 
Like most other discharge lamps, low-pressure sodium lamps cannot be run directly on the mains electricity supply. Effectively, they have no electrical resistance of their own and so the current flow would rapidly increase until the lamp was destroyed. Gear (or ‘control gear’) is required for the safe operation of the lamp (ref: James Hooker) 

Inside the lamp are such parts as the ballast, capacitor and igniter, which together make up the gear. The photocell may also be thought of as part of the gear. [Spelling note: I often see "igniter" spelt as "ignitor" with an "o" — on this page I have used the -er spelling from the Concise Oxford English Dictionary, although -or may also be prefectly reasonable]

Above: Another Wrexham SOX lamp (2004). If you thought lamp-posts were always a boring shape, think again...

The ballast is there to regulate/stabilise the electric current, as any fluctuations away from a safe constant value would have a runaway effect on the discharge and could destroy a discharge lamp. The ballast is a large heavy rectangular object, rather like a brick.

There is also a capacitor, which is a device that can store electric charge (typically between two parallel plates or cylinders), and looks like a small white or silver canister in a light fitting. The capacitor ensures that the current is ‘in harmony’ with the voltage.

The igniter, as the name suggests, is there to get the lamp going when starting. It is also either a canister- or rectangular-shaped object. It produces the high voltage required to start the lamp when cold, as discussed earlier. However, the igniter's job is not entirely done once the lamp has switched on, as it continues to provide high-voltage pulses to the lamp until the arc has fully stabilised. (ref: Colin Grimes)

The gear can either be integral (i.e. contained within the body of the lamp), or remote (i.e. in the base of a lamp-post, or elsewhere).

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© Matthew Eagles 2005. Last updated 22nd August 2009.