Philips Master SOX-E and SOX

High efficiency Low Pressure Sodium Lamp with tubular bulb for outdoor lighting with 18 to 131W power rating (SOX-E) and 35 to 180W power rating (SOX standard) Features Long, clear tubular outer bulb with heat-reflective coating Internal low pressure sodium discharge tube BY22d base Color rendering index Ra of -44 (on all lamps) Typical lifetime of 18.000 hours Luminous efficacy of 129 to 176 lumens/watt (SOX standard) Luminous efficacy of 100 to 200 lumens/watt (SOX-E) Warm white colour designation (1700 K colour temperature) Horizontal operating position Lamp control gear Requires external gear and appropriate wiring: Reactive inductive ballast rated as the lamp May require power factor correction capacitor for balancing line load

Best retro purpose

Due to their yellow, monochromatic light, these lamps are rarely more than an option for lighting exterior areas. Although they are very efficient light sources, their light is unpleasant for just about any application. Their BY22d base is not as popular as others but large amount of lamps, fixtures and gears can still be found, used, on Great Britain, as this was a very large market. Fixtures can be easily adapted for low pressure sodium lamps, especially if they are made for linear flourescent lamps, as their gear can also operate them well, if rated at nearly the same wattage.

Understanding manufacturer data

Lamp light output is always measured in lumens. This is a way of measuring by averaging light output at a distance of 1 meter in an integrating sphere. This was carried out in lab environments and this information was mentioned in lamp datasheets and catalogues. While this information is accurate it should be considered only across similar lamp types.

Lamp life is presented in thousands of hours. It does not point at a specific moment when an installed lamp will not operate anymore but a statistical point at which some of the lamps may not operate, giving a rough estimate of useful life.

Ra8 colour rendering index, or simply colour rendering index is a way of expressing the typical colour rendering capability of a lamp. While it may be non-intuitive this is a computed average of brightness of certain coloured samples that are light by the lamp. The brighter they are, the more efficient is the lamp in this task. Most lamps do not have a continuous colour spectra so only some specific colours might look very bright and others look very dull. Colour samples are not intense reds, greens and blues but intermediately vivid colours that are focused on human skin colours and some fabrics or surfaces, meaning that a only very high colour rendering indexes are desirable for indoor home lighting. Sun light renders colours almost perfectly, having a value of 100 while typical lamps have a rendering index between 50 and 80, a good value being above 80 and a poor one below 50.

Colour temperature is another important detail. The value is presented in Kelvins and follows a theory that boils down to the fact that light can be produced by heating a metal up to a specific temperature. A camp fire releases light as the flame reaches around 1500-2000 degrees Celsius and a slightly higher value expressed in Kelvin. The designation is warm white for a value of around 2700 Kelvin, natural white with a value of around 3200-3500 Kelvin, cool white for a value of around 4000-4500 Kelvin, daylight for a value between 5000 and 5500 Kelvin, and cool daylight for a value above 6000-6500 Kelvin. There are cultural preferences that make some warm colour temperatures preferred in colder climates and cool colour temperatures in warmer climates. The most popular home lighting worldwide is mostly warm white, due to a comfortable, pleasant atmosphere that is close to the old incandescent lamp.

Technical details

Note: due to the way the colour rendering index is calculated, negative values are possible. Basically, every value below 0 means that the light is basically monochromatic and that no colour distinction can be made and objects are white if not saturated and black if fully saturated.

Designation	Base	Lamp wattage	Luminous flux	Colour appearance	Colour Temperature	Colour Rendering Index (Ra8)	Service Life (to 50% failures)
Master SOX-E 18W By22d 1SL	BY22d	18 W	1800 lm	Warm White	1700 K	-44	18000 hours
Master SOX-E 26W By22d SLV	BY22d	26 W	3600 lm	Warm White	1700 K	-44	18000 hours
Master SOX-E 36W By22d SLV	BY22d	36 W	6200 lm	Warm White	1700 K	-44	18000 hours
Master SOX-E 66W By22d SLV	BY22d	66 W	10500 lm	Warm White	1700 K	-44	18000 hours
Master SOX-E 91W By22d SLV	BY22d	91 W	17500 lm	Warm White	1700 K	-44	18000 hours
Master SOX-E 131W By22d SLV	BY22d	131 W	26200 lm	Warm White	1700 K	-44	18000 hours
SOX 35W By22d 1SL	BY22d	35 W	4550 lm	Warm White	1700 K	-44	18000 hours
SOX 55W By22d 1SL	BY22d	55 W	7800 lm	Warm White	1700 K	-44	18000 hours
SOX 90W By22d 1SL	BY22d	90 W	13700 lm	Warm White	1700 K	-44	18000 hours
SOX 135W By22d 1SL	BY22d	135 W	22600 lm	Warm White	1700 K	-44	18000 hours
SOX 180W By22d 1SL/SLV	BY22d	180 W	32000 lm	Warm White	1700 K	-44	18000 hours

Durability and Repair-ability

Generally, sodium vapour lamps were very durable but these special lamps are less durable due to their higher discharge tube stress. As sodium permeates the discharge tube and electrodes are worn quicker, the lamp's output and colour rendering degrades faster than on standard high pressure sodium vapour lamps.

Compared with modern LED lamps, these sodium lamps compare favourably but they have a much poorer light output, efficiency and require careful wiring. In most cases operational cost is lower as less powerful LED luminaries can be used since all useful light is directed downwards compared with the inefficient fixtures used on most sodium vapour lamps. Ballast and ignitors can survive tens of years if properly protected against direct rain or snow exposure. A complex circuitry means that lamp servicing is difficult and there are major risks due to improper wiring on servicing as large voltages may be present. No lamp wattage seem to have been particularly popular as the whole range was used sparingly in commercial centers. Ignitors can be potentially repaired but the lack of schematics means that they can be repaired only by experienced persons and are, thus, mostly discarded when not working.

Greatest features & flaws

Features	Flaws
High light output in very compact package	Moderate risk of glare
Pleasant warm light, highly suitable for indoor use	Not very high efficiency and service life
Mostly omni-directional light	Long restrike time, hard to find lamp base
Reasonable lamp start time	Requires bulky, heavy, complex lamp gear

Review

These high pressure sodium vapour lamps are compact light sources that are viable for indoor lighting. Their very good colour reproduction and warm light appearance made them very well suited for shopping areas. Even if home applications could have been viable, the high cost of lamps, associated gear and a lack of attractive fixtures meant that this market was never represented. Creating an even warmer atmosphere than even typical incandescent lamps, with a better showcase of reds and oranges, this light source fared very well in terms of visual comfort. The only downside of SDW-T lamps was the lack of an instant restrike design, which meant that this lamp should not be switched on or off too frequently, as this would also affect light. On the few commercial applications were they were most likely applied, they fared very well. Groceries such as vegetables, bread and other food products were very well and appealingly presented. The lamps themselves and gears were quite long lived. There was also a more compact SDW-T lamp design that had similar dimensions to low-wattage ceramic metal hallide lamps that was especially attractive in the early 2000s as the entire fixture was very compact and light control was very good. Of course, the advent of LED lamps that could create any light appearance with lower maintenance, heat and better energy control meant that SDW-T lamps were not required.

Fixtures are generally compact, considering the light output made available, but are generally heavy due to the bulky electromagnetic gear being used. There are directional and omni-directional fixtures that are suited to various purposes. Fixtures were compact, having a boxy design, with a symmetrical or asymmetrical lamp placement that was suited for flood or spot lighting. Generally, due to the cost of the lamp and application, the few fixtures that were designed or adapted from other compact metal hallide ones, had very good optical systems, with high quality mirrors and lenses.

This type of lamps, due to their technology and way of operation could be successfully used both with cheaper conventional magnetic gears as well as high performance electronic ones. Light is mostly created as a result of thermal emission of hot sodium vapours, which translates into a high inertia that hides most flickering and light variations that are unavoidable in high pressure discharge lamps. This meant that SDW-T lamps were excellent for indoor uses even on inexpensive lamp gear. Despite an overall excellent lamp, the only drawbacks were high lamp cost and lack of highly marketable fixtures that detracted even from some residential luxury lighting applications as well as general perception that was mostly shaped by the typical street lighting application of cheaper lamps, setting lower expectations about the technology. Of course, manufacturers were also to blame as there was no advertising on this lamp and its benefits except occasional installation showcases.

Operation

It should always be remembered that low pressure sodium lamps must always be operated only in horizontal position. Leaving them on in other positions increase the risk of the lamp having a premature failure due to sodium loss as well as longer or improper operation, which also affects life. Their construction allows quick restarts with minimal reduction in light output. However, frequent starts do stress the lamp electrodes and wear do increase the risk of sodium being released slower than it should, reducing service life. The lamp should be carefully handled as it is quite fragile, especially for the higher wattage ratings, their length making them vulnerable to breaking. If a lamp breaks it should be known that sodium is very reactive and immediately rejects any water, being on a surface or in the humidity of the surrounding air.

Suitability

In starglazing, these lamps were appreciated due to the fact that their light is easy to filter out. As a security light, or as a light source on pedestrian or car traffic, they are fine. These lamps release low amounts of heat so they are better suited for enclosed fixtures. Most of the time, however, such lamps can be used on outdoor areas, especially since they attract less insects in the summer months and they are not affected by light scattering in foggy or snowy weather.

History

This lamp technology was successfully developed in the 1930s and improved upon in later decades. The most important achievements were higher quality arc tubes and gas fillings. The same improvements were made on the outer envelope, ensuring reduced heat losses and sodium retention. The low pressure sodium lamp was actually at the center of development on the much more successful and attractive high pressure sodium lamp, some decades later. This lamp technology has the special distinction of being the most efficient light source ever devised, reaching over 200 luments/watt in the case of the last designs, such as on Philips SOX-E.

The most important competition, early on, was the incandescent lamp, since its light was warm as well while energy efficiency was much improved. The only drawback that sodium vapour lamps had was the requirement for an additional component, the ballast. In general, due to the warm yellow light, this lamps was highly successful in colder climates such as Great Britain and other northern hemisphere countries. Streetlighting was a very common application. As the oil crisis of the 1970s was taking place, there was a strong demand for improved energy efficiency. One research goal was improving the already successful lamp, reducing heat losses of the lamp, as it was already producing reduced light at low temperatures. Special coatings could be applied on the outer envelope, further reducing heat losses and improving efficiency. Of course, this goal was not very easy to achieve as the coating had to survive long term use, absorb as little light as possible and not affect the appearance of the lamp. Special lamp gears were also designed, that had electronic circuitry. This meant that higher energy efficiency was achievable through a combination of better power management, high frequency operation and smoother starts and warmups, giving rise to SOX-E lamps.

The market slowly shrinked as high pressure sodium and metal hallide lamps became successful.

was an improvement of previously developed high pressure sodium lamps, in the early 70s. It was noticed that increasing the pressure of sodium vapours in a discharge improves light output by creating a spectrum that is more appealing. As the emission broadens to reds and greens, colour reproduction improves tremendously, while efficiency slightly drops. Of course, highly resistant ceramic tubes had to be used to contain the sodium vapour discharge and its intensity. Both heat and the corrosive, permeating nature of sodium pose challenges to long service life and good operation. In order to further increase pressure and temperature to improve colour rendering the discharge tube is broadened and shortened. This increases the intensity of the discharge but increases thermal and electrical losses of electrodes and the design of the lamp gets more complex. As usual, sodium still leaks out of the discharge tube at high temperatures so the lamp design had to compensate for these losses with a special construction and gas filling. All these requirements meant that producing such a lamp was highly costly and good manufacturing consistency was mandatory.

The market for such lamps was small, favouring some indoor aplications especially in colder climate countries such as Great Britain, where sodium lamps were already appreciated. In commercial applications they were more successful when the typical tungsten halogen lamps very low energy efficiency and simple use was no longer acceptable, in late 90s and early 2000s. Development of improved arc tubes was still possible but was considered not viable as the market was not expanding to cover such research and production expenses compared with the more flexible application compact ceramic metal hallide lamps. These lamps were much more energy expansion and have much broader appeal than high pressure sodium lamps.

Manufacturing specificities

Most manufacturers have been releasing lamps for many decades. Lamps produced in the 70s and early 80s are probably not commonly found today. In the 90s and early 2000 Philips was manufacturing lamps in Holland and other countries, while in the last years they were doing the same thing in China. Quality was not severely affected as manufacturing quality was maintained and applications were demanding, used in wealthier as well as poorer countries. It should be noted that sodium vapour lamps were mass manufactured and considered mainstream products, meaning that quality and reliability had to be consistent.

Products manufactured by Philips were similar to General Electric and Osram, with very good quality and consistency and Japanese manufacturers such as IWASAKI deserve a special mention as they were creating excellent lamps. Philips lamps, however, were much more popular and are easier to find.