Philips Master SOX-E and SOX

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High efficiency Low Pressure Sodium Lamp with tubular bulb for outdoor lighting with 18 to 131W power rating (SOX-E or SOX Economy) and 35 to 180W power rating (SOX standard)


  • 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:
    • Compatible reactive inductive ballast with matching power rating and starter
    • Compatible high-frequency electronic ballast (preferably)
    • May require power factor correction capacitor for reactive ballasts

Philips Master SOX-E 18W

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.

Fixtures that work quite well and were reasonably plentiful for sodium vapour lamps are outdoor lighting ones for compact fluorescent lamps, especially the Philips PL-L line or compatible products from other manufacturers, as their shape is similar. These fixtures can be easily recognized due to their rectangular shape and prismatic, clear diffuser. The lamp base is obviously incompatible and the power rating of the ballast or lamp gear may not be entirely suited but retrofitting is entirely possible.

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

Designation Base Lamp wattage Luminous flux Colour appearance Colour Temperature Colour Rendering Index
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
SOX 35W By22d 1SL BY22d 35 W 4550 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
SOX 55W By22d 1SL BY22d 55 W 7800 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
SOX 90W By22d 1SL BY22d 90 W 13700 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 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

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.

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
Very high light output Relatively bulky yet fragile lamps
Highest efficiency light source, suitable for outdoor use Partially directional, monochromatic light
Warmest light appearance Must be operated only horizontally, hard to find lamp base
Reasonable lamp warmup time Requires bulky, heavy, complex lamp gear


The low pressure sodium vapour lamps are quite an odd light source today. Few people remember or have experienced its widespread application in streetlighting except for northern European countries. They seem out of place, a specialist item, focused on the highest possible light output with many compromises. The most obvious compromise is colour rendering as the monochromatic light being generated cannot be pleasantly used to showcase any indoor setting except as a complementary accent light. On outdoor applications, the lamp is quite large, especially for the higher wattage models, making it difficult to find optimal fixtures. Handling must also be done carefully due to the risk of breakage and the lamp must be operated only horizontally due to the risk of sodium migration and failure. All these shortcomings mean that this lamp range mandates attention in servicing and handling, negating, for most purposes, its advantages.

In the end, despite all its energy efficiency, the low pressure sodium lamp is difficult to use and maintain, while colour rendering remains a very serious issue. Of course, if all year-round performance is required, in harsh, foggy or snowy environments is required, these lamps are perfect, but most use scenarios are far from being a good enough match for such a technology.

The situation is changed dramatically if the lamp is considered in terms of scientific or technical applications. For instance, it can be used for spectroscope or spectrometer calibration purposes, or where monochromatic light can be useful, such as in microscopy. In the case of light spectrum analysis, the closely spaced emission lines of the pure sodium vapour discharge, spaced only .6 nanometers apart, are a true performance test for any good spectrometer or spectroscope as no other light source poses such a challenge. For this reason alone, not counting other purposes, a low pressure sodium lamp is worthy in a collection.


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.


These lamps are mostly suited for outdoor applications as well as special purpose lighting in technical or scientific circumstances, due to the near monochromatic light that is being emitted and the well-known, very closely spaced, doublet of sodium vapour discharge.

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.


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. This lamp technology experienced some competition from mercury vapour lamps early on. While sodium lamps of that time fared slightly better in terms of efficiency, they were shorter lived than their counterparts. To the disadvantage of very poor skin rendition of the mercury vapour lamps, the sodium lamped fared slightly better with its monochromatic light. The only issue that would always remain in this competition is that low pressure sodium lamps were never as compact as mercury vapour ones so more efficient directional lighting could theoretically be achieved with mercury vapour lamps. Of course, this was not realized in practice as the low cost of mercury vapour lamps was also a criteria for fixtures selection which meant that system efficiency was always very poor on mercury vapour and only slightly less so on low pressure sodium lamps.

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 shrank, by the late 2000s, as high pressure sodium and metal hallide lamps became successful or when LED lighting became popular. It may seem strange that LED technology replaced the more energy efficient high pressure and low pressure sodium lamps but some aspects have to be considered. A good LED lamp design centers on efficient light distribution, that is almost 99% downwards, right from the light source. The development of electronic components brought many potential improvements in service life and output but these lamp gears were not as popular as conventional, magnetic ones, due to cost and labour requirements for updating fixtures. This meant that most light fixtures were used continuously with outdated technology, many decades. This is just one of the many reasons for which outdoor lighting was far from its theoretical potential and what made it seem that LEDs brought a revolution in this respect.

A typical lamp fixture is a compromise as all previous lamps that were generating light almost omnidirectional or slightly directional, as in the case of low pressure sodium lamps. If the fixture's diffuser was not maintained in a clean state and if water, dirt may get inside the fixture, as it commonly happens, the lighting system efficiency is also reduced. In practice, only about 60% of the designed light reaches the desired target with a new lamp and a 5-7 year old fixture. LED lighting fixtures can maintain virtually all the initial light output efficiency, as the very compact lens system is entirely sealed and remains sealed for the entire lamp life.

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 mostly in wealthier 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.